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Chapter XVII: The Oxidation of Olefins and Diolefins
498
Chapter XVII THE OXIDATION OF OLEFINS AND DIOLEFINS The Oxidation of Ethylene The main directions of the oxidation of ethylene (the simplest representative of the olefins) will be:
°2 = C2H4O C2H + 1 ° = CH 4 3CHO 2 2 C2H + 1 4 2
(XVII.1) (XVII. 2 )
C2H + °2 4
= CH 3COOH
(XVII.3)
C2H 4 + 02
= 2HCHO
(XVII.4)
C2H4 + 202
= 2CO
C2H 4 + 302
= 2C02
+ 2H 2O
+ 2H 2O
(XVII.5 ) (XVII.6 )
Processes (XVII.1) to (XVII.4) lead to valuable oxygen containing products: ethylene oxide, acetaldehyde, acetic acid and formaldehyde. Because of that, the choice of selective catalysts for the above reactions is of great practical importance. The synthesis of ethylene oxide is of special interest. This product is used for production of many valuable suhstances /1/. The hydration of C2H results in ethylene glycol which is an 40 antifreeze, a solvent, a plasticiser and a monomer for synthetic fibre. The interaction of ethylene oxide with alkylphenols, aliphatic alcohols, acids and mercaptans leads to surface-active substances. Other derivatives of C2H (for example, ethanolamines) 40 are employed in the chemical and oil-processing industries. Ethylene oxide readily enters various polymerization and co-polymerization reactions. Many pharmaceutical preparations and other chemical substances are produced using ethylene oxide. The possibility of employing C2H or its polymers as fuels for jet engines 40 is being examined.
499
A predominant method for the production of ethylene oxide in industry is the partial oxidation of ethylene over silver catalysts. This method was discovered in 1931 /2/. The complete oxidation of C2H into CO 2 and H20 on Pt was firstly observed in 4 1817 /3/. The hydrogen atoms in the H2C=CH2 molecule are bound to the bond is carbons by strong 6' -bonds. The energy of the C2H 3-H 1 104 kcal mol- /4/. The carbon atoms are connected by a double bond of energy 145 kcal mol- 1• One of these two bonds is a 6-bond, the second is a 6-bond. The latter is relatively weak, its energy being 63 kcal mol- 1 /5/. The ionization potential of ethylene (10.5-10.6 eV /4/) is lower than that of ethane (11.65 eV) while the polarizability of C2H 4 (4.2 cm3) exceeds that of C2H6 (2.6 cm3 ) /6/; i.e. ethylene is ionized or polarized more radily than corresponding paraffins. The elevated reactivity of ethylene (in comparison with the paraffins) is attributed to its unsaturation. The breaking of ~-C-C-bond or its polarization gives rise to the fast interaction of C2H with 4 catalysts and to the formation of various coordinative surface compounds.
The Oxidation of Ethylene Over Metal Catalysts The catalytic activity of metal films for the oxidation of C2H 4 (PO = 23, Pc H = 2.] Torr) decreases in the order: Pt:>Pd:>Rh> 224 >Au>W /7, 8/. The reaction on Pt proceeded at 50_100°C, on Pd at 50 0-1400C and on Rh at 120 0-155 0C; a slow process starts on Au above 200°C and on W above 230°C. Under the conditions mentioned, the activity of platinum exceeds that of palladium due to the somewhat lower activation energy (11.7 against 14.3 kcal mol-1). The main products with all the metals were CO and H In addition, 2 20. up to 3% acetic anhydride and acetic acid were formed on Pd /8/. The formation of CH on Pt was observed in earlier studies /3/. 3COOH The activity of metals supported on silica gel with respect to the oxidation of C2H4 decreases in the sequence: Pt »Pd >Cu >Ag /9/. Thus, the copper subgroup metals are less active than the platinum metals. The reaction kinetics on Pd are described by the equation:
500
r= kP
!
P
0
C2 Hi; 02
(XVII.7)
Some deviations from Eq. (XVII.7) and a retardation of the reaction are observed at higher conversions due to the catalyst poisoning by acetic anhydride and especially by acetic acid. Phis effect decreases with increasing temperature. The addition of CO 2 and H20 does not influence the reaction /8/. If the Pd surface is covered with Cl--ions, the initial rate of C2H oxidation at 155 0C becomes 5 times less than that on a 4 pure film while the CH yield increases to 15%. With Pt, par3COOH tial oxidation products were not detected but a progressive poisoning was also observed /7/. The following reaction mechanism has been proposed in references /7/ and /8/. The adsorption of 02 results in its fast dissociation into atoms on the surface. The catalytic reaction takes place between chemisorbed (0) species and gaseous (or weakly adsorbed) C2H molecules in two directions: 4
Ethylene is assumed to be oxidized rapidly into CO 2 and converted slowly into CH and then into (CH3CO)20, the latter being trans3CHO formed into CH by means of hydrolysis. 3COOH This scheme does not explain the nature of the breaking of the bonds in C2H (C-C or C-H) and the composition of the surface in4 termediates leading to deep or partial oxidation. An initial cleavage of one of the C-C-bonds in C2H with the formation of 4
501
is supposed in reference /8/; reacting with (0), this complex releases hydrogen (converted into OH-groups) and gives new C-O- and C=O-bonds. The mechanism of the oxidation of C2H can be discussed in more 4 detail. It is possible to assume that the intermediate (I 1) is transformed into more fully oxidized surface compounds; their desorption results in the final products of both mild and deep oxidation, i.e. the following consecutive chain of irreversible reactions takes place on a surface:
H CHr (0) ..
°2
-.LL(O )
2
(CH
3COOH)
r
JQ..4.(I
2
2)
(XVII. 9 )
~2(0)
fast
In the first step, the initial rupture of one C-c-bond in C2H 4 is likely to be preferable since its energy (63 kcal mol- 1) is significantly lower than'that of the C-H-bond (104 kcal mol-1) and this difference is not compensated by the difference in the energies of the new bonds formed. If the complex (I 1) contains one oxygen atom, "one-point" adsorption of the (CH 2CH20) radical is probable, for example, in the form:
This last complex can be isomerized into acetaldehyde or ethylene oxide. However, the adsorbed (CH or (C2H species formed 40) 3CHO) can readily be oxidized into (CH before they have time to 3COOH) be desorbed as CH or C2H If such isomerization takes place, 3CHO 40. the transformations (CH + (0)~(CH3COOH)~CH3COOH are 3CHO) similar to those /10/ in the surface oxidation of benzaldehyde into benzoic acid: (C6H + (0)~(C6HSCOO) + (H)----C 6H SCHO) SCOOH.
502
The species (1 2) can be formed from (CH as a result of 3COOH) several steps involving carboxylate and carbonate complexes, the initial interaction of (CH with (0) being the slowest step. 3COOH) The acetic anhydride found in the products is thought to be formed by means of the dehydration of acetic acid. Scheme (XVII.9) proposed here is not in contradiction with the parallel Scheme (XVII.B) because according to Scheme (XVII.9), CO 2 originates from C2H but not from gaseous CH 4 3COOH. Thus, ,'Ie can write the following reaction mechanism:
3
(XVII.10)
4) (CH
3COOH)
---CH
3COOH
+ (
o
)
5 )(CH )-(1 ) - - - 2C0 2 2+2H20+5 ( 3COOH)+(0
0
Assuming that the reaction occurs in an ideal adsorbed layer and that the (0) species are the predominant intermediates, one obtains for the above scheme:
(XVII.11 )
503
where r is the rate of the overall process, SCH COOH and SCO are :3 2 selectivi ties and g is the surface coverage with oxygen. At high excesses of O2 , when 8 ~1, Eq.(XVII.11) is reduced to Eq. (XVII.7) which describes the reaction kinetics on Pd. Since in the second step, the oxygen-catalyst bond is breaking, one should expect a decrease in catalytic activity with increase in qs. Fig.156 shows that this relationship is observed. Eqs (XVII.11) to (XVII.14) suggest a decrease in r and an increase in the selectivity to CH:3COOH with decreasing 8. The results of reference /7/ on the initial rates and selectivities for the oxidation of C2H over Pd partially poisoned with Cl--ions 4 support this conclusion. If the surface coverage not only of (0) species but also that of the (CH:3COOH) species, 8', is taken into account, one should expect decreasing values r with increasing values of 8'. In the 01
Fig. 156 - The dependence of the catalytic activities of the metals for the oxidation of C2H 4 /7, 8/ on qs: 1 - Pt, 2 - Pd, :3 - Rh, 4 - w.
700 400 500 500
'--_--I-_----''--_-I-_ _l....-_....!
o
40
20
extreme case, when adsorption equilibrium is established for acetic acid in the fourth step, the rate equation will be:
~~~
r=
~'0
+
V~ ~h'f
"'1
~ '2 ~ /t (! + bf/AC ~AC
)
(XVII.15 )
where bHAC is the adsorption coefficient for CH and PHAC is 3COOH its partial pressure. According to Eq. (XVII.15), the reaction rate
504
will decrease with increasing values of PHAC as is observed in practice. The second step may be a complex one involving, for example, the weak adsorption of ethylene on a metal surface free of oxygen and the SUbsequent interaction of the adsorbed C2H with (0). 4 As was mentioned above, the oxidation of ethylene over silver is a unique system giving high yields of ethylene oxide. In other cases (for example, when propylene is oxidized on Ag /11/ or pentenes are oxidized on V 20 /12/), the olefin oxides are produced 5 in small amounts. In industry, the oxidation of ethylene into C2H 40 is carried out at 2000_300 0C and 10-30 atm over silver catalysts supported on nonporous carriers (A120 , SiC, etc.). Air or pure oxygen are 3 employed as oxidants, C2H + 02 being mixed with recirculating gas 4 containing the ethylene which has not been reacted. Electronegative additives such as halogens promote the selectivity of Ag towards C2H the latter reaching 60-70% /13, 14/. The oxidation of C2H 4 40, over pure silver at 180°_220°C (3% C2H in air) leads to a selec4 tivity of 41-48% in C2H /15/. Various methods of preparation of 40 silver catalysts are described in reference /1/. The rates of oxida~ion of C2H over the (110), (111) and (211) 4 planes of a single crystal of Ag are close (differ by a factor of no more than 2.4) /16/. Silver films with oriented structure are recrystallized during catalysis, so that the catalyst acquires a disordered structure peculiar to the steady-state conditions of catalysis where the differences in initial activities of the planes disappear. The specific surface areas of Ag samples prepared by various methods (Table 73) differ significantly /15/. Despite careful purification of reagents, the silver prepared /15/ contains some impurities, especially of Cl-, which affect the reaction rates (Table 73). Chlorine was removed by repeated treatment of samples with ammonia solution. The oxidation of C2H was 4 made at 1800_220 0C (3% C~4 in air, 1 atm). The rate constant in the equation:
r=kP ~~
P
-aO
t.?~O
was used as a measure of the catalytic activity under steady-state conditions /15/.
Silver foil
4300-46000
Reduction of Ag2C03 with: diethylene glycol formaldehyde hydrogen glucose C2H + O2 mixture 4 Reduction of Ag20 with: hydrogen hydrogen peroxide in alkali solution Cathode deposition Decomposition of AgN0 2 in vacuum Silver heated in air at
Method of preparation
0.045
0.16
-
0.18
-
0.213
1.30
1.30 0.27
0.008
-
-
-
0.25
-
0.040 0.034
-
0.40
1.10
12.5 5.2
5.3
1.0 7.9 3.0
5.1
0.114 0.122
10.8
Not detected
-
0.28 0.16 0.20
'o-PEl
(l)~
al -P El
-
0.07
<1>1 ~-P
'H~(l)
I
Oal-P 'H(l)q
~-P
I
2.0 0.1
-
5.6
5.8
9.9 19.1
7.6 7.6
21.2
37.9 33.4 4.5 18.8 7.0
'o-PS
(l)~(l)
~-P
<1>1
Oal-P 'H(l)q
11 .1
12.1
10.9 5.3 6.3 12.3
al -P El
'H~
('(l-P -P(l)q
~-P
1
a
28.2 11.2
42.0
28.1
44.0
37.8 35.0 31.2 29.3 28.0
(l)al-P -P(l)q 'HM ttl -P
~-P
Rate constant Rate constant per 19 of Ag per m2 of Ag
al-P -P q
Content of C1, mg
0.28 0.15 0.22 0.42 0.43
Oal-P 'Hq (l)~ 'o-P S
~-P
<1>1
Specific surface area, 1m2 g-1
The Oxidation of Ethylene at 200 0C Over Ag Samples Prepared by Various Methods (Before and After Their Treatment with Ammonia Solution) 1151
TABLE 73
83 53
62
50
49
54 51 79 51 53
(I)
El
(l)~
.04-'>
~-P
Oal-P 'H(l)q
<1>1
I
-
48
42
48
43
43 44 43 41 46
ttl 4-'> El
'H~(l)
(l)('(l-P -P(l)q
~-P
Selectivity in C2H I % 4
C1l C1l
0
506
Table 73 shows that the rate constant per 1 g of i~ differs significantly from sample to sample. If the activity is related to unit surface of purified silver, the specific rates turn out to be nearly constant. This is an evidence in favour of the Boreskov rule on the constant specific activity of catalysts with the same cbemical composition /18/. The small differences observed in the k values should be attributed to some impurities (for example, in the Ag foil)which cannot be eliminated by the treatment with ammonia. It can be seen from Table 73 that chlorine decreases the activity and increases the selectivity of Ag. This electronegative additive as well as others (Cl, Br, I, S, Sc, Te, P and Bi) which also affect catalytic properties of Ag /1, 14, 19/ can be introduced into the gas mixture in the form of compounds decomposed during the catalysis (so-called gaseous modification). Thus, the chlorine-containing organic compounds are adsorbed and oxidized on silver giving AgCl /20/. Such a method results in a nonuniform distribution of additives along the catalytic layer. It is better to introduce an additive during the course of the catalyst preparation, for example, by way of coprecipitation of Ag2C03 and silver sulfide, sulfate, selenate, tellurate or chloride /21/. . The tracer method has been used to show /21/ that 40-100% of additives are accumulated on the catalyst surface in the last case. The above mentioned non-metals are likely to be present in the working catalyst in the form of anions (Cl-, s2-, S02-, Se0 2-,
Teo~-,
etc.). The dO~ing
4
4
of Ag with silver halides by their ad-
sorption from solutions or by way of gaseous promotion /22/ results in the formation of a phase of silver halide. Most of additives introduced into silver catalysts are mobile; these additives can move from the bulk of the catalyst to the surface, migrate along the surface and then be removed into the gas phase as a result of chemical reactions. The tracer method shows that the migration rates decrease in the order: I:> Cl-> SO~-/23/. Chlorine is removed into the gas phase quite easily while selenium is still retained at 220 0-3000C /21/. These effects should be taken into consideration when the reaction kinetics are being studied. Very small amounteof S, Se and Cl enhance the catalytic activity of Ag with respect to the formation of C2H40 (Te has a slight
507
effect) (Figs 157 and 158). At higher concentrations (at surface coverages with S > 1%, Te > 4%, Se> 10% and Cl> 7-8%), poisoning takes place. The optimum activity corresponds to small surface concentrations of additives: 0.1 % for S, 1-3% for Se, "" 1 % for Te and 5% for Cl (see Fig. 158, b). A decrease in the catalytic activity at high concentrations of Se, Te and Cl (,/ 10%) is accompanied by an increase in the selectivity to C2H which becomes higher than that for pure Ag (Figs 40 157, 158). According to reference /24/, an increase in selectivity takes place at rather high concentrations of additives of F, Cl, Br and I. In some cases, barium or calcium compounds are added
Fig. 157 - The activity and selectivity of Ag with additives of sulfur (1), selenium (2) and tellurium (3): k 1/(k1 )0 is the ratio of the rate constant of the reaction (XVII.1) for a given sample to that for Ag without additive; ,6S = (S-8 o) where S and So are the selectivities for Ag with and without additives /21/.
4-
2 !
4
2 - (,1(fft %) I
OJ
/
/0
/00
sarrace carerape/% to silver /1/. Electropositive elements (K, Na, Be, Ca and Ba) enhance the catalytic activity of Ag and lessen its selectivity to ethylene oxide /25/. In the course of the oxidation of ethylene, slow dissolution of oxygen in surface layers of silver occurs, affecting the steady-state composition and the activity of the catalyst /26/. The following important features of the reaction mechanism have been revealed. 1. In the general case, the oxidation of C2H proceeds by way 4 of a parallel-consecutive scheme:
(XVII. 16 )
508
Under typical conditions in laboratory studies and in industry, the parallel route prevails, the consecutive oxidation to CO 2 being significant only at elevated temperatures /1, 14, 19, 27/. 2. The overall reaction is inhibited by ethylene oxide and by CO 2 and H20 /27/. 3. At the temperatures which are close to those required for catalysis, ethylene is not adsorbed on pure silver but is adsorbed either on silver covered with oxygen or when the C2H + 02 4 mixture is admitted to the Ag /28/. The interaction of C2H with 4 adsorbed oxygen to form C2H 40 is reversible /29/. Under conditions of catalysis, not only the initial reagents but also the reaction products are adsorbed on the oxidized surface of silver /30-32/. 4. A lot of data suggest the existence of several forms of oxygen adsorbed on silver. Oxygen can be adsorbed in both the atomic and molecular forms which are negatively charged /19/. In references /33/ to /37/, the adsorbed oxygen species are described as surface oxides of silver, Ag~s)On where Ag(s) is surface atom of Ag and n is a number of oxygen atoms bound with the neighbouring Ag atoms. These oxides are randomly distributed on the surface and can be regarded as a two-dimensional solid solution. When [0]/ [Ag(s)]<~ , the oxide Ag(~)O )redOminates and the heat of adsorption is great. When [ 0 ] / [ Ag(s}~ , the above oxide is converted into higher oxides, Ag~s)02
or Ag~s)03'
The heats
of the conversion are small and practically do not depend on 8 • The reactivity of Ag~s)o towards reducing agents (H2 or CO) is low due to strong oxygen-silver bonds. In contrast to that, higher oxides at 200 o-300oC can rapidly oxidize hydrogen or carbon monoxide. Similar to the bulk oxides, the states of oxidation of (s)O 2' + 1 an d + 3 Ag are.• i n Ag(s)O 2 ' + 1,. '1n Ag(s)O 2 3' + 3,. '1n Ag2 (i.e. it may be depicted as AgIAgIII 02). 5. The distribution of surface silver oxides and of the heats of adsorption of O2 depen~on the additives. Thus, sulfur additives lessen the heat of adsorption of O2, decreasing the fraction of oxygen which is adsorbed in the form of Ag~s)o /33/. 6. IRS studies suggest /30/ that the rupture of the C-C-bond in ethylene occurs during its adsorption on Ag covered with oxygen; this leads to the formation of a surface compound (-CH -GH 2 2-0-). The latter is assumed to be an intermediate in the formation of C2H 40.
509
Milo /5 III
o
~ -J -to
LlS/% .~- - - - - -e
to
s:IJ
;:~:~
I
4-
-!ST, %Cf
J I
!
0.1
(
I
10
Fig. 158 - The activity and selectivity of Ag with various amounts of additives of Cl- (see Fig. 157). The conditions of pretreatment of the samples and the conditions of rate measurements are different in the cases "a" and "b" /21/
~
7. The selectivity is independent of Po /P R over quite a wide 2 range of reaction mixture composition. According to reference /29/, oxygen is adsorbed on Ag in the form of negative ions; a collision of C2H with ODe atom of adsor4 bed oxygen leads to C2H while the interaction with two (0) spe40 cies results in the formation of CO 2 including HCHO as an intermediate. A consecutive route for the reaction is also supposed: it is assumed that C2H is converted into CO 2 and H20 by isome40 rization and oxidation of CH by the formation and further 3CHO, oxidation of adsorbed organic residue or by direct oxidation of C2H The above mechanism belongs to type (XIV.28) which suggests 40. that a decrease in the selectivity to C2H will occur with increa40 sing Po /P R• This is in contradiction with the experimental data 2
obtained. In some papers, different routes for the oxidation of C2H are 4 attributed to different forms of adsorbed oxygen. Thus, C2H is 40 associated with (02) while CO 2 is associated with (0) species /19, 38, 39/. Just this peculiarity is assumed to explain the specificity of silver as a unique catalyst for the synthesis of C~40 since only with Ag are the (02) species produced in significant concentrations at the temperatures of the catalytic oxidation of C2H In contrast to this, it is supposed in reference 4• /40/ that C~40 is formed from (0) particles while (02) species lead to CO 2• Finally, many mechanisms involve /1/ only (02) species as intermediates for both C2H and CO 2• 40 Thus, a number of "mixed" mechanisms have been proposed,all of them including the participation of different forms of adsorbed oxygen (and other surface compounds). The mechanism proposed by M. Temkin et ale /35, 37, 41, 42/
510
seems to be the most comprehensive one as it is supported by various experimental evidence (primarily, by kinetic data). In the earlier papers of these authors /35,41,42/, the surface oxide Ag~s)02 was regarded as a major intermediate. The higher oxide Ag~s)03 produced by way of the O2 adsorption:
(XVII. 17) was considered to be decomposed readily according to the equation: (XVII.18) The overall mechanism /35,41,42/ will be:
N(I) 1 1) C2H 4 + (0)~(C2H40) 2) C2H + (0)~(CH2CHOH) 4 2') (CH 2CHOH) + 5 ( 0 ) - 2C02 + 2H 2 0 + 6 ( ) 0 1 3) C2H 40 + ( )~(C2H40)
°
4) 02 + (
) -(02)
4') (02) + (
) -2(0)
5) C2H + (0) ~ (C2H 402) 40 6) H20 + (0) ~ (20H) 7) CO 2 + (0) ~
N(I I ) •• C2H4 + .302
(C0.3)
= 2CO 2
o
1/2
.3
1/2
.3
o
°o
°o
(XVII. 19 )
°
+ 2H 20
The reaction is assumed to follow two routes, the chemical equations of which are (XVII.1) and (XVII.6). Oxygen is adsorbed irreversibly. The (0) species in (XVII.19) correspond to Ag~s)02' then (
) refers to Ag~s)o
when [0 ] / [ Ag(s)] "7 ~
and (0 2) corresponds to Ag~s)03. Since, , the heats of the adsorption of 02 are in-
511
dependent of 8 and the surface is considered to be uniform. This allows one to use the theory of kinetics for an ideal adsorbed layer. Scheme (XVII.19) reflects the fact that both C2H and the oxi4 dation products are bound with the oxidized surface of the Ag. The adsorption of products is reversible. The scheme does not conflict with the fact that, in the adsorption of C2H on silver 4 covered with oxygen, the C-C-bond is broken and C-o-bonds are formed. In accordance with the data mentioned above, the first step in Scheme (XVII.19) is reversible. The above mechanism corresponds to a parallel scheme of the formation of C2H which agrees with the experi.uental data. Partial 40 oxidation proceeds via (C 2H 40) species while deep oxidation involves the formation and the fast decomposition (step 2')) of a hypothetical vinyl alcoholate complex (CH 2CHOH). The mechanism discussed leads /35, 42/ to the following equationifor the rate of oxidation of C2H into C and CO 2H 40 2, reI) 4 and r(II) respectively:
(XVII. 20)
(XVII. 21 ) where
+L
(XII.22)
512
(XVII.24)
In these equations, k. and k ; are the rate constants for the ~ -... k i-th step in forward and reverse directions and K.( = ~ ) is the ~
i\..-i
equilibrium constant for that step. The selectivity for oxygen is determined by the equation:
-.!-,JIJ S. = 2 _ V2 1 (lJ) T" + 3r
)
.n
(XVII.25)
and shows the fraction of oxygen consumed for the production of CH The selectivity for ethylene deduced from Eq. (XVII.25) 2 40. will be:
1+ 65.
(XVII.26)
~
Investigations of the reaction kinetics using silver catalysts doped with Cl or Se at atmospheric /41, 42/ and elevated (up to 16 atm) pressures /35/ over a wide range of the reaction mixture compositions have shown the validity of Eqs (XVII.20) to (XVII.26). The calculated values of k 1 and k 2 are approximately constant, the scatter of values being rather large but random. These constants are practically independent of pressure /35/. Fig. 159 shows a on Pc H O/P O as follows from Eq.(XVII. 2 4 2' the relationship will be
linear dependence of So 25). At 240
0C,
P0140 ~ = 0.3'1 - 0.11 '2
f;,
'Z
2
Fig. 159 - The plot of selectivity vs Pc H O/PO for 242 the oxidation of C2H on 4 Ag promoted by selenium (255°C) /42/. 0/
02
as
0.4
0.5
f'czN"O
p;;-
(for the Se-doped Ag) and
So, = 0.28 - 0. 09 '2
Pt2!11 0 p
0
(for the Cl-doped Ag) /35/. The activation energy for the first step is shown to be lower than that for the second one. Thus, for the Cl-doped catalyst, E1 = 8 and E2 = 14 kcal mol- 1 /41/ while, for the Se-doped one, E1 = 19 and E2 = 21.5 kcal mol- 1 /42/. This suggests that the rate of deep oxidation increases with increasing temperature more sharply than that of mild oxidation. Later /37/ somewhat different mechanism was proposed. Its differences from the above scheme involve the following points. On the basis of oxygen exchange data, it is supposed that Ag~s)02 is rapidly converted into other oxides: 2,4.($)0. '92 L
(XVII.27)
(this reaction is the reverse of that in ~q. (XVII.18». Since is poorly reactive, the higher oxide Ag(S)03 is assumed to as was presumed be the major intermediate, in contrast tOAg~s~2' in Scheme (XVII.19). The overall mechanism will be /37/: N(I) NCII) N(III)
Ag~s)o
1) C2H + (° 2 ) ----C 2H + (0) 4 40 2) C2H4 + (02)----CH + (0) 3CHO 2') CH.3 CHO + 5(02)-2C0 2 + 2H 20 + 5 (0)
101
o o
1
o o
514
N(I) N(II) N(III) 3) C2H + ( )~(C2H40) 40 4) 02 + ( )~(02)
° 1/2
° 3
1/2
3
5) C2H 40 + (° 2) ~ (C2H40o ° 2) 6) H (H 2Oo02) 20 + (02)~ (C02. °2) 7) cO 2 + (02)~
°
°
°
°
°
°
8) (C2H + (0) 40)---C 2H 4
°
°
4') 2(0)-(02) + (
)
N(I): C2H + ~ 02 4
= C2H40
N(II): C2H + 302 4
= 2C0 2
N(IlI):
+ 2H
1 °
° ° (XVII. 28: ° 1
20
°= °
In many features, this mechanism is similar to Scheme (XVII.19). Tbedifferences mentioned, referring to the stability of the surface oxides of silver (i.e. of (0) and (02»' contradict the supposition that adsorbed products are formed from C2H and the high4 er silver oxide. It was assumed that the first two steps result in gaseous organic species. The intermediate which is rapidly converted into CO2 and H20 will be acetaldehyde and not adsorbed vinyl alcohol as was considered earlier. The immediate decomposition (Eq. (XVII.27» of (0) species formed in the first step of Scheme (XVII.28) leads to irreversibility of this step. The decomposition of (C 2H 40) (the reverse of the first step in Scheme (XVII.19» becomes a special step (the last step in Scheme (XVII.28». Mechanism (XVII.28) involves three basic routes, the last one being "empty". Scheme (XVII.28) leads /37/ to rate equations, the form of which coincides with that for Scheme (XVII.19» except that, instead of Eq. (XVII.23), one will have:
M= 318 ~ 1'1
(XVII.29 )
515
Consequently, Scheme (XVII.2S), like Scheme (XVII.19), agrees with the experimental rate data but the former is more acceptable with respect to the forms of adsorbed oxygen taking part in catalysis on silver. The mechanism discussed refers also to a parallel scheme for the formation of C2H and CO 2• A decrease in selectivity with in40 creasing ratio Pc H O/PO (Eq. (XVII.25) and Fig. 159) is caused by step S). If thfs4step2is excluded (i.e. k S = 0), one will have M = 0, and the selectivity becomes independent of the composition of the reaction mixture (Eq. (XVII.25)) /37/. Then, the mechanism proposed in reference /37/ becomes similar to Scheme (XIV .23). Let us consider the reaction kinetics at low conversions. In this case,
r=
1, + K2 A
1.:? fI
= PH 2
Lj
° = 0,
the value of A will be:
f !T
I
+
2~
01
2
P 11 0 1 Po'2
(see Eqs (XVII.22) and (XVII.24)) so that:
(XVII. 30)
This equation can be also obtained on the basis of Scheme (XIV • 23) (see Eqs (XIV .24) and (XIV .27)). One must take into account the fact that k , k 1 and k 2 in Eq. (XVII.2S) correspond to k , k 1 2 4 and k respectively in Eq. (XIV .23). The overall stoichiometric 4 coefficient of oxygen, Y, is then equal to (SC yC + 2H40
+ SCO
YCO ) where Si are selectivities; and
2H40
Pi are the stoichi-
2 2 ometric coefficients of oxygen in Eqs (XVII.1) and (XVII.G). Since
~
S =--C~ k1 T k,?
(XVII. 31 )
516
(designations refer to Eq. (XVII.28»
with
and
Hence:
17
T
+J~
= r(J! +12 )
(XVII•.32 )
(XVII•.3.3) which is identical to Eq. (XIV .24) (taking into account also Eq. (XIV.27) and the above-mentioned difference in the numbering of the steps). According to reference /4.3/, if Eq. (XVII.3.3) is approximated by the power rate law:
(XVII • .34 )
the reaction orders in the case of mechanism (XIV .2.3) will be: m=(J
and
n=1-8
Then the values of m and n should change within the range 0 - 1 and
m+ n
=1
(XVII• .36)
The same consequences should be true for the rates of formation of C2H and CO 2, as the selectivity (Eq. (XVII.31» does not depend
40
517
on the reaction mixture composition:
(XVII.37»
The kinetics on Ag at low conversions have been studied in reference /44/. It was found that at 240 0C, m = 0.37 and n = 0.67; at 260 0C, m = 0.32 and n = 0.68, so that m + n = 1.04 (240 0C) and m + n = 1.00 (260 0C). One can see that both consequences are valid. Mechanism (XVII.28) suggests a qUalitative explanation of the effect of additives on silver catalysts in the oxidation of C2H 4• Non-metallic additives (S, Cl, etc.) decrease the oxygen-silver bond strength /21/. According to (XVII.28), the rate of formation of C2H will be k 1PC H 8 , where 8 is surface concentra40 tion of Ag;s)03. Since in th; iirst two steps, catalyst - oxygen bonds are broken, their loosening results in two opposite effects (an increase in k 1 and k 2 and a decrease in 8). Competition between these effects leads to the rates passing through a maximum. At high concentrations of additives (at low values of qs)' the second effect prevails. The above additives, being in the form of negative species (Cl-, S2-, SO~-, etc.), enhance the work function. Since adsorbed oxygen is also negatively charged, the non~etallic additives increase electrostatic repulsion andloosen oxygen-eatalyst bonds. An opposite effect is caused by electropositive species (compounds of alkali and alkaline earth metals). This explains the relationship between catalytic properties of modified silver catalysts and their work function values /25/. Since loosening the oxygen-silver bonds is associated with greater coverages by the higher surface oxides of silver /33/, nonmetallic additives enhance the fraction of oxygen adsorbed in the form which reacts di~ectly with C2H 4• Thus, Scheme (XVII.28) is likely to reflect the essential features of the oxidation of C2H on silver. At the same time, a num4 ber of significant questions remain open, in partiCUlar: 1. What is the reason for the dual reactiVity of ethylene exhibited in the first two steps of Scheme (XVII.28)? 2. What is the true composition of the intermediate leading to
518
CO 2 ? 3. What is the reason for the unique ability of silver to catalyze the formation of ethylene oxide? It seems rather interesting to study the distribution of products with various catalysts at low temperatures and very low conversions. In the oxidation of C2H on Pd-Ag alloys /45/, the rate of the 4 formation of C2H decreases with decreasing concentration of Ag; 40 at ~40 at.% Pd, ethylene oxide is not produced. Deep oxidation is sharply enhanced at high (780 at.%) concentrations of Pd. One can put forward the following hypotheses concerning the oxidation of ethylene on metal catalysts. There exist several reaction routes, depending on the catalyst and conditions employed. They can be expressed as a combination of Scheme (XVII. 10) and (XVII.28). With Pt and Pd, atomic anions of adsorbed oxygen are weakly bound and very reactive, and this results in a mechanism of type (XVII. 10). With Ag, the (0) species are more strongly bound and less reactive, but the highest surface oxide of the metal is present in rather high concentrations, this oxide being very reactive. Oxidation of Ethylene Over Oxide Catalysts The specific catalytic activity of simple metal oxides (supported on SiC) in the oxidation of C2H at 300 0C and excess of 02 4 decreases in the order (Table 74): C03047Mn027Cu07Cr203/,NiO» '» Fe 20 3» Ce02• Under these conditions, Pd is more than two orders of magnitude more active than C0 /46/. 304 The activity pattern of the oxides supported on Si0 2 or Al 20 3Si0 2 (in the excess of 02) /47/ will be: C0304/'cr203»Ag2o»Mn203> »CuO »NiO» V20 , CdO»Fe 20 /> W0 ZnO. This sequence 5 3/Mo03 3/Ti027 agrees with reference /46/ (the exception is cr 20 3). All the catalysts mentioned lead to CO 2 and H20 . In the oxidation of C2H over a Bi-Mo-oxide catalyst which is poorly active, 4 30% CO is formed at 500 0C and with excess of C2H in the initial 4 mixture /48/. There are some indications of mild oxidation of ethylene over oxides. Thus, with a P-Mo-oxide catalyst on Si0 2 at 290 0c, acetic acid is produced (with a selectivity of 74% at 39% conversion) /49/.
519
The overall kinetics are usually expressed by (XVII.34) (see Table 74). According to reference de is not reduced in the course of catalysis. In C2H over CuO doped with KOH /50/ with excess of 4 340 0C, the values of m is 0.6 and E = 20-27 kcal Al 20 (1:1), m = 0.5 /51/. 3 TABLE
a power rate law /27/, cupric oxithe oxidation of O2 and at 217 0 mol- 1• With CuO-
74
The Catalytic Activity of Some of C2H /46/ 4
Oxides in the Oxidation Order of reaction in
Catalyst
C0 304 Mn° 2 CuO Cr 20 3 NiO Fe 20 3 Ce02
Temperatures
19 r*
E/kcal mol- 1
200-251 229-279 238-282 240-292 268-316 280-320 342-395
-5.93 -6.21 -6.86 -7.07 -7.63 -7.77 -9.48
30.5 19.6 24.5 27.7 26.3 29.3 24.4
ethylene
0.14 0.52 0.39 0.50 0.38 0.58 0.67
oxygen
0.21 0034 0.27 0.30 0.39 0.47 0.32
* r is the reaction rate (molecules m-2 s-l)at 300°C and 2% C2H 4, 50% 02 and 48% N 2•
The reaction mechanism is likely to be reflected by Scheme (XVII.10).Deep oxidation of ethylene over copper oxide involves carboxylate and formate surface complexes /52/ which can be associated with (1 2 ) in Scheme (XVII. 10). At 200 0C, these complexes are destroyed only in the presence of 02' At 3000-4000C, they can be converted into CO 2 and H20 in the absence of 02; under these conditions, the rates of catalysis and surface reduction with ethylene are equal, and a reduction-reoxidation mechanism is valid. The above mechanism suggests that there should be a 19 r - qs correlation and this is observed in practice (Fig. 160). The greater catalytic activity of the metals (Pd) in comparison with the oxides can be due to the preliminary activation of the C~4 on a metal surface which is free of oxygen; this activation
520
results in substantial loosening (or even rupture) of the C=C and facilitates the sUbsequent interaction of bond in the C 2H4 the ethylene with (0) leading to (CH 2CH20). /1/ r
-4
7e:,
-5
-6 -7
Fig. 160 - The dependence on qs of the specific catalytic activities for the oxidation of ethylene /46/: 1 - NiO; 2 - Mn02; 3 - Co 0 ; 3 4 4 - CuO; 5 - Fe 20 3; 6 - Ce02 ; 7 - Pd.
-8
Eqs (XVII.13) and (XVII.14) for selectivity, deduced from Scheme (XVII. 10), suggest that there is an increase in the yield of 0 20 40 ~s/lrcal(If-{ltOr' CH with qs. This predic3COOH tion is in a qualitative accordance with the fact that significant amounts of acetic acid can be obtained only with a P20 - Mo0 catalyst possessing high 3 5 values of qs' In the case discussed, the oxygen-catalyst bond strength is not the only determining factor, since in the steps 4) and 5) of Scheme (XVII. 10), the catalyst - acetic acid bonds are formed and destroyed. Hence, the selectivity should also depend on the surface acidity, the P-Mo-O system being appropriate in this respect for it is employed in some other reactions for the synthesis of organic acids. -9
The Oxidation of Propylene Cuprous oxide catalysts for the partial oxidation of propylene into acrolein , were discovered /53, 54/ and employed in industry in the 1940 s. With these catalysts, the reaction is carried out at 300o-400oC and with rather high C 6 : 02 ratios, close to 3H stoichiometric. The catalyst is usually supported on SiC or some
521
other inert carrier. Compounds of transition metals of Group VI or non-metals (S, Se, Te and I) were proposed as promoters /55/. The selectivity to acrolein reached a level of 70-80%. In 1960, a selective catalyst of a new type, the Bi-Mo-P-oxide system, was proposed /66/. With this catalyst, the reaction proceeds at higher temperatures ( ~4500C) and with excess of 02. Among oxidative heterogeneous catalytic reactions, the oxidation of propylene is of special importance. Most pUblications refer only to this process, this being caused both by the practical significance of the reaction and by a possibility of studying many fundamental problems of the theory of catalysis, such as: 1. The problem of a large variety of possible mechanisms, depending on the nature of the catalyst, the reaction conditions, etc. On varying these factors, the oxidation of C 6 can be directed 3H into the production of acrolein, acrylic acid, acetone, acetic acid, benzene, hexadiene or carbon oxides. 2. The problem of a large variety of factors which can affect the activity and selectivity. 3. The mechanism of catalytic action of complex and doped catalysts. 4. The effect of the reaction mixture on the composition of the steady-state catalyst (containing copper oxides and so on). The major reactions of the oxidation of propylene can be expressed by the equations: 1 C 6 + '2 °2 = C3H6O 3H
(XVII. 38)
C 6 + 1. ° = CH 3H 3COCH3 2 2
(XVII. 39 )
C 6 + 02 = CH 2=CHCHO + H2O 3H
(XVII.40)
C 6 + 1 _1 02 = CH 2=CHCOOH + H2O 3H 2
(XVII.41)
C 6 + 202 = CH + CO 2 + H2O 3H 3CHO
(XVII.42)
1 = CH C 6 +2_0 + CO 2 + H2O 3H 3COOH 2 2
(XVII.43)
C 6 + 302 = 3CO + )H2O 3H 1 _ C 6 + 4 ~ 02 - 3C0 2 + )H2O 3H
(XVII.44) (XVII.45)
522
Reactions (XVII.40) and (XVII.41), as well as reaction (XVII.4J), are of the greatest practical importance. In addition, the oxidative dehydrodimerization of C 6 into JH 1,5-hexadiene or benzene is possible: (XVII.46 )
(XVII. 47 )
The simplest unsaturated aldehyde, acrolein, is used for the production of glycerol, glutaric aldehyde, glycidol, allyl ether, nicotinic acid, etc. A valuable monomer employed in a number of polymerization processes is acrylic acid 1191. The Oxidation of Propylene Over Metal Catalysts The activity pattern of metal films in the deep oxidation of C 6 with excess of 02 (Pt:;>Pd;;>Rh/Au/"W) is similar to that in JH the oxidation of C2H 17/. With Pt and Pd, small amounts (up to 1 4 and 3%, respectively) o~ acetone were formed; with Rh, only traces were detected. The oxidation of propylene over Pd/C at 125 0C and at low conversions (J.6%) results in a rather selective production of acrolein 156/. At higher temperatures (J20 0C), the latter is formed on copper (selectivity ""'90%) 127/. The kinetic characteristics of the deep oxidation of C 6 over 3H Pt and Pd are listed in Table 75. Differences in the reaction orders in the equation: (XVII.48)
observed for metal films at low pressures and small conversions 171 and for supported metals at atmospheric pressure 1571 can be due to different methods and conditions of the experiments. The reaction orders for Ag/SiC are close to those for Pt and Pd 157/; silver is likely to be present in the form of metal (the initial catalyst was Ag20). Carbon dioxide and water were also reaction products with silver. The propylene oxide yield is poor 127/. There is evidence 111
523
on the promotion of Ag by gold, alkaline and alkaline earth metals for the production of C 60. Thus, selectivity of silver for 3H the oxidation of propylene is significantly different from that in the oxidation of ethylene. TABLE 75 The Kinetic Characteristics of the Oxidation of Propylene Over Some Metals
Catalyst
Pt film Pt/SiC Pt/SiC Pd film Pd/SiC Pd/SiC Ag/SiC
'I'empera ture rteaction mixture range/ °c composition PC Po 2 3H 6 /Torr /Torr 76-130 122-147 159-198 84-128 143-178 190-226 210-258
2.3 15.2 152 2.) 15.2 152 152
24 380 114 24 380 114 114
19 r at 30 00C
E 19 r /kcal /mol m mol- 1 m-2 s-1
In (I)
n
0
s::(I)
H (I)
'Pi (I)
0::
1 r±o. 7 21.1 19.3 20.8 -3.96 24.8 -4.95 25.4 -5.50 32.4 -3.69 -4.94
3.3 4.)1 2.39 5.6 5.46 4.70 6.80
-0.2 -0.74 -0.15 1 -0.47 -0.57 -0.61
0.5 1.45 1.16 0 0.85 1.50 1.52
7 57 57 7 57 57 57
The retardation of the reaction by propylene on Pt is likely to suggest that adsorption of the hydrocarbon or of the products of its interaction with oxygen occurs during the course of catalysis. The formation of such surface complexes is supposed in reference /58/. At the same time, the surface coverage with oxygen, as indicated by the values of n, is small. A similar picture is observed with Pd and Ag supported on SiC, but with Pd film, the situation is different (C 6 is weakly adsorbed while the adsorp3H tion of O2 is strong). Initial activation of the olefin can occur during the course of its adsorption on free metal sites and the adsorbed hydrocarbon species then interact with adsorbed oxygen /7/. During this interaction (which determines the rate of catalysis), rupture or a significant loosening of metal-oxygen bonds takes place. As a result, the catalytic activity decreases with an increase in this
524
bond strength (Fig. 161).
r,.lff Fig. 161 - The dependence on qs of the catalytic activity of the metals in the oxidation of propylene (Tr is initial temperature of the reaction /7/): 1 - Pt, 2 - Pd , J - Rh, 4 - Vi.
400 I 500 000 100'::--J----..J.---:L--:--'--
fO
40
00
fJs jlrcrrl(!l-rrtotl
One can assume that the rupture of ~-C-C bonds in propylene occurs simultaneously and this leads to the intermediate: H C-CH-CH
21
I
J
° °
II/l/Il
Further rapid oxidation of this gives rise to carboxylate - carbonate species which are converted into the deep oxidation products. On a silver surface which is covered significantly with the oxide Ag~s)03' the adsorption of the above intermediate can result in small amounts of propylene oxide. Acetone is not an intermediate for the production of CO 2 and H20 /7/. It is likely to be formed by a parallel route according to a mechanism involving the preliminary hydration of C 6• Such 3H a route will be discussed below in connection with the low temperature catalysis over oxides /56/. The high selectivity towards acrolein observed with Cu can be due to the fast oxidation of the metal into Cu20, a good catalYst for the oxidation of C 6 to acrolein /27/. 3H According to reference /56/, the mechanism of the formation of acrolein over Pd supported on charcoal is quite different from that with oxide catalysts. Tracer experiments showed that oxygen from water but not from 02 is introduced into the acrolein in the former case. Hence, it was assumed that the allylic radical CH2~CH~H2
525
formed on Pd/C is attacked by OH-groups generated by the adsorption of H The surface compound formed (CH 2=CH-CH20H) then un20. dergoes oxidative dehydrogenation to acrolein. Under favourable conditions, the hydrocarbon-oxygen species formed on metal surfaces can be desorbed into the gas phase in the form of radicals which then bring about a chain process /59/. Such a heterogeneous-homogeneous mechanism was observed for the oxidation of C 6 over Pt/BaS0 at low pressures and at 100 0C /58-60/. 4 3H With large void spaces and at elevated temperatures (350 0-4500C), the last mechanism is also typical of Ag /61/. In this case, with small amounts of catalyst, the contribution of the homogeneous process is high and the reaction products contain C2H acrolein, 4, CO, CH C and HCHO. Increamng amounts of catalyst favour the 4, 3H60 heterogeneous oxidation with a different selectivity, the major product being CO 2 and HCHO (together with C2H 4). The Oxidation of Propylene Over Simple Metal Oxides This is one of the most comprehensively stUdied reactions. In reference /62/, the oxidation of C 6 was studied over 13 metal 3H oxides (supported on nonporous SiC) with excesses of C 6 employ3H ing the flow-circulation technique. The temperatures of attaining equal specific rates were used as a measure of the catalytic activity. It was assumed that the correction connected with a fraction of the free carrier surface which does not participate in catalysis is of minor importance with respect to the activity pattern. As seen from Table 76, the sequence of activities is: Mn0 2» CuO;> » Cr 20 3 "7 C0304":> Fe 20 3/ U308":> CdO/ V2°5/ NiO/Mo0 3/, Zr0 2 » PbO/W0 3• Similar catalysts were used in reference /57/, the experiments being made in a differential reactor with two reaction mixtures (Table 77). With excess of 02' the activity decreases in the order: C0 CuO »Mn0 2/ cr 20 "»CdO»V205 /Fe 20 Th0 2 NiO,/Ce0 2,/ Al 20 3 3/ 304/ 3"» while with excess of C 6: Cu07IVln02,/C0304"»cr203/CdO,/Ce02'/ 3H 7V205/ Fe 20 "7 NiO,/Th0 2• In addition, extremely low activities 3 were observed for BaO, MgO and CaO. According to reference /63/ (Table 78), the activity order with excess of 02 at 500 0C is: C0304/CuO/In203,/NiO,/Sn02/V205/znO» "7 Fe 20 3"7 cr 20 3"7 Ge027 CdO,/Bi 20 3-;;::.Mn0 2 '/ Si0 2/, Ti0 27W0 37Ca07 7 Mo0 37 Sb 20 3• Under non-steady conditions, the corresponding activity pattern
526
TABLE 76 The Catalytic Activity of Some Metal Oxides for the Oxidation of 03H6 1621 Initial Mixture (%): C 6 - 20, 02 - 5 and N2 - 75 3H
Oxide
Specific surface areal m2 cm- 3
Tr/K* 10 3/T r
Oxide
Mn° 2 CuO Or 20 3 c0 304 Fe 20 3 U 8 30 OdO
0.610 0.130 0.170 0.070 0.130 0.330 0.040
598 604 618 627 640 655 679
V
1.67 1.65 1.62 1.59 1.56 1.52 1.47
20 5
NiO
Mn°3 Zr0 2 PbO ViO 3
Specific surface areal m2 cm- 3
Tr/K*
0.170 0.230 0.130 0.210 0.260 0.310
683 707 729 741 773 773
10 3/Tr
1.46 1.41 1.37 1.35 1.29 1.29
* T is the absolute temperature at which a rate of 1.5 x 10-6 r mol 02 m-2 s-1 is attained.
was found 164/, using the pulse technique (10% 03H6 + 90% 02)' to be: Mn0 2> 00 2°3 >OuO;>Ni 20 ;>or 20 >NiO >Fe 20 V205 ;>Sn0 2 :>Ti0 2/ 3 3 3/ ,>Sb "7 Bi 20 ZnO ;>Vi0 Mo0 (Table 79). The same tech20 3;> 3/'U308 3/' 3 nique was employed in reference 1651 for measurements of the steady-state activities and the following order was obtained (Table 80): 00 cuo 7 Mn20 3/> or 20 3> Oe02 7 Fe 20 3 />Tl 20 3/> Sn0 2/> NiO?, Ti0 2/' 3°4/ /> OdO,>U308?In203;>Mo03:>Th02:>V205;>W03:>Bi2037Ta205 /'La 20 3;> /> ZnO,> Zr0 2/Nb20 5;> Pb304/Ga203/Te027 Sb 20 4• The above formulas reflect the true phase compositions of the oxides as was shown by the X-ray method; the exceptions are: Pb (which is a mixture of 304 Pb 304 with PbO and 0(-Pb0 2), OdO (some other phase, possibly, ca, was also found) and La 20 (which also contains some admixture). 3 Silver oxide was unstable under the conditions of catalysis. In general, the activity orders established are similar for the various reaction mixture as well as for steady and non-steady states of the catalysts. In most cases, higher activity is exhibited by C0304' Mn0 2 (Mn 20 ) , OuO and Or 20 The relatively inactive ca3 3• talysts are Mo0 3, W0 3 , Zr0 2, ThO 2, Al 20 PbO (Pb Sb203(Sb204) 3, 304),
527
TABLE 77 Rate Characteristics of the Oxidation of C 6 Over l\letal 3H Oxides at 300°0 /57/
Oxide
21~
'I'emperature range /oC
E/
n
m
kcal mol- 1
C 6, 50;G O2, 48% N2 3H
0°3 04
ceo
Mn0 2 Or 20 3 CdO V20 5 Fe 20 3 NiO Oe02 Al 20 3 Th0 2
215-240 272-311 203-259 255-294 283-353 300-351 276-324 274-324 240-305 380-441 285-355
20% C 6, 15;'& 3H CuO Mn0 2 c0 0 3 4 cr20 3 CdO Ce02 V20 5 Fe 20 3 NiO Th0 2
°2 ,
-
5.78 6.04 6.34 6.91 7.07 7.42 7.44 7.45 8.10 8.60 8.85
33.8 2903 22.7 30.7 21.5 14.9 30.8 18.9 17.7 2103 26.8
-
5.85 5.88 6.18 6.95 7.07 7.17 7.45 7.58 7.10 8.38
36.8 39.0 35.1 34.1 32.4 30.9 40.0 41.1 32.8 28.8
- 0.16 - 0.29 0.39 0.36 0.74 1.00 0.42 0.48 0.87 0.88 0.70
0.49 0.21 0.28 0.32 0.12 0.15 0.13 0037 0.00 0.47 0.41
65% N2
242-276 230-261 240-270 268-301 277-323 224-274 280-330 281-311 287-329 292-339
-
° °0.24 0.17 0 0.54 -0.40 0.17
°0.56
0.40 0.64 0.55 0.71 0.70
°0.95 0.24 0.56 0.41
and CaO. The other oxides studied display an intermediate activity. At the same time, definite deviations are evident. They are likely to be caused primarily by the different compositions of the reaction mixtures, resulting in changes in activity patterns
528
TABLE 78 The Catalytic Activity of Metal Oxides for the Oxidation of C 6 at 5000C /63/ 3H Selectivity/ % ** ) 19 r* Oxide /mmol CO 2+ acro- 1,5-hem-2 s-1 CO lein xadiene + benzene C0 304 CuO In 20 3 NiO Sn0 2 V205 ZnO Fe 20 3 Cr 20 3 Ce02 CdO Bi 20 3
-3.09 -3.38 -3.5 -4.08 -4.9 -5.14 -5.19 -5.25 -5.7 -6.0 -6.0 -6.2
96.2 2.2 92.5 3.8 4.1 93.4 0.8 75.7 2.3 2.5*** 91.5 3.2 81.8 14.2 49.4 0.5 48.2 86.9 12.2 traces 4.0 69.3 2.1 99.9 traces 0.1 98.5 0.8 9.6 89.4
°
° ° °
°
°
Selectivity / % ** ) 19 r* Oxide /mmol CO 2+ acro- 1,5-hem- 2 s-1 CO lein xadiene + benzene
Mn° 2 Si0
2 Ti0 2 W0 3 CaO Mo0 3 Sb 20 4 Al 20 3 MgO PbO T120 3
-6.2 -6.4 -7.05 -7.12 -7.3 -7.42 -7.6
0 0.) 99.7 8.6 traces 46.7 85.8 12.0 0.3 0.9 90.0 6.3 86.5 74.6 25.1 63.3 16.3 38.5 42.1 94.0 traces 11 69.5 11 96.7
°
° ° °° °° °
*) Conversion of propylene is 25% (with the oxides of Al, Mg, Pb and Tl, it is below 25%) **) By-products are C2H CH etc. 3CHO, 4, ***) 0.7 for C6H6 and 1.8 for 1,5-hexadiene due to different rate equations, by various effects of the reaction mixture on the chemical composition of the oxides, etc. In some cases, sharp deviations are observed, ususally towards lower activity. The reasons for these are not quite clear. For example, in reference /62/, the activity of NiO, in reference /57/, the activities of Ce02 and Th0 2, in reference /63/, the activity of Ti0 2, cr20 and Mo0 and in reference /64/, the activity of W0 3 3 3 are all likely to be underestimated. The high activity of T120 in 3 reference /63/ is in contrast to the data of reference /65/. The selectivity of the simple oxides for the oxidation of pro-
529
TABLE 79 The Catalytic Activity of Metal Oxides for the Oxidation of C 6 3H (10% C 6 + 90% O2) Under Non-Steady Conditions /64/ 3H
Oxide
Mn° 2 c020
CuO Ni 20
3
3 Cr 2 3** NiO Fe 20 3 V20 5 Sn0 2 Ti0 2 Bi 20 3 U 8 30 Sb 20 3 ZnO W0 3 MoO3
°
r x 1010 at 300°C /mol m-2 s-l
1.15 x 10 6 9.78 x 10 4 2.82 x 10 4 1.02 x 10 4 4.78 x 10 3 3.98 x 10 2 3.02 x 103 1.82 x 10 3 1.07x10 3 9.23 x 10 2 6.02 x 10 2 5.62 x 10 2 1.74 x 106 5.02 x 10 1 4.47 x 10 1 1.29 x 10 1
Tr* /oC
149 167 181 214 247 253 274 265 295 301 321 321 400 523 467
E/ kcal mol- l
22.4 14.7 14.7 15.7 17.8 19.0 27.0 11.1 20.0 30.0 16.2 18.7 0.91 22.3 12.6 21.8
Amounts of chemisorbed C 6/ 3H cm3 m-2
The reaction temperature in adsorption measurements /oC
0.04 0.198 0.178
87 120 254
0.036
192
0.034 0.086 0.099 0.073
263 339 320 263
0.058 0.080 0.020
232 357 330
0.150
400
pylene has been studied extensively. With most oxides, the major product of mild oxidation is acrolein. A higher activity with respect to reaction (XVII.40) is exhibited by cuprous oxide catalysts, which act at 2400-4000C /19, 27/. These catalysts were first used in practice /14/. Under industrial conditions, the conversion of C 6 (per one passage through the catalyst) with the above cata3H lysts is equal to 10-20% (with excess of C 6), the selectivity in 3H acrolein being 60-85% /66/. By-products are CO 2 and H20 (with small amounts of propionaldehyde, acetone and allylic alcohol) /14/. The selectivity of cuprous oxide catalysts can be enhanced by
530
TABLE 80 The Catalytic Activity of Metal Oxides for the Oxidation of C 6 (10% C 6 in a mixture with 02 + N2) /65/ 3H 3H
Oxide
c0 0
3 4 CuO Mn 20 3 Cr 20 3 Ce02 Fe 20 3 Ag20* T120 3 Sn0 2 NiO TiO CdO UP8 In2°.3 Mo0.3 ThO2 V
20 5
W0 3 Bi 20 3 Ta 20 5 La 0 2 3 ZnO zr0
Temperature (oC) required to attain a reaction rate of 3.5 x 10-4 mol m-2 h- 1 at various concentrations of 02: 5%
10%
18%
20%
30;i~
225 240 260 295 300 300 3.30 330 370 345 338 .350 365 378 .395 445 440 387 445 435 505 447 420 438 500 515 510 590 625
220 224 255 285 295 285 305 320 345 324 325 345 357 .360 37.3 420 425 .385 435 422 470 430 415 425 490 490 510 565 600
200 204 230 278 280 290 295 295 306 310 310 .340 .345 351 360 .381 390 398 402 405 410 415 420 420 436 465 510 550 578
210 210 242 270
200 212 240 270 280 265 265 290 320 310
270 270
310 340 347 410 400 380 409 440 405 420 460
.305 337 342 .345 360 405 395 380 405 396 430 412 405 416 450 459 510 540 555
2 Nb 20 * 5 Pb 20 4 Ga20 3 Te02 Sb 20 4 *) The oxide is unstable under conditions of catalysis
E/
kcal mol- 1
19 15 19 32 19 28 19 18 23 22 24 27 25 27 26 12 34 23 23 26 19 28 30 27 16 26 36 8 8
531
nonmetallic additives (Cl, S, P or Bi) /19/.
r/10 -51!l(J{ 0 ·,t(~r!
2,,"-
(1)-2 .-J ()-4
/
I /
/
/
e
,,~
,,/ / /
/
0.2 //
:
a
JOO
1.2
(O-!
?-
08
¢-f
e e
J tl.Joo"""-- ......,
e
~~.
.
~~-<}.:
" ---6
9'-"
,
0.4¢
0
4 "
J50
400
450
t/°C
Fig. 162 - The dependence on temperature of the amounts of acrolein formed (continuous curces) and its rates of formation (dashed curves) in the oxidation of C)H over CuO (1), Co)04 (2), Fe 20) (), NiO (4) and V20 (5) / 6 62 / • 5 .An especially pronounced effect is caused by Se0 or Se (5-10 2 wt.%). With these catalysts, the reaction is carried out at great excess of 02; high conversions of C)H6 (80-90% at )100_))00C) are associated with high selectivities to acrolein (- 90%) /67/. The selectivity of cuprous ~xide catalysts increases when water vapour is added to the reaction mixture /27/. Under steady-state conditions, acrolein is produced on the oxides of Cu, Co, Fe, Ni and V /62/. On increasing the temperature, the rate of process (XVII.40) passes through maximum (Fig. 162). This is explained by the further oxidation of acrolein at elevated temperatures. According to reference /62/, the rate of reaction (XVII.40) decreases in the sequence: CuO, Co)04?-Fe 20)?-NiO?-V20 5• We have calculated the temperatures, T;, to attain equal reaction rates for the formation of acrolein (Jx10- 7 mol 02 m- 2 s-1) using the data of reference /62/ which apply to parallel scheme. Fig. 16) shows that the overall activity (Table 76) correlates with the activity for mild oxidation. The selectivity towards acrolein at 500 0C of the various oxides /6)/ decreases in the order: Al20)?-MoO)/'Sb204>V205>Fe20»Si02> ~ WO) ?- CuO ,>Sn0 /' NiO">,Co)O4 '> Cr 2 20) ":;>CdO, In20),> ZnO":;>Mn0 2/, MgO, PbO, Tl 20), Ge02, CaO, Bi 20). At the same time it was observed that other products of partial oxidation are formed besides acrolein (1,5-hexadiene and benzene); on the oxides of Zn, Bi, In,
532
Cr, Sn, W, Ti and Cd, the oxidative dimerization of propylene (reactions(XVII.46) and (XVII.47» takes place (see Table 78). These catalytic processes were first discovered with complex oxide
Fig. 163 - A correlation between the activity of some oxides for the overall reaction of the oxidation of C 6 and that for its oxidation 3H into acrolein (the data of reference /62/ were used for the correlation): 1 - V20 2 - NiO, 3 - Fe 20 3, 4 5, CuO (Cu20), 5 - c0 304'
1.4 1.4
S([lt/%
40
JO
20
10
1.8
0/2
Fig. 164 - A correlation between the activity in the oxidation of C3~ and.t.he selectivity towards " allylic" oXidation (0) and that for the oxidation into acrolein (e) (Table 78): 3 - Ti0 2; 4 1 - Mo0 2 - Sb20 4; 3, 6 - Bi 20 5 - Fe 20 7 V20 3, 3; 5; 10 - Sn0 2, Si0 2; 8 - W0 9 - cr 20 3; 3; 11 - Ge02; 12 - ZnO; 13 - CdO; 14 In20);15 - CuO; 16 - C0 304; 17 NiO; 18 - Mn0 2•
catalysts /68/. At high conversions (60-70%), the selectivity towards benzene can reach 10-18% at 500°C with the oxides of Zn, In, Sn and Cd. The selectivity to 1,5-hexadiene and benzene decreases in the order: ZnO»Bi2037In203»sn02»CdO /63/. The yield of 1,5-hexadiene decreases in the order: TI 20) ,/,CdO»Bi 20 » Sb 20)/" In 20 "> Sn0 2• The 3 3 formation of benzene over Tl 20 and In 20 was observed in reference 3 3 170/. The oxidation of propylene into acrolein occurs usually at lower temperatures than the oxidative dimerization of C 6, which 3H takes place at 500°-550°C. One can thus formulate the following empirical relationships.
533
The major product of the mild oxidation of propylene over the transition metal oxides is acrolein while the oxidative dimerization is peculiar for the oxides of metals of the main groups II-V. The selectivity towards mild (" allylic")oxidation of C 6 3H decreases, in general, with increasing overall activity (Fig.164). At the same time, the rate of mild oxidation changes in parallel with the overall activity (Fig. 163). These relationships are qualitatively supported by the data/65/ (Table 81) obtained by the pulse method under steady-state conditions. The main product of partial oxidation is acrolein, the highest selectivity being exhibited by the oxides of copper and tellurium. The catalysts in Table 81 are listed according to a decrease in their activity. On the whole, poorly selective oxides are seen to be in the upper half of the Table, i.e. an inverse relation between activity and selectivity is observed. Cuprous oxide is an evident exception. The oxidative dimerization proceeds mainly over nontransition metal oxides (Ga20 Sn0 2, In 20 as well as over La 20 Ta20 and 5 3) 3, 3, The formation of CH (T1 20 and Ag20), CH (In 20 Cr 20 3 3CHO 3• 3COCH3 3), C2H4 (Te0 2 and Sb 20 is also noticed. 4) Over some oxides (mostly of nontransition metals), CO is formed together with CO 2 (see Table 71). According to reference /57/, great amounts of CO are produced over Th02, V20 and Al 20 Gene3• 5 rally, the latter product is peculiar to poorly active catalysts which are the oxides of the polyvalent metals of groups III-VI. The selectivity towards mild oxidation usually decreases with temperature. With the higher oxides of V, Mo, Vi and U, acetaldehyde is formed /19/. The changes of activity and selectivity on going from nonsteady to steady state conditions are of considerable interest. Cupric oxide catalysts are reduced during catalysis /19/ and cuprous oxide is an active component of these catalysts /27/. If metallic copper is the initial substance, it is oxidized into Cu20 /19, 27/. During the course of the reduction of CUO, the selectivity to acrolein decreases progressively /27/. At lower temperatures (below 310 0C), the activity decreases and selectiVity increases with the number of pulses (Fig. 165). At higher temperatures (~3350C), the steady state is already established in the first pulse. In reference /71/. the irreversible chemisorption of C 6 on a 3H Sb-O-catalyst was observed during the establishment of the steady
534
TABLE 81 The Selectivity of Metal Oxides in the Oxidation of C 6 3H (10% C 6, 18% O2 and 72% N2) /65/ 3H Oxide C0 304 CuO Mn20 3 Cr 20 3 Ce02 Fe 20 3 .Ag20** 1'12°3 Sn0 2 NiO 1'i0 2 CdO U 8 30 In20 3 MoO 3 1'h02
V 20 5 ViO
3 Bi 20 3 1'a20 5 La20 3 ZnO zr0
2 Nb 20 5 Pb 304 Ga20 3 1'e02 Sb20 4
l' /oC*
r
200 204 230 278 280 290 295 295 270 310 310 340 345 351 360 381 390 398 402 405 410 415 420 420 436 462 465 510 550
Selectivity towards other towards acproducts rolein/% 0 80 0 ° °
(C6 ) ***
8
6 ° 22 45 13 8 60 3 52 50 ° 46 12 5.3 10 ° 11 1 18 6.5 28 87.5 70
35% CH 3CHO 8% CH 3CHO 3.5% C6
(C6' CH 3COCH 3)
The ratio CO/CO 2 0 0 0 0 0 0 0.14 0.06 0.7 003 ° 0.6 0.15 0.3 0.45 ° 1.8 0.75 0.15 0
(C6 ) 6% C6
5.3% C6 (C2H 4) (C2H 4)
° ° 0 0.6 0.36 0.08 0.8 003 2
*) 1'r is the temperature to attain an overall rate of 3.5x10-4mol m -2 h -1 **) The oxide is unstable under the conditions of catalysts ***) C6 hydrocarbons; the parentheses refer to trace amounts
535
3/% 75
lriff ref :: II I I II II
50
I
I
I I
II I I II II I I ill I II~I
if
II
II I
o /
.7
L __ ~I /0
JO
I I
I I
I I I
I~H 1111 I I iv I IV I I I-I 1 1-
r-r l
!I
x/%
I I
1 11/0
I
I
I I I 11 1 1
/5
20
Fig. 165 - The conversion of C 6 over CuO, X(o), and the 3H selectivity towards acrolein, See), versus the number, N, 0 of pUlses on CuO/65A 1 - 285 C, II - 310 0C, III - 335°C, IV 35800, V - 386°C
0
20 If {ll
Fig. 166 - (a) The activity,r, and the selectivity for mild oxidation, Sea), and (b) the amounts of irreversibly adsorbed propylene versus the number of pulses (N) for a Sboxide catalyst (calculated from the data of reference /71/).
r
b
at;.
40 state, the amount of propylene ad- - - - _....120 sorbed decreasing with an increase in the number of pulses (Fig.166). 0 Simultaneous reduction of the ca!0---J~---5~--~-7 tV talyst takes place. These procesa ses are accompanied by a decreasing activity and an increasing selectivity towards mild oxidation. (See our calculations based on the data of reference /71/ in Fig. 166). Similar phenomena were observed with the oxides of Cr, Mo, Mn and Sn /71/. (See also references /72/ and /73/ and Table 79). The nonsteady-state behaviour of 30 oxides has been studied in reference /65/ by the pulse technique. A typical observation for such systems is a partial removal of oxygen from metal oxides accompanied by the irreversible adsorption of the hydrocarbons. The rates of the processes mentioned depend on the reaction mixture composition and temperature. Higher temperatures and C 6/02 ratios favour the above reactions. Paral3H lel to this, the catalytic activity increases while the selectivi-
536
ty towards mild oxidation decreases (or remains constant when it is low). The reaction kinetics and mechanism have been studied most comprehensively with cuprous oxide catalysts. It was shown that a parallel scheme is valid at low temperatures ( ~ 320 0C), while further oxidation of acrolein takes place /27/ at higher temperatures ("-380 0C). A similar picture was observed with other metal oxides. By freezing the product from the reaction cycle, it was found that the oxidation of propylene on cuprous oxide is retarded with acrolein and water vapour (but not with CO 2 ) /27/. At the same time, the selectivity towards acrolein increases when the concentration of H20 grows /27, 74, 75/. On the basis of kinetic studies using the flow-circulation method, the following empirical rate equation for the formation of acrolein (r 1 ) and that for the formation of CO 2 (r 2 ) were proposed /27/: 1
I
0
k1 Ca Cc. II.
r. = t
3 0'
;Z
0
1+ 0, ( Cc !/, '3 If I
~
-tV
eft 0 )
+
'2
1
tts ns en u
(cCJI4IJ where k
(XVII.49)
1, k 2,
u2
+ C
~IJ
)
(XVII. 50)
b 1 , b 2 are constants and Ci are concentrations .*)
*) The proposed in reference /27/ empirical equation:
as
r=
k,Ca Cc :z
..,
If.
6'
(1+ !J 'CC fI. (J) '5 If
( 1+ t'Ii; 0 )11.5
(1+
01 C17u 0) 2
describing the kinetics at high concentrations of H20 can be valid only when CH is expressed in mole fractions since, when other units are 2 used, one must not add CH to 1 in the term (1 + C )0.5 2
°
H
20
•
°
537
The measurements were made at J17 o-J80oC and with excess of C 6, JH The activation energies for the formation of acrolein (JO ± 2 kcal mol- 1) and CO 2 (J6 ± 2 kcal mol- 1) obtained at constant composition of the reaction mixture were higher than those measured under conditions when the products were removed (20 ± 1 and 26 ± 1 kcal mol- 1). The treatment of the reaction mechanism proposed in reference /27/ based on a Langmuir-Hinshelwood model involving a molecular form of adsorbed oxygen and two sorts of active sites, is doubtful. In particular, the above scheme suggests that the adsorption coefficients in Eq. (XVII.49) are equal to those in Eq. (XVII. 50); however, b1~ b 2• The kinetics of the oxidation of propylene on other simple oxides have not been studied so comprehensively. According to reference /76/, the formation of aldehydes, CO and CO 2 (and the overall transformation of C H6) over vanadium oxide catalyst obeys the J equation: (XVII.51)
m
r
02
o
0.4
Fig. 167 m (o) with /57/): 1 7 - NiO, 8
0.5
0.8
n
A correlation between the reaction orders in propylene,
those for oxygen, n (~) (experimental data of reference Ce02 , 2 - V20 5, 3 - CdO, 4 - Fe 20 , 5 - CUO, 6 - Mn0 2' 3 - cr 2 0 , 9 - Co 0 3 4• 3
The activation energy for the formation of CO 2 (22 kcal mol- 1) was much higher than that for the formation of aldehydes and CO (2-4 kcal mol- 1). The same rate equation was observed with PbO /70/.
538
More information has been obtained under conditions when only the deep oxidation of C 6 takes place. Table 77 presents the daJH ta /57/ which can be described using the power rate law (XVII.48). For transition metal oxides, the order in C 6 decreases with inJH creasing order in 02 (Fig. 167). The oxidation of C 6 over NiO is retarded by water vapour. The JH kinetics follow Eq. (XIV.52) (Chapter XIV) with m = 0.23, n = 0.53 and 1 = - 0.27 at 250 0-400 0C, the activation energy being 25 kcal mol- 1 /72/. It should be noted that the experiments quoted in references /57/ and /72/ were made in a differential reactor but only in reference /72/ were the concentrations of all reagents, including H20, kept constant (H20 was added to the mixture). If PH is not 2
°
constant along the catalyst layer (as was so in reference /57/), the reaction orders in C 6 and 02 are underestimated /72/. JH The data on m and n in Table 77 are therefore likely to be lower than those which would be observed under gradientless conditions. However, they reflect the trends of the changes in the reaction orders on going from one catalyst to another. Thus, the rate law for many oxides can be expressed by Eq.(XIV. 52) which is reduced to Eq. (XVII.48) or (XVII.51) in particular cases. Using the pulse technique, it was shown in reference /71/ that the rate of catalysis exceeds that of the surface reduction of the oxides with C 6• This fact is hardly to be regarded as an argument 3H against the mechanism of alternating surface reduction-reoxidation since the surface composition of the catalysts under conditions of catalysis and reduction in the experiments of reference /71/ were significantly different. The rate of the catalytic oxidation of C H6 over CU2 0 at 200 0C 3 is nearly 5 times higher than that of surface reduction of the catalyst in experiments carried out after catalysis. Under these conditions, carbonate-carboxylate complexes formed in the interaction of C 6 with CU20 are slowly destroyed in the absence of 02 in the 3H gas phase. In the presence of 02' the rate of catalysis is equal to the rate of oxidation of the above surface complexes. At elevated temperatures, the latter are rapidly converted into CO 2 and H20 in the absence of 02' so that the rates of catalysis and surface reduction are equal. This suggests that 0 2- species participate in the catalysis /52/.
539
The same conclusion was also reached on the basis of thermodesorption data obtained during the partial oxidation of propylene into acrolein over the oxides of Cu, Cr and Mo /78-80/. The highest yield of the partial oxidation products was obtained under conditions where the anion-radicals, 02' were absent on the surface of the catalyst. In references /77/ and /81/, a cuprous oxide catalyst was used for the oxidation of propylene in which the CII - group was labelled 3 with C1 3. One half of the label was found in the carbonyl group of the acrolein formed (the isomerisation of C 6 did not occur 3H under the conditions of the experiments). The result obtained suggests that a symmetric allylic complex (CH 2 = CH =- CH 2) is a surface intermediate in the reaction. This conclusion was supported by stUdying the oxidation of propylene labelled by deuterium /82, 8J/. The model involving an allylic surface intermediate has been also confirmed by a tracer technique in reference /84/. Further development of the model was given in references /78//80/ with the help of thermodesorption data. Propylene was shown to be adsorbed on metal oxides in two forms: 1 - weakly (and reversibly) bonded, II - strongly bonded. The maximum rate of desorption for I occurred at 110 0 ± 200C and for II at N 280 0C. With copper-containing samples (CUO, Cu20 and CuO - MgO), acrolein is desorbed in the same temperature region as is the form I. At the same time, form II corresponds mainly to deep oxidation products. Similar results were obtained with cr20 and MoO It J/Al 20 J• J was found that strongly bound propylene on deep oxidation catalysts (CuO and Cr 20 ) is predominant while weakly bound olefin prevails 3 ) . Fig. 168 illuson mild oxidation catalysts (CU 20 and MOO J/Al 20 3 trates the data for Cr 20 and MoO In the last case, the 3 J/A1 20 J• thermodesorption products contain not only acrolein but also acetaldehydeo It is noteworthy that CHJCHO is also formed (together with CH 2=CHCHO) in the catalytic oxidation of C 6 JH It is natural to believe that form I leads to mild oxidation products of catalysis while the form II results in deep oxidation. The analysis of experimental data on the IR-spectra of adsorbed propylene has led to the conclusion /78-80/ that the weakly bound and reversible form I of adsorbed propylene can be attributed to :J{ - and (3 -allylic complexes while the strong form II corresponds to carbonate-carboxylate type species. On the surfaces of cr 20 3 0
540
and MOO)' only 6' -allylic complexes were detected. J.r-allylic complexes are syrrunetrical and are attached to a surface by .1! -bonds:
1/7/ / / / / 7/71/
The adsorption in this case is likely to be of the two-point one type, the complexes being parallel to the surface. 5-allylic
l/mv
I
a
b
Fig. 168 - The thermodesorption of propylene from Mo0 (a) 3/A1 20 J and Cr 20 (b) /80/ J
complexes are nonsymmetric and are bound by one dicular to the surface:
6' -bond perpen-
II
H2
CH
I
CIt 2
J; In comparison with allylic species, carbonate-carboxylate species are more stable; above 200 0C, they are destroyed and converted into CO 2 and H20. Irreversibly chemisorbed propylene is also supposed to include stable %-complexes. In order to discriminate between %- and 6-allylic complexes,
541
the thermodesorption of individual supported 6-allylic complexes of Mo and of Jr -allylic complexes of Ni, as well as the desorption and IR-spectra of allyl bromide, were studied in references /78/ and /79/. It was shown that on copper- or nickel-containing catalysts, Jr-allylic complexes are most important. With Cr 20 and MoO 3' 6 -allylic complexes pre domina t e , 3 Since form I can be detected in rather high quantities even at 300-1300C, it was assumed in references /78/-/80/ that, in this temperature region, the partial oxidation of propylene with almost 100% selectivity can take place (Form II is removed at elevated temperatures). Indeed, when pUlses of C 6 + 02 (1 : 1) were 3H injected onto CuO-MgO and cr 20 at 80 0_1200C, the oxidation of 3 C 6 into acrolein with 100% selectivity was observed. 3H The above facts allow us to propose a mechanism for the catalytic oxidation of propylene over metal oxides under conditions when carbonate-carboxylate complexes are easily destroyed and the rates of catalysis and surface reduction are equal. The proposed scheme should be considered as a tentative one. In a shortened form, the mechanism of the parallel formation of acrolein and CO 2 will be: ( )
1) °2
)-(°2)-2(0) fast 2) C 6 + (0)-(1 1) (O) .. CHO + H2O 3H fast 3 4
+
7(0) 3) C 6 + 2(0)-(1 2) 3H fast + 4) C )~(C3H40) 3H40 5) H20 +
3C02 + 3H2O
(XVII.52)
) ~(H20)
The correspondence between the rates (and selectivities) of catalysis and of surface reduction of the oxides by propylene, as well as of the formation of acrolein by the desorption of adsorbed propylene, suggest that atomic anions of surface oxygen, (0), are important intermediates in the reaction. The interaction of C 6 with (0) leads to a surface compound 3H (1 1), resulting in the formation of acrolein, while the interaction of propylene with two (0) species leads to complex (1 which 2), is converted into deep oxidation products. The species (1 are li1)
542
kely to be allylic complexes and species (1 2) may be precursors of the carbonate-carboxYlate structures. In the C 6 molecule, the 3H energies of cleavage of the ff-c-c-bond (63 kcal mol- 1 ) and that of the C-H-bond in the allylic position (77 kcal mol- 1 ) are close, so that both directions of reaction are probable. The first results in the formation of allylic complex; the liberated H-atom reacts with (0), giving the (OH)-group. The second leads to a complex of the type:
which is converted into the carbonate-carboxylate structures. This is in accordance with the fact /85/ that an intermediate leading to acrolein requires fewer O-atoms than that leading to CO 2 • In the interaction with a surface, propylene is an electron donating agent while 02 is an electron acceptor /19/. Since acrolein can be reversibly adsorbed on copper oxides, its equilibrium chemisorption is possible (step The same applies to water vapour (step 5». The corresponding rate equation will be:
4».
~ f r=-P ( 1 - 8 ) - - - - - - - - - I ~ r- b P + b n
~/{f{J
f3~IJ
If 0 I"!I;IJ
(XVII.53)
The equations for the selectivities towards acrolein (s(I» and CO 2 (S(II» and for the rates of formation of these products (r(I), r(II»are: s(!):
4 '3
(V9+
6' (XVII.55 )
543
I,
T P0
(!-IY)
(XVII. 56)
fJ%/I,!O
+
°1f0 P~O
(XVII. 57 )
where b i are adsorption coefficients. The value of mined by the expression:
8
is deter-
B=(XVII.58)
where
1 (XVII.59)
The value of
8
decreases when
decreases and when
PC
° or bH2OPH2° 3 4
bC H oPc H 3 4
_P0 2
3H 6 increases. When"" - 0 , 8 - 0 , while
when ~ -"""", 8 1. The kinetics of the oxidation of C 6 over Cu20 were studied 3H /27/ with excess of propylene and rather high concentrations of products. Therefore, the value of & in these experiments was small. At B «1, Eqs (XVII.56) and (XVII.57) reduce to:
I,
-
-PJ!
I{£)=
Iro t;~o
P ~h10
0
(!)
S
fJ hpO !f?O
+-0
(XVII.60)
544
i1 Y
/f b
P
P
°2
t.J If'l 0 C,/If 0
+
0
10
(XVII.61 )
fl
!(z 0
The formsof these equations coincide with the empirical Eqs (XVII. 49) and (XVII.50) which describe the reaction kinetics if one assumes that k 1 and k in Eqs (XVII.49), (XVII.50) are given by:
2
f II (/) f =-,f
1
P
The analysis of the data of reference /27/ shows that these relations are valid (see, for example, Fig. 169).
s/%
Fig. 169 - The selectivity towards acrolein against in Eq. (XVII.49) for catalysis on Cu20
k;
05 00
!.2
fA.
f.6
If!
Eq. (XVII.54) suggests that the selectivity towards acrolein should increase with decreasing tl. As the latter decreases with increasing PH 0' one should expect an 2
enhancement of the selectivity with increasing pressure of water vapour as is also observed. Finally, one should expect an increase in the selectivity with increase in Pc H since 8 decreases with increase in Pc H ; this 3 6 3 6 is also observed /27/.
545
Mechanism (XVII.52) thus permits an explanation of all the experimental data obtained with cuprous oxide catalyst. At be HOPe H 0 + bH oPH 0 ~ 1 and 8 « 1, Eq. (XVII.53) redu3 4
3 4
2
2
ces to Eq. (XVII.51) which describes the reaction kinetics on vanadium and lead oxide catalysts. When mild oxidation products are not formed, Eq. (XVII.53) reduces to:
i,
1
q ( 1 -18+ )of!,-()-PI!- -0
T=-j}
I
'2
(XVII.64)
2'
and instead of (XVII.58) and (XVII.59), we have:
8= (XVII.65 )
!
~ u1 = _2_. fl
'31
1+
_
0/f?(} P1?O
(XVII.66)
Power rate equations (V.49) observed for deep oxidation on many oxide catalysts are likely to be an approximation of Eq. (XVII.64). If this is so,
8 2-8
m=--,
2(1-& ) 17=
,
2-8
f
2
l =/J-2-8
(XVII.67 )
where B is the surface coverage with oxygen and 8 f is that with H20 /43/. One should then expect opposite changes in m and n what is observed (Fig. 167). Due to the dependence of m and n on 8 , the reaction orders correlate with surface oxygen bond energies /43/. A similar relationship is noted in /57/ but it is there interpreted on the basis of a Langmuir-Hinshelwood mechanism. The overall reaction rate according to Scheme (XVII.52) can be
546
!
tffr -J
-5
/0
-4-
-0
02
-1
-5
-8
02
_g
-0
04-
.5/%
a
5D 40
-1
.f0
2D fO
o
20
40
o
60 ~s/Kcat(;-atOr
L-.:.ow-o:£:J_...l...-~_.l...
Fig. 170 - The specific catalytic Fig. 171 - A correlation betactivities of metal oxides in the ween qs and the catalytic actioxidation of C3H 6 (Table 78) ver- vity towards mild (I) and deep sus qs: 1 - c0 (II) oxidation of C 6 (a); and , 2 - CUO, 3 3H 30 4 NiO, 4 - Mn0 2' 5 - Fe 20 6 - Sn0 2,the selectivity towards mild 3, 7 - V 8 - CdO, 9 _ ZnO, 10 _ oxidation of C3H6(b) (based on 20 5, the kinetic data of reference W0 3, 11 - Mo0 3• /63j): 1 - c0 , 2 - Mn0 2' 30 4 3 - NiO, 4 - CuO, 5 - Fe 20 3, 6 7 - Ti0 2, 8 - ZnO, 9 V20 5, MoO)' 10 - W0 3• written in a different way: r= r "2
+
r. .. j P 3
.?
t.J1Io
8
+
*3 Pc3//6 8 !
(XVII.68)
1.1
At high oxygen coverages ( 8
~
1)
Since in steps 2) and 3), oxygen-catalyst bonds are broken, one should expect decreasing values of r with increase in q s • This relationship is observed in practice (Figs 170 and 171). The maximum activity is exhibited by C0 for which qs is close to 304'
547
(q) s op t • At q s L... (q) s op t' tne catalytic activity decreases since the surface coverage with oxygen is small at low values of qs. The rates of mild and deep oxidation, r m and r d, at 17 ~ 1 will be given by:
r =- r. m 2
=- J~
c
Pc
u
'3 no
(XVII.70)
(XVII.71) In each of steps, 2) and 3), oxygen-catalyst bonds are broken which results in decreasing values of r m and r d with increasing values of qs (Fig. 171 a). At the same time, more oxygen-catalyst bonds are destroyed in the third step and this leads to a sharper decrease in r d compared with r m• A direct consequence of this is an increase in the selectivity with qs (Fig. 171 b) /86/. The above changes in r, r m, r d and S with qs can explain the parallel changes in rand r m (Fig. 163) as well as the inverse changes in the activity and selectivity (Fig. 164). The determining role of qs allows us to interpret some phenomena observed in the non-steady period of the reaction over oxides. During this period, two processes occur: the reduction of the metal oxide and the irreversible adsorption of propylene. Both processes result in a decrease in 8 and (if the surface is nonuniform) in an increase in qs. One should therefore expect a decrease in the catalytic activity and an increase in the selectivity with time, as takes place in practice (Fig. 165 and 166). If the values of 8 and qs change slightly, the catalytic properties should be practically unchanged. The data of reference /65/ show that in most cases investigated, both of these two situations are observed. Since the mild oxidation rate increases with decreasing qs while the selectivity decreases, a definition of I·the optimum catalyst for mild oxidation" is rather difficult. When the further oxidation of the partial oxidation product is small, its highest rates of formation are attained on catalysts with small q values; s these substances are Usus ally considered to be typical catalysts for deep oxidation. An example is given in Fig. 162. According to reference /86/, the following feature is essential for selective oxidation when a parallel scheme is valid. Owing
548
to the fact that more oxygen-catalyst bonds are broken in deep oxidation than in mild oxidation, the activation energy for the first process on each catalyst should be higher than that for the second one. This is observed with Cu- and V-containing catalysts. As a result, the selectivity decreases with increasing temperature. This effect is enhanced by the progressive oxidation of mild oxidation product. Thus, Scheme (XVII.52) can serve as a basis for discussing the various experimental facts concerning the catalytic oxidation of propylene over oxides. It should be noted that the absolute catalytic activity of transition metals in this reaction is higher than that of the metal oxides (Fig. 1J4). This is likely to be due to the higher rates of the specific activation of C 6 on metals, resulting in faster JH rupture of the J.r-C-C-bond. This is why, on transition metals, the prevailing reaction route leads to surface species of the type: HC-~-~
2
1
I
J
o 0 I I resulting in deep oxidation. This route is likely to involve intermediates the desorption of which gives acetone (on metals), acetaldehyde and CO (on oxides). On the other hand, the "allylic" route (which is predominant on selective oxide catalysts) can include dimerization and oxidative condensation of the allylic complexes with the formation of C6-products (benzene; 1,5-hexadiene, etc.). These products appear mainly on the oxides of nontransition metals which suggests the importance of acid-base properties in this case. At elevated temperatures, metal oxides (like metals) can initiate a heterogeneous-homogeneous radical-chain process. According to reference /88/, the C 6 oxidation on Cu20 at JOO o-J70 0C JH proceeds only on the surface of the catalyst while homogeneous continuation of the process is observed at 400 0C. The Oxidation of Propylene Over Complex Oxide Catalysts A rather high selectivity towards acrolein is shown by complex copper-containing oxide catalysts which operate (like Cu20) with
549
the excesses of C 6 /89/. The selectivity of CuI-chromite at 3H 350 0C is considerably higher than that of CuII-chromite /90/ which is similar to the relations between Cu20 and CuO. The catalytic activity of solid solutions of CuO in MgO increases with copper concentration /78-80/. The introduction of P20 into copper oxide 5 catalyst (copper phosphate is formed) decreases the activity. At 20 at.% of P, the selectivity increases. The activation energy for the formation of acrolein on the catalysts is lower than that for the formation of CO 2 /19/. At the same time, complex oxide catalysts containing such metals as Co or Mn, like C0 and Mn0 2, catalyse the complete com304 bustion of propylene. The kinetics of this process on Col~204 and MnC0 20 follow Eq. (XVII.51). Additives of 1i, Ti and Cu oxides 4 to these catalysts decrease their activity /91/. For the synthesis of acrolein from propylene, complex oxide catalysts based on Mo0 (Bi-Mo-O and similar systems) are employed 3 in practice. In contrast to Cu20, they operate at elevated temperatures and with excesses of 02' With Bi at 450 0C(and 9PMo052/Si02 in the presence of water vapour), a selectivity of almost 60% towards acrolein (at 92.5% conversion) is attained /66/. By-products are CH CHJCOOH an~ CH 2=CH-COOH (overall selectivity reaches 3CHO, 20%) with a balance of carbon oxides. With Bi 20 /92/ at 460°C and with an 02/CJH6 ratio of 1.0 3"Mo0J to 1.5 (in the initial mixture), a selectivity of 90% is achieved at 20-40% conversions. (At a conversion of 85%, S ~ 68%). The selectivity passes through a maximum at 460°-520°C. The parallelconsecutive scheme is valid under these conditions, the rate equation being:
(XVII.72) (The products do not affect the reaction rate). A detailed study of the kinetics of the oxidation of C 6 on 3H Bi 20 catalysts has been made in references/93/ and /94/ at J-Mo03 430°-465°0 and with excesses of 02 using the flow-circulation technique. The specific activity of the above catalysts is not an additive quantity; it passes through a maximum at Bi:Mo ~1. In this system, new chemical compounds are formed which are bismuth molybdates
550
Bi 20 and Bi 20 3'2Mo0 the last being the most 3, 3'Mo03 active. The rate law with this catalyst at quite high values of Po is given by Eq. (XVII.72); at low pressures of 02' the rate in6reases with Po ; the process is retarded by acrolein. Bi203'~~o03'
2
Table 82 shows that the specific rate constant of Eq. (XVII.72) is almost independent of the method of catalyst preparation. The activity decreases in the order: Bi 20 2Mo0 :::-Bi 20 )Mo03> Bi 20 3· 3 3· 3• 'M00 In this case (in contrast to previous ones), the selectivi3• ty towards mild oxidation changes in parallel with the overall activity. On Bi 20 /19, 92/, the selectivity is changed sli3·2Mo03 ghtly with temperature (470 0-525 0C) and with a C 6/02 ratio of 3H 0.7 to 3.0. Interesting behaviour is displayed by Co-Mo-O and Sn-Mo-O catalysts. At elevated temperatures ( ~ 450 0C), the major product of the partial oxidation over these catalysts is acrolein. On the Co-Mo-O catalyst with excess of C~6' a 50% selectivity is attained (conversions are 4.5-5.5%). An H additive increases the 3P04 selectivity up to 60%. Increased concentrations of propylene (in contrast to 02) favour high yields of acrolein. The optimum Co:Mo ratio for the formation of this product is 0.4-0.5. In this case, a selectivity of 67% is attained at 450 0C and 3-4% conversions /95/. Acrylic acid is produced on a Co-Mo-catalyst in addition to acrolein /95/. At 3800C and 27% conversion, the selectivity to CH 2=CHCOOH is 18% /19/. For the production of acrylic acid from propylene, Co-Mo-oxide catalysts (with additives of P, Sn, Te and Ni compounds) are recommended; temperatures are 3500-3700C and ]0-40% H20 is added to the propylene - air mixtures /19/. A different picture is observed with the same Co-Mo catalysts at lower temperatures (200 0_3000C). Under these conditions, acetone is a main product of partial oxidation /96-98/. C0 is an active catalyst for the full oxidation of propyle304 ne. Small amounts of MoO] (10 at.% Mo) introduced into the Co]04 reduce the activity sharply and change the selectivity. Acetone becomes the major product on Co-Mo-eatalysts; acetic acid, CO 2, acrylic acid and acrolein are by-products (Table 8). At elevated temperatures, the selectivity to CH decreases while that to 3COCH3 acrolein and acrylic acid increases; the selectivity to CH]CHO passes through a maximum. A concentration of 10-]0 at.% of Mo is optimal for the production of CH MoO) is relatively inactive, 3COCH).
( !
Bi20
-phaae )
3'Mo03
Precipitation Fusion
Precipi ta -. tion Fusion
Bi 20
3'2Mo0 3 ( fi-phase)
Precipitation Fusion
Method of preparation
Bi203'3M003 ( ci.. -pnaae )
Catalyst
16.9
11 .1
11.3
16.8
9.35 17.2
17.2
4.5
2.7
4.3
2.7
4.5
16.9
11.3
9.5
2.7
H2O
17.2
°2
9.5
C 6 3H
67.2
69.6
67.2
0.046
0.14
0.19
1.19 9.4
-
1.00
0.395 0.55
0.006 7.94
0.51
5.70
-
0.16
67.2 69.6
5.29
0.52
acrolein
7.14
acrolein + CO 2
0.51
1.23
1.06
9.17
1.38 10.78
1.85
CO 2
k·10- 2/ min- 1 m-2
0.60
Distribution of products/ voI , % acro- CO 2 lein
69.6
N2
Initial mixture/vol.%
The Oxidation of C 6 on Bismuth Molybdates (475 0C; Contact Time 0.8 s) /19/ 3H
TABLE 82
52.0
86.5
87.0
74.0
Selectivity to acrolein/%
CJ< CJ< .....
552
TABLE 83 : Mo = 9). The Oxidation of Propylene Over Co 304-Mo03(CO Reaction Mixture is: C 6 - 20, 02 - 30, H20 - 30, N2 - 20 vol.% 3H /98/.
Catalytic ties
195°C
Conversion /% Product distribution (%) :
2.0
CH
80.9
210°C
280°C
4.0
18.0
7503
20.0
traces
1.3
3.0
CH 2=CH-COOH
0.5
0.6
2.0
2/3 CH
9.9
11.4
6.0
0.8
9.0
10.6
60.0
3COCH3
CH 2=CHCHO
3COOH
1/3 CO 1/3 CO 2
8.7
operates at high temperatures and does not give acetone (the main product being acrolein). The overall activity decreases with increasing concentration of Mo (Table 84, Fig. 172). The kinetics are described by Eq.(XVII.48). The order in C 6 (m) increases and the order (n) with respect to 3H O2 decreases with an increase in the concentration of ~m02 (Fig. 172).
In the co system, phases of CoMo0 are formed, the lat304-Mo03 4 ter existing in several modifications. According to reference /97/, a high selectivity to acetone should be attributed to some optimal ratio between Co and CoMo0 At the same time, the observed ac304 4• tivity at different Co : Mo ratios does not correspond to the activities of mechanical mixtures of co + Mo0 or Co + CoMo0 304 4• 3 304 It is interesting that with the co + Mo0 mixtures (Co : Mo = 4 304 3 or 9), small amounts of acetone are formed. X-ray analysis does not detect any chemical compounds. Preliminary heating at 550 0C results
553
TABLE 84 The Kinetic Characteristics of the Oxidation of Propylene Over c0 0 - Mo0 /97/ 3 4 3 Co : Mo ratio
E/ kcal mol- 1
19 k o/
ml m-2 s -1
T /K*) r
10 3K/Tr
10/0 9/1 8/2 7/3 6/4 5/5 4/6 3/7 2/8 1/9 0/10
31.0 14.2 13.9 15.7 12.1 13.0 15.0 13.0 12.6 16.1 13.1
10.2 1.7 1.6 1.9 0.2 0.] 0.5 -0.1 -0.] 0.6 -0.3
483 565 571 606 676 695 769 794 826 820 847
2.07 1.77 1.75 1.65 1.48 1.44 1.30 1.26 1.21 1.22 1.18
Reaction order in C O 2 3H 6 (n) (m )
0 0.50 0.64 0.70 0.70 0.65
0.66 0.20 0.12 0.20 0.15 0.15
0.80
0.03
0.82 0.91
0.10 0.02
*) Tr is the temperature required to attain an overall reaction rate of 2 x 10-4 ml m- 2 s-1.
in a sharp increase in the yield of acetone, a CoMo0 phase being 4 detected simultaneously. Thus, complex Co-Mo oxide catalysts accelerate reactions (XVII. 39)-(XVII.45) leading to acetone, acrolein, acrylic acid, acetic acid and carbon oxides. The selectivity depends upon the temperature (and other conditions). At lower temperatures, acetone and acetic acid prevail, while at elevated temperatures, acrolein, acrylic acid, CO and CO 2 predominate. In the intermediate region, all these products can be obtained simultaneously. Such experiments were made at 200 0-450°0 /99/. Fig. 173 shows that a Co : Mo ratio of 1.0 is optimum for the formation of acrolein, while for acrylic and acetic acids, the optimum ratio is close to 2.0. The highest rate of the formation of acetone is achieved on catalysts enriched by Co. According to reference /99/, the activity for the production of
554
Fig. 172 - The catalytic activity (1) and the reaction orders in C 6 (2) 3H and 02 (3) for Co-Mo-O catalysts /97/.
the acid is accounted for by the presence of one of the CoMo0 modifica4 tions; the Co in the latter has an octahedral environment. The concentration of this phase increases with the 0.5 Mo0 excess up to the point where 3 Mo : Co ratio becomes equal to ~ 2. Three-component catalysts C0 304• J 20 40 00 JO;}1/l/tIt.% Mo0 3-MeXO y' where Me = Ni, Zn, Fe, Cr, Mn, Cu, Ti or V, are less active than C0 in the oxidation of 304-Mo03 C 6 to CH 2=CHCOOH. This is due to the interaction of MexO y with 3H On Co-Mo-Cr-O, Mo0 which decreases the concentration of CoMo0 4• 3, mainly acetic acid is formed; on the addition of the oxides of Ti, V, Cr, Mn or Cu to Co-Mo-O, the rate of deep oxidation increases while the additives of NiO or ZnO decrease it. Tin-molybdenum oxide catalysts behave like cobalt-molybdenum ones. The former are more active and, operating at lower temperatures, are more effective in the formation of acetone from propylene /98/. Table 85 indicates that the selectivity towards CH at low 3COCH3 temperatures reaches 90%, acrolein and acrylic acid also being in the reaction products. On elevating the temperature, the acetone yield decreases and that of the carbon oxides increases;the selectivity to CH passes through a maximum /98/. 3COOH The product distribution in the oxidation of C 6 on Sn0 2-Mo0 3 3H (Sn : Mo = 9) under a wider region of conditions /56/ is given in Fig. 174. The observed pattern is similar to that obtained on C0304 - Mo0 3• The catalytic properties of Sn0 2 - Mo0 catalysts supported on 3 porcelain at 380°-480°C (when CH 2=CHCHO (CH CH 2=CHCHO, 3CHO), CH 2=CHCOOH and carbon oxides are produced) have been studied in references /100/ and /101/. The reaction mixture was: C 6 - 11, 3H 02 - 11, N2 - 78 vol. % and conversions did not exceed 20%. On 1.0
555
r(
25 20 /5
/0
.5 /rfo
I
0
0 200
JOO
400
t/oC
Fig. 173 - The rates of formation of CO 2(1), acrylic acid (2), acetic acid (3), acrolein (4) and acetone (5) in the oxidation of C 6 over Co-Mn-oxide catalysts /99/. 3H Fig. 174 - The rates of formation of different products in the oxidation of C 6 over Sn0 2-Mo0 (Sn : Mo = 9 5) /56/: 3 3H 1 - CO, 2 - CO 2, 3 -.CH 2=CHCHO, 4 - CH 5 - CH 6 3COOH, 3COCH3, CH 2=CHCOOH, 7 - CH 3CHO. changing the Sn : Mo ratio from 9 : 1 to 1 : 9, the overall selectivity towards partial oxidation was 73-85%; for the acids, 5-20% and for the acrolein, 24-77%. The dependence upon catalyst composition of the rates of production of the different products and the selectivities is given in Fig. 175. According to references /100/ and /101/, the most active Sn-Mo-O catalyst (Sn : Mo = 9) is solid solution of Mo0 in 3 Sn0 2• It is rather selective towards acrolein. The acrylic acid yield on the Sn-Mo-O catalysts is favoured by elevated temperatures and high concentrations of H20 in the mixture. The process was studied under these conditions in references /102/ and /103/, using both a massive catalyst and supported ones (the carriers were porcelain, carborundum and porous corundum). With Sn-Mo-O/porcelain, on increasing the temperature from 200°C to 500°C, the concentrations of acrolein, acrylic acid and CO 2 increase while those of CO and acetic acid pass through a maximum
556
TABLE 85 The Oxidation of Propylene on sn02~~003. Reaction Mixture is C 6 - 20, 02 - JO, N2 - 20, H20 - JO vol.% /98/. JH
Temperature Catalytic properties
115°C
Conversion /% 2.9 Production distribution(%): CH 90.0 3OOOH 3 2/3 OH traces 3OOOH 1/300 1/30°2
124°C
135°C
175°0
3.9
9.0
22.0
90.0
85.3
65.0
traces
2.6
2.0
traces
3.0
12.1
30.0
10.0
10.0
at around 400°0. The selectivity to acrylic acid is 28.4% at 400°0 and 42.6% at 480°0. At 460°0, the selectivities to acrolein and acrylic acid increase with Po H and decrease with increasing Po • Growing con3 6 2 centrations of water vapour decrease the selectivity to acrolein and enhance the selectivity to acrylic acid. The temperature dependences of product distribution for other molybdenum oxide catalysts are similar. Thus, with the molybdate of iron (or Or or Mo) at low temperatures, acetone is formed /98/, at 360°-390 00, the oxidation of C 6 leads to acrolein /104, 105/ 3H and to small amounts of acrylic acid /19/. Table 86 shows that the molybdates of the II group metals give (at 350 00) the same products as with the Sn0 2-MoO catalysts. As seen from Fig. 176, the selecJ tiVity of the above molybdates for "allylic" oxidation decreases with an increase in their overall actiVity. The Sb 20 - !ili)03 systems, like Bi 20 Mo0 exhibit a high se4 3, lectivity towards acrolein at 380°-480 0 /101/. The Sb : Mo ratio affects the catalytic properties slightly (the exception being a sharp increase in specific rates on going from Mo : Sb = 1 : 9 to
6
-
557
.5,% 100
r·/(7 8
72,_ 6*f~ 28
75
/
50 50 20
25
0 100 I
0
!
20 JTl/% I I 80 Mo/% 0
I
Fig. 175 - The rates of formation of different products (a) and selectivities (b) in the oxidation of C 6 over Sn0 2-Mo0 at 3 3H 430 0C: a) 1 - acrolein, 2 - CO 2, 3 - acids, 4 - CO; b) 1 - partial oxidation products, 2 - acrolein, 3 - CO 2' 4 - acids, 5 - co /100/. Fig. 176 - A comparison of the specific catalytic activity and selectivity towards "allylic" oxidation of C 6 (experimental data 3H of reference /106/): 1 - BeO-Mo0 2 - ZnMo0 3 - BaMo0 4 3, 4, 4, Mo0 5 - MgMo0 6 - CaMo0 7 - CdMo0 3, 4' 4, 4• The selectivity towards CH 2 = CHCHO + CH is 80-90~. Sb 20 4). 3CHO These catalysts involve phases of Sb 20 Mo0 and antimony molyb4, 3 date. Their catalytic properties are similar to those of Mo0 /101/. 3 /101/. The complex oxide catalysts, Cu-Mo-O and Sn-Sb-Mo-O, are also rather active and selective /107/. At 360 0C, the overall activity of the molybdates decreases in the sequence: Bi 2(MoO 4)3 /" Fe 2(MoO 4)3/ CoMoO 4-;>PbMoO4. The selectivi ty decreases in the order: Bi 2 (MoO 4) 3"7 CoMoO 4;> Fe 2 (MoO 4) 3;> PbMo0 4 /104, 105/. The V20 S - MoO] /19/ and P20 S - Mo0 /19, 9S/ systems have also 3 been studied. The addition of V20 and P to Mo0 enhances its 20 S S 3 activity and decreases its selectivity for the formation of acrolein. A catalyst containing S-10% ABSOS' 10% Nb 20 and 20% MoO], supS
BeO'Mo0 3 MgrdoO 4 CaMo0 4 ZnMo0 4 SrMo0 4 CdMo° 4 BaMo° 4
Catalysts
0.4 20.2 71.9 46.7 64.5 8703 15.3
> 0
ri ri CIl H Ql
Specif~c
s::!o ,,-l CIl
~H
s::! -r-l Ql ri 00
0 0.4 0.4 0.4 0.4 1.4 0.8
0 ,,-l r-l o l»'d ~ H',-l s::!oo ,,-l CIl CIl
5.6 2.5 1.2
7.1
4.6
2.1
0
s::!o ,,-l CIl
~Ql
O~
a> s::! 0
0 0 10.7 0 16.5 0 0
.t'
I ria> CIl'd o ~ ~a> s::!OQl ,,-l CIl'd Ql',-l s::loo ,,-l CIl CIl
0 5.4 7.0 8.7 4.7 1.6 3.7
~
O~'d
0 ..-t C\l s::lO ,,-lO
0 10.8 42.8 15.5 31.8 76.0 7.3
~
0
' Immol m-2-1 rate of propy1 ene converS10n s
0.4 1.5 6.37 15.5 5.5 5.8 203
,
9.5 34.1 9.2 8.2 20.3
9.1
100
Selectivity to acrolein + acrylic acidl %
The Oxidation of Propylene Over Molybdates of the II Group Metals (350 0C; Contact Time 0.8 s) 1106/.
TABLE 86
00
on on
559
ported on 3i0 2, provides a 50~ yield of acrylic acid at 400°C /108/. Systems based on W0.3 are also rather selective (see Table 87). P additives decrease the activity and increase the selectivity 205 of W0.3 /19/. Bismuth tungstates (Table 87) are similar to bismuth molybdates (Table 82). The activity of Bi 20.3'2W0.3 is close to that of Bi 20j .2Mo0.3 but the latter is more selective. High activity is found TABLE 87 The Oxidation of C.3H6 Over Bi 20 Contact Time 0.8 s; J-W0.3(4800C; Reaction Mixture is: C.3H6 - 11.2, 02 - 16.8, H20 - 4 •.3, N2 - 67 • .3 vol. %) /19/. 10- 2k/ min- 1 m-2 Selectivity to acro- CO 2 CO CH.3 CHO Over- acrolein lein all /% process
Catalyst
Bulk composition after catalysis
Bi 2°.3'5 W0.3
W0.3 + oc-phaee
4.6
1.5 1.0
0 • .3
7.4
62.0
Bi 2°.3'JWO J
ex -phase
5.9
2.8 0.8
0.2
9.7
61.0
Bi 2°.3' 2W0.3 (preheated at 600 0C) Bi 20.3' 2W0.3 ex: - + ! -phase
7 • .3 6.2
2.9 0.9 .3.1 1.4
0.2 0.2
11 • .3 10.9
64.6 57.0
!
5 • .3
.3.4 2 •.3
11.0
48.0
Bi 2°.3'W0.3
-phase
+ W0.3
with Bi-V-O-catalysts but they are poorly selective /19/. The addition of P20 to V20 results in effects which are similar to 5 5 those in the W-P-O systems /19/. Some data have been published concerning the salts of transition metals and anions such as phosphate, arsenate, ant imonate, sulfate /19/. At .370 0C, the phosphates of Co and Fe are poorly active and do not produce acrylic acid. With iron phosphate, carbonyl compounds are formed while the phosphate of Co only catalyses deep oxidation. A rather high selectivity for the "allylic" oxidation of propylene is exhibited by some complex catalysts which do not contain the oxides of transition metals. Usually, these involve the
560
elements of the main subgr-oups IV and V of the Periodic Table (Sn, Sb and Bi). The latter can change valency easily. Propylene is oxidized over the Sn-Sb-O catalysts at 380 0-480 oC into aldehydes and CO 2, the selectivity to aldehydes being 93-95%, to acrolein 70-95% /109/. The catalytic properties of these systems (except regions of composition near Sb 20 and Sn0 2) are al4 most independent of the Sn : Sb ratio /109, 110/. The above catalysts consist of solid solutions; at rather high Sb contents, two phases (antimony oxide and solid solution) are present. The former phase is assumed to affect slightly the catalytic behaviour of the complex catalyst /109/. According to reference /110/,the rate equation is:
r=kP
%Iia
B
(XVII.73)
where 8 is determined by the conditions of adsorption equilibrium on a homogeneous surface. With Bi 20 - Sb 20 catalysts, when Bi : Sb ~ 1, only deep oxi4 3 dation takes place. Sb-enriched catalysts are poorly active but highly selective towards acrolein /111/. Over a Bi - Sn catalyst (Bi : Sn = 1), propylene is oxidized only into CO 2 and H20, which is quite different from the similar system Sb - Sn /101/. The oxidation of propylene on Bi-P-O catalysts /63, 111/ at elevated temperatures gives CO 2, acrolein and benzene as the predominant products; small amounts of CO, CH and 1,5-hexadiene 3CHO are also formed. Thus, the catalyst accelerates the oxidative dimerization of C 6 (reactions (XVII.46) and (XVII.47)). Fig. 177 3H shows that the highest rates of the production of benzene correspond to Bi : P = 2, the selectivity being 40%. The ratio Bi : P = = 1 is the most favourable for acrolein formation. The catalyst with the ratio Bi : P = 2 is a mixture of the phases BiP0 4, 2Bi203oP205 and !-Bi 20 while the sample with Bi : P = 1 is a 3, monazite-type phase (BiP0 Deep oxidation is accounted for main4). ly by 0" -Bi 20 /111/. 3 Benzene (with acrolein) is also formed on other salts of bismuth in a similar way /111/. Thus, with Bi 20 - Sb 20 and Bi 20 34 3 - Sn0 2 below 500 0C, the products are CO 2 and acrolein, while benzene appears at elevated temperatures. It is interesting that the last product is not formed on Bi-molybdates (see Table 88).
°
°
5
[Bi: P = 2]
Bi 2(Mo04)3 [Bi : Mo =
2/~
Bi(Bi)(Mo0 4)2 [Bi : Mo = 1]
~ 5.9
23.3
37.7
0
0
0
9.1
43.0
BiP0
4 (BiO)2Mo04 [Bi : Mo =
10.1
26.9
33.8
49.0
61.0
79.8
69.0
80.4
18.0
(high-temperature)
P20
45.7
4
4
3'
-
94.9
91.7
66.1
38.6
4.0
0.3
6.6
° 5.8
Selectivity 1 10 to benzene to acrolein
Bi 20 . 2Ti0 2 3 (BiO)2 S04
BiP0
BiAs0
2Bi 20
Catalyst
Conversion of C3H61 %
The Oxidation of Propylene Over Bismuth Salts 1631
TABLE 88
en 0)
......
562
o
x/% 40
/
.
0/% 20
JO
15
20
III
/0
5
0
---
--
The conversions of propylene into different products Fig. 177 over Bi 20 - P205 at 500 0C /111/: 1 - CO 2, 2 - C6H6, 33 CH 2=CHCHO. Fig. 178 - Conversions (1) and selectivities to C6H6 (2) and acrolein (3) for the oxidation of C 6 over Sn0 2 with additives of 3H P205 and Na20 (550°C) /63/. The promotion of Sn0 2 with Na20 enhances the selectivity to benzene while P 20 additives favour the formation of acrolein 5 (Fig. 178). Mo0 and other anhydrides act like P20 /63/. 5 3 Complex catalysts based on As20 (As-Cu-O, As-Ca-o, As-Cd-O, 5 As-Bi-O, As-Fe-O, etc.) display high (above 60%) selectivity to acrolein (410 0-4800C, 14-18% C 6 in air) /112/. 3H Thus, the following empirical relationships can be formulated for the oxidation of C 6 over complex oxide catalysts: 3H 1. The most pronounced changes in catalytic properties on going from the individual oxides to complex oxide systems are usually connected with the deep chemical interactions within these systems, reB~lting in the formation of new chemical compounds (daltonides) or solid solutions (bertholides). Their quantitative (sometimes qUilitative) characteristics differ significantly from those of mechanical mixtures. Nevertheless, the following rule is likely to be valid: at least one of the elements in a selective complex catalyst corresponds to a simple oxide which is also selective. For example, molybdates contain Mo (MoO] is a selective
563
°
catalyst), the Bi - Sb systems include Sb (Sb 20 is a selec4 tive catalyst) and so on. On the other hand, the spinels Co~~-O and ~m-Co-O, accelerating only the oxidation of C 6 to CO 2, con3H tain Co and Mn which give typical catalysts for deep oxidation and Mn0 2 ) . (C0 304 A catalyst is often a heterophase system (solid solution + chemical compound). This characteristic complicates the comparison of catalytic and physicochemical properties since each phase can be considered as a catalyst with its own special behaviour. The above correlation should be made for homogeneous solid systems. TABLE 89
The Catalytic Properties of Complex and Simple Oxides in the Oxidation of Propylene (450 oC; Reaction Mixture is: C 6 - 11, 3H 02 - 11, N2 - 78 vol. %) /101/ Catalytic activity.1 0 7 / mol C -2 6 m s-1 3H
Catalyst
Bi 20 3 SnO 2 Sb 20 4 Mo0 3 Bi-Mo-O Sn-Mo-O Sn-Sb-O Bi-Sb-O Mo-Sb-O Bi-Sn-O
(Bi (Sn (Sn (Bi (Mo (Bi
Mo Mo Sb Sb Sb Sn
1)
=1 =9 =9 = =
1) 1)
3) 1 3.5 : 6.5) 1) 1
.
11.65 6.80 0.13 0.46 21.64 13.88 1.40 0.24 0.64 2.26
Selectivityj)G to acro- to all the products lein of partial oxidation
°
19 87 76 86 67 87 79 81 4
°
26 87 87 94 79 94 81 86 7
2. Selective complex catalysts include at least one element with an alternating valency; this element is not necessarily a transition metal. (It can be Bi, Sb, Sn, etc.). 3.Intbeoverall oxidation process, a selective complex catalyst is either poorly active or (like the molybdates of Bi and Sn) is moderately active. Table 89 indicates that the activities of Bi 20 and Sn-Mo-O (Sn : Mo = 9) are close to those of 3n0 2 3'2Mo0 3 and Bi 20 Antimony-containing catalysts are similar (as to their 3•
564
activity) to the poorly active oxides Mo0 or Sb 20 3 4• 4. Complex catalysts allow us to carry out all the thermodynamically permitted reactions (XVII.39)-(XVII.47). The enhanced selectivity to a definite product is attained by the choice of an appropriate catalyst and reaction conditions. Thus, with Bi 20 ' J '2Mo0J' acrolein is a main product; with C0 - hlo0J' acetone is 304 formed at low temperatures; and so on. Such catalysts as Sn0 2 Mo0 give the possibility of realizing almost all the reaction rou3 tes. At low temperatures (up to 280 0C), acetone is a major product; at higher temperatures, CH CH and, especially, acrolein 3CHO 3COOH, become the predominant products. The formation of acrolein proceeds rapidly at 400 0C, when acrylic acid also appears. All the reactions are accompanied by oxidation into CO and CO 2, At elevated temperatures (above 5000C), some catalysts selectively accelerate the oxidative dimerization of propylene (into C6 products). These catalysts (Bi-P-O and others) do not contain transition metals. 5. Complex catalysts are more effective for mild oxidation than simple oxides. The latter do not allow us to attain a combination of a high selectivity to acrolein with a rather high ~ctivity as is observed with the molybdates of bismuth or tin. Acetone can also be produced (with high selectivity) only on complex systems such as Sn-Mo-O. The selectivity of the Bi-P-O and other complex catalysts for the formation of benzene is much higher than that of the best individual oxides. 0-/ V-a <1-2 ~-7 Y-J e-8 0-4- ()-!l
S/% 100
\11
<1
"* -5 ·-/0
o
o
o
*
10
/5
Fig. 119 - A correlation between activity and selectivity of Sn-Mo-o (1), Sn-Sb-o (2), Sb-Mo-O (3), Bi 20 (4), Bi-Sn-O 3'2Mo0 3 (5), Bi-Sb-O (6), Sb 20 (7), Mo0 (8), Bi 20 (9), Sn0 2 (10) for 4 3 3 the oxidation of C 6 at 450 0C (using the data of reference /101/). 3H
565
s/% 100
04-
.5'/% 75
f
IS
0" 00
5'0
50
10 25
/
25
o
J 0'-----'-----'-----"---'"
25
50
75'
Il
X/%
o
/00
50
III
/5 t;r-/O
Fig. 181 - Selectivity plotted against activity for the oxidation of C 6 over molybdates 3H (using the data of reference /105/): 1 - CoMo0 , 2 - PbMo0 , 4 4 3 - Fe 2(Mo04)3' 4 - Bi2(Mo04 )3.
0-1
.-2
0
0.5'
Fig. 180 - Correlation between activity and selectivity (X is the conversion of C H6 /107/) 3 for: 1 - M00 3-1, 2 - 1140°3-11, 3 - Sn-Sb-Mo-O, 4 - Bi-Mo-O, 5 - Bi-W-O, 6 - Cu-Mo-O, 7 - Co-Mo-O, 8 - Mn0 2 •
¥%
o
2
0
Fig. 182 - Activity-selectivity plots for BiMo (2) and Bi - W (2) oxide catalysts (using the data of reference /19/) •
0
25
"
0
s/%
4
Fig. 183 - Activity-selectivity plots for tungsten (441 0 C ) and molybdenum (5600 C ) oxide catalysts for the oxidation of C3H 6: 1 - W0 + 43% P, 3 2 - W0 + 20% P, 3 - W0 3 3, 5 - 11400 + 20% ~ 4 - 11400 3, 3 6 - 114003 + 43% P (using the data of reference
/19/).
566
r
s/%
to-/!;,mzote: 0'#r/l
(j
75 50
25
cm
'!1li /l
{[.toj!-,mO{fCCJ#6 (m 2
[
4-
2
2
/.0 0.8 0.4-
1.0 2 0.0 0/; 1.0 0.8 0.2 0.4
0.2
OJ
at
o.Of 0./
If
0.2
0 50
/00.v%
Fig. 184 - A comparison of activities of Bi-oxide catalysts with their selectivities towards acrolein (1) and benzene (2) (see Table 88).
MgO 20
40
00 81l C"O/at %
Fig. 185 - The rates of acrolein formation (1), adsorption of C H6 in a reversible form 3 (2) and intensity of f -allylyc complex absorption band (3) against catalyst composition /113/.
6. From the point of view of the theoretical considerations given in Chapter XIV, it is interesting to examine relations between activity, r, and selectivity, S. As it follows from Figs 179184 (constructed using data from the literature), there is, in general, a trend of decreasing S with increasing r. The following feature is noteworthy: the best catalysts for mild oxidation (Bimolybdate and tungstate and Sn-molybdate) deviate sharply from these correlations. 7. The activation energies for mild oxidation (for the formation of acrolein) are usually lower than those for deep oxidation (for the formation) of CO and CO 2). The data which are necessary for formulating the reaction mechanism and explaining the above regularities will now be discussed. The formation of an allylic complex as intermediate in catalysis on the Bi-Mo-o /82, 83, 107/, Bi-Mo-O /113/ and Sn-Sb-O /110/ systems has been shown by the tracer method. The results coincide with those obtained on CU 20, suggesting a similarity between the reaction mechanisms on simple and complex oxide catalysts. Investigations of the thermodesorption of propylene adsorbed on magnesium molybdate together with the IR-spectra of adsorbed spe-
567
cies /106/ have resulted in conclusions which are very similar to those obtained with simple oxides. Weakly adsorbed propylene, in which the ~-C-C-bond is retained, gives allylic complexes (and a J.{-complex) and is oxidized into acrolein while strongly adsorbed propylene, giving surface carbonate-carboxylate species, is converted into CO 2 and H20. On a CUO-MgO catalyst (Fig. 185), the rate of formation of acrolein correlates with the amounts of propylene adsorbed in a weak form and with intensity of the IR-band for the Jr-allylic complex (1440 cm- 1) /113/. The participation of surface 0 2- ions in mild oxidation is indicated by the equality of the rates of catalysis and surface reduction; the last result was obtained with the molybdates of Bi, Co, Fe and Pb /104, 105/. Experiments using 0 18 /114/ show fast diffusion of 0 2- from the bulk of "the bismuth molybdate to the surface; these anions thus participate in catalysis. The same oxygen species probably also participate in deep oxidation since, according to references /104/ and /106/, the rates of catalysis and surface reduction do not differ more than 2-3 times in this case. Adsorption meas~ements on the catalysts for mild oxidation (Sn-8b-o, Sn-Mo-o and Bi-Mo-O) and on deep oxidation catalysts (Cu-Mn-O and Cu-Cr-O) show that oxygen-hydrocarbon surface complexes in the first case contain less oxygen than in the second case /19, 115, 116/. The complexes leading to mild oxidation are charged positively (and the complexes leading to deep oxidation are charged negatively) since propylene is an electron donor while 02 is an electron acceptor. The results discussed suggest that the mechanism of the oxidation tion of C 6 on simple and complex oxide catalysts is similar. 3H Therefore, Scheme (XVII.52) can be used to interpret the experimental data. New facts found with complex catalysts allow us to expand the above scheme (for parallel routes):
568
(XVII. 74)
According to Scheme (XVII.74), propylene can react by three main pathways. The first one involves the rupture of the C-H-bond in the methyl group and the formation of the intermediate allylic complex, (1 1 ) ; the latter is oxidized on the surface into the species (Ii) which lead to acrolein; surface oxidation of (Ii) results in the complex (I~') (probably of the acrylate-type) which is converted into acrylic acid. The interaction of the allylic complex with propylene gives the species (1~") leading to the products of oxidative dimerization. The second route involves rupture of the jf -C-C-bond in propylene, with the formation of the species (1 2) leading to CH?CHO, CH and carbon oxides. The complexes (I;), (I~I) and (I; ') 3COOH are of the carbonate-carboxylate type. Lastly, the third direction, including the intermediate hydration of olefin /56, 98/, results in the formation of acetone. (This process will be discussed later). The rate of the overall reaction will be:
r
r
r: -:r: +r 2
J
't
where r 2 and r are determined by the equations: 3
569
If, in the hydration
of the olefin, the equilibrium:
with the constant Khydr • is established,
~
= kif
j(
byar.
P
liz 0
P
Cj 110
8 '" k rtf p
~
0
p
~ 1-
8
(LVII. 75)
Then: 2 ff/ 8 r= ~ Pc, fI. 8 +- ~ Pc. II 8 -I- K ~ 0 fi.. Ii .
"3'0
'36
(LVII. 76)
'1!c
At elevated temperatures, acetone is not formed
(r4~r2
+ r
J) and Eq, (XVII.68) is valid. At 8 =1, 'sq. (XVII.58) should be observed, which turns out to be so in the case of Bi-molybdate /92-94/. At 8 < 1, accorJing to Eqs (XVII.58) and (XVII.59), the reaction products are expected to inhibit the process which also takes place /93, 94/. At low temperatures on Sn-Mo-O or Co-Mo-O, acetone is the main product, deep oxidation being a side reaction. Under these conditions (r 2«r + r 4): J
r=(k38+-k"/~O)~1I '2
'3
a
8.
(X-VII.77)
Since at 8 ~ 1 in the steps determining the values of r2' rJ and r (and consequently r), oxygen-catalyst bonds are broKen, 4 a decrease in r should be expected with increasing qs. Such a relationship is shown in Fig. 186. Under conditions where acetone is not produced, one should expect (according to reference /86/), an increase in the selectivity to acrolein with increase in qs. This correlation is observed (Fig. 186) which agrees with Scheme (XVII.74). The reason for this is that in the steps leading to acrolein, fewer oxygen-catalyst bonds are broken than in the deep oxidation of propylene. The same
570
reason /86/ results in hiGher activation ener:;ies for the formation of CO and CO 2 in comparison with those for the formation of acrolein.
x/%
..y'%
/00
100
2
15
15
o
50
30
25
25
o........
_ _-J...,
~_..L.-
to
l5 {[
10.1
20
0 1.0
TrK
Fig. 186 - The relationship between the catalytic activities of complex oxide catalysts for the oxidation of C 6 (a), their seJH lectivities to acrolein (b) and oxygen-catalyst bond strength (Tr is an absolute temperature to attain equal reduction rates) /107/: 1 - Mn0 2' 2 - Bi-Mo-O, J - Bi-W-O, 4 - MoOJ-II, 5 - Mo0 2-I, 6 - CU-Mo-o, 7 - Sn-Sb-lVlo-o, 8 - Co-iVIo-o.
The inverse relation between the activities and selectivities (Figs 176, 179, 180 and 182-184) should be attributed to the above dependences between qs' and rand S. The correlations discussed are approximate. A distinct exception is bismuth molybdate (Figs. 180-182 and 186) which is the best catalyst for the oxidation of C 6 to acrolein. The sume JH concerns Sn0 2-MoO (Sn : Idn = 9) which is also an active and seJ lective catalyst (Fig. 179). These exceptions are important and will be discussed latter. Scheme (XVII.74) suggests that under conditions when CH 2=CHCHO, CH 2=CHCOOH and deep oxidation products are formed predominantly, the selectivities to acrolein and acrylic acid (3 acr. ,S a.a. ) will be given by:
(XVII.78)
571
5a.a
U/Ya J
2
+
(XVII.79)
k B J
1)
where k 2 is the rate constant for the desoprtion of (1 with the formation of acrolein while (k;)* is that for surface oxidation of (1~) into (1 leading to acrylic acid. The above equations show that the values of B determine the selectivities to acrolein and acrylic acid. Using the tracer method with a Sn-Mo-O catalyst /56/, it was shown that in the low-temperature oxidation of propylene into acetone, the source of oxygen inserted into propylene is water and not surface oxygen. This result indicates /56/ the importance of the preliminary hydration of C 6 with the formation of adsorbed 3H isopropanol, i.e. the complex (I in Scheme (XV11.74) is J) (CH Acetone is formed by the further oxidative dehyd3CH(OH)CH3). rogenation of the above complex. This conclusion is supported by the fact that acetone is obtained in the oxidation of isopropanol over Sn0 2 - MoO with high J rates and selectivities under the same conditions /98/. A similar result has been observed with co - ~bOJ' It should be noted J0 4 that small amounts of isopropanol are detected in the products of the oxidation of C 6 over these catalysts /56/. JH The hydration of propylene is assumed /56/ to involve a carbonium ion as an intermediate, with the subsequent addition of the OH-group. Since the alcohol is oxidized into acetone more slowly than is the olefin, the rate-determining step is considered to be hydration /98/. We believe that the hydration is a fast and equilibrium process while the oxidation of the alcohol is a rate-limiting step. Such a model leads to EQ. (XVII.75). In this equation, the constant k"' is equal to k4Khydr. The value of Khydr• (equilibrium constant for hydration) decreases with a rise in temperature which leads to decreasing rates of the formation of acetone. Higher rates of the alcohol oxidation (in comparison with those of olefins) and the small amounts of isopropanol detected in the products of the oxidation of C 6 can be interpreted in the following way. JH rhe Jesorption of the species (1 (i.e. adsorbed CHJCHOHCH)pro3) ceeds more slowly than the oxidation of (I In the presence of J). large amounts of alcohol, it is rapidly adsorbed, so the surface
1' ),
572
coverage by (1 8 , increases significantly. Since the rate 3 3), of the formation of acetone is proportional to 8 CH is 3, 3COCH3 produced from isopropanol with higher rates than from propylene. Eq. (XV1I.75) accounts for the observed changes in the reaction orders and accounts for the correlation of m and n with r (Fig. 172). Since the mechanism discussed invloves the hydration step, there is a correlation between the acid-base properties and the rate of formation of the acetone. The latter increases with the concentration of acidic sites on the Sn0 2-Mo0 catalysts /56/. 3 Acid-base properties are also important in the oxidative dimerization of propylene /63/. According to reference /117/, one should also expect an influence of these properties on the formation of acrylic acid since the complex (1~) is likely to be a saltlike compound. Thus, in the selective oxidation of propylene other characterictics of catalysts (besides qs) are of great significance. The bond energy of surface oxygen, qs' is of primary importance, as follows from Scheme (XVI1.74) and from the above correlations between qs and the values of r, S, m and n. At the same time, however, sharp and significant deviations (towards increased activity and selectivity) are observed for the best catalysts of mild oxidation (Bi 20 Sn0 2 - Mo0 (9:1), etc.). It is interesting that in 3, 3 3 the case of the individual metal oxides, the Ig r va qs and Si vs qs correlations (Fig. 171) do not include the results for ZnO which is the beat catalyst (among these simple oxides) for the oxidative dimerization of propylene. Complex catalysts for mild oxidation exhibit a very high selectivity for many products (acetone, acrylic acid and benzene) which cannot be attained with the simple oxides. This fundamental fact cannot be explained fully if qs is considered to be the only factor determining catalytic properties, because the values of qs lie within the region of the bond energies of the simple oxides. The necessity to consider other factors besides qs follows directly from mechanism (XVII.74). Changes in the rate constant k 2, which determines the rate of the oxidation of C;H6 to CH 2=CHCHO, depend (according to the Bronsted - Temkin relation) upon the heat change, q2' of the step for the formation of the allylic complex, (1 1), The value of q2 involves qs and the allyl-catalyst bond energy, qall' The assumption of 02Mo0
573
the constancy of qall' which was used when we deduced the Ig r vs qs or Si vs qs correlations, is unlikely to be valid in general case if the chemical natures of the catalysts compared are greatly changed. The bond energy of a catalyst with the allyl radical and the reactivity of the latter should depend on the chemical structure of the solids. There is probably an optimal value of qall corresponding to such systems as Bi-Mo-O or Sn-Mo-O which exhibit elevated selectivity towards acrolein. The complex (1 2), leading to deep oxidation, involves C-O-catalyst (or, maybe, H-O-catalyst) bonds. Their energies are likely to be little changed so that the heat change for the step
depends mainly upon the values of qs. The above considerations can be regarded as a definition of the concept /118/ on the "specific" interaction of the oxidized molecule with a catalyst which is essential for mild (but not for deep) oxidation. The validity of such a "specific" interaction is demonstrated /104, 105/ by elevated ratios of the rates of the mild oxidation of propylene to the rates of the oxidation of CO or CH 4 over bismuth molybdate, in contrast to the similar results for lead molybdate. According to references /93/, /104/ and /105/, the allylic radical is formed on Bi 3+ ions in catalysis over Bi-Mo-oxide systems; this radical then interacts with oxygen attached to Mo 6+ resulting in the formation of acrolein. The following scheme for the step C 6 + (0) ---- (1 1 ) in mecha3H nism (XVII.74) can be imagined: 0 2-
<, Me n+
m+/
+ Mel
./
~02-/ CH2.~
-
OH -. .-.. '. 'Mem+.~ 1
C~2
0 2-
OH-
+
I
Me(n-1)+ II
II
--
574
The electron from propylene passes to the Mef~ cation, reducing its degree of oxidation. The Me~~ cation promotes the rupture of the C-H-bond in the allylic position and binds the allylic radical by the ff-bond. It follows from such a model that one active site of the catalyst should contain two cations, One of them, Me~~, must be reduced rather easily by accepting (n-1 )+ an electron from the propylene; the reduced form, Mil ' must 6 be readily oxidized by 02' The Me~~ ion is M0 + must activate the C-H-allylic bond in the propylene The Me~+ion and bind the allylic radical with a moderate bond energy, so that its sUbsequent interaction with surface oxygen will proceed rather fast. The Me~+ cation is Bi 3+. The rates of the redox process
correlate with the values of qs since the redox potential is involved in the value of qs. The optimal values of qs can be estimated, as was shown in Chapter XIV. The Me I I species must be an element with alternating valencies but is not necessarily a transi tion metal. Experimental data show that the best Me~+ species are Bi 3+ and Sn4+, i.e. cations of nontransition metals which belong to Group IV and V of the Periodic Table and can change their valency. Transition metal ions are apparently less suitable as Me~+ since unpaired d-electrons cause too strong binding of the allylic complex. It is difficult at present to define those properties of Me~+ which determine the value of q2 and the reactivity of the allylic complexes. As a rough approximation, the electronegativity, X, can be used to indicate the ability to withdraw electrons from the allyl species. The limitation should be added that Mem+should not be a transition metal. For Bi 3+, ~ = 1.8 /119/. Value~ of ;t 2+, 2+, 4+, 3+, 3+, from 1.7 to 1.9 are peculiar to Cd sn sn Tl Sb In 3+ and Cu+. Let us take Me n+ to be Mo 6+. For the oxidation of C3H 6 to acrolein, the systems Sn-Mo-O, Sb-Mo-O and CU-Mo-O, like Bi-Mo-O, are highly selective, but the selectivity of Cd molybdate is low. Other properties (besides qs) can be also important in other steps in the reaction. We have noted that acid-base properties
Catalyst amount /g
228.0
221.4
0.9611 0.3138 0.5643 0.5761 0.6575 0.0792 0.0759
0.5132
Ag(l) - Y 300
Ni (II) - Y 300 0.9155
135.3 221.4 157.0 157.0 232.0 288.0 300.0
Flow rate/ ml min- 1
200 250 280 300 320 310 310
cu (II) - Y
t/oC
18.1
19.0
22.5 18.1 22.0 22.0 17 .3 17 .4 3.0
C 6 3H
28.0
28.4
37.0 28.0 36.4 36.4 26.8 17.4 50.0
O2
-
45.3
43.7
-
34.0 45.2 29.3 29.3 47.4
H2O
8.7
8.9
6.5 8.7 12.3 12.3 8.5 65.2 47.0
N2
Initial mixture / vol. %
The Oxidation of Propylene on Metal-Y-Zeolites /121/
TABLE 90
2.1
20.1
1.11 19.5 23.0 21.9 24.7 0.83 3.9
CO 2
2.67
0.52 6.32 6.67 7.20 6.12
acrolein
Yield / %
0.58
isopropanol
CJ'
on
...,
576
are of great significance in the formation of acetone or CH 2=CH-COOH. One can suppose /117/ that amphoteric nature is preferable in the last case since it permits rather easy formation and decomposition of the acrylate complexes. According to reference /63/, if the allyl species is attached to a site with high acidity, electrons are shifted to this site; the allyl species is charged positively, favouring its reaction with 0 2-. If the active site has weak acidity, the radical nature of the allyl species is preserved which favours dimerization. The evidence for this is shown in Fig. 178. The different dependences of the selectivities to acrolein and to C6 products upon the acidity accounts for the inverse correlation (Fig. 184). Complex catalysts provide a greater probability of attaining the optimal Me~+-Me~! combination than simple oxides and this explains the higher selectivities of the former types of catalysts. In the case of Cu20, the Cu+ ion corresponds to Me~+ while n+ Me ~ MJn-1)+ corresponds to Cu2+ + e~Cu+. It is inteII II resting that the electronegativity of Cu+ is equal to that of Bi 3+ /119/ which causes increased selectivities of the Cu and 20 Bi-Mo-O catalysts towards acrolein. The Oxidation of Propylene Over Zeolites Y type molecular sieves containing Cu (II) actively catalyse the oxidation of C 6 at temperatures which are close to those for 3H the case of CuO. The Cu (II) - Y zeolite is not destroyed during catalysis /120, 121/. Table 90 shows that in the presence of water, propylene is oxidized into acrolein and CO 2• At low temperature (200°C), small amounts of isopropanol are formed which indicates that the hydration of propylene takes place. Acrolein is also produced on the Ag(I) - Y zeolite while Ni(II)Y accelerates deep oxidation (Table 90). The data obtained so far are insQfficient for more detailed discussion of the mechanisms of the oxidation of C 6 over zeolites. 3H The Oxidation of the Butenes The 04 olefins exist in the form of several isomers: but-1-ene; but-2-ene, which can be in cis- or trans-form; and isobutene
5'77
(2-methylprop-1-ene). In the absence of a catulyst, isomcrisation proceeds slowly; many solids accelerate these reactions. For example, but-1-ene (in tb the presence of air) is rapidly transformed at 500°C into but-2-ene over the oxides of V, Ni, Mo, W, Zn, Sb and P /122/. This process is catalysed at 425°C by complex oxides: Bi-Mo-o, Fe-Mo-O and Bi-Fe-Mo-O /123/; the isomerisation occurs over Sn0 2-Mo0 at 135°3 210°C, over c0 0 -Mo0 at 240°-270°C /124/; V-Mo-o catalysts acce3 4 3 lerate the reaction at 350°C /125/, and so on. These facts should be taken into account in discussing the mechanism of an oxidation reaction which proceeds simultaneously with isomerisation. There are many thermodynamically permitted oxidation reactions of the butenes. The main ones are:
H 2C=CH-GH2CH 3
(XVII.80)
CH3-GH=CH-GH3
(XVII.81) (XVII.82)
(XVII.85) (XVII.86) H2C=CH-GH2CH + 1 3
i
02
= HC-CH 11 I
+ 2H 20
(XVII.87)
HC CH
\/
°
H2C=CHCH2CH + 202 =CH + CO 2 + H20 3COCH3 3
(XVII.88) (XVII.89)
H2C=CHCH2CH 3 + 2 CH
3CH=CHCH3
i 02 =
Cil5 COOH + CO 2 + H20
1 + 2 "2 02 === C2H + CO 2 + H20 5COOH
(XVII. 90) (XVII.91 )
578
(XVII.92 ) (XVII.93)
(XVII.94)
o 3° 2 ,",
HC-C" 1\
,o
"0+3H 20
(XVII.95 )
HC-C""'"
(XVII.96) (XVII. 97 ) (XVII.98) (XVII. 99) (XVII. 100 ) (XVII. 101 ) Among the above reactions, processes (XVII.80), (XVII.81), (XVII.86), (XVII.94) and (XVII.95) leading to divinyl, methacrolein and maleic anhydride are of the greatest practical importance. The synthesis of divinyl (the monomer employed for the production of synthetic rubber) is carried out over bismuth-molybdenum oxide catalysts /14, 126/. These catalysts are also effective for the oxidation of isobutene to methacrolein, used as a monomer in polymerization and co-polymerization. Previously, copper oxide catalysts were employed for the selective oxidation of isobutene and but-1-ene /14/.
579
Complex vanadium oxide catalysts, as well as Co-molybdate, catalyse the oxidation of normal butenes into maleic anhydride /95/.
The Oxidation of Butenes Over Metal Catalysts The oxidation of butene on metals has not been adequately studied. On platinum films, isobutene can be converted into CO 2 and H20 at 80 0C /7/. Kinetic characteristics of the compltee oxidation of isobutene over Pt and Pd supported on SiC /46/ are given in Table 91. First (or near to first) order in oxYgen and retardation by the hydrocarbon are observed. This is evidence for significant adsorption of the isobutene and weak adsorption of O2 on the above metals during the course of catalysis. TABLE 91 The Oxidation of Isobutene on Metals (C 8 - 2, 02 - 50, N2 4H 48 vol.%) /46/; r is in mol m- 2 s -1 at 300 0C
Catalyst
Temperatures/oC
19 r
Ig r o
E/ Reaction order kcal mol- 1 in C H in O 2 4 8
Pt
92-118
5.58
- 0.41
Pd
132-153
10.14
- 0.22
Over silver at 200°C, deep oxidation products from the butenes predominate, the mild oxidation products being produced in trace amounts. The oxidation of isobutene leads to small amounts of acetone /127/. The Oxidation of
Butenes Over Simple Oxide Catalysts
The oxidation of but-1-ene was studied /122/ at 300 0-5500C and with a ratio of C 8 : air = 0.5 over supported oxides. The high4H est activity is exhibited by the oxides of metals with alternating valency: Ni, Co, Sn, Ag, Mn and CU; less active catalysts are the oxides of Bi, In, V and Cr. The oxides of Zn, Cd, La, Pb, Wand Mo and those of the elements of the main sUbgroups of the Periodic
580
Table (1i, K, Cs, Be, Mg, Ca, Sr, Ba, B, AI, Ga, P and Sb) are relatively inactive. TABLE 92
The Oxidation of Isobutene Over Metal Oxides (C 8 - 2, 02 - 50, 4H N - 48 vol. %) /46/; r is in mol m- 2 s-1 at 300°C 2
Catalyst
Mn0 2 C0 304 cr 20 3 Oe02 NiO Fe 20 3 Th0 2
Temperatures /oC 193-238 220-262 242-270 207-250 263-305 270-312 273-312
E
Ig r
19 r o
/kcal mol- 1
-6.01 -6.30 -6.64 -7.37 -7.45 -7.66 -8.37
5.88 8.35 5.15 2.71 2.02 6.56 -2.72
36.4 38.4 30.9 23.8 24.8 37.3 14.8
Reaction order in C 8 in °2 4H
-0.25 -0.09 0.25 1.00 0.15 0.21
0.54 0.75 0.47 0.00 0.55 0.60
The nonstoichiometric oxides, MoOx (where x = 2.65-2.89), are also relatively inactive /128/. The specific activity of the metal oxides decreases in the order: Pe 20 Bi /123/. The acti3>Mo03> 20 3 vation energies for oxidation of but-1-ene have been measured for many oxides /129/. In the oxidation of isobutene (Table 92), the activity decreases in the sequence: Mn02>C0304>cr203>oe02/NiO>Fe203/Th02. The most active metals (Pt and Pd) exhibit 3-4 orders of magnitude higher activities than those of Mn0 2 or 00 (compare Tables 91 3°4 and 92). On most of the simple oxides, the mild oxidation of but-1-ene leads to divinyl. At 500°0, the selectivity of the oxides of Mn, Sn, Cr, Bi and In exceeds 30%; the oxides of Ni, Co, V and Cu are less selective /122/. Copper oxide is a selective catalyst for tIle partial oxidation of the butenes. Over ou 20, the normal butenes are converted mainly into methylvinylketone, while isobutene gives methacrolein /14, 27, 130/. At 375°-400°C, the yield of the mild oxidation products on copper oxide decreases in the order: methylvinylketones > ace-
581
taldehyde> acrolein> divinyl> crotonic aldehyde /131/. Divinyl (a product of oxidative dehydrogenation) is formed on Cu20 only at high concentrations of 02. The catalyst accelerates the isomerization of but-1-ene into but-2-ene /131/. At 350 0-3S00C on Cu20 (with 02 : C S = 0.5), the selectivity 4H to methacrolein approaches 60%; addition of water vapour enhances the selectivity (attaining a level of SO% at 340°C) /27/. In the presence of V20 at 350 0C, but-1-ene and but-2-ene 5/Al 20 3 are oxidized into acetic acid, maleic anhydride, methylvinylketone, acetaldehyde, formaldehyde, CO and CO 2; the yield of maleic anhydride increases with the 02 : C S ratio. The oxidation of iso4H butene at 370 0-390 0C results in acetic acid (3070) and methylacrolein (12/0 /127/. According to reference /132/, the n-butenes are oxidized over V 20 into divinyl, maleic anhydride, furan, acetic 5 acid and acetaldehyde. The conversion of isobutene to maleic anhydride is improbable since it demands skeletal isomerization. The selectivity to divinyl formed from the n-butenes over Mo oxides decreases in the order: M020 > Jilo0 2/, Mo0 / lVl0 20 'rhe reac5 3 3• tion mixture greatly affects chemical composition of the above oxides /133/. This phenomenon is also found with many other systems. Under steady-state-conditions during the oxidation of but-1-ene, ferric oxide is converted into Fe The selectivity to divinyl 304• (at 365°C) is 6570. The overall conversion decreases and the selectiVity increases on removing oxygen from the Fe 20 /134/. 3 Thus, the distribution of the products during the oxidation of butene depends on its structure, catalysts and the reaction conditions. On catalysts for the mild oxidation of but-1-ene, "a Ll.y Ld.c " hydrogen (in the ~-position to the C=C-bond) participates in the reaction. Over Cu20-catalysts, an unsaturated ketone (methyl-vinylketone) is formed, i.e. oxidative dehydrogenation takes place. For V-O-catalysts, the production of maleic anhydride (and furan) is peculiar. The oxidation of but-2-ene over Cu20 leads also to unsaturated carbonyl compounds (methylvinylketone, crotonic aldehyde) while on vanadium oxide catalysts, maleic anhydride is formed. The similarity of the reaction products obtained from but-1-ene and but-2-ene is likely to be due to the fast isomerization of the latter olefin. The "allylic" oxidation of isobutene to butadiene or maleic anhydride would require skeletal izomerization~ In this case, only u.nsaturated carbonyl compound, o<-methylacrolein, is formed (on
582
cu20-containing catalysts). Mild oxidation over most of the catalysts studied is accompanied by destructive oxidation (to acetic or acrylic acid, acetaldehyde, acrolein, formaldehyde, etc.) and by deep oxidation. In general, a parallel-consecutive scheme is accepted; at low temperatures, it reduces to a parallel one /14, 27, 125/. The overall process kinetics on a range of sirnple oxides can be described by the equation:
m
n
r= k~
II Po .
and n
= 1.
(AVII.102)
f 8 '2 The reaction orders are given in Table 92. For PbO /135/, m = 0
The following empirical rate equations have been obtained for the oxidation of isobutene over Cu20 (330 0-365 0 C ) /27/:
(jJ/ pal 1
q,
C'flle
r = --=-----:.---=.--m !+~(Pefl.+Puo.J '1 '8 /'2
J.:fpa8
(XVII.103)
»"
r = "2 0 C'I fh d Irb.?
(XVII. 1 04)
-p ~o
where r m is the rate of mild oxidation (to methylacrolein); r d is that for deep oxidation; k 1, k 2, b i and b 2 are constants. The rates of the oxidation of but-1-ene to each product (methylvinylke tone , acrolein, crotonic aldehyde, acetaldehyde and CO 2 ) on CU 20 at 375 0 C obey Eq. (XVII.102) with m = 0-0.2 and n = 0.9-1.0 (only for divinyl, m = - 0.5). Hence, the total rate is practically independent of Pc H and is directly proportional to Po /131/. The oxidation 4 8 2 of but-1-ene to methylvinylketone is iW1ibited by the reaction products. The rates of the surface reduction of Fe with but-1-ene 304 (365°C), of catalyst reoxidation and of catalysis under steady-state conditions are close; the selectivities for reduction and catalysis are also equal. The mechanism of the alternating reductionoxidation process with the participation of atomic oxygen anions, . der1ved . / 0 2 - , 1S from the above data / 134. At high concentrations of O2, the steady-otate catalyst is
583
OC-Fe 20 In this case, the selectivity for reduction exceeds 3• considerably that for catalysis, and the reoxidation rate is lower than that for catalysis. 'rhis SUG6c.;Sts that a different mechanism is valid under these conditions /134/. Adsorption and infrared studies /136/ showed that but-1-ene is adsorbed irreversibly on CuO (a deep oxidation catalyst) with the formation of the carbonates and carboxylates of copper. The adsorption of but-1-ene on CU 2 0 (a mild oxidation catalyst) is mainly reversible. In this case, the C=C-bond is preserved and the C=C-bond appears. The above results are similar to those obtained with propylene, indicating that a scheme of type (XVII. 53) is valid for the mechanism of the oxidation of butene. In the case of isobutene, the scheme will be: ---.. 2(0)
fast
(0 )
fast ... C4H6 0 + H2 0 10(0)
fast
,. 4CO
2
+ 4H 0 2
(XVII. 105 )
The intermediate (1 1 ) is associated with the allylic complex: CH I 3 CH 2:-;-:-;- C:7:"CH2 /777777777
while the (1 2 ) species are associated with:
yH 3 CH -C-CH 3 I I 2
o
0
1//7 171 771
whf.cn precede the formation of carbonate-carboxylate species. The
584
corresponding overall rate equ.ations is: (XVII. 106)
while the rates of the formation of methacrolein and CO 2, r(I)and r (II), will be:
kf p-
r
(f)
70
=
(/-8)
P "C'f flo 0 el( 110 0
1+0.
+
Sri)
(XVII. 107)
bII 0 PI/" 0 '2
'2
and
r
(ff )
kf Po (1- B)
=
JI
'2
P + b P Zfh6 0 C'!I10 0 If 0 /(ZO
1+ ()
(Ii )
S
(XVII.108)
where b i are adsorption coefficients, 8 is surface coverage by oxygen and SCI) and S(II) are selectivities. The selectivity to methacrolein is given by:
With 8«1, Eq. (XVII.107) reduces to
(XVIL110 )
The experiments of reference /271 were made under conditions when the selectivity changes slightly, and Eq. (XVII.110) almost coincides (in its form) with Eq. (XVII.103) obtained for Cu20. According to Eq. (XVII.109), the selectivity increases with
585
decreasing
{1. Since the value of
8
decreases when PH G Lnc.cea2
ses (see Eqs (XVII.65) and (XVII. 66 », one should expect an enhancement in SCI) with inc~ease in PH 0' as is actually observed
/27/.
2
Power rate laws of type (XVII.102) are likely to be approximations of Eq , (XVII.106) (at 17,7 biP i). I f this is so, ac co r-Id.ng to reference /43/, relations (XVII.67) should be valid. (In this case, 8' = 0 and 1 = 0). As a result, one should expect opposing changes in m and n as observed (Fig. 187). When 8:::. 0, Vie have: m ~ 0 and n ~ 1; this situation is realized in the oxidation of isobutene on PbO. Because m increases (n decreases) with increasing 8, one should expect a correlation of the orders with qs' which is observed in practice /46/. Scheme (XVII.105) and Eq.(XVII.106) predict a dependence of the catalytic activity, r, upon qs. Fig. 188 shows that the correlation
m
tfr
/.0
-2
/ 1:::.2 I-6--l
-4 -5
6
o
6
-8 -/0 0.5
!.On
Fig. 187 - A comparison of the reaction orders /46/ for isobutene (m) and oxygen (n): 1 - Ce02, 2 - cr 20 J - Fe 20 3, 3, 4 - NiO, 5 - C0 0 4 , 6 - PbO, 3 7 - Mn02·
I
20
40
60 IJs/Kcfl/(g--atOr
Fig. 188 - The correlation of Ig r with qs for the oxidation of isobutene (rate data of reference /46/): 1 - Pt, 2 - Pd~ J - NiO, 4 - Mn0 2' 5 - c0 30 4 , b - Cr 2 0 J , 7 - Fe 20 , 8 - Ce02• J
is valid. The much higher activity of the transition metals in comparison with the metal oxides (Fig. 188) is likely to be caused by
586
elevated rates of activation of the olefin on the metal surface, which is free of oxygen. Thus, 0cheme (AVII.105) reflects the important features of the mechanism of the oxidation of isobutene. A similar scheme can be assumed for the oxidation of n-butene. The rate equation observed in the case of Cu20 /131/, as well as the correlation of E with qs /129/, are in accordance with this assumption. A scheme of type (XVII.105) is simplified; in particular, i t describes the formation of only one mild oxidation product. It can be expanded (for example, in the case of the oxidation of but-1ene) using the following asslli~ptions: 1. The olefin molecule, before its reaction with surface oxygen, is adsorbed weakly and reversibly on the catalyst. During this process, the molecule is polarized; a labile J'!-complex is also formed /131/. Since coverage of the surface with the olefin is low (B R = bIlR), the reaction kinetics will be similar to those when C S reacts from the gaseous state. 4H 2. Two main reaction routes are realized, each of them involving a chain of consecutive transformations on the surface. The first one (" allylic") includes cleavage of the C-H-bond in the allylic position and results in C products (divinyl, methylvinyl4 ketone, crotonic aldehyde, furan and maleic anhydride) /137/. The second route involves rupture of the J.r-C-C-bond in the olefin and the formation of intermediate carbonate-carboxylate complexes, leading to destructive and deep oxidation (acids and aldehydes (C C and carbon oxides). 3- 1) 3. The interaction of weakly adsorbed but-1-ene with (0) results in a methylallylic radical and OH-groups:
'I'he last step in the inverse direotion leads to the isomerization of but-1-ene into but-2-ene /137/. The expanded soheme oan be expressed as follows:
587
In the first route, the methylallylic complex can react with (0) in two ways, leading either to divinyl or methylvinylketone. Low values of qs are likely to favour O-addition (i.e. ketone formation) but this is not the only factor since a range of other oxides, besides Cu20, with low values of qs (ldn02, C0 liiO and 304, Ag20) produce divinyl. The part of Scheme (XVII.111) reflecting the formation of furan and maleic anhydride is based on the data of references /125/, /130/ and /132/ according to which these products are generated from n-butene and divinyl. Divinyl, furan and maleic anhydride can also undergo further oxidation. According to reference /136/, the adsorption of methylvinylketone on Cu20 results in carbonate-carboxylate intermediates which lead to deep oxidation; this effect is also taken into, account in Scheme (XVII.111). The precise structure of the majority of the surface intermediates participating in reaction mechardsm (XVII.111) is not clear. Nevertheless, one can believe that: the adsorbed olefin is positively polarized; the methylallylic complex is attached to the metal ca tion; the complex (1m) is likel y to be a surface rnalea te; and the (0) species are negetively charged. The rate equation corresponding to Scheme (XVII.111) will coincide with Eq. (XVII.106), referring to Scheme (XVII.105). Scheme (XVII.111) suggests that the selectivity will be deter-
588
mined not only by the oxygen-catalyst bond energy, but also by the catalyst-methylallyl bond energy, qall' and by the surface acidity (in the case of formation of maleic anhydride). The electron work function of the catalyst /129/ or its activity in isomerization (migration of double bond) /122/ have been proposed as an approximate measure of qall. However, this problem has not been solved, and this makes prediction of selectivity difficult, in particular, it is not clear why methylvinylketone is formed on CU 20 while divinyl is produced on .1<'e 20 and ina Lei,c an3 hydride is formed on V20 5• It is nevertheless possible to explain changes in the activity and selectivity of a given oxide when the value of & varies. ThUS, a decrease in 8 leads to an increasing selectivity to divinyl and a decreasing activity in catalysis over Fe 20 /134/. Ac3 cording to Eq. (XVII.109), one should expect increasing selectivity with decreasing values of 8 as is observed in practice. I'he reason for this is that the second route (leading to deep oxidation) requires more (0) atoms than the first route (leading to mild oxidation). The effect is enhanced by increasing values of qs with decreasing values of 8. 'rhe overall rate equation will be:
(XVII.112 ) from which it is seen that r decreases with deurease in 8. This decrease should be also intensified by a decrease in k 2 and k with 3 increasing values of qs. It should be taken into consideration that under certain conditions quite a different mechanism is probable. According to reference /134/, the catalytic oxidation of but-1-ene over Fe 20 at 3 high concentrations of 02 procee~by way of the interaction of 02 with adsorbed olefin. The Oxidation of Butenes Over Complex Oxide Catalysts Complex systems are more effective for the mild oxidation of butenes than simple oxides. 'rhe selectivity of Cu20 in the oxidation of n-butene to methylvinylketone can be increased by the addition of the oxides of Mo or W /130/. However, different catalysts are of major importance in the mild oxidation of the butenes.
589
fABLE 93 The Oxidation of Butenes Over Bi-Mo-O at 460°C /139/
Oxidized molecule
But - 1 - ene
Conversion /%
20 40 80
But-2-ene
20 40 80
Isobutene
10 40 70
Selectivity / %: to divinyl to methacrolein 95 95 90 90 90 85 72 72 72
High activity and selectivity are exhibited by Bi-Mo-o catalysts above 400°C /14, 139, 140/; these are materials which are inindustrial importance. Some data for a Bi~~o-O catalyst with a Bi : Mo ratio of 1.0 are represented in Table 93. The oxidation of but-1-ene is accompanied by its isomerization into but-2-ene /140/. By-products in the case of the n-butenes are CH CH 2= 3CHO, =CH-CHO, furan, methylvinylketone and carbon oxides. The selectivity towards divinyl passes through a maximum at 450 0-550 0C /140/. A bismuth-molybdenum oxide catalyst is less active than a copper oxide one. The former operates at higher temperatures and with 02 in excess (cu 20 - with the olefin in excess). Over Bi-Mo-o, the main product of the oxidation of n-butene is divinyl while over Cu20 the product is methylvinylketone. The total selectivity for mild oxidation is higher in the case of Bi-Mo-o (Table 94). Since the oxidative dehydrogenation of isobutene is complicated by its structure, this olefin is converted into methacrolein over Cu20 and Bi-Mo-o. The transformation of n-C S-0 2 mixtures into di4H vinyl is oxidative dehydrogenation (H2 is not present in the products) and not dehydrogenation followed by oxidation of the H /140, 2 When Bi 20 is added to Mo0 the optimum yield of divinyl corre3 3, sponds to a Bi:Mo ratio of approximately 1.0. Molybdena is poorly
1.56
0.78
0.78
400 425 450
400 425 450
Bi-I>4o-0 (Bi : Mo = 1, Ssp= 0.46 m2 .g-1 )
Sn - Sb - 0 (Sn : Sb = 4, S = 22 m2.g -1 ) sp
p..,
0
en ~
C\I
350 375 400
-P
<,
~
0
Cu20 on carrier (Ssp=1.32 m2 g-1)
Catalyst
0 0
77.9 87.7 92 .3
78.6 85.1 89.1
22.2 41.7 47.5
4.8 3.3 2.9
4.9 3.1 2.5
7.1 3.5 2.8
-P'
~C\I
ttl ,
~~ q, 0 0
~
o:l
0 0..1
•
6.5 4.3 2.9
6.7 4.2 4.2
15.5 9.5 7.9
o
°M ,
I -P ::l .oQ) Iq mQ)
C\I
50.5 49.6 48.8
48.3 46.6 47.7
4.0 3.0 3.6
on> 'd
°M
~
rl
I Q)
0 0 3.7 4.7 5.1 4.4 5.1 4.5
0 0.6 0.8 0 0 0 0 0 0
1.3 0.7 0.5
3.9 7.0 2.2
~Ef 0
'H
H ::l
ell
20.2 22.] 21.3
q
I rl ttl QQ) O'd -P>" 0.J:l HQ) tl'd
>0 rl-P >"Q) .J:l~ -Prl
0..1 q
Selectivity (%) to I -P ::l .aI Q) o:lq q o
!=l
The Oxidation of But-1-ene Over Cu20, Bi-Mo-O and Sn-Sb-O /131/
TABLE 94
19.8 20.0 18.3
16.3 18.5 21.0
0 10.2 10.6
ell
g~
2.7 3.0 2.9
5.8 6.1
3.2 9.5 9.3
o
s:l Q) rl 0 H
°M
I Q) 'd rl a:l -PQ) Q)'d
C\I
10.0 13.0 19.2
11.2 10.1 11.2
50.0 41.4 43.7
0 0
0
01 CD
3
3
= 1) (Bi : Mo = j) (Bi : Mo = 1) 2 (Fe : Mo = 3) (Fe: Mo = 1)
~~
- 0 (Bi:Fe:Mo
= 1:1:3) = 1:1:2) 81
44
10
12
21
18
0.0
4.1
0.0
8.9
3.4
12
40
43
50
57
10
2.2
13
30
1
0.0
0.6
0.0
1.6
5.5
6.5
4.0
5.0
0.3
1.3
3
k
k2
k1
2
129.9
90.4
72
82
33.2
49.6
5.5
11.2
72.4
48.7
13.9
14.6
6303
36.3
0.0
35.6
0.0
19.7
6.6 4.0
S/%
k
to divinyl **) Data for 365°C
3
3;
*) k i are first order specific rate constants: k for oxidative dehydrogenation, 1 k 2 for deep oxidation and k for isomerization; k = k + k + k S is the selectivity
Bi : Fe : Mo - 0 (Bi:Fe:Mo
Bi - Fe -
Fe - Mo - 0
Fe - Mo - 0
Bi - Mo - 0
Bi - lIlo - 0
Fe 0 **) 2 3 Bi - Fe - 0 (Bi : Fe
Bi 20
Mo0
Catalyst
Oxidation of But-1-ene Over Catalysts Containing Mo, Bi and Fe *) (425°C, C 8 : air = 1 : 5) /123/ 4H
TABLE 95
~
'" ,...
592
active and catalyses mainly the isomerization of but-1-ene; Bi 20 J is also poorly active and accelerates deep oxidation (Table 95). The chemical compounds Bi 20 and Bi 20 exhibit low acJ'MoOJ 3'JMoOJ tivity /12J, 128, 141/. Systems with a Bi : Mo ratio close to 1 contain significant amounts of a phase of Bi 20 but this compound itself shows 3-2MoO J low activity. Increasing the concentration of MoO in the Bi-Mo-o J system leads to higher rates of the isomerization of n-butene (C=C-bond shift). The ratio of Ci8- to trans-isomers of but-2-ene in the course of the reaction is close to unity /141/. The oxidation of but-2-ene over Bi-Mo-o (Bi : Mo ~ 1) proceeds like the oxidation of but-1-ene /131/. In some cases, Bi-Mo-o catalysts are promoted, for example, by the oxides of P or Fe. Bi-Mo-o catalysts with the additives of P 20 catalyse the oxidative dehydrogenation of n-butenes at 420 0 550 /1J3/ and the oxidation of isobutene into methacrolein /142/. In the last case, the selectivity to acrolein is 60-75~ at 5JOoC (i - C 8 : air: H20 = 1 : 4.8 : 1) and at 40-50% conversion /142/, 4H Addition of iron oxide to Bi-Mo-o increases the activity in the oxidation of but-1-ene to divinyl (Table 95). The optimal compositions for the catalyst without a carrier is Bi : Fe : Mo = 1 : 1:2 and for the supported catalyst is Bi: Fe : Mo = 2 : 1 : 1. The Bi - Fe - Mo catalysts do not contain the Bi 20 phase. J'2Mo0 J At the composition Bi : Fe : Mo = 1 : 1 : 2, a Bi 20 phase J.]MoOJ 3 was formed in which Bi + was partially substituted for the Fe J +. These catalysts accelerate actively the isomerization of but-1-ene /12J/. Additives of BiP0 and Cr 20 (10 mol.%) also promote Bi 20 4 J J' .]MoOJ /14J/. The Bi - Fe - Mo systems can also be considered as a product of the substitution of Fe J + for Bi J + in ferric molybdate. Fe - Mo catalysts are more active but less selective than Bi - Mo ones. The former predominantly accelerate the isomerization of n-butene. The penetration of Bi J + enhances the activity and (mainly) the selectivity to divinyl /123/. Tellurium oxide (up to 30 at. %Te) promotes the selectivity of Fe - Mo to methacrolein formed from isobutene. At the composition Fe : Te : Mo = 1 : 0.85 : 1 and at 400°-420°0 (C 8 : air: 4H H20 = 1 : 1.5 1), the selectivity is 10-80% at a conversion of 50-60% /1J8/. The Fe - Bi system is relatively inactive ana catalyses only deep oxidation (Table 95) but a combination of the oxides of
3C
°
°
°
°
°
°
593
Fe and Sb results in a selective catalyst for the oxidation of n-butene into divinyl. At 425 0C and with Fe : Sb = 1 : 2, the selectivity attains a level of 94-95% /144/. The behaviour of the Sn-Sb-O system is similar to that of the Bi-Mo-O system (Table 94). The properties of various catalysts based on the oxides of ~o and Sb are presented in Table 96. The specific rates of the oxidative dehydrogenation of n-butene over Fe-Sb-O, Co-Sb-O and Sn-Sb-O decrease with increase in the Sb oxide content /129/. TABLE 96
The Selectivity to Divinyl in the Oxidation of But-1-ene at 400°C (1 % C4H8' 0.5% 2 ) /45/.
°
0 I
0 I
~
i'
,:Q
;z,
~0
'(
0
0
Catalyst
-.-/
Phase composition Surface areal m2 g-1 Selectivity/ %
Bi 2(Mo04)3
Q)
tX--CoMo0 Fe 2(Mo0 4)3 4
1.8 98
0
7.3 20 ()OOoC )
0 I
0 I
,0 IZl
,0
<"'\
I
7
0
IZl
;z,
;z,
i:l
Solid solution
11.0
76.0
76
97
Q)
-
(\J Q)
tX.-Fe 20 3
18.0 96
25 (365°C)
In the oxidation of but-1-ene over molybdates of the alkaline earth metals and Zn and Cd, the selectivity to divinyl is low ( ( 15%). Acetic and acrylic acids are formed together with CO 2 and the isomerization of but-1-ene takes place. The specific activity at 350°C decreases in the sequence: BeO.Mo0 3>SrMo04>MgMo04> >BaMo0 CaMo0 ZnMo0 while the selectivity for oxida4>CdM00 4> 4> 4, tive dehydrogenation decreases in the order: SrMo0 > MgMo0 BeO. 4 4> MoO 3 > CaMoO4> CdMo0 4 > BaMo0 ZnMo0 4 /106/. 4 The conversion of n-butene into divinyl, furan and maleic anhydride is typical of complex oxide catalysts based on v20 • 5 The V-P-O and V~o-O systems are effective catalysts for the synthesis of maleic anhydride from n-butenes at elevated tempe-
>
594
ratures. The cis- to trans-butene ratio in catalysis on V-i.lo-'ri-O is close to the equilibrium value /95/. The optimal V : P ratio is 1 : (1.6 - 1.2); at 450 0C, the yield of maleic anhydride reaches 75 wt.% /95/. Additives of P205 to Y205 decrease the activity and increase the selectivity (from 20 to 42%) /132/. Small amounts of 3i0 2 added to the V - P - 0 system increases the selectivity to maleic anhydride sharply (from 52 to 72 mol. % at 400°-440°C) /146/. The activity of the Y20 - Mo0 systems for the oxidation of 5 3 but-2-ene (350°C) passes through a maximum at 20 at.% Mo. The oxidation is accompanied by the isomerization of cis-but-2-ene into trans-but-2-ene and into but-1-ene. With catalysts containing less than 20% Mo, the selectivity towards maleic anhydride is low ( < 20%) and depends slightly on the MoO) content. Catalysts enriched by Mo show a high selectivity (up to 24%) independent of the Mo0 content. Thus, the highest yield of maleic anhydride is at3 tained on the most active catalyst (20% Mo). The yields of divinyl and furan are lower than those on Y205-P205' The selectivity towards the destructive oxidation products (CH)CHO and CH is 3COOH) ~ 36% and is almost independent of the Y : Mo ratio. At high conversions, the yields 9f CO and CO 2 increase /125/. In the oxidation of but-1-ene over Y20 - Ti0 2 (400°C, excess 5 of air and conversions <, 30%), the selectivity to C products pas4 ses through two maxima (at 20 and 65% Ti0 2) and one minimum (at 35% Ti0 2). The main contribution to C is given by maleic anhyd4 ride, the optimum selectivity being 47-48% and the minimum 25%. X-ray analysis shows only phases of Y20 and Ti0 2; at high Ti con5 centrations,the anatase phase includes y4+ ions. The activity of the Fe - Mo - 0 system for the oxidation of n-butene to maleic anhydride passes through a maximum at low concentrations of iron /129/. The oxidation of butenes over Sn0 2 - Mo0 and 00)0 - Mo0 at 3 4 3 low temperatures ( < 300 00) exhibits a special feature (Tables 97-99). But-1-ene gives significant amounts of the saturated aldehyde, methylethylketone. Acetic acid is also formed. Simultane'ously, the isomerization of but-1-ene to but-2-ene takes place. A similar picture is observed with but-2-ene. In comparison with 3n0 2 - MoO), the co)04 - MoO) catalyst produces less ketone but more CH)COOH. The interaction of isobutene with 02 over Sn0 2-Mo0 3 results in poor yields of the mild oxidation products but leads to a high selectivity towards the hydration product (tert-butanol at
595
90°-100°C) and to diisobutene (at 140 0-1S0 0C). On c0 0 -Mo0 , 3 4 3 methacrolein is mainly formed, acetone and acetic acid also being produced /124/. At elevated temperatures, the oxidation of the n-butenes over Co-Mo-O results in maleic anhydride /95/, while Sn-Mo-O produces divinyl /122/. A high selectivity in the oxidation of but-1-ene to acetic and acrylic acids (70~ at 300°C) is obtained on Mg-molybda te /106) • A rather high yield of divinyl from but-1-ene at 450 0-5500C was obtained on W- or V-containing catalysts (Bi-W-O, Sn-W-O, In-W-O and In-V-O) /122/. Complex chromium oxide catalysts accelerate deep oxidation. Thus, barium chromate at 200°-400°C actively catalyses the full combustion of isobutene /147/; deep oxidation of n-butenes proceeds over Co-Cr-O /95/. In general, one can notice a close similarity of the catalytic properties of complex oxide catalysts in the oxidation of propylene and butenes. The richer spectrum of mild oxidation prOducts formed in the latter case is accounted for by the more complicated structure of the butenes. The oxidative dehydrogenation of propylene is impossible while the n-butenes can be converted not only into unsaturated carbonyl compounds but also into divinyl (and furan or maleic anhydride). The oxidation of n-butene is accompanied by i8Omerizations. At rather high temperatures, the oxidation of butene proceeds by parallel-consecutive schemes. Thus, for the oxidation of n-butene over Bi-Mo-O, the following scheme is discussed:
C4HS
k
1. C H
______
~~
4 6
k~C02~
k
2
.. furan
~
k
5
(k i are the first order rate constants) where k k2 + k
k
1
4
= 0.05
3/k1
= 0.05 and
/14/. On Fe-Sb-o, the divinyl is further oxidized
into carbon oxides /148/; on V-P-O or V-Mo-O, divinyl and furan are converted into maleic anhydride, etc. /95, 125, 132/. At low temperatures, the contribution of the consecutive routes becomes
3
38.7 2.0 2.9 2.8 8.7 10.6 34.3 31.4
6.1
60.3 3.9 2.3 3.9 5.2 3.6 20.8
28.0
4.8
85.3 traces
16.4
2.2 2.6 traces 9.9
"
8.4
160°C
135°C
180°C
Sn0 2 - Mo0
37.0
24.7 1.4 1.5 2.4 4.9 12.5 52.6
17.5
210°C
I
I
20.8 ***
24.8 16.5 ***
29.2 ***
6.9 24.8 8.7 31.8 7.0 13.6 5.5 29.6 7.2 7.2
-
27.8
5.2
44.3
3.0
1.2
274°C
60.8
255°C
240°C
c030 4 - Mo0 3
*) Without isomerization. **) The percent of but-1-ene isomerized into a given product. ***) Cis- and trans-isomers (mixture).
Conversion 110 Selectivity (10)*) for oxidation into: CH 3COC 2H5 3/4CH 3COCH3 3/4C 2H 5COOH 1/2CH 3CHO 1/2CH 3COOH 1/4CO 1/4C0 2 Selectivity (10) **) for isomerization into cis-but-2-ene
Catalytic properties
The Oxidation of But-1-ene Over Sn02-Mo03 and c030 4 - Mo0 3 (C 8-20, 02 - 30, N2 - 20, H20 - 30 vol. %) 11241 4H
TABLE 97
~
J>
CJl
Selectivity (~) for isomerization into: but-1-ene cis- or trans-but-2-ene
Conversion /% Selectivity for oxidation into: CH)COC 2H5 )/4CH)COCH) )/4C 2H 5COOH 1/2CH 3CHO 1/2CH)COOH 1/4CO 4/4C0 2
58.8 3.1 traces 11.1 12.9 2.1 12.0
60.6 traces
9.7 )1.0
3.6
18.0
"
14.5
)2.8 2.1 traces 7.1 6.8 9.8 40.8
8.)
9.7
38.0 6.5 )0.6
10.4
-
266°C 5.0
cis-isomer Co)04 - MoO) 200°C 20.0
traces
15.2 18.0 traces 6.2
"
155°C 9.2
1)00C 4.0
trans-isomer Sn0 2 - MoO)
The Oxidation of But-2-ene Over Sn0 2 - MoO) and co) - MoO) (C 8 - 20, 02 - )0, N2 - 20, H20 - )0 vol.~) /124/ 4H
TABLE 98
-
traces
8.0
18.) 22.) 9.7 )2.)
17.4 traces
25)oC 4.0
traces
16.9 18.2 6.7 )1.5
-
26.1 traces
2))oC 2.5
9.6
12.1 )5.4 9.1 )0.6
-
-
12.9
275°C 7.0
trans-isomer Co)04 - MoO)
-
traces
2.0
9.7 30.4 10.) )2.3
14.) ).0
210°C 6.1
...J
'" '"
Conversion /% Selectivity (%) for oxidation into CH)C(CH 3)20H CH 2=C(CH))CHO C8H16 )/4CH)COCH) 1/2CH)CHO 1/2CH 3COOH 1/4CO 1/4C0 2
Catalytic properties
17.4
traces
-
-
-
82.6
100
-
4.8
105°C
2.7
90°C
2.5
71.9 2.1 3.5 0.7
-
19.)
7.2
1)6°C
Sn0 2 - MoO)
65.4 5.8 2.6 1.3 3.7 15.0
-
7.1
7.5
178°C
The Oxidation of Isobutene Over Sn0 2-MoO) and Co)04 - MoO) (C 8 - 20, 02 - )0, N2 - 20, H20 - )0 vol.%) /124/ 4H
TABLE 99
47.2 8.] 2.0 1.7 9.8 31.0
11.) 5.5 8.5 24.9
5.1 9.) 21.5 5.7 5.4 21.4
49.8
5.0
264°C
16.2
traces 47.9
3.8
254°C
co)04-MoO)
15.9
I traces 51.6
traces
I
1.9
I
10.0
-
2)2 0C
I
195°C
I
4.6 8.1 25.0
11.8
50.5
8.5
280°C
00
ss:
(J'
599
less significant. The overall rate at rather high concentrations of O2 is given by Eq. (XVII.102) with m = 1 and n = O. This was found to be so for Bi - Mo - 0 /129, 139, 140/, Bi - Mo - P - 0 /133) and Co - Mo - 0 /129/. In some cases, m < 1 and n) 0: for Bi - Fe 0 - Mo - 0, m = 0.7 /127/; for Fe - Sb - 0 and Sn - Sb - 0, n /131/. The process on Bi - Mo - 0 is not retarded by the reaction products /139, 140/ while on Fe - Sb - 0 the equation:
>
(XVII.113)
was obtained for the oxidation of but-1-ene. Similar inhibition was found on Fe - Te - Mo - 0 /138/. The selectivity to divinyl in the oxidative dehydrogenation of n-butenes on Fe - Sb - 0 /144/ and Bi - Mo - P - 0 /133/ increases with decreasing Po and the ratio Po fPC H in the reaction 2 2 4 8 mixture. The mechanism of alternating surface reduction-repxidation with participation of (0) anions has been proved to apply for Bi - Mo - 0 /128/, Bi - Mo - P - 0 /133/ and Fe - Sb - 0 /144/. Work function and electrical conductivity measurements for Bi - Fe - Mo - 0 suggest that there is a negative charge on the adsorbed oxygen and a positive one on the adsorbed n-butene /123/. Thermodesorption and infrared studies of but-1-ene on Mg-molybdate /106/ show that there are two major forms of adsorption. The first (weak and reversible) is attributed to f-complexes and Jr-allylic complexes of butenes and the second one (strong and irreversible) to carboxylate - carbonate species. The former species lead to mild oxidation while the latter result in deep oxidation as on Cu20 catalysts. The chemisorption of but-1-ene on a partially reduced V - P - 0 catalyst /136/ leads to a significant loosening of C=C and C-H bonds and to the formation of C=O-bonds. This can be accounted for by the ability of the V - P - 0 catalyst (in contrast to Cu20)
600
to accelerate the oxidation of n-butene into maleic anhydride. The above results suggest that Scheme (XVII.111) is valid for complex catalysts as well. The rate data are in agreement with the scheme mentioned. The equations of type (XVII.102) are approximations of Eq. (XVII.112) which corresponds to Scheme (XVII.111). Eq. (XVII.11) can be deduced on the basis of the following reaction mechanism /14S/: 1) C
4HS
+ (0)-(C
4H sO)
2) (C HSO)-(C H6 ) + H20 + ( C 4 4 fast 4H 6
4) C + (0) ~ 4H6
5) (C4H60 6)(
(C
(XVII.114)
4H60)
+x2(0) ' 1 2C02 +
)+02-(02)
i
Y2 H20 + (x 2 + 1) (
)
+(}2(0) fast
Scheme (XVII.114) is a particular case of Scheme (XVII.111). Parallel and consecutive formation of CO 2 is here taken into account. Eqs (XVII.109) and (XVII.112) predict a decrease 1n r and an increase in selectivity with decrease in 67 (and Po fPC H ). 2 4 S These relationships observed on Fe - Sb (Fig. 189) and other complex catalysts /145/, can be also due to energetic heterogeneity of the surface oxygen. The latter effect is caused by the fact that more oxygen-eatalyst bonds are broken in deep oxidation than in mild oxidation (leading to an increase in S with increase in qs); simultaneously, the overall reaction rate, r, decreases with increase ill qs. The low-temperature oxidation of butenes over Sn02 - MoO) and co)04 - MoO) /124/ gives evidence for an additional reaction route leading to the formation of saturated carbonyl compounds. The mechanism of this reaction /124/ involves the hydration of the olefin followed by the oxidation of a surface alcoholate. There are following facts in favour of the above mechanism.
°
601
s/%
r-!01Z·m-Z.,r!
/00
/0
2
/5 J
2
/
1
20 0
0
10
20
JO
40
SO It
Fig. 189 - The dependence of the overall rate of the oxidation of but-1-ene(1) and the selectivity to divinyl (2) on the number, N, of pulses of C 8 + 4H + 02 + He mixture over Fe - Sb /144/.
°
Secondary alcohols, which can be obtained by hydration of n-butene, are easily oxidized into saturated ketones under conditions of the direct oxidation of the butenes into ketones. The interaction of isobutene with 02 + H20 mixture results in hydration, as the tertiary alcohol formed cannot be converted into the ketone. Molecular hydrogen does not appear in the reaction products; hence, the conversion of the alcohol to the ketone is a true oxidative dehydrogenation. It is supposed /124/ that the hydration of the olefin is slow while the surface oxidation of the alcoholate is fast; in the last process, (0-) or (0 2-) species take part. The C=C - bond of initial butene simultaneously reacts with the surface, resulting in destructive oxidation (the formation of acids, C1 - C aldehydes and carbon oxides). This process oc3 curs more intensively with but-2-ene than with but-1-ene because of its stronger adsorption. In the low temperature oxidation of butenes (as in that of propylene), one can assume that the hydration equilibrium is shifted towards dehydration with increase in temperature. As a result, "allylic" mild oxidation and destructive oxidation become predominant routes at elevated temperatures. A generalized mechanism for the oxidation of but-1-ene taking into account low-temperature transformations, will be:
602
Products at mifd oxidat ion (C4 ) HZC=CH-GH=CH z HC-GH H~- C~OO ~ II II /
t
HC-C-,::-:O
'::I HC,/H
~
t
t
(0) (0) (HzC=CH-CH=CHz)-(If) ..
0/ H3C-CH=CH-CHJ--(Hlk:CH:: CH-CH3)~
(OH)
\
~
H C=CHCCH
I
2
(0) route I
+~
HzC=CH-CHz-CH3 -::::'(H2C=GHCH zCH3)~ (0)
maleate
I
Products ot aestruc) / tire ox!dtItiol7
3
(HlC=CH CH3 0
n
(C ( ) 1- 3 CH CHO CH WOH
route!!
HzO (Hz ~-VH-GHzGH3) route m O O
3
(0) -
eH COCH CH 3 Z 3
(CH3-~H-CH2CHJ)---
t,
3
(0)
t
CD
II
t
2
(I z)_··- (I z)---(COJ )
cartonuote carbo!late comptexes
t
-
(11/1)
(I;)
DH
aiconotate (XVII. 115)
2(OH) H20
~HzO
+ (0) + ( ) O2
+ ( ) ~(H20)
()
+ ( ) -(Oz) t{[st •
2(0)
In catalysis over complex catalysts, it is important to know on which cations the activation of the olefin takes place. For example, on the Bi - Mo - 0 catalyst, Bi J + - M0 6+ doublet is assumed to be the active site (which is why the optimum catalyst corresponds to Bi : Mo z 1) /141/. Some authors suppose that the ~-allylic complex is formed on Mo-ione; Bi J+ is assumed to facilitate surface dehydration since, in the steps leading to divinyl: )
C a + (0) 4H
• (C
4H7)
+ (OH),
) + ( 0 ) - C 6 + (OH) + ( (C 4H1 4H
),
the OH-groups are formed. Thus, the 811y1ic H-atom which is abstracted is thought to be attached to the oxygen of the bismuthylion:
603
In addition, the bismuth (by loosening the Mo - 0 bond) facilitates electron transfer from the olefin to the catalyst (surface reduction). The reduction of both the M0 6 + (to M0 4+ ) and the Bi 3 + (to Bi 2 + ) ions is assumed. FinallY, Bi 3+ ions inhibit the isomerization of the butene by decreasing the OH-- group concentration (since Bi 3+ catalyses surface dehydroxylation) and this enhances the selectivity to divinyl /128, 141/. The partial substitution of Bi 3+ for Fe 3+ ions causes the Mo-O and Mo-olefin bond strengths to approach the optimal values /123/. The results obtained suggest the necessity to revise the above point of view since it is more probable that the Bi ions have a dehydrogenating function. This means that a methyl-allyl radical is formed and localized on the Bi (but not on the Mo) ions; the hydrogen which is abstracted is attached to the Mo-O group. A correlation between the oxygen-catalyst bond energy and the catalytio properties of different complex catalysts is observed /106/ in aocordance with Soheme (XVII.115). The selectivity also increases with increase in qs on each ca'talyst (Sa - Sb - 0, Co - Mo - 0 and Fe - Mo - 0) when surface oxygen is removed. An increase in qs is caused by surfaoe heterogeneity /145/; this effect is partially attributed to decreasing values of 8 itself. Nevertheless, for the above catalysts, which differ significantly in their values of qs' there is no correlation between the seleotivity and the values of qs or the mobility of bulk oxygen /145/, In the oxidation of butene on V - Ti - 0 catalysts, there is no oorrelation between qs and the selectiVity to maleic anhydride /149/. /149/. These results indioate that the surfaoe oxygen bond energy is not the only faotor determining the catalytic properties of complex catalysts. In particular, Soheme (XVII.115) suggests that the olefin-oatalyst bond energy (espeoially, the methallyl-catalyst bond energy, qall) has an important role. We oannot at present estimate the qall values and predict the optimum qall value. It is however noteworthy that most of the active and selective oatalysts for mild oxidation contain ions of the p-elements with alternating valency (Bi, Sn and Sb). The presence of unfilled p-orbitals is likely to correspond to the optimum qall values. In the formation of acidic products (maleio anhydride, etc.),
604
the acid-base properties of catalysts should be also of great importance /117/. They are also significant for the isomerization of n-butene /124/. The reasons for the branching of the first route (Scheme (XVII. 115», leading either to oxidative dehydrogenation (Bi - Me catalyst and other complex catalysts) or to the formation of unsaturated carbonyl compounds (Cu20), are not clear. The lower oxygen bond energy in Cu20 is likely to be favourable for the latter direction.
°
The Oxidation of Butenes Over Molecular Sieves The but-1-ene and but-2-ene adsorbed on molecular sieves can be oxidized at 25 0_900C /150/. The gaseous products are methylethylketone, crotonaldehyde and but-2-en-1-o1. There is thus a similarity between the above process and the low-temperature oxidation of butene on Sn - Mo - 0 and Co - Mo - O,which involves olefin hydration. This is also evidence for the significance of surface acidity in the low-temperature oxidation of olefins. The Oxidation of Higher Olefins The much greater number of possible isomers for the higher olefins results in a significant increase in the number of thermodynamically possible reactions. For the pentenes, typical mild oxidation processes will be: a) in the case of pent-1-ene: H2C=CH-C 7 + "21 02 = CH 3H 3COC 3H7 H2C=CH-C 3H7 + ~ 02 =C 4HgCHO 1
H2C=CH-C 3H7 + '2 02
= H2C=CH-CH=CH-CH3
+ H20
605
b) in the case of pent-2-ene: CH 3CH=CH-C 2H S CH 3CH=CH-C 2HS CH 3CH=CH-C 2H S
1 + 2'
=
02
CH 3CH=CH-CH=CH2
CH + 02 3CH=CH-C 2H S
= CH
CH + 02 3CH=CH-C 2HS
=
CH 3CH=CH-C 2HS
+
1
3CH=CH-COCH3
~
+
+ H20
C + H 20 2H SCH=CH-CHO
02
=
H
2C=CH-CH=CH-CHO
c) in the case of 2-methylbut-1-ene:
r
H3
H2C=C-C 2H S
+
1
H3
1
'2
02
= CHO-CH-C 2HS r H3
rH3 H2C=C-C 2H S
+~
02
= H2C=C-CH=CH2 + H20
?H 3 H
2C=C-C 2HS
?H3
+
02
= H2C=C-CO-CH 3
+ H20
1H3
?H 3
H2C=C-C 2H + 02 = H2C=C-C2H + H2 0 S S
r
H3
H2C=C-C 2H + 1 S
~
02
=
H 20
H2C=9-CH=C3 2 + 2H CHO
20
+
2H
20
606
2-methylbut-2-ene: 1
+ '2 02
CH 3
=
CH
I
3-CH-CO.illi3
CfH3 CH
3-C=CH-CH3 CH I 3
CH 3-C=CH-CH3
The above reactions lead to saturated carbonyl compounds, to dienes and to unsaturated carbonyl compounds in which the C=O-bond is conjugated with the C=C-bond. Normal hexenes can also be oxidized into trienes or benzene (oxidative dehydrocyclization):
The Oxidation of the Higher Olefins Over Metals These reactions have been poorly studied. According to reference /151/, Pt, Ag and Au supported on Al 20 catalyee the complete oxi3 dation of pent-2-ene; platinum and silver are much more active than gold. Platinum on Al 20 (0.5 % Pt) accelerates at 300°-4000C the oxi3 dative dehydrocyclization of hept-2-ene converting into toluene. In the absence of 02' the olefin undergoes dehydrocyclization but the yield of toluene is significantly lower than in the presence of 02 /152/. The Oxidation of the Higher Olefins Over Simple Oxide Catalysts In the complete oxidation of pent-2-ene with excess of 02' the following activity pattern was obtained /151, 153/: C0 ~ Mn 0 "'? 2 3 304 7 Cr 20 7 Ni07W0 Ce02 /" Ti0 2» Fe 20 Al 20 Th02/" CuO ~V205/" Si0 2:::3/ 3? 3 3/" »BeO >Pb MgO?ZnO?Zr027 CaO (Table 100). This order is not 3047
607
precise as the reactor employed was not differential and the surface areas of the catalysts were different. Nevertheless, it 1s clear that the oxides of Co, Mn, Cr and Ni are the most active ones as in the oxidation of the other olefins. Mild oxidation of the pentenes is catalysed by CU20 /154, 155/. On Cu20/SiC at 2800_}600C /155/, pent-1-ene and pent-2-ene are converted into C dienes and d,~-unsaturated C carbonyl compounds 5 5 (pent-1-ene-}-one, pent-2-ene-4-one, pent-3-ene-5-al, pentadienal). Thus, the major direction of mild oxidation is an "allylic" one. The C diene yield is 2-4 times higher than that of 05 carbonyl 5 compounds. Oxidative dehydrogenation is favoured by increasing the 02 : C H10 ratio (from 0.3 to 2) and the temperature (from }20 0 to 5 }60 0C). The destructive oxidation of n-pentene to 04-02 carbonyl compounds takes place at the same time as full oxidation to CO 2 and H20. The overall selectivity to these carbonyl species reaches 10-}0% (at }200-34000). In the oxidation of pent-1-ene, the selectivity decreases in the order: pent-1-ene-3-one '7 pent-2-ene-4-one, aero Lein "7 propional dehyde "7 pentadi enal '> pent - }-ene-5-al; in the oxidation of pent-2-ene, the sequence of selectivities is: acrolein "7 propionaldehyde "7 pent-1-ene-3-one." pentadien,al "7 pent-2-ene-4-one;> pent-}-ene-5-al. By-products are methylethylketone, but-1-ene-4-al, pent-1-ene-5-al, pent-2-ene-5-al. Partial oxidation of branched pentenes (mixtures of 2-methylbut-1-ene, 2-methylbut-2-ene and 3-methylbut-1-ene) over copper oxide catalyst at }140-361 00 /154/ leads to isoprene, acetone, ethylacrolein and methylisopropenylketone. Acrolein, methylketene, 2-methylbut-2-ene-4-al, 2-methylbut-}-ene-1-al are formed in small amounts. Thus, in this case, mild oxidation also leads to 05 products of "allylic" oxidation, oxidative dehydrogenation exceeding the formation of unsaturated carbonyl compounds. Simultaneously, destructive and complete oxidation takes place. It is interesting that isovaleric aldehyde (a saturated carbonyl compound) was also detected in the reaction products (in small amounts). On vanadium pentoxide (supported on pumice) at 2000-40000 /12/, branched pentenes (2-methylbut-2-ene, 2-methylbut-1-ene and 3~e thylbut-1-ene) are converted into acetaldehyde, acetone, 2,}-epoxy-2-methylbutane and 3-methylbutan-2-one. Simultaneously, the isomerization of 2-methylbut-1-ene and }-methylbut-1-ene into 2-methylbut-2-ene occurs. ThUS, among all the possible 05 products
608
of mild oxidation, only 3-methylbutan-2-one is formed. The authors of reference /12/ believe that the reaction is completely heterogeneous. However, it is noteworthy that there is a definite similarity between qualitative product distribution for the oxidation of the pentenes over V20 and that for the gas-phase oxidation 5 without a catalyst. TABLE 100 The Oxidation of Pent-2-ene Over Simple Metal Oxides /151-153/ Specific surface area / m2 g-1
Catalyst
C0
304
~OJ
+ Mn 0
cr20 3 NiO W0 3 Ce02 Ti0 2 0( -Fe 2O J q -Al 2OJ Th02
3 4
1103 3.2 14.0 6.2 3.6 9.J 10.2 13.9 87.6 55.7
Temperature (OC) to attain 80% conversion 180 225 2J5 255 270 275 310 322 JJ2 365
Catalyst
CuO V20 5 Si02 BeO Pb J0 4 MgO ZnO Zr0 2 CaO
Specific surface area / m2 g-1
0.5 1.0 421 0.6 9003 0.5 0.7
'remperature(OC) to attair 80% conversion
387 400 450 485 495 5J2 545 545 560
The data obtained show that the oxidation of pentenes over cu 20 proceeds via a parallel-consecutive scheme /155/. The chemical composition of the catalysts used suggests that a mechanism of type (XVII.115) is valid for the pentene oxidation process. Both the oxidative dehydrogenation and the formation of C unsaturated car5 bonyl compounds are likely to involve allylic intermediates while saturated 0 carbonyl compounds are formed via the hydration of 5 olefins. In reference /12/, the mechanism of alternating surface reduction-reoxidation is postulated. The latter leads to a correlation of activity with qs as shown in Fig. 190. According to reference /62/, the rate of catalysis in the deep oxidation of olefins on O~O at 200°0 exceeds that of reduction
*
f
14.3
10.S
7
8
Hept-1-ene
Oct-1-ene
11.6
1S.5
9
6
Hex-1-ene
12
20.8
6
4
Butene
11.5
rR
30.7
3
Propylene
3
=1.Sn
~=
4.5
2
n
Ethylene
R
Olefin
140
150
100
93
140
125
138
34.5
rR
93
83
92
23
r CO 2
90 70
12.9 8.8
64
10S
135
96
120
80
20.2 10.7
137
91
54
3003
36
18
-
r CO 2
rR
Catalysis (ri·10- 1O) at PR=0.02 Torr at PR=O.OS Torr
11
-
12
11
14
1S
12
- 10 - -
Reduction by R: r CO . 10 -10 2 at PR= O.OS 0.02 r Torr R Torr
The Full Oxidation of Olefins Over Cu20 (2000C, Po = 0.3 Torr) /62/: 2 CnH2n +~02 = nC02 + nH20 ; rR = r C02 = r 02 (in molecules cm-2 s-1).
TABLE 101
106 64
86
S9
86 106
94
0.17
0.12
0.17
0.18
0.2
80 60 43
0.28 32
O.OS Torr
Ratio of the rates of reduction and catalysis at PR=O.OS Torr
13
0.02 Torr
Oxidation of adsorbed R after catalysis at PR=
~
m
o
610
(Table 101). Under these conditions, a carbonate-carboxylate complexes are destroyed (with the rate equal to that of catalysis) only in the presence of 02' At higher temperatures, the above compFig. 190 - The correlation of activity with qs for the oxidation of pent-2-ene (over oxides with specific surface areas within the range 9-14 m2 g-1, Table 100): 1 - c0 0 , 2 3 4 4 - Ce02, 3 - Fe 20 Ce20 3, 3, 5 - Ti0 2•
400 500 20
40
50
fls/IrCtIL(;-atO;-'
lexes are decomposed in the absence of 02 and the reduction-reoxidation mechanism becomes true. The rate of formation of carbonyl compounds and of CO 2 on Cu20 is proportional to P02' both processes being retarded by water vapour /154,155/. Hence, the surface coverage with oxygen, 8, under these conditions is low. The selectivity to CO 2 (on Cu20) increases with increase in the ratio Po fPC H and the activation energy for the formation of 2 5 10 CO 2 exceeds that for the carbonyl compounds /154, 155/. These facts can be interpreted on the basis of the proposed mechanism, since the latter suggests that there is more rupture of oxygen-catalyst bonds in deep oxidation than in mild oxidation. The Oxidation of Higher Olefins Over Complex Oxide Catalysts Bismuth molybdate is an effective catalyst for the mild oxidation of the higher olefins /139/. The distribution of the reaction products is presented in Table 102 (for PR = 0.2 atm). The experiments were made at low conversions (below 10%) when isomerization was poor. (It is enhanced at high conversions). One can see that mild oxidation results mainly in dienes and d,fl-unsaturated carbonyl compounds, i.e. "allylic" oxidation takes place. The former route, oxidative dehydrogenation, prevails.Diene carbonyl compounds are produced by way of the combination of oxidative de-
611
TABLE 102 The Oxidation of Olefins Over Bismuth Molybdate at 460 0C /139/
Olefin
Propylene
Conversion/%
Selectivity/ %
Olefin
10 40 80
To acrolein: 90 86 73
3-Methylpentene
Conversion/Io
10 20
Selectivity/ ;0 To C 12: 7H 85 73 (to Cr 10: 5; 8)
To butadiene:
But-1-ene 20 40 80
2-Methylprop-1-ene 10
40 70 Pent-i-ene 10
20 60 Pent-2-ene
95 95 90
Hex-1-ene
To methacrolein: 72
Hex-2-ene
72 72
To pentadiene: 93 87 38
CO 2 + H2 0 are only formed 20 40 70
5 10
79
20
To isoprene:
3-Methylbut-1-ene 40
35
60
(to C 80: 5H 10, to C 60: 3) 5H
24 18 10
(to hexatriene: 5; 6; 7; to benzene: 15;22;30) To methylpentadiene: 55 49 38 (to C6H100: 9; to C6H 80:
5)
80
(to C5H60 : 4)
To hexadiene: 71 59 32 (to hexatriene: 2;5;7; to benzene: 0;7;32)
20 40 70
2-Methylpent-1-ene
88
5
15
2-Methylbut-1-ene
Cyclopentene
3-Methylpent-1-ene 10
50 80
ro methylpentadiene: 90 54 27 (to vinylbutadi.ene: 10; 16; 12)
612
TABLE 102 (continuation)
Olefin
Conversion/%
Selectivity/ %
20 40 70
73 62 44 (to C aO: 5H 9, to C5H 6O: 4;9;16)
2-Methylbut-2-ene
3,3-Dimethylbut-1-ene 2 5
4,4-Dimethylpent-1-ene
Hept-3-ene
Olefin
Conversion/%
4-Methylpent-1-ene
To 2,3-dimethylbutadiene: 60 2-Methylpentadiene 56 (to °6H10: 3; to C6HaO: 6; a-, to C6H602: 1; 2)
2 5 10
70 59 40 (to C6HaO: 6;6;5)
5 10 20
67 60 44 (to C6H10O: 3; to C6HaO: 5)
To 2,3-dimethylpentadiene: Hept-1-ene 50 2 5
74 53
40
20 (to C : 7HlO 7; to C7H a: 24; to CraO: 5)
Selectivity/ %
2-Ethylhex-1 -1-ene
To Cr 12: 27 (to C 7HlO: 6; to C a: 4; 7H to C7H 6O: 2) 'fo
5 15
CaH14: 66 53 (to CaH12: 3; 7; to CaH10: 0;6; to CaH a: 0', 2)
613
hydrogenation and of allylic oxidation of the diene formed. The structure of propylene and isobutene prevents them from undergoing oxidative dehydrogenation; because of this, only carbonyl compounds are formed in this case. Nevertheless, when the CH 3-group is weakly bound to a tertiary C-atom, the branched olefin can undergo skeletal isomerization and be converted into dienes. This is why dienes are formed from 2,2-dimethylbut-1-ene (together with a,fi-unsaturated carbonyl compounds). In the oxidation of 4,4-dimethylpent-1-ene, skeletal isomerization is likely to occur after abstraction of the allylic H-atom. According to reference /139/, the formation of unsaturated aldehyde from higher olefins requires the presence of the CH 3-group attached to the vinyl C-atom. This agrees with the data for Bi but does not correspond to the picture obtained /155/ with - Mo Cu20, where unsaturated aldehydes were produced from normal pentenes. The selectivity to the above aldehydes is likely to depend markedly upon the catalyst (as well as upon the structure of the initial olefin). The oxidation of higher olefins with long chains (hex-1-ene and hex-2-ene) leads to dienes and trienes as well as to benzene, a product of oxidative dehydrocyclization. The major product of the mild oxidation of branched pentenes at 420 0-5500C on Bi - Mo (Bi : Mo~1) promoted with P205 (and also without the promoter) is isoprene /133/. With excess of 02' normal pentenes are converted into maleic anhydride on Co - Mo and V - P catalysts. At 450 0C on V - P - 0, a yield of 55 wt.% of maleic anhydride is achieved /95/. The 3n0 2 - Mo0 catalyst accelerates the oxidation of pent-1-ene 3 to methylisopropylketone at low temperatures: i.e. a saturated ketone is produced. By-products are CO 2, CO, C 2 saturated acids, 4-C acetaldehyde, diethylketone and acetone. Fast isomerization of pent-1-ene to pent-2-ene takes place under these conditions (Table 103) /124/. Oxidation of higher olefins on the above catalysts proceeds via a parallel-consecutive scheme. The contribution of the consecutive routes (leading to destructive oxidation) is enhanced by increasing the temperature /95, 124, 133, 139/. The product distribution and the composition of the catalysts employed suggest that a mechanism of type (XVII.115) is valid for the oxidation of higher olefins. At elevated temperatures, a predominant route is the first one
°
°
°
°
614
involving the formation of intermediate allyl species. On Bi - Mo - 0 and Bi - Mo - P - 0 catalysts, this leads to oxidative dehydrogenation (dehydrocyclization) and to unsaturated carbonyl compounds, while on Co - Mo - 0 and V - P - 0, maleic anhydride is formed. At low temperatures on Sn - Mo - 0, the third route prevails; it includes intermediate hydration and oxidation of surface alcoholates. Both the routes mentioned are accompanied by destructive and deep oxidation (the second route). The rate of mild oxidation of higher olefins over Bi - Mo - 0 catalyst obeys /139/ the equation:
r", = f 1!7 ~ 8
(XVII. 116 )
where PR is the partial pressure of olefin. The surface coverage with oxygen, 8, is determined /139/ by the Langmuir isotherm: 17.6 0.0 8=
b()
Po
'2 '2 17.6 (J.6 /1- b~ P + ~
0
I!o
i.e. the process i8 inhibited by product (P p is its partial pressure and b p is adsorption coefficient of the diene or unsaturated carbonyl compound). The value of b p is approximately the same for pentadiene and other dienes. Unsaturated carbonyl compounds cause stronger retardation (the values of bp for these are two orders of magnitude higher). The kinetics of the oxidative dehydrogenation of branched pentenes on Bi - Mo - P - o are described by Eq , (XVII.116) at 8 ~ 1 /154/. A scheme of type (XVII.115) leads to the following rate equations for mild (I'm) and deep (I'd) oxidation. The surface coverage wi th the olefin, 8 R' in the Henry region will be: 8 R = bJl R. The rate of mild oxidation (the first route) is equal to the rate of the reaction between the weakly adsorbed olefin and one anion of surface oxygen:
r :: I.- 8 8= k .P 8 171 Ilf mR
615
rUBLE 103
The Oxidation of Pent-1-ene on Sn0 2 - Mo0 3 (C5H 10 - 3, O2 - 30, N2 - 37, H20 - 30 vol.~) 158
Conversion /% Selectivity (%) for the oxidation into: CH + C 2H 5COC 2H 5) 3COC 3H7( 4/5 C3H7COOH 3/5 CH
3COCH3
3/5 C2H 5COOH 2/5 CH 3CHO 2/5 CH 3COOH 1/5 CO 1/5 CO 2 Selectivity (%) for the isomerization into 2-G 10 5H
/124/
185
8.0
22.0
65.4
52.6
1.0
1.4
traces
2.2
II
2.1
1.0
1.4
2.0
3.0
2.0
9.9
25.6
27.4
57.4
70.4
where km = k 1bR• Eq. (XVII.117) coincides with Eq. (XVII.116) which is adequate to explain the experimental data. The rate of deep oxidation (the second route) is equal to the rate of interaction of the olefin with two (0) species:
(XVII.118) where k d = k 2bR• The participation of (0) in the process is proved by data /133/ on the activity and selectivity of Bi - Mo - P - 0 during catalysis and surface reduction. Eq. (XVII.116) shows that the rate decreases with decreasing 8 . Such a relationship was observed in the reduction of Bi -
616
- Mo - P - 0 by pentenes /133/. Eqs (XVII.117) and (XVII.118) lead to the following expressions for selectivity:
S =--m r +r m d (XVII.119 ) Hence, the seleltivity should increase with decreasing values of 8. This is really observed on Bi - Mo - P - catalyst /133/. The relative yield of the mild oxidation products decreases with increasing conversion. This is due /139/ to an increasing retardation by the mild oxidation products and to their further oxidation.
°
The Oxidation of Dienes The simplest diene is allene, H2C=C=CH 2• Of main practical importance is the oxidation of dienes with conjugated C=C-bonds. The major reactions of diene oxidation are:
H2C=C=CH2 + 402 = 3C0 2 + 2H2 0 , 1 H2C=CH-CH=CH2 + 2' 02 = H2C=CH-COCH 3 ,
1 02 = CH H2C=CR-CH=CH2 + 2' 3-CH-CHCHO,
H2C=CH-CH=CH 2 + 02
== CH 3-CH=CHCOOH,
HC-CR H2C=CH-CH=CH2 + 02 == /I /I + H20, HC CH
'01
H2C=CH-CH=CH2 + 202
=- CH 2=CHCHO
H2C=CH-CH=CH2 + 2 ~ 02
/0 =- Hy,-C'-.,. HC-C'-o
'0
+ CO 2 + H 20, + 2H 20,
1 H2C=CH-CH=CH2 + 2 2' 02 = CH 2=CH-COOH + CO 2 + H20,
617
H2C=CH-CH=CH2 + J0 2
=
CHJCHO + 2C0 2 + H20,
1 02 = CHJCOOH + 2C0 + H H2C=CH-CH=CH2 + J '2 20, 2
H2C=CH-GH=CH2 + 3
2'1
02 -= 4CO + )H2 0,
H2C=CH-CH=CH2 + 5
'21
02 = 4 C0 2 + )H2 0,
H + 02 2C=CH-CH=CH-CH 3
== H2C=CH-CH=CH-CHO
1 H 2C=CH-CH=CH-CH J + 4 '2 02
== CH 3CHO
+ H20,
+ 3C0 2 + 2H20,
H2C=CH-CH=CH-CH + 702== 5C0 2 + 4H 2 0 , J H2C=CH-C=CH2 + 02 I CH 3
== H2C=CH-C=CH2 I CHO
+ H20,
1 H2C=CH-G=CH 2 + 4 '2 02 = CH + 3C0 2 + 2H20, 3CHO I CH 3
=
5C0 2 + 4H 2 0 , H2C=CH-C=CH2 + 702 H 3 CH + 02 = CH 3-GH=CH-CH=CH-CHO + H20, 3-CH=CH-CH=CH-CH3
9
CH CH CH
3-CH=CH-CH=CH-CH3 3-CH=CH-CH=CH-CH3 3-CH=CH-CH=CH-CH3
+ 202 = CHO-CH=CH-CH=CH-CHO + 2H20, + 602 == CH
3CHO
+ 4C0 2 + )H2 0,
+ 8 ~ 02 = 6C0 2 + 5H2 0•
The Oxidation of Dienes Over Simple Metal Oxide Catalysts
The specific activity, r, in the oxidation of allena (Table 104) at 300 0C decreases in the order: CuO 7Ivln0 co Ni07 2/ 30 4/ 7Fe2037Cr203/zn07V2057'fi02. The reaction products are CO 2, H20 and unknown mild oxidation products. The range of specific activities for deep oxidation is nearly the same as in the overall process, the selectivity to mild oxidation being in the reverse order: ZnO,;:>Ti0 V Cr '7 Mn0 NiO;::.Fe /156/. 2, 20 5/ 20 3 27 20 3::::-Cu0'7C0 30 4 One can assume that the (0) ion participates in the reaction, more oxygen-catalyst bonds being broken in deep oxidation then in mild oxidation. As a result, the overall reaction rate decreases
V20 5 'riO 2
ZnO
3
Fe 20 3
cr 20
0( -
C0 30 4 lIiO
Mn0 2
CuO
Catalyst
7.9 x 10 11 6.1 x 10 11
8.5 x 10 12 3.2 x 10 12
2.9 x 10 14 4.2 x 10 13 2.2 x 10 13
7.1 x 10 14 4.4 x 10 14
-
1.1
1.0
0.6
-
-
-
0.6
20
0.4
0.5
22
-
-
-
0.5
1.2
26
0.9
-
23
-
-
0.4
0.4
0.]
0.1
0.5
0.]
0.]
0.9
0.8
50
50
90
42
27
31
0
34
15
Selectivity (~) r(molecules cm- 2 s-1) E/ kcal mol- 1 Heaction order --::,---to mild oxida0C at 300 and Cc H = --~~.~----~ota~ con- Deep oxi- in allene, m 'I'oral conDeep tion at 300°C 3 4 version dation version oxi=1 % dation
The Oxidation of Allene on Metal Oxides with Excess of 02 /156/
TABLE 104
en 00
.....
619
with increasing q s (Fig. 191), while the selectivity to mild oxidation increases with q s (Fig. 192). The consequence of the above two interrelationships is a correlation between activity and selectivity (Fig. 193). On V (at 310 0-420 0C and with air: diolefin = )5 : 126), di20 5 vinyl in oxidized into maleic anhydride /127/. The yield of this product reaches 58% (at 34!o conversion) and decreases with increasing temperature and contact time which indicates that further oxidation of maleic anhydride occurs. By-products of the reaction are carbon oxides and formaldehyde (with a yield of 4%). At )50 0C and with a concentration of divinyl of 0.6-1.0% in air, the selectivity to maleic anhydride reaches 4)%. Small amounts of furan, CH)CHO and CH)COOH are also formed. A parallel-consecutive scheme is assumed. The parallel route involves the oxidation of divinyl into a surface intermediate leading to furan or to another complex resulting in the formation of maleic anhydride /157/.
Igr 15 10
/4
0
S/%
6
15
o} 0
2 50 .J
1J
0
25
12 11
0
20
40
60 fs!tcO't(g--crtOj-'
Fig. 191 - The correaltion of specific activity with q s for the oxidation of allene (data of reference /156/): 1 - Mn0 2' 2 - C0 0 , 3 - CuO, 4 - Fe 2 0 ) , 3 4 5 - ZnO, 6 - Ti0 2, 7 - V20 5•
0
20
40
60 fJs/kcO't(i-crtOl'
Fig. 192 - The correlation of selectivity with qs for the oxidation of allene (data of reference /156/): 1 - Co)04' 2 CuO, 3 - Mn0 2' 4 - Fe 20 3 , 5 V20 5, 6 - ZnO, 7 - Ti0 2•
620
05'/.%
10
Pig. 193 - The correlation of the activity with oelectivity for the oxidation of allene over metal oxides (the data are taken from fable 104): 1 - ZnO, 2 - V 20 5, 3 - Cr 203' 4 - NiO, 5 - Fe 20 6 - wm0 2 , 7 3, CuO, 8 - C0 304•
15
50
25 01.-----'-------1.---'---0---'-/f 12 13 /4
The Oxidation of Dienes Over Complex Oxide Catalysts The oxidation of allene on CuO - A120 with excess of 02 re3 sults in the complete combustion of the hydrocarbon /51/. On CU 20 catalysts promoted by Mo0 and Vi0 ( « 1;(;), dienes with 3 3 conjugated C=C-bonds are selectively oxidized into unsaturated carbonyl compounds, i.e. "allylic" oxidation takes place (see Table 105). In addition to carbon oxides, acetaldehyde is a by-product /103/. On Co - Mo and V - p catalysts, divinyl is converted selectively into maleic anhydride. Over the former catalyst, the product yield reached 88 wt. % (with respect to divinyl passed) /85/. The addition of P205 to V 20 results in a sharp decrease in 5 the activity but does not influence the selectivity. 'l'hus, the catalyst with a V : P ratio of 1 : 1.6 is 30 times less active than V 20 its selectivity being only 3% higher than that of V 20 5 5, 0C, (350 0.6 - 1% C 6 in air). By-products on V - P - 0 are car4H bon oxides, furan, CH and CH /157/. 3CHO 3COOH In the catalytic oxidation of divinyl on V - P at 430 0-4500C, homogeneous steps were detected. They do not affect the rate of formation of maleic anhydride /158/. Bi - Mo catalyst is relatively inactive for the oxidation of divinyl (490 0C). The reaction products are furan (selectivity 25% at 36% conversion) and carbon oxides /140/. For the oxidation of diene, a parallel-consecutive scheme is usually accepted /95, 130, 140, 157, 158/. The following mechanism (in the case of a parallel scheme) can be assumed:
°
°
°
°
621
fUTa1!
maleic tI1!nydnd8 (0) .. (12)
(t;)
(0) / HzC=CH-CH=CH z
+(
__
route I
) /
)----(H 2C=CH-CH=CHz ~
2f~~11
~oute
routeIII
HzC=CH-COCHr
Products ot
_
destrtfct/ve oXldation,CO, CO2
JI
t
1/
(It )~···--(I2)
H
20
(Ill) --- (1(")1
The first route leads to mild oxidation, the second - to deep oxidation. The first route involves an allylic type complex (H ~ 2C ~CH:--:-;-CH-;-:-:CH2). Its oxidation leads to (I~) which gives f'unan , Further co~version of (I~) on the surface results in a surface maleate, (1 2 ) , its desorption giving maleic anhydride. TABLE 105 The Oxidation of Dienes Over Cu20-Catalysts Promoted with Oxides of ~o and W /130/ (400°-420°C; diene : O2 : N2 = 1 : 1.2 : 1.8 vol.%)
Initial diene
Main reac tion product
Piperylene
Penta-2,4-dien-1-al Vinylacrolein Hexa-2,2-dien-1-al
Isoprene Dipropenyl
Diene conversion/ %
Selectivity/ %
Activation energy/ kcal mol- 1 for for mild oxi- deep oxidation dation
7.6 6.)
65 60
13 11
23 23
8.6
48
15
19
The second route include the rupture of C=C-bonds and leads to 1/ destructive and deep oxidation via (Ii ) complexes of the carbonate-carboxylate type.
622
I'he third route is similar to the oxidation of monoolefins into saturated carbonyl compounds and involves surface hydration leading to an alcoholate,(I;"), and its sUbsequent oxidation. In the case of higher diolefins (piperylene, isoprene, etc.), the first direction is similar to the "allylic" oxidation of olefins and leads to dielllils. Selective catalysts for the process are Cu20-containing systems. The observed first order in O2 and zero order in diolefin /130/ suggests that there are small surface coverages by oxygen. The greater activation energies for deep oxidation in comparison with those for mild oxidation (Table 105) may be caused by the greater number of oxygen-catalyst bonds broken in route II in contrast to route I. On a V - P - 0 catalyst, a zero order dependency on 02 was obtained /158/ which suggests that high 8 values occur, in accordance with the higher values of qs (compared with those for V20 5). RDFEB.j~NCES
2 3
4
5 6 7 8 9 10
11 12
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626
90 S.H. lnami and H. Wise, J. Catal., 18(1970)343. 91 V.P. Linde, Thesis (in Russ.), Ens t , Chem , Phy s , , AN SSSH,liloscow, 1962. 92 C.H. Adams, H.H. Voge, C.Z. Llorgan and W.F. Armstrong, J. Catal., 3( 1964 )379. 93 A.I. Gelbstein, N.V. Kulkova, S.S. Stroeva, Yu.M. Bakshi and V.L. Lapidus, Probl. Kinetiki i Kataliza, 11(1966)146. 94 A.I. Gelbstein, Yu.M. Bakshi and S.S. Stroeva, Kinetika i Kataliz, 6(1965)1025. 95 Yu.D. Kernos, Thesis (in Russ.), Leningr. Techn. Inst., 1962. 96 Y. Moro-oka, S. Tan and A. Ozaki, J. Catal., 12(1968)291. 97 Y. Moro-oka, S. Tan and A. Ozaki, J. Catal., 18(1970)125. 98 Y. Moro-oka, S. Tan and A. Ozaki, J. Catal., 18(1970)132. 99 N.K. Allakhverdova, Thesis (in Russ.), Azerb. lnst. Nefti i Khimii, Baku, 1973. 100 V.I. Lazukin, M. Ya. Rubanik and Ya. V. Zhigailo, Kataliz i Katalizatory, 2(1966)50. 101 V.I. Lazukin, Thesis (in Russ.), lnst. Phys. Chern., AN Ukr. SSR, Kiev, 1966. 102 K.M. Kholyavenko, I.M. Baryshevskaya, N.A. Chernukhina and lVl.Ya. Rubanik, Kataliz i Katalizatory, 2(1966)67. 103 K.M. Kholyavenko, LM. Baryshevskaya, U.A. IJaksimova and V.I. Lazukin, Kataliz i Katalizatory, 6(1960)61. 104 A.R. Kuliev, V.D. Sokolovskii, T.V. Andrushkevich, G.K. Boreskov and O.A. Kozmenko, Dokl. AN SSSR, 211(1973)897; 212(1373) 121105 A.R. Kuliev, Thesis (in Russ.), Inst. Catal. SO AN SSSH, Novosibirsk, 1974. 106 Yu.N. Kafarov, Thesis (in Russ.), Azerb. Inst. Nefti i Khimii, Baku, 1974. 107 W.M.H. Sachtler and N.H. de Boer, in Proc. 3rd Intern. Congr. Catal., Vol. I, North-Holl. Publ. Co., lUnsterd&~, 1965, p.252. 108 W.E. Campbell, E.L. McDaniel, W.H. Heece and J.E. liilliams, Ind. Eng. Chern., Proc. Des. Dev., 9(1970)325. 109 V.I. Lazukin, M. Ya. RUbanik, Ya.V. Zhigailo and A.A. Kurganov, Kataliz i Katalizatory, 3(1967)54. 110 G.M. Godin, C.C. McCain and E.A. Porter, in Proc. 4th Intern. Congr. Catal., Vol. I (in Russ.), Nauka, Moscow, 1970, p. 232. 111 T. Sakamoto, M. Egashira and T. Seiyama, J. Catal., 16(1970) 407. 112 T.lshikawa and K. Youkasha, J. Chern. Soc. Jap., 16(1964)407.
627
113 V.G. Mikhalchenko, Thesis (in Rues , }, Ens t , Cat aI , SO AlI 03311, Nov03ibirsk, 1974. 114 C.C. McCain, G. Gough and G.M. Godin, Nature, 198(1963)989. 115 A.V. Krylova, in Proc. 4th Intern. Congr. Catal. SymposiUlll "Mechanism and Kinetics of Complex Catal. Reac tions" (in l{USS.), Moscow, 1968, pre print N 18. 116 L.E. Derlyukova, Thesis (in Rus s , }, Lns t , Cherl1. 1'hY8., AN 3SSrt, Moscow, 1970. 117 G.I. Golodets, Dokl. AN SSSR, 184(1969)1334. 118 G.K. Boreskov, Kinetika i Kataliz, 14(1973)7. 119 S.S. Batsanov, Electronegativity of Elements and Chemical Bond (in Russ.), Izdat. SO AN SSSR, Novosibirsk, 1962, p. 65. 120 J. Mochida, S. Hayata, A. Koto and T. Seiyama, J. Catal., 15(1969)314. 121 J. Mochida, S. Hayata, A. Koto and T. Seiyama, J. Catal., 19 (1970)405. 122 T.G. Alkhazov, M.S. Belenkii and R.I. Alekseeva, in Proc. 4th Intern. Congr. Catal., Vol. I (in Russ.), Naulca, Moscow, 1970, p. 212. 123 LB. Annenkova, Thesis (in Russ.), Azerb. Inst. Nefti i Khimii, Baku, 1970. 124 S. Tan, Y. Moro-oka and A. Ozaki, J. Catal., 17(1970)132. 125 M. Ai, Bull. Chem. Soc. Jap., 44(1971)761. 126 V.K. Skarchenko, Uspekhi Khimii, 37(1968)3. 127 R.H. Bretton, Shen-Wu Wan and B.F. Dodge, Ind. Eng. Chem., 44(1952)594. 128 Ph.A. Batist, C.J. Kapteijns, B.C. Lippens and G.C.A. Schuit, J. Catal., 7(1967)33. 129 A.I. Gelbstein, S.S. Stroeva, Yu.M. Bakshi and Yu.A. Mishchenko, in Pro c , 4th Intern. Congr. Catal., Vol. I (in H.uss) , Nauka, Moscow, 1970, p. 251. 130 N.I. Popova, Thesis (in Russ.), Inst. Phys. Chern. Al~ Ukr. SSR, Kiev, 1968; Probl. Kinetiki i Kataliza, 11(1966)153. 131 S.B. Radzhabli, Thesis (in Russ.), Azerb. Inst. Nefti i iGlimii, Baku, 1973. 132 M. Ai, Bull. Chem. Soc. Jap., 43(1970)3490. 133 E.A. Melnik, Thesis (in Russ.), Sektor Neftekhimii AN Ukr.SSR., Kiev, 1971. 134 N.N. Sazonova, S.A. Venyaminov and G.K. Boreskov, Kinetiku i Kataliz, 14(1973)1169. 135 J.R. Anderson and A.B. Swanson, J. Catal., 8(1967)41.
628
136 Ya.D. Go:eokhovatskii and S.V. Gerei, in Pr-o c s E Soviet-Japanese Seuinar on Catal., Inst. Catal. SO lUf SSSR, Novosibirsk, 1971, preprint N 21. 137 :,I.Ai, P. Boutry, P. Llo n t ar'na.L and G.rhomas, Bull. Soc. Ch.Lm, ffrance (1970)2783. 138 V.d. Zhiznevskii, D.K. rolopko and B.V. Fedevich, Kataliz i ~atalizatory, 3(1367)65; 75. 139 C.lt. Adams,in Pr-o c , 3rd Intern. Congr. Catal., Vol. I , NorthHoll, Publ, Co., Amc t er-dam , 1965, p , 240. 140 Cd. Adurus , H.H. Voge, C.Z. i;lorgan and Vi.F. Armstrong, J. Catal.,3(1964)379. 141 Ph.A. Datist, B.C. Lippens and G.C.A. Schuit, J. Catal., 5 (1966)55. 142 V.ri. Zhiznevskii, A.I. Kluchkovskii, D.K. Tolopko and E.V. Fedevich, Zhur. Prikl. Khim., 39(1966)2540. 143 Ph.A. Batist, C.G.M. Moesdijk, I. Matsuura and G.C.A. Schuit, J. Catal" 20(1971 )40. 144 V.P. Shchukin, S.A. Venyaminov and G.K. Boreskov, Kinetika i Kataliz, 12(1971)621. 145 G.K. Boreskov, S.A. Venyaminov, N.N. Sazonova, Yu.D. Pankratyev and A.N. Pitayeva, in Proc. I All-Union Conf. on Mechanism of Heterogeneous Catal. Reactions (in Russ.), Inst. Chem. Phys. AN SSSR, Moscow, 1974, preprint N 103. 146 L.S. Akimova, B.R. Serebryakov and P.K. Kolchin, Neftekhimiya, 11(1971)545. 147 J. Economi, D.T. Meloon and R.L. Ostrozynski, J. Catal., 4 ( 1965 )446. 148 V.P. Shchukin, S.A. Venyaminov and G.K. Doreskov, Kinetika i Kataliz, 11(1970)1236. 149 M. Blanchard, G. Longuet, J. Rivassean and J.C. Delgrauge, Bull. Soc. Chim. France (1973)3071. 150 D.F. van Sickle and M.L. Prest, J. Catal., 19(1970)209. 151 K.C. Stein, J.J. Feenan, L.J.E. Hofer and R.B. Anderson, Bull. Bur. Mines, US Dept. Inter., 1962, p. 4. 152 IiI. Jouy and J.C. Balaceanu, in Actes du 2me Co ngr-, Intern. Catal" Vol. I, Ed. Technip, Paris, 1961, p , 345. 153 E.C. Stein, J.J. Feenan, G.P.Thompson and J.F. Schultz, Ind. ~nG' Chem., 52(1960)671. 154 Ya.B. Gorokhovatskii, E.V. Rozhkova and A.I. Pyatnitskaya, Kinetika i Kataliz, 9(1968)332.
629
155 Ya.B. Gorokhovatskii, A.I. Pyatnitskaya, A.N. Korol, E.lI. Popova and E.V. Hozhkova, Kinetika i Kataliz, 9(1968)81. 156 O.N. Kimkhai, V.V. Popovskii and G.K. Boreskov, Kinetika i Kataliz, 13(1972)908. 157 M. Ai, Bull. Soc. Chim. Jap., 43(1970)3490. 158 B.l. Ostroushko, Yu.D. Kernos and 1.1. Ioffe, Neftekhimiya, 11(1971)693.
Chapter XVII THE OXIDATION OF OLEFINS AND DIOLEFINS The Oxidation of Ethylene The main directions of the oxidation of ethylene (the simplest representative of the olefins) will be:
°2 = C2H4O C2H + 1 ° = CH 4 3CHO 2 2 C2H + 1 4 2
(XVII.1) (XVII. 2 )
C2H + °2 4
= CH 3COOH
(XVII.3)
C2H 4 + 02
= 2HCHO
(XVII.4)
C2H4 + 202
= 2CO
C2H 4 + 302
= 2C02
+ 2H 2O
+ 2H 2O
(XVII.5 ) (XVII.6 )
Processes (XVII.1) to (XVII.4) lead to valuable oxygen containing products: ethylene oxide, acetaldehyde, acetic acid and formaldehyde. Because of that, the choice of selective catalysts for the above reactions is of great practical importance. The synthesis of ethylene oxide is of special interest. This product is used for production of many valuable suhstances /1/. The hydration of C2H results in ethylene glycol which is an 40 antifreeze, a solvent, a plasticiser and a monomer for synthetic fibre. The interaction of ethylene oxide with alkylphenols, aliphatic alcohols, acids and mercaptans leads to surface-active substances. Other derivatives of C2H (for example, ethanolamines) 40 are employed in the chemical and oil-processing industries. Ethylene oxide readily enters various polymerization and co-polymerization reactions. Many pharmaceutical preparations and other chemical substances are produced using ethylene oxide. The possibility of employing C2H or its polymers as fuels for jet engines 40 is being examined.
499
A predominant method for the production of ethylene oxide in industry is the partial oxidation of ethylene over silver catalysts. This method was discovered in 1931 /2/. The complete oxidation of C2H into CO 2 and H20 on Pt was firstly observed in 4 1817 /3/. The hydrogen atoms in the H2C=CH2 molecule are bound to the bond is carbons by strong 6' -bonds. The energy of the C2H 3-H 1 104 kcal mol- /4/. The carbon atoms are connected by a double bond of energy 145 kcal mol- 1• One of these two bonds is a 6-bond, the second is a 6-bond. The latter is relatively weak, its energy being 63 kcal mol- 1 /5/. The ionization potential of ethylene (10.5-10.6 eV /4/) is lower than that of ethane (11.65 eV) while the polarizability of C2H 4 (4.2 cm3) exceeds that of C2H6 (2.6 cm3 ) /6/; i.e. ethylene is ionized or polarized more radily than corresponding paraffins. The elevated reactivity of ethylene (in comparison with the paraffins) is attributed to its unsaturation. The breaking of ~-C-C-bond or its polarization gives rise to the fast interaction of C2H with 4 catalysts and to the formation of various coordinative surface compounds.
The Oxidation of Ethylene Over Metal Catalysts The catalytic activity of metal films for the oxidation of C2H 4 (PO = 23, Pc H = 2.] Torr) decreases in the order: Pt:>Pd:>Rh> 224 >Au>W /7, 8/. The reaction on Pt proceeded at 50_100°C, on Pd at 50 0-1400C and on Rh at 120 0-155 0C; a slow process starts on Au above 200°C and on W above 230°C. Under the conditions mentioned, the activity of platinum exceeds that of palladium due to the somewhat lower activation energy (11.7 against 14.3 kcal mol-1). The main products with all the metals were CO and H In addition, 2 20. up to 3% acetic anhydride and acetic acid were formed on Pd /8/. The formation of CH on Pt was observed in earlier studies /3/. 3COOH The activity of metals supported on silica gel with respect to the oxidation of C2H4 decreases in the sequence: Pt »Pd >Cu >Ag /9/. Thus, the copper subgroup metals are less active than the platinum metals. The reaction kinetics on Pd are described by the equation:
500
r= kP
!
P
0
C2 Hi; 02
(XVII.7)
Some deviations from Eq. (XVII.7) and a retardation of the reaction are observed at higher conversions due to the catalyst poisoning by acetic anhydride and especially by acetic acid. Phis effect decreases with increasing temperature. The addition of CO 2 and H20 does not influence the reaction /8/. If the Pd surface is covered with Cl--ions, the initial rate of C2H oxidation at 155 0C becomes 5 times less than that on a 4 pure film while the CH yield increases to 15%. With Pt, par3COOH tial oxidation products were not detected but a progressive poisoning was also observed /7/. The following reaction mechanism has been proposed in references /7/ and /8/. The adsorption of 02 results in its fast dissociation into atoms on the surface. The catalytic reaction takes place between chemisorbed (0) species and gaseous (or weakly adsorbed) C2H molecules in two directions: 4
Ethylene is assumed to be oxidized rapidly into CO 2 and converted slowly into CH and then into (CH3CO)20, the latter being trans3CHO formed into CH by means of hydrolysis. 3COOH This scheme does not explain the nature of the breaking of the bonds in C2H (C-C or C-H) and the composition of the surface in4 termediates leading to deep or partial oxidation. An initial cleavage of one of the C-C-bonds in C2H with the formation of 4
501
is supposed in reference /8/; reacting with (0), this complex releases hydrogen (converted into OH-groups) and gives new C-O- and C=O-bonds. The mechanism of the oxidation of C2H can be discussed in more 4 detail. It is possible to assume that the intermediate (I 1) is transformed into more fully oxidized surface compounds; their desorption results in the final products of both mild and deep oxidation, i.e. the following consecutive chain of irreversible reactions takes place on a surface:
H CHr (0) ..
°2
-.LL(O )
2
(CH
3COOH)
r
JQ..4.(I
2
2)
(XVII. 9 )
~2(0)
fast
In the first step, the initial rupture of one C-c-bond in C2H 4 is likely to be preferable since its energy (63 kcal mol- 1) is significantly lower than'that of the C-H-bond (104 kcal mol-1) and this difference is not compensated by the difference in the energies of the new bonds formed. If the complex (I 1) contains one oxygen atom, "one-point" adsorption of the (CH 2CH20) radical is probable, for example, in the form:
This last complex can be isomerized into acetaldehyde or ethylene oxide. However, the adsorbed (CH or (C2H species formed 40) 3CHO) can readily be oxidized into (CH before they have time to 3COOH) be desorbed as CH or C2H If such isomerization takes place, 3CHO 40. the transformations (CH + (0)~(CH3COOH)~CH3COOH are 3CHO) similar to those /10/ in the surface oxidation of benzaldehyde into benzoic acid: (C6H + (0)~(C6HSCOO) + (H)----C 6H SCHO) SCOOH.
502
The species (1 2) can be formed from (CH as a result of 3COOH) several steps involving carboxylate and carbonate complexes, the initial interaction of (CH with (0) being the slowest step. 3COOH) The acetic anhydride found in the products is thought to be formed by means of the dehydration of acetic acid. Scheme (XVII.9) proposed here is not in contradiction with the parallel Scheme (XVII.B) because according to Scheme (XVII.9), CO 2 originates from C2H but not from gaseous CH 4 3COOH. Thus, ,'Ie can write the following reaction mechanism:
3
(XVII.10)
4) (CH
3COOH)
---CH
3COOH
+ (
o
)
5 )(CH )-(1 ) - - - 2C0 2 2+2H20+5 ( 3COOH)+(0
0
Assuming that the reaction occurs in an ideal adsorbed layer and that the (0) species are the predominant intermediates, one obtains for the above scheme:
(XVII.11 )
503
where r is the rate of the overall process, SCH COOH and SCO are :3 2 selectivi ties and g is the surface coverage with oxygen. At high excesses of O2 , when 8 ~1, Eq.(XVII.11) is reduced to Eq. (XVII.7) which describes the reaction kinetics on Pd. Since in the second step, the oxygen-catalyst bond is breaking, one should expect a decrease in catalytic activity with increase in qs. Fig.156 shows that this relationship is observed. Eqs (XVII.11) to (XVII.14) suggest a decrease in r and an increase in the selectivity to CH:3COOH with decreasing 8. The results of reference /7/ on the initial rates and selectivities for the oxidation of C2H over Pd partially poisoned with Cl--ions 4 support this conclusion. If the surface coverage not only of (0) species but also that of the (CH:3COOH) species, 8', is taken into account, one should expect decreasing values r with increasing values of 8'. In the 01
Fig. 156 - The dependence of the catalytic activities of the metals for the oxidation of C2H 4 /7, 8/ on qs: 1 - Pt, 2 - Pd, :3 - Rh, 4 - w.
700 400 500 500
'--_--I-_----''--_-I-_ _l....-_....!
o
40
20
extreme case, when adsorption equilibrium is established for acetic acid in the fourth step, the rate equation will be:
~~~
r=
~'0
+
V~ ~h'f
"'1
~ '2 ~ /t (! + bf/AC ~AC
)
(XVII.15 )
where bHAC is the adsorption coefficient for CH and PHAC is 3COOH its partial pressure. According to Eq. (XVII.15), the reaction rate
504
will decrease with increasing values of PHAC as is observed in practice. The second step may be a complex one involving, for example, the weak adsorption of ethylene on a metal surface free of oxygen and the SUbsequent interaction of the adsorbed C2H with (0). 4 As was mentioned above, the oxidation of ethylene over silver is a unique system giving high yields of ethylene oxide. In other cases (for example, when propylene is oxidized on Ag /11/ or pentenes are oxidized on V 20 /12/), the olefin oxides are produced 5 in small amounts. In industry, the oxidation of ethylene into C2H 40 is carried out at 2000_300 0C and 10-30 atm over silver catalysts supported on nonporous carriers (A120 , SiC, etc.). Air or pure oxygen are 3 employed as oxidants, C2H + 02 being mixed with recirculating gas 4 containing the ethylene which has not been reacted. Electronegative additives such as halogens promote the selectivity of Ag towards C2H the latter reaching 60-70% /13, 14/. The oxidation of C2H 4 40, over pure silver at 180°_220°C (3% C2H in air) leads to a selec4 tivity of 41-48% in C2H /15/. Various methods of preparation of 40 silver catalysts are described in reference /1/. The rates of oxida~ion of C2H over the (110), (111) and (211) 4 planes of a single crystal of Ag are close (differ by a factor of no more than 2.4) /16/. Silver films with oriented structure are recrystallized during catalysis, so that the catalyst acquires a disordered structure peculiar to the steady-state conditions of catalysis where the differences in initial activities of the planes disappear. The specific surface areas of Ag samples prepared by various methods (Table 73) differ significantly /15/. Despite careful purification of reagents, the silver prepared /15/ contains some impurities, especially of Cl-, which affect the reaction rates (Table 73). Chlorine was removed by repeated treatment of samples with ammonia solution. The oxidation of C2H was 4 made at 1800_220 0C (3% C~4 in air, 1 atm). The rate constant in the equation:
r=kP ~~
P
-aO
t.?~O
was used as a measure of the catalytic activity under steady-state conditions /15/.
Silver foil
4300-46000
Reduction of Ag2C03 with: diethylene glycol formaldehyde hydrogen glucose C2H + O2 mixture 4 Reduction of Ag20 with: hydrogen hydrogen peroxide in alkali solution Cathode deposition Decomposition of AgN0 2 in vacuum Silver heated in air at
Method of preparation
0.045
0.16
-
0.18
-
0.213
1.30
1.30 0.27
0.008
-
-
-
0.25
-
0.040 0.034
-
0.40
1.10
12.5 5.2
5.3
1.0 7.9 3.0
5.1
0.114 0.122
10.8
Not detected
-
0.28 0.16 0.20
'o-PEl
(l)~
al -P El
-
0.07
<1>1 ~-P
'H~(l)
I
Oal-P 'H(l)q
~-P
I
2.0 0.1
-
5.6
5.8
9.9 19.1
7.6 7.6
21.2
37.9 33.4 4.5 18.8 7.0
'o-PS
(l)~(l)
~-P
<1>1
Oal-P 'H(l)q
11 .1
12.1
10.9 5.3 6.3 12.3
al -P El
'H~
('(l-P -P(l)q
~-P
1
a
28.2 11.2
42.0
28.1
44.0
37.8 35.0 31.2 29.3 28.0
(l)al-P -P(l)q 'HM ttl -P
~-P
Rate constant Rate constant per 19 of Ag per m2 of Ag
Content of C1, mg
0.28 0.15 0.22 0.42 0.43
Oal-P 'Hq (l)~ 'o-P S
~-P
<1>1
Specific surface area, 1m2 g-1
The Oxidation of Ethylene at 200 0C Over Ag Samples Prepared by Various Methods (Before and After Their Treatment with Ammonia Solution) 1151
TABLE 73
83 53
62
50
49
54 51 79 51 53
(I)
El
(l)~
.04-'>
~-P
Oal-P 'H(l)q
<1>1
I
-
48
42
48
43
43 44 43 41 46
ttl 4-'> El
'H~(l)
(l)('(l-P -P(l)q
~-P
Selectivity in C2H I % 4
C1l C1l
0
506
Table 73 shows that the rate constant per 1 g of i~ differs significantly from sample to sample. If the activity is related to unit surface of purified silver, the specific rates turn out to be nearly constant. This is an evidence in favour of the Boreskov rule on the constant specific activity of catalysts with the same cbemical composition /18/. The small differences observed in the k values should be attributed to some impurities (for example, in the Ag foil)which cannot be eliminated by the treatment with ammonia. It can be seen from Table 73 that chlorine decreases the activity and increases the selectivity of Ag. This electronegative additive as well as others (Cl, Br, I, S, Sc, Te, P and Bi) which also affect catalytic properties of Ag /1, 14, 19/ can be introduced into the gas mixture in the form of compounds decomposed during the catalysis (so-called gaseous modification). Thus, the chlorine-containing organic compounds are adsorbed and oxidized on silver giving AgCl /20/. Such a method results in a nonuniform distribution of additives along the catalytic layer. It is better to introduce an additive during the course of the catalyst preparation, for example, by way of coprecipitation of Ag2C03 and silver sulfide, sulfate, selenate, tellurate or chloride /21/. . The tracer method has been used to show /21/ that 40-100% of additives are accumulated on the catalyst surface in the last case. The above mentioned non-metals are likely to be present in the working catalyst in the form of anions (Cl-, s2-, S02-, Se0 2-,
Teo~-,
etc.). The dO~ing
4
4
of Ag with silver halides by their ad-
sorption from solutions or by way of gaseous promotion /22/ results in the formation of a phase of silver halide. Most of additives introduced into silver catalysts are mobile; these additives can move from the bulk of the catalyst to the surface, migrate along the surface and then be removed into the gas phase as a result of chemical reactions. The tracer method shows that the migration rates decrease in the order: I:> Cl-> SO~-/23/. Chlorine is removed into the gas phase quite easily while selenium is still retained at 220 0-3000C /21/. These effects should be taken into consideration when the reaction kinetics are being studied. Very small amounteof S, Se and Cl enhance the catalytic activity of Ag with respect to the formation of C2H40 (Te has a slight
507
effect) (Figs 157 and 158). At higher concentrations (at surface coverages with S > 1%, Te > 4%, Se> 10% and Cl> 7-8%), poisoning takes place. The optimum activity corresponds to small surface concentrations of additives: 0.1 % for S, 1-3% for Se, "" 1 % for Te and 5% for Cl (see Fig. 158, b). A decrease in the catalytic activity at high concentrations of Se, Te and Cl (,/ 10%) is accompanied by an increase in the selectivity to C2H which becomes higher than that for pure Ag (Figs 40 157, 158). According to reference /24/, an increase in selectivity takes place at rather high concentrations of additives of F, Cl, Br and I. In some cases, barium or calcium compounds are added
Fig. 157 - The activity and selectivity of Ag with additives of sulfur (1), selenium (2) and tellurium (3): k 1/(k1 )0 is the ratio of the rate constant of the reaction (XVII.1) for a given sample to that for Ag without additive; ,6S = (S-8 o) where S and So are the selectivities for Ag with and without additives /21/.
4-
2 !
4
2 - (,1(fft %) I
OJ
/
/0
/00
sarrace carerape/% to silver /1/. Electropositive elements (K, Na, Be, Ca and Ba) enhance the catalytic activity of Ag and lessen its selectivity to ethylene oxide /25/. In the course of the oxidation of ethylene, slow dissolution of oxygen in surface layers of silver occurs, affecting the steady-state composition and the activity of the catalyst /26/. The following important features of the reaction mechanism have been revealed. 1. In the general case, the oxidation of C2H proceeds by way 4 of a parallel-consecutive scheme:
(XVII. 16 )
508
Under typical conditions in laboratory studies and in industry, the parallel route prevails, the consecutive oxidation to CO 2 being significant only at elevated temperatures /1, 14, 19, 27/. 2. The overall reaction is inhibited by ethylene oxide and by CO 2 and H20 /27/. 3. At the temperatures which are close to those required for catalysis, ethylene is not adsorbed on pure silver but is adsorbed either on silver covered with oxygen or when the C2H + 02 4 mixture is admitted to the Ag /28/. The interaction of C2H with 4 adsorbed oxygen to form C2H 40 is reversible /29/. Under conditions of catalysis, not only the initial reagents but also the reaction products are adsorbed on the oxidized surface of silver /30-32/. 4. A lot of data suggest the existence of several forms of oxygen adsorbed on silver. Oxygen can be adsorbed in both the atomic and molecular forms which are negatively charged /19/. In references /33/ to /37/, the adsorbed oxygen species are described as surface oxides of silver, Ag~s)On where Ag(s) is surface atom of Ag and n is a number of oxygen atoms bound with the neighbouring Ag atoms. These oxides are randomly distributed on the surface and can be regarded as a two-dimensional solid solution. When [0]/ [Ag(s)]<~ , the oxide Ag(~)O )redOminates and the heat of adsorption is great. When [ 0 ] / [ Ag(s}~ , the above oxide is converted into higher oxides, Ag~s)02
or Ag~s)03'
The heats
of the conversion are small and practically do not depend on 8 • The reactivity of Ag~s)o towards reducing agents (H2 or CO) is low due to strong oxygen-silver bonds. In contrast to that, higher oxides at 200 o-300oC can rapidly oxidize hydrogen or carbon monoxide. Similar to the bulk oxides, the states of oxidation of (s)O 2' + 1 an d + 3 Ag are.• i n Ag(s)O 2 ' + 1,. '1n Ag(s)O 2 3' + 3,. '1n Ag2 (i.e. it may be depicted as AgIAgIII 02). 5. The distribution of surface silver oxides and of the heats of adsorption of O2 depen~on the additives. Thus, sulfur additives lessen the heat of adsorption of O2, decreasing the fraction of oxygen which is adsorbed in the form of Ag~s)o /33/. 6. IRS studies suggest /30/ that the rupture of the C-C-bond in ethylene occurs during its adsorption on Ag covered with oxygen; this leads to the formation of a surface compound (-CH -GH 2 2-0-). The latter is assumed to be an intermediate in the formation of C2H 40.
509
Milo /5 III
o
~ -J -to
LlS/% .~- - - - - -e
to
s:IJ
;:~:~
I
4-
-!ST, %Cf
J I
!
0.1
(
I
10
Fig. 158 - The activity and selectivity of Ag with various amounts of additives of Cl- (see Fig. 157). The conditions of pretreatment of the samples and the conditions of rate measurements are different in the cases "a" and "b" /21/
~
7. The selectivity is independent of Po /P R over quite a wide 2 range of reaction mixture composition. According to reference /29/, oxygen is adsorbed on Ag in the form of negative ions; a collision of C2H with ODe atom of adsor4 bed oxygen leads to C2H while the interaction with two (0) spe40 cies results in the formation of CO 2 including HCHO as an intermediate. A consecutive route for the reaction is also supposed: it is assumed that C2H is converted into CO 2 and H20 by isome40 rization and oxidation of CH by the formation and further 3CHO, oxidation of adsorbed organic residue or by direct oxidation of C2H The above mechanism belongs to type (XIV.28) which suggests 40. that a decrease in the selectivity to C2H will occur with increa40 sing Po /P R• This is in contradiction with the experimental data 2
obtained. In some papers, different routes for the oxidation of C2H are 4 attributed to different forms of adsorbed oxygen. Thus, C2H is 40 associated with (02) while CO 2 is associated with (0) species /19, 38, 39/. Just this peculiarity is assumed to explain the specificity of silver as a unique catalyst for the synthesis of C~40 since only with Ag are the (02) species produced in significant concentrations at the temperatures of the catalytic oxidation of C2H In contrast to this, it is supposed in reference 4• /40/ that C~40 is formed from (0) particles while (02) species lead to CO 2• Finally, many mechanisms involve /1/ only (02) species as intermediates for both C2H and CO 2• 40 Thus, a number of "mixed" mechanisms have been proposed,all of them including the participation of different forms of adsorbed oxygen (and other surface compounds). The mechanism proposed by M. Temkin et ale /35, 37, 41, 42/
510
seems to be the most comprehensive one as it is supported by various experimental evidence (primarily, by kinetic data). In the earlier papers of these authors /35,41,42/, the surface oxide Ag~s)02 was regarded as a major intermediate. The higher oxide Ag~s)03 produced by way of the O2 adsorption:
(XVII. 17) was considered to be decomposed readily according to the equation: (XVII.18) The overall mechanism /35,41,42/ will be:
N(I) 1 1) C2H 4 + (0)~(C2H40) 2) C2H + (0)~(CH2CHOH) 4 2') (CH 2CHOH) + 5 ( 0 ) - 2C02 + 2H 2 0 + 6 ( ) 0 1 3) C2H 40 + ( )~(C2H40)
°
4) 02 + (
) -(02)
4') (02) + (
) -2(0)
5) C2H + (0) ~ (C2H 402) 40 6) H20 + (0) ~ (20H) 7) CO 2 + (0) ~
N(I I ) •• C2H4 + .302
(C0.3)
= 2CO 2
o
1/2
.3
1/2
.3
o
°o
°o
(XVII. 19 )
°
+ 2H 20
The reaction is assumed to follow two routes, the chemical equations of which are (XVII.1) and (XVII.6). Oxygen is adsorbed irreversibly. The (0) species in (XVII.19) correspond to Ag~s)02' then (
) refers to Ag~s)o
when [0 ] / [ Ag(s)] "7 ~
and (0 2) corresponds to Ag~s)03. Since, , the heats of the adsorption of 02 are in-
511
dependent of 8 and the surface is considered to be uniform. This allows one to use the theory of kinetics for an ideal adsorbed layer. Scheme (XVII.19) reflects the fact that both C2H and the oxi4 dation products are bound with the oxidized surface of the Ag. The adsorption of products is reversible. The scheme does not conflict with the fact that, in the adsorption of C2H on silver 4 covered with oxygen, the C-C-bond is broken and C-o-bonds are formed. In accordance with the data mentioned above, the first step in Scheme (XVII.19) is reversible. The above mechanism corresponds to a parallel scheme of the formation of C2H which agrees with the experi.uental data. Partial 40 oxidation proceeds via (C 2H 40) species while deep oxidation involves the formation and the fast decomposition (step 2')) of a hypothetical vinyl alcoholate complex (CH 2CHOH). The mechanism discussed leads /35, 42/ to the following equationifor the rate of oxidation of C2H into C and CO 2H 40 2, reI) 4 and r(II) respectively:
(XVII. 20)
(XVII. 21 ) where
+L
(XII.22)
512
(XVII.24)
In these equations, k. and k ; are the rate constants for the ~ -... k i-th step in forward and reverse directions and K.( = ~ ) is the ~
i\..-i
equilibrium constant for that step. The selectivity for oxygen is determined by the equation:
-.!-,JIJ S. = 2 _ V2 1 (lJ) T" + 3r
)
.n
(XVII.25)
and shows the fraction of oxygen consumed for the production of CH The selectivity for ethylene deduced from Eq. (XVII.25) 2 40. will be:
1+ 65.
(XVII.26)
~
Investigations of the reaction kinetics using silver catalysts doped with Cl or Se at atmospheric /41, 42/ and elevated (up to 16 atm) pressures /35/ over a wide range of the reaction mixture compositions have shown the validity of Eqs (XVII.20) to (XVII.26). The calculated values of k 1 and k 2 are approximately constant, the scatter of values being rather large but random. These constants are practically independent of pressure /35/. Fig. 159 shows a on Pc H O/P O as follows from Eq.(XVII. 2 4 2' the relationship will be
linear dependence of So 25). At 240
0C,
P0140 ~ = 0.3'1 - 0.11 '2
f;,
'Z
2
Fig. 159 - The plot of selectivity vs Pc H O/PO for 242 the oxidation of C2H on 4 Ag promoted by selenium (255°C) /42/. 0/
02
as
0.4
0.5
f'czN"O
p;;-
(for the Se-doped Ag) and
So, = 0.28 - 0. 09 '2
Pt2!11 0 p
0
(for the Cl-doped Ag) /35/. The activation energy for the first step is shown to be lower than that for the second one. Thus, for the Cl-doped catalyst, E1 = 8 and E2 = 14 kcal mol- 1 /41/ while, for the Se-doped one, E1 = 19 and E2 = 21.5 kcal mol- 1 /42/. This suggests that the rate of deep oxidation increases with increasing temperature more sharply than that of mild oxidation. Later /37/ somewhat different mechanism was proposed. Its differences from the above scheme involve the following points. On the basis of oxygen exchange data, it is supposed that Ag~s)02 is rapidly converted into other oxides: 2,4.($)0. '92 L
(XVII.27)
(this reaction is the reverse of that in ~q. (XVII.18». Since is poorly reactive, the higher oxide Ag(S)03 is assumed to as was presumed be the major intermediate, in contrast tOAg~s~2' in Scheme (XVII.19). The overall mechanism will be /37/: N(I) NCII) N(III)
Ag~s)o
1) C2H + (° 2 ) ----C 2H + (0) 4 40 2) C2H4 + (02)----CH + (0) 3CHO 2') CH.3 CHO + 5(02)-2C0 2 + 2H 20 + 5 (0)
101
o o
1
o o
514
N(I) N(II) N(III) 3) C2H + ( )~(C2H40) 40 4) 02 + ( )~(02)
° 1/2
° 3
1/2
3
5) C2H 40 + (° 2) ~ (C2H40o ° 2) 6) H (H 2Oo02) 20 + (02)~ (C02. °2) 7) cO 2 + (02)~
°
°
°
°
°
°
8) (C2H + (0) 40)---C 2H 4
°
°
4') 2(0)-(02) + (
)
N(I): C2H + ~ 02 4
= C2H40
N(II): C2H + 302 4
= 2C0 2
N(IlI):
+ 2H
1 °
° ° (XVII. 28: ° 1
20
°= °
In many features, this mechanism is similar to Scheme (XVII.19). Tbedifferences mentioned, referring to the stability of the surface oxides of silver (i.e. of (0) and (02»' contradict the supposition that adsorbed products are formed from C2H and the high4 er silver oxide. It was assumed that the first two steps result in gaseous organic species. The intermediate which is rapidly converted into CO2 and H20 will be acetaldehyde and not adsorbed vinyl alcohol as was considered earlier. The immediate decomposition (Eq. (XVII.27» of (0) species formed in the first step of Scheme (XVII.28) leads to irreversibility of this step. The decomposition of (C 2H 40) (the reverse of the first step in Scheme (XVII.19» becomes a special step (the last step in Scheme (XVII.28». Mechanism (XVII.28) involves three basic routes, the last one being "empty". Scheme (XVII.28) leads /37/ to rate equations, the form of which coincides with that for Scheme (XVII.19» except that, instead of Eq. (XVII.23), one will have:
M= 318 ~ 1'1
(XVII.29 )
515
Consequently, Scheme (XVII.2S), like Scheme (XVII.19), agrees with the experimental rate data but the former is more acceptable with respect to the forms of adsorbed oxygen taking part in catalysis on silver. The mechanism discussed refers also to a parallel scheme for the formation of C2H and CO 2• A decrease in selectivity with in40 creasing ratio Pc H O/PO (Eq. (XVII.25) and Fig. 159) is caused by step S). If thfs4step2is excluded (i.e. k S = 0), one will have M = 0, and the selectivity becomes independent of the composition of the reaction mixture (Eq. (XVII.25)) /37/. Then, the mechanism proposed in reference /37/ becomes similar to Scheme (XIV .23). Let us consider the reaction kinetics at low conversions. In this case,
r=
1, + K2 A
1.:? fI
= PH 2
Lj
° = 0,
the value of A will be:
f !T
I
+
2~
01
2
P 11 0 1 Po'2
(see Eqs (XVII.22) and (XVII.24)) so that:
(XVII. 30)
This equation can be also obtained on the basis of Scheme (XIV • 23) (see Eqs (XIV .24) and (XIV .27)). One must take into account the fact that k , k 1 and k 2 in Eq. (XVII.2S) correspond to k , k 1 2 4 and k respectively in Eq. (XIV .23). The overall stoichiometric 4 coefficient of oxygen, Y, is then equal to (SC yC + 2H40
+ SCO
YCO ) where Si are selectivities; and
2H40
Pi are the stoichi-
2 2 ometric coefficients of oxygen in Eqs (XVII.1) and (XVII.G). Since
~
S =--C~ k1 T k,?
(XVII. 31 )
516
(designations refer to Eq. (XVII.28»
with
and
Hence:
17
T
+J~
= r(J! +12 )
(XVII•.32 )
(XVII•.3.3) which is identical to Eq. (XIV .24) (taking into account also Eq. (XIV.27) and the above-mentioned difference in the numbering of the steps). According to reference /4.3/, if Eq. (XVII.3.3) is approximated by the power rate law:
(XVII • .34 )
the reaction orders in the case of mechanism (XIV .2.3) will be: m=(J
and
n=1-8
Then the values of m and n should change within the range 0 - 1 and
m+ n
=1
(XVII• .36)
The same consequences should be true for the rates of formation of C2H and CO 2, as the selectivity (Eq. (XVII.31» does not depend
40
517
on the reaction mixture composition:
(XVII.37»
The kinetics on Ag at low conversions have been studied in reference /44/. It was found that at 240 0C, m = 0.37 and n = 0.67; at 260 0C, m = 0.32 and n = 0.68, so that m + n = 1.04 (240 0C) and m + n = 1.00 (260 0C). One can see that both consequences are valid. Mechanism (XVII.28) suggests a qUalitative explanation of the effect of additives on silver catalysts in the oxidation of C2H 4• Non-metallic additives (S, Cl, etc.) decrease the oxygen-silver bond strength /21/. According to (XVII.28), the rate of formation of C2H will be k 1PC H 8 , where 8 is surface concentra40 tion of Ag;s)03. Since in th; iirst two steps, catalyst - oxygen bonds are broken, their loosening results in two opposite effects (an increase in k 1 and k 2 and a decrease in 8). Competition between these effects leads to the rates passing through a maximum. At high concentrations of additives (at low values of qs)' the second effect prevails. The above additives, being in the form of negative species (Cl-, S2-, SO~-, etc.), enhance the work function. Since adsorbed oxygen is also negatively charged, the non~etallic additives increase electrostatic repulsion andloosen oxygen-eatalyst bonds. An opposite effect is caused by electropositive species (compounds of alkali and alkaline earth metals). This explains the relationship between catalytic properties of modified silver catalysts and their work function values /25/. Since loosening the oxygen-silver bonds is associated with greater coverages by the higher surface oxides of silver /33/, nonmetallic additives enhance the fraction of oxygen adsorbed in the form which reacts di~ectly with C2H 4• Thus, Scheme (XVII.28) is likely to reflect the essential features of the oxidation of C2H on silver. At the same time, a num4 ber of significant questions remain open, in partiCUlar: 1. What is the reason for the dual reactiVity of ethylene exhibited in the first two steps of Scheme (XVII.28)? 2. What is the true composition of the intermediate leading to
518
CO 2 ? 3. What is the reason for the unique ability of silver to catalyze the formation of ethylene oxide? It seems rather interesting to study the distribution of products with various catalysts at low temperatures and very low conversions. In the oxidation of C2H on Pd-Ag alloys /45/, the rate of the 4 formation of C2H decreases with decreasing concentration of Ag; 40 at ~40 at.% Pd, ethylene oxide is not produced. Deep oxidation is sharply enhanced at high (780 at.%) concentrations of Pd. One can put forward the following hypotheses concerning the oxidation of ethylene on metal catalysts. There exist several reaction routes, depending on the catalyst and conditions employed. They can be expressed as a combination of Scheme (XVII. 10) and (XVII.28). With Pt and Pd, atomic anions of adsorbed oxygen are weakly bound and very reactive, and this results in a mechanism of type (XVII. 10). With Ag, the (0) species are more strongly bound and less reactive, but the highest surface oxide of the metal is present in rather high concentrations, this oxide being very reactive. Oxidation of Ethylene Over Oxide Catalysts The specific catalytic activity of simple metal oxides (supported on SiC) in the oxidation of C2H at 300 0C and excess of 02 4 decreases in the order (Table 74): C03047Mn027Cu07Cr203/,NiO» '» Fe 20 3» Ce02• Under these conditions, Pd is more than two orders of magnitude more active than C0 /46/. 304 The activity pattern of the oxides supported on Si0 2 or Al 20 3Si0 2 (in the excess of 02) /47/ will be: C0304/'cr203»Ag2o»Mn203> »CuO »NiO» V20 , CdO»Fe 20 /> W0 ZnO. This sequence 5 3/Mo03 3/Ti027 agrees with reference /46/ (the exception is cr 20 3). All the catalysts mentioned lead to CO 2 and H20 . In the oxidation of C2H over a Bi-Mo-oxide catalyst which is poorly active, 4 30% CO is formed at 500 0C and with excess of C2H in the initial 4 mixture /48/. There are some indications of mild oxidation of ethylene over oxides. Thus, with a P-Mo-oxide catalyst on Si0 2 at 290 0c, acetic acid is produced (with a selectivity of 74% at 39% conversion) /49/.
519
The overall kinetics are usually expressed by (XVII.34) (see Table 74). According to reference de is not reduced in the course of catalysis. In C2H over CuO doped with KOH /50/ with excess of 4 340 0C, the values of m is 0.6 and E = 20-27 kcal Al 20 (1:1), m = 0.5 /51/. 3 TABLE
a power rate law /27/, cupric oxithe oxidation of O2 and at 217 0 mol- 1• With CuO-
74
The Catalytic Activity of Some of C2H /46/ 4
Oxides in the Oxidation Order of reaction in
Catalyst
C0 304 Mn° 2 CuO Cr 20 3 NiO Fe 20 3 Ce02
Temperatures
19 r*
E/kcal mol- 1
200-251 229-279 238-282 240-292 268-316 280-320 342-395
-5.93 -6.21 -6.86 -7.07 -7.63 -7.77 -9.48
30.5 19.6 24.5 27.7 26.3 29.3 24.4
ethylene
0.14 0.52 0.39 0.50 0.38 0.58 0.67
oxygen
0.21 0034 0.27 0.30 0.39 0.47 0.32
* r is the reaction rate (molecules m-2 s-l)at 300°C and 2% C2H 4, 50% 02 and 48% N 2•
The reaction mechanism is likely to be reflected by Scheme (XVII.10).Deep oxidation of ethylene over copper oxide involves carboxylate and formate surface complexes /52/ which can be associated with (1 2 ) in Scheme (XVII. 10). At 200 0C, these complexes are destroyed only in the presence of 02' At 3000-4000C, they can be converted into CO 2 and H20 in the absence of 02; under these conditions, the rates of catalysis and surface reduction with ethylene are equal, and a reduction-reoxidation mechanism is valid. The above mechanism suggests that there should be a 19 r - qs correlation and this is observed in practice (Fig. 160). The greater catalytic activity of the metals (Pd) in comparison with the oxides can be due to the preliminary activation of the C~4 on a metal surface which is free of oxygen; this activation
520
results in substantial loosening (or even rupture) of the C=C and facilitates the sUbsequent interaction of bond in the C 2H4 the ethylene with (0) leading to (CH 2CH20). /1/ r
-4
7e:,
-5
-6 -7
Fig. 160 - The dependence on qs of the specific catalytic activities for the oxidation of ethylene /46/: 1 - NiO; 2 - Mn02; 3 - Co 0 ; 3 4 4 - CuO; 5 - Fe 20 3; 6 - Ce02 ; 7 - Pd.
-8
Eqs (XVII.13) and (XVII.14) for selectivity, deduced from Scheme (XVII. 10), suggest that there is an increase in the yield of 0 20 40 ~s/lrcal(If-{ltOr' CH with qs. This predic3COOH tion is in a qualitative accordance with the fact that significant amounts of acetic acid can be obtained only with a P20 - Mo0 catalyst possessing high 3 5 values of qs' In the case discussed, the oxygen-catalyst bond strength is not the only determining factor, since in the steps 4) and 5) of Scheme (XVII. 10), the catalyst - acetic acid bonds are formed and destroyed. Hence, the selectivity should also depend on the surface acidity, the P-Mo-O system being appropriate in this respect for it is employed in some other reactions for the synthesis of organic acids. -9
The Oxidation of Propylene Cuprous oxide catalysts for the partial oxidation of propylene into acrolein , were discovered /53, 54/ and employed in industry in the 1940 s. With these catalysts, the reaction is carried out at 300o-400oC and with rather high C 6 : 02 ratios, close to 3H stoichiometric. The catalyst is usually supported on SiC or some
521
other inert carrier. Compounds of transition metals of Group VI or non-metals (S, Se, Te and I) were proposed as promoters /55/. The selectivity to acrolein reached a level of 70-80%. In 1960, a selective catalyst of a new type, the Bi-Mo-P-oxide system, was proposed /66/. With this catalyst, the reaction proceeds at higher temperatures ( ~4500C) and with excess of 02. Among oxidative heterogeneous catalytic reactions, the oxidation of propylene is of special importance. Most pUblications refer only to this process, this being caused both by the practical significance of the reaction and by a possibility of studying many fundamental problems of the theory of catalysis, such as: 1. The problem of a large variety of possible mechanisms, depending on the nature of the catalyst, the reaction conditions, etc. On varying these factors, the oxidation of C 6 can be directed 3H into the production of acrolein, acrylic acid, acetone, acetic acid, benzene, hexadiene or carbon oxides. 2. The problem of a large variety of factors which can affect the activity and selectivity. 3. The mechanism of catalytic action of complex and doped catalysts. 4. The effect of the reaction mixture on the composition of the steady-state catalyst (containing copper oxides and so on). The major reactions of the oxidation of propylene can be expressed by the equations: 1 C 6 + '2 °2 = C3H6O 3H
(XVII. 38)
C 6 + 1. ° = CH 3H 3COCH3 2 2
(XVII. 39 )
C 6 + 02 = CH 2=CHCHO + H2O 3H
(XVII.40)
C 6 + 1 _1 02 = CH 2=CHCOOH + H2O 3H 2
(XVII.41)
C 6 + 202 = CH + CO 2 + H2O 3H 3CHO
(XVII.42)
1 = CH C 6 +2_0 + CO 2 + H2O 3H 3COOH 2 2
(XVII.43)
C 6 + 302 = 3CO + )H2O 3H 1 _ C 6 + 4 ~ 02 - 3C0 2 + )H2O 3H
(XVII.44) (XVII.45)
522
Reactions (XVII.40) and (XVII.41), as well as reaction (XVII.4J), are of the greatest practical importance. In addition, the oxidative dehydrodimerization of C 6 into JH 1,5-hexadiene or benzene is possible: (XVII.46 )
(XVII. 47 )
The simplest unsaturated aldehyde, acrolein, is used for the production of glycerol, glutaric aldehyde, glycidol, allyl ether, nicotinic acid, etc. A valuable monomer employed in a number of polymerization processes is acrylic acid 1191. The Oxidation of Propylene Over Metal Catalysts The activity pattern of metal films in the deep oxidation of C 6 with excess of 02 (Pt:;>Pd;;>Rh/Au/"W) is similar to that in JH the oxidation of C2H 17/. With Pt and Pd, small amounts (up to 1 4 and 3%, respectively) o~ acetone were formed; with Rh, only traces were detected. The oxidation of propylene over Pd/C at 125 0C and at low conversions (J.6%) results in a rather selective production of acrolein 156/. At higher temperatures (J20 0C), the latter is formed on copper (selectivity ""'90%) 127/. The kinetic characteristics of the deep oxidation of C 6 over 3H Pt and Pd are listed in Table 75. Differences in the reaction orders in the equation: (XVII.48)
observed for metal films at low pressures and small conversions 171 and for supported metals at atmospheric pressure 1571 can be due to different methods and conditions of the experiments. The reaction orders for Ag/SiC are close to those for Pt and Pd 157/; silver is likely to be present in the form of metal (the initial catalyst was Ag20). Carbon dioxide and water were also reaction products with silver. The propylene oxide yield is poor 127/. There is evidence 111
523
on the promotion of Ag by gold, alkaline and alkaline earth metals for the production of C 60. Thus, selectivity of silver for 3H the oxidation of propylene is significantly different from that in the oxidation of ethylene. TABLE 75 The Kinetic Characteristics of the Oxidation of Propylene Over Some Metals
Catalyst
Pt film Pt/SiC Pt/SiC Pd film Pd/SiC Pd/SiC Ag/SiC
'I'empera ture rteaction mixture range/ °c composition PC Po 2 3H 6 /Torr /Torr 76-130 122-147 159-198 84-128 143-178 190-226 210-258
2.3 15.2 152 2.) 15.2 152 152
24 380 114 24 380 114 114
19 r at 30 00C
E 19 r /kcal /mol m mol- 1 m-2 s-1
In (I)
n
0
s::(I)
H (I)
'Pi (I)
0::
1 r±o. 7 21.1 19.3 20.8 -3.96 24.8 -4.95 25.4 -5.50 32.4 -3.69 -4.94
3.3 4.)1 2.39 5.6 5.46 4.70 6.80
-0.2 -0.74 -0.15 1 -0.47 -0.57 -0.61
0.5 1.45 1.16 0 0.85 1.50 1.52
7 57 57 7 57 57 57
The retardation of the reaction by propylene on Pt is likely to suggest that adsorption of the hydrocarbon or of the products of its interaction with oxygen occurs during the course of catalysis. The formation of such surface complexes is supposed in reference /58/. At the same time, the surface coverage with oxygen, as indicated by the values of n, is small. A similar picture is observed with Pd and Ag supported on SiC, but with Pd film, the situation is different (C 6 is weakly adsorbed while the adsorp3H tion of O2 is strong). Initial activation of the olefin can occur during the course of its adsorption on free metal sites and the adsorbed hydrocarbon species then interact with adsorbed oxygen /7/. During this interaction (which determines the rate of catalysis), rupture or a significant loosening of metal-oxygen bonds takes place. As a result, the catalytic activity decreases with an increase in this
524
bond strength (Fig. 161).
r,.lff Fig. 161 - The dependence on qs of the catalytic activity of the metals in the oxidation of propylene (Tr is initial temperature of the reaction /7/): 1 - Pt, 2 - Pd , J - Rh, 4 - Vi.
400 I 500 000 100'::--J----..J.---:L--:--'--
fO
40
00
fJs jlrcrrl(!l-rrtotl
One can assume that the rupture of ~-C-C bonds in propylene occurs simultaneously and this leads to the intermediate: H C-CH-CH
21
I
J
° °
II/l/Il
Further rapid oxidation of this gives rise to carboxylate - carbonate species which are converted into the deep oxidation products. On a silver surface which is covered significantly with the oxide Ag~s)03' the adsorption of the above intermediate can result in small amounts of propylene oxide. Acetone is not an intermediate for the production of CO 2 and H20 /7/. It is likely to be formed by a parallel route according to a mechanism involving the preliminary hydration of C 6• Such 3H a route will be discussed below in connection with the low temperature catalysis over oxides /56/. The high selectivity towards acrolein observed with Cu can be due to the fast oxidation of the metal into Cu20, a good catalYst for the oxidation of C 6 to acrolein /27/. 3H According to reference /56/, the mechanism of the formation of acrolein over Pd supported on charcoal is quite different from that with oxide catalysts. Tracer experiments showed that oxygen from water but not from 02 is introduced into the acrolein in the former case. Hence, it was assumed that the allylic radical CH2~CH~H2
525
formed on Pd/C is attacked by OH-groups generated by the adsorption of H The surface compound formed (CH 2=CH-CH20H) then un20. dergoes oxidative dehydrogenation to acrolein. Under favourable conditions, the hydrocarbon-oxygen species formed on metal surfaces can be desorbed into the gas phase in the form of radicals which then bring about a chain process /59/. Such a heterogeneous-homogeneous mechanism was observed for the oxidation of C 6 over Pt/BaS0 at low pressures and at 100 0C /58-60/. 4 3H With large void spaces and at elevated temperatures (350 0-4500C), the last mechanism is also typical of Ag /61/. In this case, with small amounts of catalyst, the contribution of the homogeneous process is high and the reaction products contain C2H acrolein, 4, CO, CH C and HCHO. Increamng amounts of catalyst favour the 4, 3H60 heterogeneous oxidation with a different selectivity, the major product being CO 2 and HCHO (together with C2H 4). The Oxidation of Propylene Over Simple Metal Oxides This is one of the most comprehensively stUdied reactions. In reference /62/, the oxidation of C 6 was studied over 13 metal 3H oxides (supported on nonporous SiC) with excesses of C 6 employ3H ing the flow-circulation technique. The temperatures of attaining equal specific rates were used as a measure of the catalytic activity. It was assumed that the correction connected with a fraction of the free carrier surface which does not participate in catalysis is of minor importance with respect to the activity pattern. As seen from Table 76, the sequence of activities is: Mn0 2» CuO;> » Cr 20 3 "7 C0304":> Fe 20 3/ U308":> CdO/ V2°5/ NiO/Mo0 3/, Zr0 2 » PbO/W0 3• Similar catalysts were used in reference /57/, the experiments being made in a differential reactor with two reaction mixtures (Table 77). With excess of 02' the activity decreases in the order: C0 CuO »Mn0 2/ cr 20 "»CdO»V205 /Fe 20 Th0 2 NiO,/Ce0 2,/ Al 20 3 3/ 304/ 3"» while with excess of C 6: Cu07IVln02,/C0304"»cr203/CdO,/Ce02'/ 3H 7V205/ Fe 20 "7 NiO,/Th0 2• In addition, extremely low activities 3 were observed for BaO, MgO and CaO. According to reference /63/ (Table 78), the activity order with excess of 02 at 500 0C is: C0304/CuO/In203,/NiO,/Sn02/V205/znO» "7 Fe 20 3"7 cr 20 3"7 Ge027 CdO,/Bi 20 3-;;::.Mn0 2 '/ Si0 2/, Ti0 27W0 37Ca07 7 Mo0 37 Sb 20 3• Under non-steady conditions, the corresponding activity pattern
526
TABLE 76 The Catalytic Activity of Some Metal Oxides for the Oxidation of 03H6 1621 Initial Mixture (%): C 6 - 20, 02 - 5 and N2 - 75 3H
Oxide
Specific surface areal m2 cm- 3
Tr/K* 10 3/T r
Oxide
Mn° 2 CuO Or 20 3 c0 304 Fe 20 3 U 8 30 OdO
0.610 0.130 0.170 0.070 0.130 0.330 0.040
598 604 618 627 640 655 679
V
1.67 1.65 1.62 1.59 1.56 1.52 1.47
20 5
NiO
Mn°3 Zr0 2 PbO ViO 3
Specific surface areal m2 cm- 3
Tr/K*
0.170 0.230 0.130 0.210 0.260 0.310
683 707 729 741 773 773
10 3/Tr
1.46 1.41 1.37 1.35 1.29 1.29
* T is the absolute temperature at which a rate of 1.5 x 10-6 r mol 02 m-2 s-1 is attained.
was found 164/, using the pulse technique (10% 03H6 + 90% 02)' to be: Mn0 2> 00 2°3 >OuO;>Ni 20 ;>or 20 >NiO >Fe 20 V205 ;>Sn0 2 :>Ti0 2/ 3 3 3/ ,>Sb "7 Bi 20 ZnO ;>Vi0 Mo0 (Table 79). The same tech20 3;> 3/'U308 3/' 3 nique was employed in reference 1651 for measurements of the steady-state activities and the following order was obtained (Table 80): 00 cuo 7 Mn20 3/> or 20 3> Oe02 7 Fe 20 3 />Tl 20 3/> Sn0 2/> NiO?, Ti0 2/' 3°4/ /> OdO,>U308?In203;>Mo03:>Th02:>V205;>W03:>Bi2037Ta205 /'La 20 3;> /> ZnO,> Zr0 2/Nb20 5;> Pb304/Ga203/Te027 Sb 20 4• The above formulas reflect the true phase compositions of the oxides as was shown by the X-ray method; the exceptions are: Pb (which is a mixture of 304 Pb 304 with PbO and 0(-Pb0 2), OdO (some other phase, possibly, ca, was also found) and La 20 (which also contains some admixture). 3 Silver oxide was unstable under the conditions of catalysis. In general, the activity orders established are similar for the various reaction mixture as well as for steady and non-steady states of the catalysts. In most cases, higher activity is exhibited by C0304' Mn0 2 (Mn 20 ) , OuO and Or 20 The relatively inactive ca3 3• talysts are Mo0 3, W0 3 , Zr0 2, ThO 2, Al 20 PbO (Pb Sb203(Sb204) 3, 304),
527
TABLE 77 Rate Characteristics of the Oxidation of C 6 Over l\letal 3H Oxides at 300°0 /57/
Oxide
21~
'I'emperature range /oC
E/
n
m
kcal mol- 1
C 6, 50;G O2, 48% N2 3H
0°3 04
ceo
Mn0 2 Or 20 3 CdO V20 5 Fe 20 3 NiO Oe02 Al 20 3 Th0 2
215-240 272-311 203-259 255-294 283-353 300-351 276-324 274-324 240-305 380-441 285-355
20% C 6, 15;'& 3H CuO Mn0 2 c0 0 3 4 cr20 3 CdO Ce02 V20 5 Fe 20 3 NiO Th0 2
°2 ,
-
5.78 6.04 6.34 6.91 7.07 7.42 7.44 7.45 8.10 8.60 8.85
33.8 2903 22.7 30.7 21.5 14.9 30.8 18.9 17.7 2103 26.8
-
5.85 5.88 6.18 6.95 7.07 7.17 7.45 7.58 7.10 8.38
36.8 39.0 35.1 34.1 32.4 30.9 40.0 41.1 32.8 28.8
- 0.16 - 0.29 0.39 0.36 0.74 1.00 0.42 0.48 0.87 0.88 0.70
0.49 0.21 0.28 0.32 0.12 0.15 0.13 0037 0.00 0.47 0.41
65% N2
242-276 230-261 240-270 268-301 277-323 224-274 280-330 281-311 287-329 292-339
-
° °0.24 0.17 0 0.54 -0.40 0.17
°0.56
0.40 0.64 0.55 0.71 0.70
°0.95 0.24 0.56 0.41
and CaO. The other oxides studied display an intermediate activity. At the same time, definite deviations are evident. They are likely to be caused primarily by the different compositions of the reaction mixtures, resulting in changes in activity patterns
528
TABLE 78 The Catalytic Activity of Metal Oxides for the Oxidation of C 6 at 5000C /63/ 3H Selectivity/ % ** ) 19 r* Oxide /mmol CO 2+ acro- 1,5-hem-2 s-1 CO lein xadiene + benzene C0 304 CuO In 20 3 NiO Sn0 2 V205 ZnO Fe 20 3 Cr 20 3 Ce02 CdO Bi 20 3
-3.09 -3.38 -3.5 -4.08 -4.9 -5.14 -5.19 -5.25 -5.7 -6.0 -6.0 -6.2
96.2 2.2 92.5 3.8 4.1 93.4 0.8 75.7 2.3 2.5*** 91.5 3.2 81.8 14.2 49.4 0.5 48.2 86.9 12.2 traces 4.0 69.3 2.1 99.9 traces 0.1 98.5 0.8 9.6 89.4
°
° ° °
°
°
Selectivity / % ** ) 19 r* Oxide /mmol CO 2+ acro- 1,5-hem- 2 s-1 CO lein xadiene + benzene
Mn° 2 Si0
2 Ti0 2 W0 3 CaO Mo0 3 Sb 20 4 Al 20 3 MgO PbO T120 3
-6.2 -6.4 -7.05 -7.12 -7.3 -7.42 -7.6
0 0.) 99.7 8.6 traces 46.7 85.8 12.0 0.3 0.9 90.0 6.3 86.5 74.6 25.1 63.3 16.3 38.5 42.1 94.0 traces 11 69.5 11 96.7
°
° ° °° °° °
*) Conversion of propylene is 25% (with the oxides of Al, Mg, Pb and Tl, it is below 25%) **) By-products are C2H CH etc. 3CHO, 4, ***) 0.7 for C6H6 and 1.8 for 1,5-hexadiene due to different rate equations, by various effects of the reaction mixture on the chemical composition of the oxides, etc. In some cases, sharp deviations are observed, ususally towards lower activity. The reasons for these are not quite clear. For example, in reference /62/, the activity of NiO, in reference /57/, the activities of Ce02 and Th0 2, in reference /63/, the activity of Ti0 2, cr20 and Mo0 and in reference /64/, the activity of W0 3 3 3 are all likely to be underestimated. The high activity of T120 in 3 reference /63/ is in contrast to the data of reference /65/. The selectivity of the simple oxides for the oxidation of pro-
529
TABLE 79 The Catalytic Activity of Metal Oxides for the Oxidation of C 6 3H (10% C 6 + 90% O2) Under Non-Steady Conditions /64/ 3H
Oxide
Mn° 2 c020
CuO Ni 20
3
3 Cr 2 3** NiO Fe 20 3 V20 5 Sn0 2 Ti0 2 Bi 20 3 U 8 30 Sb 20 3 ZnO W0 3 MoO3
°
r x 1010 at 300°C /mol m-2 s-l
1.15 x 10 6 9.78 x 10 4 2.82 x 10 4 1.02 x 10 4 4.78 x 10 3 3.98 x 10 2 3.02 x 103 1.82 x 10 3 1.07x10 3 9.23 x 10 2 6.02 x 10 2 5.62 x 10 2 1.74 x 106 5.02 x 10 1 4.47 x 10 1 1.29 x 10 1
Tr* /oC
149 167 181 214 247 253 274 265 295 301 321 321 400 523 467
E/ kcal mol- l
22.4 14.7 14.7 15.7 17.8 19.0 27.0 11.1 20.0 30.0 16.2 18.7 0.91 22.3 12.6 21.8
Amounts of chemisorbed C 6/ 3H cm3 m-2
The reaction temperature in adsorption measurements /oC
0.04 0.198 0.178
87 120 254
0.036
192
0.034 0.086 0.099 0.073
263 339 320 263
0.058 0.080 0.020
232 357 330
0.150
400
pylene has been studied extensively. With most oxides, the major product of mild oxidation is acrolein. A higher activity with respect to reaction (XVII.40) is exhibited by cuprous oxide catalysts, which act at 2400-4000C /19, 27/. These catalysts were first used in practice /14/. Under industrial conditions, the conversion of C 6 (per one passage through the catalyst) with the above cata3H lysts is equal to 10-20% (with excess of C 6), the selectivity in 3H acrolein being 60-85% /66/. By-products are CO 2 and H20 (with small amounts of propionaldehyde, acetone and allylic alcohol) /14/. The selectivity of cuprous oxide catalysts can be enhanced by
530
TABLE 80 The Catalytic Activity of Metal Oxides for the Oxidation of C 6 (10% C 6 in a mixture with 02 + N2) /65/ 3H 3H
Oxide
c0 0
3 4 CuO Mn 20 3 Cr 20 3 Ce02 Fe 20 3 Ag20* T120 3 Sn0 2 NiO TiO CdO UP8 In2°.3 Mo0.3 ThO2 V
20 5
W0 3 Bi 20 3 Ta 20 5 La 0 2 3 ZnO zr0
Temperature (oC) required to attain a reaction rate of 3.5 x 10-4 mol m-2 h- 1 at various concentrations of 02: 5%
10%
18%
20%
30;i~
225 240 260 295 300 300 3.30 330 370 345 338 .350 365 378 .395 445 440 387 445 435 505 447 420 438 500 515 510 590 625
220 224 255 285 295 285 305 320 345 324 325 345 357 .360 37.3 420 425 .385 435 422 470 430 415 425 490 490 510 565 600
200 204 230 278 280 290 295 295 306 310 310 .340 .345 351 360 .381 390 398 402 405 410 415 420 420 436 465 510 550 578
210 210 242 270
200 212 240 270 280 265 265 290 320 310
270 270
310 340 347 410 400 380 409 440 405 420 460
.305 337 342 .345 360 405 395 380 405 396 430 412 405 416 450 459 510 540 555
2 Nb 20 * 5 Pb 20 4 Ga20 3 Te02 Sb 20 4 *) The oxide is unstable under conditions of catalysis
E/
kcal mol- 1
19 15 19 32 19 28 19 18 23 22 24 27 25 27 26 12 34 23 23 26 19 28 30 27 16 26 36 8 8
531
nonmetallic additives (Cl, S, P or Bi) /19/.
r/10 -51!l(J{ 0 ·,t(~r!
2,,"-
(1)-2 .-J ()-4
/
I /
/
/
e
,,~
,,/ / /
/
0.2 //
:
a
JOO
1.2
(O-!
?-
08
¢-f
e e
J tl.Joo"""-- ......,
e
~~.
.
~~-<}.:
" ---6
9'-"
,
0.4¢
0
4 "
J50
400
450
t/°C
Fig. 162 - The dependence on temperature of the amounts of acrolein formed (continuous curces) and its rates of formation (dashed curves) in the oxidation of C)H over CuO (1), Co)04 (2), Fe 20) (), NiO (4) and V20 (5) / 6 62 / • 5 .An especially pronounced effect is caused by Se0 or Se (5-10 2 wt.%). With these catalysts, the reaction is carried out at great excess of 02; high conversions of C)H6 (80-90% at )100_))00C) are associated with high selectivities to acrolein (- 90%) /67/. The selectivity of cuprous ~xide catalysts increases when water vapour is added to the reaction mixture /27/. Under steady-state conditions, acrolein is produced on the oxides of Cu, Co, Fe, Ni and V /62/. On increasing the temperature, the rate of process (XVII.40) passes through maximum (Fig. 162). This is explained by the further oxidation of acrolein at elevated temperatures. According to reference /62/, the rate of reaction (XVII.40) decreases in the sequence: CuO, Co)04?-Fe 20)?-NiO?-V20 5• We have calculated the temperatures, T;, to attain equal reaction rates for the formation of acrolein (Jx10- 7 mol 02 m- 2 s-1) using the data of reference /62/ which apply to parallel scheme. Fig. 16) shows that the overall activity (Table 76) correlates with the activity for mild oxidation. The selectivity towards acrolein at 500 0C of the various oxides /6)/ decreases in the order: Al20)?-MoO)/'Sb204>V205>Fe20»Si02> ~ WO) ?- CuO ,>Sn0 /' NiO">,Co)O4 '> Cr 2 20) ":;>CdO, In20),> ZnO":;>Mn0 2/, MgO, PbO, Tl 20), Ge02, CaO, Bi 20). At the same time it was observed that other products of partial oxidation are formed besides acrolein (1,5-hexadiene and benzene); on the oxides of Zn, Bi, In,
532
Cr, Sn, W, Ti and Cd, the oxidative dimerization of propylene (reactions(XVII.46) and (XVII.47» takes place (see Table 78). These catalytic processes were first discovered with complex oxide
Fig. 163 - A correlation between the activity of some oxides for the overall reaction of the oxidation of C 6 and that for its oxidation 3H into acrolein (the data of reference /62/ were used for the correlation): 1 - V20 2 - NiO, 3 - Fe 20 3, 4 5, CuO (Cu20), 5 - c0 304'
1.4 1.4
S([lt/%
40
JO
20
10
1.8
0/2
Fig. 164 - A correlation between the activity in the oxidation of C3~ and.t.he selectivity towards " allylic" oXidation (0) and that for the oxidation into acrolein (e) (Table 78): 3 - Ti0 2; 4 1 - Mo0 2 - Sb20 4; 3, 6 - Bi 20 5 - Fe 20 7 V20 3, 3; 5; 10 - Sn0 2, Si0 2; 8 - W0 9 - cr 20 3; 3; 11 - Ge02; 12 - ZnO; 13 - CdO; 14 In20);15 - CuO; 16 - C0 304; 17 NiO; 18 - Mn0 2•
catalysts /68/. At high conversions (60-70%), the selectivity towards benzene can reach 10-18% at 500°C with the oxides of Zn, In, Sn and Cd. The selectivity to 1,5-hexadiene and benzene decreases in the order: ZnO»Bi2037In203»sn02»CdO /63/. The yield of 1,5-hexadiene decreases in the order: TI 20) ,/,CdO»Bi 20 » Sb 20)/" In 20 "> Sn0 2• The 3 3 formation of benzene over Tl 20 and In 20 was observed in reference 3 3 170/. The oxidation of propylene into acrolein occurs usually at lower temperatures than the oxidative dimerization of C 6, which 3H takes place at 500°-550°C. One can thus formulate the following empirical relationships.
533
The major product of the mild oxidation of propylene over the transition metal oxides is acrolein while the oxidative dimerization is peculiar for the oxides of metals of the main groups II-V. The selectivity towards mild (" allylic")oxidation of C 6 3H decreases, in general, with increasing overall activity (Fig.164). At the same time, the rate of mild oxidation changes in parallel with the overall activity (Fig. 163). These relationships are qualitatively supported by the data/65/ (Table 81) obtained by the pulse method under steady-state conditions. The main product of partial oxidation is acrolein, the highest selectivity being exhibited by the oxides of copper and tellurium. The catalysts in Table 81 are listed according to a decrease in their activity. On the whole, poorly selective oxides are seen to be in the upper half of the Table, i.e. an inverse relation between activity and selectivity is observed. Cuprous oxide is an evident exception. The oxidative dimerization proceeds mainly over nontransition metal oxides (Ga20 Sn0 2, In 20 as well as over La 20 Ta20 and 5 3) 3, 3, The formation of CH (T1 20 and Ag20), CH (In 20 Cr 20 3 3CHO 3• 3COCH3 3), C2H4 (Te0 2 and Sb 20 is also noticed. 4) Over some oxides (mostly of nontransition metals), CO is formed together with CO 2 (see Table 71). According to reference /57/, great amounts of CO are produced over Th02, V20 and Al 20 Gene3• 5 rally, the latter product is peculiar to poorly active catalysts which are the oxides of the polyvalent metals of groups III-VI. The selectivity towards mild oxidation usually decreases with temperature. With the higher oxides of V, Mo, Vi and U, acetaldehyde is formed /19/. The changes of activity and selectivity on going from nonsteady to steady state conditions are of considerable interest. Cupric oxide catalysts are reduced during catalysis /19/ and cuprous oxide is an active component of these catalysts /27/. If metallic copper is the initial substance, it is oxidized into Cu20 /19, 27/. During the course of the reduction of CUO, the selectivity to acrolein decreases progressively /27/. At lower temperatures (below 310 0C), the activity decreases and selectiVity increases with the number of pulses (Fig. 165). At higher temperatures (~3350C), the steady state is already established in the first pulse. In reference /71/. the irreversible chemisorption of C 6 on a 3H Sb-O-catalyst was observed during the establishment of the steady
534
TABLE 81 The Selectivity of Metal Oxides in the Oxidation of C 6 3H (10% C 6, 18% O2 and 72% N2) /65/ 3H Oxide C0 304 CuO Mn20 3 Cr 20 3 Ce02 Fe 20 3 .Ag20** 1'12°3 Sn0 2 NiO 1'i0 2 CdO U 8 30 In20 3 MoO 3 1'h02
V 20 5 ViO
3 Bi 20 3 1'a20 5 La20 3 ZnO zr0
2 Nb 20 5 Pb 304 Ga20 3 1'e02 Sb20 4
l' /oC*
r
200 204 230 278 280 290 295 295 270 310 310 340 345 351 360 381 390 398 402 405 410 415 420 420 436 462 465 510 550
Selectivity towards other towards acproducts rolein/% 0 80 0 ° °
(C6 ) ***
8
6 ° 22 45 13 8 60 3 52 50 ° 46 12 5.3 10 ° 11 1 18 6.5 28 87.5 70
35% CH 3CHO 8% CH 3CHO 3.5% C6
(C6' CH 3COCH 3)
The ratio CO/CO 2 0 0 0 0 0 0 0.14 0.06 0.7 003 ° 0.6 0.15 0.3 0.45 ° 1.8 0.75 0.15 0
(C6 ) 6% C6
5.3% C6 (C2H 4) (C2H 4)
° ° 0 0.6 0.36 0.08 0.8 003 2
*) 1'r is the temperature to attain an overall rate of 3.5x10-4mol m -2 h -1 **) The oxide is unstable under the conditions of catalysts ***) C6 hydrocarbons; the parentheses refer to trace amounts
535
3/% 75
lriff ref :: II I I II II
50
I
I
I I
II I I II II I I ill I II~I
if
II
II I
o /
.7
L __ ~I /0
JO
I I
I I
I I I
I~H 1111 I I iv I IV I I I-I 1 1-
r-r l
!I
x/%
I I
1 11/0
I
I
I I I 11 1 1
/5
20
Fig. 165 - The conversion of C 6 over CuO, X(o), and the 3H selectivity towards acrolein, See), versus the number, N, 0 of pUlses on CuO/65A 1 - 285 C, II - 310 0C, III - 335°C, IV 35800, V - 386°C
0
20 If {ll
Fig. 166 - (a) The activity,r, and the selectivity for mild oxidation, Sea), and (b) the amounts of irreversibly adsorbed propylene versus the number of pulses (N) for a Sboxide catalyst (calculated from the data of reference /71/).
r
b
at;.
40 state, the amount of propylene ad- - - - _....120 sorbed decreasing with an increase in the number of pulses (Fig.166). 0 Simultaneous reduction of the ca!0---J~---5~--~-7 tV talyst takes place. These procesa ses are accompanied by a decreasing activity and an increasing selectivity towards mild oxidation. (See our calculations based on the data of reference /71/ in Fig. 166). Similar phenomena were observed with the oxides of Cr, Mo, Mn and Sn /71/. (See also references /72/ and /73/ and Table 79). The nonsteady-state behaviour of 30 oxides has been studied in reference /65/ by the pulse technique. A typical observation for such systems is a partial removal of oxygen from metal oxides accompanied by the irreversible adsorption of the hydrocarbons. The rates of the processes mentioned depend on the reaction mixture composition and temperature. Higher temperatures and C 6/02 ratios favour the above reactions. Paral3H lel to this, the catalytic activity increases while the selectivi-
536
ty towards mild oxidation decreases (or remains constant when it is low). The reaction kinetics and mechanism have been studied most comprehensively with cuprous oxide catalysts. It was shown that a parallel scheme is valid at low temperatures ( ~ 320 0C), while further oxidation of acrolein takes place /27/ at higher temperatures ("-380 0C). A similar picture was observed with other metal oxides. By freezing the product from the reaction cycle, it was found that the oxidation of propylene on cuprous oxide is retarded with acrolein and water vapour (but not with CO 2 ) /27/. At the same time, the selectivity towards acrolein increases when the concentration of H20 grows /27, 74, 75/. On the basis of kinetic studies using the flow-circulation method, the following empirical rate equation for the formation of acrolein (r 1 ) and that for the formation of CO 2 (r 2 ) were proposed /27/: 1
I
0
k1 Ca Cc. II.
r. = t
3 0'
;Z
0
1+ 0, ( Cc !/, '3 If I
~
-tV
eft 0 )
+
'2
1
tts ns en u
(cCJI4IJ where k
(XVII.49)
1, k 2,
u2
+ C
~IJ
)
(XVII. 50)
b 1 , b 2 are constants and Ci are concentrations .*)
*) The proposed in reference /27/ empirical equation:
as
r=
k,Ca Cc :z
..,
If.
6'
(1+ !J 'CC fI. (J) '5 If
( 1+ t'Ii; 0 )11.5
(1+
01 C17u 0) 2
describing the kinetics at high concentrations of H20 can be valid only when CH is expressed in mole fractions since, when other units are 2 used, one must not add CH to 1 in the term (1 + C )0.5 2
°
H
20
•
°
537
The measurements were made at J17 o-J80oC and with excess of C 6, JH The activation energies for the formation of acrolein (JO ± 2 kcal mol- 1) and CO 2 (J6 ± 2 kcal mol- 1) obtained at constant composition of the reaction mixture were higher than those measured under conditions when the products were removed (20 ± 1 and 26 ± 1 kcal mol- 1). The treatment of the reaction mechanism proposed in reference /27/ based on a Langmuir-Hinshelwood model involving a molecular form of adsorbed oxygen and two sorts of active sites, is doubtful. In particular, the above scheme suggests that the adsorption coefficients in Eq. (XVII.49) are equal to those in Eq. (XVII. 50); however, b1~ b 2• The kinetics of the oxidation of propylene on other simple oxides have not been studied so comprehensively. According to reference /76/, the formation of aldehydes, CO and CO 2 (and the overall transformation of C H6) over vanadium oxide catalyst obeys the J equation: (XVII.51)
m
r
02
o
0.4
Fig. 167 m (o) with /57/): 1 7 - NiO, 8
0.5
0.8
n
A correlation between the reaction orders in propylene,
those for oxygen, n (~) (experimental data of reference Ce02 , 2 - V20 5, 3 - CdO, 4 - Fe 20 , 5 - CUO, 6 - Mn0 2' 3 - cr 2 0 , 9 - Co 0 3 4• 3
The activation energy for the formation of CO 2 (22 kcal mol- 1) was much higher than that for the formation of aldehydes and CO (2-4 kcal mol- 1). The same rate equation was observed with PbO /70/.
538
More information has been obtained under conditions when only the deep oxidation of C 6 takes place. Table 77 presents the daJH ta /57/ which can be described using the power rate law (XVII.48). For transition metal oxides, the order in C 6 decreases with inJH creasing order in 02 (Fig. 167). The oxidation of C 6 over NiO is retarded by water vapour. The JH kinetics follow Eq. (XIV.52) (Chapter XIV) with m = 0.23, n = 0.53 and 1 = - 0.27 at 250 0-400 0C, the activation energy being 25 kcal mol- 1 /72/. It should be noted that the experiments quoted in references /57/ and /72/ were made in a differential reactor but only in reference /72/ were the concentrations of all reagents, including H20, kept constant (H20 was added to the mixture). If PH is not 2
°
constant along the catalyst layer (as was so in reference /57/), the reaction orders in C 6 and 02 are underestimated /72/. JH The data on m and n in Table 77 are therefore likely to be lower than those which would be observed under gradientless conditions. However, they reflect the trends of the changes in the reaction orders on going from one catalyst to another. Thus, the rate law for many oxides can be expressed by Eq.(XIV. 52) which is reduced to Eq. (XVII.48) or (XVII.51) in particular cases. Using the pulse technique, it was shown in reference /71/ that the rate of catalysis exceeds that of the surface reduction of the oxides with C 6• This fact is hardly to be regarded as an argument 3H against the mechanism of alternating surface reduction-reoxidation since the surface composition of the catalysts under conditions of catalysis and reduction in the experiments of reference /71/ were significantly different. The rate of the catalytic oxidation of C H6 over CU2 0 at 200 0C 3 is nearly 5 times higher than that of surface reduction of the catalyst in experiments carried out after catalysis. Under these conditions, carbonate-carboxylate complexes formed in the interaction of C 6 with CU20 are slowly destroyed in the absence of 02 in the 3H gas phase. In the presence of 02' the rate of catalysis is equal to the rate of oxidation of the above surface complexes. At elevated temperatures, the latter are rapidly converted into CO 2 and H20 in the absence of 02' so that the rates of catalysis and surface reduction are equal. This suggests that 0 2- species participate in the catalysis /52/.
539
The same conclusion was also reached on the basis of thermodesorption data obtained during the partial oxidation of propylene into acrolein over the oxides of Cu, Cr and Mo /78-80/. The highest yield of the partial oxidation products was obtained under conditions where the anion-radicals, 02' were absent on the surface of the catalyst. In references /77/ and /81/, a cuprous oxide catalyst was used for the oxidation of propylene in which the CII - group was labelled 3 with C1 3. One half of the label was found in the carbonyl group of the acrolein formed (the isomerisation of C 6 did not occur 3H under the conditions of the experiments). The result obtained suggests that a symmetric allylic complex (CH 2 = CH =- CH 2) is a surface intermediate in the reaction. This conclusion was supported by stUdying the oxidation of propylene labelled by deuterium /82, 8J/. The model involving an allylic surface intermediate has been also confirmed by a tracer technique in reference /84/. Further development of the model was given in references /78//80/ with the help of thermodesorption data. Propylene was shown to be adsorbed on metal oxides in two forms: 1 - weakly (and reversibly) bonded, II - strongly bonded. The maximum rate of desorption for I occurred at 110 0 ± 200C and for II at N 280 0C. With copper-containing samples (CUO, Cu20 and CuO - MgO), acrolein is desorbed in the same temperature region as is the form I. At the same time, form II corresponds mainly to deep oxidation products. Similar results were obtained with cr20 and MoO It J/Al 20 J• J was found that strongly bound propylene on deep oxidation catalysts (CuO and Cr 20 ) is predominant while weakly bound olefin prevails 3 ) . Fig. 168 illuson mild oxidation catalysts (CU 20 and MOO J/Al 20 3 trates the data for Cr 20 and MoO In the last case, the 3 J/A1 20 J• thermodesorption products contain not only acrolein but also acetaldehydeo It is noteworthy that CHJCHO is also formed (together with CH 2=CHCHO) in the catalytic oxidation of C 6 JH It is natural to believe that form I leads to mild oxidation products of catalysis while the form II results in deep oxidation. The analysis of experimental data on the IR-spectra of adsorbed propylene has led to the conclusion /78-80/ that the weakly bound and reversible form I of adsorbed propylene can be attributed to :J{ - and (3 -allylic complexes while the strong form II corresponds to carbonate-carboxylate type species. On the surfaces of cr 20 3 0
540
and MOO)' only 6' -allylic complexes were detected. J.r-allylic complexes are syrrunetrical and are attached to a surface by .1! -bonds:
1/7/ / / / / 7/71/
The adsorption in this case is likely to be of the two-point one type, the complexes being parallel to the surface. 5-allylic
l/mv
I
a
b
Fig. 168 - The thermodesorption of propylene from Mo0 (a) 3/A1 20 J and Cr 20 (b) /80/ J
complexes are nonsymmetric and are bound by one dicular to the surface:
6' -bond perpen-
II
H2
CH
I
CIt 2
J; In comparison with allylic species, carbonate-carboxylate species are more stable; above 200 0C, they are destroyed and converted into CO 2 and H20. Irreversibly chemisorbed propylene is also supposed to include stable %-complexes. In order to discriminate between %- and 6-allylic complexes,
541
the thermodesorption of individual supported 6-allylic complexes of Mo and of Jr -allylic complexes of Ni, as well as the desorption and IR-spectra of allyl bromide, were studied in references /78/ and /79/. It was shown that on copper- or nickel-containing catalysts, Jr-allylic complexes are most important. With Cr 20 and MoO 3' 6 -allylic complexes pre domina t e , 3 Since form I can be detected in rather high quantities even at 300-1300C, it was assumed in references /78/-/80/ that, in this temperature region, the partial oxidation of propylene with almost 100% selectivity can take place (Form II is removed at elevated temperatures). Indeed, when pUlses of C 6 + 02 (1 : 1) were 3H injected onto CuO-MgO and cr 20 at 80 0_1200C, the oxidation of 3 C 6 into acrolein with 100% selectivity was observed. 3H The above facts allow us to propose a mechanism for the catalytic oxidation of propylene over metal oxides under conditions when carbonate-carboxylate complexes are easily destroyed and the rates of catalysis and surface reduction are equal. The proposed scheme should be considered as a tentative one. In a shortened form, the mechanism of the parallel formation of acrolein and CO 2 will be: ( )
1) °2
)-(°2)-2(0) fast 2) C 6 + (0)-(1 1) (O) .. CHO + H2O 3H fast 3 4
+
7(0) 3) C 6 + 2(0)-(1 2) 3H fast + 4) C )~(C3H40) 3H40 5) H20 +
3C02 + 3H2O
(XVII.52)
) ~(H20)
The correspondence between the rates (and selectivities) of catalysis and of surface reduction of the oxides by propylene, as well as of the formation of acrolein by the desorption of adsorbed propylene, suggest that atomic anions of surface oxygen, (0), are important intermediates in the reaction. The interaction of C 6 with (0) leads to a surface compound 3H (1 1), resulting in the formation of acrolein, while the interaction of propylene with two (0) species leads to complex (1 which 2), is converted into deep oxidation products. The species (1 are li1)
542
kely to be allylic complexes and species (1 2) may be precursors of the carbonate-carboxYlate structures. In the C 6 molecule, the 3H energies of cleavage of the ff-c-c-bond (63 kcal mol- 1 ) and that of the C-H-bond in the allylic position (77 kcal mol- 1 ) are close, so that both directions of reaction are probable. The first results in the formation of allylic complex; the liberated H-atom reacts with (0), giving the (OH)-group. The second leads to a complex of the type:
which is converted into the carbonate-carboxylate structures. This is in accordance with the fact /85/ that an intermediate leading to acrolein requires fewer O-atoms than that leading to CO 2 • In the interaction with a surface, propylene is an electron donating agent while 02 is an electron acceptor /19/. Since acrolein can be reversibly adsorbed on copper oxides, its equilibrium chemisorption is possible (step The same applies to water vapour (step 5». The corresponding rate equation will be:
4».
~ f r=-P ( 1 - 8 ) - - - - - - - - - I ~ r- b P + b n
~/{f{J
f3~IJ
If 0 I"!I;IJ
(XVII.53)
The equations for the selectivities towards acrolein (s(I» and CO 2 (S(II» and for the rates of formation of these products (r(I), r(II»are: s(!):
4 '3
(V9+
6' (XVII.55 )
543
I,
T P0
(!-IY)
(XVII. 56)
fJ%/I,!O
+
°1f0 P~O
(XVII. 57 )
where b i are adsorption coefficients. The value of mined by the expression:
8
is deter-
B=(XVII.58)
where
1 (XVII.59)
The value of
8
decreases when
decreases and when
PC
° or bH2OPH2° 3 4
bC H oPc H 3 4
_P0 2
3H 6 increases. When"" - 0 , 8 - 0 , while
when ~ -"""", 8 1. The kinetics of the oxidation of C 6 over Cu20 were studied 3H /27/ with excess of propylene and rather high concentrations of products. Therefore, the value of & in these experiments was small. At B «1, Eqs (XVII.56) and (XVII.57) reduce to:
I,
-
-PJ!
I{£)=
Iro t;~o
P ~h10
0
(!)
S
fJ hpO !f?O
+-0
(XVII.60)
544
i1 Y
/f b
P
P
°2
t.J If'l 0 C,/If 0
+
0
10
(XVII.61 )
fl
!(z 0
The formsof these equations coincide with the empirical Eqs (XVII. 49) and (XVII.50) which describe the reaction kinetics if one assumes that k 1 and k in Eqs (XVII.49), (XVII.50) are given by:
2
f II (/) f =-,f
1
P
The analysis of the data of reference /27/ shows that these relations are valid (see, for example, Fig. 169).
s/%
Fig. 169 - The selectivity towards acrolein against in Eq. (XVII.49) for catalysis on Cu20
k;
05 00
!.2
fA.
f.6
If!
Eq. (XVII.54) suggests that the selectivity towards acrolein should increase with decreasing tl. As the latter decreases with increasing PH 0' one should expect an 2
enhancement of the selectivity with increasing pressure of water vapour as is also observed. Finally, one should expect an increase in the selectivity with increase in Pc H since 8 decreases with increase in Pc H ; this 3 6 3 6 is also observed /27/.
545
Mechanism (XVII.52) thus permits an explanation of all the experimental data obtained with cuprous oxide catalyst. At be HOPe H 0 + bH oPH 0 ~ 1 and 8 « 1, Eq. (XVII.53) redu3 4
3 4
2
2
ces to Eq. (XVII.51) which describes the reaction kinetics on vanadium and lead oxide catalysts. When mild oxidation products are not formed, Eq. (XVII.53) reduces to:
i,
1
q ( 1 -18+ )of!,-()-PI!- -0
T=-j}
I
'2
(XVII.64)
2'
and instead of (XVII.58) and (XVII.59), we have:
8= (XVII.65 )
!
~ u1 = _2_. fl
'31
1+
_
0/f?(} P1?O
(XVII.66)
Power rate equations (V.49) observed for deep oxidation on many oxide catalysts are likely to be an approximation of Eq. (XVII.64). If this is so,
8 2-8
m=--,
2(1-& ) 17=
,
2-8
f
2
l =/J-2-8
(XVII.67 )
where B is the surface coverage with oxygen and 8 f is that with H20 /43/. One should then expect opposite changes in m and n what is observed (Fig. 167). Due to the dependence of m and n on 8 , the reaction orders correlate with surface oxygen bond energies /43/. A similar relationship is noted in /57/ but it is there interpreted on the basis of a Langmuir-Hinshelwood mechanism. The overall reaction rate according to Scheme (XVII.52) can be
546
!
tffr -J
-5
/0
-4-
-0
02
-1
-5
-8
02
_g
-0
04-
.5/%
a
5D 40
-1
.f0
2D fO
o
20
40
o
60 ~s/Kcat(;-atOr
L-.:.ow-o:£:J_...l...-~_.l...
Fig. 170 - The specific catalytic Fig. 171 - A correlation betactivities of metal oxides in the ween qs and the catalytic actioxidation of C3H 6 (Table 78) ver- vity towards mild (I) and deep sus qs: 1 - c0 (II) oxidation of C 6 (a); and , 2 - CUO, 3 3H 30 4 NiO, 4 - Mn0 2' 5 - Fe 20 6 - Sn0 2,the selectivity towards mild 3, 7 - V 8 - CdO, 9 _ ZnO, 10 _ oxidation of C3H6(b) (based on 20 5, the kinetic data of reference W0 3, 11 - Mo0 3• /63j): 1 - c0 , 2 - Mn0 2' 30 4 3 - NiO, 4 - CuO, 5 - Fe 20 3, 6 7 - Ti0 2, 8 - ZnO, 9 V20 5, MoO)' 10 - W0 3• written in a different way: r= r "2
+
r. .. j P 3
.?
t.J1Io
8
+
*3 Pc3//6 8 !
(XVII.68)
1.1
At high oxygen coverages ( 8
~
1)
Since in steps 2) and 3), oxygen-catalyst bonds are broken, one should expect decreasing values of r with increase in q s • This relationship is observed in practice (Figs 170 and 171). The maximum activity is exhibited by C0 for which qs is close to 304'
547
(q) s op t • At q s L... (q) s op t' tne catalytic activity decreases since the surface coverage with oxygen is small at low values of qs. The rates of mild and deep oxidation, r m and r d, at 17 ~ 1 will be given by:
r =- r. m 2
=- J~
c
Pc
u
'3 no
(XVII.70)
(XVII.71) In each of steps, 2) and 3), oxygen-catalyst bonds are broken which results in decreasing values of r m and r d with increasing values of qs (Fig. 171 a). At the same time, more oxygen-catalyst bonds are destroyed in the third step and this leads to a sharper decrease in r d compared with r m• A direct consequence of this is an increase in the selectivity with qs (Fig. 171 b) /86/. The above changes in r, r m, r d and S with qs can explain the parallel changes in rand r m (Fig. 163) as well as the inverse changes in the activity and selectivity (Fig. 164). The determining role of qs allows us to interpret some phenomena observed in the non-steady period of the reaction over oxides. During this period, two processes occur: the reduction of the metal oxide and the irreversible adsorption of propylene. Both processes result in a decrease in 8 and (if the surface is nonuniform) in an increase in qs. One should therefore expect a decrease in the catalytic activity and an increase in the selectivity with time, as takes place in practice (Fig. 165 and 166). If the values of 8 and qs change slightly, the catalytic properties should be practically unchanged. The data of reference /65/ show that in most cases investigated, both of these two situations are observed. Since the mild oxidation rate increases with decreasing qs while the selectivity decreases, a definition of I·the optimum catalyst for mild oxidation" is rather difficult. When the further oxidation of the partial oxidation product is small, its highest rates of formation are attained on catalysts with small q values; s these substances are Usus ally considered to be typical catalysts for deep oxidation. An example is given in Fig. 162. According to reference /86/, the following feature is essential for selective oxidation when a parallel scheme is valid. Owing
548
to the fact that more oxygen-catalyst bonds are broken in deep oxidation than in mild oxidation, the activation energy for the first process on each catalyst should be higher than that for the second one. This is observed with Cu- and V-containing catalysts. As a result, the selectivity decreases with increasing temperature. This effect is enhanced by the progressive oxidation of mild oxidation product. Thus, Scheme (XVII.52) can serve as a basis for discussing the various experimental facts concerning the catalytic oxidation of propylene over oxides. It should be noted that the absolute catalytic activity of transition metals in this reaction is higher than that of the metal oxides (Fig. 1J4). This is likely to be due to the higher rates of the specific activation of C 6 on metals, resulting in faster JH rupture of the J.r-C-C-bond. This is why, on transition metals, the prevailing reaction route leads to surface species of the type: HC-~-~
2
1
I
J
o 0 I I resulting in deep oxidation. This route is likely to involve intermediates the desorption of which gives acetone (on metals), acetaldehyde and CO (on oxides). On the other hand, the "allylic" route (which is predominant on selective oxide catalysts) can include dimerization and oxidative condensation of the allylic complexes with the formation of C6-products (benzene; 1,5-hexadiene, etc.). These products appear mainly on the oxides of nontransition metals which suggests the importance of acid-base properties in this case. At elevated temperatures, metal oxides (like metals) can initiate a heterogeneous-homogeneous radical-chain process. According to reference /88/, the C 6 oxidation on Cu20 at JOO o-J70 0C JH proceeds only on the surface of the catalyst while homogeneous continuation of the process is observed at 400 0C. The Oxidation of Propylene Over Complex Oxide Catalysts A rather high selectivity towards acrolein is shown by complex copper-containing oxide catalysts which operate (like Cu20) with
549
the excesses of C 6 /89/. The selectivity of CuI-chromite at 3H 350 0C is considerably higher than that of CuII-chromite /90/ which is similar to the relations between Cu20 and CuO. The catalytic activity of solid solutions of CuO in MgO increases with copper concentration /78-80/. The introduction of P20 into copper oxide 5 catalyst (copper phosphate is formed) decreases the activity. At 20 at.% of P, the selectivity increases. The activation energy for the formation of acrolein on the catalysts is lower than that for the formation of CO 2 /19/. At the same time, complex oxide catalysts containing such metals as Co or Mn, like C0 and Mn0 2, catalyse the complete com304 bustion of propylene. The kinetics of this process on Col~204 and MnC0 20 follow Eq. (XVII.51). Additives of 1i, Ti and Cu oxides 4 to these catalysts decrease their activity /91/. For the synthesis of acrolein from propylene, complex oxide catalysts based on Mo0 (Bi-Mo-O and similar systems) are employed 3 in practice. In contrast to Cu20, they operate at elevated temperatures and with excesses of 02' With Bi at 450 0C(and 9PMo052/Si02 in the presence of water vapour), a selectivity of almost 60% towards acrolein (at 92.5% conversion) is attained /66/. By-products are CH CHJCOOH an~ CH 2=CH-COOH (overall selectivity reaches 3CHO, 20%) with a balance of carbon oxides. With Bi 20 /92/ at 460°C and with an 02/CJH6 ratio of 1.0 3"Mo0J to 1.5 (in the initial mixture), a selectivity of 90% is achieved at 20-40% conversions. (At a conversion of 85%, S ~ 68%). The selectivity passes through a maximum at 460°-520°C. The parallelconsecutive scheme is valid under these conditions, the rate equation being:
(XVII.72) (The products do not affect the reaction rate). A detailed study of the kinetics of the oxidation of C 6 on 3H Bi 20 catalysts has been made in references/93/ and /94/ at J-Mo03 430°-465°0 and with excesses of 02 using the flow-circulation technique. The specific activity of the above catalysts is not an additive quantity; it passes through a maximum at Bi:Mo ~1. In this system, new chemical compounds are formed which are bismuth molybdates
550
Bi 20 and Bi 20 3'2Mo0 the last being the most 3, 3'Mo03 active. The rate law with this catalyst at quite high values of Po is given by Eq. (XVII.72); at low pressures of 02' the rate in6reases with Po ; the process is retarded by acrolein. Bi203'~~o03'
2
Table 82 shows that the specific rate constant of Eq. (XVII.72) is almost independent of the method of catalyst preparation. The activity decreases in the order: Bi 20 2Mo0 :::-Bi 20 )Mo03> Bi 20 3· 3 3· 3• 'M00 In this case (in contrast to previous ones), the selectivi3• ty towards mild oxidation changes in parallel with the overall activity. On Bi 20 /19, 92/, the selectivity is changed sli3·2Mo03 ghtly with temperature (470 0-525 0C) and with a C 6/02 ratio of 3H 0.7 to 3.0. Interesting behaviour is displayed by Co-Mo-O and Sn-Mo-O catalysts. At elevated temperatures ( ~ 450 0C), the major product of the partial oxidation over these catalysts is acrolein. On the Co-Mo-O catalyst with excess of C~6' a 50% selectivity is attained (conversions are 4.5-5.5%). An H additive increases the 3P04 selectivity up to 60%. Increased concentrations of propylene (in contrast to 02) favour high yields of acrolein. The optimum Co:Mo ratio for the formation of this product is 0.4-0.5. In this case, a selectivity of 67% is attained at 450 0C and 3-4% conversions /95/. Acrylic acid is produced on a Co-Mo-catalyst in addition to acrolein /95/. At 3800C and 27% conversion, the selectivity to CH 2=CHCOOH is 18% /19/. For the production of acrylic acid from propylene, Co-Mo-oxide catalysts (with additives of P, Sn, Te and Ni compounds) are recommended; temperatures are 3500-3700C and ]0-40% H20 is added to the propylene - air mixtures /19/. A different picture is observed with the same Co-Mo catalysts at lower temperatures (200 0_3000C). Under these conditions, acetone is a main product of partial oxidation /96-98/. C0 is an active catalyst for the full oxidation of propyle304 ne. Small amounts of MoO] (10 at.% Mo) introduced into the Co]04 reduce the activity sharply and change the selectivity. Acetone becomes the major product on Co-Mo-eatalysts; acetic acid, CO 2, acrylic acid and acrolein are by-products (Table 8). At elevated temperatures, the selectivity to CH decreases while that to 3COCH3 acrolein and acrylic acid increases; the selectivity to CH]CHO passes through a maximum. A concentration of 10-]0 at.% of Mo is optimal for the production of CH MoO) is relatively inactive, 3COCH).
( !
Bi20
-phaae )
3'Mo03
Precipitation Fusion
Precipi ta -. tion Fusion
Bi 20
3'2Mo0 3 ( fi-phase)
Precipitation Fusion
Method of preparation
Bi203'3M003 ( ci.. -pnaae )
Catalyst
16.9
11 .1
11.3
16.8
9.35 17.2
17.2
4.5
2.7
4.3
2.7
4.5
16.9
11.3
9.5
2.7
H2O
17.2
°2
9.5
C 6 3H
67.2
69.6
67.2
0.046
0.14
0.19
1.19 9.4
-
1.00
0.395 0.55
0.006 7.94
0.51
5.70
-
0.16
67.2 69.6
5.29
0.52
acrolein
7.14
acrolein + CO 2
0.51
1.23
1.06
9.17
1.38 10.78
1.85
CO 2
k·10- 2/ min- 1 m-2
0.60
Distribution of products/ voI , % acro- CO 2 lein
69.6
N2
Initial mixture/vol.%
The Oxidation of C 6 on Bismuth Molybdates (475 0C; Contact Time 0.8 s) /19/ 3H
TABLE 82
52.0
86.5
87.0
74.0
Selectivity to acrolein/%
CJ< CJ< .....
552
TABLE 83 : Mo = 9). The Oxidation of Propylene Over Co 304-Mo03(CO Reaction Mixture is: C 6 - 20, 02 - 30, H20 - 30, N2 - 20 vol.% 3H /98/.
Catalytic ties
195°C
Conversion /% Product distribution (%) :
2.0
CH
80.9
210°C
280°C
4.0
18.0
7503
20.0
traces
1.3
3.0
CH 2=CH-COOH
0.5
0.6
2.0
2/3 CH
9.9
11.4
6.0
0.8
9.0
10.6
60.0
3COCH3
CH 2=CHCHO
3COOH
1/3 CO 1/3 CO 2
8.7
operates at high temperatures and does not give acetone (the main product being acrolein). The overall activity decreases with increasing concentration of Mo (Table 84, Fig. 172). The kinetics are described by Eq.(XVII.48). The order in C 6 (m) increases and the order (n) with respect to 3H O2 decreases with an increase in the concentration of ~m02 (Fig. 172).
In the co system, phases of CoMo0 are formed, the lat304-Mo03 4 ter existing in several modifications. According to reference /97/, a high selectivity to acetone should be attributed to some optimal ratio between Co and CoMo0 At the same time, the observed ac304 4• tivity at different Co : Mo ratios does not correspond to the activities of mechanical mixtures of co + Mo0 or Co + CoMo0 304 4• 3 304 It is interesting that with the co + Mo0 mixtures (Co : Mo = 4 304 3 or 9), small amounts of acetone are formed. X-ray analysis does not detect any chemical compounds. Preliminary heating at 550 0C results
553
TABLE 84 The Kinetic Characteristics of the Oxidation of Propylene Over c0 0 - Mo0 /97/ 3 4 3 Co : Mo ratio
E/ kcal mol- 1
19 k o/
ml m-2 s -1
T /K*) r
10 3K/Tr
10/0 9/1 8/2 7/3 6/4 5/5 4/6 3/7 2/8 1/9 0/10
31.0 14.2 13.9 15.7 12.1 13.0 15.0 13.0 12.6 16.1 13.1
10.2 1.7 1.6 1.9 0.2 0.] 0.5 -0.1 -0.] 0.6 -0.3
483 565 571 606 676 695 769 794 826 820 847
2.07 1.77 1.75 1.65 1.48 1.44 1.30 1.26 1.21 1.22 1.18
Reaction order in C O 2 3H 6 (n) (m )
0 0.50 0.64 0.70 0.70 0.65
0.66 0.20 0.12 0.20 0.15 0.15
0.80
0.03
0.82 0.91
0.10 0.02
*) Tr is the temperature required to attain an overall reaction rate of 2 x 10-4 ml m- 2 s-1.
in a sharp increase in the yield of acetone, a CoMo0 phase being 4 detected simultaneously. Thus, complex Co-Mo oxide catalysts accelerate reactions (XVII. 39)-(XVII.45) leading to acetone, acrolein, acrylic acid, acetic acid and carbon oxides. The selectivity depends upon the temperature (and other conditions). At lower temperatures, acetone and acetic acid prevail, while at elevated temperatures, acrolein, acrylic acid, CO and CO 2 predominate. In the intermediate region, all these products can be obtained simultaneously. Such experiments were made at 200 0-450°0 /99/. Fig. 173 shows that a Co : Mo ratio of 1.0 is optimum for the formation of acrolein, while for acrylic and acetic acids, the optimum ratio is close to 2.0. The highest rate of the formation of acetone is achieved on catalysts enriched by Co. According to reference /99/, the activity for the production of
554
Fig. 172 - The catalytic activity (1) and the reaction orders in C 6 (2) 3H and 02 (3) for Co-Mo-O catalysts /97/.
the acid is accounted for by the presence of one of the CoMo0 modifica4 tions; the Co in the latter has an octahedral environment. The concentration of this phase increases with the 0.5 Mo0 excess up to the point where 3 Mo : Co ratio becomes equal to ~ 2. Three-component catalysts C0 304• J 20 40 00 JO;}1/l/tIt.% Mo0 3-MeXO y' where Me = Ni, Zn, Fe, Cr, Mn, Cu, Ti or V, are less active than C0 in the oxidation of 304-Mo03 C 6 to CH 2=CHCOOH. This is due to the interaction of MexO y with 3H On Co-Mo-Cr-O, Mo0 which decreases the concentration of CoMo0 4• 3, mainly acetic acid is formed; on the addition of the oxides of Ti, V, Cr, Mn or Cu to Co-Mo-O, the rate of deep oxidation increases while the additives of NiO or ZnO decrease it. Tin-molybdenum oxide catalysts behave like cobalt-molybdenum ones. The former are more active and, operating at lower temperatures, are more effective in the formation of acetone from propylene /98/. Table 85 indicates that the selectivity towards CH at low 3COCH3 temperatures reaches 90%, acrolein and acrylic acid also being in the reaction products. On elevating the temperature, the acetone yield decreases and that of the carbon oxides increases;the selectivity to CH passes through a maximum /98/. 3COOH The product distribution in the oxidation of C 6 on Sn0 2-Mo0 3 3H (Sn : Mo = 9) under a wider region of conditions /56/ is given in Fig. 174. The observed pattern is similar to that obtained on C0304 - Mo0 3• The catalytic properties of Sn0 2 - Mo0 catalysts supported on 3 porcelain at 380°-480°C (when CH 2=CHCHO (CH CH 2=CHCHO, 3CHO), CH 2=CHCOOH and carbon oxides are produced) have been studied in references /100/ and /101/. The reaction mixture was: C 6 - 11, 3H 02 - 11, N2 - 78 vol. % and conversions did not exceed 20%. On 1.0
555
r(
25 20 /5
/0
.5 /rfo
I
0
0 200
JOO
400
t/oC
Fig. 173 - The rates of formation of CO 2(1), acrylic acid (2), acetic acid (3), acrolein (4) and acetone (5) in the oxidation of C 6 over Co-Mn-oxide catalysts /99/. 3H Fig. 174 - The rates of formation of different products in the oxidation of C 6 over Sn0 2-Mo0 (Sn : Mo = 9 5) /56/: 3 3H 1 - CO, 2 - CO 2, 3 -.CH 2=CHCHO, 4 - CH 5 - CH 6 3COOH, 3COCH3, CH 2=CHCOOH, 7 - CH 3CHO. changing the Sn : Mo ratio from 9 : 1 to 1 : 9, the overall selectivity towards partial oxidation was 73-85%; for the acids, 5-20% and for the acrolein, 24-77%. The dependence upon catalyst composition of the rates of production of the different products and the selectivities is given in Fig. 175. According to references /100/ and /101/, the most active Sn-Mo-O catalyst (Sn : Mo = 9) is solid solution of Mo0 in 3 Sn0 2• It is rather selective towards acrolein. The acrylic acid yield on the Sn-Mo-O catalysts is favoured by elevated temperatures and high concentrations of H20 in the mixture. The process was studied under these conditions in references /102/ and /103/, using both a massive catalyst and supported ones (the carriers were porcelain, carborundum and porous corundum). With Sn-Mo-O/porcelain, on increasing the temperature from 200°C to 500°C, the concentrations of acrolein, acrylic acid and CO 2 increase while those of CO and acetic acid pass through a maximum
556
TABLE 85 The Oxidation of Propylene on sn02~~003. Reaction Mixture is C 6 - 20, 02 - JO, N2 - 20, H20 - JO vol.% /98/. JH
Temperature Catalytic properties
115°C
Conversion /% 2.9 Production distribution(%): CH 90.0 3OOOH 3 2/3 OH traces 3OOOH 1/300 1/30°2
124°C
135°C
175°0
3.9
9.0
22.0
90.0
85.3
65.0
traces
2.6
2.0
traces
3.0
12.1
30.0
10.0
10.0
at around 400°0. The selectivity to acrylic acid is 28.4% at 400°0 and 42.6% at 480°0. At 460°0, the selectivities to acrolein and acrylic acid increase with Po H and decrease with increasing Po • Growing con3 6 2 centrations of water vapour decrease the selectivity to acrolein and enhance the selectivity to acrylic acid. The temperature dependences of product distribution for other molybdenum oxide catalysts are similar. Thus, with the molybdate of iron (or Or or Mo) at low temperatures, acetone is formed /98/, at 360°-390 00, the oxidation of C 6 leads to acrolein /104, 105/ 3H and to small amounts of acrylic acid /19/. Table 86 shows that the molybdates of the II group metals give (at 350 00) the same products as with the Sn0 2-MoO catalysts. As seen from Fig. 176, the selecJ tiVity of the above molybdates for "allylic" oxidation decreases with an increase in their overall actiVity. The Sb 20 - !ili)03 systems, like Bi 20 Mo0 exhibit a high se4 3, lectivity towards acrolein at 380°-480 0 /101/. The Sb : Mo ratio affects the catalytic properties slightly (the exception being a sharp increase in specific rates on going from Mo : Sb = 1 : 9 to
6
-
557
.5,% 100
r·/(7 8
72,_ 6*f~ 28
75
/
50 50 20
25
0 100 I
0
!
20 JTl/% I I 80 Mo/% 0
I
Fig. 175 - The rates of formation of different products (a) and selectivities (b) in the oxidation of C 6 over Sn0 2-Mo0 at 3 3H 430 0C: a) 1 - acrolein, 2 - CO 2, 3 - acids, 4 - CO; b) 1 - partial oxidation products, 2 - acrolein, 3 - CO 2' 4 - acids, 5 - co /100/. Fig. 176 - A comparison of the specific catalytic activity and selectivity towards "allylic" oxidation of C 6 (experimental data 3H of reference /106/): 1 - BeO-Mo0 2 - ZnMo0 3 - BaMo0 4 3, 4, 4, Mo0 5 - MgMo0 6 - CaMo0 7 - CdMo0 3, 4' 4, 4• The selectivity towards CH 2 = CHCHO + CH is 80-90~. Sb 20 4). 3CHO These catalysts involve phases of Sb 20 Mo0 and antimony molyb4, 3 date. Their catalytic properties are similar to those of Mo0 /101/. 3 /101/. The complex oxide catalysts, Cu-Mo-O and Sn-Sb-Mo-O, are also rather active and selective /107/. At 360 0C, the overall activity of the molybdates decreases in the sequence: Bi 2(MoO 4)3 /" Fe 2(MoO 4)3/ CoMoO 4-;>PbMoO4. The selectivi ty decreases in the order: Bi 2 (MoO 4) 3"7 CoMoO 4;> Fe 2 (MoO 4) 3;> PbMo0 4 /104, 105/. The V20 S - MoO] /19/ and P20 S - Mo0 /19, 9S/ systems have also 3 been studied. The addition of V20 and P to Mo0 enhances its 20 S S 3 activity and decreases its selectivity for the formation of acrolein. A catalyst containing S-10% ABSOS' 10% Nb 20 and 20% MoO], supS
BeO'Mo0 3 MgrdoO 4 CaMo0 4 ZnMo0 4 SrMo0 4 CdMo° 4 BaMo° 4
Catalysts
0.4 20.2 71.9 46.7 64.5 8703 15.3
> 0
ri ri CIl H Ql
Specif~c
s::!o ,,-l CIl
~H
s::! -r-l Ql ri 00
0 0.4 0.4 0.4 0.4 1.4 0.8
0 ,,-l r-l o l»'d ~ H',-l s::!oo ,,-l CIl CIl
5.6 2.5 1.2
7.1
4.6
2.1
0
s::!o ,,-l CIl
~Ql
O~
a> s::! 0
0 0 10.7 0 16.5 0 0
.t'
I ria> CIl'd o ~ ~a> s::!OQl ,,-l CIl'd Ql',-l s::loo ,,-l CIl CIl
0 5.4 7.0 8.7 4.7 1.6 3.7
~
O~'d
0 ..-t C\l s::lO ,,-lO
0 10.8 42.8 15.5 31.8 76.0 7.3
~
0
' Immol m-2-1 rate of propy1 ene converS10n s
0.4 1.5 6.37 15.5 5.5 5.8 203
,
9.5 34.1 9.2 8.2 20.3
9.1
100
Selectivity to acrolein + acrylic acidl %
The Oxidation of Propylene Over Molybdates of the II Group Metals (350 0C; Contact Time 0.8 s) 1106/.
TABLE 86
00
on on
559
ported on 3i0 2, provides a 50~ yield of acrylic acid at 400°C /108/. Systems based on W0.3 are also rather selective (see Table 87). P additives decrease the activity and increase the selectivity 205 of W0.3 /19/. Bismuth tungstates (Table 87) are similar to bismuth molybdates (Table 82). The activity of Bi 20.3'2W0.3 is close to that of Bi 20j .2Mo0.3 but the latter is more selective. High activity is found TABLE 87 The Oxidation of C.3H6 Over Bi 20 Contact Time 0.8 s; J-W0.3(4800C; Reaction Mixture is: C.3H6 - 11.2, 02 - 16.8, H20 - 4 •.3, N2 - 67 • .3 vol. %) /19/. 10- 2k/ min- 1 m-2 Selectivity to acro- CO 2 CO CH.3 CHO Over- acrolein lein all /% process
Catalyst
Bulk composition after catalysis
Bi 2°.3'5 W0.3
W0.3 + oc-phaee
4.6
1.5 1.0
0 • .3
7.4
62.0
Bi 2°.3'JWO J
ex -phase
5.9
2.8 0.8
0.2
9.7
61.0
Bi 2°.3' 2W0.3 (preheated at 600 0C) Bi 20.3' 2W0.3 ex: - + ! -phase
7 • .3 6.2
2.9 0.9 .3.1 1.4
0.2 0.2
11 • .3 10.9
64.6 57.0
!
5 • .3
.3.4 2 •.3
11.0
48.0
Bi 2°.3'W0.3
-phase
+ W0.3
with Bi-V-O-catalysts but they are poorly selective /19/. The addition of P20 to V20 results in effects which are similar to 5 5 those in the W-P-O systems /19/. Some data have been published concerning the salts of transition metals and anions such as phosphate, arsenate, ant imonate, sulfate /19/. At .370 0C, the phosphates of Co and Fe are poorly active and do not produce acrylic acid. With iron phosphate, carbonyl compounds are formed while the phosphate of Co only catalyses deep oxidation. A rather high selectivity for the "allylic" oxidation of propylene is exhibited by some complex catalysts which do not contain the oxides of transition metals. Usually, these involve the
560
elements of the main subgr-oups IV and V of the Periodic Table (Sn, Sb and Bi). The latter can change valency easily. Propylene is oxidized over the Sn-Sb-O catalysts at 380 0-480 oC into aldehydes and CO 2, the selectivity to aldehydes being 93-95%, to acrolein 70-95% /109/. The catalytic properties of these systems (except regions of composition near Sb 20 and Sn0 2) are al4 most independent of the Sn : Sb ratio /109, 110/. The above catalysts consist of solid solutions; at rather high Sb contents, two phases (antimony oxide and solid solution) are present. The former phase is assumed to affect slightly the catalytic behaviour of the complex catalyst /109/. According to reference /110/,the rate equation is:
r=kP
%Iia
B
(XVII.73)
where 8 is determined by the conditions of adsorption equilibrium on a homogeneous surface. With Bi 20 - Sb 20 catalysts, when Bi : Sb ~ 1, only deep oxi4 3 dation takes place. Sb-enriched catalysts are poorly active but highly selective towards acrolein /111/. Over a Bi - Sn catalyst (Bi : Sn = 1), propylene is oxidized only into CO 2 and H20, which is quite different from the similar system Sb - Sn /101/. The oxidation of propylene on Bi-P-O catalysts /63, 111/ at elevated temperatures gives CO 2, acrolein and benzene as the predominant products; small amounts of CO, CH and 1,5-hexadiene 3CHO are also formed. Thus, the catalyst accelerates the oxidative dimerization of C 6 (reactions (XVII.46) and (XVII.47)). Fig. 177 3H shows that the highest rates of the production of benzene correspond to Bi : P = 2, the selectivity being 40%. The ratio Bi : P = = 1 is the most favourable for acrolein formation. The catalyst with the ratio Bi : P = 2 is a mixture of the phases BiP0 4, 2Bi203oP205 and !-Bi 20 while the sample with Bi : P = 1 is a 3, monazite-type phase (BiP0 Deep oxidation is accounted for main4). ly by 0" -Bi 20 /111/. 3 Benzene (with acrolein) is also formed on other salts of bismuth in a similar way /111/. Thus, with Bi 20 - Sb 20 and Bi 20 34 3 - Sn0 2 below 500 0C, the products are CO 2 and acrolein, while benzene appears at elevated temperatures. It is interesting that the last product is not formed on Bi-molybdates (see Table 88).
°
°
5
[Bi: P = 2]
Bi 2(Mo04)3 [Bi : Mo =
2/~
Bi(Bi)(Mo0 4)2 [Bi : Mo = 1]
~ 5.9
23.3
37.7
0
0
0
9.1
43.0
BiP0
4 (BiO)2Mo04 [Bi : Mo =
10.1
26.9
33.8
49.0
61.0
79.8
69.0
80.4
18.0
(high-temperature)
P20
45.7
4
4
3'
-
94.9
91.7
66.1
38.6
4.0
0.3
6.6
° 5.8
Selectivity 1 10 to benzene to acrolein
Bi 20 . 2Ti0 2 3 (BiO)2 S04
BiP0
BiAs0
2Bi 20
Catalyst
Conversion of C3H61 %
The Oxidation of Propylene Over Bismuth Salts 1631
TABLE 88
en 0)
......
562
o
x/% 40
/
.
0/% 20
JO
15
20
III
/0
5
0
---
--
The conversions of propylene into different products Fig. 177 over Bi 20 - P205 at 500 0C /111/: 1 - CO 2, 2 - C6H6, 33 CH 2=CHCHO. Fig. 178 - Conversions (1) and selectivities to C6H6 (2) and acrolein (3) for the oxidation of C 6 over Sn0 2 with additives of 3H P205 and Na20 (550°C) /63/. The promotion of Sn0 2 with Na20 enhances the selectivity to benzene while P 20 additives favour the formation of acrolein 5 (Fig. 178). Mo0 and other anhydrides act like P20 /63/. 5 3 Complex catalysts based on As20 (As-Cu-O, As-Ca-o, As-Cd-O, 5 As-Bi-O, As-Fe-O, etc.) display high (above 60%) selectivity to acrolein (410 0-4800C, 14-18% C 6 in air) /112/. 3H Thus, the following empirical relationships can be formulated for the oxidation of C 6 over complex oxide catalysts: 3H 1. The most pronounced changes in catalytic properties on going from the individual oxides to complex oxide systems are usually connected with the deep chemical interactions within these systems, reB~lting in the formation of new chemical compounds (daltonides) or solid solutions (bertholides). Their quantitative (sometimes qUilitative) characteristics differ significantly from those of mechanical mixtures. Nevertheless, the following rule is likely to be valid: at least one of the elements in a selective complex catalyst corresponds to a simple oxide which is also selective. For example, molybdates contain Mo (MoO] is a selective
563
°
catalyst), the Bi - Sb systems include Sb (Sb 20 is a selec4 tive catalyst) and so on. On the other hand, the spinels Co~~-O and ~m-Co-O, accelerating only the oxidation of C 6 to CO 2, con3H tain Co and Mn which give typical catalysts for deep oxidation and Mn0 2 ) . (C0 304 A catalyst is often a heterophase system (solid solution + chemical compound). This characteristic complicates the comparison of catalytic and physicochemical properties since each phase can be considered as a catalyst with its own special behaviour. The above correlation should be made for homogeneous solid systems. TABLE 89
The Catalytic Properties of Complex and Simple Oxides in the Oxidation of Propylene (450 oC; Reaction Mixture is: C 6 - 11, 3H 02 - 11, N2 - 78 vol. %) /101/ Catalytic activity.1 0 7 / mol C -2 6 m s-1 3H
Catalyst
Bi 20 3 SnO 2 Sb 20 4 Mo0 3 Bi-Mo-O Sn-Mo-O Sn-Sb-O Bi-Sb-O Mo-Sb-O Bi-Sn-O
(Bi (Sn (Sn (Bi (Mo (Bi
Mo Mo Sb Sb Sb Sn
1)
=1 =9 =9 = =
1) 1)
3) 1 3.5 : 6.5) 1) 1
.
11.65 6.80 0.13 0.46 21.64 13.88 1.40 0.24 0.64 2.26
Selectivityj)G to acro- to all the products lein of partial oxidation
°
19 87 76 86 67 87 79 81 4
°
26 87 87 94 79 94 81 86 7
2. Selective complex catalysts include at least one element with an alternating valency; this element is not necessarily a transition metal. (It can be Bi, Sb, Sn, etc.). 3.Intbeoverall oxidation process, a selective complex catalyst is either poorly active or (like the molybdates of Bi and Sn) is moderately active. Table 89 indicates that the activities of Bi 20 and Sn-Mo-O (Sn : Mo = 9) are close to those of 3n0 2 3'2Mo0 3 and Bi 20 Antimony-containing catalysts are similar (as to their 3•
564
activity) to the poorly active oxides Mo0 or Sb 20 3 4• 4. Complex catalysts allow us to carry out all the thermodynamically permitted reactions (XVII.39)-(XVII.47). The enhanced selectivity to a definite product is attained by the choice of an appropriate catalyst and reaction conditions. Thus, with Bi 20 ' J '2Mo0J' acrolein is a main product; with C0 - hlo0J' acetone is 304 formed at low temperatures; and so on. Such catalysts as Sn0 2 Mo0 give the possibility of realizing almost all the reaction rou3 tes. At low temperatures (up to 280 0C), acetone is a major product; at higher temperatures, CH CH and, especially, acrolein 3CHO 3COOH, become the predominant products. The formation of acrolein proceeds rapidly at 400 0C, when acrylic acid also appears. All the reactions are accompanied by oxidation into CO and CO 2, At elevated temperatures (above 5000C), some catalysts selectively accelerate the oxidative dimerization of propylene (into C6 products). These catalysts (Bi-P-O and others) do not contain transition metals. 5. Complex catalysts are more effective for mild oxidation than simple oxides. The latter do not allow us to attain a combination of a high selectivity to acrolein with a rather high ~ctivity as is observed with the molybdates of bismuth or tin. Acetone can also be produced (with high selectivity) only on complex systems such as Sn-Mo-O. The selectivity of the Bi-P-O and other complex catalysts for the formation of benzene is much higher than that of the best individual oxides. 0-/ V-a <1-2 ~-7 Y-J e-8 0-4- ()-!l
S/% 100
\11
<1
"* -5 ·-/0
o
o
o
*
10
/5
Fig. 119 - A correlation between activity and selectivity of Sn-Mo-o (1), Sn-Sb-o (2), Sb-Mo-O (3), Bi 20 (4), Bi-Sn-O 3'2Mo0 3 (5), Bi-Sb-O (6), Sb 20 (7), Mo0 (8), Bi 20 (9), Sn0 2 (10) for 4 3 3 the oxidation of C 6 at 450 0C (using the data of reference /101/). 3H
565
s/% 100
04-
.5'/% 75
f
IS
0" 00
5'0
50
10 25
/
25
o
J 0'-----'-----'-----"---'"
25
50
75'
Il
X/%
o
/00
50
III
/5 t;r-/O
Fig. 181 - Selectivity plotted against activity for the oxidation of C 6 over molybdates 3H (using the data of reference /105/): 1 - CoMo0 , 2 - PbMo0 , 4 4 3 - Fe 2(Mo04)3' 4 - Bi2(Mo04 )3.
0-1
.-2
0
0.5'
Fig. 180 - Correlation between activity and selectivity (X is the conversion of C H6 /107/) 3 for: 1 - M00 3-1, 2 - 1140°3-11, 3 - Sn-Sb-Mo-O, 4 - Bi-Mo-O, 5 - Bi-W-O, 6 - Cu-Mo-O, 7 - Co-Mo-O, 8 - Mn0 2 •
¥%
o
2
0
Fig. 182 - Activity-selectivity plots for BiMo (2) and Bi - W (2) oxide catalysts (using the data of reference /19/) •
0
25
"
0
s/%
4
Fig. 183 - Activity-selectivity plots for tungsten (441 0 C ) and molybdenum (5600 C ) oxide catalysts for the oxidation of C3H 6: 1 - W0 + 43% P, 3 2 - W0 + 20% P, 3 - W0 3 3, 5 - 11400 + 20% ~ 4 - 11400 3, 3 6 - 114003 + 43% P (using the data of reference
/19/).
566
r
s/%
to-/!;,mzote: 0'#r/l
(j
75 50
25
cm
'!1li /l
{[.toj!-,mO{fCCJ#6 (m 2
[
4-
2
2
/.0 0.8 0.4-
1.0 2 0.0 0/; 1.0 0.8 0.2 0.4
0.2
OJ
at
o.Of 0./
If
0.2
0 50
/00.v%
Fig. 184 - A comparison of activities of Bi-oxide catalysts with their selectivities towards acrolein (1) and benzene (2) (see Table 88).
MgO 20
40
00 81l C"O/at %
Fig. 185 - The rates of acrolein formation (1), adsorption of C H6 in a reversible form 3 (2) and intensity of f -allylyc complex absorption band (3) against catalyst composition /113/.
6. From the point of view of the theoretical considerations given in Chapter XIV, it is interesting to examine relations between activity, r, and selectivity, S. As it follows from Figs 179184 (constructed using data from the literature), there is, in general, a trend of decreasing S with increasing r. The following feature is noteworthy: the best catalysts for mild oxidation (Bimolybdate and tungstate and Sn-molybdate) deviate sharply from these correlations. 7. The activation energies for mild oxidation (for the formation of acrolein) are usually lower than those for deep oxidation (for the formation) of CO and CO 2). The data which are necessary for formulating the reaction mechanism and explaining the above regularities will now be discussed. The formation of an allylic complex as intermediate in catalysis on the Bi-Mo-o /82, 83, 107/, Bi-Mo-O /113/ and Sn-Sb-O /110/ systems has been shown by the tracer method. The results coincide with those obtained on CU 20, suggesting a similarity between the reaction mechanisms on simple and complex oxide catalysts. Investigations of the thermodesorption of propylene adsorbed on magnesium molybdate together with the IR-spectra of adsorbed spe-
567
cies /106/ have resulted in conclusions which are very similar to those obtained with simple oxides. Weakly adsorbed propylene, in which the ~-C-C-bond is retained, gives allylic complexes (and a J.{-complex) and is oxidized into acrolein while strongly adsorbed propylene, giving surface carbonate-carboxylate species, is converted into CO 2 and H20. On a CUO-MgO catalyst (Fig. 185), the rate of formation of acrolein correlates with the amounts of propylene adsorbed in a weak form and with intensity of the IR-band for the Jr-allylic complex (1440 cm- 1) /113/. The participation of surface 0 2- ions in mild oxidation is indicated by the equality of the rates of catalysis and surface reduction; the last result was obtained with the molybdates of Bi, Co, Fe and Pb /104, 105/. Experiments using 0 18 /114/ show fast diffusion of 0 2- from the bulk of "the bismuth molybdate to the surface; these anions thus participate in catalysis. The same oxygen species probably also participate in deep oxidation since, according to references /104/ and /106/, the rates of catalysis and surface reduction do not differ more than 2-3 times in this case. Adsorption meas~ements on the catalysts for mild oxidation (Sn-8b-o, Sn-Mo-o and Bi-Mo-O) and on deep oxidation catalysts (Cu-Mn-O and Cu-Cr-O) show that oxygen-hydrocarbon surface complexes in the first case contain less oxygen than in the second case /19, 115, 116/. The complexes leading to mild oxidation are charged positively (and the complexes leading to deep oxidation are charged negatively) since propylene is an electron donor while 02 is an electron acceptor. The results discussed suggest that the mechanism of the oxidation tion of C 6 on simple and complex oxide catalysts is similar. 3H Therefore, Scheme (XVII.52) can be used to interpret the experimental data. New facts found with complex catalysts allow us to expand the above scheme (for parallel routes):
568
(XVII. 74)
According to Scheme (XVII.74), propylene can react by three main pathways. The first one involves the rupture of the C-H-bond in the methyl group and the formation of the intermediate allylic complex, (1 1 ) ; the latter is oxidized on the surface into the species (Ii) which lead to acrolein; surface oxidation of (Ii) results in the complex (I~') (probably of the acrylate-type) which is converted into acrylic acid. The interaction of the allylic complex with propylene gives the species (1~") leading to the products of oxidative dimerization. The second route involves rupture of the jf -C-C-bond in propylene, with the formation of the species (1 2) leading to CH?CHO, CH and carbon oxides. The complexes (I;), (I~I) and (I; ') 3COOH are of the carbonate-carboxylate type. Lastly, the third direction, including the intermediate hydration of olefin /56, 98/, results in the formation of acetone. (This process will be discussed later). The rate of the overall reaction will be:
r
r
r: -:r: +r 2
J
't
where r 2 and r are determined by the equations: 3
569
If, in the hydration
of the olefin, the equilibrium:
with the constant Khydr • is established,
~
= kif
j(
byar.
P
liz 0
P
Cj 110
8 '" k rtf p
~
0
p
~ 1-
8
(LVII. 75)
Then: 2 ff/ 8 r= ~ Pc, fI. 8 +- ~ Pc. II 8 -I- K ~ 0 fi.. Ii .
"3'0
'36
(LVII. 76)
'1!c
At elevated temperatures, acetone is not formed
(r4~r2
+ r
J) and Eq, (XVII.68) is valid. At 8 =1, 'sq. (XVII.58) should be observed, which turns out to be so in the case of Bi-molybdate /92-94/. At 8 < 1, accorJing to Eqs (XVII.58) and (XVII.59), the reaction products are expected to inhibit the process which also takes place /93, 94/. At low temperatures on Sn-Mo-O or Co-Mo-O, acetone is the main product, deep oxidation being a side reaction. Under these conditions (r 2«r + r 4): J
r=(k38+-k"/~O)~1I '2
'3
a
8.
(X-VII.77)
Since at 8 ~ 1 in the steps determining the values of r2' rJ and r (and consequently r), oxygen-catalyst bonds are broKen, 4 a decrease in r should be expected with increasing qs. Such a relationship is shown in Fig. 186. Under conditions where acetone is not produced, one should expect (according to reference /86/), an increase in the selectivity to acrolein with increase in qs. This correlation is observed (Fig. 186) which agrees with Scheme (XVII.74). The reason for this is that in the steps leading to acrolein, fewer oxygen-catalyst bonds are broken than in the deep oxidation of propylene. The same
570
reason /86/ results in hiGher activation ener:;ies for the formation of CO and CO 2 in comparison with those for the formation of acrolein.
x/%
..y'%
/00
100
2
15
15
o
50
30
25
25
o........
_ _-J...,
~_..L.-
to
l5 {[
10.1
20
0 1.0
TrK
Fig. 186 - The relationship between the catalytic activities of complex oxide catalysts for the oxidation of C 6 (a), their seJH lectivities to acrolein (b) and oxygen-catalyst bond strength (Tr is an absolute temperature to attain equal reduction rates) /107/: 1 - Mn0 2' 2 - Bi-Mo-O, J - Bi-W-O, 4 - MoOJ-II, 5 - Mo0 2-I, 6 - CU-Mo-o, 7 - Sn-Sb-lVlo-o, 8 - Co-iVIo-o.
The inverse relation between the activities and selectivities (Figs 176, 179, 180 and 182-184) should be attributed to the above dependences between qs' and rand S. The correlations discussed are approximate. A distinct exception is bismuth molybdate (Figs. 180-182 and 186) which is the best catalyst for the oxidation of C 6 to acrolein. The sume JH concerns Sn0 2-MoO (Sn : Idn = 9) which is also an active and seJ lective catalyst (Fig. 179). These exceptions are important and will be discussed latter. Scheme (XVII.74) suggests that under conditions when CH 2=CHCHO, CH 2=CHCOOH and deep oxidation products are formed predominantly, the selectivities to acrolein and acrylic acid (3 acr. ,S a.a. ) will be given by:
(XVII.78)
571
5a.a
U/Ya J
2
+
(XVII.79)
k B J
1)
where k 2 is the rate constant for the desoprtion of (1 with the formation of acrolein while (k;)* is that for surface oxidation of (1~) into (1 leading to acrylic acid. The above equations show that the values of B determine the selectivities to acrolein and acrylic acid. Using the tracer method with a Sn-Mo-O catalyst /56/, it was shown that in the low-temperature oxidation of propylene into acetone, the source of oxygen inserted into propylene is water and not surface oxygen. This result indicates /56/ the importance of the preliminary hydration of C 6 with the formation of adsorbed 3H isopropanol, i.e. the complex (I in Scheme (XV11.74) is J) (CH Acetone is formed by the further oxidative dehyd3CH(OH)CH3). rogenation of the above complex. This conclusion is supported by the fact that acetone is obtained in the oxidation of isopropanol over Sn0 2 - MoO with high J rates and selectivities under the same conditions /98/. A similar result has been observed with co - ~bOJ' It should be noted J0 4 that small amounts of isopropanol are detected in the products of the oxidation of C 6 over these catalysts /56/. JH The hydration of propylene is assumed /56/ to involve a carbonium ion as an intermediate, with the subsequent addition of the OH-group. Since the alcohol is oxidized into acetone more slowly than is the olefin, the rate-determining step is considered to be hydration /98/. We believe that the hydration is a fast and equilibrium process while the oxidation of the alcohol is a rate-limiting step. Such a model leads to EQ. (XVII.75). In this equation, the constant k"' is equal to k4Khydr. The value of Khydr• (equilibrium constant for hydration) decreases with a rise in temperature which leads to decreasing rates of the formation of acetone. Higher rates of the alcohol oxidation (in comparison with those of olefins) and the small amounts of isopropanol detected in the products of the oxidation of C 6 can be interpreted in the following way. JH rhe Jesorption of the species (1 (i.e. adsorbed CHJCHOHCH)pro3) ceeds more slowly than the oxidation of (I In the presence of J). large amounts of alcohol, it is rapidly adsorbed, so the surface
1' ),
572
coverage by (1 8 , increases significantly. Since the rate 3 3), of the formation of acetone is proportional to 8 CH is 3, 3COCH3 produced from isopropanol with higher rates than from propylene. Eq. (XV1I.75) accounts for the observed changes in the reaction orders and accounts for the correlation of m and n with r (Fig. 172). Since the mechanism discussed invloves the hydration step, there is a correlation between the acid-base properties and the rate of formation of the acetone. The latter increases with the concentration of acidic sites on the Sn0 2-Mo0 catalysts /56/. 3 Acid-base properties are also important in the oxidative dimerization of propylene /63/. According to reference /117/, one should also expect an influence of these properties on the formation of acrylic acid since the complex (1~) is likely to be a saltlike compound. Thus, in the selective oxidation of propylene other characterictics of catalysts (besides qs) are of great significance. The bond energy of surface oxygen, qs' is of primary importance, as follows from Scheme (XVI1.74) and from the above correlations between qs and the values of r, S, m and n. At the same time, however, sharp and significant deviations (towards increased activity and selectivity) are observed for the best catalysts of mild oxidation (Bi 20 Sn0 2 - Mo0 (9:1), etc.). It is interesting that in 3, 3 3 the case of the individual metal oxides, the Ig r va qs and Si vs qs correlations (Fig. 171) do not include the results for ZnO which is the beat catalyst (among these simple oxides) for the oxidative dimerization of propylene. Complex catalysts for mild oxidation exhibit a very high selectivity for many products (acetone, acrylic acid and benzene) which cannot be attained with the simple oxides. This fundamental fact cannot be explained fully if qs is considered to be the only factor determining catalytic properties, because the values of qs lie within the region of the bond energies of the simple oxides. The necessity to consider other factors besides qs follows directly from mechanism (XVII.74). Changes in the rate constant k 2, which determines the rate of the oxidation of C;H6 to CH 2=CHCHO, depend (according to the Bronsted - Temkin relation) upon the heat change, q2' of the step for the formation of the allylic complex, (1 1), The value of q2 involves qs and the allyl-catalyst bond energy, qall' The assumption of 02Mo0
573
the constancy of qall' which was used when we deduced the Ig r vs qs or Si vs qs correlations, is unlikely to be valid in general case if the chemical natures of the catalysts compared are greatly changed. The bond energy of a catalyst with the allyl radical and the reactivity of the latter should depend on the chemical structure of the solids. There is probably an optimal value of qall corresponding to such systems as Bi-Mo-O or Sn-Mo-O which exhibit elevated selectivity towards acrolein. The complex (1 2), leading to deep oxidation, involves C-O-catalyst (or, maybe, H-O-catalyst) bonds. Their energies are likely to be little changed so that the heat change for the step
depends mainly upon the values of qs. The above considerations can be regarded as a definition of the concept /118/ on the "specific" interaction of the oxidized molecule with a catalyst which is essential for mild (but not for deep) oxidation. The validity of such a "specific" interaction is demonstrated /104, 105/ by elevated ratios of the rates of the mild oxidation of propylene to the rates of the oxidation of CO or CH 4 over bismuth molybdate, in contrast to the similar results for lead molybdate. According to references /93/, /104/ and /105/, the allylic radical is formed on Bi 3+ ions in catalysis over Bi-Mo-oxide systems; this radical then interacts with oxygen attached to Mo 6+ resulting in the formation of acrolein. The following scheme for the step C 6 + (0) ---- (1 1 ) in mecha3H nism (XVII.74) can be imagined: 0 2-
<, Me n+
m+/
+ Mel
./
~02-/ CH2.~
-
OH -. .-.. '. 'Mem+.~ 1
C~2
0 2-
OH-
+
I
Me(n-1)+ II
II
--
574
The electron from propylene passes to the Mef~ cation, reducing its degree of oxidation. The Me~~ cation promotes the rupture of the C-H-bond in the allylic position and binds the allylic radical by the ff-bond. It follows from such a model that one active site of the catalyst should contain two cations, One of them, Me~~, must be reduced rather easily by accepting (n-1 )+ an electron from the propylene; the reduced form, Mil ' must 6 be readily oxidized by 02' The Me~~ ion is M0 + must activate the C-H-allylic bond in the propylene The Me~+ion and bind the allylic radical with a moderate bond energy, so that its sUbsequent interaction with surface oxygen will proceed rather fast. The Me~+ cation is Bi 3+. The rates of the redox process
correlate with the values of qs since the redox potential is involved in the value of qs. The optimal values of qs can be estimated, as was shown in Chapter XIV. The Me I I species must be an element with alternating valencies but is not necessarily a transi tion metal. Experimental data show that the best Me~+ species are Bi 3+ and Sn4+, i.e. cations of nontransition metals which belong to Group IV and V of the Periodic Table and can change their valency. Transition metal ions are apparently less suitable as Me~+ since unpaired d-electrons cause too strong binding of the allylic complex. It is difficult at present to define those properties of Me~+ which determine the value of q2 and the reactivity of the allylic complexes. As a rough approximation, the electronegativity, X, can be used to indicate the ability to withdraw electrons from the allyl species. The limitation should be added that Mem+should not be a transition metal. For Bi 3+, ~ = 1.8 /119/. Value~ of ;t 2+, 2+, 4+, 3+, 3+, from 1.7 to 1.9 are peculiar to Cd sn sn Tl Sb In 3+ and Cu+. Let us take Me n+ to be Mo 6+. For the oxidation of C3H 6 to acrolein, the systems Sn-Mo-O, Sb-Mo-O and CU-Mo-O, like Bi-Mo-O, are highly selective, but the selectivity of Cd molybdate is low. Other properties (besides qs) can be also important in other steps in the reaction. We have noted that acid-base properties
Catalyst amount /g
228.0
221.4
0.9611 0.3138 0.5643 0.5761 0.6575 0.0792 0.0759
0.5132
Ag(l) - Y 300
Ni (II) - Y 300 0.9155
135.3 221.4 157.0 157.0 232.0 288.0 300.0
Flow rate/ ml min- 1
200 250 280 300 320 310 310
cu (II) - Y
t/oC
18.1
19.0
22.5 18.1 22.0 22.0 17 .3 17 .4 3.0
C 6 3H
28.0
28.4
37.0 28.0 36.4 36.4 26.8 17.4 50.0
O2
-
45.3
43.7
-
34.0 45.2 29.3 29.3 47.4
H2O
8.7
8.9
6.5 8.7 12.3 12.3 8.5 65.2 47.0
N2
Initial mixture / vol. %
The Oxidation of Propylene on Metal-Y-Zeolites /121/
TABLE 90
2.1
20.1
1.11 19.5 23.0 21.9 24.7 0.83 3.9
CO 2
2.67
0.52 6.32 6.67 7.20 6.12
acrolein
Yield / %
0.58
isopropanol
CJ'
on
...,
576
are of great significance in the formation of acetone or CH 2=CH-COOH. One can suppose /117/ that amphoteric nature is preferable in the last case since it permits rather easy formation and decomposition of the acrylate complexes. According to reference /63/, if the allyl species is attached to a site with high acidity, electrons are shifted to this site; the allyl species is charged positively, favouring its reaction with 0 2-. If the active site has weak acidity, the radical nature of the allyl species is preserved which favours dimerization. The evidence for this is shown in Fig. 178. The different dependences of the selectivities to acrolein and to C6 products upon the acidity accounts for the inverse correlation (Fig. 184). Complex catalysts provide a greater probability of attaining the optimal Me~+-Me~! combination than simple oxides and this explains the higher selectivities of the former types of catalysts. In the case of Cu20, the Cu+ ion corresponds to Me~+ while n+ Me ~ MJn-1)+ corresponds to Cu2+ + e~Cu+. It is inteII II resting that the electronegativity of Cu+ is equal to that of Bi 3+ /119/ which causes increased selectivities of the Cu and 20 Bi-Mo-O catalysts towards acrolein. The Oxidation of Propylene Over Zeolites Y type molecular sieves containing Cu (II) actively catalyse the oxidation of C 6 at temperatures which are close to those for 3H the case of CuO. The Cu (II) - Y zeolite is not destroyed during catalysis /120, 121/. Table 90 shows that in the presence of water, propylene is oxidized into acrolein and CO 2• At low temperature (200°C), small amounts of isopropanol are formed which indicates that the hydration of propylene takes place. Acrolein is also produced on the Ag(I) - Y zeolite while Ni(II)Y accelerates deep oxidation (Table 90). The data obtained so far are insQfficient for more detailed discussion of the mechanisms of the oxidation of C 6 over zeolites. 3H The Oxidation of the Butenes The 04 olefins exist in the form of several isomers: but-1-ene; but-2-ene, which can be in cis- or trans-form; and isobutene
5'77
(2-methylprop-1-ene). In the absence of a catulyst, isomcrisation proceeds slowly; many solids accelerate these reactions. For example, but-1-ene (in tb the presence of air) is rapidly transformed at 500°C into but-2-ene over the oxides of V, Ni, Mo, W, Zn, Sb and P /122/. This process is catalysed at 425°C by complex oxides: Bi-Mo-o, Fe-Mo-O and Bi-Fe-Mo-O /123/; the isomerisation occurs over Sn0 2-Mo0 at 135°3 210°C, over c0 0 -Mo0 at 240°-270°C /124/; V-Mo-o catalysts acce3 4 3 lerate the reaction at 350°C /125/, and so on. These facts should be taken into account in discussing the mechanism of an oxidation reaction which proceeds simultaneously with isomerisation. There are many thermodynamically permitted oxidation reactions of the butenes. The main ones are:
H 2C=CH-GH2CH 3
(XVII.80)
CH3-GH=CH-GH3
(XVII.81) (XVII.82)
(XVII.85) (XVII.86) H2C=CH-GH2CH + 1 3
i
02
= HC-CH 11 I
+ 2H 20
(XVII.87)
HC CH
\/
°
H2C=CHCH2CH + 202 =CH + CO 2 + H20 3COCH3 3
(XVII.88) (XVII.89)
H2C=CHCH2CH 3 + 2 CH
3CH=CHCH3
i 02 =
Cil5 COOH + CO 2 + H20
1 + 2 "2 02 === C2H + CO 2 + H20 5COOH
(XVII. 90) (XVII.91 )
578
(XVII.92 ) (XVII.93)
(XVII.94)
o 3° 2 ,",
HC-C" 1\
,o
"0+3H 20
(XVII.95 )
HC-C""'"
(XVII.96) (XVII. 97 ) (XVII.98) (XVII. 99) (XVII. 100 ) (XVII. 101 ) Among the above reactions, processes (XVII.80), (XVII.81), (XVII.86), (XVII.94) and (XVII.95) leading to divinyl, methacrolein and maleic anhydride are of the greatest practical importance. The synthesis of divinyl (the monomer employed for the production of synthetic rubber) is carried out over bismuth-molybdenum oxide catalysts /14, 126/. These catalysts are also effective for the oxidation of isobutene to methacrolein, used as a monomer in polymerization and co-polymerization. Previously, copper oxide catalysts were employed for the selective oxidation of isobutene and but-1-ene /14/.
579
Complex vanadium oxide catalysts, as well as Co-molybdate, catalyse the oxidation of normal butenes into maleic anhydride /95/.
The Oxidation of Butenes Over Metal Catalysts The oxidation of butene on metals has not been adequately studied. On platinum films, isobutene can be converted into CO 2 and H20 at 80 0C /7/. Kinetic characteristics of the compltee oxidation of isobutene over Pt and Pd supported on SiC /46/ are given in Table 91. First (or near to first) order in oxYgen and retardation by the hydrocarbon are observed. This is evidence for significant adsorption of the isobutene and weak adsorption of O2 on the above metals during the course of catalysis. TABLE 91 The Oxidation of Isobutene on Metals (C 8 - 2, 02 - 50, N2 4H 48 vol.%) /46/; r is in mol m- 2 s -1 at 300 0C
Catalyst
Temperatures/oC
19 r
Ig r o
E/ Reaction order kcal mol- 1 in C H in O 2 4 8
Pt
92-118
5.58
- 0.41
Pd
132-153
10.14
- 0.22
Over silver at 200°C, deep oxidation products from the butenes predominate, the mild oxidation products being produced in trace amounts. The oxidation of isobutene leads to small amounts of acetone /127/. The Oxidation of
Butenes Over Simple Oxide Catalysts
The oxidation of but-1-ene was studied /122/ at 300 0-5500C and with a ratio of C 8 : air = 0.5 over supported oxides. The high4H est activity is exhibited by the oxides of metals with alternating valency: Ni, Co, Sn, Ag, Mn and CU; less active catalysts are the oxides of Bi, In, V and Cr. The oxides of Zn, Cd, La, Pb, Wand Mo and those of the elements of the main sUbgroups of the Periodic
580
Table (1i, K, Cs, Be, Mg, Ca, Sr, Ba, B, AI, Ga, P and Sb) are relatively inactive. TABLE 92
The Oxidation of Isobutene Over Metal Oxides (C 8 - 2, 02 - 50, 4H N - 48 vol. %) /46/; r is in mol m- 2 s-1 at 300°C 2
Catalyst
Mn0 2 C0 304 cr 20 3 Oe02 NiO Fe 20 3 Th0 2
Temperatures /oC 193-238 220-262 242-270 207-250 263-305 270-312 273-312
E
Ig r
19 r o
/kcal mol- 1
-6.01 -6.30 -6.64 -7.37 -7.45 -7.66 -8.37
5.88 8.35 5.15 2.71 2.02 6.56 -2.72
36.4 38.4 30.9 23.8 24.8 37.3 14.8
Reaction order in C 8 in °2 4H
-0.25 -0.09 0.25 1.00 0.15 0.21
0.54 0.75 0.47 0.00 0.55 0.60
The nonstoichiometric oxides, MoOx (where x = 2.65-2.89), are also relatively inactive /128/. The specific activity of the metal oxides decreases in the order: Pe 20 Bi /123/. The acti3>Mo03> 20 3 vation energies for oxidation of but-1-ene have been measured for many oxides /129/. In the oxidation of isobutene (Table 92), the activity decreases in the sequence: Mn02>C0304>cr203>oe02/NiO>Fe203/Th02. The most active metals (Pt and Pd) exhibit 3-4 orders of magnitude higher activities than those of Mn0 2 or 00 (compare Tables 91 3°4 and 92). On most of the simple oxides, the mild oxidation of but-1-ene leads to divinyl. At 500°0, the selectivity of the oxides of Mn, Sn, Cr, Bi and In exceeds 30%; the oxides of Ni, Co, V and Cu are less selective /122/. Copper oxide is a selective catalyst for tIle partial oxidation of the butenes. Over ou 20, the normal butenes are converted mainly into methylvinylketone, while isobutene gives methacrolein /14, 27, 130/. At 375°-400°C, the yield of the mild oxidation products on copper oxide decreases in the order: methylvinylketones > ace-
581
taldehyde> acrolein> divinyl> crotonic aldehyde /131/. Divinyl (a product of oxidative dehydrogenation) is formed on Cu20 only at high concentrations of 02. The catalyst accelerates the isomerization of but-1-ene into but-2-ene /131/. At 350 0-3S00C on Cu20 (with 02 : C S = 0.5), the selectivity 4H to methacrolein approaches 60%; addition of water vapour enhances the selectivity (attaining a level of SO% at 340°C) /27/. In the presence of V20 at 350 0C, but-1-ene and but-2-ene 5/Al 20 3 are oxidized into acetic acid, maleic anhydride, methylvinylketone, acetaldehyde, formaldehyde, CO and CO 2; the yield of maleic anhydride increases with the 02 : C S ratio. The oxidation of iso4H butene at 370 0-390 0C results in acetic acid (3070) and methylacrolein (12/0 /127/. According to reference /132/, the n-butenes are oxidized over V 20 into divinyl, maleic anhydride, furan, acetic 5 acid and acetaldehyde. The conversion of isobutene to maleic anhydride is improbable since it demands skeletal isomerization. The selectivity to divinyl formed from the n-butenes over Mo oxides decreases in the order: M020 > Jilo0 2/, Mo0 / lVl0 20 'rhe reac5 3 3• tion mixture greatly affects chemical composition of the above oxides /133/. This phenomenon is also found with many other systems. Under steady-state-conditions during the oxidation of but-1-ene, ferric oxide is converted into Fe The selectivity to divinyl 304• (at 365°C) is 6570. The overall conversion decreases and the selectiVity increases on removing oxygen from the Fe 20 /134/. 3 Thus, the distribution of the products during the oxidation of butene depends on its structure, catalysts and the reaction conditions. On catalysts for the mild oxidation of but-1-ene, "a Ll.y Ld.c " hydrogen (in the ~-position to the C=C-bond) participates in the reaction. Over Cu20-catalysts, an unsaturated ketone (methyl-vinylketone) is formed, i.e. oxidative dehydrogenation takes place. For V-O-catalysts, the production of maleic anhydride (and furan) is peculiar. The oxidation of but-2-ene over Cu20 leads also to unsaturated carbonyl compounds (methylvinylketone, crotonic aldehyde) while on vanadium oxide catalysts, maleic anhydride is formed. The similarity of the reaction products obtained from but-1-ene and but-2-ene is likely to be due to the fast isomerization of the latter olefin. The "allylic" oxidation of isobutene to butadiene or maleic anhydride would require skeletal izomerization~ In this case, only u.nsaturated carbonyl compound, o<-methylacrolein, is formed (on
582
cu20-containing catalysts). Mild oxidation over most of the catalysts studied is accompanied by destructive oxidation (to acetic or acrylic acid, acetaldehyde, acrolein, formaldehyde, etc.) and by deep oxidation. In general, a parallel-consecutive scheme is accepted; at low temperatures, it reduces to a parallel one /14, 27, 125/. The overall process kinetics on a range of sirnple oxides can be described by the equation:
m
n
r= k~
II Po .
and n
= 1.
(AVII.102)
f 8 '2 The reaction orders are given in Table 92. For PbO /135/, m = 0
The following empirical rate equations have been obtained for the oxidation of isobutene over Cu20 (330 0-365 0 C ) /27/:
(jJ/ pal 1
q,
C'flle
r = --=-----:.---=.--m !+~(Pefl.+Puo.J '1 '8 /'2
J.:fpa8
(XVII.103)
»"
r = "2 0 C'I fh d Irb.?
(XVII. 1 04)
-p ~o
where r m is the rate of mild oxidation (to methylacrolein); r d is that for deep oxidation; k 1, k 2, b i and b 2 are constants. The rates of the oxidation of but-1-ene to each product (methylvinylke tone , acrolein, crotonic aldehyde, acetaldehyde and CO 2 ) on CU 20 at 375 0 C obey Eq. (XVII.102) with m = 0-0.2 and n = 0.9-1.0 (only for divinyl, m = - 0.5). Hence, the total rate is practically independent of Pc H and is directly proportional to Po /131/. The oxidation 4 8 2 of but-1-ene to methylvinylketone is iW1ibited by the reaction products. The rates of the surface reduction of Fe with but-1-ene 304 (365°C), of catalyst reoxidation and of catalysis under steady-state conditions are close; the selectivities for reduction and catalysis are also equal. The mechanism of the alternating reductionoxidation process with the participation of atomic oxygen anions, . der1ved . / 0 2 - , 1S from the above data / 134. At high concentrations of O2, the steady-otate catalyst is
583
OC-Fe 20 In this case, the selectivity for reduction exceeds 3• considerably that for catalysis, and the reoxidation rate is lower than that for catalysis. 'rhis SUG6c.;Sts that a different mechanism is valid under these conditions /134/. Adsorption and infrared studies /136/ showed that but-1-ene is adsorbed irreversibly on CuO (a deep oxidation catalyst) with the formation of the carbonates and carboxylates of copper. The adsorption of but-1-ene on CU 2 0 (a mild oxidation catalyst) is mainly reversible. In this case, the C=C-bond is preserved and the C=C-bond appears. The above results are similar to those obtained with propylene, indicating that a scheme of type (XVII. 53) is valid for the mechanism of the oxidation of butene. In the case of isobutene, the scheme will be: ---.. 2(0)
fast
(0 )
fast ... C4H6 0 + H2 0 10(0)
fast
,. 4CO
2
+ 4H 0 2
(XVII. 105 )
The intermediate (1 1 ) is associated with the allylic complex: CH I 3 CH 2:-;-:-;- C:7:"CH2 /777777777
while the (1 2 ) species are associated with:
yH 3 CH -C-CH 3 I I 2
o
0
1//7 171 771
whf.cn precede the formation of carbonate-carboxylate species. The
584
corresponding overall rate equ.ations is: (XVII. 106)
while the rates of the formation of methacrolein and CO 2, r(I)and r (II), will be:
kf p-
r
(f)
70
=
(/-8)
P "C'f flo 0 el( 110 0
1+0.
+
Sri)
(XVII. 107)
bII 0 PI/" 0 '2
'2
and
r
(ff )
kf Po (1- B)
=
JI
'2
P + b P Zfh6 0 C'!I10 0 If 0 /(ZO
1+ ()
(Ii )
S
(XVII.108)
where b i are adsorption coefficients, 8 is surface coverage by oxygen and SCI) and S(II) are selectivities. The selectivity to methacrolein is given by:
With 8«1, Eq. (XVII.107) reduces to
(XVIL110 )
The experiments of reference /271 were made under conditions when the selectivity changes slightly, and Eq. (XVII.110) almost coincides (in its form) with Eq. (XVII.103) obtained for Cu20. According to Eq. (XVII.109), the selectivity increases with
585
decreasing
{1. Since the value of
8
decreases when PH G Lnc.cea2
ses (see Eqs (XVII.65) and (XVII. 66 », one should expect an enhancement in SCI) with inc~ease in PH 0' as is actually observed
/27/.
2
Power rate laws of type (XVII.102) are likely to be approximations of Eq , (XVII.106) (at 17,7 biP i). I f this is so, ac co r-Id.ng to reference /43/, relations (XVII.67) should be valid. (In this case, 8' = 0 and 1 = 0). As a result, one should expect opposing changes in m and n as observed (Fig. 187). When 8:::. 0, Vie have: m ~ 0 and n ~ 1; this situation is realized in the oxidation of isobutene on PbO. Because m increases (n decreases) with increasing 8, one should expect a correlation of the orders with qs' which is observed in practice /46/. Scheme (XVII.105) and Eq.(XVII.106) predict a dependence of the catalytic activity, r, upon qs. Fig. 188 shows that the correlation
m
tfr
/.0
-2
/ 1:::.2 I-6--l
-4 -5
6
o
6
-8 -/0 0.5
!.On
Fig. 187 - A comparison of the reaction orders /46/ for isobutene (m) and oxygen (n): 1 - Ce02, 2 - cr 20 J - Fe 20 3, 3, 4 - NiO, 5 - C0 0 4 , 6 - PbO, 3 7 - Mn02·
I
20
40
60 IJs/Kcfl/(g--atOr
Fig. 188 - The correlation of Ig r with qs for the oxidation of isobutene (rate data of reference /46/): 1 - Pt, 2 - Pd~ J - NiO, 4 - Mn0 2' 5 - c0 30 4 , b - Cr 2 0 J , 7 - Fe 20 , 8 - Ce02• J
is valid. The much higher activity of the transition metals in comparison with the metal oxides (Fig. 188) is likely to be caused by
586
elevated rates of activation of the olefin on the metal surface, which is free of oxygen. Thus, 0cheme (AVII.105) reflects the important features of the mechanism of the oxidation of isobutene. A similar scheme can be assumed for the oxidation of n-butene. The rate equation observed in the case of Cu20 /131/, as well as the correlation of E with qs /129/, are in accordance with this assumption. A scheme of type (XVII.105) is simplified; in particular, i t describes the formation of only one mild oxidation product. It can be expanded (for example, in the case of the oxidation of but-1ene) using the following asslli~ptions: 1. The olefin molecule, before its reaction with surface oxygen, is adsorbed weakly and reversibly on the catalyst. During this process, the molecule is polarized; a labile J'!-complex is also formed /131/. Since coverage of the surface with the olefin is low (B R = bIlR), the reaction kinetics will be similar to those when C S reacts from the gaseous state. 4H 2. Two main reaction routes are realized, each of them involving a chain of consecutive transformations on the surface. The first one (" allylic") includes cleavage of the C-H-bond in the allylic position and results in C products (divinyl, methylvinyl4 ketone, crotonic aldehyde, furan and maleic anhydride) /137/. The second route involves rupture of the J.r-C-C-bond in the olefin and the formation of intermediate carbonate-carboxylate complexes, leading to destructive and deep oxidation (acids and aldehydes (C C and carbon oxides). 3- 1) 3. The interaction of weakly adsorbed but-1-ene with (0) results in a methylallylic radical and OH-groups:
'I'he last step in the inverse direotion leads to the isomerization of but-1-ene into but-2-ene /137/. The expanded soheme oan be expressed as follows:
587
In the first route, the methylallylic complex can react with (0) in two ways, leading either to divinyl or methylvinylketone. Low values of qs are likely to favour O-addition (i.e. ketone formation) but this is not the only factor since a range of other oxides, besides Cu20, with low values of qs (ldn02, C0 liiO and 304, Ag20) produce divinyl. The part of Scheme (XVII.111) reflecting the formation of furan and maleic anhydride is based on the data of references /125/, /130/ and /132/ according to which these products are generated from n-butene and divinyl. Divinyl, furan and maleic anhydride can also undergo further oxidation. According to reference /136/, the adsorption of methylvinylketone on Cu20 results in carbonate-carboxylate intermediates which lead to deep oxidation; this effect is also taken into, account in Scheme (XVII.111). The precise structure of the majority of the surface intermediates participating in reaction mechardsm (XVII.111) is not clear. Nevertheless, one can believe that: the adsorbed olefin is positively polarized; the methylallylic complex is attached to the metal ca tion; the complex (1m) is likel y to be a surface rnalea te; and the (0) species are negetively charged. The rate equation corresponding to Scheme (XVII.111) will coincide with Eq. (XVII.106), referring to Scheme (XVII.105). Scheme (XVII.111) suggests that the selectivity will be deter-
588
mined not only by the oxygen-catalyst bond energy, but also by the catalyst-methylallyl bond energy, qall' and by the surface acidity (in the case of formation of maleic anhydride). The electron work function of the catalyst /129/ or its activity in isomerization (migration of double bond) /122/ have been proposed as an approximate measure of qall. However, this problem has not been solved, and this makes prediction of selectivity difficult, in particular, it is not clear why methylvinylketone is formed on CU 20 while divinyl is produced on .1<'e 20 and ina Lei,c an3 hydride is formed on V20 5• It is nevertheless possible to explain changes in the activity and selectivity of a given oxide when the value of & varies. ThUS, a decrease in 8 leads to an increasing selectivity to divinyl and a decreasing activity in catalysis over Fe 20 /134/. Ac3 cording to Eq. (XVII.109), one should expect increasing selectivity with decreasing values of 8 as is observed in practice. I'he reason for this is that the second route (leading to deep oxidation) requires more (0) atoms than the first route (leading to mild oxidation). The effect is enhanced by increasing values of qs with decreasing values of 8. 'rhe overall rate equation will be:
(XVII.112 ) from which it is seen that r decreases with deurease in 8. This decrease should be also intensified by a decrease in k 2 and k with 3 increasing values of qs. It should be taken into consideration that under certain conditions quite a different mechanism is probable. According to reference /134/, the catalytic oxidation of but-1-ene over Fe 20 at 3 high concentrations of 02 procee~by way of the interaction of 02 with adsorbed olefin. The Oxidation of Butenes Over Complex Oxide Catalysts Complex systems are more effective for the mild oxidation of butenes than simple oxides. 'rhe selectivity of Cu20 in the oxidation of n-butene to methylvinylketone can be increased by the addition of the oxides of Mo or W /130/. However, different catalysts are of major importance in the mild oxidation of the butenes.
589
fABLE 93 The Oxidation of Butenes Over Bi-Mo-O at 460°C /139/
Oxidized molecule
But - 1 - ene
Conversion /%
20 40 80
But-2-ene
20 40 80
Isobutene
10 40 70
Selectivity / %: to divinyl to methacrolein 95 95 90 90 90 85 72 72 72
High activity and selectivity are exhibited by Bi-Mo-o catalysts above 400°C /14, 139, 140/; these are materials which are inindustrial importance. Some data for a Bi~~o-O catalyst with a Bi : Mo ratio of 1.0 are represented in Table 93. The oxidation of but-1-ene is accompanied by its isomerization into but-2-ene /140/. By-products in the case of the n-butenes are CH CH 2= 3CHO, =CH-CHO, furan, methylvinylketone and carbon oxides. The selectivity towards divinyl passes through a maximum at 450 0-550 0C /140/. A bismuth-molybdenum oxide catalyst is less active than a copper oxide one. The former operates at higher temperatures and with 02 in excess (cu 20 - with the olefin in excess). Over Bi-Mo-o, the main product of the oxidation of n-butene is divinyl while over Cu20 the product is methylvinylketone. The total selectivity for mild oxidation is higher in the case of Bi-Mo-o (Table 94). Since the oxidative dehydrogenation of isobutene is complicated by its structure, this olefin is converted into methacrolein over Cu20 and Bi-Mo-o. The transformation of n-C S-0 2 mixtures into di4H vinyl is oxidative dehydrogenation (H2 is not present in the products) and not dehydrogenation followed by oxidation of the H /140, 2 When Bi 20 is added to Mo0 the optimum yield of divinyl corre3 3, sponds to a Bi:Mo ratio of approximately 1.0. Molybdena is poorly
1.56
0.78
0.78
400 425 450
400 425 450
Bi-I>4o-0 (Bi : Mo = 1, Ssp= 0.46 m2 .g-1 )
Sn - Sb - 0 (Sn : Sb = 4, S = 22 m2.g -1 ) sp
p..,
0
en ~
C\I
350 375 400
-P
<,
~
0
Cu20 on carrier (Ssp=1.32 m2 g-1)
Catalyst
0 0
77.9 87.7 92 .3
78.6 85.1 89.1
22.2 41.7 47.5
4.8 3.3 2.9
4.9 3.1 2.5
7.1 3.5 2.8
-P'
~C\I
ttl ,
~~ q, 0 0
~
o:l
0 0..1
•
6.5 4.3 2.9
6.7 4.2 4.2
15.5 9.5 7.9
o
°M ,
I -P ::l .oQ) Iq mQ)
C\I
50.5 49.6 48.8
48.3 46.6 47.7
4.0 3.0 3.6
on> 'd
°M
~
rl
I Q)
0 0 3.7 4.7 5.1 4.4 5.1 4.5
0 0.6 0.8 0 0 0 0 0 0
1.3 0.7 0.5
3.9 7.0 2.2
~Ef 0
'H
H ::l
ell
20.2 22.] 21.3
q
I rl ttl QQ) O'd -P>" 0.J:l HQ) tl'd
>0 rl-P >"Q) .J:l~ -Prl
0..1 q
Selectivity (%) to I -P ::l .aI Q) o:lq q o
!=l
The Oxidation of But-1-ene Over Cu20, Bi-Mo-O and Sn-Sb-O /131/
TABLE 94
19.8 20.0 18.3
16.3 18.5 21.0
0 10.2 10.6
ell
g~
2.7 3.0 2.9
5.8 6.1
3.2 9.5 9.3
o
s:l Q) rl 0 H
°M
I Q) 'd rl a:l -PQ) Q)'d
C\I
10.0 13.0 19.2
11.2 10.1 11.2
50.0 41.4 43.7
0 0
0
01 CD
3
3
= 1) (Bi : Mo = j) (Bi : Mo = 1) 2 (Fe : Mo = 3) (Fe: Mo = 1)
~~
- 0 (Bi:Fe:Mo
= 1:1:3) = 1:1:2) 81
44
10
12
21
18
0.0
4.1
0.0
8.9
3.4
12
40
43
50
57
10
2.2
13
30
1
0.0
0.6
0.0
1.6
5.5
6.5
4.0
5.0
0.3
1.3
3
k
k2
k1
2
129.9
90.4
72
82
33.2
49.6
5.5
11.2
72.4
48.7
13.9
14.6
6303
36.3
0.0
35.6
0.0
19.7
6.6 4.0
S/%
k
to divinyl **) Data for 365°C
3
3;
*) k i are first order specific rate constants: k for oxidative dehydrogenation, 1 k 2 for deep oxidation and k for isomerization; k = k + k + k S is the selectivity
Bi : Fe : Mo - 0 (Bi:Fe:Mo
Bi - Fe -
Fe - Mo - 0
Fe - Mo - 0
Bi - Mo - 0
Bi - lIlo - 0
Fe 0 **) 2 3 Bi - Fe - 0 (Bi : Fe
Bi 20
Mo0
Catalyst
Oxidation of But-1-ene Over Catalysts Containing Mo, Bi and Fe *) (425°C, C 8 : air = 1 : 5) /123/ 4H
TABLE 95
~
'" ,...
592
active and catalyses mainly the isomerization of but-1-ene; Bi 20 J is also poorly active and accelerates deep oxidation (Table 95). The chemical compounds Bi 20 and Bi 20 exhibit low acJ'MoOJ 3'JMoOJ tivity /12J, 128, 141/. Systems with a Bi : Mo ratio close to 1 contain significant amounts of a phase of Bi 20 but this compound itself shows 3-2MoO J low activity. Increasing the concentration of MoO in the Bi-Mo-o J system leads to higher rates of the isomerization of n-butene (C=C-bond shift). The ratio of Ci8- to trans-isomers of but-2-ene in the course of the reaction is close to unity /141/. The oxidation of but-2-ene over Bi-Mo-o (Bi : Mo ~ 1) proceeds like the oxidation of but-1-ene /131/. In some cases, Bi-Mo-o catalysts are promoted, for example, by the oxides of P or Fe. Bi-Mo-o catalysts with the additives of P 20 catalyse the oxidative dehydrogenation of n-butenes at 420 0 550 /1J3/ and the oxidation of isobutene into methacrolein /142/. In the last case, the selectivity to acrolein is 60-75~ at 5JOoC (i - C 8 : air: H20 = 1 : 4.8 : 1) and at 40-50% conversion /142/, 4H Addition of iron oxide to Bi-Mo-o increases the activity in the oxidation of but-1-ene to divinyl (Table 95). The optimal compositions for the catalyst without a carrier is Bi : Fe : Mo = 1 : 1:2 and for the supported catalyst is Bi: Fe : Mo = 2 : 1 : 1. The Bi - Fe - Mo catalysts do not contain the Bi 20 phase. J'2Mo0 J At the composition Bi : Fe : Mo = 1 : 1 : 2, a Bi 20 phase J.]MoOJ 3 was formed in which Bi + was partially substituted for the Fe J +. These catalysts accelerate actively the isomerization of but-1-ene /12J/. Additives of BiP0 and Cr 20 (10 mol.%) also promote Bi 20 4 J J' .]MoOJ /14J/. The Bi - Fe - Mo systems can also be considered as a product of the substitution of Fe J + for Bi J + in ferric molybdate. Fe - Mo catalysts are more active but less selective than Bi - Mo ones. The former predominantly accelerate the isomerization of n-butene. The penetration of Bi J + enhances the activity and (mainly) the selectivity to divinyl /123/. Tellurium oxide (up to 30 at. %Te) promotes the selectivity of Fe - Mo to methacrolein formed from isobutene. At the composition Fe : Te : Mo = 1 : 0.85 : 1 and at 400°-420°0 (C 8 : air: 4H H20 = 1 : 1.5 1), the selectivity is 10-80% at a conversion of 50-60% /1J8/. The Fe - Bi system is relatively inactive ana catalyses only deep oxidation (Table 95) but a combination of the oxides of
3C
°
°
°
°
°
°
593
Fe and Sb results in a selective catalyst for the oxidation of n-butene into divinyl. At 425 0C and with Fe : Sb = 1 : 2, the selectivity attains a level of 94-95% /144/. The behaviour of the Sn-Sb-O system is similar to that of the Bi-Mo-O system (Table 94). The properties of various catalysts based on the oxides of ~o and Sb are presented in Table 96. The specific rates of the oxidative dehydrogenation of n-butene over Fe-Sb-O, Co-Sb-O and Sn-Sb-O decrease with increase in the Sb oxide content /129/. TABLE 96
The Selectivity to Divinyl in the Oxidation of But-1-ene at 400°C (1 % C4H8' 0.5% 2 ) /45/.
°
0 I
0 I
~
i'
,:Q
;z,
~0
'(
0
0
Catalyst
-.-/
Phase composition Surface areal m2 g-1 Selectivity/ %
Bi 2(Mo04)3
Q)
tX--CoMo0 Fe 2(Mo0 4)3 4
1.8 98
0
7.3 20 ()OOoC )
0 I
0 I
,0 IZl
,0
<"'\
I
7
0
IZl
;z,
;z,
i:l
Solid solution
11.0
76.0
76
97
Q)
-
(\J Q)
tX.-Fe 20 3
18.0 96
25 (365°C)
In the oxidation of but-1-ene over molybdates of the alkaline earth metals and Zn and Cd, the selectivity to divinyl is low ( ( 15%). Acetic and acrylic acids are formed together with CO 2 and the isomerization of but-1-ene takes place. The specific activity at 350°C decreases in the sequence: BeO.Mo0 3>SrMo04>MgMo04> >BaMo0 CaMo0 ZnMo0 while the selectivity for oxida4>CdM00 4> 4> 4, tive dehydrogenation decreases in the order: SrMo0 > MgMo0 BeO. 4 4> MoO 3 > CaMoO4> CdMo0 4 > BaMo0 ZnMo0 4 /106/. 4 The conversion of n-butene into divinyl, furan and maleic anhydride is typical of complex oxide catalysts based on v20 • 5 The V-P-O and V~o-O systems are effective catalysts for the synthesis of maleic anhydride from n-butenes at elevated tempe-
>
594
ratures. The cis- to trans-butene ratio in catalysis on V-i.lo-'ri-O is close to the equilibrium value /95/. The optimal V : P ratio is 1 : (1.6 - 1.2); at 450 0C, the yield of maleic anhydride reaches 75 wt.% /95/. Additives of P205 to Y205 decrease the activity and increase the selectivity (from 20 to 42%) /132/. Small amounts of 3i0 2 added to the V - P - 0 system increases the selectivity to maleic anhydride sharply (from 52 to 72 mol. % at 400°-440°C) /146/. The activity of the Y20 - Mo0 systems for the oxidation of 5 3 but-2-ene (350°C) passes through a maximum at 20 at.% Mo. The oxidation is accompanied by the isomerization of cis-but-2-ene into trans-but-2-ene and into but-1-ene. With catalysts containing less than 20% Mo, the selectivity towards maleic anhydride is low ( < 20%) and depends slightly on the MoO) content. Catalysts enriched by Mo show a high selectivity (up to 24%) independent of the Mo0 content. Thus, the highest yield of maleic anhydride is at3 tained on the most active catalyst (20% Mo). The yields of divinyl and furan are lower than those on Y205-P205' The selectivity towards the destructive oxidation products (CH)CHO and CH is 3COOH) ~ 36% and is almost independent of the Y : Mo ratio. At high conversions, the yields 9f CO and CO 2 increase /125/. In the oxidation of but-1-ene over Y20 - Ti0 2 (400°C, excess 5 of air and conversions <, 30%), the selectivity to C products pas4 ses through two maxima (at 20 and 65% Ti0 2) and one minimum (at 35% Ti0 2). The main contribution to C is given by maleic anhyd4 ride, the optimum selectivity being 47-48% and the minimum 25%. X-ray analysis shows only phases of Y20 and Ti0 2; at high Ti con5 centrations,the anatase phase includes y4+ ions. The activity of the Fe - Mo - 0 system for the oxidation of n-butene to maleic anhydride passes through a maximum at low concentrations of iron /129/. The oxidation of butenes over Sn0 2 - Mo0 and 00)0 - Mo0 at 3 4 3 low temperatures ( < 300 00) exhibits a special feature (Tables 97-99). But-1-ene gives significant amounts of the saturated aldehyde, methylethylketone. Acetic acid is also formed. Simultane'ously, the isomerization of but-1-ene to but-2-ene takes place. A similar picture is observed with but-2-ene. In comparison with 3n0 2 - MoO), the co)04 - MoO) catalyst produces less ketone but more CH)COOH. The interaction of isobutene with 02 over Sn0 2-Mo0 3 results in poor yields of the mild oxidation products but leads to a high selectivity towards the hydration product (tert-butanol at
595
90°-100°C) and to diisobutene (at 140 0-1S0 0C). On c0 0 -Mo0 , 3 4 3 methacrolein is mainly formed, acetone and acetic acid also being produced /124/. At elevated temperatures, the oxidation of the n-butenes over Co-Mo-O results in maleic anhydride /95/, while Sn-Mo-O produces divinyl /122/. A high selectivity in the oxidation of but-1-ene to acetic and acrylic acids (70~ at 300°C) is obtained on Mg-molybda te /106) • A rather high yield of divinyl from but-1-ene at 450 0-5500C was obtained on W- or V-containing catalysts (Bi-W-O, Sn-W-O, In-W-O and In-V-O) /122/. Complex chromium oxide catalysts accelerate deep oxidation. Thus, barium chromate at 200°-400°C actively catalyses the full combustion of isobutene /147/; deep oxidation of n-butenes proceeds over Co-Cr-O /95/. In general, one can notice a close similarity of the catalytic properties of complex oxide catalysts in the oxidation of propylene and butenes. The richer spectrum of mild oxidation prOducts formed in the latter case is accounted for by the more complicated structure of the butenes. The oxidative dehydrogenation of propylene is impossible while the n-butenes can be converted not only into unsaturated carbonyl compounds but also into divinyl (and furan or maleic anhydride). The oxidation of n-butene is accompanied by i8Omerizations. At rather high temperatures, the oxidation of butene proceeds by parallel-consecutive schemes. Thus, for the oxidation of n-butene over Bi-Mo-O, the following scheme is discussed:
C4HS
k
1. C H
______
~~
4 6
k~C02~
k
2
.. furan
~
k
5
(k i are the first order rate constants) where k k2 + k
k
1
4
= 0.05
3/k1
= 0.05 and
/14/. On Fe-Sb-o, the divinyl is further oxidized
into carbon oxides /148/; on V-P-O or V-Mo-O, divinyl and furan are converted into maleic anhydride, etc. /95, 125, 132/. At low temperatures, the contribution of the consecutive routes becomes
3
38.7 2.0 2.9 2.8 8.7 10.6 34.3 31.4
6.1
60.3 3.9 2.3 3.9 5.2 3.6 20.8
28.0
4.8
85.3 traces
16.4
2.2 2.6 traces 9.9
"
8.4
160°C
135°C
180°C
Sn0 2 - Mo0
37.0
24.7 1.4 1.5 2.4 4.9 12.5 52.6
17.5
210°C
I
I
20.8 ***
24.8 16.5 ***
29.2 ***
6.9 24.8 8.7 31.8 7.0 13.6 5.5 29.6 7.2 7.2
-
27.8
5.2
44.3
3.0
1.2
274°C
60.8
255°C
240°C
c030 4 - Mo0 3
*) Without isomerization. **) The percent of but-1-ene isomerized into a given product. ***) Cis- and trans-isomers (mixture).
Conversion 110 Selectivity (10)*) for oxidation into: CH 3COC 2H5 3/4CH 3COCH3 3/4C 2H 5COOH 1/2CH 3CHO 1/2CH 3COOH 1/4CO 1/4C0 2 Selectivity (10) **) for isomerization into cis-but-2-ene
Catalytic properties
The Oxidation of But-1-ene Over Sn02-Mo03 and c030 4 - Mo0 3 (C 8-20, 02 - 30, N2 - 20, H20 - 30 vol. %) 11241 4H
TABLE 97
~
J>
CJl
Selectivity (~) for isomerization into: but-1-ene cis- or trans-but-2-ene
Conversion /% Selectivity for oxidation into: CH)COC 2H5 )/4CH)COCH) )/4C 2H 5COOH 1/2CH 3CHO 1/2CH)COOH 1/4CO 4/4C0 2
58.8 3.1 traces 11.1 12.9 2.1 12.0
60.6 traces
9.7 )1.0
3.6
18.0
"
14.5
)2.8 2.1 traces 7.1 6.8 9.8 40.8
8.)
9.7
38.0 6.5 )0.6
10.4
-
266°C 5.0
cis-isomer Co)04 - MoO) 200°C 20.0
traces
15.2 18.0 traces 6.2
"
155°C 9.2
1)00C 4.0
trans-isomer Sn0 2 - MoO)
The Oxidation of But-2-ene Over Sn0 2 - MoO) and co) - MoO) (C 8 - 20, 02 - )0, N2 - 20, H20 - )0 vol.~) /124/ 4H
TABLE 98
-
traces
8.0
18.) 22.) 9.7 )2.)
17.4 traces
25)oC 4.0
traces
16.9 18.2 6.7 )1.5
-
26.1 traces
2))oC 2.5
9.6
12.1 )5.4 9.1 )0.6
-
-
12.9
275°C 7.0
trans-isomer Co)04 - MoO)
-
traces
2.0
9.7 30.4 10.) )2.3
14.) ).0
210°C 6.1
...J
'" '"
Conversion /% Selectivity (%) for oxidation into CH)C(CH 3)20H CH 2=C(CH))CHO C8H16 )/4CH)COCH) 1/2CH)CHO 1/2CH 3COOH 1/4CO 1/4C0 2
Catalytic properties
17.4
traces
-
-
-
82.6
100
-
4.8
105°C
2.7
90°C
2.5
71.9 2.1 3.5 0.7
-
19.)
7.2
1)6°C
Sn0 2 - MoO)
65.4 5.8 2.6 1.3 3.7 15.0
-
7.1
7.5
178°C
The Oxidation of Isobutene Over Sn0 2-MoO) and Co)04 - MoO) (C 8 - 20, 02 - )0, N2 - 20, H20 - )0 vol.%) /124/ 4H
TABLE 99
47.2 8.] 2.0 1.7 9.8 31.0
11.) 5.5 8.5 24.9
5.1 9.) 21.5 5.7 5.4 21.4
49.8
5.0
264°C
16.2
traces 47.9
3.8
254°C
co)04-MoO)
15.9
I traces 51.6
traces
I
1.9
I
10.0
-
2)2 0C
I
195°C
I
4.6 8.1 25.0
11.8
50.5
8.5
280°C
00
ss:
(J'
599
less significant. The overall rate at rather high concentrations of O2 is given by Eq. (XVII.102) with m = 1 and n = O. This was found to be so for Bi - Mo - 0 /129, 139, 140/, Bi - Mo - P - 0 /133) and Co - Mo - 0 /129/. In some cases, m < 1 and n) 0: for Bi - Fe 0 - Mo - 0, m = 0.7 /127/; for Fe - Sb - 0 and Sn - Sb - 0, n /131/. The process on Bi - Mo - 0 is not retarded by the reaction products /139, 140/ while on Fe - Sb - 0 the equation:
>
(XVII.113)
was obtained for the oxidation of but-1-ene. Similar inhibition was found on Fe - Te - Mo - 0 /138/. The selectivity to divinyl in the oxidative dehydrogenation of n-butenes on Fe - Sb - 0 /144/ and Bi - Mo - P - 0 /133/ increases with decreasing Po and the ratio Po fPC H in the reaction 2 2 4 8 mixture. The mechanism of alternating surface reduction-repxidation with participation of (0) anions has been proved to apply for Bi - Mo - 0 /128/, Bi - Mo - P - 0 /133/ and Fe - Sb - 0 /144/. Work function and electrical conductivity measurements for Bi - Fe - Mo - 0 suggest that there is a negative charge on the adsorbed oxygen and a positive one on the adsorbed n-butene /123/. Thermodesorption and infrared studies of but-1-ene on Mg-molybdate /106/ show that there are two major forms of adsorption. The first (weak and reversible) is attributed to f-complexes and Jr-allylic complexes of butenes and the second one (strong and irreversible) to carboxylate - carbonate species. The former species lead to mild oxidation while the latter result in deep oxidation as on Cu20 catalysts. The chemisorption of but-1-ene on a partially reduced V - P - 0 catalyst /136/ leads to a significant loosening of C=C and C-H bonds and to the formation of C=O-bonds. This can be accounted for by the ability of the V - P - 0 catalyst (in contrast to Cu20)
600
to accelerate the oxidation of n-butene into maleic anhydride. The above results suggest that Scheme (XVII.111) is valid for complex catalysts as well. The rate data are in agreement with the scheme mentioned. The equations of type (XVII.102) are approximations of Eq. (XVII.112) which corresponds to Scheme (XVII.111). Eq. (XVII.11) can be deduced on the basis of the following reaction mechanism /14S/: 1) C
4HS
+ (0)-(C
4H sO)
2) (C HSO)-(C H6 ) + H20 + ( C 4 4 fast 4H 6
4) C + (0) ~ 4H6
5) (C4H60 6)(
(C
(XVII.114)
4H60)
+x2(0) ' 1 2C02 +
)+02-(02)
i
Y2 H20 + (x 2 + 1) (
)
+(}2(0) fast
Scheme (XVII.114) is a particular case of Scheme (XVII.111). Parallel and consecutive formation of CO 2 is here taken into account. Eqs (XVII.109) and (XVII.112) predict a decrease 1n r and an increase in selectivity with decrease in 67 (and Po fPC H ). 2 4 S These relationships observed on Fe - Sb (Fig. 189) and other complex catalysts /145/, can be also due to energetic heterogeneity of the surface oxygen. The latter effect is caused by the fact that more oxygen-eatalyst bonds are broken in deep oxidation than in mild oxidation (leading to an increase in S with increase in qs); simultaneously, the overall reaction rate, r, decreases with increase ill qs. The low-temperature oxidation of butenes over Sn02 - MoO) and co)04 - MoO) /124/ gives evidence for an additional reaction route leading to the formation of saturated carbonyl compounds. The mechanism of this reaction /124/ involves the hydration of the olefin followed by the oxidation of a surface alcoholate. There are following facts in favour of the above mechanism.
°
601
s/%
r-!01Z·m-Z.,r!
/00
/0
2
/5 J
2
/
1
20 0
0
10
20
JO
40
SO It
Fig. 189 - The dependence of the overall rate of the oxidation of but-1-ene(1) and the selectivity to divinyl (2) on the number, N, of pulses of C 8 + 4H + 02 + He mixture over Fe - Sb /144/.
°
Secondary alcohols, which can be obtained by hydration of n-butene, are easily oxidized into saturated ketones under conditions of the direct oxidation of the butenes into ketones. The interaction of isobutene with 02 + H20 mixture results in hydration, as the tertiary alcohol formed cannot be converted into the ketone. Molecular hydrogen does not appear in the reaction products; hence, the conversion of the alcohol to the ketone is a true oxidative dehydrogenation. It is supposed /124/ that the hydration of the olefin is slow while the surface oxidation of the alcoholate is fast; in the last process, (0-) or (0 2-) species take part. The C=C - bond of initial butene simultaneously reacts with the surface, resulting in destructive oxidation (the formation of acids, C1 - C aldehydes and carbon oxides). This process oc3 curs more intensively with but-2-ene than with but-1-ene because of its stronger adsorption. In the low temperature oxidation of butenes (as in that of propylene), one can assume that the hydration equilibrium is shifted towards dehydration with increase in temperature. As a result, "allylic" mild oxidation and destructive oxidation become predominant routes at elevated temperatures. A generalized mechanism for the oxidation of but-1-ene taking into account low-temperature transformations, will be:
602
Products at mifd oxidat ion (C4 ) HZC=CH-GH=CH z HC-GH H~- C~OO ~ II II /
t
HC-C-,::-:O
'::I HC,/H
~
t
t
(0) (0) (HzC=CH-CH=CHz)-(If) ..
0/ H3C-CH=CH-CHJ--(Hlk:CH:: CH-CH3)~
(OH)
\
~
H C=CHCCH
I
2
(0) route I
+~
HzC=CH-CHz-CH3 -::::'(H2C=GHCH zCH3)~ (0)
maleate
I
Products ot aestruc) / tire ox!dtItiol7
3
(HlC=CH CH3 0
n
(C ( ) 1- 3 CH CHO CH WOH
route!!
HzO (Hz ~-VH-GHzGH3) route m O O
3
(0) -
eH COCH CH 3 Z 3
(CH3-~H-CH2CHJ)---
t,
3
(0)
t
CD
II
t
2
(I z)_··- (I z)---(COJ )
cartonuote carbo!late comptexes
t
-
(11/1)
(I;)
DH
aiconotate (XVII. 115)
2(OH) H20
~HzO
+ (0) + ( ) O2
+ ( ) ~(H20)
()
+ ( ) -(Oz) t{[st •
2(0)
In catalysis over complex catalysts, it is important to know on which cations the activation of the olefin takes place. For example, on the Bi - Mo - 0 catalyst, Bi J + - M0 6+ doublet is assumed to be the active site (which is why the optimum catalyst corresponds to Bi : Mo z 1) /141/. Some authors suppose that the ~-allylic complex is formed on Mo-ione; Bi J+ is assumed to facilitate surface dehydration since, in the steps leading to divinyl: )
C a + (0) 4H
• (C
4H7)
+ (OH),
) + ( 0 ) - C 6 + (OH) + ( (C 4H1 4H
),
the OH-groups are formed. Thus, the 811y1ic H-atom which is abstracted is thought to be attached to the oxygen of the bismuthylion:
603
In addition, the bismuth (by loosening the Mo - 0 bond) facilitates electron transfer from the olefin to the catalyst (surface reduction). The reduction of both the M0 6 + (to M0 4+ ) and the Bi 3 + (to Bi 2 + ) ions is assumed. FinallY, Bi 3+ ions inhibit the isomerization of the butene by decreasing the OH-- group concentration (since Bi 3+ catalyses surface dehydroxylation) and this enhances the selectivity to divinyl /128, 141/. The partial substitution of Bi 3+ for Fe 3+ ions causes the Mo-O and Mo-olefin bond strengths to approach the optimal values /123/. The results obtained suggest the necessity to revise the above point of view since it is more probable that the Bi ions have a dehydrogenating function. This means that a methyl-allyl radical is formed and localized on the Bi (but not on the Mo) ions; the hydrogen which is abstracted is attached to the Mo-O group. A correlation between the oxygen-catalyst bond energy and the catalytio properties of different complex catalysts is observed /106/ in aocordance with Soheme (XVII.115). The selectivity also increases with increase in qs on each ca'talyst (Sa - Sb - 0, Co - Mo - 0 and Fe - Mo - 0) when surface oxygen is removed. An increase in qs is caused by surfaoe heterogeneity /145/; this effect is partially attributed to decreasing values of 8 itself. Nevertheless, for the above catalysts, which differ significantly in their values of qs' there is no correlation between the seleotivity and the values of qs or the mobility of bulk oxygen /145/, In the oxidation of butene on V - Ti - 0 catalysts, there is no oorrelation between qs and the selectiVity to maleic anhydride /149/. /149/. These results indioate that the surfaoe oxygen bond energy is not the only faotor determining the catalytic properties of complex catalysts. In particular, Soheme (XVII.115) suggests that the olefin-oatalyst bond energy (espeoially, the methallyl-catalyst bond energy, qall) has an important role. We oannot at present estimate the qall values and predict the optimum qall value. It is however noteworthy that most of the active and selective oatalysts for mild oxidation contain ions of the p-elements with alternating valency (Bi, Sn and Sb). The presence of unfilled p-orbitals is likely to correspond to the optimum qall values. In the formation of acidic products (maleio anhydride, etc.),
604
the acid-base properties of catalysts should be also of great importance /117/. They are also significant for the isomerization of n-butene /124/. The reasons for the branching of the first route (Scheme (XVII. 115», leading either to oxidative dehydrogenation (Bi - Me catalyst and other complex catalysts) or to the formation of unsaturated carbonyl compounds (Cu20), are not clear. The lower oxygen bond energy in Cu20 is likely to be favourable for the latter direction.
°
The Oxidation of Butenes Over Molecular Sieves The but-1-ene and but-2-ene adsorbed on molecular sieves can be oxidized at 25 0_900C /150/. The gaseous products are methylethylketone, crotonaldehyde and but-2-en-1-o1. There is thus a similarity between the above process and the low-temperature oxidation of butene on Sn - Mo - 0 and Co - Mo - O,which involves olefin hydration. This is also evidence for the significance of surface acidity in the low-temperature oxidation of olefins. The Oxidation of Higher Olefins The much greater number of possible isomers for the higher olefins results in a significant increase in the number of thermodynamically possible reactions. For the pentenes, typical mild oxidation processes will be: a) in the case of pent-1-ene: H2C=CH-C 7 + "21 02 = CH 3H 3COC 3H7 H2C=CH-C 3H7 + ~ 02 =C 4HgCHO 1
H2C=CH-C 3H7 + '2 02
= H2C=CH-CH=CH-CH3
+ H20
605
b) in the case of pent-2-ene: CH 3CH=CH-C 2H S CH 3CH=CH-C 2HS CH 3CH=CH-C 2H S
1 + 2'
=
02
CH 3CH=CH-CH=CH2
CH + 02 3CH=CH-C 2H S
= CH
CH + 02 3CH=CH-C 2HS
=
CH 3CH=CH-C 2HS
+
1
3CH=CH-COCH3
~
+
+ H20
C + H 20 2H SCH=CH-CHO
02
=
H
2C=CH-CH=CH-CHO
c) in the case of 2-methylbut-1-ene:
r
H3
H2C=C-C 2H S
+
1
H3
1
'2
02
= CHO-CH-C 2HS r H3
rH3 H2C=C-C 2H S
+~
02
= H2C=C-CH=CH2 + H20
?H 3 H
2C=C-C 2HS
?H3
+
02
= H2C=C-CO-CH 3
+ H20
1H3
?H 3
H2C=C-C 2H + 02 = H2C=C-C2H + H2 0 S S
r
H3
H2C=C-C 2H + 1 S
~
02
=
H 20
H2C=9-CH=C3 2 + 2H CHO
20
+
2H
20
606
2-methylbut-2-ene: 1
+ '2 02
CH 3
=
CH
I
3-CH-CO.illi3
CfH3 CH
3-C=CH-CH3 CH I 3
CH 3-C=CH-CH3
The above reactions lead to saturated carbonyl compounds, to dienes and to unsaturated carbonyl compounds in which the C=O-bond is conjugated with the C=C-bond. Normal hexenes can also be oxidized into trienes or benzene (oxidative dehydrocyclization):
The Oxidation of the Higher Olefins Over Metals These reactions have been poorly studied. According to reference /151/, Pt, Ag and Au supported on Al 20 catalyee the complete oxi3 dation of pent-2-ene; platinum and silver are much more active than gold. Platinum on Al 20 (0.5 % Pt) accelerates at 300°-4000C the oxi3 dative dehydrocyclization of hept-2-ene converting into toluene. In the absence of 02' the olefin undergoes dehydrocyclization but the yield of toluene is significantly lower than in the presence of 02 /152/. The Oxidation of the Higher Olefins Over Simple Oxide Catalysts In the complete oxidation of pent-2-ene with excess of 02' the following activity pattern was obtained /151, 153/: C0 ~ Mn 0 "'? 2 3 304 7 Cr 20 7 Ni07W0 Ce02 /" Ti0 2» Fe 20 Al 20 Th02/" CuO ~V205/" Si0 2:::3/ 3? 3 3/" »BeO >Pb MgO?ZnO?Zr027 CaO (Table 100). This order is not 3047
607
precise as the reactor employed was not differential and the surface areas of the catalysts were different. Nevertheless, it 1s clear that the oxides of Co, Mn, Cr and Ni are the most active ones as in the oxidation of the other olefins. Mild oxidation of the pentenes is catalysed by CU20 /154, 155/. On Cu20/SiC at 2800_}600C /155/, pent-1-ene and pent-2-ene are converted into C dienes and d,~-unsaturated C carbonyl compounds 5 5 (pent-1-ene-}-one, pent-2-ene-4-one, pent-3-ene-5-al, pentadienal). Thus, the major direction of mild oxidation is an "allylic" one. The C diene yield is 2-4 times higher than that of 05 carbonyl 5 compounds. Oxidative dehydrogenation is favoured by increasing the 02 : C H10 ratio (from 0.3 to 2) and the temperature (from }20 0 to 5 }60 0C). The destructive oxidation of n-pentene to 04-02 carbonyl compounds takes place at the same time as full oxidation to CO 2 and H20. The overall selectivity to these carbonyl species reaches 10-}0% (at }200-34000). In the oxidation of pent-1-ene, the selectivity decreases in the order: pent-1-ene-3-one '7 pent-2-ene-4-one, aero Lein "7 propional dehyde "7 pentadi enal '> pent - }-ene-5-al; in the oxidation of pent-2-ene, the sequence of selectivities is: acrolein "7 propionaldehyde "7 pent-1-ene-3-one." pentadien,al "7 pent-2-ene-4-one;> pent-}-ene-5-al. By-products are methylethylketone, but-1-ene-4-al, pent-1-ene-5-al, pent-2-ene-5-al. Partial oxidation of branched pentenes (mixtures of 2-methylbut-1-ene, 2-methylbut-2-ene and 3-methylbut-1-ene) over copper oxide catalyst at }140-361 00 /154/ leads to isoprene, acetone, ethylacrolein and methylisopropenylketone. Acrolein, methylketene, 2-methylbut-2-ene-4-al, 2-methylbut-}-ene-1-al are formed in small amounts. Thus, in this case, mild oxidation also leads to 05 products of "allylic" oxidation, oxidative dehydrogenation exceeding the formation of unsaturated carbonyl compounds. Simultaneously, destructive and complete oxidation takes place. It is interesting that isovaleric aldehyde (a saturated carbonyl compound) was also detected in the reaction products (in small amounts). On vanadium pentoxide (supported on pumice) at 2000-40000 /12/, branched pentenes (2-methylbut-2-ene, 2-methylbut-1-ene and 3~e thylbut-1-ene) are converted into acetaldehyde, acetone, 2,}-epoxy-2-methylbutane and 3-methylbutan-2-one. Simultaneously, the isomerization of 2-methylbut-1-ene and }-methylbut-1-ene into 2-methylbut-2-ene occurs. ThUS, among all the possible 05 products
608
of mild oxidation, only 3-methylbutan-2-one is formed. The authors of reference /12/ believe that the reaction is completely heterogeneous. However, it is noteworthy that there is a definite similarity between qualitative product distribution for the oxidation of the pentenes over V20 and that for the gas-phase oxidation 5 without a catalyst. TABLE 100 The Oxidation of Pent-2-ene Over Simple Metal Oxides /151-153/ Specific surface area / m2 g-1
Catalyst
C0
304
~OJ
+ Mn 0
cr20 3 NiO W0 3 Ce02 Ti0 2 0( -Fe 2O J q -Al 2OJ Th02
3 4
1103 3.2 14.0 6.2 3.6 9.J 10.2 13.9 87.6 55.7
Temperature (OC) to attain 80% conversion 180 225 2J5 255 270 275 310 322 JJ2 365
Catalyst
CuO V20 5 Si02 BeO Pb J0 4 MgO ZnO Zr0 2 CaO
Specific surface area / m2 g-1
0.5 1.0 421 0.6 9003 0.5 0.7
'remperature(OC) to attair 80% conversion
387 400 450 485 495 5J2 545 545 560
The data obtained show that the oxidation of pentenes over cu 20 proceeds via a parallel-consecutive scheme /155/. The chemical composition of the catalysts used suggests that a mechanism of type (XVII.115) is valid for the pentene oxidation process. Both the oxidative dehydrogenation and the formation of C unsaturated car5 bonyl compounds are likely to involve allylic intermediates while saturated 0 carbonyl compounds are formed via the hydration of 5 olefins. In reference /12/, the mechanism of alternating surface reduction-reoxidation is postulated. The latter leads to a correlation of activity with qs as shown in Fig. 190. According to reference /62/, the rate of catalysis in the deep oxidation of olefins on O~O at 200°0 exceeds that of reduction
*
f
14.3
10.S
7
8
Hept-1-ene
Oct-1-ene
11.6
1S.5
9
6
Hex-1-ene
12
20.8
6
4
Butene
11.5
rR
30.7
3
Propylene
3
=1.Sn
~=
4.5
2
n
Ethylene
R
Olefin
140
150
100
93
140
125
138
34.5
rR
93
83
92
23
r CO 2
90 70
12.9 8.8
64
10S
135
96
120
80
20.2 10.7
137
91
54
3003
36
18
-
r CO 2
rR
Catalysis (ri·10- 1O) at PR=0.02 Torr at PR=O.OS Torr
11
-
12
11
14
1S
12
- 10 - -
Reduction by R: r CO . 10 -10 2 at PR= O.OS 0.02 r Torr R Torr
The Full Oxidation of Olefins Over Cu20 (2000C, Po = 0.3 Torr) /62/: 2 CnH2n +~02 = nC02 + nH20 ; rR = r C02 = r 02 (in molecules cm-2 s-1).
TABLE 101
106 64
86
S9
86 106
94
0.17
0.12
0.17
0.18
0.2
80 60 43
0.28 32
O.OS Torr
Ratio of the rates of reduction and catalysis at PR=O.OS Torr
13
0.02 Torr
Oxidation of adsorbed R after catalysis at PR=
~
m
o
610
(Table 101). Under these conditions, a carbonate-carboxylate complexes are destroyed (with the rate equal to that of catalysis) only in the presence of 02' At higher temperatures, the above compFig. 190 - The correlation of activity with qs for the oxidation of pent-2-ene (over oxides with specific surface areas within the range 9-14 m2 g-1, Table 100): 1 - c0 0 , 2 3 4 4 - Ce02, 3 - Fe 20 Ce20 3, 3, 5 - Ti0 2•
400 500 20
40
50
fls/IrCtIL(;-atO;-'
lexes are decomposed in the absence of 02 and the reduction-reoxidation mechanism becomes true. The rate of formation of carbonyl compounds and of CO 2 on Cu20 is proportional to P02' both processes being retarded by water vapour /154,155/. Hence, the surface coverage with oxygen, 8, under these conditions is low. The selectivity to CO 2 (on Cu20) increases with increase in the ratio Po fPC H and the activation energy for the formation of 2 5 10 CO 2 exceeds that for the carbonyl compounds /154, 155/. These facts can be interpreted on the basis of the proposed mechanism, since the latter suggests that there is more rupture of oxygen-catalyst bonds in deep oxidation than in mild oxidation. The Oxidation of Higher Olefins Over Complex Oxide Catalysts Bismuth molybdate is an effective catalyst for the mild oxidation of the higher olefins /139/. The distribution of the reaction products is presented in Table 102 (for PR = 0.2 atm). The experiments were made at low conversions (below 10%) when isomerization was poor. (It is enhanced at high conversions). One can see that mild oxidation results mainly in dienes and d,fl-unsaturated carbonyl compounds, i.e. "allylic" oxidation takes place. The former route, oxidative dehydrogenation, prevails.Diene carbonyl compounds are produced by way of the combination of oxidative de-
611
TABLE 102 The Oxidation of Olefins Over Bismuth Molybdate at 460 0C /139/
Olefin
Propylene
Conversion/%
Selectivity/ %
Olefin
10 40 80
To acrolein: 90 86 73
3-Methylpentene
Conversion/Io
10 20
Selectivity/ ;0 To C 12: 7H 85 73 (to Cr 10: 5; 8)
To butadiene:
But-1-ene 20 40 80
2-Methylprop-1-ene 10
40 70 Pent-i-ene 10
20 60 Pent-2-ene
95 95 90
Hex-1-ene
To methacrolein: 72
Hex-2-ene
72 72
To pentadiene: 93 87 38
CO 2 + H2 0 are only formed 20 40 70
5 10
79
20
To isoprene:
3-Methylbut-1-ene 40
35
60
(to C 80: 5H 10, to C 60: 3) 5H
24 18 10
(to hexatriene: 5; 6; 7; to benzene: 15;22;30) To methylpentadiene: 55 49 38 (to C6H100: 9; to C6H 80:
5)
80
(to C5H60 : 4)
To hexadiene: 71 59 32 (to hexatriene: 2;5;7; to benzene: 0;7;32)
20 40 70
2-Methylpent-1-ene
88
5
15
2-Methylbut-1-ene
Cyclopentene
3-Methylpent-1-ene 10
50 80
ro methylpentadiene: 90 54 27 (to vinylbutadi.ene: 10; 16; 12)
612
TABLE 102 (continuation)
Olefin
Conversion/%
Selectivity/ %
20 40 70
73 62 44 (to C aO: 5H 9, to C5H 6O: 4;9;16)
2-Methylbut-2-ene
3,3-Dimethylbut-1-ene 2 5
4,4-Dimethylpent-1-ene
Hept-3-ene
Olefin
Conversion/%
4-Methylpent-1-ene
To 2,3-dimethylbutadiene: 60 2-Methylpentadiene 56 (to °6H10: 3; to C6HaO: 6; a-, to C6H602: 1; 2)
2 5 10
70 59 40 (to C6HaO: 6;6;5)
5 10 20
67 60 44 (to C6H10O: 3; to C6HaO: 5)
To 2,3-dimethylpentadiene: Hept-1-ene 50 2 5
74 53
40
20 (to C : 7HlO 7; to C7H a: 24; to CraO: 5)
Selectivity/ %
2-Ethylhex-1 -1-ene
To Cr 12: 27 (to C 7HlO: 6; to C a: 4; 7H to C7H 6O: 2) 'fo
5 15
CaH14: 66 53 (to CaH12: 3; 7; to CaH10: 0;6; to CaH a: 0', 2)
613
hydrogenation and of allylic oxidation of the diene formed. The structure of propylene and isobutene prevents them from undergoing oxidative dehydrogenation; because of this, only carbonyl compounds are formed in this case. Nevertheless, when the CH 3-group is weakly bound to a tertiary C-atom, the branched olefin can undergo skeletal isomerization and be converted into dienes. This is why dienes are formed from 2,2-dimethylbut-1-ene (together with a,fi-unsaturated carbonyl compounds). In the oxidation of 4,4-dimethylpent-1-ene, skeletal isomerization is likely to occur after abstraction of the allylic H-atom. According to reference /139/, the formation of unsaturated aldehyde from higher olefins requires the presence of the CH 3-group attached to the vinyl C-atom. This agrees with the data for Bi but does not correspond to the picture obtained /155/ with - Mo Cu20, where unsaturated aldehydes were produced from normal pentenes. The selectivity to the above aldehydes is likely to depend markedly upon the catalyst (as well as upon the structure of the initial olefin). The oxidation of higher olefins with long chains (hex-1-ene and hex-2-ene) leads to dienes and trienes as well as to benzene, a product of oxidative dehydrocyclization. The major product of the mild oxidation of branched pentenes at 420 0-5500C on Bi - Mo (Bi : Mo~1) promoted with P205 (and also without the promoter) is isoprene /133/. With excess of 02' normal pentenes are converted into maleic anhydride on Co - Mo and V - P catalysts. At 450 0C on V - P - 0, a yield of 55 wt.% of maleic anhydride is achieved /95/. The 3n0 2 - Mo0 catalyst accelerates the oxidation of pent-1-ene 3 to methylisopropylketone at low temperatures: i.e. a saturated ketone is produced. By-products are CO 2, CO, C 2 saturated acids, 4-C acetaldehyde, diethylketone and acetone. Fast isomerization of pent-1-ene to pent-2-ene takes place under these conditions (Table 103) /124/. Oxidation of higher olefins on the above catalysts proceeds via a parallel-consecutive scheme. The contribution of the consecutive routes (leading to destructive oxidation) is enhanced by increasing the temperature /95, 124, 133, 139/. The product distribution and the composition of the catalysts employed suggest that a mechanism of type (XVII.115) is valid for the oxidation of higher olefins. At elevated temperatures, a predominant route is the first one
°
°
°
°
614
involving the formation of intermediate allyl species. On Bi - Mo - 0 and Bi - Mo - P - 0 catalysts, this leads to oxidative dehydrogenation (dehydrocyclization) and to unsaturated carbonyl compounds, while on Co - Mo - 0 and V - P - 0, maleic anhydride is formed. At low temperatures on Sn - Mo - 0, the third route prevails; it includes intermediate hydration and oxidation of surface alcoholates. Both the routes mentioned are accompanied by destructive and deep oxidation (the second route). The rate of mild oxidation of higher olefins over Bi - Mo - 0 catalyst obeys /139/ the equation:
r", = f 1!7 ~ 8
(XVII. 116 )
where PR is the partial pressure of olefin. The surface coverage with oxygen, 8, is determined /139/ by the Langmuir isotherm: 17.6 0.0 8=
b()
Po
'2 '2 17.6 (J.6 /1- b~ P + ~
0
I!o
i.e. the process i8 inhibited by product (P p is its partial pressure and b p is adsorption coefficient of the diene or unsaturated carbonyl compound). The value of b p is approximately the same for pentadiene and other dienes. Unsaturated carbonyl compounds cause stronger retardation (the values of bp for these are two orders of magnitude higher). The kinetics of the oxidative dehydrogenation of branched pentenes on Bi - Mo - P - o are described by Eq , (XVII.116) at 8 ~ 1 /154/. A scheme of type (XVII.115) leads to the following rate equations for mild (I'm) and deep (I'd) oxidation. The surface coverage wi th the olefin, 8 R' in the Henry region will be: 8 R = bJl R. The rate of mild oxidation (the first route) is equal to the rate of the reaction between the weakly adsorbed olefin and one anion of surface oxygen:
r :: I.- 8 8= k .P 8 171 Ilf mR
615
rUBLE 103
The Oxidation of Pent-1-ene on Sn0 2 - Mo0 3 (C5H 10 - 3, O2 - 30, N2 - 37, H20 - 30 vol.~) 158
Conversion /% Selectivity (%) for the oxidation into: CH + C 2H 5COC 2H 5) 3COC 3H7( 4/5 C3H7COOH 3/5 CH
3COCH3
3/5 C2H 5COOH 2/5 CH 3CHO 2/5 CH 3COOH 1/5 CO 1/5 CO 2 Selectivity (%) for the isomerization into 2-G 10 5H
/124/
185
8.0
22.0
65.4
52.6
1.0
1.4
traces
2.2
II
2.1
1.0
1.4
2.0
3.0
2.0
9.9
25.6
27.4
57.4
70.4
where km = k 1bR• Eq. (XVII.117) coincides with Eq. (XVII.116) which is adequate to explain the experimental data. The rate of deep oxidation (the second route) is equal to the rate of interaction of the olefin with two (0) species:
(XVII.118) where k d = k 2bR• The participation of (0) in the process is proved by data /133/ on the activity and selectivity of Bi - Mo - P - 0 during catalysis and surface reduction. Eq. (XVII.116) shows that the rate decreases with decreasing 8 . Such a relationship was observed in the reduction of Bi -
616
- Mo - P - 0 by pentenes /133/. Eqs (XVII.117) and (XVII.118) lead to the following expressions for selectivity:
S =--m r +r m d (XVII.119 ) Hence, the seleltivity should increase with decreasing values of 8. This is really observed on Bi - Mo - P - catalyst /133/. The relative yield of the mild oxidation products decreases with increasing conversion. This is due /139/ to an increasing retardation by the mild oxidation products and to their further oxidation.
°
The Oxidation of Dienes The simplest diene is allene, H2C=C=CH 2• Of main practical importance is the oxidation of dienes with conjugated C=C-bonds. The major reactions of diene oxidation are:
H2C=C=CH2 + 402 = 3C0 2 + 2H2 0 , 1 H2C=CH-CH=CH2 + 2' 02 = H2C=CH-COCH 3 ,
1 02 = CH H2C=CR-CH=CH2 + 2' 3-CH-CHCHO,
H2C=CH-CH=CH 2 + 02
== CH 3-CH=CHCOOH,
HC-CR H2C=CH-CH=CH2 + 02 == /I /I + H20, HC CH
'01
H2C=CH-CH=CH2 + 202
=- CH 2=CHCHO
H2C=CH-CH=CH2 + 2 ~ 02
/0 =- Hy,-C'-.,. HC-C'-o
'0
+ CO 2 + H 20, + 2H 20,
1 H2C=CH-CH=CH2 + 2 2' 02 = CH 2=CH-COOH + CO 2 + H20,
617
H2C=CH-CH=CH2 + J0 2
=
CHJCHO + 2C0 2 + H20,
1 02 = CHJCOOH + 2C0 + H H2C=CH-CH=CH2 + J '2 20, 2
H2C=CH-GH=CH2 + 3
2'1
02 -= 4CO + )H2 0,
H2C=CH-CH=CH2 + 5
'21
02 = 4 C0 2 + )H2 0,
H + 02 2C=CH-CH=CH-CH 3
== H2C=CH-CH=CH-CHO
1 H 2C=CH-CH=CH-CH J + 4 '2 02
== CH 3CHO
+ H20,
+ 3C0 2 + 2H20,
H2C=CH-CH=CH-CH + 702== 5C0 2 + 4H 2 0 , J H2C=CH-C=CH2 + 02 I CH 3
== H2C=CH-C=CH2 I CHO
+ H20,
1 H2C=CH-G=CH 2 + 4 '2 02 = CH + 3C0 2 + 2H20, 3CHO I CH 3
=
5C0 2 + 4H 2 0 , H2C=CH-C=CH2 + 702 H 3 CH + 02 = CH 3-GH=CH-CH=CH-CHO + H20, 3-CH=CH-CH=CH-CH3
9
CH CH CH
3-CH=CH-CH=CH-CH3 3-CH=CH-CH=CH-CH3 3-CH=CH-CH=CH-CH3
+ 202 = CHO-CH=CH-CH=CH-CHO + 2H20, + 602 == CH
3CHO
+ 4C0 2 + )H2 0,
+ 8 ~ 02 = 6C0 2 + 5H2 0•
The Oxidation of Dienes Over Simple Metal Oxide Catalysts
The specific activity, r, in the oxidation of allena (Table 104) at 300 0C decreases in the order: CuO 7Ivln0 co Ni07 2/ 30 4/ 7Fe2037Cr203/zn07V2057'fi02. The reaction products are CO 2, H20 and unknown mild oxidation products. The range of specific activities for deep oxidation is nearly the same as in the overall process, the selectivity to mild oxidation being in the reverse order: ZnO,;:>Ti0 V Cr '7 Mn0 NiO;::.Fe /156/. 2, 20 5/ 20 3 27 20 3::::-Cu0'7C0 30 4 One can assume that the (0) ion participates in the reaction, more oxygen-catalyst bonds being broken in deep oxidation then in mild oxidation. As a result, the overall reaction rate decreases
V20 5 'riO 2
ZnO
3
Fe 20 3
cr 20
0( -
C0 30 4 lIiO
Mn0 2
CuO
Catalyst
7.9 x 10 11 6.1 x 10 11
8.5 x 10 12 3.2 x 10 12
2.9 x 10 14 4.2 x 10 13 2.2 x 10 13
7.1 x 10 14 4.4 x 10 14
-
1.1
1.0
0.6
-
-
-
0.6
20
0.4
0.5
22
-
-
-
0.5
1.2
26
0.9
-
23
-
-
0.4
0.4
0.]
0.1
0.5
0.]
0.]
0.9
0.8
50
50
90
42
27
31
0
34
15
Selectivity (~) r(molecules cm- 2 s-1) E/ kcal mol- 1 Heaction order --::,---to mild oxida0C at 300 and Cc H = --~~.~----~ota~ con- Deep oxi- in allene, m 'I'oral conDeep tion at 300°C 3 4 version dation version oxi=1 % dation
The Oxidation of Allene on Metal Oxides with Excess of 02 /156/
TABLE 104
en 00
.....
619
with increasing q s (Fig. 191), while the selectivity to mild oxidation increases with q s (Fig. 192). The consequence of the above two interrelationships is a correlation between activity and selectivity (Fig. 193). On V (at 310 0-420 0C and with air: diolefin = )5 : 126), di20 5 vinyl in oxidized into maleic anhydride /127/. The yield of this product reaches 58% (at 34!o conversion) and decreases with increasing temperature and contact time which indicates that further oxidation of maleic anhydride occurs. By-products of the reaction are carbon oxides and formaldehyde (with a yield of 4%). At )50 0C and with a concentration of divinyl of 0.6-1.0% in air, the selectivity to maleic anhydride reaches 4)%. Small amounts of furan, CH)CHO and CH)COOH are also formed. A parallel-consecutive scheme is assumed. The parallel route involves the oxidation of divinyl into a surface intermediate leading to furan or to another complex resulting in the formation of maleic anhydride /157/.
Igr 15 10
/4
0
S/%
6
15
o} 0
2 50 .J
1J
0
25
12 11
0
20
40
60 fs!tcO't(g--crtOj-'
Fig. 191 - The correaltion of specific activity with q s for the oxidation of allene (data of reference /156/): 1 - Mn0 2' 2 - C0 0 , 3 - CuO, 4 - Fe 2 0 ) , 3 4 5 - ZnO, 6 - Ti0 2, 7 - V20 5•
0
20
40
60 fJs/kcO't(i-crtOl'
Fig. 192 - The correlation of selectivity with qs for the oxidation of allene (data of reference /156/): 1 - Co)04' 2 CuO, 3 - Mn0 2' 4 - Fe 20 3 , 5 V20 5, 6 - ZnO, 7 - Ti0 2•
620
05'/.%
10
Pig. 193 - The correlation of the activity with oelectivity for the oxidation of allene over metal oxides (the data are taken from fable 104): 1 - ZnO, 2 - V 20 5, 3 - Cr 203' 4 - NiO, 5 - Fe 20 6 - wm0 2 , 7 3, CuO, 8 - C0 304•
15
50
25 01.-----'-------1.---'---0---'-/f 12 13 /4
The Oxidation of Dienes Over Complex Oxide Catalysts The oxidation of allene on CuO - A120 with excess of 02 re3 sults in the complete combustion of the hydrocarbon /51/. On CU 20 catalysts promoted by Mo0 and Vi0 ( « 1;(;), dienes with 3 3 conjugated C=C-bonds are selectively oxidized into unsaturated carbonyl compounds, i.e. "allylic" oxidation takes place (see Table 105). In addition to carbon oxides, acetaldehyde is a by-product /103/. On Co - Mo and V - p catalysts, divinyl is converted selectively into maleic anhydride. Over the former catalyst, the product yield reached 88 wt. % (with respect to divinyl passed) /85/. The addition of P205 to V 20 results in a sharp decrease in 5 the activity but does not influence the selectivity. 'l'hus, the catalyst with a V : P ratio of 1 : 1.6 is 30 times less active than V 20 its selectivity being only 3% higher than that of V 20 5 5, 0C, (350 0.6 - 1% C 6 in air). By-products on V - P - 0 are car4H bon oxides, furan, CH and CH /157/. 3CHO 3COOH In the catalytic oxidation of divinyl on V - P at 430 0-4500C, homogeneous steps were detected. They do not affect the rate of formation of maleic anhydride /158/. Bi - Mo catalyst is relatively inactive for the oxidation of divinyl (490 0C). The reaction products are furan (selectivity 25% at 36% conversion) and carbon oxides /140/. For the oxidation of diene, a parallel-consecutive scheme is usually accepted /95, 130, 140, 157, 158/. The following mechanism (in the case of a parallel scheme) can be assumed:
°
°
°
°
621
fUTa1!
maleic tI1!nydnd8 (0) .. (12)
(t;)
(0) / HzC=CH-CH=CH z
+(
__
route I
) /
)----(H 2C=CH-CH=CHz ~
2f~~11
~oute
routeIII
HzC=CH-COCHr
Products ot
_
destrtfct/ve oXldation,CO, CO2
JI
t
1/
(It )~···--(I2)
H
20
(Ill) --- (1(")1
The first route leads to mild oxidation, the second - to deep oxidation. The first route involves an allylic type complex (H ~ 2C ~CH:--:-;-CH-;-:-:CH2). Its oxidation leads to (I~) which gives f'unan , Further co~version of (I~) on the surface results in a surface maleate, (1 2 ) , its desorption giving maleic anhydride. TABLE 105 The Oxidation of Dienes Over Cu20-Catalysts Promoted with Oxides of ~o and W /130/ (400°-420°C; diene : O2 : N2 = 1 : 1.2 : 1.8 vol.%)
Initial diene
Main reac tion product
Piperylene
Penta-2,4-dien-1-al Vinylacrolein Hexa-2,2-dien-1-al
Isoprene Dipropenyl
Diene conversion/ %
Selectivity/ %
Activation energy/ kcal mol- 1 for for mild oxi- deep oxidation dation
7.6 6.)
65 60
13 11
23 23
8.6
48
15
19
The second route include the rupture of C=C-bonds and leads to 1/ destructive and deep oxidation via (Ii ) complexes of the carbonate-carboxylate type.
622
I'he third route is similar to the oxidation of monoolefins into saturated carbonyl compounds and involves surface hydration leading to an alcoholate,(I;"), and its sUbsequent oxidation. In the case of higher diolefins (piperylene, isoprene, etc.), the first direction is similar to the "allylic" oxidation of olefins and leads to dielllils. Selective catalysts for the process are Cu20-containing systems. The observed first order in O2 and zero order in diolefin /130/ suggests that there are small surface coverages by oxygen. The greater activation energies for deep oxidation in comparison with those for mild oxidation (Table 105) may be caused by the greater number of oxygen-catalyst bonds broken in route II in contrast to route I. On a V - P - 0 catalyst, a zero order dependency on 02 was obtained /158/ which suggests that high 8 values occur, in accordance with the higher values of qs (compared with those for V20 5). RDFEB.j~NCES
2 3
4
5 6 7 8 9 10
11 12
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