Oxidative dehydrodimerization of alkenes

343

PART 2 Oxidative dehydrodimerization

of alkenes

Typical examples of the heterogeneous catalytic reactions of oxidative dehydromerization of alkenes are the conversions of propene to lJ-hexadiene (diallyl) and of isobutene to 2,5- dimethylhexa-1,5-diene which, under certain conditions, can transform further to aromatic hydrocarbons:

CH;

CH-CH3%“,= CH-C”,-C”~c”=CH,-- O2 Q

+ Hp

CH,=C-CH3$CH2=\-CH,-CH2-\=CH4CH3@H3+b0 :H,

CH3

(2-I) (2.11)

CH3

The products of these reactions differ fundamentally from those of the well studied partial oxidation reactions to the corresponding aldehydes and acids [ 1,2] due to variations in the composition of the catalysts used. 2.1. Catalystsfor the oxy&hydrodimerimtion of alkmes

Table 2.1 summarizes the most efficient catalysts for the oxidative dehydromerization of short-chained alkenes. The majority of them contain metal oxides. The use of noble metals such as silver [3,4] and palladium [5] for these reactions has not been developed to any great extent due to their high expense and the absence of any essential advantage in comparison with oxide catalysts. Performance of the process in the presence of such metals as bismuth, tin [6] or molten salts such as lithium iodide [7] is technically rather complicated. Simple metal oxides are as a rule quite inefficient in such processes. Among the numerous catalysts studied in ref. 8 the oxides of bismuth, cadmium, zinc and tin are mentioned as being comparatively selective. According to the patent [9] the selectivity for the required products reaches a value of 60% on bismuth oxide, the fractional conversion of initial alkene being 28%. Indium and thallium oxides are both relatively selective under conditions of great alkene excess in the initial mixture. The general selectivity for diallyl and benzene attains 6O-70% with a propene conversion of less than 1% [lo]. Lead oxide is characterized by similar indices [ 11,121. Besides the low selectivity, almost all the individual oxides mentioned have low activity. Lead and thallium oxides are characterized by an unstable activity caused by the reduction

344

TABLE 2.1 The catalysts for oxidative dehydromerization of aIkenes Catalyst

Reaction COditiOllS'

AIkene conversion (%)

Selectivity (%)

Refs.

by diaIkene** by aromatics***

MetaIIic Ag doped 5OtPC,C3Hs: by oxides t&=39:1, of aIkaIine 7=0.56s metals

1.1

61.9

Metallic BiSn and 593”C,C3&: air = 1:2, their alloys in liquid state 7 = 0.2-3 s

16.6

22.4

54.4

6

Melted 98%LiI+ 2% LiOH

480°C i-CjHs:Cz = 4.2:10,7 = 0.1-5 s

36.5

-

68.8

7

Bismuth oxide

575”c, C&xirzHe = 2:5:10,7 = 2.1 s

28.5

57.0

5.4

9

Indium oxide

480°C C3H&2=5:1, 7=1.2s

-

22.6

40.0

10

ThaIhUm oxide

SUPC, c3I-I6:02 = lo:1

-

78.0

5.6

76

394

10

7=4s

Lead oxide supported on AI203

7=0.04s

Bismuth-tin Bi:Sn = 1:l

47o”C,C3Hs:@ = 1.4:1,7=3s

24.5

Bismuth-tin Bi:Sn=2:3

575”c, C3Hg:akHe = 2:5:10,7 = 1.1 s

18

Bismuth phosphorus Bi:P=2:1

500”c,c*02 = 1:1.6,7 = 1.0 g.s/mI

85.8

Bismutbphosphorus BtP=Zl doped with NQO

550"c,c3H6:02= 5.6:1,7=0.36 s

Bismuth titanimu Bi:Ti= 1:l

55OY,C3Hs:O2:Nz = k2:B.l 7 = 2.0 g.slmI

493T, cJH6:02=

-

11,12

61.5

l5,16

27.5

18

44.9

23

l5.7:1

6.9

81.9

24.2

47.0

25

29.1

27

345

Catalyst

Alkene conversion (%)

Reaction conditions

Selectivity (%) by dialkene

Refs.

by aromatics

Bismuth titanium Bi:Ti = 4:3

575”c, C&j:airzHe = 2:5:10, t = 1.1 s

20.2

26.8

24.6

18

Biimuthtitanium Bi:Ti=2:1

57YC, 7 = 2.1 s, C*:air:He = 2:5:10

22.0

38.4

17.5

9

Biimutlwzinc Bi:Zn=(l-3): (3-l)

swc, r = 0.36 s, c3I-I6:02 = 5.61

3.3

58.5

Bismuth-indimn Bi:In = 7~3, supported

5ooT, 2 = l-15 s iQHs:O2 = 1:(0.3-l)

26.2

10.5

50.3

31

Bismuthantimony Bi:Sb = 1:l

58YC, T= 0.36 s c3I-I&02 = 5.61

10.0

50.0

-

19

Bismuth antimony doped with Na, Cu,Zr oxides

42cPc, T = 0.45 s i-C&Is:02 = 4:l

32.0

65.0

Bismuth-nickel Bi:Ni = 1:l

47o”c, r = 7.5 s C3I&:airzH20 = l&3

51.2

25.3

46.5

34

Bismuth-copper Bi:Cu=lzl

47VC, T= 7.5 s, i-QI-Is:airzH20 = 1:4:3

51.5

30.5

45.6

34

Bismuth-iron Bi:Fe = 1:l

575”C, 7 = 1.1 s, C&:airHe = 2:5:10

16.1

20.7

14.6

18

BismutIAron BkFe=2:1

5wc,

6.8

49.0

9.8

35

Bismuthcadmium Sn:Cd = 5~3.5

5OOV,7=4.35 C3Hb(i-C+Hs): airzH20 = l&2

2Q.O(17.0)

Bismuth oxide doped with I<20 (2.8%) on ahmdum

5xX, C3H&xNz = 10.8rlz71.5

12.0

77.0

Bismuthzirconium BtZr = 2:3

575”c, r = 1.1 s C3Hs:airHe = 2:5:10

18.0

28.0

1=

0.36 s,

19

32

Cg-Il$~ = 5.11 -

55.2(56.3)

36

44

14.8

18

346

Catalyst

Biimuth-

Reaction conditions

Alkene conversion (%)

Selectivity (%) by diakene

Refs.

by aromatics

2.6

28.2

27.8

18

Bi:Nb=M

575”c, T= 2.1 s C&:tiHe = 2:5:10

Bismuthtungsten Bi:W=2:1

57x, T= 2.1 s C~Ha:air:He= 2:5:10

14.4

6.1

19.9

9

Bismuth-

5OOT,r = 2 g.s/ml 69 CJHa:O2:N2= 1:2:&l

33.8

27

7 = 2 g.s/ml Cj&:O2:N2 = 1:2:&l

10.1

27

IliObiUtll

arseniC

Bi:As=kl Biimuthyl &oxide (BiO)zSO4

500-C,

Manganesesodium on AlzO3,Mn:Na = lz2.3

627=C,T=0.1-S s c3H6zo2 = 4:l with HBr dopes

Tm:cadmium Sn:Cd = 0.33:

61

9.0

7.4

46.6

48

57OT, 7 = 4.3 s i-C4Hg& = 1:2

62.5

13.4

65.5

51

Lead-tin Pb:Sn = 1:l

540%

51.0

7.7

56.5

54

c3H6:02

=

Palladium dimethyl sulfoxide supported onaflask

WC, C&:02

58.2

75.6

11.0

5457

= 1:l

I.9

* Here T stands for contact time. ** Dialkene signifies 1,5-hexadiene in the case of propene and 2,5-dimethylhexa-1,5-diene in isobutene oxidation. l ** Benzene is the major aromatic product of propene conversion, and n-xylene of isobutene conversion.

of lead to the inactive metallic state [13] and by the volatility of the monovalent thallium compounds formed in the catalytic process [ 141. Bicomponent oxide systems are more active in the oxidative reactions of hydrocarbon dehydrocoupling. The most numerous group is represented by the contact catalysts based on bismuth oxide. Combinations of the latter with the oxides of tin [15-201, phosphorus [23-261, titanium [9,18,27,28], zinc [19,29,30], indium [30,31], antimony [19,32,33], nickel [34], copper [32,34], iron [18,21,34,35], cadmium [27,36,37], lead [36], manganese [38], cerium [29] and other metals are quite efficient. The higher activity of these oxides seems to be provided by the formation of compounds having a definite catalytic activity. Thus, a study of the phase composition of oxide systems of bismuth-tin [39-42], bismuth-iron [35] and bismuth-titanium [43] revealed the presence

347

’

0 SnOf

I 100

20

40

60

I 1 80

1 I 60

1 40

80 B&O, 1

1 20

1

1 0

Fig. 2.1. The influence of bismuth-tin catalyst composition on the rate (1) and selectivity (2) of propene oxidehydrodimerization at 500°C and on the content of bismuth pyrostannate in the catalyst (3) [41]. of pyrostannate (BhSnz07), orthoferrite (BiFeO3) and pyrotitanate (BizTizOs), respectively. These catalysts possess the maximum activity in the reaction of oxidative dehydrodimerization of propene in areas of phase diagram representing the same ratio of elements. A direct comparison of the reaction rate with the results of phase analysis has been performed for the bismuth-tin system in ref. 41 (see Fig. 2.1). The rate and selectivity of propene oxydehydrocoupling were found to change proportionally to the amount of pyrostannate in the bismuth-tin catalyst composition. The concentration of pyrostannate depends on the bismuth:tin ratio, as well as the method of preparation. Catalysts synthesized by the thermal decomposition of bismuth and tin hydroxides were found to contain a comparatively low Bi&rz07 fraction [42]. The reason for this is that at the stage of bismuth and tin hydroxide coprecipitation a considerable fraction of the bismuth is fixed as catalytically inactive bismuthyl chloride formed via hydrolysis of bismuth nitrate to the basic salt, followed by a subsequent exchange reaction with tin tetrachloride. Favorable conditions for bismuth pyrostannate formation are provided in the absence of chloride ions. A catalyst having an increased BizSn207 content can be obtained by the precipitation of bismuth hydroxide on the tin dioxide. This catalyst possesses a higher activity (Table 2.2). According to data provided by X-ray photoelectron spectroscopy the bismuth-tin catalyst synthesized by this procedure is characterized by increased effective cation charges and a similarity between the surface and bulk compositions in comparison to a sample obtained from tin chloride, the surface of which is tin enriched (Table 2.2). Unlike the bismuth-tin system the most active catalysts from the bismuth-phosphorus system are heterophase mixtures. An analysis of the data presented in Fig. 2.2 [44] confirms this supposition. The maximum yield of the products of dehydrocoupling of propene

348

TABLE 2.2 The infhtence of preparation procedure upon the activity,phase and surface composition of bismuth-tin oxide catalyst Bi:Sn = 1[41,42] Preparation procedure

r*lo-16 (molecules/m2 s)

Bi2Snz07 content, (rel. units)

Bi:Sn on the surface

2.2 4.2 3.7 11.7

traces 0.42 0.52 4.42

1:1.4 to.9

Initial reagent:

Oxides Hydroxides. Hydroxides Metallic tin l

Sample prepared from tin dichloride.

(benzene, 1,Shexadiene) is observed with the catalyst having a Bi:P ratio of 2 which, according to X-ray phase analysis, is a mixture of y-BizO3,2 BhO3*P205,3 Bi203 *PzOSand of three forms of bismuth phosphate BiP04 (hexagonal, monoazyth and a high temperature form). The compounds 2 Bi203*PzOs and BiP04 of hexagonal structure are supposed to provide the major contribution. The partial oxidation of propene to acrolein occurs most effectively on a catalyst having a Bi:P ratio of 1, which is a mixture of monoazyth and high temperature forms of bismuth phosphate. These suppositions have recently been confirmed by the same authors [27] during their studies of the catalytic properties of different metal phosphates, including bismuth phosphate, with different structures. The data presented in Table 2.3 show that bismuth phosphates are most active in the mild oxidation of propene, the compound 2 Biz03.PzOs and the high temperature form of BiP04 being active in oxidative dehydrocoupling, while the monoazyth form of BiP04 is active in the partial oxidation to acrolein.

3

12 Atomic

2 ratio

1 Bilp

u3

112

Fig. 2.2. The relationship between acrolein (l), benzene (2) and CO2 (3) yields in the process of oxidative propene conversion at 500°C and the composition of bismuth-phosphorus catalysts [44].

349

TABLE 2.3 Oxidative conversion of propene on the phosphates of different metals at 500°C and mixture composition of c3H6:02 = 1:2 [n] Catalyst

m(po4)2

CrP04 FePO4 coJ(po4)2

WPW ti3(m4)2 h(po4)2

CeP04 BiP04 (high temperature form) BiP04 (monoazyth form) 2 Bi203 aP205

Yield (%) co2

w-m

cd6

17.1 5.2 31.3 4.8 1.7 39.6 4.6 2.5 51.0

0 0 8.5 0 0.2 5.1 0.2 0.4 5.7

0 0 0 0 0 0 0 0 23.1

193 45.0

19.2 0

4.5 35.4

The addition of phosphorus oxide to bismuth oxide causes a change in the composition of the reaction products, besides an increase in the total activity. If dialkene hydrocarbon forms mainly in the presence of Biros, the aromatic hydrocarbons are the major products of oxydehydrodimerization on Bi-P catalysts. The same result is observed for bismuth-containing catalysts in association with zinc oxide [29], indium oxide [31] and tin dioxide [40] as the second component. A noticeable increase in the selectivity of bismuth-containing catalysts is provided by doping them with alkaline metal oxides [20,25,32,45,46]. According to ref. 45 the introduction of 2.8% potassium oxide into the supported bismuth oxide provides an increase of selectivity to 1,Shexadiene from 70 to 77% at the same fractional conversion of propene. A considerable decrease of activity is observed when the content of the alkaline metal oxides approaches that of the main component and when it is present in an amount sufficient for the formation of the corresponding compounds. That is probably why sodium bismuthate is characterized by a rather low activity in comparison with bismuth oxide while its selectivity is rather high. To conclude our consideration of bismuth-containing oxide catalysts we should note that the introduction of zirconium [18], niobium [9], tungsten [9], arsenic [8,27], sulfur [8], or aluminium [27] into the bismuth oxide catalyst composition does not give an essential increase in their activity and sometimes even causes a deterioration in their catalytic properties. Among the other oxide systems mention should also be made of the manganese-sodium system [48,49]. At the average selectivity of the products of oxydehydrodimerization (5055%) these catalysts, however, are characterized by low activity even at high temperatures: at 627°C the conversion of propene on these catalystst does not exceed 5-10% (Table 2.1). Cerium oxide catalysts doped with sodium, bismuth and zinc [50,51] possess similar properties.

350

PbO

0

20

I I 100

I 80

I

I 60

80

60

40 I

I

40

I

I

20

100 I

1

0

Fig. 2.3. The rate (1) and selectivity (2) of total diallyl and benzene formation from propene at 5OOT on lead-tin oxide catalysts of different compositions [55].

The tin-cadmium system is more active [52]. The conversion of isobutene on these oxide catalysts with a Sn:Cd ratio of 0.4 reaches 65-67% at 570°C and the selectivity ofp-xylene formation is 5865%. The activity of these catalysts increases noticeably upon the introduction of chromium oxide [53,54]. The combination of tin and lead oxides is also rather efficient and has been studied in detail in refs. 55,56. The data from ref. 56 are presented in Fig. 2.3, which shows that mixed tin and lead oxides are more active and selective than the individual oxides in the reaction of oxidative propene dehydrodimerization. The most active is the sample with a composition of Pb:Sn = 1, its selectivity towards the required products under optimum conditions being ca. 6065% at 60% propene conversion [55]. The catalysts prepared by the calcination of PbO and SnO2 mixtures consist of lead oxide phases in the red and yellow modifications,

3.51

1.5

2.0

2.5

3.0

35 1.5

2.0

2s

3.0

35 d/n

Fig. 24. Diagrams of the X-ray phase analysis of the lead-tin oxide system [%I. tin dioxide (ruthyl) and lead orthostannate PbzSnO4 (Fig. 2.4). The latter is present in all bicomponent catalysts, but its concentration is highest in samples containing Xl-80 at.-% lead. The dependence of activity upon the composition of the tin-lead system has a maximum at the same region of compositions (Fig. 2.3). On the other hand, catalysts enriched with tin dioxide are noticeably more active than are samples containing considerable amounts of lead oxide in the free state. The combination of PbzSnO4 and Snob phases is likely to provide the heightened activity of the tin-lead system in oxidative dehydrodimerization reactions of propene. Note that an additional phase of lead metastannate PbSnOs is observed in the catalysts prepared by other methods. This phase, however, has only a low activity in the reactions under consideration [56]. Attempts to apply “heterogenized” metal complex catalysts have been made [57,58]. It was shown that diallyl and benzene are formed at lower temperatures (200-350°C) and in larger quantities in the presence of palladium- and iron-containing complexes supported on natural mineral silica than in the case of bismuth oxide. However, the problem of the stabilization of the activity of these catalysts has yet to be solved. The selectivity toyards the products of oxidative dehydrocoupling of many catalysts considered is not sufficiently high (less than 6&70%) due to the essential contribution made by the parallel process of deep oxidation of hydrocarbon by dioxygen present in the gaseous phase. According to patents [ 12,59-63], in order to decrease this contribution it is recommended that these processes be performed in the absence of gaseous dioxygen and that the hydrogen of the alkenes evolved during dehydrodimerization and cyclization be bound by the oxide oxygen, which implies that the catalysts are used as an oxidant. The simple oxides of bismuth, cadmium, lead and their various combinations with each other and other metal oxides are usually used for this purpose. In these procedures dehydrocoupling is achieved

352

by passing a mixture of the hydrocarbon together with nitrogen gas or water vapor through the contact layer. Under such conditions the selectivity to the required products can attain 75-58% at 50-70% conversion of the initial hydrocarbon. However, catalyst reduction occurs during the reaction and so there is a periodic requirement to reoxidize the catalysts. This necessitates the organization of the process in reaction-regeneration cycles. The interesting suggestion was made in ref. 64 to design the catalysts as a membrane, which separates the reactor into a reaction zone and an oxidant supply zone. Hydrocarbon is introduced into the first zone and a dioxygen-containing gas (air) into the other. Bismuthlanthanum systems, for instance, designed in this way, allow the selectivity of propene dehydrodimerization to reach 77% (52% for 1,5-hexadiene and 25% for benzene) at 600°C and 3.2% alkene conversion. It is important to note that these parameters did not change during one day of reaction. In order to increase the yield of aromatic hydrocarbons from alkenes one has to apply a combined catalyst. In refs. 65-67 a combination of dehydrodimerization catalysts supported on bismuth or indium oxides together with a cyclicizing catalyst (platinum on aluminium oxide or alumochromium) is suggested for the process of oxidative dehydroaromatization of isobutene top-xylene. To conclude our consideration of the known catalysts for the oxidative reactions of alkenes dehydrocoupling we should note the variety in their qualitative composition. At the same time they’differ from catalysts used for the partial oxidation of hydrocarbons which contain, as a rule, oxides of molybdenum, tungsten, vanadium, antimony, or tellurium as the main components. These are not typical of dimerization catalys.ts. This is why the announcement in ref. 68 of the activity of a uranium-antimony catalyst in the dehydrodimerizaton reaction was rather unexpected, since it was known from earlier work to be efficient in partial oxidation [69,70] and alkene oxidative dehydrogenation [71]. Molybdenum, antimony, vanadium and other multivalent metals are the acid forming elements when used as components of the catalysts for the mild oxidation of alkenes, while the majority of the oxydehydrodimerization catalysts are combinations of the oxides of amphoteric or weak basic elements. This confirms that the direction of alkene oxidative conversion definitely depends on the acid-base properties of the material used. 2.2 Activity and acid-base properties of the catalysts

One of the first studies concerning the role of the acid-base properties of the catalysts used in oxidative alkene dehydrodimerization was the report which Seyama presented at the First Soviet-Japanese Seminar on Catalysis in 1971[8]. Assuming this type of reactions to proceed as the partial oxidation of alkenes to unsaturated aldehydes via surface compounds of an ally1 structure and using the electronegativity of the metal cation (xi) as the main parameter governing the acid-base properties of the catalyst, Seyama demonstrated for numerous simple oxides, that a noticeable conversion of propene to hexadiene-1J and benzene occurs on basic or slightly acid oxides, with electronegativities in the range 5 10) the a considerable oxidation to acrolein is observed. This qualitative observation corresponds with subsequent results on the determination of the catalytic activity of the series of bicomponent oxide systems. Fig.

353

L

6

10

14

16

ELectronegativity Fig. 2.5. The relationship between selectivity of propene conversion into diallyl and benzene (open points) and acrolein (closed points) and the average electronegativityof metal cations on cadmium-umtaining catalysts [72]: 1 - CdO-CaO, 2 - CdO, 3 - 2CdO-In& 4 - CdO-SnOz, 5 - CdO-WO3,6 - CdO-MoOs.

2.5 demonstrates, for instance, data [72] which confirm that the increase of average electronegativity of metal cations in cadmium oxide catalysts CdO-MeA& decreases the selectivity of the oxidative conversion of propene to diallyl and benzene and increases the selectivity to acrolein. Similar dependencies were obtained for the system ZnO-Me& 1721. The electronegativity or ionization potential of the metal cation characterizes the bulk properties of the catalyst. The data analysis presented in ref. 73 shows that these values correlate with the experimental values of the surface acidity for systems of one type, and, thus, one may use them in a first approach as the parameters of the acidity force of the solids. The fact that similar dependencies are revealed upon comparison of catalytic and acidic properties has been determined experimentally. Thus, in refs. 27 and 74 the force and concentration of acidic centers for bismuth-containing catalysts was measured by n-butylamine titration using a Hammet indicator and the results were then correlated with the activity and selectivity of these catalysts in the reaction of oxydehydrocoupling of propene and isobutene. The data presented in Table 2.4 [27] show that in this case acidification of the catalyst surface decreases the selectivity of oxydehydrocoupling and increases that of partial alkene oxidation. The rate of the latter increases with the increase of the acid center concentration [27]. The intense color of the majority of tin-containing catalysts such as Sn02-Me& did not allow the authors of ref.75 to measure their acidity by a titrimetric method. The activity of the samples in a reaction of the acid-base type was thus used as the characteristic of the surface acidity. This assumption is based on the fact that the catalytic properties of the compounds in the reaction promoted by acids (isomerization, dehydration, cracking, etc.) correlate generally with their acidic properties [76]. The advantages of this method are: (i) the possibility to determine the acidity under catalytic or near catalytic conditions; (ii) one can measure the acidity over a large region of values, including the acidity of catalysts with

354

TABLE 2.4 Catalytic properties and acidity of bismuth containing catalysts [27j Catalyst

Propene conversion (%)

2 BizOs P205 BiAs04 BiPO4 (high temp.) Bi203 *2TiO2 (BiO)zS04 BiP0.t (monoazyth) BizMoO6 BizMozOy BizMoJOn

88.4 69.0 79.8 45.7 61.0 43.0 37.7 23.7 5.9

Selectivity (%) CfjPbj c3H40 0

49.0 33.8 26.9 18.0 10.1 9.1 0 0 0

5.8 6.6 0.3 4.0 38.6 66.1 91.7 94.9

Acidity, Ho

+7.1+6.8+6.8+7.1+6.8+ 1.5 +3.3+ 1.5 + 1.5 -

+6.8 +4.0 +4.0 +6.8 +4.0 -3.0 +1.5 -3.0 -3.0

undeveloped surfaces, with a variation of the contact time; (iii) all the limitations imposed by the color of the catalysts are removed. In ref. 75 the isomerization of the double bond in butene was chosen as the test reaction, because its acidic mechanism on the majority of oxides is not in doubt. The only exclusions are CaO [78] and MgO [79] for which the carbanion mechanism has been postulated. The information upon which center (Bronsted or Lewis) is responsible for double bond migration in the alkene can be extracted from a correlation of the &- and trm-isomers of butene-2 formed [80]. The integral acidity of SnOz-Me& (Sn:Me = 1) catalysts, estimated on the basis of their isomerization ability, is compared in Fig. 2.6 with their activity in the oxidative conversion of propene. Relationships similar to those obtained for zinc, cadmium and bismuth containing catalysts have also been found. Thus, a comparison of the catalytic properties with the acidic ones measured by different techniques demonstrates that the correlation is rather general for oxide systems: an increase in the acidity of the contact surface results in the inhibition of oxydehydrocoupling of alkenes and an increase in the rates of their partial oxidation to unsaturated aldehydes. This

I2

l.3

1.3

1.7

21

25

l/T*lo3

Pii. 2.6. The relationship between the rate of oxidative dehydrodiierization (a) and partial oxidation (b) of propene at 500°C and the ability of the tin containing oxide catalysts to cause isomerization (inverse temperature of the.given rate of butene-1 isomerization) [751.

3.55

25 20 15 10 5 0 15 10 5 0

5 1015 20

Fig. 2.7. The influence of the sodium and phosphorus oxide content

in tin dioxideon the conversionof

propaneto acrolein (l), benzene (2) and COz (3) at 550°C [27J. gives us the possibility to regulate the acidic and, hence, catalytic properties of the catalysts via their modification with dopants of different characters. As illustrated in Fig. 2.7, the introduction of alkaline metal oxide into Sn02 in quantities which maintain the homogeneity of the system increases the selectivity to the oxydehydrodimerization of propene, while doping it with an acidic oxide (P2Os) inhibits this reaction and stimulates the partial oxidation of propene to acrolein [27]. Similar effects were discovered on the introduction of Na+ and F ions into the bismuth-phosphorus catalyst [81] and the promoting of a lead-tin catalyst with potassium oxide [82]. The interpretation of the correlation between the selectivity and acidity assumes [27,75,81] that oxydehydrodimerization of alkene proceeds in the same way as the oxidation to aldehyde via intermediate ally1compounds, which is either charged slightly positively or is neutral. Then, on the “acid”adsorption position considered as a Lewis acid this intermediate becomes similar to a cation, due to localization of the ally1 radical electron on such a center, and it is thus sensitive to the addition of an oxygen anion. On positions with a lower acidity the intermediates are of a radical nature, which favors their dimerization. Under this hypothesis the promoting effect of basic and acidic oxides upon the catalytic properties of tin dioxide, which we considered above, is explained by the change of the electronic state of the adsorption center for alkene (Sn4+)through the Sn4+-OS-Me”+ bond due to electronegativity equalization. The introduction of a cation, Me”+, which is more electronegative than Sn4’ makes the adsorption centers more acidic and vice versa. Simultaneously with the changes in the electronic state of the ally1 fragment, depending on the electron accepting properties of the adsorption center, the strength of ally1binding with the catalyst surface can change [73,75]. One would expect a lower binding strength and an increased mobility of ally1 radicals in the samples with low acidity, which provides an increase in the probability of their recombination. And, vice versa, the ally1 compound bonded strongly to the surface has a greater chance to convert to acrolein or the products of total oxidation. The activity in the test reaction of double bond migration in 1-butene is suggested to represent a relative standard for the strength of alkene-catalyst binding [83]. This is possible if isomerization is realized with the assistance of aprotic centers in regimes close to the catalytic one [77]. These conditions were strictly observed in ref. 75, where a determination

356

Fig. 2.8. IR spectra of pyridine adsorbed on the bismuth-tin catalyst [73]: 1 - background, 2 - pyidine adsorption at UPC, 3 - vacuum treatment at WC, 4 - desorption under vacuum at lOOT, 5 - desorption under vacuum at 200°C.

of the isomerizational ability of Sn-Bi, Sn-Pb and Sn-Ti catalysts in the reaction of alkene oxydehydrocoupling was performed. In these samples isomerization proceeds with the participation of aprotic acidic centers, which is confirmed by the ratio of the cfi and tram forms of 2-butene formed (close to 2) [80] and by IR spectra of adsorbed pyridine presented in Fig. 2.8. Absorption maxima at 1575 and 1450 and a shoulder at 1490 cm-’ are observed in the spectra of Sn-Bi catalysts, their intensities decreasing with the increase of the temperature of heating under vacuum. These maxima correspond to pyridine coordinated to acidic Lewis centers [84]. At the same time there are no absorption maxima at 1640,162O and 1545 cm-’ characteristic of pyridinium ion, which indicates the absence of proton centers. The undeveloped surface and insufficient transparency in the 1000-2000 cm-’ region did not allow similar measurements to be obtained for Sn-Pb and Sn-Ti catalysts 1731. The mutual change in the rates of 1-butene isomerization and propene partial oxidation on these catalysts (Fig. 2.6) proves that the stabilization of adsorbed alkene occurs on the same surface centers in both reactions. The discovered aprotic centers, the strength of which determine the strength of alkene-catalyst binding, may serve as such surface centers. Due to the increase of their electron accepting properties one should expect a strengthening of adsorbatexatalyst binding, an increase in its dergee of deformation and, thus, of the oxidation probability. The aprotic centers of average strength are evidently needed for partial alkene oxidation, the adsorption of which is not followed by the rupture of a double bond and results in the formation of surface compounds of allylic type. Mutatis mutandis, a lowered binding strength of the latter with the contact surface is required for alkene oxydehydrocoupling. One should expect the realization of similar conditions in samples with weak electron accepting properties, which is significant for basic and amphoteric metal oxides. It is not only the acidic properties but also the basic which should be optimal for the efficient oxidative dehydrodimerization of alkenes. This assumption is proved by the data

357

0 2 Basicity

4

6

8 IO 12

. ‘IO’,moles &l-is COOH/,#

Fig. 2.9. The relationship between the rate of oxidative propene (open points) and isobutene (closed points) dehydrodimerizatiou at 500°C and the basicity of Sn-Bi (l), Sn-Pb (2), Sn-Ti (3) and Sn-In (4) oxide catalysts WI.

given in Fig. 2.9 [85] on the dependence of the rate of propene and isobutene oxydehydrocoupling on the basicity of the surface in tin-containing catalysts, measured by the method of reverse benzoic acid titration. The simultaneous presence of acidic as well as of basic centers on the surface of the bismuth-phosphorus catalyst is necessary for propene-to-benzene conversion, as is noted in ref. 81. The nucleophilic 0%ions usually serve as basic centers on oxide surfaces. The localization factor of the negative charge may serve as a standard of nucleophility. This can be estimated from the position of the O-KLL Auger-peak and the 0-1s value in X-ray photoelectron spectra. In the general case the bond energy of electrons on the 0-1s level should decrease [86,87], while the kinetic energ of Auger electrons of the 0-KLL transition should increase when the negative charge on the oxygen ion is increasing [37]. Table 2.5 presents data [88] on the bond energy of 0-1s electrons and the kinetic energy of 0-KLL Auger electrons for tin dioxide and bicomponent catalysts for the oxydehydrocoupling of propene. The tin-molybdenum surface for the partial oxidation of propene to acrolein is also presented for comparison. At the transition from the SnOz system to Sn-Mo the decrease of E 0-1s values and the increase of E 0-KLLvalues occur, which corresponds to the rise of oxygen nucleophility. The rate of ally1 oxidation of propene rises in the same order. Samples promoting oxidative dehydrodimerization and catalysts of partial oxidation show the same dependence of activity on nucleophility. The basicity of the centers on the surface of oxides is a function of the chemical nature of the metal. By changing the electronic state of the latter one can regulate the nucleophility of the oxygen ion bonded to it. It has been stated [89], in particular, that the introduction of bismuth oxide into tin dioxide provides a noticeable increase of effective charge on both

358

TABLE 2.5 Energies of 0-1s and Auger electrons of tin-containing oxide catalysts and the rates of alIy1oxidation of propene at 500°C 1881

catalyst,

Ebo-1s

Mel/Me2 = 1

(ev)

SnOz Sn-Pb-0 Sn-Bi-0 (I, from hydroxides) Sn-Bi-0 (II, from metal Sn) Sn-MO-0

530.7 530.5 530.6

511.2 511.3 512.0

19.5 19.2 18.6

traces 3.20 4.27

530.5

512.5

17.8

11.7

530.2

512.8

17.4

Eti,O-KLL (ev)

A(Et,-Eb,,) (ev)

r*lO-‘” C3Ha molecules/m2 s Acrolem DlaUyl

-

3.4 19.7

metals. At the same time an increase of the localization factor of negative charge on the oxygen ions is observed, which is revealed in the decrease of bond energy of 0-1s electrons and an increase of the kinetic energy of 0-KLL Auger electrons. The data considered give evidence for the important role of the basic center in process of oxidative alkene dehydromerization. Their participation in the mild hydrocarbon oxidation is explained [90,91] by the realization of the heterolithic principle of C-H bond dissociation, according to which the hydrogen split out as a proton is bonded with nucleophilic oxygen, while the carbanion is stabilized near the metal cation on the surface of the oxide catalyst. In other words, the primary interaction of the hydrocarbon with the surface occurs on the cation-anion pair, the properties of which are really co-dependent. This mechanism of oxidized compound activation, which demands an optimum composition of acid-base properties in the catalyst, is also valid for the processes of alkene oxydehydrocoupling as well. 2.3 Selectivityand strength of oxygen binding in catalysts The acid-base properties of the catalysts are not the only factor responsible for their activity and selectivity in oxidative hydrocarbon conversions. Acidity, as was mentioned above, can determine the strength of hydrocarbon binding with the catalyst. The strength of oxygen binding with the contact surface is a factor of the same importance. Thus, for many processes of partial oxidation of hydrocarbons it has been shown that an increase of the energy of the oxygen-catalyst bond lowers the rate and increases the selectivity to mild oxidation product formation [ 1,2]. An attempt was made in ref. 8 to revise the possibility of the same dependencies for the reaction of oxidative dehydroaromatization of propene to benzene. Thermodynamic heats of individual oxide formation were chosen by the authors as comparative standards of the oxygen-catalyst bond. These values, compared with the rates of deep (CO2 + CO) and mild (benzene + acrolein) oxidative propene conversion seem to form two groups which, in the author’s opinion, differ according to the type of oxygen exchange occurring in them. The rather great deviation of the points is likely to be caused by the fact that the heats of formation characterize the bulk properties of the catalysts rather than the surface ones [92].

359

L

m

0 2 4 6 Ti I fl0

I

8 10 12 14 16 [gK FeCd Cu CO IIIJI I I I II Zn SnCr Ni Mn

V Bi

Fig. 2.10. The relationship between the rate (mol/m2 h) of COx (l), of total benzene and acrolein formation (2) and selectivity (3) [S] and the mobiity of oxygen on the surface of oxides (constants of homomolecular oxygen exchange (molecules/cm2 s) [92]).

The heats of surface oxygen desorption [93] or the rate of heteromolecule isotope exchange (heteroexchange) of the oxygen in the catalysts with gaseous phase oxygen:

(“0) + ‘*@ + (‘80) + 1a0’80 seem to be more appropriate. The kinetic parameters of this process under conditions of equilibrium oxygen concentration at the near surface contact layer, when the adsorptiondesorption stage is the limiting one, may characterize the energy of the oxygen bond [94]. The lower the rate of reaction, the greater the energy of exchange activation, and ,the stronger this bond is. Fig. 2.10 presents data [S] on the deep and mild conversion of propene on simple oxides as compared with the rates of isotope oxygen exchange taken from ref. 92. If the latter are used to characterize the bond energy of the surface oxygen, the separation of oxides into different groups disappears and the correlation becomes more distinct. The catalytic activity of oxides in the reactions of ally1oxidation of propene increases with the decrease of oxygen binding on the surface less than in the case of total oxidation. Consequently, the selectivity of total benzene to acrolein conversion rises with the strength of the oxygen bond. This seems to be correlated with the fact that a greater number of oxygen-catalyst bonds are broken at the limiting stage of deep oxidation, than in dehydroaromatixation, as is supposed for the processes of total and partial hydrocarbon oxidation [95,96]. Zinc oxide does not fit in with this correlation of selectivity with oxygen binding. The different morphology of the samples used in refs. 8 and 92, in which a determination of activity in the reactions of propene oxydehydrocoupling and isotope oxygen exchange was

360

400 g 300 2

200

0

40

80

120

160

0, % monolayer Fii. 2.11. The relationship between bond energy of bismuth-tin catalyst oxygen and degree of reduction [1011.

performed, could be the reason for this. Thus, a needle form of zinc oxide [97] was shown to be rather active in propene to benzene conversion, having 40% selectivity, while on granular and tabular samples the selectivity did not exceed 15%. The excess zinc content in a zinc oxide catalyst depends on the preparation procedure and the treatment of the sample before measurement, and this seems to have an essential influence on the acid-base and catalytic properties of the sample [98-NO]. It was found, in particular, that the selectivity of zinc oxide in oxidative propene dehydroaromatization is larger in samples having a higher concentration of conduction electrons [98]. The data analysis presented in Fig. 2.10 [8] shows that the correlation for mild conversion is correct due to the contribution of the samples in the catalysis of the oxidation of propene to acrolein. This is why a particular study of the character of the relationship between rate and selectivity on the strength of surface oxygen bonds of catalysts for oxidative alkene dimerization seemed to be rather interesting. Since acid-base properties of the surface can vary greatly in relation to the oxygen bond energy for different catalysts, it seemed rather reasonable to investigate the influence of the oxygen bond energy on the selectivity of the same catalyst while changing the energy value via partial removal of oxygen from the catalyst’s surface. Fig. 2.11 presents the relationship found with the oxygen bond energy of a bismuth-tin oxide catalyst [loll, measured by calorimetry. Forty percent of the oxygen monolayer is homogeneous. When 80% of the oxygen monolayer was removed the oxygen bond energy increased sharply and changes only slightly upon further reduction, as a result of the high rate of oxygen diffusion in this catalyst. The oxidation of propene on bismuth-tin catalysts at different levels of reduction was studied in order to investigate the influence of oxygen bond energy on the rate and selectivity of oxidative dehydrocoupling. The removal of oxygen from the catalyst up to the limit of one monolayer is assumed to result generally in a decrease of surface filling with oxygen and in the increase of its bond energy, while the acid-base properties of the catalyst are considered to change insignificantly. The results of the experiments carried out at low conversions of propene, when diallyl and carbon dioxide form in parallel, are given in Fig. 2.12 [102]. The rate of propene oxidation on the oxidized contact surface is rather high due to the participation of weakly bound oxygen. With increasing degrees of oxygen removal the rate

361

I

0

I

20

I

f

I

40 60 80 9 ,% monolayer

Fig. 2.12. The relationship behveen rate and selectivity of catalytic diallyl (1,2,5,6) and CO2 (3,4,7,8) formation and the quantity of oxygen removed from Bi-Sn contact surface at 450°C (closed points) and at Mo”C (open points) [NE].

of CO2 formation decreases and the rate of alkene dimerization increases slightly, as is clear from Fig. 2.12. The same relationships are observed for the interaction of propene with the oxidized surface of the catalyst in the absence of gaseous oxygen [ 1021.These relationships can be explained by the dependence of the strength of the oxygen bond upon its surface concentration, as presented in Fig. 2.11. The bond energy of oxygen on the surface of the BiSn catalyst rises from 200 to 360 kJ/mol when the degree of reduction is less than 80%. The most marked decrease in the total oxidation rate occurs over this area of the surface, which is not observed for the rate of diallyl formation. These relationships show that it is weakly bonded oxygen which participates in total propene oxidation. Moreover, its presence on the surface has a negative effect on the process of oxydehydrodimerixation, which is demonstrated by the slight increase of the rate of diallyl formation in the region of degrees of reduction under consideration. Both reactions are likely to proceed, under these circumstances at least, via the same surface compound, an alkene n-complex for example, which converts to the products of deep oxidation on the oxidized surface in the presence of weakly bonded oxygen. Insofar as this oxygen is consumed upon reduction of the surface the fraction of totally oxidized products is reduced and, correspondingly, the fraction of dimerized complexes increases. Consequently, the selectivity towards CO2 is decreased and there is a contrasting increase in selectivity towards diallyl. Thus, summarEng all the data considered, one can note that the rate of oxidative dehydrocoupling is determined mainly by the character and energy of the alkene’s interaction with the catalyst surface and depends rather weakly upon the bond energy of oxygen on the contact surface. The rate of the parallel process of deep oxidation, by contrast,

362

depends on the energy of the surface oxygen binding and decreases with the bond strength. This means that the necessary condition for’the selective action of the catalysts for oxidative alkene dehydrocoupling is the absence of weakly bonded surface oxygen responsible for the total alkene oxidation. 2.4. Mechanisms of the oxidutive dehydrodimenktion of alkenes The problem of the mechanism of alkene oxidation on the surface of solid catalysts includes three important aspects: activation of molecular oxygen, activation of an organic molecule usually via dissociation of one or several of its bonds, and their interaction to form the reaction products. The processes of reagent activation proceed on the catalyst surface, while the interaction of their activated forms may occur either on the surface (heterogeneous mechanism) or in the gas phase over this surface (homogeneous-heterogeneous mechanism). It is generally considered [103-1061 that the activation of molecular oxygen on the partially reduced surface of oxide systems occurs via the scheme: O2 (gas) -

02 (ads) -% 02-z

2 O-2

2 0% (lattice)

(2.111)

This scheme assumes the formation of 0~~ and O- ion-radicals under the conversion of 02 into lattice 0%. These may be formed independently on different centers; i.e. on centers which transfer one or two electrons, respectively [107]. The existence of radical oxygen forms on the oxide surface is probable at low temperatures, at which their transformation to the diamagnetic 0% form is complicated, and also in systems which are greatly diluted by dielectric, when the metal ions are separated and the electrical conductivity of the solid is rather low. Under the real conditions of oxidative catalysis on semiconductors the transfers occur rather quickly at high temperatures and the regular 0% form is the dominant one. All these forms of oxygen can participate in oxidation reactions. Thus, for many simple and complex oxide catalysts, the mild oxidation of hydrocarbons to oxygen-containing compounds [ 1,2] proceeds via a cyclic reduction-oxidation mechanism, when the reaction products form by the interaction of hydrocarbon with the surface OS, while molecular oxygen is responsible only for reoxidation of the reduced surface. This mechanism is also called a staged redox reaction or the separated interaction mechanism. The deep oxidation of hydrocarbons on the same catalysts occur, at low temperatures in particular, via an associative (concerted) mechanism with conjugated oxygen transfer [ 1081. The main difference from the redox mechanism is that the products form at the stage of surface reoxidation, which proceeds simultaneously with the oxidation of the surface hydrocarbon compound. The participation of the oxygen of the catalysts in the reaction, the coincidence of the rates of catalyst reduction measured independently and its reoxidation in the steady state with the rate of catalytic reaction, the formation of the same products with the same selectivity in the processes of both reduction and catalysis are the main indications that the

363

reaction proceeds via the sequential reduction-oxidation reaction. The participation of the oxygen of the oxide contact surface can be determined by the isotope method, introducing “heavy” oxygen into the reaction mixture or into the catalyst and following its distribution in the oxidation products [ 1091.However, one has to account for the presence of isotope exchange between the molecular oxygen, catalyst and reaction products, which complicates the experimental procedure. Moreover, if the concerted mechanism proceeds via conjugated transfer, then a transfer of oxygen isotopes to the reaction products will occur [llO], which does not allow to distinguish this mechanism from the staged one. The method of independent measurement and the comparison of the kinetic characteristics of the catalytic reaction and its individual stages [ 11l] is more simple and reliable. This approach is widely used in studies of the mechanism of partial and total hydrocarbon oxidation [1,2,91,108]. It was applied in the series of papers dedicated to the investigation of the mechanism of the oxidative dehydrodimerization of alkenes. Table 2.6, based on data published in ref. 112, presents a comparison of the rates of oxidative dehydrodimerization and deep propene oxidation in the combined and separate interaction of propene and oxygen with bismuth-tin oxide catalysts and at hydrocarbon conversions less than 15%. Under these conditions diallyl and CO2 are the products of the reaction, benzene not being formed. The satisfactory coincidence of diallyl formation via catalysis and reduction by propene prove that the oxydehydrodimerization proceeds via a staged mechanism involving the separate interaction of the reagents with the contact surface. The major part of the CO2 forms from the oxygen of the catalyst. At the same time, the essential excess of the rates of catalytic total propene oxidation over the rates under reduction conditions indicates that there is a noticeable contribution from the fusion mechanism under the conditions considered, the contribution decreasing as the temperature rises. A comparison of the rates of the overall catalytic reaction and its separate stages is given in Table 2.6 for the catalyst under steady-state conditions. The data on the interaction of pulses of propene-oxygen mixtures with a Bi-Sn catalyst displayed in Fig. 2.12 show that the activity of the catalyst is stabilized at a degree of surface TABLE 2.6 The rates of catalytic reaction, reduction and oxidation on a bismuth-tin catalyst in steady-state conditions [112] Reaction

Temperature (“C)

Composition of mixture (vol.-%)

Rate r*10-16 (molecules/m2 s)

C3H6

C3H6

02

02

w-ho

co2

Catalysis Reduction Oxidation

450 450 450

0.67 0.70

0.38 0.40

1.87 1.49 -

5.16 3.59

0.29 0.40 -

4.05 2.01 -

Catalysis Reduction Oxidation

500 500 500

0.66 0.67 -

0.40 0.38

3.35 2.45 -

8.45 3.69

0.62 0.65 -

6.09 3.52

Reaction

time, h

Fig. 2.13.The decline in weight and degree of reduction (0) of a bismuth-tin oxide catalyst in the process of propene oxydehydrodimerizationat 450°C [lU]. reduction of 70-80%. This state of the contact surface is characterized by the presence of only strongly bonded oxygen having a bond energy of 360-370 kJ/mol. This corresponds to the steady state value under the pulse regime conditions in which the experiments on the comparison of catalysis and reduction rates were carried out (Table 2.6.). The steady state of the catalyst is characterized by a similar degree of reduction in the dynamic regime too. The data [ 1131presented in Fig. 2.13 show that, if the catalytic reaction is performed under flow condition that are as close as possible to the conditions of the pulse regime, the contact weight is reduced due to the reduction process. (It was demonstrated by special experiments that there is no noticeable coke deposition on the catalysts surface in the reaction process [113].) The difference in catalyst weight in the oxidized and steady (constant weight and activity) states is 89% of one monolayer of oxygen. The phase composition of the contact surface seems to remain unchanged. But it has been noticed that the surface undergoes an essential change under the influence of the reaction mixture. This conclusion follows from a comparison of X-ray photoelectron spectra of fresh and steadystate samples (Fig.2.14): there is a negative shift in the bismuth maxima for the latter. The bismuth cations are supposed to be in a two-valent state and the tin ions are in a four-valent one on the surface of the catalysts in the steady state. The absence of weakly bonded oxygen on the surface of bismuth-tin catalysts in the steady state is confirmed by the data on temperature programmed reoxidation given in Fig. 2.15 [114]. The oxidation of the contact surface, previously reduced with propene, by oxygen shows the presence of both oxygen forms: weakly and strongly bonded. The former is produced in the low temperature region 175-200°C with an activation energy of 97+8 kJ/mol and is followed by Sn” to Sn4’ and Bi” to Bi+ and/or Bi2+ transitions. The latter is formed at higher temperatures (370-4Oo”C) and has an activation energy of 189 +4 kJ/mol and is the result of Bi+(Bi2+) to Bi3+ oxidation. There is only one high temperature peak on the curve of temperature-programmed reoxidation of the contact surface brought to its steady state in the catalytic reaction and then cooled rapidly under an inert atmosphere. The absence of the former peak indicates that, under the reaction conditions, only weakly bonded oxygen is removed from the surface (observed as a high temperature peak under reoxidation), but not strongly bonded oxygen. It is likely that weakly bonded oxygen, which is removed from the catalyst surface by the reaction mixture, is practically absent under steady state reaction conditions.

365

Sn

I

I

I

I

175

I

165 155

I

1

145

I

I

575

I

I

I

I

I

I

I

495

I

I

475

E,eV Fig. 2.14. XF’E spectra of the freshly prepared (1) and steady-state (2) samples of bismuth-tin [113].

oxide catalyst

The oxygen of the bismuth-tin oxide catalyst also participates in the reaction of oxidative propene to benzene dehydrocyclodimerization. In refs. 39,115 the pulse technique was used to demonstrate that a considerable amount of benzene, formed via oxidative diallyl dehydroaromatization, is observed in the products of catalyst reduction upon the interaction of propene with the contact surface in the absence of gaseous oxygen at high degrees of propene conversion.

0.4 2 2 0e3.5 . 0.2-e a o*l-A I

I

300

500

1 I T:C

700

Fig. 2.15. The curves of temperature programmed reoxidation of the reduced (3 wt.-%, 1) and steady-state (2) samples of bismuth-tin oxide catalyst [114].

366

IO a"e +

. 54 .>

2 0 300

500 700 T;C

Fii. 2.16. The temperature dependence of diallyl (l), benzene (2) and CO2 (3,4) yield under catalytic conditions (open points) and Bi-Ti surface reduction with propene (closed points) [116].

Similar results were obtained with pulse studies of the reaction on a bismuth-titanium oxide catalyst [116] presented in Fig. 2.16. The yields of dimerization products - diallyl and benzene - under catalysis and contact surface reduction are very close (within one curve), which means that they are likely to be formed via the staged redox mechanism. The deep propene oxidation proceeds via the associative mechanism under the conditions studied, because CO2 yields upon reduction are many times lower than they are in the catalytic reaction [ 1171.The same products - benzene and carbon dioxide - are produced in both processes at similar selectivities. Comparison of these results with the results of phase analysis allowed the authors of ref. 117 to suppose the reaction of propene oxycyclodimerization on Bi-P catalysts to be accompanied by Bi3+ * Bi” transitions. The same products (diallyl, benzene, carbon dioxide) are formed via the interaction of propene with individual oxides of bismuth [112-1211, tin [115,120,121] zinc [121,122] or indium [121] as in the catalytic reaction. Moreover, the rates and selectivity of their formation on the initial stage of reduction are very close to the analogous characteristics of the catalytic processes. The increase of the degree of oxide reduction essentially affects the distribution of reaction products. The rate of propene to carbon dioxide conversion decreases most sharply and the rate of benzene formation less sharply for SnOz as is evident from data presented in Fig. 2.17 [121]. The rate of diallyl formation, which depends on the quantity of the oxygen removed from the surface, goes through a maximum. The most distinct maximum was observed for ZnO [ 1211.These relationships [ 1221are explained by the fact that the number of centers on which the mild alkene activation occurs increases along with the decrease of 0 2- ions consumed in deep propene oxidation on the oxide surface. The reduced surface areas containing coordinationally unsaturated metal cations accessible for adsorption of hydrocarbon molecules on them may serve as these centers. These data on reduction indicate that a definite part of the products of oxidative dehydrodimerization and total propene oxidation is formed via direct participation of the oxygen of the metal oxides. The contribution of this mechanism to the catalytic process can be

367

0

25

50 9,%

75

100

monolayer

Fig. 2.17. The influence of the tin dioxide surface reduction on the rate of CO2 (1). benzene (2) and dialiyl (3) formation after reduction with propene at 500°C [121]. estimated by comparison of the reaction rate with the rates of the processes of the reactants separated interaction with the catalyst in its steady state condition [112,123]. All the points mentioned above can be summarized by the following general scheme of the redox mechanism of alkene RH oxydehydrodimerization (parentheses signify the adsorbed condition):

(0RI-J + (0) + O-(R) (2)2(R)+R-R

+ U-9

+ 20

(2.IV)

(3) 2 (OH) -, II20 + () + (0)

where (0) and ( ) represent oxidized and reduced points of catalyst surface. Stages l-3 are the reduction, while stage 4 is reoxidation of the contact surface. This scheme reflects the case where the reaction proceeds via surface dimerization of partially dehydrogenated forms (R) of the alkene. Other variants of the mechanism are possible, which differ in the character of stage 2. We shall discuss them further on. Let us now consider the nature and structure of the (R) particles. All the investigations consider the oxidative dehydrodimerization of alkenes as proceeding via the formation of ally1 structure:

This unanimous consideration is based on the following facts. Parallel to propene oxydehydrodimerization the formation of acrolein is observed on some oxide catalysts [44,124] and this is known to occur through the surface ally1 compound [125]. On the bismuth-tin oxide catalyst both ally1 chloride and bromide convert into benzene, while 2-bromopropene is totally oxidized [XX]. And, finally, the composition and structure of

100

300 T:C

500

Fig. 2.18. The curves of propene (1,2) and bromallyl (3) thermodesorption (2) surfaces of zinc dioxide (1281.

from oxidiid

(1,3) and reduced

dialkene and the aromatic hydrocarbons formed do not conflict with the ally1 mechanism [ 1271. Besides such indirect arguments there is also direct evidence for n-ally1 compounds on the surface of the oxydehydrodimerization catalysts. Zinc oxide has been studied in more detail in refs. 128-132. It was shown by the method of programmed temperature desorption [ 1281291 that propene is adsorbed on the oxidized ZnO surface in two forms - weakly and strongly bonded. The former, as can be seen from the data in Fig. 2.18 [128] desorbs as propene in a temperature region of 5CL15O”Cwith 70 kJ/mol activation energy, and the latter as CO2 and Hz0 at 250450°C with 176 kJ/mol activation energy. On the reduced surface propene is adsorbed only in a reversible weakly bonded form. Comparison of these datawith IR spectra of adsorbed propene allows one to suppose the weakly bonded propene to be in the form of a n-complex or n-ally1 on the surface. At room temperature the

Fii 2.19. IR spectra of zinc oxide before (dotted line) and after (continuous line) adsorption of propene

Pw

z-complex is usually formed, which transforms to a-ally1 at higher temperatures (75125°C). The formation of the latter is confirmed by the appearance of the corresponding absorption line at 1545 cm-’ and the line of the surface hydroxyl group at 3593 cm-’ (Fig. 2.19) and also by adsorption of deuterated propene [ 1301. An analogous situation is observed with the adsorption of 1-butene on ZnO [ 1331.At low temperatures the IR spectra of adsorbed alkene show the presence of only a n-complex, but with increasing temperature the lines corresponding to surface OH-groups and n-ally1 compounds appear. Similar results were obtained in ref. 134, in which propene adsorption on Bi203 was studied, except for the presence of a middle temperature peak on the curve of thermodesorption corresponding to the desorption of the products of partial propene oxidation: acrolein and acetone. The results on the oxidative conversion of the propene labelled by radioactive “C in its methylene group on silver give evidence in favor of the ally1mechanism [ 1351.The data on radiometric analysis of the diallyl formed in this reaction and of the products of its oxidation by potassium permanganate-succinic acid and CO2 are given in Table 2.7. The succinic acid and CO2 seem to have equal radioactivity. This is possible in the case if oxidative propene dehydrodimerization occurs through symmetric n-ally1 compound by the scheme [135]:

&=CH-CH$,CH=CH, ZC)il,=CH-CH,

C$=CH-&CH,CH=CJ

(2-V

C\=CH-C?i,-c"H,-CH=CH,

The state of ally1 compounds on the surface of the alkene oxydehydrodimerization catalysts differs essentially from the one on the contact surface of partial oxidation catalysts. This is first of all confiied by the spectral characteristics. Thus, if the frequency of the antisymmetric oscillations of the propene ally1 fragment on ZnO is 1545 cm-’ [130], it changes in the interval of 1400-1460 cm-’ on copper-containing and other catalysts [ 1361391.These deviations can be provided by the possibility of the stabilization on the surface TABLE 2.7 The distributionof diallylradioactivityin the productsof its oxidationby potassiumpermanganate11351 Reaction products

Radioactivity

Diallyl Carbon dioxide Succinicacid

73 112 50

Relatwe 100 51 46

370

Fii. 2.20. ESR spectra of ZnO samples [142]: (a) oxidized at 4oo”c; (b) containing propene adsorbed at WC, (c) after oxygen adsorption at 25°C on the surface with the previously adsorbed propene.

of n-ally1 complexes in different charge states - negatively charged, radical and positively charged. It is supposed [ 128,140,141], that in the case of dissociative propene adsorption on ZnO the anion-like ally1 compounds are formed, while on C&O, MOOJ and other partial oxidation catalysts the positively charged forms are developed, which are capable of nucleophilic interactions with the surface oxygen to form the oxygen containing products. The latter is confirmed by the results of a study on the adsorption of ally1 bromide on zinc oxide. Fig. 2.18 represents the spectrum of thermodesorption of bromallyl from a ZnO surface, taken from ref. 128. The desorption products, besides the initial reagent, propene and carbon dioxide also contain acrolein (peak with maximum at 18OT), which was not discovered in the experiments on propene thermodesorption. This fact indicates the ability of bromallyl during its adsorption on zinc oxide to form positively charged ally1compounds due to the presence of a strongly polarized C-Br bond in bromallyl. The frequency of ally1 fragment oscillation in the IR spectra of adsorbed GI-IsBr is 1470 cm-‘, which is close to its characteristics on propene to acrolein oxidation catalysts. It is evident that, for the efficient performance of the stage of ally1particle dimerization, not charged but radical forms are necessary. Their presence on the surface of the same zinc oxide was proved by RSR [ 1421.The spectra obtained are given in Fig. 2.20. The adsorption of propene at 25°C on the oxidized surface was shown to provide the spectrum with a hyperfine structure consisting of five lines. This spectrum, which is attributed to the n-ally1 radical, is registered at -196°C and disappears reversibly as the temperature rises to ambient. The radicals are quite stable and are destroyed only after the sample is heated up to 12&13O“C. The intensity of their signals also decreases due to oxygen adsorption. Simultaneously the ion-radicals Ot appear on the surface (Fig. 2.20 c). The data confirming the possibility for analogous compounds to exist on the surface of other catalysts for oxydehydrocouplingwere not referred to. Nevertheless most researchers [44,88,143] assume the oxidative alkene dimerization to occur through w- ally1 radicals. As was mentioned above, the ally1 compounds form via dissociation of a C-H bond in the a-position to an unsaturated bond. Their radical forms may be provided either by the direct homolytic rupture of the bond indicated or by heterolytic rupture with carbanion

371

formation followed by electron transfer from carbanion to the catalyst. One more type of activation is known when heterolytic dissociation of a C-H bond occurs with the formation of carbocation and negatively charged hydrogen ion [90]. However, in this case it is very difficult to assume the possibility of carbocation conversion to a radical via the electron transfer from the catalyst due to the competing reaction of the oqproduct formation after the oxygen ion nucleophilic attack. The high polarizability of C-H bonds in the propene methyl group due to the hyperconjugation effect and the basic properties of zinc oxide, supported by the data of IR and ESR spectroscopy mentioned above, are in favor of the heterolytic mechanism of z-ally1 radical formation through carbanion: CFCH-CH, I I I I I Z”*+

1 r I

\

(2-W

I

;2-

According to this scheme, protonation of methyl hydrogen is rovided by nucleophilic 0% ion. The carbanion formed as a result is stabilized on the Zn f+ cation, transforming later into a radical via electron transfer to the center of stabilization. The electron transfer is likely to occur to the catalyst bulk as well. This variant is assumed in [142], where the formation of z-ally1 radicals was shown by ESR to be accompanied by a decrease of the intensity of the V-center signals. The heterolytic type of alkene activation is also observed on the surface of several tin-containing catalysts (Sn-Bi, Sn-Pb, Sn-Ti, Sn-In) for which a dependence of the rates of propene and isobutene dimerization on the concentration of the basic centers has been observed (Fig. 2.9). The functions of these centers in fixing the proton split out of the hydrocarbon molecule is performed by nucleophilic O*- ions as has been shown in Section 2.2. The stabilization of carbanions and their conversion to radicals is likely to proceed on TABLE 2.8 Catalytic oxidation of propene on binary oxides at 450°C Catalyst

W-k

composition

conversion (%)

BiMo = 1:l Sn:Mo = 1:l Sn:Sb = 1:l In:Mo = 2:3 Mo:Sb = 1:2 Bi:Sn = 1:l Bi:In = 1:l In:Sn = 1:l

10.7 23.1 18.9 25.0 20.0 24.0

Rate, rd6 C3H6molecules/ m*s 130 15.8 1.6 12.3 3.8 5.3 8.9 2.4

Selectivity (%) Aerolein Diallyl + benzene

Refs.

86

0

144

40 43 31 81 0 0 0

0 0

145 146 147 144 146 146 146

traces 0 30 18 3

372

the cations of tin, bismuth, indium and other low-valent metals. This assumption is based on the data concerning the catalytic properties of complex oxide systems. Table 2.8 presents the results concerning propene oxidation on binary oxides containing tin, bismuth, indium, molybdenum and antimony in different combinations. The samples can be divided according to their catalytic properties into two groups. One is represented by oxides which are sufficiently selective in relation to acrolein formation. They usually contain molybdenum or antimony. The catalysts of the other group, which are various combinations of bismuth, indium and tin oxides, promote the formation of noticeable amounts of diallyl and benzene instead of acrolein - the products of oxidative dehydrodimerization and aromatization of propene. These results give indirect evidence that the formation via propene adsorption of n-ally1 compounds proceeds on the centers containing bismuth, tin or indium. If centers of another nature are not present, the ally1 compounds dime&e, producing 1,5-hexadiene with its subsequent dehydroaromatization to benzene. If there are some places on the surface which can provide for the oxidation of the ally1 compound, then acrolein formation is observed. The oxygen polyhedra of more electronegative anion-forming elements, such as molybdenum, tungsten, antimony, vanadium, are likely to serve as such centers. Thus, in ref. 148 the MoOa octahedra are assumed to be responsible for the oxidation of the ally1 complex to acrolein a bismuth-molybdenum catalyst, the MO = 0 bond being the most important. This conclusion is confirmed by the results on the activity of bismuth-tin-antimony catalysts BiSbt-An-0 [146]. This series also includes tin-antimony contact, which are rather active in partial oxidation but which do not catalyze the oxidative dehydrodimerization of propene. As x increases an exchange of antimony ions by bismuth cations occurs. This event is accompanied by a monotonous decrease of acrolein formation and by a simultaneous increase of activity in the oxydehydrodimerization reaction (Fig. 2.21). Acrolein is not observed on the bismuth-tin catalyst, but noticeable amounts of diallyl and benzene are produced. These relationships also demonstrate that the formation and stabilization of highly active ally1 compounds occurs on tin and bismuth cations.

0 Sb-Sn

0.25

0.50

0.75

l.bO Bi -57

Fig. 2.21. The dependence of rate and selectivity of acrolein (closed points) and diallyl (open points) formation at 450°C on the composition of bismuth-tin-antimony oxide catalysts [146].

373

0.6 $1 - 0.4 3

0.2 0

400 600 800 1000

Fq. 2.22. The influence of temperature on the relative content of a&l radicals in the gas phase in a control experiment (1) and on the interaction of propene with bismuth-molybdenum (2), manganese oxide (3), zinc oxide (4) catalysts (bl/I42-ratio of currents of corresponding masses in mass spectrometer) [Iso]. Ally1 radicals were also found in the gaseous phase over alkene oxidative dehydrodimerization catalysts. These data are referred to in refs. 149-151 for manganese oxide, [ 1521 for bismuth oxide and [153] for sodium-manganese contact. The concentration of free radicals over ZnO and MnO2 was found by mass-spectrometry to increase with rising temperature. At the same time, temperature has no influence on the free radical content over bismuth molybdate (Fig. 2.22), which is a typical catalyst for partial alkene oxidation. These results are in good agreement with the earlier assumption on the different states of n-ally1 compounds on the contact surfaces of dimerization and oxidation catalysts. The radical content over the catalyst also depends on the composition of the gas phase. The ESR method together with the freezing technique [152] showed that the introduction of small quantities of dioxygen into the gas phase over BizO3 results in an increase of the free ally1 radical concentration, the essential dioxygen additives leading to allylperoxide radical formation. The latter are also observed under similar conditions over the ZnO surface [ 1511.Their quantity increases with rising temperature and increasing contact time. The activation energy of allylperoxide radical formation is very close to the heat of R’ + 02 = ROi reaction. This fact and the similar properties of the peroxide radical formation under non-catalytic propene oxidation conditions [154] allow us to assume that they are generally formed in the bulk from the ally1 radicals and do not participate in the alkene dehydrodimerization. It is interesting that the ally1 radicals obtained via pyrolysis of 1,5-hexadiene are totally indifferent to the nature of the catalyst. Only acrolein is formed on interaction with the oxydehydrodimerization catalysts (ZnO, BizO$, partial (Bi-M-O, Mo03) and total (Co304, NiO) alkene oxidation [ 150,155]. At the same time the desorption of allylperoxide radicals from the surface is observed [156], these may be intermediates in the propene to acrolein oxidation. These data show that the ally1 radicals formed on propene adsorption and upon diallyl pyrolysis differ greatly. The latter are likely to transform to peroxide compounds after dioxygen adsorption on the surface ions. The presence of free radicals over the surface of the catalyst may indicate that the reaction proceeds via their dimerixation in the gas phase:

374

TABLE 2.9 The dependence of the product composition of propene oxidation at WC surface [l57j

Diitance

Conversion Product concentration C&j ic6HlO -6 (o/o)

fromZn0 (c-m) 0.05

32.3

1.10

32.4 0.0080

0.0047

(l)RH + (0) + 0-W WW+R’ (3)2R’+ (902

CO2

HZ

ROz (rcl.%)

0.06

1.5

1.3

7.8

80

0.06

1.5

1.3

7.8

21

+ W-0

+0 R-R

(4)2(m)+

co

on the distance from the ZnO

(2.W) ~~0 + (0)

+ ()

+2()-,2(O)

This scheme reflects the general heterogeneous-homogeneous mechanism of dehydrodimerization processes. In the case of propene reactions it signifies that x-ally1 radicals formed on the contact surface desorbing into the bulk recombine to lJ-hexadiene. This conclusion was drawn by the authors of ref. 152 after a study of the reaction on bismuth oxide. The Armenian researchers [157] who also discovered the ejection of radicals into the bulk from a ZnO surface at 560°C and maintained the influence of the variation of the bulk volume over the catalyst bed on the performance of propene oxidation came to the same conclusions. They demonstrated that at greater distances from the surface the concentration of peroxide radicals, formed from the ally1 ones, decreases and, thus, the content of 1,5-hexadiene increases (Table 2.9). The filling of the reactor with a glass packing causes a disappearance of the radicals in the gas phase and dramatically decreases the propene conversion and diallyl yield as well. The composition of the reaction products also changes dramatically, the carbon monoxide content is decreased, and propene oxide is totally absent. Assuming all that has been mentioned above one may represent the heterogeneous-homogeneous oxidation of propene assisted by ZnO as follows: (1) CHJ_CH=CH2 + (0) + () + (CH2.XH.XH2) + (OH) (2) 2(CHr,CH,CH2) -B CH2 = CH-CHz-CHKH = CH2 + 2( ) (3) 2(OH) -, I-I20 + (0) + ( ) (4) 02 + 2( ) -D 2(O) (5) (CHr.:CH.:CH2) -+ ‘CHz-CH = CH2 + ( ) (6) 2’CH2_cH = CH2 + CH2 = CH-CHz-CHz-CH = CH2 (2.vIII) (7) ‘CHX-CH = CH2 + 02 -, CH2 = CH-CHzOO (8) CH2 = CH-CHrOO + CHS-CH = CH2 -, CHKH-CH2 + CH2 = CH-CHzO \J 0

375

(9) CI-I2=CH-CH200 -, HCHO + ‘CHr-CHO + Hz0 + CO + ‘CH=CH2 (10) CI-&=CH-CH200 (ll)CHKH=CH2 + 2(O) -, (CHz=CH-CHrOO) + () (12) (CT-I2= CH-CH200) -, CH2 = CH-CHO + (OH) (13) &I-I2 = CH-CHzOO) --, CH2 = CH-CHrOo’ + ( ) The ally1 radical like compounds formed after the dissociative propene adsorption may also dimerize on the surface to lJ-hexadiene by a heterogeneous mechanism (stages 1.4) or they may desorb into the gas phase. In the second case free radicals are formed which are capable of reacting in two main ways - recombination to 1,Shexadiene or oxidation to peroxide radicals. The latter can interact with propene providing propene oxide (reaction 8) or can decompose via reactions 9 and 10, producing formaldehyde and carbon monoxide with water, respectively. The radicals CH2=CH-CH20’, ‘CHtcHO and ‘CH=CH2 formed as the result of these transformations are subsequently oxidized to acetaldehyde, formaldehyde and carbon oxides by dioxygenvia a chain mechanism. The stages 11-13 given at the end of the scheme reflect the possibility for ally1radicals to adsorb, the latter either transforming to acrolein on the surface or desorbing into the bulk as peroxide radicals according to the data considered above on the interactions of ZnO with the radicals obtained via pyrolysis. At the same time there are data which can hardly be explained by the heterogeneoushomogeneous mechanism. Thus, it was found for the bismuth-tin catalyst [158] that the varying of the bulk volume over this catalyst does not in practice effect the performance of propene oxydehydrodimerixation in the 4%530°C temperature range. The equality of the rates of 1,5-hexadiene formation under catalysis and reduction on this (Table 2.6) and bismuth-titanium (Fig. 2.16) catalysts gives evidence against the transfer of the reaction into the bulk, because the surface reduction of oxides usually occurs via the heterogeneous mechanism [159]. Thus, for these contacts it is more reasonable to assume that dimerixation proceeds on their surface. The surface dimerization may proceed in two ways. The first is the recombination of ally1 radicals. A possible mechanism for this process is represented above by scheme (2.IV), according to which dehydrodimers form as a result of the coupling of radicals stabilized on separate centers. This variant, accepted in refs. 143 and 160 assumes a high radical mobility. Trimm [ 1611 considers the possibility for two ally1 radicals to stabilize on the same center (metal cation) of the contact surface with their subsequent recombination. The other way in which surface dimerization may occur, suggested in ref. 162, is the interaction of alkene molecules from the gas phase with the adsorbed ally1 radical by the scheme:

Cl)= + (0) + 0-W + (0) (2)RH + W + (0) -, R-R+(OH)+() (3) 2(OH) + Hz0 + ( ) + (0) (4)02 + 2()-, 2(O)

(2.1X)

376

This mechanism, which can be named a ‘shock’ one due to its similarity to well known Tiviggs mechanism [163] of ethylene oxidation, is more likely to be realized on catalysts with a low mobility of adsorbed radicals (R). This path does not contradict the possibility for mechanism (2.IV) to proceed. Moreover, the mechanisms can coexist. Their contributions to the general rate of the process will probably be determined by the mobility of the ally1compounds on the surface, which depends upon the conditions of the experiment and the nature of the catalyst. In particular, one would expect an increase in the mobility of the radicals as the temperature increases and thus an enlarged contribution from mechanism (2.W). The same effect may be provided by the weakening of the electron-accepting properties of the radical stabilizing centers, which is usually achieved by doping the catalyst with strongly basic compounds. These factors are also likely to influence the ratio of contributions by the heterogeneous and homogeneous mechanisms. It is no accident that the transfer of the reaction to the bulk is observed at rather high temperatures (SWC and higher), the concentration of ally1 radicals in the gas phase over the catalyst with basic properties (zinc, bismuth and manganese oxides) being essentially higher at high temperatures than in the systems with acidic properties (bismuth molybdate, molybdenum trioxide). This gives us the opportunity to regulate the performance of the reaction via one mechanism or the other. 2.5. Kineticsof reactions The kinetics of oxidative dehydrodimerization of alkenes have been most intensively studied for the reactions of propene and isobutene. The measurements were carried out on both simple and complex oxide catalysts. Similar kinetic regularities were observed for propene oxydehydrodimerization on the individual oxides of thallium [14] and indium [ 1241.At low hydrocarbon conversions the main products are hexadiene and carbon dioxide; acrolein formation is also observed on indium oxide. Under these conditions rates of diallyl and CO2 formation are found which are proportional to the 0.4-0.5 power of the concentrations of propene and dioxygen and these can be described satisfactorily by empirical equations of the polynomial type. At high conversions benzene is found in noticeable amounts, together with traces of hexatriene and 1,3cyclohexadiene among the reaction products. For In.203 typical relationships between the main product yields upon the contact time are given in Fig. 2.23. One can see from this figure that benzene is formed from diallyl. Its yield depends scarcely at all on the reaction mixture composition and increases to a marked degree as the temperature rises. Benzene has no inhibiting effect, while the introduction of diallyl into the initial mixture decreases the total rates of propene conversion and acrolein formation. The latter also retards the formation of diallyl and benzene. The data on the kinetics of propene dimerization obtained in refs. 14 and 124 are described by the Langmuir-Hinshelwood equation (2.1):

r = (1 + &I-I6 &H6+ K&HI0PC&,0 + KQH40’Pc3H,ci)2’(1+ d7-Gm2

(2.1)

377

8-

Fig.2.23. The influenceof contact time on diallyl(l), benzene(2) acrolein(3) and COz (4) yieldson 111203 at WC

(1241.

where k is the rate constant of the surface reaction, KandP (with the corresponding indexes) are the constants of adsorption equilibrium and partial pressures of propene, dioxygen, diallyl and acrolein. According to the model considered, the limiting stage of the process is the formation of 1,Shexadiene via the interaction of two propene molecules and dissociated oxygen adsorbed on the surface. Propene, diallyl and acrolein adsorb competitively at the same sites, while dioxygen adsorbs on the others. Three valent thallium and indium ions serve as catalytic centers capable of forming n-complexes with alkene. Two molecules of propene, which dissociate to proton and carbanion CSHS- under the influence of nucleg philic OS, are assumed to adsorb on one center. Then the transfer of two electrons (one from each carbanion) to the metal cation occurs with subsequent ally1 radical formation, followed by their further recombination to lJ-hexadiene. A similar kinetic model was accepted for the oxidative conversion of isobutene to 2,5-dimethyl-1,5-hexadiene andp-xylene on indium oxide [M-165]. At low conversion of the initial alkene the formation rates of these compounds are described by similar polynomial equations and have almost the same activation energies: 126 and 130 kJ/mol, respectively. At high contact times (Fig. 2.24) the processes of demethylation of the required products become noticeable, resulting in the formation of toluene and benzene. A comparison of the data on the kinetics of propene and isobutene dimerization on indium oxide given in ref. 164 shows that the isobutene reaction is first order in alkene, with dialkene, rather than an aromatic hydrocarbon being formed. The Langmuir-Hinshelwood model was also applied to the interpretation of the kinetics of propene dehydrodimerization on bismuth oxide. Diallyl is supposed [ 1661to form as a

378

result of the surface reaction of two propene molecules with oxygen, adsorbed in equilibrium on the same centers, in contrast to the In203 model. This stage, in which the ally1 C-H bond breaks, is considered to be the limiting one. It is based on the equilibrium value of 1.7 for the kinetic isotope effect &&D obtained for dimerization of ordinary propene and propene deuterated on the methyl group. Note that a closely similar (1.3kO.3) kinetic isotope effect is observed under similar conditions for the process of ally1radical formation over the surface of Biro3 [ 1671. This variant of the reaction mechanism is presented by the kinetic equation: Wc& r = (1 + &H,

&rJ2 PC*

Ko$‘oz +

KoZp02)~

(2.2)

which attains the following linear form after integration:

m21n(l-x) + 3m2_4x -t y

(&-x2) + $ (3&.x2 + x3) = k(Kcm f+&J2 Ko2t (2.3)

here m = l+Kca PC*, A =Kozp002; pc3~a andP0o, are the partial pressures of propene and oxygen entering the catalyst bed, x is the oxygen conversion, and t- is the contact time. A plot of eqn. (2.3) is presented in Fig. 2.25. Here the left side is the ordinate, and the right one is the abscissa. It is evident that the experimental data obtained at different initial

00 f

60

E -0‘ ‘: 40 0, : .20

Fig. 2.24. The influence of wntad time on the 2,5-dimethylhexadiene-1,5(l), p-xylene (2). toluene (3), benzene (4) and COZ@yields under oxidativeisobuteneconversion conditions on indium oxide at X?OT[164].

379

Fii. 2.25. The kinetics of propene oxidehydrodiierization on Bi203 at 5OCKin coordinates of eqn. (2.3) (lfX] C(C3H6) = 1.9 moVm3,triangles - C(O2) = 1.5 movm3, circles - C(Oz) =0.8 movm3.

II

’

o

0.12

0

1’

024

11 0.36

TS Fig. 2.26. The relationship between propene conversion (1) and benzene (2h co2

(3) , co (4) and

dihydrogen (5) yields on zinc oxide at 500°C and the contact time (1511.

concentrations of dioxygen correspond to the straight line required by Eq. (2.3). The other kinetic models, the model of Mars-van Krevelen in particular, based on the redox mechanism, do not agree so well with the experimental findings [ 1661. The kinetics of the oxidative conversion of propene on ZnO were studied in ref. 151. The main reaction products in the temperature range of 480-500°C are benzene, carbon oxides and hydrogen, their yields rising as the contact time increases (Fig. 2.26). When the temperature is raised the propene conversion increases as well as benzene and CO2 yields, the hydrogen yield decreases, and the CO yield passes through a maximum. The activation energies of the total process and of COz, CO and benzene formation are 84 and 71,42,189 kJ/mol, respectively. The reaction orders for propene and dioxygen have been determined. Their values were found to be the same for the processes of general propene conversion and its total oxidation: the order is 0.5 in hydrocarbon, while the oxygen order changes from 1 to 0 as the oxygen concentration increases in the reaction mixture. The rate of benzene formation rises with a change of propene content. from 1 to 3 vol.-% to first order and to second order at 3-6 vol.-% propene concentration. The reaction is first order in oxygen over the whole range of concentrations studied (1.5-15 vol.-%).

TABLE 2.10 Kineticcharacteristicsof oxydehydrodimerization of alkeneson a Bi-Sn catalyst[160] Alkene

Reaction order by Cd3211 by 02

Propene Isobutene 1-Butene cis-Butene trans-2-Butene

1.6 1.5 2.0 1.9 1.9

0 0 0 0 0

E’ (kJ/mol) 1% 207 182 2Q3 214

kz** (mu&? s) 0.0036 0.0043 1.0 0.16 0.16

Kl** @Pa-‘) 470 750 30 180 140

*E: observedactivationenergy **kz, K1 constantsof eqn. (2.4).

The kinetics of alkene oxydehydrodimerization on bismuth-tin oxide catalysts have been studied in detail. Several publications have been dedicated to these studies. The reactions of oxidative dehydrocoupling of propene, isobutene and butene isomers have been investigated in ref. 160. The measurements were obtained at low conversion of the initial hydrocarbon, when the formation of dimers and carbon dioxide occurs in parallel ways. The main kinetic characteristics of these reactions are given in Table 2.10. The rate of dehydrodimerization is shown to be proportional to the 1.5-2.0 power of the partial pressure of alkene and does not depend on the oxygen content in the mixture. For the deep oxidation the orders in hydrocarbon and oxygen were found to be close to unity and ca. 0.5, respectively. The results of kinetic measurements were considered by the authors of ref. 160, as in refs. 124 and 164, according to the Langmuir-Hinshelwood model for an ideally adsorbed layer. It is assumed that the n-complex is formed on absorption of the alkene, which interacts weakly through its hydrogen atom in the a-position to an unsaturated bond with the oxygen atom located nearby on the catalyst surface. Then the disconnection of this hydrogen atom occurs with subsequent formation of a metal ion complex with two ally1 radicals. These considerations may be reflected by the following kinetic scheme, in which for the reaction of propene (z- is the metal cation of the catalyst): (1) c&j

+

zo &dko

(2) 2W-LjzO 3

GH5zC3H5

+ HOzOH

(3) c3HszGH5 - fast GHlo + z fast (4) HOzOH H20 + z0 (5)G

+

(2-W

2z%o

The order in alkene close to 2 and the inverse dependence of the reaction rate constant upon the energy of C-II bond dissociation in the ally1 position permits us to assume that stage 2 is the limiting one. Stage 1 proceeds under equilibrium, stages 3,4, and 5 are fast. The rate equation, following from this scheme:

381

Iryi 150

c 1

100

l!?? 2

50

0

20

40

Fig. 2.27. The kinetics of the oxidative dehydrodimerization of propene (1) and lsobutene(2) on Bi-Sn catalystsat 500°Cin coordinatesof eqn. (2.4)[Ml].

r=

KI k2

[

1+

2

&Ha

KI PC&,, I

(2.4)

gives a rather good description of the experimental data on propene and isobutene dimeri-

zation. As can be seen from Fig. 2.27 they fit the straight lines in l/G - ~/Pc,H~~coordinates of the linearized eqn. (2.4). In the reaction of a mixture of propene and isobutene the dimerization of each one to the corresponding dialkene occurs simultaneously with dehydrocoupling to 2-methyl-1,5hexadiene. An increase in the partial pressure of one of the alkenes in the presence of a constant amount of the other one raises the rates of dimerization and codimerization of the former, but decreases the dimerization rate of the second alkene. The kinetics of these processes can also be described by Langmuir-Hinshelwood equations. The following equation holds for the dehydrocoupling of propene with isobutene: k&H6 r =

*Kc~H~ *PC*

(1 + Kc~H~ &16

*&Ha

+ &&Is

PQHs)~

(2.5)

and for the dimerization of either of them in the mixture the following equations are valid: 2 Kcmj

PC-

1 + Kc~H~PCJH~+ KQ~ PQH~ I

r’ = k’

(2-Q

1 2

r ,,

_

- k

&HSPWS

,,

1+

KCSH~ &He

+ KCSHS PCAHS

(2.7)

382

Isobutene has a decelerating effect on propene dimerization, which is caused by the higher value of the equilibrium constant of its adsorption in comparisonwith propene (Table 2.10). Rather different kinetic characteristics were found for the reaction of propene oxidative dehydrodimerization on bismuth-tin catalyst of the same composition (Bi/Sn = 1) in ref. 168. It was found, first, that there is an essential decrease of the reaction order in alkene as the temperature rose: from 2.0 at 450°C to 1.2 at 530°C. Second, there is a negative relationship between the rate of diallyl formation and the oxygen concentration in the reaction mixture, as shown in Fig. 2.28. This relationship was obtained over a rather large (0.05-20 kPa) range of oxygen partial pressures. At oxygen partial pressures lower than 0.05 kPa the activity of the catalyst is rather unstable and decreases rapidly due to its reduction. A definite minimum oxygen content in the gas phase is required for the steady-state performance of this reaction. The concrete value of this concentration depends on the temperature and partial propene pressure. Thus, at 500°C and PC3& =3.8 kPa it is ca. 10.04 kPa. Under the conditions studied (temperature 45&53OT, propene conversion less than 10%) virtually no further diallyl oxidation occurs. This first becomes noticeable, according to the data presented in refs. 168 and 169, at diallyl partial pressures higher than 1 kPa. The steady state concentration in the experiments under consideration did not exceed 0.1 kPa. The reaction products (diallyl, COZ, water) had no decelerating effect on the propene oxydehydrodimerization. These relationships were interpreted in ref. 168 in terms of the following kinetic scheme:

a

0

16

8

P, ,$Pa

24

0

8

16

24

Paz,kf+

3

Fig. 2.28. The relationship between the rate of diallyl formation on a Bi-Sn catalyst and the partial pressures of propene (a) and dioxygea (b) at 450 (l), 485 (2), 500 (3) and 530°C (4). Curves - calculated from eqn. (2.9), points - experiment [WI.

383

kl

(1)z +

02-202

(2)z +

20249220

K2

(3) z0 + GI&

(2.W

GHs zOH

(4) C3I-IzzOH + C3Ha-

k4

C&o + H20 + z

The first stage is the irreversible adsorption of oxygen on a reduced active center on the surface. An investigation of the oxidation of the reduced catalyst by the oxygen demonstrated [ 1581that the rate of this process actually increases proportionally to the reduction of the surface. The second (quasi-equilibrium) stage reflects the redistribution of the centers with different degrees of oxidation on the surface. In the third stage an intermediate surface compound of the ally1type is formed via the interaction of propene with the oxidized z0 center: the intermediate produces diallyl interacting with propene from the gas phase in the fourth stage. This scheme corresponds to the kinetic expression: u(i K2d t &~Po2 r=Kzk:k&a6+2k~Po2(K

2 @P 3 c3ng+Kzk3k4Pc~+2klk4Poz)

(2.8)

Under the conditions studied the surface of the catalyst in its steady state is reduced up to 7080% [102]. In this range of degrees of reduction the process of reoxidation proceeds rather rapidly with an activation energy of 15-20 kJ/mol, which means that kl > k&. As a result of this, expression (2.8) is simplified to: K2tiW’&

k’k’ ‘P&

’ = K2 k3 (k3 + k4)&3Hg+ 2kW’o3 = k’ PC* + k”Po2

(2.9)

where k’ = K2k3 2L2k~and k” = kdcdk3 f ka. Eqn.(2.9) gives a satisfactory description of the experimental data (Fig. 2.28). The other forms of kinetic equations, including those of the Langmuir-Hinshelwood type mentioned above, were not in such good agreement with the experimental results. The kinetic data and their interpretation obtained for the Bi-Sn catalyst in ref. 160 differ essentially from those found in ref. 168. These disagreements are likely to be caused by the different procedures used for the preparation of the samples, which determine the phase content and surface properties of the catalyst. Thus, it is mentioned in ref. 170 that two samples of bismuth-tin contact surface, synthesized via different techniques, demonstrate different selectivities in propene oxydehydrodimerization. Moreover, the activation energy of the sample with higher selectivity is lower by 84 kJ than that of the nonselective sample, which is caused by the presence of more active centers for allyl-hydrogen splitting of propene on the surface. Several kinetic dependences have been studied for propene oxydehydrodimerization on bismuth-phosphorus oxide catalysts [171]. It was found, in particular, that at low contact times 1,5-hexadiene and benzene are formed in comparable quantities, and 1,3-cyclohexadiene in essentially smaller amounts. The increase of this parameter of the reaction provides

384

4 60. is

20

*i 0.5

0

1.0

T,g-h/mol

1.5

K

1

o.5f&y< 0

OS

, ID

1.5 7,s

Fig. 2.29. The influence of contact time on the conversion of propene to lJ-hexadiene (l), cyclohexadiene (2), benzene (3) and carbon oxides (4) on Bi-P oxide catalyst at 550°C [171]. Fig. 2.30. The dependence of steady-state concentrations of 1,Shexadiene (l), benzene (2), cyclohexadiene1,3 (3) and CO2 (4) on the propene-dioxygen mixture contact time with Bi-Fe catalyst at MST [ET].

increase of benzene and 1,3-cyclohexadiene yields, while the yield of 1,Shexadiene passes through a maximum (Fig. 2.29), which provides evidence for the sequential conversion route. The yield of these substances also increases with the rise of the partial pressures of propene and oxygen in the mixture. At the same time the order in terms of the alkene for the process of 1,5-hexadiene accumulation is found to be less than unity, and that for the benzene formation is greater than unity. In terms of dioxygen the reaction was found to be first order for both cases. The same measurements were performed for a bismuth-iron oxide catalyst [172]. On this contact surface either 1,3-cyclohexadiene or benzene are formed subsequent to 1,5-hexadiene, that is supported by the character of the dependence of their concentrations upon the contact time given in Fig. 2.30. Under the conditions studied the rates of 1,5-hexadiene and 1,3cyclohexadiene accumulation, and of benzene formation, can be expressed by the corresponding equations: an

rl = kl P&&

(2.10)

r-2= kz P&m

(2.11) (2.12)

r3 = Q

PCJH~

The deep oxidation of propene is of order zero in hydrocarbon and 0.6 in oxygen. These equations are empirical and are not supported by the corresponding reaction mechanism. To smnmarize the material discussed above, let us note that the kinetic characteristics of the processes of oxidative dehydrodimerixation of alkenes are determined to a great extent by the catalyst composition. This is related first of all to the dependence of the reaction rates on concentration, which appears to be of a rather variant nature. Two approaches became widely used for their interpretation, based on the well known assumptions of

385

Langmuir-Hinshelwood and Mars-van Krevelen. In the first one the dimerization is assumed to proceed in the adsorption layer via simultaneous interaction of two alkene molecules with an oxygen atom or molecule. This model provides a satisfactory fit to the experimental data for several catalysts in spite of the fact that the probability of this triple collision seems to be rather low. The second approach is more reasonable and in accord with the experimental facts, and is based on the consideration that the reaction occurs via a cyclic reduction+xidation of the contact surface. Various types of kinetic model are possible, depending on the nature of the catalyst and the conditions under which the process is performed, which reflects the cases of either pure heterogeneous or heterogeneous-homogeneous mechanisms for the dimerization stage. Aromatic hydrocarbons are formed on many catalysts, along with the dialkenes. An increase in the contact time causes their yields to rise, together with a decrease of selectivity to dehydrodimers, as has been demonstrated above, and this gives evidence for the sequential conversion of dialkenes to aromatic hydrocarbons. In particular, for the reaction of propene it is assumed [46,127] that benzene formation proceeds via oxidative dehydrogenation of lJ-hexadiene to hexatriene, followed by oxidative dehydrogenation of the latter to the aromatic compound. The results of 1,5-hexadiene and 1,3-cyclohexadiene oxidation to a mixture of the corresponding 1,3,5-hexatriene and benzene [127] are evidence in favor of this scheme. Besides this, 1,3-cyclohexadiene - the only cyclic dialkene found among reaction products - was observed to form on several catalysts [9,18,118]. Trimm and co-workers [124,164] suggest a somewhat simplified scheme for hexadiene aromatization which excludes the stage of hexatriene formation from consideration. These authors assume, since they did not discover the latter among the reaction products, that the hexadiene located on the catalyst surface converts to 1,3cyclohexadiene, which can further migrate to the catalytic center, thus providing for the formation of benzene. However, the absence of hexatrienes among the reaction products cannot be a sufficient reason for excluding them from the formal reaction scheme. One may assume that the hexatriene developed ‘cycloisomerizes’ easily and rapidly to cyclohexadienes due to its high reactivity, as occurs during the G-dehydrocyclization of alkanes [ 1731. Indeed, the authors of refs. 169-174, who carried out a detailed study of the transformations of 1,5-hexadiene, cyclohexene, 1,3-cyclohexadiene and benzene on a bismuth-tin oxide catalyst, have found that hexatriene formed under conditions of propene oxidative dehydrodimerization undergoes cyclization and aromatization at a high rate. They also found that 1,5-hexadiene is unlikely to cyclize mainly through cyclohexene. All the details mentioned above demonstrate that the oxidative dehydroaromatization of alkenes is a complicated sequential reaction, which can be described by the following scheme: 1 2 3 4 propene + 1,5-hexadiene + 1,3,5-hexatriene + 1,3-cyclohexadiene + benzene. All the stages of oxidative dehydrogenation (1,2,4) are catalytic ones, the cyclization stage (3) is thermal, similar to the process of dehydrocyclization [173].

386

TABLE 2.11 Oxidation of diallyl on Bin

oxide catalyst at WC

r *lo-‘6

WPa) ~HIO 0.34 1.09 1.20 2.56 4.36

[X8]

02

(molecules Q,Hla/mz s)

co2

0.40 1.40 1.78 2.10 2.33

0.010 0.094 0.119 0.141 0.156

9.2 8.8 8.5 8.8 8.8

Table 2.11 presents data on the oxidation of 1,5-hexadiene under closely similar conditions to those ones of propene oxydehydrodimerization. This process becomes noticeable as the diallyl partial pressures exceed 1 kPa, which corresponds to propene fractional conversions of 3O-40% and more. At low values of alkene conversion (less than 10%) the 1,5-hexadiene partial pressures do not exceed 0.1 kPa and one may disregard its deeper oxidation in comparison with the total rate of CO2 formation. Under these conditions the deep oxidation is competitive with oxydehydrodimerization on the other centers, since it involves a lower strength of the bond between the oxygen with the catalysts surface, according to the following mechanism [73]: (1) 22’ +

02

g12zto

(2) Z’O + C3HsA (3) (C3H6)Oz’ (4) Z’2cO3-

fast

+

(C3H6)OZ

llz’0 fas.t 3Z’zcO3 + 3H20 + 62’

(2X1)

(202 + Z’O + Z’

(5) 22’2cO3 + 02-

fast

2C02 + 42’0

The limiting stage is that of propene adsorption, which is then oxidized by the oxygen of z’0 centers to the surface compounds of carbonate type. The destruction of the latter occurs either thermally (the mechanism of separate interaction) or else it is assisted by dioxygen from the gas phase (associative mechanism). Both routes of CO2 formation are likely to occur, as can be seen from the data of Table 2.6. The equation

(2.13) corresponds to scheme (2X1) which is in accordance with the experimental data. The deviations between rates calculated on the basis of this equation and the experimentally determined rates of deep propene oxidation are illustrated in Fig. 2.31. The heat of oxygen adsorption calculated from the temperature dependence of Kl is 19O-210 kJ/mol. This

387

b

a

16

8

0

PC,k Pa

24

0

8

16

24

POpa

Fig. 2.31. The relationship between the rate of CO2 formation on the Bi-Sn catalyst and the partial pressures of propene (a) and dioxygen (b) at 450 (l), 475 (2), 500 (3) and 530°C (4). Carves - calculated from eqn. (2.13), points - experiment.

v

sp/o 40

20

1

_--3

4 5 L 0

0.4

0.0

1.2

Ts Fig. 2.32. The influence of contact time on the selectivity of oxidative lJ-hexadiene

conversion to benzene (l), lJ+yclohexadiene (2), CO2 (3), propene (4) and ethylene (5) on BiSn catalyst at 575°C [169].

value is in good agreement with the results of pulse and calorimetric measurements described in Section 2.4, according to which the deep propene oxidation on this catalyst is provided by the oxygen which is relatively weakly bonded to the surface with a bond energy of ca. 200 kJ/mol. The conversion of l,S-hexadiene at appropriate partial pressures - 2 kPa and more have been studied in detail in [169]. The data given in Fig. 2.32 demonstrate that, at 575°C more than 30% of the transforming diallyl undergoes deep oxidation. The intensity of this process increases with rising temperature and increasing oxygen content in the mixture. 1,3Cyclohexadiene and benzene are more stable in relation to deep oxidation. According to [174] data at 550-575°C 80-90% of cyclohexadiene is dehydrogenated to benzene, the remainder being oxidized to carbon dioxide. The conversion of benzene to CO2 under these conditions is 1%.

Thus, the formal scheme of oxidative propene conversion on a bismuth-tin oxide catalyst is as follows [174]: C~CH-CHfC~=~iH,Cy~~CH~~C~=C~H.C~~~~Q40 4

6

1

I3

9

(2xIII)

(02 At low propene conversions the further oxidation of intermediates and products by reactions 6,7,8 and 9 does not proceed inpractice, allowing the scheme to be significantly simplified. The scheme (2X11) is of a rather general character and is likely to accord with reactions of oxidative dehydrodimerization of other alkenes. REFEXENCES

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