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On Principles of Catalyst Choice for Selective Oxidation
G. Centi and F. Trifiro’ (Editors), New Deuelopments in Selectiue Oxidation 0 1990 Elsevier Science PublishersB.V., Amsterdam - Printed in The Netherlands
699
OX PR1:’JCIPLES OF CATALYST C H O I C E FOR SELECTIVE OXIDAT103 G .I.
GOLOBETS The L.V.Pisarzhevskii I n s t i t u t e of Physical Chemistry, Academy o f Sciences of t h e Ukrainian SSR, Prospekt Ifauki 31,Miev, USSR A3STHACT
Elementary s t e p s o f heterogeneous c a t a l y t i c processes o f o x i d a t i o n have been c l a s s i f i e d on t h e basis o f redox and a c i d base i n t e r a c t i o n s of r e a c t a n t s w i t h c a t a l y s t . General types o f mechanisms have been distinguished. Nain f a c t o r s ( e n e r g e t i c , s t r u c t u r a l , e t c . ) determining c a t a l y t i c p r o p e r t i e s o f metal oxides f o r each type o f t h e mechanism a r e examined.
-
IM9’RODUCTI 011 A progress i n t h e s e l e c t i v e oxidation r e q u i r e s an i n t e n s i v e development of i t s theory. The proposed paper r e p r e s e n t s t h e res u l t s i n t h i s f i e l d obtained by t h e author with h i s coworkers. I n t h i s connection, some remarks should be made. ( 1 ) As a consequence o f t h e s e l e c t e d aim, we s h a l l d i s c u s s here mainly our own d a t a (not t h i n k i n g , of course, t h a t they a r e b e t t e r than those o f many o t h e r authors which are summarized, f o r instance, i n Refs. /1,2/). ( 2 ) We s h a l l analyse not only our recent r e s u l t s b u t a l s o some e a r l i e r ones t o e x h i b i t a whole system of our views. ( 3 ) Nevertheless our approach i s f a r of completeness, we hope i t t o be worthy o f discussion.
C L A S S I F I C A T T O : : (IF ELETUWl’ARY STEPS AND LIECIIld~LSLlS OF TIIE 0XII)ATIOl\l PROCXSSES O‘JER OXIDE CATALYSTS It was e a r l i e r shown / 3 / t h a t i n t h e oxidation c a t a l y s i s n o t only redox but a l s o acid-base i n t e r a c t i o n s of reagents R ( o r products BO,) w i t h c a t a l y s t s a r e e s s e n t i a l . Therefore, i t i s n a t u r a l
t o c l a s s i f y elementary s t e p s using t h i s principle.Typica1 redox s t e p s a r e t h e reduction of surfaoe oxides, iCm+02’, with R and t h e r e o x i d a t i o n o f t h e i r reduced form, ( -is a n oxygen vacancy) by 02. Acid base i n t e r a c t i o n s a r e assumed i n a wide sense involving t h e formation of complexes with t h e Lewis o r Brznsted a c t i v e s i t e s , s a l t - l i k e compounds,x- complexes, e t c . A comprehensive system o f such a c l a s s i f i c a t i o n i s given i n Ref./2/. Using i t , one can c o n s t r u c t t h e majority of known mechanisms o f s e l e c t i v e and deep oxidation /?/. T t can be i l l u s t r a t e d
-
PiT(m-’’+a - a
694
by the methanol oxidation / 2 / :
iICHO
1I
In this scheme step 8) is purely redox one; steps 1 ) , 3 ) are purely acid-base stages; steps 2),5),7) are I1mixedt1 ones involving the both types of interaction; steps 4),6) include the migration of oxygen or organic intermediates. The mechanism of a catalytic reaction on a given catalyst can change significantly with temperature (Table). At moderate temperatures which are typical of iiidustrial catalysis, mechanisms of alternating surface reduction-reoxidation involving 02- species predominate. These reaction pathways (Type I) have been studied far better than other ones. For example, the Type I mechanism of the o-xylene oxidation over V-oxide catalysts has been proved by the ESR method "in situ" /4/, by com?arison of kinetics of separate staps and the overall reaction /5/, by the nethod 3f competing reactions /6/. Similar evidences have been obtained for selective oxidation of ammonia at eJ15O-35O0C /7/. A t low temperatures, when endothermal desorption of the intermediates ( 2 0 , e t c ) is especially retarded, the mechanisms with conplex reoxidation of a surface (Type IIa) become advantageous; in this case the formation of final products occurs simultaneously with exothermal surface reoxidation /8/. In catalysis over diluted layers of supported metal ions, when the reduction o f O2 to 2 02- is inhibited, the mechanisms involving reactive adsorbed species are profitable /9/ (Type I I b ) . At elevated temperatures the desorption of radicals or atoms initiating the gas-phase reaction (heterogeneous-homogeneous catalysis / l o / ) becomes possible. Such mechanisms (Type 111) are typical, f o r instance, of the CH4 oxidative coupling /11, 12/.
.
6-
695
TA2IU
C l a s s i f i c a t i o n o f t y p i c a l mechanisms of oxidation t,OC
700
t
Type 111. Heterogeneouohomogeneous radical-chain me chani sms
500 -Type I. Bechadsms o f al-
ternating surface reduction-reoxidation
300 -Type IIa. Nechanisms with 100
-
complex reoxidation o f surface
r a d i c a l s o f oxygen ON PHINCIPLES OF CATALYST SXLECYIOB FOR VARIOUS
rypm OP GCHANISM
Iliechanisms o f Type I. Since a dominating intermediate i s 02; c a t a l y t i c p r o p e r t i e s should depend on i t a bond energy expressed a s heat, Q,, o f the process o2 + 2 M ( ~ - ' ) +I I -+2 PP+O'I n simple cases l i k e the :I2 oxidation an exact r e l a t i o n s h i p between 61, and s p e c i f i c catalyt i c a c t i v i t y , r , 13 observed ( I n r decreases with growing Us since I,!-0 bonds a r e broken i n a s l o w s t e p ) / I , 13/. Since i n the format i o n o f deep oxidation products more number of P-Q bondv a r e broken than i n p a r t i a l oxidation, s e l e c t i v i t y towards mild oxidation increases with Q , /7/. Xowever, i n the majority of reactions t y p i c a l deviations appear i n the region o f high Q s values (see Fig. la). This i a observed i n the oxidation of aromatics, o l e f i n s , a l cohols, acrolein, etc. / l / . The oxides exhibiting "elevated" acNb5+, Yo6+, W6+, i.e. ions vrrith e l e c t r o n t i v i t y contain T i 4 + , ,'5V configuration o f do which a r e strong Lewis acids. Bar such catalysts,
.
I n r = In r ( Q s + ) I n r(QA), where In r ( Q s )i s a contribution determined by (-4, (i.e. by the energy o f redox processes l i k e Urn+ + e *li(m-l)) while l n r ( Q A )
696
-
-
.Fig* 1. P l o t s of lg r 4,(a> and lg r(Q,) a c i d i t y ( b ) f o r the CH OH oxidation: 1-Co304, 2-Mn02, 3-NiO, 4-Cr203, 5-Fe203, 6 3 CuXo04, 7-Mo03, 8-V205, 9-ZnM004, 10-CoNo04, 11-NIId004, 12-'Pi02, I 3-Bi2 MOO^)^, 14-bM004, 15-Cr2 (Moo4 13, 16-Pe2 ( ~ 1 0 0 /2/ ~)~ i s a f r a c t i o n of a c t i v i t y dependine; ~n the energy of acid-base i n t e r a c t i o n s , .,Q U s i n g mechanisms l i k e ( I ) , one can divide an influence o f these two f a c t o r s / 2 / . The values of ln r ( Q , ) correspond t o p o i n t s on the s t r a i g h t e q, u a l l i n g t3 l i n e of I n r Q (Fig. la). The values of In r ( Q A ) 3 v e r t i c a l d e v i a t i o n s f r o m tile l i n e , can be expressed a s
-
In
1 = [r ( 1 -a)u,,
3
OH + OL QHC,,]
+
const,
where Qi a r e adaorption h e a t s o f CH30H and HCHO; d i a a t r a n s f e r c o e f f i c i e n t i n the Zr
697
p e r t i e s a r e determined not only by energy f a c t o r s but a l s o by o ther charac t e r i a t i c s A s t o e l e c t r o n i c s t r u c t u r e , the behaviour o f ions of t r a n s i t i o n and non-transition metals i n many cases i s q u i t e d i f f e r e n t / I T , 18/. I n the oxidative coupling, s p e c i f i c properties a r e exh i b i t e d by the oxides o f p-elements. On the other hand, i n deep oxidation, the oxides o f d- and p-elements behave a s a common group o f c a t a l y s t s /2/. I n m a n y cases, the geometric ( s t r u c t u r a l ) c h a r a c t e r i s t i c s of a c t i v e s u d a c e are o f primary importance. Let us consider two aspects of the problem. When the formation o f key intermediate requires multi-point adsorption, a distance between a c t i v e s i t e s influences s e l e c t i v i t y , S o , i n the low-temperature oxidation of ammonia i n t o N20, the two-point adsorption o f HNO-species i s necessary. An increase i n the distance between active c e n t r e s causes low p r o b a b i l i t y of existence of the complexes and, a s a r e s u l t , N2 becomes a d o m i n a t i n g product /7/. With the mechanism l i k e ( I ) , f o r the formation o f the l e a s t oxidized product, RO, a s i n g l e a c t i v e s i t e , iyIm+02", i s s u f f i c i e n t . But f o r the more oxidized product, R02, double " c l u s t e r s " .Mm'02~fm+02-. a r e necessary. Hence, one should expect that the s e l e c t i v i t y towards m i l d oxidation w i l l increase when the conc e n t r a t i o n o f the "clusterst1 (determined by the concentration o f an a c t i v e component, CM, supported on i n e r t c a r r i e r ) i s decreaaed. The experimental data on the o-xylene oxidation over V 0 / A 1 0 2 5 2 3 (Fig. 2 ) q u a n t i t a t i v e l y confirm this conclusion /19/.
.
..
..
w
pig, 2. P l o t o f 0 ( f r a c t i o n o f V-ions united i n the " c l u s t e r s " 1 and s e l e c t i v i t i e s S towards o-tol u i c aldehyde (OTA) and phthalic anhydride (PA) va surface concentr a t i o n o f V C(), /19/
0.1 0.2 0.3 04 ,c
698
-Uechanisms T f i e y Ir
of T~J~-.IJ& s u a l l y c o e x i s t with the mechanisms o f Type I. 'The f o l l o w i n g r a t h e r g e n e r a l r e g u l a r i t y was deduced / 2 0 / : m i l d o x i d a t i o n products appear only v i a mechanism I while t h e more oxidized p r o ducts a r e formed v i a the both mechanisms, I and 1Ia.Por i n s t a n c e , bensaldehyde f r o m toluene i s formed only v i a mechanism I , while maleic anhydride and C02 a r e produced via mechanism I I a (C02 a l s o v i a mechanism I ) . The same i s observed i n the o x i d a t i o n o f d e r i v a t i v e s o f toluene, i n t h e CH4 oxidation, e t c . Hence, the following t y p i c a l scheme o f conversion o f h y d r o carbon, R, i n t o mild o x i d a t i o n products, RO ( f o r example, aldehyd e s ) and i n t o more oxidized producte, R02(anhydrides o f organic a c i d s , C 0 2 ) can be proposed: 5
8 -ROZ 20
O2
0
7 R.2
-R02Z2
zo
Complex HOZ i s an adsorbed aldehyde bound t o M(m-l)+ by means o f unshared e l e c t r o n p a i r o f the C=O group; RO2Z2 i B a carboxylate o r carbonate complex. Applying the Brznsted-Temkin r e l a t i o n s h i p t o the r a t e c o n s t a n t s , one can deduce the equation f o r s e l e c t i v i t y /21/:
where Q i s h e a t of the R02 adsorption; A and B a r e constant ( a t a R02given composition of the r e a c t i n g mixture). If i n t h e s e r i e s of compared c a t a l y s t s the values of ,,Q change much l e s g than those o f Q,, we o b t a i n ( f o r competing 2mechanisms I , I I a ) :
Hence i t follow6 t h a t on i n c r e a s i n g Q, s e l e c t i v i t y should p a s s through a maximum. This is r e a l l y observed i n the o x i d a t i o n of CH o r toluene (Fig. 3 ) and in t h e naphthalene o x i d a t i o n /22/. 4 A t small v a l u e s of Q,, the mechanism of Type I predominates,, and s e l e c t i v i t y grows with Q,; a t high v a l u e s o f Q, the mechanism of Type IIa p r e v a i l s and s e l e c t i v i t y decreases with an i n c r e a s e i n Q,. I n a more s t r i c t examination one should a l s o take i n t o account the values o f Q i.e. e n e r g i e s o f acid-base i n t e r a c t i o n . A'
699
"
100 L?,
200
,kJ/mool
Fig. 3. Dependence between Q, and selectivity: (a) in the CH4 oxidation on phosphates (1Cr, 2-Mn, 3-Niy 4Fe , 5 -B ,6-U,7-Mg 8-Co); (b) in the toluene oxidation on oxides (1-C0,2Cu, 3-Mn,4-Ni,5-Cr,
6-Fe,7-V,840,9-w, 10-Ti, 11-Nb) /21/
Mechanisms of Type IIb There are few evidences in favour of these mechanisms /9/. The partial oxidation o f methane over diluted layers of supported metal oxidea is likely to belong to such reaction schemes /23/.In this case the catalytic activity in total process decreases with an increase in the energy bond of metal ions with 6-(Qi)r selectivity towards mild oxidation (into methanol and formaldehyde) increases with growing Qi values /24/. Mechanisms of Type I11 These mechanism have been studied insufficiently. In Ref. /25/ the relationship between probability of heterogeneous-homogeneous mechaniem and the chemical nature of oxide catalysts have been clarified for the model reaction of hydrogen oxidation. It was ahown that in the presence of active heterogeneous catalysts (the oxidee of Co, Cu, V, etc.) the lower ignition limit, P1, of the detonating gas increaaee in comparison with the P1 value in a quartz vessel (i.e. in the presence of Si02, see Fig. 4 ) .
Pig. 4. Comparison o f values of the 1st ignition limit of detonating gaa and the values of q for metal oxides: P 1 -v205 3 2-CO-304, 3-CUO 4-MO03, 5-Ti02, 6-Si02 /25/
700
Since in the course o f reaction the catelysts were reduced, it was suiposed that an intensive chain termination takes place by way of the irreversible reduction of UmOn with €I-atoms (which are known to be the main intermediates in the radical-chain combustion of H2). Then one should expect P1 to be greater (heterogeneous-homogeneous mechanism to be less probable) when reducibility of the Dxide increases. R e a l l y , as one can see in Big.4, the P1 value increases with a decrease in the heat of dissociation, q , of metal oxides (i.e. with increasing heats o f reducP tion), The same was observed fQr the oxidation of methane, carbon monoxide and some other processes, 8 3 that the above mentioned relationship is likely to be rather wideopread. LWPEIM~’TCES
G,I. riglodeta, Ileterogcneous Catalytic h3aCtiOnS Involving Molecular O,xygen, Elsevier, Amsterdam, 1383 2 G.I. Golodets, Problemy Kinetiki i Kataliza 19(1985)28 3 G.I. Golodets, Doklcrdy Akad. Nauk SSSR, 184f1969)1334 4 V.X. Vorotyntsev, Yu.1. Pyatnitsky, G.I. Golodets,Teor.Eksper. Khim., 12( 1 976)488 5 Yu.1. Pyatnitsky V.M. Vorotyntsev, G.I. Golodets, Kataliz i Katalizatory, 1 2 f 1974128 6 Yu.1. Pyatnitsky, T.G. Skorbilina, Kinet.i Kata1.,21(1980)451 7 IT.1. Ilchenko, Uspekhi Khimii, 45(1976)2168 8 V.D. Sokolovsky, l e o r . P r o b l . K a t a l i z a , N o v o s i b i r s k , 1977, p.33 9 V.B. Kazansky, Problew Khim.Kinet., Nauka, MOSCOW, 1979,p.232 10 M.V. Polyakov, Kataliz i Katalizatory, 1(1965)35 11 X.Yu.Synev, V.N.Xorchak,O.V.Krylov,Kinet .i Kataliz,,28( 1987) 1376 12 N.I.llchenko,L.W.Xaevskaya,A.I. Bostan, G.I. Golodets, Teor. Zkspcr. Khim., 24(1988)638 1 3 G.K.Boreskov,Heterogeneous Catalysis (in Eluss.),Mauka,!doscow, 1986 14 TOG.dlkhazov, K.Yu.Adzhamov, I3 .A.IBamedov, V.P.Vislovsky, Kinet , i Kataliz., 20(1979)118 15 G.I. Golodets, Teor. Eksper. Khim., 18(1982)37 16 G.I.Golodets, V.V. Borovik, V.111. Vorotyntsev, Teor.Eksper. Khim., 22(1986)252 17 O.V. Krylov, V.F.Kiselev, Adsorption and Catalysis on Tranaition Metals and Their Oxides(in Russ. ),Khimia,Moscow,1981,p.288 18 J. Haber, N. Sochacka, B. Grzybovaka, A.Golzbiewskii, J. Xolec. Catal , 1 ( 1975135 19 Yu I Pyatnit s k y ,GoI.Golode t a ,React.Kinet .Catal .Le t t ,5 ( 1 976) 345 Katal., 20 G.I.Golodets, Yu.I,Pyatnitsky,L.N.Raevskaya,Kinet.i 1
. ..
25(1984)571
.
21 G.I.Golodeta, Kinet. i Katal., 28(1987)837 22 G.I.Golodets, Teor. Eksper.Khim., 1(1965)756 23 N.I.Ilchenko, A.I.Bostan,L.Yu.Dolgikh, G.I.Golodets,Teor.Eksp. Khim., 23(1987)641 24 N.I,Ilchenko, A.I.Boatan, G.I.Golodets, Teor.Ekaper.Khim., 24( 19881455 25 G.I.Golodets, React.Kinet.Cata1. Lett., 28(1985)131
699
OX PR1:’JCIPLES OF CATALYST C H O I C E FOR SELECTIVE OXIDAT103 G .I.
GOLOBETS The L.V.Pisarzhevskii I n s t i t u t e of Physical Chemistry, Academy o f Sciences of t h e Ukrainian SSR, Prospekt Ifauki 31,Miev, USSR A3STHACT
Elementary s t e p s o f heterogeneous c a t a l y t i c processes o f o x i d a t i o n have been c l a s s i f i e d on t h e basis o f redox and a c i d base i n t e r a c t i o n s of r e a c t a n t s w i t h c a t a l y s t . General types o f mechanisms have been distinguished. Nain f a c t o r s ( e n e r g e t i c , s t r u c t u r a l , e t c . ) determining c a t a l y t i c p r o p e r t i e s o f metal oxides f o r each type o f t h e mechanism a r e examined.
-
IM9’RODUCTI 011 A progress i n t h e s e l e c t i v e oxidation r e q u i r e s an i n t e n s i v e development of i t s theory. The proposed paper r e p r e s e n t s t h e res u l t s i n t h i s f i e l d obtained by t h e author with h i s coworkers. I n t h i s connection, some remarks should be made. ( 1 ) As a consequence o f t h e s e l e c t e d aim, we s h a l l d i s c u s s here mainly our own d a t a (not t h i n k i n g , of course, t h a t they a r e b e t t e r than those o f many o t h e r authors which are summarized, f o r instance, i n Refs. /1,2/). ( 2 ) We s h a l l analyse not only our recent r e s u l t s b u t a l s o some e a r l i e r ones t o e x h i b i t a whole system of our views. ( 3 ) Nevertheless our approach i s f a r of completeness, we hope i t t o be worthy o f discussion.
C L A S S I F I C A T T O : : (IF ELETUWl’ARY STEPS AND LIECIIld~LSLlS OF TIIE 0XII)ATIOl\l PROCXSSES O‘JER OXIDE CATALYSTS It was e a r l i e r shown / 3 / t h a t i n t h e oxidation c a t a l y s i s n o t only redox but a l s o acid-base i n t e r a c t i o n s of reagents R ( o r products BO,) w i t h c a t a l y s t s a r e e s s e n t i a l . Therefore, i t i s n a t u r a l
t o c l a s s i f y elementary s t e p s using t h i s principle.Typica1 redox s t e p s a r e t h e reduction of surfaoe oxides, iCm+02’, with R and t h e r e o x i d a t i o n o f t h e i r reduced form, ( -is a n oxygen vacancy) by 02. Acid base i n t e r a c t i o n s a r e assumed i n a wide sense involving t h e formation of complexes with t h e Lewis o r Brznsted a c t i v e s i t e s , s a l t - l i k e compounds,x- complexes, e t c . A comprehensive system o f such a c l a s s i f i c a t i o n i s given i n Ref./2/. Using i t , one can c o n s t r u c t t h e majority of known mechanisms o f s e l e c t i v e and deep oxidation /?/. T t can be i l l u s t r a t e d
-
PiT(m-’’+a - a
694
by the methanol oxidation / 2 / :
iICHO
1I
In this scheme step 8) is purely redox one; steps 1 ) , 3 ) are purely acid-base stages; steps 2),5),7) are I1mixedt1 ones involving the both types of interaction; steps 4),6) include the migration of oxygen or organic intermediates. The mechanism of a catalytic reaction on a given catalyst can change significantly with temperature (Table). At moderate temperatures which are typical of iiidustrial catalysis, mechanisms of alternating surface reduction-reoxidation involving 02- species predominate. These reaction pathways (Type I) have been studied far better than other ones. For example, the Type I mechanism of the o-xylene oxidation over V-oxide catalysts has been proved by the ESR method "in situ" /4/, by com?arison of kinetics of separate staps and the overall reaction /5/, by the nethod 3f competing reactions /6/. Similar evidences have been obtained for selective oxidation of ammonia at eJ15O-35O0C /7/. A t low temperatures, when endothermal desorption of the intermediates ( 2 0 , e t c ) is especially retarded, the mechanisms with conplex reoxidation of a surface (Type IIa) become advantageous; in this case the formation of final products occurs simultaneously with exothermal surface reoxidation /8/. In catalysis over diluted layers of supported metal ions, when the reduction o f O2 to 2 02- is inhibited, the mechanisms involving reactive adsorbed species are profitable /9/ (Type I I b ) . At elevated temperatures the desorption of radicals or atoms initiating the gas-phase reaction (heterogeneous-homogeneous catalysis / l o / ) becomes possible. Such mechanisms (Type 111) are typical, f o r instance, of the CH4 oxidative coupling /11, 12/.
.
6-
695
TA2IU
C l a s s i f i c a t i o n o f t y p i c a l mechanisms of oxidation t,OC
700
t
Type 111. Heterogeneouohomogeneous radical-chain me chani sms
500 -Type I. Bechadsms o f al-
ternating surface reduction-reoxidation
300 -Type IIa. Nechanisms with 100
-
complex reoxidation o f surface
r a d i c a l s o f oxygen ON PHINCIPLES OF CATALYST SXLECYIOB FOR VARIOUS
rypm OP GCHANISM
Iliechanisms o f Type I. Since a dominating intermediate i s 02; c a t a l y t i c p r o p e r t i e s should depend on i t a bond energy expressed a s heat, Q,, o f the process o2 + 2 M ( ~ - ' ) +I I -+2 PP+O'I n simple cases l i k e the :I2 oxidation an exact r e l a t i o n s h i p between 61, and s p e c i f i c catalyt i c a c t i v i t y , r , 13 observed ( I n r decreases with growing Us since I,!-0 bonds a r e broken i n a s l o w s t e p ) / I , 13/. Since i n the format i o n o f deep oxidation products more number of P-Q bondv a r e broken than i n p a r t i a l oxidation, s e l e c t i v i t y towards mild oxidation increases with Q , /7/. Xowever, i n the majority of reactions t y p i c a l deviations appear i n the region o f high Q s values (see Fig. la). This i a observed i n the oxidation of aromatics, o l e f i n s , a l cohols, acrolein, etc. / l / . The oxides exhibiting "elevated" acNb5+, Yo6+, W6+, i.e. ions vrrith e l e c t r o n t i v i t y contain T i 4 + , ,'5V configuration o f do which a r e strong Lewis acids. Bar such catalysts,
.
I n r = In r ( Q s + ) I n r(QA), where In r ( Q s )i s a contribution determined by (-4, (i.e. by the energy o f redox processes l i k e Urn+ + e *li(m-l)) while l n r ( Q A )
696
-
-
.Fig* 1. P l o t s of lg r 4,(a> and lg r(Q,) a c i d i t y ( b ) f o r the CH OH oxidation: 1-Co304, 2-Mn02, 3-NiO, 4-Cr203, 5-Fe203, 6 3 CuXo04, 7-Mo03, 8-V205, 9-ZnM004, 10-CoNo04, 11-NIId004, 12-'Pi02, I 3-Bi2 MOO^)^, 14-bM004, 15-Cr2 (Moo4 13, 16-Pe2 ( ~ 1 0 0 /2/ ~)~ i s a f r a c t i o n of a c t i v i t y dependine; ~n the energy of acid-base i n t e r a c t i o n s , .,Q U s i n g mechanisms l i k e ( I ) , one can divide an influence o f these two f a c t o r s / 2 / . The values of ln r ( Q , ) correspond t o p o i n t s on the s t r a i g h t e q, u a l l i n g t3 l i n e of I n r Q (Fig. la). The values of In r ( Q A ) 3 v e r t i c a l d e v i a t i o n s f r o m tile l i n e , can be expressed a s
-
In
1 = [r ( 1 -a)u,,
3
OH + OL QHC,,]
+
const,
where Qi a r e adaorption h e a t s o f CH30H and HCHO; d i a a t r a n s f e r c o e f f i c i e n t i n the Zr
697
p e r t i e s a r e determined not only by energy f a c t o r s but a l s o by o ther charac t e r i a t i c s A s t o e l e c t r o n i c s t r u c t u r e , the behaviour o f ions of t r a n s i t i o n and non-transition metals i n many cases i s q u i t e d i f f e r e n t / I T , 18/. I n the oxidative coupling, s p e c i f i c properties a r e exh i b i t e d by the oxides o f p-elements. On the other hand, i n deep oxidation, the oxides o f d- and p-elements behave a s a common group o f c a t a l y s t s /2/. I n m a n y cases, the geometric ( s t r u c t u r a l ) c h a r a c t e r i s t i c s of a c t i v e s u d a c e are o f primary importance. Let us consider two aspects of the problem. When the formation o f key intermediate requires multi-point adsorption, a distance between a c t i v e s i t e s influences s e l e c t i v i t y , S o , i n the low-temperature oxidation of ammonia i n t o N20, the two-point adsorption o f HNO-species i s necessary. An increase i n the distance between active c e n t r e s causes low p r o b a b i l i t y of existence of the complexes and, a s a r e s u l t , N2 becomes a d o m i n a t i n g product /7/. With the mechanism l i k e ( I ) , f o r the formation o f the l e a s t oxidized product, RO, a s i n g l e a c t i v e s i t e , iyIm+02", i s s u f f i c i e n t . But f o r the more oxidized product, R02, double " c l u s t e r s " .Mm'02~fm+02-. a r e necessary. Hence, one should expect that the s e l e c t i v i t y towards m i l d oxidation w i l l increase when the conc e n t r a t i o n o f the "clusterst1 (determined by the concentration o f an a c t i v e component, CM, supported on i n e r t c a r r i e r ) i s decreaaed. The experimental data on the o-xylene oxidation over V 0 / A 1 0 2 5 2 3 (Fig. 2 ) q u a n t i t a t i v e l y confirm this conclusion /19/.
.
..
..
w
pig, 2. P l o t o f 0 ( f r a c t i o n o f V-ions united i n the " c l u s t e r s " 1 and s e l e c t i v i t i e s S towards o-tol u i c aldehyde (OTA) and phthalic anhydride (PA) va surface concentr a t i o n o f V C(), /19/
0.1 0.2 0.3 04 ,c
698
-Uechanisms T f i e y Ir
of T~J~-.IJ& s u a l l y c o e x i s t with the mechanisms o f Type I. 'The f o l l o w i n g r a t h e r g e n e r a l r e g u l a r i t y was deduced / 2 0 / : m i l d o x i d a t i o n products appear only v i a mechanism I while t h e more oxidized p r o ducts a r e formed v i a the both mechanisms, I and 1Ia.Por i n s t a n c e , bensaldehyde f r o m toluene i s formed only v i a mechanism I , while maleic anhydride and C02 a r e produced via mechanism I I a (C02 a l s o v i a mechanism I ) . The same i s observed i n the o x i d a t i o n o f d e r i v a t i v e s o f toluene, i n t h e CH4 oxidation, e t c . Hence, the following t y p i c a l scheme o f conversion o f h y d r o carbon, R, i n t o mild o x i d a t i o n products, RO ( f o r example, aldehyd e s ) and i n t o more oxidized producte, R02(anhydrides o f organic a c i d s , C 0 2 ) can be proposed: 5
8 -ROZ 20
O2
0
7 R.2
-R02Z2
zo
Complex HOZ i s an adsorbed aldehyde bound t o M(m-l)+ by means o f unshared e l e c t r o n p a i r o f the C=O group; RO2Z2 i B a carboxylate o r carbonate complex. Applying the Brznsted-Temkin r e l a t i o n s h i p t o the r a t e c o n s t a n t s , one can deduce the equation f o r s e l e c t i v i t y /21/:
where Q i s h e a t of the R02 adsorption; A and B a r e constant ( a t a R02given composition of the r e a c t i n g mixture). If i n t h e s e r i e s of compared c a t a l y s t s the values of ,,Q change much l e s g than those o f Q,, we o b t a i n ( f o r competing 2mechanisms I , I I a ) :
Hence i t follow6 t h a t on i n c r e a s i n g Q, s e l e c t i v i t y should p a s s through a maximum. This is r e a l l y observed i n the o x i d a t i o n of CH o r toluene (Fig. 3 ) and in t h e naphthalene o x i d a t i o n /22/. 4 A t small v a l u e s of Q,, the mechanism of Type I predominates,, and s e l e c t i v i t y grows with Q,; a t high v a l u e s o f Q, the mechanism of Type IIa p r e v a i l s and s e l e c t i v i t y decreases with an i n c r e a s e i n Q,. I n a more s t r i c t examination one should a l s o take i n t o account the values o f Q i.e. e n e r g i e s o f acid-base i n t e r a c t i o n . A'
699
"
100 L?,
200
,kJ/mool
Fig. 3. Dependence between Q, and selectivity: (a) in the CH4 oxidation on phosphates (1Cr, 2-Mn, 3-Niy 4Fe , 5 -B ,6-U,7-Mg 8-Co); (b) in the toluene oxidation on oxides (1-C0,2Cu, 3-Mn,4-Ni,5-Cr,
6-Fe,7-V,840,9-w, 10-Ti, 11-Nb) /21/
Mechanisms of Type IIb There are few evidences in favour of these mechanisms /9/. The partial oxidation o f methane over diluted layers of supported metal oxidea is likely to belong to such reaction schemes /23/.In this case the catalytic activity in total process decreases with an increase in the energy bond of metal ions with 6-(Qi)r selectivity towards mild oxidation (into methanol and formaldehyde) increases with growing Qi values /24/. Mechanisms of Type I11 These mechanism have been studied insufficiently. In Ref. /25/ the relationship between probability of heterogeneous-homogeneous mechaniem and the chemical nature of oxide catalysts have been clarified for the model reaction of hydrogen oxidation. It was ahown that in the presence of active heterogeneous catalysts (the oxidee of Co, Cu, V, etc.) the lower ignition limit, P1, of the detonating gas increaaee in comparison with the P1 value in a quartz vessel (i.e. in the presence of Si02, see Fig. 4 ) .
Pig. 4. Comparison o f values of the 1st ignition limit of detonating gaa and the values of q for metal oxides: P 1 -v205 3 2-CO-304, 3-CUO 4-MO03, 5-Ti02, 6-Si02 /25/
700
Since in the course o f reaction the catelysts were reduced, it was suiposed that an intensive chain termination takes place by way of the irreversible reduction of UmOn with €I-atoms (which are known to be the main intermediates in the radical-chain combustion of H2). Then one should expect P1 to be greater (heterogeneous-homogeneous mechanism to be less probable) when reducibility of the Dxide increases. R e a l l y , as one can see in Big.4, the P1 value increases with a decrease in the heat of dissociation, q , of metal oxides (i.e. with increasing heats o f reducP tion), The same was observed fQr the oxidation of methane, carbon monoxide and some other processes, 8 3 that the above mentioned relationship is likely to be rather wideopread. LWPEIM~’TCES
G,I. riglodeta, Ileterogcneous Catalytic h3aCtiOnS Involving Molecular O,xygen, Elsevier, Amsterdam, 1383 2 G.I. Golodets, Problemy Kinetiki i Kataliza 19(1985)28 3 G.I. Golodets, Doklcrdy Akad. Nauk SSSR, 184f1969)1334 4 V.X. Vorotyntsev, Yu.1. Pyatnitsky, G.I. Golodets,Teor.Eksper. Khim., 12( 1 976)488 5 Yu.1. Pyatnitsky V.M. Vorotyntsev, G.I. Golodets, Kataliz i Katalizatory, 1 2 f 1974128 6 Yu.1. Pyatnitsky, T.G. Skorbilina, Kinet.i Kata1.,21(1980)451 7 IT.1. Ilchenko, Uspekhi Khimii, 45(1976)2168 8 V.D. Sokolovsky, l e o r . P r o b l . K a t a l i z a , N o v o s i b i r s k , 1977, p.33 9 V.B. Kazansky, Problew Khim.Kinet., Nauka, MOSCOW, 1979,p.232 10 M.V. Polyakov, Kataliz i Katalizatory, 1(1965)35 11 X.Yu.Synev, V.N.Xorchak,O.V.Krylov,Kinet .i Kataliz,,28( 1987) 1376 12 N.I.llchenko,L.W.Xaevskaya,A.I. Bostan, G.I. Golodets, Teor. Zkspcr. Khim., 24(1988)638 1 3 G.K.Boreskov,Heterogeneous Catalysis (in Eluss.),Mauka,!doscow, 1986 14 TOG.dlkhazov, K.Yu.Adzhamov, I3 .A.IBamedov, V.P.Vislovsky, Kinet , i Kataliz., 20(1979)118 15 G.I. Golodets, Teor. Eksper. Khim., 18(1982)37 16 G.I.Golodets, V.V. Borovik, V.111. Vorotyntsev, Teor.Eksper. Khim., 22(1986)252 17 O.V. Krylov, V.F.Kiselev, Adsorption and Catalysis on Tranaition Metals and Their Oxides(in Russ. ),Khimia,Moscow,1981,p.288 18 J. Haber, N. Sochacka, B. Grzybovaka, A.Golzbiewskii, J. Xolec. Catal , 1 ( 1975135 19 Yu I Pyatnit s k y ,GoI.Golode t a ,React.Kinet .Catal .Le t t ,5 ( 1 976) 345 Katal., 20 G.I.Golodets, Yu.I,Pyatnitsky,L.N.Raevskaya,Kinet.i 1
. ..
25(1984)571
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21 G.I.Golodeta, Kinet. i Katal., 28(1987)837 22 G.I.Golodets, Teor. Eksper.Khim., 1(1965)756 23 N.I.Ilchenko, A.I.Bostan,L.Yu.Dolgikh, G.I.Golodets,Teor.Eksp. Khim., 23(1987)641 24 N.I,Ilchenko, A.I.Boatan, G.I.Golodets, Teor.Ekaper.Khim., 24( 19881455 25 G.I.Golodets, React.Kinet.Cata1. Lett., 28(1985)131