16 R e a c t i v i t y of C a t a l y s t s D e r i v e d f r o m Organometallics Directly Deposited on Supports
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T. J. THOMAS, DENNIS A. HUCUL, and ALAN BRENNER Wayne State University, Department of Chemistry, Detroit, MI 48202 Static and reactive characterization of organometallics directly deposited on supports delineate five parameters which are important in controlling catalytic activity: coordinative unsaturation, oxidation state, dispersion, immobilization, and cluster size. The means of measuring and controlling these experimental parameters is described and the catalysts are contrasted to more traditional heterogeneous and homogeneous catalysts. Unlike immobilized homogeneous catalysts, these catalysts strongly interact with the support and generally do not retain their molecular character. Numerous activity data from model reactions (ethylene hydrogenation, ethane hydrogenolysis, and methanation) are given. In a number of cases the directly deposited organometallics have a much higher catalytic activity than their homogeneous counterparts, and sometimes also possess significantly higher dispersions and activity than traditional (salt derived) heterogeneous catalysts. A simple structure-activity relationship allows the predictions of the optimal pretreatment of a supported organometallic and when the resulting catalyst is likely to be significantly more active than a traditional heterogeneous catalyst. During the l a s t s e v e r a l years our r e s e a r c h group has been involved i n the development o f a new c l a s s o f heterogeneous c a t a l y s t s : organometallics d i r e c t l y deposited on h i g h surface area r e f r a c t o r y supports such as alumina, s i l i c a , and molecular s i e v e s . These m a t e r i a l s p h y s i c a l l y l i e at the f r o n t i e r between t r a d i t i o n a l homogeneous and heterogeneous c a t a l y s t s (note that the metal i s not i n s u l a t e d from the support by a c h a i n o f l i g a n d s and these are not immobilized homogeneous c a t a l y s t s ) and i n f a c t can combine the b e t t e r f e a t u r e s o f both types o f c a t a l y s t s , Table I .
0097-6156/82/0192-0267 $6.00/0 © 1982 American Chemical Society Miller; Chemically Modified Surfaces in Catalysis and Electrocatalysis ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
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Table I Advantages of Organometallics D i r e c t l y Deposited on R e f r a c t o r y Supports
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1. 2. 3« 4. 5.
Many c a t a l y s t p r e c u r s o r s Unusual c a t a l y s t c o n f i g u r a t i o n s (such as c l u s t e r s ) Easier characterization Immobilization of c o o r d i n a t i v e l y unsaturated s i t e s , High thermal and chemical s t a b i l i t y .
Over the l a s t few years a number of papers have been p u b l i s h e d d e a l i n g with the c h a r a c t e r i z a t i o n ( e s p e c i a l l y by i n f r a r e d spectroscopy) of supported organometallics ( e s p e c i a l l y carbonyl complexes) (JL). There have a l s o been s c a t t e r e d r e p o r t s i n d i c a t i n g higher d i s p e r s i o n s (2) than t h e i r t r a d i t i o n a l analogs (made by aqueous impregnation with metal s a l t s followed by c a l c i n a t i o n and reduction) and can have improved a c t i v i t y f o r s e v e r a l r e a c t i o n s i n c l u d i n g the hydrogénation of o l e f i n s ( 3 , 4 ) , metathesis (5,6), metnanation (7,8), and F i s c h e r - T r o p s c h s y n t h e s i s ( 9 ) . In t h i s r e p o r t s t a t i c and r e a c t i v e c h a r a c t e r i z a t i o n techniques are combined to achieve a systematic understanding of the surface chemistry of these new c a t a l y s t s and to d e r i v e a simple s t r u c t u r e - a c t i v i t y r e l a t i o n s h i p which i s i n v a l u a b l e f o r the development of improved c a t a l y s t s * Experimental S e c t i o n C a t a l y s t s were prepared by p h y s i c a l l y d i s p e r s i n g a carbonyl complex on y-alumina (Conoco Catapal SB, u s u a l l y c a l c i n e d at 500 °C) by impregnation from pentane s o l u t i o n or s u b l i m a t i o n of the s o l i d c a r b o n y l . The c a t a l y s t s were a c t i v a t e d and p a r t i a l l y c h a r a c t e r i z e d by temperature programmed decomposition (TPDE). B r i e f l y , TPDE i n v o l v e s r a i s i n g the temperature at a l i n e a r r a t e of about 5 °C/min as He i s swept through a g l a s s r e a c t o r and the e v o l u t i o n of gases ( p r i m a r i l y CO and H ) i s continuously monitored with a p a i r of thermal c o n d u c t i v i t y d e t e c t o r s . Other gases formed during TPDE ( p r i m a r i l y CH and C0 ) are analyzed s e p a r a t e l y a f t e r TPDE by b a c k f l u s h i n g through a trap of s i l i c a g e l which was h e l d at -196 °C during the run. D e t a i l s of the r i g o r o u s l y a i r f r e e c a t a l y s t p r e p a r a t i o n , the h i g h p u r i t y r e a c t i o n system, and TPDE technique have been p r e v i o u s l y p u b l i s h e d (10,11). D i s p e r s i o n measurements were u s u a l l y done at 25 °C. A very accurate pressure transducer was used which allows the measurement of about 0.001 cm of adsorbed gas. D e t a i l s of the chemisorption methodology have been p u b l i s h e d (12)• A l l a c t i v i t y measurements were done with a flow system immediately a f t e r c a t a l y s t a c t i v a t i o n and without removing the 2
4
2
3
Miller; Chemically Modified Surfaces in Catalysis and Electrocatalysis ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
16.
THOMAS ET AL.
Organometallics Deposited on Supports
269
c a t a l y s t or r e a c t o r from the r e a c t i o n l i n e . Flows were c o n t r o l l e d and monitored with an e l e c t r o n i c mass flow c o n t r o l l e r (Brooks Instrument). The e f f l u e n t from a r e a c t o r was analyzed at roughly 1 min i n t e r v a l s using a motor d r i v e n gas sampling valve ( C a r l e ) . Standard methods of gas chromatographic a n a l y s i s were used, with the peak i n t e g r a t i o n s being done by a Spectrophysics System I integrator. A c t i v i t y measurements are expressed as a formal turnover frequency, N . N i s the number of molecules r e a c t i n g per u n i t time per metal atom on the c a t a l y s t . Note that N does not c o r r e c t f o r the d i s p e r s i o n of a metal, and i n f a c t N = D*N, where N i s the normal turnover frequency and D i s the f r a c t i o n a l d i s p e r s i o n of the c a t a l y s t . N and N are seen to be complimentary concepts. N~ i s more u s e f u l f o r measuring the true e f f i c i e n c y of a c a t a l y s t since a r t i f i c a l l y h i g h a c t i v i t i e s are not generated by a c a t a l y s t with a very low d i s p e r s i o n . f
f
f
f
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f
Synthetic Methodology For a v a r i e t y of reasons l a r g e l y r e l a t e d to experimental expediency (such as ease of a v a i l a b i l i t y , simple s t o i c h i o m e t r y , and the small a d s o r p t i o n of CO(g) on a support), these s t u d i e s have s p e c i f i c a l l y focused on supported t r a n s i t i o n metal carbonyl complexes. However, i t i s f e l t that carbonyls are merely models f o r low-valent organometallics and i t i s l i k e l y that the observed p a t t e r n s of surface chemistry and a c t i v i t y have general v a l i d i t y . The method of c a t a l y s t s y n t h e s i s i s r a d i c a l l y d i f f e r e n t from that f o r a t r a d i t i o n a l heterogeneous c a t a l y s t , as i n d i c a t e d by F i g u r e 1. Whereas t r a d i t i o n a l methodology i n v o l v e s impregnation with an aqueous s o l u t i o n of a h i g h v a l e n t s a l t and r e q u i r e s a h i g h temperature r e d u c t i o n , the carbonyl route s t a r t s with a zero v a l e n t complex and i n e r t solvent (or no solvent at a l l ) and o f t e n r e q u i r e s only m i l d thermal a c t i v a t i o n . As suggested by F i g u r e 1, except f o r the more noble group 8 metals many elements are d i f f i c u l t to reduce when supported, so the t r a d i t i o n a l route of synthesis can confine a c a t a l y s t to only the h i g h v a l e n t s t a t e s . T h i s r e s t r i c t i o n need not operate with supported o r g a n o m e t a l l i c s . T h i s i n t u r n suggests that there might be a strong dichotomy i n the nature of supported c a r b o n y l s , with complexes of d i f f i c u l t to reduce metals y i e l d i n g v e r y d i f f e r e n t c a t a l y s t s than t h e i r t r a d i t i o n a l analogs. Static Characterization In t h i s phase of the study emphasis has been p l a c e d on s e v e r a l p r o p e r t i e s which can be d i r e c t l y r e l a t e d to c a t a l y t i c a c t i v i t y : stoichiometry, o x i d a t i o n s t a t e , and d i s p e r s i o n . Since most carbonyl complexes are c o o r d i n a t i v e l y s a t u r a t e d , i t i s c l e a r that ligands must d i s s o c i a t e to develop a c t i v e s i t e s . F u r t h e r , since the a d s o r p t i o n of CO(g) i s quite small on the supports and CO i s a r a t h e r s t a b l e molecule, i t f o l l o w s that the development of
Miller; Chemically Modified Surfaces in Catalysis and Electrocatalysis ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
CHEMICALLY MODIFIED SURFACES
270
c o o r d i n a t i v e u n s a t u r a t i o n should c o r r e l a t e with the e v o l u t i o n of C0(g) during a c t i v a t o n * However, i t has a l s o been shown that during a c t i v a t i o n i n c i p i e n t zero v a l e n t metal formed by the decomposition of a complex can undergo a redox r e a c t i o n with hydroxyl groups which are on the surface of most supports, equation 1 (13)• M(C0)j + n(a-OH)
A
»
(£, W(CO) , R u ( C 0 ) , O s ( C O ) , and the Fe carbonyls have been found t o be at l e a s t 1 0 - f o l d more a c t i v e than t h e i r t r a d i t i o n a l c o u n t e r p a r t s . The dichotomy between the more d i f f i c u l t to reduce metals and those e a s i e r to reduce i s b e a u t i f u l l y e x e m p l i f i e d by the data i n Tables IV and V. It i s seen that c a t a l y s t s d e r i v e d from the more d i f f i c u l t to reduce metals show b e t t e r a c t i v i t y a f t e r a c t i v a t i o n i n the ß r e g i o n , whereas c a t a l y s t s d e r i v e d from more noble metals are more a c t i v e a f t e r a h i g h temperature r e d u c t i o n . I t might a l s o be noted that these c a t a l y s t s are orders o f magnitude more a c t i v e than immobilized carbonyl complexes ( 1 6 ) . As a l r e a d y noted, TPDE on a dehydroxylated alumina should a f f o r d a route t o completely decarbonylated but s t i l l low v a l e n t c a t a l y s t s of d i f f i c u l t t o reduce metals (1J,18). As expected, the a c t i v i t i e s o f Cr(CO)^, Mo(C0)g, and W(C0) a f t e r TPDE t o 600 °C are extremely h i g h (ft* at 0 °C i s about 1, 27, and 1 s , r e s p e c t i v e l y ) . E s p e c i a l l y f o r Mo and W these are by f a r the most a c t i v e hydrogénation c a t a l y s t s of these metals ever r e p o r t e d (excepting evaporated metal f i l m s o f low surface a r e a ) • On the other hand, when supported on a more conventional alumina ( p r e t r e a t e d at 500 ° C ) , TPDE t o 600 °C r e s u l t s i n c a t a l y s t s o f very low a c t i v i t y because the average o x i d a t i o n s t a t e i s about 5. Table VI i l l u s t r a t e s s t i l l another important p r o p e r t y of supports v i s - a - v i s homogeneous carbonyl complexes; the a b i l i t y to immobilize c o o r d i n a t i v e l y unsaturated s i t e s . I t i s noteworthy that i n s o l u t i o n Fe(C0> maintained a s l i g h t a c t i v i t y only i n the presence of continued i r r a d i a t i o n , whereas a l l measurements on the supported c a t a l y s t s were made i n the dark a f t e r p h o t o a c t i v a t i o n . 6
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6
3
J 2
3
l 2
6
5
Table IV A c t i v i t y f o r the Hydrogénation of Ethylene N
1
f
(s' )
Catalyst
Activated 200 °C, He
Activated 600 °C,
(H /He) Activity
Cr(C0)
6
0.04
0.006
0.15
Mo(C0)
6
0.05
0.03
0.6
0.016
0.0007
0.04
0.00009
0.00008
0.9
0.013
0.0009
0.07
W(C0)
6
Mn (CO) 2
Fe (C0) 3
1 0
1 2
2
T = 0°C, B ^ / C ^ = 4, P = 1 atm.
Miller; Chemically Modified Surfaces in Catalysis and Electrocatalysis ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
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Table V A c t i v i t y f o r the Hydrogénation of Ethylene
Activated 200 °C, He
Catalyst Re (CO)
1 0
0.13
Ru (CO)
1 2
1.8
Os (CO)
1 2
0.16
Rh (CO)
1 2
8
Ir (CO)
1 2
0.057
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2
3
3
4
4
T = 0°C, H /C H 2
2
4
Activated 600 °C, E2
(H^He) Activity 56
7.3 225
125
11
69
140
18 132
7.5
- 4, P = 1 atm.
Table VI A c t i v i t y * of Photoactivated C a t a l y s t s f o r Monoolefin Hydrogénation Complex
Homogeneous C a t a l y s t
Cr(C0)
