Homogeneous Catalysis of the Water Gas Shift ... - ACS Publications

8 Homogeneous Catalysis of the Water Gas Shift Reaction Using Simple Mononuclear Carbonyls R. B . K I N G , A . D . K I N G , JR., and D . B .

YANG

Downloaded by MONASH UNIV on December 15, 2015 | http://pubs.acs.org Publication Date: May 5, 1981 | doi: 10.1021/bk-1981-0152.ch008

Department of Chemistry, University of Georgia, Athens, G A 30602

The water gas shift reaction is used extensively in industry to increase the hydrogen content of water gas (synthesis gas) through the reaction of carbon monoxide (CO) with water according to the following equation:

Current industrial practice for carrying out this reaction involves heterogeneous catalysts at relatively high temperatures, e.g. Fe O /Cr O above 300°C (1). However, relatively recent work has shown that the water gas shift reaction can also be carried out at considerably lower temperatures (below 200°C) using various metal carbonyl complexes as homogeneous catalysts. Thus a variety of platinum metal derivatives are active water gas shift reaction catalysts including ruthenium carbonyls (2, 3, 4), rhodium carbonyls (3,5,6,7), platinum-tin complexes (8), and phosphine-platinum(0) complexes (9). In 1978 we reported (10) that several carbonyl derivatives of more abundant metals (iron, chromium, molybdenum, and tungsten) reacted with base to give active water gas shift reaction catalysts. Subsequent work led to a detailed study on the kinetics of the water gas shift reaction catalyzed by Fe(CO) in the presence of base (11). More 2

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r e c e n t l y we have extended such d e t a i l e d k i n e t i c s t u d i e s t o s i m i l a r c a t a l y s t s d e r i v e d from the Group VI metal carbonyls M(C0) (M = Cr, Mo, and W) (L2) . T h i s paper summarizes the r e s u l t s obtained with the mononuclear carbonyls o f i r o n and the Group VI t r a n s i t i o n metals and compares the k i n e t i c s of these two water gas s h i f t c a t a l y s t systems. fi

Experimental Techniques The water gas s h i f t r e a c t i o n s were c a r r i e d out i n v e r t i c a l l y mounted type 304 s t a i n l e s s s t e e l autoclaves having an i n t e r n a l volume o f 700 ml. The autoclaves were heated e l e c t r i c a l l y and

0097-6156/81/0152-0123$05.00/0 © 1981 American Chemical Society In Catalytic Activation of Carbon Monoxide; Ford, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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s t i r r e d m a g n e t i c a l l y using a Teflon-coated magnetic s t i r r i n g bar. The temperature was c o n t r o l l e d t o w i t h i n ± l C by a p r o p o r t i o n a l c o n t r o l l e r with a thermocouple sensor mounted i n a thermocouple w e l l extending i n t o the i n t e r i o r of the autoclave. A d i g i t a l thermocouple read-out meter provided a continuous temperature reading. The pressure was monitored using a 0-3000 p s i g t e s t gauge accurate t o 0.25% of f u l l s c a l e which was attached t o each autoclave through the c l o s u r e at the top. Analyses of the gases i n the bomb (CO, CO^/ Ar, H«) were performed using a F i s h e r Model 1200 gas p a r t i t i o n e r with a 6.5 f t . 80-100 mesh Columnpak PQ column and an 11 f t 13 X molecular sieve column i n s e r i e s . Helium was used as a c a r r i e r gas i n a l l determinations. Care was taken t o i n s u r e that a l l hydrogen analyses were performed at concentrations w i t h i n the l i n e a r response region of the s e n s i t i v i t y curve f o r t h i s gas. Argon was used as an i n t e r n a l standard. A V a r i a n CDS-III d i g i t a l i n t e g r a t o r was used t o i n t e g r a t e the output from the gas p a r t i t i o n e r . Gas samples were taken by r e l e a s i n g a p o r t i o n of the i n t e r i o r gas mixture i n t o a sample chamber which uses a small b a l l o o n t o maint a i n a low p o s i t i v e pressure against a septum. T h i s chamber was purged three times with the gas mixture before removing a sample f o r i n j e c t i o n i n t o the gas p a r t i t i o n e r by means of a PressureLok s y r i n g e . The f o l l o w i n g two methods were used t o compute gas composit i o n s expressed as p a r t i a l pressures a f t e r determining the e x t e r n a l s e n s i t i v i t y f a c t o r s f o r each gas: (a) The pressure i n the autoclave was recorded at the time the gas sample was taken. S u b t r a c t i o n of the p r e v i o u s l y determined solvent vapor pressure gave the t o t a l pressure of non-condensible gases. Dalton's law was then used t o determine the i n d i v i d u a l p a r t i a l pressures of the three gases of i n t e r e s t (CO, C 0 , H ) using the gas mole f r a c t i o n s obtained i n the gas a n a l y s i s . (b) The CO i n i t i a l l y loaded i n the autoclave was mixed with argon f o r use as an i n t e r n a l standard. The composition of t h i s gas mixture was then checked by a gas a n a l y s i s . The r e s u l t i n g computed p a r t i a l pressure of argon was c o r r e c t e d to the e l e v a t e d temperatures at which the k i n e t i c data were obtained, thereby a l l o w i n g p a r t i a l pressures of the gases of i n t e r e s t to be computed d i r e c t l y using argon as an i n t e r n a l standard. The gas phase compositions obtained using methods (a) and (b) agreed with each other i n every i n s t a n c e . Further d e t a i l s on the experimental techniques used i n t h i s work are given e l s e where ( 1 1 , 1 2 ) .


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Results In a t y p i c a l experiment the autoclave was loaded with a methanol-water s o l u t i o n c o n t a i n i n g d i s s o l v e d base (potassium hydroxide or sodium formate) and metal carbonyl ( F e ( C 0 ) or M(CO)^ where M = Cr, Mo, or W) and charged with a CO/argon gas 5

In Catalytic Activation of Carbon Monoxide; Ford, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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mixture. T h e a u t o c l a v e was t h e n h e a t e d r a p i d l y w i t h s t i r r i n g t o t h e d e s i r e d t e m p e r a t u r e where p e r i o d i c p r e s s u r e r e a d i n g s a n d g a s a n a l y s e s were made. S i n c e one m o l e o f H was p r o d u c e d f o r e a c h mole o f CO consumed i n e q u a t i o n 1, t h e p a r t i a l p r e s s u r e o f CO + H r e m a i n e d e s s e n t i a l l y c o n s t a n t t h r o u g h o u t t h e r e a c t i o n e x cept f o r minor sampling l o s s e s . T h i s i n d i c a t e s t h e absence o f s i g n i f i c a n t s i d e r e a c t i o n s c o n s u m i n g CO w i t h o u t p r o d u c i n g E^. However, i n t h e e x p e r i m e n t s u s i n g ΚΟΗ a s t h e b a s e t h e i n i t i a l p r e s s u r e o f CO o r CO + H a t t h e r e a c t i o n t e m p e r a t u r e was l e s s t h a n t h e l o a d i n g p r e s s u r e o f CO b y a n amount c o r r e s p o n d i n g t o the q u a n t i t a t i v e f o r m a t i o n o f formate a c c o r d i n g t o t h e f o l l o w i n g equation: 2

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CO + 0 H ~

> HC0 " 2

(2)

T h i s e x p l a n a t i o n f o r t h i s p r e s s u r e d i s c r e p a n c y was v e r i f i e d b y e x p e r i m e n t s u s i n g d i f f e r e n t amounts o f ΚΟΗ a n d b y t h e l a c k o f such p r e s s u r e d i s c r e p a n c i e s i n experiments u s i n g formate as a base. T h e s e o b s e r v a t i o n s c l e a r l y i n d i c a t e t h a t when a b a s e s t r o n g e r than formate i s used i n t h e water gas s h i f t r e a c t i o n , the a c t u a l base p r e s e n t i n t h e r e a c t i o n i s formate produced a c c o r d i n g t o e q u a t i o n 2. During the course o f a t y p i c a l water gas s h i f t r e a c t i o n c a t a l y z e d b y F e ( C 0 ) _ i n t h e p r e s e n c e o f b a s e t h e pH s t a r t s a t 8 . 6 a n d f a l l s g r a d u a l l y t o 7 . 4 as d e t e r m i n e d f r o m f r e s h l i q u i d s a m p l e s w i t h d r a w n p e r i o d i c a l l y f r o m t h e bomb. Thus t h e f o r m a t e a c t s a s a b u f f e r t o k e e p t h e pH o f t h e w a t e r gas s h i f t r e a c t i o n system i n a r e l a t i v e l y narrow range a l m o s t i n d e p e n d e n t o f t h e amount o f b a s e ( e . g . ΚΟΗ) o r i g i n a l l y l o a d e d into the autoclave. The w a t e r g a s s h i f t r e a c t i o n s were a l s o r u n i n m e t h a n o l w a t e r m i x t u r e s o f v a r y i n g c o m p o s i t i o n s u s i n g ΚΟΗ a s t h e b a s e . I n t h e c a s e o f t h e s y s t e m d e r i v e d f r o m F e ( C O ) _ , a 25% w a t e r 75% m e t h a n o l m i x t u r e g a v e t h e f a s t e s t r a t e (11) whereas i n t h e cases o f t h e systems d e r i v e d from M(CO) (M = C r , Mo, a n d W), a 10% w a t e r - 9 0 % m e t h a n o l m i x t u r e g a v e t h e f a s t e s t r a t e s ( 1 2 ) . T h e s e optimum m e t h a n o l - w a t e r m i x t u r e s a s s o l v e n t s f o r t h e w a t e r gas s h i f t r e a c t i o n s r e p r e s e n t compromises between a h i g h c o n c e n t r a t i o n o f t h e r e a c t a n t water and a h i g h c o n c e n t r a t i o n o f m e t h a n o l t o s o l u b i l i z e t h e CO a n d m e t a l c a r b o n y l s . Furthermore, a l l o f t h e s o l v e n t m i x t u r e s u s e d i n t h i s work c o n t a i n amounts o f w a t e r w h i c h a r e l a r g e r e l a t i v e t o t h a t consumed i n t h e w a t e r g a s shift reaction. T h e r e f o r e , t h e c o n c e n t r a t i o n o f w a t e r may be regarded as a constant d u r i n g t h e water gas s h i f t r e a c t i o n s c o n ducted i n t h i s research p r o j e c t . 6

The r a t e s o f t h e w a t e r g a s s h i f t r e a c t i o n s were compared u s i n g d i f f e r e n t amounts o f t h e m o n o n u c l e a r m e t a l c a r b o n y l p r e c u r s o r f o r a l l f o u r c a s e s (Fe(CO) a n d M(CO)^ where M = C r , M o , and W). I n a l l c a s e s t h e r a t e s o r h y d r o g e n p r o d u c t i o n were f o u n d t o d o u b l e a s t h e c o n c e n t r a t i o n o f t h e m e t a l c a r b o n y l was doubled. Thus a l l o f t h e w a t e r gas s h i f t r e a c t i o n s i n v e s t i g a t e d



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i n t h i s work a r e f i r s t o r d e r w i t h r e s p e c t t o t h e m o n o n u c l e a r metal carbonyl precursor. Major d i f f e r e n c e s were n o t e d between the systems d e r i v e d from Fe(CO)^ and M ( C O ) (M = C r , Mo, and W) w i t h r e s p e c t t o t h e e f f e c t o f t h e b a s e c o n c e n t r a t i o n on t h e r e a c t i o n r a t e . Thus i n t h e case o f t h e c a t a l y s t system d e r i v e d from Fe(CO)^ t r i p l i n g t h e amount o f KOH w h i l e k e e p i n g c o n s t a n t t h e amounts o f t h e o t h e r r e a c t a n t s h a d no s i g n i f i c a n t e f f e c t on t h e r a t e o f H prod u c t i o n (11). However, i n t h e case o f t h e c a t a l y s t s y s t e m d e r i v e d f r o m W ( C O ) t h e r a t e o f H p r o d u c t i o n i n c r e a s e d as t h e amount o f b a s e was i n c r e a s e d r e g a r d l e s s o f w h e t h e r t h e b a s e was KOH, s o d i u m f o r m a t e , o r t r i e t h y l a m i n e ( 1 2 ) . T h i s i n c r e a s e may be i n t e r p r e t e d as a f i r s t o r d e r d e p e n d e n c e on b a s e c o n c e n t r a t i o n p r o v i d e d some s o l u t i o n n o n - i d e a l i t y i s assumed a t h i g h b a s e concentrations. S i m i l a r o b s e r v a t i o n s were made f o r t h e b a s e dependence o f H p r o d u c t i o n i n c a t a l y s t systems d e r i v e d from the o t h e r m e t a l h e x a c a r b o n y l s Cr(CO) and M o ( C O ) (12). Thus t h e water gas s h i f t c a t a l y s t system d e r i v e d from F e ( C O ) h a s an a p p a r e n t z e r o o r d e r base dependence whereas t h e w a t e r gas s h i f t c a t a l y s t systems d e r i v e d from M(CO) (M = C r , Mo, and W) h a v e an approximate f i r s t o r d e r base dependence. Any s e r i o u s m e c h a n i s t i c p r o p o s a l s must accommodate t h e s e o b s e r v a t i o n s . 6

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M a j o r d i f f e r e n c e s were a l s o n o t e d b e t w e e n t h e c a t a l y s t systems d e r i v e d from F e ( C O ) and those d e r i v e d from M ( C O ) (M = C r , Mo, a n d W) w i t h r e s p e c t t o t h e e f f e c t o f CO p r e s s u r e on t h e reaction rate. I n t h e s y s t e m d e r i v e d f r o m Fe(CO),- t h e r a t e o f H p r o d u c t i o n i n t h e e a r l y s t a g e s o f t h e r e a c t i o n was i n d e p e n d e n t o f t h e CO l o a d i n g p r e s s u r e i n t h e r a n g e 10 t o 40 a t m o s p h e r e s (11) . H o w e v e r , H p r o d u c t i o n u s i n g t h i s c a t a l y s t s y s t e m was f o u n d t o c e a s e a b r u p t l y when enough CO was consumed s o t h a t t h e CO p a r t i a l p r e s s u r e f e l l t o a t h r e s h o l d v a l u e b e t w e e n 3 a n d 7 atmospheres. Two i n d e p e n d e n t e x p e r i m e n t s c o n d u c t e d w i t h CO l o a d i n g p r e s s u r e s around 1 atmosphere i n d i c a t e d excess H prod u c t i o n s r e l a t i v e t o t h e CO consumed o f 5 . 7 - 0 . 1 m o l e H /mole Fe(C0) . T h e s e o b s e r v a t i o n s i n d i c a t e t h a t a minimum t h r e s h o l d p r e s s u r e o f CO i s n e e d e d i n o r d e r t o p r e v e n t t h e i r o n c a r b o n y l c a t a l y s t system from decomposing t o c a r b o n y l - f r e e c a t a l y t i c a l l y i n a c t i v e i r o n ( I I ) d e r i v a t i v e s (11). The o b s e r v a t i o n t h a t 5 . 7 moles o f e x c e s s H a r e p r o d u c e d f o r each mole o f F e ( C 0 ) may be i n t e r p r e t e d on t h e b a s i s o f F e ( C O ) ^ a c t i n g as an a v e r a g e 1 1 . 4 e l e c t r o n r e d u c i n g agent f o r water under the r e a c t i o n c o n d i t i o n s i n a c c o r d w i t h r e p o r t e d ( 1_3) o b s e r v a t i o n s t h a t F e (CO) in alkal i n e s o l u t i o n i s an a v e r a g e 1 0 . 8 e l e c t r o n r e d u c i n g a g e n t f o r t h e reduction of nitrobenzene to a n i l i n e . 5

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The r a t e s o f t h e w a t e r g a s s h i f t r e a c t i o n s c a t a l y z e d b y t h e s y s t e m s d e r i v e d f r o m M(CO) (M = C r , Mo, and W) were f o u n d t o be i n v e r s e l y p r o p o r t i o n a l t o t h e CO p r e s s u r e as i n d i c a t e d b y straight l i n e p l o t s of rates of H production versus ^/ ^init p

2

(12) . F u r t h e r m o r e , the c a t a l y s t s d e r i v e d from M(CO) Mo, and W) r e t a i n t h e i r c a t a l y t i c a c t i v i t i e s a t l o w e r

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p r e s s u r e s than t h e c a t a l y s t d e r i v e d from F e i C O ) ^ . Thus, the w a t e r g a s s h i f t c a t a l y s t s d e r i v e d f r o m H(CO)^ (M = C r , M o , a n d W) a p p e a r t o b e more r o b u s t t h a n t h o s e d e r i v e d f r o m F e ( C O ) ^ . The i n c r e a s e d c h e m i c a l s t a b i l i t y o f t h e c a t a l y s t systems d e r i v e d from M ( C O ) r e l a t i v e t o t h o s e d e r i v e d from F e ( C O ) also r e s u l t i n a n i n c r e a s e d t o l e r a n c e f o r s u l f u r , an i m p o r t a n t c h a r a c t e r i s t i c f o r a p r a c t i c a l water gas s h i f t c a t a l y s t system b e cause o f t h e p o s s i b i l i t y o f u s i n g s y n t h e s i s gas f e e d s t o c k s d e r i v e d from h i g h s u l f u r c o a l s . In order t o evaluate the s u l f u r t o l e r a n c e o f water gas s h i f t c a t a l y s t systems, t h e c a t a l y t i c r e a c t i o n s were c a r r i e d o u t a s above b u t u s i n g s o d i u m s u l f i d e r a t h e r t h a n p o t a s s i u m h y d r o x i d e o r sodium formate as t h e base to generate the c a t a l y t i c a l l y active species (14). Aqueous s o d i u m s u l f i d e i s a s t r o n g enough b a s e t o g e n e r a t e f o r m a t e through the following reactions:


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S ~ 2

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+ H 0 + CO ~ 2

HS~ + H 0 + CO 2

* HS~ + H C 0 ~

(3a)

H S + HC0 "

(3b)

2

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The H S b y - p r o d u c t , r e p r e s e n t i n g a r e l a t i v e l y r e d u c e d f o r m o f s u l f u r , i s a r e a s o n a b l e model f o r t h e s u l f u r i m p u r i t i e s i n t h e s y n t h e s i s gas o b t a i n e d from s u l f u r - r i c h c o a l . T h i s sodium s u l f i d e t e s t o f s u l f u r r e s i s t a n c e o f water gas s h i f t c a t a l y s t s y s tems g e n e r a t e d i n b a s i c s o l u t i o n s i s a v e r y s e v e r e t e s t s i n c e t h e q u a n t i t i e s o f s u l f u r i n v o l v e d a r e much l a r g e r t h a n t h o s e l i k e l y t o b e f o u n d i n s y n t h e s i s g a s made f r o m a n y s u l f u r - r i c h coals. 2

T h e w a t e r g a s s h i f t c a t a l y s t s y s t e m d e r i v e d f r o m F e t C O ) ^ was found t o be r e l a t i v e l y s e n s i t i v e towards s u l f u r p o i s o n i n g s i n c e an a q u e o u s m e t h a n o l s o l u t i o n g e n e r a t e d f r o m s o d i u m s u l f i d e a n d F e ( C O ) u s i n g an S / F e r a t i o o f 26 was c o m p l e t e l y i n a c t i v e a s a c a t a l y s t f o r the water gas s h i f t r e a c t i o n . This sulfur poisoni n g o f t h e F e ( C O ) c a t a l y s t s y s t e m may a r i s e f r o m t h e f o r m a t i o n o f t h e i r o n c a r b o n y l s u l f i d e F e ^ ( C O ) S w h i c h was d e t e c t e d i n t h e s e r e a c t i o n m i x t u r e s ( 1 4 ) . H o w e v e r , aqueous m e t h a n o l s o l u t i o n s g e n e r a t e d from sodium s u l f i d e and t h e m e t a l h e x a c a r b o n y l s M(CO) (M = C r , Mo, a n d W) r e t a i n e d 21% (M = C r ) t o 67% (M = W) of t h e c a t a l y t i c a c t i v i t y o f t h e c o r r e s p o n d i n g c a t a l y s t systems g e n e r a t e d f r o m KOH a n d t h e same m e t a l h e x a c a r b o n y l s e v e n when S/M r a t i o s a s h i g h a s 400 were u s e d . Thus t h e w a t e r g a s s h i f t c a t a l y s t s y s t e m s d e r i v e d f r o m M ( C 0 ) ^ , p a r t i c u l a r l y M = W, h a v e a high sulfur tolerance. Therefore they a r e p o t e n t i a l l y very u s e f u l f o r p r o c e s s i n g s y n t h e s i s gas d e r i v e d from h i g h s u l f u r coals. 5

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The a c t i v a t i o n e n e r g i e s o f t h e s e w a t e r g a s s h i f t c a t a l y s t s y s t e m s were d e t e r m i n e d b y r a t e m e a s u r e m e n t s a s a f u n c t i o n o f temperature. T h u s on t h e b a s i s o f r a t e m e a s u r e m e n t s a t t h e f i v e t e m p e r a t u r e s 1 8 0 , 1 6 0 , 1 5 0 , 1 4 0 , a n d 130°C t h e a c t i v a t i o n e n e r g y o f t h e c a t a l y s t s y s t e m d e r i v e d f r o m F e ( C O ) ^ was e s t i m a t e d a t



128

22 k c a l / m o l e . S i m i l a r measurements f o r the c a t a l y s t systems d e r i v e d from M ( C O ) gave e s t i m a t e d a c t i v a t i o n e n e r g i e s o f 35, 35, a n d 32 k c a l / m o l e f o r M = C r , Mo, a n d W, r e s p e c t i v e l y . These l a t t e r numbers a r e r o u g h l y s i m i l a r t o t h e a c t i v a t i o n e n e r g i e s o f 3 9 , 3 1 , and 40 k c a l / m o l e r e p o r t e d (15) f o r t h e r e p l a c e m e n t o f CO b y t r i p h e n y l p h o s p h i n e f o r C r ( C 0 ) , M o ( C 0 ) , a n d W ( C 0 ) , respectively. 6

6

Table

The c o m b i n e d r e s u l t s I.

of

these

6

studies

6

are

summarized

in


Discussion The f o l l o w i n g c a t a l y t i c c y c l e (_3,13) i s c a p a b l e o f e x p l a i n i n g o u r e x p e r i m e n t a l o b s e r v a t i o n s (11) on t h e w a t e r g a s s h i f t r e a c t i o n c a t a l y z e d by Fe(CO)^ i n the p r e s e n c e o f a b a s e :

Fe(C0)

k

+ OH

5

i

—-—>

-

k

2

k

HFe(C0)

+ H 0 k

4

2

4

(4a)

2

2

Fe(CO) C0 H 4

Fe(CO) C0 H

> 3

HFe(C0)

+ C0

4

> H F e (CO) 2

(4b)

2

+ OH

(4c)

4

H Fe(CO) 2

4

> Fe(C0) + H 5 > Fe(CO) 4

(4d)

2

k

Fe(C0)

4

+ CO

(4e)

5

The n u c l e o p h i l i c a t t a c k o f a m e t a l - b o n d e d c a r b o n y l g r o u p w i t h h y d r o x i d e t o g i v e a m e t a l - b o n d e d c a r b o x y l g r o u p ( e q u a t i o n 4a) i s w e l l e s t a b l i s h e d i n m e t a l c a r b o n y l c a t i o n chemistry (16,17,18) and h a s r e a s o n a b l e e x p e r i m e n t a l s u p p o r t f r o m s t u d i e s on t h e rhodium c a r b o n y l i o d i d e c a t a l y z e d water gas s h i f t r e a c t i o n (3). F u r t h e r m o r e , c a r b o x y l groups d i r e c t l y bonded t o t r a n s i t i o n m e t a l s s i m i l a r to that i n the proposed intermediate Fe(CO) C0 H are w e l l known (17_,18_, 1 9 , 2 0 ) t o u n d e r g o f a c i l e d e c a r b o x y l a t i o n t o g i v e t h e c o r r e s p o n d i n g m e t a l h y d r i d e e x a c t l y as i n e q u a t i o n 4 b . The H F e ( C 0 ) i n t e r m e d i a t e formed by p r o t o n a t i o n o f H F e ( C 0 ) ~ ( e q u a t i o n 4 c f i s n e c e s s a r y t o a c c o u n t f o r t h e o b s e r v a t i o n made b y P e t t i t and c o w o r k e r s (3) t h a t b o t h Reppe h y d r o f o r m y l a t i o n s and w a t e r g a s s h i f t r e a c t i o n s c a t a l y z e d b y i r o n c a r b o n y l s do n o t p r o c e e d t o any m e a s u r a b l e e x t e n t a t a pH g r e a t e r t h a n 1 0 . 7 . The r e m a i n i n g s t e p s o f t h e c a t a l y t i c c y c l e i n v o l v e r e d u c t i v e e l i m i n a t i o n of H ( e q u a t i o n 4d) a n d c o o r d i n a t i v e s a t u r a t i o n ( e q u a t i o n 4e) c o m p l e t e l y a n a l o g o u s t o s t e p s f o u n d i n many t y p e s of c a t a l y t i c cycles (21). 4

2

2

4

2

A s t a n d a r d k i n e t i c a n a l y s i s o f t h e mechanism 4a-4e u s i n g the steady s t a t e approximation y i e l d s a r a t e equation c o n s i s t e n t with the experimental o b s e r v a t i o n s . T h u s s i n c e e q u a t i o n s 4a t o 4e f o r m a c a t a l y t i c c y c l e t h e i r r e a c t i o n r a t e s must be e q u a l f o r t h e c a t a l y t i c s y s t e m t o be b a l a n c e d . The r a t e o f H2 p r o d u c t i o n



Comparison of

car-

22

3-7

none

Minimum CO p r e s s u r e r e q u i r e d to maintain c a t a l y s t activity (atm)

Catalytic activity in presence o f s u l f i d e i o n ( e x p r e s s e d as p e r c e n t o f a c t i v i t y under s u l f u r f r e e conditions)

zero

zero

first

25

Fe(CO)

order

order

order

Cr,

W)

first

10

W(CO)^

Mo,

order

21

none

35

first

order

59

35

67

32

approximately f i r s t order

(M

approximately f i r s t order

order

6

approximately

first

10

Mo(CO)

and M ( C O )

inverse f i r s t order

5


order

from F e ( C O )


first

10

Cr(CO),

C a t a l y s t Systems D e r i v e d

Temperature dependence o f r a t e e x p r e s s e d as a c t i v a t i o n energy (kcal/mole)

R a t e d e p e n d e n c e on a d d e d b a s e (formate) c o n c e n t r a t i o n .

R a t e d e p e n d e n c e on c a r b o n monoxide p r e s s u r e .

Rate dependence on m e t a l bonyl concentration.

2

I.

Optimum S o l v e n t C o m p o s i t i o n (% H 0 , V/V i n m e t h a n o l )

Table



130

b y e q u a t i o n 4d (namely k [ H F e ( C O ) ]) must be e q u a l t o t h e r a t e o f C 0 p r o d u c t i o n by e q u a t i o n 4b (namely [Fe(CO) CO^H"])which a f t e r a p p l y i n g the steady s t a t e approximation t o Fe(CO) C02H" d[Fe(CO) C O J - ] (namely = 0) leads t o the f o l l o w i n g expression dt for H production: 4

2

2

4

4

2

d[H ] 2

= k

dt

[ F e ( C O ) ] [OH"]

"1

--^-'

(5)

c

5

The r a t e o f H p r o d u c t i o n u s i n g t h e c a t a l y s t s y s t e m d e r i v e d from F e ( C O ) i s t h u s s e e n t o h a v e a f i r s t o r d e r d e p e n d e n c e on Fe(CO)j_ c o n c e n t r a t i o n and t o be i n d e p e n d e n t o f CO p r e s s u r e i n a c c o r d w i t h the e x p e r i m e n t a l o b s e r v a t i o n s o u t l i n e d above. Fur t h e r m o r e , t h e f o r m a t e b u f f e r s y s t e m g e n e r a t e d b y r e a c t i o n o f CO w i t h t h e b a s e by e q u a t i o n 2 k e e p s t h e O H " c o n c e n t r a t i o n e s s e n t i a l l y i n d e p e n d e n t o f t h e amount o f b a s e i n t r o d u c e d i n t o t h e system. Therefore the r a t e of H p r o d u c t i o n u s i n g the c a t a l y s t system d e r i v e d from F e ( C O ) ^ , a l t h o u g h h a v i n g a f i r s t o r d e r d e p e n d e n c e on OH c o n c e n t r a t i o n , i s e s s e n t i a l l y i n d e p e n d e n t o f the base c o n c e n t r a t i o n . 2


5

The f o l l o w i n g r a t h e r d i f f e r e n t t y p e o f c a t a l y t i c c y c l e i n v o l v i n g f o r m a t e d e c o m p o s i t i o n e x p l a i n s o u r o b s e r v a t i o n s on t h e w a t e r g a s s h i f t r e a c t i o n s c a t a l y z e d b y M(CO)^ (M = C r , Mo, and W) i n the presence o f a base : M(CO)

+ HC0

5

M(CO) OCOH

—>

2

-

k

2

5

HM(CO)

H M(C0) o

Ζ

In

a d d i t i o n the

2

k c

D

k

6

Generation of

formate

HM(CO)

+ C0

5

> H M(C0)

0

Ζ

+ M(CO)

5

(6b)

2

+ OH

(6c) (6d)

c

D

two e x t e r n a l

the m e t a l

M(CO)^

(b)

H

(6a)

5

2

4 —>

following

c y c l e are needed: (a) D i s s o c i a t i o n o f

"3

+ H 0

5

>

M(CO) OCOH

steps

to this

catalytic

hexacarbonyl:

a

=±M(CO),. k a

+ CO

f r o m CO and

_ k CO + OH — — > H C 0 Q

(6e)

5

hydroxide

(2)

2

E q u a t i o n 6e f o l l o w e d by e q u a t i o n 6 a i s a n a l o g o u s t o a r e p o r t e d (22) p r e p a r a t i o n o f t h e t r i f l u o r o a c e t a t e W(CO) O C O C F ~ b y treatment of W(CO) w i t h tetraethylammonium t r i f l u o r o a c e t a t e at 3

6


8.


KING ET AL.

131

elevated temperatures. E q u a t i o n 6b r e p r e s e n t s an unknown r e a c t i o n b u t was shown t o b e r e a s o n a b l e b y o b s e r v i n g CO^ a s a p r o d u c t f r o m t h e p y r o l y s i s a t 110°C o f [ ( C H ^ P N P i C H ^ ] [W(CO) OCOH], w h i c h was p r e p a r e d b y a s t a n d a r d method (22) using the r e a c t i o n o f (CO) -^Q with s i l v e r formate. Equations 6c and 6 d f o r t h e c a t a l y s t s y s t e m s d e r i v e d f r o m M(CO)^ a r e c o m p l e t e l y a n a l o g o u s t o e q u a t i o n s 4 c a n d 4d f o r t h e c a t a l y s t s y s t e m d e r i v e d from F e i C O ) ^ . 5


The k i n e t i c a n a l y s i s o f t h e m e c h a n i s m 6 a - 6 e , 2 i s more c o m p l i c a t e d t h a n t h a t o f t h e mechanism 4 a - 4 e b e c a u s e o f t h e e x t e r n a l r e a c t i o n 6e b u t n e v e r t h e l e s s i s f e a s i b l e u s i n g t h e steady state approximation. By a p r o c e d u r e s i m i l a r t o t h e d e r i v a t i o n o f e q u a t i o n 5 t h e f o l l o w i n g e q u a t i o n c a n be d e r i v e d : d[H

] = k [H M(C0) ] 4

2

5

= k [ M ( C 0 ) ] (HC0 1

5

2

]

(7)

H o w e v e r , t h e s t e a d y s t a t e c o n c e n t r a t i o n o f M (CO),, d e p e n d s upon t h e c o n c e n t r a t i o n s o f t h e s t a b l e s p e c i e s M ( C O ) ^ , H C 0 ~ , a n d CO as w e l l a s H M ( C O ) i n t h e c y c l e . Thus a p p l y i n g t h e s t e a d y s t a t e a p p r o x i m a t i o n t o [H(CO)^] one o b t a i n s t h e f o l l o w i n g e q u a t i o n where k = k^/k : eq d a 2

2

d[H

5

]

— •(Wki

[M(CO)

] [HCO ~]

mr—= W i

[M(CO)

] [HCO

— Ï C Ô Ï

~] (

8

)

T h i s m e c h a n i s t i c scheme a g r e e s w i t h t h e e x p e r i m e n t a l o b s e r v a t i o n s o f t h e f i r s t o r d e r d e p e n d e n c e s o n M(CO) and formate c o n c e n t r a t i o n s a n d t h e i n v e r s e f i r s t o r d e r d e p e n d e n c e on CO p r e s s u r e f o r the r a t e o f H p r o d u c t i o n i n t h e water gas s h i f t r e a c t i o n c a t a l y z e d b y M(COf (M = C r , Mo a n d W) i n t h e p r e s e n c e o f a b a s e s u f f i c i e n t l y s t r o n g t o g e n e r a t e f o r m a t e f r o m CO b y e q u a t i o n 2 . Acknowledgment The a u t h o r s w o u l d l i k e t o e x p r e s s a p p r e c i a t i o n f o r t h e p a r t i a l support f o r t h i s r e s e a r c h p r o v i d e d by t h e D i v i s i o n o f B a s i c S c i e n c e s o f t h e U . S . Department o f Energy under C o n t r a c t EY-76-S-09-0933.



132


Literature Cited 1. Thomas, C. L. "Catalytic Processes and Proven Catalysts," Academic Press, New York, 1970. 2. Laine, R.M.; Rinker, R. G.; Ford, P. C. J. Am. Chem. Soc., 1977, 99, 252. 3. Kang, H.; Mauldin, C. H.; Cole, T.; Slegeir, W.; Cann, K.; Pettit, R. J . Am. Chem. Soc., 1977, 99, 8323. 4. Ungerman, C.; Landis, V.; Moya, S. Α.; Cohen, H.; Walker, Η. Pearson, R. G.; Rinker, R. G.; Ford, P. C. J . Am. Chem. Soc., 1979, 101, 5922. 5. Cheng, C.-H.; Hendriksen, D. E.; Eisenberg, R. J. Am. Chem. Soc., 1977, 99, 2791. 6. Laine, R. M. J. Am. Chem. Soc., 1978, 100, 6451. 7. Baker, E. C.; Hendriksen, D. E.; Eisenberg, R. J . Am. Chem. Soc., 1980, 102, 1020. 8. Cheng, C.-H.; Eisenberg, R. J . Am. Chem. Soc., 1978, 100, 5968. 9. Yoshida, T.; Ueda, Y.; Otsuka, S. J. Am. Chem. Soc., 1978, 100, 3941. 10. King, R. B.; Frazier, C. C.; Hanes, R. M.; King, A. D. J. Am. Chem. Soc., 1978, 100, 2925. 11. King, A. D.; King, R. B.; Yang, D. B. J . Am. Chem. Soc., 1980, 102, 1028. 12. King, A. D.; King, R. B.; Yang, D. B. submitted for publi cation. 13. Pettit, R.; Cann, K.; Cole, T.; Mauldin, C. H.; Slegeir, W. Advan. Chem. Ser., 1979, 173, 121. 14. King, A. D.; King, R. B.; Yang, D. B. Chem. Comm., 1980, 529. 15. Werner, H.; Prinz, R. Chem. Ber., 1966, 99, 3582. 16. Kruck, T.; Noack, M. Chem. Ber., 1964, 97, 1693. 17. Darensbourg, D. J.; Froelich, J . A. J. Am. Chem. Soc., 1977, 99, 4726. 18. Darensbourg, D. J.; Baldwin, B. J.; Froelich, J . A. J . Am. Chem. Soc., 1980, 102, 4688. 19. Hieber, W.; Kruck, T. Z. Naturforsch. B, 1961, 16, 709. 20. Clark, H. C.; Dixon, K. R.; Jacobs, W. J . Chem. Comm. 1968, 548. 21. Tolman, C. A. Chem. Soc. Revs., 1972, 1, 337. 22. Schlientz, W. J.; Lavender, Y.; Welcman, N.; King, R. B.; Ruff, J. K. J. Organometal. Chem., 1971,33,357. RECEIVED December

8, 1980.


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