9 Homogeneous Catalytic Reduction of Benzaldehyde with Carbon Monoxide and Water Applications of the Water Gas Shift Reaction 1
WILLIAM J. THOMSON and RICHARD M. LAINE Physical Organic Chemistry Department, SRI International, Menlo Park, CA 94025
With the advent of the energy crisis, the study of homogeneous catalytic CO hydrogenation has become very popular because CO re presents one of the cheapest, readily available C building blocks for production of synfuels and because homogeneous catalysts can be very efficient hydrogenation catalysts. To date, the majority of the research has been devoted to catalytically hydrogenating CO directly to hydrocarbons (Fischer-Tropsch synthesis) with l i t t l e attention paid to other CO reduction reactions that could also be useful for synfuel production (1,2,3,4). For example, two areas that could be more thoroughly explored are CO homologation [re action (1)] and methanol synthesis [reaction (2)], which can be catalyzed homogeneously (5,6,7,8). 1
With the recent development of zeolite catalysts that can effi ciently transform methanol into synfuels, homogeneous catalysis of reaction (2) has suddenly grown in importance. Unfortunately, aside from the reports of Bradley (6), Bathke and Feder (7), and the work of Pruett (8) at Union Carbide (largely unpublished), very l i t t l e is known about the homogeneous catalytic hydrogenation of CO to methanol. Two p o s s i b l e mechanisms f o r m e t h a n o l f o r m a t i o n are suggested by l i t e r a t u r e d i s c u s s i o n s o f F i s c h e r - T r o p s c h c a t a l y s i s ( 9 - 1 0 ) . T h e s e a r e shown i n Schemes 1 and 2.
1
Οn s a b b a t i c a l leave f r o m t h e University o f I d a h o .
0097-6156/81/0152-0133$05.00/0 © 1981 American Chemical Society
134
C A T A L Y T I C
/y° ΜΗ + CO
MH
O F
C A R B O N
M O N O X I D E
H —
• M-C + CH3OH
^MH
A C T I V A T I O N
2
i
^ —
MH + C H 0 2
M-CH 0H
^
2
= homogeneous m e t a l h y d r i d e c o m p l e x . Scheme 1
MH ++ CO M n (JO
v X
Ρ
2H^——••
M-c' M-C
•
3
'.
MH + CH3OH -
2
2
„. HM=C
H M = C
y
X
M-CH 0H
\
^
2
Scheme 2 We a r e i n t e r e s t e d i n homogeneous c a t a l y t i c m e t h a n o l s y n t h e s i s b e c a u s e o f o u r p r e v i o u s w o r k on t h e Reppe r e a c t i o n s ( 1 1 , 1 2 , 1 3 ) : R-CH=CH + 3C0 + 2 H 0 2
R
h
2
e
^ ^
1
6
/
K
Q
H
»
RCH CH CH 0H + 2C0 2
2
(3)
2
2 / Κ ° Η ^ R ( C H ) - N R + 2 C 0 (4) MeOH A common theme i n r e a c t i o n s ( l ) - ( 4 ) and Schemes 1 and 2 i s t h e hydrogénation o f c a r b o n - o x y g e n m u l t i p l e bonds t o s p e c i e s w i t h c a r b o n - o x y g e n o r c a r b o n - n i t r o g e n s i n g l e bonds. In undertaking t h e w o r k p r e s e n t e d h e r e i t was o u r i n t e n t i o n t o d e t e r m i n e w h e t h e r o r n o t i n f o r m a t i o n o b t a i n e d i n t h e s t u d y o f t h e hydrogénation o f a l d e h y d i c c a r b o n - o x y g e n d o u b l e bonds i s a p p l i c a b l e t o u n d e r s t a n d i n g (a) r e d u c t i o n o f c a r b o n m o n o x i d e t o m e t h a n o l , (b) CO r e d u c t i o n to F i s c h e r - T r o p s c h p r o d u c t s , (c) h o m o l o g a t i o n as i n r e a c t i o n ( 1 ) , and (d) t h e v a l i d i t y o f e i t h e r Scheme 1 o r 2. We c h o s e t o i n v e s t i g a t e t h e CO/H 0/KOH and Rh 6 (CO)i 6 c a t a l y z e d r e d u c t i o n o f b e n z a l d e h y d e and s u b s t i t u t e d b e n z a l d e h y d e s b e c a u s e :
R-CH=CH + 3C0 + H 0 2
2
+ R NH
R u ?
2
(
5°^
2
2
3
2
2
2
•
•
Benzaldehydes a r e not s u b j e c t to base c a t a l y z e d a l d o l c o n d e n s a t i o n s , and u n d e r t h e r e a c t i o n c o n d i t i o n s Cannizarro r e a c t i o n s are not important. From thermodynamic c o n s i d e r a t i o n s CO/H 0 must be a b e t t e r r e d u c t a n t than H , thus m i l d e r r e a c t i o n cond i t i o n s m i g h t be p o s s i b l e . The c a t a l y s t d e r i v e d f r o m Rh 6 (CO)i 6 i s t h e most a c t i v e c a t a l y s t f o r a l d e h y d e r e d u c t i o n o f a l l t h e group 8 m e t a l s we have s t u d i e d ( 1 2 , 1 3 ) . £-Substitution o f t h e b e n z a l d e h y d e s s h o u l d p r o v i d e i n f o r m a t i o n about the e l e c t r o n d e n s i t y a t the c a r b o n y l carbon d u r i n g the a d d i t i o n of metal h y d r i d e to the c a r b o n - o x y g e n d o u b l e bond. 2
2
•
•
9.
THOMSON
A N D
Experimental
Reduction of Benzaldehyde
LAINE
135
Procedures
G e n e r a l M e t h o d s . M e t h a n o l u s e d i n k i n e t i c r u n s was d i s t i l l e d from sodium methoxide o r c a l c i u m h y d r i d e i n a n i t r o g e n atmosphere b e f o r e u s e . F r e s h l y d i s t i l l e d c y c l o h e x a n o l was added t o t h e m e t h a n o l i n t h e r a t i o 6.0 m l c y c l o h e x a n o l / 2 0 0 m l MeOH and was u s e d a s a n i n t e r n a l s t a n d a r d f o r g a s c h r o m a t o g r a p h i c (GC) a n a l y s i s . B e n z a l d e h y d e was d i s t i l l e d u n d e r vacuum and s t o r e d u n d e r n i t r o g e n a t 5 ° . O t h e r a l d e h y d e s ( p u r c h a s e d f r o m A l d r i c h ) were a l s o d i s t i l l e d b e f o r e u s e . The c o r r e s p o n d i n g a l c o h o l s ( p u r c h a s e d f r o m A l d r i c h ) w e r e d i s t i l l e d and u s e d t o p r e p a r e GC s t a n d a r d s . A l l m e t a l c a r b o n y l c l u s t e r c o m p l e x e s w e r e p u r c h a s e d f r o m S t r e m Chem i c a l Company a n d u s e d a s r e c e i v e d . T e t r a h y d r o f u r a n (THF) was d i s t i l l e d f r o m s o d i u m benzophenone u n d e r n i t r o g e n b e f o r e u s e . A n a l y t i c a l M e t h o d s . A l l t h e a n a l y s e s w e r e done b y g a s chroma tography. L i q u i d p r o d u c t s were a n a l y z e d on a H e w l e t t - P a c k a r d M o d e l 5711 g a s c h r o m a t o g r a p h e q u i p p e d w i t h F I D u s i n g a 4.0 m b y 0.318 cm c o l u m n p a c k e d w i t h 5% Carbowax a c i d - w a s h e d Chromosorb G. Gas p r o d u c t s w e r e a n a l y z e d w i t h a H e w l e t t - P a c k a r d M o d e l 5750 g a s c h r o m a t o g r a p h e q u i p p e d w i t h a 3 m b y 0.318 cm, 150/200 P o r o p a k Q c o l u m n a n d a 1.8 m b y 0.318 cm t y p e 13A m o l e c u l a r s i e v e c o l u m n . H y d r o g e n a n a l y s i s was a c h i e v e d b y i n j e c t i n g i n t o t h e m o l e c u l a r s i e v e c o l u m n o p e r a t e d w i t h a 8.5% h y d r o g e n - i n - h e l i u m c a r r i e r g a s . O t h e r g a s e o u s components w e r e a n a l y z e d i n t h e P o r o p a k c o l u m n u s i n g a helium c a r r i e r gas. I n t h e p r o c e d u r e u s e d f o r s t u d y i n g b e n z a l d e h y d e r e d u c t i o n , 6.0 m l o f t h e M e O H / c y c l o h e x a n o l s o l u t i o n d e s c r i b e d above w e r e u s e d a s s o l v e n t . A l d e h y d e was added t o t h e r e a c t o r v i a a 5-ml g l a s s s y r i n g e , a n d 1.0 m l o f 3 Ν KOH s o l u t i o n was added b y means o f a p i p e t t e . We w e r e c a r e f u l t o a d d e i t h e r t h e KOH o r t h e a l d e h y d e j u s t b e f o r e p r e s s u r i z i n g w i t h CO t o m i n i m i z e t h e a l d e h y d e r e d u c t i o n r e s u l t i n g f r o m t h e n o n c a t a l y t i c C a n n i z a r r o r e a c t i o n . The r e a c t o r was t h e n s e a l e d a n d f l u s h e d t w i c e w i t h 6 0 0 - p s i CO b e f o r e b r i n g i n g t h e r e a c t o r up t o t h e d e s i r e d CO p r e s s u r e . The r u n was i n i t i a t e d a t t h e t i m e t h e r e a c t o r was immersed i n t h e t e m p e r a t u r e b a t h . On c o m p l e t i o n o f t h e r u n , t h e r e a c t o r was quenched i n c o l d w a t e r and t h e p r e s s u r e a t 24°C was r e c o r d e d . The p r i m a r y a n a l y s i s was c o n d u c t e d o n t h e l i q u i d s o l u t i o n ; h o w e v e r , some r u n s w e r e a l s o s u b j e c t e d t o gas phase a n a l y s i s . I n every experiment, l i q u i d s a m p l e s w e r e c o l l e c t e d i n 1 0 - c c s a m p l e v i a l s and s t o r e d i n a r e frigerator. I n most c a s e s c h r o m a t o g r a p h i c a n a l y s e s w e r e c o n d u c t e d w i t h i n 6 h o u r s o f t h e t e r m i n a t i o n o f t h e r u n . The maximum t i m e t h a t a n y sample was s t o r e d b e f o r e a n a l y s i s was 72 h o u r s . To have a b a s i s f o r c o m p a r i s o n , we e s t a b l i s h e d a s t a n d a r d r u n f o r aldehyde r e d u c t i o n c o n s i s t i n g o f 0.1 mmole R h ( C 0 ) 30 mmole C H C H 0 1 m l 3 Ν KOH 6 m l (MeOH + c y c l o h e x a n o l ) 6
6
5
i 6
136
CATALYTIC ACTIVATION OF CARBON MONOXIDE
Ρ = 800 p s i T = 125°C R e a c t i o n time = 1 hour C a t a l y t i c a c t i v i t y was m e a s u r e d a s a f u n c t i o n o f t u r n o v e r f r e quency [moles p r o d u c t / ( m o l e c a t a l y s t ) ( h o u r ) ] . The s t a n d a r d r u n has a t u r n o v e r f r e q u e n c y o f 105±10. A l l t h e p a r a m e t e r s i n v e s t i g a t e d w e r e p e r t u r b e d a b o u t t h i s s t a n d a r d and i n c l u d e d the e f f e c t s o f c a t a l y s t , a l d e h y d e , KOH a n d w a t e r c o n c e n t r a t i o n , i n i t i a l CO p r e s s u r e , and r e a c t i o n t i m e . I n a d d i t i o n , a few s e l e c t e d runs were a l s o conducted t o examine the e f f e c t s o f hydrogen i n the gas phase as w e l l as the r e l a t i v e ease w i t h w h i c h o t h e r aldehydes c o u l d be reduced. L U
Results C a t a l y s t s and C a t a l y s t C o n c e n t r a t i o n . The most a c t i v e c a t a l y s t f o r b e n z a l d e h y d e r e d u c t i o n a p p e a r s t o b e r h o d i u m [Rh 6 (C0)i 6 p r e c u r s o r ] , b u t i r o n [as and r u t h e n i u m [as R u ( C 0 ) ] 3 2 w e r e a l s o e x a m i n e d . The r e s u l t s o f t h e s e e x p e r i m e n t s a r e shown i n T a b l e 1. C o n s i s t e n t w i t h e a r l i e r r e s u l t s ( 1 2 ) , t h e r h o d i u m c a t a l y s t i s b y f a r t h e most a c t i v e o f t h e m e t a l s i n v e s t i g a t e d a n d t h e ruthenium c a t a l y s t has almost zero a c t i v i t y . The l a t t e r i s c o n s i s t e n t w i t h the f a c t t h a t ruthenium produces o n l y aldehydes during hydroformylation. Note t h a t a s y n e r g i s t i c e f f e c t o f mixed m e t a l s does n o t a p p e a r t o b e p r e s e n t i n a l d e h y d e r e d u c t i o n a s c o n t r a s t e d w i t h the n o t i c e a b l e e f f e c t s observed f o r the water-gas s h i f t r e a c t i o n (WGSR) a n d r e l a t e d r e a c t i o n s ( 1 3 ) . The e f f e c t o f t h e c o n c e n t r a t i o n o f R h ( C 0 ) o n t h e number o f t u r n o v e r s was e v a l u a t e d b y u s i n g 0.01 mmole t o 0.10 mmole o f c a t a l y s t , a n d t h e s e r e s u l t s a r e shown i n F i g u r e 1. The t u r n o v e r s i n crease with decreasing c a t a l y s t precursor. This i s i n d i c a t i v e o f c a t a l y s i s by c l u s t e r f r a g m e n t a t i o n . The r e s u l t s o f o u r e a r l i e r w o r k (12) a r e a l s o shown i n F i g u r e 1. A l t h o u g h t h e t u r n o v e r s a r e h i g h e r than those observed here, i t s h o u l d be noted t h a t a h i g h e r temperature and a s h o r t e r r e a c t i o n time were used p r e v i o u s l y . H i g h e r t e m p e r a t u r e s h o u l d p r o d u c e more t u r n o v e r s , b u t s h o r t e r r e a c t i o n times would be e x p e c t e d t o produce l e s s , depending on the existent reaction orders. T h i s i s c o m p l i c a t e d f u r t h e r by t h e e f f e c t o f C 0 p r o d u c t i o n , w h i c h t e n d s t o consume OH", a n d a l s o b y t h e n o n i s o t h e r m a l h e a t - u p p e r i o d (5 m i n ) , w h i c h i s a s i g n i f i c a n t f r a c t i o n o f t h e 0.5-hr r e a c t i o n t i m e u s e d p r e v i o u s l y .
Fe (C0)i ]
3
6
1 2
1 6
2
KOH C o n c e n t r a t i o n S t u d i e s . The e f f e c t o f KOH c o n c e n t r a t i o n o n b e n z a l d e h y d e r e d u c t i o n was e x a m i n e d , and t h e r e s u l t s a r e shown i n F i g u r e 2 a l o n g w i t h our p r e v i o u s r e s u l t s f o r ruthenium c a t a l y z e d hydroformylation (12). The E f f e c t o f R e a c t a n t C o n c e n t r a t i o n . Several experiments were conducted t o q u a n t i f y the e f f e c t o f r e a c t a n t c o n c e n t r a t i o n on t h e a l d e h y d e r e d u c t i o n r a t e . The i n i t i a l CO p r e s s u r e was v a r i e d
THOMSON
AND
Table I.
137
Reduction of Benzaldehyde
LAINE
Benzaldehyde Reduction with Various Catalysts CATALYST
RH (C0)
1 6
FE (C0)
1
RU (C0)
1 2
6
3
RH (C0) 6
RU3(C0)
M0LES
B
0.05
OF
TURNOVERS
A
105
30
3
A
NO,
PRECURSOR
2
9
1 6
1 2
/FE (C0) 3
/FE3(C0)
93
B 1 2
A L C O H O L FORMED MOLES
OF
21
B 1 2
PER MOLE
OF T O T A L
CATALYST.
EACH.
700
0.02
0.04
m moles R h ( C O ) 6
Figure 1.
0.08
0.06
0.10
16
The effect of catalyst concentration on benzaldehyde reduction: (O) 125°C, 1 h; ( ) 150°C, 0.5 h (\2)
CATALYTIC ACTIVATION OF CARBON MONOXIDE
138
b e t w e e n 1 0 0 a n d 1200 p s i , t h e H 0 c o n c e n t r a t i o n b e t w e e n 55 a n d 167 mmole, a n d t h e C H C H 0 c o n c e n t r a t i o n b e t w e e n 10 a n d 40 ramole. Because o f s o l u b i l i t y problems, the experiments a t the h i g h e s t H 0 and h i g h e s t C H C H 0 c o n c e n t r a t i o n s w e r e a n a l y z e d a f t e r a d d i n g 3 m l THF t o t h e m i x t u r e r e c o v e r e d a t t h e e n d o f t h e r u n . F i g u r e 3 shows t h e r e s u l t s o f v a r y i n g t h e CO p r e s s u r e . The maximum a c t i v i t y a p p e a r s t o l i e n e a r 600 p s i f o r b e n z a l d e h y d e r e duction. F i g u r e 3 i s an attempt t o e l u c i d a t e an apparent r e a c t i o n o r d e r w i t h r e s p e c t t o t h e a r i t h m e t i c a l l y a v e r a g e d CO p r e s s u r e . A t p r e s s u r e s l e s s t h a n 400 p s i , t h e o r d e r i s n e a r l y f i r s t o r d e r . The s i t u a t i o n a t h i g h e r p r e s s u r e s i s not c l e a r ; however, i t i s r e a s o n a b l e t o s p e c u l a t e t h a t the dominant a s p e c t s o f the k i n e t i c s s h i f t at these pressures. The d a t a s u g g e s t t h e s h i f t i s t o z e r o - o r d e r dépendance. S t u d i e s a n a l y z i n g the e f f e c t s o f the remaining r e a c t a n t s , H 0 and C H C H 0 i n d i c a t e t h a t t h e r e a c t i o n a p p e a r s t o be z e r o o r d e r w i t h respect t o both reactants. I t i s i n t e r e s t i n g that i n previous w o r k we a l s o f o u n d s i m i l a r b e h a v i o r f o r H 0 i n r u t h e n i u m c a t a l y z e d h y d r o f o r m y l a t i o n ( 1 2 ) , a s d i d Ungermann e t a l . w i t h t h e WGSR ( 1 4 ) . 2
6
5
2
6
s
2
6
5
2
The E f f e c t o f R e a c t i o n Time. The p r o b l e m a s s o c i a t e d w i t h t i m e v a r y i n g 0H~ c o n c e n t r a t i o n s h a s a l r e a d y b e e n m e n t i o n e d . The d i f f i c u l t y a s s o c i a t e d w i t h the i n f l u e n c e o f d i s s o l v e d C 0 can be a p p r e c i a t e d b y r e f e r r i n g t o F i g u r e 4, w h i c h shows t h e r e s u l t s o f two experiments. I n one, s a m p l e s w e r e t a k e n e v e r y h o u r a n d i n t h e o t h e r s a m p l i n g o c c u r r e d e v e r y two h o u r s . However, t h e i m p o r t a n t f a c t o r i s t h a t t h e r e a c t o r was r e c h a r g e d w i t h CO a f t e r e a c h s a m p l e . N o t e t h a t t h e e f f e c t i v e r e a c t i o n r a t e i s l o w e r when two h o u r s e l a p s e between samples, presumably due t o the b u i l d u p o f C 0 , w h i c h consumes 0H~. I n f a c t , o n e e x p e r i m e n t was c o n d u c t e d a t 94°C f o r 17 h o u r s a n d o n l y 27% c o n v e r s i o n t o a l c o h o l o c c u r r e d , t h e same c o n v e r s i o n e x p e r i e n c e d a f t e r 3 h o u r s when f r e s h CO was added h o u r ly. 2
2
R e d u c t i o n o f O t h e r A l d e h y d e s . We examined t h e r e d u c t i o n o f a n i s a l d e h y d e , p-CH 0C H CH0 and t o l u a l d e h y d e , p - C H ( C H ) C H 0 t o examine t h e e f f e c t o f e l e c t r o n d e n s i t y o n a l d e h y d e r e d u c t i o n . I n a d d i t i o n , we a l s o i n v e s t i g a t e d o n e k e t o n e , a c e t o p h e n o n e , C H C 0 C H . The r e s u l t s o f t h e s e e x p e r i m e n t s a r e g i v e n i n T a b l e 2. 3
6
A
3
6
A
6
5
Discussion Because o f the c o m p l e x i t y o f the r h o d i u m - c a t a l y z e d r e d u c t i o n o f b e n z a l d e h y d e t o b e n z y l a l c o h o l w i t h CO a n d H 0 , i t i s n o t p o s s i b l e t o f u l l y e l u c i d a t e t h e mechanism o f c a t a l y t i c r e d u c t i o n g i v e n the extent o f the k i n e t i c s t u d i e s performed t o date. Howe v e r , t h e r e s u l t s do a l l o w u s t o draw s e v e r a l i m p o r t a n t c o n c l u s i o n s a b o u t t h e r e a c t i o n mechanism f o r b e n z a l d e h y d e hydrogénation and s e v e r a l r e l a t e d r e a c t i o n s . We r e c e n t l y d e s c r i b e d (12,15) t h e u s e o f c a t a l y s t c o n c e n t r a 2
3
THOMSON
AND
LAINE
0
0.5
Reduction of Benzaldehyde
1.0
1.5 2.0 mmoles KOH
2.5
3.0
3.5
Figure 2. The Effect of KOH: (--O --) KOH; (A) 1.56 mmol K COy, (hydroformylation (12) 2
ι—ι—I
100
200
I I I I
500 p
co ~
P
1000
2000
si
Figure 3. Benzaldehyde reduction turnovers vs. P o C
140
CATALYTIC ACTIVATION OF CARBON MONOXIDE
Table II.
Reduction of Other Reactants
REACTANTS
1
TURNOVER
C H CH0 6
PCH C H 3
6
Z 4
PCH 0C H 3
6
CH0
Î 4
C H C0CH 6
5
FREQUENCY
105
5
78
CH0
63
j
3
24
ι (STANDARD OF
Figure 4.
CONDITIONS,
30
MMOLES
REACTANTS)
Time varying benzaldehyde reduction Τ = 94°C: (O) purge every 1 h, (A) purge every 2 h
9.
THOMSON
A N D
141
Reduction of Benzaldehyde
LAINE
t i o n studies t o help i d e n t i f y a c t i v e c a t a l y s t species, e s p e c i a l l y where c l u s t e r c a t a l y z e d r e a c t i o n s a r e s u s p e c t e d . I n the present work, c h a n g e s i n t h e amount o f r h o d i u m added t o t h e r e a c t i o n s o l u t i o n have dramatic e f f e c t s on t h e t u r n o v e r f r e q u e n c y . A s shown i n F i g u r e 1, d e c r e a s i n g t h e r h o d i u m c o n c e n t r a t i o n r e s u l t s i n c o n s i d e r able increases i n turnover frequencies. These changes a r e i n d i c a t i v e o f e q u i l i b r i a i n v o l v i n g rhodium c l u s t e r complexes t h a t fragment r e v e r s i b l y t o s m a l l e r c l u s t e r complexes o r mononuclear complexes w h e r e i n t h e a c t i v e s p e c i e s a r e t h e fragments. I n t h e r e a c t i o n s o l u t i o n s where b e n z a l d e h y d e r e d u c t i o n o c c u r s we h a v e o b s e r v e d ( 1 2 ) , b y I R , b o t h t h e R h ( C O ) o ~ and R h ( C 0 ) " complexes. R e c e n t w o r k b y C h i n i and c o w o r k e r s (16) s u g g e s t s a t l e a s t one p l a u s i b l e e q u i l i b r i u m : 2
1 2
Rh (CO) 6
1 6
+ OH"
6
6
6
2 l s
6
2 1 5
1 2
2
3 O
" + 15C0
3 0
1 5
(5)
2
" + H 0
(6)
2
2
• Rh (C0)
1 5
- + /.Rh (CO) 12
1 5
• Rh (C0)
1 5
2Rh (C0) H~ Rh (C0)
5
• Rh (C0) H~ + C0
R h ( C 0 ) H ~ + OH" 6
3
"" + H
(7)
2
6Rh (C0) " 5
(8)
1 5
V i d a l a n d W a l k e r (17) h a v e o b s e r v e d t h a t Rh(CO)
