21 Mechanistic Aspects of the Homogeneous Water Gas Shift Reaction Downloaded by KTH ROYAL INST OF TECHNOLOGY on August 24, 2015 | http://pubs.acs.org Publication Date: May 5, 1981 | doi: 10.1021/bk-1981-0152.ch021
W. A . R. S L E G E I R , R . S. S A P I E N Z A , and B . E A S T E R L I N G Catalysis Group, Department of Energy and Environment, Brookhaven National Laboratory, Upton, NY 11973
The homogeneous water gas shift reaction (WGSR),
has been the subject of a number of reports in the recent literature (1-19). The reaction is used to provide hydrogen for processes such as ammonia synthesis, as well as to match synthesis feedgas ratios to consumption ratios in a number of downstream processes, such as hydroformylation (H :00 = 1), methanol synthesis (H :CO = 2) and methanation (H :CO = 3). Operating temperature plays a significant role in the effectiveness of a particular catalyst system, since the thermodynamically limiting conversion decreases as temperature increases (Kp 400°K 1550; Kp,700°K = 9.5 (20)). Currently heterogeneous catalysts are employed in industry for the WGSR, but these operate at high temperature (21). In this chapter, some homogeneous catalyst systems with high activity at modest temperatures will be described. Whether a homogeneous system will supplant currently used heterogeneous systems is dependent on a number of engineering process considerations. However, interest in this reaction centers about its deceptive simplicity, a better understanding of which should enhance the knowledge of catalytic reactions of carbon oxides. F u r t h e r m o r e , t h i s 2
2
2
=
r e a c t i o n i s r e l a t e d t o a number o f o t h e r c a t a l y t i c r e a c t i o n s w i t h p o t e n t i a l s y n f u e l s i m p o r t a n c e i n c l u d i n g t h e Reppe h y d r o h y d r o x y methylation r e a c t i o n (4,22),
the K o l b e l - E n g e l h a r d t
reaction (23),
0097-6156/81/0152-0325$05.00/0 © 1981 American Chemical Society In Catalytic Activation of Carbon Monoxide; Ford, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
C A T A L Y T I C ACTIVATION OF
326
and o t h e r v a r i a t i o n s o f t h e F i s c h e r - T r o p s c h s y n t h e s i s from CO and H 0 (1),
CARBON
MONOXIDE
r e a c t i o n , methanol
2
3C0
+ 2H 0
as w e l l as r e d u c t i o n s (15,24),
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ArN0
2
+ 3C0
*>CH OH + 2 C 0
2
3
of o r g a n i c
+ H0 2
(4)
2
substrates using
» ArNH
2
CO
and
H0 ?
+ 3C0 -
(5)
2
T h i s d i s c u s s i o n w i l l t r e a t two p a r t i c u l a r c a t a l y t i c s y s t e m s a f f o r d i n g r a p i d r a t e s i n b a s i c m e d i a . The r u t h e n i u m c a r b o n y l t r i m e t h y l a m i n e s y s t e m , e x c e e d i n g l y s e l e c t i v e f o r t h e WGSR, i s t h e most e f f e c t i v e known homogeneous s y s t e m . Cluster dissociation and f o r m a t i o n c o m p l i c a t e t h i s s y s t e m , b u t e v i d e n c e s t r o n g l y i n d i c a t e s an a s s o c i a t i v e - t y p e r e a c t i o n mechanism. On t h e o t h e r h a n d , t h e Group V I m e t a l c a r b o n y l s e x h i b i t a r a t h e r s t r a i g h t f o r w a r d d i s s o c i a t i v e - t y p e mechanism. The a p p l i c a t i o n o f t h i s u n d e r s t a n d i n g may be h e l p f u l i n s e l e c t i n g c a t a l y t i c s y s t e m s f o r CO/H 0 reactions. 2
The
R u t h e n i u m C a r b o n y l System
The r e a c t i o n s w i t h r u t h e n i u m c a r b o n y l c a t a l y s t s w e r e c a r r i e d out i n p r e s s u r i z e d s t a i n l e s s s t e e l r e a c t o r s ; g l a s s l i n e r s had l i t t l e e f f e c t on t h e a c t i v i t y . When t r i m e t h y l a m i n e i s u s e d as b a s e , R u ( C O ) , H ^ R u ^ C O ) ^ and H 2 R u 4 ( C O ) i 3 l e a d t o n e a r l y i d e n t i c a l a c t i v i t i e s i f the r a t e i s normalized to the s o l u t i o n conc e n t r a t i o n o f r u t h e n i u m . These r e s u l t s s u g g e s t t h a t t h e same a c t i v e s p e c i e s i s formed u n d e r o p e r a t i n g c o n d i t i o n s f r o m e a c h of these c a t a l y s t precursors. The a m b i e n t p r e s s u r e i n f r a r e d s p e c t r u m o f a t y p i c a l c a t a l y s t s o l u t i o n ( p r e p a r e d f r o m Ru3(CO)i2> t r i m e t h y l a m i n e , w a t e r , and t e t r a h y d r o f u r a n and sampled f r o m t h e r e a c t o r ) i s r e l a t i v e l y s i m p l e ( v _ : 2080(w), 2 0 2 0 ( s ) , 1 9 9 7 ( s ) , 1 9 6 5 ( s h ) and 1958(m) c m ~ l ) . However, t h e s p e c t r u m depends on t h e c o n c e n t r a t i o n of r u t h e n i u m i n s o l u t i o n . The u s e of Na2C03 as base leads to comparable s p e c t r a . A l t h o u g h t h i s s p e c t r u m d o e s n o t c o r r e s p o n d t o any p a r t i c u l a r ruthenium c a r b o n y l complex, i t i s c o n s i s t e n t w i t h the p r e s e n c e of one or more a n i o n i c r u t h e n i u m c a r b o n y l c o m p l e x e s , p e r h a p s a l o n g with neutral species. Work i s i n p r o g r e s s w i t h a v a r i a b l e p a t h l e n g t h , h i g h p r e s s u r e i n f r a r e d c e l l d e s i g n e d by P r o f . A. K i n g , t o p r o v i d e b e t t e r c h a r a c t e r i z a t i o n of s p e c i e s a c t u a l l y present under reaction conditions. A f t e r r e a c t i o n , e v a p o r a t i o n of t h e s o l v e n t from t h e R u ( C O ) 2 / 3 s o l u t i o n , f o l l o w e d by p r o t o n a t i o n w i t h H3PO4 y i e l d s p r i n c i p a l l y E^Ru^iCO)^* w i t h some R u ( C O ) and H ^ u ^ i C O ) ^ . I s o l a t i o n o f t h e a c t i v e a n i o n i c s p e c i e s has n o t b e e n s u c c e s s f u l . 3
1 2
c
Q
N M e
3
1
3
1 2
In Catalytic Activation of Carbon Monoxide; Ford, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
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21.
SLEGEIR ET AL.
327
Homogeneous Water Gas Shift
A l l e v i d e n c e i s c o n s i s t e n t w i t h t h e r e a c t i o n b e i n g homoge n e o u s . Upon c o o l i n g o f t h e r e a c t i o n t o 0°C, t h e o r i g i n a l l y homo geneous s o l u t i o n n o r m a l l y c o n s i s t s o f two p h a s e s . The p h a s e s e p a r a t i o n r e s u l t s from t h e f o r m a t i o n o f i o n i c s p e c i e s ( f o r m a t e s and c a r b o n a t e s ) d u r i n g t h e c o u r s e o f t h e r e a c t i o n w h i c h " s a l t o u t " t h e o r g a n i c phase. The l o w e r ( a q u e o u s ) phase i s c l e a r and colorless. The u p p e r ( o r g a n i c ) p h a s e i s p a l e y e l l o w t o p i n k i s h r e d , d e p e n d i n g on r u t h e n i u m c o n c e n t r a t i o n . V i s u a l i n s p e c t i o n a n d i n f r a r e d s p e c t r a i n d i c a t e t h a t the ruthenium c a r b o n y l s p e c i e s a r e p r e s e n t i n t h e u p p e r phase. N e i t h e r p h a s e i s t u r b i d . Filtration l e a v e s no o b s e r v a b l e r e s i d u e and t h e f i l t r a t e a f f o r d s WGSR r a t e s comparable t o those o f f r e s h s o l u t i o n s . N e i t h e r phase i s p a r a m a g n e t i c , and n e i t h e r c o n t a i n s a p p r e c i a b l e suspended m a t e r i a l a s e v i d e n c e d b y s h a r p NMR s i g n a l s . The RU3(CO)ΐ2/ΝΜ 3 c a t a l y s t s y s t e m i s v e r y s p e c i f i c f o r t h e WGSR. A l t h o u g h h e t e r o g e n e o u s r u t h e n i u m i s a v e r y e f f e c t i v e c a t a l y s t f o r m e t h a n a t i o n ( 2 0 ) , no methane o r h i g h e r h y d r o c a r b o n s c o u l d be d e t e c t e d . Homogeneous r u t h e n i u m has been shown t o c a t a l y z e m e t h a n o l s y n t h e s i s a t v e r y h i g h t e m p e r a t u r e s and p r e s s u r e s ( 2 5 ) , b u t o n l y t r a c e s o f m e t h a n o l a r e d e t e c t e d u n d e r WGSR c o n d i tions. S m a l l amounts o f t h e f o r m a t e i o n a r e f o r m e d , b u t t h i s seems u n a v o i d a b l e i n r e a c t i o n s i n v o l v i n g b a s e and CO, θ
CO + OH
•HC0 ". 2
(6)
The r o l e o f f o r m a t e i n t h e WGSR w i l l be d i s c u s s e d b e l o w . Gener a l l y more H t h a n CO2 i s o b s e r v e d a t t h e end o f t h e r e a c t i o n . E x p e r i m e n t s (26) s u g g e s t t h a t t h i s i s due t o t h e s o l u b i l i t y o f CO2 i n t h e s o l v e n t s y s t e m . 2
E f f e c t o f C o n c e n t r a t i o n and CO P r e s s u r e s o n t h e R u t h e n i u m C a r b o n y l - T r i m e t h y l a m i n e WGSR System. As shown i n F i g u r e 1, t h e R U 3 ( C O ) / 3 WGSR s y s t e m d e m o n s t r a t e s a n e a r l y f i r s t - o r d e r r a t e dependence on CO p r e s s u r e a t 0.5 mM Ru3(CO)^2 c o n c e n t r a t i o n . (Throughout t h i s d i s c u s s i o n , the t o t a l ruthenium c a r b o n y l concen t r a t i o n i s e x p r e s s e d a s m o l e s R u 3 ( C O ) i 2 added p e r l i t e r o f s o l u t i o n ; t h i s s h o u l d n o t be c o n s t r u e d t o be t h e a c t u a l s o l u t i o n c o n c e n t r a t i o n o f the t r i m e r under o p e r a t i n g c o n d i t i o n s . ) Here t h e i n i t i a l r a t e s o f H p r o d u c t i o n a r e 14.6 mmol ^ / h r a t 415 p s i CO and 46.0 mmol ^ / h r a t 1200 p s i . T h u s , w i t h i n e x p e r i m e n t a l u n c e r t a i n t y , a t h r e e f o l d i n c r e a s e i n CO p r e s s u r e l e a d s t o a t h r e e fold increase i n rate. F o r d and c o - w o r k e r s (_7) h a v e r e p o r t e d a f i r s t - o r d e r r a t e dependence on CO p r e s s u r e i n t h e Ru^(CO)-j^/KOH s y s t e m and a s c r i b e d t h i s e f f e c t t o CO p a r t i c i p a t i o n i n a r a t e - l i m i t i n g e l i m i n a t i o n o f h y d r o g e n f r o m a c l u s t e r s p e c i e s . T h i s e x p l a n a t i o n does n o t f i t our o b s e r v a t i o n s , b e c a u s e i f l o s s o f H 2 w e r e r a t e - l i m i t i n g , t h e u s e o f KOH a n d NMe3 as b a s e s w o u l d be e x p e c t e d t o l e a d t o c o m p a r a b l e r a t e s f o r t h e WGSR. A c o m p a r i s o n o f a c t i v i t i e s ( L a i n e ( 9 ) : 2.3 m o l H p e r mol R u ( C O ) p e r h r u s i n g KOH/MeOH a t 10 mM R u ( C O ) , 800 p s i CO, 135°; S l e g e i r ( 2 6 ) : N M e
1 2
2
2
3
3
1 2
l 2
In Catalytic Activation of Carbon Monoxide; Ford, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
328
CATALYTIC ACTIVATION OF CARBON MONOXIDE
5000 m o l H p e ^ m o l R u ( C O ) p e r h r u s i n g NMe^/THF a t 0.02 mM, 765 p s i CO, 125 ) shows t h a t t h e a c t i v i t y i s g r e a t e r by more t h a n t h r e e o r d e r s o f m a g n i t u d e when t r i m e t h y l a m i n e i s u s e d a s b a s e . We b e l i e v e t h e r e i s a b e t t e r e x p l a n a t i o n f o r t h i s a c t i v i t y d e p e n dence o n CO p r e s s u r e i n t h e R u ( C O ) ^ / N M e s y s t e m . 2
3
1 2
3
2
3
TABLE I
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R a t e Dependence on CO P r e s s u r e p
co'
p
s
mmol H
i
350 690 760
a t 0.10 mM R u ( C O ) 3
%
2
conversion
86 90 82
tR
1 2
2
8600 9000 8200
43 24 18
Conditions: 0.010 mmol R u ( C 0 ) , 20 g 25% a q . NMe~, d i l u t e d t o ~3' 100 mL w i t h THF, 100°, 10 h r , 0. 31 L r e a c t o r , Ηο mol H /mol R u ( C O ) 3
1 2
ϋ
2
3
1 2
As shown i n T a b l e I , a t 0.1 mM R u ( C 0 ) c o n c e n t r a t i o n , CO p r e s s u r e h a s l i t t l e i f a n y e f f e c t on a c t i v i t y . On t h e o t h e r h a n d , a t f i x e d p r e s s u r e , t h e c o n c e n t r a t i o n of r u t h e n i u m c a r b o n y l has a d r a m a t i c e f f e c t on a c t i v i t y ( s e e F i g u r e 2 ) . A t 0.1 mM R u ( C O > , r u t h e n i u m c a r b o n y l i s v e r y a c t i v e f o r t h e WGSR, s m a l l d e c r e a s e s in catalyst concentration lead t o substantial increases i n a c t i v i t y , a n d no a c t i v i t y dependence on CO p r e s s u r e i s o b s e r v e d . A t c o n c e n t r a t i o n s o f 0.5 mM o r more, l e s s a c t i v i t y i s o b s e r v e d , changes i n c o n c e n t r a t i o n c a u s e s m a l l e r e f f e c t s i n a c t i v i t y and r a t e dependence on p r e s s u r e i s m a n i f e s t e d . Diffusion effects h a v e been shown t o be u n i m p o r t a n t ( 2 6 ) . I t i s p r o p o s e d t h a t t h e r a t e dependence o n c o n c e n t r a t i o n and p r e s s u r e i n v o l v e s c l u s t e r d i s s o c i a t i o n and t h a t t h e monomeric s p e c i e s , Ru(CO)^, i s r e s p o n s i b l e f o r t h e h i g h a c t i v i t y o f t h i s system. D i s s o c i a t i o n i s w e l l known f o r r u t h e n i u m c a r b o n y l c l u s t e r s (25,27-31). P i a c e n t i and c o - w o r k e r s (31) h a v e demon s t r a t e d t h a t a t t e m p e r a t u r e s above 80° and CO p r e s s u r e s g r e a t e r t h a n 150 p s i , monomeric r u t h e n i u m c a r b o n y l i s o b s e r v e d i n s i g n i f i c a n t q u a n t i t i e s due t o t h e e q u i l i b r i u m , 3
2
3
Ru (CO) 3
1 2
+ 3C0
-3Ru(C0),
1 2
(7)
Thus, i n c r e a s e s i n CO p r e s s u r e f a v o r c l u s t e r d i s s o c i a t i o n and t h e f o r m a t i o n o f l a r g e r q u a n t i t i e s o f 2. High d i l u t i o n should a l s o favor the formation o f r e s u l t i n g i n g r e a t e r Ru(CO),. t o c l u s t e r r a t i o s and g r e a t e r WGSR a c t i v i t y . A s s u m i n g t h e WGSR h a s a f i r s t - o r d e r dependence on R u ( C O ) ^ c o n c e n t r a t i o n and t h a t o n l y t r i m e r i c and monomeric s p e c i e s a r e p r e s e n t , i t c a n b e shown t h a t r a t e « P ( 2 6 ) , i n accord w i t h C
Q
In Catalytic Activation of Carbon Monoxide; Ford, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
21.
SLEGEIR
E T
329
Homogeneous Water Gas Shift
AL.
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300 H
TIME, HR Figure 1. Rate of H production as a function of CO pressure at 0.50mM added Ru (CO) concentration; 0.0507 mmol Ru (CO) , 5 g NMe,, 15 g Η,Ο, solution diluted to 100 mL with THF, 100°C; (Φ) 1200 psi CO, (W 415 psi CO 2
s
12
s
H 0
1
1
0.5
1.0
i2
Γ-
r-
1.5
20
[Ru (CO)| ], MM 3
2
Figure 2. H production as a function of added Ru (CO) concentration; 5 g NMe , 15 g H 0, solution diluted to 100 mL with THF, 415 psi CO, 100°C, 5 h. The abscissa reflects the Ru carbonyl concentration based on initial catalyst load ings; clearly it does not reflect true [Ru (CO) ]. 2
s
s
n
2
3
î2
In Catalytic Activation of Carbon Monoxide; Ford, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
CATALYTIC ACTIVATION OF CARBON
330
MONOXIDE
e x p e r i m e n t s a t 0.5 mM R u ^ C O ) ] ^ ' However, i f t e t r a m e r i c s p e c i e s a r e i n e q u i l i b r i u m w i t h 2^ t h e n r a t e ^*^isolation of H ^ R u ^ C O ) ^ from a c i d i f i e d r e a c t i o n m i x t u r e s supports the e x i s tence o f t e t r a m e r i c species a t higher ruthenium concentrations. E x p e r i m e n t s a t 2.07 mM Ru3(CO)^2 c o n c e n t r a t i o n i n d i c a t e t h a t r a t e « PÇQ * , s u g g e s t i n g t h a t c l u s t e r s l a r g e r t h a n t h e t r i m e r may exist. Œ
p
T
n
e
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r n
E f f e c t o f S o l v e n t a n d B a s e on t h e R u t h e n i u m C a r b o n y l / T r i m e t h y l a m i n e System. S o l v e n t p l a y s an i m p o r t a n t r o l e i n t h e r a t e of h y d r o g e n p r o d u c t i o n . The i d e a l s o l v e n t s a r e t e t r a h y d r o f u r a n , d i g l y m e , and d i m e t h o x y e t h a n e . A l c o h o l s a r e o n l y s l i g h t l y l e s s effective. A p p a r e n t l y t h e s o l v e n t must be m i s c i b l e w i t h w a t e r , promote i o n f o r m a t i o n , and b e c a p a b l e o f w e a k l y c o o r d i n a t i n g w i t h t h e c o o r d i n a t e l y u n s a t u r a t e d s p e c i e s formed i n t h e c o u r s e o f t h e reaction. S m a l l amounts o f h y d r o c a r b o n s added t o t h e n o r m a l t e t r a h y d r o f u r a n o r d i g l y m e s o l v e n t s y s t e m r e s u l t i n i m p r o v e d WGSR a c t i v i t y , b u t l a r g e r q u a n t i t i e s i n h i b i t the r e a c t i o n (Table I I ) . When 1-butene o r 1-hexene i s u s e d , h y d r o f o r m y l a t i o n competes w i t h t h e WGSR ( 4 ) , b u t t h e r a t e o f t h i s p r o c e s s i s s m a l l compared w i t h the r a t e of H p r o d u c t i o n . W i t h p e n t a n e , no o l e f i n o r a l d e h y d e p r o d u c t s c o u l d be d e t e c t e d . C a l d e r a z z o (29) h a s r e p o r t e d t h a t R u ( C O ) ^ i s t h e p r i n c i p a l p r o d u c t when t h e a c e t y l a c e t o n a t e o f ruthenium i s t r e a t e d w i t h s y n t h e s i s gas i n heptane, 2
Ru(acac)
Q
3
+ CO + H (200
h
e
p
0
2
t
a
n
e
i80o
» Ru(CO) , 5
atm)
(8)
ς
51%
Table I I E f f e c t o f H y d r o c a r b o n A d d i t i o n s on WGSR A c t i v i t y 0.10
mM[Ru (C0) ] 3
H y d r o c a r b o n (g) none 1-butene (5.2) p e n t a n e (10) p e n t a n e (18)
1 2
0.50
mM[Ru (C0) ] 3
1 2
t ^
H y d r o c a r b o n (g)
tji
9300 11700 10700 7300
none 1-butene (5.2) 1-hexene (7.7) p e n t a n e (13) 1-butene (15.5) p e n t a n e (65)
4900 5280 6800 6440 5080
1
2
pro
(
'
9
)
76%
I t i s thought t h a t s m a l l a d d i t i o n s of hydrocarbon s o l v e n t s tend t o enhance t h e f o r m a t i o n o f Ru(CO)^, w h e r e a s l a r g e r c o n c e n t r a t i o n s s e r i o u s l y decrease the d i e l e c t r i c c o n s t a n t o f the s o l v e n t so t h a t the formation o f i o n i c s p e c i e s i n s o l u t i o n i s suppressed. The b a s e h a s a v e r y i m p o r t a n t e f f e c t o n t h e e f f i c i e n c y o f r u t h e n i u m c a r b o n y l f o r t h e WGSR ( s e e T a b l e I I I ) . Amines w e r e f o u n d t o p r o v i d e much b e t t e r a c t i v i t y t h a n B r o n s t e d b a s e s , and t r i m e t h y l a m i n e appears t o be the base o f c h o i c e , a f f o r d i n g r a t e s more t h a n two o r d e r s o f m a g n i t u d e g r e a t e r t h a n t h o s e o f B r o n s t e d bases. Table I I I E f f e c t o f Base on t h e R u t h e n i u m C a r b o n y l - C a t a l y z e d W a t e r Gas S h i f t R e a c t i o n Amt
Base 3
3
3
2
3
2
3
3
H 0,g 2
15 15 15 15 15 16 15 20 20 20 20 20
5 5 5 5 5 4 6 3 2 0.2 0.03 0.0
NMe NEt NBu N-Me P y r r o l i d i n e NHMe Pyridine NH Na C0 Li C0 Me4N0H BU4NOH None 2
Amt
Base,g
ϋ
Η2 5740 860 540 ^2400 2200 ^ 300 420 < 50 < 50 < 50 < 50 < 50
Conditions: 0.05 mmol R u ( C O ) , 92 mmol 1-butene, b a s e a n d w a t e r d i l u t e d t o 100 mL w i t h d i g l y m e , 750 p s i CO, 100°, 10 h r , 0.31 L r e a c t o r . 3
1 2
The n u c l e o p h i l i c r e a c t i o n o f h y d r o x i d e w i t h c a r b o n y l l i g a n d s of t r a n s i t i o n metal complexes,
M=C=0 + OH"
0
•M-C^' 0H
C
~ °2»ÏÏ-H
X
In Catalytic Activation of Carbon Monoxide; Ford, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
(10)
332
CATALYTIC ACTIVATION OF CARBON
MONOXIDE
i s a w e l l - k n o w n r e a c t i o n ( 3 3 ) , f r e q u e n t l y employed i n t h e p r e p a r a t i o n o f m e t a l h y d r i d e c o m p l e x e s ( 3 4 ) . However, B r o n s t e d b a s e s a r e i n v o l v e d i n a number o f r e a c t i o n s w h i c h l o w e r t h e n u c l e o p h i l e concentration: CO + OH"
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C0
+ 0H~
2
H C 0 ~ + OH" 3
» HC0 "
(6)
*-HC0 ~
(11)
2
3
^C0
2 3
~ + H 0
(12)
2
B e c a u s e o f t h e s e s i d e r e a c t i o n s , we h a v e n o t been a b l e t o m a i n t a i n t h e a p p a r e n t pH above 10.0 f o r any s i g n i f i c a n t p e r i o d o f t i m e ; t h e r e f o r e t h e maximum s u s t a i n a b l e h y d r o x i d e n u c l e o p h i l e c o n c e n t r a t i o n i n an experiment o f s e v e r a l hours i s about 1.0 χ 1 0 ~ M. Amines h y d r o l y z e , 4
NR
3
+ H O^Z?HNR 2
+ 3
+ 0H~,
(13)
t o p r o v i d e h y d r o x i d e c o n c e n t r a t i o n s c o m p a r a b l e t o t h o s e o f weaker B r o n s t e d b a s e s (0.5 M NMe i s pH 12.2; 0.5 M N a C 0 i s pH 12.1; a t room t e m p e r a t u r e i n w a t e r ) . Thus amines may p a r t i c i p a t e i n t h e WGSR v i a h y d r o x i d e a t t a c k on c a r b o n y l l i g a n d s , as i s e v i d e n t i n t h e r h o d i u m c a r b o n y l s y s t e m ( 2 6 ) . However, d i r e c t amine a t t a c k on c a r b o n y l l i g a n d s i s known. E d g e l l (35,36) h a s r e p o r t e d t h a t p r i m a r y and s e c o n d a r y a m i n e s r e a c t w i t h i r o n p e n t a c a r b o n y l t o form z w i t t e r i o n i c metallocarboxamides, which i n t h e presence o f t r a c e s o f w a t e r a r e r a p i d l y h y d r o l y z e d ( p r e s u m a b l y by n u c l e o p h i l i c a t t a c k by w a t e r on t h e a c t i v a t e d c a r b o n y l c a r b o n ) t o t h e hydride anion: 3
2
+J
Fe(C0)
5
+ R NH^ ^R N-C-Fe(CO) 2
=
2
4
3
HO =-#>HFe(C0)
4
(14)
Nesmeyanov h a s p r o v i d e d i n t e r e s t i n g examples o f a p p a r e n t i n t r a m o l e c u l a r n u c l e o p h i l i c a t t a c k by a m i n e on c a r b o n y l l i g a n d s ( 3 7 ) . A n g e l i c i (38,39) h a s d e m o n s t r a t e d t h a t amine a t t a c k o n c a t i o n i c m e t a l c a r b o n y l complexes i s a g e n e r a l r e a c t i o n r e s u l t i n g i n t h e f o r m a t i o n of carbamoyl complexes:
MC0
+
+ NHK . ^
^
M-l-NHK
~&-»>Μ-ft-NR
2
(15)
Ammonia, p r i m a r y a n d s e c o n d a r y a m i n e s a r e known t o u n d e r g o s i d e r e a c t i o n s u n d e r WGSR c o n d i t i o n s :
In Catalytic Activation of Carbon Monoxide; Ford, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
21.
CO + HNR C0 C0
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333
Homogeneous Water Gas Shift
SLEGEIR E T A L .
2
2
+ HNR
(16)
2
• HOCONR
2
+ 2HNR
• R NCONR
2
2
(17)
2
(18)
2
These r e a c t i o n s s e r v e t o d e p l e t e t h e a v a i l a b l e n u c l e o p h i l e c o n centration. Furthermore, the formation o f a r e l a t i v e l y s t a b l e c a r b a m o y l c o m p l e x may s e r v e t o l o w e r b o t h t h e m e t a l c a r b o n y l a n d the n u c l e o p h i l e c o n c e n t r a t i o n s . A n g e l i c i and B r i n k ( 4 0 ) h a v e f o u n d t h a t i n t h e r e a c t i o n s o f amine w i t h t r a n s - M ( C O ) , ( P P h M e p ) (Μ = Mn o r R e ) , t h e r a t e o f carbamoyl formation f o l l o w s t h e order, n-butylamine > c y c l o h e x y l amine j> i s o p r o p y l a m i n e > s e c - b u t y l a m i n e >> t e r t - b u t y l a m i n e , i m p l y i n g a strong s t e r i c e f f e c t i n carbamoyl formation. A similar o r d e r h a s been o b s e r v e d i n t h e r a t e o f r e a c t i o n o f o r g a n i c e s t e r s w i t h amines t o f o r m amides ( 4 1 ) . The d a t a i n T a b l e I I I i n d i c a t e t h a t a s t e r i c e f f e c t may b e o p e r a t i v e i n t h e R u ^ ( C O ) / N R 3 - c a t a l y z e d WGSR, s i n c e w i t h t e r t i a r y amines t h e r a t e f o l l o w s t h e o r d e r , NMe~ > MeNC.H > N E t > N B u , w h i c h d o e s n o t r e f l e c t t h e b a s i c i t y +
2
1 2
0
r
λ
4
Ο
ό
0
J
of t h e s e amines. A n g e l i c i (38) has c o r r e l a t e d t h e r e a c t i v i t y o f m e t a l c a r b o n y l c o m p l e x e s t o w a r d a m i n e s w i t h C-0 s t r e t c h i n g f o r c e c o n s t a n t s . A p p l i c a t i o n o f h i s e m p i r i c a l r u l e t o t h e WGSR i n d i c a t e s t h a t on e l e c t r o n i c grounds, Ru(C0)5 and R L ^ C O ) ] ^ s h o u l d b e comparably subject to carbonyl n u c l e o p h i l i c attack. Inl i g h t of the s t e r i c e f f e c t observed i n n u c l e o p h i l i c a t t a c k by amines, mononuclear s p e c i e s a r e t h o u g h t t o b e more e f f e c t i v e t h a n c l u s t e r s p e c i e s i n t h e r u t h e n i u m c a r b o n y l - a m i n e c a t a l y z e d WGSR. When t h e n u c l e o p h i l e i s t h e much more s t e r i c a l l y compact h y d r o x i d e g r o u p , t h e r e may b e l i t t l e s t e r i c b i a s i n t h e b a s e a t t a c k s t e p . The u s e o f a m i n e s a l l o w s much h i g h e r n u c l e o p h i l e c o n c e n t r a t i o n s t h a n t h o s e a c h i e v a b l e w i t h B r o n s t e d b a s e s . We h a v e u s e d s o l u t i o n s a s c o n c e n t r a t e d a s 6 Μ ΜββΝ. T h i s v a s t d i f f e r e n c e i n a v a i l a b l e n u c l e o p h i l e c o n c e n t r a t i o n p a r t i a l l y e x p l a i n s t h e huge i n c r e a s e i n r a t e a f f o r d e d b y NMe o v e r t h e r a t e w i t h B r o n s t e d bases. V e r y l a r g e c o n c e n t r a t i o n s o f h y d r o x i d e may p r o m o t e t h e b a s e a t t a c k s t e p b u t c a n d e c r e a s e t h e r a t e o f t h e WGSR due t o i n h i b i t i o n of the protonation of the metal hydride species. 3
A d d i t i o n a l Comments R e g a r d i n g t h e R u t h e n i u m C a r b o n y l - T r i m e t h y l a m i n e WGSR System. A p o t e n t i a l m e c h a n i s t i c pathway f o r a WGSR s y s t e m i n v o l v e s t h e p r o d u c t i o n o f f o r m a t e , f o l l o w e d b y i t s c a t a l y t i c decomposition: CO + OH
HCO, 2
9
In Catalytic Activation of Carbon Monoxide; Ford, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
(6)
334
CATALYTIC ACTIVATION OF CARBON
HC0
+ H 0
2
**H
2
2
MONOXIDE
+ C 0 + OH .
(19)
2
R u t h e n i u m c a r b o n y l decomposes t h e f o r m a t e i o n i n b a s i c m e d i a , b u t a t a r a t e s l o w e r t h a n t h e r a t e o f t h e WGSR. A t 100° and 0.10 mM R u ( C O ) i 2 , u n d e r 3 atm N , t h e r a t e o f d e c o m p o s i t i o n o f t r i m e t h y l ammonium f o r m a t e t o H and C 0 i s 0.6 mmol/hr. U n d e r 5 a t m CO t h e r a t e i s s l o w e r (0.4 mmol/hr. A t t h i s low CO p r e s s u r e , t h e r a t e o f H p r o d u c t i o n d i r e c t l y f r o m CO and H 0 i s more t h a n t h r e e times that from formate decomposition. Furthermore, since i n c r e a s e s i n CO p r e s s u r e r e s u l t i n i m p r o v e d H p r o d u c t i o n r a t e s (10 mmol/hr a t 50 atm CO), w h i l e a p p a r e n t l y i n h i b i t i n g t h e r a t e o f f o r m a t e d e c o m p o s i t i o n , i t may be c o n c l u d e d t h a t f o r m a t e decom p o s i t i o n h a s l i t t l e m e c h a n i s t i c s i g n i f i c a n c e i n t h e WGSR a c t i v i t y of Ru (CO) /NMe . On t h e b a s i s o f t h i s d i s c u s s i o n , we p r o p o s e t h a t t h e R u ( C O ) i / N M e - c a t a l y z e d WGSR f o l l o w s t h e mechanism shown i n F i g u r e 3. A s i m i l a r mechanism, i n v o l v i n g n u c l e o p h i l i c a t t a c k by h y d r o x i d e i n s t e a d o f amine, h a s b e e n p r o p o s e d b y P e t t i t a n d c o w o r k e r s (4) f o r the Fe(C0)5/base system. The d i h y d r i d e o n c e formed s h o u l d e l i m i n a t e H r e a d i l y ; i t decomposes r a p i d l y a t 0° (42) o r above 20° u n d e r 300 atm H (27). T h i s s h o u l d be c o n t r a s t e d w i t h t h e s t a b i l i t y a n d , p r e s u m a b l y , t h e c a t a l y t i c a c t i v i t y of the c l u s t e r h y d r i d e , H ^ R u ^ C O ) j . Kaesz (43) h a s i n d i c a t e d t h e c l u s t e r s p e c i e s 1^and are i n 3
2
2
2
2
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2
2
2
3
3
1 2
2
3
3
2
2
2
Ru (CO) 3
1 2
^z=^H Ru (CO) 4
4
(20)
1 2
e q u i l i b r i u m and may be i n t e r c o n v e r t e d . Conditions as mild as 1 a t m H a t 80° a l l o w t h e q u a n t i t a t i v e c o n v e r s i o n of ^ t o ^ ( 4 4 ) . F u r t h e r m o r e , H4Ru4(CO)^2 r e p o r t e d t o b e s t a b l e u n d e r a 1:1 m i x t u r e o f CO and H a t 100° (_45) . I f a n i m p o r t a n t s t e p i n t h e r u t h e n i u m c a r b o n y l - c a t a l y z e d WGSR i n v o l v e d e l i m i n a t i o n o f H f r o m Sj h y d r o g e n p r e s s u r e s h o u l d t e n d t o i n h i b i t t h e WGSR; i f e l i m i n a t i o n o f H o c c u r r e d w i t h t h e monomer^6, l i t t l e i n h i b i t i o n o f t h e WGSR s h o u l d b e o b s e r v e d . The e f f e c t o f h y d r o g e n p r e s s u r e o n t h e r a t e o f H p r o d u c t i o n was t e s t e d b y c a r r y i n g o u t a WGSR a t 0.10 mM R u ( C O ) ^ w i t h a n i n i t i a l p r e s s u r e o f 310 p s i H ( C 0 : H = 2 ) ; t h e r a t e o f H p r o d u c t i o n was i d e n t i c a l t o t h a t i n t h e a b s e n c e o f added Η · By j u d i c i o u s a d j u s t m e n t o f c o n d i t i o n s , t h e r a t e o f t h e R u ( C O ) ^ / N M e 3 - c a t a l y z e d WGSR c o u l d be s i g n i f i c a n t l y i m p r o v e d . As m e n t i o n e d e a r l i e r , t h o s e f a c t o r s w h i c h f a v o r f o r m a t i o n o f R u ( C O ) ^ — d e c r e a s e s i n c o n c e n t r a t i o n and i n c r e a s e s i n CO p r e s s u r e — f a v o r h i g h e r t u r n o v e r numbers i n t h e WGSR. I n c r e a s e s i n a m i n e c o n c e n t r a t i o n and i n t e m p e r a t u r e a l s o i m p r o v e t h e r a t e s o f H production. T h u s , a t 155°, 0.0082 mM R u ( C O ) a n d 1080 p s i 2
i
s
2
2
2
2
3
2
2
2
2
2
3
2
2
3
1 2
In Catalytic Activation of Carbon Monoxide; Ford, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
21.
SLEGEiR E T A L .
335
Homogeneous Water Gas Shift
i n i t i a l CO p r e s s u r e , t h e n e t h y d r o g e n t u r n o v e r number i s 270,000 o v e r a 10 h r p e r i o d . T h i s c o r r e s p o n d s t o a r a t e o f 7.5 m o l H2 p e r mol R u ( C O ) ^ 2 P s e c , an improvement o f n e a r l y t h r e e o r d e r s o f m a g n i t u d e o v e r t h e r a t e i n a n y o t h e r r e p o r t e d homogeneous s y s t e m . e r
3
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The
Group V I M e t a l C a r b o n y l
System
The Group V I m e t a l c a r b o n y l s d e m o n s t r a t e good a c t i v i t y i n t h e WGSR, b u t d i f f e r s i g n i f i c a n t l y f r o m r u t h e n i u m c a r b o n y l i n s e v e r a l ways. T a b l e s I V and V summarize some WGSR e x p e r i m e n t s w i t h c h r o mium and t u n g s t e n c a r b o n y l s i n a t e t r a h y d r o f u r a n - w a t e r s o l v e n t system. TABLE I V Cr(C0)
A
Base (mmol)
a s WGSR C a t a l y s t i n THF/HO g H 0
mmol H
7.5 15 20
9.5 2.4