Mechanistic Aspects of the Homogeneous Water Gas Shift Reaction

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),

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

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^RuîCO)^* 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.


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


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


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


21.

SLEGEIR

E T

329


AL.


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


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 ^*îsolation 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


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


(10)

332


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"


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 :


21.

CO + HNR C0 C0


333


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


(6)

334


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 1ând are i n 3

2

2

2

2


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


21.

SLEGEiR E T A L .

335


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


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

Mechanistic Aspects of the Homogeneous Water Gas Shift Reaction

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