12 C h a i n - L e n g t h C o n t r o l i n the C o n v e r s i o n of S y n g a s over C a r b o n y l C o m p o u n d s A n c h o r e d into a Zeolite M a t r i x
Catalytic Activation of Carbon Monoxide Downloaded from pubs.acs.org by TUFTS UNIV on 07/11/18. For personal use only.
D. BALLIVET-TKATCHENKO, N. D. CHAU, H. MOZZANEGA, M. C. ROUX, and I. TKATCHENKO Institut de Recherches sur la Catalyse, 2 avenue Einstein, F- 69626 Villeurbanne Cedex, France It is now superflous to point out the renewed interest for the Fischer-Tropsch (F-T) synthesis (1) i.e. the conversion of CO+H mixtures into a broad range of products including alkanes, alkenes, alcohols. Recent reviews (2,3,4,5) emphasized the central problem in F-T synthesis: selectivity or more precisely chain-length control. Recent patents and publications reported the use of modified "classical" F-T catalysts or new supported metal catalysts. Ruhrchemie disclosed (6) iron catalysts modified by additives like Ti, Mn and Mo, which are claimed to produce ca 50% of C -C alkenes. Ichikawa, by using rhodium carbonyl compounds deposited on various supports, was able to produce selectively (7) C -C alcohols and hydrocarbons. Nijs et al. reported (8) that Ru(III) ions exchanged into Y-zeolites reduced with hydrogen lead to selective catalysts for the synthesis of hydrocarbons i n the C -C range. Similarly, Fraenkel and Gates have shown (9) that catalysts prepared by reduction with cadmium of Co(II) ions exchanged into A- and Y-zeolites may produce propylene as the only hydrocarbon product under well-defined conditions. Blanchard et al., by using di-cobalt octacarbonyl supported on alumina, have observed (10) that good selectivities for C -C hydrocarbons could be obtained when the support presents a mean pore size of 5nm. The aim of this work is to prepare better defined catalytic systems by combining components with well characterized structures and properties such as zeolites and transition metal molecular complexes. Among these complexes, clusters are potential candidates for selective F-T catalysts since neighboring atoms with unique topologic and electronic features may help "hydro-oligomerization" of carbon monoxide. However, u n t i l now only very low activities have been achieved ( VI ). Zeolites are well defined, crystalline alumino-silicates (12) which may offer stabilization of metal particles (13) and shape selectivity {]h) owing to their geometrical frame properties. From the point of view of reactivity and catalysis, hope is s t i l l high that such adducts w i l l be efficient as/or more efficient than classical heterogeneous 2
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0097-6156/81/0152-0187$05.00/0 © 1981 American Chemical Society
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c a t a l y s t s . The i n t e r a c t i o n between t h e s u p p o r t and t h e c l u s t e r and t h e r e d o x b e h a v i o u r o f t h e s e p a r t n e r s a r e i m p o r t a n t f a c t o r s w h i c h w i l l i n f l u e n c e t h e t y p e o f c a t a l y s t f o r m e d . E a r l i e r work f r o m t h i s L a b o r a t o r y ( 15» 1_6) h a s shown t h a t t h e a d s o r p t i o n o f c e r t a i n t r a n s i t i o n m e t a l c a r b o n y l s i n t o a n H Y - z e o l i t e framework and a p p r o p r i a t e t h e r m a l d e s o r p t i o n l e a d e i t h e r t o i o n s (Mo, F e ) or t o m e t a l ( R e , R u ) . However t h e o x i d a t i o n r e a c t i o n c o u l d be p r e v e n t e d b y t h e use o f n o n - a c i d i c z e o l i t e s l i k e t h e Na-Y t y p e ( l j ) . We r e p o r t h e r e r e s u l t s r e l a t e d t o t h e c a t a l y t i c b e h a v i o u r o f d o d e c a c a r b o n y l - t r i - i r o n and t r i - r u t h e n i u m , b i s ( c y c l o p e n t a d i e n y l d i c a r b o n y l i r o n ) and o c t a c a r b o n y l - d i - c o b a l t d e p o s i t e d on Y - z e o l i t e s u n d e r F-T c o n d i t i o n s . The i n f l u e n c e o f t h e n a t u r e o f t h e z e o l i t e and o f t h e m e t a l , t h e d i s p e r s i o n o f t h e m e t a l and t h e r e a c t i o n c o n d i t i o n s upon a c t i v i t y and p r o d u c t s d i s t r i b u t i o n were i n v e s t i gated. Experimental. M a t e r i a l s . The NaY f a u j a s i t e was s u p p l i e d b y L i n d e Co (SK kO S i e v e s ) . A c o n v e n t i o n a l exchange w i t h NH^Cl p r o v i d e s a NH^Y sample ( u n i t c e l l c o m p o s i t i o n : (NH^)^gNa^QAI5gSi135038^) · H e a t i n g t h i s sample f o r 15h i n oxygen and 3h i n v a c u o ( I 0 " 5 t o r r ) a t 350°C l e a d s t o t h e h y d r o g e n f o r m HY. The s i l i c a - a l u m i n a was s u p p l i e d b y K e t j e n w i t h a 13$ a l u m i n a c o n t e n t . I t was v a c u u m - t r e a t e d a t k^O°C (I0"~5torr) f o r 15hrs before anchoring the cluster-. T r i - i r o n d o d e c a c a r b o n y l was p r e p a r e d a c c o r d i n g t o ( l 8 ) ; b i s ( c y c l o p e n t a d i e n y l d i c a r b o n y l i r o n ) was p r e p a r e d a c c o r d i n g t o (19); t r i - r u t h e n i u m d o d e c a c a r b o n y l and d i - c o b a l t o c t a c a r b o n y l were s u p p l i e d by Strem C h e m i c a l s . The Fe3(C0)-|2~Y a d d u c t s a r e p r e p a r e d u n d e r a r g o n a t m o s p h e r e w i t h t h e HY and NaY z e o l i t e s p r e v i o u s l y h e a t e d u n d e r vacuum a t 350°C. The i m p r e g n a t i o n o f t h e s u p p o r t i s p e r f o r m e d e i t h e r f r o m p e n t a n e s o l u t i o n a t 25°C o r f r o m d r y m i x i n g o f t h e c a r b o n y l a n d t h e z e o l i t e t o a v o i d any c o m p l i c a t i o n f r o m t h e s o l v e n t . I n t h i s l a s t p r e p a r a t i o n t h e sample s t a n d s i n v a c u o f o r 2kh a t 60°C i n order t o favour the s u b l i m a t i o n o f the carbonyl i n t o the pores o f the z e o l i t e . The Ru3(C0)-|2~ a d d u c t s a r e s i m i l a r l y p r e p a r e d b o t h t h r o u g h impregnation ( s o l v e n t : cyclohexane) o r dry mixing (heating a t 90°C u n d e r vacuum. The |CpFe(CO)o|2~ a d d u c t s a r e p r e p a r e d i n t h e same way b u t w i t h h e a t i n g a t UO°C u n d e r vacuum when t h e d r y m i x i n g p r o c e d u r e i s used. The Co2(C0)3~NaY a d d u c t s a r e p r e p a r e d b y t h e d r y m i x i n g procedure. In a n y i n s t a n c e , t h e amount o f m e t a l a n c h o r e d i s d e t e r m i n e d by c h e m i c a l a n a l y s i s . The l o a d i n g s c o r r e s p o n d t o 6-12 m e t a l atoms per unit c e l l . Y
Y
12.
BALLIVET—TKATCHENKO
ET
Syngas Conversion over Carbonyls
AL.
189
C a t a l y t i c e x p e r i m e n t s . The r u n s a r e p e r f o r m e d i n a 300mL s t a t i c r e a c t o r ( A u t o c l a v e E n g i n e e r s M o d e l AE 300) f o r 1 5 h r s u n d e r an i n i t i a l 20 b a r p r e s s u r e w i t h a sample w e i g h t l e a d i n g t o O.h mg-atom o f m e t a l . N e i t h e r t h e u n l o a d e d z e o l i t e s n o r t h e m o l e c u l a r c l u s t e r s a r e a c t i v e i n CO hydrogénation u n d e r o u r experimental conditions. The p r o d u c t s a r e a n a l y s e d by gas c h r o m a t o g r a p h y u s u a l l y on five different columns i n o r d e r t o d e t e c t CO, H , C 0 , H 0 and C^-C h y d r o c a r b o n s ( a l k a n e s , a l k e n e s ) and a l c o h o l s . The mass b a l a n c e f o r c a r b o n , b a s e d on CO c o n s u m p t i o n , g e n e r a l l y l i e s w i t h i n 85-95$. 2
2
2
n
R e s u l t s and D i s c u s s i o n . I t s h o u l d be e m p h a s i z e d t h a t t h e r e s u l t s were i n t e n d e d t o demonstrate q u a l i t a t i v e trends r a t h e r than q u a n t i t a t i v e k i n e t i c d a t a w i t h t h e s e t y p i c a l c a t a l y s t s . M o r e o v e r t h e h i g h CO c o n v e r s i o n l e v e l s a c h i e v e d i n t h e p r e s e n t work w i l l n o t be t h e l i m i t i n g f a c t o r t o o b s e r v a t i o n o f s i d e r e a c t i o n s and l o n g - c h a i n h y d r o carbons . T r i - i r o n d o d e c a c a r b o n y l z e o l i t e s a d d u c t s . The a d d u c t s F e ^ C O ) 12~HY and Fe-^(CO) -j ~NaY were u s e d as s t a r t i n g m a t e r i a l s . The c a t a l y t i c r u n s a r e p e r f o r m e d w i t h t h e s e a d d u c t s o r w i t h t h e m a t e r i a l s r e c o v e r e d f r o m t h e i r t o t a l d e c a r b o n y l a t i o n a t 200°C u n d e r vacuum. The F e ^ C O j ^ - H Y a d d u c t e x h i b i t s no c a t a l y t i c a c t i v i t y i n t h e t e m p e r a t u r e r a n g e s t u d i e d (200-300°C). D u r i n g t h e s t u d y o f t h e d e c a r b o n y l a t i o n u n d e r vacuum, s e v e r a l s t o i c h i o m e t r i c r e a c t i o n s t a k e p l a c e as e v i d e n c e d by mass s p e c t r o m e t r y and i n f r a r e d s p e c t r o s c o p y ( 1_6). The o x i d a t i o n o f F e ( 0 ) i n t o F e ( l l ) s p e c i e s by t h e z e o l i t e p r o t o n s , t h e w a t e r - g a s s h i f t r e a c t i o n and t h e hydrogén a t i o n o f C 0 a c c o u n t t o some e x t e n t f o r t h e f o r m a t i o n o f H, Fe C 0 , H 0 , CHl| and h i g h e r h y d r o c a r b o n s . The sample t h u s obtained, i . e . F e - H Y , i s as i n a c t i v e i n t h e c a t a l y t i c s y n g a s c o n v e r s i o n as a r e s t a n d a r d F e ^ - N a Y and F e ^ - N a Y e x c h a n g e d z e o l i t e s . Therefore Fe^ '^ -Y z e o l i t e s i n contrast to Ru -Y z e o l i t e s (8_) a r e n o t c a t a l y s t p r e c u r s o r s . I t i s w o r t h m e n t i o n i n g t h a t m o l e c u l a r hydrogen i s unable t o reduce F e - Y z e o l i t e even under d r a s t i c c o n d i t i o n s (20). C o n v e r s l y , t h e Fe3(CO)-|2~NaY a d d u c t i s a c t i v e f o r s y n g a s c o n v e r s i o n . A non-decomposed sample e x h i b i t s a s i g n i f i c a n t a c t i v i t y a t 230°C w h e r e a s t h e c a t a l y t i c e f f i c i e n c y f o r t h e d e c a r b o n y l a t e d one a l r e a d y a p p e a r s a t 200°C. I n f r a r e d e x p e r i m e n t s show an i n c r e a s e i n t h e s t a b i l i t y o f t h e F e ^ ( C 0 ) u n i t s upon t h e r m a l t r e a t m e n t u n d e r CO a t m o s p h e r e so t h a t t o t a l c a r b o n monoxide e v o l u t i o n o n l y t a k e s p l a c e a t 230°C t h u s s u g g e s t i n g t h a t the c a t a l y s t i s c e r t a i n l y not Fe^CO)-^* This c l u s t e r has t o be t r a n s f o r m e d i n t o h i g h e r n u c l e a r i t y s p e c i e s w h i c h b i n d l e s s s t r o n g l y w i t h c a r b o n monoxide upon CO r e - a d s o r p t i o n (17)· 2
2
2 +
2
5
2
2
2 +
+
+
+
+
n +
2 +
1 2
190
CATALYTIC
ACTIVATION
OF
CARBON
MONOXIDE
E f f e c t s o f t h e i n l e t H / C 0 r a t i o and r e a c t i o n t e m p e r a t u r e on H + C 0 c o n v e r s i o n and p r o d u c t s s e l e c t i v i t y were s t u d i e d a t c o n s t a n t i n i t i a l p r e s s u r e and r e a c t i o n t i m e w i t h non-decomposed s a m p l e s o f F e 3 ( C O ) ~ N a Y a d d u c t (k%Fe). I n a l l e x p e r i m e n t s , H 0 , C 0 a l k a n e s a n d a l k e n e s (up t o C-j ) a r e p r o d u c e d . L i n e a r a l c o h o l s a r e a l s o d e t e c t e d t o m i n o r amounts. At i n c r e a s i n g r e a c t i o n temperatures (230-350°C) the product s e l e c t i v i t y i s s h i f t e d t o w a r d s C-j-C^. The a l k e n e t o a l k a n e r a t i o d e c l i n e s a t h i g h e r r e a c t i o n temperatures whereas t h e branched t o l i n e a r a l k a n e r a t i o i n c r e a s e s a s w e l l as C 0 f o r m a t i o n . These observations are e n t i r e l y c o n s i s t e n t w i t h the behaviour o f c l a s s i c a l F-T c a t a l y s t s ( T a b l e 1 ) . A r e a c t i o n t e m p e r a t u r e o f 250°C was u s e d t o s t u d y t h e o t h e r r e a c t i o n parameters. A decrease i n the H /C0 r a t i o i n c r e a s e s the c o n s u m p t i o n o f CO w h e r e a s t h e p e r c e n t c o n v e r t e d d e c r e a s e s . C o n v e r s l y the H c o n v e r s i o n i n c r e a s e s but i t s consumption remains c o n s t a n t . I n f a c t t h i s means t h a t h i g h e r m o l e c u l a r w e i g h t p r o d u c t s are formed under low hydrogen p a r t i a l p r e s s u r e : as i n d i c a t e d i n T a b l e 2 , q u a n t i t a t i v e a n a l y s e s o f t h e p r o d u c t s show indeed a decrease i n Cj-C^ y i e l d and a concomitant i n c r e a s e i n C 3 " h y d r o c a r b o n s . The s e l e c t i v i t y f o r C 0 r e m a i n s c o n s t a n t w h i c h a p p a r e n t l y i n d i c a t e s t h a t C 0 i s a t l e a s t i n t h e case o f i r o n a p r i m a r y p r o d u c t i n t h e F-T s y n t h e s i s . We have c h e c k e d t h a t t h e i r o n / z e o l i t e c a t a l y s t a c t i v i t i e s f o r hydrogénation and t h e w a t e r g a s - s h i f t a r e n o t s i g n i f i c a n t w i t h t h e p r e c u r s o r F e 3 ( C 0 ) -j /NaY. The a l k e n e t o a l k a n e r a t i o g r e a t l y v a r i e s w i t h ( i ) H / C 0 a n d ( i i ) t h e c h a i n l e n g t h . An i n c r e a s e i n hydrogen p a r t i a l p r e s s u r e i n c r e a s e s t h e a l k a n e p r o d u c t i o n a s a l k e n e hydrogénation i s a s e c o n d a r y r e a c t i o n w h i c h t a k e s p l a c e w i t h F-T c a t a l y s t a n d , i n t h i s work, w i t h t h e z e o l i t e system. F o r a steady syngas i n l e t , C 5 / C 3 r a t i o i s c o n s i s t e n t l y much l o w e r t h a n t h e C 3 / C 3 one. Cj^/C^ r a t i o i s c o m p l i c a t e d by the e x i s t e n c e o f the butene isomers and isobutene (Table 2 ) . A l t h o u g h e t h y l e n e i s more r e a d i l y h y d r o g e n a t e d t h a n t h e o t h e r a l k e n e s , i t has b e e n r e p o r t e d ( 2 j ) t o p a r t i c i p a t e t o t h e f o r m a t i o n o f h i g h e r m o l e c u l a r w e i g h t h y d r o c a r b o n s u n d e r F-T c o n d i t i o n s . This i s observed too f o r F e 3 ( C 0 ) ~ N a Y c a t a l y s t s . I f t h e F-T s y n t h e s i s i s p e r f o r m e d w i t h e t h y l e n e as a c o - r e a c t a n t , s i g n i f i c a n t changes i n s e l e c t i v i t y are found f o r C 0 , C 3 and f o r t h e i - C ^ / n - C ^ r a t i o . The v a r i a t i o n s f o r CH^ a n d C^/C^ r a t i o can a l s o be a t t i b u t e d t o t h e H / C 0 r a t i o m o d i f i c a t i o n ( 3 . 5 / 1 i n s t e a d o f h/λ) i f one t a k e s i n t o a c c o u n t t h e h y d r o g e n consumed f o r e t h y l e n e hydrogénation ( T a b l e 3 ) . I t a p p e a r s t h a t a s C 0 seems t o be a p r i m a r y p r o d u c t ( v i d e s u p r a ) the decrease f o r i t s s e l e c t i v i t y i n the presence o f ethylene s u g g e s t s t h a t CO c o n s u m p t i o n now o c c u r s p a r t l y t h r o u g h a r e a c t i o n l e a d i n g t o more h y d r o c a r b o n s , e s p e c i a l l y C 3 , and no C 0 - S u c h a r e a c t i o n p a t h w a y c o u l d i n v o l v e t h e i n s e r t i o n o f CO i n t o a m e t a l e t h y l b o n d a s a l r e a d y w e l l documented i n c o o r d i n a t i o n c h e m i s t r y and homogeneous c a t a l y s i s o r t h e i n s e r t i o n o f a s u r f a c e c a r b e n e 2
2
1 2
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2
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2
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T a b l e 1. E f f e c t o f t h e r e a c t i o n t e m p e r a t u r e on CO a n d H c o n v e r s i o n s , a n d p r o d u c t s e l e c t i v i t i e s ( e x p r e s s e d as mole p e r c e n t o f CO c o n v e r t e d i n t o t h e d e s i r e d p r o d u c t ) . E x p e r i m e n t a l c o n d i t i o n s : c a t a l y s t = F e ^ C O ) ~ N a Y (h%Fe) ; H /C0 = i n i t i a l p r e s s u r e = 20 b a r ; r e a c t i o n t i m e = 15hrs. 2
12
2
T°C
Hp conv.
CO conv.
%
%
200
8.7
C0 %
CH^
2
%
r
c -c % 2
k
36
5-5
10.3
7.6
16
19
57. k
19
2k
kk 20 35
21+0
1*0
82
255
1*3
85
13
70
295
29
Ik
20
26
3k
365
37
88
19
19
27
T a b l e 2. E f f e c t o f t h e H /C0 r a t i o on CO a n d H c o n v e r s i o n s and p r o d u c t s e l e c t i v i t i e s ( e x p r e s s e d a s mole p e r c e n t o f CO converted i n t o the desired product). E x p e r i m e n t a l c o n d i t i o n s : c a t a l y s t = Fe-^CO)-| ~NaY {h%Fe) ; i n i t i a l p r e s s u r e = 20 b a r ; r e a c t i o n t e m p e r a t u r e = 250°C; r e a c t i o n t i m e = 15hrs. 2
2
2
H /C0 2
H
2
conv. CO conv. % %
co
%
*%
20.6
9
2
%
2
c /c
%
%
C /Co %
%
9-9
-
0.8
-
£
3
2
2
3
U/1
39
73
2/1
57
6k
9-5
7-U
k.k 7-7
U.5
k5
16
1/1
66
1+8
10.1+
5-2
2.8
7.1
kl
19-5
10
5-3
192
CATALYTIC
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CARBON
MONOXIDE
i n t o t h e a d s o r b e d e t h y l e n e ( 2 2 ) . However, i f e t h y l e n e c a n p a r t i c i p a t e t o t h e F-T r e a c t i o n s t h e p r e s e n c e o f CO i s e s s e n t i a l f o r c h a i n g r o w t h as an e t h y l e n e + H f e e d m a i n l y g i v e s e t h a n e i n t h e present r e a c t i o n conditions. The f o r m a t i o n o f C h y d r o c a r b o n s f r o m s y n g a s i n v o l v e a c h a i n g r o w t h mechanism ( 2 3 ) . The m o l e c u l a r w e i g h t d i s t r i b u t i o n w i l l depend on ( i ) t h e p r o p a g a t i o n t o t r a n s f e r r a t e r a t i o a n d ( i i ) t h e s i d e r e a c t i o n s . F r o m t h e r e a c t i o n mechanism s t a n d p o i n t , t h e s e s e c o n d a r y r e a c t i o n s (£.£. hydrogénation, i s o m e r i z a t i o n a n d h y d r o g e n o l y s i s ) a r e m a s k i n g t h e p r i m a r y g r o w t h p r o c e s s a n d make d i f f i c u l t s u c h an a p p r o a c h . A non-decomposed Fe-^CO) ^ " N a Y a d d u c t (h%Fe) p r o v i d e s a t y p i c a l m o l e c u l a r w e i g h t d i s t r i b u t i o n w h i c h i s r e p o r t e d on F i g u r e 1, c u r v e 1. A n e x p o n e n t i a l d e c r e a s e o f t h e mole p e r c e n t o f CO consumed t o f o r m C i s o b s e r v e d f r o m C-j t o C9 w i t h a c o n s i s t e n t l y l o w e r v a l u e f o r C2« H y d r o c a r b o n s h i g h e r t h a n C9 a r e o n l y p r e s e n t i n t r a c e amounts. T h i s d i s t r i b u t i o n i s i n d e p e n d e n t o f t h e H2/CO r a t i o a n d o f t h e r e a c t i o n temper a t u r e . O n l y t h e s l o p e o f t h e s t r a i g h t l i n e s l i g t h l y changes i n d i c a t i n g a v a r i a t i o n i n the value o f the chain-growth p r o b a b i l i t y . I n order t o assess the p e c u l i a r d i s t r i b u t i o n reported i n F i g u r e 1, c u r v e 1, a n o t h e r Fe-NaY c a t a l y s t p r e c u r s o r was p r e p a r e d i n s u c h a way t h a t m e t a l l i c p a r t i c l e s on t h e e x t e r n a l s u r f a c e o f the z e o l i t e a r e o b t a i n e d . T h i s i s p e r f o r m e d by h e a t i n g t h e F e ( C O ) - N a Y a d d u c t (UjtFe) i n vacuum f r o m 25 t o 250°C w i t h i n 1 h r a n d f u r t h e r e v a c u a t i o n a t 250°C f o r 1 5 h r s . P a r t i c l e s o f 20-30nm i n d i a m e t e r a r e t h u s o b t a i n e d . H i g h e r CO, H2 c o n v e r s i o n s and C0 y i e l d ( 1 3 ? ) a r e f o u n d w i t h t h i s Fe-NaY s a m p l e . The c h a i n l e n g t h d i s t r i b u t i o n ( F i g u r e 1, c u r v e 2) shows t h a t a d r a s t i c change o c c u r s i n t h e Cg-C-jQ domain. I t c a n be c o n c l u d e d f r o m t h e s e e x p e r i m e n t s t h a t t h e Fe3(C0) 2-NaY p r e c u r s o r i n d u c e s a p e c u l i a r s e l e c t i v i t y f o r t h e h y d r o c a r b o n c h a i n - l e n g t h . As t h e Fe^CO)-^ l o c a t e d i n t h e supercages and s i n c e l a r g e i r o n p a r t i c l e s are n o t d e t e c t e d by X-ray t e c h n i q u e s a f t e r t h e c a t a l y t i c r u n s , t h i s h y d r o c a r b o n d i s t r i b u t i o n c a n be a c h i e v e d ( i ) by e n c a p s u l a t i o n ( i . e . s t a b i l i z a t i o n ) o f s m a l l i r o n p a r t i c l e s and ( i i ) b y a cage e f f e c t s i n c e t h e Cg l e n g t h f i t s t h e s u p e r c a g e d i a m e t e r . M o l e c u l a r w e i g h t d i s t r i b u t i o n a s d e p i c t e d i n F i g u r e 1, c u r v e 1 must be r e l a t e d w i t h t h e p r e s e n c e o f s m a l l m e t a l p a r t i c l e s . This r e l a t i o n s h i p i s demonstrated f o r ruthenium c a t a l y s t s (vide i n f r a ) . P a r a l l e l s t u d i e s b y J a c o b s ( 8 ) , G a t e s (£) , B l a n c h a r d ( 1J0) and t h e i r c o - w o r k e r s p o i n t o u t t h e same e f f e c t o f t h e p o r o s i t y o f t h e i n o r g a n i c m a t r i x on t h e u p p e r l i m i t o f t h e h y d r o c a r b o n c h a i n - l e n g t h . However a c l e a r - c u t between p o i n t s ( i ) a n d ( i i ) i s d i f f i c u l t t o assess since the p a r t i c l e s i z e o f the c a t a l y s t can be l i m i t e d b y t h e p o r e d i a m e t e r i n w h i c h i t i s e n t r a p p e d . 2
n
n
3
1 2
2
1
a
r
e
As a l r e a d y m e n t i o n e d , t h e a c i d i t y o f t h e HY z e o l i t e precludes i t s use as a support f o r Fe-^CO)^* then i r o n p a r t i c l e s . The same b e h a v i o u r i s o b s e r v e d f o r F e ^ (CO) -| 2"" s i l i e a - a l u m i n a s y s t e m . T h i s m a t e r i a l , when decomposed a t 200°C i s n o t an e f f i c i e n t c a t a l y s t f o r F-T s y n t h e s i s a n d o n l y C^-C^ p r o d u c t s a r e
12.
BALLIVET—TKATCHENKO
ET AL.
Syngas Conversion over Carbonyls
193
T a b l e 3. E f f e c t o f e t h y l e n e as a c o - r e a c t a n t on CO, H c o n v e r s i o n s , C^ f o r m a t i o n a n d C 0 , C-j-C^ s e l e c t i v i t i e s ( e x p r e s s e d as mole p e r c e n t o f CO c o n v e r t e d i n t o t h e d e s i r e d product). Experimental c o n d i t i o n s : c a t a l y s t = Feo(CO) ~NaY (U#Fe); i n i t i a l p r e s s u r e = 20 b a r ; C|/H /C0 = 1/5/1; r e a c t i o n t e m p e r a t u r e = 250°C; r e a c t i o n t i m e = 15hrs. 2
2
12
2
H
Co-reactants
conv. CO conv.
Q
2
H C
= 2
2
+ CO
+ H
%
%
39
73
10
+ CO
2
I
1
1 2
1 3
1
1
k
5
1 6
%
% 20.6
7-6
73
Ί
iC^/nCii
CO,
%
11
—ι 7
1
8
9.9
0.8
It.8
13.2
16.7
1.6
»
'
9
10
1
11
Figure 1 A Schulz-Flory-type representation for catalysts derived from Curve 1, in situ decomposition of the Fe. (CO) -NaY adduct; Curve 2, ex situ decomposition of the Fe (CO) ~NaY adduct leading to large Fe crystallites (see text): initial pres sure, 20 bar; HJCO = 4/1; reaction temperature, 250°C t
3
12
12
194
CATALYTIC ACTIVATION OF CARBON MONOXIDE
o b t a i n e d ( 2 3 ) . However, t h e c h a i n - l e n g t h s e l e c t i v i t y c a n be a l t e r e d b y t h e a d d i t i o n o f an a c i d i c s u p p o r t , e . g . HY z e o l i t e , to the Fe3(C0)-|2" z e o l i t e i n t h e a u t o c l a v e . A d r a s t i c change i s o b s e r v e d i n t h e Cg-C-jQ r a n g e ( F i g u r e 2 , c u r v e 2 ) . T h i s b e h a v i o u r i s e x p l a i n e d by t h e occurence o f secondary r e a c t i o n s l i k e i s o m e r i z a t i o n a n d c r a c k i n g o w i n g t o t h e a c i d i t y o f t h e HY z e o l i t e . E x p e r i m e n t s p e r f o r m e d u n d e r h y d r o g e n p r e s s u r e w i t h HY z e o l i t e a l o n e show t h a t η-octane i s t r a n s f o r m e d i n t o C ( t r a c e s ) , C 3 , ( p r e d o m i n a n t , i - C ^ / n - C j ^ ) , C5 and C g whereas η-butane i s t r a n s f o r m e d i n t o C ( t r a c e s ) , C - j j i - C ^ C5 a n d Cg b u t , i n t h i s l a t t e r c a s e , t h e a c t i v i t y i s q u i t e l o w . The p r e s e n c e o f t h e HY z e o l i t e l e a d s t o a n i n c r e a s e i n t h e C^-C,- f r a c t i o n w i t h a c o n c o mitant decrease o f h i g h e r hydrocarbons. Thus, t h i s experiment f u r t h e r s u p p o r t s t h e r o l e o f NaY e n t r a p p e d i r o n p a r t i c l e s as a c t i v e s i t e s f o r t h e F-T s y n t h e s i s . N a Y
2
2
B i s ( c y c l o p e n t a d i e n y l d i c a r b o n y l i r o n ) - z e o l i t e s a d d u c t s. The a d d u c t s | C p F e ( C 0 ) | - H Y a n d | C p F e ( C O ) | ~ N a Y were u s e d as s t a r t i n g m a t e r i a l s . They a r e n o t decomposed b e f o r e t h e c a t a l y t i c r u n . I n f r a r e d s p e c t r a o f t h e s e m a t e r i a l s show t h a t t h e i n t e g r i t y o f t h e m o l e c u l a r c o m p l e x i s r e t a i n e d ; i t i s e x p e c t e d t h a t owing t o i t s s i z e , which i s smaller than that o f F e 3 ( C 0 ) , | C p F e ( C 0 ) | l i e s i n s i d e t h e c a v i t i e s . I n f r a r e d , UV-VIS and ESR d a t a w i l l p r o v i d e r e l e v a n t i n f o r m a t i o n on t h e f a t e o f t h i s compound u n d e r t h e r m a l t r e a t m e n t {2k) . The | C p F e ( C 0 ) | - H Y a d d u c t e x h i b i t s no c a t a l y t i c a c t i v i t y i n t h e t e m p e r a t u r e r a n g e s t u d i e d (200-300°C). However c y c l o p e n t e n e a n d c y c l o p e n t a n e a r e d e t e c t e d t h r o u g h GC m o n i t o r i n g o f t h e gas-phase. A redox r e a c t i o n F e ( 0 ) / H i s s t i l l o c c u r r i n g as f o r t h e Fe3(C0)-| -HY a d d u c t ; t h e e v o l v e d h y d r o g e n a l l o w s t h e reduction o f t h e cyclopentadienyl ligands. This behaviour already p r o v i d e s e v i d e n c e f o r t h e l o c a t i o n o f t h e F e ( l ) complexes w i t h i n the large c a v i t i e s o fthe z e o l i t e . The | C p F e ( C 0 ) I - N a Y a d d u c t i s a c t i v e f o r syngas c o n v e r s i o n . U n d e r t h e s t a n d a r d c o n d i t i o n s t h e CO c o n v e r s i o n i s q u i t e s i m i l a r t o t h a t o b s e r v e d f o r Fe^(CO)-j -NaY ( T a b l e k). However h y d r o g e n c o n v e r s i o n i s h i g h e r and t h i s i s r e f l e c t e d i n t h e c h a i n - l e n g t h d i s t r i b u t i o n w h i c h shows a b e t t e r s e l e c t i v i t y f o r l i g h t h y d r o carbons (Figure 3 ) . 2
2
2
2
1 2
2
2
2
2
+
2
2
2
2
D i c o b a l t octacarbonyl-NaY z e o l i t e adducts.Only t h e adducts C o ( C 0 ) g - N a Y were u s e d a s s t a r t i n g m a t e r i a l s . No d e c o m p o s i t i o n t r o u g h t h e r m a l t r e a t m e n t was a t t e m p t e d b e f o r e t h e c a t a l y t i c r u n s . These a d d u c t s a r e more a c t i v e t h a n t h e i r o n ones i n t h e c o n v e r s i o n o f s y n g a s . A t 250°C, a h i g h e r y i e l d o f methane i s o b s e r v e d ( T a b l e h) and c a r b o n d i o x i d e i s p r o d u c e d i n s m a l l e r amounts. I n s p e c t i o n o f T a b l e 5 s u m m a r i z i n g t h e i n f l u e n c e o f t h e H /CO r a t i o on p r o d u c t s s e l e c t i v i t y a l s o i n d i c a t e s a h i g h e r production o f saturated hydrocarbons. This behavior i s t y p i c a l f o r c o b a l t c a t a l y s t s i n F-T s y n t h e s i s (.2,25)· The c h a i n - l e n g t h d i s t r i b u t i o n i s s i m i l a r t o that observed f o r c a t a l y s t s derived 2
2
12.
BALLIVET—TKATCHENKO ET AL.
195
Syngas Conversion over Carbonyls
Figure 2. The effect of added H Y zeolite on the hydrocarbon distribution; Curve 1, Fe (CO) NaY + HY; Curve 2, Fe (CO) -NaY: initial pressure, 20 bar; HJCO = 4/1; reaction temperature, 250°C s
lfr
3
12
T a b l e h. C o m p a r a t i v e p e r f o r m a n c e s o f t h e F e ( C O ) , | C p F e ( C 0 ) | and C o ( C 0 ) Q - N a Y s y s t e m s (C-| s e l e c t i v i t i e s e x p r e s s e d as mole p e r c e n t o f CO c o n v e r t e d i n t o t h e d e s i r e d p r o d u c t ) . L x p e r i m e n t a l c o n d i t i o n s : i n i t i a l p r e s s u r e = 20 b a r ; H /C0 = U/1; r e a c t i o n t e m p e r a t u r e = 250°C; r e a c t i o n t i m e = 1 5 h r s . 3
1 2
2
2
2
Metal precursor
H„ c o n v . 2
Fe (C0) 3
1 2
|C Fe(C0) | P
2
CO (C0)Q 2
2
CO
conv.
%
COp
CH,
%
39
73
10
20.6
50
75
10
22
100
0.8
U2.5
2
CATALYTIC ACTIVATION OF CARBON MONOXIDE
Figure 3. A Schulz-Flory-type plot for catalyst derived from \CpFe(CO) \g-NaY adduct: initial pressure, 20 bar; H CO = 4/1; reaction temperature, 250°C 2
2
12.
BALLIVET—TKATCHENKO ET AL.
2
Syngas Conversion over Carbonyls
197
+
Figure 4. A Schulz-Flory-type plot for catalyst derived from Co (CO) -NaY adduct: Curve 1, HJCO — 4/1; Curve 2, HJCO = 1/1; initial pressure, 20 bar; reaction temperature, 250°C 2
g
T a b l e 5 . E f f e c t o f t h e H / C 0 r a t i o o n CO a n d H c o n v e r s i o n s a n d p r o d u c t s e l e c t i v i t i e s ( e x p r e s s e d a s mole p e r c e n t o f CO c o n v e r t e d into the desired product). Experimental conditions : catalyst = C o ( C 0 ) Q - N a Y (k%Fe)- i n i t i a l p r e s s u r e = 20 b a r ; r e a c t i o n tempe r a t u r e = 250°C ; r e a c t i o n t i m e = 1 5 h r s . 2
9
2
H /C0 2
Hp conv. CO c d n v .
%
Vi 1/1
2
59-5
%
CO, r
+
Cti
k
Γ
C
2
%
100
0.8 1*2.5 8
3h
6.1 11.3
k.k
C
3
c c-/c
c-/c
r 15
33.7
12
66
5.8 13.5
CATALYTIC ACTIVATION OF CARBON MONOXIDE
198
N a Y
from F e 3 ( C 0 ) - | 2 " adducts, i . e . ethylene i s again produced i n s m a l l e r amounts ( F i g u r e 3) a n d c h a i n - l e n g t h i s p r a c t i c a l l y l i m i t e d t o C^-C-JQ* However, c o m p a r i s o n o f F i g u r e s 1 a n d 3 i n d i c a t e s a g r e a t e r decrease i n hydrocarbon content a t the Cg-Cj l e v e l f o r c o b a l t c a t a l y s t s t h a n f o r i r o n c a t a l y s t s . These r e s u l t s a g r e e w e l l w i t h t h e a l r e a d y r e p o r t e d g r e a t e r hydrogénation a n d h y d r o g e n o l y s i s a b i l i t i e s o f c o b a l t c a t a l y s t s w i t h r e s p e c t t o i r o n ones (2). Thus b e s i d e s t h e i n t r i n s i c shape s e l e c t i v i t y o f t h e Y z e o l i t e , t h e n a t u r e o f t h e m e t a l and p r e s u m a b l y t h e s i z e o f t h e m e t a l p a r t i c l e s are a l s o important f a c t o r s i n l i m i t i n g the chain-growth process. I n t e r e s t i n g l y , C o ( I I ) e x c h a n g e d NaY z e o l i t e i s a l s o o p e r a t i v e u n d e r t h e F-T c o n d i t i o n s (26). However, l a r g e amounts o f methane a n d h i g h e r h y d r o c a r b o n s a r e p r o d u c e d . T h i s b e h a v i o u r i s r e m i n i s c e n t o f " c l a s s i c a l " F-T c a t a l y s t s ( 2 , 2 5 ) a n d s u g g e s t s t h e occurence o f c o b a l t c r y s t a l l i t e s outside the Y z e o l i t e c a v i t i e s . T r i r u t h e n i u m d o d e c a r b o n y l - z e o l i t e s adducts. Ruthenium-Y z e o l i t e s have a l r e a d y b e e n r e p o r t e d as c a t a l y s t s f o r s e l e c t i v e F-T s y n t h e s i s . S t a r t i n g f r o m R ^ i C O ) ^ i n s t e a d o f R u ( l l ) c o m p l e x e s (Q) on HY z e o l i t e , c a t a l y s t s c o u l d be o b t a i n e d p r o v i d e d t h a t the m a t e r i a l s are t h o r o u g h l y decarbonylated. A study o f the t h e r m a l d e c o m p o s i t i o n o f t h e Ru-^CO) -| ~HY z e o l i t e shows t h a t up t o 200°C, t h r e e c a r b o n y l g r o u p s p e r R u ^ u n i t e v o l v e a n d a p l a t e a u o c c u r s up t o 320°C. The r e m a i n i n g c a r b o n y l l i g a n d s e v o l v e b e t w e e n 320 a n d U20°C (15). Syngas c o n v e r s i o n o n l y o c c u r s w i t h s a m p l e s p r e t r e a t e d a t 3 2 0 o r U00°C. T h i s t r e a t m e n t l e a d t o s m a l l r u t h e n i u m p a r t i c l e s o f c_a 1.5~2nm a s i n d i c a t e d b y e l e c t r o n m i c r o s copy ( ] T ) . I f t h e t h e r m a l d e c o m p o s i t i o n f r o m 60 t o 320°C i s o p e r a t e d i n vacuum w i t h i n 1 h r a n d i s f o l l o w e d b y f u r t h e r e v a c u a t i o n a t 320°C, p a r t i c l e s o f 10 t o 100 nm a r e o b t a i n e d . T h e s e c a t a l y s t s a r e more a c t i v e t h a n t h e c o r r e s p o n d i n g i r o n o n e s , i^.e^ carbon monoxide h y d r o c o n d e n s a t i o n s t a r t s b e l o w 200°C. A l l t h e c a t a l y t i c r u n s were p e r f o r m e d a t 200°C a s methane i s s e l e c t i v e l y p r o d u c e d a t 250°C. T a b l e 6 a n d F i g u r e 5 show t h e d r a m a t i c d i f f e rence i n b e h a v i o u r between t h e s m a l l p a r t i c l e s - and l a r g e p a r t i c l e s - c o n t a i n i n g c a t a l y s t s . A h i g h y i e l d o f methane i s o b t a i n e d f o r Ru-Y 10 nm. The s e l e c t i v i t y o f t h e Ru-Y 1.5 nm c a t a l y s t s i s s i m i l a r t o t h a t o b s e r v e d f o r t h e Fe-NaY-HY a f u n c t i o n a l c a t a l y s t ( F i g u r e s 5 a n d 2 ) . However t h e c o m p a r i s o n c a n o n l y be q u a l i t a t i v e s i n c e t h e r e a c t i o n t e m p e r a t u r e s a r e s i g n i f i cantly different. 2
A f t e r t h e c a t a l y t i c r u n s no m o d i f i c a t i o n o f mean p a r t i c l e s i z e i s observed f o r t h i s l a s t system. C o n v e r s l y , R u - ^ C O ) ^ d e p o s i t e d o n s i l i c a - a l u m i n a i s r e a d i l y decomposed a t 200°C t o m e t a l l i c p a r t i c l e s o f 1 nm mean s i z e w h i c h a r e a l s o c a t a l y s t s f o r t h e F-T s y n t h e s i s . The c a t a l y t i c a c t i v i t y a t 200°C i s c a one t e n t h o f t h e Y z e o l i t e s u p p o r t e d ones and methane i s p r a c t i c a l l y the o n l y hydrocarbon formed. E l e c t r o n m i c r o s c o p y examination o f the c a t a l y s t a f t e r r e a c t i o n reveals a d r a s t i c s i n t e r i n g o f the
12.
BALLIVET—TKATCHENKO ET
AL.
Syngas Conversion over Carbonyls
199
T a b l e 6. E f f e c t o f t h e r u t h e n i u m p a r t i c l e s i z e on CO a n d H c o n v e r s i o n s and p r o d u c t s e l e c t i v i t i e s ( e x p r e s s e d a s mole p e r c e n t o f CO c o n v e r t e d i n t o t h e d e s i r e d p r o d u c t ) . E x p e r i m e n t a l c o n d i t i o n s : i n i t i a l p r e s s u r e = 20 b a r ; H /C0 = r e a c t i o n t e m p e r a t u r e = 200°C; r e a c t i o n t i m e = 15hrs.
2
2
C a t a l y s t a n d mean particle size
H
Q
2
conv. CO c o n v . CO %
S3
%
Ru-Y 10 nm
6b
100
0
1*1
Ru-Y 1.5 nm
38
37
3
23
10.7
6.9
/
c
1*1.1*
2.6 11*.6 56.8
Figure 5. The influence of the Ru particles size on the hydrocarbon selectivities: Curve 1, distribution for a mean-particle size of 1.5 nm; Curve 2, distribution for a mean-particle size of 10 nm; initial pressure, 20 bar; HJCO = 4/1; reaction temperature, 200°C
200
CATALYTIC
ACTIVATION
OF
CARBON
MONOXIDE
r u t h e n i u m a g g r e g a t e s : w e l l c r y s t a l l i z e d h e x a g o n a l p l a t e s o f 10 t o 100 nm a r e now p r e s e n t . T h i s r e s u l t p o i n t s o u t t h e s t a b i l i z i n g e f f e c t o f t h e z e o l i t e s u p e r c a g e s upon s i n t e r i n g o f t h e s m a l l a g g r e g a t e s a n d s e r v e s a s an i n d i r e c t e v i d e n c e f o r t h e o c c u r e n c e o f s m a l l aggregates i n t h e case o f t h e ( a i r - s e n s i t i v e ) i r o n and cobalt catalysts. Conclusion. F i s c h e r - T r o p s c h s y n t h e s i s c o u l d be " t a i l o r e d " b y t h e u s e o f i r o n , c o b a l t a n d 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 d e p o s i t e d on f a u j a s i t e Y-type z e o l i t e as s t a r t i n g m a t e r i a l s f o r t h e p r e p a r a t i o n o f c a t a l y s t s . S h o r t c h a i n h y d r o c a r b o n s , i . e . i n t h e C - J - C Q range a r e o b t a i n e d . I t appears t h a t the f o r m a t i o n and the s t a b i l i z a t i o n of s m a l l m e t a l l i c aggregates i n t o the z e o l i t e supercage are t h e p r e r e q u i s i t e t o induce a chain length l i m i t a t i o n i n the hydroc o n d e n s a t i o n o f c a r b o n m o n o x i d e . However, t h e c o n t r o l o f t h i s s e l e c t i v i t y through e i t h e r a d e f i n i t e p a r t i c l e size o f the metal o r a shape s e l e c t i v i t y o f t h e z e o l i t e i s s t i l l a m a t t e r o f s p e c u l a t i o n . F u r t h e r work i s n e e d e d t o s o l v e t h i s d i l e m n a . I n a more g e n e r a l c o n t e x t , m e t a l c a r b o n y l s on z e o l i t e s c a n be a u n i q u e way t o p r e p a r e h i g h l y d i s p e r s e d m e t a l c a t a l y s t s . I n t h e p r e s e n t w o r k , t h i s i s e s p e c i a l l y t h e c a s e f o r i r o n as no o t h e r m i l d methods a r e o p e r a t i v e . I t i s e x p e c t e d t h a t t h e method c o u l d be a p p l i e d t o t h e p r e p a r a t i o n o f b i - a n d p o l y m e t a l l i c c a t a l y s t s even though t h e s t a r t i n g m a t e r i a l are n o t b i - o r p o l y m e t a l l i c c l u s t e r s , b u t more c o n v e n i e n t l y h o m o m e t a l l i c c l u s t e r s . Acknowledgements. We a r e g r e a t l y i n d e b t e d t o t h e C e n t r e N a t i o n a l de l a R e c h e r c h e S c i e n t i f i q u e (ATP g r a n d 2333 a n d GRECO Oxydes de C a r b o n e ) f o r s u p p o r t o f t h i s work.
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