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Hydrogenation of CO Over Molybdenum-Zeolites
D.M. Bibby, C.D. Chang, R.F. Howe and S. Yurchak (Editors), Methane Conversion 0 1988 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
503
HYDROGENATION OF CO OVER MOLYBDENUM-ZEOLITES YOU-SING YONG and R.F. HOWE Chemistry Department, University of Auckland, Private Bag, Auckland (New Zeal and) ABSTRACT The molybdenum-zeolites prepared by adsorption and decomposition of M o ( C O ) ~ have been shown to be active CO hydrogenation catalysts. The product distribution varies with the charge-compensating cation in the zeolite and Si/A1 ratio. An LPG selectivity of up to 57% has been observed. INTRODUCTION The scarcity of liquid fuel and the relative abundance of coal have refocussed on the possibility of synthesising hydrocarbons via the Fischer-Tropsch synthesis (FTS). In this process, the product distribution is usually broad due to the fact that the reaction is essentially a polymerization process (ref. 1). An improvement in the selectivity of the desired products is thus essential. The use of zeolite as support has been reported to improve the selectivity due to the "cage effect" (refs. 2-4). Jacobs gal. (ref. 4) have reported a sharp cut-off at around C9-Cl0 in the CO hydrogenation over ruthenium-zeolite catalyst. Further, the selectivity can be improved by the use of alkali metals, either as charge-compensating cations (refs. 5-6) or dopants (refs. 7-8) for non-zeolite supports. For example, Murchison (ref. 7) has reported a 70% selectivity to LPG (C2-C4) product for a potassium-promoted molybdenum catalyst supported on saran charcoal. Brenner et al. (ref. 8) have also reported a high selectivity to LPG product for a potassium-promoted molybdenum-alumina catalyst. In our previous papers (refs. 9-10), we described the activity and selectivity of some molybdenum-zeolite catalysts in the FTS reaction at sub-atmospheric and moderate temperature reaction conditions. In this paper, an extension of the earlier work will be described, paying particular attention to the influence of the charge-compensating cations, the pore size and aluminium content on the product distribution. EXPERIMENTAL The synthetic zeolites NaX and NaY were obtained from Strem Chemicals Inc. CsY, LaY and HY were prepared by repeated ion-exchange of NaY as described elsewhere (ref. 11). The unit cell composition of the zeolites was determined by flame spectroscopy and EDX. The preparation of the molybdenum-zeolites and the CD hydrogenation experimental procedures have been reported earlier (refs.
504
9-10,
12).
RESULTS AND D I S C U S S I O N I n t h e absence o f molybdenum, t h e b l a n k dehydrated z e o l i t e s showed no CO I n c o n t r a s t , measurable q u a n t i t i e s o f
h y d r o g e n a t i o n a c t i v i t y even up t o 400OC.
a l i p h a t i c hydrocarbons were d e t e c t e d o v e r t h e molybdenum-zeolite c a t a l y s t s a t 300°C and above.
F i g s . 1-2 show t h e t i m e dependence o f CO c o n v e r s i o n o v e r
HY and M o ~ ~CsY. ~a t 300OC.
The c o n v e r s i o n and p r o d u c t d i s t r i b u t i o n were
dependent on t h e r e a c t i o n c o n d i t i o n s , T a b l e 1.
a typical set o f results i s illustrated i n
The molybdenum-zeolites prepared b y a d s o r p t i o n and decomposition o f
M O ( C O ) ~resembled c l o s e l y t h e alumina-supported molybdenum c a t a l y s t s p r e p a r e d b y The r e s u l t s o b t a i n e d p r e s e n t l y c o u l d
decomposing M o ( C O ) ~on a l u m i n a ( r e f . 1 3 ) . n o t match t h e f i g u r e s r e p o r t e d b y Brenner
Ct. ( r e f .
8), b u t t h i s c o u l d be due
t o t h e s i g n i f i c a n t d i f f e r e n c e s i n t h e r e a c t i o n c o n d i t i o n s used b y t h e above authors.
However, a comparison w i t h t h e s i l i c a - m o l y b d e n a c a t a l y s t ( p r e p a r e d b y
i m p r e g n a t i o n o f ammonium molybdate) c l e a r l y i n d i c a t e s t h a t t h e molybdenumz e o l i t e s were more a c t i v e on p e r molybdenum b a s i s .
The improved a c t i v i t y i s due
t o t h e presence o f z e r o v a l e n t molybdenum ( f o r LaY and HY, r e s i d u a l z e r o v a l e n t molybdenum were r e s p o n s i b l e f o r t h e a c t i v i t y ) .
As shown i n T a b l e 1, t h e a l k a l i metal-exchanged z e o l i t e s were s i g n i f i c a n t l y more s e l e c t i v e t o b o t h LPG and alkenes t h a n LaY and HY. s t r o n g molybdenum-cation i n t e r a c t i o n i n t h e z e o l i t e s .
This c l e a r l y r e f l e c t s a I t has been e s t a b l i s h e d
t h a t t h e exchange o f sodium i o n s w i t h s m a l l e r , l a r g e r o r m u l t i v a l e n t i o n s and an i n c r e a s e i n t h e Si/A1 r a t i o a l t e r t h e a c i d s t r e n g t h o f t h e z e o l i t e s ( r e f . 14). Due t o t h e s t r o n g p e r t u r b a t i o n o f t h e z e o l i t e environment, t h e molybdenum i n t h e supercages becomes e l e c t r o n - d e f i c i e n t .
T h i s e l e c t r o n d e p l e t i o n depends on t h e
s u p p o r t and i n g e n e r a l , t h i s c h a r a c t e r i n c r e a s e s w i t h i n c r e a s i n g a c i d s t r e n g t h .
As a . r e s u l t ,
t h e metal-C0 bond weakens.
hydrogen c h e m i s o r p t i o n .
T h i s i s p a r a l l e l e d by an i n c r e a s e i n
I n f r a r e d evidence showed t h a t CO d i s p r o p o r t i o n a t e s
r e a d i l y o v e r t h e s e molybdenum-zeolites a t 200°C and above.
I n addition,
c h a r a c t e r i s t i c bending v i b r a t i o n s o f CHx ( x = 1-3) segments were observed between 1445 and 1325 cm-’.
These bands grew i n i n t e n s i t y w i t h time, s u g g e s t i n g
t h a t t h e c h a i n growth o c c u r r e d v i a t h e i n s e r t i o n o f CHx species.
However, an
i n c r e a s e i n adsorbed hydrogen causes an i n c r e a s e i n t h e r a t e o f h y d r o g e n a t i o n o f t h e s u r f a c e carbon s p e c i e s . and HY c a t a l y s t s . Jacobs
9
z. ( r e f .
Hence, l e s s LPG and alkenes were observed f o r LaY
S i m i l a r r e s u l t s have been r e p o r t e d b y L e i t h ( r e f s . 5-6) and
4) over ruthenium-faujasite c a t a l y s t s .
Under t h e l o w p r e s s u r e and moderate t e m p e r a t u r e r e a c t i o n c o n d i t i o n s , t h e l a c k o f h i g h e r hydrocarbons i n t h e p r o d u c t stream i s n o t s u r p r i s i n g s i n c e t h e s e
products are thermodynamically unfavourable.
A l t h o u g h t h e p o r e s t r u c t u r e s show
no d r a m a t i c s t e r i c i n f l u e n c e on t h e p r o d u c t d i s t r i b u t i o n s , t h e y a r e s u f f i c i e n t
505
L3
20 C
._ m c u
15
V 0 W
Is)
3 C
10
5t n
L
TIME Fig. 1:
(
MINUTE
)
Time-dependent CO conversion over Mo11.8 HY at 3OO0C, CO/H2 = 1, Ptotal = 0.28 atm., 23 hr reaction
8 C
0 ._
n al
2
u
6
V 0 0)
cn
3
4
0 W
a W
2
n
0
300
600
TIME
900 (
1200
1500
MINUTE 1
Fig. 2: Time-dependent CO conversion over M015.1 CsY at 3OO0C, CO/H2 = 1, Ptotal = 0.28 atm., 23 hr reaction.
1
I
M015. 4Na71X
b, M O ~ 8La12H13Na2Y ~ .
M012. qLalZH U N a Z Y
1
I
52.0
59.1
48.9
15.8
41.4
21.0
56.8
66.4
I
42.5
44.5
%
I
I
30.5
32.7
31.8
31.8
30.9
1.0
tr
tr
tr
I- 1
48.0
14.8
13.9
51.1
40.9
33.7
l-. 9
8.2
43.2
57.4
LPG
12.0
Product Fraction ( % CO Conversion )
Activity and Product Distribution of CO Hydrogenation over Molybdenum-Zeolite Catalysts.
M024. 3H46Na5Y b,
Mo11.8H46Na5Y
Catalyst
TABLE 1:
507 t o induce a non-Schulz-Flory p r o d u c t d i s t r i b u t i o n .
These r e s u l t s a r e con-
s i s t e n t w i t h t h e e a r l i e r r e s u l t s observed o v e r c o b a l t ( r e f . 2 ) , i r o n ( r e f . 3 ) and r u t h e n i u m ( r e f . 4 ) - z e o l i t e c a t a l y s t s .
I n a l l t h e c a t a l y t i c runs, no
oxygenates were d e t e c t e d . CONCLUSION The above s t u d i e s showed t h a t t h e molybdenum-zeolites were a c t i v e CO hydrog e n a t i o n c a t a l y s t s due t o t h e presence o f z e r o v a l e n t molybdenum.
Although, t h e
p o r e s i z e o f t h e z e o l i t e s d i d n o t i n f l u e n c e t h e p r o d u c t spectrum, t h e enhanced s e l e c t i v i t y t o LPG and alkenes was due t o t h e c l o s e molybdenum-zeolite association. ACKNOWLEDGEMENT We thank t h e Donors o f t h e Petroleum Research Fund o f t h e American Chemical S o c i e t y f o r t h e s u p p o r t o f t h i s work. REFERENCES 1 2 3
4 5 6 7 8 9 10 11 12 13 14
G. H e n r i c i - O l i v g and S. O l i v g , Agnew. Chem. I n t . Ed. Engl., 15 (1976), 136. B.G. Gates and D. Fraenkel. J. Amer. Chem. Soc., 102 (1980). 2478. L.F. Nazar, G.A. Ozin, F. Hugues, J. Godber and-D. Rancourt; Agnew. Chem. I n t . Ed. Engl., 22 (1983), 624. P.A. Jacobs, H.H. N i j s and J.B. Uytterhoeven, J. Chem. SOC. Chem. Comm., (1979), 1095. I.R. L e i t h , J. Chem. SOC. Chem. Commun., 93 (1983). I.R. L e i t h , J. Catal., 91 (1985), 283. C.B. Murchison i n " F o u r t h I n t e r n a t i o n a l Conference Uses and Chemistry o f H.F. B a r r y and P.C.H. M i t c h e l l Eds.", Climax Molybdenum Molybdenum Company, Ann Arbor, Michigan, (1982), 197. A. Brenner, S. Sivasanker, E.P. Yesodharan, C. Sudhakar and C.B. Murchison, J. Catal., 87 (1984), 514. Y.S. Yong and R.F. Howe, J. Molec. Catal., 38 (19861, 323. Y.S. Yong and R.F. Howe, Stud. S u r f . S c i . Catal., 28 (1986), 883. Y.S. Yong, Ph.D. Thesis, U n i v e r s i t y o f Auckland (1987). Y.S. Yong and R.F. Howe, J. Chem. SOC. Faraday Trans. 1, 82 (1986), 2887. A. Brenner and R.L. B u r w e l l , J. Catal., 52 (1978), 353. P.A. Jacobs i n 'Carboniogenic A c t i v i t y i n Z e o l i t e s " , E l s e v i e r , Amsterdam, (1977), 57.
-
-
503
HYDROGENATION OF CO OVER MOLYBDENUM-ZEOLITES YOU-SING YONG and R.F. HOWE Chemistry Department, University of Auckland, Private Bag, Auckland (New Zeal and) ABSTRACT The molybdenum-zeolites prepared by adsorption and decomposition of M o ( C O ) ~ have been shown to be active CO hydrogenation catalysts. The product distribution varies with the charge-compensating cation in the zeolite and Si/A1 ratio. An LPG selectivity of up to 57% has been observed. INTRODUCTION The scarcity of liquid fuel and the relative abundance of coal have refocussed on the possibility of synthesising hydrocarbons via the Fischer-Tropsch synthesis (FTS). In this process, the product distribution is usually broad due to the fact that the reaction is essentially a polymerization process (ref. 1). An improvement in the selectivity of the desired products is thus essential. The use of zeolite as support has been reported to improve the selectivity due to the "cage effect" (refs. 2-4). Jacobs gal. (ref. 4) have reported a sharp cut-off at around C9-Cl0 in the CO hydrogenation over ruthenium-zeolite catalyst. Further, the selectivity can be improved by the use of alkali metals, either as charge-compensating cations (refs. 5-6) or dopants (refs. 7-8) for non-zeolite supports. For example, Murchison (ref. 7) has reported a 70% selectivity to LPG (C2-C4) product for a potassium-promoted molybdenum catalyst supported on saran charcoal. Brenner et al. (ref. 8) have also reported a high selectivity to LPG product for a potassium-promoted molybdenum-alumina catalyst. In our previous papers (refs. 9-10), we described the activity and selectivity of some molybdenum-zeolite catalysts in the FTS reaction at sub-atmospheric and moderate temperature reaction conditions. In this paper, an extension of the earlier work will be described, paying particular attention to the influence of the charge-compensating cations, the pore size and aluminium content on the product distribution. EXPERIMENTAL The synthetic zeolites NaX and NaY were obtained from Strem Chemicals Inc. CsY, LaY and HY were prepared by repeated ion-exchange of NaY as described elsewhere (ref. 11). The unit cell composition of the zeolites was determined by flame spectroscopy and EDX. The preparation of the molybdenum-zeolites and the CD hydrogenation experimental procedures have been reported earlier (refs.
504
9-10,
12).
RESULTS AND D I S C U S S I O N I n t h e absence o f molybdenum, t h e b l a n k dehydrated z e o l i t e s showed no CO I n c o n t r a s t , measurable q u a n t i t i e s o f
h y d r o g e n a t i o n a c t i v i t y even up t o 400OC.
a l i p h a t i c hydrocarbons were d e t e c t e d o v e r t h e molybdenum-zeolite c a t a l y s t s a t 300°C and above.
F i g s . 1-2 show t h e t i m e dependence o f CO c o n v e r s i o n o v e r
HY and M o ~ ~CsY. ~a t 300OC.
The c o n v e r s i o n and p r o d u c t d i s t r i b u t i o n were
dependent on t h e r e a c t i o n c o n d i t i o n s , T a b l e 1.
a typical set o f results i s illustrated i n
The molybdenum-zeolites prepared b y a d s o r p t i o n and decomposition o f
M O ( C O ) ~resembled c l o s e l y t h e alumina-supported molybdenum c a t a l y s t s p r e p a r e d b y The r e s u l t s o b t a i n e d p r e s e n t l y c o u l d
decomposing M o ( C O ) ~on a l u m i n a ( r e f . 1 3 ) . n o t match t h e f i g u r e s r e p o r t e d b y Brenner
Ct. ( r e f .
8), b u t t h i s c o u l d be due
t o t h e s i g n i f i c a n t d i f f e r e n c e s i n t h e r e a c t i o n c o n d i t i o n s used b y t h e above authors.
However, a comparison w i t h t h e s i l i c a - m o l y b d e n a c a t a l y s t ( p r e p a r e d b y
i m p r e g n a t i o n o f ammonium molybdate) c l e a r l y i n d i c a t e s t h a t t h e molybdenumz e o l i t e s were more a c t i v e on p e r molybdenum b a s i s .
The improved a c t i v i t y i s due
t o t h e presence o f z e r o v a l e n t molybdenum ( f o r LaY and HY, r e s i d u a l z e r o v a l e n t molybdenum were r e s p o n s i b l e f o r t h e a c t i v i t y ) .
As shown i n T a b l e 1, t h e a l k a l i metal-exchanged z e o l i t e s were s i g n i f i c a n t l y more s e l e c t i v e t o b o t h LPG and alkenes t h a n LaY and HY. s t r o n g molybdenum-cation i n t e r a c t i o n i n t h e z e o l i t e s .
This c l e a r l y r e f l e c t s a I t has been e s t a b l i s h e d
t h a t t h e exchange o f sodium i o n s w i t h s m a l l e r , l a r g e r o r m u l t i v a l e n t i o n s and an i n c r e a s e i n t h e Si/A1 r a t i o a l t e r t h e a c i d s t r e n g t h o f t h e z e o l i t e s ( r e f . 14). Due t o t h e s t r o n g p e r t u r b a t i o n o f t h e z e o l i t e environment, t h e molybdenum i n t h e supercages becomes e l e c t r o n - d e f i c i e n t .
T h i s e l e c t r o n d e p l e t i o n depends on t h e
s u p p o r t and i n g e n e r a l , t h i s c h a r a c t e r i n c r e a s e s w i t h i n c r e a s i n g a c i d s t r e n g t h .
As a . r e s u l t ,
t h e metal-C0 bond weakens.
hydrogen c h e m i s o r p t i o n .
T h i s i s p a r a l l e l e d by an i n c r e a s e i n
I n f r a r e d evidence showed t h a t CO d i s p r o p o r t i o n a t e s
r e a d i l y o v e r t h e s e molybdenum-zeolites a t 200°C and above.
I n addition,
c h a r a c t e r i s t i c bending v i b r a t i o n s o f CHx ( x = 1-3) segments were observed between 1445 and 1325 cm-’.
These bands grew i n i n t e n s i t y w i t h time, s u g g e s t i n g
t h a t t h e c h a i n growth o c c u r r e d v i a t h e i n s e r t i o n o f CHx species.
However, an
i n c r e a s e i n adsorbed hydrogen causes an i n c r e a s e i n t h e r a t e o f h y d r o g e n a t i o n o f t h e s u r f a c e carbon s p e c i e s . and HY c a t a l y s t s . Jacobs
9
z. ( r e f .
Hence, l e s s LPG and alkenes were observed f o r LaY
S i m i l a r r e s u l t s have been r e p o r t e d b y L e i t h ( r e f s . 5-6) and
4) over ruthenium-faujasite c a t a l y s t s .
Under t h e l o w p r e s s u r e and moderate t e m p e r a t u r e r e a c t i o n c o n d i t i o n s , t h e l a c k o f h i g h e r hydrocarbons i n t h e p r o d u c t stream i s n o t s u r p r i s i n g s i n c e t h e s e
products are thermodynamically unfavourable.
A l t h o u g h t h e p o r e s t r u c t u r e s show
no d r a m a t i c s t e r i c i n f l u e n c e on t h e p r o d u c t d i s t r i b u t i o n s , t h e y a r e s u f f i c i e n t
505
L3
20 C
._ m c u
15
V 0 W
Is)
3 C
10
5t n
L
TIME Fig. 1:
(
MINUTE
)
Time-dependent CO conversion over Mo11.8 HY at 3OO0C, CO/H2 = 1, Ptotal = 0.28 atm., 23 hr reaction
8 C
0 ._
n al
2
u
6
V 0 0)
cn
3
4
0 W
a W
2
n
0
300
600
TIME
900 (
1200
1500
MINUTE 1
Fig. 2: Time-dependent CO conversion over M015.1 CsY at 3OO0C, CO/H2 = 1, Ptotal = 0.28 atm., 23 hr reaction.
1
I
M015. 4Na71X
b, M O ~ 8La12H13Na2Y ~ .
M012. qLalZH U N a Z Y
1
I
52.0
59.1
48.9
15.8
41.4
21.0
56.8
66.4
I
42.5
44.5
%
I
I
30.5
32.7
31.8
31.8
30.9
1.0
tr
tr
tr
I- 1
48.0
14.8
13.9
51.1
40.9
33.7
l-. 9
8.2
43.2
57.4
LPG
12.0
Product Fraction ( % CO Conversion )
Activity and Product Distribution of CO Hydrogenation over Molybdenum-Zeolite Catalysts.
M024. 3H46Na5Y b,
Mo11.8H46Na5Y
Catalyst
TABLE 1:
507 t o induce a non-Schulz-Flory p r o d u c t d i s t r i b u t i o n .
These r e s u l t s a r e con-
s i s t e n t w i t h t h e e a r l i e r r e s u l t s observed o v e r c o b a l t ( r e f . 2 ) , i r o n ( r e f . 3 ) and r u t h e n i u m ( r e f . 4 ) - z e o l i t e c a t a l y s t s .
I n a l l t h e c a t a l y t i c runs, no
oxygenates were d e t e c t e d . CONCLUSION The above s t u d i e s showed t h a t t h e molybdenum-zeolites were a c t i v e CO hydrog e n a t i o n c a t a l y s t s due t o t h e presence o f z e r o v a l e n t molybdenum.
Although, t h e
p o r e s i z e o f t h e z e o l i t e s d i d n o t i n f l u e n c e t h e p r o d u c t spectrum, t h e enhanced s e l e c t i v i t y t o LPG and alkenes was due t o t h e c l o s e molybdenum-zeolite association. ACKNOWLEDGEMENT We thank t h e Donors o f t h e Petroleum Research Fund o f t h e American Chemical S o c i e t y f o r t h e s u p p o r t o f t h i s work. REFERENCES 1 2 3
4 5 6 7 8 9 10 11 12 13 14
G. H e n r i c i - O l i v g and S. O l i v g , Agnew. Chem. I n t . Ed. Engl., 15 (1976), 136. B.G. Gates and D. Fraenkel. J. Amer. Chem. Soc., 102 (1980). 2478. L.F. Nazar, G.A. Ozin, F. Hugues, J. Godber and-D. Rancourt; Agnew. Chem. I n t . Ed. Engl., 22 (1983), 624. P.A. Jacobs, H.H. N i j s and J.B. Uytterhoeven, J. Chem. SOC. Chem. Comm., (1979), 1095. I.R. L e i t h , J. Chem. SOC. Chem. Commun., 93 (1983). I.R. L e i t h , J. Catal., 91 (1985), 283. C.B. Murchison i n " F o u r t h I n t e r n a t i o n a l Conference Uses and Chemistry o f H.F. B a r r y and P.C.H. M i t c h e l l Eds.", Climax Molybdenum Molybdenum Company, Ann Arbor, Michigan, (1982), 197. A. Brenner, S. Sivasanker, E.P. Yesodharan, C. Sudhakar and C.B. Murchison, J. Catal., 87 (1984), 514. Y.S. Yong and R.F. Howe, J. Molec. Catal., 38 (19861, 323. Y.S. Yong and R.F. Howe, Stud. S u r f . S c i . Catal., 28 (1986), 883. Y.S. Yong, Ph.D. Thesis, U n i v e r s i t y o f Auckland (1987). Y.S. Yong and R.F. Howe, J. Chem. SOC. Faraday Trans. 1, 82 (1986), 2887. A. Brenner and R.L. B u r w e l l , J. Catal., 52 (1978), 353. P.A. Jacobs i n 'Carboniogenic A c t i v i t y i n Z e o l i t e s " , E l s e v i e r , Amsterdam, (1977), 57.
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