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Selective Hydrogenation of Unsaturated Aldehydes over Zeolite-Supported Metals
M . Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals II 0 1991 Elsevier Science Publishers B.V., Amsterdam
145
SELECTIVE HYDROGENATION OF UNSATURATED ALDEHYDES OVER ZEOLITESUPPORTED METALS
D.G. BLACKMOND,' A. WAGHRAY,' R. OUKACI,', B. BLANC,' and P. GALLEZOT' 'Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, PA 15261 (USA) *Institut de Recherches sur la Catalyse, CNRS, 2 Avenue Albert Einstein, 69626 Villeurbanne Cedex (FRANCE)
SUMMARY The hydrogenation of 3-methyl crotonaldehyde was investigated over Ru supported on NaY and KY zeolites in both liquid- and gas-phase reactions. Significant effects of the nature of the support on the product selectivity were observed. It was suggested that increased basicity of the zeolite resulted in increased selectivity towards the unsaturated alcohol product. INTRODUCTION Selectivity in the hydrogenation of =,p-unsaturated aldehydes has become an important topic in heterogeneous catalysis (refs. 1-4). Unsaturated alcohols, important in the synthesis of fine chemicals, may be produced selectively over certain supported group VIII-metal catalysts (refs. 5,6), but the general problem of the selective intramolecular hydrogenation of carbonyl groups remains a challenging task. Studies by this group have suggested (ref. 7) that an electronic interaction between a functionalized graphite support and small Pt or Ru particles enhances the selectivity of these metals towards the production of unsaturated alcohols, the desired product. Comparison with results f o r these metals on a carbon support demonstrated the significance of the support in this electronic effect. This paper discusses an extension of those studies to investigate the selectivity of Ru supported on NaY and Kexchanged NaY zeolites in the hydrogenation of 3-methyl
146
crotonaldehyde. Support effects have been observed for similar Ru/Y catalysts in the Fischer-Tropsch synthesis (ref. 8 ) , where modifications in the zeolite resulted in changes in catalyst activity for hydrogenation of olefins formed as primary products. EXPERIMENTAL Supported Ru catalysts were prepared using NaY zeolite (Strem Chemicals) as a support. The Ru/NaY catalyst was prepared using the zeolite directly as received. Potassium-exchanged NaY (KY) was prepared by ion-exchange of the NaY zeolite with potassium nitrate (Alpha Products, ultrapure) (refs. 10,ll). Ru-loaded catalysts were then prepared by ion-exchange of the two zeolite supports with Ru(NH,),Cl, (Strem Chemicals) to a nominal weight loading of 3% as described in detail in (9,ll). Samples were pretreated by heating in hydrogen at 75 cc/min to 573 K at 0.5 K/min and holding there for two hours. Samples were passivated and stored in air until use. Crystallite size was examined by TEM (JEOL 100 CX equipped with high-resolution pole pieces). The high pressure, liquid-phase hydrogenation of 3-methyl crotonaldehyde was carried out in a well-stirred batch autoclave under 4 MPa H, (Air Liquide, 99.995% purity) pressure using 0.1 mol of 3-methyl crotonaldehyde (UAL) (Merck) and 0.6 g catalyst. Isopropanol (37.5 cc) was used as a solvent. The catalyst was activated by stirring under 4 MPa H, pressure at 373K for two unsaturated aldehyde UAL hours prior to introduction of the reactant at the same temperature. The reaction products were monitored by repetitive sampling and gas chromatographic analysis. Since this was a batch reaction, data are reported as selectivity vs. conversion. Time of reaction to reach about 30% conversion was close to 6 0 minutes for Ru/NaY and 150 minutes for Ru/KY. Gas phase studies were performed at 0.1 MPa total pressure with a flow rate of 100 cc/min H, and 100 cc/min He (Linde). The helium stream was diverted through a saturator containing the liquid reactant UAL (Aldrich) such that its volume fraction in the inlet to the reactor was 0.5%. The gas hourly space velocity was 120 lh'lg'' catalyst. Prior to reaction, the catalyst was pretreated at 673 K for at least 2 hours following a 1 K/min rise to the reduction temperature. The reactor was cooled under
147
flowing hydrogen to 313 K prior to introduction of the UALsaturated He stream. Reaction products were analyzed by repetitive sampling and chromatographic analysis. After one hour on stream, the catalysts reached a steady state of about 1-3% conversion of UAL to three products, the unsaturated alcohol (UOL), the saturated alcohol (SOL), and the saturated aldehyde (SAL)
.
RESULTS AND DISCUSSION Characterization of the Ru/KY and Ru/NaY catalysts used in this study by transmission electron microscopy showed that the metal was well-dispersed within the zeolite as particles small enough to fit inside the supercages, less than 1 nm in diameter. Figure 1 compares the selectivities of hydrogenation products as a function of UAL conversion for liquid-phase reaction over the two RU catalysts. Both catalysts produced significant amounts of SAL. However, selectivity towards UOL, the desired product, increased threefold for the K-exchanged catalyst compared to Ru/NaY, Results of the continuous flow gas-phase reaction of UAL over the two Ru zeolite catalysts are given in Figure 2 , where the product selectivity is plotted against time of reaction. In this reaction mode the differences in product selectivity between the two catalysts were even more striking than those observed in the liquid-phase reactions described above. Once again the K-exchanged catalyst offered higher selectivity towards UOL compared to Ru/NaY. In fact, UOL was the major product for Ru/KY, surpassing SAL in contrast to the results for the liquid-phase reaction on these catalysts. Modification of the zeolite appears to have affected the selectivity of Ru in these hydrogenation reactions. Exchange of K cations for Na cations in Y zeolite increases the basicity of the support (ref. 9). In Fischer-Tropsch reactions over similar catalysts, Ru/Y catalysts so modified yielded significant increases in -the olefinic product fraction at the expense of paraffins. Olefins are believed to be primary products in F-T synthesis, with paraffins being produced from olefins in secondary hydrogenation reactions. In an analogous fashion, the Ru/KY catalyst used in the present study might also be expected to
148
inhibit intramolecular C=C hydrogenation reactions in molecules such as 3-methyl crotonaldehyde. This was indeed the case for both gas- and liquid-phase reactions, as can be seen from Figures 1 and 2. In accordance with interpretation of the F-T data, the increased basicity of the KY compared to Nay, possibly resulted in a transfer of charge from the support to the metal which in turn decreased the capability of C=C hydrogenation. Similar electronic effects were invoked in previous work by this group (7) to explain high selectivity of cinnamaldehyde to cinnamyl alcohol over Ru supported on functionalized graphite. A promoting effect on both the activity and selectivity for the production of unsaturated alcohol was also observed previously by this group (refs. 13,14) for hydrogenation of cinnamaldehyde over Pt-Fe catalysts. It was suggested that a dual-site mechanism operated in the bimetallic system whereby cationic Fe electron acceptor species preferentially activated the C=O bond while reduced Pt sites provided hydrogen for hydrogenation of the aldehyde to the alcohol. A similar mechanism might explain the increased UOL formation over Ru/KY. If the small crystallites of Ru are located within the zeolite supercages in close proximity to the K' neutralizing cations, the result may be a system with both metallic sites to provide hydrogen and a cationic site to activate the CO bond. Interactions between CO and alkali species whereby a direct K--O=C interaction causes weakening of the C=O bond have been suggested for alkali-promoted single crystal (refs. 15,16) and supported metal (refs. 17-19) systems. Recent gas-phase studies of crotonaldehyde hydrogenation over Pt/TiO, by Vannice and Sen (ref. 20) suggested a similar mechanism for TiO, moieties interacting with CO on Pt. The reaction network for hydrogenation of the unsaturated aldehyde crotonaldehyde was studied extensively by Simonik and Beranek ( 2 0 ) . They observed isomerization of the unsaturated alcohol to the saturated aldehyde at a reaction temperature of 433 K:
149
80
. A
-D
SAL
-+SOL
-
4ucc
91
Q u)
V
T
.
0
1
10
.
I
20
-
I
30
-
I
40
’
I
513
Conversion (“A)
. B 60
Q
SAL
+see 4 L K x
Figure 1. Product selectivities as a function of conversion in liquid-phase hydrogenation of 3-methyl crotonaldehyde over A ) Ru/NaY: and B) Ru/KY. UOL = unsaturated alcohol: SOL = saturated alcohol: SAL = saturated aldehyde. Pressure = 4 MPa; Temperature = 373K.
150
0
I
I
I
I
I
60
100
160
200
260
300
Tlrne (rnlna) too
M
13
10 -
90
ro
a
-
A A
-
UOL SOL SAL
Figure 2. Product selectivities as a function of time in gas-phase hydrogenation of 3-methyl crotonaldehyde over A ) Ru/NaY; and B) Ru/KY. UOL unsaturated alcohol; SOL = saturated alcohol; S A L = saturated aldehyde. Pressure = 0.1 MPa, Temperature = 313K.
151
In the present study, liquid-phase reactions over both NaY and KY showed significant production of SAL compared to the gas-phase studies. The higher temperature of the liquid-phase work might account for this increased SAL production through UOL isomerization as shown above. Another possibly important factor is the residence time of the products in the batch liquid-phase reactions. In the continuous flow mode, products may not be in contact with the catalyst long enough to undergo significant secondary isomerization reactions, a limitation which does not exist in batch reactions. Further work is underway in both liquid- and gas-phase hydrogenation studies aimed at achieving a better understanding of both the kinetics of the reaction network and the role of the zeolite support in altering the product selectivity of the metal. CONCLUSIONS Preliminary results for Nay- and KY-supported Ru catalysts demonstrate significant effects of the nature of the zeolite cation for the selectivity of 3-methyl crotonaldehyde hydrogenation. It was suggested that increased basicity of the zeolite resulted in increased selectivity toward the unsaturated alcohol product. These results agree with earlier suggestions that the nature of the support can have significant influence on the product distribution in the hydrogenation of -,p-unsaturated aldehydes. ACKNOWLEDGMENTS Support from NATO Scientific Affairs Division (Brussels, Belgium), the Centre International des Etudiants et Stagiaires (C.I.E.S., Paris, France) and the U.S. National Science Foundation (Presidential Young Investigator Program, CBT-8552656) is gratefully acknowledged.
152
REFERENCES
9
P.N. Rylander, IIHydrogenation MethodsI1l Academic Press, London, 1985. J.E. Germain, I1Catalytic Conversion of Hydrocarbons,I1 Academic Press, New York, 1969. R.L. Augustine, Catal. Rev. Sci.-Eng., u,285 (1976). R.L. Augustine, in IIAdvances in CatalysisI1l (D.D. Eley, P.W. Selwood, and P.B. Weisz, Eds.), V o l . 25, p. 56, Academic Press, New York, 1976. M.L. Khidekel, A.S. Bakhanov, A.S. Astakhova, K.A. Brikenshtein, V.I. Savchenko, I.S. Monakhova, and V.G. Dorokov, Izv. Akad. Nauk. SSR, Ser. Chem. (1970), p. 499. P.N. Rylander, and D.R. Steele, Tetr. Lett., 1579 (1969). A . Giroir-Fendler, D. Richard, and P. Gallezot, in tlHeterogeneousCatalysis and Fine Chemicals,1o (M. Guisnet et al., Eds.), p.171, Elsevier, Amsterdam, 1988. F.A.P. Cavalcanti, D.G. Blackmond, R . Oukaci, A. Sayari, A. Erdem-Senatalar, and I. Wender, J. Catal. , 113,1 (1988). Y.W. Chen, H.T. Wang, and J.G. Goodwin, Jr., J. Catal., B,
10
R. Oukaci,
11
J.Z. Shyu, E.T. Skopinski, A . Sayari, and J.G. Goodwin, Jr., Appl. Surf. Sci., a,297 (1985). Jacobs et al. JCS Far. 1 7 4 , 403 (1980). D. Richard, J. Ockelford, A. Giroir-Fendler, and P. Gallezot, submitted to Catal. Lett. D. Richard, P.Fouilloux, and P. Gallezot, in IICatalysis: Theory to Practice,Il Proc. 9th Intl. Congr. Catal., (M. J. Phillips and M. Ternan, Eds.), Vol. 3, p. 1074, The Chemical Institute of Canada, Ottawa, 1988. D. Lackey, M. Surman, Jacobs, S . Grider, D. and D.A. King, Surf. Sci. , 152-153, 513 (1985). K.J. Uram, L. Ng, and J.T. Yates, Jr., Surf. Sci., 177, 253
1 2 3 4 5 6 7 8
12 13 14
15 16 17 18 19 20 21
499 (1984).
A.
102, 126 (1985).
.
Sayari, and J.G. Goodwin,
Jr., J. Catal.,
(1986) S. Kesraoui, R. Oukaci, and D.G. Blackmond, J. Catal., 432 (1987).
105,
P.A.J. Angevaare, H.A.C. Hendrickx, and V. Ponec, J. Catal. , 110, 11 (1988). P.A.J. Angevaare, H.A.C. Hendrickx, and V. Ponec, J. Catal. , 110, 18 (1988). M . A . Vannice, and B. Sen, J. Catal., 115, 65 (1989). J. Simonik, and L. Beranek, Collection Czechoslov. Comm.,
z, 353
(1972).
145
SELECTIVE HYDROGENATION OF UNSATURATED ALDEHYDES OVER ZEOLITESUPPORTED METALS
D.G. BLACKMOND,' A. WAGHRAY,' R. OUKACI,', B. BLANC,' and P. GALLEZOT' 'Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, PA 15261 (USA) *Institut de Recherches sur la Catalyse, CNRS, 2 Avenue Albert Einstein, 69626 Villeurbanne Cedex (FRANCE)
SUMMARY The hydrogenation of 3-methyl crotonaldehyde was investigated over Ru supported on NaY and KY zeolites in both liquid- and gas-phase reactions. Significant effects of the nature of the support on the product selectivity were observed. It was suggested that increased basicity of the zeolite resulted in increased selectivity towards the unsaturated alcohol product. INTRODUCTION Selectivity in the hydrogenation of =,p-unsaturated aldehydes has become an important topic in heterogeneous catalysis (refs. 1-4). Unsaturated alcohols, important in the synthesis of fine chemicals, may be produced selectively over certain supported group VIII-metal catalysts (refs. 5,6), but the general problem of the selective intramolecular hydrogenation of carbonyl groups remains a challenging task. Studies by this group have suggested (ref. 7) that an electronic interaction between a functionalized graphite support and small Pt or Ru particles enhances the selectivity of these metals towards the production of unsaturated alcohols, the desired product. Comparison with results f o r these metals on a carbon support demonstrated the significance of the support in this electronic effect. This paper discusses an extension of those studies to investigate the selectivity of Ru supported on NaY and Kexchanged NaY zeolites in the hydrogenation of 3-methyl
146
crotonaldehyde. Support effects have been observed for similar Ru/Y catalysts in the Fischer-Tropsch synthesis (ref. 8 ) , where modifications in the zeolite resulted in changes in catalyst activity for hydrogenation of olefins formed as primary products. EXPERIMENTAL Supported Ru catalysts were prepared using NaY zeolite (Strem Chemicals) as a support. The Ru/NaY catalyst was prepared using the zeolite directly as received. Potassium-exchanged NaY (KY) was prepared by ion-exchange of the NaY zeolite with potassium nitrate (Alpha Products, ultrapure) (refs. 10,ll). Ru-loaded catalysts were then prepared by ion-exchange of the two zeolite supports with Ru(NH,),Cl, (Strem Chemicals) to a nominal weight loading of 3% as described in detail in (9,ll). Samples were pretreated by heating in hydrogen at 75 cc/min to 573 K at 0.5 K/min and holding there for two hours. Samples were passivated and stored in air until use. Crystallite size was examined by TEM (JEOL 100 CX equipped with high-resolution pole pieces). The high pressure, liquid-phase hydrogenation of 3-methyl crotonaldehyde was carried out in a well-stirred batch autoclave under 4 MPa H, (Air Liquide, 99.995% purity) pressure using 0.1 mol of 3-methyl crotonaldehyde (UAL) (Merck) and 0.6 g catalyst. Isopropanol (37.5 cc) was used as a solvent. The catalyst was activated by stirring under 4 MPa H, pressure at 373K for two unsaturated aldehyde UAL hours prior to introduction of the reactant at the same temperature. The reaction products were monitored by repetitive sampling and gas chromatographic analysis. Since this was a batch reaction, data are reported as selectivity vs. conversion. Time of reaction to reach about 30% conversion was close to 6 0 minutes for Ru/NaY and 150 minutes for Ru/KY. Gas phase studies were performed at 0.1 MPa total pressure with a flow rate of 100 cc/min H, and 100 cc/min He (Linde). The helium stream was diverted through a saturator containing the liquid reactant UAL (Aldrich) such that its volume fraction in the inlet to the reactor was 0.5%. The gas hourly space velocity was 120 lh'lg'' catalyst. Prior to reaction, the catalyst was pretreated at 673 K for at least 2 hours following a 1 K/min rise to the reduction temperature. The reactor was cooled under
147
flowing hydrogen to 313 K prior to introduction of the UALsaturated He stream. Reaction products were analyzed by repetitive sampling and chromatographic analysis. After one hour on stream, the catalysts reached a steady state of about 1-3% conversion of UAL to three products, the unsaturated alcohol (UOL), the saturated alcohol (SOL), and the saturated aldehyde (SAL)
.
RESULTS AND DISCUSSION Characterization of the Ru/KY and Ru/NaY catalysts used in this study by transmission electron microscopy showed that the metal was well-dispersed within the zeolite as particles small enough to fit inside the supercages, less than 1 nm in diameter. Figure 1 compares the selectivities of hydrogenation products as a function of UAL conversion for liquid-phase reaction over the two RU catalysts. Both catalysts produced significant amounts of SAL. However, selectivity towards UOL, the desired product, increased threefold for the K-exchanged catalyst compared to Ru/NaY, Results of the continuous flow gas-phase reaction of UAL over the two Ru zeolite catalysts are given in Figure 2 , where the product selectivity is plotted against time of reaction. In this reaction mode the differences in product selectivity between the two catalysts were even more striking than those observed in the liquid-phase reactions described above. Once again the K-exchanged catalyst offered higher selectivity towards UOL compared to Ru/NaY. In fact, UOL was the major product for Ru/KY, surpassing SAL in contrast to the results for the liquid-phase reaction on these catalysts. Modification of the zeolite appears to have affected the selectivity of Ru in these hydrogenation reactions. Exchange of K cations for Na cations in Y zeolite increases the basicity of the support (ref. 9). In Fischer-Tropsch reactions over similar catalysts, Ru/Y catalysts so modified yielded significant increases in -the olefinic product fraction at the expense of paraffins. Olefins are believed to be primary products in F-T synthesis, with paraffins being produced from olefins in secondary hydrogenation reactions. In an analogous fashion, the Ru/KY catalyst used in the present study might also be expected to
148
inhibit intramolecular C=C hydrogenation reactions in molecules such as 3-methyl crotonaldehyde. This was indeed the case for both gas- and liquid-phase reactions, as can be seen from Figures 1 and 2. In accordance with interpretation of the F-T data, the increased basicity of the KY compared to Nay, possibly resulted in a transfer of charge from the support to the metal which in turn decreased the capability of C=C hydrogenation. Similar electronic effects were invoked in previous work by this group (7) to explain high selectivity of cinnamaldehyde to cinnamyl alcohol over Ru supported on functionalized graphite. A promoting effect on both the activity and selectivity for the production of unsaturated alcohol was also observed previously by this group (refs. 13,14) for hydrogenation of cinnamaldehyde over Pt-Fe catalysts. It was suggested that a dual-site mechanism operated in the bimetallic system whereby cationic Fe electron acceptor species preferentially activated the C=O bond while reduced Pt sites provided hydrogen for hydrogenation of the aldehyde to the alcohol. A similar mechanism might explain the increased UOL formation over Ru/KY. If the small crystallites of Ru are located within the zeolite supercages in close proximity to the K' neutralizing cations, the result may be a system with both metallic sites to provide hydrogen and a cationic site to activate the CO bond. Interactions between CO and alkali species whereby a direct K--O=C interaction causes weakening of the C=O bond have been suggested for alkali-promoted single crystal (refs. 15,16) and supported metal (refs. 17-19) systems. Recent gas-phase studies of crotonaldehyde hydrogenation over Pt/TiO, by Vannice and Sen (ref. 20) suggested a similar mechanism for TiO, moieties interacting with CO on Pt. The reaction network for hydrogenation of the unsaturated aldehyde crotonaldehyde was studied extensively by Simonik and Beranek ( 2 0 ) . They observed isomerization of the unsaturated alcohol to the saturated aldehyde at a reaction temperature of 433 K:
149
80
. A
-D
SAL
-+SOL
-
4ucc
91
Q u)
V
T
.
0
1
10
.
I
20
-
I
30
-
I
40
’
I
513
Conversion (“A)
. B 60
Q
SAL
+see 4 L K x
Figure 1. Product selectivities as a function of conversion in liquid-phase hydrogenation of 3-methyl crotonaldehyde over A ) Ru/NaY: and B) Ru/KY. UOL = unsaturated alcohol: SOL = saturated alcohol: SAL = saturated aldehyde. Pressure = 4 MPa; Temperature = 373K.
150
0
I
I
I
I
I
60
100
160
200
260
300
Tlrne (rnlna) too
M
13
10 -
90
ro
a
-
A A
-
UOL SOL SAL
Figure 2. Product selectivities as a function of time in gas-phase hydrogenation of 3-methyl crotonaldehyde over A ) Ru/NaY; and B) Ru/KY. UOL unsaturated alcohol; SOL = saturated alcohol; S A L = saturated aldehyde. Pressure = 0.1 MPa, Temperature = 313K.
151
In the present study, liquid-phase reactions over both NaY and KY showed significant production of SAL compared to the gas-phase studies. The higher temperature of the liquid-phase work might account for this increased SAL production through UOL isomerization as shown above. Another possibly important factor is the residence time of the products in the batch liquid-phase reactions. In the continuous flow mode, products may not be in contact with the catalyst long enough to undergo significant secondary isomerization reactions, a limitation which does not exist in batch reactions. Further work is underway in both liquid- and gas-phase hydrogenation studies aimed at achieving a better understanding of both the kinetics of the reaction network and the role of the zeolite support in altering the product selectivity of the metal. CONCLUSIONS Preliminary results for Nay- and KY-supported Ru catalysts demonstrate significant effects of the nature of the zeolite cation for the selectivity of 3-methyl crotonaldehyde hydrogenation. It was suggested that increased basicity of the zeolite resulted in increased selectivity toward the unsaturated alcohol product. These results agree with earlier suggestions that the nature of the support can have significant influence on the product distribution in the hydrogenation of -,p-unsaturated aldehydes. ACKNOWLEDGMENTS Support from NATO Scientific Affairs Division (Brussels, Belgium), the Centre International des Etudiants et Stagiaires (C.I.E.S., Paris, France) and the U.S. National Science Foundation (Presidential Young Investigator Program, CBT-8552656) is gratefully acknowledged.
152
REFERENCES
9
P.N. Rylander, IIHydrogenation MethodsI1l Academic Press, London, 1985. J.E. Germain, I1Catalytic Conversion of Hydrocarbons,I1 Academic Press, New York, 1969. R.L. Augustine, Catal. Rev. Sci.-Eng., u,285 (1976). R.L. Augustine, in IIAdvances in CatalysisI1l (D.D. Eley, P.W. Selwood, and P.B. Weisz, Eds.), V o l . 25, p. 56, Academic Press, New York, 1976. M.L. Khidekel, A.S. Bakhanov, A.S. Astakhova, K.A. Brikenshtein, V.I. Savchenko, I.S. Monakhova, and V.G. Dorokov, Izv. Akad. Nauk. SSR, Ser. Chem. (1970), p. 499. P.N. Rylander, and D.R. Steele, Tetr. Lett., 1579 (1969). A . Giroir-Fendler, D. Richard, and P. Gallezot, in tlHeterogeneousCatalysis and Fine Chemicals,1o (M. Guisnet et al., Eds.), p.171, Elsevier, Amsterdam, 1988. F.A.P. Cavalcanti, D.G. Blackmond, R . Oukaci, A. Sayari, A. Erdem-Senatalar, and I. Wender, J. Catal. , 113,1 (1988). Y.W. Chen, H.T. Wang, and J.G. Goodwin, Jr., J. Catal., B,
10
R. Oukaci,
11
J.Z. Shyu, E.T. Skopinski, A . Sayari, and J.G. Goodwin, Jr., Appl. Surf. Sci., a,297 (1985). Jacobs et al. JCS Far. 1 7 4 , 403 (1980). D. Richard, J. Ockelford, A. Giroir-Fendler, and P. Gallezot, submitted to Catal. Lett. D. Richard, P.Fouilloux, and P. Gallezot, in IICatalysis: Theory to Practice,Il Proc. 9th Intl. Congr. Catal., (M. J. Phillips and M. Ternan, Eds.), Vol. 3, p. 1074, The Chemical Institute of Canada, Ottawa, 1988. D. Lackey, M. Surman, Jacobs, S . Grider, D. and D.A. King, Surf. Sci. , 152-153, 513 (1985). K.J. Uram, L. Ng, and J.T. Yates, Jr., Surf. Sci., 177, 253
1 2 3 4 5 6 7 8
12 13 14
15 16 17 18 19 20 21
499 (1984).
A.
102, 126 (1985).
.
Sayari, and J.G. Goodwin,
Jr., J. Catal.,
(1986) S. Kesraoui, R. Oukaci, and D.G. Blackmond, J. Catal., 432 (1987).
105,
P.A.J. Angevaare, H.A.C. Hendrickx, and V. Ponec, J. Catal. , 110, 11 (1988). P.A.J. Angevaare, H.A.C. Hendrickx, and V. Ponec, J. Catal. , 110, 18 (1988). M . A . Vannice, and B. Sen, J. Catal., 115, 65 (1989). J. Simonik, and L. Beranek, Collection Czechoslov. Comm.,
z, 353
(1972).