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Catalytic hydrogenation on raney nickel catalyst modified by chromium hydroxide deposition
Applied Catalysis, 49 (1989) 91-99 Elsevier Science Publishers B.V., Amsterdam
- Printed
91
in The Netherlands
Catalytic Hydrogenation on Raney Nickel Catalyst Modified by Chromium Hydroxide Deposition T. KOSCIELSKI,
J.M. BONNIER,
J.P. DAMON*
and J. MASSON
Laboratoire d%tudes Dynamiques et Structurales de la SklectivitC (L.E.D.S.S. Joseph Fourier, BP 53 X, 38041 Grenoble Ctdex (France) (Received 6 July 1988, revised manuscript
l), University
received 19 December 1988)
ABSTRACT The catalytic properties of Raney nickel catalysts modified by chromium hydroxide deposition were investigated. The presence of the hydroxide phase does not influence the hydrogenation of cyclohexene, but a significant increase in acetophenone and glucose hydrogenation activity was observed. These chromium-doped Raney nickel catalysts also possess a high selectivity for the hydrogenation of the acetophenone carbonyl group; their activity for the competitive hydrogenation of the phenyl group and for the successive hydrogenolysis of the C-OH bond is very low.
INTRODUCTION
Raney-type metal catalysts are commonly used in hydrogenation, hydrogenolysis, oxidation and cyclization reactions of different kinds of compounds on both the laboratory and industrial scales. The technological superiority of many alloy catalysts in these processes requires a rational explanation and a search for further effective alloy catalysts. The influence of chromium as a dopant on the activity and selectivity of Raney nickel catalysts has been investigated for certain reactions [l-6]. For example, a significant change in selectivity was observed for competitive hydrogenation of cyclohexene and acetophenone [ 4,6]. In previous papers [ 6-81, we described the catalytic and physico-chemical properties of catalysts obtained from precursor nickel alloys, Ni,-,Cr,Al,, where it was assumed that nickel had been homogeneously replaced by chromium in the nickel phase. After an alkali leaching process, chromium in the catalyst was present as a chromium oxide phase and strongly segregated to the surface. The modifications of the catalytic properties were attributed to the chromium oxide and to the residual metallic aluminium. It was difficult to elucidate the separate roles of each of the two compounds, as the residual aluminium content always increases when chromium is present in the starting alloy. Nevertheless, it was proposed that chromium oxide promotes the hydrogenation of the carbony1 groups and to a lesser extent the ethylenic groups, whereas residual me-
0166-9834/89/$03.50
0 1989 Elsevier Science Publishers
B.V.
92
tallic aluminium seems to inhibit both reactions, especially the reduction of the ethylenic groups. In order to clarify the role of aluminium and chromium oxide, we investigated other approaches to the preparation of chromium-doped Raney nickel catalysts. These catalysts were prepared by the deposition of increasing amounts of chromium hydroxide on a standard Raney nickel sample so that the aluminium content was constant for all the catalysts. The catalytic properties of these promoted nickel catalysts were studied in the hydrogenation of cyclohexene, acetophenone and glucose. EXPERIMENTAL
Catalyst preparation The standard Raney nickel catalyst (RNi) was obtained by leaching a commercial NiAl alloy (Prolabo) with 20% sodium hydroxide solution at boiling temperature [9]. The prepared RNi catalyst was kept under 4% sodium hydroxide solution. Five chromium-doped RNi catalysts (xCr-RNi, where x is the theoretical Cr/Ni atomic ratio in the catalyst), were prepared according to the following procedure. RNi was washed with distilled water until the washings were neutral. To each weighed sample in distilled water was added an amount of chromium chloride calculated to give the desired chromium loading. The solution was well stirred, then an appropriate volume of 4% sodium hydroxide solution was added according to the stoichiometry of the reaction 3NaOH+CrC1,+Cr(OH)3+3NaC1
(1)
Characterization of catalysts The bulk composition of the catalyst was determined by chemical analysis
[101.
The nickel surface area was obtained by hydrodesulphurization of S-methylthiophene (3-MeTh) in the liquid phase. The stoichiometry of the reaction was taken as S/Ni= 1:2 [ 111. The reaction was performed at 363 K under a hydrogen pressure of 1.1 MPa, using 0.6 g of the catalyst and 1.85 mmol of 3MeTh in 150 ml of cyclohexane (concentration of 3-MeTh 12.3 mmol 1-l). After 2 h, which is sufficient for the complete reactive adsorption of 3-MeTh on the catalyst surface, the amount of 3-MeTh remaining in reaction mixture was determined by gas chromatography (GC) with a (20%) Hallcomid on Chromosorb W column with n-octane as internal standard. The total surface area was measured by the BET method. Before measuring the total surface area, SBET, by liquid nitrogen adsorption, the catalyst was washed three times with distilled water and three times with distilled metha-
93
nol, then the solvent was slowly evaporated at room temperature and desorbed at 373 K for 2 h. Hydrogenation reactions The hydrogenation reactions of cyclohexene and acetophenone were carried out in cyclohexane solution (135 ml of solvent) in the presence of about 0.19 g of the catalyst. When cyclohexene was used as a substrate, n-octane was applied as an internal standard. The hydrogenation of glucose was carried out in distilled water solution (150 ml of solvent) in the presence of about 0.45 g of the catalyst. The reactions were performed in a 250-ml static reactor under a constant hydrogen pressure and at a stirring speed such that no perturbation due to mass transfer was observed. Before introduction of the substrate, the reactor containing a suspension of the catalyst in the solvent was purged with a flow of hydrogen, followed by pretreatment at room temperature for 1 h with stirring under a hydrogen pressure of 1.0 MPa. The experimental conditions were as follows: for cyclohexene, concentration 0.36 mol l-l, temperature 288 K, hydrogen pressure 0.9 MPa; for acetophenone concentration 0.32 mol l-l, temperature 353 K, hydrogen pressure 0.9 MPa; for cyclohexene and acetophenone in a competitive reaction, concentrations 0.29 and 0.33 mol l-i, respectively, temperature 333 and 353 K, hydrogen pressure 0.9 MPa; and for glucose, concentration 0.67 mol l-l, temperature 373 K, hydrogen pressure 3.0 MPa. Cyclohexane (Aldrich, 99.9% ) was treated for 2 h under reflux with stirring in the presence of Raney nickel catalysts and then distilled. Acetophenone (Aldrich, 99.9% ) was distilled before use. Cyclohexene (Aldrich, 99.9% ) was further purified by passage through an acidic alumina bed (Merck) as described by Madon et al. [ 121. In all instances, kinetic and selectivity measurements were made by determining the amount of hydrogen consumed and/or by GC measurements on the samples in solution. The conditions for the GC measurements were as follows: for cyclohexene hydrogenation, packed column (4 m x { in. O.D.) with 20% of Hallcomid on Chromosorb W (80-100 mesh), carrier gas nitrogen at a flowrate of 30 ml min-‘, column temperature isothermal at 338 K and n-octane as internal standard; for acetophenone hydrogenation, a packed column (3 m x t in. O.D.) with 10% of diethylene glycol succinate on Chromosorb G (SO-100 mesh), nitrogen as carrier gas at a flow-rate of 30 ml min-’ and column temperature isothermal at 423 K. RESULT AND DISCUSSION
Catalyst characterization Table 1 presents the bulk atomic compositions, the BET surface areas and the metallic surface areas. The first column corresponds to the chromium sam-
94
TABLE 1 Characterization of the catalysts xCr-RNi catalyst
RNi 0.72Cr-RNi 1.40Cr-RNi 2.03Cr-RNi 2.71Cr-RNi 5.35Cr-RNi
Bulk composition, Cr/Ni (at.). lo2
0 0.45 0.80 1.70 2.40 3.80
Surface area (m* g-l) BET
Metallic (Ni )
80 76 76 82 68 76
60 47 45 43 38 30
ples xCr-RNi where x is the theoretical chromium-to-nickel bulk atomic ratio and the second column gives the real chromium-to-nickel atomic ratios obtained by chemical analysis. It can be seen that not all the chromium has been deposited on the RNi surface and part of it has been lost during the precipitation process. We also measured the amount of aluminium and of sodium in the samples. The aluminium-to-nickel atomic ratio for all five catalysts was found to be about 0.1, which corresponds to those obtained with the RNi catalyst described previously [8]. A very low content of sodium compared with the chromium content was obtained for all the xCr-RNi samples (sodium-to-chromium atomic ratio about 0.04). This result confirms that chromium is present in the form of chromium hydroxide according to reaction (l), and not in the form of NaCrO,. The total surface area, SBET, was almost constant for all the doped catalysts, and similar to the RNi surface area (80 m2 g-l). We found that chromium hydroxide itself, prepared in the same way as for catalytic deposition, is unreactive with 3-methylthiophene. Therefore, the surface areas measured with the xCr-RNi samples represent only the nickel surface area. The nickel surface area decreases from 60 m2 g-l (characteristic of the reference RNi) to 30 m2 g-l (characteristic of the 5.35Cr-RNi catalyst). As the amount of chromium oxide increases, fewer nickel sites are accessible. A 60 m2 g-’ nickel surface area (the reference RNi) corresponds to about 10% of exposed nickel atoms. The 5.35Cr-Ni sample contains a Cr/Ni bulk ratio of 3.8. 10e2 (at. ) of residual chromium. As all the chromium is deposited on the surface, the ratio of the chromium atoms to the exposed nickel atoms is 0.38. It is interesting that for this sample, the nickel surface area represents only 50% of the reference. This could mean that, to a first approximation, for one chromium atom deposited on the surface one exposed nickel atom disappears. Nevertheless, we must point out that chromium is in the hydroxide form, so any interpretation of the chromium dispersion based oniy on the evolution
95
of the nickel surface is not sufficient. A more detailed characterization of the surface is therefore necessary. Catalytic properties
Before testing the catalytic properties of the chromium-doped nickel, we measured the activity of pure chromium hydroxide prepared by the same method of catalyst deposition. The reaction rates for all hydrogenation reactions were zero. Therefore the catalytic activity of chromium hydroxide itself is negligible under the experimental conditions used. The rates of cyclohexene hydrogenation as a function of the chromium content in the catalysts are presented in Table 2. The rates of cyclohexene hydrogenation is not altered when chromium is added to nickel. With the series of xCr-RNi samples we obtained a constant turnover number, which is completely different to the results obtained with the previous series of chromium-doped nickel catalysts Ni-Cr,, for which a marked decrease in the turnover number was observed when chromium oxide was present [ 61. For both series, the catalytic surfaces are constituted by the juxtaposition of the oxide phase with the nickel sites. However, the residual metallic aluminium content is much higher for the Ni-Cr, series than for the zCr-RNi series. These results confirm the previous suggestion that a high residual amount of aluminium probably inhibits cyclohexene hydrogenation. We have seen (Table 1) that when the chromium content is increased the number of accessible nickel sites decreases, that is the number of nickel ensembles decreases. In a recent study, Boudart and Cheng [13,14] showed that cyclohexene hydrogenation is a structure-insensitive reaction. Therefore, the constant rate obtained in this work indicates that hydrogenation of cyclohexene is insensitive not only to the ensembles of nickel sites, but also to perturTABLE 2 Initial hydrogenation rates of cyclohexene (V,,,) respectively
and acetophenone (V,,,)
Solvent cyclohexane; hydrogen pressure, 0.9 MPa. xCr-RNi catalyst
v,=,-102 (mm01 mNi-* min-‘)
v,=; lo2 (mm01 mNi-* min-‘)
RNi 0.72Cr-RNi 1.40Cr-RNi 2.03Cr-RNi 2.71Cr-RNi 5.35Cr-RNi
38.5 39.4 37.5 38.7 38.7 38.0
9.0 7.6 22.2 23.7 7.7 9.9
at 288 and 353 K,
96
bations of the sites themselves. These perturbations caused by chromium oxide may be due to a modification of the electronic density of nickel and/or changes in the adsorption properties of the substrate. When hydrogenation of acetophenone was performed, the following compounds were detected by GC: 1-phenylethanol, ethylbenzene, methyl cyclohexyl ketone and l-cyclohexylethanol. Depending on the experimental conditions, some of these compounds were also found when hydrogenation of acetophenone was carried out on other types of nickel catalysts [ 15-191. Therefore, the following scheme can be proposed: Q-i-cHs
b
Q-cHsw
Qc&
The initial hydrogenation rates of acetophenone to 1-phenylethanol obtained with the different samples are given in Table 2. It can be seen that when chromium is added to nickel there is an increase in activity with a maximum that corresponds to the 2.03Cr-RNi sample. In contrast to the results obtained with cyclohexene, hydrogenation of acetophenone is enhanced with the chromium-doped nickel catalysts. So far the mechanism of the hydrogenation of the carbonyl group has not been well established. To explain the increase in acetophenone hydrogenation, it is therefore difficult to define which plays the major role, electronic effects or modifications of the ensembles of the nickel sites. Further, the chromium hydroxide phase at the surface may also constitute a site of adsorption. To evaluate the selectivity for reduction of the C = 0 function in acetophenone compared with side reactions, (hydrogenation of the ring, successive hydrogenolysis of the C-OH bond), the variation in the composition of the mixture was determined and the results are plotted in Fig. 1. The results obtained with the 1.40Cr-RNi sample (Fig. la) were representative of the results obtained with all the chromium-doped catalysts. For comparison, the results obtained with pure Raney nickel were also plotted (Fig. lb). It can be seen that the yield of 1-phenylethanol is much higher with the xCrRNi catalyst (about 96%) than that obtained with the RNi catalyst (about 85% ). Further the large amount of alcohol obtained after the complete hydrogenation of the C = 0 function with the doped catalyst remains almost constant after a prolonged reaction time, whereas a marked drop is observed with the undoped nickel catalyst, giving ethylbenzene and cyclohexylethanol. The high selectivity obtained with xCr-RNi is directly correlated with the decrease in the hydrogenolysis of the C-OH bond and in ring hydrogenation. The competitive reduction of cyclohexene and acetophenone was also studied on the chromium-doped Raney nickel and the results are presented in Fig.
97
0
0
20
40
60
20
40
60
80
80
100
120
100
120 t.min
t,min
Fig. 1. Hydrogenation of acetophenone: product distribution (% ) as a function of reaction time. ( l ) Acetophenone; ( + ) 1-phenylethanol; ( X ) cyclohexyl methyl ketone; (o ) l-cyclohexylethanol; (A )ethylbenzene. Temperature, 353 K. Reactions performed on (a) 1.40Cr-RNi and (b) RNi.
2. With increasing amounts of chromium, the initial rate of cyclohexene hydrogenation (V’,=,) is constant but the initial rate of acetophenone hydrogenation (I”,,,) increases. The ratio V’c=O/V’c=c, in relation to the chromium content, is shown in Fig. 3 for two different reaction temperatures. Whatever the temperature, this ratio with the chromium-doped nickel catalyst is much higher than that with pure Raney nickel. The activities of the catalysts were also evaluated for the reaction of acetal functions. Glucose was used as the substrate in distilled water as solvent at 373 K and a hydrogen pressure of 3.0 MPa and the 2.71Cr-RNi sample was used as a representative catalyst of the chromium-doped nickel series. The results, expressed as first-order rate constants (k) in lo3 h-l mNi_‘, were as follows: RNi, 2.8; 2.71Cr-RNi, 10.4; Ni-Cr,,O,, 9.7; and Ni-Cr,,,,, 8.2. It can be seen that chromium-doped catalysts are at least four times more active than the undoped RNi. This enhancement of activity is observed no matter how the chromium is added to nickel, by the alloy (Ni-Cr,) or by deposition of the oxide phase xCr-RNi.
98
0.2
0.0
0.4
0.2
Cr / Ni
at. x
10’
“/Ni
at.r102
of cyclohexene ( V’,=,) Fig. 2. Initial rates (mmol min’ rnNie2 ) of competitive hydrogenation and acetophenone (V’,=,) as a function of chromium content, Cr/Na (at.).lO*. Solvent, cyclohexane; Temperature, 333 K, hydrogen pressure, 0.9 MPa. Fig. 3. Ratio V’,JV’,=, for competitive hydrogenation of acetophenone and cyclohexene as a function of chromium content. Cr/Ni (at.).102. Solvent, cyclohexane; temperature, (a) 333 K; (b) 353 K.
We have postulated in a previous paper [ 5 ] that the increase in nickel activity for glucose hydrogenation in aqueous media could be correlated with the presence of certain metallic compounds. According to their redox potentials, metals such as aluminium could maintain a clean surface of metallic nickel during the reaction by protecting the nickel from oxidation. It was more difficult to establish precisely the role of a metal oxide phase (such as chromium oxide). It is clear from the results obtained in this work that chromium oxide markedly promotes the activity of nickel for glucose reduction.
CONCLUSION
The results reveal a significant effect of chromium oxide on the catalytic properties of nickel. The most important results were increases in the reduction properties of the carbonyl and acetal functions in acetophenone and glucose, respectively. In contrast, the reduction of the ethylenic function of cyclohexene was not affected. Consequently, the selectivity ratio of the carbonyl to the ethylenic function reduction was enhanced. These chromium-doped catalysts were much more selective than pure Raney nickel for the reduction of the C =O function in acetophenone. A very low
99
activity for hydrogenation of the phenyl group and for hydrogenolysis of the C-OH bond was observed. By comparison of the results obtained with chromium-doped Raney nickel prepared in different ways (chromium added in the precursor alloy or deposited on the surface), it was possible to confirm that large amounts of aluminium can inhibit the hydrogenation of cyclohexene. The catalysts prepared in this work have the great advantage of facile preparation. They constitute a good system for hydrogenating carbonyl functions avoiding secondary reactions such as hydrogenolysis. The results obtained in this work call for more detailed catalyst characterizations and such experiments are our future target.
REFERENCES 1
T.N. Nalibaev, B.K. Almashev, A.B. Fasman and G.S. Tolipov, Russ. J. Phys. Chem., 48 (1974) 511. 2 P.S. Gradeff and G. Formica, Tetrahedron Lett., 51 (1976) 4681. 3 S.R. Montgomery, in W.R. Moser (Editor), Catalysis of Organic Reactions, Vol. 5, Chemical Industries, Marcel Dekker, New York, 1981. 4 T. Dawood, J.P. Damon, J. Masson and J.M. Bonnier, C.R. Acad. Sci., 299 (1984) 1317. 5 J. Court, J.P. Damon, J. Masson and P. Wierzchowski, in M. Guisnet, J. Barrault, C. Bouchoule, D. Duprez, C. Montassier and G. Perot (Eds.), Heterogeneous Catalysis and Fine Chemicals (Studies in Furface Science and Catalysis, Vol. 41), Elsevier, Amsterdam, 1988, p. 189. 6 J.M. Bonnier, J.P. Damon and J. Masson, Appl. Catal., 42 (1988) 285. 7 C. Kordulis, B. Doumain, J.P. Damon, J. Masson, J.L. Dallons and F. Delannay, Bul. Sot. Chim. Belg., 94 (1985) 371. 8 J.M. Bonnier, J.P. Damon, B. Delmon,B. Doumain and J. Masson, J. Chim. Phys., 84 (1987) 889. 9 S. Sank, J.M. Bonnier, J.P. Damon and J. Masson, Appl. Catal., 9 (1984) 69. 10 M. Khaidar, C. Allibert, J. Driole and P. Germi, Mater. Res. Bull., 17 (1982) 329. 11 E. De Plaen, Thesis, University Louvain La Neuve, Belgium, 1979. 12 R.J. Madon, J.P. O’Connell and M. Boudart AIChE J., 24 (1978) 904. 13 M. Boudart and W.C. Cheng, J. Catal., 106 (1987) 134. 14 E.E. Gonzo and M. Boudart, J. Catal., 52 (1978) 462. 15 D.V. Mushenko, N.S. Barinov and E.G. Lebedeva, Zh. Org. Khim., 7 (1971) 314. 16 N.S. Barinov, D.V. Mushenko, E.G. Lebedeva and A.A. Balandin, Dokl. Akad. Nauk SSSR, 172 (1967) 1109. 17 D.V. Mushenko, E.G. Lebedeva, V.S. Chagina, V.P. Khimich and N.S. Barinov, Zh. Prikl. Khim., 39 (1966) 2769. 18 L.Kh Freidlin, N.V. Borunova, S.S. Danielova, A.S. Nekrasov and L.I. Gvinter, Izv. Akad. Nauk SSSR, Ser. Khim., (1969) 2290. 19 N.S. Barinov, D.V. Mushenko and E.G. Lebedeva, Zh. Prikl. Khim., 42 (1969) 2398.
- Printed
91
in The Netherlands
Catalytic Hydrogenation on Raney Nickel Catalyst Modified by Chromium Hydroxide Deposition T. KOSCIELSKI,
J.M. BONNIER,
J.P. DAMON*
and J. MASSON
Laboratoire d%tudes Dynamiques et Structurales de la SklectivitC (L.E.D.S.S. Joseph Fourier, BP 53 X, 38041 Grenoble Ctdex (France) (Received 6 July 1988, revised manuscript
l), University
received 19 December 1988)
ABSTRACT The catalytic properties of Raney nickel catalysts modified by chromium hydroxide deposition were investigated. The presence of the hydroxide phase does not influence the hydrogenation of cyclohexene, but a significant increase in acetophenone and glucose hydrogenation activity was observed. These chromium-doped Raney nickel catalysts also possess a high selectivity for the hydrogenation of the acetophenone carbonyl group; their activity for the competitive hydrogenation of the phenyl group and for the successive hydrogenolysis of the C-OH bond is very low.
INTRODUCTION
Raney-type metal catalysts are commonly used in hydrogenation, hydrogenolysis, oxidation and cyclization reactions of different kinds of compounds on both the laboratory and industrial scales. The technological superiority of many alloy catalysts in these processes requires a rational explanation and a search for further effective alloy catalysts. The influence of chromium as a dopant on the activity and selectivity of Raney nickel catalysts has been investigated for certain reactions [l-6]. For example, a significant change in selectivity was observed for competitive hydrogenation of cyclohexene and acetophenone [ 4,6]. In previous papers [ 6-81, we described the catalytic and physico-chemical properties of catalysts obtained from precursor nickel alloys, Ni,-,Cr,Al,, where it was assumed that nickel had been homogeneously replaced by chromium in the nickel phase. After an alkali leaching process, chromium in the catalyst was present as a chromium oxide phase and strongly segregated to the surface. The modifications of the catalytic properties were attributed to the chromium oxide and to the residual metallic aluminium. It was difficult to elucidate the separate roles of each of the two compounds, as the residual aluminium content always increases when chromium is present in the starting alloy. Nevertheless, it was proposed that chromium oxide promotes the hydrogenation of the carbony1 groups and to a lesser extent the ethylenic groups, whereas residual me-
0166-9834/89/$03.50
0 1989 Elsevier Science Publishers
B.V.
92
tallic aluminium seems to inhibit both reactions, especially the reduction of the ethylenic groups. In order to clarify the role of aluminium and chromium oxide, we investigated other approaches to the preparation of chromium-doped Raney nickel catalysts. These catalysts were prepared by the deposition of increasing amounts of chromium hydroxide on a standard Raney nickel sample so that the aluminium content was constant for all the catalysts. The catalytic properties of these promoted nickel catalysts were studied in the hydrogenation of cyclohexene, acetophenone and glucose. EXPERIMENTAL
Catalyst preparation The standard Raney nickel catalyst (RNi) was obtained by leaching a commercial NiAl alloy (Prolabo) with 20% sodium hydroxide solution at boiling temperature [9]. The prepared RNi catalyst was kept under 4% sodium hydroxide solution. Five chromium-doped RNi catalysts (xCr-RNi, where x is the theoretical Cr/Ni atomic ratio in the catalyst), were prepared according to the following procedure. RNi was washed with distilled water until the washings were neutral. To each weighed sample in distilled water was added an amount of chromium chloride calculated to give the desired chromium loading. The solution was well stirred, then an appropriate volume of 4% sodium hydroxide solution was added according to the stoichiometry of the reaction 3NaOH+CrC1,+Cr(OH)3+3NaC1
(1)
Characterization of catalysts The bulk composition of the catalyst was determined by chemical analysis
[101.
The nickel surface area was obtained by hydrodesulphurization of S-methylthiophene (3-MeTh) in the liquid phase. The stoichiometry of the reaction was taken as S/Ni= 1:2 [ 111. The reaction was performed at 363 K under a hydrogen pressure of 1.1 MPa, using 0.6 g of the catalyst and 1.85 mmol of 3MeTh in 150 ml of cyclohexane (concentration of 3-MeTh 12.3 mmol 1-l). After 2 h, which is sufficient for the complete reactive adsorption of 3-MeTh on the catalyst surface, the amount of 3-MeTh remaining in reaction mixture was determined by gas chromatography (GC) with a (20%) Hallcomid on Chromosorb W column with n-octane as internal standard. The total surface area was measured by the BET method. Before measuring the total surface area, SBET, by liquid nitrogen adsorption, the catalyst was washed three times with distilled water and three times with distilled metha-
93
nol, then the solvent was slowly evaporated at room temperature and desorbed at 373 K for 2 h. Hydrogenation reactions The hydrogenation reactions of cyclohexene and acetophenone were carried out in cyclohexane solution (135 ml of solvent) in the presence of about 0.19 g of the catalyst. When cyclohexene was used as a substrate, n-octane was applied as an internal standard. The hydrogenation of glucose was carried out in distilled water solution (150 ml of solvent) in the presence of about 0.45 g of the catalyst. The reactions were performed in a 250-ml static reactor under a constant hydrogen pressure and at a stirring speed such that no perturbation due to mass transfer was observed. Before introduction of the substrate, the reactor containing a suspension of the catalyst in the solvent was purged with a flow of hydrogen, followed by pretreatment at room temperature for 1 h with stirring under a hydrogen pressure of 1.0 MPa. The experimental conditions were as follows: for cyclohexene, concentration 0.36 mol l-l, temperature 288 K, hydrogen pressure 0.9 MPa; for acetophenone concentration 0.32 mol l-l, temperature 353 K, hydrogen pressure 0.9 MPa; for cyclohexene and acetophenone in a competitive reaction, concentrations 0.29 and 0.33 mol l-i, respectively, temperature 333 and 353 K, hydrogen pressure 0.9 MPa; and for glucose, concentration 0.67 mol l-l, temperature 373 K, hydrogen pressure 3.0 MPa. Cyclohexane (Aldrich, 99.9% ) was treated for 2 h under reflux with stirring in the presence of Raney nickel catalysts and then distilled. Acetophenone (Aldrich, 99.9% ) was distilled before use. Cyclohexene (Aldrich, 99.9% ) was further purified by passage through an acidic alumina bed (Merck) as described by Madon et al. [ 121. In all instances, kinetic and selectivity measurements were made by determining the amount of hydrogen consumed and/or by GC measurements on the samples in solution. The conditions for the GC measurements were as follows: for cyclohexene hydrogenation, packed column (4 m x { in. O.D.) with 20% of Hallcomid on Chromosorb W (80-100 mesh), carrier gas nitrogen at a flowrate of 30 ml min-‘, column temperature isothermal at 338 K and n-octane as internal standard; for acetophenone hydrogenation, a packed column (3 m x t in. O.D.) with 10% of diethylene glycol succinate on Chromosorb G (SO-100 mesh), nitrogen as carrier gas at a flow-rate of 30 ml min-’ and column temperature isothermal at 423 K. RESULT AND DISCUSSION
Catalyst characterization Table 1 presents the bulk atomic compositions, the BET surface areas and the metallic surface areas. The first column corresponds to the chromium sam-
94
TABLE 1 Characterization of the catalysts xCr-RNi catalyst
RNi 0.72Cr-RNi 1.40Cr-RNi 2.03Cr-RNi 2.71Cr-RNi 5.35Cr-RNi
Bulk composition, Cr/Ni (at.). lo2
0 0.45 0.80 1.70 2.40 3.80
Surface area (m* g-l) BET
Metallic (Ni )
80 76 76 82 68 76
60 47 45 43 38 30
ples xCr-RNi where x is the theoretical chromium-to-nickel bulk atomic ratio and the second column gives the real chromium-to-nickel atomic ratios obtained by chemical analysis. It can be seen that not all the chromium has been deposited on the RNi surface and part of it has been lost during the precipitation process. We also measured the amount of aluminium and of sodium in the samples. The aluminium-to-nickel atomic ratio for all five catalysts was found to be about 0.1, which corresponds to those obtained with the RNi catalyst described previously [8]. A very low content of sodium compared with the chromium content was obtained for all the xCr-RNi samples (sodium-to-chromium atomic ratio about 0.04). This result confirms that chromium is present in the form of chromium hydroxide according to reaction (l), and not in the form of NaCrO,. The total surface area, SBET, was almost constant for all the doped catalysts, and similar to the RNi surface area (80 m2 g-l). We found that chromium hydroxide itself, prepared in the same way as for catalytic deposition, is unreactive with 3-methylthiophene. Therefore, the surface areas measured with the xCr-RNi samples represent only the nickel surface area. The nickel surface area decreases from 60 m2 g-l (characteristic of the reference RNi) to 30 m2 g-l (characteristic of the 5.35Cr-RNi catalyst). As the amount of chromium oxide increases, fewer nickel sites are accessible. A 60 m2 g-’ nickel surface area (the reference RNi) corresponds to about 10% of exposed nickel atoms. The 5.35Cr-Ni sample contains a Cr/Ni bulk ratio of 3.8. 10e2 (at. ) of residual chromium. As all the chromium is deposited on the surface, the ratio of the chromium atoms to the exposed nickel atoms is 0.38. It is interesting that for this sample, the nickel surface area represents only 50% of the reference. This could mean that, to a first approximation, for one chromium atom deposited on the surface one exposed nickel atom disappears. Nevertheless, we must point out that chromium is in the hydroxide form, so any interpretation of the chromium dispersion based oniy on the evolution
95
of the nickel surface is not sufficient. A more detailed characterization of the surface is therefore necessary. Catalytic properties
Before testing the catalytic properties of the chromium-doped nickel, we measured the activity of pure chromium hydroxide prepared by the same method of catalyst deposition. The reaction rates for all hydrogenation reactions were zero. Therefore the catalytic activity of chromium hydroxide itself is negligible under the experimental conditions used. The rates of cyclohexene hydrogenation as a function of the chromium content in the catalysts are presented in Table 2. The rates of cyclohexene hydrogenation is not altered when chromium is added to nickel. With the series of xCr-RNi samples we obtained a constant turnover number, which is completely different to the results obtained with the previous series of chromium-doped nickel catalysts Ni-Cr,, for which a marked decrease in the turnover number was observed when chromium oxide was present [ 61. For both series, the catalytic surfaces are constituted by the juxtaposition of the oxide phase with the nickel sites. However, the residual metallic aluminium content is much higher for the Ni-Cr, series than for the zCr-RNi series. These results confirm the previous suggestion that a high residual amount of aluminium probably inhibits cyclohexene hydrogenation. We have seen (Table 1) that when the chromium content is increased the number of accessible nickel sites decreases, that is the number of nickel ensembles decreases. In a recent study, Boudart and Cheng [13,14] showed that cyclohexene hydrogenation is a structure-insensitive reaction. Therefore, the constant rate obtained in this work indicates that hydrogenation of cyclohexene is insensitive not only to the ensembles of nickel sites, but also to perturTABLE 2 Initial hydrogenation rates of cyclohexene (V,,,) respectively
and acetophenone (V,,,)
Solvent cyclohexane; hydrogen pressure, 0.9 MPa. xCr-RNi catalyst
v,=,-102 (mm01 mNi-* min-‘)
v,=; lo2 (mm01 mNi-* min-‘)
RNi 0.72Cr-RNi 1.40Cr-RNi 2.03Cr-RNi 2.71Cr-RNi 5.35Cr-RNi
38.5 39.4 37.5 38.7 38.7 38.0
9.0 7.6 22.2 23.7 7.7 9.9
at 288 and 353 K,
96
bations of the sites themselves. These perturbations caused by chromium oxide may be due to a modification of the electronic density of nickel and/or changes in the adsorption properties of the substrate. When hydrogenation of acetophenone was performed, the following compounds were detected by GC: 1-phenylethanol, ethylbenzene, methyl cyclohexyl ketone and l-cyclohexylethanol. Depending on the experimental conditions, some of these compounds were also found when hydrogenation of acetophenone was carried out on other types of nickel catalysts [ 15-191. Therefore, the following scheme can be proposed: Q-i-cHs
b
Q-cHsw
Qc&
The initial hydrogenation rates of acetophenone to 1-phenylethanol obtained with the different samples are given in Table 2. It can be seen that when chromium is added to nickel there is an increase in activity with a maximum that corresponds to the 2.03Cr-RNi sample. In contrast to the results obtained with cyclohexene, hydrogenation of acetophenone is enhanced with the chromium-doped nickel catalysts. So far the mechanism of the hydrogenation of the carbonyl group has not been well established. To explain the increase in acetophenone hydrogenation, it is therefore difficult to define which plays the major role, electronic effects or modifications of the ensembles of the nickel sites. Further, the chromium hydroxide phase at the surface may also constitute a site of adsorption. To evaluate the selectivity for reduction of the C = 0 function in acetophenone compared with side reactions, (hydrogenation of the ring, successive hydrogenolysis of the C-OH bond), the variation in the composition of the mixture was determined and the results are plotted in Fig. 1. The results obtained with the 1.40Cr-RNi sample (Fig. la) were representative of the results obtained with all the chromium-doped catalysts. For comparison, the results obtained with pure Raney nickel were also plotted (Fig. lb). It can be seen that the yield of 1-phenylethanol is much higher with the xCrRNi catalyst (about 96%) than that obtained with the RNi catalyst (about 85% ). Further the large amount of alcohol obtained after the complete hydrogenation of the C = 0 function with the doped catalyst remains almost constant after a prolonged reaction time, whereas a marked drop is observed with the undoped nickel catalyst, giving ethylbenzene and cyclohexylethanol. The high selectivity obtained with xCr-RNi is directly correlated with the decrease in the hydrogenolysis of the C-OH bond and in ring hydrogenation. The competitive reduction of cyclohexene and acetophenone was also studied on the chromium-doped Raney nickel and the results are presented in Fig.
97
0
0
20
40
60
20
40
60
80
80
100
120
100
120 t.min
t,min
Fig. 1. Hydrogenation of acetophenone: product distribution (% ) as a function of reaction time. ( l ) Acetophenone; ( + ) 1-phenylethanol; ( X ) cyclohexyl methyl ketone; (o ) l-cyclohexylethanol; (A )ethylbenzene. Temperature, 353 K. Reactions performed on (a) 1.40Cr-RNi and (b) RNi.
2. With increasing amounts of chromium, the initial rate of cyclohexene hydrogenation (V’,=,) is constant but the initial rate of acetophenone hydrogenation (I”,,,) increases. The ratio V’c=O/V’c=c, in relation to the chromium content, is shown in Fig. 3 for two different reaction temperatures. Whatever the temperature, this ratio with the chromium-doped nickel catalyst is much higher than that with pure Raney nickel. The activities of the catalysts were also evaluated for the reaction of acetal functions. Glucose was used as the substrate in distilled water as solvent at 373 K and a hydrogen pressure of 3.0 MPa and the 2.71Cr-RNi sample was used as a representative catalyst of the chromium-doped nickel series. The results, expressed as first-order rate constants (k) in lo3 h-l mNi_‘, were as follows: RNi, 2.8; 2.71Cr-RNi, 10.4; Ni-Cr,,O,, 9.7; and Ni-Cr,,,,, 8.2. It can be seen that chromium-doped catalysts are at least four times more active than the undoped RNi. This enhancement of activity is observed no matter how the chromium is added to nickel, by the alloy (Ni-Cr,) or by deposition of the oxide phase xCr-RNi.
98
0.2
0.0
0.4
0.2
Cr / Ni
at. x
10’
“/Ni
at.r102
of cyclohexene ( V’,=,) Fig. 2. Initial rates (mmol min’ rnNie2 ) of competitive hydrogenation and acetophenone (V’,=,) as a function of chromium content, Cr/Na (at.).lO*. Solvent, cyclohexane; Temperature, 333 K, hydrogen pressure, 0.9 MPa. Fig. 3. Ratio V’,JV’,=, for competitive hydrogenation of acetophenone and cyclohexene as a function of chromium content. Cr/Ni (at.).102. Solvent, cyclohexane; temperature, (a) 333 K; (b) 353 K.
We have postulated in a previous paper [ 5 ] that the increase in nickel activity for glucose hydrogenation in aqueous media could be correlated with the presence of certain metallic compounds. According to their redox potentials, metals such as aluminium could maintain a clean surface of metallic nickel during the reaction by protecting the nickel from oxidation. It was more difficult to establish precisely the role of a metal oxide phase (such as chromium oxide). It is clear from the results obtained in this work that chromium oxide markedly promotes the activity of nickel for glucose reduction.
CONCLUSION
The results reveal a significant effect of chromium oxide on the catalytic properties of nickel. The most important results were increases in the reduction properties of the carbonyl and acetal functions in acetophenone and glucose, respectively. In contrast, the reduction of the ethylenic function of cyclohexene was not affected. Consequently, the selectivity ratio of the carbonyl to the ethylenic function reduction was enhanced. These chromium-doped catalysts were much more selective than pure Raney nickel for the reduction of the C =O function in acetophenone. A very low
99
activity for hydrogenation of the phenyl group and for hydrogenolysis of the C-OH bond was observed. By comparison of the results obtained with chromium-doped Raney nickel prepared in different ways (chromium added in the precursor alloy or deposited on the surface), it was possible to confirm that large amounts of aluminium can inhibit the hydrogenation of cyclohexene. The catalysts prepared in this work have the great advantage of facile preparation. They constitute a good system for hydrogenating carbonyl functions avoiding secondary reactions such as hydrogenolysis. The results obtained in this work call for more detailed catalyst characterizations and such experiments are our future target.
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