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Liquid phase catalytic hydrogenation of benzophenone: Role of metal support interaction, bimetallic catalysts, solvents and additives
M. Guisnet et al. (Editas), Heterogeneous Cutulysis and Fine Chemicnls 111 1993 Elsevier Science Publishers B.V. All rights reserved.
Q
251
Liquid phase catalytic hydrogenation of benzophenone: Role of metal support interaction, bimetallic catalysts, solvents and additives P.S. Kumbhara and the late R.A. Rajadhyaksha Department of Chemical Technology, University of Bombay, Matunga, Bombay 400 019, India. *Present address: Laboratoire de Chimie Organique Physique e t CinCtique Chimique Appliqubes, URA 418 CNRS, Ecole Nationale Supbrieure de Chimie, 8 rue de 1'Ecole Normale, 34053 Montpellier Cedex 1, France.
Abstract
Liquid-phase hydrogenation of benzophenone was studied over well characterised supported monometalic Ni and bimetallic Ni-Cu and Ni-Fe (75:25, composition by mass) catalysts. Interesting changes in selectivity were obtained over the bimetallic catalysts. Different strategies such as use of Ti02 as a support, change in solvent and use of NaOH were tested to improve the activity and selectivity to benzhydrol. A Ni-Fe(75:25)/TiOZ catalyst combined with NaOH and methano1:water as solvent was found to be the most efficient system to selectively obtain benzhydrol. 1. INTRODUCTION
The activity and selectivity of metallic catalysts is known to be influenced by alloying and metal support interaction (MSI) (1).However, these concepts have been rarely applied in liquid phase hydrogenation, which are commonly encountered i n fine chemical synthesis. The present work stemmed from our results on hydrogenation of acetophenone (2) and was undertaken t o investigate the effect of alloying, MSI, solvents and additives on the hydrogenation of benzophenone over Ni based catalysts. Product of the reaction, benzhydrol, is a n important drug intermediate. The undesired product, diphenylmethane, is obtained by hydrogenolysis of benzhydrol. Conventionally, the reaction is carried out by using Zn/NaOH as the catalytic system. However, this process produces environmentally unwanted sludge and is expensive. Recently, the reaction has been carried out using Pd catalysts poisoned with Pb or modified by phosphine ligand (3.4).
252
2.1. Catalysts Hydrogenation of benzophenone was investigated using Ni, Ni-Cu(75:25) and Ni-Fe(75:25) (composition by mass) supported on S i0 2 (Aerosil 200) and T i 0 2 (P-25) of Degussa. The catalysts were prepared by impregnation of the respective nitrates i n a n appropriate proportion by the incipient wetness technique. The total metal loading was kept constant at 20% by weight. The catalysts were calcined i n air at 723 K for 4 h and were stored in dessicator until1 further use. Prior to use required amount of the catalyst to be tested was reduced i n Hz at 523 K for 2 h followed by another 2 h a t 723 K in a specially designed reducer (5) that prevented exposure of the catalyst to air. The catalysts were characterised by H2 chemisorption, degree of reduction measurements (D.R.), temperature programmed reduction(TPR), X-ray diffraction(XRD) of r e d u c e d - p a s s i v a t e d c a t a l y s t s , j n - s i t u e x te n d e d a b s o r p t i o n fine structure(EXAFS) (Ni-Fe) and Mossbauer spectroscopy (Ni-Fe). Details of these studies will be published elsewhere(6). 2 8 Reactionpdure Hydrogenation of benzophenone (0.065 mol) (Loba Chemie, A.R. grade) was carried out in a 100 ml Parr autoclave using appropriate solvents (35.5 ml, A.R. grade) and pre-reduced catalyst (0.24 g unless otherwise specified). The hydrogenation was carried out a t hydrogen partial pressure of 60.5 kg/cm2 and 408 K. The speed of agitation was 1550 rpm a n d was chosen after confirming the absence of diffusional limitations. Samples were periodically withdrawn and analysed on a gas chromatograph (Perkin Elmer) using a 2 m x 0.33 cm carbowax (KOH washed) on chromosorb W column with a flame ionisation detector. Authentic pure samples were Lsed for product identification and quantification. In some cases th e products were separated by column chromatography an d were characterised by infra-red (IR), H-NMR a n d mass spectral (MS) analysis.
3. RESULTS AND DISCUSSION
3.1. Characterisation of catalysts
The XRD pattern indicated that Ni-CdSiOz was in a complctely FCC alloy phase. EXAFS data indicated t h a t major portion of Ni-Fe catalyst was in a FCC alloy phase with small am o u n t of unreduced FeO. Mossbauer spectroscopy showed t h a t th e Ni-Fe catalysts exist a s a mixture of supermagnetic an d ferromagnetic alloy with small amounts of Fe2+. TPR studies showed that both Cu and Fe improve the reducibility of NiO. Ni-CdSiOz was i n completely reduced state (100 % D.R.) as compared to partially reduced Ni/SiOa (76% D.R.). Compared to Ni/SiO2 both Ni-Cu a n d Ni-Fe catalysts showed decreased H2 chemisorption capacities which is characteristic of surface enrichment of Ni by Cu an d Fe respectively. Ni/"iO2 and Ni-Fe/TiOz
253
catalysts also showed suppression in H2 chemisorption, characteristic of the SMSI state (1). Details of these studies will be published elsewhere (6).
32. Hydrogenation of benzophenone in methanol
Hydrogenation of benzophenone was first studied using methanol as solvent. A typical concentration profile for the reaction over Ni/SiO2 catalyst is shown i n Figure 1.
Q
* *
0
Benzophenone Benzhydrol
1-methoxy-1,l-diphenylmethane
100
200
300
Time, min
Figure 1. Hydrogenation of benzophenone over 20% Ni/SiOz catalyst. Reaction conditions: Benzophenone, 0.065 mol, Ni/SiOz, 0.24 g; Methanol, 35.5 ml; 308 K; H2 , 60.5 kg/cin2 I n addition to th e expected hydrogenation products (benzhydrol a n d diphenylmethane), th ere was formation of a by-product i n appreciable quantities. The by-product was separated by column chromatography (silica column) a nd was identified by IR, NMR and MS as a n ether, 1-methoxy-1,ldiphenyl methane. The reaction was further studied by using SiO2 supported Ni-Cu and Ni-Fe an d Ti02 supported Ni an d Ni-Fe catalysts. The results a re summarised i n Table 1. The results are truly remarkable. There was a n increase in ether formation over Ni-Cu/Si02 an d Ni/Ti02, whereas it was negligible over Ni-Fe/SiOz. Ni/TiOe catalyst showed increased activity in agreement with th e results of
254
Vannice et al. (7) for carbonyl group hydrogenation. However, the catalyst showed increased ether formation. The most active a n d selective catalyst for formation of benzhydrol was Ni-Fe (75:25)/TiO2. The increased activity over NiA'iOz is probably due to the creation of highly active sites at the metal support interface (7). Further, increase i n activity over Ni-FeRiOz is due to synergetic metal support interaction a n d electronic effect of Fe as reported earlier for acetophenone hydrogenation (2). Table 1 Comparison of various catalysts for hydrogenation of benzophenone in presence of methanol ________---_______--____________________----------------
Catalyst
Time h
Conversion o/o
Selectivity to products (mol%l)a
.........................................
I
I1
68.3 52.0 46.4 84.5 84.5
19.7 37.5 43.7 4.6 1.7
________________________________________-------_----_---
Ni/SiOa 4.5 Ni/TiO2 2.5 Ni-Cu (75:25)/SiO$ 6.0 Ni-Fe (75:25)/Si02 4.5 Ni-Fe (75:25)/"302 2.0
89.3 92.6 83.8 85.9 86.7
________________________________________---_------------
I11
12.0 10.5 9.9 10.9 13.9
I:benzhydrol;II:ether(l-methoxy-l,l-diphenylmethane~; 111: diphenylme t hane b Catalyst weight : 0.48 g Reaction conditions : same as in Fig. 1. a
To check the ether formation pathway, hydrogenation of benzhydrol was carried out in presence of methanol under similar conditions over selected catalysts. The selectivity to ether varied a s follows: Ni-Cu (75:25)/SiO2 (highest) >> Ni/SiOa >> Ni-Fe (75:25)/Si02 (lowest) From these results i t is clear th at the ether formation occurs due to the bimolecular reaction between benzhydrol and methanol in presence of catalyst and Ha. Based on this the overall reaction scheme can be depicted a s :
255
OH
i' e
*!-o 0
n
i
y
c
1+"3OH OCH3
Benzophenone
" i i
\ e
C
H
4
Diphenylmethane
Ether, 1-metohxy- 1,l -diphenylmethane
Mechanism of ether formation
Formation of eth er (methoxycyclohexane) over Pd/C catalyst d u r in g hydrogenation of cyclohexanone in methanol was first reported by Nishimura e t al. (8). This scheme was used to prepare 4,4-dicyclohexyl methyl ether in 84% yield using Pd/C as the catalyst (9). Pines (10) in a n exhaustive study reported ether formation from various alcohols in presence of hydrogen over Ni based catalysts. He suggested th at ether formation was due to two types of sites present on Ni; one basic and the other having a n acidic character. These sites are created because of incomplete reduction of the metal. However, this reason was contradicted by Ponec et a1.(12) who showed th a t there was no co-relation between the ether formation and the reducibility of the metals, as Pt and Pd, which were completely reduced showed ether formation. I n the present case, contrary to t he arguments of Pines (lo), the competely reduced Ni-Cu/SiOz (D.R. loo%), showed increased eth er formation a s compared to partially reduced Ni/SiOz (D.R. 76% 1. Based on the present d a ta we propose the following mechanism for ether formation :
I0 : /\ 9" I 0 I CH3
Ph
r-
M I , M2 : Metal sites
MI
M2
We believe t hat ether formation is likely to occur as a result of nucleophilic attack of a methoxy species on the carbinol carbon as shown above. In Ni-Cu and Ni-Fe catalysts, the surface is known to be enriched by Cu and Fe, which was found to be also true in the present case from the hydrogen chemisorption dat a . T hus i n th e bimetallic catalysts, Ni atoms will be preferentially
256
surrounded by Cu or Fe atoms. Therefore, the above reaction is likely to involve a methoxy species bonded to Fe or Cu on the bimetallic catalysts. Evidence for this hypothesis is drawn from the d ata on heat of adsorption of methoxy species on these metals, which are as follows (12) : Fe: -87, Ni: -60, Cu: -8 kJ/mol. According to this d ata ease of formation of the methoxy species which is related to the yield of ether will be in the order Cu>Ni>Fe which indeed agrees with the results obtained i n th e present study. Similarly, increased e th e r formation on NiPTiO2 is also linked to the ease of formation of methoxy species over this catalyst (13). Another interesting point is the very small difference in the yield of the hydrogenolysis product diphenylmethane, over all the catalysts (Table 1). This is contrary to the results on C-C bond hydrogenolysis over bimetallic and Ti02 supported catalysts (1). This indicates that the sites for C - 0 hydrogenolysis are probably different from C-C bond hydrogenolysis.
35.Meet of solvents and additive
Solvents a nd additives a r e known to influence activity a n d selectivity in hydrogenation reactions. In the present study we used four solvents having different dielectric constants (methanol, i-propanol, cyclohexane a n d 10% water in methanol) to check their influence on activity a n d selectivity over the most active Ni-FeA'iO2 catalyst. The results are summarised in Table 2.
Table 2 Effect of solvents and additive (NaOH) on activity and selectivity to benzhydrol over 20%Ni-FeA'iOz catalyst. Solvent/ additive
Initial rate x 103 g m o lh g of cat.
Conversion of benzophenone o/o
Selectivity Dielectric to benzhydrol constant % of solvent
---_------------_---------------------------------------
Cyclohexane 2.63 84.0 58.8 i-Propanol 6.30 86.0 80.0 methanol 2.00 86.7 84.5 methanol+watera 4.11 90.1 91.0 methanol+NaOHb 0.61 86.2 94.3 (methanol+wate+ +NaOHb) 1.70 88.0 98.4 ________________________________________-------------_-a Water: 10% by volume of the solvent b NaOH: 0.1%by weight of benzophenone Reaction conditions : same a s in Fig. 1.
2.0 18.3 32.6 >>32.6
__
257
T he selectivity to benzhydrol is dependent on t h e solvent a n d can be correlated with the dielectric constant of the solvent; similar to earlier findings of Masson et al. (14) for acetophenone hydrogenation. Addition of NaOH results in an increase in the selectivity at the expense of drop in activity. However, this can be partly compensated by using methano1:water as a solvent. Using this combination i t was possible to achieve high selectivity to benzhydrol (98.4% selectivity at 88% conversion) at reasonable activity. 4. CONCLUSION
Interesting change in selectivity for ether ( 1-methoxy-1,l-diphenylmethane) formation during the hydrogenation of benzophenone in methanol is obtained over SiOa -supported bimetallic Ni-Cu and Ni-Fe catalysts. The formation of the ether is due to the bimolecular reaction between the product, benzhydrol a n d methanol in presence of catalyst and hydrogen. Ni-Cu (75:25)/&02 showed a n increase in ether formation whereas, Ni-Fe (75:25)/Si02 showed a suppression. The results suggest t h a t the methoxy species, h a s a possible role i n this reaction a nd i t s formation depends on the metal used. No suppresion in hydrogenolysis of the C - 0 bond was observed over bimetallic, as well a s Ti02 supported catalysts, indicating involvement of different type of sites than C-C bond hydrogenolysis. Ni-Fe (75:25)/Ti02 showed a maximum in activity a n d selectivity to benzhydrol due to the synergetic effect of MSI and the electronic effect of Fe. The nature of the solvent modifies activity and selectivity and depends on its dielectric constant. Addition of NaOH in controlled a mo u n ts improved selectivity. Benzhydrol can be obtained selectively ( > 98% ) a t high conversion of benzophenone (88%)over Ni-Fe (75:25)/Tio2 catalyst using methanol: 10% water as a solvent and NaOH(O.l%;by weight of benzophenone) as additive.
ACKNOWLEDGEMENTS Financial assistance from DST, Govt. of India is gratefuly acknowledged. PSK is thankful to UGC, Govt. of India, for senior research fellowship. PSK wishes to dedicate this paper to the memory of late Professor R.A. Rajadhyaksha.
1. 2.
3. 4. 5.
G.L. Haller and D.E. Resasco in Advances in Catalysis, 36 (1989) 173. P.S. Kumbhar, M.R. Kharkar, G.D. Yadav and R.A. Rajadhyaksha, J.Chem.Soc.Chem.Commun. (1992) 584. L.W. Gosser, US Patent No. 4302345 (1982). K. Kunerk, Ger.Offen. 2837022 (1980). P.S. Kumbhar, Ph.D. (Tech.) thesis, UDCT, Bombay (1992).
258
6.
7. 8. 9. 10.
11. 12. 13. 14.
P.S. Kumbhar, M.R. Kharkar, G.D. Yadav and R.A. Rajadhyaksha, to be forwarded for publication. M.A. Vannice and B. Sen, J.Catal., 113 (1989) 82. S. Nishimura and I. Takashi, J.Chem.Soc.Chem.Commun., (1967) 422. P.N. Rylander i n Catalytic Hydrogenation in Organic Synthesis, Academic Press New York, 1978. H. Pines in Advances in Catalysis, 35 (1987) 323 and references therein. V.Ponec, A.Vanderberg, J.Doombois and N.J. Ken, J.Catal., 54 (1978) 243. R.J. Madix, Catal.Rev.Sci.Engg., 26 (1984) 281. J . Falconer, B. Chen and L. Chang, J.Catal., 127 (1991) 732. J.Masson, P. Cividino, J. Bonnier and P. Fouilloux in Studies in Surface Science and Catalysis 59 (Heterogeneous Catalysis and Fine Chemicals 111, M. Guisnet et al. (eds.), Elsevier, Amsterdam, 1991, pp. 245-252.
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251
Liquid phase catalytic hydrogenation of benzophenone: Role of metal support interaction, bimetallic catalysts, solvents and additives P.S. Kumbhara and the late R.A. Rajadhyaksha Department of Chemical Technology, University of Bombay, Matunga, Bombay 400 019, India. *Present address: Laboratoire de Chimie Organique Physique e t CinCtique Chimique Appliqubes, URA 418 CNRS, Ecole Nationale Supbrieure de Chimie, 8 rue de 1'Ecole Normale, 34053 Montpellier Cedex 1, France.
Abstract
Liquid-phase hydrogenation of benzophenone was studied over well characterised supported monometalic Ni and bimetallic Ni-Cu and Ni-Fe (75:25, composition by mass) catalysts. Interesting changes in selectivity were obtained over the bimetallic catalysts. Different strategies such as use of Ti02 as a support, change in solvent and use of NaOH were tested to improve the activity and selectivity to benzhydrol. A Ni-Fe(75:25)/TiOZ catalyst combined with NaOH and methano1:water as solvent was found to be the most efficient system to selectively obtain benzhydrol. 1. INTRODUCTION
The activity and selectivity of metallic catalysts is known to be influenced by alloying and metal support interaction (MSI) (1).However, these concepts have been rarely applied in liquid phase hydrogenation, which are commonly encountered i n fine chemical synthesis. The present work stemmed from our results on hydrogenation of acetophenone (2) and was undertaken t o investigate the effect of alloying, MSI, solvents and additives on the hydrogenation of benzophenone over Ni based catalysts. Product of the reaction, benzhydrol, is a n important drug intermediate. The undesired product, diphenylmethane, is obtained by hydrogenolysis of benzhydrol. Conventionally, the reaction is carried out by using Zn/NaOH as the catalytic system. However, this process produces environmentally unwanted sludge and is expensive. Recently, the reaction has been carried out using Pd catalysts poisoned with Pb or modified by phosphine ligand (3.4).
252
2.1. Catalysts Hydrogenation of benzophenone was investigated using Ni, Ni-Cu(75:25) and Ni-Fe(75:25) (composition by mass) supported on S i0 2 (Aerosil 200) and T i 0 2 (P-25) of Degussa. The catalysts were prepared by impregnation of the respective nitrates i n a n appropriate proportion by the incipient wetness technique. The total metal loading was kept constant at 20% by weight. The catalysts were calcined i n air at 723 K for 4 h and were stored in dessicator until1 further use. Prior to use required amount of the catalyst to be tested was reduced i n Hz at 523 K for 2 h followed by another 2 h a t 723 K in a specially designed reducer (5) that prevented exposure of the catalyst to air. The catalysts were characterised by H2 chemisorption, degree of reduction measurements (D.R.), temperature programmed reduction(TPR), X-ray diffraction(XRD) of r e d u c e d - p a s s i v a t e d c a t a l y s t s , j n - s i t u e x te n d e d a b s o r p t i o n fine structure(EXAFS) (Ni-Fe) and Mossbauer spectroscopy (Ni-Fe). Details of these studies will be published elsewhere(6). 2 8 Reactionpdure Hydrogenation of benzophenone (0.065 mol) (Loba Chemie, A.R. grade) was carried out in a 100 ml Parr autoclave using appropriate solvents (35.5 ml, A.R. grade) and pre-reduced catalyst (0.24 g unless otherwise specified). The hydrogenation was carried out a t hydrogen partial pressure of 60.5 kg/cm2 and 408 K. The speed of agitation was 1550 rpm a n d was chosen after confirming the absence of diffusional limitations. Samples were periodically withdrawn and analysed on a gas chromatograph (Perkin Elmer) using a 2 m x 0.33 cm carbowax (KOH washed) on chromosorb W column with a flame ionisation detector. Authentic pure samples were Lsed for product identification and quantification. In some cases th e products were separated by column chromatography an d were characterised by infra-red (IR), H-NMR a n d mass spectral (MS) analysis.
3. RESULTS AND DISCUSSION
3.1. Characterisation of catalysts
The XRD pattern indicated that Ni-CdSiOz was in a complctely FCC alloy phase. EXAFS data indicated t h a t major portion of Ni-Fe catalyst was in a FCC alloy phase with small am o u n t of unreduced FeO. Mossbauer spectroscopy showed t h a t th e Ni-Fe catalysts exist a s a mixture of supermagnetic an d ferromagnetic alloy with small amounts of Fe2+. TPR studies showed that both Cu and Fe improve the reducibility of NiO. Ni-CdSiOz was i n completely reduced state (100 % D.R.) as compared to partially reduced Ni/SiOa (76% D.R.). Compared to Ni/SiO2 both Ni-Cu a n d Ni-Fe catalysts showed decreased H2 chemisorption capacities which is characteristic of surface enrichment of Ni by Cu an d Fe respectively. Ni/"iO2 and Ni-Fe/TiOz
253
catalysts also showed suppression in H2 chemisorption, characteristic of the SMSI state (1). Details of these studies will be published elsewhere (6).
32. Hydrogenation of benzophenone in methanol
Hydrogenation of benzophenone was first studied using methanol as solvent. A typical concentration profile for the reaction over Ni/SiO2 catalyst is shown i n Figure 1.
Q
* *
0
Benzophenone Benzhydrol
1-methoxy-1,l-diphenylmethane
100
200
300
Time, min
Figure 1. Hydrogenation of benzophenone over 20% Ni/SiOz catalyst. Reaction conditions: Benzophenone, 0.065 mol, Ni/SiOz, 0.24 g; Methanol, 35.5 ml; 308 K; H2 , 60.5 kg/cin2 I n addition to th e expected hydrogenation products (benzhydrol a n d diphenylmethane), th ere was formation of a by-product i n appreciable quantities. The by-product was separated by column chromatography (silica column) a nd was identified by IR, NMR and MS as a n ether, 1-methoxy-1,ldiphenyl methane. The reaction was further studied by using SiO2 supported Ni-Cu and Ni-Fe an d Ti02 supported Ni an d Ni-Fe catalysts. The results a re summarised i n Table 1. The results are truly remarkable. There was a n increase in ether formation over Ni-Cu/Si02 an d Ni/Ti02, whereas it was negligible over Ni-Fe/SiOz. Ni/TiOe catalyst showed increased activity in agreement with th e results of
254
Vannice et al. (7) for carbonyl group hydrogenation. However, the catalyst showed increased ether formation. The most active a n d selective catalyst for formation of benzhydrol was Ni-Fe (75:25)/TiO2. The increased activity over NiA'iOz is probably due to the creation of highly active sites at the metal support interface (7). Further, increase i n activity over Ni-FeRiOz is due to synergetic metal support interaction a n d electronic effect of Fe as reported earlier for acetophenone hydrogenation (2). Table 1 Comparison of various catalysts for hydrogenation of benzophenone in presence of methanol ________---_______--____________________----------------
Catalyst
Time h
Conversion o/o
Selectivity to products (mol%l)a
.........................................
I
I1
68.3 52.0 46.4 84.5 84.5
19.7 37.5 43.7 4.6 1.7
________________________________________-------_----_---
Ni/SiOa 4.5 Ni/TiO2 2.5 Ni-Cu (75:25)/SiO$ 6.0 Ni-Fe (75:25)/Si02 4.5 Ni-Fe (75:25)/"302 2.0
89.3 92.6 83.8 85.9 86.7
________________________________________---_------------
I11
12.0 10.5 9.9 10.9 13.9
I:benzhydrol;II:ether(l-methoxy-l,l-diphenylmethane~; 111: diphenylme t hane b Catalyst weight : 0.48 g Reaction conditions : same as in Fig. 1. a
To check the ether formation pathway, hydrogenation of benzhydrol was carried out in presence of methanol under similar conditions over selected catalysts. The selectivity to ether varied a s follows: Ni-Cu (75:25)/SiO2 (highest) >> Ni/SiOa >> Ni-Fe (75:25)/Si02 (lowest) From these results i t is clear th at the ether formation occurs due to the bimolecular reaction between benzhydrol and methanol in presence of catalyst and Ha. Based on this the overall reaction scheme can be depicted a s :
255
OH
i' e
*!-o 0
n
i
y
c
1+"3OH OCH3
Benzophenone
" i i
\ e
C
H
4
Diphenylmethane
Ether, 1-metohxy- 1,l -diphenylmethane
Mechanism of ether formation
Formation of eth er (methoxycyclohexane) over Pd/C catalyst d u r in g hydrogenation of cyclohexanone in methanol was first reported by Nishimura e t al. (8). This scheme was used to prepare 4,4-dicyclohexyl methyl ether in 84% yield using Pd/C as the catalyst (9). Pines (10) in a n exhaustive study reported ether formation from various alcohols in presence of hydrogen over Ni based catalysts. He suggested th at ether formation was due to two types of sites present on Ni; one basic and the other having a n acidic character. These sites are created because of incomplete reduction of the metal. However, this reason was contradicted by Ponec et a1.(12) who showed th a t there was no co-relation between the ether formation and the reducibility of the metals, as Pt and Pd, which were completely reduced showed ether formation. I n the present case, contrary to t he arguments of Pines (lo), the competely reduced Ni-Cu/SiOz (D.R. loo%), showed increased eth er formation a s compared to partially reduced Ni/SiOz (D.R. 76% 1. Based on the present d a ta we propose the following mechanism for ether formation :
I0 : /\ 9" I 0 I CH3
Ph
r-
M I , M2 : Metal sites
MI
M2
We believe t hat ether formation is likely to occur as a result of nucleophilic attack of a methoxy species on the carbinol carbon as shown above. In Ni-Cu and Ni-Fe catalysts, the surface is known to be enriched by Cu and Fe, which was found to be also true in the present case from the hydrogen chemisorption dat a . T hus i n th e bimetallic catalysts, Ni atoms will be preferentially
256
surrounded by Cu or Fe atoms. Therefore, the above reaction is likely to involve a methoxy species bonded to Fe or Cu on the bimetallic catalysts. Evidence for this hypothesis is drawn from the d ata on heat of adsorption of methoxy species on these metals, which are as follows (12) : Fe: -87, Ni: -60, Cu: -8 kJ/mol. According to this d ata ease of formation of the methoxy species which is related to the yield of ether will be in the order Cu>Ni>Fe which indeed agrees with the results obtained i n th e present study. Similarly, increased e th e r formation on NiPTiO2 is also linked to the ease of formation of methoxy species over this catalyst (13). Another interesting point is the very small difference in the yield of the hydrogenolysis product diphenylmethane, over all the catalysts (Table 1). This is contrary to the results on C-C bond hydrogenolysis over bimetallic and Ti02 supported catalysts (1). This indicates that the sites for C - 0 hydrogenolysis are probably different from C-C bond hydrogenolysis.
35.Meet of solvents and additive
Solvents a nd additives a r e known to influence activity a n d selectivity in hydrogenation reactions. In the present study we used four solvents having different dielectric constants (methanol, i-propanol, cyclohexane a n d 10% water in methanol) to check their influence on activity a n d selectivity over the most active Ni-FeA'iO2 catalyst. The results are summarised in Table 2.
Table 2 Effect of solvents and additive (NaOH) on activity and selectivity to benzhydrol over 20%Ni-FeA'iOz catalyst. Solvent/ additive
Initial rate x 103 g m o lh g of cat.
Conversion of benzophenone o/o
Selectivity Dielectric to benzhydrol constant % of solvent
---_------------_---------------------------------------
Cyclohexane 2.63 84.0 58.8 i-Propanol 6.30 86.0 80.0 methanol 2.00 86.7 84.5 methanol+watera 4.11 90.1 91.0 methanol+NaOHb 0.61 86.2 94.3 (methanol+wate+ +NaOHb) 1.70 88.0 98.4 ________________________________________-------------_-a Water: 10% by volume of the solvent b NaOH: 0.1%by weight of benzophenone Reaction conditions : same a s in Fig. 1.
2.0 18.3 32.6 >>32.6
__
257
T he selectivity to benzhydrol is dependent on t h e solvent a n d can be correlated with the dielectric constant of the solvent; similar to earlier findings of Masson et al. (14) for acetophenone hydrogenation. Addition of NaOH results in an increase in the selectivity at the expense of drop in activity. However, this can be partly compensated by using methano1:water as a solvent. Using this combination i t was possible to achieve high selectivity to benzhydrol (98.4% selectivity at 88% conversion) at reasonable activity. 4. CONCLUSION
Interesting change in selectivity for ether ( 1-methoxy-1,l-diphenylmethane) formation during the hydrogenation of benzophenone in methanol is obtained over SiOa -supported bimetallic Ni-Cu and Ni-Fe catalysts. The formation of the ether is due to the bimolecular reaction between the product, benzhydrol a n d methanol in presence of catalyst and hydrogen. Ni-Cu (75:25)/&02 showed a n increase in ether formation whereas, Ni-Fe (75:25)/Si02 showed a suppression. The results suggest t h a t the methoxy species, h a s a possible role i n this reaction a nd i t s formation depends on the metal used. No suppresion in hydrogenolysis of the C - 0 bond was observed over bimetallic, as well a s Ti02 supported catalysts, indicating involvement of different type of sites than C-C bond hydrogenolysis. Ni-Fe (75:25)/Ti02 showed a maximum in activity a n d selectivity to benzhydrol due to the synergetic effect of MSI and the electronic effect of Fe. The nature of the solvent modifies activity and selectivity and depends on its dielectric constant. Addition of NaOH in controlled a mo u n ts improved selectivity. Benzhydrol can be obtained selectively ( > 98% ) a t high conversion of benzophenone (88%)over Ni-Fe (75:25)/Tio2 catalyst using methanol: 10% water as a solvent and NaOH(O.l%;by weight of benzophenone) as additive.
ACKNOWLEDGEMENTS Financial assistance from DST, Govt. of India is gratefuly acknowledged. PSK is thankful to UGC, Govt. of India, for senior research fellowship. PSK wishes to dedicate this paper to the memory of late Professor R.A. Rajadhyaksha.
1. 2.
3. 4. 5.
G.L. Haller and D.E. Resasco in Advances in Catalysis, 36 (1989) 173. P.S. Kumbhar, M.R. Kharkar, G.D. Yadav and R.A. Rajadhyaksha, J.Chem.Soc.Chem.Commun. (1992) 584. L.W. Gosser, US Patent No. 4302345 (1982). K. Kunerk, Ger.Offen. 2837022 (1980). P.S. Kumbhar, Ph.D. (Tech.) thesis, UDCT, Bombay (1992).
258
6.
7. 8. 9. 10.
11. 12. 13. 14.
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