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Alkali promoted regio-selective hydrogenation of styrene oxide to β-phenethyl alcohol
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
533
ALKALI P R O M O T E D REGIO-SELECTIVE HYDROGENATION OF STYRENE OXIDE TO [3-PHENETHYL A L C O H O L C.V. Rode*, M. M. Telkar and R.V.Chaudhari Homogeneous Catalysis Division, National Chemical Laboratory, Pune 411 008, India. Fax: +91 20 5893260, e-mail: [email protected] The selective hydrogenation of styrene oxide to 2-phenyl ethanol (~-Phenethyl alcohol) has been investigated using different catalysts and supports. The effect of reaction conditions such as H 2 pressure, agitation speed, concentration of substrate and temperature on the initial rate of reaction was investigated. The complete conversion of styrene oxide was obtained using 1% Pd/C, as a catalyst, under milder temperature (313K) and pressure (2.048 MPa) conditions. 2- phenyl ethanol was selectively formed when alkali was used as a promoter. A plausible mechanistic pathway has also been proposed for the hydrogenation of the styrene oxide to 2- phenyl ethanol. 1. INTRODUCTION 2-Phenyl ethanol (13-Phenethyl alcohol, PEA) is extensively used in perfumery and deodorant formulations as it possesses faint but lasting odour of rose petals[ 1]. The conventional synthetic methods for PEA involve Grignard synthesis starting from ethylene oxide and Friedel craft alkylation of benzene in presence of A1C1312]. Both these processes are multistep and suffer from the following drawbacks: Formation of side products (biphenyl) leading to poor selectivity of PEA Handling of hazardous chemicals like diethyl ether, ethylene oxide Tedious work up and recovery of pure PEA which is critical for perfumery applications. Generation of appreciable quantities of wastes due to use of A1C13. PEA can also be prepared by reduction of styrene oxide using different reducing agents in stoichiometric quantities and major side product formed in such reduction processes is secondary alcohol (phenyl carbinol)[3-6]. Recently, single step catalytic hydrogenation of styrene oxide has been reported using Raney Ni and other metal catalysts in a temperature range of 120150 oC with selectivity of PEA in the range of 60-87%. Thus, all of these routes give one or the other side products along with PEA, posing serious problems in the recovery of pure PEA which is crucial for the perfumery applications. In this paper we report a single step catalytic hydrogenation of styrene oxide using alkali promoted supported metal catalyst which gives complete conversion of styrene oxide with PEA selectivity as high as 99.9 %. The aim of our work was to screen various transition metal catalysts on different supports and study the effect of temperature, hydrogen pressure and concentration of substrate on conversion of styrene oxide and selectivity of PEA. 2. E X P E R I M E N T A L 2.1 Materials All the chemicals were procured from Aldrich, Co Ltd, USA, and the various catalysts were prepared by the procedure given elsewhere[7]. Hydrogen gas of > 99.9% purity was supplied by Indian Oxygen Ltd., Bombay.
534 2.2 Experimental set-up and procedure All the hydrogenation experiments were carried out in a 300 ml capacity SS-316 autoclave (Parr, USA) the details of which are described else where [8]. In a typical experiment, known quantities of styrene oxide, solvent, catalyst along with the promoter were charged into the autoclave and the contents were flushed twice with nitrogen and then the system was pressurised with H2 to the required pressure. The reaction was then continued at a constant pressure by supply of hydrogen from the reservoir vessel. The consumption of H2 was recorded as a function of time. The liquid samples were analysed by GC for reactant and products. 3. RESULTS AND DISCUSSION
Some initial experiments on hydrogenation of styrene oxide using 1% Pd/C catalyst showed that the selectivity of PEA was only 51% due to the formation of side products such as 1-methoxy ethyl benzene, dimethoxy ethane and 2-methoxy benzene ethanol. These side products were identified by GCMS and GCIR. In order to enhance the selectivity of PEA, a systematic study on catalyst screening, role of support, and promoter was undertaken. Table 1: Screening of catalysts Catalyst Conversion Used (%) I%Pt/C 70 1%Pd/C 100 10%Ni/C 60 10%Ni/HY 10 2%Ru/C 82
Selectivity (%) 88 99 85 -87
3.1 Screening of Catalyst
Several transition metal catalysts such as Pd, Pt, Ni, Ru were tested for their activity and selectivity for hydrogenation of styrene oxide at 313 K and 2.048 MPa pressure in presence of NaOH as a promoter and the results are presented in Table 1. It can be seen from this Table that I%Pd/C catalyst selectively gives Temp :313 K, Pressure : 2.048 MPa, Solvent: PEA as the product, with 100% conversion of MeOH, Conc of Catalyst:0.375 Kg/m~, Conc of styrene oxide. In case of other catalysts, the Styrene oxide: 0.4166 Kmol/m3, Conc of NaOH : conversion of styrene oxide was much less than 0.013 kg/m~ that for 1%Pd/C with low selectivity to 2- phenyl ethanol. The other side product formed was found to be 1- methoxy ethyl benzene which, was identified by GCMS. Pt/C and Ni/C catalysts showed almost comparable activity (70% and 60% conversion respectively), while Ni/HY catalyst showed lowest activity (10% conversion). In case of Ni/HY catalyst, from the consideration of pore size of support and the particle size of supported metal, almost all metal is expected to be on the outer surface of HY zeolite, leading to a very small surface area for the supported metal causing the lowest catalyst activity [9]. It is interesting to note that the formation of neither deoxygenated (e.g. ethyl benzene) nor any isomerisation products (e.g. 1phenethyl alcohol) was observed in the present work. The formation of such products has been reported in earlier work for other epoxy compounds [10] in which mostly the gas phase hydrogenation experiments were conducted in the temperature range of 120-180~ The absence of any deoxygenated product, in this work suggests that the metal-styrene oxide interaction is weaker particularly, for the Pd catalyst. Isomerization products were also not observed because of addition of alkali, which neutralises the acid sites responsible for the isomerisation which is said to be a parallel reaction with hydrogenation of epoxy compounds [ 11 ]. 3.2 Effect of supports In this work the role of support such as carbon, alumina, silica and zeolite-ZSM-5, was investigated for 1% Pd catalyst and the results are shown in Figure 1. For all the supports
535 studied, 100% selectivity to 2- phenyl Cony ethanol was obtained in presence of NaOH looSel. while the catalytic activity varied in the order C >A1203 >SiO2 >ZSM-5. It is known that in 0~80a basic medium, (pH range of this work was 11-12) the activated carbon support is stable ~> but alumina and silica are likely to dissolve ~ 60undergoing structural changes[ 12]. This may be the reason for decrease in activity of the fi 40 catalysts when supported on alumina and silica [13]. When zeolite was used as a support, the channel dimension of ZSM-5 20 (5.4 x 5.6 and 5.8 x 5.2 A ~ does not allow the penetration of styrene oxide due to its 0 Carbon Alumina silica ZSM-5 larger diameter (7.32A ~ to get adsorbed on Fig 1 Effect of supports on conversion and selectivity the entire surface (external + pore surface) of Temp: 313 K, Pressure: 2.048 MPa, Solvent: MeOH, the catalyst[13]. This is a probable explanation Conc. of Styrene oxide: 0.4166 kmol/m3, Conc. of catalyst: 0.375 Kg/m3,Conc.ofNaOH:0.013Kg/m3 and more work is required for a clear understanding of the observed trends. Since 1%Pd/C was the most active catalyst, detailed investigation on the effect of solvent, promoter, its concentration, temperature, hydrogen pressure, etc. was carried out using this catalyst, and the results are discussed in the following sections. .,I
O
3.3 Effect of solvents Solvents such as methanol, hexane and 1-4 dioxane were screened for hydrogenation of styrene oxide. In a protic solvent, methanol the conversion obtained was 100% while, in aprotic solvents such as hexane and 1-4 dioxane the conversion obtained was 75% and 33% respectively. This can be explained in two ways, i) Solubility of hydrogen is higher in methanol hence, highest conversion was obtained in methanol ii) and the protonated diol gets attacked by hydride to give 2-phenyl ethanol. As the protonation increases the hydride attack is easier therefore, leading to highest conversion of styrene oxide in methanol. 3.4 Effect of Promoters Nucleophilic promoters are believed to play a key role in the hydrogenation of epoxides. The role of various organic and inorganic promoters was investigated and the results are given in Table 2. It was found that in absence of a promoter and methanol as a solvent though, the conversion of styrene oxide obtained was 99%, the selectivity of PEA was only 51%. Besides PEA, other side products obtained were 1-methoxy ethyl benzene and 1,2- dimethoxy ethyl benzene. For all the promoters studied in this work the selectivity to 2-phenyl ethanol achieved was above 95% and in some cases even >99% however, the level of conversion of styrene oxide varied, giving complete conversion with only sodium hydroxide as a promoter.
3.5Effect of H2 Pressure Figure 2 shows the effect of pressure of hydrogen on the initial rate of reaction, for different temperatures, I%Pd/C catalyst and NaOH as the promoter. It was observed that initially as the pressure increases the rate of reaction also increases to a maximum (3.44 MPa) and then
536
drops down with further increase in H 2 pressure indicating the possibility of hydrogen inhibited kinetics at higher pressure. At low pressure (< 3.44 MPa) both styrene oxide and H2 would be chemisorbed on the catalyst surface with some free active sites also available. As the H2presure increases the rate would increase until all surface sites are occupied by hydrogen. Further increase in the H 2 pressure, would cause the adsorbed styrene oxide to be swept away which 4 would result in decrease in hydrogenation rate. "7 9 40~ O t~
Table 2: Screening of promoters Promoter used
Conv.
Selec.
(%)
(%)
NaOH
99.9 Na2CO 3 47.0 Quinoline 36.8 Pyridine 64.3 Triethylamine 70.2 Diethylamine 47.5 Dimethylamine 55.0 Without 99.0 promoter . . . . . . . . . . . . . . .
50~
x
9
-'63
......
99.9 97.6 96.9 94.3 99.6 99.9 99.8 51.2
Temp. 313 K, Pressure : 2.048MPa, Solvent : MeOH, Conc. of styrene oxide : 0.4166 Kmol/m 3 Conc. of catalyst: 0.375 Kg/m 3, Conc. of promoter :0.013 Kg/m 3.
X
0
i lib
0
,
i
1
9
i
9
i
9
i
2 3 4 Pressure, MPa
9
i
,
5
6
Fig 2 Effect of pressure on initial rate of reaction. Temp. 313 K, Solvent : MeOH, Conc. of styrene oxide : 0.4166 Kmol/m 3 Conc. of catalyst : 0.375 Kg/m 3, Conc. of NaOH :0.013 Kg/m 3.
3.6 Effect of substrate concentration The effect of concentration of substrate on the initial rate of reaction was studied and the results are shown in Figure 3. Initially, the rate of reaction increases as the concentration of styrene oxide increases upto 1.2 x 1 0 - 4 K m o l / m 3 beyond which the rate decreases, with further increase in substrate concentration. This effect is more pronounced at higher temperature (333K). Similar observation made for H2 effect on rate of hydrogenation indicate that the adsorption of both styrene oxide as well as Hz is important and need to be considered. 3.7 Effect of Temperature The effect of temperature on both selectivity of PEA and the rate of hydrogenation was studied in a temperature range of 313-333 K. The selectivity of PEA was found to be unaffected at all the temperatures
.7~ I .~E3
~~~,,~
B 40~ X 50~ 60~
o 0
1 2 Concentration of substrate, kmoCm3
3
Fig 3 Effect of concentration of substrate on initial rate of reaction. Temp.: 313 K, Pressure: 2.0148 MPa, Solvent: MeOH, Conc. of catalyst: 0.375 kg/m 3, con. of NaOH: 0.013 kg/m 3
537 while, the initial rate of hydrogenation increased with increase in the temperature and the activation energy evaluated from the Arrhenius plot was found to be 55.4 KJ/mol. 4. P R O P O S E D M E C H A N I S M
The reactions of epoxy compounds with H 2 in the presence of supported metal catalysts are known to give deoxygenated, isomerised and hydrogenated products. In our work, formation of ethyl benzene or styrene (deoxygenated products) was not observed hence, the strong adsorption of oxygen to the catalyst surface is not expected. Notheisz, et al. also reported that the metal-epoxy oxygen interaction was found to be weaker in case of Pd catalysts [14]. Moreover, the presence of NaOH on the catalyst surface decreases the adsorptivity of the epoxy oxygen resulting in higher selectivity of the desired alcohol (PEA). The absence of isomersied products is due to the neutralisation of acid centres (if any) by added alkali. Many authors have described mechanism of ring opening of oxiranes. Among them Bartok described the opening ofoxacycloalkanes in acidic medium to give different products with secondary alcohol as a major product [ 10]. Mitsui, et al. have explained deoxygenation of styrene oxide involving the radical cleavage reaction [15]. They have suggested two different mechanisms for explaining the formation of PEA, one on the basis of radical cleavage and the other involving SN 2 mechanism. In their work none of the intermediates could be separated or characterised and also the role of NaOH as a promoter in the reaction mechanism was not clearly understood. In our work the formation of only 2- phenyl ethanol indicates the regio selective opening of the C-O bond which is less hindered i.e. distant from the subsituents which is normally observed in the case of Pd and Pt metal catalysts [ 16]. The addition of NaOH is also responsible for the formation of PEA, because it neutralises the acidic sites reponsible for isomersization products (ketone in this case) which after hydrogenation give secondary alcohol. The regioselective formation of 2- phenyl ethanol can be explained based on two different reaction pathways as shown in schemes I and II. Scheme I, involves formation of n benzyl complex from the adsorbed styrene oxide. The rc benzyl complex (2) yields an alkoxide ion (3), which is stabilised by NaOH. The alkoxide ion on protonation with a solvent like methanol gives selectively 2-phenyl ethanol. According to this mechanism, the cleavage of C-O bond is postulated to be from the more substituted side, which is normally not the case for Pd catalyst. However this has been proposed by Mitsui, et al [ 10]. Scheme I
(1)
(2)
(3)
In scheme II S N 2 attack of OH- is proposed, leading to the cleavage of C-O bond from the less hindered side. The secondary alkoxide ion (4) formed in this ease then yields an intermediate 2-phenyl ethane diol (5) which on hydride attack gives selectively 2- phenyl Scheme II ,.
~
,
H
(4) (5) ethanol. Both the mechanistic pathways may contribute simultaneously to regioselective formation of PEA. However, the probability of Scheme II operating seems to be more because
538 i) the cleavage of C-O bond is from the less hindered side. ii) C2 carbon of 2- phenyl ethane diol is more electropositive than C, carbon atom due to the electronegativity of phenyl ring hence, the hydride attack on C2 atom is favored to give selectively PEA. iii) In a separate experiment, styrene oxide was refluxed in aqueous sodium hydroxide for 3 hours to give 2- phenyl ethane diol (5) which was separated and well characterized. This diol was isolated and then further hydrogenated using 1%Pd/C catalyst, in methanol as a solvent to give 2-phenyl ethanol. 5. CONCLUSIONS The hydrogenation of styrene oxide in presence of Pd/C catalyst and NaOH as a promoter under very mild conditions was found to be regioselective to give only 2- phenyl ethanol as the product. A systematic study on screening of catalysts, promoters, solvents and the effect of major reaction parameters such as H z pressure, substrate concentration and temperature on the catalyst activity and selectivity was carried out. Speculative reaction pathways have been proposed for the regioselective formation of 2- phenyl ethanol. REFERENCES
1. B. D. Mookherjee and R. A Wilson, Kirk othmer (eds.) Encyclopedia of chemical technology, 4 ed, John Wiley, New York, Vol. 4, 1996. 2. E.T. Theimer, Fragrance chemistry, Academic Press, New York, 1982. 3. E. L. Eliel and D.W. Delmonte, J. Am. Chem. Soc.,78 (1956) 3226. 4. M. L Mihailovic, V. Andrejevic and J. Milovanoic, Helv. chim. Acta., 69 (1976) 2305. 5. A. Okawa and H. K. Soai, Bull. Chem. Soc. Jpn., 60 (1987) 1813. 6. S. Krishnamurthy, R. M. Schubert and H. C. Brown, J. Am. Chem. Soc., 95 (1973) 8486. 7. R. Mozingo and E. C. Homing, (eds) Organic Synthesis collective volume, 3, John Willey, London, 1956. 8. C. V. Rode, S. P. Gupte, R. V. Chaudhari, C. D. Pirozhkov and A.L Lapidus, J. Mol. Cat., 91 (1994) 195. 9. M. V. Rajshekharam, C.V. Rode, M. Arai, S.G. Hegde and R.V. Chaudhari, Appl. Cat. A: General 195 (2000) 1. 10. M. Bartok, F. Notheisz, A. G. Zsigmond and G.V. Smith, J. Cat. 100 (1986) 39. 11. H. Davidova and M. Kraus, J. Cat. 61 (1980) 1. 12. M. Bartok, Catalyst supports and supported catalyst: Theoretical aspects and applied concepts, (eds). Alvin Stiles Butterworth, New York, (1987). 13. R. Augustine, Heterogenous catalyst for synthetic chemist, Marcel Dekker, New York, 1996. 14. R.E. Malz and H. Heinemann (eds), Marcel Dekkar, New York, 1996. 15. S. Mitsui, S. M Imaizumi and Y. Sugi, Tetrahedron, 29 (1973) 4093. 16. F. Notheisz, A. Molnar, A.G. Zsigrnond and M. Bartok, J. Catal, 131 (1986) 98.
533
ALKALI P R O M O T E D REGIO-SELECTIVE HYDROGENATION OF STYRENE OXIDE TO [3-PHENETHYL A L C O H O L C.V. Rode*, M. M. Telkar and R.V.Chaudhari Homogeneous Catalysis Division, National Chemical Laboratory, Pune 411 008, India. Fax: +91 20 5893260, e-mail: [email protected] The selective hydrogenation of styrene oxide to 2-phenyl ethanol (~-Phenethyl alcohol) has been investigated using different catalysts and supports. The effect of reaction conditions such as H 2 pressure, agitation speed, concentration of substrate and temperature on the initial rate of reaction was investigated. The complete conversion of styrene oxide was obtained using 1% Pd/C, as a catalyst, under milder temperature (313K) and pressure (2.048 MPa) conditions. 2- phenyl ethanol was selectively formed when alkali was used as a promoter. A plausible mechanistic pathway has also been proposed for the hydrogenation of the styrene oxide to 2- phenyl ethanol. 1. INTRODUCTION 2-Phenyl ethanol (13-Phenethyl alcohol, PEA) is extensively used in perfumery and deodorant formulations as it possesses faint but lasting odour of rose petals[ 1]. The conventional synthetic methods for PEA involve Grignard synthesis starting from ethylene oxide and Friedel craft alkylation of benzene in presence of A1C1312]. Both these processes are multistep and suffer from the following drawbacks: Formation of side products (biphenyl) leading to poor selectivity of PEA Handling of hazardous chemicals like diethyl ether, ethylene oxide Tedious work up and recovery of pure PEA which is critical for perfumery applications. Generation of appreciable quantities of wastes due to use of A1C13. PEA can also be prepared by reduction of styrene oxide using different reducing agents in stoichiometric quantities and major side product formed in such reduction processes is secondary alcohol (phenyl carbinol)[3-6]. Recently, single step catalytic hydrogenation of styrene oxide has been reported using Raney Ni and other metal catalysts in a temperature range of 120150 oC with selectivity of PEA in the range of 60-87%. Thus, all of these routes give one or the other side products along with PEA, posing serious problems in the recovery of pure PEA which is crucial for the perfumery applications. In this paper we report a single step catalytic hydrogenation of styrene oxide using alkali promoted supported metal catalyst which gives complete conversion of styrene oxide with PEA selectivity as high as 99.9 %. The aim of our work was to screen various transition metal catalysts on different supports and study the effect of temperature, hydrogen pressure and concentration of substrate on conversion of styrene oxide and selectivity of PEA. 2. E X P E R I M E N T A L 2.1 Materials All the chemicals were procured from Aldrich, Co Ltd, USA, and the various catalysts were prepared by the procedure given elsewhere[7]. Hydrogen gas of > 99.9% purity was supplied by Indian Oxygen Ltd., Bombay.
534 2.2 Experimental set-up and procedure All the hydrogenation experiments were carried out in a 300 ml capacity SS-316 autoclave (Parr, USA) the details of which are described else where [8]. In a typical experiment, known quantities of styrene oxide, solvent, catalyst along with the promoter were charged into the autoclave and the contents were flushed twice with nitrogen and then the system was pressurised with H2 to the required pressure. The reaction was then continued at a constant pressure by supply of hydrogen from the reservoir vessel. The consumption of H2 was recorded as a function of time. The liquid samples were analysed by GC for reactant and products. 3. RESULTS AND DISCUSSION
Some initial experiments on hydrogenation of styrene oxide using 1% Pd/C catalyst showed that the selectivity of PEA was only 51% due to the formation of side products such as 1-methoxy ethyl benzene, dimethoxy ethane and 2-methoxy benzene ethanol. These side products were identified by GCMS and GCIR. In order to enhance the selectivity of PEA, a systematic study on catalyst screening, role of support, and promoter was undertaken. Table 1: Screening of catalysts Catalyst Conversion Used (%) I%Pt/C 70 1%Pd/C 100 10%Ni/C 60 10%Ni/HY 10 2%Ru/C 82
Selectivity (%) 88 99 85 -87
3.1 Screening of Catalyst
Several transition metal catalysts such as Pd, Pt, Ni, Ru were tested for their activity and selectivity for hydrogenation of styrene oxide at 313 K and 2.048 MPa pressure in presence of NaOH as a promoter and the results are presented in Table 1. It can be seen from this Table that I%Pd/C catalyst selectively gives Temp :313 K, Pressure : 2.048 MPa, Solvent: PEA as the product, with 100% conversion of MeOH, Conc of Catalyst:0.375 Kg/m~, Conc of styrene oxide. In case of other catalysts, the Styrene oxide: 0.4166 Kmol/m3, Conc of NaOH : conversion of styrene oxide was much less than 0.013 kg/m~ that for 1%Pd/C with low selectivity to 2- phenyl ethanol. The other side product formed was found to be 1- methoxy ethyl benzene which, was identified by GCMS. Pt/C and Ni/C catalysts showed almost comparable activity (70% and 60% conversion respectively), while Ni/HY catalyst showed lowest activity (10% conversion). In case of Ni/HY catalyst, from the consideration of pore size of support and the particle size of supported metal, almost all metal is expected to be on the outer surface of HY zeolite, leading to a very small surface area for the supported metal causing the lowest catalyst activity [9]. It is interesting to note that the formation of neither deoxygenated (e.g. ethyl benzene) nor any isomerisation products (e.g. 1phenethyl alcohol) was observed in the present work. The formation of such products has been reported in earlier work for other epoxy compounds [10] in which mostly the gas phase hydrogenation experiments were conducted in the temperature range of 120-180~ The absence of any deoxygenated product, in this work suggests that the metal-styrene oxide interaction is weaker particularly, for the Pd catalyst. Isomerization products were also not observed because of addition of alkali, which neutralises the acid sites responsible for the isomerisation which is said to be a parallel reaction with hydrogenation of epoxy compounds [ 11 ]. 3.2 Effect of supports In this work the role of support such as carbon, alumina, silica and zeolite-ZSM-5, was investigated for 1% Pd catalyst and the results are shown in Figure 1. For all the supports
535 studied, 100% selectivity to 2- phenyl Cony ethanol was obtained in presence of NaOH looSel. while the catalytic activity varied in the order C >A1203 >SiO2 >ZSM-5. It is known that in 0~80a basic medium, (pH range of this work was 11-12) the activated carbon support is stable ~> but alumina and silica are likely to dissolve ~ 60undergoing structural changes[ 12]. This may be the reason for decrease in activity of the fi 40 catalysts when supported on alumina and silica [13]. When zeolite was used as a support, the channel dimension of ZSM-5 20 (5.4 x 5.6 and 5.8 x 5.2 A ~ does not allow the penetration of styrene oxide due to its 0 Carbon Alumina silica ZSM-5 larger diameter (7.32A ~ to get adsorbed on Fig 1 Effect of supports on conversion and selectivity the entire surface (external + pore surface) of Temp: 313 K, Pressure: 2.048 MPa, Solvent: MeOH, the catalyst[13]. This is a probable explanation Conc. of Styrene oxide: 0.4166 kmol/m3, Conc. of catalyst: 0.375 Kg/m3,Conc.ofNaOH:0.013Kg/m3 and more work is required for a clear understanding of the observed trends. Since 1%Pd/C was the most active catalyst, detailed investigation on the effect of solvent, promoter, its concentration, temperature, hydrogen pressure, etc. was carried out using this catalyst, and the results are discussed in the following sections. .,I
O
3.3 Effect of solvents Solvents such as methanol, hexane and 1-4 dioxane were screened for hydrogenation of styrene oxide. In a protic solvent, methanol the conversion obtained was 100% while, in aprotic solvents such as hexane and 1-4 dioxane the conversion obtained was 75% and 33% respectively. This can be explained in two ways, i) Solubility of hydrogen is higher in methanol hence, highest conversion was obtained in methanol ii) and the protonated diol gets attacked by hydride to give 2-phenyl ethanol. As the protonation increases the hydride attack is easier therefore, leading to highest conversion of styrene oxide in methanol. 3.4 Effect of Promoters Nucleophilic promoters are believed to play a key role in the hydrogenation of epoxides. The role of various organic and inorganic promoters was investigated and the results are given in Table 2. It was found that in absence of a promoter and methanol as a solvent though, the conversion of styrene oxide obtained was 99%, the selectivity of PEA was only 51%. Besides PEA, other side products obtained were 1-methoxy ethyl benzene and 1,2- dimethoxy ethyl benzene. For all the promoters studied in this work the selectivity to 2-phenyl ethanol achieved was above 95% and in some cases even >99% however, the level of conversion of styrene oxide varied, giving complete conversion with only sodium hydroxide as a promoter.
3.5Effect of H2 Pressure Figure 2 shows the effect of pressure of hydrogen on the initial rate of reaction, for different temperatures, I%Pd/C catalyst and NaOH as the promoter. It was observed that initially as the pressure increases the rate of reaction also increases to a maximum (3.44 MPa) and then
536
drops down with further increase in H 2 pressure indicating the possibility of hydrogen inhibited kinetics at higher pressure. At low pressure (< 3.44 MPa) both styrene oxide and H2 would be chemisorbed on the catalyst surface with some free active sites also available. As the H2presure increases the rate would increase until all surface sites are occupied by hydrogen. Further increase in the H 2 pressure, would cause the adsorbed styrene oxide to be swept away which 4 would result in decrease in hydrogenation rate. "7 9 40~ O t~
Table 2: Screening of promoters Promoter used
Conv.
Selec.
(%)
(%)
NaOH
99.9 Na2CO 3 47.0 Quinoline 36.8 Pyridine 64.3 Triethylamine 70.2 Diethylamine 47.5 Dimethylamine 55.0 Without 99.0 promoter . . . . . . . . . . . . . . .
50~
x
9
-'63
......
99.9 97.6 96.9 94.3 99.6 99.9 99.8 51.2
Temp. 313 K, Pressure : 2.048MPa, Solvent : MeOH, Conc. of styrene oxide : 0.4166 Kmol/m 3 Conc. of catalyst: 0.375 Kg/m 3, Conc. of promoter :0.013 Kg/m 3.
X
0
i lib
0
,
i
1
9
i
9
i
9
i
2 3 4 Pressure, MPa
9
i
,
5
6
Fig 2 Effect of pressure on initial rate of reaction. Temp. 313 K, Solvent : MeOH, Conc. of styrene oxide : 0.4166 Kmol/m 3 Conc. of catalyst : 0.375 Kg/m 3, Conc. of NaOH :0.013 Kg/m 3.
3.6 Effect of substrate concentration The effect of concentration of substrate on the initial rate of reaction was studied and the results are shown in Figure 3. Initially, the rate of reaction increases as the concentration of styrene oxide increases upto 1.2 x 1 0 - 4 K m o l / m 3 beyond which the rate decreases, with further increase in substrate concentration. This effect is more pronounced at higher temperature (333K). Similar observation made for H2 effect on rate of hydrogenation indicate that the adsorption of both styrene oxide as well as Hz is important and need to be considered. 3.7 Effect of Temperature The effect of temperature on both selectivity of PEA and the rate of hydrogenation was studied in a temperature range of 313-333 K. The selectivity of PEA was found to be unaffected at all the temperatures
.7~ I .~E3
~~~,,~
B 40~ X 50~ 60~
o 0
1 2 Concentration of substrate, kmoCm3
3
Fig 3 Effect of concentration of substrate on initial rate of reaction. Temp.: 313 K, Pressure: 2.0148 MPa, Solvent: MeOH, Conc. of catalyst: 0.375 kg/m 3, con. of NaOH: 0.013 kg/m 3
537 while, the initial rate of hydrogenation increased with increase in the temperature and the activation energy evaluated from the Arrhenius plot was found to be 55.4 KJ/mol. 4. P R O P O S E D M E C H A N I S M
The reactions of epoxy compounds with H 2 in the presence of supported metal catalysts are known to give deoxygenated, isomerised and hydrogenated products. In our work, formation of ethyl benzene or styrene (deoxygenated products) was not observed hence, the strong adsorption of oxygen to the catalyst surface is not expected. Notheisz, et al. also reported that the metal-epoxy oxygen interaction was found to be weaker in case of Pd catalysts [14]. Moreover, the presence of NaOH on the catalyst surface decreases the adsorptivity of the epoxy oxygen resulting in higher selectivity of the desired alcohol (PEA). The absence of isomersied products is due to the neutralisation of acid centres (if any) by added alkali. Many authors have described mechanism of ring opening of oxiranes. Among them Bartok described the opening ofoxacycloalkanes in acidic medium to give different products with secondary alcohol as a major product [ 10]. Mitsui, et al. have explained deoxygenation of styrene oxide involving the radical cleavage reaction [15]. They have suggested two different mechanisms for explaining the formation of PEA, one on the basis of radical cleavage and the other involving SN 2 mechanism. In their work none of the intermediates could be separated or characterised and also the role of NaOH as a promoter in the reaction mechanism was not clearly understood. In our work the formation of only 2- phenyl ethanol indicates the regio selective opening of the C-O bond which is less hindered i.e. distant from the subsituents which is normally observed in the case of Pd and Pt metal catalysts [ 16]. The addition of NaOH is also responsible for the formation of PEA, because it neutralises the acidic sites reponsible for isomersization products (ketone in this case) which after hydrogenation give secondary alcohol. The regioselective formation of 2- phenyl ethanol can be explained based on two different reaction pathways as shown in schemes I and II. Scheme I, involves formation of n benzyl complex from the adsorbed styrene oxide. The rc benzyl complex (2) yields an alkoxide ion (3), which is stabilised by NaOH. The alkoxide ion on protonation with a solvent like methanol gives selectively 2-phenyl ethanol. According to this mechanism, the cleavage of C-O bond is postulated to be from the more substituted side, which is normally not the case for Pd catalyst. However this has been proposed by Mitsui, et al [ 10]. Scheme I
(1)
(2)
(3)
In scheme II S N 2 attack of OH- is proposed, leading to the cleavage of C-O bond from the less hindered side. The secondary alkoxide ion (4) formed in this ease then yields an intermediate 2-phenyl ethane diol (5) which on hydride attack gives selectively 2- phenyl Scheme II ,.
~
,
H
(4) (5) ethanol. Both the mechanistic pathways may contribute simultaneously to regioselective formation of PEA. However, the probability of Scheme II operating seems to be more because
538 i) the cleavage of C-O bond is from the less hindered side. ii) C2 carbon of 2- phenyl ethane diol is more electropositive than C, carbon atom due to the electronegativity of phenyl ring hence, the hydride attack on C2 atom is favored to give selectively PEA. iii) In a separate experiment, styrene oxide was refluxed in aqueous sodium hydroxide for 3 hours to give 2- phenyl ethane diol (5) which was separated and well characterized. This diol was isolated and then further hydrogenated using 1%Pd/C catalyst, in methanol as a solvent to give 2-phenyl ethanol. 5. CONCLUSIONS The hydrogenation of styrene oxide in presence of Pd/C catalyst and NaOH as a promoter under very mild conditions was found to be regioselective to give only 2- phenyl ethanol as the product. A systematic study on screening of catalysts, promoters, solvents and the effect of major reaction parameters such as H z pressure, substrate concentration and temperature on the catalyst activity and selectivity was carried out. Speculative reaction pathways have been proposed for the regioselective formation of 2- phenyl ethanol. REFERENCES
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