Stereoselective hydrogenation of p-tert-butylphenol over supported rhodium catalyst

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Abstract

p-tert-Butylcyclohexanol is used as an intermediate for manufacturing esters with lower carboxylic acids, which are important in perfumery, soap and detergents manufacturing. Stereoselective hydrogenation of p-tert-butylphenol (PTBP) to cis and trans-p-tert-butylcyclohexanol (PTBCH) has been carried out by using supported metal catalysts such as rhodium supported on carbon (2% Rh/C), Raney nickel, nickel supported on silica (20% Ni/SiO2), and palladium supported on carbon (5% Pd/C). Among the variety of catalysts used 2% Rh/C exhibited 100% conversion of PTBP and 100% selectivity of the cis-isomer of PTBCH when methanesulphonic acid was used as the co-catalyst. The reaction was 100% selective towards the product. The effects of various parameters on the rates of reaction were studied systematically and a kinetic model was built. The reaction was found to be kinetically controlled. The formation of cis-isomer was explained theoretically.

Introduction

Stereoselective hydrogenation of aromatic compounds is a problem of great importance in the production of fine chemicals. The stereochemistry of different heterogeneous catalytic reactions has remained a subject of considerable interest throughout the years [1]. In such catalytic systems, the activity and stereoselectivity has been seen to depend on the nature of the catalyst and of the compound to be hydrogenated, reaction conditions, solvents, etc. Several studies have focused on the liquid-phase catalytic hydrogenation of monoalkylphenols. In this case, industrial interests attaches only to the cis-form of the corresponding monoalkylcyclohexanols as they serve as important intermediates for the fragrance industry [2].

4-tert-Butylcyclohexanol (PTBCH), a chemical of wide industrial importance, is conventionally prepared by catalytic hydrogenation of 4-tert-butylphenol (PTBP). It is used as an intermediate for manufacturing esters with lower carboxylic acids, which are important in perfumery, soap and detergent manufacturing [3]. There are various reports in literature on hydrogenation of 4-tert-butylphenol using supported and unsupported metal catalysts. Konuspaev et al. [4] synthesized PTBCH with a cis/trans ratio of 1:2 by hydrogenation of PTBP over unsupported rhodium catalyst. Sekiguchi and Tanaka [5] have claimed 93.4% PTBCH with a cis/trans ratio of 89.9:10.1 in the presence of Rh/C and HCl. In another claim, Sekiguchi and Tanaka [6] have reported 93.1% PTBCH with a cis/trans ratio of 91.1:8.9 over Rh/C and HBr. Vogtman et al. [7] hydrogenated PTBP to give 99.3% PTBCH containing 28% cis-isomer in the presence of nickel supported on alumina. Furumoto et al. [8] synthesized 99.2% PTBCH at 150 °C and 3 MPa in the presence of modified nickel catalyst. Morikawa [9] has obtained 97% PTBCH containing 58% cis in presence of Raney Ru catalyst. In another claim, 95.5% PTBCH containing 82.1% cis-isomer was obtained over Rh/C and HClO4 [10].

This work is concerned with experimental data on cis selectivity and the corresponding kinetic modeling for 4-tert-butylphenol hydrogenation over a supported rhodium catalyst.

Section snippets

Chemicals and catalysts

p-tert-Butylpnenol, methanesulphonic acid and isopropanol were obtained from M/s S.D. Fine Chemicals, Mumbai. p-tert-Butylcyclohexanol was obtained from M/s Hindustan Polyamides and Fibres, Mumbai, as a gift sample. Rhodium supported on carbon (2% Rh/C) was procured from M/s S.K. Agarwal, Mumbai. Raney nickel was obtained from Kalin Industries, Mumbai. Supported Ni catalyst (20% Ni/SiO2) was procured from M/s Monarch Chemicals, Mumbai. All chemicals were analytical grade reagents and used

Effect of different catalysts

The published information suggested that an acid should be used as a co-catalyst to improve the selectivity of the cis-isomer and acids such as H2SO4, HCl, HBr and HClO4 were employed. It was thus thought prudent to employ an organic acid such as carboxylic acids or sulphonic acids. The preliminary experiments indicated that MSA was a good co-catalyst which was used to evaluate the efficacy of all catalysts. A further discussion on the role of MSA will be provided later.

The catalysts used were

Kinetics of the reaction

The addition of MSA on 2% Rh/C changes the cis/trans ratio and in fact, as the MSA concentration is increased, the trans content goes on reducing and finally only cis-isomer is found. Thus, the role of MSA is to alter the activity of the sites particularly for the adsorption of PTBP. There should be two different types of sites S1 and S2 on the catalyst surface; one the sites covered by MSA and other the metal sites.

Conclusions

The kinetics of liquid-phase hydrogenation of 4-tert-butylphenol over metal catalysts was investigated and a kinetic model is built. In the hydrogenation of 4-tert-butylphenol with Rh/C only cis- and trans-4-tert-butylcyclohexanol were produced and the production of corresponding cyclohexanone was not observed. Among the variety of catalysts used 2% Rh/C exhibited 100% conversion of PTBP and 100% selectivity of the cis-isomer of PTBCH when methanesulphonic acid was used as the co-catalyst. The

Acknowledgements

PKG thanks the University Grants Commission for awarding a Senior Research Fellowship. GDY thanks the Darbari Seth Professorship Endowment.

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