Selective hydrogenation of guaiacol to 2-methoxycyclohexanone over supported Pd catalysts†

Selective hydrogenation of guaiacol to 2-methoxycyclohexanone was investigated with various Pd catalysts. This reaction is much more difficult than the hydrogenation of phenol to cyclohexanone, namely in terms of the low reactivity of guaiacol and the reduced selectivity of 2-methoxycyclohexanone due to the demethoxylation reaction. Pd/TiO₂ catalysts were found to be superior to other supported Pd catalysts in terms of activity and selectivity to 2-methoxycyclohexanone. The Pd dispersion did not affect the selectivity of Pd/TiO₂ catalysts. Meanwhile, the increase of Pd dispersion decreased the turnover frequency, and the optimum Pd dispersion was about 25%. The presence of residual chloride ions had a negative effect on the selectivity to 2-methoxycyclohexanone. The optimal Pd/TiO₂ catalyst gave 65% yield of 2-methoxycyclohexanone. The catalyst was reusable after washing with toluene solvent to extract residual organic species from the catalyst surface. The catalyst was capable of hydrogenating various phenolic compounds, namely methoxyphenols, into the corresponding cyclohexanone derivatives.