Effects of Alcoholic Solvents on the Formation of Cyclohexanones in the Hydrogenation of Phenols over Pd–C Catalysts

Abstract

Abstract The catalytic hydrogenation of phenol and cresols (methylphenols) with commercial Pd on active carbon (Pd–C) and acid-treated Pd–C (Pd–C–A) has been studied kinetically in t-pentyl alcohol (t-PeOH) and cyclohexane as solvents under 5.0 MPa hydrogen pressure at 120 °C. With both of the catalysts the maximum yields of the cyclohexanone intermediates as well as the hydrogenation rates of the phenols decreased markedly in t-PeOH, compared with hydrogenation in cyclohexane. With Pd–C–A in t-PeOH the decrease in the yields of cyclo-hexanone intermediates was shown to be due only to the increase in the relative reactivity of the cyclohexanones to phenols (K). On the other hand, with Pd–C the marked decreases were attributed not only to the increases in the values of K but also to the decreases in the selectivity for the formation of the ketone intermediates (f). The rates decreased in the order phenol > m- > p- > o-cresol, irrespective of the catalysts and solvents. The relative reactivities and strengths of the adsorption of phenol to isomeric cresols were evaluated by both individual and competitive hydrogenation. It is suggested that the phenols are adsorbed on the catalysts both at the benzene nucleus and the hydroxyl group, and that the remarkable effects of alcoholic solvents over basic Pd–C result from the interaction of the hydroxyl groups as phenoxide ions with alcoholic solvents which may weaken adsorption at the phenoxide ions.