Catalytic Hydrogenation of Alkylphenols for Renewable Caprolactone

Selective hydrogenation of lignin-derived alkyl-phenol to alkyl-cyclohexanone is a key step in the synthesis of renewable caprolactone from lignin-derived monomers. Selective hydrogenation of p-cresol, a model compound for lignin-derived monomers, to 4-methyl-cyclohexanone was conducted using Pd/γ-Al₂O₃ in a continuous three-phase flow reactor with side products of 4-methyl cyclohexanol. High conversion (85%) and selectivity (>93%) were demonstrated at ambient reaction pressure. Evaluation of the hydrogenation at different reaction parameters showed that the reaction selectivity was determined by the surface coverage of p-cresol and 4-methyl cyclohexanone. High selectivity to 4-methyl cyclohexanone was also attributed to the lower apparent activation barrier of p-cresol hydrogenation (67 ± 2 kJ mol^-1) compared to that of hydrogenation 4-methyl cyclohexanone (92 ± 11 kJ mol^-1), with a faster rate of initial hydrogenation of p-cresol relative to carbonyl hydrogenation of 4-methyl-cyclohexanone.