Studies Concerning the Factors Affecting the Formation of Cyclohexanone Intermediates in the Catalytic Hydrogenation of Phenols. I. Hydrogenation of p-Cresol over Various Pd-C Catalysts

Article abstract of DOI:10.1246/bcsj.65.824

The catalytic hydrogenation of p-cresol with commercial Pd on active carbon (Pd-C), water-washed Pd-C (Pd-C-N), acid-treated Pd-C (Pd-C-A), and base-treated Pd-C (Pd-C-B) has been kinetically studied in cyclohexane as a solvent under 0.15-8.0 MPa hydrogen pressure at 80 deg C.Under each reaction condition, the hydrogenation rate constants for the cresol and the ketone intermediate, k₁+k₃ and k₂, respectively, the relative reactivity of the ketone to the cresol (K) and the ratio of the adsorption coefficient of the ketone to that of the cresol (b₂/b₁) were determined of the basis of a Langmuir-Hinshelwood model.Regardless of the difference in the nature of the catalysts, hydrogenations with these catalysts always gave high selectivities of 0.85-1.0 for the formation of the ketone intermediate, and small values of 0.014-0.62 for the relative reactivity (K).The small values of K over Pd-C and Pd-C-B were shown to be mostly due to the small values of k₂/(k₁+k₃).In contrast, even smaller values of K over Pd-C-N and Pd-C-A were found due to a great difference in the strength of adsorption between the cresol and the ketone intermediate.In general, both the values of K and k₂/(k₁+k₃) increase with increasing hydrogen pressure.All of the experimental results show that both the alkaline impurities associated with catalyst preparation and the hydrogen pressure are the most important factors affecting the formation of the ketone intermediate.