HYDROGENATION OF PHENOL TO CYCLOHEXANONE OVER PALLADIUM AND ALKALI-DOPED PALLADIUM CATALYSTS

Palladium catalysts supported on Al2O3 and MgO were prepared and used in the gas-phase hydrogenation of phenol to cyclohexanone. In all range of conditions investigated, selectivity to cyclohexanone was higher on Pd/MgO than on Pd/Al2O3. On Pd/Al2O3 the reaction rates were found to be of first order with respect to the phenol partial pressure and second order with respect to the hydrogen pressure. On Pd/MgO, instead, the order of reaction with respect to phenol was - 1 and the order with respect to hydrogen was about 1. Addition of alkali or alkaline earth metals (Ca, K, Cs) to Pd/Al2O3 increased the activity and the selectivity to cyclohexanone without modifying the orders of reaction. On the basis of the results of this study, a reaction mechanism involving adsorption of phenol on the sites of the support at the interface of the palladium metal particles is proposed. The higher selectivity to cyclohexanone found on Pd/MgO compared to Pd/Al2O3, is explained by two different forms of adsorbed phenol.