Selective phenol hydrogenation to cyclohexanone over a dual supported Pd-Lewis acid catalyst.

Cyclohexanone is an industrially important intermediate in the synthesis of materials such as nylon, but preparing it efficiently through direct hydrogenation of phenol is hindered by over-reduction to cyclohexanol. Here we report that a previously unappreciated combination of two common commercial catalysts-nanoparticulate palladium (supported on carbon, alumina, or NaY zeolite) and a Lewis acid such as AlCl3-synergistically promotes this reaction. Conversion exceeding 99.9% was achieved with >99.9% selectivity within 7 hours at 1.0-megapascal hydrogen pressure and 50 degrees C. The reaction was accelerated at higher temperature or in a compressed CO(2) solvent medium. Preliminary kinetic and spectroscopic studies suggest that the Lewis acid sequentially enhances the hydrogenation of phenol to cyclohexanone and then inhibits further hydrogenation of the ketone.