Influence of the Charge Transfer Capacity of Alkali and Alkaline Earth Metals as Promoters in Thehydrogenation of Phenol over Palladium and Nickel Catalysts

The gas phase hydrogenation (523-573 K) of phenol has been studied over 1 wt.% Pd/Al₂O₃ and 1 wt.% Ni/SiO₂ catalysts doped with Group I and II promoter oxides. A direct correlation between catalytic activity and the charge transfer capacity of the promoters is presented where hydrogenation is favored by increasing electron donation from the promoter. The Pd catalysts generated cyclohexanone (selectivity > 97%) as the predominant product; selectivity was unaffected by the presence of the alkali or alkaline earth dopants. The Ni system exhibited appreciable hydrogenolysis behavior and charge transfer from the dopants limited the degree of hydrodeoxygenation to favor complete hydrogenation to cyclohexanol.