Gas phase hydrogenation/hydrogenolysis of benzaldehyde and o-tolualdehyde over Ni/SiO2

The gas phase hydrogen treatment of benzaldehyde and o-tolualdehyde was studied over a Ni/SiO2 catalyst prepared by homogeneous precipitation/deposition. The reactions were conducted in the absence of diffusion limitations and reproducible turnover frequencies are presented. The products generated resulted from the hydrogenation and hydrogenolysis of the substituent C=O bond and from the hydrogenolysis of the aryl-carbonyl C-C. bond, the aromatic ring remaining intact. The temperature dependencies of the rate constant and product selectivity are illustrated and apparent activation energies for hydrogenation to the aromatic alcohol are given. The influence of the ortho-substituted methyl group on the reactivity of the carbonyl function is discussed. The reaction of benzyl alcohol and 2-methylbenzyl alcohol over the same catalyst was investigated and the overall reaction pathway is identified.