The hydrogenation of o-, m-, and p-xylene over Ni/SiO2

The gas phase hydrogenation of o-, m-, and p-xylene was studied over a Ni/SiO2 catalyst prepared by homogeneous precipitation/deposition. The hydrogenation of each xylene yielded steroisomeric product mixtures of the saturated dimethylcyclohexane. The stereospecificity of the reaction is related to the nature of the reactant/catalyst interaction which governs the mode of addition of hydrogen to the carbons bearing the methyl substituents. The appearance of a common well-defined reversible maximum (T-max) in the rate vs. temperature plots is reported and discussed. Reaction orders with respect to xylene partial pressures are plotted as a function of reaction temperature. The temperature dependence of the rate constants was fitted to an Arrhenius equation and generated positive (T less than or equal to T-max) and negative (T greater than or equal to T-max) activation energies. The derivation of true activation energies and heats of adsorption from the kinetic data is presented. Turnover frequencies at a particular temperature decreased in the order p-xylene > m-xylene > o-xylene. The respective roles of steric and electronic effects in determining the strength of adsorption and surface reactivity are discussed. A compensation effect, which is established for the experimentally determined or apparent kinetic parameters, is attributed to variations in the temperature dependencies of the surface concentration of the reactive species. (C) 1997 Academic Press.