The hydrogenation of toluene and o-, m-, and p-xylene over palladium II. Reaction model

Abstract

The similarity among activation energies and partial pressure dependencies for the hydrogenation of benzene, toluene, and the three xylene isomers allowed the use of our previous model for benzene hydrogenation, which could be simplified for the hydrogenation of toluene and the xylenes because there was no evidence of deactivation or significant coverages of hydrogen-deficient surface inhibitors. Thus a Langmuir-Hinshelwood model involving two sets of active sites, one on the Pd surface and the other composed of acid sites in the Pd-support interfacial region supplied by spilled-over hydrogen, readily describes the kinetics for these four aromatic hydrocarbons as well as benzene. The presence of these acid sites, presumed to be Brønsted sites, was confirmed by the parallel reaction of o-xylene isomerization to m-xylene, and rate enhancements in this reaction were nearly identical to those observed for the hydrogenation of o-xylene. The derived rate expression was computer fitted to the data and the obtained values of the equilibrium adsorption constants for H 2 and these aromatic compounds are consistent with assumptions in the model and provide trends in agreement with results from the literature.