HYDROGENATION OF AROMATIC-HYDROCARBONS OVER SUPPORTED PT CATALYSTS .3. REACTION MODELS FOR METAL-SURFACES AND ACIDIC SITES ON OXIDE SUPPORTS

Sixteen different reaction models were evaluated for benzene and toluene hydrogenation between 317 and 364 K over a family of supported Pt catalysts. For benzene hydrogenation on the Pt surface, only one model was consistent with all the data-that which invoked the addition of the first H atom to the aromatic ring as the rate-determining step (RDS) as well as the concurrent formation of a predominant H-deficient surface species. This model also described toluene hydrogenation on Pt. and the H-deficient species was indicated to be the phenyl (Or tolyl) group. To explain the higher rates obtained with acidic supports, a similar model involving spilled-over hydrogen and aromatic molecules adsorbed on acid sites was considered, and it accurately fit the rate data attributed to the support surface. However, another model proposing addition of the second H atom as the RDS on these acid sites and no inhibition by any H-deficient species could not be discounted. The similar activation energies of 12 ± 2 kcal/mol for benzene. toluene, and xylene hydrogenation on Pt and Pd surfaces as well as on acid sites is attributed to the formation of a cyclohexadiene intermediate; for example. 1.3-cyclohexadiene has a positive free energy of formation of 12.4 kcal/mol. This model appears to be general enough to describe the hydrogenation of numerous aromatic molecules over Group VIII metals. © 1993 Academic Press, Inc.