Gas-phase hydrogenation of o-xylene over Pt/alumina catalyst, activity, and stereoselectivity

The kinetics of the gas-phase hydrogenation of o-xylene over Pt/alumina catalysts was studied at 430-520 K, a hydrogen partial pressure of 0.19-0.36, and an o-xylene partial pressure of 0.04-0.10. The catalysts were characterized by H₂/o-xylene TPD, H₂ chemisorption, energy-dispersive X-ray analysis, and XPS. The stereoisomers, cis- and trans-1,2-dimethylcyclohexane, were the only reaction products. A reversible maximum in the o-xylene hydrogenation activity vs temperature was observed; it was explained by a decrease in the surface concentration of o-xylene at higher temperatures. The hydrogenation rate was independent of the o-xylene concentration, whereas the reaction orders with respect to hydrogen varied from 0.9 to 3 in the temperature range investigated. The stereoselectivity of the products was found to depend on temperature, reactant concentrations, and platinum precursor. The catalyst prepared from a chlorine-containing precursor exhibited a lower hydrogenation activity and selectivity toward the trans isomer. Chlorine remained on the catalyst surface, even after reduction at 673 K. Dehydrogenation and configurational isomerization of cis- to trans-1,2-dimethylcyclohexane took place at the same time as hydrogenation. Dehydrogenation and configurational isomerization reactions are enhanced by the presence of residual chlorine on the catalyst surface. The reaction pathway is proposed.