Henrik Backman

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

Abstract 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 H 2 / o -xylene TPD, H 2 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.

Kinetics and modeling of o-xylene hydrogenation over Pt/γ-Al2O3 catalyst

Gas-phase hydrogenation of o-xylene to cis- and trans-dimethylcyclohexane (DMCH) was studied in a differential microreactor at atmospheric pressure and 430–520 K over a Pt/-Al2O3 catalyst. Rapid reversible deactivation of the freshly reduced catalyst prompted reactivation before every kinetic measurement and extrapolation of the steady-state activity to initial activity. Reaction orders were close to zero for o-xylene and ranged from 1.5 (430 K) to 3.0 (520 K) for hydrogen. A rate maximum for the production of the saturated compounds was observed at about 460 K. The results were in line with previously reported results on xylene hydrogenation on Ni/-Al2O3 catalyst.