Gas phase hydrogenation of o- and p-xylene on Ni/Al 2O 3 — Kinetic behaviour

Abstract

The gas phase hydrogenation kinetics of o-xylene and p-xylene on a Ni/Al 2O 3 catalyst was investigated at temperatures between 403 K and 493 K in a differential fixed bed reactor. The main products, the respective cis- and trans-dimethylcyclohexanes, were formed in non-equilibrium ratios; the amount of the trans-isomer was clearly below the equilibrium one. The hydrogenation also yielded partially saturated cyclic olefins in non-negligible amounts ( < 13% of the main products). Strong catalyst deactivation, which was reversible in hydrogen flow at elevated temperatures, was observed. Overall hydrogen reaction orders above two were calculated from extrapolated initial reaction rates at the highest experimental temperatures. The overall aromatic reaction order was always close to zero. A rate maximum in the overall reaction rate as function of temperature was observed. The results indicate that the reaction proceeds through consecutive hydrogen addition steps on the catalyst surface, the cyclic olefin being the key intermediate.