GAS-PHASE HYDROGENATION OF O-XYLENE AND P-XYLENE ON NI AL2O3 - KINETIC-BEHAVIOR/

The gas phase hydrogenation kinetics of o-xylene and p-xylene on a Ni/ Al2O3 catalyst was investigated at temperatures between 403 K and 493 K in a differential fixed bed reactor. The main products, the respecti ve cis- and trans-dimethylcyclohexanes, were formed in non-equi libriu m ratios; the amount of the trans-isomer was clearly below the equilib rium one. The hydrogenation also yielded partially saturated cyclic ol efins in non-negligible amounts (< 13% of the main products). Strong c atalyst deactivation, which was reversible in hydrogen flow at elevate d temperatures, was observed. Overall hydrogen reaction orders above t wo were calculated from extrapolated initial reaction rates at the hig hest experimental temperatures. The overall aromatic reaction order wa s always close to zero. A rate maximum in the overall reaction rate as function of temperature was observed. The results indicate that the r eaction proceeds through consecutive hydrogen addition steps on the ca talyst surface, the cyclic olefin being the key intermediate.