Kinetics and stereoselectivity of o-xylene hydrogenation over Pd/Al ₂O₃

Gas-phase hydrogenation of o-xylene was investigated over 1 wt.% Pd/ alumina catalyst at 430-520 K. The catalyst was characterised by H ₂-temperature programmed desorption (TPD), o-xylene-TPD, H ₂-chemisorption, scanning electron microscope (SEM)/energy dispersive X-ray analyser (EDXA) and X-ray photoelectron spectroscopy (XPS). A reversible maximum in the o-xylene hydrogenation activity versus temperature was observed which owes to decreased surface aromatic concentration as indicated by o-xylene-TPD experiments. The reaction orders with respect to o-xylene were slightly negative (ca. -0.3) in the temperature range investigated, whereas orders with respect to hydrogen increased by temperature from 1.3 (at 440 K) to 2.5 (at 520 K). The catalyst exhibited a higher selectivity towards the formation of the trans-isomer of 1,2-dimethylcyclohexane. The product stereoselectivity was found to depend on the operation temperature and reactant concentrations. Concurrent dehydrogenation and configurational isomerisation of the product took place at considerable rates at the temperatures investigated. A reaction mechanism which involves the dehydrogenation and isomerisation reactions is proposed.