GAS-PHASE HYDROGENATION OF ORTHO-XYLENE OVER PALLADIUM CATALYSTS

Gas phase hydrogenation of o-xylene over palladium supported catalysts was studied. Catalysts were characterized by hydrogen and carbon monoxide chemisorption and hydrogen-oxygen titrations. The supports used were silica, γ-alumina and silica-alumina. The products of the reaction were cis- and (trans-1,2-dimethylcyclohexane and -1,2-dimethylcyclohexene. The selectivity was modified by temperature, metal dispersion, and acidity of the support, the production of the trans isomer increasing with an increase in each of these three factors. On the contrary, the presence of electron-donor molecules, such as pyridine, increased the selectivity to the cis isomer. The production off trans-1,2-dimethylcyclohexane may be adequately explained by the 'roll-over' of the intermediate species. It is proposed that the electron density of palladium is the determinant factor for the stereoselectivity of the catalysts. © 1990.