KINETICS OF TOLUENE HYDROGENATION ON NI/AL2O3 CATALYST

The gas phase hydrogenation of toluene to methylcyclohexane on a commercial Ni/Al2O3 catalyst was investigated in a differential reactor operating at atmospheric pressure and temperatures between 150 and 210-degrees-C. The results revealed that the hydrogenation kinetics is of the order 1-3 with respect to hydrogen at the actual temperature interval and that the reaction order increases with temperature. The reaction order with respect to toluene is negative. The reaction rate exhibited a maximum at approximately 170-degrees-C. The rate maximum is explained by the escape of catalytically active hydrogen from the Ni-surface at the highest reaction temperatures, which was confirmed by temperature-programmed desorption studies and chemisorption studies of hydrogen. The kinetics was modelled with an empirical power-law rate expression and with three mechanistic rate models. The latter were based on the assumption of rapid competitive adsorption steps of toluene and hydrogen and rate determining surface reaction steps involving addition of hydrogen atoms to adsorbed toluene and partially hydrogenated intermediate molecules. The best fit to the experimental data were provided by two models; one implying simultaneous addition of hydrogen atoms to adsorbed toluene and the other, being more probable from a mechanistic point of view, implying sequential addition of hydrogen atoms to adsorbed toluene.