KINETICS OF TOLUENE HYDROGENATION ON NI AL2O3 CATALYST/

The gas phase hydrogenation of toluene to methylcyclohexane on a comme rcial Ni/Al2O3 catalyst was investigated in a differential reactor ope rating at atmospheric pressure and temperatures between 150 and 210-de grees-C. The results revealed that the hydrogenation kinetics is of th e order 1-3 with respect to hydrogen at the actual temperature interva l and that the reaction order increases with temperature. The reaction order with respect to toluene is negative. The reaction rate exhibite d a maximum at approximately 170-degrees-C. The rate maximum is explai ned by the escape of catalytically active hydrogen from the Ni-surface at the highest reaction temperatures, which was confirmed by temperat ure-programmed desorption studies and chemisorption studies of hydroge n. The kinetics was modelled with an empirical power-law rate expressi on and with three mechanistic rate models. The latter were based on th e assumption of rapid competitive adsorption steps of toluene and hydr ogen and rate determining surface reaction steps involving addition of hydrogen atoms to adsorbed toluene and partially hydrogenated interme diate molecules. The best fit to the experimental data were provided b y 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 adso rbed toluene.