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

A mechanistic rate model was developed to describe the gas phase hydro genation kinetics of o-xylene and p-xylene on a Ni/Al2O3 catalyst at t emperatures between 403 and 493 K. The mechanistic scheme comprises co nsecutive and pairwise hydrogen addition steps to the aromatic molecul es adsorbed on the catalyst surface. Competitive as well as non-compet itive quasi-equilibrated adsorption of the reactants were accounted fo r. An experimentally observed cyclic olefin is assumed to be the key i ntermediate, determining the stereoselectivity of the main products (c is- and trans-dimethylcyclohexanes) by its adsorption properties. All the main features of the observed reaction kinetics, including a rate maximum in the overall rate as a function of temperature as well as th e production rates of the main products and the intermediate cyclic ol efin, were satisfactorily predicted by the rate model. The calculated model parameters were consistent with thermodynamics and previously re ported parameters for elementary steps.