On the Mechanism of Xylene Isomerization and its Limitations as Reaction Test for Solid Acid Catalysts

The kinetics of the gas-phase isomerization of m-xylene, over a series of ultraestable Y zeolites, has been studied in a fixed-bed flow reactor under initial conditions. Using deuterated p-xylene as reactant it has been seen that more than 20% of the m- and o-xylene obtained, are formed via a bimolecular mechanism. The relative proportion of uni- to bimolecular mechanism depends on the zeolite composition. The relation between the turnover number, calculated from the kinetic rate constant for the unimolecular mechanism, and the framework Al content of the zeolite, was coincident with the topological zeolite model. Besides, the values of the adsorption constants indicate that dealumination has an influence on the adsorption capacity of the zeolite for m-xylene, specially visible at high Si/Al ratios, coincident with the result of direct adsorption experiments of m-xylene, which in turn could explain the observed changes in the ratio uni- to bimolecular xylene isomerization upon dealumination.