Role of external surface sites in shape-selective catalysis over zeolites

Abstract

A simple kinetic model is developed having an analytic solution that explains and quantifies isomer shape selectivity in zeolite catalysis. The model, based on the assumption of existence of both shape-selective (e.g., para-selective) internal catalytic sites and nonselective (e.g., non-para-selective) external sites, addresses the preferred overall formation of the isomer that diffuses more rapidly in the intracrystalline space, as well as the decline in shape selectivity with conversion. Exemplified for the production of xylene from toluene over HZSM-5 catalysts, the model gives a very good fit with experimental data of toluene disproportionation. Lack or loss of para selectivity is thus explained as resulting from reactions occurring on the external surface of the zeolite. The model predicts maxima in {ital p}-xylene production, subject to system variables, as a function of reaction time.