Analysis of Diffusion Limitation in the Alkylation of Benzene over H-ZSM-5 by Combining Quantum Chemical Calculations, Molecular Simulations, and a Continuum Approach

A continuum model based on the Maxwell−Stefan (M-S) equations in combination with the ideal adsorbed solution theory has been used to analyze the influence of adsorption thermodynamics and intraparticle diffusional transport on the overall kinetics of benzene alkylation with ethene over H-ZSM-5. The parameters appearing in the M-S equations were obtained from molecular dynamics simulations, and pure component adsorption isotherms were obtained from configurational-bias Monte Carlo simulations in the grand canonical ensemble. Rate coefficients for the elementary steps of the alkylation were taken from quantum chemical calculations. The intrinsic kinetics of two different reaction schemes were analyzed. The simulations show that all apparent rate parameters of the alkylation are strongly dependent on the reaction conditions. By taking diffusional limitation into account, experimentally determined reaction rates and the orders in the partial pressures of reactants can be reproduced. The results of this study show that empirical power law rate expressions become inappropriate when used to correlate kinetic data over a broad range of conditions. In addition, it is demonstrated that the usual approaches to determine effectiveness factors for reactions in porous media, which assume a constant effective diffusivity, may lead to substantial deviations from rigorous simulations, whereas the simulation model developed here can be used to predict the effectiveness factor for zeolite particles for any set of reaction conditions.