Modeling the concentration dependence of diffusion in zeolites. II. Kinetic Monte Carlo simulations of benzene in Na-Y

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Abstract
We have performed kinetic Monte Carlo simulations of benzene diffusion in Na-Y at finite loadings for various temperatures to test the analytical theory presented in Paper I, immediately preceding this paper. Our theory and simulations assume that benzene molecules jump among SII and W sites, located near Na+ ions in 6-rings and in 12-ring windows, respectively. The theory exploits the fact that supercages are identical on average, yielding Dθ=16kθaθ2=κaθ2/6〈τ1〉[1+Keq(1→2)], where kθ is the cage-to-cage rate coefficient, Keq(1→2) is the W→SII equilibrium coefficient, 〈τ1〉 is the mean W site residence time, and κ is the transmission coefficient for cage-to-cage motion. The simulations use fundamental rate coefficients calculated at infinite dilution for consistency with the theory in Paper I. Our theory for kθ, Keq(1→2) and 〈τ1〉 agrees quantitatively with simulation for various temperatures and loadings. The simulated transmission coefficient is nearly 12 for all but the highest loadings, qualitatively validating our mean field approximation. Comparison between our theory and experimental data shows excellent qualitative agreement with tracer zero-length column data, but also shows qualitative disagreement with both pulsed field gradient NMR and frequency response data.