Kinetic Monte Carlo simulations were made of benzene diffusion in Na-Y at finite loadings and various temperatures in order to test an analytical theory. The theory and simulations assumed that benzene molecules jumped between SII and W sites, located near to Na+ ions in 6-ring and in 12-ring windows, respectively. The theory exploited the fact that super-cages were identical on average, yielding Dθ = (1/6)kθaθ2 = Kaθ2/6<τ1>[1 + Keq(1→2)], where kθ was the cage-to-cage rate coefficient, Keq(1→2) was the W→SII equilibrium coefficient, <τ1> was the mean W site residence time, and κ was the transmission coefficient for cage-to-cage motion. The simulations used fundamental rate coefficients calculated at infinite dilution, for consistency with theory. The latter theory for kθ, Keq(1→2) and <τ1> agreed quantitatively with simulations for various temperatures and loadings. The simulated transmission coefficient was nearly 1/2 for all but the highest loadings; thus qualitatively validating the mean field approximation. Comparison between the present theory and experiment revealed excellent qualitative agreement with tracer zero-length column data, but also showed qualitative disagreement with both pulsed field gradient NMR and frequency response data.

Modeling the Concentration Dependence of Diffusion in Zeolites. II. Kinetic Monte Carlo Simulations of Benzene in Na-Y. C.Saravanan, S.M.Auerbach: Journal of Chemical Physics, 1997, 107[19], 8132-7