Journal of Chemical Physics, Vol.112, No.3, 1531-1540, 2000
Correlation effects in molecular diffusion in zeolites at infinite dilution
Molecular diffusion in zeolites is often resumed to a random walk between specific adsorption sites within the channels and cavities of the materials. Several types of correlations between the steps of the walk come to precise this assumption: kinetic correlations due to the incomplete relaxation of the molecule in its final site, vacancy correlations arising at high loading because molecules are blocking each other, and geometrical correlations because zeolite channels and cages can boast nonsymmetric sites. The first and last correlation effects can be observed at infinite dilution. In this article we present a way of calculating an exact diffusion coefficient at infinite dilution as a function of the microscopic rate constants, taking into account both geometric and kinetic correlation effects. This is achieved by cutting the molecular motion into uncorrelated sequences of jumps, where all jumps inside one sequence are correlated to each other. This method is applied to study geometrical correlations of benzene in NaY, comparing with kinetic Monte Carlo data of Saravanan [C. Saravanan, F. Jousse, and S. M. Auerbach, J. Chem. Phys. 108, 2162 (1998)], and both kinetic and geometrical correlations of ethane in silicalite, comparing with molecular dynamic simulations of Karger [J. Karger, P. Demontis, G. B. Suffritti, and A. Tilocca, J. Chem. Phys. 110, 1163 (1999)].