Modeling Jump Diffusion in Zeolites: II. Applications

We review recent applications of jump models for diffusion in zeolites. We describe the results of a coarse-grained model of the interplay between zeolite anisotropy and disorder, finding that certain disorder patterns can change how anisotropy controls membrane permeation. We show the results of a lattice model for single-file diffusion in zeolite membranes, demonstrating how single-file motion is manifested in anomalous mean-square displacements at short times, and in nonintensive Fickian self-diffusion coefficients at later times. We discuss a normal-mode analysis approach for treating framework flexibility for tightfitting zeolite-guest systems, showing that simulations allowing for framework flexibility can converge is less CPU time than those that keep the framework rigid. We then explore models of the loading dependence of self-diffusion in zeolites, with emphasis on benzene in NaX and NaY. We enumerate the decisions that need to be made when modeling such systems, and indicate the choices/approximations we have made for modeling benzene in NaX and NaY. We report kinetic Monte Carlo results for the loading dependence of benzene diffusion in NaX, which is found in reasonable agreement with NMR data, but in poor agreement with tracer ZLC results. We then speculate on the possibility of having a subcritical fluid adsorbed in a nanoporous material, and how such a thermodynamic state would impact diffusion in such a system. We close with a review of outstanding problems in modeling jump diffusion in zeolites. Submitted for the Proceedings of the NATO Advanced Study Institute on “Fluid Transport in Nanopores,” Eds. J. Fraissard, W.C. Conner, Jr., and V. Skirda, La Colle Sur Loup, France, June 16-27, 2003.