Dynamic Monte-Carlo simulations of binary self-diffusion in zeolites: effect of strong adsorption sites

Dynamic Monte-Carlo simulations are performed to examine binary self-diffusion on heterogeneous cubic lattices and in ZSM-5 (MFI), which includes both strong and weak adsorption sites due to the presence of Al atoms in addition to Si. It is observed that the diffusion behavior in such zeolites strongly depends on the pore network topology, the fraction of strong adsorption sites, and the differing relative adsorption strengths of the diffusing species on different types of sites. The Maxwell-Stefan approach previously proposed to predict diffusivities in silicalite-1, the all-Si version of MFI, is reexamined and generalized to include the effect of strong adsorption sites. It is concluded that this approach is qualitatively and quantitatively inaccurate when correlation effects are large, as a result of zeolite heterogeneity. An alternative theory based on an effective medium approximation and other mean-field arguments is more successful for the case where differences in relative adsorption strengths of the species are very high. This theory can be further refined to include intermolecular correlations. (C) 2004 Elsevier Ltd. All rights reserved.