Dynamic Monte-Carlo and mean-field study of the effect of strong adsorption sites on self-diffusion in zeolites

Self-diffusion in zeolites is studied using a simple mean-field theory and dynamic Monte-Carlo simulations of diffusion on a lattice model of the zeolite pore space. Our method is powerful, because of its simplicity, flexibility and ability to study the influence of various factors: the topology of the pore network, the fraction of strong adsorption sites, the relative strengths of weak and strong sites, the number of sites per zeolite cage, and the ratio of intracage to extracage hopping probabilities. Notwithstanding the simplicity of the modeling, our results confirm the qualitative trends observed in pulsed-held gradient NMR experiments, and are able to explain them. For ZSM-5, we also found a new possible type of functional dependency of the diffusivity on occupancy, with two inclination points. (C) 1999 Elsevier Science Ltd. All rights reserved.