Dynamic Monte-Carlo and mean-"eld study of the e!ect of strong adsorption sites on self-di!usion in zeolites

Self-di!usion in zeolites is studied using a simple mean-"eld theory and dynamic Monte-Carlo simulations of di!usion on a lattice model of the zeolite pore space. Our method is powerful, because of its simplicity, #exibility and ability to study the in#uence 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 con"rm the qualitative trends observed in pulsed-"eld gradient NMR experiments, and are able to explain them. For ZSM-5, we also found a new possible type of functional dependency of the di!usivity on occupancy, with two inclination points. ( 1999 Elsevier Science Ltd. All rights reserved.