Model for analysing diffusion in zeolite crystals

Abstract

A model is presented which describes the concentration dependence of the effective diffusion coefficient in a zeolite crystal on the basis of a statistical thermodynamic equilibrium analysis. If there is a distribution of cavities occupied by differing numbers of molecules at any average level of adsorption, a large number of rate parameters is involved, since intrinsic diffusion coefficients from one cavity to another are able to vary with the number of molecules adsorbed per cavity. The experimentally determined effective diffusivity depends on concentration in a manner determined by the values of the intrinsic diffusion coefficients and the thermodynamic parameters. Only if rather restrictive assumptions are made does our model reduce to Darken's law. Adsorption rate data reported in the literature are interpreted by our correlation.