The Maxwell-Stefan diffusivity in zeolites exhibited various dependences upon the molecular loading or occupancy. The loading dependence was caused by a number of factors, including zeolite topology, connectivity and molecule-molecule interactions; which led to a decrease or increase in the energy barrier to diffusion. Using the quasi-chemical theory of Reed and Ehrlich (1981) for surface diffusion on a square lattice as a basis, a simple model was developed to describe the loading-dependence of the Maxwell-Stefan diffusivity for a lattice topology with an arbitrary coordination number. The model was validated by kinetic Monte Carlo simulations of square, cubic and MFI zeolite topologies. Published molecular dynamics simulations of the loading dependence of Maxwell-Stefan and self-diffusivities in a variety of zeolite topologies could be modelled by using this approach. The Maxwell-Stefan formulation permitted accurate prediction, of the transport and self-diffusivities in binary mixtures, using only pure-component diffusion data.

Modelling the Occupancy Dependence of Diffusivities in Zeolites. Krishna, R., Paschek, D., Baur, R.: Microporous and Mesoporous Materials, 2004, 76[1-3], 233-46