Effect of multiple occupancy upon the diffusivity in zeolites

The traditional hopping model of diffusion in zeolites predicts a concentration-independent diffusivity D-r whereas observations almost always yield D(theta) increasing strongly with Increasing average occupancy theta of the adsorption sites. An explanation is attempted in terms of a model allowing double occupancy of adsorption wells or ''cages.'' It is assumed that the repulsion between molecules adsorbed in one well lifts its bottom, whereas the level of the ''windows'' between wells remains unchanged. The potential barrier from well to window is thereby lowered so that jumps become more frequent at higher density, when double occupancy is more probable. The resulting expression for D(theta) is compared with the main features of existing experimental curves. Although it is possible to fit the observed low density behavior of D[theta), the model is not able to reproduce the steep approach toward saturation.