EFFECT OF MULTIPLE OCCUPANCY UPON THE DIFFUSIVITY IN ZEOLITES

The traditional hopping model of diffusion in zeolites predicts a conc entration-independent diffusivity D-r whereas observations almost alwa ys yield D(theta) increasing strongly with Increasing average occupanc y 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 we ll lifts its bottom, whereas the level of the ''windows'' between well s remains unchanged. The potential barrier from well to window is ther eby lowered so that jumps become more frequent at higher density, when double occupancy is more probable. The resulting expression for D(the ta) 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.