ABSTRACT
The important uses of porous crystalline zeolites in selective sorption, molecule sieving, and catalysis involve the migration of sorbates within the crystals. The important factors which control this migration and its relationship between molecular size, shape, and the topologies of the open framework within which the molecules move must be understood. The phenomenon of diffusion in zeolites is perhaps the least well understood. However, diffusion in zeolites differs from that in other conventional catalysts in many ways and hence these concepts are not applicable. In Knudsen diffusion the pores are sufficiently small so that the mean free path is much greater than the pore size. Furthermore, the activation energy of zeolite diffusion is substantially greater than Knudsen diffusion and bulk diffusion. Most diffusion studies made in the configurational regime have involved zeolite powders and weakly sorbed gases. Sorption occurs either at constant pressure or at constant volume with variable pressure.
