Diffusion in Partially Ion Exchanged Molecular Sieves

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Abstract
Results of a simple idealized numerical simulation are presented showing the variation of sorbate diffusivity with degree of ion exchange in a system such as the Na–Ca A zeolites. Theoretical curves showing the expected variation of diffusivity, diffusional activation energy, and pre-exponential factor are given. The pattern of behavior predicted depends on the ratio of the diffusivities for the extreme ionic forms. When this ratio is small there is a gradual variation in diffusivity, activation energy, and pre-exponential factor over the entire range of ionic compositions but for large ratios there is an abrupt change at a particular ionic composition. Limited experimental data for the diffusion of n-butane in partially Ca2+ exchanged 4A sieve are presented and interpreted in terms of the theoretical model.

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