Diffusion in type A zeolites: New insights from old data

The extensive kinetic data accumulated, over many years, from a series of experimental studies of the kinetics of adsorption/desorption of a wide range of different sorbates in type A zeolites are reviewed and analyzed in an attempt to develop a coherent understanding of the behavior of these systems. Kinetic data for large laboratory synthesized crystals and small commercial crystals, measured under similar conditions, have been studied in detail. In well dehydrated crystals the sorption rates are generally controlled by intracrystalline diffusion but exposure to traces of water leads to the development of surface resistance and a pronounced reduction in the sorption rate. Zeolite samples of different origin show widely different sorption rates but the diffusional activation energies (for a given sorbate) are essentially constant. The differences between the different samples appear to be due mainly to differences in the cation distribution caused by differences in the initial dehydration procedure. The ideal cation distribution (in 5A) in which all window sites are unoccupied is realized only in very carefully dehydrated samples in which the water was removed slowly at gradually increasing temperature under a high vacuum. In the small commercial 5A crystals many of the windows are blocked, even though all the cations could theoretically be accommodated in the 6-ring sites. This is probably due to cation hydration reactions which are likely to occur when the dehydration conditions are not carefully controlled.

Structure of zeolite A showing exchangeable cation sites.Figure optionsDownload as PowerPoint slideHighlights
► Sorption rates in type A zeolites vary widely depending on sample pre-treatment.
► The form of the transient sorption curve indicates the rate controlling process.
► Traces of water cause a transition from internal diffusion to surface rate control.
► Constant activation energies with large changes in D∞ suggest window blocking.