Intracrystalline Diffusion in Zeolites

A new experimental technique (ZLC) has been developed and applied to study the diffusion of a range of hydrocarbons (xylene, benzene, cyclohexane and linear paraffins) in unaggregated crystals of zeolites A and X. The validity of the method was confirmed by varying the crystal size and the nature and flowrate of the purge gas. The method has advantages of speed and simplicity but the major advantage is that the intrusion of extraneous heat and mass transfer resistances is much less significant than in conventional uptake rate measurements. As a result, the new method can be applied to systems in which diffusion is too rapid to follow in a conventional sorption experiment. Several of the systems studied have also been investigated by NMR, as well as by uptake and exchange rate measurements using large crystals. For all sorbates in both A and X zeolites the results obtained by the new ZLC method are consistent with our earlier sorption rate data but the diffusivities are much smaller than the values obtained by the NMR PFG method. Agreement between ZLC and NMR measurements was found only for butane in large 5A crystals. The large difference in diffusivity between large laboratory synthesized crystals and small commercial crystals is also confirmed for this system.