EFFECTIVE DIFFUSIVITIES IN ZEOLITES .3. EFFECTS OF POLARITY, CATION DENSITY, AND SITE OCCUPANCY IN ZSM-5

The effective diffusivity that determines the catalytic reaction inhibition (utilization factor) in the steady-state process of catalysis, Dss, differs from that derived from nonsteady-state sorption kinetics, Dns, (e.g. “uptake diffusivity”). Dss is obtainable from Dns, by a transformation based on the equilibrium ratio cT/co of the total sorbed concentration lathe applied vapor phase concentration co. Dss is several orders of magnitude larger than Dns. For o-xylene in siliceous ZSM-5, Dss was found to be virtually invariant with temperature. Introducing a polar NH2 substituent in place of CH3 does not alter the temperature independence, i.e., it does not introduce or alter “activation energy.” In fact, a larger diffusivity results, presumably from the slightly smaller effective molecular size. Introducing a high sodium cation density in the zeolite structure creates a temperature dependence of Dss. However, it is shown to be a result of the high percentage of occupancy of transition (jump) sites by sorbed molecules which diminishes with increasing temperature. These several observations allow important conclusions to be drawn concerning the real or apparent nature of “activated” diffusion in the zeolite. © 1993 Academic Press, Inc.