EFFECTIVE DIFFUSIVITIES IN ZEOLITES .2. EXPERIMENTAL APPRAISAL OF EFFECTIVE SHAPE-SELECTIVE DIFFUSIVITY IN ZSM-5 CATALYSIS

A method is described for experimentally probing the molecular shape-sensitive diffusivity of a molecular species effective during the steady-state process of catalysis in a zeolite. It does not require the use of catalyst samples of differing and known particle (i.e., crystallite) sizes but with otherwise identical properties. It can be carried out on a single catalyst sample. The method makes use of hydrogen-deuterium transfer as the catalytic reaction. Instead of comparing reaction rates on different crystallite sizes, it makes use of altering the magnitude of a pseudo-first-order reaction rate constant to probe for the effect of diffusion inhibition. The method is used here to examine the magnitude of the catalytically effective diffusivity of ortho-xylene is ZSM-5, in comparison to the “uptake diffusivity” derived from sorption measurements. © 1993 Academic Press, Inc.