XE-129 NMR SELF-DIFFUSION MEASUREMENTS - A NOVEL METHOD TO PROBE DIFFUSIONAL BARRIERS ON THE EXTERNAL SURFACE OF ZEOLITE CRYSTALLITES

129Xe n.m.r. pulsed-field gradient measurements have been carried out to probe the surface permeability of zeolites NaX, NaCaA, and ZSM-5. Although for zeolite NaX the passage through the external surface of the crystallite is found to be of minor importance for the observed exchange rates, in zeolites NaCaA and ZSM-5, this process is found to be significantly retarded. Similar experiments with methane do not provide any stringent indication of the existence of diffusional barriers on the external surface ("surface barriers"). One has to conclude, therefore, that owing to its somewhat larger diameter in comparison to methane, xenon represents a sensitive tool for probing structural distortion in the surface layer of zeolites. © 1990.