Xenon-129 NMR was used to probe macroscopic distributions of aromatic molecules adsorbed in a packed bed of 1μm NaY zeolite particles. Relative rates of guest transport through the intracrystalline (micro) and intercrystalline (macro) pores play a unique role in the axial distribution of sorbate molecules, such as hexamethylbenzene, in a zeolite powder. Xenon-129 NMR spectra show that a sharp hexamethylbenzene adsorption front advances through a bed of dehydrated NaY crystallites at 523K. However, at 573K or in the presence of coadsorbed water, hexamethylbenzene species disperse through the bed without forming a sharp boundary between adsorption zones. When guest transport was controlled by pseudo steady-state diffusion in the macropores, axial penetration of the bed by vapor-phase guest species occurs in a sharp adsorption front. A shrinking-core transport model then quantitatively estimates the intracrystalline diffusivities of hexamethylbenzene in dehydrated and partially hydrated NaY zeolite of 10-11 and 10-13m2/s, respectively, at 523K. Xenon-129 NMR proves to be a powerful tool for probing adsorbed guest distribution in zeolites, allowing relative time scales to be established for transport of molecular guests in NaY powders.

Transport of Aromatic Molecules in NaY Zeolite Powders. B.F.Chmelka, J.V.Gillis, E.E.Petersen, C.J.Radke: AIChE Journal, 1990, 36[10], 1562-8