TRANSPORT OF AROMATIC-MOLECULES IN NAY ZEOLITE POWDERS

Xenon‐129 NMR is used to probe macroscopic distributions of aromatic molecules adsorbed in a packed bed of 1‐μm 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 HMB adsorption front advances through a bed of dehydrated NaY crystallites at 523 K. However, at 573 K or in the presence of coadsorbed water, HMB species disperse through the bed without forming a sharp boundary between adsorption zones. When guest transport is controlled by pseudosteady‐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 HMB in dehydrated and partially hydrated NaY zeolite of 10−11 and 10−13 m2/s, respectively, at 523 K. 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. Copyright © 1990 American Institute of Chemical Engineers