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.