Transport of aromatic molecules in NaY Zeolite powders

Abstract

Xenon-I29 NMR is used to probe macroscopic distributions of aromatic molecules adsorbed in a packed bed of 1-pm 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 B. F. Chmelkaadsorption front advances through a bed of dehydrated NaY crystallites at 523 K. However, at 573 K or in the presence of coadsorbed water, J. V. GillisHMB species disperse through the bed without forming a sharp E. E. Petersenboundary between adsorption zones. C. J. RadkeWhen guest transport is controlled by pseudosteady-state diffusion in Department of Chemical Engineering the macropores, axial penetration of the bed by vapor-phase guest University of California species occurs in a sharp adsorption front. A shrinking-core transport Berkeley, CA 94720 model then quantitatively estimates the intracrystalline diffusivities of HMB in dehydrated and partially hydrated NaY zeolite of lo-" and 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.Correspondence mncerning this papcr should he addressed to C. J. Radke. 1562October 1990 sponsible for the macroscopic adsorbate concentration profiles observed. Adsorbate Distributions by '"Xe NMRAs a sensitive, nonreactive probe of supercage cavities within the crystalline zeolite matrix, room-temperature 129Xe N M R spectroscopy provides unique information on interparticle adsorbate heterogeneities in NaY powders. By monitoring the chemical shift of physisorbed 12'Xe, distinct regions containing different concentrations of adsorbed guests can be identified within NaY zeolite samples.All zeolite samples were prepared in 10-mm N M R tubes connected to a vacuum apparatus through a Kontes highvacuum stopcock. NaY zeolite was synthesized according to the procedure of Breck (1982), with crystallinity confirmed by X-ray diffraction studies. Separate scanning electron microscopy experiments establish NaY crystallite grain diameters of 1-2 Mm (Chmelka, 1990). Before adding the adsorbate species, the NaY samples were dehydrated in the sample tubes by heating a t 723 K under vacuum (ca. lo-' torr or 1.3 x Pa)

129Xe-NMR of physisorbed xenon used as a probe for the study of microporous solids

“…The rapid movement of xenon atoms through large-pore zeolites at room temperature usually limits resolvable structural details to macroscopic dimensions. Nevertheless, Chmelka et al have measured macroscopic adsorbate distribution heterogeneities in NaY zeolite which, when coupled with a crystallite size scaling analysis, make ir possible to estimate diffusivities of coadsorbed organic molecules [29]. In such cases, other complementary experimental methods are often needed to check interpretations of lZ9Xe-NMR results [30][31][32][33].…”

Adsorption in bidisperse‐pore systems

“…A recent study (Chmelka et al, 1990) of the adsorption of hexamethylbenzene in NaY zeolite using '29Xe hMR demonstrates how the behavior of the system depends critically on the numerical magnitude of 6.…”

129Xe NMR overview of xenon physisorbed in porous solids

“…Nevertheless, Chmelka et al have measured macroscopic adsorbate distribution heterogeneities in NaY zeolite, which, when coupled with a crystallite size scaling analysis, make it possible to estimate diffusivities of coadsorbed organic molecules. 17 To minimize the averaging effect due to diffusion, one can decrease the experimental temperature, or coat the external surface of particles with large molecules. 18 NaA zeolite has received special attention because diffusion of xenon between˛-cavities is very slow due to the blockage of the apertures by Na cations.…”