¹²⁹Xe NMR Study of Adsorption and Dynamics of Xenon in AgA Zeolite^†

The adsorption of xenon in AgA zeolite has been studied by ¹²⁹Xe NMR spectroscopy, yielding information on the silver distribution, Xe cluster size and distribution, and xenon exchange dynamics. The exchange of xenon is slow between the α-cages of AgA treated in a vacuum at 380 or 410 K (yellow AgA), and separate lines appear in the ¹²⁹Xe NMR spectra for cages containing different xenon populations. Up to 10 different Xe_n clusters with n between 1 and 8 can be distinguished; the Xe₇ and Xe₈ clusters appear to be present in two different states Xe₇‘ + Xe₇‘‘ and Xe₈‘ + Xe₈‘‘, reflecting cage differences apparent only at high xenon loading. The population distribution, studied over a broad range of xenon loadings, cannot be described in terms of the simple hypergeometric distribution except at loading levels below n = 4. The Xe_n cluster lines for “yellow” AgA zeolite are all shifted uniformly to low field relative to the clusters in NaA zeolite, a feature which currently has no firm explanation. Because large xenon clusters up to Xe₈ can reside in the α cage, size constraints lead one to conclude that any charged silver clusters must be located inside the sodalite (β) cages. The uptake and redistribution of xenon in the zeolite is relatively slow (hours to weeks). The slow passage of xenon atoms through the 8-rings can be attributed to the presence of a hydrated silver ion at the aperture. This blocking effect disappears for samples annealed at higher temperatures to give “orange” AgA. Xenon exchange dynamics between the cages has been studied in detail by applying 2D-EXSY NMR methods. All of the exchange constants can be obtained directly from the analysis of the 2D-NMR data, and from variable-temperature 2D EXSY experiments. The activation energy of xenon transfer between the different cages can be estimated to be 45 ± 10 kJ/mol, significantly lower than for NaA zeolite. Cage-to-cage exchange rate constants increase with the degree of loading above ca. n = 4, reflecting decreased sorption energies at higher loading.