XENON ADSORPTION IN NAA ZEOLITE CAVITIES

Adsorption of xenon atoms in the a-cages of NaA zeolite has been studied using Xe-129 NMR spectroscopy to probe directly the distribution and configuration of molecules in confined, microporous environments. The Xe-129 NMR spectrum is sensitive to subtle changes in xenon environment, so relative populations of alpha-cages containing different numbers of xenon guests can be determined and the effects of other co-adsorbed species monitored. On the basis of Xe-129 NMR spectra, the distribution of xenon atoms among NaA alpha-cages is shown to exhibit a marked dependence on the pressure at which the xenon guests are introduced. Xe-129 NMR spectra recorded at 200 K reveal that xenon atoms in the NaA alpha-cages experience diminished mobility (resembling condensation phenomena) at higher temperatures than in the bulk gas of equivalent density. Thus, the chemical potential of adsorbed xenon can be investigated experimentally as a function of both temperature and guest density. The density dependence of the Xe-129 chemical shift in Xe/NaA and in bulk xenon gas shows that Xe-Xe interactions in the proximity of the NaA cage wall are important in alpha-cages containing more than five xenon guests. This trend is linked to entropic effects which may enhance xenon adsorption in the confined environment of the NaA alpha-cages.