Characterization of adsorbed xenon in Zeolite-A, X, and Y by high-pressure 129Xe NMR spectroscopy

Abstract

The local structure of confined xenon in A-, X- and Y-type zeolites (molecular sieve 5A, 13X and zeolite NaY) was investigated by in situ high-pressure ¹²⁹Xe NMR spectroscopy. Xenon confined in dehydrated samples gives a resonance peak in the chemical shift range from 50 to 100 ppm at 0.1 MPa, and the chemical shift values increase as pressure increases. The pressure dependence of ¹²⁹Xe chemical shift on xenon confined in the micropores can be approximately described by the Langmuir adsorption model, resulting in the parameters being able to describe pore diameter, adsorption ability of the pore, and the local structure of the confined xenon atoms. Using these parameters, the pore structure, the mobility and the cluster size of xenon in the micropore are discussed for A- and X-type zeolites. Furthermore, the effect of pre-adsorbed water and the binder for pellet preparation on the pore structure is also examined. In the Y-zeolite, a chemical exchange of xenon between the outside and inside of the micropores was observed above 0.75 MPa. Spectral simulation assuming a simple two-site exchange model revealed the exchange rate constant and the population ratio of xenon in the exchange between the outside and the inside the micropores. The exchange of xenon is optimum around the supercritical point of the bulk xenon (∼6 MPa), and decreases above 6 MPa, and this originates from the cooperative phenomenon in the supercritical fluid.