Variable temperature ¹²⁹Xe NMR studies of xenon adsorbed on mesoporous MCM-41 molecular sieves

A comprehensive variable temperature ¹²⁹Xe NMR study of xenon adsorbed on mesoporous MCM-41 molecular sieves with varied pore sizes, morphology and aluminum concentration has been made. The observed 1Xe NMR chemical shifts (8) of the adsorbed Xe exhibit a strong dependent on Xe loading (ρ) and temperature (T). At low Xe loading, the ¹²⁹Xe chemical shifts show a concave upward dependence on Xe density which vary with temperature and pore size of the adsorbent. Whereas at high loading, a nearly linear relation between the observed chemical shifts and effective Xe density was found whose slopes of δ vs ρ_eff is temperature dependent. Regressional fitting based on a two-site exchange model enable us to fit the observed δ(T, ρ) data in a least squares sense to second-order polynomial with ρ. The temperature variations of the Xe-wall interaction at zero density as well as the Xe-Xe interactions were derived explicitly and compared with the results available for bulk Xe, Xe adsorbed on microporous materials and on polymer surfaces.