Preferential occupation of xenon in zeolite MCM-22 as revealed by Xe-129 NMR spectroscopy

Adsorption of xenon in zeolite MCM-22, a zeolite containing two separate po re systems (the 10-membered ring (MR) sinusoidal channels and 12-MR superca ges interconnected by 10-MR window system), has been investigated in detail with Xe-129 NMR spectroscopy as a function of Xe loadings over a wide rang e of temperatures (120-350 K). NMR results suggest that Xe atoms are prefer entially adsorbed in the supercages of the zeolite at low Xe pressure (less than a few atmospheres), while Xe atoms can penetrate into the two-dimensi onal sinusoidal channels at high Xe pressure. No direct exchange between th e Xe in the supercages and the sinusoidal channels was observed in variable -temperature (VT) Xe-129 NMR experiments for all of the samples, which is i n line with the crystal structure of the zeolite. However, when the Xe adso rption pressure is greater than 6 atm, two different kinds of Xe exchange w ere observed in VT experiments over the temperature range. The exchange of Xe at the different adsorption sites in the same supercage (i.e., Xe atoms in the two pockets and those in the central part of the supercage) occurs a t lower temperatures (170-122 K), which is demonstrated by two-dimensional Xe-129 NMR exchange spectroscopy. The exchange between Xe in the supercage and the sinusoidal channel takes place through gaseous Xe in the interparti cle space at higher temperatures (280-350 K).