Investigation Of Porous Materials By ～(129)Xe NMR Technique

In the past twenty years, ¹²⁹Xe NMR has been extensively used for studying a variety of porous materials. ¹²⁹Xe NMR spectroscopy is an important and very useful technique for investigating the pore structure of porous materials and it can provide many structural information that is difficult to obtain using other spectroscopy techniques. In this dissertation, ¹²⁹Xe NMR technique has been investigated in both theory and experiment and two novel porous materials, microporous zeolite MCM-22 and mesoporous materials MAS-7, are characterized by ¹²⁹Xe NMR spectroscopy.1. Based on confined one-dimensional collision model, the coefficientδ_Xe-Xearising from Xe-Xe collisions measured by ¹²⁹Xe NMR experiment is related to the pore size ( D = 1 .681δ_Xe-Xe^1/2, indicating a new approach to the determination of the size of narrow channels in various microporous materials.2. Adsorption of xenon in zeolite MCM-22, a zeolite containing two separate pore systems (the 10-membered ring (MR) sinusoidal channels and 12-MR supercages interconnected by 10-MR window system), has been investigated in detail by variable temperature (VT), magic-angle spinning (MAS), and co-adsorption ¹²⁹Xe NMR spectroscopy as a function of Xe loadings over a wide range of temperatures (120 - 350 K) in different magnetic field B₀. NMR results suggest that Xe atoms are preferentially adsorbed in the supercages of the zeolite at low Xe pressure (less than a few atmosphere), while a few Xe atoms can penetrate into the two-dimensional sinusoidal channels at high Xe pressure revealed by MAS ¹²⁹Xe NMR. When the Xe adsorption pressure is larger than 6 atm, two different kinds of xenon exchange were observed by VT experiments over the temperature range. The direct exchange of Xe at the different adsorption sites in the same supercage occurs at lower temperatures (170– 122 K). The indirect exchange between xenon in different adsorption sites takes...