Study of Xenon Mobility in the Two Forms of MIL-53(Al) Using Solid-State NMR Spectroscopy

Abstract

The Al-based metal–organic framework (MOF) MIL-53­(Al) exhibits a structural transition between a large-pore (<i>lp</i>) form and a narrow-pore (<i>np</i>) one. Such change is induced by temperature, external pressure, or the adsorption of guest molecules. ¹²⁹Xe solid-state NMR experiments under static and magic-angle spinning (MAS) conditions have been used to study the <i>lp</i>–<i>np</i> transition in MIL-53­(Al) initially loaded with xenon gas under a pressure of 5 × 10⁴ Pa (at room temperature). The conversion of the <i>lp</i> form into the <i>np</i> one when the temperature decreases from 327 to 237 K and the reopening of the pores below 230 K are then observed. Furthermore, ¹H → ¹²⁹Xe cross-polarization under MAS (CPMAS) experiments demonstrate the possibility to observe the <i>np</i> phase at <i>T</i> ≤ 230 K, while the <i>lp</i> one is unseen because the xenon residence time is too short for successful cross-polarization transfer. Moreover, even for the <i>np</i> phase at 199 K, the xenon atoms still exhibit significant motion on time scale faster than a few milliseconds. We prove the exchange of Xe atoms between the <i>lp</i> and <i>np</i> forms at room temperature with the two-dimensional (2D) ¹²⁹Xe EXchange SpectroscopY (EXSY) NMR method. Using ¹²⁹Xe selective inversion recovery (SIR) experiments, the rate for this exchange has been measured at 43 ± 6 s^–1