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 (lp) form and a narrow-pore (np) 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 lp–np transition in MIL-53(Al) initially loaded with xenon gas under a pressure of 5 × 104 Pa (at room temperature). The conversion of the lp form into the np 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 np phase at T ≤ 230 K, while the lp one is unseen because the xenon residence time is too short for successful cross-polarization transfer. Moreover, even for the np phase at 199 K, the xenon atoms still exhibit significant motion on time scale faster than a few millisec...