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

International audienceThe 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. 129 Xe 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 × 10 4 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, 1 H → 129 Xe 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 milliseconds. We prove the exchange of Xe atoms between the lp and np forms at room temperature with the two-dimensional (2D) 129 Xe EXchange SpectroscopY (EXSY) NMR method. Using 129 Xe selective inversion recovery (SIR) experiments, the rate for this exchange has been measured at 43 ± 6 s −1