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

The Al-based metal organic framework (MOF) MIL-53(Al) exhibits a structural transition between a large-pore (1p) form and a narrow-pore (np) one. Such change is induced by temperature, external pressure, or the adsorption of guest molecules. Xe-129 solid-state NMR experiments under static and magic-angle spinning (MAS) conditions have been used to study the 1p np transition in MIL-53(Al) initially loaded with xenon gas under a pressure of 5 x 10(4) Pa (at room temperature). The conversion of the 1p 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, H-1 -> Xe-129 cross-polarization under MAS (CPMAS) experiments demonstrate the possibility to observe the np phase at T <= 230 K, while the 1p 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 1p and np forms at room temperature with the two-dimensional (2D) Xe-139 EXchange SpectroscopY (EXSY) NMR method. Using Xe-129 selective inversion recovery (SIR) experiments, the rate for this exchange has been measured at 43 +/- 6 s(-1).