Tracking the evolution and differences between guest-induced phases of Ga-MIL-53 via ultra-wideline 69/71 Ga solid-state NMR spectroscopy.

Ga-MIL-53 is a metal-organic framework (MOF) that exhibits a "breathing effect," in which the pore size and overall MOF topology can be influenced by temperature, pressure, and host-guest interactions. The phase control afforded by this flexible framework renders Ga-MIL-53 a promising material for guest storage and sensing applications. In this work, the structure and behavior of four Ga-MIL-53 phases (as, ht, enp and lt), along with CO2 adsorbed within Ga-MIL-53 at various loading levels, has been investigated using 69/71 Ga solid-state NMR (SSNMR) experiments at 21.1T and 9.4T. 69/71 Ga SSNMR spectra are observed to be very sensitive to distortions in the octahedral GaO6 secondary building units within Ga-MIL-53; by extension, Ga NMR parameters are indicative of the particular crystallographic phase of Ga-MIL-53. The evolution of Ga NMR parameters with CO2 loading levels in Ga-MIL-53 reveals that the specific CO2 loading level offers a profound degree of control over the MOF phase, and the data also suggests that a re-entrant phase transition is present. Adsorption of various organic compounds within Ga-MIL-53 has been investigated using a combination of thermal gravimetric analysis (TGA), powder X-ray diffraction (pXRD) and 69/71 Ga SSNMR experiments. Notably, pXRD experiments reveal that guest adsorption and host-guest interactions trigger unambiguous changes in the long-range structure of Ga-MIL-53, while 69/71 Ga SSNMR parameters yield valuable information regarding the effect of the organic adsorbates on the local GaO6 environments. This approach shows promise for the ultra-wideline investigation of other quadrupolar metal nuclei in MIL-53 (e.g., In-MIL-53) and MOFs in general, particularly in regards to adsorption-related applications.