Mechanisms Governing Noble Gas Adsorption in a Copper-Based Metal-Organic Framework

Metal-organic frameworks (MOFs) are promising sorbents for the capture and separation of noble gases from off-gases at nuclear reprocessing sites and nuclear waste repositories. However, little is known about the degradation mechanism of MOFs under ionizing radiation. Herein, CuBTC was exposed to high doses of ⁶⁰Co gamma irradiation, between 1 and 5 MGy, and characterized, to elucidate the impact of radiation. Postirradiation, the N₂ gas adsorption of CuBTC decreased, but unexpectedly, gas uptake and surface area at the moderate dose of 3 MGy were lower than at a dose of 5 MGy. Xenon (Xe) gas uptake also decreased upon irradiation, but the adsorption loss was proportional to the irradiation dose received. In contrast, the krypton (Kr) uptake increased until 5 MGy indicating that pore constriction had occurred. Comparison of the reflection intensities of the (200) and (220) aligned with observed surface area trends postirradiation. Fourier transform infrared spectroscopy was used to probe the local structure of the framework, and the noncoordinated linker bonding moieties increased then decreased. This suggested that gamma irradiation caused coordination defects that blocked the pores of the MOF, aligning with the gas adsorption and X-ray analyses. Combined, these findings indicate that though the bulk framework remains stable up to 5 MGy, the pore properties of the MOF are acutely sensitive to irradiation. Altogether, this work illustrates a methodology to comprehend the porosity changes of an MOF upon exposure to high-dose ionizing radiation.