New Discovery of Metal-Organic Framework UTSA-280: Ultrahigh Adsorption Selectivity of Krypton over Xenon

Treatment of radioactive noble gases, such as Xe and Kr, has attracted special attention in the context of used nuclear fuel (UNF). In recent years, metal-organic frameworks (MOFs) are being actively investigated on adsorption separation of Xe/Kr to get high-purity Xe. However, a few reports about experimental and hypothetical MOFs on the selective adsorption of Kr/Xe to get high-purity Kr can be found, in spite of the special importance of Kr. In this work, ultramicroporous MOF [Ca(C4O4) (H2O)] (UTSA-280) with one-dimensional rigid channels and pore size of 3.806 Å, which had been formally fabricated for the adsorption and separation of ethane and ethene, was sieved out from 30 MOFs by GCMC simulation, and it is found that there is a very large selectivity of Kr/Xe of 72.1 on UTSA-280 with a high Kr uptake of 1.4832 mmol/g. This represents the first study of MOF of selective Kr/Xe separation at normal temperature and pressure. The plotted adsorption isostere indicates a strong Kr-Kr interaction at high loading compared to Xe-Xe. Molecular dynamics (MD) simulation, density functional theory (DFT), and spatial probability density (SPD) calculations all reveal that the exceptionally high Kr uptake capacity and Kr/Xe selectivity result from the synergy of the confinement effect and van der Waals interaction of UTSA-280. Further energy decomposition analysis (EDA) at the symmetry-adapted perturbation theory (SAPT) shows that the main contribution of the adsorption of Kr on UTSA-280 are induction and dispersion interactions. In addition, it is shown that the Kr uptake on UTSA-280 with different metal centers is positively correlated with the largest cavity diameters (LCDs) and porosities of UTSA-280-M (M = Cu, Zn, Co, Ni, and Ca). Importantly, UTSA-280 has a high water stability and is easily synthesized at a large scale under environmentally friendly and economically efficient conditions. The present study may provide valuable information for the synthesis of superior materials for the entrapment of Kr from the Kr/Xe mixture.