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.

The separation of radioactive noble gases, such as Xe and Kr, has attracted special attention in the context of used nuclear fuel (UNF). In this study, 180 metal–organic frameworks (MOFs) formally used for selective adsorptions of ethane and ethylene, with a similar kinetic diameter to Kr and Xe, were initially screened for the Kr/Xe separation using the grand canonical Monte Carlo (GCMC) method. Then, the structure–adsorption property relationships were generalized, that is, the MOFs of higher Kr/Xe selectivity are with the porosity at 0.2–0.4 and the ratio of the largest cavity diameter/pore limiting diameter at 1.0–2.4. Based on the relationships, six reported MOFs with large Kr uptakes and Kr/Xe selectivities were experimentally screened out and validated by GCMC simulations within the CoRE-MOF database, which are higher than most reported MOFs under conditions pertinent to nuclear fuel reprocessing of an 80/20 v/v mixture of Kr/Xe at normal temperature and pressure. Further simulations reveal that higher Kr uptakes and Kr/Xe selectivities of six MOFs result from the confinement effect of the pores. Molecular dynamic simulations showed that the six MOFs are ideal membrane separation materials of Kr from Xe, which are driven by adsorption and diffusion. Analyses of electronic structure-based density functional theory calculations showed that the main interaction between Kr and the six MOFs is van der Waals force dominated by dispersion and induction interactions. Therefore, the generalized structure–adsorption property relationships may assist the screening of MOFs for the separation and production of Kr/Xe from UNF industrially.

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