Impact of Ligand in Nickel-Based Metal–Organic Frameworks on Selectively Adsorbing 85Krypton from N2-enriched Radioactive Off-Gas

Removing radioactive ⁸⁵Kr effectively during spent fuel reprocessing is still a significant challenge due to the low absorption capacity and poor ⁸⁵Kr/N₂ selectivity of the currently available adsorbents. Herein, three types of nickel-based metal-organic frameworks (MOFs) were synthesized, including Ni-FA, Ni(ina)₂, and JUC-86, to investigate the impact of ligands on selective ⁸⁵Kr adsorption via experimental and quantum calculations, aiming to explore the critical structural properties that enhance Kr adsorption capabilities. Analysis of the experimental characterization reveals that the interaction between Kr and MOFs was influenced by the properties of ligands, independent of the specific surface area. Quantum computing results indicate that aromatic structured ligands can form multiple Kr···π interactions with Kr, thereby enhancing the affinity between the framework and Kr, and ligands containing N-heterocycles can further augment electron polarization between Kr and the framework, providing a unique affinity for Kr through van der Waals forces. JUC-86 features both aromatic and nitrogen-heterocyclic structures, exhibiting the highest ligand polarity, which enables it to achieve a Kr uptake of 2.71 mmol/g, surpassing those of Ni(ina)₂ (1.62 mmol/g), Ni-FA (0.76 mmol/g), and all previously reported adsorbents. More importantly, the JUC-86 demonstrated the best Henry constant of 8.72 mmol/g/bar for Kr, and the selectivity of Kr/N₂ was calculated to be 9.03, showing an excellent Kr affinity that indicates potential for capturing trace ⁸⁵Kr from N₂-rich off-gas streams. These findings revealed the key structural characteristics for ⁸⁵Kr adsorption of three Ni-MOFs and indicated that higher ligand polarity and extra functional groups are key factors in improving ⁸⁵Kr affinity, aside from pore size.