Interplay between noble gases and MOFs: Insights from ¹²⁹ Xe and ⁸³ Kr NMR spectroscopy

Efficient separation and storage of radioactive noble gas isotopes [krypton-85 (⁸⁵Kr) and xenon-131m/133/135 (^131m/133/135Xe)] from off-gas during spent nuclear fuel reprocessing are vital for environmental protection and noble gas recovery. Metal-organic frameworks (MOFs) are promising adsorbents, yet atomic-level insights into Xe and Kr adsorption remain limited. Here, we report the first application of high-field solid-state ⁸³Kr nuclear magnetic resonance (NMR), alongside extensive ¹²⁹Xe NMR, to investigate gas adsorption in eight MOFs representing three design strategies: ultramicroporous one-dimensional-channel MOFs with pore sizes matching noble gas diameters, functionalized MOFs, and MOFs with open metal sites. Single-gas and coadsorption studies reveal distinct adsorption sites, guest-host interactions, and gas dynamics in each MOF. Most MOFs retain crystallinity after exposure to 60-kilogray γ radiation. Several radiation-sensitive MOFs exhibit enhanced stability in the presence of Xe, suggesting that Xe adsorption enhances framework stability and may broaden the range of MOFs usable under γ radiation in nuclear off-gas separation. These findings offer valuable molecular-level design insights.