Nonpolar Pore Confinement within Metal–Organic Frameworks for Xe/Kr Separation

Adsorptive separation of Xe/Kr is challenging due to their similar properties. A notable difference between Xe and Kr lies in their polarizability, with Xe being much “softer” (having a higher polarizability). By taking advantage of this, in this work, nonpolar methyl groups were incorporated into metal–organic frameworks (MOFs) in a programmable manner to provide a synergistic effect derived from the pore confinement and nonpolar pore environment. The pillar-layered MOF, NNM-30, can capture Xe with an exceptionally high capacity of 3.07 mmol/g at 298 K and 20 kPa, a more than 7-fold increase compared to that achieved with pristine MOF (Co-DMOF). A high Xe/Kr selectivity (16.56) was also observed. The excellent separation capacity under dry or humid conditions for a 20:80 Xe/Kr mixture was confirmed by breakthrough experiments. Additionally, Co-DMOF-(CH₃)₄ showed efficient Xe capture at an ultralow concentration (400 ppm), which indicates it is promising for Xe removal from the used nuclear fuel reprocessing off-gas.