Taming Flexibility: Synergistic Pore and Polarity Engineering in a MOF for High-Efficiency Xe/Kr Separation

The separation of xenon (Xe) and krypton (Kr), two inert gases with nearly identical physicochemical properties, remains a formidable industrial challenge. Flexible metal-organic frameworks (MOFs) offer guest-specific recognition via structural responsiveness; however, their application in low-concentration separations is limited by intrinsic flexibility and high gate-opening pressures, as exemplified by ZIF-7. Herein, we propose and experimentally substantiate a synergistic regulation strategy that concurrently modulates the pore size and polarity of the framework, thereby overcoming the intrinsic flexibility of MOFs and enabling exceptional Xe/Kr separation performance. By incorporating 20% chlorine into the framework (ZIF-7-Cl(20)), the pore environment is effectively polarized, promoting highly selective Xe recognition while preserving adsorption capacity. The optimized material exhibits an IAST selectivity of 30.8 for Xe/Kr (20/80, v/v). Grand Canonical Monte Carlo simulations reveal strong Xe─Cl interactions as the origin of the enhanced selectivity. Dynamic breakthrough experiments further demonstrate excellent separation performance, affording high-purity Kr (>99.9%) with a productivity of 129 L kg^{- 1} and a retention time of 84 min g^{- 1}. This work provides a generalizable strategy for converting flexible MOFs into high-performance adsorbents for challenging gas separations.