Nanoporous Metal Organic Frameworks with Open Metal Sites for Gas Separation Membranes

Metal–organic frameworks (MOF) featuring coordinatively unsaturated sites (CUS) present a transformative opportunity for membrane-based gas separations, enabling a shift from passive molecular sieving to active, affinity-driven transport. These specific recognition centers are engineered through distinct synthetic strategies, ranging from the thermal removal of labile solvent ligands in stoichiometric frameworks to the precision introduction of missing-linker or missing-cluster defects via modulator-controlled synthesis. This review provides a comprehensive analysis of CUS-MOF based membranes, detailing how open metal nodes facilitate specific recognition mechanisms─such as electrostatic interactions and pi-complexation─to achieve superior selectivities for energy-intensive, industrially relevant mixtures like CO2 and olefin/paraffin separation. Furthermore, we elucidate the dual function of these sites: beyond separation, they act as critical molecular anchors at the filler–polymer interface, suppressing nonselective voids and enhancing plasticization resistance in mixed-matrix membranes.