Uniform Covalent Polymerization of Zirconium–Organic Cages for High-Loading CO ₂ Separation Membranes

Metal-organic cages (MOCs), with their well-defined molecular structures and uniform porosity, hold great promise for application in gas separation membranes. However, the aggregation of MOCs in membranes at high loadings during conventional physical blending drastically deteriorates the separation performance. Herein, this study presents a strategy of MOC site-specific polymerization for fabricating uniform, high-loading, and selective membranes for CO₂ separation. An amino-functionalized zirconium MOC (ZrTNH₂) is synthesized as the building block and subsequently polymerized with poly(ethylene glycol)diisocyanate (NCO-PEG-NCO) through extensive nucleophilic addition reactions. The resulting ZrTNH₂-PEG membrane exhibits a remarkable loading of up to 75 wt %, while preserving the discrete structural characteristics of ZrTNH₂. It exhibits a high CO₂ permeability of 577 Barrer and an exceptional CO₂/N₂ selectivity of 50 at 298 K and 2 bar, surpassing the latest upper bound and many previously reported MOC-based membranes. This membrane design greatly advances the mixed-matrix concept in membranes and serves as a new starting point for next-generation separation materials.