Alkyl-chain engineered ionic COFs eliminate interfacial defects and transcend the permeability-selectivity trade-off for high performance CO2/CH4 separation

Mixed matrix membranes (MMMs) incorporating porous fillers offer transformative potential for gas separations, yet their practical implementation is often hindered by interfacial defects and the inherent permeability–selectivity trade-off. Herein, we propose a covalent organic framework (COF) engineering strategy that integrates ionic functionalization with alkyl chain length modulation (C2, C8, C12) to fabricate high-performance 6FDA-DAM-based MMMs. By grafting bromoalkanes onto the pyridinic sites of TpDp COF, the resulting ionic COFs (iCOF TpDp–CnBr) synergistically eliminate interfacial voids through Br − -mediated electrostatic attraction and alkyl-dependent van der Waals interactions, while the bromide anions establish CO2-philic nanochannels via ion–quadrupole interactions. At 10 wt% loading, the 6FDA-DAM/TpDp–C8Br MMM achieves a CO 2 permeability of 363.7 Barrer and a CO 2 /CH 4 selectivity of 48.7, representing a 2.4-fold increase in permeability and a 2.7-fold enhancement in selectivity over the pristine polymer. This dual improvement transcends the 2008 Robeson upper bound, driven by optimized interfacial adhesion and tailored redistribution of free volume. Long-term stability tests (35 °C, 0.2 MPa) confirm structural integrity under the evaluated conditions. Density functional theory simulations reveal that the C8 alkyl chain minimizes interfacial energy by balancing steric hindrance and polymer–filler compatibility, establishing a charge–chain molecular design paradigm for next-generation membranes with promising application in natural gas purification.