Engineering Fractional Free Volume in Cross-Linked MOF-Polyimide Membranes for Gas Separation

Mixed matrix membranes (MMMs) are an energy efficient and highly tunable approach to gas separation, achieved by integrating the distinct transport properties of polymers and nanomaterials. However, MMMs commonly face challenges such as trade-off between permeability and selectivity, as well as CO 2 plasticization. In particular, polyimide membranes suffer from low resistance to CO 2 plasticization, typically below 15 bar. In this study, we present a synergistic approach to overcome the permeability-selectivity trade-off, and enhance CO 2 plasticization resistance through the rational engineering of the fractional free volume (FFV) of 6FDA-durene polyimide-based MMMs. A highly permeable aluminum-based metal-organic framework (MOF), Christian-Albrechts-Universität (CAU-10-H) was first incorporated into the 6FDA-durene polyimide matrix to enhance the FFV, thereby improving gas permeability. A small loading of CAU-10-H increased the CO 2 permeability by 98.8% (e.g., from 501 to 996 Barrer), with only a slight decrease in CO 2 /N 2 selectivity (e.g., from 15 to 13.4). Subsequently, sub-nanometer transport control was achieved by introducing amine-based cross-linkers to regulate the expanded polyimide matrix and strengthen the polymer-MOF interface. The optimized 6D/CAU(3):EDA-2h membrane demonstrated a H 2 /CO 2 selectivity of 8.5 while maintaining a high H 2 permeability of 34 Barrer. This represents an 8.5-fold improvement in selectivity compared to the uncross-linked MMM, surpassing the 2008 Robeson upper bound. Furthermore, the membrane demonstrated improved resistance to CO 2 plasticization up to 30 bar and moderate resistance to physical aging, exhibiting a 43% decrease in H 2 permeability over 100 days. These findings highlight the potential of synergistic filler-matrix-cross-linker integration as an effective strategy for advancing MMMs toward next-generation gas separation membranes.