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Bimetallic Zeolitic Imidazolate Frameworks-containing Mixed Matrix Membranes for C3H6/C3H8 Separation Performance Enhancement


Mixed matrix membranes (MMMs) are essentially hybrid membranes consisting of dispersed molecular sieves and continuous polymer matrix. Zeolitic imidazolate frameworks (ZIFs) are the cutting-edge microporous crystalline materials consisting of tetrahedrally-coordinated transition metal ions linked by imidazolate organic linkers. ZIFs are attractive molecular sieving materials due to versatile tunability on both their pore structures and chemical properties. In this work, three different ZIFs including ZIF-8, Co-substituted ZIF-8 (ZIF-67), and equimolar Zn:Co-mixed ZIF-8 (ZIF-8-67) were incorporated into 6FDA-DAM (PI) polyimide matrix for C3H6/C3H8 separation performance enhancement. All three MMMs improved C3H6/C3H8 separation performance compared to that of the pristine PI membrane due to the appropriate molecular sieving behavior of ZIFs. More interestingly, the bimetallic ZIFs (ZIF-8-67) containing MMM exhibited the highest extent of enhancement in C3H6 permeability by as much as 213.7% compared to that of the pristine PI despite the lowest extent of C3H6/C3H8 selectivity improvement among three different MMMs. Furthermore, In comparison to PI, the C3H6 permeability of PI/ZIF-8-67 (28.9 wt%) showed significant enhancement (ca. 272%) without scarifying C3H6/C3H8 selectivity (ca. 60% enhancement). Such unexpected C3H6/C3H8 separation performance in PI /ZIF-8-67 MMMs was ascribed to the largest pore limiting diameter of ZIF-8-67, which was confirmed by Rietveld refinement-based aperture size estimation. Also, the temperature dependent transport properties in all three MMMs improved C3H6/C3H8 energetic selectivity compared to that of the pristine PI membrane at the sacrifice of entropic selectivity due to the breathable behavior of the ZIF pore structure, thereby enhancing the diffusivity selectivity.