6FDA-Based Co-Polyimide Membranes Incorporating Modulated MOF-808s for Olefin/Paraffin Gas Separations

MOF-808 was synthesized using different (perfluoro)carboxylic acid modulators, including acetic acid (AA), butyric acid (BA), trifluoroacetic acid (TFAA) and heptafluorobutyric acid (HFBA). These samples were incorporated into co-polyimide 6FDA-DAM:DABA (6FDD), and the performance of the resulting MMMs was assessed for C₂ and C₃ olefin/paraffin separation. Enhanced permeability was observed for both C₂H₄/C₂H₆ and C₃H₆/C₃H₈ mixtures thanks to the introduced porosity upon filler incorporation in all cases. Due to the large pore size of MOF-808, diffusion-selective permeation through the polymer phase of the MMMs determined the eventual selectivity for C2 gases, leading to separation factors similar to that of the unfilled 6FDD membrane. For C₃H₆/C₃H₈ separation, the incorporation of fluorinated MOFs significantly improved separation performance, unlike their non-fluorinated counterparts. The unfilled 6FDD membrane exhibited a C₃H₆/C₃H₈ separation factor of 7.4 with a C₃H₆ permeability of 22 Barrer, while the incorporation of MOF-808-TFAA and MOF-808-HFBA led to C₃H₆/C₃H₈ separation factors of 13.1 and 13.5 with corresponding improved C₃H₆ permeabilities of 42 Barrer and 33 Barrer, respectively. Considering that these MMMs showed C₃H₆ permeabilities similar to those of MMMs containing their non-fluorinated MOF counterparts that exhibited no enhancement in membrane selectivity, the improved C₃H₆/C₃H₈ separation factor was attributed to the preferential adsorption of C₃H₈ over C3H6 on the fluorinated MOFs, acting as a trap for C₃H₈ and reducing its diffusivity. These results highlight the significance of matching the permeation characteristics of the selected polymer-filler pair on MMM performance for different gas pairs.