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

Abstract

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.