2D conjugated microporous polymer membranes for organic solvent nanofiltration

Two-dimensional conjugated microporous polymer (CMP) membranes have gained increasing attention in the nanofiltration field owing to their tunable pore structure and solvent stability. Nevertheless, fabricating freestanding thin films with accessible porosity is challenging. In this study, we fabricated four freestanding CMP membranes based on the benzobisoxazole linkage and investigated their performance for organic solvent nanofiltration (OSN). The effect of pore size on the OSN performance was studied by investigating three CMP structures (B-BOP, TPPy-BOP, and TPB-BOP)-based membranes with various theoretical pore sizes (1.4, 1.6, and 2.8 nm, respectively). The effect of pore functionalization on the molecular sieving properties was evaluated by comparing two otherwise equivalent CMP structures with and without fluorine functionalization (TPB-BOP and TPB-F-BOP, respectively). The inherent control on the membranes’ molecular sieving properties is evidenced by the proportionality between the pore sizes of the CMP structures and their filtration performances. The molecular weight cutoff (MWCO) values for B-BOP, TPPy-BOP, TPB-BOP, and TPB-F-BOP were 676, 760, 1759, and 1402 g mol−1, respectively, which were consistent with the expected trend based on theoretical pore sizes. The experimental results are consistent with pore flow model predictions, indicating that the pore structure remains intact despite the materials lack long-range order. All the CMP membranes demonstrated long-term stability, exhibiting a constant acetone flux (212 L m−2 h−1 for B-BOP and 879 L m−2 h−1 for TPB-BOP) and Rose Bengal rejection (100 % for B-BOP and 67 % for TPB-BOP) over 120 h of continuous operation at 30 bar.