Hydroxyl-functionalized Covalent Organic Framework Membranes: Fast Organic Solvent Nanofiltration

Two-dimensional covalent organic framework (COFs) membranes have shown promise for organic solvent nanofiltration applications. However, the ability to modulate the chemical properties of the membranes and their effects on the molecular transport process has not yet been explored. Here, we demonstrate the synthesis of two COF membranes (TFP-MP_OHF and TFP-MPF) with the same scaffold structures but different internal chemical properties. The presence of hydroxyl groups in the TFP-MP_OHF membranes resulted in a significant improvement in polar solvent permeability. In contrast, the hydrophobic TFP-MPF membranes offered excellent permeability to nonpolar solvents, which was 130 - 235% higher than the TFP-MP_OHF and commercial polymeric membranes. In addition, both COF membranes exhibited precise molecular sieving capacity with an apparent molecular weight cutoff (MWCO) of 800 g mol^-1 and excellent stability. A deviation from the pore-flow model was observed for the TFP-MP_OHF membranes, which was due to the specific interactions between solvent molecules and polar channel walls.