Reducing the Pore Size of Covalent Organic Frameworks in Thin-Film Composite Membranes Enhances Solute Rejection

Three imine-linked covalent organic framework (COF) films are incorporated as active layers into separate thin-film composite (TFC) membranes and tested for their ability to reject an organic pollutant surrogate and salt from water. The synthesized membranes consist of a polyacrylonitrile (PAN) support and a TAPB-PDA-H, TAPB-PDA-Me, or TAPB-PDA-Et COF thin film. The latter two COFs direct six methyl and ethyl substituents per tiled hexagon into the pores, respectively, while maintaining the same topology across the series. These substituents decrease the effective pore size of the COF compared to the parent TAPB-PDA-H COF. The TFC TAPB-PDA-Me membrane rejects Rhodamine-WT (R-WT) dye and NaCl better than the TFC TAPB-PDA-H membrane, and the TFC TAPB-PDA-Et membrane exhibits the best rejection overall. The solution-diffusion model used to analyze this permeation behavior indicates that there is a systematic difference in rejection as subsequent pendant groups are added to the interior of the COF pores. These findings demonstrate the concept of tuning the selectivity of COF membranes by systematically reducing the effective pore size within a given topology.