Covalent organic framework composite membranes with skeleton charge for efficient desalination

Covalent organic frameworks (COFs) have emerged as promising candidates for membrane desalination due to high porosity and well-defined channel structure. However, the COFs with relatively large pore size cannot effectively separate small ions through size sieving effect, resulting in low rejection. Herein, we fabricate two COF membranes with different surface charge properties, namely Tp-PaBr-COF (negative electricity) and Tp-Pa-COF (near-neutral), directly on porous ceramic tubes using solvothermal method. The introduction of bromine atoms into the skeleton of Tp-PaBr-COF membrane leads to significant negative-charges, resulting in enhanced ion repulsion and improved ion rejections in the desalination. The Tp-PaBr-COF membrane exhibits a higher ion rejection (99.9%) than the Tp-Pa-COF membrane (98.2%), while maintaining a comparable water flux (23.3 L m−2 h−1) during the desalination of 3.5 wt.% NaCl solution at 80 °C. It indicates that the surface charge of Tp-PaBr-COF membrane contributes to high ion rejection via the Donnan exclusion effect.