Aqueous-Phase Preparation of High-Quality COF Nanofiltration Membranes for Dye/Salt Separation

Covalent organic frameworks (COFs) membranes have shown huge potential in low-cost nanofiltration treatment of saline industrial wastewater with high permselectivity. However, their practical application is often limited by complicated preparation processes, particularly the extensive use of toxic solvents such as 1,4-dioxane and dichloromethane. Herein, we present a facile and environmentally-friendly strategy to synthesize high-quality COF membranes exclusively in aqueous system instead of any organic reagents. The procedure involves depositing water-synthesized TpPa-1 onto a porous substrate via vacuum filtration, followed by cross-linking with epichlorohydrin (ECH). This method significantly mitigates health and safety risks by eliminating the use of hazardous organic solvents throughout the entire process. The green metrics assessment further confirmed the environmental advantages of aqueous synthesis strategy. The resultant TpPa-1-ECH membrane exhibited an excellent nanofiltration performance for dye desalination. The separation factor of Congo red/NaCl can reach 139, while the permeation flux can reach 450 L m -2 h -1 MPa -1 . The overall performance not only far surpasses most polymeric membranes but is also comparable to other reported COF membranes prepared with toxic organic solvents.