Polydopamine-modulated covalent organic framework membranes for molecular separations

Covalent organic frameworks (COFs) are emerging porous crystalline materials with regular and permanent channels for constructing high-performance molecular sieving membranes. However, fabrication of COF membranes on porous supports in a facile manner remains a great challenge. Herein, a polydopamine (PDA)-modulated bottom-up approach was proposed to fabricate an ultrathin sulfonated imine-linked COF layer (SCOF, synthesized by Schiff-base reaction of 1,3,5-triformylphloroglucinol with 2,5-diaminobenzenesulfonic acid) on a polyacrylonitrile (PAN) support. The PDA layer deposited on the support acted as a molecular linker that attracted and bound COF building monomers, thus facilitating SCOF nucleation and growth of a defect-free SCOF layer. An optimized PDA deposition time of 1 hour provides sufficient adsorption sites for COF monomers without introducing additional water transport resistance. The resultant SCOF/PDA/PAN membrane shows an excellent water permeance of up to 1346 L m−2 h−1 MPa−1 with desirable dye rejection (e.g., Eriochrome black T and Congo red, >99.0%), which is nearly 7–12 times higher than that of most membranes with comparable rejection in the literature. This facile and universal PDA-modulated approach can be extended to a broad range of COF membranes and many other kinds of framework-based membranes.