A two-dimensional cationic covalent organic framework membrane for selective molecular sieving

Abstract

The search of new-type membrane materials with ideal molecular sieving caused wide interest in both academia and industry. Covalent organic frameworks (COFs) are excellent candidates for efficient molecular separation because of their well-defined pore structure and fine-tuned pore size. However, existing synthetic approaches of COFs mainly result in an insoluble and unprocessable powder, which severely restricts their widespread applicability. In this work, a facile bottom-up interfacial crystallization approach to obtain a two-dimensional (2D) cationic COF, EB-COF:Br nanosheets, is reported. Then a layer-by-layer restacking process is performed to fabricate a continuous and dense 2D ionic COF membrane with a tunable thickness by simple vacuum filtration. The 2D COF membrane shows much higher solvent permeability than graphene-oxide membranes and commercial nanofiltration membranes because of its high porosity. Moreover, due to there being abundant positive charge sites in its pore walls, the EB-COF:Br membrane demonstrates highly selective sieving performance for dye molecules/ions with different charges and sizes. The EB-COF:Br membrane can efficiently reject ∼>98% of anionic dye molecules/ions, while maintaining high solvent permeability. The cationic 2D COF membrane far outperforms other nanofiltration membranes in terms of excellent selective molecular/ionic sieving and superior solvent permeability. The result suggested that the ionic COF membrane can offer a new avenue for separation technology.