Rapid synthesis of ultrathin covalent organic polymer membranes with subnanometer pores for efficient organic solvent nanofiltration

Covalent organic polymers (COPs) with abundant micropores are deemed as promising materials for building molecular separation membranes. In spite of this potential, fabricating COP separation membranes through an efficient and scalable method remains a significant challenge. Herein, we report a simple and efficient strategy for the synthesis of COP membranes with three-dimensionally interconnected micropores. Utilizing transition-metal nitrates as a catalyst, we achieve fast polymerization of amorphous COPs with relatively uniform micropores (0.7 nm in diameter). The rational design of organic-aqueous interface allows for the direct fabrication of robust COP membranes with an ultrathin thickness of ∼20 nm on porous polyacrylonitrile substrates. The resulting hydrophobic but pore-uniform frameworks in these membranes permit fast permeation of organic liquids with a notable molecular weight cutoff of 388 g mol⁻¹ in ethanol. To demonstrate scalability, we show that our strategy can produce large-size COP membranes with a prominent area of 200 cm², which exhibit performances similar to that of small membrane coupons. The present study offers a potentially scalable method for producing highly microporous COP membranes toward efficient molecular separation in organic liquids.