The Ionic Liquid–H₂O Interface: A New Platform for the Synthesis of Highly Crystalline and Molecular Sieving Covalent Organic Framework Membranes

Covalent organic frameworks (COFs) are highly porous crystalline polymers with uniform pores and large surface areas. Combined with their modular design principle and excellent properties, COFs are an ideal candidate for separation membranes. Liquid–liquid interfacial polymerization is a well-known approach to synthesize membranes by reacting two monomers at the interface. However, volatile organic solvents are usually used, which may disturb the liquid–liquid interface and affect the COF membrane crystallinity due to solvent evaporation. Simultaneously, the domain size of the organic solvent–water interface, named the reaction zone, can hardly be regulated, and the diffusion control of monomers for favorable crystallinity is only achieved in the water phase. These drawbacks may limit the widespread applications of liquid–liquid interfacial polymerization to synthesize diverse COF membranes with different functionalities. Here, we report a facile strategy to synthesize a series of imine-linked freestanding COF membranes with different thicknesses and morphologies at tunable ionic liquid (IL)–H2O interfaces. Due to the H-bonding of the catalysts with amine monomers and the high viscosity of the ILs, the diffusion of the monomers was simultaneously controlled in water and in ILs. This resulted in the exceptionally high crystallinity of freestanding COF membranes with a Brunauer–Emmett–Teller (BET) surface area up to 4.3 times of that synthesized at a dichloromethane–H2O interface. By varying the alkyl chain length of cations in the ILs, the interfacial region size and interfacial tension could be regulated to further improve the crystallinity of the COF membranes. As a result, the as-fabricated COF membranes exhibited ultrahigh permeance toward water and organic solvents and excellent selective rejection of dyes.