Highly crystalline and robust covalent organic framework membranes for predictable solvent transport and molecular separation

Covalent organic frameworks (COFs) with well-ordered nanopores and stable structures have excited exponential interest as potent membrane platforms for molecular separation in organic solvent systems. However, the fabrication of defect-free and crystalline COF membranes remains extremely challenging due to the poor processibility of COF powders. Herein, we report the facile in situ synthesis of highly crystalline COF membranes on the confined surface of porous substrates via contra-diffusion interfacial polymerization. The continuous imine-linked COF membranes with high structural rigidity exhibited robust chemical stability in organic solvents. The variable membrane thickness was achieved by tailoring the initial monomer concentration. Notably, the synthesized COF membranes displayed tunable solvent permeance and solute rejection with a linear dependence on their thickness. The linear relationship between the membrane thickness and pore tortuosity was further demonstrated, enabling the reasonable prediction of solvent permeance based on the pore-flow model. Owing to the regular hexagonal nanochannels, the COF membrane exhibited precise molecular sieving capability toward molecules with varying dimensions in the organic solvent. This work showcases the immense potential of crystalline COF membranes for molecular separation in harsh organic solvents.