Covalent Organic Framework Membranes through Sequential Imine Exchange for Precise Molecular Separation

Covalent organic framework (COF) membranes with high crystallinity hold great promise in precise molecular separation, but often suffer from the intercrystalline defects and thus poor membrane-formation ability. This study reports a sequential imine exchange strategy to fabricate highly crystalline, defect-free COF membranes for precise molecular separation. Two functional amines (aromatic and aliphatic amines) are employed in highly reversible imine exchange reaction to separately conduct crystallization and defect remedy processes according to their different energy gaps. Aromatic amine, which serves as COF framework building unit, undergoes the first-step imine exchange for high crystallinity because of its lower energy gap. Afterward, the hyperbranched aliphatic amine with abundant amino groups undergoes the second-step imine exchange, affording the tight connections between adjacent crystals and the excellent membrane-formation ability. Accordingly, the COF membrane exhibits high permeance (344 L m^-2 h^-1 bar^-1 for water, 462 L m^-2 h^-1 bar^-1 for methanol) and rejection (> 99.9% for Congo red and Alcian blue). Meanwhile, the membrane is endowed with an ultrahigh separation factor (> 528) for mixed dye aqueous solutions and large-scale processability (> 290 cm²). This work offers a new strategy to fabricate highly crystalline and defect-free COF membranes, revealing their large potential in diverse practical applications.