Ammonia modulated synthesis of covalent organic framework (COF) membranes for efficient dye separation

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Traditional solvothermal synthesis of COFs requires stringent conditions, including high temperature, free of oxygen and prolonged time. Regulating the growth process by monofunctional inhibitors via slow imine-exchange can produce COFs and COF membranes under mild conditions. However, commonly used inhibitors form stable imine-linked ​with aldehyde monomers, resulting in long-time imine-exchange process ​and introducing​membrane defects. This study proposed a new strategy utilizing ammonia modulation for the synthesis under mild conditions. Ammonia could be easily removed, which accelerates the imine-exchange process to promote the growth and assembly of COF crystallites. The ammonia-modulated synthesis achieved much higher yield (86%) of crystalline COFs than aniline-modulated synthesis did (34%). This mechanism enabled to establish two methods of Volatilization-driving method and Solvent-growth method for preparing COF membranes. The uniform growth and strong assembly of COFs under ammonia modulation ensured the dense structure and high performance of the COF films synthesized by both methods. Typically, the optimized COF membrane exhibited 100% rejection to dyes of Congo red, Direct blue, Brilliant blue R250, and Alcian blue with excellent permeance, highly competitive compared with other state-of-art membranes. The proposed synthesis strategy provides a viable option and corresponding theoretical guidance for facile preparing high-performance COF films.