Pressure-modulated synthesis of self-repairing covalent organic frameworks (COFs) for high-flux nanofiltration

Covalent organic frameworks (COFs), with their inherent merits of specific pore size and uniform channels, have been extensively employed to produce nanofiltration (NF) membranes. However, COF-based NF towards precise separations, especially for ion separations, are often unsatisfying due to the intercrystalline defects in selective layers. Herein, a pressure-modulated synthesis method has been explored to prepare crystalline and defect-free COF membranes on seeded substrates for fast NF. Amine and aldehyde solutions are separately placed with a level interval to create vertical hydraulic pressure, which can regulate the mobility of the amine monomers and thus results in the self-repairing of defects in the COF planes and remediation of intercrystalline gaps. The abundant nucleation sites on seeded substrates promote the confined growth of COF crystallites in top layers, leading to an ultrathin selective layer with improved permeance. The resultant COF membrane shows tight methyl orange (~90.4%) and Na2SO4 (~63.6%) rejections with a pronounced water permeance of up to ~44.2 L m−2 h−1 bar−1, which is ~2–10 times higher than other NF membranes with similar rejections. This pressure-modulated synthetic strategy establishes not only the self-repairing synthesis of COFs but also the controllable preparation of defect-free COF-based NF membranes, thus enabling precise and fast separation of molecules and ions. [Display omitted] •A pressure-modulated synthesis method was built up to fabricate highly crystalline COF nanofiltration membranes.•The external pressure precisely regulates the mobility of precursors and eliminates the nonselective intercrystalline gaps.•Abundant nucleation sites promote the conformal growth of COF crystals.•The membranes exhibit excellent water permeance and rejections to objects with a size above 1.1 nm.