Aqueous Synthesis of Ionic Covalent Organic Framework Nanosheets towards Membrane-Based Li+/Mg2+ Separation

Covalent organic framework (COF) nanosheets are ideal building blocks for fabricating high-performance membranes. The production of COF nanosheets in water is a significant step towards sustainable synthesis but remains an unresolved challenge. Herein, we report an aqueous synthesis method for COF nanosheets via a group capping-decapping strategy. The in-plane-dominated COF growth is driven by two factors: first, capping of aldehyde groups with secondary amines enhances monomer solubility in water for homogeneous nucleation; second, the presence of ionic groups on amine monomers suppresses out-of-plane stacking via electrostatic repulsion. By adjusting Brønsted acids or bases concentrations, the decapping rate and the subsequent COF polymerization rate can be tuned, enabling precise control over the reaction kinetics. Consequently, highly crystalline ionic COF nanosheets can be mildly synthesized in water with high volumetric yield and at a gram scale. The COF composite membranes assembled from nanosheets and polyethylenimine exhibit high selectivity and outstanding Li+ recovery efficiency for Li+/Mg2+ separation. Furthermore, the excellent processability of COF nanosheets is demonstrated by fabricating the tubular membrane module.