Three-Dimensional Covalent Organic Framework Membranes: Synthesis by Oligomer Interfacial Ripening and Application in Precise Separations

Covalent organic framework (COF) membranes are viable to tackle intractable separations, thanks to their uniform configurations and devisable functions. However, in stark contrast to the intensively studied two-dimensional COF membranes, three-dimensional (3D) COF membranes with unique merits remain largely unexplored because of formidable synthetic obstacles. Herein, we report a novel method to produce robust 3D COF membranes with interpenetrated nanochannels by ripening presynthesized oligomers at the organic-water interface. With the amphiphilicity and appropriate size, oligomers are prone to be entrapped at the designed interface. The ripening of interface-captured oligomers generates self-supporting crystalline 3D COF membranes with controllable thicknesses. The resulting membranes are provided with uniformly interpenetrated nanochannels, of which the aperture sizes ranging from ∼0.8 to 1.5 nm can be tailor-made by monomer design. We further identify the prominent competence of these 3D COF membranes in the high-precision fractionation of ions and molecules. Particularly, the sub-nanometer channels of the 3D COF-1 membrane allow for the efficient sieving of mono- and multivalent ions, attaining a K⁺/Al³⁺selectivity of >800. This work reports a facile but efficient strategy to synthesize COF membranes and opens up the applications of 3D COFs in the precise separation of ions and fine molecules.