Masking covalent organic frameworks (COFs) with loose polyamide networks for precise nanofiltration

Covalent organic frameworks (COFs) with uniform, tunable nanoporosities are promising alternatives to prepare nanofiltration membranes with homogeneous nanochannels for high-resolution molecular sieving. Due to the poor membrane-forming ability as well as the relatively inadequate strategies for pore engineering, thus far, the preparation of continuous and defect-free COF membranes with tunable pore sizes is still challenging. Herein, we demonstrate an ingenious strategy to prepare the heterostructured membranes featuring the loose polyamide networks masked COF nanofilms for precise sieving of small molecules. The loosely structured polyamides are created by the interfacial polymerization to mask the chemically stitched (chemically interlinked) ultrathin COF nanosheets. The masked polyamide with flexible, loose networks not only enhance the membrane-forming ability of the natively rigid ultrathin COF nanofilms, but also provide a masking effect that can effectively tune the pore size of the heterostructured COF membranes. Benefitting from the synergistic enhancement of the membrane-forming ability and the pore size tunability, thus-prepared membranes exhibit excellent molecular sieving performances toward various small molecules. This work is expected to provide a guidance for the rational design and controllable preparation of COF membranes for precise nanofiltration.