Covalent organic framework membrane with hourglass-shaped nanochannels for ultrafast desalination

Covalent organic framework (COF) holds great potential as next-generation high-performance desalination membrane material owing to their uniform nanochannels (homo-nanochannels) and abundant functional groups, and the hierarchical structures of nanochannels should be rationally designed to break the trade-off between water permeability and ion rejection. Here, a kind of COF membrane with hourglass-shaped nanochannels is fabricated by installing amino-cyclodextrin nanoparticles (CDN) onto the mouth of COF membrane via sequential assembly. The resulting hetero-nanochannels consist of a hydrophilic conical entrance (~1.6 nm) and a hydrophobic spout (~0.5 nm), created by the CDN specific cavity, onto the homo-nanochannels of COF with intrinsic nanopores (~1.4 nm). The hydrophilic conical entrance facilitates the entry of water molecules, whereas the hydrophobic spout and the homo-nanochannels collectively enable fast water transport. Meanwhile, the amino groups on CDN endow the hetero-nanochannels with pH-responsive ability to dynamically regulate their effective size and charge. Accordingly, the optimum COF-CDN membrane exhibits high desalination performance, with a water flux of 98 L m^-2 h^-1, and rejection of 94% for Na₂SO₄ and 92% for NaCl. The COF-CDN membrane also exhibits superior operational stability (7 days) and pH cycle stability, validating the utilization of COF membrane in efficient desalination.