Fluorinated Covalent Organic Framework Membranes for Pervaporation Desalination

Pervaporation membrane process, which involves liquid-vapor phase transition, holds great promise for high-salinity water desalination. The exploitation of advanced membrane materials and the active manipulation of water transport behavior within membrane nanochannels remain highly challenging. Here, we report fluorinated covalent organic framework (COF) membranes, which are assembled from COF nanosheets with tunable fluorine density for pervaporation desalination. The optimized membrane delivers an exceptional water flux of 435.75 kg m-2 h-1 (3.5 wt % NaCl, 70 °C) with 99.99% salt rejection and maintains 176.38 kg m-2 h-1 under 15 wt % hypersalinity, along with superior chlorine tolerance. Experimental characterization and simulations reveal a fluorination-governed mechanism for phase-transition-coupled water transport. Fluorinated nanochannels disrupt the hydrogen-bond network of confined water, generating a low-coordination, weakly bonded state. Such water structural transformation lowers the energetic barrier for water evaporation within nanochannels, facilitating liquid-vapor phase transition. Meanwhile, the weakened water-wall interactions reduce transport resistance, enabling efficient water permeation. The electronegativity of fluorinated nanochannels establishes an effective electrostatic barrier against hydrated salt ions. This work demonstrates the potential of fluorinated COF membranes in high-performance pervaporation desalination and provides insights into fluorination-facilitated water transport in membrane processes involving phase transition.