Scalable and Versatile Fabrication of Free-Standing Covalent Organic Framework Membranes with Tunable Microstructure for Molecular Separation

Covalent organic framework (COF) membranes hold significant promise for applications in separation, catalysis, and energy conversion; however, their industrial adoption has been hindered by the lack of scalable and efficient fabrication methods. Here, we present a fast, versatile, and broadly applicable strategy for fabricating free-standing and flexible COF membranes by casting precursor suspensions, followed by heat treatment under controlled humidity. This approach enables the fabrication of COF membranes with lateral dimensions up to several square decimeters and thicknesses that are tunable down to submicron levels within 1 h. It demonstrates remarkable versatility for producing a family of ketoenamine-linked COF membranes through the condensation of 1,3,5-triformylphloroglucinol with various amine monomers differing in length, side groups, and geometry. The resulting crack-free COF membranes exhibit high mechanical strength, with ultimate tensile strength up to 60 MPa and Young's modulus up to 1.7 GPa, as well as exceptionally high porosity, with Brunauer-Emmett-Teller (BET) surface areas reaching up to 2226 m² g^-1. More importantly, the morphology, porosity, and crystallinity of the membranes can be finely tuned by modulating the heating conditions. The membranes with optimized microstructures demonstrate excellent separation performance, achieving over 99% rejection in nanofiltration of aqueous dye solutions, and a separation factor of 11 with an H₂ permeance of 2857 GPU in H₂/CO₂ gas separation. This approach provides a scalable and effective pathway toward large-scale COF membrane manufacturing for advanced molecular separations and other membrane-based technologies.