Microwave-Assisted Fabrication of Highly Crystalline, Robust COF Membrane for Organic Solvent Nanofiltration

Fabrication of crystalline, robust covalent organic framework (COF) membranes based on disorder-to-order strategy is promising yet highly challenging. Herein, a microwave-assisted method for fabricating COF membranes is proposed. Initially, monomers polymerize rapidly on the surface of porous Al 2 O 3 substrate at room temperature to form an amorphous pristine membrane. Subsequently, a microwave field is exerted to trigger fast crystallization, acquiring a crystalline COF membrane within 60 min. The amorphous pristine membrane exhibits a high dissipation factor, indicating excellent microwave absorption capability, which accelerates the dynamic reversible reactions during the microwave treatment and thus ensures a rapid transition from the amorphous to the crystalline state. Owing to the high-crystallinity and robust structure, the COF membranes exhibit high rejection rates for solute molecules with molecular weights exceeding 700 Da (e.g., Evans blue: 98.7%) and high solvent permeance for organic solvents (e.g., ethanol: 87.8 Lm −2 h −1 bar −1 , n-hexane: 222.3 Lm −2 h −1 bar −1 ). Surprisingly, the COF membranes exhibit superior mechanical properties, with Young's modulus of 33.91 ± 3.94 GPa, outperforming previously reported polycrystalline COF membranes and are close to those for inorganic zeolite membranes. The microwave-assisted COF crystallization method opens a new avenue to fabricating a variety of crystalline membranes for advanced separations.