Strong Acid and Organic Solvent Resistant Highly Crystalline Ethylene‐Bonded COFs Nanofiltration Membranes for Precise Separation of Organic Small Molecules

Abstract Conventional reversible covalent bonds in covalent organic frameworks (COFs) are prone to hydrolytic degradation under harsh conditions, compromising their stability. This work addresses this by replacing them with irreversible, high‐energy ethylene linkages. Herein, highly crystalline ethylene‐bridged 2D COFs membranes are synthesized via Knoevenagel condensation between benzaldehyde and 2,4,6‐trimethylpyridine. Using an economical potassium hydroxide catalyst, the process yields COFs with well‐defined pores, high specific surface areas, and remarkable crystallinity. The resulting TFB TMP‐COFs demonstrate a high pure water permeance of 248 L·m −2 ·h −1 ·bar −1 , a good salt/dye separation factor of 16.33, and superior molecular sieving capabilities. These membranes reject over 90% of active pharmaceutical ingredients and over 95% of dye molecules. Furthermore, the TFB TMP‐COFs composite membranes exhibit excellent chemical stability, even in strong acids and organic solvents like tetrahydrofuran, methanol, and ethanol. Crucially, they show significantly enhanced solvent resistance compared to commercial P84 membranes. This work establishes a foundation for employing highly crystalline and ultra‐stable vinyl‐bridged COFs nanofiltration membranes in organic small molecule purification.