Hydrazone-linked covalent organic frameworks for membrane separation

Covalent organic frameworks (COFs), which are porous crystalline materials built using reticular and dynamic covalent chemistry, are attracting significant interest in advanced membrane separations. Their appeal stems from their higher mass transport efficiency and superior precision sieving, enabled by their ordered and modifiable pore channels, high porosity, and designable structure. Hydrazone-linked COFs, a subclass of Schiff base COFs, have emerged as promising membrane materials due to their large surface area, structural flexibility, and abundant heteroatomic sites. The versatility of their structure allows for precise tuning of pore size, architecture, and functionality by selecting specific building blocks or through post-modification, enabling the development of customized membranes for targeted separations. This review provides a comprehensive examination of the synthesis methods and applications of hydrazone-linked COF-based membranes, highlighting how their chemical stability, pore characteristics, and heteroatomic functionalities govern their performance. We analyze various fabrication techniques—including mixing, interfacial polymerization, covalent nanosheet stacking, and in situ growth—and discuss their impact on membrane performance. The applications in gas separation, water treatment, membrane catalysis, and energy storage are systematically evaluated, with a comparative analysis against conventional membrane materials. Finally, we identify persistent challenges related to scalability and long-term stability and outline future research directions to facilitate the practical implementation of these advanced membranes.