Designing and Molding Covalent Organic Frameworks for Separation Applications

ConspectusSeparation processes hold a pivotal role not only in the modern chemical industry but also in our daily lives. Yet, conventional separation methods have been synonymous with excessive energy consumption, exacerbating global environmental concerns. Therefore, exploring alternative separation technologies with low energy consumption and environmental friendliness is of great significance and in urgent demand. Porous materials have emerged as a beacon of hope in this regard, offering superior energy efficiency, minimal environmental impact, and scalability. Presently, porous polymeric materials have found extensive use in separation applications. However, a significant drawback lies in the amorphous nature of traditional polymers, making precise control over their pore architecture at the molecular level a daunting task. Additionally, deciphering their intricate structures for a comprehensive understanding of the structure–function relationship presents challenges. Recently, porous framework polymers represented by covalent organic frameworks (COFs) have emerged as a new class of crystalline porous polymers that features low densities, well-defined structures, high surface areas, adjustable pore sizes, and facilely tailored functionality, thus exhibiting huge potential in the field of separations. Furthermore, the defined crystal structures and precise regulation of the pore environment render COFs perfect platforms for investigating separation mechanisms and structure–function relationships. Regrettably, despite their immense potential in separation applications, COFs face significant challenges. (1) Many reported COFs have pores larger than 1 nm or lack the specific binding sites essential for separating small molecules, particularly gases, which limits their effectiveness in gas separation processes. (2) Conventional synthesis methods, like solvothermal techniques, often yield powdery COF samples, posing difficulties in molding them for practical applications. (3) The large-scale, cost-effective production of high-quality COFs that meet the rigorous demands of industrial applications remains an underexplored frontier. These persistent challenges underscore the need for innovative research and development efforts to unleash the full potential of COFs in addressing critical separation challenges and advancing industrial processes.Addressing the above challenges, in this Account, we mainly summarize the recent research progress achieved by our team, including the following: (i) designing various COFs as highly efficient separators via synthesis strategies, such as generating a penetrated structure, introducing binding sites, and adjusting the pore size; (ii) creating various molding strategies including a cross-linking strategy, nanotape strategy, and melt polymerization strategy to fabricate COFs into membranes or monoliths; (iii) exploring the separation application scopes of COFs, such as gas separation, pollutant removal, and water treatment. And, we point out the existing challenges and future development directions for the COF field.