Engineering Covalent Organic Framework Membranes

CONSPECTUS: Membrane technology plays an increasingly important role for sustainable development of our society owing to its huge capability to tackle the energy crisis, water scarcity, environmental pollution, and carbon neutrality. To fully unlock the potential of membranes, it is in high demand to develop advanced membrane materials that significantly outperform conventional polymer membrane materials in separation performance and long-term stability. The emergent covalent organic frameworks (COFs) have been deemed as potent membrane materials because of their unique structure and properties in comparison with polymers, zeolites, and metal organic frameworks (MOFs). (i) First, the highly tunable and ordered crystalline pore structure, high porosity, and excellent stability render COFs an ideal membrane material. COFs are more stable than MOFs and, in some cases, are even more stable than zeolite. Moreover, it is easier to introduce functional groups into the COF nanochannels compared with zeolite and MOFs. Further, COFs are ideally suitable for constructing ordered nanochannels with size in the range of 0.6-3 nm which is difficult to be realized by other materials. (ii) Second, along with the unremitting discovery of diverse platform chemistries such as reticular chemistry, the in-depth understanding of nucleation/growth mechanisms of COFs as well as the rapid progress of manufacturing technologies and various routes to fabricating COF membranes with favorable physical and chemical structures inside the nanochannels are being actively exploited. COFs generally show better membrane-formation ability owing to their abundant 2D structures, which make it easier to fabricate ultrathin membranes compared with zeolite and MOFs. (iii) Last, a great number of COF membranes exhibit exceptionally high separation performance and stability, establishing their position as the next-generation membranes. In this Account, we discuss three types of engineering toward COF membranes based on Schiff base reaction for high-efficiency molecules/ion separations, i.e., reticular engineering, crystal engineering, and nanochannel engineering. First, we discuss the reticular engineering of COF membranes with a focus on the bond types, chemical structure, and architecture design. The membrane-formation ability and methods of COFs are also analyzed. Second, we discuss the crystal engineering of COF membranes with a focus on the key thermodynamical and kinetic factors to drive the disorder-to-order transition where we attempt to dig deeper into the crystallization habit of COF membranes. Third, we discuss nanochannel engineering of COF membranes with a focus on the construction and modulation of the physical and chemical microenvironments of nanochannels for efficient and selective transport of molecules/ions. Last, we conclude with a perspective on the opportunities and major challenges in the R&D of COF membranes, targeting at identifying the future directions.