Crystalline Covalent Triazine Frameworks and 2D Triazine Polymers: Synthesis and Applications

Covalent triazine frameworks (CTFs) are a novel class of nitrogen-rich conjugated porous organic materials constructed by robust and functional triazine linkages, which possess unique structures and excellent physicochemical properties. They have demonstrated broad application prospects in gas/molecular adsorption and separation, catalysis, energy conversion and storage, etc. In particular, crystalline CTFs with well-defined periodic molecular network structures and regular pore channels can maximize the utilization of the features of CTFs and promote a deep understanding of the structure-property relationship. However, due to the poor reversibility of the basic reaction for constructing the triazine unit and the traditional harsh synthesis conditions, it remains a considerable challenge to synthesize crystalline CTFs with diverse molecular structures, and there is still a significant lack of understanding of their polymerization mechanism, which limits their precise structural design, large-scale preparation, and practical applications. As the basic building block of bulk crystalline CTFs, two-dimensional triazine polymers (2D-TPs) which ideally have single-atom thickness have also aroused intensive interest due to their ultrathin 2D sheet morphology with structural flexibility, a fully exposed molecular plane and active sites, and excellent dispersibility and processability. However, the efficient and scalable production of high-quality 2D-TPs and the investigation of their unique properties and functions remain largely unexplored.In this Account, we summarize our recent contributions to the synthesis and application exploration of crystalline CTFs and 2D-TPs. We first introduce the design, synthesis, and polymerization mechanism of the crystalline CTFs. In order to synthesize high-quality CTFs, we have successively used a series of new synthetic methods including a solution polymerization strategy, microwave-assisted superacid-catalyzed polymerization strategy, polyphosphoric acid-catalyzed polymerization strategy, and solvent-free FeCl3-catalyzed polymerization strategy, achieving the production of highly crystalline layered CTFs from the gram level to the hundred-gram level and then to the kilogram level and realizing new CTF molecular structures. We also reveal a direct ordered 2D polymerization mechanism that provided meaningful guidance for the controllable preparation of functional CTFs. Next, we introduce the design, synthesis, and formation mechanism of 2D-TPs. We have developed effective bottom-up and top-down strategies to prepare 2D-TPs for different needs. On one hand, we have established the dynamic interface polymerization method, the monomer-dependent method, and the solvent-free salt-catalyzed polymerization strategy for the direct synthesis of ultrathin 2D-TPs with thickness down to the single-layer limit and provided important insights into the 2D polymerization mechanism. On the other hand, we have opened up the physical and chemical exfoliation of crystalline layered CTFs such as liquid sonication and ball milling exfoliation and covalent and noncovalent modification exfoliation for the large-scale production of 2D-TPs. Then, we present the application progress of crystalline CTFs and 2D-TPs in various batteries, photo/electrocatalysis, and adsorbents with an emphasis on their unique and outstanding performance and structure-property relationship. Lastly, the main challenges faced by crystalline CTFs and 2D-TPs in practical applications and future research directions are discussed in detail. We hope that this Account will provide valuable insights and practical strategies for promoting the development of functional organic framework materials and 2D polymer materials.