Covalent organic frameworks (COFs) are a class of crystalline organic porous materials. Through the design of pore structure and surface functionality, such materials can be used in fields such as adsorption and catalysis. In the process of COFs synthesis, monomers usually have a rigid structure, and the synthesis of COFs with a flexible skeleton has always been a research difficulty. Recently, the research group of Professor Liu Honglai of East China University of Science and Technology has made progress in the construction of covalent organic frameworks based on flexible crown ethers. The related results were published in Angew. Chem. Int. Ed. under the title of "Construction of Covalent Organic Frameworks with Crown Ether Struts" ( DOI: 10.1002/anie.202101163). As the first-generation supramolecular host compound, crown ethers have been widely studied in the field of coordination chemistry and supramolecular chemistry since they were reported in the 1960s. There are many types of crown ether molecules. The common feature is that they have a cavity ring that can encapsulate guest molecules. Depending on the ring size, they can form stable complexes with different metal ions, anion (cation) ions, and have significant affinity and selection. Therefore, crown ether compounds have application potential in the field of adsorption and separation. However, due to its flexible molecular chain cavity and based on the cyclic crown ether as the skeleton, it is still a challenging problem to design and synthesize a porous framework with high crystallinity. The researchers synthesized COFs with a controllable cavity size by using new crown ether units. Powder X-ray diffraction, high-resolution transmission electron microscopy, and nitrogen absorption and desorption curve tests showed that the material has high crystallinity, permanent porosity and High specific surface area. In addition, the excellent acid-base and chemical stability of the two crown ether COFs also lay a good foundation for water treatment applications. The adsorption experiments show that the materials have excellent alkali metal ion adsorption and separation performance.





In this work, the author accurately constructed two porous covalent organic frameworks with high crystallinity based on the flexible crown ether molecules, which achieved efficient adsorption and separation performance of alkali metal ions, thus woven porous frameworks with specific adsorption behavior. Materials provide new ideas. An Shuhao, a postdoctoral fellow in the School of Chemistry and Molecular Engineering, East China University of Science and Technology, is the first author, Professor Liu Honglai is the corresponding author, and the co-corresponding authors are Associate Researcher Xu Qing from Shanghai Institutes for Advanced Study and Zhai Lipeng, a young teacher from Zhongyuan Institute of Technology. And many help from Professor Hu Jun. The research results have been supported by the National Natural Science Foundation of Chinas major research projects and the National Natural Science Foundation of Chinas Youth Science Fund project.