Design,Synthesis,Characterization,and Application Of Pyridine-based Covalent Organic Frameworks

Covalent organic frameworks(COFs)are a new type of crystalline porous organic materials constructed via covalent bonds.COFs have been attracting intensive research interests due to their long-range ordered structures,well-defined pores,high surface areas,and easy to be modified organic skeletons.These merits have rendered the COF materials with great potential in diverse applications,such as gas storage and separation,catalysis,optoelectronics,and sensing.However,the applications of the functionalized COFs are still in their infancy.Pyridine is one of the most important N-heterocycles,which plays a vital role in constructing functionalized molecules or materials.Accordingly,this thesis mainly focuses on the design,synthesis,and applications of the pyridine-embedded COFs with different structures and linkages.In Chapter 1,we reviewed the background of the thesis from two respects.Firstly,we summarized the emergence,development,and recent progress in COF materials.Secondly,we introduced the pyridine unit and its wide applications in functionalized structures.In Chapter 3,a new 3D dia-structured COF(LZU-301)has been synthesized via the solvothermal condensation of a pyridine-based linear monomer and a tetrahedral structural monomer.LZU-301 displayed a guest-responsive crystalline structure,which could occur structural transformation upon guests inclusion and removal.The flexibility of this crystalline structure was further identified by the PXRD analysis,solvent sorption measurements,and ¹²⁹Xe NMR spectroscopy.Additionally,LZU-301 showed high CO₂ adsorption and separation abilities due to the incorporation of pyridine units.In Chapter 4,based on the 3D dia-structure of LZU-301,shape-selective catalysis of LZU-301has been realized in the Knoevenagel condensation reaction of aldehydes and malononitrile.Benefiting from the 1D straight channels and the evenly distributed catalytic sites,the remarkable shape-selective performance was observed via varying the molecular sizes of aldehydes.Notably,the?secondary shape selectivity?which has been discovered in zeolite systems was also observed in the competing experiments.Moreover,a 1D nonporous material(LZU-1D)has been synthesized and used as a comparison of LZU-301.The poor shape-selectivity of LZU-1D further indicated the confinement effect of LZU-301.In Chapter 5,a?-ketoenamine linked COF(LZU-28)has been synthesized via the condensation of the pyridine-based amine and 1,3,5-triformylphloroglucinol.Owing to the good chemical stability,the pyridine units in LZU-28 can be modified via the methylation and protonation,resulting in two ionic COFs,LZU-28-Me and LZU-28-H.These ionic COF materials possess the positive skeletons with a lot of anions inside the channels.Accordingly,these ionic COF materials were used as anion dyes adsorbents with high capacity and selectivity.In Chapter 6,we have synthesized two pyridine-based dimethyl acetal monomers,and developed a new strategy to synthesize–C=N–linked COFs directly from dimethyl acetals and amines.Three new COFs(imine-linked LZU-20,hydrazone-linked LZU-21,and azine-linked LZU-22)have been successfully constructed through this new approach.All the synthesized COF materials are highly crystalline and exhibit good thermal stability.Compared with aldehyde monomers,dimethyl acetals can exhibit improved stability and good solubility.Therefore,our strategy not only allows the facile construction of–C=N–linked materials via the direct use of dimethyl acetals,but also opens up a new approach for constructing application-oriented COFs due to the improved stability and availability of actals.