| Covalent organic frameworks(COFs)have attracted much attention since they were proposed in 2005.COFs are a kind of two-dimensional or three-dimensional porous crystalline material based on the accurate connection of the constituent units by strong covalent bonds.,mainly composed of light atoms,such as C,N,H,O,and so on.Benefit from the characteristics of lightweight,large specific surface areas,crystalline porous structure,high designability,and strong stability,COFs have broad application potential in various areas,such as heterogeneous catalysis,batteries,biological transport,gas separation,etc.Therefore,it has attracted many researchers to conduct in-depth researches.Various structures are the basis of wide applications of COFs in various fields,and functionalization,the main way to enrich the structure pool of COFs,is one of the most important directions in COFs researches.Halogen,a kind of functional group with strong electronegativity,can not only regulate the reaction process,but also stabilize the structure,regulate the charge,anchor or provide substitution sites,and adjust the hydrophobicity.This thesis focuses on halogen functionalized COFs and explores the role of halogen in the synthesis and post-modification of materials.Firstly,fluorine and bromine atoms were selected as functional groups,and then two kinds of halogen functionalized COFs were designed and synthesized based on the Schiff base reaction.The active aldehyde monomers were constructed by introducing strong electronegative F and Br atoms respectively,which improved the reaction efficiency and break the limitations of traditional catalysts.Halogen COFs with large surface area,high crystallinity,and stability were obtained without catalysis.In addition,the specific surface area of FCOF-1 reached 697 m2 g-1 within 5 minutes of the reaction and 1602 m2 g-1 within 5 hours.Secondly,we explored the application potential of halogen COFs in post-synthesis modification based on BrCOFs.A post-synthesis functional pathway based on the Suzuki-Miyaura coupling reaction was designed by using the abundant Br atoms in BrCOFs as active sites.4-methylthiophenylboronic acid was used as a post-synthesis modification monomer to react with BrCOFs in the presence of Pd(PPh3)4.The COFs with methylthio side chain were obtained successfully,and the conversion was as high as 59%.In addition,various groups such as cyano,ester,and aldehyde groups were introduced into COFs by covalent bonding.The results showed that the Suzuki-Miyaura coupling reaction successfully constructed a new and universal post-synthesis modification platform,which is of great significance for the synthesis of functionalized COFs.Because the boric acid monomers needed for the Suzuki reaction have been highly commercialized,this new post-synthesis modification method has the advantages of a large monomer library,mild conditions,high conversion rate,high group tolerance,non-toxic and harmless.And it overcomes the defects of low modification rate and limited selection of reaction monomer in the current commonly used functionalization methods.Thirdly,we studied the application of functionalized COFs in the adsorption and separation of sulfur hexafluoride(SF6).Because of its stability,abundant and regular pores,large surface area,COFs are universally applied in gas adsorption.Sulfur hexafluoride has important applications in the fields of plasma etching and insulating gas but possesses a strong greenhouse effect and long atmospheric life.However,the separation and recovery of SF6 by COFs have not been studied.Based on the designability of COFs,functional COFs with different pore sizes and side-chain groups were selected to explore the influencing factors of SF6 adsorption.The results show that small pore size plays an important role in SF6 adsorption.At the same time,the strong interaction between F atoms and sulfur hexafluoride can effectively realize the adsorption and selectively separation of SF6 gas.The maximum adsorption capacity reaches 2.61 mmol g-1.The above research provides a reference for the application of COFs in the SF6 adsorption field.In conclusion,two kinds of halogen COFs with high crystallinity and large specific surface area,namely FCOFs and BrCOFs,were designed and synthesized without catalyst.Based on the characteristics of Br atoms,an efficient functionalization pathway was constructed.And then the functionalized COFs were applied in the field of SF6 gas adsorption and separation. |
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