Progressive Ionization Modification And Catalytic Properties Of MIL-101 In The Cycloaddition Of CO₂-epoxides

As a greenhouse gas,carbon dioxide（CO₂）plays a vital role in maintaining the Earth’s suitable climate and environment.However,the excessive CO₂ produced by the burning of fossil fuels has caused negative effects such as global warming and seawater acidification.Fortunately,CO₂ is a kind of C₁ resource with high abundance,low cost,non-toxicity and renewable.Catalytic conversion of CO₂ into value-added chemical products can not only alleviate environmental problems,but also achieve the maximum utilization of resources,which is a very attractive research field in sustainable chemistry.The CO₂-epoxides cycloaddition（CEC）reaction not only has100%atomic economy,but also produces cyclic carbonate as an important chemical product.Therefore,the CEC reaction has attracted extensive research interest.In order to overcome the chemical inertness of carbon dioxide,various catalysts have been developed to lower the barrier of carbon dioxide cycloaddition reaction.However,the solid catalyst of CEC reaction often requires a large amount of halide anions as cocatalyst,which reduces the heterogeneous and eco-friendly properties of the catalyst.Among them,metal-organic frameworks（MOFs）are a new kind of porous coordination polymers composed of metal ions or ion clusters and organic ligands,which have large specific surface area,high porosity,structural and functional diversity,and have attracted extensive attention.According to the above content,the classic MIL-101（Cr）was selected as the precursor in this thesis.IMOFs with high bromide content were obtained by introducing pyridinium and triazolium into the skeleton through two successive post-modification"click"reaction（alkynyl-azidonium cycloaddition and N-alkylation）.The catalytic performance of the material for CEC reaction was investigated.The thesis is devoted to the following two subjects.1.Synthesis and characterization of IMOFs of pyridinium and triazolium with different ligandsWith the help of solvothermal method,MIL-101-NO₂ and MIL-101-R/NO₂（R=H,COOH,the ratio of NO₂/R is about 7:3）were obtained by adjusting the ratio of linking ligands（2-nitroterephthalic acid,terephthalic acid and trimellitic acid）.Subsequently,the conversion of the nitro-amino-azide group was achieved through a series of post-modification methods to obtain MIL-101-R/N₃（R=H,COOH,NH₂,N₃/R ratio is about 7:3）.Finally,a two-step click reaction is used to obtain ionic metal-organic frameworks with different ionization degrees:（i）Cu（I）catalyzed the[2+3]cycloaddition click reaction between the azide group in MIL-101-N₃/R and3-ethynylpyridinium to obtain triazole and pyridinium modified ionic MOF,namely MIL-101-R/pmtz-X（X=Tf O^-,Br^-）;（ii）Through the N-alkylation reaction of triazole,more positive charges are introduced into the MOF skeleton,while more counter anions are encapsulated in the pores,and MIL-101-R/pmtzm-X（X=Tf O^-,Br^-）containing triazolium and pyridinium on the skeleton is obtained.The structure and composition of the above IMOFs were characterized by infrared spectroscopy,PXRD,XPS,¹H NMR,ion chromatography,N₂ adsorption and CO₂ adsorption tests.The test results confirmed the successful synthesis of IMOFs.The structure of IMOFs after multi-step modification is still intact,showing large specific surface area and good adsorption capacity for CO₂.The Br/Cr value of MIL-101-H/pmtzm-Br is relatively high（0.94）.2.Catalytic properties of IMOFs for the CO2-epoxides cycloaddition（CEC）The cycloaddition of CO₂ to epichlorohydrin was used as the model reaction to investigate the catalytic properties of various mixed-linker IMOFs with different coexistent functional groups including different degree of N-quaternization and/or different anions.In the series of MOF with the same R,the catalytic activity sequence was MIL-101-R/pmtzm-Tf O（almost no activity）<MIL-101-R<MIL-101-R/pmtz-Tf O<MIL-101-R/pmtz-Br<MIL-101-R/pmtzm-Br;In MIL-101-R/pmtz-Br and MIL-101-R/pmtzm-Br containing different R,COO^-<COOH<NH₂<H.Combined with other control experiments,the results are as follows:（1）Neutral 1,2,3-triazole groups have certain catalytic activity,and Br^-ions have more outstanding catalytic activity,while triazolium,pyridinium and Tf O^-ions have no catalytic activity;（2）The further introduction of Br^-ions through N-alkylation of triazole can improve the catalytic activity;（3）The porous framework of MIL-101 has a synergistic catalytic effect with triazole groups or Br^-ions;（4）The introduction of COOH（or COO^-）and NH₂ into the ion frame is not conducive to catalytic reaction.The optimum reaction conditions（temperature,carbon dioxide pressure,dosage of IMOF）and catalytic kinetics of MIL-101-H/pmtzm-Br were studied.The results show that the catalyst can catalyze the CEC reaction without co-catalyst and solvent.The catalytic efficiency is better than most recently reported ionic catalyst materials without the aid of co-catalyst.The catalyst has good cyclic performance.Its structure and activity remain basically unchanged after four catalytic cycles.Substrate expansion experiments show that MIL-101-H/pmtzm-Br has a good catalytic effect on a variety of epoxides,and has a good universality.