Design Of Catalysts For Cycloaddition Reaction Of CO₂ And Epoxides And Study On Catalytic Performance

Carbon dioxide（CO₂）is the main greenhouse gas,and the climate change caused by its excessive emission has become a global environmental problem.To achieve CO₂emission reduction,China has put forward the goals of"emission peak"and"carbon neutrality".The conversion of CO₂into high value-added chemicals is an important route to achieving this goal.Among them,the synthesis of cyclic carbonates from the cycloaddition reaction of CO₂and epoxides exhibits 100%atom efficiency,which is considered to be an efficient,economical,and environmentally friendly method.In this cycloaddition reaction,the ring-opening of epoxide is the rate-determining step.The research shows that the synergistic catalysis of Lewis acid and Lewis base is the key to realizing the reaction under mild conditions.At present,most of the reported catalysts have some problems,such as low catalytic activity,harsh reaction conditions,or difficult separation of catalysts.Given these problems,this thesis developed a variety of highly efficient homogeneous and heterogeneous catalysts through molecular structure design and studied their catalytic performance and catalytic mechanism.The main research contents are as follows:1.A N-confused zinc porphyrin（Zn（NCP）Cl）was designed and synthesized.X-ray structural analysis of Zn（NCP）Cl reveals that Zn²⁺was coordinated with three nitrogen atoms from the N-confused porphyrin and with an axially coordinated chlorine.Due to the strong electron-withdrawing ability of chloride,zinc shows strong Lewis acidity,thereby enhancing its ability to activate epoxide and making Zn（NCP）Cl have excellent catalytic activity.The turnover frequency（TOF）value reached 266 667 h^-1for the conversion of CO₂with epichlorohydrin into cyclic carbonate at 120°C and an initial CO₂pressure of 1.7 MPa within 3 h.A series of epoxides have been converted into the corresponding cyclic carbonates in high yields（92-99%）and selectivity（>99%）.The mechanism of Zn（NCP）Cl catalyzed cycloaddition reaction was proposed based on experiments and density functional theory.2.The catalytic performance of triethanolamine borate（TEOAB）for the CO₂cycloaddition reaction was studied.TEOAB is an environmentally friendly,nontoxic,cheap,and water-soluble chemical.As TEOAB contains one Lewis pair（boron as Lewis acid,tertiary amine as Lewis base）,Lewis acid could activate the epoxide,and Lewis base could open the ring of the epoxide.The unique acid-base structure makes TEOAB a novel bifunctional catalyst for CO₂cycloaddition reaction.The results show that the synergistic effect of Lewis acid-base pairs makes TEOAB show good catalytic activity and high selectivity（99%）without co-catalyst.When the dosage of TEOAB is 1 mol%,the yield of cycloaddition reaction of CO₂and ECH catalyzed by TEOAB reaches 99%.The catalytic activity can be further improved by using TBAB as co-catalyst.When the amount of TBAB is 0.25 mol%,only 0.05 mol%TEOAB can make the cycloaddition reaction to 99%yield.A series of epoxides also have been converted into the corresponding cyclic carbonates in high yields.As triethanolamine borate and TBAB are water-soluble,they can be easily removed by water washing to obtain the pure products with little loss for water-insoluble cyclic carbonates.The activation effect of boron on epichlorohydrin was verified by NMR,and based on the experimental results,the mechanism of TEOAB catalyzed cycloaddition reaction was proposed.3.Aβ-diketone zinc pyridyl poly ionic liquids（Zn（acacpy）₂-IL）were prepared by solvothermal method from quaternary ammonium reaction betweenβ-diketone zinc pyridyl and 1,4-bis（bromomethyl）-benzene.The Zn（acacpy）₂-IL has both a zinc Lewis acid center and Br^-nucleophilic center that can synergistically catalyze CO₂cycloaddition reaction.Meanwhile,the porous structure and flexible chain network facilitate the transport of substrate molecules.Zn（acacpy）₂-IL showed high catalytic activity without co-catalyst.Under mild conditions（60°C,atmospheric CO₂）,the yield of chloropropene cyclic carbonate reached 98%within 36 h.Zn（acacpy）₂-IL also has good catalytic performance for other epoxides（yield:93-98%）.In addition,Zn（acacpy）₂-IL remained high activity and high selectivity after five cycles.4.A covalent organic framework material（Dha Tat-COF-IL）grafted with imidazole-based ionic liquids was designed and synthesized.The synergistic effect of nucleophilic anions（Br^-）and acidic protons（C2-H）in imidazole ionic liquids have good catalytic performance for cycloaddition.Dha Tat-COF-IL has a mesoporous framework,high BET surface areas,and dense imidazolium and Br-active sites.Their mesoporous and ordered open channels contribute to the exposure of active sites and favor the fast transportation of the substrates.The combination of imidazolium salts endows high enrichment of CO₂near the active sites,thus promoting the rapid conversion of the reactants.The catalytic activity of Dha Tht-COF-IL for CO₂and ECH in 1 h（yield:85%）was significantly stronger than that of imidazole ionic liquid homogeneous catalyst（yield:59%）at 120°C and 1.7 MPa without co-catalyst.The enhanced catalytic efficiency is attributed to the highly dispersed active sites of the imidazolium salts within the framework of Dha Tht-COF-IL.Moreover,under mild conditions（80°C,atmospheric CO₂）,Dha Tht-COF-IL was applied to the synthesis of cyclic carbonates with excellent yields（93-97%）for the cycloaddition of various epoxides with CO₂.Dha Tat-COF-IL could be reused at least ten times without significant loss of catalytic activity.5.The coupling of CO₂and epoxides is usually required harsh conditions（high temperature and high pressure）.Herein,a core-shell Bi⁰@β-Bi₂O₃encapsulated in porous carbon（Bi⁰@β-Bi₂O₃@PC-600）is fabricated by a bismuth-based MOF（CAU-7-TATB）pyrolysis at 600°C.The CO₂activated by photoelectron is easier to cycloaddition with epoxides activated by the photogenerated hole.The results showed that a series of epoxides were converted into corresponding cyclic carbonates with92-99%yield and high selectivity（99%）within 12 h at ambient conditions and under visible light.The photocatalyst remained high activity after five cycles.Combined with the experiment,the mechanism of photocatalytic CO₂cycloaddition reaction was proposed.The excellent photocatalytic activity is attributed to the delayed recombination of photoelectron-hole pairs by nitrogen-rich porous carbon and the dispersion of Bi⁰@β-Bi₂O₃nanoparticles in porous carbon.In addition,the SPR effect generated by metal Bi⁰can further improve the photoelectric transmission efficiency ofβ-Bi₂O₃.The introduction of photoactivation in the reaction can effectively reduce the inertness of the CO₂reaction.