Synthesis And Catalysis Research For Cycloaddition Reaction Of CO₂ Of Protic Pyrazolium Ionic Liquid

With the acceleration of industrialization,the carbon dioxide?CO₂?emitted into the atmosphereis greatly increased resulting in lots of negative effect on the environment.At the same time,CO₂ is also abundant,non-toxic,and renewable C1 resources.Utilization of CO₂ is not only beneficial to refining the environment but also helpful to alleviate the energy crisis.Synthesis of cyclic carbonate from cycloaddition reaction of CO₂ and epoxides is one of the most important pathways.However,it could not be completed in a benign condition due to the kinetic inertness of CO₂.The involvement of catalyst would not only accelerate the reaction but also perform it in a more benign condition.More importantly,it is suitable for the large scale application.Ionic liquids?ILs?stands out from numerous catalysts due to its unique physical and chemical properties and excellent catalytic activity.The catalytic activity of imidazolium ILs is excellent for the cycloaddition of CO₂ and epoxides,which is attributed to the unique five-member ring in imidazolium ILs.As the molecular isomer,pyrazolium ILs also have a similar five-member ring,However,less attention has been focused on the latter.Pyrazolium ILs have never been employed to catalyze the fixation of CO₂,especially for the protic pyrazolium ILs.A series of protic pyrazolium ILs were firstly synthesized by us.Then,they were taken as the catalysts for the cycloaddition reaction of CO₂ and epoxides.The influence of different substituted position and alkyl chain length is explored.The optimal reaction conditions are confirmed.And their reusability and generality are also considered.Finally,the structure-activity relationship of the catalyst was studied by density functional theory?DFT?.Pyrazolium ILs studied in this thesis could be easily synthesized with cheap raw materials.However,their catalytic activity could be compared with the imidazolium ILs.They would be a new potential catalysts for the fixation of CO₂.The details include the following parts:1.A series of protic pyrazolium ILs with different substituted positions,1-methylpyrazoliumbromide?HMPzBr?,1,3-dimethylpyrazolium bromide?HMM₃PzBr?,and 1,5-dimethylpyrazolium bromide?HMM₅PzBr?,and 1,3,5-trimethylpyrazolium bromide?HTMPzBr?were synthesized by one step.Their structures were characterized by ¹H NMR,¹³C NMR,MS?ESI?and other methods.They present excellent catalytic activity for the coupling reaction of CO₂ and epoxides in the absence of metal and solvent.HMM₅PzBr has the best catalytic performance.The influence of catalyst dosage,reaction temperature,carbon dioxide initial pressure,and reaction time on the synthesis of propylene carbonates?PC?is investigated.The optimal reaction conditions are catalyst dosage of 1.0 mol%,reaction temperature of130?,CO₂ initial pressure of 1.5 MPa,and reaction time of 3.0 h.The highest yield of PC is 87.80%under the optimal reaction condition.HMM₅PzBr has great generality since the catalyst presents excellent catalytic activity for the cycloaddition reaction of CO₂ and other epoxides.HMM₅PzBr could be used for five times without obvious loss of catalytic activity.Finally,the reaction mechanism is discussed by theoretical calculation to further elucidate the structure-activity relationship.2.A series of protic pyrazolium ionic liquids with different alkyl chain length,1-ethylpyrazoliumbromide?HEPzBr?,1-propylpyrazolium bromide?HPPzBr?,1-butylpyrazolium bromide?HBPzBr?,and1-amylpyrazolium bromide?HAPzBr?were synthesized by two steps.The structure of ionic liquids was characterized by ¹H NMR,¹³C NMR,MS?ESI?and other methods.The catalytic performance of these ionic liquids for the reaction of CO₂ and epoxides in the metal-and solvent-free was studied.1-ethyl pyrazolium bromide?HEPzBr?has the best catalytic performance.The influence of catalyst dosage,reaction temperature,carbon dioxide pressure,and reaction time on the synthesis of cyclic carbonates is investigated to identify the optimal reaction conditions.The optimal reaction conditions are a dosage of 1.0mol%,reaction temperature of 140?,CO₂ pressure of 2.0 MPa,and reaction time of 4.0 h.The highest yield of PC is 94.52%under the optimal conditions.HEPzBr has great generality since the catalyst presents excellent catalytic activity for the cycloaddition reaction of CO₂ and other epoxides.HEPzBr could be used for five times without obvious loss of catalytic activity.Finally,the reaction mechanism is discussed by theoretical calculation to further elucidate the structure-activity relationship.The calculated sequence of barrier heights is totally consistent with the experimental results.Moreover,the influence of non-covalent interaction is further discussed by the AIM and NCI methods,which is also confirmed by the ¹H NMR measurement.