Synthesis And Catalysis Research For Cycloaddition Reaction Of CO₂ Of Pyrazolium Ionic Liquids

Carbon dioxide（CO₂）is not limited to be greenhouse gas but also the non-toxic,inexpensive,and abundant C1 building block.The conversion and chemical fixation of CO₂ to high-value compounds is extraordinary attractive in both environmental and social issues.There are many pathways to utilize the CO₂ with the development of modern chemical technology.Up to date,the cycloaddition of CO₂ with epoxides（CCE reaction）is regarded as one of the most efficient methods to synthesize cyclic carbonate because of the 100%atom utilization and negligible side product.Moreover,the cyclic carbonate has been widely applied in numerous regions.However,the thermodynamic stability and kinetic inertness of CO₂ impede its various conversions.The incorporation of catalyst has been testified to be an efficient route to promote the CO₂ conversion.Various catalysts have been developed for the cycloaddition of CO₂ and epoxides including transition metals,metal complexes,molecular sieves,functional polymers,MOFs,etc.However,high catalyst loading,poor recyclability,and inclusion of metal and solvent are still inevitable in these systems.Therefore,the development of robust and single-component catalyst is a great challenge for academic researchers.The emergence of ionic liquids（ILs）offers us a novel choice that it is not only the catalyst but also the solvent.Moreover,there are some ubiquitous characteristics for ILs due to their distinct structures.If there is no need for the involvement of co-catalyst,especially for metal,it would be a green and "perfect" catalyst.In the past decades,a large number of homogeneous and heterogeneous catalytic systems based on ionic liquids have been developed to promote the CCE reaction.However,there are still some problems including high cost of the catalyst and harsh reaction conditions for large-scale industrial applications.The high viscosity of ionic liquids is not favourable for practical operation.More importantly,the poor stability of some ionic liquids has negative effect for recycle,especially for the homogeneous ionic liquid catalysts.Pyrazolium-based ionic liquids have similar chemical properties with imidazolium-based ionic liquids,such as,low viscosity and low melting point in electrochemical applications.In this thesis,a series of pyrazolium ionic liquids with or without functional group are synthesized and used to catalyze the cycloaddition reaction of CO₂ with epoxides.The main contents are described as follows:1)Synthesis of symmetric dialkylpyrazolium ionic liquids and research of their catalytic activity for the cycloaddition reaction of CO₂.A series of symmetric dialkylpyrazolium ionic liquids were synthesized by simple one-step reaction.Their structures were characterized by elemental analysis,HRMS,NMR,IR,etc.Their physical and chemical properties were tested by TG and DSC.They were firstly used to catalyze the cycloaddition reaction of CO₂ with epoxides.The structure-activity relationship was studied by variation of the length of alkyl chain（C₁-C₁2）and anions.The mechanism of catalytic reaction was studied by both experimental and theoretical methods.Both the non-bond interaction and electrostatic interaction lead to the difference of catalytic activity for different catalysts.And diethylpyrazolium iodided ionic liquid has the best catalytic activity.The yield of 96%was obtained under the reaction conditions of 120°C,2.0 MPa CO₂ pressure and 1 mol%catalyst loading for 4 h.The ionic liquid has good substrate applicability and thermal stability.Moreover,the catalyst can be reused for five cycles with no significant loss of activity.These ionic liquids are potential catalysts for large scale application due to their simple synthesis method,low cost,low viscosity,good recyclability,and excellent catalytic activity.2)Synthesis of asymmetric dialkylpyrazolium ionic liquids and research of their catalytic activity for the cycloaddition reaction of CO₂.In order to investigate the effect of the chain length of each N-alkyl substituted group on the catalytic performance of pyrazolium ionic liquids,a series of asymmetric dialkylpyrazolium ionic liquids were synthesized and utilized to catalyze the cycloaddition of CO₂ and epoxides.They present good catalytic activity for the cycloaddition reaction of CO₂.The 85%of product yield and the 99%of selectivity could be obtained with 1-ethyl-2-propylpyrazolium bromide（EPPzBr）as catalyst under the reaction conditions of 130°C,2.0 MPa CO₂ pressure and 1 mol%catalyst loading for 4 h.The catalyst has good substrate applicability and recycling performance.The mechanism of the catalytic reaction was studied by experiments and quantum chemical calculation.The effect of non-bond interaction in the transition state is investigated.The order of catalytic activity estimated by Double-IL model is in agreement with the experimental results.Although the catalytic activity of asym-dialkylpyrazolium ionic liquids is slightly lower than that of sym-dialkylpyrazolium ionic liquids,their catalytic activity is still superior to that of many other ionic liquids used in the cycloaddition reaction of CO₂.3)Synthesis of hydroxyl-functionalized pyrazolium ionic liquids and research of their catalytic activity for the cycloaddition reaction of CO₂.On the basis of previous studies,the catalytic activity of ionic liquids would be greatly improved when enhanced the ability of electrophilic activation.A series of hydroxyl-functionalized pyrazolium ionic liquids were synthesized and utilized to catalyze the cycloaddition of CO₂ and epoxides.Moreover,the optimal reaction conditions were explored.The experimental results show that under the optimum conditions of 110°C,1.0 MPa CO₂ pressure,4 h reaction time and 1 mol%catalyst loading,product yield of 91%can be obtained with HEEPzBr.In addition,this catalyst has good substrate applicability and better recycling performance.The catalytic activity of hydroxyl-functionalized pyrazolium ionic liquids is obviously higher than that of non-functionalized pyrazolium ionic liquids.Exploitation of functionalized ionic liquids would be the important research content in the future.