Research On Fixation Of CO₂ Catalyzed By Imidazolium And Pyrazolium Ionic Liquids

The combustion of fossil fuels produces a large amount of carbon dioxide?CO₂?,which is emitted into the atmosphere resulting in serious environmental problems.However,CO₂ is also an inexpensive,nontoxic and abundant C1 resource that can be used as raw material to produce various chemical products.The coupling reaction of CO₂ with propylene oxide?PO?to propylene carbonate?PC?is one of the most efficient ways.Moreover,it is a rare pathway to be realized in industrial production.However,the title reaction is difficult to be completed in a benign condition because of the thermostability and kinetic inertness of CO₂.The involvement of catalyst is an effective way to promote the reaction.Various catalysts have been developed for this reaction,but all of them have some common disadvantages including harsh reaction conditions,low catalytic efficiency,and the requirement of organic solvents.Ionic liquids could be taken as catalyst and solvent.Therefore,the development of various ionic liquids has been widely focused on by researchers.To clarify the reaction mechanism is the prerequisite for the development and design of catalysts,and it is also a difficult and important problem for the CO₂ fixation reaction,especially for the mixed catalysts.There are two parts in this thesis:The first part is to clarify the reaction mechanism of CO₂and propylene oxide?PO?catalyzed by amino-functionalized imidazolium ionic liquid,to improve the reliability of theoretical prediction by refining the catalytic model,and to provide theoretical basis for the design of ionic liquids with desired properties.The other part is to design the pyrazolium ionic liquid and then to synthesize the selected ionic liquid with the ultimate goal of developing new catalysts from theoretical design to experimental synthesis.The main contents of this paper are as follows:?1?Although the mechanisms of coupling reaction of CO₂ with propylene oxide?PO?catalyzed by various room-temperature ionic liquids or task functionalized ionic liquids have been theoretically elucidated in previous literature,it is not totally suitable for the amino-functionalized ionic liquid.First,the1-?3-aminopropyl?-3-methylimidazolium chloride?[APmim]Cl?would react with CO₂ to produce the1-?3-carbamic acid propyl?-3-methylimidazolium chloride?[CAPmim]Cl?.[APmim]Cl,[CAPmim]Cl,and combination of them would be employed as the catalysts.In order to consider two different catalysts in the same reaction,the catalytic mechanism of amino-functionalized imidazolium ionic liquids was studied by the Double-IL model?catalytic model of two molecular ionic liquids and one molecular of substrate?at the M06/6-311+G?2d,2p??PCM?//B3PW91/6-31G?d,p?level.Two different cations would simultaneously activate the oxygen atoms of PO.Alternatively,one ionic liquid was employed as electrophile,while the other ionic liquid stabilized the primary one by hydrogen bonds.According to different ring-opening modes,nine possible routes were located.Besides the nucleophilic activation of the Cl^-anion,the[CAPmim]⁺cation was employed to activate the O atom of PO directly leading to the ring-opening of PO.The[APmim]⁺cation was utilized to stabilize the[CAPmim]⁺,which is the most favorable route.The theoretical results showed that the[CAPmim]Cl played an important role in the catalytic processes while the influence of the[APmim]Cl could not be omitted.The noncovalent interactions?NCI?plot was employed as a tool to analyze the transition states of the ring-opening step of top three favorable routes.The hydrogen bond played an important role in the catalytic reaction,even more important than the electrostatic interaction.?2?Although a variety of catalysts have been explored for the coupling reaction of CO₂ and PO to produce cyclic carbonate,single-component catalyst with high efficiency is still rare in the absence of organic solvents and cocatalyst.We first designed three pyrazolium ionic liquids,1,2-diethylpyrazolium bromide?DEPzBr?,1,2-diethyl-3-methylpyrazoliumbromide?DEMPzBr?,and1,2-diethyl-3,5-dimethylpyrazolium bromide?DEDMPzBr?.Their catalytic mechanism was studied by both Single-IL model?catalytic model of a molecular ionic liquid and a molecular substrate?and Double-IL model at the M06/6-311+G?2d,2p??PCM?//B3PW91/6-31G?d,p?level.The theoretical calculations showed that the order of catalytic activity is DEPzBr>DEDMPzBr>DEMPzBr.In addition,the structure-activity relationship of ionic liquids was analyzed by NCI,atoms in molecule?AIM?and natural bond orbital?NBO?methods.The analysis showed that both electrostatic interaction and hydrogen bond promoted the opening of PO.Then,they were synthesized and employed to catalyze the coupling reaction of CO₂ and PO.The propylene carbonate?PC?yields are in a range of 82.7%-88.7%under a benign condition,120°C,2.0 MPa initial CO₂ pressure and 4 h with 0.5 mol%catalysts loading.The sequence of catalytic activity determined in experiment was completely consistent with the theoretical prediction.In this work,three new efficient catalysts for the reaction of CO₂ and PO were developed by us with benign reaction conditions in the absence of solvent and metal.More importantly,it is helpful to establish the reliable structure-activity relationship and provide the possibility to develop ionic liquids from theoretical design to experimental synthesis in the future.