Cycloaddition Reaction Of CO₂ Catalyzed By Task Functionalized Pyrazolium Ionic Liquid

The extensive emission of carbon dioxide?CO₂?has resulted in lots of negative effect on the environment.However,CO₂ is not limited to be greenhouse gas but also the non-toxic,inexpensive,and abundant C1 building block.The conversion of CO₂ to high-value compounds is not only beneficial to refining the environment but also helpful to alleviate the energy crisis.Although there are many pathways to utilize the CO₂,the thermodynamic stability and kinetic inertness of CO₂ limit its various conversions.Synthesis of cyclic carbonate from cycloaddition reaction of CO₂ and epoxides is one of the most important pathways.The high pressure and high temperature are necessary items to accomplish the coupling reaction.The involvement of catalyst is one of the most favorable methods to ensure the reaction to be performed under the mild conditions.There are numerous catalysts for this reaction since twenty years ago.Ionic liquids?ILs?stand out from them due to the distinct physical and chemical properties.The negative vapor pressure would result in the less pollution as compared with the organic species.More important,the structures of ionic liquids are easy to be tuned by variation of cation and anion.Ionic liquids are both catalyst and solvent,in which no additional organic solvent is required.Therefore,ionic liquids are regarded as "green" catalysts.Up to now,lots of single-component ionic liquids have been applied for this reaction to be catalyst.However,the reaction condition could not be regarded as mild because of high temperature and pressure.In addition,the dosage and reusability of ionic liquids should also be considered to develop an excellent catalyst.Four amino-functionalized pyrazolium ILs and ten carboxyl-functionalized pyrazolium ILs have been synthesized in this thesis,which are employed to catalyze the title reaction.As compared with the non-functionalized pyrazolium ILs and imidazolium ILs,the reaction conditions are greatly refined along with the reasonable product yield.The amino-functionalized pyrazolium ILs and carboxyl-functionalized pyrazolium ILs would be synthesized by simple steps with high synthetic yield.The main contents are described as follows:1)A series of amino-functionalized pyrazolium ILs,2-?2-aminopropyl?-1-methyl-pyrazolium bromide?APMPzBr?,2-?2-aminoethyl?-1-methyl-pyrazolium bromide?AEMPzBr?,2-?3aminoethyl?-1-ethyl-pyrazolium bromide?AEEPzBr?,and 2-?3-aminopropyl?-1-ethyl-pyrazolium bromide?APEPzBr?are synthesized with two steps,allkylation reaction and neutralization reaction.Their structures are characterized by ¹H NMR,¹³C NMR,MS?ESI?and other methods.They present excellent catalytic activity for the coupling reaction of CO₂ and epoxy propane?PO?without co-catalyst and solvent under 110?,1.5 MPa CO₂ initial pressure,catalyst amount 1.0 mol%,and reaction time 4 h.Among them,APEPzBr has the best performance with the product yield of 94%.When the alkyl group substituted on N1atom in pyrazolium ring is fixed,the catalytic activity is enhanced with the increase of alkyl chain length substituted on N2 atom in pyrazolium ring with the following sequence:APMPzBr?92%?>AEMPzBr?89%?and APEPzBr?94%?>AEEPzBr?76%?.If the substituted alkyl group connected with-NH₂ group is fixed,the ethyl group is more suitable to be the other substituted group rather than methyl group.The influence of catalyst dosage,reaction temperature,CO₂ initial pressure and reaction time is further optimized one by one.APEPzBr would be reused by five cycles without significant loss of activity.Moreover,APEPzBr present excellent suitability for most of epoxides with the exception of cyclohexene oxide.Finally,the catalytic mechanism is elucidated by density functional theory.According to the barrier height of the most optimal reaction channel,the catalytic performance of various catalysts is compared.There is good agreement between theoretical and experimental result.non-covalent interaction?NCI? and atoms in molecule?AIM? methods are employed to analyze the role of weak interactions played in the catalysis.2)A series of carboxyl-functionalized pyrazolium ionic liquids are synthesized and utilized to catalyze the cyclouddition of CO₂ and epoxides including 2-?caboxymethyl?-1;-methyl-pyrazolium bromide?CMMPzBr?,2-?carboxymethyl?-1-ethyl-pyrazolium bromide?CMEPzBr?,2-?carboxyethyl?-1-methyl-pyrazolium bromide?CEMPzBr?,2-?2-carboxyethyl?-1-ethyl-pyrazolium bromide?CEEPzBr?,2-?3-carboxypropyl?-1-methyl-pyrazolium bromide?CPMPzBr?,2-?caboxypropyl?-1-ethyl-pyrazolium bromide?CPEPzBr?,2-?4-carboxybutyl?-1-methyl-pyrazolium bromide?CBMPzBr?and 2-?4-carboxybutyl?-1-ethyl-pyrazolium brmide?CBEPzBr?.Their structures are characterized by ¹H NMR,¹³C NMR,MS?ESI?and other methods.They present excellent catalytic activity with the product yield over 61%under the reaction conditions of 110?,2.0 MPa CO₂ pressure,1.0 mol% catalyst loading,and 4 h.CMEPzBr is the best with the product yield of 99.4%.In previous report,the long alkyl chain is an insurance for the excellent catalytic activity of carboxyl-functionalized ionic liquids?normally larger than carboxyl propyl?.However,the reaction condition to synthesize ionic liquids is harsh along with low product yield.In the other word,the carboxyl-functionalized pyrazolium ionic liquids with short alkyl chain could also have acceptable product yield.The catalytic activity of CMEPzBr is not decreased after it is utilized by 7 times with the PC yield around 99%.More important,the catalytic activity of CMEPzBr would be kept under more benign conditions 70?,1.0 MPa CO₂ pressure,1.0 mol% catalyst loading,and 24 h with 90% product yield.The product yield could reach 90%.The reaction mechanism is carried out at the M06/6-311+G?2d,2p??PCM?//B3PW91/6-31G?d,p?level of theory.The reason of high catalytic activity for CMEPzBr is elucidated from microscopic items.