Study On The Preparation Of Cyclic Carbonates From CO₂ And Epoxides Catalyzed By Bifunctional Organocatalysts

Carbon dioxide(CO2),one of the most well-known greenhouse gases and abundant C1 feedstock on the earth,has the advantages of being non-toxic,non-flammable,stable,easily available and renewable.Therefore,from the standpoint of environmental protection and resources utilization,the chemical fixation of CO2 into value-added chemicals has seen an explosion of interest.Of these,the coupling of epoxides with CO2 to form five-membered cyclic carbonates has long been considered to be one of the most promising routes for the utilization of CO2.Ionic liquids(ILs),widely reported as efficient cycloaddition catalysts,nevertheless,are frequently encountered with low catalytic activity,harsh reaction conditions and tedious preparation process.To solve the above problems,in this study,a series of protic ionic liquids(PILs)based on pyridine and its derivatives and nitrogenous organic bases as well as choline chloride-derived deep eutectic solvents(DES)were designed to catalyze the synthesis of cyclic carbonates upon the principle of CO2 and epoxides activation.A series of bifunctional catalysts-PILs,with double catalytic sites-Lewis base and Br(?)nsted acid,were designed and synthesized.From catalysts screening,we find that delocalized cations,hydrogen bonds and nucleophilic anions are important elements of PILs with high catalytic activities.Based on the systematic study and comparison of congeneric catalysts,4-dimethylaminopyridine bromide([DMAPH]Br)was found to be the optimal catalyst for subsequent studies.The catalysts reported in the previous literatures usually work under harsh reaction conditions(high temperature and high pressure),while[DMAPH]Br can efficiently catalyze terminal epoxides transform into the corresponding cyclic carbonates with high conversions and high selectivities under mild reaction conditions(0.1 MPa,120 ?,1 mol%,4 h).Furthermore,the catalyst could be recycled over five times without appreciable loss of catalytic activity.Although[DMAPH]Br enjoys higher catalytic activity under high pressure compared with atmospheric pressure,in view of potential operation dangers and strict requirements for equipments of high-pressure reaction,reaction under ambient condition is more preferable.It is particularly worth mentioning that under optimal conditions and using a mixture of 15%CO2 and 85%N2 to mimic flue gas in industry,[DMAPH]Br is able to convert SO into SC with excellent conversion,which,to our knowledge,represents the first example of PILs catalyzed synthesis of cyclic carbonate with diluted CO2.Finally,based on the experimental results and previous reports,the synergistic mechanism of[DMAPH]+ and Br-is proposed.A series of ChCl-derived deep eutectic solvents were designed and synthesized to be novel,environmentally friendly and inexpensive catalysts for the cycloaddition reaction from epoxides and CO2.The presence of hydrogen bond(HBD)could greatly enhance catalytic performance of ChCl and moderate acidity of HBD is favorable to the reaction.The catalytic activity is in the following sequence:polyacid>phenol>polyol>p-toluenesulfonic acid and amide.Among the catalysts tested,ChCl/malonic acid(1:1)DES is optimal.The reaction parameters on catalytic activity were optimized and substrate generality of the catalyst were also investigated as well.Finally,based on the experimental results and previous reports,the synergistic mechanism of choline cation and Cl-is proposed.