Hydrogen Bond Donor Contained Ionic Liquids For Carbon Dioxide Coupling Reaction To Synthesize Cyclic Carbonate

To solve the environmental problems caused by CO₂,the optimal way is resource utilization by chemical catalytic conversion of CO₂.Coupling reaction of CO₂and epoxides to cyclic carbonates has been paied much attention,because the reaction is100%atomic economy reaction,and the cyclic carbonate products have broad applications.Considering the dynamic inertness and thermodynamic stability,the exploitation of efficient catalyst is needed to realize the catalytic conversion of CO₂.Ionic liquids possess the advantages of low vapor pressure,non-volatile,non-flammable,good stability and strong dissolution ability.Ionic liquid exhibited excellent catalytic performance when acted as catalyst.Therefore,the introduction of hydrogen bond donor(amino,hydroxyl and carboxyl)and nucleophilic group(such as I^-,Br^-,Cl^-and other nucleophiles)guaranteed the excellent catalytic activity of ionic liquid during the CO₂ coupling reaction.Therefore,ionic liquid exhibited potential applications in the industrialization of CO₂ catalytic conversion.During the CO₂ coupling reaction processes,amino and hydroxyl groups acted as hydrogen bond donor,could form hydrogen bond with epoxides.Lewis acid Zn²⁺could form adduct with epoxides,nucleophilic group could attack the?carbon to promote the ring-opening reaction of epoxides.Based on the above mentioned factors,three kinds of 1,8-diazabicyclo[5.4.0]undec-7-ene(DBU)-based ionic liquids were facilely synthesized,which could cooperate with co-catalyst Zn I₂ for coupling of CO₂to propylene oxide.The effects of reaction conditions were discussed systematacially,and an optimized reaction condition(120 ^oC,3.0 MPa,2 h)was obtained.The reusability and substrate universality were also discussed.Combining the experimental results and literature reports,a feasible reaction mechanism was proposed,where epoxides were activated by amino,hydroxyl and Zn²⁺synergistically,ring-opening was promoted by I^-as well as CO₂ was activated by basic group.Additionally,amidinothiourea-Zn I₂(ATUI)eutectic-based ionic liquid was synthesized by solvent method.ATUI possessed many specific functional groups,such as hydrogen bond donor(amino,secondary amino),Lewis acid Zn²⁺,and nucleophilic group I^-.Therefore,ATUI could catalyze the coulping of CO₂ with propylene oxide efficiently under 110 ^oC,1.0 MPa,4 h,without the employment of solvent or co-catalyst.The propene carbonate yield could reach to 95%,selectivity was 99%.ATUI exhibited outstanding reusability and substrate universality.ATUI also exhibited excellent activity in the catalytic experiments of CO₂ with many other epoxides even under 90 ^oC,1.0 MPa,6 h.Based on the experimental results and reported literature,a plausible reaction mechanism was proposed.And then the kinetic model was performed to discuss the effect of hydrogen bond donor number on coupling reaction.In order to avoid the employment of transition metals,carboxylic functionalized polymerized ionic liquids anchored on vinyl functionalized mesoporous silica catalysts(mSiO₂-V-n,n=1,2,3,4)were synthesized.First,the strucutre of mSiO₂-V-n were characterized.Then,the catalytic activity of mSiO₂-V-n for CO₂ coupling reaction was discussed,mSiO₂-V-2 showed excellent catalytic activity because of the proper polymerized ionic liquid loading and specific surface area.Next,reaction conditions were optimized,the reusability and substrate universality were also investigated under the optimized condition.There was no obvious propene carbonate yield decrease after 10 cycles over mSiO₂-V-2 catalyst.However,the propene carbonate yield reduced from 95%to 60%in 3 cycles of carboxylic functionalized polymerized ionic liquids modified SBA-15 catalyst.The experimental results indicated the excellent stability of catalysts anchoring polymeric ionic liquids on vinyl functionalized mesoporous silica by copolymerization.Kinetic equation of the CO₂coupling reaction and propylene oxide catalyzed by mSiO₂-V-2 was obtained.Combining the experiment results and reported literature,a probable reaction mechanism was proposed,where carboxyl,silicon hydroxyl and C₂-H proton of imidazole ring activating epoxides,tertiary nitrogen of imidazole ring activating CO₂,nucleophilic group assisting the ring-opening of epoxides.In order to integrate the advantages of homogeneous catalysts and heterogeneous catalysts,four kinds of amino-functionalized bis-imidazolium based polymerized ionic liquids(PBIL-m,m=1,2,3,4)were synthesized.Solid–liquid transition behavior occurred around 50 ^oC of PBIL-m resulting in homogeneous catalysis above50 ^oC and heterogeneous separation of the catalyst at room temperature after the reaction.Amino could activate CO₂,and C₂-H proton of imidazole ring could activate epoxides without the employment of transition metals.PBIL-3 showed the best catalytic activity among PBIL-m because of the proper bridge chain length.An optimized reaction condition was obtained(100 ^oC,2.0 MPa,4 h),the reusability and substrate universality of PBIL-3 were discussed under the optimized condition.Meanwhile,the substrate universality of PBIL-3 was also discussed under 80 ^oC,1.0MPa,24 h.Amino group of PBIL-3 could activate CO₂was verified by FT-IR spectra,and C₂-H proton of imidazole ring could activate propylene oxide was verified by ¹H NMR,hence,a CO₂ coupling reaction mechanism was proposed.In conclusion,ionic liquids,which are rich in hydrogen bond donor,could activate CO₂ and epoxides in CO₂coupling reaction.Therefore,ionic liquids exhibited excellent catalytic activity.The design of catalysts mSiO₂-V-n and PBIL-m have certain guiding significance for the design of subsequent catalysts.