Synthesis Of Functionalized Amino Acid Ionic Liquids And Their Applications In CO₂ Cycloaddition Reactions

The cycloaddition reaction between CO₂ and epoxides is one of the important ways to achieve CO₂ emission reduction and alleviate global warming as well as other related climate problems.Amino acid ionic liquids（AAILs）,as highly functionalized ionic liquids with good functional designability,have been used as catalysts for CO₂ cycloaddition due to activative groups to CO₂ and epoxide both on their anions and cations.However,some defects also limit the application of AAILs in the CO₂ cycloaddition to epoxides such as the high viscosity,weak nucleophilicity,and the need to separate from the reaction system after the reaction.In the meanwhile,as the homogeneous catalyst,AAILs also requires complex separation and purification after reactions,resulting in additional energy consumption.In order to solve the above problems,different functional modifications were modified to AAILs to reduce their high viscosity and improve the weak nucleophilicity.Furthermore,AAILs could be loaded on porous materials to be transformed into heterogeneous catalysts.The improved functionalized AAILs show a good application prospect in the CO₂ cycloaddition.In order to solve the drawback of high viscosity,AAILs were modified with functionalization of ether groups.Four ether-functionalized imidazolyl AAILs,[MOBMIM][AA]were synthesized by the two-step method and used as catalysts for the CO₂ cycloaddition reaction to epoxides.The introduction of ether-based functional groups successfully reduced the viscosity of AAILs by one-third,accelerated the transfer and exhibited good CO₂ affinity.The absorption of CO₂by 1-butyl-3-methoxyimidazol glycine（[MOBMIM][Gly]）could reach 2.36 mmol（mmol-IL）^-1.The CO₂ cycloaddition with propylene oxide（PO）to produce propylene carbonate（PC）was catalyzed under the conditions of 110℃,2 MPa and 12 h.The 94%yield along with 99%PC selectivity were obtained and this catalytic system was environmentally friendly and biocompatible without the addition of cocatalysts and organic solvents.The good catalytic activity was attributed to the synergistic effect of the H-C（2）group on the cationic imidazole ring,the carboxyl-COO^-and the amino group-NH₂ on the amino acid anion.During CO₂ cycloaddition process,H-C（2）group activated the epoxide,-COO^-attacked theβ-C-O bond of the epoxide to open the ring,and at the same time the-NH₂ and H-C（2）groups were able to activate CO₂.To further improve the catalytic activity of AAILs,nucleophilic halogens were introduced into AAILs,and six AAILs composed of 1,1,3,3-tetramethylguanidine,amino acids and halogens,[HTMG][His][X]and[HTMG][Lys][X]（X=Cl,Br,I）were synthesized using a simple one-step protonation reaction.The synergistic interaction among the three different ions resulted in high catalytic activity without additional cocatalysts.Among them,[HTMG][His][I]achieved 99%PC yield and 99%selectivity in the cycloaddition reaction of CO₂ to PO under the very mild conditions of 30℃,20 h and 1 MPa CO₂ pressure.When the conditions changed to the ambient temperature and pressure,96%PC yield and 99%selectivity could be obtained after the reaction for 72 h.The mechanism of the catalysis for CO₂ cycloaddition reaction by[HTMG][His][I]was investigated by ¹H NMR,¹³C NMR and FT-IR characterization.The results showed that the abundant hydrogen bond donor（HBD）group-NH₂⁺on the tetramethylguanidine cation could activate the epoxide,and the iodide ion as a nucleophilic reagent effectively promoted the ring opening procedure of epoxides.In addition,the–NH₂ and H–C（2）groups from histidine anions could simultaneously lead the CO₂ activation.The synergistic effect of above-mentioned functional components realized the efficient catalysis of CO₂cycloaddition reaction by AAILs at room temperature and pressure.As homogeneous catalysts,AAILs should be separated through extracting after each reaction,which brought additional energy consumption.Taking advantage of supercritical CO₂ fluid（Sc CO₂）as the medium,the transformation from homogeneous to multiphase catalysts could be succesfully realized by loading the[HTMG][His][X]（X=Cl,Br,I）into SBA-15 to obtain[HTMG][His][X]/SBA-15（X=Cl,Br,I）.The loading of[HTMG][His][X]into[HTMG][His][X]/SBA-15 was 20.03 wt%using Sc CO₂ medium,which was 2.94-folds as that of[HTMG][His][X]/SBA-15（Im）prepared by impregnation synthesis method.Among the synthesized catalysts,[HTMG][His][I]/SBA-15 showed the best performance in catalyzing the cycloaddition reaction of CO₂ with PO without an additional halogen source co-catalyst.It achieved 95%yield and 99%PC selectivity under 100℃,6 h and 2 MPa CO₂ conditions.An equivalent amount of unloaded[HTMG][His][I]only obtained 69%PC yield under the same conditions.It was due to the mesopores from SBA-15 which promote the mass transfer and diffusion of reactants that enhance the catalytic activity of AAILs for the CO₂cycloaddition reaction.Moreover,the[HTMG][His][X]/SBA-15（X=Cl,Br,I）catalyst successfully combined the advantages of both homogeneous and heterogeneous catalysts.[HTMG][His][X]/SBA-15（X=Cl,Br,I）could not only achieved the high CO₂cycloaddition catalytic activity but also the easy separation from the catalytic system.