Research On The Mechanism Of CO₂ Coupling Reaction Catalyzed By Pyrazolium And Quaternary Ammonium Ionic Liquids

In the past century,the concentration of CO₂ in the atmosphere is increased year after year resulting in the serious green house effect.The utilization of CO₂ has attracted much attentions from various communities.CO₂ is an important C1 resources with cheap and non-toxic advantageous.Numerous valuable compounds could be synthesized with the CO₂ as the substrate.It is not only be beneficial to reducing the negative influence of CO₂ on the environment but also be favorable to discovering the new carbon resources.Among various utilized pathways,the coupling reaction of CO₂ and epoxides to produce propylene carbonate?PC?is one of the most efficient methods.However,catalysts are essential items to prompt the reaction under the benign conditions due to the kinetic inertness and thermal stability of CO₂.Ionic liquids stand out from various catalysts because of numerous advantageous.Besides the experimental studies,the theoretical investigations are required to elucidate the reaction mechanism and critical items to control the reactions,which would provide helpful clues to develop new robust catalysts.Although the reaction mechanism of CO₂ and epoxides catalyzed by ionic liquids have been studied by numerous theoretical studies,the catalytic activity predicted by the theoretical studies greatly deviates from the corresponding experimental results.One possible reason is that the interactions among ionic liquids are omitted in the previous theoretical studies.In addition,some novel catalytic performance appears with the application of new ionic liquids indicating that there is new reaction mechanism.To solve above problems,the following studies have been performed by combination of theoretical and experimental studies:1.To consider the influence of interactions among ionic liquids on the catalytic activity,the dimer of ionic liquids is employed to be catalyst for the coupling reaction of CO₂.At the M06/6-311+G?2d,2p??PCM?//B3PW91/6-31G?d,p?level of theory,the rate-determining barrier height is calculated for four different quaternary ammonium ionic liquids.The corresponding results are NEt4Br?19.14 kcal/mol?>[HTe A]Br?15.92 kcal/mol?>(NEt₃?HE?Br?15.46 kcal/mol?> [HMEA]Br?15.26 kcal/mol?,which is different from the experimental result.Although the interactions between ionic liquids have been involved in the dimer,it plays the limited role in refining the theoretical accuracy.It is necessary to consider more complicated situations.On the basis of the molecular dynamics results,there are six ionic pairs around thecenter catalytic region,which would have great influence on the catalytic features.Therefore,the rate-determining steps of four reactions are calculated again by the ONIOM method?B3PW91/6-31G?d,p?:HF/3-21G?with the aforementioned transition sates catalyzed by dimer as QM region and six ionic pairs around them as MM region.The calculated results are totally consistent with the measured product yields,i.e.,[HTe A]Br?31.67 kcal/mol?> [HMEA]Br?18.58 kcal/mol?> NEt4Br?16.63kcal/mol?> NEt₃?HE?Br?11.39 kcal/mol?.Finally,some important interactions in above four transition states are considered by natural bond orbital?NBO?,electron localization function?ELF?and atoms in molecules?AIM?to further elucidate the essential items to affect the catalytic activity.The above results provide the possibility to determine the catalytic sequence of different ionic liquids by theoretical method.2.By the same method,the catalytic activity for a series of pyrazolium ionic liquids is investigated.To further improve the accuracy,six ionic pairs are considered as catalyst in the center region,i.e.,QM region,rather than two ionic pairs.Then,seven ionic pairs are included around QM region to be MM region,to consider the influence resulted by the micro-environment of ionic liquids.The determined sequence of catalytic activity is consistent with the experimental measurements.Moreover,the deviations of catalytic activity among the different ionic liquids are further enlarged when the seven ionic pairs are included in MM region.Therefore,it is reasonable to determine the catalytic sequence for a series of ionic liquids by the above method.In addition,the catalytic sequence of 1,2-diethylpyrazolium bromide?DEPz Br?,2-?3-aminopropyl?-1-ethyl-pyrazolium bromide?APEPz Br?,and 1-carboxypropyl imidazolium bromide?HCPIm Br?are compared by the same method,which is in good agreement with the experimental results.3.In this section,four new ammonium-functionalized pyrazolium ionic liquids are synthesized by us to explore the influence of alkyl group substitution.The structures of synthesized ionic liquids are determined by 1H NMR,13 C NMR,and MS methods.Then,they are employed as the catalysts for the coupling reaction of CO₂ without any solvent and co-catalyst.The catalytic activity of [EPz PNH3]Br2 is the best with the product yield of PC under PO 0.1 mol,[EPz PNH3]Br2 1.0 mol%,70?,0.5 MPa CO₂ pressure,and reaction 24 h.Moreover,[EPz PNH3]Br2 is suitable for various substrates.Its catalytic activity could be kept even after reused 5 times.As compared with other similar ionic liquids,such as,pyrazolium ionic liquids,imidazolium,ionic liquids and quaternary ammonium ionic liquid,the catalytic activity of[EPz PNH3]Br2 is much better with the greatly lowered reaction temperature,which is attributed to the new reaction mechanism.A new mechanism is proposed by density functional theory?DFT?.One[EPz PNH3]Br2 could absorb CO₂ to form the intermediate,[EPz PNH2?COOH?]Br2,followed by the ring-opening of PO and other steps.The catalytic performance could be improved with the enhanced ability of CO₂ adsorption,which provides the valuable clues to develop the new ionic liquids with excellent catalytic performance.