Density Functional Theory Study On The Absorption Mechanism Of CO₂ By Task-specific Ionic Liquids

The problem of greenhouse effect caused by the excessive emission of CO₂ has at-tracted worldwide attention,and the efficient capture and separation of CO₂ has become a current research hotspot.Task-specific ionic liquid（TSIL）is considered to be a promising CO₂ absorbent,but it has the problems of low absorption capacity,high viscosity and high cost.In order to break this bottleneck which restricts the industrial application,it is necessary to deeply investigate the absorption mechanism of TSIL,master the matching rules of anions and cations,and then directionally design TSIL absorbents with high absorption performance.To this end,this article uses density functional theory（DFT）to systematically investigate the microstructural characteristics and interaction mechanisms of aprotic and protic ionic liquids,and reveals the reaction mechanism of CO₂ absorption,hoping to provide theoretical guidance for the development and design of new TSIL absorbents.For the aprotic TSIL,1-ethyl-3-methylimidazolium（[Emim]⁺）and 1-butyl-3-methylimidazolium（[Bmim]⁺）with different side chain length are selected as cations and acetate（[OAc]^-）as the anion respectively.The microstructure characteristics before and after the formation of ion pairs are comparatively analyzed.With the help of electrostatic potential analysis,natural bond orbital analysis,and quantum theory of atoms in molecules,the nature of interactions between anions and cations are revealed.It is clarified that the hydrogen bonds（H-bonds）formed by[OAc]^-with H protons on the imidazolium ring are medium strength,while that formed with protons on the side chain are weak strength.In addition,the C1-H2…O red-shifted H-bond contributes the most to the stability of ion pairs.Energy decomposition analysis results reveal that the interactions of ion pairs are mainly electrostatic dominant,charge transfer and orbital overlap contributions are also significant.For the protic TSIL,dimethylethylenediamine（[DMEDAH]⁺）is chosen as cation,[OAc]^-and substituted phenolic group（[4-F-PhO]^-）as anions respectively.Based on the solvation model,the interaction mechanism between diamine cation and different anions is explored,and the possible interaction sites between them and CO₂ are predicted.The results show that the solvent water molecules have a promoting effect on proton transfer.In addition to the electrostatic force,there are also moderate H-bonds in the stable ion pair configurations.When the diamine protic TSIL absorbs CO₂,the N atom on the primary amine group in the cation and the O atom in the anions are likely to interact with CO₂,while the dominant position of the interaction between the phenolic anion and CO₂ is more obvious.In this paper,the DFT method is used to investigate the mechanism of CO₂absorption by[DMEDAH][4-F-PhO]and[DMEDAH][OAc].Based on the different active sites of the interaction of anions and cations with CO₂,multiple reaction paths are designed.Through the microstructure analysis of reactant,transition state,product,and the calculation of bond order,bond energy,electrostatic potential,charge,as well as the Gibbs free energy barrier and enthalpy barrier of different reaction paths,it is found that the anion absorption path of[DMEDAH][4-F-PhO]is advantageous in kinetics,while the cation absorption path is thermodynamically favorable,while[DMEDAH][OAc]follows the cation absorption mechanism.