Theoretical Study On Ion-pair Structure And CO₂ Absorption Mechanism In Dual Amino Imidazolium-Based Amino Acid Ionic Liquids

A large amount of greenhouse gases such as CO₂emitted in industrial activities has caused serious environmental problems.In current studies,green amino acid ionic liquids（AAILs）with low ecotoxicity,high bioactivity and high reactivity to CO₂have attracted extensive attention of researchers.In this paper,the structure and interaction of ion pairs in ionic liquids composed of dual amino functionalized imidazolium cations and amino acid anions were studied by using density functional theory（DFT）,and the results were compared with the analogues with non-amino functionalized imidazolium cation.We also discussed the catalytic mechanism of H₂O for the CO₂absorption reaction of the AAILs based on dual amino functionalized imidazolium cations.The main conclusions of this paper are as follows:1.The structures of isolated cations[Apaeim]⁺and[C4C3im]⁺were optimized by B3LYP-D3 method combined with 6-311++G（D,P）base set and their electrostatic potential analyses were performed.The results show that the presence of-NH₂in[Apaeim]⁺changes the charge distribution of cations,and the most positive electrostatic potential distributes around down-side of imidazolium ring in[Apaeim]⁺,but concentrates around upper-side of imidazolium ring in[C4C3im]⁺.2.The structures of[Apaeim][AA]and[C4C3im][AA]（AA=Gly、Ala、Val）ion pairs were optimized by B3LYP-D3/6-311++G（D,P）.The results show that in the most stable configurations,the O atoms of the carboxyl group of amino acid anions contact directly with C-H of upper-side of imidazolium ring and with N-H/C-H of amino/alkyl of side-chains with a nearly coplanar configurations.There are multiple intermolecular hydrogen bonds in[Apaeim][AA]ion pairs,in which C2-H7···O9,N1-H8···O9 and N4-H5···O10 are moderately strong hydrogen bonds,and the others are weak hydrogen bonds.However,in[C4C3im][AA]ion pair,only C3-H8···O11 is a moderately strong hydrogen bond,and other hydrogen bonds are weak hydrogen bonds.3.The binding energies,interaction energies and deformation energies of[Apaeim][AA]and[C4C3im][AA]（AA=Gly、Ala、Val）were calculated by B3LYP-D3/6-311++G（D,P）combined with counterpoise method.The results show that the intermolecular interaction energies of diamino functionalized ion pairs[Apaeim][AA]is about 30 k J·mol^-1more negative than that of non functionalized ion pairs[C4C3im][AA],but the deformation energies of[Apaeim][AA]is 30 k J·mol^-1greater than that of[C4C3im][AA].The difference between the interaction energies and deformation energies offsets each other,resulting in the very similar binding energies of the two kinds of ion pairs.4.In order to simulate the application of ILs in CO₂absorption,the reaction mechanism of CO₂absorbtion of imidazolium cation[Apaeim]⁺and three amino acid anions[Gly]^-、[Ala]^-、[Val]^-were calculated by using the SMD continuum solvation model in combination with the M062x method and 6-31++G（D,P）basis set.The results show that the reaction energy barrier the CO₂absorption by[Apaeim]⁺is more than 130 k J·mol^-1,thus the reaction cannot occur at lower than 100 Celsius from the perspective of kinetics.In addition,there are two reaction paths when CO₂reacts with amino acid anions（[Gly]^-、[Ala]^-and[Val]^-）.The more favourable paths with proton transfer from-NH₂to O atom of the carboxyl have low energy barriers of18.43,18.17 and 29.39 k J·mol^-1for three anions,respectively.The energy barrier data show that the absorption of CO₂by three amino acid anions can finish quickly at room temperature.The above calculation results show that amino acid anions plays a pivotal role in the absorption of CO₂by pure[Apaeim][AA]ionic liquid.5.Under the same conditions,the reactions of[Apaeim]⁺,three amino acid anions with CO₂in the presence of additional H₂O were calculated.The results show H₂O can play a medium role in the course of proton transfer,and consequently,reduced the reaction energy barrier by 50～80 k J·mol^-1in CO₂absorbtion of imidazolium cation[Apaeim]⁺.On the other hand,the presence of H₂O does not change significantly the reaction mechanism of CO₂with[Gly]^-、[Ala]^-、[Val]^-.In conclusion,in the presence of H₂O,both the cations[Apaeim]⁺and the amino acid anions（[Gly]^-、[Ala]^-、[Val]^-）in ionic liquids[Apaeim][AA]can participate in the CO₂absorption process,thus greatly increasing the CO₂absorption efficiency of ILs.