DFT Study on the Chemical Absorption Mechanism of CO₂ in Diamino Protic Ionic Liquids

Diamino protic ionic liquids (DPILs) possess a wide application prospect in the field of acid gas absorption. In this work, two representative DPILs, that is, dimethylethylenediamine 4-fluorophenolate ([DMEDAH][4-F-PhO]) and dimethylethylenediamine acetate ([DMEDAH][OAc]), which had been proved to display favorable CO₂ absorption performance in experiments, were selected. Based on the solvation model, the different mechanisms of CO₂ absorption by [DMEDAH]⁺ cations combined with different anions were investigated using the dispersion-corrected density functional theory method. Above all, the possible active sites of the reaction between DPILs and CO₂ were analyzed by electrostatic potential (ESP) and electronegativity, and the transition states in each path were searched and verified by frequency calculation and intrinsic reaction coordinate calculation. Furthermore, the Gibbs free energy and reaction heat of each path were calculated, and the free energy barrier and enthalpy barrier diagrams were shown. It was found that the absorption path by the anion of [DMEDAH][4-F-PhO] was favorable in kinetics, while the absorption path by the cation was thermodynamically beneficial. In addition, [DMEDAH][OAc] only showed the possibility of cation absorption, and the mechanism of the transfer of active protons to weak acid anions and the formation of acetic acid molecules was more favorable. Moreover, through the structural analysis, bond order and bond energy calculation, ESP analysis of the ion pair absorption configuration, and comparison with the products of CO₂ absorbed by isolated ions, it was found that the interaction between anions/cations and CO₂ could weaken or enhance the interaction between anions and cations in different reaction steps. Hopefully, this study is helpful to understand the absorption mechanism of CO₂ by DPILs and provides a theoretical basis for the R&D of multi-active site functionalized ILs.