Design of novel dual functional ionic liquids and DFT study on their CO2 absorption mechanism

Dual functional ionic liquids with the combinations of diethylenetriamine cation ([DETAH]+) or 1-ethanolamine-ethylenediamine cation([1-AOEt-EDAH]+) and 4-fluorobenzoate anion ([4-F-PhO]−) were explored for efficient CO2 capture in this work, and the absorption mechanisms of isolated ions and ion pairs were investigated using the dispersion corrected density functional theory (DFT) method. Above all, the possible reaction sites and reaction activity of TSILs were inferred by electrostatic potential (ESP) and Frontier molecular orbital analysis, then the reaction pathways for absorbing CO2 were proposed, and the transition state search of each elementary reaction by cation, anion and ion pair absorption was performed. Furthermore, the Gibbs free energy and reaction heat of each path were separately calculated, and the Gibbs free energy barrier and enthalpy barrier diagrams were drawn. In addition, the change of structural parameter, bond order and ADCH charge of reactants (R), transition states (TS), intermediates (IM), and products (P) were compared. The results showed that the absorption reaction of [DETAH]+ was the most likely to occur in kinetics, while that of [1-AOEt-EDAH]+ was favorable in thermodynamics. For the [DETAH][4-F-PhO], ion pair binding competed strongly with the CO2 chemisorption, while for [1-AOEt-EDAH][4-F-PhO], anions improved the stability of the whole structure, leading to the decrease of the Gibbs free energy barrier. Hopefully, the results of this work are helpful to provide a theoretical basis for designing of novel functional ionic liquids.