Design and Synthesis of Novel Dual-Functional Protic Ionic Liquids with a Superior High CO₂ Absorption Efficiency

As a predictive tool, quantum chemical calculations can be used to design protic ionic liquids (PILs) and predict the result. By adding anionic negative potential sites, two dual-functional PILs diethylenetriamine-barbituric acid [C₄H₁₄N₃]₂[C₄H₂N₂O₃] and diethylenetriamine-ethylenolactonium [C₄H₁₄N₃]₂[C₃H₂N₂O₂] were designed. The simulation results indicated that multisite absorption of anions and cations resulted in an expected absorption ratio exceeding 3:1 (mol CO₂:mol ILs). Furthermore, the Gibbs free energy and enthalpy barrier were calculated. Based on this, the two PILs were synthesized in a controlled manner, and the experimental results demonstrated that 0.25 mol/L [C₄H₁₄N₃]₂[C₄H₂N₂O₃] and [C₄H₁₄N₃]₂[C₃H₂N₂O₂] exhibited a superior CO₂ absorption capacity of 3.152 and 3.466 mol CO₂/mol ILs, respectively. After five adsorption-desorption experiments, the regeneration rates of [C₄H₁₄N₃]₂[C₃H₂N₂O₂] were all higher than 90%. Finally, the reaction mechanism for CO₂ capture in these PILs was revealed that the significant increase in capacity could be attributed to the combined absorption of double negative potential N atoms on anions and primary and secondary amines on cations by using ¹³C NMR.