Carbon Dioxide Absorption by the Imidazolium-Amino Acid Ionic Liquids, Kinetics, and Mechanism Approach

The kinetics and mechanism of CO₂ absorption by ionic liquids (ILs) were studied, theoretically. The studied ILs are composed of 1-ethyl-3-methylimidazolium [Emim]⁺ as the cation with a general formula of the [Emim][X] (X = Gly^-, Ala^-, Lys^-, Arg^-). To investigate the alkyl chain length and the number of the amine group effects on the CO₂ absorption, different amino acid anions were chosen. On the basis of the enthalpy changes during CO₂ capture, a chemisorption nature is confirmed. An increase in the number of amine (-NH₂) groups in the ILs structures, facilitates the CO₂ absorption. According to kinetic results, the rate of CO₂ absorption by [Emim][Gly] is higher than that of [Emim][Ala]. This can be interpreted by a higher steric hindrance in [Emim][Ala] due to an additional methyl group in the amino acid chain. Donor-acceptor interactions and C-N bond formation were investigated by natural bond orbital analysis. Moreover, topological studies show a covalent nature for the C-N bond critical point that showing CO₂ capture is a chemisorption process. Finally, on the basis of kinetic energy results, donor-acceptor interaction and topological analysis, [Emim][Arg] is proposed as the best candidate for CO₂ absorption from the kinetic and thermodynamic viewpoints.