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.