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

The kinetics and mechanism of CO 2 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 CO 2 absorption, different amino acid anions were chosen. On the basis of the enthalpy changes during CO 2 capture, a chemisorption nature is confirmed. An increase in the number of amine (−NH 2 ) groups in the ILs structures, facilitates the CO 2 absorption. According to kinetic results, the rate of CO 2 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 CO 2 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 CO 2 absorption from the kinetic and thermodynamic viewpoints.