Reaction Mechanism of CO2 with Choline-Amino Acid Ionic Liquids: A Computational Study

Carbon capture and sequestration are the major applied techniques for mitigating (Formula presented.) emission. The marked affinity of carbon dioxide to react with amino groups is well known, and the amine scrubbing process is the most widespread technology. Among various compounds and solutions containing amine groups, in biodegradability and biocompatibility perspectives, amino acid ionic liquids (AAILs) are a very promising class of materials having good (Formula presented.) absorption capacity. The reaction of amines with (Formula presented.) follows a multi-step mechanism where the initial pathway is the formation of the (Formula presented.) bond between the (Formula presented.) group and (Formula presented.). The added product has a zwitterionic character and can rearrange to give a carbamic derivative. These steps of the mechanism have been investigated in the present study by quantum mechanical methods by considering three ILs where amino acid anions are coupled with choline cations. Glycinate, L-phenylalanilate and L-prolinate anions have been compared with the aim of examining if different local structural properties of the amine group can affect some fundamental steps of the (Formula presented.) absorption mechanism. All reaction pathways have been studied by DFT methods considering, first, isolated anions in a vacuum as well as in a liquid continuum environment. Subsequently, the role of specific interactions of the anion with a choline cation has been investigated, analyzing the mechanism of the amine- (Formula presented.) reaction, including different coupling anion-cation structures. The overall reaction is exothermic for the three anions in all models adopted; however, the presence of the solvent, described by a continuum medium as well as by models, including specific cation- -anion interactions, modifies the values of the reaction energies of each step. In particular, both reaction steps, the addition of (Formula presented.) to form the zwitterionic complex and its subsequent rearrangement, are affected by the presence of the solvent. The reaction enthalpies for the three systems are indeed found comparable in the models, including solvent effects.