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

Abstract

Carbon capture and sequestration are the major applied techniques for mitigating CO2 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 CO2 absorption capacity. The reaction of amines with CO2 follows a multi-step mechanism where the initial pathway is the formation of the C−N bond between the NH2 group and CO2. 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 CO2 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–CO2 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 CO2 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.

Adsorption of Choline Phenylalanilate on Polyaromatic Hydrocarbon-Shaped Graphene and Reaction Mechanism with CO2: A Computational Study

“…Quick intramolecular proton transfer leads, therefore, to the formation of two isomers that have energies nearly equivalent and are found at about 28 kJ/mol lower than the initial zwitterionic complex. The reaction of addition of CO 2 to the [Phe] anion and subsequent intramolecular arrangement is therefore significantly exothermic with a reaction energy of about 64 kJ/mol, in agreement with the previous study carried out at the M062X/6-311++G** level …”

“…The energetic profile calculated for the whole reaction in the liquid bulk (Figure ) is very similar to that calculated on the graphene surface (Figure ) although the proton transfer from the NH 2 group through the TS structure (Figure ) requires higher energy. However, the barrier height is again quite low (19 kJ/mol), comparable with the values calculated for CO 2 -adducts with other anions like glycinate and l -prolinate, and does not hinder the formation of carbamic acid. In conclusion, the reaction of the [Phe] anion with CO 2 and the subsequent rearrangement to give carbamic derivatives are overall exothermic although with a value of energy (50 kJ/mol) lower than in the bulk liquid (64 kJ/mol).…”

“…The pattern presented in Figure 12 12 and confirms the exothermicity of the reaction (38 kJ/mol) although ion pairing with cations seems to be a factor that reduces the reaction energy and affects the subsequent rearrangement of the carbamic acids, in agreement with that observed for the reaction in the bulk liquid. 55 Ion pairing is predicted in the bulk as well as on the graphene surface. The analysis of the electron density and atomic charge before and after adsorption on graphene reveals that cations tend to deplete the electron charge of graphene, whereas anions make the surface more negative.…”

“…In the case of the glycinate anion, often taken as a benchmark for modeling larger AA anions, − the zwitterionic intermediate undergoes structural rearrangement through intramolecular proton transfer from the NH 2 group to one of the CO 2 – groups and subsequent internal rotation to give carboxylic acids as products. A few studies have described the reaction of the anions with CO 2 by including the cationic contribution. , Recently, we have applied quantum mechanical methods to study the reaction of three AA anions, glycinate, phenylalanilate, and prolinate, in vacuum and in liquid environments by using the polarizable continuum model (PCM) in the absence and presence of the choline cation . The introduction of the cation and the specific interactions with the anion change the energies of the two reaction steps, especially those involving intramolecular proton transfer.…”

“…52,54 Recently, we have applied quantum mechanical methods to study the reaction of three AA anions, glycinate, phenylalanilate, and prolinate, in vacuum and in liquid environments by using the polarizable continuum model (PCM) in the absence and presence of the choline cation. 55 The introduction of the cation and the specific interactions with the anion change the energies of the two reaction steps, especially those involving intramolecular proton transfer. As mentioned above, most studies focused on the liquid bulk and did not investigate whether the CO 2 capture capacity of ILs changes upon the adsorption of ionic species on graphene surfaces.…”