Carbon Dioxide in 1-Butyl-3-methylimidazolium Acetate. I. Unusual Solubility Investigated by Raman Spectroscopy and DFT Calculations

Abstract

The unusual solubility of carbon dioxide in 1-butyl-3-methylimidazolium acetate (Bmim Ac) has been studied by Raman spectroscopy and DFT calculations. It is shown that the solubility results from the existence of two distinct solvation regimes. In the first one (CO(2) mole fraction ≤ 0.35), the usual Fermi dyad is not observed, a fact never reported before for binary mixtures with organic liquids or ionic liquids (IL). Strong experimental evidence complemented by effective DFT modeling shows that this regime is dominated by a chemical reaction leading to the carboxylation of the imidazolium ring accompanied by acetic acid formation. The reactive scheme proposed involves two concerted mechanisms, which are a proton exchange process between the imidazolium cation and the acetate anion and the carboxylation process itself initiated from the formation of "transient" CO(2)-1-butyl-3-methylimidazole 2-ylidene carbene species. In that sense, CO(2) triggers the carboxylation reaction. Moreover, this dynamic picture circumvents consideration of a long-lived carbene formation in dense phase. The second regime is characterized by the detection of the CO(2) Fermi dyad showing that the carboxylation reaction has been strongly moderated. This finding has been interpreted as due to the interaction of the acetic acid molecules with the COO group of acetate anions involved in monodentate forms with the cation. The observation of the Fermi doublet allows us to infer that CO(2) essentially preserves its linear geometry and that the nature and strength of the interactions with its environment should be comparable to those existing in organic liquids and other IL as well. These results have been supported by DFT calculations showing that the CO(2) molecule interacts with energetically equivalent coexisting structures and that its geometry departs only slightly from the linearity. Finally, we find that the CO(2) solvation in Bmim Ac and 1-butyl-3-methylimidazolium trifluoroacetate (Bmim TFA) cannot be straightforwardly compared neither in the first regime due to the existence of a chemical reaction nor in the second regime because CO(2) interacts with a variety of environments not only consisting of ions pairs like in Bmim TFA but also with carboxylate and acetic acid molecule.

Intermolecular interactions in mixtures of 1-n-butyl-3-methylimidazolium acetate and water: Insights from IR, Raman, NMR spectroscopy and quantum chemistry calculations

“…In particular, the interest in the properties of 1-n-butyl-3-methylimidazolium acetate (BmimAc)/water system is motivated by the fact that this mixture dissolves lignin (and not cellulose) which is one of the principal polymeric components of wood and this mixture is considered as one of the routes to achieve the dissolution of cellulose in a BmimAc/water/ CO 2 medium in supercritical conditions of CO 2 [31][32][33]. Furthermore, acetate anion hydration is an important issue in solution chemistry.…”

Pathways of the Chemical Reaction of Carbon Dioxide with Ionic Liquids and Amines in Ionic Liquid Solution

“…In addition, FTIR and Raman spectroscopy have been utilized. [40][41][42] This paper focuses specifically on certain ionic liquids that are capable of acting as chemisorbents for CO 2 in cyclic carbon capture separation processes in which the CO 2 can be easily desorbed from the ionic liquid which is then recycled to the absorption step. Specific ionic liquids have been found to be effective at low pressures by a process of chemisorption in the liquid thus facilitating exceptionally high overall system CO 2 uptake efficiency.…”

Understanding chemical reactions of CO2 and its isoelectronic molecules with 1-butyl-3-methylimidazolium acetate by changing the nature of the cation: The case of CS2 in 1-butyl-1-methylpyrrolidinium acetate studied by NMR spectroscopy and density functional theory calculations

“…[14][15][16][17][18][19][20][21][22][23] A reactive mechanism has been proposed in which the proton bounded to carbon atom 2 of the imidazolium cation is released leading to the generation of a carbene intermediate, whereas the proton interacts with the acetate anion to form acetic acid (Fig. 1).…”

“…Other works, although considering the formation of carbene and acetic acid, differ slightly from this initial interpretation, arguing that the reactions are triggered by the introduction of carbon dioxide itself in the IL and that the carbene and carboxylate formation results from a concerted cooperative process between the cation, the anion, and the CO 2 molecule. [20][21][22] More recently, it was theoretically inferred that it is the introduction of neutral molecules like CO 2 in the IL charged network which cancels partially the charge-stabilizing effect existing in the neat IL and that this so-called inverse ionic liquid effect facilitates the carbene formation and, thus, the chemical absorption. 24,25 Very recently, new insights in the understanding of solute-solvent interactions within IL were provided from a theoretical study on the solvation of a carbene 1-ethyl-3-methylimidazole-2-ylidene in 1-ethyl-3-methylimidazolium acetate.…”