Molecular Approach toward Gases Absorption by Ionic Liquids

Abstract

This thesis reports the theoretical studies of gas absorption in complex systems. These systems, namely ionic liquids, are considered to be promising candidates to substitute conventional gas capture systems which violate principles of Green Chemistry. The primary focus of this work is to understand gas absorption in ionic liquids on the molecular level using first principles methods such as ab initio molecular dynamics simulations and static quantum chemical calculations combined with statistical thermodynamics and predictive thermodynamic models. These methods allow for the derivation of predictive estimates for the knowledge-based design and tuning of ionic liquids for gas capture. <br /> A simple protocol is suggested in the first part of the thesis to distinguish physical from chemical absorption of carbon dioxide (CO₂) in ionic liquids with varying anion basicity. This protocol includes a simple geometric optimization with a continuum solvation model of anion-CO₂ complexes. A value of the O-C-O angle in these optimized structures defines the possible kind of CO₂ absorption in the ionic liquid with the corresponding anion. The anions showing chemical absorption are further narrowed down to those promising for reversible chemical absorption based on the calculated reaction Gibbs free energies. The suggested approaches are easily transferable to other systems if the possible reactions are known. In case of complex reaction mechanisms such as the absorption of CO₂ in amino acid ionic liquids, the application of ab initio molecular dynamics simulations allows for the determination of the possible reactions in the system. The subsequent variation of the structure of the cation and the anion reveals the change of Gibbs free energies and barriers in the reactions. These small structural changes affecting the kinetics and energetics of the CO₂ absorption can be used as recipes for the precise tuning of the absorption rate and the total CO₂ capacity in ionic liquids. <br /> The solvation mechanisms of CO₂ and sulfur dioxide (SO₂) gases in ionic liquids are analyzed in the second part of the thesis. The solvation shells of both gases contain groups donating weak interactions, e.g., a $pi$-system of the cation or alkyl hydrogen atoms, indicating the high importance of the weak interactions in the CO₂ and SO₂ solvation. The essential difference between the solvation mechanisms of CO₂ and SO₂ is the interaction with an extended cation-anion network of an ionic liquid. CO₂ molecules do not incorporate into the cation-anion network, tending to be solvated by nonpolar groups of an ionic liquid. Instead, SO₂ acts as a linker and incorporates into the cation-anion network, which corresponds to the formation of anion-solute-cation structures. From this qualitative picture, the significantly higher solubility of SO₂ than CO₂ in ionic liquids can be explained. <br /> The last part of the thesis deals with the chemical absorption of CO₂ by carbenes available in some ionic liquids. The ability of the 1-ethyl-3-methylimidazolium acetate ionic liquid to form a carbene varies from gas to bulk phases in presence or absence of CO₂. While CO₂ suppresses the carbene formation in the gas phase, the opposite effect is observed in the bulk phase. This occurs due to an inverse ionic liquid effect, when an introduction of the neutral molecules in the pure ionic liquid might induce the formation of neutral species like carbenes. The detailed insight into the subsequent reactions of CO₂ with carbene and the transformation of the formed carbene-CO₂ adduct into an isomeric adduct has been performed theoretically in close collaboration with experimentalists. The experimental evidence supported by static quantum chemical calculations and ab initio molecular dynamics simulations allows for the establishment of a possible generalized mechanism of the isomerization of the carbene-CO₂ adduct. The driving force for such isomerization is the high basicity of the system. <br /> The presented theoretical approaches from the combination of first principles methods can be routinely applied to investigate the absorption of other gases in unstudied ionic liquids. The gathered knowledge can be further used for the optimization and precise tuning of the ionic liquids for gas absorption.