Influence of water on the CO₂ capture mechanism, capacity and thermodynamic properties of aprotic heterocyclic anion ionic liquids
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Ionic liquids (ILs) are promising alternative solvents for carbon capture applications, including aprotic heterocyclic anion (AHA) ILs. These ILs were specifically designed for this application. They bind with CO2 in a 1 to 1 mole ratio, with a modest enthalpy of reaction. Water is an important impurity present in the flue gas, and understanding its effect is critical in designing the separation process. Water is of particular importance for ILs because it can change their physical properties and the chemistry of their reaction with CO2. In this dissertation, a comprehensive study regarding the reaction mechanism, CO2 solubility and equilibrium values of the IL-CO2-H2O system is presented. The changes in the reaction mechanism of AHA ILs with CO2 in the presence of water were investigated using NMR spectroscopy methods. It was established that in addition to the anion reacting with CO2 to form carbamate, the anion reacts with water and is reprotonated, leaving hydroxide to react with CO2 to form bicarbonate. The amount of reprotonated anion, carbamate and bicarbonate formed depends strongly on the nature of the anion and substituent groups on it. Additionally, the enthalpy, entropy and equilibrium constants for the reaction between the IL and CO2 in the presence of water were estimated. This was done by fitting experimental data of the CO2 absorption isotherms of AHA ILs and water mixtures to a Langmuir model. The results revealed that the enthalpy of the reaction leading to bicarbonate formation is larger in magnitude than the value obtained for the reaction of the neat IL with CO2 (only carbamate formation). An increase in the overall enthalpy of reaction could increase the amount of energy needed for the IL regeneration process and, subsequently, the cost of carbon capture. Nevertheless, it was concluded that water present in the post-combustion flue gas will not hinder the use of AHA ILs, in CO2 capture applications.