The separation of CO2 is an important topic in thermal separation science. The scrubbing of carbon dioxide from exhaust gases has gained more interest in the last years to reduce the emissions of green for counteracting global warming but is also an important process step in biogas upgrade to produce methane from regenerative sources. A commonly used process is absorption with following regeneration. Often amine solutions are used as solvents, which react with carbon dioxide and thus can bind CO2 with a high capacity and selectivity. This comes along with a huge energy demand for regeneration and losses of solvent due to irreversible reaction products.
Novel solvents should be able to overcome these disadvantages but provide comparable separation performance to optimize the carbon dioxide scrubbing. Ionic liquids are a promising solvent family. Their modular setup of anion and cation makes the development of a task specific solvent possible.
In this work ionic liquids with an amino acid anion are investigated systematically in their ability as CO2 absorbents. They can bind carbon dioxide physically as well as chemically due to the reaction of the amine functional group of the amino acid with CO2. Both influences were screened with a-priori quantum chemical based methods. The change of free Gibbs enthalpy and reaction enthalpy of the reactions were predicted with DFT-calculations. Thereby the different equilibrium conversion and the tendency towards consecutive reactions were investigated. The anions were coupled with various cations in a screening of the physical solubility of CO2 in the ionic liquids. The solubility is predicted with the COSMO-RS model. The combination of the chemical and phase equilibrium leads to the CO2 loading as a function of the pressure. It can be seen that carbon dioxide is mainly bound chemically but the variation of the cation leads to a difference in the CO2 loading of up to 10% to 15 % only due to different physical solubility. From these results the hypothetical solvents were benchmarked based on their predicted properties, whereas high carbon dioxide capacity and low reaction energy for an energetic favorable regeneration are desired. The most promising candidates were synthesized to measure the carbon dioxide solubility and validate the predictions. This approach shows a systematic solvent development based on quantum chemical methods which reduces experimental time and saves development costs.
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