(388f) Mechanism, Energetics and Molecular Descriptors Governing CO2 Chemisorption in Choline Based Eutectic Solvents

Eutectic solvents based on choline salts with amine functionalized anions as hydrogen bond acceptors (HBAs), and three different hydrogen bond donors (HBDs), namely ethylene glycol (EG), propylene glycol (PG), and monoethanolamine (MEA) were developed as alternative solvents for CO₂ capture at low partial pressures. Composition-dependent physical and thermal properties as well as CO₂ chemisorption mechanism and energetics were investigated. Further, simple structural descriptors of choline salts and HBDs were defined using DFT calculations and utilized in the quantitative structure–property relationship (QSPR) analysis. Accordingly, multi-linear models were developed to determine viscosity, density, and CO₂ capacity of the eutectic solvents; these models can serve as a starting point for future design of improved eutectic sorbents. In parallel, NMR analysis and DFT calculations were performed to examine CO₂ binding and reaction energetics. It was found that in eutectics with EG and PG, CO₂ binds to multiple sites including the hydroxyl oxygens on choline and ethylene glycol, and the amine on the anion. For binding to the anion, the calculated reaction enthalpies are on the order of -21.8 kJ/mol to -65.7 kJ/mol with the inclusion of the HBD. However, in the case of eutectics with MEA, it was found that the binding of CO₂ to the amine on MEA was proceeded with a hydrogen transfer from the MEA⁺-CO₂^- zwitterion intermediate to the anion. This was in place of the known proton transfer from the zwitterion to another free MEA, thus achieving equimolar uptake of CO₂ per MEA in the eutectic solvent. The binding energies, which include the interaction energy between MEA and the HBA anions, ranged from -94.7 kJ/mol to -151.8 kJ/mol for CO₂ binding to MEA, with the different binding energies dependent on the proton transfer energy from MEA to the HBA anions. The lower the proton transfer energy, the more downhill CO₂ binding for MEA. As the MEA-MEA interactions are weakened by the presence of the choline salts, the extent of hydrogen bonding is lowered. This is specifically true compared to the EG and PG where CO₂ diffusivities, as examined by MD simulations, are lower than the MEA based eutectic. This presentation will discuss the roles of the HBAs and the HBDs on the CO₂ reaction mechanisms for applications in CO₂ separations.