(454g) The Effect of Water on CO2 Capture By AHA Ionic Liquids

Gabriela Avelar, University of Notre Dame

Oscar Morales, University of Notre Dame

Joan Brennecke, University of Notre Dame

Key words: Ionic liquids, water, CO₂ capture.

In the context of postcombustion CO₂ capture, ionic liquids (ILs) are a promising alternative to traditional absorbents such as aqueous monoethanolamine solutions. One key advantage to ILs is their tunable enthalpy of reaction for CO₂ binding. ILs with aprotic heterocyclic anions (AHA) have been specifically designed for this application^1,2. The AHA ILs are very promising, because they react stoichiometrically with CO₂, with modest enthalpy of reaction; and they do not present any increase in viscosity after CO₂ bonding, in contrast to the traditional ILs that chemically bind with CO₂.

Water is a very important impurity to consider in this process, since the flue gas contains a considerable amount of it, which is further complicated by the ability of water to change the IL viscosity, in addition to changing the chemistry of the CO₂ binding reaction. A full understanding of the reaction mechanism, kinetics and equilibrium values in the IL-CO₂-H₂O system is therefore crucial to postcombustion CO₂ capture by AHA ionic liquids.

We propose a reaction mechanism where in addition to the reaction of the anion with CO₂ to form carbamate, the anion reacts with water, and is reprotonated, leaving CO₂ to react with hydroxide to form bicarbonate, as previously suggested for aminoacid³, triazolide⁴ and phenolic⁵ ionic liquids. According to the proposed mechanism a decrease in the amount of water will correspond to the amount of reprotonated anion.

In order to quantify the amount of reprotonation, a proton NMR spectroscopy method was developed. The change in the amount of water was measured for different ionic liquids and water mixtures exposed to CO₂ by comparison of the integration of the water peak before and after CO₂ exposure. The rate of the IL-CO₂-H₂O reaction was studied by proton NMR spectroscopy, as well. Finally, the CO₂ solubility in the AHA ILs in the presence of water and reversibility of the reaction were investigated using a volumetric method.

It was found that several AHA anions undergo reprotonation in the presence of water and CO₂, although the reaction appears to be reversible. The rate and equilibrium values, as well as the CO₂ uptake values in the presence of water, vary significantly with the nature of the anion.

References:

1. Seo S., Quiroz-Guzman M., De Silva A.M., Lee T.B., Huang Y., Goodrich B.F., Schneider W.F, Brennecke J.F. Chemically Tunable Ionic Liquids with Aprotic Heterocyclic Anion (AHA) for CO₂ Capture. J. Phys. Chem. B. 2014; 118:5740-5751.
2. Gurkan B., Goodrich B.F., Mindrup E.M., Ficke L.E., Massel M., Seo S., Senftle T.P., Wu H., Glaser M.F., Shah J.K., Maginn E.J., Brennecke J.F., Schneider W.F. Molecular Design of High Capacity, Low Viscosity,Chemically Tunable Ionic Liquids for CO₂ Capture. J. Phys. Chem. B. 2010; 1:3494-3499.
3. McDonald J.L, Sykora R.E., Hixon P., Mirjafari A., Davis J.H. Impact of water on CO₂ capture by amino acid ionic liquids. Environ. Chem. Lett. 2014; 12:201-208.

1. Thompson R.L., Shi W., Albenze E., Kusuma V.A., Hopkinson D., Damodaran K., Lee A.S., Kitchin J.R., Luebke D.R, Nulwala H. Probing the effect of electron donation on CO2 absorbing 1,2,3-Triazolide ionic liquids. The Royal Society of Chemistry. 2014; 4:12748-12755.
2. Wang C., Luo H., Li H., Zhu X., Yu B. and Dai S. Tuning the Physicochemical Properties of Diverse Phenolic Ionic Liquids for Equimolar CO2 Capture by the Substituent on the Anion. Chem. Eur. J. 2012; 18: 2153 – 2160.