Ionic liquids and carbon dioxide

In 1999 Blanchard et al. reported a good solubility of carbon dioxide in l-butyl-3-methylimidazolium hexafluorophosphate at high pressures, while the ionic liquid did not dissolve in carbon dioxide. Therefore, supercritical carbon dioxide is suited to extract organic solutes from ionic liquids, and also continuous flow homogeneous catalysis in ionic liquids carbon dioxide systems is possible. First spectroscopic studies show that the anion dominates the interactions with carbon dioxide by Lewis acid-base interactions. However, the strength of carbon dioxide anion interactions did not correlate with carbon dioxide solubility. Thus, strong anion-carbon dioxide interactions were excluded as major cause for the carbon dioxide solubility in ionic liquids. Instead, a correlation of carbon dioxide solubility and the ionic liquid molar volume was observed. Additionally, a significant volume decrease of dissolved carbon dioxide was 

Based on these observations a space-filling mechanism 

Additionally to physisorption, chemisorption can contribute to the solubility of carbon dioxide in ionic liquids. One example is l-alkyl-3-methylimidazolium acetate. The basicity of the anion allows the abstraction of the most acidic proton of the cation, forming a carbene and acetic acid. Subsequently, the former cation is carboxylated. The availability of carbenes in l-alkyl-3-methylimidazolium acetate was used already 

2- 3 orders of magnitude and decrease translational dynamics as well. As a result, a gel-like network is formed. Based on these results, Gurkan et al. developed low viscosity ionic liquids for carbon dioxide uptake. Static quantum chemical calculations identified pyrrolide and pyrazolide anions suitable for carboxylation. Subsequent viscosity measurements of the synthesized compounds showed a nearly unaffected viscosity by carbon dioxide uptake. In agreement with this, translational and rotational dynamics are similar in the neat and carbon dioxide reacted ionic liquid due to the absence of a hydrogen-bond network.  

In summary, physisorption of carbon dioxide is mainly driven by a spacefilling mechanism. Weak cation-anion interactions seem to 

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M. C. Kroon, Combined Reactions and Separations Using Ionic Liquids and Carbon Dioxide, Delft University of Technology, Delft, 2006. 

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