Separation of alcohols from aqueous solutions usually requires energy intensive processes. We explore, here, the use of ionic liquids to remove alcohols from aqueous solutions. Ionic liquids are low melting salts with negligible volatility at ambient conditions. Therefore, the recovered alcohol could simply be evaporated from the ionic liquid solvent. To evaluate various ionic liquids for this application, this study details the phase behavior of binary and ternary systems of ionic liquids with water and alcohols.
The mutual solubilities of various ionic liquids with water were determined in order to study the effect of the structure of the ionic liquid on the solubility. An increase in the alkyl chain length or in the substitution decreases the mutual solubility of the ionic liquid with water. The ionic liquids with the diacyanoamide anion have the largest mutual solubility with water, followed by ionic liquids with trifluoromethanesulfonate tetrafluoroborate, tetracyanoborate, hexafluorophosphate, bis(trifluoromethylsulfonyl)imide, tris(trifluoromethylsulfonyl)methide, and tris(pentafluoroethyl)trifluorophosphate anions. Further, the mutual solubilities of various ionic liquids with 1-hexanol and 1-octanol were measured, which confirmed published trends1-6 on how the IL structure modifies the mutual solubility. We also demonstrated that adding a hydroxyl group on the cation alkyl chain length decreases the mutual solubility with alcohols, and that ILs with the tris(pentafluoroethyl)trifluorophosphate anion have a smaller mutual solubility than those with the bis(trifluoromethylsulfonyl)imide anion but greater than ILs with the tetrafluoroborate anion. The second component of our work is to improve ionic liquids for various liquid-liquid separations. Necessary to design separation processes, ternary diagrams of systems with water and alcohols for various ionic liquids were developed. Further, distribution coefficients, selectivities and the number of stages for a liquid-liquid extractor are calculated using this data.
In order to compare the energy requirement with other processes, this study also measured excess enthalpies for mixtures of ionic liquids and alcohols. Using these enthalpies, the energy required to evaporate the alcohol from the ionic liquid was calculated. We found that this energy intensive step requires approximately 50 kJ/molAlcohol while current processes require between 100-2000 kJ/molAlcohol 7, 8. We conclude that this method could reduce the energetic requirement in separating alcohols from water.
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