Bioethanol and biobutanol are two important energy additives and chemicals, while most of them are produced so far by fermentation using sugars as carbon source. Separation of these alcohols from their fermentation broth requires up to 6 % of the energetic value of the compounds themselves [86]. Conventionally, alcohols and water were separated by distillation or membrane technology, but these technolo­gies are energetically costly, or are not mature for large scale application. The tenability of miscibility and solubility of ILs to water, and other organic compounds offers significant advantages for separation of alcohols from water [87].

In an early study [88], ILs were tested for extraction of BuOH from aqueous solutions. It was found that BuOH distribution coefficient in 1-Butyl-3-methylimi — dazolium hexafluorophosphate ([BMIM][PF6])-water system was 0.85, which was very close to the predicted value. 1-octyl-3-methylimidazolium hexafluoro­phosphate ([OMIM][PF6]) had increased molecular dimensions of the alkylimi- dazolium cation gave lower mutual solubility of the ionic liquid and water, resulting in higher extractive selectivity for BuOH. Pervaporative BuOH recovery from 1 wt% aqueous solution and [OMIM][PF6] was investigated using commercial polydimethylsiloxane membrane MEM-100. Although the viscosity of [OMIM] [PF6]-water-BuOH solution was about 100 fold higher than the viscosity of the BuOH-water mixture, the flux rate through the membrane was only 0.6 fold lower at higher selectivity. BuOH-water ratio in the permeation was close to that in ionic liquid feed, suggesting that the membrane did not improve the separation. Distilla­tion is thought to be more economical for BuOH recovery from ILs [89].

It was demonstrated that the solubility of water in the hydrophobic IL 1-alkyl-3- methylimidazolium hexafluorophosphates could be significantly increased in the presence of ethanol as a so-solute. It was found that 1-hexyl-3-methylimidazolium hexafluorophosphate and 1-octyl-3-methylimidazolium hexafluorophosphate are completely miscible with ethanol, and immiscible with water, whereas 1-butyl-3- methylimidazolium hexafluorophosphate is totally miscible with aqueous ethanol only between 0.5 and 0.9 mole fraction ethanol at 25 °C. At higher and lower mole fraction of ethanol, the aqueous and IL components were only partially miscible and a biphasic system was obtained upon mixing equal volumes of the IL and aqueous ethanol. These observations indicated that ILs may be exploited as an extraction solvent for bioethanol recovery from fermentation broth.

As the hexafluorophosphate anion are partially hydrolyzed in the presence of water and thus generating corrosive HF [90], following up research in this area was moved to ILs with anions bis(trifluoromethylsulfonyl)imide and tetracyanoborate. These ILs are more stable in water and are more hydrophobic than those of hexafluorophosphate anion based ILs. Studies have led to the development of ternary diagrams, separation coefficients of 1-hexyl-3-methylimidazolium bis (trifluoromethylsulfonyl)imide/ethanol/water, and 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide/1-butanol/water. It was found that 1-hexyl-3- methylimidazolium bis(trifluoromethylsulfonyl)imide can be used successfully to separate 1-butanol from water. Although it can also be used for ethanol separation, the solvent/feed ratio has to be unreasonably high [91, 92].

The tetracyanoborate-based ILs, 1-hexyl-3-methylimidazolium tetracya — noborate, 1-decyl-3-methylimidazolium tetracyanoborate, and trihexyltetradecyl- phosphonium tetracyanoborate, have also been studied. A complete miscibility in the binary liquid systems of 1-butanol with these ILs was observed. The presence of imidazolium cation gave lower selectivity and distribution ratio than those with phosphonium cation. The ILs with the longer alkyl chains at the cation showed higher selectivity and distribution ratio. Regarding to performance of the imidazolium based ILs, the choice of anion was shown to have a large impact on the upper critical solution temperatures of the system. The relative alcohol affinity for the different anions was (CN)2 N > CF3SO3 > (CF3SO2)2 N > BF4 > PF6 [93, 94].

Liquid-liquid extraction of 1-butanol from water employing nonfluorinated task-specific ILs (TSILs) has also been described recently [95]. Tetraocty — lammonium 2-methyl-1-naphthoate [TOAMNaph] was identified as the best IL which had butanol distribution coefficient of 21 and selectivity of 274. These data were substantially better than those of the benchmark solvent oleyl alcohol, which had butanol distribution coefficient of 3.4 and selectivity of 192. The conceptual design study showed that butanol extraction with [TOAMNaph] requires 5.65 MJ/kg BuOH, which is 73 % less than that by conventional distillation.

To establish a more economic separation technology, studies were carried out to get in depth understanding of the vapor-liquid equilibrium of ethanol-water-ILs [96], 1-butanol-water-ILs, and high pressure CO2-induced phase changes [97]. The results suggested that ILs were capable of breaking the binary azeotrope ethanol-water, opening a new possibility as entrainer for this system, while the 1-hexyl-3-methyl imidazolium chloride moved the azeotrope composition to a smaller fraction of ethanol. In addition, hexane assisted ILs extraction of ethanol [98], and phosphonium and ammonium ionic liquid-based supported liquid mem­branes have also been investigated [99].