Aqueous biphasic systems comprising analogues of glycine-betaine ionic liquids: Toward greener separation platforms

Abstract

The use of biobased and bioactive compounds has boosted the development of sustainable extraction and purification processes, in which ionic liquids have been playing a major role. Ionic-liquid-based aqueous biphasic systems (IL-based ABS) have been raising significant interest as advantageous extraction and purification platforms; however, imidazolium-based ILs have been the primary option as phase-forming agents. Aiming the development of more sustainable IL-based ABS, in this work, six novel analogues of glycine-betaine ILs (AGB-ILs) were investigated. These ILs were synthesized and characterized in terms of purity, physico-chemical properties, ecotoxicity and ability to form ABS with tri-potassium citrate. The investigated AGB-ILs are composed of tri(n-alkyl[2-ethoxy-2-oxoethyl]ammonium/phosphonium and tri(n-alkyl[4-ethoxy-4-oxobutyl]ammonium/phosphonium cations with ethyl, n-propyl and n-butyl substituents, and the tosylate ([Tos]-) anion. The synthesized ILs present degradation temperatures in the range from 220 to 265 °C, and are classified as relatively harmless regarding their ecotoxicity. The studied AGB-ILs display slightly lower thermal stability, but yet a lower ecotoxicity compared to the commercial IL 1-butyl-3-methylimidazolium tosylate. Their ability to form ABS was assessed, allowing to conclude that an expansion in the alkyl side chain length and spacer, as well as the presence of phosphonium instead of ammonium as central atom, increase the ability for phase separation in presence of tri-potassium phosphate aqueous solutions. The extraction performance of the studied AGB-ILs-based ABS was finally assessed with furfural, a platform chemical. In the majority of the studied systems, furfural preferentially migrates to the AGB-IL-rich phase, obtaining extraction efficiencies in a single step to the IL-rich phase up to 98%. These results reveal that novel ABS comprising AGB-ILs can be potentially used as separation platforms of bioactive compounds.