Ionic Liquid-Mediated Liquid-Liquid Extraction

Abstract

Liquid-liquid extraction is an important kind of separation method that is based on the distribution of chemicals between two different liquid phases. Compared to other separation methods, liquid-liquid extraction often has unique advantages for the separation of chemicals that have high or similar boiling points, with relatively large capacity and low consumption of material and energy (Treybal, 1951). However, the kinds of extractants that could be used for liquid-liquid extraction process are relatively small at present, so only limited separation efficiency could be achieved for the separation of many mixtures, especially those having similar structures. Besides, the volatility of existing extractants or extraction solvents not only can bring contamination to the environment, but also can lead to difficulty on the recovery of extractant and subsequent purification of products when the solutes are also volatile. These problems have limited the application of liquid-liquid extraction to more separation processes. Using ionic liquids (ILs) as extractants may be a prospective solution to the above problems. The physicochemical properties of IL could be designed and adjusted task-specifically, so large separation selectivity may be achieved for various mixtures that need to be separated (Han & Armstrong, 2007). Moreover, the cohesive energy of ILs is always very large, so ILs are easy to form various immiscible liquid-liquid biphasic systems with other solvents (Marcus, 2010). Besides, the vapor pressures of ILs are extremely low, so ILs could be regarded as green extractants and the separation of ILs with volatile solutes could also be simplified (Welton, 1999). These characteristics have made ILs appropriate to be used as extractants in liquid-liquid extractions. Liquid-liquid extraction using IL as extractant has been studied by many researchers in recent years since the partition of substituted-benzene derivatives between water and a hydrophobic IL 1-butyl-3-methylimidazolium hexafluorophosphate ([C4mim]PF6) was initially reported (Huddleston et al., 1998). These studies include the removal of sulfides and nitrides from diesel and gasoline (Holbrey et al., 2008; Xie et al., 2008), the separation of aromatics from aliphatics (Meindersma & de Haan, 2008), the removal of pollutants from water (Egorov et al., 2008; Vijayaraghavan et al., 2006), the isolation of biological substances from aqueous mixtures (Wang et al., 2007), the extraction of glycerol from biodiesel (Abbott et al., 2007), the extractive essential oil terpenless (Arce et al., 2006), and so on. From those works, ILs have been revealed to have the strong ability of interacting with organic