Improved separation efficiency using ionic liquid–cosolvent mixtures as the extractant in liquid–liquid extraction: A multiple adjustment and synergistic effect

This article tries to provide a feasible approach to solving current problems of ionic liquid (IL) mediated liquid–liquid extraction such as large viscosity, relatively high polarity and considerable cost. The performance of IL–cosolvent mixtures instead of pure ILs as extractant was systematically studied for the first time, taking the extractive separation of tocopherol homologs as an example. The adjustment of separation efficiency through both the structures of IL and cosolvent and their relative amount was conducted. Moreover, the synergistic effect between IL and cosolvent was revealed. A large separation selectivity, adequate distribution coefficient, low viscosity, and low consumption of IL can be simultaneously achieved in the IL–cosolvent–hexane biphasic system. For example, while using the 2:98 [bmim]Cl–acetonitrile mixture (molar ratio) as the extractant (viscosity was only about 0.44 mPas) at 303.15 K, the selectivity of δ-tocopherol to α-tocopherol was 11.3, four times larger than that using pure acetonitrile, and the distribution coefficients of δ-, β&γ-, and α-tocopherol were 4.07, 2.02, and 0.36, respectively, at least eighteen times larger than those using pure [bmim]Cl. A quantitative correlation between the selectivities of different tocopherol homologs and the hydrogen-bond basicity of IL–cosolvent mixtures was established with a linear equation. Overall, this work demonstrated that improved separation efficiency might be achieved by using IL–cosolvent mixtures as extractant compared to using pure IL or pure cosolvent.

► Synergistic effect between ionic liquid and cosolvent was revealed. ► Quantitative correlation of selectivity with hydrogen-bond basicity was created. ► Separation efficiency could be adjusted by multiple approaches. ► Satisfying separation was achieved when the molar faction of [bmim]Cl was only 2%.