Revisiting greenness of ionic liquids and deep eutectic solvents

Green solvents are one of the hot topics of green chemistry. Ionic liquids (ILs) and deep eutectic solvents (DESs) are deemed as green solvents to a certain extent, and they have been applied in many areas, such as dissolution, separation, catalysis, electrochemistry and material synthesis. However, the greenness of ILs and DESs should be revisited because more and more evidences have shown that they are not always green. In this perspective, besides the reported merits, the disadvantages and problems of some ILs and DESs, such as instability, volatility, hygroscopicity, toxicity, flammability, regenerability, cost, energy consumption and impurities are discussed. Moreover, 13 strategies to avoid the disadvantages of ILs and DESs and to increase the greenness are proposed. Comparison of the greenness of ILs and DESs is further conducted. This perspective provides some new viewpoints on the greenness of ILs and DESs.