Recovery of ionic liquids with aqueous two-phase systems induced by carbon dioxide

Recovery is a very important factor for the industrial application of ionic liquids (ILs). In this work, a novel method is presented for the recovery of ILs by using carbon dioxide (CO₂-induced formation of aqueous two-phase systems (ATPSs). It was found that, in the presence of amines, introduction of CO₂ into aqueous IL solutions leads to the formation of ATPSs at 25 °C and atmospheric pressure, in which the upper phase is ammonium-salt-rich and the lower phase is IL-rich. Thus, the ILs in aqueous solutions can be significantly enriched, and the amines can be regenerated by heating and bubbling Ar or N₂ in the salt-rich phase. To better understand the recovery of ILs, the phase diagrams of the ATPSs were measured at 25 °C, and the effects of the molecular structure of the ILs and the amines and temperature of the systems on the recovery efficiency of the ILs were investigated. It was shown that the single-step recovery efficiency of the ILs could be as high as 99 % in the presence of primary or secondary amines. Therefore, this new method could potentially be sustainable, efficient, and attractive to industry.