Task-Specific Ionic Liquid for Solubilizing Metal Oxides

Protonated betaine bis(trifluoromethylsulfonyl)imide is an ionic liquid with the ability to dissolve large quantities
of metal oxides. This metal-solubilizing power is selective. Soluble are oxides of the trivalent rare earths,
uranium(VI) oxide, zinc(II) oxide, cadmium(II) oxide, mercury(II) oxide, nickel(II) oxide, copper(II) oxide,
palladium(II) oxide, lead(II) oxide, manganese(II) oxide, and silver(I) oxide. Insoluble or very poorly soluble
are iron(III), manganese(IV), and cobalt oxides, as well as aluminum oxide and silicon dioxide. The metals
can be stripped from the ionic liquid by treatment of the ionic liquid with an acidic aqueous solution. After
transfer of the metal ions to the aqueous phase, the ionic liquid can be recycled for reuse. Betainium bis(trifluoromethylsulfonyl)imide forms one phase with water at high temperatures, whereas phase separation
occurs below 55.5 °C (temperature switch behavior). The mixtures of the ionic liquid with water also show
a pH-dependent phase behavior:  two phases occur at low pH, whereas one phase is present under neutral or
alkaline conditions. The structures, the energetics, and the charge distribution of the betaine cation and the
bis(trifluoromethylsulfonyl)imide anion, as well as the cation−anion pairs, were studied by density functional
theory calculations.