Ionic liquids with polychloride anions as effective oxidants for the dissolution of UO₂

Herein, polychloride ([Cl₃]^- or/and [Cl₅]^-) ionic liquids (ILs) were prepared from their imidazolium chloride precursors by the addition of chlorine gas. The highest storage ability of Cl₂ was found in the [Bmim]Cl IL among the six imidazolium chlorides [Rmim]Cl (Rmim = Emim, 1-ethyl-3-methylimidazolium; Bmim, 1-butyl-3-methylimidazolium; Pmim, 1-propyl-3-methylimidazolium; Pnmim, 1-pentyl-3-methylimidazolium; Hmim, 1-hexyl-3-methylimidazolium; and Omim, 1-octyl-3-methylimidazolium). When a mixed IL of [Bmim][Tf₂N] (1-butyl-3-methylimidazolium bistriflimide) and [Bmim]Cl was used, the IL cation could maintain good stability during the chlorination process, and the imidazolium cation [Bmim]⁺ could retain almost the same structure after the chlorine gas was introduced into the mixed IL according to ¹H NMR spectroscopy. It has been found that the synthesized polychloride ILs not only can successfully dissolve UO₂, but can also decrease the consumption of Cl₂ and increase the chlorine efficiency. Spectroscopy studies indicate that [UO₂Cl₄]^2- is the principal product in the IL reaction solution. The dissolved uranyl species can be easily recovered from the ILs by crystallization in the form of [Bmim]₂[UO₂Cl₄]. Moreover, even if 57.1 wt% of the lanthanide element, when compared with the total amount of dissolved uranium and lanthanide elements, was dissolved in the IL mixture, only uranium-containing crystals could be isolated from the system.