Cobalt(II) Complexation in the [C4mim][Tf2N] Ionic Liquid
Martina SANADAR1, An?ela KOVA?EVI?1, Marilena TOLAZZI1, Andrea MELCHIOR1
1Dipartimento Politecnico di Ingegneria e Architettura, Laboratorio di Tecnologie Chimiche, Università di Udine, Via Cotonificio 108, 33100, Udine, Italy
Critical Raw Materials (CRMs) are essential resources for strategic sectors of the European industry, which present a high supply risk coupled to an increasing demand in the next years. A possible strategy to mitigate the dependence upon import is to recover them from secondary sources (such as e-waste). The increasing demand for Li−ion batteries for electric vehicles shed light upon Co(II) supply chain. The metal is crucial to the cathode of these batteries. Due to the increasing consumption for the transportation market, the batteries might be considered a secondary source of Co(II).
As far as the separation and recycling of Co(II) is concerned, the use of hydrometallurgical processes where the separation occurs between an aqueous phase and a hydrophobic ionic liquid (IL) has been proposed. The main advantage of this extraction process is that the volatile organic compounds can be avoided. Among ILs which are in the liquid state at room temperature (RTILs), those based on the N,N’−alkylimidazolium ([CnCmim]+) cation and bis(trifluoromethylsulfonyl)imide ([Tf2N]−) anion have been extensively studied for metal extractions. It is also noticeable that few papers report the complexation enthalpy in ILs, despite the key−role of this parameter for the understanding of the strength of the metal−ligand interaction and the role of the process occurring in solution upon complexation.
The first objective of this study was to define the speciation and thermodynamics of Co(II) complex formation with chloride and nitrate anions in [C4mim][Tf2N]. The second objective was to obtain structural information on the species formed. The interest in the Co(II) speciation with nitrate and chloride anions is due to the fact that the liquid samples containing the metal to be recovered usually contain high concentrations of these anions.
The complex formation of Co(II) ion with nitrate and chloride ions in dry [C4mim][Tf2N] IL was studied by means of UV−Visible spectrophotometry, isothermal titration calorimetry (ITC), molecular dynamics (MD) simulations and density functional theory (DFT) calculations. By means of UV−Vis titrations in dry [C4mim][Tf2N] it was possible to determine the speciation and stability constants. ITC allowed us to obtain the complete thermodynamic characterization (stoichiometry and enthalpy). DFT and MD were useful for determine the possible structures of the different species.