Critical Investigation of Betaine Hydrochloride‐Based Deep Eutectic Solvent for Ionometallurgical Metal Production

Abstract

The applicability of a deep eutectic solvent (DES) consisting of betainium hydrochloride, urea and glycerol is examined with respect to ionometallurgical metal extraction and compared with the ionic liquid (IL) betainium bis(trifluoromethylsulfonyl)imide ([Hbet][NTf2]). The DES dissolves numerous metal oxides, where not only betaine and chloride act as stabilizing ligands, but also nascent ammonia seems to be essential. From such solutions, cobalt, copper, zinc, tin, lead, and even vanadium can be electrodeposited, demonstrating the feasibility of ionometallurgy. However, repeated recycling of the DES is not conceivable. NMR spectroscopy and mass spectrometry identify numerous decomposition reactions taking place at 60 °C already. The by‐products that are formed not only make recycling more difficult, but also pose a toxicity problem. The opportunities and obstacles of DESs and ILs for their application in ionometallurgy are critically discussed. It is shown that a thorough understanding of the underlying chemical processes is critical.

Ionometallurgy: an academic exercise or promising approach?

“…Thus, the (beneficial) properties reported for the pure DES change and might be lost. 19,110,111 This effect is less pronounced for ILs.…”

“…Specifically ammonia evolution during decomposition can have significance in the solubility of MOs. 111…”

“…Investigations under isothermal conditions should therefore be performed as a routine when evaluating the stability of ILs and DESs. 111 NMR spectroscopy can be a suitable, fast and convenient method to identify decomposition products. However, it has to be taken into account, that the detection limit is relatively high and that especially the presence of paramagnetic metal ions significantly affects the quality of the spectra.…”