p-Toluenesulfonic Acid-Based Deep-Eutectic Solvents for Solubilizing Metal Oxides

Combinations of p-toluenesulfonic acid monohydrate with different hydrogen-bond acceptors (i.e., choline chloride, tetraethylammonium chloride, tetrabutylammonium chloride, betaine hydrochloride, β-alanine, tetrabutylphosphonium chloride, ethylammonium chloride, methyltriphenylphosphonium bromide, and benzyltriphenylphosphonium chloride) were tested for deep-eutectic solvent (DES) formation. Only p-toluenesulfonic acid monohydrate:choline chloride, p-toluenesulfonic acid monohydrate:tetrabutylammonium chloride, and p-toluenesulfonic acid monohydrate:tetrabutylphosphonium chloride at a 1:1 molar ratio remained liquid at room temperature. The obtained deep-eutectic solvents were characterized by determining their density and viscosity as a function of temperature and their thermal operational window (i.e., decomposition temperature and melting point/glass transition temperature). Moreover, IR spectroscopy was used to elucidate the formation of hydrogen bonds. The solubility of different metal oxides in the DES p-toluenesulfonic acid monohydrate:choline chloride was experimentally determined. The effect of the hydrogen-bond donor:hydrogen-bond acceptor molar ratio (2:1, 1:1, and 1:2) on metal oxide solubility was explored. A comparison was made with metal oxide solubilities in other choline chloride-based DESs and in acidic aqueous solutions.