Effect of Hydrophobicity of Ionic Liquids on the Leaching Selectivity of Platinum from a Spent Automotive Catalyst

Abstract

Recycling platinum group metals from secondary resources such as spent automotive exhaust catalysts is promising for the circular economy. However, the selective separation of Pt over massive amounts of impurity metals such as Mg and Al is particularly challenging. In this study, non-aqueous direct leaching of platinum from a spent automotive catalyst (SAC) using hydrophobic ionic liquids, namely, trihexyl(tetradecyl)phosphonium chloride (P 66614 Cl) and trioctyl(dodecyl)phosphonium chloride (P 88812 Cl) with the aid of pre-loading hydrochloric acid/hydrogen peroxide was proposed. The more hydrophobic P 88812 Cl exhibited more efficient and selective leaching of Pt over Mg and Al. The recovery of Pt from the metal-loaded P 88812 Cl, and the reusability of the IL for SAC leaching were also demonstrated.

Platinum Group Metal Recycling from Spent Automotive Catalysts Using Reusable Hydrophobic Deep Eutectic Solvent

“…19,20 Notably, metal leaching with these nonaqueous solvents often enables higher metal extraction selectivity compared with traditional aqueous systems. 21 Moreover, because hydrophobic solvents do not mix with water, they can contact aqueous solutions to allow for facile recovery of metals extracted from the hydrophobic solvent into the aqueous solution. 22,23 Oxidative leaching of precious metals involves adding oxidizing agents to organic solvents.…”

“…For example, trihalide ILs, synthesized by adding chlorine, bromine, or iodine to halogen-containing ILs, − have been used to leach precious metals and SAC powders near room temperature . Additionally, phosphonium-based hydrophobic ILs loaded with aqueous HCl/H 2 O 2 were employed to leach PGMs from a SAC . Similarly, hydrophilic DESs comprising choline chloride and p -toluenesulfonic acid, with nitric acid or H 2 O 2 as an additive, have been used to leach automotive catalyst powders. , However, efficient leaching of PGMs in non-aqueous media (i.e., while minimizing simultaneous leaching of unwanted metals, such as Al and Mg) remains challenging.…”

“…DESs are solvents that comprise hydrogen bond donors (HBD) and acceptors (HBA) and exhibit a unique solid-to-liquid transition to form liquid at room temperature. − ILs and DESs are considered environmentally friendly solvents owing to their low volatility. Numerous studies have explored metal processing with non-aqueous solvents, such as ILs or DESs, via solvometallurgy or ionometallurgy. , Notably, metal leaching with these non-aqueous solvents often enables higher metal extraction selectivity compared with traditional aqueous systems . Moreover, because hydrophobic solvents do not mix with water, they can contact aqueous solutions to allow for facile recovery of metals extracted from the hydrophobic solvent into the aqueous solution. , …”