Thiodiphenol-Based n‑Dialkylamino Extractants for Selective Platinum Group Metal Separation from Automotive Catalysts

Five kinds of dialkylamino-modified thiodiphenols (2–6) were synthesized to elucidate the extraction of Pd­(II) and Pt­(IV) in HCl media by thiodiphenol-based n-dialkylamino extractants. Although the synthesized species 2–6 showed good solubility in hydrocarbon-based diluents such as kerosene, ShellSol D70, and ISOPAR M, the addition of n-octanol to the platinum group metals (PGMs) extraction was needed to inhibit third layer formation. All the 2–6 extract Pd­(II) and Pt­(IV) were much more effective than the commercial extractants, 2-hydroxy-5-nonylacetophenone oxime (LIX84-I), tri-n-butylphosphate (TBP), and tri-n-octylamine (TOA). Increasing the hydrophobicity of the extractants by increasing the alkyl chain length had an effect on the Pd­(II), Pt­(IV), and Rh­(III) extractabilities, which increased in the order from 2 to 6. Extractant 6 could also be used for the selective extraction of Pd­(II) and Pt­(IV) (E% > 99%) from the leach liquors of automotive catalysts containing Pd­(II), Pt­(IV), Rh­(III), La­(III), Ce­(III), Y­(III), Zr­(IV), Ba­(II), and Al­(III) in HCl media. The effective stripping of Pd­(II) and Pt­(IV) was achieved from an extracted organic phase containing 6 using 0.5 M thiourea in 1.0 M HCl. In addition, 6 was reusable and exhibited a high E% for Pd­(II) and Pt­(IV) (>96%) after five extraction cycles, indicating potential usefulness for the selective recovery of Pd­(II) and Pt­(IV) from leachates in platinum group metal refineries.