Solvent extraction and separation of palladium from platinum group elements: Synthesis and characterization of 2-mercaptopyridine N-oxide-palladium (II) complex

Abstract

Platinum group elements (PGEs) are known for their similar chemical properties, which complicates their isolation and separation from mineral ores and recycled products such as autocatalysts. Therefore, this study investigated the separation of PGEs (except Os) in a synthetic autocatalyst mixture using 2-mercaptopyridine N-oxide sodium salt (NaPT) as an extractant. Solvent extraction was performed with an aqueous mixture containing Pt, Pd, Rh, Ir and Ru and NaPT in the presence of toluene, which produced a pink organic layer. An ICP analysis indicated a complete absence of Pd in the orange aqueous layer. The extractions were repeated with different [HCl] and [PT−] and the analytical results confirmed the complete extraction of Pd from the mixture. The formation of a Pd(PT)2 complex in the pink product was confirmed with IR, 1H and 13C NMR. Pd(OAc)2 reacted with NaPT in DMF to form pink crystals, which were suitable for crystallography. The pink crystals were also characterized with IR, 1H and 13C and found to be identical to the product isolated after solvent extraction. The crystal structure indicated the isolation of the square planar Pd(PT)2·DMF complex, which crystalized in the monoclinic P21/n space group with four molecules per unit cell. The two PT− ligands in the complex bonded with the sulfur atoms (and also the oxygen atoms) cis with respect to each other, with the S–Pd–S and O–Pd–O bond angles at 96.27(5) and 91.08(14)° respectively. The two Pd–S bonds distances were 2.2443(13) and 2.2416(15) Å. The selective and complete isolation of Pd from a mixture of PGE using NaPT can be extremely useful in the recovery of Pd from autoctalaysts and other mixed PGE waste materials.