Rapid and selective extraction of Pd() ions using the SCS type pincer ligand 1,3-bis(dimethylthiocarbamoyloxy)benzene, and its Pd() extraction mechanism

• Published in: RSC advances 2016-01, Vol.6 (2), p.1243-1252
• Main Authors: Gandhi, Muniyappan Rajiv, Yamada, Manabu, Kondo, Yoshihiko, Shibayama, Atsushi, Hamada, Fumio
• Format: Article
• Language: English
• Subjects: Aluminum; Catalysts; Ligands; Liquid-liquid extraction; Media; Palladium; Phase separation; Platinum
• ISSN: 2046-2069; 2046-2069
• DOI: 10.1039/c5ra23146a

We report the salient metal complexation properties of the SCS pincer ligand 1,3-bis(dimethylthiocarbamoyloxy)benzene ( 1 ), and its Pd( ii ) extraction properties in liquid-liquid extraction. Rapid and selective extraction by 1 of >99% Pd( ii ) ions from an automotive catalyst leach liquor containing Rh, Pd, Pt, Zr, Ce, Ba, Al, La, and Y in Cl − media was observed. The extractability ( E %) of 1 toward Pd was higher than that of the analogous compounds 1,4-bis(dimethylthiocarbamoyloxy)benzene ( 2 ), 1,4-bis(dimethylthiocarbamoyloxy)2,5-di- tert -butylbenzene ( 3 ), and 1,2-bis(dimethylthiocarbamoyloxy)benzene ( 4 ), and the commercial Pd( ii ) extractant di- n -hexyl sulfide (DHS). In o -dichlorobenzene diluent, 1 showed a high E % Pd (99.9-97.0%) in Cl − media (0.1-8.0 M) in 30 min, and excellent Pd( ii ) extraction and good phase separation were obtained with the various diluents studied. Pd( ii ) ions were stripped from the extractants using 0.1 M thiourea in 1.0 M HCl, enabling the reuse of 1 . Loading capacity measurements revealed a Pd/extractant 1 ratio of 1. Palladation is promoted at the benzene ring and dimethylthiocarbamoyl moieties via formation of stable fused six-membered chelate rings. The extractant 1 -Pd( ii ) complex was characterized using FT-IR spectroscopy, NMR spectroscopy, elemental analysis, single-crystal XRD study, and MALDI-TOF MS analysis. Extractant 1 displayed a high E % Pd (99%) even after five extraction cycles, indicating that it is very stable in acidic media and may be useful for the rapid and selective recovery of Pd( ii ) from catalyst solutions in platinum group metal refineries. Extractant 1 extracts Pd( ii ) ions via pincer type S-C-S tridentate ligands, with the formation of fused six-membered chelate rings. Rapid and selective extraction by 1 of >99% Pd( ii ) ions from an automotive catalyst leach liquor was observed.