Highly selective extraction of palladium from spent automotive catalyst acid leachate using novel alkylated dioxa-dithiacrown ether derivatives

[Display omitted] •New cyclic Pd2+ extractants dimethoxy-dioxadithia-benzo-14-crown-4 ethers (3g–i).•Ideal crown ether cavity size for Pd2+ forming preferential S4 coordination geometry.•High stability and high separation efficiencies in highly concentrated acidic feed.•Applicable for repeated extraction and stripping cycles with consistent performance.•High Pd2+ extraction efficiency and purity from spent automobile leachate. Novel macrocyclic extractants were synthesized and employed in liquid–liquid extraction (LLE) for palladium/platinum (Pd/Pt) separation in aqueous feed source. Dioxadithia-benzo-crown ether diols (BCES2O2) 2g–2i were prepared through bis-epoxide cyclization with aqueous 1,2-benzenedithiol under mild basic conditions. For improved miscibility and stability in the organic diluent (i.e. toluene), 2g–2i were alkylated to afford dimethoxy-BCES2O23g–3i. LLE experiments reveal their high stability, extraction efficiency, and separation performance (βPd2+/Pt2+=104−105) in highly acidic feed solutions (∼6M HCl). The most suitable extractant 3i followed a 1:1 3i-Pd2+ complex with stability constant Kex∼105. Density functional theory calculations reveal that the rigid cavity size (1.58–1.65Å) of 3i is ideal for Pd2+ (1.56Å) in forming S4 coordination geometry. The Pd2+ can be repeatedly extracted by 3i [in toluene] and stripped by 1M HCl+0.15M thiourea solution without performance deterioration. Each cycle can be accomplished