Multidentate thia-crown ethers as hyper-crosslinked macroporous adsorbent resins for the efficient Pd/Pt recovery and separation from highly acidic spent automotive catalyst leachate

Abstract

Multidentate thia-crown ether (CE) diols containing different number of sulfur heteroatoms (2S-4S) were developed as ligands for Pd and Pt. Seven thia-CE diols (denoted as: 2g-2m) were synthesized at 63–93% yields through ring-opening cyclization of bis-epoxide intermediates with 1,2-benzenedithiol. Each thia-CE contains 2 –OH groups as reactive sites for adsorbent fabrication. Initial screening of thia-CE diols through liquid–liquid extraction and density functional theory (DFT) calculations reveal that bidentate (2S, 2O) dithia-CE diol 2i with cavity size Ø2i = 1.61 Å is most selective towards Pd (ØPd2+=1.56 Å) and tetradentate (4S) tetrathia-CE diol 2m (Ø2m = 1.57 Å) to Pt (ØPt2+=1.48 Å). DFT calculations indicate that size-match relationship and denticity difference dictated the coordination stability of 2i with Pd and 2m with Pt, which ultimately defined their respective selectivities. Thia-CEs 2i and 2m were subsequently fabricated as macroporous adsorbent resins (2i-X and 2m-X) via crosslinking of their bis-epoxide derivatives with ethylenediamine in porogenic PEG 400 solvent. Metal ion uptakes were Langmuir-type with high capacities (2i-X: QPd = 212 mg g−1; 2m-X: QPt = 345 mg g−1) and kinetic rates follow the pseudo-second order rate model. Metal ion uptakes are mainly due to neutral coordination with the thia-CEs (84–86%) and to some extent, due to anion complexation with ammonium groups (14–16%). Recovery of Pd by 2i-X and Pt by 2m-X can be carried out effectively and repeatedly in highly acidic feed (6 M HCl) without performance deterioration. Sequential adsorption of Pd by 2i-X and Pt by 2m-X are highly selective in the presence of base metal ions (Mg2+, Al3+, Cr3+, Mn2+, Fe3+, Ni2+, and Pb2+) making these resins ideal for the treatment of highly acidic spent auto-catalyst leachate.