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

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 empty set(2i) = 1.61 angstrom is most selective towards Pd (empty set(Pd)(2+)=1.56 angstrom) and tetradentate (4S) tetrathia-CE diol 2m (empty set(2m) = 1.57 angstrom) to Pt (empty set(Pd)(2+) =1.48 angstrom). 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: Q(Pd) = 212 mg g(-1); 2m-X: Q(Pt) = 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.

Automated summary

Multidentate thia-crown ether diols with different numbers of sulfur heteroatoms were developed as ligands for Pd and Pt, with bidentate dithia-CE diol 2i and tetradentate tetrathia-CE diol 2m showing high selectivity. These ligands were successfully fabricated into macroporous adsorbent resins for metal ion uptake, demonstrating high capacities and effective recovery of Pd and Pt in highly acidic conditions. Sequential adsorption of Pd by 2i-X and Pt by 2m-X showed high selectivity in the presence of other metal ions, making these resins ideal for treating acidic spent auto-catalyst leachate.