(381r) Experimental and Theoretical Characterization of Thiacrown Ether Epoxy-Polymer Adsorbents for the Selective Recovery of Pd and Pt from Spent Auto-Catalyst Leachate

Palladium (Pd) and Platinum (Pt) along with other platinum-group metals (PGMs) play an important role for technological advancement due to their high catalytic reduction and oxidation property, superior resistivity, high temperature and electrical stability, and resistance to tarnish. Hence, PGMs have been widely used in automotive emission control catalyst, industrial chemical catalyst, electronic devices, biomedical devices and jewelry. However, PGMs especially Pd and Pt are extremely expensive due to their low concentration on the earth’s crust and the complex processes required for their extraction and refining. Currently, the separation and purification of Pd and Pt are carried out using liquid-liquid extraction (LLE) with commercial extractants (e.g. sulfur containing, oxime, thioglycoamides, ammonium and phosphonium functional groups) that require multiple stages of extraction process to achieve the desired purity. In terms of economic and technical view, adsorption is the most suitable method for the recovery of PGMs because of its high efficiency and low cost. Thus, many studies have attempted to incorporate thiourea or thioglycoamides in polymeric adsorbents but did not show similar LLE selectivities due to the non-selective multi-dendate coordination involved. Therefore, the development of a highly selective adsorbent will the improve sustainable supply PGMs especially if recovered from secondary sources such as spent automotive catalysts, industrial catalyst and electronic waste.

Herein, novel selective thiacrown ether ligands were engineered based on fine tuning the coordination specificity by changing the number of sulfur heteroatoms, structure of crown ether (CE) and cavity size (Ø_CE). The performance of each thiacrown ether was evaluated with the aid of experimental extraction and theoretical density functional theory (DFT) studies. Dihydroxy-dithia-, -trithia-, and -tetrathia-14-crown-4 ether derivatives (2g-2m) were effectively synthesized by bis-epoxide intermediate ring-opening cyclization reaction with 1,2-benzenedithiol using a mild base triethylamine and LiCl as template in water-tetrahydrofuran solvent. The resulting bis-epoxide cyclization reaction gave CEs with free di-hydroxyl groups necessary for further chemical modifications and subsequent polymerization.

Initial experimental liquid-liquid extraction in chloroform-water system and density functional theory (DFT) calculations using the ab initio Jaguar program B3LYP with LACVP** basis set reveal that the CE cavity size and denticity of the synthesized thiacrown ethers affected their selectivity (β) towards Pd and Pt. Among the thiacrown ether derivatives, bidentate dihydroxy-dibenzo-dioxadithia-14-crown ether (2i) with a DFT calculated cavity size of 1.58-1.65 Ǻ was most selective towards Pd (β_Pd/Pt=10⁵). Meanwhile, tetradentate dihydroxy-benzo-dimethyl-tetrathia-14-crown-4 ether (2m) with a cavity size of 1.31-1.60 Ǻ was selective towards Pt (β_Pt/Pd=10³). This is due to the CE cavity size-match relationship where the ionic size of Pd (1.56 Ǻ) or Pt (1.48 Ǻ) is suitable for the CE cavity. DFT also showed that the Pd and Pt are situated perfectly inside the CE cavity with an S₄ CE-Mⁿ⁺ complex coordination geometry and have the lowest calculated binding energies (ΔE_CE-Mⁿ⁺). The complex 2i-Pd²⁺ showed the most stable conformer with ΔE_CE-Mⁿ⁺=-69.10 kcal/mol whereas complex 2m-Pt²⁺ geometry with ΔE_CE-Mⁿ⁺=-75.27 kcal/mol.

The synthesized dihydroxy-CEs 2i and 2m where further modified with epoxide groups by reacting with epichlorohydrin under strong basic conditions. The resulting CE bis-epoxide intermediates (2i-ep or 2m-ep) where cured with ethylenediamine in polyethylene glycol (PEG) at 150^oC to afford porous thiacrown ether epoxy-polymerized adsorbents (EP-2i or EP-2m) that are selective towards Pd or Pt. The well-defined macroporous structure of the epoxy-polymer adsorbents were achieved by in situ polymerization in porogenic solvent PEG. Initially, the mixture of CE bis-epoxides and ethylenediamine in PEG is homogenous but with increasing cross-linking density, phase separation of the PEG occurs which leads to nucleation forming the porous structures. To evaluate the performance of each thiacrown ether epoxy-polymer adsorbent, the effects of HCl concentration was investigated to determine the optimum conditions for Pd and Pt separation as well as the adsorbent stability in very acidic solutions. Furthermore, the effects of the initial concentration of Pd or Pt, kinetics, and recyclability were also investigated to established optimum operating conditions. Lastly, the recovery of Pd and Pt was done using simulated auto-catalyst leachate containing high concentration of other competing metals (Al³⁺, Ce³⁺, Mg²⁺, Pb²⁺, Fe³⁺, Ni²⁺, Mn²⁺, and Cr³⁺) and showed excellent recovery of Pd and Pt.

This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No. 2017R1D1A1B03028102) and by the Ministry of Science, ICT and Future Planning (No. 2017R1A2B2002109).