(275g) Liquid-Liquid Extraction and Theoretical Binding Studies of Crown Ether Derivatives with Mixed Heteroatoms Towards Platinum Group Metals

Platinum-group metals (PGM) especially palladium (Pd) and platinum (Pt) are extremely scarce in comparison to other precious metals due to their low natural abundance and the complex processes required for their extraction and refining. Currently, the separation and purification of PGMs are carried out using commercial extractant having low selectivity, which require multiple stages of extraction process. Therefore, development of effective extractants with high selectivity will improve its sustainability especially for PGMs recovered from secondary sources such as spent automotive catalysts, industrial catalyst and electronic waste.

Herein, the design of novel selective extractant was based on fine tuning the coordination specificity by changing the heteroatoms, structure of crown ether (CE) and cavity size (Ø_CE) was realized with the aid of experimental extraction and theoretical Density Functional Theory (DFT) studies. Efficient green synthesis of 15-14 membered crown ethers with mixed heteroatoms was achieved via intramolecular cyclization of bulky or rigid bis-epoxides with aromatic N or S di-nucleophiles using trimethylamine (Et₃N) and lithium chloride template in water to give crown ether derivatives with mixed heteroatoms (O, N, S). Initial experimental liquid-liquid extraction and DFT studies were implemented to evaluate the performance of each synthesized CE towards Pt/Pd extraction. Among the CEs synthesized, mixed thiacrown ether having dibenzo moieties, 2HDBS₂14C4, with Ø_CE=1.58 Å shows highest selectivity towards Pd (Ø_Pd=1.52 Å) even in the presence of other platinum (Pt, Ru and Rh) or precious group metals (Sn, Sb, Hf, Ir and Au). In order to determine the effect of the structure and the heteroatoms towards Pd or Pt coordination, optimized DFT structures of the free CE using the ab initio Jaguar program B3LYP with LACVP** basis set was employed. From the DFT results, the cavity size match relationship, where the CE cavity size is similar to the metal diameter, as well as changing the heteroatoms plays major role in synthesizing Pd selective CEs.

The novel thiacrown ethers synthesized were applied in liquid-liquid extraction process using simulated PGM solutions. The thiacrown ethers were first methylated to increase its solubility in the extraction solvent toluene prior to liquid-liquid extraction studies. The effects of HCl and thiacrown ethers concentration to the distribution of Pd and Pt were performed subsequently to determine the optimum HCl concentrations, CE-Pd stoichiometry, and extraction equilibrium coefficients (K_ex). Re-extraction of Pd in the solvent phase using thiourea-HCl and its recyclability through repeated extraction and re-extraction were done to determine its stability and applicability in real extraction system. Finally, the thiacrown ethers were used as extractants to recover Pd and Pt from simulated spent automotive catalyst leach solutions.

This work was supported by NRF funded by the Korea government funded by the Ministry of Science and ICT (2017R1D1A1B03028102 and 2017R1A2B2002109) and Ministry of Education (2009-0093816 and 22A20130012051(BK21Plus)).

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