Synthesis of pincer-type extractants for selective extraction of palladium from PGMs: An improved liquid-liquid extraction approach to current refining processes

Environmentally friendly Pd(II) recovery from spent automotive catalysts using resins impregnated with a pincer-type extractant

Extractant-impregnated resins have potential for recovering platinum group metals selectively and efficiently. Herein, 1,3-bis(2-(octylthio)propan-2-yl)benzene (1), a pincer-type extractant, was impregnated in Amberlite XAD-7 resin (1-EIR), and the batch Pd(II) sorption conditions, including impregnated amount, shaking time, Pd(II) concentration, HCl concentration, and Pd(II) desorption reagents, were optimized. The maximum Pd(II) sorption capacity of 1-EIR was 49 mg g⁻¹ after 24 h in a 700 ppm Pd(II) solution. Over 20 adsorption–desorption cycles, 1-EIR showed good reusability, with a sorption percentage (S%) of > 92%. However, not all Pd(II) was desorbed from 1-EIR. Complete Pd(II) collection was achieved by combining desorption with flaking of the Pd–extractant complex from Pd(II)-loaded 1-EIR by Soxhlet extraction, as ~ 13 mg g⁻¹ remained after the 20th adsorption–desorption cycle by absorptiometric method. The sorption mechanism was elucidated based on the Langmuir isotherm model, thermodynamic parameters, and sorption kinetics. Pd(II) sorption by 1-EIR was spontaneous and endothermic, and the sorption kinetics followed a pseudo-second-order model. Notably, 1-EIR also exhibited high selectivity for Pd(II) from a simulated mixed metal solution and a spent automotive catalyst leachate (S% = 98% and > 99%, respectively). Thus, this extractant-impregnated system is promising for selective Pd(II) recovery from spent automotive catalysts and other secondary resources.

Highly Selective Rh(III) Recovery from HCl Solutions Using Aromatic Primary Diamines via Formation of Three-Dimensional Ionic Crystals

A new Rh(III) separation method using metal-containing hydrochloric acid (HCl) solutions has been developed. This method includes Rh(III) precipitation with high selectivity using aromatic primary diamines as precipitants. The compound p-phenylene diamine dihydrochloride (PPDA) successfully precipitates only Rh(III) from HCl solutions containing Pd(II), Pt(IV), and Rh(III). Furthermore, highly selective Rh(III) recovery from the simulated spent catalyst leach solution, comprising Pd, Pt, Rh, Ce, Al, Ba, Zr, La, and Y in 5 M HCl, was achieved using PPDA. Single-crystal X-ray analysis revealed that the Rh(III)-containing precipitate using PPDA forms three-dimensional ionic crystals comprising the [RhCl₆]^3-/ammonium form of PPDA/chloride anion/H₂O at a 1:2:1:2 ratio. Formation of these unique ionic crystals plays a key role in the highly selective Rh(III) recovery. This Rh(III) recovery method will be promising for use in the purification process of Rh as well as the practical Rh recovery from spent catalysts.

Rapid and selective recovery of palladium from platinum group metals and base metals using a thioamide-modified calix[4]arene extractant in environmentally friendly hydrocarbon fluids

A novel macrocyclic calix[4]arene extractant having a long alkyl chain thioamide, 25,26,27,28-tetrakis(N-n-octylthiocarbamoyl)methoxy-5,11,17,23-tetra-tert-butylcalix[4]arene (1), was synthesized from 25,26,27,28-tetrakis(N-n-octylcarbamoyl)methoxy-5,11,17,23-tetra-tert-butylcalix[4]arene (2) using Lawesson's reagent. Extractant 1 was characterized using ¹H NMR, ¹³C NMR, FT-IR spectroscopy, and elemental analysis. The Pd(II) extraction abilities of 1 and 2 were studied in high-boiling-point and environmentally friendly hydrocarbon diluents. Pd(II) extraction experiments were conducted using single-metal Pd(II) solutions, simulated mixed palladium group metal (PGM) solutions, and acid-leached automotive catalyst residue solutions. Different experimental conditions, including the shaking time, HCl/HNO₃ concentration, Pd(II) concentration, extractant concentration, and the organic/aqueous phase ratio, were studied systematically. Extractant 1 showed very selective (> 99.9%) Pd(II) extraction from the mixed PGM/base metal solutions and the acid-leached automotive catalyst residue solution. Conversely, the Pd(II) extraction ability of extractant 2 was found to be negligible. Extractant 1 showed very fast extraction kinetics and a high extraction capacity as compared to those of the commercial extractant di-n-octyl sulfide. Effective stripping of Pd(II) from 1 was performed using HCI, HNO₃, NH₃, and HCl-thiourea solutions. Furthermore, 1 was successfully recycled over five extraction/stripping cycles. The Pd(II) extraction mechanism of 1 was studied using FT-IR spectroscopy. Extractant 1 exhibited very selective Pd(II) extraction and high acid stability, demonstrating its industrial applicability for the extraction of Pd(II) from leached automotive catalyst liquors containing PGMs and base metals.