Abstract
Pt, Pd, and Rh platinum group metals (PGMs) are utilized in automotive catalytic converters to reduce the level of harmful gas emissions. To evaluate the separation efficiencies of PGMs from the leach liquors of automotive catalysts, their extractabilities from both single-component solutions and leach liquors were determined using three p-dialkylaminomethylcalix[4]arenes (alkyl: n-hexyl~n-octyl; 3–5) as extractants, and the obtained results were compared with the data of acyclic p-di-n-octylaminomethyldimethylphenol (6) and those previously reported for macrocyclic calix[4]arene- and thiacalix[n]arene-based amine and amide extractants. Various extraction parameters, including the contact time, HCl concentration, and concentrations of Pd(II) and Pt(IV) ions, were examined, and the distributions and stoichiometry ratios of the recovered Pd(II) and Pt(IV) species were calculated. All macrocyclic extractants 3–5 exhibited high and selective extractabilities of Pd(II) and Pt(IV) ions contained in the leach liquors of automotive catalysts. The E% values of 3–5 from the leach liquors were 94.2, 93.0, and 97.7 for Pd(II) and 91.7, 94.0, and 92.5 for Pt(IV), respectively. Acyclic compound 6 did not perform well even though higher ratios of extractant used. Furthermore, the results of reusability testing demonstrated relatively large extractability values obtained for these two metals even after five extraction/stripping cycles.
Highlights
Calix[n]arenes have been used in catalysis, molecular recognition, ion separation, sensors, and so on [1,2,3]
The concentration of 6 was four times higher than that of 5, the extractabilities of 5 and 6 determined for the three platinum group metals (PGMs) were significantly different: 66.9% for Pd(II), 80.4% for Pt(IV), and 0% for Rh(III). These results suggest that the macrocyclic extractant 5 and PGMs formed highly stable chemical species owing to assembly of metal recognition sites by modification of the amino moieties located along the annular structures of the calixarene upper rims
The E% magnitudes determined for the seven other metal ions present37 in the leach liquors of automotive catalysts were below 6%
Summary
Calix[n]arenes have been used in catalysis, molecular recognition, ion separation, sensors, and so on [1,2,3]. By performing chemical modifications of the functional groups located at the calixarene “upper” and “lower” rims, many functionalized calix[4]arenes were synthesized, and their ability to extract precious metals was intensively studied. As compared with the commercially available extractants, certain polydentate-functionalized calix[n]arenes can efficiently and selectively extract platinum group metals (PGMs) from solutions. Owing to their scarcity in the Earth’s crust and eccentrically located resources, PGMs (Pd, Pt, Rh, Ru, Ir, and Os) represent valuable key components of many industrial applications.
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