Amide-functionalised phosphonium-based ionic liquids as ligands for rhodium(iii) extraction.

Wataru Yoshida,Masahiro Goto

doi:10.1039/d1ra00489a

Abstract

Seven new amide-functionalised phosphonium-based ionic liquids (APILs) with chloride anions are synthesised and applied to extraction of rhodium(iii) from HCl solution. The effects of structural modification of the APILs on the extraction performance are examined by liquid–liquid extraction using toluene as a diluent and the results compared with those for trihexyltetradecylphosphonium chloride ([P66614][Cl]), a typical commercial extractant. The performance of the APILs as rhodium(iii) extractants is influenced by three main factors: (1) the length of the alkyl chains attached to the P atom; (2) the length of the linker between the amide and phosphonium moiety; and (3) the type of amide group. A novel ligand, [3°C2P444][Cl], had outstanding performance in the effective recovery of rhodium(iii). Extraction of rhodium(iii) from a 1.0 mol dm−3 HCl solution with 0.5 mol dm−3 [3°C2P444][Cl] proceeded quantitatively (>98%) and the extraction efficiency was higher than that of the commercial extractant [P66614][Cl]. The mechanism of rhodium(iii) extraction by [3°C2P444][Cl] was investigated by slope analysis, UV-vis, and FT-IR spectroscopy. These results indicate that [RhCl4(H2O)2]− in aqueous solution is extracted by [3°C2P444][Cl] through an anion-exchange mechanism and slowly converted into a dimer, [Rh2Cl9]3−, in the organic phase.

Highlights

Rhodium (Rh) is an important metal in the automobile industry as a catalyst and the demand and price of the metal have rapidly increased in recent years.[1,2,3] In 2014, Nuss and Eckelman reported that global warming potential and pollution metrics attributed to Rh production are the highest among all metals.[4]
We found that the extraction of Rh(III) with amide-functionalised phosphonium-based ionic liquids (APILs) is in uenced by three main factors: (1) the length of alkyl chains attached to the P atom; (2) the length of the linker between the amide and the phosphonium; and (3) the type of the amide group
APILs were synthesised in two steps by the general synthetic outlined in Scheme 1

Summary

Introduction

Rhodium (Rh) is an important metal in the automobile industry as a catalyst and the demand and price of the metal have rapidly increased in recent years.[1,2,3] In 2014, Nuss and Eckelman reported that global warming potential and pollution metrics attributed to Rh production are the highest among all metals.[4]. A marked improvement of extraction of inert PGMs has been reported on introduction of a neutral donor such as an amide or a urea.[48,49] Tasker et al developed amidefunctionalised amine ligands and reported that the amide group enhanced the extraction of chlorometallate anions.[50,51,52,53,54,55] Narita et al reported that amide containing tertiary amines extracted Rh(III) more efficiently than the tri-n-octylamine (i.e., an analogous unsubstituted tertiary amine).[27,28,29] On this basis, we designed seven new amide functionalised phosphonium-based ionic liquids (APILs) as potential extractants for the uptake of Rh(III) from HCl solutions. Slope analysis and absorption spectroscopy were conducted to clarify the extraction mechanism

Results and discussion

Conclusion

Extraction procedures

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