Proton Chelating Ligands Drive Improved Chemical Separations for Rhodium.

Area of Science:

• Inorganic Chemistry
• Materials Science
• Sustainable Chemistry

Background:

• Current rhodium extraction methods have significant environmental drawbacks.
• Limited understanding of rhodium's complex chemistry hinders the development of efficient extraction processes.
• No commercial extractants currently exist for rhodium due to its challenging speciation in hydrochloric acid.

Purpose of the Study:

• To investigate the molecular-level chemistry of rhodium extraction.
• To develop and characterize a novel reagent for selective rhodium(III) separation.
• To understand the mechanism of rhodium(III) transport and selectivity.

Main Methods:

• Synthesis and characterization of the diamidoamine reagent N-n-hexylbis(N-methyl-N-n-octylethylamide)amine.
• Solvent extraction experiments in aqueous HCl.
• Characterization using slope analysis, FT-IR, NMR spectroscopy, EXAFS, SANS, and ESI-MS.
• Computational modeling to elucidate the extraction mechanism.

Main Results:

• The diamidoamine reagent successfully transports Rh(III) into an organic phase.
• Rhodium is transported as the monoaquated dianion [RhCl5(H2O)]2- via an outer-sphere assembly.
• High selectivity for Rh(III) over chloride ions was achieved.
• A comprehensive understanding of the anion recognition mechanism was established.

Conclusions:

• The developed diamidoamine reagent provides a promising avenue for selective rhodium extraction.
• The study highlights the importance of multi-technique approaches for understanding complex solution-phase chemistry.
• This knowledge can guide the design of future extractants for sustainable rhodium recovery from mining and recycling sources.