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.