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.