Ionic Couple-Driven Palladium Leaching by Organic Triiodide Solutions

Pd dissolution capabilities of a variety of organic triiodides (OrgI₃) in organic solvent, where Org⁺ = 3,5-bis(phenylamino)-1,2-dithiolylium [(PhHN)₂DTL⁺], 3,5-bis(morpholino)-1,2-dithiolylium (Mo₂DTL⁺); tetrabuthylammonium (TBA⁺); and tetraphenylphosphonium (Ph₄P⁺), toward the crude metal and model-spent three-way catalyst (TWC), are described here. Enhanced Pd-leaching yields from TWC were obtained using OrgI₃ solutions (up to 98%) in spite of the fully inorganic KI₃ one (38%) in the same mild conditions. The reaction products were isolated and characterized as Org₂[Pd₂I₆]. Crystallographic and DFT studies highlighted the presence of several ion-pair secondary interactions in the products, which can explain the improved effectiveness of the Pd etching by OrgI₃. For comparison purposes, the gold leaching by using R₂DTLI₃ and the obtained Au complexes were investigated. Preliminary results addressed to recover the metal and the reagents from the etching product showed that (PhHN)₂DTLI₃ is the most promising reagent to improve sustainability in the whole process.