Speciation-Dependent Solvent Extraction of Polyoxopalladates Toward Separation of Critical Materials

Innovating robust separation methods for critical elements extraction and reuse is important to sustain the global consumption of microelectronics, energy, and pharmaceuticals. Effective separations via liquid-liquid extraction (LLE) requires molecular-level understanding and optimization of solution speciation. Here, we design LLE for palladium based on polyoxopalladates (POPs), where Pd is critical for microelectronics, catalysts, and drug production. Mild solution conditions are designed to evaluate the role of templating heterometals and ligands (arsenate, phenylarsonate, phenylphosphonate, acetate, phosphate) that drive POP assembly. Small-angle X-ray scattering, electrospray ionization mass spectrometry, UV-vis spectroscopy, along with compositional analysis, respectively described speciation and extraction efficiency (including separation factors for competitive Pd-Ni separation). Most effective LLE of Pd (>99.9%) are arsenate/phenylphosphonate/acetate capped hexamers or heptamers without templating metals. These fragments of the larger, prior-reported Pd₁₂/Pd₁₅/Pd₈₄ POPs represent simple formulations, important for translation to scaled-up processes. Comparing alkali-acetate buffers highlight that potassium is more effective than lithium or sodium, presumably due to strong ion-pairing between Pd-oxoanions and the larger alkali, facilitating transport across the aqueous-organic interface. Pd and Au-Pd LLE studies yielded the first K⁺-charge balanced and K⁺-templated POPs, illustrating simply swapping the alkali from Na⁺ (usually employed) to K⁺ can enable isolation of new topologies and inspire new applications.