Solvent extraction in extended hydrogen bonded fluids – separation of Pt(iv) from Pd(ii) using TOPO-based type V DES

Abstract

In this work, the challenging solvent extraction (SX) separation of Pt(IV) from Pd(II) is investigated using non-ionic hydrophobic type V deep eutectic solvents (DES) incorporating trioctylphosphine oxide (TOPO) as a constituent. Type V DES are highly structured solvents characterised by the existence of an intermolecular hydrogen bonded network, the strength of which dictates their non-ideality and varies with the eutectic composition. The extraction efficiency and selectivity were found to strongly vary with the TOPO molar fraction (x_TOPO) and nature of the hydrogen bond donor, presenting an antagonism between the SX mechanism and the intermolecular interactions of the DES components. However, the antagonistic effect with increased hydrogen bonding does not influence all extracted species proportionally: whilst Pt(IV) extraction remained largely unaffected (%EE ≥ 90%), extraction of Pd(II) dropped sharply allowing for a ten-fold increase in selectivity compared to traditional TOPO-based SX systems. The higher TOPO concentration in DES enabled maximum loading concentrations significantly superior to those in extractant–organic diluent systems. Furthermore, the extensive number of hydrogen bonded configurations suppresses the formation of a third phase which limits traditional SX using non-ionic extractants, even after extraction of 19.5 g L⁻¹ of Pt(IV) from 8.0 mol L⁻¹ HCl in the TOPO + decanoic acid eutectic. The experimental and molecular dynamics simulation results presented herein identify the criteria for rational type V DES selection and application, providing a new life to otherwise poorly selective extractants through their inclusion in these systems whilst removing the need for organic diluents.