Effect of the hydrogen bond donor nature on the extraction ability of a tributyl phosphate-based deep eutectic solvent

Abstract

The use of hydrophobic deep eutectic solvents (DESs) as extractants for metal ion separation is largely due to the ease of their preparation and the possibility of modifying them depending on the composition of the metal mixture to be separated. However, the most common approach to DES modification involves the replacement of the extractant, while the second component is often regarded merely as an inert diluent that affects only physical properties and extractant concentration. In this study, we demonstrate that this view is incomplete. The extraction of zirconium(IV) ions was performed using five DESs based on tributyl phosphate (a hydrogen bond acceptor) to evaluate the effect of structural features of the hydrogen bond donor, which itself lacks extractive properties. The donor components included a series of structurally related phenols: phenol, 4-tert-butylphenol, ionol, thymol, and a nonplanar analog of the latter, the terpenoid alcohol menthol. It was found that under identical experimental conditions, the extraction efficiency of zirconium(IV) ions may vary significantly depending on the donor nature, ranging from 67% to 30% at 0.5 mol L⁻¹ H⁺ and from 38% to 80% at 6 mol L⁻¹ H⁺ for menthol and ionol, respectively. Furthermore, the binding energy in donor:acceptor molecular clusters, calculated by computational methods, correlates with the extractive ability of the DES. Using 2D NMR spectroscopy and quantum-chemical calculations, we show that hydrogen bond donors directly compete with the metal ion and HCl for interaction with the PO group of tributyl phosphate. Varying the hydrogen bond donor not only changes the extraction efficiency over a wide range but also inverts the extraction order with increasing acidity. Based on the results obtained, an approach was proposed for controlling the extraction properties of tributyl phosphate-based deep eutectic solvents through rational selection of the hydrogen bond donor, establishing a clear correlation between donor structure, intermolecular interaction energy, and extraction performance.