Effect of Structural\nModification in β Diketone\non the Mutual Separation of Nd and Pr into Room-Temperature Ionic\nLiquid

A novel approach for mutual separation between Nd and\nPr has been\ndemonstrated by the selective dissolution of Nd<sub>2</sub>O<sub>3</sub> into an ionic liquid (C<sub>4</sub>mim·NTf<sub>2</sub>) containing\ntwo β diketones, viz., acetyl trifluoroacetone (ATA) and benzoyl\ntrifluoroacetone (BTA), directly from a physical mixture of their\nmost stable oxides, i.e., Nd<sub>2</sub>O<sub>3</sub> and Pr<sub>6</sub>O<sub>11</sub>, avoiding their cumbersome aqueous phase dissolution\nand the subsequent aqueous phase chemistry. The controlled release\nof the H<sup>+</sup> ion by the β diketones to form the enolate\nconformation has been used to hold the dissolved Nd<sup>3+</sup> ion\nin the ionic liquid phase as Nd (enolate of β diketone)<sub>4</sub><sup>–</sup> species. The selective dissolution of\nNd<sub>2</sub>O<sub>3</sub> over Pr<sub>6</sub>O<sub>11</sub> can\nbe achieved by utilizing one of the prime driving forces of their\ndifference in lattice energy of formation (<i>U</i><sub>Nd<sub>2</sub>O<sub>3</sub></sub> ∼ −3292.86 kJ/mol\nand <i>U</i><sub>Pr<sub>6</sub>O<sub>11</sub></sub> ∼\n−7360.45 kJ/mol, respectively). The extent of dissolution was\nfound to be greater with BTA for both Nd and Pr as compared to that\nseen with ATA. However, the selectivity was much better with ATA.\nA maximum separation factor of β<sub>Nd/Pr</sub> ∼ 217\nwas achieved using 0.1 M ATA in C<sub>4</sub>mim·NTf<sub>2</sub> containing 1.5% water for 15 min of biphasic equilibration with\na solid-to-liquid ratio of 5.33 g L<sup>–1</sup> from the physical\nmixture of Nd<sub>2</sub>O<sub>3</sub>/Pr<sub>6</sub>O<sub>11</sub> ∼ 2:1. The separation factor, reported in this study, was\nfound to be more than 100 times better than that reported for the\nconventional route of hydrometallurgical separation. The exothermic\nnature of Nd dissolution was evidenced from the calorimetric experiments,\nwith the estimated enthalpy of dissolution for ATA and BTA being −169.23\nand −126.46 J/g, respectively. The subsequent quantitative\nback extraction of Nd by 0.5 M HNO<sub>3</sub> provides the opportunity\nfor recycling of the ionic liquid for subsequent dissolution. The\neffects of water content, ATA/BTA concentration, relative ratio for\nNd<sub>2</sub>O<sub>3</sub>/Pr<sub>6</sub>O<sub>11</sub>, dissolution\ntemperature, etc., have been studied to understand their separation\ncharacteristic and hence to achieve the best mutual separation of\nthe two rare earth metal ions.