Density Functional Theoretical Modeling of Selective Ligand for the Separation of Zr and Hf Metal Oxycations (ZrO2+and HfO2+)

The structure and energetics of TBP (tri-butyl phosphate), THPO (Tri-hexyl phosphine oxide), and CYANEX925 (bis (2, 4, 4-trimethylpentyl) octylphosphine oxide) and their complexes with Hf and Zr oxycation (ZrO2+ and HfO2+) with BP86 density functional employing TZVP basis set have been presented. The calculated gas phase binding energy was shown to follow the order CYANEX925 > THPO > TBP >. The calculated gas phase binding energy for Hf oxycation is found to be higher than that of Zr oxycation for all the ligands which is contradictory to the observed experimental trend. The experimental selectivity trend was recovered by inclusion of the second solvation sphere of the metal ion both in gas and aqueous phase for a particular ligand. Both the implicit and explicit solvation model fail to predict the high selectivity of ZrO2+ with CYANEX925 over TBP in the absence of nitrate anion. The presence of nitrate anion along with the second solvation shell (n = 24 water molecules) around the oxycation, either in the gas or solvent phase shows consistent results for selectivity as observed in the solvent extraction experiments. Further, the preferential selectivity of Hf4+ cation over Zr4+ cation using liquid MIBK (methyl isobutyl ketone) in the thiocyanate medium is also captured through DFT calculation. The experimental separation factor [SF(Zr/Hf or Hf/Zr)] was found to be well correlated with the extraction energy difference between two metal ions for a particular ligand.