Molecular modelling of tantalum penta-halides during hydrolysis and oxidation reactions

Abstract

The transition metals tantalum (Ta) and niobium (Nb), which are usually found together in nature, have similar chemical and physical properties, making their separation challenging. There are various methods available for the separation of these two metals, including reduction, fluorination, chlorination and solvent extraction (SX) (Ayanda and Adekola, 2011). In a recent study investigating the suitability of SX for the separation of Ta and Nb, it was shown that speciation data would be required to help explain the distribution data obtained. Since traditional speciation techniques cannot be readily applied for Ta and Nb, it was decided to determine the suitability of molecular modelling for this purpose. To investigate the suitability of modelling for this application a case study was selected where it was hypothesised that when TaF5 is dissolved in water, it could react stepwise with water to finally form tantalum penta-hydroxide (Ta(OH)5) and other oxyfluoride species including TaOF3. Due to the fact that literature on TaF5 reactions with water is limited, TaCl5 and its reactions was used to develop the model (method). As part of the model development and verification, DFT was used to calculate the energy needed for these reactions, comparing different functionals and basis sets. The validated model was then applied to TaF5 as a case study. From the results it was confirmed that the reaction of TaX5 (X = Cl or F) with water to form Ta(OH)5 and Ta2O5 is an endothermic reaction, while the formation of Ta(H2O)F5 and TaF4OH was exothermic