Molecular Modelling of the Hydrolysis of Tantalum and Niobium Pentafluoride

Abstract:

Tantalum (Ta) and niobium (Nb) are two metals with similar chemical and physical properties and are found together in nature. One form of Ta is tantalum pentafluoride, which is stable in reducing environments, is corrosive resistant and stable under harsh conditions. Ta is currently used in nuclear reactors with a wide variety of uses and advantages. For these applications, pure Ta is needed to insure high value catalysts, contrary the higher the purity grade the more expensive the production of these high value catalysts. One way of ensuring an economic viable process for the production of high purity Ta, is to find a cost effective way to separate Ta from Nb. Ungerer et al. studied the separation of Ta and Nb, using safer chemicals and techniques for the environment in a solvent extraction (SX) process. During this study, separation was achieved in a sulphuric acid (H₂SO₄) medium with the extractants diiso-octyl phosphinic acid (PA) and di-(2-ethylhexyl) phosphoric acid (D2EHPA). The main obstacle during this study was the speciation of Ta and Nb, springing the question of why separation occurred with some extractants and not with the others. One method for determining the speciation of the compounds in a reaction mixture is by using computational techniques for molecular modelling. Several molecular modelling programs are available which uses various mathematical equations and approximations. Progress in computational chemistry over the last 20 years has made quantum mechanical calculations on large molecules, chemical systems as well as on macromolecule reactions possible. Calculations based on the density-functional theory (DFT) are now, not only used on light elements and small molecules, but also on metal complexes, heavy metals and especially on metal separation in SX. With these models at hand, SX processes were modelled within realistic margins to fit the experimental setup in a small scale laboratory. It is anticipated that the advances from this work will provide the possibility to determine, with good approximation, the outcome not only of the proposed Ta SX experiments, but also SX in general, before embarking on expensive, time consuming experiments and environmental unfriendly waste generation. In this paper molecular modelling was used to compile a partial energy profile for a proposed reaction mechanism for the reaction of tantalum- and niobium pentafluoride (TaF₅, NbF₅) with water to form tantalum- and niobium hydroxides. In the process, possible species that may form during the reaction were identified and evaluated.

• Periodical: Advanced Materials Research (Volume 1019)
• Pages: 406-411
• DOI: https://doi.org/10.4028/www.scientific.net/AMR.1019.406
• Online since: October 2014
• Authors: Marietjie J. Ungerer, Derik J. van der Westhuizen, Henning M. Krieg*, Cornie Van Sittert
• Keywords: Molecular Modelling, Niobium, Tantalum

References

[1] M.J. Ungerer, et al., Separation of Tantalum and Niobium by Solvent Extraction, in Natural Sciences. 2012, North-West University: Potchefstroom. p.104.

[2] B. M Matthew and C. Anastasio, A chemical probe technique for the determination of reactive halogen species in aqueous solution: Part 1 - bromide solutions. Atmos. Chem. Phys., 2006. 6: pp.2423-2437.

DOI: 10.5194/acp-6-2423-2006

[3] M. Pesavento, G. Alverti, and R. Biesuz, Analytical methods for determination of free metal ion concentration, labile species fraction and metal complexation capacity of environmental waters: a review. Anal. Chim. Acta, 2009. 631(2): pp.129-141.

DOI: 10.1016/j.aca.2008.10.046

[4] M. Baghdadi, and F. Shemirani, In situ solvent formation microextraction based on ionic liquids: A novel sample preparation technique for determination of inorganic species in saline solutions. Anal. Chim. Acta, 2009. 634(2): pp.186-191.

DOI: 10.1016/j.aca.2008.12.017

[5] Y.K. Chau, P.T.S. Wong, and G.A. Bengert, Determination of methyltin(IV) and tin(IV) species in water by gas chromatography/atomic absorption spetrophotometry. Anal. Chem., 1982. 54: pp.246-249.

DOI: 10.1021/ac00239a023

[6] J. Narbutt, and M. Czerwinski, eds. Chapter 16: Computational chemistry in modelling solvent extraction of metal ions. Solvent extraction principles and practice, ed. J. Rydberg, C. Musikas, and G.R. Choppin. (1992).

DOI: 10.1201/9780203021460.ch16

[7] A. Agulyansky, The chemistry of tantalum and niobium fluoride compounds. 2004: Elsevier, Amsterdam.

[8] M. Siodmiak, G. Frenking, and A. Korbin, On the mechanism of chemical vapor deposition of Ta2O5 from TaCl5 and H2O. An ab initio study of gas phase reactions. Materials Science in Semiconductor Processing, 2000. 3(6): pp.65-71.

DOI: 10.1016/s1369-8001(00)00010-x

[9] Material Studio Modelling Environment. 2012, Accelrys Software Inc.: San Diego.

[10] J.P. Perdew and Y. Wang, Accurate and simple analytic representation of the electron-gas correlation energy. Physical Review B, 1992. 45(23): pp.13244-13249.

DOI: 10.1103/physrevb.45.13244

[11] B. Delley, Ground-state enthalpies: evaluation of electronic structure approaches with emphasis on the density functional method. The Journal of Physical Chemistry A, 2006. 110 (50): pp.13632-13639.

DOI: 10.1021/jp0653611

[12] B. Delley, ed. Modern Density Functional Theory: A tool for chemistry. Theoretical and Computational Chemistry, ed. J.M. Seminario and P. Politzer. Vol. 2. (1995).

[13] F. Marchetti, G. Pampaloni, and S. Zacchini, 19F NMR spectroscopy as useful tool for determining the structure of coordination compounds of MF5 (M = Nb, Ta). Journal of Fluorine Chemistry, 2010. 131(1): pp.21-28.

DOI: 10.1016/j.jfluchem.2009.09.014

[14] A.J. Edwards, The structures of niobium and tantalum pentafluorides. Journal of Chemical Society, 1964(0): pp.3714-3718.

[15] A. Zupan and M. Causa, Density functional LCAO calculations for solids: Comparison among Hartree-Fock, DFT local density approximation, and DFT generalized gradient approximation structural properties. International Journal of Quantum Chemistry, 1995. 56(4): pp.337-344.

DOI: 10.1002/qua.560560419