Toward Redox- and Photoredox-Based Niobium/Tantalum Separations

Niobium (Nb) and tantalum (Ta) present a challenging chemical separation due to their similar reactivities. Modern Nb/Ta separations employ hydrofluoric acid, relying on speciation differences between the two metals in fluoride-containing media. The environmental, health and safety, and economic implications of hydrofluoric acid use present an opportunity for the development of alternative separation techniques. In this study, we introduce two fluoride-free approaches for NbVCl5/TaVCl5 differentiation. Our electrochemical-based technique leverages the more accessible NbV/IV redox couple and results in a selective precipitation, yielding a separation factor, SNb/Ta = 13.1 ± 2.9. This result represents a considerable improvement compared to previously reported redox-driven methods (SNb/Ta = 6 ± 2). In addition, we present a photochemical method that selectively produces NbIVCl4 solids from a NbV/TaVCl5 mixture, achieving a separation factor of SNb/Ta = 16.1 ± 4.0 by a photoredox-driven separation. This technique similarly manifests due to the orbital energy differences between NbV and TaV, albeit in the excited state. Our findings not only highlight the electronic structure differences between Nb and Ta but also indicate new prospective, fluoride-free Nb/Ta separation avenues.