Oxo-Cluster-Based Zr/Hf^IV Separation: Shedding Light on a 70-Year-Old Process

Zirconium and hafnium in the tetravalent oxidation state are considered the two most similar elements on the periodic table, based on their coexistence in nature and their identical solid-state chemistry. However, differentiating solution phase chemistry is crucial for their separation for nuclear applications that exploit the neutron capture of Hf and neutron transparency of Zr. Here we provide molecular level detail of the multiple factors that influence Zr/Hf separation in a long-exploited, empirically designed industrial solvent-extraction process that favors Hf extraction into an organic phase. In the aqueous solution, both Hf and Zr form an oxo-centered tetramer cluster with a core formula of [OM₄(OH)₆(NCS)₁₂]^4- (OM-NCS, M = Hf, Zr). This was identified by single-crystal X-ray diffraction, as well as small-angle X-ray scattering (SAXS), of both the aqueous and organic phase. In addition to this phase, Zr also forms (1) a large oxo-cluster formulated [Zr₄₈O₃₀(OH)₉₂(NCS)₄₀(H₂O)₄₀] (Zr) and (2) NCS adducts of OZr-NCS. Zr was identified first by SAXS and then crystallized by exploiting favorable soft-metal bonding to the sulfur of NCS. While the large Zr likely cannot be extracted due to its larger size, the NCS adducts of OZr-NCS are also less favorable to extraction due to the extra negative charge, which necessitates coextraction of an additional countercation (NH₄⁺) per extra NCS ligand. Differentiating Zr and Hf coordination and hydrolysis chemistry adds to our growing understanding that these two elements, beyond simple solid-state chemistry, have notable differences in chemical reactivity.