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

Abstract

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 4 -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 48 ) and (2) NCS adducts of OZr 4 -NCS. Zr 48 was identified first by SAXS and then crystallized by exploiting favorable soft-metal bonding to the sulfur of NCS. While the large Zr 48 likely cannot be extracted due to its larger size, the NCS adducts of OZr 4 -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.

Structure, Properties, and Reactivity of Polyoxocationic Zirconium and Hafnium Clusters: A Computational Investigation

“…The nucleation of [Hf(H 2 O) 8 ] 4+ into [Hf 4 (μ 2 -OH) 8 (H 2 O) 16 ] 8+ is presented in Figure . The nucleation process is expected to favor olation over oxolation since no μ 2 –O groups have been experimentally observed in tetrameric cyclic polyoxocationic zirconium and hafnium clusters to date. ,, The formation of the dimer from the hydrated hafnium monomer is exergonic by 24.0 kcal/mol. The addition of another monomeric unit to the dimer can lead to the formation of either a linear trimer or a bent trimeric cluster.…”

Fatty acid capped, metal oxo clusters as the smallest conceivable nanocrystal prototypes

“…20,21 Subtle differences in cluster chemistry between Zr and Hf are exploited in the industrial separation of Zr from Hf. 22 Titanium oxo clusters exhibit exciting photo-redox properties. 23 Group 4 MOFs are based on group 4 metal oxo clusters that act as secondary building units (or nodes) in the MOF framework.…”

Metal–Oxo Cluster Formation Using Ammonium and Sulfate to Differentiate MIV (Th, U, Ce) Chemistries

“…Excluding all other influencing factors, the self-assembly of oxo clusters proceeds by the following fundamental steps: While these reactions are described as a function of the pH, inner-sphere (metal–ligand) and outer-sphere (counterion) interactions can significantly alter or compete with the simple M IV -aqua reactions described above. For example, ligation can differentiate and direct aqueous phase chemistry of these metals, including the very similar Zr/Hf IV . − The most-readily observed species in solution and in the solid state is a hexamer; generally formulated [M IV 6 O 4 (OH) 4 ] 12+ . The hexamer (M 6 ) has the same topology as the core building unit of fluorite-type M IV O 2 , marking the most common anchor point across the M IV cations (and the building unit for the above-mentioned UiO-66).…”