Emergent Hf-Selective Precipitation of Aqueous (Zr,Hf) Thiocyanate Molecules through Nuclearity Control

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Abstract Naturally occurring Zr with ∼2% Hf must be separated to high levels of purity for both nuclear and semiconductor industries. Since the dawn of the nuclear age, this process is executed on an industrial scale by selective Hf-isothiocyanate (N═C═S) extraction into methyl isobutyl ketone (MIBK). Yet SCN-MIBK mixtures yield toxic, noxious byproducts, also giving rise to performance variability. Here, we eliminate the MIBK and instead precipitate Hf-rich species via inclusion of choline, a common food additive, in the aqueous phase. Augmentation with sulfate, acid, and ammonium finely tunes Zr/Hf nuclearity and coordination, increasing separation factors (SF) from the industrial standard of 6/7 to 33, with Zr(Hf)(N═C═S)6/72/3– and Zr(Hf)(N═C═S)5(SO4)3 – as key Hf-rich coordination complexes. Solid-state (powder and single-crystal X-ray diffraction) and solution (Raman spectroscopy and small-angle X-ray scattering) characterization show increasing acid decreases nuclearity from tetrameric to trimeric to dimeric to monomeric Zr/Hf complexes by simultaneously decreasing oxyhydroxide ligation and increasing N═C═S ligation. In agreement with experiment, computation predicts increasing thermodynamic favorability for lower-nuclearity Hf species (monomer) compared to Zr species. This study demonstrates that precise atomic-level understanding and manipulation of both metal coordination and noncovalent interactions can yield competitive precipitation-based separations that have been notoriously inferior to their solvent extraction counterparts.