The role of titanium-oxo clusters in the sulfate process for TiO₂ production

TiO₂ is manufactured for white pigments, solar cells, self-cleaning surfaces and devices, and other photocatalytic applications. The industrial synthesis of TiO₂ entails: (1) the dissolution of ilmenite ore (FeTiO₃) in aqueous sulfuric acid which precipitates the Fe while retaining the Ti in solution, followed by (2) dilution or heating the Ti sulfate solution to precipitate the pure form of TiO₂. The underlying chemistry of these processing steps remains poorly understood. Here we show that the dissolution of a simple Ti^IV-sulfate salt, representative of the industrial sulfate process for the production of TiO₂, immediately self-assembles into a soluble Ti-octadecameric cluster, denoted as {Ti₁₈}. We observed {Ti₁₈} in solution by small-angle X-ray scattering and Ti extended X-ray absorption fine structure (Ti-EXAFS) analysis, and ultimately crystallized it for absolute identification. The {Ti₁₈} metal-oxo cluster was previously reported as a polycation; but shown here, it can also be a polyanion, dependent on the number of sulfate ligands it carries. After immediate self-assembly, the {Ti₁₈}-cluster persists until TiO₂ precipitates, with no easily identified structural intermediates in the solution or solid state, despite the fact that the atomic arrangement of {Ti₁₈} differs vastly from that of titania. The evolution from solution phase {Ti₁₈} to precipitated TiO₂ nanoparticles was detailed by X-ray scattering and Ti-EXAFS. We offer a hypothesis for the key mechanism of complete separation of Fe from Ti in the industrial sulfate process. These findings also highlight the emerging importance of the unusual Ti(Ti)₅ pentagonal building unit, featured in {Ti₁₈} as well as other early d⁰ transition metal-oxo clusters including Nb, Mo and W. Finally, this study presents an example of crystal growth mechanisms in which the observed "pre-nucleation cluster" does not necessarily predicate the structure of the precipitated solid.