Within supported metal oxides, a few select materials seem to grab all the glory: titania-silica and vanadia-alumina readily come to mind. In contrast, my research group often finds itself exploring catalytic materials in some of the less-travelled corners of the periodic table. When beginning several projects examining periodic trends in supported oxides for biomass conversion, epoxidation with H2O2, and deamination, we quickly encountered work on niobia-silica carried out by Beata Kilos during her PhD. We then proceeded to undertake a 6-year collaboration with Dr. Kilos, but niobium was rarely featured! Therefore, I will take this opportunity to discuss work inspired by Dr. Kilos, but rarely discussed with her. I will specifically focus on the exceptional activity of supported Nb-SiO2, particularly when prepared using precursors that ensure high dispersion of the final oxide. When compared in an isostructural series of Ti-, Zr-, Hf-, Nb- and Ta-SiO2, Nb-SiO2 displays the highest turnover frequency in cyclohexene epoxidation using H2O2, consistent with its strong electronegativity. Its high activity is also due in part to its known water tolerance and its lack of significant activity towards allylic oxidation. In a detailed study of precursor effects on Nb-SiO2 structure and catalytic rates, we found relatively weak correlations between X-ray absorption spectral features, UV-visible absorption edges, and catalytic reactivity. Instead, there is a stronger correlation between numbers of sites titrated by phenylphosphonic acid and rate, suggesting that different syntheses primarily result in different fractions of active sites, rather than fundamentally different active sites. Using this knowledge, we have also demonstrated the utility of Nb-SiO2 in sulfoxidation, explored how selectivity in epoxidation and sulfoxidation are controlled by titration of specific sites on Nb-SiO2, shown that the water-tolerance of Nb-SiO2 enables its use in exotic applications such as the utilization of electrochemically-derived H2O2, and created inverse SiO2@Nb2O5 materials that provide a route into selective Brønsted acid catalysis. Select examples from other research groups will also be included in the discussion on these useful materials that have been often overlooked (but not, of course, by Beata Kilos).

Niobium: A love story

255th National Meeting (2018)

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