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Hydrated niobium oxides are used as strong solid acids with a wide variety of catalytic applications, yet the correlations between structure and acidity remain unclear. New insights into the structural features giving rise to Lewis and Brønsted acid sites are presently achieved. It appears that Lewis acid sites can arise from lower coordinate NbO 5 and in some cases NbO 4 sites, which are due to the formation of oxygen vacancies in thin and flexible NbO 6 systems. Such structural flexibility of Nb–O systems is particularly pronounced in high surface area nanostructured materials, including few-layer to monolayer or mesoporous Nb 2 O 5 · n H 2 O synthesized in the presence of stabilizers. Bulk materials on the other hand only possess a few acid sites due to lower surface areas and structural rigidity: small numbers of Brønsted acid sites on HNb 3 O 8 arise from a protonic structure due to the water content, whereas no acid sites are detected for anhydrous crystalline H-Nb 2 O 5 .
Kreissl et al. (Mon,) studied this question.