Silica–niobia oxides as viable acid catalysts in water: Effective vs. intrinsic acidity

By dispersing a niobia phase in/on silica matrix it was possible to maintain the catalytic activity in the dehydration of fructose in water for longer time without any deactivation, compared with the fast deactivation observed on bulk niobium oxide hydrate. The surface acidity of the bulk niobium oxide hydrate and silica-diluted niobia materials was responsible of the different catalytic performances observed. As the dehydration reaction can run in water, in agreement with the development of a green process, the acid properties of the catalysts were comparatively studied in water too, to find sound relations between acid and catalytic properties. On all the catalysts, the effective acidity, determined in water, was much lower compared with the intrinsic acidity determined in cyclohexane, as expected. A relation between the catalytic activity in the dehydration of fructose and the amount of effective acid sites of the samples was evidenced.