Effect of Adsorbed Water Molecules on the Surface Acidity of Niobium and Tantalum Oxides Studied by MAS NMR

Niobium oxides and tantalum oxides are typical solid acid catalysts that exhibit high activity and high water-resistance in the aqueous environment. To understand their acid catalytic performance, several niobium oxides and tantalum oxides and their hydrates were prepared, and the effects of adsorbed water on the acidity were investigated by 31P MAS NMR under controlled atmospheres. The results show that the introduction of water can convert the strong Lewis acid sites of Nb2O5·nH2O, Nb2O5-350, Ta2O5·nH2O, and Ta2O5-350 into weak Lewis acid sites with the induction of new Brønsted acid sites, whereas the strong Lewis acid sites of Nb2O5-450 and Ta2O5-450 convert into weak Lewis acid sites only. The obtained information may provide a unique insight into the role of water in many heterogeneous catalytic reactions that directs toward the rational design of water-resistant catalysts.