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

Abstract

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 31 P MAS NMR under controlled atmospheres. The results show that the introduction of water can convert the strong Lewis acid sites of Nb 2 O 5 • nH 2 O, Nb 2 O 5 -350, Ta 2 O 5 •nH 2 O, and Ta 2 O 5 -350 into weak Lewis acid sites with the induction of new Brønsted acid sites, whereas the strong Lewis acid sites of Nb 2 O 5 -450 and Ta 2 O 5 -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.

MAS NMR Studies on the Formation and Structure of Oxygen Vacancy on the CeO2 {110} Surface under a Reducing Atmosphere

“…TMP may adsorb on electron-deficient sites, and its chemical shift is sensitive to the change of electron density of its P nucleus. Therefore, 31 P MAS NMR with TMP as the probe molecule has been successfully used in the study of acidity of solid catalysts. − This technique may be applied in the study of oxygen vacancy because oxygen vacancies and Ce sites on the surface are electron-deficient Lewis acid sites and may affect the electron density of the P atom of adsorbed TMP molecules.…”

A critical assessment of the roles of water molecules and solvated ions in acid-base-catalyzed reactions at solid-water interfaces

“…Contrary to such cases, some Lewis acidic transition metal oxides have occasionally been found to acquire higher activities in the presence of water [54]. A solid state 31 P MAS NMR study [55] on water adsorption over representative water-resistant oxides, specifically niobium oxides and tantalum oxides, showed that adsorbed water on low-coordinated structural units (MO4 and MO5) could induce a 1:1 conversion of strong LAS to weak LAS and newly generated BAS, or of strong LAS to weak LAS without forming new BAS, as probed by trimethylphosphine. The fact that such changes depended on the hydration and calcination temperature of these oxides suggests that water effects could vary significantly in these compositionally similar systems and the level of water pressure could be key to the manifestation being an inhibition or enhancement.…”

Superhydrophobic MOF/polymer composite with hierarchical porosity for boosting catalytic performance in an humid environment

“…3 Moreover, in some cases, many heterogeneous Lewis acid catalysts are unstable in moisture systems and tend to hydrolyze. 3,4 These disadvantages severely limit the performance of this kind of catalyst in some practical applications. Consequently, the fabrication of hydrophobic, especially superhydrophobic catalysts that keep water molecules away from catalytic sites and reactants, is a promising method for resolving these problems.…”