Stability and surface acidity of niobium(V) oxide grafted on a silica gel surface

Abstract

Silica gel with a specific surface area, S_BET, of 322 m² g^–1, and an average pore diameter of 6 nm, has been chemically modified with Nb^V oxide by a grafting reaction. The quantity of attached atoms was 8.9 × 10^–2 mol g^–1 with a surface density of 1.7 atoms nm^–2. The XPS technique showed that the atomic Nb/Si ratio of ca. 0.03 remained unchanged when the samples were heated from 423 to 1573 K. At high calcination temperatures, i.e. T 1473 K, the specific surface area of Nb₂O₅/SiO₂ was nearly reduced to zero as consequence of a crystallization process. Spectroscopic and X-ray diffraction data showed that Nb₂O₅ and SiO₂ crystallized as separated phases at this temperature. Heating the material up to 973 K did not affect its transition-metal exchange capacity. Lewis and Brønsted acidities were also studied by the pyridine sorption technique. The Lewis acid sites were stable in samples precalcined up to 973 K and the Brønsted acid sites could be detected for samples precalcined up to 773 K.