STABILITY AND SURFACE-ACIDITY OF NIOBIUM(V) OXIDE GRAFTED ON A SILICA-GEL SURFACE

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 x 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 greater-than-or-equal-to 1473 K, the specific surface area of Nb2O5/SiO2 was nearly reduced to zero as a 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 Bronsted acidities were also studied by the pyridine sorption technique. The Lewis acid sites were stable in samples precalcined up to 973 K and the Bronsted acid sites could be detected for samples precalcined up to 773 K.