Preparation of highly dispersed niobium oxide on silica by equilibrium adsorption method

Two series of Nb₂O₅/SiO₂ catalysts are prepared by an equilibrium adsorption method (type A) and a conventional evaporation to dryness method (type E). The loading amounts of type A catalysts were low ( ˜ 0.2 wt% as Nb₂O₅) and those of type E catalysts were varied from 0.1 to 10 wt%. UV/VIS diffuse reflectance spectra of the catalyst samples show that micro particles of Nb₂O₅) are present on the catalysts with high-loading and monomeric or oligomeric niobate species are present on the catalysts with low-loading. By analyzing luminescence spectra of the low-loading samples, we conclude that the type A catalyst contains monomeric NbO₄ tetrahedra and the type E catalyst oligomeric NbO₄ tetrahedra. Propene photo-oxidation on type A catalysts yields propene oxide selectively, formed on monomeric NbO₄, whereas the photo-oxidation on the type E catalyst with low-loading yields propanal selectively. Propanal is the product in decomposition of propene oxide on oligomeric NbO₄ tetrahedra.