Titania–silica doped with transition metals via flame synthesis: structural properties and catalytic behavior in epoxidation

The influence of trace amounts of transition metals (Cr, Mn, Fe, Co) on the structural and catalytic properties of silica and 1 wt% titania–silica nanoparticles prepared by flame synthesis has been investigated. The transition metal concentration was varied from 40 to about 2000 ppm by admixing the corresponding transition metal precursor to the silica and titania–silica precursor mixtures. These components were fed into a methane–oxygen diffusion flame, affording a single-step flame synthesis of the mixed oxide materials. The specific surface areas of all the powders thus prepared ranged from 60 to 240 m² g⁻¹, with oxygen flow being the most influential parameter. Diffuse reflectance UV-VIS and FT-IR spectroscopy showed that the Ti sites in the doped nanoparticles were unaffected by chromium, manganese, and cobalt, while iron led to the formation of a dual site with considerable acidity. Epoxidation of 2-cyclohexenol by tert-butylhydroperoxide (TBHP) was used to probe the effect of the transition metal dopants on the activity and selectivity of the titania–silica. A significant loss in selectivity to the epoxide was observed for all doped mixed oxides. The chromium-doped material was highly active, favoring radical processes. Even at a doping level of 40 ppm, considerable amounts of ketone by-products were formed. Manganese doping led to slow decomposition of the TBHP, but had no significant influence on the alkene conversion at Mn contents of up to 2000 ppm. Incorporation of iron afforded Lewis acid sites, leading to dehydration activity, as demonstrated by the occurence of water abstraction from the reactant. Cobalt-doped titania–silica showed only a weak tendency toward radical reactions, and no reactions due to Lewis acid sites were observed. Among the metal dopants, only chromium underwent significant leaching under reaction conditions.