Molecular Design of Supported Niobium Oxide Catalysts

The current investigation demonstrates that it is possible to molecularly design supported niobium oxide catalysts with the assistance of molecular characterization methods such as Raman spectroscopy. The formation and location of the surface niobium oxide species are controlled by the surface hydroxyl chemistry, and the surface niobium oxide species are located in the outermost layer of the catalysts as an overlayer. The catalyst composition is a critical parameter since it affects the presence of different niobium oxide species (several different surface species and crystalline phases), and the reactivity also varies somewhat with surface niobium oxide coverage. The preparation method is not a critical parameter since it does not appear to influence the structure or reactivity of the surface niobium oxide species. However, for silica supported niobium oxide catalysts the preparation method does affect the amount of niobium oxide that can be dispersed as a two-dimensional overlayer. Calcination temperature is an important parameter that controls the activation and deactivation of the supported niobium oxide catalysts, but calcination temperature is not critical if moderate temperatures, 400-500°C, are used. The specific oxide support is a critical parameter since it dramatically affects the reactivity of the surface niobium oxide species and determines if the surface niobium oxide site is active for redox or acid catalysis. Thus, the critical parameters that affect the catalytic properties of the supported niobium oxide catalysts are the specific oxide support and catalyst composition.