IN-SITU RAMAN-SPECTROSCOPY STUDIES OF BULK AND SURFACE METAL-OXIDE PHASES DURING OXIDATION REACTIONS

Bulk V-P-O and model supported vanadia catalysts were investigated wit h in situ Raman spectroscopy during n-butane oxidation to maleic anhyd ride in order to determine the fundamental molecular structure-reactiv ity/selectivity insights that can be obtained from such experiments. T he in situ Raman studies of the bulk V-P-O catalysts provided informat ion about the bulk crystalline phases, the hemihydrate precursor and i ts transformation to vanadyl pyrophosphate. However, the Raman experim ents could not provide any molecular structural information about the amorphous and surface phases also present in this bulk metal oxide cat alyst because of the strong Raman scattering from the crystalline phas es. In contrast, in situ Raman studies of the model supported vanadia catalysts, where the active phase is present as a two-dimensional surf ace metal oxide overlayer, provided new insights into this important h ydrocarbon oxidation reaction. In addition, the surface properties of the supported vanadia catalysts could be molecularly engineered to pro be the role of various functionalities upon the structure-reactivity/s electivity relationship of n-butane oxidation to maleic anhydride. The se fundamental studies revealed that the oxidation of n-butane require d only one surface vanadia site and that the critical rate determining step involved the bridging V-O-P or V-O-support bonds. The selective oxidation of n-butane to maleic anhydride could occur over one surface vanadia site as well as multiple adjacent surface vanadia sites, but the reaction is more efficient with multiple sites. The n-butane oxida tion TOF increased with the introduction of both surface Bronsted and Lewis acid sites, but only the surface Lewis acid sites increased the maleic anhydride selectivity.