Spectroscopic and Structural Characterization of Low-Level Alkali Doping Effects on Mo/Silica−Titania Catalysts

The structural characteristics of supported MoO x species inherent over a high TiO 2 content silica−titania mixed-oxide support (1:1 molar ratio), combined with the structural effects that are induced by an alkali promoter, are the primary focus of this investigation. The catalysts were prepared using a “one-pot” modified sol−gel/coprecipitation technique that distributes the molybdenum precursor throughout the Si/Ti support network as it forms during gelation. The Si/Ti 1:1 mixed-oxide support is in a state of nanodispersed titania (anatase) over silica. With the introduction of molybdenum and successive levels of alkali promoter, the dispersion of titania decreases as does the interaction of TiO x species with silica. Concurrent with this change is the observation of an enhanced interaction of the MoO x species with silica. Correlation is found among the propane ODH reactivity, the surface coverage of oxidic molybdenum species, and the nature of the Si/Ti support. 1 H- 29 Si CP-MAS NMR data and DRIFT spectra taken in the hydroxyl stretching region (3000−3900 cm -1 ) suggest that, with the addition of alkali, the three dimensionality of the silica support is changing to accommodate the change in MoO x surface structure. The presence of potassium significantly alters the dispersion of titania and the electronic structure of the surface MoO x domains, even before the formation of K-molybdate species is observed at higher K/Mo molar ratios. This electronic interaction is observed readily in the ESR spectrum of Mo(V) that is present in the dehydrated samples. Data suggest that surface-supported species, present as distorted octahedral MoO x, become the most distorted at low alkali levels (K/Mo = 0.07). The MoO x species at this level of alkali promotion, which experience a decrease in Lewis acidity, could tend to be more reactive toward electronegative Si−O - support ligands on the Si/Ti 1:1 support. The observed increase in propane ODH reactivity at low K/Mo molar ratios may be ascribed to a highly distorted structure sharing both titania and silica ligands.