Unusual Phase Evolution in MoVTeNb Oxide Catalysts Prepared by a Novel Acrylamide-Gelation Route

The phase evolution of MoVTeNb mixed oxides derived from a novel acrylamide-gelation approach combined with subsequent washing by hydrogen peroxide and/or annealing in argon was investigated by using powder X-ray diffraction and scanning electron microscopy in conjunction with energy-dispersive X-ray spectroscopy. The calcination of acrylamide-gelation derived carbon-based xerogels in air at 410 °C afforded at unexpected low temperature the formation of a phase mixture containing a significant amount of small MoVTeNb oxide crystallites consisting of the M1 phase (ICSD no. 55097). Hydrogen peroxide treatment drastically changed the phase evolution upon annealing in argon at 600 °C. Whereas further enrichment of the M1 phase took place for the untreated precursor, annealing of the washed material resulted in complete collapse of the M1 phase, yielding a ternary MoVNb mixed oxide with tetragonal Mo5O14-type structure (ICSD no. 27202). The reducing effect of organic residues on phase evolution during calcination of the carbon-inorganic xerogel in air is discussed. The presence of side phases and the deficiency in tellurium upon annealing have a critical impact on the phase composition of the final product. The catalysts exhibit significant activity in partial oxidation of propane to acrylic acid. The analysis of the product distribution suggested that the activation of both propane and propylene intermediates proceed exclusively over the M1 phase, while side phases do not play a significant role in the reaction