Characterization and kinetic studies on the highly active ammoxidation catalyst MoVNbTeOx

The catalytic performance and structures of multicomponent MoVNbTe oxides have been studied to find the correlation among the performance, active phases, and preparation conditions. The catalyst with the composition MoV0.4Nb0.1Te0.2Ox showed high activity and selectivity for propane ammoxidation reaction (ca. 50% selectivity to acrylonitrile at ca. 90% conversion) when it was pretreated at 830-900 K for 2 h under O-2-free or less than 100 ppm O-2-contaminated N-2 or He flow conditions. Active phases were characterized by XRD, which gave 2 theta = 14.1 degrees, 24.9 degrees, 26.9 degrees, 29.1 degrees, 32.4 degrees, 33.2 degrees, and 35.4 degrees in addition to the peaks at 22.1 degrees, 28.2 degrees, 36.2 degrees, 44.7 degrees, and 50.0 degrees reported previously. XPS revealed the presence of Mo5+ in addition to Mo6+. In the presence of more than 1000 ppm O-2 contamination during the pretreatment, alpha-MoO3 was formed at the surface region, resulting in reduction of the activity for propane ammoxidation. The activation energy for propane conversion was 32.0 kcal/mol, which was a little smaller than the 36.1 kcal/mol for propene ammoxidation. The reaction rate (v) for propane ammoxidation on the MoVNbTe oxide catalyst at 693 K is given by v = k [propane](0.8) [O-2](0.4) [NH3](-0.4). The active phases and reaction mechanism are discussed on the basis of the characterization by XRD and XPS and the kinetic data. (C) 2000 Academic Press.