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 ph ases, and preparation conditions. The catalyst with the composition MoV0.4N b0.1Te0.2Ox showed high activity and selectivity for propane ammoxidation r eaction (ca. 50% selectivity to acrylonitrile at ca. 90% conversion) when i t 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 d egrees reported previously. XPS revealed the presence of Mo5+ in addition t o 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 red uction of the activity for propane ammoxidation. The activation energy for propane conversion was 32.0 kcal/mol, which was a little smaller than the 3 6.1 kcal/mol for propene ammoxidation. The reaction rate (v) for propane am moxidation on the MoVNbTe oxide catalyst at 693 K is given by v = k [propan e](0.8) [O-2](0.4) [NH3](-0.4). The active phases and reaction mechanism ar e discussed on the basis of the characterization by XRD and XPS and the kin etic data. (C) 2000 Academic Press.