ABSTRACT
Vanadium Phosphorus Oxide (VPO) catalysts are commercially used for the selective oxidation of n-butane to maleic anhydride (MA) (Centi 1993). Among the various crystallographic phases making up the complex VPO catalytic system, vanadyl pyrophosphate (VO^P^ 7 is believed to be the most active and selective phase for this mild oxidation reaction (Bordes 1987), whereas various VOPO4 polymorphs are thought to result in over-oxidation to carbon oxides. The recent patent literature shows that VPO catalysts are often promoted with low concentrations of transition metal ions to improve their activity. Fe and Co are the most commonly used dopants investigated to date (Zazhigalov et al. (1993), Hutchings et al. (1996), Sajip et al. (2001)). Their introduction in the VPO system results in various morphological changes, depending on the precise catalyst preparation method and the amount of the dopant added. The precise mechanisms by which these transition metal dopants promote this catalyst system are not yet fully understood.
