Selective Oxidation of n-Butane over Vanadium-Phosphorus Oxide: Oxygen Activation and Dynamics

The complex microkinetics of the selective oxidation of n-butane to maleic anhydride (MAN) over vanadyl pyrophosphate-based (VPP) catalysts has been widely investigated so far, however, they are still under debate. We present novel results of kinetic analyses and isotope labeling to shed light on the activation of gas-phase oxygen and the dynamics of oxygen species on the catalyst surface and in the bulk. We suggest the existence of at least two catalytically relevant surface oxygen species responsible for selective and unselective activation of n-butane, which are formed by adsorption and consecutive dissociation of O-2 at the active site. The formation of selective [O] from unselective [O-2] is likely promoted by co-fed H2O, which increases the MAN selectivity. Regarding the VPP bulk serving as a reservoir for active [O] species it is concluded that the dynamic exchange is limited to the amorphous surface layer forming under reaction conditions, whereas the crystalline bulk is not involved into the reaction. Labelling studies are heavily influenced by vital O-16/O-18 scrambling of MAN isotopes with the reaction product H2O.