The Structure of Active Sites in Me–V–O Catalysts (Me = Mg, Zn, Pb) and Its Influence on the Catalytic Performance in the Oxidative Dehydrogenation (ODH) of Propane

Me–V–O catalysts (Me = Mg, Zn, Pb) of different Me : V ratios were characterized with respect to phase composition, structure and valence state of vanadium sites as well as composition and acid–base properties of the surface using XRD, TEM, TPR, TPD/TPRS, FTIR, potentiometric titration, XPS, EPR, and UV/VIS-DRS. The latter two techniques were also applied in situ under catalytic reaction conditions. The following structure–reactivity relationships were derived by comparing the results of catalyst characterization with those of catalytic tests: Both V5+ and V4+ catalyze the ODH of propane; however, V4+ seems to be more selective though less active than V5+. V sites in octahedral or square pyramidal coordination are more active but less selective than VO4 tetrahedra. Isolated VO4 and V2O7 units as present in crystalline metal ortho- and pyrovanadates, respectively, are more selective but less active than VOx species in amorphous clusters or even in crystalline chain- or layer-like structures. The differences in the catalytic properties arising from different metal cations are governed not only by the oxidation potential of the latter but also to a major degree by the acid–base properties, the crystal size and the extent of structural disorder. Catalytic activity and selectivity of the Me–V–O catalysts decrease in the order Mg>Pb>Zn. The lower activity of Zn–V–O and Pb–V–O catalysts in comparison with Mg–V–O samples of similar composition is assumed to be due to large crystallites with low surface defect concentration.