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. (C) 2001 Academic Press.