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 characte rized with respect to phase composition, structure and valence state of van adium sites as well as composition and acid-base properties of the surface using XRD, TEM, TPR, TPD/TPRS, FTIR, potentiometric titration, XPS, EPR, an d UV/VIS-DRS. The latter two techniques were also applied in situ under cat alytic reaction conditions. The following structure-reactivity relationship s were derived by comparing the results of catalyst characterization with t hose of catalytic tests: Both V5+ and V4+ catalyze the ODH of propane; howe ver, V4+ seems to be more selective though less active than V5+. V sites in octahedral or square pyramidal coordination are more active but less selec tive than VO4 tetrahedra. Isolated VO4 and V2O7 units as present in crystal line metal ortho-and pyrovanadates, respectively, are more selective but le ss active than VOx species in amorphous clusters or even in crystalline cha in-or layer-like structures. The differences in the catalytic properties ar ising from different metal cations are governed not only by the oxidation p otential of the latter but also to a major degree by the acid-base properti es, the crystal size and the extent of structural disorder. Catalytic activ ity 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 M g-V-O samples of similar composition is assumed to be due to large crystall ites with low surface defect concentration. (C) 2001 Academic Press.