Effect of additives on properties of V2O5/SiO2 and V2O5/MgO catalysts: II. Structure and physicochemical properties of the catalysts and their correlations with oxidative dehydrogenation of propane and ethane

Effect of additives (A) of main group elements (P and K) and of transition metal ions (Ni, Cr, Nb and Mo) introduced to VOx/SiO2 (VSiA) and VOx/MgO (VMgA) catalysts on their structure and physicochemical properties, has been studied with the use of XRD, XPS, Raman spectroscopy, H2-TPR and TPD of adsorbed propane. Acido-basic properties were determined from IR spectra of adsorbed pyridine and from isopropanol decomposition. The results of the characterization have been correlated with catalytic data for oxidative dehydrogenation (ODH) of propane and ethane reported in Part I. The additives do not affect markedly the catalysts’ structure, but modify their acido-basic properties and reducibility. For both VSi and VMg catalysts number of acidic sites increases with the electronegativity of the additive. For VSiA catalysts, in which V2O5 is the main phase and acidic properties dominate over dehydrogenating, the selectivity to propene increases with the decrease in acidity and with the increase in the MO bond energy. For VMgA catalysts, containing magnesium orthovanadate, in which dehydrogenating properties dominate over acidic ones, the selectivity to propene does not depend clearly on the acido-basic properties, but appears to be influenced by the properties of the catalysts’ oxygen. Selectivity to ethene in ODH of ethane does not depend on acidic properties for both series of the catalysts.