SPECTROSCOPIC CHARACTERIZATION OF MAGNESIUM VANADATE CATALYSTS .2. FTIR STUDY OF THE SURFACE-PROPERTIES OF PURE AND MIXED-PHASE POWDERS

The surface chemistry of the three stable monophasic powders magnesium orthovanadate Mg3(VO4)2, magnesium pyrovanadate Mg2V2O7 and magnesium metavanadate MgV2O6, as well as of an amorphous oxide catalyst with a n Mg : V atomic ratio of 3 : 2, has been investigated by FTIR spectros copy. The bulk structure of the amorphous catalyst has been characteri zed by FT-FIR and Raman spectroscopies. The nature of the surface hydr oxy groups, as well as the adsorption of molecular probes such as pyri dine, acetonitrile and CO2, has been investigated. The samples show we ak Lewis acidity and no Bronsted acidity. Moreover, they show signific ant but not extreme basic character. They weakly adsorb oxygenate comp ounds like alcohols and ketones and interact only at high temperatures with alkanes and alkenes. These materials, active as alkane oxydehydr ogenation catalysts, are much less reactive than other vanadia-based s elective oxidation catalysts, like V2O5-TiO2 and (VO)2P2O7, towards or ganic molecules. This is attributed to the basic environment generated by the MgO component in catalysts belonging to the MgO-V2O5 system, w hich causes a parallel decrease of the Lewis acidity and of the oxidiz ing power of the active vanadium ions.