Spectroscopic characterization of magnesium vanadate catalysts. Part 2.—FTIR study of the surface properties of pure and mixed-phase powders

Abstract

The surface chemistry of the three stable monophasic powders magnesium orthovanadate Mg₃(VO₄)₂, magnesium pyrovanadate Mg₂V₂O₇ and magnesium metavandate MgV₂O₆, as well as of an amorphous oxide catalyst with an Mg : V atomic ratio of 3 : 2, has been investigated by FTIR spectroscopy. The bulk structure of the amorphous catalyst has been characterized by FT-FIR and Raman spectroscopies. The nature of the surface hydroxy groups, as well as the adsorption of molecular probes such as pyridine, acetonitrile and CO₂, has been investigated. The samples show weak Lewis acidity and no Brønsted acidity. Moreover, they show significant but not extreme basic character. They weakly adsorb oxygenate compounds like alcohols and ketones and interact only at high temperatures with alkanes and alkenes. These materials, active as alkane oxydehydrogenation catalysts, are much less reactive than other vanadia-based selective oxidation catalysts, like V₂O₅–TiO₂ and (VO)₂P₂O₇, towards organic molecules. This is attributed to the basic environment generated by the MgO component in catalysts belonging to the MgO–V₂O₅ system, which causes a parallel decrease of the Lewis acidity and of the oxidizing powder of the active vanadium ions.