PREPARATION OF V-MG-O CATALYSTS - NATURE OF ACTIVE SPECIES PRECURSORS

A series of V-Mg-O catalysts have been prepared using MgO or magnesium oxalate and aqueous solutions of vanadyl-oxalate or ammonium metavana date as vanadium sources. After calcination, large differences in the V/Mg surface atomic ratios were observed on the different catalysts, i ndicating differences in the V-Mg interaction, which are related to th e catalysts preparation procedure. In addition, a new preparation proc edure of V-Mg-O catalysts which allows an homogeneous dispersion of va nadium along the catalyst is presented. By X-ray diffraction, IR, UV-V IS and X-ray photoelectron spectroscopic characterization of the sampl es before and after the calcination step, different Mg- and V-compound s have been observed. Before the calcination step, Mg(OH)(2) and/or ma gnesium-oxalate, as wen as V5+ and/or V4+ species, depending on vanadi um sources and vanadium content, were observed. After the calcination step, the formation of magnesium vanadates depends only on the vanadiu m content of the catalysts. Mg3V2O8 is formed at low vanadium content of the catalyst and Mg3V2O8+alpha-Mg2V2O7 at high vanadium content. Ho wever, the crystallinity of the magnesium vanadate phases, as well as the distribution of vanadium along the support strongly depends on the preparation procedure.