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 metavanadate as vanadium sources. After calcination, large differences in the V/Mg surface atomic ratios were observed on the different catalysts, indicating differences in the V-Mg interaction, which are related to the catalysts preparation procedure. In addition, a new preparation procedure of V-Mg-O catalysts which allows an homogeneous dispersion of vanadium along the catalyst is presented. By X-ray diffraction, IR, UV-VIS and X-ray photoelectron spectroscopic characterization of the samples before and after the calcination step, different Mg- and V-compounds have been observed. Before the calcination step, Mg(OH)2 and/or magnesium-ozalate, as well as V5+ and/or V4+ species, depending on vanadium sources and vanadium content, were observed. After the calcination step, the formation of magnesium vanadates depends only on the vanadium content of the catalysts. Mg3V2O8 is formed at low vanadium content of the catalyst and Mg3V2O8+α-Mg2V2O7 at high vanadium content. However, the crystallinity of the magnesium vanadate phases, as well as the distribution of vanadium along the support strongly depends on the preparation procedure.