Preparation of V-Mg-O catalysts: nature of active species precursors

Abstract

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 V 5+ and/or V 4+ 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. Mg 3V 2O 8 is formed at low vanadium content of the catalyst and Mg 3V 2O 8+α-Mg 2V 2O 7 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.