The impact of redox properties on the reactivity of V2O5/Al2O3 catalysts

The redox properties of 5 and 15 wt.% V2O5/Al2O3 catalysts calcined at 753 and 973 K were characterized by measuring their composition as a function of P-O2 at 748 K using coulometric titration. The redox isotherms showed that the 5 wt.% catalyst calcined at 753 K underwent gradual reduction from V+5 to V+4 at P-O2 between 10(-4) and 10(-19) atm, and stepwise reduction from V+4 to V+3 at a P-O2 of 10(-24) atm corresponding to a Delta G(red) of 343 kJ/mol Of O-2. This sample was also found to exhibit relatively high thermal stability. The redox isotherm for a 15 wt.% catalyst calcined at 753 K was similar to that obtained for a 5 wt.% catalyst. This sample was much less thermally stable, however, and recrystallized upon calcination at 973 K, forming a highly reducible AlVOx species that underwent reduction from V+5 to V+3 in a single step at a P-O2 of 10(-6) arm at 748 K with a Delta G(red) of 86 kJ/mol. In spite of the differences in the redox properties, all the catalysts were found to exhibit a similar reactivity for the selective oxidation of methanol. The catalysts also exhibited a similar propane ODH activity except for the 15 wt.% catalyst calcined at 973 K which had a lower activity, was unselective, and produced Only CO2. (C) 2009 Elsevier Inc. All rights reserved.