Supported vanadium oxide-based catalysts for the oxidehydrogenation of propane under cyclic conditions

Heterogeneous catalysts were prepared by deposition of vanadium oxide on different supports, namely alumina, titania, titania–alumina cogel and silica. Catalysts were tested in the reaction of propane oxidehydrogenation to propylene under co-feed and under redox-decoupling conditions (i.e., by alternating the reducing and the re-oxidation steps). Under cyclic conditions, the selectivity to propylene was higher than under co-feed conditions when the catalyst was treated for longer reduction times, but this was due to the fact that in the former case the mechanism was dehydrogenative rather than oxidative. On the contrary, on oxidized catalysts the mechanism was an oxidative one even in the absence of molecular oxygen, and conversions were higher than 20%; however, in this case the improvement in selectivity to propylene with respect to the co-feed operation was low. Catalysts made of vanadium oxide either dispersed inside a high-surface-area silica gel, or supported over silica, allowed higher instantaneous propane conversions than catalysts in which vanadium oxide was deposited over alumina or titania, and also gave the greater improvement of selectivity to propylene with respect to the co-feed conditions. These differences were attributed to the nature of the vanadium species which develop when silica is the support.