Oxidative Dehydrogenation of Propane over V₂O₅/MoO₃/Al₂O₃ and V₂O₅/Cr₂O₃/Al₂O₃: \nStructural Characterization and Catalytic Function

Abstract

The structure and catalytic properties of binary dispersed oxide structures prepared by sequential deposition\nof VO<i>_x</i> and MoO<i>_x</i> or VO<i>_x</i> and CrO<i>_x</i> on Al₂O₃ were examined using Raman and UV−visible spectroscopies,\nthe dynamics of stoichiometric reduction in H₂, and the oxidative dehydrogenation of propane. VO<i>_x</i> domains\non Al₂O₃ modified by an equivalent MoO<i>_x</i> monolayer led to dispersed binary structures at all surface densities.\nMoO<i>_x</i> layers led to higher reactivity for VO<i>_x</i> domains present at low VO<i>_x</i> surface densities by replacing\nV−O−Al structures with more reactive V−O−Mo species. At higher surface densities, V−O−V structures\nin prevalent polyvanadates were replaced with less reactive V−O−Mo, leading to lower reducibility and\noxidative dehydrogenation rates. Raman, reduction, and UV−visible data indicate that polyvanadates\npredominant on Al₂O₃ convert to dispersed binary oxide structures when MoO<i>_x</i> is deposited before or after\nVO<i>_x</i> deposition; these structures are less reducible and show higher UV−visible absorption energies than\npolyvanadate structures on Al₂O₃. The deposition sequence in binary Mo−V catalysts did not lead to significant\ndifferences in structure or catalytic rates, suggesting that the two active oxide components become intimately\nmixed. The deposition of CrO<i>_x</i> on Al₂O₃ led to more reactive VO<i>_x</i> domains than those deposited on pure\nAl₂O₃ at similar VO<i>_x</i> surface densities. At all surface densities, the replacement of V−O−Al or V−O−V\nstructures with V−O−Cr increased the reducibility and catalytic reactivity of VO<i>_x</i> domains; it also led to\nhigher propene selectivities via the selective inhibition of secondary C₃H₆ combustion pathways, prevalent in\nVO<i>_x</i>−Al₂O₃, and of C₃H₈ combustion routes that lead to low alkene selectivities on CrO<i>_x</i>−Al₂O₃. VO<i>_x</i> and\nCrO<i>_x</i> mix significantly during synthesis or thermal treatment to form CrVO₄ domains. The deposition sequence,\nhowever, influences catalytic selectivities and reduction rates, suggesting the retention of some of the component\ndeposited last as unmixed domains exposed at catalyst surfaces. These findings suggest that the reduction\nand catalytic properties of active VO<i>_x</i> domains can be modified significantly by the formation of binary\ndispersed structures. VO<i>_x</i>−CrO<i>_x</i> structures, in particular, lead to higher oxidative dehydrogenation rates and\nselectivities than do VO<i>_x</i> domains present at similar surface densities on pure Al₂O₃ supports.