Catalytic Properties of Supported MoO3 Catalysts for Oxidative Dehydrogenation of Propane

The effects of MoOx structure on propane oxidative dehydrogenation (ODH) rates and selectivity were examined on Al2O3-supported MoOx catalysts with a wide range of surface density (0.4-12 Mo/nm), and compared with those obtained on MoOx/ZrO2. On MoOx/Al2O3 catalysts, propane turnover rate increased with increasing Mo surface density and reached a maximum value for samples with ~ 4.5 Mo/nm. All MoOx species are exposed at domain surfaces for Mo surface densities below 4.5 Mo/nm. Therefore, the observed trends reflect an increase in ODH turnover rates with increasing MoOx surface density. As Mo surface densities increase above the polymolybdate monolayer value (~ 4.5 Mo/nm), ODH turnover rates decreased with increasing Mo surface density, as a result of the formation of MoO3 crystallites with inaccessible MoOx species. The ratio of rate constants (k2/k1) for propane combustion (k2) and for propane ODH reactions (k1) decreased with increasing MoOx surface density and then remained constant for values above 5 Mo/nm. Propene combustion rate constants (k3) also decreased relative to those for propane ODH (k1) as two-dimensional structures formed with increasing Mo surface density. These Mo surface density effects on k2/k1 and k3/k1 ratios were similar on MoO3/Al2O3 and MoO3/ZrO2, but the effects of Mo surface density on ODH turnover rates for samples with submonolayer MoOx contents were opposite on the two catalysts. A comparison of ODH reaction rates and selectivity among MoO3/Al2O3, MoO3/ZrO2, bulk MoO3, ZrMo2O8, and Al2(MoO4)3 suggests that the behavior of supported MoOx at low surface densities resembles that for the corresponding bulk compounds (ZrMo2O8, and Al2(MoO4)3), while at high surface density the behavior approaches that of bulk MoO3 on both supports.