Alkali Effects on Molybdenum Oxide Catalysts for the Oxidative Dehydrogenation of Propane

The effects of alkali oxides on the structure and on the catalytic behavior of MoOx/ZrO2 were examined for the oxidative dehydrogenation (ODH) of propane. X-ray diffraction, Raman and X-ray absorption spectroscopy, and estimates of the MoOx surface density are consistent with the predominant presence of two-dimensional polymolybdate domains in samples with ∼4 Mo/nm2 surface density and (alkali/Mo)at. ratios of 0–0.2 treated in air at 773 K. The presence of alkali (Cs, K, Li) did not affect the structure of MoOx domains, but influenced their electronic and catalytic properties. Propane ODH turnover rates decreased monotonically with increasing A:Mo atomic ratio and with increasing basicity of the alkali oxide (Cs>K>Li). These basic oxides inhibit the initial reduction of MoOx in H2 by strengthening Mo–O bonds and increasing reduction activation energies. As a result, rate-determining C–H bond activation steps, which involve the local reduction of Mo6+ centers, and the overall ODH catalytic sequence proceed more slowly when alkali oxides modify the MoOx domains. The ratio of the rate constant for primary propane combustion (k2) to that for propane ODH (k1) is small (∼0.1) and it increased slightly with increased alkali content. The ratio of the propene combustion rate constant (k3) to the propane ODH rate constant (k1) is large (15–25); it decreased with the addition of small amounts of alkali (alkali/Mo ∼0.05) and then remained constant at higher alkali contents. The stronger inhibition of secondary propene combustion reactions relative to that of primary ODH steps reflects the weaker Lewis acidity of Mo6+ cations modified by the strongly basic alkali oxide species. These effects decrease the binding energy of propene on the sites required for C–H bond activation of both propene and propane. They also account for the lower reaction rates and for the higher selectivity observed at a given propane conversion on alkali-modified MoOx ODH catalysts.