Oxidative dehydrogenation of propane over vanadia-based catalysts supported on high-surface-area mesoporous MgAl 2O 4

Abstract

The oxidative dehydrogenation of propane to propene was investigated over a series of novel vanadia-based catalysts supported on high-surface-area magnesium spinel. A mesoporous MgAl 2O 4 support was synthesized via a low-temperature sol–gel process involving the heterobimetallic alkoxide precursor, Mg[Al(O i Pr) 4] 2. A high-purity catalyst support was obtained after calcination at 1173 K under O 2 atmosphere and active vanadia catalysts were prepared from the thermolysis of OV(O tBu) 3 after grafting onto the spinel support. MgAl 2O 4-supported catalysts prepared in this manner have BET surface areas of 234–245 m 2/g. All of the catalysts were characterized by X-ray powder diffraction, and Raman, solid-state NMR, and diffuse-reflectance UV–vis spectroscopy. At all vanadium loadings the vanadia supported on MgAl 2O 4 exist as a combination of isolated monovanadate and tetrahedral polyvanadate species. As the vanadium surface density increases for these catalysts the ratio of polyvanadate species to isolated monovandate species increases. In addition, as the vanadium surface density increases for these catalysts, the initial rate of propane ODH per V atom increases and reaches a maximum value at 6 VO x /nm 2. Increasing the vanadium surface density past this point results in a decrease in the rate of propane ODH owing to the formation of multilayer species in which subsurface vanadium atoms are essentially rendered catalytically inactive. The initial propene selectivity increases with increasing vanadium surface density and reaches a plateau of ∼95% for the V/MgAl catalysts. Rate coefficients for propane ODH ( k 1), propane combustion ( k 2), and propene combustion ( k 3) were calculated for these catalysts. The value of k 1 increases with increasing VO x surface density, reaching a maximum at about 5.5 VO x /nm 2. On the other hand, the ratio ( k 2/ k 1) for V/MgAl decreases with increasing VO x surface density. The ratio ( k 3/ k 1) for both sets of catalysts shows no dependence on the vanadia surface density. The observed trends in k 1, ( k 2/ k 1), and ( k 3/ k 1) are discussed in terms of the surface structure of the catalyst.