Oxidative dehydrogenation of propane over Mg-V-O oxides supported on MgO-coated silica: Structural evolution and catalytic consequence

Supported Mg-V-O oxides were prepared by co-impregnation of magnesium and vanadium citrates with MgOcoated mesoporous silica (SBA-15). Their structures were characterized by complementary techniques including powder X-ray diffraction, Raman spectroscopy, diffuse reflectance UV vis spectroscopy and transmission electron microscopy. These catalysts were examined in oxidative dehydrogenation (ODH) of propane to propylene, and the reaction kinetic parameters of k(1)/k(2) and kilki were analyzed, which represent the ratios of the rate constants for primary propane ODH (k(1)) to propane combustion (k(2)) and that for secondary propylene combustion (k(5)) to kb respectively. Characterization results show the structural evolution of the vanadium species from isolated VO4 monovanadates to Mg3V2O5 crystallites, without formation of crystalline V205, with increasing the vanadium content from 1.7 wt% to 25.6 wt%. Dispersed monovanadates exhibited higher propane ODH rates than Mg3V2O5 crystallites, in line with their higher reducibility probed by temperature-programed reduction in H2, but they were less selective to propylene due to the favorable propylene combustion over its formation, as reflected by the higher kilki ratios. The supported Mg3V2O5 crystallites with sizes of about 15-30 nm, relative to those of above 30 nm, were found to be intrinsically more selective for the propane ODH, consistent with their higher ki/k2 and lower kilki ratios. Consequently, they offered the superior selectivities and yields of propylene at higher propane conversions. Such understanding on the intrinsic catalytic properties of the Mg-V-O oxides and particularly Mg3V2O5 crystallites provides new insights into the structural requirement for the propane ODH reaction, beneficial to design of more efficient Mg-V-O-based catalysts for the production of propylene.