Effects of pH and vanadium concentration during the impregnation of Na-SiO2 supported catalysts for the oxidation of propane

The linear and non-linear effects of the impregnation pH and of the concentration of vanadium in solution over the properties of Na-SiO2 supported catalysts used for the oxidation of propane under light-off/light-out tests at oxidizing conditions were studied. The listed factors had no net effects over the loading of vanadium, the surface area, and the porous structure of the catalysts. Depending on the pH, sodium vanadates of different particle sizes were formed; namely, NaVO3 microcrystals and particles made of α-NaVO3 and β-NaVO3 phases were formed at acidic pH, whereas sodium metavanadate nanoparticles were formed at pH=9.0. For the catalysts synthesized at pH=3.8, i.e., near the point of zero charge of the support, and at pH=9.0, the α-NaVO3 phase prevailed over the β-NaVO3 phase after dehydration and catalytic testing. The catalyst synthesized at pH=9.0 and at the highest concentration of vanadium in the impregnating solution showed normal hysteresis behavior while being the best in terms of the selectivity to propene and the conversions of propane and oxygen which were at the same level as the ones found for a VOx/SiO2 benchmark. The correlation of the catalytic trends with the physicochemical properties of the catalysts allowed concluding that increasing the impregnation pH favors the formation of less reducible and more acidic V4+ species that favor the reactivity and the selectivity to propene over the Na-SiO2 supported sodium vanadate catalysts synthesized herein. the most selective to propane oxidative dehydrogenation at a propane conversion comparable. Kinetic and thermodynamic arguments were discussed to elucidate the origin the catalytic trends.