Relating the Synthesis Method of VO X /CeO 2 /SiO 2 Catalysts to Red-Ox Properties, Acid Sites, and Catalytic Activity for the Oxidative Dehydrogenation of Propane and n-Butane

Four V-Ce/SiO 2 catalysts were synthesized, using V grafting via VO(O-iC 3 H 7 ) 3 on Ce-loaded Aerosil support, with Ce incorporation either by grafting with Ce(O-iC 3 H 7 ) 4 (VOx/CeO 2 /S-GA and VOx/CeO 2 /S-GB) or by incipient wetness impregnation with Ce salt (VOx/CeO 2 /S-IA and VOx/CeO 2 /S-IB). Vanadium loadings were similar, while Ce was loaded to two different levels. Raman and X-ray diffraction (XRD) analysis indicated significant differences in VOx speciation: Vanadium pentoxide (V 2 O 5 ) nano-crystals were prevalent on those catalysts synthesized via Ce impregnation, and absent from the catalysts prepared with grafted Ce. Cerium vanadate (CeVO 4 ) phase was detected in catalyst prepared via Ce grafted at one theoretical monolayer coverage. Microcalorimetry results show that V 2 O 5 species found on VOx/CeO 2 /S-IA are related to acid sites (acid strength > 90 kJ/mol) responsible for the oxidative dehydrogenation (ODH) reaction of n-butane. Catalyst acidity in VOx/CeO 2 /S-IA, combined with the redox properties of the vanadium species on the same material, promoted a high selectivity to butadiene and cis- and trans-2-butenes, which subsequently reacted in a Diels-Alder addition to produce benzaldehyde and total oxidation products. In contrast, the lack of acidity in VOx/CeO 2 /S-GA led to very low selectivity (< 20%) to dehydrogenation products in both the ODH of butane and propane.