Catalytic function of VOx/Al2O3 for oxidative dehydrogenation of propane: support microstructure-dependent mass transfer and diffusion

Abstract

The intrinsic effect of the support microstructure on the catalytic function of VO_x/Al₂O₃ in the oxidative dehydrogenation of propane (ODHP) was studied. The results of characterization indicate that similar vanadia species are supported on Al₂O₃ with different microstructures, but that the interactions between VO_x and alumina are slightly modified. In this case, the microstructure-dependent catalytic function in the ODHP reaction is rigorously revealed. The flower-like morphology with non-compact and non-monolithic particles assembled as nanoribbons exhibits inter-particle stacking pores, which contribute to a severe extra-granular diffusion limitation (EGDL). The sphere-like morphology with compact and monolithic particles assembled as nanosheets possesses both inter-particle and intra-particle pores, which lead to a moderate EGDL. However, neither catalyst exhibits an intra-granular diffusion limitation (IGDL). Conversely, the bulk-like morphology with monolithic particles and without the assembly of secondary granules only exhibits intra-particle pores, which results in a slight EGDL and IGDL. Moreover, these two disturbances of diffusion can be excluded by means of controlling the reaction parameters. A comparison of the apparent and intrinsic kinetics, together with the structure–function relationship, determines that the EGDL might play an important role in influencing the ODHP efficiency. An in situ FT-IR study shows that the adsorption-activation of propane and that of propylene proceed via different initial intermediates and transformation processes. The former initially involves surface propoxide species that will be transformed into aldehydic and ketonic intermediates at the reaction temperature. The latter directly involves π-allyl alkoxidic and σ-allyl aldehydic species, as well as vanadia-carboxylate species with a quasi-anhydride structure at ambient temperatures, at which these intermediates are prone to catalytic combustion.