Anomalous reactivity of supported V2O5nanoparticles for propane oxidative dehydrogenation: influence of the vanadium oxide precursor

Abstract

The oxidative dehydrogenation (ODH) of propane to propylene by supported vanadia catalysts has received much attention in recent years, but different reactivity trends have been reported for this catalytic reaction system. In the present investigation, the origin of these differing trends are investigated with synthesis of supported V/SiO₂, V/TiO₂, and V/Al₂O₃ catalysts prepared with three different vanadium oxide precursors (2-propanol/vanadyl triisopropoxide [VO(O-Pri)₃] (VTI), oxalic acid/ammonium metavanadate [NH₄VO₃] (AMV), and toluene/vanadyl acetylacetonate [VO(C₅H₇O₂)₂] (VAA)) in order to elucidate the influence of the precursor on supported vanadia phase and propane ODH activity. In situ Raman spectroscopy revealed that the choice of vanadium precursor does not affect the dispersion of the supported vanadium oxide phase below 4 V nm⁻² (0.5 monolayer coverage), where only isolated and oligomeric surface VO₄ species are present, and only the AMV precursor favors crystalline V₂O₅ nanoparticle (NP) formation below monolayer coverage (8 V nm⁻²). The propane ODH specific reactivity trend demonstrated that there is no significant difference in TOF for the isolated and oligomeric surface VO₄ sites. Surprisingly, V₂O₅ NPs in the ∼1–2 nm range exhibit anomalously high propane ODH TOF values for the supported vanadia catalysts. This was found for all supported vanadium oxide catalysts examined. This comparative study with different V-precursors and synthesis methods and oxide supports finally resolves the debate in the catalysis literature about the dependence of TOF on the surface vanadium density that is related to the unusually high reactivity of small V₂O₅ NPs.