Catalysis science of the solid acidity of model supported tungsten oxide catalysts

  • Wachs I
  • Kim T
  • Ross E
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A series of supported WO3catalysts were synthesized by incipient wetness impregnation of ammonium metatungstate aqueous solutions onto Al2O3, TiO2, Nb2O5, and ZrO2supports as a function of tungsten oxide loading. The resulting solid acid catalysts were physically characterized with in situ Raman and UV-vis spectroscopy and chemically probed by methanol dehydration to dimethyl ether (CH3OH-TPSR and steady-state CH3OH dehydration). The molecular structures of the dehydrated supported tungsten oxide phase were determined to be monotungstate and polytungstate surface WOxspecies below monolayer surface coverage (9 W/nm2). The electronic structure for the different tungsten oxide species, Eg, was independent of the specific support and decreased monotonically with increasing tungsten oxide domain size (W/nm2). The solid acid catalytic activity, however, did not correlate with either the molecular or electronic structures because of the dominant contribution by the surface WOxspecies to the overall catalytic performance of the supported WO3catalysts and its reactivity dependence on the specific support. For supported WO3/Al2O3, the surface WOxmonolayer was more active than the crystalline WO3phases and, consequently, the TOF decreased with increasing surface W/nm2density. For tungsten oxide supported on Nb2O5, TiO2and ZrO2, the surface WOxmonolayer is less active than the crystalline WO3phases and, consequently, the TOF increases with surface W/nm2density. These reactivity trends reflect the influence of the specific support cation electronegativity on the acid character of the bridging W-O-Support bond. © 2006 Elsevier B.V. All rights reserved.

Author-supplied keywords

  • Alumina
  • Catalysts
  • Dimethyl ether
  • Edge Energy
  • Methanol
  • Niobia
  • Oxidative dehydration
  • Raman
  • Reaction
  • Spectroscopy
  • Supported
  • TPSR
  • Titania
  • Tungsten oxide
  • UV-vis
  • Zirconia

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