Genesis and Structure of WOx/ZrO2Solid Acid Catalysts

Zirconia-supported tungsten oxide catalysts were prepared by suspending hydrous zirconium oxide in aqueous solutions of ammonium metatungstate at their natural pH. The suspensions were refluxed at 383 K followed by evaporation of the water, drying and calcination. The WO 3 loadings were chosen between 3.6 and 32 wt % and the calcination temperatures were in the range 773 ≤ T ≤ 1273 K. The resulting materials were structurally characterized by X-ray diffraction, differential thermal analysis, and surface area measurements, by laser Raman and Fourier transform infrared (FTIR) spectroscopy, and by UV−vis diffuse reflectance spectroscopy. Their acidic properties were tested by FTIR spectroscopy using carbon monoxide as a probe molecule. The experimental results indicate that the presence of WO x retards the crystallization of the zirconia material and stabilizes the tetragonal ZrO 2 phase and the intrinsic BET surface area. W−O−W linkages are detected even at the lowest WO 3 loadings indicating that oligomeric WO x clusters are initially anchored to the ZrO 2 surface which grow in extension with increasing loading. Near the theoretical monolayer coverage a relatively dense overlayer has formed that eventually forms a three-dimensional network of interconnected WO 6 octahedra. W O groups are present and presumably connected with peripheral WO 6 octahedra. This structural model is supported by the number of nearest W neighbors, which varies between two and five depending on the WO 3 loading as determined from the edge position in UV−Vis diffuse reflectance spectra. It is inferred that the WO 6 network may be structurally described as a pseudo-heteropolytungstate that might have some Zr 4+ polyhedra incorporated. FTIR spectroscopy showed an extremely broad quasi-continuous absorption in the O−H stretching region, which is suggestive of the presence in the overlayer network of delocalized protons. These are considered to be responsible for the detected protonic acidity of the materials.