Genesis and structure of WOx/ZrO2 solid 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 WO3 loadings were chosen between 3.6 and 32 wt % and the calcination temperatures were in the range 773 less than or equal to T less than or equal to 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 mono;dde as a probe molecule. The experimental results indicate that the presence of WO, retards the crystallization of the zirconia material and stabilizes the tetragonal ZrO2 phase and the intrinsic BET surface area. W-O-W linkages are detected even at the lowest WO3 loadings indicating that oligomeric WOx clusters are initially anchored to the ZrO2 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 WO6 octahedra. W=O groups are present and presumably connected with peripheral WO6 octahedra. This structural model is supported by the number of nearest W neighbors, which varies between two and five depending on the WO3 loading as determined from the edge position in W-Vis diffuse reflectance spectra. It is inferred that the WO6 network may be structurally described as a pseudo-heteropolytungstate that might have some Zr4+ 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.