Investigation of the structure and activity of VOx/ZrO2/SiO2 catalysts for methanol oxidation to formaldehyde

High surface area silica-supported bilayered VOx/ZrO2/SiO2 catalysts were prepared with a constant vanadium surface density of 0.5 V nm−2 and zirconium surface coverages ranging from 0.0 to 2.1 Zr nm−2. In all cases, the zirconia layer was predominantly amorphous in nature. The vanadia existed as isolated tetrahedral OV(OM)3 (M = Si, Zr) regardless of zirconia surface density. At least two distinct tetrahedral vanadia environments were identified by 51V NMR on the support: OV(OSi)3 and OV(OZr)3, with up to 35% of all V in the latter site at the highest Zr loading. The fraction of V bound to Zr as determined by 51V NMR agrees with an independent determination of the fraction of sites reduced by methanol at 600 K, a temperature too low for significant reduction of vanadia on silica. The turnover frequency for methanol oxidation increased by nearly two orders of magnitude as the Zr loading was increased. When normalized by the number of OV(OZr)3 sites determined from 51V NMR and UV–Visible, the turnover frequency for methanol oxidation to formaldehyde was constant with zirconia surface coverage. It is proposed that the much higher activity of OV(OZr)3 compared with OV(OSi)3 sites is attributable to differences in the mechanism by which H-abstraction from VOCH3 groups in the rate-limiting step leads to formaldehyde formation associated with the two types of sites.

Methanol oxidation on bilayered VOx/ZrO2/SiO2 catalysts was investigated. The catalysts were shown to exist in both OV(OSi)3 and OV(OZr)3 environments with an increasing fraction of the latter with increasing Zr surface density. A constant TOF was obtained when the rate was normalized to the number of OV(OZr)3 sites, which are the active sites for methanol oxidation on these bilayered catalysts.Figure optionsDownload high-quality image (83 K)Download as PowerPoint slideHighlights
► Bilayered VOx/ZrO2/SiO2 catalysts were studied for methanol oxidation.
► TOF increased with increasing Zr surface density.
► Both OV(OSi)3 and OV(OZr)3 exist on the surface.
► OV(OZr)3 improves reaction rate through an alternative H-abstraction step.