Investigation of the structure and activity of VO_x/ZrO ₂/SiO₂ catalysts for methanol oxidation to formaldehyde

High surface area silica-supported bilayered VO_x/ZrO ₂/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.