Mechanistic Investigation On the Role of Zirconium in the Oxidation of Methanol to Formaldehyde On Bilayered VOx/ZrOx/SiO2 Catalysts

Supported vanadium oxides have been the subject of much research for the oxidation of alcohols to aldehydes.  Recent studies have shown that the specific activity of vanadium in such catalysts is a strong function of the support composition. For example isolated vanadate groups on zirconia and titania exhibit nearly 10³ times the activity of vandate species on silica.  The aim of the present study was to examine the structure and performance of catalysts comprised of a high surface area, mesoporous silica support containing a fractional monolayer of zirconia and isolated vanadate groups.  Several supports with zirconium loadings from 0 to 1.5 Zr/nm², and low vanadium loading (0.4 V/nm²) were characterized with BET, x-ray absorption spectroscopy (XAS), infrared (IR), Raman spectroscopy, and ⁵¹V NMR to determine the structure of the active site.  Temperature programmed desorption/oxidation (TPD/TPO) and temperature programmed reaction (TPR_x) were performed on the catalyst to determine the kinetic data for the reaction.  The results of the experimental studies show a systematic change in the activity of the supported vanadate groups for methanol synthesis as a function of zirconium loading.  The origins of this change can be traced to subtle changes in the local composition and structure of the vanadate groups.  The results of the VO_x/ZrO_x/SiO₂ catalysts will be discussed in detail and compared to similar bilayered catalysts with titanium present.