Effect of vanadia loading on the acidic, redox and catalytic properties of V2O5–TiO2 and V2O5–TiO2/SO42− catalysts for partial oxidation of methanol

V2O5/TiO2 and V2O5/TiO2–SO42− catalysts with various vanadia loadings (5, 15 and 25 wt% V2O5) were prepared by successive incipient wetness impregnations and the effect of doping with SO42− was studied. The catalysts were characterized by XRD, BET, XPS, Raman spectroscopy, redox cycles (TPR1/TPO/TPR2), ammonia adsorption microcalorimetry and tested in the selective oxidation of methanol to dimethoxymethane (DMM). The Raman spectra revealed the formation of crystalline V2O5 particles when the vanadia coverage was higher than 15 wt%. Redox cycles revealed that the monomeric vanadia species were easier to reduce but more difficult to oxidize than the polymeric and crystalline species. The supported vanadia species were found to be composed of stoichiometric V2O4 and V2O5, as shown by XPS. The ammonia adsorption microcalorimetric study evidenced acid sites with adsorption heats Qdiff > 110 kJ mol−1, in amounts which increased upon the addition of SO42− but decreased with increasing vanadia loading. Furthermore, the results of the methanol oxidation reaction suggest that the addition of SO42− on highly dispersed supported vanadia catalysts promoted the catalytic activity.