Effect of oxomolybdate species dispersion on direct methanol oxidation to dimethoxymethane over MoO_x/TiO₂ catalysts

The one-step selective oxidation of methanol to dimethoxymethane (DMM) was demonstrated over titania-supported molybdenum oxide catalysts, containing different amounts of molybdenum and prepared using two different impregnation techniques, namely wet impregnation and incipient wetness impregnation. The corresponding catalysts exhibited both acidic and redox properties, which are necessary for the oxidation of methanol to formaldehyde with subsequent condensation of the latter with excess methanol to finally yield DMM. The formation of well-dispersed polyoxomolybdate species on the catalyst surface was evidenced using IR-Raman, X-ray diffraction, and nitrogen physisorption. Varying the amount of these polyoxomolydate species was associated with a modulation of the acidic and redox properties, as shown by NH₃-TPD and H₂-TPR. With respect to the catalytic performances, the best balance between acid and redox properties was observed over the samples containing 8 wt.% Mo, which corresponds to a theoretical MoO_x species coverage close to a monolayer.