Nature of surface sites of image catalysts and reactivity in selective oxidation of methanol to dimethoxymethane

The selective oxidation of methanol to dimethoxymethane (DMM) over sulfated vanadia–titania catalysts, prepared by co-precipitation and calcined at different temperatures, was studied in the 393–473 K interval under steady state conditions. The catalysts were characterized by X-ray diffraction (XRD), Brunauer–Emmett–Teller isotherms (BET), inductively coupled plasma optical emission spectroscopy (ICP–OES), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), and Raman spectroscopy. The redox and acidic properties were examined using temperature programmed reduction (TPR), isopropanol probe reaction, ammonia adsorption calorimetry, and pyridine adsorption FTIR techniques. As evidenced by pyridine adsorption FTIR, some Brönsted acid sites transformed to Lewis sites upon removal of sulfate species by washing the samples with deionized water. A high sulfur content increased the number of Brönsted acid sites but reduced their strength. The best catalyst revealed the presence of amorphous polymeric VOx species with terminal Vdouble bond; length as m-dashO bonds, and both redox and Brönsted acid sites, resulting from an adequate balance between the calcination temperature and the sulfate concentration. These are the key parameters for optimizing the DMM production.