Abstract
Conventional aqueous impregnated W/HZSM-5 catalysts have been modified by treatment with an 1 M ammonia aqueous solution prior to the calcination step, obtaining tungsten loading in the range 0.02–0.2 wt.% UV diffuse reflectance spectroscopy (UV DRS), X-ray diffraction (XRD), temperature-programmed reduction (TPR), temperature-programmed desorption of ammonia (TPDA), surface area measurement, scanning electron microscope (SEM) and TEM were used for the characterization of the samples. The catalysts obtained were almost exclusively constituted of monomeric tungsten species. The characterization findings let us draw a model in which these species are chemically joined to the Brönsted acid sites of the HZSM-5 zeolite and, therefore, well dispersed on the support. These catalysts were tested in the partial oxidation of methane to formaldehyde at atmospheric pressure. The obtained formaldehyde yields were up to three times higher than those of the conventional aqueous impregnated W/HZSM-5 catalysts.
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