Modified W/HZSM-5 catalysts: structure and catalytic properties

Conventional aqueous impregnated W/HZSM-5 catalysts have been modified by t reatment with an 1M ammonia aqueous solution prior to the calcination step, obtaining tungsten loading in the range 0.02-0.2 wt.% W diffuse reflectanc e spectroscopy (W DRS), X-ray diffraction (XRD), temperature-programmed red uction (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 exc lusively constituted of monomeric tungsten species. The characterization fi ndings let us draw a model in which these species are chemically joined to the Bronsted acid sites of the HZSM-5 zeolite and, therefore, well disperse d on the support. These catalysts were tested in the partial oxidation of m ethane to formaldehyde at atmospheric pressure. The obtained formaldehyde y ields were up to three times higher than those of the conventional aqueous impregnated W/HZSM-5 catalysts. (C) 2001 Elsevier Science B.V. All rights r eserved.