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Molybdenum species have been deposited on a high surface area silica by the impregnation method and further calcination at 500 "C in air. Mo loading was varied from 0-10 wt YO Mo. I.r., u.v.-visible, XPS and EDX-STEM techniques have been used to characterize the nature of the molybdenum species as a function of Mo loading. At low M o content ( < 3 wt YO Mo) interaction between silanol groups and molybdate ions occurred resulting in the formation of monomeric tetrahedral MOO:species with a U.V. band near 245 nm. For intermediate loadings (3-8 wt %) polymeric octahedral molybdate species were identified with a U.V. band near 340 nm. At high Mo content crystallites of MOO, were also observed. Catalytic properties for both isopropyl alcohol conversion in air at 100 "C and propene oxidation at 400 "C were studied as a function of Mo loading. It was observed that the catalysts are acidic as evidenced by isopropyl alcohol dehydration into propene at low Mo content and their activity increased with Mo content following the amount of Moot-species. Polymeric molybdate species were observed to exhibit mainly redox-type properties as evidenced by isopropyl alcohol oxidative dehydrogenation into acetone and by propene oxidation into propanal (electrophilic attack) and into acrolein (allylic-type reaction). MOO, crystallites were observed to exhibit usual properties of unsupported MOO, catalysts.