REACTIONS OF PROPENE ON SUPPORTED MOLYBDENUM AND TUNGSTEN-OXIDES

Alumina-supported tungsten and molybdenum oxide catalysts with a wide range of MO(3) loadings were studied in the metathesis of propene, The catalysts were activated by several procedures in order to correlate the oxidation state of Mo or W, as characterized by XPS, and the acidic character, as determined by IR of adsorbed pyridine, with the main metathesis reaction and side olefin reactions. The catalytic activity of the samples under standard conditions was found to be invariant with the pretreatment (air or He), This leads to the conclusion that the activation of catalysts takes place under the propene atmosphere during the first minutes of reaction, XPS studies of fresh and spent catalysts showed that the active sites for metathesis seem to be W(VI) or Mo(VI) species, Both MoO3/Al2O3 and WO3/Al2O3 catalysts are active in parallel reactions such as polymerization, isomerization and cracking. The product distribution for the metathesis reaction differed greatly from that expected, Thus, for all catalysts the ethene/butene ratio is lower than 1 and isobutene was the main component of the C-4 fraction, its proportion increasing with the metal oxide content. Studies by IR of adsorbed pyridine showed the Bronsted acid sites generated on MoO3 or WO3 in samples with high metal oxide content to be responsible for the lateral acid-catalyzed reactions.