INVESTIGATIONS OF CATALYTIC MECHANISMS FOR SELECTIVE PROPENE OXIDATION IN THE PRESENCE OF STEAM

A study was carried out on propene partial oxidation in the presence a nd absence of steam over a mixed transition metal oxide. Its objective was to explore mechanisms and key rate controlling steps for this par tial oxidation in the presence of added water. The reaction was studie d in the concentration ranges (by volume %) C-3(-) less than or equal to 20%; 5% < O-2 < 20%; 0.5% < H2O < 15% with N-2 as a diluent. An Sb/ Sn/V oxide catalyst was used. Three techniques were used: (1) transien t response of reaction products to a step-change in feed composition, (2) temperature-programmed desorption (TPD) for reactant and product a dsorbates, and (3) isotopic transient experiments using O-18(2). TPD a nd isotopic experiments suggest that water addition disturbs the adsor bate concentrations on the surface through competitive adsorption and creates new sites, leading to a slow increase in the rate of partial o xidation. These experiments also establish that oxygen is not strongly chemisorbed but oxygen exchange with the surface is possible. The exi stence of at least three groups of adsorption and/or reaction sites wa s postulated, corresponding to weakly bonded propene at 140 degrees C, strongly bonded propene at 210 degrees C and sites on which propene i s converted to oxygenates up to 400 degrees C. only the strongly bondi ng sites contribute to acid formation, C-C bond scission and total oxi dation. With water present, further oxidation of oxygenates is suppres sed and carbon dioxide forms only from C-C bond scission. Water may pa rticipate in acid formation.