(544fk) Temperature Programmed Surface Reaction and in-Situ IR Studies of the Oxidative Scission of Methyl Ketones over ?-Al2O3 supported Vanadium Oxide
In order to provide insight into the mechanism and reaction kinetics governing ketone scission, we have studied the oxidative cleavage of methyl ketones over VOx/Î³-Al2O3 catalysts using Temperature Programmed Reaction Spectroscopy (TPSR). Specifically, we have investigated how perturbations in substitution at the carbonyl alpha carbon; extent of polymerization in the vanadate surface phase; gas phase oxygen pressures; and the identity and reducibility of the metal oxide support impact the kinetics and selectivity of ketone oxidation. Macroscopically, ketone oxidation will generally produce, as primary products, an aldehyde fragment and a carboxylic acid fragment. Under aerobic conditions, TPSR experiments indicate that peak rates of aldehyde formation occur at roughly 340K, while production of the carboxylic acid co-product is observed at roughly 500K. This may suggest that the surface carboxylates are more strongly bound than surface aldehydes or that the formation of surface-bound aldehydes is kinetically more facile than the formation of surface carboxylic acid precursors. In contrast, TPSR under anaerobic conditions reveals that aldehyde formation is shifted to a substantially higher temperature (500K), while the carboxylic acid co-product is observed only in trace quantities and at significantly higher temperatures (673K). Instead, we observe a dramatic shift in selectivity toward gas-phase carbon oxides, which suggests that, in the absence of gas-phase molecular oxygen, surface bound carboxylic acid fragments (or precursors thereof) undergo combustion side reactions. It is interesting to observe an inverse correlation between combustion selectivity and gas-phase oxygen pressure. This suggests there is a critical difference between atomic âsurfaceâ oxygen provided by dissociative oxygen chemisorption and âlatticeâ oxygen provided by reduction of V-O-X bonds. It appears that chemisorbed oxygen can facilitate oxidation of surface species under mild conditions where it is possible to control selectivity, whereas lattice oxygen only becomes available at higher temperatures, where it is difficult to prevent combustion. These observations suggest a complex reaction mechanism wherein surface bound hydrocarbons react with both chemisorbed oxygen atoms (e.g., Langmuir-Hinshelwood mechanism) and lattice oxygen atoms (e.g., Mars-van-Krevelen mechanism). Our ongoing investigations in this area are aimed at further resolving the exact mechanism of oxidative ketone scission using in-situ or operando experiments and micro kinetic analysis.
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