(638e) The Contribution of Acid and Redox Sites during the Catalytic Oxidation Scission of Ketones over Supported Vanadium Oxide: An in Situ FTIR Study.

We have considered contributions of Mars-van Krevlen (MVK) and Eley-Rideal (ER) to the vanadium-catalyzed, aerobic, oxidative scission of methyl ketones to produce carboxylic acid and carbonyl fragments. Our prior work reveals that oxidation activity scales with surface acidity and cation reducibility; however, the specific mechanistic contributions of Lewis sites, Bronsted sites, and redox sites remain unclear. To resolve the roles of acid and redox sites, we perform selective site-poisoning experiments during the aerobic oxidation of 3-methyl-2-butanone on VO_x/Al₂O₃, and we characterize both surface coverage and reactivity using in situ FTIR and packed bed reactors.

FTIR spectra of pyridine saturated VO_x/Al₂O₃ samples show two distinct peaks corresponding to Lewis (≈ 1450 cm^-1) and Bronsted (≈ 1540 cm^-1) sites. When co-feeding pyridine during the oxidation of 3-methyl-2-butanone, its competitive adsorption at Lewis and Bronsted sites results in a decreased coverage of ketone and acetate species and reduced oxidative scission rates compared to normal steady-state conditions. Ketone could substitute weakly adsorbed pyridine at Lewis sites, and we observe partial recovery of acetate coverage upon removal of the titrant co-feed. In contrast, pyridine remains bound at Bronsted sites after removal of the pyridine co-feed. These observations suggest that Lewis sites are involved directly in the ketone oxidation, whereas Bronsted sites appear to bind spectator species. Introducing an ammonia co-feed during the oxidative scission of 3-methyl-2-butanone results in the formation of a distinct set of reaction products that include acetone, acetamide, and acetonitrile. Under these conditions, we observe the appearance of V-O-N bands at 930 cm^-1 and 965 cm^-1, which are consumed rapidly upon stopping the ammonia co-feed. These observations suggest that incorporation of nitrogen into the vanadium oxide lattice leads to the formation of acetamide and acetonitrile, helping to resolve the role of lattice oxygens during the oxidative scission of 3-methyl-2-butanone on vanadium oxides.