(100f) The Role of Acids and Redox Sites during Oxidative Scission of Ketones over VOx/?-Al2O3

We have considered the contributions of Mars-van Krevlen and Eley-Rideal to the vanadium-catalyzed, aerobic, oxidative scission of methyl ketones to produce carboxylic acid and carbonyl fragments. However, the specific mechanistic contributions of Lewis acid sites (LA), Bronsted acid sites (BA), 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 (3M2B) on VO_x/γ-Al₂O₃ using pyridine, 2,6-ditert-butyl-pyridine (DTBP), and ammonia as titrants. We characterize both surface species and reactivity using in situ Fourier Transformed Infrared Spectroscopy (FTIR) and Packed Bed Reactors (PBR).

Co-feeding of pyridine and DTBP in PBR results in an identical rate decrease despite DTBP only bonding with BA while pyridine bonding with both LA and BA. Interestingly, acetate species are not generated in FTIR experiment of 3M2B on pyridine and DTBP saturated VO_x/γ-Al₂O₃ at 40 °C, unlike the unpoisoned surface, but only start to show up after extensive desorption of BA adsorbed titrants at 140 °C. This suggests that the poisoning of BA sites makes the catalyst behave similar to γ-Al₂O₃ at low temperature, and BA are involved directly in acetic acid production. In contrast, 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 of equal molar acetone and acetonitrile, indicating the production of carboxyl and carbonyl fragments from different steps and surface redox sites are responsible only for the formation of acetic acid. Water is proven to promote the reaction rate by cleaning surface acetate, providing more accessible hydroxyl groups/BA, and acting as an additional proton source. Based on all observations, we can uncover the function of acid and redox sites as well as the mechanism of selective oxidative scission of methyl ketone on VO_x/γ-Al₂O₃.