(671c) Elucidating the Roles of Lattice and Molecular Oxygen during the Oxidative Scission of Methyl Ketones over Supported Vanadium Oxides

At the macro-scale, it is clear that both molecular and lattice oxygen play significant roles in facilitating the scission of ketones over VO_X/γ-Al₂O₃. At a mechanistic level, however, their contributions remain unresolved. In order to probe their individual impacts, we have employed Temperature Programmed Surface Reaction (TPSR) spectroscopy. Specifically, we examine trends in product formation during temperature-programmed reactions of chemisorbed 3- methyl-2-butanone over γ-Al₂O₃ and VO_X/γ-Al₂O₃. Under steady state conditions, this reaction generates two oxidation products, acetone and acetic acid, which are easily resolved by online mass spectrometry.

The role of molecular oxygen was investigated by conducting TPSR experiments over γ-Al₂O₃ under both anaerobic (He) and oxidizing conditions (15% O₂ in He). Our results indicate that γ-Al₂O₃ is a non- reducible material. Interestingly, despite its non-reducible lattice, oxidative ketone scission does occur over γ-Al₂O₃ under aerobic environments. This indicates direct participation of molecular O₂ in the oxidative scission mechanism.

We additionally examined the role of lattice oxygen by performing analogous TPSR experiments over VO_x/γ-Al₂O₃ under both anaerobic and aerobic conditions. Results indicate that lattice oxygen from reducible VO_x can also initiate oxidative scission.

A final contrast between oxidative scission over γ-Al₂O₃ and VO_x/γ-Al₂O₃ is that, although we observe evidence of oxidative scission of 3-methyl-2-butanone on γ-Al₂O₃ under aerobic conditions, we only observe production of the acetone fragment. Co-production of acetic acid is only observed over VO_x/γ-Al₂O₃. These results suggests that molecular oxygen initiates the scission reaction to form acyl and carboxylate surface species; the former will desorb as acetone without requiring further reduction of the lattice, whereas the latter can only desorb upon reduction of a V-O-X bond. We therefore conclude that both molecular and lattice oxygen participate directly in the oxidative scission of methyl ketones, indicating a mechanism that combines both Langmuir-Rideal and Mars-van-Krevelen pathways.