(602b) Methyl Ketone Oxidative Scission over ?-Al2O3 Supported Vanadium Oxides: An Interpretation of Roles of Lattice Oxygen and Molecular Oxygen from Spectroscopic Insight

Both lattice and gas phase oxygen play important roles in facilitating the oxidative cleavage of methyl ketones on vanadium oxide surfaces; however, their mechanistic contributions remain unclear. To provide insight into the contributions of each we employ an online mass spectroscopic analysis of reaction products during Temperature Programmed Surface Reactions (TPSR) and transient analysis of products. Specifically, we examine trends in the oxidative scission of 3-methyl-2-butanone (3M2B) during TPSR experiments over γ-Al₂O₃ and VO_X/γ-Al₂O₃ under anaerobic (He) and aerobic (15% O₂/He) conditions, and we consider the approach to steady state in a packed bed reactor.

Oxidative scission does not occur over γ-Al₂O₃ under He, which is consistent with its non-reducible lattice; however, we do observe oxidative scission products over γ-Al₂O₃ under aerobic environments. This indicates that dioxygen can facilitate ketone oxidation through Eley-Rideal or Langmuir-Hinshelwood type pathways. In contrast, oxidative scission is observed over VO_X/γ-Al₂O₃ under both anaerobic and aerobic environments, which indicates that lattice oxygen of VO_X can facilitate the oxidative scission of 3M2B. During transient kinetic experiments over γ-Al₂O₃ and VO_X/γ-Al₂O₃ under aerobic conditions, we observe that both materials produce the oxidative scission product acetone; however, its stoichiometric co-product acetic acid is only observed over VO_X/γ-Al₂O₃. Further, γ-Al₂O₃ becomes inactive for oxidative scission after completing roughly 1 turnover, whereas VOx/γ-Al₂O₃ approaches a steady state turnover rate after roughly 60 minutes on stream. FTIR spectra obtained under identical conditions reveal that surfaces are dominated by carboxylates; consequently, we conclude that strong binding of acetate fragments on γ-Al₂O₃ render it inactive, whereas lattice reducibility at VO_X centers facilitates acetate desorption, resulting in an active material. Since both molecular and lattice oxygen plays significant roles in the oxidative scission of 3-methyl-2-butanone, we conclude that the oxidative scission of ketones on vanadium oxide surfaces has elements of Eley-Rideal and Mars-van Krevelen pathways.