A Spectroscopic Study of the Reaction Mechanism for Oxidative Scission of Methyl Ketones over Supported Vanadium Oxides

Aerobic oxidative scission of ketones occurs over supported vanadium oxides to produces a carboxylic acid fragment and a carbonyl fragment—an aldehyde or a ketone. In order to elucidate the mechanism of oxidative scission, particularly for interactions between adsorbed molecules and the Lewis acidic (V^x+ and Al³⁺) and basic (O^2-) surface sites, and build understanding of structure-function relations in catalytic oxidation, we consider the oxidative scission of 3-methyl-2-butanone (3M2B) over vanadium oxides supported on g-Al₂O₃. We interrogate surface interactions using in situ temperature programmed transmission FTIR and Diffuse Reflectance UV-Vis spectroscopies under both anaerobic (He) and aerobic (15% O₂ in He) conditions.

Transient in situ FTIR spectra obtained during surface saturation with 3M2B at 313K indicate that gas-phase ketones bind through their carbonyl oxygen at surface Lewis sites. As reaction temperatures increase from 313K to 593K, we observe the formation of enolate-like structures prior to the onset of C-C scission in the presence and absence of gas phase oxygen. In the aerobic condition, we observe low-temperature scission of the C-C bond, which forms bidentate adsorbed acetate. In contrast, under anaerobic environments, oxidative C-C scission is delayed to a higher temperature and the surface is saturated by adsorbed acetate species, which do not desorb as acetic acid; rather, they remain bound until the onset of combustion reactions above 533K. Interestingly, we observe perturbation to the vanadyl band (1015 cm^-1) at elevated temperatures with the onset of combustion. This suggests the vanadyl bond (V=O) does not participate directly in oxidative ketone scission, and that deep reduction of the vanadium center does not occur until elevated temperatures. This conclusion is supported by DR-UV-vis spectroscopy under aerobic reaction conditions from 323K to 473K, where we observe a clear increase in d-d transitions, corresponding to a partially reduced vanadium cation under steady state oxidative scission conditions.