Ketonization of acetic acid vapour over polycrystalline magnesia: in situ Fourier transform infrared spectroscopy and kinetic studies

Ketonization of acetic acid vapour over polycrystalline MgO was examined both kinetically and spectroscopically from room temperature to 480 °C. The kinetic study was performed with the use of a flow-reactor and gas chromatography, whereby the reaction rate and the catalyst selectivity were determined. The spectroscopic study was carried out by means of in situ infrared (IR) absorption measurement. Nature and products of adsorptive and absorptive interactions with the catalyst not only of the acid, but also of products of the reaction (acetone and CO2) were identified by in situ IR spectroscopy, and quantified by thermogravimetric analysis. Moreover, adsorption and surface reactions of methylbutynol molecules were observed by IR spectroscopy and utilized to probe specifically the availability on MgO surfaces of strong, reactive base (O2−) sites. The results could help to reveal the strong catalyst tendency towards absorption of the acid and CO2 molecules, and consequent formation of magnesium acetate and carbonate bulk phases, respectively. This unexpected behaviour of MgO, which enjoys high lattice (MgO bond) energy, was considered to be driven by the strong catalyst basicity (ionicity). Accordingly, the ketonization of acetic acid over MgO was found to occur via overlapping catalytic and pyrolytic routes. Reactive species and reaction pathways involved in each route were proposed.