(114c) Mechanism of Ketonization of Acetic Acid Over Reducible Oxides and Zeolites

The conversion of acetic acid to acetone is a reaction of primary importance to the upgrading of bio oil and bio oil fractions. The conversion of corrosive acetic acid improves stability of the bio oil, while acetone can be utilized as a chemical building block for subsequent aldol condensation to liquid fuel range products.  In this contribution, we discuss the mechanism of vapor phase ketonization over three different catalysts, TiO₂, Ru/TiO₂, and HZSM-5. A detailed Langmuir-Hinshelwood kinetic model was fit to the data over TiO₂ and Ru/TiO₂. Results indicate a second order rate dependence of acetic acid on the ketonization mechanism, as well as a strong inhibition of acetone on the reaction. Water and CO₂ also inhibit the ketonization, but to a lesser extent than acetone. For example, the adsorption constants of Water, CO₂, and Acetone at 280°C were found to be 0.029 and 0.016, and 0.403 Torr^-1, respectively. The activation entropies will be discussed as well to support the proposed reaction mechanism. The role of a β-keto-acid intermediate as well as the possible importance of α-hydrogen in an enolization step will be discussed.

Ketonization is also catalyzed by zeolites, such as HZSM-5, although the mechanism is not well understood partially due to the complications arising from subsequent aldol condensation reactions and excessive cracking at high temperatures. Conditions have been identified that favor high selectivity to ketones, and isotope labeling experiments have been conducted to propose the ketonization mechanism that occurs over zeolites and identify the importance of the α-hydrogen. Finally, the mechanism of ketonization over HZSM-5 will be contrasted with the mechanism over reduced TiO₂.