Fundamental Understating of Aqueous-Phase Acetic Acid Ketonization over Zirconia

Catalytic upgrading of bio-oils is an important step in the production of transportation fuels and valuable chemicals from lignocellulosic biomass resources. The biocrude generated by hydrothermal  liquefaction (HTL) generally contain a large amount of oxygenates such as phenolic compounds, aldehydes, ketones, and alcohols, especially a substantial concentration of carboxylic acids. Among various catalytic upgrading routes, ketonization of carboxylic acids provides a very efficient method for removing the oxygen content and increasing the H/C ratio simultaneously. Up to now, although ketonization of carboxylic acids has been widely investigated over a series of metal oxide catalysts in vapor phase, very little is known about the ketonization reactivity and reaction mechanisms of these metal oxide catalysts in aqueous condensed phase. Fundamental understanding of the condensed aqueous phase effects on the ketonization is essential for developing efficient catalysts for aqueous phase bio-oil conversion.  In the present work, ab initio based molecular dynamics (AIMD) simulations and density function theory (DFT) calculations were performed to study aqueous phase ketonization of acetic acid over zirconia catalysts. We found that the condensed aqueous environment only slightly affects the stabilities of reaction intermediates and transition states along the reaction pathways. However, a new ketonization route via the hydrolysis of surface zirconia acetate phase formed in the presence of liquid water will be the major reaction pathway.