(589g) Insights into the Mechanism of Acetic Acid Ketonization over Zeolites

The corrosive nature of acetic acid leads to storage and transport limitations of biomass pyrolysis oils while its acidity catalyzes the polymerization of other species present in the pyrolysis oil to undesired products. The conversion of acetic acid to the more stable chemical building block, acetone, is of significant interest. The vast majority of mechanistic studies concerning this reaction occur over reducible oxides and basic catalysts. This reaction can readily occur with high selectivity over protonated zeolites as well, but the details of the mechanism are unclear. In this contribution we use a combination of reaction kinetics coupled TPD experiments and in-situ FTIR to separate the role of surface acyl and anhydride species from adsorbed carboxylates on the ketonization reaction over HZSM-5. In addition, a series of reactants with various degrees of substitution and the number of alpha hydrogens are used both pure and co-fed to demonstrate the critical role of the alpha hydrogen for this reaction. We propose a mechanism involving the coupling of an enolate with a dehydrated surface intermediate over protonated zeolites based on these exerimental results. Because most feedstocks that contain acetic acid also contain water, a kinetic study was conducted to identify the rate determining step and the inhibiting role of water on this reaction.