Carboxylic acids are one of the major intermediates derived from pyrolysis or hydrolysis of biomass. Ketonization of carboxylic acids (Eqn. 1), catalyzed by metal oxides (e.g., TiO2), is an important reaction for biomass conversion, as it produces ketones which are building blocks to undergo further aldol condensation to expand the fraction of molecules for fuel/chemical production. R1COOH + R2COOH → R1COR2 + CO2 + H2O (R1, R2= alkyl group) (1) Water, either carried over from biomass feedstocks or used as reaction medium, always presents in the biomass conversion process. Although the mechanism of ketonization on metal oxides has been widely studied, the kinetic consequences of the water on the ketonization reaction have not been well understood. Previous work (Pham, et al. Top. Catal. 2014, 57, 706-714) showed that the presence of water decreased the activity of carboxylic acid ketonization on TiO2 catalysts and simply ascribed it to the competitive adsorption of water with carboxylic acid on Ti cation sites. However, in this work, by using acetic acid and propanoic acid as model compounds, our kinetic measurement shows that H2O not only decreases the rate but also increases the intrinsic activation barrier of carboxylic acid ketonization reaction on TiO2 (Figure 1a). By combining approaches of nuclear magnetic resonance spectroscopy (NMR), infrared spectroscopy (DRIFTS), and density function theory (DFT) calculations, we demonstrate that there are multiple species present on TiO2 surface upon carboxylic acid adsorption, including molecular carboxylic acid, monodentate carboxylate, and chelating/bridging bidentate carboxylates (Figure 1b). The presence of H2O increases the coverage of the less active bridging bidentate carboxylate, leading to the decrease of the ketonziation activity.

Effects of Water on Carboxylic Acid Ketonization over TiO2 Catalysts: An Insight into the Reaction Intermediates

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