(435d) Kinetics and Mechanism of Ketonization of Acetic Acid on HZSM-5 Catalyst

Acetic acid is the most abundant and problematic compound among the light oxygenates resulting from the pyrolysis of biomass^1-2. The acidity imparted to biomass streams from acetic acid promotes polymerization of other species that result in humin formation and subsequent catalyst deactivation in subsequent upgrading attempts. The common upgrading strategy of hydrotreating converts these light acids to low value light gases. The selective ketonization eliminates this acidity while forming acetone, which is a building block to produce longer-chain hydrocarbons via aldol condensation/hydrogenation^3-4. In this contribution we report conditions necessary to selectively convert acetic acid to acetone over HZSM5 in the gas phase at a wide variety of temperatures ranging from 250-320°C. The reaction is investigated via a combination of temperature programmed desorption (TPD) techniques, IR of surface adsorbates, detailed reaction kinetics and Density Functional Theory (DFT) calculations. A reaction mechanism is proposed involving the formation of surface acyl species followed by coupling with an activated acid with an apparent barrier of 67 kJ/mole. TPD of acetic acid indicates an initial dehydration step in the reaction pathway, which is further confirmed by IR analysis of the catalyst surface. Isotope labeling experiments reveal the absence of a primary kinetic isotope effect involving hydrogen,⁵ which when combined with the experimentally observed 2^nd order Langmuir Hinshelwood rate behavior demonstrates that C-C coupling of acyl species with an activated carboxylic acid is the rate-determining step. DFT calculations reveal that acetic acid first adsorbs on HZSM-5 by forming H-bonds with Brønsted acid sites, which subsequently dehydrate to form acyl species. A second carboxylic acid is then activated via a tautomerization step followed by coupling with the adsorbed acyl species, which is the rate limiting transition state. Finally a detailed single site Langmuir-Hinshelwood model is presented in agreement with experimental and theoretical results. The influence of water on catalyst deactivation rates, competition for active sites, and surface intermediates is also discussed.

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