VAPOR PHASE UPGRADING OF RENEWABLE CARBOXYLIC ACIDS AND OXYGENATES OVER BRØNSTED ZEOLITES

Abstract

Ketonization of acetic acid is a powerful route to convert these problematic acids into stable building blocks to produce fuels and chemicals. This reaction is investigated over HZSM-5 via a combination of temperature programmed techniques (TP), IR, and reaction kinetics. TP experiments demonstrate the generation of stable acyl intermediates that require higher temperatures to facilitate C-C coupling with a second acetic acid molecule. Isotope labeling experiments reveal that the rate-determining step is the formation of the C-C bond rather than the activation of the second acid. A detailed Langmuir Hinshelwood kinetic modelling revealed the thermodynamic and kinetic parameters involved in the reaction. All the experimental data is verified with theoretical calculations using density functional theory. Though ketonization is an effective C-C coupling technique the inevitable production of CO2 during the reaction reduces carbon yield and increase greenhouse gas emissions. This study report the direct acylation of methylfuran with acetic acid in the presence of water, all of which can be readily produced from biomass. This direct coupling limits unwanted polymerization of furanics while producing acetyl methyl furan. Reaction kinetics and density functional theory calculations illustrate that the dehydration of the acid to form surface acyl species is rate limiting. We show that furanic species effectively stabilize the charge of the transition state, therefore lowering the overall activation barrier. These results demonstrate a promising new route to C-C bond forming reactions to produce higher value products from biomass.