(205f) Direct Carbon-Carbon Coupling of Furanics with Acetic Acid over BrøNsted Zeolites

Torrefaction of biomass at mild thermal conditions can selectively decompose hemicellulose, which yields a blend of furanic and carboxylic acid species that are difficult to separate. The acidity introduced by acetic acid facilitates the polymerization of furanic species at room temperature, creating obvious storage and transportation challenges¹. One promising route to eliminate these acids was by converting them to ketones via decarboxylative ketonization over HZSM-5², but this results in the loss of carbon in the form of is CO₂. Here we report direct acylation of methylfuran with acetic acid as acylating agent over HZSM-5 zeolite to produce acetyl methyl furan with very high selectivity. A reaction mechanism is proposed involving the dehydration of acetic acid as rate-determining step based on a combination of temperature programmed desorption (TPD) studies, reaction kinetics, and Denisty Functional Theory (DFT) calculations. TPD experiments to generate surface acyl groups followed by pulses of methylfuran demonstrate the direct C-C coupling of the two species to form acetyl methyl furan. The apparent activation energy for acylation was estimated to be 24.5 kJ/mole over the temperature range of 220-250 °C at under steady state flow conditions while the reaction behaves as first order with respect to acetic acid. DFT calculations reveal the generation of acyl species requires a true activation energy barrier of 104 kJ/mole, with a corresponding first order an apparent barrier of 16 kJ/mole. The barrier for the formation of acetyl methyl furan is low due to its ability to stabilize the charge of the acyl group in the transition state, and thus the overall barrier is determined by the rate of acyl formation. The presence of water is inevitable in all the streams produced from biomass, and the introduction of water in the feed reavealed an inhibition on acylation reaction rate by an order of -0.7 with a positive effect on catalyst stability. This study illustrates a new path of C-C coupling to produce higher value chemicals from biomass without sacrificing carbon as CO₂in the process.

Reference

1. Stocker, M. Biofuels and Biomass-To-Liquid Fuels in the Biorefinery: Catalytic Conversion of Lignocellulosic Biomass using Porous Materials. Angewandte Chemie-International Edition 47, 9200-9211, (2008).
2. Gumidyala, A., Sooknoi, T. & Crossley, S. P. Title: Selective ketonization of acetic acid over HZSM-5: The importanceof acyl species and the influence of water. Journal of Catalysis, (Accepted).