Kinetics and Mechanism of Acetic Acid Esterification with Ethanol On Zeolites

Esterification can be used as a mode of biomass deoxygenation and has been studied here using acetic acid and ethanol as biomass surrogates. Steady state, transient, and isotopic-exchange reaction studies of acetic acid and ethanol were used to identify co-adsorbed ethanol–acetic acid dimers as surface intermediates within specific elementary steps involved in the esterification of acetic acid with ethanol on zeolites. A reaction mechanism involving two dominating surface species, an inactive ethanol dimeric species adsorbed on Brønsted sites inhibiting ester formation and a co-adsorbed complex of ethanol and acetic acid on the active site reacting to produce ethyl acetate, was shown to describe the reaction rate as a function of temperature, acetic acid partial pressure, and ethanol partial pressure. Measured differences in rates as a function of zeolite structure and the rigorous interpretation of these differences in terms of esterification rate and equilibrium adsorption constants will be discussed.