Catalytic coupling of carboxylic acids by ketonization as a processing step in biomass conversion

Abstract

Carboxylic acids, common intermediate products in biomass conversion processes, can be converted into ketones via ketonization reactions over a ceria–zirconia catalyst. Reaction kinetics studies were carried out using hexanoic acid, as a representative carboxylic acid, in the presence of 1-pentanol and 2-butanone, as representative biomass-derived alcohol and ketone species. Studies were carried out at temperatures from 448 to 623 K, and employing a range of hexanoic acid partial pressures from 0.05 to 0.3 atm. Two different reactions were observed to take place at these reaction conditions: esterification and ketonization, both consuming hexanoic acid. Product inhibition by water and carbon dioxide was observed and studied by co-feeding these components to the reactor. Hexanoic acid adsorption on the catalyst surface is an important step in the reaction, and the rate of ketonization shifts from second order to zero order as the partial pressure of acid increases. The measured activation energy for the ketonization of hexanoic acid (132 kJ/mol) is higher than the esterification reaction (40 kJ/mol), such that the irreversible ketonization reaction is favored at higher temperatures (>573 K) compared to the reversible esterification reaction. Direct ketonization of esters does not take place in the presence of acids, and instead takes place by hydrolysis with water followed by ketonization of the corresponding acid. The results of this study can be described by a simple kinetic model including site blocking by adsorbed hexanoic acid, carbon dioxide, and water.