Subcritical Aqueous-Phase Oxidation Kinetics of Acrylic, Maleic, Fumaric, and Muconic Acids

Abstract

Unsaturated carboxylic acids such as acrylic, maleic, fumaric, and muconic acids have been observed as intermediate products in wet oxidation reactions of phenols. Oxidation kinetics of these compounds dissolved in water were studied in a titanium autoclave at a temperature range of 180 and 280 °C and oxygen partial pressures between 10 and 55 bar. The acid-decay data are well-represented by a three-half power-law kinetics for all acids. The disappearance rate of acrylic and fumaric acids is proportional to a square root of the oxygen partial pressure; for maleic acid the order takes a very low value (0.12). Thermal decomposition of acrylic acid can be neglected at temperatures employed, but it was found highly significant for maleic, fumaric, and muconic acids. These three acids completely decomposed at 280 °C with formic acid as a major intermediate product, whereas acetic acid was formed only in the presence of oxygen. The conversion of acrylic acid into CO2 and other acid intermediates at 290 °C was found to be about 72%. The lumped TOC (total organic carbon) concentration in solutions reduced by a first-order process for all acids investigated, but the order with respect to oxygen was between 0.3 and 1.0. The activation energy for the thermal decomposition of maleic and fumaric acids was 56.6 and 71 kJ/mol, respectively, whereas for the total decomposition (oxidative and thermal) of acrylic, maleic, and fumaric acids it was 94.3, 99.2, and 83.6 kJ/mol, respectively. The data for TOC reduction of acrylic and muconic acids gave the activation energy of 102 and 87.4 kJ/mol, respectively, whereas for maleic and fumaric acids its values were found to be 104.7 and 84.2 kJ/mol for the first step and 83.4 and 54.4 kJ/mol for the second step, respectively.