Subcritical aqueous-phase oxidation kinetics of acrylic, maleic, fumaric, and muconic acids

Unsaturated carboxylic acids such as acrylic, maleic, fumaric, and muconic acids have been observed as intermediate products in wet oxidation reaction s of phenols. Oxidation kinetics of these compounds dissolved in water were studied in a titanium autoclave at a temperature range of 180 and 280 degr ees C and oxygen partial pressures between 10 and 55 bar. The acid-decay da ta are well-represented by a three-half power-law kinetics for all acids. T he disappearance rate of acrylic and fumaric acids is proportional to a squ are 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 neglect ed at temperatures employed, but it was found highly significant for maleic , fumaric, and muconic acids. These three acids completely decomposed at 28 0 degrees C with formic acid as a major intermediate product, whereas aceti c. acid was formed only in the presence of oxygen. The conversion of acryli c acid into CO2 and other acid intermediates at 290 degrees C was found to be about 72%. The lumped TOC (total organic carbon) concentration in soluti ons reduced by a first-order process for all acids investigated, but the or der with respect to oxygen was between 0.3 and 1.0. The activation energy f or the thermal decomposition of maleic and fumaric acids was 56.6 and 71 kJ /mol, respectively, whereas for the total decomposition (oxidative and ther mal) of acrylic, maleic, and fumaric acids it was 94.3, 99.2, and 83.6 kJ/m ol, respectively. The data for TOC reduction of acrylic and muconic acids g ave 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, respectiv ely.