Hydrothermal stability of aromatic carboxylic acids

The chemical behavior of aromatic carboxylic acids in water at elevated temperatures has implications for industrial chemistry, fuel processing and waste treatment processes. Therefore, the hydrothermal reactions of six different aromatic carboxylic acids were investigated from 250–410 °C at times up to 240 min. Benzoic acid was the most stable of the six, showing negligible degradation after 1 h of hydrothermal treatment at 350 °C. Terephthalic acid, 2,6-naphthalene dicarboxylic acid and isophthalic acid were stable after 1 h at 300 °C, but they decarboxylated to form monoacids in 10–15% yields at 350 °C. Trimellitic anhydride decomposed completely after 30 min at 350 °C, but showed no appreciable decomposition after 30 min at 250 °C. Terephthalic acid and isophthalic acid were the main degradation products, but o-phthalic acid was also formed in small amounts at 350 °C. The o-phthalic acid conversion to benzoic acid was 73% after 60 min at 300 °C, but the diacid remained stable at 250 °C for 1 h. An autocatalytic kinetics model provided a good description of the trimellitic anhydride and terephthalic acid decomposition at 300 and 410 °C, respectively.