| Large-scale chemical processes to produce terephthalic acid via p-xylene partial oxidation typically use acetic acid as the reaction medium. We have conducted an in-depth investigation of terephthalic acid synthesis in an alternative, environmentally benign reaction medium, high-temperature water (HTW).;In this experimental study, we determined the influence of temperature, catalyst identity, supercritical water density, and p-xylene, oxygen, and catalyst initial concentrations on the molar yields of terephthalic acid and intermediate products. First, we conducted experiments in 1.54 mL stainless steel batch reactors with H2O2 as the oxidant source. MnBr2 was the most effective catalyst of those we assessed. The highest yield of terephthalic acid obtained in these experiments was 57 +/- 15% at 380°C after 7.5 min with MnBr2 as the catalyst.;We conducted further p-xylene oxidation experiments in a 440 mL Hastelloy autoclave batch reactor. We used air as the oxidant and monitored O2 consumption and CO and CO2 formation. The maximum terephthalic acid yield, which is the highest observed to date for p-xylene partial oxidation in high-temperature liquid water, was 94%. This yield occurred at 300°C, [p-xylene]0 = 0.02 M, [O2]0 = 0.10 M, [MnBr 2] = 0.007 M, and t = 6 min. The CO and CO2 yields at t = 6 min were 7% and 1%, respectively. We are the first to report COx yields from this reaction in HTW.;We also investigated the hydrothermal reactions of terephthalic acid and five additional aromatic carboxylic acids from 250°C to 410°C for times up to 240 min. Terephthalic acid was stable after one hour at 300°C, but decarboxylated to form benzoic acid in 10% yield at 350°C. An autocatalytic kinetics model provided a good description of terephthalic acid decomposition at 410°C.;Furthermore, we assessed the economics and environmental impact of an HTW-based terephthalic acid synthesis process. We determined that an HTW-based process operating at 300°C with air separation had a capital investment equivalent to the current process and lower energy and pollutant intensities. Therefore, HTW is a technically, economically, and environmentally promising solvent for terephthalic acid synthesis. |
