High-temperature liquid water: A viable medium for terephthalic acid synthesis

We report new information concerning the effect of oxygen concentration and catalyst concentration and identity [MnBr₂, CoBr₂, ZrBr₄, and Mn(OAc)₂] on the partial oxidation of p-xylene in dense water from 250 to 380 °C. Water is a more environmentally benign replacement for the acetic acid solvent used commercially. We used a 440 mL Hastelloy batch reactor for all experiments and monitored O₂ consumption and product (including CO_x) formation. Increasing the catalyst concentration at 300 °C significantly increased terephthalic acid yields. MnBr₂ was the most active catalyst of those we assessed. Increasing the initial O₂ concentration beyond a modest excess did not significantly alter the terephthalic acid yield, but it increased the CO₂ yield. Injecting supplemental O₂ midreaction, however, did cause the terephthalic acid yield to increase. The highest terephthalic acid yields (>80%) occurred at 300 °C, [p-xylene]₀ = 0.02 M, [O₂]₀ = 0.10 M, [Br] = 0.014 M, and t = 5-15 min. These yields are the highest reported to date from this reaction in high-temperature liquid water. Moreover, under these conditions and t = 15 min, CO_x yields were below 2% and reaction intermediates were not detected.