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 degrees 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 O-2 consumption and product (including CID,) formation. Increasing the catalyst concentration at 300 degrees C significantly increased terephthalic acid yields. MnBr2 was the most active catalyst of those we assessed. Increasing the initial O-2 concentration beyond a modest excess did not significantly alter the terephthalic acid yield, but it increased the CO2 yield. Injecting supplemental O-2 midreaction, however, did cause the terephthalic acid yield to increase. The highest terephthalic acid yields (> 80%) occurred at 300 degrees C, [p-xylene](0) = 0.02 M, 10[O-2]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, COxs yields were below 2% and reaction intermediates were not detected.