High-temperature liquid water (HTW) as a reaction medium is attractive for organic chemical synthesis because water is inexpensive, abundant, non-toxic, renewable, environmentally benign, and has desirable chemical properties. A particularly suitable large scale industrial reaction system for a HTW process is p-xylene oxidation to terephthalic acid. We have reported that high terephthalic acid yield and very low byproduct yields are available in HTW around 300 ºC. Though the technical feasibility of a HTW process for terephthalic acid synthesis has been demonstrated, one barrier that accompanies the HTW process at present is the low p-xylene concentrations (below 0.02 mol/L) that have been used experimentally. Low p-xylene concentrations mean low production of terephthalic acid per unit reactor volume. In this work, we conducted synthesis at higher concentrations of p-xylene, 0.2 mol/L, which is 10 times concentration of our previous work. To work at higher concentrations, though, involves running the reaction as a two-phase aqueous suspension since higher concentrations exceed the solubility of p-xylene in HTW.

We report new information concerning the effect of MnBr₂ catalyst concentration and oxygen feed method on the partial oxidation of p-xylene in HTW at 300 ºC. We used a 440 mL Hastelloy batch reactor for all experiments. Pure oxygen gas (O₂) was the oxidant and was fed into the batch reactor by syringe pump. Increasing the catalyst concentration slightly increased the reaction rate of terephthalic acid formation. The difference of oxygen feed method such as feed speed and amount significantly affected the p-xylene conversion and the terephthalic acid yield. The highest terephthalic yields (>90%) occurred at [p-xylene] = 0.2 mol/L, [O₂] = 1.8 mol/L, [Br] = 0.056 mol/L, and t = 130 min. These results indicate the feasibility of HTW for terephthalic acid synthesis at higher concentrations.