Terephthlic Acid Synthesis in Supercritical Water

We have demonstrated the feasibility of terephthalic acid synthesis from p-xylene in super-critical water at 380°C and explored the effects of key process variables. Reactions were carried out batchwise and isothermally in 1.54 mL stainless steel vessels. Hydrogen peroxide served as the oxidant and manganese bromide as the catalyst. We determined the effects of batch holding time, water density, and catalyst, oxidant, and p-xylene initial concentrations on the yield of terephthalic acid and the product distribution. The highest yield of terephthalic acid obtained was 57 ± ·15% at a water density of 400 kg m^-3, a batch holding time of 7.5 min, and initial concentrations of p-xylene, catalyst, and oxidant of 0.07 M, 7.7 × 10^-3 M, and 0.58 M, respectively. At reaction times longer than 10 minutes, terephthalic acid yields decreased due to decarboxylation of terephthalic acid. Water density had little effect on the yield of terephthalic acid. When catalyst and oxidant were present in low levels, increasing their molar equivalents augmented the terephthalic acid yield. There was little additional increase in the terephthalic acid yield, however, after the levels of catalyst and oxidant reached 0.1 and 8 equivalents, respectively. The experimental results provided some insight into the reaction mechanism.