TEREPHTHALIC ACID (HOOCC₆H₄COOH)

The catalyzed oxidation of p-xylene produces terephthalic acid (TPA). Cobalt acetate promoted with either NaBr or HBr is used as a catalyst in an acetic acid medium. Reaction conditions are approximately 200°C and 15 atmospheres. The yield is about 95%:

Special precautions must be taken so that the reaction does not stop at the p-toluic acid stage. One approach is to esterify toluic acid as it is formed with methanol. This facilitates the oxidation of the second methyl group. The resulting dimethyl terephthalate (DMT) may be hydrolyzed to terephthalic acid. Another approach is to use an easily oxidized substance such as acetaldehyde or methylethyl ketone, which, under the reaction conditions, forms a hydroperoxide. These will accelerate the oxidation of the second methyl group. The DMT process encompasses four major processing steps: oxidation, esterification, distillation, and crystallization.

Figure 1.1 shows a typical p-xylene oxidation process to produce terephthalic acid or dimethyl terephthalate.

Figure 1.1. A typical p-xylene to dimethyl terephthalate process.

The main use of TPA and DMT is to produce polyesters for synthetic fiber and film.