Production
Terephthalic acid is produced by oxidation of p-xylene by oxygen in air:
This reaction proceeds through a p-toluic acid intermediate which is then oxidized to terephthalic acid. In p-toluic acid, deactivation of the methyl by the electron withdrawing carboxylic acid group makes the methyl one tenth as reactive as xylene itself, making the second oxidation significantly more difficult. The commercial process utilizes acetic acid as solvent and a catalyst composed of cobalt and manganese salts, with a bromide promoter. The yield is nearly quantitative. The most problematic impurity is 4-formylbenzoic acid, which is removed by hydrogenation of a hot aqueous solution. This solution is then cooled in a stepwise manner to crystallize highly pure terephthalic acid.
Despite optimized yields greater than 95% with excellent purity, the synthesis has proven to have shortcomings. Due to high reaction temperature, approximately 5% of the acetic acid solvent is lost by decomposition or ‘burning’. Solvent burning is a significant economic factor in the oxidation process. In addition, product loss by decarboxylation to benzoic acid is common. The high temperature diminishes oxygen solubility in an already oxygen starved system. Pure oxygen cannot be used in the traditional system due to hazards of flammable organic-O2 mixtures. Atmospheric air can be used in its place, but once reacted needs to be purified of toxins and ozone depleters such as methylbromide before being released. Additionally, the corrosive nature of bromides at high temperatures requires the reaction be run in expensive titanium reactors.
Alternatively, but not commercially significant, is the so-called "Henkel process" or "Raecke process", named after the company and patent holder, respectively. This process involves the rearrangement of phthalic acid to terephthalic acid via the corresponding potassium salts. Terephthalic acid can be prepared in the laboratory by oxidizing various para-disubstituted derivatives of benzene, including caraway oil or a mixture of cymene and cuminol with chromic acid.
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