Purified Terephthalic Acid

Terephthalic acid is the organic compound with formula C6H4(COOH)2. This colourless solid is a commodity chemical, used principally as a precursor to the polyester PET, used to make clothing and plastic bottles. Several billion kilograms are produced annually. It is one of three isomeric phthalic acids.
Properties
Terephthalic acid is poorly soluble in water and alcohols, consequently up until around 1970 most crude terephthalic acid was converted to the dimethyl ester for purification. It sublimes when heated.
Production
Terephthalic acid is produced by oxidation of p-xylene by oxygen in air:
The oxidation is conducted using acetic acid as solvent and a catalyst composed of cobalt and manganese salts, using 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. The solution is then cooled in a stepwise manner to crystallize highly pure terephthalic acid.