Preparation of esters of aromatic acids

An alkyl ester of an aromatic carboxylic acid is obtained by introducing a gas containing molecular oxygen into a reaction zone containing a liquid mixture of an alkyl aromatic hydrocarbon and an alkyl ester of an aliphatic carboxylic acid, together with an oxidation catalyst dissolved in the mixture, maintaining the reaction zone at 150-300 DEG C. and under a pressure sufficient to maintain at least 1.25 mole equivalents of the alkyl ester per alkyl group of the alkyl aromatic hydrocarbon in the liquid phase, and withdrawing the oxidised mixture from the reaction zone. The process is particularly applicable to the production of dimethyl terephthalate from p-xylene. Toluene and durene are examples of other alkyl aromatic hydrocarbons which may be used, and methyl acetate, formate, propionate, 3.5.6- trichloroperfluoro-hexanoate, and perfluoroacetate, and dimethyl oxalate and perfluoroglutarate, are examples of alkyl esters. Ethyl, propyl, or butyl esters may also be used. Some free carboxylic acid may be present and in the case of a perhalogenated acid it may serve as catalyst or catalyst promoter of the oxidation. Oxides, hydroxides, inorganic and organic acid salts of cobalt, lead, manganese, chromium, iron, cerium, and vanadium may be used as oxidation catalyst. In an example, p-xylene is oxidised with air in an autoclave in the presence of cobalt naphthenate and in the presence of methyl acetate. The reaction product contains dimethyl terephthalate, methyl hydrogen terephthalate, terephthalic acid, methyl p-toluate, and p-toluic acid. Unreacted and partially reacted material may be returned to the reaction vessel. Free aliphatic acid is preferably re-esterified before returning the residual liquor to the reaction zone.