GB770224A - Improvements in or relating to the preparation of terephthalic acid - Google Patents

Abstract

Terephthalic acid is prepared by oxidizing in the liquid phase p-diacetylbenzene or a mixture of p-diacetylbenzene with a minor proportion of one or more other substances, with oxygen or an oxygen-containing gas in the presence of a saturated aliphatic carboxylic acid containing from 2 to 8 carbon atoms per molecule and a catalyst comprising a compound of a metal capable of exhibiting variable valency characteristics. The diacetylbenzene may be prepared by oxidizing p-di-alkyl-benzenes such as p-diethyl-benzene, p-di-sec.-butylbenzene, or p-di-isopropylbenzene, with oxygen or an oxygen-containing gas in the presence of a catalyst comprising a compound of a metal capable of exhibiting variable valency characteristics. Both oxidations may be continuous. p-Diacetylbenzene is oxidized by passing the oxidizing gas such as oxygen or air into a solution consisting of p-diacetylbenzene dissolved in the ratio of from 10 : 1 to 1 : 20 by weight in a saturated aliphatic carboxylic acid such as acetic, propionic or butyric acid, and containing the catalyst which is a manganese compound such as manganese acetate in the proportion of from 0.1 to 5 per cent by weight of the p-diacetylbenzene. A promoter such as barium acetate may be present. The reaction is conducted at from 90 DEG to 180 DEG C. under atmospheric pressure or at from 120 DEG to 300 DEG C. under pressures from 10 to 45 kg/sq. cm. The terephthalic acid with some catalyst precipitates from the reaction mixture is filtered off and the filtrate containing p-diacetylbenzene and p-acetylbenzoic acid is recycled. The p-diacetylbenzene is prepared by passing the oxidizing gas such as oxygen or air into p-di-isopropylbenzene or its solution in a hydrocarbon such as tert.-butyl-benzene or in a halogenated hydrocarbon such as monochlorobenzene, in the presence of the catalyst which may be a cobalt compound such as cobalt naphthenate or cobalt acetate or a manganese compound, at atmospheric pressure between 90 DEG and 150 DEG C. or at superatmospheric pressure. Crude p-diacetylbenzene crystallizes from the reaction mixture on cooling, is filtered off and the filtrate containing p-di-isopropylbenzene and p-isopropylacetylbenzene may be recycled or large amounts of p-isopropylacetylbenzene may be separately oxidized to diacetylbenzene. The reaction products may also be separated by distillation. The crude p-diacetylbenzene which may contain catalyst may be purified before further oxidation by recrystallization or by complex formation with orthophosphoric acid. In a modified process, p-di-isopropylbenzene is oxidized to terephthalic acid in the two stages described but without isolation of the p-diacetylbenzene. Thus p-di-isopropylbenzene is oxidized as before but a saturated aliphatic carboxylic acid containing 2 to 8 carbon atoms per molecule is used as solvent, when the p-diacetylbenzene does not separate from the reaction mixture. The catalyst for the oxidation of p-diacetylbenzene is added and the mixture oxidized as described, the terephthalic acid precipitating from the mixture as before. In examples: (1) portions of p-di-isopropylbenzene are oxidized by air at various flow-rates and temperatures without a solvent and in the presence of cobalt naphthenate with stirring for various reaction times and the products separated by distillation to give p-diacetylbenzene, the p-isopropylacetylbenzene fractions being combined and oxidized with air and a cobalt naphthenate catalyst to p-diacetyl benzene, the combined p-diacetylbenzene products dissolved in propionic acid with manganese acetate and air passed through to give p-acetylbenzoic acid and terephthalic acid with some formaldehyde; (2) p-di-isopropylbenzene is oxidized with oxygen in the presence of cobalt naphthenate to give p-isopropylacetylbenzene and p - diacetylbenzene which is oxidized as in (1) above to p-acetylbenzoic acid and terephthalic acid; (3) p-diisopropylbenzene is dissolved in propionic acid and cobalt acetate added, oxygen is passed through until absorption has ceased when manganese acetate is added and oxygen passed through the mixture to give terephthalic acid. Specification 737,439 is referred to.