GB801562A - Improvements in the production of benzene dicarboxylic acids - Google Patents

Abstract

Benzene dicarboxylic acids are made by treating the corresponding dialkylbenzenes in a known manner with oxygen or oxygen-containing gases at raised temperature in the presence of an oxidation catalyst soluble in the dialkylbenzene until the absorption of oxygen has ended, then treating the resulting crude oxidation product in an aqueous caustic alkali medium with oxygen or oxygen-containing gases while maintaining the alkaline reaction at raised temperature under pressure in the presence of an emulsifier, if desired in the presence of a water-soluble oxidation catalyst, until the absorption of oxygen is completed and liberating the free carboxylic acid from the resulting alkali-metal salt solution by acidification. Dialkylbenzenes specified are xylenes, diethylbenzenes, diisopropylbenzenes and cymene. The first oxidation step may be conducted at 145-150 DEG C. initially under atmospheric pressure in the presence of about 0.05-1.0 per cent by weight of a cobalt naphthenate or manganese stearate catalyst, the temperature being raised towards the end of the reaction. When the starting material is a xylene, the crude oxidation product is mainly a toluic acid, but with dialkylbenzenes which have two or more carbon atoms in a side chain diacetyl benzenes or acetylbenzoic acids are obtained. The oxidation product is comminuted and subjected to the second oxidation step in the presence of sufficient aqueous caustic alkali to ensure a permanent alkaline reaction, an alkyl amyl sulphonate emulsifier such as an alkyl benzene or alkyl naphthalene sulphonate in amounts of about 1 per cent by weight of the original dialkylbenzene, and 0.5 to 5 per cent by weight of the original dialkylbenzene of a water-soluble catalyst such as an alkali-metal salt of an oxyacid of the 5th and 6th groups of the periodic system, such as vanadic, molybdic, tungstic or chromic acid. The aqueous solution of alkali-metal salt of the benzene dicarboxylic acid obtained is purified by treatment with an oxidizing agent such as sodium hypochlorite in presence of an emulsifying agent. In examples: (1) p-diethylbenzene is oxidized with air in the presence of cobalt naphthenate to give initially diacetyl benzene and then acetylbenzoic acid which is comminuted, aqueous caustic soda, sodium bichromate and di-isobutyl-naphthalene sulphonate added thereto and treated with air under pressure to give terephthalic acid, purified as the sodium salt by oxidation with sodium hypochlorite and di-isobutyl-naphthalene sulphonate; (2) p-xylene is similarly oxidized to give terephthalic acid. Specifications 679,400 and 786,747 are referred to.