GB762644A - Process for the production of terephthalic acid and derivatives thereof - Google Patents

Abstract

Terephthalic acid is obtained from its alkali-metal salt prepared by heating an alkali-metal salt of isophthalic acid at temperatures between 340 DEG C. and the decomposition temperature of the salt. Alkali-metal salts specified are dipotassium isophthalate and the sodium, rubidium and lithium isophthalates. Catalysts such as mercury or mercury compounds, or oxides of zirconium, titanium, arsenic, tin, iron, lead, or zinc may be used. In place of pure alkali-metal isophthalates as starting-materials, reaction mixtures yielding potassium isophthalate, mixtures of isophthalates with potassium salts of other organic acids such as mixtures with potassium phthalate obtained by the oxidation of xylene mixtures, or mixtures of alkali-metal isophthalates with inert substances such as sand, coke, powdered metals and metal turnings and fragments or inert salts such as potassium carbonate, sulphate or chloride may be used. The reaction may be conducted in the absence of oxygen and water under a carbon dioxide atmosphere under atmospheric or increased pressure and may be effected by heating with agitation or by heating thin layers of the starting material in narrow reaction chambers formed by division of the reaction space by metal partitions between the top and bottom and/or between the sides. Terephthalic acid is obtained by acidification of an aqueous solution of the crude alkali metal terephthalate formed in the heating step, extraction of the filtered precipitate with boiling water and reprecipitation of the residue from alkali. Benzoic and trimesic acids are obtained as by-products. Terephthalyl chloride and dimethyl terephthalate may be prepared by known methods from the product of the heating step. In examples, dipotassium isophthalate is heated under CO2 (1) at various temperatures and pressures without a catalyst, (2) under pressure in the presence of iron oxide, zirconium dioxide, stannous oxide, titanium dioxide, arsenic oxide, stannic oxide and litharge, (3) at atmospheric pressure in the presence of a mixture of iron oxide and litharge; a mixture of dipotassium terephthalate and dipotassium phthalate is heated at atmospheric pressure under CO2 in the presence of a mixture of iron and lead oxides. Specification 747,204 is referred to.