GB811899A - Process for the preparation of terephthalic acid - Google Patents

Abstract

Terephthalic acid is prepared by heating a salt of another mononuclear aromatic carboxylic acid consisting principally of the potassium salt, dissolving the reaction product in water or a water-containing solvent to give a saturated solution of terephthalate, which is then treated with sodium carbonate, sodium hydroxide or a sodium salt of the carboxylic acid which is being re-arranged to precipitate sodium terephthalate. The sodium terephthalate is then separated and the remaining solution evaporated, if necessary after the addition of fresh carboxylic acid to be rearranged so as to obtain a residue containing the potassium salt of said acid, which is recycled to the heat treatment process. The terephthalic acid is liberated from the separated sodium terephthalate by acidifying with acids stronger than terephthalic. The potassium salts of the following acids are suitable as starting materials, benzoic, phthalic, isophthalic, hemimellitic, trimellitic, trimesic, mellophanic, prehnitic, pyromellitic and mellitic, or mixtures thereof. The heat treatment may be carried out by heating the salts at 350-450 DEG C. in the presence of carbon dioxide or carbon dioxide-nitrogen mixtures, in some cases under pressure and preferably in the presence of 0.1 to 10 per cent by weight of, e.g. oxides, carbonates, halides or organic salts of zinc cadmium or divalent iron, especially the salts of those carboxylic acids serving as starting materials. A suitable water-containing solvent in which to obtain the saturated solution of the potassium terephthalate is methanol. This operation is preferably carried out at ordinary or increased pressure in the hot, e.g. at boiling temperature. Alternatively the saturated solution may be prepared by making a dilute solution and then concentrating it to saturation point. The saturated solution may then be freed from undissolved residues before addition of the sodium salt or mixture of such salts, preferably in the hot in an amount such that the content of the sodium ions is at least equal to that of the potassium ions present or a little more, e.g. 20 per cent over the calculated amount. The precipitate of disodium terephthalate is separated, e.g. by filtering or centrifuging, preferably in stages, e.g. by filtering the hot mixture, whilst keeping it at the operating temperature, allowing the filtrate to cool, and filtering off the disodium terephthalate which separates out, and the remaining filtrate is evaporated and recycled. Examples are given.