GB822516A - Process for the preparation of aromatic di- and poly-carboxylic acids - Google Patents

Abstract

Alkali and alkaline-earth metal salts of monocarboxylic acids, in which the carboxyl groups are attached to aromatic ring systems or to heterocyclic ring systems of aromatic structure, are converted into salts of di- or poly-carbooxylic acids of aromatic structure by heating, to a temperature above 300 DEG C. and at a pressure less than 400 atm., the salt of the monocarboxylic acid in the presence of a salt of oxalic acid. The reaction is advantageously carried out in the presence of carbon dioxide or an inert gas, and with addition of catalytically active oxides or salts of metals, especially of cadmium. Particularly good results are obtained if the catalyst is present in a very finely-divided state. This may be achieved by dissolving the catalyst in a potassium cyanide melt and adding the melt, if desired after cooling and pulverization, to the charge. In Example 1, a mixture of potassium benzoate, potassium oxalate and cadmium fluoride is intimately ground-up in a ball mill, and then charged to a rotating autoclave. The autoclave is then charged with carbon dioxide at 50 atm., and heated for one hour at 405 DEG C. The maximum pressure during the heating amounts to 180 atm. The reaction product is dissolved in water; and the solution is filtered. The filtrate is acidified with hydrochloric acid, whereupon terephthalic acid is precipitated. Examples 3 and 4 describe the isolation of orthophthalic acid from the product of heating a mixture of sodium benzoate, sodium oxalate, and cadmium fluoride. The product is suspended in water, the oxalate is destroyed with permanganate, and the solution is extracted with ether. The ether-extract is evaporated; and the residue is separated into orthophthalic acid and benzoic acid by digestion with chloroform. In Example 7, a mixture of potassium benzoate, potassium oxalate and a melt prepared from potassium cyanate and zinc carbonate is intimately ground, and heated in a rotating autoclave for one hour at 430 DEG C. under a pressure of 152 atm. of purified nitrogen. The product is dissolved in water, and the solution is acidified to precipitate terephthalic acid. Examples 8 and 9 describe the preparation of terephthalic acid by heating potassium benzoate mixed with potassium oxalate and calcium oxalate, or zinc oxalate, respectively. In Example 10, a mixture of the potassium salt of nicotinic acid, potassium oxalate and calcium chloride is heated at 350 DEG C. in a carbon dioxide atmosphere for 10 hours; and pyridine-2,5-dicarboxylic acid is isolated from the product. Example 12 describes the isolation of trimesic acid and orthophthalic acid from the product obtained by heating, at 440 DEG C., for 5 hours, a mixture of sodium benzoate, sodium oxalate and cadmium fluoride. Other starting materials referred to are salts of a - and b -naphthoic acids, and diphenylmonocarboxylic acids; and also salts of monocarboxylic acids in which the carboxyl group is attached to the ring system of anthracene, terphenyl, diphenylmethane, benzophenone, quinoline, isoquinoline, a -pyran, furan, thiophene, thiazole, indole, benzthiazole or benzimidazole. Instead of the salts of the monocarboxylic acids, reaction mixtures can also be used which give rise to such salts. For example, the acid anhydrides or esters may be heated with an alkali metal carbonate.