GB747204A - Process for the production of terephthalic acid or derivatives thereof from phthalic acid - Google Patents

Abstract

Terephthalic acid and derivatives thereof are prepared from phthalic acid by a process wherein a potassium salt of phthalic acid is heated to a temperature above 340 DEG C. in the presence of an inert atmosphere and a potassium salt of terephthalic acid is obtained. Preferred starting materials are dipotassium phthalate or a mixture of phthalic anhydride and potassium carbonate. The upper limit of reaction temperature is 500 DEG C., above which excessive carbonization occurs. Oxygen is excluded from the process by carrying it out in an inert gas, preferably carbon dioxide, and generally at superatmospheric pressure. The reaction may be carried out in an autoclave, fitted with a stirrer or a removable insert divided by intermediate walls so that when charged the reaction mixture is in thin layers. Water is avoided as even trace quantities reduce the yield. The reactants may be mixed with inert substances such as sand, coke, carbon or metals which do not influence the reaction, or salts such as potassium carbonate, sulphate or chloride. Certain metal compounds, for example mercuric chloride, copper oxide, lead oxides, antimony oxide, barium oxide, chromium (IV) oxide and zirconium dioxide have a catalytic effect. Terephthalic acid is separated by dissolving the reaction product in water, precipitating the acids by the addition of a mineral acid, treating the precipitate with boiling water, filtering off the insoluble terephthalic acid and removing the last traces of phthalic acid as the anhydride by drying at 120 DEG C. Minor amounts of benzoic and trimesic acids are also formed. The terephthalic acid is converted into the corresponding acid chloride by treatment with phosphorus pentachloride and thence by treatment with methyl alcohol to the dimethyl ester. In example (1) dipotassium phthalate was heated at 350-354 DEG C. for 6 hours under a maximum pressure of carbon dioxide of 69 atmospheres, the terephthalic acid being recovered as described above. Numerous examples describe similar processes with, for example, the addition of potassium carbonate or using mono-potassium phthalate. Further features exemplified are, carrying out the process under normal pressure in a stream of carbon dioxide, under pressure of nitrogen or carbon monoxide, using potassium pyrophosphate as an inert salt, or carrying out the reaction in the presence of boric acid anhydride, sulphur, manganese, iron oxide, titanium dioxide, mercury, lead hydroxide, stannic oxide, arsenic trioxide or pentoxide, tungstic acid anhydride or iron scale.