Patent GB-776274-A (GB)
Title: Process for the preparation of terephthalic acid
Filed: 1954-03-11
Abstract: A process for preparing terephthalic acid comprises reacting in the liquid phase at an elevated temperature para-diisopropylbenzene with molecular oxygen in the presence of 0.1 to 10 per cent by weight of a heavy metal compound as catalyst based on the weight of the reaction mixture and benzoic acid, the ratio of the benzoic acid being in the range of 1 to 8 mols. per mol. of para-di-isopropylbenzene.  The process may be carried out in the presence of water or an aliphatic monocarboxylic acid having from 1 to 6 carbon atoms in the molecule, e.g. acetic or caproic acid, or in the presence of both.  The use of any other monocarboxylic acids liquid at room temperature, stable in the reaction system and preferably free from hydrogen atoms attached to tertiary carbon atoms or mixtures thereof together with the benzoic acid is mentioned.  Elevated pressure may be employed e.g. to maintain a liquid phase of the lower aliphatic acid or to maintain a water content of at least about 10 per cent of saturation or both.  A desired water concentration may be maintained by condensing water-vapour removed and returning it to the reactor.  The para-di-isopropyl benzene employed as starting material may be pure or may contain some meta-di-isopropylbenzene, lower or higher alkylated benzenes and saturated aliphatic hydrocarbon material relatively resistant to oxidation in the system.  Pure oxygen or mixtures of gases, e.g. air, may be employed in the process.  In examples: an agitated mixture of para-di-isopropylbenzene, benzoic acid and manganese cumate is treated with air at elevated temperature, crude terephthalic acid is filtered off and washed with acetic acid, the acetic acid washings may be distilled, and the residue processed to recover isophthalic acid or mixed with the filtrate and condensate from the reactor exit gases and distilled and the distillates recycled to the reactor; cobalt cumate is employed as catalyst in a similar process (1); a similar process is carried out in the presence of water and at elevated pressure (2); a process similar to (1) is carried out in the presence of caproic acids using manganese and cobalt cumates as catalysts (3).  The crude terephthalic acid may be converted to the dimethyl ester by direct esterification in the presence of an acid, e.g. hydrochloric, phosphoric or sulphuric acid or it may be converted to the acid chloride by reaction with thionyl chloride and reacted with methanol. Crude oxidation mixtures may be washed with glacial acetic acid to separate isophthalic acid. Additional catalysts mentioned include manganese carboxylates, e.g. of monocarboxylic acids having from 2 to 10 carbon atoms, lead and silver cumates, cobalt octanoate, zinc acetate, mercuric oxide and vanadium oxide.  Metal carboxylate catalysts are prepared by dissolving an appropriate organic acid in caustic soda and adding an aqueous solution of the appropriate metal acetate; manganese cumate is so prepared and when dried in air and then over calcium chloride at low pressure is somewhat soluble in para-di-isopropylbenzene.  Specifications 737,439, 762,129, 762,849 and 774,902 are referred to.