Patent GB-816691-A (GB)
Title: A process for working up alkali metal terephthalates
IPC: C07C51/41
Filed: 1957-01-03
Abstract: Terephthalic acid is obtained from dialkali metal terephthalates which have been prepared by thermal rearrangement of salts of benzene carboxylic acids, by reacting the dialkali metal terephthalate in solution or suspension with a benzene carboxylic acid, separating from the reaction product a residue containing substantially monoalkali metal terephthalate and a filtrate containing substantially salts of the benzene carboxylic acid, reacting the residue with further benzene carboxylic acid in solution or suspension and so obtaining terephthalic acid and returning the acid salts of the benzene carboxylic acid to the treatment of dialkali metal terephthalates and the substantially neutral salts of the benzene carboxylic acid to the initial thermal rearrangement. Terephthalic acid is similarly obtained by treating the dialkali metal terephthalate in solution or suspension with an acid salt of a benzene carboxylic acid, separating from the reaction product a residue containing substantially mono-alkali metal terephthalate and a filtrate containing a substantially neutral salt of the benzene carboxylic acid, reacting the residue in solution or suspension with a benzene carboxylic acid and so obtaining terephthalic acid and acid salts of the benzene carboxylic acid and returning the latter portion to the initial thermal rearrangement reaction and the acid salts formed in the reaction of the residue with the benzene carboxylic acid to the initial treatment of the dialkali metal terephthalates. By benzene carboxylic acid is meant any benzene carboxylic acid other than terephthalic acid which can be thermally rearranged in the form of its salt to a salt of terephthalic acid.  The preferred benzene carboxylic acids are phthalic acid (H2P) which may be prepared in situ using phthalic anhydride and water, and benzoic acid (HB).  It is possible to use a mixture of benzoic acid and a benzoate as an acid salt of benzoic acid.  When phthalic acid is used the cycle in diagram 1 is followed; when monopotassium phthalate is used the cycle in diagram 2 is followed; when benzoic acid is used the cycle in diagram 3 is followed; and when a mixture of benzoic acid and potassium benzoate is used the cycle in diagram 4 is followed. Diagram 1. <PICT:0816691/IV (b)/1>  Diagram 2. <PICT:0816691/IV (b)/2>  Diagram 3. <PICT:0816691/IV (b)/3>  Diagram 4. <PICT:0816691/IV (b)/4>  0.5 to 5 equivs. of acid or acid salt may be used in the first stage and at least 1 and preferably 2 equivs. of acid are used in the second stage.  The reaction may take place in water and/or a water-soluble organic solvent, at about -10 DEG  to 200 DEG  C., preferably at 0-50 DEG  C. and at such a concentration that the terephthalic acid and monoalkali terephthalate are completely undissolved, e.g. solutions containing about 1 to 10 per cent dialkali metal terephthalate are treated, although more concentrated solutions may be used.  In the examples: (1) dipotassium terephthalate is treated with phthalic anhydride and water, the residue treated with phthalic anhydride and water and this second filtrate added to further terephthalate; (2) dipotassium terephthalate is treated with monopotassium phthalate obtained as the first filtrate in (1); (3) dipotassium terephthalate is treated with phthalic anhydride and water, the residue with phthalic anhydride and water and the filtrate with further dipotassium terephthalate; (4), (5), (6) monopotassium terephthalate is treated with benzoic acid; (7) dipotassium terephthalate is treated with benzoic acid, the residue treated with further benzoic acid and this second filtrate with further terephthalate, the process is repeated a further four times; (8) similar to (7) using a mole. ratio of terephthalate to benzoic acid of 1:2, the results of 15 cycles are tabulated; (9) a mixture of dipotassium and disodium terephthalate is treated with benzoic acid; (10) dipotassium terephthalate is treated with monopotassium phthalate and the residue treated with phthalic anhydride and water, the second filtrate is treated with further phthalic anhydride and water, four further cycles are described; (11) dipotassium terephthalate is treated with benzoic acid, the residue treated with further benzoic acid and the filtrate treated with more terephthalate, four further cycles are described. Specifications 809,755 and 810,552 are referred to.