DE1089746B - Process for purifying terephthalic acid - Google Patents

Description

Process for Purification of Terephthalic Acid The invention relates to a method for purifying terephthalic acid.

Commercially available terephthalic acid often contains, depending on its origin colored or color-forming impurities. For the production of high polymer polymethylene terephthalates A terephthalic acid of very high purity is required, as the latter are raw materials of considerable technical importance in the manufacture of threads, fibers and films are.

It has been found that the recrystallization of crude terephthalic acid from benzoic acid in an inert atmosphere is a particularly effective purification process. In contrast to purification processes using other solvents, such as that of French patent specification 1136 560, which uses dimethylformamide as a solvent, the purified terephthalic acid can be obtained from benzoic acid in almost quantitative yield. The recrystallization of crude terephthalic acid from benzoic acid does not give a purer product than that which is obtained by the process of British patent specification 777,782, but it is much less laborious and does not require the addition of a precipitant. Benzoic acid is much less toxic than dimethylformamide and can be recovered without difficulty. The quality of the terephthalic acid purified by the process according to the invention is comparable to that of the terephthalic acid obtained by the specified process and, like this, results in an almost colorless polyethyleneterephthalate when reacted directly with ethylene glycol.

According to the invention, crude terephthalic acid, for example in a Nitrogen atmosphere, dissolved in benzoic acid and the resulting solution to approx 150 ° C cooled.

Since terephthalic acid is no longer soluble in benzoic acid at 150 ° C, it is expedient to cool to this temperature before the separation of the crystallized out Terephthalic acid- takes place. The dissolving can take place at normal pressure if one Wants to produce a 3o / o solution of terephthalic acid in benzoic acid, and finds at Overpressure takes place when a 50% solution is obtained at 270 ° C.

The cleaning is advantageous in an atmosphere of inert gas, such as nitrogen, the terephthalic acid-benzoic acid mixture, at least until the terephthalic acid has dissolved, is shaken.

Before the cleaning according to the procedure takes place, the crude terephthalic acid can be used according to well known methods, e.g. B. by simply recrystallizing from water pre-purify at overpressure or by extraction with suitable solvents.

Known chemical cleaning processes, z. B. an oxidation, reduction. Treatment with periodic acid (H J 04) or by Adding lead tetraacetate or by absorbing the impurities on activated carbon be connected.

The benzoic acid used as a solvent is made by simple distillation recovered for further use.

The process can be carried out in batches or continuously. It has also been found that aqueous solutions of benzoic acid can also be used of the method according to the invention are suitable. However, then the cleaning effect is less.

The invention is illustrated by the following examples in which parts and percentages are based on weight.

Example 1 24 parts of impure terephthalic acid, prepared by air oxidation of p-xylene, was mixed with 800 parts of benzoic acid and heated until the benzoic acid melted and stirring was started. The suspension was heated until the temperature reached 250 ° C and a clear, yellow solution was obtained. This solution was filtered and the filtrate was slowly cooled to 150 ° C. Then the crystallized terephthalic acid (22.5 parts) was collected on a filter, washed with boiling water until it was free of benzoic acid and dried. The effect achieved is shown in Table I. Table I. Uncleaned, cleaned Measurement terephthalic terephthalic acid acid Color .................... pale white pale yellow Degree of purity ............ 88.3 ° / o 97.6 ° / o Light transmission ...... 0.08 0.02 Example 2 24 parts of impure terephthalic acid, prepared by air oxidation of p-xylene, were recrystallized as described in Example 1, from 800 parts of benzoic acid, but in a nitrogen atmosphere. 23 parts of terephthalic acid were obtained. The cleaning effect is shown in Table II. Table II Uncleaned, cleaned Measurement terephthalic terephthalic acid acid Color .................... pale white pale yellow Degree of purity ............ 95.0 % 99.50 / 0 Light transmission ...... 0.22 0.01 In the following Examples 3 to 5, 24 parts each of terephthalic acid, which had been prepared as indicated in Table III, were recrystallized from 800 parts of benzoic acid, as described in Example 2. The effect of the cleaning measures according to the invention is emphasized by specifying comparative values. Table III Purity light color of the color of the Example type of extraction natural color polyethylene No. of terephthalic acid of the material of the acid degree through-esterified tere- / o leakage product phthalares 3 Air oxidation of p-diisof impure pale yellow 98.4 0.75 yellow yellow propylbenzene purified white 99.3 0.06 colorless colorless 4 Air oxidation of p-xylene j impure yellow 94.1 0.15 yellow yellow l purified white 98.8 0.03 colorless colorless 5 Thermal disproportionate unclean white 98.5 0.17 pale yellow pale green Purified white potassium benzoate 98.9 0.04 colorless colorless Example 6 48 parts of impure terephthalic acid, prepared by air oxidation of p-xylene, were mixed with 800 parts of benzoic acid and placed in a closed vessel purged with nitrogen. The mixture was heated at 130'C, then begun with stirring and further heated until the temperature of 285 'C reached. A clear, yellow solution was obtained. The developed pressure was 2.8 kg / cm2. The mixture was then slowly cooled to 150 ° C., the container opened, the crystallized terephthalic acid (42 parts) collected on a filter, washed with acetone (56 ° C.) until it was free of benzoic acid and dried. The cleaning effect is shown in Table IV. Table IV Raw Purified Measurement terephthalic terephthalic acid acid Acid color ............... pale yellow I white Degree of purity, ° / o ......... 90.4 98.5 Light transmission ...... 0.25 0.06 Color of the esterified product duktes ................. yellow colorless Color of polyethylene terephthalate ............ I yellow colorless The light transmittance of the terephthalic acid solutions (4% terephthalic acid in ammonium hydroxide f 1 part 0.88 ammonia, 1 part water, w / v)] was measured at 380 m in 4 cm cells.

A particular advantage of using benzoic acid as a cleaning medium is its stability under the conditions of the procedure according to the invention, such as is shown by the following experiment.

The gases evolved during the cleaning process of Example 6 were collected and analyzed as carbon dioxide. The carbon dioxide present indicated that no more than 0.01% of the benzoic acid had decomposed. The mother liquors which precipitated were distilled at 150 ° C. in vacuo and pure benzoic acid corresponding to 98% of the raw material was recovered.

Claims (1)

PATENT CLAIM: Process for the purification of terephthalic acid, which was produced by catalytic oxidation of dialkylbenzenes or by thermal disproportionation of potassium benzoate, by recrystallization from an organic solvent, characterized in that benzoic acid is used as the solvent. References considered British Patent No. 777,782; French patent specification No. 1 136560.