FR2459220A1 - Terephthalic acid purificn. - by dissolving in water and then hydrogenating and recrystallising in presence of alkaline material - Google Patents

Abstract

Purificn. of terephthalic acid (I) comprises dissolving crude (I) (obtd. by oxidising p-tolualdehyde with O2 in the liq. phase using water as solvent and a bromine cpd. as catalyst) in water, hydrogenating using H2 in the presence of a hydrogenation catalyst and recrystallising. Novelty is that hydrogenation and recrystallisation are carried out by adding an alkaline material. Highly pure (I) is produced easily by the H2 reduction in spite of using water as the solvent for oxidation, and it is possible to prevent the lowering of activity of the hydrogenation catalyst. Pref. p-tolualdehyde is supplied to water, contg. 0.8-10 wt.% of bromine ions, continuously and oxidised with O2 at 180-240 degrees C at a raw material supplying velocity of 0.1-0.35 kg/hr.l continuously. The obtd. crude (I) is sepd. and dissolved in water. After adding an alkaline material such as a hydroxide, carbonate or bicarbonate of Na, K, Ca, Ba, etc. to the soln. aq. (I) is contacted with H2 at 200-330 degrees C for 0.005-10 hrs in the presence of catalyst to obtain highly pure (I).

Description

 The present invention due to KOMATSU Makoto, ODA Ryoichi relates to a process for the purification of terephthalic acid which has been obtained by oxidation of p-tolualdehyde in the liquid phase, in the presence of a bromine compound as catalyst , using water as the solvent.

 Patent JA No. 2666/59 describes the oxidation reaction of p-xylene in the presence of heavy metal salts such as cobalt, manganese, etc. and a bromine compound as catalyst, using a lower aliphatic monocarboxylic acid as solvent, to obtain terephthalic acid, this process is widely used on an industrial scale.

 Also known from patents JA 1392/64 or GB 833 438 is the oxidation reaction of an aromatic compound containing an alkyl substituent or a partially oxidized alkyl substituent, in an aqueous solvent, in the presence of an it bromine for get terephthalic acid.

 In addition, the production of terephthalic acid by o:: ydation of p-toluic acid in an aqueous solvent containing hydrobromic acid is described in patent application E "- Blessed JA 4019/71 and in the patent US 3,678,106.

 However, the terephthalic acid which is produced by any of these prior art processes which usually takes 4-carboxy-benzaldehyde, which will be referred to below as "4-CBA", an intermediate reaction product or d2 .

colored or coloring matter of unknown structure and it is colored yellow. Therefore, the terephthalic acid, as obtained, cannot be used directly for a reaction with Li-cols to form polyesters and must be purified later to obtain a terephthalic acid which is laughed at once.

for direct polymerization.

 On the other hand, processes for the production of high purity terephthalic acid are known, by purification of crude terephthalic acid, containing impurities, by the ars J 16P60 / 66 and 33189/74, according to which dissolve terephthalic cide in water as a crystallization solvent, rl, carry out a hydrogenation or decarbonylation reaction, in the presence of a catalyst during the dissolution in the water, in order to transform the materials reaction intermediates t dyestuffs, from a material which is easy to purify, terephthalic acid is recrystallized: ue to obtain high purity terephthalic acid.

The Applicant has carried out studies for the purification of terephthalic acid obtained by the process with aqueous solvent according to the technique described in the patent
JA 16860/66, and she discovered that the purification of terephthalic obtained by the aqueous solvent process is more difficult than that of terephthalic acid obtained by a process with acetic acid solvent .

As a result of intensive research to solve the problem, the Applicant has discovered that the terephthalilue acid obtained by the aqueous solvent process contains a very small amount of bromine compounds, while the terephthalic acid obtained by the acetic acid solvent process does not contain analyzable amounts of bromine compounds. It results from other studies on the behavior of bromine compounds in the hydrogenation reaction made by the Applicant, that the.

Bromine compounds are transformed into hydrogen bromide by the action of hydrogen. This means that it has been found that when the crude terephthalic acid formed in the acetic acid solvent is dissolved in water and that the hydrogenation is carried out at elevated temperature according to the process described in br <- vet JA supra 16860/66, a mother liquor resulting from the crystallization of terephthalic acid does not contain an analyzable amount of hydrogen bromide, while when the crude terephthalic acid formed in the aqueous solvent is hydro generated in the same way as above, the mother liquor resulting from the recrystallization of terephthalic acid contains a very small amount of hydrogen bromide or bron.hy dric acid.

 Assuming that this hydrogen bromide weakens the activity of the hydrogenation catalyst, the Applicant has added sodium hydroxide containing at least one equivalent by weight of sodium relative to the bromine atoms contained in the crude terephthalic acid in the aqueous solution of terephthalic acid to be purified, and then hydrogenation and recrystallization * are carried out, and it has been found that no weakening of the activity of the hydrogenation catalyst is observed .

The present invention is based on this discovery and provides a process for the purification of crude terephthalic acid obtained by oxidation of p-tolualdehyde in the liquid phase with molecular oxygen in the presence of a bromine compound as a catalyst using water as solvent, by dissolving the crude terephthalic acid in water and carrying out the hydrogenation of the resulting aqueous solution, in the presence of a hydrogenation catalyst with hydrogen.

molecular and recrystallization of prG terephthalic acid from the solution which consists in adding an alkaline material to the terephthalic acid before the hydrogenation.

 The amount of alkaline material to be added must be at least one equivalent by weight relative to the bromine atoms contained in the crude terephthalic acid to be purified. Kid; the amount of alkaline material to be added is in excess compared to the bromine atoms in equivalent weight, the catalytic activity itself is not adversely affected or rather, it can sometimes be obtained a preferable result, because it seems that it is possible to prevent a very small amount of polymeric impurities from accumulating in the pores of activated carbon as a support for the hydrogenation catalyst, but the exact reason has not yet been elucidated. it goes without saying that it is not preferable to increase the quantity of material to be supplied to such an extent that this decreases the efficiency of the purification of terephthalic acid.

 The alkaline material which is used according to the present invention comprises hydroxides, carbonates, bicarbonates, etc., of sodium, potassium, calcium, barium, etc.

The catalyst which is used during the oxidation of p-tolualdehyde according to the present invention is a bromine compound capable of generating bromine ions under the reaction conditions, such as hydrogen bromide (hydrobromic acid), ethyl bromide, sodium bromide plus hydrogen chloride (hydrochloric acid), etc. Subways; heavy materials such as cobalt, manganese, etc. are not always necessary.

The amount of bromine compound to be added is from Ci, 8 to 10% by weight in the form of bromine atoms relative to water as solvent. When the amount of bromine compound is less than 0.8.0 by weight, the starting material, p-tolualdehyde undergoes a vigorous combustion reaction, which weakens the yield of terephthalic acid. When the amount of bromine compound is too large on the other hand, the oxidation reaction takes place quickly and one cannot keep pure white terephthalic acid and secondary products such as 4- CBA increases, which decreases the yield of desired terephthalic acid. The amount of bromine compound which is preferable for having the best yield tt, t from 2 to 7% by weight as bromine atoms relative to the water as a solvent.

The temperature of the oxidation reaction is in the range of 180 to 2500C, preferably 200 to 240 "C. When the oxidation reaction temperature is too low, the oxidation reaction does not take place sufficiently , while if the temperature is too high the build-up reaction occurs quickly and pure white terephthalic acid cannot be obtained.

 The pressure of the oxidation reaction generally depends automatically on the step consisting in keeping the reaction temperature constant by evaporation of the aqueous solvent and in carrying out condensation and reflux operations, it is possible to keep the reaction pressure at a value desired constant using an external heat exchange device. The pressure range is not particularly rigorous, insofar as the reaction solution can be retained in the liquid phase, but it is usually from 10 to 70 kg / cm 2 on the pressure gauge.

An oxidizing agent is either oxygen or air, but economically, air is used advantageously.

 The solvent which is used in the oxidation reaction is water, and it is necessary to add water as a solvent to the reaction system in an amount which is at least 1.5 times by weight that of the starting material, p-tolualdehyde.

 The terephthalic acid which results from the oxidation reaction is a crude terephthalic acid containing 0.01 to 1.0% by weight, in particular 0.05 to 0.3% by weight of 4-CBA, and after having been separated from the aqueous solvent, the crude terephthalic acid is redissolved in water and the hydrogenation is carried out in the presence of a hydrogenation catalyst. various hydrogenation catalysts can be used for this purpose, and, for example, noble metals from Group VIII of the Periodic Table of the Sloweners, such as ruthenium, rhodium, palladium, osnium, iridium, platinum, etc., and iron, cobalt, nickel, etc., on a support such as activated carbon, diatomaceous earth, etc. used in the form of a very fine powder or as salt or in the form of Raney nickel or dfJ black platinum or palladium.

 The hydrogenation temperature should be at least one temperature convenient for the complete dissolution of the terephthalic acid, and is usually between 200 and 3300C, preferably between 225 and 300 "C.

 The hydrogenation pressure is not particularly rigorous insofar as the aqueous solution can be kept in the liquid phase and it is usually from 16 to 100 kg / cm 2 ac pressure gauge.

The duration of the hydrogenation depends on the purity desired for the purified terephthalic acid and on the conditions of contact with hydrogen, and is usually from 0.005 to 10 hours, preferably from 0.01 to 2 hours.

When carrying out the practice of the present invention, p-tolualdehyde is continuously supplied in an aqueous solvent containing 0.8 to 10% by weight of bromine ions in the form of bromine atoms or in a reactor with the aqueous solvent and continuously oxidizes with molecular oxygen under oxidation reaction conditions which are
Reaction temperature 180 to 240 "C
P-tolualdehyde loading rate 0.1 to 0.35 kg / h. Liter
(LSV) (relative to the solvent)
the resulting terephthalic acid is separated from the reaction solution and redissolved in water to completely dissolve the precipitated crude terephthalic acid, then an alkaline material is added to the aqueous terephthalic acid solution and brought in contact the aqueous solution of terephthalic acid with hydrogen in the presence of a hydrogenation catalyst at a temperature of 200 to 330 "C, for a contact time of 0.005 to 10 hours. The aqueous hydrogenated solution is cooled and l the precipitated pure terephthalic acid is separated from the solution, and the mother liquor is recycled to the oxidation reaction for reuse as a solvent.

During the oxidation reaction, water is used as solvent, but high purity terephthalic acid can be produced quickly by hydrogenation according to the present invention while it can be effectively prevented at the same time decreasing the activity of the hydrogenation catalyst.

EXAMPLE
P-tolualdehyde is continuously charged in an autoclave (net capacity of 2 liters) filled with 900 g of water as solvent, containing 3% by weight of hydrogen bromide (hydrobromic acid) at a rate of 300 g per hour at 2300C for one hour, and the oxidation is carried out semi-continuously. After cooling, the resulting terephthalic acid is removed from the autoclave, washed with water at room temperature and dried.

The properties of terephthalic acid are as follows
4-CBA 3,030 ppm
OD340 1.23 OD340 represents the absorption in 340 mu obtained by measurement, by placing a solution of 2 g of terephthalic acid in 25 ml of
KOH 2N, in a 50 mm cell.

 The resulting terephthalic acid contains 2100 ppm of bromine compounds as bromine atoms.

300 g of net crude terephthalic acid (containing 0.63 g of bromine atoms, ie 7.9) are loaded into an autoclave with a net capacity of 2 liters and fitted with an agitator. 3 gram equivalent of bromine atoms) 0.35 g of sodium hydroxide (containing 8.75. 10 gram equivalent of sodium atoms) and 900 g of water.

 Then the autoclave is filled with hydrogen under 2 pressures of 15 kg / cm2 on the pressure gauge and it is heated to 2800c. When the autoclave has reached this temperature, a small rack is filled, filled with 27 g of palladium on activated carbon catalyst whose initial activity has been excited by a prior hydrogenation reaction and whose activity is stable, in the terephthalic acid solution in the autoclave, following an external signal and kept as is at 2800C for one hour.

After completion of the hydrogenation reaction, the autoclave is slowly cooled to precipitate terephthalic acid.

The properties of the resulting purified terephthalic acid are given below
4-CBA 10 ppm
or 340 0.088
COMPARATIVE EXAMPLE
The crude terephthalic acid obtained is purified under the same oxidation conditions as those of the example, in the same way as in the example except that sodium hydroxide is not added.

The properties of terephthalic acid-purified are as follows
4-C8A 35 ppm OD340 0, 119
REFERENCE EXAMPLE
Purified in the same way as in the example, crude terephthalic acid obtained from p-xylene as starting material, using as solvent acetic acid and cobalt, manganese and bromine as catalysts, on an industrial scale.

The properties of crude terephthalic acid are as follows
4-CBA 1320 ppm OD340 - O, 670
The properties of purified terephthalic acid are as follows
4-CBA 3.5 ppm
OD340 0.045
When the purification of the terephthalic acid obtained by oxidation is carried out, using water as solvent, without adding alkaline material to it, it is observed that the activity of the catalyst decreases, but this decrease in activity can be prevented of the catalyst by the addition of the alkaline material

Claims (3)

 1. Process for the purification of crude teraphthalic acid obtained by oxidation of p-tolualdehyde in the liquid phase with molecular oxygen in the presence of a bromine compound as catalyst, using water as solvent , by dissolving the crude terephthalic acid in water, hydrogenating the resulting aqueous solution of terephthalic acid with molecular hydrogen in the presence of a hydrogenation catalyst and recrystallizing the terephthalic acid from of the aqueous hydrogenation solution, characterized in that it consists in adding an alkaline material to the aqueous solution of terephthalic acid before the hydrogenation. -2. Process according to Claim 1, characterized in that the alkaline material is a hydroxide, a carbonate or a bicarbonate of sodium, potassium, calcium or barium. 3. Method according to claim 1, characterized in that there is added to the aqueous solution of terephthalic acid at least one equivalent by weight of alkaline material relative to the bromine atoms contained in the crude terephthalic acid.