DE1940319C3 - Process for obtaining naphthalene-2,6-dicarboxylic acid from the product of the thermal rearrangement or disproportionation of alkali salts of suitable naphthalenecarboxylic acids, which product contains a dialkali salt of naphthalene-2,6-dicarboxylic acid - Google Patents

Description

The invention relates to a process for obtaining naphthalene-2,6-dicarboxylic acid from a dialkali salt the naphthalene-2,6-dicarboxylic acid-containing product of thermal storage or disproportionation of alkali salts of other naphthalene mono- or dicarboxylic acids than naphthalene-2,6-dicarboxylic acid.

It is known, for example naphthalene-α- or - / i-monocarboxylic acid or naphthalene-1,8-dicarboxylic acid with hydroxides, carbonates or bicarbonates of alkali metals in an aqueous medium To implement heating with formation of the corresponding alkali salts.

It is also known by heating the alkali metal salts obtained in this way or mixtures thereof to high temperatures in carbon dioxide or another inert gas atmosphere (German patent 953 072) or by heating dialkalinaphthalene-1,8-dicarboxylates (German patents 932 125 and 1002 316) crude dialkali salt of To produce naphthalene-2,6-dicarboxylic acid. These known conversions are referred to below as rearrangements designated.

It is also known to separate the by-products from the rearrangement product and the dialkali salt to convert the naphthalene-2,6-dicarboxylic acid into the free naphthalene-2,6-dicarboxylic acid. In which known processes is the rearrangement product which, in addition to the desired dialkali salt of naphthalene-2,6-dicarboxylic acid z. B. alkali salts of naphthalene mono- or z. B. contains naphthalene-2,3-dicarboxylic acid, dissolved in water and filtered off water-insoluble material.

Mineral acid is then added to form the naphthalenecarboxylic acids. The then free acids obtained by filtration are mixed with an organic solvent, e.g. B.

a lower aliphatic alcohol, such as methanol or ethanol, or lower aliphatic carboxylic acids, such as acetic acid or propionic acid, heated in order to separate the naphthalene-2,6-dicarboxylic acid, which is insoluble in the organic solvent, from the other dicarboxylic acids on the basis of their solubility. The known process described above is disadvantageous in that the alkali metal salt of the mineral acid (strong acid) formed as a by-product cannot be reused for the preparation of the starting material for the rearrangement, ie the alkali metal salt of naphthoic acid. Furthermore, the naphthalene-2,6-dtcarboxylic acid obtained in this way contains alkali salts of the mineral acid used and the starting carboxylic acids and, if it is not subjected to complicated cleaning measures, shows purities of about 98 to 97%, which corresponds to an optical density of about 0.20 (in a aqueous NH ₄ OH solution in a ratio of 1: 1) at 400 mμ. This insufficient purity makes the product suitable as a material for the direct production of polyalkylene-2,6-naphthalenedicarboxylates which correspond to the following formula

0- ( 0 Il
-C — 0-- -R - 3 Fi -C- V / \

and by reacting the naphthalene-2,6-dicarboxylic acid with an alkylene glycol, such as ethylene glycol, Propylene glycol and the like are unsuitable.

As a result, the crude naphthaIin-2,6-dicarboxylic acid obtained by conventional methods must also be used a lower aliphatic alcohol such as methanol

or ethanol, esterified and ζ. B. by recrystallization, Distillation and the like. Purified. The purified alkyl ester is either as such or after Conversion into the narhthalene-2,6-dicarboxylic acid reacted with the alkylene glycol.

The object of the invention was to create a process for obtaining a naphthalene-2,6-dicarboxylic acid high purity, used for the production of high quality polyalkylene-2,6-naphthalenedicarboxylates with good reproducibility is suitable that enables reuse of the alkali and furthermore free from the disadvantages occurring in the customary production of naphthalene-2,6-dicarboxylic acid is.

It must be taken into account that it is difficult to use the monoalkali salt of naphthalene-2,6-dicarboxylic acid to be converted into the corresponding free acid.

[In the context of the invention it was found that the monoalkali salt of naphthalene-2,6-dicarboxylic acids slightly disproportionate when heated in water or an aqueous organic solvent, whereby the desired free dicarboxylic acid precipitates as such and the dialkali salt is formed as a by-product. Of a similar behavior also of terephthalic acid on naphthalenedicarboxylic acids and in particular It was not obvious to conclude 2,6-dicarboxylic acid since it is known that chemical reactions take place even with the isomeric phthalic acids, for example Terephthalic acid and isophthalic acid. For example, the disproportionation of monopotassium terephthalate is in progress according to German patent specification 959 184 smoothly, while in the case of isophthalic acid this reaction does not occur despite the same structure. For the naphthalene-2,6-dicarboxylic acid monoalkali salt, which has a largely different structure than the phthalic acids and also has the carboxyl group contains in two different rings, it is therefore by no means possible to predict with certainty whether and which reaction occurs.

It was found that the dialkali salt formed as a by-product for reuse the preparation of the monoalkali salt of naphthalene-2,6-dicarboxylic acid, which is used as the starting material recycled and thereby the maphthalene-2,6-dicarboxylic acid can be produced industrially advantageously can.

The process according to the invention for the recovery of naphthalene-2,6-dicarboxylic acid from the dialkali salt containing naphthalene- ^ o-dicarboxylic acid Product of the thermal rearrangement or disproportionation of alkali salts of other naphthalene

mono- or -dicarboxylic acids than the naphthalene-2,6-dicarboxylic acid is characterized in that a) a filtered aqueous solution of the dialkali salt containing product so far that when the concentrated solution is cooled to room

temperature at least 70% of the dialkali salt precipitate, or b) an alkali metal carbonate or bicarbonate is admixed with the starting solution and this mixture constricts; gaseous through the aqueous solution of the dialkali salt deposited according to a) or b)

Carbon dioxide conducts the precipitate thus obtained separates the monoalkali salt of naphthalene-2,6-dicarboxylic acid, this by heating in water or an aqueous organic solvent in naphthalene-2,6-dicarboxylic acid and the dialkali

salt of naphthalene ^ o-dicarboxylic acid is disproportionated, the precipitated naphthalene-2,6-dicarboxylic acid is separated off and the mother liquor is returned to the carbon dioxide conversion stage.
The method according to the invention is preferred

characterized in that the according to a) or b) remaining mother liquor with that from the carbon dioxide conversion stage obtained mother liquor combined and used to produce the starting material for the rearrangement or disproportionation.

It is favorable from the carbon dioxide conversion stage mother liquor obtained returned to the mixed concentration stage.

The disproportionation according to the invention in the presence of the solvent can be achieved by the

reproduce the following formula, in which the use of the potassium salt is given as an example, where however, of course, the sodium salt can be used in the same way:

2KOOC

COOH

COOK

(»Aq.« Means: in dissolved form) - HOOC

- COOH1

(desired product)

The above reaction is carried out in water or an aqueous organic medium.

Its course is determined by the "concentration" of the monoalkaliL · ::! of naphthalene-2,6-dicarboxylic acid affected in water. As the monopotassium naphthalene-2,6-dicarboxylate is hardly soluble in water, the term "concentration" here denotes the weight ratio of the total monopotassium naphthalene-2,6-dicarboxylate including the insoluble monopotassium naphthalene-2,6-dicarboxylate: to the amount of the total mixture.

This concentration is appropriately selected in accordance with the temperature and pressure conditions in the liquid phase disproportionation, and the lowest concentration at which a sufficient reaction rate is achieved is selected. When the disproportionation ζ. B. is carried out at about 100 ⁰ C and atmospheric pressure, the reaction proceeds at higher monoalkali salt concentrations than i0% from little more. As a result, it is advantageous in this case not to use concentrations higher than 10%, in particular 8%. The preferred reaction temperature is not below 60 ° C., preferably not below 80 ^° C. The reaction rate is accelerated at higher temperatures. At the boiling point, the reaction is complete in 30 minutes. Atmospheric pressure can be used, but the reaction can also be carried out at elevated pressures and temperatures above 100 "C to be carried out. In this case, the monoalkali salt concentration can be increased to values higher than 10%.

The simultaneous presence of the naphthalene-2,6-dicarboxylic acid is in no way harmful, however the presence of the dialkali salt of naphthalene-2,6-dicarboxylic acid can cause problems give.

That is why it is particularly beneficial to use the dialkali salt concentration (based on water) should not be kept higher than 5%. To do this, one can the dialkali salt of naphthalene-2,6-dicarboxylic acid, which is formed in the course of the reaction, with React pressurized carbon dioxide gas and thus convert it into the corresponding monoalkali metal.

It is also possible to carry out the reaction in a mixture of water and organic solvent to carry out the impurities contained in monoalkalinaphthalene-2,6-dicarboxylate can dissolve. In this case the favorable weight ratio of water to organic solvent lies usually in the range of 1: 0 to 1: 1, although this may e.g. B. with the nature of the organic Solvent or the purity of the starting material! serving monoalkali salt varies. When suitable organic solvents are e.g. B. called methanol and ethanol.

The mother liquor containing the dialkaline naphthalene dicarboxylate can after separation of the free acid for the production stage of the monoalkalinaphthalene-2,6-dicarboxylate serving as the starting material be recycled with fresh dialkalinaphthalene-2,6-dicarboxylate.

The preparation of the monoalkalidicarboxylate from the dialkali salt can be carried out in the following manner are: The dialkali salt is mixed with any other naphthalenedi- or monocarboxylic acid than naphthalene-2,6-dicarboxylic acid, preferably the same naphthoic acid as used for the preparation of the starting material was used for the rearrangement (z. B. naphthalene - /? - monocarboxylic acid), in an aqueous Medium reacted with heating and the resulting precipitate was filtered off. In this procedure can the by-product (e.g. monoalkalinaphthalene - /? - monocarboxylate) obtained by concentration and subjected to thermal rearrangement to form dialkalinaphthalene-2,6-dicarboxylate will.

An example of the above embodiments is given below:

In the precipitation stage for the preparation of the monoalkalinaphthalir-2,6-dicarboxylate by "double conversion" the concentration of the dialkali salt of naphthalene-2,6-dicarboxylic acid is favorable in the water not higher than 20 percent by weight, preferably. ^ t about 10 percent by weight. It is still cheap, no less than the equimolar amount, however not more than 1.5 times the molar amount of naphthalene monocarboxylic acid to use the dialkai salt of naphthalene-2,6-dicarboxylic acid and the implementation at temperatures not below 90 C and at atmospheric pressure for at least 30 minutes to execute. If desired, higher temperatures and pressures can be used, whereby the response time is shortened accordingly.

According to another alternative, the monoalkali salt of naphthalene-2,6-dicarboxylic acid used as starting material can be used can be produced by reaction with carbon dioxide gas.

Gaseous carbon dioxide is dissolved in the aqueous solution of the dialkali salt of naphthalene-2,6-dicarboxylic acid introduced. The resulting monoalkali salt precipitates and is separated off. Below is this Stage referred to as the carbon dioxide reaction stage.

Since the mother liquor, from which the precipitate was separated, from an aqueous, alkali metal carbonate containing solution, it can be used to prepare the starting material for the thermal rearrangement can be used in the following manner: One of naphthalene-2,6-dicarboxylic acid Different naphthalenedi- or monocarboxylic acid, preferably the same naphthalenecarboxylic acid that was used for the original production of the starting material used in the rearrangement reaction becomes the aqueous solution added and evaporated to dryness with heating.

It is also possible to use the gaseous carbon dioxide, which is a by-product in the above production step for the starting material for the thermal rearrangement arises, in the carbon dioxide conversion stage due, whereby both the alkali and the carbon dioxide are used favorably will.

The above embodiment is also shown below by a scheme, wherein the dipotassium salt of naphthalene-1,8-dicarboxylic acid is used as the starting material for the rearrangement will.

HOOC COOH

KOOC COOK

+ 2KHCO ₃
aq.

[Production stage of the starting material
the rearrangement reaction]

+ 2CO ₂ + 2H ₂ O
aq.

KOOC COOK

[Carbon dioxide conversion stage for the production of the monoalkali salt of naphthalene-2,6-dicarboxylic acid]

KHCO ₃

aq.

² KOOC \

Heating in H ₂ O or H ₂ O COOH + organic solvent

[Disproportionation level J.

HOOC

COOK

The suitable working conditions for the reaction with gaseous carbon dioxide for the preparation of the monoalkali salt of naphthalene-2,6-dicarboxylic acid are as follows: The concentration of the dialkali salt of naphthalene-2,6-dicarboxylic acid in water can be in the range from 5 to 30 percent by weight, preferably 15 to 20 Percent by weight, it being possible for the dialkai salt to be partially suspended; the gaseous carbon dioxide is not higher introduced into the system at temperatures above 70 ° C, in particular O to 40 ⁰ C, under atmospheric pressure or slightly elevated pressure. The carbon dioxide is expediently introduced from the lower part of the reaction vessel in such a way that fine carbon dioxide

940

bubbles are dispersed in the liquid system. The reaction time varies depending on the mode of initiation, the stirring speed or the Reaction temperature, however, is normally with a continuous introduction of 60 to 120 minutes the etching practically ended. The precipitated monoalkyl salt of naphthalene-2,6-dicarboxylic acid is filtered off while the reaction product is kept at a temperature not higher than 70 ° C will.

The crude dialkali salt of naphthalene-2,6-dicarboxylic acid. which was obtained from the rearrangement reaction already mentioned can also be used here are cleaned after it is through a special concentration stage or mixed concentration stage would. This results in a very favorable combination of all levels and full utilization of by-products achieved.

In the following the "special concentration level" explained in more detail.

First, the alkali salt of the naphthalene mono or -dicarboxylic acid under pressure in a carbon dioxide or other inert gas atmosphere at high Temperatures, preferably in the range from 350 to 450 ° C. 2 to 3 hours in the presence known per se

Catalysts, preferably cadmium iodide, heated. The rearrangement product obtained is then in optionally warm water to separate insoluble materials such as carbides and catalysts by filtration. The filtrate can with

Treated to activated carbon and then concentrated to a concentration in the range listed below whereupon the alkali salt of naphthalene-2,6-dicarboxylic acid precipitates. This allows the dialkali naphthalene-2,6-dicarboxylate with high purity

effectively separated and recovered while practically all by-products and unreacted Alkali salts of naphthalenecarboxylic acid, which are about 5 to 10 percent by weight in the rearrangement reaction product present, dissolved in the mother liquor

remain.

The term "degree of concentration" is defined by the following equation:

Degree of concentration =

Total weight of the precipitate in the solution
Total weight of the solution

100%,

To determine the yield or degree of purity, crude dipotassium naphthalene ^ .o-dicarboxylate was used. which was obtained in the above rearrangement reaction, dissolved in water, and from this carbide and catalysts removed by filtration. The filtrate was concentrated to the various levels of concentration shown in Table I and the concentrates cooled to room temperature. The yield (percent by weight j of dipotassium naphthalene-2,6-dicarboxylate after precipitation to all of the dipotassium naphthalene-2,6-dicarboxylate present and the purity of the precipitate from each concentrate is also shown in Table 1.

45

Table I. purity <degree of concentration Precipitation yield 99.7
99.6
99.4
96.0 40
50
60
75 57
70
81
91

It follows that starting from crude dipotassium naphthalene-2,6-dicarboxylate is conveniently concentrated to 50 to 60%. Most of the dipotassium naphthalene-2,6-dicarboxylate is within this range precipitated and virtually all of the by-products remain in solution.

If the concentration is above 60%, some of that of dipotassium naphthalene ^ o-dicarboxylate falls different potassium naphthalene carboxylates with from, so that the purity of the dicaIium naphthalene-2,6-dicarboxylate precipitate deteriorates and the product becomes unsuitable for the objects of the invention. If the concentration level is below 50%, on the other hand, significant amounts of the dipotassium salt remain in the mother liquor, thereby reducing the yield is decreased.

These relationships can also be determined empirically for the other alkali salts and the optimal one Concentration range can be chosen accordingly. The loss booty should be less than 100% but not less than 70%, but the purity of the precipitate exceed 97%.

The mixed concentration stage already mentioned above is explained below.

At this stage, the rearrangement reaction product becomes the aqueous solution, resulting in the insoluble Materials such as carbide, catalyst and the like have been removed. Alkali carbonate or bicarbonate, for example potassium carbonate, sodium carbonate, potassium bicarbonate or sodium bicarbonate are preferred the carbonate or bicarbonate of the dialkali salt of naphthalene-2,6-dicarboxylic acid formed in the rearrangement reaction contained alkali metal, added and the mixture to a degree of concentration concentrated, in which the yield of precipitated dialkali salt of naphthalene-2,6-dicarboxylic acid after cooling at room temperature at least 70 percent by weight and the purity of the precipitated Dialkali salt exceeds 97%. The dialkali salt of naphthalene-2,6-dicarboxylic acid can also do this can be obtained in high yield and high purity.

As an alkali carbonate or bicarbonate (solution), part or all of the mother liquor obtained from the carbon dioxide conversion stage (after separation of the precipitated monoalkali naphthalene-2,6-dicarboxylate) used.

As an example, potassium carbonate was added to the aqueous solution of the crude dipotassium naphthalene-2,6-dicarboxylate, from which carbide, catalyst, etc. had been removed by filtration, in various concentrations added and for each mixture the amounts of the in the aqueous potassium carbonate solution Dissolved (number of grams dissolved in 100 g of the solution) and the impurities (mainly Kaliuw-2-naphthoat) determined at different temperatures. The results are summarized in Table II.

K ₂ CO ₃ (percent by weight)

13.2
19.4
31.1

Table II

Dipotassium

naphlhalin-

2,6-dicarboxylate

33
9
4th
Nothing

Potassium 2-naphthoate

86

23

Nothing

Temperature ("C)

M) Dissolved substances

Dipotassium naphtrmline

2,6-dicarboxylate

35

11th

nothing potassium-2-naphllioate

100

55

35

Nothing

80

Dipotassium

naphlhalin-

2,6-dicarboxylate

38
13th

nalium-2-niiphthnal

117
80
60
19th

It follows that in this case the concentrations of the solutes increase and eventually DikaHum-naphthalene- ^ o-dicarboxylate precipitates and that the solubilities of dipotassium naphthalene-2,6-dicarboxylate are particularly in the region of 80 ° C and the impurities differ greatly. This makes it possible to use a dipotassium naphtha-2,6-dicarboxylate of high purity and only a very low content of discolouring impurities to obtain. The corresponding values can also be determined for other alkali salts.

Typical embodiments of the present method are described below with reference to FIG the drawings explained.

According to the flow sheet of FIG. 1, the alkali salt of the starting carboxylic acid becomes that already described Subjected to rearrangement and the product dissolved in water or warm water and becomes insoluble Materials filtered out therefrom. Then the aqueous solution is as previously described on a certain Concentration restricted. The precipitate, which is obtained from the purified dialkali salt of naphthalene-2,6-dicarboxylic acid exists, is fed to the carbon dioxide conversion. Contains the remaining mother liquor mainly the alkali salt of naphthoic acid as a by-product of the rearrangement or alkali salt not converted during the rearrangement.

On the other hand, the monoalkali salt of naphthalene-2,6-dicarboxylic acid precipitated by carbon dioxide becomes fed to the disproportionation stage. The remaining mother liquor consists of an aqueous one Solution of alkali bicarbonate containing a small amount of the unreacted dialkali salt of naphthalene-2,6-dicarboxylic acid contains.

The two mother liquors mentioned are combined with that for the production of the starting material for the rearrangement serving carboxylic acid and a rearrangement catalyst added, with heating evaporated and returned to the rearrangement reaction.

The precipitated naphthalene-dicarboxylic acid can be washed with a small amount of hot water, if necessary. After drying, this gives a product of high purity. The remaining mother liquor, containing the dialkali salt of naphthalene-2,6-dicarboxylic acid, can be combined with the washing liquid mentioned and used to dissolve the precipitate obtained in the concentration stage, whereupon the solution obtained is subjected to a _{5% CO 2 treatment.}

In the case of the flow diagram according to FIG. The process shown in Figure 2 is the concentration stage of the process according to FIG. 1 replaced by the mixed concentration stage described; the nature of the repatriation of both Mother liquors, which are used to produce the starting material for the rearrangement, is opposite the method according to Fi g. 1 slightly modified

Here, the remaining mother liquor, after the precipitate from the rearrangement product containing concentrated liquid was removed with one of naphthalene-2,6-dicarboxylic acid different starting acid and mixed with catalyst, brought to dryness and for the rearrangement as used in FIG. On the other hand, that obtained from the carbon dioxide conversion step is After the precipitate has been separated off, mother liquor is returned to the mixed concentration stage.

According to the present invention, a naphthalene-2,6-dicarboxylic acid with particularly high Purity of not less than 99% with an optical density determined with an aqueous ammonia solution in a ratio of 1: 1 at 400 πΐμ (Οϋ400πΐμ). of not higher than 0.002 in high yield with good reproducibility in terms of quality and purity obtained by using the monoalkali salt obtained in the carbon dioxide conversion stage, especially when using one from the mixed concentration stage or the special concentration stage obtained dialkali salt of naphthalene-2,6-dicarboxylic acid in the carbon dioxide reaction stage.

Then some practical examples are given.

example 1

Gaseous carbon dioxide was replaced by 200 g of a 15% strength aqueous solution of dipotassium naphthalene-2,6-dicarboxylate passed from room temperature and obtained 18 g of precipitate. The acid number of the precipitate was 219 (the acid number of the Monopotassium naphthalene-2,6-dicarboxylate is 220). By infrared spectral analysis and determination Based on the potassium content, the precipitate was identified as pure monopotassium naphthalene-2,6-dicarboxylate. The yield was 67%. The remaining solution could be concentrated further, and at The introduction of gaseous carbon dioxide was further '; Obtained monopotassium salt.

5.08 g of the monopotassium naphthalene-o-dicarboxylate were suspended in water, stirred at the boiling point for 120 minutes, filtered and the obtained Precipitate washed with a small amount of water. This gave 2.14 g of solid had an acid number of 520 (the acid number of naphthalene-2,6-dicarboxylic acid is 518). The Infra-

rnt-Spektralanulyse confirmed that the solid is out pure naphthalene- ^ o-dicarboxylic acid. The yield based on the starting material monopotassium naphthalene-2,6-jicarboxylate, was 99%.

The filtrate contained 2.94 g of dipotassium naphthalene-2,6-dicarboxylate. It was concentrated to a concentration of 15% and again with gaseous carbon dioxide implemented, whereby 1.78 g of monopotassium naphlhalin-2,6-dicarboxylate were obtained.

Example 2

This is due to the rearrangement of potassium fi-naphthoate Crude dipotassium naphthalene-2,6-dicarboxylate obtained with heating was dissolved in water, filtered and decolorized. This gave 220 g of a clear, 20% strength obtained aqueous solution of dipotassium naphthalene-2,6-dicarboxylate. 25.4 g of i-naphthoic acid added to the solution and heated to boiling for 2 hours. 38.2 g of precipitate were obtained which had an acid number of 222 and based on infrared spectral analysis as practically pure monopolyium - naphthalene - 2,6 - dicarboxylate ientified. The yield was quantitative.

5.08 g of monopotassium naphthalene-2,6-dicarboxylate were suspended in 95 g of water and the ^{mixture was stirred at 80 °} C. for 120 minutes. The precipitate formed was filtered off and washed with a small amount of water. 2.72 g of precipitate with an acid number of 398 were obtained. According to the acid number and the result of infrared spectral analysis, the precipitate contained 60% naphthalene-2,6-dicarboxylic acid and 40% monopotassium naphthalene-2,6-dicarboxylate. The yield was 80%.

Example 3

A monopotassium naphthalene-2,6-dicarboxylate obtained by the same procedure as in Example 1 was used as the starting material.

10.16 g each of the monopotassium naphthalene-2,6-dicarboxylate were processed into an aqueous suspension with a concentration of 5, 7 and 9% respectively. The suspensions were heated to boiling with stirring for one hour and the suspensions obtained After cooling, precipitates are filtered off. The results of the Infraroi speciral analyzes and the acid numbers of the precipitates are listed in Table III.

Table III

concentration

(Weight percent)

. composition mono- Weight number (Weight percent) potassium- of the down Naphtha naphthalene- Blow ihalin- 2,6-di- 2,6-di- carboxylate 522 carbon 0 (G) 450 acid 13.9 4.28 348 100 39.0 4.80 86.1 6.14 61.0

yield

Example 4
(With reference to the flow diagram of Fig. 1)

10 kg of mixture of potassium 2-naphthoal and 4 mole percent cadmium iodide were 4 hours in a pressurized atmosphere of carbon dioxide of 30 kg / crn heated to 38O ⁰ C. ² 8.27 kg of a rearrangement product were obtained. This was dissolved in 18.01 water and heated with stirring for 1 hour, cooled to room temperature and filtered, with 2.77 kg of naphthalene and 0.35 kg of carbide and catalyst being separated off. The filtrate was treated with 65 g activated charcoal, placed in an evaporator, and concentrated until 14,631 water had distilled off. This gave (a) 3.90 kg of crystalline dipotassium naphthalene-2,6-dicarboxylate and (b) a filtrate. The crystals (a) were in a

To the reaction vessel equipped with a gas inlet tube and stirrer, 22 liters of water were added and the mixture was stirred at room temperature and atmospheric pressure, with gaseous carbon dioxide being introduced for 2 hours. After filtering, (c) 2.85 kg of crystalline monopotassium naphthalene-2,6-dicarboxylate and a filtrate (d) were obtained. The solid product (c) was placed in a vessel with a stirrer evaporation apparatus, treated with 541 water and disproportionated 2 hours at 10O ⁰ C with stirring. The mixture was then filtered hot to give a filter cake (Π and a filtrate (e). The cake (f) was further washed with 1.5 liters of hot water and dried. 1.35 kg of the desired naphthalene-2,6- dicarboxylic acid obtained.

step (e) was combined with the washing liquid and used to dissolve the crystalline dipotassium naphthalene-2,6-dicarboxylate (a'h that was obtained on the next pass, returned.

The naphthalene-2,6-dicarboxylic acid had a pure

unit of 99.4% and an optical density, determined as an aqueous ammonia solution in a ratio of 1: 1 400 πΐμ, from 0.005. The product could directly with Ethylene glycol with the formation of a polyethylene-2,6-naphthalenedicarboxylate be implemented of high quality.

The filtrates (b) and (d) were combined, treated with 2-naphthoic acid and cadmium iodide and evaporated to dryness. The dry product underwent the rearrangement under the same conditions as above and converted the reaction product into naphthalene-2,6-dicarboxylic acid as described. These cycle operations were 10 times repeated.

In all tests the purity of the naphthalene-2,6-dicarboxylic acid obtained was 99.4 bis 99.6%: the optical density of the aqueous ammonia solution at 400 ηΐμ was between 0.005 and 0.006.

Example 4

4 mol percent, based on potassium salts, of cadmium iodide were added to a mixture of potassium 1-naphthoate and potassium 2-naphthoate, and the mixture was heated at 380 ° C. for 4 hours ^{in a pressurized carbon dioxide atmosphere of 30 kg / cm 2.} The resulting rearrangement product was dissolved in warm water and the insoluble material was filtered off. The filtrate was decolorized with activated charcoal. This gave 100 kg of an aqueous solution of the crude dipotassium naphthalene-2,6-dicarboxylate which contained 18.5 kg of dipotassium naphthalene-o-dicarboxylate and 1.5 kg of other potassium salts of naphthalenecarboxylic acids. The purity of the dipotassium naphthalene-2,6-dicarboxylate was 92.4 percent by weight.

An aqueous solution containing 20 percent by weight of potassium hydrogen carbonate was added to this 100 kg aqueous solution added, in a vaporizer

I 940 319

mixed and concentrated. The solid cake formed was filtered off at 80 to 90 C, with the following Results (Table IV) were obtained:

Table IV

Weight of the aqueous
KHCO

KHCOj solution
of 20 percent by weight

(kg)

30th

41

*) Explanation of the expression has already been given.

The corresponding values with the cake obtained by identical procedures, but with the addition of potassium hydrogen carbonate are shown in Table IV for comparison.

Table IV

17.4 kg of the dipotassium naphthalene-2,6-dicarboxylate obtained by mixed concentration with the addition of potassium hydrogen carbonate were processed into an aqueous solution of 15 percent by weight, decolorized with activated carbon and kept for 1 hour at 20 ^° C. while introducing gaseous carbon dioxide. The resulting white precipitate was filtered and consisted of monopotassium naphthalene-2,6-dicarboxylate. The filter cake was made into an aqueous slurry having a concentration of 5 percent by weight and refluxed for 1 hour to disproportionate. The cake obtained by hot filtration of the white precipitate obtained consisted of 5.3 kg of naphthalene-2,6-dicarboxylic acid. The purity of the acid was 99.9%, the optical density of its aqueous ammonia solution (ratio 1: 1, measured at 400 ΐτίμ) was 0.002.

Example 5

To 100 kg of the aqueous solution of the crude obtained by the same procedure as in Example 5 Dipotassium naphthalene-2,6-dicarboxylate was 99 kg of the in the carbon dioxide reaction stage according to Example 5 obtained mother liquor, from which the precipitate

ίο had been filtered off, added, mixed in an evaporator and concentrated. When the concentration was 53%, the precipitate yield was 96% and the purity of the precipitate was 99.5%.
Furthermore, 250 kg of the filirate (Mutler liquor), which had been obtained in the disproportionation according to Example 5, were added to the dipotassium naphthalene-2,6-dicarboxylate and concentrated to an aqueous solution of 15 percent by weight. The solution was decolorized and carbon dioxide was passed in for 1 hour at 20 ^° C. with stirring. The cake obtained after filtration was made into an aqueous slurry of 5% by weight, which was disproportionated by refluxing for 1 hour. By hot filtration, 8.1 kg of a solid cake consisting of naphthalene-2,6-dicarboxylic acid was obtained. The product was 99.9% pure and the optical density (as determined above) was 0.003.

- ₀ B eis ρ ie 1 6

The four-stage work cycle of the relocation. Mixture concentration, carbon dioxide treatment and disproportionate with the three in FIG. 2 was repeated nine times. The following results were obtained: On 20 kg of the crude dipotassium naphtha! In-2,6-dicarboxylate, which remained in warm water after the insoluble matter was separated off 12.9 kg of naphthalene-2,6-dicarboxylic acid obtained on the ninth pass. Fraud in all attempts the purity of the product 99.9%. and the optical density (as above) was between 0.002 and 0.004. the Naphthalene-2,6-dicarboxylic acid was polycondensed directly with ethylene glycol. A polyethylnaphthalene-zodicarboxylate was thereby used with the same degree of brightness as a polyethylene terephthalate obtained from directly polymerizable terephthalic acid.

1 sheet of drawings

Claims (3)

Patent claims: 1. Process for the production of naphthalene-2,6-dicarboxylic acid from the product containing a dialkali salt of naphthalene-2,6-dicarboxylic acid the thermal rearrangement or thermal disproportionation of alkali salts of others Naphthalene mono- or dicarboxylic acids than the naphthalene-2,6-dicarboxylic acid, thereby characterized in that a) a filtered aqueous solution of the dialkali salt containing Concentrates product so far that when the concentrated solution is cooled to room temperature at least 70% of the dialkali salt precipitate, or b) an alkali metal carbonate or mix bicarbonate and concentrate this mixture; by the aqueous solution of according to a) or b) deposited dialkali salt passes gaseous carbon dioxide, the resulting precipitate separates the monoalkali salt of naphthalene-2,6-dicarboxylic acid, this by heating in water or an aqueous organic solvent in naphthalene-2,6-dicarboxylic acid and the dialkali salt of naphthalene-2,6-dicarboxylic acid is disproportionated, the precipitated naphthalene-2,6-dicarboxylic acid separated and the mother liquor returned to the carbon dioxide conversion stage. 2. The method according to claim 1, characterized in that that the mother liquor remaining after a) or b) together with that from the carbon dioxide conversion stage The mother liquor obtained is used to produce the starting material for the rearrangement. 3. The method according to claim 1 or 2, characterized in that one proceeds according to b) and the mother liquor obtained from the carbon dioxide conversion stage is returned to the mixed concentration stage.