CS227320B2 - Method of recovering oxidation catalysts on the basis of heavy metals in the production of dimethyl terephtalate according to witten - Google Patents

Abstract

1. Process for the recovery and reuse of heavy metal oxidation catalyst from the Witten process for the production of dimethyltherephthalate proceeding from high boiling distillation residues with a cobalt content of 1 to 10 g/kg and possibly a manganese content of 0.1 to 5 g/kg residue and/or a nickel content of 0.1 to 5 g/kg of residue, which accumulate in the oxidation of p-xylene and/or p-toluic acid methyl ester-containing mixtures in the liquid phase with oxygen or an oxygen-containing gas at a pressure of 4 to 8 bar and a temperature of 140 to 170 degrees C in the presence of dissolved heavy metal oxidation catalyst, subsequent esterification of the oxidation product with methanol at a pressure of 20 to 30 bar and a temperature of 230 to 280 degrees C and distillative separation of the esterification product into a p-toluic acid methyl ester-rich fraction, a dimethylterephthalate-rich fraction and a high boiling distillation residue, by extraction of the heavy metal oxidation catalyst with water, diluted mineral acids, aqueous low molecular aliphatic monocarboxylic acid with 1 to 4 C-atoms or alcohols with heating, optionally after combustion of the high boiling distillation residues, characterized in that a) the aqueous, acidic, heavy metal oxidation catalyst-containing extract with a cobalt content of 0.2 to 20 g/l, where appropriate a manganese content of 0.05 to 10 g/l and where appropriate a nickel content of 0.05 to 10 g/l is treated with a strongly acid cation exchanger resin in alkali metal form, for example Na**+- or K**+-form at elevated temperature up until its exchanger capacity is attained, b) the cation exchanger resin is washed at elevated temperature and is regenerated at room temperature with Na**+- or K**+-acetate-containing solutions with displacement of the catalyst components and receipt of an aqueous acetic acid catalyst solution containing the catalyst components, and c) the regenerated catalyst solution is employed again directly in the oxidation of p-xylene and/or p-toluic acid methyl ester.

Description

The present invention relates to a process for the recovery of heavy metal oxidation catalysts for the production of dimetherephthalate by the Wiiten method. It is based on high-boiling distillations of residues with a cobdl content of between 1 and 10 g / kg of residue and optionally with a manganese content of between 0,1 and 5 g / kg of residue and / or with a nickel content of between 0,1 and 5 g / kg residue which odpadej during the oxidation of mixtures of p-xylene obsOihuících and / or metylesttr p-toluate in the liquid phase _ч pomooí gases enveloping oxygen. This oxidation is carried out at a pressure in the range of 140 to 170 ° C in the presence of a dissolved oxidation catalyst, followed by an ester of the oxidation product with methanol at a pressure in the range of 2-3 M at a temperature in the range of 230-280 ° C and distillative. separating the product into a fraction rich in methyl p-toluyl ester, a fraction rich in dimethyl terephthalate and high-boiling distillates.

Dirnethylteftephthalate is needed as a raw material for the production of polyester by reaction of ethylene glycol or tetra-ethylene glycol. This polyester is used for the production of fibers, fillers, films or moldings. Dimethyl tthi phthalate is produced in various large-scale plants by known methods according to Witten or Witten as well.

The industrial process is such that the mixture contains! In the absence of solvents and halogen compounds, the p-xylene ester of p-toluylic acid is reacted in the presence of cobalt and manganese compounds dissolved in the reaction mixture to form an oxidate consisting predominantly of p-olefinic acid, loolooltetetetlate and tetetilic acid. at a temperature in the range of from 230 to 280 ° C and a pressure in the range of from 2 to 3 MPa of the ester. The heavy metal oxidation catalyst system is preferably added in an amount of about 70 to 200 ppm of cobalt and 7 to 100 ppm of hydrogen, based on the weight of the catalyst. and the metal content. The ester ester product is distilled in the so-called distillation of the crude ester

227320 2 is separated into a p-toluylic acid methyl ester-rich fraction, a dimethyl terephthalate-rich fraction and a high-boiling distillation residue. The methyl-rich p-toluyl ester fraction is returned to oxidation, and the dimethyl-terephthalate-rich fraction is subjected to subsequent purification and further processing. The high-boiling residue contains, in addition to the organic constituents, compounds of a heavy metal catalyst system, such as cobalt and manganese compounds.

Technically, it is possible to bring high-boiling distillation residues of alkylaromate oxidation, from which no further usable products are to be recovered either by isolation or conversion, to incineration, possibly using combustion heat, and to obtain heavy metal ash from flue gas after combustion, for example U.S. Patent No. 3,341,470).

The ash of this type comprises catalyst components which have been present in the high-boiling distillation residue, if not previously removed by extraction, parts of the maaerial of the used equipment released by corrosion, as well as components of the added fuel, such as black oil or heavy fuel. oils.

The ash of this kind can be treated with mineral acids and the heavy metals precipitate out of solution as nitrides or hydroxides (DE-OS 22 60 498). Separation of such precipitates by filtration or centrifugation, as well as the removal of adhering residues of the corresponding mineral acids by means of a hydration cake, however, present technical difficulties.

The complete removal of inorganic acid residues is a prerequisite for the re-desert of heavy metals coming from high-boiling distillation residues as oxidation catalysts for the oxidation of alkyl aromatics in the liquid phase by air oxygen.

The process of the present invention further provides that in such ashes resulting from the combustion of high-boiling distillation residues from the liquid phase oxidation process in the presence of three heavy metal catalysts, the content of heavy metals, such as iron, is increased, chromium, vanadium, molybdenum, me3 and titanium, which are derived from the maaerials of the equipment used and from fuels used for the combustion of residues. These heavy metals substantially reduce or inhibit the activity of the cobalt, nickel or mimganic catalysts or mixtures thereof when reintroduced into the oxidation reaction.

For the production of dimethyl terephthalate, it is extremely advantageous to recover the oxidation catalyst, i.e. a mixture of cobalt and manganese compounds and / or nickel compounds, optionally after burning the distillation high-boiling residue and reuse it to oxidize the p-xylene and the meetylester.

It is therefore an object of the present invention to recover the catalyst from the high-boiling distillation residues from the distillation of the crude ester. These separated components may be treated directly, directly into the oxidation, or valuable catalyst components in the form of aqueous solutions may be obtained, after as simple processing as possible without evaporation or the presence of trimellitic acid and the trimethyloloyl ether of trimellitic acid present. for other povjiií.

The present invention is based on obtaining from the acid extract a ratate regenerate which is practically completely free from interfering organic constituents, in particular trimetalic acid and the trimellitic acid movers, as well as metal compounds formed from corrosion plant parts.

BE patent application No. P 29 23 681 proposes a process for recovering an oxidation catalyst from distillate residues which is free from the catalyst and which is dispensed with in the production of dimethyl terephthalate and the reuse of the recovered catalyst in the process. as in the fresh catalyst desert. In this process, it has been shown that during the extraction of the catalyst residues from the distillation of the crude ester, trimellitic acid and trimellitic moonmethyl ether are dissolved together with the catalyst, whereby trimeeitic acid and trimellitic acid monomethyl ester can substantially degrade the oxidation reaction. Therefore, in the extract, the weight ratio of total trimellitic acid to trimonitic acid monoethyl ether to multiplication is measured in the extract. a heavy metal catalyst to a msudmin value of 1.8: 1.

According to the present invention, the content of trimellitic acid and the momomethylsulfate of trimellitic acid in the extract by distillation of the residues can be many times higher, for example five times higher than the content of cobeate and manganese, thus achieving practically three times the ratio of total trimellitic acid and mononetylee. The amounts of heavy metal catalysts admitted in the aforementioned DE patent application No. P 29 23 681.

The content of trimellitic acid and of the trimethylolone monoethylether in the extract depends on the type of treatment of the crude ester and hence on the chemical composition of the high-boiling distillation residue. With increasing content of trimellitic acid. and trimellitic acid monooeeyl ether 'in the extract, an increased consumption of fresh heavy metal catalyst is needed to guarantee a flawless oxidation reaction during the recovery of the extracted catalyst.

It is also an object of the present invention to recover cobalt or cobalt and manganese compounds together with nickel compounds.

The above-mentioned drawbacks have been overcome by providing a process according to the present invention which consists in extracting the high-boiling distillation residues directly or after burning with water, dilute mineral acids, aqueous solutions of low (< -1 >). aliphatic mono-carbonic acids having 1 to 4 carbon atoms or alcohols and an aqueous acidic extract containing cobalt in an amount of 0.2 to 20 g / liter, or manganese in a multiplication of 0.05 to 10 g / liter and optionally nickel in O, 05-10 g / liter, is treated with a strongly acidic cloning resin in an alkali metal cycle, for example a sodium or potassium cycle, at a temperature in the range of 20 to 95 ° C, preferably at 70 ° C, until the ion exchange and cation exchange resin exchange capacity is then washed at a temperature in the range of 20 to 95 ° C, preferably at 70 ° C, and regenerated at room temperature with sodium acetate or vinegar solution. potassium.

The residues from the high-boiling distillation of heavy metal residues are dissolved in eosels of acids, for example hydrochloric acid, with the addition of oxidizing agents such as hydrogen peroxide solution which serves to oxidize, inter alia, divalent iron ions and impurities accumulated in the solution. they are precipitated from the small equipment and from the spent fuel, such as iron, chromium, vanadium, copper and titanium, by adjusting the pH of the solution to 6 or, for example, precipitated as hydroxides with aqueous sodium hydroxide and separated off with insoluble ash .

The purified solution was worked up by addition of linear aliphatic nízkneonelcU.árních ^eenoncl, booxylových acids having 1 to 4 carbon atoms, for example by adding acetic acid to adjust pH to 5 or below. The sodium ions contained in the solution are removed by treatment with a strongly acidic cationic exchange resin, which is heavily occupied with cobalt, magnesium and / or magnesium ions. The acetic acid acidified heavy metal solution is then treated with a strongly acidic cationic resin in a sodium or potassium cycle until the ion exchanger capacity is reached, then washed at elevated temperature and regenerated at room temperature with sodium acetate or potassium acetate solution to displace the metal to form an aqueous solution acidified with acetic acid containing metal ions as part of the catalyst.

By treating the combustion products of the high-boiling residue residues of the present invention, it is possible in a simple way to obtain a water-soluble organic catalyst solution, free of mineral acid residues and contaminants that reduce the activity of the catalyst components.

The aqueous catalyst solutions obtained contain cobalt acetate and mngirnyl acetate containing about 5 to 70 g / l of cobalt, 1 to 35 g / l of mwngane and optionally nickel (II) vine in an amount of about 1 to 35 g / l.

These aqueous solutions, enveloping catalyst components such as acetates, are preferably recycled directly to the oxidation of the mixture containing the methyl ester of p-xylene and / or p-toluylic acid.

In addition to the ion exchanger in an alkali metal cycle, preferably sodium or potassium, it can be used to absorb an ion exchanger catalyst that operates in a hydrogen cycle.

It is advantageous to regenerate the ion exchanger saturated with the metal ions of the catalyst with dilute aqueous sodium acetate solutions, since the reaction equilibrium is used when strongly acidic ion exchangers are used.

R- (C ²⁺ ) + 2 Na ⁺ <----> R- (Nei ⁺ ) 2 + Co ²⁺ in comparison and reaction equilibrium equation

R- (C ²⁺ ) + 2H ⁺ <-----> R- (H ⁺ ) ₂ + Co ²⁺ in which R denotes a stationary ion-exchange matrix, shifted more to the right side of the reaction plane.

In the surprisingly simple manner, the process of the present invention makes it possible to concentrate the catalyst content of the extract to about 20 times by treating with strong acidic cation exchange resin. higher than the starting concentration, without comparable high contents of trimellitic acid and trimethylitic acid methyl ether interfering with the formation of complexes of the metal ion exchange of the catalyst with, for example, sodium ion exchange ion.

Extraction of the extract by evaporation, which leads to losses of the catalyst metal by the formation of flakes and precipitates at elevated levels of trimeeitic acid and monimethyl ester of trimmeitic acid, can therefore be omitted according to the invention. Conversely, the separation of trimethitic acid and momimethyl ester as well as other accompanying organic salts is achieved in a simple manner. Notwithstanding the disturbances that cause higher trimeeitic acid and trimethylitic acid smear levels in the oxidation of bolds containing p-xylene and / or P'-toluye acid methyl ester, this result is particularly significant

The process of the present invention is carried out in a preferred form by cooling the acidic aqueous extracts at a temperature of about 95 ° C to a temperature of one minute and separating the precipitated organic components. This is followed by reheating to about 70 ° C to avoid further precipitation. When followed cm processing on strong acid ion exchange pryskyHci in the sodium at a temperature of about 70 ° C to extract still present application rates of organic souččzsí, especially acid Уiimeelitové, monimttylleteru acid УiimetliУové _l of УtitfУalové, monomtet ^ y ^ (Beretet ^ l-carboxylate and podobrých , they do not absorb on the ion exchange, but remain in the aqueous phase and pass through the ion exchanger without inhibition.

In the event that the waste water thus formed, which is predominantly alkali metal ions and organic compounds, can not be transferred to biological treatment but has to be subjected to a pelvic treatment, the alkali metal ions can be enriched in the ion exchanger by treatment with a strongly acidic cation exchange resin. and eluting with a strong acid, preferably hydrochloric acid, to remove the process as an aqueous solution of a neutral salt such as sodium chloride.

Upon reaching the exchange capacity of the ion exchanger with cobalt ions and mefigamers, the saturated ion exchanger is treated with fully desalinated water as a washing liquid which is preheated to a temperature of about 70 ° C. This treatment serves to remove organic components that have been absorbed onto the ion exchange resin and which form a precipitated film on the matrix surface, which would substantially reduce the resin exchange capacity. The effluent living water is expediently introduced into the extraction.

At the subsequent regeneration, which is carried out at a temperature, about

The first bed volumes of the aqueous sodium acetate ion exchanger retained in the previous regeneration cycle as Drip and which are mixed with the anterior of the subsequent regeneration cycle are placed on a cobalt-ion-exchanged ion exchanger whose bed was initially filled with fully deminrolized water. About 50% of the bed volume is withdrawn as a solution free of cobalt and meaigcaate ions. Thereafter, about 15% of the bed volume was seized as a front with a low content of cobalt and manganese. ions. About 135% of the bed volume is taken as the PiOL fraction as a solution of cobalt and maauannate acetate, hereinafter referred to as the conceninate.

About 80% bed volume of about 15% aqueous sodium acetate solution containing about 10-15 g / l of free acetic acid is used to completely displace the catalyst component from the ion exchanger followed by about 80% bed volume by demim of settled water to remove sodium acetate solution containing cobalt and associated ions from the noitomanic bed. The solutions obtained in an amount of about 160% of the bed volume are withdrawn up to approx

After the concentration of cobalt and meauanate ions has been carried out, the solution is further withdrawn in an amount of 5 to 10% and combined with the concentrate. The inflowing fractions in an amount of about 150-155% of the bed volume are withdrawn as a drip, combined with the first withdrawn ancestor fraction and are used in the next displacement cycle.

№ ^ s ^ 1 ^ <^ <^^^ ^{C UII:} I Examples serve to illustrate the present invention.

The incineration residue of Examples 3 and 4 is obtained by burning high-boiling distillation residues from the production of Pioelytteeeffttu from the Pie Bitena process at 800 to 1200 ° C and separating from the spia-in by means of 95% of the homogeneous high-boiling residues. addition of 5% by weight of heavy fuel oil. ВезН! ^! the residues to be incinerated contain about 0.1 to 1.0% of the heavy metal ash forming components of the heavy metals. '

Example 1

100 kg of the residue from the distillation of the crude ester obtained in an industrial plant for the production of výrobuΡ β β Η ta ta ta ta ta ta ta společ společ společ nou nou společ společ nou nou společ of oxygen as oxidizing agent, at a pressure of about 0.8 M M and at a temperature in the range of 150 to 170 ° C, using a catalyst in the form of a solution of cobalt acetate and meoeaeate acetate in aqueous acetic acid, with a constant concentration in the oxidation product of about 90 ppm cobalt and 10 ppm of manganese, followed by a continuous esterification of the oxidation product with methyl alcohol at a temperature of about 250 degrees Celsius and a pressure of 2.5 MPa and a continuous distillation of the product by vacuum distillation, the fraction rich in the head being collected. p-toluylic acid, which is returned after the oxidation, and the bottom product is passed through following a distillation column, where a fraction rich in Pioellyl terephthalate is collected in the head and high-boiling distillation residues with a cobalt content of 2.3 g / kg and a manganese content of 0.2 g / kg remain in the heel, 60 l of reaction water from dimethylterephthalate production is extracted with an acid content of about 3%, calculated as acetic acid, at a temperature of about 95 ° C up to a residual cobaltate content of 20 ppm.

After decnitation, 56 l of an extract containing cobalt and meaigeate ions is obtained, the cobalt ion content being 3.8 g / l and the manganese ion content 0.3 g / l. The hot extract was cooled to 20 ° C and the precipitated organic layers were removed by filtration.

The titanium dioxide solution is heated to 70 ° C to prevent additional precipitation and is introduced from below into a tube filled with a soda-acid ion-exchange resin in the sodium cycle labeled Lewaatt S 100. The resin volume is 1.1 titers. This introduction is only performed. until the ion exchanger capacity is exhausted.

Of the 56 titanium cobalt and meangian extracts produced, 19 liters at 70 ° C are passed through the cation exchange resin, corresponding to a total cobalt and meaigeanate ion content of 78.4 g, or 2.66 cobalt ion rolls. Then, the cation exchanger was washed from above with one titre of fully demineralized water at 70 ° C. Then, the elution is terminated by pouring the cationic resin from above with 2.2 titers of a 10% aqueous sodium acetate solution, corresponding to 1.3 wad of sodium ions per solution titre, followed by changing the gradient with one 100% of completely mineralized water at room temperature. 0.4 titer of front, 2.0 titre of concentrate and 0.8 µtr of the drip are obtained.

The aqueous concentrate contains the following:

cobalt ions mengimate ions sodium ions acetic acid organic impurities

30.1 g / l

2.6 g / l

1,0 gi

12.0 g / 1 undetectable (polarographically)

In order to reduce the loss of cobalt and mengenate ions, the front and the droplets are combined with cobalt, mengmiate and sodium ions and reused as an elution solution.

Example 2

In a continuously operating extraction apparatus, 300 kg / h of high-boiling distillation residues are extracted from the crude ester ester obtained in the same manner as described in Example 1 at 95 ° C. The extraction was carried out with 150 kg / h of acidic reaction water from the production of dimethyl terephthalate, whose origin and acid content were the same as in Example 1. The extraction was carried out with stirring.

After separation of the organic phase, the aqueous solution obtained contains 4.6 g / l of cationic ions and 0.4 g / l of cationic ions. The aqueous solution was cooled to about 20 ° C, precipitating organic products which were filtered off and returned to the process. The aqueous solution is collected in a reservoir. In order to prevent further precipitation of organic fats, the aqueous solution is heated to 70 ° C and 650 l / h of this solution is passed at 70 ° C through a gradient filled with 180 Htry sieve acidic cation exchange resin under the trade name Lewaatt S 100 sodium cycle. The ion exchange flow is stopped after 3 hours.

In order to remove the organic surfactants from the slope, this is then treated with 400 l of hot fully demineralized water. Then nuu! the catalyst ions with a solution of sodium acetate containing 10-15 g / l of free acetic acid, which is partly derived from the ancestor and is dripped from a previous elution. This solution contains 15% sodium acetate, at room temperature. A total of 515 liters of solution was used for elution. After elution, the ion exchanger column was washed with 170 liters of fully demineralized water.

Four fractions are obtained in this way: ‘liters of cobalt and msaigsaate-free solution, liters of the front

240 liters of endetret and

320 liters of drip

The fermented endate contains:

37.0 g / l of cobalt ions a

3.1 · g / l mEaigenate ions.

Organic components except acetic acid are not detectable by polarographs. The front and the drip are joined and reused in the next loop. The concentrate obtained is directly introduced into the oxidation described in Example 1. The attentivity of this endenate is the same as that of a fresh catalyst with the same concentration of cobalt acetate and manganese acetate.

Example 3

50.5 g of residual residue from the diimelte ether acetate process were dissolved in 300 ml of dilute hydrochloric acid solution (12%) and 2 ml of 30% hydrogen peroxide solution for 2 hours at 95 ° C.

The sample used contains:

50.7% of the cobalt content

5.4% by weight of manganese

0.37% w / w iron

0.13% nickel nickel

100 ppm chromium

000 ppm molybdenum

100 ppm vanadium

100 ppm copper and

00 ppm titanium.

Finally, the solution is diluted with completely demineralized water and mixed with about 40% sodium hydroxide solution to pH 7. 9 ml of sodium hydroxide solution are used. The reaction mixture was then heated at 95 ° C for one hour and finally filtered through a pleated filter.

The filtrate is diluted to 8 liters with demineralized water and adjusted to pH 4 by addition of 5 ml of concentrated acetic acid.

This solution contains:

2.9 g / l of cobalt

0.2 g / l of manganese ppm nickel <5 ppm iron <5 ppm chromium, molybdenum, vanadium, copper, titanium.

The solution obtained is passed through a column of 250 ml of strongly acidic cation exchange resin, which is saturated with cobalt and meaigrate ions, to completely remove the sodium ions.

The solution obtained is passed through a column packed with 250 ml of a strongly acidic cation exchange resin with the trade name Lewattt S 100 in a sodium cycle. The effluent obtained at the outflow contains:

ppm of cobalt and ppm of methane.

The above solution is passed through the column until the ion exchanger is exhausted when 300 ppm of cobalt is contained in the waste water. This way the column is switched. 3.5 liters of the pellets were spread

After depletion of the ion exchange resin, the column is washed with 250 ml of deminiralized water.

The elution of the cations is determined by the following solutions:

400 ml of pooled ancestor and drip fractions from the previous experiment

200 m of 18% aqueous sodium acetate solution with 15 g / l of free acetic acid

200 m deImnijot.izovtné vody.

Elution yields the following fractions:

ml of ancestor fraction

400 µl of main sodium-poor fraction

340 m sodium ion rich.

The fraction and drip fractions were combined and used to elute in the next experiment.

The main fraction contains the following solutions:

30.9 g / l of cobalt

2.1 g / l manganese ppm nickel <5 ppm amine, molybdenum, vanadium, mmdi and tin

175 ppm sodium.

The main fraction thus obtained can be used as a catalyst solution in the process for the preparation of dimethyl tertiary oil.

Example 4

50.1 g of the residue from the high-temperature decomposition residues were dissolved in 95 ml of 12% hydrochloric acid and 2 ml of 30% hydrogen peroxide solution at 95 DEG C. for 2 hours.

Inset residual ash contains the following quantities of individual components:

40.6% cobalt

4.3% manganese 19.8% nickel

970 ppm iron <100 ppm chromium

800 ppm molybdenum <100 ppm vanadium ppm copper <100 ppm muzzle

280 ppm sodium.

The pH of the mixture is adjusted to 6.2 with 7 ml of about 40% aqueous sodium hydroxide solution. After one hour, the solution, heated to 95 ° C, is filtered through a pleated filter. The filtrate was diluted with demineralized water to a total volume of 10 liters and adjusted to pH 3.9 by addition of 10 mL of concentrated acetic acid. This solution contains the following amounts of individual components:

1.64 g / l of cobalt

0.16 g / l of manganese

0.87 g / l nickel <5 ppm iron <5 ppm chromium <5 ppm molybdenum <5 ppm vanadium <5 ppm copper <5 ppm titanium.

The solution obtained is passed through a column of 250 ml of strongly acidic cation exchange resin saturated with cobalt, manganese and nickel ions to remove a substantial portion of the sodium ions.

This solution is now passed through a column packed with 250 ml of strongly acidic Lewatit S 100 cation exchange resin in a sodium cycle. The waste water at the outflow contains:

ppm cobalt ppm manganese ppm nickel.

4.7 liters of this solution are consumed to saturate the ion exchanger until it is exhausted.

After the ion exchanger has been exhausted, it is washed with 250 ml of demineralized water.

The following solutions are used to elute cobalt, manganese and nickel ions:

390 ml of pooled ancestor and drip fractions from the previous experiment

200 ml of 18% sodium acetate solution containing 15 g / l of free acetic acid

210 ml demineralized water ·

The following solutions are obtained by elution:

ml of ancestor fraction

400 ml main fraction, low in sodium ions

320 ml sodium ion-rich drip fraction.

The main fraction contains the following components:

21.5 g / l of cobalt

1.9 g / l of manganese

10.2 g / l nickel <5 ppm iron <5 ppm chromium <5 ppm molybdenum <5 ppm vanadium <5 ppm copper <5 ppm titanium 240 ppm sodium.

Claims (3)

SUBJECT OF THE INVENTION Process for recovering heavy metal oxidation catalysts for the production of Witten dimethyl terephthalate starting from high-boiling distillation residues containing catalytically active metal compounds, which distillation residues originate from the oxidation of mixtures containing p-xylene and / or meetylate. p-toluyl, in the presence of a dissolved heavy metal catalyst, followed by esterification of the resulting oxidation product with methanol and a destitative separation of the esterification product into a p-toluylic acid-rich fraction, a dimethyl-terephthalate-rich fraction and a high-boiling distillation residue. the high-boiling distillation residues are extracted directly or after combustion with water, dilute mineral acids, aqueous solutions of low molecular weight aliphatic monocarboxylic acids having 1 to 4 carbon atoms or alcohols and aqueous an acidic extract containing cobalt in an amount of 0.2 to 20 g / liter or manganese in an amount of 0.05 to 10 g / liter and optionally nickel in an amount of 0.05 to 10 g / liter to treat a strongly acidic cation exchange an alkali metal resin, for example a sodium or long cycle, at a temperature in the range of 20 to 95 ° C, preferably at 70 ° C until the ion exchange capacity is reached, and the cation exchange resin is then at a temperature in the 20 to 95 ° C, preferably at 70 ° C, washed and regenerated at room temperature with sodium acetate or potassium acetate solution. 2. A process according to claim 1, characterized in that the high-boiling distillation residues are extracted with water, aqueous solutions of carbon monoxide (C 1-C 4) alcohols or alcohols and aqueous acidic extract, cobeat in the range of 0.2 to 20 g / liter. or a mengao of 0.05 to 10 g / liter and optionally nickel in an amount of 0.05 to 10 g / liter, is treated with a strongly acidic cationic resin in an icic metal cycle, for example in a sodium or an aryld cycle, at a temperature in the range 20 to 95 ° C, preferably 70 ° C, until the exchange capacity of the ion exchanger and the ion exchange resin is then washed at a temperature in the range of 20 to 95 ° C, preferably at 70 ° C, and regenerated at room temperature with sodium or potassium acetate solution. Process according to claim 1, characterized in that the ash is extracted after the combustion of high-boiling distillation residues by means of aqueous mineral acid solutions with the addition of oxidizing agents at a temperature of 95 ° C, then diluted with water, alkaline pH and precipitated iron and chromium hydroxides. and insoluble ash is filtered off, the filtrate is diluted. with water and acidification, the linear, low-carbon monohydroxy-monohydric acid having from 1 to 4 carbon atoms and the extract obtained is lyophilized. cobalt in the range of 0.2 to 20 g / liter, or possibly in the range of 0.05 to 10 g / liter, and optionally an oid in the range of 0.05 to 10 g / liter, is treated with a strongly acidic caustic ^, leached through the cycle. an alkali metal, for example in a sodium or sodium cycle, at a temperature in the range of 20 to 95 ° C, preferably at 70 ° C, until the exchange capacity of the ion exchanger and the exchange resin is reached at 20 to 95 ° C, preferably washed at 70 ° C and regenerated at room temperature with sodium or potassium acetate solution ·