DE2525135A1 - Dimethyl terephthalate prepn. by oxidn. process - comprising methanol treatment of recycled distn. residue or extract contg. catalyst - Google Patents

Abstract

Dimethyl terephthalate, (I), prepn. by oxidn. of p-xylene and/or Me p-toluate with O2 or an O2-contg. gas in the presence of a heavy metal catalyst, esterification of oxidn. reaction mixt. with MeOH, distn. of esterification mixt. to separate (I) from fractions having a lower b. pt. and recycling to oxidn. step of (A) distn. residue contg. the heavy metal catalyst and/or (B) heavy metal catalyst -contg. extract obtd. after further extn. of (A), takes place by process in which (A) and/or (B) is contacted with MeOH before recycling. (I) is used as starting material in prepn. of fibre- and film-forming polyesters. Contact of (A) or (B) with MeOH before recycling to oxidn. step, (i) increases yield of (I), (ii) reduces colour of oxidn. reaction mixt. and (iii) facilitates subsequent purification.

Description

It improves manufacturing process for dimethyl terephthalate die The invention relates to an improved process for the preparation of I) imethyl terephthalate. In particular, the invention relates to a method wherein p-xylene (PX) and / or methyl ptoluate (MPT) with either molecular oxygen or one containing molecular oxygen Gas can be oxidized in the presence of a heavy metal catalyst and methyl esterification takes place of the oxidation product to form an esterification reaction mixture, the resulting reaction mixture for the separation of dimethyl terephthalate (DMT) and the fractions with lower boiling points than Die is distilled and afterward the heavy metal oxidation catalyst contained in the distillation residue to the Oxidation level is returned in a high state of activity and in it reused: Dimethyl terephthalate (DMT) is a compound that is used as a starting material for the production of fiber and film-forming polyesters is valuable and becomes technical manufactured on a large scale.

Although a large number of processes for the production of DMT known are the SD method and the Witten method, which are described below are the two methods that are technically used to the greatest extent.

According to the SD; process, PX is made with a molecular oxygen-containing one Gas in the presence of a heavy metal catalyst and a bromine compound as a promoter in a lower fatty acid solvent such as acetic acid to form terephthalic acid (TA) oxidizes, whereupon the TA continues to esterify with methanol to form DMT (see, for example, U.S. Patent No. 2,833,816).

TA can thus after the SD processing by single-stage oxidation of PX, however, the bromine compound used as a promoter are and the acetic acid used as a solvent is highly corrosive. That's why the Apparatus must necessarily be made of expensive materials such as titanium and nevertheless, the service life of the equipment is short. Furthermore, there is a large amount of the Acetic acid to be used as a solvent is required and must also, Since TA is insoluble in acetic acid and cannot be melted itself, slurries and solids are handled in the process. Apart from that of this there are still numerous disadvantages if the point of view of the technical operation It is contemplated1 so that difficulties arise in purifying the TA.

Another widely used process is called the Witten process designated. In this process, PX is contained with a molecular oxygen Formation of gas in the liquid phase in the presence of a heavy metal catalyst oxidized by p-2oluic acid (PT-A), which esterifies with methanol to form MPT whereupon the obtained MPT in the liquid phase with a molecular oxygen contained gas in the presence of a heavy metal catalyst for conversion into nionomethyl terephthalate (MMT) is oxidized and then the MMT is mixed with methanol to form DMT esterified (for example, reference is made to British patent specification 727 989 referenced). Following this procedure, the PX will undergo the following four reaction stages supplied: The oxidation (PTA), the esterification (MPT), the oxidation (MMT) and the Esterification (DMT). There is also a proposal to oxidize a mixture of PX and MPT and then to esterify the oxidation product. In the latter case it will Mixture of PX and MPT in the liquid phase with one containing molecular oxygen Gas is oxidized in the presence of a heavy metal catalyst, whereupon the resulting oxidation reaction mixture is esterified from PTA and MMT with methanol. DMT is made from the esterification product formed in the process recovered and fresh PX becomes the remaining reaction mixture, which is mainly is built up from MPT, which is then again with the formation of MMT and PTA is oxidized (see, for example, British Patent 809 730 referenced).

- The DMU-containing esterification reaction mixture obtained by this Witter process is usually subjected to distillation to produce both DMT and To separate Eomponenten with a lower boiling point than DMT, for example the different Intermediate products which are converted into DMT during reoxidation and methyl esterification are capable, such as methyl p-toluate, ethyl p-formylbenzoate and p-tolualdehyde. on the other hand became the tar-like by-product with a higher boiling point than DiST, which is used as distillation residue which is the heavy metal catalyst used in the oxidation reaction contained, so far as it was obtained, to the oxidation state without separation of the heavy metal catalyst or after concentrating part of it. Alternatively, the heavy metal catalyst was separated off and removed from the still lotion residue recovered and then fed back to the oxidation stage. These were the methods previously used for the recycling of the catalyst and the like. Applied became.

According to the available investigations, however, it was found that, if, as described above, the distillation residue or the one after it Concentration obtained residue or from the distillation residue, for example Heavy metal catalyst separated by extraction to the oxidation stage without Treatment of the same, but returned in the directly received state, the disadvantage was obtained that compared with the case where a fresh heavy metal catalyst was used, the yield of DMT was low and the color of the oxidation reaction mixture continued was high, so that difficulties arose in its subsequent cleaning.

- It has now been found that if the above distillation return or the heavy metal catalyst separated from it by extraction the oxidation state after the previous contact treatment of the same with methanol is returned, an increase in the yield of DMT compared to the case obtained where the recycling of the still residue or the heavy metal catalyst carried out without this methanol treatment e and that the additional coloring of the oxidation reaction mixture was lower and thus the subsequent purification was relieved. In this way it was found that roughly comparable results to those where a fresh catalyst was used could be obtained.

Thus, in the process for the production of dimethyl terephthalate, where p-xylene and / or methyl ptoluate with molecular oxygen or a molecular Oxygen-containing gas oxidized in the presence of a heavy metal catalyst are, the resulting oxidation reaction mixture is esterified with methanol and then the esterification reaction mixture obtained for the separation of dimethyl terephthalate and fractions having a lower boiling point than dimethyl terephthalate are distilled and then to the above oxidation stage (A) of the heavy metal catalyst containing distillation residue and / or (B) the heavy metal catalyst containing Extrak + part, which after further extraction treatment of the distillation residue (A) was obtained, is recycled, according to the invention, an improved process, which by a contact treatment of this distillation residue (A) or des Extract part (B) is labeled with methanol prior to recycling the same.

In the improved process according to the invention, the oxidation of PX and / or MPT and the esterification of the oxidation reaction mixture as well as the Distillation of the esterification reaction mixture according to the previously known methods be performed. For example, these operations can be carried out according to the in British Patents 809,730 and 1,313,083; U.S. Patent 2,894 978 and German Offenlegungsschrift 2,311,209 will.

Both PX and MPT can be used individually as the starting material however, it is usually advantageous and beneficial to use a mixture of PX and EPT. PX and MPT are favorably in a ratio of common mixed between 2: 1 and 1: 4 on a weight basis.

Also the heavy metal catalyst used in the oxidation stage is one of the kind used as an oxidation catalyst in the Witten process will. For example, a class of heavy metals or heavy metal compounds, as described in UK Patent 809,730 or US Patent, supra 2,894,978 are listed, can be used individually or as oxidation catalysts of the CoS or Ni-P type as described in British Patent 1,313,083 or US Pat German OgS 2 311 209 can be used. The use of catalysts Co-Mn or Ni-Mn type is particularly preferred.

Heavy metal compounds can be those which are insoluble or are difficultly soluble in the reaction mixture, as well as those which are soluble in it are to be used. Examples of such compounds are the various inorganic compounds such as carbonates, oxides, Hydroxides and Like., of cobalt, nickel and manganese. The preferred compounds are the cobalt, Nickel and manganese salts of organic acids, of which acetates are the strongest, Benzoates, toluates and naphthenates of cobalt, nickel and manganese are preferred, since these are easily accessible and still have a gate solubility in the reaction mixture demonstrate.

The oxidation reaction can usually take place at a temperature im Range from 140 to 2500 C.

The oxidation conditions can be within a wide range within the framework of the known methods listed above according to the classes of starting materials and catalysts and the like. Vary. A preferred method according to the invention, wherein the catalysts of Co-Mn and Ni-Mn type are used will be dealt with briefly below. If, for example, a catalyst Co-Mn type is used, oxidation of a mixture is favorably used of PX and MPT with molecular oxygen or one containing molecular oxygen Gas at a temperature of 140 to 2400 C and preferably 160 to 2200 C in the presence a reaction system from a catalyst mixture, which consists of Sobaltmetall or consists of a cobalt compound and a manganese metal or a manganese compound, wherein the atomic ratio of Co to Mn is in the range from 99.9: 0.1 to 1:99 and preferably 99: 1 to 10:90, in such a concentration that the total amount of these materials, calculated as manganese and cobalt metals, 50 to 1500 ppm and preferably 80 to 500 ppm, based on the total amount of the reaction mixture, amounts to. If, on the other hand, the oxidation state using a catalyst is performed by the Ni-Mn type, the operation becomes most favorable by Oxidation of the mixture of PX and MPT with molecular oxygen or a molecular one Oxygen-containing gas at a temperature of 160 to 250 ° C and in particular 180 to 2300 C in the presence of a catalyst mixture consisting of a nickel metal or a nickel compound and a manganese metal or a manganese compound, wherein the atomic ratio of Ni to Mn is 95: 5 to 0.5: 99.5 and preferably 90 : 10 to 2: 98, in such a concentration that the total amount of these metals, calculated as nickel and manganese metals, from 20 to 5000 ppm and preferably 80 to 800 ppm, based on the total amount of the reaction mixture, is carried out.

The methyl esterification of the resulting oxidation reaction mixture and the distillation can be carried out according to the known ones described above Procedures are carried out.

The essential features of the present invention reside in the Considerations that the distillation residue containing the heavy metal catalyst (A), which was obtained after the above-described distillation, or the heavy metal catalyst containing extraction part (B), which after a further extraction treatment of (A) was obtained with methanol for activation the heavy metal catalyst are treated, whereupon the reactivated catalyst is recycled to the oxidation stage. The present invention is particular suitable for activating oxidation catalysts of the Co, Mn, Co-Mn and Ni-Mn types.

Under the term "distillation residue (A)", which according to of the invention is contacted with methanol, it is understood that the residue component is included, which is carried out after the separation by distillation practically of all or a major part of the crude DMT and the fractions with lower Boiling point as DMT from the esterification reaction mixture in the case of the one discussed above so-called Witten process remains, or the concentrated product of this residue component, the concentration of which was carried out in the following manner. The one after the separation distillation residue remaining from DMT and fractions with a lower boiling point than DMT is to temperatures of, for example, 260 to 4000 C and preferably 280 to 3800 C and then after the recovery of active components, such as DMT, the are relatively easy to distill off, a concentrated product remains. The method according to the invention can also be applied to this concentrated product will.

Furthermore, according to the invention, the catalyst containing the heavy metal catalyst can be used Extract part (B), which after the treatment of the distillation residue (A) with extraction treatment as well as contact treatment be subjected to methanol.

For the extraction and separation of the heavy metal catalyst from the The above distillation residue is favorably an aqueous solvent used.

In addition to water, solvent mixtures can be used as such aqueous solvents of lower aliphatic alcohols such as methanol and water, and mixed solvents of lower fatty acids and water. The extraction can also be done after all such liquid-liquid extraction systems, such as single batch extraction, concurrent multi-stage extraction, counter-rotating multi-stage extraction and continuous counter-rotating extraction systems are carried out. The extract obtained in the process can then, in the state as obtained, the following methanol treatment to be added or, if necessary, after an éil or the total amount the extraction solvent was removed.

There are no strict restrictions on carrying out the methanol treatment according to the present invention, provided that it is carried out in such a way that intimate contact between the distillation residue listed above (A) or the extraction part (B) and methanol is carried out.

For example, the temperature at which the contact between the Distillation residue (A) or the extract part (B) and the methanol carried out is not critical and the temperature can be adjusted accordingly within a wide range carried out under the applied oxidation, esterification and distillation conditions will. However, a temperature is usually in the range of 100 to 4500 C, preferably 150 to 4000 C, and more preferably 170 to 3500 C, usually suitable.

If a temperature lower than 1000 C is used, it occurs no great improvement in the activity of the recovered catalyst, so that the activation effects attributable to the methanol treatment are low are. On the other hand, if the temperature exceeds 4500 C, it is not either cheap. Although the activation effects are not affected, an increase occurs the equipment costs due to the increased reaction pressure, the loss Methanol due to the formation of dimethyl ether and the like.

The amount of methanol used in the contact treatment becomes influenced by the class of the distillation residue (A) or the extract part (B) and can thus cannot be specifically specified. Usually however, the methanol is used in an amount of at least 0.02 part by weight for each Part by weight of the distillation residue (&) or the extract part (B) is used. Although there is no particular limitation on the upper limit, is the use of methanol in too large an amount is not economical. she should therefore not more than 20% by weight. Especially in the case of the distillation residue (A) an amount in the range from 0.05 to 10 parts by weight is preferred. on the other hand in the case of the extraction part (B), methanol is favorably in an amount of at least 0.2 parts by weight and preferably in the range from 0.5 to 1000 parts by weight used.

Although the length of time with which the still residue (A) or the Extraction part (B) is contacted with methanol according to the invention, favorably should be selected in accordance with such conditions as contact temperature and the like usually 0.1 to 20 hours will be sufficient. If the contact time is too short is, the effects obtained by contact with methanol are insufficient. On the other hand, if the period of time is too long, it is also not favorable because it is is not only uneconomical, but also a loss of methanol due to it of side reactions takes place.

The method to be used according to the invention for contacting the distillation residue (A) or the extract part (B) with methanol is not particularly limited.

However, such a method is advantageously used as z. B. one where the single batch contact is made in the batch the distillation residue (A) or the extraction part (B) and the methanol consists of a closed pressure vessel and stirring the contents while heating, or the one where a continuous countercurrent contact of the liquid Distillation residue (A) or the extract part (B) with gaseous methanol using a staged column, bubble tray; column or packed column is carried out.

The distillation residue (A) or the extract part (B) that is on Treated in this way with methanol can be returned directly to the oxidation stage can, however, in the case of the methanol treatment of the distillation residue (A) if necessary also a heavy metal catalyst extraction treatment, as described above, are subjected, whereupon the obtained, the heavy metal catalyst containing part of the extract or after the removal of part or the whole of the extraction solvent remaining extraction residue to the oxidation stage can be traced back.

On the other hand, the methanol, which after firm contact with the Distillation residue (A) or the extract part (B) is present before recycling the residue (A) or the extract part (B) may or may not be removed will.

There are thus several excellent technical advantages by recycling the methanol-treated distillation residue (A) or of the extraction part (B) to the oxidation stage according to the process according to the invention. Not only is it possible to make a marked improvement in the yield of DMT as opposed to it the usual method to achieve it, but it is also the extent of the discoloration the oxidation reaction mixture obtained from the oxidation stage is far lower, so that the cleaning of the DMT is facilitated in the subsequent stage.

The following examples are provided for further explanation the present invention, it being understood that these examples are merely illustrative serve the invention without limiting it.

Examples 1 and 2 A mixture of PX and MPT in a weight ratio of 1: 1.4 was continuously in the liquid phase with air during a residence time of 4.5 hours at 1650 C and under an overpressure of 3.9 kg / cm2 in the presence oxidized by cobalt acetate and manganese acetate, so that a Gx dation product that predominantly consisted of PTA and M2IT.

This was followed by an esterification of the oxidation product obtained with methanol to form an ester mixture, which is predominantly as MPT and DMT passed.

This ester mixture was subjected to continuous distillation a column top pressure of 110 mm Hg and a column bottom temperature of 2400 C subjected to the separation of DMT and the products of lower boiling point than DMT.

500 g of this distillation residue CA) and 125 g of methanol were fed into a 1000 cc stainless steel autoclave equipped with a stirrer. After the inside of the autoclave was purged with nitrogen, was the contact of the residue (A) with the methanol during the periods given in Table I. Periods of time and carried out at the temperatures specified therein with stirring.

After the ethanol treatment, the mixture was withdrawn as a whole and the methanol is removed by evaporation.

400 g of the resulting methanol-treated residue (B) and 400 g water was placed in a 3-necked flask and during Stirred at 900 ° C. for 2 hours. After the extraction was completed, the two layers were separated and a pink water layer was obtained. After evaporation This water layer to dryness became a recovered catalyst, the cobalt and manganese contained in the amounts shown in Table I.

The oxidation reaction was then recovered using this Catalyst carried out. One with reflux condenser, stirrer and gas inlet inlet 500 cm3 stainless steel autoclave equipped with 60 g PX, 140 g EPT, 5 g of PTA and the catalyst recovered above are charged in such an amount that that its co-concentration, calculated as metallic Co, is 0.01% by weight of the liquid chemicals brought in. The reaction was then continued for 3.5 hours a reactor temperature of 1650 C and an overpressure of 10 kg / cm2 with high-speed stirrers by blowing in air in such an amount that the flow velocity of the same at the outlet was 1500 cm3 / minute.

After the reaction had ended, the reaction product was cooled and removed. Its acid number was determined and this was used as a measure of the conversion taken. Furthermore, the yields of the various components were determined by analysis determined by its composition. The yields of DMT and the various effective components, d. H.

the intermediate products for the production of DMT (compounds, which can be converted into DMT by oxidation and esterification, such as PTA, NZG, TA, p-methylbenzyl alcohol, p-tolualdehyde, methyl p-formylbenzoate and p-formylbenzoic acid) were calculated in the following way: Effective yield formed of effective - components (mol) x 100 components (mol%) - amount of PX consumed (Mole) + MPT (mole) Furthermore, the amounts of coloring impurities were compared 1.0 g of the oxidation product dissolved in 45 cm) dimethylformamide and placed in a color comparison tube brought and determined the Hazen number.

These results are listed together in Table I.

On the other hand, a distillation residue was used for comparison purposes, who has not received the above methanol treatment, in the same way as the Subjected to catalyst extraction treatment to obtain a recovered catalyst, which is used in the same way to carry out the same oxidation test became. The catalysts obtained in this case are shown in Table I as the results listed in comparison 1.

Furthermore, an identical oxidation test was carried out for comparison purposes carried out, where fresh cobalt acetate and manganese acetate are used as catalysts became. The results obtained in this case are in Table I as the results of reference test 1 listed.

Table I Ver Methanol Methanol Metal Content Amount of Acid Yield Coloring search treatment treatment of the recovered at the oxi number of the efficiency no. temperature time nenen catalysis use oxidation seeds (Hazen- (° C) (Hours) tors of the catalytic reaction component number) Co (%) Mn (%) sator's product ten Co (mg) Mn (mg) (mg / KOH / g) (%) Ex. 200 0.5 5.4 0.11 20.5 0.42 314 92.6 60 1 Example 280 0.5 3.6 0.076 20.5 0.43 307 92.3 50 2 Comparison - - 9.7 0.234 20.5 0.49 209 84.4 250 1 reference attempt - - - - 20.5 0.45 313 92.7 50 1 example 3 Since the distillation residue (A) obtained in Example 1 still contains some DMT, he continued to undergo continuous distillation at column top pressure of 35 mm Hg and a column bottom temperature of 270 C to obtain such Components such as DMT subjected, so that a distillation residue (A ') is obtained became.

The methanol treatment of this distillation residue (A ') was for 8 hours at 270 ° C. using the same procedure as in Example 1 below Use of 300 g of the distillation residue (A ') and 300 g of methanol carried out. After the methanol treatment, the treated residue (B ') and water became one 3-neck flask in a ratio of 2 parts by weight of the former to 1 part by weight of water added, followed by extraction according to the procedure as in Example 1 for recovery of the heavy metal catalyst was carried out.

For comparison, the heavy metal catalyst was recovered by the extraction of the same from the still residue (A ') by exactly the same Procedure as in Example 1 was carried out, but without the distillation residue (A ') was subjected to methanol treatment. This extracted and recovered Catalyst was treated as in Example 1 to recover the catalyst to obtain. The cobalt and manganese content in these catalysts is given from the following table II.

These recovered catalysts were used; and the oxidation reactions was carried out where as in Example 1. The yields of active components, the extent of the discoloration and the acid numbers were also as in Example 1 determined. The results obtained are shown in Table II.

Table II Methanol- Methanol Distil- Metal content of the amount of the Acid yield extent treatment- treatment- recovered the oxidation number d. Effective digestion residue / oxidizer used in the catalyst Color temperature time methanol Co (%) Mn (%) catalyst components (Hazen- (° C) (Hrs.) (Weight ratio Co (mg) Mn (mg) react- (%) number) ratio) tion product (mgKOH / g) Example 270 8.0 1/1 7.0 0.14 20.6 0.41 243 89.9 90 3 Comparison - - - 13.57 0.3- 20.5 0.45 116 78.6 400 2 Example 4 A still residue (A ') was obtained by operating in the same manner as in Example 3. This distillation residue contained 0.0865% by weight of cobalt and 0.0018% by weight of manganese as a heavy metal catalyst. 300 g of this distillation residue (A ') and 150 g of methanol were contacted for 2 hours at 2600 C as in Example A, see above that a treated residue (') was obtained.

The oxidation reaction was carried out using the resultant treated residue (B2) carried out. That bites, an autoclave of the same As in Example 1 was with 60 g -Xylene, 140 g p-methyltoluate, 5 g p-toluic acid and 27 g of the treated residue (B ') were charged and the oxidation reaction was carried out carried out under identical conditions as in Example 1. After the Oxidation reaction was the acid number of the reaction product and its degree of coloration as determined in Example 1. The acid number was 247 mg KOH / g and the degree of coloration was 150.

If for comparison the oxidation reaction works in exactly the same way as above, but using the distillation residue (A ') directly was carried out as an oxidation catalyst without treating it with methanol, the oxidation reaction stopped in 20 minutes and a satisfactory reaction product could not be obtained.

Example 5 A mixture of PX and MPT in a weight ratio of 1: 1.3 became in the liquid phase with air continuously during a residence time of 4 hours at 2000 C and an overpressure of 10 kg / cm2 in the presence oxidized by manganese acetate and nickel acetate and obtained an oxidation product, which consisted predominantly of PTA and MMT.

This was followed by an esterification of the oxidation product obtained with methanol and it became an ester mixture, which mainly consists of MPT and DMT existed, received.

This ester mixture was distilled under a pressure of 110 mm Hg and after completion of the distillation at a point where there is no further distillation took place at a column temperature of 2400 C were DMT and components of separated lower boiling point than DMT.

This distillation residue was then treated with methanol and extracted as in Examples 1 and 2, so that the recovered catalyst was obtained.

The oxidation test was then obtained using the resultant Recovered catalyst carried out.

An identical catalyst as in Examples 1 and 2 was with 60 g PX, 140 g MPT, 5 g PTA and the recovered catalyst in such an amount, that its Ni concentration is 0.01% by weight of the liquid chemicals introduced scam, loaded. The reaction was then carried out for 2.5 hours at a reaction temperature of 200 ° C and an overpressure of 10 kg / cm2 with high-speed agitation Injection of air in such an amount that its flow velocity at the outlet 1500 cm3 / minute was carried out.

The analyzes and calculations were carried out as in Examples 1 and 2 and the results obtained are shown in Table III.

On the other hand, a similar catalyst extraction operation was used for comparison purposes with a distillation residue performed that of the above described methanol treatment had not been subjected to a recovered Obtain catalyst, which is used to carry out the same oxidation test was used (comparison 4).

For reference purposes, an oxidation test was continued in the same Carried out wisely, but using fresh manganese acetate and nickel acetate as catalysts (reference experiment 2).

Table III Experiment - Methanol - Methanol - Amount of the acid no. treatment-treatment-oxidation used the temperature time catalyst reaction reactants (Hazen number) (° C) (hour) Mn (mg) Ni (mg) th (mgKOH / g) (%) Ex. 250 0.5 20.4 20.5 273 93.7 50 5 Comparison - - 20.6 20.5 168 82.2 350 3 reference attempt - - 20.5 20.5 273 93.8 40 2 example 6 A mixture of PI and MPT in a weight ratio of 1: 1.4 was used in the liquid phase continuously with air for a residence time of 5 hours oxidized at 150 ° C and an overpressure of 3.9 kg / cm2 in the presence of cobalt acetate and an oxidation product composed predominantly of PTA and MMT.

This oxidation product has undergone esterification and distillation Subsequent treatment with methanol and extraction with water subjected to the to obtain recovered catalyst.

The oxidation treatment was then carried out using the resultant Recovered catalyst carried out.

An autoclave of the same type as used in Examples 1 and 2 was made with 70 g PX, 130 g MPT, 5 g PTA and the recovered catalyst in such an amount that its Co concentration, based on the liquid introduced Chemicals, 0.03 wt% was charged.

Then the reaction was continued for 4.5 hours at a reaction temperature of 160 ° C and an overpressure of 5 kg / cm2 with high-speed agitation Injection of air in such an amount that its flow velocity at the outlet 600 cm3 / min. was carried out.

Analyzes and calculations were carried out in the same way as in Examples 1 and 2, the results being shown in Table IV.

For comparison purposes, on the other hand, a distillation residue, who had not received the above-described methanol treatment, in the same way Way of a catalyst extraction operation to obtain a recovered Catalyst subjected, which then to carry out the same oxidation experiment was used (Comparison 4).

TABLE IV Experiment: methanol-methanol- amount of acid number yield at degree of color no. treatment treatment of the oxidation of one of the oxidizing agents (Hazen number) temperature time set catalytic reaction components (° C) (hour) production (%) (Co) (mg) product (mgKON / g) Example 6 270 0.5 61.5 194 88.0 70 Comparison - - 61.5 115 81.5 300 4 Example 7 One equipped with a stirrer A 500cc stainless steel autoclave was made with 2g des after exactly the same Procedure as in comparison 1 recovered catalyst and 100 g of methanol loaded. After purging the interior of the autoclave with nitrogen, the Methanol treatment of the recovered catalyst while stirring the contents of the Autoclave carried out at 2000 C for 1 hour.

After the completion of the methanol treatment, the mixture was withdrawn as a whole. The methanol was removed by evaporation and the catalyst recovered.

The oxidation test was carried out using the catalyst thus obtained as carried out in Example 1, the results listed in Table V being obtained became. Table V Experiment Amount of the acid number in the oxidation Yield of degree of coloration no. Used catalyst of the oxide-active compound (Hazen number) Co (mg) Mn (mg) reaction reactants production (%) product (mgKON / g) Example 20.5 0.43 311 92.4 60 7 Compared to 20.5 0.49 209 84.4 250 1

Claims (10)

Process for the production of dimethyl terephthalate by oxidation of p-xylene and / or methyl p-toluate with molecular oxygen or a gas containing molecular oxygen in the presence of a heavy metal catalyst, Esterification of the resulting oxidation reaction mixture with methanol, followed by Treatment of the esterification reaction mixture obtained by distillation for Separation of dimethyl terephthalate and fractions with a lower boiling point than Dimethyl terephthalate, and subsequent recycling to the oxidation state of a (A) distillation residue containing the heavy metal catalyst, and / or (B) of the extraction fraction containing the heavy metal catalyst, which after further extraction treatment of the distillation residue (A) was obtained, characterized in that the distillation residue (A) and / or the extract part (B) is contact treated with methanol prior to recycling. 2. The method according to claim 1, characterized in that the contact treatment is carried out with methanol at a temperature of 100 to 4500 C. 3. The method according to claim 2, characterized in that the contact treatment is carried out with methanol at a temperature of 150 to 4000 C. 4. The method according to claim 1 to 3, characterized in that the contact treatment using methanol in an amount of at least 0.02 parts by weight for each part by weight of the still residue (A) or des Extraction portion (B) is carried out. 5. The method according to claim 4, characterized in that the contact treatment using methanol in an amount of 0.05 to 10 parts by weight is carried out on each part by weight of the distillation residue (A). 6. The method according to claim 4, characterized in that the contact treatment using methanol in an amount of 0.5 to 1000 parts by weight to each Part by weight of the extraction portion (B) is carried out. 7. The method according to claim 1 to 6, characterized in that a mixture of p-xylene and methyl p-toluate is used as the starting material. 8. The method according to claim 7, characterized in that a weight ratio applied from p-xylene to methyl p-toluate in the mixture in the range of 2: 1 to 1: 4 will. 9. The method according to claim 1 to 8, characterized in that the heavy metal catalyst consists of a Co-Mn or Ni-Iln type catalyst. 10. The method according to claim 1 to 9, characterized in that the oxidation reaction is carried out at a temperature in the range from 140 to 2500 C. will.