DE2327773A1 - PROCESS FOR THE PRODUCTION OF DIMETHYLTEREPHTHALATE AND ITS INTERMEDIATES - Google Patents

Description

Process for the production of dimethyl terephthalate and its intermediates The invention relates to a process for the production of dimethyl terephthalate and its intermediates from the distillation residue, which at. one procedure is obtained in the p-xylene and / or slethyl-p-toluate with maleular oxygen or a molecular oxygen-containing gas in the liquid phase in the presence of manganese metal or a manganese compound at least partially in the reaction system is soluble (component A), the resulting reaction mixture is oxidized with methanol esterified and the esterified product is distilled to separate crude dimethyl terephthalate and the products with lower boiling points than diethyl terephthalate The term "intermediates" used here is to be understood as meaning the compounds which consist of dimethyl terephthalate in the oxidation and / or methyl esterification form, e.g.

Methyl p-toluate, methyl p-formyl benzoate and methyl p-methoxymethyl benzoate. As used herein, the term "reaction system" means the reaction mixture to understand that not only the starting material, i.e. p-xylene and / or methyl p-toluate, but also the main oxidation products, i.e.

p-Toluic acid and / or monomethyl terephthalate and in the Oxidation process of the compound-forming intermediates, such as terephthalic acid, p-methylbenzyl alcohol, p-tolualdehyde, methyl p-formylbenzoate, p-formylbenzoic acid and the like, which ultimately result from further oxidation and / or methyl esterification can be converted into dimethyl terephthalate, and others through side reactions Contains formed impurities. The reaction mixture can also be the diluent which may be used.

The process comprising the steps of: oxidation of p-xylene and / or Methyl p-toluate in the liquid phase with molecular oxygen or a molecular one Oxygen-containing gas in the presence of a catalyst, esterification of the process obtained reaction mixture with methanol and distillation of the esterified product for the production of crude dimethyl terephthalate is known. Such a procedure is occasionally referred to as the Witten or Witten-Hercules method.

As typical embodiments of the oxidation of p-xylene and / or Methyl p-toluate according to the Witten or Witten-Hercules process are in particular the following three are known: (i) Four-step "process in the manufacture of dimethyl terephthalate p-xylene is first converted into p-xylene in the liquid phase with a molecular oxygen containing gas is oxidized in the presence of a catalyst to form the oxidation product, which consists mainly of p-toluic acid (first oxidation stage). Then that will Product with Esterified methanol and the methyl p-toluate thus formed will again be similar to. oxidized above (second oxidation stage); the product will re-esterified in a similar manner and the resulting crude dimethyl terephthalate is separated from the reaction mixture (see e.g. British patent specification 727 989).

(ii) "Two Step" Process: In the manufacture of dimethyl terephthalate from p-xylene by the process normally referred to as the "two-step" process is a mixture of p-xylene with methyl p-toluate in the liquid phase with a molecular oxygen-containing gas oxidizes to form the oxidation product, which consists mainly of p-toluic acid and monomethyl terephthalate, which then is esterified with methanol and of the resulting reaction mixture that consists mainly of methyl p-toluate and dimethyl terephthalate, the crude Separated dimethyl terephthalate (see e.g. British patent specification 809 730).

(iii) Improved Two-Step Process: This process provides an improvement of the above two-step process (ii), in which a Mixture of p-xylene and methyl p-toluate with one containing molecular oxygen Gas is partially oxidized in the liquid phase in the first stage and in the the second and subsequent stages, with further addition of p-xylene, the oxidation is continued continuously; the oxidation product obtained is with Methanol is esterified and the crude dimethyl terephthalate formed is removed from the reaction mixture separated (see e.g. British patent specification 864 106).

In the above processes (i), (ii) and (iii), the dimethyl terephthalate formed is usually separated by distillation. As a result, the intermediates with boiling points below that of dimethyl terephthalate, such as methyl p-toluate, methyl p-formylbenzoate and p-tolualdehyde and the like, also separated with dimethyl terephthalate by distillation. The intermediates thus separated can be converted into dimethyl terephthalate by further oxidation and methyl esterification will. So far, after the separation of dimethyl terephthalate and the at lower temperatures boiling intermediate products remaining distillation residue either discarded or at best used as fuel. As a catalyst that opted for the above oxidation of p-xylene and / or methyl p-toluate with molecular Oxygen is suitable, the most diverse metals and metal compounds are in the literature, which may be polyvalent (see e.g. British patent specification 809,730 and U.S. Patent 2,894,978). In practice, however, was essentially as the only viable catalyst cobalt metal or a cobalt compound that soluble in the reaction system such as cobalt acetate is used.

In examining the above-described oxidation process of p-xylene and / or methyl p-toluate with molecular oxygen has now been found that manganese metal or a manganese compound at least partially in the reaction system is soluble (component A) has excellent catalytic activity. It it was also found that when cobalt metal or a cobalt compound, the is at least partially soluble in the reaction system (component B) at the same time in addition to the component A is used, the valuable products that are mainly consist of p-toluic acid and / or monomethyl terephthalate, with a greater reaction rate and can be formed in higher yields compared to using the usual catalyst and that the products are free from undesirable Discoloration are. A process in which the two-component catalyst system, which consists of the Components A and B is used, is in the patent ........

(Patent application P 21 44 920.7) described. This procedure exists by using p-xylene and / or methyl p-toluate, e.g., a mixture of p-xylene and methyl p-toluate in a mixing ratio within the range 2: 1 to 1: 4 (by weight) with molecular oxygen or a molecular one Oxygen-containing gas in the presence of a catalyst as defined above Components A and B in a ratio within the range 0.1: 99.9 to 99: 1, preferably from 1:99 to 90:10 (g atomic ratio of manganese metal to cobalt metal) contains the total amount of manganese metal and cobalt metal in the catalyst 50 to 1500, preferably 80 to 500 ppm, based on the total reaction mixture 2ie from 540 to 240 ° C., preferably from 0 160 to 220 ° C., is oxidized. The procedure is special advantageous when the catalytic components A and B are used in such amounts that when they are converted into manganese metal or cobalt metal, the concentration of manganese metal in the reaction mixture within the range from 0.3 to 40, preferably from 0.5 to 20 ppm and the concentration of cobalt metal is within the range of 50 to 500, preferably 80 to 300 ppm because as a result, even more colorless valuable products are obtained in a higher yield can be.

It has also been found that when nickel metal or a nickel compound, which is at least partially soluble in the reaction system (component C), in addition to the above-mentioned catalytic component A is used, the valuable product, which consists mainly of p-toluic acid and / or monomethyl terephthalate, with obtained a greater reaction rate and in higher yield will than when using the usual catalyst.

The product is also more colorless than the usual product. -The procedure in which the two-component catalyst is used, the components A and C is the subject of the German patent ....... (patent application P 23 11 209.6).

This latter process relates to the oxidation of p-xylene and / or methyl p-toluate, in particular methyl p-toluate or one. mixture of which with p-xylene at 160 to 250, preferably 180 to 230C, in the presence of one Catalyst which contains components C and A in such an amount that then, when the components are converted to nickel metal and manganese metal, respectively, the weight ratio from nickel metal to manganese metal in the catalyst within the range of 95: 5 to 0.5: 99.5. This process has the advantage that no carboxylic acid solvent, like acetic acid or a corrosive accelerator like bromine or a bromine compound, is used.

In the usual oxidation process, in which the well-known cobalt containing catalyst is used is the range of reaction temperature practically limited to 140 to 160 ° C.

At higher temperatures, the formation of by-products increases rapidly to, although a greater reaction speed is achieved thereby, whereby the Yield is reduced and, moreover, the product obtained thereby becomes discolored strong.

Although usually no reaction temperatures of more than 160 ° C can be used without incurring extreme practical disadvantages the method proposed above, that is, the method using a catalyst containing at least manganese or a manganese compound is the more advantageous Carrying out the oxidation reaction within a high temperature range of for example 160 to 2500C and is therefore not in the usual range of 140 limited to 1600C. There is also the great advantage the in the two above-mentioned patent applications described inventions in that they obtain colorless valuable products with an extremely high reaction rate and in high yield within such high reaction temperature ranges, such as 160 to 220, preferably 170 to 200 ° C, if the components A and B contain Two component catalyst is used and within the range of 160 to 250, preferably from 180 to 230C, if the catalytic components A and C should be used.

When studying the reaction products of those mentioned above Patent applications described method using one of the component A, in particular the specified components A and B or A and C containing catalyst it has been found that in these processes for the oxidation of p-xylene and / or methyl p-toluate using a catalyst containing component A, in which the reaction product is esterified with methyl and then distilled, the amount of after the separation (Production) of dimethyl terephthalate or of dimethyl terephthalate and the oxidized and methyl esterified products with boiling points below that of dimethyl terephthalate (e.g., methyl p-toluate, methyl pformyl benzoate, and the like) is considerably larger than they normally do in similar oxidation processes using a conventional one Cobalt catalyst is obtained. It was found that the reason for the increase in amount of the still residue that is that using the oxidation process the component A, in particular the components A and B or A and C containing Catalyst the amounts of carbon monoxide, carbon dioxide and aliphatic carboxylic acids, such as formic acid and acetic acid, which are by-products in the oxidation reaction arise compared to the usual method using only the component B can be reduced as a catalyst, but that the formation of higher compared to boiling products as by-products in the first-mentioned process the last-mentioned method increases by about 20 to 30% or even more. It is It is therefore clear that the usefulness of the oxidation process in which a component A containing catalyst is used, can be greatly improved if the Still bottoms could be converted into valuable products.

The aim of the present invention is therefore to provide an improved method to convert such a distillation residue back into valuable products to specify. Another object of the invention is to provide a method of formation of dimethyl terephthalate and the valuable intermediate products thereof from the distillation residue and to indicate the extraction (separation) of the valuable products. Other goals Features and advantages of the invention will appear from the following description.

It has now been found that the above objects and advantages according to the invention can be achieved by the distillation residue, which is a Process is obtained in which p-xylene and / or methyl p-toluate in the liquid phase with molecular oxygen or a gas containing molecular oxygen in Presence of manganese metal or a manganese compound, at least partially is soluble in the reaction system <component A) is oxidized, the resulting Esterified reaction mixture with methanol and distilled the esterified product to separate the crude dimethyl terephthalate and the intermediates with boiling points below that of dimethyl terephthalate, with methanol at one temperature within the range of 260 to 4000C contacted to form dimethyl terephthalate and the intermediate products thereof and the valuable products wins (separates). the Separation of dimethyl terephthalate and the valuable intermediate products, such as methyl p-toluate and methyl p-formyl benzoate and the like, of which with methyl esterified product by distillation has hitherto been carried out by normally the bottom temperature of the distillation column at 200 to 240, at most at 2500C held under a reduced pressure of 50 to 200 mmHg.

The reason for the separation of the valuable products mainly consist of dimethyl terephthalate, by distillation at reduced pressure, is that thereby the separation of dimethyl terephthalate is facilitated, while the discoloration of the product is prevented, as well as that that thereby the conversion the metal component in the oxidation catalyst into the metal oxide or into the free metal, especially at high temperatures, on the heat transfer surfaces of the heat exchanger fail and reduce the heat transfer efficiency and lead to constipation, is prevented. In addition, the use of excessively high temperatures for the separation of dimethyl terephthalate by Distillation is thermally uneconomical.

It has now been found, however, that if the above distillation residue, after the separation of the valuable products, such as dimethyl terephthalate, methyl p-toluate and the like, from the product esterified by methyl remains by distillation, at from 260 to 400, preferably from 270 to 350.degree. C., by the process according to the invention is contacted with methanol, surprisingly large amounts of valuable products, such as dimethyl terephthalate and those by-products caused by oxidation and / or Methyl esterification can be converted into dimethyl terephthalate, such as methyl p-toluate, Methyl p-formyl benzoate, methyl p-methoxymethyl benzoate and like that obtained from the distillation residue or

can be separated. The distillation residue is within the temperature range specified above contacted with methanol, because at temperatures below 270, preferably below 260 ° C, the recovery ratio of said valuable components is low and at temperatures above 380, preferably undesirable side reactions occur above 4000C, which not only reduces the recovery ratio is decreased, but also various disadvantages such as loss of methanol by the formation of dimethyl ether and an increase in the investment costs for the Apparatus occur because of the higher reaction pressures. Therefore is the preferred one Temperature range for the methanol treatment from 270 to 350 ° C. The application of Temperatures in excess of 350 ° C lead to no noticeable benefit.

It was found that after the separation of the valuable products, such as dimethyl terephthalate, from the methyl esterified oxidation product produced by Oxidation of p-xylene and / or methyl p-toluate in the presence of a cobalt compound (Component B) according to the usual practice and the subsequent methyl esterification of the product is obtained, distillation residue remaining by distillation Surprisingly, has a composition that is clearly different from that of the similar distillation residue that differs in the oxidation under Use of a catalyst is obtained which contains a manganese compound (component A), in particular components A and B, or component A and a nickel compound (Component C) contains. As in the examples and control examples below is shown, when contacting the residues with methanol under identical Conditions only about 5 percent by weight of the first-mentioned distillation residue converted into valuable products, which mainly consist of dimethyl terephthalate exist.

In contrast, in the case of the last-mentioned distillation residue 20 to t0 weight percent or even more converted back into valuable products when contacted with methanol under appropriate conditions. this is very surprising indeed.

The methanol treatment used in the present invention is extraordinary advantageous when used in combination with the oxidation process described above is carried out in which a catalyst is used which at least the component A contains. Especially when p-xylene and / or methyl ptoluate are present a catalyst consisting of components A and B at relatively high Temperatures, e.g.

160 to 2200C, preferably 170 to 2000C, or one of the components A and C containing catalyst at 160 to 2500C, preferably 180 to 230C is oxidized and the oxidation product is methylesterified, the distillation residue decreases still quantitatively too with the advantage that an even higher conversion into valuable Products can be obtained when the distillation residue of the invention The methanol treatment of the present invention is accordingly subjected to methanol treatment particularly advantageous when used in combination with the high-temperature oxidation reaction, as indicated above, with a considerable increase the overall yield of dimethyl terephthalate is achieved from p-xylene.

The composition of the valuable products after separation, like crude dimethyl terephthalate, from the methyl esterified oxidation product that in the oxidation using one containing at least component A. Catalyst is obtained, distillation residue obtained by distillation has not yet been precisely determined.

It was found, however, that the residue was most likely from different and immovable products that boil at high temperature, such as. Diphenyl compounds such as dimethyl 4,4'-diphenyldicarboxylate and trimethyl 2,4 ', 5-diphenyltricarboxylate, Benzocoumarin compounds such as dimethyl 3,4-benzocoumarin dicarboxylate, benzyl benzoate compounds such as Dimethyl 4,4'-benzylbenzoate dicarboxylate, and trimethylbenzene-1,2,4-tricarboxylate and the like. In addition to the above-mentioned compounds, the residue contains presumably large amounts of high molecular weight compounds that contain at least three benzene rings in one molecule, and colored, tarry, boiling at high temperature By-products whose structures are unknown. When such a still residue is contacted with methanol at such high temperatures according to the invention, is believed that then, although the details of the reaction mechanism are still up to now have not been clarified, reactions occur, such as decomposition, esterification, Etherification and transesterification and the like, with very complicated mechanisms, e.g. the methanolysis of the benzyl benzoate compounds, the methyl etherification of the compounds formed aromatic alcohols, the methyl esterification of aromatic carboxylic acids, such as Monomethyl terephthalate, and the decomposition of trimethylbenzene-1,2,4-tricarboxylate to form dimethyl terephthalate and the like. The contact time of the distillation residue with methanol varies according to the invention depending on the contact conditions, such as temperature and the like, but it is usually within the Range 0.1 to 20 hours. If the contact time is too short, the recovery is of the valuable components insufficient. On the other hand, there is an excessively long contact time not only uneconomical, but there is also the risk that disadvantages, such as the reduction in the recovery ratio and a loss of methanol Side reactions occur.

As a distillation residue to be treated according to the invention, i.e. as a residue, which after the separation of the valuable products, such as crude dimethyl terephthalate, What remains is the one who has the valuable products, such as dimethyl terephthalate and the intermediates thereof, e.g.

p-Toluic acid monomethyl terephthalate, terephthalic acid and the like, in of a lesser degree, usable in the same way as that which contains practically free from.

Is dimethyl terephthalate.

In a preferred embodiment of the invention, the esterification takes place of the reaction mixture that occurs in the oxidation of p-xylene and / or methyl p-toluate is obtained according to the invention, incomplete to such a degree that the acid number of the esterified product is not below 3 mgKOH / g, preferably not below 5 mgKOH / g, decreases and the incompletely esterified product becomes dimethyl terephthalate and the other valuable intermediates separated, and the remaining one Distillation residue is treated with methanol under the conditions already given contacted. By this incomplete esterification and subsequent treatment with methanol of the distillation residue are particularly advantageous when the Oxidation reaction ore in accordance with appropriate high temperatures in the presence a catalyst that contains at least component A (manganese content, metal or a manganese compound) / preferably at 160 to 2500C in the presence of a catalyst, which contains components A and B or A and C, is carried out.

If the esterification is carried out to such a degree, in which the acid number of the oxidation reaction mixture is reduced to a value of less than 5 mgKOH / g, preferably less than 3 mgKOH / g, is reduced for the esterification takes an extremely long time, which is a disadvantage because this reduces the production speed per apparatus. aside from that is the total yield of Dimethyltex: ephthalate and valuable intermediates after the subsequent methanol treatment of the distillation residue of the esterified The product is practically the same as the one that results from a more complete esterification until the acid number of the oxidation reaction product is reduced to below 5 mgKOH / g, preferably below 3 mgKOH / g, is obtained.

In particular if the oxidation according to the invention is within the The high temperature range specified above from 160 0 to 250 C is carried out, the rate of oxidation increases, thereby increasing the productivity of the oxidation apparatus increases. Due to the incomplete esterification of the oxidation product described above it is therefore possible to increase the productivity of the esterification apparatus as well. The total reaction time can thus be advantageously shortened without that the ultimately obtainable overall yield of dimethyl terephthalate and valuable Intermediate products is reduced. For example, if an oxidation product with an acid number of 200 mgKOH / g @@ is esterified, the dimethyl terephthalate productivity increases an esterification apparatus to reduce the acid number of the obtained Meresterified product to 3, 5 and 10 mgKOH / g by 9, 18 and 308 respectively those to reduce the acid number to 1 mgKOH / g, if all other conditions be kept constant.

The distillation residue which can be used according to the invention normally contains about 0.05 to 0.5 percent by weight of the metal compounds used as oxidation catalyst, i.e. the manganese compound and optionally the cobalt or nickel compound than the corresponding pure metals. These remaining metal compounds can are left in the residue during the methanol treatment according to the invention or they can, for example, partially by extraction before the methanol treatment or practically completely removed. These metal compounds obtained in this way can be reused as an oxidation catalyst. According to the invention, the Residue with the remaining catalytic components in the specified high temperatures of 260 to 400 ° C with methanol, but none adverse results, such as reduction and precipitation of the catalytic components, and other related disadvantages occur, unlike in the case in which the valuable products, such as dimethyl terephthalate, from the methyl esterified product are distilled off. If the residue is at relatively low temperatures, such as 260 to 2900C, is contacted with methanol, but accelerate the in the Residual remaining metal compounds result in the formation of usable products. This is another advantage achieved by the present invention.

The methanol treatment of the present invention can be carried out in the presence of a known one Transesterification catalyst, such as in the presence of compounds of zinc, titanium and the like, carried out in addition to the above-mentioned catalytic components will. The amount of methanol used in the treatment varies depending on on the composition of the distillation residue and the way of contacting - the residue with the methanol. Usually, however, at least 0.02, preferably 0.05 to 10 parts by weight of methanol are used per part by weight of residue, whereby beneficial results are obtained. The way in which the still bottoms is contacted with methanol is not critical. The residue and the methanol can for example be introduced together into a closed pressure vessel and are contacted with one another in batches with stirring and with heating, or Gaseous methanol and liquid residue can advantageously be stored in a plate column continually be contacted with one another in countercurrent.

In the process according to the invention, when the distillation residue is contacted a small amount of water was formed with methanol at 260 to 4000C. The water can continuously removed from the system during the methanol treatment, this but is not essential. Usually the still bottoms are preferred just as it is in contact with methanol, however, it is also possible to use this one To subject pretreatment, for example an oxidation, hydrolysis and the like.

The methanol-treated product obtained according to the invention is excess methanol and the water formed, for example by evaporation and then distilled. Thereafter, dimethyl terephthalate and the other intermediates produced by oxidation and / or methyl esterification in Dimethyl terephthalate can be converted, such as methyl p-toluate, methyl p-formylbenzoate, Methyl p-methoxymethyl benzoate and the like, easily separated and recovered, leaving the residue that is not converted into these valuable components has been. A method of separation other than that by distillation can also be used be applied.

The methanol-treated product can be used in a suitable stage the above-described preparation of dimethyl terephthalate from p-xylene according to the Witten process either as is or after removal of methanol and water are introduced.

That as one of the valuable intermediates in the invention Process formed methyl p-methoxymethylbenzoate causes the oxidation of the Methyl-substituted aromatic compounds, such as p-xylene, is accelerated, what is exemplified in British patent specification 961 474, and it also represents a valuable organic ether itself, the by Oxidation can be converted into monomethyl terephthalate.

Another advantage of the method according to the invention is that that such a compound with excellent properties is made and by can be separated from the distillation residue.

Although the invention has been preferred with reference to FIG Embodiments explained in more detail, but it is clear that these in many ways altered and modified without departing from the scope of the present Invention is abandoned.

Example A A 1: 1,2 mixture (by weight) of p-xylene with methyl p-toluate, the liquid phase was continuously oxidized with air with an average residence time of 4.5 hours at 1720C and 3.8 kg / cm (G) in the presence of cobalt acetate and manganese acetate. This is how you got 1930 parts of a consisting mainly of p-toluic acid and nonomethyl terephthalate Oxidation product The amounts of cobalt and manganese in the oxidation product were each calculated as pure netall, 0.0095 or 0.0002%. Danr.s became the product of oxidation Esterified with 520 parts of methanol to form 1985 parts of a mixture with an acid number of 5.4 mg IIOH / g, consisting mainly of methyl p - toluate and dimethyl terephthalate (this process is described in the German patent. ... ... (Patent application P 21 44 920.7) described).

This esterified mixture was in a column at a peak pressure of 95 mm Hg and a bottom temperature of 223 C continuously distilled to therefrom dimethyl terephthalate and the compounds with boiling points below that to be separated from dimethyl terephthalate. Because the distillation residue obtained in the process still contained a substantial amount of dimethyl terephthalate, it became at the column top pressure of 15 mm Hg and further distilled at a bottom temperature of 2350C. To this Wise were the useful components such as dimethyl terephthalate and methylNp-toluate and the like, continuously-distilled from the system, with 43 parts of one Distillation return.

stood behind, practically devoid of any of the useful Components was with the exception of 0.8% dimethyl terephthalate and 2.0% monomethyl terephthalate. There was no terephthalic acid in it or contain p-toluic acid. The cobalt and manganese residues in the residue, calculated as pure metals, were 0.092% and 0.002%, respectively.

200 g of the distillation residue were together with 50 g of methanol in a stainless steel autoclave with a 3 capacity of 500 cm, the was equipped with a stirrer, introduced. After the atmosphere of the autoclave was replaced by nitrogen, the methanol treatment was carried out with stirring varying temperatures within varying periods of time, as in the following Table I indicated, carried out. After that, all of it was treated with methanol. Product.

a distillation with a pressure at the top of the column of 20 mm Hg and subjected to a soil temperature of 255OC. The amounts of this way separated and recovered usable products, i.e. dimethyl terephthalate, Methyl p-toluate, methyl p-formyl benzoate and p-methoxymethyl benzoate are in the Table I below is also given. Before contacting with methanol, were the contents of valuable components in 200 g of the residue 1.6 g of dimethyl terephthalate and 4.0 grams of monomethyl terephthalate.

Table I Ver. Methanol Treatment Valuable Distillation Products search temp. maxima- time dimethyl- methyl- methyl- methyl- total no. (° C) (Stunterephtha- p-tolu- p-for- p-meth- velvet rich) lat (g) at (g) mylben- oxyme- (g) ter zoat (g) thylben-pressure zoat (g) (kg / cm2G) A-1 (Kon- 240 32 1.0 8.4 2.2 1.0 - 11.6 troll test) A-2 (control 240 38 8.0 8.6 2.6 2.4 0.5 14.1 troll test) A-3 260 35 1.0 26.5 5.2 4.6 1.2 37.5 A-4 260 40 8.0 26.6 5.6 6.2 3.6 42.0 A-5 270 46 8.0 29.5 6.1 9.2 5.4 50.2 A-6 280 50 8.0 30.4 6.2 9.7 8.2 54.5 A-7 290 56 8.0 31.0 6.8 10.1 8.7 56.6 A-8 300 53 3.0 30.6 6.4 10.1 6.4 53.4 A-9 300 58 7.0 31.1 6.8 12.6 11.3 61.8 A-10 320 55 0.5 36.6 8.9 13.5 6.8 65.8 A-11 320 88 5.0 27.4 11.6 12.0 10.2 61.2 A-12 350 68 0.5 32.6 12.2 13.2 7.6 65.6 A-13 350 105 3.0 27.3 18.5 9.3 3.6 58.7 The distillation residue had just before the Methanol treatment has a viscosity of 100 cP at 1200C. In contrast, the Viscosity of the after the valuable components have been separated off by distillation in the experiment no. A-7 remaining residue 95 cP at 120 ° C. The residue of Run No. 7 was suitable for use as a fuel.

Example B A 1: 1,4 mixture (by weight) of p-xylene and methyl p-toluate was in the liquid phase with air at varying temperatures, as indicated in Table II below 2, under a pressure of 4 kg / cm (G) in the presence of various oxidation catalysts, as they are also in are given in Table II below, oxidized. Oxidation products were thereby produced obtained, which consisted mainly of p-toluic acid and monomethyl terephthalate. The amount of catalyst remaining in each of the oxidation products, based on on the corresponding pure metal is also given in Table II below.

Each of the oxidation products were similar to Example A with methanol esterified and the product was distilled. After that, crude dimethyl terephthalate became separated and a distillation residue was obtained, which is practically none contained more valuable products. 200 g of each of the distillation residues were with 50 g of methanol each at varying temperatures over varying periods of time, as indicated in the following Table II, treated similarly as in Example A Die Residues treated with methanol were then distilled, with the valuable Components were obtained in the amounts given in Table II below. Tabel II Ver Oxyda- Oxydationska- Wert- Methanol- Valuable distillation products Bemersuch tion catalyst full treatment dim- methyl- methyl- methyl- total kings no. temp. Type methyl compo- temp. Time ethyl p-tolu p-for- p-meth- yield (° C) conc- nenten (° C) (hrs.) terephate (g) mylben- oxyme- (g) tration in the thalat zoat thylbenin the back- (g) (g) zoat Oxyda- stand (g) tion- before the product metha - (%) Nol treatment (g) Manering compliance level was 0.0102. Example B-1 180 5.1 300 6.0 30.5 6.5 9.1 10.8 56.9 Nickel acetate 0.0101 0.0102 B-2 190 "5.8 300 6.0 38.2 7.9 12.2 13.2 71.5 "0.0101 0.0102 B-3 200" 5.4 300 6.0 48.1 10.4 16.3 16.2 91.0 "0.0101 Manganese-B-4 180 naph- 0.0156 4.9 290 1.0 23.2 4.6 3.6 2.8 34.2" thenat continuation from Table II Ver Oxyda- Oxydationska- Value- Methanol- Valuable distillation products Trial catalyst full treatment dim- methyl- methyl- methyl- total kings no. temp. Compothyl p-tolu p-for- p-meth- yield type methyl (° C) nents Temp. Time terephthalate (g) mylben- oxyme- (g) concen in dem (° C) (hours) thalate zoate ethylbentration return- (g) (g) zoate in the stand (g) Oxyd before the ion methaproduct nolbe - (%) treatment (g) B-5 180 manganese 0.0156 4.9 290 7 24.1 4.7 3.8 8.1 40.7 naph- according to thenat Example B-6 Cobalt-Kon-160 0.0120 5.6 290 1.0 6.9 1.2 0.5 0.4 9.0 naphthetrollnat example B-7 160 "0.0120 5.6 290 8.0 7.0 1.5 1.0 2.0 11.5" cobalt Konnaphthetrollnat 0.0134 Example B-8 160 5.0 280 6.0 6.9 1.8 0.9 0.8 10.4 Nickel naphthenate 0.0014 Continuation of Table II. Oxyda- Oxydationska- Wert- Methanol- Valuable distillation products search catalyst full treatment No. temp. Compo- dime- methyl- methyl- methyl- total Bemer-type methyl- (° C) nenten Temp. Time thyl- p-tolu-p-for- p-meth- yield concen in the (° C) (hrs.) terephthalate (g) mylben- oxyme- (g) tration back thalate zoat thylbenin dem stand (g) (g) zoat Oxydavor der (g) ion methaproduct nolbe - (%) action (g) cobalt- Control naphtha 0.0134 B-9 160 5.0 300 6.0 7.1 1.9 2.1 1.4 12.5 nickel 0.0014 naphthenate cobalt 0.0131 naphthenate B-10 160 5.2 280 6.0 6.7 1.5 1.2 0.9 10.3 "Chromium 0.0012 naphthenate 0.0131 B-11 160" 5.2 300 6.0 7.2 1.6 1.9 1.8 12.5 "0.0012 cobalt 0.0198 invention naphthenate according to B-12 160 5.1 300 6.0 25.0 4.9 7.2 8.8 45.9 Example Manganese 0.0010 Naphthenate B-13 170 "0.0198 5.0 300 6.0 31.9 6.7 11.8 10.9 61.3 "0.0010 From a comparison of the results of Tables I and II above it can be seen that when the oxidation catalyst no manganese; but e.g. only a cobalt compound, a cobalt compound and a nickel compound or a chromium compound contained from the still residue separable valuable component was only less quantitative. If, on the other hand, oxidation catalyst systems that contained manganese compounds could contain very large amounts of valuable components can be obtained from the distillation residue.

Example C 3 kg of the distillation residue used as starting material of Example A were mixed with 6 kg of water and stirred for two hours heated to 90 ° C. Immediately thereafter, the system was split into two phases. The larger proportions of cobalt and manganese contained in the still residue were extracted into the aqueous phase and recovered. That way it was an extraction residue is obtained which contains 0.003% cobalt, calculated as pure metal, and contained practically no manganese. In each case 200 g of this extraction residue were similar to Example A at varying temperatures within varying For periods as indicated in Table III below, contacted with 50 g of methanol. Thereafter, each of the resulting products was distilled and the valuable ones Components were separated. The results obtained are similar to given in Example A in Table III below. Before the methanol treatment the extraction residue contained only 1.7 g of dimethyl terephthalate and 3.5 g of monomethyl terephthalate in 200 g of the feed introduced into the autoclave.

Table III Experiment with methanol treatment Valuable distillation products No. Temp. Time dimethyl- methyl- methyl- methyl- total- (° C) (hrs.) Terephtha- p-tolu- p-for- p-meth- yield lat (g) at (g) mylben- oxyme- an consumptionzoate (g) ethylbenable comzoate (g) components (g) C-1 (control 240 8.0 6.7 1.6 0.6 0.3 9.2 troll test) C-2 260 8.0 18.8 1.8 0.7 0.8 22.1 C-3 270 0.5 19.0 2.0 0.8 - 21.8 C-4 270 8.0 21 ,1 3.4 3.8 5.2 33.5 C-5 290 4.0 26.7 6.0 8.6 5.8 47.1 C-6 290 8.0 30.9 6.4 9.3 8.6 55.2 C-7 320 4.0 27.9 12.5 12.6 9.9 62.9 C-8 350 3.0 29.1 20.8 10.4 3.2 63.5 example D A plant for the production of dimethyl terephthalate from p-xylene according to the Witten-Hercules process, consisting of the oxidizing apparatus for oxidizing a mixture of p-xylene and Methyl ptoluate in the liquid phase with air, the esterification apparatus for esterification the oxidation product consisting mainly of p-toluic acid and monomethyl terephthalate with methanol, the distillation apparatus for a distillation under reduced Pressure of the esterified product for the separation of crude dimethyl terephthalate and the compounds with lower boiling points than dimethyl terephthalate from the distillation residue, the Reiligungsapparat for cleaning the crude dimethyl terephthalate, the one Distillation apparatus for distillation under reduced pressure for separation the remaining valuable components from the distillation residue of the esterified Product for the recirculation of the same in the main reaction system with formation a distillation residue that has practically no valuable component as waste contains, and the methanol treatment apparatus for treating the latter Distillation residue by the process according to the invention and for recycling of the valuable components formed in the main reaction system, was via a longer period of time varying from those given in Table IV below Conditions operated. The daily yield of refined dimethyl terephthalate, the molar yield of the refined dimethyl terephthalate formed the p-xylene consumed and the amount of distillation residue formed per day, as waste in the distillation apparatus during the distillation under reduced Pressure was obtained and was practically free of valuable components. When the methanol treatment apparatus was not in use, the distillation residue became withdrawn from the system and discarded, while if it was in operation, the Arrears at 29,000 for an average Dwell time of Was contacted with methanol for 7 hours.

Then the system was started at the column peak pressure of 30 mm Hg and the bottom temperature of 2700C distilled. The one after the separation of the valuable Components remaining residue was discarded.

Table IV- Oxidation- Oxide- Acid- Methanol- Dimethyl- Molars Formed- Destil- Bemersuch catalysis- number treated- terephtha- exploiters Deslation no. Temp. of the pulmonary system (%) tilla residue / (° C) Veratur Yield of tional dimethyl ester (parts / day) back-terephthalate prostate lat ducts (parts / (mgKOH / g) day) cobalt acetate invention-D-1 and 180 5.0 in operation 214 89.9 16.5 0.077 according to through manganese acetate management example D-2 "180 1.1" 180 89.8 14.2 0.079 "D-3" 180 5.2 not in 210 85.8 16.9 0.080 control operation Example D-4 only cobalt 150 1.4 in service 84 81.0 5.4 0.064 "acetate D-5" 150 1.0 "83 80.8 5.2 0.063" The results of Table IV above clearly show that when cobalt acetate and manganese acetate simultaneously, as Oxidation catalyst were used, the reaction rate of the oxidation was markedly increased over the use of wobalt acetate alone and that as a result larger amounts of dimethyl terephthalate were formed, so that a very much higher one molar yield was achieved. In addition, when cobalt acetate and Manganese acetate were used at the same time, the methanol treatment according to the invention the extraction of large amounts of valuable components from the distillation residue, whereby the molar yield was improved even further. The difference in the Yields between Runs Nos. D-1 and D-3 were 4.1%. Da dimethyl terephthalate is manufactured on a very large technical scale, this difference is economically extremely important in yields. If on the other hand cobalt acetate alone was used as an oxidation catalyst, the methanol treatment did not result in any substantial improvement in molar yield.

A comparison of experiments D-1 and D-2 shows that the incomplete Esterification the molar yield was not significantly reduced, while the The rate of formation of dimethyl terephthalate was markedly increased.

Example E In the oxidation of a 1: 1.2 mixture (based on the Weight) of p-xylene and methyl p-toluate similar to Example A became an oxidation product containing 0.0142% cobalt and 0.003% manganese, each calculated on the pure metals. The oxidation product was esterified with methanol to form an esterified mixture consisting mainly of methyl p-toluate and dimethyl terephthalate O Due to the smaller size of the esterification apparatus used in comparison compared to that of Example A, the esterified mixture had a very high acid number of 18.3 mgKOH / g.

The mixture was at a column peak pressure of 95 mm Hg and a Bottom temperature of 2200C continuously distilled, taking dimethyl terephthalate and the compounds having boiling points lower than dimethyl terephthalate were separated. The distillation residue remaining in the column contained still be considerable amounts of dimethyl and monomethyl terephthalate and he was therefore further distilled to obtain the valuable components that can be distilled, such as dimethyl terephthalate.

A distillation residue was obtained, the 1 1.3 Z dimethyl terephthalate and contained 12.1% monomethyl terephthalate as valuable components.

In each case 200 g of this distillation residue were used with varying Amounts of methanol, as given in the following Table V, to the same Way as in Example A contacted at 270 ° C for two hours and then distilled, where the results also given in Table V below were obtained valuable products were received. For comparison, 200 g of the same distillation residue were used Heated to 270 ° C. for two hours in the absence of methanol and then heated distilled at a column top pressure of 40 mm Hg and a bottom temperature of 230 ° C and the valuable components have been separated.

The results obtained are also given in Table V below to Ver (SU) distillation products d such as O dimethyl- methyl- methyl- methyl- p-toluyl- total No. terephtha- p-tolu-p-formyl- p-meth- acid (g) velvet lat (g) at (g) benzoate oxy-me- (g) estilltions- (g) In cU o N 4 N ~~~~ ----- oat (). ~ N r3 In ono \ o a Uk N nssss u, (Control attempt) E-2 4 32.8 III 11 37.8 0.02 I # ~ , U 48.5 5.6 7.6 2.4 o Ck vC) do IIIA u 8 cd cV N cr); m XX P4O U rq 1 Lc- (I Oh Oh A hh A CL -, <g U boms 4 O Oh so E-6 200 X 6.1 7.8 3.2 1 Cr, 1 P-, Y- (E3v v> clra ri “Ll UH so <S <60 00 O Ct aD cs H o , 1 4 4 ON eu) s sD tn U QXU <5E u o = = bo e oo e un rs mo > 4 mv ~ ua) as O cs 4 oo O o 3 g SNX. 4 4. 4th O U o. 4 o -O o O o @ 5: l H 4 OO o <t c> 3 O EO o I = 4 u) S o H ak NX 4 m ç b eg () Q) lll | bl :> to Z SqU PXX Ikl Ail lxa N1

Claims (12)

Claims process for the production of dimethyl terephthalate and the intermediate products thereof, characterized in that there is a distillation residue at 260 to 40,000 contacted with methanol and the dimethyl terephthalate formed and separating the intermediates therefrom, using a distillation residue as a starting material used, which is obtained by oxidation of p-xylene and / or methyl p-toluate in the liquid phase with molecular oxygen or a molecular oxygen containing gas in the presence of manganese metal or a manganese compound that is at least partially soluble in the reaction system (catalytic component A), esterification of the resulting oxidation reaction mixture with methanol and Distilling the esterified product to separate off the crude dimethyl terephthalate formed and other products with boiling points lower than dimethyl terephthalate. 2. The method according to claim 1, characterized in that the distillation residue is contacted with methanol at 270 to 350 ° C. 3. The method according to claim 1 or 2, characterized in that the Distillation residue with at least 0.02 parts by weight of ethanol per weight steep Residue is contacted. 4. The method according to claims 1 to 3, characterized that the distillation residue with 0.05 to 10 parts by weight of methanol per part by weight Residue is contacted. 5. Process according to claims 1 to 4, characterized in that that the distillation residue Osl contacted with methanol for 1 to 20 hours will 6. The method according to claims 1 to 5, characterized in that that the oxidation reaction mixture is incompletely esterified, so that its acid number is reduced to not less than 3 mg KO / g. 7. The method according to claims 1 to 5, characterized in that that the oxidation reaction mixture is incompletely esterified, so that its acid number is reduced to not below 5 mg KOH / g. 8. The method according to claims 1 to 7, characterized in that that the oxidation of p-xylene and / or methyl p-toluate in the presence of the component A and of cobalt metal or a cobalt compound which is at least partially in the reaction system is soluble (component B), or the component A and nickel metal or a nickel compound which is at least partially soluble in the reaction system (component C) is carried out. 9. The method according to claims 1 to 8, characterized in that that the starting mixture consisting of p-xylene and methyl ptoluate in a weight ratio from 2: 1 to 1: 4, is oxidized at 140 to 2400C in the presence of a catalyst, The consists of components A and B, being the total amount of metal components within the range of 50 to 150 ppm based on the weight of the total Reaction mixture, calculated as manganese metal or cobalt metal, and the g atomic ratio from manganese metal to cobalt metal within the range 0.1: 99.9 to 99: 1 lies. 10. The method according to claim 9, characterized in that the 0> 'ydation the starting mixture is carried out at 160 to 2200C. 11. The method according to claims 1 to 8, characterized in that that the oxidation of methyl p-toluate or a mixture thereof 0 with p-xylene as Starting mixture carried out at 160 to 250 ° C in the presence of a catalyst , which consists of components C and A, in a weight ratio of 95: 5 to 0.5: 99.5, calculated as nickel metal: manganese metal. 12. The method according to claim 11, characterized in that the oxidation the starting mixture is carried out at 180 to 230 ° C.