DE3011175C2 - Process for the production of dimethyl terephthalate and intermediate products of DMT production - Google Patents

Description

The invention relates to a process for the production of dimethyl terephthalate (DMT) and intermediates the DMT production from the higher-boiling bottom products in the generic term of the patent claim described type.

DMT is required as a raw material for synthetic fibers and foils on an industrial scale. Intermediates DMT production, e.g. B. p-Toluic acid methyl ester (PTE) and tcrephthalaldehyde acid methyl ester (TAE) are used in technology by oxidation and subsequent esterification with methanol in DMT converted.

In addition, there is rapid production of DMT and of Intermediate products of DMT production from the low-value, high-boiling tar-like products mentioned Residues of high technical interest.

DMT is made by taking a mixture of p-xylene (p-X) and PTE in the absence of solvents and halogen compounds in the liquid phase at elevated pressure and elevated temperature Oxygen or an oxygen-containing gas in & o Is oxidized in the presence of dissolved heavy metal oxidation catalysts. Serve as oxidation catalysts in particular mixtures of compounds of cobalt and manganese (DE-PS 20 10 137).

Then, the oxidation product which ^h> is composed predominantly of monomethyl terephihalat (MMT) and p-toluic acid (PTS), esterified with methanol. The esterification product (crude ester) is separated in a crude ester distillation into a PTE fraction, a DMT fraction and a high-boiling tar-like residue.

According to the teaching of DE-PS 23 10 824 DMT and intermediates of DMT production can be obtained from this high-boiling tar-like residue by subjecting this high-boiling tar-like residue to a heat treatment and distilling off the DMT formed and the intermediates. Suitable temperature ranges are 260 to 400 ° C, preferably 280 to 38O ⁰ C, indicated in particular 300 to 370 ° C; the heat treatment can take place in the presence of the oxidation catalysts that have accumulated in the residue; In addition to the oxidation catalysts, the residue may also contain known transesterification catalysts, such as compounds of zinc, titanium and the like.

The oxidation catalysts can also be removed from the distillation residue before the heat treatment be partially or essentially completely separated off, for example by extraction. After separation the oxidation catalysts are according to the teaching of DE-PS 23 10 824 with advantage temperatures applied above 300 ° C.

It is advantageous to use the oxidation catalysts, in particular the valuable cobalt component, from the Recover distillation residues and reuse them in the oxidation. A recovery by extraction before the heat treatment is more favorable, as the viscosity of the residue by the Heat treatment increases sharply, so that the extraction is then hindered by too high a viscosity.

However, the procedure according to DE-PS 23 10 824 has disadvantages if the oxidation catalysts are used removed after the heat treatment by extraction. Then namely the reaction rates lower, and there are temperatures above to set sufficient reaction rates of 300 ° C is necessary. These high temperatures have the disadvantage that higher costs arise because common and easy to handle heat transfer fluids at temperatures of 300 ° C and more have a reduced lifespan. These heat transfer fluids become all the faster thermally degraded, the higher temperatures are used.

The present invention is based on the object of a method for the production of DMT and of intermediate products of DMT manufacture with high yield and increased reaction rate by heat treatment of the high-boiling tarry distillation residues of DMT production, after the oxidation catalysts partially or essentially completely from these residues were separated, and at the reaction temperatures of less than 300 ° C are sufficient for To make available.

This object is achieved in that the heat treatment in a method of the specified type the bottom products in the presence of sufficiently soluble compounds of titanium, zirconium, manganese, Iron, nickel, zinc, aluminum, silicon, boron, germanium, tin or lead as fission catalysts in Amounts of the metal components from 0.02 to 4 wt .-%, based on the weight of the high boiling point Is carried out residue and the recovered DMT and the intermediates together in the Oxidation or esterification stage of the process

rens can be traced back.

Compounds of iron, manganese, titanium and aluminum are preferred because they are used during the inventive heat treatment very particularly have high catalytic activities. Of these, compounds of manganese are particularly suitable because this element is sufficiently catalytically active during the heat treatment and because the aromatics oxidation is not adversely affected by manganese components, which are carried away by the products that are distilled off and in the aromatic oxidation are introduced.

It is surprising that iron has a particularly high catalytic activity in the heat treatment, although iron according to the prior art is by no means a particularly active transesterification catalyst. From this and from the different quantity distribution of the DMT intermediates when used different catalysts shows that not transesterifications, but cleavage reactions are unknown Nature are decisive for the formation of large quantities of useful products.

It is also surprising that cobalt has a comparatively low cleavage activity, so that when Process according to the invention DMT and intermediate products of DMT production at higher speeds arise than in the presence of equivalent amounts of cobalt.

It is not critical which compounds of the catalytically active elements are used as cleavage catalysts are used provided these compounds are sufficiently soluble in the higher-boiling distillation bottoms are. For example, acetylacetonates or acetates or the xylene-soluble salts of longer-chain, aliphatic, saturated, monobasic, branched or straight-chain fatty acids.

Appropriate solutions of the compounds used as catalysts with the higher-boiling distillation bottoms mixed or dispersed in it; however, in special cases also solid compounds of the catalysts in the residue be solved.

If too little catalyst is present in the heat treatment, the reaction will be little accelerated; therefore no less than 200 ppm of catalyst, calculated as metal, and based on the ■ »■> Weight of the high-boiling residue used, preferably 1500 ppm to 10,000 ppm. Higher catalyst concentrations more than 4% by weight should generally not be used.

The temperature of the residue during the heat treatment according to the invention is from 220 to 300 ° C, preferably 250 to 290 ° C.

The pressure during the heat treatment is not critical; however, it is advisable to set it so that that DMT and intermediate products of DMT production « distill off during the heat treatment; therefore, reduced pressure is appropriate.

The duration of the heat treatment depends on the type and amount of catalyst as well as the reaction temperature and the rate at which DMT and intermediate products of DMT production are distilled off. The duration of the heat treatment is appropriately chosen so that no significant amounts of DMT and intermediates of the DMT manufacture bs occur more at a further extension of the heat treatment. Whether this condition is met can easily be determined by taking samples and heat treating the samples in the laboratory. Of course, a longer or shorter duration of the heat treatment can be set if this is necessary for operational reasons. However, too short a heat treatment leads to losses of DMT and intermediate products of DMT production, while too long a heat treatment requires large equipment and a very high viscosity the residue as well as encrustation of the equipment and pipes.

The heat treatment can be carried out either batchwise or continuously; preferred will be a continuous implementation.

In addition to DMT, the heat treatment according to the invention mainly produces the following useful products:

PTE, PTS, TAE and MMT. All useful products are expediently taken together from the high-boiling residue distilled off. "Then either is distillative or by fractional crystallization DMT separated from the intermediates and the intermediates are returned directly or indirectly to the oxidation or esterification stage of the process; you can also direct all of the products distilled off from the high-boiling tar-like residue or return it indirectly to the oxidation or esterification stage of the process.

The process according to the invention can be particularly advantageous for such high-boiling residues are used that originate from an air oxidation process that is combined with a combined oxidation catalyst system, ζ. B. works with a mixture of cobalt and manganese compounds.

The applicability of the method according to the invention is guaranteed in the same way whether the Distalization residues to be used according to the invention have a significant residual content due to their manufacture of DMT or not.

The advantages that can be achieved with the method according to the invention are also evident if there is still one there is a substantial residual content of DMT in the residue used, because this DMT is additional is won. Even from distillation residues, the DMT content of which is higher than the content of high-boiling tar-like constituents, DMT and DMT precursors according to the invention Method can be obtained advantageously because of the already existing amount of DMT and the additional resulting amount of DMT and DMT precursors can be obtained at the same time.

The advantages of the invention are that from the distillation bottom of the crude ester distillation larger Quantities of DMT and intermediate products of DMT production with shorter reaction times and lower temperatures than in the previously known processes can be obtained. This will be great Reactor volumes and special heating measures avoided. These benefits are particularly evident after partial or complete removal of the oxidation catalysts achieved.

The following examples serve to further illustrate the invention.

Example 1 (comparative example)

In an industrial plant for DMT production, PX and PTE were continuously oxidized ^{together in the liquid phase at 6-8 bar pressure and 150-17O 0 C reaction temperature;} A freshly prepared solution of cobalt acetate and served as the oxidation catalyst

Manganese acetate in 2% aqueous acetic acid. This solution was added continuously, so that a steady-state concentration of 90 ppm cobalt and 9 ppm manganese pin was established in the oxidation product. In addition to the catalyst mentioned and the carboxylic acids formed, the oxidation product contained unconverted feedstock and various intermediate and by-products. This oxidation product was continuously esterified with methanol at temperatures of about 250 ^° C. and pressures between 20 and 30 bar. The esterification product was continuously separated by vacuum distillation; that fraction which boiled higher than DMT was distilled off under vacuum in a reboiler until it contained 7.0% by weight DMT. It was then extracted with 4% aqueous acetic acid solution at elevated temperature until it contained less than 20 ppm cobalt and less than 5 ppm manganese. Subsequently, water was drawn off from the high-boiling fraction at 100 ^{° C. under reduced pressure.}

100 g of this largely metal- and water-free, high-boiling fraction were subjected to a heat treatment ^{at 0.05 bar and 290 ° C. for 7 hours.} The portions distilling off were condensed in a descending condenser, weighed and analyzed by gas chromatography. From 100 g of the fraction used, 10.6 g DMT, 3.1 MMT, 0.6 g PTE, 0.6 g PTS and 0.3 g TAE were obtained, i.e. a total of 15.2 g usable products.

Example 2

Iron (III) acetylacetonate was added to 100 g of the extracted and anhydrous, high-boiling fraction, which is described in Example 1, so that it contained 0.5% by weight of iron. The iron-containing residue was bar for 7 hours at 0.05 and subjected to 29o C ⁰ to a heat treatment. The fractions to be distilled off were condensed in a condenser, weighed and analyzed by gas chromatography. 42.1 g of usable products were now obtained; they consisted of 22.4 g DMT, 5.6 g MMT, 8.0 g PTE, 4.0 g PTS and 2.1 g TAE

Example! 3

100 g of the extracted and anhydrous high-boiling fraction described in Example 1 were used mixed with titanium (III) acetylacetonate so that they contained ίο 0.5% by weight of titanium. They then became like in Example 2 treated. 40.7 g of usable products were now obtained; they consisted of 23.0 g DMT, 4.5 g MMT, 7.4 g PTE, 3.2 g PTS and 2.5 g TAE.

Example 4

was carried out as in Example 3, but with manganese (III) acetylacetonate. There were 30.1 g usable Products received, they consisted of 24.6 g DMT, 1.5 g MMT, 2.6 g PTE, 1.2 g PTS, 0.2 g TA E.

Example 5

was carried out as in Example 3, but with zinc acetate. 30.1 g of usable products were obtained. she consisted of 23.6 g DMT, 1.4 g MMT, 3.1 g PTE, 1.4 g PTS, 0.6 g TA E.

Example 6 (comparative example)

was carried out as in Example 3, but with cobalt (II) acetylacetonate. There were 26.8 g of usable products obtain. They consisted of 20.5 g DMT, 3.2 g MMT, 1.7 g PTE, 1.3 g PTS, 0.1 g TAE.

Example 7 (comparative example)

was carried out with non-extracted residue, otherwise as in Example 1. There were 29.8 g usable Products received. They consisted of 21.5 g DMT, 3.7 g MMT, 1.8 g PTE. 1.5 g PTS and 1.3 g TAE.

Claims (2)

Patent claims: 1. Process for the production of dimethyl terephthalate (DMT) and intermediate products of DMT production from the higher-boiling bottom products that result from the common air oxidation of p-xylene and p-toluic acid methyl ester in the presence of heavy metal oxidation catalysts at 140 to 180 ° C and 3 to 11 bar in the liquid phase, subsequent esterification of the resulting Sau- ίο ren with methanol and distillative separation of the esters formed and extraction of the oxidation catalysts obtained from the bottom products by heat treating these bottom products at 220 to 300 ° C and separation of the DMT and the intermediate products, characterized in that that the heat treatment of the bottom products in the presence therein sufficiently soluble Compounds of titanium, zirconium, manganese, iron, nickel, zinc, aluminum, silicon, boron, germanium, Tin or lead as cracking catalysts in amounts of the metal component from 0.02 to 4% by weight, based on the weight of the high-boiling residue, is carried out and that recovered DMT and the intermediates together in the oxidation or in the esterification stage of the proceedings. 2. Process for the production of dimethyl terephthalate (DMT) and intermediate products of DMT production from the higher-boiling bottom products according to claim 1, characterized in that the recovered DMT and the intermediates separately and only the intermediates in the Oxidation or can be returned to the esterification stage of the process.