NL7905873A - IMPROVED METHOD FOR PREPARING DIMETHYL TERPHALTALATE BY OXYDATION OF P-TOLUYLIC ACID METHYL ESTER OR A MIXTURE THEREOF WITH P-XYLENE. - Google Patents

Description

* · * 'Τ VO 7615 _ DYFJLMIT ÏTOBEL AKTIEWCrESELLSCHAFT,

Troisdorf / Bez. Cologne,

Federal Republic of Germany*

Improved method for preparing dimethyl terephthalate by oxidation of p-toluic acid methyl ester or a mixture thereof with p-xylene.

The invention relates to an improved method of preparing dimethyl terephthalate (DMT) by oxidation of p-toluic acid methyl ester (PTE) or a mixture of PTE with p-xylene. (PK) in the presence of a lower aliphatic alcohol-the liquid phase under elevated pressure and elevated temperature with oxygen or an oxygen-containing gas in the presence of oxidation catalysts, subsequent esterification of the acids formed with methanol and separation of the esterification products.

10 D2 T is used as the starting material for preparing fiber and film-forming polyesters by reaction with ethylene glycol. It is prepared in many technical installations by the above-described method (Witten or Katzschmann process, compare German patent 1,041,945).

Although many of these methods of preparing DMT have been proposed, besides the Witten or Katzschmann process, only those processes have acquired industrial significance, in which PK is oxidized directly to a tetraphthalic acid (TPS) step ( eg, the SD process) and the crude terephthalic acid obtained is subsequently esterified with methanol. According to the SD process (cf. U.S. Pat. No. 2,833,816), PX is oxidized with a molecular oxygen-containing gas in acetic acid as a solvent in the presence of heavy metal catalysts and a bromine compound to form TPS.

The crude TPS obtained contains impurities which must be removed prior to further reaction with ethylene glycol to prepare fiber and film-forming polyesters. One way of removing these by-products is to subsequently esterify the crude TPS with methanol to DMT and separate the DMT by distillation of the impurities from the TPS. The bromine compound required by the SD process as a catalyst is highly corrosive on the usual reactor materials. As a result, the reactors must necessarily be made of special materials »e.g. titanium, are manufactured; nevertheless they only have a shortened life span. Furthermore, large amounts of the acetic acid required as a solvent are lost by combustion. Since TPS is only slightly soluble in acetic acid and is itself not meltable, large amounts of suspensions and solids must be maintained in the process.

In the Witten (Katzschmann) process, a mixture of PX and PTE is oxidized in the absence of solvents and halogen compounds in the liquid phase at elevated pressure and elevated temperature with oxygen or an oxygen-containing gas in the presence of oxidation catalysts the reaction mixture obtained which is predominantly from monomethyl terephthal

late (MMT) and p-toluic acid (PTS) are reacted with methanol in a separate esterification step and the DMT is separated from the esterification product. In addition to the DMT, an ester mixture consisting mainly of PTE is obtained. Fresh PX is added to this ester mixture and renewed to form MMT

and PTS oxidized.

In an older embodiment of the Witten process, PX and PTE were also each separately oxidized and the resulting acids separated in two esterification steps with methanol-esterified, whereby DMT was obtained in the last reaction step.

Due to the absence of bromine compounds, stainless steel as a reactor material is sufficient. Failure to maintain large quantities of suspensions and solids will cease to apply, 790 58 73 ίτ 3 ** *

Losses due to oxidation of the solvent do not occur in the fitting process.

In contrast, the UTitten process does not convert a certain part of the PX into DMT, but into unwanted by-products. Such by-products are e.g. carbon monoxide, carbon dioxide, high boiling tar-like residue and readily volatile acids, e.g. acetic acid and formic acid. These readily volatile acids accumulate in the oxidation waste water and can then lead to corrosion problems and environmental impacts.

10 The above-mentioned disadvantages have been successfully reduced by improvements in the Witten process. E.g. the specific combustion loss * can be reduced by a special cobalt and manganese-containing oxidation catalyst (compare German Ausleges 2,010,137 and 2,163,031). Specific combustion loss is here understood to mean the amount of CO2, CO, acetic acid and formic acid, which is formed during the oxidation, which is related to the reaction.

Torch can be reduced by an appropriate treatment of the high-boiling tar-like residue by converting a portion of this high-boiling tar-like residue into lower boiling useful products, eg, in DMT and PTE (compare German Patent 2,244 * 662, and German Offenlegenschrift 2,427 * 875) ►

Processes were also described in which the number of reactors required for the DlfP preparation was reduced by oxidation of PX and PTE and simultaneous esterification of the formed acids with methanol in the same reactor (compare German Auslegeschriften 1,048,571 »1 * 104,939 and 1,418,175 (German Offenlegungsschrift 2,425 * 763 and U.S. Pat. No. 2,879 * 289).

These processes for the simultaneous oxidation and esterification have not technically continued. It is believed in the art that if methanol is supplied during the oxidation it is not possible to obtain a smooth course of the reaction and that there are major disadvantages compared to the Witten process and 790 58 73. 4 process difficulties occur, compare Ewald Katzschmann, Chemie-Ingenieur-Technik, J58, (1966), 1-10.

Such drawbacks are, for example, the undesired oxidation of large amounts of methanol, as well as the presence of large amounts of methanol in the oxidation waste air. The amounts of methanol thus entered into the oxidation waste air are condensed together with the reaction water and the PX present in the oxidation waste air and reduce the density of the aqueous phase. This makes the separation of the aqueous phase of the 10 PX more difficult.

These disadvantages are also not eliminated by the improved process for simultaneous oxidation and esterification described in German Offenle-gungsschrift 2,425 * 763.

German Offenlegungsschrift 2 * 425 * 763 describes a process for the direct preparation of alkyl esters of aromatic carboxylic acids by catalytic conversion of aromatic compounds, which compounds contain at least one methyl group and / or the oxidation derivatives thereof with a molecular oxygen-containing gas in the liquid phase in the presence of a lower aliphatic alcohol at a concentration of the lower aliphatic alcohol in the reaction mixture present in the liquid phase at 0.5-15 wt% and in the presence of special catalyst systems at special concentrations of the individual catalyst components. However, in this method, which, in contrast to the method according to the invention, the oxidation and the esterification reaction proceed simultaneously in the same reactor, the influence of the concentration of the low aliphatic alcohol in the reaction mixture on the selectivity of the oxidation of PX to PTS and from PTE to MMT 30 not studied.

In the process according to the invention, the oxidation and the subsequent esterification with methanol take place in separate steps.

An object of the present invention therefore consists in the development of an improved process for the preparation of oxanes by oxidation of PX and PTE without the use of solvents and halogenated compounds, by subsequent treatment of the reformed acids. and separation of the DI.iT from the esterification mixture, whereby, compared to the methods known hitherto, smaller groups of whores are readily obtained from aerosolic acids, without oxidizing interfering wholes of methanol and in the oxidation waste air or the oxidation waste water. To get.

A further preparation of the invention consists in the invention of a suitable process which yields smaller quantities of high-boiling residues which cannot be converted into DMT and PTE compared to the prior art.

Tender cooking and judgments appear from the description.

These yarns are solved in a surprising manner by the fact that in a process for preparing dimethyl terephthalate by oxidation of p-toluic acid methyl ester or a mixture of p-tolyl acid methyl ester with p-xylene in the presence of a low aliphatic alcohol in the liquid phase under high pressure and elevated temperature with oxygen or an oxygen-containing gas 20 in the presence of oxidation catalysts, subsequent retention of the reformed acids with methanol and separation of the resterification products, and the restoration of the residues obtained is readily runny acids and not in DMT and PTE residue to be reacted, in the course of the oxidation of the liquid phase to be oxidized and a noticeable residual amount of an adequate residual amount of methanol is stirred in, so that a methanol concentration of between 400 and 4000 ppm is maintained in the reaction mixture.

Within the scope of the invention, in stepped agitation of the oxidation in a cascade of oxidation reactors, the methanol concentration in each oxidation step can be adjusted separately.

Due to the low methanol feed rate resulting from oxidation, which leads to the remainder of the residual whores being easily obtained from aerosolic acids and from residues which cannot be converted into DMT and PTE, the quantity of DMT yielded on xylene is simultaneously increased.

The acids formed by oxidation-oxidation are then esterified under oxygen exclusion in the esterification reactor and with a sufficient amount of methanol and DMT is separated from the esterification mixture.

The improvements obtained according to the invention are particularly surprising, since it has hitherto been assumed in the prior art that when methanol is fed during the oxidation it is not possible to achieve a smooth course of the reaction and that disadvantages and difficulties in the course of the reaction of the reaction would occur. Therefore, there was an important technical prejudice to overcome.

In the process of the invention, PX and PTE can be oxidized separately in different reactors, while a small amount of methanol is added in one or more of the oxidation reactors. However, with particular advantage the method according to the invention can be modified if 'PX and PTE are oxidized jointly in the same oxidation reactors. In the case where PX and PTE are oxidized jointly in a reactor cascade, the low methanol addition according to the invention is generally carried out in each reactor, with particular advantage, however, it is carried out in the second reactor of the cascade and in the The second reactor followed the reactors.

For the process of the invention, those oxidation conditions which are known in the art are used for a process for the common oxidation of PX and PTE without the use of solvents and halogen compounds in the liquid phase at elevated pressure and elevated temperature - »temperature with oxygen or an oxygen-containing gas, subsequent esterification of the acids formed and separation of the DMT from the esterification mixture.

A cobalt and manganese-containing oxidation catalyst is used with particular advantage, but other oxidation catalysts can also be used.

The oxidation can be carried out at higher temperatures at a higher rate, however, the selectivity of the oxidation at high temperatures is less, at lower temperatures, on the other hand, difficulties can arise due to high-melting oxidation products. The oxidation is therefore carried out at temperatures between 140 and 240 ° C, preferably between 150 and 170 ° C.

At higher pressure, the oxidation can be carried out at a faster rate. Therefore, the reaction pressure should be between 3 and 25 resin, preferably between 5 and 9 resin.

Essential to the process of the invention is the appropriate methanol concentration in the reaction mixture. The effect achieved decreases with decreasing methanol concentration, too high a methanol concentration, too much methanol enters the waste

air and in wastewater and oxidation may become unstable. Such oxidation instabilities can often be eliminated by increasing the temperature, however at too high temperatures the selectivity of the main reaction in the Witten-20 process, i.e. the oxidation of PX to PTS and of PTE decreases

to MMT.

Therefore, the methanol concentration in the reaction mixture should be between 400ppm and 400ppm, preferably between 700-2000ppm.

The methanol concentration in the reaction product depends on the amount of methanol supplied, on the amount of air supplied, on the reaction pressure and on the reaction temperature. It is therefore expedient to draw samples from the reactor, determine the methanol concentration in these samples and adjust them to the above values. "

The process according to the invention is particularly advantageous if the oxidation is started without supplying methanol and after the oxidation has started the methanol supply starts. The oxidation can be carried out continuously or stepwise.

35 During the oxidation according to the method according to the invention / 7905873 v-8 ...

predominantly acids and poor small amounts of methyl ester are formed. Therefore, the oxidation products are esterified with methanol under oxygen exclusion. This esterification takes place in a manner known per se, for example, the reaction product can be continuously esterified in countercurrent with gaseous methanol at high temperatures and pressures.

The separation of the DMT from the esterification mixture also takes place in a manner known per se, e.g. by distillation, place.

The reduction in the concentration of readily volatile acids in the oxidation waste water obtained by the process according to the invention is achieved, for example, by the fact that the readily volatile acids are esterified with methanol, because also the shipping equivalent of the waste water is the method according to the invention is reduced.

The invention is further elucidated by means of the examples below.

In all examples, the saponification equivalent of the oxidation waste water was determined by mixing a measured sample of waste water with an excess of 1N NaOH solution, leaving the mixture at room temperature for 16 hours, then heating at 60 ° C for 20 minutes and titration with 1-N-HCl solution against phenolphthalein.

The residue not convertible to DMT was determined by reacting a sample with a methanol excess at 250 ° C in a closed autoclave with subsequent gas chromatographic analysis. Analyzed on a packed column impregnated with Dexsil with a temperature program of 110-320 ° C. Methyl stearate was used as the internal standard.

50 Calibrated with pure 2,5 '4'-tricarbomethoxybiphenyl for quantitative determination. The thus determined concentration of residue not to be converted into DMT was based on the acid number of the oxidized product (= reaction mixture) reached during the oxidation; the size thus calculated is called "specific residue formation" 55.

7905873 9

The methanol content of the oxidized product (= reaction mixture) was determined by adding 8 wt.% PX to this product, then distillation with continuous stirring to a top temperature of 155 ° C at normal pressure, then distilling the methanol with PX pyridine was added to the distillate until a single homogeneous liquid phase was present, after which the hydroxyl number of this homogeneous mixture was determined and the amount of methanol distilled off was calculated therefrom. The process according to the invention is described below with reference to Examples I and II opposite the known methods (comparative example).

In the examples, as well as in the comparative example, the following reaction conditions are always maintained: The oxidation was carried out continuously in a stainless steel reactor. This orydator had a useful volume of 1.5 and was equipped with an air supply tube, a double jacket for heating and cooling, a water separator, measuring and regulating devices for the supply of PX and of PTE, as well as dosing pumps 20 for dosing catalyst solution and of methanol.

95.5 kg PX and 122.5 kg PTE were pumped into the head of this reactor per hour. At the same time, liquid was continuously discharged by means of a position control through a bottom valve, so that the liquid content of the reactor was always 1.3 m 2. The reactor temperature was maintained at 162 ° C by temperature control.

A mixture of cobalt acetate and manganese acetate was used as the catalyst. Both acetates were dissolved in 1.57% aqueous acetic acid to such an extent that the solution contained 30 g / l cobalt and 3 g / l manganese. This solution was continuously pumped into the reactor head so much that a stationary concentration of 100 ppm cobalt and 10 ppm manganese were established in the reactor contents. This concentration was checked by regular sampling and analysis of samples,

165 air per hour was introduced into this reactor. Gas was continuously discharged through a release valve in the reactor head, so that the pressure in the gas space of the reactor was 6.5 bar. The condensable parts dragged from the reactor with the oxidation waste gas were condensed in a condenser and this condensate was collected in a graduated separator for the oxidation waste water provided with sight glasses.

Comparison example

After achieving a stable operating state in the oxidizing device described above, 3 samples of the oxidized product and the oxidation waste water were analyzed in each case. In the oxidized product the following components were

at 23 * 5 mass- $ MMT

31 µl "-1 ° PTE

18.8 "- $ PTS

15.0 TPS

15 2.9 "-f * PX

0.8 "- $> DMT

determined.

The methanol concentration in the oxidized product was less than 400 ppm, the acid number of the oxidation product was 253 mg KOH / g; the oxidized product contained, on average, 8200 ppm of residue not convertible into DMT; a specific residue formation of 32.4 ppm g / mg KOH can be calculated from this. In the oxidation effluent, the acid concentration and saponification equivalent were 1300 mg equivalent / l.

Example I

The comparative example was repeated with the only difference that 4 kg / h of methanol were continuously fed to the reactor pool. 3 samples of the oxidized product and the water of reaction were also analyzed each time.

3D In the oxidized product, the following components were added

at 23.2 mass $ MMT

34.5 "-1 ° PTE

19.7 "- $ PTS

13.8 "-1» tps 35 3.0 "- # PX ·

1.2 "- <fo DMT

790 5 8 73 11 4 determined.

The methanol concentration in the oxidized product averaged 900 ppm, the acid value averaged 241 mg KOH / g; the oxidized product contained on average 5500 ppm of residue not convertible into DMT 5? a specific residue formation of 22.8 ppm. g / mg KOE is calculated therefrom. Acid concentration and saponification equivalent in the oxidation waste water each averaged 900 mg equivalent / l. The oxidation waste water contained an average of 5.8% by weight of methanol and was readily separated from the organic phase of the vapor condensate.

7 Example II

Example I was repeated with the only difference that 1.9¾ methanol was continuously added to the reactor pool. After analyzing 3 samples each, the oxidized product was found to have an average methanol concentration of 700 ppm, an acid value of 248 mg KOH / g, a concentration of non in DM? convertible residue of 69ΟΟ ppm and a specific residue formation of 27.8 ppm.g / mg KOH.

In the oxidized product, the following components were added at 23.0 mass MMT

32.0 "- $ PTE

17.7 «- $ PIS

14.7 "-96 TPS

2.4 "- $ PX

25 0.8 '»- $ DM3? determined.

Acid concentration and saponification equivalent in the oxidation water averaged 1100 mg equivalent / l. The oxidation waste water contained an average of 3.0 wt.% Methanol and could be separated well from the organic phase of the vapor condensate.

In Example I, a reduction of the "specific residue formation" of 30% was achieved with the method according to the invention compared to the comparative example. The corresponding reduction in residue formation under the conditions of Example 2 is 14%.

790 58 73

Claims (4)

2 V 12 1. Process for preparing dimethyl terephthalate by oxidation of p-toluic acid methyl ester or a mixture of p-tolylnurmethyl ester with p-xylene in the presence of a low aliphatic alcohol in the liquid phase under elevated pressure and elevated temperature with oxygen or an oxygen -containing gas in the presence of oxidation catalysts, subsequent digestion of the acids formed with methanol and separation of the esterification products, characterized in that to reduce the amounts of readily volatile acids obtained and to not in DM3. and PTE residue to be converted, in the course of the oxidation, an amount of methanol insufficient for any appreciable esterification is fed to the liquid phase to be oxidized, so that a methanol concentration of between 400 and 4000 ppm is maintained in the reaction mixture. Process according to claim 1, characterized in that the methanol concentration in the reaction mixture is maintained at 2000 to 2000 ppm. 5. Process according to claim 2, characterized in that, in the case of cascading oxidation of oxidation reactors, the methanol concentration in each oxidation step is adjusted separately. 7905873