DE2703161C2 - Process for the production of terephthalic acid - Google Patents

Description

The present invention relates to a process for the production of terephthalic acid, in particular on a process for the production of high quality terephthalic acid, which is used as the starting material is suitable for the production of polyethylene terephthalate by direct esterification.

In the past it was polyethylene terephthalate made from dialkyl terephthalate and ethylene glycol. Lately interest has turned to one procedure for the polycondensation of terephthalic acid and ethylene glycol; the terephthalic acid starting material this However, the process must be of high purity; H. the content of 4-carboxybenzaldehyde (one of the in Terephthalic acid present impurities, which is referred to below as 4-CBA) must be under 500 ppm.

The conventional liquid-phase oxidation of p-dialkylbenzene usually contains terephthalic acid a large amount of impurities including various reaction intermediates, such as 4-CBA, p-toluic acid and other compounds with unknown chemical structure. To achieve a highly pure, for polyethylene terephthalate suitable product, the terephthalic acid slurry prepared in this way must therefore undergo a post-oxidation by passing molecular oxygen through them (cf. e.g. GB-PS 9 83 677). This post-oxidation removes the intermediate reaction product as well as a trace of other impurities, mainly of a coloring nature, caused by the Oxidation in the liquid phase does not remove these undesirable materials in the mother liquor the slurry are present; this post-oxidation increases the quality of terephthalic acid improved. It was found, however, that the amount of impurities removed by the post-oxidation is closely related to the conditions under which the oxidation of p-xylene in the liquid phase occurs, and the intended effect of the post-oxidation is often not achieved, which is evident from the conditions the oxidation in the liquid phase depends.

It is already a Process for the production of terephthalic acid has been proposed in which p-xylene is catalytically oxidized in the liquid phase to form a terephthalic acid slurry, wherein the 4-CBA content of the mother liquor of the slurry is kept below 800 ppm, whereupon the The resulting slurry is post-oxidized by passing molecular oxygen-containing gas through it passes through and thus receives highly pure terephthalic acid with a 4-CBA content below 500 ppm, which for polycondensation with ethylene glycol is suitable. Of the different procedures, the 4-CBA content in Keeping the mother liquor below 800 ppm is the most economical from an economic point of view, in which the reaction time of the oxidation of the liquid phase is extended.

However, this method has the inherent disadvantage that when working on a large scale due to the so longer reaction time a lot of acetic acid that the Reaction solvent, is oxidized ("burned"), so that the process becomes uneconomical. Therefore a process was now sought by which the 4-CBA content in the mother liquor of the Terephthalic acid slurry is reduced and the oxidation of the acetic acid solvent is prevented or is significantly reduced. According to the invention it has now been found that in the oxidation of p-xylene in In the liquid phase, a special catalyst system reduces the oxidation of acetic acid and reduces the reaction time shortened, while the quality of the Te rephthalsäurepruduktes is improved.

The subject matter of the present invention is defined by the preceding claims. 6ϊ In the first stage of the method according to the invention p-xylene in acetic acid is used as an oxidation solvent in the presence of the specified catalysts subject to such a degree that a conversion of

p-xylene in terephthalic acid of at least 95%, preferably more than 98% and in particular more than 99.5% is achieved.

The oxidation in the liquid phase used according to the invention, in which the p-xylene is oxidized in acetic acid in the presence of the catalyst described above with a gas containing molecular oxygen, is known as the "SD" process (cf., for example, US Pat 28 33 816, 29 62 36i, 30 92 658, 31 15 520 and ?, \ ! 9 860), F> \? Processes in these patents can be used with success, provided that the catalyst used and the oxidation conditions meet the requirements set out in the claim. The process according to the invention can be carried out continuously or semi-continuously, the best results being achieved with the continuous process.

The amount of acetic acid is usually 0.5-20 parts, preferably 1-10 parts, per part by weight of the additive oxidizing p-xylene. The acetic acid should be less than 30% water or anrfire organic materials, such as Contain aldehydes or ketones.

The gas containing molecular oxygen which is suitable according to the invention contains 5-100% by volume of oxygen. Usually air is used for the sake of simplicity. The amount of the molecular oxygen-containing gas supplied to the reaction system is, in terms of O2, _{1-100 mol 1,} preferably 3-100 mol per mol of p-xylene.

The pressure to carry out the oxidation is not critical as long as the reaction system is liquid State is maintained, and is usually up to 200 bar, preferably 10-50 bar.

The catalyst used in the present invention is a Co-Mn-Br system. As a cobalt compound according to the invention, for. B. cobalt acetate, naphthenate or -bromide can be used. The amount of the cobalt compound to be used is in terms of Co on the basis of the solvent, 200-600 ppm, preferably 250-500 ppm and especially 300-400 ppm.

According to the invention, as a manganese compound z. B. manganese acetate, naphthenate or bromide is used will. The amount of manganese compound to be used is, in terms of Mn based on Co, 0.5 to 1.5 times, preferably 0.7 to 13 times, the weight. Quantities outside of these specified Limits degrade the quality of the terephthalic acid produced.

According to the invention, the bromine compound is hydrogen bromide, Manganese or cobalt bromide can be used. It has been found that other bromine compounds, such as sodium, potassium or ammonium bromide and tetrabromoethane, show only low activity and therefore fail to achieve the intended effect.

The amount of the bromine compound to be used is in terms of Br on the basis of the Solvent, 400-2000 ppm, preferably 700-1500 ppm. Too much will improve it Quality of terephthalic acid is not and is uneconomical as it is used to recover the bromine compound makes re-use necessary. On the other hand, too small quantities lead to poorer quality of terephthalic acid. In order to produce high quality terephthalic acid, the amount of Bromine compound can be selected within the above range so that it, in terms of Br the basis of Co. more than the 2.0-fold c. preferably more than 2.5 times the weight at Using cobalt or manganese bromide, these act both as a bromine compound and as cobalt or Manganese compound.

The temperature for conducting the liquid phase oxidation is 190-225 ° C, in particular 205-220 ⁰ C This higher reaction temperature is critical in the present invention. In the known prior art, the oxidation of p-Xy! OI ii.

Terephthalic acid can be carried out in the liquid phase at a temperature below 200 ^° C. in order to avoid unnecessary oxidation of the acetic acid solvent and a deteriorated quality of the terephthalic acid product. According to the invention, by using the catalyst system mentioned in the above, it is possible to carry out the oxidation in the liquid phase at a higher temperature and thus shorten the time required and noticeably reduce the oxidation of the solvent.

The oxidation in the liquid phase can be carried out continuously or semi-continuously. It is decisive, the 4-CBA concentration in the mother liquor of the terephthalic acid slurry formed below 800 ppm, preferably below 700 ppm. At a 4-CBA concentration above 800 ppm, a large amount of impurities carried in the precipitated terephthalic acid; therefore this is one Purification by post-oxidation inferior end product and not for direct polycondensation suitable

For the purpose of reducing the 4-CBA concentration in the mother liquor to the desired value 800 ppm, it is advisable to control the residence time of the starting material in the oxidation vessel; however, other measures can also be used, such as B. an increase in the amount of solvent, Reduction of the water content in the reaction mixture, increase of the partial pressure of oxygen in the reaction vessel and improved dispersion of p-xylene and / or containing molecular oxygen Gas in the reaction mixture.

So z. B. the following measures are recommended:

1. In a continuous process with a weight ratio of solvent to p-xylene from 3: 1 the residence time is about 1.5-2.5 hours;

2. in a continuous process with a residence time of one hour, the solution is as follows average / para-xyiol ratio about 6: 1; and

J. in a continuous process, the water content in the mother liquor of the reaction mixture increases 5-14% by weight regulated.

The above numbers vary depending on other parameters of the reaction conditions and cannot be standardized. The numbers given are therefore only examples and other conditions to maintain a special 4-CBA content, you can use the above guidelines Attempts to be determined.

Then the terephthalic acid slurry thus prepared is subjected to post-oxidation. main goal the same is the oxidation of various reaction intermediates, such as 4-CBA, in the mother liquor. Therefore, the presence of a large amount of unreacted p-xyloI starting material! in the Slurry to be avoided, albeit somewhat

p-xylo! q also removed during post-oxidation will.

The post-oxidation can be continuous or semi-continuous take place. With continuous monitoring the reaction mixture is poured into a post-Gxiuationsgefäß or transferred to a crystallization tank, and before or during deep cooling of the mixture Precipitation of the terephthalic acid crystals becomes a molecular oxygen-containing gas through the slurry passed through. With the semi-continuous » Process is a molecular oxygen-containing gas directly into the oxidation vessel Termination of the oxidation in the liquid phase introduced

For a post-oxidation that is as complete as possible, it is advisable to use oxygen that no longer contains molecular oxygen Gas introduced as a means of oxidizing the reaction intermediates present in the slurry necessary is.

If too much gas is introduced, the solvent and water will evaporate, resulting in Reduction of the post-oxidation temperature leads; this in turn requires additional heating, making the process uneconomical. By regulating the speed of the Oxygen-containing gas, the post-oxidation proceeds smoothly without further heating.

Since the amount of the impurities to be oxidized in the post-oxidation is small, the amount of the molecular oxygen-containing gas to be introduced is between «» to ¹ A ₀ compared to the oxidation in the liquid phase. Usually the flow rate of the gas is adjusted so that the exhaust gas from the post-oxidation has an oxygen concentration of 0.5-8% by volume, preferably 1.5-4% by volume. If the exhaust gas from the liquid phase oxidation contains sufficient oxygen, it can be fed directly to the post-oxidation stage, or it can be fed in after it has been topped up with free gas containing molecular oxygen. The temperature for performing the post-oxidation is between the temperature of the liquid phase oxidation (which is referred to as T ° C) and (T-30) ° C.

Setting a 4-CBA level of less than 800 ppm in the mother liquor of the terephthalic acid slurry is e.g. B. obtained in particular by that in a continuous process the residence time of the starting material in the oxidation vessel controlled, the proportion of solvent to the Auspangsmaierial to be oxidized increased, the water content of the reaction system is reduced, the partial pressure of oxygen in the oxidation vessel is increased and the p-xylene and the molecular oxygen-containing gas is uniformly dispersed in the reaction medium. There is also the Adjustment of the residence time in the oxidation stage (a) is essential.

The slurry containing the solid terephthalic acid is then conveniently subjected to crystallization subjected and the precipitated terephthalic acid removed and washed with acetic acid, the Washing of the terephthalic acid with acetic acid is preferably carried out so that the solid Terephthalic acid suspended in acetic acid and the terephthalic acid slurry a solid / liquid separation subject.

GB PS 9 83 677 describes the production of terephthalic acid in the presence of catalysts which contain smaller amounts of cobalt and broinide than is required according to the invention. Furthermore, in this process the post-oxidation biri hor.e: vrt T .: -auv j'iiten uurehg -.- leads as the main oxidation. This leads to a larger amount of by-products and thus to a terephthalic acid of reduced purity. About the importance, with as little as possible; Nothing is mentioned in this reference when it stops working with 4-CBA.

In DE-OS 18 02 433 p-Xyioi in the presence of Kobaithromid, Magnesiumbromid i_nd others Salts of the two metals mentioned are oxidized. On the The importance of a specific ratio of these catalyst components is not indicated. Of the The actual catalyst used in this reference has a manganese: cobalt ratio of 0.47 and thus does not fall under the present invention. About the importance of temperatures for the main oxidation and the post-oxidation or the 4-CBA content are not in this literature reference Executions made.

In DE-OS 22 16 208 an oxidation of p-xylene described, the oxidation being interrupted and the reaction mixture in a crystallizer is transferred, in which the Nc.ehoxidation then takes place. By doing this, the Oxygen concentration changed so that no uniform oxidation during the entire oxidation process and therefore no continuous reaction is possible. General information about the Composition of the catalyst are not made, the actually used ratios of Manganese: Cobalt are outside the claimed range.

In DE-AS 21 54 147 are at a corresponding Reaction uses catalyst systems that have a lower ratio of manganese: cobalt than is required according to the invention. The known method also contains no reference to post-oxidation and the importance of a low 4-CBA concentration are not mentioned.

Furthermore, there is the advantage over the known method that according to the invention with less solvent can be used and no such large amounts of catalyst are required. For the rest, the following is also used in this regard Comparative examples pointed out.

In US-PS 30 69 462 a similar oxidation process is used described, in which, however, the ratio of manganese: cobalt in the examples is considerable is higher than is required according to the invention. More general information about any Catalyst systems are not to be found in this literature.

GB-PS 8 48 284 also describes only very generally d ^: c use of cobalt biomide and magnesium bromide catalysts in corresponding oxidations, without the necessary proportions of the catalyst components being specified in more detail. No post-oxidation is carried out in this known process either.

DE-AS 12 67 677 describes a multi-stage process for the oxidation of p-xy'ol in terephthalic acid described in de <n barium bromide catalysts. KobaiiSsiizen and manganese salts can be used. According to example 1 of the literature:,;.: '' '- becomes .lie Oxidation carried out in three stages and in ucr first Reaction stage a conversion rate of 5i to 62.2%

achieved. In the mother liquor must therefore still there are very large amounts of intermediates including 4-CBA and the inventive c and essential feature, namely to keep the 4-CBA content of the mother liquor below 800 ppm, cannot> can be achieved. According to the invention, a practically complete conversion of reached p-xylene and the post-oxidation only serves for the oxidation of traces of unwanted intermediates, in

According to the invention, terephthalic acid can be produced with a quality comparable to that of the best product of the Prior art (/. B. the product of Example 16) is comparable, the residence time being shorter and the Oxidation of acetic acid are less and less i> Catalyst is used.

The 4-CBA concentration in the mother liquor as well as the permeability and mentioned in the examples Oxidation of acetic acid were determined as follows:

Determination of 4-carboxybenzaldehyde

A sample of the slurry containing terephthalic acid is withdrawn from the reaction vessel and the solids contained in the slurry are filtered off. I g of FiItrats is then mixed with 20 ml of methanol 2ϊ and 2 ml of 89% sulfuric acid and the mixture heated for 2 hours at 70 ⁰ C to obtain an esterification of the acid components such as terephthalic acid, 4-carboxybenzaldehyde, p-toluic acid and acetic acid is effected .

N-Butyl benzoate as an internal standard and 2 ml of dichloromethane are added to this reaction product, the mixture then shaken thoroughly with dichloromethane to produce the esterified products and the Extract n-butyl benzoate and then let the mixture stand so that it turns into an aqueous and an Solution -! Layer can separate.

The dichloromethane phase is then subjected to gas chromatography subject, where the solid phase "Celite®" and the liquid phase 5% of a silicone with -to Methylphenyl groups and 0.1% polyethylene glycol with a molecular weight of 6000 (each based on the weight of "Celite") is used to determine the concentration of 4-carboxybenzaldehyde.

Permeability (T340)

The transmittance was aqueous with a solution of 15 parts of terephthalic acid in 100 parts of 2 N potassium hydroxide measured in a spectrophotometer ^s, o using a quartz cell of a inner size of 1 cm and at a wavelength of 340 μΐπ.

oxidation

The oxidation was based on the amounts of CO and CO2 (which are decomposition products of acetic acid) calculated in the exhaust gas from the reaction vessel of each experiment and the results with those of Example 1 as compared to 1.

The following examples illustrate the eo invention without limiting it. All parts are parts by weight.

U: is ρ ic I 1
Continuous seduction

The process was carried out in a device consisting of an autoclave made of titanium for oxidation in the liquid phase, the one with reflux condenser. Stirrer. Heating means, inlet for the starting material. Inlet for the solvent-containing catalyst, inlet for the molecular oxygen-containing gas, outlet for the Exhaust gas and outlet for the reaction product were provided; the post-oxidation vessel was an autoclave Titan, with reflux condenser, stirrer. the inlet, inlet for the tercphthalic acid slurry, outlet for the Exhaust gas and outlet for the reaction product were provided; the crystallizer \ v, ir with coolants Mistake.

The following materials were continuously fed into the liquid phase oxidation vessel in the specified form Quantities introduced:

p-xylene

acetic acid

Cobalt acetate tetrahydrate

Manganese acetate tetrahydrate

47 ⁿ / o aqueous hydrogen bromide

water

Splits

2.85

0.004

0.004

0.006

0.15

The Aiutile at Co. Mn and Br .iuf of the base of Acetic acid were .530 ppm, 330 ppm and 1000 ppm, respectively. The reaction took place at 20 ° C. and 25 bar, with air at such a rate; the reaction mixture It was found that the exhaust gas contained an oxygen concentration of 4% by volume. The dwell was set to 120 minutes.

The Teiephthskaun'-ui'si ^ 'iiiii.-ung hergesceüte was transferred to &, · post-oxidation ^{vessel and oxidized at 190 0} C and 17.2 bar for a residence time of 85 minutes, a mixture of air and the exhaust gas from the Oxidation vessel (O? Concentration of the mixture = 10 vol .-%) was introduced at such a rate that the exhaust gas contained 4 vol .- 'Vo oxygen.

Subsequently, the Terephthalsäureaufschlämmung was in the crystallizer for crystallization at IOO ^c C'übe ^r gslühn.

The separated terephthalic acid crystals have been .ohen bsi by mixing with a 4-fold amount by weight of acetic acid 8O ⁰ C for 20 minutes for. '., He washed and recovered stirring the washed terephthalic acid and dried.

The permeability of the terephthalic acid produced in this way. the 4-CBA concentration in the mother liquor and the oxidation of acetic acid are shown in Table 1.

Example 2 to 5

Example 1 was with the reaction temperatures and / or residence times given in Table 1 liquid phase oxidation repeated, the results listed they were achieved.

Table I.

* Vergleidisheispide
"~ Ierephth.iKiurc

ίο

Example no Temp. ⁰ C Ver * .- 4CBA • _r , ₄₀ μ, 4-CBA oxidation / hurry ppm (ppm / TPA "ι min 1 210 120 310 91.5 270 1.0 2 210 95 500 90.0 400 0.8 3 220 85 280 90.5 240 1.0 4 * 210 45 1000 78.0 800 0.5 5 * 210 40 1300 68.0 1100 0.4

Example h to IJ

Example I was made with those shown in Table 2 ge: it has the essential active components in the catalyst system repeated, where ilii-: i ifiirfiihripn! ^ "'jbnis.se he / ied.

Example 14 to Ib

Example I was with the modified active components mentioned in Table 3 in the Catalvsatorsy-

v.ieiierh;: !! were achieved.

Table 2

Dragonfly 3

Ueisp. Co Mn rephthalic acid Br Mn / Co Use 4-CBA T-. ₄ "(4) 4- (BA ( ) xklation Time ppm ppm / TPA ** min 6th 50 (1 500 1500 1.0 120 300 91.0 270 .1 7th 330 300 1000 0.91 120 300 91.0 270 .0 8th 330 250 KKK) 0.758 120 310 91.0 280 , 0 9 330 420 1000 1.27 120 290 91.5 260 .0 10 * 330 600 1000 1.82 120 290 84.0 260 .1 I I * 330 100 1000 0.33 120 900 55.0 700 ( 12 * 1000 100 3000 0.1 120 450 91.0 400 , 5 13 * 1000 100 1500 0.1 120 500 85.0 450 .1 * Comparative example ** ~ ~ τ ^

Example Co Mn Br Bromine compound Stay.-
Time; min 4-CBA
ppm T ₁₄₁ , (%) 4-CBA
ppm / TAP " oxidation 14 330
15 330
16 * 330
* Comparative example
** = terephthalic acid 330
330
600 900
960
1000 CoBr ₂
MnBr ₂
NaBr 120
120
175 350
340
300 91.0
91.0
93.0 300
290
300 1.0
1.0
1.6

Comparative examples with respect to DE-OS 21 54 147

A. Example 7 was repeated, the ratio of Μη / Co being reduced to 02. There were the following

Reached values that also contain the results of Example 1, which also represents a comparative example:

Cobalt / conc. (Ppm)

Mn / Co

4CBA

• 'ppm / TPA)

Example 7
Ex. !!
Comparative example A.

330 330 330

270

700

1000

91.0 55

50

example 1
Comparative Example B

270
500

"1.5

87

B) In a further experiment, example 1 was repeated, with the difference that no post-oxidation was carried out. The ones obtained in this experiment Results are compared to the results of Example 1 in the table below:

After- 4-C ^- BA T _14n ("· '■■)

oxidation (ppm / TRAl

From these two experiments it follows that the enveloping d'c: “er conditions is critical. and that the Catalyst compositions disclosed in DE-OS 21 54 147 are unsuitable for the present process are.

C. Furthermore, example 1 was reworked, mn Exception that Μη / Co ratio 0.2 and the Br / C'o Ratio of 3.0 in the catalyst system used, no post-oxidation was carried out and the reaction was carried out until the 4 / CBA content of 270 ppm / TBA of Example 1 was achieved. The results of this experiment are the Results of Example I are compared in the table below

Response time (min)

Co-cieh. ! ppi "!

Owdation Nachowdalion

Owcl.ition

Example 1 330 120

Comparative example C 2000 160

^l »1.5

1.0

D) According to the invention, it is also possible to use significantly lower amounts of catalyst than according to the method of DE-OS 21 54 147 work, as can be seen from the following comparison:

Co-cieh Mn <ieh. ISr- (I eh Ciesamtgeh. (ppm I (ppm) (ppm I (ppm ι Example 1* 330 330 1 (X) O 1 660 Example 1** 8 (X) 40 2,400 3 240 Example 2 ** 2,000 100 6000 8HX) P'is game 3 " 5 (XXl 250 15 (XXi 20 250 'sorl. invention ** Document I) H-OS 21 5414 "

Claims (3)

Patent claims: I. Process for the preparation of terephthalic acid, wherein one (A) p-xylene in acetic acid as a solvent of a one-stage oxidation with a molecular oxygen-containing Gu> in Λ; r; .unit of an oxidation catalyst which essentially consists of (1) a cobalt compound in terms of Co in an amount of 200 to 600 ppm of the solvent, (2) a manganese compound in terms of Mn in an amount by weight of 0.5 to l, 5 times the weight of Co and (3) at least one bromine compound in an amount of 400 to 2000 ppm based on the solvent, consists, in the liquid phase at elevated temperature and at a pressure that the reaction system obtained in the liquid state, subjected to the formation of a terephthalic acid slurry and b) post-oxidation the terephthalic acid slurry by passing a molecular oxygen-containing gas and the post-oxidation at lower or at most the same temperature as the oxidation is carried out, characterized in that the oxidation of stage (a) is carried out at a temperature of 190 to 225 ° C and at least one compound from the group of hydrogen bromide or manganese or cobalt bromide is used as the bromine compound, the 4-carboxybcrizaldehyde concentration in the mother liquor keeps the resulting from the oxidation stage (a) slurry below 800 ppm and in stage (b) a temperature between the oxidation temperature T ⁰ C of stage (a) and a temperature of (T-30) ° C is maintained. 2. The method according to claim 1, characterized in that that in the continuous procedure by regulating the residence time of the starting material a 4-carboxybenzaldehyde content of the mother liquor of the terephthalic acid slurry in the reaction vessel of less than 800 ppm. 3. The method according to claim 1, characterized in that that as a molecular oxygen-containing gas in stage (b) as exhaust gas from stage (a) Used alone or mixed with fresh air.