DE3149021C1 - Process for the purification of terephthalaldehydic acid esters - Google Patents

Abstract

Terephthalaldehydic acid esters with contents of impurities of, particularly, aromatic carboxylic acids, are purified by reaction with orthocarboxylic acid esters and subsequent distillation.

Description

The invention relates to a method for the purification of terephthalaldehydic acideslern of low molecular weight, of allphatic alcohols, in particular of the methyl or ethyl ester, of aromatic ones contained as impurities Carboxylic acids such as p-toluic acid and / or Terephthalic acid and / or terephthalaldehyde acid. The process is based on the selective esterification of these Carboxylic acids by reaction with an orthoester and a subsequent fractional distillation.

Without remaining restricted to this, the process according to the invention was specially developed to purify a methyl terephthalaldehyde ester (TAE) isolated from the production of oil methyl terephthalate by the "Katzschmann process" and contaminated, among other things, by p-toluic acid (PTS). The "Katzschmann process" for the production of dimethyl terephthalate (DMT) (cf., for example, Ullmanns Enzyclopadle der technischen Chemie, Vol. 16 (1965), pp. 752 and 753) is a multistage, continuous process which Essentially consists of an oxidation column and an esterification column. In the oxy-gallon stage, p-xylene and p-toluic acid methyl ester (PTE) are converted into p-toluic acid (PTS) and terephthalic acid methyl ester by catalytic air oxidation, which are then reacted in the subsequent esterification stage with methanol to form PTE and DMT . The DMT is isolated by distillation and the PTE- reverberant resl product is stirred back into the oxidation stage. In addition to this residual product, other product streams, for example from cleaning stages, also occur in the DMT process, some of which are also fed into the oxidation stage.

ίο The product streams mentioned now also contain a large number of different intermediate and by-products of the DMT product line. One of the intermediate products of the DMT process is methyl terephthalaldehyde ester (TAE), which is difficult to access by direct means, and can be enriched from such mixtures by fractional distillation.

The isolation and purification of the TAE from product streams of the DMT process creates considerable difficulties, however, since it occurs in these product mixtures only in relatively low concentrations of, for example, < 10% and among the numerous other products there is always one or more with extremely similar boiling ratios Is. These interfering products with almost the same vapor pressure include p-toluic acid (PTS) in particular. The similar boiling behavior of TAE and PTS means that in the fractional distillation to isolate TAE from circulatory products of the DMT process, PTS is also enriched together with the TAE.

jo For example, as part of the DMT process from a mixture that mainly consists of the three isomers of dimethyl phthalate ( > 85%) and in addition to approx. 4 to 8% TAE also contains 1 to 3% PTS, a distillate with a

r> TAE content of> 50% and a PTS content of approx. 15% can be obtained.

In a -wide distillation stage, the TAE content increased to 80 to> 90% and at the same time the PTS concentrailon can be reduced to approx. 5 to 10%. Already

• ίο these distillation stages are with a high technical level and cell expenditure, and a considerable part of the TAE already goes into the Pre- and post-runs of the distillations and thus lost for the TAE acquisition. Another destlllatlve up-

• f> concentration of the TAE on > 95% or even> 98% would only be possible with such a high level of technical effort or to achieve such a low TAE yield that a TAE recovery in this way is less economical Would not be justifiable.

w A considerably more favorable situation would be given if if it succeeds, PTS from TAE-containing mixtures or concentrates on a different than the destillative To cut off ways. There has therefore been no lack of such attempts.

"In DE-OS 28 08 867 a method for removal of p-toluic acid and possibly other aromatic carboxylic acids from TAE or TAE-containing mixtures described. It is based on the separation of these acids in the form of salts, which were previously prepared by adding basic

*> <> see means to a TAE-containing melt or Solution have been prepared. This process has the disadvantage that the aromatic carboxylic acids separated off as salts, In the case of PTS, monomethyl terephthalate, Terephthalic acid and terephthalic

^hr > aldehydic acid (TAS) is a valuable intermediate stage in the DMT process and is withdrawn from the process. In addition, the process has the required filtration stage and / or solvent recovery

technically very complex, whereby the necessary technical effort would be increased considerably, they wanted return the aromatic carboxylic acids separated off as salts to the DMT process.

The present invention was therefore based on the object, in one rich to TAE, from a product stream of the DMT process obtained by distillation the PTS-üehalt after a technically possible simple method to reduce so that then TAE in a simple way, for example by frktionierle Distillation, in the desired purity of z. B.> 96% can be obtained. PTS and possibly in addition to this in the same process step as well with separated other aromatic carboxylic acids such as MMT (Monomelhylterephthat), TPS or TAS for the Gcsamiprozefi are preserved.

According to the method according to the invention, the problem posed becomes particularly simple and technical easily realizable way solved in that a corresponding, Fraction with a relatively high TAE content obtained from a product stream of the DMT process distilled with the lowest possible concentration of PIS and other carboxylic acids at elevated temperature mil in particular orthoformic acid methylene or ortho-acetic acid methylene reacted. Be there the aromatic carboxylic acids contained in the relevant fraction in addition to TAE into the corresponding Mclhyl ester transferred, which are easily separated from TAE by distillation and returned to the DMT process can.

As recognizable Tcrephlhalaldchydsäurcestcr arbitrary saturated aliphatic alcohols with up to 8 carbon atoms, preferably up to 4 carbon atoms, and very preferably up to 2 carbon atoms, regardless of the origin and regardless of the ArI of the contaminating Ammalic Carboxylic acids are to be purified in the same way, the invention relates to such a process.

The invention relates to a method for cleaning one by p-toluic acid and / or tercphthalic acid and / or terephthalic acid and optionally its methyl or ethyl esters and other impurities contaminated Tercphlhalaldehydsäurcesters, which is characterized in that the contaminated Tercphthalaldehydic acid cster at temperatures of 160 up to 300 ° C with trimethyl orthoformate or orthoformate or with orthoformic acid or ürihoessigsäure-triethyl ester under Abdesllllicrcn des resulting. Methanol or ethanol and formic acid or acetic acid ester-containing Tielsicdergemisches is treated, with per as an impurity contained carboxylic acid equivalent 1 to 3 moles of orthoester can be used and the orthoester under normal pressure or under a weak vacuum of up to 300 mbar directly into the melt of the terephthalic acid ester is fed and thoroughly distributed and then the residue is subjected to fractional distillation will.

The starting material from contaminated terephthalaldehydesler (TAE) can be any content of TAE, carboxylic acids and possibly their methylsier or Contain ethyl ester. Contents of carboxylic acids, their esters and other impurities are possible, insofar as this is due to the conversion into the ester according to the invention or because it is present as an ester anyway fractional distillation of IAE are separable.

Starting materials with 50 to 98% by weight of TAE, preferably 70 to 95% by weight, are particularly suitable. particularly preferably 75 to 90% by weight, IAE and corresponding amounts of impurities. In starting materials mil A low TAE content comes as impurities in addition to carboxylic acid, low-boiling and high-boiling Esters before, including DMT and its isomers, dit at the fractional distillation after purification in one or more fractions of TAE are separated.

However, contaminated material is also used as the starting material

TAE of other origin in question, the impurities mentioned or others, in particular other or

ίο can be the same carboxylic acids and / or their esters. If the reaction is carried out with triethyl orthoformic acid or onhoacetic acid esters, the temperatures are high preferred from 230 ° C. The fraction of the separated ethyl esters is used outside of the DMT process.

Although the product streams of the DMT process often contain others besides TAE together with PTS aromatic carboxylic acids such as terephthalic acid, lerephthalaldehyde acid (TAS) and terephthalic acid monomethyl ester the method according to the invention in principle not slören, but in the sense of a Kalalyiischen Accelerating the implementation of the orthoester can even be beneficial, is a high aromatic content Carboxylic acids are disadvantageous in that they are accordingly large amounts of orthoesters are required to convert these carboxylic acids to methylsieres. In the special case of Terephlhalaldehydsäuie (TAS) However, this increases the amount of TAE, so that even a higher TAS content is not necessarily than Would be considered a disadvantage.

The esterification of carboxylic acids by reaction with orthoesters is a known reaction, and the application of this process to the solution of the problem could therefore be regarded as obvious. In practice, however, there were considerable difficulties resulting from secondary and secondary reaclions of the orthoester with TAE, and not only the methyl terephthalic acid methyl ester dimethylacetal
(IAEA), but also, after reactions that have not yet been clarified, I) Ml and several non-tidy lilies. high-boiling by-products were obtained. These side reactions sometimes led to a very considerable reduction in the 1 AE content. For example, a raw 1 AE with a TAE content of approx. 82 "is obtained with an external PTS noise of approx. 7% molar amount of trimethoxymethane in a closed vessel at 150 ° C for 4 hours. The PTS concentration only decreases to 5.5% of the TAE level but to 74 ", while on the other hand,. 9s, IAEA arise. If the same experiment is carried out at 200 C, the PTS sound is reduced to 3 "" and the TAE content to approx. 76%. In this case, only 1.3% TAEA. DMT is formed at a temperature of 250 C in an analogous experiment

«PTS is fully esterified, but it drops 4 hours of the TAE-Gehall aul · 57 ",,. While beside 1.4% TAEA and approx. 6% DMT now approx. 13 ", non-identified. high-boiling by-products result.

The decisive prerequisite for the inherent

^In accordance with the procedure, the unexpected finding was that, under certain, very special conditions, the conversion of the aromatic carboxylic acids contained in the crude TAE to be purified with Orihoeslern trol / the very large Ί AE advance with

¹ ^ a surprisingly high selectivity, i.e. h is possible with only relatively low 1 AU losses

1. the reaction with the orthoester at temperatures above! 50 ° C, preferably above 200 ° C and especially in a temperature range of 22 (1 to 260 C,

2. Door to remove the as reaction products as quickly as possible formed Tielsieder is taken care of by not completing the implementation in a eii.em Vessel under the increased self-pressure caused by the low-pressure boilers formed. but under normal pressure or optionally under a weak vacuum between normal pressure and about 300, especially between Normal pressure and about 400 mbar will.

3. The orthoester is fed directly into the melt and not into the gas space above.

4. Rapid and thorough distribution of the Orthoesler by stirring or pumping the melt is taken care of.

To carry out the procedure according to the invention The required amount of orthoester depends on the amount of aromatic carboxylic acids in the product to be cleaned TAE group. Based on these carboxylic acids, a 1 to 3 fold, preferably an I bis l.Slach molar amount of orthoester as sufficient and proved useful. If less than the molar amount of orthoester corresponding to the carboxylic acids is used, for example, PTS is not converted into the methyl ester to the desired extent and to a certain level from '-.0,5 ",. reduced. On the other hand, however, is also a larger one An excess of orthoester is disadvantageous since it favors the undesired side reactions with TAt will.

As the experiments already described show, increases the degree of dissolution of PTS with that of 160 to 250 "C The quantitative PTS conversion at 250 ° C. is, however, extraordinary accompanied by a strong reduction in the TAE content. Surprisingly, it has now been found that this is then considerable Can be improved if the implementation is not in a closed pressure vessel under the through formed low boilers such as methanol and formic acid or acetic acid methylsier increased autogenous pressure, but is carried out under normal pressure, the low boilers quickly escaping, even under these conditions the PTS turnover increases with increasing temperature, but the TAE turnover decreases. so that a temperature increase has a positive effect and at temperatures of approx. 250 "C a sufficient PTS conversion with relatively low TAE losses. It has therefore proven to be particularly advantageous that the Treatment of the crude TAE with orthoester directly in a still and the low boilers formed to be distilled off via a column. TAE can then be followed by fractional distillation in the desired purity can be obtained.

To the orthoester at the reaction temperatures above its boiling point, in particular at the preferred, relatively high temperature of 230 "C To use it as optimally as possible, this must be inserted directly into the Introduced melt and ensured thorough distribution by stirring or pumping the melt will. The orthoester can be added continuously or in portions. It is among the preferred reaction conditions, a metering and post-reaction time of 0.5 to 2 hours each is sufficient.

If, under these conditions, the raw TAE with the aforementioned composition (82 ".. TAE. 6.8., PTS) at 250 C with that related to the PTS content 1.21 'by molar amount of trimethovinielhan added, see above the PTS content is reduced to ·. 1.5 ", the TAE-V Salary however only to 81 "., While only 0.2", TAEA are formed. After increasing the amount of onhoester to the a total of 1.3 times the molar amount of the original PTS content, the PTS concentration drops to 0.5 · .. while the TAE content drops to 78.4 ", and the ίο TAEA concentration rises to 0.7".

If the experiment described above is carried out with the method described in Pl S related 1.2-fold molar amount 1 rimethoximeihan 150 C repeated, the PTS concentration is only reduced to 5.3 "... but the TAE content to 73.3", reduced !. where 9.3 ■ ',. TAEA to be formed. With increasing reaction temperature Acetal formation is now suppressed in favor of PTS esterification. Already at 190 C could with a raw TAE with approx. 6 "., PTS and ^ 89" ,, TAE by adding the 2lach molar amount of trimethoxime methane based on the PTS content to determine the PTS concentration to - ^ 1 ".. At the same time, however, the TAE Gehall reduced to approx. 82 ".

Although these conditions are less favorable than those at higher temperatures because of the higher orthoester consumption and greater expenditure of time, they are, on the other hand, technically easier to implement. The formation of TAEA is not a fundamental obstacle to carrying out the TAE cleaning process under these conditions, since TAEA can then easily be converted back into TAE by hydrolysis. For example, if the obtained in the previously described test by treatment with Trimethoximethan crude TAE with a related to the ■ J5 TAE content of about b% 2-fold molar amount of water added at 19O ⁰ C, the TAEA content to 0.2 "drops ". while the TAE content rises again to 87", but the reduced PTS concentration remains unchanged. Since this two-step process for cleaning TAE. In which a treatment with orthoester is carried out first and then one with water, which can be carried out at lower temperatures and is therefore easier to implement under limited technical requirements, this treatment with water may be provided to reduce increased levels of TAEA.

The after treatment with Orthoester, but before fractional distillation possible treatment with water takes place at temperatures of 150 to 250 ° C. preferably from 170 to 220 ° C. The amount of water is 1.0 to 3.0, preferably 1.5 to 2.5 and very preferred 1.8 to 2.2 moles per mole of TAEA.

Although the process of the invention for cleaning developed a obtained from the DMT PRODUCTlON Terephthalaldehydsäuremethylester-Fraktlon, it is in principle not limited to, but can also apply for cleaning a contaminated with aromatic carboxylic acids, but obtained by other means Terephthalaldehydsäuremethylesiers ver ^b0. In addition, the process according to the invention can also be used with other terephthalaldehyde esters, for example the ethyl ester, in order to selectively convert aromatic carboxylic acids contained as impurities into esters. It is particularly suitable to add a residual content of terephthalaldehyde acid (TAS) in the corresponding methyl or ethyl ester by reaction with trimethyl ortho-malelic acid or orlo-acetic acid in the case of tere-

phthalaldehydsauremelhyleslers, or with Orihoamelsen · acid-oiler ortho-acetic acid-triäthylesiei im I alle ilcs Terepliihalaldeliydsiiureillhylestors in the opposing Methyl or allyl esters of Terephlhalaldeh.ulsüuiv / Li overlooked. In this lall, the indulgent in all \ experience an increase in howling .111 I erephtlial aldehyde ester.

example 1

/ u " ¹ µm) g of a traction rich in terephthalaldehyde-containing metals (TAt;) of the composition (sample 1) given in table I Dosing pump via an immersion tube while vigorously stirring the melt at 250 C within

I)

half of 30 minutes 4X5 g Trimethoxitnelhan (4.0 mol) / metered. The mixture was then stirred at 250 ° C. for 1 hour. That formed during the reaction from methanol. Meihylformiai and residual Trimethoximethan existing Ί was lefsledergemisch abdesiilliert a l'üllkörpe ^r kolonne. After the total reaction time X ¹ OH 1.5 hours, the melt had the composition given in Table 1, Sample 2). A further 47 g of trimethovimeihan were then added within 3 minutes at 250 ° C. and the mixture was stirred at the stated temperature for 2.5 hours, after which the composition changed in the manner indicated (sample 31 195 ° C 5640 g TAI-: obtained with a purity of 97%. A residue of 510 g with a TAIi content of 73% remained in the still

Tabel

Hdspiel

\ ergleidisbeispicl

I'mbe

TAI:

PTS Pl I:

UMT

TAhA Ims

1 2

! 4th

2 6

3 7

4 8
Sample 1 - raw material

I ΛΙ · .. I erephlhalaldehyilsiluremethylester

PTS: p-tüluylic acid

PTL: p-Toluic acid methyl ester

UMT: Dimethyl terephthalate (Terephlhalsauredimethylestcr) TAhA: Terephlhalaldehνdsäuremeihylesier-t-line iriylacetal MS: unidentified oil boilers

82.1 6.8 6.8 1 .0 0.2 - 81.0 1.4 13.2 I. ,8th 0.7 78.4 0.4 15.1 2 , 5 9.3 - 73.3 5.3 8.2 1 , 4 12.3 _ 70.4 4.7 8.5 1 .5 7.8 _ 73.9 5.5 7.7 1 , 6 1.3 !, 4 70.3 2.7 12.1 3 , 2 1.4 1.3 56.6 _ 15.4 7th ,1 12.8

\ ei-ieich example 1

The experiment described in Example 1 was repeated, except that 485 g of trimethoxime methane were metered in at 150 ° C. to 7730 g of crude TAE of the composition given in Table 1 (sample 1) over the course of 105 minutes and then stirred at 150 ° C. for 2 hours. The composition of the melt then obtained is shown in Table 1 (sample 4). A further 485 g of trimethoxime methane were then ^{fed in at 150 ° C.} within 1.5 hours and stirred for 1 hour, the composition of the melt having changed in the manner indicated (sample 5).

Comparative example 2

500 g of crude TAE of the composition given in Table 1 (sample 1) were heated together with 33 g of trimethoxime methane in a 1-liter autoclave thoroughly flushed with nitrogen at 150 ^° C. for 4 hours while stirring. The pressure was then released and the composition of the melt (sample 6. Tab. I) was determined.

Comparative example 3

The experiment given as comparative example 2 was ^{repeated at a temperature increased to 200 °} C., the composition changing in the manner indicated (sample 7, table 1).

Comparative example 4

The one described in Comparative Examples 2 and 3

The experiment was carried out at a temperature of 250 ° C with the the result shown in Table 1 (sample 8) was repeated.

Example 2

600 g of trimethoxymethane were metered into 7600 g of crude TAE of the composition given in Table 2 (sample 9) at 190 "C. in the manner described in Example 1 under nitrogen over the course of 135 minutes, and the melt was stirred at 190" C. for 2 hours. The composition had then changed in accordance with the information in Table 2 (sample 10). A further 120 g of trimethoxime methane were then fed in at 190 ° C. over the course of 30 minutes and the mixture was stirred for 90 minutes (sample II). Now 80 g of water was metered in by means of a Doslerpumpe via the dip tube and stirred for 1 hour at 190 ° C within 30 minutes at 19O ⁰ C. The composition of the melt had then changed in the specified catfish (sample 12).

Sample 9 =. Raw material

IO

Table 2 \ 1ΛΕ % Pl S ",. Pll ·. ■ · .. I) Ml ■ '"IAKA sample 89.3 6.3 1.5 0.4 _ q 83.3 1.3 6.4 0.9 4.9 10 81.9 0.7 7.0 1.1 5.7 11th 87.1 0.7 7.2 1.2 0.2 12th

Example 3

For 7690 g of crude TAt with 86.4%, TAE, 11.2 ",. TAS and 0.3"., PTS, 500 g of trimethoxyniethane were added within 90 minutes ^{at 25O = C under nitrogen and with stirring.} wherein the low boilers formed distilled off via a column. After a further 90 minutes at 250 "C, the TAL content had risen to 90.2" and the TAS content had fallen to 5.8 ". After adding a further 400 g of trimelhoxymelhan within 60 minutes, the TAK content had risen to 93.3%. While the TAS Ciehail had fallen to 1.2 "" and the PTS content to 0.1 ", \% DMT, 0.6" \, TALiA and 1.8 ", unidentified oil boilers were formed.

Claims (4)

Patent claims: 1. A method for purifying a by p-toluic acid and / or terephthalaldehyde and / or terephthalic acid and g ^ f. their methyl or ethyl esters and other impurities contaminated terephthalaldehyde ester, characterized in that the contaminated terephthalaldehyde ester at temperatures of 160 to 300 ° C with trimethyl orthoformate or orthoformate or triethyl orthoformate with distilling off the resulting. Methanol or Aelhanol and formic acid or Essigsäureester Tlefsiederge- containing treated-mixture, wherein per as an impurity contained carboxylic acid equivalent of I to 3 moles Orthoester are used and the Orthoester under normal pressure or under a weak vacuum to 300 fed mbar directly into the melt of Terephthalaldehydsäureesters and is thoroughly distributed and then the residue is subjected to fractional distillation. 2. The method according to claim I, characterized in that the orthoester treatment above 200 ° C, preferably at 230 to 260 "C is carried out. 3. The method according to claim 1 or 2, characterized in that the melt after the orthoester treatment but before the fractional distillation, first a treatment with water Temperatures of 150 to 250 ° C is subjected, per mole of the terephthalaldehyde acetal contained in the melt I to 3 moles of water are used. 4 method according to any one of claims 1 to 3, characterized in that a single ethyl terephthalaldehyde ester, that by fractional distillation from a product stream of the production of Tcrephthalsiluredlnicthylester was isolated. Is used in the process.