DE2363414A1 - PROCESS FOR REDUCING THE CONTENT OF TEREPHTHALDEHYDIC ACID METHYLESTER IN DIMETHYLTEREPHTHALATE - Google Patents

Description

The invention is a method for reducing the Content of methyl terephthalaldehyde or its dimethylacetals in dimethyl terephthalate by conversion to methyl benzoate and dimethyl terephthalate.

Dimethyl terephthalate is a well-known raw material for the production of synthetic fibers. For paser purposes, a special pure dimethyl terephthalate is required, so the dimethyl terephthalate must be brought into a form unusually pure for a large-scale product before use. The methyl terephthalaldehyde ester is particularly difficult to separate off (Methyl p-formyl benzoate) and also its dimethyl acetal, which are typically in proportions of about 0.05 to about 3 percent by weight in the crude dimethyl terephthalate are contained, which is known via an oxidation of p-xylene with molecular oxygen Procedure is won. Since these aldehyde components increase the stability of the dimethyl terephthalate even in very small concentrations greatly reduce, so that, inter alia, a disruptive color occurs, it is necessary to remove it as far as possible.

Dimethyl terephthalate can be reduced by rectification Pressure to be cleaned. As is well known, it is relatively easy to remove some of the impurities, such as those in clearly lower temperature boiling methyl esters of benzoic acid and p-toluic acid to be separated. But mostly even higher

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Fractions contained terephthalaldehyde ester is so similar in vapor pressure to dimethyl terephthalate that a separation by conventional rectification is very difficult, if not practically impossible.

In order to achieve the necessary purity of the dimethyl terephthalate, cleaning by rectification is therefore usually at least as good one more cleaning stage ahead. For example, dimethyl terephthalate is used recrystallized from methanol. This method leads to products with a thoroughly satisfactory purity for one subsequent rectification, however, is extremely complex in terms of process technology and has, due to the not insignificant solubility, of the dimethyl terephthalate in methanol result in significant losses. In addition, problems arise when working up the Mother liquor on.

Since the crystallization is extremely complex, there has been no lack of attempts to transfer the crude dimethyl terephthalate to others Way to free from the terephthalaldehyde acid methyl ester in order to carry out the rectification without prior crystallization can. For example, methyl terephthalaldehyde is known in the molten dimethyl terephthalate after the addition of catalytically active metal salts which are soluble in the melt, if necessary also using additional activators such as bromine compounds to oxidize (US-PS 3,576,842, GB-PS 1,043,289). These methods remain unsatisfactory in the result despite the considerable effort. Only the addition of the melt-soluble catalysts as new impurities already pose additional problems. The same applies to all proposals, dimethyl terephthalate Reagents that react specifically with the aldehyde component, e.g. adding thiosulphates (Japanese Pat. Application 495/68). Farther a process is known which converts the aldehyde component to powder)! Cu / Cr oxide or Raney copper to p-toluic acid methyl ester

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hydrogenate (GB-PS 955 516). Everyone can do this Additives can be removed relatively easily, but there are pressures from 10 to 70 at and reaction times of 2 hours applied so that expensive equipment is necessary.

Another very important disadvantage of all known processes what is common is that the harmful Terephthalaldehyde ester is removed, - the conversion products however inferior substances compared to dimethyl terephthalate are. '.

The object of the invention was to provide a method for reducing the Content of methyl terephthalaldehyde or its dimethylacetals to be found in dimethyl terephthalate, in which new products which are only known to be easily separated by distillation are formed, in which no losses of dimethyl terephthalate occur and which is characterized by particular simplicity and in which additional proportions of methyl terephthalaldehyde or its dimethylacetals be converted into the valuable dimethyl terephthalate.

This object was achieved by passing the crude dimethyl terephthalate in the presence of methanol at elevated temperatures of up to about 550 ^° C. in the gas phase over a solid palladium catalyst.

The troublesome impurity, methyl terephthalaldehyde or its dimethyl acetal becomes practically completely two different Compounds implemented: once by decarbonylation of the benzoic acid methyl ester, which is due to its Boiling point can easily be separated from dimethyl terephthalate by distillation, and on the other hand results from a dehydrating Esterification with the methanol present an additional amount of dimethyl terephthalate:

Pd

+ -CH-OH Pd ^ CH, -OC -> / VV-CO-CH, + H

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The proportion of methanol contained in the gaseous reaction mixture can be selected within wide limits. So are special Methanol concentrations from 1 to 95 vol- $; based on the whole gaseous reaction mixture, suitable. Below these concentrations, the proportion of dimethyl terephthalate formation decreases in the implementation more and more. The total conversion of the methyl terephthalaldehyde ester also depends on the methanol concentration dependent, because surprisingly the methanol is not ' only absolutely necessary for the process of the dehydrating esterification, but it also clearly favors the process .. of the decarbonylation. An increase in the methanol concentration of E.g. 1 in 10 or 50 Vol- ^ in the reaction mixture leads to first stronger with further increasing concentration leveling off increase of the conversion of methyl terephthalaldehyde ester. The upper limit of the methanol addition is in no way critical and becomes only through economic considerations, e.g. costs for the Separation of the methanol from the dimethyl terephthalate determined. It has surprisingly been found that the decomposition of Only a very small amount of methanol in the reaction described achieved, although under the same reaction conditions without the presence of the dimethyl terephthalate to be purified, a much higher degree of methanol decomposition (to CO + 2 Hp) occurs. The process according to the invention is carried out in the gas phase and thus allows a problem-free separation of the products from the solid catalyst, in particular without recognizable palladium losses. This basically sets it apart from procedures in the Liquid phase, in which · a complete separation of the catalysts would cause considerable difficulties.

The reaction temperature can be varied within wide limits, without the conversion of the terephthalaldehyde ester being decisive · The lower limit for the reaction temperature is because of the for an economical conversion in the gas phase required minimum vapor pressure of the dimethyl

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terephthalate practically, although below this temperature for an appreciable reaction can be observed at about 15O ⁰ C. Since the degree of undesired methanol decomposition increases with increasing reaction temperature, an upper limit of approx. 350 ° C. is given due to this and the incipient thermal instability of the dimethyl terephthalate. It is particularly advantageous to work in the range from 180 to 520 ° C. The reaction preferably takes place at approximately normal pressure. Although the reaction can also be carried out under positive or negative pressure, a substantial deviation from normal pressure is inexpedient, since the use of positive or negative pressure equipment only results in increased costs. It is possible to work with inert carrier gases, but this is not necessary.

A solid palladium catalyst is used as the catalyst. For economic reasons, it is preferred to use supported palladium catalysts in which the palladium is located on an inert support material which offers sufficient surface area for Pd distribution. In order not to let the catalyst become too sensitive to contact poisons, it is advisable not to keep the palladium content of the catalyst below 0.01 percent by weight. The palladium concentration on the catalyst essentially influences the conversion of methyl terephthalaldehyde, less the selectivity of the reaction. Catalysts with 0.01 to 1.0% palladium appear to be particularly suitable for the purification of dimethyl terephthalate, the lower limit being given by the falling conversion, the upper limit being given primarily for economic reasons, but also with regard to the methanol concentration, which increases somewhat with the palladium concentration. Decomposition should not be chosen unnecessarily high. In order to avoid undesired side reactions, the catalyst support should be catalytically inactive for decomposition reactions of dimethyl terephthalate and methanol. This condition is met by a large number of commercially available catalyst supports, such as silica gel, aluminum oxide or activated carbon; however, it is recommended

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check the activity of the support prior to making the finished catalyst to ensure that none is catalytic active impurities are present in the carrier, which cause undesirable side reactions. The preparation of the catalyst takes place according to the usual methods, e.g. by Impregnating the inert support with a solution of a palladium compound. The catalyst, prior to its use, can be used with e.g. Hydrogen can be reduced, but it can also be used directly, since it under the reaction conditions in the catalytically active form.

The external shape of the catalyst is not a critical factor for the process. For example, the catalyst can be used in tablet, Be spherical, strand or piece shape. The particle size of the catalyst should be chosen so that between the advantageous surface-dependent growing with decreasing grain size Activity and the disadvantageously increasing flow resistance at the same time, a practical compromise is found.

Although the methanol decomposition in the process according to the invention is only very low, it has surprisingly been shown that with Addition of small amounts of water to the methanol, the decomposition can be suppressed even further. It has also been shown that that part of the terephthalaldehyde ester which is in the form of the diacetal is significantly faster in the presence of water vapor and is implemented more fully. The water is generally used in amounts up to 20 vol- $, based on the total gaseous Reaction mixture, added. A significantly higher addition of water is possible, but does not bring any significant additional advantages with himself.

The method according to the invention can then be used with particular advantage apply when terephthalic acid is esterified with methanol in the gas phase or when dimethyl terephthalate from a liquid phase esterification together with methanol and the water released during the esterification for separation from the still unesterified Acid is continuously withdrawn in vapor form. In these

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Cases, the gas mixture crude dimethyl terephthalate / methanol / Water passed directly over the catalyst without intermediate condensation and so cleaned. No further addition of methanol or steam is then required for cleaning the other costs are also low.

The method according to the invention allows a comparatively simple one Way, dimethyl terephthalate · of terephthalaldehyde acid methyl ester and releasing related compounds with a previously unknown degree of effectiveness, being in addition to the easily separable Benzoic acid methyl ester - as a particular advantage of the process - additional dimethyl terephthalate is formed. the Use of a heterogeneous catalyst in the gas phase reaction does not pose any separation problems. The rapid reaction allows the removal of the technically most important and in one step most unpleasant impurity of the dimethyl terephthalate practically to any desired size.

In the following examples, which are intended to illustrate the invention are used, a number of abbreviations are used, which have the following meanings:

DMT = dimethyl terephthalate

TASME = methyl terephthalaldehyde ester

BSME = methyl benzoate

TSME = toluic acid methyl ester

example 1

Production of a supported palladium catalyst with a content of 0.1 percent by weight of Pd, based on the total finished catalyst

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A 'for the decomposition of DMT and methanol largely inactive commercially available carrier material, silica gel or aluminum oxide in Kugelbzw. ^{The cylindrical shape was heated to 230 °} C. for 4 hours, mainly to remove adsorbed water, a stream of dry air being continuously passed through the carrier material. 0 · 583 g of dry PdCIp were concentrated in 2.5 ml. Hydrochloric acid dissolved with heating. This solution was diluted with 750 ml of acetone in a 2 liter round bottom flask. 350 g of the dried carrier material were then added to the solution while swirling. After standing for one hour, the solvent was evaporated off in a rotary evaporator initially at normal pressure and later in vacuo. The catalyst was then heated for 1 hour at 23O ⁰ C in a nitrogen flow and the Pd (II) finally at 230 ° C for two hours, passing a hydrogen stream to reduce metal.

Example 2 .

For the laboratory tests, a normal pressure gas phase apparatus was used, consisting of a liquid evaporator, a DMT gas saturator, a quartz reactor filled with a catalyst. And a condensation device, which were also connected in series in this order. In the evaporator heated to 70 C 20.0 g / h of methanol were added via a microdosing pump and evaporated into a nitrogen stream of 10.0 l / h. This gas flow got into a thermostated at 180 ° C DMT gas saturator, which at the start of the experiment with 300 g of a mixture of 98 percent by weight of fiber-pure DMT and 2.0 percent by weight was filled with pure TASME. Under these conditions, 10.0 g / h of a mixture of DMT and TASME from the was approximately constant Saturator evaporates. After about eight hours of testing, the amount of DMT / TASME mixture that had evaporated was replenished. The TASME content in the vaporized TASΜΕ / DMT mixture changed slightly depending on the duration of the experiment and was therefore analytically calibrated.

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The quartz reactor, provided with a bottom frit for better Gas distribution and as a catalyst support, was thermostatted with the aid of a surrounding salt bath. Sr was filled with 50 ml a commercially available per I-shaped silica gel carrier (type B of Kali-Chemie Engelhard catalysts GmbH) with an average diameter of about 5 mm, which, as described in Example 1, 0.1 ^ Pd had been occupied. The gas mixture from the saturator was passed into the reactor. The contact time at a reaction temperature of 300 ° C. was 3 * 5 seconds (calculated under reaction conditions with reference to the bulk catalyst volume).

Product samples were taken between the 7x and 8x hours of operation each

The experiment was taken and dissolved in chloroform.

The dissolved products were analyzed by gas chromatography. The end gas analyzes were also carried out by gas chromatography carried out.

Table 1 gives the results of a series of experiments on gas phase cleaning of crude DMT in the apparatus described on the above Catalyst at a reaction temperature of 300 C again.

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Table 1: Conversion of a TASME / DMT mixture on a Pd (0.1 fo silica gel catalyst at 300 C and a contact time of 3 x 5 seconds in the presence of methanol.

Total operating time
of the catalyst hour 8th 16 O.
2.
97. 00
63
37 32 .00
.56
.44 40 .00
.50
.50 48 .00
.46
.54 Reactor insert (wt. Fo) 100. 00 .00 .00 .00 BSME '
TASME
DMT 0.00
2.85
97.15 0.00
2.72
97.28 0
2
97 O
2
97 O
2
97 100.00 100.00 1.
O ₀
98 63
07
30th 100 .55
.10 100 .06
.41 100 .07
.50 Product (weight iQ 100. 00 .00 .00 .00 BSME
TASME
DMT I.70
O.O9
98.21 1.61
0.06
98.33 97. 5 * 1
0
_28 1
0
98 .5 1
0
98 100.00 100.00 77 100 100 100 TASME sales {% of
Mission) 97 98 23 96 97 97 Decarbonylation
(Mole $ of sales) 74 73 76 76 72 dehydr. Esterification
(Mole $ of sales) 26th 27 24 24 28

The artificially produced by adding pure TASME to pure DMT Instead of the DMT technically contaminated with TASME, the mixture was initially used as the starting material for the experiments preferred, as the lack of other components enables a clear assessment of the test sequence.

The values show that a TASME conversion of approx. 97% (based on use), which remained constant within the experimental and analytical error limits, was found over the entire duration of the experiment; Furthermore, that BSME was formed as the main product of this reaction by a decarbonylation reaction (approx. 75 % based on conversion), but that also an essential one

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Part of the TASME (approx. 25 % based on conversion) has been converted into DMT by dehydrogenative esterification. This last reaction is of course particularly advantageous, since it creates additional proportions of the desired main product from a disruptive by-product.

The gas analyzes showed that in addition to the CO produced by the decarbonylation of TASME, further CO was formed by the decomposition of methanol to CO + 2 Hp. The values determined for this decomposition were, however, under the selected reaction conditions in each case below 1.0 % of the methanol input and are therefore insignificant. The formation of COp. Could not be detected.

Example 3

The test procedure in this example corresponds to that of Example 2 with the exception of the changes in the catalyst composition which are evident from Table 2.

Table 2 shows the influence of the Pd concentration on the product composition in the purification of raw DMT.

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Table 2: Implementation of a TASME-DMT mixture at 300 Presence of methanol

in

Carrier material .-Si) Al ^ O O Silica gel 0.10 0.25 Pd conc. ( Weight-fo) 0.10 .025 Reactor insert (wt O
2
97 0.00
2.85
97.15 0.00
2.78
97.22 BSME
TASME
DOT vo
-arc ο
...
OVDO
00ΓΟ O .00
.70
.30 Product (weight- fo) 1
O
98 1.70
0.09
98.21 1.65 '
0.03
98.32 BSME
TASME
DMT I.83
0.12
98.O5 90 .54
.27
.19 97 99 TASME sales (fo from
Mission) 96 76 74 72 Decarbonylation
(Mole $ of sales) 79 24 26th 28 dehydr. Esterification
(Mole $ of sales) 21

It turns out that the carrier material is not significant Exerts influence on the course of the reaction.

For comparison, the same supports without Pd coating were used under the same reaction conditions. There was no conversion whatsoever. In addition, for comparison purposes, fiber-pure DMT was again passed under the same test conditions over silica gel and aluminum oxide, each covered with 0.10 fo Pd. Here, too, no trace of the formation of by-products from DMT could be found.

Example 4 . ·

The only undesirable side reaction of the purification of the crude DMT of TASME became a small one, but one that increased with temperature Decomposition of the methanol promoting the reaction

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(see, inter alia, Example 5) to CO H- 2 H _? established. It was found that small amounts of water added to the methanol markedly inhibit the decomposition of methanol without the TASME conversion being noticeably affected. Here, too, there was no COp. In addition, if part of the TASME was present in the reactor insert in the form of dimethylacetal, this was implemented more quickly and much more extensively in the presence of water. Table 5 shows these relationships. Reaction conditions that are not specifically stated again correspond to those of Example 2.

Table J5r decomposition of methanol to CO + ■ H _p and conversion of

TASME dimethylacetal in the conversion of a TASME / DMT mixture over a Pd (0.1 %) silica gel catalyst depending on the water content of the methanol used

Reaction temp. ( ⁰ C)
HpO in methanol (wt. %) 0 320 ⁰ C
10 0 280 ⁰ C
10 Reactor use
TASME (wt. %}
TASME acetal (wt .- ^) 2.80
0.00 2.68
0.00 2.26
0.94 2.12
O.82 product
TASME (wt .- ^)
TASME acetal (wt .- ^) 0.07
0.00 0.08
0.00 0.07
0.05 O.O9
0.002 TASME sales (moles- $ from a
sentence) based on aldehyde +.
Acetal
Methanol decomposition too
CO + Hp (weight- fo from use) 97.5
1 OO 97
0.46 96.5
0.28 97
0.10

Example 5 .

In this example the reaction temperature and the input TASME concentration were varied. According to the results of Example 4 were. 10 % of the methanol input replaced by water (18 g / h methanol + 2.0 g / h water). The other reaction conditions not listed correspond to those of Example 2. The results obtained are shown in Table 4.

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Table 4: Conversion of a TASME / DMT mixture in the presence of methanol / water at different reaction temperatures and different use TASME concentrations.

Pd concentrations
Reaction temp. (° C) (Weight f ₀ ) Mission) O.
240 10
28O 320 0 .25
300 0.20
23 TASME / concentration ⁴ "
in the reactor insert. (wt. %)
in the product (ppm) CO + 2H ₂ 2.58
428 2.65
681 2.68
787 2.78
265 O.52
76 99 TASME sales ($ from 98.5 97-5 97 99 98.5 0.51 CH ^ OH decomposition to
(^ from the mission) 0.04 0.12 0.46 0.42 0.44

+! based on the D?> Tr / TASM3 mixture

The TASME use concentration has within the range of interest Area does not have a significant influence on the TASME urine rate. The last column of the table shows that at a TASME use concentration of 0..20 percent by weight under the selected reaction conditions the result is a product whose TASME content is already within the normal tolerance for fiber-pure DMT.

By lengthening the contact time or, for example, by increasing the Pd concentration on the catalyst, the TASME final concentration can in all cases always be further reduced to the desired value according to the relationships that are in principle well known to any person skilled in the art. -

Example 6

In this example, the role of methanol in the implementation should be examined. For this purpose, the methanol was gradually replaced by an amount of nitrogen equivalent to the gas volume; eventually the nitrogen was replaced with an equal amount of other carrier gases. (In Wahl'von helium to a helium flow of 25.3 l / h at 25 ⁰ C gave so, said I5.3 l / h instead of the methanol and 10.0 l / h was used instead of nitrogen.)

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The amounts used otherwise corresponded to those of Example 2. Table 5 shows the influence of the carrier gas selected in each case

the TASME conversion in the DMT purification on a Pd catalyst with 0.10 % Pd at 280 ° C.

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Table 5: Influence of different methanol concentrations and

Carrier gases on the TASME conversion when converting a TASME / DMT mixture over a Pd (0.1 %) SiIicage! Catalyst at 280 ° C and the same contact time

Share in gas Reaction mixture methanol (VoI-J!.

Carrier gas (VoI-J *) 94 (N _g ) 91 (Ng) 85 (N ₂ ) 70 (N _g ) 57 (Ng) - 95 (N ₂ ) 95 (He) 95 (H ₂ )

Raw DMT

TASME-Ümsa'tz (# from the mission)

55 56

ο co cn

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The. Values show that the TASME conversion depends heavily on the methanol Concentration is dependent. Already 1 vol- $ methanol in the feed gas mixture increases the conversion rate compared to the use of pure nitrogen or helium as inert carrier gas considerably. An increase in the methanol concentration initially results in a further significant increase in sales finally laughs.

If inert carrier gases are selected, cleaning is based exclusively on decarbonylation. Under the reaction conditions used and reacted for 8% of 9.5 ° f ^the converted TASME in this Rdchtjung: only in hydrogen as a carrier gas, the hydrogenation of TASME to TSME takes place in addition to the decarboxylation of a minor extent. If, after working for a long time with hydrogen as the sole carrier gas, a switch was made to the methanol (+ water ^ nitrogen mixture), the high TASME conversion values typical for the counterpart of methanol were obtained again after some time Hydrogen as a carrier gas is therefore specific for hydrogen and is not based on irreversible damage to the catalyst.

Example 7

A crude DMT was obtained by quantitative esterification of technically occurring crude terephthalic acid and under conditions used according to Example 2 for DMT purification according to the invention described. For the input material or for the converted The composition of the product was determined by gas chromatography:

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(Wt .- ^
(Wt -.%
(Wt -.%
(Wt -.% ] Reactor use product BSME
TSME
TASME
DMT 0.22
0.55
1.61
97.84 1.20
0.55
0.05
98.42 100.00 100.00

According to this, the TASME conversion was 97 %, the proportion of the decarbonylation was 76 mol- $ and the proportion of the dehydrogenative esterification was 24 mol- # of the converted TASME. Like DMT, p-Toluic acid ester (TSME) was not attacked. -

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Claims (1)

Pat ent claims 1 · Process for reducing the content of methyl terephthalaldehyde! Ester or its dimethyl acetal in dimethyl terephthalate, by conversion to methyl benzoate, which can be easily separated by distillation, and to dimethyl terephthalate, characterized in that the crude dimethyl terephthalate is prepared in the presence of methanol at elevated temperatures of up to 550 ^° C. in the gas phase via a
solid palladium catalyst conducts 2. The method according to claim 1, characterized in that one Methanol in a concentration of 1 to 95 vol- $, based on applies the entire gaseous reaction mixture. 5 »Method according to claims 1 and 2, characterized, . that one works at temperatures between 180 and 520 ⁰ C. 4. The method according to claim 1 to J> ,, characterized in that one works in the presence of steam. 5 method according to claim 4, characterized in that one 1 to 20 vol- # water vapor, based on the total gaseous Reaction mixture «. 6. The method according to claim 1 to 4, characterized in that one a supported catalyst with a palladium content of 0.01 to 1.0 percent by weight, based on the total finished catalyst, used. 509827/0 96 4 - 20 -. ' OZ 2755 Ίο.12.1975 7. Method according to claims 1 to 6, characterized by * that one vapor mixtures of reaction products and resulting from the esterification of terephthalic acid with methanol Directs methanol directly over the catalyst. 509827/0 964