DE2363414C3 - Process for the purification of dimethyl terephthalate by reducing the content of methyl terephthalaldehyde or its dimethylacetals - Google Patents

Description

The invention relates to a method for j ', Reduction of the content of terephthalaldehyde acid methyl ester or its dimethyl acetal 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. A particularly pure dimethyl terephthalate is required for fiber purposes., so that the dimethyl terephthalate before its use in a for a large-scale product unusually pure form must be brought. r>

The methyl tercphthalic acid are particularly difficult to separate (Methyl p-formyl benzoate) and also its dimethyl octal. the typical in Proportions from about 0.05 to about 3 percent by weight in the crude dimethyl terephthalate are contained, the üher -w an oxidation of p-xylene with molecular oxygen is obtained by known methods. Since these aldehyde compounds were already present in very small concentrations the stability of dimethyl tercphthalate is strong decrease so that ti. a. a disturbing color; niflntl. π it is necessary to remove them as far as possible.

Dimethyl terephthalate can be purified by rectification under reduced pressure. It succeeds known to be relatively easy to remove some of the impurities such as /. B. those at significantly lower temperatures> » boiling methyl ester of benzoic acid and p-toluic acid. to separate. The tercphthalic acidic acid, which is usually contained in even higher proportions, is in the Vapor pressure so similar to dimethyl terephthalate that it can be separated by conventional rectification >> is difficult, if not practically impossible.

In order to achieve the necessary purity of the dimethyl terephthalate, purification by rectification is usually preceded by at least one further purification stage. So becomes /. B. recrystallized the dimethyl terephthalate from mi methanol. Although this method leads to / u products with a thoroughly satisfactory purity for a subsequent rectification, it is extremely complex in terms of process technology and, due to the not insignificant solubility of the dimethyl terephthalate in methanol, results in significant losses. In addition, problems arise in the work-up of the mother liquor.

Since the crystallization is extremely complex, there has been no lack of attempts to use the crude dimethyl terephthalate otherwise from the methyl terephthalaldehyde ester to free in order to carry out the rectification without prior crystallization can. So is z. B. known, the terephthalaldehyde acid methyl ester in the molten dimethyl terephthalate, after addition, catalytically active, more soluble in the melt Metal salts, if necessary also with the use of additional activators, such as bromine compounds, oxidize zi (US-PS 35 76 842, GB-PS 1043 213). These Despite the considerable effort, methods remain unsatisfactory as a result. Just the addition of the Melt-soluble catalysts as new impurities already pose additional problems. Same thing applies to all suggestions that the dimethyl terephthalate reacts specifically with the aldehyde component Reagents, e.g. B. add thiosulfates (Jap. PaL application 495/68). Furthermore, a method is known, the aldehyde component on powdered Cu / Cr oxide or on Raney copper to form p-toluic acid methyl ester to hydrogenate (GB-PS 9 55 516). With this method, all additives can be made relatively easily are removed again, but pressures of 10 to 70 atm and reaction times of 2 hours are used, so that expensive equipment is necessary.

Another very significant disadvantage of all known method is common is that Although the harmful tercphthalaldehyde acid ester is removed, the conversion product is in the Substances are inferior in comparison to dimethyl terephthalate.

It was the object of the invention. a method for reducing the content of methyl alcoholic acid phthalaldehyde or its dimethylacetals in dimethylterephthalate, in which only dcstilkitiv is known easily separable products are formed in which no losses of dimethyl terephthalate occur and which is characterized by particular simplicity and with the additional proportions of the tertiary aldehyde acid methyl ester b / w. its dimethyl acetal can be converted into the valuable dimethyl terephthalate.

This task became clear. that the crude dimethyl terephthalate in the presence of 1 to 95 Vf) I. Vn methanol, based on the total gaseous Reaklionsgcmiscii. at temperatures of 150 to 50 "C in the gas phase over a solid Killailiiiinkatalysator. The troublesome impurity Tcrcphthalaldehydsäiircmethylcstcr or its Dimclhylacclal is practically completely converted into two different compounds Its boiling point can be easily separated from dimethyl terephthalate, and / or another amount of dimethyl terephthalate is formed by a hydrochloric esterification with the methanol present:

(II, O C <

Il

I'd

Il cn, ο c

> ico

O O

Il / -χ Il

C'Hj - Ο -CC VCO-CH ₃ -1- H,

The proportion of methanol contained in the gaseous reaction mixture can be selected within wide limits. Thus, methanol concentrations of from 1 to 95% by volume, based on the total gaseous reaction mixture, are particularly suitable. Below these concentrations, the proportion of dimethyl terephthalate formation in the conversion decreases more and more. The total conversion of the methyl terephthalaldehyde ester is also dependent on the methanol concentration, because, surprisingly, the methanol is not only absolutely necessary for the course of the dehydrating esterification, but it also promotes the course of the decarbonylation. An increase in the methanol concentration of e.g. B. 1 to 10 or 50% by volume in the reaction mixture leads to an initially strong increase in the conversion of methyl terephthalaldehyde ester, which increases as the concentration increases. The upper limit of the methanol addition is in no way critical and is only determined by economic considerations, e.g. B. The cost of separating the methanol from the dimethyl terephthalate is determined. Surprisingly, it has been found that the decomposition of methanol in the reaction described reaches only a very small extent, 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 H ₂ ) occurs.

The inventive method is in the Carried out gas phase and thus allows a problem-free separation of the products from the solid catalyst, especially without any discernible palladium losses. This basically sets it apart from Procedure in the liquid phase. in which a complete separation of the catalysts is considerable Would cause trouble.

The reaction temperature can be varied within wide limits without the conversion of the phthalaldehyde acidic acid being decisively influenced. The Untcrgren / .e for Reüktionstcmpcralur is due to the time necessary for an economical implementation in Ciasphase Mindcsldampfclruckcs of Dimclhylterephlhalals at 150 ⁰ C, although below this temperature for an appreciable reaction can be observed. Since the degree of undesirable Mcth: inol - /. Replacement rises with increasing reaction temperature, this and the incipient thermal instability of the diynylicrephthalal give an upper limit of 350 "C. One works particularly favorably in the range from 180 to 20" C. The reaction preferably takes place at about normal pressure. Although the reaction can also be carried out at elevated or low pressure, a substantial deviation from normal pressure is inexpedient, since the use of high or low pressure equipment only incurs low costs. If necessary, inert carrier gases can be used, but this is not necessary.

A solid palladium calcium analyzer is used as the catalyst used. Due to economic considerations, palladium substrate analyzers are preferably used, in which the palladium is on an inert

Carrier material is located that is sufficient for Pd distribution Surface offers. In order not to make the catalyst too sensitive to contact poisons, it is advisable not to fall below a palladium content of 0.01 percent by weight in the catalyst. The palladium concentration on the catalyst essentially influences the conversion Terephthalaldehyde acid methyl ester, less the selectivity the reaction. Catalysts appear to be particularly suitable for purifying dimethyl terephthalate with 0.01 to 1.0% palladium, the lower limit being due to the falling conversion, the upper limit is given primarily for economic reasons, but also with regard to the Palladium concentration slightly increasing methanol decomposition should not be chosen unnecessarily high should. The catalyst support should be catalytically inactive in order to avoid undesired side reactions be for decomposition reactions of dimethyl terephthalate and methanol. This + v condition is followed by a A variety of commercially available catalyst supports met, such. B. silica gel, alumina or activated carbon; it recommends however, to check the activity of the carrier before the preparation of the finished catalyst in order to ensure that there are no catalytically active impurities in the carrier which are undesirable Cause side reactions. The catalyst is prepared according to the usual methods Methods, e.g. B. by impregnating the inert support with a solution of a palladium compound. Of the Catalyst can prior to its use with e.g. B. hydrogen, but it can also can be used directly, as it changes into the catalytically active form under the reaction conditions.

The external shape of the catalyst is not a critical parameter for the process Catalyst z. B. in tablet, ball, strand or bar form. Here is the particle size of the catalyst should be chosen so that between the growing advantageously with decreasing grain size surface-dependent activity and the disadvantageously growing flow resistance at the same time practical compromise is found.

Although the mclhanol reduction in the case of the present invention Procedure is very low. it has surprisingly been found that less when added Amounts of water to which the methanol decomposition can be suppressed even further. Also has shown that the part of the terephthalaldehyde acid ester. which is in the form of the diacelal, significantly faster and more complete in the presence of water vapor is implemented. The water is generally used in amounts of up to 20% by volume. based on the whole gaseous Ueaklionsgcmisch, added. One essential higher addition of water is possible, but does not bring any significant additional advantages.

The final process can then be followed by mil Use particular advantage 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

is continuously withdrawn in vapor form from the still unesterified acid. In these cases it can Gas mixture crude dimethyl terephthalate / methanol / Water can be passed directly over the catalyst without intermediate condensation and purified in this way. In the No further addition of methanol or steam is then required for cleaning, and neither is the rest Costs become low.

The inventive method allows on comparatively easy way to dimethyl terephthalate from terephthalaldehyde acid methyl ester and related To free compounds with a previously unknown degree of effectiveness, being in addition to the easily separable Methyl benzoate - as a particular advantage of the process - additional dimethyl terephthalate is formed will. The use of a heterogeneous catalyst in the gas phase reaction does not pose any separation problems on. The rapid reaction allows the removal of the technically most important and in one step most unpleasant contamination of the dimethyl terephthalate to practically any desired extent.

In the following examples, which are intended to illustrate the In order to serve the invention, a number of abbreviations are used, which have the following meanings: DMT = dimethyl terephthalate TASME = terephthalaldehyde acid methyl ester BSME = methyl benzoate TSME = methyl toluate

Production of the catalyst Production of a supported palladium catalyst with a content of 0.1 percent by weight of Pd

on the entire finished catalyst A commercially inactive carrier material, silica gel or aluminum oxide in spherical or cylindrical shape, largely inactive for the decomposition of DMT and methanol, was ^{heated to 230 °} C. for 4 hours, with a continuous stream of dry air was passed through the carrier material. 0.583 g of dry PdCb was dissolved in 2.5 ml of concentrated hydrochloric acid with heating. This solution was diluted with 750 ml of acetone in a 2 liter round bottom flask. 350 g of the dried support 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 ^{im at 230 °} C. for 1 hour

Table 1

Conversion of a TASME / DMT mixture on a Pd (OJ%) silica gel catalyst at 300 C and a contact time of 3.5 seconds in the presence of methanol

Siicksioffslrom tempered and the Pd (II) finally reduced at 23O ⁰ C by passing a hydrogen stream for two hours to the metal.

example 1

A normal pressure gas phase apparatus was used for the laboratory tests, consisting of a liquid evaporator, a DMT gas saturator, a quartz reactor filled with catalyst and a condensation device, which were also connected in series in this order. In the. evaporators heated to 70 ^° C. were introduced via a micrometering pump 20.0 g / h of methanol and evaporated into a nitrogen stream of 10.0 l / h. This gas stream passed into a ^C to 180 C thermostated DMT gas saturators, the TASME beginning of the experiment with 300 g of a mixture of 98 weight percent fiber of pure DMT and 2.0 weight percent pure was filled. Under these conditions, 10.0 g / h of a mixture of DMT and TASME were evaporated from the saturator almost constantly. The amount of DMT / TASME mixture that had evaporated was replenished after each approximately eight-hour test. The TASME content in the vaporized TASME / DMT mixture changed slightly depending on the duration of the experiment and was therefore analytically calibrated.

The quartz reactor, provided with a bottom frit for better gas distribution and as a catalyst support. was thermostated with the help of a surrounding sali bath. It was filled with 50 ml of a commercially available pearl-shaped silica gel carrier with a medium-sized one Diameter of approx. 5 mm, which, as described in connection with the preparation of the catalyst, has 0.1% Pd had been. The gas mixture from the saturator was passed into the reactor. The contact time turned out to be at a reaction temperature of 300 "C 3.5 seconds (calculated under reaction conditions with reference to the bulk catalyst volume).

Product samples were taken between the 7th and 8th operating hours of each trial and placed in Dissolved chloroform.

The dissolved products were analyzed by gas chromatography. The Endgasanaly.en were also carried out by gas chromatography.

Table 1 shows the results of a series of experiments for the gas phase purification of crude DMT in the apparatus described on the catalyst mentioned at a reaction ^{temperature of 300 °} C. again.

Total Belricbsdaucr 0.00 16 of the catalyst - Hour 40 4 « 8th 2.85 24 32 Reactor input (% by weight) 97.15 0.00 0.00 0.00 BSME 100.00 2.72 0.00 0.00 2.50 2.46 TASME 97.28 2.63 2.56 97.50 97.54 DMT 1.70 100.00 97.37 97.44 100.00 100.00 0.09 100.00 100.00 Product (wt .-%) 98.21 1.61 1.53 1.43 BSME 0.05 1.63 1.55 0.06 0.07 TASME 98.33 0.07 0.10 98.41 98.50 DMT 98.30 98.35

100.00

100.00 100.00

100.00

100.00

ino.no

continuation

Overall usage of the catalyst! - hours 8 16 24 .1

40

TASME sales. (% from the
Mission)

Decarbonylation
(Moi-% of sales)
Dehydr. \ esterification
(MoI- ";. From 1 Imsat / l

97 98 97.5 96 97.5 97 74 73 77 76 76 72 2 (i 27 23 2t 24 2 H

Diis artificially by adding pure TASME to it The mixture produced with pure L) MT was used instead of the DMT which was technically contaminated with TASMR / uilrfCnM «ils Einsai / maiei iai preferred to attempts, since the lack of other components enables a clear accounting of the test sequence.

The values show that, over the entire duration of the experiment, one within the experimental and analytical error limits of constant TASME-lIiiT-etch of approx. 97% (based on use) was found: furthermore, that as the main product of this reaction BSMf: formed by a decarbonylation reaction has been (approx. 75% based on sales), but that next to it also a substantial part of the TASMF (approx. 25% based on conversion) by dehvdrierendc esterification in DMT has been convicted. This last reaction is of course particularly advantageous because it is a disturbing one Nehenproduk'i additional proportions of the desired Main products emerge.

The gas analyzes showed that, in addition to the CO produced by the decarboxylation of TASMF, further CO caused by the decomposition of methanol to CO + 2 W; has been formed. The values determined for this decomposition were, however, under the chosen reaction conditions in each case below 1.0% of the methanol input and are therefore insignificant. The formation of CO: could not be detected.

Example 2

The experimental procedure in this example corresponds to cL. of Example 1 with the exception of that from Table 2 shows changes in the catalyst composition.

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

Table 2

Implementation of a TASMF-DMT mixture at 3 (K) C in the presence of methanol

I r.iuemi.jieri.il

Al-U-. Silicaid

il.ld 0. (05

(l. in

11.25

R ca k gates in sat / (weight- ",, ι 0.00 0. (H) 0.00 0. (X) BSMI: 2.02 2.7 (1 2.85 2.78 TASMl 97.08 97.30 97.15 97.22 DMT Product (weight) 1.83 1.54 1.70 1.65 IiSMF 0.12 0.27 0.09 0.03 TASME 98.05 98.19 98.21 98.32 DMT 96 90 97 99 TASME sales
(ι from the mission! 79 76 74 72 Decarbonylation
IMoI- "',) from sales) 21 24 26th 28 Dehydr. Esterification
(MoI- ",, from sales)

It turns out that the carrier material does not exerts a significant influence on the course of the reaction.

For comparison, the same supports without Pd coating were made under the same reaction conditions used. There was no conversion whatsoever. In addition, fiber-pure DMT was again used for comparison under the same test conditions, over silica gel and aluminum oxide, each covered with 0.10% Pd. directed.

9 10

Here, too, no trace of the formation of any minor water additions to the methanol could be found By-products from DMT are detected. Significantly inhibit methanol decomposition without the

TASME turnover is noticeably influenced. No CO2 was produced here either. In addition, when part of the

Example 3. j ^ SME in the reactor insert in the form of Dimethylace-

The only undesirable side reaction of the Reinitals occurred, this more rapidly in the presence of water gung u'fs raw DMT from TASME, a much more extensive one was implemented. Table 3 shows this minor decomposition relationships that increase with temperature. Not specified reaction of the methanol promoting the reaction (see conditions again correspond to those of 11.il. Example 4) / 11 CO + 2II · determined. Hs became m example !.

Decomposition of methanol / u CO * II; and limsat / from TA.SMK-Dimethylatelal at the
Implementation of a TASMI7I) MT mixture on a Pil (OI "") silica gel catalyst in
Depending on the water content of the methanol used

Reaction temperature (()
.12 (1 (2S (l (

HiO in the methanol Kieu.- "..)
Il IO O

2.XO 2. GX 2.2fi 2.12 0. (K) 0.00 0, ¹ M. 0.X2 0.07 o.ox 0.07 0. (W 0. (K) 0.00 0.05 0.002 ) 7.5 97 9fi.5 97

Reactor insert /
IASMi: (Ciew.- "") TASMM-HCCtIiI Kiev.- ¹¹ ;,) product

IASMf. ((ic «.- ¹ :,) lASMH-ncetal ((iew .- 'ί, ι

TASM Ii-I imsat / (MoI- ¹ O vom liinsat / he /, on aldehyde + acetal

Methanol-Zcrset / ung / u CO Hl. UO 0.4fi 0.2X 0.10

(CiCW ^{. - 1.} From I insat /)

Example 4 j _replacing methanol insert by water (18 g / h

In this example, the reaction temperatures were .r m methanol + 2.0 g / h water). The rest of the

UMV! UIIU Ulli.lüi /.- I A. 31VI I-'IXW Il / .U 11 ί I UMWII VcIMlCIt. 1.111 "IU 111 ί UIII \ U ü Γ. I I Wl 1.1UUU I Mg U 11 g UIIU 11 i."> | JI U V. 11 V. II UUIIl-II V (JII

speaking of the results? of Example 3, 10% of Example I. The results obtained are shown in Table 4.

Table 4

Umsel / ιιημ a LASMIi / DMT mixture in the presence of methanol / water at different reaction temperatures and different Kinsiitz-TASMIi con / enlrations

l'd-Kim / enlralionen Kiev .- "/ ..)

(I. IO (1.25

Rciiktionstempcralur ((»

165 **) 240 2X0 320 .150 -> (X)

TASMH / con / entration *)

in the reactor single salt (Cicw .-%) 2.42 2.58 2.65 2, fiX 2.47 2.78 0.52 0.20

in the product (ppm) 521 428 (, 8I 787 1770 2fi5 7 (. 23

TASME sales (% of Hiring rate) 98 98.5 97.5 97 93 99 98.5 99

CMjOH decomposition to CO + 2II, 0, (X) 0.004 0.12 0.4fi 2.15 0.42 0.44 0.51
(% of stake)

*) Based on the IIMT / TASMH mixture.

") To reduce the dew point, the Ciuxsinmj after the DMT-Salliger 25 l / h iN \ ivorerhii / ii / ugcscl / i.

The TASME use concentration has shown that with a TASME use concentration of

area of interest no significant influence 0.20 percent by weight under the selected reaction

on TASME sales. The last column of the conditions table results in a product whose TASME-Gc-

Il

stop already lies within the usual tolerance for fiber-pure DMT.

By extending the contact time or z. By increasing the Pd concentration on the catalyst In all cases, the TASME final concentration can in principle be well known to any person skilled in the art Connections can always be further reduced to the desired value.

Example * 5

In this example, the role of methanol should be for the implementation will be checked. This

Replaced methanol in a staggered manner by an amount of ar nitrogen equivalent to the gas volume; in the end the nitrogen was replaced with an equal amount of other carrier gases. (Choosing helium resulted in this a helium flow of 25.3 l / h at 25 ° C, where 15.3 l / h instead of methanol by 10.0 l / h instead of Nitrogen were used.)

The quantities used corresponded to those of Example I. Table 5 shows the influence of the carrier gas selected in each case on the TASM EI Itnsiitz in the DMT purification on a Pd-Kalysalor with 0.1O ¹ Vl) Ptl at 280 C.

Table 5

Influence of various methanol con / entral ions and carrier gases aiii the TASME conversion in the environment a TASME / DMI mixture on a Pci (0.1 7,.) - Silieagel-Kalalysator at 280 C and the same contact time

Share in gas.
Reaction mixture

Methanol (VoI- ",,)
Triiger gas (VoI - "/ ,,)
Raw DMT (VoI- ¹ V !,)

TASME sales /
("'" From the mission)

94 (N,)
5

91 (N,)
5

85

IO

85 (N,)
s

91.5

The values show that the TASME rate is strongly influenced by depends on the methanol concentration. Already I vol .-% methanol in the feed / .gasgcmisch increases the Dismantling speed compared to the use of pure nitrogen or helium as the inert carrier gas is considerable. An increase in the methanol concentration initially results in a further significant increase in sales, which ultimately results flattens.

If inert carrier gases are selected, cleaning is based exclusively on decarbonylation. Only at Hydrogen as a carrier gas takes place in addition to the decarbonylation to a lesser extent also the llydriei iiiig of TA.SiviR / U TSfvif · .: under the rccaktionsbedingiingcn 8% of the TASME used and 9.5% of the implemented TASME reacted in this direction. Has been after working with hydrogen as the sole carrier gas for a longer period of time on the methanol (+ water / nitrogen mixture switched, so after some time were again those for the presence of methanol typical high TASME sales valuec obtained. The low conversion with hydrogen as a carrier gas is thus specific for hydrogen and is not based on irreversible damage to the catalyst.

58

37 (N;)

97.5

95

98

95 (N,)

SI

95 (He)

5
83

Example b

A crude DMT was obtained by quantitative esterification of crude terephthalic acid obtained industrially and under conditions according to Example I for DMT purification according to the invention described used. For the starting material or for the converted product, the gas chromatography was used the following composition is determined:

(CiCW .- "/.)
(Ciew.- "!,)
(CiCW.- ".,) Reagent feed / product IJVM I. (I T) ι "> n rSME
IASME
I) MT 0.33
1.61
97.84 0.33
0.05
98.42

100.00

100.00

After that, TASME sales were 97%, the proportion the decarbonylation 76 mol% and the proportion of the dehydrogenative esterification 24 mol% of the converted TASME. Like DMT, p-Toluic acid ester (TSME) was not attacked.

Claims (3)

Patent claims: 1. A process for the purification of dimethyl terephthalate by reducing the content of terephthalaldehyde methyl ester or its dimethylacetals, by conversion to methyl benzoate and dimethyl terephthalate, characterized in that the crude dimethyl terephthalate in the presence of 1 to 95 vol .-% methanol, based on the total Gaseous reaction mixture, at temperatures from 150 to 350 ⁰ C in the gas phase passes over a solid palladium catalyst 2. The method according to claim 1, characterized in that temperatures between 180 and 320 ⁰ C are used. 3. The method according to claim 1 and 2, characterized in that in the presence of Steam works.