DE2127737C3 - Process for the production of monomethyl terephthalate - Google Patents

Description

Mol p-xylene become known up to a p-xylene conversion.

between 15 and 60% occurs as starting materials for polyester production

become present on an industrial scale

b) p-toluic acid to p-toluic acid methyl- 25, for example dimethyl terephthalate and terephthalate is esterified, acid is used, with the terephthalic acid being used

a dihydric alcohol such as B. ethylene glycol,

c) the oxidation of the toluic acid methyl ester with esterified or dimethyl terephthalate with such a temperature between 140 and 170 ° C, alcohol is transesterified. Both the pressure between 1 and 3 bar and with 30 dimethyl terephthalate and the terephthalic acid cobalt as an oxidation catalyst must be in an extremely pure form. The representation concentration between 10 ^s and 10 " ³ grams of terephthalic acid in the extremely high purity required for polyester production atom cobalt / mol p-toluic acid methyl ester, in particular up to a p-toluic acid methyl ester conversion with regard to color impurities, prepares between 20 and 45 % is led, 35 however, major technical difficulties.

Dimethyl terephthalate, which is lighter in its pure form

d) and which can be obtained by oxidation of p-Toluylsäure- than terephthalic acid has, as methylesters present reaction mixture, but Polyeste ^r precursor the disadvantage that in which the monomethyl terephthalate is crystallized by formation of the polyester monomers two methanol-cooling for, first with 40 ester groups based on a polyvalent methanol must be transesterified at a leaching temperature of alcohol, with a significant maximum of 30 ° C or chloroform with a borrowed amount of methanol as an undesirable secondary leaching temperature of at most 30 to product. Especially in the event that the 40 ° C is washed out, then the polyester starting material for the polymerization of a crystalline monomethyl terephthalate is delivered to another location, dietary fiber is in separation from the solution at elevated temperature the intermediate product for reasons of transport costs in temperature one of the solvents methanol, unfavorable. It is particularly disadvantageous if this acetone, ethyl acetate or acetoacetic ester dissolved dietary fiber becomes valuable, still usable constituents, then the dissolved Monomethyltere- have such. B. the methyl alcohol obtained in the transesterification of phthalate to remove impurities 50 dimethyl terephthalate, which is then passed through a solid adsorbent activated carbon during the monomer formation and the pure monomethyl or for further processing are transported away terephthalate - after Must crystallize.

coming from the adsorption The invention is based on the object of pure

Solution through cooling - on mechanical 55 monomethyl terephthalate economically in large-scale Paths are separated. on a large scale in the highest possible yield

to win over the starting material p-xylene. In particular, the object of the invention is also to see monomethyl terephthalate as a pre-

60 products for the production of polyester.

It is part of the task according to the invention to use monomethyl terephthalate for such a bearing transportable polyester intermediate product to be

The invention relates to a method for NEN that in the monomer formation as possible production of high-purity monomethyl terephthalate 65, small amounts of by-products occur, with ί p-xylene, especially as a starting product for the by-product water that occurs in addition to methanol

Conversion to suitable monomers for is.
lyester education. The oxidation of p-xylene with air or a

J>

A nitrogen-oxygen mixture to form p-toluic acid is known per se as an intermediate step in the production of terephthalic acid or its alkyl esters. The oxidation is mostly carried out in the presence of dissolved salts of polyvalent heavy metals as a catalyst in bubble column reactors at elevated pressure and elevated temperature. The Reakt ^; on is known to proceed via various intermediate products, from which p-toluic aldehyde and p-toluic alcohol can be isolated as stable intermediate stages, to p-toluic acid. In a subsequent reaction, p-toluic acid and p-toluyl alcohol are converted into p-toluic acid-p-toluyl ester; furthermore, oxidation of the remaining free methyl group of p-toluic acid usually forms not inconsiderable amounts of terephthalaldehyde and terephthalic acid. In addition, polynuclear aromatic oxidation products often occur.

These known oxidation reactions, however, do not lead to the solution of those stated according to the specification Task, starting from p-xylene, pure monomethyl terephthalate to produce in the highest possible yield. Because for this purpose it is necessary that most of the p-xylene reacted in the oxidation reacts to p-toluic acid. According to the However, the experience gained so far from oxidation reactions did not appear to be possible to guide the reaction in such a direction. It had previously been assumed that it wasn't possible, in the oxidation of p-xylene, the formation of p-toluic acid-p-toluyl ester and terephthalic acid as well as polynuclear aromatic oxidation products as secondary products of p-toluic acid largely suppressed.

The reaction conditions specified according to the invention but surprisingly lead to the opposite, not previously expected result. this can be explained in retrospect among other things by the fact that in addition to that as an intermediate product on the way p-toluic aldehyde formed from p-xylene to p-toluic acid also the p-toluyl alcohol, which is also formed as an intermediate, contrary to previous assumptions reacted further to p-toluic acid under certain conditions revealed by the invention. Furthermore, it was not to be foreseen that when the oxidation was carried out under the conditions according to the invention The reaction between p-toluic alcohol and p-toluic acid to the conditions is possible undesired secondary product p-toluic acid p-toluyl ester largely suppressed.

This object is achieved with the invention. It consists essentially in the fact that in one Process for the preparation of monoesters of aromatic dicarboxylic acids by oxidation of dialkylated ones aromatic compounds with oxygen or oxygen-containing gases in the liquid phase afterwards Esterification of the carboxyl groups formed and further oxidation of the monoesters formed to the Monoesters of aromatic dicarboxylic acids

60

a) the oxidation of p-xylene at temperatures between 90 and 130 ° C, pressures between 3 and 15 bar and an oxygen partial pressure of less than 3 bar and cobalt as an oxidation catalyst in a concentration between 10 ~ ⁶ and ΙΟ " ² gram atom cobalt / mol p-xylene takes place up to a p-xylene conversion between 15 and 60%,

b) p-toluic acid to p-toluic acid methyl ester is esterified,

c) the oxidation of the toluic acid methyl ester at temperatures between 14G and 170 ⁰ C, pressures between 1 and 3 bar and with cobalt as oxidation catalyst in a concentration between 10 ~ ^s and 10 " ³ gram atom cobalt / mole p-toluic acid methyl ester to a p-toluic acid methyl ester -The turnover is between 20 and 45%,

d, in which the monomethyl terephthalate is crystallized out by cooling), and the present by oxidation of the p-Toluylsäuremethylesters reaction mixture is first washed with methanol at a Auswaschtemperatur of at most 30 ° C or chloroform at a Auswaschternperatur of at most 30 to 4O ⁰ C, then the crystalline monomethyl terephthalate is dissolved after separation from the solution at elevated temperature in one of the solvents methanol, acetone, ethyl acetate or acetoacetic ester, then the dissolved monomethyl terephthalate is passed over a solid adsorbent activated carbon to remove impurities and the pure monomethyl terephthalate - after crystallization from the of the solution coming from the adsorption by cooling - is separated by mechanical means.

In particular, the invention provides this method for obtaining monomethyl terephthalate to lead that the p-xylene used in a yield of over 90% to monomethyl terephthalate is implemented.

The inventive oxidation of p-xylene, for example with air or another oxygen-containing Gas to p-toluic acid is converted into a bubble acid reactor at temperatures between 90 and 140 ° C, particularly advantageously between 90 and 130 ° C, and pressures between 1 and 15 bar, especially advantageously between 3 and 15 bar, and an oxygen partial pressure of less than 3 bar as well with an oxidation catalyst in the form of p-xylene-soluble salts of a polyvalent heavy metal and with a p-xylene conversion between 15 and 60% and recycling of the unreacted p-xylene and the intermediate products obtained in addition to p-toluic acid carried out in the oxidation reactor.

The safe procedure in the intended direction results from the fact that in the invention The formation of p-toluic acid-p-toluyl ester by increasing pressure and temperature ranges Pressure and falling temperature is reduced, since the solubility of the water of reaction formed increases in the reaction mixture and thus the position of the chemical equilibrium in favor of p-toluic acid and the p-toluyl alcohol is shifted. At lower temperatures, the secondary products formed by the oxidation of p-toluic acid are also formed, such as the already mentioned compounds terephthalaldehydic acid and terephthalic acid, only to a lesser extent; however, their formation increases with increasing oxygen partial pressure. Regarding the concentration of the catalyst, which can also be manganese, for example, it should be noted that lower catalyst concentrations otherwise

the same reaction conditions to a lower dation of the p-xylene obtained with p-toluic acid

Formation of the undesired secondary products tere-methanol to p-toluic acid methyl ester is known,

phthalic acid and terephthalaldehyde acid. It is known, for example, the esterification

The oxidation of p-xylene to p-toluic acid can take place in the liquid phase below the critical temperature

under the inventive conditions diskpn- 5 of the methanol, which is located at 240 ⁰ C, in pressure vessels

to be carried out continuously and preferably continuously. With this procedure, however

be guided. In the continuous process - no complete conversion of p-toluic acid is achieved,

the reaction is wise in the specified since the chemical equilibrium is in the range of

Reaction areas to be designed in such a way that practically realizable reaction conditions are not

that the intermediate products as largely as possible completely on the part of the p-toluic acid methyl ester

react to p-toluic acid and the formation of lies. Another known practice exists

Secondary products is still low. in that the esterification above the critical temperature

In the case of discontinuous operation, the reaction of the methanol must be carried out or, according to the invention, at the point at which maximum achievable pressure and temperature combinations such that selectivity, ie moles of p-toluic acid formed per 1% of methanol, is in vapor form in any case. This vapor-converted mole of p-xylene must be terminated; the shaped methanol is carried out in a suitable reactor, so oxidation is only carried out to an incomplete p-xylene conversion due to the solid or molten p-toluic acid. This point corresponds to the minimum of the sum of the selectivities of secondary esters passed through their solution or suspension. The latter method has the products. The p-toluic acid is by a suitable 20 advantage that the water of reaction formed by the thermal or mechanical separation processes of vaporous methanol is stripped 'and zutrennen daden remaining components in the reaction mixture to completely by chemical equilibrium. Unreacted p-xylene and the intermediate in favor of the desired ester are shifted products p-toluene aldehyde and p-toluene alcohol, which can.

can be further oxidized to p-toluic acid, 25 In principle, it is in the preparation according to the invention according to the invention again possible for the oxidation in the position of monomethyl terephthalate, the employed a following approach. Esterification of p-toluic acid to p-toluic acid

In the case of continuous operation, it is the methyl ester of the invention after the second of the two listed

according to further to carry out the proportion of intermediate and known methods, whereby the ge

By-products in the reaction mixture, the reac- 30 melted p-toluic acid in a pressure column

gate leaves to keep as low as possible, required. in countercurrent to vaporous methanol from

A residence time that is as uniform as possible in the reactor. flows up and down and the ester formed

d. H. a narrow dwell time spectrum - as shown below.

for example, generally by cascade-like arrangement In a particularly advantageous manner, the

tion of several ideally mixed, continuous esterification of p-toluic acid with methanol, but in

flow through reaction vessels can be obtained the following combination of per se, such as

- the running through the reactor continuously flowing through, each known process steps take place

Realize reaction mixture. gen, through

Both fresh p-xylene are fed into the reactor

as well as 40 a) separated from the reaction mixture, incomplete esterification of p-toluic acid and recycled p-xylene and p-toluyl alcohol and, after dissolving in liquid methanol in the autoclave, p-toluic aldehyde. The narrow residence time b) initiation of the reaction spectrum obtained according to a) can according to the invention by a multi-mixture - consisting of p-toluic acid methyl-stage bubble column reactor, in which the mixing ester, methanol, unconverted p-toluic acid of the liquid reactor contents along the reactor 45 and reaction water - in a downstream pressure reactor, that is, in its flow direction, as far as possible ended pressure reactor and there is more completely suppressed, or by several one-esterification by leading in countercurrent flow of stage bubble column reactors, which are connected as a cascade up and down to gaseous methanol, one behind the other, so that the liquid reaction mixture, the individual reactors in succession 50, the first incomplete esterification should vordurchströmt without it comes to a back-mixing preferably in a stirred autoclave, or a cascade between the individual reactors, verwirk- of stirred autoclave and the subsequent light we rden. The mean residence time of the liquid complete esterification with gaseous methanol phase in the reaction system for the oxidation is carried out in a pressure column, the conversion being selected so that the conversion 55 can be carried out continuously at the reactor outlet, the maximum selectivity that can be achieved is discontinuous which is present in the incomplete borrowed mode of operation. After the esterification has been separated off, unreacted methanol before p-toluic acid is removed, unconverted p-xylene and the subsequent complete conversion of the intermediate products p-toluyl alcohol and p-toluyl esterification are separated off.

aldehyde is continuously returned to the oxidation

Detailed descriptions of a particularly favorable results discontinuously to conditions of a continuously guided Oxidationspro- when setting the subsequent reaction and · zesses are in the embodiments 1 and 2 _a) temperatures from 150 to 230 ⁰ C and

Dfefa of the sequence according to the invention of the process * ^{b) in a D} ™ * range of 15 and 60 bar ashes.

rens steps for the production of monomethyltere- The maximum achievable yield of p-toluyl-

The esterification given by the phthalate from the oxy acid methyl ester is determined by the experimental

7 8

Temperature dependence of chemical equations that p-toluic acid in considerable amounts in

weight constant K given: vaporous methanol is discharged from the column and is not esterified, as is the case with F.insntz

Ig κ - u 3.67 due to a fine melt of p-toluic acid

[7 * ° K] 5 theirs at the esterification temperatures already relative

• f high vapor pressure would be the case.

Next is the one required according to the invention

_K __ (p-toluic acid methyl ester) (water) Oxidation of p-toluic acid methyl ester to mono-

Consider (p-toluic acid) · (methanol) 'methyl terephthalate. As part of the Gc-

lo samerfindung is to solve the subtask that

where the brackets indicate the concentration of the oxidation reaction under such conditions

individual components per unit volume show that monomethyl terephthalate in as possible

put. large and constant yield is obtained.

The higher temperatures are to be preferred. In particular, care must be taken to ensure that the formation

because of the weak endothermicity of the 15 by-products and by-products as much as possible

Reaction - the enthalpy of reaction is +7.3 kcal, is suppressed.

Mol - according to the equation for equilibrium with those known from your prior art

constant the location of the chemical equilibrium procedure, according to which it is known. that with the Oxi

with increasing temperature in favor of the formation of dation of p-toluic acid methyl ester using

methyl p-toluate shifts. 20 preparation of a soluble salt that acts as a catalyst

In order to actually use a polyvalent heavy metal monomethyltcrct in the or the autoclave optimal yield of p-toluic acid methyl phthalate is formed, the reester according to the invention can be should not solve the reaction in any way. With the previously known Temperatures and pressures in the range between oxidation processes are the pressure and Tempc-200 and 23O ° C or about 40 to 60 bar maintained 25 temperature conditions in such a range that in will. to a considerable extent a thermal conversion of the

In the complete esterification of p-toluic acid monomethyl terephthalate to dimethyl terephthalate

the temperature in the pressure column is convenient and terephthalic acid takes place. Likewise form

moderately in a range between 220 and thereby regularly larger amounts of for the crfin-

260 ° C, while the pressure is adjusted so that 30 proper process sequence undesirable Nc-

in each case according to the benproducts selected in the individual case, such as, for example, p-to-

Temperature methanol mostly in vapor form preluic acid. for the monomethyl terephthalate end

located. product, especially if it is directly

In the esterification with vaporous methanol, raw materials for the monomers of the polyester are used

the resulting water of reaction is driven off and is said to be extremely harmful. Furthermore arise with the

so that the chemical equilibrium is completely due to the oxidation reactions carried out so far as well

in favor of the formation of p-toluic acid methyl ester, those compounds which even in traces the Rcak-

postponed. strongly discolouration product and presumably indicate

This esterification process offers opposite and / or fluorenone structure. The colored ones

known methods have considerable advantages: * o Contamination, which is particularly

Most of the implementation can take place in or processing the monomethyl terephthalate to poly

several consecutive, continuous esters are harmful, can be caused by the usual

auto cleaning processes through which a liquid reaction mixture flows, such as washing out and / or

clave as a pure liquid phase reaction with higher crystallize not removed to a sufficient extent

Speed, d. H. in less time than the corresponding 45. Sublimation or distillation is also considered

Appropriate steam (methanol) liquid [to exclude molten cleaning processes, since the for this

or dissolved (p-toluic acid) reaction], which in a required temperature already has a considerable

Pressure column would be carried out. thermal conversion of the monomethyl terephthalate

whereby, per time and reactor volume, larger mellates to dimethyl terephthalate and terephthalic acid

genes of p-toluic acid can be esterified 50 takes place, as shown in the table below

For the termination of the esterification until it becomes full, constant conversion of p-toluic acid is because of

for this only a short response time a comparison - Table 1

wise to the possible implementation of the total thermal conversion of monomethylterephtha

esterification in the pressure column only a small 55 lat; Starting substance: monomethyl terephthalate Reactor volume required in the downstream pressure column. That in the invention Combination of autoclave and pressure column resulting total reactor volume is at the given Performance less than when performing the overall 60 esterification in a thin column. - When alone The use of one or more autoclaves arranged like a cascade can achieve a complete conversion of the p-toluic acid without technically complex Removal of the water of reaction because of location 65 not to be educated in chemical equilibrium. This applies to both continuous and discontinuous modes of operation. - Furthermore,

T t MMT DMT TPS ( ⁰ Q (H) (Mole percentage) (Mole percent) (Mole percent) 180 5 92.6 3.7 3.7 200 5 91.1 4.4 4.4 225 2 90.4 4.8 4.8 225 8th 72.3 13.8 13.8 240 1 93.8 3.1 3.1 240 2 87.0 6.5 6.5 240 4th 80.6 9.7 9.7 240 8th 70.1 14.9 14.9 609 684/15

For the reasons mentioned, the oxidation reactions are suitable in those previously known Molds not for the production of pure monomethyl terephthalate in high according to the invention Yield.

However, as has surprisingly been shown, the aim according to the invention can be achieved when the oxidation is carried out in compliance with the reaction conditions below, namely at temperatures between 130 and 200 ^° C., particularly advantageously between 140 and 170 ^° C., and pressures between 1 and 6 bar, particularly advantageously between 1 and 3 bar, and the presence of a salt of a polyvalent heavy metal soluble in p-toluic acid methyl ester as an oxidation catalyst, such as manganese or cobalt, particularly advantageously with cobalt as an oxidation catalyst in the form of a salt soluble in p-toluic acid methyl ester in a concentration between about 10 ^ ⁵ and 10 ~ ³ gram atom of cobalt'mol p-toluic acid methyl ester and a conversion of p-toluic acid methyl ester to monomethyl terephthalate of about 20 to 45%, whereby unconverted p-toluic acid methyl ester and intermediate products, which arise in the oxidation of p-toluic acid methyl ester to monomethyl terephthalate , e.g. can be used for re-oxidation.

The oxidation process can be carried out so that of the converted p-toluic acid methyl ester at least 90% reacts to monomethyl terephthalate

The oxidation of the p-toluic acid methyl ester to be carried out under the conditions according to the invention to monomethyl terephthalate can be carried out continuously in a bubble column reactor.

However, care must be taken to ensure that the mixing of the reaction mixture flowing through the reactor along their direction of flow, d. H. from adding to removing, to achieve a uniform Residence time is kept as short as possible.

A cascade of bubble column reactors can also be used to achieve a uniform residence time will.

Has to carry out the oxidation under the inventively stated reaction conditions, with the result that the emerging as an important intermediate Terephthalaldehydsäuremethylester almost completely τ \ monomethyl terephthalate is reacted and that the formation of secondary products such as dimethyl terephthalate and terephthalic acid, the thermal by the above conversion from mono-methyl terephthalate (see. Table 1) are formed, and p-toluic acid, which arises as a decomposition product and is pushed back to extremely low proportions by high-boiling or summing polynuclear aromatic compounds.

The inventive oxidation of p-toluic acid methyl ester to monomethyl terephthalate is im individually described in the exemplary embodiments 3 and 4.

The monomethyl terephthalate obtained by oxidation from p-toluic acid methyl ester must then still be subjected to cleaning. The cleaning can e.g. B. in a known manner Wash out with a solvent. It would also be possible to use monomethyl terephthalate ; as well as that obtained from p-xylene via intermediate stages Recrystallize several times dimethyl terephthalate, which is to serve as a starting material for polyester and then rectify. However, all these known measures lead to monomethyl terephthalate, in particular with regard to the purity requirements for further processing into polyester, not to a satisfactory result.

To completely purify the monomethyl terephthalate, it is proposed according to the invention that

a) that in the oxidation of p-toluic acid methyl ester resulting reaction mixture which has cooled to such an extent that monomethyl terephthalate predominates is crystalline, washed out with a solvent, its dissolving power for monomethyl terephthalate is as low as possible, while it is p-toluic acid methyl ester as well as the completely dissolves other products,

b) separating the crystalline monomethyl terephthalate from the solution and

c) then at elevated temperature in a solvent with a strong temperature dependence to dissolve for the solubility of the monomethyl terethalate and then

d) to remove any impurities that are still present, in particular color impurities,

to pass over a solid adsorbent and

e) the solution coming from the adsorption for the crystalline separation of the pure monomethyl terephthalate to cool and the pure monomethyl terephthalate by a mechanical separating

process to separate from the remaining solution.

Solvents with low dissolving power for monomethyl terephthalate are suitable for washing out for example methanol or chloroform, their solubility for monomethyl terephthalate at low temperatures of up to 20 ° C is shown in Table 2 below.

Table 2

Solubility of monomethyl terephthalate

Solubility of monomethyl terephthalate (g) in 100 g of methanol 100 g of chloroform

0 1.08 0.19 10 1.25 0.22 20th 1.87 0.27

The temperature for washing out is preferably 0 ° C. when using methanol or lower, if chloroform is used, preferably at 10 ° C or lower.

The reaction mixture coming from the oxidizer, which is essentially made up of Composed of monomethyl terephthalate, methyl p-toluate and methyl terephthalaldehyde, can be worked up according to the invention in two different ways.

Wegl

The reaction mixture is either in crystallized form or, preferably, as a suspension of already crystallized monomethyl terephthalate in a melt of p-toluic acid methyl ester, the has still dissolved proportions of monomethyl terephthalate and of methyl terephthalaldehyde ester, with mixed with a suitable solvent so that

all components except for monomethyl terephthalate, that remains suspended.

Way 2

The monomethyl terephthalate is first enriched by cooling the reaction mixture so far is that the monomethyl terephthalate in a remaining, mainly from p-Toluylsäuremethylestcr existing melt as much as possible crystallized out. Melt and crystals are through a mechanical separation process separated from each other. The crystals are in turn with a suitable solvent, as described in route 1, washed out.

The suspension obtained on the path 1 or 2 is broken down by a mechanical solid-liquid separation, such as centrifuging, into crystalline monomethyl terephthalate and liquid. When using the enrichment of the monomethyl terephthalate according to route 2, the washing can be coupled with the mechanical separation of crystals and melt carried out there, so that the suspension of the crystals and the subsequent mechanical solid-liquid separation are omitted.

The solution obtained during washing is used by distillation in solvent on the one hand and p-toluic acid methyl ester and methyl terephthalaldehyde on the other hand, which are used again for the oxidation be separated; any residual amount of undesired by-products that is present must be discharged will. The solvent can be used again for washing out.

The monomethyl terephthalate obtained after washing still contains traces of colored accompanying substances. These are removed by the adsorption provided according to the invention. The monomethyl terephthalate is dissolved in a solvent, which shows a strong temperature dependence of the solubility for monomethyl terephthalate, at an elevated temperature. As the solvent are, for example methanol, acetone, ethyl acetate and ethyl acetoacetate, but preferably methanol is used, which is already used elsewhere in the process. When using methanol, the dissolving temperature should preferably be between 60 and 80 ° C .; Significantly higher temperatures are to be avoided, since a noticeable esterification of the monomethyl terephthalate to dimethyl terephthalate begins here. The resulting solution of monomethyl terephthalate is treated with a solid adsorbent such as activated carbon, fuller's earth or diatomaceous earth to remove the colored impurities. This is expediently done in a tower filled with the solid adsorbent, through which the solution flows at a temperature above its saturation point for monomethyl terephthalate in good contact with the adsorbent. By cooling the solution after the end of the adsorption, the monomethyl terephthalate is then precipitated out of the solution in crystalline form and separated from the liquid by a mechanical separation process. The monomeric methyl terephthalate obtained in this way, freed from solvent after drying, is white and has an acid number of 311.5 ± 1 and an esterification number of 311.5 ± 1; the theoretical values are each 311.4.

That when the pure mono crystallizes out

Methyl terephthalate accumulating solvents can be used again to dissolve monomethyl terephthalate the adsorption can be used. The adsorbent is according to the invention when its absorption capacity for the colored impurities is exhausted by driving off these impurities with overheated Water vapor or methanol vapor is regenerated.

The cleaning according to the invention is on hand

of the embodiment 5 explained in more detail.

ίο The monomethyl terephthalate obtained according to the invention is to be used in particular as a starting product for the production of suitable monomers for polyester formation.
The pure monomethyl terephthalate obtained according to the invention can also be used as a preliminary or intermediate product in the production of pure dimethyl terephthalate by esterifying it with methanol to give dimethyl terephthalate. Furthermore, it can be purely from the according to the invention

»Ο representable monomethyl terephthalate by hydrolysis make pure terephthalic acid.

The process sequence according to the invention for the production of pure monomethyl terephthalate from p-xylene is explained below using a flow diagram shown in the drawing.

p-xylene α and air or another oxygen-containing gas b flow continuously into the oxidation device 1, where they react with one another. The reaction mixture k formed there, which in addition to p-toluic acid and p-xylene also contains p-toluic aldehyde, p-toluic alcohol and other by-products, goes into device 2, in which k is separated mechanically and / or thermally. The stream e, consisting of p-xylene, p-toluene alcohol and p-toluic aldehyde and not separated p-toluic acid is returned to 1; p-Toluic acid / enters the esterification device 3. From the exhaust gas d leaving the oxidation device 1, p-xylene α and, if necessary, water of reaction c are separated off in the separating device 16; p-xylene a is returned to 1.

In the esterification device 3, the p-toluic acid is converted into p-toluic acid methyl ester with steadily flowing methanol g; the liquid reaction mixture j formed in this way is passed into the separating device 4, where thermal separation of j into p-toluic acid methyl ester h, by-products m to be discharged and, if necessary, unreacted p-toluic acid / takes place. Unreacted p-toluic acid / is returned to the esterification device 3; the p-toluic acid methyl ester A enters the oxidation device 5 continuously. In the separating device 18, p-toluic acid methyl ester A is separated off from the methanolic evaporation of the esterification device 3 and passed to 4. In the device 19, water of reaction c. Methanol g and any p-xylene c present separated from one another; α is returned to 1 and g to 3; the water of reaction c goes off together with any inert gas present.

For the oxidation of the p-toluic acid methyl ester A flowing into the oxidation device f, air or another oxygen-containing gas b likewise continuously flows into 5. The out of 5 out The reaction mixture which occurs and which is composed mainly of unreacted methyl p-toluate monomethyl terephthalate and methyl terephthalaldehyde ester is used in the separating

13 '14

device 6, after having previously fed most of the. The monomethyl terephthalate used as the oxidizing agent is crystallized out, mechanically preheated air to the reaction temperature, which separates the niche; the separated melt o, which flowed continuously through the reactor, was divided into small gas bubbles by mainly composed of p-toluic acid methyl ester next to a sieve plate. The gas mixture leaving the reactor 5, which is composed of the methyl terephthalate / aldehyde, was returned to the oxidation device 5 before it was used. Relaxation to normal pressure for separation The crystallizate, which consists essentially of monomethyl from the p-xylene carried along and the resulting reac- terephthalate ρ , is passed into the wash-out water through a condenser; Austenung 7. - P-xylene condensed by bypassing the separating device 6 was returned to the reactor, but the reaction mixture η can also be passed directly, while the separated water is passed out of the 7. - That the oxidation pre-reaction system has been removed. For the separation of direction 5 leaving exhaust gas d is in the separation of residual amounts of p-xylene, the exhaust gas was through device 17 of p-toluic acid methyl ester Λ of the two cold traps, the temperature of which is in each case O ⁰ C returned to 5, and possibly reac, and an adsorption water c filled with activated carbon. 15 application spectrum.

The washing out of the reaction product from the In the reactor described were 2000 g of oxidation device 5 in the device 7 p-xylene, the 200 mg cobalt in Foi m from in p-xylene with the solvent q. - The reaction product of soluble cobalt salts of the C ₆ to C ₁₀ fatty acids can, under certain circumstances, also be used in the form of a mixture. The p-xylene was washed out from separator 6; first heated in this 20 in Srickstoffstrcm to 120 ⁰ C. In this case, devices 7 and 8 have been omitted. Thereafter, the Mickstoffstrom was replaced by an air-containing suspension r of crystalline monomethyl stream of 1145 l / h; the temperature was terephlhalat in the resulting solution is fed into the during the entire oxidation 120 ± 1 ° C and mechanical separation device 8, where a pressure of 8 bar in the reactor.

Separation into crystalline monomethyl terephthalate ρ 25 After a corresponding preliminary test

and the solution s, which had essentially been made p-toluic acid, that with a conversion of the

methyl ester and methyl terephthalaldehyde ester p-xylene of greater than about 32% among those mentioned

contains, takes place. The solution s is in the thermal conditions after a reaction time of about

Separating device 15 worked up; the back 120 min already larger amounts of the undesired

recovered solvent α is again after 7 30 secondary products terephthalaldehyde acid and tere-

out, the resulting mixture «formed from p-toluic acid phthalic acid, the oxidation was after 120 min

methyl ester and the methyl terephthalaldehyde terminated. The reaction mixture was as follows

ester is again in the oxidizer 5 Composition:

used, while a possibly remaining reverse component mol percent

stand ν is discharged. The crystalline mono- 35 n-xylene 12 89

methyl terephthalate ρ of 8 is used in the solution- n-toluyl alcohol .. 043

direction 9 dissolved in the solvent t . The solution of ρ n-toluene aldehyde ............. 6.92

in / is either in the adsorption device 10α ρ toluic acid 4 43

or lOb given, in which the colored impurities plToluic acid-p-Toluyiester ''.'.'.'.'.'.'.'. θ ', Ο2

be separated. After the Adsorp- 40 terephthalaldehydic acid 0.01

tion agent by absorbing a certain amount of terephthalic acid 0 05
When the colored impurities are exhausted, there will be other by-products ''.'.'.'.'.'.'.'.'.'.'. 6.12
regenerated by driving off the adsorbed substances with superheated steam w . It follows from this that 74% of the reacted from the adsorption device 10a or 106 exiting., 5 p-xylene react to p-toluic acid; The solution is taken into account in the cooling device 11 until it is possible for the crystalline separation of p-toluene alcohol and p-toluene aldehyde present in the reaction mixture to be cooled again in a monomethyl terephthalate; the crystalline approach used for the oxidation to p-toluic acid monomethyl terephthalate will be able to be from the liquid in, then the effective from the mechanical separation device 12 increases and 50 volume of p-toluic acid to 96.6%; the yield of the terephthalaldic device 13 for the removal of the solvent in the drying- the undesired secondary products, from which the end product hydric acid and terephthalic acid amounts to a total of 1%. Monomethyl terephthalate leaks p. The solvent. .
vapors t are condensed in device 14, for example
and with the liquid / from the mechanical 55 The continuous oxidation of p-xylene combined into separating device 12 in the dissolving device 9 p-toluic acid was returned to a V4A made of V4A. vertical tubular bubble column reactor

In the following, the individual inventions with an inner diameter of 10 cm are carried out.

according to process steps Embodiments 1 A heat exchanger was installed in the reactor

to 5 specified. 60 built; Heat could also be dissipated through the pipe wall.

. -I ₁ i ^{) zrw} - are supplied. The oxidizing agent

Example 1 turned air in the same manner as in

The discontinuous oxidation of p-xylene described in Example 1, passed into the reactor; as well

p-Toluic acid was collected in a V4A steel, the exhaust gas leaving the reactor was after

saturated, vertical bubble column reactor, des- 65 in Example 1 explained method treated. Through

sen internal diameter was 10 cm. Floor-type internals was achieved that of

A heat exchanger was built into the reactor; continuously flowing into the top of the reactor

Heat could also be dissipated via the pipe wall and the liquid returned above the gas distributor

/ IQ

: n was, along the stream was mixed and with it m for the continuously

the

p-xylene dissolved cobalt salts of the Q- to C, -fat- "SYt Ä ^D

viTlSo
Airflow from

! f ⁷ ^ ¹¹ Pf ^ratur

tionseemisch im

Component weight percent

p-xylene _{89 3}

P ^Tolu y ^Lalk ° hol 2.6

p-Toluylaldehyde ₃ ' ₉

p-toluic acid ₃ ' ₅

Terephthalaldehyde + tere- '

phthalic acid ₀ , l

p-Toluic acid p-toluyl ester 0.4

Other by-products 0 ^ 2

After that '

(Filtrate I) separated. The Knstallisat was with fresh p-xylene, the temperature of which was 15 ° C (Filtrate II). The combined filtrates I and II were quasi-continuously with additional fresh p-xylene, d. H. with short interruptions the one carried out for the discontinuous Separation of crystals and liquid required time were required, with a diaphragm pump in the

n5v ο 1 enKnTrh _a T ^ V joined

p-XyloI corresponded au mo arer basis of the sum of ÄTSSJfJi ^ vT. "" ί ^Tere P ^hthalsaure

trwfph SSi Γ I l \ ' ^die u ^{in the crystallizate} . After the reactor had reached a steady operating state after about 150 min, the composition of the

Reaction mixture, the following average values

^ge

Component weight percent

p-xylene 780

p-Toluyl alcohol 2 ^ 5

p-Toluylaldehyde 5J5

p-Toluic acid \\ '4

p-Toluic acid p-Toluyl ester 1 ^ 2

Terephthalaldehyde o] l

Terephthalic acid 0 ^ 4

Other by-products l '

The output was about 3700 g / h. This resulted in 372 g of p-toluic acid + 15 g of terephthalic acid per Hour separated. About 3315 g / h of reaction mixture, of which the p-toluic acid was separated, recycled and an additional 300 g / h of fresh p-xylene together with small amounts Amounts of catalyst (500 mg cobalt / 1000 g p-xylene) fed.

^During the steady-state operating state, which was maintained over ³⁶ ° ^mm , 93% of the p-xylene set reacts to p-toluic acid and 2.3% to terephthalic acid; the remainder is distributed among various by-products.

B e i s ρ i e 1 3

^{The disco} ^ inuierliche oxidation of p-toluyl- ^ acid methyl ester to monomethyl terephthalate was

'° ™ ^einCm ™ ^{c in the example} * ^ written Rtaktions ^svste m carried out. 1500 g of p-toluic acid methyl ester, the 150 mg of cobalt in the form of p-toluic acid-

^m ^ _h ^{Lester 1Ösliche} "cobalt salts DETC _e - L C were added ₁₀ -Fettsauren, were m the reactor

^d - ^NEXT ™ ^{nitrogen stream} brought ^{to th} e te reaction temperature of 200 ⁰ C. The oxidation was then initiated by switching to an air flow of 420 Nl / h; the pressure ^{in the} reactor was about 1 bar. In a corresponding

In the preliminary test, it was found that with a p-toluic acid methyl ester conversion of greater than about 35 ⁰ ^, which was reached after a reaction time of about 360 min, larger amounts of undesirable high-boiling by-products were formed and the ther-

^{I have} already mixed conversion of monomethyl ^{terephthalate to} dimethyl terephthalate and terephthalic acid

3. ^ p

Component mole percent

methyl p-toluate 6.44

Methyl terephthalaldehyde ester ... 0.18

35 monomethyl terephthalate 3.17

Dimethyl terephthalate 0.01

Terephthalic acid 0.01

Other by-products 0.14

^{From this it} * ^{ies that 89} % of the implemented

40 methyl p-toluate to form monomethyl terephthalate reacted; one takes into account that the ter-

Phthalaldehydsäuremethylestfr can be used in a subsequent oxidation approach, then the effective yield of Monomethykerephtha-45 Iat94 ^{o /}

Example 4

^{In analogy to} the method described in Example 3, 1950 g of methyl p-toluate,

Were added to CJJ-fatty acids, at 160 ⁰ C and 4 bar with air ⁽⁴ ^ 5 Nl / h) oxidized - 50 to 230 mg of cobalt in the form of soluble in p-ToIuylsäuremethylester salts of C. ₆ This experiment was terminated at a p-toluic acid methyl ester conversion of 27% after 55,240 min, since the increased pressure resulted in a noticeable formation of the undesired products (cf. Example 3) despite the low temperature above this conversion.

The reaction mixture thus obtained had the following

60 Composition:

Component mole percent

p-Toluic acid methyl ester. 9 44

Terephthalaldehyde methyl ester .. 0 * 25

6 ₅ monomethyl terephthalate 3 03

Dimethyl terephthalate .. θ'θ4

Terephthalic acid 0 ^ 04

Other by-products 0J9

6096847154

17 18

It follows that 85% of the converted temperature with a flow rate of

p-ToIuylsäuremethylesters to Monomethylterephtha- 1500 ml / h through a granulated activated carbon Iat responded; one takes into account that the Tere-filled adsorption tower is conducted. The dwell time

methyl thalaldehyde ester ίη, a subsequent oxide solution in the adsorption tower was 20 min; after

dation approach can be used again, then 6 the exit from the tower it became a crystalline

is the effective yield of monomethylnene deposition of the monomethyl terephthalate

terephthalate 92.5 %. 0 ⁰ C cooled. The crystals were suction filtered

_R. -ic ^and dried. Thus 257 g became white

Example 5 obtained monomethyl terephthalate, its acid number

1000 g of an amount obtained by air oxidation of p-toluyl- io was 311.5 ± 1, 44 g of monomethyl terephthalate remained

methyl acid ester was dissolved in the filtrate; 10 g monomethyl terephthalate

those for the combined washing and adsorption stay in the adsorption tower or went to the

technology used; lost the composition of these mi separations.

Schung was as follows: The obtained monomethyl terephthalate (I) and

Component percent by weight ¹ S ^the filtrate (A) was checked for possible contents of far-

p-toluate investigated 6012 S ^{bi en} impurities out by a

Terephthalic acid methyl ester. '.' 0.94 ^{solution of} ° ' ² S monomethyl terephthalate (I) in

Monomethyl terephthalate 35.36 ^{20 ml of} dimethylformamide as well as the one obtained

Dimethyl terephthalate 0 30 methanolic filtrate with monochromatic light

Terephthalic acid 0 30 ²⁰ ^ ⁴ ^ ^nm ) and the light absorption

p-Toluic acid 114 was measured. The thickness of the cuvette used

Other by-products f ^betru 93 g · ^mm 4 ° ^AIs comparison substance for the obtained

Monomethyl terephthalate was a multiple after

The solidified reaction mixture was comminuted by adsorption of distilled monomethyl terephthalate (II) and stirred intensively with 800 g of methanol at 0 ° C. for 1 hour to remove dimethyl terephthalate. The undissolved crystalline chloroform was then washed out several times, line monomethyl terephthalate was suction filtered and used; as a comparison solution for the filtrate (A) 200 g of methanol, the temperature of which was 0 ° C., pure methanol was used. The measured carefully rewashed. The mono-transparencies of the solutions obtained in this way, together with the methyl terephthalate, the amount of which was 311 g, had corresponding values for monomethyl terephthalate, an acid number of 312 ± 1.5; by gas chromatography (III), which, according to the described washing-out technique, graphic analysis, no foreign substances were recrystallized once in methanol, but no longer detectable, but the product showed adsorption, and the corresponding pale yellow coloration. In the methanolic solution, the methanolic mother liquor (B) remained together with the other 35 mentioned above:
Products return 44 g of monomethyl terephthalate (for
If chloroform is used, this proportion of sample transparency,%

at a washing ^{temperature of 0 °} C. to about 3 g of monomethyl terephthalate (I) 98 ± 1

humiliate). Monomethyl terephthalate (II) 98 ± 1

Of the mono- 40 monomethyl terephthalate (III) obtained after washing out ... 89 ± 1

methyl terephthalate were used to remove the staining filtrate (A) 100 ± 1

borrowed impurities from adsorption 310 g with mother liquor (B) 77 ± 1

63 ° C dissolved in 4120 g of methanol and in this methanol, 100 ± 1

1 sheet of drawings

Claims (1)

'f An industrially applicable process for the production of pure monomethyl terephthalate is Patent claim ^{not yet} known. It is just ^v ' known, e.g. B. Swiss patent 3 15 580, 5 that monomethyl terephthalate in certain reac- Process for the production of monoesters aroma processes as a subsequent or intermediate product matic dicarboxylic acids by oxidation di- the further processing of oxidation products of the alkylated aromatic compounds with acid p-xylene is obtained. However, there are still no known reactants or oxygen-containing gases in liquid processes that exclusively affect the win phase subsequent esterification of the ion formed from pure monomethyl terephthalate in as much as possible Carboxyl groups and further oxidation of the large yield compared to the use of p-xylene deten monoesters are more aromatic in relation to the monoesters. So far there are none at all Dicarboxylic acids, thereby marked- information on whether and under which Reakn et that tion conditions the conversion of p-xylene in 15 Monomethyl terephthalate with extensive training a) the oxidation of p-xylene at temperatures switching of secondary and secondary products possible between 90 and 130 ⁰ C, pressures between is. 3 and 15 bar and one oxygen partial The production of monomethyl terephthalate in pressure of less than 3 bar and cobalt as an industrial scale as a starting material for the Oxidation catalyst in a concentration of 20 Polyester production for the formation of monomers between 10 " ^s and 10- * gram atom of cobalt / for polyester formation is therefore not yet available