DE2127737B2 - PROCESS FOR THE PRODUCTION OF MONOMETHYL TEREPHTHALATE - Google Patents

Description

Nitrogen-oxygen mixture to p-toluic acid is called Intermediate step in the production of terephthalic acid or its alkyl esters is known per se. the Oxidation is usually carried out in the presence of dissolved salts of polyvalent heavy metals as a catalyst carried out in bubble column reactors at elevated pressure and temperature. The reaction is known to proceed via various intermediates, one of which is p-toluene aldehyde and p-toluene alcohol can be isolated as stable intermediates, to p-toluic acid. In a subsequent reaction, p-toluic acid is formed and p-toluyl alcohol of p-toluic acid p-toluyl ester, furthermore, the remaining free methyl group of p-toluic acid is formed by oxidation mostly not inconsiderable amounts of terephthalic acid and terephthalic acid. Also kick often polynuclear aromatic oxidation products.

However, these known oxidation reactions do not lead to the solution of those presented according to the invention 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 a process for the preparation of monoesters more aromatic Dicarboxylic acids, by oxidation of dialkylated aromatic compounds with oxygen or those containing oxygen Gases in the liquid phase followed by esterification of the carboxyl groups formed and Further oxidation of the monoesters formed to the monoesters of aromatic dicarboxylic acids

a) the oxidation of p-xylene at temperatures between 90 and 130 ⁰ 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 10 ~ ^a gram atom cobalt / mol p-xylene 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 methyl methyl ester at temperatures between 140 and 170 ° C, pressures between 1 and 3 bar and with cobalt as the oxidation catalyst in a concentration between 10 ~ ^s and 10 ~ ³ gram atom of cobalt / mole of methyl p-toluate to a methyl p-toluate -LJmsatz between 20 and 45% ίο is carried out,

d) and the reaction mixture present after oxidation of the p-ToIuylsäuremethylester, in which the monomethyl terephthalate has crystallized by cooling, is first washed with methanol at a leaching temperature of 30 ° C or less or chloroform at a temperature of 30 to 40 ° C at most, then the crystalline monomethyl terephthalate is dissolved after separation of ao 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 * 5 activated carbon to remove impurities and the pure monomethyl terephthalate - after the crystallization from the solution coming from the adsorption by cooling - is separated by mechanical means.
30th

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-xy! ol obtained p-toluic acid mi

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

phthalic acid and terephthalaldehyde acid. So it is known, for example, the Veresteruni

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

under the inventive conditions discontinuously 5 of the methanol, which is located at 240 ⁰ C, in Druckgefäßei

to be carried out continuously and preferably continuously. In this procedure, abe

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

The reaction is wise in the specified since the chemical equilibrium is in the de

To design reaction areas in this way. practically feasible reaction conditions not

that the intermediates as largely as possible completely on the part of the methyl p-toluate

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

Secondary products is still low. therein, the esterification above the critical Tem

In the case of a discontinuous procedure, the reaction of the methanol must be carried out or be

tion according to the invention at the point of maximum achievable such pressure and temperature combinations, dat

Selectivity, i.e. moles of p-toluic acid formed per 15% of methanol is in each case in vapor form. This steam

converted moles of p-xylene are terminated; the shaped methanol is in a suitable Reaktoi

Oxidation is only incomplete due to the solid or molten p-toluic acid

p-xylene conversion led. This point corresponds to or through their solution or suspension in the ready!

Minimum of the sum of the selectivities of secondary esters formed. The latter method has the

products. The p-toluic acid has a suitable advantage that the water of reaction formed with the

thermal or mechanical separation process is driven off from vaporous methanol and there

the other components in the reaction mixture to completely due to the chemical equilibrium

separate. Unreacted p-xylene and the intermediate in favor of the desired ester are shifted

products p-toluene aldehyde and p-toluene alcohol that can.

can be further oxidized to p-toluic acid, 35 In principle, it is in the Her

are again possible according to the invention 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 perform the portion of intermediate and known methods, the ge By-products in the reaction mixture, the reac- 30 melted p-toluic acid in a pressure column

Tor leaves to keep as low as possible, required in countercurrent to vaporous methanol before

A residence time that is as uniform as possible in the reactor, flows from the top to the bottom and the Estei formed

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

for example, generally by cascade-like arrangement several ideally mixed, continuous esterification of p-toluic acid with methanol, however, ir

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-Toluylaldehyd used. The narrow residence time b) initiation of the reaction spectrum obtained according to a) can, according to the invention, be achieved by a multiple mixture - consisting of p-toluic acid methyl stages Bubble column reactor in which the ester, methanol and unreacted p-toluic acid are mixed of the liquid reactor contents along the reactor - 45 as well as reaction water - in a downstream axis, d. H. in its direction of flow, if possible dead pressure reactor and there complete egg is largely suppressed, or by several one-esterification by guiding in countercurrent stage bubble column reactors, which as a cascade downstream from top to bottom to gaseous methanol, are interconnected so that the liquid reaction mixture flows through the individual reactors one after the other without back-mixing, preferably in a stirred autoclave or a cascade between the individual reactors comes from stirred autoclaves and the subsequent ones become light. The mean residence time of the liquid complete esterification with gaseous methanol The phase in the reaction system for the oxidation is carried out in a pressure column, with the conversion to be chosen so that the conversion 55 can be carried out continuously at the reactor outlet, The maximum selectivity that can be achieved is the discontinuous. This is useful for the incomplete Borrowed mode of operation is present After separation of the esterification, unconverted methanol before p-toluic acid are unreacted p-xylene and lead to the subsequent complete Verdie Intermediate products p-toluyl alcohol and p-toluyl ester were separated off

aldehyde is continuously returned to the oxidation process. esterification achieved in one or more autoclaves

Detailed descriptions of a discontinuous one when setting the following reaction and a continuously carried out oxidation process particularly favorable results: process are in the working examples 1 and 2 "\ τ _ * ■,, ™. ^ «O ^. ■ e _ntna ] _th 65 'temperatures between 150 and 230 ⁰ C and

The in 'the sequence according to the invention of the method ^{b) in the pressure range} between 15 and 60 bar.

rens steps for the production of Monomethyltere- The maximum achievable yield of p-Toluyl-

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

Temperature dependence of the chemical equilibrium constant ΛΓ given:

Ig κ - -

+ 3.67

[T ° K]

(methyl p-toluate) (water)
(p-toluic acid) (methanol)

where the expressions in brackets represent the concentration of the individual components per unit volume.

The higher temperatures are to be preferred because of the weak endothermicity of the Reaction - the enthalpy of reaction is +7.3 kcal / mol - according to the equation for the equilibrium constants the position of chemical equilibrium with increasing temperature in favor of the formation of methyl p-toluate shifts.

In order to obtain an actually optimal yield of p-toluic acid methyl ester in the autoclave (s), the temperatures and pressures during the reaction should be kept in the range between 200 and 230 ^° C. or about 40 to 60 bar.

In the complete esterification of p-toluic acid in the pressure column, the temperature is expediently in a range between 220 and 260 ^° C., while the pressure is set so that in each case, depending on the temperature selected in the individual case, methanol is largely in vapor form.

During the esterification with vaporous methanol, the water of reaction formed is driven off and thus the chemical equilibrium completely in favor of the formation of p-toluic acid methyl ester postponed.

This esterification process offers considerable advantages over the known methods:

Most of the reaction can take place in one or more autoclaves connected in series, through which liquid reaction mixture flows, as a pure liquid phase reaction at a higher rate, ie in a shorter time, than the corresponding vapor (methanol) liquid [molten or dissolved (p-toluic acid) reaction ], which would be carried out in a pressure column, as a result of which larger amounts of p-toluic acid can be esterified per time and reactor volume. To complete the esterification until the p-toluic acid has been completely converted, only a small reactor volume is required in the downstream pressure column because of the short reaction time for this, compared to the possible implementation of the total esterification in the pressure column. The total reactor volume resulting from the combination according to the invention of autoclave and pressure column is smaller for a given output than when the total esterification is carried out in a pressure column. - If only one or more autoclaves arranged in a cascade are used, complete conversion of the p-toluic acid cannot be achieved without the technically complex removal of the water of reaction because of the position of the chemical equilibrium. This applies to both continuous and discontinuous modes of operation. - Further, that p-toluic acid is discharged in significant amount in vaporous methanol from the column and not esterified, as would be ^emer when using fine melt of p-toluic acid, because of their already relatively high at the esterification vapor pressure of the case is avoided.

Next is the oxidation of p-toluic acid methyl ester to monomethyl terephthalate, which is required according to the invention considered. In the context of the overall invention, the sub-task is to be solved, which Oxidation reaction to lead under such conditions that monomethyl terephthalate in as possible large and constant yield is obtained. In particular, care should be taken to ensure that the education by by-products and by-products is suppressed as far as possible.

With the methods known from the prior art, according to which it is known that during the oxidation of methyl p-toluate using a soluble salt of a polyvalent heavy metal monomethyl terephthalate which acts as a catalyst arises, the task according to the invention can in no way be solved. With the previously known Oxidation processes, the pressure and temperature conditions are in such a range that in to a considerable extent a thermal conversion of the monomethyl terephthalate to dimethyl terephthalate and terephthalic acid takes place. Likewise, larger amounts of that according to the invention are regularly formed Process sequence of undesired by-products such as, for example, p-toluic acid, those for the monomethyl terephthalate end product, especially if it is used directly as a starting material to serve for the monomers of the polyester, are extremely harmful. Furthermore arise with the Oxidation reactions carried out up to now also include those compounds which, even in traces, are the reaction product strongly discolour and presumably have a benzil and / or fluorenone structure. The colored ones Impurities that occur in particular in the further processing of the monomethyl terephthalate to polyester harmful can be caused by the usual cleaning processes such as washing out and / or recrystallization not removed sufficiently. Sublimation or distillation is also considered Exclude cleaning processes, since at the temperatures required for this already a considerable thermal conversion of monomethyl terephthalic acid to dimethyl terephthalate and terephthalic acid takes place as shown in the table below.

Table 1

Thermal conversion of monomethyl terephthalate; Starting substance: monomethyl terephthalate

T t MMT DMT TPS ( ⁰ Q (H) (Mole percent) (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

609 524/51

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

The aim of the invention can, however, as surprisingly «; has shown mnter by carrying out the oxidation limit of the following reaction conditions to achieve, and at temperatures from 130 to 200 ⁰ C, particularly advantageously between 140 to 170 ° C, and pressures between 1 and 6 bar, particularly advantageously 1-3 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 " ⁶ and 10 ^-3 gram atoms 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%, with unconverted p-toluic acid methyl ester and intermediates, which are formed during the oxidation of p-toluic acid methyl ester to monomethyl terephthalal, being used for renewed oxidation.

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

The oxidation of 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.

To achieve a uniform dwell time can a cascade of bubble column reactors can also be used.

Carrying out the oxidation under the reaction conditions specified according to the invention has As a result, the methyl terephthalaldehyde ester, which occurs as an essential intermediate, is almost completely is completely converted to monomethyl terephthalate and that the formation of secondary products such as Dimethyl terephthalate and terephthalic acid obtained by the above-mentioned thermal conversion from monomethyl terephthalate (see Table 1) are formed, and p-toluic acid, which is formed as a decomposition product as well as of high-boiling or subliming inehrkernigen aromatic compounds on extremely low shares are pushed back.

The inventive oxidation of p-toluene methyl ester to monomethyl terephthalate is detailed in the examples 3 and 4 described.

The monomethyl terephthalate obtained by oxidation from p-toluic acid methyl ester must then be used 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 intermediate stages obtained from p-xylene. »To recrystallize dimethyl terephthalate, which is to serve as the starting material for polyester, several times 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 because that monomethyl terephthalate predominantly is present in crystalline form, to be washed out with a solvent, the solvent properties of which for monomethyl terephthalate is as low as possible.

while it completely dissolves p-toluic acid methyl ester and the 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 7Ui crystalline separation of the pure monomethyl Cool terephthalate and the pure monomethyl terephthalate by a mechanical separation process separated from the remaining solution.

Solvents with low dissolving power for monolivic acid rephihaate 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

T ( ⁰ C)

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

0
10
20th

1.08
1.25
1.87

0.19
0.22
0.27

The temperature for washing out is preferably ⁰ ° C. or lower ^{when using methanol, and preferably 10 °} C. or lower when using chloroform.

The one coming out of the oxidizer

Reaction mixture consisting essentially of monomethyl terephthalate, methyl p-toluate
and methyl terephthalaldehyde ester can be worked up according to the invention in two different ways.

Wegl

The reaction mixture is either crystallized in Form or preferably as a suspension of already crystallized monomethyl terephthalate in a melt of p-toluic acid methyl ester, which still contains shares of monomethyl terephthalate and of Terephthalaldehydsäuremethylester has dissolved, mixed with a suitable solvent, so that

all components except for monomediyl 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 composed of p-toluic acid methyl ester <: r 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 route 1 or 2 is through a mechanical solid-liquid separation, such as centrifugation, into crystalline monomethyl terephthalate and liquid disassembled. When using the enrichment of monomethyl terephthalate after path 2, the washing can be coupled with the mechanical separation of crystals and melt carried out there so that the crystals are suspended and the subsequent mechanical solid-liquid separation not applicable.

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 out 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. Suitable solvents are, for example, methanol, acetone, ethyl acetate and acetoacetic ester, but preference is given to using methanol which has already been used elsewhere in the process. The solution temperature should preferably be between 60 and 80 ⁰ C when methanol is used; 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 monomethyl 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 happens when the pure monomethyl terephthalate crystallizes out Accumulated 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.

to The monomethyltere obtained according to the invention Phthalate should be used in particular as a starting product for the production of suitable monomers for di < Polyester formation can be used.

The pure monomethyl terephthalate obtained according to the invention can also be used as a preliminary or intermediate can be used in the manufacture of pure dimethyl terephthalate by using mi Methanol is esterified to ÄDimethylterephthalat. Furthermore, it can be purely from the according to the invention

ao representable monomethyl terephthalate by hydrolysis make pure terephthalic acid.

The process sequence according to the invention for the production of pure monomethyl terephthalate au; p-xylene is used in the following on the basis of one in de Drawing illustrated flow diagram explained.

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 resulting reaction mixture k, 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 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 in methanol; 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, byproducts m to be discharged and, if necessary, unreacted p-toluic acid f takes place. Unreacted p-toluic acid / is returned to the esterification device 3; the p-toluic acid methyl ester h TRITL continuously in the oxidation device 5 a in the separation device 18 is p-Toluylsäuremethylestern from the exhaust steam of the methanolic · Ver esterungsvorrichtung 3 deposited and Gelei tet after 4 hours. In the device 19, water of reaction c, methanolg and any p-xylene present are separated from one another; α becomes after 1 and g after; returned; the water of reaction c goes off together with any inert gas present.

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

13 ^g 14

device 6, after zvor most of the are fed. The monomethylterephthakis used as the oxidizing agent is crystallized, raecha-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 p-toluic acid melhyiestrt- next to a sieve plate. The gas mixture which composes the methyl terephthalaldehyde ester 5 was returned to the oxidizing device 5 before it was discharged from the reactor. Relaxation to normal pressure for separation The crystallizate, which consists essentially of monomethyl from the p-xylene carried along and the reaction phthalate / 7 formed, goes into the washing device water through a condenser conduit: Ausung 7. - P-xylene condensed by bypassing the separating device 6 if the reaction mixture was returned to the reactor, the reaction mixture λ can also be passed immediately, while the separated water is passed out of the after 7. - That the oxidation pre-reaction system has been removed. For the deposition device 5 exhaust gas leaving d is in the separation of residual amounts of p-xylene, the exhaust gas by device 17 of p-ToIuylsäuremethylester was h of the two cold traps whose temperature jeweiis CrC is returned after 5, and was optionally reaction, and a adsorption water c filled with activated carbon separately 15 tion spectrum sent

In the reactor described, 2000 g of oxidation device 5 were washed out in device 7 p-xylene, the 200 mg cobalt in general in the form of p-xylene with the solvent q. - The reaction product of soluble cobalt salts of the C ₆ - to C, _e- FeUic acids can, under certain circumstances, but have already been mixed, used. The p-xylene was washed out from the separating device 6; in this 20 initially ^{heated to 120 °} C. in a stream of nitrogen. In this case, the devices 7 and 8 would be omitted. Thereafter, the nitrogen flow was replaced by an air-containing suspension r of crystalline monomethyl flow of 1145 Ni / h; the temperature was terephthalai in the resulting solution is fed into the during the entire oxidation 120 ± TC and mechanical separation device 8, where one of the pressure 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 q is again after 7 30 secondary products terephthalaldehyde acid and tere-

out, the resulting mixture u 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 p-xylene 12 89

methyl terephthalate / »of 8 is n-Toluylalkohoi in the solution 0 43

direction 9 dissolved in the solvent t . The solution of ρ p-toluene aldehyde '.' 0 * 92

in / either in the adsorption device 10a £ T _{o] U} yj _s äure 443

or 100 given, in which the colored impurities plToluic acid p-Tolüyiester ''.'.'.'.'.'.'.'. θ! θ2

be separated. After the Adsorp- 40 terephthalaldehydic acid 0.01

medium by taking in a certain amount of terephthalic acid 0 05
the colored impurities is exhausted, it becomes _son £ by-products "'.'.'.".'.'.'.'.'.'. 042
regenerated by driving off the adsorbed substances with superheated steam w . It follows from this that 74% of the converted 45 p-xylene emerging from the adsorption device 10a or 106 react to p-toluic acid; The solution is taken into account in the cooling device 11 up to the point that the crystalline separation of p-toluene alcohol and p-toluene aldehyde present in the reaction mixture as completely as possible is cooled again in a monomethyl tercphthalal; 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. 17th
vapors t are condensed in the device 14 Example 2
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.

_R. 11 ^DZW · are supplied. The oxidizing agent

a c 1 s ρ 1 el turned air was in the same way 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. By

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

was continuously decreased, along the stream

If the direction of flow was mixed only a little and thus maintained for 360 minutes, 93% of the respondents

a narrow residence time spectrum for the continuously converted p-xylene to p-toluic acid and 2.3%

flowing through liquid was realized. Terephthalic acid; the rest was spread over

In the reactor described, 5 other by-products were initially produced.

2000 g p-xylene, the 200 mg cobalt in the form of in.

p-xylene dissolved cobalt salts of the Q- to C ₁₀ -fat example ^

acids were added, used. The p-xylene The discontinuous oxidation of p-toluyl-

was first in a stream of nitrogen to a methyl ester to monomethyl terephthalate

temperature of 140 ° C, then was on an io in a reaction as described in Example 1

Air flow of 480 Nl / h switched; the temperature system carried out. 1500 g p-toluic acid methyl ester,

was the 150 mg cobalt in the form of p-toluic acid during the entire subsequent oxidation

140 ± 2 ° C, the pressure 8 bar. The reaction was methyl ester soluble cobalt salts of the C ₆ - to Q ₀ -

initially run discontinuously until the reac fatty acids were added to the reactor

tion mixture used in the reactor after 60 min the following 15 and initially in a stream of nitrogen on the

Had reached Composition: desired reaction temperature brought 200 ⁰ C.

Component Weight Percent ^{Then, the} oxidation by switching on ^was

p-xyloi 89 3 introduced an air stream of 420 Nl / h; the pressure

p-Toluylaikohoi 26 ' ^m R ^^ or was about 1 bar. In a corresponding

p-Toluylaldehyde 3 9 ^{20 the preliminary tests} had been found that in

p-Toluic acid 35 a p-toluic acid methyl ester conversion of greater

Terephthalaldehydic acid + tere- 'than about 35% after a reaction time of about

phthalic acid 01 ° ³⁶ ""^{"erre 'cnt was} - large quantities of unwanted

p-Toluic acid p-Toluoyiester ". '.'. '.'. '.'. '.'. o ', 4 high-boiling by-products were formed and the ther-

Other by-products 0.2 ^{35 mixed} . ^e Conversion of monomethyl terephthalate

on dimethyl terephthalate and terephthalic acid already

j The reaction mixture then ran off continuously to a measurable extent; therefore the dis-

»Taken; this was cooled to 15 ° C and continuous oxidation attempt after 350 min.

• I am p-toluic acid which separates out in crystalline form and is broken. The reaction mixture obtained had

Terephthalic acid from the remaining liquid 30 has the following composition:

(Filtrate I) separated. The crystals were fri component mol percent

For example, p-xylene had a temperature of 15 ° C, after methyl p-toluate it was 6.44

washed (filtrate II) The combined filtrates I and terephthalaldehyde acid methyl ester ... 0.18

II were quasi-monomethyl terephthalate 3.17 with additional fresh p-xylene

continuous, d. H. with short interruptions, 35 dimethyl terephthalate 0.01

the terephthalic acid carried out discontinuously by the / ur the 0 01

Separation of crystals and liquid required _so J _ti by-products''.".'.'.'.'.'.'.'.'.'.'.O', 14
Time was limited, with a diaphragm pump in the

Reactor returned. That contained in the filtrate II shows that 89% of the converted

p-xylene, together with the additionally added 40 p-toluic acid methyl ester to form monomethyl terephthalate

p-xylene, on a molar basis, corresponded to the sum of lat reacted; one takes into account that the ter-

ebbated p-toluic acid and terephthalic acid phthalaldehyde acid methyl ester in a following

+ the residual moisture in p-xylene, which can be used in the crystallizate oxidation mixture, then

j remained. After the reactor is about 150 minutes later, the effective yield of monomethylterephthalate

j had reached a steady operating state, 45 lat 94%.

ΐ were continuously used for the composition of Example 4
j from the reactor, in which there is a liquid

volume of about 2700 ml, discharged in analogy to that described in Example 3

! Reaction mixture following average values solid method were 1950 g of methyl p-toluate,

put: 50 to 230mg cobalt in the form of in p-toluic acid

Component Percentage by weight of methyl ester soluble salts of Q to C ₁₀ fatty acids

_χ j _{ol 78} q were mixed in at 160 C and 4 bar with air

P-Toiuylaikohoi V5 (485 Nl / h) oxidized. This attempt was made at a

p-toluene aldehyde 5 * 5 p-toluic acid methyl ester conversion of 27% after

ρ joiuyic acid 114 55 240 min canceled because of the increased pressure

p-toluic acid p-TÖluyiester '.'. '.'. '.'. '.'. '. l', 2 "" ^e appreciable formation of the undesired product

Terephthalaldehyde 0.1 ^duct f. Ft. Example 3) despite the low temperature

»Terephthalic acid 0.4 ^whether ? ^{rhalb dl} ? ^se , ^s sales began

Other by-products 1 ^{The reaction mixture thus obtained} had the following

60 Composition:

The output was about 3700 g / h. From this became

372 g p-toluic acid + 15 g terephthalic acid per component mol percent

Hour separated. About 9.44 p-toluic acid methyl ester was added to the reactor

3315 g / h reaction mixture, of which the p-toluyl-terephthalaldehyde acid methyl ester ... 0.25

acid was separated off, and an additional 65 Monomethylterephthalat 3.03

300 g / h of fresh p-xylene together with a small amount of dimethyl terephthalate 0.04

Amount of catalyst (500 mg cobalt / 1000 g p-xylene) terephthalic acid 0.04

fed in. Other by-products 0.19

17 18

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

p-Toluylsäuremethylesters to Monomethylterephtha- 1500 ml / h through a with granulated activated charcoal

lat responded; one takes into account that the Tere-filled adsorption tower is conducted. The dwell time

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

dation approach can be used again, then 5 the exit from the tower it became the crystalline

is the effective yield of monomethylnene deposition of the Mcnomethylterephthalat

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

. ic ^and dried. Thus 257 g became white

Example 5 obtained monomethyl terephthalate, its acid number

1000 g of a 311.5 db 1 by air oxidation of p-toluyl- io, 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-Toluic acid methyl ester 6012 ^bi .? ^en impurities examined by a

Methyl terephthalaldehyde ^ester. 0.94 solution 8 of 0.2 g of monomethyl terephthalate (I) m

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

Dimethyl terephthalate 0 30 methanolic filtrate with monochromatic light

Terephthalic acid 030 ^ao ^^ ^nm ^ was irradiated and the light absorption

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

Other by-products 1 * 93 ^ carried ^ ^mm · ^{As a} comparative substance for the obtained

Monomethyl terephthalate was a multiple after

The solidified reaction mixture was comminuted adsorption of distilled monomethyl terephthalate (II), and ^{stirred intensively with 800 g of methanol at 0 °} C. for 1 hour as that 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 chroniatol (III), which, according to the graphic analysis described, 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 i 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)

1 2 An industrially applicable process for the production of pure monomethyl terephthalate is Claim: not yet known. It is just known, e.g. B. Swiss patent 3 15 580, 5 that monomethyl terephthalate in certain reac- Process for the production of mono-esters aromatization 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 formed ion of pure monomethyl terephthalate in 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 terephihalate with extensive support a) the oxidation of p-xylene at temperatures switching of secondary and secondary products possible between 90 and 130 ° C, pressures between is. 3 and 15 bar and an oxygen partial The production of monomethyl terephthalate in pressure less than 3 bar and cobalt as an industrial-scale as a starting material in the Oxidationskatalysatot in a concentration »o polyester recovery for the formation of monomers is between 10 ^-5 and. IC * - ² gram atom of cobalt / for the formation of polyester, therefore, no mol of p-xylene has been 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 2u p-toluic acid methyl- 35, 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 ³ 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 of between 10 ^-5 and IO ³ gram of terephthalic acid in the polyester required for the recovery of cobalt atom / mole of p-toluic acid methyl ester extremely high purity, in particular to a p-toluic acid methyl ester conversion with respect to color contamination, prepares between 20 and 45% is guided, 35 however, major technical difficulties. Dimethyl terephthalate, which is lighter in its pure form d) and which can be obtained as terephthalic acid after oxidation of the p-toluic acid, has the disadvantage that the reaction mixture present as methyl ester, polyester intermediate, however, is that in which the monomethyl terephthalate is crystallized from two methanol-based ester groups, initially with 40-based ester groups, due to the formation of the polyester monomers must be transesterified with a polyvalent methanol at a leaching temperature of alcohol, with a maximum of 30 ⁰ C or chloroform with a borrowed amount of methanol as an undesirable secondary leaching temperature of at most 30 to product. Particularly in the case that the 4O is washed ⁰ C, then the polyester starting material for the polymerization of a crystalline monomethyl terephthalate after completion 45 different location is delivered, fibers are temperature in separation from the solution at elevated tempera- the precursor for reasons of transport costs in 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 eliminate impurities 50 dimethyl terephthalate, which is then processed via a solid adsorbent activated carbon during the monomer formation and the pure monomethyl or transported away for further processing terephthalate - after which 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 ■ - to manufacture 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 process for the generation of high-purity monomethyl terephthalate 65 small amounts of byproducts during the formation of monomers, with p-xylene, in particular as a starting product for the byproduct water produced in addition to methanol, conversion to suitable monomers for she is.
Polyester formation. The oxidation of p-xylene with air or a