DE2505831B2 - Process for the purification of crude dimethyl 2,6-naphthalenedicarboxylate - Google Patents

Description

WjF 2 H) '' ¹ ",!, - 1/160 · /" r.

met what

W / F means the remaining ratio (unit: dimensionless),

W means the withdrawal ratio of the liquid soil phase (unit: kg / h) or the amount of liquid soil phase (unit: kg),

F means the ratio with which the crude 2,6-naphthalenedicarboxylic acid dimethyl ester is introduced into the distillation zone (unit: r> kg / h), or means the amount that is introduced by the ester (unit: kg),

Ai is the acid number of the crude dimethyl 2,6-naphthalenedicarboxylate introduced into the distillation zone, which is 1.5 to 40 (unit: numerical value, expressed in mg of KOH required to neutralize the carboxyl groups present in 1 g of the crude

2,6-naphthalenedicarboxylic acid dimethyl ester
are included), and r>

Tb means the temperature of the liquid bottom phase (unit: numerical value, given in ⁰ C).

The invention relates to a process for the purification of crude dimethyl 2,6-naphthalenedicarboxylate by vacuum distillation of the crude dimethyl 2,6-naphthalenedicarboxylate using heat according to the claim.

Purified dimethyl 2,6-naphthalenedicarboxylate is obtained high quality not only in good ίο yields and with satisfactory quality reproducibility, but also operationally advantageous, with the formation of disadvantageous by-products is avoided while performing the distillation. Γι

The dimethyl ester of 2,6-naphthalenedicarboxylic acid (2,6-NDA) is the valuable starting compound for the Manufacture of polyesters that have various excellent physical properties such as for example of polyalkylene-2,6-naphthalenedicarbot> oxylate. This 2,6-NDA-dimethylester can according to various Process are produced. For example, it can be obtained by using the 2,6-NDA, the obtained by oxidation of 2,6-dialkylnaphthalene, esterified with methanol or by converting the 2,6-NDA, b5 obtained by heat disproportionation and / or heat rearrangement of naphthalene monocarboxylic acids and / or naphthalenedicarboxylic acids, with the exception of 2,6-NDA, are esterified with methanol (cf., for example, the U.S. Patent 30 42 709). DE-OS 22 43 158, JA patent publication 2666/1959 and DE-PS 9 53 072 and 11 35 906 deal with the manufacture of 2,6-NDA; US Pat. No. 1,454,532 relates to the manufacture of a polyester.

The crude 2,6-NDA-dimethyl ester produced in this way contains color impurities and is thus usually yellowish-brown or similar in color. If such a crude 2,6-NDA-dimethylester for the Used in the manufacture of polyester, the color of the resulting polyester suffers. The paint impurities must therefore be removed by cleaning.

The purification methods can be crystallization, adsorption and distillation or combinations of these Procedures are used. Of these methods, the distillation method is most preferred due to its implementation and costs.

When performing purification by distillation at reduced pressure of crude 2,6-NDA-dimethy'ester with application of heat occurs Presents a number of difficulties that need to be resolved. One of the biggest difficulties is that the The melting and boiling points of 2,6-NDA-dimethyl ester are significantly higher than those of dimethyl terephthalate. It is therefore necessary to carry out the distillation at higher temperatures and at higher pressures, with the result that new paint impurities and impurities such as methyl naphthoate, which are mainly formed by thermal decomposition of 2,6-NDA-dimethyl ester, formed during the distillation will. As there is no easy and practical way to prevent this deleterious by-product formation To avoid during the distillation, it is very difficult to use the purified 2,6-NDA-dimethyl ester to produce the desired high quality and quality reproducibility through a distillation process, a procedure that is advantageous in terms of performance and cost.

If, as in known processes, an esterified reaction product becomes so high its degree of esterification is subjected to distillation as much as possible with the intention of increasing the yield of 2,6-NDA-dimethyl ester To increase during the esterification reaction, the distilled 2,6-NDA-dimethyl ester contains dyes. Surprisingly, it has now been found that the desired product with high quality and good Quality reproducibility can be obtained when crude 2,6-NDA-dimethyl ester, its degree of esterification is kept at a slightly lower value and has an acid number within a specified one Area owns, is distilled. It has also been found that when the distillation is among such Conditions is carried out that the remaining amount or rate of 2,6-NDA-dimethyl ester within of the distillation zone is selected in relation to certain factors of liquid bottom phase temperature and if the acid number has at least a specified value, the deposition of 2,6-NDA in the liquid bottom phase can be prevented, thereby reducing the formation of deposits or Crusts in the bottom of the distillation column is prevented, with the result that all the disadvantages that occur in the Implementation of the distillation process occur as the aggravation of heat conduction, decrease in the amount of evaporation and increased heat loss can be avoided. It continued to be

found that the desired high quality product can be obtained more easily and with better quality reproducibility if the purification by distillation is carried out under such conditions that specific acid number conditions are met ; are, and preferably under such conditions that the acid number conditions and the aforementioned residual quantity conditions are met.

The present invention is therefore based on the object of providing a method for purifying crude in 2,6-NDA-dimethyl ester to be created by distillation • at reduced pressure with the application of heat, the process being said to be operationally advantageous and Purified 2,6-NDA-dimethyl ester of high quality with good quality reproducibility. ι.

Although the crude 2,6-NDA-dimethyl ester used in the purification process of the present invention can be prepared by any method, for example, that obtained by oxidizing 2,6-dialkylnaphthalene as described above is advantageously used , 2,6-NDA being obtained, which is then methylesterified in the usual way, or that which is obtained by heat disproportionation and / or heat rearrangement of naphthalene monocarboxylic acids and / y, or naphthalenedicarboxylic acids, with the exception of 2,6-NDA, is used, wherein 2,6-NDA is obtained, which is then methylesterified in the usual way.

The crude 2,6-NDA-dimethyl ester, which is distilled according to the invention, is one whose acid number (A) or in acid value, expressed in mg KOH, which are necessary to neutralize the carboxyl groups present in 1 g of the crude ester is 1.5 to 40, preferably about 2 to about 30, and most preferably about 3 to about 20, y As can be seen from the following comparative experiments, difficulties arise when the acid value (A) of the crude raw material used 2,6-NDA-dimethyl ester is on the high side outside the above range, and it is difficult to prevent the adverse formation of by-products during the distillation process. On the other hand, if the acid number is too low, the separation of the color components is poor; in either case, it is impossible to obtain a highly purified -> 2,6-NDA-dimethyl ester with good quality reproducibility without substantial thermal decomposition to methyl naphthoate, etc. occurring. As can be seen from the results shown later comparative experiments, these colored by- \ a can-products that are formed during the distillation even if the resulting purified 2,6-NDA dimethyl ester is further distilled hardly be removed, unless the acid value (A ) ranges from 1.5 to 40. It is therefore practically impossible to produce a highly purified 2,6-NDA-dimethyl ester in a technically advantageous manner if the formation of these by-products is not prevented during the distillation by using crude 2,6-NDA-dimethyl ester, the the aforementioned acid number condition (A) is met, distilled.

For the preparation of crude 2,6-NDA-dimethyl ester, which is used as a starting material in the process of the present invention, the following process is suitable. Methyl esterification of 2,6-NDA is carried out in such a way that the acid number is within the range given above, ensuring that a small amount of 2,6-NDA and / or 2,6-NDA-monomethyl ester remains. This can be achieved by determining the acid number of the esterification product while the esterification reaction is proceeding and terminating the reaction at the point in time when the desired acid number value (A) has been reached. If the acid number is too low, it can be adjusted by adding 2,6-NDA and / or 2, b-NDA monomethyl ester.

2,6-NDA-dimethyl ester of significantly higher quality can be obtained by performing rectification instead of simple distillation.

The distillation can be carried out either continuously or batchwise, from process and For reasons of economy, the continuous distillation process is preferred. The use of well-known Distillation equipment is sufficient. For example, you can use a vacuum still, distillation towers use with floors etc.

A preferred distillation temperature is in the range of about 200 to about 290 ⁰ C, and especially from about 210 to about 280 ° C.

Accordingly, the inventive distillation is carried out in such a way that the following conditions are met: The temperature of the liquid bottom phase of the distillation zone is maintained at about 200 to about 300 ° C, and the residual ratio (W / F) of the dimethyl ester in the distillation zone satisfies the following relationship:

WjF ä 1 () '"«, 1, - 1/160 · / · "

wherein

W / F means the remaining ratio (unit: dimensionless)

W means the extraction ratio of the liquid soil phase (unit: kg / h) or the amount of liquid soil phase (unit: kg),
F is the ratio with which the crude 2,6-NDA-dimethyl ester is introduced into the distillation zone (unit: kg / h) or the amount of ester introduced (unit: kg),

Af denotes the acid number of the crude 2,6-NDA-dimethyl ester introduced into the distillation zone, which is 1.5 to 40 (unit: a numerical number as indicated by the KOH mg / g crude 2,6-NDA-dimethyl ester ), and

Tb means the temperature of the liquid bottom phase (unit: a numerical number given in "C),

in this regard, when W denotes the withdrawal rate or ratio of the bottom liquid phase, F denotes the rate or ratio at which the crude 2,6-NDA-dimethyl ester is introduced, and when W denotes the amount of Liquid bottom phase means, F iJie amount means that is introduced of ester.

The upper limit of the residual ratio (W / F) mentioned above is preferably about 0.9.

When the distillation is carried out, the free dicarboxylic acids and the monomethyl esters remain in the crude 2,6-NDA-dimethyl ester are contained in the liquid bottom phase as high-boiling components. The liquid collected from the bottom of the still (or high boiling fractions) is removed can be recycled to the esterification stage and be reused.

The 2,6-NDA-dimethyl ester, that of the upper part the distillation zone is distilled off and collected can, if desired, be further purified by it is subjected to repeated distillations or by other purification processes such as immersion using solvents, etc. used.

As indicated above, in the process according to the invention, crude 2,6-NDA-dimethyl ester with an acid number (Af) of 1.5 to 40 is subjected to distillation at reduced pressure with the application of heat. These reduced pressure conditions can be appropriately varied according to the heating conditions and other distillation conditions. For example, reduced pressures can range from about 5 to about 250 mm Hg abs. can be used to advantage.

In the method according to the invention, any

III a crude 2,6-NDA-dimethyl ester which satisfies the aforementioned acid number ("/ ^ - conditions can be used. An ester is preferably used which, after removal from the esterification zone, is preferred at temperatures of 200 to 300.degree from 220 to 290 ⁰ C and most preferably from 230 to 280 "C for a certain time, the time varying according to the temperature conditions mentioned above the ester i, n is introduced into the distillation zone can be controlled by providing a storage tank at a suitable point in the line through which the reaction product is conveyed into the distillation zone the following functions a and b are fulfilled:

(Standing time) X (Stancilempcratur - 180) > 50 (standing time) X (Slancltcmpcratur - 200) <200

(Function a) (function b)

Most preferably, the standing tcm and the standing time are chosen so that the following functions a ' and b 'are met:

(Standing time) X (standing temperature - 180)> 70 (standing time) X (standing temperature - 200) <180

(Function a ') (function b')

Most preferably, the stand temperature and the stand line are chosen so that the functions at the same time a "and b" are met:

(Standing time) X (standing temperature - 180)> 100 (standing time) X (standing temperature - 200) <150

(Function a ") (function b")

45

In the functions mentioned above, the time unit is hour, the temperature unit is ⁰ C, and the calculation is made by entering these numerical values in the functions mentioned above. The service life has the value given above.

According to the method according to the invention, color impurities, their removal from crude 2,6-NDA-dimethyl ester is difficult by other purification methods, very easy and effective removed by a simple process and obtained 2,6-NDA-dimethyl ester with a particularly good color. That The method of the invention is also effective in removing the esterification catalyst.

Since there are no deposits or incrustations inside the distillation column when performing the Distillation of crude 2,6-NDA-dimethyl ester can be formed by the process according to the invention, difficulties arise such as a difficulty in heat conduction or a reduction in the amount of vaporized material and an increase in heat loss does not occur, with the result that the distillation process is highly efficient can run and that purified 2,6-NDA-dimethyl ester of increased quality in good yield with good quality reproducibility can be obtained.

In contrast to this, US Pat. No. 2,938,837, in which the crude dimethyl ester is used, occurs in the method of operation a sufficient amount of alkali, such as sodium bicarbonate

60

b5 nat, neutralized and then distilled in vacuo, considerable impairments, such as the Comparative Example 11 below can be seen.

The following examples illustrate the invention.

The degree of discoloration as used in the examples is determined in the following manner.

A 1.0 cm long cell is with a solution of 2,6-NDA-dimethyl ester in o-chlorophenoI in a Concentration of 10% by weight filled. A comparison cell is only filled with o-chlorophenol. With a Spectrophotometer (Model 101, manufactured by Shimazu Seisakusho Co., Ltd., Japan) then takes the measurement of optical density at a wavelength of 500 microns to indicate the degree of discoloration.

Examples 1 to 4
and Comparative Examples 1 to 5

Crude 2,6-NDA-dimethyl ester with the acid number (Af) as listed in Table 1 below is subjected to simple distillation, the distillation conditions given in Table I being used. The distillation is carried out until the temperature at the top of the distillation column can be maintained at 255 ° C. The results obtained are shown in Table I below.

1 abcllc I

V ₁ -IM. I.

VgI-B. 2

.iU'ii.iI wage 2. (.- NI) A-UiIiKlIn lcslci

Sum i '- /: ihl

(A ₁ )

Wiliiihunt "'

Wed ^- Hn I-

ii.iplilhiiiil

(ii-hall

0.7 I, 3S

0.8

1.5 (1

Ex. I. 1.9 1.52 0 Ex. 2 9.8 1.34 (I. Ex. 3 *) 17.8 1.41 0 Ex. 4th 19th 1.40 0 Cf.-B. 3 55 1.50 0 Cf.-B. 4th 75 1.45 0 Cf.-B. 5 **) 0.24 0.17 0

Di'sl HImI iiinsbcdinpunpcn pressure 117 / \ u '"> A _r AuI- Cru- (icrciiiif ! CtT 7, (.- NI) A- Mi ^- HnI limp. Mim I / IWl / ,, scliliim- SlClI- dimclhyli'slii naplilhoal il lliissi- 111 munps- education)! Vi ^- I- (irhall I ¹ I ^- Ii Hd- Section. hildunp l'aili- ((H ^- W .-%) iK'iiphasc prad 0.01 I <I 30th 0.3 0.018 no or ? 55 no 0.14 less (bright dcspl. 30th 0.3 0.020 no yellow) 255 no 0.17 dcspl. 30th (1.3 0.048 no (yellow) the same 255 30th 0.3 0.25 no no 0.032 the same 255 30th 0.5 0.45 no no 0.02 (1st the same 255 30th 0.5 0.48 no no 0.03 1 0.29 255 30th 0.5 1.4 Yes no 0.025 3.4 255 30th 0.5 1.9 Yes Yes 0.086 255 Yes 0.12 (bright 0.01 30th 0.3 0.006 no yellow) or 255 no 0.091 less (bright yellow)

*) The starting material used contains crude 2,6-NDA-dimclhyleslcr, which in Verglcichsbeisp. 2 changes were made, for which one adds 0.07 (in other words, parts 2, f) -NDA-moiiomethylcslcr / part of the ester.

**) The purified 2,6-NDA-dimethylcr, which was obtained in comparison example 2, was further distilled under the same destination conditions.

Examples 5 and 6 and Comparative Example 6

Crude 2,6-NDA-dimethyl ester with the acid number (Aj) given in Table II below was subjected to a simple distillation under the distillation conditions given in Table II. The results obtained are shown in Table II.

Table II Starting matcria I rawer Available methyl Distillation conditions pressure W / F 10 '"!! / I \ _r up- Kru Purified 2,6-NDA Melhyl No. 2,6-N DA-dimethyl ester exercise naphthoate Temp. mm 1/160 • 7ft slippery sten dimethyl ester naphthoate acid Degree salary d. liquid Ed mingling education Ver Gchalt number (Wt .-%) bow Section. education förb. (Weight%) 1.40 0 dcnphasc Degree 0.01 (A ₁ ) 1.48 0 CC) 100 0.5 0.27 no 0.01 19th 1.50 0 295 100 0.6 0.54 no no 0.024 2.7 Example 5 38 295 100 0.8 0.79 no no 0.032 Example 6 55 295 no 0.095 Cf. ex. 6th

Example 7
and Comparative Examples 7 and 8

A crude 2,6-NDA-dimethyl ester, which was identical to that used in Example 4 (acid number 19, degree of discoloration 1.4, methyl naphthoate content 0%), was continuously at reduced pressure at a column head pressure of 30 mm Hg abs, a Column top temperature of 255 ° C and liquid bottom phase temperature of 275 ° C using a ten tray distillation column. A purified 2,6-NDA-dimethyl ester was obtained, the degree of discoloration of which was 0.015, an extremely low value.

No formation of methylnaphthoate was noticed during rectification.

For comparison, Example 7 was repeated, but using crude 2,6-NDA-dimethyl ester, which was identical to that used in Comparative Example 4 (acid number 75, degree of discoloration 1.45, methyl naphthoate content 0%). The purified 2,6-NDA-dimethyl ester had a degree of discoloration of 0.053 (light yellow-white) and its methyl naphthoate content was 4.2%. In this case, it was very difficult to conduct the experiment, and frequent clogging of the gas-liquid separator, vacuum lines, vacuum pump, etc. was observed.

For comparison purposes, the rectification was carried out again as in Example 7, but one used the same crude 2,6-NDA-dimethyl ester, as used in Comparative Example 2 (acid number 0.8, degree of discoloration 1.5, methyl naphthoate content 0%). The degree of discoloration of the resulting 2,6-NDA dimethyl ester was 0.15 (light yellow).

Example 8

Koher 2,6-NDA-dimethyl ester with an acid number of 4.2, a degree of discoloration of 1.48 and a methylnjphthoate content of 0% was rectified as described in Example 7 , with the result that a purified 2,6-NDA- dimethyl ester with an extremely low degree of discoloration of 0.017 was obtained.

K)

Γι

During the cectification there was no formation of Methyl miphthoate observed.

Examples 9 to 12 and Comparative Examples 9 and 10

Crude 2,6-NDA-diniethyl ester with an acid number (A) of 9.2, a degree of discoloration of 1.15 and a methyl naphthoate content of 0% was subjected to the simple destinations under the silencing conditions given in Table III below . The distillation was carried out in such a way that the residual ratio WiF. as indicated in FiIhHc Hl was obtained. The eih.ilii-i.ni r'ig results are given in Table III.

Fable III

No. Distillation conditions pressure η ι 10 '" ¹ M, - AuI- Kruslcn- (kreinigler 2,6-Nl) A- Methyl Temp. D. mm Hg 1 / 160- r _h slim education dimethylcsler naphthoate liquid : Section. education Vetla'rbungs- (i old Floor phasc manure gr.id 0 (C) 8th 0.50 0.45 0 Example 9 210 8th 0.45 0.15 no no 0.02 0 Example IO 210 18th 0.35 0.2) moderate no 0.021 0 Example 11 240 18th 0.27 0.29 no no 0.024 0 Example 12 240 8th 0.0? 0.039 moderate no 0.025 Comparative 210 Yes Yes 0.19 0.31 Example 9 *) 18th 0.95 1.74 Comparative 240 Yes Yes 0.052 Example 10 **)

*) Crude 2,6-NDA-dimethylester with an acidity (I /) of 0.8, a degree of Verliirhung voTi l, i and a Melhyliuph-

thoat content of 0% was used. *) Crude 2,6-NDA-dimethyl ester with an acid / ahl (.

lho.it (i.ilt of 0% was used.

Example 13

Crude 2,6 NDA dimethyl ester with an acid number of 13.5, a degree of discoloration of 1.45 and a methyl naphthoate content of 0% was distilled continuously under reduced pressure. The still used had a column diameter of 100 mm and 10 sieve trays with a ratio of the hole area of 8%. The distillation was carried out while the temperature of the bottom liquid phase portion was 260 ° C. A residual W / F ratio of 0.5 was maintained. The value '»» A, "■« »· T _b was 0.32. Working in this way, the distillation can easily be carried out continuously without a slurry forming or depositing The resulting purified 2,6NDA dimethyl ester has a low degree of discoloration of 0.015 and becomes pure white in the form

-Tl

Obtain crystals showing no discoloration. The formation of methyl naphthoate by thermal decomposition was not observed.

Comparative example II

Using a raw material consisting of 100 parts of the same crude 2,6-NDA-dimethyl ester (acid number 19, degree of discoloration 1.4, methyl naphthoate content 0%) as used in Example 7, and from 4.75 files of sodium bicarbonate (excess of 2 4% with respect to the theoretical value of the acid number), a purification procedure was carried out in the same manner carried out as in Example 7.

The 2,6NDA dimethyl ester purified in this way had a degree of discoloration of 0.072, contained 8.6% methyl naphthoate and often caused clogging of the Vacuum tubes and the vacuum pump, which made operation difficult.

Claims (1)

Claim: Process for the purification of crude 2,6-naphthalenedicarboxylic acid dimethyl ester by vacuum distillation '> la tion of the crude 2,6-naphthalenedicarboxylic acid dimethyl ester using heat, characterized in that this distillation takes place at a temperature of the liquid bottom phase of the distillation zone of about 200 to about m 300 ⁰ C is carried out under such conditions that the residual ratio (W / F) of the dimethyl ester in the distillation zone has the relationship