DE1057090B - Process for the preparation of dimethyl terephthalate - Google Patents

Description

GERMAN

It is known that the complete esterification of terephthalic acid with low molecular weight alcohols, like methanol, presents certain difficulties that are associated with the water formed during the esterification in are directly related. It has already been proposed to use a large excess of methanol and to esterify in the presence of sulfuric acid, as sulfuric acid is known to be involved in esterification reactions binds the water formed. The sulfuric acid can be dispensed with if, after the process of the German Auslegeschrift 1 000 019 in a closed autoclave at temperatures of at least 150 ° C is esterified. The excess methanol cannot be dispensed with in this process. According to the words of this patent application, the smallest excess of methanol required can be 20 mol be used per mole of terephthalic acid, and Example 2 proves that at a molar ratio of 12 or only 6 moles of methanol per 1 mole of terephthalic acid 12 or even 81 parts of terephthalic acid also after 5 hour reaction time are not implemented at all.

It has now surprisingly been found that the esterification of terephthalic acid with methanol in the closed autoclave in just as short a time and with the greatest success can be done by in the case of esterification above about 215 ° C., at most 20 mol, preferably less than 10 mol, alcohol 1 mole of terephthalic acid to act. This result is likely to be all the more surprising as it is not only the aforementioned explanatory document, but also one A number of older patents expressly emphasize that the smallest excess 20 moles per mole of terephthalic acid should be. Since, moreover, according to the above-mentioned interpretation, with 6 moles of methanol 1 mole of terephthalic acid after 5 hours of heating at 200 ° C. results in only 19% esterification of terephthalic acid is achieved, was not to be expected that the slight temperature increase of only about 20 ° C would already bring about a complete esterification with the same reaction time.

The advantages of an esterification with an excess of methanol of not more than 20 moles of alcohol per mole of terephthalic acid are obvious. The throughput per esterification device are a multiple, and the work-up with water diluted, unreacted alcohol requires a lot less effort since the total amount of using Water diluted alcohol is considerably less.

The possibility of practically complete esterification using only a relatively small excess of alcohol is all the more astonishing since, as is known, the proportion of water in the reaction mixture increases rapidly with increasing degree of esterification
of dimethyl terephthalate

Applicant:

Mining Association GmbH,
Essen-Kray, Dortmunder Str. 151

Dr. Anton Benning, Dortmund-Eving,

Dr. Otto Grosskinsky, Dortmund-Kirchhörde,

and Dr. Arthur W. Krause, Herne (Westphalia),

have been named as inventors

and the proportion of water in the process according to the invention takes a much higher percentage in the reaction mixture than with an esterification using a high excess of methanol.

Based on previous publications and general experience in the field of esterification of carboxylic acids it must be assumed that an esterification with a much lower methanol excess comes to a standstill early on.

Therefore, an excess of 20 moles of alcohol per mole of terephthalic acid has hitherto been the lowest possible recommended and usually even terephthalic acid with 40 mol and more alcohol esterified. That regardless of this, the esterification is complete with a much smaller excess of methanol runs, must therefore be regarded as extremely surprising.

example 1

1 mole of terephthalic acid was combined with 7 moles of methanol in a stainless steel rotary autoclave Heated 225 ° C and held this temperature for 5 hours. The resulting pressure fluctuated between 80 and 100 atm. The conversion was 95% of theory. The crude ester still contained 1% monoester, see above that after distillation 94% of the purest dimethyl terephthalate were obtained. Melting point: 140.6 ° C.

Example 2

1 mole of terephthalic acid was mixed with 7 moles of methanol in the same autoclave under a nitrogen atmosphere Heated to 255 ° C for 5 hours. The pressure was kept between 100 and 150 atmospheres. The conversion was

909 510/465

Claims (1)

98.3 per cent of theory. The crude ester obtained still contained 0.7% monoester. Example 3 1 mole of terephthalic acid was mixed with 0.1 mole of dimethyl terephthalate and heated 7 · mol of methanol in the same autoclave to 225 ° C for 4 hours. The resulting pressure fluctuated between 95 and 120 atmospheres. The conversion was 98.5% of theory. The crude ester obtained still contained 0.3 to 0.5% monoester. ■ '■' claim: Process for the preparation of dimethyl terephthalate by esterification of terephthalic acid with methanol at elevated pressure and elevated temperature, characterized in that the esterification is carried out above 215 ° C and to 1 mol of terephthalic acid at most 20 mol, preferably less than 10 moles of alcohol. © 909 510/465 5.59