CH498081A - Preparing pure colourless diphenyl terephthalate from - Google Patents

Abstract

Pure, colourless phenyl terephthalate is prepared by heating, to above 150 deg.C, a dialkyl terephthalate with at least an equivalent amount of phenyl acetate, under an inert atmosphere, with the addition of 1-5 weight % of active carbon, then adding a titanate ester as transesterification catalyst, when the liberation of alkyl acetate is complete, the crude ester conaining carbon is stirred in an aromatic hydrocarbon, preferably xylene, under an inert gas, and the carbon-containing crystallisate which separates is recrystallised once more, possibly with the addition of further adsorbent. - The diphenyl terephthalate is suitable for the preparation of polycondensates.

Description

       

  Process for the production of pure, colorless diphenyl terephthalate According to Swiss patent 435 312, uniform diaryl esters of aromatic dicarboxylic acids, the carboxyl groups of which are not adjacent, can be produced in such a way that the acids or their alkyl esters with 1 to 6 carbon atoms in the alkyl group equivalent amounts or a slight excess of a phenol and acetic anhydride in the presence of transesterification catalysts heated to temperatures above 140 ° C. and the acetic acid released and, if appropriate, the alkyl acetate removed from the reaction mixture as quickly as possible.



  In the preparation of the aryl esters of terephthalic acid, titanic acid esters are preferably used as catalysts, since they far exceed all other catalysts in their effectiveness in terephthalic acid. The crude diphenyl terephthalate obtained in the presence of titanium compounds is deep red in color. This color cannot be completely removed by recrystallization of the ester from solvents, preferably from aromatic hydrocarbons, using adsorbents such as coal or fuller's earth, even after repeated implementation. However, since it is essential for many applications of diphenyl terephthalate, for example for the production of polycondensation products, that it is colorless, it was previously necessary to use vacuum distillation.

   However, due to the very high melting point of diphenyl terephthalate of 199 to 200 C, this is associated with particular difficulties and is very difficult to carry out.



  The present invention relates to a process for the production of pure, colorless diphenyl terephthalate, by heating dialkyl terephthalate, the alkyl groups of which contain 1 to 6 carbon atoms, with an equivalent amount or a slight excess of phenyl acetate above 150 ° C. in an inert atmosphere with the addition of 1 to 5% by weight of activated carbon and butyl titanate and with the quickest possible removal of the released alkyl acetate, the butyl titanate only being added when the reaction mixture has been heated to over 150 ° C,

   and after the end of the alkyl acetate cleavage, the carbon-containing crude ester is stirred into an aromatic hydrocarbon, preferably xylene, under inert gas and the carbon-containing crystals which have separated out are recrystallized again, optionally with the addition of fresh adsorbent.



  The alkyl esters of terephthalic acid and lower alkanols with 1 to 6 carbon atoms in the alkyl radical can be used as starting materials.



  At least one mole of phenyl acetate is used for each carbalkoxy group to be reacted. In order to achieve a quantitative conversion, it can be advantageous to use a small excess of phenyl acetate. In general, however, for economic reasons, an excess of 250 / o over the equivalent amounts will not be exceeded.



  The catalytically active titanium compounds used are preferably ortho-titanic acid esters, such as, for example, tetrabutyl titanate or tetraphenyl titanate.



  The dialkyl terephthalate and an at least equivalent amount of phenyl acetate are, after the addition of 1 to 5% by weight of activated carbon, heated to 150 ° C. in an inert atmosphere and kept at this temperature for a few minutes with stirring to remove air. Gases which essentially consist of nitrogen or carbon dioxide are suitable as inert gas. The catalytically active titanium compound is then added in an amount of 0.1 to 5.0% by weight and the temperature in the reaction mixture is increased until the elimination of alkyl acetate begins and remains briskly in progress. The temperature rises in the course of the transesterification. Since the diphenyl terephthalate is thermally very stable, an upper temperature limit is not important.

   Towards the end of the transesterification, it may be useful to apply a vacuum in order to quickly remove the last amounts of alkyl acetate and to avoid excessive overheating of the reaction mixture.



  After the reaction has ended, any excess phenyl acetate used is removed by distillation. The reaction product, the ester portion of which consists almost quantitatively of diphenyl terephthalate, is stirred into an aromatic hydrocarbon at room temperature, preferably xylene, under an inert gas. The precipitated carbon-containing crystals are recrystallized again from suitable solvents, it being possible to use a fresh adsorbent. The colorless crystals are separated from the mother liquor by suction or centrifugation, washed with the pure solvent and dried.



  <I> Example </I> 194 parts by weight of dimethyl terephthalate (1 mol) and 326 parts by weight of phenyl acetate (2.4 mol) are melted under inert gas in a flask equipped with a stirrer, thermometer, gas supply and column, after adding 5.2 parts by weight of activated carbon and heated with stirring to 150 ° C. for 10 minutes while a stream of inert gas is passed through the apparatus. Then 1.3 parts by weight of butyl titanate are added at 160.degree. The calculated amount of methyl acetate is split off within 80 minutes. The carbon-containing reaction product is stirred into 1 liter of xylene at 22 ° C. under inert gas, the crystals are filtered off with suction and recrystallized from xylene with the addition of 1% by weight of activated carbon.

   The dried crystals melt at 199 to 200 ° C., have a saponification number of 353 (calc. 353) and are completely colorless. The filtrate has a Hazen color number of 60.


    

Claims (1)

PATENT CLAIM Process for the production of pure, colorless diphenyl terephthalate, by heating dialkyl terephthalate, the alkyl groups of which contain 1 to 6 carbon atoms, with an equivalent amount or a small excess of phenyl acetate above 150 C in an inert atmosphere with the addition of 1 to 5% by weight of activated carbon oil and butyl titanate and with the quickest possible removal of the released alkyl acetate, the butyl titanate only being added when the reaction mixture has been heated to over 150 C, and after the end of the elimination of the alkyl acetate, the carbonaceous crude ester is stirred into an aromatic hydrocarbon under inert gas and the deposited carbonaceous crystals are recrystallized once more. SUBClaims 1..A method according to claim, characterized in that xylene is used as the aromatic hydrocarbon. 2. The method according to claim, characterized in that the recrystallization is carried out with the addition of fresh adsorbent. <I> Note of </I> Eig. <I> Office for Intellectual Property: </I> If parts of the description are not in accordance with the definition of the invention given in the patent claim, it should be remembered that according to Art. <B> 51 </B> of the patent law of Claim is decisive for the material scope of the patent.