3,594,414 United States Patent Office Patented July 20, 1971 2 3,594,414 terephthalate in a transesterification reaction with ethylene PROCESS FOR THE PREPARATION OF FIBER glycol for producing polyesters. This is because dimethyl GRADE TEREPHTHALIC ACID Ewald Katzschmann, Witten-Bommern, Germany, as terephthalate (DMT) can be prepared virtually free from signor to Dynamit Nobel Aktiengesellschaft, Troisdorf, all impurities by recrystallization and distillation. Hence, (Germany in one embodiment of the present invention, for the No Drawing. Filed Sept. 1, 1967, Ser. No. 664,948 production of extremely pure terephthalic acid, it is ad Claims priority, application Germany, Sept. 3, 1966, vantageous to use highly pure DMT and to subject the D 51,020 same to hydrolysis, instead of purifying crude tereph Int, C. C07e 63/26 thalic acid by way of one of the involved and costly U.S. C. 260-515R 15 Claims O processes. A comparative principle of this aspect of the invention is the basis of U.S. Pat. 2,968,674, wherein extremely pure adipic acid is prepared by the hydrolysis ABSTRACT OF THE DISCLOSURE of its ester in the presence of inert gases, in particular, A process for the preparation of fiber-pure terephthalic CO2. acid which comprises hydrolyzing dimethyl terephthalate 5 Hydrolysis as such as a known reaction and is employed at a temperature of from about 180° to 280 C., pref by the chemical industry often and very successfully. erably 200-250° C. The process is advantageously car One example is the hydrolysis of fats, accomplished ried out under pressure in a corrosion-proof vessel with basically under pressure at higher temperatures and by an aqueous solution of a neutral water-soluble inorganic the use of suitable catalysts. Such conditions apply to salt, said salt being used in concentrations of about 5 20 esters which are normally susceptible to hydrolysis. In to 50% by weight. The preferred salts employed are certain cases, these conditions do not suffice to attain sodium chloride, potassium chloride and calcium chloride. easily a degree of conversion which would be worthwhile The process may be carried out as a continuous process with respect to the economics of the situation. These with the obtained terephthalic acid being further puri latter considerations are of particular pertinence especial fied by subsequent washings or by steam distillation. Left 25 ly in the case of terephthalic acid esters, all of which can over filtrates and washings are recycled to the reaction be hydrolyzed only with difficulty. This fact is well known solution for a more efficient operation. So far as polyethylene terephthalate is concerned. How ever, dimethyl terephthalate also possesses a great re sistance to hydrolysis, a fact demonstrated by French BACKGROUND OF THE INVENTION 30 Pat. 1,296,465, which proposes a steam distillation at temperatures near 170° C. as a purification process for This invention relates to a process for the preparation DMT. U.S. Pat. 3,120,561 describes the hydrolysis of of fiber-grade terephthalic acid. More particularly, it polyethylene terephthalate at temperatures ranging from relates to a process for the preparation of terephthalic 230 to 250° C. As far as the hydrolysis of DMT is acid having a very high degree of purity such that it may 35 concerned, there are only published some tests relating be used for producing fibers and films. Even more par to water-dioxane and water-acetone mixtures. ticularly, the invention relates to a process for the prep The application of the various specifications used for aration of fiber-grade terephthalic acid by the neutral hydrolysis reactions, as known from the prior art, to hydrolysis of dimethyl terephthalate. dimethyl terephthalate has led to unsatisfactory results Terephthalic acid with a very high degree of purity, 40 so far as the rate of conversion, purity and crystal form unusually high for industrial purposes, is required for are concerned. The alkaline separation leads to the alkali the preparation of polyesters which are to be used as salt of terephthalic acid, which in turn must react with a fiber and film material. Such polyesters are usually made mineral acid in order to become free terephthalic acid. from terephthalic acid and ethylene glycol by direct esteri The employment of very pure mineral acids is costly, fication and subsequent condensation polymerization. At 45 and so is the equipment required for such a process. the present time, there are available on the market certain The acidic separation results in impure terephthalic acid "pure-fiber' grades of terephthalic acid. However, these which can be made non-acidic only at great expenditure. materials do not possess the attainable optimum of purity Waste water problems arise with both of these processes, and therefore fail to meet many of the specific require thereby making the same more complicated and costly. ments in the industry. 50 The neutral conversion, proposed by U.S. Pat. 3,120,561, The removal of all impurities found in a terephthalic whereby polyethylene terephthalate is heated with water, acid as a result of its preparatory process is very difficult results in a low and uneconomical 60% to 65% rate and has been feasible heretofore only by a combination of conversion. This is possibly caused by the fact that of purification steps, the reason being the heterogeneous the polyethylene terephthalate contains impurities, for structure and the diverse chemical properties of the im example, titanium dioxide, which exert a catalytic effect, purities. These difficulties have led to the use of dimethyl while the DMT is absolutely free of such substances. 3,594,414 3. 4. Accordingly, a hydrolytic process for DMT must meet economics of further processing (such as transporting and the following requirements in order to be feasible for handling thereof). industrial and economic use: The method proposed by the present invention is ac (1) Industrially available materials must be employed complished in the following exemplary manner. Fiber in the reaction which have inert properties during the grade DMT, together with a two- to four-fold quantity of reaction relative to the terephthalic acid. a 5-50% by weight neutral salt solution in distilled (2) A high, if possible total, rate of conversion must water, is heated in a titanium autoclave, or in an auto be attained. clave made from an equivalent material such as tantalum, (3) If catalysts or additives for the purpose of specific with stirring or shaking to temepratures of 180 to 250 effects are employed, it is necessary that these substances O C. for periods ranging from one to six hours, preferably can be separated easily from the free terephthalic acid. two to three hours. The liberated methanol is withdrawn (4) The terephthalic acid must be obtained in the prop continuously by means of a condenser. The pressure, deter er crystalline form. mined empirically, is approximately 30 to 50 atmospheres The hydrolysis attempted by the use of 0.25 to 2 moles gauge pressure. Upon the termination of the reaction, of dimethyl terephthalate and 200 to 500 cc. of distilled 5 which can be determined by the amount of methanol water in a 1 liter titanium shaking autoclave at tempera withdrawn, the reaction substance is cooled to 60-80 C. tures ranging from 200 to 250 C., pressures of 30 to 45 and then filtered or centrifuged. The filtrate is always re atmospheric gauge pressure and reaction times of 1 to 6 turned or recycled as a hydrolysis component, thus re hours without catalysts, as well as with the use of catalysts plenishing any actual losses of the component. such as zinc or titanium salts, and/or the admixture of 20 An alternate execution of the method proposed by the solvents such as methanol, acetone, xylene, dioxane, gla present invention is the hydrolysis without the removal cial acetic acid and the like, led to unsatisfactory results. of the methanol by distillation. In this case, the reaction The rate of conversion ranged from 60-90% of the theoret time is determined empirically. The methanol, and then a ical. This yield could be increased only by uneconomi portion of the water, is removed by distillation upon com cally long reaction periods, which also resulted in a deteri 25 pletion or termination of the reaction. Any existing traces oration of the quality of the terephthalic acid. Decisive as of DMT and terephthalic acid monomethyl ester, which far as the rejection of the known methods was the fact are volatile to water vapor, will become part of the distil that the terephthalic acid so obtained showed a titanium late together with the water vapors. It is advantageous to positive reaction which could not be subsequently elimi return the thusly removed DMT and monoester, or the nated. The acid could be filtered with difficulty only be 30 entire distillate, to the reaction process. cause of its excessively fine grain size. Measurements of The terephthalic acid, separated by filtration or centri the transparency of a 7.5% by weight solution of teleph fuging, will still contain a small amount of neutral salt thalic acid in 2 N KOH by means of a 50 mm. cell with solution. By separate washing with cold, or preferably 380 mu, or a 20 mm. cell with 340 mp, respectively, hot, water, or by washing in the filter or in the centrifuge, showed values ranging from 70 to 90%, which fell short 35 the terephthalic acid is freed of any remaining salt, a of the required values. step which can be effectively accomplished without diffi Accordingly, one of the objects of the present invention culty. In order to increase its purity, the slightly aqueous is to provide an improved process for the preparation of terephthalic acid can also be washed with acetone or with terephthalic acid which overcomes the disadvantages and some other water-insoluble solvent of dimethyl tereph deficiencies of the prior art methods.