US2894978A - Process of producing esters of phthalic acids by oxidation of xylene and toluic acid ester mixtures - Google Patents

Description

PROCESS OF PRODUCING .ESTERS F PHTHALIC ACIDS BY OXIDATION OF XYLENE AND TOLU- IC ACID ESTER MIXTS No Drawing. Application June 28, 1957 Serial No. 668,606

Claims priority, application Germany July 2, 1956 12 Claims. (Cl. 260-475) The present invention relates to the production of esters of phthalic acids and more particularly to the production of esters of phthalic acids from mixtures of xylenes and esters of toluic acids with monovalent alkanols having 1 to 4 carbon atoms by oxidation by means of oxygen-containing gases at elevated temperature.

It is one object of the present invention to provide a simple and highly advantageous process of producing esters of phthalic acids by oxidation of mixtures of xylenes in mixture with esters of toluic acids whereby substantially all of the xylene starting material is converted into oxidation products and whereby the formation of undesirable by-products is considerably reduced.

Other objects of the present invention and advantageous features thereof will become apparent as the description proceeds.

The process according to the present invention consists in using as starting material a mixture of xylenes and esters of toluic acids with alkanols having 1 to 4 carbon atoms in the proportion by weight between about 1:3 and, at the most, 1:1. Said mixture is oxidized by means of oxygen or oxygen-containing gases, such as air, at a temperature between about 80 C. and about 250 C. and preferably between about 140 C. and about 160 C. at ordinary or increased pressure for a prolonged period of time, at least for about four hours. Thereby first all the xylenes are converted into toluic acids. At the same time the toluic acid esters present in the starting mixture are, at least partly, oxidized to phthalic acid monoesters. The resulting toluic acids sufier intermediarily an ester radical interchange reaction with the phthalic acid monoesters formed on said oxidation so that toluic acid esters are produced. The resulting mixture of toluic acid esters and phthalic acids is then further oxidized until as much toluic acid ester is oxidized as corresponds to the toluic acids which were obtained by the initial xylene oxidation. In this manner a degree of oxidation is achieved which permits to produce, after subsequent esterification of the resulting oxidation mixture, 40% to 60% of diesters of phthalic acids, calculated for the total amount of oxidation products.

As stated above, the oxidation is carried out at elevated temperature between about 80 C. and about 250 C. and preferably between about 140 C. and about 160 C. It is possible to operate at atmospheric pressure. Preferably, however, oxidation is effected under pressure. A pressure between about 1.5 atm. and about 5.0 atm.. has proved to be sufficient in order to obtain satisfactory results. A higher pressure may, of course, also be used although it requires more complicated apparatus and a careful supervision of the reaction. Oxidation is preferably effected in the presence of catalysts. Especially suitable catalysts are compounds of multivalent metals, i.e. of metals which, like cobalt or manganese, occur in several stages of valency. Such metal compounds atcnt O must be soluble in the reaction mixture. Especially suitable are, for instance, salts of cobalt with fatty acids having 6 to 12 carbon atoms, such as the first runnings of the distillation of coconut oil and the like fatty acids or cobalt salts with aromatic carboxylic acids, such as toluic acids or benzoic acid.

The process according to the present invention can be carried out in batches as well as in continuous operation. When carrying out the process in continuous operation, it is advisable to effect oxidation in a number of apparatus units in order to ensure proper proceeding of the reaction. In continuous operation care must be taken that initially oxidation is carried out in a xylene. contalmng phase and that subsequently the intermediate oxidation mixture is further oxidized in a xylene-free phase.

One of the advantages of the simultaneous oxidation of xylenes and toluic acid esters according to the present invention consists in yielding a mixture of oxidation products at the end of the oxidation which mixture contains unreacted toluic acid esters of a considerably higher dissolving power for the oxidation products than possessed by the xylenes.

Another advantage of the simultaneous oxidation of xylenes with an excess of toluic acid esters according to the present invention consists in the decrease in the formation of by-products which are usually obtained in oxidation of xylenes.

Still another advantage of the process according to the present invention consists in a considerable acceleration of the reaction.

In principle, the oxidation of p-xylene to p-toluic acid in the presence of the methyl ester of p-toluic acid is carried out as follows:

When operating in batches, a solution of p-xylene in the methyl ester of p-toluic acid, for instance, a solution containing 35 parts by weight of xylene and 65 parts by Weight of the ester, is subjected to oxidation by means of air in the presence of a cobalt compound. The gases escaping from the reaction vessel are cooled and the condensate obtained thereby is separated from the water present therein and is returned to the reaction vessel. When operating at a temperature between about C. and about C. under slightly increased pressure, for instance, under a pressure between about 1.5 atm. and about 3.0 atm., initially oxidation of p-xylene proceeds slightly more rapidly than that of the methyl ester of p-toluic acid. However, as soon as the xylene concentration has been reduced considerably, vigorous oxidation of the methyl ester of p-toluic acid sets in. Oxidation is continued until an amount of the methyl ester of p-toluic acid has been oxidized to the monomethyl ester of terephthalic acid which corresponds substantially to the molal amount of p-toluic acid that has been formed by oxidation of p-xylene. Said amount can readily be determined by analysis of samples taken from time to time. The resulting oxidation mixture which is free of xylene is then esterified with methanol, the dimethyl ester of terephthalic acid formed thereby which is insoluble in methanol is filtered ofi, excess methanol is removed, and the non-oxidized methyl ester of p-toluic acid, the amount of which corresponds approximately to that charged initially, is again subjected to oxidation after addition of p-xylene in an amount sufficient to maintain the xylene-p-toluic acid equilibrium, required according to the present invention.

When carrying out the process acccording to the pres ent invention in continuous operation, oxidation is effected according to the countercurrent principle in a reaction tower or in a number of oxidation vessels.

An essential characteristic of the process according to the present invention is the specific proportion of xylene Patented July 14, 1959 to toluic acid ester as well as the considerably prolonged reaction duration. Said reaction duration is dependent upon the reaction temperature, the pressure, the amounts of starting mixture subjected to oxidation, and the amount of oxidation agent introduced. It should not be less than about 4 hours. As stated above, the proportion of xylene to toluio acid ester must be maintained between about 1:3 and, at the most, 1:1 expressed in proportions by weight. With other proportions there is no assurance that the above mentioned yield of phthalic acid ester is obtained at the end of the oxidation procedure.

It is furthermore very important that at the end of the oxidation process considerable amounts of non reacted toluic acid ester are present in the oxidation mixture. Said toluio acid esters serve as solvents or, respectively, distributing agents for the oxidation products produced thereby. The lower the amount of ester remaining as solvent for the oxidation product, after oxidation is completed, the higher is the solidification point of the oxidation mixture and as a result thereof, the more difficult is its transport and handling in the liquid or, respectively, pastelike state.

The amounts of phthalic acids which are formed during the oxidation of the xylenes are of no importance in carrying out the process according to the present invention since they are dissolved or, respectively, suspended in the toluic acid esters present in the oxidation mixture and acting as solvents. Such formed phthalic acids are also esterified and thus separated from the oxidation mixture, after oxidation is completed.

According to the present invention it is, of course, possible to use not only the various isomer xylenes as such but also their mixtures with each other. Likewise, the various isomer toluic acid esters can be used as such or in mixture with each other. starting mixture employed, there are obtained the various isomer phthalic acid esters, after oxidation is completed.

The phthalic acid esters obtained according to the present invention can be separated from the other compounds obtained on oxidation either by distillation or by crystallization. They are useful either as plasticizers or as starting material for polycondcnsation products, for instance, for producing fibers, filaments, foils, films, bristles, and the like plastic materials. Furthermore they are valuable additives to plastics, resins, and the like products.

The following examples serve to illustrate the present invention without, however limiting the same thereto.

Example 1 A mixture of 1.2 kg. of 96% p-xylene and 3.6 kg. of pure methyl ester of p-toluic acid is filled into a pressure-resistant oxidation vessel of liters capacity which is provided with reflux cooler and water separator, 10 g. of the cobalt salt of the first runnings obtained on distillation of coconut oil fatty acids are added thereto as a catalyst. Air is introduced into the mixture at a temperature of 160 C. and a pressure of 5 atmospheres. 1.08 cu. m. of air, measured at atmospheric pressure, are introduced per hour. Oxidation is continued for 8 hours. Thereafter, a sample is taken. it shows that a product with an acid number of 157 and a saponification number of 428 is obtained. The mixture contains about 2.5% of non-oxidized xylene. Oxidation is continued for 4 more hours. A sample shows that the resulting oxidation product has an acid number of 197 and a saponification number of 468 and that all the xylene is oxidized. Oxidation is continued for 8 more hours. The acid number of the oxidation mixture, after hours total oxidation time, has increased to 250. 5.2 kg. of oxidation mixture are obtained.

The mixture is esterified by heating with 20 liters of methanol in the presence of 180 g. of 98% sulfuric acid at 65 C. for 30 hOlllS.' After cooling, the crystallized dimethyl ester of terephthalic acid is filtered ofi, the filtrate Depending upon the i is freed of excess methanol, and the residue is washed free of sulfuric acid. 2.1 kg. of the dimethyl ester of terephthalic acid with a saponification number of 578, an acid number of 0.6, and a melting point of 140.5 C. are obtained in this manner.

The residue consists of 3.6 kg. of the methyl ester of toluic acid of the saponification number of 402 and the acid number of 6. 1.2 kg. of fresh xylene are added to said ester and the mixture is again oxidized in the same manner as described hereinabove after addition of 5 g. of the same catalyst. After 18 hours, the oxidation mixture has an acid number of 288 and a saponification number of 495. The mixture is again esterified as described hereinabove and yields 2.75 kg. of the dimethyl ester of terephthalic acid.

The remaining methyl ester of p-toluic acid is made up to 4.8 kg. by the addition of p-xylene and the mixture is again oxidized as described hereinabove. Oxidation, esterification, separation of the phthalic acid ester, and restoration of the initial proportion of xylene to toluic acid ester by the addition of xylene may be repeated many times.

Example 2 A mixture of 10 kg. of 96% p-xylene and 20 kg. of technical grade methyl ester of p-toluic acid are filled into a pressure-resistant oxidation vessel of a capacity of 60 liters which is provided with reflux condenser and water separator. 60 g. of the cobalt salt of the lower fatty acids obtained as first runnings on distillation of coconut oil fatty acids are added thereto as catalysts. 5.4 cu. m. of air are introduced into the reaction mixture per hour at a pressure of 1.5 atm. and at a temperature of C. After 6 hours the oxidation mixture has an acid number of and a saponification number of 427 and still contains 5.6% of non-oxidized xylene. After 10 hours no more non-oxidized xylene can be determined. After a total oxidation duration of 18 hours, the acid number of the reaction mixture has increased to 305. 32.25 kg. of oxidation mixture are obtained. The mixture is esterified by means of methanol under pressure. The esterification product is added to methanol in order to precipitate the dimethyl ester of terephthalic acid. 14.3 kg. of dimethyl ester of terephthalic acid are filtered 01f. Its acid number is 1.2, its saponification number 576', and its melting point 140 C. The remaining ester which consists mainly of the methyl ester of p-toluic acid is made up to 30 kg. by the addition of p-xylene and the resulting mixture is again oxidized as described hereinabove.

It is, of course, also possible to employ, in place of p-xylene and of the methyl ester of p-toluic acid, equimolecular amounts of o-xylene or m-xylene and of the methyl esters of o-toluic acid or m-toluic acid. Like- Wise, a mixture of xyleneS, such as a commercial product containing, for instance, 66% of m-xylene, 33% of p-xylene, and the remainder being o-xylene and ethyl benzene, as well as mixtures of the methyl esters of mtoluic acid and p-toluic acid with small amounts of otoluic acid may also be employed as starting material. The procedure is the same as described hereinabove in the examples.

Esterification of the oxidation mixture to form the diesters of the resulting phthalic acid and the esters of' toluic acids is effected, for instance, by heating the oxidation mixture with anhydrous methanol in an autoclave at a temperature between about 200 C. and about 250 C. for a period of 2 to 3 hours. Thereafter the autoclave is vented and the same amount of anhydrous methanol is added thereto and heating to a temperature between about 200 C. and about 250 C. is continued for 2 to 3 hours. In this manner, the methyl esters'of said toluic acids and the dimethyl esters of the phthalic acids are obtained with an acid number as low as 10 and less.

Esterification of the oxidation mixture may also be efiected by refluxing the oxidation mixture with an excess of methanol and with the addition of hydrochloric acid or other conventional esterification catalysts whereby the water produced during esterification is continuously removed from the distillate while the methanol is returned into the reaction vessel.

In place of the catalyst used in the preceding examples, there may be employed other catalysts such as oobaltous and manganous caproate, caprylate, caprinate, l'aurate and the like. The concentration of the catalyst employed is comparatively low and is usually in the range of about 0.001% to about 0.1% of cobalt or manganese metal based on the total weight of the reaction mixture.

Of course, many changes and variations in the composition of the starting material and in the proportion of xylene and the lower alkyl esters of toluic acid, in the reaction conditions such as temperature, pressure, and duration, in the catalysts used, in the methods of esterifying the oxidation mixture and of separating the resulting esters, in the manner in which the esterified oxidation mixture is purified and the 'like may be made by those skilled in the art in accordance with the principles set forth herein and the claims annexed hereto.

Although the methyl esters of toluic acids are especially suitable for the oxidation process according to the present invention, it is also possible to use other esters of said acids, for instance, the esters with ethanol, npropanol, isopropanol, n-butanol, isobutanol, tertiary butanol. Likewise, esterification of the oxidation mixture, after oxidation is completed, may be carried out with such other lower alkanols. The procedure is, in general, the same as that described in the preceding examples.

I claim:

1. In a process of producing esters of phthalic acids from mixtures of xylene and esters of toluic acids with monovalent alkanols having 1 to 4 carbon atoms, the steps which comprise oxidizing a mixture of xylenes and esters of toluic acids with alkanols having 1 to 4 carbon atoms in the proportion by weight between about 1:3 and, at the most, 1:1 in the liquid phase with an oxygen-containing gas at a temperature between about 80 C. and about 250 C. in the presence of an oxidation catalyst for a prolonged period of time until the xylenes are substantially completely oxidized and continuing oxidation of the resulting intermediate, sub stantially xylene-free oxidation mixture until as much toluic acid ester is oxidized as corresponds to the amounts of toluic acids formed on xylene oxidation.

2. The process according to claim 1, wherein oxidation is carried out at a temperature between about 140 C. and about 160 C.

3. The process according to claim 1, wherein oxidation is carried out under pressure.

4. The process according to claim 1, wherein a mixture of xylenes and the methyl esters of toluic acids is used as starting material.

5. The process according to dation catalyst is the cobalt having 6 to 12 carbon atoms.

6. In a process of producing esters from mixture of xylenes and esters of monovalent alkanols having 1 to 4 carbon atoms,

claim 1, wherein the oxisalt of an alkanoic acid of phthalic acids toluic acids with the steps which comprise oxidizing a mixture of xylenes and esters of toluic acids with monovalent alkanols having 1 to 4 carbon atoms in the proportion by weight between about 1:3 and, at the most, 1:1 with an oxygen-containing gas at a temperature between about C. and about 250 C. in the presence of an oxidation catalyst for a prolonged period of time until the xylenes are substantially completely oxidized, thereafter continuing oxidation of the resulting oxidation mixture until an amount of toluic acid esters is oxidized which corresponds to the amount of toluic acids formed on xylene oxidation, esterifying the resulting oxidation mixture, separating and removing the phthalic acid esters formed during oxidation from the toluic acid esters present in the oxidation mixture, adding to said toluic aci esters an amount of xylenes sufiicient to restore the proportion by weight of xylenes and esters of toluic acid to a proportion between about 1:3 and, at the most, 1:1, and repeating oxidation of the mixture of xylenes and esters of toluic acids.

7. The process according to claim 6, wherein oxidation is carried out at a temperature between about C. and about C.

8. The process according to claim 6, wherein oxidation is carried out under pressure.

9. The process according to claim 6, wherein the catalyst is the cobalt salt of an alkanoic acid having 6 to 12 carbon atoms.

10. The process according to claim 6, wherein the ester of toluic acids is the methyl ester of said acids.

11. The process according to claim 6, wherein oxidation and esterification are carried out in continuous operation.

12. In a process of producing the dimethyl ester of terephthalic acid, the steps which comprise oxidizing a mixture of p-xylene and the methyl ester of p-toluic acid in the proportion by weight between about 1:3 and, at the most, 1:1, at a temperature between about 140 C. and about 160 C. with air under a pressure between about 1.5 atm. gauge and about 5 atm. gauge with the addition of the cobaltous salt of the fatty acids obtained as first runnings in the distillation of coconut oil fatty acids for a prolonged period of time until the xylene is substantially completely oxidized, continuing oxidation of the resulting oxidation mixture until an amount of the methyl ester of p-toluic acid corresponding to the amount of p-toluic acid formed on oxidation of p-xylene, is oxidized, esterifying the reaction mixture by heating with methanol in the presence of an esterification catalyst, cooling the esterification mixture, removing, by filtration, the crystallized dimethyl ester of terephthalic acid, evaporating excess methanol from the filtrate, admixing to the evaporation residue substantially consisting of the methyl ester of p-toluic acid, p-xylene in an amount suflicient to restore the proportion by weight of p-xylene and the methyl ester of p-toluic acid to a proportion between about 1:3 and, at the most, 1:1, and repeating oxidation of the resulting mixture of p-xylene and the methyl ester of p-toluic acid.

References Cited in the file of this patent UNITED STATES PATENTS 2,772,305 Levine et a1. Nov. 27, 1956

Claims (1)

1. IN A PROCESS OF PRODUCING ESTERS OF PHTHALIC ACIDS FROM MIXTURES OF XYLENE AND ESTERS OF TOLUIC ACIDS WITH MONOVALENT ALKANOLS HAVING 1 TO 4 CARBON ATOMS, THE STEPS WHICH COMPRISE OXIDIZING A MIXTURE OF XYLENES AND ESTERS OF TOLUIC ACIDS WITH ALKANOLS HAVING 1 TO 4 CARBON ATOMS IN THE PROPORTION BY WEIGHT BETWEEN ABOUT 1:3 AND, AT THE MOST, 1:1 IN THE LIQUID PHASE WITH AN OXYGEN-CONTAINING GAS AT A TEMPERATURE BETWEEN ABOUT 80*C. AND ABOUT 250*C. IN THE PRESENCE OF AN OXIDATION CATALYST FOR A PROLONGED PERIOD OF TIME UNTIL THE XYLENES ARE SUBSTANTIALLY COMPLETELY OXIDIZED AND CONTINUING OXIDATION OF THE RESULTING INTERMEDIATE, SUBSTANTIALLY XYLENE-FREE OXIDATION MIXTURE UNTIL AS MUCH TOLUIC ACID ESTER IS OXIDIZED AS CORRESPONDS TO THE AMOUNTS OF TOLUIC ACIDS FORMED ON XYLENE OXIDATION.