US2880237A - Process for production of aromatic polycarboxylic acids - Google Patents

Description

PROCESS FOR PRODUCTION OF AROMATIC POLYCARBOXYLIC ACIDS James 0. Knobloch, Hobart, Ind., assignor to Standard Oil Company, Chicago, 11]., a corporation of Indiana No Drawing. Application May 20, 195 Serial No. 660,049.

11 Claims. (Cl. 260-524) This invention relates to a processfor the preparation of aromatic acids, and particularly to a method for oxidizing an alkylated aromatic monocarboxylic acid to produce aromatic polycarboxylic acids. It espectially relates to a process for the oxidation of alkylated benzoic acids to produce terephthalic acid and isophthalic acid.

With the development of various new polyester resins suitable for film and fiber use, and containing aromatic polycarboxylic acids as essential ingredients, a need has arisen for an efficient commercial process for producing the various aromatic polycarboxylic acids. In the past, the preparation of these acids has been ditficult to achieve on a commercial scale, and commercial utilization of the acids, particularly of isophtalic acid and terephthalic acid, has been hindered.

While commercial processes for the oxidation of alkylated benzenes to the corresponding substituted benzoicacids have been known, it has been generally recognized that oxidation of any alkyl group on the aromatic nucleus is dilficult to achieve in the presence of an already formed carboxylic acid group. Thus the art has resorted to various multiple-staged operations in order to accomplish this oxidation successfully, and such processes involving conversion of alkylated benzoic acids to more readily oxidizable intermediates, such as the salts or esters thereof are known. Such processes are complex and expensive, and considerable research effort has been expended in seeking a method for direct oxidation of alkyl substituted benzoic acids to polycarboxylic acids such as isophthalic and terephthalic acid.

It is an object of the present invention to provide an economical and practical method for oxidizing alkyl substituted aromatic acids in the liquid phase with oxygen containing gases to produce aromatic polycarboxylic acids. Another object is to provide a method for carrying out such oxidation reactions in the presence of novel, oxidation-resistant solvents to obtain relatively high yields of the desired polycarboxylic acids. It is a further object of this invention to provide improvements in the process for the preparation of phthalic acids by the oxidation of alkyl substituted benzoic acids e.g. by

the oxidation of the methyl group of toluic acids, with,

Other objects molecular oxygen in the liquid phase. and advantages of the present invention will be' apparent to those skilled in the art from the more detailed description which follows.

It has now been discovered that alkyl substituted this invention, an alkylated benzoic acid may be converted by oxidation in the liquid phase at elevated temperatures with an oxygen containing gas to a dibasic d-.. has qes dq qy. be. 2 3M 19 sa d i United States PatentO a product of the oxidation reaction, or may comprise the neutral ester of an isomer of said acid or neutral esters of mixtures of isomeric acids. For purposes of simplicity and ease of operation, itis preferred that the solvent be an ester of the polycarboxylic acid produced in the oxidation reaction particularly in order to facilitate the recovery of the products of oxidation as will be =further described hereinafter.

The charge storck to the oxidation process comprises an aromatic monocarboxylic acid possessing at least one oxidizable alkyl group as an additional ring substituent. The oxidizable group is one which has at least one hydrogen atom on the carbon atom attached to the aromatic ring. The alkyl group need not be restricted in chain length, but for practical purposes, lower alkyl groups of 1 to 4 carbon atoms are preferred. The charge stock may comprise a singe compound, or may consist of a mixture of alkylated aromatic acids such as mixtures of isomeric toluic acids obtained, for example, by partial oxidation of mixed xylenes. The aromatic acids may contain one or more partially oxidized alkyl substituents such as acetyl groups, etc. which may be effectively further oxidized by the method of the invention. The process may not only be applied to various aromatic acids obtained by oxidation of one alkyl group of a hydrocarbon such as xylene, mesitylene, pseudocumene, durene, cymene and the like, but also to alkylated polynuclear aromatic acids such as methyl naphthoic acid and the like.

The ester solvent utilized in the invention is preferably a neutral lower alkyl ester of the aromatic acid to be produced in the oxidation reaction. Thus in the oxidation of toluic acids it is preferred to utilize lower alkyl ester of phthalic acid, which ester is further preferably an ester of the same isomeric acid to be formed in the oxidation. When p-toluic acid is to be oxidized, the preferred ester is dirnethyl terephthalate. Other esters which will serve equally well are for example, neutral esters of the methyl phthalic acids, neutral esters of tbutyl phthalic acid, trimethyl trimesate, tetramethyl prehnitate, tetramethyl pyromellitate. While the methyl esters are preferred, esters. of other alcohols, for example, aliphatic alcohols of 2 to 8 carbon atoms may be employed. The ester solvent may comprise a single compound, or mixtures of esters may be utilized. The only requirement for the solvent is that its melting point lie below the oxidation temperature, and to this end esters which form eutectic mixtures with each other may be advantageously utilized to insure liquid phase reaction conditions at the oxidation temperature.

While the proportions of alkylated aromatic acid and aromatic ester solvent are not critical, at least 10% by volume of ester solvent should be employed, and preferably a larger amount for example, 20 to volume percent is present. The lower concentrations of ester solvent, can be employed, but the oxidation mixture containing substantial amounts of aromatic dibasic acid may be difiicult to handle due to precipitation of the dicarboxylic acid formed by oxidation. When higher concentrations of oxidizable charge stock are employed, the conversion need not be carried to completion, but the oxidation may be interrupted to yield a solution or slurry of polybasic acid which may be more readily processed for recovery of the desired polybasic aromatic acid.

or semi-continuous manner. tinuous manner, provision may be made for removal of The oxidation step is conducted by contacting a mixture of aromatic diester and alkylated aromatic acid with a free oxygen containing gas at elevated temperature in an oxidation zone to produce a mixture of polybasic aromatic acid and starting ester. Surprisingly, it has been found that the diester solvent is particularly resistant to oxidation, and may be recovered and recycled to the oxidation zone without appreciable loss. The aromatic acids so formed may be separated from the ester solvent or converted to the corresponding esters and recovered as such.

The oxidation step may be conducted in the presence of a known metallic catalyst if desired, although in many cases a catalyst may not be necessary, depending-- upon the resistance of the charged aromatic acid to oxidation.

For this purpose, the compounds of metals having molecular weights from about 50 to about 210, and especially the heavy metals, may be utilized. Compounds 'ofcobalt, vanadium, manganese, chromium, nickel, lead, cerium, barium and the like, are all effective catalysts in the process, the cobalt compounds being preferred. The metallic catalysts may be utilized singly, or as a mixture of one or more metallic compounds. The metals may be utilized in the form of their oxides, or hydroxides in the form of oil soluble compounds. Examples of catalysts which give good results are cobalt salts of organic acids, for example, salts of lower fatty acids, e.g. acetates; of higher fatty acids, e.g. caprylates or linoleats; of cyclic aliphatic acids, e.g. naphthenates; and of aromatic acids, e.g. benzoates or toluates. 'Concentrations in the range of about 0.0010.5% by weight of metal based on total weight of the substituted aromatic acid to be oxidized are generally effective.

As has been mentioned, the mixture of alkyl aromatic acid and diester solvent is contacted inthe liquid phase at elevated temperature with oxygen or an oxygen containing gas, for example with air or air which has been enriched by admixture with oxygen. The oxidation is conducted at temperatures of from about 150 "to about 300 C., depending upon the resistance of the particular feedstock to oxidation, the lower temperatures being preferred to avoid decomposition of the desired dibasic acids which leads to reduced yields of impure products.

It will be recognized by those skilled in the art that time and temperature are interrelated variables, the oxidation at more elevated temperatures generally requiring less time to achieve a yield comparable with that obtained over a longer reaction period at lower temperatures. Generally, a reaction time'of from 2 to 24 hours will be suificient, and the choice of reaction time will be dictated not only by the rate of oxidation, but also by the'degree of conversion desired. The oxidation is advantageously interrupted where the product of reaction is insoluble or partially insoluble in the reaction medium at reaction temperature to avoid formation of thick slurries of dibasic acid which are difiicult to agitate effectively and thus cause inefficient utilization of oxygen in the later stages of the oxidation.

The oxidation can be conducted at atmospheric pressure, or under elevated pressures for example, up to about 100 atmospheres. -The use of elevated pressure is preferred and results in a faster rate of oxidation, thus reducing the time during which the reaction mixture must be kept at elevated temperatures.- It will be realized that reaction temperature and pressure are interrelated variables, the preferred temperature being lower within the range indicated when superatmospheric pressure is employed.

-'The oxidation can be conducted in batch, continuous When operated in a condibasic acid formed during the reaction by cooling and filtering the reactor effluent continuously and for addition of alkyl benzoic acid to the recovered diester solvent which may be recycled to the reactor. When operating in a continuous manner, it may be preferable to operate theo'x'idation reaction so as to obtain only partial conversion of the alkyl benzoic acid feedstock in order to facilitate separation of the dibasic acid and recycle of the ester solvent.

The separation and extraction of the dibasic acids from the oxidation reaction effluent may be carried out in any one'of anumber of ways. Generally the reactor effluent will contain a mixture of some unconverted alkyl benzoic acid feedstock, and the diester solvent, as well as the desired dicarboxylic acid. The reaction mixture may be treated with dilute alkali to remove all acidic constituents,'and*the acids'recovered by treatment of the alkaline solution withstrong mineral acids. The mixture of organic acids which is thus recovered may be further treated, for example, by extraction with hot water, to achieve a separation of the mono basic acids from the less soluble dibasic acids.

Alternatively, the reaction mixture may be esterified by conventional procedures to produce a mixture of esters which may be separated by fractional distillation. Where the dibasic acid formed corresponds to the acid component of the'diester solvent utilized in the oxidation, the esterified mixture may be fractionated to remove esters of mono basic acids, a portion of the residual diester being taken as product, and the remainder recycled directly to the oxidation reactor as solvent for subsequent oxidations. Other methods of recovery of the desired acid will occur to one skilled in the art, for example, by simple filtration 'where the desired acid is insoluble or partially insoluble in the cooled oxidation efiluent, or 'by saponification of the total reactor efiiuent, followed by recovery of a mixture of acids and separation in conventional manner. The latter method is particularly effective Where the dibasic acid formed by oxidation is the same as theacid component of the ester solvent utilized, no further separation of the dibasic acid being required.

The following examples are presented as an illustration of the practice of the process of the invention.

'EXAMPLE 1 A mixture of 3.4 grams m-toluic acid and 30.6 grams dimethylisophthalate with 0.2 wt. percent cobaltous acetate and 0.9 wt."percent yellow lead oxide (PbO) was oxidized in aglass liner installed in a 500 ml. rocker bomb. Oxygen (0.368 mole) was pressured in. The bomb was rocked and the temperature was raised stepwise over a 3 hour period to 200 C., held at 200 C. for 30 minutes, at 225 C.for 30 minutes and at 250 C. for minutes. Maximum pressure was 515 psi. at 250 C. The product was saponified by refluxing with 2.5 g.'KOH in ml.' ethanol; water (100 ml.) was added and alcohol distilled to a boiling point of 99 C. The residue was acidified with 96% sulfuric acid, digested 20 minutes on the steam bath, filtered and washed with water. The solid acid and soluble organic acid (recovered from the filtrate by ether extraction) were combined, dried under vacuum (28.1 g.) and analyzed by a chromatographic technique similar to that of Marvel and Rands (J. Am. Chem. Soc., 1950, 72, 264-2). This indicat ed that 88.1% of the m-toluic acidhad been oxidized;

92.9 of the organic acids had been accounted for.

EXAMPLE 2 500 p.s.i."oxyg"en '(0.106'mole) for six hours at 250 C.

The bomb was cooled, vented and-repressured with oxygen (0.095 mole). -The oxidation was continued for six hours at250 -C. The product was saponified by re fluxing with 200' g. 10% NaOH (aq.). Methanol was distilled 'to-a boiling point of 99 C. The residue was ass as? filtered through fritted glass and the filtrate acidified with sulfuric acid. The suspension was digested on a steam bath. The precipitated acid was filtered and washed with water. This solid acid and the soluble organic acid (recovered from the filtrate by ether extraction) were combined and dried under high vacuum. Analysis revealed that 79.7% of the m-toluic acid had been oxidized to isophthalic acid. A material balance of 91.4% was realized on the organic acids.

EXAMPLE 3 A mixture of 3.0 g. m-toluic acid and 12.0 g. dimethyl isophthalate was oxidized without catalyst in a glass liner installed in a 200 ml. rocker bomb. Two five hour oxidation periods (0.107 and 0.100 mole respectively) at 250 C. and 500 p.s.i. gave a product that on saponification (as in Example 2) revealed that 82.3% of the m-toluic acid had been oxidized to isophthalic acid. The organic acid material balance was 95.0%.

EXAMPLE 4 A series of single stage oxidations was carried out under identical conditions. Each run was made in a glass liner installed in the 200 ml. rocker bomb. The table lists the conditions employed and the results obtained. The products were worked up as in Example 2. It can be seen that both mand p-toluic acids are more easily oxidized than the methyl toluates. It is also apparent from the table that methyl m-toluate has been more easily oxidized than methyl p-toluate contrary to what would have been expected had no solvent been employed.

Table.0xidations in dimethyl isophthalate (DMI) Toluic Acid Methyltoluate oxidant (A') metaparametapara- Mole percent in DMI:

Toluic Acid 26. 3 26. 3 Methy 26. 3 26. 3 Mole Ratio, 022 A 4. 66 4. 66 4. 62 4. 66 Time at 250 0. (hrs.) 5% 5% 5% 5% Wt. percent Cobaltous n-caprylate 0.2 0.2 0.2 0.2 Percent of Toluic acid (or ester) Oxidized 87. 2 92. 9 65. 9 64. 6 Material Balance on organic acid content 96. 4 99. 7 97. 8

Dimethyl isophthalate was employed in the above experiments because of its low melting point (64-65% C.). However, dimethylterephthalate (M.P. 140 C.), dimethyl o-phthalate (liquid), trimethyl hemimellitate (M.P. 100 C.), trimethyl trimellitate (liquid), trimethyl trimesate (M.P. 143 C.), tetramethyl prehnitate (M.P. IDS-109 C.), tetramethyl pyromellitate (M.P. 138 C.), or eutectic mixtures of similar solvents might also be employed for the oxidation of toluic acids or more highly alkylated benzoic acids. The only requirement for a solvent would be that its melting point lies below the oxidation temperature.

The above examples are illustrative only and are not intended to limit the invention in any way. Numerous modifications of the invention will occur to those skilled in the art. The reaction vessel may be in the form of a stirred autoclave or in any other suitable form, such as a tower or a horizontal tubular reactionunit. The process may be operated batchwise or continuously, and in continuous operation the reactants may be passed through the oxidizer concurrently or countercurrently. Any suitable method may be employed for obtaining adequate contact between the oxidizing gas and the liquid reaction medium. Since numerous methods for practicing the invention will occur to those skilled in the art, it will be understood that I do not limit myself except as set forth in the following claims.

What I claim is:

1. A process for the preparation of an aromatic dicarboxylic acid which comprises oxidizing an aromatic monocarboxylic acid having at least one oxidizable alkyl substituent on the aromatic ring in a solvent comprising a neutral lower alkyl ester of a polycarboxylic aromatic acid in the liquid phase with a gas containing molecular oxygen at a temperature of from about 150 C. to about 300 C. and at a pressure of from about 1 to 100 atmospheres.

2. The process of claim 1 wherein the oxidation is carried out in the presence of a metallic oxidation catalyst.

3. A process for the preparation of a benzene dicarboxylic acid which comprises oxidizing a benzene monocarboxylic acid having at least one oxidizable alkyl substituent on the benzene ring in a solvent comprising a neutral lower alkyl ester of a benzene polycarboxylic acid in the liquid phase With a gas containing molecular oxygen at a temperature of from about 150 C. to about 300 C. and at a pressure from about 1 to 100 atmospheres, said mixture containing from about 10% to about by volume of benzene monocarboxylic acid.

4. The process of claim 3 wherein the oxidation is carried out in the presence of from about 0.001 to about 0.5% by weight of a metallic oxidation catalyst based on the weight of the benzene monocarboxylic acid.

5. The process of claim 4 wherein the oxidation is carried out in the presence of a cobalt salt as oxidation catalyst.

6. A process for the preparation of a phthalic acid which comprises oxidizing a toluic acid in the liquid phase with a gas containing molecular oxygen at a temperature of from about 150 C. to about 300 C. and a pressure of from about 1 to atmospheres in the presence of at least 10% by volume of a neutral lower alkyl ester of phthalic acid.

7. The process of claim 6 wherein the neutral lower alkyl ester of phthalic acid is the dimethyl ester.

8. The process of claim 7 wherein the toluic acid is para-toluic acid.

9. The process of claim 7 wherein the toluic acid is meta-toluic acid.

10. In a process for the preparation of terephthalic acid by the liquid phase oxidation of para-toluic acid with molecular oxygen in the presence of a metallic oxidation catalyst at a temperature of from about C. to about 300 C. and at a pressure of from about 1 to 100 atmospheres, the improvement which comprises conducting the oxidation in the presence of from about 10 to 90% by volume of the reaction mixture of dimethyl terephthalate as solvent.

11. In a process for the preparation of isophthalic acid by the liquid phase oxidation of meta-toluic acid with molecular oxygen in the presence of a metallic oxidation catalyst at a temperature of from about 150 C. to about 300 C. and at a pressure of from about 1 to 100 atmospheres, the improvement which comprises conducting the oxidation in the presence of from about 10 to about 90% by volume of the reaction mixture of dimethyl isophthalate as solvent.

Toland Nov. 21, 1950 Toland Nov. 21, 1950

Claims (1)

1. A PROCESS FOR THE PREPARATION OF AN AROMATIC DICARBOXYLIC ACID WHICH COMPRISES OXIDIZING AN AROMATIC MONOCARBOXYLIC ACID HAVING AT LEAST ONE OXIDIZABLE ALKYL SIBSTITUENT ON THE AROMATIC RING IN A SOLVENT COMPRISING A NEUTRAL LOWER ALKYL ESTER OF A POLYCARBOXYL AROMATIC ACID IN THE LIQUID PHASE WITH A GAS CONTAINING MOLECULAR OXYGEN AT A TEMPERATURE OF FROM ABOUT 150*C. TO ABOUT 300*C. AND AT A TEMPERATURE OF FROM ABOUT 1 TO 100 ATMOSPHERES.