US3546274A - Process for preparing esters - Google Patents

Description

United States Patent 3,546,274 PROCESS FOR PREPARING ESTERS Walter L. Borkowski, Media, and William D. Vanderwerif, West Chester, Pa., assignors to Sun Oil Company, Philadelphia, Pa., a corporation of New Jersey No Drawing. Continuation-impart of application Ser. No. 337,547, Jan. 14, 1964. This application Jan. 27, 1967, Ser. No. 612,096

Int. Cl. C07c 69/76 US. Cl. 260-475 8 Claims ABSTRACT OF THE DISCLOSURE Improvements in the preparing of dialkyl esters of naphthalene carboxylic acids from dialkyl naphthalenes in which separation of the intermediate carboxylic acid and the subsequent esters from by-products is facilitated in the respective stages by using at least some lithium chromate as the alkali metal chromate oxidizing agent and carrying out both the oxidation and the esterification under a C0 atmosphere. The separation is facilitated because of the formation of water soluble carboxylic acid salts and water insoluble inorganic salts as by-products in the oxidation step and the formation of alcohol soluble esters and alcohol insoluble by-product salts in the esterification step of the process. The dialkyl naphthalenes can be obtained as a gas oil extract. The oxidation is carried out at about 100 to 300 C. The esterification is carried out with an alcohol of about 1 to 12 carbon atoms and at about 50 to 200 C.

This application is continuation-in-part of Ser. No. 337,547 filed on Jan. 14, 1964, now US. Pat. 3,330,862, by the present inventors based on matter divided out of that application.

This invention relates to a novel process for the preparation of naphthalenedicarboxylic esters, and more particularly an an improved process for the production of 2,6-naphthalenedicarboxylic esters by the oxidation of 2,6-dialkylnaphthalene.

Naphthalenedicarboxylic acids and their lower alkyl esters, particularly dimethyl 2,6-naphthalenedicarboxylate, are useful in the preparation of polyester fibers and films. The monomer acids desired as such or as an intermediate to the esters may be prepared by direct oxidation of either substantially pure 2,6-dimethylnaphthalene, or the more readily obtainable mixtures of alkylnaphthalene isomers such as methylnaphthalenes, dimethylnaphthalenes, trimethylnaphthalenes, ethylnaphthalenes and the like, such as those found in catalytic gas oil fractions obtained by solvent extraction of a catalytic gas oil distillation fraction boiling in the range of from about 485 to 515 F. to form the corresponding carboxylic acids. Thus, for example, one method of preparing naphthalenedicarboxylic acids (sometimes referred to as NDCA) from dialkylnaphthalenes has been to oxidize a dimethylnaphthalene (sometimes referred to as DMN) with sodium dichromate in accordance with the following equation:

While this is generally a satisfactory method, it is nec essary to employ a buffered reaction system since the NaOI-I produced during the oxidation will convert the remaining dichromate to unreactive chromate:

Na2CI'207+2NaOH'- 2Na CrO -i-H O In practice, a convenient buffer is a 50% excess of the dichromate oxidizing agent; the reaction then becomes:

'ice

This excess of dichromate is not only costly, but it thereafter makes the resulting separation of the product and spent reagent, as well as the recovery of the sodium chromate, technically and economically unattractive.

Therefore, it is an object of the present invention to provide a method of oxidizing alkylnaphthalenes, and particularly dialkylnaphthalenes such as 2,6-dimethylnaphthalenes to the corresponding naphthalenecarboxylic acids and particularly the esterification thereof with substantial economy and simplicity of operation over prior processes.

It has now been found, in accordance with the present invention, that these and other objects may be achieved by oxidizing alkylnaphthalenes with a suitable oxidizing agent in the presence of carbon dioxide, the cation of said oxidizing agent having the property of forming a water-soluble salt with the naphthalenecarboxylic acid but not with the resulting carbonic acid, followed by the esterification of said acid with an alcohol of from about 1 to 12 carbon atoms, in the presence of carbon dioxide.

This process is most advantageous in that the carbon dioxide not only functions as the buffering agent, but in addition, it precipitates the selected cation as the insoluble carbonate; thus the salts of the product naphthalenecarboxylic acids are the only water-soluble species present at the completion of the oxidation reaction and their isolation requires only filtration and evaporation of the water and in certain markedly dilferent preferred embodiments in the preparation of the esters, the solubility-insolubility (in alcohol) of product and by-product are also used to facilitate recovery of the ester. While this method is preferably employed in the oxidation of a single alkylnaphthalene, nevertheless, it is entirely suitable in the case of mixed isomers for the preparation of mixed carboxylate salts followed by esterification whereby a mixture of esters obtains.

The cations of the oxidizing agents useful in this process form insoluble carbonates, yet they can be readily recovered and regenerated to the corresponding chromate by known methods for further reuse in the oxidation process.

In one embodiment of this process, the alkylnaphthalene starting material may be oxidized to form the corresponding carboxylate salt in accordance with the following equation:

This reaction is desirably carried out in an aqueous medium at a temperature of about -300 C. and preferably at about 250 C., for about 2-6 hours, at a pH of from about 4 to 8, the carbon dioxide having been introduced under pressure into the reaction vessel before the reaction is initiated. The reaction mixture is then cooled and filtered to remove the precipitated Cr O and Li CO the filtrate is extracted with a suitable organic solvent such as ether, or a low boiling hydrocarbon solvent to remove any unreacted starting material. The remaining filtrate is then boiled for an additional 1-3 hours to convert any soluble LiHCO to Li CO which is then removed by filtration.

The lithium salt may then be directly esterified with an alcohol of about 1 to 12 carbon atoms, as for example with methanol, ethanol, butanol, hexanol, nonanol, and dodecanol, to form the corresponding dialkylnaphtha lenedicarboxylate. This esterification can be carried out in accordance with the following illustrative equation wherein the preferred alcohol, methanol, is used:

This reaction, which is characterized by the recovery of additional lithium carbonate suitable for regeneration to lithium chromate, is carried out at a lower temperature than the original oxidation on the order of 50200 C. preferably at about IOU-150 C., the CO in the specific case of the lithium carboxylate being introduced at a pressure of from 100 to 1000 p.s.i. As in the case of the first step of oxidizing the hydrocarbon to the carboxylate, the reaction is not pressure dependent; however, pressure is needed to contain the required amount of CO as well as the alcohol at the high temperatures employed.

In a further embodiment of this process, it has been found that in an atmosphere of carbon dioxide, if a combination of oxidizing agents is employed in which one of the cations is lithium and the other cation is either sodium, potassium, cesium, or a mixture thereof, the latter named cations will react with the carboxylic acid formed by the oxidation to form the desired Watersoluble carboxylate salt and this in turn will result in facilitated separation. Thus, for example, potassium chromate, sodium chromate, cesium chromate or the like can be utilized in combination with lithium chromate in accordance with the following equation:

where M is potassium, sodium or cesium. The resulting 2,6-naphthalenedicarboxylate salt, or mixture of isomeric naphthalenedicarboxylate salts, depending on the aromatic charge, may then be esterified. In this case the ester is soluble in the alcohol medium used in the estrification but the potassium, sodium or cesium carbonate product in this case, like the lithium carbonate in the first step, is insoluble so that the ester is easily separated and recovered. Of course, it can now be appreciated that the water formed in the esterification is removed as formed to maintain the reaction relatively anhydrous. A clear excess of CO over that required to form the carbonate is required to maintain an acidic medium. The reaction conditions employed when combinations of oxidizing agents are used are essentially as described hereinabove with respect to the first embodiment. Thus following the introduction of carbon dioxide and heating the reaction mixture for from 2 to 6 hours, the precipitated oxides and carbonates are removed by filtration and the filtrate extracted with an ether to remove unreacted starting material. In the case where crude aromatic charge stocks are used as starting materials, other substances such as decalins, tetrahydronaphthalenes, acenaphthenes and the like which may be present are converted to the corresponding tetralones, quinones, etc. and are likewise removed by the ether extraction step. If the initial reaction mixture is heated sufficiently prior to filtering off the precipitates, additional heating to convert the intermediate LiHCO is not necessary, in which case the reaction product may readily be recovered by evaporating the filtrate to dryness.

It will be recognized by those skilled in the art that the amounts of the reactants and reaction conditions will be varied in accordance with the nature of the starting material, depending upon the relative proportions of mono-, diand trialkylnaphthalenes in the charge stock as well as non-naphthalenic materials. It will be appreciated from the foregoing description that one of the unique advantages of this modified dialkylnaphthalene oxidation and esterification process for the preparation of naphthalenedicarboxylic esters is that the precipitated carbonate may be regenerated conveniently, and in some cases at least without prior separation or purification, to reform the original oxidizing agent suitable for reuse. Thus, for example, when the only cation employed is lithium, the resulting mixture of lithium carbonate and chromic oxide may be regenerated by kiln roasting, pressure oxidation or other well-known means to form lithium chromate. Similarly, where cesium, sodium, or potassium dichromate or chromate is employed, the cation which is removed in the form of the carboxylate salt is readily replenished by the addition of the corresponding hydroxide, carbonate or other suitable form to the precipitated solids prior to regeneration, and the lithium, cesium, sodium and/or potassium chromate mixture or the like is recovered. The use of lithium chromate alone is not preferred in the first step because, although it facilitates separation of. the intermediate carboxylic salt due to the insolubility of the M 00 it is more expensive than the corresponding sodium and potassium compounds.

The esterification is conveniently represented and easily understood from the following equation:

where M, as in Equation 6, is sodium, potassium or cesium.

The following are examples given by way of illustration and are not to be regarded as limitations of this invention:

EXAMPLE 1 28.4 g. of a catalytic gas oil extract containing 20% by weight of methylnaphthalenes, 8% ethylnaphthalenes, 53% dimethylnaphthalenes, 9% trimethylnaphthalenes and 10% other aromatics is charged to a one-liter rocking autoclave with 29.3 g. (0.15 mole) of potassium chromate, 58.5 g. (0.45 mole) of lithium chromate and 200 ml. of water. The autoclave is then pressured with 480 p.s.i. of carbon dioxide and subsequently heated at 250 C. for four hours. The crude reaction mixture is filtered, the solid washed with ether and the filtrate extracted with ether. Evaporation of the ether washes and extracts gives 8.4 g. of unreacted hydrocarbons. The solids consist of 49.5 g. of chromic oxide and 10.9 g. of lithium carbonate. The aqueous filtrate is boiled for two hours, then filtered hot to obtain 17.4 g. of precipitated lithium carbonate. Evaporation of the filtrate gives 31.4 g. of potassium salts of mixed naphthalenecarboxylic acids (98.5% of theory) and 5.4 g. of lithium carbonate.

The recovered lithium carbonate is combined with the filter cake from the first filtration, which consists of a mixture of chromic oxide and lithium carbonate, for regeneration of the lithium chromate potassium chromate mixture by roasting with added potassium carbonate.

EXAMPLE 2 27.8 g. of catalytic gas oil extract is charged to a oneliter rocking autoclave with 58.5 g. of lithium chromate (0.45 mole), 57.3 g. of cesium chromate (0.15 mole), and 200 ml. of water. The autoclave is then pressured with 480 p.s.i. of carbon dioxide and then heated at 250 C. for four hours. The crude reaction mixture is boiled vigorously for two hours and filtered hot, the solids washed with ether and the filtrate extracted with ether. The solids consist of 49.5 g. of chromic oxide and 17.4 g. of lithium carbonate. The ether washes and extracts contain 8.3 g. of unreacted hydrocarbons. Evaporation of the aqueous filtrate gives a 98% yield of cesium salts of mixed naphthalenecarboxylic acids containing 4.0 g. of lithium carbonate.

The recovered lithium carbonate is combined with the filter cake from the first filtration, which consists of a mixture of chromic oxide and lithium carbonate, for regeneration of the lithium chromate cesium chromate mixture by roasting with added cesium carbonate.

EXAMPLE 3 To a one-liter rocking autoclave is charged 27.5 g. of an aromatic extract of catalytic gas oil containing approxi mately 90% by weight of a mixture of methyl-, ethyl-, dimethyland tn'methyl-naphthalenes, and 10% other aromatics, 58.5 g. (0.45 mole) of lithium chromate, 200 ml. of water and 480 p.s.i. of carbon dioxide. The autoclave is then heated at 250 C. for four hours, cooled and the reaction mixture filtered. The filter cake and filtrate are both washed with ether. Evaporation of the latter gives 7.5 g. of unreacted hydrocarbons. The aqueous filtrate is boiled vigorously to convert the lithium bicarbonate to insoluble lithium carbonate which is then filtered off hot. The aqueous filtrate is dried to yield 24.9 g. of the lithium salts of the mixed naphthalenecarboxylic acids.

Acidification with hydrochloric acid of the above aqueous filtrate of lithium salts, in lieu of drying the filtrate yields 21.5 g. of mixed naphthalenecarboxylic acids; an additional 2.2 g. of water-soluble acids is recovered from solution by extraction with ether. The total yield of acids is 98.5% of theory.

EXAMPLE 4 31.2 g. (0.20 mole) of 2,6-dimethylnaphthalene is charged to a one-liter rocking autoclave with 24.3 g. (0.15 mole) of sodium chromate, 58.4 g. (0.45 mole) of lithium chromate, 200 ml. of water and 480 p.s.i. of carbon dioxide and heated for four hours at 250 C. The crude reaction mixture is filtered and the solids washed with ether and the filtrate extracted with ether. The filtrate is then boiled vigorously for two hours and filtered hot to remove the precipitated lithium carbonate. Evaporation of the aqueous filtrate yields 69.8 g. of the disodium salt of 2,6-naphtha1enedicarboxylic acid (97% yield based on dimethylnaphthalene reacted).

EXAMPLE 5 To a one-liter rocking autoclave is charged 31.2 gms. 0.20 mole) of 2,6-dimethylnaphthalene, 103.9 gms. (0.80 mole) of lithium chromate, 200 ml. of water, and 480 p.s.i. of carbon dioxide. The autoclave is heated at 250 C. for three hours. The cooled reaction mixture is then filtered and both the filter cake and filtrate are washed with hexane. Evaporation of the washes yields 0.8 gm. unreacted 2,6-dimethylnaphthalene. The aqueous filtrate is boiled vigorously for three hours and filtered hot to recover the pricipitated lithium carbonate. The filtrate from the second filtration is evaporated to yield 43.2 gms. of the lithium salt of 2,6-naphthalenedicarboxylic acid.

EXAMPLE 8 32.4 gms. of the potassium salts of mixed naphthalenecarboxylic acids prepared according to Example 1 are charged to a one-liter rocking autoclave with 150 ml. of methanol. The autoclave is then pressured with about 400 p.s.i. of carbon dioxide and subsequently heated at about 125 C. for three hours. The crude reaction mixture is filtered, and evaporation of the methanol gives a good yield of dimethyl naphthalenedicarboxylate in a high state of purity.

We claim:

1. A process for the preparation of dialkyl esters of naphthalene dicarboxylic acids which comprises oxidizing catalytic gas oil extract containing a mixture of isomeric alkyl naphthalenes with an alkali metal chromate oxidizing agent consisting essentially of a combination of lithium chromate and a member selected from the group consisting of sodium chromate, potassium chromate, cesium chromate, and mixtures thereof, in the presence of carbon dioxide at a temperature of about to 300 C. to form an alkali metal naphthalene-carboxylate salt, esterifying said alkali metal naphthalene-carboxylate salt with an alkanol containing from 1 to about 12 carbon atoms at from 50 C. to 200 C. in the presence of carbon dioxide and recovering a naphthalene-carboxylate ester.

2. A process according to claim 1 wherein the esterification temperature is in the range of about 100-150 C.

3. A process according to claim 1 wherein the carbon dioxide pressure is in the range of about 100 to 1000 p.s.i.

4. A process according to claim 1 wherein the esterification temperature is in the range of about l00150 C., the carbon dioxide pressure is in the range of about 100 to 1000 p.s.i. and the second said chromate oxidizing agent is sodium chromate.

5. A process according to claim 1 wherein the esterification temperature is in the range of about 100-150 C. the carbon dioxide pressure is in the range of about 100 to 1000 p.s.i. and the second said chromate oxidizing agent is potassium chromate.

6. A process according to claim 4 wherein the catalytic gas oil extract is 2,6-dimethylnaphthalene.

7. A process according to claim 5 wherein the catalytic gas oil extract is 2,6-dimethylnaphthalene.

8. A process according to claim 1 wherein the lithium chromate is employed in a mole ratio to said other metal chromate of about 3:1 of said other metal chromates.

References Cited UNITED STATES PATENTS 2,005,774 6/1935 Demant 260--524 LORRAINE A. WEINBERGER, Primary Examiner E. J. SKELLY, Assistant Examiner U.S. Cl. X.R. 260-524