MXPA99008688A - Process for preparing 2,6-naphthalenedicarboxylic acid - Google Patents

Abstract

A process for producing 2,6-naphthalenedicarboxylic acid by the liquid phase, exothermic oxidation of 2,6-dimethylnaphthalene comprising adding to a reaction zone oxidation reaction components comprising 2,6-dimethylnaphthalene, a source of molecular oxygen, a solvent comprising an aliphatic monocarboxylic acid, and a catalyst comprising cobalt, manganese and bromine components wherein the atom ratio of cobalt to manganese is at least about 1:1 and the total of cobalt and manganese, calculated as elemental cobalt and elemental manganese added to the reaction zone, is less than about 0.40 weight percent based on the weight of the solvent added to the reaction zone;maintaining the contents of the reaction zone at a temperature and pressure sufficient to cause the oxidation of 2,6-dimethylnaphthalene to 2,6-naphthalenedicarboxylic acid and the vaporization of at least a portion of the reaction solvent while maintaining a liquid phase reaction mixture;condensing the vaporized solvent and returning an amount of the condensed solvent to the reaction zone to maintain the amount of water in the reaction zone at no more than about 15 weight percent based on the weight of solvent in the reaction zone;and withdrawing from the reaction zone a mixture comprising 2, 6-naphthalenedicarboxylic acid.

Description

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PROCEDURE FOR PREPARING THE ACID 2,6- NAFTALENDICARBOXILICO FIELD OF THE INVENTION This invention relates to a process for the production of 2,6-naphthalenedicarboxylic acid by the oxidation of the liquid phase of 2,6-dimethylnaphthalene with a gas containing molecular oxygen. More particularly, this invention relates to a continuous process for the production of 2,6-naphthalenedicarboxylic acid in high yield by the oxidation of the liquid phase of 2,6-dimethylnaphthalene with a gas containing molecular oxygen in the presence of low levels. of a catalyst comprising heavy metal and bromide components.

BACKGROUND OF THE INVENTION 2,6-naphthalenedicarboxylic acid (2,6-NDA) is a useful monomer for the preparation of high performance polymeric materials such as polyesters and polyamides. Polyethylene 2,6-naphthalate (PEN) is such a high performance polyester and is REF: 031352 prepares, for example, by the condensation of either 2, 6-naphthalenedicarboxylic acid or dimethyl-2,6-naphthalene dicarboxylate with ethylene glycol. The fibers and films made of PEN have improved thermal and strength properties relative to, for example, fibers and films made of polyethylene terephthalate. High-strength fibers made of PEN can be used to make rim yarn, and PEN-made films are advantageously used to make magnetic recording tape and electronic components. Also, because of their superior resistance to gas diffusion and particularly to the diffusion of carbon dioxide, oxygen and water vapor, films made of PEN are useful for making food containers, particularly food containers of the type commonly referred to as " hot filled ". Polyesters made from mixtures of terephthalic acid and 2,6-naphthalenedicarboxylic acid or dimethyl-2,6-naphthalenedicarboxylate have also been found to have unique and desirable properties such as resistance to gas diffusion, which makes them suitable for manufacturing, for example, of beverage containers or other containers for food products.

In order to prepare high quality polyesters suitable for the aforementioned applications, it is desirable to start with purified 2,6-naphthalenedicarboxylic acid or purified dimethyl-2,6-naphthalenedicarboxylate (DM-2, 6-NDC). Since dimethyl-2,6-naphthalenedicarboxylate is typically prepared by the esterification of 2,6-naphthalenedicarboxylic acid using methanol, a purer form of 2,6-naphthalenedicarboxylic acid provides the purest dimethyl-2,6-naphthalene dicarboxylate. Therefore, it is advantageous to have 2,6-naphthalenedicarboxylic acid of higher purity. The 2,6-naphthalenedicarboxylic acid is more conveniently prepared by the oxidation of liquid phase, heavy metal catalyzed by 2,6-dimethylnaphthalene using molecular oxygen, and particularly air, as the oxygen source for the oxidation reaction. During this oxidation, the methyl substituents on the 2-6-dimethylaphthalene naphthalene ring are oxidized to the carboxylic acid substituents. Methods for oxidizing 2,6-dimethylnaphthalene to 2,6-naphthalenedicarboxylic acid by such a liquid phase reaction are known. For example, U.S. Patent No. 5,183,933 issued to Harper et al. Describes a continuous process for oxidizing 2,6-dimethylnaphthalene to 2,6-naphthalenedicarboxylic acid using high levels of metal catalysts for the oxidation of manganese and cobalt added to the mixture of the oxidation reaction. During the oxidation of the liquid phase of 2,6-dimethylnaphthalene to 2,6-naphthalene dicarboxylic acid using a catalyst comprising cobalt, manganese and bromine components, various by-products are usually produced. For example, trimellitic acid (TMLA) is produced by the oxidation of one of the rings of the 2-molecule., 6-dimethylnaphthalene. 2-For-il-6-naphthoic acid (FNA) is also produced, a result of the incomplete oxidation of one of the methyl groups of the 2,6-dimethylnaphthalene molecule. Bromination of the naphthalene ring during the oxidation reaction results in the formation of bromonaphthalenedicarboxylic acid (BrNDA). Additionally, the loss of a methyl (or carboxylic acid) substituent during the oxidation reaction results in the formation of 2-naphthoic acid (2 -NA). These side products, as well as a collection of other unidentified by-products, are undesirable because they contaminate the product of 2,6-naphthalene dicarboxylic acid. It has also been determined that when high levels of catalytic metals are used to oxidise 2,6-dimethylnaphthalene to 2,6-naphthalenedicarboxylic acid, these metals tend to remain with the 2,6-naphthalenedicarboxylic acid product making it difficult to purify 2,6-naphthalenedicarboxylic acid in subsequent purification operations. For example, when 2-6-naphthalenedicarboxylic acid is esterified to produce dimethyl-2,6-naphthalene dicarboxylate, the catalytic metals, residual in the product of 2,6-naphthalene dicarboxylic acid, contaminate the heat exchangers and other equipment used for the manufacture of dimethyl-2,6-naphthalenedicarboxylate. Also, any catalytic metal removed in such esterification processes usually results in a loss of valuable product because the metals tend to remain complexed for or suspended in 2,6-naphthalenedicarboxylic acid or dimethyl-2,6-naphthalenedicarboxylate. . Therefore, it is desirable to use low levels of metal catalysts for oxidation not only from the cost point of view of these catalysts, but also to reduce the complexity and cost of the downstream purification procedures required to prepare the acid 2, 6- Naphthalenedicarboxylic or dimethyl-2,6-naphthalenedicarboxylate sufficiently pure. The technique requires a procedure for the liquid phase oxidation of 2,6-dimethylnaphthalene suitable for large-scale commercial operations, which can produce 2,6-naphthalenedicarboxylic acid in high yield and which has low levels of impurities, and which create a product which is easily purified. The present invention provides such a method.

BRIEF DESCRIPTION OF THE INVENTION A process for producing 2,6-naphthalenedicarboxylic acid is provided by the exothermic liquid phase oxidation of 2,6-dimethylnaphthalene which comprises adding a reaction zone to the components of the oxidation reaction comprising 2, 6-dimethylnaphthalene, a source of molecular oxygen, a solvent comprising a monocarboxylic acid, aliphatic, and a catalyst comprising components of cobalt, manganese and bromine wherein the ratio of cobalt to manganese atoms is at least about 1: 1 and the total cobalt and manganese, calculated as elemental cobalt and elemental manganese, is less than about 0.40 weight percent based on the weight of the solvent added to the reaction zone; maintaining the contents of the reaction zone at a temperature and pressure sufficient to cause the oxidation of 2,6-dimethylnaphthalene to 2,6-naphthalene dicarboxylic acid and vaporization of at least a portion of the reaction solvent while maintaining a mixture of liquid phase reaction; condensing the vaporization solvent and returning a quantity of the condensed solvent to the reaction zone to maintain the amount of water in the reaction zone at no more than about 15 weight percent based on the weight of the solvent in the reaction zone; and withdrawing from the reaction zone a mixture comprising 2,6-naphthalenedicarboxylic acid. A process for producing 2,6-naphthalenedicarboxylic acid by the exothermic oxidation of the liquid phase of the 2,6-dimethylnaphthalene in a reaction mixture comprising a carboxylic acid, aliphatic, low molecular weight and water, a catalyst comprising the cobalt and manganese components and a molecular oxygen source comprising maintaining the reaction mixture in a reaction zone at a temperature and pressure sufficient to cause the oxidation of 2,6-dimethylnaphthalene to 2,6-naphthalenedicarboxylic acid; withdrawing from the reaction zone a mixture of the reaction product comprising 2,6-naphthalene dicarboxylic acid and reaction mother liquor; adding water and aliphatic, low molecular weight carboxylic acid to the reaction mixture of the product removed from the reaction zone to form a mixture of the diluted reaction product; and separating the 2,6-naphthalenedicarboxylic acid from the reaction mixture of the product, diluted.

DETAILED DESCRIPTION OF THE INVENTION The oxidation reaction in the process of this invention is a liquid phase reaction wherein a catalyst comprising components of cobalt, manganese and bromine is used to catalyze the oxidation of the methyl substituents in 2,6- dimethylnaphthalene to carboxylic acid substituents. A gas containing molecular oxygen supplies the oxygen for the oxidation reaction and also produces water and carbon oxides. The reaction is conducted typically and preferably in a continuous manner wherein the reaction components comprising the 2,6-dimethylnaphthalene starting material, catalytic components, the molecular oxygen source and the solvent are continuously added to an area of the reaction of oxidation under reaction conditions and predetermined addition rates. In a continuous oxidation process, a mixture of the reaction product containing the desired 2,6-naphthalenedicarboxylic acid is typically and continuously removed from the reaction zone. During the start of a continuous oxidation process, the composition of the reaction mixture in the zone of the oxidation reaction changes as the reaction proceeds. However, after a period of time, permanent state conditions are achieved and the composition of the reaction mixture in the reaction zone becomes constant, that is, conditions commonly called "traced" are obtained. Due to its insolubility, most of the acid product 2, 6-naphthalenedicarboxylic acid is typically in solid form in the reaction mixture in the form of a slurry and can be separated from the liquid part of the reaction mixture of the product, commonly called the mother liquor of the oxidation reaction, by any Appropriate method for dividing solids from liquids. Before separating the mother liquor from 2,6-naphthalenedicarboxylic acid, the slurry of the reaction mixture is preferably cooled in one or more catalyst vessels, preferably varying in series, to crystallize the 2,6-naphthalenedicarboxylic acid dissolved in the mother liquor of the oxidation reaction whereby the recovery of the desired 2,6-naphthalenedicarboxylic acid is maximized and also to reduce the temperature of the oxidation reaction mixture such that the 2,6-naphthalenedicarboxylic acid contained therein is You can separate using conventional separation equipment. The preferred hydrocarbon feedstock for the continuous oxidation process of this invention is 2,6-dimethylnaphthalene. This raw material can be isolated from refinery streams containing naphthalene including those commonly called tar fractions, or from one or more of several "distillation residues" produced during crude oil refining procedures. However, the concentration of 2,6-dimethylnaphthalene in these refinery streams is generally low and it is therefore difficult to obtain large quantities of the desired raw material of 2,6-dimethylnaphthalene suitably. An alternative and currently preferable source of 2,6-dimethylnaphthalene is one or more of the known synthetic processes for the preparation of 2,6-dimethylnaphthalene. One such pathway starts with o-xylene and butadiene wherein o-xylene is alkenylated in the liquid phase with butadiene in the presence of an alkali metal catalyst such as sodium and / or potassium to form 5-ortho-tolyl pentene. Such an alkenylation reaction is described in U.S. Patent No. 3,953,535 issued to Shima et al. The 5-ortho-tolyl pentene is subsequently cyclized to form the 1,5-dimethyltetraline, which is then dehydrogenated to form the 1,5-dimethylnaphthalene. The 1,5-dimethylnaphthalene is isomerized to form 2,6-dimethylnaphthalene which can be isolated as a solid product. A suitable process for conducting these cyclization, dehydrogenation and isomerization reactions is described in U.S. Patent No. 4,950,825 issued to Sikkenga et al. Another process for preparing 2,6-dimethylnaphthalene starting from m-xylene, propylene and carbon monoxide is described in U.S. Patent No. 5,023,390 issued to Takafumi et al. Any method or process for preparing or isolating 2,6-dimethylnaphthalene is suitable as a source of the 2,6-dimethylnaphthalene used in the process of this invention. Preferably, the 2,6-dimethylnaphthalene is about 98.5% to the monkeys, and most preferably at least about 99% pure, by weight. However, it has surprisingly been determined that the process of this invention can be used to oxidise 2,6-dimethylnaphthalene of still relatively low purity, for example, 2,6-dimethylnaphthalene having purity as low as about 85% in weigh. Thus, one of the advantages of the invention is the ability to use low purity 2,6-dimethylnaphthalene. The molecular oxygen source employed in the liquid phase oxidation in the process of this invention can vary from pure oxygen to a gas containing about 0.1 weight percent molecular oxygen, with the remaining gas being a stabilizing gas such as nitrogen which is inert in the oxidation of liquid phase. More preferably, for reasons of economy, the source of molecular oxygen is air. In order to avoid the formation of explosive mixtures, the gas containing molecular oxygen that is introduced into the reaction zone must be added in an amount such that the mixture of exhaust gas leaving the reaction zone contains approximately 0.5 to 8 volume percent oxygen measured in a solvent-free base. The solvent used for the liquid phase oxidation reaction comprises a carboxylic acid, aliphatic, low molecular weight having 1 to 6 carbon atoms, a mixture of two or more such low molecular weight carboxylic acids, or a mixture of one or more such low molecular weight carboxylic acids with water, for example, about 1 to about 10 weight percent water. Suitable solvents include, for example, acetic acid, propionic acid, n-butyric acid and mixtures of one or more of these acids with water. Preferably, due mainly to cost and availability, the oxidation solvent added to the reaction mixture comprises acetic acid containing water, for example about 1 to about 10, and preferably about 5 percent by weight of water. Additionally, water is formed as a product of the oxidation reaction.

The oxidation reaction is an exothermic reaction and the heat generated is partly dissipated by the vaporization of the solvent from the oxidation reaction. Typically, a pon of the vaporized solvent or leaving vapors is removed from the reaction zone, cooled to condense the vapor and the cold liquid, resulting in return to the oxidation reaction mixture. The steam is typically cooled and condensed in a condensing vapors. This vapor is a mixture of water and acetic acid when acetic acid is used as the aliphatic monocarboxylic acid solvent. By separating the water from the acetic acid before it returns to the reaction zone, the water level in the reaction zone can be adjusted, to a degree, to lower levels than those that would otherwise develop in the zone. of reaction due to the formation of water during the oxidation reaction. It was found to be advantageous to operate at low water levels in the oxidation reaction mixture, for example, water levels of not more than about 15 weight percent of the total reaction mixture in the reaction zone, in the form of more preferably less than about 10 weight percent. However, it was determined that rather than separating the water from the acetic acid present in the condensed vapor, it is preferable to direct all or a pon, for example, at least about 10 weight percent, more preferably at least about 25 weight percent. by weight and much more preferably at least about 40 weight percent, of such a stream condensed to the slurry of the oxidation reaction after such a slurry is removed from the oxidation reaction zone and preferentially to one or more of the aforementioned crystallizers used to cool the thickened mixture containing 2,6-naphthalenedicarboxylic acid after the thickened slurry is removed from the zone of the oxidation reaction. The addition of such a condensed stream, which contains acetic acid and water, to the slurry of the oxidation reaction gives a purer 2, 6-naphthalenedicarboxylic acid after 2,6-naphthalene dicarboxylic acid is separated from the mother liquor. the oxidation reaction, diluted. In paular, this serves to reduce the levels of catalytic metals and trimellitic acid in the product of 2,6-naphthalenedicarboxylic acid. The condensed stream typically comprises acetic acid and water wherein the weight ratio of acetic acid to water is in the range of about 1.5: 1 to about 8: 1 and more preferably in the range of about 2.3: 1 to about 6.5 :1. The amount of condensed stream added to the slurry of the oxidation reaction removed from the oxidation zone is suitably about 1 to about 200 weight percent of the slurry of the oxidation reaction, preferably about 20 to about 150 weight percent and more preferably about 50 to about 100 weight percent. This addition of the condensed stream to the slurry of the oxidation reaction preferably occurs in a continuous manner, i.e., the condensed stream is continuously added to the slurry of the oxidation reaction as the slurry is removed from the slurry. oxidation reactor. After removing the 2,6-naphthalenedicarboxylic acid from the mother liquor which has been diluted preferentially with the condensed stream, mentioned above, the mother liquor can be treated, typically by distillation, to recover the acetic acid to recirculate it. the mixture of the oxidation reaction.

A pon of the mother liquor can also be made to recirculate to the oxidation reaction mixture. The weight ratio of the solvent of monocarboxylic acid, aliphatic to 2,6-dimethylnaphthalene for the reaction of liquid phase oxidation, that is, the solvent ratio, is suitably in the range of about 2: 1 to about 12: 1, preferably in the range of about 3: 1 to about 6: 1, respectively. The low ratios of the monocarboxylic acid solvent to 2,6-dimethylnaphthalene, ie, 2: 1 to 6: 1, are advantageous because larger quantities of 2,6-naphthalenedicarboxylic acid can be produced per reactor volume. The solvent ratio, as used herein, means the amount of the solvent, by weight, in the slurry of the oxidation reaction removed from the reaction zone divided by the amount, by weight, of 2,6-dimethylnaphthalene. added to the zone of the oxidation reaction. The catalyst used in the liquid phase oxidation according to the process of this invention comprises cobalt, manganese and bromine components. Each of the components of cobalt and manganese can be provided in any of their known ionic or combined forms that provide soluble forms of cobalt and manganese in the oxidation reaction solvent. For example, one or more of carbonate, bromide or cobalt acetate and / or manganese tetrahydrate may be employed. The bromine component of the oxidation catalyst is provided by a suitable source of bromine including, for example, elemental bromine, i.e., Br, ionic bromide such as HBr, NaBr, KBr, NH4Br, etc. or organic bromides which are known to provide bromide ions at the operating temperature of the oxidation such as, for example, benzyl bromide, mono- and dibromoacetic acid, bromoacetyl bromide, tetrabromoethane, ethylene dibromide, etc. It has been determined that 2,6-dimethylnaphthalene can be oxidized to 2,6-naphthalenedicarboxylic acid having low levels of FNA, BrNDA and 2-NA using low levels of cobalt and manganese catalytic metals added to the reaction mixture of oxidation - provided that a high ratio of cobalt to manganese is used in the oxidation reaction mixture. Thus, in the process of this invention, the atomic ratio of cobalt to manganese in the oxidation reaction mixture is at least about 1: 1, preferably at least about 2: 1, more preferably at least about 2.5: 1 and much more preferably at least about 3: 1. As used herein, the "atomic ratio" is the atomic ratio of the catalytic components, for example, the ratio of milligram atoms of elementary cobalt to milligram atoms of elemental manganese, or, as discussed below, the milligram atoms of bromine measured as atomic bromine to the total of the milligram atoms of cobalt and the milligram atoms of manganese. The ratio of total cobalt and manganese catalyst metals to 2,6-dimethylnaphthalene added to the reaction mixture in gram atoms of cobalt and manganese (the total cobalt and manganese calculated on the basis of elemental cobalt and elemental manganese) moles of 2,6-dimethylnaphthalene is suitably not greater than about 0.15: 1, preferably not greater than about 0.10: 1 and much more preferably not greater than about 0.06: 1. The total amount of cobalt and manganese, calculated as elemental cobalt and elemental manganese added to the oxidation reaction mixture, is less than about 0.40 weight percent, preferably no greater than about 0.35 weight percent, and much more preferably less than about 0.30 weight percent based on the weight of the solvent added to the oxidation reaction zone. The atomic ratio of the bromine component in the catalyst for the oxidation process of this invention to the total of the cobalt and manganese components, is in the range of about 0.3: 1 to about 0.8: 1, and more preferably about 0.4. : 1 to about 0.7: 1. If the atomic ratio of bromine to total cobalt and manganese exceeds 0.8: 1, a large amount of brominated products such as BrNDA can be formed. The individual catalyst components can be introduced into the reaction zone where the liquid phase oxidation is occurring either separately or in one or more combinations, and these can be introduced in any convenient way, for example, as a solution in water or a mixture of water and the oxidation solvent of monocarboxylic acid, or another suitable solvent. In the process of this invention it is advantageous to remove the solvent from the oxidation reaction mixture by removing at least a portion of the outgoing, condensed vapors, preferably to return all of the outgoing vapor, condensate or condensed liquid to the mixture of the oxidation reaction. As discussed hereinabove, it is advantageous to use the outgoing, condensed vapor to dilute the slurry of the oxidation product leaving the zone of the oxidation reaction. The amount of such solvent removed is preferably an amount which provides a concentration of catalytic metal of cobalt and manganese in the reaction mixture, calculated as elemental cobalt and elemental manganese, of at least about 0.10 weight percent, in the form preferably at least about 0.20 percent by weight, and more preferably at least about 0.30 percent by weight based on the weight of the solvent in the reaction zone. As discussed hereinabove, the removal of the condensed liquid from the outgoing steam also serves to achieve the desired low levels of water in the oxidation reaction mixture. The reaction temperature for the liquid phase oxidation according to the process of this invention is suitably in the range from about 188 ° C (370 ° F) to about 216 ° C. (420 ° F), and preferably in the range of about 193 ° C (380 ° F) to about 213 ° C (415 ° F). Reaction temperatures higher than about 216 ° C (420 ° F) or lower than about 188 ° C (370 ° F) generally cause reduced yields of the desired 2,6-naphthalenedicarboxylic acid. The apparatus used to drive the oxidation reaction can be a deposit reactor (preferably stirred), a piston-type expense reactor, a compartment reactor or a combination of two or more of these reactors. For example, the apparatus may consist of two or three agitated tank reactors, ordered in series. Optionally, a piston-type expense reactor can be used suitably to mix and preheat reagents before they enter the reactor or stirred tank reactors. In operation, the minimum pressure at which the oxidation reaction is maintained is preferably a pressure which will maintain at least 50 percent by weight and more preferably at least 70 percent by weight of the solvent in the reaction zone of oxidation in the liquid phase. When the solvent is a mixture of acetic acid and water, the suitable reaction pressures are from about 0.1 absolute atmospheres to about 35 absolute atmospheres and typically in the range of about 10 absolute atmospheres to about 30 absolute atmospheres. During the oxidation reaction of this invention, 2,6-dimethylnaphthalene can be added to the zone of the oxidation reaction at various rates. The rate at which the 2,6-di-ethylnaphthalene is added is related to the ratio of the solvent and the residence time in the reactor. As mentioned above, the ratio of the solvent is the amount of the solvent, by weight, in the slurry of the oxidation removed from the reaction zone divided by the amount, by weight, of 2,6-dimethyl Inaftalene added to the zone of the oxidation reaction. The residence time in the reactor in minutes is the purged weight of the oxidation reactor in pounds divided by the effluent velocity of the reaction mixture in pounds per minute. The ratio of the solvent and the residence time are related to a value called "total hydrocarbon yield" or HCTP. The HCTP, as used herein, is pound moles of 2,6-dimethylnaphthalene added per cubic foot of the reaction solvent in the reactor per hour, and is a productivity measurement for the oxidation reactor. The HCTP is suitably in the range of about 0.02 to about 0.20, preferably about 0.04 to about 0.16, and more preferably about 0.05 to about 0.16. The mixture of the oxidation reaction produced in the reaction zone during the liquid phase oxidation reaction is preferably removed continuously from the reaction zone typically in the form of a slurry of acid 2, 6 solid naphthalenedicarboxylic acid in the mother liquor of the reaction mixture. The mother liquor typically comprises the low molecular weight monocarboxylic acid solvent, water, dissolved impurities and catalyst components. The deacetylated 2,6-naphthalenedicarboxylic acid is separated from the mother liquor by one or more suitable methods for dividing a solid from a liquid phase such as, for example, centrifugation, filtration, sedimentation, etc. As discussed hereinabove, before this division step, the oxidation reaction mixture can be cooled. The cooling can be carried out by any convenient method, for example, a tube and a cylindrical body-type heat exchanger can be used, or the reaction mixture can be cooled in a vessel equipped with cooling coils or a reactor jacket, cooled , Exterior. Alternatively, the reaction mixture can be added to a vessel at a pressure lower than that used by the oxidation reaction. At reduced pressure, the solvent of the oxidation reaction embols, which cools the reaction mixture. A protruding steam condenser can be used to cool, condense and return the outgoing steam to the container to further assist in cooling. Two or more of these containers may be used in series, each at a somewhat lower temperature than the previous vessel, to cool the reaction mixture in a stepwise manner. The oxidation reaction mixture is typically cooled to about 121 ° C (250 ° F) or below before splitting 2,6-naphthalenedicarboxylic acid from the mother liquor of the oxidation reaction. After the mixture of the oxidation reaction leaves the zone of the oxidation reaction, but before the division of the 2,6-naphthalenedicarboxylic acid from the mother liquor, it is also desirable to put the reaction product mixture in contact again with a gas containing oxygen in the absence of 2,6-dimethylnaphthalene recently added. This treatment of the oxidation reaction mixture with a gas containing molecular oxygen in the absence of recently added 2,6-dimethylnaphthalene can be conducted at any time after the reaction mixture leaves the zone of the oxidation reaction. , and can be conducted in any suitable manner whereby the molecular oxygen containing gas is contacted with the reaction mixture at an elevated temperature and preferably at a temperature in the range of about 66 ° C (150 ° F) ) at approximately 232 ° C (450 ° F). However, more preferably, the oxidation reaction mixture, as it leaves the zone of the oxidation reaction, is contacted directly with an oxygen-containing gas in one or more suitable reactor vessels such as a reactor. of deposit or a compartment reactor. Advantageously, a tank reactor is used, with or without an agitator, and the gas containing molecular oxygen is sprayed into the reactor, preferably at a point at the bottom of the reactor. The temperature is suitably in the range of about 177 ° C (350 ° F) to about 232 ° C (450 ° F). Although the rate of introduction of the oxygen-containing gas is not critical, there must be sufficient molecular oxygen present to oxidize the FNA formyl group within a residence time of about 0.25 hours to about 2 hours at the temperature used. As described above, the composition of the exhaust gas must be controlled to prevent the formation of explosive mixtures. It is also possible to treat the reaction mixture with the oxygen-containing gas when the oxidation reaction mixture must be cooled, as described above. In this way, for example, while the reaction mixture is maintained at a reduced pressure to provide cooling of the reaction mixture, the oxygen-containing gas is sprayed through the reaction mixture. The oxygen-containing gas may contain from about 0.1 weight percent molecular oxygen to pure oxygen, with the remaining gas being an inert, stabilizing gas such as nitrogen. In one embodiment of the present invention water and preferably water and acetic acid (or other low molecular weight aliphatic carboxylic acid) are added to the effluent from the oxidation reaction zone in order to increase the solubility of the catalyst metals of the oxidation, trimellitic acid and the products that are formed by the formation of trimellitic acid complexes with the catalytic metals of the oxidation of cobalt and manganese. If the optional oxygen-containing gas treatment is used, the addition of water or a combination of water and acetic acid can occur either before or after optional treatment with the oxygen-containing gas. The addition of acetic acid and water decreases the amount of metals and trimellitic acid that would otherwise be incorporated into the acid., 6-naphthalenedicarboxylic when it is divided, in the manner described hereinabove, of the mother liquor. The amount of acetic acid (or other carboxylic acid, aliphatic, low molecular weight) and water added to the slurry of the effluent from the oxidation reaction zone is an amount that provides a purer form of the acid 2,6- Naphthalenedicarboxylic acid after dividing it or separating it from the oxidation reaction mixture. The acetic acid and water added to the slurry of the effluent is suitably from about 1 to about 200 weight percent of the slurry, preferably from about 20 to about 150 weight percent and more preferably about 50 to about 100 weight percent. The weight ratio of acetic acid to water added to the slurry of the effluent is suitably from about 0.1: 1 to about 10: 1, more preferably from about 0.2: 1 to about 7.1. As described hereinabove, a preferred source of water and acetic acid for adding the slurry to the effluent of the oxidation reaction is the mixture of acetic acid and water resulting from the condensation of the vapors projecting from the oxidation reaction. liquid phase. The source of acetic acid and water can also be obtained from a purifier or an absorber used to remove the acetic acid from that part of the steam leaving the oxidation reactor which does not condense in the condenser of the outgoing steam. In this purifier or absorber, water is used to remove or purify the acetic acid from the steaming vapors of the non-condensed, gaseous oxidation reactor. Although other sources of water, such as deionized water, and other sources of acetic acid, such as fresh acetic acid, can be added to the slurry leaving the oxidation reactor, it is advantageous to use the water and the acetic acid of the absorber or of the steam leaving the oxidation reactor, condensed because such a procedure does not require the use of solvent sources from the outside of the process and also because no additional water is added to the process that must be separated from the acetic acid in order to be possible the reuse of valuable acetic acid. The acetic acid and water are preferably added continuously to the slurry of the effluent. In another embodiment, the 2,6-naphthalenedicarboxylic acid, after its separation from the mother liquor of the reaction mixture, can be redispersed or re-mixed in a suitable solvent such as water, a low-weight carboxylic acid or a mixture of water and a low molecular weight carboxylic acid in a weight ratio of about 0.1 to about 1 part of 2,6-naphthalenedicarboxylic acid by the solvent. Preferably, at least a portion of the solvent used to re-disperse or reconstitute the 2,6-naphthalene dicarboxylic acid in this manner is the condensate product of the vapor leaving the oxidation reaction mixture. After this step is remixed, the 2,6-naphthalenedicarboxylic acid can be separated from the solvent in the manner described hereinabove. The remixing step provides a purer 2,6-naphthalenedicarboxylic acid. The separate solvent comprising water and acetic acid can, for example, be returned, at least in part, to the oxidation reactor or it can be distilled, at least in part, to recover the acetic acid to recirculate to the oxidation reactor. The mother liquor that separates from the oxidation reaction mixture contains more of the oxidation metal catalytic components. However, the mother liquor also contains byproducts of undesirable reaction such as trimellitic acid. However, this mother liquor is valuable because it can be recirculated, either before or after dilution as described hereinabove, to the zone of the oxidation reaction as a source of acetic acid and, more importantly, as a source of catalytic, active metals. The mother liquor may be recirculated to the oxidation reaction zone in an amount in the range of about 1 weight percent of the mother liquor to about 100 weight percent. Preferably, about 5 to about 50 weight percent of the mother liquor is recirculated, the remaining portion is typically treated to recover the acetic acid and catalytic metals to recirculate to the oxidation reaction mixture. It has also been determined that a preferred method for recirculating the catalytic metals valuable to the oxidation reaction zone comprises removing metals from the mother liquor using methods known to those of skill in the art, such as carbonate precipitation, precipitation. of oxalate or by ion exchange processes such as that described in U.S. Patent No. 4,162,991. The procedures of Mobile Process Technology, Inc. to remove catalytic metals from the process stream are also suitable. Additionally, the mother liquor can be concentrated to recover the acetic acid solvent and the residue containing the oxidation catalyst metals can be incinerated. The cobalt and manganese catalytic metals from the resulting ash can be recirculated to the reaction mixture.

The present invention will be understood more clearly from the following examples. However, it is understood that these examples are presented only to illustrate the embodiments of the present invention and are not proposed to limit the scope thereof.

EXAMPLES The continuous oxidations described in Examples 1 to 4 in Table I were conducted in a titanium plated pressure reactor, equipped with an agitator and condensers of the outgoing steam and lines to add the reaction components and remove the product mixture, as well as a portion of the product stream condensed from the outgoing steam. The reactor was maintained at a level of approximately 55% full-thickness vented suspension. The oxidation raw material of 2,6-di-ethylnaphthalene was maintained above 110 ° C (230 ° F) to maintain it in the liquid state, and the upper level of the slurry in the ventilated reactor was added to the reactor below. The solvent (acetic acid containing about 5 weight percent water) and the catalyst components (as a solution in aqueous acetic acid) and recirculated mother liquor (for Examples 1-3) obtained from the separation of the acid solids 2,6-naphthalenedicarboxylic was also pumped into the reactor at controlled rates to achieve the values set forth in Table I. Compressed air was added at a rate to achieve approximately 2.5-3.5 volume percent oxygen in the gas stream of reactor output. The pressure in the reactor was adjusted to maintain the desired reaction temperature, typically 209 ° C (408 ° F), while allowing the heat of the reaction to be removed by means of vaporization of the solvent. The vaporized solvent was condensed in the outgoing steam condensers and returned to the oxidation reactor as reflux. The reaction mixture in the form of a slurry of 2,6-naphthalenedicarboxylic acid in the mother liquor was continuously removed from the reactor and directed to a series of crystallizers where the temperature was reduced upon release of the pressure. The 2,6-naphthalenedicarboxylic acid was separated from the mother liquor of oxidation using a centrifugal machine. For Examples 1-2, water was added to the slurry in the crystallizer to dissolve the catalytic, residual, and organic, soluble impurities. This water was added either as a direct addition of deionized water or as a portion of the bottom stream of the absorber which was used to remove residual acetic acid from the malodorous gas from the reactor by means of water purification. For examples 3 and 4, in addition to the water from the absorber, a portion of the product stream condensed from the vapors projecting from the reactor to the crystallizers was also sent to provide water and acetic acid to dissolve and dilute the catalytic, residual metals and impurities inorganic, soluble solids of 2,6-naphthalenedicarboxylic acid. The data shown in Examples 1-4 were obtained as average values for periods of time from about 1 week to 1 month of operation. The products of the organic reaction were sampled after solid / liquid separation and drying. The organic reaction products were analyzed using liquid chromatography. The catalytic metals and bromide concentrations were measured by X-ray fluorescence spectroscopy. A retrograde stream of malodorous gas from the reactor was also continuously analyzed to determine the concentrations of solvent-free malodorous gas of oxygen and carbon dioxide. The recent cobalt and manganese catalyst components were added as an aqueous solution of their hydrated acetate salts, ie, Co (OAc) 2 • 4H20 and Mn (OAc) 2 • 4H20. Bromide was added as an aqueous solution of hydrobromic acid. In Examples 1-9, the NDA is 2,6-naphthalenedicarboxylic acid, the DMN is 2,6-dimethylnaphthalene, the NDA isomers are other naphthalenedicarboxylic acids, LC means liquid chromatography, XRF means X-ray fluorescence spectroscopy and EGC. means gas chromatography of esterification with which the sample is treated to form the methyl ester of any of the carboxylic acid groups present so that the sample can be analyzed by gas chromatography.

TABLE I Effect of the Ratio of Co: Mn and Concentration of Aqua a Based on all the catalyst and the solvent contains the current that feeds the oxidation reactor, excluding the reflux of the condenser from the outgoing steam. b Based on the catalyst and solvent in the slurry of the reactor effluent, excluding any solvent removed from the outgoing steam condenser to reduce the concentration of water in the slurry in the oxidation reactor. c Defined as pounds / hour of the solvent in the slurry of the oxidation reactor effluent divided by the pounds / hour of the DMN feeding the oxidation reactor. d Defined as the molar feed rate of DMN in lb mol / hr divided by the volume of the solvent in the oxidation reactor in pie3. e Defined as the purged weight of the oxidation reactor in lbs divided by the speed of the slurry effluent of the oxidation reactor in Ibs / min. f Measured in a solvent-free base. g The NDA solids were recovered in a centrifugal machine with an undrilled basket, dried in a rotary dryer, and analyzed by means of LC and XRF. h Estimated based on 2.5 x% by weight of 2.7-NDA measured by LC analysis.

The data in Table II show the results of a series of continuous oxidation reactions performed in a manner similar to Examples 1 -. Table I lists the results of a series of continuous oxidation runs conducted using the different ratios of cobalt to manganese catalytic metals and using different amounts of catalytic metals added to the oxidation reaction mixture. These examples demonstrate that 2,6-dimethylnaphthalene can be successfully oxidized to 2,6-naphthalenedicarboxylic acid using low levels of catalytic metals. This is demonstrated by a comparison of the results for Example 1 with the results of Examples 2-4. In Example 1, the ratio of total catalytic metals (ie, cobalt and manganese) to 2,6-dimethylnaphthalene was 0.114 while in Examples 2-4, the ratio ranged from 0.098 to one below 0.031. The product analysis showed that, except for the TMLA, the amount of by-products were approximately the same or in most cases were lower for Examples 2-4 compared to Example 1. In this way, acid 2, 6-naphthalenedicarboxylic acid was produced using a substantially reduced amount of the catalyst for the amount of oxidized 2,6-dimethylnaphthalene. The results for Examples 3 and 4 demonstrate the benefits of removing the oxidation reaction mixture from a portion of the vaporized oxidation solvent produced during the exothermic oxidation reaction. In these two examples approximately one-half of the solvent added to the reaction mixture was removed by not returning to the reaction mixture of the solvent that was vaporized and condensed. In this way, after the vaporized solvent condensed, only a portion was returned to the oxidation reactor. This procedure was used to adjust the water concentration in the reaction mixture to the low levels reported in Table I.

TABLE l Effect of the Ratio of Co: Mn Based on all the catalyst and the solvent contain the streams that feed the oxidation reactor, excluding the reflux of the condenser of the outgoing steam. Based on the catalyst and the solvent in the slurry of the reactor effluent, excluding any solvent removed from the condenser of the outgoing vapor to reduce the concentration of water in the slurry of the oxidation reactor. Defined as pounds / hr of the solvent in the slurry of the oxidation reactor effluent divided by the pounds / hr of the DMN feeding the oxidation reactor. Defined as the molar feed rate of the DMN in lb mol / hr divided by the volume of the solvent in the oxidation reactor in pie3.

- Defined as the purged weight of the oxidation reactor in lbs divided by the effluent velocity of the slurry of the oxidation reactor in lbs / min. f Measured in a solvent-free base. g Calculated as 100 minus the sum of organic by-product yields measured by LC and EGC analyzes and divided by DMN feed purity.

The mixture of acetic acid and water removed from the oxidation reaction mixture in this manner was added to the crystallization to dissolve the catalytic metals and TMLA and to dilute the mother liquor. In this way, 2,6-naphthalenedicarboxylic acid having low levels of metals was produced. A comparison of these results shows that when using relatively high ratios of cobalt to manganese, that is, ratios of 1: 1 or greater, the amount of metal catalysts that remain with the product of 2,6-naphthalenedicarboxylic acid is greatly reduced . For example, a comparison of the results for Example 5 to Example 9 in Table II shows that when using a cobalt to manganese ratio of 3: 1 the amount of catalytic metals in the filtered 2,6-naphthalenedicarboxylic acid was reduced by approximately 44%. The concentration of TMLA was also substantially reduced. Although these data show that the amount of 2-NA produced in the oxidation reaction is greater for the run of the oxidation reaction with a 3: 1 ratio of cobalt to manganese (Example 5, Molar Reactor Performance data), the 2-NA is removed from the product of 2,6-naphthalenedicarboxylic acid after it was filtered from the mother liquor.

It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention.

Having described the invention as above, the content of the following claims is claimed as property.

Claims (10)

1. A process for producing 2,6-naphthalenedicarboxylic acid by the exothermic oxidation of the liquid phase of 2,6-dimethylnaphthalene, characterized in that it comprises adding to the reaction zone the components of the oxidation reaction comprising 2,6-dimethylnaphthalene, molecular oxygen source, a solvent comprising an aliphatic monocarboxylic acid, and a catalyst comprising components of cobalt, manganese and bromine wherein the atomic ratio of cobalt to manganese is at least about 1: 1 and the total cobalt and manganese, calculated as elemental cobalt and elemental manganese added to the reaction zone, is less than about 0.40 weight percent based on the weight of the solvent added to the reaction zone; maintaining the contents of the reaction zone at a temperature and pressure sufficient to cause the oxidation of 2,6-dimethylnaphthalene to 2, β-naphthalenedicarboxylic acid and the vaporization of at least a portion of the reaction solvent while maintaining a mixture of liquid phase reaction; condensing the vaporized solvent and returning a quantity of the condensed solvent to the reaction zone to maintain the amount of water in the reaction zone in no more than about 15 weight percent based on the weight of the solvent in the reaction zone; and withdrawing from the reaction zone a mixture comprising 2,6-naphthalenedicarboxylic acid.

2. The process according to claim 1, characterized in that the atomic ratio of cobalt and manganese is at least about 2: 1.

3. The process according to claim 2, characterized in that the total cobalt and manganese added to the reaction zone is no more than about 0.35 weight percent based on the weight of the solvent added to the reaction zone.

4. The process according to claim 1, characterized in that the ratio of the total cobalt and manganese catalytic metals to 2,6-dimethylnaphthalene added to the reaction zone in gram atoms of cobalt and manganese to 2,6-dimethylnaphthalene moles is no more than about 0.15: 1.

5. A process for producing 2,6-naphthalenedicarboxylic acid by the exothermic oxidation of the liquid phase of 2,6-dimethylnaphthalene in a reaction mixture comprising a carboxylic acid, low molecular weight aliphatic and water, a catalyst comprising the components of cobalt and manganese where the atomic ratio of cobalt to manganese is at least about 1: 1, and a source of molecular oxygen, characterized in that it comprises keeping the reaction mixture in a reaction zone at a temperature and pressure sufficient to cause oxidation of 2,6-dimethylnaphthalene to 2,6-naphthalenedicarboxylic acid and vaporization of at least a portion of the reaction solvent while maintaining a liquid phase reaction mixture; withdrawing from the reaction zone a mixture of the reaction product comprising 2,6-naphthalenedicarboxylic acid and the mother liquor of the oxidation reaction; adding at least a portion of the vaporized reaction solvent to the product mixture to form a mixture of the diluted reaction product; and separating the 2,6-naphthalenedicarboxylic acid from the mixture of the diluted reaction product.

6. The process according to claim 5, characterized in that the total cobalt and manganese added to the reaction mixture is no more than about 0.35 weight percent based on the weight of the solvent added to the reaction mixture.

7. The process according to claim 5, characterized in that the ratio of total cobalt and manganese catalytic metals to 2,6-dimethylnaphthalene added to the reaction mixture in gram atoms of cobalt and manganese to moles of 2,6-dimethylnaphthalene is no more than about 0.15: 1.

8. The process according to claim 5, characterized in that the carboxylic acid, aliphatic, is acetic acid.

9. The method according to claim 5, characterized in that it is a continuous process.

10. The process according to claim 9, characterized in that the vaporized reaction solvent is condensed to a liquid before adding it to the product mixture, wherein the amount of the condensed reaction solvent added to the product mixture is about 1 to about 200 weight percent of the product mixture, wherein the condensed reaction solvent comprises acetic acid and water, and wherein the weight ratio of acetic acid to water is at least about 1.5: 1. SUMMARY OF THE INVENTION A process for producing 2,6-naphthalenedicarboxylic acid by exothermic liquid phase oxidation of 2,6-dimethylnaphthalene which comprises adding to a reaction zone the components of the oxidation reaction comprising 2,6-dimethylnaphthalene, a source of molecular oxygen, a solvent comprising a monocarboxylic, aliphatic acid and a catalyst comprising the components of cobalt, manganese and bromide wherein the atomic ratio of cobalt to manganese is at least about 1: 1 and the total cobalt and manganese, calculated as elemental cobalt and elemental manganese added to the reaction zone, it is less than about 0.40 weight percent based on the weight of the solvent added to the reaction zone; maintaining the contents of the reaction zone at a temperature and pressure sufficient to cause oxidation of 2,6-dimethylnaphthalene to 2,6-naphthalenedicarboxylic acid and vaporization of at least a portion of the reaction solvent while maintaining a mixture of liquid phase reaction; condensing the vaporized solvent and returning a quantity of the condensed solvent to the reaction zone to maintain the amount of water in the reaction zone in no more than about 15 weight percent based on the weight of the solvent in the reaction zone, and withdrawing from the reaction zone a mixture comprising 2,6-naphthalene dicarboxylic acid.