| PATENT NUMBER | This data is not available for free |
| PATENT GRANT DATE | February 27, 2001 |
| PATENT TITLE |
Method of producing aromatic carboxylic acids by oxidizing alkyl aromatic hydrocarbons or partially oxidized intermediates thereof |
| PATENT ABSTRACT | An improved production method of aromatic carboxylic acid products of significantly improved yield and quality by oxidizing alkyl aromatic substrates or their partially oxidized intermediates in a conventional MC-type catalyst system modified to contain additional components such as alkali metal or alkaline earth metal in an acetic acid medium in a feed gas containing oxygen and optionally carbon dioxide. Since carbon dioxide functioned as a co-oxidant along with oxygen in the oxidation reaction, the oxidation reaction proceeds more selectively to produce the carboxylic acid product much faster under milder reaction conditions over the conventional MC-type oxidation. In particular, the oxidation of para-xylene carried out by the novel present method enabled production of terephthalic acid of higher yield and enhanced quality, which were improved far more than the extent that generally could be expected by current PTA producers. The present invention also provides an effective purification process to produce highly pure terephthalic acid or isophthalic acid by the oxidation of impurities such as 4-carboxybenzaldehyde and para-toluic acid or 3-carboxybenzaldehyde and meta-toluic acid contaminated in crude terephthalic acid and isophthalic acid product, respectively. |
| PATENT INVENTORS | This data is not available for free |
| PATENT ASSIGNEE | This data is not available for free |
| PATENT FILE DATE | December 3, 1999 |
| PATENT FOREIGN APPLICATION PRIORITY DATA | This data is not available for free |
| PATENT REFERENCES CITED |
J. Yoo, "Selective Gas-Phase Oxidation at Oxide Nanoparticles on Microporous Materials, " Catalysis Today, vol. 41 (1998), pp. 409-432. J. Yoo, "Gas Phase Oxygen Oxidation of Alkylaromatics over CVD Fe/Mo/Borosilicate Molecular Sieve, Fe/Mo/DBH. VII. Oxidative Dehydrogenation of Alkylaromatics," Applied Catalysis A: General, vol. 142 (1996), pp. 19-29. J. Yoo, "The CVD Fe/Mo/DBH (Deboronated Borosilicate Molecular Sieve) -Catalyzed Oxidation Reactions," Applied Catalysis A: General, vol. 143 (1996), pp. 29-51. J. Yoo, "Gas-Phase Oxygen Oxidations of Alkylaromatics over CVD Fe/Mo/Borosilicate Molecular Sieve. VI. Effects of para-Substituents in Toluene Derivatives ," Allied Catalysis A: General, vol. 135 (1996), pp. 261-271. J. Yoo, et al., "Gas-Phase Oxygen Oxidations of Alkylaromatics over CVD Fe/Mo/Borosilicate Molecular Sieve. II. The Role of Carbon Dioxide as a Co-Oxidant," Applied Catalysis A: General, vol. 106 (1993), pp. 259-273. G. Zajac, et al., "Characterization and Oxidation Catalysis of Alkylaromatics over CVD Fe/Mo/Borosilicate Molecular Sieve: Fe/Mo/DBH," Journal of Catalysis, vol. 151, No. 2, Feb. 1995, pp. 338-348. W. Partenheimer, "Methodology and Scope of Metal/bromide Autoxidation of Hydrocarbons, " J. Chem Soc. Chem. Commun., vol. 23 (1995), vol. 23 pp. 69-158. M. Aresta, et al., "Carbon Dioxide as Modulator of the Oxidative Properties of Dioxide in the Presence of Transition Metal System," J. Chem. Soc. Chem. Commun., 1992, pp. 315-317. |
| PATENT CLAIMS |
What is claimed is: 1. A method of producing an aromatic carboxylic acid, said method comprising the steps of: oxidizing, with a feed gas comprising oxygen and optionally carbon dioxide, an alkyl aromatic compound or a partially oxidized intermediate thereof, using a catalyst comprising (a) cobalt, manganese, and bromine, and (b) an alkali metal or alkaline earth metal dissolved in a solvent comprising an aliphatic carboxylic acid having 1 to 6 carbon atoms. 2. A method according to claim 1, wherein the catalyst further comprises a transition metal or lanthanide metal. 3. A method according to claim 2, wherein the transition metal or lanthanide metal is selected from the group consisting of zirconium, hafnium, cerium, iron, molybdenum, chromium, and tungsten. 4. A method according to claim 2, wherein the feed gas comprises carbon dioxide. 5. A method according to claim 4, wherein the concentration of carbon dioxide is 1% to 80%, by volume, of the feed gas. 6. A method according to claim 5, wherein the concentration of carbon dioxide is 5% to 50%, by volume, of the feed gas. 7. A method according to claim 6, wherein the feed gas lacks an inert diluent. 8. A method according to claim 1, wherein the feed gas comprises carbon dioxide. 9. A method according to claim 8, wherein the concentration of carbon dioxide is 1% to 80%, by volume, of the feed gas. 10. A method according to claim 9, wherein the concentration of carbon dioxide is 5% to 50%, by volume, of the feed gas. 11. A method according to claim 10, wherein the feed gas lacks an inert diluent. 12. A method according to any of claims 4 through 11, wherein the carbon dioxide is added with a gas sparging device into one or more zones of a reactor in a gas phase or liquid phase either periodically, intermittently, or in a continuous manner. 13. A method according to any of claims 4 through 11, wherein the carbon dioxide is mixed into the feed gas, or carbon dioxide remaining in reaction vent gas is recovered by condensation and is recycled to provide carbon dioxide required for the oxidation reaction. 14. A method according to claim 1, wherein the alkyl aromatic compound is selected from the group consisting of para-xylene, meta-xylene, ortho-xylene, pseudocumene (1,2,4-trimethylbenzene), mesitylene (1,3,5-trimethylbenzene), durene (1,2,4,5-tetramethylbenzene), pentamethylbenzene, hexamethylbenzene, dimethylnaphthalene, 4,4'-dimethylbiphenyl, and toluene. 15. A method according to claim 1, wherein the partially oxidized alkyl aromatic intermediate is selected from the group consisting of para-toluic acid, meta-toluic acid, ortho-toluic acid, para-tolualdehyde, meta-tolualdehyde, ortho-tolualdehyde, 4-carboxybenzaldehyde, 3-carboxybenzaldehyde, and 2-carboxybenzaldehyde. 16. A method according to claim 15, wherein the partially oxidized alkyl aromatic intermediate is selected from the group consisting of 4-carboxybenzaldehye, 3-carboxybenzaldehyde, para-toluic acid, and meta-toluic acid. 17. A method according to claim 1, wherein the aromatic carboxylic acid is selected from the group consisting of terephthalic acid, isophthalic acid, phthalic acid, phthalic anhydride, naphthalene dicarboxylic acid, trimellitic acid, trimellitic anhydride, trimesic acid, pyromellitic dianhydride, benzene pentacarboxylic acid, benzene hexacarboxylic acid, 4,4'-biphenyldicarboxylic acid, and benzoic acid. 18. A method according to claim 17, wherein the aromatic carboxylic acid is selected from the group consisting of terephthalic acid, phthalic acid, isophthalic acid, trimesic acid, trimellitic acid, trimellitic anhydride, and pyromellitic dianhydride. 19. A method according to claim 18, wherein the aromatic carboxylic acid is terephthalic acid. 20. A method according to claim 1, wherein the alkali metal or alkaline earth metal is selected from the group consisting of Li, Na, K, Rb, Cs, Be, Mg, Ca, Sr, and Ba. 21. A method according to claim 20, wherein the alkali metal or alkaline earth metal is selected from the group consisting of Na, K, and Cs. 22. A method according to claim 21, wherein the alkali metal or alkaline earth metal is K. 23. A method according to any of claims 1 and 20 through 22, wherein the alkali metal or alkaline earth metal is provided by a metal compound selected from the group consisting of acetate, acetate hydrate, bromide, chloride, fluoride, iodide, carbonate, carboxylate, alkoxide, azide, naphthenate, oxalate, octanoate, acetylacetonate, hydroxide, nitrate, borate, and oxide. 24. A method according to claim 1, wherein the mole ratio of alkali metal or alkaline earth metal to bromine is from 0.001 to 3. 25. A method according to claim 1, wherein the solvent comprises from 2% to 25%, by weight, of water. 26. A process of purifying crude terephthalic acid products or crude isophthalic acid products containing partially-oxidized intermediates of alkyl aromatic compounds as impurities to obtain substantially pure terephthalic acid and isophthalic acid by using the method according to claim 15. 27. A process according to claim 26, wherein the partially-oxidized intermediate impurities are selected from the group consisting of 4-carboxybenzaldehyde and 3-carboxybenzaldehyde. 28. A process comprising the steps of: obtaining the product of a liquid phase oxidation of an alkyl aromatic hydrocarbon using a cobalt-manganese-bromine catalyst; and thereafter, as a post-oxidation step, purifying the product obtained in said obtaining step to remove impurities therein using the method according to claim 1. 29. A method of producing terephthalic acid by oxidizing para-xylene or para-toluic acid, said method comprising the steps of: oxidizing, using a feed gas comprising oxygen and from 5% to 50% by volume of the feed gas of carbon dioxide, para-xylene or para-toluic acid, in the presence of a catalyst prepared by combining cobalt acetate tetrahydrate, manganese acetate tetrahydrate, hydrogen bromide, and potassium acetate. 30. A method according to claim 29, wherein the only source of bromine in the catalyst used in said oxidizing step is one compound, the hydrogen bromide. 31. A method according to claim 29, wherein para-xylene is oxidized in said oxidizing step. 32. A method according to claim 29, wherein para-toluic acid is oxidized in said oxidizing step. 33. A method of producing phthalic acid or phthalic anhydride by oxidizing ortho-xylene, said method comprising the steps of: oxidizing, using a feed gas comprising oxygen and from 5% to 50% by volume of the feed gas of carbon dioxide, ortho-xylene, in the presence of a catalyst prepared by combining cobalt acetate tetrahydrate, manganese acetate tetrahydrate, hydrogen bromide, and potassium acetate. 34. A method according to claim 2, wherein the transition metal or lanthanide metal is selected from the group consisting of zirconium, hafnium, and cerium. 35. A method according to claim 1, wherein the mole ratio of alkali metal or alkaline earth metal to bromine is from 0.05 to 2. |
| PATENT DESCRIPTION |
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention discloses an improved process for oxidizing alkyl aromatic hydrocarbons and/or their partially oxidized intermediates to produce aromatic carboxylic acids. The process involves the liquid phase oxidation in the presence of a catalyst of cobalt-manganese-bromine and alkali metal or alkaline earth metal, in an aliphatic carboxylic acid having 1.about.6 carbon atoms such as acetic acid as a solvent with a feed gas containing oxygen either with or without carbon dioxide. In particular, one or more than one type of alkali metal or alkaline earth metal components are preferably added to the catalyst system, and furthermore, an additional transition metal or lanthanide series metal is introduced to the catalyst system, cobalt-manganese-bromine, when it is deemed necessary. The rate of the oxidation reaction of an alkyl aromatic substrate was remarkably increased in the present process over the conventional MC-type process (i.e., a liquid phase oxidation reaction using a cobalt-manganese-bromine catalyst). The yield and quality of the carboxylic acid product were also significantly improved by the present process. Thus, terephthalic acid of improved yield and purity is produced by carrying out the oxidation of para-xylene in the presence of a catalyst containing the additional components such as potassium and/or transition metal in the co-presence of carbon dioxide with oxygen, at relatively mild reaction conditions. With the present invention, highly pure terephthalic acid or isophthalic acid can be produced by oxidizing impurities such as 4-carboxybenzaldehyde, para-toluic acid, 3-carboxybenzaldehyde, and meta-toluic acid contaminated in crude terephthalic acid and crude isophthalic acid, respectively. 2. Description of the Related Art As discussed below, methods of manufacturing aromatic carboxylic acids are well known and are widely used commercially. For example, a method of manufacturing of aromatic carboxylic acids such as terephthalic acid (TPA), isophthalic acid (IPA), phthalic acid, phthalic anhydride, naphthalene dicarboxylic acid, trimellitic acid, trimellitic anhydride, trimesic acid, pyromellitic dianhydride, 4,4'-biphenyldicarboxylic acid and benzoic acid by oxidizing alkylaromatic compounds or the oxidized intermediates thereof, in the presence of cobalt-manganese-bromine, from such alkylaromatic compounds as para-xylene, para-tolualdehyde, para-toluic acid, 4-carboxybenzaldehyde (4-CBA), meta-xylene, meta-tolualdehyde, meta-toluic acid, 3-carboxybenzaldehyde, ortho-xylene, dimethylnaphthalene, pseudocumene (1,2,4-trimethylbenzene), mesitylene (1,3,5-trimethylbenzene), durene (1,2,4,5-tetramethylbenzene), pentamethylbenzene, hexamethylbenzene, 4,4'-dimethylbiphenyl and toluene is well known (for example, U.S. Pat. Nos. 2,833,816 and 5,183,933). Such aromatic carboxylic acids are used as raw materials for manufacturing polyester after appropriate purification such as hydrogenation, etc. (U.S. Pat. No. 3,584,039). Also, polyester is widely used as a synthetic fiber, film, etc. There were continuous endeavors to develop a catalyst system with high efficiency and enhanced reactivity to manufacture aromatic carboxylic acids. The newly developed technologies, however, were not practical due to the increase of side reactions, price of catalyst, difficulty of operation, and precipitation of catalyst, etc. Improvements of the efficiency of the reaction and the catalyst in the manufacturing of aromatic carboxylic acids are very significant because they may improve productivity, quality and cost competitiveness due to the reduction in the reaction time and side reactions. In other words, it is highly desirable to develop a technology to increase the efficiency of the oxidation reaction of alkyl aromatic compounds and the oxidized intermediates thereof by means of an improvement in the catalyst system or other reaction processes. There were many attempts to increase the efficiency by adding a third metal catalyst to the cobalt-manganese-bromine catalyst system which is the basic catalyst system, to enhance the catalyst efficiency during the manufacturing of aromatic carboxylic acids. The added metals were mainly transition metals, and by adding, for example, hafnium, zirconium, molybdenum, etc., the reactivity therein was increased (U.S. Pat. No. 5,112,992). Further, as an example of an attempt to add an alkali metal component, a catalyst system was used, in which alkali metal components such as lithium, sodium and potassium were added to the cobalt-manganese-nickel-bromine catalyst system, in the presence of two or three types of bromine compounds. There, the method involved manufacturing of terephthalic acid of a monomer grade by a two-step process of oxidation and re-crystallization (WO96/41791). In that method, there is a disadvantage in that the catalyst system is very complicated, since nickel must be added to cobalt-manganese-bromine for the catalyst system and since more than two types of bromine compounds are necessary (both compounds having an ionic bond and those having a covalent bond are needed). The newly developed technologies, however, were not practical due to the increase of the side reactions, price of catalyst, difficulty of operation, and precipitation of catalyst, etc. even though there were many attempts to develop a catalyst system for aromatic carboxylic acids with high efficiency and enhanced reactivity. On the other hand, an oxygen containing gas such as air was mainly used as an oxidant during the manufacturing of aromatic carboxylic acids. Carbon dioxide was not used as an oxidant due to its chemical stability. Yet, in the research for improving the process efficiency, there was a case in which chemically stable carbon dioxide, recycled from the reaction vent gas, was injected to the reactor to increase the stability in the process by mitigating the problematic possibilities of explosion due to oxygen when using pure oxygen or gas containing pure oxygen or oxygen enriched gas as an oxidant (U.S. Pat. No. 5,693,856). Nevertheless, the case is not known in which carbon dioxide was added to improve the reaction efficiency. In summary, the basic oxidation technologies for aromatic carboxylic acids manufacture, especially for TPA manufacture, have been extensively developed. The basic process technology is now approaching a point of diminishing returns, and further major breakthroughs i.e., new catalyst systems, raw materials, and basic unit operations, are not anticipated. The leading PTA (purified terephthalic acid) producers are expected to have greater optimization and energy integration across the entire production complex and more advanced control schemes. However, surpassing the current general expectation, this invention made a remarkable breakthrough to achieve improved catalyst activity and selectivity toward aromatic carboxylic acids, especially for PTA, in the aforementioned catalyst composition under milder oxidation conditions. SUMMARY OF THE INVENTION As a result of research for resolving the above problems, the inventors herein added one or more than one type of alkali metal or alkaline earth metal components (i.e., reagents or compounds comprising alkali metal or alkaline earth metal) to the catalyst of cobalt-manganese-bromine, in which a transition metal or lanthanide metal was also added as deemed necessary, during the manufacturing of aromatic carboxylic acids. The inventors also found that when an appropriate amount of carbon dioxide was added to the oxygen containing gas, which was supplied as an oxidant in the oxidation reaction, the reactivity not only dramatically increased, but the color properties of the product also increased along with reductions in the side reactions and the amount of impurities. Based on such findings, the present invention has been perfected. In view of the foregoing, in one aspect, the present invention relates to a method of producing an aromatic carboxylic acid, the method comprising the steps of oxidizing, with a feed gas comprising oxygen and optionally carbon dioxide, an alkyl aromatic compound or a partially oxidized intermediate thereof, using a catalyst comprising (a) cobalt, manganese, and bromine, and (b) an alkali metal or alkaline earth metal dissolved in a solvent comprising an aliphatic carboxylic acid having 1 to 6 carbon atoms. In another aspect, the present invention relates to a method of producing terephthalic acid by oxidizing para-xylene or para-toluic acid, the method comprising the steps of oxidizing, using a feed gas comprising oxygen and from 1% to 80% by volume of the feed gas of carbon dioxide, para-xylene or para-toluic acid, in the presence of a catalyst prepared by combining cobalt acetate tetrahydrate, manganese acetate tetrahydrate, hydrogen bromide, and potassium acetate. In yet another aspect, the present invention relates to a method of producing phthalic acid or phthalic anhydride by oxidizing ortho-xylene, the method comprising the steps of oxidizing, using a feed gas comprising oxygen and from 1% to 80% by volume of the feed gas of carbon dioxide, ortho-xylene, in the presence of a catalyst prepared by combining cobalt acetate tetrahydrate, manganese acetate tetrahydrate, hydrogen bromide, and potassium acetate. In a still further another aspect, the present invention relates to a process of purifying crude terephthalic acid products or crude isophthalic acid products containing partially-oxidized intermediates of alkyl aromatic compounds as impurities to obtain substantially pure terephthalic acid and isophthalic acid by using an above-discussed method. In a still further aspect, the present invention relates to a method of producing an aromatic carboxylic acid, the method comprising the steps of oxidizing an alkyl aromatic compound or a partially oxidized intermediate thereof, with a gas (e.g., a feed gas or a reaction gas) comprising oxygen and at least 1% by volume of the gas of carbon dioxide, using a catalyst comprising (a) cobalt and manganese and (b) an alkali metal or alkaline earth metal. In another aspect, the present invention relates to a method of producing terephthalic acid by oxidizing para-xylene or para-toluic acid, the method comprising the steps of oxidizing para-xylene or para-toluic acid, with a gas comprising oxygen and at least 1% by volume of the gas of carbon dioxide, in the presence of a catalyst prepared by combining cobalt acetate tetrahydrate, manganese acetate tetrahydrate, hydrogen bromide, and potassium acetate. In yet another aspect, the present invention relates to a method of producing terephthalic acid by oxidizing para-xylene or para-toluic acid, the method comprising the steps of using carbon dioxide as a co-oxidant along with oxygen to oxidize para-xylene or para-toluic acid in the presence of a cobalt-manganese or nickel-manganese catalyst comprising an alkali metal or alkaline earth metal, thereby producing terephthalic acid, wherein the carbon dioxide is present in the gas phase in an amount of at least 1% by volume of the gas phase. In still another aspect, the present invention relates to a method of producing phthalic acid or phthalic anhydride by oxidizing ortho-xylene, the method comprising the steps of oxidizing ortho-xylene, with a gas comprising oxygen and at least 1% by volume of the gas of carbon dioxide, in the presence of a catalyst prepared by combining cobalt acetate tetrahydrate, manganese acetate tetrahydrate, hydrogen bromide, and potassium acetate. In a yet further aspect, the present invention relates to a method of producing an aromatic carboxylic acid, the method comprising the steps of oxidizing an alkyl aromatic compound or a partially oxidized intermediate thereof, with a gas comprising oxygen and an effective amount of carbon dioxide, the effective amount being an amount sufficient to exhibit action by the carbon dioxide as a co-oxidant, using a catalyst comprising (a) at least one transition metal and (b) an alkali metal or alkaline earth metal, thereby producing the aromatic carboxylic acid. In another aspect, the present invention relates to a liquid phase O.sub.2 oxidation of alkylaromatics such as p-, m-, and o-xylenes to the corresponding terephthalic acid, isophthalic acid, and phthalic anhydride with the MC-type catalyst, Co--Mn--Br, or a modified version of that catalyst further containing an alkali metal or alkaline earth metal, in the co-presence of CO.sub.2, e.g., ##EQU1## In another aspect, the present invention relates to a method of producing an aromatic carboxylic acid, the method comprising the steps of oxidizing an alkyl aromatic compound or a partially oxidized intermediate thereof, with a gas comprising oxygen with or without carbon dioxide, using a catalyst comprising (a) at least one transition metal and (b) an alkali metal or alkaline earth metal. Where the gas comprises oxygen and carbon dioxide, preferably the carbon dioxide is present in an effective amount (e.g., an amount greater than that present in air and sufficient to exhibit action by the carbon dioxide as a co-oxidant (e.g., as in the examples herein below)). More preferably the carbon dioxide is present in an amount of at least 1% by volume of the gas phase, and still more preferably at least 5% by volume of the gas phase. Other preferred ranges for carbon dioxide include at least 7%, at least 14%, and at least 28% by volume. The catalyst may comprise, e.g., (a) cobalt and manganese (which may be without, e.g., bromine or nickel) or (b) nickel and manganese. Further, where the catalyst comprises bromine, the mole ratio of the alkali or alkaline earth metal to bromine is preferably 0.001-5, more preferably 0.05-2, and most preferably 0.1-1. In all of the foregoing aspects, the catalyst may comprise a conventional MC-type catalyst (e.g., a cobalt-manganese-bromine catalyst). In yet another aspect, the present invention relates to a process of purifying crude terephthalic acid products or crude isophthalic acid products containing, as an impurity, a partially oxidized intermediate of an alkyl aromatic compound, to obtain substantially pure terephthalic acid or isophthalic acid by using an above-discussed method. In a still further aspect, the present invention relates to a process comprising the steps of (a) obtaining the product of a liquid phase oxidation of an alkyl aromatic hydrocarbon using a cobalt-manganese-bromine catalyst, (b) thereafter, as a post-oxidation step, purifying the product obtained in the obtaining step to remove impurities therein using an above-discussed method. In other aspects, the present invention relates to (a) an aromatic carboxylic acid prepared by an above-discussed method, (b) polyester made using the aromatic carboxylic acid, and (c) a product made using the polyester. These and other aspects, objects, advantages, and features of the present invention will become apparent from the following detailed description of preferred embodiments thereof. Unless otherwise stated, in this application, the concentration of gas is in volume %, the concentration of the catalyst is in weight ppm (by total weight of the reaction mixture), and the concentration of the product, and any other unspecified %, is in weight %. Description of the Preferred Embodiments The present invention relates to a production method of aromatic carboxylic acids, wherein alkylaromatic compounds or the oxidized intermediates thereof are oxidized by an oxygen containing gas, with an aliphatic carboxylic acid having 1-6 carbon atoms as a solvent, in the presence of a catalyst of cobalt-manganese-bromine with addition of a transition metal or lanthanide metal as deemed necessary. In the process, one or more than one type of alkali metal or alkaline earth metal components are added to the catalyst system with the process also featuring the addition of carbon dioxide to the oxygen containing gas, which is supplied as an oxidant. Starting substances, i.e., alkylaromatic compounds or the oxidized intermediates thereof, to be oxidized in the present invention are preferably the compounds of benzene, naphthalene or similar aromatic compounds having one or more than one substituted alkyl groups (or a functional group having an oxidized alkyl group), such as para-xylene, para-tolualdehyde, para-toluic acid, 4-carboxybenzaldehyde, meta-xylene, meta-tolualdehyde, meta-toluic acid, 3-carboxybenzaldehyde, ortho-xylene, dimethylnaphthalene, pseudocumene (1,2,4-trimethylbenzene), mesitylene (1,3,5-trimethylbenzene), durene (1,2,4,5-tetramethylbenzene), 4,4'-dimethylbiphenyl, and toluene. More specifically, preferred alkyl aromatic compounds include para-xylene, meta-xylene, ortho-xylene, pseudocumene (1,2,4-trimethylbenzene), mesitylene (1,3,5-trimethylbenzene), durene (1,2,4,5-tetramethylbenzene), pentamethylbenzene, hexamethylbenzene, dimethylnaphthalene, 4,4'-dimethylbiphenyl, and toluene, while the partially oxidized alkyl aromatic intermediates preferably are para-toluic acid, meta-toluic acid, ortho-toluic acid, para-tolualdehyde, meta-tolualdehyde, ortho-tolualdehyde, 4-carboxybenzaldehyde, 3-carboxybenzaldehyde, or 2-carboxybenzaldehyde, and more preferably 4-carboxybenzaldehyde, 3-carboxybenzaldehyde, para-toluic acid, or meta-toluic acid. The intended substances of the present invention, i.e., aromatic carboxylic acids, are preferably the compounds of benzene, naphthalene or similar aromatic compounds having one or more than one substituted carboxylic acid groups (or anhydrides with the removal of water by condensation of the carboxylic groups), such as terephthalic acid, isophthalic acid, phthalic acid, phthalic anhydrides, naphthalene dicarboxylic acid, trimellitic acid, trimellitic anhydrides, trimesic acid, pyromellitic dianhydride, 4,4'-biphenyldicaroboxylic acid, and benzoic acid, more preferably selected from the group consisting of terephthalic acid, phthalic acid, isophthalic acid, trimesic acid, trimellitic acid, trimellitic anhydride, and pyromellitic dianhydride, and most preferably terephthalic acid. As for the basic catalyst in the present invention, a cobalt-manganese-bromine catalyst system was used. If deemed necessary, a transition metal or lanthanide metal component may be added. In the basic catalyst, the atomic weight ratio of manganese/cobalt is preferably 0.1.about.10, or more preferably 0.5.about.5. The atomic weight ratio of bromine/(manganese+cobalt) is preferably 0.1.about.10, or more preferably 0.5.about.2. The concentration of cobalt is preferably 20.about.10,000 ppm of the weight of the reactants (i.e., the substitute (the starting substance to be oxidized), the solvent, and the catalyst), or more preferably 50.about.1,000 ppm. As for the source of bromine, it could be a bromine compound, such as hydrogen bromide, potassium bromide, tetrabromoethane, etc. As for the source of manganese and cobalt, a compound which is soluble in solvents, such as acetate, carbonate, acetate tetrahydrate, bromide, etc. can be used, or more preferably, as a source of cobalt, manganese, bromine, respectively, are Co(OAc).sub.2 H.sub.2 O, Mn(OAc).sub.2 4H.sub.2 O, and hydrogen bromide. Compounds of Ce, Zr, Hf, Mo, Cr, Fe, W, etc. are preferred for transition metals or lanthanide metals, which are added if necessary. The weight ratio of the added transition metal or lanthanide metal/manganese is appropriately 0.001.about.1. Further, the present invention can be applied to an oxidation reaction by a cobalt-manganese catalyst without bromine as well as a nickel-manganese-bromine catalyst. The additive alkali metal or alkaline earth metal components used in the present invention can be any alkali metal or alkaline earth metal components. Specific examples include one or more than one type selected from Li, Na, K, Rb, Cs, Be, Mg, Ca, Sr or Ba, more preferably Na, K, and Cs, and most preferably K. These additive alkali metals or alkaline earth metals can be used in the forms of compounds having solubility in the solvents as used. Compounds such as acetate, acetate hydrate, bromide, chloride, fluoride, iodide, carbonate, carboxylate, alkoxide, azide, naphthenate, oxalate, acetylacetonate, hydroxide, nitrate, borate and oxide can be used. Among these, an acetate compound is most preferred. The mole ratio of the additive alkali metal or alkaline earth metal to bromine is appropriately 0.001.about.3, or more preferably 0.05.about.2, or most preferably 0.1.about.1. If the mole ratio is less than 0.001, the effect based on the addition of alkali metals or alkaline earth metals is not expected. If the mole ratio is more than 5, the retardation effect on the reaction is prevailing. The solvent used in the present invention can be any aliphatic acids of C.sub.1.about.C.sub.6, such as formic acid, acetic acid, propionic acid, n-butyric acid, isobutyric acid, pentanoic acid, hexanoic acid, trimethylacetic acid, etc, or more preferably acetic acid or the mixture of acetic acid and water. Preferably, the solvent comprises from 2% to 25%, by weight, of water. The amount of solvent should be 1.about.10 times the weight of an alkylaromatic compound or the oxidized intermediate compound thereof. Further, the present invention can be applied to the oxidation reaction in water as a solvent. As for the reaction gas used in the present invention, oxygen, or a gas mixture of oxygen and an inert gas such as nitrogen can be used, or more preferably, a gas mixture of oxygen and carbon dioxide can be used. Preferably, the reaction gas or feed gas lacks an inert diluent. The minimal pressure of the reaction is such that some portion of an alkylaromatic compound or the oxidized intermediate thereof and the solvent is maintained as liquid. The reaction pressure is appropriately 0.about.35 atm in terms of the gauge pressure or more preferably 8.about.30 atm. The amount of carbon dioxide should be 1.about.80% (by volume) of the gas, or more preferably 5.about.50%. As for the method of adding carbon dioxide, it can be supplied in the gas phase at the upper part of the reactor or in the reactants of liquid phase, either periodically or continuously. (For example, carbon dioxide may be added with a gas sparging device into one or more zones of a reactor in a gas phase or liquid phase either periodically, intermittently, or in a continuous manner.) As for the method of supplying carbon dioxide to the reactor, carbon dioxide can be mixed into the reaction gas. Alternatively, the method of recycling the reacted vent gas to the reaction gas can be used for the purpose of utilizing carbon dioxide and oxygen remaining in the vent gas. (For example, carbon dioxide remaining in reaction vent gas may be recovered by condensation and recycled to provide carbon dioxide required for the oxidation reaction.) When it is supplied to the reactants in liquid, a dip tube etc. can be used for supplying via bubbling. The production method of aromatic carboxylic acids of the present invention could be carried out by a batch type process or a continuous process. The appropriate reaction temperature should be 100.about.255.degree. C., or more preferably 175.about.235.degree. C., or most preferably 180.about.210.degree. C. If the reaction temperature is too low, it is impractical since the reaction rate is too slow. If the reaction temperature is too high, it is non-economical due to the excessive side reactions. As a reactor, general CSTR (continuous stirred tank reactor) or LOR (liquid oxygen reactor) specially designed to mix liquid oxygen and liquid hydrocarbon substrates without appreciable loss of unreached oxygen into the overhead vapor space can be used. According to the present invention, the reaction time is decreased at the same reaction temperature for obtaining the same conversion. At the same reaction time, the present invention requires a lower reaction temperature for a given conversion. The productivity and quality such as chemical impurities and color properties can be improved due to the decreased side reactions with the present invention. |
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