Analysis of Foreign Patents on Pd/C Catalysts for Hydrogenation Refining of Terephthalic Acid

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Analysis of Foreign Patents on Pd/C Catalysts for the Hydrogenation Refining of Terephthalic Acid. Purified terephthalic acid (PTA) is a key raw material in the polyester industry, used primarily in the production of polyethylene terephthalate (PET), as well as various polyester fibers, polyester bottles, polyester films, and engineering plastics such as polybutylene terephthalate (PBT). PTA produced on an industrial scale is obtained by the oxidation of p-xylene, but it contains high levels of oxidation by-products such as 4-carboxybenzaldehyde (4-CBA). Polymerization with ethylene glycol can lead to chain scission and even a darkening of color; therefore, crude PTA cannot be directly polymerized with ethylene glycol to produce PET. In 1964, the American company Amoco developed a catalytic hydrogenation refining process. In this process, the TPA solution containing 4-CBA was subjected to hydrogenation using a Pd/C catalyst under high temperature and pressure, which reduced 4-CBA to p-formic acid, which is soluble in water. The resulting substance was then crystallized and separated to yield pure PTA. At present, most PTA manufacturers around the world use this method, that is, hydrogenation refining with a palladium-on-carbon catalyst to remove impurities such as 4-CBA. The main companies abroad that are engaged in research and development of hydrorefining technologies and catalysts include the American company Amoco, Standard Oil Company, Engelhard Corporation; as well as the German companies Sud Chemile Mt S.R.L. and Degussa ； Italian Chimet Corporation, as well as Japanese companies such as Mistui Petrochemical Industries and Mitsui Chemicals Co., Ltd. In recent years, world PTA production has grown at a very rapid pace, with the global demand for PTA expected to increase at an average annual rate of 6.4% by 2008. The global demand for PTA (equivalent amount of PTA) in 2008 will reach 82.81 million tons. The further expansion of global PTA demand provides ample opportunities for the development of Pd/C catalysts for purification. To this end, relevant foreign institutions have been continuously focusing on research into Pd/C catalysts for the hydrogenation refining of PTA, and have filed numerous patents. I. ENGELHARD IND INC filed U.S. Patent US3138560 in 1964, which was the world’s first Pd/C catalyst suitable for hydrogenation. The preparation method of this catalyst involves impregnating a carbon carrier with an aqueous solution of a divalent palladium compound containing halogens such as chlorine and bromine; the solution may also contain a peroxide oxidant, and the pH is ≥4.5 when the carbon carrier is dispersed in pure water. As palladium undergoes hydrolysis on the carbon, the amount of palladium on the carbon decreases. II. HITACHI LTD’s publicly disclosed patent JP10202104 (AD: 1997.01.20, PD: 1998.08.04) relates to a hydrogenation catalyst for the purification of terephthalic acid. It uses activated carbon as a carrier, with palladium as the main active component; by giving the activated carbon a certain pore size and a certain cumulative micropore volume, the catalyst exhibits high activity at relatively high space velocities. This invention uses activated carbon with a pore size of less than 2 nm and a cumulative micropore volume of 0.40–0.55 cm3/g as the carrier. The invention states that it is preferable to use activated carbon that has been ground into powder with a particle size of 4–8 mesh; the content of active palladium in such carbon should be 0.1–5%. The activated carbon is immersed in a solution containing palladium salts, allowing the palladium salts to be adsorbed onto the carbon. A reducing agent is then added to reduce the palladium salts, thereby depositing palladium on the activated carbon. When the Pd content is 0.5%, the concentration of 4-CBA can be reduced from 3000 ppm to 5 ppm, with a conversion rate of 99.8%. The company’s published patent WO97/47384 (AD: 1997.3.19, PD: 1997.12.18) describes a Pd/C hydrogenation catalyst that exhibits high activity and a long service life in hydrogenation reactions. According to this invention, by adjusting the pH of the palladium salt solution during catalyst preparation, a larger proportion of palladium can be loaded on the surface of the carbon carrier, where the diameter of the target particles on the surface is greater than that of the palladium particles inside it. For example, in a hydrogenation catalyst using activated carbon as the carrier and palladium as the main active component, the cumulative pore volume of activated carbon with a pore diameter ≤ 2 nm is at least 0.4 cm3/g and at most 0.55 cm3/g. As a result, initial activity is improved, and long lifespan is achieved. In the example implementation, a catalyst containing 0.5 weight% Pd was used, enabling the concentration of 4-CBA to be reduced from 3000 ppm before the test to 5 ppm after the test, with a conversion rate of 99.8%. III. Amoco Corporation: Its publicly disclosed patent US4791226 (AD: 1984.03.09, PD: 1988.12.13) relates to a method for refining crude terephthalic acid and its catalyst. The catalyst used consists of catalytically active palladium microcrystals supported on a porous activated carbon carrier; the surface area of the activated carbon particles is ≥600 m2/g, and the longitudinal length of the microcrystals is less than 35 angstroms. The preparation of the catalyst involves reacting an active carrier with an aqueous-free solution of palladium salts in an organic solvent in the presence of hydrogen; the organic solvent penetrates into the carrier material, and the palladium salts are reduced to palladium metal microcrystals. In the examples, by using the catalyst of this invention, the 4-CBA concentration could be reduced from 7900 ppm before treatment to 23–306 ppm after treatment, resulting in a conversion rate of 96%–99.7%. IV. Standard Oil Company: Its publicly disclosed patent US4421676 (AD: 10.29.1981, 12.20.1983) relates to a method for preparing PD/C catalysts used in the purification of crude terephthalic acid. This method involves adsorbing active palladium microcrystals onto a porous carbon carrier; the surface area of the activated carbon particles is ≥600 m2/g, and the longitudinal length of the microcrystals is less than 35 angstroms. The specific procedure involves reacting the carrier with Na2Pd(NO2)4, after which Na2Pd(NO2)4 is adsorbed onto the carrier and reduced to metallic palladium. Na2Pd(NO2)4 is obtained by reacting nitrites with palladium halides in a molar ratio of 4:1. The experimental results show that with this catalyst, the concentration of 4-CBA can be reduced from 7900 ppm before treatment to 39–380 ppm after treatment, achieving a conversion rate of 95.0%–99.5%. The company’s published patent US4415479 (AD: 10.29.1981, PD: 11.15.1983) relates to a PD/C catalyst for the purification of crude terephthalic acid. The preparation method involves adsorbing active palladium microcrystals onto a porous carbon carrier; the surface area of the activated carbon particles is ≥600 m2/g, the longitudinal length of the microcrystals is less than 35 angstroms, and the palladium content is less than 1.0%. The specific procedure includes reacting activated carbon particles with an amine solution and a palladium salt in the presence of an organic carboxylic acid. The concentration of the amine must be sufficient to dissolve the palladium salt, and the molar ratio of acid to amine is ≥0.75. Palladium reacts with carbon to yield the catalyst. V. SudChem Company (Milan, Italy): The patent CN1205244 published by this company (application date: May 18, 1998; publication date: January 20, 1999) describes a hydrogenation catalyst that includes metal palladium supported on activated carbon, where less than 50% of the Pd by weight is located in the surface layer up to a depth of 50 μm, while the remaining Pd is found in a layer at a depth of 50–400 μm. The activated carbon used in this invention is preferably coconut charcoal, which is pre-treated with an acidic aqueous solution containing 1-5 weight% concentrated HCl before spraying the Pd compound solution, in order to enhance the acidity of the catalyst. Catalysts can be specifically used for the purification of crude terephthalic acid produced via the oxidation of p-xylene. In the embodiment of this invention, a catalyst with a Pd content of 0.498 wt% was used, resulting in a 4-CBA conversion rate of 96.7% after 45 minutes. VI. Samsung General Chemical Co., Ltd. and Berosskov Catalyst Institute: The patent CN1283521 (application date: July 28, 2000; publication date: February 14, 2001) disclosed by these two organizations relates to a catalyst composition used for purifying terephthalic acid, its preparation method, and the process for purifying terephthalic acid. The catalyst composition used for purifying terephthalic acid contains catalytically active palladium deposited on carbon material, or crystals of palladium and at least one metal from Group VIII of the periodic table; the carbon material is a graphite-like material with mesoporous structure, having an average mesopore size of 40–400 Å, with mesopores accounting for at least 0.5 of the total pore volume, and a graphitism degree of not less than 20%. The metal crystals are distributed within numerous particles of this carbon material, in layers located at a distance from the outer surface of the particles equal to 1–30% of their radius. VII. Mitsui Petrochemical Industries, Ltd. The patent CN1058583 disclosed by this company (application date: July 4, 1991; publication date: February 12, 1992) relates to a method for producing refined terephthalic acid, which involves the catalytic oxidation of p-xylene in a liquid phase to produce crude terephthalic acid with 4-carboxybenzaldehyde as the main impurity ； And in the reactor, crude terephthalic acid is treated with hydrogen in the presence of a hydrogenation catalyst, thereby producing refined terephthalic acid containing a fixed amount of 4-carboxybenzaldehyde in a stable manner. Its improvements enable the treatment process to reach a stable state rapidly after partially replacing the deactivated catalyst with a new one. The improvements include adding crude terephthalic acid containing a larger amount of 4-carboxybenzaldehyde than in the stable treatment state to the reactor, and treating the terephthalic acid until a stable state is reached. This invention uses granular palladium as a catalyst supported on activated carbon, achieving a 4-CBA conversion rate of 99.8%. VIII. SUED CHEMIE MT SRL (IT): The patent US6066589 published by this research institution (Application date: 1998.05.19, Patent grant date: 2000.05.23) relates to a hydrogen catalyst that can be used for the purification of terephthalic acid. The catalyst consists of metal palladium loaded on activated carbon, with less than 50% of the palladium located in the surface layer at depths greater than 50 microns, while the remaining portion is found in the surface layer at depths between 50 and 400 microns. When a palladium-carbon catalyst with a palladium content of 0.498% was used, the conversion rate of 4-CBA was 96.7%. IX. COSMO SOGO KENKYUSHO KK – The patent JP2000037633 (AD: 1998.07.22, PD: 2000.02.08) disclosed by this company relates to a method for purifying crude aromatic dicarboxylic acids and the catalysts used in such a process. The hydrogenation catalyst mentioned in this patent is one in which group 8 noble metals such as palladium are loaded on granular activated carbon; the total surface area of the activated carbon is at least 800 m2/g, its micropore volume is at least 0.6 ml/g, its macropore volume is at least 0.2 ml/g, and its sulfur content is at least 1000 ppm. X. HYOSUNG CORP: The patent disclosed by this company, KR2005069180 (AD: 12/31/2003, PD: 07/05/2005), relates to a Pd/C hydrogenation catalyst for the purification of terephthalic acid. The preparation method involves dissolving palladium nitrate or palladium halides in an organic solvent containing nitric acid to obtain a palladium precursor solution; porous activated carbon is then impregnated with this solution and dried ； Among them, the specific surface area of the activated carbon is 600 m2/g, the sulfur content is ≤1000 ppm, the concentration of nitric acid in the organic solvent is 0.01–10 M; the organic solvent can be one or more types of organic solvents selected from alcohol solvents with 1–10 carbon atoms, propylene solvents with 3–10 carbon atoms, and ester solvents with 3–10 carbon atoms. References: 1. ENGELHARD IND INC, Process for producing palladium on carbon catalysts, US3138560 2. HITACHI LTD, CATALYST FOR HYDROGENATION, PRODUCTION THEREOF AND TEREPHTHALIC ACID-REFINING PROCESS, JP10202104 3. HITACHI LTD, Catalyst for hydrogenation and method of manufacturing same, WO97/47384 4. Amoco Corporation, Catalyst and process for purification of crude terephthalic acid, US4791226 5. Standard Oil Company, Process for preparation of palladium on carbon catalysts used in the purification of crude terephthalic acid, US4421676 6. Standard Oil Company, Palladium on carbon catalyst for purification of crude terephthalic acid, US4415479 7. Shude Chemical Company (Milan, Italy), Hydrogenation catalysts, CN1205244 8. Samsung General Chemical Co., Ltd., Catalyst composition, its preparation method and method for purifying terephthalic acid, CN1283521 9. Mitsui Petrochemical Industries, Ltd., Method for producing refined terephthalic acid, CN1058583 10. BORESKOVA INST KATALIZA SIBIR, Catalytic composition, method for manufacturing thereof and method for the purification of terephthalic acid, US6753290 11. SUED CHEMIE MT SRL (Italy), Hydrogenation catalysts, US6066589 12. COSMO SOGO KENKYUSHO KK, METHOD FOR PURIFYING CRUDE AROMATIC DICARBOXYLIC ACID AND CATALYST TO BE USED IN PURIFICATION, JP2000037633 13. HYOSUNG CORP, Method for production of Pd/C hydrogenation catalyst and method for purifying terephthalic acid using the catalyst, KR2005069180 Translated by Qu Liman. Last edited by zxh6267 on 2009-4-1 16:50]