【Basic Knowledge of Reaction Mechanisms for the “Polymer” Major - 003】Basic Knowledge of the Polymer PTA Reaction Mechanism

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PTA production principle: The PTA production process consists of two stages – oxidation and purification. 1. Principle of the oxidation reaction: In this oxidation process, cobalt acetate and manganese acetate are used as catalysts, while liquefied hydrogen serves as a promoter. Using terephthalene as the raw material, oxygen from the air is utilized in an acetic acid solvent to oxidize terephthalene into terephthalic acid under conditions of 1.15 MPaG and 188°C. The chemical reaction equation is as follows: C8H10 + 3O2 → C6H4(COOH)2 + 2H2O + 318.7 Kcal/mol. The side reactions that occur during the oxidation of terephthalene are more complex than the main reaction. Because the PX oxidation reaction takes place under high-temperature peroxide conditions. While the main reaction is taking place, PX and acetic acid in the system undergo partial combustion. Carbon monoxide, carbon dioxide, and water are generated according to the following equations: C8H10 + O2 → CO2 + H2O; CH3COOH + O2 → CO2 + H2O. Additionally, if the proportions in the oxidation reaction are not correct, or if impurities are introduced due to the impurity in the p-xylene raw material, auxiliary materials, and chemicals, some side reactions occur, resulting in the formation of more by-products. In the end, they are all carried into the oxidation stream as impurities. 2. Principle of the hydrogenation reaction: Crude terephthalic acid is not suitable for producing polyester products due to its impurity content. The main impurities are intermediate products resulting from the oxidation of terephthalic acid, including 4-carboxybenzaldehyde (4-CBA), p-toluenesulfonic acid (PT-acid), and colored substances. Crude TA with a purity of 99.7% must be purified to 99.98% pure terephthalic acid in order to be suitable as a raw material for polyesters. The basic reactions in the hydrorefining process are all based on the reverse principle of the PX oxidation reaction. The main oxidation sequence is as follows: p-xylene → p-methylphenol → PT acid → 4-CBA → terephthalic acid. During the reaction, due to the effect of equilibrium concentrations, PX cannot be completely oxidized to TA; certain concentrations of oxidation intermediates and colored substances remain in the oxidation solution. Since the structure of 4-CBA is essentially identical to that of TA molecules, it continuously forms co-crystals with TA, ending up being incorporated into TA as impurities, among which 4-CBA has the greatest impact. To eliminate 4-CBA, taking advantage of the reverse reaction that produces 4-CBA, the crude TA obtained in the oxidation process is dissolved in deionized water at a high temperature of 286°C and under high pressure of 76 Barg. Under the action of a palladium-carbon catalyst, it reacts with hydrogen, causing 4-CBA in the crude TA to be reduced to PT acid, while the colored substances are also decomposed. The reaction equation is as follows: 4-CBA + 2H2 → PT acid + H2O + 43.7 Kcal/mol. This is because the solubility of PT acid in water is much higher than that of terephthalic acid (TA) in water. When TA crystallizes out from the aqueous solution, PT acid remains dissolved in the water; therefore, at a temperature of around 150°C, PT acid can be easily separated from TA through pressure filtration, thereby indirectly removing 4-CBA from the product. Side reactions: Mainly side reactions resulting from the hydrogenation of PTA due to excessive hydrogenation. PTA + H2 → 4-CBA + H2O; PTA + H2 → cyclohexane. The mechanism of the hydrogenation reaction of PTA + H2 involves palladium (Pd) in the palladium/carbon catalyst, which activates the aldehyde group (-CHO) in 4-CBA, turning it into a reactive group. At the same time, it also activates hydrogen molecules, turning them into reactive groups as well, thereby enabling the hydrogenation reaction to proceed smoothly. 4CBA+H2→CH2OHC6H4COOH→CH3C6H4COOH