Terephthalic Acid Powder CAS 100-21-0

This method was first proposed by Medieval Corporation in the United States and Bonham Chemical Industries in the United Kingdom in 1955, and industrialized by Amoco Chemicals in 1958. The overall reaction equation is (Figure 1):

But the actual process is much more complex, and some people believe that it goes through the following steps (Figure 2):

Due to the second methyl group being difficult to oxidize, the reaction process is easily stopped at the para methylbenzoic acid or para carboxybenzaldehyde stage. In order to continue the oxidation reaction, Amoco Chemical Company adopts a process of high temperature and adding co catalyst bromide (commonly tetrabromoethane) to the cobalt acetate manganese acetate catalyst (Figure 3).

Bromine produced by bromide can trigger a chain reaction of free radical oxidation. Oxidation reactions are generally carried out in tower reactors. The reaction temperature is 175-230 ℃, but most of them are above 200 ℃. Higher temperatures can accelerate the reaction, reduce intermediate products, but also increase the by-products obtained from decomposition. Due to the fact that the heat of reaction is transferred away by the water and solvent acetic acid generated by the evaporation reaction, the reaction pressure is related to the evaporation amount, generally ranging from 1.5 to 3.0 MPa. The residence time is 0.5-3 hours. Increasing the concentration of cobalt acetate and manganese acetate can shorten the residence time or lower the reaction temperature. The high-temperature oxidation process can achieve a yield of over 90% based on p-xylene. Due to the high reaction temperature and the presence of bromine, which has a strong corrosive effect, titanium or titanium lined materials are required for the reactor.
PTA has low solubility in acetic acid, and the oxidation product is in the form of a slurry. After centrifugal separation and drying, the solid crude PTA is obtained, and the most harmful impurity is p-carboxybenzaldehyde (content 1000-5000ppm). Crude PTA can be used to produce polyester through dimethyl terephthalate, but a better method is purification, using refined PTA directly as the raw material for polyester. The commonly used refining method is the hydrogenation method adopted by Amoco Company, which involves dissolving crude PTA in water at high temperature and high pressure, then hydrogenating impurities in the presence of palladium catalyst, followed by crystallization and filtration to obtain fiber grade (purity specification suitable for spinning) refined PTA. The content of para carboxybenzaldehyde in the product can be less than 25ppm. The yield of terephthalic acid during the refining process is greater than 97%. In addition to hydrogenation, there are also methods such as sublimation for refining.
The low-temperature oxidation method for xylene generally has a reaction temperature below 150 ℃, and although cobalt acetate is also used as a catalyst, bromide is not used. At this point, in order to convert the second methyl group into a carboxyl group, it is generally necessary to add a co oxide that is prone to producing peroxides during the oxidation reaction. For example, Mobile Chemical Company in the United States uses methyl ethyl ketone, Eastman Kodak Company in the United States uses acetaldehyde, and Toray Company in Japan uses formaldehyde. After oxidation, these substances also generate acetic acid, which is the solvent used during oxidation. Taking the Dongli method as an example, the reaction conditions are: temperature of 120-150 ℃, pressure of 3MPa, and yield of 96%. The low-temperature oxidation method does not require titanium materials in the reactor due to the absence of bromide and low reaction temperature.

The patent of the Federal German company Henkel, also known as the Henkel I law. Industrialization was achieved by Emperor Corporation of Japan. This method first converts phthalic anhydride into dipotassium phthalate, which is then translocated to obtain dipotassium terephthalate, and then acidified (or acid precipitated) to obtain PTA. The most difficult of these steps is the transposition reaction, which uses cadmium or zinc catalysts, with a reaction temperature of 350-450 ℃ and a pressure of 1-5MPa. The reactor structure is also very complex. The potassium sulfate generated after acidification with sulfuric acid is difficult to convert into potassium hydroxide for recycling and can only be used as potassium fertilizer. The Henkel I method has expensive raw materials and complex processes, so although it has been industrialized, it has not been widely promoted.

The patent of Henkel GmbH in the Federal Republic of Germany (processes 11, 12, 13, 16 in Figure 4), also known as Henkel I method. Industrialization was achieved by Emperor Corporation of Japan. This method first converts phthalic anhydride into dipotassium phthalate, which can be converted into dipotassium phthalate through transposition reaction, and then acidified (or acid precipitated) to obtain terephthalic acid powder. The most difficult step among these is the transposition reaction, which requires cadmium or zinc catalysts, reaction temperature of 350-450 ℃, pressure of 1-5MPa, and a complex reactor structure. The potassium sulfate generated after acidification with sulfuric acid is difficult to convert into potassium hydroxide for recycling and can only be used as potassium fertilizer. The Henkel I method has expensive raw materials and complex technology, so although it has been industrialized, it has not been widely promoted.

Also known as the Henkel II method (i.e. processes 1, 12, 14, 16 in Figure 4). Using toluene as raw material, it is first oxidized to produce benzoic acd, and its potassium salt is dismutated to produce benzene and dipotassium terephthalate, which is then acidified to form PTA. The most critical one is the dismutation reaction, which takes place at 400 ℃, 2MPa, and in the presence of carbon dioxide. This method was industrialized by Mitsubishi Chemical Industries in Japan in 1963. Due to high costs, it was discontinued in 1975. However, due to the fact that the raw material toluene is much cheaper than xylene, some companies in certain countries are still researching and improving this method.