Terephthalic Acid (PTA) was discovered in the 19th century. It was not until 1949 when Britain's benemen chemical industry company found that PTA (or its derivative dimethyl terephthalate) was the main raw material for polyester production that it began to be widely produced. In 1981, the world PTA output reached 3.485mt. The first industrialized production method was nitric acid oxidation. With the development of the polyester industry, a method for producing PTA from a variety of raw materials and through a variety of ways has been developed (Fig. 1). The most economical and widely used method is the high-temperature liquid-phase oxidation method using p-xylene as raw material (see the color chart), which has high yield and short process. The low-temperature oxidation of p-xylene has mild reaction conditions and little corrosion, but the process is long and is only used in a few factories. It has also been proposed that p-xylene is first ammoniated and oxidized to produce p-phenylenenitrile, and then hydrolyzed to produce PTA. However, this method has not been produced on a large scale. Due to the high cost of separating p-xylene from mixed xylene, some methods starting from other raw materials have also been developed. Although some of these methods have been industrialized for a long time, they have not been developed, while others are only in the intermediate experimental stage.
High temperature liquid phase oxidation of p-xylene:
This law was first proposed by the American medieval company and the British bnemen chemical industry company in 1955, and was industrialized by the American Amoco chemical company in 1958. The total reaction formula is (Fig. 1):
However, the actual process is much more complicated. Some people think that it is through the following steps (Fig. 2):
since the second methyl group is not easy to be oxidized, the reaction process is easy to stop at the stage of p-methylbenzoic acid or p-carboxybenzaldehyde. In order to continue the oxidation reaction, Amoco chemical company adopts the process of high temperature and adding cocatalyst bromide (commonly tetrabromoethane) to cobalt acetate manganese acetate catalyst.
Bromine produced by bromide can trigger the chain oxidation reaction that produces free radicals. The oxidation reaction is generally carried out in a tower reactor. The reaction temperature is 175-230 ℃, but most of them are higher than 200 ℃. Higher temperature can accelerate the reaction and reduce the intermediate products, but the by-products from decomposition also increase. Since the heat of reaction is removed by the water and solvent acetic acid produced by the evaporation reaction, the reaction pressure is related to the evaporation amount, generally 1.5-3.0mpa. The residence time is 0.5 ~ 3H. The increase of the concentration of cobalt acetate and manganese acetate can shorten the residence time or reduce the reaction temperature. The yield of p-xylene in the high temperature oxidation process can reach more than 90%. Due to the high reaction temperature and the presence of bromine, which has a strong corrosion effect, the reactor needs titanium or titanium lining material.
PTA has little solubility in acetic acid, and the oxidation product is in the form of slurry. After centrifugation and drying, solid crude PTA is obtained. 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 as the raw material of polyester directly. The commonly used refining method is the hydrogenation method adopted by Amoco, that is, the crude PTA is dissolved in water under high temperature and pressure, then the impurities are hydrogenated in the presence of palladium catalyst, and then crystallized and filtered to obtain fiber grade (purity specification suitable for spinning). The content of p-carboxybenzaldehyde in the product can be less than 25ppm. The yield of terephthalic acid in the refining process is more than 97%. In addition to hydrogenation, the refining methods include sublimation.
Low temperature oxidation of p-xylene the reaction temperature of this method is generally lower than 150 ℃. Although cobalt acetate is also used as catalyst, bromide is not used. At this time, 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 produce peroxide during the oxidation reaction. For example, the American Mobil Chemical Company uses methyl ethyl ketone, the American Eastman Kodak company uses acetaldehyde, and the Japanese Toray company uses trimethyl acetaldehyde. These substances also produce acetic acid after oxidation, and acetic acid is the solvent used for oxidation. The reaction conditions are as follows: the temperature is 120 ~ 150 ℃, the pressure is 3Mpa, and the yield is 96%. The low-temperature oxidation method has no bromide and low reaction temperature, so the reactor can not use titanium material.
Phthalic anhydride transposition method:
The patent of Henkel company (processes 11, 12, 13 and 16 in Fig. 4) is also called Henkel I method. Industrialization was realized by Japanese Teijin company. In this method, phthalic anhydride is first converted into dipotassium phthalate, dipotassium terephthalate can be obtained through transposition reaction, and then PTA can be obtained through acidification (or acid precipitation). In these steps, the most difficult is the transposition reaction. Cadmium or zinc catalyst is used in this reaction. The reaction temperature is 350-450 ℃, the pressure is 1-5mpa, and the reactor structure is also very complex. It is very difficult to convert the potassium sulfate generated after acidification with sulfuric acid into potassium hydroxide for recycling, so it can only be used as potassium fertilizer. Henkel I process is expensive in raw materials and complicated in technology. Therefore, although it has been industrialized, it has not been popularized.
Toluene oxidation disproportionation method:
Also known as Henkel II method (i.e. 1, 12, 14 and 16 processes in Fig. 4). That is, toluene is used as raw material, and benzoic acid is prepared by oxidation first, and its potassium salt is disproportioned to produce benzene and dipotassium terephthalate, which is acidified to form PTA. The most critical one is disproportionation reaction, which is carried out at 400 ℃, 2MPa and the presence of carbon dioxide. This law was industrialized in Japan by Mitsubishi Chemical Industry Corporation in 1963. It was discontinued in 1975 due to high cost. However, since the raw material toluene is much cheaper than p-xylene, some companies in some countries are still studying and improving this method.