1 Introduction

In 1851, O L Erdman discovered the anhydride in the thermal decomposition of mellitic acid. In 1947, California Research Corp. used the tetramethylbenzene as the raw material for the first time to obtain pyromellitic dianhydride by vapor phase catalytic oxidation of V2O5 composite oxide catalyst. In 1960, DuPont of the United States established the production equipment for the production of pyromellitic dianhydride by liquid phase nitric acid oxidation with tetramethylbenzene as the raw material. In 1969, Furukawa Electric Co., Ltd. established a production unit using nitric acid oxidation and liquid phase air oxidation. In 1970, Deviba Chemical Company changed the air oxidation method to establish a 500 t/a production unit [1-3].

 In the 1960s, China began to conduct experimental research and trial production. The initial use of 1,2,4-trimethylbenzene chloromethylation, nitric acid oxidation and potassium permanganate oxidation process was completed. 15t/a production unit. In the 1970s, the production of mesitylene was carried out by using meta-trimethylbenzene as a raw material, and then tetramethylbenzene was catalyzed by air oxidation, and pyromellitic dianhydride was prepared by air oxidation. The research work of two process routes of pyromellitic dianhydride is carried out by the partial oxidation of trimethylbenzene and then by air oxidation [4-6]. In the early 1990s, the construction of a 200t/a production plant for the gasification of tetramethylbenzene by gas phase catalytic oxidation of tetracycline was carried out, and its completion will bring China's uniform production to a new level. At present, there are many enterprises producing perchloric anhydride, but the annual output is less than 10,000 tons, and it has to be imported a lot every year. The development and utilization prospects are broad.

 Pyromellitic dianhydride (1,2,4,5-benzenetetracarboxylic dianhydride) has four carboxylic acid groups in the molecule, and all of them are listed, which can be esterified, acylated, hydrogenated, amidated, Various chemical reactions such as imidization and nitrilation. Pyromellitic dianhydride is an important raw material for organic synthesis and a basic raw material for the development of new chemical materials and high value-added fine chemical products. In recent years, the use of pyromellitic dianhydride has been expanding, and polyimides can be synthesized by reacting a homo-anhydride with 4,4-diaminodiphenyl ether. Polyimide is a new type of synthetic material that is resistant to high temperature, low temperature, radiation, impact and has excellent electrical and mechanical properties. It has an irreplaceable and important use in other engineering plastics in the aerospace industry, atomic energy industry and electromechanical industry. As the market for polyimides continues to expand, pyromellitic dianhydride is the main raw material for synthetic polyimides, and its demand is increasing.

 Therefore, improving the quality of pyromellitic dianhydride, increasing the yield of pyromellitic dianhydride, developing a high yield and high conversion ratio of pyromellitic dianhydride synthesis process route not only has a potential, huge social economy. Benefits; and can meet the needs of China's aerospace industry for new engineering materials, which is of great strategic importance to our economic and national defense construction.

2 Properties and main uses of pyromellitic dianhydride

2.1 Properties of pyromellitic dianhydride Pyromellitic dianhydride (1,2,4,5-benzenetetracarboxylic dianhydride), commonly known as homo-anhydride, English name is 1,2,4,5-benzene tetracarboxylic dianhydride or Pyromellitic dianhydride (PMDA) is an important chemical raw material with a molecular weight of C10H2O6 and a molecular weight of 218.12.

The pure product of pyromellitic dianhydride is white crystal. The industrial product is powder or needle crystal according to the preparation process. The needle crystal is not easy to absorb water and is easier to store. The melting point is 284-286 °C and the boiling point is 380- 400 °C, vapor pressure at 290 °C is 8.4 kPa, relative density at 1.60 °C is 1.680, soluble in dimethyl sulfoxide, dimethylformamide, acetone, methyl ethyl ketone, N-methylpyrrolidone, methyl Ketone, methyl isobutyl ketone and ethyl acetate, insoluble in chloroform, diethyl ether, petroleum ether, benzene, n-hexane and cold soda solution.

2.2 The main use of pyromellitic dianhydride The pyromellitic dianhydride is an important organic synthetic industrial raw material, and is also the basic raw material for the development of new chemical materials and high value-added fine chemical products, mainly used for the production of polyimide. The monomer can also be used as a curing agent for epoxy resins and a crosslinking agent for polyester resins, and is widely used for producing indigo blue dyes and some important derivatives. The main use of the anhydride is [7-11]:

(1) Raw materials used as polyimides Polyimides are generally produced by a two-step process using both drunken and aromatic diamines. The first step is to form a high molecular weight polylysine which is usually separated from the solution in the form of a film; then the second step, a dehydration cyclization reaction, is carried out at a certain temperature. The homogen anhydride is condensed with 4,4'-diaminodiphenyl ether to form a new high temperature resistant engineering plastic polyimide (PI). Polyimide products have outstanding heat resistance, radiation resistance, impact resistance and electrical insulation. They are a new type of engineering plastic developed in the 1960s. They can be used as substitutes for metals, ceramics and polymer materials. Heat-resistant enameled wire lacquer, heat-resistant powder coating, heat-resistant electrical insulating film and printed circuit board, etc., are called "golden film" and are widely used in electronics, electrical machinery, machinery manufacturing, atomic energy and aerospace. The homo-phenylene type polyimide, N,N'-(4,4'diphenyl ether), is the most commonly used shrink-chain aromatic imide in the industry. In 1961, DuPont of the United States realized industrial production. The film is usually used as a heat-resistant insulating gasket or wrapping material for making H- or C-class motors and cables or as a flexible circuit board substrate, and can also be used as a mold material for the use of atomic reactors and space stations. Materials, and jet engine tubing materials that work at 200-232 °C. When pyromellitic dianhydride and aromatic tetraamine are reacted, polybenzimidazole pyrrolidone can be obtained, and its radiation resistance, oxidation resistance and performance under acid conditions are stronger than polyimide, but alkali resistance is poor. Most of them are currently used in the field of space technology.

(2) Used as an epoxy resin curing agent Epoxy resin cured by pyromellitic dianhydride (PMDA), which has high heat resistance and can reach 200-250 °C, and can be used as a motor material for casting and lamination. In particular, motor materials with good insulation properties have unique advantages for large castings and castings that must withstand a wide range of temperature changes. Using pyromellitic dianhydride as a curing agent for epoxy resin adhesives, it can be quickly bonded to produce an impact-resistant transient adhesive. For the same resin, the thermal stability of the different anhydride type curing agents was increased in the following order: succinic anhydride <maleic anhydride <phthalic anhydride <tetrahydrophthalic anhydride <hypoalhydride. The use of the homogen anhydride as the epoxy resin curing agent also has some disadvantages, such as being difficult to dissolve, difficult to be mixed with the resin, and being expensive, but it is being widely used due to the above-mentioned unique properties and the fact that no irritating gas is generated during curing.

 (3) Used as a polyester resin cross-linking agent and coating additive. Polyester resin can be used as a thermosetting polyester mold. Adding a small amount of PMDA to the unsaturated polyester can make the obtained mold have good shrinkage resistance and high resistance. Impact strength and high temperature resistance. PMDA is used as a crosslinker for powder coating polyesters to improve the properties of the coating. It has good magnetic properties and low curing temperature. Its coatings have chemical resistance and good thermal resistance, and can increase its hardness and impact strength, and it does not produce toxic gases during operation.

 (4) Synthetic plasticizer tetrakis(2,2-dimethylpentyl) pyromellitate (TPP) and tetraoctyl pyromellitate (TOPM) prepared by the reaction of the phthalic anhydride and the corresponding alcohol have good Electrical insulation and heat resistance, can be used to produce impregnating agents and high temperature insulating varnishes, widely used in electrical parts, automotive wiring, semiconductors, etc.; used as automotive cables, moisture and heat resistant epoxy Capsule component, anti-fog polyethylene resin component and anhydrous dye component of fiber and thermoplastic; used as car seat cushion, artificial leather, blinds, sealing material and filler; etc.; also used in car dashboard, refrigerator gasket And PVC plasma bags, infusion bags and various plastic tubes, can also be used for PVC heat-resistant calendered film and production of beach tents and air cushions. Tetrakis(2-ethylhexyl) pyromellitate obtained by esterification of alkanoic anhydride with 2-ethylhexanol is a polyvinyl chloride heat-resistant plasticizer which can be used to produce heat-resistant cables of 102-120 °C. Special durable heat-resistant plastic products, polyvinyl chloride products used in medicine and food. In addition, pyromellitic dianhydride can also be used as a binder, a surfactant, a water treatment agent, a metal corrosion inhibitor, an additive for leather tanning agents, a low temperature performance improver for diesel, an electrophotographic color tone improver, an electrode material. , hot melt road marking paint, azo dyes and high temperature lubricants, dyes, etc.