Preparation method of pyromellitic acid

There are various methods for preparing pyromellitic acid, and the following are two common preparation methods:
Method 1:
It is obtained by oxidation of 1,2,4,5-tetratoluene (or 5-isopropyl-1,2,4-trimethylbenzene), or by chloromethylation and oxidation of xylene.
In the production process, certain steps are carried out in the presence of carbon dioxide, which helps to improve the yield of pyromellitic acid and reduce the content of impurities in the product.
Method 2:
Gas phase catalytic oxidation reaction was carried out on a fixed bed reactor using xylene and air as raw materials.
After the gas produced by the reaction is cooled by heat exchange in a heat exchanger, it passes through three traps in sequence.
Dissolve the captured gas, cool and crystallize it, filter it three times, and dry it to obtain phthalic anhydride.
Add phthalic anhydride to water, undergo hydrolysis reaction at 90-100 ℃ and 0.2MPa, and then undergo thermal filtration to obtain phthalic acid filtrate.
Crystallize the filtered solution of pyromellitic acid at 40-45 ℃ and atmospheric pressure for 6 hours, then centrifuge and separate.
Dry under negative pressure at 70-80 ℃ to obtain crude pyromellitic acid.
The crude product of pyromellitic acid is prepared by heating it to 220-230 ℃ for 12 hours in a vacuum environment, and then heating it to 250-270 ℃ for 0.5-2 hours to obtain the finished product of pyromellitic acid.
Both of the above methods can be used to prepare pyromellitic acid, but the specific process parameters and equipment may vary. In practical operation, appropriate preparation methods and process parameters should be selected according to the specific situation.