Clean synthesis method of pyromellitic diimide

According to the invention, urea and choline chloride are prepared into liquid urea in advance, namely urea-choline chloride DES (2: 1), pyromellitic dianhydride is used as a raw material, a urea-choline chloride eutectic solvent DES is used as a solvent, and pyromellitic diimide is synthesized at a lower temperature by using urea as a catalyst. The method overcomes the defect that the pyromellitic diimide PMDI prepared by a pyromellitic dianhydride and urea solid-phase synthesis method is low in efficiency, and the solvent DES can be recycled and reused. Due to the introduction of liquid urea DES, the use of a volatile organic solvent VOCs is avoided, so that the whole preparation process is more environment-friendly. And filtering the PMDI crude product, sequentially washing with acetone and water, and drying to obtain the PMDI product with the purity of more than 99%. The process is easy to operate and mild in condition, decomposition of urea is reduced due to the low reaction temperature, raw material los