Preparation method of diethyl maleates  is mainly made by esterification of maleic anhydride and ethanol in the presence of sulfuric acid. This process has two kinds of atmospheric pressure with benzene esterification and negative pressure without benzene esterification.
(1) Atmospheric pressure with benzene esterification
Add a certain amount of benzene and ethanol to the esterification reaction pot, put in maleic anhydride, add concentrated sulfuric acid dropwise under stirring, heat through the jacketed steam, and make the reactants undergo esterification at about 75℃. The generated water is removed by ternary azeotropic distillation with benzene and ethanol, and the upper layer of benzene and ethanol liquid is refluxed to the reaction pot. About 13 ~ 14h later, when the distillation tower temperature rises to 68.2℃, the separator lower water level is no longer rising, indicating that all the water in the reaction pot has been evaporated, the esterification reaction is complete. Stop reflux, continue distillation to 95 ~ 100℃, distillation of benzene and ethanol. Cool down to 50℃, neutralize with 5% aqueous sodium carbonate solution, wash with water and then remove the residual benzene and ethanol under vacuum to obtain the product diethyl maleic acid.
(2) Negative pressure benzene-free esterification
Esterification of maleic anhydride and ethanol under the action of sulfuric acid is carried out under a certain vacuum and temperature to bring out the ethanol and the water generated by the reaction in a gaseous state, and then the ethanol is separated through the fractionation column to reflux the esterification, so that the reaction tends to be complete. This method can shorten the reaction cycle, improve the yield and product quality, improve the operating environment, most domestic production plants use this method.
In addition, cation exchange resin can also be used as a catalyst for exchange conversion to produce diethyl maleic acid.
Refining method: Washing with dilute potassium carbonate solution, drying with anhydrous potassium carbonate or sodium sulfate and distillation under reduced pressure.