Study on the Isomerization of Maleic Acid to Fumaric Acid without Catalyst

Fumaric acid is an important food additive and industrial intermediate compound. The traditional methods of producing fumaric acid were catalyzed by maleic acid isomerization. In this study, isomerization of maleic acid in water without catalyst was investigated at elevated temperature, which addressed the problem of sewage discharge. HPLC analysis showed maleic acid converted into fumaric acid and small amount of malic acid simultaneously. The effects of concentration of maleic acid, reaction temperature, reaction time, and stirring on the yield of fumaric acid were investigated. The optimum reaction conditions were also explored. The results showed the isomerization of maleic acid reached equilibrium at about 1 h, stirring did not affect the reaction rate, and conversion due to the monomolecular mechanism. In order to achieve “zero emission,” the recycle of filter liquor was also studied. The pH of filter liquor changing signifies the lower pH was in favor of the conversion of maleic acid to fumaric acid. The high yield and recyclability suggested that this process had promising application in fumaric acid production.