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.

In recent years, the increase in demand for fumaric acid from industry has resulted in an increased need for a high-selectivity process for the conversion of maleic acid to fumaric acid. A highly selective conversion of fumaric acid was achieved without a catalyst by a simple one-step hydrothermal reaction. In addition, the competitive conversion of maleic acid, fumaric acid, and malic acid was first systematically investigated in detail without using a catalyst. The products were characterized by X-ray diffraction and Fourier transform infrared, which demonstrated that the product was fumaric acid. The highly selective conversion of fumaric acid was achieved, and the yield of fumaric acid could reach 92%. Furthermore, a reaction kinetic model was put forward to study the competitive transformation process. The kinetic model predictions were found to agree well with the experimental data. The kinetic parameters were used to explain the changes in the content of every substance at different reaction temperatures and reaction times. In addition, the initial maleic acid concentration in the reaction was also considered as an influencing factor. These results can facilitate the conditional control and product control of industrial processes for the production of fumaric acid or malic acid using latter without a catalyst.

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