A set of chemical kinetic and phase-transition experiments was performed to investigate the competing conversions of fumaric acid into maleic and malic acids by isomerization and hydration, respectively. The reactions were carried out in sealed stainless steel batch reactors operated up to ∼1600 kPa and jacketed with glycerol at 398, 423, 448, and 473 K. The catalyst used was 0.97 M hydrochloric acid. To avoid excessive data near equilibrium, the reaction time was reduced from 720000 to 12600 s as the temperature of the heating fluid was increased. To ensure a homogeneous reaction, the solubility of fumaric acid in water was determined in the temperature range from 293 to 473 K using a high-pressure variable-volume sapphire view cell. The variation in the species concentration with time was determined by HPLC analysis of 12 reaction mixture samples per reaction run conducted twice under identical conditions. The parameters of the Arrhenius equation for the isomerization and hydration reactions were tuned on the kinetic experimental data by applying the Simplex method of optimization (k 20 = 1.27 × 10–7 s–1, k 30 = 1.29 × 107 s–1, E a2/R = −2815 K, E a3/R = 11260 K).

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Fumaric acid (FA) is an important commodity in the food and polymer industries; its main route of production is the chemical synthesis from maleic acid (MA). In this work, the isomerization of MA to FA catalyzed by thiourea was investigated. The experiments were performed in an automated reactor with temperature and stirring control, using methanol as the solvent to ensure a homogeneous system and to avoid a possible parallel hydration reaction to malic acid. The reaction kinetics was monitored in real time by attenuated total reflectance Fourier-transform infrared spectroscopy (ATR/FT-IR). Studies on the influence of the catalyst concentration (1−20% of the initial molar concentration of MA) and the temperature (20−60 °C) were carried out in reaction times between 2 and 5 h. The kinetics of the reaction was modeled using an integral method over the infrared data, assuming a reaction mechanism that yielded a partial order of 1.41 for the MA concentration and a 0.99 order for the thiourea. These orders were similar to the ones found by the proposed mechanism for the acid-catalyzed isomerization, in which monomaleate is a nucleophilic reagent to react with the protonated MA. This may present a path for further studies regarding the correct reaction mechanism. The kinetic model provided an excellent correlation with the experimental data, and the observed activation energy of the reaction was calculated to be 43.1 kJ mol −1 . At higher temperatures, such as 60 °C, and with a thiourea concentration of 10% mol/mol (0.0502 mol L −1 ), the isomerization conversion was around 90% in 60 min. Accordingly, thiourea proved to be an appropriate catalyst for FA synthesis, and ATF/FT-IR proved to be a reliable technique for the kinetic study.