Kinetic Modeling of Maleic Acid Isomerization to Fumaric Acid Catalyzed by Thiourea Determined by Attenuated Total Reflectance Fourier-Transform Infrared Spectroscopy

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.