Microwave-assisted synthesis of malic acid involving hydrochloric acid as catalyst

The competing acid catalyzed reactions of fumaric acid (F) isomerization to form maleic acid (Mx) (reaction R1), and the hydration of F to malic acid (M) (reaction R2) were investigated under microwave heating. The kinetic experiments were performed in a sealed microwave-irradiated batch reactor operated at pressures between 615 and 1715 kPa, in the temperature range from 433 to 478 K, and for reaction times up to 21,600 s. Hydrochloric acid at three different concentrations (C H = 0.65, 1.30 and 1.94 M) was used as the catalyst. The concentrations of F, Mx and M were periodically determined by HPLC analyses. The Arrhenius parameters for the forward rate constants of the reactions R1 and R2 were tuned on the experimental data of species concentrations (k 10 = 5.63 ± 0.04 M−1 s−1, E a1/R = 4837 ± 121 K, k 20 = (6.9 ± 0.3) × 107 M−2 s−1, E a2/R = 13,532 ± 212 K). The tuning procedure involved the Simplex method of optimization, and a kinetic model represented by a system of four ordinary differential equations from material balances on species F, Mx, M and HCl. The model was able to reproduce the variation in species concentrations with time for all experimental conditions investigated, just with a partial set of the currently obtained data used to adjust the model parameters (C H = 0.65 M and 1.94 M). The current results at C H of 1.30 M and analogous data from the literature under quite different reaction conditions (e.g.; C H = 0.97 M, non-isothermal reactor externally heated with glycerol) confirm the reliability of the modeling approach. The catalyst concentration had the expected significant effect on the reaction rates, but the influence of microwave irradiation on the kinetics of the conversion of F and on the selectivity of M was small.