Kinetic Study of CaO-Catalyzed Transesterification of Cyclic Carbonates with Methanol

Simultaneous catalytic contributions of CaO in the liquid phase (homogeneous) and in the solid phase (heterogeneous) for transesterification are considered in the kinetic analysis, because sparingly soluble CaO has significant catalytic activity. Experimental concentration-time profiles were obtained at different catalyst loadings, propylene carbonate (PC) concentrations, and methanol concentrations in a temperature range of 20-60 degrees C in a stirred batch reactor for both the homogeneous and heterogeneous reactions. Kinetic models based on power-law rate forms were evaluated and discriminated to fit the experimental data. Reaction orders of 1 for PC, 2 for methanol, and 1 for both DMC and PG fit the experimental data best for the homogeneous reaction, whereas orders of 0.35 for PC, 1.9 for methanol, and 0 for both DMC and PG fit the experimental data best for the heterogeneous reaction. The heterogeneous CaO catalyst (E-a = 56.8 kJ/mol) was found to be more active than the homogeneous CaO catalyst (E-a = 82.4 kJ/mol). The reaction-order and activation-energy differences may be attributed to different reaction pathways for homogeneous- and heterogeneous-CaO-catalyzed transesterifications. Different reaction mechanisms were proposed for the homogeneous and heterogeneous reactions, respectively. It is observed that kinetic models based on mechanisms fit better than power-law models, especially for the heterogeneous reaction. The kinetic model for the homogeneous reaction suggests the formation of active species CH3O-. Then, PC gets activated by the CH3O- to form the first intermediate, which then forms the second intermediate by reaction with one mole of methanol. This is followed by a reaction of the second intermediate with another molecule of methanol to form the final products, DMC and PG. However, for the heterogeneous-CaO-catalyzed reaction, adsorption of PC and dissociative adsorption of methanol on the surface of the active species is assumed to occur simultaneously. Then, the adsorbed PC and methanol react to directly form the final products, DMC and PG. The good fits of the kinetic models with the experimental data suggest that the proposed homogeneous- and heterogeneous-reaction mechanisms represent the transesterification of propylene carbonate satisfactorily.