Xinhao Li and Jiaxi Xu* Pages 158 - 162 ( 5 )
Background: Microwave couples directly with the molecules that are present in the reaction mixture, leading to a rapid, efficient, and instantaneous heating process. For a specific reaction system, the level of this internal heating is depended on the microwave power.Methods: The acid-catalyzed esterification of benzoic acid with butanol was selected as a model reaction to study the influence of the microwave power on the reaction rate. The kinetic behavior of the reaction was studied in a preheated temperature-controlled oil bath. The formation of butyl benzoate was measured as a function of time at six different temperatures between 60°C to 120°C. The concentrations of butyl benzoate present in solution were determined by means of HPLC with naphthalene as an internal standard. The plots of the natural logarithms of the concentrations versus time at each temperature and reaction rates were obtained, generating Arrhenius equation of the reaction. The reaction rates of microwave-assisted reactions under different power modes were determined similarly. The actual reaction temperature under microwave conditions were derived from Arrhenius equation. Results: The results indicate that the microwave power can affect the heating rate of the reaction mixture and cause the superheating phenomena at the initial reaction stage. When performing the esterification at a setup microwave power of 40 W or higher, the more microwave power, the more obvious superheating phenomena. An approximate linear relationship between the initial microwave power and the actual temperature of reactions was observed. When using the appropriate power mode, such as switching the Powermax mode off, with the appropriate heating rate, the superheating at the initial reaction stage disappears. Conclusion: The application of simultaneous cooling would lengthen the heating time but increase the actual temperature of the reaction. When using a significantly lower initial microwave power of 10 W, the actual temperature of the reaction abnormally increased.
Esterification heating rate, microwave power, simultaneous cooling, superheatin.
State Key Laboratory of Chemical Resource Engineering, Department of Organic Chemistry, Faculty of Science, Beijing University of Chemical Technology, Beijing 100029, Department of Organic Chemistry, Faculty of Science, Beijing University of Chemical Technology, Beijing 100029