A comparative kinetics study on the isothermal heterogeneous acid-catalyzed hydrolysis of sucrose under conventional and microwave heating

Abstract

The isothermal kinetics of sucrose hydrolysis at the acidic ion-exchange resin type IR-120 H under conventional (CH) and microwave heating (MWH) was investigated. Isothermal kinetics curves in the temperature range from 303 to 343K for both CH and MWH were determined. By application the model-fitting method, it was recognized that the kinetics of sucrose hydrolysis can be described by a first-order chemical reaction for both heating modes. The values of the activation energy (Ea) and pre-exponential factor (lnA) for sucrose hydrolysis were found to be lower under MWH than under CH. Application of the differential isoconversional method showed that sucrose hydrolysis was kinetically an elementary reaction. It is found that the increased rate of hydrolysis observed under MWH was not a consequence of overheating. A new explanation of the established effects of microwave heating based on a model of selective energy transfer during the chemical reaction is suggested. The established decreases in the activation energy and in the pre-exponential factor under MWH in comparison to CH is explained by an increase in the energy of the ground vibrational level of the –OH out-of-plane deformation in the sucrose molecule and with a decrease in the anharmonicity factor, which is caused by the selective resonant transfer of energy from the catalyst to the –OH oscillators in the sucrose molecules.