Applying a unified process kinetic equation to advanced materials process analysis: Characterization of the kinetics of isothermal microwave‐assisted chemical syntheses

Abstract

Rate-enhancement of any isothermal, isobaric chemical synthesis conducted under resonant microwave (RM) irradiation versus the same process activated by conventional field-free heating has been attributed to a reduction in activation enthalpy of the process. This report applies a unified process kinetic equation (UPKE) to demonstrate and characterize non-thermal microwave effects (NTME) on kinetics-enhancements observed in isothermal microwave-assisted chemical syntheses (IMACS). The UPKE, derived from a mesoscopic irreversible thermodynamic model, pinpoints that the rate of any high-affinity chemical reaction is effectively independent of the affinity of the process as described by the mass-action rate law. Energetically, activation enthalpy reduction observed in IMACS is considered the major NTME, which causes dominant process-rate enhancements. This NTME results from RM-induced enthalpy variation during the reaction: RM energy-input first promotes the molar enthalpy of the irradiated reactant(s) at temperature, which consequently motivates an activation enthalpy reduction for rate-enhancement. Conversely, frequency coefficient lowering is another common NTME occurring in IMACS, causing an adverse yet compensable setback to process-kinetics as predicted by the UPKE. Applicability of the UPKE-proposed rationale and methodology for IMACS kinetic characterization is fully confirmed by relevant data in the literature.