Kinetic study of multiphase reactions under microwave irradiation: A mini-review

Abstract

Microwave (MW) heating is rapid, selective, and volumetric, and it is a compelling non-conventional heating approach for driving chemical reactions. The effect of MW irradiation on the kinetics of thermal/catalytic reactions is still under debate. A group of researchers reported that the effect of MW heating on reaction kinetics is highlighted through the non-thermal effects of MWs on kinetic parameters and reaction mechanisms in addition to the thermal effect. However, another group attributed the observations to the thermal effect only. In the present work, we summarized and critically synthesized available information in the literature on the subject. It can be concluded that MW heating has solely the thermal effect on gas-solid reactions, and the variations of kinetic parameters are related to the direct and indirect impacts of that. Temperature measurement limitations, physical structure variation, and non-uniform temperature distribution are the primary sources of the discrepancy in previous studies. In ionic liquid-solid reactions, the presence of electromagnetic fields can affect the movement of ions/polar molecules which can be considered a non-thermal effect of MWs. However, the effect of MW absorption by solid/catalyst, and the formation of hot spots must be taken into account to avoid potential discrepancy. Therefore, further theoretical/experimental studies are required to clarify the effect of MWs on liquid-solid reactions. In addition, developing reliable temperature measurement methods and isothermal reaction domain are required for an accurate kinetic study during MW irradiation.

Temperature Distribution Assessment in Gas–Solid Reactive and Nonreactive Systems Heated by Microwaves

“…Some studies reported that kinetic parameters change under MW heating due to the nonthermal effect of MW heating in addition to the thermal effect . However, results presented in Figure show that nonuniformity in heat dissipation inside the reactor can result in the underestimation/overestimation of temperature.…”

“…In addition, the presence of elements/compounds with different dielectric properties in the solid phase and applying hybrid heating, i.e., employing an MW absorber, such as SiC or activated carbon, for heating materials that have poor interactions with MWs, can result in microscale hotspots. They can, accordingly, affect apparent kinetic parameters …”

“…Variations of the solid-phase physical structure under MW heating can affect apparent kinetic parameters. When the internal mass transfer is the reaction rate-limiting step, the appearance of microfractures inside solid particles can cause lower apparent activation energy than what can be obtained for conventional heating . Therefore, minimizing micro- and macroscale hotspots is a promising approach for estimations of intrinsic reaction kinetic parameters.…”

“…In addition, both the increase and decrease of the pre-exponential factor of reaction kinetics under MW irradiation were reported in the literature. However, since MWs can potentially improve agitation and the pre-exponential factor represents the collision frequency in a specific reaction, its reduction under MW heating is questionable. ,, Moreover, no theoretical proof and accurate experimental observations were reported to accurately answer the MW effect on chemical reaction improvement. , Interested readers are encouraged to check out the recent review by Adavi et al for further details.…”

“…The impedance of EMWs is the ratio of the transverse components of the electrical field to those of the magnetic field. Transverse components are those perpendicular to the direction of EMW propagation . The impedance of the cavity can be affected by variations in the voidage, diameter, and height of the loaded sample (i.e., the gas–solid mixture in a gas–solid reactor, also called the bed), as well as its electrical conductivity.…”