The fundamentals and mechanisms of microwave irradiation are fundamentals of microwave technology. Two mechanisms of the microwave technology, electric field and magnetic field heating, which make microwave irradiation unique and potentially viable in numerous fields are included as well. These aspects are essential to be understood prior to investigating microwave-absorbing catalysts for the production of biofuel. With the outstanding benefit of microwave heating, the active sites of catalysts or entire catalysts can be heated selectively to enhance the catalytic performance. In the case of microwave-unabsorbing material, selective heating of the catalyst will result in heat transfer to organic solvents and eventually increases the conversion yield. The kinetic rate by microwave irradiation was higher than conventional heating by 1.15 times.
Microwaves are a form of electromagnetic waves consisting of two perpendicular components (electrical and magnetic fields) that their electromagnetic spectrum fall between radio waves and infrared . The microwave frequencies are ranged from 0.3 to 300 GHz which correspond to the wavelength of 1 m to 1 mm. Microwave is generally utilized in phones, radar signals, navigational applications, satellite communications, food preparation, drying materials or medical treatments. In chemical industries, the microwave irradiation is mainly used for materials heating purposes, where the materials’ responses varied due to the magnetic and electric fields associated with microwaves . As microwave irradiation is mainly utilized for telecommunications, the regulations of the wavelengths for microwave equipment in research, industrial, domestic and medical field are required and the major operating frequency is 2.45 ± 0.05 GHz for most of the countries .
To generate microwave irradiation, the basic components required for industrial microwaves are the generator, waveguide and applicator. The usage of generator is the main component to convert the electrical energy into microwave energy and it often composed of transformer, solenoid and magnetron tube. Microwave irradiation is emitted from magnetron tube, while solenoid is used to wrap the magnetron tube for regulating the microwave power and high voltage is produced using transformer . The waveguide is used to guide the microwave to the applicator, usually built up of aluminum due to its lightweight. The applicator is a place where microwave irradiation meets the load or samples to be investigated.
Several important factors have to be considered in microwaves are the frequency used, product mass, ionic content, dielectric properties, specific heat and density of the target object. The properties of materials as regard to microwave irradiation are described as followed. Their complex permittivity (ε*) is described in Equation (1) :
ε* = ε′ − jε″ = ε0 (εr′ − jε″eff)
where ε′ is the real term to quantify the material’s ability in storing electrical energy (also known as dielectric constant) and ε″ is the imaginary term of a loss factor that the material’s ability in dissipating electrical energy (also known as dielectric loss); while the complex permeability (µ*) is described in Equation (2):
µ* = µ′ − jµ″
where µ′ is the real term to symbolic the amount of magnetic energy stored in the material and µ″ is the imaginary term of the magnetic energy that can be converted into heat energy. The ratios of imaginary to real terms for the complex permittivity and permeability are stated as below:
tan δ = ε″/ε′
tan δµ = µ″/µ′
where tan δ is the loss tangent (for complex permittivity) and tan δµ is the magnetic loss tangent (for complex permeability) . The dielectric constant and dielectric loss varied with respect to the type of materials, which subsequently influence the loss tangent.
As compared to radio waves, microwave has higher frequencies which contains higher energy. From literatures, the higher microwave frequency significantly affects the penetration depth of microwave irradiation and their relationship can be expressed using wavelength and penetration depth as shown in following equation .
Dp = (λ/2π)·[(ε′)1/2/ε″]
where Dp represents the depth of penetration which the incident energy absorbed is 63%, λ represents the wavelength of microwave, and π is the value of 3.1415.
The power loss density per unit volume, P, for both electric field and magnetic field heating are stated in Equations (6) and (7), respectively.
P = ω·ε″eff ·ε0·E2rms
P = ω·µ″eff ·µ0·H2rms
where ω is the angular frequency (also equals to 2πf, where f is frequency), ε″eff is the effective dielectic loss factor, ε0 is the permittivity of free space, Erms is the local value of electric field for Equation (6); while µ″eff is the effective magnetic loss factor, µ0 is the magnetic permeability of vacuum and Hrms is the local value of the magnetic field .
By exposing product to the microwave irradiation, the product mass is a concern especially for material heating. As the heat exchange is solely depending on the product mass, makes no difference for microwave heating and conventional heating through the definition of isobaric heat capacity (ΔQ = m·Cp·ΔT), and changing only based on temperature difference and Cp. However, both methods possess varied heating efficiencies and lead to different energy consumption. The properties of the object being microwaved can influence the efficiency and uniformity of microwave irradiation. Ionic content of the subject influences the occurrence of ionic conduction when irradiate with microwaves while the dielectric properties of subject governs on the ability of a material to interact with the electromagnetic field of microwave .
2. Mechanism of Microwave Irradiation
Catalytic reaction performed through microwave irradiation has gained much attention due to their outstanding performance and enhanced reaction efficiency. The mechanisms of microwave irradiation are the main factor for their wide range of application in chemical processes. The energy absorbed during the process was dissipated as heat, hence the microwave heating is internal rather than external which different from the conventional heating methods. The even internal heating resulted by the conversion of electromagnetic energy into heat at molecular level is believed to assist in the enhancement of catalyst stability and decrement of coke formation .
In general, the heating mechanisms of microwave energy consist of electric field heating and magnetic field heating, which are governed by different mechanisms. For electric field heating, there are two major mechanisms, namely dipolar polarization and ionic conduction, while the combination of these two mechanisms is interfacial polarization . The dipolar polarization is a mechanism involves the rotation of polar molecules aligning themselves with microwave’s electric field, where the heat and friction were generated due to the continuous alignment of polar molecules as the electric field was regularly oscillating as shown in Figure 1. As for ionic conduction, the electric current is produced due to the oscillation of ions in forth and backward due to the electric force of microwaves. The produced current undergoes internal resistance, in which the collisions between the charged molecules with their neighboring molecules heat up the materials .
Figure 1. Electric field heating mechanisms under microwave. (a) Dipolar polarization which involves the rotation of polar molecule to align itself with electric field. (b) Ionic conduction that shows an ion moving back and forth if the molecule is charged.
However, another dominant mechanism known as magnetic losses would be advantageous to the selective heating of particular catalyst zones, due to the losses of eddy currents, hysteresis and residual loss occurred. Heat generation was observed during hysteresis losses as the magnetic dipoles of the materials could not keep pace with altering magnetic poles leading to a friction. Eddy current losses occur when there are resistances towards the circulating currents within the magnetic materials created by the generated electromagnetic field; while residual losses refers to other types of losses other than hysteresis and eddy current losses .
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