Chapter 1
General Introduction to Microwave Chemistry

Satoshi Horikoshi and Nick Serpone

1.1 Electromagnetic Waves and Dielectric Materials

The common characteristics of electromagnetic waves (light and radio waves) are that they (i) have no mass, (ii) are free to move at the speed of light, (iii) cannot collide with each other, (iv) possess significant energy, and (v) are reflected on a metal surface. By contrast, when microwaves (MWs) are used to irradiate dielectric materials, various phenomena occur according to the nature of the electromagnetic waves. The influence of electromagnetic waves on dielectric materials in various ranges of the electromagnetic spectrum is summarized in Figure 1.1 [1], which also shows the various analytical instruments and their usefulness in the ranges indicated.

1. Atomic species: If a dielectric is placed in an alternating electric field of the electromagnetic waves, the positions of the atomic nucleus (protons) and of the electrons that constitute the dielectric tend to follow the course of the electric field. The spatial relationship of a proton and an electron in an atom becomes distorted when subjected to the electric field (referred to as electronic polarization). Since the electric dipole that reflects the electric deviation (a proton and an electron) inside an atom is very small, a resonance phenomenon will occur with light of short wavelengths such as X-rays and ultraviolet rays.
2. Molecular species: The distortion of the electric charge by electromagnetic waves also occurs in a molecule with an electric dipole moment and in a salt. For example, in the structure of NaCl salt, composed of Na+ and Cl- ions, distortion of the structure arises when subjected to an alternating electric field (referred to as ionic polarization). An electric dipole moment of a molecule is large compared with that of an atom, so that resonance occurs in the infrared domain, that is, at the longer wavelengths. The magnitude of ionic polarization is dictated by the interionic distance and the types of ions involved. Moreover, changes in bond lengths in an organic molecule occur through stretching and bending vibrations. These phenomena occur at various wavelength regions absorbed, depending on the chemical structure; they can be observed by means of an infrared absorption spectrophotometer.
3. Molecular assemblies: Orientation polarization of a dielectric possessing an electric dipole takes place on interaction with the electromagnetic waves in the microwave range. Moreover, as for the case of ions in a solution, Joule heating takes place by space-charge polarization. For example, when microwave heating an electrolyte/water solution, dielectric heating and Joule heating occur simultaneously compared with pure water, and thus the exothermic efficiency becomes remarkably high. Accordingly, dielectric heating by orientation polarization of water and resistance heating by the Joule process are enhanced in electrolyte/water media.

Figure 1.1 Resonance of dielectric to electromagnetic waves and positioning of the analytical equipment.

Reproduced from Ref. [1] Copyright 2013 by Wiley-VCH Verlag GmbH.

1.2 Microwave Heating

Microwave radiation is electromagnetic radiation spanning the frequency range 30 GHz-300 MHz (i.e., from a wavelength of 1 m to 1 cm). It is used widely in communications and in heating processes, especially in the heating of foodstuff. Historically, the powerful interaction of microwaves with materials was discovered in 1946 from the melting of chocolate, a process attributed to microwave heating, whereas the first commercial microwave oven was developed in 1952 by the Raytheon Company [2]. This discovery is frequently taken up as an example of serendipity. Before the discovery of microwaves, high-frequency induction heating was commonly used. The patent of dielectric heating by means of high-frequency induction was issued in 1933 [3]. In the 1970s, the microwave generator was re-engineered by Japanese scientists into a domestic microwave oven (using a simple, reliable, and inexpensive magnetron), which allowed for simple food and mass processing [4]. Domestic and industrial microwave ovens generally operate at a frequency of 2.45 GHz corresponding to a wavelength of 12.24 cm and an energy of 1.02 × 10-5 eV, or around 900 MHz with a longer wavelength of 37.2 cm, which can provide up to 100 kW in larger process heating applications [5]. Since then, microwave chemistry has become a rapidly developing branch in numerous fields of research, in industry, and in its applications.

A molecule is formed when atoms combine to share one or more of their electrons. This rearrangement of electrons may cause an imbalance in charge distribution, thereby creating a permanent electric dipole moment. These moments are oriented in a random manner in the absence of an electric field so that no polarization exists. Electric dipole moments in a polar molecule undergo group movement so that a mutual electric dipole may be left without. When a microwave is applied to this state, an electric field has an influence on the electric dipole (Figure 1.2). The microwaves' electric field (E field) will exercise a torque (N) on the electric dipole, and the dipole will consequently rotate to align itself with the electric field thus causing orientation polarization to occur [6]. If the field changes direction, the torque will also change. The orientation polarization changes by a vibration in a microwave electric field. A time difference is caused between the frequency of the microwave electric field and the electric dipole of molecules. At the general frequency of 2.45 GHz, the microwaves vibrate 2 450 000 000 times/s; a molecular cluster cannot follow this vibration through the power chain with the surrounding molecules. This delay changes to heat energy (kinetic energy) as loss of the electromagnetic wave energy. The friction accompanying the orientation of the dipole will contribute to dielectric losses. The dipole rotation causes a variation in both ?r´ (the relative dielectric constant) and ?r? (the relative dielectric loss) at the relaxation frequency, which usually occurs in the microwave region. Water is an example of a substance that exhibits a strong orientation polarization.

Figure 1.2 Image of dipole rotation for polar molecule in an electric field (E field).

By contrast, a solid substance with a partial electric charge (a dielectric material) is also an insulator. The slight distortion of the atomic position (lattice point) in a structure lattice, and the lattice strain of a crystal of solid substances cannot follow the changing time of the microwave electric field. Microwave heating of a solid substance progresses by these phenomena [7]. On the other hand, the heating of solid substances with a magnetic dipole moment occurs by the microwaves' magnetic field component. The heating process is the same phenomenon as by the microwaves' electric field [8]. Generation of heat by magnetic loss heating is expected only in magnetic (solid) materials. Joule heating progresses by the interaction of an electric or a magnetic field with solid particles of activated carbon and with solids possessing conductivity-like metallic properties [9].

1.3 The Various Types of Microwave Heating Phenomena

Polarization phenomena are generated regardless of the polar or nonpolar nature of the molecules. On the other hand, alignment polarization (molecular alignment) occurs only when the polarity of molecules is affected by the electric field because of their permanent dipoles. Thus, the role of microwaves (and radio waves) is not simply as a heating source. With regard to the latter, we must recognize three types of heating phenomena caused by microwaves: (i) conduction loss heating, (ii) dielectric heating, and (iii) magnetic loss heating. In this regard, the thermal energy P produced per unit volume originating from microwave radiation is given by Eq. (1.1) [10]:

where |E| and |H| denote the strength of the microwaves' electric and magnetic fields, respectively; s is the electrical conductivity; f is the frequency of the microwaves; ?0 is the permittivity in vacuum; ?r? is the relative dielectric loss factor; µ0 is the magnetic permeability in vacuum; and µr? is the relative magnetic loss.

The first term in Eq. (1.1) expresses conduction loss heating; the second term denotes dielectric loss heating, whereas magnetic loss heating is given by the third term. Microwave heating of solutions is governed mostly by dielectric loss heating, whereas conduction loss heating involves mostly, but not exclusively (see later), solid materials. Therefore, microwave heating of materials is dictated by the electrical, dielectric, and magnetic properties. Moreover, when a change of frequency occurs, their behavior also changes. It is possible to design catalyzed reactions based on these properties and build innovative microwave chemistry processes. A catalyzed reaction occurring on a solid catalyst is expected to show characteristic differences relative to the solution bulk because the reacting substrates in the liquid medium may be adsorbed onto the solid's surface. Temperature rise in systems...