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Microwave chemistry is based on the efficient heating of materials in most cases solvents by dielectric heating effects. Dielectric heating works by two major mechanisms:. Schematic illustration of the two main dielectric heating mechanisms: Gases cannot be heated under microwave irradiation, since the distance between the rotating molecules is too wide. Similarly, solid materials like ice are nearly microwave transparent, since the water dipoles are bound in the crystal lattice and cannot move as freely as in the liquid state.
However, some conductive solid materials, like silicon carbide, where electrons can move freely, are excellent microwave absorbers and therefore heat very quickly. With dielectric heating, electric energy is converted into kinetic energywhich is ultimately converted into heat. The heating characteristics of a particular material e. If, however, the mixture is non-polar, passive heating elements can be added to aid the heating process.
In general, the interaction of microwave irradiation with matter is characterized by three different processes: While highly dielectric materials, like polar organic solvents, lead to strong absorption of microwaves and consequently to a rapid heating of the medium, non-polar microwave transparent materials show only small interactions with microwaves transmission.
Microwaves pass through such materials. This makes them suitable as construction materials for reactors. If microwave radiation is reflected by the material surface, there is almost no introduction of energy into the system. Interaction of different materials with microwaves: Graphical illustration of heat introduction and temperature distribution in to a reaction mixture for a conventional heating and b microwave heating.
While conventionally the heat comes from the outside and goes into the reaction mixture by convection currents resulting in a very hot vessel wall , microwaves go through the almost microwave-transparent vessel wall and directly heat the reaction mixture on a molecular basis. Traditionally, organic synthesis is carried out by refluxing a reaction mixture using a hot oil bath as a heat source. However, this way of heating a reaction mixture is comparatively slow and energy inefficient, since first the heat energy is transferred from the hot oil bath to the surface of the reaction vessel, and then the hot surface heats the content of the reaction vessel see Figure 6, entry a.
Furthermore, the hot surface can lead to local overheating and to decomposition of sensitive material. Microwaves pass through the almost microwave-transparent vessel wall see Figure 5, transmission and heat the reaction mixture on a molecular basis — by direct interaction with the molecules solvents, reagents, catalysts, etc. Furthermore, the conversion of electromagnetic energy into heat energy works highly efficiently and results in extremely fast heating rates — not reproducible with conventional heating.
Due to the rapid heating to the target temperature, the formation of byproducts is suppressed. This is another huge advantage of microwave heating, since it means that higher product yields can be achieved and the work-up is simplified. In the early days of microwave synthesis, household microwave ovens were extensively used in chemical laboratories.
Although nowadays dedicated instrumentation is available , there are still chemists who use kitchen microwave ovens for scientific purposes. However, most major scientific journals no longer accept manuscripts wherein domestic ovens have been described as a heating source, [6] since there are serious scientific and also practical reasons why dedicated instrumentation should be preferred:. Domestic microwave ovens have exclusively been developed for household purposes. Therefore, such ovens do not provide any active safety features for chemical synthesis.
If nevertheless used for chemical reactions, household ovens will not protect the chemist in case of unexpected reaction behavior.
As it is the highest priority of an instrument manufacturer to provide the utmost safety for the user, dedicated instrumentation has numerous safety features implemented in order to allow safe processing, even under extreme temperature and pressure conditions. Superheating of solvents in sealed vessels is the key advantage in microwaves synthesis, since due to the Arrhenius law, the reaction times can be shortened considerably! In open vessel systems, the potential time-saving benefit is limited.
Therefore, microwave synthesis in sealed vessels gives access to a much wider range, applying temperatures far above the boiling point of the used solvent s. Domestic microwave ovens do not allow reaction parameter control, since this is not necessary for simply heating food to eatable temperatures. A kitchen microwave oven therefore does not provide any possibility to measure the temperature of the reaction mixture.
Microwave Assisted. Organic Synthesis. Danielle L. Jacobs. Crimmins Group Organic rxns in mids, but only recently exploded. • Lack of. In this context microwave-assisted organic synthesis (MAOS) is a very attractive option. The microwave technology applied in organic synthesis was introduced.
However, this is the key parameter for chemical synthesis! In contrast to domestic microwave ovens, dedicated microwave reactors are usually equipped with IR sensors for reaction temperature control, pressure sensors to monitor the reaction pressure in the closed vessels and a magnetic stirrer to enable proper agitation. Often optional immersing temperature probes for more accurate internal measurement of the reaction temperature are available. Figure 7 shows an example heating profile of a typical microwave experiment, in which the parameters are accurately measured and recorded throughout the whole experiment process.
The instrument records temperature, pressure and power during the whole experiment. A typical experiment process consists of three steps: Since time equals money, tools for improving the economic efficiency are always highly appreciated. Besides the great advantages that result from microwave heating per se, dedicated microwave instrumentation allows for an additional efficiency improvement, since in contrast to domestic microwave ovens they enable you to adapt two valuable approaches for your laboratory workflow:.
Parallel techniques from high-throughput reaction screening to parallel scale-up as well as exploiting the economic advantages of automated sampling units are well-established methods nowadays.
These techniques cannot be served by domestic ovens in such a convenient and efficient way. In contrast to domestic ovens, dedicated microwave reactors feature an in-built magnetic stirrer, which is of great importance since without proper agitation the temperature distribution within the reaction mixture will not be uniform and the measured temperature will be dependent on the position of the temperature sensor, as clearly demonstrated in Figure 8.
Reprinted with permission from: Copyright American Chemical Society. As a consequence, even completely homogeneous solutions need to be stirred when employing microwave heating, since otherwise efficient agitation cannot be ensured and temperature gradients may develop. In cases of e. Domestic microwave ovens usually provide a pulsed irradiation mode for heating. This means that a setting of e.
This way of heating is sufficient for household applications. However, it can be very dangerous when used for chemical synthesis, since such power peaks force the formation of hot spots and the risk of spontaneous non-controllable exotherms. In order to reduce the occurrence of hot spots as well as to minimize the risk of thermal runaways, dedicated reactors provide continuous power output Figure 8b.
This allows you to apply exactly the amount of microwave power which is needed in order to reach or hold the set reaction temperature. Applied microwave power profile for an experiment at W a in a pulsed mode application domestic microwave oven , and b in a reactor providing the possibility of continuous power output. Besides scientifically and economically important advantages, dedicated instruments also provide advantages in terms of handling and programming.
They are usually equipped with an intuitive user interface that is operated via touchscreen, where the software supports on-screen monitoring and editing of running experiments, automatic data recording as well as data management on the instrument or on a PC. Get your free copy now. Here is a list of interesting and relevant scientific publications on microwave synthesis of the last decade.
Anton Paar Research Award Are you developing new methods or applications in instrumental analytics or material characterization? General aspects The bottleneck of conventional synthesis is typically the optimization, i. History While fire is nowadays rarely used in order to perform chemical synthesis it was not until Robert Bunsen invented the burner in that the energy from this heat source could be applied to a reaction vessel in a focused manner.
Principles of microwave heating The electromagnetic spectrum Figure 2: The spectrum of electromagnetic waves. Microwave dielectric heating Microwave chemistry is based on the efficient heating of materials in most cases solvents by dielectric heating effects. Dielectric heating works by two major mechanisms: Dipolar polarization see Figure 3 For a substance to be able to generate heat when irradiated with microwaves it must be a dipole, i. Since the microwave field is oscillating, the dipoles in the field align to the oscillating field.
This alignment causes rotation, which results in friction and ultimately in heat energy. Ionic conduction see Figure 3 During ionic conduction, dissolved completely charged particles usually ions oscillate back and forth under the influence of microwave irradiation.
Microwave heating in the laboratory began to gain wide acceptance following papers in , [6] although the use of microwave heating in chemical modification can be traced back to the s. Conventional heating usually involves the use of a furnace or oil bath, which heats the walls of the reactor by convection or conduction.
The core of the sample takes much longer to achieve the target temperature, e. Acting as internal heat source, microwave absorption is able to heat the target compounds without heating the entire furnace or oil bath, which saves time and energy. However, due to the design of most microwave ovens and to uneven absorption by the object being heated, the microwave field is usually non-uniform and localized superheating occurs.
Microwave volumetric heating MVH overcomes the uneven absorption by applying an intense, uniform microwave field.
Different compounds convert microwave radiation to heat by different amounts. This selectivity allows some parts of the object being heated to heat more quickly or more slowly than others particularly the reaction vessel. Microwave chemistry is applied to organic chemistry [8] and to inorganic chemistry. A heterogeneous system comprising different substances or different phases may be anisotropic if the loss tangents of the components are considered.
As a result, it can be expected that the microwave field energy will be converted to heat by different amounts in different parts of the system. This inhomogeneous energy dissipation means selective heating of different parts of the material is possible, and may lead to temperature gradients between them. Nevertheless, the presence of zones with a higher temperature than others called hot spots must be subjected to the heat transfer processes between domains.
Where the rate of heat conduction is high between system domains, hot spots would have no long-term existence as the components rapidly reach thermal equilibrium. In a system where the heat transfer is slow, it would be possible to have the presence of a steady state hot spot that may enhance the rate of the chemical reaction within that hot zone.
On this basis, many early papers in microwave chemistry postulated the possibility of exciting specific molecules, or functional groups within molecules. However, the time within which thermal energy is repartitioned from such moieties is much shorter than the period of a microwave wave, thus precluding the presence of such 'molecular hot spots' under ordinary laboratory conditions.
The oscillations produced by the radiation in these target molecules would be instantaneously transferred by collisions with the adjacent molecules, reaching at the same moment the thermal equilibrium.
Processes with solid phases behave somewhat differently. In this case much higher heat transfer resistances are involved, and the possibility of the stationary presence of hot-spots should be contemplated. A differentiation between two kinds of hot spots has been noted in the literature, although the distinction is considered by many to be arbitrary.
Macroscopic hot spots were considered to comprise all large non-isothermal volumes that can be detected and measured by use of optical pyrometers optical fibre or IR.