Microwave chemistry is the science of applying microwave radiation to chemical reactions.

Microwaves act as high frequency electric fields and will generally heat any material containing mobile electric charges, such as polar molecules in a solvent or conducting ions in a solid. Microwave heating occurs primarily through two mechanisms: dipolar polarization and ionic conduction. Polar solvents because their dipole moments attempt to realign with the oscillating electric field, creating molecular friction and dielectric loss. The phase difference between the dipole orientation and the alternating field leads to energy dissipation as heat. Semiconducting and conducting samples heat when ions or electrons within them form an electric current and energy is lost due to the electrical resistance of the material. Domestic and commercial microwave systems typically operate at a frequency of 2.45 GHz, which allows effective energy transfer to polar molecules without quantum mechanical resonance effects. although the use of microwave heating in chemical modification can be traced back to the 1950s. Although occasionally known by such acronyms as MAOS (microwave-assisted organic synthesis), MEC (microwave-enhanced chemistry) or MORE synthesis (microwave-organic reaction enhancement), these acronyms have had little acceptance outside a small number of groups.

Microwave chemistry is applied to organic chemistry and to inorganic chemistry.

Microwave effect

There are two general classes of microwave effects:

  • Thermal microwave effects, which involves the heating of a component. This includes the specific microwave effects which cannot be (easily) emulated through conventional heating methods. Examples include: (i) selective heating of specific reaction components, (ii) rapid heating rates and temperature gradients, (iii) the elimination of wall effects, and (iv) the superheating of solvents. Microwave-specific effects tend not to be controversial and invoke "conventional" explanations (i.e. kinetic effects) for the observed effects.
  • Non-thermal microwave effects, which were proposed in order to explain unusual observations in microwave chemistry. As the name suggests, the effects are supposed not to require the transfer of microwave energy into thermal energy. Such effects are controversial.

A review has proposed this definition and examples of microwave effects in organic chemistry have been summarized.

Heating effect

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.g. when heating a large sample of ceramic bricks.

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. and in-situ methods. Some theoretical and experimental approaches have been published towards the clarification of the hot spot effect in heterogeneous catalysts.

A different specific application in synthetic chemistry is in the microwave heating of a binary system comprising a polar solvent and a non-polar solvent obtain different temperatures. Applied in a phase transfer reaction a water phase reaches a temperature of 100 °C while a chloroform phase would retain a temperature of 50 °C, providing the extraction as well of the reactants from one phase to the other. Microwave chemistry is particularly effective in dry media reactions.

Catalysis

Application of microwave heating to heterogeneous catalysis reactions has not been explored intensively due to presence of metals in supported catalysts and possibility of arcing phenomena in the presence of flammable solvents. However, this scenario becomes unlikely using nanoparticle-sized metal catalysts. MAE can be open (exposed to air), dynamic (with a pump), nitrogen-protected, done under vacuum, or even combined with ultrasound assisted extraction. An 2012 study from Australia found that brewing a teabag in freshly boiled water for 30 seconds followed by 1 minute of MAE in a microwave oven improves the extraction of catechins and caffeine, compared to the "common household method" of just letting the tea rest in water. (The tea may taste bitterer and more astringent as a result of improved extraction.) ABC Radio Sydney reported on this study in 2017, making the imprecise (but since well-circulated claim) that microwaving is the "best" way to brew tea.

References

  • AMPERE (Association for Microwave Power in Europe for Research and Education)
  • Microwave Synthesis @ organic-chemistry.org
  • homepages uconn.edu site with microwave equipment and research