Microwave chemistry

Microwave chemistry is the science of applying microwave irradiation to chemical reactions ^{[1] [2] [3] [4]}. Microwaves act as high frequency electric fields and will generally heat anything with a mobile electric charge. Polar solvents are heated as their component molecules are forced to rotate with the field and lose energy in collisions. 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. The concept was introduced in 1986 ^[5].

MORE synthesis stands for Microwave-organic Reaction Enhancement.

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Heating effect

See also: microwave effect

See also: non-thermal microwave 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.

Microwave heating is able to heat the target compounds without heating the entire furnace or oil bath, which saves time and energy. It is also able to heat an object throughout the volume (instead of through its outer surface), in theory producing more uniform heating. However, due to the design of most microwave ovens and to absorption by the object being heated, the microwave field is usually non-uniform and localized superheating occurs.

Some compounds absorb microwave radiation differently than others. This selectivity allows some parts of the object being heated to heat more quickly or more slowly than surrounding parts.

Microwave heating can have certain benefits over conventional ovens:

• reaction rate acceleration
• milder reaction conditions
• higher chemical yield
• lower energy usage
• different reaction selectivities

Selective heating

A heterogeneous system (composed by different substances or different phases) is anisotropic if regarded to the loss tangent. As a result a different dissipation of the electric field into heat in different system domains can be expected. A different dissipation means a Selective heating of different parts of the material, leading theoretically 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. Under conditions where a high amount of heat could be transferred between system domains a possible hot spot would be canceled by the exchange of heat from the hot zones to the cold zones until reaching the thermal equilibrium. Only in a system where the heat transfer would be hindered, it would be possible to have the presence of a steady state hot spot able to enhance the rate of the chemical reaction happening in its surrounding.
As a result, the presence of molecular hot spots by coupling of the radiation with certain reactants in a homogeneous liquid phase reaction reported by some authors seem to have no scientific basis. 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. Another kind of discussion is necessary if processes with solid phases are considered. 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 was done in literature. Under the designation of macroscopic hot spots were considered all large non-isothermalities which can be detected and measured by use of optical pyrometers (Optical fibre or IR). Some authors detected using these analytical techniques thermal gradients inside solid phases under microwave irradiation. Microscopic hot spots were considered the second Hot-spot category. They are non measurable non-isothermalities in the micro-nanoscale (e.g. supported metal nanoparticles inside a catalyst pellet) or in the molecular scale (e.g. a polar group on a catalyst structure). This microhotspot effect has been up to date just postulated in several gas-phase catalytic reactions, since no direct experimental measurements are possible. 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 an apolar 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.

References

Organic synthesis

Inorganic synthesis

• Martín-Gil J, Martín-Gil FJ, José-Yacamán M, Carapia-Morales L and Falcón-Bárcenas T. "Microwave-assisted synthesis of hydrated sodium uranyl oxonium silicate". Polish J. Chem, 2005, 1399-1403.

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