Green Chemistry Principles - Process Intensification for Green Chemistry - page 52

Figure 2.10 The electromagnetic spectrum.

Microwave heating operates in a different manner to conventional oil baths. Microwaves
interact with dipoles or ions and create 'molecular heating' by causing dipole rotations or
ionic conduction. Both of these ways of receiving energy are caused by molecules/ions
attempting to align with the rapidly oscillating microwave field. In a simplistic view,
microwave reactors are capable of enhancing reaction rates because they allow more
molecules to have sufficient energy to overcome the activation barrier of the reaction.
These high-energy molecules are created by preventing them from relaxing from the
excited state: kinetic relaxation occurs in 10 5 seconds, whereas microwaves apply energy
in 10 9 seconds, which creates a non-equilibrium state [77].
The acceptance of microwave heating as a chemistry tool is demonstrated by the
exponential growth in microwave-related publications since the mid 1980s, from initial
articles involving organic synthesis in 1986 [78]. Original studies involved the use of
domestic ovens: these produced hot and cold spots due to an uneven microwave field,
and led to unpredictable results and numerous explosions [79]. Subsequently, a number
of companies (CEM Corporation [80], Biotage [81] and Anton Paar Gmbh [82] are
among the market leaders) began producing systems designed for chemistry, using
monomode microwave generators and laboratory-scale apparatus(5-100mL).This
provided further encouragement to researchers to trial reactions under microwave
irradiation. Reported microwave-assisted chemistry benefits include decreased reaction
times, reduced overall energy consumption and improved yield and selectivity. This is
summarized by Leadbeater, who stated that microwave technology is 'enabling a wide
range of reactions to be performed easily and quickly'[83].