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Microwave Reactors for Chemical Synthesis
17 Март 2014

Summary: Microwave reactors have been used in R&D for a number of years but are now gaining commercial importance as an alternative to conventional batch reactors. For some reactions they offer the potential for significant energy reductions as well as yield and selectivity improvements and the opportunity for carrying out solvent free reactions with the subsequent environmental benefits.

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Most batch chemical processes use jacketed reactors and hot oil or steam to heat up the reactor contents. This process is not selective; besides heating up molecules we want to react it is also heating up solvent, the high thermal mass reactor and the surrounding environment.

Microwaves are a much more selective means of getting energy into target molecules, without wasting energy heating reactors and surroundings. Microwaves specifically heat up molecules with a strong dipole moment like water. This has proved useful in synthesising heat sensitive materials through using a two phase system such as water / hydrocarbon, with the hydrocarbon layer remaining cool in a microwave reactor, into which the product dissolves quenching the reactor. The actual energy efficiency of a microwave reactor will therefore depend on the substrate and will be much more suitable for polar molecules. Non-polar molecules may react if, for example, they are dissolved in a more polar solvent, but this will negate much of the energy efficiency to be gained by targeting specific molecular types.

The technique is widely used in pharmaceutical R&D laboratories in making a large number of small scale samples for screening; this is because reactions can often be completed in seconds rather than hours. However scale up has had a number of problems not lease associated with the drop in microwave power with distance from the source. Several companies are now producing commercial scale reactors which overcome this problem, for example through having the reactants continuously passing through the microwave source. The application of microwave irradiation to provide enhanced reaction rate and improved product yield is providing quite successful in the formation of a variety of carbon-heteroatom bonds.

In conventional jacketed or coiled reactors, unless reactants are liquids, solvents are required to act as a heat transfer medium, and frequently need to be removed subsequent to the reaction, often using energy intensive techniques such as distillation. Many microwave reactions can be carried out directly in the solid state, which may save energy, depending on work up procedures.

There has been relatively little direct comparison made of the energy consumed in a microwave reactor against a conventional reactor for the same chemistry, but there are obvious analogies to be made between a microwave and conventional cooking oven. One reported example shows that energy utilization and specific energy requirements for microwave based biodiesel synthesis are better than conventional techniques. Another study, on the Suzuki coupling reaction, at pilot plant scale suggests a continuous microwave uses just 11% of the energy of a conventional steam heated reactor.

Microwave reactors or not suitable for all reaction types but in addition to energy improvements may offer higher yields and selectivities compared to conventional reactor technologies and should be considered at the R&D stage as an alternative technology for imputing energy.

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