Synthesis | Solids in Microwave Reactors?

Although the majority of chemical reactions take place in the liquid state employing a solvent or a liquid reagent, there are requirements from time to time to heat solid reaction mixtures like powders or even larger solid particles and parts. While microwave heating of liquid reaction mixtures is state of the art, microwave heating of powders/solids bears some issues.

The big advantages of microwave heating in chemical synthesis have been proven with thousands of respective publications, review articles and books in the past two decades. Nevertheless certain things have to be kept in mind for successful processing under microwave irradiation.

In the early days of microwave chemistry microwave heating in dedicated scientific reactors was thought to be a very homogeneous way of heating.

However, it turned out that agitation of the vial content is a crucial factor, since otherwise hotspots lead to local overheating while other areas of the mixture stay comparably cold. This has been demonstrated with 3 fiber-optic probes and one IR sensor, simultaneously measuring the temperature of a solvent which is heated in the microwave field.

This figure shows that fiber optic (FO) and IR temperature probes in different positions of the reaction mixture detect completely different temperatures if the reaction mixture is not homogenized.

Only with stirring all temperature probes show a similar, representative reaction temperature.

Based on this phenomenon, this report gives an insight into the issues of heating solid mixtures in a dedicated microwave reactor. Furthermore it also helps to distinguish between different applications which are often mixed up when it comes to discussions about solid phase chemistry.

  • Solid reaction mixtures / dry media reactions
  • Solid phase chemistry
  • Solvent-free / solventless / neat reactions

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