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In an essay article in Angewandte Chemie, Oliver Kappe from the University of Graz, Austria, is trying to lay to rest the idea that microwave reactors can accelerate chemical reaction by doing anything other than heating.
The main thrust of the argument is that it is essentially impossible to accurately measure the temperature of a reaction mixture without a direct, internal fibre-optic probe. Using the external infrared sensors fitted to most microwave reactors simply doesn’t cut it if you really want to work out whether what you’re seeing is really a special effect of microwave irradiation, or just an artefact of differences in heating.
To illustrate the point, Kappe and his team repeated experiments from two recent publications. The first came from Gregory Dudley at Florida State University in Tallahassee, US, and was covered in Chemistry World at the time.
Dudley and his team made great efforts to try and separate the bulk heating effect of microwaves from specific interactions with certain molecules in their reaction mixture, taking inspiration from Kappe’s own work in the experimental design. They compared reactions run in the microwave to reactions at the same temperature using conventional heating baths, and the results seemed to indicate that the microwave reactions were more efficient.
However, Kappe asserts that the external IR sensors employed by Dudley to monitor the reaction temperature were inadequate. When Kappe and his team repeated the experiment, using their internal fibre-optic probe, they found no difference between the microwave and conventionally heated reactions, and also that they needed less microwave power (on the same type of reactor) to maintain the reaction temperature, indicating that Dudley’s microwave reactions were probably running at a higher temperature, which is what was causing the rate enhancement.
Similarly in the second example, when Kappe’s team tried to reproduce experiments monitored using external IR sensors with their internal fibre-optic probe, the apparent non-thermal microwave effect disappeared.
So what is the message? Kappe is certainly not trying to discourage researchers from using microwaves – they are a great tool for speeding up chemistry, allowing reactions to proceed in superheated solvents at enhanced reaction rates.
However, when it comes to claims that these rate enhancements are due to anything other than thermal effects, he is simply saying that extraordinary claims require extraordinary evidence, and that means you really need to be able to measure temperature accurately. Kappe suggests that the best way to do this is to combine internal fibre optic probes with external IR measurements, and any paper in which measurements are not made with internal fibre optic probes should be treated with scepticism.
This technology is already available on the latest generation reactors, but they are expensive pieces of kit, so how many labs are likely to upgrade? In industrial labs, where simply being able to make a compound for testing is often of greater importance than the exact conditions used, this may be less of an issue. And how many academic researchers can afford it?