Microwave chemistry – should we blame the tools?
Posted by Phillip on Wed 9 Jan 2013Categories: News | [3] Comments
— Microwaves – not magic
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?
Phillip Broadwith
Thu 10 Jan 2013 at 3:14 pm
I agree with Kappe that there is no strong evidence for non-thermal microwave effects. Microwaves just heat reaction components BUT they can heat things in a way that is difficult or impossible to replicate. For example in two phase reactions with a solid catalyst such as Suzuki coupling the catalyst can be heated much faster than the surrounding liquid. In a batch reactor the temperature equilibrates but in a flow reactor the result is a much higher temperature at the reaction site than measured (by fiber optic) in the reaction mixture. This can and does give much faster reactions than the measured temperature would suggest and gives the possibility of less homogeneous side reactions in the cooler reaction mixture away from the catalyst surface
Fri 5 Apr 2013 at 8:33 pm
We have worked for many years in the field of microwave chemistry and have followed all the debates about temperature measurement in microwave cavities and microwave effects with great interest.
Regarding reaction enhancement, our results completely support the general views of the community – in that microwave heating allows for a rapid, precise and controllable heating. This therefore allows for both high and low heating rates as well as a narrow control on the reaction temperature window – all of which allows more effective chemistry to be carried out.
With regards to the temperature measurement, temperature probes inside the cavity, such as IR, may well be heated preferentially over the material. Also the IR probe essentially measures the temperature of the vessel wall making that there is indeed substantial doubt concerning the effective temperature within the system. Below is a link to our work in the this area; like many we wanted to ensure our observations were true to the temperature being observed.:
Microwave-mediated pyrolysis of macro-algae. Vitaliy L. Budarin, Yizhe Zhao, Mark J. Gronnow, Peter S. Shuttleworth, Simon W. Breeden, Duncan J. Macquarrie and James H. Clark; Green Chem., 2011, 13, 2330; DOI: 10.1039/c1gc15560a
In this work we have added polymer beads to the reaction medium allowing for a correlation between their softening, melting and decomposition temperatures and the actual temperature measured. Additionally, this work also contains a comparison between the use of an internal fibre optic versus a surface IR measurement. These results were in good correlation with previous work by Stankiewicz in Delft. Furthermore, we have found that there are a number of mechanisms to even out the temperature distribution inside biomass during MW pyrolysis. With water being the major product of pyrolysis an efficient mechanism exists to remove excess energy from hotspots cooling them down. From another point of view water vapour has a high heat capacity and thus holds the potential to distribute energy evenly throughout the biomass. Moreover char obtained at temperatures below 220°C is a weak absorber of microwave radiation and cannot be overheated.
Regarding hydrothermal experiments we wish to point out that, besides IR and fibre optic measurement devices, also the pressure reading can be used as an effective means, through the well-known steam tables, to determine the real temperature inside the vessel.
Mon 20 May 2013 at 3:05 am
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