Microwave Heating for Organic Teaching Labs and Undergraduate Research
Elizabeth Blue (2013 cCWCS ALOC Workshop in Charlotte, NC & 2011 cCWCS Green Chemistry Workshop in Eugene, Oregon), is always on the lookout for ways to improve and "green" the Organic Chemistry Teaching labs at Campbell University in Buies Creek, NC. In the spring of 2013, she sought quotes on laboratory-grade microwaves, since she had read an increasing number of journal articles about how microwave heating was being utilized in organic synthesis to increase yields, lower energy use, decrease reaction times, and in some cases, improve purity. In addition, she had run a few experiments in conventional (kitchen-grade) microwaves at workshops. A kitchen microwave was considered, but the features available in laboratory-grade microwaves seemed worth the added expense: the increased safety features (since flammable solvents would be used), temperature control, and the ability to run multiple reactions simultaneously for the teaching labs.
A CEM MARS 5 "demo" unit (most similar to CEM's current MARS 6 synthesis unit) with temperature control and 20 GlassChem reaction vessels was purchased for $12,342.32 (a similar new unit would have run ~$20,000). This unit had been lent out to laboratories to try out, but was offered with a 1-year repair warranty. Other companies that also sell laboratory microwaves are Milestone, Anton-Parr, and numerous others. The MARS 5 is particularly useful for teaching laboratories, as it can hold up to 24 reactions at a time. The thick-walled reaction tubes have pressure relief caps, limiting the pressure to 200 psi. The particular model purchased has a fiber-optic cable with a temperature sensor at the tip. The fiber-optic cable is threaded down into the microwave and fits down into a sapphire well of one of the reaction vessels for temperature control during the experiment. The maximum capacity of the GlassChem reaction vessels is 14 mL, and each tube must contain the same (or very similar) composition and volume so that heating can be as uniform as possible. There are models that also offer pressure control, if that feature is needed.
All microwaves have a minimum working load (the minimum amount of solvent required to safely run the microwave without damaging the magnetron), which varies depending on the solvent being used. Roughly speaking, the more polar the solvent, the more readily it responds to microwave heating. For the MARS 5, with most solvents and experiments, this usually means more than one reaction vessel must be run. For high absorbing solvents (such as low molecular weight alcohols), the minimum working load is 10 mL total solvent; for medium absorbing solvents (such as water or acetone) or low absorbing solvents (such as ethyl acetate or THF), the minimum working load is 50 mL total solvent (these values are specific to the MARS 5).
The unit was provided with a lab manual of experiments, but the particular experiment currently used in the teaching laboratories at Campbell is a Diels-Alder experiment modified from experiments in J. Chem. Ed. articles by Baar, et. al. (DOIs: 10.1021/ed800001x, 10.1021/ed082p1393). The Diels-Alder reaction nicely demonstrates the concept of atom economy and the students compare/contrast the effectiveness of a conventional (90 minute reflux) reaction to the microwave reaction. The microwave heating takes a total of 15 minutes (5 min ramp to 130 °C and 10 min holding at 130 °C). The students usually set up the reflux first, but have the microwave reaction worked up before the conventional reflux is complete. For most students, the % yield of the microwave reaction is higher, and the purity is similar to the conventional reflux (both quite high purity). However, occasionally for some groups, the purity of the conventional heating is slightly better. There is also the possibility of having students do calculations to compare energy usage in the two experiments in order to extend the green chemistry concepts. There is research to suggest that some of the advantage seen with microwave heating is due to the higher pressures that are achieved using the thick-walled reaction vessels. (https://dx.doi.org/10.1021/ar300318c) However, teaching lab investigations at Campbell on this particular Diels-Alder reaction have still resulted in better conversion, yields, and purity with the microwave heating when compared to pressure tubes in 130 °C silicon oil baths for time frames similar to the microwave reaction (15 min total heating time).
In addition to use in the teaching labs, this microwave has been used for numerous undergraduate student research projects and in the research techniques section of the organic teaching laboratory. Students have used the microwave to cut down their reaction times and obtain higher yields or higher purity in many instances. The minimum volume requirements discussed above can be a disadvantage when running research experiments; nonetheless, the microwave has overall been very useful in our research endeavors. If one were looking for a microwave to use only or primarily for research, or for very small teaching labs, a single-vessel microwave reactor like the CEM Discover SP model (or a similar model from another manufacturer) might be more useful.
In terms of maintenance, the purchase of a "demo" unit has probably led to higher maintenance than a new unit would. One circuit board had to be replaced during the 1-year warranty period due to corrosion. This repair was covered by the warranty. It is suspected that this corrosion was due to the microwave having likely been used for acid digestion when it was a demo model. Since the warranty period ended, the RAM battery has been replaced a couple of times at minimal cost (watch style battery), and the display stopped functioning and was replaced (cost of parts only ~$630, free advice and assistance from the CEM Technical Support Staff). This spring, the unit seemed to stop stirring properly during the Diels-Alder runs and resulted in poor conversion, so this will be another repair that needs to be seen to at the 4 year mark. There are many resources available on microwave heating, and many experiments published in the literature. Liz is happy to answer any questions about her experiences and experiments with the microwave at blue@campbell.edu.