Safety Considerations for Microwave Synthesis

The best microwave safety device is a trained and knowledgeable operator.

Microwave safety may seem like a simple topic, but the rapid transfer of energy associated with microwave-enhanced chemistry does create safety issues. Is microwave irradiation safe? The answer is yes, but only with equipment that has been properly designed for its specific use. Closed vessel microwave systems have been in common use in laboratories since 1985. They have become the preferred method of sample preparation for many laboratory analyses. During that period, there have been documented cases of vessel failures. Reasons for these failures vary from exceeding the load limit of the vessel to using vessels well beyond their serviceable lifetime, or exceeding the pressure or temperature rating of the vessels. Frequently, such events are caused by the chemist being unfamiliar with the kinetics of the reaction. The best microwave safety device is a trained and knowledgeable operator. When performing microwave synthesis, a chemist should also be aware of the equipment being used and the stability of solvents at high temperatures. This chapter discusses these issues in hopes of increasing microwave safety awareness.

Equipment

Using the correct hardware for microwave synthesis is imperative for personal safety. DO NOT PURCHASE A DOMESTIC MICROWAVE OVEN FROM AN APPLIANCE STORE! It may seem to be a more economical solution, but these ovens are not designed for the rigors of laboratory usage. There are no safety controls or monitoring of power, temperature, or pressure. Acids and solvents corrode the interiors quickly, and the cavities are not designed to withstand the resulting explosive force from a vessel failure in runaway reactions. In addition, safety interlocks have been compromised allowing the unit to continue producing microwave energy even though the door has been opened.

The majority of the research discussed in the applications chapter has been executed in multi-mode domestic microwave ovens. In the 1980s, laboratory instrument companies began to address the specific microwave safety issues. These instruments featured corrosion-resistant stainless steel cavities with reinforced doors. In the event of a vessel failure, the vessel and its contents would be contained within the cavity. Venting mechanisms were added for vapor accumulation in order to prevent potential explosions. Power, temperature, and pressure monitoring, with automatic safety controls, were also installed.

Single-mode microwave instrumentation is now available. These cavities are designed to provide a more consistent energy distribution with reproducible, stable energy patterns. The instruments are equipped with the same precautionary mechanisms as multi-mode cavities and both the temperature and pressure input values are used as safety parameters. Microwave power is automatically lowered just before either value is reached. Just as with any variable controller, power is cycled to maintain the operator-set parameter of pressure and/or temperature. Single-mode laboratory systems also act as a containment in the event of a vessel failure. The operator should always be sure to utilize the certified pressure tubes and accessories supplied by the original manufacturer. Placing any item inside a microwave cavity, which has not been designed, tested, and certified for use in that specific cavity, most assuredly will result in a failure of the equipment and/or the reaction.

Chemical Applications and Safety

Another important safety issue in microwave synthesis is the actual chemistry being performed. The chemist must be aware of the potential kinetics of the reaction to be accomplished. They should also be aware of the stability of their reagents at high temperatures. Many solvents and reagents decompose to hazardous components from prolonged exposure to high temperatures. This information is provided in Section 10 (Stability and Reactivity) of the Material Safety Data Sheet (MSDS) for each chemical. Potentially risky chemistries include those that are also unsafe under conventional heating conditions. Both azide and nitro groups have been known to cause explosions with thermal heat. Precautions should be taken when using microwave irradiation with compounds containing these functional groups. Additionally, any exothermic reaction should be treated carefully because of the fast energy transfer associated with microwave irradiation. An exothermic reaction is uncontrolled. It will only stop when the available fuels are expended. The production of pressure and heat happens at an alarmingly fast rate and can exceed the ability of the designed vent mechanisms on the vessel to safely relieve the condition. When this happens a vessel failure is imminent. A laboratory microwave system will contain the energy of the resulting failure. A well-designed system will not sustain damage. It will also be able to be cleaned and placed back into service in a matter of minutes.

A question that is always raised is whether transition metals can be used as catalysts in a microwave-assisted reaction. Absolutely! Microwave irradiation can greatly enhance organometallic reactions. When using a metal catalyst, only small amounts of ground material are needed, and this will not cause arcing within the microwave field. Conversely, metal filings and other ungrounded metals within the microwave field should be avoided, as they do provide a potential arc source.

Microwave chemists should also be aware of the potential for localized superheating. This can occur in a viscous sample when there is not proper stirring. When performing transition metal catalyzed reactions, a metallic coating on the vessel wall may result. The coating absorbs energy extremely well, heats quickly, and could melt the reaction tube. This can also occur in solvent-free reactions, especially when the reagents are adsorbed onto a mineral oxide. To reduce this occurrence, ensure adequate stirring with a heavier stir bar in pressurized reactions or with a mixer for open vessel, solvent-free experiments.

If you come away with one thing from this chapter, it should be to use equipment designed for the task and receive training on its utilization! Microwave irradiation provides more energy than thermal heat — there is no limit. If you are unsure about a particular reaction, then start small: use small amounts of reagents and start with a low power level and temperature. You can always increase the temperature or power level after observing the results. Work with chemicals in a laboratory hood to eliminate inhaling toxic fumes that can result from reagents and solvents exposed to high temperatures. Becoming familiar with microwave instrumentation and the hardware associated with it is also necessary. Used correctly, microwave technology provides a very safe way to perform chemistry, as the reaction vessel is more contained than when utilizing conventional heating methods.