“Cooling-while-Heating” or “Enhanced Microwave Reactions”—What really happens!

There have been some reports of ‘Enhanced microwave reactions’, which is described to take place when a reaction mixture is cooled while it is simultaneously being heated with microwaves. This has been assigned to the increased amount of microwave power going into the reaction. Since the amount of microwave power is after all directly proportional to temperature, an investigation was performed to study the temperature changes during “Cooling-while-Heating” more accurately.

An experimental set-up was designed whereby the temperature of the reaction vessel (5 ml vial) was measured simultaneously by two independent methods:

1) Using an infra-red pyrometer, a highly efficient sensor for measuring the surface temperature of the vessel. The performance of this device is somewhat more accurate and rapid compared to a conventional IR sensor to be found in commercial microwave synthesis reactors, but otherwise the location of measurement is identical: they both measure the surface temperature of the vessel wall which faces the sensor aperture.

2) the second temperature measurement device was a fiber-optic temperature sensor immersed in the vessel contents. This measured the actual temperature of the contents of the vial, and it therefore senses the true temperature of the vessel contents.




During temperature measurement, the vessel was irradiated by microwaves while a cooling air stream passed over the vessel wall. This situation is identical to the situation called “Enhanced Microwave Synthesis” or “PowerMAX”. Temperature measurements were made using the two sensors simultaneously and independently.



Fig 1. Results of a cooling experiment using 5 ml of DMF

Fig 1. shows the results of a cooling experiment using 5 ml of DMF. The target temperature was set to 180 °C. When the temperature had reached the target temperature (180 °C), cooling air was applied, whilst the microwave irradiation was still on. After about 80 seconds, the cooling and heating were switched off. A short time later, the cooling was switched on to bring the DMF down to room temperature. The internal temperature of the DMF in the vial was 55 °C higher than the temperature that the IR sensor was recording.

The temperature differentials between the outside wall of the vial and the reaction mixture core can vary substantially depending on the composition of the reaction mixture and concentration.

This investigation was further validated when the Diels-Alder reaction that was previously reported to have been successful using “Enhanced Microwave Synthesis” techniques afforded comparable yields when repeated at the internal temperature of the reaction mixture.



Fig 2. Diels-Alder Reaction

Time/min        Temp/°C                     Power/W         Conversion
5                      200 without cooling        100                   21%
5                      120 with cooling*           250                   75 %
5                      155 without cooling        variable             72 %

* 155ºC measured in the reaction mixture with fiber optic temperature probe