• Basics
• History
• Projects

Application of Microwaves in Chemistry

Today microwaves are widely applied for heating foodstuff almost in every modern kitchen. From some point of view, this process can be called "chemical", because the chemical composition of the fresh and prepared food may be quite different. First experiments on such heating were done by Percy Spenser.

During and after The Second World War he worked in the area of radar-related research, and tasting a new magnetron, he noted that a chocolate bar in his pocked was melted. His further experiments showed also, that maize kernels placed near working magnetron were quickly transferred into popcorn, and an egg was exploded. He started to study the food heating effect, and by late 1947 the firs commercial microwave oven appeared on the market. The first microwave oven, the Radarange, was about .8 m tall and weighed 340 kg. It was water-cooled and consumed 3000 watts, about three times as much as modern microwave ovens. The costing was about $5000 each.

On the other hand, since the Stone Age the humanity used fire to prepare food, and nobody speaks about Chemistry in this period. So in the case of microwave heating for chemical application, the first experiments should not be referred to Percy Spenser’s results.

Besides domestic application the microwave power was applied then for freeze-drying of food. This process called also "lyophilization" is used to preserve a perishable material or make the material more convenient for transport. The freeze-drying is performed by microwave heating of the frozen material to allow the water to sublime directly from the solid phase to gas. The pioneer research in this field is done by David Compson, who published a book Microwave heating: in freeze-drying, electronic ovens, and other applications.

Westport, 1962.

Concerning chemical applications, especially, microwave-assisted organic chemistry, two pioneering publications from the groups of R. Gedye and R.J. Giguere (1986) are cited as the first experimental works in this field. The authors described several reactions that were carried out in sealed glass of Teflon vessels in domestic microwave oven. In particular it was shown that hydrolysis of benzamide in such conditions proceeds faster and gives higher yield comparing with conventional thermal heating:

Today it is clear, that these two works had the most impact upon the Microwave Chemistry intensive development in future. However, were these publications really first?

As early as 1973 in Zhurnal Vsesoyuznogo khimicheskogo obshchestva im. D.I. Mendeleeva. (1973, 18. p.34–42) a topic problem review "Application of Microwave frequency Irradiations for Stimulation of Chemical Processes" was published. The review by A.N. Ponomarev and V.A. Tarasenko contains 32 references, it was in Russian, and published 13 years earlier then the pioneering publications by R. Gedye and R.J. Giguere. In 70th of the past century.

Ardalyon N. Ponorev worked in the group of V.L. Tolroze, and they studied different physical methods of stimulations of chemical reactions. In their experiments with microwave irradiation they studied rubber vulcanization and polymerization reactions of methyl methacrylate and styrene in benzene in glass vessels or directly in the space of the metal microwave resonator. In the case of the metal vessels they observed twofold reaction acceleration at the beginning of the polymerization. They work resulted in development of microwave installation for tire vulcanization.

Today, the controlled microwave irradiation is widely used for acceleration and chemo-control of the polymerization reactions. However, for those days the Ardalyon N. Ponomarev publication was truly predictive. It raised the questions, that looks so common today:

• Mechanism for the interaction of microwave irradiation with substance
• "Non-thermal" or "specific" microwave effect
• Advantages of microwave diapason of radio frequencies for stimulation of chemical reactions and possible ways of the microwave cavities design
• Calculation of kinetic parameters of the microwave assisted chemical reaction depending on dielectric properties of the reaction mixture
• Modeling of microwave scale-up reactions

Thus, authors formulated the basic questions concerned the microwave stimulation of chemical processes. In the conclusion authors expressed their hope that their work will attract attention of scientists to the microwave assisted chemical processes. Unfortunately, this work left unmet in those time, and the rise of interest to the Microwave Chemistry began only fifteen year later.

In our days the short history of the Microwave Chemistry is quickly expanded by novel exiting results, that is demonstrated by exponential growth of the publication number in the scientific literature.

Discuss the article on Forum.

References:

• Gallawa, J. C. The Complete Microwave Oven Service Handbook — Operation, Maintenance, Troubleshooting and Repair, Prentice Hall College Div, 2007
• Kappe, C. O.; Stadler, A. Microwaves in Organic and Medicinal Chemistry, Wiley-VCH, Weinheim, 2005
• Microwaves in Organic Synthesis (Loupy, A., Ed.), Wiley-VCH, Weinheim, 2006
• Gedye, R.; Smith, F.; Westaway, K.; Ali, H.; Baldisera, L.; Laberge, L.; Rousell, J. Tetrahedron Lett., 1986, 27, 279 –282.
• Giguere, R. J.; Bray, T. L.; Duncan, S. M.; Majetich, G. Tetrahedron Lett., 1986 ,27, 4945 –4958.