Microwave Chemistry

Giancarlo Cravotto, Diego Carnaroglio, Giancarlo Cravotto, Diego Carnaroglio

• Publisher: De Gruyter
• Year: 2017
• ISBN:
  9783110480023
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Organic Chemistry

Microwave Chemistry has changed the way to work in chemical laboratories and is an established state-of-the-art technology to accelarate and enhance chemical processes. This book not only gives an overview of the technology, its historical development and theoretical background, but also presents its exceptionally broad spectrum of applications. Microwave Chemistry enables graduate students and scientist to learn and apply its methods successfully.

Giancarlo Cravotto and Pedro Cintas

1Microwave chemistry: history, development and legacy

1.1Introduction

The reader may be surprised to see an introductory chapter on the history of microwaves (MW) in this textbook on MW chemistry; after all, MWs are hardly ancient history! In fact, MWchemistry is very much an active research field and one for which a definitive model of chemical reactivity has yet to be established, despite the fact that the discipline has grown exponentially since the now-distant, late 1980s. The exact nature of the thermal, and any potential nonthermal effects of MWs remain the subject of intense debate, and particular mechanisms that account for specific or anomalous cases are proposed from time to time. Despite all this, we still had the strange feeling that writing this sort of memoir should be undertaken by the field’s pioneers, who have since retired from science, as they take us through the developments they experienced and contributed to. However, you have us – authors who are currently engaged in MW chemistry and can therefore view this growing subject from up close.

However, far from being solely a comprehensive historical survey, this chapter’s purpose is twofold: (1) to describe the historical breakthroughs that originally launched MW-based science and (2) to highlight a series of instrumental developments in MW chemistry. In doing so, there will be inevitable omissions, especially in the applied domain, which will be covered by other contributors throughout this book. Emphasis is placed on seminal findings and conceptual toeholds whose significance was often only unveiled in retrospect. We feel that these subjects should be of interest to younger readers and experienced MW practitioners alike. As is often the case in science, we will see that while many breakthroughs in MW science and technology came about thanks to chance, all have been invariably driven by curiosity.

1.2How it all began

MW radiation does not lie in the most energetic zone of the electromagnetic spectrum. It falls between radio frequencies and infrared (IR) radiation, at frequencies of between around 300 MHz (0.3 GHz) and 300 GHz, while most commercial devices operate at a standard 2.45 GHz. The corresponding wavelengths (between 1 m and 1 cm), clearly indicate that MWs and matter do not interact at either the atomic or molecular levels. The existence of electromagnetic waves was a prediction forged from Maxwell’s equations, formulated in 1873, although experimental verification was not accomplished until much later, in the 1880s, by Heinrich Hertz, who was able to produce and detect MWs (ca. 1 GHz) using a rudimentary generator [1]. For a few decades, up until the invention of radio transmitters, these so-called shortwaves were little more than an academic curiosity, even if their salient properties, such as heating, were immediately noticeable [2]. Figure 1.1 summarizes these physical properties.

Fig. 1.1: Physical properties of MW radiation.

The use of high-frequency radio waves to heat substances, and particularly for cooking, dates back to the 1920s and can be traced to the invention of vacuum tube radio transmitters. However, a curious note appeared in an American magazine for radio experimenters entitled “Cooking by ultra short waves” as early as 1933 [3]. The single-column note contained the following statement:

“Food can be cooked by means of the ultra short wave radio transmitter. The food is heated by internally passing high frequency current through it. This is probably the only basic advance in the art of preparing food for human consumption since cavemen, thousands of years ago, first burned meat over a fire and heated vegetables in crude vessels of boiling water.

For cooking, the ultra high frequency current is made to pass from one pan-shaped electrode to another. The uncooked food is placed between two electrodes, directly in the path of the radio transmitter’s power.

Bread is toasted in a half dozen seconds or so, steaks, potatoes, and other solid meats and vegetables require several minutes, as does the boiling of water for making coffee or cooking vegetables.”

Both Westinghouse and Bell Laboratories released patent applications in the late 1930s, demonstrating the cooking of foods between metal plates attached to a shortwave device working at ca. 60 MHz [4].

Although the heating effect of electromagnetic fields at both radio and MW frequencies is caused by dielectric heating, i.e., by the reorientation of polarized molecules in a rapidly alternating electric field, the use of an MW power generator inside a cavity (the magnetron, as in modern domestic ovens) was not originally intended for cooking. The different versions of magnetrons that were developed between 1920 and 1940 were designed for radar transmitters and radar devices for air defense, which, of course, became instrumental during World War II [2]. There is a widespread consensus that Percy L. Spencer should be credited with the invention of MW cooking. Spencer, a self-taught man, became an expert on radio technology and was contracted by Raytheon to develop radar equipment for military purposes. One day, while Spencer was standing in front of an MW source, he noticed that a candy bar had melted in his pocket. Inspired by this serendipitous finding, Spencer and his colleagues attempted to heat other food, such as the world’s first microwaved popcorn. Whether or not these and other eureka moments actually occurred as they are described in popular science narratives may now be irrelevant. The observation marked the beginning of MW cooking, and Raytheon filed the first related patent in 1945, and subsequent improvements followed [5]. The first commercially available oven was an MW chamber weighing around 750 pounds (ca. 340 kg) and was about 6 feet (ca. 180 cm) tall, sold at the prohibitive price of US$ 5000 a unit. Affordable kitchen ovens (less than US$ 500) did not become available until 1967. At this point, food processing became the first industrial application of MW technology and ranged from cooking itself to sterilization and drying [6].

1.3From cooking to chemistry

If you consider the fact that that MW cooking is essentially the conversion of electromagnetic energy into heat, you would be forgiven for thinking that MW-activated chemical reactions emerged soon after the application of MWs to food preparation. After all, thermal stimulation is the oldest means of inducing chemical transformations. However, this transition was lengthier than one might expect. Unlike conventional conductive heating, dielectric heating is a complex issue based on physical properties that require microscopic interpretation and in-depth mathematical formalism, incomprehensible jargon to most chemical practitioners. For many years, the use of MW ovens was a lab trick. The late 1980s saw in the chemical literature descriptions of numerous examples of activated reactions that overlooked accurate temperature estimations and other technical details. As a consequence, the systematic abuse and misuse of domestic ovens often led to irreproducible results that varied from lab to lab.

Take, for example, a talk in the mid-1990s at a meeting of organic chemists given by Jack Hamelin, one of the pioneers of synthetic MW chemistry, which was received with both interest and skepticism. Even scientists familiar with other forms of radiation found it difficult to understand how thermal effects arose from the action of electromagnetic waves. In the year 2000, however, at the biannual meeting of the European Society of Sonochemistry, the inaugural lecture by MW expert D. M. P. Mingos was greeted by applause and relatively few questions about the mechanism of action. Change had come.

In a broad sense, the first chemical applications of MW heating lay in sample pretreatment, such as digestion and ashing [7–10]. The technique has now become routine and provides fast and simplified protocols. One of the earliest, if not the first, application of MW frequencies to synthetic chemistry was a 1969 Dow Chemical Co. patent that reported MW-induced emulsion polymerization of vinyl monomers using pulsed radiation (from the radio frequency to MW range) [11]. However, this application still used a two-electrode device, not characteristic magnetron power. In 1973, a paper coauthored by Ponomarev and Tarasenko, dealt with the activation of chemical processes by MW irradiation [12]. These authors described both rubber vulcanization and the polymerizations of methyl methacrylate and styrene in benzene. Reactions were either conducted in glass vessels or directly inside the metal MW resonator; twofold acceleration was observed at the beginning of polymerization. The paper, published in Russian in a journal not readily accessible to the international scientific community, was largely ignored, even if it pave...