Microwave heating principles

S. M. Bradshaw, E. J. van Wyk and J. B. de Swardt, Microwave heating principles and the application to the regeneration of granular activated carbon. Journal of the South African Institute of Mining and Metallurgy, 98, 201-210 (1998). 

Theoretical and applied aspects of microwave heating, as well as the advantages of its application are discussed for the individual analytical processes and also for the sample preparation procedures. Special attention is paid to the various preconcentration techniques, in part, sorption and extraction. Improvement of microwave-assisted solution preconcentration is shown on the example of separation of noble metals from matrix components by complexing sorbents. Advantages of microwave-assisted extraction and principles of choice of appropriate solvent are considered for the extraction of organic contaminants from solutions and solid samples by alcohols and room-temperature ionic liquids (RTILs). 

In conclusion, metal nanoclusters in DMF interact strongly with microwaves. In reactions catalysed by these clusters, the microwave heating may be tantamount to preferentially heating the catalytic site, which can lead to more effective catalysis. Such cluster-catalysed reactions can be in principle screened in parallel in multimode m/w ovens reducing both time and operational costs. However, the ovens must be adapted so that the parallel reactors are uniformly heated. 

Another important reaction principle in modem organic synthesis is carbon-hydrogen bond activation [159]. Bergman, Ellman, and coworkers have introduced a protocol that allows otherwise extremely sluggish inter- and intramolecular rhodium-catalyzed C-H bond activation to occur efficiently under microwave heating conditions. In their investigations, these authors found that heating of alkene-tethered benzimidazoles in a mixture of 1,2-dichlorobenzene and acetone in the presence of di-//-... 

As an illustration of this principle, a volatile polar molecule is a byproduct, eliminated as a result of the microwave heating (Eq. 4), and the equilibrium is shifted to completion. The second effect of irradiation is activation of the alkylation step itself (Eq. 5). All the reagents can be used in the theoretical stoichiometry. Some indicative results are given in Tab. 5.4 [9]. 

Tinga, W.R., Design principles for microwave heating and sintering, Electromagn. Energy Rev., 1988,1,1. 

This section reviews the basic principles of physics pertaining to microwave heating. 

The proof-of-principle of the MAMEF technique, which couples the benefits of MEF with low power microwave heating to kinetically accelerate the bioassays, was first demonstrated with a model protein-fluorophore system,[l] with biotinylated-BSA and fluorophore-labeled streptavidin, as shown in Figure 7.3A. The... 

SEME is a recent method of extraction, patented in 2004, with the specific objective of obtaining essential oil from plant material [45, 46]. Based on a relatively simple principle, SFME involves placing the vegetable material in a microwave reactor without addition of solvent or water. SFME is a combination of microwave heating and distillation, and is performed at atmospheric pressure. In terms of quality and quantity, SFME seems to be more competitive and economic than classical methods such as hydro or steam distillation [59, 60]. 

The book examines the scale-up of microwave-promoted chemistry and describes the various approaches and the current commercially available microwave reactors available for scale-up. It also discusses microwave heating as a tool for teaching in the undergraduate organic chemistry laboratory. It explores curriculum, the broad range of reactions that can be performed, and the incorporation of green chemistry principles. 

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