Photochemical Reactor and Microwaves

The photochemical reactor used for microwave-assisted experiments is an essential tool for experimental work. Such equipment enables simultaneous irradiation of the sample with both MW and UV-visible radiation. The idea of using an electrodeless lamp, in which the discharge is powered by the MW field, for photochemistry was born half a century ago [53, 62]. The lamp was originally proposed as a source of UV radiation only, without considering the effects of microwaves on photochemical reactions. The first applications of EDL were connected with the construction of a high-intensity source of UV radiation for atomic fluorescence flame spectrometry [88-90]. 

Gunning, Pertel, and their coworkers reported the photochemical separation of mercury isotopes [92-95] in a flow reactor which consisted of a microwave-operated discharge lamp [52, 96] cooled by a flowing film of water. A filter cell and a circulation system, to prevent heating of the filter solution and the cell, were placed concentrically and coaxially with the lamp. A similar reactor, for small-scale laboratory photolysis of organic compounds in the solution or gas phase, has been proposed by Den Besten and Tracy [91]. In this arrangement the EDL was placed in a reaction solution and was operated by means of an external microwave field from a radio or microwave-frequency transmitter (Fig. 19.11). The quantum output of the lamp was controlled by changing the output of the trans- 

Filling material Excited Main emission bands, i [nm] Refs 

antenna B, transmitter Q, capacitor C2, variable capacitor D, jacketed flask E, EDL E, reaction mixture G, circulating coolant. Adapted from Ref [91]. 

The use of a domestic microwave oven appeared in a patent [97], according to which gaseous reactants were irradiated with microwave and UV-visible radiation to produce desired photoproducts (the EDL was positioned inside the MW cavity, although outside the reaction vessel). Several similar reactors have been proposed for UV sterilization [98-100] or for treatment of waste water containing organic pollutants [101-103]. 

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As the energy intensity of a process becomes more of an issue on cost and environmental grounds we shall start to see processes being developed using microwave, ultrasonic, electrochemical and photochemical reactors. For some reactions these techniques will not only deliver energy efficiency but may also lead to higher selectivities and atom efficiencies. 

Chemat and his coworkers [92] have proposed an innovative MW-UV combined reactor (Fig. 14.7) based on the construction of a commercially available MW reactor, the Synthewave 402 (Prolabo) [9[. It is a monomode microwave oven cavity operating at 2.45 GHz designed for both solvent and dry media reactions. A sample in the quartz reaction vessel could be magnetically stirred and its temperature was monitored by means of an IR pyrometer. The reaction systems were irradiated from an external source of UV radiation (a 240-W medium-pressure mercury lamp). Similar photochemical applications in a Synthewave reactor using either an external or internal UV source have been reported by Louerat and Loupy [93],... 

Kl4n, P., H4jek, M. and Cirkva, V., The electrodeless discharge lamp a prospective tool for photochemistry, Part 3 the microwave photochemistry reactor, /. Photochem. Photobiol, A Chem., 2001, 140, 185. 

In this chapter we report and analyze examples of asymmetric organocatalytic reactions under non-classical conditions high pressure, microwave heating, ultrasound irradiation, and ball milling. Organocatalytic processes based on photochemical or electrochemical activation as well as applying continuous-flow reactors are not included. 

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