Microwave-ultrasound reactor

Chemat et al. have reported several microwave reactors, including systems that can be used in tandem with other techniques such as sonication [68], and ultraviolet radiation [69]. With the microwave-ultrasound reactor, the esterification of acetic acid with n-propanol was studied along with the pyrolysis of urea. Improved results were claimed compared with those from conventional and microwave heating [68]. The efficacy of the microwave-UV reactor was demonstrated through the rearrangement of 2-benzoyloxyacetophenone to l-(2-hydroxyphenyl)-3-phenylpropan-l,3-dione [69]. 

A combination of different techniques can frequently improve yields of final compounds or synthetic conditions, for example a reunion of direct electrochemical synthesis and simultaneous ultrasonic treatment of the reaction system [715]. Reunion of microwave and ultrasonic treatment was an aim to construct an original microwave-ultrasound reactor suitable for organic synthesis (pyrolysis and esterification) (Fig. 3.7) [716], The US system is a cup horn type the emission of ultrasound waves occurs at the bottom of the reactor. The US probe is not in direct contact with the reactive mixture. It is placed a distance from the electromagnetic field in order to avoid interactions and short circuits. The propagation of the US waves into the reactor occurs by means of decalin introduced into the double jacket. This liquid was chosen by the authors of Ref. 716 because of its low viscosity that induces good propagation of ultrasonic waves and inertia towards microwaves. 

Chemat, F., Poux, M., Di Martino, J.-L. and Berlan, J., An original microwave-ultrasound combined reactor suitable for organic synthesis application to pyrolysis and esterification,. Microwave Power Electromag. 

Lagha, A., Chemat, S., Bartels, P.V. and Chemat, F., Microwave-ultrasound combined reactor suitable for atmospheric sample preparation procedure of biological and chemical products, Analusis, 1999, 27, 452. 

The implementation of in-line ultrasound for sample preparation in flow analysis is analogous to microwave or UV irradiation. The coiled reactor (or a mini-chamber) is immersed in a water bath to which ultrasound is applied. As mechanical waves are involved, the inner walls of the tubing should be thin and flexible to avoid wave damping, which would decrease the efficiency of the ultrasound. Alternatively, an ultrasound probe can be placed near the ultrasound reactor [143]. The use of an... 

Chemat, F., M. Poux, J. L. Martino, and J. Berlan, An Original Microwave-Ultrasound Combined Reactor Suitable for Organic Synthesis Apphcation to Pyrolysis and Esterification, J. Microwave Pofwer and Electromag. Energy, 31, 2P(1996). 

Cravotto G, Di Carlo S, Curini M, Tumiatti V, Rogerro C (2007) A new flow reactor for the treatment of polluted water with microwave and ultrasound. J Chem Tech Biotech... 

By coupling an ultrasonic probe with a microwave reactor and propagating the ultrasound waves into the reactor via decalin introduced into their double jacket design, Chemat et al. studied the esterification of acetic acid with propanol and the pyrolysis of urea to afford a mixture of cyanuric acid, ameline and amelide (Scheme 9.19)136. Improved results were claimed compared to those obtained under conventional and microwave heating. The MW-US technique was also used to study the esterification of stearic acid with butanol and for sample preparation in chemical analysis137,138. 

In particular, two main streams of PI applications have been identified (i) PI innovations for reactors (e.g., microreactors, monolith reactors, spinning disc reactors, reactive separations) and (ii) PI technologies for more efficient energy transfer (e.g., ultrasound, pulse, plasma, microwave). Several PI technologies offer important potential, but require important fundamental/strategic research to reach proof-ofconcept on the laboratory scale. These PI technologies are ... 

Efforts have been made to accelerate the reaction. Ionic liquids can be obtained in < 1 min using microwave-assisted preparation, and ultrasound-assisted synthesis leads to the rapid isolation of ionic liquids with high chemical purity. If a microwave reactor with reaction condition control mechanisms is used, ionic liquids can be conveniently prepared even on a large scale (up to 2 moles), both in high yields and with short reaction times. ... 

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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