Sonochemical reactions heterogeneous

In organometallic chemistry, the use of ultrasound in liquid-liquid heterogeneous systems has been limited to Hg. The emulsification of Hg with various liquids dates to the very first reports on sonochemistry (3,203,204). The use of such emulsions for chemical purposes, however, was delineated by the extensive investigations of Fry and co-workers (205-212), who have reported the sonochemical reaction of various nucleophiles with a,a -dibromoketones and mercury. The versatility of this reagent is summarized in Eqs. (30)-(36). 

Torok, B., Balazsik, K., Torok, M., Szollosi, G., Bartok, M. (2000) Asymmetric sonochemical reactions. Enantioselective hydrogenation of a-ketoesters over platinum catalysts. Ultrasonics Sonochem. 7, 151-155. Torok, B., Balazsik, K., Torok, M., Felfoldi, K., Bartok, M. (2002) Heterogeneous asymmetric reactions 20. Effect of ultrasonic variables on the... 

In some books and papers (Suslick et Mason et Luche et many examples are given of heterogeneous sonochemical reactions. In the present section, only heterogeneous catalytic reactions, studied under sonication, will be briefly considered. The reader s attention is confined to supported catalysts (i.e., active metal on alumina, silica, carbon, polymer, etc.). Raney nickel will be considered as an example of a porous structure. 

Horst C, Chen Y-S, Kunz U, Hoffmann U (1996) Design, modeling and performance of a novel sonochemical reactor for heterogeneous reactions. Chem Eng Sci 51 1837-1846... 

The ultrasonic cleaning bath is clearly the most accessible source of laboratory ultrasound and has been used successfully for a variety of liquid-solid heterogeneous sonochemical studies. There are, however, several potential drawbacks to its use. There is no means of control of the acoustic intensity, which will vary from bath to bath and over the lifetime of a single cleaning bath. In addition, their acoustic frequencies are not well controlled and differ from one manufacturer to another, and reproducibility from one bath to another may therefore suffer. Reproducible positioning of the reaction flask in the bath is critical, since standing waves... 

Frequency and Intensity. Most ultrasonic baths operate in the 30 -80 kHz range. Frequency is rarely an important factor, provided the frequency is low enough to permit cavitation. The cell disruptors normally adapted for sonochemical uses operate at 20 kHz. The intensity must be enough to produce cavitation. Beyond that, optimum intensities for heterogeneous reactions have not been determined. 

Rule 2 applies to heterogeneous systems where a more complex situation occurs and here reactions proceeding via ionic intermediates can be stimulated by the mechanical effects of cavitational agitation. This has been termed false sonochemistry although many industrialists would argue that the term false may not be correct because if the result of ultrasonic irradiation assists a reaction it should still be considered to be assisted by sonication and thus sonochemical . In fact the true test for false sonochemistry is that similar results should, in principle, be obtained using an efficient mixing system in place of sonication. Such a comparison is not always possible. 

Ultrasound is known for its capacity to promote heterogeneous reactions (Ley and Low, 1989) mainly through greatly increased mass transport, interfacial cleaning and thermal effects. In addition, homogeneous chemical reactions have been reported to be modified (Suslick et ai, 1983 Luche, 1990 Colarusso and Serpone, 1996) for example the sonochemical generation of radical species in aqueous media is important in environmental detoxification (Kotronarou et al., 1991 Serpone et al., 1994). 

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