Hielscher Ultrasonics: Sonocatalysis


Courtesy of Hielscher Ultrasonics GmbH

In many chemical manufacturing processes catalysis is involved to increase the reaction rate. The use of ultrasound is a potential way to speed up the chemical catalysis. During the sonication of liquids, the accoustic wavelength range between approx. 110 and 0.15 mm for frequencies of 18kHz and 10Mhz. This is significantly higher than those of molecular dimensions, resulting in that there is between the molecules and the accoustic field no direct coupling of chemical species. The effects of sonication are mostly generated by the ultrasonic cavitation in liquids. Therefore, for the ultrasound catalysis at least one reagent in a liquid phase is necessary. Cavitation erosion on particle surfaces generates unpassivated, highly reactive surfaces. Short periods of high temperatures and pressures contribute to a molecular decomposition and increase the reactivity of many chemical substances. Ultrasonic irradiation can also be used to prepare the catalysts, e.g. to produce aggregates of fine-size particles.
In heterogeneous catalysis, the catalyst and reagents are in different phases. Therefore the heterogeneous catalysis is limited to the phase boundary, because this is the only place where the reagent and the catalyst are present. The exposure of the reactants and of the catalyst is a key factor for many multi-phase chemical reactions. For this reason, the specific surface area of the phase boundary becomes influential for the chemical rate of reaction. Ultrasound is used in many processes for the effective dispersion of solids and the fine emulsification of liquids. As is reduced by the particle or droplet size, increases while the total surface area of the phase boundary layer. In correlation to the phase boundary, the chemical reaction rate is increased.
Using an in-line ultrasonic reactor, fine particles or droplets can be achieved during the initial mixing of the reactants and the catalyst in a very short time and at high flow rates. This method can also be used for high viscosity materials. Ultrasound contributes to the heterogeneous and homogeneous catalysis in many ways. By customizing the ultrasonic amplitude and the fluid pressure individual effects of the catalytic process can be actively increased or reduced.



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