Ultrasonication boosts heterogeneous reactions

Paul Boughton
Chemical reactions require the interaction of the reactant molecules. For heterogeneous or multi-phase reactions and catalysis the interaction is limited to the contact surface of the dispersed reactant. Therefore, the phase boundary is a rate determining factor.
Ultrasonic cavitational mixing is an effective means to increase the exposure of the reactants without the need to increase their concentration. Cavitation generates high shear forces that reduce the particle size of reactants resulting in higher specific surface area and increased exposure. Finally, when reagents react at a phase boundary, the products of the chemical reaction accumulate at the contact surface and block other reagent molecules from interacting.
Cavitational and acoustical streaming results in turbulent flow that transports material from and to the reactant surface. In the case of droplets, the ultrasonication leads to the coalescence and subsequent formation of new droplets. As the chemical reaction progresses over time, a repeated sonication, eg two-stage or recirculation, may be required to maximize the exposure of the reagents.
Ultrasonic cavitation is a unique way to put energy into chemical reactions. A combination of high speed liquid jets, high pressure (>1000atm) and high temperatures (>5000K), enormous heating and cooling rates (>109Ks-1) occur locally concentrated during the implosive compression of cavitational bubbles. For many chemical reactions, this increases reaction rates or changes the reaction pathway significantly.
Hielscher Ultrasonics produces ultrasonic devices for the application at any scale from small test tubes to commercial production. For initial studies, a UP400S (400 watts) is very suitable. The UIP1000hd (1000 watts) is the more sophisticated research tool, as it allows for the sonication at wide range of process parameters, such as amplitude, pressure or temperature. This device can be used either for batch sonication or for inline processing using flow cell reactors.
Hielscher offers extensive technical support during application development, e.g. in their process laboratory.
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Hielscher Ultrasonics GmbH is based in Teltow, Germany. www.hielscher.com