Sonocatalysis – The Effects of Sound Generated Waves on Catalytic Chemistry
a report by
Kathrin Hielscher Hielscher Ultrasonics GmbH
During the sonication of liquids, the acoustic wavelength range is between approximately 110 and 0.15 mm for frequencies of 18 kHz and 10 Mhz. This is significantly higher than those of molecular dimensions, showing that between the molecules and the acoustic field there is no direct coupling of chemical species. The effects of sonication are mostly generated by the ultrasonic cavitation in liquids (see Figure 1). 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, for example, to produce aggregates of fine-size particles.
The application of power ultrasound is a well-known tool to create extremely fine emulsions and dispersions. In chemistry such extremely fine-size emulsions and dispersions are used to enhance chemical reactions. This means that the interfacial contact area between two or more immiscible liquids or between liquid and solid becomes dramatically enlarged and provides a better, more complete and/or faster course of the reaction. Other useful effects, which can be achieved by ultrasonication, are the break-up of bondages and the creation of free radicals.
For catalysis – the same as for other chemical reactions – enough kinetic energy is needed to start the reaction. High-power ultrasound couples enough energy into liquids to drive chemical reaction, for example, catalytic reactions.
Ultrasonic forces have various positive effects regarding the chemical reaction, including:
•
• • • • •
a chemical reaction that will normally not occur because of its low kinetic energy can get started by ultrasonication;
chemical reactions can be accelerated by ultrasonically-assisted catalysis; reduction or complete avoidance of catalyst; raw materials can be used more efficiently; by-products can be reduced; and
replacement of cost-intensive hazardous strong base with inexpensive inorganic base.
Kathrin Hielscher is Marketing Manager at the Hielscher Ultrasonics GmbH, a family owned company that specialises in the development and manufacture of high-power ultrasonic devices. She was born in 1983 in Lauingen, Germany and after finishing her studies in 2009, she joined the company, founded by her father-in-law in 1992, full time.
E:
Kathrin@Hielscher.com
Homogeneous and Heterogeneous Catalysis Depending on the presence of catalyst and reactant in the same or different phase, a distinction is made between homogeneous and heterogeneous catalysis. For the homogeneous catalysis, the catalyst and reactants are in the same phase, whereas for the heterogeneous catalysis, the catalyst and reactants are in different phases.
In homogeneous systems, the ultrasound forces produce a fine and even mix of both materials, the catalyst and the reactant. As both are in the same phase, this could be also achieved by the use of a conventional stirrer, but besides the even mixing, which leads to a faster and more complete reaction, sonochemical effects – such as the breaking of bonds and the creation of radicals – can occur.
The effects of ultrasound in aqueous solutions are quite well
explored. The primary products are H2 and H2O2 but there is strong evidence for various high-energy intermediates, including HO2, H•, OH• and perhaps e−(aq). The OH• radicals produced from the sonolysis of water are able to attack essentially all organic compounds
including halocarbons, pesticides and nitroaromatics and through a series of reactions, oxidise them completely. The advantage of sonolysis for such remediation, for example, waste water treatment, lies in its low maintenance requirements and the low energy/volume efficiency of alternative techniques.
In nonaqueous solutions, the sonochemical effect of sonolysis can also be observed. The reaction equation of the sonolysis of volatile organometallic complexes is shown below (Equation 1). Under ultrasonic irradiation, some volatile organometallics whose thermal and photochemical reactivities were well characterised, show unusual reactivities, including controlled multiple ligand substitution and clusterification.1
Fe(CO)5
Fe(CO)5 +L
)))
(n=1,2,3;L = Lewis base) )))
Fe3 Fe3
(CO)12 (CO)5_n
Ln (1)
Heterogeneous catalysis often is slow due to its limited phase boundary caused by catalyst and reactant in different phases. As the phase boundary is the only place where catalyst and reactant are available, the reaction is limited to the phase boundary. To enhance the reaction, the surface area of particles and therefore the phase boundary must be enlarged. This can be achieved by ultrasonically assisted emulsification or dispersing. Ultrasonic cavitation is a very effective method of creating a very fine and even emulsification of liquids and dispersion of solids. By particle or droplet size reduction, the total surface area of the phase boundary increases at the same time. Figure 2 shows the correlation between particle/droplet size and surface area in the case of spherical particle/droplets. Simultaneously,
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Catalysts
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