Sonochemistry and Sonochemical Reactors

Sonochemistry is the field of chemistry where high-intensity ultrasound is used to induce, accelerate and modify chemical reactions (synthesis, catalysis, degradation, polymerization, hydrolysis etc.). Ultrasonically generated cavitation is characterized by unique energy-dense conditions, which promote and intensify chemical reactions. Faster reaction rates, higher yields and the use of green, milder reagents turn sonochemistry into a very advantageous tool in order to obtain improved chemical reactions.

Sonochemistry

Sonochemistry is the research and processing field in which molecules undergo a chemical reaction because of the application of high-intensity ultrasonication (e.g., 20 kHz). The phenomenon responsible for sonochemical reactions is acoustic cavitation. Acoustic or ultrasonic cavitation occurs when powerful ultrasound waves are coupled into a liquid or slurry. Due to the alternating high-pressure / low-pressure cycles caused by power ultrasound waves in the liquid, vacuum bubbles (cavitational voids) are generated, which grow over several pressure cycles. When the cavitational vacuum bubble reaches a certain size where it cannot absorb more energy, the vacuum bubble implodes violently and creates a highly energy-dense hot spot. This locally occurring hot spot is characterised by very high temperatures, pressures and micro-streaming of extremely fast liquid jets.

Acoustic Cavitation and Effects of High-Intensity Ultrasonication

Acoustic cavitation, often also called ultrasonic cavitation, can be distinguished into two forms, stable and transient cavitation. During stable cavitation, the cavitation bubble oscillates many times around its equilibrium radius, whilst during transient cavitation, in which a short-lived bubble undergoes dramatic volume changes in a few acoustic cycles and terminates in a violent collapse (Suslick 1988). Stable and transient cavitation may occur simultaneously in the solution and a bubble undergoing stable cavitation may become a transient cavity. The bubble implosion, which is characteristic for transient cavitation and high-intensity sonication, creates various physical conditions including very high temperatures of 5000–25,000 K, pressures of up to several 1000 bar, and liquid streams with velocities of up to 1000m/s. Since the collapse/implosion of cavitation bubbles occurs in less than a nanosecond, very high heating and cooling rates in excess of 10¹¹ K/s can be observed. Such high heating rates and pressure differentials can initiate and accelerate reactions. Regarding the occurring liquid streams, these high-speed microjets show especially high benefits when it comes to heterogeneous solid–liquid slurries. The liquid jets impinge upon the surface with the full temperature and pressure of the collapsing bubble and cause erosion via interparticle collision as well as localized melting. Consequently, a significantly improved mass transfer in the solution is observed.

Ultrasonic cavitation is most effectively generated in liquids and solvents wit low vapour pressures. Therefore, media with low vapour pressures are favourable for sonochemical applications.
As result of ultrasonic cavitation, the intense forces created can switch pathways of reactions to more efficient routes, so that more complete conversions and/or the production of unwanted by-products are avoided.
The energy-dense space created by the collapse of cavitation bubbles is called hot-spot. Low-frequency, high-power ultrasound in the range of 20kHz and the ability to create high amplitudes is well established for the generation of intense hot-spots and the favourable sonochemical conditions.

Ultrasonic laboratory equipment as well as industrial ultrasonic reactors for commercial sonochemical processes are readily available and proven as reliable, efficient, and environmental-friendly on lab, pilot and fully-industrial scale. Sonochemical reactions can be carried out as batch (i.e., open vessel) or in-line process using a closed flow cell reactor.

Sono-Synthesis

Sono-synthesis or sonochemical synthesis is the application of ultrasonically generated cavitation in order to initiate and promote chemical reactions. High-power ultrasonication (e.g., at 20 kHz) shows strong effects on molecules and chemical bondings. For instance, the sonochemical effects resulting from intense sonication can result in splitting molecules, creating free radicals, and/or switching chemical pathways. Sonochemical synthesis is therefore intensely used for the fabrication or modification of a wide range of nano-structured materials. Examples for nanomaterials produced via sono-synthesis are nanoparticles (NPs) (e.g., gold NPs, silver NPs), pigments, core-shell nano-particles, nano-hydroxyapatite, metal organic frameworks (MOFs), active pharmaceutical ingredients (APIs), microsphere decorated nanoparticles, nano-composites amongst many other materials.
Examples: Ultrasonic transesterification of Fatty Acid Methyl Esters (biodiesel) or the transesterification of polyols using ultrasound.

TEM image (A) and its particle size distribution (B) of silver nanoparticles (Ag-NPs), which have been sonochemically synthesized under optimum conditions.

Also widely applied is the ultrasonically promoted crystallization (sono-crystallization), where power-ultrasound is a used to produce supersaturated solutions, to initiate crystallization / precipitation, and control crystal size and morphology via ultrasonic process parameters. Click here to learn more about sono-crystallization!

Sono-Catalysis

Sonicating a chemical suspension or solution can significantly improve catalytic reactions. The sonochemical energy reduces reaction time, improves heat and mass transfer, which subsequently results in increased chemical rate constants, yields, and selectivities.
There are numerous catalytic processes, which benefits drastically from the application of power ultrasound and its sonochemical effects. Any heterogeneous phase transfer catalysis (PTC) reaction involving two or more immiscible liquids or a liquid-solid composition, benefits from sonication, the sonochemical energy and the improved mass transfer.
For instance, the comparative analysis of silent and ultrasonically-assisted catalytic wet peroxide oxidation of phenol in water revealed that the sonication reduced the energy barrier of the reaction, but had no impact on the reaction pathway. The activation energy for the oxidation of phenol over RuI₃ catalyst during sonication was found to be 13 kJ mol^-1, which was four times smaller in comparison to the silent oxidation process (57 kJ mol^-1). (Rokhina et al, 2010)
Sonochemical catalysis is successfully used for the fabrication of chemical products as well as the manufacturing of micron- and nano-structured inorganic materials such as metals, alloys, metal compounds, non-metal materials, and inorganic composites. Common examples of ultrasonically assisted PTC are the transesterification of free fatty acids into methyl ester (biodiesel), hydrolysis, the saponification of vegetable oils, sono-Fenton reaction (Fenton-like processes), sonocatalytic degradation etc.
Read more about sono-catalysis and specific applications!
Sonication improves click chemistry such as azide-alkyne cycloaddition reactions!

Other Sonochemical Applications

Due to their versatile usage, reliability and simple operation, sonochemical systems such as the UP400St or UIP2000hdT are valued as efficient equipment for chemical reactions. Hielscher Ultrasonics sonochemical devices can be easily used for batch (open beaker) and continuous inline sonication using a sonochemical flow cell. Sonochemistry including sono-synthesis, sono-catalysis, degradation, or polymerization are widely used in chemistry, nanotechnology, materials science, pharmaceuticals, microbiology as well as in other industries.

High-Performance Sonochemical Equipment

Hielscher Ultrasonics is your top supplier of innovative, state-of-the-art ultrasonicators, sonochemical flow cell, reactors and accessories for efficient and reliable sonochemical reactions. All Hielscher ultrasonicators are exclusively designed, manufactured and tested at the Hielscher Ultrasonics headquarter in Teltow (near Berlin), Germany. Besides highest technical standards and outstanding robustness and 24/7/365 operation for highly efficient operation, Hielscher ultrasonicators are easy and reliable to operate. High efficiency, smart software, intuitive menu, automatic data protocolling and browser remote control are just a few features that distinguish Hielscher Ultrasonics from other sonochemical equipment manufacturers.

Precisely Adjustable Amplitudes

The amplitude is the displacement at the front (tip) of the sonotrode (also known as ultrasonic probe or horn) and is the main influencing factor of ultrasonic cavitation. Higher amplitudes mean more intense cavitation. The required intensity of cavitation strongly depends on the reaction type, chemical reagents used and targeted results of the specific sonochemical reaction. This means the amplitude should be precisely adjustable in order to tune the intensity of acoustic cavitation to the ideal level. All Hielscher ultrasonicators can be reliably and precisely adjusted via an intelligent digital control to the ideal amplitude. Booster horns can be additionally used to decrease or increase the amplitude mechanically. Ultrasonics’ industrial ultrasonic processors can deliver very high amplitudes. Amplitudes of up to 200µm can be easily continuously run in 24/7 operation. For even higher amplitudes, customized ultrasonic sonotrodes are available.

Precise Temperature Control During Sonochemical Reactions

In the cavitation hot-spot, extremely high temperatures of of many thousands of degrees Celsius can be observed. However, these extreme temperatures are limited locally to the minute interior and surrounding of the imploding cavitation bubble. In the bulk solution, the temperature rise from the implosion a single or few cavitation bubbles is negligible. But continuous, intense sonication for longer periods can cause an incremental increase of the bulk liquid’s temperature. This increase in temperature contributes to many chemical reactions and is often considered as beneficial. However, different chemical reactions have different optimum reaction temperatures. When heat-sensitive materials are treated, temperature control may be necessary. In order to allow for ideal thermal conditions during sonochemical processes, Hielscher Ultrasonics offers various sophisticated solutions for precise temperature control during sonochemical processes, such as sonochemical reactors and flow cells equipped with cooling jackets.
Our sonochemical flow cells and reactors are available with cooling jackets, which support an effective heat dissipation. For continuous temperature monitoring, Hielscher ultrasonicators are equipped with a pluggable temperature sensor, which can be inserted into the liquid for constant measuring of the bulk temperature. Sophisticated software allows the setting of a temperature range. When the temperature limit is exceeded, the ultrasonicator automatically pauses until the temperature in the liquid has lowered to a certain set point and starts automatically sonicating again. All temperature measurements as well as other important ultrasonic process data are automatically recorded on a built-in SD card and can be revised easily for process control.
Temperature is a crucial parameter of sonochemical processes. Hielscher’s elaborated technology helps you to keep the temperature of your sonochemical application in the ideal temperature range.

The table below gives you an indication of the approximate processing capacity of our ultrasonicators: