Ultrasound as a catalyst in aqueous phase reactions

What is ultrasound ?

What is ultrasound?

• Ultrasound is an oscillating sound pressure wave with a frequency greater than the upper limit of the human hearing range.

• Ultrasound is defined by the American National Standards Institute as "sound at frequencies greater than 20 kHz.

How does it propagate ?Sound is transmitted through the medium by

inducing vibrational motion of the molecules through it travels .

This motion can be seen as the ripples produced when a pebble is dropped into a pool of still water .The waves move but the water molecules which constitute the wave come back to their original position after the wave has passed .

Present day uses :

medical

Chemical processes

weapons

Uses in chemical processes :

ultrasound

humidifier

cleaningcatalyst

Where it all began? The influence of sonic waves traveling through liquids was

first reported by Robert Williams Wood and Alfred Lee Loomis in 1927.

Experiment: The frequency of the energy that it took for sonic waves to "penetrate" the barrier of water.

They came to the conclusion that sound does travel faster in water, but because of the water's density compared to our earth's atmosphere it was incredibly hard to get the sonic waves into the water.

After lots of research they decided that the best way to disperse sound into the water was to make loud noises into the water by creating bubbles that were made at the same time as the sound.

Sonochemistry experienced a renaissance in the 1980s with the advent of inexpensive and reliable generators of high-intensity ultrasound.

Working principle : acoustic cavitation

Effects of cavitation bubbles on different phasesThe cavitation bubble has a variety of effects within the

liquid medium depending upon the type of system in which it is generated.

Homogeneous liquid

Heterogeneous liquid/liquid

Heterogeneous solid/liquid

Homogeneous liquid phase reactions

In the bulk liquid immediately surrounding the bubble where the rapid collapse of the bubble generates shear forces which can produce mechanical effects.

In the bubble itself where any species introduced during its formation will be subjected to extreme conditions of temperature and pressure on collapse leading to chemical effects.

Heterogeneous powder - liquid reactions

Acoustic cavitation can produce dramatic effects on powders suspended in a liquid

Surface imperfections or trapped gas can act as the nuclei for cavitation bubble formation on the surface of a particle and subsequent surface collapse can then lead to shock waves which break the particle apart.

Cavitation bubble collapse in the liquid phase near to a particle can force it into rapid motion.

Under these circumstances the general dispersive effect is accompanied by interparticle collisions which can lead to erosion, surface cleaning and wetting of the particles and particle size reduction.

Heterogeneous liquid/liquid system

In heterogeneous liquid/liquid reactions, cavitational collapse at or near the interface causes disruption and mixing, resulting in the formation of very fine emulsions.

The collapse of a cavitation bubble on or near to a surface is unsymmetrical because the surface provides resistance to liquid flow from that side.

The result is an inrush of liquid from the side of the bubble remote from the surface resulting in a powerful liquid jet being formed, targeted at the surface.

The effect is equivalent to high pressure jetting and is the reason that ultrasound is used for cleaning.

This effect can also activate solid catalysts and increase mass and heat transfer to the surface by disruption of the interfacial boundary layers.

Cavitation near a surface

Ultrasound to enhance a liquid/liquid reaction

CH3COOC5H11 + NaOH C5H11OH + CH3COONa

A + B C + DWe know that , MH = if MH > 2 ,instantaneous reactionIf 0.02<< MH < 2 , intermediate reaction ratesIf MH < 0.02 , slow reaction rates

Here ,MH >> 0.02i.e. an intermediate reaction .The temperature and concentration of the

reactants were chosen such that the reaction was influenced by the mass transfer .

Concentration of B was chosen as 300 mol/m3

,which was quite low and thus the reaction was a bit slower .

Modelling :

The reaction was modelled with simple double – film model with the following assumptions :

The reaction was 2nd order overall .As the reaction was slow ,it was assumed to take

place in the bulk of the continuous aqueous phase .

Mass transfer volumetric coefficients Kla were supposed to be constant during the course of the reaction and were same for both A and C .

Procedure :The two liquids were mixed together using

mechanical agitation at different speeds and the data were noted down .

After determining the optimum mechanical stirring speed ,ultrasound was added to it and the results were noted down .

To determine the influence of temperature along with the effect of ultrasound ,the reaction was carried out at various temperatures and data were noted down .

Moreover ,for observing whether ultrasound power have any effects on the reaction ,experiments were done at various powers of ultrasound .

ResultsThe mechanical agitation at various rpms was

used and the curves were plotted .

Contd.Ultrasound Power :Increasing the ultrasound power results in an

increase of the reaction rate .But ,here again this increase is limited by

reaching the chemical regime .

Conclusion :Therefore, it can be concluded that ultrasound is

very effective in improving the reaction rate, but that high powers are not necessary for that reaction; in fact, when ultrasound power is increased above 20W, cavitation increases but its effect on mass transfer cannot be observed anymore because the reaction rate is no more limited by mass transfer.

Advantages Initialization of reactions that are otherwise not feasible Increase of yieldReduction of reaction timeLower reaction temperaturesSimplified preparationUse of cheaper raw materials, especially oxidantsShift of the selectivitySupresses side reactions

ConclusionThe chemical consequences of acoustic cavitation

are far reaching. Bubble collapse in liquids creates unique high-energy

conditions to drive chemical reactions in otherwise cold liquids.

The utility of sonochemistry has been explored, and important applications have been developed for synthesis of unusual inorganic and biomedical materials.

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