Article

www.elsevier.com/locate/apcata

Applied Catalysis A: General 279 (2005) 1221.6.5. External temperature. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

* Corresponding author. Tel.: +358 2 215 4985; fax: +358 2 215 4479.

E-mail address: [email protected] (D.Yu. Murzin).

0926-860X/$ see front matter # 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.apcata.2004.10.0441. Sonochemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.2. History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.3. Applications of ultrasound. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.4. Acoustic cavitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.4.1. Homogeneous sonochemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.4.2. Heterogeneous sonochemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.5. Sonoluminescence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.6. Factors effecting cavitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.6.1. Ultrasonic frequency. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.6.2. Acoustic power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.6.3. Solvent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.6.4. Gas type and content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4Contentsnew developments in equipment, processing techniques and operational methods. This development aims at more compact, safe, energy

efficient and environmentally friendly process.

Several unconventional processing techniques rely on alternative forms of energy. Chemistry under extreme and non-conventional

conditions is an actively studied topic in applied research and industry. Alternatives to conventional synthetic procedures promise

enhancement of reaction rates, yields, selectivity and also bear promise of milder reaction conditions in chemical synthesis. During the

last few decades, chemical application of ultrasound (US) and microwave (MW) irradiation has received a lot of attention and widespread

research is going on in these areas. Significant enhancement of selectivities, rates and yields in chemical reactions has been achieved by means

of US and MW irradiation. The popularity of US and MW irradiation as chemical laboratory techniques is rapidly growing, based on the

number of publications, presentations and meetings, demonstrating their vast potential. Other less exploited methods are solar and plasma

reactors. These, however, will not be touched in the present review, which focuses on the use of ultrasound and microwaves as sources of

energy, mainly in catalytic applications.

# 2004 Elsevier B.V. All rights reserved.

Keywords: Ultrasound irradiation; Microwave irradiation; Hetrogeneous catalytic reactionProcess intensification has become a very interesting approach, transforming current practices in chemical engineering and bringing forthReview

Utilization of electromagnetic and acoustic irradiation in enhancing

heterogeneous catalytic reactions

B. Toukoniitty, J.-P. Mikkola, D.Yu. Murzin*, T. Salmi

Laboratory of Industrial Chemistry, Process Chemistry Centre, Abo Akademi,

Biskopsgatan 8, 20500 Turku, Finland

Received 8 June 2004; received in revised form 25 October 2004; accepted 27 October 2004

Available online 15 December 2004

Abstract

. . .

. . .

. . .

. . .

. . .

. . .

. . .

. . .

. . .

. . .

. . .

. . .

. . .

. . .

. . .

. . .

y . .

. . .

. . .

. . .

. . .

. . .

. . .

1.1. Introduction published the first paper on chemical effects of ultrasound

talysUltrasound is simply sound pitched above the frequency

bond of human hearing. It is a part of sonic spectrum that

ranges from 20 kHz to 10 MHz and corresponds to the

wavelengths from 10 to 103 cm.The application of ultrasound, in connection to chemical

reactions, is called sonochemistry. The range from 20 kHz to

around 1 MHz is used in sonochemistry, since acoustic

cavitation in liquids can be efficiently generated within this

frequency range [1]. However, common laboratory equip-

ment utilizes the range between 20 and 40 kHz.

1.2. History

Chemical application of ultrasound has become an

exciting new field of research during the last decade,

although the interest in ultrasound and cavitational effect

dates back over 100 years. The first report on cavitation was

published in 1895 by Thornycroft and Barnaby when they

noticed that the propeller of their submarine, the H.M.S.

Daring, was pitted and eroded [2]. The first commercial

application appeared in 1917 when Paul Langevin invented

and developed echo-sounder. The original echo-

sounder, later on, became underwater SONAR for

submarine detection during the World War II. The same

[3]. In 1980, Neppiras used for the first time the term

sonochemistry in a review of acoustic cavitation [4].

Since that time, sonochemistry has become so popular that

in 1986 the first international meeting on sonochemistry

took place at Warwick University, UK. In the 1980s, the

renaissance of sonochemistry research has accelerated.

1.3. Applications of ultrasound

Household use of ultrasound is widespread: ultrasound is

applied in dog whistles, burglar alarms and jewellery cleaners.

Also, high frequency ultrasound, at around 5 MHz, is

applied for medical purposes: to remove kidney stones

without surgery, to image fetal development during

pregnancy, to treat cartilage injuries (such as tennis

elbow), for dental descaling and in physiotherapy [5].

In industry, ultrasound is important in extraction, impreg-

nation, crystallization, filtration, emulsifying cosmetics and

foods, welding plastics, powder production, cutting alloys,

cleaning and disinfections of medical instruments as well as

in food processing equipment [6].

1.4. Acoustic cavitation

The origin of sonochemical effects in liquids is the1.6.6. External pressure . . . . . . . . . . . . . . . . . . .

1.7. Heterogeneous catalyzed systems . . . . . . . . . . . . . .

1.8. Other applications . . . . . . . . . . . . . . . . . . . . . . . .

1.8.1. Polymerization reactions . . . . . . . . . . . . . .

1.9. Ultrasonic systems . . . . . . . . . . . . . . . . . . . . . . . .

1.9.1. Ultrasonic bath. . . . . . . . . . . . . . . . . . . . .

1.9.2. Horn (probe) system . . . . . . . . . . . . . . . . .

2. Microwave chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.2. History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.3. Industrial application . . . . . . . . . . . . . . . . . . . . . .

2.4. Microwaves in chemistry. . . . . . . . . . . . . . . . . . . .

2.4.1. Microwave spectroscopy . . . . . . . . . . . . . .

2.4.2. Dielectric polarization. . . . . . . . . . . . . . . .

2.5. How materials interact with microwaves . . . . . . . . .

2.6. Microwave heating of liquids and solids . . . . . . . . .

2.7. Microwaves specific and athermal effects in chemistr

2.8. Microwaves application in heterogeneous catalysis . .

2.9. Microwave equipment . . . . . . . . . . . . . . . . . . . . . .

2.9.1. Multi-mode cavities . . . . . . . . . . . . . . . . .

2.9.2. Single-mode cavities . . . . . . . . . . . . . . . . .

3. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Sonochemistry

B. Toukoniitty et al. / Applied Ca2year Lord Rayleigh published the first mathematical model. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

for cavitational collapse, predicting enormous local tem-

peratures and pressures. In 1927, Richards and Loomis

is A: General 279 (2005) 122phenomenon of acoustic cavitation. Ultrasound is trans-

atalysmitted through a medium via pressure waves by inducing

vibrational motions of the molecules, which alternately

compress and stretch the molecular structure of the medium

due to a time-varying pressure [5]. Molecules start to

oscillate around their mean position and if the strength of the

acoustic field is sufficiently intense, cavities are created in

liquids. This will happen if the negative pressure exceeds the

local tensile strength of the liquid.

The cavities are also called cavitation bubbles and the

process itself is referred to as cavitation. Two types of

cavitation are known: stable and transient. Stable cavities are

bubbles, which form and oscillate around their equilibrium

position over several rarefaction/compression cycles, before

collapsing or never collapsing at all [2].

A transient cavity exists for only a few acoustic cycles. It

means that the bubbles grow during the expansion phase of

the sound wave, followed by a shrinking phase and,

consequently, a violent collapse (Fig. 1). This collapse

generates very high pressures and temperatures. The

chemical and physical effects of ultrasound are the results

of both stable and transient cavities.

Two different theories exist about cavitation: the hot-spot

and the electrical theory. The hot-spot theory assumes that

short-lived, localized hot-spots are generated in liquids.

Such a hot-spot can reach a temperature of roughly around

5000 8C, a pressure of 1000 atm, a lifetime less than amicrosecond and heating and cooling rates above 109 8C s1

B. Toukoniitty et al. / Applied C

Fig. 1. Cavitation process [5]: formation, growth, and violent collapse of

bubbles, which generates very high pressures and temperatures. This

process is called cavitation.[1].

According to the electrical theory, an electrical charge is

created on the surface of a cavitation bubble and enormous

electrical field gradients are formed across the bubble, being

able to break bonds upon collapse [7]. However, in 1996,

Lepoint-Mullie [8] discredited the electrical theory as a valid

mechanism behind sonoluminescence (SL) and sonochem-

istry. It seems very unlikely that SL arises from the

mechanism of bubble fragmentation during which ejected

microbubbles could accumulate electrical charges as

claimed by Margulis.

1.4.1. Homogeneous sonochemistry

1.4.1.1. Aqueous systems. The effect of ultrasound on

aqueous solutions has been studied for many years.Sonification of aqueous solutions leads to the sonolysis of

the various components in the solution. For example,

sonolysis of water produces two radicals: H2O ! OH + H[9]. Cavitational implosion produces free radicals in most

solvents and these radicals can recombine or react with other

species, to induce secondary reduction and oxidation

reactions [10].

Several reports dealing with utilization of acoustic

irradiation in hazardous chemical destruction and waste

water treatment were published in the recent years [1115],

since, ultrasonically generated free radicals in solvents can

be used to degrade toxic compounds.

Beneficial impact of acoustic irradiation on chemical

synthesis (heterogeneous or homogeneous) can be utilized in

connection to free radical formation under ultrasound, as it

promotes additional reaction pathways. Serpone and Terzian

[16] investigated ultrasonic induced dehalogenation and

oxidation of 2-, 3-, and 4-chlorophenol in air-equilibrated

media based on sonolysis mechanism of aqueous solution.

1.4.1.2. Organic systems. In case of organic systems, the

enhancing effect of ultrasound is not necessarily directly

related to thermal effects like in aqueous systems, but it is

rather a result of single-electron-transfer (SET) process

acceleration. The SET-step is required as the initial step in

some reactions, for example cycloadditions involving

carbodienes and heterodienes [17]. In systems where the

reaction mechanism does not require a SET-step, ultrasound

has a minor or no effect on the reaction [2].

1.4.2. Heterogeneous sonochemistry

1.4.2.1. Liquidliquid systems. Ultrasound forms very fine

emulsions in systems of two immiscible liquids. It means

that the surface area available for the reaction between two

phases is significantly increased and, therefore, also the

reaction rate increases [2]. Emulsions produced by

sonification are more stable than those formed convention-

ally [18].

1.4.2.2. Liquidsolid systems. When a cavitation bubble

collapses violently near a solid surface, liquid jets are

produced and high-speed jets of liquid are driven into the

surface of a particle (Fig. 2). These jets and shock waves can

cause surface coating removal, produce localized high

temperatures and pressures as well as improve the liquid

solid mass transfer [19]. Moreover, surface pitting may

result.

1.5. Sonoluminescence

When a liquid is irradiated by ultrasound, light is emitted

in some cases. This light-emission is called sonolumines-

cence (SL), which is basically emission of light associated

with cavitation. Sonoluminescence was first discovered in

1934 by Frenzel and Schultes in water [21]. There are two

is A: General 279 (2005) 122 3forms of sonoluminescence: single bubble sonolumines-

talyscence (SBSL) and multi-bubble sonoluminescence (MBSL)

[22]. The duration of a light flash is much more shorter in the

case of SBSL (100400 ps) than upon MBSL (3000 ps) [23].

No general consensus has been reached as to what is the

cause of this light emission although several theories have

been published. Walton and Reynolds believe that SL is

caused by recombination of free radicals generated within

cavitation bubbles during the collapse [24]. At the same

time, Suslick et al. favors an alternative theory claiming that

the light emission is caused by thermally created chemi-

luminescence [25].

1.6. Factors effecting cavitation

The ambient conditions of the reaction system (fre-

quency, acoustic power, solvent, ultrasonic intensity, system

vapor pressure, external temperature and pressure, presence

and nature of dissolved gases, sample preparation, buffer)

can effect the intensity of cavitation, which influences the

reaction rates and/or yields [2]. In the following, these

factors are discussed in more detail.

B. Toukoniitty et al. / Applied Ca4

Fig. 2. Collapse of a cavitation bubble near to a solid surface [20].1.6.1. Ultrasonic frequency

Frequency has a significant influence on cavitation since

by increasing the frequency the cavitation effect is

decreased. There are two explanations for this: (1)

rarefaction cycle of the sound wave produces a negative

pressure which is not sufficient in its duration and/or

intensity to initiate cavitation [2], (2) rarefaction and

compression cycles are too short to permit bubbles to reach a

sufficient size to collapse [26]. This is the reason why many

sonochemical reactions are carried out applying frequencies

between 20 and 50 kHz.

1.6.2. Acoustic power

With increasing power input to the reaction mixture, the

reaction rates increase to a maximum and then decrease with

additional power increase [27]. A probable explanation for

the observed decrease of the reaction rate, at high acousticpowers, is the formation of a dense cloud of cavitation

bubbles near the surface of the horn tip, which acts as a

screen and reduces the amount of acoustic energy

transmitted to the fluid [28]. The optimum power input is

also dependent on the operating frequency [29]. Henglein

and Gutierrez [30] investigated the effect of ultrasonic power

on formation of iodine (Fig. 3) by using a commercial

20 kHz sound generator (KLN system 582, horn diameter

14 mm). Initially, the rate was increasing linearly, reaching a

plateau and sharply dropping at high power.

1.6.3. Solvent

Solvents with low vapor pressure and high surface tension

undergo intense cavitational effects, although cavities are

more easily formed in solvents with high vapor pressure, low

surface tension and low viscosity [2]. Solvents with high

vapor pressure decrease the effect of bubble collapse and

cavitation is inhibited. Since it is required that the negative

pressure in the rarefaction region must overcome the natural

cohesive forces acting in liquid and therefore it is more

difficult to produce cavitation in viscous liquids [26].

is A: General 279 (2005) 122

Fig. 3. The effect of ultrasonic power on reaction rate of iodine formation

[30].The effect of solvent on sonic enantioselective hydro-

genation of 1-phenyl-1,2-propanedione (Fig. 4) over 5 wt.%

Pt/silica fiber (SF) was investigated (Table 1). The highest

enhancement of the rate and enantiomeric excess, compared

to silent conditions, was observed in mesitylene (the solvent

with the lowest vapor pressure).

The enantiomeric excess (ee) of B is defined as followsee = ([1R] [1S])/([1R] + [1S]), where [R] and [S] are theconcentrations of (R)- and (S)-1-hydroxy-1-phenylpropa-

none, respectively.

1.6.4. Gas type and content

In general, dissolved gases with high specific heat ratio

usually give better cavitational effects than a gas with low

specific heat ratio since they convert more energy upon

cavitation [2]. For this reason, monoatomic gases such as

helium, argon and neon are preferred to diatomic gases such

as nitrogen and oxygen [26].

formation of the bubbles more difficult. On the other hand, at

lower temperatures higher rates are obtained in sonochem-

ical processes [32]. In certain reactions systems, an optimum

reaction temperature may lead to more favorable results [2].

1.6.6. External pressure

With raising external pressure (Ph) cavitation threshold

and intensity of bubble collapse are increased. It is assumed

that there will no longer be a resultant negative pressure

phase of the sound wave (since Ph PA > 0), therefore

B. Toukoniitty et al. / Applied Catalysis A: General 279 (2005) 122 5The sonochemical effect also depends on thermal

conductivity of a gas: the greater the thermal conduction

of a gas, the more heat (formed in the bubble during the

Fig. 4. Reaction scheme of 1-phenyl-1,2-propanedione hydrogenation: (A)1-phenyl-1,2-propanedione, (B) (R)-1-hydroxy-1-phenylpropanone, (C)(S)-1-hydroxy-1-phenylpropanone, (D) (S)-2-hydroxy-1-phenylpropanone,(E) (R)-2-hydroxy-1-phenylpropanone, (F) (1R,2S)-1-phenyl-1,2-propane-diol, (G) (1S,2S)-1-phenyl-1,2-propanediol, (H) (1S,2R)-1-phenyl-1,2-pro-panediol, (I) (1R,2R)-1-phenyl-1,2-propanediol.collapse) will be dissipated to the surrounding liquid

resulting into an increased heat loss [31].

Gases with very high solubility in the reaction mixture

may reduce the intensity of a cavitation by possible

redissolving of the cavitational bubble before collapse [2].

1.6.5. External temperature

Generally, an increase in the ambient reaction tempera-

ture results in overall decrease in the sonochemical effect.

With increasing reaction temperature, the equilibrium vapor

pressure increases, which leads to easier bubble formation.

However, the cavitation bubbles which are formed, contain

more vapor [2]. The vapor reduces ultrasonic effect by

decreasing violent cavitation bubble collapse. Lower

temperature causes a higher viscosity, which makes the

Table 1

Solvent effect on the initial hydrogenation rate and on enantiomeric excess (ee)

Solvent Pvp (15 8C)(bar)

DvapH (25 8C)(kJ/mol)

Silent

Initial rate

( 103 mol/Methyl acetate 0.1814 32.29 1.6

Toluene 0.0220 38.01 2.6

Mesitylene 0.0016 47.50 0.3

Pvp is the vapor pressure of solvent at reaction temperature 15 8C; DvapH the molaexcess (after 2 h of reaction).cavitation bubbles cannot be created. However, a sufficiently

large increase in the intensity (I) of the applied ultrasonic

field could produce cavitation, even at higher overpressures,

since it will generate larger values of Pa (Eq. (1)) making

Ph PA < 0. In that Pm (the pressure of collapsing bubble)is approximately Ph + PA, increasing the value of Ph will

lead to a more rapid (Eq. (2)) and violent (Eq. (3)) collapse

[20]:

Pa PA sin2pft (1)where Pa is the applied acoustic pressure, t the time, f the

frequency and PA is the pressure amplitude

t 0:915Rm rPm

1=21 Pvg

Pm

(2)

where t is the collapse time, Rm the radius of the cavity at the

start of collapse, r the density of the medium and Pm is the

pressure in the liquid

Pmax P PmK 1P

K=K1(3)

where Pmax is the maximum pressure developed in the

bubble, Pm the pressure in the liquid at the moment of

transient collapse, P the pressure in the bubble at its max-

imum size and K is the polytropic index of the gas mixture.

Moulton et al. [33] observed, when investigating

hydrogenation of soybean oil at high hydrogen pressure

(14 bar), negligible enhancement of the catalyst activity

while at lower hydrogen pressure (8.5 bar) the ultrasonic

effect was significantly increased.

Torok et al. [34] observed a similar trend when studding

cinamaldehyde hydrogenation over Pt/SiO2 catalyst. At

30 bar hydrogen pressure the enhancement of the catalyst

activity under ultrasound was almost negligible. When the

Ultrasound

L gcat min)

ee (%) Initial rate

( 103 mol/L gcat min)ee (%)

40 2.1 39

18 3.3 37

35 1.3 55

r enthalpy (heat) of vaporization at temperature 25 8C; ee the enantiomeric

talyspressure was decreased to 1 bar the catalyst activity was

significantly enhanced by ultrasound.

Comprehensive study of the influence of several

parameters such as hydrostatic pressure, nature of dissolved

gases frequency in degradation of phenol was reported by

Berlan et al. [15].

1.7. Heterogeneous catalyzed systems

Heterogeneous catalytic reaction systems are industrially

the most important class of reactions. A large number of

products ranging from bulk chemicals to alimentary and

pharmaceuticals are produced by means of heterogeneous

catalysis. This is the reason why quite many heterogeneous

systems could be found in the literature, in order to probe the

effect of ultrasound. Table 2 contains several representative

examples, which demonstrate the positive effects of

ultrasound on activity and selectivity. Some of the results

are commented below.

Lindley et al. investigated Ullmann [35] coupling of

2-iodonitrobenzene over Cu-powder. The positive effect of

copper powder presonification was reported. Under ultra-

sound the yield increased from 1.5 to 70.4%.

Li et al. [36] carried out a reaction of chalcones with

ethyl cyanoacetate catalyzed by KF/basic alumina in

anhydrous ethanol. The mixture was irradiated in a water

bath of an ultrasonic cleaner, at 2534 8C. Shorter reactiontime and higher yield was observed compared to silent

conditions.

Shin and Han [37] hydrogenated diphenylacetylene over

palladium hydrazine. The mixture was exposed to sonifica-

tion in common ultrasound laboratory cleaning bath at

25 8C. A higher yield was observed under ultrasound.Torok et al. [38] investigated sonochemical enantiose-

lective hydrogenation of ethyl pyruvate over 3 wt.% Pt/SiO2catalyst in ethanol and toluene. The hydrogenation reaction

was carried out at room temperature (25 8C). The catalystwas activated in a closed system by stirring it for 1 h or in an

ultrasonic bath for 30 min under atmospheric hydrogen

pressure. After the activation period ethyl pyruvate was

injected into the reactor. The experimental results indicated

that the acoustic irradiation of catalyst modifier system

priori to the reaction is a highly efficient method to enhance

enantioselectivity. However, such enhancement was very

solvent specific, as the catalyst activity was increased by

sonification in ethanol solvent, only.

Cains et al. [39] carried out hydrogenation of oct-1-ene

over 10% Pd/C catalyst, at 25 8C, in absolute ethanol.Presonification of this catalyst led to a considerable

enhancement in activity resulting in 100% conversion after

only 30 min for the sonicated system. Only 66% conversion

was achieved under silent conditions.

Sulman [40] studied hydrogenation of a carbonyl group

of ethyl 9-(2,3,4-trimethoxy-6-methylbenzoyl)nonanoate on

Pt/sibunit catalyst. The catalyst was subjected to the

B. Toukoniitty et al. / Applied Ca6ultrasonic irradiation (ultrasonic flux density 1 W cm2)for 30 s. A yield enhancement was observed by use of

ultrasonic pretreated catalyst. The increase in the ultrasonic

flux density from 0.1 to 1 W cm2 favored the catalystactivity, while its further increase to 3 W cm2 resulted inthe decrease of activity.

Ando et al. [41] investigated ultrasonic irradiation of a

mixture of benzyl bromide, potassium cyanide and alumina

in an aromatic solvent. When benzyl bromide was treated

with potassium cyanide and alumina in toluene under

mechanical agitation at 50 8C, only a mixture o- and p-benzyltoluene was obtained in 75% yield. Unexpectedly,

alumina had not promoted the cyanide substitution reaction

but enhanced the FriedelCrafts reaction of benzyl bromide

with toluene. In contrast, when the same reaction mixture

was irradiated by ultrasound (ultrasonic bath) at 50 8C, thesubstitutions did take place to afford benzyl cyanide in 71%

yield. Thus, ultrasound completely switched the reaction

course from the FriedelCrafts reaction to nucleophilic

substitutions.

Han and Boudjouk [42] studied Reformatsky reaction

over Zn at 2530 8C under ultrasonic irradiation (ultrasonicbath). A significant enhancement of the yield under

ultrasound was observed. Several examples of different

Reformatsky reactions and solvents comparisons were

reported in this paper.

Fuentes et al. [43] investigated synthesis of chalocones by

ClaisenScmidt condensation over activated barium hydro-

xide C-200, in ethanol at 25 8C. Substantial improvement ofthe yield by catalyst activation in the presence of ultrasound

(ultrasonic bath) was noticed.

Boudjouk and Han [44] carried out hydrogenations of

olefins using formic acid as the hydrogen transfer agent over

10% Pd/C, at 25 8C, with toluene solvent under ultrasoundirradiation (ultrasonic cleaning bath). They reported

significant enhancement of the hydrogenation rate in the

presence of sonic waves.

Szollosi et al. [45] investigated selective hydrogenation

of cinnamaldehyde to cinnamyl alcohol, at a reaction

temperature of 60 8C and hydrogen pressure of 1 bar, inisopropyl alcohol. Several heterogeneous Pt catalysts on

different supports were tested: 5% Pt/C, 5% Pt/Al2O3,

EuroPt-1 (6.3% Pt/SiO2), and 3% Pt/SiO2. The catalysts

were presonificated in an ultrasonic bath, at 25 8C, for10 min. As the data indicates, the reaction rates decreased

over 5% Pt/C and 5% Pt/Al2O3, while the increase was

negligible over EuroPt-1 (6.3% Pt/SiO2) and significant over

3% Pt/SiO2. Parallel, the cinnamyl alcohol became the

major product of the hydrogenation over 3% Pt/SiO2.

Han and Boudjouk [46] studied sonically accelerated

hydrosilation of alkenes (several reactions were reported).

The experiments were carried out in a single-necked flask

submerged to an ultrasonic laboratory cleaner (Bransonic

model 220). Their results showed that ultrasonic waves

dramatically increased the activity of Pt/C in the hydro-

silation reactions, giving yields comparable to those

is A: General 279 (2005) 122obtained with a homogeneous catalyst. In each case, the

B. Toukoniitty et al. / Applied Catalysis A: General 279 (2005) 122 7

Table 2

Organic synthesis over heterogeneous catalysts studied in the presence of ultrasound

Reference Reaction Conventional conditions Ultrasonic conditions

[35] Ullman coupling of 2-iodonitrobenzene:

(copper power presonificated for 15 min)

Stirring; temperature: 63 8C;reaction time: 2 h; yield: 1.5%

Probe system;

power: 135 W;

temperature: 63 8C;reaction time: 2 h;

yield: 70.4%

[36] Michael reaction:

(catalyst: KF/basic alumina)

Bath system; temperature:

2534 8C; reaction time:1.5 h; yield: 86%

[37] Hydrogenation of diphenylacetylene: Magnetic stirring; temperature: 25 8C;reaction time: 1 h; yield: 24%


25 8C; reaction time:1 h; yield: 100%

[38] Enantioselective hydrogenation of ethyl pyruvate:

(solvent: ethanol, toluene; catalyst: 3 wt.% Pt/SiO2)

Stirring; temperature: 25 8C;solvent: ethanol; yield: 95% (600 min);

ee: 20.2%; solvent: toluene;

yield: 95% (73 min); ee: 24.8%


25 8C; solvent: ethanol;yield: 100% (52 min);

ee: 34.2%; solvent:

toluene; yield: 100%

(110 min); ee: 50.5%

[39] Hydrogenation of oct-1-ene:

(catalyst: 10 wt.% Pd/C)

Stirring; temperature: 25 8C;solvent: ethanol; reaction time:

30 min; yield: 66%

Probe system; temperature:

25 8C; solvent: ethanol;reaction time: 30 min;

yield: 100%

[40] Hydrogenation of ethyl

9-(2,3,4-trimethoxy-6-methylbenzoyl)nonanoate:

(catalyst: Pt/sibunit)

Stirring; no catalyst pretreatment;

reaction time: 80 min; yield: 55%

Ultrasonic catalyst

pretreatment; reaction

time: 80 min; Y: 85%

(1 W cm2); Y: 65%(0.1 W cm2); Y: 75%(3 W cm2)

[41] Change in pathway from the FriedelCrafts reactions to

nucleophilic substitutions:

Stirring; temperature: 50 8C;product: o- and p-benzyltoluene;

yield: 75%

Bath system; power:

200 W; product:

benzyl cynanide;

yield: 71%

[42] Reformatsky reaction: Stirring; temperature: 80 8C;reaction time: 12 h; yield: 50%

Bath system;

temperature: 2530 8C;power: 150 W; reaction

time: 30 min; yield: 98%

B. Toukoniitty et al. / Applied Catalysis A: General 279 (2005) 1228

Table 2 (Continued )

Reference Reaction Conventional conditions Ultrasonic conditions

[43] Synthesis of chalcones by ClaisenSchmidt condensation:

(catalyst: activated barium hydroxide C-200)

Stirring; temperature: 25 8C;solvent: ethanol; catalyst mass:

1.0 g; reaction time: 60 min; yield: 5%

Bath system;

temperature: 25 8C;solvent: ethanol;

catalyst mass: 0.1;

reaction time: 10 min;

yield: 76%

[44] Hydrogenation of olefins:

(catalyst: 10 wt.% Pd/C)

Stirring; temperature: 25 8C;reaction time: 1 h; yield:

maximum 70%

Bath system;

temperature: 25 8C;solvent: ethanol;

reaction time: 1 h;

yield: 100%

[45] Selective hydrogenation of cinnamaldehyde to cinnamyl alcohol:

(solvent: isopropyl alcohol; catalyst: 5% Pt/C,

5% Pt/Al2O, EuroPt-1, 3% Pt/SiO2)

Stirring; temperature: 25 8C;rate (5% Pt/C):

60 103 mol g1Pt h1; rate (5%Pt/Al2O3):40 103 mol g1Pt h1; rate(EuroPt-1): 25 103 mol g1Pt h1;rate (3%Pt/SiO2): 31 103 mol g1Pt h1;selectivity (5% Pt/C): 42 mol%; selectivity

(5%Pt/Al2O3):47 mol%; selectivity

(EuroPt-1): 58 mol%; selectivity

(3%Pt/SiO2): 40 mol%

Bath system; presonifi-

cation 10 min;

temperature:

25 8C; rate (5% Pt/C):36 103 mol g1Pt h1;rate(5%Pt/Al2O3):

32 103 mol g1Pt h1;rate (EuroPt-1):

29 103 mol g1Pt h1;rate (3%Pt/SiO2):

72 103 mol g1Pt h1;selectivity (5% Pt/C):

37 mol%; selectivity

(5%Pt/Al2O3):47 mol%;

selectivity (EuroPt-1):

78 mol%; selectivity

(3%Pt/SiO2): 79 mol%

[46] Hydrosilation reaction:

(catalyst: Pt/C)

Bath system;

temperature:

30 8C; reaction time:1.5 h; yield: 94%

[47] Dechlorination of polychlorinated biphenyls:

(catalyst: 10% Pd/C)

temperature: 25 8C; solvent: hexane;reaction time: 45 min; yield:

maximum 100%

Bath system;

temperature:

25 8C; solvent: hexane;reaction time: 15 min;

yield: 100%

[48] FriedelCrafts acylation of 2-methoxynaphtalene:

(catalyst: Amberlyst-36, Amberlyst-15,

Indion-130, K-10 clay, Filtrol-24)

Conventional method; Amberlyst36:

conversion (20 min): 25%;

Amberlyst15: conversion (20 min):

22%; Indion-130: conversion (20 min):17%

Probe system;

temperature: 25 8C;Amberlyst36:

conversion (20 min):

65%; Amberlyst15:


50%; Indion-130:


60%

[49] Hydrogenation of citral to citronellal:

(catalyst: RaNi)

Stirring; temperature: 70 8C;solvent isopropanol; reaction time:

50 min; yield: maximum 50%

Probe system;

temperature: 70 8C;solvent: isopropanol;


yield: 60%

acoustic streaming improves mass transfer between the

liquid phase and the surface, thus increasing the observed

reaction rates [1]. Finally, sonification of aqueous solutions

leads to the sonolysis of the various components in the

atalysis A: General 279 (2005) 122 9

Conventional conditions Ultrasonic conditions

Stirring; temperature: 15 8C;rate (5%Pt/SF): 0.3 103 mol g1Pt h1;ee (2 h): 35%; rate (5% Pt/Al 2O3):

1.7 103 mol g1Pt h1;ee (5%Pt/Al2O3): 52%

Probe system;

temperature: 15 8C;rate (5%Pt/SF):

1.3 103 mol g1Pt h1;ee (2 h): 55%; rate

(5% Pt/Al2O3):

2.0 103 mol g1Pt h1;ee (5%Pt/Al2O3): 52%

Fig. 5. SEM image of used RaNi catalyst treated under ultrasound.

Fig. 6. SEM image of used RaNi catalyst without sonification.reaction proceeded smoothly and without vigorous

exotherm.

Gonzalo Rodrguez and Lafuente [47] found a new

heterogeneous catalytic method for the complete dechlor-

ination of PCBs using an ultrasound source. They studied

dechlorination of 2,4,5-trichlorobiphenyl (PCB-29) with

solid hydrazine hydrochloride and sodium carbonate over

Pd/C catalyst in hexane, at 60 8C (silent conditions) and25 8C (ultrasonic irradiation). The dechlorination of PCB-29by ultrasonication at 487 kHz showed remarkably high

efficiency.

Yadav and Mujeebur Rahuman [48] investigated sonic

intensification of FiedelCrafts acylation of 2-methoxy-

naphtalene with acetic anhydride under several acid treated

clays (such as K-10, Filtrol-24) and cation exchange resins

(such as Amberlyst-36, Amberlyst-15 and Indion-130) [48].

The reaction was carried out in a probe system at a

temperature of 25 8C. The results indicated that ultrasoundeffectively promotes the acylation of 2-methoxynaphtalene

in the presence of a solid acid catalyst. Sonification did not

alter the selectivity but only intensified the reaction rates.

Mikkola et al. [49] studied hydrogenation of citral over

RaNi catalyst in isopropanol, at the operating pressure

50 bar and at the temperature of 70 8C. The results indicatedthat the reaction rate, as well as the selectivity to the desired

end products; citronellol and citronellal were enhanced by

the use of ultrasound irradiation.

Toukoniitty et al. [50] carried out enantioselective

hydrogenation of 1-phenyl-1,2-propanedione over cinchona

alkaloid modified 5 wt.% Pt/silica fiber (SF) and 5 wt.% Pt/

Al2O3 catalyst. The reaction was carried in mesitylene at

15 8C and at the hydrogen pressure of 10 bar. In the case of5 wt.% Pt/silica fiber (SF) catalyst, an acceleration of the

reaction rate was observed and the enantiomeric excess was

significantly higher under ultrasound. However, with Pt/

B. Toukoniitty et al. / Applied C


Reference Reaction

[50] Enantioselective hydrogenation of 1-phenyl-1,2-propandione:

(solvent: mesitylene; catalyst:

5 wt.% Pt/ silica fiber (SF), 5 wt.% Pt/Al 2O3)Al2O3 application of ultrasound resulted only in a moderate

enhancement of the reaction rate.

In general, it can be concluded, that acoustic irradiation

was found to be a beneficial tool for enhancing the reaction

rate and selectivity. There are several explanations for

observed phenomenon. The catalyst activity is increased due

to the deformation of the catalyst surface that exposes fresh,

highly active surface and reduces the diffusion length in the

catalyst pores (Figs. 5 and 6). Furthermore, ultrasound is

capable of cleaning and smoothening the catalyst surface

(Figs. 7 and 8). The local turbulent flow associated with

Fig. 7. SEM image of used 5 wt.% Pt/silica fibre (SF) catalyst treated under

ultrasound.

talyssolution and free radicals are formed. Ultrasonically

generated free radicals in solvents can be used to degrade

toxic compounds and, therefore, acoustic irradiation is

widely used in waste water treatment and oxidation

reactions.

Acoustic irradiation can also be utilized in the process of

catalyst preparation, in order to improve the catalyst activity

and enhance the crystallization process.

Synthetic zeolites are prepared hydrothermally by

heating aqueous solutions of sodium silicate and sodium

aluminate, at a temperature range of 25300 8C for fewhours to several days. Lindley [51] found out that in

synthesis of zeolite NaA, the use of ultrasound led to

substantial reductions in nucleation time (1 h, probe

system) and overall completion times (3.4 h, probe system)

compared to control reactions (5 h nucleation time and 10 h

completion time).

Bianchi et al. [52] investigated the activity of ultra-

sonically prepared Co/SiO2 catalyst in FisherTropsch

synthesis. The catalyst was prepared by direct addition of

the aqueous solution of cobalt nitrate to the silica and

followed by sonification for 1 h at room temperature and

then reduced in flowing hydrogen. Catalyst prepared under


Fig. 8. SEM image of used 5 wt.% Pt/silica fibre (SF) catalyst without

sonification.US showed smaller crystallite size (38 A) and higher metal

dispersion compared to the catalyst prepared by classical

incipient wetness impregnation method (crystallite

size = 69 A). The catalyst prepared by using US showed

quite good stability deactivating by less than 10% in 4 days.

On the contrary, the catalyst prepared by a classical

impregnation method deactivated rapidly with the loss in the

conversion of 40% in 4 days.

Li and Inui [53] studied the effect of acoustic irradiation

on copper/zinc/alumina oxides catalyst, during co-precipi-

tation and aging. The catalytic activity of the catalyst was

evaluated in methanol synthesis. Irradiation of the suspen-

sion during co-precipitation and aging steps, in the

preparation of copper/zinc/alumina catalyst, appreciably

enhanced the activity for methanol synthesis. This enhance-

ment is proposed to stem from the promotion by the

ultrasound of the formation of the hydrotalcite-like phase(56.4%) in the precursor of the catalyst compared to the

classical method (37.5%).

Pillai et al. [54] exposed vanadium phosphorus oxide

(VPO) to acoustic irradiation in one of the steps of the

catalyst preparation (precipitation) and studied its phase

composition and activity for hydrocarbons oxidation. It

was demonstrated that active VPO catalyst could be

prepared in a relatively short time (36 h) by employing

ultrasound compared to a conventional method (CM)

(20 h). At the same time, there was no deterioration of the

catalytic properties: both catalysts had similar compositions

and surface areas (US = 10.3 m2 g1cat , CM = 10.2 m2 g1cat )

and displayed comparable oxidation activity for liquid

phase hydrogenperoxide oxidation of cycloalkanes in

acetonitrile.

1.8. Other applications

1.8.1. Polymerization reactions

The effect of ultrasound on polymerization reactions has

been studied for about 30 years. Most studies deal with the

radical polymerization of vinyl monomers where sonifica-

tion makes the addition of thermal initiators unnecessary and

allows for better control over the molecular weight, tacticity

and polydispersity [55].

Ultrasound accelerates emulsion polymerization because

of its ability to create fine emulsions.

Not only emulsion, but suspension and polymerization as

well, is enhanced by sonification since ultrasound prevents

agglomeration of monomer droplets [56].

Price [57] has shown that the reaction rate in step-growth

reaction can be accelerated by the use of high intensity

ultrasound. In this case, the source of enhancement seems to

be related to local heating produced by collapsing cavitation

bubbles.

1.9. Ultrasonic systems

For laboratory research purposes, two basic methods of

applying acoustic power to liquid loads are used: low

intensity systems 12 W cm2 (an ultrasonic bath) and highintensity systems affording hundreds of W cm2 (a hornsystem).

1.9.1. Ultrasonic bath

Ultrasonic baths were originally produced for cleaning

purposes. However, nowadays, ultrasonic baths are widely

used in sonochemistry because lower acoustic intensities are

often required in liquidsolid heterogeneous systems. For

these reaction systems, common ultrasonic cleaning baths

are sufficient [58]. They are easily available and rather

inexpensive.

1.9.2. Horn (probe) system

Horn systems (Fig. 9) are getting more popular in

is A: General 279 (2005) 122sonochemical laboratory research. These systems are able to

B. Toukoniitty et al. / Applied Catalys

Fig. 9. High-pressure autoclave with in situ ultrasonic irradiation systemdeliver large energy intensities of ultrasound power directly

to the reaction mixture, which can be regulated by varying

the amplitude delivered by the transducer [2]. A disadvan-

tage of using a horn system is the possibility of

contamination of the reaction solution caused by erosion

and pitting of the horn tip [59].

2. Microwave chemistry

2.1. Introduction

Microwave (MW) irradiation is a form of electromag-

netic energy. Microwaves consist on an electric component

as well as a magnetic one (Fig. 10). The microwave region of

(horn system).the electromagnetic spectrum is situated between infrared

radiation and radio frequencies. Microwave irradiation

ranges from 30 GHz to 300 MHz and corresponds to the

Fig. 10. Electromagnetic irradiation: electric and magnetic components

[60].wavelengths of 1 cm to 1 m. Microwave heaters use specific,

fixed frequencies 2.45 GHz (wavelength 12.2 cm) or

900 MHz (wavelength 33.3 cm), in effort to avoid inter-

ferences with RADAR (wavelength from 1 to 25 cm) and

telecommunication applications. All domestic microwave

ovens operate at the frequency of 2.45 GHz.

In comparison to the conventional heating, energy

transfer does not occur by convention and conduction but

by dielectric loss in case of microwave heating.

2.2. History

During the World War II, Randall and Booth working at

the University of Birmingham, in connection to the

development of RADAR, designed a magnetron to generate

microwaves [61].

As with many other great inventions, the microwave oven

was a by-product of another technology. In 1946, Percy

Spencer realized that a candy bar in his pocket melted during

the tests of a vacuum tube called magnetron [62]. The

widespread use of domestic microwave ovens appeared

during 1970s, in the United States and Japan.

Microwave ovens have been used for cooking and

defrosting of food for past three decades. Gedye et al. [63]

published the first pioneering report about utilizing

microwave irradiation in chemical synthesis, in 1986.

During the last decade, microwave heating has been

increasingly applied in carrying out organic synthesis.

2.3. Industrial application

Most of the industrial applications of electromagnetic

heating are found when a change of state of non-conductive

matter is involved (e.g. defrosting, dehydration with the

change of state of water) [64]. Other important use of MW is

sintering and fusion of solids. Microwave heating is also

used in food industry for drying fruits, berries, corn, etc.

[65]. In medicine, microwaves are utilized in resonance

therapy and microwave thermography.

2.4. Microwaves in chemistry

Microwave irradiation, in connection to, chemistry is

applied in two different areas, namely microwave spectro-

scopy and microwave dielectric heating.

2.4.1. Microwave spectroscopy

There is a growing interest in new analytical instruments

to quantify and identify the constituents of gas samples

obtained from automobile exhausts, factory emissions, and

process streams. Microwave spectroscopy offers high

sensitivity, accurate quantification, and unambiguous spe-

cies identification and, furthermore, it is compact and easy to

use.

Microwave spectroscopy is a major method in physical

is A: General 279 (2005) 122 11chemistry to study the molecules in the gas phase.

Generally, the more polar a molecule is, the more effectively

it will couple with microwaves.

2.4.2.2. Ionic conduction. Ionic conduction is just mini-

mally different from dipole interaction. In the presence of

the electric field, ionic species will migrate in one or the

other direction, depending on the electric field. During this

migration, energy is transferred from the dielectric field

causing ionic interactions that speed up the heating of the

talysis A: General 279 (2005) 122Microwave spectra arise when the rotational energy of a

compound is changed. These energy changes are rather

small, only few joules per mole. The absorption peaks are

unusually narrow and frequency measurements can routi-

nely be carried out with an accuracy of as much as 710

significant digits. From these moments, it is possible to

calculate the geometrical structure of a molecule more

accurately than with any other method.

Microwave spectroscopy provides excellent finger-

printing technique for identifying molecules in the gas

phase. The most important analytical application of

microwave spectroscopy so far has been to confirm the

presence of a wide range of molecules in outer space

[61].

2.4.2. Dielectric polarization

Microwave dielectric heating depends on the ability of an

electric field to polarize charges in materials and their

inability to follow rapid changes of an electric field space

[61].

The total polarization is a sum of several components:

at ae aa ad ai (4)where ae is the electronic polarization, aa the atomic polar-

ization, ad the dipolar polarization and ai is the interfacial

polarization (MaxwellWagner effect).

The time scale of the electronic and atomic polarization/

depolarization is much smaller than microwave frequencies,

therefore they do not contribute to microwave dielectric

heating.

Microwave energy can effect molecules in two principal

ways: (a) dipolar polarization and (b) by ionic conduction. A

third mechanism (c) interfacial polarization, can also occur,

although it has often limited importance.

The dielectric loss tangent defines the ability of a material

to convert electromagnetic energy into heat energy, at

certain frequency and temperature:

tan d e00

e0(5)

where e0 is the dielectric constant, describes the ability of amolecule to be polarized by electric field and e00 is thedielectric loss, describes the efficiency with which the

energy of the electromagnetic irradiation can be converted

into heat.

Both parameters, the dielectric loss and dielectric

constant are measurable properties.

2.4.2.1. Dipolar polarization. When microwave energy is

passing through the matter, molecules of the matter having

dipole moments will rotate and try to align themselves with

the electric field [61]. Polar molecules have stronger

interactions with the electric field. Polar ends of the

molecules tend to align themselves and oscillate in step with

the oscillating electric field of microwaves. Collisions and

B. Toukoniitty et al. / Applied Ca12friction between the moving molecules results in heating.solution [66]. Ionic conduction increases with temperature,

thus allowing ionic solutions to become stronger absorbers

of microwave energy as they are heated.

For example, if pure water is heated in microwave oven,

mainly the polarization mechanism occurs and the heating

rate is significantly lower than in a case when a salt is added

and both mechanisms occur and contribute to the heating

effect.

2.4.2.3. Interfacial polarization. A suspension of conduct-

ing particles in a non-conducting medium is an inhomo-

geneous material whose dielectric constant is frequency

dependent. The loss relates to the build up of charges

between interfaces and is known as the MaxwellWagner

effect. As the concentration of conducting phase is

increased, a point is reached when individual conducting

areas contribute: this has been developed by Maxwell

Wagner in the two-layer capacitor model [66].

2.5. How materials interact with microwaves

Materials interact with microwaves in three ways

(Fig. 11). Metals are good conductors because they tend

to reflect microwave energy and they do not warm up that

well. Transparent materials are good insulators because they

are transparent to microwave energy and they do not warm

up. Absorbing materials receive microwave energy and are

heated [66]. These different materials interactions with

microwaves enable selective heating.

The advantage of microwave irradiation as an energy

source for heterogeneously catalyzed systems is that

microwaves do not in many cases substantially heat up

the adsorbed organic layers but interact directly with the

metal sites on catalyst surface and hot-spots might be

created. The temperature of the reactive sites was calculated

to reside 918 K above the bulk temperature [67].

Fig. 11. Interaction of transparent (insulator), absorbing (dielectric),reflecting (conductor) materials with microwave energy [68].

2.6. Microwave heating of liquids and solids

B. Toukoniitty et al. / Applied Catalys

Table 3

Temperatures of several liquids (50 mL) after heating from room tempera-

ture for 1 min at 560 W and 2.45 GHz [61]

Liquids t (8C) Bp (8C)

Water 81 100

Methanol 65 65

1-Propanol 78 78

Acetic acid 110 119

Ethyl acetate 73 77

Acetone 56 56

Hexane 25 68

Heptane 26 98

CCl4 28 77The rate of temperature increase due to the dielectric field

of microwave radiation is determined by the following

equation [61]:

dT

dt k e

00fE2r:m:s:rcp

(6)

where E2r:m:s: is the r.m.s. field intensity, r the density, cp the

specific heat capacity, k the constant, e00 the dielectric lossand f is the frequency.

Depending on the parameters in Eq. (6) the temperature

increase can be quite substantial.

The main advantage of microwave heating is the

instantaneous heating of solids and liquids, which is not

the case of conventional heating (see Tables 3 and 4).

Organic and inorganic solvents with low molecular

weights and high dipole moments (e.g. H2O, MeOH, EtOH)

couple effectively with microwaves at 2.45 GHz. At the

same time, non-polar organic solvents have negligible

dielectric losses and therefore they do not effectively couple

with microwaves [61]. If polar solvents are used, the main

interactions occur between microwaves and polar molecules

and it would be expected that any specific microwave effects

related to the reactants and products would be masked by the

solventmicrowave interaction. The reaction rates should,

Table 4

Temperatures of several solids (25 g) heated from room temperature at1 kW oven and 2.45 GHz with 1000 mL vented water load [61]

Solids t (8C) Time (min)

Al 577 6

C 1283 1

CO2O3 1290 3

Ni 384 1

SnCl2 476 2

CaO 83 30

PbO2 182 7

TiO2 122 30

WO3 532 0.5

Authors comment [61]: most of these values were obtained in multi-mode

cavity with operation cycles (domestic microwave oven) and cannot there-

fore, be considered to provide more than indicative values of the tempera-

tures reached in a single-mode equipment with a homogeneous and

continuous field strength.therefore, be basically the same as those observed under the

conventional conditions. This hypothesis has been con-

firmed recent study of esterification reactions, where no

enhancement by microwaves irradiation was observed

compared with the conventional reaction at the same

reaction temperature [69].

In 1992, Baghurst and Mingos showed that microwaves

heat solvents above their normal boiling points. Water

heated by microwaves reaches 105 8C before boiling. Evenhigher temperature difference (38 8C above the normalboiling point) can be achieved with acetonitrile, since

superheating to 120 8C was reported. This superheating isone of the reasons why reactions proceed faster [70].

Microwave effects are most likely to be observed under

solvent free reaction conditions [71] as absorption of

microwave irradiation is limited only to the reactive species.

The possible specific effects will not be moderated or

impeded by solvents.

Microwave effects depend on microwavematerial

interactions, the more polar is a molecule, more significant

is the microwave effect when the rise in temperature is

considered. In terms of kinetics and reactivity, the specific

heat has therefore be considered according to reaction

mechanism and, particularly, how polarity of the system is

altered during the progress of the reaction [72]. Specific

microwave effects can be expected for a polar mechanism,

when the polarity of reaction is increased during the reaction

from the ground state toward the transition state [73]. The

outcome is essentially dependent on the medium and

reaction mechanism.

Several articles contain examples of increased selectivity

in which the chemo- or regioselectivity can be increased

under the microwave dielectric heating compared to

conventional heating [74,75]. It can be foreseen that

microwave effects could be important in determining the

selectivity of some reactions. When competitive reactions

are involved, the ground state is common for both processes.

The mechanism occurring via the more polar transition state

could, therefore, be favored under the dielectric heating [72].

Finally, it is important to note, that microwave heating is

rapid because energy is absorbed directly by the material.

The temperature of the reaction mixture is uniform,

especially in comparison with conventional heating [76].

2.7. Microwaves specific and athermal effects in chemistry

Specific microwave effects could basically be of two

origins: those, which are not purely thermal, and a special

thermal effect connected with possible intervention of hot-

spots [72]. One of the few theoretical papers trying to

explain acceleration under microwaves has been published

by Miklavc [77]. The author claims that large increases in

chemical reaction rates occur due to the effects of rotational

excitation and on collision geometry.

Concerning microwave dielectric heating, there is a

is A: General 279 (2005) 122 13controversy, whether specific athermal effects exist. Several

talysquestions still await detailed theoretical consideration. Are

electromagnetic fields able to enhance or to modify

collisions between reactants? Do reactions proceed at the

same reaction rate under microwave or conventional heating

for the same bulk temperature? What changes are caused by

electromagnetic field on the molecular level? Since the

beginning of the use of microwaves in chemical synthesis,

athermal effects have been assumed. However, recent

experiments with proper temperature measurements indicate

a failure of these athermal effects. Many of these prejudices

have arisen from inaccurate knowledge of the temperature

distribution due to the difficulty in measuring temperature in

irradiated medium [78].

The pioneering report claiming athermal effects was

published by Sun et al. [79] on hydrolysis of phospho

anhydride bonds in nucleotide triphosphates. The reaction

temperature seemed to be the same for microwave and

conventional heating. However, the hydrolysis rate was

almost 15 times higher under microwaves. These results

could prove that it is possible to obtain localized absorption

of microwaves without significant increase of temperature.

But later on, Jahngen et al. [80] referred that hydrolysis rate

enhancement observed by Sun et al. was caused by thermal

gradients and not special athermal effects.

Raner and Strauss [81] clearly pointed out the problem of

kinetic investigation. In order to perform kinetic studies the

exact temperature must be known, and thermal gradient

must be modeled. Esterification reaction was carried out

under microwave dielectric heating and conventional

heating (oil bath). Additional results showed [81] that the

final yield of ester depended only on temperature but not on

the mode of heating.

These examples clearly demonstrate that it is very

important to carefully monitor reaction temperature directly

in the electromagnetic field.

According to Gedye et al. [82,83] it is possible to observe

different product composition under microwave and con-

ventional heating. However, probable explanation for this

phenomenon is that microwave heating significantly

increases the reaction temperature and it is possible that

the reaction temperature in case of dielectric heating could

exceed the ignition temperature for an additional reaction,

which is not possible at the lower temperatures achieved by

conventional heating.

Although it seems that at present moment there are no

athermal effects in connection to microwaves dielectric

heating, electromagnetic irradiation in chemical reactions

still remains largely unknown and all the potentials of

microwaves in chemical and catalytic applications have to

be discovered in the future.

2.8. Microwaves application in heterogeneous catalysis

Several examples, highlighting application of micro-

B. Toukoniitty et al. / Applied Ca14waves in heterogeneous catalysis are presented in Table 5.Pillai et al. [84] studied MW-assisted olefin epoxidation

over hydrotalcite catalyst, using excess of acetonitrile.

Apparently, the selective epoxidation of a variety of olefins

occurs more rapidly upon microwave irradiation than by

conventional heating in an oil bath. This is presumably due

to the polar nature of the reaction intermediates that couple

efficiently with microwaves and causes the dramatic rate

acceleration. This approach significantly minimizes the

longer reaction times required in conventional heating of

olefins with H2O2 and avoids the use of large excess of

volatile organic solvent usually employed.

Hajek et al. [85] investigated rearrangement of 1,1,3-

triphenylpropargyl alcohol under microwave irradiation

(700 W) as well as conventional conditions in oil bath

(100 8C), in the presence of 10 g neutral alumina (Al2O3).Conversion of 1,1,3-triphenylpropargyl alcohol in non-

stirred sample (temperature gradient 18 8C) was higher(three times) under microwave exposure compared to the

conventional heating. Conversion in stirred sample was

almost the same under microwave and conventional heating.

Local superheating effect is responsible for the acceleration

of the reaction as indicated by these results.

Huber and Jones [86] studied the acceleration of an Ortho-

Ester Claisen rearrangement by clay-catalyzed microwave

thermolysis. Conventional thermolysis was carried out in

three-necked round bottom flask. The assembly was flushed

with dry nitrogen gas and then triethyl orthoacetate was

introduced, followed by 2-cyclohepten-1-ol and, finally,

propionic acid. The mixture was heated using an oil bath to

140 8C. In the case of microwave theromolysis, montmor-illonite KSF was suspended in dry DMF in a flame dried

sealed tube. 2-Cyclohepten-1-ol was added under stream of

nitrogen, followed by triethyl orthoacetate. The mixture was

exposed to irradiation for 9 min (500 W) in a commercial

microwave oven. Under traditional conditions, only moderate

yields of the product were recovered, contaminated with

varying amounts of starting alcohol and their derived acetates

even after forcing conditions (12 h, 180 8C). By microwavethermolysis, high yields of clean product were obtained.

Application of microwave heating accelerates many ring

opening reactions. For example, Villemin and Labiad [87]

investigated the opening of phenylthio dichlorocyclopropanes

in the presence of AgBF4 in ethanol, under conventional

heating and, alternatively, without solvent (dry reaction),

catalyzed by AgBF4/Al2O3 under microwave irradiation

(microwave oven, 650 W). Microwave heating allows us

working under solvent free conditions and has potential for

significant acceleration of the reaction rates.

Calinescu et al. [88] carried out catalytic dechlorination

of chlorobenzene in a solution of NaOH in 2-propanol, at

boiling temperature. It was found that Pd (supported alumina

or aluminum silicate) is a very active catalyst. The

microwave heating had very favorable effect: the conver-

sions were almost double in comparison to classical heating.

Banik et al. [89] carried out hydrogenation of b-lactam in

is A: General 279 (2005) 122ethylene glycol and ammonium formate over 10% Pd/C

B. Toukoniitty et al. / Applied Catalysis A: General 279 (2005) 122 15

Table 5

Heterogeneous catalyzed organic reactions enhanced by microwave irradiation

Reference Reaction Conventional conditions Microwave conditions

[84] Olefin epoxidation:

(catalyst: hydrotalcite (Mg6Al2CO3(OH)164H2O)

Oil bath; solvent:

acetonitrile; reaction

time: 5 h;

conversion: 60%

Multi-mode; power:

720 W; solvent:

acetonitrile; reaction time:

5 min; conversion: 96%

[85] Rearrangement of 1,1,3-triphenylpropargyl alcohol: Oil bath; stirring;

temperature: 100 8C;reaction time: 15 min;

yield: 6.2%

Multi-mode; no stirring;

power: 700 W; temperature:

100 8C; reaction time:15 min; yield: 21.5%

[86] Ortho-Ester Claisen rearrangement:

(catalyst: montmorillonite KSF clay)

Oil bath; temperature: 140 8C;solvent: DMF; reaction time:

012 h; yield: 68%

Multi-mode; power:

500 W; solvent: DMF;


yield: 100%

[87] Opening of cyclopropanes:

(catalyst: AgBF4 on Al2O3)

Solvent: water and ethanol;

reaction time: 24 h;

yield: 20%

Multi-mode; power:

650 W; in absence of

solvent; reaction time:

9 min; yield: 75%

[88] Dechlorination of chlorobenzene:

(catalyst: Pd/AlSi, Pd/Al2O3)

Temperature: 80 8C;reaction time: 60 min;

conversion: 62%

Multi-mode; reaction time:

60 min; conversion: 100%

[89] b-Lactam formation:

(catalyst: 10% Pd/C)

Multi-mode; solvent:

ethylene glycol;


yield: 90%

[90] Dehydrogenation of citronellal:

(catalyst: Y-zeolite)

Temperature: 140 8C;reaction time: 5 h;

conversion (2): 50%;

conversion (3): 22%;

conversion (4): 4.5%

Multi-mode; power:

500 W; reaction time:

3 min; conversion (2):

89%; conversion (3):

6%; conversion (4): 5%

B. Toukoniitty et al. / Applied Catalysis A: General 279 (2005) 12216


Reference Reaction Conventional conditions Microwave conditions

[91] Deprotection of benzyl esters: Oil bath; temperature:

135 8C; solvent:dichloromethane; reaction


Multi-mode; temperature:

130 ! 140 8C; solvent:dichloromethane; reaction


[92] Reduction of SO2 and NOx: Multi-mode; SO2 removal:

99%; NO removal: 99%

[93] Oligomerization of methane (proposed mechanism)

CH4 ! :CH2 + H2; :CH2 ! :CH + :H; 2CH4 ! C2H4 + 2H2;2CH4 ! C2H4 + 3H2; C2H4 + H2 ! C2H6; 3C2H2 ! C6H6(catalyst: RaNi)

Single-mode (continuous

flow operating quartz

reactor); power: 400 W;


conversion: 24%

[94] Esterification of propionic acid with ethanol:

(catalyst: ion-exchange resin (Amberlyst 15))

Conventional heating;

temperature: 105 8C;reaction time: 13 h;

Concentration

(ethyl propion.):

0.64 mol/L; Keq. = 4.9

Single-mode; temperature:

105 8C (500 W); reactiontime: 13 h; Concentration

(ethyl propion.): 0.63 mol/L;

Keq. = 4.43

[95] Selective oxidation of alkylarenes:

(catalyst: KMnO4/montmorillonite K10)

Conventional conditions:

stirring; temperature: 25 8C;reaction time: 20 h; yield:

90%; sonic conditions: reaction

time: 2 h; yield: 86%

Not specified; solvent:

in absence reaction time:

10 min; yield: 87%

[96] Oxidation of alcohols:

(catalyst: clayfen [K10/Fe(NO3)3])

Oil bath; temperature:


Multi-mode; in absence

of solvent; reaction time:

30 s; yield: 87%

[67] Esterification reaction:

(catalyst: Fe2(SO4)3 with montmorillonite)

Temperature: 140 8C;reaction time:

2 h; yield: 83%

Single-mode; temperature:


[97] Condensation of 1,4-diacetylpiperazine-2,5-dione with aldehydes:

(catalyst: KF/Al2O3)

Conventional condition;

temperature: 25 8C;solvent: DMF; reaction

time: 16 h; yield: 90%

Multi-mode; power:

240 W; in absence of

solvent; reaction time:

15 min; yield: 94%

concentration was observed under microwaves compared to

conventional heating. The fact that the equilibrium remained

unaffected was, nevertheless, expected since the laws of

thermodynamics should prevail. Spent catalysts (Amberlyst

15) were characterized by scanning electron microscopy

(SEM). Slight melting of catalyst particle occurred under

microwave irradiation compared to conventional heating

(Figs. 12 and 13). Cracking of the catalyst surface was

observed under microwave irradiation, as well, which could

not be obtained under conventional heating (Figs. 14 and

15).

atalysis A: General 279 (2005) 122 17

Fig. 12. SEM image of spent ion-exchange resin catalyst (Amberlyst 15)

subjected to conventional heating.

Fig. 13. SEM image of spent an ion-exchange resin catalyst (Amberlyst 15)

subjected to microwave irradiation.

Fig. 14. SEM image of spent ion-exchange resin catalyst (Amberlyst 15)(1 g). This reaction mixture was irradiated for 2 min, at low

power setting, and then worked up after removing the

catalyst by filtration. The filtrate was diluted with water and

extracted with ethyl acetate. Evaporation of the solvent from

organic layer gave about 90% yield of the product.

Ipaktschi and Bruck [90] carried out citronellal dehy-

drogenation over Y-zeolite under microwave irradiation

(microwave oven, 500 W) and conventional heating. The

reaction rate and selectivity were dramatically enhanced

under microwave exposure, compared to the reaction under

conventional thermal conditions. Several other Claisen

rearrangements under microwaves were studied, as well.

Varma et al. [91] studied deprotection of benzyl benzoate

over acidic alumina. The catalyst was added to a solution of

benzyl ester, dissolved in a minimum amount of dichloro-

methane (12 mL), at room temperature, and the reaction

mixture was thoroughly mixed using a vortex mixer. The

adsorbed material was dried in air and placed in an alumina

bath inside a microwave oven. Pure benzoic acid was obtained

(yield = 92%). The observed yield was three times higher

under milder reaction conditions by using microwave irradia-

tion, compared to the conventional heating in an oil bath.

Tse et al. [92] investigated decomposition of SO2 and NOxover several nickel and copper catalysts under microwave

irradiation. For the majority of the gas phase experiments

either 5% SO2 in air or 25% NO in an inert gas was passed

through a packed bed of commercially available supported

catalyst pellets while the system was exposed to microwave

irradiation. For SO2, the most efficient catalyst was Ni-1404;

more than 99% of the SO2 was removed from the effluent gas

after reaction over this catalysts over prolonged periods of

use. Experiments with 25% NO in He over several catalysts

indicated efficient decomposition under microwave exposure.

Interestingly, some copper containing catalysts, which had

shown activity in the SO2 experiments removed essentially

none of the NO, while Ni catalyst afforded 99% of NO

removal. These results indicate potential of the microwave

induced catalytic technology to the problems of removal of

high concentrations of undesirable pollutant gases.

Conde et al. [93] studied the oligomerization of methane

under microwave heating over Raney nickel catalyst. The

maximum conversion achieved in this reaction was 24%, at

400 W and 10 min of irradiation. Oligomers, such as

ethylene, benzene and ethane have been prepared selectively

under different conditions. Ethylene was the major product

and observed selectivity for ethylene was 70% at 530 W and

5 min, whereas benzene formation was favored (13%) when

an intermediate power (400 W) was used. Pretreatment of

the catalyst with hydrogen reduction increases the formation

of ethane (54%), at low and intermediate power.

Toukoniitty et al. [94] investigated esterification reaction

of propionic acid with ethyl alcohol over an ion-exchange

resin (Amberlyst 15) as a catalyst. The experiments were

carried out under microwave irradiation and conventional

heating in microwave loop reactor (Fig. 16). No significant

B. Toukoniitty et al. / Applied Cacceleration of reaction rate, neither shift in equilibrium subjected to conventional heating.

talysShaabani et al. [95] carried out selective oxidations of

alkylarenes over KMnO4/montmorillonite K10, under sonic

and microwave irradiation. As can be seen from the results

presented in Table 5, the utilization of ultrasound in this

reaction decreases the time required to obtain good yields by

a factor of about 10. Moreover, the use of microwave

irradiation decreases the time required by another order of

magnitude.

Varma and Dahiya [96] studied the oxidation of alcohols in

solvent free conditions using clayfen (montmorillonite K10

clay-supported iron(III) nitrate) under microwave exposure.

Clayfen-mediated solvent free microwave thermolysis was

found to be a convenient, selective and environmentally

friendly process in comparison to the conventional solution

phase or heterogeneously catalyzed reaction pathways.

Chemat et al. [67] carried out esterification reaction

between stearic acid and n-butanol over Fe2(SO4)3montmorillonite under microwave and conventional heating

conditions. The yield and the rate of heterogeneously

catalyzed esterification increased with microwave heating,

in comparison to the conventional heating under similar

reaction conditions (temperature, concentration, pressure).

Villemin and Alloum [97] investigated dry condensation

of 1,4-diacetylpiperazine-2,5-dione with aldehyde over KF/

Al2O3 under microwave exposure. 1,4-Diacetylpiperazine-


Fig. 15. SEM image of spent ion-exchange resin catalyst (Amberlyst 15)

subjected to microwave heating.2,5-dione and benzaldehyde were absorbed on the solid base

KF/Al2O3 and irradiated for 15 min. The reaction took place

at room temperature as well, when 1,4-diacetylpiperazine-

2,5-dione and benzaldehyde were dissolved in DMF, reacted

with KF/Al2O3. In this case, the obtained yield was poorer

than in dry condensation under microwaves.

As a summary, significant acceleration of a wide range of

chemical reactions under microwave irradiation in compar-

ison to the conventional heating has been reported in this

review. However, enhancement in several cases can be due to

higher reaction temperature in the dielectric field, although

main part of the reported studies were carried out in

microwave reactors equipped with efficient temperature

control, which enables exact temperature monitoring during

the reaction. Feasible explanation for observed increase of

activity and selectivity is selective heating of the catalystparticles leading to the creation of hot-spots, moreover, when

the reaction mechanism occurs via the more polar transition

state, dielectric heating can be favored. Rapid superheating of

the solvents improves activity and selectivity as well. A very

important advantage of microwave irradiation is the

possibility to carry out many chemical syntheses rapidly

and with good yields in solutions as well as in the absence of a

solvent. This leads to enhancement of selectivity and gives

raise to the inherently greener chemical production.

Microwave irradiation can also be utilized in catalyst

preparation in order to enhance crystallization process,

improve catalyst activity, significantly shorten reaction time

and enable dry synthesis to achieve very high batch yield

[98]. However, conventional thermal methods are not

replaced by microwave heating and might in some cases

be preferable. Most of the attention was paid to the field of

zeolite synthesis, the comprehensive review with detailed

description of the area was published by Cundy [99].

Prasad et al. [100] prepared alumina- and silica-supported

palladium catalyst by conventional and microwave heating.

The catalysts were characterized and tested for benzene

hydrogenation activity. Catalysts containing 10 wt.% Pd were

prepared by impregnating commercial alumina and silica with

aqueous solutions of palladium nitrate. The catalysts were

evaporated to near dryness and dried in oven at 120 8C for12 h. A portion of catalyst was subsequently calcinated in air

for 5 h and the remaining portion was irradiated in microwave

oven for 5 min. In the case of Pd/Al2O3 catalyst, microwave

heating degreases the total surface area (MH = 117 m2/g,

CH = 152 m2/g), but a slight increase of total surface area by

microwave heating was observed with Pd/SiO2 catalyst

(MH = 499 m2/g, CH = 462 m2/g). The particle size was

significantly increased by microwave heating (Pd/Al2O3:

MH = 229 A, CH = 47 A and Pd/SiO2: MH = 160 A,

CH = 135 A). The preparation of samples by microwave

heating leads to enhanced crystallite size and increased

hydrogenation activity.

Bond et al. [101] studied Ni/Al2O3 catalyst, utilizing

microwave heating for catalyst drying. Ni/Al2O3 was

prepared by impregnation to obtain catalysts with 5 and

10% loading of the active metal. The catalysts precursors

were prepared by wetting the pellets in aqueous solutions of

nickel nitrate of the required concentrations to provide

desired metal loading. The catalysts were dried by

microwave or conventional heating. Drying times of catalyst

precursors were reduced by using microwave irradiation

compared to conventional heating. In the case of 10% Ni/

Al2O3 catalyst the pellet strength was investigated. The

results confirmed that the microwave dried samples are

significantly stronger than those dried conventionally. The

microwave dried catalyst also showed much more uniform

metal distribution on the support.

Synthesis of TS-1 zeolite under microwave irradiation

has been studied by Uguina et al. [102]. In this work, TS-1

zeolite was synthesized by utilizing both conventional and

is A: General 279 (2005) 122microwave heating. The use of microwave irradiation allows

the synthesis of highly crystalline TS-1 zeolite in a very

short time (2030 min), compared to conventional heating

(around 15 h). Synthesis of TS-1 zeolite under microwave

irradiation leads to larger average crystal sizes than under

conventional heating, but they posses more uniform shapes

and sizes.

Somani et al. [103] synthesized ZSM-5 zeolite using two

different techniques: first by conventional hydrothermal

method and second by exposing the hydrogel to the

microwave irradiation followed by conventional method.

The crystallinity of ZSM-5 was tested by using X-ray

diffractometer. Zeolite synthesized under microwave irra-

diation took half time (MH = 18 h) to crystallization of

100% crystalinity compared to conventional method

(CH = 36 h). The scanning electron micrograph showed

that size of the crystals prepared under microwave

irradiation were larger (MH = 56 mm) compared toconventional method (CH = 34 mm).

2.9. Microwave equipment

on the behavior of chemical reactions. This approach makes

it feasible to extrapolate the results to industrial scale.

2.9.1. Multi-mode cavities

Domestic microwave oven is a typical example of a

multi-mode device. Such devices comprise a large micro-

wave cavity with reflective walls that are necessary to

prevent leakage of radiation and to increase the efficiency of

the oven. It is obvious that domestic microwave ovens are

not intended for chemical applications. For instance, they do

not allow application of high pressures since explosions can

occur [104]. Moreover, they are not compatible with

corrosive and inflammable compounds; this is being another

disadvantage of them. Domestic microwave ovens operate

on duty cycles, with the intervals between zero and full

power reaching several seconds (e.g. 600 Woven30 s dutycycle). Large duty cycles are undesirable in chemical

applications since samples may cool dramatically between

switching steps [61]. However, self-modified microwave

ovens were utilized in chemical applications already a

decade ago since commercially produced equipment was not

B. Toukoniitty et al. / Applied Catalysis A: General 279 (2005) 122 19Microwave equipment can be divided into two cate-

gories: (a) multi-mode and (b) single-mode cavities.

Considering an empty metallic volume, the electric field

repartition into that volume is very heterogeneous, if the

dimensions of that volume are too big compared to the

wavelength. This is the case for multi-mode applicators

(such as a domestic microwave oven). The repartition is well

mastered and stable if the applicator dimensions are close to

the single-mode structure. The use of wave-guides emitting

the fundamental mode at fixed frequency, allows to master

and, above all control, the power transmission when the aim

is to study the influence of microwave electromagnetic fieldFig. 16. Single-mode Measily available.

2.9.2. Single-mode cavities

Single-mode units have a smaller cavity. These types of

equipment have greater energy efficiency and more even

temperature distribution in the reaction vessel than multi-

mode units. Single-mode resonant heater allows a sample to

be placed at a position of much higher electric field strength

than can be obtained in a multi-mode oven [61].

Single-mode equipment (Figs. 16 and 17) needs to have a

built-in protection of the magnetron and working with small

samples does not, therefore, cause problems [105]. Single-W loop reactor.

talys

nd thmode equipment has a variable microwave power input,

which means that the resulting temperature can be measured

accurately and the pre-set temperature can be achieved. Such

approach requires a recording unit to read the signal from the

temperature measuring device. It should be noted that in

fact, temperature measuring within microwave field is quite

problematic and hence the temperature has to be measured

by special technique, like infrared pyrometry or fiber-optic

devices. The level of power emitted by generator coupled to

a load, must be determined from simple thermodynamic

dependences as follows [106]: suppose that for a certain

mass, m (kg), of material, with specific heat capacity, cp (kJ/

kg K), an increase of temperature DT is desired within acertain time interval t (s). The necessary theoretical power is

obtained from the following equation:


Fig. 17. Side-view of the microwave cavity (heating zone) aPtheor mcpDT

t(7)

where Ptheor denotes the theoretical power. The material to

be treated must be located into the vessel within the cavity.

The vessel must be microwave transparent (Teflon, quartz)

chemically inert and tolerate temperatures reached. More-

over, high pressures operating conditions induced by micro-

wave irradiation are very attractive for some chemical

applications, therefore the reaction vessel should be able

to accommodate high pressures. For temperatures up to

180 8C, Teflon is a convenient choice. However, if highertemperatures are required, a quartz vessel is preferred.

The single-mode microwave generator (Fig. 17) consists

of a boxed high volt

Documents

Article