Microwave Activation of Catalysts and Catalytic Processes

ISSN 0036�0244, Russian Journal of Physical Chemistry A, 2010, Vol. 84, No. 10, pp. 1676–1694. © Pleiades Publishing, Ltd., 2010.Original Russian Text © L.M. Kustov, I.M. Sinev, 2010, published in Zhurnal Fizicheskoi Khimii, 2010, Vol. 84, No. 10, pp. 1835–1856.

1676

INTRODUCTION

Various radiation types (gamma irradiation, elec�tron and neutron beams, RF/microwave or sound�range waves, and irradiation by UV and visible light)are extensively used in material preparation and for theinitiation and stimulation of chemical processes. Inrecent years, much attention has been given to theaction of microwave radiation on matter, in particular,to the influence of microwave heating on the prepara�tion of various materials and catalytic reactions.

Microwaves, or hyperhigh�frequency electromag�netic waves, are electromagnetic waves 1 mm–1 mlong. Their frequency range is 200 MHz–300 GHz,and they are intermediate in the spectrum of electro�magnetic radiation between radiofrequencies andinfrared radiation. Waves 1–25 cm long are extensivelyused in radar applications, and other waves, in long�haul communications. To avoid interferences to civilcommunications and military and naval hardware, aset of frequencies was fixed for use in industrial,research, and medical equipment [1]. The 915 MHzfrequency (33.3 cm) is used in industrial devices,2.45 GHz (12.2 cm) is used in microwaves, and, inaddition, devices working at 0.433, 5.8, and 24.125 GHzare allowed in certain countries.

The basic elements of microwave devices are mag�netron generators invented during World War II byRandal et al. at Birmingham University for use in radi�olocation. The first commercial microwave furnaceswere produced by the “Raytheon” corporation (USA)and for the first time appeared at USA markets in the1950s. At world market, they appeared in the 1970s–1980s simultaneously with the development of themass production of magnetrons.

The key advantage of energy transfer with micro�wave fields over convective methods for heating is thetransfer of energy through radiation rather than heattransfer or convection. This ensures fast penetration ofenergy into the volume of materials transparent tomicrowave fields, that is, almost instantaneous heating

(and cooling when the field is switched off) of sub�stances. Ideally, the transformation of electromagneticfield energy into heat occurs simultaneously andequally over the whole object volume absorbing micro�wave radiation, which results in high heating rates.The spatial temperature distribution in the object isthen different from that observed under traditionalconvective or contact heating conditions. The mostimportant differences related to the appearance oftemperature gradients and nonequilibrium conditionsare observed when a reaction medium or material (forinstance, a catalyst) consists of several phases with dif�ferent abilities to be heated by microwave radiation.

Examples of using microwave radiation for the exe�cution of chemical processes are given in [2]. One ofthe spheres of industrial applications of microwavefields is microwave drying technologies. This problemwas for the first time studied by Levinson [3], who useda carbonaceous material as a receiver of microwavesfor heating materials to be dried to the required tem�perature.

The existing examples are largely related to organicsynthesis under the conditions of microwave irradia�tion. The first research works on the use of microwavefields in organic synthesis were published in 1986 [4,5]. The purpose of these works was to decrease reac�tion duration, increase yields and selectivity, anddecrease energy and reagent consumption.

Starting with the beginning of the 1980s, interest inthe use of microwave fields as an energy source for theactivation of chemical reactions or separation of sub�stances has been increasing. The first patent dates to1982; it describes gas�phase destruction of chlorine�containing organic compounds on heterogeneous cat�alysts (paramagnetic and ferromagnetic powders)heated by a microwave field at a 2.45 GHz frequencyto reaction temperature during a short time [6].Another patent dated to 1985 describes the catalytictransformation of methane into ethylene on nickeland iron catalysts activated by microwave field pulses

CHEMICAL KINETICS AND CATALYSIS

Microwave Activation of Catalysts and Catalytic ProcessesL. M. Kustov and I. M. Sinev

Zelinskii Institute of Organic Chemistry, Russian Academy of Sciences, Leninskii pr. 47, Moscow, 119991 Russiae�mail: [email protected] April 2, 2010

Abstract—The results obtained by using microwave activation in catalytic processes such as the conversionof lower alkanes, the removal of volatile organic compounds and sulfur� and nitrogen�containing compoundsfrom air, and the hydrogenation of aromatic compounds and dehydrogenation of naphthenes are discusses.

DOI: 10.1134/S0036024410100055

RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 84 No. 10 2010

MICROWAVE ACTIVATION OF CATALYSTS AND CATALYTIC PROCESSES 1677

about milliseconds wide [7]. According to [8], the useof microwave energy pulses provides additional energysaving.

The number of the corresponding publicationssubstantially increased starting with 1995. Althoughthe first objects of study were deposited metallic cata�lysts, currently, the efforts of researchers are largelyfocused on the use of perovskite�like mixed oxidesbecause of their high catalytic activity and high abilityto absorb electromagnetic radiation energy. With fewexceptions, so called single�mode generators workingat a 2.45 GHz frequency are used for heating catalysts.Multi�mode systems, in which a larger number of dif�ferent modes are generated, were largely used for thepreparation of catalysts.

As applied to catalysis, microwave technologiescan be used for both catalyst preparation and prelimi�nary activation and direct execution of catalytic pro�cesses. In this review, we concentrate on the use ofmicrowave activation directly in catalytic processes.

Pioneers in the use of microwave heating in heter�ogeneous catalysis were Wan et al. In [9], they usedferro� and paramagnetic metallic particles distributedin a liquid reaction medium, which was supposed toaccelerate the reaction, because the majority oforganic reagents and solvents did not absorb micro�wave field to a substantial extent. Another advantage ofthis approach becomes clear when catalytically activecomponents absorbing microwave radiation are placedinto a carrier matrix transparent to microwave field.This approach is known as “microwave catalysis.”Wan as a rule worked with ferromagnetic and para�magnetic metallic catalysts that well absorbed micro�wave energy and had high catalytic activity.

Wan’s team discovered that the use of pulsedmicrowave action allowed catalyst temperature andprocess selectivity to be controlled more accurately. Ata high absorbing activity of catalysts, a short micro�wave power pulse was sufficient for heating it to reac�tion temperature. During “dead time” (cycle partwithout microwave action), heat released in the vol�ume of a catalyst was indeed scattered in the matrixsurrounding it and transparent to microwave field(reagents or carrier). Because of a lower temperatureof the system as a whole, side reactions could be sup�pressed.

Wan’s team used a single�mode microwave unitwith a maximum output power of 3 kW. Apart fromworking magnetron power, dead time between pulseswas regulated. Therefore, the selectivity of productformation and process temperature were controlled.These studies showed that metals “set fire” to micro�wave plasmas at decreased temperatures and pres�sures, which could successfully be used in catalyticreactions [10]. Short plasma discharges could, how�ever, appear also when systems containing metals orpure oxide catalysts were used at reduced pressures,

which caused catalyst coking and, as a consequence, adecrease in catalyst activity in certain reactions.

Apart from metallic powders, Wan et al. studiedother commercially available materials containingnickel and copper, wires, nets, and disperse metalliccatalysts distributed in nonconducting materials. Theystudied reactions of various types, including thehydrogenation and hydrocracking of hydrocarbons[11, 12], methane decomposition [13], the oxidationof hydrocarbons, the reduction of sulfur and nitrogenoxides [14], the synthesis of acetylene [15–18], thecatalytic decomposition of halogenated hydrocarbons[19], olefins [20], oil sand from Alberta province(Canada), and bitumens [21], and the synthesis ofhydrogen cyanide [22].

According to Perry [23, 24] and Thomas [25, 26],substantial temperature gradients between catalyti�cally and microwave active materials (for instance,platinum) and environment transparent to microwavefields could not be obtained. This was theoreticallysubstantiated by Perry for a simplified model based onequilibrium heat exchange between a catalyst particleand gas. On the other hand, it is assumed that catalyt�ically active system regions can be heated selectivelywhen pulsed energy supply is used. At the same time,Mingos and Zhang [27] reported a temperature differ�ence of ~100–200 K between a catalytically activecenter and its nearest environment in the endothermicdecomposition of H2S on MoS2/γ�Al2O3 at constantheating by a microwave field.

Roussy et al. [28] used high�dispersity platinumabsorbing microwave radiation on Al2O3 transparentto microwave field for the isomerization of hexane and2�methylpentane and hydrogenolysis of methylcyclo�pentane. They also studied the isomerization of2�methyl�2�pentene on pure oxide catalysts catalyzedby acids and the oxidative condensation of methane ontransition metal oxides. They found that the use ofmicrowave fields in situ could lead to the results iden�tical to or quite different from those obtained usingthermal activation. Apart from presenting their exper�imental data, Roussy makes an important method�ological conclusion. The temperature dependences ofconversion extensively used in the literature con�cerned with catalysis are not informative because ofstrong differences in the methods of heating and diffi�culties in determining the true catalysis temperatureunder microwave field conditions. To obtain informa�tion that can more easily be interpreted, it was sug�gested to compare “conversion–selectivity” depen�dences rather than temperature curves.

PHYSICAL BASICS OF MICROWAVE FIELD INTERACTION WITH SUBSTANCES

In the gas phase at low pressures, the lifetimes ofstates obtained in the excitation of a particular rota�tional mode are long, and microwave spectrum lines

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KUSTOV, SINEV

are then fairly narrow. It follows that the spectrum of afree molecule in the microwave range is a set of narrowlines over the frequency range 3–60 GHz. At pressureshigher than 10–1 torr, the frequency of collisionsincreases. Accordingly, the lifetime of an excited statedecreases. As a consequence, according to the Heisen�berg uncertainty principle, spectral lines broaden. Themean velocity (energy) of the motion of moleculesand, therefore, gas temperature increase when rota�tional excitation relaxes and its energy is transferred totranslational degrees of freedom. This explains themechanism of observed gas heating under the action ofmicrowave radiation.

In liquids and, especially, solids, substance mole�cules cannot rotate independently. For this reason,other specific mechanisms of heating by microwavefields operate for substances in condensed states. Oneof the possible mechanisms is dielectric polarization,which is a relaxation process. Thermal energy releaseoccurs in the relaxation of fixed charges (for instance,dipoles or charged point defects) from the polarizedinto stationary state. The second mechanism involvescurrents of free charges excited in solids and contrib�uting to heating because of Ohmic loss; this mecha�nism is characteristic of solids with substantial con�ductivity (metals and semiconductors). The thirdmechanism that should also be taken into account isloss of vortex currents excited by magnetic fields.

We can confidently rule out the possibility thatmicrowave fields can initiate chemical reactions byexciting and, especially, breaking chemical bonds, asin photochemical processes under the action of elec�tromagnetic radiation quanta at far shorter waves. Theenergy of microwave photons is ~1 J/mol, which isincomparable with activation energies of the majorityof chemical processes. For instance, let us comparethe energies of molecular motions and chemical bonds(I, Brownian movement; II, H�bonds; III, covalentbonds; and IV, ionic bonds) with the energy of micro�wave photons (V), kJ/mol,

THE EFFECTS AND MECHANISMS OF DIELECTRIC HEATING

OF CONDENSED SUBSTANCES

Substances in the condensed state can be heated byhigh�frequency electromagnetic waves. The reason forheating is the ability of the electric electromagneticfield component to act on electric charges. If there aremobile charge carriers in substances (both metals andsemiconductors), currents appear under the action ofelectric fields. If charge carriers are, however, fixed incertain regions, their movement is limited by counterforces, and dielectric polarization can appear. Bothdielectric polarization and conductivity are micro�

I II III IV V

1.64 3.8–42 480 730 1 × 10–3

wave heating mechanisms. Microwave heatingdepends on the frequency and power of the fieldapplied, but, since it is not based on quantum effects,it can be considered in terms of classical electrody�namics. The theory of microwave heating was devel�oped by several authors, among which Debye (1928),Cole and Cole (1941), and Hill (1962) should be men�tioned.

The rate of heating of liquids and solids under theaction of the electric electromagnetic field componentis determined by the equation [29]

, (1)

where Eef is the acting electric field strength value, ρis the density of the substance, Cp is the isobaric heatcapacity, ε'' is the dielectric loss coefficient, and f isthe field frequency. Radiation loss is determined bythe equation [30]

, (2)

where е is the emissivity of the body, α is the Stefan–Boltzmann constant, and А/V is the ratio between theouter surface area and volume of the body. It followsthat heating is determined by the dielectric loss, ther�mal radiation of the body, and the strength of the elec�tric component of the electromagnetic field. All thesephysical properties depend on temperature, whichstrongly complicates a complete theoretical analysis ofdielectric heating.

One of few examples of a detailed study of thedielectric properties of a substance [31] is a study ofthe temperature dependence of the dielectric constantand dielectric loss coefficient of CuO. It was found forCuO that the dielectric loss coefficient sharplyincreased at ~700°C, which in turn caused a sharpincrease in temperature. The data on heating of vari�ous liquids and solids in household and special (forlaboratory studies) microwave furnaces were reportedin several works. Some results obtained for liquid andsolid substances [32, 33] are summarized in Tables 1and 2, respectively.

It is easy to notice that nonpolar liquids (dimethylether, hydrocarbons, and CCl4) remain almostunheated by microwave fields under given conditions,which complicates the accomplishment of catalyticprocesses in these media (when these substances playthe role of solvents or catalytic reaction substrates).Similarly, solids with a high conductivity (carbon andmetals) are rapidly and effectively heated by micro�wave radiation, whereas insulators or high energy�gapsemiconductors (some ionic chlorides, CaO, La2O3,and CeO2) are characterized by only weak heating.Attention should be given to materials and oxides usedas components of carriers and catalysts. Whereas car�bon and transition metal oxides reach high tempera�tures, titanium and zirconium oxides are heated tomoderate temperatures, and aluminum, magnesium,

δ δ = × ε ρef/ const /2'' pT t fE C

( ) 4

p

T e A Tt C V

δ α= −

δ ρ rev



and silicon oxides cannot be heated above 100–150°С. On the whole, the results presented in Tables 1and 2 only give a qualitative idea of the rules governingheating of pure substances in microwave fields. Someof them, especially the data on heating of semicon�ducting oxides and solids with a complex dependenceof properties on stoichiometry, phase composition,temperature, and other parameters (transition metaloxides), require a more detailed analysis, which isbeyond the scope of the present review.

One of the most frequently discussed problems ofheating by microwave fields is temperature measure�ments, which are complicated by the presence of high�intensity electromagnetic fields. Temperature sensorsmost frequently used in laboratories (metallic thermo�couples, resistance thermometers, semiconductingelements) should be thoroughly screened andgrounded to avoid sparking and obtainment of dis�torted data. Such precautions are especially necessaryif volatile and easily inflammable substances areheated in microwave fields. The temperature of thesurface can be measured up to 3000°С with the use ofinfrared and optical pyrometers. The use of devicesthat allow radiation intensity to be measured at severalwavelengths simultaneously is especially effective; thisallows the influence of indeterminate and/or changingoptical properties of solids to be avoided (“graynesscoefficient,” that is, discrepancy between the temper�ature dependence of radiation intensity and the lawsgoverning blackbody radiation).

Let us consider particular mechanisms of micro�wave field interactions with substances (microwaveheating mechanisms).

Dielectric Polarization

One of the reasons for microwave heating is theability of electric fields to polarize charges in sub�stances and the inability of these charges to follow fastchanges in field parameters. The total polarization isthe sum of several separate effects,

αt = αe + αa + αd + αi, (3)

where αe is the electronic polarization, which is a con�sequence of the rearrangement of electrons withrespect to the atomic nucleus; αa is the atomic polar�ization, which appears when the relative arrangementof atomic nuclei in a molecule changes or when thecharge in a molecule is distributed nonuniformly; αd isthe dipole polarization, which appears when the ori�entation of a constant dipole in space changes underthe action of the electric field; and αi is the interphasepolarization, which appears at the boundary betweentwo phases with different permittivities (the Maxwell–Wagner effect).

The behavior of a substance in an alternating elec�tromagnetic field depends on the ratio between thecharacteristic orientation and relaxation times and thefrequency of field oscillations. The characteristic

Table 1. Temperatures of various liquid samples (50 ml) af�ter the action of a microwave field, power 560 W, at a2.45 GHz frequency for 1 min (Tmw) (heating from roomtemperature) and boiling temperatures (Tb)

Liquid Tmw, °C Tb, °C

Water 81 100

Methanol 65 65

Ethanol 78 78

1�Propanol 97 97

1�Butanol 109 117

1�Pentanol 106 137

1�Hexanol 92 158

1�Chlorobutane 76 78

1�Bromobutane 95 101

Acetic acid 110 119

Ethyl acetate 73 77

Chloroform 49 61

Acetone 56 56

Dimethylformamide 131 153

Dimethyl ether 32 35

Hexane 25 68

Heptane 26 98

CCl4 28 77

Table 2. Heating of solids by microwave field; samples 25 g,heating from 20°C in a household microwave furnace, pow�er 1 kW, frequency 2.45 GHz (τ is heating duration)

Sub�stance Tmw, °C τ, min Sub�

stance Tmw, °C τ, min

Al 577 6 CaO 83 30

C 1283 1 CeO2 99 30

Co3O3 1290 3 CuO 701 0.5

CuCl 619 13 Fe2O3 88 30

FeCl3 41 4 Fe3O4 510 2

MnCl2 53 1.75 La2O3 107 30

NaCl 83 7 MnO2 321 30

Ni 384 1 PbO2 182 7

NiO 1305 6.25 Pb3O4 122 30

SbCl3 224 1.75 SnO 102 30

SnCl2 476 2 TiO2 122 30

SnCl4 49 8 V2O5 701 9

ZnCl2 609 7 WO3 532 0.5

1680


KUSTOV, SINEV

times of atomic and electronic polarization are con�siderably shorter than the period of field oscillations inthe microwave range, and these types of polarizationtherefore do not contribute to dielectric heating. Con�versely, the characteristic times of dipole and inter�phase polarization are comparable with the period offield oscillations and therefore make the major contri�bution to substance heating by microwave fields. Theyshould be considered separately.

Dipole Polarization

Dipole polarization αd appears because substancemolecules have dipole moments. At low frequencies,time during which the electric field changes its direc�tion is longer than time required for a change in theorientation of dipoles, and polarization is in phasewith electric field. Electromagnetic field providesenergy necessary for a change in the orientation ofdipoles. The energy transferred is very small, and thetemperature of the substance almost does not change.If the frequency of the electromagnetic field is fairlyhigh, it changes its direction more rapidly than dipolescan change their orientation. Since dipoles do notshift, field energy transforms into heat.

In the microwave range of frequencies, time duringwhich a field changes its direction is comparable withtime necessary for changes in the orientation ofdipoles. Dipoles rotate because of the action of electricfield rotational momentum, but the resulting polariza�tion lags behind field changes. When electric field ismaximum and has a direct orientation, polarizationcan still be low. It continues to grow while fieldstrength already decreases. Such a lag is evidence thata substance absorbs field energy and is heated.

The dielectric properties of a substance are deter�mined by two constants. The dielectric constant ε'describes the ability of a molecule to be polarized by anelectric field. It is maximum at low frequencies, whenmaximum field energy can be stored in a substance.The dielectric loss coefficient ε'' determines the effec�tiveness of the transformation of electromagnetic fieldenergy into heat by a substance. The ε'' value passes amaximum as the dielectric constant decreases. The

dielectric loss slope ( = ε''/ε') determines thedegree of the transformation of electromagneticenergy into heat in a substance at given field frequencyand temperature.

The dielectric characteristics of any substancechange fairly substantially as a function of frequency atgiven temperature [34]. For instance, for water (as asolvent in catalytic processes or a substance participat�ing or formed in a catalytic process), the dielectric losscoefficient has a fairly large value over a broad fre�quency range and passes a maximum at about 20 GHz,rather than at 2.45 GHz, as is usually believed. Thedepth at which field power decreases by two times isanother important parameter for planning microwave

δtan

experiments or in the development of an industrialprocess with the participation of a microwave field. Atsmall ε'' values, the depth of electromagnetic fieldpenetration into a substance Dp can be approximatelydescribed by the equation

, (4)

where λ is the electromagnetic field wavelength.Theoretical studies of the frequency dependences

of ε'' and ε' are based on the Debye equations [35]

, (5)

, (6)

where and determine the high�frequency andstatic dielectric constant asymptotes and ω and τ arethe frequency and relaxation time characterizing thetime of polarization appearance and decrease. Equa�tions (5) and (6) are applicable to both liquids and sol�ids, although they are obtained using different models.An interesting feature of the Debye equations is theindependence of the maximum ε' and ε'' values fromfield frequency and relaxation time,

. (7)

It is as a rule assumed that dipoles in liquids canhave arbitrary spatial orientations and constantlychange them because of thermal motion. The Debyeinterpretation of relaxation is based on the existence ofviscosity forces in a substance. Debye used the Stokestheorem to obtain the following equation for the relax�ation time of spherical dipoles:

τ = 4πr3η/kT, (8)

where η is the viscosity of medium, r is the radius ofthe dipolar molecule, and k is the Boltzmann con�stant. In solids, the thermal motion of atoms and ionsis minimum, and a dipole has a set of equilibrium posi�tions. They are separated by potential barriers, whichshould be overcome by the dipole that changes its ori�entation. In the simplest case, there are two such posi�tions, and the dipole moments of a molecule in themare anticollinear.

According to the Boltzmann statistics, the numberof transitions from one state into another is propor�tional to (1 – e–t/τ), where t is the time and τ is therelaxation time. We therefore obtain the followingequation relating the relaxation time and dielectricconstants:

. (9)

Here, 1/n is the time of a single vibration in the poten�tial well and Ua is the difference between the energiesof dipole states. The absorption value for such a modelwas obtained by Frohlich [36], who suggested that itshould be equal to that obtained for liquids. It wasproved that the following equation for the Onsager

( )1/2' "pD = λ ε ε

d / 2 20' ' ' '( ) (1 )

∞ ∞ε = ε + ε − ε + ω τ

d / 2 20'' ' '( ) (1 )

∞ε = ε − ε ωτ + ω τ

∞ε' ε 0'

max max/2 /20 0'' ' ' ' ' '( ) , ( )∞ ∞

ε = ε − ε ε = ε + ε

a 0'exp( )( 2)'( 2)

U kTn

∞

ε +τ =

ε +



model [37] was applicable to many liquids and solids[38–40]:

, (10)

where N is the number of molecules and m is theirmass. Importantly, energy absorption decreases as thetemperature increases in this model.

It was found in practice that most of polar liquids,for instance, aliphatic ketones with long chains andmany ethers, had small dielectric loss values becauseof large differences between equilibrium positionenergies in the condensed state. In the majority ofother cases, for instance, for many ethers with longchains, dielectric loss values are large at room temper�ature and decrease as the temperature increases inagreement with the Onsager equation.

Interphase Polarization

A disperse system consisting of conducting parti�cles in a nonconducting medium, for instance, a cata�lyst is a nonuniform material whose dielectric proper�ties exhibit pronounced frequency and, primarily,phase composition dependences. Loss in such materi�als is related to the aggregation of charges at interphaseboundaries and is known as the Maxwell–Wagnereffect. However, its influence on substance heating inthe microwave range has not been determined conclu�sively as yet. Energy absorption according to thismechanism is concentrated at frequencies of ~109 Hz.Wagner showed that, for the simplest model of con�ducting spheres in a nonconducting medium, thedielectric loss coefficient for the volume material frac�tion ν is determined by the equation

, (11)

where σ is the conductivity of the conducting phaseand ε' is its dielectric constant. Correctness of Eq. (11)was proved experimentally [41].

Conductivity Loss

As mentioned above, conductivity and relatedOhmic loss determine one of the microwave heatingmechanisms that can operate in catalytic systems.When a microwave field acts on a medium containingmobile charge carriers, conductivity currents appearin the system. Taking into account loss related tomedium resistance, the complex permittivity of a sub�stance takes the form

. (12)

The real part of this value is determined by the Debyetheory. The loss part determines not only relaxationbut also Ohmic loss. The contribution of conductivity

∞

∞

∞

π ε ε +ε − ε =

ε + ε

2 20

0

0

' '4 ( 2)' '' '9 (2 )

Nm

kT

( )( )νε ωτε =

× σ + ω τ

max10 2 2

9 '''1.8 10 1

if

∞

∞

ε − ε σε = ε + −

+ ω ωε

0

0

' '( )* '(1 )

ij

j t

loss primarily depends on conductivity. For high�con�ductivity materials (catalysts), there are such conduc�tivity values that conductivity loss is much larger thandipole relaxation effects if these values are exceeded.

For alumina at room temperature, conductivityloss becomes noticeable at low frequencies only. Lossin the microwave range largely occurs because ofdipole relaxation. The conductivity of a materialincreases as the temperature grows, and the fraction ofconductivity loss in the microwave range increases andbecomes comparable with polarization effects. Anincrease in alumina conductivity as the temperaturegrows is related to thermal activation of electrons,which pass from the 2p oxygen valence zone to the3s3p conduction zone. In addition, material electronicconductivity as a rule increases in the presence ofdefects, which create additional levels in the forbiddenband. The concentration of defects can also substan�tially increase at high temperatures.

THE USE OF MICROWAVE EFFECTSIN CATALYSIS

Heterogeneous catalysis is a basis of many science�intensive technologies used in such industrially impor�tant processes as the utilization and reprocessing ofwastes of many chemical and petrochemical pro�cesses, purification of thermal station and car exhaustgases, waste�free syntheses of valuable chemical prod�ucts, oil and gas processing, and the production ofhigh�octane fuels. The activation of heterogeneouscatalysts both at the stage of their preparation and dur�ing use is of the greatest importance for increasing theactivity and selectivity of catalytic processes. Tradi�tional methods for the activation of heterogeneouscatalysts, such as thermal treatment and oxidation orreduction at high temperatures in a medium of variousgases, often do not provide the necessary catalystactivity, selectivity, or stability, because such treat�ments generate a wide spectrum of active centers ofvarious natures and strengths, including active centersthan catalyze undesirable side reactions. In addition,currently, ever increasing use in industry is found bycatalysts that differ from systems used earlier by morecomplex compositions, the organization of the struc�ture and active centers at the molecular level, andpolyfunctionality of action. This makes the problem ofthe development of new nontraditional methods oftheir activation and regeneration still more pressing.

If microwave activation is used in catalytic pro�cesses, it should be taken into account that the objectsof study are nonuniform multiphase systems includingcatalysts and gases in which volumetric structural andproperty changes occur under the action of electro�magnetic radiation. The main idea of microwavecatalysis is to exert volumetric controllable electro�magnetic action on the catalyst–reagents system. Thisaction should change the state of the system andincrease the effectiveness of the work of catalysts, the

1682


KUSTOV, SINEV

selectivity of raw material conversion into valuableproducts, and catalyst stability. One of the main direc�tions toward solving this problem is a decrease in thetemperature and other reaction parameters by processexecution under mild conditions in an electromag�netic microwave field with the retention of or increasein process efficiency.

Numerous data show that, under microwave acti�vation conditions, higher reaction rates are obtainedthan in thermally activated processes, contact timedecreases, and the yields of final products increase.These advantages are often ascribed to so�called“microwave effects” without performing a detailedanalysis of the corresponding reaction conditions. It islikely that such a microwave effect was for the firsttime observed by Sun in his work on the hydrolysis ofadenosinetriphosphate [42]. The results obtained bySun and the conclusions drawn by him were later cor�rected by Jahngen, who found that an increase in thereaction rate by 12–15 times under microwave fieldaction conditions was likely caused by temperaturegradient [43]. Since then, it is accepted to separatethermal and nonthermal (electromagnetic) microwaveeffects. Mingos and Baghurst discussed the theoreticalbasics of dielectric heating in microwave fields andgave numerous examples of its use [44].

THERMAL EFFECTS

A typical thermal microwave effect was obtained forthe example of the catalytic sulfonation of naphtha�lene [45]. The regioselectivity of the introduction ofsulfo groups can be controlled by the power of micro�wave action and the resulting rate of heating. Theexperimental data closely agree with a theoreticalanalysis based on the hypothesis of the existence ofsuperheated regions on the surface of the catalyst [46].

A solution to the Arrhenius equation with the temper�ature of the superheated region increased by 70 K gavean almost twofold increase in the concentration ofproducts compared with uniform temperature distri�bution over the catalyst during the first seconds ofheating (figure). The calculations were performedusing the k = 1.45 × 1013 mol–1 s–1 preexponential fac�tor and E = 80 kJ/mol activation energy.

The next example of thermal microwave effects isthe formation of so�called “hot points.” Traditionally,regions with a temperature higher than that of theenvironment are called hot points. They appear as aresult of stronger interaction with a microwave fieldand low�effectiveness heat exchange. Hot points aredifficult to observe in heterogeneous catalysts of gasphase reactions, because these zones can have atomicsizes in such typical cases as metallic catalysts or oxy�gen defects in oxides. Because of poor resolution ofinfrared pyrometers, direct temperature measure�ments cannot be performed, and the real temperatureof hot points can only be determined by indirect, as arule, kinetic methods.

Perry et al. [23, 24] discuss the presence of substan�tial differences between microwave and thermal acti�vation for the example of the oxidation of CO onPt/Al2O3 and Pd/Al2O3. They studied the kinetics ofthe reaction, specifically, Arrhenius curves (the depen�dences of the rate of formation of СО2 on inverse tem�perature), to find that, for both catalysts, the rate ofthe reaction at a given temperature was higher undermicrowave activation conditions compared with ther�mal activation. At the same time, the form of thedependence of the reaction rate on reagent concentra�tions and activation energy did not depend on themethod of heating. Note that the authors of [23, 24]used a fairly doubtful procedure for temperature mea�surements, they introduced a thermocouple into thevolume of the catalyst after switching microwave fieldoff. However, they performed a more detailed analysiswith corrections in temperature to draw the conclu�sion that there was no difference between microwaveand thermal activation. Moreover, Perry advanced thehypothesis that isolated microparticles of materials(including metals) absorbing microwave fields on asubstrate transparent to electromagnetic waves couldnot be heated to temperatures substantially higherthan the mean temperature of the environment. Thishypothesis was theoretically substantiated by Perry[24] by calculations of heat release in spherical metal�lic nanoparticles and subsequent heat transfer into thegas phase under stationary conditions. The differencesbetween the temperatures of the surface of the metaland the surface of the carrier calculated for 1 and100 nm particles were of 1.1 × 10–10 and 1.6 × 10–10 K,respectively.

Mingoos and Zhang studied the catalytic decom�position of H2S on 30% MoS2/γ�Al2O3 to find [27]that, under the action of a microwave field, conversion

80

40

0 4 8 12 16τ, s

1

η, %

2

60

20

Time dependence of product yield (η) under the condi�tions of (1) uniform temperature distribution and (2) thepresence of superheated regions on the catalyst of naph�thalene sulfonation [45].



was higher that predicted by analyzing thermody�namic equilibrium at a mean measured system tem�perature and higher than that obtained in experimentswith usual thermal activation. X�ray patterns showedthat the catalyst after microwave activation containedthe α�Al2O3 phase, which could be formed fromγ�Al2O3 only at temperatures above 1000°С, whereasthe mean measured temperature in experiments didnot exceed 800°С. On the other hand, initially amor�phous molybdenum disulfide with 150–170 nm parti�cles transformed into hexagonal crystals under micro�wave field action. Because the melting temperature ofMoS2 is 1185°С, this substantiates the formation ofhot points including both active phase and carrier par�ticles in the catalyst.

NONTHERMAL EFFECTS

As distinct from thermal microwave effects, non�thermal effects appear much rarer. Parmon et al. useda specially designed unit to study the epoxidation ofethylene with separating thermal from nonthermaleffects [47]. The catalyst was heated not only by amicrowave field but also by pumped hot air. Todecrease convective heat loss, the reactor and part ofair introduction line situated in a microwave furnacewere placed into an evacuated shell.

The specific action of a microwave field was onlyobserved for the Ag/α�Al2O3 catalyst subjected toreducing treatment before measurements. In an oxi�dative atmosphere, no differences were observedbetween experiments performed with the use of amicrowave field and in its absence. This effect is dis�cussed taking into account another observation,according to which the formation of defects in silvervolume and on the surface is decelerated in a reducingatmosphere. According to Roussy, structural changesor “electron behavior” effects are one of the possiblereasons for an increase in selectivity in the isomeriza�tion of 2�methylpentane on Pt/Al2O3 [48, 49].

Another nonthermal effect was considered for theBaBiO3 – x catalyst of the oxidative condensation ofmethane [50]. An increase in selectivity with respect toC2 products when microwave activation was used was

explained by a decrease in the rate of carbanion oxidation because of a decrease in the concentrationof oxygen adsorbed by the catalyst. Different behaviorsof catalysts activated by microwave field and heatingwere substantiated by measurements of the real andimaginary complex permittivity components undermicrowave field conditions (single�mode high�powermicrowave field heating catalysts) and in a classic res�onator (broad�band conductivity measurements [51]with a small microwave power supply after samplethermal treatment). When the catalyst reached a tem�perature higher than 500°С, the imaginary permittiv�ity component increased in oxygen and air. Con�versely, dielectric loss decreased in an inert atmo�

−CH3

sphere (helium). The authors explain these effects bydeceleration of redox processes under microwave fieldaction. It is stated that metallic behavior correspondsto a maximum degree of BaBiO3 – x system reduction(x = 0.49), and the conclusion is drawn that electro�magnetic field decreases the rate of oxygen absorptionat arbitrary oxygen pressure.

PROMISING DIRECTIONS OF STUDY OF THE USE OF MICROWAVE FIELDS

IN THE CATALYSIS OF GAS�PHASE PROCESSES

Among gas�phase catalytic reactions that most fre�quently were subjected to stimulation by microwavefields, the following processes should be mentioned:

—oxidative condensation of methane;—partial oxidation of hydrocarbons;—purification of gases from noxious impurities,

including purification of car exhausts;—hydrogenation of aromatic compounds;—dehydrogenation of naphthene compounds.In the first two reactions, which are of great impor�

tance for applications, the key effectiveness factor isthe attainment of high selectivity with respect to thedesired products at equal degrees of reagent conver�sion. The catalytic neutralization (purification) of var�ious gases and gas exhausts is also a rich field for stud�ies and developments. High heating rates combinedwith easy power control when a microwave field is usedbroaden the possibility of a decrease in the content ofvarious contaminants in gases. In addition, in the lastcase, the use of microwave heating performed almostinstantaneously solves the “cold start” problem, whichusually arises with inertial thermal heating.

The Oxidative Condensation of Methane

The oxidative condensation of methane, the maindesired products of which are ethylene and ethane,was for the first time described in the early 1980s.Since then, this reaction has been the object of seriousscientific interest. Currently, the practical importanceof this process again increases, because the use of nat�ural and casing�head oil gases as raw materials forchemical industry is an acute problem under the con�ditions of constantly decreasing oil reserves and asharp increase in oil cost.

Numerous studies of this process showed that thereis a kinetics limit of the yield of the desired products(20–25%). This limit is determined by the ratiobetween methane and С2 hydrocarbon reactivities. Atcomparatively low conversions (≤10–15%), a 80–85%selectivity can be reached. Selectivity decreases con�siderably as the conversion of methane increases. Freeradical reactions substantially influence the process,because the primary reaction product, ethane, isformed in the recombination of methyl radicals gener�

1684


KUSTOV, SINEV

ated as a result of methane activation with the partici�pation of catalyst surface active centers [52].

According to several authors, the use of microwaveactivation can cause a decrease in the temperature ofthe gas phase and catalyst volume. As a consequence,it has an advantage over convective heating activationbecause of suppression of deep oxidation reactions.The first studies in this direction were performed byBond [53] with the use of sodium aluminate as a cata�lyst. On the whole, selectivities with respect to С2hydrocarbons obtained under thermal and microwaveheating conditions did not differ strongly. But reactiontemperatures differed by more than 400 K. Work [53],pioneering for microwave catalysis, was not free ofcertain shortcomings. Apart from different methodsfor temperature measurements under different activa�tion conditions, the authors discuss the results givingno process characteristics other than selectivities withrespect to С2 hydrocarbons and deep oxidation prod�ucts (CO and СО2). Apart from a substantial decreasein reaction temperature, the composition of reactionproducts changed, selectivity with respect to СО2decreased, and selectivity with respect to COincreased. Under microwave activation conditions,the ethylene/ethane ratio increased, whereas ethanewas the main product under thermal activation condi�tions.

Roussy also studied several mixed oxides and com�pared their activity in a microwave field with that undertraditional heating conditions. These oxides includedLa2O3, La2Zr2O7, SmLiO2, (SmLiO2)0.8(CaOMgO)0.2,Li/MgO, and BaBiO3 – x [54]. No difference betweenthermal and microwave heatings was obtained forLa2Zr2O7, whereas La2O3 showed higher selectivitieswith respect to the desired products at equal conver�sions under thermal heating conditions. SmLiO2 and(SmLiO2)0.8(CaOMgO)0.2 showed opposite depen�dences of selectivity on conversion: at low conver�sions, selectivity with respect to С2 hydrocarbonstended to 100% under microwave activation condi�tions and to zero when traditional heating was used. Athigh methane conversions (30–40%), selectivitiesbecame close to each other (about 50–60%).

The authors of [54] explain the observed effects bydifferent ratios between the rates of homogeneous andheterogeneous stages leading to the formation of thedesired and side products under different heating con�ditions. The kinetic patterns of the process were ana�lyzed using a simplified scheme,

The k4 and k5 values were substantially higherunder traditional heating conditions, which corre�sponded to the observed substantial difference inselectivity with respect to the desired products. It wasassumed that the reason was the occurrence of deep

CH4 C2H6 C2H4

COx

k1 k3

k2

k5k4

oxidation of С2 hydrocarbons predominantly in the gasphase, and the measured temperature of the gas phasewas substantially lower under microwave activationconditions. It was also assumed that active centersresponsible for the formation of СН3 radicals werespecifically and selectively activated by a microwavefield in samples characterized by high dielectric lossvalues (SmLiO2 and (SmLiO2)0.8(CaOMgO)0.2).

The influence of the temperature of the gas phasewas studied in detail for the Li/MgO and BaBiO3–x

catalysts, for which an important role was played bydifferent mechanisms of the formation of methyl rad�icals, which dimerized to yield ethane [55]. ForBaBiO3–x, the existence of surface oxygen ions O2–(s)is supposed. These ions can detach a proton from the

methane molecule with the formation of the surface ion. Carbanions interact with molecular oxy�gen to produce methyl radicals [56]. The secondmechanism was established by [57] for the Li/MgOcatalyst. It was based on the existence of O– ions onthe surface of the catalyst. These ions could detach ahydrogen atom from methane to produce methyl rad�icals, which dimerized in the gas phase to yield ethane.It was assumed that the formation of undesirable prod�ucts (carbon oxides) largely occurred heterogeneouslyand homogeneously when the reaction was governedby the first and second mechanisms, respectively. Theinfluence of the gas phase oxidation of methyl radicalswas proved for the Li/MgO catalyst by diluting it witha material (silica) inert with respect to microwavefields. Microwave activation of the diluted sample gaveselectivity with respect to С2 products comparablewith that obtained using traditional heating. Toexplain this result, the authors advanced the hypothe�sis according to which microwave activation increasedselectivity with respect to С2 hydrocarbons as a resultof a decrease in the rate of the gas�phase oxidation ofmethyl radicals, because gas surrounding the catalystremained comparatively cold. For the BaBiO3–x cata�lyst, dilution with an inert substance did not influenceselectivities with respect to С2 hydrocarbons of differ�ently activated reactions, which, according to [57],could be evidence of a lower contribution of gas�phasereactions on this catalyst.

Catalysts with perovskite structures were studied byChen et al. [58–60]. Depending on their compositionand structure, perovskites can have very different elec�tromagnetic properties; they can be insulators, metal�lic conductors, superconductors, materials with giantmagnetic resistance (these materials change their elec�tric resistance by orders of magnitude in the presence ofa magnetic field), and dielectrics [61]. Studies were per�formed for oxides classified by the authors as oxygenconducting materials, namely, SrCe0.95Yb0.05O3,BaCe0.93La0.07O3, and Li2SO4/BaCe0.93La0.07O3. It wasassumed that the detachment of hydrogen from themethane molecule played a key role in the oxidativecondensation of methane. In addition, the perovskites

−CH s3( )



specified were easily heated by a microwave field toreaction temperatures. A decrease in reaction temper�ature by ~200 K was observed, the other conditionsbeing equal (see Table 3). The distribution of productsfor microwave and thermally activated processeschanged insignificantly.

The oxidative condensation of methane in thepresence of mixed oxide catalysts (Bi2O3)1–x(WO3)x(x = 0.2, 0.3, and 0.4) was studied in [63]. Accordingto the authors (see Table 4), equal conversions wereattained when microwave activation was performed ata lower temperature compared with the thermal pro�cess. In addition, selectivity with respect to С2 hydro�carbons increased under microwave conditions,mainly because of a decrease in the contribution ofdeep ethane oxidation to CO and СО2. Note that theethane : ethylene ratios were substantially different fortwo methods of activation of the oxidative methanecondensation process. The results were discussed onthe assumption of interrelation between the mobilityof lattice oxygen and catalytic properties [64] on theone hand and the influence of microwave fields on thetransport of ions [65, 66] on the other.

On the whole, the results obtained in studies of theinfluence of microwave activation on the oxidativecondensation of methane are contradictory. On theone hand, effects described in some publications arefairly substantial. On the other hand, some of theresults contradict the general rules governing the pro�cess (see above), such as the existence of the kineticslimit independent of the particular mechanism and

localization of its separate stages. A decrease in thetemperature required to reach equal conversions by200–300 K under microwave field conditions with theretention of and even increase in selectivity withrespect to С2 hydrocarbons is also questionable.According to the current concepts of the heteroge�neous�homogeneous character of oxidative methanecondensation (e.g., see [67–69]), a decrease in tem�perature below 650°С should cause a sharp decrease inselectivity in the presence of molecular oxygen in thegas phase. This is caused by a shift to the right of equi�librium

and subsequent formation of oxygen�containing prod�ucts (up to carbon oxides) through CH3O2 radicals.Methyl radicals are then removed from the process,and the formation of С2 hydrocarbons is blocked.

It is hardly probable that the use of microwavefields changes the mechanism of methane activation(see above the conclusion of the impossibility ofchemical bond dissociation and/or electronic excita�tion of molecules under the action of microwave radi�ation). Therefore, the probability of the mechanismwith the generation of primary methyl radicalsremains fairly high. Taking into account difficulties oftemperature measurements (especially, separate mea�surements for a catalyst and a free gas in a granularlayer), it should be stated that the results cited aboveneed independent verification before drawing conclu�sions about the special and effective action of a micro�

2 3CH O CH O3 2+ ⇔i i

Table 4. Influence of the method of heating on the oxidative condensation of methane on oxygen conducting mixed oxidecatalysts

Methodof heating Т, °С Conversion

of СН4, %

Selectivity, %С2Н4/С2Н6

СО СО2

Thermal 840 20 62 20 18 2.8

Microwave 525 20 72 11 17 1.3

Microwave 580 30 65 12 23 1.8

C2+

Table 3. Influence of temperature and method of heating (I, microwave and II, thermal) on the oxidative condensation ofmethane on proton conducting perovskite�like catalysts

Catalyst T, °C Conversion of СН4, %

Selectivity, %Method

C2H2 C2H4 C2H6 CO CO2

SrCe0.95Yb0.05O3 580 20 1 23 30 9 37 I

SrCe0.95Yb0.05O3 775 20 0 29 32 4 35 II

BaCe0.93La0.07O3 590 25 2 30 29 6 33 I

BaCe0.93La0.07O3 825 25 0 38 26 1 35 II

Li/SrCe0.95Yb0.05O3 590 14 0 27 49 1 23 I

Li/BaCe0.95Yb0.05O3 620 18 0 28 45 1 26 I

Li/SrCe0.93Yb0.07O3 590 18 0 34 42 2 22 I

1686


KUSTOV, SINEV

wave field on the catalysts of oxidative methane con�densation and the corresponding process.

Partial Oxidationand Other Hydrocarbon Conversion Processes

The catalytic partial oxidation of organic com�pounds of various classes is a basic reaction of manyfundamental organic synthesis and petrochemistryprocesses. In the past decades, such processes with theparticipation of hydrocarbons, largely saturated hydro�carbons, have attracted special attention because ofrise in the cost of oil and the necessity of the involve�ment of other raw material resources in the productionof valuable chemical products and half�finished prod�ucts, in the first place, of natural and casing�headgases. These processes lie close to various variants ofthe transformation of hydrocarbons and other organicsubstances into synthesis gas and/or hydrogen oxida�

tive, vapor, and carbon dioxide conversions and theirvarious combinations.

The first experiments with the use of microwavefields in such processes were described by Wan [13].That work contained the primary data on vapor con�version with various catalysts, reaction conditions,and initial hydrocarbons (see Table 5). Water vaporwas added into the reaction mixture by water vaporiza�tion or adding water�containing salts, such as CuSO4 ·2H2O [70], to catalysts. According to the authors,microwave fields initiated the dissociation of adsorbedwater to the ОН• and Н• radicals. Although the yieldsof oxygen�containing compounds were fairly low(~5%) under microwave methane activation condi�tions, the authors indicate the potential possibility ofusing water as a cheap and ecologically pure oxidizerin such reactions.

Bi and Dai studied the partial oxidation of methaneto CO and H2 on 10% Co/ZrO2, 10% Ni/La2O3, and10% Co/La2O3 [71, 72]. They explain the difference of50–250 K between the temperatures of reaching simi�lar conversion values under microwave and thermalactivation conditions by the formation of hot points onoxygen defects. A comparative review of the resultsobtained is given in Table 6.

Liu and Iin [73, 74] describe the partial oxidationof о�xylene and toluene on vanadium catalysts depos�ited on TiO2 and SiO2. Reactions were performed in amodified household microwave furnace. At this stage,only these authors comprehensively described experi�ments in a multimode furnace. They, however, used aprocedure in all the experiments that could not butraise natural questions. It is not quite clear why a mer�cury thermometer is used and what is then the accu�racy of temperature measurements under microwavefield conditions. It is also clear from [74] that a peri�odic microwave activation mode was used, but processparameters (power, duration, and pulse ratio) were notreported. One way or another, the authors report aboutan increase in the conversion of о�xylene under micro�wave activation conditions compared with thermalheating. Selectivity with respect to phthalic anhydridedecreases as the temperature increases. These observa�tions are explained by stronger interaction of V2O5

Table 5. Microwave activated partial oxidation of C1–C6 hydrocarbons

Hydrocarbon Catalyst Reaction conditions Products

Methane Ni�1404* H2O (vapor) Methanol, acetone, dimethyl ether

Methane Ni (microstructured) Pulsed microwave activation, H2O (vapor) Acetone, C2, C3, methanol, ethanol

Propane Ni (1 µm) H2O (vapor) Methanol, butanols, propanols

Propylene CuO H2O (vapor) Propanol, ethanol, acetone, propylene oxide

n�Hexane СuO H2O (vapor) Methanol, propanol, hexanone

Cyclohexane V2O5 H2O2 Cyclohexanol, cyclohexanone

* Commercial sample of the Ni/NiO composition.

Table 6. Influence of temperature and method of heating(I, microwave and II, thermal) on the partial oxidation ofmethane according to [71, 72]

Method T, K Conversion of СН4,%

Selectivity, %

СО СО2

10% Co/ZrO2

I 723 48 79 21

I 1073 100 99 1

II 873 63 66 34

II 1073 94 93 7

10% Ni/La2O3

I 673 58 74 26

I 973 100 100 <1

II 873 40 77 23

II 1073 91 92 8

10% Co/La2O3

I 723 34 57 43

I 1073 100 100 <1



particles, which are the active phase of the catalyst,with microwave field compared with the substrate.Their temperature is therefore higher than the meancatalyst temperature. Microwave field prevents activephase particles from segregation observed under ther�mal heating conditions. The distribution of the V2O5active phase over the surface of SiO2 becomes moreuniform, and dispersity increases. As a result, thenumber of catalytically active V=O centers on the sur�face of the catalyst grows. When the temperature of theactive phase exceeds 370°С, deep oxidation processesbegin to prevail, which decreases selectivity withrespect to phthalic anhydride. According to theauthors, this is caused by the superheating of V2O5 par�ticles and the formation of electric discharges on thesurface of the catalyst.

The Removal of Sulfur and Nitrogen Compounds

The removal of SO2 and NOx from air is one moreheterogeneous catalysis area that has been extensivelyexplored during the past decades. It spite of certainsuccesses achieved, the problem is still far from com�plete solution, especially because of a great number ofvarious objects in which these contaminants areformed and conditions of their removal. Strictly, a sub�stantial part of works concerned with the removal ofsulfur and nitrogen compounds from gas mixturescannot be treated as exclusively catalytic, becausecomplex technological approaches are used in them.This is a combination of the adsorption, catalysis, andstoichiometric reduction of oxides; the stimulatingaction of microwave fields is used at various stages ofcontaminated gas and/or solid catalyst processing.

Currently, the technology of the selective catalyticreduction of nitrogen oxides (SCR�DeNOx) holds theleading position in the removal of nitrogen oxides fromwaste gases of power stations. Nitrogen oxides arereduced by ammonia or urea at temperatures of from300 to 400°С on MoO3/WO3/V2O5 mixed oxidesdeposited on TiO2. This is a highly effective process,but it requires heavy material expenditures because ofcorrosion activity of ammonia and the occurrence ofside reactions. For instance, ammonia at high temper�atures is oxidized to NO. In addition, in the presenceof sulfur�containing compounds in a fuel, ammoniumsulfide and sulfur oxides are formed. Their removal isa nontrivial problem. However, during the pastdecade, technologies were developed for abandoningthe use of ammonia as a reducing agent in favor of lessexpensive and more friendly ecologically hydrocar�bons. Currently, there are no catalysts stable towardmoisture contained in all waste gases.

The first works on the use of microwave activationin these processes were published by Wan [13]. Exper�iments with the removal of sulfur and nitrogen oxideswere performed on catalysts similar to those describedabove for partial oxidation. Gas mixtures consisting of9% SO2 or 25% NO diluted with an inert gas (nitrogen

or helium, respectively) were used. With the Ni�1404catalyst, 99% of SO2 was removed during 3 h. Similarresults were obtained for all the other catalysts exceptcommercial Ni�1600 nickel�oxide catalyst, which wasrapidly poisoned by sulfur compounds. The other cat�alysts were successfully regenerated in a flow of nitro�gen in a microwave field and could be repeatedly usedup to 10 times without a noticeable decrease in activ�ity. When NO was decomposed on nickel catalystsunder microwave activation conditions, a more than99% conversion of initial nitrogen oxide was observed.Catalysts remained active for 5 h, however, after 4 h ofwork, N2O was observed at the exit of the reactor inaddition to N2 and O2; the content of N2O increasedwith time. Note that the authors of [13] give no com�ments concerning the results obtained in microwaveactivation of the removal of sulfur and nitrogen oxides.Moreover, they give no suggestions concerning themechanism of microwave field action on these pro�cesses. In addition, data are incomplete in the workcited: the results are not compared with the thermalactivation data, and such important parameters as thetemperature of catalysts and microwave activationconditions are not given.

Starting with the mid�1990s, Cha has been study�ing the destruction of air contaminants in microwavefields [75, 76]. He obtained several patents in this field[77–79]. As distinct from Wan, Cha used various car�bon adsorbents with different structures. Equal adsor�bent samples were saturated with a mixture of1000 ppm NO and 610 ppm NO2 in a humid air (watervapor content 0.95%). All the samples were thentreated by a microwave field (power 480 W, frequency2.45 GHz). During treatment, desorbed substanceswere recorded. After the desorption stage (30 min),weight loss and specific surface area were measured forall the adsorbents. According to the authors, theregeneration of carbon adsorbents saturated withnitrogen oxides activated by a microwave fieldincreased both sorption capacity and the rate ofadsorption for all the samples except GAC 830(Atochem) commercial activated carbon. The pres�ence of moisture in the initial mixture contributed tothe transformation of NO into NO2 and HNO3, whichwere then adsorbed. According to the authors, the rea�son for the improvement of the sorption properties ofthe samples was a higher ability of the centers of NO2and HNO3 adsorption to absorb microwave field com�pared with the surrounding volume (it is, however, notclear from the paper why the authors arrive at such aconclusion). These centers then participated in thereduction of nitrogen oxides.

Although such a method for the removal of nitro�gen oxides is not completely catalytic, it has severalobvious advantages. When carbon adsorbents areused, the presence of reducing agents (such as tradi�tionally used very toxic ammonia) is unnecessary, andthe reduction of nitrogen oxides proceeds at a high rateunder these conditions. In addition, because of the

1688


KUSTOV, SINEV

formation of HNO3, carbon sorbents are additionallyactivated, which can be a source of a valuable “side”product (activated carbon). The authors also note thatthis method is applicable to the purification of gasexhausts from comparatively small sources, such ashousehold diesel generators and heaters, when thetransformation of nitrogen oxides and nitric acid intovaluable products is impossible.

The first studies of the reduction of nitrogen oxidesin a microwave field with methane in the presence ofoxygen on zeolite catalysts were described by Chang[80]. The author used Co�NaZSM�5 and Co�HZSN�5and the H forms of the same zeolites. He comparedmicrowave and convective heating regimes. Theexperimental data show that the reduction of NO pro�ceeds much more actively and at lower temperaturesunder microwave activation conditions. For instance,the H�ZSM�5 and Co�HZSM�5 catalysts were almostinactive under convective heating conditions at tem�peratures below 400°С. The Co�NaZSM�5 catalystcatalyzes the deep oxidation of methane without theinvolvement of NO into the process at temperaturesbelow 420°С. At higher temperatures, the reduction ofNO begins, but the oxidation of methane with oxygenproceeds more actively, as previously. Conversely, theaction of microwave field at the same temperaturesresults in high degrees of the transformation of NO onall the three catalysts. On both cobalt catalysts, theconversion of NO exceeds 68%, whereas the H form isless active in the reduction of NO. These data lead theauthor of [80] to conclude that a high degree of theabsorption of microwave energy by catalysts changesreaction paths and results in the predominant activa�tion of the reduction of nitrogen oxide by methane.Among possible mechanisms of microwave activationof NO, the author of [80] mentions the formation ofnitrogen dioxide, which is known to more activelyreact with organic substances. Another possible mech�anism of the destruction of the N–O bond is interac�tion with the methyl radical formed in the homolyticabstraction of hydrogen from the methane moleculeby surface oxygen.

Zhang studied other catalysts of the same type,In/HZSM�5 and In�Fe2O3/HZSM�5 (1 : 8 : 20 byweight) [81]. Under thermal heating conditions, thetwo catalysts show equal activities, whereas, undermicrowave activation conditions, the In/HZSM�5catalyst is absolutely inactive. The catalyst containingiron oxide was active in a microwave field; moreover,nitrogen oxide conversion reached 100% if a water�free mixture was used. According to the authors, ironoxide well absorbs microwave field energy, whereas theIn/HZSM�5 catalyst almost does not absorb electro�magnetic waves, which explains the effect observed.Since Fe2O3 is a good chemisorbent of nitrogen oxides,the latter are activated more easily. Note that theobserved temperature of catalyst volume necessary forthe complete conversion of NO decreased by 200 K inmicrowave compared with convective heating experi�

ments. Mingos et al. [82] observed the formation ofsuperheated regions 0.09–1.0 mm in diameter underthe action of microwave radiation on MoS2/γ�Al2O3during the endothermic decomposition of H2S. Thecatalyst volume temperature at which equal processrate was achieved decreased by 150–200 K comparedwith convective heating.

One of the possible methods for creating adsorp�tion�catalytic systems for the protection of the envi�ronment, in particular, for the removal of volatileorganic compounds, is the addition of componentsstrongly interacting with electromagnetic fields toadsorbents and catalysts already studied under thermalactivation conditions [83]. Several patents [84] andpublications [85] are concerned with the developmentof catalytic systems for the purification of car exhaustsunder “cold start” conditions. The influence of micro�wave radiation on the catalytic purification of carexhaust gases was studied with varying radiation powerfrom 0 to 200 W [86]. The catalyst was the reduced Pt–Pd–Rh/Al2O3 system on a ceramic monolith. Micro�wave and thermal heating conditions were compared.Microwave radiation allowed the temperature of reac�tion initiation to be decreased substantially even in thepresence of water vapor and catalyst activity at lowtemperatures to be increased. The mechanism of theeffect was not studied and explained, but the sugges�tion was made that adsorbed particles were activatedby microwaves directly or through catalyst particles.Similar data were published in [74].

It was shown in [24, 86] that temperature gradientswithin a catalyst granule were not very large, and thatthe temperatures of active particles and substrate wereequal, for instance, in the oxidation of CO onPd/Al2O3, even when the substrate itself did not absorbmicrowave energy. According to the authors, this con�clusion is valid for a wide range of metal particle sizes.Studies performed led the authors to conclude that, atreliable temperature measurements for the oxidationof CO, the reaction proceeded identically and did notdepend on the source of heat (microwave or usualheating). Thomas [26] did not agree with this conclu�sion. According to Thomas, substantial temperaturegradients between metal particles and substrates canbe obtained by varying particle size and microwaveradiation frequency. An important conclusion that fol�lows from the discussion on this point in the literatureis the necessity to take great care with communica�tions in which the authors assert that they were able todecrease reaction temperature with increasing orretaining activity or increase activity at a constanttemperature.

Other Catalytic Processes with Microwave Activation

Other directions in catalysis related to the use ofmicrowaves were discussed in reviews [87, 88]. In [89]and several other publications by the same authors,microwave activation of the Pd–Fe/Al2O3 catalyst of



hydrodechlorination was studied. It was shown thatthe size of metallic deposited catalyst particleschanged under the action of microwaves, and thatmetal particles could form alloys. The possibility ofincreasing the activity of reduced catalysts of chlo�robenzene hydrodechlorination was shown. Micro�wave heating had the same effect (enlargement of par�ticles) on the Pd/Nb2O5 catalyst of the same reaction[90], which increased the activity and stability of thecatalytic system. Similar results were obtained formicrowave�irradiated Pd/Al2O3 and Pd/SiO2 catalystsof the hydrogenation of benzene.

Conde et al. [91] showed that microwave radiationfrequencies heavily influenced the effects related tomicrowaves in catalysis. They performed the oligo�merization of methane on nickel catalysts. Changes inmicrowave frequencies changed field distribution andthe set of observed reaction products. At all the fre�quencies studied, an increase in radiation powerincreased the conversion of methane and decreasedselectivity with respect to С2 hydrocarbons. At a4.6 GHz frequency, an increase in power decreasedselectivity with respect to ethane and ethylene butincreased selectivity with respect to acetylene andbenzene; at 2.45 GHz, acetylene did not form. Anincrease in frequency at a constant power increasedselectivity with respect to benzene. The use of a gasdiluent (helium) also changed the picture of selectivi�ties under microwave conditions. The results obtainedin studies of the influence of radiation frequency ledthe authors to conclude that the most probable reasonfor changes in process characteristics was a change inthe real temperature of the process in the zone oftransformations rather than some specific microwaveactivation of reaction participants or catalyst. Thisconclusion was also substantiated by the insufficiencyof microwave energy for splitting chemical bonds inthe system in question. The ability of a material totransform microwave energy into heat was shown topass a maximum as the frequency increased. It followsthat frequency variations can be used to influence theamount of heat released and the real temperature ofcatalyst active centers.

Microwave heating was also used to perform otherreactions, namely,

—the hydrogenation of chlorinated phenols onPt/C [92];

—the isomerization of 2�methylpentane onPt/Al2O3 [93];

—the epoxidation of ethylene on Ag/Al2O3 [94];

—the hydrogenation of olefins, hydrocracking ofcyclic hydrocarbons, and water shift reaction CO +Н2О = СО2 + Н2 [11];

—arylation according to Heck on palladium cata�lysts deposited on Al2O3, C, MgO, and CaCO3 [95];

—the oxidation of toluene to benzoic acid onV2O5/TiO2 [73];

—and several other processes.The decomposition of 4�chlorophenol to СО2,

HCl, and Н2О by oxidation with air on nickel oxidecatalysts (pH 7, 70°C, t = 5 min) was performed undermicrowave activation conditions in situ in [96]. Sev�eral examples of successful use of microwave radiationin the hydrogenation and hydrogenolysis of alkenesand alkanes and water shift reaction are given in [97].

We thoroughly studied the dehydrogenation of sev�eral naphthene and polycyclic (including heterocy�clic) compounds on deposited metallic catalysts underthe action of microwave heating [98]. The dehydroge�nation of such compounds was suggested as one of twostages of safe keeping of hydrogen; it is based on cyclicreversible reactions of the hydrogenation of aromatic(heteroaromatic) compounds followed by the dehy�drogenation of the saturated substance obtained. Insuch cycles, capacity no less than 7–7.5 wt % of thecomposite material, including the organic substrateand catalyst, was reached. This is much higher thanthe capacity of intermetallic compounds traditionallyused for hydrogen storage (3–4 wt %) or carbon nano�tubes suggested earlier (1–2 wt %).

Oxidation reactions. Interesting data on the use ofmicrowave activation in the oxidation (epoxidation) ofstyrene were reported in [99]. The Co–Y–ZrO2 sul�fated catalyst was used. The optimum microwave radi�ation power for it was found to be ~400 W; it decreasedthe time of the reaction to several minutes, while theconversion of styrene and selectivity with respect toepoxide remained high (91% at 120°C). An increase inpower to 800 W resulted in the formation of a newproduct, styrene glycol.

Effective use of microwave heating in the oxidationof alkenes and alcohols under the action of hydrogenperoxide was described in [100]. The catalyst wasMSM�41�type mesoporous titanium silicate modifiedby organic compounds. It was recycled several timeswithout activity loss. A positive effect of microwaveradiation was obtained in the oxidative dehydrogena�tion of ethylbenzene to styrene on iron oxide depos�ited on multilayer carbon nanotubes [101]. Anincrease in selectivity with respect to styrene wasobserved under microwave activation conditions(380–450°C); it was found that part of Fe2O3 trans�formed into Fe0 nanocrystals.

The oxidation of benzene to phenol with the use ofnitrous oxide as a mild oxidizer is a very promisingone�stage method for the production of phenols.Microwave activation of the zeolite catalyst of thisreaction (Fe�ZSM�5) was used to decrease the contri�bution of coking [102]. Although some increase inselectivity with respect to phenol and a decrease in therate of catalyst deactivation were observed, on thewhole, no substantial improvement of process charac�teristics was obtained. The influence of microwaveradiation on the formation of acrylonitrile in theammoxidation of glycerol (a side product of the pro�

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duction of biodiesel fuel from vegetable oils) under theaction of hydrogen peroxide and ammonia on V–Sb–Nb oxide catalysts was studied in [103, 104]. Selectiv�ity with respect to acrylonitrile was quite substantialunder microwave heating conditions (83.8%) at a46.8% conversion. For comparison, the reactionunder thermal activation conditions gives selectivity ofonly ~60% at a conversion not exceeding 15%.

Organic synthesis. Organic synthesis of com�pounds of various classes is the most extensively devel�oping area of microwave technology applications[105–107]. In these studies, both homogeneous (e.g.,see [108]) and heterogeneous (e.g., see [109]) catalystswere used. For instance, in work [108] concerned withthe addition reactions of CCl4 and CCl3COOEt, reac�tion rates increased by 3–20 times under microwaveactivation conditions compared with thermal heating.The authors of [109] studied the decomposition oftrichloroethylene on a deposited Pt/Ni catalyst. Theyfound that optimum process was characterized by acatalytic layer thickness of 1–3 µm. A decrease orincrease in thickness decreased catalyst activity orcaused coke accumulation.

Waste and renewable raw material processing. Sev�eral examples of the use of microwave heating in bio�catalysis, in the first place, in the hydrolysis of cellu�lose for plant raw material processing were described[110]. Among diverse reactions of side product andwaste processing, the pyrolysis of glycerol into synthe�sis gas [111] and the oxidation of compounds model�ing lignin on the Co(salen)/SBA�15 mesoporous cat�alyst [112] can be mentioned. In the last case, thecomplete conversion of apocynol (model compound)under the action of hydrogen peroxide was obtained in40 min, whereas the usual heating regime gave conver�sion no higher than 60% and only in ~24 h. Accordingto [113], a reactor with microwave activation could beused for acid hydrolysis of cellulose and cellulosewastes with phosphoric acid. A 90% yield of glucosewas obtained.

A review of the methods for the utilization of wasteswith the use of microwave technologies was given in[114, 115]. The processes considered included repro�cessing of old tires and plastic wastes and restoration of(removal of harmful contaminants from) soils andunderground water. It is, however, noted that there arescaling problems with the passage from laboratorystudies to microwave technologies on an industrialscale. The use of microwave methods for the produc�tion of biogas with a low content of CO2 and CH4 byprocessing dead�water precipitates was described[116]. Compared with thermal pyrolysis, the micro�wave method allows a higher yield (~94%) of synthesisgas (CO + Н2) with a lower content of CO2 and CH4to be obtained.

Review [117] considers the use of microwave tech�nology in processing of tar sand and oil products, inparticular, for the extraction of bitumens, conversion

of heavy residues, the removal of sulfur and nitrogencompounds, and heating tar sand for decreasing theviscosity of bitumens. One more example of usefulconversion of wastes is the biotechnological produc�tion of hydrogen under microwave processing condi�tions [118]. Recently, microwave technologies havebegun to be applied for liquefaction of coal [119].

PROSPECTS FOR THE USE OF MICROWAVE RADIATION IN CATALYSIS

To summarize, there are good grounds in certaininstances for believing that catalytic processes occurunder the action of microwave fields differently thanunder traditional thermal activation with convective orconductive heating. The energy of microwave radia�tion quanta is, however, insufficient for directly break�ing chemical bonds in reacting molecules. A necessarycondition for the action of a microwave field on a pro�cess is therefore the possibility of interaction of a solid(a catalyst or its separate components) with this field.

Among various mechanisms of the action of micro�wave field on solids, several classes of catalysts andprocesses that offer promise for use in practice can beidentified. For instance, the Maxwell–Wagner inter�phase polarization mechanism can likely operate inmixed oxide catalysts (such as extensively studied cat�alysts of partial oxidation based on vanadium andmolybdenum oxides). For such catalysts, “nontrivial”and “nonthermal” effects can most probably beexpected; these effects are of obvious interest for thefundamental science of catalysis.

Among heterogeneous catalysts, there are alsomany systems representing massive conductors (met�als and activated carbons) and systems containing adeposited active metallic phase. Strictly, the Maxwell–Wagner mechanism can also be valid for metallic cat�alysts deposited on a carrier inert with respect tomicrowave fields. Heat exchange between a depositedconducting phase and a carrier transparent to micro�wave radiation is of great importance for heterostruc�tures of the type metal + carrier transparent to electro�magnetic field. If the characteristic time of thermalrelaxation is larger than reaction duration, such a sys�tem is of great interest for bifunctional catalysis withthe separation of functions between a metal and a car�rier active in the given case, for instance, a carrier pos�sessing acid properties or acting as a donor of activelattice oxygen. The main special feature of heating ofconductors is the propagation of electromagnetic fieldat a near�light velocity. This can be of obvious advan�tage for processes that require fast and selective cata�lyst heating. In these instances, microwave field can beused to solve technological problems.

Importantly, clear�cut separation between thermaland nonthermal effects is necessary for revealing thereal action of a microwave field on catalytic (and, onthe whole, chemical) reactions. Even with compara�



tively simple reactions, errors can arise if experimentaldata are analyzed without due care. Nontrivial micro�wave effects are largely exclusively thermal, and thereason for a change in reaction path is the ability ofvarious catalyst components to absorb electromag�netic field better than their environment.

For instance, an analysis performed by Perry forthe oxidation of CO on Pt/Al2O3 and Pd/Al2O3showed [22, 23] that there can only be insignificantdifferences between catalytic processes performed in amicrowave field and under thermal heating condi�tions. Because of inaccuracy of temperature measure�ments in a strong electromagnetic field, these differ�ences can be leveled when experimental conditions areimproved. In addition, the formation of superheatedregions can be excluded for exothermic reactions onmetallic deposited catalysts under the continuousaction of a microwave field.

Only a small number of researchers present proofsthat the observed phenomena have a nonthermal ori�gin. Roussy repeatedly mentions that a comparison ofactivation by microwave fields and thermal heatingcannot be based exclusively on measurements andcomparison of reaction temperatures because ofstrong differences in methods for heating and difficul�ties of accurate temperature measurements. Otherreaction parameters (conversions, selectivities, andthe type and distribution of microwave�active materi�als in a catalyst) also require detailed consideration.

Currently, the procedure for selecting an appropri�ate catalyst for a heterogeneous gas phase catalyticprocess that is ideally suitable for activation in micro�wave fields is as previously not quite clear. The selec�tion is complicated because the catalyst should, first,have substantial catalytic activity and, secondly, pos�sess certain electrophysical properties (dielec�tric/magnetic properties and conductivity). To obtaina maximum positive effect of microwave radiation ona catalytic system, comparatively inert componentsstrongly interacting with electromagnetic fields areintroduced into catalysts.

The advantages of microwave technologies canmore effectively be used if catalytically and electro�magnetically active catalyst components are placed ona substrate transparent to microwave fields. This isdone to form superheated regions only in the volumeor on the surface of the active catalyst phase, whichdecreases the temperature of the catalyst as a wholeand the gas phase. This offers the possibility of sup�pressing side reactions and decreasing energy expendi�tures for heating inert materials that do not directlyparticipate in the catalytic process.

Apart from the selection of objects of study and theformation of structures providing optimum combina�tions of adsorption, catalytic, and electrophysicalproperties, it is necessary to develop a methodologicalapproach to studies of the influence of microwave acti�vation on the processes under study. We should rule out

(or minimize) the possibility of incorrect interpreta�tion of experimental data. This in the first place relatesto temperature measurements. It would be ideal toseparately measure the temperature of various solidcatalyst components (adsorbent, carrier, and activecatalyst phases) and the gas phase. Such procedures doexist. In particular, it is possible in principle to deter�mine the temperature of various complex system com�ponents by measuring their optical spectra. Howeverin practice, such procedures are difficult to use in themajority of cases. In addition, they require a fairlycomplex and expensive equipment.

For this reason, the development of additional cri�teria for revealing specific effects that appear under theaction of microwave fields on reacting systems is animportant problem. In the first place, this relates tochanges in kinetic characteristics and dependences(reaction orders with respect to components and acti�vation energies) that are less sensitive to errors in tem�perature measurements than a comparison of activitiesor general transformation rates. Changes in processselectivity with respect to certain products at equalconversions and different methods of reacting systemactivation are most interesting and informative.

It is also necessary to track changes in the state of asolid under the action of microwave fields. In certaininstances, the appearance of structures that are notformed in the absence of such action can be expected.If they have (as expected) catalytic properties and/orreactivity different from those observed usually, theappearance of the corresponding nontrivial effects inchemical reactions is possible.

Lastly, a special position is held by so�calledbifunctional catalysis, in which the occurrence of var�ious stages of the process as a whole takes place on var�ious catalytic system components. A typical example istransformations of hydrocarbons on metallic catalystsdeposited on acid carriers. A metallic component (as arule, a Pt family metal) activates the initial moleculewith breaking bonds and/or charge transfer (dehydro�genation, hydrogenolysis), whereas carbon skeletontransformations (alkylation and isomerization) occuron acid carrier centers. Simultaneous hydrogena�tion/dehydrogenation, cyclization, aromatization,etc. processes can occur on both complex catalystcomponents in various combinations. Microwaveradiation creating temperature gradients in a hetero�geneous system whose components differently interactwith electromagnetic fields can then cause substantialchanges in the general rate and, which is especiallyimportant, distribution of reaction products.

Technologically, the advantages of the use ofmicrowave activation of heterogeneous gas phase cat�alytic reactions compared with thermal activationbecome obvious when fast heating or cooling andpulsed energy supply are required. The mobileremoval of contaminants from air is a suitable area forthe use of microwave technologies, because they easily

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provide high�intensity pulses and solve the cold startproblem. Apart from the search for and synthesis ofsuitable catalysts, the development of new microwaveequipment for applications on pilot and industrialscales is necessary.

Effects observed in catalysis under the action ofmicrowaves are still not well understood and have beenpoorly studied, although it is clear that irradiation ofmicrowave�absorbing materials (catalysts, carriers,and reaction medium) can cause rapid volume heat�ing, effectively remove moisture from solids, and mod�ify surface properties. This distinguishes the action ofmicrowaves from traditional thermal treatment, whichis used in the preparation of catalysts and in reactions.

Solid catalytic materials can be divided into threegroups according to their character of interaction withmicrowave radiation. The first group includes metalswhose smooth surface fully reflects microwave rays.Metals then are not heated, because there is almost nomicrowave radiation loss into their volume. However,if the surface is rough, microwave radiation can causean arc discharge on it. The second group includesdielectrics transmitting microwave radiation almostunchanged through their volume. These are alumi�num and silicon oxides, fused quartz, various glasses,porcelain and faience, polyethylene, polystyrene, andfluoroplastics (Teflon etc.). Lastly, the third groupinclude dielectrics that absorb microwave radiation,which is in particular accompanied by sample heating.In practice, mixtures containing substances thatweakly and strongly absorb microwave radiation areoften used for microwave heating. By varying the com�position of such mixtures, we can control the maxi�mum temperature of mixture heating and the compo�sition of reaction products.

Clearly, the use of microwaves in catalysis will bedeveloped in two directions. The first direction isrelated to the preparation and preliminary activationof catalysts with the use of microwaves for traditionalreactors, where heating is performed by traditional(thermal) methods [120–122]. The second directionis the action of microwaves on catalysts and reactionmedia (if they absorb microwaves) during chemicalcatalytic reactions [123–128].

The literature data considered can be generalizedas follows. The use of microwave action on heteroge�neous catalysts during their preparation sometimesallows catalysts with a more uniform distribution ofparticles to be obtained, the preparation of catalysts tobe accelerated, and uniform catalyst volume phaseheating to be performed. In certain instances, we canobtain catalysts with the required dispersity by varyingradiation frequency. With catalysts consisting of sev�eral phases, the replacement of traditional with micro�wave heating can contribute to the preferable forma�tion of separate phases. When microwave activationduring a catalytic reaction is used, a decrease in thetemperature of reaction beginning, an increase in

activity or selectivity, and a change in the distributionof reaction products are often observed. A comparisonof a process performed under traditional conditionswith the same process in the presence of a microwavefield is, however, possible not always because of diffi�culties related to true process temperature measure�ments in the latter case. Most likely, we can expectboth purely thermal microwave action effects andeffects related to the transformation of the active cata�lyst component because of the simultaneous action onit of microwaves and reaction medium.

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Microwave Activation of Catalysts and Catalytic Processes