Local thermal nonequilibrium on solid and liquid interface generated in a microwave magnetic field

Article abstract of DOI:10.1246/cl.2012.1409

Dehalogenation of (2-haloethyl)benzene using Fe particles was carried out in the electric and magnetic fields in a microwave single mode cavity. Microwave irradiation in a magnetic field enhanced the conversion of (2-haloethyl)benzene compared with heatin

Full text of DOI:10.1246/cl.2012.1409

doi:10.1246/cl.2012.1409
Published on the web October 27, 2012
1409
Local Thermal Nonequilibrium on Solid and Liquid Interface Generated
in a Microwave Magnetic Field
Dai Mochizuki,* Masashi Shitara, Masato Maitani, and Yuji Wada*
Department of Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology,
2-12 Ookayama, Meguro-ku, Tokyo 152-8852
(Received June 9, 2012; CL-120517; E-mail: daim@apc.titech.ac.jp)
Dehalogenation of (2-haloethyl)benzene using Fe particles
was carried out in the electric and magnetic fields in a
microwave single mode cavity. Microwave irradiation in a
magnetic field enhanced the conversion of (2-haloethyl)benzene
compared with heating by a mantle heater and microwave in an
electric field, while the ethylbenzene selectivities were com-
parable. Larger Fe particles generated sparks under microwave
irradiation, resulting in the increase of ethylbenzene selectivities.
These data suggested the reaction acceleration by a local thermal
nonequilibrium in the solid-liquid system by the selective
microwave heating of the solid.
Figure 1. Experimental setup in the single-mode microwave
resonator.
eration of local thermal nonequilibrium in metal particles
irradiated by the microwave magnetic field in comparison with
acceleration by sparks.
A single-mode irradiation device is shown in Figure 1. In a
waveguide, traveling waves and returning waves interfere with
each other and form a standing wave, producing a microwave
intensity distribution in the waveguide. The phase difference
between the electric and magnetic fields of the standing wave
is ³/2. Where the electric field intensity is the strongest, the
magnetic field intensity is the weakest, and vice versa. The
electric field and magnetic field intensity distributions of
microwaves due to these standing waves are calculated
theoretically in the Supporting Information.25 Therefore, micro-
wave irradiation, primarily by the electric or magnetic field, can
be realized by changing the location at which the specimen is
inserted, although it is not possible to provide irradiation purely
by the electric field or the magnetic field alone. Magnetic
material, which has a large magnetic loss coefficient, is heated
rapidly in the magnetic field, whereas solvents are not easily
heated by microwaves because their magnetic loss coefficient
is relatively small. That is, local thermal nonequilibrium is
expected to occur at the interfacial surface of the solvent and the
magnetic metal. In this study, we use iron(II) particles as the
magnetic metal and verify the local thermal nonequilibrium
effect by microwave irradiation with the magnetic field in a
single-mode microwave device.
The microwave system is shown in Figure 1. A magnetron
was used as a microwave power source at the frequency of
2.45 GHz. The electric field intensity distribution in the wave-
guide showed sinusoid pattern (Figure S1),25 indicating the
formation of standing wave in the cavity. Fe particles (1.125 g)
were dispersed in decalin (10.6 mL) in a three-necked test tube
and were subjected to microwave irradiation at the points where
the electric or magnetic field intensity was strongest. After they
were heated to a given temperature, (2-chloroethyl)benzene
(0.112 g, 0.7 mmol) was infused. The time of infusion was set to
0 min, and samples were taken at regular intervals thereafter.
The reactants and product materials were analyzed using gas
chromatography. The same procedure was followed using a
mantle heater for comparison. Because a magnetic stirrer cannot
stir Fe particles evenly, agitation during the experiment was
Microwave heating is applied in various areas such as
plasma generation1-3 and extraction4,5 and ceramic sintering.6-8
Since Gedye9 and Giguere10 first used microwave heating in
organic synthesis in 1986, many studies have reported chemical
syntheses using microwave heating. Many reports on reduced
reaction time and improved product selectivity in microwave-
assisted chemical syntheses have been published, in which the
authors claimed that these syntheses were conducted under the
same temperature conditions as in conventional syntheses.11-13
This effect is called the nonthermal effect and is considered to
differ from the thermal effect, which features rapid, uniform
internal heating. However, no definite proof of the existence of
such a nonthermal effect distinct from the thermal effect of
microwaves has been provided to date.
Local thermal nonequilibrium, which is defined as the
phenomenon of heating domains at much higher temperatures
than a bulk solution temperature, is one of the special features of
microwave heating.14-17 Microwaves heat materials directly, and
their heating efficiency depends on the electrical conductivity,
dielectric constant, and magnetic permeability.18 When micro-
waves are applied to a solution consisting of a solvent that is
heated inefficiently by microwaves and a solid that is heated
highly efficiently, we expect selective heating of only the solid,
resulting in local thermal nonequilibrium.19 Tsukahara et al.
directly observed such local thermal nonequilibrium by Raman
spectroscopy.20 They found that the surface temperature of
cobalt particles became 50 °C higher than that of the solvent.
Furthermore, dechlorination of halo-organic compounds was
accelerated by microwave irradiation. In general, metal particles
do not get heated in an electric field because they reflect
microwaves; however, magnetic metal particles such as cobalt
were heated by magnetic loss of microwaves. Therefore, when
the solution is irradiated by only the microwave magnetic field,
the local thermal nonequilibrium is probably accelerated.
Conversely, recent reports show that microwaves generate
sparks in a metal-liquid reaction systems and accelerate the
reaction.21-24 Thus, this study attempts to explain this accel-
Chem. Lett. 2012, 41, 1409-1411
© 2012 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

1410
Figure 2. Temperature profiles for a sample of decalin heated
by microwave irradiation in (a) magnetic and (b) electric field
with Fe particles and (c) magnetic and (d) electric field without
Fe particles.
Figure 3. Conversions of (2-chloroethyl)benzene by micro-
wave heatings in magnetic ( ) and electric field ( ) and a
mantle heatings ( ).
Table 1. Conversions and ethylbenzene selectivities of
(2-chloroethyl)benzene by changing particle size and heating
method
achieved by a mechanical stirrer (800 rpm). Moreover, to
consider the differences between materials, similar experiments
were performed with (2-bromoethyl)benzene and (2-iodoethyl)-
benzene.
Ethylbenzene
Particle size
/¯m
Conversion
/%
Heating mode
selectivity
To examine the heating properties in the area where the
electric or magnetic field intensity is strongest, a test tube
containing Fe particles and decalin was inserted, and the
temperature profile associated with microwave heating was
measured (Figure 2). A test tube containing decalin only was
heated to 58 °C under electric field irradiation but only to 22 °C
under magnetic field irradiation after microwave irradiation
for 5 min. When Fe particles were added to the decalin, the test
tube was heated to 110 °C under electric field irradiation and
162 °C under magnetic field irradiation, indicating that the
heating efficiency improved significantly. Because the increase
in heating efficiency resulted from heat generated by the Fe
particles, formation of nonequilibrium local heating under
magnetic field irradiation was confirmed. The increase in
heating efficiency under electric field irradiation is thought to
result from the weak magnetic field that intrudes around the
point where the electric field intensity is strongest.
To obtain the dechlorination reaction, while controlling
the microwave intensity and keeping the solution temperature
at 190 « 3 °C, a reactant was added after the solvent temper-
ature reached 190 °C to avoid possible variations in reactivity
because of different heating rates (Figure 3). Dechlorination of
(2-chloroethyl)benzene progressed linearly, indicating a stable
reaction rate. After 45 min of reaction, the conversion was 0.6%
with mantle heater heating, 2.4% with electric field irradiation,
and 5.3% with magnetic field irradiation. Ethylbenzene was
detected as the main product, and its selectivity after 45 min was
calculated to be 72%, 71%, and 69% with mantle heater heating,
electric field irradiation, and magnetic field irradiation, respec-
tively (Table 1). The similar conversions suggest that the
reaction mechanism is the same for microwave heating and
mantle heating. Conversely, the significant increase in the
dechlorination conversion is the result of acceleration of the
reaction by microwave heating, supporting the occurrence of
nonequilibrium local heating.
/%a
45
Mantle
0.6
2.4
5.3
0.7
7.7
8.3
72
71
69
72
89
88
MW(E field)
MW(H field)
Mantle
MW(E field)
MW(H field)
150
aEthylbenzene selectivity was calculated by dividing the ethyl-
benzene yield by the conversions of (2-chloroethyl)benzene.
to bromine or iodine. In addition, the reaction rate was highest
under magnetic field irradiation for all the reaction substrates.
This was attributed to the predicted selective heating of iron
particles by the microwave magnetic field, which caused a high
temperature on their surfaces. Furthermore, for (2-chloroethyl)-
benzene and (2-bromoethyl)benzene, the reaction was also
accelerated under electric field irradiation. This is attributed to
the effect of selective heating, i.e., heating of iron particles by
the electric field and the weak magnetic field coexisting at the
maximum point of the electric field.
Because of the low yield and linearity of the reaction
regardless of the heating method, the reaction rate is propor-
tional to the initial concentration. The reaction rate equation is,
therefore, expressed as in eq 1.
v ¼ k½Feꢀ0½ð2-iodoethylÞbenzeneꢀ0
ð1Þ
A kinetic analysis based on the Arrhenius plot was applied to
(2-iodoethyl)benzene, which showed the highest reaction rate
(Figure 4). Regardless of the heating method, the activation
energy in the reaction was the same. Thus, the reaction
mechanism did not change under different heating methods.
The reaction under the magnetic field was accelerated because
Fe particles were selectively heated, and their surface temper-
ature exceeded the bulk temperature. Local thermal nonequili-
brium showed a temperature difference of about 15 °C according
to a calculation of the Arrhenius parameter. Previous work on
cobalt particles showed a higher temperature difference; the
The effect of differences in the reaction substrate was
examined by changing the halogen groups. Regardless of the
heating method, the reaction conversion rate exhibited an
increasing trend as the halogen atom was changed from chlorine
Chem. Lett. 2012, 41, 1409-1411
© 2012 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

1411
magnetic field accelerates the reaction at the liquid-solid
interface.
References and Notes
1
M. Kamo, Y. Sato, S. Matsumoto, N. Setaka, J. Cryst.
Growth 1983, 62, 642.
2
K. Kobashi, K. Nishimura, Y. Kawate, T. Horiuchi, Phys.
Rev. B 1988, 38, 4067.
3
4
Y. Wu, P. Qiao, T. Chong, Z. Shen, Adv. Mater. 2002, 14, 64.
C. S. Eskilsson, E. Björklund, J. Chromatogr., A 2000, 902,
227.
T−1 × 103/K−1
5
6
S. Mutyala, C. Fairbridge, J. R. J. Paré, J. M. R. Bélanger, S.
Ng, R. Hawkins, Fuel Process. Technol. 2010, 91, 127.
D. E. Clark, W. H. Sutton, Annu. Rev. Mater. Sci. 1996, 26,
299.
M. J. Mayo, Int. Mater. Rev. 1996, 41, 85.
R. Roy, D. Agrawal, J. Cheng, S. Gedevanishvili, Nature
1999, 399, 668.
Figure 4. Arrhenius plots for the dehalogenation reaction of
(2-iodoethyl)benzene by microwave heatings in magnetic ( )
and electric field ( ) and a mantle heatings ( ).
7
8
reason for the difference was probably that Fe particles have a
lower magnetic loss than cobalt particles.
9
R. Gedye, F. Smith, K. Westaway, H. Ali, L. Baldisera, L.
Laberge, J. Rousell, Tetrahedron Lett. 1986, 27, 279.
In recent years, accelerated reactions have been attributed to
sparks produced by microwave irradiation on the solid sur-
face.21-24 In this study, the effect of sparks was also examined by
changing the particle size. Because it is known that sparks tend
to occur as the particle size increases,23 we conducted an
experiment in which sparks were intentionally produced by
increasing the iron particle size from 45 to 150 ¯m. An
observation window located beside the waveguide was used.
No sparks were observed when 45-¯m iron particles were used,
but sparks were seen every few seconds with 150-¯m particles.
In the conversion of (2-chloroethyl)benzene using 150-¯m iron
particles by mantle heating, little change occurred compared
with that in conversion using 45-¯m iron particles; however,
irradiation of the 150-¯m iron particles by microwave magnetic
heating and microwave electric heating enhanced conversion by
7.7% and 8.3%, respectively. Sparks were often accompanied
by a change of the morphology and surface appearance of the
metal, resulting in the enhancement of the reactions.21-24 Use of
150-¯m iron particles also increased the ethylbenzene selectivity
for microwave heating to 89% and 88%, respectively. The
increase in the ethylbenzene selectivity compared with the
results for small particles indicates that the reaction mechanism
changed owing to spark production. Conversely, the ethyl-
benzene selectivity with 45-¯m particles was the same for
microwave heating and mantle heater heating. This can be
understood as a result that little or no spark was produced
therein.
10 R. J. Giguere, T. L. Bray, S. M. Duncan, G. Majetich,
Tetrahedron Lett. 1986, 27, 4945.
11 C. O. Kappe, Chem. Soc. Rev. 2008, 37, 1127.
12 C. O. Kappe, Angew. Chem., Int. Ed. 2004, 43, 6250.
13 P. Lidström, J. Tierney, B. Wathey, J. Westman, Tetrahedron
2001, 57, 9225.
14 A. de la Hoz, Á. Díaz-Ortiz, A. Moreno, Chem. Soc. Rev.
2005, 34, 164.
15 D. Bogdal, M. Lukasiewicz, J. Pielichowski, A. Miciak, S.
Bednarz, Tetrahedron 2003, 59, 649.
16 C. Blanco, S. M. Auerbach, J. Am. Chem. Soc. 2002, 124,
6250.
17 H. Jobic, J. E. Santander, W. C. Conner, Jr., G. Wittaker, G.
Giriat, A. Harrison, J. Ollivier, S. M. Auerbach, Phys. Rev.
Lett. 2011, 106, 157401.
18 A. C. Metaxas, R. J. Meredith, Industrial Microwave
Heating, Peter Peregrinces, Institution of Electrical Engi-
neers, Exeter, 1983.
19 Y. Suttisawat, S. Horikoshi, H. Sakai, P. Rangsunvigit, M.
Abe, Fuel Process. Technol. 2012, 95, 27.
20 Y. Tsukahara, A. Higashi, T. Yamauchi, T. Nakamura, M.
Yasuda, A. Baba, Y. Wada, J. Phys. Chem. C 2010, 114,
8965.
21 B. H. P. van de Kruijs, M. H. C. L. Dressen, J. Meuldijk,
J. A. J. M. Vekemans, L. A. Hulshof, Org. Biomol. Chem.
2010, 8, 1688.
Dehalogenation of (2-haloethyl)benzene using Fe particles
was performed in the electric or magnetic field in a microwave
single-mode cavity. The microwave magnetic field enhanced the
dehalogenation; this result is attributed to the presence of local
thermal nonequilibrium. The dehalogenation mechanism in
microwave heating is comparable to that in conventional
heating, whereas sparks generated by microwave irradiation of
larger Fe particles produced a different reaction mechanism. The
local thermal nonequilibrium produced by the microwave
22 B. Gutmann, A. M. Schwan, B. Reichart, C. Gspan, F. Hofer,
C. O. Kappe, Angew. Chem., Int. Ed. 2011, 50, 7636.
23 W. Chen, B. Gutmann, C. O. Kappe, ChemistryOpen 2012,
1, 39.
24 S. Horikoshi, A. Osawa, M. Abe, N. Serpone, J. Phys. Chem.
C 2011, 115, 23030.
25 Supporting Information is available electronically on the
CSJ Journal web site http://www.csj.jp/journals/chem-lett/
index.html.
Chem. Lett. 2012, 41, 1409-1411
© 2012 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/