S. Inagaki et al.
Bull. Chem. Soc. Jpn. Vol. 84, No. 10 (2011) 1137
promoted chemistry is based on the efficient heating of
materials by microwave dielectric heating. The heating
characteristics of particular materials under microwave irradi-
a mixture of Si(OEt) , nonionic surfactant (Pluronic P123,
4
(EO) (PO) (EO) ) in aqueous HCl solution and then
2
0
70
20
calcined at 550 °C. A solution of 1.25 g of sucrose, 1.25 g of
distilled water, and 0.14 g of concentrated H SO was added
4
3
ation conditions are dependent on the dielectric properties.
2
4
The ability of a specific material to convert electromagnetic
energy into heat at a given frequency and temperature is
determined by the loss factor, tan ¤. This loss factor is
expressed as the quotient tan ¤ = ¾¤¤/¾¤, where ¾¤¤ is dielectric
loss, which is indicative of the efficiency with which electro-
magnetic radiation is converted into heat, and ¾¤ is the
dielectric constant describing the ability of molecules to be
polarized by the electric field. A reaction medium with a high
tan ¤ value is required for efficient absorption and consequently
for rapid heating. In general, solvents can be classified as high
dropwise to 1.0 g of calcined SBA-15. The moist SBA-15 was
manually stirred for a few minutes and then dried at 100 °C for
3 h followed by heating at 160 °C for 4 h to give a brownish
powder. Then, a solution of 0.8 g of sucrose, 1.6 g of distilled
water, and 0.09 g of concentrated H SO was added dropwise to
the brownish powder, followed by repeating the drying and
heating procedure once more. Carbonization of the obtained
powder was carried out at 950 °C in vacuo (<200 Pa) and then
silica etching of the carbonized products (ca. 1.4 g) was
performed by using 5.0 g of 10 wt % HF solution to give
CMK-3 (ca. 0.5 g).
2
4
(tan ¤ > 0.5), medium (0.1 < tan ¤ < 0.5), and low microwave
absorbing (tan ¤ < 0.1): for instance, tan ¤ values of ethylene
glycol, water and toluene at 2.45 GHz and 20 °C are 1.350,
0
Impregnations of Pd (1.0 wt %) on CMK-3 and activated
carbon were performed by evaporation-to-dryness. An appro-
priate amount of bis(acetylacetonato)palladium, [Pd(acac)2],
was dissolved in acetone to prepare 1.3 mM Pd solution. Next,
500 mg of porous carbon was added and the whole suspension
was stirred at room temperature for 4 h. After removing the
solvent from the suspension in vacuo, the residual solid was
recovered and dried in an air oven at 100 °C overnight to
give 1.0 wt % [Pd(acac) ]/porous carbon catalyst. [Pd(acac) ]/
.123, and 0.040, respectively.44 Since polar solvents with high
tan ¤ values show rapid and efficient heating by microwave
irradiation, microwave-assisted Pd-catalyzed coupling reac-
tions have often been attempted in a polar medium such as
poly(ethylene glycol)26 or water.
24,27,37
As solid materials are also efficiently heated by microwave
irradiation, microwave heating of heterogeneous catalyst can
realize a rapid and efficient catalytic reaction by selective
heating of catalyst or catalyst support solids. Wada and co-
2
2
SBA-15 was also prepared by the same procedure as mentioned
above. All the catalysts were used without reduction pretreat-
ment in this study.
4
5
workers have observed nonequilibrium local heating occur-
ring in dimethyl sulfoxide (DMSO) in the proximity of Co
particles under microwave irradiation by real-time in situ
Raman spectroscopy. They also demonstrated the rate-enhance-
The porous solids and Pd-supported porous solids
were characterized by powder X-ray diffraction (XRD,
Ultima-IV, Rigaku), nitrogen adsorptiondesorption measure-
ment (Belsorp-Mini, BEL JAPAN Inc.), and field-emission
scanning electron microscopy (FE-SEM, JSM-7001F, JEOL).
Collecting the Temperature Profiles under Microwave
Irradiation. All microwave irradiation experiments in this
study were performed using a single-mode Discover Labmate
system (2.45 GHz, CEM Corp.) using a 20-mL two-necked
round-bottom flask made of Pyrex glass. Figure 1 illustrates the
equipment for monitoring the temperature of the reaction
mixture and testing the catalytic reactions. The flask equipped
with a condenser was filled with argon, supplied from an argon-
filled balloon connected to the condenser. A fiber-optic probe
sensor was introduced through a rubber septum that caps the
narrow-neck of the flask, immersed in the reaction mixture, and
fixed at 7 mm from the bottom of the flask and at the horizontal
center of the flask. The level of the reaction mixture (ca. 5 mL)
was maintained at 13 mm from the bottom of flask under
static conditions. During the heating, the reaction mixture was
carefully stirred by a magnetic stirrer bar (14 mm © 2 mmº),
making the level of the reaction mixture slightly higher but
constant. In all experiments, the microwave power was fixed
at 50 W (not a temperature-control mode). The temperature
increase of the reaction mixture was monitored by a fiber-
optic probe sensor and recorded every second at a resolution of
3
ment in the catalytic dehalogenation of the C(sp )Cl bond of
2
-chloroethylbenzene and 4-phenylbutyl chloride caused by
nonequilibrium local heating of the surface of Co and/or Fe
catalyst particles under microwave irradiation.45 This finding
provides a strong basis for realizing the construction of a highly
efficient reaction field on the surface of heterogeneous catalyst
solid by microwave irradiation.
Since carboneous materials also are efficiently heated by
microwave irradiation, they are good candidates for catalyst
support in microwave-assisted heterogeneous catalytic reac-
tions. In particular, microporous activated carbon (AC) has
been utilized for microwave-promoted reactions as follows:
37
Pd/AC for SuzukiMiyaura coupling reaction or hydrogen-
4
648
49
ation,
and Pt/AC for dechlorination.
5
0,51
We have used the ordered mesoporous carbon CMK-3
instead of activated carbon as a catalyst support. In this study,
we have attempted microwave-assisted SuzukiMiyaura cou-
pling catalyzed by Pd/CMK-3 or several related catalysts with
careful observation of the reaction temperatures, and inves-
tigated the microwave dielectric heating effects of reactants,
solvents, and catalyst supports on the temperature of the
reaction mixture and the product yield.
Experimental
1
°C. For safety, when the temperature reached the boiling
Preparation of Pd/Porous Carbon Catalysts. Ordered
mesoporous carbon, CMK-3, and activated carbon, YP50F
point of the solvent, the microwave power was to be automati-
cally shut down.
(
Kuraray Chemical), were used as a catalyst support. CMK-3
The temperature-monitoring experiments under microwave
irradiation at 50 W were performed under three different
conditions as follows: (1) a solvent only; (2) a substrate or a
50,51
was prepared as follows:
porous silica SBA-15 was hydrothermally synthesized from
hexagonally ordered meso-