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70
S. Horikoshi et al. / Journal of Catalysis 289 (2012) 266–271
product yields. Heating the reaction mixture by the conventional
method of an oil bath (CH), the chemical yield of 4-MB with Pd/
ACs was 1.2-fold higher compared to the Pd/CMCs (Fig. 4).
Evidently, no selective heating of the catalysts occurred by the
CH method. Therefore, we ascribe the greater yield of the product
with the Pd/ACs relative to Pd/CMCs from conventional heating to
the larger surface area of the AC support.
An interesting contradiction to expectations is worth noting in
Fig. 4, as we expected the ACs support to be more effective than
the CMCs support under otherwise identical temperature condi-
tions by microwave irradiation of Pd/CMCs catalyst and Pd/ACs
catalyst. As the Fig. 4 emphasizes, the Pd/CMCs catalyst was far
more effective. In an earlier study [7], we observed formation of
hot spots (i.e., arcing or so-called microplasma) on the Pd/ACs sur-
face for the Suzuki–Miyaura coupling reaction in the toluene sol-
vent under high microwave electric field conditions. Related to
these observations, Menéndez and coworkers [14] identified two
different types of microplasmas formed on microwave heating of
activated carbon: ball lightning plasma and arc discharge plasma.
Apparently, ball lightning plasmas were more abundant initially
when the temperature of the carbon bed was still relatively low
hot spots were seen on the Pt/AC surface. Once again, no hot spot
formed when using the Pt/CMCs system, at least none were ob-
served. However, in spite of the formation of the microplasma
and plausible aggregation of the Pt on the ACs support, the extent
of dehydrogenation of tetralin with Pt/AC under microwave irradi-
ation is relatively significant (ca. 16%) but lower than with Pt/CMCs
(ca. 25%). Under conventional heating, the yields were significantly
lower, respectively, 3% and 5%. As the reaction temperature of the
tetralin dehydrogenation was considerably higher (boiling point,
207 °C) than that of the Suzuki–Miyaura coupling reaction (bp. of
toluene, 110 °C), the high-temperature microplasma generated on
the ACs surface likely aided in the heating of the tetralin. In the
dehydrogenation of tetralin with Pt/CMC, ca. 98% of the reaction
products were composed of hydrogen and naphthalene (reaction
(2)) with the remaining 1–2% consisting of hydrocarbon impurities
(substrates), whereas using the conventional heating mantle meth-
od to drive the reaction, the amount of hydrocarbon substrates was
ca. 2%. No doubt generation of hot spots contributed to formation
of impurities; however, the increase was negligible under our
conditions. The amount of hydrocarbon impurities increased
somewhat to ca. 5% when using the Pt/AC catalyst.
(
<400 °C), whereas arc discharge plasmas were seen at higher
temperatures (400–700 °C) in accord with our observations [7].
The high-temperature hot spots had a deleterious effect on the
product yields from the Suzuki–Miyaura coupling reaction as they
caused the metal catalyst Pd on the ACs support to aggregate and
reduce reaction efficiency [7]. In the current study, generation of
hot spots was confirmed through photographs taken with a high-
speed camera for the Pd/ACs system (see Fig. 5). By contrast, no
arc discharge plasma was observed for the Pd/CMCs system. It is
plausible that the smaller electrical conductivity of the CMCs
4
. Conclusions
The present study has demonstrated that carbon microcoils
(
CMCs) are indeed effective catalyst supports in microwave organic
chemistry, despite the fact that the available surface area is signif-
icantly less than the activated carbon particles. In revenge, how-
ever, the carbon microcoils proved to be better microwave
absorbers than the ACs and thus optimal for the selective heating
of the metal catalysts. In addition, somehow the CMCs were able
to prevent generation of the microplasmas that have proven to im-
pact negatively on reaction yields [7]. Recent years have witnessed
fabrication of equipment for the continuous synthesis of CMC in
industry. Production of CMCs continues to improve every year. If
CMCs were to attract attention as a catalyst support in chemical
syntheses, no doubt the manufacturing technique would see fur-
ther improvement.
(
6.4-fold smaller) relative to the ACs (see Table 1) controls the
generation of such microplasma. Another factor that could impact
formation of the plasmas is the helical structure of the CMCs that
may be unsuitable in forming the hot spots. The dielectric polariza-
tion induced by the microwaves’ E field on the CMCs with low elec-
trical conductivity is smaller than that of the ACs and thus
moderates the electric discharge. Additionally, the generation of
hot spots on the Pd/AC catalyst can cause the aggregation of the
metal catalyst on the support (Fig. 5), so that hot spots can have
a negative influence on the progress of the reaction [7].
Acknowledgments
The temporal course of the dehydrogenation of tetralin using
Pt/CMCs and Pt/ACs catalysts is shown in Fig. 6. After 90 min of
microwave irradiation, the conversion yield in the presence of Pt/
CMCs was 50% greater than the yield with Pt/ACs. In this case also,
Financial support from the Japan Society for the Promotion of
Science (JSPS) through a grant-in-aid for young scientists to S.H.
(
No. B-23750247) is gratefully appreciated. This research was also
supported by CLUSTER (second stage) of the Ministry of Educa-
tion, Culture, Sports Science and Technology, Japan. One of us
(
N.S.) thanks Prof. Albini of the University of Pavia for his hospital-
ity during the many semesters in his Laboratory. We are grateful
to Prof. Nikawa (Kokushikan University, Tokyo) for the measure-
ment of dielectric parameters and also to Dr. Motojima of Toyota
Physical and Chemical Research Institute for the supply of the
CMCs.
3
2
1
0
0
0
0
References
[1] X. Chen, S. Motojima, Kona 24 (2006) 222.
[
[
2] S. Motojima, S. Hoshiya, Y. Hishikawa, Carbon 41 (2003) 2653.
3] M. Hájek, Microwaves catalysis in organic synthesis, in: A. Loupy (Ed.),
Microwaves in Organic Synthesis, Wiley-VCH Verlag, Weinheim, Germany,
2006, p. 615 (Chapter 13).
[4] S. Horikoshi, A. Osawa, Y. Suttisawat, M. Abe, N. Serpone, Org. Process Res. Dev.
14 (2010) 1453.
0
50
100
[
5] P. Makowski, A. Thomas, P. Kuhn, F. Goettmann, Energy Environ. Sci. 2 (2009)
80.
Irradiation time (min)
4
[
6] S. Motojima, X. Chen, in: H.S. Nalwa (Ed.), Encyclopedia of Nanoscience and
nanotechnology, vol. 6, American Scientific Publishers, CA, 2004, p. 775.
Fig. 6. Temporal course of the dehydrogenation of tetralin in the presence Pt/CMCs
and Pt/ACs catalysts subjected to microwave dielectric heating (MW) and conven-
tional mantle heating (CH) at a temperature of 207 °C (closed circles, Pt/CMC-MW;
open circles, Pt/CMC-CH; solid triangles, Pt/AC-MW; open triangles, Pt/AC-CH).
[7] S. Horikoshi, A. Osawa, M. Abe, N. Serpone, J. Phys. Chem. C 115 (2011) 23030.
[8] Y. Suttisawat, S. Horikoshi, H. Sakai, P. Rangsunvigit, M. Abe, Fuel Process.
Technol. 95 (2012) 27.