Microwave-assisted hydrogenation of chlorinated phenols for complete dechlorination

Article abstract of DOI:10.1246/cl.2000.632

Microwave-assisted hydrogenation of chlorinated phenols, such as 4-chlorophenol and pentachlorophenol, using platinum supported on activated carbon (Pt/C) as a catalyst, leads to rapid dechlorination, giving chlorine-free chemicals such as phenol, cyclohexanol, and other chlorine-free highly reduced compounds, which provides a facile and complete detoxification method for robust chlorinated aromatics.

Full text of DOI:10.1246/cl.2000.632

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Chemistry Letters 2000
Microwave-Assisted Hydrogenation of Chlorinated Phenols for Complete Dechlorination
Yuji Wada, Hengbo Yin, Takayuki Kitamura, and Shozo Yanagida*
Material and Life Science, Graduate School of Engineering, Osaka University, Yamada-oka, Suita, Osaka 565-0871
(Received February 23, 2000; CL-000183)
Microwave-assisted hydrogenation of chlorinated phenols,
min followed by a cooling time of 1 min under magnetic stir-
ring. The temperature was raised up to 373 K under MW irra-
diation for 1 min and went down to 363 K during the cooling
time (1 min). For solvent-free dechlorination, i.e., without
using water as a solvent, pentachlorophenol or 4-chlorophenol
was impregnated on the Pt/C catalyst using its methanol solu-
tion. Methanol was removed from the resulting dispersion with
a rotary vacuum evaporator. The solvent-free system was con-
nected to a U-shaped glass tube trap filled with methanol. After
MW irradiation, the products in the reaction system were
washed out by methanol, combined with those in the trap, and
analyzed by gas chromatography and GC-MS with a capillary
column (PEG 20 M).
Figure 1 clearly shows that the dechlorination rate of 4-
chlorophenol in water under MW irradiation was larger than that
under the conventional heating. Under MW irradiation, 4-
chlorophenol was completely dechlorinated to phenol within 40
min in water. The conventional heating experiment was carried
out at 368 K for comparison, but the conversion of 4-chlorophe-
nol was only 50% after 40 min of heating. In addition, the Pt/C
catalyst showed steady activity under MW irradiation while its
activity was gradually decreased under conventional heating
(Figure 1). The sustainable catalysis under MW irradiation can
be explained as being due to high temperature of the catalyst that
such as 4-chlorophenol and pentachlorophenol, using platinum
supported on activated carbon (Pt/C) as a catalyst, leads to rapid
dechlorination, giving chlorine-free chemicals such as phenol,
cyclohexanol, and other chlorine-free highly reduced com-
pounds, which provides a facile and complete detoxification
method for robust chlorinated aromatics.
Dechlorination processes of polychlorinated aromatics
have been paid much attention in view of detoxification of
halogen-containing highly toxic compounds, such as pen-
tachlorophenol (PCP), dichlorodiphenyltrichloroethane (DDT),
and polychlorinated biphenyl (PCB). Oxidative degradation of
chlorinated chemicals, i.e., combustion or photo-oxidation into
CO , HCl, and H O, faces difficulty because 1) the more halo-
2
2
gen atoms the compounds contain, the more resistant to oxida-
tion they are, and 2) the formation of thermally stable and toxic
intermediates or byproducts such as dioxins is unavoidable in
the processes.1 Reductive dechlorination of polychlorinated
aromatics is regarded as an alternative dechlorination method to
2
-6
avoid the above-mentioned problems.
On the other hand,
microwave(MW)-assisted chemistry has rapidly blossomed in
7
8-10
7,11,12
organic and inorganic syntheses,
catalysis,
and haz-
8
, 13-15
ardous waste treatment
due to its rapid heating ability
depending on the dielectric loss of substances. Interestingly,
surfaces of carbon black and related carbon powder can be
heated up to 1556 K under MW irradiation in a short time (e.g.,
1
min).9 Recently Jou reported oxidative destruction of
trichloroethylene in the presence of activated charcoal under
MW irradiation.14 Abramovitch et al. showed reductive decom-
position of PCB in the presence of graphite or metal as a reduc-
ing reagent under MW irradiation.15
We now report efficient dechlorination of chlorinated phe-
nols as robust chlorinated aromatics by MW-assisted hydrogena-
tion in the presence of activated carbon-supported platinum(Pt/C)
catalyst. The Pt catalyst on activated carbon can hardly be poi-
soned under MW irradiation because catalytic poisonous ele-
ments like chloride anions formed during hydrogenation must be
removed under high temperature and reductive atmosphere.
A domestic MW oven (Sanyo Company, max. 650 W and
2.45 GHz) was modified by installing a condenser and a ther-
mosensor through the holes on the ceiling. The reaction mixture
could be stirred by a magnetic stirrer installed at the bottom of
the oven. The temperature of solutions was monitored by a fiber
sensor. Platinum supported on activated carbon (Pt/C, 5wt% Pt)
was purchased from Wako Pure Chemical Industries, LTD. Into
a Pyrex glass flask (200 mL) with a condenser, an aqueous solu-
tion of 4-chlorophenol with the dispersed Pt/C catalyst was
-1
placed and hydrogen gas (1 atm, 20-30 mL min ) was introduced
and kept flowing through the flask. In order to avoid violent
boiling, the reaction mixture was intermittently irradiated for 1
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
633
may suppress the poisoning of Pt by adsorption of the increasing
chloride anion in the system. In fact, the pH of the water system
decreased rapidly with increasing the MW irradiation time
because of the formation of HCl in the hydrogenation process.
The temperature change of hexane by MW irradiation was
measured in the presence of dispersed Pt/C or platinum black
under stirring (Figure 2). Hexane containing dispersed Pt/C was
heated up to its boiling point within 1.2 min under MW irradia-
tion. However, hexane containing dispersed sole platinum black
was slowly warmed up in the same manner as pure hexane on
continuous MW irradiation. These facts indicate that MW irra-
diation should selectively raise the temperature of the activated
carbon alone, not platinum black nor hexane, in a short time.
Since activated carbon is heated selectively and rapidly as men-
9
tioned above, it should supply heat to supported Pt as a catalyst.
We also assumed that the temperature of Pt/C should be higher
than 373 K (the boiling point of water) under MW irradiation,
since the microwave dielectric heating is too fast to attain the
thermal equilibrium between the catalyst and the surrounding
water.
tems is attributed to higher temperature of the catalyst surface.
These results suggest that MW-assisted reductive dechlorina-
tion of chlorinated materials using activated carbon-supported
platinum(Pt/C) as a catalyst would be applicable to complete
detoxification of the halogenated aromatics.
The authors wish to thank Mr. Hiromitsu Kuramoto and Ms.
Miwa Saito for their technical advice on the experiments. The
present study was supported in part by the Japan Society for the
Promotion of Science as part of the "Research for the Future
Program" and also by a Grant-in-Aid for Scientific Research from
the Ministry of Education, Science, Sports, and Culture of Japan.
References and Notes
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