Dechlorination of polychlorinated dibenzo-p-dioxins catalyzed by noble metal catalysts under mild conditions

Article abstract of DOI:10.1016/S0045-6535(01)00170-9

Dechlorination of polychlorinated dibenzo-p-dioxins such as 2,7-dichlorodibenzo-p-dioxin (2,7-DCDD) and 1,2,6,7-tetrachlorodibenzo-p-dioxin (1,2,6,7-TCDD) was carried out in a solution of NaOH in 2-propanol in the presence of carbon-supported noble metal

Full text of DOI:10.1016/S0045-6535(01)00170-9

Chemosphere 46 (2002) 507–510
www.elsevier.com/locate/chemosphere
Dechlorination of polychlorinated dibenzo-p-dioxins
catalyzed by noble metal catalysts under mild conditions
*
Yuji Ukisu , Tatsuo Miyadera
National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba West,
1
6-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan
Received 1 December 2000; received in revised form 19 June 2001; accepted 9 August 2001
Abstract
Dechlorination of polychlorinated dibenzo-p-dioxins such as 2,7-dichlorodibenzo-p-dioxin (2,7-DCDD) and 1,2,6,7-
tetrachlorodibenzo-p-dioxin (1,2,6,7-TCDD) was carried out in a solution of NaOH in 2-propanol in the presence of
carbon-supported noble metal catalyst (Pd/C or Rh–Pt/C) at temperatures between 23 and 35 °C. At initial concen-
trations of 140–240 lg=ml, 2,7-DCDD and 1,2,6,7-TCDD were efficiently converted to a chlorine-free product, di-
benzo-p-dioxin (DD), in high yield (60–80%). The conversion of 2,7-DCDD and 1,2,6,7-TCDD and the yield of DD
2
were hardly affected by the atmosphere (N or air). We postulate that the displacement of aromatic chlorines by hy-
drogen selectively occurs on the catalyst, involving hydrogen-transfer from 2-propanol to the substrates. Ó 2002
Elsevier Science Ltd. All rights reserved.
Keywords: Hydrodechlorination; Organic chlorides; Catalytic reaction; 2-Propanol; Sodium hydroxide
1
. Introduction
A number of methods for degrading PCDD/Fs have
been examined on the laboratory scale. Supercritical
water systems are capable of oxidizing PCDD/Fs con-
tained in fly ash (Sako et al., 1997), but high temperature
and high pressure (>374.1 °C, >22.12 MPa) are required
to generate the supercritical conditions. PCDD/Fs in
solution or on particles can be decomposed under am-
bient conditions by irradiation with UV-light (Crosby
et al., 1971; Kieatiwong et al., 1990), sunlight (Dobbs
and Grant, 1979; McPeters and Overcash, 1993), and
c-rays (Hilarides et al., 1994), but the prolonged irradi-
ation is necessary to reduce toxicity. A chemical reagent
(KPEG) produced by mixing solid potassium hydroxide
with a polyethylene glycol formulation is found to de-
chlorinate PCDD/Fs at temperatures between 70 and
The emission of polychlorinated dibenzo-p-dioxins
PCDDs) and polychlorinated dibenzofurans (PCDFs)
(
at municipal and hazardous waste incinerators is strictly
regulated to reduce pollution. Although the firing sys-
tems have been designed and operated to minimize the
formation of PCDD/Fs, the integration of disposal pro-
cedures such as adsorption and dust collection is in-
dispensable at present facilities in order to achieve the
stringent limiting value for PCDD/F emissions in the
exhaust gases. Catalytic oxidation is often adopted to
decompose PCDD/Fs in the exhaust gases, where the
reaction is performed with vanadium-based catalysts at
temperature around 200 °C (Ide et al., 1996).
1
50 °C and applied for disposal of activated carbon and
soils contaminated by PCDD/Fs (Tiernan et al., 1989).
Recently, it is reported that a multiphase catalytic sys-
tem with a noble metal catalyst (Pt/C or Pd/C) allows
the efficient hydrodechlorination of PCDD/Fs at 50 °C
(Tundo et al., 2000).
*
Corresponding author. Tel.: +81-298-618219; fax: +81-298-
18208.
E-mail address: y-ukisu@aist.go.jp (Y. Ukisu).
6
0
045-6535/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved.
PII: S0045-6535(01)00170-9

508
Y. Ukisu, T. Miyadera / Chemosphere 46 (2002) 507–510
We demonstrate here that catalytic dechlorination of
HP-5MS (30 m ꢀ 0:25 mm, 0:25 lm film-thickness). n-
PCDDs proceeds rapidly with a very high conversion in
a solution of NaOH in 2-propanol in the presence of
carbon-supported noble metal catalysts under ambient
conditions. The PCDDs used in this study were 2,
Dodecane was used as the internal standard.
7
-dichlorodibenzo-p-dioxin (2,7-DCDD) and 1,2,6,
-tetrachlorodibenzo-p-dioxin (1,2,6,7-TCDD). Two car-
3. Results and discussion
7
bon-supported catalysts (Pd/C and Rh–Pt/C) were
examined, because these catalysts exhibit very high ac-
tivities for the dechlorination of polychlorinated biphe-
nyls (PCBs), chlorobenzenes, and chlorotoluenes in the
same dechlorination system (Ukisu et al., 1996, 1998,
The time profile for the dechlorination of 2,7-DCDD
in a solution of NaOH in 2-propanol with Rh–Pt/C
catalyst at 35 °C is shown in Fig. 1. The concentration of
2,7-DCDD in the solution decreased sharply, while the
concentration of monochlorodibenzo-p-dioxin (MCDD)
first increased to a maximum value and then fell to zero.
The concentration of chlorine-free product, dibenzo-
p-dioxin (DD), was gradually increased, implying that
dechlorination of 2,7-DCDD proceeded stepwise; the
ultimate yield of DD in the solution was 62%. After
completion of the reaction, the catalyst and liquid phase
were separated by centrifugation, and the substrates and
products adsorbed on the catalyst were extracted with
toluene. The amount of DD adsorbed on the catalyst
was equivalent to a yield of 18%, which is too large to
be negligible; trace amounts of 2,7-DCDD and MCDD
were detected.
2
000; Ukisu and Miyadera, 1997).
2
. Experimental
2
.1. Materials
The substrates, 2,7-DCDD and 1,2,6,7-TCDD, were
obtained from AccuStandard. The carbon-supported
Rh-based catalyst powder (Rh–Pt/C), prepared by a
previously described method (Ukisu et al., 1998), con-
tained 3 wt% Rh and 1 wt% Pt. The carbon-supported
Pd catalyst powder, Pd/C (5 wt% Pd) was obtained
from N. E. Chemcat. Prior to use, Rh–Pt/C and Pd/C
Table 1 summarizes the experimental results for de-
chlorination of 2,7-DCDD and 1,2,6,7-TCDD at tem-
peratures between 23 and 35 °C for 3 h. The conversions
and yields were evaluated by summing the amounts in
the liquid phase and adsorbed on the catalyst except
Runs 3 and 4. With Pd/C catalyst at 23 °C, 2,7-DCDD
was efficiently converted to DD in excellent yields
2 2
were heated under a flow of N and H , respectively,
at 200 °C for 0.5 h. All chemicals and gases were
high-purity grade and were used without further purifi-
cation.
(
(
>75%) under both aerobic and anaerobic conditions
Runs 1 and 2). The tolerance to air has already been
2
.2. General procedure
observed in the dechlorination of chlorotoluenes (Ukisu
et al., 2000). When Rh–Pt/C was used as the catalyst, a
slightly higher temperature (35 °C) was required to
The PCDD substrate (0.8–1.4 mg) and a large excess
of NaOH (12 mg) were dissolved in 2-propanol (6.0 ml)
and the mixture was placed in a Pyrex test tube with a
rubber cap. The initial concentrations of PCDD sub-
strate and NaOH were equivalent to 0.4–0.9 and
5
0 lmol=ml, respectively. If desired, the air in the test
tube was replaced with nitrogen. An appropriate
amount of noble metal catalyst (10 mg) was then added
to start the dechlorination reaction. The reaction mix-
ture was stirred vigorously with a magnetic stirrer;
temperature was controlled by immersion in a water
bath. Before and during the reaction, an aliquot of the
reaction mixture was taken for GC-MS analysis. After
completion of the reaction, the reaction mixture was
centrifuged, followed by analysis of the liquid phase.
The recovered catalyst was placed in an ultrasonic gen-
erator and extracted with 3 ml of toluene for 5 min. The
extraction was repeated three times, and the extract was
concentrated for analysis. The concentrations of sub-
strates and products were determined by GC-MS (GC:
HP6890, MS: HP5973) equipped with a column of
Fig. 1. Time profile for dechlorination of 2,7-DCDD with
Rh–Pt/C catalyst. Reaction conditions: substrate, 172 lg=ml
ð0:68 lmol=mlÞ; 2-propanol, 6.0 ml; NaOH, 2000 lg=ml
ð50 lmol=mlÞ; Rh–Pt/C, 10 mg; 35 °C; in air.

Y. Ukisu, T. Miyadera / Chemosphere 46 (2002) 507–510
509
Table 1
Catalytic dechorination of polychlorinated dibenzo-p-dioxins
a
b
b
Run Substrate
Initial concentration Catalyst
ðlg=mlÞ
Atmosphere Temperature Conversion
(°C)
Yield (%)
(%)
c
Light-CDD
DD
1
2
3
4
5
6
7
8
a
2,7-DCDD
2,7-DCDD
2,7-DCDD
2,7-DCDD
2,7-DCDD
2,7-DCDD
1,2,6,7-TCDD
1,2,6,7-TCDD
215
236
196
213
213
209
167
139
Pd/C
Pd/C
N
2
23
23
30
30
23
35
23
35
99.9
0.02
0.02
<0.1
0.9
80.1
74.5
66.9
94.4
70.0
69.6
74.1
59.9
Air
Air
Air
99.9
>99.9
>99.9
97.6
d
d
Pd/C
Pd/C
Rh–Pt/C
Rh–Pt/C
Pd/C
N
N
N
N
2
2
2
2
7.6
0.1
99.8
99.9
<0.01
0.06
Rh–Pt/C
99.8
Reaction conditions; 2-propanol (6.0 ml), NaOH ð2000 lg=mlÞ, catalyst (10 mg), 3 h.
b
Estimated by summing the amounts in the liquid phase and adsorbed on the catalyst except Runs 3 and 4.
Chlorinated dibenzo-p-dioxin with a chlorine number less than that of the substrate.
After Run 3 was completed and the catalyst was recovered, Run 4 was done. Conversion and yield were determined by analysis of
c
d
solution.
complete the dechlorination of 2,7-DCDD; high yields
of DD and a small amount of MCDD were detected
Cl
O
(
Runs 5 and 6). The dechlorination of 1,2,6,7-TCDD
Cl
O
Pd/C or
Rh-Pt/C
O
was also completed using Pd/C or Rh–Pt/C at temper-
atures below 35 °C (Runs 7 and 8), although the
yields of DD were slightly lower than those for 2,7-
DCDD.
2,7-DCDD
Cl
O
2-propanol
+ NaOH
O
Cl
DD
23-35˚C
The halogen-free products besides DD could not be
detected by GC-MS. The discrepancies in mass balance
in Table 1 probably reflect the small amount of product
still remaining on the catalyst after extraction with tol-
uene. In order to minimize the effect of adsorbed DD,
dechlorination of 2,7-DCDD was repeated after the
catalyst was recovered; DD yield in the solution in-
creased from 66.9% to 94.4% (Runs 3 and 4).
O
Cl
Cl
1,2,6,7-TCDD
CH3
H3C
C
O
H
Cl
O
A previous study revealed that the catalytic dechlo-
rination of chlorobenzenes in a solution of NaOH in 2-
propanol involved hydrogen transfer from 2-propanol
to the aromatic chlorines (Ukisu and Miyadera, 1997).
Also, the catalytic transfer hydrogenation of ketone or
aldehyde with alcohols is well known as a Meerwein–
Pondorf–Verley reaction, which includes a hydride
transfer from the alkoxide to carbonyl carbon of ketone
or aldehyde in the presence of homogeneous or hetero-
geneous catalysts (Ivanov et al., 1994). On the basis of
these results, we postulate a surface intermediate as il-
H
Cl
O
Catalyst
Fig. 2. Outline of dechlorination of PCDDs and postulated
surface intermediate.
drogen is often necessary in a conventional reductive
dechlorination method with transition metal and noble
metal catalysts (LaPierre et al., 1978). Moreover, sepa-
ration and recovery of catalyst is easily accomplished in
this heterogeneous solid catalyst system. In combination
with solvent extraction, the disposal of fly ash and
activated carbon contaminated by PCDD/Fs might
to be possible; KPEG system has been successful in
detoxification of contaminated soil and activated carbon
(Tiernan et al., 1989).
ꢁ
lustrated in Fig. 2; hydride ðH Þ produced from 2-pro-
panol displaces a chloride anion from the aromatic ring
of PCDD. The released HCl is neutralized by NaOH
dissolved in the solution or adsorbed on the catalyst
2
surface, resulting in the formation of NaCl and H O.
For polychlorinated species, the displacement of chlo-
rine is repeated until DD is formed.
This catalytic hydrogen-transfer dechlorination sys-
tem using 2-propanol as a hydrogen source does not use
molecular hydrogen, having a potential advantage in
practical use, although high pressure of molecular hy-
In summary, we have achieved the catalytic dechlo-
rination of PCDDs such as 2,7-DCDD and 1,2,6,7-
TCDD with very high conversion and excellent yields
under mild conditions (<35 °C). Further optimization of

510
Y. Ukisu, T. Miyadera / Chemosphere 46 (2002) 507–510
the catalytic system for dechlorination of PCDFs as well
as PCDDs is in progress in our laboratory.
Sako, T., Sugeta, T., Otake, K., Sato, M., Tsugumi, M., Hiaki,
T., Hongo, M., 1997. Decomposition of dioxins in fly ash
with supercritical water oxidation. J. Chem. Eng. Jpn. 30,
7
44–747.
Tiernan, T.O., Wagel, D.J., Garrett, J.H., VanNess, G.F.,
Solch, J.G., Harden, L.A., Rogers, C., 1989. Laboratory
and field tests to demonstrate the efficancy of KPEG reagent
for detoxification of hazardous wastes containing polychlo-
rinated dibenzo-p-dioxins (PCDD) and dibenzofurans
(PCDF) and soils contaminated with such chemical wastes
18, 835–841.
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