Enhancement of the catalytic hydrodechlorination of tetrachloroethylene in methanol at mild conditions by water addition

Article abstract of DOI:10.1016/j.tetlet.2005.03.178

The dechlorination of tetrachloroethylene (PCE) over carbon-supported palladium catalyst (Pd/C) in methanol (MeOH) at mild conditions was enhanced through the addition of water to the reaction mixture. The dechlorination of PCE was accelerated by increasing the amount of water in the mixture from 0% to 50%, and beyond which the reaction slowed down, however. The presence of water in the mixture enhanced the adsorption of PCE onto the Pd/C but compromised the solubility of H₂ gas in the mixture. It was also noted that the selectivity of the HDC reaction was improved with the increase in the amount of water in the mixture as the formation of trichloroethylene (TCE) was completely eliminated when the HDC was carried out in mixtures with 50% water or more. Other chlorinated intermediates were not detected in all the reactions.

Full text of DOI:10.1016/j.tetlet.2005.03.178

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 3613–3617
Enhancement of the catalytic hydrodechlorination
of tetrachloroethylene in methanol at mild conditions by water
addition
Neil C. Concibido,a,* Tetsuji Okuda,b Yoichi Nakano,b
Wataru Nishijimab and Mitsumasa Okadaa
aDepartment of Material Science and Engineering, Faculty of Engineering, Hiroshima University, 1-4-1 Kagamiyama,
Higashi-Hiroshima 739-8527, Japan
bEnvironmental Research and Management Center, Hiroshima University, 1-5-3 Kagamiyama, Higashi-Hiroshima 739-8513, Japan
Received 24 January 2005; revised 28 March 2005; accepted 29 March 2005
Available online 11 April 2005
Abstract—The dechlorination of tetrachloroethylene (PCE) over carbon-supported palladium catalyst (Pd/C) in methanol (MeOH)
at mild conditions was enhanced through the addition of water to the reaction mixture. The dechlorination of PCE was accelerated
by increasing the amount of water in the mixture from 0% to 50%, and beyond which the reaction slowed down, however. The pres-
ence of water in the mixture enhanced the adsorption of PCE onto the Pd/C but compromised the solubility of H2 gas in the mixture.
It was also noted that the selectivity of the HDC reaction was improved with the increase in the amount of water in the mixture as
the formation of trichloroethylene (TCE) was completely eliminated when the HDC was carried out in mixtures with 50% water or
more. Other chlorinated intermediates were not detected in all the reactions.
Ó 2005 Elsevier Ltd. All rights reserved.
PCE, a suspected human carcinogen, is widely used in
dry cleaning, metal degreasing and paint stripping
processes and has thus become one of the most widely
detected chlorinated organic compounds (COCs) in the
environment, especially in groundwater. In Japan,
PCE was detected at levels above the environmental
quality standard (0.01 mg/L) in 17 out of 4225 ground-
water wells surveyed in the year 2000, making it the
second most widely occurring COC in groundwater.1
Therefore, the detoxification of PCE is an important
environmental and social concern. Conventionally, PCE,
along with other chlorinated ethylenes are removed
from groundwater by air-stripping followed by adsorp-
tion on activated carbon. However, subsequent inciner-
ation of the spent activated carbon could bring about
other environmental problems such as dioxin forma-
tion.2–4 Our laboratory had proposed a process whereby
chlorinated ethylenes adsorbed on activated carbon or
air-sparged from contaminated water were concentrated
in organic solvents and later decomposed by ozonation.
This method was effective for TCE and cis-1,2-DCE but
not for PCE, for which very slow decomposition rates
were achieved.5
Catalytic hydrodechlorination (HDC) is one of the most
promising alternatives for the degradation of COCs.3
HDC is usually carried out in the gas phase at high
temperatures and pressures.3,6 Ordon˜ez et al.7 studied
the HDC of chlorinated ethylenes in heptane over noble
metal catalysts at 50 bar and 300 °C. They suggested
that the dechlorination of PCE occurred first with the
catalytic hydrogenation of PCE to produce 1,1,2,2-tetra-
chloroethane (TTCE) then the pyrolytic hydrodechlori-
nation of TTCE (Scheme 1).
There is a growing trend in using milder conditions6 for
HDCs that efficient methods using organic solvents and
mild conditions have been reported.3,8–12 In our labora-
tory, the HDC of PCE over Pd/C in organic solvents at
20 °C and atmospheric pressure was investigated.2 The
degradation of 1.2 mM PCE was found fastest in MeOH
in which a rate constant of 0.6 minꢀ1 gꢀ1 Pd/C was
Keywords: Tetrachloroethylene; Hydrodechlorination; Pd/C catalyst;
Water–MeOH mixtures.
*
Corresponding author. Tel./fax: +81 824 246195; e-mail: koons@
hiroshima-u.ac.jp
0040-4039/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.03.178

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N. C. Concibido et al. / Tetrahedron Letters 46 (2005) 3613–3617
NaOH was added to have an initial concentration of
9.6 mM to neutralize the HCl that will be produced by
the reaction. The reactor was first saturated with N2
gas and then with H2 gas. Fifty millilitres of PCE solu-
tion in the reaction solvent was added to have an initial
concentration of 1.2 mM in the reactor. The reaction
mixture was stirred at 250 rpm and was maintained at
20 0.5 °C under atmospheric pressure. Concentrations
of PCE and chlorinated intermediates were analyzed
with Shimadzu QP 5050 GC/MS, whereas chloride con-
centrations were determined with Dionex DX-500 Ion
Chromatograph. The catalyst and chemicals were pur-
chased from Wako Pure Chemicals, Inc. and the water
used throughout the study was ultrapure water pro-
cessed through Advantec Water Distillation Apparatus.
Scheme 1. HDC of PCE over noble metal catalysts in heptane at
300 °C and 50 bar.
The amounts of chloride produced from the HDC of
PCE in different water–MeOH mixtures are shown in
Figure 1. Compared to the HDC in pure MeOH, chlo-
ride was produced faster in 20% and 50% water. Within
20 min, the amounts of chloride produced from HDC in
50% water and in 20% water were already 4.26 mM and
3.92 mM, respectively, whereas that from HDC in
MeOH was barely half of these values (1.87 mM). The
chloride produced from the HDC in MeOH, however,
was almost similar to that from the HDC in 80% water
and almost twice as much as that from the HDC in pure
water.
obtained. PCE was completely dechlorinated to ethane
within minutes. Contrary to the mechanism shown in
Scheme 1, TTCE and other chlorinated intermediates
were not detected except for low levels of short-lived
TCE (Scheme 2). The method could also be applied to
the destruction of TCE and cis-1,2-DCE, which were
completely dechlorinated with rate constants of 2.13
and 7.0 minꢀ1 gꢀ1 Pd/C, respectively.
The HDC of PCE in MeOH may be further improved
by adding water to the reaction solvent as some
researchers have discovered that the addition of water
to alcohols as reaction solvents improved the HDC of
COCs. The lifetime of Pd/C catalyst was improved when
a 50/50 (v/v) water/ethanol mixture was used as solvent
instead of pure ethanol for the HDC of chlorophenol at
35 °C and 35 psi.10 Similarly, the HDC of triclosan at
50 °C and atmospheric pressure was enhanced when
the same mixture was used instead of pure ethanol.11
Furthermore, the HDCs of chlorobenzene and chloro-
toluene at 25 °C and atmospheric pressure were much
improved in 80/20 (v/v) water–ethanol mixture com-
pared to when pure ethanol was used.12 In this letter,
we report a significant improvement in the dechlorina-
tion of PCE over Pd/C in MeOH at mild conditions
by the addition of water to MeOH as reaction solvent.
Figure 2 shows the total ion chromatograms of samples
from HDC of PCE in 50% water and in pure MeOH. In
30 min, PCE could not be detected in 50% water
whereas a substantial amount of PCE still remains in
pure MeOH. This signifies a remarkable enhancement
in the HDC of PCE by the addition of water.
The initial rate constants for PCE dechlorination under
H2 were calculated from the chloride production data
(Fig. 1) and are shown in Figure 3. The rate of PCE
MeOH
20% water
4
50% water
Twenty-five milligrams of pre-activated13 commercial
Pd/C catalyst (10% w/w) was mixed with 250 mL of
reaction solvent in a three-necked glass reactor. Pure
MeOH, pure water and water–MeOH mixtures (20–
80% water in MeOH) were used as reaction solvents.
80% water
100% water
3
2
1
0
H
H
Cl
Cl
Cl
Cl
H2, NaOH
H
H + NaCl + H2O
Pd/C, MeOH
20°C, atm P
H
H
0
5
10
time
15
20
Cl
Cl
H
Figure 1. Chloride production from HDC in different water–MeOH
mixtures. All reactions were carried out using the following: initial
PCE concentration, [PCE]0 ffi 1.2 mM; 25 mg 10% Pd/C catalyst;
300 mL reaction mixture; 9.6 mM NaOH; H2 atmosphere; 20 °C and
atmospheric pressure.
Cl
Scheme 2. HDC of PCE over Pd/C in MeOH at 20 °C and
atmospheric pressure.

N. C. Concibido et al. / Tetrahedron Letters 46 (2005) 3613–3617
3615
Figure 2. GC–MS Chromatograms of samples from HDC of PCE in 50% water and in pure MeOH.
6
4
2
0
increasing the amount of water to 80% and 100% water
eventually lead to a decline in the rate.
The increase in PCE dechlorination rate brought about
by the increase in the water content of the mixture may
be explained by how water affects the adsorption of PCE
onto the Pd/C catalyst. This was investigated by carry-
ing out the reactions under N2 gas. Figure 4 shows the
results for pure MeOH, 50% water and pure water.
Within just 1 min, PCE concentrations decreased rap-
idly to less than 2% in pure water and to less than 5%
in 50% water. Contrastingly, only a slight decrease in
PCE concentration was noted in pure MeOH. Small
amounts or no chloride at all were measured in all the
solvents implying that the sharp decreases in PCE levels
in pure water and in 50% water were due to the adsorp-
tion of PCE onto Pd/C.
0
20
40
60
80
100
% water in mixture
Figure 3. Initial PCE degradation rate constants of HDC in different
water–MeOH mixtures.
The increase in the adsorption of PCE brought about by
the increase in the amount of water in the reaction sol-
vent is in agreement with an earlier study which stated
that the adsorption of a solute on activated carbon
increases as its solubility in a solvent decreases.14 In
dechlorination in MeOH improved 3-folds when the
reaction was carried out in 20% water. It further im-
proved almost five times when the amount of water in
the mixture was increased to 50%. However, further

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N. C. Concibido et al. / Tetrahedron Letters 46 (2005) 3613–3617
25
1
8
6
20
4
MeOH
2
15
MeOH
20% water
50% water
100% water
0.1
8
6
10
4
5
0
2
0.01
0
5
10
15
20
0
5
10
time
15
20
time, mins
Figure 5. TCE evolution from the HDC of PCE in different water–
MeOH mixtures. All reactions were carried out using the following:
[PCE]0 ffi 1.2 mM; 25 mg 10% Pd/C catalyst; 300 mL reaction mixture;
9.6 mM NaOH; hydrogen atmosphere; 20 °C and atmospheric
pressure.
Figure 4. Dechlorination of PCE in different water–MeOH mixtures
under N2. All reactions were carried out using the following:
[PCE]0 ffi 1.2 mM; 25 mg 10% Pd/C catalyst; 300 mL reaction mixture;
9.6 mM NaOH; N2 atmosphere; 20 °C and atmospheric pressure.
our study, as the amount of water in the reaction solvent
was increased, the solubility of PCE decreased,15 and
thus its adsorption on the catalyst was enhanced. Since
the HDC process proceeds first with the adsorption of
PCE on the catalyst and then its eventual dechlorination
on the catalyst surface,3 better adsorption brought
about by the addition of water to MeOH favours the
dechlorination of PCE. Addition of water also brings
about better ionization of the added NaOH, enhancing
its ability to neutralize the HCl produced in the HDC
reaction.11,12 Moreover, addition of water promotes bet-
ter removal of adsorbed reaction products, especially
NaCl, from the catalyst surface.4,11,12
On the other hand, the dechlorination of TTCE ap-
peared to be a two-step reaction where it was immedi-
ately converted quantitatively to TCE which was then
dechlorinated to ethane with a rate constant of
7.4 minꢀ1 per gram Pd/C. Both dechlorination reactions
proceeded to completion in less than 30 min at rates fas-
ter than that of PCE.
In summary, we were able to improve our method of
dechlorinating PCE over Pd/C in MeOH at mild condi-
tions by adding up to 50% (v/v) of water to the reaction
solvent. The rate of PCE dechlorination was enhanced
as the amount of water in the reaction solvent was
increased from 0% to 50%, at which a 5-fold increase
in the rate was observed. Beyond 50% water, the rate
declined. Moreover, the selectivity of the reaction
improved as the formation of TCE was completely
eliminated in HDCs in mixtures with 50% water or
more. Application of the procedure for the dechlorina-
tion of TCE or TTCE produced remarkable results as
these compounds were dechlorinated even faster than
PCE.
On the other hand, addition of water results in a lower
solubility of H2 in the HDC mixture since the solubility
of H2 in water at the reaction temperature of 20 °C is
more than 10 times lower than in MeOH.16 A decrease
in the solubility of H2 in the reaction medium results
to a decrease in HDC rates.3,6 Thus, when the amount
of water was further increased to 80% and 100%, the
rate of PCE dechlorination eventually decreased.
It was also noted that addition of water had a profound
effect on the selectivity of the reaction. Figure 5 shows
that carrying out the HDC of PCE in 20% water results
to lower levels of less persistent TCE as compared to
that in pure MeOH. TCE was the only chlorinated inter-
mediate found in the HDC in MeOH but it was com-
pletely eliminated in mixtures with 50% water or more
(Fig. 2). Thus, for HDCs in the latter, the side reaction
in Scheme 2 disappears and PCE is directly dechlori-
nated to ethane. The coexistence of TCE or other chlo-
rinated intermediates with PCE is a concern since it can
cause the rate of PCE degradation to decline.2
Acknowledgements
The main author wishes to express his deepest gratitude
to the Ministry of Education, Culture, Sports, Science
and Technology of the Japanese Government for the
research scholarship provided.
References and notes
1. Environmental Agency of Japan, 2001. 2000 Report of the
General Survey of Groundwater Quality in Japan, Tokyo.
2. Nishijima, W.; Ochi, Y.; Tsai, T. Y.; Nakano, Y.; Okada,
M. Appl. Catal. B 2004, 51, 133–138.
3. Aramendia, M. A.; Borau, V.; Garcia, I. M.; Jimenez, C.;
Lafont, F.; Marinas, A.; Marinas, J. M.; Urbano, F. J. J.
Mol. Catal. A: Chem. 2002, 184, 237–245.
The HDC reactions of two other chlorinated organic
compounds, namely TCE and TTCE, were carried out
in 50% water–MeOH mixture over Pd/C at 20 °C and
atmospheric pressure. TCE was dechlorinated directly
to ethane with a rate constant of 8.14 minꢀ1 gꢀ1 Pd/C.

N. C. Concibido et al. / Tetrahedron Letters 46 (2005) 3613–3617
3617
4. Ukisu, Y.; Kameoka, S.; Miyadera, T. Appl. Catal. B
2000, 27, 97–104.
10. Hoke, J. B.; Gramiccioni, G. A.; Balko, E. N. Appl. Catal.
B 1992, 1, 285–296.
5. Nakano, Y.; Okawa, K.; Nishijima, W.; Okada, M. Water
Res. 2003, 37, 2595.
6. Roy, H. M.; Wai, C. M.; Yuan, T.; Kim, J. K.;
11. Xia, C.; Xu, J.; Wu, W.; Luo, Q.; Chen, J.; Zhang, Q.;
Liang, X. Appl. Catal. B 2003, 45, 281–292.
12. Xia, C.; Xu, J.; Wu, W.; Luo, Q.; Chen, J.; Zhang, Q.;
Liang, X. Catal. Commun. 2004, 5, 383–386.
13. Hattori, K.; Sajiki, K.; Hirota, K. Tetrahedron 2001, 57,
4817.
14. Dobruskin, V. K. Carbon 2000, 38, 1271–1277.
15. Ladaa, T. I.; Lee, C. M.; Coates, J. T.; Falta, R. W., Jr.
Chemosphere 2001, 44, 1137–1143.
Marshall, W. D. Appl. Catal.
143.
A 2004, 271, 137–
7. Ordon˜ez, S.; Sastre, H.; Diez, F. Appl. Catal. B 2000, 25,
49.
8. Sajiki, H.; Kume, A.; Hattori, K.; Hirota, K. Tetrahedron
Lett. 2002, 43, 7247–7250.
9. Sajiki, H.; Kume, A.; Hattori, K.; Hirota, K. Tetrahedron
Lett. 2002, 43, 7251–7254.
16. The Chemical Society of Japan. Kagaku Binran Kisohen,
Maruzen Publishing: Tokyo, 1993, II-156.