Hydrodechlorination of chlorinated hydrocarbons over metal–carbon
composite catalysts prepared by a modified carbothermal reduction method
N. Lingaiah, Md. A. Uddin, A. Muto and Yusaku Sakata*
Department of Applied Chemistry, Faculty of Engineering, Okayama University, Okayama, 700-8530, Japan.
E-mail: yssakata@cc.okayama-u.ac.jp
Received (in Cambridge, UK) 25th May 1999, Accepted 19th July 1999
A highly stable and active Pd–Fe carbon composite catalyst
system for hydrodechlorination of chlrohydrocarbons is
obtained by a novel modified carbothermal reduction
method using ion exchange resins.
The XRD pattern of monometallic Pd/C shows peaks arising
from metallic Pd. In bimetallic catalysts, the addition of more
iron leads to the formation of Fe
catalyst, most of the iron exists as Fe
3
C. In the monometallic Fe/C
C.
3
The HDC of chlorobenzene over mono- (Pd, Fe) and
bimetallic (Pd–Fe/C) catalysts resulted in the formation of
benzene as the only organic product. Percentage conversion
values obtained after the catalysts reached steady state condi-
tions at a reaction temperature of 150 °C are presented in Fig. 1.
Note that the monometallic Pd/C catalyst shows very low
activity at steady state conditions. The Fe/C catalyst does not
show any activity in this reaction. In the bimetallic Pd–Fe/C
catalysts the addition of Fe to Pd leads to a substantial increase
in the activity, even though Fe is inactive for this reaction. The
increased activity of the bimetallic catalysts relative to the
monometallic catalysts is in contrast to earlier findings, where a
decrease in activity in the cases of bimetallic Pd–Sn, Pd–Rh and
Pd–Fe catalysts due to the second metal diluting the catalyst
surface was reported.6
Chlorinated hydrocarbons are one of the most widespread and
persistent toxic pollutants. The disposal of these organic wastes
is a vital environmental issue. Among the several methods
proposed for their destruction, catalytic hydrodechlorination
(HDC) is of increasing interest because it excludes the
formation of more toxic compounds such as dioxins and has a
comparatively low reaction temperature.1 Noble metals are
the catalysts of choice for this reaction, which is carried out in
either the liquid or the gas phase. Chlorobenzene is usually
studied as a model compound for the HDC reaction since it
represents the halogen species found in many organic wastes.
Deactivation of the catalyst by HCl produced during the
–3
4
reaction is a commonly encountered problem. The develop-
,9
ment of highly stable and active catalysts for this reaction
remains as a challenging task. Recently, more attention has been
focused on bimetallic catalysts over monometallic catalysts in
Preparation by the CTR method may result in a different
morphology of the catalyst surface. SEM–EDX images of Pd
and Fe for a cross-sectional view of a cylindrical granule of the
bimetallic 2Pd–2Fe/C catalyst are shown in Fig. 2. It can be seen
from Fig. 2(a) that the majority of the Pd is present on the
surface whereas most of the iron [Fig. 2(b)] is dispersed inside
the catalyst particle. During the CTR the Pd might be
5
order to improve longevity and activity. However, it has been
reported that the use of bimetallic catalysts instead of mono-
metalic ones can result in a decrease of HDC activity.6
In this work we report for first time the preparation of Pd and
Pd–Fe carbon composite catalysts by a novel modified
carbothermal reduction (CTR) method, and their activity in
chlorobenzene HDC. We have been developing a modified
carbothermal reduction (CTR) method to prepare highly
dispersed and thermally stable metal or metal compound-based
carbon composite catalysts.7 In the CTR process, first an
organic ion exchange resin is exchanged with the required metal
ions, followed by thermal treatment in the temperature range
Table 1 Physical characteristics of the mono- and bimetallic Pd–Fe carbon
composite catalysts
Fe
Pd
Catalyst content/ content/
S
g
BET/m2
p
V /ml
code
g gcat21
g gcat21
21
g
cat21
XRD Peak
5
00–800 °C. Since the metallic ions adsorbed on the resin are
2
4
2
2
2
Pd
Pd
0
0
0.069
0.11
0.068
0.06
0.054
0
140
209
331
333
370
200
0.071
0.149
0.331
0.334
0.377
0.327
Fe, Fe
Pd
Pd
Pd
Pd, Fe, Fe
3
C
highly dispersed, the catalysts prepared by this method are
highly dispersed.
Pd2Fe 0.036
Pd4Fe 0.079
Pd8Fe 0.111
In the preparation of the catalysts, first the required amounts
2 3 3
of aqueous solutions of Pd and Fe (PdCl and Fe(NO )
3
C
3
precursors used respectively) were added to a commercial
chelate type of ion exchange resin (spherical granules) under
constant stirring with a magnetic rod. The stirring was
continued for about six hours then the solution was filtered and
the metal exchanged resin dried at room temperature for about
2Fe
0.29
Pd, Fe, Fe C
1
2 h. The dried samples were then subjected to carbothermal
2
1
2
reduction at 800 °C in an N flow (300 ml min ) for 3 h. After
CTR the catalysts retained their spherical shape but were
reduced in size.
The catalyst tests were performed in a fixed bed microreactor
at atmospheric pressure as described elsewhere.8
The physical characteristics of the catalysts along with their
compositions are shown in Table 1, which shows that the
surface areas and pore volumes are increased in the bimetallic
catalysts in comparison with the corresponding monometallic
catalysts. In monometallic catalysts pore blocking due to
segregation during the CTR process might be the cause of the
decrease in these values. The presence of Fe in the bimetallic
catalysts may result in a reduction of the Pd segregation.
Fig. 1 Hydrodechlorination of chlorobenzene on monometallic Pd and Fe
and bimetallic Pd–Fe carbon composite catalysts (reaction temperature:
2
1
150 °C; space velocity: 7680 h ).
Chem. Commun., 1999, 1657–1658
1657
Lingaiah
