Catalytic hydrodechlorination reaction of chlorophenols by Pd nanoparticles supported on graphene

Article abstract of DOI:10.1007/s11164-015-2368-8

Chlorophenols are widely used as industrial chemicals such as herbicides, insecticides, wood preservatives, and disinfectants. However, chlorophenols are very toxic materials and they have become the cause of current environmental issues. Hydrodechlorination (HDC) reaction is a more environmentally friendly removal method of chlorophenols than other methods. In this paper, Pd-modified graphene was prepared and applied to HDC reaction. Pd supported on graphene (refer to Pd/G) was prepared using the recently reported microwave irradiation method. The Pd(II)/GO was made by impregnation methods of palladium precursors in solution phase and was subsequently reduced to Pd/G by microwave irradiation. The morphological and chemical structure of the Pd/G was characterized by XRD, SEM, ICP-MS, EDS, and TEM. It was found that the graphene-based Pd catalyst showed the highest catalytic performance among Pd/Y, Pd/MCM-41, and Pd/G catalysts. This is attributed to the smaller particle size and higher dispersions of Pd nanoparticles on the graphene surface. The catalytic HDC of chlorophenols was investigated. For HDC reaction, 100 ppm solution of chlorophenols such as 4-chlorophenol, six isomers of dichlorophenol, and 2,3,5-trichlorophenol in isopropanol was treated with catalyst and base, such as Na₂CO₃ and K₂CO₃, under a hydrogen gas at ambient pressure. The progress of the chlorophenol decomposition was analyzed with GC. All chlorophenols were completely decomposed within 2 h in the 3 % Pd/G catalyst. The reaction pathway of chlorophenols was elucidated from the conversion of chlorophenols and selectivities of products. The reuse of the catalyst was also studied. The performance of the recycled catalyst in HDC reaction up to six successive runs was observed.

Full text of DOI:10.1007/s11164-015-2368-8

Res Chem Intermed
DOI 10.1007/s11164-015-2368-8
Catalytic hydrodechlorination reaction
of chlorophenols by Pd nanoparticles supported
on graphene
Wonghil Chang1 Hojun Kim2 Ga Young Lee2
Byoung Joon Ahn2
•
•
•
Received: 3 July 2015 / Accepted: 17 November 2015
Ó Springer Science+Business Media Dordrecht 2015
Abstract Chlorophenols are widely used as industrial chemicals such as herbi-
cides, insecticides, wood preservatives, and disinfectants. However, chlorophenols
are very toxic materials and they have become the cause of current environmental
issues. Hydrodechlorination (HDC) reaction is a more environmentally friendly
removal method of chlorophenols than other methods. In this paper, Pd–modified
graphene was prepared and applied to HDC reaction. Pd supported on graphene
(refer to Pd/G) was prepared using the recently reported microwave irradiation
method. The Pd(II)/GO was made by impregnation methods of palladium precursors
in solution phase and was subsequently reduced to Pd/G by microwave irradiation.
The morphological and chemical structure of the Pd/G was characterized by XRD,
SEM, ICP-MS, EDS, and TEM. It was found that the graphene-based Pd catalyst
showed the highest catalytic performance among Pd/Y, Pd/MCM-41, and Pd/G
catalysts. This is attributed to the smaller particle size and higher dispersions of Pd
nanoparticles on the graphene surface. The catalytic HDC of chlorophenols was
investigated. For HDC reaction, 100 ppm solution of chlorophenols such as
4-chlorophenol, six isomers of dichlorophenol, and 2,3,5-trichlorophenol in iso-
propanol was treated with catalyst and base, such as Na2CO3 and K2CO3, under a
hydrogen gas at ambient pressure. The progress of the chlorophenol decomposition
was analyzed with GC. All chlorophenols were completely decomposed within 2 h
in the 3 % Pd/G catalyst. The reaction pathway of chlorophenols was elucidated
&
Byoung Joon Ahn
ahnbj@jbnu.ac.kr
Wonghil Chang
wchang@jj.ac.kr
1
2
Department of Nano and Advanced Materials Engineering, Jeonju University, Jeonju 560-759,
Korea
Department of Chemistry, Education Institute of Fusion Science, Chonbuk National University,
Jeonju 561-756, Korea
123

W. Chang et al.
from the conversion of chlorophenols and selectivities of products. The reuse of the
catalyst was also studied. The performance of the recycled catalyst in HDC reaction
up to six successive runs was observed.
Keywords Hydrodechlorination Á Chlorophenol Á Dichlorophenols Á
Trichlorophenol Á Pd–graphene
Introduction
Chlorophenols are usually used as the intermediates for organic synthesis, and are
extensively employed as pesticides, herbicides, disinfectants, and wood preserva-
tives. Chlorophenols and their derivatives are carcinogenic, persistent, and bio-
accumulative [1]. Chlorinated phenols have been detected in groundwater, surface
water, air, and soils as a result of their improper disposal, leaching from landfills,
and the incineration of chlorinated wastes. Contamination of water and soil by
chlorine–substituted phenolic compounds now represents a significant environmen-
tal burden due to their widespread industrial use, inherent toxicity, and persistence
in the environment [2, 3]. The toxic chlorophenols have become current
environmental concerns. The toxicity of these compounds is connected with the
chlorine content, and their biodegradability is strongly affected by chlorine
substitution in the molecules. The group of chlorophenols comprises of 19 different
chlorinated-phenol isomers, including pentachlorophenol, three tetrachlorophenols,
six trichlorophenols, six dichlorophenols, and three monochlorophenols. Most
chlorophenols are listed as priority pollutants by the U.S. EPA [4, 5]. Under certain
conditions, chlorophenols occurred in the aqueous environment can be transformed
to more toxic polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans.
Highly chlorinated aromatic compounds are recalcitrant towards disposal. Biolog-
ical treatment of low concentration of such materials in aqueous waste streams often
leaves the chlorinated compounds untreated or required several days. Therefore
chlorophenols, even at a trace level, need to be removed from water [6]. Several
methods have been developed for treating chlorophenol-contaminated media,
including thermal treatment, sonification, ozonation, electrochemical treatment,
Fenton-type oxidative degradation, advanced oxidation, bioremediation, phy-
todegradation, reductive dechlorination by zero valence metals and bimetallics
known as dissolving metal reductions, adsorption on diversified materials, and
catalytic hydrodechlorination (HDC) [7].
Hydrodechlorination (HDC) reaction by novel metals such as palladium and
rhodium is more environmentally friendly than other methods [1, 8–10]. Supports
used for palladium catalysts are carbon, silica, alumina, zeolite, and meso porous
materials. Although these supports allow efficient recycling, a decrease in the
activity of catalysts is frequently observed. Therefore, the development of
heterogeneous palladium nanoparticles that combine high activity, stability, and
recyclability is an important goal of nanomaterials research that is likely to have a
considerable impact on chemical and pharmaceutical industries in the future.
Recently, graphene has been considered as a potential support system for palladium-
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Catalytic hydrodechlorination reaction of chlorophenols…
catalyzed reactions [11]. Graphene has a one-atom-thick planar sheet of sp2-
hybridized carbon atoms and because of the excellent mechanical flexibility,
graphene is a very appealing matrix to encapsulate nanoparticles against their
physical and chemical instability during use. Since graphene has excellent electron
capture and transport properties, large specific surface area, good affinity toward
organic molecules, and high thermal, chemical, and mechanical stability, it
represents desirable characteristics as supports for metallic nanoparticles in efficient
and recyclable catalysts that allow for industrial applications within environmen-
tally benign processes [12].
In this study, palladium nanoparticles supported on graphene (refer to Pd/G) were
prepared by microflow H2 reduction [13], sodium borohydride (NaBH4) reduction,
or microwave irradiation method [14]. Microwave irradiation has been demon-
strated for the synthesis of a variety of nanoparticles including metals, metal oxides,
bimetallic alloys, and semiconductors with controlled size and shape without the
need for high temperature or high pressure. Unlike conventional thermal heating,
microwave irradiation allows better control of the extent of graphene oxide (GO)
reduction by NaBH4 or hydrazine hydrates as both the microwave irradiation power
and time can be adjusted to yield a nearly complete concurrent reduction of GO and
the palladium salt. These Pd/G catalysts were utilized in adsorption and HDC of
chlorophenols. The catalytic HDC of chlorophenols, such as 4-chlorophenol, six
isomers of dichlorophenol, and 2,3,5-trichlorophenol, by Pd/G were investigated.
The recyclability of the catalysts was also studied using 4-chlorophenol as a model
compound.
Experimental
Materials and methods
High-purity graphite powder (99.99 %) and palladium acetate [Pd(OAc)2, 99.99 %]
were purchased from Sigma-Aldrich and used as received. 4-Chlorophenol, 2,3-
dichlorophenol, 2,4-dichlorophenol, 2,5-dichlorophenol, 2,6-dichlorophenol, 3,4-
dichlorophenol, 3,5-dichlorophenol, and 2,3,5-trichlorophenol were purchased from
Sigma-Aldrich and used as received. Powder XRD analyses were performed on a
multi-purpose high-performance X-ray diffractometer D/max-2500 (Rigaku). The
diffraction data were recorded for 2h angles between 6.0° and 80°. SEM
measurements were performed using a S-4700 FE-SEM (Hitachi). TEM measure-
ments were performed using a JEM-200FS (HT) FE-TEM (JEOL). ICP-MS data
were obtained from Agilent 7500A and EDS data were measured using EDS 1600
(Shimadzu).
Preparation of Pd/G nanoparticles and other catalysts
Graphene oxide (GO) was prepared by the oxidation of high-purity graphite powder
with H2SO4/KMnO4 according to the method of Hummers and Offeman [15].
Briefly, graphite (2.0 g) was added to concentrated H2SO4 (50 ml) in an ice bath,
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W. Chang et al.
and NaNO3 (1.0 g) and KMnO4 (7.0 g) were slowly added under continuous
stirring. After 2 h, the suspension was removed from the ice bath and warmed to
35 °C. The whole batch was carefully poured into a 1-l flask in an ice bath, and
H2O2 (30 %) was added until gas was no longer detected. Then, the suspension was
filtered and washed with 0.1 M HCl solution and water. After repeated washing of
the resulting yellowish-brown cake with hot water, the powder was dried at room
temperature under vacuum overnight. For the preparation of palladium nanopar-
ticles supported on graphene (3 % Pd/G), 500 mg of the dried GO in 250 ml of
deionized water and then an added 32 mg of palladium acetate were sonicated until
a homogeneous yellowish brown dispersion was obtained. The solution was placed
inside a conventional microwave after adding 100 ll of the reducing agent
hydrazine hydrate. The microwave oven (Daewoo MW8119SB) was then operated
at full power (1000 W), 2.45 GHz, in 30-s cycles (on for 10 s, off and stirring for
20 s) for a total reaction time of 60 s. The yellowish brown solution of Pd(OAc)2–
GO changed to a black color, indicating the completion of the chemical reduction to
reduced-graphene. The Pd/G sheets were separated by using an Eppendorf 5804
centrifuge operated at 5000 rpm for 15 min and dried overnight under vacuum.
For comparison of the catalytic activity of Pd/G with other Pd-based catalysts,
Pd–MCM-41 and Pd–Y were prepared. The preparation and characterization of
these catalysts were described elsewhere [16]. Pd–MCM-41 was synthesized by
hydrogen-flow reduction of [Pd(NH3)4]2?-modified MCM-41, which was prepared
by impregnation method using an aqueous solution of [Pd(NH3)4]Cl2 in water. Pd–
Y was prepared by ion-exchange method of zeolite Y followed by hydrogen-flow
reduction. Palladium contents of these catalysts were determined by EPMA as
*3 wt%.
Reaction procedure for hydrodechlorination of chlorophenols
For HDC reaction, 10 ml of 100 ppm solution of 4-chlorophenol in isopropanol was
treated with catalyst (50 mg of 3 % Pd/G), base (K2CO3, 100 mg) under a hydrogen
gas at ambient pressure. The progress of the chlorophenol-decomposition was
checked with GC. In order to find the effect of base in the reaction, various solid
bases, such as Na2CO3, NaOAc, or KF/Al2O3, were added for HDC reaction instead
of K2CO3 [17].
Results and discussion
Recently, the synthesis of metal nanoparticles by the chemical reduction of metal
salts has been widely studied because of their applicability to catalysis. Among
them, palladium-catalyzed hydrogenation reactions and cross-coupling reactions
have drawn much attention in organic synthesis due to its industrial potential for the
synthesis of chemicals and their intermediates. Although homogeneous palladium
catalysts exhibit higher activity and selectivity for cross-coupling reactions, the uses
of homogeneous Pd catalysts are restricted in large-scale reactions due to the
contamination of products [18]. Tedious separation and recycling of the palladium
123

Catalytic hydrodechlorination reaction of chlorophenols…
catalyst are other major drawbacks of the homogeneous catalysts. In order to
address these problems, heterogeneous Pd catalysts have been employed in recent
years. Palladium has been supported on materials such as activated carbon,
polymers, zeolites, silica, molecular sieves, clays, carbon nanotubes, and graphene.
Among them, graphene has been considered as a potential support system for
palladium-catalyzed reactions [12]. Although graphene allows efficient recycling, a
decrease in the activity of catalysts is frequently observed. Therefore, the
development of heterogeneous palladium nanoparticles supported on graphene that
combine high activity, stability, and recyclability is an important goal of
nanomaterials research that is likely to have a considerable impact on the chemical
and pharmaceutical industries in the future. In this study, Pd nanoparticles supported
on graphene (Pd/G) were prepared using the recently reported microwave
irradiation method. The Pd(II)/GO was made by impregnation methods of palladium
precursors in solution phase and were subsequently reduced to Pd/G.
The morphological and chemical structure of the Pd/G was characterized by
XRD, SEM, ICP-MS, EDS, and TEM. The morphologies of the Pd/G were found by
SEM and TEM. The structure of the catalyst was determined by XRD. The broad
peak of 24.8° shows most of the GO was reduced into graphene. The 39.1° peak is
characteristic for Pd(111) and other peaks represent Pd(200) and Pd(220) plane.
Palladium content in the catalyst was measured by EDS and ICP. As depicted in
Fig. 1, a lot of palladium nanoparticles responded as black dots in the image of the
Pd/G catalyst. The average particle diameter of Pd NPs by hydrogen reduction was
Fig. 1 a XRD patterns of Pd/G, b EDS analysis of Pd content in Pd/G, c TEM image of Pd/G (H2 gas
flow reactor reduction), and d TEM image of Pd/G (NaBH4 reduction)
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W. Chang et al.
determined to be 2.0 0.5 nm where more than 50 nanoparticles were randomly
chosen and measured in an effort to achieve the desired statistical significance. The
diameter of Pd NPs by sodium borohydride reduction was determined to
3.0 0.5 nm, and the sizes of the catalysts were 5–8 lm indicated by SEM
images. Palladium content of 3 % Pd/G was measured by EDS (3.3 %) and ICP-MS
(3.2 %).
In order to test the degradation efficiency of the Pd/G catalyst in HDC reaction,
variable amounts of the catalyst were added in the HDC reaction. Thus, 10 ml of
100 ppm solution of 4-chlorophenol in isopropanol was treated with catalyst (50,
100, or 150 mg of 3 % Pd/G), base (K2CO3, 100 mg) under a hydrogen gas at
ambient pressure. The progress of the chlorophenol-decomposition was checked
with GC/FID. As shown in Table 1, with 50 mg of 3 % Pd/G, 41.4 % of
4-chlorophenol was decomposed within 1 h and 4-chlorophenol had completely
disappeared in 2 h. With 100 mg of 3 % Pd/G, 85.4 % of 4-chlorophenol was
decomposed within 1 h and 4-chlorophenol had completely disappeared in 2 h.
With 150 mg of 3 % Pd/G, 4-chlorophenol had completely disappeared within 1 h.
From these results, we decided that 50 mg of catalyst was sufficient for degradation
of 100 ppm chlorophenol solution. Next, in order to find the effect of base in the
HDC reaction, various solid bases, such as Na2CO3, NaOAc, or KF/Al2O3, were
tested for HDC reaction instead of K2CO3. As shown in Table 2, other bases
Na2CO3, NaOAc, and KF/Al2O3, were also effective in HDC reaction. It is found
that the degradation rate is faster with Na2CO3 than with K2CO3. Interestingly,
when NaOAc was employed as a base, no cyclohexanone conversion was detected.
It seems that chlorophenol substrate structures and base may change the mechanism
of HDC reaction. The catalytic performance of Pd/G for the HDC of 6
dichlorophenol isomers was also investigated and the results are shown in Table 3.
These Pd/G catalysts were also showed excellent catalytic activities for the HDC of
dichlorophenols with reaction time of 2 h in the same condition. It seems that
dechlorination rate was increasing with the increasing number of chlorine
substituents. The dechlorination reaction rate appeared to be increased in the order
of para- or ortho- [ meta-position to hydroxyl group because of the electronic and
steric hindrance of the substrate. The authors also prepared the palladium catalysts
Table 1 Hydrodechlorination of 4-chlorophenol (100 ppm) in 10 ml of isopropanol with various
amounts of 3 % Pd/G and 100 mg of K2CO3
Time (h) 50 mg
100 mg
150 mg
Conv. (%) XaPhOH XbCone Conv. (%) XPhOH XCone Conv. (%) XPhOH XCone
1
2
3
6
a
41.4
93.1
100
100
100.0
85.2
78.2
57.1
0.0
14.8
21.8
42.9
85.4
100.0
100.0
100.0
100.0
65.1
54.8
41.4
0.0
34.9
45.2
58.6
100.0
100.0
100.0
100.0
81.6
49.2
37.6
26.5
18.4
50.8
62.4
73.5
XPhOH: % selectivity of phenol
b
XCone: % selectivity of cyclohexanone
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Catalytic hydrodechlorination reaction of chlorophenols…
Table 2 Hydrodechlorination of 4-chlorophenols (100 ppm) in 10 ml of isopropanol with 50 mg of 3 %
Pd/G and 100 mg of base
Entry
Base
1 h
2 h
Conv. (%)
XaPhOH
XbCone
Conv. (%)
XPhOH
XCone
1
2
3
4
a
K2CO3
41.4
91.7
100.0
85.1
0.0
14.9
18.5
0.0
93.1
100.0
100.0
100.0
85.2
79.2
14.8
20.8
29.6
0.0
KF/Al2O3
Na2CO3
NaOAc
100.0
87.4
81.5
70.4
100.0
100.0
XPhOH: % selectivity of phenol
b
XCone: % selectivity of cyclohexanone
Table 3 Hydrodechlorination of dichlorophenols (100 ppm) in 10 ml of isopropanol with 50 mg of 3 %
Pd/G and 100 mg of K2CO3
Entry
Compound
1 h
2 h
Conv. (%)
XaPhOH
XbCone
Conv. (%)
XPhOH
XCone
1
2
3
4
5
6
7
a
4-Chlorophenol
41.4
100.0
100.0
59.2
100.0
82.4
0.0
17.6
0.0
93.1
100.0
100.0
100.0
100.0
100.0
90.9
85.2
59.1
24.4
72.0
61.5
56.3
67.7
14.8
40.9
75.6
28.0
38.5
43.7
32.3
2,3-Dichlorophenol
2,4-Dichlorophenol
2,5-Dichlorophenol
2,6-Dichlorophenol
3,4-Dichlorophenol
3,5-Dichlorophenol
100.0
100.0
100.0
100.0
100.0
0.0
100.0
82.0
0.0
0.0
49.8
0.0
XPhOH: % selectivity of phenol
b
XCone: % selectivity of cyclohexanone
Table 4 Comparison of degradation efficiency of the catalysts (Pd/G, Pd–Y, and Pd–MCM-41)
Entry
Compound
1 h
2 h
Pd/G
Pd–Y
Pd–MCM-41
Pd/G
Pd–Y
Pd–MCM-41
1
2
3
4
4-Chlorophenol
41.4
100
100
82.0
35.1
2.6
19.4
0
100
100
100
100
65.5
15.7
100
100
55.7
9.8
2,4-Dichlorophenol
2,6-Dichlorophenol
3,4-Dichlorophenol
42.1
56.6
15.3
4.0
28.7
8.9
HDC reaction condition: Chlorophenols (100 ppm) in 10 ml of isopropanol were treated with 50 mg of
catalysts using K2CO3 as base, and the degradation efficiency was checked after 1 and 2 h of reaction
time, respectively
supported on zeolite (Pd–Y) and mesoporous material (Pd–MCM-41) for compar-
ison of degradation efficiency of Pd/G catalyst with other Pd based catalysts. The
results of HDC reaction efficiency of the catalysts was listed in Table 4 [17]. For
comparison same palladium content (3 %) of Pd–Y and Pd–MCM-41 catalysts were
prepared, and chlorophenols (100 ppm) in 10 ml of isopropanol were treated with
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W. Chang et al.
50 mg of catalysts using K2CO3 as base, and the degradation efficiency was
checked after 1 and 2 h of reaction time, respectively. In the case of 4-chlorophenol,
41.4 % of 4-chlorophenol was decomposed within 1 h and 4-chlorophenol was
completely disappeared in 2 h with Pd/G catalyst. However, 65.5 % of 4-chlorophe-
nol was decomposed with Pd–Y and 55.7 % of 4-chlorophenol was decomposed
with Pd–MCM-41 in 2 h, respectively. Activity of Pd/G catalyst in decomposition
of dichlorophenols, such as 2,4-dichlorophenol, 2,6-dichlorophenol, and 3,4-
dichlorophenol, was also better than decomposition of dichlorophenols with Pd/Y
or Pd/MCM-41 under the same reaction conditions. The graphene-based Pd catalyst
showed the highest catalytic performance among Pd/Y, Pd/MCM-41, and Pd/G
catalysts. It seems that these results were due to high dispersity of palladium
nanoparticles on the graphene surface, the higher specific surface area, and good
affinity toward organic molecules especially aromatic compounds [17].
In order to find out the application of Pd/G catalyst to the environmentally
polluted chlorophenol samples, mixture of dichlorophenol isomers (100 ppm, 10 ml
in isopropanol) was decomposed under the same reaction conditions. As shown in
Fig. 2, 2,3-dichlorophenol, 2,4-dichlorophenol, 2,5-dichlorophenol, and 2,6-
dichlorophenol disappeared within 2 h but 3,4-dichlorophenol and 3,5-dichlorophe-
nol decomposed in 3 h because chlorine atoms in the meta position of phenol may
be reduced very slowly in an HDC reaction, and monochlorophenols were not
detected because produced monochlorophenols were simultaneously dechlorinated
to phenol, and finally phenol was converted to cyclohexanone. In order to compare
the Pd content of the catalyst in trichlorophenol HDC reaction, 1, 2, 3, 4, and 5 %
Pd/G were prepared. When trichlorophenol were tried to decompose with these
catalyst, 2,3,5-trichlorophenol was also degraded in 2 h with 2, 3, 4, or 5 % Pd/G.
However, when 1 % Pd/G was used as a catalyst, dechlorination reaction took much
Fig. 2 Hydrodechlorination reaction of six isomers of dichlorophenol (100 ppm) in 10 ml of isopropanol
with 50 mg of 3 % Pd/G. Dichlorophenols were completely decomposed within 3 h and phenol and
cyclohexanone were produced
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Catalytic hydrodechlorination reaction of chlorophenols…
Table 5 Hydrodechlorination
of 2,3,5-trichlorophenols
Catalyst
Time (h)
Conv. (%)
XaPhOH
XbCone
(100 ppm) in 10 ml of
isopropanol with 50 mg of Pd/G
catalyst and 100 mg of K2CO3
1 % Pd/G
1
2
3
5
1
2
3
5
1
2
3
5
1
2
3
5
1
2
3
5
45.3
49.9
0.0
0.0
0.0
100.0
100.0
100.0
71.0
53.7
0.0
66.1
0.0
2 % Pd/G
3 % Pd/G
4 % Pd/G
5 % Pd/G
54.8
29.0
21.0
0.0
94.1
79.0
100.0
100.0
70.0
100.0
100.0
52.0
0.0
48.0
3.5
100.0
100.0
100.0
79.6
96.5
0.0
100.0
100.0
60.5
0.0
39.5
2.2
98.4
97.8
100.0
100.0
92.9
0.0
100.0
100.0
57.3
0.0
42.7
0.0
a
XPhOH: % selectivity of
100.0
100.0
100.0
100.0
100.0
100.0
phenol
b
XCone: % selectivity of
cyclohexanone
0.0
0.0
Scheme 1 Feasible mechanism
of HDC reaction of
chlorophenols
time (Table 5). It is found that HDC reaction is finished through hydrogenolysis of
C–Cl bond and hydrogen-substitution resulting phenol, and further hydrogenation of
phenol yielding cyclohexanone [18]. This result was similar to reports of Dong
group [19]. The feasible mechanism of HDC reaction with Pd/G is shown in
Scheme 1.
Recovery and reuse of the catalyst and leaching of palladium nanoparticles in
reaction medium are important issues when HDC reactions are carried out under
heterogeneous conditions for industrial applications. Since it has been reported that
the catalytic activity of the Pd NPs can be gradually poisoned by HCl formed during
HDC reaction in spite of the addition of base, after the first cycle of the reaction the
recovered catalyst was washed with alcohol and dried thoroughly for recycling
study. It has been reported that the remaining oxygen functional groups of reduced
graphene oxide can avoid the agglomeration and leaching of the Pd nanoparticles
through interaction between the nanoparticles and the functional groups of graphene
sheets, guaranteeing the high activity of the Pd/G. The performance of the recycled
123

W. Chang et al.
100
80
60
40
20
0
2 hr
1 hr
1st
2nd
3rd
4th
5th
6th
Fig. 3 Degradation yield of the recycled catalyst Pd/G
Fig. 4 a TEM image of Pd/G catalyst after first run and b TEM image of Pd/G catalyst after sixth run
catalyst in HDC reaction up to six successive runs is shown in Fig. 3. The
reusability of the Pd/G nanocatalyst indicates that no obvious loss of activity with
decreased reaction rate was detected for 4-CP HDC after the catalyst was reused six
times without leaching of Pd but aggregation of Pd NPs was observed after six-
recycle of the catalyst (Fig. 4).
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Catalytic hydrodechlorination reaction of chlorophenols…
Conclusions
For HDC reaction of 4-chlorophenol, 10 ml of 100 ppm solution of 4-chlorophenol
in isopropanol was treated with catalyst (50 mg of 3 % Pd/G) and base (K2CO3,
100 mg) under hydrogen gas at ambient pressure. After 1 h, 41.4 % of
4-chlorophenol was decomposed and 4-chlorophenol completely disappeared in
2 h. It was found that all bases including K2CO3, KF/Al2O3, Na2CO3, and NaOAc
are applicable for this reaction. These Pd/G catalysts also showed excellent catalytic
activities for the HDC of dichlorophenols with the reaction time of 2 h in the same
conditions. It seems that the reaction rate for dechlorination increased in the order of
para- or ortho- [ meta-position to hydroxyl group because of the electronic and
steric hindrance of the substrate. Trichlorophenols were tried to decompose with
these catalysts, 2,3,5-trichlorophenol were also degraded in 2 h with 2, 3, 4, or 5 %
Pd/G. However, when 1 % Pd/G was used as a catalyst, dechlorination reaction took
a lot of time. Final product was detected to be cyclohexanone in all Pd/G catalysts.
The reusability of the Pd/G nanocatalyst indicates that no obvious loss of activity
with decreased reaction rate was detected for 4-chlorophenol HDC after the catalyst
was reused six times without leaching of Pd but aggregation of Pd nanoparticles was
observed after six-recycle of the catalyst.
Pd nanoparticles of 1, 2, 3, 4, and 5 % supported on graphene (Pd/G) were easily
prepared by H2 reduction or NaBH4 reduction. The HDC reaction of chlorophenols
was catalyzed by Pd/G with base additives such as K2CO3 or Na2CO3 at room
temperature. 4-Chlorophenol (100 ppm) was completely decomposed with Pd/G in
2 h. 2,3,5-TCP was also decomposed within 2 h. The reaction process of HDC was
monitored by GC/FID and can be explained feasible degradation process with GC
detection of degradation intermediates. It is found that Pd/graphene was an efficient
catalyst for remediation of chlorophenols and can be recycled several times without
decreasing activity.
Acknowledgments Financial support from Jeonju University Research Council and Chonbuk National
University was greatly acknowledged.
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