Hydrodechlorination of chlorophenols over Pd catalysts supported on zeolite Y, MCM-41 and graphene

Article abstract of DOI:10.1007/s11164-018-3387-z

Hydrodechlorination (HDC) reaction of chlorophenols was carried out using Pd catalysts supported over zeolite Y, MCM-41 or graphene. Pd-MCM-41 and Pd-Y zeolite were prepared by impregnation and ion-exchange method, respectively. Pd-graphene (Pd-G) was prepared by hydrazine hydrate reduction of palladium ion dispersed on graphene oxide. The catalysts were characterized by several analytical tools such as X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS). These catalysts were subjected to HDC reaction of chlorophenols, such as 4-chlorophenol (4-CP), 2,4-dichlorophenol (2,4-DCP), 2,6-dichlorophenol (2,6-DCP) and 3,4-dichlorophenol (3,4-DCP). The reaction rate of HDC of chlorophenols catalyzed by Pd catalysts with various solid bases, such as KF/Al₂O₃ (alumina), sodium acetate (NaOAc) and K₂CO₃ was compared. First, Pd-MCM-41 and Pd-Y catalysts were compared. 2,4- and 3,4-DCPs were completely decomposed within 6?h, in the case of Pd-MCM-41 with NaOAc. Using Pd-Y instead of Pd-MCM-41 with NaOAc, much faster decomposition was observed. Faster decomposition of 4-CP and DCPs was observed with NaOAc base than K₂CO₃ or KF/Al₂O₃ under the same condition. In the case of Pd-Y with KF/Al₂O₃, slower decomposition of 4-CP and DCPs was observed. These base effects were interpreted using the solubility of NaCl and KCl in alcohol and the basic sites of KF/Al₂O₃. Because the solubility of NaCl is known to be larger than KCl solubility in alcohol, byproduct NaCl could be easily dissolved and ionized in solvents. For Pd-Y with KF/Al₂O₃, the small pore size of Y zeolite can interfere with the diffusion of HCl to KF/Al₂O₃ basic site. Second, three catalysts, including Pd-graphene, were compared. 2,4-DCP was decomposed within 2?h using Pd-G with either K₂CO₃, NaOAc or KF/Al₂O₃. Pd-G catalyst showed the highest catalytic activity among Pd-G, Pd-MCM-41 and Pd-Y catalysts. The high activity and stability of the Pd-G could be attributed to the strong metal–support interaction with an electron-deficient site and a critical Pd particle size (ca. 3.5?nm) of Pd-G nanocatalyst with a stronger resistance to the deactivation and good affinity toward aromatic organic molecules, especially phenols. The progress of HDC reaction was monitored by gas chromatography with flame ionization detection (GC/FID), and a feasible degradation process could be explained by analyzing the degradation products such as phenol, cyclohexanone and cyclohexanol from resulting GC chromatograms. The effect of reaction temperature on HDC in Pd-G catalyst was also discussed. In conclusion, Pd-G is an efficient catalyst for decomposition of chlorophenols and can be applied to remediation of chlorophenol-contaminated water under mild conditions.

Full text of DOI:10.1007/s11164-018-3387-z

Res Chem Intermed
https://doi.org/10.1007/s11164-018-3387-z
Hydrodechlorination of chlorophenols over Pd catalysts
supported on zeolite Y, MCM-41 and graphene
Wonghil Chang1 Hojun Kim2 Jihyeon Oh2 Byoung Joon Ahn2
•
•
•
Received: 9 September 2016 / Accepted: 18 January 2017
Ó Springer Science+Business Media B.V., part of Springer Nature 2018
Abstract Hydrodechlorination (HDC) reaction of chlorophenols was carried out
using Pd catalysts supported over zeolite Y, MCM-41 or graphene. Pd-MCM-41 and
Pd-Y zeolite were prepared by impregnation and ion-exchange method, respec-
tively. Pd-graphene (Pd-G) was prepared by hydrazine hydrate reduction of palla-
dium ion dispersed on graphene oxide. The catalysts were characterized by several
analytical tools such as X-ray diffraction (XRD), scanning electron microscopy
(SEM), transmission electron microscopy (TEM) and energy-dispersive X-ray
spectroscopy (EDS). These catalysts were subjected to HDC reaction of
chlorophenols, such as 4-chlorophenol (4-CP), 2,4-dichlorophenol (2,4-DCP), 2,6-
dichlorophenol (2,6-DCP) and 3,4-dichlorophenol (3,4-DCP). The reaction rate of
HDC of chlorophenols catalyzed by Pd catalysts with various solid bases, such as
KF/Al2O3 (alumina), sodium acetate (NaOAc) and K2CO3 was compared. First, Pd-
MCM-41 and Pd-Y catalysts were compared. 2,4- and 3,4-DCPs were completely
decomposed within 6 h, in the case of Pd-MCM-41 with NaOAc. Using Pd-Y
instead of Pd-MCM-41 with NaOAc, much faster decomposition was observed.
Faster decomposition of 4-CP and DCPs was observed with NaOAc base than
K2CO3 or KF/Al2O3 under the same condition. In the case of Pd-Y with KF/Al2O3,
slower decomposition of 4-CP and DCPs was observed. These base effects were
interpreted using the solubility of NaCl and KCl in alcohol and the basic sites of KF/
Al2O3. Because the solubility of NaCl is known to be larger than KCl solubility in
alcohol, byproduct NaCl could be easily dissolved and ionized in solvents. For Pd-Y
&
&
Wonghil Chang
wchang@jj.ac.kr
Byoung Joon Ahn
ahnbj@jbnu.ac.kr
1
2
Department of Carbon and Nano-Materials Engineering, Jeonju University, Jeonju 55096,
Korea
Department of Chemistry Education, Chonbuk National University, Jeonju 54896, Korea
123

W. Chang et al.
with KF/Al2O3, the small pore size of Y zeolite can interfere with the diffusion of
HCl to KF/Al2O3 basic site. Second, three catalysts, including Pd-graphene, were
compared. 2,4-DCP was decomposed within 2 h using Pd-G with either K2CO3,
NaOAc or KF/Al2O3. Pd-G catalyst showed the highest catalytic activity among Pd-
G, Pd-MCM-41 and Pd-Y catalysts. The high activity and stability of the Pd-G
could be attributed to the strong metal–support interaction with an electron-deficient
site and a critical Pd particle size (ca. 3.5 nm) of Pd-G nanocatalyst with a stronger
resistance to the deactivation and good affinity toward aromatic organic molecules,
especially phenols. The progress of HDC reaction was monitored by gas chro-
matography with flame ionization detection (GC/FID), and a feasible degradation
process could be explained by analyzing the degradation products such as phenol,
cyclohexanone and cyclohexanol from resulting GC chromatograms. The effect of
reaction temperature on HDC in Pd-G catalyst was also discussed. In conclusion,
Pd-G is an efficient catalyst for decomposition of chlorophenols and can be applied
to remediation of chlorophenol-contaminated water under mild conditions.
Keywords Hydrodechlorination Á 4-Chlorophenol Á 2,4-Dichlorophenol Á 2,6-
Dichlorophenol Á 3,4-Dichlorophenol Á Pd-MCM-41 Á Pd-Y zeolite Á Pd-graphene
Introduction
Chlorophenols, which are carcinogenic, persistent and bio-accumulative organic
pollutants, have been detected in groundwater, surface water, air and soils as a result
of their improper disposal, leaching from landfills and the incineration of wastes.
Contamination of water and soil by chlorophenolic compounds now represents a
significant environmental burden [1]. Chlorophenols are comprised of 19 different
chlorinated phenols. Among the methods for degradation of chlorophenols, such as
thermal treatment, sonification, ozonation, electrochemical treatment, Fenton-type
oxidative degradation, advanced oxidation, bioremediation, phytodegradation,
reductive dechlorination by zero valence metals and bimetallics known as
dissolving metal reductions, adsorption on diversified materials, and catalytic
hydrodechlorination, hydrodechlorination (HDC) reaction is more environmentally
friendly than other methods. The traditional catalyst for HDC reaction is usually
platinum oxide (PtO2) or palladium on charcoal (Pd/C). Recently palladium- or
rhodium-supported catalysts have been developed in order to increase the catalytic
activity. Supports for metal catlayst include active carbon, silica, alumina, zeolite
and mesoporous materials [2]. HDC reaction of chlorophenols in aqueous solution
by palladium nanoparticles (NPs) supported on various supports has been reported
[3].
For HDC of chlorophenols, palladium catalysts supported on mesoporous
material MCM-41 and microporous Y zeolite were prepared and their catalytic
activity was investigated. MCM-41 is a promising catalyst support in liquid-phase
reactions because of its well-defined mesoporous structure in combination with a
˚
high surface area. Typical pore diameter of MCM-41 is ca. 35 A, which makes large
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Hydrodechlorination of chlorophenols over Pd catalysts…
organic molecules able to diffuse relatively easily to the catalytic active sites located
inside the pores. MCM-41 has been used in many applications such as production of
intermediates and fine chemicals, petrochemical reactions, optical applications,
development of sensors, and different industrial reactions, such as oxidation,
condensation and enzymatic reactions [4]. MCM-41 is better option than
microporous Y zeolites for some applications with diffusional problems, especially
when large reacting molecules are involved. Recently, graphene has been elucidated
as an interesting support material for palladium-catalyzed reactions. Since graphene
has large specific surface area, good conductivity, good affinity toward organic
molecules and low cost, it represents good characteristics as supports for metallic
NPs. Herein, Pd supported on graphene can be prepared by microflow H2 reduction,
hydrazine hydrate reduction or microwave irradiation method. In this study, the
catalytic activity of Pd-Y and Pd-graphene (Pd-G) were compared to that of Pd-
MCM-41 in order to provide an experimental condition in the view of green
chemistry [5].
Experimental
Materials and method
4-Chlorophenol (4-CP), 2,4-dichlorophenol (2,4-DCP), 2,6-dichlorophenol (2,6-
DCP) and 3,4-dichlorophenol (3,4-DCP). were purchased from Sigma-Aldrich and
used as received. High-purity graphite powder (99.99%), cetyltrimethylammonium
bromide and palladium acetate [Pd(OAc)2, 99.99%] were purchased from Sigma-
Aldrich and used as received. Scanning electron microscopy (SEM) measurements
were performed using an S-4700 field-emission scanning electron microscope
(Hitachi). Transmission electron microscopy (TEM) measurements were carried out
using a JEM-200FS (HT) field-emission transmission electron microscope (JEOL).
Inductively coupled plasma mass spectrometer (ICP-MS) data were obtained using
an Agilent 7500 A. And energy dispersive X-ray spectroscopy (EDS) data were
measured using an EDS 1600 (Shimadzu).
Preparation of Pd-MCM-41 and Pd-Y
The preparation and characterization of Pd-MCM-41 were described in a previous
report [6]. A mixture of cetyltrimethylammonium bromide and ammonium
hydroxide was stirred, and a 1 M NaOH solution in Ludox HS40 was added
slowly to the mixture. The mixture was stirred at room temperature, then an aqueous
sodium aluminate solution was added with vigorous stirring. The resulting mixture
was stirred for 1 h and heated in an oven at 100 °C for 48 h. Upon cooling to room
temperature, the pH of the mixture was adjusted to 11. This procedure was repeated
several times and then the precipitate was filtered, washed and dried in an oven at
100 °C for 24 h. The product was then calcined for 8 h at 550 °C. [Pd(NH3)4]2?
-
modified MCM-41 was prepared by impregnation using an aqueous solution of
[Pd(NH3)4]Cl2 in water or in acetonitrile and followed by hydrogen-flow reduction.
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W. Chang et al.
Pd-Y catalyst was also prepared by an ion-exchange method of zeolite Na-Y
followed by hydrogen-flow reduction. The catalysts were characterized using SEM,
TEM and EDS.
Preparation of Pd-G catalyst
Graphene oxide (GO) was obtained by the oxidation of high-purity graphite powder
with H2SO4/KMnO4 according to the method of Hummers and Offeman [7]. Briefly,
2.0 g of graphite was added to 50 mL of concentrated H2SO4 in an ice bath, and
1.0 g of NaNO3 and 7.0 g of KMnO4 were slowly poured under continuous
agitation. After the removal of suspension and warming, 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, after filtering and washing, the powder was dried at
room temperature under vacuum overnight. For the preparation of Pd-graphene (Pd-
G), 500 mg of the dried GO was added in 250 mL of deionized water, and 32 mg of
palladium acetate were then added and 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 Pd/G sheets were separated by using an Eppendorf 5804
centrifuge operated at 5000 rpm for 15 min and dried overnight under vacuum. Pd-
G was characterized using TEM, XRD and EDS.
General Procedure for hydrodechlorination of chlorophenols by Pd-MCM-
41, Pd-Y and Pd-G
For hydrodechlorination reaction, 10 mL of 100 ppm solution of 4-CP in
isopropanol was treated with Pd-MCM-41 or Pd-Y or Pd-G catalyst (50 mg), and
base (K2CO3 or KF/Al2O3 or NaOAc) under a hydrogen gas at ambient pressure.
The progress of chlorophenol decomposition was checked with gas chromatography
with flame ionization detection (GC/FID) and the degradation efficiency was
checked after 30 min or 1 h of reaction time, respectively.
Results and discussion
The morphological and chemical structure of the Pd catalysts were characterized by
XRD, SEM, ICP-MS, EDS and TEM. The morphologies of the catalysts were found
by SEM and TEM. The structure of the catalyst was determined by XRD and
reported in a previous paper [8].
Palladium contents in the catalysts were measured by EDS and ICP/MS. As
depicted in Fig. 1, many Pd NPs responded as black dots in the image of the Pd-
MCM-41 and Pd/G catalyst. It can be seen that Pd NPs were decorated on graphene
and MCM-41 uniformly with a mean diameter of 3.0 0.5 nm by sodium
borohydride reduction and 4.0 1.0 nm in MCM-41, respectively. Pd contents
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Hydrodechlorination of chlorophenols over Pd catalysts…
Fig. 1 EDS analysis of Pd content in Pd-Y (a), Pd-MCM-41 (b) and Pd-graphene (c), SEM image of Pd-
Y (d) and TEM image of Pd-MCM-41 (e) and Pd- graphene (f)
were measured by EDS as * 8 wt% (Pd-Y), 3.8 wt% (Pd-MCM-41) and 3.3 wt%
(Pd-G).
As a model reaction of HDC reaction, decomposition of 4-CP was attempted
using Pd-MCM-41 and Pd-Y zeolite catalysts with various solid bases, such as KF/
Al2O3, NaOAc and K2CO3. The disappearance of 4-CP from the solution containing
Pd catalysts may be due to HDC reactions and/or sorption to mesoporous material-
related surfaces. As shown in Fig. 2, whereas 4-CP was completely removed within
2 h with NaOAc, it was removed slowly within 10 h with KF/Al2O3 or K2CO3. In
the case of KF/Al2O3 base, Pd-MCM-41 catalyzed the HDC reaction much faster
than Pd-Y under the same condition. Then dichlorophenols (DCPs) were applied to
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W. Chang et al.
Fig. 2 Degradation efficiency of HDC reaction of 4-chlorophenol (100 ppm) in 10 mL of isopropanol
with 50 mg of Pd-MCM-41 (solid line) and Pd-Y (broken line) catalysts and 100 mg of K2CO3 (circle),
KF/Al2O3 (triangle) and NaOAc (square)
HDC reaction using Pd-MCM-41 and Pd-Y catalysts. The results of HDC reaction
of 2,4-DCP and 3,4-DCP catalyzed by Pd-MCM-41 and Pd-Y with various solid
bases, such as KF/Al2O3, NaOAc and K2CO3, are depicted in Fig. 3.
2,4-DCP and 3,4-DCP were completely decomposed within 6 h when the
reaction was catalyzed by Pd-MCM-41 with NaOAc. When the reaction was carried
out by Pd-Y instead of Pd-MCM-41 with NaOAc, much faster decomposition was
observed. However, complete decomposition of 3,4-DCP with KF/Al2O3 required
6 h. Faster decomposition of 4-CP and DCPs was observed with NaOAc base than
K2CO3 or KF/Al2O3 under the same condition. In order to eliminate HCl poison
produced during the reaction process, some bases, such as NaOAc, KF/Al2O3 and
K2CO3, are used in these experiments. Reaction byproduct NaCl is formed by the
reaction of HCl and NaOAc, whereas KCl is produced in KF/Al2O3 or K2CO3. The
solubility of NaCl is 1.375 g/100 g in methanol and 0.055 g/100 g in ethanol at
25 °C, whereas the solubility of KCl is 0.539 g/100 g in methanol and 0.034 g/
100 g in ethanol at 25 °C [9]. Because the solubility of NaCl is known to be larger
than KCl solubility in alcohol, NaCl could be easily dissolved and ionized in
solvents. However, KCl would deposit on the catalyst surface or block the pore of
catalysts. In the case of Pd-Y with KF/Al2O3, slower decomposition of 4-CP and
DCPs was observed. 2,4-DCP was completely decomposed in 6 h using Pd-MCM-
41 catalyst and was decomposed in 12 h using Pd-Y catalyst with KF/Al2O3. 2,4-
DCP was almost completely decomposed in 3 h when the reaction was catalyzed by
Pd-Y with NaOAc or K2CO3. It is known that K3AlF6 is formed by the reaction of
aqueous KF and alumina. So, the coordinately unsaturated F- ions seem to be
important as the basic sites on KF/Al2O3 [10]. It seems that the small pore of Y
zeolite can interfere with the diffusion of HCl to KF/Al2O3 basic site.
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Hydrodechlorination of chlorophenols over Pd catalysts…
Fig. 3 Degradation efficiency of HDC of 2,4-DCP (solid line) and 3,4-DCP (broken line) (100 ppm) in
10 mL of isopropanol with 50 mg of Pd-MCM-41 (top) and Pd-Y (bottom) catalysts and 100 mg of
K2CO3 (circle), KF/Al2O3 (triangle) and NaOAc (square)
Although Pd-MCM-41 and Pd-Y showed an efficient catalytic activity, a
decrease in the activity of the immobilized catalysts is frequently observed.
Therefore, an important goal of nanomaterials research may be the development of
heterogeneous Pd nanocatalysts that combine high activity, stability and recycla-
bility that is likely to have a considerable impact on the chemical and
pharmaceutical industries in the future. Only recently, graphene and graphene
oxide have been considered as potential support systems for palladium-catalyzed
coupling applications. Because of the higher specific surface area and thermal
stability of graphene when compared to graphene oxide, G-based nanocatalysts
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W. Chang et al.
Fig. 4 Degradation efficiency
of HDC reaction of 2,4-DCP
(100 ppm) in 10 ml of
isopropanol with 50 mg of Pd-Y
(circle), Pd-MCM-41 (square)
and Pd-G (triangle) catalysts and
100 mg of K2CO3 (top), KF/
Al2O3 (bottom) and NaOAc
(middle)
123

Hydrodechlorination of chlorophenols over Pd catalysts…
Table 1 Comparison of the degradation efficiency of the catalysts for 2,4-DCP
Entry
Base
1 h
2 h
Pd-G
Pd-MCM-41
Pd-Y
Pd-G
Pd-MCM-41
Pd-Y
1
2
3
K2CO3
100.0
87.9
75.1
0.0
29.0
3.7
5.39
70.0
7.0
100.0
100.0
100.0
9.8
63.2
18.3
44.5
92.3
28.6
NaOAc
KF/Al2O3
might be expected to exhibit superior activity compared to GO-based nanocatalysts.
For this reason, we chose to evaluate the HDC reaction activity of Pd-G
nanomaterials prepared by a microwave-assisted reduction procedure developed in
our group. In order to evaluate the catalytic activity of the Pd-G in HDC reaction,
the reaction was carried out under the same reaction condition and the results were
compared with that of Pd-MCM-41 and Pd-Y catalysts. 2,4-DCP was decomposed
using Pd-G, Pd-MCM-41 and Pd-Y catalyst with various solid bases, such as KF/
Al2O3, NaOAc and K2CO3. 2,4-DCP was removed within 2 h using Pd-G with
either K2CO3, NaOAc or KF/Al2O3. Pd-Y and Pd-MCM-41 catalyzed the HDC
reaction slower than Pd-G under the same condition. The results of HDC reaction of
2,4-DCP catalyzed by Pd-G, Pd-MCM-41 and Pd-Y with various solid bases are
depicted in Fig. 4.
The Pd-G catalyst showed an excellent catalytic activity for the HDC reaction of
2,4-DCP in 2 h of reaction time under the same condition (Table 1). For
comparison, Pd-G, Pd-MCM-41 and Pd-Y catalysts were prepared, and 100 ppm
chlorophenols in 10 mL of isopropanol solution were treated with 50 mg of the
catalysts using K2CO3 as a base. The degradation efficiency was checked after 1 and
2 h of the reaction time. It was found that Pd-G catalyst is more efficient for HDC
reaction than either Pd-MCM-41 or Pd-Y. The graphene-based Pd catalyst showed
highest catalytic activity among Pd-G, Pd-MCM-41 and Pd-Y, maybe because of
the high dispersity and stability of Pd NPs on the graphene surface. The catalytic
performance of Pd-G for HDC reaction of other chlorophenols was also
investigated.
As indicated in the GC chromatograms of Fig. 5, DCPs were converted directly
to phenol and then were changed into cyclohexanone, then to cyclohexanol. From
analyzing the results of the catalytic performance of Pd-G for the HDC reaction, we
could find that ortho- and para-substituted chlorine is removed faster than meta-
substituted chlorine to a hydroxyl group in HDC reaction. 4-CP, 2,4-DCP, 2,6-DCP
and 3,4-DCP (100 ppm each) in 10 mL of isopropanol, respectively, were
completely removed within 2 h using 50 mg of Pd-G with 100 mg of K2CO3 base.
Decomposed products identified with GC were phenol and cyclohexanone.
Although cyclohexanol is also proposed to be a decomposition product by other
research groups, cyclohexanol was detected in only trace amounts in our GC
conditions. The feasible degradation process of chlorophenols are depicted in Fig. 6.
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W. Chang et al.
Fig. 5 GC chromatogram of hydrodechlorination of 4-chlorophenol, 2,4-dichlorophenol, 2,6-
dichlorophenol and 3,4-dichlorophenol (100 ppm) in 10 mL of isopropanol with 50 mg of Pd-
graphene catalyst and 100 mg of K2CO3
123

Hydrodechlorination of chlorophenols over Pd catalysts…
Fig. 5 continued
Recently, catalytic HDC reaction of chlorophenols by Pd NPs supported on
graphene has been published by our group. Recovery and recycling of Pd-G catalyst
for HDC reaction was also reported [8]. The reusability of the Pd-G nanocatalyst
indicated that no obvious loss of activity with decreased reaction rate was detected
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W. Chang et al.
Fig. 6 Feasible degradation process of the chlorophenols
Table 2 Temperature effect of hydrodechlorination of 4-chlorophenol (100 ppm) in 10 mL of iso-
propanol with 50 mg of Pd-graphene and 100 mg of K2CO3
Temp. (°C) 1 h
2 h
3 h
Conv. (%) XaPhOH XbCone Conv. (%) XPhOH XCone Conv. (%) XPhOH XCone
0
3.6
11.4
9.2
–
–
–
–
–
–
0
0
21.6
88.9
61.4
56.2
73.7
100.0
18.9
0
38.6
42.0
54.4
0
45.6
100.0
85.4
25
35
45
55
65
43.8 100.0
26.3 100.0
–
74.9
14.6
26.1
0
10.7
49.1
56.7
–
63.3
0
100.0
73.9
100.0
100.0
100.0
100.0
81.1 100.0
100.0 100.0
100.0
100.0
0
aXPhOH: % selectivity of phenol
bXCone: % selectivity of cyclohexanone
for HDC reaction of 4-CP after Pd-G catalyst was reused six times. The high
activity and stability of the Pd-G could be attributed to the strong metal–support
interaction with an electron-deficient site and a critical Pd particle size (ca. 3.5 nm)
of Pd-G nanocatalyst with a stronger resistance to the deactivation [11].
For the industrial remediation of chlorophenol-contaminated water, reaction
temperature should be not be too high. Table 2 shows the temperature effect of
HDC reaction of 4-CP. At 25 °C, 4-CP was completely disappeared after 3 h. The
degradation efficiency of 4-CP was greater in higher temperatures, such as 55 and
65 °C, than in ambient temperature. However, ambient temperature is a better
choice for the removal of chlorophenols using Pd-G catalyst in the view of green
chemistry. In summary, it seems that Pd-G catalyst can be applied to the industrial
remediation of chlorophenol-contaminated water under mild conditions.
Conclusion
The HDC reaction of chlorophenols (such as 4-CP, 2,4-DCP, 3,4-DCP and 2,6-
DCP) was catalyzed by Pd-G, Pd-MCM-41 or Pd-Y zeolite. First, Pd-MCM-41 and
Pd-Y catalysts were compared. 2,4-DCP was completely decomposed within 6 h
when the reaction was catalyzed by Pd-MCM-41 with NaOAc. A slightly faster
decomposition of 2,4-DCP was observed when the reaction was carried out by Pd-Y
instead of Pd-MCM-41 with NaOAc, and complete decomposition required 6 h.
2,4-DCP was completely decomposed within 6 * 12 h when the reaction was
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Hydrodechlorination of chlorophenols over Pd catalysts…
catalyzed by Pd-MCM-41 or Pd-Y with KF/Al2O3. These base effects were
interpreted using the solubility of NaCl and KCl in alcohol and the basic sites of KF/
Al2O3. Because the solubility of NaCl is known to be larger than KCl solubility in
alcohol, NaCl is easily dissolved and ionized in solvent. However, KCl is deposited
on the catalyst surface or blocks the catalyst pores. Second, three catalysts,
including Pd-graphene, were compared. 2,4-DCP was decomposed within 2 h using
Pd-G with either K2CO3, NaOAc or KF/Al2O3. The graphene-based Pd catalyst
showed the highest catalytic activity among Pd-G, Pd-MCM-41 and Pd-Y. The high
activity and stability of the Pd-G could be attributed to the strong metal–support
interaction with an electron-deficient site and a critical Pd particle size (ca. 3.5 nm)
of Pd-G nanocatalyst with a stronger resistance to the deactivation and good affinity
toward aromatic organic molecules, especially phenols. The progress of HDC
reaction was monitored by a GC spectrometer. From GC chromatograms, a feasible
degradation process can be achieved by analyzing the degradation products, such as
phenol, cyclohexanone and cyclohexanol. The effect of reaction temperature on
HDC in Pd-G catalyst was also discussed. In conclusion, Pd-G is an efficient
catalyst for decomposition of chlorophenols and can be applied to the remediation
of chlorophenol-contaminated water.
Acknowledgements Financial support from Jeonju University Research Council and Chonbuk National
University is greatly acknowledged.
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