Effective hydrodechlorination of 4-chlorophenol over pd deposited on multiwall carbon nanotubes

Article abstract of DOI:10.1080/15533174.2013.801862

Multiwall carbon nanotubes (MWCNTs) supported Pd nanoparticles were prepared by impregnation method and characterized by TEM, XRD, XPS, and FT-IR. The as-prepared catalyst Pd/MWCNTs exhibited excellent catalytic property for the hydrodechlorination (HDC) of 4-chlorophenol (4-CP) in aqueous medium without any additives under mild conditions. The results indicated that the choice of support and the introduction of metal ions played a significant role in the catalytic property for HDC of 4-CP. The high catalytic performance of Pd/MWCNTs was attributed to the highly dispersed Pd nanoparticles on the support MWCNTs and the interaction between support and metal actives. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/15533174.2013.801862

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Effective Hydrodechlorination of 4-Chlorophenol Over
Pd Deposited on Multiwall Carbon Nanotubes
Hongying Denga, Guangyin Fana & Yinhu Wanga
a Chemical Synthesis and Pollution Control Key Laboratory of Sichuan Province, College of
Chemistry and Chemical Engineering, China West Normal University, Nanchong, P. R. China
Accepted author version posted online: 06 Jan 2014.Published online: 17 Apr 2014.
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To cite this article: Hongying Deng, Guangyin Fan & Yinhu Wang (2014) Effective Hydrodechlorination of 4-Chlorophenol Over
Pd Deposited on Multiwall Carbon Nanotubes, Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry,
44:9, 1306-1311, DOI: 10.1080/15533174.2013.801862
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Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 44:1306–1311, 2014
Copyright ꢀ Taylor & Francis Group, LLC
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ISSN: 1553-3174 print / 1553-3182 online
DOI: 10.1080/15533174.2013.801862
Effective Hydrodechlorination of 4-Chlorophenol Over
Pd Deposited on Multiwall Carbon Nanotubes
Hongying Deng, Guangyin Fan, and Yinhu Wang
Chemical Synthesis and Pollution Control Key Laboratory of Sichuan Province, College of Chemistry
and Chemical Engineering, China West Normal University, Nanchong, P. R. China
environmentally friendly and highly efficient route for decom-
posing these compounds under relatively mild conditions with-
out the formation of the toxic by-products. Various heteroge-
neous metal catalysts (Pd,[2–5] Pt,[6,7] Rh,[8–11] Ni[12–15]) have
been applied either in bulk or supported forms for the treatment
of wastewaters containing chlorophenols. Pd nanoparticles are
reported to be among the most promising ones because the
HDC reaction can be performed at ordinary temperature and
atmospheric pressure.[16]
Multiwall carbon nanotubes (MWCNTs) supported Pd
nanoparticles were prepared by impregnation method and char-
acterized by TEM, XRD, XPS, and FT-IR. The as-prepared cat-
alyst Pd/MWCNTs exhibited excellent catalytic property for the
hydrodechlorination (HDC) of 4-chlorophenol (4-CP) in aqueous
medium without any additives under mild conditions. The results
indicated that the choice of support and the introduction of metal
ions played a significant role in the catalytic property for HDC
of 4-CP. The high catalytic performance of Pd/MWCNTs was at-
tributed to the highly dispersed Pd nanoparticles on the support
MWCNTs and the interaction between support and metal actives.
Recent researches focused on the development of Pd cata-
lysts have demonstrated that the choice of support has significant
influence on the dispersion and electronic properties of the Pd
species,[3] which affects the reaction rates of the catalytic HDC
of chlorophenols. Among the commonly used supports, it has
been reported that the use of Al2O3 can improve the stability
of active species of the catalyst. However, this metal oxide sup-
port is more sensitive to HCl produced during the HDC process.
Furthermore, the support activated carbon (AC) also has been
extensively investigated for the HDC of chlorophenols because
of its high surface area and particularly physical and chemical
properties, but their applications should be carefully consid-
ered due to its strong adsorption properties based on surface
area, pore size distribution, and surface composition. Very re-
cently, the synthesis of new carbon materials or their composites
has been exploited for the removal of wastewaters containing
chlorophenols. For example, Jin et al.[2] reported the catalytic
HDC of 4-CP over mesoporous silica–carbon nano-composite
(MSCN) supported palladium catalyst, which attained 100%
removal efficiency at 258–313 K under ordinary hydrogen pres-
sure by using triethylamine (Et3N) as a base additive. The cat-
alytic activity of the Pd/MSCN catalyst was much higher than
that of the commercial active carbon supported Pd. The catalytic
HDC over Pd catalysts often requires the presence of base to
neutralize hydrochloric acid formed in the reaction in order to
prevent the deactivation of the catalyst. However, the addition
of additives needs extra work to segregate the additives from
the reaction mixture with the purpose of avoiding the secondary
pollution to the environment.
Keywords 4-chlorophenol, hydrochlorination, multiwall carbon nan-
otubes, palladium nanoparticles
INTRODUCTION
Chlorophenols are known as commercially important chem-
icals and are widely applied as intermediates or end products in
the production of pesticides, herbicide and disinfector.[1] The
residue of these compounds is harmful to the environment
and human beings since these chlorinated phenols are persis-
tent toxic pollutants and recalcitrant to chemical and biologi-
cal degradations. Therefore, removal of these compounds is an
emerging process. Great efforts have been put into detoxifying
the aqueous wastes containing chlorophenols through method-
ologies based on photochemical oxidation, thermal combustion,
biodegradation, and incineration, which generate various by-
products and pose many problems for the environment. The
treatment of these toxic pollutants facilitated by liquid phase
dechlorination has attracted investigative attention since this
process is judged as simple, safe, and effective. Especially,
the HDC of chlorophenols using noble metal as catalysts in-
volving molecular hydrogen as a hydrogen source, currently
receive a constant growth of interest. The catalytic HDC is an
Received 15 April 2013; accepted 29 April 2013.
Address correspondence to Guangyin Fan, Chemical Synthesis and
Pollution Control Key Laboratory of Sichuan Province, College of
Chemistry and Chemical Engineering, China West Normal University,
Nanchong, P. R. China. E-mail: fanguangyin@cwnu.edu.cn.
Carbon nanotubes, with high tensile strength, large sur-
face area, and high electric and thermal conductivity,[17] is
considered to be an attractive material for a wide range of
1306

EFFECTIVE HDC OF 4-CP OVER Pd/MWCNTs
1307
application especially as a catalyst support. Thus, the carbon
nanotubes supported metals have been widely investigated as
catalysts and have shown promising applications in cataly-
sis.[17–19] Compared to carbon black and porous carbons, car-
bon nanotubes supported catalyst has the advantages of excel-
lent resistance against corrosion and better contact between the
reactant and the catalytic component.[17] But there are few re-
searches concerning the employment of the carbon nanotubes
supported catalysts for the treatment of chlorinated organic com-
pounds. Recently, Song et al.[18] prepared NH3 modified carbon
nanofibers (CNFs) supported Pd catalyst for the catalytic HDC
of chlorobenzene. Although the Pd/CNFs catalyst showed high
FIG. 1. TEM morphology of the Pd/MWCNTs catalyst and the mapping
activity and stability, one limitation of the reaction system is that pattern of the Pd/MWCNTs.
the treatment procedure must be conducted in NH3 atmospheric
at 1123 K.
To date, it is still a big challenge over most of the pre-
(Agilent 7890A) with a FID detector and PEG-20M supelco
column (30 m×0.25 mm, 0.25 um film) and nitrogen was used
as a carrier gas.
vious heterogeneous catalysts in view of developing a highly
active and recyclable system for the HDC of chlorophenols un-
der mild conditions without any additives. Herein we prepared
Pd/MWCNTs catalyst by an impregnation method using PdCl2
as precursor and MWCNTs as support. The as -prepared cata-
lyst Pd/MWCNTs was applied for the HDC of 4-CP under mild
conditions without any promoters. Effects of supports, organic
solvents and metal ions were also investigated. The catalyst
Pd/MWCNTs showed much higher removal efficiency than the
Pd/C catalyst and can be recycled six times without loss of any
activity.
Preparation of Pd/MWCNTs
Typically, 3.5 mg/mL PdCl2 (10 mL) aqueous solution was
placed into a 50 mL round-bottom flask and 500 mg MWCNTs
was also added into the flask. The aqueous solution of NaBH4
(75 mg, 5 mL) was slowly dropped into the solution under
vigorous stirring. After stirring for 3 h, the product was washed
with distilled water several times and dried under vacuum at
323 K for 12 h. The Pd loading was 1.0 wt% measured by ICP.
Activity Test
The HDC reaction was carried out in a 25 mL round-bottom
flask. A typical procedure for the HDC of 4-CP is as follow-
ing: The desired amounts of catalyst (5 mg), 4-CP (2.5 g/L),
and solvent (5.0 mL) were charged to the flask equipped with a
balloon. The reactor was vacuumed and flushed with pure hy-
drogen. When the designated reaction temperature was reached,
the stirring rate was adjusted to 1200 rpm, and reaction time was
accounted. Many experimental data were obtained by repeating
the reaction two or three times and they had the good repeata-
bility.
EXPERIMENTAL
Materials
Palladium chloride (≥59.5%) was purchased from the Kun-
ming Institute of Precious Metals, China. MWCNTs (500 m2/g)
were purchased from the Chengdu Institute of Organic Chem-
istry, Chinese Academy of Sciences. Ethylene glycol (EG, 99%)
and sodium borohydride (NaBH4) were purchased from Alfa
Aesar and used as received. Other solvents were analytical grade
and used as received.
Instruments
RESULTS AND DISCUSSION
Transmission electron microscopy (TEM) measurements
were carried out on a JEOL model 2010 instrument operated
at an accelerating voltage of 200 kV. X-ray diffraction (XRD)
Characterization of Pd/MWCNTs
Transmission electron microscopy (TEM) experiments have
patterns were recorded on a Rigaku X-ray diffractometerD/max- been carried out to detect a possible structure of the Pd parti-
2200/PC equipped with Cu Kα radiation (40 kV, 20 mA) over cles (Figure 1). These images show that the Pd nanoparticles
the range of 10–90◦X-ray photoelectron spectroscopy (XPS, dispersed well on the support MWCNTs with a mean par-
Kratos XSAM800) spectra was obtained by using Al Ka ra- ticle size of 2.1 nm. The compositional analysis carried out
diation (12 kV and 15 mA) as an excitation source (hv = by energy-dispersive X-ray spectroscopy (EDS) measurements
1486.6 eV) and Au (BE Au4f = 84.0 eV) and Ag (BE Ag3d = demonstrates that the presence of Pd element and the Pd loading
386.3 eV) as reference. All binding energy (BE) values were is about 1.1 wt%, which agrees well with the ICP result.
referenced to C1s peak of contaminant carbon at 284.6 eV.
The crystal structure of the purified MWCNTs and
A Fourier transform infrared spectrum was recorded with a Pd/MWCNTs were characterized by powder X-ray diffraction
Nicolet 6700 (resolution 0.4 cm−1) infrared spectrometer. All (XRD) and the results were shown in Figure 2. It can be seen
hydrogenation samples were analyzed by gas chromatography that the diffraction peak at 2θ = 25.8◦ is estimated to be the

1308
H. DENG ET AL.
δ+
Pd03d3/2
Pd
δ+
3d5/2
Pd
3d3/2
Pd03d5/2
Pd(111)
(b)
(a)
350 348 346 344 342 340 338 336 334 332 330
10
20
30
40
50
60
70
80
90
2
theta (degrees)
Binding Energy(eV)
FIG. 2. XRD patterns of the MWCNT (a) and Pd/MWCNT (b) catalysts.
FIG. 3. XPS pattern of the Pd/MWCNT catalysts.
(002) plane of graphite (Figure 2). The characteristic peaks
of concentrated nitric acid treated MWCNTs show no differ-
ence compared with the pristine MWCNTs. The diffraction
peak at 2θ = 40.1◦ is attributed to the (111) plane of the Pd
(Figure 2).
has been proved to be an effective way to evaluate the removal
efficiency of noble metal nanocatalysts. The HDC of 4-CP was
negligible without catalyst or in the presence of pure MWCNTs
at the same conditions, which shows that the presence of palla-
dium is indispensable for high catalytic activity. Generally, the
HDC of 4-CP results in the production of three products, that
are phenol, cyclohexanol, and cyclohexanone.[2] Compared with
the reactant 4-CP, the three products detected are lowly toxic
and useful as intermediates in production of high value-added
chemicals. However, only phenol was detected as product in the
present system.
The progression of the reductive HDC process at various
temperatures is shown in Figure 5. As expected, the HDC rates
of the reactions were clearly increased with the elevation of the
reaction temperature. The reaction time for complete conversion
of 4-CP was 30 min and 40 min at the temperature of 293 K
and 298 K in water under atmospheric pressure, respectively;
The electronic state of the Pd species on the MWCNTs was
measured with the XPS. The pattern of Pd/MWCNTs catalyst
is illustrated in Figure 3. The Pd3d peaks in XPS spectra show
that besides the binding energy values at 336.0 eV and 341.1 eV,
which are attributed to the 3d5/2 and 3d3/2 peaks of metallic pal-
ladium, the binding energy values at 337.5 eV and 342.7 eV are
assigned to the 3d5/2 and 3d3/2 peaks of palladium oxide, respec-
tively. The presence of palladium oxide should be owing to the
oxidation of air to nano-palladium particles and the incomplete
reduction of palladium oxide. Pd(0) was the active species for
the reaction because the catalyst was not active to catalyze the
HDC prior to reduction and the palladium oxide also can be
reduced during the HDC process.
Fourier transforms infrared (FTIR) spectra of MWCNTs
and Pd/MWCNTs were also measured. Figure 4 shows the IR
spectrum of MWCNTs, the characteristic absorption peaks at
3425 cm−1 and 1383 cm−1 are assigned to the O-H stretching
mode; the peaks at 1584 cm−1 and 1046 cm−1 are ascribed to the
C-O stretching modes, respectively, indicating that the surface of
MWCNTs was decorated with carboxyl groups, thus providing
some reactive and anchoring sites for nucleation and growth of
Pd particles. An almost identical IR spectrum of Pd/MWCNTs
was observed after immobilization of palladium on the sur-
face of MWCNTs (Figure 4), but the characteristic peaks of
MWCNTs in Pd/MWCNTs are shifted to higher wavenumbers
(1584–1630 cm−1 and 3425–3438 cm−1), which are attributed
to the interaction between Pd and MWCNTs.[20]
(a)
2925.88
1383.48
1584.11
1046.72
3425.79
(b)
2923.68
1389.97
1046.72
1630.93
MWCNT
Pd/MWCNTs
3438.56
3500 3000 2500 2000 1500 1000 500
σ/cm-1
Optimization of Reaction Conditions
The catalytic properties of the Pd/MWCNTs catalysts were
investigated using the HDC of 4-CP as a model reaction, which
FIG. 4. FTIR morphology of (a) MWCNT and (b) Pd/MWCNT catalysts.

EFFECTIVE HDC OF 4-CP OVER Pd/MWCNTs
1309
Pd/MWCNTs
100
80
60
40
20
0
100
80
60
40
20
0
Pd/Al2O3
Pd/C
Pd/Fe3O4
Pd/TiO2
20 oC
25 oC
30 oC
0
5
10 15 20 25 30 35 40 45 50
Reaction time(min)
0
5
10
Reaction time(min)
FIG. 7. Effect of supports on 4-CP hydrodechlorination.
15
20
25
30
FIG. 5. Effect of reaction time on 4-CP hydrodechlorination.
when the reaction temperature was increased to 303 K, the 4-
CP was completely transformed to phenol and cyclohexanone
within 25 min. The results indicate that higher temperature fa-
complete removal of 4-CP increased with increasing initial 4-CP
concentrations, the absolute removal amount increased.
cilitated the breaking down of the Cl–C bond to accelerate the Effect of Supports
reaction.
The effect of initial concentration of 4-CP removal efficiency
It is reported that the choice of the solid support has a sig-
nificant influence on the catalytic activity and selectivity. We
also compared the activity of Pd/MWCNTs with those of Pd
deposited on Al2O3, C, TiO2 and Fe3O4 prepared by a similar
method and the results were illustrated in Figure 7. The results
reveal that the Pd/MWCNTs catalyst possess very high activity
in aqueous medium, approaching almost 100% conversion of
4-CP over a period of 20 min, whereas the Pd/TiO2 catalyst
exhibits lower activity in terms of about 28% conversion within
a period of 30 min. The Al2O3, TiO2, and Fe3O4 supported Pd
catalysts show a moderate activity. In this region of study, the
activity of the catalysts varied in the order: Pd/MWCNTs >
Pd/C > Pd/Fe3O4 > Pd/Al2O3 > Pd/TiO2.
was investigated in the range from 2.0 g/L to 3.5 g/L over
the Pd/MWCNTs catalyst. The results presented in Figure 6
show that the removal percentages of 4-CP reached 94.3%,
89.1%, 74.6%, and 67.4% each after 10 min, and then reached
100%, 100%, 96.6%, and90.5%, respectively, after 20minofthe
reaction within the investigated range. Moreover, the reaction
time for achieving the complete removal efficiency increased
with the increase of the initial concentration. For 2.0 g/L 4-
CP, it was completely hydrodechlorinated within 15 min, while
it needed 40 min when the concentration of 4-CP increased
to 3.5 g/L. The time for complete hydrodechloriation of 4-CP
were 15, 20, 30, and 40 min, respectively. Though the time for
Effect of Different Solvents
The effects of different solvents on 4-CP removal efficiency
are presented in Figure 8. As shown in Figure 8, the removal per-
centages of 4-CP was 100% within 15 min when using water as
the reaction medium. In contrast, the addition of organic solvents
both protic solvent such as methanol, ethanol, and n-propanol,
and aprotic solvent such as cyclohexane and 1,4-dioxane re-
sulted in decrease of removal efficiency under identical reaction
conditions. As can be seen in Figure 8, when using the polar
solvents methanol, ethanol, and n-propanol as the solvents, the
removal percentages decreased to 49.6%, 37.5%, 27.8%, and
27.2% within 60 min, respectively. In contrast, when using non-
polar cyclohexane and 1,4-dioxane as solvents, the conversion
of 4-CP was 24.3% in cyclohexane, and the lowest removal
efficiency of 3.6% was obtained in 1,4-dioxane with the same
reaction time. Compared to the HDC process in aqueous solu-
tion, the Pd/MWCNTs catalyst exhibitedrelativelylowactivities
in the investigated organic solvents which was ascribed to the
100
80
60
2.0 g/L
40
2.5 g/L
3.0 g/L
20
0
3.5 g/L
0
5
10
15
20
25
30
Reaction time(min)
FIG. 6. Effect of initial concentration on 4-CP hydrodechlorination.

1310
H. DENG ET AL.
100
80
60
40
20
0
water
methanol
ethanol
n-propanol
cyclohexane
1, 4-dioxane
100
80
60
40
20
0
CuCl
2
H O
2
(a)
ZnCl
2
NiCl
2
FeCl
3
NH Cl
4
0
5
10
15
20
25
30
0
10
20
30
40
50
60
Reaction time(min)
Time(min)
FIG. 8. Effect of solvent on 4-CP hydrodechlorination.
FIG. 9. Effect of metal ions on 4-CP hydrodechlorination.
Stability of the Catalyst
probably adsorption of organic solvents on the surface of the
catalysts and blocking some of active sites.
The reusability of the Pd/MWCNTs catalyst was tested us-
ing 4-CP as a substrate at the same reaction conditions. After
completion of the reaction, the catalyst was separated from the
reaction system by centrifugation, and the supernatant liquid
was removed. The recovered catalyst was thoroughly washed
for three times with ethanol and dried in vacuum at room tem-
perature for next use. Reusing the Pd/MWCNTs catalyst in six
consecutive cycles showed that the catalytic activity remained
without a significant loss of removal efficiency (Figure 10).
Moreover, Metal leaching measurement investigated by ICP
analysis of the catalyst indicates that the Pd leaching was negli-
gible. The composition of the Pd/MWCNTs catalyst remained
virtually unchanged as the fresh catalyst, which are attributed
to the high recyclability of the catalyst.
Effects of Metal Ion in Aqueous Solutions
The influence of metal ions on the catalytic HDC of 4-CP
in aqueous media also was investigated by introducing a se-
ries of metal ions into the reaction mixture. Figure 9 shows
the effects of some metal ions on the HDC of 4-CP in water.
It was clearly indicated that the presences of metal ions such
as Cu2+, Ni2+, Fe3+, and Zn2+ were disadvantageous to the
HDC of 4-CP. Only about 39.5% and 18.9% 4-CP were hy-
drogenated after 5 min when FeCl3 or ZnCl2 (0.6 mmol) was
added into the reaction mixture; however 65.6% 4-CP was con-
verted with the same reaction time in the absence of metal ions.
Compared with FeCl3 and ZnCl2, the effects of NiCl2 or NH4Cl
were weak, but the complete reactions still needed about 25 min,
25 min, and 30 min, respectively, while the complete conversion
in water only needed 20 min. However, we found that with the
addition of the same amount of CuCl2 into water, the consider-
ably remarkable deactivation of the catalyst was observed within
30 min. Our results showed that the addition of metal ions into
the reaction system suppressed the activity of the catalyst and
led to the decrease of the HDC rate. This phenomenon was
possibly attributed to the adsorption of metal ions on the active
sites of Pd/MWCNTs to lead to the deactivation of the catalyst
as reported by Xia et al.[21] The inhabitation action of heavy
metal ions, such as Fe3+, Ni2+, Cu2+, and Zn2+, was assigned
to a metallic ions connecting with their orbitals and d-electrons,
their d-electrons take part in the intermetallic bonds between
the toxic metal and the catalyst and resulted in highly toxicity to
the Pd catalyst. However, the Pd catalyst is not poisoned if the
metal ions, such as Na+, have no d-electron to form the bond
between the metal and the catalyst.[21]
100
80
60
40
20
0
1
2
3
4
5
6
Recycle times
FIG. 10. Stability of the Pd/MWCNTs.

EFFECTIVE HDC OF 4-CP OVER Pd/MWCNTs
1311
CONCLUSION
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ions into the mixture led to the deactivation of the catalyst to
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FUNDING
This work was financially supported by the National Natural
Science Foundation of China (21207109), Scientific Research
Fund of Sichuan Provincial Education Department (11ZA034), 15. Feng, J.-T.; Lin, Y.-J.; Evans, D.G.; Duan, X.; Li, D.-Q. J. Catal. 2009, 266,
351–358.
and the Opening Project of Key Laboratory of Green Catalysis
of Sichuan Institutes of High Education (No. LZJ1205).
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