Effective hydrodechlorination of 4-chlorophenol catalysed by magnetic palladium/reduced graphene oxide under mild conditions

Article abstract of DOI:10.1039/c4ra01799d

Magnetic palladium/reduced graphene oxide nanocomposites (Pd/rGO-Fe ₃O₄) were prepared by depositing Pd nanoparticles on an rGO-Fe₃O₄ magnetic support. The as-prepared nanocomposites were investigated as catalysts for the hydrodechlorination of 4-chlorophenol in the aqueous phase. The complete conversion of 4-chlorophenol (with a concentration as high as 2.5 g L^-1) to phenol was obtained in a reaction time of 40 min at room temperature and balloon hydrogen pressure without any additives. The excellent catalytic activity of the Pd/rGO-Fe ₃O₄ can be attributed to the small particle size of Pd, and an electron-deficient state of Pd in the catalyst as a result of the strong interaction between the active sites and the oxygen of the oxygen-containing groups on the rGO-Fe₃O₄ support.

Full text of DOI:10.1039/c4ra01799d

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PAPER
Effective hydrodechlorination of 4-chlorophenol
catalysed by magnetic palladium/reduced
graphene oxide under mild conditions
Cite this: RSC Adv., 2014, 4, 25440
a
a
b
b
b
Yanlin Ren, Guangyin Fan,* Weidong Jiang, Bin Xu and Fuan Liu
Magnetic palladium/reduced graphene oxide nanocomposites (Pd/rGO-Fe
depositing Pd nanoparticles on an rGO-Fe magnetic support. The as-prepared nanocomposites were
investigated as catalysts for the hydrodechlorination of 4-chlorophenol in the aqueous phase. The
3 4
O ) were prepared by
3 4
O
ꢀ1
complete conversion of 4-chlorophenol (with a concentration as high as 2.5 g L ) to phenol was
obtained in a reaction time of 40 min at room temperature and balloon hydrogen pressure without any
Received 28th February 2014
Accepted 9th April 2014
additives. The excellent catalytic activity of the Pd/rGO-Fe
of Pd, and an electron-deficient state of Pd in the catalyst as a result of the strong interaction between
the active sites and the oxygen of the oxygen-containing groups on the rGO-Fe support.
3 4
O can be attributed to the small particle size
DOI: 10.1039/c4ra01799d
www.rsc.org/advances
3 4
O
detoxifying chlorinated phenols under mild conditions without
any additives.
1
. Introduction
Chlorinated phenols are valuable chemicals with wide appli-
Besides the catalytic nature of the metal nanoparticles (NPs)
cations in the elds of agriculture and industry. However, themselves, it has been proposed that the support also plays a
residual chlorinated phenols show signicant associations with critical role in enhancing the catalytic performance of the metal
adverse effects on the environment and human beings, due to catalyst. Bearing this in mind, a variety of supports including
1
6
7–11
4,5,11–16
their high toxicity and resistance to biodegradation. Among the ZrO
, Al
O
2
,
and carbon
have been investigated and
2
3
technologies for detoxifying these poisonous pollutants, cata- applied in the detoxication of wastewater containing chlori-
lytic hydrodechlorination (HDC) has attracted great attention nated phenols. Among these supports, metal oxides can
because this method is known to be highly efficient, occur improve the stability of the active sites of the catalysts. Never-
under relatively mild conditions and yield less-toxic products theless, they are more sensitive to the HCl produced from the
such as phenol, cyclohexanol, and cyclohexanone. In particular, HDC reaction, resulting in a loss of catalytic activity. Compar-
the HDC of chlorinated phenols catalyzed by supported noble atively, carbon materials as supports are more resistant to acid
metal catalysts, through the scission of carbon–chlorine bonds and thus have provoked extensive interest from researchers in
on the metal surface using molecular hydrogen as a hydrogen assessing their usefulness for the catalytic decomposition of
source, is among the most investigated in recent years. When in chlorophenols. Activated carbon (AC), as a typical carbon
the metallic phase, palladium has been the subject of intensive carrier, has been demonstrated to have a strong ability to
2
research because of its special activity in the HDC reaction.
uniformly distribute the metal active sites, due to its high
However, the HDC activity is always prohibited by severe cata- surface area and porosity. However, the application of AC as a
lyst deactivation, caused by the surface adsorption of chlorine support in the HDC reaction, depending on its pore size
3
from the carbon–chlorine bonds and the aggregation of the distribution and surface composition, may cause the adsorp-
4,5
active Pd particles. To overcome these drawbacks, a large tion of the substrate as well, which results in poor recycling
amount of additives, such as bases, were introduced into the properties. Several additives were also employed to assist these
reaction systems, meaning extra work is needed to separate the carbon materials in achieving a better dispersion of the metal
Pd particles from the reaction mixture in order to avoid NPs and thus a better catalytic performance. For example, by
secondary pollution. Up to now, it has still remained a modifying the carbon nanobers with a pyridinic basic species
3
tremendous challenge to seek stable and recyclable catalysts for via the treatment of carbon nanobers in a stream of NH gas,
Liu et al. successfully improved the catalytic activity and
stability of Pd NPs on carbon nanobers for the HDC of chlor-
ophenol. However, very high temperature is required to intro-
a
Chemical Synthesis and Pollution Control Key Laboratory of Sichuan Province,
College of Chemistry and Chemical Industry, China West Normal University,
17
duce such basic species. Aerwards, Jin and co-workers
reported Pd/MSCN (mesoporous silica–carbon nano-
Nanchong 637009, P. R. China
b
a
Key Laboratory of Green Catalysis of Sichuan Institute of High Education, Zigong
composite) catalyst which exhibited pronounced activity for the
6
43000, P. R. China
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catalytic HDC of 4-CP at room temperature under ordinary 2.3 Catalyst characterization
hydrogen, but this used triethylamine (Et N) as a base addi-
3
The morphology and particle sizes of the catalysts were char-
acterized using transmission electron microscopy (TEM), which
was carried out on a JEOL model 2010 instrument operated at
an accelerating voltage of 200 kV. Powder X-ray diffraction
18
tive. Therefore, it is a big challenge to explore appropriate
supports with a good stability in an acidic environment and a
strong capability to interact with metal catalysts for the recy-
clable conversion of chlorinated phenols, under mild condi-
tions and without additives.
(XRD) patterns were recorded on a Rigaku X-ray diffractometer
D/max-2200/PC equipped with Cu Ka radiation (40 kV, 20 mA).
In a previous work, we found that the catalyst Pd/rGO
exhibited an excellent performance for the HDC of 4-chlor-
ꢁ
The samples were scanned over the range of 10–90 . X-ray
photoelectron spectroscopy (XPS, Kratos XSAM800) spectra
were obtained using Al Ka radiation (12 kV and 15 mA) as an
excitation source (hn ¼ 1486.6 eV) and Au (BE Au4f ¼ 84.0 eV)
and Ag (BE Ag3d ¼ 386.3 eV) as references. All binding energy
19
ophenol (4-CP). However, lengthy centrifugation is required to
separate the catalyst. To this end, we prepared a magnetic rGO-
supported palladium catalyst and applied it for the catalytic
HDC of 4-CP. The as-prepared catalyst showed an excellent
catalytic performance with a yield of phenol as the sole product.
The catalyst can be isolated from the reaction mixture by an
external magnet and can be reused four times without any
obvious loss of activity.
(BE) values were referenced to the C1s peak of contaminant
carbon at 284.6 eV.
2.4 Activity test
The HDC of 4-CP was chosen as a model reaction, to estimate
the catalytic properties of the Pd/rGO-Fe O nanocomposites.
Typically, the HDC reaction was conducted in a two-necked
2
. Experimental
3 4
2
.1 Materials
Na PdCl
Precious Metals, China. FeCl
round-bottomed ask equipped with a hydrogen balloon. The
2
4
was purchased from the Kunming Institute of reaction temperature was controlled by a thermal controller.
$4H O, FeCl $6H O and graphite Specically, an appropriate amount of catalyst (ca. 5.0 mg) and
2
2
3
2
ꢀ
1
powder were provided by Chengdu Kelong Chemical Reagent an aqueous solution of 4-CP (ca. 2.5 g L , 5.0 mL) were trans-
Factory. Other chemicals and solvents (analytical grade ferred into the ask, followed by the treatment of the reactor
reagents) were supplied from the Aladdin Industrial Corpora- under vacuum and the charging of it with highly pure hydrogen.
tion, China, and were used as received without further Aer that, the reactor was put into a water bath. The stirring rate
purication.
was adjusted to 1200 rpm and the reaction time was noted when
the setting reaction temperature was achieved. To monitor the
process of the HDC reaction, samples were periodically with-
drawn from the reactor with a syringe. The removal efficiency of
2.2 Catalyst preparation
The GO used in this procedure was synthesized by the oxidation
Pd/rGO-Fe
with a FID detector and a PEG-20M capillary column (30 m ꢂ
.25 mm, 0.25 mm lm), and nitrogen was used as a carrier gas.
3 4
O for the 4-CP was analyzed by GC (Agilent 7890A)
20
of graphite, according to the improved Hummer's method. For
the synthesis of rGO-Fe FeCl $4H (411 mg) and
FeCl $6H O (1118 mg) were dissolved in deionized water (25
3
O
4
,
2
2
O
0
3
2
mL). Then, the as-prepared mixture was transferred into a
yellow-brown suspension of GO, which had been rstly
prepared through the addition of GO (100 mg) into deionized
water (75 mL) under ultrasonication. Immediately, the pH value
3
. Results and discussion
3 4
3.1 Characterization of Pd/RGO-Fe O
of the mixture was adjusted to 11 by adding ammonia, and this The particle sizes and morphologies of the rGO-Fe
was followed by the reduction of the mixture at 363 K for 4 h rGO-Fe nanocomposites were characterized by TEM, and the
using hydrazine hydrate (4 mL) as a reducing agent. Aer results are illustrated in Fig. 1. As shown in Fig. 1a and c, the
cooling down to room temperature, the black solid was washed Fe NPs have been successfully immobilized on the surface of
with water and ethanol several times, and then dried under the rGO with various kinds of shapes, such as spheres and
vacuum at 343 K for 12 h, to yield the rGO-Fe O .
quadrilaterals. The particle sizes of the spheres were in a range
The rGO-Fe O -stabilized Pd NPs (Pd/rGO-Fe O ) were from about 4 to 15 nm, and the average edge length of the
3 4
O and Pd/
3 4
O
3 4
O
3
4
3
4
3 4
fabricated as follows: the desired amount of rGO-Fe O (100 mg) quadrilaterals was about 15 nm. From the EDX result, it can be
3
4
ꢀ
1
and an aqueous solution of Na
2
PdCl
4
(2.78 mg mL , 5 mL) seen that carbon, oxygen and Fe were detected. During the
were mixed in a 50 mL round-bottomed ask under ultra- process of depositing the Pd NPs on the rGO-Fe
sonication for 30 min. Aerwards, the ask was put into an ice- spherical Fe particles disappeared and were replaced by
(5.6 mg mL , 5 larger ones, because of the aggregation of particles during the
3 4
O , the smaller
3 4
O
ꢀ1
water bath, and an aqueous solution of NaBH
4
mL) was slowly added into the mixture under vigorous stirring. reduction of the Pd precursor. Interestingly, the Pd NPs were
Aer stirring for 4 h, the black solid produced was collected well dispersed, not only on the surface of the rGO but also on
with a magnet, washed with deionized water several times, and the Fe O , with an average particle size of 1.8 nm and a narrow
3
4

nally dried under vacuum at 343 K for 12 h. (The content of Pd size distribution of 1.1–2.8 nm (Fig. 1b and d). The EDX char-
was 5.0 wt%, estimated by inductively coupled plasma spec- acterization showed that the Pd NPs were successfully deposited
2
+
troscopy (ICP)).
3 4
on the surface of the rGO-Fe O through the reduction of Pd
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3 4
Fig. 2 XRD patterns for graphite (a), GO (b), rGO-Fe O (c) and Pd/
rGO-Fe (d).
3 4
O
22
attributed to hematite, were also detected, indicating an impu-
rity in the sample. Nevertheless, the presence of hematite also can
Fig. 1 TEM images of rGO-Fe
O
3 4
3 4
(a and c) and Pd/rGO-Fe O (b and
d); EDX spectra of rGO-Fe
mapping of Pd/rGO-Fe
3
O
(g).
4
(e) and Pd/rGO-Fe
O
3 4
(f); elemental enhance the magnetic properties of the catalyst to some extent. A
ꢁ
3
O
4
new peak at 40.1 , which belonged to the (111) Bragg diffraction
of Pd (JCPDS no. 88-2335), demonstrated the successful immo-
bilization of palladium on the rGO-Fe
XPS proles of GO and the Pd/rGO-Fe
and are illustrated in Fig. 3. An XPS survey scan performed on the
The XRD patterns of graphite, GO, rGO-Fe O and Pd/rGO- sample of GO showed the existence of oxygen and carbon on the
O
3 4
support.
using NaBH
4
as a reducing agent, which was conrmed by
O
3 4
were documented
elemental mapping (Fig. 1g).
3
4
Fe O are shown in Fig. 2. The graphite exhibited a very strong surface of the GO sheets. The C1s spectrum is deconvoluted into
3
4
ꢁ
and sharp peak at 26.4 which was assigned to the (002) crystal four peaks at 284.5, 285.5, 287.0 and 288.3 eV (Fig. 3a) which are
plane of graphite. As can be observed, this diffraction peak dis- attributed to the C–C, C–OH, C–O–C and HO–C]O groups,
ꢁ
21
3 4
appeared, accompanied by the appearance of a new peak at 10.0 , respectively. In the case of the Pd-RGO-Fe O , aside from
aer the oxidation of graphite to GO. This phenomenon could carbon and oxygen, iron and palladium were also distinctively
possibly be attributed to the insertion of various oxygen-con- observed. Compared to the C1s spectrum of the GO, it can be
taining functional groups (epoxy, hydroxyl, carboxyl and seen that the peak intensities for the oxygen-containing func-
21
carbonyl) between the layers of the GO. As shown in Fig. 2c, this tionalities are substantially reduced, indicating that GO was
peak was eventually removed though the treatment of the GO and successfully reduced during the procedure. In particular, it
iron precursors using hydrazine hydrate as the reducing agent. should be noted that the peak corresponding to the C–O–C
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
Meanwhile, the peaks located at 30.1 , 35.5 , 43.1 , 53.5 , 57.0 , groups disappeared, resulting in an enhancement of the intensity
ꢁ
and 62.6 matched well with the (220), (311), (400), (422), (511), of the C–C groups. The residues of hydroxyl groups, such as OH
and (440) Bragg diffractions of Fe
respectively, conrming the formation of the Fe
3
O
4
(JCPDS no. 76-0956), and COOH, on the surface of the catalyst not only improve its
crystalline hydrophilicity but also stabilize the Pd NPs as anchoring sites,
phase. However, weak peaks at 32.9 and 49.3 which were ensuring a good dispersion of the Pd NPs on the rGO-Fe O .
3 4
O
ꢁ
3
4
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be seen that the magnetic saturation value of the rGO-Fe
3
O
4
was
ꢀ
1
9
8
1.4 emu g , while the value of the Pd/rGO-Fe O decreased to
2.2 emu g , due to the immobilization of Pd NPs on the
3
4
ꢀ
1
surface of the support. This is in agreement with the literature,
24
as reported by Ma et al. However, the decrease in saturation
magnetization did not affect the separation of the catalyst from
the reaction system, since the catalyst can be easily separated
from the reaction mixture by a magnet.
3
.2 Catalytic properties of Pd/rGO-Fe
The HDC of 4-CP was chosen as a model reaction to investigate
the catalytic properties of the Pd/rGO-Fe catalyst. Apart from
3 4
O
3 4
O
the main product, phenol, only trace amounts of cyclohexanol
and cyclohexanone were detected during the reaction process.
From the results listed in Table 1, it can be seen that no
3 4 3 4
Fig. 3 XPS spectra of GO (a), rGO-Fe O (b) and Pd/rGO-Fe O (c).
3 4
conversion of 4-CP was observed using rGO or rGO-Fe O as a
catalyst at 303 K and balloon hydrogen pressure. This indicates
that the Pd provided the active sites for the HDC of 4-CP. For
As shown in Fig. 3c, the XPS spectrum of Pd was also char- comparison, a Pd/rGO catalyst without magnetic properties was
acterized, to ascertain the oxidation state of Pd. As can be also prepared, and used for the HDC reaction. It exhibited a
3 4
3 4
observed, the Pd binding energy of the Pd/rGO-Fe O showed comparable activity to the Pd/rGO-Fe O catalyst towards the
two strong peaks centered at 341.3 and 336.0 eV, which were HDC of 4-CP. However, high speed centrifugation is required to
attributed to the Pd3d3/2 and Pd3d5/2 peaks respectively. This separate the catalyst from the reaction mixture, due to the
2
+
indicated that the Pd has been completely reduced during the hydrophilicity of the Pd/rGO ascribed to the functional oxygen-
process. However, it should be noted that the metallic Pd was in containing groups on the surface of the rGO. Therefore, the Pd/
an electron-decient state, since the binding energy of Pd3d5/2 rGO-Fe O nanocomposite was chosen as the catalyst for HDC.
3
4
was 0.6 eV higher than the standard spectrum. This was
The effect of catalyst loading on the catalytic performance of
ascribed to a reduction of the electron density of the Pd atoms, the Pd/rGO-Fe O catalyst was rstly investigated at 303 K and
3
4
caused by an interaction between the metallic Pd phase and the balloon hydrogen pressure. From the results illustrated in
high electronegativity of the oxygen of the oxygen-containing Fig. 5, it can be seen that the conversion of 4-CP to phenol
groups in the GO. Interestingly, the electrondecient Pd was increased with an increase in Pd loading. Specically, the times
assumed to be responsible for the improved performance of the for complete conversion of 4-CP were 50 and 30 min when the
3,23
catalysts for the HDC reaction,
which was proved by the Pd loading was 3.0 wt% and 5.0 wt%, respectively, while only
excellent catalytic activity of the Pd/rGO-Fe O at room 76.4% conversion of 4-CP was obtained in 60 min with a Pd
3
4
temperature and balloon hydrogen pressure in our work.
The magnetic properties of the samples of rGO-Fe
Pd/rGO-Fe
the rGO-Fe
loading of 1.0 wt%. Therefore, 5 wt% loading of Pd was selected
and to further investigate the catalytic properties of the Pd/rGO-
O were also investigated, and the hysteresis loops of Fe O catalyst towards the HDC of 4-CP.
3 4
3 4
3
O
4
3
O
4
and Pd/rGO-Fe
3
O
4
are illustrated in Fig. 4. It can
The effect of catalyst concentration on the HDC reaction
efficiency with the Pd/rGO-Fe catalyst was studied, with the
3 4
O
Table 1 Hydrodechlorination of chlorophenols catalyzed by different
a
catalysts
Conversion
(%)
Selectivity
(%)
Catalysts
rGO
Time (min)
Substrates
25
25
25
25
30
20
40
50
120
4-CP
4-CP
4-CP
4-CP
2-CP
3-CP
2,4-DCP
2,6-DCP
2,4,6-TCP
—
—
—
—
3
rGO-Fe O
Pd/rGO
4
99.8
100.0
100.0
100.0
100.0
100.0
100.0
99.0
99.2
99.5
98.1
>99.8
99.9
99.9
Pd/rGO-Fe
Pd/rGO-Fe
Pd/rGO-Fe
Pd/rGO-Fe
Pd/rGO-Fe
Pd/rGO-Fe
3
3
3
3
3
3
O
O
O
O
O
O
4
4
4
4
4
4
a
Fig. 4 Room temperature magnetization curves of rGO-Fe
Pd/rGO-Fe
O
3 4
and
Reaction conditions: catalyst, 5.0 mg; temperature, 303 K; pressure, 1
1
ꢀ
O
3 4
.
atm; solvent, water (2.5 g L , substrate concentration).
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Fig. 7 Effect of 4-CP concentration on the performance of the Pd/
rGO-Fe O catalyst.
3 4
Fig. 5 Effect of Pd loading on the HDC of 4-CP catalyzed by Pd/rGO-
Fe O .
3 4
ꢀ
1
the change of 4-CP concentration did not affect the product
variation of the catalyst concentration from 0.5 to 1.5 g L . The
results are shown in Fig. 6. One can see that the conversion of 4-
CP increased with an increase in catalyst concentration. For
instance, the conversion of 4-CP was 93.7% in 60 min with a low
distributions, since only phenol was observed with an increase
ꢀ1
of 4-CP concentration from 1.5 to 3.0 g L . Consequently, the
HDC of 4-CP catalyzed by our Pd/rGO-Fe O catalyst maintains
3
4
ꢀ
1
high activity under mild conditions without any additives.
The effect of temperature on the catalytic properties of the Pd/
3 4
rGO-Fe O catalyst was investigated under different tempera-
tures, keeping other reaction parameters the same. The results
are illustrated in Fig. 8. As expected, the conversion of 4-CP
increased gradually with an elevation of reaction temperature
from 293 to 308 K. In terms of the selectivity, the product
distributions did not obviously change in the investigated
temperatures. It has been reported that the catalytic HDC of 4-CP
is a pseudo-rst-order reaction. Thus, the activation energy for
catalyst concentration of 0.5 g L . To our surprise, a complete
conversion of 4-CP to phenol was achieved in 30 min when the
ꢀ
1
catalyst concentration was 1.0 g L . The reaction time for
complete conversion of 4-CP was 15 min when the catalyst
ꢀ1
concentration was increased to 1.5 g L . With regards to the
product distributions, the increase in catalyst concentration did
not change the nal product, in which phenol was detected as
the main product. Therefore, the catalyst concentration was set
ꢀ
1
to 1.0 g L to further investigate other factors that affected the
catalytic performance of the Pd/rGO-Fe catalyst.
3 4
O
3 4
the HDC of 4-CP with the use of Pd/rGO-Fe O as a catalyst can be
The effect of 4-CP concentration on the HDC of 4-CP with the
evaluated. According to the Arrhenius equation as follows,
Pd/rGO-Fe O catalyst was explored. As shown in Fig. 7, an
3
4
increase in 4-CP concentration led to a decrease in conversion
rate. The conversion of 4-CP was accomplished at short reaction
times of 15 and 20 min when the concentration of 4-CP was 1.5
obs ¼ Aeꢀ
a
E /RT
k
the activation energy (E
a
) was calculated. In the equation, kobs
represents the estimated rst-order rate constant, A reveals the
ꢀ1
and 2.0 g L , respectively. The reaction times for complete
conversion of 4-CP increased to 25 and 40 min when 2.5 and 3.0
ꢀ
1
g L of 4-CP were introduced to the reaction system. Moreover,
Fig. 6 Effect of catalyst concentration on the HDC of 4-CP catalyzed Fig. 8 Effect of reaction temperature on the HDC of 4-CP with the
by Pd/rGO-Fe Pd/rGO-Fe catalyst.
3
O
4
.
3 4
O
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frequency factor, and R and T express the ideal gas constant and have a lower solvation ability and H-bonding (if any), or are
2
6
temperature, respectively. As shown in Fig. 8 inset, the rate without these properties, reached a lower catalytic activity.
constants originating from the slope of the straight line were Similarly, our results as discussed above conrm this argu-
.1118, 0.1725, 0.2488, and 0.3138 at 293, 298, 303, and 308 K ment. Therefore, a higher value of 3, and a lower value of v
0
2
respectively. The coefficient of determination (R ) value was should be taken into consideration to achieve a higher
0.99 in all tests. Consequently, the activation energy for the reaction rate.
HDC of 4-CP over the Pd/rGO-Fe catalyst is approximately We also investigated the catalytic performance of the Pd/
rGO-Fe catalyst, and the results are shown in Table 1. It
As reported in previous literature, the solvent plays an can be seen that the complete conversions of 3-CP, 2-CP, and
important role in the catalytic properties of the catalyst, as 4-CP could be accomplished in different reaction times due to
>
3 4
O
ꢀ1
52.0 ꢃ 4.2 kJ mol
.
3 4
O
2
5,26
well as the product distributions of the HDC reaction.
To the electronic and steric hindrance of the substrates. More-
this end, different solvents such as water, ethanol, i-prop- over, the catalyst also showed excellent catalytic properties
anol, methanol, HF and hexane were introduced to test the for the HDC of dichlorophenols (2,4-DCP and 2,6-DCP) and
catalytic properties of the Pd/rGO-Fe O catalyst, and the trichlorophenol (2,4,6-TCP) with an increase in reaction time.
3 4
results are illustrated in Fig. 9. It was observed that the initial Under the same conditions, the HDC rate follows the order:
reaction rate of the 4-CP followed the order: water > methanol monochlorophenols > dichlorophenols > trichorophenol.
>
ethanol > i-propanol > THF > hexane. In particular, the HDC The reusability of the catalyst was also investigated. As shown
of 4-CP reached as high as 100% within 40 min in water in Fig. 10, the results indicate that the catalyst was quite
under the same reaction conditions. When the water was stable and could be recycled four times without any loss of
replaced by protic solvents such as methanol, ethanol and i- activity. Moreover, the catalyst can be easily separated from
propanol, the conversion of 4-CP was obviously decreased, the reaction medium with a magnet and Pd leaching was
and even lower 4-CP conversion was observed in aprotic negligible, as analyzed by ICP (<0.1%). Such phenomena
solvents such as THF and hexane. It has been reported that proved the good stability of Pd/rGO-Fe
the dielectric constant (3) and molar volume (v) are two of the HDC of 4-CP. The excellent catalytic properties of the Pd/rGO-
most important properties which can affect the catalytic Fe can probably be attributed to the following points.
3 4
O for the catalytic
3
O
4
performance of the catalyst in the HDC reaction. For Firstly, the electron-decient state of Pd, caused by the
example, the initial rate of the HDC of 2,4-DCP can be interaction between the metallic Pd and the oxygen of the
inuenced by the solvent polarity and structure, which oxygen-containing groups in the rGO, plays an important role
corresponds to the dielectric constant (3) and molar in improving the catalytic properties of the catalyst towards
2
6
volume (v). The initial rate of the 2,4-DCP was higher the HDC of the chlorinated compounds, in agreement with
2
7
with an increased strength in the ionic forces due to solva- reported literature. Moreover, a smaller Pd particle size is
tion. On the other hand, a lower molar volume of the solvent more tolerable to the HCl during the HDC process. Finally,
enables more solvent molecules to interact with the charge the magnetic properties of the catalyst, when it was recycled
2
6
reaction intermediate, strengthening the HDC reaction. In in a facile collection of catalysts, were immune from the loss
particular, water has the highest and lowest values of 3 and usually caused by mechanical recycling. This advanced
v respectively, and can form well organized structures composite material provides a novel kind of effective catalyst,
through the formation of an H-bond between the dissolving with great promise for the catalytic HDC of 4-CP in practical
ions, thus exhibiting the highest catalytic activity for the applications.
HDC reaction. The solvents, such as THF and hexane, which
3 4
Fig. 9 Effect of solvents on the HDC of 4-CP with the Pd/rGO-Fe O
catalyst.
3 4
Fig. 10 Reusability of the Pd/rGO-Fe O catalyst.
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7
S. Srinivas, P. S. Prasad and P. K. Rao, Catal. Lett., 1998, 50,
4
. Conclusion
77–82.
In summary, a Pd/rGO-Fe
3
O
4
catalyst was synthesized and
8 S. G ´o mez-Quero, F. C ´a rdenas-Lizana and M. A. Keane, Ind.
applied for the HDC of 4-CP in aqueous phase, at room
temperature and balloon hydrogen pressure. Through charac-
terization, it can be seen that the Pd NPs were of a narrow size
Eng. Chem. Res., 2008, 47, 6841–6853.
9 E. D ´ı az, J. A. Casas, A. F. Mohedano, L. Calvo, M. A. Gilarranz
and J. J. Rodr ´ı guez, Ind. Eng. Chem. Res., 2008, 47, 3840–3846.
´
distribution and were dispersed well on the surface of the 10 G. Yuan and M. A. Keane, Appl. Catal., B, 2004, 52, 301–314.
magnetic rGO-Fe O support. The results indicate that the 11 Z. M. de Pedro, E. Diaz, A. F. Mohedano, J. A. Casas and
3
4
magnetic catalyst Pd/rGO-Fe O4 showed excellent catalytic
J. J. Rodriguez, Appl. Catal., B, 2011, 103, 128–135.
properties for 4-CP HDC. This high catalytic activity could be 12 A. Wiersma, E. Van de Sandt, M. Makkee, C. Luteijn, H. Van
attributed to the smaller particle size of the Pd combined with
Bekkum and J. Moulijn, Catal. Today, 1996, 27, 257–264.
3
the functional groups on the surface of rGO, which resulted in 13 G. Yuan and M. A. Keane, Chem. Eng. Sci., 2003, 58, 257–267.
an electron-decient Pd from the strong metal-support inter- 14 Y. Mitoma, M. Kakeda, A. M. Simion, N. Egashira and
action and the presence of hydroxyl groups on the surface of the
support. Furthermore, the catalyst exhibited quite stable prop- 15 L. Calvo, M. A. Gilarranz, J. A. Casas, A. F. Mohedano and
erties and can be recycled at least four times without signicant
J. J. Rodr ´ı guez, J. Hazard. Mater., 2009, 161, 842–847.
deactivation. It was easily separated from the reaction mixture 16 G. Yuan and M. A. Keane, Catal. Commun., 2003, 4, 195–201.
C. Simion, Environ. Sci. Technol., 2009, 43, 5952–5958.
without ltration, due to its magnetic properties.
17 Q. Liu, Z.-M. Cui, Z. Ma, S.-W. Bian and W.-G. Song, J. Phys.
Chem. C, 2008, 112, 1199–1203.
1
8 Z. Jin, C. Yu, X. Wang, Y. Wan, D. Li and G. Lu, J. Hazard.
Mater., 2011, 186, 1726–1732.
Acknowledgements
This work was nancially supported by the National Natural 19 H. Deng, G. Fan, C. Wang and L. Zhang, Catal. Commun.,
Science Foundation of China (21207109), the Applied Basic
2014, 46, 219–223.
Research Programs of Science and Technology Department of 20 D. C. Marcano, D. V. Kosynkin, J. M. Berlin, A. Sinitskii,
Sichuan Province (no. 2014JY0107), and the Opening Project of
Key Laboratory of Green Catalysis of Sichuan Institutes of High
Education (no. LZJ1205).
Z. Sun, A. Slesarev, L. B. Alemany, W. Lu and J. M. Tour,
ACS Nano, 2010, 4, 4806–4814.
21 G. Fan, W. Huang and C. Wang, Nanoscale, 2013, 5, 6819–
6825.
2
2
2
2 X. Liang, X. Wang, J. Zhuang, Y. Chen, D. Wang and Y. Li,
Adv. Funct. Mater., 2006, 16, 1805–1813.
3 C. Molina, A. Pizarro, J. Casas and J. Rodriguez, Appl. Catal.,
B, 2014, 148, 330–338.
4 F. Zhang, J. Jin, X. Zhong, S. Li, J. Niu, R. Li and J. Ma, Green
Chem., 2011, 13, 1238–1243.
Notes and references
1
2
M. A. Keane, ChemCatChem, 2011, 3, 800–821.
H. Rong, S. Cai, Z. Niu and Y. Li, ACS Catal., 2013, 3, 1560–
1563.
3
4
5
6
J. A. Baeza, L. Calvo, M. A. Gilarranz and J. J. Rodriguez,
Chem. Eng. J., 2014, 240, 271–280.
P. Albers, J. Pietsch and S. F. Parker, J. Mol. Catal. A: Chem.,
25 S. G ´o mez-Quero, E. D ´ı az, F. C ´a rdenas-Lizana and
M. A. Keane, Chem. Eng. Sci., 2010, 65, 3786–3797.
26 S. G ´o mez-Quero, F. C ´a rdenas-Lizana and M. A. Keane, AIChE
J., 2010, 56, 756–767.
2001, 173, 275–286.
C. Xia, Y. Liu, S. Zhou, C. Yang, S. Liu, J. Xu, J. Yu, J. Chen and
X. Liang, J. Hazard. Mater., 2009, 169, 1029–1033.
Y. Shao, Z. Xu, H. Wan, H. Chen, F. Liu, L. Li and S. Zheng,
J. Hazard. Mater., 2010, 179, 135–140.
2
7 L. M. Gomez-Sainero, X. L. Seoane, J. L. G. Fierro and
´
A. Arcoa, J. Catal., 2002, 209, 279–288.
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