Controllable synthesis of a Pd/PdO nanocomposite as a catalyst for hydrogenation of nitroarenes to anilines in water

Article abstract of DOI:10.1039/c6ra06900b

A heterogeneous catalytic assembly of the different Pd/PdO ratio nanoparticles supported on oxide carbon nanotubes (OCNTs) can be controllably synthesized by a one-pot gas-liquid interfacial plasma (GLIP) method via adjusting the glow produced parameter. The Pd/PdO nanoparticles have uniform particle size distribution. It is found that the catalysts with different Pd/PdO ratios could affect catalytic activity during the reduction of 4-nitrophenol (4-NP) in water by NaBH₄. The best turn over frequency (TOF) value is up to 3000 h^-1, which is much higher than the case only using Pd nanoparticles as catalysts. Moreover, the catalysts allowed hydrogenation of nitroarenes in water by atmospheric pressure H₂, and it displayed remarkable activity toward the hydrogenation of nitroarenes using low Pd loading in good yields. The catalyst can be simply and efficiently used for ten consecutive runs without significant decrease in activity.

Full text of DOI:10.1039/c6ra06900b

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PAPER
Controllable synthesis of a Pd/PdO nanocomposite
as a catalyst for hydrogenation of nitroarenes to
anilines in water†
Cite this: RSC Adv., 2016, 6, 52620
a
a
b
a
Fan Yang,‡ Sen Dong,‡ Chunxia Wang‡ and Yongfeng Li*
A heterogeneous catalytic assembly of the different Pd/PdO ratio nanoparticles supported on oxide carbon
nanotubes (OCNTs) can be controllably synthesized by a one-pot gas–liquid interfacial plasma (GLIP)
method via adjusting the glow produced parameter. The Pd/PdO nanoparticles have uniform particle
size distribution. It is found that the catalysts with different Pd/PdO ratios could affect catalytic activity
during the reduction of 4-nitrophenol (4-NP) in water by NaBH
4
. The best turn over frequency (TOF)
value is up to 3000 h , which is much higher than the case only using Pd nanoparticles as catalysts.
Moreover, the catalysts allowed hydrogenation of nitroarenes in water by atmospheric pressure H , and
ꢀ1
Received 16th March 2016
Accepted 23rd May 2016
2
it displayed remarkable activity toward the hydrogenation of nitroarenes using low Pd loading in good
yields. The catalyst can be simply and efficiently used for ten consecutive runs without significant
decrease in activity.
DOI: 10.1039/c6ra06900b
www.rsc.org/advances
atomic efficiency, and compatibility with industrial process. On
one hand, catalysts (e.g., Pd/C) of higher selectivity require high
Introduction
2
Functionalized anilines are highly valuable chemical interme- reaction temperature and high H pressure because of their
diates widely used in the production of pharmaceuticals, poly- lower intrinsic activity, leading to high energy input and diffi-
1
mers, pesticides, explosives, bers, dyes and cosmetics.
culties in H handling. On the other hand, chemical reactions
2
Current industrial production of aniline involves the catalytic using water as solvent is expected to receive more attention in
ꢁ
hydrogenation of nitrobenzene performed at 200–300 C and 1– organic synthesis from both a practical and an economic
3
MPa with copper, palladium or palladium–platinum sup- standpoint due to its harmfulless, inexpensive, safety and
2
32
ported on carbon or inorganic oxides as the catalysts. Obvi- segregative. Therefore, the development of more effective and
ously, the high temperature and pressure; toxic, ammable, handle-convenient catalysts, less toxic solvent, low temperature
environmentally hazardous organic solvents; unavoidable and pressure, high selective reaction system for this molecular
formation of harmful azo- and azoxy-derivatives are the main transformation reaction is still highly desirable.
disadvantages. For their importance, extensive effort has been
In the past several years, our group has focused on the
dedicated to exploring the effective greener reduction systems, development of green catalysts and environmentally friendly
including hydrogenation promoted by various heterogeneous procedures to various organic molecular transformations reac-
3
4–8
9
10
11
12
13
14
catalysts, such as Pt, Pd, Rh Ni , Rh, Cu, Au, Ru, Ni , tions by a series of carbon materials supported Pd and Au
3
1
15
33–38
Ni–NiO; catalytic reduction with various reducing reagents, for nanoparticles.
These catalysts can be successfully applied in
3
–6,8–10,16
17–21
22,23
example, H ,
hydrazine,
and others;
silanes,
sodium borohy- catalyst reactions in water due to the functionalized carbon
2
24–29
11,30,31
dride,
reduction process in various solvent, materials, showing good hydrophobic/hydrophilic property.
including water (H
2
O), ethanol (EtOH), tetrahydrofuran (THF), Moreover, the catalysts are fabricated by a gas–liquid interfacial
ethyl acetate (EtOAc). Among them, catalytic hydrogenation is of plasma method (GLIP), which shows many merits for synthesis
particular interest owing to its environmental friendliness, of metal nanoparticles, such as high process rate, preparation
of nanomaterials in large scale, avoiding the use of toxic
a
stabilizers and reducing agents, ambient reaction temperature,
State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing,
Changping 102249, China. E-mail: yi@cup.edu.cn; Fax: +86-010-89739028; Tel: +86- no need to stir during the nanoparticle formation process, and
10-89739028
39–41
0
produced the glow along with the emission of argon.
The
b
Beijing National Laboratory for Molecular Sciences, Key Laboratory of Analytical
Chemistry for Living Biosystems, Institute of Chemistry, The Chinese Academy of
Sciences, Beijing 100190, China
schematic illustration of the GLIP method for synthesis of metal
nanoparticles is shown in Fig. 1. Most recently, we have revealed
that Pd(NO ) $2H O can be decomposed to PdO as well as
3
2
2
†
1
‡
Electronic supplementary information (ESI) available. See DOI:
0.1039/c6ra06900b
36,37
reduced to Pd under glow condition.
Therefore, we
successful fabricate the Pd/PdO nanoparticles which show
These authors contributed equally.
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Fabrication of the Pd/PdO nanoparticles decorated OCNTs
The Pd/PdO nanoparticles decorated OCNTs catalyst was
prepared by a GLIP method with Pd(NO $2H O, OCNTs at
3
)
2
2
room temperature for 10 min. The glow plasma was generated
between the top at stainless steel (SUS) and the bottom ionic
liquid electrode. Ar gas was introduced and used as the plasma-
reaction gas. The power source was supplied from CLASSMAN
HIGH VOLTAGE INC. with the model as FJ1P120. The VDC
¼
260–270 V was applied to a stainless steel electrode in gas phase
for the generation of an Ar plasma, where the discharge current
I was xed to 3–44 mA and the Ar gas was introduced up to
a pressure of 140–500 Pa. 14 mg palladium salt dissolved in 1
mL ionic liquid 1-butyl-3-methylimidazolium tetrauoroborate
Fig. 1 Schematic illustration for synthesis of metal nanoparticles
supported on OCNTs by the GLIP method.
4
([bmim]BF ), 48 mg OCNTs were added to the stainless steel
better catalytic activity than Pd nanoparticles due to the
reactor, then the palladium solution was added to the reactor
incubation 15 min. For the formation of Pd nanoparticles,
electrons were irradiated toward the ionic liquid for 10 min, and
then the mixture was sonicated in ethanol to remove the excess
impurities and extracted from the ionic liquid by a centrifuge
process. Pd/PdO nanoparticles decorated OCNTs were prepared
with the Pd/PdO ratio decreased, and the respective materials
were called Pd-n, (n ¼ 1, 2, 3, 4), the detail parameter and the Pd/
PdO ratio were summarized in Table 1.
boundary effect of Pd and PdO component. In continuation of
36
our previous work, it is the rst time for us to report
a controllable synthesis of different ratios Pd/PdO nanoparticles
supported on the oxidized multi-walled carbon nanotube by the
GLIP method via adjusting the glow produced parameter, and
the catalytic performance of different ratios Pd/PdO nano-
particle are examined by the hydrogenation of nitroaromatics to
aromatic amines.
Experimental
General information
The typical procedure for catalytic reduction of 4-NP
All chemicals and solvents were purchased from commercial In a typical catalysis reaction, 2.1 mg Pd-1 catalyst was used,
suppliers. Transmission electron microscopy (TEM, Tecnai G2, corresponding to a percentage of Pd of 2 mol% with respect to 4-
F20) combined with an energy dispersive X-ray spectroscopy NP, and 14 mg 4-NP (0.1 mmol) were mixed together in 1 mL
(EDS) at an acceleration voltage of 200 kV were used to measure milliQ-water, then stirred on 600–800 rpm for 1 min in order to
the size, morphology, size distribution and element content of mix thoroughly in a micro-reaction vial. Subsequently, 378 mg
Pd catalysts. X-ray diffraction (XRD, Bruker D8 Advance Ger- NaBH4 (10 mmol) in 3 mL milliQ-water was added into the
many) was applied to characterize the crystal structure of the above solution by vigorous magnetic stirring. Aer that, 12 mL of
hybrid materials, and the data were collected on a Shimadzu the reaction mixture was collected and diluted to 3 mL with
XD-3A diffractometer using Cu Ka radiation. The X-ray photo- milliQ-water for UV-vis measurement every 30 s. UV-vis
electron spectroscopy (XPS, Thermo Fisher K-Alpha American absorption spectra were recorded to monitor the change in
with an Al Ka X-ray source) was used to measure the elemental the reaction mixture over the range from 200 to 500 nm at room
composition of samples. The Pd loading in the OCNTs samples temperature. The catalytic reaction by using the other three Pd-
were determined by inductively coupled plasma optical emis- n catalysts for the conversion of 4-NP to 4-AP were carried out
sion spectrometer (ICP-OES). UV-vis absorption spectra of the under the same conditions.
samples were recorded at room temperature on a TU-1900
1
13
apparatus. H NMR and C NMR spectra were recorded on
JNM-LA300FT-NMR for checking the nal product from the
hydrogenation of nitroaromatics reaction.
Table 1 The synthetic parameters and characterization for Pd/PdO
nanoparticles
Fabrication of the OCNTs
Sample
Pd-1
Pd-2
Pd-3
Pd-4
The carbon nanotubes used in our experiments were purchased
from Beijing Cnano Technology Limited (purity: >95%, average
Pressure
Electricity (mA)
140
0.3
140
0.5
190
1.4
500
4.4
length: 10 mm, average diameter: 11 nm). OCNTs were prepared Voltage (V)
by slight oxidation of 1 g puried CNTs with cooled piranha Pd-n size (nm)
268
4.2
12.0
11.2
61/39
265
3.5
10.0
10.6
45/55
270
3.9
11.6
10.8
33/67
260
3.8
11.0
11.0
23/77
a
b
ꢁ
Pd loading (wt%)
solutions (4 : 1, vol/vol 96% H SO /30% H O ) 100 mL at 25 C
2 4 2 2
c
Pd loading (wt%)
for 100 min. The resulting mixture was diluted with ice water.
Aer that, the obtained suspension was centrifuged at 4000 rpm
for 10 min and washed with distilled water for several times
until the pH of the mixture reached 7. Finally, the precipitate
obtained was dried in a vacuum oven for 12 h before use.
b
Pd/PdO
a
b
Average size obtained from the size distribution histogram. The sum
c
of Pd and PdO loading calculated by XPS. The sum of Pd and PdO
loading calculated by ICP-OES.
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The typical procedure for the hydrogenation of 4-nitrotoluene
in water
All hydrogenation reactions were carried out under standard
ꢁ
conditions (60 C, 1 atm of H
2
). A micro-reaction vial, charged
with the Pd-1 (4 mg), corresponding to a percentage of Pd of
0
4
.75 mol% with respect to 4-nitrotoluene was used, and 68.6 mg
-nitrotoluene in 2 mL water with a magnetic stir was connected
Fig. 3 (a) A SAED image of Pd-3; (b) XRD spectra of Pd-1 (black curve),
Pd-2 (red curve), Pd-3 (blue curve) and Pd-4 (purple curve).
to a H balloon. Aer consumption of 4-nitrotoluene, which was
2
monitored by Thin Layer Chromatography (TLC), the reaction
mixture was extract with ethyl acetate, then the catalyst was
recovered by ltering off the solution, later on, the catalyst was
washed with ethanol for several times for the use in the next
run. The HCl ethyl acetate solution was added to the ltrate in
order to induce the precipitation of the amine hydrochloride
salt.
In order to certify the Pd element state and the Pd/PdO ratio
of the Pd-n, XPS, a powerful technique was used to conrm the
component of the Pd/PdO hybrid materials. Wide XPS scan
indicate that the Pd-n catalysts are composed of Pd, C, and O, as
shown in Fig. 4a. The peaks of the chemical components C 1s
and O 1s are at 284.17 and 533.17 eV, respectively. A small peak
corresponding to Pd 3p is observed at 562.08 eV. The Pd cata-
lysts show two peaks for Pd 3d5/2 and Pd 3d3/2 which are split
Results and discussion
0
into two types of Pd electronic states (Pd and PdO) centred
Initially, Pd/PdO nanoparticles with different ratios were
prepared by the GLIP method. It is necessary to emphasize that
samples with the different Pd/PdO ratios can be synthesized
through change the parameter of the power source, as described
in Table 1. The morphology of Pd-n has been examined by TEM,
as shown in Fig. 2. These results demonstrate that all the
synthesized Pd-n catalysts exhibit uniform morphologies. The
average particle diameters are described by distribution histo-
grams, and the Pd loading contents of Pd-n are calculated by
XPS and ICP-OES, which are summarized in Table 1. It indicates
that the high yield for Pd loading on the surface of the OCNTs
has been obtained, and the average particle sizes are 4.2 nm, 3.5
nm, 3.9 nm and 3.8 nm, respectively.
around 335.97, 337.87, 341.17 and 343.37 eV, as shown in
Fig. 4b–e. Moreover, the Pd/PdO ratio can be calculated by peak-
differentiation-imitating, and the results are summarized in
Table 1. These results suggest that the Pd catalyst contains Pd
and PdO active species, and the ratio of the Pd/PdO can be
controlled through adjusting the plasma parameters. These
results demonstrate that the ratio of the PdO is increased with
the higher power output, and the PdO ratio can be controlled
In addition, the SAED of Pd-n is indicated in Fig. 3a. The
lattice spacing measured from the diffraction rings are 0.34,
0.23, 0.21, 0.20, 0.17, 0.14, 0.13 and 0.12 nm, corresponding to
reections C(002), Pd(111), C(100), Pd(200), C(004), Pd(220),
C(110) and Pd(311), respectively. Fig. 3b shows the XRD patterns
ꢁ
ꢁ
ꢁ
of Pd-n, which shows the diffraction peaks at 25.9 , 34.0 , 40.4 ,
4
ꢁ ꢁ ꢁ ꢁ
3.4 , 46.5 , 68.3 and 81.8 , corresponding to the C(002),
PdO(101), Pd(111), C(100), Pd(200), Pd(220) and Pd(311). These
evidences have conrmed that the Pd/PdO possess a face-
centered crystalline structure.
Fig. 4 (a) XPS spectra of Pd-1 (black curve), Pd-2 (green curve), Pd-3
(red curve) and Pd-4 (blue curve); high-resolution XPS Pd 3d image of
Fig. 2 TEM images of Pd-1 (a), Pd-2 (b), Pd-3 (c) and Pd-4 (d).
Pd-1 (b), Pd-2 (c), Pd-3 (d) and Pd-4 (e).
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a
between 40% and 80% approximately. The synergistic action of Table 2 Hydrogenation of 4-nitrotoluene under different conditions
the Pd/PdO ratio may play an important role in highly active
catalytic species. However, there are few reports about how the
42,43
Pd/PdO ratio affects the catalytic activity.
Therefore, in our
case the hydrogenation of nitroarenes reaction was used to
evaluate the catalyst efficiency in order to reveal the affection of
Pd/PdO ratio for the reduction reaction.
ꢁ
b
Entry
Cat.
Solvent
Temperature ( C)
Yield (%)
Initially, we employed the reduction of 4-nitrophenol as the
model reaction to evaluate the Pd-n catalytic performance in
micro-reaction vial at room temperature in aqueous solution
1
2
3
4
5
6
7
8
9
Pd-1
Pd-2
Pd-3
Pd-4
Pd/OCNTs
Pd-3
Pd-3
Pd-3
Pd-3
H O
60
60
60
60
60
40
80
60
60
60
60
60
60
60
81
81
94
88
67
85
68
91
92
52
0
2
H
H
H
H
H
2
2
2
2
2
O
O
O
O
O
with NaBH as reduction agent. The evolution of UV-vis spectra
4
with reaction time for the reduction of 4-NP to 4-AP by using
different catalysts was monitored, as shown in Fig. 4. With the
time increasing, the absorption band at 400 nm was decreased,
and the intensity of the new absorption band at 300 nm was
increased, being ascribed to the formation of 4-AP. Two iso-
sbestic points are observed at 277 and 317 nm. The turnover
H O
2
EtOAc
EtOH
THF
1
1
1
0
1
2
Pd-3
OCNTs
Pd/Cc
Pd-5
H
H
2
O
O
2
37
68
60
frequency (TOF) is a signicant parameter for evaluating the 13
2
H O
H O
2
d
catalytic activity in heterogeneous catalytic reaction. The TOF 14
Pd-6
ꢀ
1
values of various catalysts are 857, 1000, 3000 and 1500 h for
a
Reaction conditions: 4-nitrotolune (0.5 mmol), cat. Pd (0.75 mol%),
b
c
Pd-1, Pd-2, Pd-3 and Pd-4 catalysts, respectively. In comparison and in 1 mL solvent. Isolated yield. Cat. Pd-5 is PdO/OCNTs
d
(
(
0.75 mol%). Cat. Pd-6 is Pd/OCNTs (0.25 mol%) and PdO/OCNTs
0.5 mol%).
with other Pd heterogeneous, the Pd-n catalyst demonstrates
44–49
obviously superior catalytic activity,
and the Pd-3 catalyst
shows the best catalytic activity. However, the reusability of the
Pd-n was not performed well, probably due to the PdO active
species was reduced by the strong reductant (NaBH ). Above Interestingly, a slightly decrease in activity was observed by
4
speculation was proved by which PdO peak was disappeared in using Pd-4 as catalyst (entry 4). We have also checked the case
the XPS spectra of the recycle Pd-n (Fig. 5).
Therefore, we try to use H as reductant for further esti- and PdO/OCNTs catalysts, and the results show much lower
only using Pd/OCNTs, PdO/OCNTs and a mixture of Pd/OCNTs
2
mating their high catalytic activity, we rst use Pd-n as catalyst activity compared with Pd-n catalysts (entry 5, 13 and 14). These
for selective hydrogenation by using p-nitrotoluene as the results demonstrate that the PdO component in the catalyst can
model substrate (Table 2). We employed the reactions with enhance the activity, and the PdO ratio is an important factor
a very low catalyst loading (0.75 mol%) under mild reaction affecting the reaction. The reaction temperature also shows
ꢁ
conditions (atmospheric H pressure and 60 C in water). The some inuence on the yield of the product, and the reaction
2
ꢁ
Pd-1 and Pd-2 catalysts show good activity for 4-nitrotoluene carried out at 60 C shows the best results (entries 3, 6 and 7).
hydrogenation reaction under current conditions (entries 1 and We have further identied several solvents, including THF,
2). To our great delight, Pd-3 gave the best results (entry 3). EtOAc, EtOH, and the reactions are carried out under the
benchmark conditions. To our surprise, the EtOAc and EtOH
are compatible with this catalytic system (entries 8 and 9), but
they still show little lower yield compared with H O. The THF
2
did not work well with this catalyst (entry 10). The blank test
without Pd/PdO nanoparticles was also carried out, and we
found that the reaction did not proceed at all, which means that
the Pd/PdO active species is necessary for this reduction reac-
tion. In sharp contrast to the case of Pd/PdO, the reaction
catalyzed by Pd/C proceeds rather slowly and stops at a quite low
conversion. This result is rationalized by the lack of a suitable
50
hydrophobic environment provided by the OCNTs. Therefore,
the PdO enhance the Pd nanoparticles activity and OCNTs
provide the hydrophilic–hydrophobic environmental, resulting
in the high activity of Pd-3 for catalytic hydrogenation of
nitroarenes in water.
With the optimized reaction conditions in hand, we have
further extended the Pd-3 catalyst to various nitroarenes to
examine the generality of the hydrogenation reactions (Table 3).
Satisfyingly, in all cases, the hydrogenation of nitrobenzene
proceeded smoothly and completely to give aniline in excellent
Fig. 5 (a) Successive absorption spectra of the conversion from 4-NP
to 4-AP with Pd-n catalysts: Pd-1 (a), Pd-2 (b), Pd-3 (c) and Pd-4 (d).
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a
Table 3 Pd-3 catalyzed hydrogenation of nitroarenes in water
b
Entry
Ar-NO
2
(1)
Ar-NH
2
(2)
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
9
4-MeOOCC
4-COOHC
4-CH OCC
4-CNC NO
4-ClC NO
NO (1f)
4-CH OC NO
4-OHC
2-COOHC
3-OHC
3-NO
8-Nitroquinoline (1l)
6
H
4
NO
NO
NO
(1d)
(1e)
2
(1a)
(1b)
(1c)
4-MeCOOC
4-COOHC
4-CH OCC
4-CNC NH
4-ClC NH
NH (2f)
4-CH OC NO
4-OHC NH (2h)
2-COOHC NH
3-OHC
3-NH
8-Aminoquinoline (2l)
6
H
4
NH
NH
NH
(2d)
(2e)
2
(2a)
(2b)
(2c)
4
18
12
18
3
87
79
91
89
69/87
98
97
95
90
6
H
4
2
6
H
4
2
3
6
H
4
2
3
6
H
4
2
6
H
4
2
6
H
4
2
6
H
4
2
6
H
4
2
C
6
H
5
2
C
6
H
5
2
4
4
3
6
H
4
2
(1g)
(1h)
3
6
H
4
2
(2g)
6
H
4
NO
2
6
H
4
2
24
18
24
15
4
6
H
4
NO
2
(1i)
6
H
4
2
(2i)
10
11
12
6
H
4
NO
NO
2
(1j)
(1k)
6
H
4
NH
NH
2
(2j)
99
98
99
2
C
6
H
4
2
2
C
6
H
4
2
(2k)
a
ꢁ
b
Reaction conditions: nitroarenes (0.5 mmol), Pd-3 (0.75 mol%), and atmospheric H
2
in 1 mL H
2
O at 60 C. Isolated yield.
isolated yields with high selectivity. The electronic features of ltered from the reaction and washed by EtOH for next run.
the nitroarenes did not affect the reaction yield or selectivity. This process was repeated for 10 cycles, giving all cycles in high
Various 4-substituted nitroarenes, bearing either electron- yield without prolong the reaction time (Fig. 6a). Aer the
donating or electron-withdrawing groups, such as –COOMe, reactions for 10 cycles, the Pd-3 was again examined by TEM
–
COOH, –COCH
sponding products in good to excellent yields (entries 1–4 and 7, shown in TEM image (Fig. 6b). The XPS spectra of recycled Pd-3
). As for the 4-chloronitrobenzene (1e), the hydrogenation show no difference compare with the fresh Pd-3. These results
3
, –CN, –OMe and –OH, provided the corre- and XPS. The particle size is a little bit increased to 4.1 nm, as
8
reaction was accompanied by dehalogenation reaction, giving illustrate that the Pd/PdO nanoparticle are stabilized in the
the 4-chloroaniline 69% yield (entry 5). Nitrobenzene (1f) also current reduction systems.
proceeded smoothly to give aniline in excellent yield (entry 6).
Based on the above results, the reason for the high activity of
Importantly, other position substituted nitroarenes group, Pd-3 catalyzed hydrogenation of nitroarenes is schematically
including 2-COOH (1i), 3-OH (1j), were compatible with the shown in Scheme 1. Initially, the Pd-3 was dispersed in H O,
2
catalysis system (entries 9 and 10). For compound with two nitro and the H
groups, 1,3-dinitrobenzene (1k), both nitro groups were reduced
to amines (entry 11). Moreover, heterocycle nitroarene 8-nitro-
quinoline (1l) was converted to 8-aminoquinoline in quantita-
tive yields (entry 12). These results indicate that the Pd-3 shows
high activity in nitroarene hydrogenation.
2
and nitro group was absorbed on the surface of the
Reusability is one of the most important features for
a heterogeneous catalyst, which is superior to a homogenous
one. First, to conrm that the reaction is indeed catalyzed by
solid Pd-3 rather than by homogenous palladium species, we
have carried out the leaching experiments. Aer the catalytic
hydrogenation of p-nitrotoluene was carried out for 1 h under
standard conditions, the Pd-3 was removed from the vessel by
centrifugation with p-toluidine produced in 56% yield. No
further reaction took place aer removing the catalyst in 3 h,
then the Pd-3 was put back into the mixture. As a result, the
hydrogenation reaction is restarted, and the product aniline
was obtained in 95% in 3 h. In addition, the ICP-OES was not
detected the Pd nanoparticles in the reaction mixture. The
potential recyclability of the catalyst Pd-3 was explored in the
model hydrogenation of 4-nitrotoluene. The reaction was
Fig. 6 (a) The yields for 10 cycles of the Pd-3 catalyzed 4-nitrotoluene
hydrogenation reaction; (b) a TEM image for recovery of Pd-3 catalyst
after 10 cycles hydrogenation of 4-nitrotoluene.
carried out under normal atmospheric pressure, and in water at
ꢁ
6
0 C for 3 h. The completion of the reaction was monitored by
TLC analysis. Aer completion of the reaction, the catalyst was
Scheme 1 A possible reaction pathway for the high activity of Pd-3.
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51
step to give the [Pd]–H species. Such species are responsible
for the rapid reduction of nitroarenes into the corresponding
anilines. It is proposed that the high catalytic activity of Pd-3 for
hydrogenation of nitroarenes is due to the synergistic effect 10 M. M. Dell'Anna, V. Gallo, P. Mastrorilli and G. Romanazzi,
between Pd and PdO crystallographic plane.
Molecules, 2010, 15, 3311–3318.
1
1
1
1
1 T. Subramanian and K. Pitchumani, ChemCatChem, 2012, 4,
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Conclusions
2 X. Liu, S. Ye, H.-Q. Li, Y.-M. Liu, Y. Cao and K.-N. Fan, Catal.
Sci. Technol., 2013, 3, 3200–3206.
3 J. H. Kim, J. H. Park, Y. K. Chung and K. H. Park, Adv. Synth.
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4 M. B. Gawande, A. K. Rathi, P. S. Branco, I. D. Nogueira,
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In conclusion, we have developed an environmentally-friendly
approach to controllable synthesis of different ratios Pd/PdO
nanoparticles functionalized OCNTs for the rst time by the
GLIP method through adjusting plasma parameter with
Pd(NO ) $2H O as a precursor. The Pd/PdO nanoparticles as
3
2
2
a new kind catalyst is applied in reduction of 4-nitrophenol
4-NP) in water by NaBH . The turn over frequency (TOF) value is
(
4
ꢀ1
15 F. Yuan, Y. Ni, L. Zhang, S. Yuan and J. Wei, J. Mater. Chem.
A, 2013, 1, 8438–8444.
up to 3000 h , showing much higher catalytic activity than Pd
nanoparticles. The catalysts with different Pd/PdO ratios
signicantly affect the catalytic activity. Moreover, the catalyst
demonstrates high catalytic activities for hydrogenation of
1
1
1
1
2
6 G. Evdokimova, S. Zinovyev, A. Perosa and P. Tundo, Appl.
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7 Q. Shi, R. Lu, L. Lu, X. Fu and D. Zhao, Adv. Synth. Catal.,
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2
2007, 349, 1877–1881.
prepared catalysts display remarkable activity toward various
nitroarenes by using low Pd loading in good yields. In addition,
the catalyst can be simply and efficiently used for ten consec-
utive runs without signicant decrease in activity. Further work
is in progress to extend such kind of catalyst for other
applications.
8 D. Cantillo, M. M. Moghaddam and C. O. Kappe, J. Org.
Chem., 2013, 78, 4530–4542.
9 S. Pagoti, S. Surana, A. Chauhan, B. Parasar and J. Dash,
Catal. Sci. Technol., 2013, 3, 584–588.
0 R. K. Rai, A. Mahata, S. Mukhopadhyay, S. Gupta, P.-Z. Li,
K. T. Nguyen, Y. Zhao, B. Pathak and S. K. Singh, Inorg.
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Acknowledgements
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2 R. J. Rahaim and R. E. Maleczka, Org. Lett., 2005, 7, 5087–
This work was supported by National Natural Science Founda-
tion of China (No. 21202203, 21576289 and 21322609), Science
Foundation Research Funds Provided to New Recruitments of
China University of Petroleum, Beijing (No. YJRC-2013-31),
Science Foundation of China University of Petroleum, Beijing
5090.
3 Y. Sunada, H. Kawakami, T. Imaoka, Y. Motoyama and
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