PVA-encapsulated Palladium Nanoparticles: Eco-friendly and Highly Selective Catalyst for Hydrogenation of Nitrobenzene in Aqueous Medium

Article abstract of DOI:10.1002/asia.201900333

In aqueous medium without any other additives, palladium (Pd) nanoparticles with water-soluble polyvinyl alcohol (PVA) as stabilizer were synthesized for the catalytic hydrogenation of nitrobenzene. Under the optimum experimental conditions, the nitrobenzene conversion and the selectivity for aniline were 99.3 % and 100 %, respectively. Comprehensive characterization methods, including TEM, UV/Vis, confocal laser scanning microscopy (CLSM), XRD and XPS allowed a better understanding of the role of PVA aggregates and the properties of Pd nanoparticles. The nitrobenzene conversion exceeded 80 % even after 6 cycles without any treatment of the catalyst. A mechanism about the hydrogenation of nitrobenzene catalyzed by Pd/PVA system was proposed. The Pd/PVA catalyst also exhibited excellent activity and selectivity, particularly to ortho-fluoronitrobenzene and ortho-nitrotoluene. This research can provide a reference for the environmentally friendly catalysis for hydrogenation of nitrobenzene and other substituted nitrobenzene compounds.

Full text of DOI:10.1002/asia.201900333

CHEMISTRY
AN ASIAN JOURNAL
www.chemasianj.org
Accepted Article
Title: PVA-encapsulated Palladium Nanoparticles: Eco-friendly and
Highly Selective Catalyst for Hydrogenation of Nitrobenzene in
Aqueous Medium
Authors: Xiaoyan Wang, Changru Huang, Xiaohao Li, Congxia Xie,
and Shitao Yu
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Asian J. 10.1002/asia.201900333
Link to VoR: http://dx.doi.org/10.1002/asia.201900333
A Journal of
A sister journal of Angewandte Chemie
and Chemistry – A European Journal

Chemistry - An Asian Journal
10.1002/asia.201900333
FULL PAPER
PVA-encapsulated Palladium Nanoparticles: Eco-friendly and
Highly Selective Catalyst for Hydrogenation of Nitrobenzene in
Aqueous Medium
[
a]
[a]
[a]
[a]
[b]
Xiaoyan Wang, Changru Huang, Xiaohao Li, Congxia Xie,* and Shitao Yu
Abstract: In aqueous medium without any other additives, palladium always thermally unstable and have high boiling points. In
Pd) nanoparticles with water-soluble polyvinyl alcohol (PVA) as addition, hydrogenation in liquid phase has higher heat transfer
(
stabilizer were synthesized for the catalytic hydrogenation of coefficient and higher volume productivity compared with vapor
nitrobenzene. Under the optimum experimental conditions, the phase.[4] Therefore, it is a better choice for the hydrogenation of
nitrobenzene conversion and the selectivity for aniline were 99.3% nitrobenzene carrying out in liquid phase. Kinds of metals, such
and 100%, respectively. Comprehensive characterization methods, as Ni,[9] Co,[10] Ru,[11] Rh,[12] Pt,[13,14] Pd,[15] Au[16] as well as bi-
including TEM, UV-Vis, CLSM, XRD and XPS allowed a better metals,[17,18] can be used for nitrobenzene hydrogenation.
understanding of the role of PVA aggregates and the properties of However, one or more disadvantages for these reaction
Pd nanoparticles. The nitrobenzene conversion exceeded 80% even processes still exist, such as the complicated preparation
after 6 cycles without any treatment of the catalyst. A mechanism method of catalyst, organic solvent as reaction medium,
about the hydrogenation of nitrobenzene catalyzed by Pd/PVA introducing toxic reductant, harsh reaction conditions and poor
system was proposed. The Pd/PVA catalyst also exhibited excellent recyclability etc.
activity and selectivity particularly to ortho-fluoronitrobenzene and
ortho-nitrotoluene.This research can provide a reference for the
environmentally friendly catalysis for hydrogenation of nitrobenzene
and other substituted nitrobenzene compounds.
NB
Acidic or
neutral
conditions
NO2
Alkaline
-
H2O + 2e,2H
conditions
Introduction
NSB
-
H2O
N
N
AOB
The production of aniline from the catalytic hydrogenation of
nitrobenzene is a vital industrial reaction.[1-3] The major product
is aniline together with some by-products such as azobenzene
NO
O
-
H2O + 2e,2H
+
2e,2H
(
AB) , hydrazobenzene (HAB) and azoxybenzene (AOB). Most
published papers about the hydrogenation of nitrobenzene have
tried to explain the reaction mechanism.[4-8] After sufficient
supplement and correction, the following mechanism has
gradually been accepted (Scheme 1). As indicated in Scheme 1,
aniline can be produced through two different reaction paths. In
acidic or neutral conditions, aniline tends to be formed via
nitrosobenzene (NSB) and N-phenylhydroxylamine (PHA), which
is also the main reaction route. The second path defined as
adverse event route occurs only in alkaline conditions and two
intermediates, AOB and AB, are formed from the condensation
of NSB with PHA and the condensation of NSB with aniline,
respectively.
PHA
- H2O
N
N
AB
NHOH
-
H2O + 2e,2H
+
2e,2H
H
AN
+ 2e,2H
N
HAB
N
H
NH2
Main reaction route
Adverse event route
The hydrogenation process is commercially conducted in
vapor phase over copper-based catalysts or in liquid phase over
Ni catalysts. It is well known that aromatic nitro-compounds are
Scheme 1. The different mechanisms of catalytic hydrogenation of
nitrobenzene.[4-8]
[
a]
Xiaoyan Wang, Changru Huang, Xiaohao Li, Prof. Congxia Xie,*
State Key Laboratory Base of Eco-chemical Engineering,
College of Chemistry and Molecular Engineering, Qingdao University
of Science and Technology
In recent years, nano-catalysts have received more and
more attention owing to their high catalytic activity and chemical
selectivity.[19] Especially, the catalytic hydrogenation of
nitrobenzene to aniline in an aqueous solution using transition or
noble metals as catalysts, is one of the most extensive studies
Zhengzhou Road 53, Qingdao, Shandong Province, China
E-mail: xiecongxia@126.com
[b]
Prof. Shitao Yu
[
13]. Metal nanoparticles have been prepared in water phase
College of Chemical Engineering, Qingdao University of Science
and Technology
with surfactants as stabilizers, such as polyvinyl pyrrolidone
(PVP),[20,21] polyoxyethylene-polyoxy-propylene-polyoxyethylene
triblock copolymer,[22] et al. These surfactant molecules can be
self-assembled into micelles. Micells with non-polar hydrophobic
Zhengzhou Road 53, Qingdao, Shandong Province, China
Supporting information for this article is given via a link at the end of
the document.
For internal use, please do not delete. Submitted_Manuscript
This article is protected by copyright. All rights reserved.

Chemistry - An Asian Journal
10.1002/asia.201900333
FULL PAPER
core feature in preventing aggregation of metal nanoparticles.[23]
Furthermore, micellar non-polar micro-environment increases
the solubility of organic substrates and accelerates organic
reaction in water phase. Polyvinyl alcohol (PVA), a hydrophilic
surfactant, is used as the stabilizer of metal nanoparticles as
well. Different from PVP and some tri-block copolymers, PVA
molecules can’t form micelles, but can form aggregates due to
the presence of hydroxyl groups in each PVA molecule. These
hydroxyl groups can interact with metal nanoparticles, and
control growth process of metal nanoparticles.[24] Nano-
ruthenium confined in PVA solution has been synthesized for the
selective hydrogenation of α-pinene by our group, exhibiting
excellent activity and selectivity.[25] Unfortunately, poor catalytic
performance was obtained for nitrobenzene hydrogenation. It
has been found that Pd catalyst is highly efficient over
hydrogenation of aromatic nitro functional group.[26] Bearing this
in mind, PVA-stabilized Pd nanoparticles could be an alternative
to improve the catalytic efficiency for hydrogenation of
nitrobenzene.
Herein, we continue to report a new catalytic system, that is,
Pd nanoparticles encapsulated in PVA aggregates (Pd/PVA) to
explore a convenient, efficient and eco-friendly technology for
the production of aniline. It is worth noting that no organic
solvent was used during the preparation of Pd/PVA catalyst and
hydrogenation of nitrobenzene. Not only preparation of catalysts
but also the hydrogenation of nitrobenzene is in water medium
under mild reaction conditions. The nitrobenzene hydrogenation
is carried out under biphasic water/substrate conditions, which
makes it much easier to separate the catalyst from products by
simple extraction. The reaction system meets with the
requirements of eco-friendliness due to the use of hydrogen (not
Figure 1. TEM images and distribution histograms of Pd nanoparticles.
(a) and (b): Pd-PVA78000, (c) and (d): Pd-PVA145000 , (e) and (f): Pd-PVA205000.
4
toxic NaBH ), water and nontoxic PVA. For a better under-
standing, a reasonable inference is made about the mechanism
of the stabilization of Pd nanoparticles and nitrobenzene
hydrogenation. Catalytic hydrogenation of other substituted
nitrobenzenes by the Pd/PVA system was investigated as well.
procedure. One of the possible reasons is that the shorter chain
length of PVA47000 aggregates can’t afford enough steric
hindrance to stabilize Pd nanoparticles.
As exhibited in Figure 1a and Figure 1b, Pd-PVA78000 has
the best dispersion and the smallest average particle size. In
contrast, Pd-PVA145000 forms larger particles and some of them
Results and Discussion
link together (Figure 1c and Figure 1d). For Pd-PVA205000
,
The effects of PVA molecular weight on Pd/PVA system
obvious agglomeration and particle sizes in a wider range can
be observed (Figure 1e and Figure 1f). With an increase in PVA
molecular weight from 78,000 to 205,000, the average particle
sizes of Pd nanoparticles increased from 4.8 nm to 8.7 nm.
Similar to Oh and coworkers’ reports, the kinetic behavior of
Compared with RuCl
the best catalytic performance was obtained when Pd(C
was used as the metal precursor under the same reaction
conditions (Table S1). Therefore, Pd(C was selected as
3 2 2 4 2
·xH O, PdCl2, NiCl , PtCl and CoCl ,
2 3 2 2
H O )
2 3 2 2
H O )
longer PVA carbon chains in water phase is more complex,
which causes that the closer capped Pd2+ within PVA
aggregates can aggregate more easily once Pd2+ is reduced and
grown.[27] Therefore, Pd-PVA205000 has a largest average size
the metal precursor to do the following experiments. We further
explored the effect of PVA molecular weight on the mean size
and morphology of Pd nanocatalysts. Pd nanoparticles stabilized
by PVA with different molecular weights (47000, 78000, 145000,
and a poorest dispersion.
205000) are abbreviated as Pd-PVA47000, Pd-PVA78000, Pd-
Previously, Dai et al. demonstrated that the average size
PVA145000 and Pd-PVA205000, respectively.
and particle distribution of nano-Pd were consistent with their
catalytic performance.[28] Such a relation in our work was
TEM characterization was conducted with Pd nanoparticles
pretreated with washing and centrifugal process (Figure 1). For
Pd-PVA47000, TEM characterization was not performed due to the
obvious agglomeration of the sample during the pretreatment
completely observed as shown in Table 1. It indicates that the
molecular weight of PVA is a key factor for the formation and
catalytic activity of Pd nanoparticles.
For internal use, please do not delete. Submitted_Manuscript
This article is protected by copyright. All rights reserved.

Chemistry - An Asian Journal
10.1002/asia.201900333
FULL PAPER
different valence of Pd species in samples (Figure 2c).
According to XPS standard library, the standard binding energy
of Pd 3d5/2 is 335.00-335.80 eV and that of PdO
336.40-336.60 eV. As indicated in Figure 2c, two peaks at
35.49 eV and 336.69 eV are assigned to Pd 3d5/2 and
x
/Pd 3d5/2 is
Table 1. The catalytic performance of 4 different nano-Pd catalysts for
nitrobenzene hydrogenation.
3
Mw. of
PVA
The average size of
PVA-stabilized Pd
nanoparticles(nm)
-
Conversion
of nitro-
benzene(%)
45.5
Selectivity
for
PdO
x
/Pd 3d5/2. The standard binding energies for Pd 3d3/2 and
/Pd 3d3/2 are 340.30-340.50 eV and 340.40-341.60 eV,
Entry
(
g/mol)
47,000
78,000
145,000
205,000
aniline(%)
100
PdO
x
1
2
3
4
respectively. In Figure 2c, two peaks located at 340.78 eV and
41.92 eV are assigned to Pd 3d3/2 and PdO /Pd 3d3/2. These
4.8 ± 0.6
84.8
100
3
x
5.7 ± 1.7
61.3
100
measured binding energies are slightly higher than the standard
8.7 ± 5.1
55.6
100
values, which might result from the highly dispersion of Pd nano-
catalysts.[32] All results can match with the standard spectrum
Reaction conditions for the preparation of Pd nanoparticles: 15 mg PVA,
.0  10-5 mol Pd(C
, 3 mL H O, 1.0 MPa H , 50 °C, 1 h. Reaction
1
2
H
3
O
2
)
2
2
2
0
library perfectly, and the molar ratio of PdO
x
to Pd is calculated
conditions for the hydrogenation of nitrobenzene: the molar ratio of catalyst
to substrate is 1:500 (0.62 g nitrobenzene, 3.02 g Pd/PVA catalyst (9.1×10-6
to be 1:11.
mol Pd)), 1.0 MPa H
2
, 45 °C, 2 h.
The CLSM technique was applied in the morphological
analysis of Pd/PVA system in depth. Two specimens were
measured by CLSM, including PVA78000 aggregates (Figure 3a)
and Pd-PVA78000 (Figure 3b). In Figure 3a, the majority of PVA
aggregates can be seen clearly with transparent cavity-like
structure, while a small amount of them is black. Additionally,
most of PVA aggregates present spherical shape. The formation
of PVA aggregates is the self-assembly of PVA molecules in
water attributed to their hydroxyl functional groups.
According to the above results, PVA (Mw.: 78,000) was
selected as the stabilizer of Pd nanoparticles.
Catalyst characterization
PVA concentration also had a remarkable effect on PVA-
encapsulated Pd nanoparticles. Accoring to their catalytic
performance for the hydrogenation of nitrobenzene, the optimal
PVA concentration was determined as 5.0 g·L-1 (Figure S1). The
2 3 2 2
original Pd(C H O ) -PVA solution was pale yellow and
translucent, and it changed to be dark brown and opaque after
reduction (Figure 2a). The UV-Vis spectra for the original
solution and hydrogenated solution (Figure S2) suggest that
most of Pd2+ ions have been reduced to Pd nanoparticles, which
can be confirmed by the subsequent XRD and XPS
characterization.
0
The XRD pattern provides more accurate evidence for the
generation of Pd0 nanoparticles (Figure 2b). Four diffraction
peaks at 40.15°, 46.40°, 68.10°, and 81.93°, can be ascribed to
four crystal faces that are Pd (111), Pd (200), Pd (220) and Pd
Figure 3. The CLSM images of (a) PVA78000 aggregates, (b) Pd-PVA78000
.
(
311). The spectrum is coincident with the standard XRD
Figure 3b is the CLSM image of PVA aggregates containing
Pd nanoparticles. Compared with PVA aggregates, there is no
obvious difference for the two samples in the size and
morphology. It suggests that Pd nanoparticles have little
influence in the morphology of Pd-PVA78000. Figure 3 also
underlines the ability of PVA to both stabilize and disperse Pd
nanoparticles. It indicates that the dispersion of PVA aggregates
patterns of metallic palladium (JCPDS: No.46-1043).[29, 30] The
XRD results also imply that nano-Pd has face-centered cubic
structure.[31]
plays
a crucial role in formation and distribution of Pd
nanoparticles.[33]
The influence of reaction conditions in nitrobenzene hydro-
genation
Figure 2. (a) Reduction process, (b) XRD spectrum and (c) XPS patterns of
As indicated in Figure 4a, the catalytic activity decreases
with an increase in the amount of substrate. When the
nitrobenzene amount exceeds a certain value (1:550), the ntro-
PVA-encapsulated Pd nanoparticles.
The XPS spectra give the proportional relationship of
For internal use, please do not delete. Submitted_Manuscript
This article is protected by copyright. All rights reserved.

Chemistry - An Asian Journal
10.1002/asia.201900333
FULL PAPER
proceeds with a molar ratio of 1:500 at 45 °C with 2.0 MPa H
h. 99.3% nitrobenzene conver-sion and 100% selectivity for
aniline have been achieved.
2
for
3
Under the optimal conditions, several different catalysts
were chosen to compare their catalytic performance with the
Pd/PVA catalyst, including commercially available Pd/C, Ru/C,
and Pd nanoparticles stabilized by other common polymers such
as polyvinyl pyrrolidone (Mw.: 58000) and polyethylene glycol
(
Mw.: 70000).
Table 2. The catalytic performance of different catalysts for nitrobenzene
hydrogenation.
Conversion of
Selectivity for
aniline(%)
Entry
Catalyst
nitrobenzene(%)
1
2
3
4
5
Pd/C (10 wt%)
Ru/C (5 wt%)
Pd-PVA78000
Pd/PEG
100.0
93.5
99.3
91.7
96.0
100
100
100
100
100
Figure 4. The influence of reaction conditions in nitrobenzene hydrogenation.
(a) the molar ratio of Pd nanoparticles to nitrobenzene (1.0 MPa H2, 45 °C, 2
h), (b) temperature (the molar ratio of catalyst to substrate is 1:500 (0.62 g
nitrobenzene, 3.02 g Pd/PVA catalyst (9.1×10-6 mol Pd)), 1.0 MPa H2, 2 h), (c)
hydrogen pressure (the molar ratio of catalyst to substrate is 1:500, 45 °C, 2 h),
Pd/PVP
(d) reaction time (the molar ratio of catalyst to substrate is 1:500, 45 °C, 2.0
MPa H2).
Reaction conditions for the hydrogenation of nitrobenzene: the molar ratio of
catalyst to substrate is 1:500, 2.0 MPa H , 45 °C, 3 h.
2
benzene conversion sharply decreases. Meanwhile, obvious
aggregation of Pd nanoparticles was observed after the reaction.
It suggests that the structure of PVA aggregates might have
been broken. Therefore, the molar ratio of 1:500 is selected for
the following experiments. It should be noted that the blank
experiment was carried out to better understand the reaction
route. When no catalyst was added, aniline was not detected.
We also did another control experiment. All the steps for both
the preparation of catalyst and hydrogenation of nitrobenzene
were the same as the experimental section described except
that palladium acetate was not added. This experiment (without
Pd) gave no activity as well. These results indicate that Pd
nanoparticles in PVA matrix show an excellent catalytic
performance for the hydrogenation of nitrobenzene. From 25 °C
to 45 °C, nitrobenzene conversion significantly increases with
the rising temperature (Figure 4b). At 45 °C, the highest
nitrobenzene conversion is obtained. Further increasing the
temperature, the conversion of nitrobenzene obviously
decreases. It indicates that in the high-temperature environment,
Pd/PVA system easily denatures and inactivates. A narrower
range of temperature (40-50 °C) is suitable for nitrobenzene
As shown in Table 2, Pd/C catalyst gave the highest activity.
However, Pd/C was seriously stuck on the inner wall of the
autoclave after the reaction and could not be recycled. For
Pd/PEG and Pd/PVP catalyst, aggregation of Pd nanoparticles
was observed after the catalytic hydrogenation. It indicates that
PVA can provide better stabilization for Pd nanoparticles. These
results have shown the advantages of PVA as the stabilizer.
The reusability of Pd/PVA catalyst on nitrobenzene
hydrogenation
The reusability for this catalytic system was explored as well.
Under the optimized conditions, Pd-PVA78000 catalyst was
recycled without any reactivation treatments. Figure 5a displays
that the conversion of nitrobenzene decreases gradually during
the recycling process. The probable reason is the mass transfer
limitation between PVA-stabilized nano-Pd in water phase and
substrates in organic phase during the hydrogenation reaction
(Scheme 2). During mass transfer process, the loss of Pd
2
hydrogenation. Gradually increasing H pressure or prolonging
nanoparticles is inevitable. When the lost Pd nanoparticles enter
into the organic phase, the PVA aggregates can not stabilize
them again. The aggregation of Pd nanoparticles is also
observed (Figure 5b). Nevertheless, nitrobenzene conversion
still exceeds 80% even after 6 cycles. It is worth mentioning that
these experimental results are gained without any additional
stabilizer. In addition, the selectivity for aniline remains 100% for
each cycle. It suggests that the backbone of PVA aggregates
has not changed, and it may still play a valuable role in
controlling the selectivity for aniline.[36, 37]
the reaction time (Figure 4c and Figure 4d), nitrobenzene
conversion rises correspondingly. The higher hydrogen pressure
results in larger hydrogen solubility, which is in favor of the
hydrogenation of nitrobenzene.[34] The increasing nitrobenzene
conversion with reaction time also demonstrates that Pd nano-
particles are well stabilized by PVA aggregates.[35] Catalytic
activity maintains the same when reaction time is prolonged to 3
2
h under 2.0 MPa H .
To sum up, the optimized reaction conditions are as follows.
Pd nanoparticles are confined in PVA (MW.: 78,000) solution with
-1
a dosage of 5.0 g·L . The hydrogenation of nitrobenzene
For internal use, please do not delete. Submitted_Manuscript
This article is protected by copyright. All rights reserved.

Chemistry - An Asian Journal
10.1002/asia.201900333
FULL PAPER
nanoparticles that are the catalytically active species (Scheme
2
). Prior to being reduced, PdII dispersed in the aqueous PVA
matrix via coordination bonds between Pd2+ and hydroxyl groups
of PVA.[38] During the reduction process with H
as the reductant,
2
a large amount of Pd nanocrystals will be gradually formed in
the water phase. As exhibited in Scheme 2, Pd nanoparticles
can also be adsorbed on the PVA aggregates by interacting with
their hydroxyl groups.[28] Meanwhile, Pd nanoparticles tend to be
gradually confined into the PVA aggregates due to their
nonpolar micro-environment. Therefore, both the hydroxyl
Figure 5. (a) Reusability of Pd/PVA system, (b) TEM characterization for the
groups of the PVA aggregates and the nonpolar micro-
environment have a strong adsorbability to Pd0 nanoparticles,
recycled catalyst after 3 runs.
which give enough protection to Pd nanoparticles through steric
hindrance.[39]
The catalytic activity of Pd/PVA in hydrogenation of
nitrobenzene to aniline were compared in detail with some
catalysts reported previously (Table S2).
It has been widely accepted that aniline can be formed
directly without other by-products via the main reaction route
under acidic or neutral conditions (Scheme 1). The pH value of
Pd/PVA system is about 6.0, which facilitates the main reaction
route for nitrobenzene hydrogenation. Herein, we propose a
reaction mechanism about hydrogenation of nitrobenzene
(Scheme 3).
As shown in Table S2, our Pd/PVA catalyst exhibits
excellent catalytic performance. It is worth noting that no organic
solvent is used during the preparation of Pd/PVA catalyst and
hydrogenation of nitrobenzene. Not only preparation of catalysts
but also the hydrogenation of nitrobenzene is in water medium
under mild reaction conditions. It is much easier to separate the
catalyst from products by simple extraction.The preparation of
4
previously reported catalysts always required toxic NaBH as
reductant and organic solvents. These comparison results
confirm that Pd/PVA catalyst is effective and eco-friendly for
hydrogenation of nitrobenzene.
Mechanism of catalytic hydrogenation of nitrobenzene
According to the comprehensive analysis of these obtained
experimental data, a mechanism about the preparation of Pd
nanoparticles (Scheme 2) and catalytic hydrogenation of
nitrobenzene (Scheme 3) can be reasonably inferred.
Scheme 3. The mechanism of nitrobenzene hydrogenation catalyzed by
Pd/PVA system.
Nitrobenzene hydrogenation occurs on the surface of Pd
nanoparticles, and hydrogen transfers from metal-hydride
complex on metal surface to nitrobenzene.[40] Metal-hydrogen
bonds can be formed between Pd nanoparticles and
hydrogen.[41] Therefore, PHA (C
6 5
H -NHOH) is still an
intermediate. The electronegative hydrogen atom will be
enriched with electrons provided by metallic Pd nanoparticles. It
might be vital for the selection of the reaction route for the
reason that it is more conducive to nucleophilic reaction. In
addition, temperature is also an important factor for the reaction
route. 40-50 °C is the suitable temperature range for this catalyst
system. The mild reaction temperature also inhibits the
occurrence of side reactions such as polycondensation reaction
and deep hydrogenation of aniline. Therefore, a synergistic
effect causes the high selectivity for aniline.
Scheme 2. The scheme for catalytic hydrogenation of nitrobenzene by
Pd/PVA system.
Initially, the Pd(Ⅱ) precursor is in situ converted into Pd0
For internal use, please do not delete. Submitted_Manuscript
This article is protected by copyright. All rights reserved.

Chemistry - An Asian Journal
10.1002/asia.201900333
FULL PAPER
We further investigated the catalytic performance of the
Pd/PVA catalyst for hydrogenation of other substituted
nitrobenzenes (Table S3). To our delight, the Pd/PVA catalyst
also exhibited excellent activity and selectivity particularly to
ortho-fluoronitrobenzene and ortho-nitrotoluene. It indicates that
the catalyst is also suitable for the hydrogenation of other
substituted nitrobenzene compounds, not just nitrobenzene.
nitrobenzene.
Hydrogenation of nitrobenzene
The Pd/PVA catalyst and nitrobenzene in certain proportion were put
into a 75 mL stainless-steel autoclave. The autoclave was then sealed
and flushed with 1.0 MPa H
2
3 times. Subsequently, the hydrogenation of
nitrobenzene was carried out at desired temperature under required H
2
pressure with continuous stirring. After the reaction, the mixture was
extracted by dichloromethane, and the organic phase was determined by
Gas Chromatography (GC, Fuli GC-9790) with a capillary column (OV-
Conclusions
1701, 50 m × 0.25 mm × 0.25 μm) and a FID detector. The catalytic
performance of Pd/PVA system was evaluated according to nitrobenzene
conversion and selectivity for aniline. For the recycling experiments, the
organic layer was collected with a pipette for GC analysis. The aqueous
Pd/PVA phase remained in the autoclave for the next cycle.
In summary, Pd nanoparticles confined in PVA aggregates
were prepared by the reduction of Pd2+ ion with H
. The particle
2
size, morphology and chemical states of Pd nanoparticles were
estimated by TEM, UV-Vis, CLSM, XRD and XPS, respectively.
Judging from the CLSM and TEM measurements, the particle
size and distribution of Pd nanoparticles can be controlled by
PVA aggregates. The Pd/PVA system exhibits excellent catalytic
performance. 99.3% nitrobenzene conversion have been
achieved together with 100% selectivity for aniline. The unique
selectivity for aniline is possibly dependent on a synergistic
effect of metal-hydrogen bonds and mild reaction temperature. It
is worth noting that the reaction system is environment-friendly,
which can be especially attributed to the good biological
compatibility of PVA, hydrogen as reductant, the mild reaction
conditions and no use of any solvent except water. In addition, it
is easy to separate the products from the catalysts by simple
extraction. The nitrobenzene conversion exceeded 80% even
after 6 cycles. The Pd/PVA catalyst also exhibited excellent
performance for the hydrogenation of other substituted
nitrobenzene compounds.
Characterization
Particle size and shape of Pd nano-catalysts were tested by
transmission electron microscopy (TEM, Hitachi-7650). Before TEM
characterization, the prepared Pd/PVA catalyst was centrifuged and the
solid residue was first washed with ultrapure water three times and then
with ethanol twice to remove any traces of PVA. After that, the pretreated
Pd nanoparticles were dispersed in ethanol for the following TEM
characterization. A drop of Pd/PVA suspension was added onto the
copper grid. More than 300 particles were counted to obtain each size
distribution histogram of Pd nanoparticles.
The reduction degree of Pd (Ⅱ) was determined by ultraviolet-visible
absorption spectra (UV-Vis). Freshly prepared Pd/PVA system was
diluted 20 times before determination.
The phase structure of nano-Pd were recorded through X-ray
diffraction (XRD, Rigaku multiflex diffractometer) with Ni filtered Cu Kα
radiation. The valence of nano-Pd was tested by using X-ray
photoelectron spectroscopy (XPS, Kratos-Axis 165). The measured
binding energies were calibrated by C1s peak at 284.60 eV. Before
determination, ethanol was evaporated under vacuum.
The photographs of PVA aggregates and Pd/PVA system were
taken by a confocal laser scanning microscope (CLSM, Leica, TCS-SP5-
II) with an ultraviolet source.
Experimental Section
Materials
Nitrobenzene (≥99%) was obtained from Chinasun Specialty
Products Co., Ltd. Pd(C
were obtained from Shanghai Civi Chemical Technology Co., Ltd. PdCl
2 3 2 2 3 2
H O ) (≥98.0%) and RuCl ·xH
O (35.0%~42.0%) Acknowledgements
2
(
>99%) was obtained from Shanghai Macklin Biochemical Co., Ltd. NiCl
≥98%), PtCl (≥99%) and CoCl
2
The authors are grateful for the financial support of the
National Natural Science Foundation of China (31270615) and
the Taishan Scholar Project of Shandong Province (ts
(
4
2
(≥99%) were purchased from Beijing lark
technology Co., Ltd. Pd/C (10wt%), Ru/C (5wt%) and polyvinyl
pyrrolidone (PVP, Mw: 58000) were purchased from Aladdin Industrial
Corporation. PVA with different molecular weights (47000, 78000,
201511033).
1
45000, 205000) were supplied by Sigma-Aldrich Co., Ltd. Polyethylene
glycol (PEG, Mw: 70000) and CH Cl (≥99.5%) were purchased from
(≥99.9%) was purchased from
2
2
Keywords: PVA aggregates • Pd nanoparticles • Water •
Shanghai Macklin Biochemical Co., Ltd. H
Qingdao Heli Co., Ltd.
2
Hydrogenation of nitrobenzene
[
1]
2]
P. Sangeetha, K. Shanthi, K.S. Rama Rao, B. Viswanathan, P. Selvam,
Appl. Catal. A-Gen. 2009, 353, 160-165.
Preparation of Pd nano-catalyst
[
E. H. Boymans, P.T. Witteb, D. Vogt, Catal. Sci. Technol. 2014, 5, 176-
2 3 2 2
A certain amount of Pd(C H O ) , PVA with different molecular
183.
weights (47000, 78000, 145000, 205000) and ultrapure water were
added into a 75 mL stainless steel autoclave, and vigorously stirred at
ambient temperature for 1 h. Afterwards, the reduction of Pd(Ⅱ) acetate
[
3]
4]
J. Zhang, G. Lu, C. Cai, Catal. Commun. 2016, 84, 25-29.
M. Turáková, T. Salmi, K. Eränen, J. Wärnå, D.Y. Murzin, M. Králik,
Appl. Catal. A-Gen. 2015, 499, 66-76.
[
2
was performed under 1.0 MPa H at 50 °C with stirring for 1 h. The dark
[
5]
6]
H. Li, Q. Zhao, Y. Wan, W. Dai, M. Qiao, J. Catal. 2006, 244, 251-254.
N. Mahata, A.F. Cunha, J.J.M. Orfao, J.L. Figueiredo, Appl. Catal. A-
Gen. 2008, 351, 204-209.
brown colloids was achieved, showing the formation of Pd nanoparticles.
The Pd/PVA catalyst can be directly applied in hydrogenation of
[
For internal use, please do not delete. Submitted_Manuscript
This article is protected by copyright. All rights reserved.

Chemistry - An Asian Journal
10.1002/asia.201900333
FULL PAPER
[7]
[8]
[9]
E.A. Gelder, S.D. Jackson, C.M. Lok, Chem. Commun. 2005, 4, 522-
24.
L.B. Kochetova, M.V. Klyuyev, Petrol. Chem. C/C of Neftekhimiia. 1997,
7, 414-421.
5
3
a) T. Wang, Zh. Dong, T. Fu, Y. Zhao, T. Wang, Y. Wang, Y. Chen, B.
Han, W. Ding, Chem. Commun. 2015, 51, 17712-17715; b) G.
Romanazzi, A. M. Fiorea, M. Mali, A. Rizzuti, C. Leonelli, A. Nacci, P.
Mastrorilli, M. M. Dell’Anna, Mol. Catal. 2018,446, 31–38.
[10] H. Jiang, Y. Dong, Chinese J. Chem. 2008, 26, 1407-1410.
[11] X. Yan, M. Liu, H. Liu, K. Liew, N. Zhao, J. Mol. Catal. A-Chem. 2001,
170, 203-208.
[12] Zh. Zhang, X. Wang, K. Yuan, W. Zhu, T. Zhang, Y. Wang, J. Ke, X.
Zheng, C. Yan, Y. Zhang, Nanoscale. 2016, 8, 15744-15752.
[
13] G. Fan, Y. Wang, C. Wang, RSC Adv. 2014, 4, 10997-11002.
14] H. Pan, X. Li, Y. Yu, J. Li, J. Hu, Y. Guan, P. Wu, J. Mol. Catal. A-Chem.
[
2015, 399, 1-9.
[
15] a) X. Yu, M. Wang, H. Li, Appl. Catal. A-Gen. 2000, 202, 17-22; b) M. M.
Dell’Annaa, S. Intini, G. Romanazzi, A. Rizzuti, C. Leonelli, F. Piccinni,
P. Mastrorilli, J. Mol. Catal. A-Chem. 2014, 395, 307–314.
[
[
[
16] C. Torres, C. Campos, J. Fierro, M. Oportus, P. Reyes, Catal. Lett.
2013, 143, 763-771.
17] X. Lu, Y. Chen, Z. Zhao, H. Deng, D. Zhou, C. Wei, R. Nie, Q. Xia, RSC
Adv. 2016, 6, 15354-15361.
18] M. Liu, W. Yu, H. Liu, J. Zheng, J. Colloid Interf. Sci. 1999, 214, 231-
237.
[
19] M. Cargnello, C.B. Murray, Science. 2013, 341, 771-773.
20] M. Zhang, Y. Yang, C. Li, Q. Liu, C.T. Williams, C. Liang, Catal. Sci.
Technol. 2014, 4, 329-332.
[
[21] M. Liu, C. Wang, W.W. Yu, J. Nanosci. Nanotechno. 2010, 10, 7715-
7720.
[
22] S. Hou, C. Xie, H. Zhong, S. Yu, RSC Adv. 2015, 5, 89552-89558.
23] G.M. Whitesides, M. Boncheva, Proc Natl Acad Sci U S A. 2002, 99,
[
4769-4774.
[
24] R. Jain, D. Dominic, N. Jordan, E.R. Rene, S. Weiss, E.D.V.
Hullebusch, R. Hübner, P.N.L. Lens, Chem. Eng. J. 2016, 284, 917-925.
25] X. Wang, F. Yu, C. Xie, S. Yu, Mol. catal. 2018, 444, 62-69.
[
[
26] P. Tomkins, E. Gebauer-Henke, W. Leitner, T.E. Müller, ACS Catal.
2
015, 5, 203-209.
27] I. Jang, K. Song, J.H. Park, M. Kim, D.W. Kim, S.G. Oh, Mater. Lett.
013, 96, 214-217.
28] H. Zhang, Y. Yang, W. Dai, D. Yang, S. Lu, Y. Ji, Catal. Sci. Technol.
012, 2, 1319-1323.
29] F. Zhang, J. Jin, X. Zhong, S. Li, J. Niu, R. Li, J. Ma, Green Chem.
011, 13, 1238-1243.
30] D. Nandi, S. Siwal, M. Choudhary, K. Mallick, Appl. Catal. A-Gen. 2016,
23, 31-38.
[
[
[
[
2
2
2
5
[
[
[
[
[
31] T. Xie, H. Liu, H. Yang, Acta Phys.-Chim. Sin. 2012, 28, 2209-2213.
32] Y. Wang, Y. Deng, F.Shi, J. Mol. Catal. A-Chem. 2014, 395, 195-201.
33] Z. Niu, Y. Li, Chem. Mater. 2014, 26, 72-83.
34] M. Yamauchi, H. Kitagawa, Synthetic Metals. 2005, 153, 353-356.
35] F.A. Harraz, S.E. El-Hout, H.M. Kill, I.A. Ibrahim, J. Catal. 2012, 286,
184-192.
[
[
[
36] B.L. Albuquerque, A. Denicourt-Nowicki, C. Mériadec, J.B. Domingos,
A. Roucoux, J. Catal. 2016, 340, 144-153.
37] S. Paganelli, R. Tassini, G.L. Sorella, O. Piccolo, F. Baldi, V.D. Rathod,
New Biotechnol. 2015, 32, 313-317.
38] M. Adlim, M. A. Bakar, K.Y. Liew, J. Ismail, J. Mol. Catal. A-Chem.
2004, 212, 141-149.
[39] L.S. Ott, R.G. Finke, Coord. Chem. Rev. 2007, 251, 1075-1100.
[40] S. Sharma, J. Colloid Interf. Sci. 2015, 441, 25-29.
[41] H. Toulhoat, P. Raybaud, J. Catal. 2003, 216, 63-72.
For internal use, please do not delete. Submitted_Manuscript
This article is protected by copyright. All rights reserved.

Chemistry - An Asian Journal
10.1002/asia.201900333
FULL PAPER
Entry for the Table of Contents (Please choose one layout)
Layout 1:
FULL PAPER
PVA-encapsulated Pd nanoparticles in
water medium were synthesized and
applied in the hydrogenation of
nitrobenzene under mild conditions,
exhibiting excellent catalytic perfor-
mance for eco-friendly production of
aniline.
Xiaoyan Wang, Changru Huang,
Xiaohao Li, Congxia Xie,* and Shitao Yu
Page No. – Page No.
PVA-encapsulated Palladium Nano-
particles: Eco-friendly and Highly
Selective Catalyst for Hydrogenation
of Nitrobenzene in Aqueous Medium
For internal use, please do not delete. Submitted_Manuscript
This article is protected by copyright. All rights reserved.