Ultra-low-loading palladium nanoparticles stabilized on nanocrystalline Polyaniline (Pd@PANI): A efficient, green, and recyclable catalyst for the reduction of nitroarenes

Article abstract of DOI:10.1002/aoc.5159

Ultra-low-loading Pd@PANI nanocomposites (0.048 w.t% Pd) were synthesized via a method that combined interfacial polymerization and in situ composite with camphor sulfonic acid ((+)-CSA) as a dopant. Transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectra, and X-ray photoelectron spectroscopy (XPS) were performed to characterize the structures. It can be used as an efficient catalyst for the reduction of nitroarenes in aqueous solution by using a smaller amount of NaBH₄ (2.5 equiv.) at room temperature with high activity (TON?=?3.4?×?10³), good stability (cycled eight times), as well as wide applicability (27 substrates). The catalyst also showed a marvelous activity in the gram-level reaction (yield?=?92%). UV–Visible spectrophotometry was used to investigate the reaction kinetics for the reduction of 4-nitrophenol to 4-aminophenol, and the results reconfirmed the excellent performance of the catalyst. The unique properties and superior performance of the prepared ultra-low-loading Pd@PANI nanocomposites lead it be an attractive alternative catalyst for conventional organic catalytic applications.

Full text of DOI:10.1002/aoc.5159

Received: 6 May 2019
DOI: 10.1002/aoc.5159
Revised: 21 June 2019
Accepted: 3 July 2019
F U L L P A P E R
Ultra‐low‐loading palladium nanoparticles stabilized on
nanocrystalline Polyaniline (Pd@PANI): A efficient, green,
and recyclable catalyst for the reduction of nitroarenes
Gang Wang
| Shuo Yuan | Zhiqiang Wu | Wanyi Liu | Haijuan Zhan |
Yanping Liang | Xiaoyan Chen | Baojun Ma | Shuxian Bi
State Key Laboratory of High‐efficiency
Ultra‐low‐loading Pd@PANI nanocomposites (0.048 w.t% Pd) were synthesized
Utilization of Coal and Green Chemical
Engineering, National Demonstration
Center for Experimental Chemistry
Education, College of Chemistry and
Chemical Engineering, Ningxia
via a method that combined interfacial polymerization and in situ composite
with camphor sulfonic acid ((+)‐CSA) as a dopant. Transmission electron
microscopy (TEM), X‐ray diffraction (XRD), Fourier transform infrared (FTIR)
spectra, and X‐ray photoelectron spectroscopy (XPS) were performed to charac-
terize the structures. It can be used as an efficient catalyst for the reduction of
nitroarenes in aqueous solution by using a smaller amount of NaBH4 (2.5
equiv.) at room temperature with high activity (TON = 3.4 × 103), good stabil-
ity (cycled eight times), as well as wide applicability (27 substrates). The cata-
University, Yinchuan 750021, P. R. China
Correspondence
Wanyi Liu, State Key Laboratory of High‐
efficiency Utilization of Coal and Green
Chemical Engineering, College of
Chemistry and Chemical Engineering,
Ningxia University, Yinchuan 750021,
People's Republic of China.
lyst also showed
a marvelous activity in the gram‐level reaction
(yield = 92%). UV–Visible spectrophotometry was used to investigate the reac-
tion kinetics for the reduction of 4‐nitrophenol to 4‐aminophenol, and the
results reconfirmed the excellent performance of the catalyst. The unique prop-
erties and superior performance of the prepared ultra‐low‐loading Pd@PANI
nanocomposites lead it be an attractive alternative catalyst for conventional
organic catalytic applications.
Email: liuwy@nxu.edu.cn Haijuan Zhan,
State Key Laboratory of High‐efficiency
Utilization of Coal and Green Chemical
Engineering, College of Chemistry and
Chemical Engineering, Ningxia
University, Yinchuan 750021, People's
Republic of China.
Email: zhanhj@nxu.edu.cn
KEYWORDS
Funding information
catalytic performance, palladium nanoparticle, polyaniline, reduction of nitroarenes in water, reused
for multiple cycles
The Major Innovation Projects for Build-
ing First‐class Universities in China's
Western Region, Grant/Award Number:
ZKZD2017003; the National First‐rate
Discipline Construction Project of
Ningxia, Grant/Award Number:
NXYLXK2017A04; National Natural Sci-
ence Foundation of China, Grant/Award
Numbers: 21862013 and 51364038
1 | INTRODUCTION
for supported catalytic materials, excellent environmental
stability, and low cost.[1–7] Moreover, metal/PANI com-
Polyaniline (PANI) is considered to be one of the most
promising multifunctional polymers due to its easy prep-
aration, high electrical conductivity, good performance
posites have been applied in organic reactions in a signif-
icant way,[8–22] which are extremely cheap and recyclable
in comparison with organometallic complexes.[23]
Appl Organometal Chem. 2019;e5159.
wileyonlinelibrary.com/journal/aoc
© 2019 John Wiley & Sons, Ltd.
1 of 12
https://doi.org/10.1002/aoc.5159

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WANG ET AL.
Among nano‐M@PANIs, nano‐Pd@PANI is one of
reactions require to use of excessive amounts of NaBH4
and additional organic auxiliaries.[40] Moreover, using
large amounts of metal as a catalyst leads to heavy‐metal
contamination and very low TON values.[41–44] Hence, an
alternative, cost‐effective, high‐activity, high‐stability,
and more environmentally friendly reduction of
nitroarenes with negligible metal contamination is of
high demand.[45]
Herein, we designed and synthesized a well‐dispersed
Pd@PANI catalyst via self‐stabilized dispersion polymeri-
zation (SSDP) method by using camphor sulfonic acid
((+)‐CSA) as a dopant. The dark golden powder was
obtained in 75% yield (Figure 1a). Inductively coupled
plasma mass spectrometry (ICP‐MS) analysis indicated
that the weight ratio of Pd NPs in the material was
0.0476%. After a series of conditions optimizations, the
Pd@PANI catalyst was successfully applied to reduce var-
ious nitroaromatics with water as the solvent and could
be reused for more than eight times without deactivation,
indicating great potential for reagent development. The
reaction kinetics were also studied in our work.
the most important catalysts due to its wide catalytic
applications in various organic reactions.[24–29] The
preparation of nano‐Pd@PANI is usually based on the
initial oxidation of aniline monomers to obtain PANI
carriers. Then, the nanoparticles are obtained by
impregnating with Pd.[28] It is worth noting that the
Yu et al. research team[24–26,30] recently reported the
catalyst obtained by oxidizing aniline via air or hydro-
gen peroxide and showed an ultra‐low metal load
(0.12–0.75%) and high stability. The catalyst has been
successfully applied for Suzuki cross‐coupling[24,31] with
a high turnover number (TON) at 4.7 × 103, which
showed the possibility of utilizing ultra‐low supported
catalyst materials.
Of late, more and more studies on metal NPs sup-
ported on a recyclable polymer‐supported palladium cat-
alyst,[32–36] such as graphene oxide[37,38] or graphene,[39]
have been reported for the reduction of nitroarenes. How-
ever, the nano‐M@PANIs materials are not widely
accepted for the reduction of nitroarenes, since the
FIGURE 1 (a) Photograph of the Pd@PANI catalyst. (b) Scanning electron microscope (SEM) image of Pd@PANI catalyst. (c–d) HRTEM
image of Pd@PANI catalyst (Pd NPs: small black dots)

WANG ET AL.
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and purified by silica gel column chromatography. The
structure of the product was characterized by proton
nuclear magnetic resonance (1HNMR) and carbon‐13
nuclear magnetic resonance (13CNMR). Characterization
data of the products are given in the Supplementary
Material.
2 | EXPERIMENTAL
2.1 | General methods
All reagents are more than 98% pure and the solvent is A.
R. It is used directly after purchase. Unless otherwise
stated, the reaction is carried out in air. 1H and 13C
NMR were recorded by Bruker 400 MHz spectrometer.
ICP‐MS measurements were conducted by Agilent 7800.
The morphology of the materials was obtained on JEOL
JEM‐2010F High Resolution Transmission Electron
Microscope (HRTEM). The powder X‐ray diffraction
(XRD) patterns were recorded by a D/max 2200
PC03030502 X‐ray diffractometer using Cu‐Karadiation.
Fourier transform infrared (FTIR) spectra were recorded
in the range of 400–4000 cm−1 using a Spectrum
Two03040404 FT‐IR spectrometer. The X‐ray photoemis-
sion (XPS) spectra were performed at room temperature
using a Thermo ESCALAB 250xi hemispherical electron
energy analyzer.
2.4 | General methods: Ultraviolet
spectrophotometric determination of
reduction reaction of 4‐Nitrophenol
4‐nitrophenol (2.6 × 10−4 mol) and sodium borohydride
(0.13 mol) were added to a 250 mL flask. Then, 100 ml
of deionized water was added and stirred for 30 min at
room temperature under the argon atmosphere. Accu-
rately remove 2 ml of solution from the reaction and
dilute to 100 ml, and the full wavelength absorbance
was measured by a ultraviolet (UV–Vis) spectrophotome-
ter, with the result was recorded as 0 min. Pd@PANI cat-
alyst (30 mg) was added to the reaction bottle, and the
above operation was repeated at certain time intervals.
The full wavelength absorbance determination was car-
ried out by a UV–Vis spectrophotometer.
2.2 | Gram‐scale preparation of Pd@PANI
Aniline (1.864 g, 20 mmol), camphor sulfonic acid
(4.686 g, 20 mmol), deionized water (40 ml), and chloro-
form (50 ml) were added to a jacket reactor. Mechanical
stirring was applied and cooling lowered the temperature
to −35 °C until the solution was a white paste. Then
PdCl2 (89.29 mg, 0.5 mmol), (NH4)2S208 (4.564 g,
20 mmol), and camphor sulfonic acid (20 mmol) were
added to the reactor. Continuous stirring continued until
the solution was completely blue‐black, and static inver-
sion was carried out at a low temperature (−35 °C). The
blue‐black Pd@PANI was obtained by filtration, washing
with deionized water for four times and ethanol for four
times, using 1 mol/L sodium hydroxide (50 ml) for the
acid removal treatment, washing with water and ethanol
to neutral pH conditions, and then vacuum drying at
60 °C for 4 hr for collection and use.
2.5 | TON and TOF value calculation
method in Table 2
Number of moles of reactant conversion
TON ¼
moles of catalyst active centers
ꢀ
ꢁ
D*Y
B
m*C
¼
B ¼
(1)
(2)
M
Reaction time
TON
TOF ¼
TON is the turnover number, indicating the molar
amount of reactants converted per unit of active site
(Equation (1)). “D” represents the mole fraction of the
reactants added (mmol), “Y” represents the conversion
rate of the reactants, “m” represents the mass of the cat-
alyst added in the reaction (mg), “C” represents the per-
centage of active sites contained in the catalyst (In this
paper, the active center of the catalyst is approximately
equal to the Pd content obtained by ICP test), “M” repre-
sents the mass fraction of metal palladium. TOF is the
turnover frequency, represents the amount of reactant
that is converted per unit time by the unit active site
(Equation (2)).[31,46]
2.3 | General procedure for the reduction
reaction
Nitrobenzene (0.5 mmol), sodium borohydride
(1.25 mmol), and 0.03 g Pd@PANI (1.4 × 10−2 mg of cal-
culated palladium) were added into a 25 ml reaction bot-
tle, and 2 mL of deionized water was added and stirred at
room temperature. TLC followed up the reaction. After
the reaction, the catalyst was collected by centrifugalizing
and was reused in the next round of reactions. The
organic phase was washed with saturated salt water and
dried with anhydrous Na2SO4. The product was separated

4 of 12
WANG ET AL.
air rather than brown (Figure 1, Image a).[31] Addition-
ally, the successful preparation strategy of ultra‐low‐
loading Pd catalysts on Pd@PANI nanoparticles has the
advantage of using much less Pd than traditional catalysts
(currently only 0.048% Pd vs. previously 0.125–0.75%[31]
and 10.0%[24] Pd).
3 | RESULTS AND DISCUSSION
3.1 | Crystal structure and morphology of
catalyst
The catalysts loaded with Pd always show excellent cata-
lytic performance for reduction reactions, but their high
price limits their application. Therefore, it is necessary
to develop a supported Pd catalyst with low Pd loads
and appreciable activity compared with conventional cat-
alysts. Recently, our research team synthesized ultra‐low‐
loading Pd@PANI nanocomposites by the combination of
interfacial polymerization and in situ composite method.
An aqueous solution of camphor sulfonic acid ((+)‐CSA)
and PdCl2 were used as raw materials and ammonium
peroxodisulfate as the oxidant during the preparation.[28]
The in situ doping and loading were completed in about
4 hr by a one‐pot method. Then, the CSA‐doped
polyaniline nanoparticles were deacid‐doped by NaOH
aqueous solution (1 M). After the CSA was completely
removed, a black Pd@PANI nanocomposites without
CSA was finally obtained. Compared with the Pd nano-
particles grown by combining aqueous nanofiber disper-
sion with a solution of palladium (II) nitrate (4:1) and
incubation for 1 d,[28] and the air stirring of PdCl2 and
aniline for 96 hr,[24] the reaction time was greatly reduced
in this work. Moreover, this method eliminated the need
for a separate reduction step. Fortunately, the black
Pd@PANI nanocomposites are quickly obtained in the
Figure 1 (b) shows a scanning electron microscope
(SEM) image of the Pd@PANI catalyst. The fiber net
of Pd@PANI, like arachnoids in the brain, can be
observed in the image. This is different from the lamel-
lar polyaniline prepared by the conventional method.
The use of camphorsulfonic acid in the polymerization
of aniline can effectively adjust the growth of
polyaniline, and linearly grow to obtain a linear net-
work of polyaniline material. Besides, linear mesh struc-
tures may also be one of the reasons for the extremely
low metal loading. The transmission electron micro-
graph (TEM) images of the Pd@PANI indicate the Pd
nanoparticles have good dispersion on the PANI support
(Figure 1, Image c), and the average size of the Pd
nanoparticles was determined to be in the range of 4–
7 nm. High‐resolution transmission electron microscopy
(HR‐TEM, Figure 1, Image d) shows Pd (111) lattice
planes with a spacing of d(111) = 0.223 nm, consistent
with the literature value for nanocrystalline Pd,[26,31]
Therefore, it was thought that the coordinate structure
of Pd and polyaniline is an asymmetric octahedron
(Figure 1(e)). Octahedral nanocrystals (Pd (111)) and
structures have the highest surface energy in the
FIGURE 2 (a) XRD of the Pd@PANI; (b) IR of the Pd@PANI; (c) XPS N 1 s of the Pd@PANI; (d) XPS Pd 3d of the Pd@PANI

WANG ET AL.
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presence of reductant and are highly active to the
reduction of nitrobenzene by NaBH4.
3.3 | Reduction of nitroarenes by
Pd@PANI
The initial conditions of the reaction are: catalyst 50 mg,
four equivalents of sodium borohydride, 100 °C, using
water as a solvent, and the yield detected by 1HNMR with
1,3,5‐trimethoxybenzene as the internal standard. To our
great delight, when using 30 mg Pd@PANI catalyst to cat-
alyze the hydrogenation of 0.5 mmol of nitrobenzene with
2 mmol of NaBH4 in H2O, 92% yield of aniline is obtained
in 4 hr (Table 1, entry 3, 7). Too much or little catalyst leads
to a decrease of yield, which may be related to the course of
the reaction. Different amounts of catalyst have different
controls of the intermediate path of the reaction, and the
by‐products involved are different (Table 1, entries 1–4).
It is interesting to note that the best yield can be obtained
with 2.5 equivalents of sodium borohydride (entry 7).
When the sodium borohydride is less than two equivalents,
the nitrobenzene cannot be completely reduced, and a
large amount of nitrogen oxybenzene by‐product is pro-
duced (entries 8–9). Next, we tried different reducing
agents, such as hydrazine hydrate (entry 10), ascorbic acid
(entry 11) and formate (entry 12), but did not get good
results. This may be because the reducing mechanism of
different reducing agents is different, and a reducing agent
such as formate amine can be decomposed under the
action of a catalyst to generate hydrogen in situ. Sodium
borohydride is a nucleophilic addition of the positive
charge center by the dissociated hydrogen anion. The reac-
tion does not require heating and have a higher yield at
room temperature (entry 16). Finally, by comparing the
activities of the reactions in different solvents, it was found
that the catalysts gave good results in methanol, ethanol,
trifluoroethanol, and hexafluoroisopropanol (entries
17–21). It was also confirmed that sodium borohydride
does not reduce nitrobenzene without the addition of a cat-
alyst (entry 23). After an optimization of the conditions
and taking into account the energy‐saving and environ-
mentally friendly green chemical production concept, it
can be concluded that the optimum reaction conditions
are 30 mg of catalyst, 2.5 equivalents of sodium borohy-
dride, at room temperature in water, with a 92% yield of
aniline obtained in 4 hr.
3.2 | Catalyst characterization
The crystalline structures of the Pd@PANI catalyst were
characterized by X‐ray diffraction (XRD), shown in
Figure 2(a). XRD patterns of PANI and Pd@PANI
revealed that the polyaniline salt was partially crystalline
and shows peaks at 2θ = 15.1, 20.1, and 24.0 correspond-
ing to the interface distances (d) of 6.1, 4.3, and
3.5 Å,[36,47] respectively. However, a new diffraction peak
appears at 40° (Pd (111)) in the curve of Pd@PANI, which
belongs to the amorphous diffraction peak of Pd. It is also
observed that the regularity of Pd@PANI is lower than
PANI, because Pd inserts into the space between PANI
chains. From ICP‐MS we can find the load of the precious
metals of the Pd@PANI catalyst is much lower than that
of the materials prepared through the aerobic oxidation
method (10.0%)[24,26] or interfacial emulsion polymeriza-
tion only by ammonium persulfate oxidation.[22] It is also
significantly lower than the materials prepared by hydro-
gen peroxide oxidation (0.75%).[31]
The molecular structure of the Pd@PANI net was
characterized by Fourier‐transform infrared spectroscopy
(FTIR) (Figure 2b), which shows the characteristic peaks
of PANI at 1584 and 1495 cm−1 (quinoid and benzenoid
ring, respectively)[26]; the ratio of quinoid and benzenoid
is determined to be ca 16:31. In addition, there is no char-
acteristic peak of CSA (C=O 1730 cm−1, S=O 1160 cm−1
,
C‐S 752 cm−1) in the infrared spectrum. It shows that
CSA has been completely removed after dedoping with
sodium hydroxide, which does not affect the catalytic
activity in the subsequent reaction. Camphor sulfonic
acid acts as a templating agent in the preparation process.
It essentially acts to regulate the molecular morphology
and size of polyaniline.
XPS results showed that only Pd2+ existed in the fresh
material. The XPS spectrum of N is deconvoluted into
three major component peaks centered at 399.7(‐NH2),
398.9, and 398.3(=NH) eV, and the linewidths are 1.1,
0.85, and 0.74 eV, respectively, in Figure 2 (c) and (d).
The binding energy of 398.9 eV suggests that the quinoid
ring of PANI is adsorbed (combined) with the plane of
the ring parallel to the metal surface of Pd (111), with
the bonding being via the π orbitals of quinoid ring or
imine in the PANI and the lone‐pair on the N atom.[24]
Additionally, for Pd spectra, two major component peaks
Most impressively, the ultra‐low‐loading palladium
loading nanoparticle (Pd@PANI) can be reused by simple
centrifugation and shows excellent stability and recycla-
bility (Table 1, entry 16e–h). Notably, this catalyst could
be used for 20 mmol‐scale without the product yield
decreasing (entry 22), showing good practicability for
organic synthesis.
We are pleased that the catalyst has broad applicability in
the nitro reduction reaction. Whether the substituent is an
electron donating group or an electron withdrawing group,
centered at 343.3(3d3/2
linewidths of 1.17 eV and 1.0 eV, respectively, which are
attributed to a divalent palladium structure.
) and 338.1(3d5/2) eV with

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WANG ET AL.
TABLE 1 Optimization of reaction conditionsa
Entry
Pd@PANI (mg)
Hydrogen source (equiv.)
Temp (°C)
Solvent
Yieldb (%)
1
50
40
30
20
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
1200
/
NaBH4 (4)
100
100
100
100
100
100
100
100
100
100
100
100
80
Water
Water
Water
Water
Water
Water
Water
Water
Water
Water
Water
CH3OH
Water
Water
Water
Water
MeOH
EtOH
i‐PrOH
TFE
82
2
NaBH4 (4)
74
3
NaBH4 (4)
92
4
NaBH4 (4)
82
5
NaBH4 (3.5)
NaBH4 (3)
74
6
76
7
NaBH4 (2.5)
NaBH4 (2)
92
8
76
9
NaBH4 (1.25)
NH2.NH2 H2O (4)
Vitamin C (4)
CHOONH4 (4)
NaBH4 (2.5)
NaBH4 (2.5)
NaBH4 (2.5)
NaBH4 (2.5)
NaBH4 (2.5)
NaBH4 (2.5)
NaBH4 (2.5)
NaBH4 (2.5)
NaBH4 (2.5)
NaBH4 (2.5)
NaBH4 (2.5)
56
10
11
12
13
14
15
16
17
18
19
20
21
22
23
N.Rc
Tranced
10
81
60
85
40
87
R.T
R.T
R.T
R.T
R.T
R.T
R.T
R.T
92,90e,91f,93g,91h
94
90
N.Rc
89
HFLP
Water
Water
92
92f
/
aNitrobenzene (51 μL, 62 mg, 0.5 mmol), 2 ml water, stir at room temperature for 4 h.
bThe yield detected by1HNMR with1,3,5‐Trimethoxybenzene as internal standard.
cNo reaction product detected.
dYield <3%.
eConversion of nitrobenzene with the reusability of catalyst.
fReaction scale enlarged to 20 mmol, isolated yield.
we can obtain a high yield. After 360 minutes of reaction,
the reduction product aniline of nitrobenzene was obtained
in a yield of 92% (Table 2, entry 2a). When the substituent
group is a hydroxyl group, the reaction time is the shortest,
and the 2‐, 3‐, 4‐substituted product can be obtained in
30 minutes. At the same time, we can find that when the
substituent group is 3‐substituted, it can get higher yield in
a shorter time (Table 2, entries 2c, 2f). When the substituent
group is a halogen element (Table 2, entries 2j–2n), the reac-
tivity of the fluorine atom is the strongest, which may be
related to the electronegativity of the atom. The fluorine
atom has strong electronegativity and strong electron‐
withdrawing ability, which makes the electron cloud
density on the entire benzene ring lower, which is more
conducive to the reduction reaction.
Interestingly, nitrobenzenes containing carbonyl
groups on the benzene ring are reduced at the same time
(Table 2, entries 2r–2 s). Similarly, styrene‐substituted
nitrobenzene, carbon–carbon double bonds, and nitro
groups are also present (Table 2, entries 2x), but haloge-
nated nitrobenzene and cyano‐substituted nitrobenzene
are reduced only by a nitro group (Table 2, entries 2j–
2n, 2u). It is worth noting that the yield of the compound
containing three nitro groups, nitro‐containing bisindole
methane derivatives and the heterocyclic are high
(Table 2, entries 2y–3a, 2z), while NaBH4 dosage was half

WANG ET AL.
7 of 12
TABLE 2 Scope of substrates catalyzed by Pd@PANIa,b
d
d
d
2a, 4 h, 92%c
TON = 3.4 × 103
TOF = 11 × 10−4 h−1
2b, 0.25 h, 96%
2c, 0.08 h, 97%
2d, 0.41 h, 92%
TON = 3.5 × 103
TOF = 0.7 × 10−4 h−1
TON = 3.6 × 103
TOF = 0.2 × 10−4 h−1
TON = 3.4 × 103
TOF = 1.2 × 10−4 h−1
2e, 1.5 h, 88%
2f, 1.5 h, 88%
2 g, 1.5 h, 90%
2 h, 0.66 h, 61%
TON = 3.3 × 103
TOF = 4.5 × 10−4 h−1
TON = 3.3 × 103
TOF = 4.5 × 10−4 h−1
TON = 3.3 × 103
TOF = 4.4 × 10−4 h−1
TON = 2.2 × 103
TOF = 2.9 × 10−4 h−1
e
2i, 0.66 h, 90%
2j, 0.5 h, 95%
2 k, 0.66 h, 85%
2 l, 0.33 h, 42%
TON = 3.3 × 103
TOF = 1.9 × 10−4 h−1
TON = 3.5 × 103
TOF = 1.4 × 10−4 h−1
TON = 3.1 × 103
TOF = 1.9 × 10−4 h−1
TON = 1.5 × 103
TOF = 2.1 × 10−4 h−1
2 m, 1 h, 34%
2n, 3 h, 80%
2o, 1.5 h, 88%
2p, 4 h, 61%
TON = 1.2 × 103
TOF = 7.9 × 10−4 h−1
TON = 2.9 × 103
TOF = 10 × 10−4 h−1
TON = 3.2 × 103
TOF = 4.5 × 10−4 h−1
TON = 2.2 × 103
TOF = 17 × 10−4 h−1
2q, 6 h, 92%
2r, 2 h, 63%
2 s, 4 h, 61%
2 t, 2 h, 64%
TON = 3.4 × 103
TOF = 17 × 10−4 h−1
TON = 2.3 × 103
TOF = 8.5 × 10−4 h−1
TON = 2.2 × 103
TOF = 17 × 10−4 h−1
TON = 2.4 × 103
TOF = 8.3 × 10−4 h−1
f
2u, 2 h, 63%
2v, 3.5 h, 44%
2w, 2 h, 74%
2x, 1 h, 74%
TON = 2.3 × 103
TOF = 8.5 × 10−4 h−1
TON = 1.6 × 103
TOF = 21 × 10−4 h−1
TON = 2.7 × 103
TOF = 7.2 × 10−4 h−1
TON = 2.7 × 103
TOF = 3.6 × 10−4 h−1
(Continues)

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WANG ET AL.
TABLE 2 (Continued)
f
f
f, g
2y, 2 h, 82%
2z, 2 h, 90%
3a, 5 h, 70%
TON = 3.0 × 103
TOF = 6.5 × 10−4 h−1
TON = 3.3 ×
103 TOF = 5.9 × 10−4 h−1
TON = 2.6 × 103
TOF = 19 × 10−4 h−1
Reaction Conditions:
aReaction Conditions: nitroarene (1 mmol), NaBH4 (1.25 mmol), Pd@PANI (0.026 mol %), distilled water (2 mL), room temperature.
bIsolated yields.
cYield detected by 1H NMR with Dibromomethane as internal standard.
dYield after work up.
e24% of 3‐Methoxyaniline.
fReaction at 100 °C.
g5 equivalents of NaBH4 was used.
of normal. Therefore, polyaniline is not only a supporting
material for metals, but also acts as a nitrogen‐containing
ligand to synergistically increase the activity of the catalyst.
The catalyst was tolerant to nitrogen and oxygen and the
reactions of 1y and 1z afforded the desired products 2y in
82% yield and 2z in 90% yield. Thus, the ability of the
Pd@PANI for nitrobenzene reduction with a high TON
value is an order of magnitude higher than other novel diva-
lent catalysts reported recently for the reduction of nitro-
benzene. Additionally, 1.4 g of the catalyst can be
produced in a single batch, which has a higher value than
the catalyst of micro‐preparation for application.
some fibers appeared agglomerated, but the obvious
pore structure was still observed in the material
(Figure 3b). It can be seen from Figure 3c that the crys-
tal form of polyaniline changes after the catalyst is used
six times, but a distinct diffraction peak of palladium
can be seen at 40.0°. In addition, we found in the infra-
red spectrum that the catalyst used 6 times showed sig-
nificant blue‐shifting at 1377 and 830 cm−1, peak
positions moved from 1377 and 830 cm−1 to 1282 and
807 cm−1, respectively.
Figure 3e shows the N 1 s XPS core‐level spectra of
Pd@PANI net catalyst that was already used six times.
The spectrum of N is deconvoluted into three major
component peaks centered at 400.1, 399.5(‐NH2), and
398.3(=NH) eV, and the linewidths are 1.18, 0.90, and
0.92 eV, respectively. However, binding energies at
400.1 shows that the nitrogen atoms of amino coordi-
nated to Pd(0)[25] via the lone‐pair on the N atom of
the amino without any π orbitals, consistent with the
rules of XPS chemical shift.[26] Figure 3f shows the Pd
3d XPS core‐level spectra of a Pd@PANI net catalyst
that was already used six times. The ratio of divalent
Pd (342.3 eV 3d3/2, 337.1 eV 3d5/2) to zero‐valent Pd
(341.1 eV 3d3/2, 335.8 eV 3d5/2) is 1:2, indicating that
3.4 | Recycle and reuse of the catalyst
In addition, we examined the reusability of the catalyst.
After each reaction, the catalyst was collected by simple
filtration, washed several times with water and ethanol,
dried in a vacuum oven and placed in the next use. We
found that the catalyst was used 8 times, the activity
was not reduced, and the yield was 93% (Figure 3a).
After repeated use of the catalyst for six times, the
original reticular fiber structure was destroyed, and

WANG ET AL.
9 of 12
FIGURE 3 (a) Recycle and reuse of the Pd@PANI catalyst; (b) SEM of Pd@PANI catalyst after recycling six times; (c) XRD pattern of 6‐
recycled Pd@PANI; (d) FT‐IR spectra of PANI, Pd@PANI catalyst and after six times; (e) and (f) XPS spectra of 6‐recycled Pd@PANI
most of the divalent palladium has been reduced to
zero‐valent palladium by sodium borohydride during
reaction. Pd0 should be the real catalytic species gener-
ated via the reduction of Pd2+ by sodium borohy-
dride.[26,31] Although morphology (Figure 3b), crystal
structure (Figure 3c) and the valency of Pd has been
changed after reaction, the yield after eight repeated
cycles is very excellent (93%).
scan using an ultraviolet–visible spectrophotometer, and
the activity of the catalyst was further compared. Usually,
the aqueous solution of 4‐nitrophenol exhibits a maxi-
mum absorption peak at 317 nm,[48] but after the addition
of the reducing agent sodium borohydride, the maximum
absorption peak is red‐shifted to 400 nm.[49] It can be
seen from the figure that after the addition of the
Pd@PANI catalyst to the reaction mixture, the maximum
absorption peak at 400 nm immediately decreases, and at
the same time, a new peak appears at 300 nm due to the
formation of the reduction product 4‐aminophenol.
It can be noticed that the reaction was carried out
immediately after the addition of the catalyst, and there
was no induction time (Figure 4a and 4c). This was
because we introduced argon into the solution before
the start of the reaction, the effect of dissolved oxygen
in the water on the reaction was excluded. Secondly, the
3.5 | The kinetics of the reduction of
nitroarenes by Pd@PANI
We investigated the kinetics of our developed catalyst for
the reduction reaction by using 4‐nitrophenol as a model
substrate. The apparent rate constant of 4‐nitrophenol
reduction was calculated by performing a full wavelength

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WANG ET AL.
FIGURE 4 (a, c) Successive UV–visible spectra of 4‐nitrophenol using: (a) First use, (c) Repeated six times catalysts. (b, d) Plot of ln (Ct/C0)
versus time for the reduction of 4‐nitrophenol with (b) First use, (d) Repeated six times catalyst
reducing agent sodium borohydride is in absolute excess,
so pseudo‐first‐order kinetic assumptions can be applied
to the 4‐nitrophenol alone (Equation (3)).[50,51] A0 is the
absorbance at time 0, At is the absorbance at time t, and
Kapp is the apparent rate constant for reaction.
found to be 11.2 × 10−4 −1. A comparison of the Kapp
s
values has been provided in Table 3. We found that the
catalyst cycled for six times had approximately twice the
apparent reaction rate constant (Kapp) than fresh catalyst.
This means that the efficiency of the catalyst enhances
while the reaction proceeds, and the ability of Pd(0) is
higher than Pd (II) in the Metal/PANI system (Table 3).
dCt
dt
lnCt
C0
lnAt
A0
¼ −KappCt or
¼ −Kapp
−
¼ −Kapp
t
or −
t
(3)
Figure 4 shows the Plot of ln (Ct/C0) versus time for
the reduction of 4‐nitrophenol with (Figure 4b) First use,
(Figure 4d) Repeated six times catalyst. From the slope of
the line obtained by plotting lnCt/C0 vs. time, the appar-
ent rate constant (Kapp) for the reduction of 4‐nitrophenol
could be obtained. For the first‐used catalyst, Kapp was
4 | CONCLUSIONS
In conclusion, a Pd@PANI net containing 0.046 w.t.% Pd
was prepared in situ by a simple protocol at a gram‐scale
and was characterized by XRD, XPS and TEM techniques.
Palladium nanoparticles with a size of about 4–7 nm were
uniformly dispersed on PANI without significant
agglomeration. Compared with previously reported cata-
lytic systems, the Pd@PANI nanoparticles have the
following three advantages: (i) The catalyst can
reduce nitroaromatic hydrocarbons to corresponding
aminoaromatic hydrocarbons in aqueous solution at
room temperature only using NaBH4 (2 equiv.) as a
hydrogen source; (ii) The catalyst has high stability and
can be recycled more than eight times. (iii) The ultra‐
low‐loading Palladium and higher TON value
(TON = 3.4 × 103).
found to be 6.4 × 10−4 −1. Repeated six times, Kapp was
s
TABLE 3 Comparison of Kapp with different usage times cata-
lysts for the reduction of 4‐nitrophenol
Catalyst
Kapp (×10−4 s−1
)
Error limits
First use
6.4
11.2
9.8
0.0108
0.0089
0.0019
R. f[20]
Repeated six times
PdCl2
PS@PANI@Au/m‐SiO2
8.3

WANG ET AL.
11 of 12
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The unique properties of this catalyst make it an
4520.
attractive alternative for conventional organic catalytic
applications. More related works are ongoing in our labo-
ratory to develop the practical green synthetic
methodologies.
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ACKNOWLEDGEMENTS
J. Mater. Chem. A 2013, 1, 13197.
This work was financially supported by the Major Inno-
vation Projects for Building First‐class Universities in
China's Western Region (No. ZKZD2017003), the
National First‐rate Discipline Construction Project of
Ningxia (No. NXYLXK2017A04) and the National Natu-
ral Science Foundation of China (No. 51364038 and No.
21862013). We thank LetPub (www.letpub.com) for its
linguistic assistance during the preparation of this
manuscript.
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