Pd nanoparticles/graphene quantum dot supported on chitosan as a new catalyst for the reduction of nitroarenes to arylamines

Article abstract of DOI:10.1007/s13738-020-02104-9

A new heterogeneous catalyst was obtained by growing graphene quantum dots on chitosan and subsequent immobilization of Pd nanoparticles. The catalyst after characterization was used in the reduction of nitroarenes to the corresponding amines by NaBH₄ as a weak reducing agent of nitro compounds. The catalyst exhibited excellent catalytic activity and selectivity under mild reaction conditions in water as a green solvent during 1?h. Additionally, the catalyst can be reused for five consecutive runs without any significant decrease in its activity and selectivity.

Full text of DOI:10.1007/s13738-020-02104-9

Journal of the Iranian Chemical Society
https://doi.org/10.1007/s13738-020-02104-9
ORIGINAL PAPER
Pd nanoparticles/graphene quantum dot supported on chitosan
as a new catalyst for the reduction of nitroarenes to arylamines
Nastaran Kalanpour1 · Saeid Nejati1 · Sajjad Keshipour1
Received: 20 April 2020 / Accepted: 15 October 2020
© Iranian Chemical Society 2020
Abstract
A new heterogeneous catalyst was obtained by growing graphene quantum dots on chitosan and subsequent immobilization
of Pd nanoparticles. The catalyst after characterization was used in the reduction of nitroarenes to the corresponding amines
by NaBH4 as a weak reducing agent of nitro compounds. The catalyst exhibited excellent catalytic activity and selectivity
under mild reaction conditions in water as a green solvent during 1 h. Additionally, the catalyst can be reused for fve con-
secutive runs without any signifcant decrease in its activity and selectivity.
Keywords Heterogeneous catalyst · Graphene quantum dot · Chitosan · Nitroarenes · Reduction
Introduction
as chitosan and cellulose are the leading support [12–14] due
to their unique features such as physical and chemical versa-
tility, ease of chemical or physical modifcation, biodegra-
dability, and suitable sorption capacity [15]. Interests about
chitosan as a support for the metal nanoparticles such as
palladium nanoparticles have been progressively increased
in recent years. Furthermore, the amine functional groups on
the surface of chitosan play a key role for increasing metals
loading on the polymer [16].
Development of industrial processes soared concerns about
the environmental safety. Nitro compounds are considered as
one of the environmental pollutants, so their reduction to the
safe amines is the project agenda for many researches [1, 2].
Arylamines have many useful applications as antioxidants,
corrosion inhibitors, agrochemicals, photographic develop-
ers, and pharmaceuticals [3, 4]. Therefore, catalytic produc-
tion of arylamines from nitroarenes is gained remarkable
attention in the feld of chemical industries. One of the key
approaches is the utilization of nanomaterials such as metal
nanoparticles in heterogeneous catalytic systems due to their
advantages of easy separation, and capability of recovery
and recycles [5]. Among various transition metal nanoparti-
cles, palladium nanoparticles are able to be used for an array
of catalytic goals [6] which its heterogeneous forms have
been employed widely in the reduction of nitroaromatics [4].
Agglomeration of metal nanoparticles is a challenge for
their successful application because of losing the surface
area. Solid supports are important tools to prevent aggrega-
tion of the nanoparticles [7]. In recent years, a wide variety
of supports have been extended for instance cellulose [8],
SBA-15 [9], zeolite [10], and titania [11]. Biopolymers such
Graphene quantum dots (GQDs) have been extensively
considered as a new class of promising organic nanomate-
rials due to their high stability, excellent biocompatibility,
interesting surface area, and low-toxicity [17, 18]. While
various nanosupports were introduced for Pd [19–22], GQDs
demonstrated special activity to increase the catalytic per-
formance of some transition metals such as Pd [4, 23]. In
continuation of our research works about heterogeneous cat-
alytic systems, herein we have investigated the preparation
and catalytic activity of Pd/GQD-Chitosan nanocomposite.
Successively, the catalytic activity of the presented catalytic
system was evaluated through the reduction of nitroarenes to
arylamines using sodium borohydride (NaBH4) at ambient
conditions.
* Sajjad Keshipour
s.keshipour@urmia.ac.ir
1
Department of Nanotechnology, Faculty of Science, Urmia
University, Urmia, Iran
Vol.:(0123456789)
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Journal of the Iranian Chemical Society
Typical procedure for the catalytic reduction
of nitroarenes in the presence of the Pd/
GQD‑Chitosan
Experimental
Materials and methods
In a typical procedure, 1 mmol nitrobenzene as precursor,
0.6 mmol NaBH4, 5 mL H2O, and 0.2 g Pd/GQD-Chitosan
as catalyst were mixed at room temperature for 1 h. The
reaction progress was followed with thin layer chromatog-
raphy (TLC). On completion, the catalyst was separated
through fltering, washed with acetone, and reused for con-
secutive runs (Scheme 1).
In this research, all of the chemicals and solvents were
purchased from Sigma-Aldrich and used without fur-
ther purifcation. Chitosan (80–90% deacetylated) with
2000 MW was obtained from Golden-shell Biochemical
(Zhejiang, China). Citric acid as a starting material for the
production of GQDs was purchased from Sigma-Aldrich
and used without further purification. Field emission
scanning electron microscopy (FE-SEM) was carried out
with a microscope of MIRA3TESCAN-XMU. Energy-
dispersive X-ray spectroscopy (EDS) was performed with
a scanning electron microscope of TSCAN Company, and
the analytical instrumentation used for transformations of
the catalyst was Fourier transform infrared spectrometer
(FT-IR) using potassium bromide. Transition electron
microscopy (TEM) micrographs were obtained with TEM
Philips EM 208S. The elemental analysis was performed
with an Elementar Analysensysteme GmbH VarioEL. Gas
chromatographic (GC) measurements were carried out in
Varian 3900 GC. The following conditions were used for
all GC analyses: injector and detector temperature, 260 °C;
initial temperature, 100 °C; temperature ramp, 3 °C/min;
fnal temperature, 280 °C. Selectivity was calculated as
follows: (peak area of the desired product/sum of the peak
areas for desired and by-product) × 100.
Results and discussion
Chitosan has a lot of amine functionalities susceptible for
modifcation by electrophiles. In this regard, chitosan was
reacted with citric acid under activation with DCC/DMAP
to give citric acid-chitosan. Since one of the most important
approaches for the synthesis of GQD is based on citric acid
self-condensation reaction [24], the loaded citric acid on
the chitosan commenced condensation reaction with unat-
tached citric acids to yield GQDs on chitosan. Finally, Pd
nanoparticles were deposited on the modifed chitosan by
the chemical reduction of Pd(II) (Scheme 2).
Characterization of Pd/GQD‑Ch catalyst
FT-IR spectroscopy was employed to approve the modifca-
tions of chitosan toward the synthesis of Pd/GQD-Chitosan
(Fig. 1). Chitosan citric acid demonstrated an absorption
peak at 1694 cm−1 for carbonyl groups and a new vibra-
tion mode at 3331 cm−1 for OH groups of citric acid. With
the transformation of citric acids to GQDs, the peak at
1694 cm−1 was disappeared and a new peak was observed
at 1706 cm−1 which attributed to the removing carbonyls of
citric acids and formation of new carbonyls on GQDs. In
addition, the OH peak at 3331 cm−1 did not observe in the
GQD-chitosan spectrum confrming the transformation. Pd
deposition of the GQD-chitosan had no signifcant changes
on the GQD-chitosan spectrum, as expected.
Preparation of the Pd/GQD‑Chitosan support
In the experimental procedure, 0.3 g of the N, N′- dicy-
clohexylcarbodiimide (DCC) and 0.05 g of the N, N′-
dimethylaminopyridine (DMAP), 1 g chitosan, and 0.1 g
citric acid were added into a round bottom fask contain-
ing 10 mL DMSO:H2O (1:1) and the mixture was stirred
at 90 °C for 3 h. Then, 20 mL acetone was added to the
mixture and the precipitate was fltered out, washed with
acetone (2 × 10 mL), and dried in an oven at 60 °C. A
beaker contains 1 g citric acid, and 1 g citric acid-chitosan
was heated in a furnace at 150 °C for 1 h. After that, 10 ml
H2O was added to the mixture and the precipitate was
fltered out, washed with acetone (2 × 10 mL), and dried
in an oven at 60 °C.
The EDS analysis was employed to obtain an ele-
mental analysis of the nanocomposite structure. These
results indicated that Pd has been successfully anchored
In order to prepare Pd/GQD-Chitosan, 0.05 g of PdCl2
was gradually added into the 0.5 g obtained GQDs-Chi-
tosan in 10 mL distilled water under continuous stirring.
The reduction of Pd(II) to Pd(0) was performed by the
addition of NaBH4 solution (0.8 g in 5 mL H2O). After 2 h,
the product was separated by centrifuging at 12,000 rpm,
washing with H2O (2 × 5 mL), and drying at 70 °C.
Scheme 1 Reduction of nitroarenes to arylamine on the surface of
the Pd/GQD-Chitosan catalyst
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Journal of the Iranian Chemical Society
Scheme 2 Preparation proce-
dure of Pd/GQD-Ch catalyst
Fig. 1 FT-IR spectra of preparation of chitosan, chitosan citric acid,
GQD-chitosan, and PdNPs/GQDs-Chitosan
Fig. 2 EDS spectra of the Pd/GQD-Chitosan catalyst
SEM image of Pd/GQD-Chitosan was provided for the
observation of the nanocomposite’s surface. Although the
image indicated the polymer particles of chitosan, elucida-
tion of GQD and PdNPs needs TEM micrographs (Fig. 4).
TEM micrographs were prepared for screening Pd
and GQD nanoparticles. Low focused TEM image of Pd/
GQD-Chitosan showed shadows on the polymer backbone
to the nanocomposite (Fig. 2). ICP-MS analysis showed
0.31 mmol Pd per 1 g of the catalyst.
XRD also is a potent analysis instrument to reveal the
composite structures. The analysis was used to investigate
the catalyst structure which showed some peaks related to
chitosan, GQD, and Pd nanoparticles (Fig. 3).
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Journal of the Iranian Chemical Society
Fig. 3 XRD pattern of Pd/GQD-Chitosan
Fig. 5 TEM micrographs of Pd/GQD-Chitosan (the scale bar for a
100 nm, b 50 nm and c 50 nm)
Fig. 4 SEM image of Pd/GQD-Chitosan nanocomposite (the scale
bar = 100 μm)
(Fig. 5a) which more focusing on the shadows (Fig. 5b)
revealed a well-ordered structure of GQD deposited on the
polymer. Also, the dispersion of Pd NPs on the composite
was confrmed with a TEM image with the maximum par-
ticle size between 6 and 8 nm (Fig. 5c).
Thermal Gravimetric Analysis (TGA) was performed
on Pd/GQD-Chitosan and was compared with chitosan’s
TGA curve (Fig. 6). The TGA curve of Pd/GQD-Chitosan
revealed that the catalyst is stable at temperatures below
258 °C. This range of temperature is convenient for catalyst
stability giving an opportunity to use the catalyst in rela-
tively high temperatures. In the meantime, Pd/GQD-Chi-
tosan demonstrated a little decrease in the decomposition
temperature compared to chitosan which obviously confrms
the Pd catalytic activity on the degradation of chitosan.
Fig. 6 TGA analyses of chitosan (a) and Pd/GQD-Chitosan (b)
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Journal of the Iranian Chemical Society
a 63% yield. While GQD cannot carry out the reduction
reaction directly, it improves the catalytic activity of Pd sig-
nifcantly which can be found from 99% conversion of the
reaction in the presence of PdNPs/GQD-Chitosan [4, 23]
(Table 3).
Reduction of nitroarenes
The catalytic activity of the generated Pd/GQD-Chitosan
composite was evaluated in the reduction of nitroarenes.
For that, preliminary optimization was performed on the
nitrobenzene reduction as the model substrate. The speci-
fed time to complete the reduction reaction was deter-
mined in the production of aniline at mild conditions
using TLC. The reduction of 1 mmol nitrobenzene needs
0.062 mmol Pd in the presence of NaBH4 (0.6 mmol)
in H2O (5 mL) at room temperature to give aniline with
99% GC conversion during 1 h. The reaction yield was
declined in low catalyst amounts and the reaction dura-
tion did not decrease using an excess catalyst (Table 1).
Then, the efect of the solvent on the reduction reaction
was investigated. In this regard, aniline was evaluated as
the sole product and its yield was determined to be 99%
in the presence of water (entry 1) and 96% in the presence
of water/ethanol with a 1:1 ratio (entry 2) as solvents. The
use of organic solvents for the reaction including etha-
nol (entry 3), methanol (entry 4), acetonitrile (entry 5),
and dichloromethane (entry 6) did not aford high yields.
Moreover, NaBH4 amounts were optimized with a mini-
mum of 0.6 mmol for obtaining a higher yield.
Recyclability of Pd/GQD-Chitosan was investigated in
the reduction of nitrobenzene. The results demonstrated
that the catalyst could be utilized for the next batch after
the separation of catalyst in the frst run from the reac-
tion medium. It is worthy that the catalytic activity of the
obtained nanocomposite was surprisingly high and the loss
of activity in the recycled catalyst was found to be negli-
gible (Fig. 7).
In addition, the FT-IR spectrum of the recycled catalyst
was prepared and compared to the spectrum of an unused
catalyst (Fig. 8). The spectra comparison evinced that the
catalyst structure did not change during the reaction.
A hot fltration test was performed on the catalyst. For
that, the catalyst was removed from the nitrobenzene reac-
tion mixture after 20 min and the progress of the reaction
was monitored by GC. It is observed that the reaction com-
pletely was quenched after removing the catalyst which con-
frms performing the reaction heterogeneously (Table 4). In
the meantime, a leaching study was carried out on the solu-
tion by ICP-MS analysis which the result showed very low
leaching, 0.2 µg/L, of Pd in the solution.
For the development of the catalyst application, the
reduction of nitrobenzene derivatives was also studied
(Table 2). Nitro compounds were converted to the corre-
sponding amines in high yields. The existence of various
substitutes for nitroarenes afected the performance of the
catalyst. Also, some of the substrates need EtOH:H2O (1:1)
as the solvent.
The proposed mechanism for the reduction of nitro com-
pounds by the catalyst was shown in Scheme 3. The mecha-
nism includes activation of nitro by Pd NPs to give I which
under the nucleophilic attack of hydride ion aforded inter-
mediate II. Intermediate II by the reaction with a H2O and
a hydride ion produced amine and Pd oxide. The prepared
Pd oxide under the reaction with hydride ion can yield Pd to
retain the catalytic cycle. The role of GQD in this reaction
is assisting to the stability of the produced Pd intermediates
because of extensive electronic system on its structure which
can distribute the charge of intermediates on its structure and
help to their stability.
The efects of the catalyst ingredients were investigated
on the reaction yield of nitrobenzene reduction. The reduc-
tion reaction in the presence of GQD-chitosan did not aford
any product and Pd-chitosan gave aniline as the product with
Table 1 Selective reduction of nitrobenzene as a model substrate
Entry Catalyst (mol %) Solvent
Time (h) Yield (%)a
1
2
3
4
5
6
7
8
9b
0.032
0.062
0.062
0.062
0.062
0.062
0.062
0.062
0.062
H2O
H2O
H2O
MeOH
MeCN
CH2Cl2
EtOH
H2O:EtOH (1:1)
H2O
1
1
0.75
1
1
1
1
1
1
81
99
84
45
32
0
51
96
87
Conclusions
Herein, we have developed an active and efective catalytic
system based on chitosan as a green support for the selective
reduction of nitroarenes to arylamines in mild condition with
signifcant yields. Chemoselectivity of the catalyst toward
ortho, meta, and para isomers of the products was also inves-
tigated about the reduction of some nitrobenzene derivatives.
By considering the reusability of the catalyst, no meaningful
loss of yield was observed during its application for fve con-
secutive runs. Finally, arylamines were successfully synthe-
sized during the reduction of nitroarenes in a mild condition
using water as a green solvent at room temperature.
Reaction conditions: nitrobenzene (1 mmol), NaBH4 (0.6 mmol), sol-
vent (5 mL), rt
aGC yield
bNaBH4 (0.5 mmol)
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Journal of the Iranian Chemical Society
Yield (%)a
99
Table 2 Selective reduction of
nitrobenzene derivatives with
PdNPs/GQDs-Chitosan
Entry Substrate
1
Product
Solvent
H2O
2
3
H2O
H2O
98
96
4
H2O
H2O
97
98
5
6b
H2O:EtOH 74
(1:1)
7b
H2O:EtOH 77
(1:1)
8b
9
H2O:EtOH 78
(1:1)
H2O
98
10
H2O
98
11
12
H2O:EtOH 66
(1:1)
H2O:EtOH 62
(1:1)
13
H2O:EtOH 65
(1:1)
14
15
H2O:EtOH 68
(1:1)
H2O:EtOH 81
(1:1)
16
H2O:EtOH 79
(1:1)
Reaction conditions: nitrobenzene (1 mmol), NaBH4 (0.6 mmol), solvent (5 mL), rt
aGC yield
bNaBH4 (1.0 mmol)
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Journal of the Iranian Chemical Society
Table 3 Investigation of the catalyst ingredients
Entry
Catalyst
Yield (%)a
1
2
3
PdNPs/GQD-Chitosan
GQD-Chitosan
PdNPs-Chitosan
99
0
63
Reaction conditions: nitrobenzene (1 mmol), NaBH4 (0.6 mmol), sol-
vent (5 mL), rt
aGC yield
Table 4 Hot fltration test for
Fig. 7 Recycling of the Pd/GQD-Chitosan catalyst for the reduction
Entry Time (min) Yield (%)a
the reduction of nitrobenzene by
of nitroarenes to arylamines
Pd/GQD-Chitosan
1
2
3
20
30
60
47
48
48
Reaction conditions: nitroben-
zene
(1
mmol),
NaBH4
(0.6 mmol), solvent (5 mL), rt
aGC yield
Fig. 8 FT-IR spectra of Pd/GQD-Chitosan (a), and the recovered Pd/
GQD-Chitosan (b)
Scheme 3 Mechanism of the nitro’s reduction by the catalyst
Acknowledgements We gratefully acknowledge fnancial support from
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(2009)
the Research Council of Urmia University.
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1238–1243 (2011)
7. H. Knözinger, K. Kochloef, T. Turek, O. Deutschmann, Ull-
mann’s Encycl. Ind. Chem. 1, 2–110 (2009)
8. S. Keshipour, M. Khezerloo, Appl. Organomet. Chem. 32, e4255
(2018)
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