Pd(0) supported on N-doped graphene quantum dot modified cellulose as an efficient catalyst for the green reduction of nitroaromatics

Article abstract of DOI:10.1039/c6ra19668c

A new efficient catalyst was introduced for the green reduction of nitroaromatics. The catalyst was obtained via modification of cellulose with N-doped graphene quantum dots and Pd nanoparticles. The new cellulose nanocomposite after characterization was applied as the catalyst in the reduction reaction of nitroaromatics using NaBH₄ at room temperature. Aromatic amines were obtained as the product of the reduction reaction over 2 h. This reaction has green reaction conditions such as mild reaction conditions, high yield, green solvent and recyclable catalyst. Also, the recovered catalyst is applicable in the reduction reaction 6 times without significant decrease in activity.

Full text of DOI:10.1039/c6ra19668c

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Pd(0) supported on N-doped graphene quantum dots modified
cellulose as an efficient catalyst for the green reduction of
nitroaromatics
Received 00th January 20xx,
Accepted 00th January 20xx
Sajjad Keshipoura and Kamran Adaka
DOI: 10.1039/x0xx00000x
A new efficient catalyst was introduced for the green reduction of nitroaromatics. The catalyst was obtained via
modification of cellulose with N-doped graphene quantum dots and Pd nanoparticles. The new cellulose nanocomposite
after characterization was applied as the catalyst in the reduction reaction of nitroaromatics using NaBH4 at room
temperature. Aromatic amines were obtained as the product of the reduction reaction during 2h. This reaction has the
green reaction conditions such as mild reaction conditions, high yield, green solvent and recyclable catalyst. Also, the
recovered catalyst is applicable in the reduction reaction for 6 times without significant decrease in activity.
www.rsc.org/
silica, cellulose, zeolite, alumina, titania and carbon nanotube
were applied. Among the supports, cellulose has been
Introduction
Reduction of organic nitro compounds is one of the most
important reactions in the organic synthesis for the preparing
of amines.1 Aromatic amines have significant application in the
preparation of dyes, antioxidants, pharmaceuticals, and
agrochemicals.2 Various approaches have been introduced for
the reduction of aromatic nitro compounds using a hydrogen
source such as alcohol, hydrazine hydrate, silane, and format.3
Reduction of nitro groups with sodium borohydride (NaBH4) in
combination with metal nanoparticles represents an important
catalytic system in modern preparative chemistry. The
reduction of nitro compounds into amines using NaBH4 under
normal conditions4 does not perform, instead, NaBH4 in
combination with a transition metal halide is able to reduce
aromatic nitro compounds successfully.5 NaBH4, in protic
moieties, reduces transition metal ions to give metal boride
nanoparticles that allow the rapid reduction of nitro
compounds into their corresponding amines.6 Recently, some
of the transition metals such as Pd and Au as the catalyst made
attracted great attentions due to some advantages such as
hydrophilicity, chirality, biodegradability and high
functionality. In recent years wide investigations were
performed on cellulose and its derivatives focusing on their
biological, chemical, as well as mechanical properties.
Cellulose and its derivatives can be used as a support since
they are renewable, biodegradable and non-toxic.9 Cellulose
supported Cu(0) for Aza-Michael addition,10 Pd(0) for Heck and
Sonagashira couplings,11 Cu(I)/Pd(0) for Click cyclization12 and
Co(II) for the oxidation reactions13 are some example of
cellulose supported catalysts.
The advancement of nanoscience and technology is indebted
discovery of various new nanomaterials for versatile
applications utilizing their size and shape dependent
properties. Quantum dots (QDs) are one of the nanomaterials
well known for their variation of bandgap with size.
Conventional QDs such as PbS, and CdSe are extremely
hazardous for processing, and there are also innumerable
challenges for disposal and recycling.14 Organic quantum dots
such as graphene quantum dots (GQDs) don’t have the
limitations of conventional QDs. GQDs with tunable emissions
are considered to be an important nanomaterials due to the
superiority in resistance to photobleaching, low toxicity,
excellent biocompatibility, low cost, and abundance of raw
materials in nature.15 They have been demonstrated potential
applications on optical bioimaging probes,16 theranostic
agent,17 detection probes,18 light-emitting diode materials,19
and efficient visible light-active photocatalysts20 and so on.
Doping of GQDs with hetero atoms (such as B, N, S, and F) is
also an effective method to tune the optical and electrical
properties of GQDs.21 Among the dopants, N is interesting for
the electronic modification due to its comparable size with
carbon. From our point of view as a catalyst designer,
a
reduction of nitro compounds with good efficiency.7
However, a great result in the reduction rate was obtained for
bimetallic systems such as Pd-Ni, Pd-Cu and Pd-Ag.8
With increasing the importance of green chemistry in human
life, heterogeneous catalysts have gained a special place in the
catalyst research. Introduction of
heterogeneous catalysts created
a
large number of
a
competition for the
selecting a best heterogenized catalytic system for a reaction.
High efficiency, non-toxicity, biocompatibility and recyclability
are some of the challenging for heterogeneous catalysts. For
affording the mentioned conditions various supports such as
a.Department of Nanochemistry, Nanotechnology Research Center, Urmia
University, G. C., P. O. Box 165-5715944931, Urmia, Iran. Email:
s.keshipour@urmia.ac.ir.
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existence of N can be assist to homogeneous distribution of nm (Figure 6a). Also, deposition of PdNPs was successfully
metal nanoparticles on the GQDs. However, N-GQDs can be performed on the cellulose with the particle size between 25-35
good support for the metal nanoparticles such as Pd(0) nm (Figure 6b).
generated in situ for the catalytic propos. Also, due to efficient
Scheme 1. Reduction of nitroaromatics with PdNPs@Cell-N-GQD
activity of graphene as a support in the reduction reaction, it
was expected that N-GQDs will be an efficient support for
PdNPs@Cell-N-GQD
Pd(0) in the catalytic reduction of nitro compounds. For
Ar NO2
Ar NH2
possibility of the N-GQDs recycling, cellulose as a green
support chemically modified with N-GQDs, then Pd(0)
nanoparticles was deposited on the modified support to give
PdNPs@Cell-N-GQD. PdNPs@Cell-N-GQD was investigated in
NaBH4
Scheme 2. Synthesis of PdNPs@Cell-N-GQD
Cellulose
H2O:DMSO (1:1)
HO
O
the reduction of nitroaromatics as
recoverable catalyst using NaBH4 as
(Scheme 1).
a
heterogeneous
DCC\DMAP
O
N-doped GQD
a
reductant agent
HO
100 °C, 24h
H2O
OH
180 °C, 4h
N-GQD was synthesized from citric acid and ethylenediamine
in H2O as the solvent in autoclave.22 For preparation of the
PdNPs@Cell-N-GQD, the carboxylic acid group of N-GQD was
activated with N,N'-dicyclohexylcarbodiimide (DCC) and 4-
dimethylaminopyridine (DMAP) and then reacted with
cellulose to give Cell-N-GQD. To achieve homogeneous
distribution of Pd(II) on the Cell-N-GQD, they were stirred at
room temperature for 24 h. Then, Pd(II) was reduced to Pd
NPs with NaBH4 to give PdNPs@Cell-N-GQD (Scheme 2).
The catalyst was characterized with FT-IR, UV-Vis
spectroscopy, Energy Dispersive X-Ray Spectroscopy (EDS), X-
Ray Diffraction pattern (XRD), Thermal Gravimetric Analysis
(TGA), Flame Atomic Absorption Spectroscopy (FAAS) and
Transmission Electron Microscopy (TEM). The FT-IR spectra for
cellulose, N-GQDs, Cell-N-GQD and PdNPs@Cell-N-GQD were
prepared (Figure 1). Absorption bands at 2856 cm-1 for C-H
band and 1577 cm-1 for C=O are characteristic peaks of N-
GQDs which observed in the FT-IR spectrum of Cell-N-GQD,
and approved the modification of cellulose with N-GQD.
One of the important behaviors of the N-GQDs is the
photoluminescence (PL) activity. The PL activity of prepared N-
GQD was studied in excitation wavelengths 350, 370, 390, 400,
410, 430 and 450 nms with strongest peak at about 400 nm.
Also, the PL peaks of N-GQD shift to longer wavelengths with
increasing the excitation wavelength (Figure 2). PL activity of
N-GQD was quenched in Cell-N-GQD which approved the
bonding of N-GQD to cellulose.
O
autoclave
HO
COOH
HO
N
HO
OH
+
H2N
COOH
OH
HOOC
HOOC
N
NH2
OH
COOH
HO
HO
N-doped GQD
COOH
OH
HO
HO
N
COOH
OH
HOOC
N
HOOC
OH
HO
HO
O
O
OH
O
HO
O
O
O
HO
OH
OH
n
1) PdCl2, H2O, 24 h
2) NaBH4
PdNPs@Cell-N-GQD
EDS is a common method for approve the metal loading on the
support. The EDS analysis showed that Pd nanoparticles were
loaded on the Cell-N-GQD surface (Figure 3). The amount of Pd
loading on the support was determined to be 0.14 mmol per
1g PdNPs@Cell-N-GQD with FAAS.
XRD pattern of PdNPs@Cell-N-GQD exhibits the diffraction
peaks of (101), (002) and (040) for cellulose, and (111), (200)
and (220) for Pd (Figure 4). The diffraction peak of the N-GQDs
was overlapped with cellulose (002).
TGA which applied for the study of thermal behavior of
materials
evidenced
PdNPs@Cell-N-GQD
starts
to
decomposition above 209 °C in air which it’s a good thermal
stability for a catalyst. Also, other mass loss was observed at
231 °C for PdNPs@Cell-N-GQD (Figure 5).
The TEM image of the catalyst indicates the N-GQDs was
deposited on the cellulose with the particle size between 4-9
Figure 1. FT-IR spectra for cellulose, N-GQD, Cell-N-GQD and
PdNPs@Cell-N-GQD
2 | RSC Adv., 2016, 00, 1-3
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The catalytic activity of PdNPs@Cell-N-GQD was evaluated in
the reduction of nitroaromatics. So, the reduction of
nitrobenzene (1a) (1 mmol) with NaBH4 was investigated for
the optimization of reaction conditions. It was found that 0.56
mol% Pd (Table 1, entries 1-3) with 1 mmol of NaBH4 (2eq) in
H2O (Table 1, entries 5-9) at room temperature is the best
reaction conditions for the reduction of nitrobenzene. The
catalyst was separated via filtration and after removing of the
solvent, the product was purified using column
chromatography with ethylacetate:n-hexane solvent. The 1H
NMR study show that the reduction of nitrobenzene
proceeded to give aniline (2a). Aniline was produced as the
sole product with 95% yield during 2h without any byproduct.
The reaction yield was decreased in the low amounts of
catalyst. The reaction did not perform in the absence of
NaBH4, and gave low yield in low amount of NaBH4 (Table 1,
entry 4). After screening a variety of solvents, H2O was
determined to be the best solvents. Also, the reaction did not
perform without the catalyst (Table 1, Entry 10).
Figure 2. The PL of N-GQD
For the investigation of Pd NPs and N-GQDs effects on the
reaction, the reduction reaction of nitrobenzene was studied
using PdNPs@Cellulose. Interestingly, the reduction reaction
was performed using PdNPs@Cellulose with aniline as the sole
product in 67% yield during 3h. Since PdNPs@Cell-N-GQD
afforded high yield in short time, these results show the high
activity of Pd in the presence of N-GQD in the nitroaromatic
reduction reaction. Also, cellulose as a support reduced the
PdNPs aggregation and increased surface area.
Figure 3. The EDS analysis of PdNPs@Cell-N-GQD
Figure 4. The XRD pattern of PdNPs@Cell-N-GQD
Figure 6. The TEM images of Cell-N-GQD (a), PdNPs@Cell-N-GQD
before use (b) and PdNPs@Cell-N-GQD after use
Figure 5. The TGA of PdNPs@Cell-N-GQD in air
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Table 1. Optimization of the reaction conditions for reduction of reaction gave corresponding amino carboxylic acid which
nitrobenzenea
shows carboxylic acid is stable versus reduction with NaBH4 in
the presence of PdNPs@Cell-N-GQD.
Entry
(Pd mol%)
Solvent
Yield (%)b
Potential Pd leaching into the reaction mixture was also
analyzed with FAAS analysis. For this purpose, sample was
taken through a syringe filter during the heterogeneous
reduction reaction of nitrobenzene, the solvent was
evaporated, and the residue was dissolved in HNO3. The
analysis of these samples with FAAS showed that the Pd
concentration in the reaction solution was less than the
detection limit. This result indicates that virtually no Pd
leaches from the surface into the solution. Also, did not
observe any changes in the FT-IR spectrum of the catalyst
recovered from the reaction (Figure 7). The TEM image of the
catalyst after the reaction shows that no aggregation was
happened for PdNPs (Figure 6c).
1
0.43
0.56
0.71
0.56
0.56
0.56
0.56
0.56
0.56
-
H2O
81
95
96
71
60
93
0
2
H2O
3
4c
H2O
H2O
5
EtOH
6
EtOH:H2O (1:1)
MeCN
MeOH
CH2Cl2
H2O
7
8
57
0
9
10
0
aReaction conditions: nitrobenzene (2 mmol), NaBH4 (1 mmol),
solvent (5 mL), room temperature, 2h. b Isolated yield. c NaBH4
(1 mmol).
Recyclability of the PdNPS@Cell-N-GQD was examined in the
reduction of nitrobenzene. After carrying out the reaction, the
catalyst was separated via filtration, washed with EtOH (2 × 5
mL) and reused. An only minor decrease in the reaction yield
was observed after six repetitive cycles for the reaction (Table
3).
Table 2. The reduction of various nitroaromatics using PdNPs@Cell-N-
GQD and NaBH4
Entry
1
R
Ph
solvent
H2O
H2O
Yield (%)b
95
91
2c
p-HOOC-Ph
o-HOOC-Ph
p-O2N-Ph
p-H2N-Ph
p-H3C-Ph
o-H3C-Ph
m-H3C-Ph
m-O2N-Ph
naphtyl
o-O2N-Ph
o-HO-Ph
m-HO-Ph
2,3-dimethyl-Ph
2,6-dimethyl-Ph
Finally, a comparison study was performed for the obtained
results with some recent reports about reduction of
nitrobenzene as a sample. Most important factors for this
reaction are yield, solvent, temperature and reaction duration.
As can be seen from Table 4, various catalysts were used for
the reduction of nitrobenzene which in all of them high yields
obtained. So, the challenge of this reaction is the gentling of
the reaction conditions not increasing the yield. A comparison
of PdNPs@Cell-N-GQD with other catalysts of Table 4 for the
reduction of nitrobenzene shows that very good conditions
resulted with PdNPs@Cell-N-GQD. H2O as the solvent at room
temperature in 2h is a convenient reaction conditions for the
reduction of nitrobenzene with PdNPs@Cell-N-GQD. Also, the
catalyst has other advantages commonly for most of
heterogeneous catalysts including easy separation and
recyclability. PdNPs@Cell-N-GQD has another advantage
3c
H2O
90
4c
H2O/EtOH
H2O/EtOH
H2O/EtOH
H2O/EtOH
H2O/EtOH
H2O/EtOH
H2O/EtOH
H2O/EtOH
H2O/EtOH
H2O/EtOH
H2O/EtOH
H2O/EtOH
88d
91
5
6
7
94
92
8
93
9c
86e
90
10
11c
12
13
14
15
89f
93
94
93
90
aReaction conditions: nitroaromatic 1 (1 mmol), NaBH4 (1
mmol), PdNPs@Cell-N-GQD (0.05 g), solvent (5 mL), room
temperature, 2h. bIsolated yield. cNaBH4 (2 mmol).dR
◌=p-
H2N-Ph. eR =m-H2N-Ph. fR
◌ ◌=o-H2N-Ph.
Table 3. Successive trials by using recoverable PdNPs@Cell-N-GQD for compared to some heterogeneous catalysts including easy
the reduction reaction of nitrobenzenea
preparation of the catalyst.
Trial
Catalyst amount (g)
Yield (%)b
1
2
3
4
5
6
0.04
0.04
0.04
0.04
0.04
0.04
95
95
94
94
94
94
aReaction conditions: nitrobenzene (1 mmol), NaBH4
(1 mmol), H2O (5 mL), r.t., 2h. bIsolated yield
For more investigation of the PdNPs@Cell-N-GQD activity, the
reaction was examined for various electron withdrawing and
electron
donating
substituted
nitroaromatics
and
dinitroaromatics. For most of the nitroaromatics H2O:EtOH
(1:1) was applied as the solvent since the reduction reaction in
H2O gave lower yields. The reduction reaction successfully
afforded aniline derivatives in high yields. Dinitroaromatics
gave the corresponding diamine in the reduction reaction
(Table 2, entries 4, 9 and 11). For nitro carboxylic acids the
Figure 7. FT-IR spectra of PdNPs@Cell-N-GQD
4 | RSC Adv., 2016, 00, 1-3
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Table 4. Comparison of the results for reduction of nitrobenzene to solvent was evaporated under vacuum and the product purified
aniline.
Entry
with column chromatography with n-hexane:ethylacetate (4:1).
Catalyst
[Ref.] Solvent Time Temp. Yield
(h)
(°C)
(%)
Acknowledgements
1
2
Pd-gCN
zinc
23
24
EtOH
Ether/H2O
4
9
70 99 (G)
70 90 (I)
powder/chelating
ethers
Co-Mo2C/AC
Ni–B
Zinc
phthalocyaninein
PEG-400
Fe3O4/
β-alanine-
acrylamide-Ni
PdNPs@Cell-N-
GQD
We gratefully acknowledge financial support from the
Research Council of Urmia University.
3
4
5
25
26
3g
EtOH
H2O/EtOH
EtOH
2
8
8
80
r.t.
100
99.6
100 99 (G)
room 98 (I)
room 95
Notes and references
1
(a) S. Chandrappa, T. Vinaya, T. Ramakrishnappa and K. S. Rangappa,
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6
7
2
-
H2O
H2O
5
2
2
3
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Perri and C. Siciliano, J. Chromatogr. A, 2005, 1066, 143-148.
Conclusions
new cellulose supported N-GQDs and PdNPs was
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ChemCatChem, 2009, 1, 210-221.
A
synthesized. Pd NPs has great activity in the vicinity of N-GQDs
for the reduction of nitroaromatics. Cellulose as a support
afforded the recyclability of the catalyst. The catalyst was
applied in the reduction of nitroaromatics to corresponding
amines selectively in high yields. The reduction reaction was
performed with NaBH4 in H2O or H2O:EtOH (1:1) as the
solvents at room temperature in 2h. Various nitrobenzenes
and dinitrobenzenes were reduced to the corresponding
amine in good yields. Mild reaction conditions and green
solvent of this reaction in combination with recyclability of the
catalyst make the presented method an interesting approach
compared to most of the reports.
Experimental
4
5
S. D. Burk and R. L. Danheiser, Handbook of Reagents for Organic
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4555-4558. (b) A. Ono, H. Sasaki and F. Yaginuma, Chem. Ind. (London)
1983, 480. (c) S. Yoo and S. Lee, Synlett, 1990, 419-420.
Preparation of PdNPs@CellꢀNꢀGQD
N
-GQD was synthesized from citric acid and ethylenediamine
in H2O in autoclave at 180 °C during 4 h.22 For preparation of
the PdNPs@Cell- -GQD, -GQD (0.3 g) was dissolved in 40
mL H2O:DMSO (1:1) and then, cellulose (2 g) was added to the
solution. N'-Dicyclohexylcarbodiimide (0.2 g) and 4-
6
(a) J. O. Osby and B. Ganem, Tetrahedron Lett., 1985, 26, 6413-6416. (b)
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I. I. Obraztsova, N. K. Eremenko, G. Yu. Simenyuk, A. N. Eremenko and B.
G. Tryasunov, Solid Fuel Chem., 2012, 46, 364-367.
N
N
7
8
N
,
dimethylaminopyridine (0.03 g) were added to the mixture and
the mixture was stirred at 100 °C. After 24 h, the reaction
mixture was filtered off and the residue washed with H2O (3 ×
9
Y. Habibi, L. A. Lucia and O. J. Rojas, Chem. Rev., 2010, 110, 3479-3500.
10 K. R. Reddy and N. S. Kumar, Synlett, 2006, 2246-2250.
11 (a) C. M. Cirtiu, A. F. Dunlop-Brière and A. Moores, Green Chem., 2011,
13, 288-291. (b) S. Keshipour, S. Shojaei and A. Shaabani, Cellulose,
2013, 20, 973-980.
12 S. Keshipour and A. Shaabani, Appl. Organometal. Chem., 2014, 28, 116-
119.
10 mL) and acetone (3 × 5 mL) to give Cell-N-GQD. Cell-N-
GQD (2 g) was dispersed in H2O (20 mL), and PdCl2 (0.04 g)
added to the mixture under stirring. Stirring was continued for
24 h and then, 10 mL solution of NaBH4 (0.01g) was added
13 (a) A. Shaabani, S. Keshipour, M. Hamidzad and M. Seyyedhamzeh, J.
Chem. Sci., 2014, 126, 111-115. (b) A. Shaabani, S. Keshipour, M.
Hamidzad and S. Shaabani, J. Mol. Catal. A Chem., 2014, 395, 494-499.
14 L. Cao, M. J. Meziani, S. Sahu and Y. P. Sun, Acc. Chem. Res., 2013, 46,
171-180.
dropwise during 2 h. Finally, PdNPs@Cell-N-GQD was filtered
off, washed with H2O (3 × 10 mL) and dried in oven (70 °C).
Typical procedure for the reduction of nitrobenzene
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Nitrobenzene (0.12 g, 1.00 mmol) was added to a round-
bottomed flask containing colloidal of PdNPs@Cell-N-GQD
(0.04 g) in H2O (5 mL) at room temperature. 5 mL of NaBH4
solution (0.6 mmol) was added dropwise to the reaction vessel
during 0.5 h. After 1.5 h, PdNPs@Cell-N-GQD was separated
with filtration and washed with acetone (2 × 5 mL). The filtrate
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6 | RSC Adv., 2016, 00, 1-3
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Graphical Abstract:
CH2OOH
OH
HO
N
HO
Pd
COOH
OH
HOOC
HOOC
N
OH
HO
HO
O
O
OH
O
HO
O
O
O
HO
OH
n
OH
ArNH2
Ar-NO2
NaBH4, H2O or H2O:EtOH (1:1), r.t., 2 h
86-95%
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