Encapsulation of silver nanoparticles into graphite grafted with hyperbranched poly(amidoamine) dendrimer and their catalytic activity towards reduction of nitro aromatics

Article abstract of DOI:10.1016/j.molcata.2012.04.001

Hyperbranched polyamidoamine (PAMAM) dendrimer have been successfully grafted on the graphite surface and silver nanoparticles (AgNPs) were readily synthesized within the graphite grafted PAMAM dendrimer templates and applied as nanocatalysts in the reduction of nitro aromatics. Three generations of PAMAM dendrimers with varying chain branches have been utilized in order to serve this purpose. The grafting of PAMAM dendrimer on graphite surface has been monitored using TGA, Raman and FT-IR spectra and the AgNPs/GR-G_1.0PAMAM, AgNPs/GR-G_2.0PAMAM, AgNPs/GR-G_3.0PAMAM nanocatalysts were characterized using XRD, UV-visible spectra, SEM, TEM and EDX spectral analysis. The prepared catalysts were found to exhibit enhanced catalytic activity towards the reduction of 4-nitrophenol and the reaction rate constant for our third generation catalyst was estimated to be 21.7 × 10^-3 s^-1, which is the highest reported heterogeneous system so far. The efficiency of the system has been further demonstrated through the reduction of halonitroarenes without dehalogenation in the halo-substituted nitro benzenes and reduction of nitro groups in the presence of imine functionalities under mild condition.

Full text of DOI:10.1016/j.molcata.2012.04.001

Journal of Molecular Catalysis A: Chemical 359 (2012) 88–96
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Encapsulation of silver nanoparticles into graphite grafted with hyperbranched
poly(amidoamine) dendrimer and their catalytic activity towards reduction of
nitro aromatics
Rajendiran Rajesh, Rengarajan Venkatesan∗
Department of Chemistry, Pondicherry University, Puducherry 605 014, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Hyperbranched polyamidoamine (PAMAM) dendrimer have been successfully grafted on the graphite
surface and silver nanoparticles (AgNPs) were readily synthesized within the graphite grafted PAMAM
dendrimer templates and applied as nanocatalysts in the reduction of nitro aromatics. Three generations
of PAMAM dendrimers with varying chain branches have been utilized in order to serve this purpose.
The grafting of PAMAM dendrimer on graphite surface has been monitored using TGA, Raman and FT-
IR spectra and the AgNPs/GR-G1.0PAMAM, AgNPs/GR-G2.0PAMAM, AgNPs/GR-G3.0PAMAM nanocatalysts
were characterized using XRD, UV–visible spectra, SEM, TEM and EDX spectral analysis. The prepared
catalysts were found to exhibit enhanced catalytic activity towards the reduction of 4-nitrophenol and the
reaction rate constant for our third generation catalyst was estimated to be 21.7 × 10−3 s−1, which is the
highest reported heterogeneous system so far. The efficiency of the system has been further demonstrated
through the reduction of halonitroarenes without dehalogenation in the halo-substituted nitro benzenes
and reduction of nitro groups in the presence of imine functionalities under mild condition.
© 2012 Elsevier B.V. All rights reserved.
Received 30 October 2011
Received in revised form 12 March 2012
Accepted 1 April 2012
Available online 10 April 2012
Keywords:
PAMAM dendrimer
Silver nanoparticles
Aromatic nitro reduction
Modified graphite nanocatalyst
1. Introduction
The highly active surface atoms of these nanoparticles could lead
to aggregation of the naked nanoparticles and consequently result-
Aromatic amines are versatile synthetic intermediates due to
their application in the preparation of dyes, pharmaceuticals,
agricultural products, surfactants and polymers [1–3]. At both lab-
oratory and industrial scale, the preparation of aromatic amines
is conventionally accomplished by the reduction of corresponding
nitro compounds using catalytic hydrogenation and a variety of
other reduction conditions [4,5]. Numerous reducing agents have
been utilized for the reduction of nitro compounds with the most
classic being metallic reagents in the presence of an acid. However,
this methodology is messy and environmentally hazardous [6] and
therefore synthetic routes involving clearer and cheaper alterna-
tives are necessary. One such methodology is to employ NaBH4 in
water as the hydride source. However, reduction of nitro groups
with NaBH4 in the absence of any catalyst is highly tedious and
hence many different metal catalysts have been used to carry out
this reduction [7–9].
ing in diminished catalytic activity. Metal nanoparticles based
on different methods have been developed to solve this issue
and utilized for the catalytic reduction of nitrophenols to amines.
Yang et al. have employed gold nanoparticles doped in meso-
porous organic gel based on polymeric phloroglucinol carboxylic
acid–formaldehyde as a nanocatalyst for the catalytic reduction
of 4-nitrophenol, with a reduction rate constant of 7.4 × 10−3 s−1
[13]. Hayakawa et al. reported that gold nanoparticles prepared
by laser irradiation methods have shown a rate constant of
1.78–13.2 × 10−3 s−1 [14] and Panigrahi et al. have reported gold
nanoparticles supported by polystyrene anion resin which exhib-
ited a rate constant of 0.16 × 10−3 s−1 [15]. Lu et al. have reported
the tree-type spherical polymer supported silver nanoparticles
(AgNPs) and their estimated rate constant was 0.33 × 10−3 s−1 [16].
However, these procedures suffer from selectivity, for instance,
reduction of halo substituted nitro aromatics resulted in dehalo-
genated amines [17,18]. To overcome this problem, Chen et al.
and Raja et al. have prepared Ag, Au, Co and Pd nanoparticles
embedded in silica matrix to achieve reduction of nitro selectivity
with high yield [19–21]. This methodology requires high temper-
atures (∼140 ◦C) and high pressure (∼120 atm) conditions which
might be due to smaller available metal sites on the surface of SiO2
matrix.
Noble metal nanoparticles have attained great attention in catal-
ysis research since they possess high surface area than that of their
bulk counterparts leading to improved catalytic activities [10–12].
∗
Corresponding author. Tel.: +91 413 2654415; fax: +91 413 2655987.
E-mail address: venkatesan63@yahoo.com (R. Venkatesan).
1381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2012.04.001

R. Rajesh, R. Venkatesan / Journal of Molecular Catalysis A: Chemical 359 (2012) 88–96
89
Therefore, the need of the day is to encapsulate the metal
nanoparticles in a heterogeneous system that permits easy pas-
sage of the substrates and products of the catalytic reaction. One
such way is to stabilize the nano particles in an organic dendrimer
matrix which provides open and flexible structure. Polyami-
doamine (PAMAM) dendrimers are highly branched, well-defined,
synthetic macromolecules available in nanometer dimensions [22].
Synthetic methodologies for dendrimer structure are difficult and
expensive [23] and hence alternative methods have been adopted
by constructing dendrimer branches on solid support such as chi-
tosan, carbon nanotubes, silica, polymer surface [24–28]. These
matrices are again found to be expensive and hence our focus
was to synthesize PAMAM dendrimer branches on graphite sur-
face. In addition to its cost-effectiveness, graphite grafted organic
dendrimer structures provide the property that can bridge the gap
between homogeneous and heterogeneous phase.
anhydrous tetrahydrofuran (THF), and dried under vacuum for
24 h at room temperature, generating acylated graphene (GR-COCl)
(0.4953 g).
2.2.3. Preparation of GR-NH2 initiator from GR-COCl
GR-COCl (0.4953 g) was mixed with ethylenediamine (20 mL)
and placed in an ultrasonic bath (40 kHz) for 5 h at 60 ◦C. The mix-
ture was stirred for another 24 h at the same temperature and the
resulting solid was separated by vacuum filtration using 0.22 ␮m
Millipore polycarbonate membrane filter and subsequently washed
with anhydrous methanol. After repeated washing and filtration,
the resulting solid was dried overnight under vacuum, generating
GR-NH2 (0.5020 g).
2.2.4. Growth of PAMAM dendrimers on the GR surface initiated
by GR-NH2
Hence, in the present investigation we have synthesized novel
nanocatalysts based on AgNPs encapsulated on three genera-
tions of PAMAM dendrimers with varying chain branches that
have been grafted on graphite surface. The efficiency of prepared
nanocatalysts towards reduction of nitro aromatics has been exam-
ined. It has been demonstrated that our third generation catalyst
shows the reduction rate constant of 21.7 × 10−3 s−1, which is
the highest among reported values. Further, the selectivity of the
nanocatalyst towards the selective reduction of nitro groups in
presence of halo- and imine-substituents has been verified.
The mixture of methylacrylate (20 mL) and methanol (50 mL)
was added to a 250 mL three-necked round-bottom flask. The
prepared GR-NH2 initiator (0.1 g) in 20 mL methanol was care-
fully dropped in to methylacrylate solution within 20 min with
continuous stirring. The solution was placed in an ultrasonic
bath (40 kHz) for 7 h at 50 ◦C and the mixture was stirred for
another 24 h. In order to ensure that no ungrafted polymer or
free reagents were present in the product, the filtered solid was
dispersed in methanol, filtered, and washed three times with
methanol. The product was dried overnight to give the den-
dritic polymer grafted graphite generation 0.5 (GR-G0.5PAMAM,
0.1011 g).
2. Experimental
After rinsing, GR-G0.5PAMAM in 20 mL methanol was dropped
into 40 mL of 1:1 methanol/ethylenediamine solution. The solution
was placed in an ultrasonic bath (40 kHz) for 5 h at 50 ◦C, and the
mixture was stirred for another 24 h at the same temperature. The
solid was then filtered and washed three times with methanol to
give generation 1.0 dendrimer modified graphite (GR-G1.0PAMAM).
Stepwise growth using methylacrylate and ethylenediamine was
repeated until the desired number of generations (up to genera-
tion 3.0) were achieved (Scheme 1). The dendrimer-modified GR
was then washed thrice with methanol (25 mL), and water (25 mL)
respectively.
2.1. Chemicals
Graphite, fine powder of particle size 50 ␮m was purchased
from Loba Chemie Pvt. Ltd., India. Sulfuric acid, potassium per-
manganate, hydrogen peroxide, sodium nitrate, thionyl chloride,
hydrazine monohydrate, ethylene diamine (EDA), methacry-
late (MA), silver nitrate, sodium borohydride, 4-nitrophenol,
nitrobenzene, 4-nitrotoluene, 2-hydroxynitrobenzene, 2-chlor-
onitrobenzene, 4-bromonitrobenzene, 4-iodonitrobenzene, 2-
aminonitrobenzene, 4-aminonitrobenzene, 4-nitrobenzylalcohol,
4-chloro-3,4-dinitrobenzene were obtained from Merck, India. All
chemicals were used without further purification. Double distilled
water was used through out the experiment.
2.2.5. Dispersion of AgNPs on the hyperbranched GR-G1.0
GR-G2.0, GR-G3.0PAMAM dendrimer grafted graphite
[AgNPs/GR-G1.0, 2.0, 3.0PAMAM]
,
A
100 mL two-neck round bottom flask with 50 mg of
hyperbranched dendrimer grafted graphite (GR-G1.0PAMAM, GR-
2.0PAMAM, and GR-G3.0PAMAM) were dispersed with 10 mL of
2.2. Systematic preparation of novel AgNPs/GR-G1.0PAMAM,
AgNPs/GR-G2.0PAMAM and AgNPs/GR-G3.0PAMAM
G
de-ionized water and then sonicated for 30 min. To this dispersion,
10 mL of 0.01 M of AgNO3 solution was added drop wise into each
of the generation dendrimer grafted graphite and the mixture was
stirred for 1 h and then finally reduced with 0.01 M NaBH4 (10 mL).
The filtered solid was washed three times with water. The prod-
uct was dried overnight under vacuum (AgNPs/GR-G1.0PAMAM,
AgNPs/GR-G2.0PAMAM, AgNPs/GR-G3.0PAMAM) (46 mg, 43 mg,
40 mg) respectively.
2.2.1. Preparation of graphene oxide [GR-COOH] from graphite
Graphene oxide (GO) was prepared using a reported procedure
of Hummers and Offeman’s method [10] with minor modifica-
tions. In a typical reaction, 1 g of graphite, 1 g of NaNO2, and 50 mL
of H2SO4 were stirred together in an ice bath. KMnO4 (3 g) was
slowly added to the stirring mixture, and the rate of addition
was controlled to prevent the temperature of reaction vessel from
exceeding 20 ◦C. The mixture was then transferred in to a 35 ◦C
water bath and stirred for 1 h, resulting in the formation of a thick
paste. Subsequently, 50 mL of de-ionized water was added gradu-
ally, resulting in an increase in temperature to 98 ◦C. After 15 min,
the mixture was further treated with 150 mL de-ionized water and
10 mL 30% H2O2 solution. The warm solution was filtered and then
washed with de-ionized water until a pH of 7 is obtained and the
GR-COOH was dried at 65 ◦C under vacuum.
2.3. Characterizations
The PAMAM dendrimer functionalizations on the graphite
surface were confirmed by micro Raman spectrometer (Witec Con-
focal Raman instrument (CRM200) with Ar ion laser 514.5 nm).
The crystalline nature of the AgNPs/GR-G1.0PAMAM, AgNPs/GR-
G2.0PAMAM, AgNPs/GR-G3.0PAMAM samples were studied by
2.2.2. Preparation of acylated graphene [GR-COCl] from graphene
oxide [GR-COCl]
powder XRD using X-pertPRO Pananalytical X-ray diffractometer
employing Cu K␣ radiation. The IR spectra of the samples were
recorded on Nicolet 6700 FT-IR spectrometer using KBr method.
Surface morphology and elemental composition of the catalyst
GR-COOH (0.5 g) was suspended in SOCl2 (30 mL) and stirred
for 24 h at 70 ◦C. This solution was filtered and then washed with

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R. Rajesh, R. Venkatesan / Journal of Molecular Catalysis A: Chemical 359 (2012) 88–96
Scheme 1. AgNPs/GR-G3.0PAMAM dendrimer.
were determined by SEM (HITACHI S-3400N Scanning Electron
Microscope) along with EDX Energy dispersive-X-ray spectroscopy
(Thermo SuperDry II attached with SEM). Tecnai F-30 field emis-
sion Transmission Electron Microscope (TEM) has been used to
capture the TEM images of AgNPs. The additional information of
PAMAM dendrimer grafting was also obtained from thermal gravi-
metric analyzer (TG-DTA-Q 600 SDT) from room temperature to
800 ◦C. Optical absorption spectra of the prepared nano catalyst
were recorded at room temperature with Ocean optics (HR4000)
spectrophotometer.
3. Results and discussion
3.1. Grafting of PAMAM dendrimer on graphite surface
The grafted content of the PAMAM dendrimer modified graphite
can be determined by measuring the weight loss between 200 ◦C
and 800 ◦C [29,30]. TGA curves in Fig. 1 demonstrate no weight loss
for pure graphite whereas the graphene oxide shows weight loss
of ∼11% between 100 and 150 ◦C corresponding to the pyrolysis of
OH and COOH groups [31–34]. In case of PAMAM dendrimers

R. Rajesh, R. Venkatesan / Journal of Molecular Catalysis A: Chemical 359 (2012) 88–96
91
100
95
90
85
80
75
70
65
60
55
50
a
e
b
c
d
d
c
a
b
c
d
e
GR
GR-COOH
b
a
GR-G1.0-PAMAM
GR-G2.0-PAMAM
GR-G3.0-PAMAM
e
10
20
30
40
50
60
70
80
Position [º2 theta]
90
180
270
360
450
540
630
720
Fig. 4. X-ray diffraction patterns of: (a) pure graphite, (b) GR-G3.0PAMAM, (c)
Temperature (ºC)
AgNPs/GR-G1.0PAMAM, (d) AgNPs/GR-G2.0PAMAM, and (e) AgNPs/GR-G3.0PAMAM.
Fig. 1. TGA thermograms of: (a) pure graphite, (b) GR-COOH, (c) GR-G1.0PAMAM,
(d) GR-G2.0PAMAM, and (e) GR-G3.0PAMAM dendrimer grafted graphite.
grafted graphite, additional weight losses from organic moieties
were also seen and the weight losses were estimated as 16%, 20%,
34%, for GR-G1.0PAMAM, GR-G2.0PAMAM, GR-G3.0PAMAM in the
temperature range 200–800 ◦C. The gradual increase in weight loss
from G1.0PAMAM to G3.0PAMAM indicates the expansion of den-
drimer network and thus the TGA analysis supports our synthetic
approach depicted in Scheme 1.
d
The linkage between graphite and PAMAM dendrimer was
investigated by recording the Raman spectra of dry samples of
graphite powder using the laser excitation of 514.5 nm. Fig. 2
represents the Raman spectra of pure graphite, GR-G1.0PAMAM,
GR-G2.0PAMAM and GR-G3.0PAMAM dendrimer (curves a–d
respectively) and all the samples were found to show two pre-
dominant peaks corresponding to D and G bands respectively
[35–38]. The D and G bands for pure graphite are observed around
1357 and 1568 cm−1 respectively which arise from the activa-
tion in the first order scattering process of sp3 and sp2 carbon
in graphite sheets respectively. The intensity ratio of D and G
bands expresses the sp2/sp3 carbon ratio manifesting the mea-
sure of disorder [39]. Generally, the D band is relatively weak or
nearly invisible in perfect graphite lattice and can arise from sym-
metry breaking at the edge. The D bands appeared at 1357 cm−1
for all the samples without any shift in absorption energy and
c
b
a
1050
1200
1350
1500
1650
1800
1950
-1
Raman shift (cm )
Fig. 2. Raman spectra of: (a) pure graphite, (b) GR-G1.0PAMAM, (c) GR-G2.0PAMAM,
(d) GR-G3.0PAMAM and dendrimer grafted graphite.
1.0
a
b
c
GR-COOH
GR-G3.0 PAMAM
AgNPs/GR-G3.0 PAMAM
f
a
b
e
0.8
0.6
0.4
0.2
0.0
d
c
c
b
a
500
1000
1500
2000
2500
3000
3500
Wavenumber (cm-1)
240
320
400
480
560
640
720
800
880
Wavelength (nm)
Fig. 3. FT-IR spectra of: (a) pure graphite, (b) GR-COOH, (c) GR-COCl, (d) GR-CONH2,
(e) GR-methylacrylate, and (f) GR-G1.0PAMAM dendrimer respectively.
Fig. 5. UV–visible spectra of: (a) GR-COOH, (b) GR-G3.0PAMAM, and (c) AgNPs/GR-
G3.0PAMAM nanocatalysts.

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R. Rajesh, R. Venkatesan / Journal of Molecular Catalysis A: Chemical 359 (2012) 88–96
Fig. 6. SEM images of: (A and B) pure graphite, (C and D) GR-G3.0PAMAM, (F–H) TEM images of AgNPs/GR-G3.0PAMAM.
the peak intensity was found to increase gradually upon the sub-
stitution of PAMAM dendrimer. The D/G ratios were found to
be 0.730, 0.987, 1.018, 1.03 for pure graphite, GR-G1.0PAMAM,
GRG2.0-PAMAM, GR-G3.0PAMAM respectively, due to increase in
the number of sp3 carbon atom reflecting the successful func-
tionalization of the PAMAM dendrimer on the graphite surfaces.
Furthermore, the G band of the pure graphite appears at 1568 cm−1
which gets shifted to 1580 cm−1, 1583 cm−1 and 1588 cm−1 in the
case of GR-G1.0PAMAM, GR-G2.0PAMAM, and GR-G3.0PAMAM den-
drimers respectively. This observed red shift and increase in the
intensity of G band again confirms the substitution induced defect
on the sp2 carbon atom of graphite sheets. These observations are
in line with the reports Kudin et al. [37]. Henceforth, the Raman
spectral analysis affirms that the PAMAM dendrimers have been
profoundly exfoliated on the graphite surface.
Further evidence of PAMAM dendrimer exfoliation on the
graphite surface has been probed by FT-IR spectroscopy studies
(Fig. 3). The FT-IR spectra of pure graphite (Fig. 3, curve a) shows
the stretching vibrations of C C and C C groups around 1650 and
1380 cm−1 to the respectively [25,40]. The spectra of GR-COOH
(Fig. 3, curve b) shows broad and intense peaks at 3436, 1734,
and 1258 cm−1 corresponding to the stretching vibration of O H,
diamine (Fig. 3, curve d) on to the graphite surface was confirmed by
the appearance of an amide band, CONH at 1645 cm−1 and NH2
band at 3435 cm−1. The formation of GR-methylacrylate (Fig. 3,
curve e) was ensured through the vanishing of 3435 cm−1 signal
and introduction of ester peak at 1670 cm−1. The confirmation of
GR-G1.0-PAMAM dendrimer (Fig. 3, curve f) could be seen via the
appearance of NH2 peak at 3435 cm−1 and C O peak at 1722 cm−1
[28]. Other PAMAM dendrimer grafted samples exhibited similar
behavior.
3.2. Encapsulation of AgNPs into the graphite grafted PAMAM
dendrimer
X-ray diffraction was carried out in order to compare the
crystallinity of the grafting PAMAM dendrimer network on the
graphite surface and the encapsulated silver nano particles. The
powder XRD spectra (Fig. 4, curves c–e) reveals that all the three
generation of PAMAM dendrimers supported silver nanoparticle
with sizes about 20 nm calculated through Scherrer’s equation
(L = 0.9ꢀ/ˇ(2ꢁ) × cos ꢁmax). The diffraction patterns suggest that
AgNPs exist in hcp and shows reflections at 2ꢁ = 38.2◦, 44.2◦, 64.6◦
and 77.4◦ corresponding to {1 1 1}, {2 0 0}, {2 2 0}, and {3 1 1} lat-
tice planes respectively [42,43]. Moreover the damage to graphite
layer structure can be seen from complete collapse of diffraction
pattern at 26.45◦ implying that the PAMAM grafted graphite sam-
ples are composed of mostly single or few layers.
C
O, and C O groups respectively [41]. As observed from the spec-
trum of acylated graphite GR-COCl in Fig. 3 (curve c), the peak
corresponding to O H stretching vibration at 3436 cm−1 disap-
peared and the
C
O stretching vibration shifted from 1734 cm−1
to 1694 cm−1 along with the introduction of a new peak at 694 cm−1
confirms the formation of C Cl bond. The anchoring of ethylene
Systematic changes were observed in UV–visible spectra while
grafting the dendrimer and also during the encapsulation of AgNPs
Fig. 7. Energy dispersive-X-ray spectrum (EDX) of AgNPs/GR-G3.0PAMAM.

R. Rajesh, R. Venkatesan / Journal of Molecular Catalysis A: Chemical 359 (2012) 88–96
93
0.5
1.4
1.2
1
B
A
0
0-sec
15-sec
30-sec
-0.5
45-sec
60-sec
-1
0.8
0.6
0.4
0.2
0
75-sec
-1.5
-2
90-sec
105-sec
120-sec
-2.5
-3
220
270
320
370
420
470
520
0
30
60
90
120
Wavelength (nm)
Time (sec)
Fig. 8. (A) UV–visible spectra for the reduction of 4-nitrophenol measured at 15 s intervals using AgNPs/GR-G3.0PAMAM nanocatalyst and (B) plot of ln A vs time (s) for the
reduction 4-nitrophenol using AgNPs/GR-G3.0PAMAM nanocatalyst.
(Fig. 5, curves a–c). In Fig. 5 (curve a) an absorption band at 240 nm,
indicative of ␲–␲* transition of graphene oxide appeared and this
band shifted to 300 nm upon grafting the PAMAM dendrimer. This
red shift is ascribed to the n–␲* transition of PAMAM dendrimer
(Fig. 5, curve b) and this confirms the successful grafting. Encapsu-
lation of AgNPs can be seen in Fig. 5 (curve c) through the strong
surface plasmon resonance absorption at 417 nm [44,45].
The scanning electron microscope images demonstrate a clear
preview of the surface of graphite samples before and after func-
tionalization with PAMAM dendrimer. Fig. 6A and B shows the
SEM images of pure graphite which shows flakes like structures
indicating multilayered morphology. In the case of PAMAM sub-
stituted graphite (Fig. 6C and D), the detectable layers have been
replaced by diffused snow like features which could be ascribed to
the distraction of graphite layers by dendrimer substitutions. Fig. 6E
represents the SEM images of third generation PAMAM dendrimer
encapsulated with AgNPs which indicates that the AgNPs are uni-
formly anchored dispersed within the PAMAM dendrimer network.
TEM images shown in Fig. 6F–H also demonstrate the success-
ful encapsulation of AgNPs within PAMAM dendrimer. AgNPs are
buried inside the dendrimer network cavities and hence the clear
crystalline morphology of AgNPs could not be visualized clearly and
they appear as black mass.
0.025
0.02
0.015
0.01
0.005
0
0
1
2
3
4
5
6
Amount of catalyst (mg)
Fig. 9. Catalytic dosage of AgNPs/GR-G3.0PAMAM.
with in the PAMAM dendrimer cavity. In the EDX spectrum,
the signals of C, O, and N are present besides the Ag signal,
indicating the AgNPs distribution with the PAMAM dendrimer
structure.
Further the catalyst was also characterized by EDX analysis
(Fig. 7). The EDX analysis demonstrates the AgNPs accumulation
Table 1
Systematic literature survey of the reduction of 4-nitrophenol and corresponding rate constant at room temperature.a
c
Entry
Sample
Carrier system
Metal
Particle size (nm)b
k (s−1
)
Ref.
1
2
3
4
5
6
7
AgNPs/GR-G3.0PAMAMd
AgNPs/GR-G2.0PAMAMd
AgNPs/GR-G1.0PAMAMd
Au-PFe
PAMAM dendrimer
PAMAM dendrimer
PAMAM dendrimer
Gel
Dendrimer
Ion-exchange resin
Polymer brush
Ag
Ag
Ag
Au
Au
Au
Ag
20 nm
20 nm
20 nm
8 nm
8.3 nm
17 nm
7.5 nm
21.7 × 10−3 s−1
10.7 × 10−3 s−1
2.4 × 10−3 s−1
7.4 × 10−3 s−1
This work
This work
This work
13
Au-PAMAM/PPIf
Au-resing
1.78–13.2 × 10−3 s−1 14
0.16 × 10−3 s−1
0.33 × 10−3 s−1
15
16
Ag-PS-PEGMAh
a
Reactions were catalyzed by AgNPs/GR-G1.0PAMAM, AgNPs/GR-G2.0PAMAM, AgNPs/GR-G3.0PAMAM.
Nanoparticles size (observed by TEM).
b
c
d
e
f
Apparent rate constant (k) s−1
.
Graphite grafted G3.0, 2.0, 1.0PAMAM dendrimer encapsulated AgNPs respectively.
Polymeric phloroglucinol carboxylic acid–formaldehyde supported Au nanoparticles.
Polypropyleneimine dendrimer supported AuNPs.
Polystyrene anion resin supported AuNPs.
Polystyrene grafted poly(ethyleneglycol) methylacrylate supported AgNPs.
g
h

94
R. Rajesh, R. Venkatesan / Journal of Molecular Catalysis A: Chemical 359 (2012) 88–96
Table 2
Catalytic reduction of various nitro aromatics over AgNPs/GR3.0-PAMAM.a,b
Entry
1
Substrate
Product
Time (min)
20
Yieldc (%)
NH2
NO2
100
NO2
NH2
NH2
OH
OH
2
3
25
25
100
100
NO2
OH
OH
NO2
NH2
NH2
NH2
4
5
20
20
100
100
NH2
NH2
NO2
NO2
6
7
25
25
100
100
HO
OH
NH2
NO2
NH2
NO2
NH2
NH2
NO2
NH2
8
9
40
10
20
64
100
100
Cl
Cl
NO2
NH2
Cl
Cl
NH2
NO2
10
Br
Br
NH2
NO2
11
20
40
100
I
I
NO2
NO2
NH2
NH2
N
N
N
N
12
61
a
Reactions conditions: 50 mg of catalyst, 1 mmol of substrate 10 mmol of NaBH4, 50 mL of water stirring at room temperature.
Reused of the catalyst after separation from the reaction mixture.
Conversion yield was monitored through GC.
b
c

R. Rajesh, R. Venkatesan / Journal of Molecular Catalysis A: Chemical 359 (2012) 88–96
95
3.3. Catalytic activity
4. Conclusions
3.3.1. Catalytic reduction of nitro arenes
In conclusion, we have demonstrated low cost alternative proce-
dure for systematic grafting of hyperbranched PAMAM dendrimer
on graphite surface using Michael addition of methacrylate and
ethylenediamine which can encapsulate and stabilize AgNPs. The
third generation graphite grafted PAMAM dendrimer encapsulated
AgNPs shows excellent catalytic activity towards the reduction of
4-nitrophenol under mild conditions. The versatility of our system
has been demonstrated through speed of the reaction and selec-
tive nitro reduction without dehalogenation in halonitroarene and
Schiff’s base ligands containing imine groups. Further investiga-
tion on the encapsulation of other noble metal nanoparticles into
our graphite grafted PAMAM dendrimer and their application in
photocatalysis and electrocatalysis are underway.
The catalytic performance of the AgNPs/GR-G1.0PAMAM,
AgNPs/GR-G2.0PAMAM, AgNPs/GR-G3.0PAMAM, was studied
through model reduction kinetic experiment using 0.01 M aqueous
solution of 4-nitrophenol and 0.1 M NaBH4 and 5 mg catalyst
[13–16]. The reduction was monitored through the absorbance
recorded at 400 nm (Fig. 8) and the pseudo first order rate constant
was estimated to be 21.7 × 10−3 s−1 (Table 1), for AgNPs/GR-
G3.0PAMAM which is the highest achieved so far to the best of our
knowledge in catalytic reduction of aromatic nitro compounds.
The enhanced catalytic activity observed with our nanocatalyst
is attributed to the hydrophobic environment created by the
dendrimer backbone which stabilizes the aryl nitro substrates
adjacent to the catalytic sites which in turn facilitates the reduction
of nitro groups [46]. The maximum catalytic activity obtained at
AgNPs/GR-G3.0PAMAM (Table 1) could be the consequence of more
number of encapsulated AgNPs catalysts in the highly expanded
network of third generation PAMAM than the others. The reduction
was found to occur only in the presence of our nanocatalyst and
no reduction occurred at pure graphite powder or third generation
dendrimer graphite in the absence of AgNPs which shows that
the catalytic reduction occurs at the surface of our nanocatalyst.
Furthermore, the catalyst could be regenerated successfully after
the reduction reaction and could be reused for at least 10 more
times under same conditions without any change in the reduction
rate.
Acknowledgments
We gratefully acknowledge the University Grants Commission
(UGC), Govt. of India, New Delhi, for funding the research project (F.
No. 39-782/2010 (SR). The authors also acknowledge the services
rendered by Central Instrumentation Facility (CIF), Pondicherry
University.
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/
j.molcata.2012.04.001.
3.3.2. Influence of catalytic dosage
Fig. 9 shows the influence of catalytic dosage with various
amount of AgNPs/GR-G3.0PAMAM catalyst. As expected with an
increasing amount of catalyst, the rate of the reduction of 4-
nitrophenol increases. The individual kinetic reactions for each
dosage are shown in Figs. S1–S5 (see electronic supplemen-
tary material).
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