Silver nanoparticles embedded over mesoporous organic polymer as highly efficient and reusable nanocatalyst for the reduction of nitroarenes and aerobic oxidative esterification of alcohols

Article abstract of DOI:10.1016/j.apcata.2014.03.014

Silver nanoparticles (Ag-NPs) have been finely dispersed at the mesoporous organic polymer via post-synthetic chemical grafting over mesoporous poly-triallylamine (MPTA-1). The resulting Ag-MPTA-1 nanomaterial has been characterized by elemental analysis, powder x-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), UV-vis diffuse reflectance spectroscopy (DRS), thermogravimetric analysis (TGA), EPR spectroscopy and AAS elemental analysis. The Ag-MPTA-1 acts as an efficient heterogeneous nanocatalyst in the reduction of substituted nitrobenzenes via transfer hydrogenation. The material also showed excellent catalytic activity in one-step catalytic oxidative esterification of primary alcohols using molecular oxygen as a green oxidant. The catalyst is air-stable, inexpensive, easy to prepare and reused several times without significant decrease in activity and selectivity.

Full text of DOI:10.1016/j.apcata.2014.03.014

Applied Catalysis A: General 477 (2014) 184–194
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Silver nanoparticles embedded over mesoporous organic polymer as
highly efficient and reusable nanocatalyst for the reduction of
nitroarenes and aerobic oxidative esterification of alcohols
Noor Salama,b, Biplab Banerjeec, Anupam Singha Roya,b, Paramita Mondala,b
,
Susmita Roya,b, Asim Bhaumikc,∗, Sk. Manirul Islama,b,∗
a Department of Chemistry, University of Kalyani, Kalyani, Nadia, 741235, W.B., India
b Department of Chemistry, Aliah University, Sector-V, Salt Lake, Kolkata, 700 091, West Bengal, India
c Department of Materials Science, Indian Association for the Cultivation of Science, Kolkata 700032, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Silver nanoparticles (Ag-NPs) have been finely dispersed at the mesoporous organic polymer via post-
synthetic chemical grafting over mesoporous poly-triallylamine (MPTA-1). The resulting Ag-MPTA-1
nanomaterial has been characterized by elemental analysis, powder x-ray diffraction (XRD), transmission
electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), UV–vis diffuse reflectance
spectroscopy (DRS), thermogravimetric analysis (TGA), EPR spectroscopy and AAS elemental analysis. The
Ag-MPTA-1 acts as an efficient heterogeneous nanocatalyst in the reduction of substituted nitrobenzenes
via transfer hydrogenation. The material also showed excellent catalytic activity in one-step catalytic
oxidative esterification of primary alcohols using molecular oxygen as a green oxidant. The catalyst is
air-stable, inexpensive, easy to prepare and reused several times without significant decrease in activity
and selectivity.
Received 20 December 2013
Received in revised form 5 March 2014
Accepted 8 March 2014
Available online 15 March 2014
Keywords:
Mesoporous material
Poly-triallylamine
Silver nanoparticles
Transfer hydrogenation
Oxidative esterification
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
high surface area-to-volume ratio, low coordination number, and
easy access to a large number of active sites [12–16]. Among the
Chemical grafting of a homogeneous metal complex on a porous
solid surface is a versatile strategy to create a site-isolated metal
complex with a regulated metal-coordination structure having
unique catalytic properties [1–5]. The advantage of a supported
metal-complex catalyst is not only in making the separation of the
catalyst from the reaction medium easy but also improves of the
activity and stability of the catalyst due to the site isolation of the
active metal site. Further, this leads to the formation of reusable
new metal coordination structure on a support surface or sup-
ported metal nanoparticles with more or less uniform particle size
distribution [6–10].
There are several methods to chemically coordinate metal com-
plexes on solid surfaces and the grafting over mesoporous polymers
bearing the donor atoms is one of the typical ways to obtain the
immobilized metallic nanoparticles [11]. These supported metallic
nanoparticles have drawn intense attention in recent years as cat-
alysts in many organic transformation reactions because of their
different noble metal nanoparticles, stabilized silver nanoparticles
(Ag-NPs) have been widely employed as catalysts for a wide range
of organic reactions. They show good catalytic activity towards
coupling, cycloaddition, sigmatropic rearrangement, cycloisomer-
ization and nitrene transfer reactions [17]. Various approaches have
been attempted in the last few years for the synthesis and fab-
rication of Ag-NPs stabilized over a wide variety of mesoporous
materials, such as alumina, titania, silica, carbon, etc. due to their
easy separation and recovery from the reaction medium [18–20].
The catalytic hydrogenation of nitro compounds is one of the
most important chemical reactions as the product amines are
potent intermediates for the synthesis of various industrial prod-
ucts like agrochemicals, pharmaceuticals, dyes, polymers, rubbers,
photographic developers, corrosion inhibitors, anticorrosion lubri-
cants, hair-dye products and many more [21–23]. The traditional
synthesis routes for reduction of nitrobenzenes proceeds through
catalytic hydrogenation, electrolytic reduction, metal mediated
reductions etc [24]. But many of these processes utilize poten-
tially explosive H2 gas, high pressure reactors, hazardous and
harmful materials like mineral acids etc. are carried out at a mod-
erately high pressure [25,26] and temperature [26,27], which lead
∗
Corresponding authors. Tel.: +91 33 2473 2805.
E-mail addresses: msab@iacs.res.in (A. Bhaumik), manir65@rediffmail.com
(Sk.M. Islam).
http://dx.doi.org/10.1016/j.apcata.2014.03.014
0926-860X/© 2014 Elsevier B.V. All rights reserved.

N. Salam et al. / Applied Catalysis A: General 477 (2014) 184–194
185
Scheme 1. Schematic diagram for the synthesis of mesoporous polymer MPTA-1 (a) and loading of Ag-NP on it at room temperature to obtain Ag-MPTA-1 nanocatalyst (b).
to low selectivity for the desired product due to nonselective
hydrogenation of other functional groups. In this context, transfer
hydrogenation is advantageous over the traditional hydrogenation
reaction in respect of selectivity, where an alcohol like isopropanol,
considered as one of the green solvents, acts as solvent as well as
the source of hydrogen and the reaction takes place at atmospheric
pressure and relatively low temperature [24,28,29]. Similarly, aro-
matic esters have always attracted a great deal of interest in organic
synthesis as intermediates such as liquid crystal polymers, cosmet-
ics, pharmaceuticals, agrochemicals, and food additives because of
their versatility [30]. Methyl esters in particular are very useful
from the viewpoints of atom economy, versatility for further trans-
formations and chemical industry specially, in odoriferous com-
ponents in flowers and fruits [31] and biodiesel intermediates [32].
The present ecological standards increase the pressure to the devel-
opment of environmentally benign methods. The use of molecular
oxygen or hydrogen peroxide in catalytic oxidation reactions has
attracted considerable attention in this context [33–35]. In this sce-
nario, a more elegant and environmentally benign method for the
preparation of methyl esters based on the one-step direct oxidative
esterification of primary alcohols is very demanding [36–38].
Herein, we wish to report the synthesis of a robust and non
air sensitive silver nanoparticles embedded mesoporous poly-
triallylamine nanocatalyst Ag-MPTA-1 and its excellent catalytic
activity in the reduction of substituted nitrobenzenes via transfer
hydrogenation and aerobic oxidative esterification of alcohols in
presence of molecular oxygen.
2.2. Synthesis of mesoporous polytriallylamine MPTA-1
Mesoporous polymer MPTA-1 was synthesized through the
polymerization of triallylamine under hydrothermal conditions by
using APS as radical initiator (Scheme 1) [39]. In a typical synthesis
1.17 g of SDS was dissolved in 25 ml of water with constant stirring
followed by the addition of 1.12 g of TAA. Then 0.47 g TEMED was
added in this mixture and conc. HCl (12 N) was also added dropwise
until a clear solution is obtained. Addition of acid helps the protona-
tion of the N-atoms of the triallylamine molecules, which facilitates
the ionic interaction with the sulfonate group of the SDS molecules.
The pH of the gel was maintained nearly around 7.0. Then 3.71 g of
APS dissolved in 8 ml of water was added quickly into the solution
under vigorous stirring. A white precipitate was appeared imme-
diately. The resultant mixture was stirred for another 1 h and then
autoclaved at 348 K for 3 days without stirring. The observed final
pH of the synthesis gel was ca 4.0–5.0. After the hydrothermal treat-
ment the product was filtered and washed with deionized water for
several times. Resultant solid was dried under vacuum to obtain
mesoporous polymer MPTA-1.
We have chosen TAA as a precursor for the synthesis of MPTA-
1 because one TAA molecule contains three olefin double bonds
in each of its branches; this polymerization process gives random
cross-linking, leading to a high stability of the organic polymer
framework of MPTA-1. MPTA-1 has mesoporosity, thus Ag-NPs can
be immobilized at the surface of mesopores and can get stabilized
through the interaction between surface of the NPs and N-atoms of
MPTA-1 framework.
2.3. Synthesis of colloidal Ag nanoparticles
2. Experimental
In a typical synthesis, 0.1 ml of an aqueous solution of 1% AgNO3
was taken in 10 ml of water containing 0.5 mg TRIS and was stirred
for 2 min. Then, 0.25 ml of an aqueous solution of NaBH4 (0.08%)
was added drop-wise under stirring. The stirring was continued for
another 10 min, and the resulting nanocolloid was stored at 4 ◦C.
2.1. Materials
Triallylamine (TAA) and tris(hydroxymethyl) aminomethane
(TRIS)
were
obtained
from
Sigma–Aldrich.
N,N,N’,N’-
tetramethylenediamine (TEMED) and ammonium persulfate
(APS) were obtained from Loba Chemie, India and used as received.
Sodium lauryl sulfate (SDS) was purchased from Loba Chemie
and used as structure-directing agent. Silver nitrate (AgNO3) was
purchased from Universal Chemicals, India. Sodium borohydride
(NaBH4) was obtained from Spectrochem, India and used as
received. All other reagents and substrates were also purchased
from Merck, India.
2.4. Synthesis of Ag-MPTA-1 nanocatalyst
In a typical synthesis of Ag-MPTA-1, 50 mg of template-free
MPTA-1 was dispersed in a 10 ml of TRIS-stabilized Ag-NPs and
stirred for 1 h at room temperature. The colour of the col-
loidal nanoparticles gradually disappeared while stirring. The

186
N. Salam et al. / Applied Catalysis A: General 477 (2014) 184–194
supernatant solution was colourless after 1 h of stirring at room
temperature, whereas the colour of MPTA-1 changed to black, indi-
cating the loading of Ag-NPs onto the surface of MPTA-1. After
centrifugation, black coloured Ag-NP loaded mesoporous polymer
Ag-MPTA-1 was obtained. This mesoporous material was washed
several times with distilled water and dried at room temperature.
The loading of Ag-NPs onto MPTA-1 was further confirmed by AAS
analysis (0.2 wt%).
2.5. General procedure for the transfer hydrogenation reduction
of nitrobenzenes
0.2 mmol of the reactant was dissolved in 20 ml isopropanol fol-
lowed by the addition of 50 mg of Ag-MPTA-1 and 20 mg NaOH
(0.5 mmol) in a 100 ml round bottom flask. The reaction mixture
was refluxed at 80 ◦C for the desired time period under a nitrogen
atmosphere. To study the progress of the reaction, the reaction mix-
tures were collected at different time intervals. The products were
quantified by a GC and identified by a GC-MS.
2.6. General procedure for the aerobic oxidative esterification of
benzylic alcohols with methanol
In a typical aerobic oxidation, a mixture of benzylic alcohol
(0.145 ml, 2.5 mmol), K2CO3 (87 mg, 0.4 mmol), methanol (6.0 ml)
and Ag-MPTA-1 catalyst (50 mg) were taken in a 50 ml RB flask. The
reaction was carried out for 12 h under O2 atmosphere at 45 ◦C.
After the completion of the reaction, the catalyst was filtered off
and washed with water followed by acetone and dried in oven. The
filtrate was extracted four times with ethyl acetate (4 × 15 ml) and
the combined organic layers were dried with anhydrous Na2SO4
by vacuum. The filtrate was concentrated and the resulting residue
was purified by column chromatography on silica gel to obtain the
final product.
2.7. Characterization techniques
Fig. 1. Small (a) and wide angle (b) powder XRD pattern of Ag -MPTA-1.
Powder x-ray diffraction (XRD) patterns of the meso-
porous polymer and Ag-MPTA-1 samples were analyzed with
a Bruker D8 Advance x-ray diffractometer using Ni-filtered Cu
K␣ (ꢀ = 0.15406 nm) radiation. Transmission electron microscopy
(TEM) images of the mesoporous polymer were obtained using
a JEOL JEM 2010 transmission electron microscope operating at
200 kV. Ag content in Ag-MPTA-1 was estimated by using a Shi-
madzu AA-6300 atomic absorption spectrometer (AAS) fitted with
a double beam monochromator. EPR measurements were per-
formed on a Bruker EMX EPR spectrometer at x-band frequency
(9.46 GHz) at room temperature (298 K). Thermogravimetric anal-
ysis (TGA) of Ag-MPTA-1 was carried out using a Mettler Toledo
TGA/DTA 851e. UV–Vis spectrum of Ag-MPTA-1 was obtained from
a Shimadzu UV-2401PC doubled beam spectrophotometer having
an integrating sphere attachment for solid samples. The reaction
products of the liquid phase reactions were quantified by using a
Varian 3400 gas chromatograph equipped with a 30 m CP-SIL8CB
capillary column and a flame ionization detector and identified
by Trace DSQ II GC-MS equipped with a 60 m TR-50MS capillary
column.
corresponding to this broad diffraction is ca 3.88 nm. Absence of
higher order diffraction peak suggested the disorder mesostructure
of the sample, same as that of parent MPTA-1. The wide angle x-ray
powder diffraction pattern for the Ag-MPTA-1 material is shown
in Fig. 1b. The formation of silver nanoparticles was confirmed
from this wide angle powder XRD pattern of Ag–MPTA-1. The sharp
diffraction peaks at 2ꢁ values 38.84◦, 45.04◦, 65.43◦, 78.08◦ can be
indexed to the diffraction planes (1 1 1), (2 0 0), (2 2 0), (3 1 1) of
pure face-centred cubic (fcc) silver [40]. This indicates the pres-
ence of face-centred cubic Ag nanoparticles bound at the surface
of the polymer matrix. In Fig. 2 HR-TEM images of the Ag-MPTA-1
materials at different magnifications are shown. In this figure silver
nanoparticles with diameter 15–20 nm are clearly observed (dark
spots) and these are uniformly distributed throughout the speci-
men grid. The FFT diffractogram of a selected area of the grid is
shown in image. Diffraction spots in this FFT pattern suggested
crystalline feature of the Ag-nanoparticles bound at the surface
of Ag-MPTA-1 and the planes are indexed. Debye–Scherrer equa-
tion for average crystallite size calculation is D = 0.9ꢀ/ˇcosꢁ, where
D is the average crystallite size (Å), ꢀ is the x-ray wavelength
˚
3. Results and discussion
(CuK␣ = 1.5406 A), ˇ is the full width at half maximum (FWHM)
in radians and ꢁ is the Bragg diffraction angle of the most intense
peak (1 1 1). Using this equation average crystallite size has been
estimated to be 15.0 nm which matches well with the HR-TEM data
[41].
In Fig. 3 the EPR spectrum of the Ag-MPTA-1 in solid state at
room temperature (298 K) is shown. It provides information on the
nature of the metal ion environment in the Ag-grafted mesoporous
3.1. Characterization of catalyst
Small angle powder XRD pattern of the sample Ag-MPTA-1 is
shown in Fig. 1a. A single and broad diffraction peak is obtained in
the small angle XRD having a 2ꢁ value of 2.241◦, indicating the pres-
ence of the mesophase in the material. The interparticle distance

N. Salam et al. / Applied Catalysis A: General 477 (2014) 184–194
187
1.2
1.1
1.0
0.9
0.8
300
400
500
600
700
Wavelength (nm)
Fig. 4. UV–vis absorbance spectra of Ag-MPTA-1.
absorption between 400 and 420 nm, corresponding to the surface
plasmon resonance (SPR) of Ag NPs [43] suggesting the formation
of Ag NPs on the MPTA-1 surface. Additional absorption bands at
240 and 300 nm could be attributed to the chromophoric organic
functionality of the polymer matrix.
The quantitative determination of the organic content and the
framework stability of the Ag-MPTA-1 samples are obtained from
the thermogravimetric (TGA) and differential thermal analysis
(DTA) under N2 flow (not shown). The TGA of this material showed
the first weight loss below 100 ◦C due to desorption of physisorbed
water. This was followed by a gradual decrease in the weight up to
200 ◦C. Further weight loss above 200 ◦C corresponds to the collapse
of the nanostructure and the polymer together with broad DTA sig-
nal. Thus, this thermal analysis result suggested that Ag-MPTA-1
sample is stable up to 200 ◦C.
3.2. Catalytic activity
Fig. 2. HR-TEM images of the Ag-MPTA-1 at different magnifications.
Supported metals are useful as heterogeneous catalysts in
a wide variety of organic transformation. Specially stabilized
nanoparticles, in comparison to bulk solids, have a significantly
high catalytic activity and demand for clean technology and sus-
tainability. Thus, we have investigated the catalytic activity of
the mesoporous polymer Ag-MPTA-1 in the transfer hydrogena-
tion reduction of substituted nitrobenzenes and one-step catalytic
oxidative esterification of primary alcohols using molecular oxygen
as a green oxidant.
3.3. Catalytic transfer hydrogenation reduction of nitrobenzenes
Among the hydrogenation reactions catalyzed by silver
nanoparticles, catalytic hydrogenation of aromatic nitro com-
pounds is very exciting. The use of silver nano-complex as effective
catalyst for transfer hydrogenation encouraged us to carry out
this type of reaction using mesoporous polymer supported silver
nanocatalyst. The effect of mesoporous Ag-MPTA-1 nanocatalyst
on their catalytic activity for the hydrogenation of nitrobenzene
was investigated as a model reaction (Scheme 2).
For the optimization of the product yield, nitrobenzene was
taken as test substrate for different conditions. We have carried
out the hydrogenation of nitrobenzene in the presence of vari-
ous solvents and base and the results are summarized in Table 1.
Firstly, we have carried out the transfer hydrogenation in the
presence of various solvents and NaOH as a base and the results
are summarized in Table 1. Methanol, ethanol, and isopropanol
were taken for investigations and isopropanol was found to be a
Fig. 3. EPR spectrum of Ag -MPTA-1.
polymer. It shows a symmetric signal and g value corresponding
to this signal is calculated to be 1.99, which could be attributed
to the presence of Ag-nanoparticles embedded into the polymer
matrix [42]. UV–vis spectrum (Fig. 4) of the as-prepared Ag-MPTA-
1 showed that the colloidal silver thus formed exhibits a strong

188
N. Salam et al. / Applied Catalysis A: General 477 (2014) 184–194
halo nitroarenes displayed somewhat poor reactivity compared
to their para-substituted counterparts (Table 2, entries 5–7 and
9–11). This proves that our catalytic system is sensitive to steric
effects. Further, the Ag-MPTA-1 showed high chemoselectivity in
the reduction of the –NO2 group without affecting the –CN and
–COCH3 groups (Table 2 entries 14 and 15). Nitroarenes possess-
ing electron-donating groups also afforded excellent yields of the
corresponding product (Table 2, entries 12 and 13).
Scheme 2. Hydrogenation of nitrobenzenes to amines in the presence of AgNPs-
MPTA-1 as nanocatalyst.
Table 1
3.4. Aerobic oxidative esterification of benzylic alcohols catalyzed
by Ag-MPTA-1
Optimization of reaction conditions.a
Entry
Alcohol
Base
Yield (%)b
1
2
3
4
5
6
7
8
MeOH
EtOH
NaOH
NaOH
NaOH
KOH
Et3N
LiOH
K2CO3
–
14
67
97
78
Trace
32
28
–
Aromatic esters have always attracted a great deal of inter-
est in organic synthesis as intermediates such as liquid crystal
polymers, cosmetics, pharmaceuticals, agrochemicals, and food
additives because of their versatility. We started our investigation
for aerobic oxidative esterification of benzylic alcohols in methanol
as solvent (Scheme 3) as model reaction in presence of silver nano
catalyst.
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
a
Reaction conditions: nitrobenzene (0.2 mmol), Ag-MPTA-1 (0.05 g), and base
(0.5 mmol), solvent (20 ml), temperature (80 ◦C), time 10 h, N2 atmosphere.
In order to optimize the reaction condition, the performance of
a silver-catalyzed aerobic oxidative esterification is known to be
governed by the number of factors such as the bases, solvents etc.
Benzylic alcohol and methanol were used in the model reaction
and molecular oxygen was employed as the oxidant (Table 3). In
presence of Na2CO3 as base it showed the good result with 71%
yield of the desired product 2a and 42% yield of the aldehyde 2b
(entry 5) as determined by GC but no selectivity desired product
obtained. In absence of base no esterification reaction in MeOH
was observed (entry 8). While a stronger base (K2CO3) gave the
best reactivity with a 95% yield of the ester, as determined by GC
and the aldehyde could not be observed (entry 1). To verify the sol-
vent effect in this reaction, a series of reactions were investigated
by taking the model reaction in different solvents. Using MeOH
as solvent gave the best result with 95% ester and no aldehyde.
However, solvents such as EtOH, H2O, THF and DMSO were not
suitable for the reaction and no satisfactory yields were obtained
(entries 2–4).
To further test the catalytic performances of the Ag-MPTA-1,
the oxidative esterification of other benzylic alcohols over Ag NPs-
MPTA-1 catalyst were carried out under O2 at 45 ◦C for 12 h. The
results are presented in Table 4. Aromatic alcohols containing both
electron-donating and -withdrawing groups were converted to
the corresponding methyl esters in good to high yields. The con-
version of benzyl alcohol was as high as 95% (Table 4, entry 1).
For the −CH3 and −OCH3 substituted benzylic alcohols, 89% and
91% conversions were achieved, respectively. For benzylic alco-
hols containing electron-withdrawing groups, such as −NO2, −Cl,
and −Br, the moderate conversions (69%, 34%, and 75%) were still
obtained (Table 4, entries 4, 6, and 5). The oxidative esterifica-
tion of 4-bromobenzyl alcohol resulted in relatively low selectivity
(60.0%). The differences in conversion reflect the influences of
substituent on the reactivity of substrates. However, an aliphatic
alcohol was not oxidized smoothly. In order to examine the limi-
tations of our catalyst, we conducted the oxidative esterification of
the less reactive straight-chain alcohols over Ag-MPTA-1 (Table 4,
b
Yield determined by GC and GCMS analysis.
suitable reducing agent for the maximum conversion of nitroben-
zene. As seen in Table 1, the use of methanol resulted in poor
conversion of the substrate (entry 1) when compared to ethanol
and isopropanol. Most gratifyingly, the reaction performed using
isopropanol (iPrOH) as the hydride donor gave in overall the high-
est yield. The effect of the bases on the catalytic performance of
this system has also been investigated. The substrate was allowed
to react with a catalytic amount of complex in the presence of
different bases like NaOH, KOH, K2CO3, LiOH and Et3N, with iso-
propanol (Table 1). It was found that the use of NaOH and KOH
leads to 78–91% of conversion (entries 3, 4). In the absence of base,
no conversion of nitrobenzene into aniline was observed (entry 8).
Similarly, the use of triethylamine as base also leads to trace con-
version (entry 5). As a result NaOH was selected as the optimum
base. From the above discussions, it can be seen that the best yield
was obtained in the reduction of nitrobenzene by using NaOH as
base in iPrOH as solvent at 80 ◦C for 10 h.
To examine the scope of the Ag-MPTA-1 catalytic system for
the reduction of the nitro group, a series of nitroarenes with struc-
turally divergent functional groups were examined. The catalytic
system was found to be very efficient regardless of the presence
of electron-withdrawing or electron-donating substituents in the
aromatic ring. The results are presented in Table 2. Simple unsub-
stituted nitrobenzene yielded the desired aniline product in 97%
yield (Table 2, entry 1). o-, m- and p-nitrophenols gave excellent
yields of the corresponding anilines (Table 2, entries 2–4). m-
Nitrophenol was found to be the most reactive among all the three
nitrophenols. Halogenated nitroarenes were selectively reduced
to their corresponding halo anilines in good to excellent yields
and no dehalogenation of the product was observed which was
often encountered with several procedures such as hydrogena-
tion (Table 2, entries 5–11). It was seen that ortho-substituted
Scheme 3. Ag-MPTA-1 catalyzed aerobic oxidative esterification reactions.

N. Salam et al. / Applied Catalysis A: General 477 (2014) 184–194
189
Table 2
Catalytic reduction of substituted nitrobenzene over Ag -MPTA-1 nanocatalysta
NO2
NH2
Ag NPs-MPAT-1
iPrOH, NaOH
X
X
Entry
Substrate
Product
Time (h)
Yield (%)b
NH2
NO2
1
2
3
10
97
96
97
NH2
NO2
10
OH
NO2
OH
NH2
OH
OH
8
NO2
NH2
4
5
6
8
12
12
98
81
85
OH
OH
NH2
NO2
Cl
Cl
NO2
NH2
I
I
NO2
NH2
Br
Br
7
8
12
12
86
83
NO2
NH2
F
F

190
N. Salam et al. / Applied Catalysis A: General 477 (2014) 184–194
Table 2 (Continued)
Entry
Substrate
Product
Time (h)
Yield (%)b
NH2
NO2
9
10
84
Cl
Cl
NH2
NO2
10
10
87
I
I
NO2
NH2
11
10
88
Br
Br
NH2
NO2
12
5
85
CH3
CH3
NO2
NH2
13
8
86
OCH3
OCH3
NO2
NH2
14
12
87
CN
CN
NO2
NH2
15
12
89
COCH3
COCH3
a
Reaction conditions: substrate (0.2 mmol), Ag-MPTA-1 (0.05 g), and NaOH (0.5 mmol), iPrOH (20 ml), temperature (80 ◦C), N2 atmosphere.
Yield determined by GC and GCMS analysis.
b

N. Salam et al. / Applied Catalysis A: General 477 (2014) 184–194
191
Table 3
Impact of reaction parameters on the oxidative esterification of benzylic alcohols with methanol.a
Entry
Solvent
Base
Yields (%)b
Ester (%) 2a
Aldehyde (%) 2b
1
2
3
4
4
5
6
7
8
MeOH
DMSO
THF
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Na2CO3
Cs2CO3
K2PO4
–
95
–
–
24
25
71
54
23
–
–
–
–
H2O
37
EtOH
MeOH
MeOH
MeOH
MeOH
13
42
21
37
–
a
Reaction conditions: benzylic alcohol (0.145 ml, 2.5 mmol), base (0.4 mmol), solvent (6 ml), 45 ◦C, oxygen balloon, time (12 h), Ag-catalyst (50 mg).
Yield of product was determined by GC and GC-MS analysis.
b
entry 7–9). For n-butanol, a 41% conversion was obtained. The con-
versions of n-pentanol and n-hexanol could reach 53% and 57%,
respectively.
3.6. Heterogeneity test
3.6.1. Hot filtration test
To examine whether silver was being leached out from the solid
support to the solution, experiment has been carried out in the
transfer hydrogenation reduction of nitrobenzene with our sup-
ported Ag-MPTA-1 nano-catalyst. A typical hot filtration test was
performed in the transfer hydrogenation reduction to investigate
whether the reaction proceeded in a heterogeneous or a homo-
geneous fashion. For the rigorous proof of heterogeneity, a test
was carried out by filtering catalyst from the reaction mixture at
80 ◦C after 4 h and the filtrate was allowed to react up to the com-
pletion of the reaction (6 h). In this case no change in conversion
was observed, which suggests that the catalyst is heterogeneous in
nature. No evidence for leaching of silver or decomposition of the
complex catalyst was observed during the catalytic reaction. It was
noticed that after filtration of the catalyst from the reactor at the
reaction temperature, hydrogenation reactions do not proceed fur-
ther. Atomic absorption spectrometric analysis of the supernatant
solution of the reaction mixture thus collected by filtration also con-
firmed the absence of silver ions in the liquid phase. Thus, results
of the hot filtration test suggested that Ag-nanoparticle/ions are
not being leached out from the solid catalyst during the transfer
hydrogenation reactions.
3.5. Recyclability of Ag-MPTA-1 nanocatalyst
For a heterogeneous catalyst, it is important to examine its ease
of separation, recoverability and reusability. The reusability of the
mesoporous Ag-MPTA-1 nanocatalyst was investigated in transfer
hydrogenation reduction of nitrobenzenes and oxidative esterifica-
tion of benzyl alcohol. After each run the filtrate was concentrated
and the resulting residue was purified. After the completion of the
reaction, the contents were centrifuged to separate the solid cat-
alyst from the reaction mixture. The catalyst was then thoroughly
washed with distilled water followed by acetone and then dried
in air before using in the next run and almost consistent activ-
ity was observed for next five consecutive cycles (Fig. 5). As seen
from the figure the catalyst can be efficiently recycled and reused
for repeating cycles without appreciable decrease in product yield.
Little decrease in the product yields during the recycling could be
attributed to the loss of the catalyst amount in the separation pro-
cess from the reaction mixture.
3.7. Comparison with other reported catalytic system
Transfer hydrogenation reduction of nitrobenzenes and oxida-
tive esterification of benzyl alcohol under heterogeneous condi-
tions over a variety of catalysts has been studied (Table 5). Table 5
provides a comparison of the results obtained for our present cat-
alytic system with those reported in the literature [44–47]. From
Table 5, it is seen that Ag-MPTA-1 showed higher catalytic activity
over these catalysts (higher TOFs than most of the related cat-
alytic systems). Although core–shell Ag@Ni magnetic nanocatalyst
showed high catalytic activity in transfer hydrogenation using iso-
propyl alcohol as hydrogen donor [45] it remained unexplored
in oxidative esterification of alcohols. Similarly Au nanoparticles
dispersed on mesoporous silica material in Au-HMS-M showed
high catalytic activity in oxidative esterification of alcohols only
[46]. Ag-NPs dispersed in the mesoporous polymeric framework of
Fig. 5. Recycling efficiency of Ag-MPTA-1 nanomaterial in transfer hydrogenation
reduction of nitrobenzenes and oxidative esterification of benzyl alcohol.

192
N. Salam et al. / Applied Catalysis A: General 477 (2014) 184–194
Table 4
Oxidative esterification of benzylic alcohols over Ag-MPTA-1 nanocatalysta
Entry
Substrates
Product
Yield (%)b
O
OH
1
95
O
O
OH
2
3
89
91
O
O
OH
O
O
O2N
B r
O
O
OH
O
4
69
O2N
O
OH
O
5
75
B r
O
OH
O
6
34
Cl
Cl
O
7
41
OH
O
O
OH
8
9
53
57
O
O
OH
O

N. Salam et al. / Applied Catalysis A: General 477 (2014) 184–194
193
Table 4 (Continued)
Entry
Substrates
Product
Yield (%)b
O
OH
O
O
10
42
NH2
NH2
O
OH
11
39
N
N
a
Reaction conditions: alcohol (2.5 mmol), K2CO3 (0.4 mmol), MeOH (6 ml), 45 ◦C, oxygen balloon, time (12 h), Ag-catalyst (50 mg).
Yield of product was determined by GC and GCMS analysis.
b
Table 5
Comparison of catalytic activity of the Ag-MPTA-1 in transfer hydrogenation reduction of nitrobenzene and oxidative esterification of benzyl alcohol with other reported
systems.
Entry
Catalyst
Reaction conditions
Transfer
Oxidative
Ref.
hydrogenation
reduction reaction
esterification
reaction
Yield (%) (TOF/h−1
)
Yield (%) (TOF/h−1
)
1
2
3
Cu nanoparticles/HCOONH4
Ag@Ni magnetic nanocatalyst
Pd/Char, Bi(NO3)3
Au-HMS-M
Ag-MPTA-1 nanocatalyst
Ethylene glycol, 120 ◦C, 12 h
86(0.03)
96(4.8)
–
–
–
[43]
[44]
[45]
[46]
Isopropyl alcohol, KOH, 80 ◦C, 2 h
KOMe, 60 ◦C, oxygen (1 atm), 2 h
K2CO3, 130 ◦C, oxygen (10 atm), 4 h
NaOH, iPrOH, 80 ◦C, 10 h (Transfer
hydrogenation) K2CO3, 45 ◦C, oxygen
balloon, 12 h (Oxidative esterification)
92(11.5)
94 (410)
95 (198)
–
4
97(19.4)
This study
Ag-MPTA-1 found highly reactive for both class of reactions and
thus have future potential for multifunctional catalytic activities.
Commission (UGC) New Delhi, India for providing support to the
Department of Chemistry, University of Kalyani under PURSE, FIST
and SAP programme.
4. Conclusions
References
From our experimental observations we can conclude that a
new Ag-nanocatalyst has been developed by embedding of Ag-
nanoparticles at the surface of mesoporous polymer MPTA-1 and it
has been successfully employed as a recyclable heterogeneous cat-
alyst for the hydrogenation of aromatic nitroarenes and oxidative
esterification of alcohols for a wide range of substrates. Both these
catalytic processes are green and offer a number of advantages,
such as easy work-up and separation of the catalyst from the reac-
tion mixture by filtration, reusability for several times with minimal
loss of catalytic activity. These key findings make the nanotechnol-
ogy based recyclable heterogeneous catalysis platform inherently
advanced, economical, green and environmentally sustainable.
[1] R.A. Sheldon, M. Wallau, I.W.C.E. Arends, U. Schuchardt, Acc. Chem. Res. 31
(1998) 485–493.
[2] P. McMorn, G.J. Hutchings, Chem. Soc. Rev. 33 (2004) 108–122.
[3] J.M. Thomas, R. Raja, D.W. Lewis, Angew. Chem. Int. Ed. 44 (2005) 6456–6482.
[4] M. Tada, Y. Iwasawa, Chem. Commun. (2006) 2833–2844.
[5] C. Copéret, J.-M. Basset, Adv. Synth. Catal. 349 (2007) 78–92.
[6] K. Yamaguchi, C. Yoshida, S. Uchida, N. Mizuno, J. Am. Chem. Soc. 127 (2005)
530–531.
[7] N. Rendón, R. Berthoud, F. Blanc, D. Gajan, T. Maishal, J.-M. Basset, C. Copéret, A.
Lesage, L. Emsley, S.C. Marinescu, R. Singh, R.R. Schrock, Chem. Eur. J. 15 (2009)
5083–5089.
[8] S. Muratsugu, Z. Weng, H. Nakai, K. Isobe, Y. Kushida, T. Sasaki, M. Tada, Phys.
Chem. Chem. Phys. 14 (2012) 16023–16031.
[9] M. Tada, S. Muratsugu, M. Kinoshita, T. Sasaki, Y. Iwasawa, J. Am. Chem. Soc.
132 (2010) 713–724.
[10] N.J. Schoenfeldt, J.M. Notestein, ACS Catal. 1 (2011) 1691–1701.
[11] D. Chandra, B.K. Jena, C.R. Raj, A. Bhaumik, Chem. Mater. 19 (2007) 6290–6296.
[12] A. Mohanty, N. Garg, R. Jin, Angew. Chem. Int. Ed. 49 (2010) 4962–4966.
[13] X.Y. Liu, Y.N. Li, J.W. Lee, C.Y. Hong, C.Y. Mou, B.W.L. Jang, Appl. Catal. A: Gen.
439 (2012) 8–14.
Acknowledgements
SMI acknowledges Department of Science and Technology (DST)
and Council of Scientific and Industrial Research (CSIR) New Delhi,
India for funding. BB is thankful to the CSIR, New Delhi, India for
CSIR-SPM fellowship. PM and NS are also thankful to the UGC
New Delhi, India for their research fellowship. AB thanks DST,
New Delhi for providing instrumental facilities through DST Unit
on Nanoscience and DST-SERB research grants. We acknowledge
Department of Science and Technology (DST) and University Grant
[14] K.-I. Shimizu, N. Imaiida, K. Sawabe, A. Satsuma, Appl. Catal. A: Gen. 421–422
(2012) 114–120.
[15] F. Dong, W. Guo, S.-K. Park, C.-S. Ha, Chem. Commun. 48 (2012) 1108–1111.
[16] J. Mondal, A. Modak, A. Bhaumik, J. Mol. Catal. A: Chem. 350 (2011) 40–48.
[17] M. Harmata, Silver in Organic Chemistry, John Wiley & Sons, Hoboken, 2010.
[18] A.B. Castle, E. Gracia-Espino, C. Nieto-Delgado, H. Terrones, M. Terrones, S.
Hussain, ACS Nano 5 (2011) 2458–2466.
´
[19] R. Grabowski, J. Słoczyn´ ski, M. Sliwa, D. Mucha, R.P. Socha, M. Lachowska, J.
Skrzypek, ACS Catal. 1 (2011) 266–278.

194
N. Salam et al. / Applied Catalysis A: General 477 (2014) 184–194
[20] S.E. Davis, M.S. Ide, R.J. Davis, Green Chem. 15 (2013) 17–45.
[21] A. Patra, A. Dutta, A. Bhaumik, Catal. Commun. 11 (2010) 651–655.
[22] X.-H. Li, X. Wang, M. Antonietti, Chem. Sci. 3 (2012) 2170–2174.
[23] K.S. Shin, Y.K. Cho, J.Y. Choi, K. Kim, Appl. Catal. A: Gen. 413–414 (2012)
170–175.
[24] S.U. Sonavane, M.B. Gawande, S.S. Deshpande, A. Venkataraman, R.V. Jayaram,
Catal. Commun. 8 (2007) 1803–1806.
[25] M. Lin, B. Zhao, Y. Chen, Ind. Eng. Chem. Res. 48 (2009) 7037–7043.
[26] H. Liu, J. Deng, W. Li, Catal. Lett. 137 (2010) 261–266.
[27] M.M. Dell’Anna, V. Gallo, P. Mastrorilli, G. Romanazzi, Molecules 15 (2010)
3311–3318.
[28] E. Yilgor, G.E. Atilla, A. Ekin, P. Kurt, I. Yilgor, Polymer 44 (2003) 7787–7793.
[29] T.T.H. Vu, P.S. Kumbhar, F. Figueras, Adv. Synth. Catal. 345 (2003) 493–496.
[30] T. Nobuta, A. Fujiya, S. Hirashima, N. Tada, T. Miura, A. Itoh, Tetrahedron Lett.
53 (2012) 5306–5308.
[31] J. Otera, Esterification: Methods, Reactions, and Applications;, Wiley VCH,
Weinheim, 2003.
[34] K. Ebitani, K. Motokura, K. Muzugaki, K. Kaneda, J. Am. Chem. Soc. 126 (2004)
10657.
[35] A. Chellamani, N.I. Alhaji, S. Rajagopal, J. Phys. Org. Chem. 20 (2007) 255.
[36] R.L. Oliveira, P.K. Kiyohara, L.M. Rossi, Green Chem. 11 (2009) 1366.
[37] E. Taarning, A. Theilgaard Madsen, J.M. Marchetti, K. Egeblad, C.H. Christensen,
Green Chem. 10 (2008) 408.
[38] F.-Z. Su, J. Ni, H. Sun, Y. Cao, H.-Y. He, K.-N. Fan, Chem. Eur. J. 14 (2008) 7131.
[39] D. Chandra, A. Bhaumik, J. Mater. Chem. 19 (2009) 1901–1907.
[40] H. Chen, Y. Liu, G. Zhao, Electron. Mater. Lett. 7 (2011) 151–154.
[41] M. Pramanik, A. Bhaumik, J. Mater. Chem. A 1 (2013) 11210–11220.
[42] G. Mitrikas, C.C. Trapalis, N. Boukos, V. Psyharis, L. Astrakas, G. Kordas, J. Non-
Cryst. Solids 224 (1998) 17–22.
[43] S. Pal, G. De, Mater. Res. Bull. 44 (2009) 355.
[44] A. Saha, B.C. Ranu, J. Org. Chem. 73 (2008) 6867–6870.
[45] M.B. Gawande, H. Guo, A.K. Rathi, P.S. Branco, Y. Chen, R.S. Varma, D.L. Peng,
RSC Adv. 3 (2013) 1050–1054.
[46] A.B. Powell, S.S. Stahl, Org. Lett. 15 (2013) 5072–5073.
[47] L.A. Parreira, N. Bogdanchikova, A. Pestryakov, T.A. Zepeda, I. Tuzovskaya, M.H.
Farías, E.V. Gusevskaya, Appl. Catal. A: Gen. 397 (2011) 145–152.
[32] N. Pal, M. Paul, A. Bhaumik, Appl. Catal. A: Gen. 393 (2011) 153–160.
[33] T. Mallat, A. Baiker, Chem. Rev. 104 (2004) 3037.