Ag-nanoparticle embedded p(AA) hydrogel as an efficient green heterogeneous Nano-catalyst for oxidation and reduction of organic compounds

Article abstract of DOI:10.1002/aoc.3917

P(AA)-Ag heterogeneous catalyst system comprised of Ag nanoparticles embedded within hydrogel matrices has been described for the selective aerobic oxidation of alcohols and reduction of nitro phenols in water. P(AA)-Ag nanocomposite was characterized by Fourier transform infrared (FT-IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermal gravimetric analysis (TGA), X-Ray photoelectron spectroscopy (XPS) and inductively coupled plasma atomic emission spectrometer (ICP). Catalytic activity of p(AA)-Ag catalyst was investigated in the aerobic oxidation of primary alcohols and reduction of nitro compounds by emphasizing the effect of different parameters such as temperature, catalyst amount, substituent effect, etc. The catalyst was easily recovered from the reaction medium and it could be re-used for other three runs without significant loss of activity.

Full text of DOI:10.1002/aoc.3917

Received: 10 March 2017
DOI: 10.1002/aoc.3917
Revised: 2 May 2017
Accepted: 11 May 2017
F U L L PA P E R
Ag‐nanoparticle embedded p(AA) hydrogel as an efficient green
heterogeneous Nano‐catalyst for oxidation and reduction of
organic compounds
Massomeh Ghorbanloo
| Ali Heydari1 | Hidenori Yahiro2
1 Department of Chemistry, Faculty of
Science, University of Zanjan, 45371‐38791
Zanjan, Iran
2 Department of Materials Science and
Biotechnology, Graduate School of Science
and Engineering, Ehime University,
Matsuyama 790‐8577, Japan
P(AA)‐Ag heterogeneous catalyst system comprised of Ag nanoparticles embedded
within hydrogel matrices has been described for the selective aerobic oxidation of
alcohols and reduction of nitro phenols in water. P(AA)‐Ag nanocomposite was
characterized by Fourier transform infrared (FT‐IR), scanning electron microscopy
(SEM), transmission electron microscopy (TEM), thermal gravimetric analysis
(TGA), X‐Ray photoelectron spectroscopy (XPS) and inductively coupled plasma
atomic emission spectrometer (ICP). Catalytic activity of p(AA)‐Ag catalyst was
investigated in the aerobic oxidation of primary alcohols and reduction of nitro com-
pounds by emphasizing the effect of different parameters such as temperature, cata-
lyst amount, substituent effect, etc. The catalyst was easily recovered from the
reaction medium and it could be re‐used for other three runs without significant loss
of activity.
Correspondence
Massomeh Ghorbanloo, Department of
Chemistry, Faculty of Science, University of
Zanjan, 45371‐38791 Zanjan, Iran.
Email: m_ghorbanloo@yahoo.com
KEYWORDS
green nano‐ catalyst, nitro phenols, oxidation, primary alcohols, reduction
1 | INTRODUCTION
interaction between Ag NPs and zeolite, which prevents effi-
cient disperse and stabilization of Ag NPs in the matrix. In
Silver nanoparticles, NPs, are versatile building blocks with
strong surface plasmon resonance, high electrical, optical
and high surface‐to‐volume ratio[1,2] that makes them an
interesting subject in electronics,[3] surface enhanced Raman
spectroscopy,[4] sensing,[5] and catalysis.[6] However, the
aggregation of these nanoparticles due to their high surface
charge and surface to volume ratio, decreases their surface
area,[7] which results in a decrease in their catalytic activity.[8]
To overcome this problem, silver NPs are usually loaded or
dispersed in a solid matrix, such as polymeric matrices e.g.
hydrogels,[9] polyelectrolyte brushes,[10] and dendrimers,[11]
or inorganic oxides e.g. silica,[12] alumina,[13] titania[14] and
zeolite.[15] The catalytic activity of silver nanoparticles is
shown to be dependent on their size, content and the interac-
tion between the Ag NPs and the matrix.[16] For example, the
catalytic activity of Ag NPs dispersed in zeolite matrix is
lower compared to the other media, because of the ineffective
contrast to the zeloits, the polymer matrixes have the ability
to tune the guest‐host interaction in order to provide well‐
defined spatial distribution. The well‐defined spatial distribu-
tion provides kinetic stability for metal nanoparticles, as well
as excellent mechanical properties for the composites.
Among all the stabilizing systems, hydrogels are the most
effective ones, particularly when stabilized metal nanoparti-
cles are used as catalysts. For instance, the surface of the
metal nanoparticles is not covered and for this reason cata-
lytic activity is not affected by stabilization. Or the speed of
catalytic reaction can be tuned by controlling the crosslinking
density of polymeric network. These composites were further
used as catalysts for oxidation of alcohols and reduction of
nitrophenols.
Nitrophenol and its derivatives are important by‐products
produced from pesticides, herbicides and synthetic
dyes.[17,18] 4‐Nitrophenol (4‐NP) can damage the kidney,
Appl Organometal Chem. 2017;e3917.
wileyonlinelibrary.com/journal/aoc
Copyright © 2017 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.3917

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GHORBANLOO ET AL.
central nervous system and liver. Thus removal of NP from
the environment is important reaction. The reduction of 4‐
NP is also necessary for the manufacture of analgesic, antipy-
retic and other drugs, in pharmaceutical industries.[19] In
addition this reaction has been widely exploited for
confirming the synthesis of efficient nanoparticle catalyst
by the study of catalytic activities of nanoparticles.[20]
The selective oxidation of alcohols to the corresponding
carbonyl compounds has been comprehensively studied,
due to their importance in production of fine chemicals and
intermediates.[21,22] As a result of the ever‐growing concerns
over green chemistry and chemical processes, many efforts
have been made to develop new catalytic protocols for the
oxidation of alcohols, especially with O2 as oxidant, in water
as green solvent.[23]
In these regards, we prepared poly(acrylic acid)
(p(AA)) hydrogel and used it as reactor as well as stabi-
lizers for the synthesis of silver metal nanoparticles. The
catalytic activity of the prepared p(AA)‐Ag composite was
investigated for the reduction of 4‐NP and oxidation of
alcohols. The effect of different parameters on the reduc-
tion and oxidation reactions were studied. In addition, the
reusability of the catalyst was also investigated for four
consecutive cycles. According to the best of our knowledge
this is for the very first time that Ag is fabricated in p(AA)
hydrogel. Also, no report exists in literature regarding the
investigation of oxidation of alcohols in the presence of
O2 as an oxidant and reduction of nitrophenol in the pres-
ence of NaBH4 as reducing agent in aqueous medium by
poly(AA)‐Ag composite.
−50 °C for 3 days to preserve their porous structure without
any collapse. After that, the dried samples were deposited
onto an aluminum stub and sputter‐coated with gold for
60 s to enhance conductivity. Transmission electron
microcopy (TEM, HITACHI S‐4800) was used to find out
the size of metal nanoparticles inside the hydrogel nanocom-
posites. To image the silver nano particles, TEM analysis was
performed on a JEM‐2100 (JEOL). The swollen hydrogel
was finely grounded with the help of a soft ball and the
resulted hydrogel nanocomposite samples were dispersed in
1 mL of ethanol and dropped on collodion film coated copper
grid for TEM analysis. Thermogravimetric analysis (TGA)
was performed on an STA409PC Netzsch thermal analyzer
and BÄHR‐Thermoanalse GmbH‐Type TAS03, Germany
in flowing N2 (60 mL min−1) at a heating rate of 10 °C
min−1. The X‐ray photoelectron spectrum (XPS) was col-
lected in a 1600E Perkin Elmer using Mg‐Kα excitation
sources. All the binding energy (BE) values were referenced
to the C 1 s line at 284.5 eV. The reaction products of oxida-
tion were determined and analyzed using an HP Agilent 6890
gas chromatograph equipped with a HP‐5 capillary column
(phenyl methyl siloxane 30 m × 320 μm × 0.25 μm).
2.2 | Preparation of hydrogels
Polymeric hydrogel was synthesized from AA via free radical
polymerization reaction techniques in mild condition (40 °C),
as shown in Scheme 1.[24,25] In the synthesis of p(AA)
hydrogels, 5 ml AA (0.0725 mol), 0.055 g MBA (0.5%)
and 10 μl TEMED were mixed with 4 ml pure water and to
this solution a separately prepared APS solution of
0.165 g APS in 1 ml water (1 mol% of AA) was added
and vortexes homogeneously. The mixture was placed into
plastic straws (~ 4 mm in diameter), and these plastic
straws were immersed in a 40 °C water bath controlled
by a thermostat for 4 h to complete polymerization and
crosslinking. Finally, the obtained 3‐D hydrogels were cut
in equal shapes, washed with approximately of 2000 ml
of water for 24 h. The wash water replenished every 2 h
to remove un‐reacted species (monomer, crosslinker, accel-
erator and initiator). After the cleaning procedure,
hydrogels were dried in an oven to a constant weight at
40 °C and kept in sealed containers for further use.
Selected FT‐IR (KBr, cm−1): (p(AA)): 3530 (br, strong),
2935 (m), 2865 (w), 1739 (vs), 1558 (m), 1462 (s), 1419 (m),
1257 (m), 1172 (s), 1118 (w), 937 (w), 805 (m), 642 (w), 491
(w).
2 | EXPERIMENTAL
2.1 | General considerations
The monomer, acrylic acid (AA) (99%, Sigma Aldrich), the
crosslinker, N,N′‐methylenebisacrylamide (MBA) (99%,
Across), the initiators, ammonium persulfate (APS) (99%,
Sigma Aldrich) and the accelerator N,N,N′,N′‐
tetramethylmethylenediamine (TEMED) (98% Across) were
used in hydrogel preparation. Ag(CH3COO) (Merck) was
used as metal ion source. Sodium borohydride (NaBH4,
98%, Merck) was used in the reduction of metal ions to pre-
pare metal nanoparticles. All the chemicals were used as
received without further purification.
FT‐IR spectra were recorded in KBr disks with a Bruker
FT–IR spectrophotometer. The exact amount of the silver in
the composites was determined by ICP‐AES, Spectro
Genesis. Morphology of swollen p(AA), hydrogel was inves-
tigated with Scanning Electron Microscopy (SEM) via
MIRA3 FEG SEM (Tescan, Czech Republic) and an acceler-
ating voltage of 10 keV. The sample was swollen and quickly
frozen in liquid nitrogen. The hydrogel was freeze‐dried at
2.3 | In situ synthesis of metal nanoparticles
within p(AA) hydrogel
For in situ fabrication of metal nanoparticles within p(AA)
hydrogel, first silver ions were loaded into hydrogel

GHORBANLOO ET AL.
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SCHEME 1 Schematic presentation of p(AA) synthesis and preparation of metal nanoparticle within p(AA) hydrogels
network by dispersing 0.1 g of the dried p(AA) hydrogel in
50 ml, 500 ppm aqueous solution of silver acetate for 24 h
at room temperature under continuous stirring. Ag(I) ions
loaded hydrogels were washed with DI to remove unbound
metal ions. Then, to reduced metal ions within hydrogel
network, metal ions loaded hydrogels were treated with
50 ml, 0.1 M NaBH4. Finally, the prepared p(AA)–Ag
hydrogel composites were filtered, washed with DI and
used for characterization, as shown in Scheme 1 and
Figure 1‐a,b and c. The amounts of metal nanoparticles
entrapped in hydrogels were calculated by ICP measure-
ments after dissolution of metal nanoparticles embedded
within p(AA) hydrogel by treating with 5 M HCl aqueous
solution.
To test the reusability of the p(AA)‐Ag composite, after
every usage, the catalyst was separated from reaction mixture
by filtration, washed with DI and reused in the same reaction
conditions again. All the experiments were repeated at least
three times and there average values are given with standard
deviations.
2.5 | General reduction procedure
Catalytic activity of p(AA)‐Ag composites was studied for
the catalytic reduction of 4‐NP. 0.01 M solution of 4‐NP
was prepared and into 50 ml of this solution 0.35 M of
NaBH4 was added and stirred at 250 rpm. Certain amount
of hydrogel composite was added as a catalyst into this
mixture. About 0.3 ml sample was taken out from reaction
mixture at different time intervals and diluted up to 15
times. Then the progress of the reduction reaction was
observed by measuring the absorption maxima at 400 nm
by UV–Visible spectrophotometer (Shimadzu 1601).
Reduction rate constant was calculated by measuring the
decrease in intensity of absorption peak at 400 nm. To
study the effect of temperature on the rate of reduction of
4‐NP, the reduction was carried out at three different tem-
peratures; 25 °C, 40 °C and 60 °C keeping the amount
of reactants and catalyst constant. Also the effect of amount
of catalyst was investigated on the reduction of 4‐NP. For
this, different amounts of catalysts were used and other
reaction conditions were kept constant. To evaluate the
reusability of catalysts, after first cycle the catalysts were
separated from reaction medium by filtration, washed with
DI water and used again for the reduction of 4‐NP under
the same conditions till fourth cycle. All the experiments
were repeated at least three times and there average values
are given with standard deviations.
2.4 | General oxidation procedure
A mixture of p(AA)‐Ag (0.036 mmol Ag) in water (5.0 ml)
was placed into a two‐necked flask equipped with a mag-
netic stirrer. The flask was evacuated and refilled with pure
oxygen (balloon filled). Then the benzyl alcohol (0.108 g,
1.0 mmol) and K2CO3 (0.166 g, 1.2 mmol) was added into
the solution with a syringe. The mixture was heated to
reach the set temperature under O2 atmosphere for 24 h.
Then the solution was introduced to continue the reaction
in the stable temperature. The resulting mixture was vigor-
ously stirred at 80 °C under O2 atmosphere for 24 h. After
the reaction, the catalyst was de‐swelled in MeOH (2.0 ml)
for 24 h. Then the organic phase was combined and
removed the solvent and analyzed immediately by GC.
The oxidation products were identified by comparing the
retention times with the literature data. For the blank test,
the oxidation reaction of benzyl alcohol was accomplished
with bare p(AA).

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GHORBANLOO ET AL.
3 | RESULTS AND DISCUSSION
3.1 | Synthesis and characterization
The FT‐IR spectra of AA and p(AA) hydrogel are presented
in Figure 1‐d and e, respectively. Figure 2‐a indicates the
O–H stretching band in 3530 cm−1 region and the C–H
stretching at 2935 and 2865 cm−1 regions, respectively.[26]
Also the bands at 1739 and 1462 cm−1 are assigned to
C = O and COO− stretching band of acrylic acid, respec-
tively.[27] In addition, the NH band of MBA is overlapped
by OH peak at about 3200 cm‐1[28] so the related peak is
disordered.
Scanning electron microscopy technique was used to ana-
lyze the morphology of hydrogels. The SEM image of
p(AA), shown in Figure 1‐f, indicates the formation of homo-
geneous and highly porous material.
To determine the thermal properties and the amount of
silver nanoparticles entrapped inside the gels network, ther-
mal gravimetric analysis (TGA) were performed for pure
and composite hydrogels. About 3 mg hydrogel was put in
the pan of the TGA analyzer and heated from 30 to 800 °C
with a heating rate of 10 °C/min under 60 mL/min N2 flow.
Figure 2 shows TGA curves of bare p(AA) (Figure 2‐a),
and p(AA)–Ag composite (Figure 2‐b). It could be seen
clearly that there were two processes of weight loss for all
samples when heating up the temperature. The first stage at
less than 250 °C was due to the release of the physically
absorbed water. The second as a major weight loss in the
range of 250–800 °C was obtained from the decomposition
of p(AA). P(AA) was thermally decomposed completely with
no carbon residue at 800 °C, which is similar to other previ-
ous reports. Upon their comparison and reduction of bare
p(AA) hydrogel from p(AA)‐Ag composite hydrogels, it
was found that the metal content of the composite was
19.7 wt.% Ag compounds. Increased in thermal stability of
p(AA)‐Ag composite in comparison with bare p(AA)
FIGURE 1 The digital camera images of a) bare p(AA), b) p(AA)‐
Ag+, c) p(AA)‐Ag0, d) FTIR spectra of acrylic acid, e) FTIR spectra
of p(acrylic acid) hydrogel, f) SEM images of p(AA) hydrogel
FIGURE 2 TGA thermogrames of a) bare p(AA) hydrogel, b)
p(AA)‐Ag composite

GHORBANLOO ET AL.
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hydrogel, can be attributed to the coordination of silver nano-
particle with functional groups of poly acrylic acid hydrogels,
because the TGA experiments were performed under N2
atmosphere, there is no possibility of presence of O2. So,
the formation of oxide compounds in the TGA experiment
can be negligible.
The amount of metal ion within the hydrogel was deter-
mined, also, by using ICP after dissolution by 5 M 100 ml
HCl treatment and according to the ICP measurement results,
0.1 g p(AA) hydrogel absorbed 194 mg Ag.
To investigate the nanostructure, the sample was exam-
ined with TEM observations. The TEM images of metal
nanoparticles‐containing p(AA) hydrogels are given in
Figure 3. As can be seen, metal nanoparticles with a uniform
spherical shape, about 20–40 nm, are distributed within
p(AA) hydrogel matrices.
To obtain further information on the Ag‐NPs in the
hydrogel matrix and in order to gain further insight into the
ionic state of the nanoparticles, we performed XPS measure-
ments on the sample p(AA)‐Ag. A high‐resolution Ag 3d X‐
ray photoelectron spectrum of the silver nanoparticles is
shown in Figure 4. Ag 3d5/2 and 3d3/2 spin‐orbit coupled core
levels are observed at 369.95 eV and 375.97 eV respectively.
This confirms the presence of Ag in metallic state.[29] In
addition, the energy splitting value of 6.0 eV for the 3d dou-
blet of Ag indicates formation of metallic silver
nanoparticles.[30,31]
3.2 | The catalytic activity of p(AA)‐Ag toward
the oxidation reaction of alcohols
The selective oxidation of alcohols, especially employing
oxygen as the terminal oxidant, has been believed one of
the most important reactions in organic synthesis.[32] With
the p(AA)‐Ag catalyst in hand, we effort to evaluate its cata-
lytic activity for the aerobic oxidation of alcohols, and some
examples are summarized in Table 1.
Initially, the oxidation reaction of the benzyl alcohol as
the model substrate was selected, and was carried out in water
at 80 °C. The blank experiments without Ag loading exhibits
a negligible benzyl alcohol conversion (<7% at 80 °C and
24 h reaction time). However, in the presence of the Ag cat-
alyst, benzyl alcohol conversion increases. Within 24 h and
with 0.018 mmol Ag, 15% isolated yield of the benzoic acid
was obtained. Within 24 h and with 0.036 mmol Ag, 79%
isolated yield of the benzoic acid and 1.96% yield of the
benzaldehyde were obtained (Table 1, entry 1). B. Zahed
et al. has been reported oxidation of benzyl alcohol in the
presence of AgNPs.[33] In comparison with their results,
p(AA)‐Ag could act as better catalytic system with high
selectivity to benzoic acid.
To establish the scope for the activity of p(AA)‐Ag, this
study was further extended to the catalytic oxidation of other
FIGURE 3 TEM images of metal nanoparticles from p(AA)‐Ag
(white spots can be considered as Ag nanoparticles)

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GHORBANLOO ET AL.
The reusability of p(AA)–Ag catalyst system was studied
by using the same catalyst in benzyl alcohol oxidation repeat-
edly up to three times. After every use, the catalyst was fil-
tered and washed with DI, and used again for the same
reaction under the same reaction conditions. The conversion
for benzyl alcohol oxidation by p(AA)–Ag are illustrated in
Table 2.
3.3 | The catalytic activity of p(AA)‐Ag toward
the reduction reaction of nitrophenol
Catalytic activity of the prepared p(AA)–Ag composite was
investigated for reduction of 4‐NP (Figure 5‐a).
Nitro compound reduction reactions were elected because
of their importance in the environments, and in the number of
manufacturing of various pharmacology products, dye and
pigment industries.[39] According to thermodynamic studies,
the reduction of nitro compounds is possible in the presence
of excess amounts of aqueous solution of NaBH4, as reducing
agent, with large kinetic barrier.[40] Fortunately, the presence
of a catalyst helps to overcome this energy barrier and makes
these reactions feasible under mild conditions such as room
temperature.
FIGURE
4
Silver 3d XPS spectrum of the silver–polymer
composite. The peaks at binding energies of 368.17 eV for 3d5/2 and
374.23 eV for 3d3/2 are indicative of metallic silver
primary benzyl alcohols with substituent such as 4‐MeO, 2‐
Br and 3‐NO2 groups (Table 1, entries 2–4). Carboxylic acids
(as major product) and aldehyde (as minor product) were
observed for all substrates investigated. The substitution
effect on aromatic substrates was studied. As shown in
Table 1, the conversion of alcohols, increased in the presence
of electron‐donating substituent, such as –OMe at the para
position of benzyl alcohol, (Table 1, entry 2), but in contrary,
withdrawing group, such as NO2 (Table 1, entry 3) decreased
the conversion.[34] This trend is in agreement with Hammett
plot results[35,36] and suggesting that the formation of the
transition state with a carbocationic character on the benzylic
carbon during the discharge of hydrogen in the rate‐determin-
ing step is involved in the oxidation pathway over the present
catalysts.[37] In addition, the oxidation of 2‐substitute benzyl
alcohol was obtained in a low yield (Table 1, entry 4), which
may be attributed to the steric hindrance by the surface of the
silver sites.[38]
A hypothesis of the possible mechanism of catalytic oxi-
dation of alcohols by p(AA)‐Ag was presented in Scheme 2.
Deprotonation of alcohol was promoted by K2CO3 to from
alkoxide on the Ag surfaces. Ag catalyzes the β‐hydride elim-
ination to produce corresponding aldehyde along with the
formation of O2 and H2O. As shown in Scheme 2, alcohol
converted to aldehyde then H2O and O2 are released. Alde-
hyde is the first product of this process whereas carboxylic
acids can generally be obtained in two steps in an aqueous
medium, via the in situ generation of geminal diol through
hydration of aldehyde followed by oxidation into the corre-
sponding carboxylic acid (Scheme 2).
In all the catalytic reduction of nitro aromatic compounds
0.036 mmol of hydrogel–Ag composites were used as
catalyst.
Reduction of nitro compound was tracked by measuring
the decrease in their absorbance peak in UV–Vis spectra taken
at various intervals of times. Only a small amount of 4‐ NP
was reduced in the absence of catalyst even after 3 hours as
shown in Figure 5‐b. But after the addition of catalyst almost
all of the 4‐NP present in the reaction mixture was reduced
within 40 minutes as demonstrated in Figure 5‐c.
As degradation of NP was carried out in large excess of
NaBH4 so this reaction was supposed as pseudo first order
and values of kapp were calculated by plotting ln(Ct/C0) vs.
time as shown in Figure 6. From the linear relations of
ln(Ct/C0) (Figure 6), we found that therate constant was
0.134( 0.0042) min−1. In order to evaluate the effect of
temperature on the catalytic activity of p(AA)–Ag compos-
ite the reduction of 4‐NP was investigated three different
temperatures; 25 °C, 40 °C, 60 °C keeping the amount of
reactant and catalyst constant. Dependence of reaction rate
on temperature is displayed in Figure 6, (ln(Ct/Co) versus
time at different temperatures). The reduction rates were
increased linearly as temperature rise. Because at higher
temperature, rate of diffusion of reactant molecules into
hydrogel composites and collision frequency of reactant
and catalyst increase due to enhance in the average kinetic
energy of molecules.
Activation parameters can be calculated from the data
obtained by the reduction reactions carried out at different
temperatures catalyzed by p(AA)‐Ag composite. Energy of

GHORBANLOO ET AL.
7 of 11
TABLE 1 Catalytic activity of p(AA)‐Ag on the oxidation of benzyl alcohola
Entry
Substrate
Catalyst amount (mmol Ag)
Conversion b (%) (σ)c
Product
Selectivity (%)
1
0.036
80.96 (1.12)
1.96
79
4
2
0.036
0.036
84.6 (0.92)
0.08
84.52
75.8 (1.36)
4.34
89
3
0.036
36.5 (0.52)
6.24
90
aCatalyst (p(AA)‐Ag), substrate (1.0 mmol), H2O, (5 ml) as solvent, K2CO3 1.2 mmol, temperature 80 °C, Time = 24 h under O2 atmosphere
bConversions are based on the starting substrate;;
cσ = Standard deviation;
activation (Ea) was calculated from Arrhenius equation
which is given below
from the intercept of plot of ln(kapp/T) vs. 1/T and was found
as −196.17 J mol−1 K−1.The negative value of ΔS# shows
that the reduction of 4‐NP is entropically unfavorable which
can be made favorable by adding catalyst.[40] The value of
ΔG# was calculated from the following equation,
ln k ¼ ln A‐ðEa=RTÞ
(1)
According the Arrhenius equation lnkapp was plotted
against 1/T as shown in Figure 7‐a, Ea was calculated from
ΔG # ¼ ΔH #−TΔS #
the slope of this plot and found to be 22.11 kJ mol−1
.
The activation enthalpy (ΔH#) and entropy (ΔS#) were
The values of ΔG# at 298 K, 313 K, 333 K were calcu-
calculated by using Eyring equations (Equation (2)).
lated as 77.92 kJ mol−1, 80.87 kJ mol−1, 84.79 kJ mol−1
,
respectively. The positive values of ΔG# shows that reduction
of 4‐NP is a non‐spontaneous process[7] and it needs an input
energy to proceed which is provided by the addition of the
catalyst.
ln k=T ¼ lnðkB=hÞ þ ΔS #=R−ΔH #=Rðl=TÞ
(2)
According to Eyring equation lnkapp/T was plotted
against 1/T as shown in Figure 7‐b. The value of ΔH# was cal-
culated from the slope of this plot and was found to be
19.47 kJ mol−1. The positive value of ΔH# shows that the for-
mation of an activated complex in the reduction of 4‐NP is an
endothermic process.[41] The value of ΔS# was calculated
The effect of catalyst amount on the reduction rate of 4‐
NP was also investigated. The reaction was studied at three
different amounts of catalyst viz. 0.018, 0.036, 0.072 mmol.
As shown in Figure 8‐a, with an increase in catalyst amount,
the increase in rate of reaction was observed, as expected. As

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GHORBANLOO ET AL.
SCHEME 2 Plausible pathways for the oxidation of alcohols in the presence of p(AA)‐Ag nano composite
with the increase in amount of catalysts, the available
catalytic sites are also increased. The presence of larger
catalytic sites in the reaction mixture increases the effective
collision frequency, and hence the rate of reaction is also
increased.
In order to investigate the effect of amount of reducing
agent on the reduction rate, different amounts of NaBH4 were
used by assuming all other parameters constants. To assess
the effect of NaBH4 on the reduction rate, different amounts
of NaBH4 (0.1–0.5 M) were used in reduction of 4‐NP. As
can be seen from Figure 8‐b, there is a linear increase in
the reduction rate of 4‐NP with the increase in the amount
of NaBH4 at 25 °C. The same trend has been reported in
several literatures.[42]
The reusability of p(AA)–Ag catalyst system was studied
by using the same catalyst in 4‐NP reduction repeatedly up to
four times. After every use, the catalysts were filtered and
washed with DI, and used again for the same reaction under
the same reaction conditions. The percent activity and
conversion for 4‐NP reduction by p(AA)–Ag is illustrated in
Figure 9. Activity was calculated by taking ratio of reduction
rate of every successive reaction to the initial reduction rate,
and as illustrated from Figure 9, 100% conversion of 4‐NP
was observed after 4th use of Ag nano catalyst system.
However, 3.5% loss in the activity of p(AA)–Ag catalyst
was observed at the end of four consecutive use.
TABLE 2 Effect of catalyst recycling on the oxidation of benzyl
a
alcohol
Conversion (%) (σ)c
Benzoic acid
selectivity (%)
Number of cycle
Fresh
80.96 (0.92)
81.5 (1.20)
80 (1.15)
79
78
80
79
1
2
3
FIGURE 5 (a) representation of the reduction of 4‐nitrophenol
(4‐NP) to 4‐aminophenol (4‐AP) by the p(AA)–Ag nanoparticle,
(b) UV–visible spectra of 4‐NP in the absence of catalyst and
presence of NaBH4, (c) UV–visible spectra for the reduction of
4‐NP catalyzed by p(AA)‐Ag composite and NaBH4. Reaction
conditions; 0.01 M 4‐NP 50 mL, NaBH4 = 0.35 M,
catalyst = 0.036 mmol, 25 °C
81 (0.95)
aCatalyst (p(AA)‐Ag), Benzyl alcohol (1.0 mmol), H2O, (5 ml) as solvent, K2CO3
1.2 mmol, temperature 80 °C, Time = 24 h under O2 atmosphere;
bConversions are based on the starting substrate;
cσ = Standard deviation;

GHORBANLOO ET AL.
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FIGURE 6 The determination of apparent rate constants (kapp) for
the reduction 4‐NP catalyzed by p(AA)–Ag composites with
temperature
FIGURE 8 a) the change in reduction rate constant of 4‐NP with the
amount of Ag catalyst inside p(AA)–Ag composites b) change in values
of kapp with the change in of concentration of NaBH4 during the
reduction of NP
FIGURE 9 The change in conversion and activity of p(AA)–Ag
catalyst systems with repetitive usage in 4‐NP reduction. [25 °C,
0.01 M 4‐NP = 50 mL, [NaBH4] = 0.35 M, catalyst = 0.036 [mmol]
FIGURE 7 The determination of apparent rate constants (kapp) for
the reduction 4‐NP catalyzed by p(AA)–Ag with temperature, a) plot
of ln(kapp) vs. 1/T b) plot of ln(kapp /T) vs. 1/T for the reduction 4‐NP
catalyzed by p(AA)–Ag with temperature
4, it is seen that p(AA)‐Ag showed higher catalytic activity
over reported catalysts. Although CPL‐Ag NCs and
AgNPs@MWCNTs‐polymer showed high reaction rate in
NP reduction and Ag@IPN, PdNPs@P2VP‐Fe3O4 showed
high catalytic activity in oxidation of benzyl alcohol. A com-
parison between the system represented here, and the previ-
ously reported systems indicates higher catalytic activities
for our system. One reason for the high catalytic activities
in our system could be the softness, flexibility, and hydro-
philic nature of the hydrogels, carriers, and stabilizers, which
were used for the preparation of the metal nanoparticles. The
other reason could be the hydrophilicity and types of
3.4 | Comparison with other reported
catalytic system
Table 3 provides a comparison of the NP reduction results
obtained (kapp and Ea) for our present catalytic system with
those reported in the literatures.[43–51] Also, oxidation of ben-
zyl alcohol under heterogeneous conditions over a variety of
catalysts has been studied[52–57] (Table 4). From Table 3 and

10 of 11
GHORBANLOO ET AL.
TABLE 3 Comparison of the rate constant and activation energy calculated in the present work with reported literature
Catalyst
Kapp (min−1
0.00216
0.1032
0.03072
0.0923
0.0563
0.12
)
Eact
Not reported
28.2
Reference
[43]
PS‐NIPA‐Ag composite
p(AMPS)‐cu hydrogel composite
PS‐PVImAgNPs
Poly‐(acrylamide glycolic acid)/Ag composites
P(AMPS)‐Ni composite
p(AMPS)‐co
PVA/PS‐PEGMA/Ag
CPL‐Ag NCs
AgNPs@MWCNTs‐polymer
P(AA)‐Ag
[44]
[45]
[46]
[47]
[48]
[49]
[50]
[51]
Not reported
33.8
25.7
27.8
0.00468
0.9
Not reported
Not reported
Not reported
22.11
0.472
0.134
This work
TABLE 4 Comparison of the benzyl alcohol oxidation in this work with reported literature
Entry
Catalyst
AgNPs
Conversion (%)
Reaction conditions
CH3CN, NHPI, 24 h, 80 °C
H2O, K2CO3, 24 h, 90 °C
H2O, K2CO3, 25 h, 80 °C,
p‐xylene, 130 °C, 12 h
H2O, K2CO3, 4 h, 80 °C, O2
H2O, K2CO3, 4 h, 90 °C, O2
H2O, K2CO3, 24 h, 80 °C, O2
Reference
[52]
1
2
3
4
5
6
7
33
[53]
[54]
[55]
[56]
[57]
Pd@U‐B20
Pd/Fe@C
43
63
Ag@IPN
98
PdNPs@P2VP‐Fe3O4
Au NPs@CLIL
P(AA)‐Ag
96
90
80.96
This work
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5235. b) L. Daniel, G. Gaël, M. Céline, C. Caroline, B. Daniel,
S. Jean‐Pierre, Nanotechnology 2013, 24, 452001.
functional groups in the polymeric network, which promotes
the overall catalytic process in the system.
[3] K. Naseem, S. Ashraf, A. Sharif, R. Rehan, Appl. Organomet.
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4 | CONCLUSION
[4] Y. C. Lai, W. X. Pan, D. J. Zhang, J. H. Zhan, Nanoscale 2011, 3,
It was demonstrated here that a facile preparation of
p(AA) hydrogels via radical polymerization was accom-
plished. The p(AA) hydrogels were then used as
templates for in situ synthesis of Ag nanoparticles to
prepare p(AA)‐Ag composites. The prepared composites
were found to be very effective catalysts toward oxidation
reaction of benzyl alcohols and reduction of nitrophenol
under green conditions such as easy separation of the
catalyst, using water as solvent and relatively low temper-
ature. This organic inorganic hybrid hydrogel was found
to be efficient and reusable catalysts in oxidation
reactions of primary alcohols in short reaction time and
good to excellent yields.
2134.
[5] V. Tharmaraj, K. Pitchumani, Nanoscale 2011, 3, 1166.
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ACKNOWLEDGMENTS
Authors are thankful to University of Zanjan for financial
support of this study.
[12] N. Iranpoor, H. Firouzabadi, A. Safavi, S. Motevalli, M. M.
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How to cite this article: Ghorbanloo M, Heydari A,
Yahiro H. Ag‐nanoparticle embedded p(AA) hydrogel
as an efficient green heterogeneous Nano‐catalyst for
oxidation and reduction of organic compounds. Appl
Organometal Chem. 2017;e3917. https://doi.org/
10.1002/aoc.3917
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