Oxidant free dehydrogenation of alcohols using chitosan/polyacrylamide entrapped Ag nanoparticles

Article abstract of DOI:10.1039/c5ra05046d

Silver nanoparticles encaged in a nanoporous chitosan/polyacrylamide interpenetrating polymer network (Ag@IPN) were synthesized and characterized by various techniques such as Fourier Transform Infra Red (FTIR), X-ray Diffraction (XRD), Scanning Electron Microscopy-Energy Dispersive X-ray Spectroscopy (SEM-EDX), Transmission Electron Microscopy (TEM), Thermogravimetric Analysis (TGA) and ICP-AES. This material exhibits good catalytic activity for the oxidant free dehydrogenation of alcohols to the corresponding carbonyl compounds. The reusability of Ag@IPN for this reaction was compared with a chitosan-silver complex under the same reaction conditions. It was found that encaging silver nanoparticles in interpenetrating polymer networks (IPN) has improved their stability by avoiding leaching and aggregation.

Full text of DOI:10.1039/c5ra05046d

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Oxidant free dehydrogenation of alcohols using
chitosan/polyacrylamide entrapped Ag
nanoparticles†
Cite this: RSC Adv., 2015, 5, 46443
Received 22nd March 2015
Accepted 6th May 2015
*
Deepak K. Kurhe, Thomson A. Fernandes, Tushar S. Deore and Radha V. Jayaram
DOI: 10.1039/c5ra05046d
www.rsc.org/advances
Silver nanoparticles encaged in
a
nanoporous chitosan/ hydrotalcite and carbon have catalyzed the oxidant free dehy-
polyacrylamide interpenetrating polymer network (Ag@IPN) were drogenation of primary and secondary alcohols into the corre-
synthesized and characterized by various techniques such as Fourier sponding aldehydes and ketones.
Transform Infra Red (FTIR), X-ray Diffraction (XRD), Scanning Electron
Recent drive towards sustainable and green chemical
Microscopy-Energy Dispersive X-ray Spectroscopy (SEM-EDX), processes has lead to the use of biopolymers as catalyst
Transmission Electron Microscopy (TEM), Thermogravimetric Anal- supports.15 Chitosan is an abundant natural hydrophilic
ysis (TGA) and ICP-AES. This material exhibits good catalytic activity for biopolymer, which is widely used in pharmaceutical, food,
the oxidant free dehydrogenation of alcohols to the corresponding biomedical16 industries and catalysis.17 As chitosan is a
carbonyl compounds. The reusability of Ag@IPN for this reaction was nontoxic, biodegradable polymer18 which has good process-
compared with a chitosan–silver complex under the same reaction ability,19 motivated its use as a support in an environment
conditions. It was found that encaging silver nanoparticles in inter- friendly heterogeneous catalysis. Also, the abundant amino and
penetrating polymer networks (IPN) has improved their stability by hydroxyl groups of the chitosan present unique affinity towards
avoiding leaching and aggregation.
most transition metal species, making it an ideal solid support
for applications in aqueous, organic and ionic liquid media.20
Kiyotomi Kaneda et al. have successfully carried out the
oxidant free dehydrogenation of alcohols under heterogeneous
condition using hydrotalcite supported silver nanoparticles.
However, considering the growing consciousness of environ-
mental issues chitosan supported silver nanoparticles catalyzed
oxidant free dehydrogenation of alcohols is of considerable
interest.
An interpenetrating polymer network (IPN) is a polymer
comprising two or more networks which are at least partially
interlocked on a polymer scale.21 Materials formed from IPN's
share characteristic properties of each network and have higher
mechanical strength and stability than individual homopol-
ymer network.22 Also, IPN suppresses the reunion of nano-
particles, improves their distribution stability and uniformity in
their networks.23
1 Introduction
Oxidation of alcohols to carbonyl compounds is a fundamental
transformation in synthetic organic chemistry.1 Recently, tran-
sition metal catalyzed oxidation of alcohols using environ-
mentally friendly oxidants such as oxygen,2 hydrogen peroxide3
has been reported. Transition metal catalyzed oxidant free
dehydrogenation of alcohols is an atom economical method to
convert alcohols into carbonyl compounds.4 Though several
homogeneous catalytic systems have been reported for this
process, they oen suffer from drawbacks such as air sensitivity,
difficult catalyst synthesis and the requirement of additives.5–7
Supported transition metal catalyzed oxidant free dehydro-
genation of alcohols is an environmental benign process as it
avoids formation of H2O,8 produces H2, which is a valuable
energy source9 and also suppresses over oxidation of reactants
to carboxylic acids.10 Transition metals such as, Ni,11 Au,12 Cu13
and Rh14 supported over various supports such as alumina,
Thus it was intended to synthesize chitosan supported silver
nanoparticles and improve the stability of silver nanoparticles
by encaging them in IPN and use it as a catalyst for the oxidant
free dehydrogenation of alcohols under oxidant free reaction
conditions.
In this study, we synthesized chitosan supported silver
nanoparticles by the reported procedure and formed inter-
penetrating polymer network with crosslinking polyacrylamide
and chitosan. This silver-IPN catalyst (referred to as Ag@IPN) is
Department of Chemistry, Institute of Chemical Technology, Matunga, Mumbai, 400
019, India. E-mail: rv.jayaram@ictmumbai.edu.in; Tel: +91 22 3361 2607
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra05046d
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characterized by various spectroscopic and thermogravimetric attributed to the –CONH2 group of chitosan. In the FTIR spec-
techniques and used for the oxidant free dehydrogenation of trum of silver nanoparticle stabilized in chitosan (b), the
alcohols under nitrogen environment. This catalyst showed the absorption band 1654.15 cmꢁ1 representing chitosan –CONH2,
improved stability of silver nanoparticles in IPN than silver disappeared and a new band appeared at 1629.78 cmꢁ1, which
chitosan complex under the same reaction conditions.
indicated the attachment of silver to nitrogen atom. The
absorption band at 1655.24 cmꢁ1 in Ag@IPN represents the
amide stretching vibration from the crosslinked polyacrylamide.
Thermal studies of Ag/chitosan and Ag@IPN have been
carried out (Fig. S2†). In case of Ag/chitosan (a) one stage
decomposition with 36.45% weight loss was observed in the
temperature range 200–400 ꢀC, while in case of Ag@IPN (b)
rst stage decomposition was observed with 15.10% weight
loss in the temperature range 200–300 ꢀC. A second stage
decomposition was observed with 30.04% weight loss in the
2 Experimental
2.1 Materials and instrumentation
Low molecular weight chitosan (deacetylation 75–85%) was
obtained from Sigma Aldrich (India) and used as received. All
chemicals used in this study were of analytical grade and used
as received without further purication. UV spectra were
obtained using Shimadzu 1650PC (E) UV-Vis. Spectrophotom-
eter. FTIR spectra were recorded using a FTIR spectrophotom-
eter (Perkin Elmer Spectrum 100) as KBR pellets.
Thermogravimetric (TGA) analysisꢁo1f the samples was carried
out with a heating rate of 10 ꢀC min and up to 700 ꢀC under N2
atmosphere with a Perkin Elmer instrument (STA 6000). Silver
content in the materials was determined by ICP-AES analysis
using a Varian AA240 analyzer. Scanning Electron Micrographs
(SEM) and Energy Dispersive X-ray Analysis (EDX) were carried
out using a FEI Quanta 200 ESEM FEG instrument. TEM
imaging was done on 200 kV instrument (JEOL JEM 2100). XRD
proles were recorded on a Bruker AXS diffractometerꢀwith Cu-
ꢀ
temperature range 320–450 C. This second stage decomposi-
tion in case of Ag@IPN can be attributed to the decomposition
of amide groups and the degradation of the polymer main
chains.25
The XRD pattern of Ag/chitosan (a) and Ag@IPN (b) is shown
in Fig. 1. The XRD peaks at 2q values around 38ꢀ, 44ꢀ, 64ꢀ and
77ꢀ could be assigned to the 111, 200, 220 and 311 crystallo-
graphic planes of the face centered cubic Ag crystals.26
The presence of well dispersed silver nanoparticles
(60–75 nm) in the Ag@IPN was conrmed by SEM analysis
(Fig. 2a). The silver content was found 4.49 wt% by EDX analysis
(Fig. 2b).
Fig. S3(a–d)† shows the transmission electron micrographs
of silver in chitosan (a&b) and chitosan-polyacrylamide IPN
networks (c&d). These TEM images show that the silver nano-
particles are well dispersed without aggregation in the chitosan
and chitosan-polyacrylamide IPN networks.
˚
Ka radiation (l ¼ 1.540562 A) over a 2q range of 0–80 .
2.2 Catalyst preparation
Chitosan supported silver nanoparticles (Ag/chitosan) were
synthesized by the reported procedure.24 Chitosan (2.0 g) was
dissolved in 50 mL acetic acid solution (1%). To this 5 mL
aqueous solution of AgNO3 (0.1 mol Lꢁ1) was added and the
solution was stirred at 30 ꢀC overnight. At this stage, freshly
prepared aqueous solution of NaBH4 (5 times that of metal salt
on mole basis) was added and stirred for 2 h. This solution was
mixed with 0.2 mL 10% H2SO4, 0.4 mL of 25% glutaraldehyde
solution and 0.4 mL 10% acetic acid solution to form solution I.
Stirring with acetone for 30 minutes resulted in the precipita-
tion of the Ag/cross linked polymer. The polymer was washed
with distilled water repeatedly and dried under vacuum. Acryl-
amide (3.91 g), N,N0-methylenebisacrylamide (0.465 g) and
potassium persulfate (0.08 g) were mixed in 10 mL distilled
water to form solution II. Solution I was prepared again and
solution I&II were mixed (chitosan : acrylamide ¼ 0.8 : 1) and
heated to 70 ꢀC for 3 h to form interpenetrating polymer
network gels which were washed with distilled water repetitively
and dried under vacuum to get Ag@IPN.
The silver content in Ag/chitosan and Ag@IPN as deter-
mined by ICP analysis was found to be 0.052 mmol gꢁ1 and
0.024 mmol gꢁ1 respectively.
3 Results and discussion
3.1 Catalyst characterization
The FTIR spectra of chitosan (a), Ag/chitosan (b) and Ag@IPN (c)
are shown in Fig. S1.† The IR spectrum of chitosan absorption
bands at 3444.49 cmꢁ1 represents the –NH2 and –OH groups.
Absorption bands at 2993.99 cmꢁ1 and 2856.11 cmꢁ1 represent
aliphatic groups. The absorption band at 1654.15 cmꢁ1 is Fig. 1 XRD patterns of Ag/chitosan (a) and Ag@IPN (b).
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The mesh size (x), or the correlation distance between two
adjacent crosslinks, provides a measure of the space available
between the macromolecular chains available for diffusion and
movement of particles. This can be determined as follows.27
ꢀ
ꢃ
1=2
ꢁ
ꢂ
2Mc
ꢁ1=3 Cn
l
Mr
x ¼ V2;s
(3)
where, Cn is the Flory characteristic ratio (Cn is 8.5 for poly-
acrylamide in water) and l is the carbon–carbon bond length
(0.154 nm).
The polymer equilibrium swelling ratio (Q) and mesh size for
polyacrylamide networks in Ag@IPN was determined by equi-
librium swelling study and found to be 0.90 and 3.13 nm
respectively. As mesh size for polyacrylamide networks in
Ag@IPN is less than the size of silver nanoparticles, poly-
acrylamide networks will help in preventing the leaching of
silver nanoparticles.
3.2 Catalytic activity
Catalytic activity of Ag@IPN was tested for the conversion
alcohols to corresponding carbonyl compounds under oxidant
free environment.
The conversion of 1-phenylethanol to acetophenone was
selected as a model reaction. This conversion was carried out
with different temperatures and quantities of the catalyst to
obtain the optimized reaction conditions (Table 1). There was
no difference in the conversion when catalyst loading was
reduced from 0.1 mol% of Ag to 0.05 mol% Ag, but with
0.025 mol% Ag a low yield was observed. Hence 0.05 mol% of Ag
was used for further studies (Scheme 1).
Aer optimizing the reaction conditions for the conversion
of 1-ethylbenzene to acetophenone, the scope of the catalyst was
tested for the reaction of various alcohols (Table 2). The catalytic
system was found to be applicable to various aliphatic, aromatic
and heteroaromatic alcohols.
Fig. 2 (a) SEM image of Ag@IPN. (b) EDX image of Ag@IPN.
The important parameters used to characterize nano-
structure of crosslinked hydrogel networks are the polymer
volume fraction in the swollen state (V2,s), molecular weight of
ꢀ
the polymer chains between two neighboring crosslinks (Mc)
and the corresponding mesh size (x).
Table 1 Optimization of reaction conditions
1
Q ¼
(1)
V2;s
Catalyst
(mol% of Ag)
Entry
Solvent
T (ꢀC)
Time (h)
Yielda,b (%)
where V2,s is the volume fraction of the polymer in the swollen
state.
1
2
3
4
5
6
0.1
0.1
0.1
0.1
0.05
0.025
Toluene
Toluene
Toluene
p-Xylene
p-Xylene
p-Xylene
90
110
110
130
130
130
12
12
24
12
12
12
72
86
93
97
97
92
Number average molecular weight between two adjacent
ꢀ
crosslinks (Mc) can be determined from the nominal cross-
linking ratio X and the molecular weight of the repeating unit
ꢀ
ꢀ
Mr of polyacrylamide (Mr ¼ 71) as
Mr
a
Reaction conditions: 1-phenylethanol (1 mmol) and p-xylene (5 mL),
catalyst (Ag@IPN). b Determined by GC.
Mc ¼
(2)
2X
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3.3 Catalyst reusability
In order to check the reusability of both Ag/chitosan and
Ag@IPN, the conversion of 1-phenylethanol to acetophenone
was repeated seven times with same amount of silver under
optimized reaction conditions (Fig. 3). The catalyst was ltered
off aer each run and washed thoroughly with organic solvents.
It was dried and fresh reaction was performed with new
Scheme
acetophenone.
1 Ag@IPN catalyzed conversion of 1-phenylethanol to
Table 2 Synthesis of carbonyl compounds from alcohols catalyzed by Ag@IPN
Entry
Reactant
Product
Time (h)
12
Yielda,b (%)
1
97
2
3
12
12
96
93
4
5
12
12
98
90
6
7
12
12
84
86
8
9
20
22
88
86
10
11
12
12
83
92
12
12
93
a
Reaction conditions: alcohol (1.0 mmol), catalyst (0.05 mol% Ag), p-xylene (5 mL), temperature (130 ꢀC). b Determined by GC.
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In summary, silver nanoparticles encaged in chitosan/
polyacrylamide interpenetrating networks. It was used as a
catalyst for the oxidant free dehydrogenation of alcohols. Its
reusability was compared with Ag/chitosan catalyst for the
oxidation of 1-phenylethanol to acetophenone under same
reaction conditions. It shows that encaging of silver nano-
particles in chitosan/polyacrylamide avoids leaching of silver
nanoparticles. Thus, Ag@IPN catalytic system was expected to
contribute to the development of environmentally benign
oxidation process.
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Author (DKK) is thankful to UGC for providing Research
Fellowship (SAP) and BSR One time grant.
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