Surface grafted hyper-branched polyglycerol stabilized Ag and AuNPs heterogeneous catalysts for efficient reduction of Congo Red

Article abstract of DOI:10.1166/jnn.2016.10655

Six types of insoluble polymer-supported beads immobilized with Ag and AuNPs nanoparticle catalysts were synthesized using newly prepared three different types of polymer-supported poly(styrene)-co-poly(vinyl benzene chloride) matrix (PS-PVBC), surface grafted with (i) triethanolamine (TEA), (ii) glycidyl trimethyl ammonium chloride (GTMAC) and (iii) hyper-branched polyglycerol (HPG) and Ag and AuNPs as a catalytic moiety and thus yield polymer-supported nanoparticle catalysts viz., PS-PVBC-TEA-AgNPs and AuNPs, PS-PVBC-g-GTMAC-AgNPs and AuNPs and PS-PVBC-g-GTMAC-AgNPs and AuNPs catalyst respectively. These bead-shaped heterogonous nanoparticle catalysts were characterized by UV-Vis, FTIR, FESEM, HRTEM and TGA techniques. The efficiency for stabilization/loading of metal nanoparticles with respect to varied intensities of hyper-branched chain grafted onto their matrix was screened by determining their comparative catalytic activity. The catalytic potential of thse catalysts was inspected through reduction of Congo Red (CR) keeping pseudo first order identical reaction condition. The observed k_obs values reveal that irrespective of metal the catalyst derived from hyper-branched polyglycerol as stabilizing agent viz., PS-PVBC-g-HPG-AgNPs and PS-PVBC-g-HPG-AuNPs shows (k_obs = 3.98× 10^-2 min^-1 and k_obs = 4.54 × 10^-2 min^-1) four and two times greater activity than the catalyst derived from TEA and GTMAC hyper-branched chain. Further, for the same reaction PS-PVBC-g-HPG-AuNPs showed more efficiency than the PS-PVBC-g-HPG AgNPs catalyst. The stability and reusability of the superior catalyst viz., PS-PVBC-g-HPG-AuNPs catalyst was observed to be good even at the sixth cycle. This catalyst can be continuously used to conduct the reduction of various dyes in continuous mode operation in industrial scale.

Full text of DOI:10.1166/jnn.2016.10655

Article
Journal of
Nanoscience and Nanotechnology
Vol. 16, 426–438, 2016
Copyright © 2016 American Scientific Publishers
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Surface Grafted Hyper-Branched Polyglycerol Stabilized
Ag and AuNPs Heterogeneous Catalysts for
Efficient Reduction of Congo Red
Eagambaram Murugan∗ and Paramasivam Shanmugam
Department of Physical Chemistry, School of Chemical Sciences, University of Madras, Maraimalai Campus,
Chennai 25, Tamil Nadu, India
Six types of insoluble polymer-supported beads immobilized with Ag and AuNPs nanoparti-
cle catalysts were synthesized using newly prepared three different types of polymer-supported
poly(styrene)-co-poly(vinyl benzene chloride) matrix (PS-PVBC), surface grafted with (i) tri-
ethanolamine (TEA), (ii) glycidyl trimethyl ammonium chloride (GTMAC) and (iii) hyper-branched
polyglycerol (HPG) and Ag and AuNPs as a catalytic moiety and thus yield polymer-supported
nanoparticle catalysts viz., PS-PVBC-TEA-AgNPs and AuNPs, PS-PVBC-g-GTMAC-AgNPs and
AuNPs and PS-PVBC-g-GTMAC-AgNPs and AuNPs catalyst respectively. These bead-shaped het-
erogonous nanoparticle catalysts were characterized by UV-Vis, FTIR, FESEM, HRTEM and TGA
techniques. The efficiency for stabilization/loading of metal nanoparticles with respect to varied
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intensities of hyper-branched chain grafted onto their matrix was screened by determining their com-
IP: 187.60.137.36 On: Thu, 11 Feb 2016 06:19:38
parative catalytic activity. The catalytic potential of these catalysts was inspected through reduction
Copyright: American Scientific Publishers
of Congo Red (CR) keeping pseudo first order identical reaction condition. The observed kobs values
reveal that irrespective of metal the catalyst derived from hyper-branched polyglycerol as stabilizing
agent viz., PS-PVBC-g-HPG-AgNPs and PS-PVBC-g-HPG-AuNPs shows (kobs = 3ꢀ98×10−2 min−1
and kobs = 4ꢀ54 × 10−2 min−1) four and two times greater activity than the catalyst derived from
TEA and GTMAC hyper-branched chain. Further, for the same reaction PS-PVBC-g-HPG-AuNPs
showed more efficiency than the PS-PVBC-g-HPG AgNPs catalyst. The stability and reusability
of the superior catalyst viz., PS-PVBC-g-HPG-AuNPs catalyst was observed to be good even at
the sixth cycle. This catalyst can be continuously used to conduct the reduction of various dyes in
continuous mode operation in industrial scale.
Keywords: Heterogeneous Nanocatalyst, Polymer Beads, Hyperbranched Polyglycerol,
Congo Red.
1. INTRODUCTION
catalysts, apart from catalytic moiety/nanoparticles, the
selection of support/matrices has also been given greater
attention in order to improve the performance of the cat-
alyst. Especially, the polymer support can have an effect
on the catalytic activity of the supported metal either by
creating reactive sites at the metal surface or by enhanced
release of the metal into the solution by leaching.7
In contrast, polymer supports have always gained sig-
nificant attention as they are inert, non-toxic, non-volatile,
stable, flexible and reusable. But these polymer-supported
catalysts have some drawbacks too. For instance, metal
leaching/aggregation may occur from the catalyst centers,
which will contaminate the valuable products. In addition,
Metallic nanoparticles are of wide interest since they have
many interesting chemical, physical and optical properties
as well as catalytic properties for potential applications
in the field of physics, chemistry, biology, material sci-
ence and other interdisciplinary fields.1 In recent years,
the design and synthesis of polymer supported nanocat-
alyst have attracted much attention in academia and
industries.2–6 For this reason, heterogenization of the cat-
alysts to solid supports is one of the interesting choices.
Generally, in the synthesis of heterogeneous nanoparticle
∗Author to whom correspondence should be addressed.
426
J. Nanosci. Nanotechnol. 2016, Vol. 16, No. 1
1533-4880/2016/16/426/013
doi:10.1166/jnn.2016.10655

Murugan and Shanmugam
Surface Grafted HPG Stabilized Ag and AuNPs Heterogeneous Catalysts
there may be reduction or complete loss of selectivity of
the catalysts when they get attached to an insoluble sup-
port. Apparently, several groups have reported that poly-
mers are the best stabilizers to protect the metal NPs.
Widdecke et al. has reported that polymer sulfonic acid
resins are important industrial catalysts and polymer sup-
ported metal complex catalyst are poised to make an
important contribution in the synthesis of both the com-
modity and specialty chemicals.8 In recent years, studies
are found in literature for the stabilization/immobilization
of metal nanoparticles using grafting of polymers onto
the insoluble polymer matrices.9ꢀ10 For example, Yang and
coworkers used the carboxylic acid functionalized polymer
microsphere for the stabilization of gold NPs (AuNPs).11
Ger and coworkers published their work of immobiliza-
tion of NPs on sulfate ion functionalized polystyrene
matrix.12
However, in the synthesis of nanoparticle catalyst the
selection of appropriate stabilizing agent plays a pivotal
role because agglomeration of metal nanoparticles can be
excluded by employing suitable agents. Further, it is also
necessary to search the methods allowing the acquisition of
efficient nano system with high storage of stability/activity.
To address this issue, polymer scientists have intro-
duced symmetrical macro molecule viz., dendrimer (hyper
branched polymer) as a “stabilizing agent” through which
activity of silver nanoparticles increases with decreasing
particle size. The maximum catalytic action was observed
for NPs with sizes as low as 5 nm in the case of cat-
alytic hydrogenation of anthracene28 or even below 1 nm
in the case of the electro-catalytical reduction of oxygen.29
Similar dependence of catalytic activity on particle size
was observed also for AgNPs anchored to inert carrier and
it was used for partial oxidation of propylene to propy-
lene oxide and acrolein.30 Interestingly, in this case, it was
shown that the size of the AgNPs influences not only the
magnitude of the catalytic action, but also the selectiv-
ity toward the production of propylene oxide, which on
the other hand rises with increasing particle size. Simi-
larly, according to the chemical stability and high resis-
tance to surface oxidation, beside the enhanced chemical
reactivity, gold nanoparticle is an important nanomaterial
for use as a reactive center or as a nanocatalyst in many
chemical reactions.31 Haruta and co-workers discovered
that small gold nanoparticles (Au NPs) supported on some
reducible oxides can be used as highly active catalyst for
CO oxidation.32 In recent years, heterogeneous catalysis
by gold has attracted much attention.13ꢀ33ꢀ34 Gold catalysts
have found many applications in inorganic reactions and
organic reactions. In particular, the application of gold cat-
alysts in the synthesis of organic chemicals has been the
subject of active research recently.35ꢀ36
metal nanoparticles are stabilized/encapsulated efficiently
and lead to active/stable nanoparticle catalyst. Den-
Congo Red is an azo dye with
a
structure
3,3 -((biphenyl)-4,4 -diylbis(azo))-bis(4-amino-1-naphthalene
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ꢀ
-
dritic hyper-branched polyglycerols are important because
of their well-defined structure and chemical versatility.
Research on dendrimer stabilized nanoparticle catalysis
has become a fast growing field due to a variety of appli-
cations. They combine the desirable physical and chemical
properties of the encapsulated Nanoparticles (NPs) with
the tunable solubility and surface reactivity of the den-
drimer templates. The first report of dendrimer encapsu-
lated metal NPs appeared in 199817 and was indicated as
an effective homogeneous catalyst. Weijian et al. devel-
oped multifunctional aqueous nano-crystals stabilized by
hyper branched polyglycerol.18 Though homogeneous cat-
alyst produces high yield and selectivity, recoveries of
such homogeneous catalyst from the reaction mixture is
tedious and possess certain limitations. To address these
drawbacks, researchers attempt to develop heterogeneous
nanoparticle catalyst using dendrimer as stabilizing agent.
Recently Murugan et al. have also reported similar type
of dendrimer stabilized gold nanoparticle catalysts for the
reduction of p-nitrophenol.13
sulphonicacid) disodium salt. It is intended primarily for
the coloration of paper products, used in medicine (as a
biological stain), textile industries and also used as an
indicator since it turns from red–brown in basic medium
to blue in acidic, it is also used to colour textile and wood
pulp.37 It is recalcitrant and act as potent carcinogen and
mutagenic because of the presence of aromatic amine
group. Minute amounts of reactive group of azo dyes
show their visual effect with adverse impact in terms of
total organic carbon (TOC) and chemical oxygen demand
(COD) thus causing severe environmental problems world-
wide. Azo dyes have long been recognized as a human
urinary bladder carcinogen and Tumorigenic in animals38
cyanogenic in fishes, reduction in seed germination and
induce dwarfism in plants. The release of those coloured
waste waters into ecosystems is a dramatic source of
esthetic pollution, eutrophication, and perturbations in
aquatic life. As a consequence, different methods such
as oxidation,39 adsorption,40ꢀ41 biological-treatment,42
flocculation-precipitation,43 membrane separation,44 photo-
catalyst45–47 and ozonation48 are frequently used to remove
these contaminants. Although different homogeneous and
heterogeneous nanoparticle catalysts are already exploited
extensively for the reduction/degradation of various dyes,
but still, search and efforts are on for the synthesis of con-
venient, stable, efficient bead-shaped insoluble polymer-
supported nanoparticle catalyst for the degradation of azo
The silver nanoparticles (AgNPs) have been gaining
significant research interest due to their unique shape
and size dependent optical19 antimicrobial20 and cat-
alytic properties.21 Nanostructures of silver such as mono-
dispersed nanoparticles,22 nanoprisms,23 nanocubes,24
nanowires25 and nano disks26 have potential applications
in optics, catalysis, and SERS detection.27 The catalytic
J. Nanosci. Nanotechnol. 16, 426–438, 2016
427

Surface Grafted HPG Stabilized Ag and AuNPs Heterogeneous Catalysts
Murugan and Shanmugam
(–N N–) dyes to be performed under continuous mode
of operation. In general, as described in the previous
paragraph, the synthesis of efficient, stable, convenient
heterogeneous nano particle catalyst mainly depends on
the nature of stabilizing agent which includes chain length
and intensities of functional group and size, shape and
chemical behavior of poly(styrene) matrix. It is in this
background, in this study we attempted to screen the three
different types of branching and functional group grafted
onto the bead-shaped insoluble poly(styrene) matrix for
better stabilization of metal nanoparticles and thus to
report the efficient/convenient heterogeneous nanoparti-
cle catalyst for quick reduction/continuous reduction of
congo red.
HITACHI S-3000H scanning electron microscope (SEM)
at the acceleration voltage of 20 keV. A strip of double
sided conducting carbon tape was applied to the metal stub
and the samples were deposited on the carbon tape after
that sample was allowed for scanning. HRTEM images
were recorded on JEOL-3010 high-resolution transmission
electron microscope (HRTEM) operated at 300 keV. Sam-
ples were prepared by placing a micro drop of the nanopar-
ticle solution on a carbon-coated Cu TEM grid, which had
been pre-treated by glow discharge to render the carbon
surface slightly hydrophilic, and then allowing the solvent
to evaporate in air and recorded with various accelera-
tion voltages from 200 keV to 300 keV. Thermo gravimet-
ric analyses (TGA) were carried out under nitrogen (N2)
atmosphere at a flow rate of 20 ml/min and at a heating
ꢁ
rate of 20 C/min in a SDT Q600 V8.0 Build 95 thermal
analyzer.
2. EXPERIMENTAL DETAILS
2.1. Chemicals
Gelatin (Lancaster), boric acid (SRL), polyvinyl alco-
hol (Lancaster), styrene (Lancaster), divinylbenzene
(Lancaster), vinylbenzylchloride (Sigma Aldrich), azo-
bisisobutyronitrile (AIBN) (Lancaster), sodium sulphate
(SRL), triethanolamine (Merck), sodium hydroxide (SRL),
glycidyltrimethylammonium chloride, glycidol (SRL),
ethanol (SRL), silver nitrate (merck), chloro auric acid
(Sigma Aldrich), THF (SRL), hydrochloric acid (Sd-fine),
2.4. Synthesis of Common Insoluble Bead-Shaped
Poly(styrene)–Poly(vinyl benzyl chloride) Matrix
(PS-PVBC)
The insoluble form of bead-shaped cross-linked
poly(styrene)-co-poly(vinyl benzylchloride) (PS-PVBC)
microsphere matrix was prepared using styrene (sup-
porting monomer, 72%), divinylbenzene (cross linking
monomer 3%), and vinyl benzylchloride (functional
monomer, 25%) by suspension polymerization method
as reported in literature.49 The resulting dried and sieved
PS-PVBC beads having mesh size 80–120 were analyzed
by FT-IR, SEM and EDAX analyses. This PS-PVBC
bead was used as the common matrix for surface-
grafting of stabilizing agent viz., TEA, GTMAC and
HPG individually and their resulting beads were used
as individual matrix for stabilization/immobilization of
Ag and AuNPs to obtain bead shaped heterogeneous
nanoparticles catalysts. The catalytic potential of these
catalysts in turn were examined by employing reduc-
tion of CR as a model reaction under pseudo-first order
condition.
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chloroform (SRL), congo red (SRL), sodium methox-
IP: 187.60.137.36 On: Thu, 11 Feb 2016 06:19:38
ide (SRL), sodium borohydride (SRL), tetrahydrofuran
Copyright: American Scientific Publishers
(Sd-fine), acetonitrile (SRL), methanol (SRL), acetone
(SRL), diethylether (SRL) and double distilled water were
used as provided.
2.2. Purification Method for the Monomers
All the monomers were purified by treating twice with
NaOH (10%) to remove the 4-tert-butylcatechol (TBC)
inhibitor and then washed three times with distilled water.
The supernatant was transferred to a column packed with
dried K2CO3 over aluminum oxide as adsorbent. Finally,
the purified monomer eluted from column was dehydrated
using anhydrous sodium sulfate before use.
2.4.1. Functionalization of Triethanolamine Onto
Bead-Shaped PS-PVBC Matrix (PS-PVBC-TEA)
2.3. Characterization
The immobilizations of silver and gold nanoparticles
onto the three different polymer matrices were confirmed
proved by UV-Visible spectrophotometry. UV-Vis spec-
tra of the samples were recorded on brucker lambda-
35 UV-Vis instrument. FTIR spectra were recorded using
Bruker Tensor-27 FTIR spectrophotometer. The sampling
was done by making use of KBr pellets with there quired
amount of sample. The background calibrations have been
carried out using pure KBr pellets, and the measure-
The insoluble form of chloride containing beads viz., PS-
PVBC were swelled initially in 50 mL round bottom
flask using acetonitrile50 (Scheme 1). The swelled PS-
PVBC beads were quaternized with excess amount of tri-
ethanolamine (20 mL) and the reaction mixture was stirred
ꢁ
for 48 h at 80 C in an inert atmosphere. The quater-
nized form of PS-PVBC beads were filtered and washed
with methanol and acetone. Then the quaternized PS-
ꢁ
PVBC beads were dried under vacuum at 60 C for 24 h
ꢁ
ments were carried out between 400–4000 cm−1 at 25 C.
leading to PS-PVBC-TEA beads. The corresponding PS-
PVBC-TEA beads were characterized with FTIR, SEM
and EDAX analyses.
The surface morphology of the polymeric bead matrix
and nanoparticle immobilized beads were analyzed with
428
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Murugan and Shanmugam
Surface Grafted HPG Stabilized Ag and AuNPs Heterogeneous Catalysts
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Copyright: American Scientific Publishers
Scheme 1. Synthesis of six different types of heterogeneous nanoparticle catalyst viz., PS-PVBC-TEA-Ag and AuNPs, PS-PVBC-g-GTMAC-Ag and
AuNPs and PS-PVBC-g-HPG-Ag and AuNPs.
2.4.2. Surface Grafting of Glycidyltrimethylammonium
Chloride (GTMAC) Onto the Bead-Shaped
2.4.3. Grafting of Hyper Branched Polyglycerol (HPG)
Onto Bead-Shaped PS-PVBC-TEA Matrix
PS-PVBC-TEA Matrix (PS-PVBC-g-GTMAC)
(PS-PVBC-g-HPG)
Grafting of HPG onto PS-PVBC-TEA matrix was per-
formed by epoxy ring opening polymerization reaction.
Initially, 1 g of PS-PVBC-TEA was taken into 100 mL
round bottom flask and swelled in dried methanol (20 mL)
for 24 h. After swelling, the PS-PVBC-TEA beads were
partially deprotonated (10%, 6.55 mL) using addition of
Grafting of GTMAC onto the PS-PVBC-TEA beads was
performed by epoxy ring opening reaction. Initially, 1 g
of PS-PVBC-TEA was taken into 100 mL round bot-
tom flask and swelled in dried methanol (20 mL) for
24 h. After swelling, the PS-PVBC-TEA beads were
partially deprotonated (10%, 6.55 mL) using the addi-
ꢁ
potassium methoxide solution at 45 C for 12 h. Then,
ꢁ
tion of potassium methoxide solution at 45 C for 12 h.
glycidol was slowly added drop by drop to the contents
of the round bottom flask and then the reaction mixture
was stirred at 95 ꢁC for 12 h. The epoxy ring open-
ing reaction was terminated with addition of 1 mM of
HCl (7 mL) and thus yielded HPG grafted PS-PVBC-
TEA matrix viz., PS-PVBC-g-HPG-AuNPs were filtered.
Then the PS-PVBC-g-HPG beads were filtered, washed
with ethanol and acetone and then dried under vacuum at
Then, GTMAC was slowly added in drops at 95 ꢁC
for 12 h and the reaction mixture was stirred for about
12 h, in order to allow the epoxy ring opening reaction.
Then the obtained GTMAC grafted PS-PVBC-TEA matrix
viz., PS-PVBC-g-GTMAC beads were filtered, washed
with ethanol and acetone and then dried under vacuum
at 60 ꢁC for 24 h. The obtained PS-PVBC-g-GTMAC
beads were characterized with FTIR, SEM and EDAX
analyses.
ꢁ
60 C for 24 h. The obtained PS-PVBC-g-HPG-AuNPs
were characterized by FTIR, SEM and EDAX analyses.
J. Nanosci. Nanotechnol. 16, 426–438, 2016
429

Surface Grafted HPG Stabilized Ag and AuNPs Heterogeneous Catalysts
Murugan and Shanmugam
2.4.4. Stabilization/Immobilization of AgNPs and
AuNPs Onto PS-PVBC-TEA,
analysis and the surface morphology and immobilization
of Ag and Au onto the respective polymer matrix was
studied with SEM, EDAX, HRTEM and TGA analyses.
PS-PVBC-g-GTMAC and PS-PVBC-g-HPG
Matrices
The stabilization/immobilization of AgNPs and AuNPs
was performed individually onto the PS-PVBC-TEA, PS-
PVBC-g-GTMAC and PS-PVBC-g-HPG matrices using
respective metal precursor in solution viz., AgNO3 and
HAuCl4 (Scheme 1). 0.5 g of PS-PVBC-TEA, PS-PVBC-
g-GTMAC and PS-PVBC-g-HPG matrices were taken
individually in six different single necked 100 mL R.B
flask and swelled in 50 mL of solvent mixture (H2O:THF,
7.5:2.5) for overnight. The respective swelled polymer
beads were purged with nitrogen atmosphere for 1 h. After
1 h, AgNO3 (5.7 × 10−3 mM) was added individually to
three different RB flask containing PS-PVBC-TEA, PS-
PVBC-g-GTMAC and PS-PVBC-g-HPG matrices, then
the solution was sꢁtirred and reflexed in an oil bath
with stirring at 80 C. Care should be taken is such a
way that the reaction mixture should be protected from
light exposure in order to avoid photochemical reduc-
tion of Ag+. After 10 min the colour of the beads was
changed from colourless to light yellow. Then, 2 equiv-
alents of NaBH4 were added to the solution, subse-
quently, after 1 h the colour was changed from light
yellow to dark yellow. Again after 1 h the flask was
stirred for 24 h at 80 ꢁC and then the resulting three
2.5. Comparative Catalytic Activity of Six Types of
Heterogeneous Nanoparticle Catalyst for the
Reduction of Congo Red (CR)
The comparative catalytic efficiency of six types
of bead-shaped heterogeneous nanoparticle catalyst
viz., PS-PVBC-TEA-AgNPs, PS-PVBC-TEA-AuNPs, PS-
PVBC-g-GTMAC-AgNPs, PS-PVBC-g-GTMAC-AuNPs,
PSVBC-g-HPG-AgNPs and PS-VBC-g-HPG-AuNPs cat-
alyst were examined separately by conducting the reduc-
tion of Congo red (CR) maintaining pseudo first order
condition (Scheme 2). That is, irrespective of the cata-
lyst, the reaction was performed in quartz cuvette (hav-
ing path length of 1 cm) kept at ambient temperature.
Exactly, 0.25 mL of (1 M) aqueous sodium borohydride
solution, 0.25 mL (0.01 M) of aqueous of CR were taken
in a cuvette to which added 2.5 mL double distilled water
and then 6 mg of catalysts were added individually into
the cuvette. The progress of each reaction was monitored
quantitatively by UV-Vis spectrophotometry by measuring
the characteristic peak of –N N– group at 496 nm. The
time required for the complete reduction of specific con-
centration of the substrate was determined. That is, pseudo
first order rate constant for the reduction of CR catalyst
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by each catalyst was calculated by measuring the decreas-
IP: 187.60.137.36 On: Thu, 11 Feb 2016 06:19:38
types of AgNPs immobilized/stabilized matrices were fil-
tered through buchner funnel, washed threꢁe times with
ethanol and dried at under vacuum at 60 C for 12 h.
Then the AgNPs immobilized PS-PVBC-TEA, PS-PVBC-
g-GTMAC and PS-PVBC-g-HPG matrices were labeled
as PS-PVBC-TEA-AgNPs, PS-PVBC-g-GTMAC-AgNPs
and PS-PVBC-g-HPG-AgNPs respectively.
Copyright: American Scientific Publishers
ing absorbance noticed at 496 nm against time. The rate
constant was calculated using the following formula irre-
spective of the reaction.
kobs = 1/t lnꢁꢂAꢂ−A0ꢃ/ꢂAꢂ−Atꢃꢄ
where, A0-is the initial absorbance, At the absorbance at
Similarly, HAuCl4 (5.7×10−3 mM) solution was added
individually to the swelled PS-PVBC-TEA, PS-PVBC-g-
GTMAC and PS-PVBC-g-HPG matrices in another three
different RB flask. It was observed that the colour of the
solution changed from colourless to yellow. After 2 h, 2
equivalents of NaBH4 was added to the respective flask
which in turn changed the colour of the beads from yel-
low to dark purple and then the beads were refluxed for
time ‘t’, A -the absorbance at infinity time.
ꢂ
The kobs values obtained for each catalyst are given in
Table III. The obtained kobs values of all the six types
ꢁ
24 h at 80 C and then were filtered through buchner fun-
nel, washed three times with ethanol and dried in vac-
ꢁ
uum oven at 60 C for 12 h and thus obtained AuNPs
immobilized PS-PVBC-TEA, PS-PVBC-g-GTMAC and
PS-PVBC-g-HPG matrices. The resulting heterogeneous
nanoparticle catalysts were labeled as PS-PVBC-TEA-
AuNPs, PS-PVBC-g-GTMAC-AuNPs and PS-PVBC-g-
HPG-AuNPs respectively.
The newly developed six types of heterogeneous
nanoparticle catalyst viz., PS-PVBC-TEA-AgNPs, PS-
PVBC-TEA-AuNPs, PS-PVBC-g-GTMAC-AgNPs, PS-
PVBC-g-GTMAC-AuNPs, PS-PVBC-g-HPG-AgNPs and
PS-PVBC-g-HPG-AuNPs were characterized with UV-Vis
Scheme 2. Reduction of Congo red using polymer-supported heteroge-
neous Ag and AuNPs catalysts.
430
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Murugan and Shanmugam
Surface Grafted HPG Stabilized Ag and AuNPs Heterogeneous Catalysts
of catalyst reveals that PS–PVBC-g-HPG–AuNPs catalyst
was found to be effective in catalytic reduction of CR. The
catalyst used for first cycle of CR reduction was washed
with water, ethyl acetate and methanol and then the cat-
alysts was dried under vacuum and could be recycled 6
times and employed the same for each cycles and the
observed rate constants are shown in Figure 13.
stretching indicate the functionalization of triethanol amine
onto the PS-PVBC matrix.
Further, grafting of GTMAC and HPG onto the PS-
PVBC-TEAmatrix was analyzed by FTIR (Figs. 1(c)
and (d)). The FTIR spectrum of PS-PVBC-g-GTMAC
(Fig. 1(c)) shows peaks at 1113.28 cm−1, 1218 cm−1
and 3416 cm−1 due to C–O–C (ether linkage), C–N+
(onium group) and –OH (hydroxyl group) respectively.
The appearance of these peaks confirmed the graft-
ing of GTMAC onto the PS-PVB-TEA matrix. The
FTIR spectrum of PS-PVBC-g-HPG (Fig. 1(d)) shows
the increasing intensity for C–O–C (1110.13 cm−1ꢃ and
–OH (3416 cm−1ꢃ and the disappearance of C–N+ꢂstrꢃ at
1218 cm−1 evidences the grafting of hyper branched
polyglycerol onto the PS-PVBC-TEA matrix.
3. RESULTS AND DISCUSSION
3.1. FTIR
The PS-PVBC beads were prepared using suspension
polymerization technique and the resulting beads were
functionalized with triethanolamine. Initially, the func-
tionalization of triethanol amine has been established by
analyzing the PS-PVBC (control) and triethanol amine
functionalized PS-PVBC-TEA matrices and the corre-
sponding FTIR spectra were shown in Figures 1(a) and
(b) respectively. In Figure 1(b), triethanol amine func-
tionalized PS-PVBC-TEA shows the characteristic peak at
1218.25 cm−1 and 3416 cm−1 corresponding to C–N and
–OH stretching frequency respectively. Similarly, another
characteristic peak viz., C-Clstretching was observed at
698 cm−1 in PS-PVBC beads, whereas, this peak was
sharply reduced in the triethanolamine functionalized PS-
PVBC matrix (Fig. 1(b)). Hence, the appearance of
C–N and –OH stretching and the disappearance of C–Cl
3.2. SEM
The surface morphology of PS-PVBC, PS-PVBC-
TEA, PS-PVBC-g-GTMAC and PS-PVBC-g-HPG were
established through SEM analyses and the obtained
images were shown in Figures 2(a)–(d) respectively. From
Figure 2(a), it is inferred that the surface morphology of
the first level matrix viz., PS-PVBC was homogeneous
and smooth with spherical shape. After functionalizing
with TEA, the smooth surfaces of PS-PVBC beads were
slightly changed into heterogeneous surface (Fig. 2(b)).
From Figures 2(c), (d), it was observed that the intensity
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of the heterogeneity increases as the number of different
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functional groups increases due to grafting of GTMAC
Copyright: American Scientific Publishers
and HPG onto the PS-PVBC-TEA matrix. Similar, sur-
face grafting study was already reported by Murugan et al.
for polymer supported dendritic palladium nanocatalyst for
Suzuki coupling reaction.13
The stabilization/immobilization of AgNPs and AuNPs
onto the PS-PVBC-TEA, PS-PVBC-g-GTMAC and
PS-PVBC-g-HPG matrices was also established with
Figure 1. FTIR spectra of (a). PS-PVBC, (b). PS-PVBC-TEA, (c). PS-
Figure 2. SEM images of (a). PS-PVBC, (b). PS-PVBC-TEA, (c). PS-
PVBC-g-GTMAC, (d). PS-PVBC-g-HPG.
PVBC-g-GTMAC, (d). PS-PVBC-g-HPG.
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431

Surface Grafted HPG Stabilized Ag and AuNPs Heterogeneous Catalysts
Murugan and Shanmugam
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Copyright: American Scientific Publishers
Figure 3. SEM images of (a). PS-PVBC-TEA-AgNPs, (b). PS-PVBC-TEA-AuNPs, (c). PS-PVBC-g-GTMAC-AgNPs, (d). PS-PVBC-g-GTMAC-
AuNPs, (e). PS-PVBC-g-HPG-AgNPs, (f). PS-PVBC-g-HPG-AuNPs.
SEM analysis and the obtained images are shown in
Figures 3(a)–(f) respectively. The obtained SEM images
reveal that the white dots/patches are noticed in PS-PVBC-
TEA-Ag and AuNPs, PS-PVBC-g-GTMAC-Ag and
AuNPs and PS-PVBC-g-HPG-Ag and AuNPs catalyst was
compared to their corresponding plain PS-PVBC-TEA,
PS-PVBC-g-GTMAC, PS-PVBC-g-HPG (Figs. 2(b)–(d))
matrices. Sreekumar et al. reported the Polystyrene-
supported poly(amidoamine) dendrimer–manganese com-
plex: Synthesis, characterization and catalysis.51 Similarly,
in earlier studies we have also reported the same
observation for polymer-supported dendritic nanocatalysts
immobilized with gold nanoparticles.13 The intensity of the
white dots/patches was found to increase in the order; PS-
PVBC-TEA < PS-PVBC-g-GTMAC < PS-PVBC-g-HPG
matrices. The PS-PVBC-g-HPG has more number of –OH
group where corresponds to TEA and GTMAC. This in
turn was influenced by the intensity of white dots/patches
from SEM images.
3.3. EDAX and TGA Analysis
The quantum of stabilizing agent viz., TEA, GTMAC and
HPG grafted onto the surface of PS-PVBC beads and their
432
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Murugan and Shanmugam
Surface Grafted HPG Stabilized Ag and AuNPs Heterogeneous Catalysts
Table I. Percentage of elements determined from EDAX analysis using
polymer supported AgNPs and AuNPs.
determined through EDAX and the observed percentage
of elements were given in Table I. On comparison, it was
observed that the percentage composition of C was found
to decrease and N, O was found to increased, after grafting
of TEA, GTMAC onto the PS-PVBC matrix. Similarly,
after grafting of HPG onto the PS-PVBC-TEA matrix,
the percentage of N decreases with increase in “O” con-
tent and this due to availability of more number of –OH
groups on the PS-PVBC-TEA matrix. In the case of their
corresponding catalyst, the percentage of Ag and AuNPs
stabilized in respective bead matrices increases gradually
in the following order of PS-PVBC-TEA < PS-PVBC-g-
GTMAC < PS-PVBC-g-HPG matrices. This order is due
to the availability of more functional group (–OH) in
PS-PVBC-g-HPG matrix than compared with PS-PVBC-
TEA (–OH, –N+Cl−) and PS-PVBC-g-GTMAC (–OH,
–N+Cl−ꢃ matrices.
Percentage of elements determined
from EDAX analysis using various
polymer-supported nanocatalyst
Name of the catalyst
C%
N%
O% Cl% Ag% Au%
PS-PVBC
72.36
–
–
27ꢅ64
–
–
–
–
–
–
–
PS-PVBC-g-TEA
PS-PVBC-g-GTMAC
PS-PVBC-g-PG
69.38 8ꢅ06 15ꢅ64 6ꢅ92
55.72 21ꢅ82 10ꢅ28 12ꢅ18
44.54 11ꢅ02 38ꢅ74 5ꢅ70
–
PS-PVBC-g-TEA-AgNPs 74.48 5ꢅ42 9ꢅ87 7ꢅ85 2ꢅ38
PS-PVBC-g-TEA-AuNPs 73.13 6ꢅ40 7ꢅ9 9ꢅ34
–
3ꢅ23
–
–
PS-PVBC-g-GTMAC-
AgNPs
61.26 17ꢅ55 7ꢅ95 7ꢅ62 5ꢅ62
PS-PVBC-g-GTMAC-
AuNPs
60.45 16ꢅ02 8ꢅ34 8ꢅ91
6ꢅ28
PS-PVBC-g-PG-AgNPs
PS-PVBC-g-PG-AuNPs
55.65 2ꢅ66 28ꢅ03 1ꢅ98 11ꢅ68
–
56.74 1ꢅ62 25ꢅ28 0ꢅ94
–
15ꢅ42
Similarly, to determine the thermal decomposition of all
the six types of catalyst they were analyzed by TGA under
nitrogen atmosphere at a heating flow rate 20 C/m. The
corresponding ability for stabilization of Ag and AuNPs
were studied extensively with EDAX. That is, the charac-
teristic elements such as C, N, Cl, O, Ag and Au avail-
able in the respective catalyst were semi-quantitatively
ꢁ
weight loss curves of all the six types of catalysts viz.,
PS-PVBC-TEA-Ag and AuNPs, PS-PVBC-g-GTMAC-Ag
and AuNPs and PS-PVBC-g-HPG-Ag and AuNPs are
shown in Figures 5(a)–(f). All the six types of cata-
lysts (Figs. 8(a)–(f)) show thermal stability in the range
of 250–400 ꢁC. The percentage of weight loss reveals
that, whether it is a quaternized with TEA, GTMAC and
(a)
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HPG, the higher branching molecules grafted/ function-
IP: 187.60.137.36 On: Thu, 11 Feb 2016 06:19:38
alized onto the polymer matrices than with their corre-
Copyright: American Scientific Publishers
sponding lower branched TEA. Increased the % of weight
loss reveals that the hyper-branched polymers contains
more number of functional group such as –OH than
with grafting of lower amount of TEA and GTMAC
molecule. Therefore, when it was analyzed with TGA,
hyper-branched polyglycerol get decomposed and thus
shown increased percentage of weight loss.
(b)
Figure 5. TGA curves of (a). PS-PVBC-TEA-AgNPs, (b). PS-PVBC-
TEA-AuNPs, (c). PS-PVBC-GTMAC-AgNPs, (d). PS-PVBC-GTMAC-
AuNPs (e). PS-PVBC-HPG-AgNPs, (f). PS-PVBC-HPG-AuNPs.
Figure 4. EDAX spectrum of (a). PS-PVBC-g-HPG-AgNPs, (b). PS-
PVBC-g-HPG-AuNPs.
J. Nanosci. Nanotechnol. 16, 426–438, 2016
433

Surface Grafted HPG Stabilized Ag and AuNPs Heterogeneous Catalysts
Murugan and Shanmugam
3.4. UV-Visible and HRTEM Studies
The immobilization/stabilization of AgNPs and AuNPs
onto the three types of matrices viz., PS-PVBC-TEA,
PS-PVBC-g-GTMAC and PS-PVBC-g-HPG were estab-
lished through UV-Vis and HRTEM analyses. Ini-
tially, the six types of bead-shaped heterogeneous
nanoparticle catalysts viz., PS-PVBC-TEA-AgNPs, PS-
PVBC-g-GTMAC-AgNPs, PS-PVBC-g-HPG-AgNPs, PS-
PVBC-TEA-AuNPs, PS-PVBC-g-GTMAC-AuNPs and
PS-PVBC-g-HPG-AuNPs were treated individually with
sodium dodecyl sulfate/ethanol and then sonicated for
an hour in order to extract Ag and AuNPs from the
PS-PVBC-TEA, PS-PVBC-g-GTMAC and PS-PVBC-g-
HPG matrices. After completion of sonication time,
the colour of the solution was changed into yellow
in the case of AgNPs and pink for AuNPs thus con-
firming the extraction of Ag and AuNPs from the
respective matrices. Further, it was also verified by
analyzing the respective metal nanoparticle solution
with UV-Vis and the observed spectra are shown in
Figures 6(a)–(c) respectively. From the UV-Vis spectra
of PS-PVBC-TEA-AgNPs, PS-PVBC-g-GTMAC-AgNPs,
PS-PVBC-g-HPG-AgNPs (Figs. 6(a)–(c)), it is observed
that, surface plasmon resonance (SPR) band appeared
at 408 nm confirming the formation of AgNPs.49
Similarly, from the UV-Vis spectrum of PS-PVBC-TEA-
Figure 7. UV-Vis spectra of (a). PS-PVBC-TEA-AuNPs, (b). PS-
PVBC-g-GTMAC-AuNPs, (c). PS-PVBC-g-HPG-AuNPs.
Further, these findings were again checked with
HRTEM analysis and the obtained images for four types
of catalysts are shown in Figures 8(a), (b) for PS-PVBC-
TEA-Ag and AuNPs and shown in Figures 7(c), (d) for
PS-PVBC-g-HPG-Ag and AuNPs and these entire images
show intense black dots, possibly due to high distribu-
tion of AgNPs and AuNPs in the range of 20–30 nm
and 10–20 nm respectively. Almost all the particles are
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AuNPs, PS-PVBC-g-GTMAC-AuNPs, PS-PVBC-g-HPG-
AuNPs (Figs. 7(a)–(c)), it is observed that, SPR band
appeared at 546 nm thus confirming the formation of
AuNPs.13 The PS-PVBC-g-HPG-Ag and AuNPs catalyst
has shown higher peak intensity as compared with The PS-
PVBC-g-GTMAC-Ag and AuNPs and PS-PVBC-TEA-Ag
and AuNPs catalyst respectively. This observation fur-
ther shows that PS-PVBC-g-HPG beads have more –OH
groups than TEA and GTMAC.
IP: 187.60.137.36 On: Thu, 11 Feb 2016 06:19:38
spherical in shape. These observations confirm the immo-
Copyright: American Scientific Publishers
bilization of AgNPs and AuNPs onto the PS-PVBC-TEA
and PS-PVBC-g-HPG matrix respectively. Polymer sup-
ported hyper branched polyglycerol is not only important
for immobilization for the nanoparticles, but it also con-
trols the size and shape of the nanoparticles, and acts
against aggregation and leaching of metal nanoparticles.
The results of HRTEM analysis confirm the immobi-
lization/stabilization of AgNPs and AuNPs onto the PS-
PVBC-TEA and PS-PVBC-g-HPG matrix respectively.
3.5. Comparative Catalytic Study of 6 Types of
Bead-Shaped Heterogeneous Nanoparticle
Catalyst for the Reduction of Congo Red
In the comparative catalytic study of the activity of the
of PS-PVBC-TEA-AgNPs, PS-PVBC-TEA-AuNPs, PS-
PVBC-g-GTMAC-AgNPs, PS-PVBC-g-GTMAC-AuNPs,
PS-PVBC-g-HPG-AgNPs and PS-PVBC-g-HPG-AuNPs
catalysts were used individually for the reduction of CR
with NaBH4 under identical pseudo first reaction condition
in aqueous medium. The occurrence of the reaction was
quantitatively monitored by measuring the disappearance
of peak intensity of CR through UV. The observed pseudo
first order rate constants of CR are given in Table II. The
absorption peak of CR noticed at 496 nm was found to
decrease with time. Control experiment (without catalyst)
was also performed and it was seen that the peak due to
–N N– at 496 nm remained unaltered even for two days,
Figure 6. UV-Vis spectra of (a). PS-PVBC-TEA-AgNPs, (b). PS-
PVBC-g-GTMAC-AgNPs, (c). PS-PVBC-g-HPG-AgNPs.
434
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Murugan and Shanmugam
Surface Grafted HPG Stabilized Ag and AuNPs Heterogeneous Catalysts
Figure 8. HRTEM images of (a) PS-PVBC-TEA-AgNPs, (b) PS-PVBC-TEA-AuNPs, (c) PS-PVBC-g-HPG-AgNPs, (d) PS-PVBC-g-HPG-AuNPs.
showing that the reaction is kinetically restricted. How-
ever, in the presence of each catalyst the peak intensity of
CR noticed at 496 nm gradually decreased with irradia-
In order to establish the right reaction condition for the
effective reduction of CR, the reaction has to be opti-
mized. Hence, this superior catalyst viz., PS-PVBC-g-
HPG-AuNPs was employed to study the kinetics of the
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tion time and complete degradation/reduction take place in
about 35–50 min (Fig. 9). The pseudo-first order rate con-
stants (kobs) were calculated by following the disappear-
ance of CR and the obtained values are given in Table II.
The kobs values indicate that although the same amount of
catalyst was used, the reduction/degradation efficiency of
CR depends on the surface grafting of TEA, GTMAC and
HPG onto the polymer beads, stabilization/immobilization
of Ag and AuNPs and size of the MNPs. This observa-
tion further confirms that the PS-PVBC-g-HPG beads con-
tain more number –OH groups than PS-PVBC-TEA and
PS-PVBC-g-GTMAC beads. Further, from Table II, PS-
PVBC-g-HPG-AuNPs catalyst shows a greater kobs value
(4.54 × 102 min−1) than the PS-PVBC-g-HPG-AgNPs
(3.98 × 102 min−1) due to the size of the nanoparticles
(AgNPs = 25 nm and AuNPs = 13 nm), lower reduction
potential of Ag (0.68 eV) compared to AuNPs (1.68 eV).
IP: 187.60.137.36 On: Thu, 11 Feb 2016 06:19:38
same reduction/degradation of CR by varying the experi-
Copyright: American Scientific Publishers
mental parameters such as [catalyst], [NaBH4] and [sub-
strate]. Further, the stability of the superior catalyst viz.,
PS-PVBC-g-HPG-AuNPs was examined through the same
reaction under identical condition and by employing the
recovered catalyst upto 6th cycle. The kobs for the recov-
ered catalyst viz., PS-PVBC-g-HPG-AuNPs has shown the
same catalytic activity as observed in the first cycle and
the plot drawn from kobs versus number of cycles (Fig. 13).
From the Figure 13, it was seen that PS-PVBC-g-HPG-
AuNPs catalyst maintains its activity without any leaching
of AuNPs from PS-PVBC-g-HPG matrix.
3.6. Detailed Kinetics of CR Using
PS-PVBC-g-HPG-AuNPs
The detailed kinetics study for the superior poly-
mer supported heterogeneous nanoparticle catalyst viz.,
PS-PVBC-g-HPG-AuNPs and the obtained kobs values
are given in Table III. In polymer supported heteroge-
neous nanoparticle catalyst the rate of reaction gener-
ally increases linearly with the amount of catalyst. With
the view to examine the effect of catalyst (PS-PVBC-g-
HPG-AuNPs) the said reduction of CR was performed
by varying the catalyst amount from 2–10 mg keeping
other parameters constant. Meijboom et al. also stud-
ied similar type of variations.52 The plot derived from
observed rate constant against the amount of catalyst is
shown in Figure 10. From the plot, it is observed that the
Table II. Comparative catalytic efficiency of 6 types of the bead-shaped
heterogeneous Ag and AuNPs catalyst derived from PS-PVBC-TEA,
GTMAC and HPG matrices for the reduction of congo red.
S. no
Name of the catalyst
kobs ×102 min−1
1
2
3
4
5
6
PS-PVBC-g-TEA-AgNPs
PS-PVBC-g-TEA-AuNPs
PS-PVBC-g-GTMAC-AgNPs
PS-PVBC-g-GTMAC-AuNPs
PS-PVBC-g-HPG-AgNPs
PS-PVBC-g-HPG-AuNPs
0.54
0.98
1.78
2.54
3.98
4.54
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435

Surface Grafted HPG Stabilized Ag and AuNPs Heterogeneous Catalysts
Murugan and Shanmugam
Table III. Detailed kinetic study using PS-PVBC-g-HPG-AuNPs cata-
lyst for the reduction of congo red.
Amount of
catalyst
(mg)
Amount of
Amount
of CR
kobs ×102 [NaBH4] kobs ×102
kobs ×102
min−1
(M)
min−1
(x 10−2 M)
min−1
2
4
6
8
3ꢅ89
4ꢅ34
4ꢅ54
5ꢅ35
13ꢅ75
0.6
0.8
1.0
1.2
1.4
4.18
4.43
4.54
3.64
5.51
0.6
0.8
1.0
1.2
1.4
7.60
6.70
4.54
5.57
4.98
10
Notes: [Substrate]: 1.0× 10−4 M; [NaBH4]: 1.0 × 10−2 M; Catalyst: 6 mg; Temp:
27 ꢁC.
rate of reaction was directly proportional to the amount
of catalyst. It is quite obvious that due to increase in
the active sites available on the catalyst surface the rate
constant of the reaction was increased. Aiqinwang et al.
also observed similar results for the catalytic reduction
of organic dyes.53 The effect of [NaBH4] was also stud-
ied by varying the concentration from 0.6 to 1.4 moles
keeping other parameters constant. The plot derived from
observed rate constant against the [NaBH4] of catalyst is
shown in Figure 11. The plot reveals that the rate constants
were proportional to the concentration of NaBH4. Simi-
lar, results with sodium borohydride was also reported by
Murutan et al.49
Figure 10. Effect of [catalyst].
The effect of [CR] in reduction/degradation was studied
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by varying its concentration from 0ꢅ6−1ꢅ4 × 10−2 mole,
IP: 187.60.137.36 On: Thu, 11 Feb 2016 06:19:38
keeping other parameters constant. The pseudo-first order
Copyright: American Scientific Publishers
rate constants were calculated and plotted against the [CR].
From plot Figure 12, it is seen that the rate decreased with
the increase in [CR]. As the [CR] is increased keeping
the amount of catalyst as constant, only lesser amount of
active sites are available, and that in turn reduced the rate
of reaction. Similar results on the rate of CR reduction
with addition of substrate was reported.54
Figure 11. Effect of [sodium borohydride].
Figure 9. UV-Vis spectra for the reduction of CR using PS-PVBC-g-
HPG-AuNPs.
Figure 12. Effect of [substrate].
J. Nanosci. Nanotechnol. 16, 426–438, 2016
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Murugan and Shanmugam
Surface Grafted HPG Stabilized Ag and AuNPs Heterogeneous Catalysts
appearance of surface Plasmon resonance peak at 408 and
546 nm in UV-Vis spectra, the change of smooth sur-
face (control) to heterogeneous surface (catalyst) noticed
in SEM, availability of oxygen, AgNPs and AuNPs load
determined by EDS have strongly confirmed that the per-
centage of AuNPs is greater where compared to AgNPs,
in the case of superior catalysts derived from PS-PVBC-g-
HPG matrix. All these catalysts were successfully acceler-
ating the reduction of CR. All these catalysts were highly
stable, easily recoverable and maintain its high activity
over a 6 recycles. Further efforts to extend application of
this system in other reactions are currently under progress
in our laboratory.
Acknowledgment: The authors thank the University of
Madras for the financial support under URF and NCNSNT
schemes, and for providing characterization facilities.
Figure 13. Recycling efficiency of PS-PVBC-g-HPG-AuNPs catalyst.
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g-AuNPs Through Reduction of CR
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Delivered by Publishing Technology to: Chinese University of Hong Kong
Received: 20 September 2014. Accepted: 17 October 2014.
IP: 187.60.137.36 On: Thu, 11 Feb 2016 06:19:38
Copyright: American Scientific Publishers
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J. Nanosci. Nanotechnol. 16, 426–438, 2016