Development of novel inorganic-organic hybrid nanocomposites as a recyclable adsorbent and catalyst

Article abstract of DOI:10.1039/c4ra03684k

Development of novel inorganic-organic hybrid nanocomposites based on the immobilization of nanoparticles on tubular clay minerals has received a lot of attention not only because of their abundance in nature and their low cost but also because they can be utilized as catalysts, as a nanoreactor to host reactants for nanosynthesis, and for the controlled release of bio-molecules. Halloysite nanotube (HNT), an important inorganic material, possesses a hollow tubular structure. Here, we demonstrate a method for the selective modification of the outer surface of the HNTs with an organosilane to make HNTs a novel solid-phase adsorbent to extract heavy metal ions from aqueous solutions. The new functionalized HNTs exhibit excellent selectivity for the extraction of mercury which shows monolayer molecular adsorption over HNTs and the adsorption of Hg(ii) follows pseudo-second order kinetics. Following the surface modification of HNTs, a two-step procedure has been developed to fabricate metal/HNT nanocomposites. With the commercially available reagents, monodispersed Au, Ag, and AuAg alloy nanoparticles (NPs) were synthesized in a hot organoamine solvent under a nitrogen atmosphere using a standard air-free technique. Metal/HNT nanocomposites are obtained by the immobilization of preformed metal NPs on HNT surfaces where the organosilane behaves as a linker molecule. Finally, the catalytic activity of these nanocomposites was determined and their use was compared in the production of anthranilic acid. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra03684k

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PAPER
Development of novel inorganic–organic hybrid
nanocomposites as a recyclable adsorbent and
catalyst†
Cite this: RSC Adv., 2014, 4, 34435
Subhra Jana* and Sankar Das
Development of novel inorganic–organic hybrid nanocomposites based on the immobilization of
nanoparticles on tubular clay minerals has received a lot of attention not only because of their
abundance in nature and their low cost but also because they can be utilized as catalysts, as a
nanoreactor to host reactants for nanosynthesis, and for the controlled release of bio-molecules.
Halloysite nanotube (HNT), an important inorganic material, possesses a hollow tubular structure. Here,
we demonstrate a method for the selective modification of the outer surface of the HNTs with an
organosilane to make HNTs a novel solid-phase adsorbent to extract heavy metal ions from aqueous
solutions. The new functionalized HNTs exhibit excellent selectivity for the extraction of mercury which
shows monolayer molecular adsorption over HNTs and the adsorption of Hg(II) follows pseudo-second
order kinetics. Following the surface modification of HNTs, a two-step procedure has been developed to
fabricate metal/HNT nanocomposites. With the commercially available reagents, monodispersed Au, Ag,
and AuAg alloy nanoparticles (NPs) were synthesized in a hot organoamine solvent under a nitrogen
atmosphere using a standard air-free technique. Metal/HNT nanocomposites are obtained by the
immobilization of preformed metal NPs on HNT surfaces where the organosilane behaves as a linker
molecule. Finally, the catalytic activity of these nanocomposites was determined and their use was
compared in the production of anthranilic acid.
Received 23rd April 2014
Accepted 21st July 2014
DOI: 10.1039/c4ra03684k
www.rsc.org/advances
9,10
the inner and outer surfaces of the HNT. The neighbouring
alumina and silica layers with their water of hydration result in
Introduction
A halloysite nanotube (HNT) is an economically viable alumi- a packing disorder and cause the nanotubes (NTs) to curve and
1,11
nosilicate clay and has become a promising candidate for roll up to form multi-layers.
The hollow tube formation
nanoconned reactions because of its well-dened hollow process is driven by the mismatch in the periodicity between the
1
,2
tubular nanostructure and abundance in nature. The envi- oxygen sharing tetrahedral SiO
sheets. Based on the state of hydration, HNTs are
sheets and the adjacent octa-
2
ronmentally friendly and biocompatible nature of HNTs makes hedral AlO
them an important nanomaterial for developing new inorganic– generally classied into two groups: hydrated halloysite-10 A˚
6
organic hybrid nanocomposite for use in many areas such as with one layer of water molecules between the multi-layer and
3
–7
˚
drug delivery, catalysis and so on.
chemical composition to that of kaolinite, and their chemical versible phase transition with loss of adsorbed water.
HNTs have a similar dehydrated halloysite-7 A which may be obtained by an irre-
12–14
HNTs
The are quite polydispersed in size; typically, the length varies from
2,8
structure can be expressed as Al Si O (OH) $nH O.
2
2
5
4
2
internal surface of the HNT is composed of a gibbsite-like array 1–3 mm and the outer and inner diameter are in the range of 30–
of Al–OH groups whereas the Si–O–Si groups present on the 70 nm and 15–30 nm, respectively.
external surface, revealing the positive and negative charges at
Because of the high length-to-diameter ratio, availability of a
nanoscale lumen, and high temperature resistance, HNTs have
become the focus of current research. Due to the availability of
the lumen, HNTs have been used as an ideal substrate for the
controlled or sustained release of drugs or bio-molecules and a
host molecule to immobilize large size enzymes unlike other
Department of Chemical, Biological & Macro-Molecular Sciences, S. N. Bose National
Centre for Basic Sciences, Sector-III, Block – JD, Salt Lake, Kolkata 700 098, India.
E-mail: subhra.jana@bose.res.in
†
Electronic supplementary information (ESI) available: Materials, detail
3,15
procedure of adsorption studies and catalysis, XRD, TGA, and FTIR spectra of porous materials.
They can also be used as anti-corrosion
HNTs and surface modied HNTs, EDS spectra and elemental mapping of pure
agents, biocides, and as a nanoreactor to host reactants for
HNTs, FESEM images of Au/HNTs, EDS spectra and XRD patterns of Ag/HNTs,
Au/HNTs, and AuAg/HNTs, UV-visible absorption spectra of Ag, Au, and
Au0.33Ag0.66 NPs, adsorption kinetics of Hg(II), recycling of o-NBA. See DOI:
16–20
nanosynthesis and biomimetic synthesis.
The high length-
to-diameter ratio and the high temperature resistance prop-
erty enables HNTs to produce high quality ceramics. These clay
1
0.1039/c4ra03684k
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NTs have also been utilized as catalyst carriers in the petroleum
rening industry for the catalytic cracking of heavy oils. Wang
Experimental
Surface modication of halloysite nanotubes
21
et al. have synthesized TiO /HNT composites which exhibit
2
22
photocatalytic activity for the degradation of methanol. HNTs The reaction was carried out under a nitrogen atmosphere
have also been used to make composites with a humidity using standard air-free techniques. The round bottomed ask
23
control ability. Magnetic molecularly imprinted polymers was tted with a condenser, rubber septum, thermocouple
using HNTs as a support for magnetic NPs were also synthe- adaptor, and quartz sheath in which a thermocouple was
24
sized by Yan et al. However, HNTs are of particular interest for inserted. Halloysite nanoclay (1.0 g) was added to a 50 mL ask
the selective removal of heavy metal contaminants, such as containing 12 mL of toluene and the reaction mixture was
ꢀ
cadmium, lead, mercury and so on, from aqueous solution and heated with a heating mantle. At 60 C, 1.0 mL of (3-mercap-
also for the reduction of nitro aromatics aer appropriate topropyl)trimethoxysilane (MPTMS) was injected to the mixture
modication of their surfaces.
in the ask and then the temperature of the reaction mixture
ꢀ
Selective modication of the naturally occurring HNTs to was increased to 120 C and reuxed at that temperature for
manipulate the chemico-physical properties through the 12 h. Once the ask was cooled down to room temperature, the
control of the chemistry of the constituents as well as the product was washed several times with toluene and then
ꢀ
morphology of these nanostructures has interested researchers ethanol, and then dried at 100 C under vacuum. Aer the
in recent years. Halloysite nanotubes, which have aluminium surface modication, the organosilane functionalized HNTs
on the innermost and silicate on the outermost surfaces, allow were referred to as Mod-HNTs.
for having different inner/outer surface chemistry. Because of
the presence of hydrophilic surfaces of HNTs, their application
in some elds is sometimes restricted. Modication of the
surfaces of the halloysite nanotubes with organosilanes repre-
sents a way to improve the dispersal of clay NTs and thus,
enhances their physical, mechanical, and chemical properties.
Yuan et al. reported a modication of HNTs with g-amino-
Synthesis of Au, Ag, and AuAg nanoparticles
The synthetic protocol for metal NPs was based on the thermal
decomposition of a metal salt in a hot organoamine solvent.
All reactions were carried out under a nitrogen atmosphere
using standard air-free techniques. The synthetic procedure was
conducted in a single ask. Gold(III) chloride trihydrate (0.235
mmol) was dissolved in a mixture of oleylamine (20 mM) and
26,27
14
propyltriethoxysilane. HNTs have also been functionalized
2
5
with N-(b-aminoethyl)-g-aminopropyltriethoxysilane.
et al. demonstrated lumen modication of HNTs with dopa-
Lvov
1-octadecene (40 mM) in a three-necked round bottomed ask.
The ask was tted with a condenser, rubber septum, thermo-
couple adaptor, and quartz sheath in which a thermocouple was
6,17
mine derivatives and octadecylphosphonic acid. There is also
a report on the functionalization of HNTs with surfactants such
as sodium dodecanoate and decyltrimethylammonium
inserted. The mixture was heated with a heating mantle to
ꢀ
1
20 C at which point, the color of the solution changed from
10
bromide. Joo et al. synthesized carboxylic acid functionalized
yellow to orange to pink indicating the formation of Au NPs.
Similarly, silver nitrate (0.235 mmol) was dissolved in a mixture
of oleylamine (20 mM) and 1-octadecene (40 mM) in a three-
necked round bottomed ask and heated to 120 C. The
yellow colored solution was then aged at 120 C for 30 min
under a nitrogen atmosphere. For the synthesis of the AuAg
alloy, gold(III) chloride trihydrate and silver nitrate were dis-
solved in a mixture of oleylamine and 1-octadecene in a three-
HNTs and studied their aggregation and dispersion behaviour
8
in acidic and basic solution. Despite the different reports on
the surface modication of HNTs and their potential applica-
tions, there has been no report, to our knowledge, on the
synthesis of metal or alloy/HNTs nanocomposites based on the
immobilization of the preformed NPs over the surface of HNTs
aer selective modication and their potential catalytic appli-
cation to synthesize anthranilic acid.
In this paper, we report the selective surface modication of
the clay NTs with an organosilane and such selective function-
alization of HNTs makes them a new type of adsorbent with
tunable properties. The new functionalized HNTs exhibit
excellent efficiency as a solid-phase adsorbent to extract heavy
metal ions from aqueous solutions. We have further established
a two-step procedure to fabricate monodispersed metal NPs
onto the surface of the HNTs. This was followed by the synthesis
ꢀ
ꢀ
ꢀ
necked round bottomed ask and heated to 120 C. The solu-
ꢀ
tion was then aged at 120 C for 1 h under a nitrogen atmo-
sphere. Particles were collected by centrifugation aer the
addition of an acetone–hexane (3 : 1) mixture and were redis-
persed in toluene and this solution then remains stable for
about couples of months or few months.
Synthesis of metal/HNTs nanocomposites
of Au, Ag, and their alloy NPs, which were then immobilized on Metal/HNTs nanocomposites were prepared as follows: 0.5 g of
the surface of modied HNTs to synthesize Au/HNTs, Ag/HNTs, Mod-HNTs was added to a beaker containing Au NPs (7 mL,
ꢁ2
and AuAg/HNTs nanocomposites where the organosilane 10 M) and the mixture was stirred for 1 h to complete the
behaves as a linker molecule. The as-synthesized nano- immobilization of Au NPs onto the surface of the Mod-HNTs. It
composites were used as a heterogeneous catalyst for the was observed that addition of the Mod-HNTs to the Au NPs
reduction of nitro aromatics. All the nanocomposites remained solution causes gradual fading of the intense pink color of the
stable and active for a couple of months or few months even at solution, which nally becomes colorless while the Mod-HNTs
the end of the reaction and can be recycled a number of times. themselves turned pink, demonstrating complete immobiliza-
tion of Au NPs. In a similar way, Ag and AuAg NPs were
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indicating that the basic structure of halloysite remains unal-
tered even aer surface modication by MPTMS. Two newly
ꢁ1
ꢁ1
emerged bands at 2931 cm and 2550 cm in Mod-HNTs are
assigned to the stretching vibration of C–H and S–H respec-
tively, suggesting that the graing of MPTMS to the surface of
HNTs has occurred. The phase and purity of HNTs before and
aer modication of the surface by X-ray diffraction (XRD) was
studied. Even aer surface modication, the XRD pattern of the
Mod-HNTs remains the same demonstrating no intercalation of
MPTMS into the interlayer of HNTs and this is in agreement
6,29
with the reported pattern of HNTs (ESI, Fig. S3†).
The morphology and the size of HNTs and Mod-HNTs were
determined using eld emission scanning electron microscopy
FESEM) and transmission electron microscopy (TEM), respec-
Scheme 1 Schematic presentation of the surface modification of
HNTs with MPTMS followed by the synthesis of metal/HNTs
nanocomposites.
(
tively. The FESEM images of HNTs (Fig. 1A and B) before and
aer modication demonstrate that they consist of cylindrical
shaped tubes with an open-ended lumen. TEM images of HNTs
and Mod-HNTs (Fig. 1C and D) demonstrate that the cylindrical
shaped tubes composed of multi-layer walls with a lumen, have
irregularity in both length and wall thickness which results in
variation in the inner and outer diameter. The length of the
tubes is 1.0 to 1.5 mm. The outer diameter of the tubes is 50–100
nm whereas lumen size is around 15–20 nm. The observed
defects on the surface of HNTs may be because of mechanical
fabricated on to Mod-HNTs and the color of the Mod-HNTs
changes to yellow and brown, respectively, leaving behind a
colorless reaction solution. Aer the immobilization of gold,
silver, and their alloy NPs, the synthesized nanocomposites are
referred to as Au/HNTs, Ag/HNTs, and AuAg/HNTs, respectively
(see Scheme 1).
14
damage or crystallographic defects.
Energy dispersive X-ray spectroscopy (EDS) further corrobo-
rates the presence of sulfur and carbon with the three main
Results and discussion
Structure and morphology of HNTs and Mod-HNTs
Because of the presence of the negatively charged outer surfaces constituents, oxygen, aluminium, and silicon in Mod-HNTs
of the HNTs, we can tune the properties by the selective modi- (Fig. 2) whereas pure HNTs contain only oxygen, aluminium,

cation of the surface through graing of an organosilane, and silicon (Fig. S4 ESI†). The elemental mapping shown in
MPTMS. As the surface defects generally occur on the external Fig. 2 was obtained from SEM-EDS mapping to show the
surface of HNTs, the hydroxyl groups at those defects become elemental composition of HNTs before and aer surface
14
available for surface modication. Unlike the untreated HNTs, modication. The elemental maps also ensured the presence of
the aqueous solution of modied HNTs does not show any
colloidal stability, indicating the hydrophobic nature of the
outermost surfaces aer the modication with organosilane. To
estimate the silane/HNT ratio, we have studied the thermal
decomposition behavior of both HNTs and Mod-HNTs using
thermogravimetric analysis (TGA) under nitrogen. TGA curves
(
ESI, Fig. S1†) of HNTs before and aer surface modication
ꢀ
represent the mass loss in the temperature range 50–600 C.
The weight loss between 50–150 C corresponds to the desorp-
ꢀ
tion of physisorbed water from the surface, and in the region of
ꢀ
1
50–250 C, is because of the loss of hydrogen bonded MPTMS
28
or removal of the residual template. Decomposition of MPTMS
which was graed on to the HNT surface occurs at around 250–
ꢀ
ꢀ
4
75 C. Above 375 C, the weight loss observed in HNTs and
Mod-HNTs is ascribed to the dehydroxylation of the residual
14
structural AlOH groups. The mass loss estimated by TGA
analysis for modied HNTs is in accordance with the amount of
ligand covalently bound to the HNTs.
Fourier-transform infrared (FTIR) spectra of HNTs and Mod-
HNTs exhibit (Fig. S2†) well-dened peaks at 1635, 3621, and
ꢁ
1
3
694 cm , demonstrating the deformation of water, O–H
stretching of inner hydroxyl groups, and O–H stretching of
inner surface hydroxyl groups, respectively. All these peaks are
Fig. 1 FESEM images of HNTs (A) before, and (B) after surface modi-
fication. TEM images of (C) HNTs, and (D) surface modified HNTs. Inset
observed with a slight decrease in intensity in the Mod-HNTs, of C and D shows a single HNT.
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Fig. 2 Element mapping by SEM and corresponding EDS spectra of
surface modified HNTs.
sulfur and carbon in the Mod-HNTs from the surface modi-
cation of HNTs by MPTMS. The concentration of sulfur in the
Mod-HNTs is 1.04 wt%, which was also conrmed by TGA
analysis. Furthermore, we have performed carbon/sulfur anal-
ysis using a carbon/sulfur analyzer to determine the exact
amount of sulfur present in the Mod-HNTs and it has been
estimated to be 1.08 wt%.
Morphological characterization of metal/HNT
nanocomposites
Fig. 3 TEM images of (A) Au, (B) Ag, and (C) AuAg NPs before and (D)
Au/HNTs, (E) Ag/HNTs, and (F) AuAg/HNTs after immobilization on
Mod-HNTs.
Based on the thermal decomposition techniques, Au, Ag, and
AuAg alloy NPs were synthesized from their metal salts. TEM
images of Au, Ag, and AuAg alloy NPs are presented in Fig. 3A–C.
All the NPs exhibit
a
nearly monodispersed spherical
30
cubic (fcc) structure (Fig. S7†). For the AuAg alloy NPs, the
morphology with an average particle size of 9 ꢂ 0.5 nm for Au
NPs, 8.5 ꢂ 0.5 nm for Ag NPs and it is 10 ꢂ 1 nm for AuAg NPs.
Aer the surface modication of HNTs, the as-synthesized
metals or alloy NPs were immobilized on the surface of the
Mod-HNTs to form metal/HNT nanocomposites where the
organosilane acts as a linker molecule. The FESEM images of
Au/HNTs nanocomposites are presented (see ESI, Fig. S5†) at
different magnications. Fig. 3D–F show the TEM micrographs
of Au/HNTs, Ag/HNTs, and AuAg/HNTs nanocomposites. From
the FESEM and TEM images, it is evident that the NPs are
immobilized exclusively over the surface of the HNTs and are
well dispersed. Even aer the fabrication of metal/HNTs
nanocomposites, all the NPs retained their size, shape, and
morphology. Furthermore, the TEM images of Au NPs before
and aer immobilization on Mod-HNTs, shown in Fig. 4,
substantiate this fact.
interplanar distance measured from the adjacent lattice fringes
is 0.24 nm, indicative of (111) planes of the fcc Au or Ag (see
Fig. S8A†).
It should be mentioned that because of the comparable
lattice constants of Au (0.408 nm) and Ag (0.409 nm), XRD
patterns and high-resolution TEM images of AuAg NPs are
similar to those of Au or Ag NPs. However, we can differentiate
them by considering their optical properties by means of
surface plasmon resonance peak shi using UV-visible
Elemental analysis by energy dispersive X-ray spectroscopy
(Fig. S6†) further demonstrates the formation of metal/
halloysite nanocomposites. Signals from Ag and Au were
observed for Ag/HNTs and Au/HNTs, respectively. The EDS
spectrum of AuAg/HNTs conrms the presence of both Au and
Ag in the nanocomposites. The crystal structure of these three
different NPs aer immobilization on the HNTs was conrmed
Fig. 4 TEM images of (A) a bunch of HNTs loaded with Au NPs and (B)
a single nanotube after immobilization of Au NPs. The inset of B shows
by powder XRD analysis which demonstrated their face centred a TEM image of Au NPs before immobilization.
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1
/n
spectroscopy (see Fig. S8B†). The peak at 425 nm represents the
formation of the Au0.33Ag0.66 alloy rather than the Au/Ag core/
shell NPs. There is a report in the literature where the Au ions where C
Q ¼ K C
e
F e
ꢁ
1
e
is equilibrium concentration of adsorbate (mg L ), Q
were reduced on the surface of the previously formed Au–NH2 is amount of adsorbate at equilibrium (mg g ), Q
chelate complex using tea polyphenols for the synthesis of Au/ maximum adsorption capacity of the adsorbent, and K
e
ꢁ
1
m
is the
is the
L
HNTs. Aer reduction, the Au NPs obtained are irregular in affinity constant or Langmuir adsorption equilibrium constant.
ꢁ
1
ꢁ1 1/n
size and shape having a diameter of 20 to 40 nm and are densely
K
F
is the Freundlich equilibrium constant (mg g )/(mg L
)
,
25
dispersed over the surface. In contrast, our synthetic approach and n is the adsorption equilibrium constant. Based on the
produces monodispersed spherical nanoparticles with precise Langmuir and Freundlich model we have analyzed the experi-
control over particle growth and morphology and retain their mental data obtained for the equilibrium adsorption isotherm
size and morphology even aer immobilization and are well of Hg(II) (Fig. 5A). The experimental data tted better with the
distributed on the Mod-HNTs surfaces.
Langmuir model than the Freundlich model, demonstrating a
monolayer molecular adsorption for the Mod-HNTs. The
ꢁ
1
maximum Hg(II) adsorption capacity of 62.19 mg g of adsor-
bent was achieved.
Adsorption of heavy metal pollutants by surface modied
HNTs
The adsorption kinetics of Hg(II) were examined using
pseudo-rst order and pseudo-second order rate equations.
The pseudo-rst order rate equation is given by:
36
37
Mercury is one of the main pollutants in water and the
maximum permissible concentration allowed in ground water
and surface water is less than 0.5 mg L according to WHO.
Inorganic mercury compounds are rapidly accumulated in the
kidneys and also cause neurological and renal disturbances.
ꢁ1
ꢁ
e
e
1
k t
Q
t
¼ Q
e
ꢁ Q
31–33
Thus, there is a need to develop a low-cost adsorbent to extract
these pollutants from ground water. A new solid-phase adsor-
bent was synthesized by the surface modication of HNTs with
MPTMS for the removal of heavy metals ions. Multiple sets of
batch experiments were performed to study the mercury
adsorption isotherm. To determine the adsorption capacity,
Mod-HNTs were added to Hg(II) solution and the reaction
mixture was equilibrated at room temperature under neutral
conditions. Aer the adsorption of Hg(II) ions by the Mod-HNTs,
the amount of unbound Hg(II) ions present in the solution was
estimated using a UV-visible spectrophotometer at different
time intervals (Fig. S9 of ESI†). The uptake of Hg(II) ions by the
Mod-HNTs as a function of time was estimated and is presented
in the ESI, Fig. S10A.† It is evident that the adsorption of Hg(II)
increases with time and nally reaches a maximum, 1 h aer
the addition of adsorbent (Fig. S10B†). However, unmodied
HNTs hardly show adsorption even aer 24 h. The calculated
Hg/S molar ratio indicates that there is a 1 : 1 complexation
mechanism between Hg(II) and thiol and also demonstrates the
presence of sufficient amounts of active adsorption sites in the
modied halloysite. Therefore, it is conrmed that all the thiol
groups are located on the external surfaces of the HNTs.
The pseudo-second order rate equation is:
The adsorption isotherm of Hg(II) on Mod-HNTs was studied
by static equilibrium adsorption to determine the binding
properties and the experimental data were tted to the
34
35
Freundlich as well as the Langmuir isotherm model. Lang-
muir isotherm theory, which assumes monolayer coverage of
the adsorbate over an homogenous adsorbent surface, can be
expressed as follows:
L m e
K Q C
Q
e
¼ 1
þ K
L e
C
The Freundlich equilibrium isotherm which is used for the
description of multi-layer adsorption with interaction between
adsorbed molecules can be expressed as follows:
Fig.
5 (A) Adsorption isotherm and (B) pseudo-second order
adsorption kinetics of Hg(II) by modified HNTs.
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2
k
2
Q
e
t
Q
t
¼ 1
þ k
2
e
Q t
ꢁ
1
where Q is the amount of adsorbate (mg g ) at time t, Q is the
t
e
ꢁ1
equilibrium adsorption capacity of adsorbent (mg g ), and k1
and k are the rate constants for the pseudo-rst order and the
2
pseudo-second order reaction, respectively. Fig. 5B represents
uptake of Hg(II) ions by Mod-HNTs as a function of time. It was
observed that the adsorption kinetics of Hg(II) follows pseudo-
2
second order rate equation (with R ¼ 0.997), assuming the
chemical adsorption process is the rate-limiting step. To
determine the chemical stability and regenerability of the
adsorbent, several adsorption/desorption tests were carried out
using 10% thiourea in 0.05 M HCl solution as eluent. Aer
successive regeneration, the adsorption efficiency of Mod-HNTs
remains almost the same up to the 5th cycle and then gradually
decreases, which is probably because of the irreversible Hg(II)
adsorption or because of the oxidation of thiol groups (Fig. S11
of ESI†). Hg(II) adsorption efficiency by Mod-HNTs was
compared with that of other reported adsorbents based on the
maximum mercury uptake capacity by MPTMS, and these
results are presented in Table 1. Its ability to restore high
mercury uptake capacity for a number of cycles even aer
regeneration and reuse makes Mod-HNTs a new alternative as
an adsorbent.
Fig. 6 Adsorption efficiency of modified HNTs for the different
adsorbates from their aqueous solution (200 mg L ).
ꢁ1
selectivity of RSH towards Hg(II) than Cd(II). The smaller ionic
radii of Cd(II) presumably make it less so than Hg(II), as larger
ionic radii is another characteristic required for a so acid.
Thus, the modied HNTs may be a new alternative for a sensor
or an adsorbent for the detection as well as adsorption of Hg(II),
Pb(II), and Cd(II) from aqueous solution.
The adsorption efficiency of Mod-HNTs has also been
determined for the adsorption of Pb(II) and Cd(II) ions from
aqueous solution. For a comparative study, the same experi-
ment was performed taking the adsorbate solution of three
different heavy metal ions. We found that out of the three
adsorbates, Mod-HNTs adsorbed Hg(II) maximally and the
adsorption efficiency follows the order: Hg(II) > Cd(II) > Pb(II)
Catalytic activity of metal/HNTs nanocomposites
The development of recyclable catalysts has fascinated
researchers as it is indeed a real interdisciplinary eld, which
not only relates chemistry to materials science and engineering
but also offers many technologies and opportunities to provide
easily recoverable and reusable catalytic systems. To study the
catalytic activity of these metal/HNTs nanocomposites, we have
considered the reduction of o-nitrobenzoic acid (o-NBA) to
produce anthranilic acid as a model reaction. Based on the
spectroscopic measurements, we have studied the reaction
kinetics at room temperature. The detailed procedure of the
catalytic reaction is described in the ESI.† Under neutral or
acidic conditions, an aqueous solution of o-NBA shows a
distinct spectral prole with an absorption maximum at 267
nm. Aer the addition of a freshly prepared ice cold aqueous
(
Fig. 6). The above observation can be explained with the help of
43
Pearson's hard and so acids and bases (HSAB) theory.
According to HSAB principles “hard acids prefer to bind with
the hard bases to give ionic complexes, whereas the so acids
prefer to bind with so bases to give covalent complexes”. As
per the HSAB theory, the Lewis acid and bases are divided into
hard, so, and borderline types. In our study, graed MPTMS
(RSH) over HNTs is a so base which prefers to bind with Hg(II),
a so acid, compared to Pb(II) which is a border line acid.
Although Cd(II) is a so acid type, we observed the higher
4
solution of NaBH , there was only a slight decrease in the
absorbance value without any shi of lmax of o-NBA and the
Table 1 Comparison of mercury(II) adsorption capacity by reported adsorbents from aqueous solution
ꢁ1
ꢁ1
Adsorbent
S content (mmol g
)
Hg(II) adsorption (mmol g
)
Reference
FMMS
5.2
2.5
32
Mg-MTMS
MCM-41
6.4
4.7
3.01
2.1
38
39
Organoceramic SOL-AD-IV
Amorphous silica powder
3.72
0.24
2.3
2.38
1.3
3.62
0.28
1.26
1.51
0.59
0.31
40
41
41
28
2
0 mol% MPTS blocks
M1 HMS
MP-HMS-9.0
Mod-HNTs
42
0.32
Present work
34440 | RSC Adv., 2014, 4, 34435–34442
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peak position remained unaltered for few days in the absence of
It is well-documented that the rate of reaction generally
any catalyst. Upon the addition of 0.02 g of Ag/HNTs nano- increases linearly with the amount of catalyst in heterogeneous/
48
composites to the reaction mixture, we observed that the peak at microheterogeneous catalysis. Keeping all the experimental
2
67 nm gradually red-shied and nally moved to 307 nm conditions the same, it was observed that the rate of reaction
44
which can be ascribed to the formation of anthranilic acid.
increases with the amount of catalyst and nally reaches a
Fig. 7A shows typical UV-visible absorption spectra for the maximum value with a large excess of catalyst ($0.03 g of Ag/
successive reduction of o-NBA by Ag/HNTs nanocomposites. HNT nanocomposites), shown in Fig. 7C. Because of the
Thus, the progress of the reaction is visualized by a gradual red advantage of heterogeneous catalysis, the catalyst particles get
shiing of the peak from 267 to 307 nm. The reaction does not separated from the product, anthranilic acid. Their efficiency as
proceed with pure HNTs under the same experimental a recyclable catalyst for this reduction reaction was studied. At
conditions.
The rate of the nanocomposite catalyzed reaction follows NBA was reduced without any addition of new catalyst and
pseudo-rst order kinetics for o-NBA (Fig. 7B) as the concen- NaBH . Aer one reaction cycle, nanocomposites were washed
the end of the reaction, we have noticed that a fresh batch of o-
4
tration of the borohydride ion largely exceeds that of o-NBA, thoroughly with distilled water, dried at room temperature and
thus it remains constant throughout the reaction. Aer the recycled for the reduction of four consecutive fresh solutions of
addition of NaBH , the metal NPs which were immobilized on o-NBA (see the ESI, Fig. S12†).
4
the HNTs surfaces started the catalytic reduction by relaying
For a comparative study, we also examined the catalytic
ꢁ
electrons from the donor BH4 to the acceptor o-NBA only aer activity of Au/HNTs and AuAg/HNTs for the reduction of o-NBA.
the adsorption of both onto the NP surfaces. The rate of electron A good linear correlation was obtained for the plot of ln A versus
transfer at the catalyst surfaces is inuenced by a two-step time (Fig. 7D) for all the nanocomposites. Among the Au, Ag,
45–47
mechanism:
(a) diffusion of o-nitrobenzoate ions to the and AuAg/HNT nanocomposites, the reaction catalyzed by Ag/
catalyst surface and (b) interfacial electron transfer and diffu- HNTs shows the fastest rate of reduction. We believe that the
sion of o-aminobenzoate ions from the surface. It is also worth higher affinity of Ag–N probably makes Ag/HNTs the best
noting that the excess of NaBH increased the pH of the reaction catalyst out of the three nanocomposites and thus, the rate of
4
medium and retarded the degradation of the borohydride ions. reduction of o-NBA decreases in the order: Ag/HNTs > AuAg/
ꢁ
The liberated hydrogen from BH4 removed the dissolved air HNTs > Au/HNTs. The efficiency of the catalyst particles has
from the solution, and thus prevented the aerial oxidation of also been studied for the reduction of different nitro aromatics,
46,49
anthranilic acid. Evolution of small bubbles of hydrogen gas such as nitrophenols
which was surrounding the catalyst particles, facilitates auto- acids.
matic stirring of the solution and the particle itself remains well
and other isomers of nitrobenzoic
distributed in the reaction mixture and thus, creates favorable
conditions for the reaction to proceed.
Conclusions
In summary, selective surface modication of the HNTs with an
organosilane was demonstrated for the adsorption of heavy
metals from aqueous solutions. The new functionalized
HNTs exhibit excellent selectivity as solid-phase adsorbent of
mercury(II) ions. Thus, this modied halloysite may be a new
alternative as a sensor for the identication and detection of
Hg(II). To utilize them in practical applications, the Mod-HNTs
can be used in the clean-up process of the industrial effluents
and also for ground water treatment in the future because of its
ability to restore mercury uptake capacity for a number of
cycles. Based on the thermal decomposition of metal salts in a
hot organoamine solvent, monodispersed Au, Ag, and AuAg NPs
were synthesized to fabricate metal/HNTs nanocomposites
through the immobilization of metal NPs over the surface of
modied HNTs. The catalytic activity of these nanocomposites
has been studied for the reduction of nitro aromatics and their
catalytic efficiency has also been compared. This procedure
offers an added advantage for the efficient separation and
Fig. 7 (A) UV-visible spectra for the successive reduction of o-NBA recycling of the catalyst at the end of the reaction which is still a
catalyzed by Ag/HNT nanocomposites, and a corresponding (B)
challenging task for homogeneous metal catalysts from both
absorbance vs. time plot. Conditions: [o-NBA] ¼ 0.1 mM; [NaBH ] ¼
4
the economical and ecological point of view and synthesis of
amino compounds on a large scale is also possible. Thus, the
new nanocomposites might have widespread use as catalysts in
many industries.
0
.1 M; Ag/HNTs ¼ 0.02 g. (C) Plot of reaction rate constant against
amount of Ag/HNTs nanocomposites. (D) ln A vs. time plot for 0.02 g
of Ag/HNTs, Au/HNTs, and AuAg/HNTs to compare their catalytic
activity.
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2
4 J. Pan, H. Yao, L. Xu, H. Ou, P. Huo, X. X. Li and Y. Yan, J.
Phys. Chem. C, 2011, 115, 5440–5449.
Acknowledgements
The work was supported by the Department of Science and 25 H. Zhu, M. L. Du, M. L. Zou, C. S. Xu and Y. Q. Fua, Dalton
Technology (DST), New Delhi through DST INSPIRE faculty
Trans., 2012, 41, 10465–10471.
grant and SN Bose National Centre for Basic Sciences, Kolkata, 26 S. Jana, J. W. Chang and R. M. Rioux, Nano Lett., 2013, 13,
India.
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2
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7 S. Jana and R. M. Rioux, Nanoscale, 2012, 4, 1782–1788.
8 B. Lee, Y. Kim, H. Lee and J. Yi, Microporous Mesoporous
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34442 | RSC Adv., 2014, 4, 34435–34442
This journal is © The Royal Society of Chemistry 2014