Synthesis of magnetic silver cyclodextrin nanocomposite as catalyst for reduction of nitro aromatics and organic dyes

Article abstract of DOI:10.1016/j.molliq.2019.112279

Nanocomposites decorated with metallic nanoparticles in their matrix are important class of heterogeneous catalyst with high catalytic activity. Functionalized polymers are low cost materials which offer excellent supports for catalysts to render stability to metallic nanoparticles. Herein we report synthesis of Silver nanocomposite using β-cyclodextrin (b-CD) maleic anhydride crosslinked polymer anchored on the surface of magnetic nanoparticles. The nanocomposite was characterized using sophisticated analytical techniques and its role as a catalyst for reduction of nitroaromatics and organic dyes was investigated. This catalytic system exhibited comparatively lower E_a value of 18 kJ mol^?1 and followed pseudo first order kinetics. Simultaneous reduction of 4-Nitrophenol and Methylene Blue could be achieved in a time interval of 7 minutes. Being magnetically separable, the catalyst exhibited high recycling efficiency (up to 5 cycles) and ease of operation under mild conditions.

Full text of DOI:10.1016/j.molliq.2019.112279

Journal Pre-proof
Synthesis of magnetic silver cyclodextrin nanocomposite as
catalyst for reduction of nitro aromatics and organic dyes
Pratik Nariya, Manita Das, Falguni Shukla, Sonal Thakore
PII:
S0167-7322(19)35333-4
DOI:
https://doi.org/10.1016/j.molliq.2019.112279
MOLLIQ 112279
Reference:
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
25 September 2019
2 December 2019
6 December 2019
Please cite this article as: P. Nariya, M. Das, F. Shukla, et al., Synthesis of magnetic silver
cyclodextrin nanocomposite as catalyst for reduction of nitro aromatics and organic dyes,
Journal of Molecular Liquids(2019), https://doi.org/10.1016/j.molliq.2019.112279
This is a PDF file of an article that has undergone enhancements after acceptance, such
as the addition of a cover page and metadata, and formatting for readability, but it is
not yet the definitive version of record. This version will undergo additional copyediting,
typesetting and review before it is published in its final form, but we are providing this
version to give early visibility of the article. Please note that, during the production
process, errors may be discovered which could affect the content, and all legal disclaimers
that apply to the journal pertain.
© 2019 Published by Elsevier.

Journal Pre-proof
Synthesis of magnetic silver cyclodextrin nanocomposite as catalyst for
reduction of nitro aromatics and organic dyes
Pratik Nariya, Manita Das, Falguni Shukla and Sonal Thakore*
Department of Chemistry, Faculty of Science, The Maharaja Sayajirao University of Baroda,
Vadodara 390 002, India
*Author for correspondence E-mail:chemistry2797@yahoo.com, drsonalit@gmail.com
Abstract
Nanocomposites decorated with metallic nanoparticles in their matrix are important class of
heterogeneous catalyst with high catalytic activity. Functionalized polymers are low cost
materials which offer excellent supports for catalysts to render stability to metallic
nanoparticles. Herein we report synthesis of Silver nanocomposite using β-cyclodextrin (b-
CD) maleic anhydride crosslinked polymer anchored on the surface of magnetic
nanoparticles. The nanocomposite was characterized using sophisticated analytical
techniques and its role as a catalyst for reduction of nitroaromatics and organic dyes was
investigated. This catalytic system exhibited comparatively lower Ea value of 18 kJ mol-1 and
followed pseudo first order kinetics. Simultaneous reduction of 4-Nitrophenol and Methylene
Blue could be achieved in a time interval of 7 minutes. Being magnetically separable, the
catalyst exhibited high recycling efficiency (up to 5 cycles) and ease of operation under mild
conditions.
Keywords:
Cyclodextrin, Magnetic nanoparticle, Cross-linked Polymer, Nanocomposite, Heterogeneous
Nanocatalyst, Nitroaromatics
1. Introduction
Over the last two decades, metal-based nanoparticles prepared through green synthesis route
without any surfactant are widely applied in both homogeneous and heterogeneous catalysis
due to their exclusive structures and properties [1]. Among them, the most important property
of metal nanoparticles is their large surface area to volume ratio which increases their
interaction with other molecules [2]. Catalytic systems comprising of silver nanoparticles
have been used for organic reactions owing to their properties like reactivity, selectivity,
stability and recyclability. Such catalytic system yield highly efficient and selective organic
transformations. The application of heterogeneous silver-based nanoparticles to general
1

Journal Pre-proof
organic reactions has proved to be an effective strategy in the development of highly efficient
organic transformations in term of efficiency and selectivity [3,4]. However, metal
nanoparticles (NPs) are thermodynamically not stable. It has tendency of aggregation to
minimize their surface area which causes significant loss of its catalytic activities [5,6]. While
magnetic separation techniques have long been in use, intensive research into super paramagnetic
nanomaterials has accelerated the development of magnetically recoverable catalysts [7]. Iron
oxide being magnetically separable gives an additional advantage over other support. It
avoids classical filtration method of separation.
In this context, an excellent opportunity come into light for application of hybrid
nanocomposites composed of Ag with magnetic oxides in catalysis. Ag nanoparticles
supported on magnetic oxides may serve to avoid suspension aggregation and catalytic
deactivation. Meanwhile, the surface coating of magnetic nanoparticles precludes the
agglomeration of superparamagnetic nanoparticles enhancing their dispersion in solution.
Since, the magnetic properties facilitate easy recovery [8]. The applications of magnetic
materials with good reproducibility in heterogeneous catalysis have been widely reported.
Surface modified magnetic nanoparticles and nanocomposites are capable of catalysing
various reaction including reduction of nitroaromatics [9,10]
The reduction of 4-nitrophenol is a very simple reaction for the analysis of reaction kinetics
and improvement of catalytic activity. Moreover, this reaction is a well-known model
reaction for determining the activity of a catalyst because of the convenience of analysing the
progress of the reaction. In general, nitrophenols are not reduced by NaBH4 in aqueous or
non-aqueous solutions without a catalyst. Moreover, the reduction of nitro compounds to the
corresponding amines is an important industrial reaction. This is because amines are the
starting compounds for several synthesis processes as intermediates such as azo, imines, and
amides and furthermore, these compounds can be converted to other important intermediates
that have many applications in agriculture, pharmaceuticals, dyes, polymers and many of
them are biologically active. However, the disadvantage of the reaction for nitro compound
reduction is the use of high temperatures, long reaction times and most importantly the use of
costly catalysts. Therefore, it is necessary to develop a method for the conversion of 4-
nitrophenol to 4-aminophenol in aqueous solution under mild conditions by using an
appropriate catalyst [11]. Many catalyst like self-assembled iron oxide coated Pt
nanocomposites [12], Ag immobilized on modified CNTs [13], Ag-Graphene oxide
nanocomposites [14] are reported for nitro to amine reduction. However these catalysts are
economically less viable. This gives a rational to improve the catalyst system to make it
2

Journal Pre-proof
economically viable. β-cyclodextrin (b-CD), a well-known host for hydrophobic molecules
and multi-hydroxyl compound can form crosslinked polymer [15]. Reports are available on
cyclodextrin decorated silver nanocatalysts for reduction of nitrophenols. However these
systems take 30 minutes or more for completion of reaction. Considering practical
applicability of the catalysts modifications are required to for completion of reaction in less
time [12,16,17]. The choice of crosslinkers is crucial to determine several important
properties of b-CD polymers [18,19]. In the past, we have successfully synthesized
nanocomposites of iron nanoparticles and b-CD crosslinked with maleic anhydride which
demonstrated remarkable adsorption capacity.
Herein, we report the synthesis and characterisation of magnetic-silver nanocomposite with
β-Cyclodextrin (b-CD) linked through urethane functionalities. Because of well-defined
lipophilic cavity of b-CD efficient catalysis would be accomplished in aqueous medium.
Being magnetically separable, the catalyst exhibited high recycling efficiency and ease of
operation under mild conditions. Then, the catalytic activity was evaluated towards the
reduction of 4-Nitrophenol to 4-Aminophenol. The application has been extended to the
reduction of organic dyes.
2. Experimental Section
2.1 Materials
Iron oxide nanoparticles (Fe3O4 MNPs), Hexamethylene Diisocyanate (HMDI) and β-
Cyclodextrin (b-CD) was purchased from Sigma Aldrich and TCI, India respectively. Silver
nitrate (AgNO3), and methylene blue (MB) was purchased from Merck Mumbai, India.
Dimethyl formamide (DMF), Maleic anhydride (MA), sodium borohydride (NaBH4),
Hydrazine hydrate (NH2NH2.H2O), 4-Nitroaniline (4-NA), 3-Nitroaniline (3-NA), 2-
Nitroaniline (2-NA), 4-Nitrophenol (4-NP), 2-Nitrophenol (2-NP), Nitrobenzene and 2,4-
Dinitrophenol (2,4-DNP) was purchased from Spectrochem, India. All the compounds were
used as received without further purification. All the solutions were prepared using double-
distilled and demineralised water.
2.2 Characterization methods
1H NMR spectra of CD-MA polymer was recorded on Bruker Avance (400MHz). FT-IR
spectra were recorded with KBr pellets on a Bruker Alpha IR spectrophotometer in range of
4000–400 cm-1. The powdered X-ray diffraction (XRD) patterns were recorded using Bruker
3

Journal Pre-proof
D8 Advance X-Ray diffractometer. The X-Rays were generated using a sealed tube and the
wavelength of x-ray was 0.154 nm (Cu K-alpha). Samples were prepared by pressing dried
powder and patterns were collected with 0.5 step per second and 2θ ranging from 10o to 90°.
Surface area and porosity of the catalyst was measured by a volumetric adsorption system
(Micromeritics Instrument corporation, USA, model ASAP 2020) using N2
adsorption/desorption isotherms at 77 K up to 1 bar. Prior to the measurements, the samples
were activated (degassed) by heating at the rate of 1 K min-1 up to 383 K under vacuum. The
temperature as well as vacuum was maintained for 10 hours prior to the measurements.
Surface area was calculated by the Brunauer–Emmet–Teller (BET) method and porosity was
calculated by the Barrett–Joyner–Halenda (BJH) method. High-Resolution Transmission
Electron Microscopy (HR-TEM) was carried out using a Jeol (Jem-2100F) electron
microscope operated at an acceleration voltage of 200 kV. For this purpose, the sample was
dried, powered and dispersed in methanol and ultra-sonication was carried out for 30
minutes. Subsequently, that sample was deposited onto a carbon-coated grid at room
temperature and it was allowed to air dry (about 6 hours) TEM images and Selected Area
Electron Diffraction patterns (SAED) were then recorded. Energy-dispersive X-ray
spectroscopy (EDX) was performed for elemental mapping using JSM-5610 LV instrument
in scanning mode. The analysis was carried out to ensure that the spectrum acquired is a
representation of the bulk of the catalyst system. The surface composition was investigated
using X-ray Photoelectron Spectroscopy (XPS) on a PHI 5000 VersaProbe III spectrometer.
Raman spectra were recorded using Mini Ram-II (B&W TEK) Raman Spectrometer with
“CLEANLAZE” laser technology (excitation at 785 nm) and TE cooled 2048 pixel CCD
array spectrometer in range of 200 cm-1 to 2500 cm-1. Thermo gravimetric analysis (TGA)
was performed using TG–DTA 6300 INCARP EXSTAR 6000 at a heating rate of 10°C/min
in the temperature range of 30–500°C, nitrogen atmosphere was maintained throughout the
measurement. AgNC catalysed reduction reactions were monitored using PerkinElmer
Lambda 35 UV-vis spectrophotometer by corresponding λmax of the analytes.
2.3 Synthesis of Silver nanocomposite (AgNC)
The synthetic route followed for the preparation of the nanocomposite is as per scheme 1.
2.3.1 Synthesis of CD-MA
CD-MA cross linked polymer was synthesized as per previously reported procedure [20].
2.3.2 Synthesis of CD-MA@FeMNPs
4

Journal Pre-proof
Fe3O4 MNPs (50 mg) were taken in a round bottomed flask and dispersed in DMF (5 mL) by
sonicating for 20 minutes. HMDI (200 µL) was added slowly and dropwise while sonicating
the mixture. The reaction was stirred for 3 hours. To this reaction mixture 250 mg of CD-
MA polymer was added and further reaction was continued at 60oC for 24h. The product was
precipitated in water, washed with acetone and separated by magnetic decantation.
2.3.3 Synthesis of Silver nanocomposite (AgNC)
To prepare AgNC, 10 ml AgNO3 (0.1M) was taken in a stoppered Erlenmeyer flask. 100 mg
of CD-MA@FeMNPs were added to the flask and the resultant solution was sonicated for 10
minutes. 25% hydrazine hydrate solution was added dropwise at an interval of 5 minutes. The
reaction was allowed to proceed for 2 hours to ensure complete reduction of silver ions. The
product was separated by magnetic decantation, washed with acetone and dried under
vacuum.
5

Journal Pre-proof
Scheme 1: Schematic for Synthesis of AgNC
6

Journal Pre-proof
2.4 Catalytic reduction of 4-Nitrophenol
The reduction of 4-NP with NaBH4 was used as a model reaction to determine the catalytic
activity of synthesized AgNC as catalyst. In a typical reaction 10 mg of AgNC was added to
3 mL 4-NP solution (0.359 mmol) under stirring conditions in 25 mL round bottom flask at
o
35 C, then freshly prepared 3 mL NaBH4 solution (5.385 mmol) was added slowly to the
above reaction mixture. Initially, a sudden change in colour from faint yellow to intense
yellow was observed which finally disappears. The progress of reaction was monitored using
a UV-Vis spectrophotometer. After complete reduction, the catalyst was separated using an
external magnet and dried under vacuum after washing with water and acetone.
3. Results and Discussion
3.1 Synthesis and characterization of silver nanocomposites (AgNC)
Silver nanocomposites were prepared in 3 step one pot synthesis as shown in scheme-1. In
the first step b-CD was modified by crosslinking with MA to prepare CD-MA polymer. An
oxoanion is generated in strong alkaline conditions which is created due to addition of NaH.
The oxoanion is formed by the de-protonation of hydroxyl group at C2 position of the
anhydrous glucose unit of CD molecule. The second step involves introduction of isocyanate
functionalities on the surface of iron oxide nanoparticles (MNPs) using Hexamethylene
Diisocyanate (HMDI)[21]. The NCO groups reacts with free hydroxyl groups of CD-MA
polymer to give CD-MA@FeMNPs. The final step involves loading of silver nanoparticles
on the CD-MA@FeMNPs. Silver Nitrate solution was used as a source of silver. Hydrazine
hydrate causes reduction of Ag+ ions to give silver nanoparticles. The free carboxylic and
hydroxyl functionalities of CD-MA polymer forms complex with silver to stabilize the silver
nanoparticles and prevent their agglomeration.
3.2 NMR Spectra Analysis
1
The modified polymer (CD-MA) was characterized using H proton NMR spectroscopy
shown in Fig. S1 [20].
3.3 FT-IR Spectra Analysis
The FT-IR spectrum of AgNC (Fig. 1a) shows broad band at 3325 cm-1 is corresponding to
the -OH groups of β-cyclodextrin (b-CD). The C=O stretching frequency observed at 1620
cm-1. The presence of N-H and C-N bond corresponding to urethane linkages were confirmed
by presence of peak at 1572 cm-1. The successful conjugation of the polymer with hydroxyl
groups of Fe3O4 nanoparticles is confirmed by the presence of peak at 455 cm-1 due to
formation of Fe-O linkages[22]. The peaks at 1477 cm-1 and 1440 cm-1 signifies the presence
7

Journal Pre-proof
C=C confirming the attachment of CD-MA polymer [23]. The successful loading of silver is
observed due to appearance of peak at 629 cm-1 which can be assigned to Ag-O vibration
[24].
Fig. 1: (a) FT-IR spectra of CD-MA@FeMNPs (b) EDX pattern of AgNC
3.4 EDX Analysis
The elemental composition of AgNC obtained by EDX analysis is shown in Fig. 1b. The
weight percentage of Ag was found to 19.24 and other elements are as shown in table 1.
EDX analysis confirms that the final product AgNC mostly consists of Ag, Fe, O, C and N
elements, due to the absence of any other signals [10,25]. The presence of elements like C, N
8

Journal Pre-proof
and Ag other than Fe and O suggest the formation of nanocomposites with desired
functionalities.
Table 1: Elemental composition of the AgNC
Material
Fe (wt %) O (wt %)
C (wt %) N (wt %) Ag (wt %)
Bare Fe3O4
nanoparticle
70.25
29.71
0
0
0
AgNC
7.93
19.06
39.71
14.05
19.24
3.5 HR-TEM Analysis
In our earlier studies on core shell Fe3O4@Ni catalytic system showed somewhat spherical
morphology with some cubic particles [10]. Similarly the High Resolution Transmission
Electron Microscopy (HR-TEM) images of AgNC exhibited rod like particles in addition to
discreet monodispersed spherical particles.
Fig. 2: HR-TEM images of AgNC at different resolutions (a) 20 nm, (b) 10 nm, (c) 5 nm,
showing particle distribution; the resolved lattice fringes and SAED pattern of AgNC are
shown in (d), (e), (f) respectively
9

Journal Pre-proof
The average particle size of the spherical nanocomposites observed was 50 ± 3 nm (Fig. 2).
The average diameter of rod like particles was 10 nm. This indicates the presence of silver
nano rods on the spherical matrix of CD-MA coated iron oxide nanoparticles. The presence
of bright spots in SAED pattern indicates crystalline nature of AgNC. The lattice spacing of
silver is found to be 0.243 nm, which corresponds to (111) diffraction plane of silver as
shown in Fig. 2e [26]. Fig. 2f shows corresponding Selected Area Electron Diffraction
(SAED) pattern of AgNC. It shows characteristic diffraction rings at (111), (200), (220) and
(311) [27]. The presence of these rings is due to superimposition of the diffraction pattern
that is created by the presence of two different metals (iron and silver) in the sample.
3.6 UV-Visible Spectrophotometric Determination:
The formation of Ag nanoparticles in the matrix of CD-MA@FeMNPs polymer is
characterized by the Surface Plasmon Resonance absorption (SPR) band which appears in the
range of 400 and 450 nm as shown in fig. S2 [12] .
3.7 Dynamic Light Scattering Analysis
The hydrodynamic diameter of AgNC was determined using Dynamic Light Scattering
(DLS) experiments at 25°C, the diameter was observed to be 51 nm (Fig. S3). Moreover, the
particles were mono-dispersed which is in agreement with the observations of HR-TEM.
3.8 X-Ray Diffractogram Analysis
Fig. 3a shows XRD pattern of AgNC. The diffraction peak at 2θ = 37.95, 44.29, 64.40,
77.34, 81.44 corresponding to (111), (200), (220), (311), (222) are the characteristic planes
for Ag [28]. The diffractogram also shows peak at 2θ = 30.28, 35.65, 57.32,
63.0corresponding to (220), (311), (511), (440) planes which are due face centred cubic
crystal system of Fe [10]. The co-existence of the diffraction pattern of Ag with Fe confirms
the loading of Ag on the nanocomposites [29]. These results corroborates with the
observations of SAED pattern thus confirming the formation of AgNC.
10

Journal Pre-proof
(a)
Fig. 3: (a) XRD pattern and (b) Raman spectra of AgNC
3.9 Raman Spectra Analysis
Fig. 3b shows the Raman spectra of AgNC. The peaks at 978, 1377, 1562 cm-1 corresponds
to C-N stretching vibrations. The N-H bending modes in the plane appears at 1347 and 1437
cm-1, which is in agreement with previous report [30]. It indicates successful conjugation of
CD-MA polymer with Fe3O4 nanoparticle by urethane linkage. It also shows characteristic
vibrational band at 668 cm-1 for Fe3O4 nanoparticles [31]. Carbonyl (C=O) vibration mode
11

Journal Pre-proof
appeared at 1775 cm-1 which shows shift compared to normal vibration. The shift is due to
conjugation of carbonyl group. The peaks at 2127 and 2200 cm-1 corresponding to =C-H
stretching vibrations, which indicates presence of maleic anhydride from CD-MA polymer.
[32,33].
3.10 X-ray photoelectron spectroscopy (XPS) analysis
The successful loading of silver onto the nanocomposites, its oxidation state, electronic
properties and the chemical environment were determined with the help of X-ray
photoelectron spectroscopy (XPS).
Fig. 4: (a) Full scan XPS spectra of AgNC; High resolution spectra of (b) Ag 3d, (c) C 1s, (d)
N 1s (e) O 1s.
12

Journal Pre-proof
Fig. 4a shows full scanned spectrum for AgNC in the range of 0-800 eV. The spectra shows
the presence of Ag, C, N, O and Fe which is in agreement with EDX results. The high
resolution XPS spectrum of Ag after deconvolution (Fig. 4b) shows intense doublet band at
367.90 eV and 373.90 eV for 3d5/2 and 3d3/2 respectively. The peaks were shifted to lower
binding energy compared to binding energy of bulk silver which is 368.2 eV and 374.2 eV
[34]. The change in environment around Ag causes decrease in binding energy which is
evident for interaction between Ag and O of hydroxyl group of cyclodextrin or carbonyl
group [35,36]. The spin energy difference of these two peaks is 6.0 eV, indicating that Ag
exists in the zero-valent state (Ag0) rather than +1 state (Ag+1) [37]. This proves successful
loading of silver on CD-MA@FeMNPs. The high resolution of XPS spectrum of C 1s is
shown in Fig. 4c. The characteristics peaks located at 284.95 eV and 288.95 eV are attributed
to -C-C and O=C-O respectively. The N 1s spectrum (Fig. 4d) also showed peaks at 399.55
eV which are due to presence of -NH- bonds. The spectrum for O 1s (Fig. 4e) showed peak at
531.5 eV corresponding to –OH [38]. These results indicates successful synthesis of desired
catalyst with silver loading.
3.11 BET Analysis
Brunauer-Emmet-Teller (BET) adsorption isotherm and surface area plot of AgNC is shown
in Fig. S4, which exhibits type II adsorption isotherm as per the IUPAC classification. It is
characteristic for nanoparticles [39]. BET specific surface area, total pore volume, and
average pore diameter are as shown in Table 2. BET surface area of AgNC was found to be
22.9518 m2/g, which demonstrates it has high surface area. The pore size and total pore
volume of AgNC were found to be 13.4081 nm and 0.076006 cm3/g respectively.
Table 2: BET parameters of AgNC
BET plot
Sample
Average pore
diameter (nm)
BET specific
Total pore volume
(cm3/g)
surface area (m2/g)
AgNC
13.4081
22.9518
0.076006
3.12 TGA Analysis
Thermal properties of AgNCs were evaluated by TGA analysis as shown in Fig. 5a. The
degradation of the material occurs in three stages. A mass loss of 0.3% was observed at
temperature below 250 oC is due to loss of adsorbed water molecules. The next weight loss of
13

Journal Pre-proof
about 51% was observed in a temperature range of 250-400 oC corresponding to the cleavage
of ester and urethane linkages (Fig. 5b) [15,40] between 250-400 oC. The final weight loss of
10% in 400-550 oC range occurs as a result of b-CD degradation [23]
Fig. 5: TGA analysis of (a) AgNC (b) CD-MA@FeMNPs
3.13 VSM Analysis
The assessment of the magnetic properties of AgNC was performed by carrying out Vibrating
Sample Magnetometer (VSM) measurements at ambient temperature. A comparison of the
magnetic properties of bare Fe3O4 nanoparticles and AgNC is shown in Fig. 6. The estimated
values of Saturation magnetization (Ms), remnant magnetization (Mr) and coercivity (Hc)
were 0.1076 emu/g, 0.424.59× 10-3 emu/g and 3.6591 Oe respectively, suggests that it has
ferromagnetic behaviour [37]. A decrease in magnetization observed in case of AgNC which
is due to coating of polymer on surface of Fe3O4 nanoparticles. However AgNC could be
easily dispersed and withdrawn with the help of an external magnet (inset: Fig. 6). This
suggests that the magnetic property of iron oxide nanoparticles at the core is retained even
after its conjugation with CD-MA polymer.
14

Journal Pre-proof
Fig. 6: VSM analysis of (a) bare Fe3O4 nanoparticle (b) AgNC (Inset: Photograph showing
separation of AgNC using an external magnet)
3.14 Catalytic reduction of 4-Nitrophenol and other aromatic nitro compounds
The reduction of Nitrophenols was chosen as a model reaction to assess the capability of
AgNC to act as catalyst. The conversion of 4-AP from 4-NP occurs by the formation of 4-
Nitrophelonate ion intermediate. Despite being a thermodynamically favourable reaction, the
reduction of 4-NP to 4-AP using aqueous NaBH4 cannot occur. This is owing to a large
potential difference which exists between donor and acceptor molecules which acts as a
kinetic barrier for the progress of the reaction and hence lowers its feasibility. Metal
nanoparticles can help to overcome this barrier by facilitating electron transfer to the acceptor
¯
4-NP from the donor BH4 ions to catalyse the reaction [41]. The absorbance measurements
of 4-NP are unaffected by the presence of catalyst due to their relatively low concentration.
After sodium borohydride solution was added, a change in colour of the solution (faint
yellow to intense yellow) was observed due to formation of 4-nitrophenolate ion. The
formation of intermediate nitrophenolate ion causes a shift in absorption peak from 317 nm to
400 nm [11]. With the progress of the reaction the absorbance at 400 nm decreases gradually
and new absorption peak at around 300 nm increases due to formation of product 4-amino
phenol (4-AP) (Fig. 7a, Table 3; entry 1) [42].
15

Journal Pre-proof
Fig. 7: UV-Vis absorption spectra of reduction of 4-NP to 4-AP in presence of (a) AgNC and
NaBH4 (b) NaBH4 only (c) AgNC only and (d) Fe3O4 and NaBH4
Table 3 shows control experiment details for reduction of 4-nitrophenol. Control reactions
were carried out in presence of only NaBH4. It was observed that even after 72 h of stirring
reaction does not take place in the absence of catalyst (Fig. 7b, Table 3; entry 3). Further,
the reduction of 4-NP was also monitored in the absence of NaBH4 by taking only 4-NP and
AgNC. In the absence of NaBH4 absorption peak at 317 nm remains unaltered (Fig. 7c, Table
3; entry 2). Further the effect of bare Fe3O4 nanoparticle in presence of NaBH4 on reduction
of 4-NP was investigated. It was found that the decrease in absorbance after 24 h, thus bare
Fe3O4 cannot function as catalyst (Fig. 7d, Table 3; entry 4).
Table 3: Control experiment for reduction of 4-NP
Sr. No.
Reactant
NaBH4 (mmol)
Catalyst
(10 mg)
Time
1.
5.45
AgNC
11 min
2.
3.
4.
-
AgNC
-
No reaction
No reaction
No reaction
4-Nitrophenol
7.93
5.45
Fe3O4
nanoparticles
16

Journal Pre-proof
It has been proved from above experiments AgNC has good catalytic activity for 4-
NP.Experiments were carried out to assess the capability of AgNC to reduce other nitro
aromatics. General schematic for reduction of nitroarenes is shown in scheme 2. Various
substrates taken for the study and the time taken for their reduction is shown in Table 4.
Scheme 2: General scheme for the reduction of nitroaromatics
Table 4: Reduction of nitro aromatics to aromatic amines using AgNC a
Sr. No.
Nitroarene
4-Nitrophenol
2-Nitrophenol
2-Nitroaniline
3-Nitroaniline
4-Nitroaniline
2,4-Dinitrophenol
Nitrobenzene
Time (min)
1
2
11
9
3
50
30
55
35
25
4
5
6b
7
a Reaction conditions: All the reactions were carried out at room temperature (35 °C), 0.359
mM of substrate, catalyst 10 mg (AgNC), 5.53 mM NaBH4. b 0.359 mM substrate, catalyst 10
mg, 11.06 mM NaBH4.
17

Journal Pre-proof
Fig. 8: UV-Vis spectra for reduction of (a) 2-Nitrophenol (b) 3-Nitroaniline (c) 2-
Nitroaniline (d) 4-Nitroaniline (e) 2,4-Dinitrophenol and (f) Nitrobenzene
Upon comparison, it was observed that 2-Nitrophenol showed the maximum reactivity and
take lowest time to give corresponding 2-Aminophenol as product (Fig. 8a, Table 4; entry
2). As reported previously, the reaction goes through the nitrophenolate ion, which is
stabilized by resonance due to the presence of an electron-withdrawing nitro group. 4-NP is
more stable than 2-Nitrophenol due to steric effect as it takes more time to reduce (Table 4;
entry 1)[43]. A similar explanation can be given for 2-Nitroaniline and 4-Nitroaniline, which
are both stabilized by resonance. However, 3-Nitroaniline takes lesser time for reduction as
there is no resonance stabilization, which makes it more reactive (Fig. 8b, Table 4; entry 4).
Among 2-ntroaniline and 4-Nitroaniline, 2-Nitroaniline is less stable than the latter due to
18

Journal Pre-proof
steric effects as it takes lesser time to reduce (Fig. 8c, Table 4; entry 3). 4-Nitroaniline has
the most stable anion in the alkaline medium, thereby making it less reactive and takes longer
time for reduction to occur (Fig. 8d, Table 4; entry 5) [44]. For 2,4-Dinitrophenol, it was
observed that with the increase in number of nitro groups the amount of required NaBH4 was
higher (11.06 mM) and time taken for reduction is also greater as compared to nitrophenols
(Fig. 8e, Table 4; entry 6). Since solubility of nitrobenzene is low in water, a water-ethanol
(95:5, v/v) solution was used as solvent (Fig. 8f, table 4; entry 7)
3.15 Kinetic studies of the reduction of 4-NP by AgNC
The kinetics of the reduction reaction was determined using AgNC. It is postulated that
initially no reaction takes place due to a time delay. This time delay is an induction period
before the start of reaction wherein a restructuring of the nanoparticle surface takes place.
The restructuring of surface atoms creates catalytically active sites like corners or edges. This
aids the initiation of the reaction [45]. It was observed that the reduction of 4-NP to 4-AP
follows pseudo first order reaction kinetics. As the concentration of NaBH4 is far greater than
that of 4-NP, the reaction rate is presumably independent of NaBH4 concentration. Hence the
rate of the reaction can be evaluated by studying the kinetics of the reaction in terms of the
concentration of 4-NP [46,47]. Fig. 9a represents UV-vis absorption spectra of 4-NP at
different time intervals in presence of AgNC and NaBH4.The following equation (1) was used
for calculating the rate constant:
C
t
ln (C0) = −푘t
------------------ (1)
Where Ct is the concentration of the 4-NP at time t (min), C0 is the initial concentration of 4-
NP at time t = 0 (min) and k is rate constant [48]. The plot of ln(Ct/C0) versus time gives good
linear relationship, which is in agreement with pseudo first order reaction kinetics as shown
in Fig. 9b. The rate constant (k) 0.6787 min-1 is obtained from slope of the linear relationship
of ln(Ct/C0) versus time. The conversion of 4-NP to 4-AP was calculated using following
equation (2):
A
Conversion = (1 − A0t) × 100%
------------------- (2)
Where At stands for UV absorbance at 400 nm, which is proportional to the concentration of
reduced 4-NP; A0 stand for initial UV absorbance at 400 nm after addition of NaBH4 [48].
The Turn over Frequency (TOF) is an important factor for evaluating the catalytic activity of
a catalyst. TOF was calculated by dividing Number of moles of 4-NP that has reacted by
19

Journal Pre-proof
Number of moles of Ag nanoparticle in the catalyst per reaction time [49]. The TOF for
catalytic reduction of 4-NP using AgNC as catalyst was observed to be 19.5 min-1. The
observed TOF was higher as compared to other silver nanoparticle based catalyst reported in
literature (Table 5) which suggests that AgNC has high catalytic efficiency.
Table 5: Comparison of catalytic activities for reduction of 4-NP with catalysts reported
in the literature
Sr.
No.
Catalyst
k
TOF
Reference
(min-1)
(min-1)
1.
AgNPs/SiNSs
Ag/TP
4.8114
0.3162
1.32
3.52
3.846
4.150
2.61
[35]
[50]
[51]
[52]
[53]
[54]
2.
3.
4.
5.
6.
Ag@TPHH-COF
Fe3O4/Ag@NFC
np-Ag-Al
1.98
0.036
-
1.91
Fe3O4@P(MBAAm-
co-MAA)@Ag
17.06
7.
AgNC
0.6787
19.5
This study
3.16 Effect of Catalyst Quantity and Temperature on reduction of 4-NP
The amount of catalyst was varied in order to determine its effect on reduction of 4NP and its
rate constant as shown in Table 6. The quantity of AgNC was varied from 3 mg to 10 mg
(Table 6; entry 1-4) and the corresponding rate constant was determined. The 3 mg AgNC
(Table 6; entry 1) shows lowest k value of 0.3644 min-1, while 10 mg AgNC shows highest k
value of 0.8921 min-1. It causes decrease in the reaction time due to higher number of active
sites. Owing to its moderate activity 5 mg of AgNC (Table 6; entry 2) was taken as the
optimized quantity to determine effect of temperature and activation energy for further
investigations. Fig. 9b and 9e shows that ln(Ct/C0) has good linear relationship with time (t)
and Fig. 9c and 9f shows percent conversion versus time at 35 oC and 45 oC respectively.
The reaction time and rate constants at 35°C and 45°C are shown in Table 7. It was observed
that as the temperature increases the value of rate constant increases and reduction time
decreases (Table 7; entry 1 and 2). The increased reaction rate at higher temperature was
due to increased diffusion speed of reactants to Ag nanoparticles, which is also consistent
with Arrhenius equation (Table 7; entry 2) [11].
20

Journal Pre-proof
Table 6: Effect of catalyst amount on reduction of 4-NP a
Sr. No. Quantity of AgNC
(mg)
Time
(min)
11
Rate constant (k)
(min-1)
1.
2.
3.
4.
3
0.3644
5
7
6
0.6747
0.7706
0.8921
7
10
4.5
a Reaction conditions: 1.5 mL 4-NP (0.12 mM), 1.5 mL NaBH4 (12 mM), room temperature
(35 °C), experiments were performed in a cuvette.
Fig. 9: (a, d) The time dependent UV-vis absorption spectra of 4-NP reduction (b, e) plot of
o
o
ln(Ct/C0) versus time (c, f) plot of % conversion of 4-NP with time 35 C and 45 C
respectively
21

Journal Pre-proof
The activation energy (Ea) for this catalytic system was calculated using following equation
(3) of Arrhenius equation.
E
a
ln푘 = ln퐴 − RT -------------------- (3)
Where Ea is activation energy, R is universal gas constant, A is constant and k is rate constant
at temperature T. The value of Ea was found to be 18 KJ mol-1. The value of Ea was observed
to be comparatively lower than earlier reported silver nanoparticles (40.25 KJ mol-1) [55] and
CNT/CS/AgNP composites (89.27 KJ mol-1)[56]. Hence AgNC have excellent catalytic
property for the reduction of 4-NP.
Table 7: Effect of Temperature on reduction of 4-NP a
Sr. No.
Temperature Time (min)
(oC)
Rate
constant (k)
min-1
1.
2.
35
45
7
0.6747
4.5
0.7789
a
Reaction conditions: 1.5 mL 4-NP (0.12 mM), 1.5 mL NaBH4 (12 mM), 5 mg AgNC,
experiment performed in cuvette.
3.17 Influence of AgNC as catalyst at higher concentration of 4-NP
Further, the influence of AgNC as catalyst on reduction of 4-NP at its higher concentration
was investigated and found that it reduces 4-NP to 4-NA at higher concentration. Effect of
temperature on reduction of 4-NP at different concentration is shown in Table 8. It was
observed that as temperature increases, rate of the reaction increases and it takes less time to
reduce 4-NP to 4-AP (Table 8; entry 1-3 and entry 5-7). However there was no change in
reduction time for lower concentration (Table 8; entry 4 and 8). As the concentration of 4-
NP increases absorbance for reduced product 4-AP increases as shown in Fig. 10a and 10b at
35oC and 45oC respectively. Fig. 10c and 10d shows bar diagram of time required for
reduction of 4-NP against concentration of 4-NP at 35oC and 45oC respectively
22

Journal Pre-proof
Table 8: Effect of Temperature on reduction of 4-NP at higher concentration a
Sr No. Temperature (oC) Concentration of
4-NP (mM)
Volume of 1 M
NaBH4 (mL)
6
Time (min)
1.
2.
3.
4.
5.
6.
7.
8.
100
11
9
50
4
3
1
6
4
3
1
35
45
25
6
10
6
100
50
6
7
25
4
10
6
a
Reaction conditions: 4.0 mL 4-NP, 10 mg AgNC, experiment performed in round bottom
flask.
Fig. 10: Absorption spectra for reduction of 4-NP with different concentration at (a) 35 oC (b)
45 oC and bar diagram of reduction time Vs Concentration at (c) 35 oC (d) 45 oC
The effect of AgNC quantity on reduction of 4-NP at different concentration was also
determined and the results are as shown in table 8. The time required for reduction of 4-NP
was lesser as the amount of AgNC increases (Table 9; entry 1-4). The reason for the
23

Journal Pre-proof
decrease in reaction time can be attributed to the higher interaction of the catalyst with the
reactants.
Table 9: Effect of catalyst quantity on reduction of 4-NP a
Sr No. Amount of AgNC
(mg)
Time (min)
1.
2.
3.
4.
10
6
9
7
5
9
3
12
a
Reaction conditions: 4.0 mL 4-NP (25 mM), 3 mL NaBH4 (1 M), room temperature (35
oC), experiment performed in round bottom flask.
3.18 Recycling study of AgNC catalyst
Considering the practical applicability of AgNC for the reduction reaction it is important to
assess the reusability of catalyst. The catalyst has inherent magnetism and hence can be easily
separated using an external magnet. It was retrieved after each run and washed with water
followed by acetone to ensure removal of unwanted remnants from the surface and dried at
o
60 C in vacuum oven. It was observed that the catalyst can efficiently reduce 4-NP up to 5
catalytic cycles without significant loss in its activity. The bar diagram for reduction time
taken for each cycle is shown in Fig. S5.
Catalyst poisoning causes a decrease in the catalytic activity after the 5th cycle. In the process
of relaying the electrons from the donor BH4¯ ions to the acceptor 4-nitrophenolate
molecules there is a gradual formation of an oxide layer on the surface of catalyst. The
subsequent co-adsorption of BH4¯ ions and 4-NP ions on the surface of catalyst causes an
adverse effect on the catalytic activity of AgNC. The HR-TEM images of the recycled
material shows agglomeration of the nanoparticles which is due to residual magnetism. (Fig.
S5b & S5c). The SAED pattern of the recovered AgNC shows a disruption in the lattice
fringes (Fig. S5e). FTIR spectra and X-ray diffractogram show no significant changes in
characteristic bands or peaks (Fig. S6a and S6b). However there is a decrease in the intensity
of the peaks which is attributed to the presence of adsorbed reactants on the surface of the
catalyst. The deposition of reactants and decrease in intensity peak was further confirmed by
Raman spectra as shown in Fig. S6c.
3.19 Proposed mechanism for reduction of nitroaromatics
24

Journal Pre-proof
Though the reduction of nitroaromatics has been frequently used to examine catalytic activity
of metal nanoparticles using sodium borohydride, the mechanism of these reactions is not
very clear yet. There could be several possible pathways for reduction of nitroaromatics by
NaBH4 in the presence of metal catalyst. The reactant and surface hydrogen species derived
from borohydride are first adsorbed onto the surface of the catalyst, a process that is rapid,
reversible, and described in terms of Langmuir isotherms as reported by Menumerov E et al
[57]. Representative mechanism for reduction of nitroaromatics is shown in Fig. 11.
Modification of Fe3O4 nanoparticle by CD-MA polymer provides a platform for host guest
interactions. Introducing b-CD molecules to the system may increase the diffusion coefficient
of reactants, and subsequently increase the reaction rate. Introducing b-CD molecules to the
system provides a supramolecular host system to facilitate the entrapment of substrate
molecules and increase their diffusion coefficient subsequently accelerating the reaction rate
which results in rapid formation of products in typically less than 15 minutes [12,58]
Hydroxyl group of b-CD forms inclusion complex with reactants via hydrogen bonding
which is reversible. It means that the guest molecule can leave the hydrophobic inner cavity
of b-CD after completion of the reaction [59]
Figure 11: Proposed mechanism for reduction of nitroaromatics using AgNC.
3.20 Catalytic reduction of Methylene Blue (MB) dye by AgNC
25

Journal Pre-proof
Organic dyes are the main contaminants along with nitroarenes in water. It is of great
importance to find a catalyst capable of catalytically reduce the organic dyes. Methylene blue
was selected to study the catalytic reduction ability of AgNC as catalyst. Methylene blue has
the ability to model the chemical behaviour of most of the dyes used in textile industries
which act as main contaminants in industrial effluent water [60]. For this study, 2.5 mL of
MB solution (0.045 mM) was treated with 10 mg AgNC in presence of 0.5 mL NaBH4 (4.5
mM). The progress of reduction was monitored by change in UV-vis absorption maximum at
λmax= 664 nm. As the reduction progressed, characteristic peak at 664 nm decreases and
nearly disappears within 12 minutes as shown in Fig. 12a. This indicates that AgNC could
catalyse the reduction reaction of –C=N– groups and destroy the large conjugated structure of
MB Fig. 12b. shows ln(Ct/C0) has good linear relationship with time (t) which indicates that
reaction follows pseudo first order kinetics. The rate constant (k) 0.1587 min-1 was obtained
from slope of ln(Ct/C0) versus time (t). Moreover the presence of silver nanoparticles in
AgNC assists the transfer of electrons once MB and borohydride ions are adsorbed on its
surface [61]. Control experiments were performed to prove that the disappearance of colour is
due to reduction and not merely due to adsorption (Fig. S7 and S8).
Figure 12: (a) Time dependent UV-vis absorption spectra and (b) plot of ln(Ct/C0) versus
time for reduction of MB, (c) Photographic representation for the reductive degradation of a
model mixture of 4-NP and MB.
26

Journal Pre-proof
The practical applicability of AgNC for reduction nitro compounds and dyes present in waste
water was evaluated. For this waste water sample was mimicked by mixing 5 mL 0.05 mM
MB and 5 mL 0.359 mM 4-NP to resemble wastewater. The resulting mixture develops a
green colour as shown in Fig. 12c. To this mixture 10 mg of AgNC and 100 mg of NaBH4
were added and kept for stirring. It was observed that after 7 minutes green colour of mixture
completely disappears. This concludes that AgNC can degrade dyes simultaneously with
nitroaromatics and hence can be potential catalytic systems for environmental wastewater
remediation. [51]
4. Conclusion
Silver nanocomposite was synthesized by surface modification of magnetic nanoparticles
with cyclodextrin maleic anhydride copolymer by means of urethane linkages. The chemical
and structural properties of the prepared silver nanoconjugates (AgNC) were characterized by
the FT-IR, BET, HR-TEM, XPS, XRD, VSM and Raman Spectroscopy. The catalytic
activity of AgNC was studied in detail by using 4-NP reduction as a model. When AgNC was
used as a catalyst, the Ea of 4-NP reduction was calculated as low as 18 KJ mol-1, and TOF
was 19.5 min-1.These results of AgNC are superior to most silver nanoparticle based catalyst.
Interestingly AgNC exhibits excellent catalytic activity for reduction of several nitroaromatic
compounds as well as methylene blue dye at ambient temperature. Further, AgNC being
magnetically separable can be recycled up to 5 cycles without losing catalytic activity.
Further work is under way for establishing the selectivity of the catalyst and improving
reduction performance. The scope can be extended to reduction of organic pollutants from
industrial waste water.
5. Conflicts of interest
There are no conflicts to declare.
6. Acknowledgement
The authors acknowledge DST-FIST for NMR and BET facilities. The authors are grateful to
DST-PURSE, Department of Physics, The Maharaja Sayajirao University of Baroda for FT-
Raman facility. The authors acknowledge SAIF, IIT-Chennai, for providing VSM facility;
SAIF, IIT-Mumbai, for HR-TEM facility; UGC-DAE CSR Indore for XRD measurements,
Institute Instrumentation Centre, IIT-Roorkee, for XPS analysis and Department of
Metallurgical and Materials engineering for EDX analysis respectively. PN is thankful to
Gujarat State Biotechnology Mission (GSBTM), Gandhinagar and MD is thankful to Solvay
27

Journal Pre-proof
Research Project sponsored by Solvay Research and Innovation Centre, Savli, Vadodara for
providing financial assistance.
28

Journal Pre-proof
References
[1] K. Sravanthi, D. Ayodhya, P.Y. Swamy, Green synthesis, characterization and
catalytic activity of 4-nitrophenol reduction and formation of benzimidazoles using
bentonite supported zero valent iron nanoparticles, Mater. Sci. Energy Technol. 2
(2019) 298–307. doi:10.1016/j.mset.2019.02.003.
[2] G. Gahlawat, A.R. Choudhury, A review on the biosynthesis of metal and metal salt
nanoparticles
by
microbes,
RSC
Adv.
9
(2019)
12944–12967.
doi:10.1039/c8ra10483b.
[3] C. Wen, A. Yin, W.L. Dai, Recent advances in silver-based heterogeneous catalysts
for green chemistry processes, Appl. Catal. B Environ. 160–161 (2014) 730–741.
doi:10.1016/j.apcatb.2014.06.016.
[4] X.Y. Dong, Z.W. Gao, K.F. Yang, W.Q. Zhang, L.W. Xu, Nanosilver as a new
generation of silver catalysts in organic transformations for efficient synthesis of fine
chemicals, Catal. Sci. Technol. 5 (2015) 2554–2574. doi:10.1039/c5cy00285k.
[5] D. Maiti, U. Manju, S. Velaga, P.S. Devi, Phase evolution and growth of iron oxide
nanoparticles: Effect of hydrazine addition during sonication, Cryst. Growth Des. 13
(2013) 3637–3644. doi:10.1021/cg400627c.
[6] M. Kaushik, A. Moores, Review: Nanocelluloses as versatile supports for metal
nanoparticles and their applications in catalysis, Green Chem. 18 (2016) 622–637.
doi:10.1039/c5gc02500a.
[7] L.M. Rossi, N.J.S. Costa, F.P. Silva, R. Wojcieszak, Magnetic nanomaterials in
catalysis: Advanced catalysts for magnetic separation and beyond, Green Chem. 16
(2014) 2906–2933. doi:10.1039/c4gc00164h.
[8] S. Saire-Saire, E.C.M. Barbosa, D. Garcia, L.H. Andrade, S. Garcia-Segura, P.H.C.
Camargo, H. Alarcon, Green synthesis of Au decorated CoFe 2 O 4 nanoparticles for
catalytic reduction of 4-nitrophenol and dimethylphenylsilane oxidation , RSC Adv. 9
(2019) 22116–22123. doi:10.1039/c9ra04222a.
[9] Q. Zhang, X. Yang, J. Guan, Applications of Magnetic Nanomaterials in
Heterogeneous
Catalysis,
ACS
Appl.
Nano
Mater.
(2019).
doi:10.1021/acsanm.9b00976.
[10] P.S. Rathore, R. Patidar, T. Shripathi, S. Thakore, Magnetically separable core-shell
iron oxide@nickel nanoparticles as high-performance recyclable catalysts for
chemoselective reduction of nitroaromatics, Catal. Sci. Technol. 5 (2015) 286–295.
doi:10.1039/c4cy00673a.
[11] S.R. Thawarkar, B. Thombare, B.S. Munde, N.D. Khupse, Kinetic investigation for the
catalytic reduction of nitrophenol using ionic liquid stabilized gold nanoparticles, RSC
Adv. 8 (2018) 38384–38390. doi:10.1039/c8ra07404f.
[12] M. Russo, A. Spinella, A. Di Vincenzo, G. Lazzara, M.R. Correro, P. Shahgaldian, P.
Lo Meo, E. Caponetti, Synergistic Activity of Silver Nanoparticles and
Polyaminocyclodextrins in Nanosponge Architectures, ChemistrySelect. 4 (2019) 873–
879. doi:10.1002/slct.201803424.
[13] H. Veisi, S. Kazemi, P. Mohammadi, P. Safarimehr, S. Hemmati, Catalytic reduction
29