Bio-synthesis and structural characterization of highly stable silver nanoparticles decorated on a sustainable bio-composite for catalytic reduction of nitroarenes

Article abstract of DOI:10.1016/j.molstruc.2019.01.057

Bio-polymers are the most significant natural alternative stabilizers compared to their synthetic counterparts for fabrication of noble metal nanoparticles because of their higher thermal stability, renewability, low cost, eco-friendliness, strong mechanical capacity, and biodegradability properties. Therefore, a new bio-composite (CMC-Pct-AG), which is consisted of sodium carboxymethyl cellulose, agar, and pectin natural biopolymers, was fabricated as an immobilizing agent in this study. Then, highly stable silver nanoparticles (Ag NPs@CMC-AG-Pct) were successfully decorated on the surface of designed CMC-Pct-AG without the use of any hazardous reducing agents, and their chemical structures were illuminated with Uv–Vis, FT-IR, TG/DTG, SEM, EDS, XRD, and ICP-OES analyses. Subsequently, the catalytic performance of Ag NPs@CMC-AG-Pct was studied in the reduction of various nitroarenes in the presence of NaBH₄ at room temperature. These tests indicate that Ag NPs@CMC-AG-Pct is an efficient catalyst which converts nitroarenes to desired amines with good yields and short reaction times. Reproducibility of the catalyst was also investigated, and it is found that Ag NPs@CMC-AG-Pct served several times as a retrievable and reusable catalyst for catalytic reduction of nitroarenes.

Full text of DOI:10.1016/j.molstruc.2019.01.057

Accepted Manuscript
Bio-synthesis and structural characterization of highly stable silver nanoparticles
decorated on a sustainable bio-composite for catalytic reduction of nitroarenes
Talat Baran
PII:
S0022-2860(19)30067-5
10.1016/j.molstruc.2019.01.057
MOLSTR 26108
DOI:
Reference:
To appear in:
Journal of Molecular Structure
Received Date:
Accepted Date:
07 November 2018
16 January 2019
Please cite this article as: Talat Baran, Bio-synthesis and structural characterization of highly stable
silver nanoparticles decorated on a sustainable bio-composite for catalytic reduction of nitroarenes,
Journal of Molecular Structure (2019), doi: 10.1016/j.molstruc.2019.01.057
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Bio-synthesis and structural characterization of highly stable silver nanoparticles
decorated on a sustainable bio-composite for catalytic reduction of nitroarenes
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Talat Baran
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Aksaray University, Faculty of Science and Letters, Department of Chemistry, 68100
Aksaray, Turkey
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Corresponding author:
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Address: Department of Chemistry, Faculty of Science and Letters, Aksaray University,
Aksaray, Turkey, 68100
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Tel.: +90 382 2882162; fax: +90 382 2882299.
E-mail address: talatbaran_66@hotmail.com
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Abstract
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Bio-polymers are the most significant natural alternative stabilizers compared to their
synthetic counterparts for fabrication of noble metal nanoparticles because of their higher
thermal stability, renewability, low cost, eco-friendliness, strong mechanical capacity, and
biodegradability properties. Therefore, a new bio-composite (CMC-Pct-AG), which is
consisted of sodium carboxymethyl cellulose, agar, and pectin natural biopolymers, was
fabricated as an immobilizing agent in this study. Then, highly stable silver nanoparticles (Ag
NPs@CMC-AG-Pct) were successfully decorated on the surface of designed CMC-Pct-AG
without the use of any hazardous reducing agents, and their chemical structures were
illuminated with Uv-Vis, FT-IR, TG/DTG, SEM, EDS, XRD, and ICP-OES analyses.
Subsequently, the catalytic performance of Ag NPs@CMC-AG-Pct was studied in the
reduction of various nitroarenes in the presence of NaBH4 at room temperature. These tests
indicate that Ag NPs@CMC-AG-Pct is an efficient catalyst which converts nitroarenes to
desired amines with good yields and short reaction times. Reproducibility of the catalyst was
also investigated, and it is found that Ag NPs@CMC-AG-Pct served several times as a
retrievable and reusable catalyst for catalytic reduction of nitroarenes.
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Keywords: silver nanoparticles; bio-polymer; composite; nitroarenes; reduction
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1. Introduction
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Aromatic amines are significant intermediates for synthesis of important chemical
products such as agriculture substances, photographic and rubber chemicals, polyurethanes,
pharmaceuticals, azo dyes, polymers, antioxidant compounds, and antiseptic agents [1-4].
Synthesis of aromatic amines have been performed under catalytic and non-catalytic methods
by using different reducing agents in the literature [5, 6]. Among these methods, selective
catalytic reduction of nitroarenes via noble metal nanoparticles (NPs) is one of the most
important, new, and rapid method [7]. There are different noble metal nanoparticles in
literature such as Rh [8], Pt [9], and Au[10], and they have been tested in reduction of
nitroarenes. Among these nanoparticles, silver nanoparticles (Ag NPs) have been preferred in
the catalytic reduction reactions due to their lower price, good conductivity, high stability, and
catalytic activity. [11-13]. However, easy oxidation and aggregation problems of Ag NPs are
inevitable after synthesis, and these drawbacks give rise to weak durability and catalytic
performance of Ag NPs [11, 12]. To overcome these problems, researchers have immobilized
nanoparticles on different inorganic materials such as zeolites [14], graphene [15], magnetic-
materials [7], activated carbon [16], and bentonite [17]. The stabilization of Ag NPs on
suitable stabilizers presents some advantages such as (i) increases in nanoparticle stability,
reactivity, reusability and selectivity, (ii) decreased agglomeration, and (iii) easy separation
and recovery from reaction media. [18].
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Biopolymers are ideal stabilizers for synthesis of metallic nanoparticles due to their
low cost, high metal interaction, and green nature [19, 20] . Additionally, bio-polymers have
been widely utilized in agriculture, biotechnology, textile, industrial, and medicinal areas due
to their unique physico-chemical and biological properties [21-24]. The most important
members of the polysaccharide family are carboxymethyl cellulose, agar, and pectin because
of their solubility in water. Additionally, these versatile biopolymers have free groups on their
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polymer chain such as carboxyl and hydroxyl functional groups. These biopolymers may
easily form complexes with different transition metal ions thanks to these functional groups
[25-27]. Then metal nanoparticles can be prepared without using any reducing agents such as
NaBH4 and hydrazine [28-30]. Therefore, these eco-friendly polymers or their composites
can be designed as support materials for synthesis of metal nanoparticle in catalytic systems.
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In this study, a novel bio-composite (CMC-Pct-AG), which is obtained from eco-
friendly sodium carboxymethyl cellulose, agar, and pectin biopolymers, was prepared as a
stabilizer for synthesis of Ag NPs. Then, Ag NPs were successfully stabilized on the designed
bio-composite without using any toxic reducing agents. Subsequently, the catalytic
performance of Ag NPs@CMC-AG-Pct was tested in catalytic reductions of different
substituted nitroarenes with NaBH4 at room temperature, and excellent reaction yields were
recorded. Additionally, Ag NPs@CMC-AG-Pct indicated superior reusability by giving a
yield of 75% even after eight successive runs. These results reveal that Ag NPs@CMC-AG-
Pct is a highly retrievable and efficient catalyst for reduction of nitroarenes.
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2. Materials and methods
2.1 Materials
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Agar, pectin, sodium carboxymethyl cellulose, PdCl2, AgNO3, NaBH4, nitroarenes,
and ethanol were obtained from Sigma-Aldrich.
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2.2 Instrumentation
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FT-IR spectra of CMC-AG-Pct and Ag NPs@CMC-AG-Pct were recorded with a
Perkin Elmer Spectrum 100 FT-IR spectrophotometer. The reduction of nitroarenes and
fabrication of silver nanoparticles was monitored with UV–vis spectroscopy (Genesys 10S
UV-VIS Spectrophotometer). X-ray pattern of Ag NPs@CMC-AG-Pct was recorded by
Rigaku smart lab system (at 40 kV, 30 mA, and 2θ with a scan angle: 10–75°). The surface
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images of CMC-AG-Pct and Ag NPs@CMC-AG-Pct and EDS mapping of Ag NPs@CMC-
AG-Pct were obtained using Zeiss Supra 55VP. Silver ion content of Ag NPs@CMC-AG-Pct
was detected with Perkin Elmer Optima 2100 DV Inductively Coupled Plasma (ICP) Optical
Emission Spectrometer (OES). Thermal stabilities of CMC-AG-Pct and Ag NPs@CMC-AG-
Pct were investigated with EXSTAR S11 7300 (nitrogen atmosphere; heating range: 30–650
°C).
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2.3 Green synthesis of Ag NPs@CMC-AG-Pct
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1 g of CMC was added to 300 mL of water and stirred at 70 °C until it was dissolved.
Then, 1 g of agar was transferred into reaction media and stirred for 4 h to ensure
homogenous mixture. Subsequently, 1 g of pectin was also added in reaction mixture, and the
resulting mixture was stirred overnight to obtain the bio-composite. After this process,
AgNO3 (50 mL, 0.08 M) was added in the reaction media and stirred at 80°C for 2 h to
provide complete reduction of silver ion (Ag+) to silver (Ag0). It was observed that, with the
addition of AgNO3 to the reaction medium, yellow color of the reaction mixture gradually
converted to black due to excitation of surface plasmon resonance [28]. This change of color
showed that formation of silver nanoparticle on bio-composites were successfully performed.
Finally, the obtained silver nanoparticles were separated by centrifugation from reaction
media and then they were dried at 50°C.
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…………………………………..Scheme 1……………………………….
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2.4 General method for reduction of nitroarenes
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The mixture of nitroarenes (1.0 mmol), freshly prepared NaBH4 (5.0 mmol), and 10
mL of EtOH/H2O (v/v = 1:1) were added in Schlenk tube and stirred for 2 min at room
temperature. Then, Ag NPs@CMC-AG-Pct (1.5 mmol) was added in reaction media and
stirred for designated time. The catalytic reduction studies were monitored using UV-Vis
spectroscopy. After the catalytic reductions, Ag NPs@CMC-AG-Pct was separated by
centrifugation from reaction media, and the obtained filtrate was extracted with ethyl acetate
for three times and then dried over anhydrous magnesium sulfate. Finally, organic solvent was
evaporated to obtain desired reduced products.
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3. Results and discussion
3.1 Characterization of Ag NPs@CMC-AG-Pct
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UV-Vis analysis of Ag NPs@CMC-AG-Pct was examined to confirm the formation of
silver nanoparticles, and its spectrum is given in Fig. 1. It was reported that Ag NPs have
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characteristic surface plasmon resonance (SPR) peak near 400–450 nm in UV-Vis spectrum,
confirming the formation of spherical nanoparticles [31, 32]. In this study, UV-Vis spectrum
of CMC-AG-Pct was examined, and no bands were observed between 400-500 nm. However,
UV–Vis spectrum of Ag NPs@CMC-AG-Pct was observed to have a SPR band at 410 nm
(Fig. 1). This band confirmed the formation of silver nanoparticles on CMC-AG-Pct and their
spherical shapes. Additionally, Ag NPs@CMC-AG-Pct was examined with UV-Vis analysis
after one month, and no clear change in the shape and position of SPR peak was observed.
This analysis indicates that Ag NPs@CMC-AG-Pct is considerably stable.
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…………………………………..Figure 1……………………………….
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FT-IR analysis was performed to indicate successful preparation of CMC-AG-Pct and
Ag NPs@CMC-AG-Pct (Fig. 2). FT-IR spectrum of CMC-AG-Pct indicated some important
bands at 3319, 2919, 1739, 1599, 1371, and 1039 cm-1 which correspond to stretching of -OH,
-CH, and -COO groups, asymmetric stretching of –COO, vibration mode of sulfate groups
and stretching of C-O-C glycosidic bond, respectively [33, 34]. As expected, the prepared Ag
NPs@CMC-AG-Pct showed peaks similar to those of its composite. Additionally, it was
observed that the characteristic peaks of Ag NPs@CMC-AG-Pct shifted to lower
wavenumbers due to the interaction between Ag with CMC-AG-Pct.
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…………………………………..Figure 2……………………………….
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In order to determine their surface morphological properties, CMC-AG-Pct and Ag
NPs@CMC-AG-Pct were analyzed with SEM, and their images are provided in Fig 3. As
seen in Fig. 3, CMC-AG-Pct has a film like structure. On the other hand, SEM images of Ag
NPs@CMC-AG-Pct was recorded at different magnifications to confirm the formation of
silver nanoparticles (Fig. 3b, c). As seen in SEM images of Ag NPs@CMC-AG-Pct, Ag NPs
were spherically immobilized on the surface of CMC-AG-Pct composite. Additionally, Ag
NPs were homogeneously dispersed on CMC-AG-Pct, and no aggregation was observed in
the images. Additionally, the particle sizes of the prepared spherical Ag NPs were determined
between the range of 44-57 nm.
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…………………………………..Figure 3……………………………….
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EDS mapping analysis of Ag NPs@CMC-AG-Pct was also carried out to verify the
formation of Ag NPs on CMC-AG-Pct (Fig. 4). As clearly seen from Fig. 4, Ag peak beside
C, N, and O are all present in the images and spectrum. These findings showed that Ag NPs
were successfully stabilized on the designed composite. Silver ion content of Ag NPs@CMC-
AG-Pct was also determined to be 14.12 wt% with EDS elemental analysis and 14.09 wt%
with ICP-OES. These values which are very close to each other showed that silver
nanoparticles were homogeneously distributed on the designed stabilizer.
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The formation of Ag NPs was investigated with X-ray diffraction analysis, and the
obtained pattern is given in Fig. 5. XRD pattern of Ag NPs@CMC-AG-Pct gave strong and
sharp peaks at 38.32°, 46.37°, 64.65°, and 77.26°. These peaks correspond to (111), (200),
(220), (311), and (222) planes of metallic silver particles and demonstrate that Ag NPs were
immobilized on the surface CMC-AG-Pct [35, 36].
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The thermal behavior of the prepared materials was explored with TG/DTG analysis,
and their spectra are displayed in Fig. 6. The results showed that CMC-AG-Pct had good
thermal resistance up to 223.4°C. On the other hand, the thermogram of Ag NPs@CMC-AG-
Pct indicates that it has more thermal stability than its composite (229.5°C). This result shows
that silver nanoparticles contributed to the thermal durability of Ag NPs@CMC-AG-Pct. In
addition, it was determined that Ag NPs@CMC-AG-Pct gave higher solid residue content
than CMC-AG-Pct. This residual content can be explained by metal-oxide residue (Ag-O),
confirming the production of Ag NPs on CMC-AG-Pct.
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…………………………………..Figure 6……………………………….
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3.2 Catalytic performance of Ag NPs@CMC-AG-Pct in catalytic reduction of nitroarenes
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Before the evaluation of the catalytic performance of Ag NPs@CMC-AG-Pct against
the reduction of various nitroarenes using NaBH4, reduction of 2-nitroaniline (2-NA) was
determined as the model reaction, and then different preliminary experiments were carried out
to find out optimum reaction condition (mole ratio of NaBH4, mole percentage of Ag
NPs@CMC-AG-Pct, selection of solvent). It is known that 2-NA has two characteristic
absorption peaks at 283 and 410 nm in UV-Vis spectrum [37, 38]. After the catalytic
reduction of 2-NA in the presence of the catalyst by using NaBH4, the peak at 410 nm
disappears and the band at 283 nm shifts to a higher wavelength, indicating the reduction of 2-
NA to phenylenediamine (2-PDA) [39-41]. Another proof of the complete reduction of 2-NA
is that the yellow color of the reaction solution becomes colorless with the completion of the
reduction process. In this study, after a series of experiments, the best reaction yield was
obtained with 1 mmol 2-NA, 5 mmol NaBH4, 1.5 mmol Ag NPs@CMC-AG-Pct, under 10
mL of ethanol:water mixture (v/v, 1:1) and at room temperature. In 5 min, the conversion
yield for the reduction of 2-NA to 2-PDA in the presence of Ag NPs@CMC-AG-Pct under
optimal reaction conditions was recorded to be 99%. As seen in Fig. 7, the band at 410 nm
vanished, and the band at 283 nm shifted to 290 nm. The color change was observed in
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parallel to the disappearance of the band at 410 nm. These observations showed that the
reduction of 2-NA was successfully performed in the presence of Ag NPs@CMC-AG-Pct.
Additionally, to show the catalytic performance of designed silver nanoparticles, reduction of
2-NA was performed using NaBH4 without any addition of Ag NPs@CMC-AG-Pct, but the
reaction did not occur. This test clearly reveals that Ag NPs@CMC-AG-Pct has critical
importance for catalytic reduction of nitroarenes.
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The catalytic efficiency of Ag NPs@CMC-AG-Pct was also investigated in reduction
of various nitroarenes under the determined optimal reaction conditions using NaBH4 as the
reductant, and the obtained results are given in Table 1. As seen in Table 1, Ag NPs@CMC-
AG-Pct gave excellent reaction yields with very short reaction time against the reduction of
nitroarenes. For example, Ag NPs@CMC-AG-Pct converted 4-nitrophenol to the
corresponding aniline in 6 min with high reaction yield (96%). The catalyst provided the
reduction of nitrobenzene to aniline with excellent yield (95%). Ag NPs@CMC-AG-Pct
produced good reaction yields against the reduction of Br, Cl, and I substituted nitroarenes
(entry 4-7). Consequently, Ag NPs@CMC-AG-Pct successfully converted various substituted
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nitroarenes to the desired amines with good yields at very short reaction time thanks to the
powerful interaction between designed bio-composite and silver ions.
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Table 1. Catalytic reduction of nitroarenes in the presence of Ag NPs@CMC-AG-Pct
Entry Substrate
Product
Reaction time (min)
Yield (%)
NH2
NO2
1
5
99
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220
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222
223
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NH2
NH2
NH2
NH2
NO2
NO2
2
3
8
6
95
96
OH
Br
OH
Br
4
5
15
15
90
93
NH2
NO2
Br
Br
NH2
I
NO2
I
6
7
25
30
94
89
NH2
NO2
Cl
Cl
NO2
NH2
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Reaction conditions: 1.0 mmol nitroarenes, 5.0 mmol NaBH4, 1.5 mmol Ag NPs@CMC-AG-Pct and 10 mL of
ethanol:water (v/v, 1:1) at room temperature
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Additionally, the reproducibility of Ag NPs@CMC-AG-Pct was also studied against
the reduction of 2-NA under the optimal reaction conditions. After the catalytic reduction,
used Ag NPs@CMC-AG-Pct was separated by centrifugation, washed with ethanol and then
dried for use in next runs. This regeneration process was repeated at the end of each catalytic
reduction. The reusability tests indicated that Ag NPs@CMC-AG-Pct is a retrievable catalyst,
and it produces a high reaction yield (75%) after even eight successive runs. Additionally, it
was found that SEM micrograms of Ag NPs@CMC-AG-Pct after the eight successive runs
showed similarity to that of the fresh catalyst in terms of the surface morphology, shape, and
size of particles (Fig. 9). These results reveal that Ag NPs@CMC-AG-Pct retains its chemical
structure after successive runs and is considerably stable for catalytic reduction reactions.
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…………………………………..Figure 9……………………………….
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4. Conclusions
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In conclusion, highly stable silver nanoparticles were embedded on the designed bio-
composite using a green and simple procedure. Then structural characterization of Ag
NPs@CMC-AG-Pct was performed by FT-IR, Uv-Vis, TG/DTG, XRD, SEM, EDS and ICP-
OES analysis. It was found that silver nanoparticles uniformly dispersed on CMC-AG-Pct,
and the average diameter of the particles were in the range of 44–57 nm. Then, Ag
NPs@CMC-AG-Pct was successfully applied to the catalytic reduction of various nitroarenes,
and the catalyst showed excellent catalytic performance with short reaction time. Moreover,
the reusability of Ag NPs@CMC-AG-Pct was tested, and the catalyst recycled eight times
with no important loss of its catalytic performance. The obtained results show that the
developed Ag NPs@CMC-AG-Pct is suitable as a catalyst for the reduction of nitroarenes,
and it can be utilized against reduction of different nitro compounds or organic dyes due to its
efficient catalytic activity.
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Figure and Scheme Captions
Fig. 1 UV-Vis spectrum of Ag NPs@CMC-AG-Pct
Fig. 2 FT-IR spectra of (a) CMC-AG-Pct and (b) Ag NPs@CMC-AG-Pct
Fig. 3 SEM images of (a) CMC-AG-Pct and (b, c) Ag NPs@CMC-AG-Pct
Fig. 4 EDS mapping and EDS spectrum of Ag NPs@CMC-AG-Pct
Fig. 5 XRD pattern of Ag NPs@CMC-AG-Pct
Fig. 6 TG/DTG spectra of (a) CMC-AG-Pct and (b) Ag NPs@CMC-AG-Pct
Fig. 7 UV-Vis spectrum of reduction of 2-NA
Fig.8 Recycling performance of Ag NPs@CMC-AG-Pct for the reduction of 2-nitroaniline
Fig. 9. SEM analysis of Ag NPs@CMC-AG-Pct after the 8th cycle.
Scheme 1. Preparation of Ag NPs@CMC-AG-Pct
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Highlights
Highly stable silver nanoparticles were successfully immobilized on the bio-composite
Ag NPs@CMC-AG-Pct showed good catalytic performance against reduction of various
nitroarenes
Silver nanoparticles were homogeneously dispersed and their particle sizes were determined
between the range of 44-57 nm.
Ag NPs@CMC-AG-Pct was successfully reused in eight successive runs