Green synthesis of the Ag/HZSM-5 nanocomposite by using Euphorbia heterophylla leaf extract: A recoverable catalyst for reduction of organic dyes

Article abstract of DOI:10.1016/j.jallcom.2016.05.278

During this paper, the Ag/HZSM-5 nanocomposite has been successfully synthesized by using an aqueous extract of Euphorbia heterophylla leaves as a stabilizing and reducing agent. The green synthesized Ag/HZSM-5 nanocomposite was characterized by FT-IR (Fourier transform infrared spectroscopy), FESEM (field emission scanning electron microscopy), EDS (energy dispersion X-ray spectroscopy), UV-vis, XRD (X-ray powder diffraction) and elemental mapping. The Ag/HZSM-5 nanocomposite was found to be efficient nanocatalyst for the reduction of organic dyes such as Methylene blue (MB), Congo red (CR), Rhodamine B (RhB) and 4-nitrophenol (4-NP) in water at room temperature. The catalytic activities of the nanocatalyst in reactions were monitored by using UV-vis spectroscopy. Interestingly, the Ag/HZSM-5 catalyst can be easily recovered and reused several times without any significant loss of catalytic efficiency.

Full text of DOI:10.1016/j.jallcom.2016.05.278

Accepted Manuscript
Green synthesis of the Ag/HZSM-5 nanocomposite by using Euphorbia heterophylla
leaf extract: A recoverable catalyst for reduction of organic dyes
Mahmood Tajbakhsh, Heshmatollah Alinezhad, Mahmoud Nasrollahzadeh, Taghi A.
Kamali
PII:
S0925-8388(16)31640-1
10.1016/j.jallcom.2016.05.278
JALCOM 37798
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 21 March 2016
Revised Date: 23 May 2016
Accepted Date: 25 May 2016
Please cite this article as: M. Tajbakhsh, H. Alinezhad, M. Nasrollahzadeh, T.A. Kamali, Green
synthesis of the Ag/HZSM-5 nanocomposite by using Euphorbia heterophylla leaf extract: A recoverable
catalyst for reduction of organic dyes, Journal of Alloys and Compounds (2016), doi: 10.1016/
j.jallcom.2016.05.278.
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ACCEPTED MANUSCRIPT
Green synthesis of the Ag/HZSM-5 nanocomposite by using Euphorbia
heterophylla leaf extract: A recoverable catalyst for reduction of organic dyes
,a
,b
Mahmood Tajbakhsh,* Heshmatollah Alinezhad,a Mahmoud Nasrollahzadeh* ,c and Taghi A.
Kamalia
aDepartment of Organic Chemistry, Faculty of Chemistry, University of Mazandaran, P. O. Box 47416-95447, Babolsar,
Iran
bDepartment of Chemistry, Faculty of science, University of Qom, P. O. Box 37185-359, Qom, Iran
cCenter of Environmental Researches, University of Qom, Qom, Iran
ABSTRACT
During this paper, the Ag/HZSM-5 nanocomposite has been successfully synthesized by using an aqueous
extract of Euphorbia heterophylla leaves as a stabilizing and reducing agent. The green synthesized Ag/HZSM-
5 nanocomposite was characterized by FT-IR (Fourier transform infrared spectroscopy), FESEM (field emission
scanning electron microscopy), EDS (energy dispersion X-ray spectroscopy), UV-vis, XRD (X-ray powder
diffraction) and elemental mapping. The Ag/HZSM-5 nanocomposite was found to be efficient nanocatalyst for
the reduction of organic dyes such as Methylene blue (MB), Congo red (CR), Rhodamine B (RhB) and 4-
nitrophenol (4-NP) in water at room temperature. The catalytic activities of the nanocatalyst in reactions were
monitored by using UV-vis spectroscopy. Interestingly, the Ag/HZSM-5 catalyst can be easily recovered and
reused several times without any significant loss of catalytic efficiency.
Keywords: Ag/HZSM-5 nanocomposite; Euphorbia heterophylla; Reduction; NaBH4; Organic dyes
1. Introduction
*Corresponding author at: Department of Organic Chemistry, Faculty of Chemistry, University of Mazandaran, P. O. Box
47416-95447, Babolsar, Iran (M. Tajbakhsh) and Department of Chemistry, Faculty of science, University of Qom, P. O.
Box 37185-359, Qom, Iran (M. Nasrollahzadeh)
E-mail addresses: tajbaksh@umz.ac.ir (M. Tajbakhsh), mahmoudnasr81@gmail.com (M. Nasrollahzadeh).
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Synthetic pigments and dyes have a wide range of applications in cosmetic, textile, drug, printing, food-
processing industries and their annual production is about ~7 × 105 tons per year [1-3]. The presence of
nitroarene compounds and toxic dyes compounds in aquatic ecosystem is of great concern as these are
chemically and biologically stable; therefore, it is difficult to eliminate them by natural degradation processes
[4,5]. Hence, there is an urgent need for the development of effective treatment protocols for waste effluents of
dye industries.
Although physical and conventional chemical techniques such as adsorption, precipitation and ozonation
have been employed for the decolorization of dye effluents, they possess inherent restrictions such as formation
of hazardous by-products, high cost and intensive energy requirements [6-8].
Nowadays, due to a higher available catalytic surface, catalysts are more and more used in the form of
nanoparticles (NPs). Generally, different synthesis techniques including quick precipitation, sol-gel,
electrochemical, sonochemical, microwave irradiation, solid state reaction and alcohothermal synthesis have
been applied to the preparation of metal nanoparticles (MNPs) [9-17]. Although, most of these routes are
effective but many of them suffer from some disadvantages such as high temperature, harsh reaction conditions,
the use of expensive reagents, long reaction times, toxic and hazardous capping agents or stabilizers to control
the size and composition of the NPs, the environmental pollution caused by use of organic solvents and low
yields of the products.
In addition, separation of the nanosized metal catalysts from the reaction mixture by centrifugation or simple
filtration is very difficult. To overcome these limitations, several inorganic materials have been used as support
for immobilization of MNPs [18-20]. Among solid supports, zeolites have been reported as efficient catalysts
and new class of promising supports for the development of environmentally friendly acidic catalysts [21-23].
As a result of its hydrothermal stability, zeolite has recently been garnering interest as a novel support [24]. In
the middle of zeolites, the HZSM-5 zeolite is a medium pore (5.1-5.6 A˚) zeolite with three dimensional
channels defined by 10-membered rings [25]. Due to its thermal stability, high acidity, unique shape selective
behavior, high surface area and structural network HZSM-5 has been shown to be a stable and effective catalyst
for organic reactions [26,27].
Compared to the chemical and physical protocols, environmentally benign production methods of MNPs by
applying various bacteria, plants extract and fungus are very attractive [28-36]. The biosynthetic procedures for
the synthesis of MNPs have several benefits over physical and chemical synthetic routes, such as simplicity,
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cost effectiveness as well as compatibility for biomedical and pharmaceutical applications. So, the use of plants
extract in the preparation of MNPs has environmental and economical advantages.
Quite recently, we reported the green synthesis of an Ag/TiO2 nanocomposite using Euphorbia heterophylla
leaf extract and its application as a recyclable and heterogeneous catalyst for the reduction of organic dyes [36].
Hence, the Ag/HZSM-5 nanocatalyst was fabricated by applying Euphorbia heterophylla leaf extract without
any surfactant or stabilizer. The catalytic activity of the Ag/HZSM-5 nanocomposite was investigated for the
reduction of 4-NP, CR, RhB and MB using NaBH4 in water at room temperature and the reactions were checked
by UV-vis spectroscopy. More significantly, the catalyst can easily be regenerated and recycled several times
without significant loss of catalytic performance. To be an eco-friendly procedure, in spite of its natural
availability, biological relevance’s and non-toxicity, Euphorbia heterophylla has never been explored for the
green synthesis of the Ag/HZSM-5 nanocomposite.
2. Experimental
2.1. Instruments and reagents
High-purity chemical reagents were purchased from the Aldrich and Merck chemical companies. HZSM-5
zeolite was prepared according to the literature [23]. FT-IR spectra were recorded on a Nicolet 370 FT/IR
spectrometer (Thermo Nicolet, USA) using pressed KBr pellets. UV-visible spectral analysis was recorded on a
double‐beam spectrophotometer (Hitachi, U‐2900) to ensure the formation of nanoparticles. Morphology and
particle dispersion was investigated by field emission scanning electron microscopy (FESEM) (TESCAN
MIRA3). The chemical composition of the prepared nanostructures was measured by EDS analysis. X-ray
diffraction (XRD) measurements were carried out using a Philips powder diffractometer type PW 1373
goniometer (Cu Kα = 1.5406 A˚). The scanning rate was 2º/min in the 2θ range from 10 to 80˚.
2.2. Preparation of the Euphorbia heterophylla leaf extract
The Euphorbia heterophylla leaf extract was prepared according to our recent work [36].
2.3. Green synthesis of the Ag/HZSM-5 nanocomposite
In a typical preparation of the Ag/HZSM-5 nanocomposite, 50 mL of aqueous solution of AgNO3 (0.004 M)
was gradually added to 25 mL of the plant extract and 1.0 g of the HZSM-5 zeolite with constant stirring at 80
°C for 2 h. After 20 min the color of the solution changed from white to black due to the excitation of surface
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plasmon resonance indicating the formation of the Ag/HZSM-5 nanocomposite. Next, the colored solution was
centrifuged at 7000 rpm for 30 min to completely precipitation of the Ag/HZSM-5 nanocomposite. The
achieved product was then washed several times with ethanol, then air dried for 24 h at room temperature. Next,
it was kept under inert argon atmosphere for more precession and protection of our research.
2.4. Catalytic reduction of the 4-NP by Ag/HZSM-5 nanocomposite
In a typical run, 25 mL of the 4-NP aqueous solution (2.5 mM) was mixed with 5.0 mg of Ag/HZSM-5
nanocomposite, stirring constantly for 2 min. Then, 25 mL of freshly prepared NaBH4 aqueous solution (0.25
M) was added; the reactants were mixed and allowed to stir at room temperature. The yellow color of the
solution gradually disappeared, indicating the formation of the 4-aminophenol (4-AP). The reaction progress
was checked by UV-vis spectroscopy. Next, the catalyst was simply separated by simple centrifugation and
washed successively with water and ethanol, dried and was used for successive runs.
2.5. Catalytic reduction of the MB by Ag/HZSM-5 nanocomposite
In a typical procedure, 5.0 mg of catalyst and 25 mL of the MB aqueous solution (3.1 × 10-5 M) were mixed in a
beaker. Then, 25 mL of freshly prepared NaBH4 aqueous solution (5.3 × 10-3 M) was added and the mixture was
allowed to stir at room temperature. The progress of the reaction could be checked by the change of the
absorption intensity in UV-vis spectrophotometer. After completion of reaction, the catalyst was simply
collected from the reaction system by centrifugation and washed successively with water and ethanol and dried
for the next run.
2.6. Catalytic reduction of the CR by Ag/HZSM-5 nanocomposite
In a typical experiment, 25 mL of the CR aqueous solution (1.44 × 10-5 M) was added to 5.0 mg of catalyst.
Then, 25 mL of prepared NaBH4 aqueous solution (5.3 × 10-3 M) was added and the mixture was allowed to stir
at room temperature. The progress of the conversion reaction was then checked by recording the time-dependent
UV-vis absorption spectra of the mixture using a spectrophotometer. For the recycling experiment, the catalyst
was simply separated from the reaction system by brief centrifugation and washed successively with water and
ethanol and dried for the next cycle.
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2.7. Reduction of the RhB by Ag/HZSM-5 nanocomposite
Typically, 5.0 mg of catalyst was added to 25 mL of the RhB aqueous solution (2.09 × 10-5 M). Next, 25 mL of
freshly prepared NaBH4 aqueous solution (5.3 × 10-3 M) was added and the mixture was permitted to stir at
room temperature. The progress of the conversion reaction was then monitored by recording the time-dependent
UV-vis absorption spectra of the mixture using a spectrophotometer. At the end of the reaction, the catalyst was
simply separated from the reaction system by brief centrifugation and washed successively with water and
ethanol and dried for the consecutive cycle.
3. Results and discussion
The stable Ag/HZSM-5 nanocomposite was fabricated via a simple and green method. Facile biosynthesis of the
Ag/HZSM-5 nanocomposite using an aqueous Euphorbia heterophylla leaf extract has been considered as an
easy; cost efficient, eco-friendly, non-toxic and efficient protocol for exploiting Euphorbia heterophylla leaf
extract. The existence of flavonoid and other phenolics in the extract could be responsible for the reduction of
the Ag+ ions and formation of the Ag NPs according the following mechanism (Scheme 1).
Scheme 1. Plausible mechanism for the green synthesis of the Ag NPs using Euphorbia heterophylla leaf
extract.
3.1. Characterization of the Ag/HZSM-5 nanocomposite
The structure of synthesized Ag/HZSM-5 nanocomposite was fully characterized by XRD, FE-SEM, EDS, UV-
vis, elemental mapping and FT-IR.
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In a typical synthesis of the Ag/HZSM-5 nanocomposite, for further monitoring, first stable the Ag NPs were
separately synthesized by using a simple biological and low-cost approach via reduction of silver nitrate solution
with the aqueous Euphorbia heterophylla leaf extract as the reducing and stabilizing agent. The characteristics
of the obtained Ag NPs were studied using UV-vis and FT-IR techniques in our recent work [36]. The reduction
of the Ag+ ions into silver particles was monitored by measuring the UV-vis spectrum by diluting a small
aliquot of the sample into distilled water. As the Euphorbia heterophylla leaf extract was added to silver nitrate
solution, the color of the solution changed from faint light to dark brown indicating Ag NPs formation.
Therefore, the progress in conversion reaction of the Ag+ ions to Ag NPs was followed by a color change and
spectroscopic techniques. UV-vis spectrograph of the colloidal solution of silver nanoparticles has been
recorded as a function of time. The absorption spectrum (Figure 1) of the dark brown silver nanoparticles
solution prepared with the proposed method showed a surface plasmon absorption band with a maximum of 440
nm [36], indicating the presence of the Ag NPs.
Figure 1. UV-vis spectrum of prepared Ag NPs using the aqueous Euphorbia heterophylla leaves extract.
Reprinted from ref. 36 with permission. Copyright (2016) Sciencedirect.
The FT-IR spectrum of the Ag/HZSM-5 nanocomposite (Figure 6) showed peaks at 1220-1088 and 800 cm-1,
which were assigned to SiO4 tetrahedral units [37]. Two absorbance bands at 3448 and 1633 cm-1 for the O-H
stretching vibration due to H2O interporous structure and H-O-H bending vibration respectively, were also
observed. The strong and broad absorbance band at 454 cm-1 was assigned to the T-O bending vibration of the
SiO4 and AlO4 internal tetrahedral [38].
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Figure 2. FT-IR spectrum of the Ag/HZSM-5 nanocomposite.
The X-ray diffraction pattern of the Ag/HZSM-5 nanocomposite is shown in Figure 3. The relative
crystallinity of the Ag/HZSM-5 was calculated based on the intensity of the peaks of angle 2θ range (22-25)
[39]. The diffraction peaks at 2θ = 38.5° and 44.47° were indexed as the (111) and (200) planes of face centered
cubic (fcc) metallic silver particles [40].
Figure 3. XRD powder pattern of the Ag/HZSM-5 nanocomposite.
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The morphology of the Ag/HZSM-5 nanocomposite is revealed by field emission scanning electron
microscopy (FESEM). It is clearly observed that the Ag grain pervaded HZSM-5 zeolite surface, which display
a good combination between HZSM-5 zeolite and Ag NPs (Figure 4). This is in agreement with that calculated
from XRD investigation. Also, FESEM analysis of the nanocatalyst displayed spherical morphology of the Ag
NPs on the outer surface of HZSM-5 zeolite.
Figure 4. FESEM images of the Ag/HZSM-5 nanocomposite.
The chemical composition of the Ag/HZSM-5 nanocomposite was determined by Energy Dispersive X-ray
Spectroscopy (EDS). In the EDS spectrum (Figure 5), peaks related to Si, Al, O and Ag are observed. The total
Ag content was found to be 29.64 wt %. EDX mapping was then performed to study the elemental distribution
of the Ag/HZSM-5 catalyst as depicted in (Fig. 6). The elemental mapping images demonstrate that Ag
nanoparticles are highly dispersed on the HZSM-5 zeolite surface in agreement with data achieved by XRD.
SiKα
4000
3500
3000
2500
2000
1500
1000
500
0
O Kα
AgLβ
AgLα
AlKα
keV
0
5
10
Figure 5. EDS spectrum of the Ag/HZSM-5 nanocomposite.
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Figure 6. EDX compositional mapping analysis for Ag/HZSM-5: Elemental maps for (Blue) O, (Green) Al,
(Black) Si and (Red) Ag.
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3.3. Catalytic efficiency and application of the synthesized Ag/HZSM-5 nanocomposite for reduction of the 4-NP
In the present work, the catalytic activity of the Ag/HZSM-5 nanocomposite was tested by the reduction of 4-
NP to 4-AP in the presence of NaBH4 in water at room temperature as a model reaction (Scheme 2).
It is well-known that 4-NP in aqueous solution exhibits a strong absorption peak at 317 nm which is
remarkably red-shifted to 400 nm when treated with a freshly prepared aqueous solution of NaBH4 (Figure 7).
The absorption peak at about 400 nm comes from the formation of dark yellow colored 4-nitrophenolate ions
(Scheme 3) in alkaline conditions upon the addition of NaBH4. It was demonstrated that the reduction reaction
did not proceed in the absence of the Ag/HZSM-5 nanocomposite and the absorption peak at 400 nm stayed
unchanged even for over 100 min. After Ag/HZSM-5 nanocomposite was added into the solution containing 4-
NP and NaBH4, the absorption peak at 400 nm gradually dropped in intensity as the reduction reaction
proceeded and a new absorption peak started to appear as a shoulder at 300 nm corresponding to 4-AP which
indicated the reduction of 4-NP to 4-AP. The absorption peak at 400 nm is fully removed after 320 s induction
period.
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Figure 7. Time-dependent UV-visible absorption spectra for the reduction of the 4-NP using Ag/HZSM-5
nanocomposite in aqueous media at room temperature
The effects of the amount of the NaBH4 and catalyst were determined for reduction reaction. As depicted in
Table 1, no reaction was achieved in the absence of the catalyst (Table 1, entry 1). So, indicating that the
catalytic reduction occurs at the surface of catalyst. The best result was achieved with 100 equivalents of the
NaBH4 and 5.0 mg of catalyst at room temperature (Table 1, entry 5). With an overall look at Table 1, it was
observed that the catalytic performance increased with an increase in the amount of the NaBH4. Moreover, in
the presence of 100 equivalents of the NaBH4 and 5.0 mg of catalyst, complete conversion was obtained within
320 s. To understand the effect of the Ag NPs on the reduction of the 4-NP, reaction was carried out in the
presence of 5.0 mg of the HZSM-5 and 100 equivalents of the NaBH4 (Table 1, entry 2). In the absence of the
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Ag NPs, under the same conditions, the HZSM-5 has a lower rate than the Ag/HZSM-5. The Ag/HZSM-5
shows the fastest rate because of the synergic interaction between Ag NPs and HZSM-5.
Table 1.
Optimization of reaction conditions for reduction of the 4-NP to 4-AP.
Entry
Catalyst (mg)
0
NaBH4 (equivalents)
Time
24 ha
540 s
640 s
440 s
320 s
320 s
1
2
3
4
5
6
50
100
50
75
100
100
HZSM-5 (5.0)
Ag/HZSM-5 (5.0)
Ag/HZSM-5 (5.0)
Ag/HZSM-5 (5.0)
Ag/HZSM-5 (7.0)
a No reaction.
The catalytic reduction of the 4-NP to 4-AP is an electron transfer (ET) process. The reaction was carried out
-
in two steps. In the first step of process, 4-NP and BH4 diffuse from aqueous solution to the surface of catalyst
-
via π-π stacking interactions. Then the hydrogen atoms, which are formed from BH4 , after ET to Ag NPs attack
4-NP molecule. Finally, the corresponding product was desorbed from the surface of the catalyst.
3.4. Catalytic reduction of the RhB, CR and MB
In the present work, we have also studied the catalytic ability of the Ag/HZSM-5 nanocomposite towards the
reduction of the MB, CR and RhB with the maximal absorption peak (λmax) at 663, 493 and 554, respectively
and results are presented in Table 2. As shown in Table 2, a short time and low amount of catalyst were needed
for the reduction of various dyes. The best results were obtained with 5.0 mg of the Ag/HZSM-5 nanocomposite
in the presence of freshly prepared NaBH4 aqueous solution (5.3 × 10-3 M) at room temperature.
Table 2.
Catalytic reduction of the RhB, CR and MB using Ag/HZSM-5 nanocomposite.
Entry
Dye (M)
Catalyst (mg)
Time (s)
100
100
200
94
NaBH4 (M)
5.3 × 10-3
5.3 × 10-3
5.3 × 10-3
5.3 × 10-3
1
2
3
4
CR (1.44 × 10-5)
CR (1.44 × 10-5)
RhB (2.09 × 10-5)
MB (3.1 × 10-5)
5.0
7.0
5.0
5.0
The catalytic reductions of the MB, CR and RhB were monitored by UV-vis absorption spectroscopy of the
reaction mixture after the addition of the NaBH4 and Ag/HZSM-5 nanocomposite. As can be seen in Figure 8,
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the absorption intensity at the absorption peak of MB, CR and RhB decreased as the reduction reaction
proceeded. It was observed that degradation of dyes occurred within 94-200 s, depending upon the dye,
indicating the Ag/HZSM-5 nanocomposite was able to reduce MB, CR and RhB.
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Figure 8. Time dependent UV-vis spectral changes of the reaction mixture of (a) MB, (b) CR, (c) RhB and
NaBH4 catalyzed by Ag/HZSM-5 NPs.
3.5. Catalyst recyclability
Recyclability and stability of the nanocatalyst for the reduction of various dyes via NaBH4 was tested. The
Ag/HZSM-5 nanocomposite can be easily separated from the reaction mixture by mild centrifugation and
washed with water and ethanol several times for the successive runs. The regenerated catalyst was recycled five
times for 100% reduction of the 4-NP. This recyclability demonstrates the turnover and high stability of catalyst
under operating conditions. The leaching of metal from the Ag/HZSM-5 nanocomposite was checked. After the
isolation of the catalyst, the reaction solution was analyzed by the AAS (Atomic Absorption Spectroscopy)
technique. The result of analysis after fifth runs showed that only a very small amount of Ag metal (less than 1.0
%) was leached into the solution.
4. Conclusions
In summary, we have demonstrated an environmentally friendly protocol to prepare stable Ag/HZSM-5
nanocomposite using Euphorbia heterophylla leaf extract without using any special capping agents. The
flavonoids present in extract of leaves of Euphorbia heterophylla act as both capping/stabilizing and reducing
agents. The present method is inexpensive, simple, easily scaled up and environmentally benign because it does
not require an extra reductant or surfactant template, hazardous materials and organic solvent. The prepared
Ag/HZSM-5 nanocomposite capped by biomolecules was characterized by using FESEM, EDS, UV-vis,
elemental mapping, XRD and FT-IR techniques. Moreover, the catalytic performance of the Ag/HZSM-5
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nanocomposite for reduction of variety of dyes in water was also considered. In addition, the catalyst could be
easily isolated from the reaction mixture and successively recovered and reused for five cycles without any
significant loss of catalytic efficiency, thus making this procedure environmentally more acceptable.
Furthermore, the synthesized catalyst by this protocol is absolutely stable and can be encouraged for a broader
range of applications.
Acknowledgments
We gratefully acknowledge the Iranian Nano Council and the Universities of Mazandaran and Qom for the
support of this work.
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ACCEPTED MANUSCRIPT
Highlights:
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Ag/HZSM-5 nanocomposite was prepared using Euphorbia heterophylla leaf extract and characterized.
First report on the use of the Ag/HZSM-5 nanocomposite for reduction of organic pollutant dyes.
The catalyst is found to be stable and reusable.
17

ACCEPTED MANUSCRIPT
Graphical Abstract
Green synthesis of the Ag/HZSM-5 nanocomposite by using Euphorbia
heterophylla leaf extract: A recoverable catalyst for reduction of organic
dyes
Mahmood Tajbakhsh,* Heshmatollah Alinezhad, Mahmoud Nasrollahzadeh* and Taghi A.
Kamali
18

ACCEPTED MANUSCRIPT
Highlights:
•
•
•
Ag/HZSM-5 nanocomposite was prepared using Euphorbia heterophylla leaf extract and characterized.
First report on the use of the Ag/HZSM-5 nanocomposite for reduction of organic pollutant dyes.
The catalyst is found to be stable and reusable.