Biosynthesis, characterization and catalytic activity of an Ag/zeolite nanocomposite for base- and ligand-free oxidative hydroxylation of phenylboronic acid and reduction of a variety of dyes at room temperature

Article abstract of DOI:10.1039/c5nj02909k

This paper reports on the synthesis and use of an Ag/zeolite nanocomposite as a separable catalyst for the ligand-free hydroxylation of phenylboronic acid to phenol and the reduction of 4-nitrophenol (4-NP), methyl orange (MO), congo red (CR), methylene blue (MB) and rhodamine B (RhB) at room temperature. The Ag/zeolite nanocomposite was prepared using the extract of a leaf of Euphorbia prolifera and characterized by FT-IR, XRD, FE-SEM, EDS and TEM analyses. The catalyst was quantitatively recovered and reused without significant loss in the catalytic activity.

Full text of DOI:10.1039/c5nj02909k

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Biosynthesis, characterization and catalytic
activity of an Ag/zeolite nanocomposite for
base- and ligand-free oxidative hydroxylation
of phenylboronic acid and reduction of a variety
of dyes at room temperature
Cite this:DOI: 10.1039/c5nj02909k
a
a
b
Arezo Hatamifard, Mahmoud Nasrollahzadeh* and S. Mohammad Sajadi
This paper reports on the synthesis and use of an Ag/zeolite nanocomposite as a separable catalyst for
the ligand-free hydroxylation of phenylboronic acid to phenol and the reduction of 4-nitrophenol
Received (in Montpellier, France)
2
0th October 2015,
Accepted 11th January 2016
(4-NP), methyl orange (MO), congo red (CR), methylene blue (MB) and rhodamine B (RhB) at room
temperature. The Ag/zeolite nanocomposite was prepared using the extract of a leaf of Euphorbia
prolifera and characterized by FT-IR, XRD, FE-SEM, EDS and TEM analyses. The catalyst was quantitatively
recovered and reused without significant loss in the catalytic activity.
DOI: 10.1039/c5nj02909k
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caused by the formation of side products. Therefore, the develop-
ment of a simple, highly efficient and environmentally benign
Introduction
The development of new methodologies for the introduction method for the base- and ligand-free oxidative hydroxylation of
of OH groups into functionalized arenes is of great interest in phenylboronic acid at room temperature under heterogeneous
organic synthesis because phenols are not only present widely conditions has been a major challenge in organic synthesis.
as a key functional group in numerous biologically active
In the last decade, the syntheses of metal nanoparticles
compounds, various natural products, and pharmaceuticals (MNPs) have been developed, since they exhibit unique surface
1
6
in monomeric or polymeric forms, but are also important properties, high-specific surface area and high catalytic activities.
2
precursors for a variety of oxygen heterocycles.
The catalytic properties of MNPs critically depend on their size,
There are several methods for the synthesis of phenols in the morphology, chemical composition, and surface structure. Due to
3
,4
literature. One group of the main methods available for the the instability of small size MNPs, various stabilizers have been
synthesis of phenols is the traditional direct hydroxylation of used to prevent their aggregation into an inactive bulk metal.
aryl halides using palladium based catalysts with phosphine Chemical methods are among the most important approaches for
3
4
7
ligands and a CuI/non-phosphine ligand at higher temperatures.
the synthesis of MNPs. However, these methods generally use
So far, several approaches have been introduced in recent hazardous and expensive organic solvents, expensive and toxic
years for the synthesis phenols from arylboronic acids under reagents as reducing and stabilizing agents, and it is very likely
7
various conditions including copper(II) sulphate/phenanthroline/ that trace amounts of unreacted reagents remain in the solution.
KOH, copper(II) chloride/amphiphilic surfactants and Cu(II)–b-CD Therefore, the environment is polluted. As a result, these drawbacks
complexes, Ru(bpy) Cl ꢀ6H O, electrocatalysts, Amberlite-IR-120 have limited large scale applications of MNPs in industry. Therefore,
3
2
2
5
resin and clay-entrapped Cu(OH)x.
one of the most essential needs in nanotechnology is to develop
Earlier reported methods for the synthesis of phenols environmentally friendly, sustainable, simple, rapid, easy to control
suffered from drawbacks such as the use of base or toxic and and energy-efficient methods for the synthesis of MNPs.
expensive ligands and homogeneous catalysts, harsh reaction
conditions, tedious work-ups and environmental pollution trees gum. The green synthesis of MNPs under environmentally
benign conditions is of greater importance to address growing
Recently, MNPs have been stabilized by plant extracts and
8
concerns for medical and biotechnological applications. Several
a
Department of Chemistry, Faculty of Science, University of Qom,
types of MNPs for catalytic reactions have recently been studied
PO Box 37185-359, Qom, Iran. E-mail: mahmoudnasr81@gmail.com;
8
in our laboratory using plant extracts and tree gums. This
Fax: +98 25 32103595; Tel: +98 25 32850953
b
method has the advantages of high yields, simple methodology,
short reaction times, easy work-up, very mild reaction conditions,
Department of Petroleum Geoscience, Faculty of Science, Soran University,
PO Box 624, Soran, Kurdistan Regional Government, Iraq
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interest in the development of stabilized nanosized catalysts
8
and green chemistry, herein we report a base- and ligand-free
silver-catalyzed synthesis of phenol by the direct hydroxylation
of phenylboronic acid using H O as an oxidant (Scheme 1). In
2
2
addition, we report the reduction of 4-NP, CR, MO, MB and RhB
using the Ag/zeolite nanocomposite in the presence of NaBH
in water at room temperature.
4
Results and discussion
Fig. 1 Image of Euphorbia prolifera leaves.
In this work, we used the leaf extract of Euphorbia prolifera as
a reducing and stabilizing agent for the green synthesis of
the Ag/zeolite nanocomposite and their catalytic activity for the
hydroxylation of phenylboronic acid to phenol and also the
reduction of 4-NP, CR, MO, MB and RB at room temperature.
The presence of antioxidant phenolic glycosides as potent reducing
agents in the extract may be responsible for the reduction of Ag(I)
ions and the formation of the corresponding Ag NPs.
avoidance of the high reaction thermodynamical parameters
(
temperature and pressure), dangerous chemicals and cost
effectiveness.
Among these nanoparticles, Ag nanoparticles (Ag NPs) have
attracted the attention of many researchers interested in the
9
field due to their catalytic activity and biological applications.
However, the agglomeration of Ag NPs is inevitable. An ideal
support is needed to decrease the agglomeration of NPs. The
immobilization of Ag NPs on solid supports is one of the best
methods to improve the efficiency and recovery of catalysts.
Among solid supports, zeolite has been used widely as an
efficient support and catalyst in organic reactions due to its
good chemical and thermal stability, low cost, low toxicity and
excellent catalytic activity, ease of handling, high reusability,
Characterization of green synthesized Ag NPs using leaf extract
of Euphorbia prolifera
The formation of Ag NPs was monitored by recording UV-vis
spectra. Moreover, the UV spectrum of the extract (Fig. 2) shows
bonds at lmax 320 nm (bond I) due to the transition localized
within the ring of the cinnamoyl system; whereas the p - p*
transition is shown by double bonds. The extract of Euphorbia
prolifera leaf was obtained in aqueous media to extract highly
polar phytochemicals like phenolics with conjugated double
bonds (in A, B and C ring of the flavone nuclei) as major plant
constituents. Then according to the literatures about the
plant and UV-vis fingerprint spectrum of its aqueous extract
1
0
and benign character.
There are several green synthetic procedures for metal NPs.
8
However, there is no report in the literature for the green
synthesis of the Ag/zeolite nanocomposite wherein plant gums
or plant extracts were used as reducing agents.
The genus Euphorbia is the largest in the plant family
Euphorbiaceae, comprising a lot of species and ranging from
annuals to trees. Euphorbia prolifera (Fig. 1) belonging to the
family Euphorbiaceae is a perennial herbaceous plant containing
latex and has unique flower structures distributed in a large part
1
2
our result supports this idea. Therefore, the absorption at
30 nm (bond II) is for the absorbance of a ring related to the
2
benzoyl system and p - p* transitions and demonstrates the
presence of phenolics.
1
1
The UV spectrum of plant leaf extract shows absorption
bands due to the transition localized within the ring of the
cinnamoyl and benzoyl system. Although they are generally
related to the p - p* transitions of double bonds but this
absorption bonds of cinnamoyl and benzoyl systems as shown
of Middle East.
In this study, we describe the use of leaf extract of Euphorbia
prolifera as the reducing and stabilizing agent in the synthesis
of the Ag/zeolite nanocomposite without using any harmful
reducing or capping agents, which could be a synthetically
alternative route that is environmentally friendly, avoiding
the use of hazardous materials. Also, as a continuation of our
Scheme 1 The new strategy for the hydroxylation of phenylboronic acid
under ligand- and solvent-free conditions in air.
Fig. 2 UV-vis spectrum of the aqueous extract of leaves of Euphorbia
prolifera.
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color of the solution immediately changed into dark with lmax
around 430 nm indicating the formation of Ag NPs as charac-
terized by the UV-vis spectrum. In general, the stability of NPs is
very important for their applications. The green synthesized Ag
NPs obtained here are stable with no significant variance in the
shape, position and symmetry of the absorption peak even after
fifteen days due to in situ bio-capping by the plant residue. The
significant change of the peak maxima and shape after 15 days
is probably for deformation processes such as oxidation of the
nanosurface.
On the basis of the above results, a possible mechanism was
+
proposed for the reduction of Ag and the formation of Ag NPs
Fig. 3 FT-IR spectrum of the aqueous extract of leaves of Euphorbia by the aqueous extract of leaves of Euphorbia prolifera (Scheme 2).
prolifera.
The presence of a range of phytochemicals especially anti-
oxidant phenolics of aglycones and glycosides such as flavonoid
aglycones and those with sugar moieties was demonstrated along
with their application as antimicrobial, antioxidant, anticancer,
antiproliferation, antifungal compounds through many studies
by the spectrum are finger print characteristic of and specific to
flavone nuclei inside the extract as reported in the literature.
The FT-IR analysis was carried out to identify the possible
biomolecules responsible for the reduction of silver ions and
capping of the bioreduced nanoparticles. The FT-IR spectrum
of the crude extract (Fig. 3) depicted some peaks at 3500 to
1
2
11
during the last decade. These phytoconstituents confirmed the
application of Euphorbia prolifera leaf extract as a suitable source
for the synthesis of nanoparticles using the reducing ability of
11
these potent antioxidants.
ꢁ1
3100, 1720, 1605, 1395 and 1100 cm which represent free OH
Furthermore, the FT-IR of Ag NPs shows demonstrative
differences in the shape and location of signals indicating the
interaction between AgNO and involved sites of phytochemicals
3
in a molecule and the OH group forming hydrogen bonds, a
carbonyl group (CQO), a stretching CQC aromatic ring and
C–OH stretching vibrations, respectively. These peaks suggested
the presence of flavonoids and other phenolics in the plant leaf
extract. Phenolic acids may be adsorbed on the surface of Ag NPs,
possibly by interaction through p-electrons or Ag–O bond for-
mation (with ꢁOH group of flavonoids and other phenolics) in
the absence of other strong ligating agents. The presence of
flavonoids and other phenolics in the extract may be responsible
for the reduction of metal ions and the formation of the
corresponding metal nanoparticles.
for the production of nanoparticles (Fig. 5). Changing the loca-
ꢁ1
tion of peaks at 3450, 1705, 1410, 1300 and 1000 cm represent
the OH functional groups, a carbonyl group (CQO), a stretching
CQC aromatic ring and C–OH stretching vibrations, respectively,
(Table 1). Polyphenolics could be adsorbed on the surface of
metal nanoparticles, possibly by interaction through p-electron
interaction in the absence of other strong ligating agents.
It seems that the synthesized Ag NPs are stabilized by an
organic layer that originated from the extract of leaves of
Euphorbia prolifera. These nanoparticles could be well stabi-
lized by the polyphenolics via the interaction of Ag NPs with the
OH functional groups.
The synthetic procedure for the preparation of the Ag/zeolite
nanocomposite was synthesized in one step without the appli-
cation of toxic reagents or surfactant templates. The aqueous
extract of leaves of Euphorbia prolifera was used for the synthesis
of the Ag/zeolite nanocomposite by treating with natural Natrolite
The UV-vis spectrum of green synthesized Ag NPs using
Euphorbia prolifera leaf extract (Fig. 4) showed significant
changes in the absorbance maxima due to surface plasmon
resonance demonstrating the formation of Ag NPs. The yellow
zeolite and AgNO in water at 60 1C for 2 h. Natural Natrolite
3
zeolite was provided from Hormak area (Sistan and Baluchestan,
Iran). The aqueous extract of the leaves of Euphorbia prolifera is
a reducing and stabilizing agent, which favors reduction and
results in the formation and distribution of Ag(0) nanoparticles
on the Natrolite zeolite without a hazardous impact on the
environment.
The morphology and structure of the Ag/zeolite nanocompo-
site was fully characterized by means of X-ray diffraction (XRD),
Fourier transformed infrared (FT-IR) spectroscopy, field emission
scanning electron microscopy (FE-SEM), energy dispersive X-ray
spectroscopy (EDS) and transmission electron microscopy (TEM).
The powder XRD patterns for the Ag/zeolite nanocomposite and
Ag NPs showed the expected crystallinity of Ag NPs. Fig. 6 shows
Fig. 4 UV-vis spectrum of green synthesized Ag NPs using aqueous
extract of leaves of Euphorbia prolifera between 4 min to 18 days.
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Scheme 2 Green synthesis of Ag NPs using the aqueous extract of the Euphorbia prolifera leaves.
were studied by FE-SEM and TEM. As shown in Fig. 7, it seems
that the Ag NPs are semispherical with some agglomerations of
the particles. Particle size distribution is represented in Fig. 9.
Fig. 8 and 9 show that the average size of the Ag(0) NPs is
around 15 nm.
The successful immobilization of Ag NPs was also confirmed
by EDS analysis (Fig. 10), which showed the presence of Ag in
addition to O, Al and Si elements. Furthermore, the presence of
Ag was approved using the elemental mapping images (Fig. 11).
Concerning the results of XRD and SEM for the Ag/zeolite nano-
composite, we can confirm that Ag NPs were exactly fabricated
and immobilized on the zeolite surface.
Fig. 5 FT-IR spectrum of Ag NPs synthesized using the aqueous extract
of leaves of Euphorbia prolifera.
Synthesis of phenol catalyzed by the Ag/zeolite nanocomposite
The catalytic activity of the Ag/zeolite nanocomposite for the
synthesis of phenol was investigated by using phenylboronic
acid as a model substrate. Initial studies were performed in
order to optimize the reaction conditions for the synthesis of
Table 1 FT-IR signals of extract and green synthesized Ag NPs
Plant extract signals
Ag NP signals
(cm
a
ꢁ1
ꢁ1
2 2
phenol via hydroxylation of phenylboronic acid by 30% aq H O
FG
(cm
)
)
in the presence of the Ag/zeolite nanocomposite at room
temperature (Table 2).
OH
CQO
Aromatic CQC
C–OH
C–H (sp )
3100 to 3500
1720
1605
1395 to 1100
2910
1391
3450
1705
1410
1300 to 1000
2890
1375 to 1450
1605
Trace amounts of the phenol product were observed, when
the reaction was carried out at room temperature in the
absence of a catalyst or a solvent (Table 2, entry 1). Various
reaction parameters, such as the influence of catalyst loading,
solvent and time, were investigated. It was observed that an
increase in the reaction time from 60 min to 40 h, resulted in an
increase in the yield of product (Table 2, entry 2). The use of
3
CH
3
bending
2
CQC alkenes (sp )
1615
a
FG: functional groups.
broad peaks of 2y values of 34.3, 43.11, 64.2, 75.3 and 84.0 degrees, an organic solvent, for example, acetonitrile or methanol did
which are assigned to the corresponding 111, 200, 220, 311 and 222 not improve the product yield in the absence of the catalyst
indices of the face centered cubic (fcc) lattice of metallic Ag. The (Table 2, entries 3 and 4). Surprisingly, the yield of product
particle size can be found by applying Sherrer’s equation and the increased dramatically to 90% when the same reaction was
average particles size is found to be 17 nm. The actual phases for performed in the presence of 5 mg of the Ag/zeolite nano-
zeolite were silicon oxide cristabolite–SiO
silicate nanozeolite.
2
(cubic) and aluminium composite for 15 min at room temperature (Table 2, entry 6).
Different solvents were also tested in this reaction system.
10f
The FE-SEM, EDS and TEM images of the prepared catalyst Nevertheless, the best results were achieved when the reaction
are shown in Fig. 7 and 8. The morphology and size of Ag NPs was performed in the absence of any solvent (Table 2, entry 10).
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Fig. 6 XRD pattern of green synthesized Ag NPs and the Ag/zeolite nanocomposite, respectively.
Fig. 7 FE-SEM images of the green synthesized Ag/zeolite nanocomposite.
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Fig. 8 TEM images of the green synthesized Ag/zeolite nanocomposite.
Fig. 9 Size distribution histogram of the green synthesized Ag/zeolite nanocomposite.
Fig. 10 EDS spectrum of the green synthesized Ag/zeolite nanocomposite.
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Fig. 11 EDS elemental mapping of the green synthesized Ag/zeolite nanocomposite.
Table 2 Optimization of reaction conditions for the hydroxylation of
a
phenylboronic acid
b
Entry
Catalyst (mg)
Solvent
Time (min)
Yield (%)
1
2
3
4
5
6
7
8
9
—
—
—
—
—
—
60
40 h
8
4
79
7
7
93
90
85
80
83
97
79
c
MeCN
MeOH
—
MeCN
MeOH
THF
DCM
—
8
Zeolite (5.0)
Ag/zeolite (5.0)
Ag/zeolite (5.0)
Ag/zeolite (5.0)
Ag/zeolite (5.0)
Ag/zeolite (5.0)
Ag/zeolite (2.0)
25
15
20
20
20
5
1
1
0
1
Scheme 3 Mechanism of the hydroxylation of phenylboronic acid.
—
13
a
Reaction conditions: phenylboronic acid (1.0 mmol), H
.5 mL); room temperature, air. Isolated yield. The reaction did not
2
O
2
(30% aq,
b
c
0
reach completion.
The Ag/zeolite nanocomposite probably has an important
role in the hydroxylation of phenylboronic acid and the plausible
mechanism is shown in Scheme 3. This reaction allows the
The use of lower catalyst loadings resulted in incomplete reactions formation of the Ag/zeolite-peroxide composite via the reaction
entry 11). Also, increasing the amount of the Ag/zeolite nano- of Ag/zeolite with H O . The reaction of the Ag/zeolite-peroxide
(
2
2
composite showed no substantial improvement in the reaction composite with phenylboronic acid facilitates the rearrange-
yield. Thus the optimized reaction conditions were found to be ment and then elimination of H O and the final product will be
2
1
3
2 2
1.0 mmol of phenylboronic acid, 0.5 mL of 30% aq H O and produced.
5
.0 mg of the catalyst for 5 min under solvent-free and aerobic
To show the advantages of the proposed method in compar-
ison with other reported methods, we summarized some of the
conditions (entry 10).
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Table 3 Comparison of present methodology with other reported methods in the hydroxylation of phenylboronic acid for the synthesis of phenol
a
Entry
Reaction conditions
CuSO , 1,10-phenanthroline, KOH, H
Time
Yield (%)
Ref.
1
2
3
4
5
6
7
8
9
4
2
O, r.t.
3 h
7 h
24 h
8 h
6 h
1 h
24 h
8 h
90
94
87
92
85
94
70
91
67
92
495
92
92
81
13a
13b
13c
13d
13e
13f
13g
13h
13i
13j
13k
13l
13m
13n
13o
Methylene blue (MB), iPr Net, hn, O , MeCN:H
ꢀH O, PEG-400, Cs CO , H O, air, 80 1C
UV-vis, N(Et) , hn, O , MeCN:H
MeNHNH , THF, air, r.t.
Flavinium catalyst, N
CuCl , Brij S-100, O , H
CuSO O, ellagic acid, MeOH, air, 60 1C
ꢀ5H
PhI, NaIO , MeCN:H O, 80 1C
PhI(OAc) , Et N, MeCN:H O, r.t., open flask
a-Fe , ambient, solar visible light irradiation, THF, open flask
Ag NP mont K-10, H , r.t.
Acidic Al , H , r.t.
[Ru(bpy)
]ꢀ6H
Pd(L)Cl
CHCl , Et
NH
OHꢀHCl, Ethanol, NaOH, r.t.
Ag/zeolite nanocomposite, H , r.t.
2
2
2
O
N
2
H
4
2
2
3
2
3
2
2
O
2
2
H
2
4
ꢀH
2
O, CF
O, r.t.
3
CH
2
OH–CH
3
OH, O
2
, r.t.
2
2
4
2
4
2
8 h
10
11
12
13
14
15
2
3
2
10 min
3.5 h
15 min
10 min
28 h
24 h
2
O
3
2 2
O
2
O
Cl
3
2
O
2
3
2
2
O, iPr
2
NEt, DMF, visible light irradiation, air
0
2
(L = N,N -bis(diphenylphosphanyl)-2-(diphenylphosphanyl)-ethanamine),
88
3
3 2
N, O , r.t.
1
1
6
7
2
26 h
5 min
68
97
13p
This work
2
O
2
a
Isolated yield.
results for the hydroxylation of phenylboronic acid in Table 3,
which shows that the green synthesized Ag/zeolite nanocomposite
is an equally or more efficient catalyst with respect to reaction time
and yield than the previously reported ones.
Compared with the other literature studies on the hydroxy-
1
3
lation of phenylboronic acid, the notable features of our
method are:
ꢂ Green synthesis of the Ag/zeolite nanocomposite in water
without using any harmful reducing or capping agents;
ꢂ The use of plant extract as an economic and effective
bioreductant represents an interesting, fast and clean synthetic
route for the large-scale synthesis of an efficient and low-cost
Fig. 12 UV-visible spectra for the catalytic reduction of 4-NP to 4-AP at
ꢁ3
Ag/zeolite nanocomposite which can be potentially applied for several intervals. Conditions: [4-NP] = 2.5 ꢃ 10 M; [NaBH
future catalytic applications in other reactions;
4
] = 0.25 M;
catalyst = 5.0 mg.
ꢂ Toxic ligands and bases are not needed;
ꢂ The catalyst can be easily recovered; and
ꢂ The yields of the products are very high.
Catalytic reduction of 4-NP, CR, MO, MB and RhB by the
Ag/zeolite nanocomposite
The catalytic efficiency of the Ag/zeolite nanocomposite was
also tested towards the reduction of 4-NP, CR, MO, MB and RhB
Scheme 4 The reduction reaction for the conversion of 4-NP to 4-AP.
in the presence of excess NaBH
4
at room temperature. The
catalysis reactions were monitored spectrophotometrically by
recording the changes in the UV-vis absorption spectrum of the of the Ag/zeolite nanocomposite. With a small amount of the
reaction mixture, which was recorded after centrifugation of Ag/zeolite nanocomposite being added, the absorption intensity
the Ag/zeolite nanocomposite.
The catalytic reduction of 4-NP was monitored spectrophoto- the appearance of a new absorption peak at 300 nm, indicating
metrically by the decrease of the absorbance peak at lmax the formation of 4-AP. The reaction was completed in the
17 nm with time. Fig. 12 shows the reduction of 4-NP to 4-AP presence of 5 mg of the Ag/zeolite nanocomposite as the catalyst
in the presence of the Ag/zeolite nanocomposite. 4-NP solution and 100 equivalents of NaBH for 160 s at room temperature.
of 4-NP at 400 nm decreased quickly with time, accompanied by
=
1
4
3
4
14
shows a strong absorption peak at 317 nm which is remarkably The effects of the concentration of the Ag/zeolite nanocomposite
shifted to 400 nm when NaBH solution was added into the 4-NP, and NaBH were also studied by carrying out reduction reactions
4
4
indicating the formation of the 4-nitrophenolate ion (Scheme 4). It in the presence of varying amounts of the catalyst and NaBH
was demonstrated that the peak at 400 nm remained unchanged (Table 4). Based on the observation of the results it was evident
for over 100 min in the absence of the Ag/zeolite nanocomposite. that a combination of the Ag/zeolite nanocomposite and NaBH
It is important to note that the reduction started after the addition was necessary for the quick and effective reduction of 4-NP to
4
4
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Table 4 Optimization of reaction conditions for the reduction of 4-NP
forms from the hybrid attacks on 4-nitrophenolate ions for the
production of 4-AP and its desorbtion on the surface of the
nanocomposite. This ET-induced hydrogenation of 4-NP occurred
spontaneously at the surface of the metal catalyst. These are in
4
-NP (mM)
NaBH
4
(mM)
Catalyst (mg)
Time
2
2
2
2
2
2
2
.5
.5
.5
.5
.5
.5
.5
250
250
250
125
187.5
250
250
Ag/zeolite (0.0)
Ag NPs (5.0)
Zeolite (5.0)
Ag/zeolite (5.0)
Ag/zeolite (5.0)
Ag/zeolite (5.0)
Ag/zeolite (7.0)
100 min
15 min
10 min
240 s
203 s
160 s
16
agreement with previous reported findings in the literature studies.
In all cases, the metal catalysts play an important role as electron
16
4
relays for the reduction of 4-NP with NaBH .
Four different dyes, namely CR, MO, MB and RhB, were also
chosen for evaluating the catalytic activity of the Ag/zeolite
nanocomposite in aqueous medium and in the presence of
160 s
4
-AP. Nevertheless, the best results were achieved in the presence excess NaBH . It is reported in the literature that the reduction
4
of 5.0 mg of the catalyst and 100 equivalents of NaBH . Also, of organic dyes will not proceed in the absence of the catalyst
4
1
5
increasing the amount of the catalyst showed no substantial even in excess of NaBH4. The catalytic reduction of dyes was
improvement in the reaction time. monitored spectrophotometrically by following the decrease
The catalytic activity of Ag was improved upon combination of the absorbance peak at lmax(lmax(CR) = 493 nm, lmax(MO)
=
with zeolite and the formation of the Ag/zeolite nanocomposite. 465 nm, lmax(MB) = 663 nm and lmax(RhB) = 554 nm) after the
In comparison of the catalytic ability of Ag NPs individually and addition of the catalysts (Fig. 13). It was observed that the
the nanocomposite, the time required to complete the reaction reduction of organic dyes occurred within 40–450 s, depending
was 15 min and 160 s, respectively which indicates the consi- on the dye.
derable improvement of the catalytic activity. This demonstration
To optimize the catalyst amount, the reduction reactions
can be referred to zeolite as a support to prevent the aggregation of were performed in the presence of various amounts of the
nanoparticles and its synergic effect. The higher surface area of the Ag/zeolite nanocomposite (Table 5).
synthesized nanocomposite leads to an increase in the adsorption
A comparison of the catalytic activities of various catalysts
and reactant concentrations which cause a highly efficient contact for the reduction of 4-NP, MB, RhB, CR and MO is given in
with Ag NPs supported on zeolite. The conversion of 4-NP to 4-AP Table 5. As shown in Table 6, with the Ag/zeolite nanocomposite
using the Ag/zeolite nanocomposite is a reduction electron transfer as the catalyst, the catalytic activity is very high. In comparison
ꢁ
16,17
process from BH4 to 4-NP by Ag NPs which the hydrogen atom with the reported catalysts in the literatures,
shorter time is
Fig. 13 UV-visible spectra of the catalytic reduction of CR (A), MO (B), MB (C) and RhB (D) by NaBH
4
in the presence of the Ag/zeolite nanocomposite.
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a
Table 5 Optimization of reaction conditions for the reduction of organic
Table 7 Reusability of nanocatalyst
dyes
b
Entry
Yield (%)
Recovery of catalyst (%)
Organic dye (M)
NaBH
4
(M)
Catalyst (mg)
Time
1
2
3
4
5
6
7
95
95
94
93
93
92
92
99
99
98
98
97
96
96
ꢁ
ꢁ
ꢁ
5
5
5
ꢁ3
a
RhB (2.09 ꢃ 10
RhB (2.09 ꢃ 10
RhB (2.09 ꢃ 10
)
)
)
5.3 ꢃ 10
5.3 ꢃ 10
5.3 ꢃ 10
5.3 ꢃ 10
5.3 ꢃ 10
5.3 ꢃ 10
5.3 ꢃ 10
5.3 ꢃ 10
5.3 ꢃ 10
5.3 ꢃ 10
5.3 ꢃ 10
Zeolite (15.0)
Ag/zeolite (7.0)
Ag/zeolite (10.0)
Zeolite (1.0)
Zeolite (5.0)
Ag/zeolite (3.0)
Zeolite (7.0)
Ag/zeolite (3.0)
Ag/zeolite (7.0)
Ag/zeolite (5.0)
Ag/zeolite (7.0)
NR
ꢁ3
ꢁ3
ꢁ3
ꢁ3
ꢁ3
ꢁ3
ꢁ3
ꢁ3
ꢁ3
ꢁ3
b
30 min
7.30 min
ꢁ
ꢁ
ꢁ
5
a
MB (3.1 ꢃ 10
MB (3.1 ꢃ 10
MB (3.1 ꢃ 10
MO (3.0 ꢃ 10
MO (3.0 ꢃ 10
MO (3.0 ꢃ 10
)
)
)
)
)
)
NR
5
5
7.06 min
50 s
ꢁ
ꢁ
ꢁ
5
9 min b
a
5
5
Reaction conditions: Ag/zeolite nanocomposite (10.0 mg), phenyl-
20 min
boronic acid (1.0 mmol), H
2
O
2
(30% aq, 0.5 mL); room temperature, air.
88 s
40 s
40 s
b
ꢁ
5
Isolated yield.
CR (1.44 ꢃ 10
)
)
ꢁ
5
CR (1.44 ꢃ 10
a
b
No reaction after 30 min. Not completed.
Table 6 A comparison of the reaction time of the present system with the
literature precedents of some recently published catalytic systems for the
reduction of 4-NP, MB, RhB, CR and MO
Substrate
-NP
Catalyst
Time
Ref.
4
GA–Pt NPs
Pd–FG
Au@PZS@CNTs
Ni–PVAm/SBA-15
8 h
16a
16b
16c
16d
16e
16f
16g
12 min
16 min
85 min
60 min
60 min
2 h
TiO
Fe
2
O
–G1%
@C@Pt
3
4
Cu NPs
HMMS–NH
2
–Pd
60 min
5.5 h
1.5 h
12 min
510 s
16h
16i
16j
16k
16l
p(AMPS)–Ni composite
PdCu/graphene
KCC-1/Au
Ag/KCC-1
Fig. 14 TEM image of the recycled Ag/zeolite nanocomposite.
Ag/zeolite nanocomposite
160 s
This work
MB
Porous Cu microspheres
SiNWAs–Cu
8 min
10 min
10 min
7.5 min
5 min
50 s
17a
17b
17c
17d
17e
This work
centrifugation, washed with methanol and deionized water,
and finally dried in an oven for the next run of the hydroxyla-
tion reaction (Table 7). The catalyst was reused several times
without any significant loss in the catalytic activity. This result
suggests that the Ag/zeolite nanocomposite is stable during
the hydroxylation reaction. The TEM analysis of the recycled
catalyst showed that particles are identical in shape and size
even after the seventh run. However, some nanoparticle aggre-
gation is observed (Fig. 14).
The catalytic efficiency of the catalyst remained almost
constant up to seven cycles of operation and the time required
for 100% reduction of MO, CR and MB was found to be almost
same up to the 7th cycle.
Au/Fe
3
O
4
@C
Ag NPs on silica spheres
Aucore–PANIshell
Ag/zeolite nanocomposite
RhB
SiNWAs–Cu
14 min
18 min
15 min
15 min
47 min
10 min
7.30 min
17b
15
17f
17g
17h
17k
This work
Fe
Au–PANI nanocomposite
Fe /Ag
3
O
4
@PANI@Au
3
O
4
Ag/HLaNb
PS/Ag
2
O
7
Ag/zeolite nanocomposite
CR
Copper nanocrystals
Cu@SBA-15
Ag/zeolite nanocomposite
500 s
7 min
40 s
17l
17i
This work
MO
Cu@SBA-15
Ag/zeolite nanocomposite
5 min
88 s
17i
This work
Conclusion
The green synthesis of the Ag/zeolite nanocomposite using the
needed for the reduction of organic dyes using the Ag/zeolite extract of leaves of Euphorbia prolifera was demonstrated in this
nanocomposite.
study. The formation of the Ag/zeolite nanocomposite by the
reduction of the Ag(I) is possibly facilitated by phenolic acid
and flavonoids present in the Euphorbia prolifera leaf extract as
Catalyst recyclability
To evaluate the recyclability of the Ag/zeolite nanocomposite both reducing and stabilizing agents. The Ag/zeolite nano-
in the reaction, phenylboronic acid was chosen as a model composite shows strong catalytic activity in the base- and
substrate. After the model reaction was carried out under the ligand-free oxidative hydroxylation of phenylboronic acid to
optimized reaction conditions, the catalyst was recovered by phenol and the reduction of 4-NP, CR, MO, MB and RhB at
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room temperature. The protocol is very simple and versatile, filtered and washed with distilled water and dried at 120 1C in
and has been used for the synthesis of phenol in high yields. an oven for 6 h. The Ag/zeolite nanocomposite catalyst was used
The catalyst was recycled several times without any loss in the for characterization and investigation of the catalysis.
catalytic activity.
Preparation of phenol via hydroxylation of phenylboronic
acid. In a 25 mL round bottom flask, the Ag/zeolite nano-
composite (5.0 mg) was added to phenylboronic acid (1.0 mmol)
Experimental
Reagents and methods
2 2
and H O (30% aq, 0.5 mL). The mixture was magnetically
stirred at room temperature for 5 min (Table 2) under aerobic
conditions. After the completion of the reaction (as monitored
by TLC), the mixture was filtered and extracted with diethyl
ether. The ethereal phase was separated to give pure products.
The product is known a compound and the spectral data and
High-purity chemical reagents were purchased from the Merck
and Aldrich chemical companies. All materials were of com-
mercial reagent grade. Melting points were determined in open
capillaries using a BUCHI 510 melting point apparatus and are
13
melting point were identical to those reported in the literature.
1
uncorrected. H NMR spectra were recorded on a Bruker Avance
DRX-400 spectrometer at 400 MHz. FT-IR spectra were recorded
on a Nicolet 370 FT/IR spectrometer (Thermo Nicolet, USA)
using pressed KBr pellets. The element analyses (C, H, N) were
obtained from a Carlo ERBA Model EA 1108 analyzer carried
out on a Perkin-Elmer 240c analyzer. TLC was performed on
silica gel polygram SIL G/UV 254 plates. X-ray diffraction (XRD)
measurements were carried out using a Philips powder dif-
fractometer type PW 1373 goniometer (Cu Ka = 1.5406 Å). The
scanning rate was 21 min in the 2y range from 10 to 901.
Transmission electron microscopy (TEM) was recorded on a
Zeiss-EM10C-80 KV. The morphology and particle dispersion
were investigated by scanning electron microscopy (SEM) (Cam
scan MV2300). The chemical composition of the prepared
catalyst was measured by EDS (Energy Dispersive X-ray Spectro-
scopy) performed in SEM. UV-visible spectral analysis was
recorded on a double-beam spectrophotometer (Hitachi, U-2900)
to ensure the formation of nanoparticles.
Reduction of 4-NP, MB, MO, CR and RhB aqueous solution
Catalytic reduction of 4-NP. In a typical run, 5.0 mg of the
catalyst was added to 25 mL of 4-NP aqueous solution (2.5 mM).
Next, 25 mL of freshly prepared NaBH aqueous solution (250 M)
4
was added and the mixture was allowed to stir at room tempera-
ture until the deep yellow solution became colorless. The reaction
progress was monitored by UV-vis spectroscopy. After the com-
pletion of the reaction, the catalyst was simply collected from the
reaction system by brief centrifugation and washed successively
with deionized water and dried for the next cycle.
ꢁ
1
Catalytic reduction of organic dyes. In a typical run, 3.0 mg
of the catalyst was added to 25 mL of MB aqueous solution
ꢁ
5
(
3.1 ꢃ 10 M). Next, 25 mL of freshly prepared NaBH aqueous
4
ꢁ3
solution (5.3 ꢃ 10 M) was added and the mixture was allowed
to stir at room temperature. After the completion of the reaction
(as monitored by UV-vis absorption spectra), the catalyst was
simply collected from the reaction system by brief centrifugation
and washed successively with deionized water and dried for the
next cycle. Similar procedures were used to catalyze the reduction
of CR, MO and RhB. For each catalytic reaction, the added
Preparation of extract, Ag NPs, Ag/zeolite nanocomposite and
phenol
Preparation of the extract of leaves of Euphorbia prolifera.
amount of NaBH was equal and in the case of MO, CR and RhB
4
5
0 g of powdered dried leaves of Euphorbia prolifera was added
the catalyst amounts were 7.0, 3.0 and 10.0 mg, respectively.
Also, MO, CR and RhB aqueous solution concentrations were
to 250 mL of double distillated water in a 500 mL flask and well
mixed. The preparation of extract was done using magnetic
heating stirrer at 70 1C for 30 min. The obtained extract was
centrifuged in 7000 rpm and filtered, and then the filtrate was
kept in a refrigerator for further use.
ꢁ
5
ꢁ5
ꢁ5
3.0 ꢃ 10 , 1.44 ꢃ 10 and 2.09 ꢃ 10 M, respectively.
Acknowledgements
Preparation of Ag NPs using prolifera leaf extract. In a 250 mL
conical flask, 50 ml of 0.003 M aqueous solution of AgNO3
We gratefully acknowledge the Iranian Nano Council and
University of Qom for the support of this work.
(99.99%) was mixed with 50 mL of the aqueous plant extract
with vigorous shaking until a gradual change in the color of the
mixture from yellow to dark brown for 4 min indicating the
formation of Ag NPs (as monitored by UV-vis and FT-IR spectra
of the solution). The well-shaked mixture was then filtered and
centrifuged at 7000 rpm for 30 min and the obtained precipitates
were washed with n-hexane and absolute ethanol to remove
possible impurities.
Preparation of the Ag/zeolite nanocomposite using prolifera
leaf extract. In a typical synthesis of the Ag/zeolite nanocompo-
site, 20 mL of the plant extract was added dropwise to 40 mL of
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