A green and cost-effective approach for the production of gold nanoparticles using corn silk extract: A recoverable catalyst for Suzuki–Miyaura reaction and adsorbent for removing of dye pollutants

Article abstract of DOI:10.1016/j.poly.2019.01.060

In this study, we have developed an eco-friendly and inexpensive method for the synthesis of gold nanoparticles immobilized on the Schiff-base functionalized SBA-15 by using of corn silk extract as a natural reducing agent. Appropriate analyses confirm the structure of this heterogeneous nanocomposite. This synthesized nanocomposite acts as a robust nanocatalyst for the selective production of biphenyls in Suzuki–Miyaura coupling reaction and a strong adsorbent for degradation of anionic and cationic dyes such as methyl orange and methylene blue. The results show that this composite leads to the formation of biphenyl derivatives in excellent yields even for less-active aryl halides such as chlorobenzene at mild conditions and efficient degradation of organic dyes (>99%) in a short time through a pseudo-first order kinetic model. Easy separation of this nanocomposite from the reaction mixture by applying an external magnet and its reusability for several times without significant loss in its activity, are the other advantages of this nanocomposite.

Full text of DOI:10.1016/j.poly.2019.01.060

Accepted Manuscript
A Green and Cost-Effective Approach for the Production of Gold Nanoparticles
Using Corn Silk Extract: A Recoverable Catalyst for Suzuki-Miyaura Reaction
and Adsorbent for Removing of Dye Pollutants
Mohammad Mehdi Khodaei, Mahsa Dehghan
PII:
S0277-5387(19)30082-8
https://doi.org/10.1016/j.poly.2019.01.060
POLY 13733
DOI:
Reference:
To appear in:
Polyhedron
Received Date:
Revised Date:
Accepted Date:
16 October 2018
25 January 2019
27 January 2019
Please cite this article as: M.M. Khodaei, M. Dehghan, A Green and Cost-Effective Approach for the Production
of Gold Nanoparticles Using Corn Silk Extract: A Recoverable Catalyst for Suzuki-Miyaura Reaction and Adsorbent
for Removing of Dye Pollutants, Polyhedron (2019), doi: https://doi.org/10.1016/j.poly.2019.01.060
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A Green and Cost-Effective Approach for the Production of Gold
Nanoparticles Using Corn Silk Extract: A Recoverable Catalyst for Suzuki-
Miyaura Reaction and Adsorbent for Removing of Dye Pollutants
Mohammad Mehdi Khodaei,[a,b]* Mahsa Dehghan[a]
[a]
Department of Organic Chemistry, Razi University, Kermanshah 67149-67346, Iran.
Nanoscience & Nanotechnology Research Center (NNRC), Razi University, Kermanshah 67149-
7346, Iran
[b]
6
*
Corresponding author. Tel.: +98-38-8067724ꢀ. Fax: +98-83-34274759. E-mail address:
mmkhoda@razi.ac.ir (M. M. Khodaei)
Abstract
In this study, we have developed an eco-friendly and inexpensive method for the synthesis of gold
nanoparticles immobilized on the Schiff-base functionalized SBA-15 by using of Corn silk extract
as a natural reducing agent. Appropriate analyses confirm the structure of this heterogeneous
nanocomposite. This synthesized nanocomposite acts as a robust nanocatalyst for the selective
production of biphenyls in Suzuki-Miyaura coupling reaction and a strong adsorbent for
degradation of anionic and cationic dyes such as methyl orange and methylene blue. The results
show that this composite leads to the formation of biphenyl derivatives in excellent yields even for
less-active aryl halides such as chlorobenzene at mild conditions and efficient degradation of
organic dyes (>99%) in a short time through a pseudo-first order kinetic model. Easy separation
of this nanocomposite from the reaction mixture by applying an external magnet and its reusability
for several times without significant loss in its activity, are the other advantages of this
nanocomposite.
Keywords: Corn silk extract; Nanoadsorbent; Magnetic; Suzuki-Miyaura reaction; Organic dyes.
1
. Introduction
1

In recent years, metal nanoparticles have attracted a lot of attention due to their unique
characteristics such as high activity, higher fermi potential, excellent performance and applications
as active catalyst for many electron-transfer reactions [1-7]. There are different methods for the
production of metal nanoparticles including photocatalytic and chemical reduction of metal salts
[
4,6,7], and solvent extraction [5]. An alternative, efficient and green method to the chemical
methods is plant extract [8].
Corn silk refers to the shiny and weak fibers that grow as a part of ears of corn. It is a waste material
in the cultivation of corn. Corn (maize) can be cultivated in the most parts of the world [9]. For
the first time, the Native Americans [10] and Chinese [11] used the corn silk extract as an effective
herb. Now, many developed countries use it as a traditional medicine. It contains proteins,
carbohydrates, vitamins, minerals and fiber [12]. Corn silk extract has shown positive therapeutic
effects in a number of cases such as bladder infections, inflammation of the urinary system,
inflammation of the prostate, kidney stones, bedwetting, diabetes, high blood pressure and high
cholesterol levels [13-25]. Its therapeutic potential is related to the presence of bioactive
components especially flavonoids and terpenoids. Flavonoids (polyphenols) are an important class
of natural antioxidants [26]. Therefore, corn silk can act as a strong, natural and safe reducing
agent for the production of metal nanoparticles.
In recent years gold nanoparticles have been used for catalyzing of various organic reactions such
as oxidation, reduction and hydrochlorination reactions [27-29] Several research works present in
which Au NPs have been used for coupling reactions [37-39]. Common catalysts for Suzuki-
Miyaura coupling reaction include palladium, nickel [30], and copper [31] complexes. Some
examples show that gold (I) complexes are strong catalysts for coupling reactions [32]. For high
efficiency, ease of separation and reusability, these complexes immobilize on solid supports as
nanoparticles [33-36]. Herein, we demonstrate that gold nanoparticles immobilized on mesoporous
silica can efficiently catalyze Suzuki coupling reaction. The SBA-15 materials, with thick walls
(around 4 nm), allow easier entry of substrates to the inner surface of the materials, which
accelerates the physical and chemical processes [40-44]. The modification of SBA-15 with
different organic compounds improves its features [44,45]. There is a large number of catalytic
applications of Schiff-base functionalized SBA-15. The Schiff-base derivatives have different
properties; hence, they can be used in different fields such as organic and bioinorganic chemistry,
catalyst, synthesis of various compounds and liquid crystal. The existence of electron pair on the
2

nitrogen atom with the ability to coordinate to metal ions or different organic groups makes Schiff-
bases highly flexible in the synthesis and modifications [46-52].
In this research, Au NPs prepare with a simple, cost-effective and nontoxic method by using of
Corn silk extract as a natural reducing agent and then, immobilize on the Schiff-base functionalized
SBA-15 (Fe
3
4
O -Au@SF-SBA-15). This magnetically separable nanocomposite plays a dual role
as a nanocatalyst for Suzuki-Miyaura coupling reaction and a nanoadsorbent for rapid degradation
of cationic and anionic dyes. Herein, we report the catalytic role of Au NPs for coupling reactions
between aryl halides and arylboronic acids for the production of biaryl compounds in excellent
yields even for less-reactive aryl halides such as chlorobenzene derivatives. In this research, the
final catalyst (Fe
3
4
O -Au@SF-SBA-15) was characterized by FESEM, CHN, EDX, Mapping,
TEM, FT-IR, BET/BJH, TGA, low-angle and wide-angle XRD techniques (Scheme 1).
Scheme 1: Preparation of Au NPs immobilized on the Schiff-base functionalized SBA-15 (Fe
3
4
O -Au@SF-
SBA-15).
2
. Experimental section
3

2
.1. Apparatus and instruments
A BRUKER ALPHA spectrophotometer was used to record FT-IR spectra (range: 400-4000 cm-
1)
. An X'PertPro diffractometer using Cu Kα radiation acquired the X–ray diffraction patterns.
Transmission electron microscopy (TEM) analysis was performed using a ZEISS-EM 10C at 80
kV acceleration voltage. Field emission Scanning electron microscopy (FESEM, Mapping) and
EDX were performed on a ZEISS SIGMA VP-500 instrument. N adsorption-desorption isotherms
2
o
were measured at -196 C using BELSORP‐mini II instrument. Thermogravimetric analysis
o
-1
(
TGA) was carried out using an STA PT- 1000 instrument at a heating rate of 10 C min in air.
The concentration of Au was estimated using inductively coupled plasma optical emission
spectrometer (ICP-OES) Varian Vista PRO Radial.
2
.2. Chemicals and materials
Pluronic P123 (Ethylene Oxide-Propylene Oxide-Ethylene Oxide), Mw = 5800 g/mol, tetraethyl
orthosilicate (TEOS), 3-chloropropyl triethoxysilane (CPTES, 99%), 1,3,5- triazine- 2,4,6- triamine
(melamine), 2-thiopene carbaldehyde, hydrogen tetrachloroaurate tetrahydrate, methyl orange (MO),
methylene blue (MB), aryl halide derivatives, phenylboronic acid derivatives, potassium carbonate,
ethanol and hydrochloric acid were supplied by Merck and Aldrich companies and were used without
further purification. Double distilled water was used throughout the experiments.
2
.3. Synthesis of Schiff-base functionalized SBA-15 (SF-SBA-15)
The SBA-15 support was prepared through surfactant method by Zhao et al. [53,54]. Then, it was
modified by Vunain et al. method [55]. Briefly, the amount of 50 mL of dry toluene, 5.0 g of SBA-15
and 5.0 mL (20.76 mmol) of 3-chloropropyl triethoxysilane (CPTES) were introduced to a 100 mL of
round-bottom flask. This solution was refluxed for 24 h and then, the solid (SBA-15-Cl) was filtered
o
and washed with toluene, dichloromethane, and methanol, and dried at 110 C [56]. At the next step,
we added the amount of 0.83 g (5.0 mmol) of potassium iodide to a solution of 1.0 g of SBA-15-Cl in
o
5
0 mL of acetonitrile at 50 C. After 1 h, 0.63 g (5.0 mmol) of melamine was added and then, refluxed
o
for 9 h. Then, we filtered the obtained solid, rinsed with ethanol and dried at 80 C. After that, the
obtained solid that was produced in previous step (1.0 g) was dissolved in 50 mL of methanol and 1.24
g (11.0 mmol) of 2-thiophenecarbaldehyde was added to it and refluxed for 9 h. After completion of the
reaction, the obtained product (SF-SBA-15) was separated.
4

2
.4. Extraction of corn silk extract
We use a simple procedure for the extraction of corn silk extract. We poured the fresh corn silks (10.0
o
g) in a 500 mL beaker containing 200 mL of deionized water. Then, we heated this mixture at 60 C for
8
h. After that, we separated the corn silks by filtration and placed the filtrate at room temperature until
the whole of its water evaporates and a concentrated substance (like honey) remains.
2
.5. Synthesis of Au@SF-SBA-15
For the synthesis of Au@SF-SBA-15, we added the amount of 0.2 g of SF-SBA-15 to an aqueous
o
solution of HAuCl
4
(1 mM) and stirred for 24 h at 40 C. Then, the temperature was increased
o
o
from 40 C to 60 C and 4 mL of corn silk extract (2.5% W/V) added to the solution and stirred
for 1 h. We separated the obtained solid, washed several times with deionized water and dried. The
loading of Au was measured to be 2.54 wt% by ICP-OES.
2
.6. Synthesis of Fe
3
4
O -Au@SF-SBA-15
In a 100 mL round-bottom flask, the as-prepared Fe
3
4
O NPs (0.1 g) was dispersed in 20 mL of
absolute ethanol by ultra-sonication (Fe NPs were synthesized by using of the method
3
O
4
described in our previous work) [57]. In another 100 mL round-bottom flask, 0.1 g of Au@SF-
SBA-15 was added to 40 mL of ethanol and stirred for 30 min at ambient temperature. Then, we
added the dispersed Fe
3
O
4
NPs to this solution and stirred for 24 h. After that, we isolated the solid
o
product by an external magnet and dried at 50 C.
2
.7. Adsorption experiments
The performance of synthesized nanocomposite investigated by the degradation of MO and MB
in the presence of NaBH as a source of hydride ion. All experiments were done at ambient
temperature as follow: NaBH aqueous solution (2 mL, 0.25 M) was added into aqueous solutions
of MO and MB (18 mL, ꢀ4 ppm) under stirring. Then, the amount of 1.0 mg of Fe -Au@SF-
4
4
3
O
4
SBA-15 nanoadsorbent (0.013 mmol% of Au) was dispersed in the above solution and changes of
the absorption peaks in UV-Vis spectrometer were recorded. In order to evaluate the effect of the
variables on the adsorption of organic dyes, the effect of pH (5.0-9.0), the amount of adsorbent
(1.0-3.0 mg) and contact time (10-300 sec) were investigated. The pH adjusted by adding aqueous
solutions of HCl (0.1 M) or NaOH (0.1 M). The concentrations of MO and MB solutions analyzed
5

by measuring their absorbance at 464 and 553 nm, respectively. These wavelengths correspond to
the maximum absorption peaks of MO and MB dyes. Calibration curves obtained by using of
standard solutions of MO and MB with known concentrations versus the absorbance. The results
of calibration experimental data were shown as a straight line with a high coefficient of
2
determination (R = 0.999). The amount of absorbed dyes by the adsorbent and the adsorption
percentage calculate by the following equations [58]:
q = ((C
0
– C
e
) V)/m
[1]
[2]
%
adsorption = (100 (C
0
– C
e
)) / C
0
Where q is the amount of absorbed dyes on per gram of adsorbent (mg/g), C
0
and C are the initial
e
and equilibrium concentrations of dyes in aqueous solutions (mg/L), respectively, V is the initial
volumes of dye solutions (L), and m is the mass of the adsorbent (g).
2
.8. General procedure for Suzuki–Miyaura coupling reaction
A mixture of aryl halide (1 mmol), phenylboronic acid (1.2 mmol), K
2
CO
3
(2 mmol), Fe
3
O
4
-
Au@SF-SBA-15 nanocatalyst (50 mg or 0.64 mmol% of Au) and 5 mL of C
2
H
5
OH/H O (1:1)
2
o
were added to a 25 mL round-bottom flask and stirred at 80 C for appropriate times. After
completion the reaction (controlled by TLC), the mixture was cooled and the catalyst was
recovered by a magnet and washed three times with ethyl acetate and ethanol. The combined
organic layer was dried over MgSO
4
. Then the solvent was evaporated and the residue was purified
1
13
by column chromatography and characterized by its H NMR, C NMR and melting point.
3
3
. Results and discussion
.1. Characterization of Fe
3
4
O -Au@SF-SBA-15
XRD patterns of Fe
3
O
4
NPs (a), Au@SF-SBA-15 (b), and Fe
3
4
O -Au@SF-SBA-15 (c) present in
Figure 1. In Figure 1a, the strong peaks at 2θ = 30.2°, 36.4°, 43.7°, 53.5°, 56.3°, 62.3°, and 73.8°
are attributed to the (220), (311), (400), (422), (511), (440) and (533) planes of highly crystalline
cubic spinel structure of Fe
3
O
4
NPs [59,60] (JCPDS no. 19–0629). In Figure 1b, a broad peak at
2
6
θ = 23° is attributed to the amorphous silica walls [61] and diffraction peaks at 2θ = 38.4°, 44.2°,
o
4.3°, and 77.7 are corresponded to the (111), (200), (220) and (311) planes of metallic Au NPs
(JCPDS no. 04-07834). The same sets of characteristic peaks in Figures 1a and 1b are also
6

observed for Fe
3
4
O -Au@SF-SBA-15 indicating the presence and stability of the crystalline phase
of Fe and Au NPs on the amorphous silica walls (Figure 1c).
3
O
4
Figure 1: The XRD patterns of the Fe
O
3 4
NPs (a), Au@SF-SBA-15 (b) and Fe
3
4
O -Au@SF-SBA-15 (c).
The low‐angle X‐ray scattering measurement can effectively identify the mesoporous materials
with hexagonal symmetry. Figure 2 shows the low‐angle XRD pattern of Fe -Au@SF-SBA-15.
It can be observed that the three distinct reflections for Fe -Au@SF-SBA-15 in the low-angle
3
O
4
3
O
4
region indexed as (100), (110) and (200), are exactly same as reflections for SBA-15 [62]. This
7

observation proves that the ordered hexagonal mesoporous structure of SBA-15 preserves its
structure after Schiff-base functionalization and immobilization of Au NPs.
Figure 2: Low-angle XRD spectrum of Fe
3
O -Au@SF-SBA-15.
4
ICP and EDS analyses confirm the presence of Au units in the Fe
3
4
O -Au@SF-SBA-15
nanocomposite. FESEM and TEM images can characterize the morphology of the synthesized
nanocomposite. TEM images clearly showed the presence of well-ordered two-dimensional
hexagonal mesoporous for the SBA-15 (Figure 3a), Au@SF-SBA-15 (Figure 3b) and Fe
Au@SF-SBA-15 (Figure 3c). TEM images for Au@SF-SBA-15 and Fe -Au@SF-SBA-15
show that Au and Fe NPs have been well-dispersed on the surface of SBA-15 and suggests that
3
4
O -
3
O
4
3
O
4
the SBA-15, despite the loading of metal NPs has sufficient structural stability to retain its
morphology. Small black dots, observed in the TEM images in Figure 3b and 3c, are assigned to
Au(0) NPs [63] and bigger black dots in the TEM image of Fe
3
4
O -Au@SF-SBA-15 (Figure 3c)
that have not appeared in the TEM image of Au@SF-SBA-15 (Figure 3b), are attributed to Fe
3
O
4
NPs.
The FESEM images of SBA-15 and Fe
3
4
O -Au@SF-SBA-15 are shown in Figure 3d-f. As can be
seen in Figure 3d, the FESEM of SBA-15 sample consists of short rods with wheat like
morphologies and rod-like structures that randomly packed bunch. In addition, these types of
morphologies have long-range parallel channels with two-dimensional hexagonal structures. The
8

morphology of SBA-15 after functionalization with Schiff-base and immobilization of Au NPs is
same as the SBA-15 (Figures 3e and 3f). The Mapping analysis and EDX pattern of Fe O -
3
4
Au@SF-SBA-15 show the presence of Au, C, N, O and S atoms, which it suggests that successful
grafting and immobilizing of Schiff-base and Au NPs on the surface of SBA-15 support.
a
b
d
e
9

g
f
h
Figure 3: TEM images of SBA-15 (a), Au@SF-SBA-15 (b) and Fe
images of SBA-15 (d) and Fe
mapping analysis of Fe -Au@SF-SBA-15 (h).
3
O
4
-Au@SF-SBA-15 (c) and FESEM
3
O
4
-Au@SF-SBA-15 (e,f), EDX pattern of Fe
3
O -Au@SF-SBA-15 (g) and
4
O
3 4
1
0

In a comparative mode, the FT‐IR spectra for each step of the synthesis of introduced
nanocomposite (Fe
-Au@SF-SBA-15) are available in Figure 4a-f. The broad band at 3450 cm−1
in Figure 4a is assigned to O―H stretching of silanol groups and the adsorbed water molecules.
O
3 4
-1
The stretching and bending vibrations of Si–O–Si can be detected at 1100 and 800 cm ,
−
1
respectively. The weak peaks at 2990 and 2850 cm in Figure 4b-e are assigned to the methylene
−
1
group vibrations in the propyl chain. The appeared peak at around 1640 cm is attributed to the
characteristic vibration of imine group of N-(thiophen-2-ylmethylene)-1,3,5-triazine-2,4,6-
−
1
triamine, while this band shifts to lower frequency (1630 cm ) in the Au@SF-SBA-15 catalyst
(Figure 4d). The shifting of this band to lower frequency confirmed the formation of metal–ligand
−
1
bonds. In addition, the band at 1540 cm is assigned to aromatic ―C=C― stretching vibration.
−
1
Figure 4e exhibits a characteristic peak at 579 cm , which is attributed to the Fe‐O stretching
vibration that the existence of this peak proves the presence of Fe NPs in the synthetic
3
O
4
nanocomposite. These results indicate that the organic moieties are successfully grafted to the
SBA-15. The FT-IR spectrum of corn silk extract presents in Figure 4f. Corn silk contains proteins,
carbohydrates, vitamins, minerals, and fiber. Corn silk is rich of flavonoids [64]. Flavonoids are strong
anti-oxidants. The presence of flavonoids in the corn silk can be proven qualitatively through FT-IR
spectroscopy. The FT-IR spectrum in Figure 4(f) shows the following characteristic peaks: 3418,
-1
2
930, 2870, 1611, 1389 and 1050 cm , respectively. The appeared peaks at 3418, 2930 and 2870
-1
cm are related to the OH group and the symmetric stretching vibration of CH
2
. The peak at 1611
-1
-1
cm is assigned to the stretching vibration of ―C=C― and the other peak at 1389 cm is
-1
corresponded to the phenolic OH. The appeared peak at 1050 cm is attributed to the stretching
vibration of C-H, which is specific for the aromatic nucleus [65].
The N
2
adsorption/desorption isotherms for SBA-15, SF-SBA-15 and Fe
3
4
O -Au@SF-SBA-15 are
shown in Figure 5. The textural properties present in Table 1. As can be seen in Figure 5, all
materials show a type IV standard IUPAC isotherm hysteresis loop at a high relative pressure
values (P/P
distribution [67]. However, their shapes are different, with a capillary condensation of N
occurring over a slightly lower P/P range when going from SBA-15 to Fe -Au@SF-SBA-15
nanoadsorbent [68]. With the same order for the adsorbed N volume, the pore volumes decrease.
0
= 0.65) [66]. It is a characteristic of mesoporous materials with a narrow pore size
2
0
3
O
4
2
The data of the isotherms with the correlation coefficient from BET fitting are used to calculate
the physicochemical properties of the materials. The reduction in the specific surface area and pore
1
1

size of functionalized SBA-15 compare to SBA-15 can be attributed to the presence of Schiff-
base, Au and Fe , which blocks the channels (Table 1)
3
O
4
4
3
3
2
2
1
1
0
0
.5
.5
.5
.5
4500
4000
3500
a
3000
2500
2000
1500
d
1000
e
500
0
-1
Wavenumber (cm )
b
c
f
Figure 4: FT-IR spectra of SBA-15 (a), SBA-15-Cl (b), SF-SBA-15 (c), Au@SF-SBA-15 (d), Fe
Au@SF-SBA-15 (e) and corn silk extract (f).
3
O -
4
700
600
500
400
300
200
100
0
a
b
c
0
0.2
0.4
0.6
0.8
1
1.2
p/p0
Figure 5: N
2
adsorption-desorption isotherm curves for SBA-15 (a), SF-SBA-15 (b) and Fe
3
O -Au@SF-
4
SBA-15 (c).
1
2

Table 1: Specific surface area, pore volume and mean pore diameters for SBA-15, SF-SBA-15 and Fe
3
O -
4
Au@SF-SBA-15 nanocomposite.
2
-1 a
3
1- b
Pore diameter (nm)c
Materials
Surface area (m g )
622.63
Pore volume (cm g )
SBA-15
0.92
0.70
0.48
7.69
7.00
5.90
SF-SBA-15
-Au@SF-SBA-15
363.75
Fe
3
O
4
274.12
a
b
c
S
BET
Total pore volume determined at P/P
Mean pore diameter determined using the BJH method.
0
= 0.99.
UV-Vis spectroscopy is a useful tool for investigating formation of gold NPs in the presence of
corn silk extract. As can be seen in Figure 6, the appeared peak at 550 nm after reduction of HAuCl
4
by the plant extract indicates the formation of Au NPs [69].
4
3
2
1
0
.5
3
.5
2
.5
1
.5
0
350
450
550
650
750
850
Wavelength (nm)
Figure 6: UV-Vis spectrum of reduction of HAuCl in the presence of corn silk extract.
4
The thermogravimetric analysis of SF-SBA-15 and Fe
3
4
O -Au@SF-SBA-15 nanocomposite
o
presents in Figure 7. The initial weight losing before 100 C might be attributed to the elimination
of physically adsorbed water. The new step of the weight losing for SF-SBA-15 between 240-620
o
o
C (39.4%) and for Fe
the decomposition of organic templates. As can be seen, by the incorporation of Fe
NPs, the weight losing decreased from 39.4% to 14.53% because of the increasing of inorganic
3
O
4
-Au@SF-SBA-15 between 100-600 C (14.53%) could be attributed to
3
O
4
and Au
1
3

o
content in the Fe
3
O -Au@SF-SBA-15. Another step of weight losing after 600 C is due to the
4
decomposition of silanol groups [70].
1
10
00
1
90
80
70
60
50
b
a
0
200
400
600
800
1000
1200
o
Temperature ( C)
Figure 7: TG curves of SF-SBA-15 (a) and Fe
3
4
O -Au@SF-SBA-15 (b).
Elemental analysis (CHN) of bulk catalysis (Fe
3
4
O -Au@SF-SBA-15) shows the percentage of C,
H, N, S and O atoms. In addition, the amounts of Fe and Au were measured by ICP-OES analysis.
The results present in Table 2.
Table 2: Elemental analysis for Fe
3
O -Au@SF-SBA-15 nanocomposite.
4
C
H
N
S
O
Au
Fe
4.63%
2.02%
2.41%
1.58%
9.45%
2.54%
8.59%
3
.2. Evaluation of the catalytic performance for the adsorption of dye molecules
MO and MB as the typical organic pollutants were selected to evaluate the catalytic performance
of the Fe -Au@SF-SBA-15 with NaBH as reducing agent. NaBH was used, since it has high
electron injection capacity of the BH
-. Due to the high importance of removing of dye pollutants
3
O
4
4
4
4
from the environment, many researchers have focused their attention on this issue [71-77].
In the absence of adsorbent or reducing agent, the degradation efficiency is very low and after 2 h
no significant changes of the peaks at 464 and 553 were observed (Figure 8a and 8b). In addition,
1
4

in the presence of SF-SBA-15 with or without NaBH
apperceived. This suggest that neither the nanocatalyst nor NaBH
swift degradation of dye molecules. Green plots in Figure 8 show the role of Fe
adsorption process. Based on the obtained results in the absence of magnetic NPs, complete
degradation of MO and MB happens. The amount of Fe NPs in the adsorbent (Fe -Au@SF-
SBA-15) is much less than that play a significant role in degradation process. A little amount of
Fe NPs only added to functionalized SBA-15 just for easy separating of nanoadsorbent at the
4
no significant changes of the peaks
alone is able to catalyze the
NPs in the
4
3
O
4
3
O
4
3
O
4
3
O
4
end of the reaction.
Based on the obtained results in Figures 8c and 8d, the degradation of organic dyes could be
complete quickly when both Fe
3
O
4
-Au@SF-SBA-15 and NaBH
4
exist together. This observed
-
enhancement of the catalytic efficiency relates to the relaying of electrons from BH
MB via the Au NPs [78]. Therefore, the Fe
catalytic activity for adsorbing of these dye molecules. Figure 9 shows the kinetic curves for the
reduction of MO and MB by Fe -Au@SF-SBA-15 nanoadsorbent. The linear variation of
ln(퐶 /퐶 ) versus reaction time indicates that the reaction follows pseudo-first-order kinetics, where
and 퐶 represent the concentration of the organic dyes after time 0 and 푡, respectively [79].
According to the formula ln(퐶
4
to MO and
3
4
O -Au@SF-SBA-15 nanocomposite has superior
3
O
4
푡
0
퐶
0
푡
−
1
푡
/퐶
0
) = −푘푡, the apparent rate constants 푘 (sec ) estimated from the
−
1
slopes of straight lines for MO and MB are 0.0137 and 0.0133 sec , respectively.
Three independent variables (pH, the amount of adsorbent and contact time) show the effect of
reaction variables on the degradation of MO and MB. There are the range and levels of the
variables in Table S1 (see supporting information). Twenty experiments were designed and the
Removal % of dyes were measured as the process responses. The results and experimental
conditions present in Tables S2 and S3 for MO and MB, respectively. The ANOVA tables
(Analysis of Variance) for effective variables on catalyst function for degradation of MO and MB
are presented in Tables S4 and S5, respectively. The significance of each of the independent
variables evaluates by F-value and P-value tests. Significant model terms and meaningful
interactions are those that P-value for them is less than 0.05. So, it is deduced from Table S4, A,
2
2
B, C, A , C , AC and BC with P-value < 0.05 are significant model terms or the most effective
variables on catalyst function for degradation of MO (A: pH, B: the amount of adsorbent and C:
contact time). Also, based on Table S5, the most effective variables on catalyst function for
2
degradation of MB are A, C, A , AB and BC. Surface and contour plots related to meaningful
1
5

interactions present in Figures S1-S4. Adjusted R-squared for degradation of MO and MB is
almost up to 99%, which shows that the proposed model for investigating the Removal % of dyes
is very suitable. Equations 3 and 4 state the relationship between meaningful experimental
variables and process responses for MO and MB, respectively (the constant value and the
coefficients for effective variables are available in Tables S4 and S7 in supporting information).
2
2
Y = -501.57+154.97A+9.68B+0.898C-11.15A -0.001C -0.051AC-0.0126BC
[3]
[4]
2
Y = -552.58+158.64A-0.260C-9.665A -2.830AB+0.052BC
2
a
Initial
1
1
1
1
.8
.6
.4
.2
1
2
2
2
2
2
h with SF-SBA-15
h with NaBH
4
h with SF-SBA-15 + NaBH
h with Fe -Au@SF-SBA-15
13 s with Au@SF-SBA-15 + NaBH
4
0
0
0
0
.8
.6
.4
.2
0
3
O
4
4
200
400
600
800
-
0.2
Wavelength (nm)
b
Initial
1
1
0
0
.9
.4
.9
.4
2
2
2
2
2
h with SF-SBA-15
h with NaBH
4
h with SF-SBA-15 + NaBH
h with Fe -Au@SF-SBA-15
75 s with Au@SF-SBA-15 + NaBH
4
3
O
4
4
-
0.1
400
500
600
700
800
Wavelength (nm)
1
6

Figure 8: Reduction of MO (a) and MB (b) by different catalytic situations and time-dependent UV-Vis
absorption spectra for the reduction of MO (c) and MB (d) by Fe -Au@SF-SBA-15 nanoadsorbent in
3
O
4
the presence of NaBH
4
.
0
.5
0
0.5
0
b
a
0
50
100
150
200
0
50
100
150
-0.5
-
0.5
-
1
-1
-
1.5
-1.5
-
2
y = -0.0137x + 0.0799
R² = 0.9934
y = -0.0133x + 0.1274
R² = 0.9928
-
2
-2.5
Time (s)
Time (s)
Figure 9: Kinetic curves for the reduction of MO (a) and MB (b) by Fe
3
O -Au@SF-SBA-15
4
nanoadsorbent.
3
.3. Mechanism of reduction of dyes by Fe
3
4
O -Au@SF-SBA-15 nanoadsorbent
The catalyst composition has a significant effect on the catalyst activity [80]. In the reduction
process of organic dyes, Au NPs supported on SF-SBA-15 surface serve as an electron relay
system [81]. Scheme 2 shows the possible mechanism for reduction of organic dyes in the presence
1
7

4-
of NaBH
4
by Au NPs. At the first step, the adsorption of dye molecules and BH occurs on the
4-
surface of adsorbent. In other words, dye molecules and BH first diffuse from the aqueous
solution to SBA-15 and then, Au NPs immobilized on the SF-SBA-15 act as a catalyst to transfer
4-
electrons from BH to the organic dyes and these events lead to the reduction and decolorization
of dye molecules.
Scheme 2: Proposed mechanism for reduction of MO and MB in the presence of Fe
and NaBH
3
4
O -Au@SF-SBA-15
4
.
3
.4. Evaluation of the catalytic activity in Suzuki-Miyaura coupling reaction
In order to evaluate the catalytic activity, the Fe -Au@SF-SBA-15 nanocatalyst was used to
3
O
4
synthesis biphenyl derivatives through Suzuki-Miyaura reaction. This catalytic system can
catalyze Suzuki coupling reaction in a short time compare to Pd NPs. Moreover, Au NPs can
efficiently catalyze the formation of C-C bond between less-reactive aryl halides (chlorobenzene)
and phenylboronic acid. A comparison table for the evaluation of the efficiency of this catalytic
system with Pd NPs presents in Table 3.
The model reaction for optimizing the effect of base, solvent, temperature and the amount of Au-
loading is the coupling reaction between iodobenzene and phenylboronic acid. The results are
available in Table 4. As can be seen, the nature of base strongly affects the efficiency of the
coupling reaction (Table 3, entries 11-15) and K
2
CO afforded the best results for this reaction and
3
1
8

allowed the biphenyl product obtained in 98% yield after 45 min. The other bases such as NEt
NaOH, K PO and NaOAc were not as effective as K CO and only afforded moderate to low
yields of the product. Another efficient factor on the catalytic performance of Fe -Au@SF-
3
,
3
4
2
3
3
O
4
SBA-15 is Au-loading (Table 4, entries 16-19). Based on the obtained results the best performance
is obtained when 50 mg of catalyst (0.64 mmol% of Au) is used but in other cases, high yields of
the C–C coupled products were not achieved. Finally, the effects of solvent and temperature were
examined and the best result was obtained in H
2
O: EtOH (1:1) at 80 °C for 45 min in the presence
CO
of 50 mg of Fe -Au@SF-SBA-15 (0.64 mmol% of Au) using 2 mmol of K
3
O
4
2
3
.
Entry 22 shows the role of magnetic nanoparticles in the coupling reaction. Based on the obtained
results in the absence of magnetic NPs, there is no change in the yield of the production of
biphenyl. Therefore, these nanoparticles would only help to easy separation of the catalyst from
the reaction mixture.
We applied optimized reaction conditions for the coupling of aryl halides with phenylboronic
acids. The results are available in Table 5.
Table 3: Suzuki coupling of chlorobenzene with phenylboronic acid catalyzed by a variety of catalysts.
Entry
Catalyst
Time Base
h)
Temp.
( C)
120
50
85
80
60
120
80
80
Yield
(%)
20
8
Ref.
o
(
1
2
3
4
5
6
7
8
Pd(II)–diimine complex
Pd–L/SBA-16
24
3
K
K
2
CO
PO
CO
CO
Na CO
CO
NaOH
CO
3
[82]
[83]
[84]
[85]
[86]
3
4
[PdBr
2
{et(impy)}
2
]
16
24
24
18
4
Cs
2
3
0
Pd(0)–MCM-41
Pd(II)–MCM-41
Molecular sieves-supported Pd catalyst
Au@PATP
K
2
3
24
60
84
87
95
2
3
K
2
3
[87]
[37]
Fe
3
O
4
-Au@SF-SBA-15
1.33
K
2
3
This work
L = 1,1,3,3-tetramethyl-2-(3-trimethoxysilylpropyl)-guanidine, et(impy) = ethylene bridged imidazo[1,5]
pyridine-3-ylidenes, PATP = poly(2-aminothiophenol).
1
9

Table 4: Optimization of the reaction conditions.a
HO
OH
I
B
+
Entry
Catalyst
Solvent
Base
Au-Loading
Temp. Time Yieldb
(
mmol%)
0.64
0.64
0.64
0.64
0.64
0.64
0.64
0.64
0.64
0.64
0.64
0.64
0.64
0.64
0.64
0.13
0.32
0.51
0.77
0.64
0.64
0.64
(o C)
reflux
reflux
reflux
reflux
reflux
100
100
100
80
(min)
300
100
100
300
100
100
100
100
45
(%)
11
54
41
13
35
56
48
63
81
89
98
78
66
83
79
25
54
83
98
trace
57
97
1
2
3
4
5
6
7
8
9
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
-Au@SF-SBA-15
-Au@SF-SBA-15
-Au@SF-SBA-15
-Au@SF-SBA-15
-Au@SF-SBA-15
-Au@SF-SBA-15
-Au@SF-SBA-15
-Au@SF-SBA-15
-Au@SF-SBA-15
-Au@SF-SBA-15
-Au@SF-SBA-15
-Au@SF-SBA-15
-Au@SF-SBA-15
-Au@SF-SBA-15
-Au@SF-SBA-15
-Au@SF-SBA-15
-Au@SF-SBA-15
-Au@SF-SBA-15
-Au@SF-SBA-15
-Au@SF-SBA-15
-Au@SF-SBA-15
toluene
K
2
CO
3
H
2
O
K
2
CO
3
EtOH
THF
K
K
K
K
K
K
K
K
K
2
2
2
2
2
2
2
2
2
CO
CO
CO
CO
CO
CO
CO
CO
CO
3
3
3
3
3
3
3
3
3
DMF
DMF:H
DMF:H
DMF:H
2
2
2
O (1:1)
O (2:1)
O (1:2)
H
H
H
H
H
H
H
H
H
H
H
H
H
H
2
2
2
2
2
2
2
2
2
2
2
2
2
2
O:EtOH (1:2)
O:EtOH (2:1)
O:EtOH (1:1)
O:EtOH (1:1)
O:EtOH (1:1)
O:EtOH (1:1)
O:EtOH (1:1)
O:EtOH (1:1)
O:EtOH (1:1)
O:EtOH (1:1)
O:EtOH (1:1)
O:EtOH (1:1)
O:EtOH (1:1)
O:EtOH (1:1)
1
1
1
1
1
1
1
1
1
1
2
2
2
0
1
2
3
4
5
6
7
8
9
0
1
2
80
45
80
45
NEt
NaOH
PO
NaOAc
3
80
45
80
45
K
3
4
80
45
80
45
K
K
K
K
K
K
K
2
2
2
2
2
2
2
CO
CO
CO
CO
CO
CO
CO
3
3
3
3
3
3
3
80
45
80
45
80
45
80
45
25
45
50
45
Au@SF-SBA-15
80
45
a
Reaction conditions: iodobenzene (1 mmol), phenylboronic acid (1.2 mmol), base (2 mmol) and 5 mL of solvent.
Isolated yield.
b
2
0

Table 5: Coupling of aryl halides with phenylboronic acid derivatives in the presence of catalytic
amounts of Fe
O
3 4
-Au@SF-SBA-15.a
HO
OH
X
B
Fe O -Au@SF-SBA-15
3
4
R
+
*
o
R
*
H O/EtOH , 80 C
R
2
R
b
c
-1
Entry
X
R
R*
Product
Time
Yield (%)
TOF (h )
(
min)
45
1
2
3
4
5
6
7
8
9
I
Br
Cl
I
H
H
H
H
1a
1a
1a
1b
1b
1b
1b
1c
1c
1c
1d
1d
1d
1e
1e
1f
98
97
95
97
94
93
90
92
90
94
97
94
95
95
87
96
93
91
204.16
151.56
111.37
165.25
146.88
134.20
76.92
60
H
H
80
4-Me
4-Me
H
H
55
Br
Br
Cl
I
H
60
4-Me
4-Me
H
65
H
110
60
4-NO
H
2
143.75
70.31
Cl
Br
I
4-NO
H
2
120
75
1
1
1
1
1
1
1
1
1
0
1
2
3
4
5
6
7
8
4-NO
2
117.5
4-OMe
H
H
60
151.56
135.64
127.18
127.18
54.38
I
4-OMe
65
Br
I
4-OMe
4-COMe
H
H
H
70
70
Cl
I
4-COMe
4-OMe
H
150
80
4-OMe
112.54
145.31
142.19
I
3,4-Dimethoxy
2-Amino
1g
1h
60
I
H
60
a
Reaction conditions: aryl halide (1 mmol), phenylboronic acid (1.2 mmol), K
and 0.05 g of Fe -Au@SF-SBA-15 (0.64 mmol% of Au).
Isolated yields.
2
CO
3
(2 mmol), H O:EtOH (5 mL)
2
3
O
4
b
c
TOF (turnover frequency) = (moles converted/mol of active site).(yield/ time).
3
.5. Reusability of Fe
3
O
4
-Au@SF-SBA-15 nanocomposite
-Au@SF-SBA-15 nanoadsorbent, after each
To examine the stability and recyclability of Fe
3
O
4
adsorption, we separated the adsorbent with a magnet, washed three times with distilled water and
ethanol and then, reused for another color degradation reaction to evaluate its recycling
performance. In addition, for investigating the catalytic performance of Fe
3
4
O -Au@SF-SBA-15,
2
1

after the reaction between iodobenzene and phenylboronic acid, the catalyst separated by a magnet,
washed three times with ethyl acetate and ethanol and then, reused for another coupling reaction.
As observed in Figure 10, the Fe
3
4
O -Au@SF-SBA-15 nanoadsorbent can be successfully reused
for fourteen times for adsorption of dye molecules and eight times for catalyzing the Suzuki-
Miyaura coupling reaction (Figure 11) only with a very small loss in its catalytic activity. These
results indicate the high stability of the Fe
3
4
O -Au@SF-SBA-15 nanocomposite. A small decrease
in its performance is due to the loss of catalyst after every recycling. After fourteen cycles for
degradation of MB, the XRD peaks of the reused nanoadsorbent match well with the fresh
nanocatalyst and it confirms the recoverability of the nanocomposite and stability of the Au NPs
during the reaction (see Figure S5 in supporting information).
100
95
90
85
80
75
70
1
2
3
4
5
6
7
8
9 10 11 12 13 14
Number of cycle
MO MB
Figure 10: The reusability of Fe
Reaction conditions for each cycle: NaBH
MB (18 mL, ꢀ4 ppm) and 1 mg of Fe -Au@SF-SBA-15 nanoadsorbent (0.013 mmol% of Au).
O
3 4
-Au@SF-SBA-15 nanoadsorbent for degradation of MO and MB.
(
4
aqueous solution (2 mL, 0.25 M), aqueous solutions of MO or
O
3 4
2
2

100
95
90
85
80
75
70
1
2
3
4
5
6
7
8
Run
Figure 11: The reusability of Fe
3
O -Au@SF-SBA-15 nanocatalyst for Suzuki-Miyaura coupling reaction
4
between iodobenzene and phenylboronic acid. (Reaction conditions for each cycle: iodobenzene (1 mmol),
phenylboronic acid (1.2 mmol), K
Au).
2
CO
3
(2 mmol) and 0.05 g of Fe
3
4
O -Au@SF-SBA-15 (0.64 mmol% of
3
.5.1. Leaching test
ICP analysis can evaluate the stability of the nanocomposite after several uses in reaction cycles.
The ICP-OES results showed that only 0.05 wt% and 0.06 wt% of Au was leached after fourteen
cycles for degradation of MO and MB, respectively and 0.04 wt% of Au was leached after eight
cycles for the production of biphenyl. The results are available in Table 6.
Table 6: ICP analysis for determination of the amount of Au and Fe
3
O NPs after several reaction cycles
4
ICP results after fourteen
cycles for degradation of MO
ICP results after fourteen ICP results after eight cycles
cycles for degradation of MB for the production of biphenyl
Au (wt%)
.49
Fe (wt%)
7.36
Au (wt%)
2.48
Fe (wt%)
7.34
Au (wt%)
2.50
Fe (wt%)
7.45
2
4
. Conclusions
In this study, we introduced a green and economic method for the synthesis of Au NPs immobilized
on SF-SBA-15 by corn silk extract. This nanocomposite successfully used for the degradation of
the chemically and biologically resistant cationic (MB) and anionic (MO) dyes. The results exhibit
that this heterogeneous composite can efficiently (>99%) degrade organic dyes within a short time.
The reduction followed pseudo-first-order kinetics. Moreover, this nanocomposite can
successfully catalyze the Suzuki-Miyaura coupling reaction between aryl halides and
2
3

phenylboronic acids. The biphenyl products obtained in excellent yields even for chlorobenzene
derivatives at mild conditions. The unique characteristics of this nanocomposite include its simple
and green preparation method, ease of separation, stability and recyclability. These features made
it an excellent and sustainable adsorbent and catalyst in comparison with other systems. On the
other hand, the high performance of this nanocomposite for degradation of various organic dyes,
such as MO and MB indicates that this nanocomposite is also favorable for other dyes, potentiating
its wide applications for industrial removal of dyes. Thus, it would be an ideal platform for the
fabrication of highly efficient catalytic systems for various catalytic applications.
Acknowledgements
The authors gratefully acknowledge Razi University for the financial support.
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