A green approach to the synthesis of Ag doped nano magnetic γ-Fe2O3@SiO2-CD core-shell hollow spheres as an efficient and heterogeneous catalyst for ultrasonic-assisted A3 and KA2 coupling reactions

Article abstract of DOI:10.1039/c7ra04635a

Using a green approach and ultrasonic-assisted template-free five-step process, a novel heterogeneous catalyst, γ-Fe₂O₃@SiO₂-CD/Ag hollow spheres (h-Fe₂O₃@SiO₂-CD/Ag), was fabricated. The d

Full text of DOI:10.1039/c7ra04635a

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A green approach to the synthesis of Ag doped
nano magnetic g-Fe2O3@SiO2-CD core–shell
hollow spheres as an efficient and heterogeneous
catalyst for ultrasonic-assisted A3 and KA2 coupling
reactions†
Cite this: RSC Adv., 2017, 7, 36807
a
b
S. Sadjadi, M. Malmirb and M. M. Heravi
*
*
Using a green approach and ultrasonic-assisted template-free five-step process, a novel heterogeneous
catalyst, g-Fe2O3@SiO2-CD/Ag hollow spheres (h-Fe2O3@SiO2-CD/Ag), was fabricated. The design of
the catalyst was based on the synthesis of core–shell h-Fe2O3@SiO2 followed by amine
functionalization, reaction with tosylated cyclodextrin and subsequent Ag(0) doping using hollyhock
flowers collected from the Banaruiyeh District, in Larestan, Iran as extract-based reducing agent. The
novel catalyst was fully characterized using SEM/EDX, BET, TGA, ICP-AES, FTIR, VSM and XRD
techniques. The hybrid catalyst was successfully applied for promoting A3 and KA2 coupling reactions
under mild and green conditions, i.e. ultrasonic irradiation at room temperature in aqueous media.
Studying the reusability of the catalyst and Ag(0) nanoparticle leaching established that the catalyst can
be recovered and reused with preserving its catalytic activity and negligible Ag(0) leaching for five
reaction runs. Upon reusing for the sixth run, however, slight leaching of Ag(0) and consequently loss of
catalytic activity was observed.
Received 25th April 2017
Accepted 20th July 2017
DOI: 10.1039/c7ra04635a
rsc.li/rsc-advances
of a novel, efficient and heterogeneous catalyst for this process
is of great interest.
Introduction
Magnetic nanoparticles, MNPs, and its core–shell structures
received ongoing interest due to their outstanding properties
such as high magnetic susceptibility and coercivity, super
paramagnetic behaviour and high surface area.9 This class of
materials has been widely used for catalysis, target-drug
delivery, hyperthermia treatment of cancer, diagnosis, MRI,
enzyme immobilization, magnetic separation and sensors.10–12
Among various uses, catalysis has gained growing interest.
Using MNPs diverse range of heterogeneous catalysts with tune-
able size and morphologies can be designed and synthesized.
Another merit of MNPs is their facile separation from the
reaction mixture, which is more efficient and economical than
conventional ltration and centrifuge approaches.13–15
Propargylamine derivatives are key intermediates which are
broadly applied for the synthesis of a diverse range of natural
products and biologically active nitrogen containing chemicals
such as herbicides, fungicides, restricted peptide isosteres,
oxotremorine analogues, indolizines, pyrroles and quino-
lines.1–4 Propargylamines can be obtained from three-
component coupling reaction of amines, aldehydes and
alkynes, referred to as the A3 coupling reaction. Using this
valuable one-pot procedure several elements can be introduced
into a molecule.5 This reaction can be promoted by using
various metal catalysts such as gold, copper, iron, silver and
zinc based catalysts3,6,7 that are able to activate the terminal C–H
bond of the alkyne. Despite developing some promising meth-
odologies, some of them require a toxic solvent such as toluene,
or expensive media, like ionic liquids.8 Therefore, development
Hollow MNPs (h-MNPs), benet from high surface areas, tiny
particle sizes and low densities. However, they may suffer from
aggregations. To circumvent this issue, the surface of h-MNPs is
usually decorated with stabilizing agents.14
aGas Conversion Department, Faculty of Petrochemicals, Iran Polymer and
Petrochemical Institute, PO Box 14975-112, Tehran, Iran. E-mail: samahesadjadi@
yahoo.com
bDepartment of Chemistry, School of Science, Alzahra University, PO Box 1993891176,
Vanak, Tehran, Iran. E-mail: mmh1331@yahoo.com; mmheravi@alzahra.ac.ir; Fax:
+98 21 88041344; Tel: +98 21 88044051
Ultrasonic irradiation is a useful tool for developing green and
clean procedures for the synthesis of organic compounds. The
notable advantageous of ultrasonic-assisted approaches are
accelerated reaction rates, waste minimizing, and simplicity of the
procedure, high yields and purities of products.16,17 The effect of
ultrasonic irradiation can be dened on the base of cavitation
effect. The ultrasonic irradiation can cause high local
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ra04635a
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temperatures and pressure inside the bubbles and accelerates catalyst was compared with some of previously reported catalytic
mass transfer and turbulent ow in the liquid.17
procedures.
In last decade, we devoted our research to develop new
heterogeneous and reusable catalysts for promoting organic
transformations.14,18–22 In continuation of our attempts, herein, we
wish to disclose a novel efficient hybrid catalyst for promoting A3
and KA2 coupling reactions under mild and green reaction
condition. The design of the catalyst is based on fabrication of g-
Fe2O3 hollow spheres, which we disclosed in our previous report,14
followed by formation of SiO2 shell. The obtained h-Fe2O3@SiO2
was subsequently amine functionalized with 3-N-(2-(trimethox-
ysilyl)ethyl)methanediamine and reacted with tosylated cyclodex-
trin to form h-Fe2O3@SiO2-CD. The latter was then doped with
Ag(0) NPs by using hollyhock ower extract as a extract-based
reducing agent (Scheme 1). The reusability and Ag(0) leaching
were also studied. Moreover, the catalytic performance of the
Results and discussion
Catalyst characterization
To conrm the formation of h-Fe2O3@SiO2-CD/Ag, the FTIR
spectrum of the catalyst was obtained and compared with h-
Fe2O3@SiO2-CD, Fig. 1. The strong absorption bands observed
at 470–590 cmꢀ1 can be assigned to Fe–O stretching vibration
and conrm the formation of magnetic NPs.14 Both spectra
exhibited the characteristic band of Si–O stretching at 1082
cmꢀ1 indicating the successful coverage of SiO2 shell. The
absorption peaks at 980, 810 and 470 cmꢀ1 were in good
agreement with the bending vibrations of the Si–O–Si bond.9
The bands at 2925 cmꢀ1 and 3493 cmꢀ1 can be attributed to the
Scheme 1 The possible formation process of h-Fe2O3@SiO2-CD/Ag hollow spheres.
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Fig. 1 The FT-IR spectra of (a) h-Fe2O3@SiO2-CD and (b) h-Fe2O3@SiO2-CD/Ag.
Fig. 2 The XRD pattern of h-Fe2O3@SiO2-CD/Ag.
–CH2 stretching and –NH functionality and prove the conjuga- 2q ¼ 3–20 (labelled as *) can be assigned to the amorphous
tion of 3-N-(2-(trimethoxysilyl)ethyl)methanediamine. silica and CD. Moreover, the observed peaks at 30.3, 35.6, 43.2,
These bands can also represent the –OH and –CH2 groups in 54.0, 57.3, 63.0 and 74.6 (labelled as F) can be attributed to the
CD. Comparing the FTIR spectra of h-Fe2O3@SiO2-CD and h- {220}, {311}, {400}, {422}, {511}, {440} and {533} planes of the
Fe2O3@SiO2-CD/Ag reveals the broadening and sharpening of typical cubic structure hematite (g-Fe2O3) (JCPDS card no. 39-
the bands upon Ag incorporation. According to the previous 1346).14 The peaks labelled as A are characteristic peaks of Ag(0)
reports,23 biological extracts contain several functional groups NPs (JCPDS card no. 04-0783). Using Debye–Scherrer equation,
that can be clearly detected in FTIR analysis. Hence the the average particle size of Ag(0) NPs was estimated to be 37 nm.
broadening and sharpening of the bands in and h-Fe2O3@SiO2-
CD/Ag can be assigned to the presence of some functionalities thermo gravimetric analysis (TGA) of h-Fe2O3@SiO2-CD/Ag was
in hollyhock ower extract such as –OH, C]N, N–H, C–H etc. carried out, Fig. 3. Two degradation steps can be detected for h-
To provide more insight into the properties of the catalyst,
The X-ray diffraction pattern, XRD, was exploited for inves- Fe2O3@SiO2-CD/Ag over the range of 40–800 ꢁC. The rst weight
tigation of the phase and composition of h-Fe2O3@SiO2-CD/Ag, loss, observed at about 180 ꢁC, is due to loss of adsorbed water
Fig. 2. According to the literature,24 the broad halo observed at molecules and/or surface hydroxyl groups. The second weight
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Fig. 3 The TGA analysis of the h-Fe2O3@SiO2-CD/Ag.
loss can be assigned to the degradation of organic moiety. Using Fe2O3@SiO2-CD and h-Fe2O3@SiO2-CD/Ag exhibited spherical
TGA analysis, the content of organic moiety in h-Fe2O3@SiO2- morphologies, indicating that functionalization of h-Fe2O3@-
CD/Ag was estimated to be about 4 w/w%. To study the SiO2 with CD and incorporation of Ag(0) NPs did not alter the
morphology of the catalyst and investigating the effect of morphology dramatically.
attachment of 3-N-(2-(trimethoxysilyl)ethyl)methanediamine,
The EDX analysis of h-Fe2O3@SiO2-CD/Ag is shown in
CD and embedding Ag(0) NPs on the morphology, the FE-SEM Fig. 4d. The presence of the Fe and O atoms can represent the g-
analysis of h-Fe2O3@SiO2 and SEM analyses of h-Fe2O3@SiO2- Fe2O3 magnetic NPs. Moreover, observation of the Si and O
CD and h-Fe2O3@SiO2-CD/Ag were obtained, Fig. 4. As expected, atoms can prove the existence of SiO2 shell. The conjugation of
the h-Fe2O3@SiO2 possessed a spherical morphology. Both h- 3-N-(2-(trimethoxysilyl)ethyl)methanediamine and CD can be
Fig. 4 The FE-SEM analysis of (a) h-Fe2O3@SiO2 and SEM analyses of (b) h-Fe2O3@SiO2-CD, (c) h-Fe2O3@SiO2-CD/Ag and (d) the EDX analysis
of h-Fe2O3@SiO2-CD/Ag.
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conrmed by the presence of N, Si, C and C and O elements,
respectively. The existence of Ag atom in EDX analysis can
indicate the incorporation of Ag.
The elemental mapping analysis of h-Fe2O3@SiO2-CD/Ag is
illustrated in Fig. 5. As obvious, the N has been distributed
uniformly in the catalyst, indicating that 3-N-(2-(trimethoxysilyl)
ethyl)methanediamine decorated the whole surface of the h-
Fe2O3@SiO2–N2 surface evenly. The good dispersion of Ag NPs
also established that Ag(0) NPs were dispersed all around the
catalyst.
To disclose the textural properties of the catalyst, the
nitrogen adsorption–desorption isotherm of the h-Fe2O3@SiO2-
CD/Ag was obtained, Fig. 6. The shape of the isotherm
conrmed the porous structure and according to IUPAC clas-
sication exhibits type II nitrogen adsorption–desorption
isotherms with H3 hysteresis loops.25 The textural properties
including specic surface area, total pore volume and average
pore diameter of h-Fe2O3@SiO2-CD/Ag are reported in Table 1.
Additionally, these parameters were compared with those of h-
Fe2O3@SiO2-CD to investigate the inuence of incorporation of
Ag(0) doping on the textural properties of nal catalyst.
Comparison of these parameters established that doping of
Ag(0) NPs increased the specic surface area and total pore
volume while decreased average pore diameter. According to
the previous reports, the increase in specic surface area can
emerge from the presence of Ag(0) NPs on h-Fe2O3@SiO2-CD.26
Fig. 6 N2 adsorption–desorption isotherms of the catalyst.
Using room temperature vibrating sample magnetometer
(VSM), the magnetic property of h-Fe2O3@SiO2-CD/Ag was
studied and compared with that of h-Fe2O3 Fig. 7. As depicted,
the maximum saturation magnetization (Ms) value of h-Fe2-
O3@SiO2-CD/Ag was calculated as 41.15 emu gꢀ1 that is slightly
lower than Ms of h-Fe2O3 (45.28 emu gꢀ1).
Fig. 5 The elemental mapping analysis of h-Fe2O3@SiO2-CD/Ag.
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Table 1 Textural property of the h-Fe2O3@SiO2-CD/Ag and h-Fe2-
O3@SiO2-CD
dene the efficiency of ultrasound irradiation on the base of
cavitation phenomena. That is, formation, growth and collapse
of the cavities which result in generation of high local temper-
atures and pressures. These can concentrate huge amount of
energy from the conversion of kinetic energy of liquid motion
into heating of the contents of the cavities. This can be
considered as a driving force for promoting the organic trans-
formation.27 It is proved that the ultrasonic irradiation not only
can accelerate the reaction rate, but also affect the reaction
through altering the reaction pathway and enhancing chemical
reactivity. Subsequently, to elucidate whether the extract-based
reduction of the silver species can affect the catalytic activity,
the catalytic activity of h-Fe2O3@SiO2-CD/Ag was compared with
that of h-Fe2O3@SiO2-CD/Ag–H (in which the silver was reduced
by hydrazine). Interestingly, h-Fe2O3@SiO2-CD/Ag–H exhibited
comparative catalytic activity. This result established that both
chemical reduction and extract-based reduction can lead to the
catalysts with high catalytic activity. Hence, both preparation
procedures can be chosen. When the green chemistry is of
focus, the green-synthesis approach is privileged while using
commercially available hydrazine as the reduction agent can
render chemical reduction more feasible.
SBET
Total pore volume Average pore
Sample
(m2 gꢀ1
)
(cm3 gꢀ1
)
diameter (nm)
h-Fe2O3@SiO2-CD/Ag 21.067
h-Fe2O3@SiO2-CD 12.139
0.1834
0.1143
34.822
37.679
This result implies that the functionalization of h-Fe2O3 and
subsequent attachment of CD and incorporation of Ag(0) NPs
did not affect the magnetic property of h-Fe2O3 dramatically.
Furthermore, it can be found that there are no evident changes
in the coercivity from the enlarged view of the central loop of the
h-Fe2O3 and h-Fe2O3@SiO2-CD/Ag. The content of silver in h-
Fe2O3@SiO2-CD/Ag was also estimated by using ICP-AES anal-
ysis. Initially, the denite amount of catalyst was digested in
concentrated hydrochloric and nitric acids solution. The
resulting extract was then analyzed by ICP-AES. The Ag content
was calculated as 0.4 w/w%.
According to the literature, the Ag(0) NPs is the dominant
catalytic active site for promoting A3 coupling reaction.28–31 Ag(0)
NPs can activate the substrate (phenyl acetylene) and promote
formation of intermediates which then transformed into the
desired product (see Reaction mechanism section). Hence, it
can be concluded that the content of Ag(0) NPs can inuence
the catalytic activity. To conrm this postulation, h-Fe2O3@-
SiO2-CD/Ag with various content of Ag(0) (0.2–0.8 w/w%) were
synthesized and their catalytic activities were compared. It was
found that small amount of silver can catalyse the reaction
efficiently. Therefore, the need for scant amount of silver can be
considered as the merit of this protocol. Moreover, it was found
that increasing the amount of silver content from 0.2 to 0.4 w/
w% can improve the catalytic activity. This can be attributed to
Catalytic activity of nanomagnetic h-Fe2O3@SiO2-CD/Ag
Considering the importance of the A3 and KA2 coupling reac-
tions, the catalytic activity of the h-Fe2O3@SiO2-CD/Ag was
studied for promoting these reactions. Initially, reaction of
benzaldehyde, phenyl acetylene and morpholine was selected as
a model reaction and carried out in the presence of catalytic
amount of the catalyst (20 mg) under conventional reux
condition in aqueous media.
The results established high catalytic activity of the h-Fe2-
O3@SiO2-CD/Ag (90%). Motivated by this result and in attempt
to achieve a green and rapid procedure, the model reaction was
performed under ultrasonic irradiation. Gratifyingly, the
desired product was furnished in high yield (97%) in a very
short reaction time. According to the previous reports,16,27 we
Fig. 7 VSM analyses of (a) h-Fe2O3 and (b) h-Fe2O3@SiO2-CD/Ag.
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Fig. 8 Effects of loading of catalyst, duration of reaction and solvent for the synthesis of propargylamine.
the increase of the catalytic active sites. However, this trend was donating and electron withdrawing functional groups could
not sustainable and further increase of silver content not only furnish the corresponding products in high yields. However,
did not improve the catalytic activity but also had a detrimental using heterocyclic aldehyde, furfural, led to slightly lower yield.
effect on the catalytic performance. This observation can be These results established the reliability of this protocol.
attributed to the aggregation of the Ag(0) NPs aggregation.
Encouraged by these results, we studied the possibility of
To explore the role of CD in the catalytic activity, the catalytic replacing aldehydes with ketones, KA2 coupling reaction (Table
activity of h-Fe2O3@SiO2-CD/Ag was compared with that of 2, products 4r and 4s). Interestingly, h-Fe2O3@SiO2-CD/Ag
Fe2O3@SiO2–N2/Ag. The result clearly demonstrated the supe- could also catalyse this reaction to furnish the corresponding
rior catalytic activity of the former, indicating the effective role products in high yields. Finally, the efficiency of this protocol
of CD in the catalytic activity. According to the literature, this and the catalytic performance of h-Fe2O3@SiO2-CD/Ag for cat-
observation can be attributed to the NPs and CD.32 Moreover, it alysing the model reaction were compared with those of some
is postulated that the presence of the CD can stabilize the NPs.33 previously reported catalytic methodologies to disclose the
Then, the reaction variables including ultrasonic irradiation, merits of this procedure (Table 3). As obvious, h-Fe2O3@SiO2-
solvent and catalyst amount were optimized by altering the CD/Ag resulted in desired product in higher or comparative
reaction variables and studying their effects on the yield of the yields. However, compared to all cases, h-Fe2O3@SiO2-CD/Ag
model product (Fig. 8). As shown in Fig. 8, increasing the amount led to the product in shorter reaction time. Moreover, the pre-
of catalyst from 10 mg to 20 mg resulted in higher yields. sented protocol does not required any harsh reaction condition,
However, dependent on the reaction condition, more increase in inert atmosphere or toxic solvent.
the catalyst amount (to 30 mg) had different effects on the yield of
the product. As depicted, in toluene, acetonitrile and EtOH, this
Reaction mechanism
had a detrimental effect on the yield of the reaction and no
product was obtained using 30 mg catalyst. This can be assigned
to the formation of various by-products. In the case of water as
a solvent, the effect of the catalyst amount is inuenced by the
power of the used ultrasonic irradiation. Using 30 mg catalyst
under low (50 W) or high (100 W) power of ultrasonic irradiation
resulted in increase of the yield of the product while using
medium (70 W) ultrasonic power reduced the yield slightly. The
optimum reaction variables were found to be using 20 mg cata-
lyst under ultrasonic irradiation of power 70 W in aqueous media
and ambient temperature.
Considering the previous reports,14,34 the plausible mechanism
for the formation of propargylamines includes activation of
terminal C–H bond of phenylacetylene by Ag NPs of the catalyst
and formation of an intermediate, Ag–acetylide I. In this regard
the suggested mechanism involves the activation of the C–H
bond of alkyne via interaction by the metal nanoparticles (M).
Thus, terminal alkyne is converted into the M/acetylide
intermediate (I). The nucleophilic addition of the amine to the
already activated aldehyde by Ag leads to generation of iminium
ion (II). Next, the M/acetylide intermediate attacks the imi-
nium ion simultaneously to provide the corresponding prop-
argylamine (III). In this process one molecule of water can be
formed from hydroxyl group and hydrogen of terminal alkyne,
Scheme 2.
Having the optimum reaction condition in hand, the
generality of this protocol was examined by using various
amines and aldehydes with different electron densities (Table
2). As shown in this table, aldehydes with both electron
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Table 2 Synthesis of three-component reaction of derivatives aldehyde or cyclic ketone, secondary-amines and terminal alkynesa
Paper
3,14,34–36
R1
R2
R3
R4
Product
Time (min)
Yieldb (%)
1a: C6H5
1b: p-Cl–C6H4
1c: p-NO2–C6H4
1d: p-Me–C6H4
1e: p-MeO–C6H4
1f: o-OH–C6H4
1g: Furfuryl
1h: p-OHCC6H4
1a
1b
1c
1d
1e
1f
1g
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
3a: –(CH2)5–
3a
3a
3a
3a
3a
3a
3a
3b: –(CH2)2–O–(CH2)2–
3b
3b
3b
3b
3b
3b
3b
4a
4b
4c
4d
4e
4f
4g
4h
4i
4j
4k
4l
4m
4n
4o
4p
4q
4r
7
10
12
15
8
10
12
10
8
97
95
90
90
91
94
85
90
97
90
85
90
92
96
90
90
88
89
90
8
12
15
10
12
20
20
15
20
15
1i: m-NO2–C6H4
1a
2a: –(CH)5–
2a
3c: C2H5
3a
3b
C2H5
4s
a
Reaction conditions: aliphatic ketone 2a (1.0 equiv.) or arylaldehydes 1a (1.0 equiv.), amines 3a (1.0 equiv.), alkynes (1.1 equiv.), H2O (20 mL) and
h-Fe2O3@SiO2-CD/Ag (0.02 g) under ultrasonic irradiation (70 W) at room temperature. b Isolated yield.
Table 3 Comparison of h-Fe2O3@SiO2-CD/Agwith other recently reported acid catalysts for the synthesis of propargylamine
Catalyst (amount)
Reaction conditions
Time (min)
Catalyst amount
Yield (%)
Ref.
CuI
H2O/r.t./U.S
45
7
60
15 mg
20 mg
10 mg
20 mg
10 mol%
10 mol%
10 mg
2 mol%
0.5 mol%
350 mg
10 mol%
98
97
96
91
89
89
95
95
91
85
89
37
h-Fe2O3@SiO2-CD/Ag
h-Fe2O3@DA/Ag
Gold nanocrystals stabilized on montmorillonite
ZnO nanoparticles
Immobilized silver on surface-modied ZnONPs
Cu2O–ZnO
CuCN
CuNPs/TiO2
Naon-NR50
ZnS
H2O/r.t./70 W
This work
S.F./90 ꢁ
C
14
38
5
30
35
39
1
Reux/toluene
Stirrer/90 ꢁ
Reux/H2O
S.F./100 ꢁ
[bmim]PF6/S.F./120 ꢁ
180
120
240
60
120
420
300
270
C
C
C
Neat/70 ꢁ
C
CH3CN/70–80 ꢁC/N2 atm
Reux/CH3CN
40
41
Reusing the catalyst for the h run, however, led to slight loss
of catalytic activity. Upon reusing the catalyst for the sixth run,
remarkable loss of catalytic activity (10%) was observed and the
desired product was furnished in lower yield. To justify this
behaviour, the leaching of Ag(0) NPs of reused catalysts were
studied using hot ltration method and ICP-AES analysis. The
results demonstrated zero leaching of Ag(0) NPs for the catalysts
reused for four times. For the catalyst reused for h run only
negligible loss of Ag(0) NPs was observed. This can justify the
Catalyst reusability
The reusability of a heterogeneous catalyst is an important
feature that makes it green and economically-efficient. To
investigate the possibility of reusing h-Fe2O3@SiO2-CD/Ag, the
yields of the model reaction in the presence of fresh and reused
catalysts in various time intervals were measured and
compared, Fig. 9.
As shown in this gure, the catalyst can be recovered and
reused for four times without any loss of catalytic activity.
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Scheme 2 Plausible mechanism for the synthesis of propargylamine catalyzed by h-Fe2O3@SiO2-CD/Ag.
cyclodextrin,
N-(2-(trimethoxysilyl)ethyl)methanediamine,
toluene, triethylamine, acetone, ethanol, ethylene glycol (EG),
urea, AgNO3, NH3$H2O and hydrazine hydrate were analytical
grade reagents, purchased from Sigma-Aldrich, and used
without further purication. The progress of the organic reac-
tions were monitored by TLC on commercial aluminum-backed
plates of silica gel 60 F254, visualized, using ultraviolet light.
Melting points were determined in open capillaries using an
1
Electrothermal 9100 without further corrections. H NMR and
13C NMR spectra were recorded on Bruker DRX-400 spectrom-
eter at 400 and 100 MHz respectively.
The catalyst characterization was performed by using various
characterization techniques including XRD, FTIR, BET, SEM/EDX,
TGA, and ICP-AES. FTIR spectra were obtained by using PERKIN-
ELMER-Spectrum 65 instrument. The BET analyses were carried
out using BELSORP Mini II instrument. Prior to BET analyses, the
samples were degassed at 423 K for 3 h. SEM/EDX images were
recorded by employing a Tescan instrument, using Au-coated
samples and acceleration voltage of 20 kV. Room temperature
powder X-ray diffraction patterns were obtained by using
a Siemens, D5000. CuKa radiation from a sealed tube.
Fig. 9 Reusability of the h-Fe2O3@SiO2-CD/Ag catalyst in A3 coupling.
slightly lower catalytic activity of this catalyst. The leaching of
Ag(0) NPs for the catalyst reused for sixth run however, were
detectable. This nding can prove that the Ag(0) NPs are the
catalytically active species. Moreover, the nature of the catalysis
is heterogeneous.
All compounds were known and were identied by compar-
ison of their physical and spectroscopic data with those of
authentic compounds and were found identical. However, we
Experimental
Materials and instruments
1
compared all compounds by their melting points. H and 13C
NMR spectra of selected compounds are reported and given as
ESI.†
All chemicals and reagents, including trisodium citrate dihy-
drate, sodium acetate trihydrate, FeCl3$6H2O, TEOS, b-
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suspension was heated under reux condition overnight. Upon
completion of the reaction, the brown product 5 was magneti-
cally collected, washed with dry toluene for three times and
Synthesis of nanomagnetic Fe2O3 hollow sphere (a)
Nanomagnetic Fe2O3 hollow sphere were prepared through
solvothermal method.14 Initially, FeCl3$6H2O (5 mmol) was
dissolved in 70 mL of ethylene glycol in a ask. Next, 30 mmol of
sodium acetate trihydrate, 1.5 mmol of trisodium citrate dehy-
drate and urea (17 mmol) were added to the aforementioned
solution. The resulting mixture was stirred vigorously for 60
minutes and then, transferred to the Teon-lined stainless-steel
ꢁ
dried in oven at 80 C for 12 h.
Preparations of hollyhocks ower extract (f)
Fresh leaves of hollyhock ower were collected around Banar-
uiyeh District, in Larestan, Iran. First, the collected hollyhock
owers (2 g) were crushed in porcelain mortar. Then, the
resulting powder was mixed thoroughly with 100 mL deionized
water and boiled for 60 min at 80 ꢁC. The extract 6 was obtained
by cooling the mixture and simple ltration.
ꢁ
autoclave (150 mL capacity) and kept at 220 C for 24 h. Upon
completion of the process, the reactor was cooled to room
temperature and the resulting brown product 1 was washed for
three times with EtOH/H2O and dried at 80 ꢁC for 24 h in oven.
Synthesis of h-Fe2O3@SiO2 core–shell NPs (b)
Extract-based synthesis of Ag NPs and their embedding into h-
Fe2O3@SiO2-CD: synthesis of h-Fe2O3@SiO2-CD/Ag (g)
The h-Fe2O3@SiO2 core–shell NPs was prepared according to
a previously reported method with little modication.42 Briey,
h-Fe2O3 (1 g) was dispersed in a mixture of 20 mL of deionized
water, 50 mL of ethanol and 6 mL of NH3. The pH value of the
suspension was kept at ca. 11 and then, the mixture was soni-
cated for 30 min at room temperature. Aer that, TEOS (2 g) was
added into the mixture, which was continuously stirred for 12 h
at room temperature in air. Finally, the obtained h-Fe2O3@SiO2
core–shell, 2, was collected by an external magnet, washed three
times with EtOH/H2O to remove blank silica, and dried at 80 ꢁC
for 12 h in oven.
h-Fe2O3@SiO2-CD (1 g) was dispersed into a solution of AgNO3
(0.1 g) in 20 mL deionized H2O under stirring condition at room
temperature for 30 minutes. The Ag+ ions were adsorbed onto
the surfaces of h-Fe2O3@SiO2-CD via the electrostatic attraction.
Aer that the fresh extract 6 (1 mL) as a reducing agent was
added into the suspension. Upon addition of the extract 6 the
colour of the mixture was changed to black which indicate the
reduction of silver to Ag(0). To immobilize Ag(0) NPs the into
the h-Fe2O3@SiO2-CD, the mixture was stirred for 12 h. Finally,
the product 7 was successfully collected by magnetic separation,
washed three times with EtOH/H2O and dried at 60 ꢁC for 12 h.
The schematic processes of synthesis of the catalyst are depic-
ted in Fig. 1. Notably, to compare the catalytic activity of the
catalyst achieved from the extract-based approach with the one
obtained from conventional chemical reduction of silver
species, the immobilization of Ag(0) nanoparticles on h-Fe2-
O3@SiO2-CD was also performed by reduction of silver
precursor with hydrazine. To this purpose, the same procedure
reported above was used except that the reduction was carried
out by using 2 mL hydrazine hydrate. This catalyst was named
as h-Fe2O3@SiO2-CD/Ag–H. h-Fe2O3@SiO2–N2/Ag was also
prepared through synthesis of h-Fe2O3@SiO2–N2 (c) and
subsequent immobilization of Ag(0) NPs via extract-based
approach similar to the one reported for the synthesis of h-
Fe2O3@SiO2-CD/Ag except that h-Fe2O3@SiO2–N2 was used as
the starting material.
Synthesis of h-Fe2O3@SiO2–N2 core–shell NPs (c)
For amine-functionalization of the surface of h-Fe2O3@SiO2-
core–shell (h-Fe2O3@SiO2) (3 g), was dispersed by sonication in
100 mL dry toluene for 1 h. Subsequently, 3-N-(2-(trimethox-
ysilyl)ethyl)methanediamine (3 mL) as functionalization group
and Et3N (3 mL) as a catalyst were added and the reaction
mixture was reuxed overnight. At the end of the reaction, the
mixture was cooled to room temperature and the amine-
functionalized h-Fe2O3@SiO2core–shell, 3, was collected by
using an external magnet and washed with toluene, and then
dried under air for 12 h.
Synthesis of b-cyclodextrin-OTs (d)
Tosylation of CD with p-toluenesulfonyl chloride was performed
according to literature procedure.43 Typically, p-toluenesulfo-
nylchloride (7.9 mmol) was added to a solution of pyridine
ꢁ
General procedure for A3 and KA2 coupling reactions
containing 15.86 mmol of CD. The mixture was cooled to 0 C
and kept for 48 h. Upon completion, distilled water was added
to the mixture and the solvent was evaporated to afford an oily
product. The latter transformed into a white precipitate upon
addition of cold water. The nal product was achieved by
ltration, recrystallization from water and drying.
To a mixture of phenyl acetylene (1.1 equiv.), amine (1.0 equiv.),
aldehyde or ketone (1.0 equiv.) in H2O (10 mL) as a green
solvent, nanomagnetic h-Fe2O3@SiO2-CD/Ag (20 mg) was
added. The mixture was subjected to ultrasonic irradiation of
power of 70 W. The progress of the reaction was monitored by
TLC. Upon completion of the reaction, the reaction mixture was
cooled to ambient temperature and diluted with hot ethanol (10
Synthesis of h-Fe2O3@SiO2-CD (e)
To synthesize the h-Fe2O3@SiO2-CD, b-cyclodextrin-OTs (0.5 g) mL). The catalyst was separated by an external magnet from the
was dispersed in dry toluene (20 mL) under stirring condition. cooled mixture, washed with ethanol, dried in air and re-used
The pH value of the suspension was kept at ca. 7–8. Then, the for a consecutive reaction run under the same reaction condi-
resulting mixture was added to the suspension of h-Fe2O3@- tions. The resulting residue was puried by recrystallization
SiO2–N2 core–shell 3 (1 g) in 20 mL dry toluene. Aer that, the (hot ethanol) to obtain the excellent yield of product.
36816 | RSC Adv., 2017, 7, 36807–36818
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RSC Advances
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Conclusions
In summary, hollyhock ower extract was used as a reducing
agent for the synthesis of Ag(0) NPs on cyclodextrin decorated h-
Fe2O3@SiO2 core–shell, obtained through
a green facile
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and heterogeneous catalyst (h-Fe2O3@SiO2-CD/Ag). The utility
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under mild and green condition, i.e. ultrasonic irradiation at
room temperature in aqueous media was proved. Hot ltration
experiments ruled out the remarkable Ag(0) leaching up to ve
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Acknowledgements
The authors are thankful to Alzahra University Research
Council S. Sadjadi and M. M. Heravi are also thankful to Iran
National Science Foundation (INSF) for supporting this
research under contract no. 95834576.
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36818 | RSC Adv., 2017, 7, 36807–36818
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