Gold nanoparticles decorated on a graphene-periodic mesoporous silica sandwich nanocomposite as a highly efficient and recyclable heterogeneous catalyst for catalytic applications

Article abstract of DOI:10.1039/c5ra00062a

Ultra-small Au nanoparticles (Au NPs) were immobilized on HS-functionalized sandwich-like periodic mesoporous silica (PMS) coated graphene (G) to form the novel nanocomposite catalyst Au NPs@HS-G-PMS. The synthesized Au NPs@HS-G-PMS nanocatalyst exhibited excellent catalytic activity in three kinds of organic reactions. The reduction of 4-nitrophenol by sodium borohydride, the Suzuki-Miyaura cross coupling reaction of aryl halides with phenyl boronic acid and the three component coupling reaction of aldehyde, alkyne, and secondary amine (A³-coupling) in water were all successfully accomplished. Most importantly, the catalyst could be used repetitively more than six times without significant deactivation.

Full text of DOI:10.1039/c5ra00062a

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Gold nanoparticles decorated on graphene-periodic
mesoporous silica sandwich nanocomposite as a
highly efficient and recyclable heterogeneous
catalyst for catalytic applications
Cite this: DOI: 10.1039/x0xx00000x
Siyavash Kazemi Movahed, Monire Shariatipour and Minoo Dabiri*
The ultra-small Au nanoparticles (Au NPs) immobilized HS-functionalized sandwich-like periodic
mesoporous silica (PMS) coated graphene (G) to form a novel nanocomposite catalyst Au NPs@HS-G-
PMS. The synthesized Au NPs@HS-G-PMS nanocatalyst exhibited excellent catalytic activity on the three
kinds of organic reactions. The reduction of 4-nitrophenol by sodium borohydride, the Suzuki- Miyaura
cross coupling reaction of aryl halides with phenyl boronic acid and three coupling reaction of aldehyde,
alkyne, and secondary amine (A3-coupling) in water were successfully done. Most importantly, the
catalyst could be used repetitively more than six times without significant deactivation.
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
while enhancing the dispersibility and stability of graphene-
supported metal nanocatalysts, which lead to preservation of
Introduction
Recently, graphene which is a single layer of sp2 carbon atoms
bonded in a hexagonal lattice has attracted great attention from
scientists all over the world. The extraordinary properties of
these exfoliated graphene sheets, such as their extremely large
surface area,1 fast charge mobility,2 remarkably high mechanical
strength and Young's modulus,3 excellent chemical stability4 and
low manufacturing cost make graphene a promising candidate to
disperse or immobilize catalytically active species for
heterogeneous catalysis.5 Graphene supported metal
nanoparticles (NPs )composites have been widely studied, and a
strong metal–graphene interaction was also revealed and may
contribute to the enhanced catalytic performance of the
supported metal NPs.6
The periodic mesoporous silica-coated graphene (G-PMS)
hybrids have shown great capabilities to circumvent the known
limitations of the applications of most graphene-supported metal
nanocatalysts.7 The aggregation of graphene sheets would be
avoided due to the restraining of the π–π stacking interactions by
amorphous PMS external layer. In addition, owing to have a
substantial number of mesopores, the PMS layers can confine
metal nanoparticles selectively based on their specific size. The
confinement in PMS and generally porous materials can prevent
metal nanoparticles from sintering into the larger clusters, which
happen under harsh reaction conditions such as at high
temperatures required for many catalytic and detoxification
processes. Therefore, novel methods to secure ultrafine metal
nanoparticles (<2 nm) onto graphene supports have been devised
catalytic activity even under harsh reaction conditions.8
As shown in Scheme 1, G-PMS were synthesized through
combining sol–gel and in situ reduction techniques. The 2D
sandwich-like graphene-based periodic mesoporous silica (G-
PMS) hybrids were prepared via soft template-assisted reducing
process. In this procedure, the cationic surfactant
cetyltrimethylammonium bromide (CTAB) was chosen to
electrostatically adsorb and self-assemble onto the surface of
highly negatively charged graphene oxide in alkaline solution.
Upon hydrazine reduction treatment and the soft-template
removing, the G-PMSs products were successfully collected
with mesoporous silica around the surface of single-layer
graphene oxide.9 PMS covered on the surface of G could act as
a stabilizing agent to prevent serious aggregation of individual G
by weakening the
π−π stacking interaction between G
nanosheets. Then, the hydroxyl groups of G-PMS were
converted to −SH groups by a silylation modification technique
on G-PMS with 3-mercaptopropyltrimethoxysilane (MPTMS).
The metal affinity groups, such as NH2, SiH, SH groups have
been post-grafted onto the surface of catalyst support to stabilize
and prevent the metal nanoparticles from aggregating.10 Among
them, SH group bind to gold surfaces with high affinity, most
frequently thiol modified ligands are used as stabilizing agents
which bind to the surface of the AuNPs by formation of Au-
sulfur bonds.11 Finally, the immobilization of ultra-small and
well dispersed Au NPs on the surface of G-PMS was conducted
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via an in situ reduction of HAuCl4 as the precursor to yield the chromatography was performed on a Trace GC ultra from the
Au NPs@HS-G-PMS hybrid.
Thermo Company equipped with FID detector and Rtx®-1
capillary column.
Preparation of Garphene Oxide
The graphite powder (2.5 g) was first treated with a mixture of
12.5 mL of concentrated H2SO4 with 2.5 g K2S2O8 and 2.5 g
P2O5. The mixture was kept at 80 ºC for 6 h. Subsequently, the
mixture was cooled to room temperature and diluted with 500
mL DI water and left overnight. The mixture was then filtered
and washed with DI water to remove the residual acid. The
product was dried under ambient conditions overnight. The pre-
oxidized graphite was then subjected to oxidation by Hummers’s
method. The pretreated graphite powder was put into cold (0 ºC)
concentrated H2SO4 (125 mL). Then KMnO4 (15 g) was added
gradually under stirring, and the temperature of the mixture was
kept below 20 ºC by cooling. The mixture was then stirred at 35
ºC for 4 h and then diluted with DI water (250 mL). Because
adding water to concentrated sulfuric acid medium releases a
large amount of heat, the dilution was carried out in an ice bath
to keep the temperature below 50 ºC. After adding all of the 250
mL DI water, the mixture was stirred for 2 h, and then an
additional 750 mL DI water was added. Shortly thereafter, 20 ml
30% H2O2 was added to the mixture and the colour of the mixture
changed into brilliant yellow and began bubbling. The mixture
was filtered and washed with 0.5 M HCl to remove metal ions,
followed by 500 mL DI water to remove the acid, and then was
dialyzed against DI water. The resulting GO solid was dried in
air.12
Scheme 1. Schematic diagram illustrating the synthesis of Au NPs@HS‐G‐PMS
nanocatalyst
Experimental section
Chemicals
All chemicals were purchased from commercial suppliers and all
solvents were purified and dried using standard procedures.
Physical measurements
XPS analysis was performed using a Gammadata-scienta ESCA
200 hemispherical analyzer equipped with an Al Ka (1486.6 eV)
X-ray source. Raman spectra of G-PMS, HS-G-PMS, and Au
NPs@HS-G-PMS hybrids were recorded on a Bruker SENTERR
(2009) with an excitation beam wavelength at 785 nm. Scanning
electron microscope G-PMS, HS-G-PMS, and Au NPs@HS-G-
PMS hybrids were performed using Hitachi S-4160 SEM and
Philips XL-30 ESEM. Diffraction data were collected on two
STOE STADI P reflection and transmission mode and Cu-Kα1
radiation (λ =1. 5406 Å). Transmission Electron Microscopy
characterization of G-PMS and Au NPs@HS-G-PMS hybrids
fresh and reused nanocomposites were performed using a
transmission microscope Philips CM-30 with an accelerating
voltage of 150 kV. Melting points of products were measured on
an Elecrtothermal 9100 apparatus and are uncorrected. The
concentration of gold was estimated using Shimadzu AA-680
flame atomic absorption spectrophotometer and inductively
coupled plasma optical emission spectrometer (ICP-OES)
Varian Vista PRO Radial. The thermal stability of the G-PMS,
HS-G-PMS, and Au NPs@HS-G-PMS hybrids were determined
using a thermogravimetric analyzer (TGA/DTA BAHR: STA
503) under air and a heating rate of 10 ºC min−1. Elemental
analysis of HS-G-PMS hybrid was performed using an
Elementar Analysensysteme GmbH VarioEL CHNS. The
nitrogen adsorption-desorption isotherms of G-PMS, HS-G-
PMS, and Au NPs@HS-G-PMS hybrids were collected by using
a Micrometrics PHS-2828(PHSCHINA) surface area analyser
with N2 at 77.3 K. UV/Vis spectra were recorded employing an
Analytik Jena Specord S600 Diode Array spectrometer. Gas
Preparation of G-PMS
79 mL the as-synthesized GO (3.8 mg mL−1) aqueous solution
was added into 602 mL water containing 6.8 g CTAB and 0.27 g
NaOH, and then ultrasonically treated for 2 h. After magnetic
stirring for 2 h at 40 °C, tetraethylorthosilicate (TEOS; 5.4 mL
dissolved in 21.7 mL ethanol) was slowly added to the above
mixture. After reaction for 12 h, 1.8 mL of hydrazine was
additionally introduced into the above mixture, and then heated
at 70ºC for 5 h. The obtained product was centrifuged and
washed with warm ethanol for three times. The product was then
dispersed in 680 mL acetone stirred at 40 °C for 24 h. The
product was collected by centrifugation and washed by warm
ethanol for three times.9
Preparation of HS-G-PMS
The (0.1 g) was ultrasonically dispersed in 630 mL of ethanol.
Then, solution of MPTMS (2 mmol in 50 mL ethanol) was added
to the G-PMS suspension under stirring, and refluxed for 12 h.
The yielded suspension was centrifuged and washed with ethanol
for several times to get a purified HS-G-PMS. The loading of
sulfur was determined to be 4.85 wt% from elementary analysis
(CHN).
Preparation of Au NPs@HS-G-PMS
The HS-G-PMS (0.2 g) was ultrasonically dispersed in 100 mL
of DI H2O. Then, 0.52 ml of HAuCl4 (0.04 M) was added to the
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RGO suspension under stirring. The mixture was kept at room the result of the first-order scattering of the E2g mode of sp2
temperature for 5 h with constant stirring. The resulted carbon domains.14 The intensity ratio of D and G peaks, ID/IG, is
precipitation was collected and repeated washing with DI H2O used as a measure of the disorder or restoration of the graphene
and Ethanol, and dried at room temperature. The ICP analysis lattice and expressing the sp2/sp3carbon ratio.15 As shown, after
gave the actual Au contents as 1.8 wt.% for Au NPs-RGO hybrid. the silylation of G-PMS with MPTMS, the ID/IG was 1.95 for the
resulting HS-G-PMS, while the it was 1.85 for G-PMS.
General procedure for the reduction of 4-NP
Considering the G band and D band that represent the sp2 and sp3
The reduction of 4-NP by NaBH4 was chosen as a model reaction C atoms, respectively, it can be anticipated that the increasing of
to investigate the catalytic performance of the Au NPs@HS-G- the sp3 hybridization structure (MPTMS) raises from the
PMS hybrid. In short, 30 mL of 4-NP (0.12 mM) were mixed successful silylation modification. The ID/IG decreases to 1.45 for
with 30 mL of a freshly prepared aqueous NaBH4 solution (0.17 the Au NPs@HS-G-PMS, possibly as a result of the Au NPs
M). Then, hybrid (0.25 mol% of Au) was added to the resulting growth at defective regions of the HS-G-PMS surface.16
solution, and the reaction was allowed to proceed until the Additionally, after conjugation of the Au NPs onto the HS-G-
solution became colorless. The reaction conversion was PMS sheets, the intensity of the Raman signals of graphene (D
determined by ultraviolet visible spectroscopy (UV-Vis).
and G bands) were enhanced relative to HS-G-PMS, because of
the surface enhanced Raman spectroscopy (SERS) of Au NPs.
SERS were obtained via an electromagnetic enhancement
General procedure for Suzuki reaction
A mixture of Au NPs@HS-G-PMS hybrid (2 mol % of Au), (excitation of localized surface plasmons involving a physical
K3PO4 (3 mmol), aryl halides (1.0 mmol), phenyl boronic acid interaction) or chemical enhancement (formation of charge-
(1.2 mmol), and H2O (3 mL) under air was stirred for 6 h at 100 transfer complexes involving chemical interaction) with
ºC. The reaction progress was monitored by GC. After enhancement factors of ~ 1012 and ~10 to 100, respectively. The
completion of the reaction, the heterogeneous mixture was low enhancement factor for the Au NPs@HS-G-PMS hybrid
cooled to room temperature and the catalyst was separated from indicates the presence of a chemical interaction or bonding
reaction medium simple filtration. Then, the reaction mixture between Au NPs and HS-G-PMS.17
was concentrated and then residue was purified by column
chromatography (SiO2, -Hexane and Ethyl acetate) to yield photoelectron spectroscopy (XPS) analysis. As shown in Fig. 2,
The electronic properties of hybrid were probed by X-ray
n
pure product. The catalysts were recovered by using a simple the peaks corresponding to Au 4f, Si 2p, S 2p, C 1s and O 1s are
filtration and washed extensively with acetone and deionized clearly observable in the XPS full spectrum. The XPS spectrum
water and drying in the air.
of Au 4f core for Au NPs@HS-G-PMS level displays main peaks
at 84.18 and 87.81 eV which correspond to the binding energy
of Au0 4f7/2 and Au0 4f5/2, respectively (Fig. 2b).18 A pronounced
General procedure for A3-coupling reaction
A mixture of Au NPs@HS-G-PMS hybrid (7.5 mol % of Au), peak at 101.91 eV (in the Si 2p XPS spectrum) which
aldehyde (1.0 mmol), amine (1.2 mmol), acetylene (1.3 mmol) corresponds to the bonding energy of Si–O–C can be easily
in H2O (3 mL) was refluxed at 100 °C for 24 h. After completion observed.19 The S 2p signal for Au NPs@HS-G-PMS hybrid
of the reaction (TLC), the heterogeneous mixture was cooled to includes two components. The first constitutes two doublets,
room temperature and the catalyst was separated from reaction situated at 162.24 and 162.93 eV, respectively, attributable to S-
medium simple filtration. Then, the reaction mixture was Au groups on G-PMS sheets. The second component located at
concentrated and then residue was purified by column 163.48 and 164.36 eV is attributable to the sulfur atoms of the
chromatography (SiO2,
pure product. The catalysts were recovered by simple filtration
and washed extensively with acetone and deionized water and characterized by means of scanning electron microscopy (SEM)
drying in the air. and transmission electron microscopy (TEM). As is seen in
All products are known compounds and were reported representative SEM micrographs (Fig. 3 a-f), G-PMS, HS-G-
n
-Hexane and Ethyl acetate) to yield SH groups.20
The microstructure and morphology of the hybrids are
previously.
PMS, and Au NPs@HS-G-PMS hybrids remain as a nanosheet-
like structures, with lateral dimensions ranging from hundreds of
nanometers to
a few microns. Additionally, the SEM
Results and discussion
micrographs of G-PMS, HS-G-PMS (Fig. 3 a-d) show increasing
layer thickness by addition of MPTMS to G-PMS surface.
Additionally, the density and distribution of elements of the Si,
S, and Au on the Au NPs@HS-G-PMS hybrid are evaluated by
quantitative energy dispersive X-ray spectroscopy (EDS)
mapping (Fig. 4b-d).
Fig. 5 a-d compares the morphologies of G-PMS and Au
NPs@HS-G-PMS, as investigated by TEM. The micrograph of
G-PMS possesses similar sheet morphology (Fig. 5a-b). As Fig.
Characterization
Raman spectroscopy is a very useful tool for investigating the
electronic and phonon of structure graphene-based materials.13
The Raman spectra of the prepared G-PMS, HS-G-PMS, and Au
NPs@HS-G-PMS hybrids are shown in Fig. 1. The characteristic
D and G bands of carbon materials are observed around 1300 and
1600 cm−1, respectively. The D band is characteristic of a
breathing mode for k-point phonons of A1g, while the G band is
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4c-d shows, the surfaces of HS-G-PMS are covered with good
dispersion of Au NPs with an average size of 2 nm.
∼
Fig. 1 Raman spectra G‐PMS, HS‐G‐PMS, and Au NPs@HS‐G‐PMS hybrids
Fig. 3 SEM micrographs of (a‐b) G‐PMS, (c‐d) HS‐G‐PMS and (e‐f) Au NPs@HS‐G‐
PMS hybrids
Fig. 4 SEM micrograph of (a) Au NPs@HS‐G‐PMS hybrids and corresponding
quantitative EDS element mapping of (b) Si, (c) S and (d) Au.
Fig 2. (a) Full range XPS spectrum of Au NPs@HS‐G‐PMS hybrid. (b) Au 4f, (c) Si 2p,
and (d) S 2p core level regions XPS spectra of Au NPs@HS‐G‐PMS hybrid,
respectively
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resistant to high temperature, G-PMS is thermally stable
compared with GO and had a little weight loss indicating SiO2
was successfully coated onto GO. The total weight lost for G-
PMS, HS-G-PMS, and Au NPs@ HS-G-PMS hybrids were only
42.20, 23.32 and 21.26% at temperatures around 450 ºC,
respectively.
The XRD pattern of the Au NPs@HS-G-PMS hybrid is
shown in Fig. 8. The diffractions for the Au crystal are hardly
detected, resulting from the small size of Au NPs.24 The G-PMS
hybrid showed diagnostic diffraction peak with
d spacing around
4.82 nm. It is assigned to 100 XRD reflection from hexagonally
d
ordered PMS.25 The peaks at 2θ=44 and 64 degrees correspond
to stainless steel sample holder of powder diffractometer.
Fig. 5 TEM micrographs of (a‐b) G‐PMS, and (c‐d) Au NPs@HS‐G‐PMS hybrids
The porous nature of G-PMS, HS-G-PMS, and Au NPs@HS-
G-PMS hybrids was evaluated by nitrogen physisorption
measurements. The specific surface area was calculated from the
adsorption isotherms based on the Brunauer–Emmett–Teller
(BET) analysis. It was found that the isotherms of G-PMS, HS-
G-PMS, and Au NPs@ HS-G-PMS hybrids are type IV
according to the IUPAC classification, associated with a well-
defined capillary condensation step suggesting a mesoporous
structure.21 The BET total surface areas of G-PMS, HS-G-PMS,
and Au NPs@HS-G-PMS hybrids were found to be 452.86,
615.43 and 648.59 m2g-1, respectively (Fig. 6a). From these
results, it can be concluded that the MPTMS and Au NPs act as
spacers between G-PMS sheets thereby preventing their
agglomeration and aggregation.22 Moreover, the volume
adsorbed was larger at high relative pressure, exhibiting the H3-
type hysteresis loop, and this kind of hysteresis loop indicated
that these hybrids are comprised of aggregates of plate-like
particles forming slit-like pores.23 The average pore size
diameters of G-PMS, HS-G-PMS, and Au NPs@HS-G-PMS
were calculated to be 2.48, 2.55 and 2.58 nm, respectively, using
Barrett-Joyner-Halenda (BJH) analysis.
Fig. 7 TGA plots of GO, G‐PMS, HS‐G‐PMS, and Au NPs@HS‐G‐PMS hybrids
Fig. 6 (a) Nitrogen adsorption–desorption isotherms of G‐PMS, HS‐G‐PMS, and Au
NPs@ HS‐G‐PMS hybrids (b) the pore size distribution curves of G‐PMS, HS‐G‐PMS,
and Au NPs@ HS‐G‐PMS hybrids
Thermogravimetric analysis (TGA) was further used to study
the thermal behavior and stability of the Au NPs@HS-G-PMS
hybrid. TGA plots of GO, G-PMS, HS-G-PMS, and Au
NPs@HS-G-PMS hybrids are shown in Fig. 7. The weight of GO
declined sharply between 100 ºC and 200 ºC, due to pyrolysis of
the labile oxygen-containing functional groups. Since SiO2 is
Fig. 8 XRD patterns of GO, G‐PMS, HS‐G‐PMS, and Au NPs@HS‐G‐PMS hybrids.
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The catalytic activity of Au NPs@HS‐G‐PMS hybrid The catalytic activity of Au NPs@HS‐G‐PMS hybrid
toward the reduction of 4‐nitrophenol by sodium toward the Suzuki–Miyaura cross‐coupling reaction
borohydride
Aryl–aryl bond formation via the palladium-catalyzed Suzuki
cross coupling reaction is one of the most important tools of
modern organic synthesis. The Suzuki cross coupling reaction is
one of the most general and powerful tools for the synthesis of
pharmaceuticals, herbicides, polymers, liquid crystals, natural
products, ligands for catalysis and advanced materials.28
Inspired by the high activity and stability of Au NPs@HS-G-
PMS hybrid, the Suzuki coupling reaction was further employed
as another model reaction to test the further performance of Au
NPs@HS-G-PMS nanocatalyst. We chose the Suzuki-Miyaura
coupling reaction of phenyl boronic acid with phenyl iodide as a
model reaction in H2O as solvent at 100 ºC in the presence of
K3PO4 as base and with a catalyst loading of 2 mol% of Au for 6
h. Under this condition, we found that the cross coupling reaction
proceeds well, affording the excellent yield (93%) of the
corresponding biphenyl.
After the careful investigation of the prepared Au NPs@HS-G-
PMS hybrid, it was employed in the catalytic reduction of 4-
nitrophenol (4-NP) in the presence of NaBH4 as a hydrogen
source, because Au NPs are known to be effective in the
conversion of 4-NP. As is well known, 4-nitrophenol (4-NP), a
well-characterized industrial pollutant that has been listed on the
“Priority Pollutant List” by the U.S. Environmental Protection
Agency, is harmful to aquatic life and human health in terms of
its toxicity, potential carcinogenicity, and mutagenicity.26
Hydrogenation of 4-NP to the corresponding 4-aminophenol (4-
AP is known as one of the fundamental reactions for the
synthesis and manufacture of fine and industrial chemicals for
the production of analgesic and antipyretic drugs, photographic
developer, corrosion inhibitor, anticorrosionlubricant, and
soon.27
We next managed to examine the scope and limitation of
Suzuki-Miyaura reaction with various types of aryl haides
derivatives and phenyl boronic acid. (Table 1). It can be seen
from Table 1 that the Suzuki–Miyaura cross-coupling reaction
with most substrates preceded in good yields. Aryl iodides and
bromides bearing electron-donating and electron-withdrawing
groups reacted well giving good yields. The hindered substrates
such as 2-iodotoluene and 2-iodo-1,3,5-trimethylbenzene
converted into the corresponding products with a moderate yield
(Table 1, entries 7 and 8).
As shown in Fig. 9a, the pure of 4-NP solution exhibited a
distinct absorption maximum at 317 nm, which shifts to 400 nm
in the presence of an alkali because of the formation of 4-
nitrophenolate ion. It was found that the intensity of the
characteristic absorption peak of 4-nitrophenolate ion at 400 nm
quickly decreases, and the characteristic absorption of 4-AP at
around 300 nm also appears rapidly (Fig. 9b). The reaction
kinetic could be analyzed and confirmed from the time-
dependent absorption spectra, which showed the gradually
decrease of 4-nitrophenolate ions. Linear relationships between
ln(Ct/C0) and reaction time were obtained in the reduction
reaction catalyzed by Au NPs@HS-G-PMS hybrid, which match
well with the first-order reaction kinetics. The rate constant k was
calculated to be 0.14 min-1 for the reaction catalyzed by Au
NPs@HS-G-PMS hybrid (Fig. 9c). The recyclability of the Au
NPs@HS-G-PMS hybrid was also examined by the reduction of
4-nitrophenol. It was found that the recovery can be successfully
achieved in ten successive reaction runs (Fig. 9d).
Table 1 Au NPs@HS-G-PMS hybrid catalyzed Suzuki–Miyaura cross-
coupling reactiona
Ar1-Ph
1
+
Ph-B(OH)2
Ar -X
Entry
1
2
3
4
5
6
7
8
Ar1
Ph
X
I
I
I
I
I
I
I
I
Yieldb
93%
87%
84%
95%
96%
91%
76%
58%
83%
79%
89%
4-Me-C6H4
4-MeO-C6H4
4-NO2-C6H4
4-MeCO-C6H4
4-CO2H-C6H4
2-Me-C6H4
1,3,5-Trimethyl-C6H2
Ph
9
10
11
Br
Br
Br
4-Me-C6H4
4-NO2-C6H4
a Aryl halide (1 mmol), phenyl boronic acid (1.2 mmol), Au NPs@HS-G-
PMS (2 mol % of Au), K3PO4 (3 mmol), 100 ºC, 6h. b Isolated yield.
The reusability of the Au NPs@HS-G-PMS hybrid was
examined in Suzuki–Miyaura cross-coupling reaction of phenyl
boronic acid with phenyl iodide. It was found that the recovery
can be successfully achieved in six successive reaction runs
(Table 2).The heterogeneous nature of the catalysis was proved
using a hot filtration test and atomic absorption spectroscopy
(AAS) analysis. To determine whether the catalyst is actually
functioning in a heterogeneous manner or whether it is merely a
reservoir for more active soluble gold species, we performed a
hot filtration test in the Suzuki–Miyaura cross-coupling reaction
Fig. 9 (a) UV‐vis spectra of (a) 4‐nitrophenol and 4‐nitrophenolate; (b) the
reduction of 4‐NP to 4‐AP over Au NPs@HS‐G‐PMS hybrid; (c) plot of ln(Ct/C0)
against the reaction time of the reduction of 4‐NP over Au NPs@HS‐G‐PMS hybrid;
(d) the catalytic stability tests of Au NPs@HS‐G‐PMS hybrid.
of phenyl boronic acid with phenyl iodide after
∼50% of the
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coupling reaction was completed. The hot filtrates were then
transferred to another flask containing K3PO4 (3 equiv.) in H2O
(2 mL) at 100ºC. Upon further heating of catalyst-free solution
for 6 h, no considerable progress (∼3% by GC analysis) was
observed. Moreover, using AAS of the same reaction solution at
the midpoint of completion indicated that no significant
quantities of gold were lost to the reaction liquors during the
process. ICP result of the used Au NPs@HS-G-PMS
nanocomposite catalyst indicate leaching of 1.3% of gold in the
Suzuki–Miyaura cross-coupling reaction of phenyl boronic acid
with phenyl iodide after the sixth cycle.
Fig. 11 Full‐range XPS spectrum of Au NPs@HS‐G‐PMS reused after six cycles.
Table 2 Reusability of the Au NPs@HS-G-PMS hybrid in Suzuki–Miyaura
cross-coupling of phenyl boronic acid with phenyl iodidea
Reaction cycle
Yieldb
1st
93%
2nd
93%
3rd
93%
4th
91%
5th
91%
6th
90%
aPhenyl iodide (1.0 mmol), phenyl boronic acid (1.2 mmol), K3PO4 (3.0
mmol), Au NPs@HS-G-PMS (2 mol% of Au), 100ºC, 6 h and H2O (2 mL).
bGC yield, n-dodecane was used as an internal standard
Moreover, no appreciable changes in textural properties after
six reactions cycles were detected, as clearly evidenced from
TEM analysis of the recycled catalyst (Fig. 10). TEM
micrographs also indicated no detectable aggregation or
agglomeration of Au NPs in the recovered Au NPs@HS-G-PMS
hybrid after 6th reaction run. Additionally, TEM micrographs of
the recovered Au NPs@HS-G-PMS hybrid showed an increase
in size of Au NPs in comparison to the fresh hybrid. There has
also been a decrease in the reaction yield about 3% from the first
run to the sixth run that indicated the little size dependence on
the catalytic activity of Au NPs@HS-G-PMS hybrid.
XPS analysis of Au NPs@HS-G-PMS hybrid after the 6th
runs revealed the presence high concentration of K and P, which
may suggest that some base remains attached to the surface of
the solid (Fig. 11). Additionally, the peaks corresponding to Au
4d, C 1s, O 1s, S 2p & 2s and Si 2p & 2s are clearly observed in
the XPS full spectrum.
Fig. 12 XRD pattern of Au NPs@HS‐G‐PMS reused after six cycles. This XRD pattern
was measured on acetate foil.
The catalytic activity of Au NPs@HS‐G‐PMS hybrid
toward the A3‐coupling reaction
The three-component coupling of an aldehyde, an alkyne and an
amine, commonly called an A3-coupling, as MCR, has received
much attention in recent years.29 The resulting propargylamine
derivatives are interesting molecules for drug screening, useful
intermediates for heterocycle synthesis, and are involved in
many different synthetic strategies.30 For this reason, much effort
has been devoted to the development of new alternatives to this
extremely useful transformation.
XRD pattern of Au NPs@HS-G-PMS hybrid after the 6th
runs show in Fig. 12. The diffractions for the Au crystal are still
hard to detect. The peak at 2θ=53 degree correspond to acetate
foil transmission X-ray powder diffractometer.
The catalytic activity of the Au NPs@HS-G-PMS hybrid as
catalyst was then tested in A3-coupling reaction. To optimize the
reaction conditions, 4-chlorobenzaldehyde, morpholine and
phenylacetylene were tested as model substrates in the presence
of various solvents (Table 3). The results indicate that reaction
temperature, solvent and loading of Au significantly influence
the product yield in the coupling reaction. After several
screening experiments with different combinations, the best ones
were proved to be Au NPs@HS-G-PMS hybrid (7.5 mol% of
Au), 4-chlorobenzaldehyde (1 mmol), morpholine (1.2 mmol),
phenylacetylene (1.3 mmol) and H2O (3 mL) at 100 ºC for 24 h.
Fig. 10 TEM micrographs of Au NPs@HS‐G‐PMS reused after six cycles in Suzuki–
Miyaura cross‐coupling of phenyl boronic acid with phenyl iodide (a) bright field
(b) dark field.
This journal is © The Royal Society of Chemistry 2012
RSC Adv., 2015, 00, 1‐3 | 7

RSC Advances
Page 8 of 9
VJieowuArrtnicalelONnlainme e
ARTICLE
DOI: 10.1039/C5RA00062A
Table 3 Screening of the A3-coupling reaction conditions
The reusability of the Au NPs@HS-G-PMS hybrid was
examined in A3-coupling reaction of 4-chlorobenzaldehyde,
morpholine and phenylacetylene. It was found that the recovery
can be successfully achieved in six successive reaction runs
(Table 5). ICP result of the used Au NPs@HS-G-PMS hybrid
indicate leaching of 4.9% of Au in A3-coupling reaction of 4-
chlorobenzaldehyde, morpholine and phenylacetylene after the
sixth cycle. The comparison between the catalysis reaction using
only Au NPs31 and Au NPs@HS-G-PMS hybrid shows that
unsupported Au NPs show sharp drop in activity and delayed
reaction time after five cycles (92% in 5 h for the first run to 63%
in 18 h for the fifth runs).
Entry
Solvent
Temp. (ºC)
Au NPs@HS-G-PMS
Yieldb
(mol%)
7.5
7.5
7.5
7.5
7.5
7.5
7.5
5
1
2
3
4
5
6
7
8
9
CHCl3
CH3CN
PhCH3
MeOH
H2O
H2O
H2O
H2O
H2O
100
100
100
100
100
80
68%
72%
51%
81%
93%
64%
56%
74%
49%
94%
Table 5 Reusability of the Au NPs@HS-G-PMS hybrid in A3-coupling
reaction of 4-chlorobenzaldehyde, morpholine and phenylacetylene a
Reaction cycle
Yieldb
1st
93%
2nd
93%
3rd
90%
4th
88%
5th
88%
6th
86%
60
a4-chlorobenzaldehyde (1 mmol), morpholine (1.2 mmol), phenylacetylene
1.3 mmol), Au NPs@HS-G-PMS (7.5 mol% of Au), H2O (3 mL) 100ºC and
24 h; b GC yield based on 4-chlorobenzaldehyde
100
100
100
2.5
10
10
H2O
a 4-chlorobenzaldehyde (1 mmol), morpholine (1.2 mmol), phenylacetylene
(1.3 mmol), solvent (3 mL), 24 h; b GC yield based on 4-chlorobenzaldehyde
Conclusion
In summary, a 2-D hybrid material has been developed by the
ultra-small Au NP immobilization on sandwich-like mesoporous
silica encapsulated G. The as-prepared Au NPs@HS-G-PMS
hybrid exhibited a high catalytic activity for the reduction of 4-
NP, the Suzuki-Miyaura cross coupling reaction for a range of
aryl bromides and iodides with phenyl boronic acid and A3-
coupling reaction of aldehyde, alkyne, and secondary amine in
water. Moreover, the catalyst was chemically stable and could be
recycled for at least six times in the corresponding reaction
without reduction in the catalytic activity. Furthermore, the TEM
image of the recovered catalyst showed the presence of well-
distributed Au NPs on the HS-G-PMS sheets without any
aggregation.
Using the optimized reaction conditions, it was then
attempted to expand the scope of benzaldehydes, secondary
amines and terminal alkynes applicable to the present A3
coupling reaction (Table 4). The benzaldehyde bearing electron-
withdrawing groups (
compared with the benzldehyde bearing electron donating group
-Me and -OMe) (Table 4, entry 1-3). Moreover, the 4-
p-Cl and p-NO2) led to higher yield
(
p
p
chlorobenzaldehyde was used in A3-coupling with piperidine or
pyrolidine and phenylacetylene (Table 4, entries 6 and 7).
Furthermore, by using various substituted phenylacetylenes with
substituents that have different electronic properties, different
yields of the final products were achieved. The aryl acetylene
bearing electron-donating groups (
compared with the aryl acetylene bearing electron withdrawing
group ( -CF3) (Table 4, entries 8 and 9), which could be a
p-Me) were more reactive
Acknowledgements
p
reflection of the reduced nucleophilicity of the acetylide anion.
We gratefully acknowledge financial support from the Research
Council of Shahid Beheshti University.
Notes and references
Table 4 Au NPs@HS-G-PMS hybrid catalyzed A3-coupling reactiona
a
Faculty of Chemistry, Shahid Beheshti University, Tehran, Islamic
Republic of Iran. E-mail: m-dabiri@sbu.ac.ir; Fax: +98 21 22431661; Tel:
+98 21 29903255.
Entry
1
2
3
4
5
6
7
8
R1
R2
Ph
Ph
Ph
Ph
Ph
Ph
Ph
R3
Yieldb
93%
89%
82%
79%
98%
96%
77%
84%
96%
82%
2
4-Cl-C6H4
Ph
Morpholine
Morpholine
Morpholine
Morpholine
Morpholine
Piperidine
Pyrolidine
Morpholine
Morpholine
Piperidine
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