Gold nanoparticles supported on mercaptoethanol directly bonded to MCM-41: An efficient catalyst for the synthesis of propargylamines

Article abstract of DOI:10.1016/j.catcom.2015.09.028

Mercaptofunctionalized MCM-41 (MCM-SH) was obtained via reaction of MCM-chloride with mercaptoethanol. Then, gold nanoparticles supported on MCM-SH (Au@HS-MCM nanocomposite) were prepared by chemical reduction of HAuCl₄ by NaBH₄. The

Full text of DOI:10.1016/j.catcom.2015.09.028

Catalysis Communications 73 (2016) 88–92
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Catalysis Communications
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Short communication
Gold nanoparticles supported on mercaptoethanol directly bonded to
MCM-41: An efficient catalyst for the synthesis of propargylamines
⁎
Afsaneh Feiz, Ayoob Bazgir
Department of Chemistry, Shahid Beheshti University G.C., Tehran 1983963113, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 11 May 2015
Received in revised form 19 August 2015
Accepted 28 September 2015
Available online xxxx
Mercaptofunctionalized MCM-41 (MCM-SH) was obtained via reaction of MCM-chloride with mercaptoethanol.
Then, gold nanoparticles supported on MCM-SH (Au@HS-MCM nanocomposite) were prepared by chemical
reduction of HAuCl4 by NaBH4. The obtained catalyst was used as an effective and reusable heterogeneous
catalyst in three-component coupling of aldehydes, alkynes, and amines (A3-coupling) in water. The catalyst
could be recovered and reused several times without a significant loss of activity.
© 2015 Elsevier B.V. All rights reserved.
Keywords:
MCM-41
Gold
A
3 coupling
Mercaptofunctionalized MCM
Nanoparticle
1. Introduction
catalyst [16,17]. Recently, several gold heterogeneous systems were
successfully used to catalyze A3-coupling reactions with reusability of
The transition-metal catalyzed multi-component reactions (MCRs),
which consist of several simultaneous bond-forming reactions in a
one-pot manner, are a powerful synthetic strategy for the synthesis of
diverse complex molecules from simple precursors [1]. Among MCRs,
the catalytic coupling reaction of aldehyde, amine and alkyne (A3
coupling) for the synthesis of propargylamine is one of the best exam-
ples. Propargylamines are versatile building blocks for the synthesis of
N-heterocycles such as pyrroles, pyrrolidines and oxazoles [2–5]. In
addition, propargylamine derivatives are key intermediates for natural
product synthesis [6,7]. Several complexes or salts of the late transition
metals (e.g., Ir, Zn, Fe, Co, Ni, Ag, Au, Ir, Ru) and Cu/Ru bimetallic systems
under homogeneous conditions have all been used for the A3 coupling
[8–10] and among which the cationic gold species showed the highest
catalytic activity [11,12]. However, besides the potential limitations of
homogeneous catalysis for achieving a sustainable catalytic process,
the rapid reduction of cationic gold species into inactive metallic
atoms is unavoidable when gold salts activate alkynes/alkenes
[13–15]. Therefore, gold nanoparticles supported on nanomaterials
may therefore be an attractive solution to this problem. The immobiliza-
tion of metal nanoparticles on high-surface-area inorganic supports
allows better stability and dispersity of the particles as well as further
exploitation of the special activity and recycling properties of the
catalysts [18–21]. However, these gold heterogeneous catalysts were
used either in toxic solvent or in the presence of expensive ionic liquids
or required drastic reaction conditions. To the best of our knowledge
studies on green A3-coupling reactions with gold heterogeneous
catalysts in water are still very limited and it is desirable to develop
highly efficient gold catalysts for A3-coupling reactions in environmen-
tally friendly medium. Most recently, Corma et al. [22] reported gold
supported on nanocrystalline CeO2 and ZrO2 and Li et al. developed
gold nanoparticles immobilized on the HS-functionalized cellulose
nanocrystallite [23] as heterogeneous catalysts for the A3-coupling
reactions in water.
The presence of a large number of silanol (Si–OH) groups on
the surface of inorganic mesoporous materials such as MCM-41 facili-
tates functionalization of MCM-41 with SH ligands. Recently, HS-
functionalization of MCM-41 and nanosilica was reported using
expensive mercaptosilane linker and the solid materials were purified
through extensive extraction by Soxhlet apparatus [24,25]. This paper
describes a new method for the functionalization of MCM-41 with SH
ligands without using any expensive 3-mercaptosilane linkers and
extensive extraction. The method can be expanded to other solid
supports containing free OH groups. Herein, a simple and efficient
method for the preparation of gold nanoparticles supported on MCM-
SH nanocomposite is reported. The Au@HS-MCM nanocomposite
was used as an efficient and reusable heterogeneous catalyst for
A3-coupling reactions in water and additionally, recycling potential
of the catalyst has been assessed.
⁎
Corresponding author.
E-mail address: a_bazgir@sbu.ac.ir (A. Bazgir).
http://dx.doi.org/10.1016/j.catcom.2015.09.028
1566-7367/© 2015 Elsevier B.V. All rights reserved.

A. Feiz, A. Bazgir / Catalysis Communications 73 (2016) 88–92
89
2. Experimental
2.1. Preparation of MCM-Cl
MCM-41 was prepared according to method previously published
[26]. Thionyl chloride (8 mL) was added to 1 g of oven dried MCM-41
(120 °C, vacuum) in a round bottomed flask equipped with a condenser
and a drying tube and refluxed for 48 h. Excess of thionyl chloride was
distilled off and the resulting product was flame-dried and stored in a
sealed vessel. This silica chloride can be used for months without losing
its activity.
2.2. Preparation of MCM-SH
2-Mercaptoethanol (3 ml) was added to 1 g MCM-Cl and was stirred
vigorously for 24 h at room temperature. Then the mixture was filtered
and washed with ethanol, acetone, methanol and water. After drying,
the loading of ligand was calculated by TGA about 0.2 mmol · g−1
The atomic percent of S (was monitored with elemental analysis) is
0.74%, and this is in accordance with the TGA.
.
Scheme 1. Schematic description for preparation of Au@HS-MCM nanocomposite.
2927 and 2578 cm−1 corresponding to the vibration of the C–H and
S–H groups in the mercaptoethanol [23,29]. Because of the coordination
of the HS-group with the Au NPs the SH signal in Au@HS-MCM nano-
composite is weaker [23].
2.3. Preparation of Au@SH-MCM
The mercapto-functionalized MCM (0.5 g) was ultrasonically
dispersed in 7 mL of deionized water and HAuCl4 (0.1 mmol) was
added and the mixture was stirred for 5 h with constant stirring. A
freshly prepared solution of NaBH4 (0.1 M) in ice cold water was then
added. The mixture was held in room temperature with vigorous
stirring. After filtration and washing with deionized water and ethanol
the final material was dried at 70 °C for 12 h. The Au concentration in
the Au@SH-MCM hybride was determined by atomic adsorption
spectroscopy (AAS) following digestion with concentrated aqua regia
Thermogravimetric analysis (TGA) was further used to study
the composition of the Au@HS-MCM nanocomposite (Fig. 2). TGA plot
of catalyst shows two main weight losses. The first one occurs at
~90 °C and was assigned to adsorbed water. Another one, above
~260 °C (and continuing to ~550 °C) is related to the decomposition
of covalently bonded organic groups from the Au@HS-MCM. It reveals
that the catalyst has good thermal stability until 260 °C. The mass loss
of 12.9% in sample Au@HS-MCM nanocomposite corresponds to
0.24 mmol · g−1 of the mercaptoethanol bound to the MCM surface
(Fig. 2). A detailed analysis of the TGA can be found in the Supporting
Information. Additionally, the atomic percent of sulfur was monitored
with elemental analysis (CHNS). The atomic percent of S is 0.74%, and
this is in accordance with the TGA. The Au content of the catalyst was
estimated to be 0.14 mmol · g−1 (2.6%W) by AAS.
A scanning electron microscopy image of Au@HS-MCM nanocom-
posite, corresponding energy dispersive X-ray spectroscopy (EDS) map-
ping and XRD pattern of the Au@HS-MCM nanocomposite are shown in
supporting information.
The morphology of fresh MCM-41 and Au@HS-MCM nanocomposite
was determined by transmission electron microscopy (TEM). Fig. 3a
shows the porous structure of fresh MCM-41. The TEM image of
the Au@HS-MCM nanocomposite (Fig. 3b) shows a distribution of
gold nanoparticles in the support with an average particle diameter of
solution at 80 °C, and was found to be 0.14 mmol · g−1
.
2.4. Typical procedure for A3 coupling reaction
Au@HS-MCM (2 mol% of Au, 143 mg) was added to the mixture of
benzaldehyde (1 mmol, 106 mg), piperidine (1.2 mmol, 102 mg) and
phenylacetylene (1.5 mmol, 153 mg) in 4 ml H2O and the mixture
was stirred at 80 °C for 24 h. After completion of the reaction the catalyst
was separated. The filtrate was extracted with CHCl3 and concentrated
to purify through a short silica gel plug with hexane/ethyl acetate
(3:1, V/V). The catalysts were recovered after washing with chloroform
and acetone and dried at 70°C in vacuo.
3. Results and discussion
3.1. Synthesis and characterization of the Au@HS-MCM nanocomposite
The process for the preparation of Au@HS-MCM nanocomposite
is schematically described in Scheme 1. The MCM-41 was reacted
with thionyl chloride to afford the corresponding chlorinated MCM
(MCM-Cl). MCM-Cl was reacted with 2-mercaptoethanol to obtain
mercaptofunctionalized MCM (MCM-SH). The final catalyst (Au@HS-
MCM nanocomposite) was prepared by deposition of gold nanoparti-
cles onto MCM-SH through chemical reduction of HAuCl4 by NaBH4
[27].
Mercapto groups (−SH) have been used as stabilizers of gold
nanoparticles in recent years. In these cases, however, weak reductants
cannot reduce gold ion owing to strong bonding of mercapto group and
gold ion. Only NaBH4 as a stronger reductant, has been able to reduce
mercapto-bonded gold ion [28].
Au@HS-MCM nanocomposite was characterized by FT-IR, CHN, TEM,
XPS and TGA. FT-IR spectra of MCM-41, MCM-SH and Au@HS-MCM
nanocomposite are shown in Fig. 1. Comparison between the FT-IR
spectra of MCM-41 and MCM-SH reveals additional bands around
Fig. 1. FT-IR spectra of MCM-41, MCM-SH and Au@HS-MCM.

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A. Feiz, A. Bazgir / Catalysis Communications 73 (2016) 88–92
network (Si–O–Si) [31]. The S 2p signal for Au@HS-MCM includes two
components. The first constitutes two doublets, situated at 161.98 and
162.23 eV respectively, attributable to S–Au groups that indicated the
covalent interaction between Au and S [32]. The second component
located at 162.23 and 162.70 eV is attributable to the sulfur atoms of
the SH groups [33].
3.2. Catalytic activity toward A3 coupling reaction
The A3 multi-component condensation reaction was used as
benchmarking reaction for the catalyst Au@HS-MCM. To optimize
the reaction conditions, benzaldehyde 1a, piperidine 2a, and
phenylacetylene 3a were selected as model substrates in different
conditions (Table 1). As shown in Table 1, different solvents were
screened in the model reaction and it was found that H2O is the optimal
solvent in the presence of 2.0 mol% catalyst (Table 1, entry 5). The reac-
tions were carried out at different amounts of Au@HS-MCM nanocom-
posite ranging from 0.5 to 2.0 mol% of Au in H2O at 80 °C for 24 h. It
was found that when the amount of the catalyst increases from 0.5 to
2.0 mol%, the yields increase from 61 to 94%, respectively (Table 1,
entry 5–7). We also performed several experiments at 60 °C, 80 °C,
and reflux conditions in H2O using 2.0 mol% catalyst for 24 h (Table 1,
entries 5, 8 and 9). The optimal reaction temperature is 80 °C (Table 1,
entry 5).
Fig. 2. TGA of Au@HS-MCM nanocomposite.
10–13 nm and also indicated no detectable aggregation or agglomera-
tion of Au nanoparticles.
The electronic properties and bonding states of hybrid were probed
by X-ray photoelectron spectroscopy (XPS) analysis. As shown in Fig. 4,
the peaks corresponding to Au 4f, Si 2p, S 2p, C 1s and O 1s are clearly
observable in the XPS full spectrum. The XPS spectrum of Au 4f core
level region for Au@HS-MCM nanocomposite displays main peaks at
84.18 and 87.86 eV which correspond to the binding energy of Au0
4f7/2 and Au0 4f5/2, respectively (Fig. 4) [30]. High resolution Si 2p XPS
spectrum of the Au@HS-MCM can be split into two sublevels. The first
one at 101.94 eV is attributed to the Si–O–C bonds, while the other
one at 103.96 eV corresponds to the silicon participant from the siloxane
After the best reaction conditions were set, various aldehydes,
amines, and alkynes were screened to explore the scope of the three-
component coupling reaction (Table 2). Aromatic aldehydes with both
electron-donating and electron-withdrawing functionalities afforded
the corresponding propargylamines in good to excellent yields. The
Fig. 3. (a) TEM images of MCM-41, (b) fresh Au@HS-MCM and (c) reused Au@HS-MCM after five cycles.

A. Feiz, A. Bazgir / Catalysis Communications 73 (2016) 88–92
91
Fig. 4. (a) Full range XPS spectrum of Au@HS-MCM. (b) Au 4f, (c) Si 2p, and (d) S 2p core level regions XPS spectra of Au@HS-MCM nanocomposite, respectively.
aromatic aldehydes possessing electron-withdrawing group (Table 2,
4f and 4g) led to higher yields compared to the aromatic aldehydes
possessing an electron-donating groups (Table 2, 4d and 4e).
Heteroaromatic aldehyde such as thiophene-2-carbaldehyde partici-
pates well in this reaction (Table 2, 4k). Different amines and aromatic
alkynes also underwent the corresponding three-component coupling
reaction smoothly and generated the products in good to excellent
yields.
coupling reaction is completed. The hot filtrates were then transferred
to another flask containing H2O at 80 °C. Upon further heating of the
catalyst-free solution for 24 h, no considerable progress (~7% 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 are lost to the reaction liquors during the process
(0.6% of Au).
The possibility of recycling the catalyst was also examined in the A3
coupling reaction. As shown in Table 3, there is no significant decrease
in catalytic activity after five successive reaction runs. Furthermore, no
significant change was observed in textural properties as is clear in
TEM of the recycled catalyst (Fig. 3c). In addition no detectable gold
In order to investigate whether the catalyst actually acts in a hetero-
geneous manner, we performed a hot filtration test in the coupling reac-
tion of benzaldehyde, piperidine and phenylacetylene after ~50% of the
Table 1
Screening of the reaction conditionsa.
Table 2
Three-component coupling of aldehyde, amine, and terminal alkynes.
Product Aldehyde 1
4
Amine 2
Alkyne 3
Yield
(%)
Entry
Solvent
Temperature
Catalyst
Yield(%)b
a
b
c
d
e
f
g
h
i
Benzaldehyde
Benzaldehyde
Benzaldehyde
4-Methylbenzaldehyde
4-Methoxybenzaldehyde
4-Fluorobenzaldehyde
4-Chlorobenzaldehyde
4-Chlorobenzaldehyde
Benzaldehyde
Piperidine
Pyrrolidine Phenylacetylene
Morpholine Phenylacetylene
Piperidine
Piperidine
Piperidine
Piperidine
Phenylacetylene
95
85
80
75
73
96
92
89
91
(mol % Au)
1
2
3
4
5
6
7
8
9
CH3Cl
Reflux
Reflux
Reflux
80 °C
80 °C
80 °C
80 °C
Reflux
60 °C
2
2
2
2
2
1
0.5
2
2
49
35
68
85
94
73
61
95
69
Phenylacetylene
Phenylacetylene
Phenylacetylene
Phenylacetylene
MeCN (Reflux)
PhCH3 (Reflux)
H2O/EtOH
H2O
H2O
H2O
Pyrrolidine Phenylacetylene
Piperidine
4-(Trifluoromethyl)
phenylacetylene
4-Methyl
H2O
H2O
j
Benzaldehyde
Piperidine
94
phenylacetylene
phenylacetylene
Phenylacetylene
a
Benzaldehyde (1 mmol), piperidine (1.2 mmol), and phenylacetylene (1.3 mmol).
Reaction Time = 24 h.
k
l
Thiophene-2-carbaldehyde Piperidine
1-Naphthaldehyde Piperidine
93
77
b
GC yield.

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A. Feiz, A. Bazgir / Catalysis Communications 73 (2016) 88–92
Table 3
Appendix A. Supplementary data
Reusability of the Au@HS-MCM nanocomposite in A3 coupling reactiona.
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.catcom.2015.09.028.
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We gratefully acknowledge the financial support from the Research
Council of Shahid Beheshti University.