Immobilized silver on surface-modified ZnO nanoparticles: As an efficient catalyst for synthesis of propargylamines in water

Article abstract of DOI:10.1016/j.molcata.2014.08.007

Silver nanoparticles were immobilized on ionic liquid modified zinc oxide nanoparticles leading to a ZnO-IL/Ag NPs catalyst, which exhibits high catalytic performance in the synthesis of propargylamines through one-pot three-component condensation reaction of aldehydes, amines, and alkynes in water. The as-prepared catalyst was characterized by ICP-AES, FT-IR, XRD, SEM, and TEM. The catalyst was easily recycled and reused at least five times without significant loss of its catalytic activity.

Full text of DOI:10.1016/j.molcata.2014.08.007

Accepted Manuscript
Title: Immobilized Silver on Surface-modified ZnO
Nanoparticles: as an Efficient Catalyst for Synthesis of
Propargylamines in Water
Author: Farnaz Movahedi Hassan Masrouri M.Z. Kassaee
PII:
S1381-1169(14)00354-9
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2014.08.007
MOLCAA 9222
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
16-6-2014
31-7-2014
1-8-2014
Please cite this article as: F. Movahedi, H. Masrouri, M.Z. Kassaee, Immobilized
Silver on Surface-modified ZnO Nanoparticles: as an Efficient Catalyst for Synthesis
of Propargylamines in Water, Journal of Molecular Catalysis A: Chemical (2014),
http://dx.doi.org/10.1016/j.molcata.2014.08.007
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*Graphical Abstract (for review)
Immobilized Silver on Surface-modified ZnO Nanoparticles: as
an Efficient Catalyst for Synthesis of Propargylamines in Water
Farnaz Movahedia*, Hassan Masrouria, M.Z. Kassaeeb
aDepartment of Chemistry and Petrochemical Engineering, Standard Research Institute, P.O.
Box 31745-139, Karaj, Iran
bDepartment of Chemistry, Tarbiat Modares University, P.O. Box 14155-4838, Tehran, Iran
Page 1 of 40

*Highlights (for review)
Research highlights
 Synthesis of Ag NPs immobilized on surface of ionic liquid modified ZnO nanoparticles
 High catalytic activity of ZnO-IL/Ag NPs in synthesis of propargylamines
 Reusability of ZnO-IL/Ag NPs without significant loss of its catalytic activity
Page 2 of 40

*Manuscript
Click here to view linked References
Revised Manuscript (clear Manuscript)
Immobilized Silver on Surface-modified ZnO Nanoparticles: as an Efficient
Catalyst for Synthesis of Propargylamines in Water
Farnaz Movahedia,, Hassan Masrouria, M.Z. Kassaeeb
aDepartment of Chemistry and Petrochemical Engineering, Standard Research Institute, P.O.
Box 31745-139, Karaj, Iran
bDepartment of Chemistry, Tarbiat Modares University, P.O. Box 14155-4838, Tehran, Iran
Abstract:
Silver nanoparticles were immobilized on ionic liquid modified zinc oxide nanoparticles leading
to a ZnO-IL/Ag NPs catalyst, which exhibits high catalytic performance in the synthesis of
propargylamines through one-pot three-component condensation reaction of aldehydes, amines,
and alkynes in water. The as-prepared catalyst was characterized by ICP-AES, FT-IR, XRD,
SEM, and TEM. The catalyst was easily recycled and reused at least five times without
significant loss of its catalytic activity.
Keywords: Ag nanoparticles, Heterogeneous catalysis, Propargylamines, Ionic liquid, ZnO
nanoparticles
Corresponding author Tel.: +98-912-7031230; Fax: +98-26-32803880; E-mail: Fzmovahedi@standard.ac.ir
1
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1. Introduction
The development of nano-sized metal particles has attracted considerable attention because
of their promising applications in diverse fields [1-3]. They show great potential as versatile
catalysts for green and sustainable organic synthesis owing to their large surface area and size-
dependent properties different from bulk metals [4-6]. The control of their size, shape, dispersity,
and in particular the combination of metal nanoparticles with a specific heterogeneous support, is
essential to prevent aggregation and/or coalescence, resulting to enhanced activity and selectivity
in a desired application [7-9]. Nanoparticles supported catalysts not only compete with their
homogeneous counterparts but also, can be considered as ‘soluble’ analogues since they are
readily dispersed and exhibit an intrinsically a high surface area, attribute that allow excellent
accessibility of substrates to the surface bound active catalytic sites. Zinc oxide nanoparticles
(ZnO NPs) have their own importance due to their vast area of applications [10–14], especially
in catalysis both as support and as active phase for hydrogenation or dehydrogenation reactions
[15]. In addition, coverage of nanostructures by ionic liquids (ILs) is advised for construction of
narrow size distribution nanoscale heterogeneous catalysts [16]. A kind of catalytic system in
which the facile recovery and reuse of ionic liquids is achieved through covalent tethering to
heterogeneous nanostructure supports [17, 18]. This has led to a new possibility for developing
environmentally friendly catalytic systems through stability in water and air, easy separation,
high catalytic efficiency and reusability, resulting in vast applications for various organic
reactions [19].
On the other hand, one-pot three-component catalytic couplings of aldehydes, amines and
alkynes via C-H activation of the terminal alkynes have attracted a considerable interest in recent
years [20]. The resultant propargylamines are major skeletons [21-23] and synthetically key
2
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intermediates [24-26] for preparation of many nitrogen containing biological active compounds,
such as β-lactams, oxotremorine analogues, conformational restricted peptides, and therapeutic
drugs [27, 28]. Three-component synthesis of propargylamines has been reported using various
transition metal catalytic systems such as Ag (І) salts [29], Au (ІІІ) salen complexes [30],Cu (І)
salts [31], Cu/Ru bimethallic system [32], heteropolyacid-supported silver (Ag-HPA) [33],
hydroxyapatite-supported copper (Cu-HAP) [34], AgY zeolite [35] as well as Ag-, Au- and Cu-
nanoparticles [36-39]. Although these types of catalysts impose their own advantages and
disadvantages to the reaction, the considerable drawback is that most of them proceed in toxic
solvents and harsh conditions. Consequently, attempts to achieve more efficient and
environmentally friendly methods for chemical synthesis of propargylamines still remains a big
challenge and continues to be the focus of intense research.
Herein, we report a new catalytic system based on Ag NPs immobilized on ZnO NPs
modified with ionic liquid groups and its application as a novel and efficient catalyst for the A3-
coupling reaction of aldehydes, amines and alkynes to produce propargylamines in water. We
anticipated that Ag and ZnO NPs as heterogeneous sources, along with the ILs as the
homogeneous matrix, would create a catalyst with efficiency beyond the customary limit.
2. Experimental
2.1.Chemicals and instruments
The reagents and solvents used in this work were obtained from Fluka or Merck and used
without further purification. Nanostructures were characterized using a Holland Philips Xpert X-
ray powder diffraction (XRD) diffractometer (Co K, radiation, λ = 0.178897 nm), at a scanning
speed of 2/min from 10 to 80 (2θ). The particle size and morphology were investigated by a
3
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transition electron microscope (TEM, JEOL JEM-2010 on an accelerating voltage of 200 kV)
and scanning electron microscope (SEM, Holland Philips XL30 microscope with an accelerating
voltage of 25 kV). FT-IR measurements were performed using KBr disc on a Thermo IR-100
infrared spectrometer (Nicolet). Elemental analyses for C, H and N were performed using a
Heraeus CHN–O–Rapid analyzer. A radial view Varian Vista-Pro simultaneous ICP-OES
(Springvale, Australia) coupled to a V-groove nebulizer and equipped with a charge coupled
1
device (CCD) was applied for determination of the silver ions. The H and 13C NMR spectra
were measured at 500.1, and 125.7 Hz, respectively, on a Bruker DRX 500-Avance FT-NMR
instrument with CDCl3 as the solvent.
2.2. Synthesis of 1-(3-trimethoxysilylpropyl)-3-methylimidazolium chloride [tmim][Cl] ionic
liquid
1-Methylimidazole (2.0 g, 25.0 mmol) and (3-chloropropyl) trimethoxysilane (4.9 g, 25.0
mmol) were dissolved in toluene (20 mL) and refluxed for three days. Then two phases formed,
with 1-(3-trimethoxysilylpropyl)-3-methylimidazolium chloride [tmim][Cl] as a viscous oil at
the bottom and the solvent on the top. They were separated and the viscous oil was washed with
toluene (5×10 mL). After drying the oil under vacuum at 60 ºC for one day, ionic liquid (3.5 g,
12.75 mmol, 51% yield) was obtained with a honey-like consistency at room temperature [40].
2.3.Surface modification of ZnO NPs with [tmim][Cl] ionic liquid
ZnO NPs were synthesized through a solid-state reaction method including Zn(OAc)2.2H2O
and sodium hydroxide in the ratio of 1:4 [41]. The obtained ZnO NPs (1.2 g) were sonicated for
one hour before being treated with [tmim][Cl] (0.277 g, 1 mmol) in anhydrous toluene under
4
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reflux condition overnight. The mixture was cooled to room temperature and the modified ZnO-
IL(Cl) was washed with EtOH several times to remove the impurities and dried under vacuum.
In order to increase the basic characteristic of the ionic liquid anchored on the surface of ZnO
NPs, an anion-exchange processing was carried out using the modified ZnO-IL(Cl) (1.0 g) and
KOH (0.06 g, 1 mmol) in CH2Cl2 (20 ml) for 2 days at room temperature. Finally, the modified
ZnO-IL(OH) was obtained after centrifuging (4000 rpm) and washing with H2O several times.
According to elemental analysis, the loading capacity of ZnO NPs was determined to be 0.54
mmolg-1 of basic ionic liquid.
2.4.Immobilization of silver NPs on Ionic Liquid-Modified ZnO NPs
To a stirred solution of the modified ZnO-IL (1.0 g) in deionized water (20 ml) in 100 ml
round bottom flask was added a solution of AgNO3 (0.1 g, 0.59 mmol) in 20 ml water, at room
temperature. After 10 minutes, N2H4.H2O (0.1 g, 2.0 mmol) was added in one portion as the
reducing agent. The agitation continued for 12 h. The resulting powder was centrifuged (6000
rpm, 5 min) and washed with deionized water and EtOH, then dried under vacuum at 60 ºC
overnight to give the corresponding ZnO-IL/Ag NPs as a catalyst.
2.5. General procedure for the synthesis of propargylamines
Catalyst (10 mol %) was added to a mixture of aldehyde (1 mmol), amine (1.1 mmol) and
phenylacetylene (1.2 mmol) in water (3 mL) at reflux conditions, and the mixture was stirred for
appropriate periods. The progress of the reaction was monitored by TLC. Consequently, the
mixture was centrifuged (4000 rpm, 5 min) till the catalyst was settled down and the solvent
containing the crude product was decanted out. The centrifugate was extracted with CH2Cl2 (2×5
5
Page 7 of 40

ml). The organic phase concentrated under reduced pressure to afford the crude product, which
after chromatography on silica gel (100–200 mesh) using hexane:ethyl acetate, 90:10, gave the
corresponding propargylamines.
3. Results and discussion
3.1. Synthesis and characterization of ZnO-IL/Ag NPs
Following our recent reports on the catalytic potentials of ZnO NPs [41, 42], surface of the
prepared ZnO NPs were modified by treatment with the as synthesized 1-(3-trimethoxy
silylpropyl)-3-methylimidazolium chloride [tmim][Cl] ionic liquid in refluxing toluene to be
engaged as a heterogeneous nanostructure support. At the next stage, the modified ZnO-IL(Cl)
were treated with potassium hydroxide in CH2Cl2 at room temperature, in order to increase the
basic catalytic characteristic of the ILs anchored on the surface of ZnO NPs. Then AgNO3 and
hydrazine monohydrate were added into the stirred solution of the modified ZnO-IL(OH) in
deionized water, simultaneously. Ag NPs were produced while embedded in to ionic liquid
networks anchored on the surface of ZnO NPs (scheme 1), in which was used as a heterogeneous
catalyst in propargylamines synthesis. The Ag content was evaluated to be 0.31 mmolg-1 by
inductively coupled plasma (ICP) analysis. Also, the loading capacity of ZnO NPs was
determined to be 0.54 mmolg-1 of the basic ILs via the elemental analysis.
6
Page 8 of 40

Scheme 1.
Successful surface modification of the ZnO NPs with ionic liquid was evidenced by Fourier
transform infrared (FT-IR) spectral analysis. (Fig. 1). In contrast to the naked ZnO NPs, the
modified ZnO-IL(OH) showed the characteristic peaks of imidazolium ring at 1570 cm-1 and
1029 cm-1, along with hydroxyl at 3417 cm-1.
7
Page 9 of 40

Fig.1: FT-IR spectra for naked ZnO NPs (top) and ZnO-IL (bottom).
The X-ray diffraction pattern of ZnO-IL/Ag NPs is shown in Fig. 2. Three additional
diffraction peaks appeared beside those attributed to the pure ZnO NPs. The XRD patterns of the
ZnO NPs could be indexed to the hexagonal wurtzite structure (space group: P63mc; a = 3.249
Å, c = 5.206 Å, JCPDS card no. 36-1451), while the three additional peaks at 2θ = 44.57◦, 51.90◦
and 76.45◦ are assigned respectively to the diffraction line planes (1 1 1), (2 0 0) and (2 2 0),
corresponding to the face-center-cubic (fcc) of Ag NPs, in accordance with the JCPDS 04-0783.
No evidence for any impurity was detected. Based on this study, the average particle size has
been estimated 30 nm for ZnO NPs and 7 nm for Ag NPs by using Debye-Scherrer equation,
Dh,k,l = kλ/β cosθ, where k is a constant (generally considered as 0.89), λ is the wavelength of Co
Ka (1.78897 Å ), β is the corrected diffraction line full-width at half-maximum (FWHM), and θ
is Bragg’s angle [43].
8
Page 10 of 40

Fig. 2: XRD pattern of ZnO-IL/Ag.
The morphology and grain size of the ZnO-IL/Ag NPs were investigated by scanning
electron microscopy (SEM) and transmission electron microscopy (TEM) (Fig. 3). Evidently, the
corresponding diffraction pattern (Figure 3c) can be indexed to the hexagonal wurtzite phase of
ZnO in the size range of 25-35 nm confirming the results extracted from the XRD pattern. Also,
it is clear that Ag NPs are distributed on the surface of ZnO NPs. The average size of Ag NPs is
estimated to be 6-10 nm, in accordance with the XRD results.
9
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Fig. 3: TEM images (a, b), electron diffraction pattern (c) and SEM image (d) of ZnO-IL/Ag NPs.
3.2. Evaluation of the catalytic activity of ZnO-IL/Ag NPs through the synthesis of
propargylamines
We investigated the catalytic activity of ZnO-IL/Ag NPs as a heterogeneous catalyst in one-pot
synthesis of propargylamines through three-component reaction of aldehydes, secondary amines
and alkynes (Scheme 2). In an effort to develop an optimal catalytic system, various reaction
parameters were probed for the condensation reaction of benzaldehyde, piperidine, and
phenylacetylene as a model reaction. Various catalysts including Bulk- and Nano-ZnO, Ag NPs,
and ZnO-IL/Ag NPs were evaluated for this model reaction, and the results are summarized in
Table 1. The reaction did not proceed in the absence of the catalyst neither in solvent-free
conditions nor in water (Table 1, entries 1, 2). Among the used catalysts, ZnO-IL/Ag NPs
rendered the highest yield. Indeed, ZnO nanoparticles as well as Ag NPs would catalyze the
10
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reaction per se, with 61 and 78% yield, respectively. Yet, their combination in the form of ZnO-
IL/Ag NPs boosts the yield to 92%.
Scheme 2.
To appraise the effects of media, we carried out the condensation of benzaldehyde,
piperidine and phenylacetylene in various organic solvents by using ZnO-IL/Ag NPs (20 mg per
1 mmol of reactants) as the catalyst (Table 1, entries 7-11 and 13). It should be noted that ZnO-
IL/Ag NPs showed good catalytic activity in a variety of organic solvents such as acetonitrile
and ethanol, where over than 80% conversion was obtained. Nevertheless, the reaction in water
is very clean and this is the solvent of choice, environmentally appreciable.
Table 1: Optimization of the ZnO-IL/Ag NPs catalyzed model reaction for the synthesis of propargylaminesa.
Entry
1
2
3
4
5
6
7
9
10
11
12
13
14
Catalyst
Solvent
Water
Solvent-free
Water
Time (h)
Yield (%)b
No catalyst
No catalyst
ZnO Bulk (20 mol%)
ZnO NPs (20 mol%)
ZnO-IL (10 mg)
Ag NPs (20 mol%)
ZnO-IL/Ag (20 mg)
ZnO-IL/Ag (20 mg)
ZnO-IL/Ag (20 mg)
ZnO-IL/Ag (20 mg)
ZnO-IL/Ag (10 mg)
ZnO-IL/Ag (20 mg)
ZnO-IL/Ag (50 mg)
12
12
8
8
8
6
6
6
6
4
3
3
3
Nil
Nil
24
61
55
78
82
35
42
81
78
92
92
Water
Water
Water
EtOH
THF
Toluene
MeCN
Water
Water
Water
aReaction condition: benzaldehyde (1 mmol), piperidine (1.1 mmol), phenylacetylene (1.2 mmol), catalyst, reflux.
bIsolated yields.
11
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To investigate the effect of the catalyst concentration, systematic studies are carried out in the
presence of different amounts of the catalyst (10, 20, 50 mg) in water, affording propargylamines
with 78%, 92%, and 92% isolated yields, respectively. Thus, the best yield is found in the
presence of just 20 mg ZnO-IL/Ag NPs, and the use of higher amounts of catalyst (50 mg) does
not improve the result to an appreciable extent (Table 1, entry 14).
To evaluate the scope and the generality of the ZnO-IL/Ag catalyst, a variety of aliphatic and
aromatic aldehydes as well as various secondary amines and alkynes was examined for
preparation of the corresponding propargylamines (Table 2). Aliphatic aldehydes, such as
isobutyraldehyde and n-butyraldehyde afforded better yields than most of the aromatic ones.
Among the aromatic aldehydes, those possessing halide groups such as 4-chlorobenzaldehyde
produced propargylamines in better yields than the aldehydes with electron-donating group like
4-methoxybenzaldehyde (Table 2, entries 2, 4). Also, aldehydes with electron-withdrawing
group such as 4-nitrobenzaldehyde did not produce any result. Among the amines, piperidine,
Pyrrolidine, and morpholine gave excellent yields of the propargylamines, whereas dibutylamine
formed the corresponding product in moderate yield. When alkylacetylenes were used in place of
phenylacetylene (Table 2, entries 23-26), the corresponding propargylamines were isolated in
excellent yields. All products were characterized on the basis of their spectroscopic data such as
FT-IR, 1H and 13C NMR spectra.
Table 2: One-pot synthesis of propargylamines by the reaction of aldehydes, amines, and TMSCN in the presence
of ZnO-IL/Ag NPs in watera.
Amines
(R2, R3)
Entry
Aldehydes (R1)
Alkynes (R4)
Time (h)
Yield (%)b
Ph
Piperidine
Piperidine
Piperidine
Piperidine
Piperidine
Piperidine
Ph
Ph
Ph
Ph
Ph
Ph
3
10
6
3
3
92
78
90
94
91
95
1
2
3
4
5
6
4-MeO-C6H4
4-Me-C6H4
4-Cl-C6H4
3-Cl- C6H4
n-Pr
3
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i-Pr
Ph
4-Me-C6H4
4-MeO-C6H4
4-Cl-C6H4
n-Pr
Piperidine
Pyrrolidine
Pyrrolidine
Pyrrolidine
Pyrrolidine
Pyrrolidine
Morpholine
Morpholine
Morpholine
Morpholine
Morpholine
Morpholine
Dibutylamine
Dibutylamine
Piperidine
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
4
3
4
10
3
3
4
5
3
3
3
10
8
8
4
4
3
3
3
94
93
88
73
93
94
89
82
90
93
93
nil
68
71
90
88
95
92
92
94
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
Ph
4-Me-C6H4
4-Cl-C6H4
4-Br-C6H4
n-Pr
4-NO2-C6H4
Ph
4-Cl-C6H4
Ph
Ph
4-Me-C6H4
4-Me-C6H4
n-Bu
n-Bu
n-Bu
n-Bu
Ph
Ph
Ph
Pyrrolidine
Piperidine
Morpholine
Piperidine
4-Cl-C6H4
n-Pr
Piperidine
3
a Aldehyde (1 mmol), amine (1.1 mmol), phenylacetylene (1.2 mmol), catalyst (20 mg), H2O, reflux.
b Isolated yields.
A plausible mechanism for the formation of the product would be as follows: the
immobilized Ag NPs on the surface of ZnO NPs insert in terminal alkyne to form the Ag
acetylide intermediate through the activation of C-H bond. On the other hand, formation of the
iminium ions from aldehydes and secondary amines could be accelerated in the presence of
Lewis acid and base sites of catalyst core, Zn2+ and O2-, respectively. Condensation of Ag
acetylide intermediate with iminium ion produced in situ, gives the corresponding
propargylamine.
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Scheme 2. Proposed mechanism for catalytic synthesis of propargylamines
The recyclability and reusability of the catalyst was investigated for the model A3-coupling
reaction of benzaldehyde, piperidine and phenylacetylene in water. After each run, the catalyst
was separated by centrifugation (4000 rpm, 5 min), washed with isopropanol and dried in an
oven, then was reused as such for subsequent experiment under similar reaction conditions. The
results showed that the catalyst (either 20 mg or the lower amounts for example 10 mg) could be
effectively used for at least five consecutive cycles without much appreciable loss in its catalytic
activity (Table 3). The Ag content in the fresh catalyst and for the used catalyst, after 5 runs, was
determined to be 0.31 and 0.28 mmolg-1 by ICP, respectively. Such an excellent reusability and
high stability of the catalyst could be attributed to the strong embedding of Ag NPs on the
surface of the modified ZnO NPs. Indeed, the ionic liquid networks anchored on the surface of
the heterogeneous nanoscale support, contribute significantly to the direct immobilization of the
Ag NPs.
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Table 3: Reusability of the ZnO-IL/Ag catalysta.
Runs
Yieldsb (%)
Yieldsc (%)
Recovery of ZnO-IL/Agd (%)
Ag content
(mmolg-1)
78
76
75
72
72
92
90
90
88
87
98
96
95
93
91
0.31
0.30
0.30
0.29
0.29
fresh
1
2
3
4
a Reaction mixture: benzaldehyde (1 mmol), Piperidine (1.1 mmol), Phenylacetylene (1.2 mmol), H2O, reflux.
b Yields are achieved in the presence of 10 mg catalyst.
c Yields are achieved in the presence of 20 mg catalyst.
d The recovery percent of ZnO-IL/Ag is achieved for 20 mg catalyst.
A comparison for the efficiency of the catalytic activity of ZnO-IL/Ag NPs with several pervious
methods is presented in Table 4. The results show that this method is superior to some of the
earlier methods in terms of yields and reaction times.
Table 4: Comparison between catalytic efficiency of ZnO-IL/Ag with several known catalysts used in
propargylamines synthesis.
Entry
Conditions
Yields (%)
Ref.
[44]
[45]
[46]
[33]
[34]
[38]
[31]
[35]
NiCl2, Toluene, 111 ºC, 9h
InBr3, Toluene, 80 ºC, 6h
CuCN, [bmim]PF6, 120 ºC, 2h
AgTPA, CH3CN, 80 ºC, 4h
CuHAP,CH3CN, reflux
Cu NPs, THF, 70-80 ºC, 5.5h
CuI, H2O, Microwave, 20 min
Ag-NaY, Neat, 100 ºC, 15h
ZnO-IL/Ag, H2O, reflux, 3h
87
80
74
98
90
98
91
73
92
1
2
3
4
5
6
7
8
9
This work
Reaction mixture: benzaldehyde (1 mmol), Piperidine (1.1 mmol), Phenylacetylene (1.2 mmol).
Valuable capability of the catalyst is indicated by producing a wide range of
propargylamines. The procedure would be in accordance with environmental norms because
water is proposed as a green solvent for the reaction in which the catalyst could be recovered
easily by centrifuging and decantation.
15
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4. Conclusion
The Ag NPs were embedded successfully in the ZnO-IL nanohybrid matrix. The as prepared
ZnO-IL/Ag nanoparticles was applied as an efficient heterogeneous catalyst for the three-
component coupling reactions of aldehydes, amines and alkynes in water to achieve
propargylamines. This method offers several advantages including high yields, short reaction
times, use of water as a green solvent, a simple work-up procedure, ease of separation and
recyclability of the catalyst, as well as the ability to tolerate a wide variety of substitutions in the
reagents.
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*Manuscript
Click here to view linked References
Revised Manuscript (highlighted version)
Immobilized Silver on Surface-modified ZnO Nanoparticles: as an Efficient
Catalyst for Synthesis of Propargylamines in Water
Farnaz Movahedia,, Hassan Masrouria, M.Z. Kassaeeb
aDepartment of Chemistry and Petrochemical Engineering, Standard Research Institute, P.O.
Box 31745-139, Karaj, Iran
bDepartment of Chemistry, Tarbiat Modares University, P.O. Box 14155-4838, Tehran, Iran
Abstract:
Silver nanoparticles were immobilized on ionic liquid modified zinc oxide nanoparticles leading
to a ZnO-IL/Ag NPs catalyst, which exhibits high catalytic performance in the synthesis of
propargylamines through one-pot three-component condensation reaction of aldehydes, amines,
and alkynes in water. The as-prepared catalyst was characterized by ICP-AES, FT-IR, XRD,
SEM, and TEM. The catalyst was easily recycled and reused at least five times without
significant loss of its catalytic activity.
Keywords: Ag nanoparticles, Heterogeneous catalysis, Propargylamines, Ionic liquid, ZnO
nanoparticles
Corresponding author Tel.: +98-912-7031230; Fax: +98-26-32803880; E-mail: Fzmovahedi@standard.ac.ir
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1. Introduction
The development of nano-sized metal particles has attracted considerable attention because
of their promising applications in diverse fields [1-3]. They show great potential as versatile
catalysts for green and sustainable organic synthesis owing to their large surface area and size-
dependent properties different from bulk metals [4-6]. The control of their size, shape, dispersity,
and in particular the combination of metal nanoparticles with a specific heterogeneous support, is
essential to prevent aggregation and/or coalescence, resulting to enhanced activity and selectivity
in a desired application [7-9]. Nanoparticles supported catalysts not only compete with their
homogeneous counterparts but also, can be considered as ‘soluble’ analogues since they are
readily dispersed and exhibit an intrinsically a high surface area, attribute that allow excellent
accessibility of substrates to the surface bound active catalytic sites. Zinc oxide nanoparticles
(ZnO NPs) have their own importance due to their vast area of applications [10–14], especially
in catalysis both as support and as active phase for hydrogenation or dehydrogenation reactions
[15]. In addition, coverage of nanostructures by ionic liquids (ILs) is advised for construction of
narrow size distribution nanoscale heterogeneous catalysts [16]. A kind of catalytic system in
which the facile recovery and reuse of ionic liquids is achieved through covalent tethering to
heterogeneous nanostructure supports [17, 18]. This has led to a new possibility for developing
environmentally friendly catalytic systems through stability in water and air, easy separation,
high catalytic efficiency and reusability, resulting in vast applications for various organic
reactions [19].
On the other hand, one-pot three-component catalytic couplings of aldehydes, amines and
alkynes via C-H activation of the terminal alkynes have attracted a considerable interest in recent
years [20]. The resultant propargylamines are major skeletons [21-23] and synthetically key
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intermediates [24-26] for preparation of many nitrogen containing biological active compounds,
such as β-lactams, oxotremorine analogues, conformational restricted peptides, and therapeutic
drugs [27, 28]. Three-component synthesis of propargylamines has been reported using various
transition metal catalytic systems such as Ag (І) salts [29], Au (ІІІ) salen complexes [30],Cu (І)
salts [31], Cu/Ru bimethallic system [32], heteropolyacid-supported silver (Ag-HPA) [33],
hydroxyapatite-supported copper (Cu-HAP) [34], AgY zeolite [35] as well as Ag-, Au- and Cu-
nanoparticles [36-39]. Although these types of catalysts impose their own advantages and
disadvantages to the reaction, the considerable drawback is that most of them proceed in toxic
solvents and harsh conditions. Consequently, attempts to achieve more efficient and
environmentally friendly methods for chemical synthesis of propargylamines still remains a big
challenge and continues to be the focus of intense research.
Herein, we report a new catalytic system based on Ag NPs immobilized on ZnO NPs
modified with ionic liquid groups and its application as a novel and efficient catalyst for the A3-
coupling reaction of aldehydes, amines and alkynes to produce propargylamines in water. We
anticipated that Ag and ZnO NPs as heterogeneous sources, along with the ILs as the
homogeneous matrix, would create a catalyst with efficiency beyond the customary limit.
2. Experimental
2.1.Chemicals and instruments
The reagents and solvents used in this work were obtained from Fluka or Merck and used
without further purification. Nanostructures were characterized using a Holland Philips Xpert X-
ray powder diffraction (XRD) diffractometer (Co K, radiation, λ = 0.178897 nm), at a scanning
speed of 2/min from 10 to 80 (2θ). The particle size and morphology were investigated by a
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transition electron microscope (TEM, JEOL JEM-2010 on an accelerating voltage of 200 kV)
and scanning electron microscope (SEM, Holland Philips XL30 microscope with an accelerating
voltage of 25 kV). FT-IR measurements were performed using KBr disc on a Thermo IR-100
infrared spectrometer (Nicolet). Elemental analyses for C, H and N were performed using a
Heraeus CHN–O–Rapid analyzer. A radial view Varian Vista-Pro simultaneous ICP-OES
(Springvale, Australia) coupled to a V-groove nebulizer and equipped with a charge coupled
1
device (CCD) was applied for determination of the silver ions. The H and 13C NMR spectra
were measured at 500.1, and 125.7 Hz, respectively, on a Bruker DRX 500-Avance FT-NMR
instrument with CDCl3 as the solvent.
2.2. Synthesis of 1-(3-trimethoxysilylpropyl)-3-methylimidazolium chloride [tmim][Cl] ionic
liquid
1-Methylimidazole (2.0 g, 25.0 mmol) and (3-chloropropyl) trimethoxysilane (4.9 g, 25.0
mmol) were dissolved in toluene (20 mL) and refluxed for three days. Then two phases formed,
with 1-(3-trimethoxysilylpropyl)-3-methylimidazolium chloride [tmim][Cl] as a viscous oil at
the bottom and the solvent on the top. They were separated and the viscous oil was washed with
toluene (5×10 mL). After drying the oil under vacuum at 60 ºC for one day, ionic liquid (3.5 g,
12.75 mmol, 51% yield) was obtained with a honey-like consistency at room temperature [40].
2.3.Surface modification of ZnO NPs with [tmim][Cl] ionic liquid
ZnO NPs were synthesized through a solid-state reaction method including Zn(OAc)2.2H2O
and sodium hydroxide in the ratio of 1:4 [41]. The obtained ZnO NPs (1.2 g) were sonicated for
one hour before being treated with [tmim][Cl] (0.277 g, 1 mmol) in anhydrous toluene under
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reflux condition overnight. The mixture was cooled to room temperature and the modified ZnO-
IL(Cl) was washed with EtOH several times to remove the impurities and dried under vacuum.
In order to increase the basic characteristic of the ionic liquid anchored on the surface of ZnO
NPs, an anion-exchange processing was carried out using the modified ZnO-IL(Cl) (1.0 g) and
KOH (0.06 g, 1 mmol) in CH2Cl2 (20 ml) for 2 days at room temperature. Finally, the modified
ZnO-IL(OH) was obtained after centrifuging (4000 rpm) and washing with H2O several times.
According to elemental analysis, the loading capacity of ZnO NPs was determined to be 0.54
mmolg-1 of basic ionic liquid.
2.4.Immobilization of silver NPs on Ionic Liquid-Modified ZnO NPs
To a stirred solution of the modified ZnO-IL (1.0 g) in deionized water (20 ml) in 100 ml
round bottom flask was added a solution of AgNO3 (0.1 g, 0.59 mmol) in 20 ml water, at room
temperature. After 10 minutes, N2H4.H2O (0.1 g, 2.0 mmol) was added in one portion as the
reducing agent. The agitation continued for 12 h. The resulting powder was centrifuged (6000
rpm, 5 min) and washed with deionized water and EtOH, then dried under vacuum at 60 ºC
overnight to give the corresponding ZnO-IL/Ag NPs as a catalyst.
2.5. General procedure for the synthesis of propargylamines
Catalyst (10 mol %) was added to a mixture of aldehyde (1 mmol), amine (1.1 mmol) and
phenylacetylene (1.2 mmol) in water (3 mL) at reflux conditions, and the mixture was stirred for
appropriate periods. The progress of the reaction was monitored by TLC. Consequently, the
mixture was centrifuged (4000 rpm, 5 min) till the catalyst was settled down and the solvent
containing the crude product was decanted out. The centrifugate was extracted with CH2Cl2 (2×5
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ml). The organic phase concentrated under reduced pressure to afford the crude product, which
after chromatography on silica gel (100–200 mesh) using hexane:ethyl acetate, 90:10, gave the
corresponding propargylamines.
3. Results and discussion
3.1. Synthesis and characterization of ZnO-IL/Ag NPs
Following our recent reports on the catalytic potentials of ZnO NPs [41, 42], surface of the
prepared ZnO NPs were modified by treatment with the as synthesized 1-(3-trimethoxy
silylpropyl)-3-methylimidazolium chloride [tmim][Cl] ionic liquid in refluxing toluene to be
engaged as a heterogeneous nanostructure support. At the next stage, the modified ZnO-IL(Cl)
were treated with potassium hydroxide in CH2Cl2 at room temperature, in order to increase the
basic catalytic characteristic of the ILs anchored on the surface of ZnO NPs. Then AgNO3 and
hydrazine monohydrate were added into the stirred solution of the modified ZnO-IL(OH) in
deionized water, simultaneously. Ag NPs were produced while embedded in to ionic liquid
networks anchored on the surface of ZnO NPs (scheme 1), in which was used as a heterogeneous
catalyst in propargylamines synthesis. The Ag content was evaluated to be 0.31 mmolg-1 by
inductively coupled plasma (ICP) analysis. Also, the loading capacity of ZnO NPs was
determined to be 0.54 mmolg-1 of the basic ILs via the elemental analysis.
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Scheme 1.
Successful surface modification of the ZnO NPs with ionic liquid was evidenced by Fourier
transform infrared (FT-IR) spectral analysis. (Fig. 1). In contrast to the naked ZnO NPs, the
modified ZnO-IL(OH) showed the characteristic peaks of imidazolium ring at 1570 cm-1 and
1029 cm-1, along with hydroxyl at 3417 cm-1.
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Fig.1: FT-IR spectra for naked ZnO NPs (top) and ZnO-IL (bottom).
The X-ray diffraction pattern of ZnO-IL/Ag NPs is shown in Fig. 2. Three additional
diffraction peaks appeared beside those attributed to the pure ZnO NPs. The XRD patterns of the
ZnO NPs could be indexed to the hexagonal wurtzite structure (space group: P63mc; a = 3.249
Å, c = 5.206 Å, JCPDS card no. 36-1451), while the three additional peaks at 2θ = 44.57◦, 51.90◦
and 76.45◦ are assigned respectively to the diffraction line planes (1 1 1), (2 0 0) and (2 2 0),
corresponding to the face-center-cubic (fcc) of Ag NPs, in accordance with the JCPDS 04-0783.
No evidence for any impurity was detected. Based on this study, the average particle size has
been estimated 30 nm for ZnO NPs and 7 nm for Ag NPs by using Debye-Scherrer equation,
Dh,k,l = kλ/β cosθ, where k is a constant (generally considered as 0.89), λ is the wavelength of Co
Ka (1.78897 Å ), β is the corrected diffraction line full-width at half-maximum (FWHM), and θ
is Bragg’s angle [43].
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