Magnetic nanoparticles entrapped in the cross-linked poly(imidazole/imidazolium) immobilized Cu(ii): An effective heterogeneous copper catalyst

Article abstract of DOI:10.1039/c4ra07817a

Anchoring of copper sulfate in layered poly(imidazole-imidazolium) coated magnetic nanoparticles provided a highly stable, active, reusable, high loading, and green catalyst for the click synthesis of 1,2,3-triazoles via a one-pot cycloaddition of alkyl h

Full text of DOI:10.1039/c4ra07817a

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Magnetic nanoparticles entrapped in the cross-
linked poly(imidazole/imidazolium) immobilized
Cu(II): an effective heterogeneous copper catalyst†
a
Cite this: RSC Adv., 2014, 4, 46418
Ali Pourjavadi, Seyed Hassan Hosseini,a Nasrin Zohrehb and Craig Bennettc
*
Anchoring of copper sulfate in layered poly(imidazole-imidazolium) coated magnetic nanoparticles
provided a highly stable, active, reusable, high loading, and green catalyst for the click synthesis of 1,2,3-
triazoles via a one-pot cycloaddition of alkyl halide, azide, and alkyne (Cu-A3C). The catalyst was
characterized by FTIR, TGA, TEM, SEM, XRD, EDAX, VSM and AAS. High selectivity, broad diversity of
alkyl/benzyl bromide/chloride and alkyl/aryl terminal alkynes, and good to excellent yields of products
were obtained using 0.7 mol% catalyst. The catalyst was readily recovered and reused up to 6 times
without significant loss of activity.
Received 30th July 2014
Accepted 9th September 2014
DOI: 10.1039/c4ra07817a
www.rsc.org/advances
several runs10 reducing the cost of production.12 Robustness,
increased air, moisture and thermal stability,14 minimization of
metal leaching,21,25 improved handling of the catalyst8 and
minimization of product contamination with copper27 are other
advantages of heterogeneous copper-based catalysts.
Introduction
Several features of copper confer a remarkably broad range of
activities allowing it to catalyze the oxidation and oxidative
union of many substrates.1
Following the Sharpless's report28 on Cu catalyzed azide-
alkyne cycloaddition (CuAAC) and considering the advantages
of heterogeneous catalysts, various heterogeneous systems have
been attained by the covalent and non-covalent immobilization
of copper onto the variety of supports for CuAAC. Most include
supported Cu0 nanoparticles on solid supports29 such as Al2O3/
ball-milling,30 chelate resins,31 silica coated maghemite,32 and
nano Fe3O4/glutathione.33 Moreover, various N- or C-ligand/CuI
complexes have been covalently immobilized on solid supports
for CuAAC.34 Magnetic or silica supported ionic liquid/
NHCs,35,36 dendrimers,37 polymer-supported azide or 2,20-biar-
ylpyridine,38,39 magnetic chelating adsorbents,40 merrield resin
supported tris(triazolyl)methanol,41 SiO2 supported immino
pyridine,42 and supported copper(I)–NHC43 are some examples.
Although, these catalysts are efficient, each of them suffers from
some drawbacks such as multi-step catalyst preparation, low
loading, long reaction time, tedious work up of catalyst and
product, and harsh reaction condition. In most cases, the low
loading problem of heterogeneous Cu-catalysts leads to the
application of a larger amount of catalyst or higher temperature
and longer reaction time. Immobilization of Cu onto the surface
of polymers is a good way to overcome this limit since a large
number of functionalities can be linked to Cu.
To date, numerous copper-based catalytic systems have been
reported for various organic transformations.1–4 However, in
recent years, heterogenization of copper has received consid-
erable attention through supporting or immobilizing the cata-
lyst into or onto the insoluble solid support. Some of the most
important heterogeneous Cu-catalyzed processes reported in
recent years, include self-condensation of amines,5 C–H aryla-
tion,6,7 asymmetric Henry reaction,8,9 terminal alkyne homo-
coupling,10,11 kinetic resolution of 1,2-diols,12,13 Ulmann type
C–X cross-coupling,14–16 chemoselective hydrogenation of cin-
namaldehyde,17 Sonogashira reaction,18,19 alcohol oxidation,20
epoxidation of alkenes,21,22 addition of terminal alkynes to
imines,23–26 and, perhaps the most important, Cu-azide-alkyne
cycloaddition (CuAAC) reaction resulting in 1,2,3-triazoles
known as a click reaction.
The primary advantages of these heterogeneous copper-
based catalysts, as opposed to homogeneous ones, are easy
catalyst recyclability and product separation, minimizing waste
of the products during the catalyst separation and its
disposal.24,16 In addition, the recycled catalyst can be reused for
aPolymer Research Laboratory, Department of Chemistry, Sharif University of
Technology, Tehran, Iran. E-mail: purjavad@sharif.edu; Fax: +98 2166165311; Tel:
+98 2166165311
Accordingly, with the aim of increased ease of recovery,
increased loading and stability of catalyst, and effective
anchoring, we have investigated the development of novel cross-
linked poly(imidazole/imidazolium)-CuII coated magnetic
nanoparticles and its efficiency in copper-catalyzed alkyl
bDepartment of Chemistry, Faculty of Science, University of Qom, P. O. Box: 37185-359,
Qom, Iran
cDepartment of Physics, Acadia University, Wolfville, Nova Scotia, Canada
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra07817a
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halide–azide–alkyne cycloaddition (Cu-A3C) as a one-pot three- was added. The mixture was sonicated for 20 min and then
component “click reaction” for synthesis of 1,2,3-triazoles.
deoxygenated by argon for another 20 min. Aerwards, AIBN
(0.1 g) was added to the mixture and the ask was equipped
with a condenser and placed in an oil bath at 70 ꢀC. Aer 18 h,
the solid products were magnetically separated and washed
three times with methanol and dried under vacuum at 50 ꢀC
(MNP@PIL, 1.37 g).
Experimental
Reagents and analysis
Ferric chloride hexahydrate (FeCl3$6H2O), ferrous chloride tet-
rahydrate (FeCl2$4H2O), copper sulfate (CuSO4), ammonia
(30%), and 3-methacryloxypropyltrimethoxy silane (MPS, 98%)
were purchased from Merck. 1-Vinylimidazole was obtained
from Aldrich and was distilled before use. 1,4-Dibromobutane
was obtained from Aldrich, while 2,20-azobisisobutyronitrile
(AIBN, Kanto, 97%) was recrystallized from ethanol.
FTIR spectra of samples were taken using an ABB Bomem
MB-100 FTIR spectrophotometer. Thermogravimetric analysis
(TGA) was performed under a nitrogen atmosphere using a TGA
Q 50 thermogravimetric analyzer. The morphology of the cata-
lyst was observed using a Philips XL30 scanning electron
microscope (SEM) and transmission electron microscopy (TEM)
images were taken using a Philips CM30 electron microscope.
The resulting powdered materials were subjected to an
anchoring (complexation) reaction. The powdered magnetic PIL
(0.5 g) was added to 30 mL water and an excess amount of
CuSO4 (1.5 g) was added to the solution. The mixture was
vigorously stirred for 3 days at 70 ꢀC. The solid products
(MNP@PILCu) were then magnetically separated, washed ve
times with water (5 ꢁ 100 mL) and twice with methanol (2 ꢁ 20
ꢀ
mL) and dried under vacuum at 50 C.
General procedure for CuAAC reaction catalyzed by
MNP@PILCu
Deionized water (2 mL) and Na-ascorbate solution (1 mL, 20
mol%) was added to a mixture of alkyne 1 (1 mmol), halide 2 (1
mmol), NaN3 (1.3 mmol) and MNP@PILCu (4 mg, 0.7 mol% Cu)
in a round-boꢀttom ask (10 mL). The reaction mixture was then
stirred at 50 C for an appropriate time indicated in Table 2.
Aer reaction completion, methanol was added and the catalyst
was magnetically separated, washed with methanol and dried
for another run. The product mixtures were extracted by hot
ethyl acetate and obtained as almost pure crystals aer removal
of the solvent under reduced pressure. The structures of the
products were conrmed by physical property measurements
and spectral data (see ESI†).
Synthesis of vinyl functionalized magnetic nanoparticles
The magnetic Fe3O4 nanoparticle coated silica (denoted as
MNP) was synthesized as previously reported method.44
1 g MNP was added to dry ethanol and then 2 mL of
ammonia solution was added to the ask. An excess amount (10
mmol) of the 3-(trimethoxysilyl)propylmethacrylate (MPS)
solution was then dropwise added and the mixture was stirred
at 60 ꢀC for 48 h. The MPS coated magnetic nanoparticles
(denoted as MNP@MPS) were magnetically separated and
washed several times with methanol and dried under vacuum at
ꢀ
50 C.
Results and discussion
Synthesis of IL monomer and cross-linker
The desired catalyst was prepared via a few steps. Magnetic
nanoparticles were prepared based on co-precipitation of a
mixture solution of Fe(II) and Fe(III) in alkaline pH. This method
produces Fe3O4 nanoparticles with sufficient magnetism to
allow easy catalyst recovery. Coating of the Fe3O4 surface with a
SiO2 shell was carried out by TEOS to increase stability against
acidic medium and to provide easier functionalization in the
next synthesis steps. In order for effective polymerization and
covalent graing of copolymers onto the surface, MNPs were
subjected to functionalization with MPS before polymerization.
The presence of functional group C]C in MPS facilitated this
approach.
Ionic liquid monomer 3-ethyl-1-vinylimidazolium tetra-
uoroborate ([EVim][BF4]) and cross-linker 1,4-butanediyl-3,30-
bis-1-vinylimidazolium bistetrauoroborate ([BBVim][BF4]2)
were prepared by quaternization of 1-vinylimidazole with ethyl
bromide followed by exchange of the bromide with BF4 anion.
The copolymerization and graing was carried out by free
radical initiation polymerization of monomers in the presence
of MNP@MPS and cross-linker (Scheme 1). Copolymerization
Typically, 1-vinylimidazole (2.82 g, 30 mmol) and ethyl bromide
(3.27 g, 30 mmol) were stirred in 10 mL methanol at 60 ꢀC for 20
h. The reaction mixture was then cooled to room temperature
and added to 250 mL of diethyl ether. The resulting translucent
solution was placed in a refrigerator for 5 h. Solid products were
separated by decantation of supernatant and washed three
ꢀ
times with diethyl ether and dried under vacuum at 50 C. For
exchange of bromide anion to tetrauoroborate, 3-ethyl-1-
vinylimidazolium bromide was stirred in an equimolar amount
of aqueous HBF4 (50%) at room temperature. The pure product
was obtained aer evaporation of water at 70 ꢀC under reduced
pressure. The same procedure was used for the synthesis of 1,4-
butanediyl-3,30-bis-1-vinylimidazolium
bistetrauoroborate
([BBVim][BF4]2) except that the molar ratio of 1-vinylimidazole
to 1,4-dibromobutane and HBF4 to 1,4-butanediyl-3,30-bis-1-
vinylimidazolium dibromide were 2 : 1. 1H and 19F NMR of the
monomer and cross-linker are cited in the ESI.†
Synthesis of the catalyst
MNP@MPS (0.4 g), 1-vinylimidazole (Vim, 1 g), IL monomer was initiated by AIBN and magnetic nanoparticles were
([EVim][BF4], 0.4 g) and cross-linker ([BBVim][BF4]2, 0.2 g) were entrapped into the cross-linked insoluble copolymers. This
loaded into a 50 mL round bottom ask and 30 mL of methanol approach efficiently endows the resulting MNP@PIL particles
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Table 1 Optimization of reaction conditions for the click Cu-A3C reactiona
Entry
Solvent
Catalyst (mg)
T (ꢀC)
Time (h)
NaN3 (eq.)
Na ascorbate (mol%)
Yieldb (%)
1
2
3
4
5
6
7
8
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
Neat
10
6
4
4
4
4
4
4
4
5
5
3
2
4
4
4
4
4
4
4
r.t
r.t
r.t
r.t
50
80
50
50
50
50
50
50
50
50
50
50
50
50
50
50
3
4
4
7
2
2
2
2.5
3
3
4
4.5
5
5
3
3
2.5
2.5
2.5
2.5
1
1
1
1
1
1
1.3
1.3
1.3
1.3
1.3
1.3
1.3
1.3
1.3
1.3
1.3
1.3
1.3
1.3
10
10
10
10
10
10
10
20
20
20
20
20
20
20
20
20
20
20
20
20
81
88
70
91
89
91
92
95
95
95
96
83
63
40
70
75
64
21
Trace
10
9
10
11
12
13
14
15
16
17
18
19
20
tBuOH
tBuOH–H2O (1 : 4)
DMF
CH3CN
CH2Cl2
THF
a
b
Benzyl bromide: 1 mmol, phenyl acetylene: 1 mmol, solvent: 2 mL. Isolated yield.
with a series of active catalyst sites, providing higher catalyst
The thermal stability and organic content of MNP@PIL was
loading. Finally, copper was loaded via complexation of copper measured by thermal gravimetric analysis (Fig. 2). The weight
sulfate and copolymer chains on the surface of MNP@PIL. This loss for all samples occurring around 150 ꢀC was completely
resulted in the desired magnetic poly(imidazole/imidazolium)- attributed to the loss of adsorbed water molecules. Comparison
immobilized CuII (MNP@PILCu).
of the weight losses in the TGA curves of MNP (Fig. 2a) and
Due to the polymer coating, the catalyst adsorbs more copper MNP@MPS (Fig. 2b) showed that loading of MPS on the surface
ions than traditional heterogeneous catalysts. Also, the polymer of MNP is 0.78 mmol gꢂ1. From the TGA curve of MNP@PIL, it
chains on the surface of the catalyst increase the spacer length was calculated that the copolymer content in the catalyst is
and the substrates can easily adsorb onto the catalyst surface. about 55 wt%. It is notable that the loading amount of the
Moreover, the ionic part of the copolymer in the MNP@PIL monomer and the cross-linker cannot be calculated individually
structure accelerates the substrate adsorption onto the catalyst by TGA due to the identical nature of monomer and cross-
ꢂ
ꢀ
surface and the reaction proceeds faster. The hydrophobic BF4
anions in copolymer matrix prevent the swelling of the catalyst which are attributed to weight losses for losing adsorbed water
in water medium and reduce the copper ion leaching. molecules, imidazolium and imidazole parts of the catalyst
linker. DTG analysis shows three peaks at 100, 320 and 380 C
FTIR was employed to characterize the catalyst at this step. structure, respectively. These results showed that catalyst is
As shown in Fig. 1, FTIR spectrum of MNP (Fig. 1a), MNP@MPS thermally stable and can be used under harsh reaction condi-
(Fig. 1b), MNP@PIL (Fig. 1c), and MNP@PILCu catalyst (Fig. 1d) tion at high temperature.
shows stretching mode vibrations of Fe–O and Si–O in the same
Fig. 3 displays the powder X-ray diffraction patterns collected
region around 640 and 1100 cmꢂ1, respectively. Coating of MPS over the two-theta range 5 to 100 degrees for MNP@PILCu. The
onto the MNP surface was conrmed by stretching mode diffraction peaks have similar positions and relative intensities
vibrations of C]C and steric C]O at 1420 and 1720 cmꢂ1 in the as the standard Fe3O4 sample (blue lines) showing that the
FTIR spectrum of MNP@MPS (Fig. 1b). Aer copolymerization, modications have not caused a phase change in the Fe3O4
MNP@PIL showed a strong band at 3100 cmꢂ1 as well as bands particles. A broad peak around 20ꢀ is attributed to the amor-
at 1610 and 1530 cmꢂ1, clearly indicating the presence of phous silica shell around the magnetic nanoparticles.
aliphatic C–H and imidazole ring. On complexation with
The magnetization curves of MNP and MNP@PILCu showed
copper, the FTIR spectrum of MNP@PILCu (Fig. 1d) showed a small coercivities, which indicated the superparamagnetic
new band at 1120 cmꢂ1 attributed to the S]O bond of CuSO4. nature of both materials (Fig. 4). The saturation magnetization
These results conrm that catalyst was successfully prepared.
of MNP@PILCu is smaller than bare MNP due to entrapment of
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Table 2 Three-component click synthesis of 1,2,3-triazoles using MNP@PILCu catalysta
Entry
1
1
2
Time (h)
Yield of 3b (%)
2.5 (X ¼ Br)
3 (X ¼ Cl)
3a: 95
3a: 94
2
3
3
3b: 91
2.5 (X ¼ Br)
3.5 (X ¼ Cl)
3c: 92
3d: 89
4
5
2
4
4
3e: 97
3f: 88
3g: 87
6
7
4 (X ¼ Br)
4 (X ¼ Cl)
3h: 84
3h: 80
8
3
3i: 90
3j: 91
9
3.5
10
2.5 (X ¼ Br)
3 (X ¼ Cl)
3k: 93
3k: 90
11
4
3l: 90
12
13
4
4
3a: 96
3m: 90c
14
15
4
4
3n: 91c
3o: 94c
16
17
3
3p: 87
3q: 90
2.5
18
3.5
3r: 88
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Table 2 (Contd.)
Entry
19
1
2
Time (h)
3
Yield of 3b (%)
3s: 90
20
3
3t: 92
a
Reaction condition: alkyl halide (1 mmol), NaN3 (1.3 mmol), acetylene (1 mmol), H2O (2 mL). b Isolated yield. c MeOH as solvent.
Scheme 1 Preparation of MNP@PILCu catalyst.
MNPs into the nonmagnetic materials. However, the magneti- activity. EDX analysis conrms the presence of copper and iron
zation is still large enough for separation of the catalyst.
The TEM image of magnetic nanoparticles and MNP@PILCu
in the catalyst.
The loading of Cu(II) ions in MNP@PILCu was calculated by
are shown in Fig. 5a and b, respectively. TEM image of the atomic absorption spectroscopy (AAS) using standard samples.
catalyst showed the presence of well dispersed spherical nano- It was found that the loading amount of Cu(II) ions in
particles into the copolymer matrix. The average size of the MNP@PILCu is 1.87 mmol gꢂ1. This result shows that catalyst
magnetic nanoparticles was observed to be 3.0 ꢃ 0.8 nm, has high loading level of copper ions.
reecting a narrow size distribution in the catalyst.
One-pot cycloaddition reaction of benzyl bromide, sodium
A typical SEM image and EDX analysis of MNP@PILCu is azide and phenyl acetylene in the presence of Na-ascorbate, was
shown in Fig. 6. A bulk polymeric structure with a rough surface selected as a click model reaction to investigate the efficiency of
is seen in SEM image. The rough surface of the catalyst is due to the MNP@PILCu catalyst. Several optimizations, such as cata-
the presence of the magnetic nanoparticles. Such a structure lyst loading, NaN3 and Na-ascorbate amount, solvent, temper-
increases the catalytic surface area and improves the catalytic ature, and time were examined to obtain the best catalytic
condition for click synthesis of N-benzyl-4-phenyl-1,2,3-triazole.
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Fig. 3 The XRD pattern of MNP@PILCu.
Fig. 1 FTIR spectra of MNP (a), MNP@MPS (b), MNP@PIL (c) and
MNP@PILCu (d).
Water clearly stands out as the solvent of choice because of its
low cost, green nature and environmental acceptability. Thus,
optimization of catalyst amount, temperature, and time were
carried out in H2O as solvent using equimolar ratio of reactant
and 10 mol% Na-ascorbate (Table 1). It was found that the
amount of catalyst, except when present in very low quantity
(entries 12 and 13), is inversely related to the reaction temper-
ature and time. Thus, in order to avoid long reaction time and
ꢀ
high temperature, the use of 4 mg MNP@PILCu at 50 C was
selected for further optimization of the model reaction. Under
this condition, increasing the amount of NaN3 and Na-ascor-
bate from 1 to 1.3 eq. and 10 to 20 mol%, enhanced the yield of
product 3a up to 95% (entry 8). Other solvents were also tested
(entries 14–20). In aprotic polar and nonpolar solvent, the
reaction gives very low yield probably because of poor solubility
of NaN3. In polar protic solvents, reactions take place at higher
rate with more than 70% yield of product. However it was found
that H2O functioned better than other protic solvents, probably
because of its high ability to dissolves NaN3.
For further investigation in catalyst activity same reaction
under optimized condition was performed in the presence of 1
mol% copper sulfate as a catalyst. The result showed that the
yield of triazole product was 12% (1 : 1 mixture of regioisom-
ers). A second reaction was also performed by copper sulfate in
Fig. 4 Magnetization curves of MNP (a) and MNP@PILCu (b).
the presence of 10 mol% 1-methylimidazol as a ligand. The
result showed that the yield of triazole product increased to 98%
(pure regioisomer). These results show that imidazole groups
improve the activity and selectivity of copper catalyst by
complexation.
Under the optimized reaction condition, the scope of the
copper catalyzed synthesis of 1,2,3-triazole with respect to the
various acetylenes and halides was explored. As indicated in
Table 2, both aromatic and aliphatic terminal acetylenes gave
the corresponding triazoles in good yields. However aliphatic
Fig. 2 TGA and DTG curves of MNP (a), MNP@MPS (b) and MNP@PIL (c).
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Fig. 5 TEM images of MNP@PILCu.
Fig. 7 Leaching experiment. Red line: reaction in the presence of
catalyst and blue line: reaction after removal of catalyst.
alkynes required longer reaction times compared to their aryl
counterparts (compare 16/1, 17/2, 20/1, 18/8). More importantly,
electron-decient acetylenes (entries 13–15) which are normally
difficult substrates for Cu-A3C, exhibited similar behavior and
led to triazole formation in high yield with acceptable reaction
times. A variety of halide substrates were also subjected to the
Cu-A3C click reaction. It was found that all reactions produced
corresponding triazoles, however, the reaction rates of chlorides
(entries 1, 3, 7, 10) were slower than bromides, so that the
reaction time must be enhanced for an equivalent trans-
formation. It was also found that the benzyl halides containing
electron-donating groups (compare entries 1, 3, 4) gave the
products in lower yields or longer reaction times. These results
indicate that the catalytic systems can be readily applied to the
combinatorial click synthesis of 1,4-disubstituted-1,2,3-
triazoles.
that no further production of 3a was observed. Aer catalyst
removal, the reaction mixture was also analyzed using induc-
tion coupled plasma (ICP-AES) and no copper species were
detected. These results can rule out any contribution of leached
Metal leaching is an important issue to consider when
working with metal-supported heterogeneous catalysts, it being
necessary to verify whether the observed catalyst activity is
derived from solid or leached metal species to the solution. To
determine whether our click reaction using MNP@PILCu
actually occurs through a heterogeneous catalytic process, the
following experiments were carried out. The model reaction was
performed under optimized condition (Table 1, entry 8), and the
MNP@PILCu catalyst was completely removed from the reac-
tion mixture by an external magnet aer 60 min. Then, the
reaction was allowed to stir for 4 hours (Fig. 7) and it was found
Fig. 8 Recycling experiment.
Fig. 6 SEM image and EDAX analysis of MNP@PILCu.
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Table 3 The comparison of MNP@PILCu with reported catalysts in the synthesis of triazoles
Copper loading
Catalyst loading
(mol%)
Catalyst
on catalyst (mmol gꢂ1
)
Time (h)
T (ꢀC)
Yield (%)
Ref.
Functionalized chitosan/Cu
Cross-linked poly(IL)/Cu
Ionic polymer/Cu
Poly(4-vinyl pyridine)/Cu
Nano ferrite-glutathione-copper
Poly(NIPAM/Im)/Cu
0.6
1.0
0.1
1.0
5
12 h
48 h
2.5 h
25 min
10 min
1.5
70
r.t
r.t
100
120 mw
50
99
98
99
89
99
99
95
45
46
47
48
49
50
0.25
1.32
0.25
0.46
1.87
13
2.5
0.25
0.7
MNP@PILCu
25 min
50
Present work
copper into the reaction medium, and conrms that the magnetic recovery and excellent reusability (up to 6 runs) is
observed catalyst is completely heterogeneous. expected to contribute to the utilization of this catalyst for the
The most important factors concerning a heterogeneous development of benign chemical processes and products.
catalyst are its stability and reusability under the reaction
conditions along with the ease of separation. To investigate
these factors for MNP@PILCu, benzyl bromide, phenyl acety-
Acknowledgements
lene and sodium azide were subjected to the click reaction in
the presence of catalyst MNP@PILCu under optimized condi-
tion. The catalyst was easily recovered and reused up to 6 times.
As depicted in Fig. 8, the catalytic system worked extremely well
even up to six subsequent runs with almost 92% yield and
without signicant loss of activity. According to the leaching
experiments, a slight decrease in the yield was attributed to the
loss of catalyst mass within the recovery and washing process.
The TEM images of the catalyst aer recovery (Fig. S1†) showed
that the structure of the catalyst was stable under the reaction
conditions with no change being observed. The FTIR spectrum
of the catalyst aer recycling showed all the characteristic peaks
of fresh catalyst, except the stretching vibration peak of the
sulfate anions of copper sulfate which are exchanged by
ascorbate.
A comparison between MNP@PILCu and other reported
supported copper catalysts is presented at Table 3. As shown in
Table 3, copper loading in MNP@PILCu is higher than other
catalysts due to imidazole groups strongly adsorb copper ions
from the solution. Also, the amount of applied catalyst in the
reaction was mostly lower than other catalysts. These results
show that the presence catalyst has some advantage than other
reported copper catalysts.
Iran National Science Foundation (INSF) is gratefully acknowl-
edged for nancial support of this work.
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