Poly(basic ionic liquid) coated magnetic nanoparticles: High-loaded supported basic ionic liquid catalyst

Article abstract of DOI:10.1016/j.crci.2013.01.003

A novel poly(ionic liquid) (PIL) coated magnetic nanoparticle was synthesized by distillation-precipitation-polymerization of 1-vinyl-3- ethyl imidazolium in the presence of surface modified magnetic nanoparticles. The resulting catalyst was used as magne

Full text of DOI:10.1016/j.crci.2013.01.003

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Full paper/Memoire
Poly(basic ionic liquid) coated magnetic nanoparticles: High-loaded
supported basic ionic liquid catalyst
*
Ali Pourjavadi , Seyed Hassan Hosseini, Seyyed Alireza AghayeeMeibody,
Seyedeh Talieh Hosseini
Polymer Research Laboratory, Department of Chemistry, Sharif University of Technology, Tehran, Iran
A R T I C L E I N F O
A B S T R A C T
Article history:
A novel poly(ionic liquid) (PIL) coated magnetic nanoparticle was synthesized by
distillation-precipitation-polymerization of 1-vinyl-3- ethyl imidazolium in the presence
of surface modified magnetic nanoparticles. The resulting catalyst was used as magnetic
heterogeneous base catalyst for the synthesis of 4H-benzo[b]pyrans in water. The
separation of the catalyst from the reaction mixture was readily achieved by simple
magnetic decantation and the catalyst could be easily recycled without appreciable loss of
catalytic activity. Because of polymer layers coated the surface of the magnetic
nanoparticles, the catalyst has a high loading level of ionic liquid.
Received 27 September 2012
Accepted after revision 11 January 2013
Available online xxx
Keywords:
Magnetic catalysts
Polymer coated magnetic nanoparticles
Poly(ionic liquid)
´
ß 2013 Published by Elsevier Masson SAS on behalf of Academie des sciences.
Brønsted base catalyst
1. Introduction
combine the advantages of ionic liquids with those
heterogeneous support materials. Many reports about
Ionic liquids (ILs) have attracted great attention in a
variety of different areas, reaching from material synthesis
to separation science as well as alternative reaction media
[1]. However, despite promising results, their widespread
use in process chemistry is still hampered by the following
practical drawbacks:
the preparation of supported ionic liquid catalysts have
been published and the ionic liquids were normally
immobilized onto the support materials, such as silica
gel [3–5], mesoporous silica [6–8], magnetic nanoparticles
[9–11] and polymers [12–14]. But in the normal immobi-
lization of ionic liquids, the amount of IL, which was loaded
onto the surface of support materials, was very low due to
surface immobilization occurring only in one surface layer.
Among the support materials, magnetic nanoparticles are
more interesting due to the large surface area-to-volume
ratio and simple recyclability and reusability.
One of the processes used in the synthesis of
pharmaceutical compounds is a base-catalyzed reaction
with a homogeneous catalyst, such as KOH or NaOH. This
reaction can produce products with high yield under mild
conditions. However, for satisfactory use of the products as
pharmaceutical compounds, the base catalyst must be
removed from it, requiring a cumbersome purification
process. Moreover, the process wastewater is environ-
mentally toxic, corrosive and the catalyst could not be
recovered. Thus, it is still a challenge to find novel
heterogeneous Brønsted-type basic catalysts with high
ꢀ
ꢀ
ꢀ
homogeneous reaction, which was difficult for product
separation and catalyst recovery;
high viscosity, which results in only a minor part of ionic
liquids take part in the catalyzed reaction;
the use of relatively large amounts of the ionic liquids,
which is costly and may cause possible toxicological
concerns [2].
To overcome these drawbacks, the concept of sup-
ported ionic liquid catalysis (SILC) has been established to
* Corresponding author.
E-mail addresses: purjavad@sharif.edu,
meghdad_hossiny@yahoo.com (A. Pourjavadi).
1631-0748/$ – see front matter ß 2013 Published by Elsevier Masson SAS on behalf of Acade´mie des sciences.
http://dx.doi.org/10.1016/j.crci.2013.01.003
Please cite this article in press as: Pourjavadi A, et al. Poly(basic ionic liquid) coated magnetic nanoparticles: High-
loaded supported basic ionic liquid catalyst. C. R. Chimie (2013), http://dx.doi.org/10.1016/j.crci.2013.01.003

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activity and selectivity, combined with simple separation
from the reaction medium. However, many Brønsted-type
basic catalysts were previously supported onto many
support materials, but they suffer problems, such as low
loading and low thermal stability [15,16].
solution pH was adjusted to 12. A black precipitate was
formed after continuously stirring for 1 h. The precipitate
was magnetically separated and washed four times with
the deionized water and two times with ethanol and was
dried under vacuum at 50 8C overnight.
Herein, we report novel heterogeneous Brønsted-type
basic catalysts based on Fe3O4 magnetic nanoparticle
(MNP) coated with the multilayers of poly(ethylvinylimi-
dazolium) hydroxide, which proved to be highly efficient
for the synthesis of 4H-benzo[b]pyrans in water.
2.3. Surface modification of magnetic nanoparticles
3 g MNP were dispersed in 40 mL of a 4/1 mixture of
ethanol/water, then 2 mL of ammonium hydroxide 25%
solution was added. Afterward, an excess amount
(10 mmol per 1 g of Fe3O4) of the MPS was added dropwise
over a period of 10 min, and the reaction mixture was
stirred at 50 8C for 48 h. The modified MNPs were washed
three times with methanol and magnetically separated to
remove any excess of reagent and salts.
The 4H-benzo[b]pyran family have attracted consid-
erable attention as an important heterocyclic compounds
because of their wide range of biological properties,
such as anti-cancer, anti-coagulant, anti anaphylactic
activities [17]. The conventional method for synthesis of
4H-benzo[b]pyrans is using organic solvents such as
DMF or acetic acid under reflux temperature, which gives
poor yields [18]. Several catalytic systems have been
used for improvement of the products yields [19–31];
however, most of these methods suffer from some
disadvantages, such as toxic reagents, long reaction
times, no recyclability of catalysts and use of toxic
solvents. Therefore, to overcome these drawbacks a great
deal of efforts is directed to develop an efficient catalyst
for the synthesis of these compounds under green
conditions.
2.4. Synthesis of ionic liquid monomer and cross-linker
5 mmol of 1-vinylimidazole, 5 mmol of ethyl bromide
were added to 30 mL of methanol in a 100 mL round
bottom flask at 0 8C. The mixture was stirred at room
temperature overnight, followed by 60 8C for 15 h. After
cooling down, the reaction mixture was added dropwise
into 250 mL of diethyl ether and a white precipitate was
formed. The precipitate was filtered off and washed with
diethyl ether again and dried at room temperature until
2. Experimental
constant weight. (1H NMR, CDCl3,
d ppm): 1.66 (t, 3H), 4.53
(q, 2H), 5.42 (d, 1H), 6.00 (d, 1H), 7.48 (dd, 1H), 7.70 (s, 1H),
2.1. Reagents and analysis
7.87 (s, 1H), 10.97 (s, 1H).
1,4-butanediyl-3,30-bis-l-vinylimidazolium dibromide
(BVD), as cross-linking agent, was prepared in the same
manner except that the molar ratio of 1-vinylimidazole to
1,4-dibromobutane was 2:1 in the reaction. (1H NMR,
Ferric chloride hexahydrate (FeCl3ꢁ6H2O), ammonia
(30%), ferrous chloride tetrahydrate (FeCl2ꢁ4H2O) and 3-
(trimethoxysilyl)propylmethacrylate (MPS, 98%) were
obtained from Merck. 1-vinylimidazole was obtained from
Aldrich and was distilled before use. Bromoethane and 1,4-
dibromobutane were obtained from Aldrich. 2,20-Azobisi-
sobutyronitrile (AIBN, Kanto, 97%) was recrystallized from
ethanol.
DMSO,
d ppm): 1.87 (s, 2H), 4.29 (s, 2H), 5.43 (d, 1H), 6.00
(d, 1H), 7.33 (dd, 1H), 7.99 (s, 1H), 8.24 (s, 1H), 9.69 (s, 1H).
2.5. Synthesis of poly(ionic liquid) coated magnetic
nanoparticles
Thin layer chromatography (TLC) was performed with
silica gel 60 F254 plates and UV light was used for
visualization. FT-IR spectra of samples were taken using an
ABB Bomem MB-100 FT-IR spectrophotometer. The
samples were powdered and mixed with KBr to make
pellets. Proton nuclear magnetic resonance spectra (1H
NMR) were recorded on a Brucker NMR 500 MHz instru-
ment. Thermogravimetric analysis (TGA) was acquired
under a nitrogen atmosphere with a TGA Q 50 thermo-
gravimetric analyzer. Transmission electron microscopy
(TEM) images were taken with a TOPCON-002B electron
microscope. The magnetic property of catalyst was
measured by a vibrating sample magnetometer (VSM)
(Model 7400).
The MNP@P[imEt][OH] was prepared by distillation-
precipitation-polymerization of IL monomer and [BVD] as
the cross-linker and AIBN as the initiator, in methanol.
Typically, 200 mg of MNP@MPS nanoparticles were
dispersed by ultrasonic in 100 mL methanol in a 250 mL
single-necked flask for 10 min. Then, a mixture of 2.5 mmol
of 1-ethyl-3-vinylimidazolum bromide, 0.5 mmol of [BVD],
and 2.5 mg of AIBN were added to the flask to initiate the
polymerization. The mixture was completely deoxygenat-
ed by bubbling purified argon for 30 min. The flask
submerged in a heating oil bath was attached with a
fractionating column, Liebig condenser, and a receiver. The
reaction mixture was heated from ambient temperature to
the boiling state within 1 h and the reaction was ended
after about 60 mL of methanol was distilled from the
reaction mixture within 5 h. The obtained MNP@
P[imEt][Br] were collected by magnetic separation and
washed two times with water and three times with
methanol to eliminate excess reactants and few generated
polymer microspheres. After separation of MNP@P[i-
mEt][Br], the catalyst was stirred in excess KOH (1 M)
2.2. Synthesis of magnetic nanoparticles
The magnetic Fe3O4 nanoparticles were prepared by
chemical co-precipitation method. An iron salt solution
was obtained by mixing 13.6 g FeCl3ꢁ6H2O and 5 g
FeCl2ꢁ4H2O in 500 mL deionized water under nitrogen at
room temperature. By adding dropwise a NH3 solution, the
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3
solution for 2 days at room temperature. Then the catalyst
was magnetically separated and washed three times with
water and two times with methanol and dried under
vacuum at 50 8C for 2 h. To ensure that all bromide ions are
exchanged with hydroxide ions in catalyst, the AgNO3 test
was used and it was found that only 7% of bromide ion was
not exchanged. The loading of the hydroxide anion in the
catalyst was calculated by titration with HCl (0.01 M) and
it was found to be 2.1 mmol/g.
2.6. General procedure for synthesis of 4H-benzo[b]pyrans
In a 10 mL round bottom flask, aldehyde (1 mmol),
malononitrile (1.2 mmol), water (5 mL), MNP@P[imEt]
[OH] (5 mol%) was added and the mixture was stirred for
15 min at room temperature. Then, dimedone (1 mmol)
was added to the flask and the mixture was refluxed for an
appropriate time. After completion of the reaction,
monitored by TLC, the reaction mixture was cooled down
to room temperature and diluted with hot ethylacetate,
then the catalyst was magnetically separated and washed
two times with methanol and dried under vacuum. The
organic layer was concentrated under vacuum and the
resulting solid products were recrystallized in ethanol.
Scheme 1. Synthetic route for ionic liquid monomer and cross-linker.
some advantages over conventional grafting of material
onto the surface of MNPs. In the normal grafting, only one
layer of surface could be grafted by the homogenous
catalyst. However, by multilayer polymer coating, the
available-surface limitation for grafting would be re-
moved. This approach causes loading of grafted homoge-
nous material to increase and the catalyst will be used in a
low weight percent compared to the substrates. This
would be useful in large-scale applications. On the other
hand, polymer chains will prevent aggregation of nano-
catalysts in high temperature applications. MNP@P[imE-
t][OH] were synthesized by the distillation–precipitation–
polymerization method in the presence of MNPs as
heterogeneous bed. The polymerization was initiated by
AIBN in the large amount of methanol at 70 8C. As
copolymers are insoluble in the methanol (because of
cross-linking), the generated copolymers will continuously
precipitate from the solution and attach to the surface of
the magnetic nanoparticles to form a multilayered shell.
For exchange of bromide with hydroxide ion, MNP@P[i-
3. Results and discussion
A co-precipitation method was used for the synthesis
of MNPs. Surface modification of MNPs with MPS before
polymer coating causes grafting of polymer chains
onto the surface of MNPs by covalent bonding. On
the other hand, modification of MNPs by methacrylate
groups cause polymerization to become easier on
the surface and all magnetic nanoparticles will be
grafted by a copolymer shell [32]. Fig. 1 shows FT-IR
spectra of MNP before (Fig. 1a) and after (Fig. 1b) surface
modification with MPS. The spectra of MNP@MPS
nanoparticles and pure MPS (Fig. 1c) indicate a signifi-
cant vibrational absorption of the Si–CH2 bond in the
fingerprint region between 1326 and 1299 cmꢂ1. Anoth-
er characteristic signal can be attributed to the
asymmetric Si–O–Si vibration between 1172 and
943 cmꢂ1. In addition, the spectrum of the methacryloxy
group-bearing particles shows the vibrational absorp-
tion of the carbonyl double bond at 1718 cmꢂ1. From
these data, it could result that MNPs were successfully
modified with MPS. From the weight loss in the TGA
curves of MNP and MNP@MPS on Fig. 2a, b respectively,
it is calculated that the loading of MPS onto the MNPs
surface was 0.54 mmol/g.
The procedure for synthesis of ionic liquid monomer 1-
vinyl-3-ethylimidazolium bromide ([VEim][Br]) and cross-
linker (BVD) is shown in Scheme 1. The ionic liquid
monomer was synthesized by the quaternization of 1-
vinylimidazole with ethyl bromide. For preparation of
[BVD], quaternization was performed by 1,4-dibromobu-
tane and two equivalent of 1-vinylimidazole.
The procedure for the synthesis of magnetic nanoparti-
cle coated multilayered poly (1-vinyl-3-ethyl imidazolium
hydroxide), noted as MNP@P[imEt][OH] is depicted in
Scheme 2. Coating of multilayers of PIL onto the MNPs has
Scheme 2. General procedure for synthesis of MNP@P[imEt][OH].
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Fig. 1. FT-IR spectrum of bare MNP(a), MNP@MPS (b), pure MPS (c), Ethylvinylimidazolium bromide (d), MNP@P[imEt][OH] (e).
Fig. 2. TG analysis of bare MNP (a), MNP@MPS (b) and MNP@P[imEt][OH] (c).
mEt][Br] was treated by a KOH solution for 48 h at room
temperature (Fig. 1).
The magnetic properties of the MNP@P[imEt][OH] and
the bare MNP were investigated by VSM at room
temperature (Fig. 5). As seen, the saturation magnetization
of the sample decreases due to the polymer coating.
The catalytic activity of MNP@P[imEt][OH] was inves-
tigated in the three-component reaction of dimedone,
aldehydes, and malononitrile (Scheme 3).
Table 1 lists the performances of various conditions
for optimization of control experiments. As seen in Table 1,
in the absence of catalyst, the yield of benzopyrene
product was only 13% after 3 h at room temperature. For
Fig. 1d shows the FT-IR spectrum of 1-ethyl-3-
vinylimidazolium bromide with a characteristic absorp-
tion peak at 1574 cmꢂ1, which is attributed to a stretching
vibration of the imidazole ring. In the FT-IR spectrum of
MNP@P[imEt][OH] (Fig. 1e), characterization peaks of the
MNP and IL monomer can be seen.
The morphology of the MNP@ P[imEt][OH] was also
investigated by TEM (Fig. 3). TEM analysis displays a dark
MNPs core coated by grey polymer shells. Size distribution
diagrams of catalyst before and after reaction are shown on
Fig. S3. As expected, after recycling of the catalyst, the size
of nanoparticles is increased because of aggregation.
The XRD pattern of the MNP@P[imEt][OH] shows
characteristic peaks and relative intensity, which match
well with the standard Fe3O4 sample (JCPDS file No. 19-
0629, Fig. 4). The broad peak from 208 to 308 is consistent
with an amorphous silica phase in the shell of the
Fe3O4@MPS in catalyst (Fig. 4).
The thermal stability of the MNP@P[imEt][OH] was
determined by the TGA (Fig. 2c). The TG curve indicates an
initial weight loss up to 100 8C, which is due to the
adsorbed water on the support. Complete loss of the
grafted PIL was seen in the temperature range from 200 to
600 8C, and the amount of organic component was about
51% of the total solid catalyst (Fig. 2).
Fig. 3. TEM image of MNP@P[imEt][OH].
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5
Fig. 4. XRD pattern of MNP@P[imEt][OH].
the sake of comparison between homogenous catalyst and
heterogonous ones, the catalytic activity of 1-ethyl-3-
vinylimidazolium hydroxide was tested and 95% product
was obtained in 20 min at room temperature. The catalytic
activity of unexchanged anion of MNP@P[imEt][Br]was also
investigated and only 38% yield was obtained after 2 h. The
optimumamountofcatalyst was foundto be5 mol%atroom
temperature. The catalyst was used under the same
conditions in various solvents and the results are shown
in Table 1. Although using ethanol–water mixture as
reaction solvent gave good yield, in the view of green
chemistry, we chose water.
Subsequently, we investigated the generality of the
MNP@P[imEt][OH] in the synthesis of 4H-benzo[b]pyrans
by the reaction of various aldehydes with malononitrile
and dimedone. As seen in Table 2, various aldehydes
Fig. 5. Vibration sample magnetometer curves of bare magnetic
nanoparticles (a) and MNP@P[imEt][OH] (b).
Scheme 3. Synthesis of 4H-benzo[b]pyrans catalyzed by MNP@P[imEt][OH] as catalyst.
Table 1
Optimization of reaction condition for synthesis of 4H-benzo[b]pyransa
Entry
Catalyst
Amount of catalyst (mol%)
Solvent
Time
Yieldb
(%)
1
2
3
4
5
6
7
8
9
10
None
–
H2O
3 h
13
95
38
95
92
73
96
73
90
87
EVim/OH
5
H2O
20 min
2 h
MNP@P[imEt][Br]
MNP@P[imEt][OH]
MNP@P[imEt][OH]
MNP@P[imEt][OH]
MNP@P[imEt][OH]
MNP@P[imEt][OH]
MNP@P[imEt][OH]
MNP@P[imEt][OH]
10
10
5
H2O
H2O
20 min
20 min
30 min
20 min
20 min
20 min
20 min
H2O
2
H2O
5
H2O–Ethanol
CHCl3
CH3CN
EtOH
5
5
5
EVim/OH: 1-Ethyl-3-vinylimidazolium hydroxide.
a
Reaction condition Benzaldehyde (1 mmol), malononitrile (1.2 mmol) and dimedone (1 mmol), solvent volume (5 mL) at room temperature.
Isolated yield.
b
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Table 2
and reused five times without any significant loss of
catalytic activity.
Synthesis of 4H-benzo[b]pyrans by MNP@P[imEt][OH].
Entry
R–CHO
Time (min)
Yielda (%)
1
Ph
20
20
20
25
30
40
25
40
20
15
25
20
20
35
15
45
25
92
97
90
92
88
84
90
81
97
95
90
93
94
89
92
87
79
2
4-(Cl)Ph
Appendix A. Supplementary data
3
2,4-di(Cl)Ph
4-(Me)Ph
4-(HO)Ph
2-(HO)Ph
4-(MeO)Ph
2-(MeO)Ph
4-(NO2)Ph
3-(NO2)Ph
2-(NO2)Ph
4-(NMe2)Ph
4-(CN)Ph
Vanillin
4
5
Supplementary data (FT-IR, and 1H NMR of products)
associated with this article can be found, in the online
version, at http://dx.doi.org/10.1016/j.crci.2013.01.003.
6
7
8
9
10
11
12
13
14
15
16
17
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catalyst was improved. Moreover, the catalyst could be
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