Facile preparation of ionic liquid functionalized magnetic nano-solid acid catalysts for acetalization reaction

Article abstract of DOI:10.1007/s10562-010-0271-x

The facile two-step preparation procedure of a novel magnetic nano-solid acid catalyst is described, which includes grafting an ionic liquid onto Fe ₃O₄ nanoparticles, followed by the sulfonation of phenyl groups in the ionic liquid. The catalytic performance of this novel material has been systematically studied in the acetal formation of benzaldehyde and ethylene glycol. The experimental results testify this catalyst possesses high catalytic activity with a yield of 97% under mild reaction conditions. Furthermore, the catalyst is readily separated using a permanent magnet and it is reusable without any significant decrease in catalytic activity.

Full text of DOI:10.1007/s10562-010-0271-x

Catal Lett (2010) 135:159–164
DOI 10.1007/s10562-010-0271-x
Facile Preparation of Ionic Liquid Functionalized Magnetic
Nano-Solid Acid Catalysts for Acetalization Reaction
Ping Wang
HaiYan Zhu
•
AiGuo Kong
•
•
WenJuan Wang
•
YongKui Shan
Received: 13 November 2009 / Accepted: 11 January 2010 / Published online: 29 January 2010
Ó Springer Science+Business Media, LLC 2010
Abstract The facile two-step preparation procedure of a
novel magnetic nano-solid acid catalyst is described, which
includes grafting an ionic liquid onto Fe O nanoparticles,
heterogeneous catalyst supports have received increasing
attention because when the diameter of the particles decrea-
ses to the nanometer scale, ample external surface area
emerged, which allows a high dispersion of the active species
and provides a large amount of the active and accessible
centers [9]. Moreover, magnetic nanoparticles (MNPs) offer
an added advantage of the separation and eliminate the fil-
tration step for recycling catalyst after completion of the
reaction [10]. However, nano-scale magnetic solid acid
materials were few reported although MNPs-supported basic
ionic liquids [11], amino group [12], composite hydroxide
materials [13] had been prepared. Recently, several different
sulfonic acids grafted onto silica-coated magnetic nanopar-
ticle supports were reported by Jones and coworkers [14], and
the synthesis of these catalysts was more complex and
expensive. It is still attractive to prepare efficient magnetic
solid acid catalysts using the cheap raw material and simple
method in the synthetic chemistry.
3
4
followed by the sulfonation of phenyl groups in the ionic
liquid. The catalytic performance of this novel material has
been systematically studied in the acetal formation of benz-
aldehydeandethyleneglycol. The experimentalresultstestify
this catalyst possesses high catalytic activity with a yield of
9
7% undermildreactionconditions. Furthermore, thecatalyst
is readily separated using a permanent magnet and it is reus-
able without any significant decrease in catalytic activity.
Keywords Magnetism Á Solid acid catalyst Á
Nanoparticle Á Ionic liquid Á Functionalization
1
Introduction
Homogenous acid catalysts including corrosive protic acids
1, 2], Lewis acids such as AlCl , FeCl [3, 4] are widely
Herein, we have prepared a magnetic nano-solid acid
catalyst via grafting an ionic liquid onto Fe O nanoparti-
[
3
3
3 4
applied in the large-scale synthesis of industrial bulk chem-
icals as well as the production of fine chemicals. However, it
is urgent to replacing highly corrosive, hazardous and pol-
luting acid catalysts with eco-friendly and renewable solid
acid catalysts for meeting green chemistry demands, which
may be a challenge of green chemistry and green engineering
cles followed by sulfonation of phenyl groups in the ionic
liquid. This catalyst exhibited high catalytic activity in the
acetalization reactions, and can be easily separated after
completion of the reaction and reused without any signif-
icant reduction of catalytic activity.
[5, 6]. Recently, the facile recovery and reuse of homoge-
neous catalysts by covalent tethering to heterogeneous sup-
port are extensively investigated, which become an important
field of catalytic research [7, 8]. Nanoparticles as
2 Experimental
2.1 Preparation of Materials
2
.1.1 Grafting an Ionic Liquid onto MNPs
P. Wang Á A. Kong Á W. Wang Á H. Zhu Á Y. Shan (&)
Department of Chemistry, East China Normal University,
200062 Shanghai, People’s Republic of China
e-mail: ykshan@chem.ecnu.edu.cn
Fe O nanoparticles were prepared by the chemical
3 4
co-precipitation method [15]. 0.18 M Fe(NO ) solution of
3
3
123

1
60
P. Wang et al.
4
00 mL and 0.12 M FeSO solution of 400 mL were added
4
plasma-atomic emission spectroscopy (ICP-AES) on a
thermo elemental PERKIN-ELMERPLASMA-2000 spec-
trometer. The amount of acid sites was quantified by acid–
base titration.
in 1,000 mL four-neck rounded bottom flask at room
temperature, and then 25 wt% NH Á H O solution of
3
2
9
6 mL was quickly added to the mixture solution at 313 K
under mechanical stirring vigorously in N2 protecting.
After 15 min, the mixtures were heated to 333 K and held
for 5 min to form stable nanoparticles (Fe O ) at the same
2.3 Catalytic Reactions
3
4
temperature. The resultant black precipitates (Fe O
3
The acetalization reaction of benzaldehyde (PhCHO) with
ethylene glycol (EG) was carried out in a 100 mL three-
necked flask, 0.06 g catalysts were added to a mixture of
30 mmol PhCHO, 90 mmol EG and 185 mmol cyclohex-
ane. The reaction was conducted with vigorous magnetic
stirring under reflux for 2 h. The products were confirmed
by GC–MS. The reaction conversion was monitored by gas
chromatography (GC) analysis in reference to a toluene
internal standard.
4
nanoparticles) were separated by using a permanent mag-
net, washed with deionized water and dried in vacuum at
3
53 K for 8 h.
The grafting an ionic liquid onto MNPs were carried out
by the procedures as follows, a 1,000 mL three-necked
round-bottomed flask equipped with a condenser, a ther-
mocouple, and mechanical stirrer were charged with
freshly prepared Fe O4 nanoparticles of 6.0 g, 3-chlor-
3
opropyltrimethoxysilane (CPTMO) of 29.8 g, 47.2 g tri-
phenylphosphine (PPh ) and 480 mL dried toluene. The
3
mixtures were kept for 48 h under reflux with continuous
stirring at 383 K. The final product was collected by
magnetic decantation, washed with ethanol, and dried in
vacuum at 353 K overnight (denoted as MNPsSi-PPh3).
3 Results and Discussion
3.1 Synthesis of Materials
Scheme 1 shows the schematic representation of the syn-
thetic pathway for MNPsSi-PPh -SO H solid acid catalysts
2
.1.2 Sulfonation of MNPsSi-PPh3
3
3
by a two-step chemical modification. In the synthesis, the
formation of ionic liquids and its grafting on Fe O nano-
The sulfonation of MNPsSi-PPh was completed by the
3
3 4
method of vapour-phase sulfonation as described [16]. The
sulfonation was performed in a Teflon-lined autoclave
particles are achieved by one-pot method using inexpen-
sive and routine chemicals (CPTMO and PPh ). Moreover,
3
where MNPsSi-PPh powders of 3 g were contacted with
3
the sulfonation of MNPsSi-PPh3 can be easily accom-
plished by contacting the samples with the vapor from
fuming sulfuric acid in a closed autoclave. In comparison
with the methods reported by Jones and coworkers, the
whole synthesis procedure of MNPsSi-PPh -SO H is sim-
the vapor from 50 mL 50 wt% SO /H SO at 353 K for
4
3
2
7
2 h. The sulfonated samples were washed with hot
deionized water ([353 K) to remove any physically
adsorbed species until the sulfate ions were not detected in
the upper layer solution. After the separation, the samples
were dried at 373 K overnight in air (denoted as MNPsSi-
PPh -SO H).
3
3
ple and inexpensive, and it is suitable for the large-scale
production of magnetic solid acid catalyst. Besides, the
ionic liquids can be tightly grafted on the surface of Fe O
3 4
3
3
2
.2 Characterization
The X-ray diffraction (XRD) patterns of all samples were
recorded with a Rigaku Corporation D/MAX2200PC dif-
fractometer using Cu-Ka radiation (k = 0.1541 nm), and
the data were collected from 10° to 70° (2h) with a scan
speed of 6°/min. Transmission electron microscopy (TEM)
images were taken on a JEOL-JEM-2010 transmission
electron microscope using an accelerating voltage of
CH3O
Fe 33 O 44
O
O
O
CH O Si
Cl
P
FF ee 33 O 44
P
3
Si
0
Cl-
CH O
110 C,toluene
3
SO3H
O
2
00 kV. Magnetic properties were measured at room tem-
Fe 3O4
O
O
Si
P
Cl-
SO3H
perature using a vibrating sample magnetometer (VSM).
Fourier transform infrared (FT-IR) spectra were recorded
on a Nexus-870 Fourier-transform spectrophotometer
using KBr pellet technique with a measuring range
SO H
3
-
1
4
00–4,000 cm .The amount of Si, P and S in the
Scheme 1 The synthesis process of MNPSi-PPh
catalysts
3
3
-SO H solid acid
synthesized samples was quantified by inductively coupled
1
23

Facile Preparation of Ionic Liquid
161
and the linked PPh provide a chance to obtain more acid
3
sites by the sulfonation. The changeable cation group may
be used for further modification of catalyst.
3
.2 X-Ray Diffraction
Figure 1 shows the large-angle XRD patterns of these
samples. The XRD pattern of Fe O clearly showed six
3
4
reflection peaks and confirmed the formation of a cubic
spinel ferrite structure, and the crystallite size was calcu-
lated from the Scherrer formula and found to be an average
diameter of about 11.3 nm (Fig. 1a). The same six reflec-
tion peaks appeared in the XRD patterns of the synthesized
samples of MNPsSi-PPh -SO H (Fig. 1b). No obvious
3
3
change of the average particle size was observed, which
suggested that the Fe O crystallite phase was still main-
3
4
tained in the MNPsSi-PPh -SO H nanoparticles.
3
3
3
.3 Transmission Electron Microscopy
The nanoparticle size and morphology of Fe O nanopar-
3
4
ticles, MNPsSi-PPh -SO H were investigated by TEM. As
3
3
shown in Fig. 2a and b, Fe O nanoparticles (Fig. 2a)
3
4
displayed an approximate spherical shape with the particle
size range from 8 to 13 nm and slight agglomeration, in
agreement with the results of XRD. Figure 2b shows the
TEM image of the synthesized samples MNPsSi-PPh3-
SO H, although aggregated phenomenon of some nano-
3
particles was also observed, the particle diameters of the
nanoparticles still remained in the range of 8–13 nm. It
indicated that the two-step chemical modification of the
Fe O was not caused the significant increase of the par-
3
4
ticle size, and the advantages of magnetic nanoparticles can
be maintained.
Fig. 2 TEM images of a Fe
samples of MNPsSi-PPh
cycle reusage
3
O
4
nanoparticles, b the synthesized
3 3
-SO H, c the synthesized samples after five
311
1
1
1
400
200
000
220
440
3.4 Magnetic Properties
5
11
4
00
8
6
4
2
00
00
00
00
0
422
(
c)
b)
The magnetic properties of these samples were character-
ized by vibrating sample magnetometer (Fig. 3). All of
the samples were suitable for supporting materials in
separation owing to their superparamagnetic properties.
(
(
a)
The saturation magnetization of Fe O nanoparticles
3
4
1
0
20
30
40
50
60
70
(Fig. 3a) and MNPsSi-PPh -SO H (Fig. 3b) was 47.0 and
3 3
Two theta(degrees)
45.1 emu/g, respectively. A slight decrease of the satura-
tion magnetization of the MNPsSi-PPh -SO H was due to
3
3
Fig. 1 Large-angle XRD patterns of (a) Fe
synthesized samples of MNPsSi-PPh
samples after five cycle reusage
3
O
4
nanoparticles, (b) the
-SO H, (c) the synthesized
the successful grafting of ionic liquids on the surface of
Fe O nanoparticles.
3 3
3
4
123

1
62
P. Wang et al.
liquid. The result implied that CPTMO without forming the
ionic liquid tag may be also supported on the surface of
Fe O nanoparticles. Although CPTMO can not be further
(
(
a)
b)
3
4
sulfonated, it resulted in the formation of an inert barrier on
the surface of Fe O nanoparticles and may inhibit the loss
3
4
of the ferrite. The molar ratio of S, P was 1.14: 1 in the
MNPsSi-PPh -SO H samples, suggesting that three phenyl
3
3
groups of PP were partly functionalized by –SO H group.
3
3
Acid–base titrations were used to measure acid capacity
that is the quantity of catalytically active ingredients of
samples as described in the literature [18]. In a typical
measurement, the samples of 0.5 g were suspended in
Fig. 3 Magnetization curves at room temperature of (a) Fe
nanoparticles and (b) MNPsSi-PPh -SO
3 4
O
3
3
H
0.02 M KCl solution of 50 mL and then was stirred for
3
.5 Fourier Transform Infrared Spectroscopy
20 min and titrated with 0.01 M KOH in the presence of
phenolphthalein. The results revealed that the samples of
MNPsSi-PPh -SO H possessed 0.51 mmol/g acid amount.
Figure 4 shows the FT-IR spectra of the Fe O nanoparti-
3
4
3
3
cles, MNPsSi-PPh , and MNPsSi-PPh -SO H. FT-IR
3
Under the same sulfonation conditions, the acidity from the
direct sulphonation of Fe O nanoparticles was not
3
3
spectrum of the MNPs showed similar spectrum to Fe O
3
4
3 4
materials as described in literature [17]. In comparison to
-
observed. However, for the sulfonation of CPTMO modi-
fied Fe O nanoparticles, the acid–base titration results
1
Fig. 4a, a strong peak near 1,100 cm (corresponding to
the Si–O stretch) and the relatively weak peaks at 2,800–
3
4
revealed 0.02 mmol/g acid amount. Generally, the sulfo-
nation of CPTMO modified Fe O nanoparticles was dif-
-
1
3
,000 cm (ascribed to the stretching vibrations of C–H)
were observed in the FT-IR spectra of MNPsSi-PPh3
Fig. 4b), which implied that CPTMO was bonded to
Fe O nanoparticles. The distinguished feature in the
3
4
ficult in this synthesis condition, which had a negligible
contribution to the total acid amount of MNPsSi-PPh3-
(
3
4
SO H materials.
3
FT-IR spectra of MNPsSi-PPh -SO H was the appearance
3
3
-
1
of a new peak at 1,032 cm , which was assigned to the
symmetric stretching vibration of S=O group (Fig. 4c).
These results were in agreement with the chemical com-
ponent of the materials prepared.
3.7 Catalytic Studies
Acetals are widely used in carbohydrate synthesis and
organic synthesis reactions of carbonyl protection in the
synthesis of the fine chemicals and pharmaceutical indus-
tries [19–21]. The acetalization reaction (Scheme 2) of
benzaldehyde with ethylene glycol as a test reaction was
carried out to investigate the catalytic properties of the
prepared magnetic solid acid materials.
3
.6 ICP-AES Analyses and Acid–Base Titrations
ICP-AES analyses testified that Si, S, P had been supported
on Fe O nanoparticles. The molar ratio of Si, P deter-
3
4
mined was 6.29: 1, higher than theoretic value in an ionic
3.7.1 Effect of the Reaction Conditions
The different reaction conditions had the remarkable effect
on the yield of acetal in the acetalization reaction (Fig. 5).
As shown in Fig. 5a, the yield of acetal increased with the
increment of the catalyst mass, and the highest yield (97%)
of acetal was obtained when the amount of catalyst used
was 0.06 g. With further increasing of the catalyst mass,
the yield of acetal decreased. The results indicated that the
suitable amount of catalyst used was important in this
-1
1
032cm
(c)
(b)
(a)
H
OH
Catal.
O
O
3000
2500
2000
1500
1000
C
O
C
H2O
cyclohexane
-1
OH
Wavenumbers (cm )
Fig. 4 FT-IR spectra of (a) Fe
c) MNPsSi-PPh -SO
3
O
4
nanoparticles, (b) MNPsSi-PPh
3
,
Scheme 2 Schematic illustration for the acetalization reaction of
benzaldehyde with ethylene glycol
(
3
3
H
1
23

Facile Preparation of Ionic Liquid
163
a)100
The optimal reaction time to get the highest yield was
h under the optimum conditions (including the appro-
(
2
8
6
4
0
priate amounts of EG, PhCHO, and catalyst). The decrease
in the yield of acetal observed in prolonged reaction time
was due to the hydrolysis of the product acetal (Fig. 5c).
Because a great quantity of water was produced in the
acetalization reactions of PhCHO with EG, the cyclohex-
ane was used as an agent entraining water in order to rise
the yield of acetal. The effect of the amount of the cyclo-
hexane used on the yield was studied. The experimental
results in Fig. 5d illustrated that when cyclohexane of
0
0
0
.00
0.02
0.04
0.06
0.08
Catal.(g)
b)100
(
8
6
4
2
0
0
0
0
0
1
85 mmol was added in the reactive system, the most of
water produced in the reaction was entrained and the
highest yield was obtained.
0
1
2
3
4
3.7.2 The Acetalization or Ketalization of Three Different
Aldehyde or Ketone with EG
Molar ratio(nEG/nPhCHO
)
(
(
c)1
00
8
6
4
2
0
0
0
0
0
Under the optimal reaction conditions, the acetalization or
ketalization of the EG with different aldehyde (or ketone)
were performed. The results were showed in Table 1. The
conversion of propionaldehyde, butanone and cyclohexa-
none was 96, 95 and 94%, respectively. It indicated that the
magnetic solid acid catalysts prepared possessed an
excellent performance in the acetalization or ketalization of
the carbonyl.
0
1
2
3
4
Reaction time(h)
d)100
9
9
9
9
9
8
6
4
2
0
3
.7.3 Recycling of the Catalyst
Reusability of the catalyst was an important character and a
performance criterion of any industrial process. In this
method reported herein, the catalyst can be separated by
simple magnetic decantation by using a permanent magnet
at the end of reaction and reused after washing with ethanol
and then drying in vacuum at 323 K for 8 h. After the fifth
5
0
100 150 200 250 300 350 400
Cyclohexane(mmol)
Fig. 5 The effect of the reaction conditions on the yield of acetal:
the amount of MNPsSi-PPh -SO H (a), molar ratio of EG/PhCHO
b), reaction time (c) and cyclohexane (d). a Reaction conditions:
3
3
(
n
reaction time: 2 h. b Reaction conditions: the catalyst: 0.06 g;
EG:nPhCHO = 3:1; cyclohexane: 185 mol; reflux temperature: 354 K;
Table 1 The acetalization or ketalization of different aldehyde or
ketone with EG
cyclohexane: 185 mmol; reflux temperature: 354 K; reaction time:
2
a
h. c Reaction conditions: the catalyst: 0.06 g; nEG:nPhCHO = 3:1;
Entry Substrate
Yield (%)
96
Conversion (%) Selectivity (%)
cyclohexane: 185 mmol; reflux temperature: 354 K. d Reaction
conditions: the catalyst: 0.06 g; nEG:nPhCHO = 3:1; reflux tempera-
ture: 354 K; reaction time: 2 h
1
2
O
96
95
94
100
100
100
acetalization reaction, and the excess acid amount may
promote the occurrence of the reverse reaction.
O
95
94
The molar ratio of EG/PhCHO played an important role
for the yield of acetal in the acetalization reaction (Fig. 5b).
The results showed that the yield of acetal was directly
proportional to the molar ratio of EG to PhCHO and the
conversion of PhCHO can be improved by increasing the
amount of EG. When the molar ratio of EG/PhCHO was
3
O
a
Yield = Conversion 9 Selectivity
Reaction conditions: the catalyst: 0.06 g; nEG:naldehyde or ketone = 3:1;
cyclohexane: 185 mmol; reflux temperature: 354 K; reaction time:
3
:1, the best yield obtained was 97%. Further raising the
2
h
amount of EG, the increase in yield was negligible.
123

1
64
P. Wang et al.
1
00
kind of novel magnetic nano-solid acid catalyst may be
wide applied in acid-catalyzed reactions.
8
6
4
2
0
0
0
0
0
Acknowledgments We thank key project of Shanghai Science and
Technology Committee (no. 05JC14070, 06DZ05025, 08JC1408600)
for financial support.
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Fig. 6 The reusability of MNPsSi-PPh
3
-SO
3
H in the acetalization
reactions of PhCHO with EG. Reaction conditions: the catalyst:
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0
5
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3
3
X-ray diffraction patterns, the appearance and the average
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1
1
1
1
1
1
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4
Conclusions
In conclusion, a magnetic nano-solid acid catalyst,
MNPsSi-PPh -SO H, is prepared by a simple method, and
1
2
2
3
3
possesses an excellent performance in the acetalization of
the carbonyl under mild reaction conditions, and it can be
reusable without significant loss in catalytic activity. This
1
23