Acetalization of carbonyl compounds catalyzed by acidic ionic liquid immobilized on silica gel

Article abstract of DOI:10.1016/j.molcata.2011.08.005

Imidazolium-silica heterogeneous catalyst (SG-[(CH₂) ₃SO₃H-HIM]HSO₄) was prepared by immobilization of acidic ionic liquid 1-(propyl-3-sulfonate) imidazolium hydrosulfate ([(CH₂)₃SO₃H-HIM]HSO₄) on silica-gel using tetraethoxysilane (TEOS) as silica source in this study. The properties of the samples were characterized by FT-IR, SEM and TG/DSC. The results suggested that [(CH₂)₃SO₃H-HIM]HSO₄ had been successfully immobilized on the surface of silica-gel and the immobilized ionic liquid catalyst SG-[(CH₂)₃SO ₃H-HIM]HSO₄ had good thermal stability. The original smooth surface of silica-gel was covered with [(CH₂) ₃SO₃H-HIM]HSO₄ and a rough surface of SG-[(CH₂)₃SO₃H-HIM]HSO₄ was formed, but the size of particles had no obvious change. Moreover, SG-[(CH ₂)₃SO₃H-HIM]HSO₄ exhibited high catalytic activity for a series of acetalization and could be recovered easily. After reused for 10 times in the synthesis of benzaldehyde ethanediol acetal, the catalyst could still give satisfactory catalytic activity.

Full text of DOI:10.1016/j.molcata.2011.08.005

Journal of Molecular Catalysis A: Chemical 348 (2011) 77–82
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Acetalization of carbonyl compounds catalyzed by acidic ionic liquid
immobilized on silica gel
Jinmei Miao, Hui Wan, Yanbing Shao, Guofeng Guan∗, Bin Xu
College of Chemistry and Chemical Engineering, Nanjing University of Technology, Nanjing 210009, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Imidazolium-silica heterogeneous catalyst (SG-[(CH2)3SO3H-HIM]HSO4) was prepared by immobiliza-
tion of acidic ionic liquid 1-(propyl-3-sulfonate) imidazolium hydrosulfate ([(CH2)3SO3H-HIM]HSO4) on
silica–gel using tetraethoxysilane (TEOS) as silica source in this study. The properties of the samples
were characterized by FT-IR, SEM and TG/DSC. The results suggested that [(CH2)3SO3H-HIM]HSO4 had
been successfully immobilized on the surface of silica–gel and the immobilized ionic liquid catalyst SG-
[(CH2)3SO3H-HIM]HSO4 had good thermal stability. The original smooth surface of silica–gel was covered
with [(CH2)3SO3H-HIM]HSO4 and a rough surface of SG-[(CH2)3SO3H-HIM]HSO4 was formed, but the size
of particles had no obvious change. Moreover, SG-[(CH2)3SO3H-HIM]HSO4 exhibited high catalytic activ-
ity for a series of acetalization and could be recovered easily. After reused for 10 times in the synthesis
of benzaldehyde ethanediol acetal, the catalyst could still give satisfactory catalytic activity.
© 2011 Elsevier B.V. All rights reserved.
Received 3 December 2010
Received in revised form 20 June 2011
Accepted 15 August 2011
Available online 22 August 2011
Keywords:
Acetalization
Heterogeneous catalysis
Silica–gel
Ionic liquids
Imidazole
1. Introduction
In order to solve these problems mentioned above, immobilized
ionic liquid catalysts combining the advantageous characteris-
Acetalization of carbonyl compounds with alcohols to form
acetals was one of the most useful methods for protecting carbonyl
compounds in organic synthesis. Conventionally, this reaction was
carried out in the presence of a protonic acid catalyst such as
sulfuric acid or hydrochloric acid. Although these homogeneous
acids showed satisfactory catalytic performance for this reaction,
they caused the problems of tedious purification process of the
product, large amount of acidic wastes, corrosion of equipments,
and severe environmental pollution [1–4]. All these disadvantages
made acetalization inefficient and uneconomical. Therefore, there
was an urgent need to develop environmentally friendly and high
efficient catalysts for acetalization.
In recent years, ionic liquids as being environmental-friendly
reaction media had attracted significant attention for their unique
properties, such as high thermal stability, negligible vapor pres-
sure, tunable acidity and selective dissolvability [5–10]. Acidic ionic
liquids including the Lewis and the Brønsted ones had proved to
be efficient catalysts for acetalization [1,11–14]. However, several
drawbacks of ionic liquids, such as unendurable viscosity, high cost,
tedious purification procedure of the product and requirements
of long reaction time for acetalization, restricted their widespread
practical applications [15,16].
tics of ionic liquids, inorganic acids and solid acids had been
proposed [17]. In 2007, Sugimura et al. [18] investigated the copoly-
merization of 1-vinylimidazolium based acidic ionic liquid with
styrene and its use as effective and reusable catalyst for acetal
formation under mild reaction condition. In 2009, this research
group further investigated the immobilization of metal chlorides
on imidazolium-styrene copolymer [19]. The catalyst (GaCl3) was
demonstrated to be an efficient heterogeneous catalyst for acetal-
ization. Recently, a novel nano-solid acid catalyst [20] was prepared
by a facile two-step preparation procedure. It possessed an excel-
lent performance for acetalization of the carbonyl under mild
reaction conditions, and could be reusable without significant loss
in catalytic activity. In all the literatures reported above, styrene
or Fe3O4 were used as solid supports. Silica–gel was one of the
most commonly used inorganic supports. It could be synthesized
easily in a controllable way by hydrolysis of TEOS [21]. And the
produced mesoporous silica–gel possessed the excellent character-
istics of stable mesoporous structure, high surface area, controllable
pore size, and good mechanical strength and thermal stability.
These excellent characteristics made the produced mesoporous
silica–gel an attractive solid support for the preparation of immobi-
lized ionic liquid catalyst [22–24]. However, there were few studies
about the preparation of silica-supported acidic ionic liquid cata-
lyst using TEOS as silica source and its application for acetalization.
And the preparation of immobilized Brønsted acidic ionic liquid
[(CH2)3SO3H-HIM]HSO4 by chemical covalent bond, to our knowl-
edge, was still absent from the literature.
∗
Corresponding author. Tel.: +86 25 8358 7198.
E-mail addresses: guangf@njut.edu.cn, njutggf@163.com (G. Guan).
1381-1169/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2011.08.005

78
J. Miao et al. / Journal of Molecular Catalysis A: Chemical 348 (2011) 77–82
O
O
S
-
SO3
N
N
HN
HN
O
N
N
HN
HN
+
+
+
H2SO4
-
SO3H
SO3
+
-
HSO4
Scheme 1. Preparation of [(CH2)3SO3H-HIM]HSO4.
NaOC2H5
N
N
Cl
N
O
HN
+
(EtO)3Si
(EtO) 3Si
O
S
O
-
SO3
N
N
N
N
+
(EtO) 3Si
(EtO)3Si
SO3H
HSO4
H2SO4
N
N
+
(EtO) 3Si
-
Scheme 2. Preparation of ClPTES-[(CH2)3SO3H-HIM]HSO4.
TEOS
SO3H
N
N
N
N
SO3H
HSO4
Si
O2
+
+
(EtO) 3Si
-
-
HSO4
Scheme 3. Preparation of SG-[(CH2)3SO3H-HIM]HSO4.
In this study, we reported our efforts at exploring silica-
2.3. Preparation of ClPTES-[(CH2)3SO3H-HIM]HSO4
supported Brønsted acidic ionic liquid catalyst SG-[(CH2)3SO3H-
HIM]HSO4 using TEOS as silica source. FT-IR, SEM and TG/DSC
were employed to characterize the properties of samples. More-
over, the catalytic activity and reusability performance of the
immobilized ionic liquid catalyst used for acetalization were also
examined.
Imidazole (6.8 g) and sodium ethoxide (6.8 g) were dissolved
in ethanol (100 mL), and the mixture was stirred at 70 ◦C for 8 h
to give sodium imidazole [26]. Subsequently, ClPTES (24.05 g) was
added drop-wise and the mixture was refluxed for 12 h under N2
atmosphere. The formed orange suspension was filtered to remove
NaCl precipitate from the solution [27]. The solvent was removed by
rotatory evaporation under reduced pressure and an yellowish oil
containing N-(3-propyltrimethoxysilane)imidazole was obtained.
1,3-Propane-sultone (12.2 g) was slowly added into the solu-
tion of equal-molar N-(3-propyltrimethoxysilane)imidazole and
ethanol (100 mL). The mixture was stirred at 50 ◦C for 8 h and
ethanol was removed by rotatory evaporation under reduced pres-
sure. Then H2SO4 was added drop-wise. The solution was stirred
at 60 ◦C for another 12 h. Finally, the formed ClPTES-[(CH2)3SO3H-
HIM]HSO4 was washed with ether for 3 times and dried in vacuum
at 50 ◦C for 5 h (Scheme 2).
2. Experimental
2.1. Chemical reagents
Imidazole
(99%),
1,3-propane-sultone
(99%),
3-
chloropropyltriethoxysilane (ClPTES, 98.8%), EO20PO70EO20
(P123, Aldrich), TEOS, ether and other chemicals (AR grade) were
commercially available and used without further purification.
2.2. Preparation of ionic liquid [(CH2)3SO3H-HIM]HSO4
2.4. Preparation of immobilized ionic liquid catalyst
SG-[(CH2)3SO3H-HIM]HSO4
First, imidazole (6.8 g) was dissolved in ethanol (100 mL), and
equal-molar 1,3-propane-sultone was slowly added. The mixture
was stirred at room temperature for 24 h and filtered to get the
white precipitate. Subsequently, the product was washed with
ethanol for several times and dried in vacuum at 60 ◦C for 6 h.
Second, the formed solid was dissolved in water, and equal-molar
sulfuric acid was slowly dropped. After the dropping was finished,
the mixture was heated up to 60 ◦C gradually and then stirred for
12 h [25]. Finally, the formed colorless liquid was washed with ether
for 3 times and dried in vacuum at 50 ◦C for 12 h (Scheme 1).
[(CH2)3SO3H-HIM]HSO4-functionalized silica–gel was synthe-
sized with the molar ratio of the mixture as follows:
TEOS:xClPTES-[(CH2)3SO3H-HIM]HSO4:0.027 P123:16.2 HCl:100
H2O, with x = 0.125, 0.25, 0.375, 0.5. In typical synthesis,
4
a
P123 (1.58 g) was dissolved in H2O (18.0 g) and HCl (5.89 g).
After TEOS (8.32 g) was added and pre-hydrolyzed for 1.5 h at
45 ◦C, ClPTES modified Brønsted acidic ionic liquid (2.46 g, x = 0.5)
was added successively. Then the mixture was carried out at

J. Miao et al. / Journal of Molecular Catalysis A: Chemical 348 (2011) 77–82
79
Table 1
Catalytic performance of various catalysts for acetalization of benzaldehyde and
ethanediol.a
(a)
(b)
Entry
x
Reaction
Yield (%)b
temperature (◦C)
1566
1566
1642
1
2
3
4
5
6
0.125
0.25
0.375
0.5
0.5
0.5
60
60
60
60
50
70
85.3
89.4
90.2
96.8
93.0
93.5
586
1058
3150
3435
586
796
1642
a
Benzaldehyde (70 mmol), ethanediol (126 mmol), catalyst (0.61 g, 4% of the total
mass of benzaldehyde and ethanediol), cyclohexane (8 mL), 110 ◦C, 1.5 h.
954
b
Yield was achieved by GC analysis.
460
500
different temperatures for a certain time to form the final prod-
uct. The whole process was operated under N2 atmosphere. The
precipitate was filtered, subsequently washed with acetone, ether
and ethanol, and then refluxed in ethanol for 24 h to remove the
template P123. Finally, the formed immobilized acidic ionic liquid
was dried in vacuum at 60 ◦C for 6 h (Scheme 3).
1058
1000
4000
3500
3000
2500
2000
1500
Wavenumber (cm-1)
Fig. 1. FT-IR spectra of [(CH2)3SO3H-HIM]HSO4 (a), and SG-[(CH2)3SO3H-HIM]HSO4
(b).
2.5. Characterization
2.6. Acetalization and analysis
Fourier transform infrared (FT-IR) spectra of the samples in the
range of 400–4000 cm−1 were carried out on a Thermo Nicolet 870
spectrophotometer with anhydrous KBr as standard. Scanning elec-
tron microscope (SEM) QUANTA200 (FEI, Holland) was employed
to characterize the average diameters and surface morphologies of
silica–gel and SG-[(CH2)3SO3H-HIM]HSO4. The thermal properties
were examined by a NETZSCH STA 409PC thermo gravimetric and
differential scanning calorimetry (TG/DSC) analyzer with a heating
rate of 10 ◦C per minute under a dynamic N2 atmosphere and the
temperature was ranging from 35 ◦C to 800 ◦C.
Taking the acetalization of benzaldehyde with ethanediol as
an example, the typical procedure was performed as follows:
benzaldehyde (7.42 g, 70 mmol), ethanediol (7.81 g, 126 mmol),
catalyst (0.61 g, 4% of the total mass of benzaldehyde and ethane-
diol), and cyclohexane (8 mL, as a water-carrying agent) were
charged successively into a 250 mL round bottom flask with a
reflux condenser, a water segregator and a magnetic stirrer. Then
the reaction was allowed to proceed at desired temperature for
1.5 h. All the reactions were analyzed using a gas chromatograph
Fig. 2. The whole and magnified SEM images of silica–gel (A and B), and SG-[(CH2)3SO3H-HIM]HSO4 (C and D).

80
J. Miao et al. / Journal of Molecular Catalysis A: Chemical 348 (2011) 77–82
HO
OH
SiO2
O
H
H
R
HO+
H
R
O+H
OH
HO
H
2
3
1
R
SiO2
H
H
H
R
H2O+
OH
HO
OH
O
O
H2O
R
5
4
SiO 2
H
H
R
6
OH
O
O
O
R
O
R
O+H
H
H
8
7
SiO2
H
Scheme 4. Mechanism of acetalization over SG-[(CH2)3SO3H-HIM]HSO4.
equipped with an FID detector (SP6800A, SE-54 capillary column
30 m × 0.25 mm × 0.3 ␮m). The same procedures were carried out
for the other acetalization of aldehydes by alcohols.
In a recycling experiment, SG-[(CH2)3SO3H-HIM]HSO4 was
recovered by decantation, and collected for catalytic reusability
test after dried under 100 ◦C for 2 h. The procedures for recycling
experiment were the same as described above.
spectroscopic studies were carried out. First, the characteris-
tic peaks of [(CH2)3SO3H-HIM]HSO4 around 586 cm−1, 1642 cm−1
,
1566 cm−1 and 1058 cm−1 could be clearly observed in Fig. 1a and
b. They were ascribed to C–H, C C, C N stretching vibrations of the
imidazole ring [29], and S O symmetric stretching vibrations of
the –SO3H group, respectively [30,31]. The band at 3150 cm−1 was
assigned to N–H stretching vibration of [(CH2)3SO3H-HIM]HSO4
[32]. Meanwhile, typical peaks of Si–O–Si of SG-[(CH2)3SO3H-
HIM]HSO4 could be observed around 460 cm−1, 769 cm−1 and
954 cm−1 in Fig. 1b. And the band at 3435 cm−1 was assigned to
the O–H of physical adsorbed water. However, the peak of N–H
at 3150 cm−1 of [(CH2)3SO3H-HIM]HSO4 which existed in Fig. 1a
totally disappeared in the spectrum of SG-[(CH2)3SO3H-HIM]HSO4
in Fig. 1b. This observation suggested that [(CH2)3SO3H-HIM]HSO4
had been immobilized on silica–gel by chemical covalent bond via
the route shown above.
3. Results and discussion
3.1. Catalytic performance of catalysts prepared under different
conditions
In order to obtain the appropriate preparation conditions for
SG-[(CH2)3SO3H-HIM]HSO4, effects of molar ratio of IL to TEOS and
reaction temperature on catalytic performance of catalysts were
investigated in the acetalization of benzaldehyde and ethanediol.
The results were shown in Table 1. The more ClPTES-[(CH2)3SO3H-
HIM]HSO4 was added, the higher yield of benzaldehyde ethanediol
acetal was obtained (Table 1, entries 1–4). When x increased up
to 0.5, the high yield of 96.8% was achieved. It was unnecessary
to further increase the amount of ionic liquid. With a rise in tem-
perature (Table 1, entries 4 and 5), the molecular movement was
intensified and the collision probability between silica–gel and
ClPTES-[(CH2)3SO3H-HIM]HSO4 increased correspondingly. How-
ever, a further increase of temperature (Table 1, entries 4 and 6) also
enhanced the collision of ClPTES-[(CH2)3SO3H-HIM]HSO4 itself
which led to a decrease in the yield of benzaldehyde ethanediol
acetal [28]. Next, the catalyst (Table 1, entry 4) was characterized
as follows.
3.2.2. SEM
Fig. 2 shows the average diameters and surface morphologies
of silica–gel and SG-[(CH2)3SO3H-HIM]HSO4. It was seen that the
spherical silica–gel particles were agglomerated together and in
the size of 2–5 ␮m (Fig. 2A). After the immobilization of ionic liquid,
the size of the obtained SG-[(CH2)3SO3H-HIM]HSO4 had no obvious
change (Fig. 2C). At a high magnification, the surface of silica–gel
was rather smooth (Fig. 2B). However, it was found that the sur-
face of SG-[(CH2)3SO3H-HIM]HSO4 was much rough (Fig. 2D). This
finding demonstrated that ionic liquid [(CH2)3SO3H-HIM]HSO4 had
been immobilized on the surface of silica–gel.
3.2.3. TG/DSC
As shown in Fig. 3, TG/DSC analysis was employed to investigate
the thermal stability of the chemically modified silica–gel since the
observed weight loss was associated with the loss of the organic
components attached to the surface.
3.2. Catalyst characterization
3.2.1. FT-IR
To confirm the immobilization of active component
[(CH2)3SO3H-HIM]HSO4 on the support silica–gel, FT-IR
The small amount of weight loss within 200 ◦C in Fig. 3a
was attributed to the desorption of physical adsorbed water and

J. Miao et al. / Journal of Molecular Catalysis A: Chemical 348 (2011) 77–82
81
Table 2
Acetalization of different aldehydes and alcohols in SG-[(CH2)3SO3H-HIM]HSO4.a
Entry
Aldehydes
Alcohols
Time (h)
Yield (%)b
Selectivity (%)c
1
2
3
4
5
6
7
8
9
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Furfural
Furfural
Furfural
Phenylacetaldehyde
Phenylacetaldehyde
Ethanediol
1.5
2
3
1
3
1.5
0.5
2.5
2.5
95.2
93.0
87.1
98.5
85.0
95.9
97.4
98.4
94.1
100
100
100
100
100
100
100
100
100
1,2-Propanediol
1,4-Butanediol
Neopentyl glycol
Ethanediol
1,2-Propanediol
Neopentyl glycol
Ethanediol
1,4-Butanediol
a
Aldehyde (70 mmol), alcohol (126 mmol), catalyst (4% of the total mass of aldehyde and alcohol), cyclohexane (8 mL), 110 ◦C.
Yield was achieved by GC analysis.
Selectivity for acetal.
b
c
100
100
(b)
(a)
0
80
60
40
20
0
95
90
85
80
-2
-4
-6
-8
0
100
200
300
400
500
600
700
800
°
Temperature ( C)
1
2
3
4
5
6
7
8
9
10
Fig. 3. TG (a)/DSC (b) patterns of silica–gel SG-[(CH2)3SO3H-HIM]HSO4.
Recycling time
Fig. 4. Recycling of SG-[(CH2)3SO3H-HIM]HSO4 for the synthesis of benzaldehyde
ethanediol acetal.
residual solvent. Correspondingly, an obvious endothermic peak
appeared around 70 ◦C in Fig. 3b. When the temperature fur-
ther increased to 350 ◦C, the weight of SG-[(CH2)3SO3H-HIM]HSO4
decreased rapidly. The reason might be that the organic compo-
nents of ionic liquid and the propyl of ClPTES separated from
the surface of silica–gel. Correspondingly, endothermic peaks at
410 ◦C appeared. Finally, it was observed that the chemically mod-
ified silica–gel exhibited good thermal stability under 350 ◦C and
the residual weight of SG-[(CH2)3SO3H-HIM]HSO4 was about 80%
around 700 ◦C.
3.4. Mechanism for acetalization
In the present work, the possible mechanism for acetalization of
aldehydes with alcohols over SG-[(CH2)3SO3H-HIM]HSO4 was con-
sistent with the reported ones [11,14]. As shown in Scheme 4, in the
first step, the carbonyl group of aldehyde 1 took a proton from the
immobilized ionic liquid catalyst. The protonated carbonyl group
2 was activated for nucleophilic addition of O atom of the alcohol.
After deprotonation, the hemiacetal 4 was formed. Then the hemi-
acetal 4 was protonated again. After loss of water, the oxonium ion
6 was formed. It accepted a second alcohol hydroxyl group to form
7 and subsequent deprotonation gave the acetal 8.
3.3. Catalytic activity of SG-[(CH2)3SO3H-HIM]HSO4 for
acetalization
4. Conclusion
The immobilized acidic ionic liquid catalyst SG-[(CH2)3SO3H-
HIM]HSO4 was prepared and applied in acetalization of aldehydes
with alcohols in this study. After characterization, it was found that
[(CH2)3SO3H-HIM]HSO4 had been successfully immobilized on the
surface of silica–gel by chemical covalent bond and the immobi-
lized ionic liquid catalyst had good thermal stability. The original
smooth surface of silica–gel was covered with [(CH2)3SO3H-
HIM]HSO4 and a rough surface of SG-[(CH2)3SO3H-HIM]HSO4 was
formed, but the size of particles had no obvious change. SG-
[(CH2)3SO3H-HIM]HSO4 showed good catalytic performance for
a series of acetalization. Satisfactory yields of 81.0–98.5% were
achieved under appropriate reaction conditions. The immobilized
ionic liquid catalyst could be recovered easily and reused for 10
times for the synthesis of benzaldehyde ethanediol acetal without
significant loss of catalytic activity. In conclusion, the immobilized
In order to investigate the scope and limitation of SG-
[(CH2)3SO3H-HIM]HSO4 as catalyst for acetalization, different
aldehydes and alcohols as the reactants were also tested and
the results were summarized in Table 2. It could be found that
SG-[(CH2)3SO3H-HIM]HSO4 could give high catalytic activity for
acetalization. Good to excellent yields ranging from 85.0% to 98.5%
were obtained in all systems under investigation. And it should be
pointed out here that selectivities to the desired products were all
100% over SG-[(CH2)3SO3H-HIM]HSO4.
The reusability of SG-[(CH2)3SO3H-HIM]HSO4 was evaluated in
the acetalization of benzaldehyde and ethanediol. The results were
shown in Fig. 4. It was apparent that SG-[(CH2)3SO3H-HIM]HSO4
showed good reusability for the reaction. The catalyst could be
reused for 10 times without significant loss of catalytic activity.

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J. Miao et al. / Journal of Molecular Catalysis A: Chemical 348 (2011) 77–82
ionic liquid catalyst SG-[(CH2)3SO3H-HIM]HSO4 prepared in this
study was proved to be an excellent recyclable heterogeneous cat-
alyst for the synthesis of acetals and showed potential application
in industry.
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