[Hmim]3PW12O40: A high-efficient and green catalyst for the acetalization of carbonyl compounds

Article abstract of DOI:10.1016/j.cclet.2010.02.004

[Hmim]₃PW₁₂O₄₀ was developed and used in the acetalization of carbonyl compounds in excellent yields. The ionic liquid-heteropoly acid hybrid compound and reaction medium formed temperature-dependent phase-separation system with the ease of product as well as catalyst separation. The catalyst was recycled more than 10 times without any apparent loss of catalytic activity.

Full text of DOI:10.1016/j.cclet.2010.02.004

Available online at www.sciencedirect.com
Chinese Chemical Letters 21 (2010) 678–681
www.elsevier.com/locate/cclet
[
Hmim] PW O : A high-efficient and green catalyst
3 12 40
for the acetalization of carbonyl compounds
a
a
b
Yan Dai , Bin Dong Li , Heng Dao Quan , Chun Xu Lu¨
a,*
a
School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China
b
National Institute of Advanced Industrial Science and Technology, Ibaraki 3058565, Japan
Received 27 September 2009
Abstract
3
Hmim] PW12O40 was developed and used in the acetalization of carbonyl compounds in excellent yields. The ionic liquid-
[
heteropoly acid hybrid compound and reaction medium formed temperature-dependent phase-separation system with the ease of
product as well as catalyst separation. The catalyst was recycled more than 10 times without any apparent loss of catalytic activity.
#
2010 Chun Xu L u¨ . Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: [Hmim]
3
PW12O40; Phosphotungstic acid; Carbonyl compounds; Acetalization; Ionic liquid
Acetalization plays an important role in organic synthesis for the protection of carbonyl groups [1], and usually is
achieved by treatment of carbonyl compounds with an alcohol or diol in the presence of Brønsted or Lewis acids [2].
However, those procedures own some drawbacks such as many by-products and serious environmental pollution.
Besides that, the separation of the products from the catalytic system is also difficult.
Recently, heteropoly acids (HPAs) especially Keggin type have attracted increasing interest due to their high
acidity, low toxicity and tunable redox properties [3]. Unfortunately, most HPAs exhibit some difficulties for product
separation in homogeneous catalytic systems. The deposition of HPAs on porous solid supports has been considered as
an effective means for improving properties and obtaining better performance in many potential heterogeneous
catalytic applications [4]. However, HPAs supported on carrier by physisorption still show a degree of solubility in the
reaction medium, which causes low recovery ratio and difficulty of separation. Thus, to develop novel HPAs and their
salts is a crucial task at present.
Ionic liquids (ILs) absorbed more and more interests of researchers and successfully used in acetalization reaction
due to low corrosion tendencies, good thermal stability and reusage [5]. In view of the advantages of ILs and HPAs, the
ionic liquid based hybrid molecular materials have been studied [6]. Some researchers have already reported IL–solid
catalytic systems. However, these ILs inevitably showed some drawbacks such as low recovery ratio, low yield or
difficulty of synthesis. In addition, they were used as electrochemical rather than as catalysts. Herein, we report a
Keggin phosphotungstate anion based Brønsted acidic ionic salt [Hmim] PW O could be used as a high-efficient,
3
12 40
reusable and environmental-friendly catalyst for the acetalization of carbonyl compounds (Scheme 1).
*
Corresponding author.
E-mail address: lucx@mail.njust.edu.cn (C.X. L u¨ ).
1
001-8417/$ – see front matter # 2010 Chun Xu L u¨ . Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2010.02.004

Y. Dai et al. / Chinese Chemical Letters 21 (2010) 678–681
679
Scheme 1.
3
Fig. 1. Illustration of the acetalization of benzaldehyde with glycol using [Hmim] PW12O40 as catalyst: (1) heterogeneous mixture before the
reaction at room temperature; (2) homogeneous mixture during the reaction at reflux; (3) near completion of the reaction; and (4) at the end of the
reaction at room temperature.
1
. Results and discussion
Hmim] PW O was obtained from phosphotungstic acid and N-methyl imidazole with 95% yield at room
[
3
12 40
temperature [7]. It is white solid with high melting point (>300 8C), which is absolutely different from conventional
ionic liquids. [Hmim] PW O can be dissolved in dimethyl sulfoxide, however, it does not dissolve readily in other
3
12 40
standard organic solvents and water at room temperature. The unique property is coincident with the requirements of a
solid catalyst.
To study the catalytic activity of [Hmim] PW O , the acetalization of benzaldehyde and glycol was chosen as
3
12 40
reaction model. The reaction was performed smoothly and 2-phenyl-1,3-dioxolane was obtained with 97% yield [8]. It
may be mentioned here that the catalyst and reaction medium formed temperature-dependent phase-separation system
in the acetalization of benzaldehyde (Fig. 1). The outstanding feature of the catalyst is that it scarcely dissolves in
standard organic solvents and water at room temperature, but can dissolve in glycol at high temperature due to the
formation of H-bonding between [Hmim] PW O and glycol. Thus, the reaction system was heterogeneous at room
3
12 40
temperature and then became homogeneous at reflux during the reaction. As the consumption of glycol, the system
became heterogeneous and induced a spontaneous self-separation of the catalyst. The unusual behavior was also
observed in the acetalization of other carbonyls and glycol.
Because glycol was one of the reactants, the catalyst layer was recycle used without any further purification.
Performance of recycling used [Hmim] PW O was investigated in the reaction of benzaldehyde with glycol. As
shown in Table 1, [Hmim] PW O could be easily recycled and reused 10 times without obvious loss of activity.
3
12 40
3
12 40
Moreover, the recovery yield of the catalyst was obtained more than 93% after reused 10 times.
In order to prove the merits of the catalyst in the present work, we compared the catalytic activity of
[
Hmim] PW O with other catalysts reported previously, and the data were listed in Table 2. Although inorganic
3 12 40
salts and Brønsted acids could catalyze the acetalization reaction, the yields were dissatisfactory (Table 2, entries 1–4).
Moreover, some of these catalysts are not stable in air. In cases of I , H SiW O , H PW O /Pan and H PW O ,
2
4
12 40
3
12 40
3
12 40
due to their shortcomings of difficult recovery and too many by-products, they could not be considered as economic
and green catalysts even though some of them displayed good catalytic capacity. As for [TMPSA]HSO4,
Table 1
3 40
The recycle of [Hmim] PW12O .
Cycles
1
2
3
4
5
6
7
8
9
10
92
a
Yields
97
98
97
96
95
94
93
93
92
a
Chemical yields of the reaction.

680
Y. Dai et al. / Chinese Chemical Letters 21 (2010) 678–681
Table 2
Comparison of the effect of catalysts in acetalization of benzaldehyde with glycol.
Entry
Catalyst
Yield (%)
1
2
3
4
5
6
7
8
9
FeCl
NSO
Bi(OTf)
Al-SBA-15
3
Á6H
2
O
58 [2g]
43 [2a]
73 [2c]
65 [2f]
92 [2h]
84 [2g]
76 [2i]
92 [2j]
77 [2k]
72 [2d]
68
H
2
3
H
3
Á4H
2
O
I
2
H
H
4
3
SiW12
PW12
O
40
O
40/PAn
IL-1000/TsOH
[TMPSA]HSO
[PSebim]HSO
PW12
[Hmim]
4
1
1
1
0
1
2
4
H
3
O
40
3
PW12
O
40
97
Reaction conditions: benzaldehyde (50 mmol), glycol (75 mmol), catalyst (0.44 mmol), toluene (8 mL), reflux for 2 h.
[
catalysts are expensive and require special efforts for preparation. [Hmim] PW O could be easily recycled and
PSebim]HSO and IL-1000/TsOH, they gave the desired products in moderate to excellent yields, however, these
4
3
12 40
reused 10 times without obvious loss of activity and weight. Thus, in view of excellent catalytic capacity, low cost,
outstanding stability and ready reutilization, [Hmim] PW O was chosen as an effective catalyst for acetalization
reaction of benzaldehyde and glycol.
3
12 40
In order to evaluate the generality of the process, we extended experiments to a series of alcohols and carbonyl
compounds. The reaction proceeded smoothly. Corresponding acetals and ketals were obtained in high yields in most
cases (Table 3). Due to its unique structure with two active sites, glycol usually reacts more actively than monohydroxy
alcohol. The yields of acetalization of glycol and benzaldehydes vary depending upon the types and positions of
substitutions. The results showed that the electron-donating groups decrease the yield, whereas electron-withdrawing
groups increase it. Due to steric factors, o-hydroxybenzaldehyde reacted more slowly than p-hydroxybenzaldehyde.
Table 3
Acetalization of carbonyl compounds.
a
Entry
Carbonyl compounds
Alcohol
Time (h)
Yield (%)
1
2
R
R
b
1
C
C
C
C
C
C
6
6
6
6
2
3
H
H
H
H
H
H
5
5
5
5
5
7
–
–
–
–
–
–
H–
H–
H–
H–
H–
H–
H–
H–
H–
H–
H–
H–
H–
H–
H–
CH
Methanol
Ethanol
Butan-1-ol
Glycol
Glycol
Glycol
Glycol
Glycol
Glycol
Glycol
Glycol
Glycol
Glycol
Glycol
Glycol
Glycol
Glycol
Glycol
2
82
79
81
97
99
96
93
78
85
62
97
96
88
95
94
89
98
90
b
2
2
b
3
3
c
4
2
c
5
2.5
2.5
3
c
6
c
7
4 9
iso-C H –
c
8
4-OH–C
3-OH–C
2-OH–C
6
H
H
H
5
5
5
–
–
–
4
c
9
6
6
4
c
1
1
1
1
1
1
1
1
1
0
1
2
3
4
5
6
7
8
6
c
c
c
c
c
c
c
c
4-NO
3-NO
2-NO
2
2
2
–C
–C
–C
6
6
6
H
H
H
5
5
5
–
1.5
1.5
4
–
–
4-Cl–C
3-Cl–C
6
H
5
–
–
1.5
1.5
5
6
H
5
C
6
H
5
–
3
–
–C
4
H
6
–
–CH
CH
2
–
2
C
2
H
5
–
3
–
3.5
b
Reaction conditions: carbonyl compound (50 mmol), alcohol ( 150 mmol; 75 mmol), toluene (8 mL), [Hmim]
c
3
PW12O
40 (0.44 mmol), reflux.
a
Detected by GC/MS. Yields were based on the starting carbonyl compounds.

Y. Dai et al. / Chinese Chemical Letters 21 (2010) 678–681
681
Because alkyl group provides more electron density and steric crowding, acetophenone has a lower reactivity than
benzaldehyde.
2
. Conclusion
In conclusion, [Hmim] PW O was facilely prepared and used as a high-efficient catalyst for the acetalization of
3 12 40
carbonyl compounds. [Hmim] PW O has several advantages: (1) it and reaction medium can form temperature-
3
12 40
dependent phase-separation system with the ease of product as well as catalyst separation; (2) it was recycled more
than 10 times without any apparent loss of weight and catalytic activity; (3) it has a wide range of application for
different substrates and the products can be obtained conveniently in good to high yields. In a word, [Hmim] PW O
3
12 40
is an excellent reusable catalyst.
Acknowledgments
We are grateful for the financial support from Nanjing University of Science and Technology.
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[
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[7] Phosphotungstic acid (5 mmol) and deionized water (15 mL) was added drop wisely into N-methyl imidazole (15 mmol) in 30 min with constant
stirring. Then the materials were stirred at room temperature for 24 h. Finally, the produced white solid was filtrated, washed and dried (95%
À1
1
yield). Mp > 300 8C; IR (KBr): n (cm ) 3330, 3159, 2871, 1585, 1548, 1438, 1384, 1330, 1151, 1080, 979, 893, 808, 740, 622, 592; H NMR
300 MHz, DMSO-d , TMS): d 3.85 (s, 3H, CH ), 7.60 (s, 1H, CH), 7.65 (s, 1H, CH), 8.92 (s, 3H, CH); Anal. Calcd. for C12 PW12 40: C
.61, H 0.68, N 2.69, P 0.97, W 68.76; found C 4.59, H 0.71, N 2.68, P 0.98, W 68.73; ESI-MS (energy: 50v, m/z, z = 3): 958.3 (48%), 701.5
100%), 470.6 (41%).
8] Representative procedure: the mixture of benzaldehyde (50 mmol), glycol (75 mmol), toluene (8 mL) and [Hmim]
(
6
3
H
21
N
6
O
4
(
[
3
PW12O40 (0.44 mmol) was
stirred at 80 8C for certain time. Water formed in the process was removed by azeotropic distillation using toluene as water-taken reagent. After
the reaction, the organic layer was separated by decantation, and the product was detected by GC–MS (GC yield: 97%). The catalyst was reused
without any treatment.