Ionic liquids containing carboxyl acid moieties grafted onto silica: Synthesis and application as heterogeneous catalysts for cycloaddition reactions of epoxide and carbon dioxide

Article abstract of DOI:10.1039/c0gc00612b

A series of carboxyl-group-functionalized imidazolium-based ionic liquids (CILs) were synthesized and grafted onto silica gel. The catalytic activity of the resulting heterogeneous catalysts toward the synthesis of cyclic carbonate via cycloaddition reactions of epoxide and CO₂ was studied. The effect on the reaction of the grafted ILs' structures and the reaction conditions such as reaction temperature, time, pressure, and the amount of catalyst used, were systematically investigated. For comparison, silica-grafted imidazolium-based ILs with and without hydroxyl groups were also used to catalyze cycloaddition reactions. The carboxylic acid group in the catalyst was demonstrated to have a synergistic effect with halide anions. A high yield of cyclic carbonates and excellent selectivity could be obtained under optimized conditions. The Royal Society of Chemistry.

Full text of DOI:10.1039/c0gc00612b

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PAPER
Ionic liquids containing carboxyl acid moieties grafted onto silica: Synthesis
and application as heterogeneous catalysts for cycloaddition reactions of
epoxide and carbon dioxide
a
a
a
b
a
Lina Han, Hye-Ji Choi, Soo-Jin Choi, Binyuan Liu and Dae-Won Park*
Received 28th September 2010, Accepted 18th February 2011
DOI: 10.1039/c0gc00612b
A series of carboxyl-group-functionalized imidazolium-based ionic liquids (CILs) were
synthesized and grafted onto silica gel. The catalytic activity of the resulting heterogeneous
catalysts toward the synthesis of cyclic carbonate via cycloaddition reactions of epoxide and CO
2
was studied. The effect on the reaction of the grafted ILs’ structures and the reaction conditions
such as reaction temperature, time, pressure, and the amount of catalyst used, were systematically
investigated. For comparison, silica-grafted imidazolium-based ILs with and without hydroxyl
groups were also used to catalyze cycloaddition reactions. The carboxylic acid group in the
catalyst was demonstrated to have a synergistic effect with halide anions. A high yield of cyclic
carbonates and excellent selectivity could be obtained under optimized conditions.
9
Introduction
salts alkali halide and imidazolium alkyl halide, immobilized on
10–13
14,15
silica
or polymer supports,
for use as heterogeneous catalysts for the synthesis of carbonates.
More recently, Sun and co-workers found that hydroxyl groups
containing an IL could activate the cycloaddition reaction.
Zhu et al. demonstrated that choline chloride/urea could greatly
have been extensively explored
Cyclic carbonates have drawn much attention and have been
widely used not only as polar aprotic solvents in organic and
polymeric synthesis but also as ingredients in pharmaceuticals
and fine chemicals in biomedical synthesis.
most effective routes for the synthesis of cyclic carbonates
is the cycloaddition of CO with epoxides. Considering the
16
1–3
One of the
improve catalytic activity for the cycloaddition reaction of CO
2
2
and epoxide because of the presence of a hydroxyl group in
desire for chemical technology that has minimal environmental
17
choline chloride. First, the catalyst’s OH group and halide
4
impact, some green ionic liquid (IL) catalysts such as 1-butyl-
anion at the Lewis basic site make a coordinated attack on
the epoxide. The coordination of the H and O atoms of the
epoxide through a hydrogen bond results in the polarization
of C–O bonds, and the halide anion makes a nucleophilic
attack on the sterically hindered b-carbon atom of the epoxide.
The carboxylic acid group is undoubtedly a stronger Brønsted
acid and hydrogen bonding donor than the hydroxyl group.
Therefore, the incorporation of carboxylic acid moieties into
ILs could provide an effective alternative for improving the
5
3
-imidazolium tetrafluoroborate, 1-octyl-3-methylimidazolium
6
7,8
tetrafluoroborate, and pyridinium or ammonium salts have
been developed, and have exhibited excellent catalytic per-
formance toward such reactions. Unfortunately, those reac-
tions were conducted in homogeneous catalytic systems, which
suffer from poor catalyst recovery and product separation.
Hence, it is crucial to identify more efficient heterogeneous
catalysts.
18
Heterogeneous IL catalysts prepared by grafting ILs onto
solid supports retain important physical and chemical features
of ILs, such as nonvolatility, nonflammability, and good ther-
mal stability, as well as high catalytic activity and selectivity.
Therefore, supported ILs are considered promising as catalysts
toward many catalytic reactions. The quaternary ammonium
catalytic performance of ILs toward the cycloaddition of CO
and epoxides.
We synthesized a series of carboxylic-acid-functionalized
imidazolium-based ILs bearing different halide anions (CILXes)
and grafted them onto a silica gel surface, forming various
2
heterogeneous catalysts for the cycloaddition reaction of CO
2
-
-
-
and epoxides. The effect of anions (Cl , Br , and I ) and
reaction parameters (temperature, pressure, and time) on the
yields and selectivity were investigated. For comparison, we
also studied silica grafted non-functionalized IL (IL1-Si) and
silica-grafted hydroxyl-group-functionalized IL (IL2-Si) in this
work.
a
Division of Chemical Engineering, Pusan National University, Busan,
09-735, Republic of Korea. E-mail: dwpark@pusan.ac.kr; Fax: +82 51
12 8563; Tel: +82 51 510 2399
6
5
b
Institute of Polymer Science & Engineering, School of Chemical
Engineering, Hebei University of Technology, Tianjin, 300130, China.
E-mail: byliu@hebut.edu.cn; Tel: +86 22 6020 4277
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Results and discussion
Characterization of catalysts
FT-IR analysis. Imidazolium-based ILs with different func-
-
-
-
tional groups bearing varied anions (Cl , Br , and I ) were
synthesized and immobilized on the surface of a commercial
silica gel. The resulting heterogeneous catalysts (CILX-Si, IL1-
Si and IL2-Si) and pure silica were characterized by FT-IR
analysis. As shown in Fig. 1, CILX-Si exhibits the characteristic
-
1
-1
bands of C O (1730 cm ) and C–O stretching (1402 cm )
of carboxylic acid. In comparison with pristine silica, all the
IL-grafted silica samples demonstrate newly-appeared C–H
-
1
stretching (3130, 2950, and 2850 cm ), and imidazolium ring
-
1
stretching bands (1560 and 1460 cm ), characteristic of the
presence of IL components in the heterogeneous catalysts. These
facts evidenced that the functionalized ILs were successfully
grafted onto the silica surface.
29
Fig. 2
Si NMR spectrum of CILBr-Si.
13
Fig. 3
C NMR spectrum of CILBr-Si.
attributed to carbon atoms connected to the imidazole ring, and
other carbon atoms show peaks from 5 to 38 ppm.
Fig. 1 FTIR spectra of pristine silica, CILX-Si, IL1-Si and IL2-Si.
Catalytic performance of grafted ILs with a functional group
To investigate the effects of molecular composition (functional
group and anion type) of grafted ILs on catalytic reactivity, a
variety of silica-grafted ILs with functional groups (-COOH,
-OH) were employed as heterogeneous catalysts in cycloaddi-
tion reactions. All reactions were performed under the same
2
9
13
Si and C MAS-NMR characterization
To investigate the efficiency of the grafting reaction, a solid-
state Si MAS-NMR analysis of CILBr-Si was conducted.
As shown in Fig. 2, two peaks centered at -103 and -112
2
9
◦
ppm can be clearly observed; they are attributed to Q
OSi) (OH)] and Q [Si (OSi) ] silicon atoms, respectively.
The peaks located at -67, -56, and -44 ppm are assigned to
[Si(OSi) R], T [Si(OSi) R(OH)], and T [Si (OSi) R(OH)
3
[Si
conditions (110 C, 0.86 MPa, 3 h) using the same amount of
(
3
4
4
catalysts and allyl glycidyl ether (AGE). As shown in Table 1 and
2, the obtained AGE conversions ranged from 65% to 76% of the
total available AGE with >93% allyl glycidyl carbonate (AGC)
selectivity. The high selectivity indicated negligible creation of
byproducts in the cycloaddition reactions. Runs 1–3 in Table 2
demonstrate the effects of halide anions located on the grafted
CILX on the catalytic reactivity in the cycloaddition reactions
T
3
3
2
2
1
3
2
]
organosiloxane, respectively. These results indicated the pres-
ence of organic functionalization moieties as part of the
silica.
Fig. 3 shows the C MAS-NMR spectrum of CILBr-Si. The
chemical shifts at 174 ppm correspond to carbon atoms in the
carboxylic acid group; those at 135 and 120 ppm correspond to
the three imidazole ring carbon atoms. The signal at 55 ppm is
1
3
between AGE and CO . CILI-Si exhibited the highest catalytic
2
activity. The AGE conversions for CILCl-Si, CILBr-Si, and
CILI-Si were 71%, 73%, and 76%, respectively (Table 2, runs 1, 2,
1
024 | Green Chem., 2011, 13, 1023–1028
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Table 1 Elemental analysis of catalyst
moieties, either in the free state or fixed by IL catalysts into the
catalytic system can greatly enhance the catalytic performance
of supported ILs toward the cycloaddition reaction.
IL grafting
amount/mmol g-
1
Sample
N (%)
C (%)
H (%)
CILCl-Si
CILBr-Si
CILI-Si
IL2-Si
1.04
1.03
1.00
1.62
3.50
6.84
6.87
6.34
8.11
9.27
3.14
3.62
3.44
2.99
2.88
0.37
0.36
0.35
0.58
1.25
Effect of temperature, pressure, and reaction time
Since CILBr-Si showed good catalytic activity, the effects of
reaction parameters (temperature, time, and CO pressure) were
2
IL1-Si
examined using a CILBr-Si-based catalytic system. First, the
relationship between AGE conversion and reaction time was
investigated. Fig. 4 shows that AGE conversion increased with
reaction duration up to a reaction time of 3 h, after which AGE
conversion did not increase and the reaction rate slowed. The
a
Table 2 Effect of grafted IL structure on cycloaddition reaction
c
b
Run Catalyst
Conversion (%) Selectivity (%) TON
1
2
3
4
5
6
7
CILCl-Si
CILBr-Si
CILI-Si
IL2-Si
IL1-Si
IL1-Si/H
71
73
76
70
65
71
93
96
98
92
95
92
96
143
156
170
89
40
41
results suggest that the fixation of CO onto AGE was completed
within 3 h. Thus, the other reaction parameters were assessed
using 3 h reaction times.
2
d
d
2
O
3
IL1-Si/CH
COOH 77
47
a
Reaction conditions: catalyst, 0.5 g; AGE, 40 mmol; CO
.86 MPa; reaction temperature, 110 C; time, 3 h; without solvent.
2
pressure,
◦
0
b
Turnover number (TON): number of moles of product per mole of
c
immobilized IL. The amount of immobilized IL was calculated by EA.
d
2
The amount of H O and acetic acid is the same as immobilized IL.
and 3). These marked differences may reflect the nucleophilicity
of the anions in the immobilized alkyl halides.
-
-
-
The ion radii of Cl , Br , and I increase and their electroneg-
ativity decreases in that order; thus, anion movement away
-
-
-
from the imidazolium cation in the order of I > Br > Cl
reflects their nucleophilicity order. Generally, anions with higher
19
nucleophilicity are favored for AGE conversion, a finding that
is supported by the increases in AGE conversion in the order of
-
-
-
I > Br > Cl observed in this study.
The CILI-Si catalyst showed not only the highest AGE
conversion (76%) but also the highest selectivity (98%) among
the tested CILX-Si catalysts. In addition, all the tested CILX-Si
catalysts showed quite high turnover numbers (TONs) ranging
from 143 to 170. For example, CILBr-Si had a TON of 156,
which was significantly higher than that reported for an IL with
no functional group immobilized on commercial silica (TON =
◦
Fig. 4 Time variant yield for CILBr-Si at 110 C and CO pressure of
1.62 MPa.
2
The results in Table 3 show that the reaction temperature
has a strong effect on AGE conversion (runs 2a, 2b, and 2c).
In catalytic reactions conducted at 90 C, 100 C, and 110 C,
the AGE conversions were 56%, 66%, and 73%, respectively.
This temperature and conversion-rate relationship is attributed
to higher reactivity at higher temperatures. However, when
the reaction temperature was increased to 120 C (run 2h),
AGE conversion decreased to 70%. Although higher reaction
system temperatures increase reactivity, the solubility of the
◦
◦
◦
10
11
40) and that of silicate MCM-41 (TON = 11.6).
The grafted IL with an OH group (IL2-Si) exhibited lower
activity (70% AGE conversion, run 4 in Table 2) than CILBr-Si.
Silica-grafted ILs (IL1-Si) with no functional group exhibited
rather lower catalytic activity (65% AGE conversion and a TON
of 40; run 5 in Table 2). This indicates that the carboxylic acid
group was responsible for the high conversion and selectivity.
For comparison, a small amount of water was introduced
into the reaction; the AGE conversion increased to 71%, but
the selectivity decreased to 92% (run 6 in Table 2). This can
be explained by the fact that the presence of water in a
catalytic system facilitates the formation of the byproduct 1,2-
◦
CO gas phase in the reaction system decreases with increasing
temperature. Therefore, in reactions performed at temperatures
2
◦
greater than 110 C, CO
2
solubility affects reactivity, leading to
a decrease in AGE conversion.
The effects of CO pressure were also investigated. As shown
in runs 2c and 2d in Table 3, when the reaction CO pressure in-
creased from 0.86 MPa to 1.34 MPa, AGE conversion increased
from 73% to 92%. At higher pressures, 1.62 MPa and 1.82 MPa,
AGE conversion increased to 98% and 99%, respectively (Table
2
2
20
glycol, which was confirmed through GC-MS spectrometry
GC-MS; Micromass, UK) by comparing the retention times
(
and fragmentation patterns with those of authentic samples.
Surprisingly, when acetic acid was added to the reaction, the
catalytic reactivity of IL1-Si was greatly enhanced, yielding a
much higher AGE conversion (77%) and selectivity (96%, run
3, runs 2e and 2f). Similar effects of CO
activity have been observed in other catalytic systems. During
such catalytic reactions, a higher CO pressure can effectively
increase the solubility of CO in AGE, enabling the reaction
2
pressure on catalytic
5,21
2
7
in Table 2). Therefore, the involvement of carboxylic acid
2
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a
a
Table 3 Effect of reaction parameter on catalytic performance
Table 4 Different epoxides react with CO
2
◦
◦
Run
T/ C
Pressure/MPa
Conversion (%)
Selectivity (%)
Epoxides
Product
T/ C
Time/h
Yield (%)
99
2
2
2
2
2
2
2
2
a
b
c
d
e
f
g
h
90
0.86
0.86
0.86
1.34
1.62
1.82
2.17
0.86
56
66
73
92
98
99
97
70
94
95
96
98
97
99
99
92
110
3
5
5
100
110
110
110
110
110
120
110
98
96
a
Reaction conditions: catalyst, CILBr-Si 0.5 g; AGE, 40 mmol; time,
h.
3
1
15
equilibrium to shift toward a carbonate formation. However,
when the catalytic reaction was conducted at a CO pressure
of 2.17 MPa, the AGE conversion dropped slightly (97%; Table
2
3
1
, run 2 g) compared with that in the reaction carried out at
.82 MPa (run 2f). The results indicate that AGE conversion
1
1
15
15
5
98
82
does not continue to increase with pressure, suggesting that a
high CO pressure may retard the interaction between AGE and
the catalyst. Such retardation may be related to a dilution effect
occurring when the reaction pressure reaches equilibrium at a
specific temperature, which has been reported to result in lower
2
24
21
AGE conversion.
The reusability of the grafted IL catalyst was also studied
with the catalyst used in Table 2, entry 2. Fig. 5 shows the
performance of the catalyst, which did not lose its catalytic
abilities significantly after five runs.
a
Reaction condition: epoxide, 40 mmol; catalyst CILBr-Si, 0.5 g; CO
pressure, 1.62 MPa.
2
cyclic carbonates in high yields with high selectivity. Compared
with the other epoxides, when cyclohexene oxide was reacted
with CO , the reaction took longer (24 h) and a relatively lower
2
yield (82%) was obtained. This fact was attributed to higher
hindrance of the reactant, which was also demonstrated by other
18,22
authors.
When other epoxides were used, the yields ranged
from 96% to 99%.
Proposed mechanism of coupling reaction
Previous research suggested that Brønsted acids can accelerate
the ring-opening reaction of epoxides by forming hydrogen
23
bonds. In our current study, grafted ILs with carboxyl and
hydroxyl functional groups demonstrated better catalytic ac-
tivities than grafted ILs with no functional group (Table 2,
entries 2, 4, 7). It can be deduced that the carboxylic acid group
and hydroxyl group can efficiently promote the cycloaddition
reaction. Furthermore, when acetic acid was introduced into the
coupling reaction, the AGC yield is 77% which is much higher
than that obtained by grafted IL with no functional group. This
further proves that the carboxylic acid group is favorable to the
cycloaddition reaction. The proposed mechanism is shown in
Scheme 1: the X anion (Lewis base) of the catalyst opens the
epoxy ring, which was activated by the carboxylic acid group
and electronic interaction with the imidazolium cation, to give
Fig. 5 AGC yield of recycled CILBr-Si.
Synthesis of cyclic carbonate from CO and other epoxides
2
The above results demonstrate that grafted ILs with a functional
group were efficient catalysts for the cycloaddition of AGE and
CO
of these catalysts to other epoxides, cyclic carbonates were
synthesized from different epoxides and CO at the optimized
2
in the absence of a solvent. To investigate the applicability
the intermediate. Then the intermediate further reacts with CO
2
2
to form the corresponding cyclic carbonate and regenerates the
catalyst. Thus, in our current catalysis systems, the coexistence
of hydrogen bond donors (–COOH) and the imidazolium cation
and halide anion showed the synergistic effect of promoting the
temperature in the presence of the synthesized catalysts. The
results are summarized in the Table 4. CILBr-Si was found to be
applicable to a variety of epoxides to produce the corresponding
1
026 | Green Chem., 2011, 13, 1023–1028
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Scheme 2 Synthesis of IL with or without functional group.
CILXes and IL2 were synthesized by using 3-chloropropionic
acid, 3-iodopropionic acid, and 2-bromoethanol, respec-
tively, instead of 3-bromopropionic acid. An IL with no
functional group, 1-ethyl-3-(3-triethoxysilypropyl) imidazolium
bromide (IL1), was synthesized according to the reported
10
procedure.
Scheme 1 The proposed reaction mechanism for cycloaddition of
epoxide and CO catalyzed by CILX-Si.
The NMR data are as follows:
2
1-Propionic acid-3-(3-triethoxysilypropyl) imidazolium bro-
1
mide (CILBr): H NMR (300 MHz, DMSO-d
6
): d 0.52 (m, 2H),
coupling reaction. This effect might be the main contribution to
their high catalytic activity and selectivity.
1
4
.05 (t, 9H), 1.82 (m, 2H), 2.95 (t, 2H), 3.45 (q, 6H), 4.35 (m,
H), 7.68 (s, 1H), 7.82 (s, 1H), 9.36 (s, 1H), 9.65 ppm (br, 1H);
1
3
C (75 MHz, DMSO-d
18.7, 121.1, 136.2, 172.3 ppm.
-Ethyl-3-(3-triethoxysilypropyl) imidazolium bromide (IL1):
H NMR (300 MHz, DMSO-d ): d 0.50 (m, 2H), 1.03 (t, 9H),
.39 (t, 3H), 1.84 (m, 2H), 3.68 (q, 6H), 4.22 (m, 4H), 7.67 (s,
6
); d 7.8, 18.2, 24.3, 34.2, 43.8, 45.9, 47.8,
Conclusions
1
1
Silica-grafted imidazolium-based ILs with carboxylic acid and
hydroxyl groups and with no functional group, have been
demonstrated as effective catalysts for the synthesis of cyclic
1
6
1
1
7
1
3
H), 7.85 (s, 1H), 9.46 ppm (s, 1H); C (75 MHz, DMSO-d
.5, 15.3, 18.4, 23.8, 43.8, 51.0, 56.0, 119.7, 122.1, 134.3 ppm.
-Hydroxyethyl-3-(3-triethoxysilypropyl) imidazolium bro-
6
); d
carbonates via the cycloaddition of epoxides with CO . Car-
2
10
boxylic acid-functionalized imidazolium-based ILs immobilized
on silica (CILX-Si) show the highest activity and selectivity,
since the -COOH group in the IL cation can greatly accelerate
the reactions and showed a synergistic effect with the halide
anions. We also found that the involvement of acetic acid can
significantly improve catalytic performance more efficiently than
water. These heterogeneous catalysts can be easily separated
from the products and reused.
1
1
mide (IL2): H NMR (300 MHz, DMSO-d
6
): d 0.53 (m, 2H),
.02 (t, 9H), 1.84 (m, 2H), 3.42 (q, 6H), 3.72 (t, 2H), 4.25 (m,
H), 7.70 (s, 1H), 7.78 (s, 1H), 9.19 (s, 1H), 9.52 ppm (br, 1H);
1
4
1
3
C (75 MHz, DMSO-d
6
); d 8.1, 17.8, 24.5, 50.1, 52.2, 54.5, 59.9,
1
18.9, 121.8, 136.8 ppm.
Synthesis of CILX-Si
Experimental
Before immobilization, silica gel was purified with hot pi-
ranha solution followed by rinsing with water and drying
under a stream of nitrogen. The pretreated silica and the
IL were co-dispersed in a flask containing a solution of
dehydrated toluene and methanol. The mixture was refluxed
for 24 h under a nitrogen atmosphere. The resulting prod-
uct was filtered and washed with dichloromethane to re-
move excess IL. After evaporating the residual solvent in a
vacuum oven, CILBr-Si was obtained. Following a similar
procedure, a series of CILX-Si, IL2-Si, and IL1-Si were also
synthesized. The preparation process of CILX-Si is shown in
Scheme 3.
Material
The following reagents were purchased from Aldrich
and used as-received: 3-chloropropionic acid, 3-bromopropionic
acid, 3-iodopropionic acid, bromoacetic acid, (3-chloropropyl)
triethoxysilane, dichloromethane, diethyl ether, allyl glycidyl
ether (AGE), and other epoxides. Silica gel (500 m g , 60 A)
was commercially obtained from Aldrich and purified with hot
2
-1
˚
piranha solution (H
2
O
2
and H
2
SO ) before use.
4
Preparation of CILXes
CILXes were synthesized according to the procedures shown
in Scheme 2. In a typical reaction, imidazole (3.4 g) and (3-
chloropropyl) triethoxysilane (12 mL) were poured in sequence
into a flask containing 50 mL of dry toluene with vigorous
stirring under an argon atmosphere. After the reaction pro-
ceeded under reflux conditions for 10 h, 8 g of 3-bromopropionic
acid were added to the flask. The mixture was held for
another 24 h. Subsequently, CILBr was obtained after the
solvent was removed. Following a similar procedure, the other
Scheme 3 Synthesis of CILX-Si, IL2-Si and IL1-Si.
Green Chem., 2011, 13, 1023–1028 | 1027
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◦
-1
Cycloaddition reaction
449 instrument with a heating rate of 10 C min at temperatures
◦
◦
ranging from 40 C to 800 C under a nitrogen atmosphere.
Allyl glycidyl carbonate (AGC) was synthesized by a coupling
reaction between AGE and CO in the presence of CILX-Si,
2
Acknowledgements
as illustrated in Scheme 4. All the reactions were carried out
in a 55 mL stainless-steel reactor with a magnetic stirrer. In a
typical reaction process, 0.5 g of catalyst was introduced into
the reactor containing 40 mmol of AGE. The reactions were
conducted under a preset carbon dioxide pressure at different
temperatures. After a reaction was complete, the reactor was
cooled to ambient temperature, and the products were analyzed
on a gas chromatograph (Agilent HP 6890 A) equipped with
a capillary column (HP-5, 30 m ¥ 0.25 mm) using a flame-
ionized detector. The purity and structure of products obtained
This work was supported by the National Research Foundation
of Korea (R01-2009-0070580) and by the National Foundation
of China (No. 50973026). The authors are also thankful for
the financial support of 2010 NRF-NSFC cooperative program
(D00020 and No. 5101140349).
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Scheme 4 Synthesis of cyclic carbonate from epoxide and CO .
2
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1
1
Characterization
2
Elemental analysis (EA) was performed using a Vario EL III
◦
instrument. 2 mg of each sample was subjected to 1100 C,
14 Y. Xie, Z. F. Zhang, T. Jiang, J. L. He, B. X. Han, T. B. Wu and K.
1
13
while sulfanilic acid acted as a standard. H NMR and
C
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Han, J. Mol. Catal. A: Chem., 2008, 284, 52.
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1
1
NMR analysis were performed on Varian 300 MHz and 75
MHz spectrometers, respectively, using DMSO as a solvent.
2
9
13
Solid-state NMR analysis was conducted at Si and
C
frequencies of 79.5 and 100.6 MHz, respectively, on an INOVA-
2
9
4
00 WB MAS probe. Si MAS spectra were measured at
room temperature under the following conditions: MAS at
kHz; p/2 pulse, 6.5 ms and a repetition delay of 60 s; 3928
1
5
1
3
scans referenced to tetramethylsilane. C cross-polarization
spectra were measured with a recycle delay of 5 s under the
following conditions: MAS at 5 kHz; p/2 pulse, 7 ms; 1024 scans
referenced to tetramethylsilane. Fourier transformation infrared
2
2
2
0 J. Q. Wang, D. L. Kong, J. Y. Chen, F. Cai and L. N. He, J. Mol.
Catal. A: Chem., 2006, 249, 143.
1 L. F. Xiao, F. W. Li, J. Peng and C. Xia, J. Mol. Catal. A: Chem.,
2
006, 253, 265.
(
FT-IR) spectra were obtained on an AVATAR 370 Thermo
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13.
23 J. W. Huang and M. Shi, J. Org. Chem., 2003, 68, 6705.
-
1
Nicolet spectrophotometer with a resolution of 4 cm . Thermal
gravimetric analysis (TGA) was performed on a Netzsch STA
7
1
028 | Green Chem., 2011, 13, 1023–1028
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