Synthesis of novel solid acidic ionic liquid polymer and its catalytic activities

Article abstract of DOI:10.1134/S0023158413060062

The novel solid acidic ionic liquid polymer has been synthesized through the copolymerization of acidic ionic liquid oligomers and resorcinol- formaldehyde (RF resin). The catalytic activities were investigated through the acetalization. The results showed that the PIL was very efficient for the reactions with the average yield over 99.0%. The procedure was quite simple with just one-step to complete both the reactions. The high hydrophobic BET surface, high catalytic activities and high stability gave the PIL great potential for green chemical processes. Pleiades Publishing, Ltd., 2013.

Full text of DOI:10.1134/S0023158413060062

ISSN 0023ꢀ1584, Kinetics and Catalysis, 2013, Vol. 54, No. 6, pp. 724–729. © Pleiades Publishing, Ltd., 2013.
Published in Russian in Kinetika i Kataliz, 2013, Vol. 54, No. 6, pp. 765–770.
Synthesis of Novel Solid Acidic Ionic Liquid Polymer
and Its Catalytic Activities1
Xuezheng Liang
Institute of Applied Chemistry, Shaoxing University, Shaoxing, 312000 China
eꢀmail: liangxuezheng@126.com
Abstract—The novel solid acidic ionic liquid polymer has been synthesized through the copolymerization of
acidic ionic liquid oligomers and resorcinolꢀformaldehyde (RF resin). The catalytic activities were investiꢀ
gated through the acetalization. The results showed that the PIL was very efficient for the reactions with the
average yield over 99.0%. The procedure was quite simple with just oneꢀstep to complete both the reactions.
The high hydrophobic BET surface, high catalytic activities and high stability gave the PIL great potential for
green chemical processes.
DOI: 10.1134/S0023158413060062
1. INTRODUCTION
tion of ILs becomes the good choice. Ionic liquids
supported with pendant ferrocenyl groups and silica
gel have been presented for the purpose [9–10]. Howꢀ
ever, the expensive reagents were employed and the
catalytic activities decreased a lot. The ions were disꢀ
persed on the surface and could not form the ion clusꢀ
ters, which decreased the synergistic effect. On the
other hand, the acid sites on the surface fell off easily,
which cause the recycled activities drop quickly.
Recently, polymeric ionic liquids (PILs) become a
new class of materials, which showed excellent propꢀ
erties as electrolytes for electrochemical devices
[11–12].
Ionic liquids (ILs) were wellꢀknown as the green
materials with many advantages such as high conducꢀ
tivity, adjustable structure, negligible volatile pressure,
wide electrochemical window and strong dissolution
ability [1]. ILs were widely applied as catalysts in
chemical synthesis [2–5]. Sulfonic acid groups funcꢀ
tionalized ionic liquids owned even higher activities
than the homogeneous catalysts such as sulfuric acid
for various acidꢀcatalytic reactions [6–8]. Although
acidic ionic liquids were efficient procedures for variꢀ
ous reactions, such as the esterification, acetalization
and alkylation, the ILs were dissoluble with the cataꢀ
lytic system, especially the polar compounds, which
Here, the novel solid acidic polymeric ionic liquid from
made the catalyst recovery difficult and added the Brønsted acidic ionic liquid [SO3H(CH2)3VIm]HSO4 and
product purification task. Therefore, the immobilizaꢀ RF resin was presented (Scheme 1).
The synthesis route of the novel acidic
ionic liquid polymer
THF, RT
72 h
SO–3
+
N
O
O
N
N
N
+
S
O
H SO
2
4
Ethanol
AIBN
(
N
N
SO3H
N
HSO4–
N
SO3H
+
( )
( )3
+
3
HSO4–
–
4
HSO
+
N
HO3S
N
OH
O
+
N
N
SO3H
(
HSO
H
H
–
4
4
OH
HSO
N
80°C, 72 h
N
N
N
SO3H
(
HO3S
HSO
4
Scheme 1.
1
The article is published in the original.
724

SYNTHESIS OF NOVEL SOLID ACIDIC IONIC LIQUID POLYMER
725
In order to form the ions clusters to retain the ionic
Transmittance, %
100
liquid conformation, the ionic liquid monomer was
polymerized first. Then, the oligomers was copolyꢀ
merized with resorcinol and formaldegyde. RF resin
was used as the high BET surface supplier, which not
only enhance the efficiency of mass transfer but also
prevent the acid sites releasing. The catalytic activities
of the novel solid acidic ionic liquid polymer were
investigated through the acetalization. The results
showed that the novel PIL was very efficient for the
reactions.
95
90
85
80
2. EXPERIMENTAL
75
All organic reagents were commercial products of
the highest purity available (>98%) and used for the
reactions without further purification.
3500 3000 2500 2000 1500 1000
Wavenumbers, cm–1
Fig. 1. The IR spectrum of the PIL.
2.1. Synthesis of the Novel Acidic Ionic Liquid Polymer
The vinyl imidazole (9.4 g, 0.1 mol), 1,3–propane
sulfonate (12.2 g, 0.1 mol) and 20 mL tetrahydrofuran
were mixed and stirred magnetically for 72 h at room
temperature. Then, a white solid zwitterion was
formed. The white solid zwitterion was filtrated and
washed repeatedly with ether. After dried in vacuum
netic stirrer and thermometer. Here a DeanꢀStark
apparatus was used to remove the water continuously
from the reaction mixture. The mixture was refluxed
for the specified time. The progress of the reaction was
monitored by GC analysis of the small aliquots withꢀ
drawn. On completion, the catalyst was recovered by
centrifugation, washing with acetone and drying in an
(110 C, 1.33 Pa), the white solid zwitterion was
°
obtained in good yield (91%). Equalmolar amount of
sulfuric acid was added to the above obtaned zwitteꢀ
oven at 80°C for about 1 h.
rion and the mixture was stirred for 4 h at 60
the ionic liquid monomer. 1H NMR for the zwitterion
(400 MHz, D2O, TMS): 2.37 (m, 2H), 2.97 (t,
7.6 Hz, 2H), 4.44 (t, JH–H = 7.2 Hz, 2H), 5.46 (d, 1H),
5.85 (d, 1H), 7.17 (m, 1H), 7.66 (s, 1H), 7.82 (s, 1H),
9.11 (s, 1H). IR(KBr): 1037 cm–1 and 907 cm–1 (–SO3H),
1166 cm–1 (C–N), 3409 cm–1 (O–H).
Monomer (3.14 g, 10 mmol), 20 mL ethanol and
0.01 g Azobisisobutyronitrile (AIBN) were mixed
together to form the solution. After stirring at 70°C for
4 h, the homogenous oligomers solution was formed.
After cooling to room temperature, resorcinol (1.1 g,
10 mmol) and 20 mL water were added to the mixture
and stirred at room temperature to form the solution.
Then, formaldehyde (1.22 g, 37 wt %, 15 mmol) was
dropped to the mixture. After stirring for 1 h, the mixꢀ
°C to form
3. RESULTS AND DISCUSSION
δ
J =
3.1. Characterization of the Acidic Ionic Liquid Polymer
The acidity of the novel solid acidic PIL was
4.2 mmol/g, which was determined through the neuꢀ
tralization titration. The PIL owned much high acidꢀ
ity compared to common heterogeneous acids. The
acidity quite agrees with the equalmolar of IL monoꢀ
mer and RF resin, which also could be adjusted
through the molar ratio. The more acidic monomer,
the higher acidity obtained. On the other hand, the
BET surface decreased with the RF content. The PIL
owned the BET surface of 323 m2/g.
The IR spectrum (Fig. 1) of the PIL showed the
sulfonic acid group absorbability at 1049 and 960 cm–1
which confirmed the dual acidic groups. FT–IR specꢀ
trum also showed that the PIL contain resident functionꢀ
alities including C–C (1150 cm–1), Ar–H (3150 cm–1),
and C=O (1740 cm–1) and OH (3400 cm–1).
ture was place in an oven at 80
organic gel was completed, the resulting black solid
was dried in an oven at 80 overnight. After cooling
°C for 72 h. After the
°C
to room temperature, the obtained black solid was
washed with water until no acidity detected in the filꢀ
trate. The novel solid acidic ionic liquid was obtained
The scanning electron microscope (SEM) images
of PIL showed that the resulting particles were irreguꢀ
lar spheres structure with small particle sizes about 1–
after drying at 120°C overnight in an oven.
2
µ
m and big particle sizes about 3–5 m as depicted
µ
in Fig. 2. The particles connected with each other
without obvious boundary. The particles changed
2.2. The Procedure for the Acetalization
In the typical procedure: carbonyl compounds greatly from the pure RF resin, which indicated the
(0.1 mol), 10 mL cyclohexane, diols (0.15 mol) and interactions between the IL and RF resin occurred
the catalyst (0.05 g) were mixed together in a three during the synthetic process. Most crossꢀlinked prodꢀ
necked round bottomed flask equipped with a magꢀ ucts with the structure connected together. The crossꢀ
KINETICS AND CATALYSIS Vol. 54
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2013

726
XUEZHENG LIANG
5
µ
m
10
µ
m
10 µm
Fig. 2. The SEM images of the PIL.
linked structure made the recovery of the PIL quite groups decreased the activities with relatively low yield
simple without specified operation except for filtraꢀ (entries 14–19). On the other hand, electronꢀwithꢀ
tion.
drawing groups enhanced the nucleophilicity of the
carbonyl groups and the aromatic aldehydes with F
and nitro groups owned higher activities (entries 20–
23). Different diols such as 1,2ꢀethanediol, 1,2ꢀpropyꢀ
lene glycol and neopentyl glycol were all transformed
to products smoothly. The reactivities reduced as folꢀ
lows: 1,2ꢀethanediol > 1,2ꢀpropylene glycol > neoꢀ
pentyl glycol for the steric effect.
3.2. The Catalytic Activities for Acetalization
Acetalization of various carbonyl compounds with
diols catalyzed by PIL was performed first (Table 1).
As was anticipated, all raw materials were successfully
transformed to the corresponding acetals (ketals). The
results shown in Table 1 clearly demonstrated that the
novel solid acidic ionic liquid polymer was very effiꢀ
cient, with high yields for most reactions. Aliphatic
aldehydes transformed to the corresponding acetals
quickly under the reaction conditions (entries 1–5).
The ketalization reactions were also examined. Cycloꢀ
hexanone obtained almost complete conversion and
selectivity for little steric hindrance of carbonyl group
(entries 6–8). The activities of linear ketones such as
butanone decreased a little with moderate yields
(entries 9–10). Aromatic aldehydes, such as benzaldeꢀ
hyde were acetalized to afford the corresponding 1,3ꢀ
acetals with high yields (entries 11–13). The substituꢀ
ents attached to aromatic ring attached much imporꢀ
tance for the reactions. Aromatic aldehydes with elecꢀ
tronꢀdonating groups such as methyl or methoxyl
3.3. The Recycled Activity of the Catalyst
One property of the PIL is the reusability. After
reactions, PIL was solid catalyst and could be recovꢀ
ered by filtration. The recovered activities were invesꢀ
tigated carefully (Fig. 3). Unlike the immobilized IL,
the acid sites dropped from the surface and the cataꢀ
lytic activities decreased quickly. The yield remained
unchanged even after PIL had been recycled for a sixth
time. The acidic IL was embedded in the RF resin,
which provide high surface and prevent the acidic sites
loss.
3.4. Comparative Study on the Catalytic Activities
of Different Catalysts
A comparative study on the catalytic activities of
the PIL with the other catalysts was carried out using
benzaldehyde and 1,2ꢀethanediol (Table 2). From this
study it can be concluded that PIL owned much
higher activity than others. For the Lewis acid ionic
liquid, the catalytic amount was more and longer reacꢀ
tion time was needed. Futhrtmore, the byproduct
water generated from the reaction decomposed the
catalyst, which decreased catalytic activities greatly.
As to the traditional –SO3H functionalized ionic liqꢀ
uid [SO3HꢀBmim][HSO4], there was some soluble
with the reaction mixture, which was difficult to sepꢀ
erate. The traditional homogeneous catalysts such as
H2SO4 and H3PO4 owned relatively low activities. H+
was also very efficient for the side reactions, which
would reduce the yield. On the other hand, the novel
solid acidic PIL owned high BET surface, which sepꢀ
arated the water from the catalytic system quickly and
Yield, %
100
80
60
40
20
0
1
2
3
4
5
6
Run times
Fig. 3. The recycled activity of PIL.
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SYNTHESIS OF NOVEL SOLID ACIDIC IONIC LIQUID POLYMER
727
Table 1. Preparation of various acetals from the corresponding carbonyl compounds
Yield, %a,b
99
Entry
Substrate
Product
Reaction time, h
O
Cl
1
0.5
Cl
CHO
CHO
O
O
Cl
2
3
4
5
0.5
0.5
1.0
1.0
99
99
99
98
Cl
Cl
O
O
Cl
CHO
CHO
CHO
O
O
O
O
O
O
O
6
7
8
0.5
1.0
1.5
99
99
99
O
O
O
O
O
O
O
O
O
9
2.0
3.0
95
94
O
O
O
10
O
O
O
11
12
1.5
2.0
97
95
CHO
CHO
O
O
KINETICS AND CATALYSIS Vol. 54
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728
XUEZHENG LIANG
Product
Тable 1. (Contd.)
Yield/%a,b
Entry
Substrate
Reaction time/h
O
O
13
3.0
94
CHO
O
14
15
16
17
18
19
H3CO
3.0
3.5
3.0
2.0
3.0
3.5
94
90
91
95
92
91
H3CO
H3CO
H3CO
H3C
CHO
O
O
O
H3CO
H3CO
CHO
CHO
CHO
CHO
CHO
O
O
O
H C
H CO
3
3
O
O
H C
H CO
H3C
3
3
O
O
H C
H CO
H3C
3
3
O
O
O
F
CHO
F
20
21
1.0
2.0
99
96
F
F
F
O
O
F
CHO
F
F
O
22
23
1.5
2.5
95
94
O2N
O2N
CHO
CHO
O2N
O
O
O2N
O
a
All reactions were carried out under Dean–Stark conditions: PIL (0.05 g), diols (0.15 mol), carbonyl compound (0.1 mol) and cycloꢀ
b
hexane (10 mL), refluxed. The yield was taken by GC according to carbonyl compounds. In all cases, the corresponding products were
exclusively obtained.
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SYNTHESIS OF NOVEL SOLID ACIDIC IONIC LIQUID POLYMER
Table 2. The comparison of different catalysts
729
Catalyst
Novel PIL
Catalyst amount, mg
Reaction time, h
Yield, %a, b
50
60
1.5
2.0
2.0
1.0
2.5
2.5
97
89
95
91
88
90
[Et3NH]Cl–AlCl3
[SO3HꢀBmim][HSO4]
H2SO4
60
70
H3PO4
80
Amberlystꢀ15
100
a
All reactions were carried out under Dean–Stark conditions: PIL (0.05 g), diols (0.15 mol), carbonyl compound (0.1 mol) and cycloꢀ
hexane (10 mL), refluxed.
b
The yield was taken by GC according to carbonyl compounds. In all cases, the corresponding products were exclusively obtained.
stopped the active sites loss. Therefore, the novel PIL
owned much higher activities for the reaction. Also,
the traditional ionic exchange resins (Amberlystꢀ15,
acidity 0.8 mmol/g) owned much lower activity for the
low acidity. It clearly showed that the novel PIL should
be one of the best choices for the operational simplicꢀ
ity, userꢀfriendly catalyst and for the scaling up purꢀ
pose.
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4. CONCLUSIONS
The novel solid acidic PIL has been synthesized
through the copolymerization of acidic ionic liquid
oligomers and RF resin. The PIL showed high activiꢀ
ties for the acetalization. The catalyst owned the
advantages of high acidity, high BET surface and high
stability, which made the novel catalyst hold great
potential for the replacement of the homogeneous catꢀ
alysts in green chemical processes.
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ACKNOWLEDGMENTS
12. Li, J.N., Wang, L.N., Qi, T., Zhou, Y., Liu, C.H.,
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The work was supported by the National Natural
Science Foundation of China No. 21103111.
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2013