Synthesis and catalytic activity of porous polymer containing ionic liquid structures

Article abstract of DOI:10.1134/S0023158416060082

A novel porous polymer containing ionic liquid (IL) structures was synthesized via quternization and condensation of 4-vinylpyridine and p-xylylene dichloride. The ionic liquid structures were incorporated in the polymeric framework and for this reason bulky IL molecules can hardly block pores and neutralize active sites. The polymer shows a high BET surface area and easily accessible active sites. Catalytically the polymer is very active in Michael additions with averaged yields over 96.0% achieved after short reaction times. The high BET surface, remarkable activity, operational simplicity, wide applicability and improved stability are the key properties of the polymer.

Full text of DOI:10.1134/S0023158416060082

ISSN 0023-1584, Kinetics and Catalysis, 2016, Vol. 57, No. 6, pp. 840–846. © Pleiades Publishing, Ltd., 2016.
Synthesis and Catalytic Activity of Porous Polymer Containing Ionic
Liquid Structures1
Junqiao Li, Wei Lu, Weifeng Li, and Xuezheng Liang*
School of Chemistry and Chemical Engineering, Shaoxing University, Shaoxing, 312000 China
*e-mail: liangxuezheng@126.com
Received November 11, 2015
Abstract– A novel porous polymer containing ionic liquid (IL) structures was synthesized via quternization
and condensation of 4-vinylpyridine and p-xylylene dichloride. The ionic liquid structures were incorporated
in the polymeric framework and for this reason bulky IL molecules can hardly block pores and neutralize
active sites. The polymer shows a high BET surface area and easily accessible active sites. Catalytically the
polymer is very active in Michael additions with averaged yields over 96.0% achieved after short reaction
times. The high BET surface, remarkable activity, operational simplicity, wide applicability and improved
stability are the key properties of the polymer.
Keywords: porous polymer with ionic liquid structures, high activity, Michael addition
DOI: 10.1134/S0023158416060082
introduced to prepare SILPCs. The commercial silica
gel was one of the most used supports. The imidaz-
olium IL with peroxotung state anions were immobi-
lized onto silica gel and the catalyst showed activity
comparable with that observed for olefin poxidation in
the presence of IL [7]. The acidic ILs anchored to sil-
ica gel also showed high activity for hydrolysis of cellu-
lose [8]. Low amounts of IL loaded on the silica gel
(below 1.0 mmol/g) resulted in low activities. To
increase the loading amounts, the multilayer structure
on the support was formed using bis-vinylimidazolium
ILs [9]. However, the ILs accommodated in the lower
layers could not function during the reactions, which
resulted in the loss of the active ILs. Compared to sil-
ica gel, mesoporous molecular sieves have higher sur-
face area and the regular system of mesopores and
because of these properties, the solids can accommo-
date high amounts of ILs. SBA-15 functionalized by
an acidic IL exhibited high activity in Knoevenagel–
Michael-cyclization reactions [10]. MCM-41 was also
used to support ILs with phosphomolybdic acid [11].
However, the use of mesoporous molecular sieves
increases the catalyst cost. The pore walls in amor-
phous solids have propensity to collapse during the
catalytic process. Bulky IL molecules can block the
entrance to the pores. To prevent this phenomenon
mesoporous organosilica with alkylimidazolium IL
incorprated in the framework was synthesized [12].
However, the use of organosilicon monomer and tem-
plate needed for the complex synthetic process raises
the cost of the catalyst. Furthermore, the expensive
linkages are needed to connect the IL and silica. The
organic polymers were also applied as supports in the
Ionic liquids (ILs) attracted considerable interest
as solvents because of their versatile properties such as
non-volatility, wide liquid range, thermal stability and
controlled miscibility [1]. Various organic synthesis
such as hydrogenation, Suzuki reaction, aldol con-
densation, Diels–Alder reactions, alkylation, Michael
addition and Schmidt reaction have been performed
using ionic liquids as environmentally friendly solvents
[2]. ILs also perform special functions in materials sci-
ence, electrochemistry and separation technology [3,
4]. Since the first use of ILs as catalysts for organic
synthesis, the potentials of ILs in synthetic applica-
tions became considerably wider [5]. Although many
efficient processes were developed, the application of
ILs in chemical industry is still limited. High solubility
favors the interaction between ILs and reactants
enhancing catalytic activity. However, it is still diffi-
cult to ensure a complete recovery of the ILs from the
homogeneous reaction systems. An excessive viscosity
is another drawback that is unfavorable for mass trans-
fer and causes the operational difficulties. Further-
more, the ILs are even more toxic than the solvents
that they were aimed to substitute. Therefore, the sup-
ported ionic liquid phase catalysts (SILPCs) became a
good choice [6]. The resulting SILPCs combined the
advantages of the solid catalysts and the ILs. As a
result, the SILPCs found various applications in het-
erogeneous catalytic processes. The nature of the sup-
port affects both the connection mode and the BET
surface; accordingly, supports play an important role
in the immobilization of IL. Various supports were
1
The article is published in the original.
840

SYNTHESIS AND CATALYTIC ACTIVITY
841
simple copolymerization immobilization process. The on the surface. The bulky ILs could considerably
IL grafted onto the highly cross-linked polymer was reduce the BET surface area and in this way signifi-
efficient for CO2 cycloaddition [13]. Xiao developed
the solvothermal synthesis of the highly porous
poly(divinyl-benzene) (PDVB) that was widely used as
a support [14, 15]. Our group was able to support the
acidic IL on PDVB and applied the catalyst to synthe-
size biodiesel from waste oils [16, 17]. However, the
cantly affect the amount of loading. On the other
hand, many active ILs could not function properly
when the support is covered with excessive amounts of
ILs due to the increased mass transfer limitations. In
order to improve the amount of IL loaded, the novel
porous hyper-cross-linked polymer with IL embedded
traditional immobilization was achieved by single in framework was synthesized from 4-vinylpyridine
chemical bonds formed by grafting the structure of IL and p-xylylene dichloride (Scheme).
Synthesis of hyper-cross-linked polymer with IL structures
Cl
Cl
DCE
SnCl4
N
Cl
+
Cl−
N+
Cl−
N
Cl
Cl
DCE − 1,2-dichloroethane
Scheme.
First, 4-vinylpyridine and p-xylylene dichloride formed. The product was triply washed with acetone.
were quaternized to form the IL structures. Then, the After drying at ambient temperature overnight and at
IL and excess p-xylylene were condensed to form the 80°C in an oven for 12 h, slightly yellow hyper-cross-
hyper-cross-linked polymer via the SnCl4-catalyzed linked polymer with IL structure was obtained. The
amount of residual SnCl4 in the polymer was exam-
alkylation. The ILs were embedded in the framework
to act as the pore wall structures and help to avoid the ined using ICP technique. The results showed the
pore blockage. A high surface area facilitates the absence of tin in the polymer indicating total removal
accessibility of active sites to reactants. The catalytic of Lewis acid from the catalyst.
activities of the polymer were investigated using the
aza-Michael addition as a test reaction. The results
The Procedure for aza-Michael Addition
indicate that the novel polymer has a higher activity
Requisite amounts of amines (20 mmol), alkenes
(24 mmol) and catalyst (20 mg) were mixed and reacted
for definite time intervals at ambient temperature. The
GC analysis was conducted to monitor the reaction
process. On completion, the catalyst was recycled by fil-
tration. The recycled catalyst was washed with ethanol
and dried at 80°C overnight. Activity of the recycled
system was carefully investigated.
than the homogeneous IL with easy recovery as an
additional advantage.
EXPERIMENTAL
All organic reagents were commercial products of
the highest purity available and they were used in the
reactions without further purification.
RESULTS AND DISCUSSION
Characterization of the Hyper-Cross-Linked Polymer
with IL Structures
Synthesis of the Hyper-Cross-Linked Polymer
with IL Structures
In a typical experiment, 4-vinylpyridine (1.05 g,
10 mmol) and p-xylylene dichloride were dissolved in
1,2-dichloroethane (DCE, 20 mL). The solution was
stirred for 4 h at 75°C and the mixture became turbid.
Then SnCl4 (1 mL) was added to the mixture and
In order to improve the IL loading amount and
BET surface, the ILs structures were incorporated into
the polymeric framework. The quaternization of vin-
ylpyridine and p-xylylene dichloride was taken to gen-
reacted at 75°C for another 12 h. The brown gel was erate the IL structures. Then, the condensation of the
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842
JUNQIAO LI et al.
1 µm
1 µm
1 µm
(а)
(b)
(c)
Fig. 1. The SEM images of the polymer with different molar ratio.
ILs and excess p-xylylene dichloride were carried out confirmed the uniform pattern of the structure. The
proper choice of IL moieties and n(vinylpyri-
dine)/n(p-xylylene dichloride) molar ratio are of great
importance for the polymer structure. The ILs imbed-
ded in the polymer bring about electrostatic interac-
tions between particles, which restrict agglomeration
and decreases the alkylation reactivity of the particles.
As a result, the particles with well-defined spherical
shape are formed. With further increase in vinylpyri-
dine amount aimed to reach n(vinylpyridine)/n(p-
xylylene dichloride) = 1.0, particles become bigger
with sizes over 200 nm (Fig. 1c). In addition, the
spherical patterns are replaced by particles with a
square morphology. Some particles stick together to
form clusters without clear boundary. A number of
particles was divided into smaller entities with regular
spherical patterns. Half of the p-xylylene dichloride
was involved in quaternization reaction leading to a
low cross-linking degree of the polymer. Only half of
aromatic rings were alkylated. The hyper-cross-linked
structure was not properly formed. The BET analysis
showed a surface area of 25.5 m2/g, which further con-
firmed a low cross-linking extent of the polymer. On
preparing the polymer IL loading was increased to
4.09 mmol/g, but the most active sites were entrapped
in the interior due to a low surface area. These active
sites could not function during the reaction. Both the
IL amount and the surface area affected the catalytic
activity. Therefore, the polymer with n(vinylpyri-
dine)/n(p-xylylene dichloride) = 0.5 was chosen,
which combined a high amount of IL and a large sur-
face area.
using SnCl4 to form the hyper-cross-linked polymer.
The IL was derived from vinylpyridine and the IL
amount could be adjusted by varying the amount of
vinylpyridine. However, the reactivity of the aromatic
rings in ILs was much lower than p-xylylene dichloride
due to the electron-withdrawing effect of pyridinium
cations. Using excessive amounts of p-xylylene dichlo-
ride caused a significant self-condensation leading to
an uneven distribution of IL moiety in polymer. The
molecular ratio of the reactants greatly affects the
polymer morphology (Fig. 1). It appears that the self-
condensation of p-xylylene dichloride occurs when
the polymer was synthesized with n(vinylpyridine)/
n(p-xylylene dichloride) = 0.20. The presence of
excessive amounts p-xylylene dichloride resulted in
considerable self-condensation and formation of the
bulky blocks, which represent a typical structure of
poly p-xylylene dichloride (Fig. 1a). The polymeriza-
tion between ILs and p-xylylene dichloride yielded
polymer in small particles. Although the polymer has
a high surface area of 1048 m2/g, the ILs were distrib-
uted unevenly in the polymer. The self-condensation
of p-xylylene dichloride favors the development of
high surface area of the polymer. The amount of IL in
the polymer was only 1.02 mmol/g. On the other
hand, a prevailing fraction of the surface area was
inactive, and could not be involved in catalytic reac-
tions. By contrast, the polymer synthesized using
n(vinylpyridine)/n(p-xylylene dichloride)
= 0.50
showed a uniform particle-size distribution (Fig. 1b).
The polymerization produced regular spherical parti-
cles with the diameter of about 50 nm without bulky
blocks, which indicated that the ILs were uniformly
incorporated into the polymeric framework. The
nitrogen sorption–desorption analysis indicated a
high surface area (667 m2/g), which could provide suf-
The FT-IR of the polymer with IL structures was
also carried out (Fig. 2). The bands at 2921–3012 cm–1
are due to stretching vibrations of Ar–H originated
from abundant aromatic rings in polymer. The strong
absorption at 1577 cm–1 was assigned to N–C stretch-
ficient space for mass transfer during the catalytic pro- ing vibrations in pyridinium IL indicating that the
quaternization of vinylpyridine smoothly occurred
during the synthetic process. The FT-IR also showed
the strong absorption at 1421 cm–1, which belonged to
the C=C bonds. In order to distinguish the double
cess. The IL moieties were easily accessible to reac-
tants when a high loading (2.87 mmol/g) was used and
the polymer with high surface area was produced. In
the agglomerates composed by spherical particles,
every particle showed a clear boundary, which further bonds in aromatic rings from those in vinylpyridine
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SYNTHESIS AND CATALYTIC ACTIVITY
the iodine value analysis was performed. The results
843
showed that the residual contains nearly no vinyl
groups. The results confirmed that the polymerization
and alkylation was well-completed using SnCl4 cata-
lyst to form the hyper-cross-linked structures. The
C–H stretching vibration at 1137 cm–1 confirmed the
presence of alkyl groups from p-xylylene dichloride
and vinylpyridine, which is another evidence that the
quaternization and alkylation occurred smoothly. In
spectra of the polymer the multiple peaks at 860, 689,
776, 648 and 619 cm–1 assignable to the multi-substi-
tuted structures of the aromatic rings were observed.
The presence of multi-substituted aromatic rings also
confirmed the hyper-cross-linked structures, which
was essential for the development of pore structure.
The resulting polymer with a high surface area greatly
improved the mass transfer efficiency.
Ar–N
C–N
C=C
C–N
4000 3500 3000 2500 2000 1500 1000 500
Wavenumbers, cm–1
Fig. 2. The FT-IR spectrum of the polymer with IL struc-
The elemental analysis gave the following results.
Found: C 79.25%, H 6.57%. Calculated for IL struc-
ture: C 79.20%, H 6.60%. The agreement between the
experimentally found and calculated values for the
structure shown in Scheme provides another evidence
that the quaternization and alkylation proceed satis-
factorily under the conditions chosen for the synthesis.
The hyper-cross-linked structures were successfully
formed to fabricate the porous polymers with IL
incorporated in the framework, which was beneficial
for the catalytic activity. The value of the polymer den-
sity (0.6 g/cm3) is also indicative of the formation of
well-developed pore structure. The polymer showed
high thermal stability with the degradation occurring
at temperatures beyond 280°C.
tures.
tively prevented a further addition to form the multi-
substituted product. On the other hand, the multi-
substituted product could be obtained using an
increased amount of methyl acrylate at a higher tem-
perature. Also, the diamine could be transformed
using the polymer. Both amino groups of ethylenedi-
amine were attacked by methyl acrylate to give 98.1%
yield in 11 min (entry 5). With hydroxylamine the
product was obtained in a good yield (96.7%) after
13 min, which indicated that the functionalities such
as hydroxyl groups remained stable during the reaction
(entry 6). The cyclic amines showed a high reactivity
due to low steric obstacles caused by the presence of
the ring structure (entries 7, 8). The presence of elec-
tron deficient alkenes influences the reaction effi-
ciency. Ethyl acrylate showed a lower reactivity than
methyl acrylate (entries 9–12). A negligible increase in
the steric obstacles caused by the presence of the ter-
minal vinyl structures accounts for weakly decreasing
yields. Although high steric hindrances could be
expected with dimethyl maleate, the polymer showed
high activity yielding 93.2% of diisopropylamine
(entry 13). Methyl piperazine showed a high yield of
98.7% in 8 min (entry 14). Due to the presence of the
piperazine ring and the ensuing electron donating
effect the cyclic amine showed high reactivity to
dimethyl maleate with an observed yield of 98.6%
achieved within 7 min. By using the polymer high activ-
ity was also observed for butyl maleate. Nevertheless, the
yields also decreased with increasing steric hindrances
caused by the presence of amines (entries 15–17). For
methyl piperazine with lower steric obstacles a high yield
of 96.7% was obtained within 11 min (entry 17). With
diisopropylamine a yield of 91.2% was observed after
21 min providing additional evidence of the high activ-
ity of the polymer (entry 16). Involvement of alkenes
The Catalytic Activity for aza-Michael Addition
The catalytic activities of the polymer were investi-
gated via the aza-Michael additions of amines with
electron deficient alkenes (Table 1). The reaction con-
dition was optimized to obtain the desired productsin
the highest yield using diisopropylamine and dibutyl
maleate as model reactants. The polymer showed very
high activity for the reactions with high yields obtained
in short reaction times. With dimethylamine signifi-
cant steric hindrance cannot be expected and in reac-
tion with methyl acrylate it was highly reactive with
99.2% yield achieved within 5 min (entry 1). The reac-
tivity decreased when more bulky molecule of diethyl-
amine was used (entry 2). A further reduction in the
yield was observed in the reaction between diisopro-
pylamine and methyl acrylate (entry 3). Nevertheless,
when the reaction time was increased to 15 min a high
yield of 95.6% was obtained. The primary amine was
efficiently transformed using the polymer. Ethylamine
showed a yield of 98.5% within 6 min (entry 4). The
single substituted product was formed under mild
reaction conditions. A steric obstacle arising from the
presence of the mono-substituted product was much also affected the yields (entries 18–21). For acryloni-
higher than in the case of ethylamine, and that effec- trile low steric hindrances can be expected and diiso-
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844
JUNQIAO LI et al.
Table 1. The aza-Michael addition of various amines with electron deficient alkenes.
R1
R2
EWG
R1
R2
EWG
cat, R.T.
NH +
N
Yield, % a, b
99.2
Entry
Amine
Alkene
Reaction time, min
H
N
1
2
5
8
COOCH3
COOCH3
98.5
95.6
H
N
3
15
COOCH3
N
98.5
98.1
4
5
6
NH2
NH2
HO NH2
COOCH3
COOCH3
11
H2N
96.7
99.4
6
7
13
4
COOCH3
COOCH3
N
N
99.3
8
4
O
N
COOCH3
98.7
97.4
94.8
H
N
9
9
COOC2H5
COOC2H5
10
13
H2N
NH2
N
11
12
13
17
5
COOC2H5
COOC2H5
N
N
99.2
93.2
N
23
COOCH3
H3COOC
98.6
14
15
7
N
N
N
N
COOCH3
COOC4H9
H3COOC
C4H9OOC
95.2
91.2
H
N
18
16
27
COOC4H9
COOC4H9
C4H9OOC
C4H9OOC
N
96.7
17
18
11
4
99.3
99.1
H
N
CN
19
20
21
5
7
3
CN
CN
CN
N
98.5
99.4
H2N
NH2
N
N
a
b
The reaction conditions: amino groups 20 mmol, alkene 24 mmol, catalyst 50 mg, RT (25°C).
The yield was calculated on GC using an internal standard.
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SYNTHESIS AND CATALYTIC ACTIVITY
Table 2. The comparison of different catalysts a, b
845
Catalyst
Catalyst amount, mg
Reaction time, min
Yield, %
Polymer
IL
50
100
120
300
27
80
91.2
84.3
82.3
70.5
[SO3H-Bmim][HSO4]
Amberlyst-15
110
210
a
The reaction conditions: amino group 20 mmol, alkene 24 mmol, catalyst 50 mg, RT (25°C).
The yield was calculated on GC using an internal standard.
b
propylamine was obtained in 99.1% yield within 5 min
(entry 19).
Comparative Study of Different Catalysts
The catalytic activities of different catalysts were
compared in the reactions between diisopropylamine
and dibutyl maleate where the effect of steric hin-
drance is highly probable (Table 2). The IL from the
quaternization of pyridine and benzyl chloride was
also used for the reaction. Table 2 indicates that the
polymer described in this work was the most efficient
catalyst for the reaction. The sample of IL containing
the same active sites as the polymer was very active
with a yield of 84.3% achieved within 80 min. It
appears that the IL readily dissolved in the reaction
system favors interactions between reactants. How-
ever, the separation of the IL from the reaction system
was quite difficult. As to the traditional acidic IL
[SO3H-Bmim] [HSO4], the catalytic activity was even
lower than that of IL. It can be suggested that strong
acid sites of IL easily enter the interaction with amines
to form ammonium salt. As a result, the acid IL
showed lower activity. The acidic resin Amberlyst-15
was also used for the reaction. However, the activity
was low due to high steric hindrance caused by the
reactants. The resin gave 70.5% yield with an
increased catalyst amount and longer reaction time.
Such behavior can be accounted for by suggestion
that the major part of active sites was inaccessible for
the reactants. The reason for inaccessibility of active
sites can be sought in a low surface area of the resin.
Therefore, a higher catalyst amounts were needed to
provide an additional portion of accessible active
sites for reactants. The polymer showed a high sur-
face area (667 m2/g) and hydrophobicity of the sur-
face. A combination of active sites embedded in the
framework and a well-developed surface provide high
rates of mass transfer. The hydrophobic surface
showed high affinity to ractants. The reactants accu-
mulated in the polymer promoted the desired reac-
tion. The active sites show a proper activity for the
reaction readily activating alkenes to complete the cat-
alytic cycle. All these factors can be taken into account
to explain, why the highest activity was observed with
the polymer.
Activity of the Recycled Polymer
In view of the possible use of the polymer as a cat-
alyst it was desirable to explore catalytic activity of the
recycled polymer. After reactions, the polymer was
recycled by filtration. The mass loss of the polymer
was quite low with the recycled yield over 90%. The
catalyst loss was caused predominantly by absorption
of the filter paper. The polymer loss was taken into
account and the amount of reactants was correspond-
ingly reduced to maintain the same reaction condi-
tions. The activity of the recycled polymer was investi-
gated in the reaction between diisopropylamine and
dibutyl maleate. The results showed that the polymer
activity remained stable with 90.3% yield after six
cycles whereas the yield with the fresh catalyst was as
high as 91.2%. In addition, it was observed that the
yields in the presence of the recycled and fresh cata-
lysts after 10 min reaction time were comparable. After
six cycles the polymer gave a yield of 65.3%, while with
a fresh polymer 66.3% yield was recorded. These
results provide another evidence that the polymer
recycled for six times retains considerable portion of
activity indicating high stability of the polymer.
Hyper-cross-linked polymeric network with IL struc-
ture incorporated in the framework provide active sites
easily accessible for reactants. The ILs embedded in
the polymer preserve at least two chemical bonds,
which effectively preclude the loss of active sites. Fur-
thermore, due to a high surface area negligible mass
transfer limitations and correspondingly insignificant
pore blockage can be expected. It can be thus inferred
that after multiple recyclization the active sites in the
polymer are still accessible for reactants. The surface
area of the polymer after six cycles of operation was
found to be 651 m2/g, fairly similar to that of the fresh
catalyst (667 m2/g). It is therefore evident that the pore
structure remains nearly intact and easily accessible
active sites are available in the recycled polymer. The
FT-IR also showed that the same bands characterize
recycled and fresh polymers. All these results con-
firmed the high stability of the polymer.
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846
JUNQIAO LI et al.
4. Baig, R.B. and Varma, R.S., Green Chem., 2013,
CONCLUSIONS
vol. 15, p. 1839.
The novel hyper-cross-linked polymer with the IL
5. Watanabe, H., Carbohydr. Polym., 2010, vol. 80,
structure was synthesized via the quaternization and
condensation of vinylpyridine and p-xylylene dichlo-
ride. Since ILs were incorporated into the polymeric
framework the pore blockage could be avoided and the
active sites were accessible for reactant. The polymer
containing 2.87 mmol/g of IL showed a high surface
area (667 m2/g). The polymer was very active in aza-
Michael additions with the average yields over 95%
achieved within several minutes. In terms of activity, the
polymer is superior to conventional ILs and has an
advantage of simple recovery. High activity, a well
developed surface and reasonable stability are the key
advantages of the polymer that has the potential replace
homogeneous catalysts in green chemical processes.
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