Polystyrene Nanometer-Sized Particles Supported Alkaline Imidazolium Ionic Liquids as Reusable and Efficient Catalysts for the Knoevenagel Condensation in Aqueous Phase

Article abstract of DOI:10.1007/s10562-017-2255-6

Abstract: Polystyrene nanometer-sized particles supported alkaline 1-propyl imidazolium ionic liquids catalysts (nano-PS-CH₂-[pIM][B]) were prepared and characterized by FT-IR, SEM, TG/DTA, BET and particle size distribution analysis. The resul

Full text of DOI:10.1007/s10562-017-2255-6

Catalysis Letters
https://doi.org/10.1007/s10562-017-2255-6
Polystyrene Nanometer-Sized Particles Supported Alkaline
Imidazolium Ionic Liquids as Reusable and Efficient Catalysts
for the Knoevenagel Condensation in Aqueous Phase
Min Ma1 · Hansheng Li1 · Wang Yang1 · Qin Wu1 · Daxin Shi1 · Yun Zhao1 · Caihong Feng1 · Qingze Jiao1,2
Received: 7 September 2017 / Accepted: 17 November 2017
© Springer Science+Business Media, LLC, part of Springer Nature 2017
Abstract
Polystyrene nanometer-sized particles supported alkaline 1-propyl imidazolium ionic liquids catalysts (nano-PS-CH2-[pIM]
[B]) were prepared and characterized by FT-IR, SEM, TG/DTA, BET and particle size distribution analysis. The results
suggested that ionic liquids were successfully loaded on the surface of polymer carriers by covalent bond with high alkali
amount. It showed excellent catalytic activities for Knoevenagel condensation especially in aqueous phase, which much
higher than that of NaOH and micro-PS-CH2-[pIM][B]. It will be expected for the potential application of organic reactions
in aqueous phase.
Graphical Abstract
Keywords Nanometer size · Supported ionic liquids · Knoevenagel condensation · Aqueous phase
1 Introduction
Ionic liquids (ILs) have increasingly attracted interest as
being green solvents and catalysts over the last few decades,
mainly due to their excellent properties such as negligible
* Hansheng Li
hanshengli@bit.edu.cn
Extended author information available on the last page of the article
Vol.:(0123456789)
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M. Ma et al.
vapor pressure, high thermal stability and high reaction
activity [1]. In the field of catalysis, ILs can be designed
into various structures, which present more opportunities
to design and optimize reaction systems [2, 3]. It has been
proved that ILs not only enhance the reaction rate to a great
extent in many reactions, such as alkylation [4], hydrogena-
tion [5], carbonylation [6], polymerization [7], and enzyme
catalysis [8], but also improve the selectivity of the objec-
tive products. However, similar to other homogeneous cata-
lysts, ILs are still hampered by several practical drawbacks,
namely, catalyst recovery and product purification. Thus,
supported ionic liquids (SILs) have been generated to over-
come problems mentioned above [9]. It is SILs that ILs
covalent on the solid supports like molecular sieves [10],
mesoporous silica gels [11], polymers [12, 13], carbon mate-
rials [14, 15] and magnetic materials [16], etc. SILs have
been extensively evaluated for their attractive features that
combine the benefits of ILs with advantages of heterogene-
ous carriers for designability, ease of handling, separation
and recycling [17]. Furthermore, it is desirable to minimize
the amount of the utilized ionic liquid in a potential process
[18].
to catalyze the Knoevenagel condensation of benzalde-
hyde and ethyl cyanoacetate. Furthermore, polystyrene
micrometer-sized particles supported alkaline 1-propyl
imidazolium ionic liquids (micro-PS-CH2-[pIM][B]) was
prepared by suspension polymerization in comparison
with nano-PS-CH2-[pIM][B] we concerned about. Atten-
tion was concentrated on the catalytic activities of these
catalysts for Knoevenagel condensation in aqueous phase.
2 Experimental
2.1 Materials
Styrene (St, > 98%), divinylbenzene (DVB, 95%, mixture
of m- and p-isomers), vinyl benzyl chloride (VBC, 95%),
and hexadecane (HD) were purchased from J&K Scientific
Ltd (Beijing, China). N-propyl imidazole, sodium hydrox-
ide (NaOH), potassium persulfate (K2S2O8), polyvinyl
pyrrolidone (PVP), ethyl cyanoacetate, methylbenzene,
2,2-azobisisobutyronitrile (AIBN), methanol and other
chemicals (AR grade) were obtained from Beijing Chemi-
cal Works (Beijing, China). St and VBC were distilled
under reduced pressure and stored in a freezer, AIBN was
recrystallized, other chemicals were used as received.
Particularly, polymer supported ionic liquids (PSILs) are
a series of attractive catalysts for their corrosion resistance to
acid and alkali, excellent stability and modifiability [19]. At
present, PSILs in the catalytic field are mainly concentrated
in two aspects. One is PSILs modified with acidic groups,
and applied for a variety of organic reactions as acidic cata-
lysts [20–23]. The other is ILs with transition metal catalyst
immobilized on polymer supports to improve the catalytic
efficiency and ameliorate stability of metal catalysts [24–30].
The previous reports have shown that polystyrene resin sup-
ported acidic ionic liquid catalysts are potential candidates
for catalyzing organic reactions like CO2 cycloaddition reac-
tion [31], Henry reaction [32], acetalization reaction [33]
and Fischer esterification [34], etc. Nevertheless, most of
their polymer carriers are designed to be amorphous, mas-
sive or micrometer-sized particles for easy recycling, but
their catalytic performances need to be further enhanced.
Provided the recovery of the catalysts, it is foreseeable that
the PSILs with nanometer-sized polymer carrier are more
likely to express excellent catalytic properties due to their
higher specific surface area, more active sites and better
dispersion.
2.2 Synthesis of PS‑CH2Cl Particles
2.2.1 PS-CH2Cl Nanometer-Sized Particles (PS-CH2Cl NPs)
The miniemulsion polymerization of cross-linked PS-
CH2Cl NPs (Scheme 1) was derived from a previous study
of styrene polymerization [35], but the recipe has been
changed based on a number of experiments. Firstly, a solu-
tion of 0.048 g of surfactant (SDBS) in 45 g of deionized
was prepared, in the meantime, 6 g of monomers mixture
(volume ratio of VBC:DVB:St = 15:4:1) with 0.325 mL
of HD were blended and stirred for 20 min. Then, two
phases were mixed for 1 h at 300 r min−1 for pre-emul-
sification. After that, the resultant mixture was sonicated
for 10 min at 400 kW by using a Scientz-IID ultrasonic
processor and turned into a white milk-like miniemulsion.
The obtained miniemulsion was placed in a 250 mL four-
necked flask, purged with the nitrogen for 30 min, and
then heated to 70 °C. The polymerization was initiated by
adding 0.12 g of KPS. The reaction usually continued for
4 h under a slow stirring. Finally, the product was filtered,
washed repeatedly with ethanol and deionized water and
dried under vacuum at 60 °C, and the white powder was
obtained.
The present work described in detail the preparation
and catalytic performances of polystyrene nanometer-sized
particles supported alkaline 1-propyl imidazole ionic liq-
uids (nano-PS-CH2-[pIM][B]). The cross-linked chlorome-
thyl functionalized polystyrene nanometer-sized particles
(PS-CH2Cl NPs) were prepared by miniemulsion polym-
erization. During the synthesis of PSILs, OH−, Ac− and
Im− as basic anions were introduced by ion exchange sepa-
rately. These three types of basic catalysts were applied
1 3

Polystyrene Nanometer-Sized Particles Supported Alkaline Imidazolium Ionic Liquids as…
Scheme 1 The procedure of miniemulsion polymerization
2.2.2 PS-CH2Cl Micrometer-Sized Particles (PS-CH2Cl MPs)
In order to study the effect of the polymer particle size on
the catalytic performance, PS-CH2Cl MPs were synthesized
by the polymerization of conventional free radical suspen-
sion according to the previous report as a comparison [36].
Firstly, 1.0 g of PVP, 3.0 g of sodium chloride were intro-
duced into a round-bottomed flask containing 100 mL of
deionized water, and stirred to become a transparent solu-
tion. Then, 6 mL of the monomer mixture (volume ratio of
VBC:DVB:St=15:4:1), and 0.5 g of AIBN were dissolved
in 7.5 mL of toluene, introduced into a beaker and stirred
to be homogeneous. Two solutions mentioned above were
Scheme 2 The procedure for preparing PS-CH2-[pIM][B] catalyst
blended, the polymerization of conventional free radical
suspension started at 75 °C and continued for 8 h under the
protection of nitrogen atmosphere. Finally, the synthesized
PS-CH2Cl MPs were filtered and washed with hot water and
methanol for three times to remove the dispersant PVP, the
product was dried under vacuum at 50 °C for 8 h at last.
in a vacuum at 50 °C, the PS-CH2-[pIM][B] catalyst was
finally obtained.
2.4 Analytical Methods
Fourier transform infrared spectrometry (FTIR) analysis
was conducted by using Shimadzu IR Affinity-1 s in a fre-
quency range of 4000–450 cm−1 with anhydrous KBr as
dispersing agent. Thermogravimetric and differential ther-
mal analysis (TG/DTA) was performed using WCT-1D by
heating the samples under N2 atmosphere with a heating
rate of 10 °C min−1 from room temperature up to 700 °C.
Scanning electron microscopy (SEM) images were obtained
with a JSM-7500F at 5 kV. The size distribution of par-
ticles was recorded on a Malvinic Matsersizer 2000. The
specific surface values and pore volume were obtained from
the BELSORP-max analyzer at 77 K. In addition, the alkali
content of the prepared catalyst was determined by the titra-
tion method as follows. A certain volume of 0.020 mol L−1
of benzoic acid in ethanol and 0.1 g of the catalyst were
stirred at room temperature for 6 h and kept filtrate, then
0.01 mol L−1 NaOH solution was used to titrate the filtrate
containing the phenolphthalein indicator. When the color of
2.3 Synthesis of PS‑CH2‑[pIM][B] Catalysts
The process for preparing PS-CH2-[pIM][B] (B: basic anion)
was shown in Scheme 2. Firstly, 1 g of PS-CH2Cl particles
were suspended in 40 mL of toluene and ultrasonicated for
1 h for well swelling. Secondly, N-propyl imidazole (triple
molar equivalent of chloromethyl) was added into the sus-
pension liquid, and the mixture was heated to 90 °C for 24 h.
The product was washed with toluene, ethanol and methanol,
successively, and dried under vacuum at 50 °C for 8 h. The
resultant product was named as PS-CH2-[pIM][Cl]. Finally,
the PS-CH2-[pIM][Cl] was dispersed in a sodium salt solu-
tion (NaOH, NaIm, or NaAc) with toluene-methanol mixed
solvent with the volume ratio of toluene to methanol of 1:3,
followed by strring it at room temperature for 6 h. This pro-
cess of ion exchange was repeated three times. After being
washed with water and methanol, successively, and dried
1 3

M. Ma et al.
filtrate changed, the volume of the NaOH solution consumed
was recorded and the alkali content of the catalyst was cal-
culated in terms of the concentration of the known standard
solutions and the mass of the catalyst.
2.5 Evaluation of Catalytic Performances
The catalytic performances of the catalysts prepared were
evaluated by the Knoevenagel condensation of benzaldehyde
with ethyl cyanoacetate. The amount of the catalyst was nor-
mally 0.2% of benzaldehyde molar amount in the reaction.
The Knoevenagel condensation of benzaldehyde with
ethyl cyanoacetate is irreversible, as such, the catalytic
performances of the catalysts were evaluated by using the
conversion of benzaldehyde. The typical procedure was per-
formed as follows. Reactants, benzaldehyde (10 mmol) and
ethyl cyanoacetate (10 mmol) were poured into a 30 mL
glass reactor containing 4 mL of solvent. The prepared cata-
lyst was then added into this solution and the reaction was
carried out at 60 °C. The agitation rate was kept at 300 rpm
to ensure catalysts contact adequately with reactants. Sam-
ples were analyzed with high performance liquid chroma-
tography (HPLC) equipped with an ultraviolet photometric
detector (with a wavenumber of 254 nm). A Kromat C18
reversed phase silica gel column was used for test and the
column temperature was maintained at room temperature.
The mobile phase consisted of methanol and deionized water
with a volumetric ratio of 9:1 at a flow rate of 0.3 mL min−1.
In recycling experiments, catalysts were recovered by fil-
tration through a 0.44 µm organic nano-filtration membrane,
washed with ethyl acetate, ethanol and solvent, successively
and then used in next time.
Fig. 1 FT-IR spectra of PS-CH2-Cl, PS-CH2-[pIM][Cl] and PS-CH2-
[pIM][OH]
observed from the absorption curves of all samples. From
the infrared absorption spectrum of the PS-CH2Cl sample, it
can be seen that the stretching vibration absorption peak of
C–Cl bonds at 671 cm−1 and the in-plane bending vibration
of C–H bonds in the methylene group linked to the chlorine
atom at 1265 cm−1, which indicated the presence of chloro-
methyl styrene structural units. However, they disappeared
in the infrared absorption spectra of PS-CH2-[pIM][Cl] and
PS-CH2-[pIM][B] samples, along with the appearance of
absorption peaks at 1560 and 1157 cm−1, which correspond-
ing to the C=C and C=N vibrations of imidazole moiety,
respectively. These proved that ILs were successfully loaded
onto cross-linked polymer particles.
Independent and complete spherical morphology of cat-
alysts were shown in Fig. 2. Nano-PS-CH2-[pIM][B] pre-
sented a particle size distribution ranging from 50 to 300 nm
and their average particle size of 137 nm, while they turned
to be 50–300 and 147 µm respectively for the micro-PS-
CH2-[pIM][B] (Fig. 2d). The miniemulsion polymerization
used in here is advantageous in a high polymerization rate
due to no existence of monomer diffusion phenomenon.
The particle size of the prepared particles mainly depends
on the size of the monomer droplets, and generally dis-
tributes between 50 and 500 nm [38, 39]. It explains the
narrow particle size distribution for the prepared particles.
The particle size distribution of the PS-CH2Cl particles
prepared in this work was relatively larger mainly due to
the high content of DVB. It has been proved by previous
report [40] that the increasing amount of the crosslinking
agent led to a wider size distribution. In comparison with
PS-CH2-Cl particles, the introduction of ILs onto polymer
particles made their surface smoother. In comparison with
fresh PS-CH2-[pIM][OH] (Fig. 2b, c), the morphology of
3 Results and Discussion
3.1 Characterization of Catalysts
FT-IR analysis was carried out to confirm the immobiliza-
tion of basic ILs on cross-linked PS-CH2Cl particles, as
shown in Fig. 1. The characteristic peaks of benzene ring
exhibited the C=C skeleton vibration at 1600, 1510 and
1450 cm−1, the stretching vibration of C–H at 3024, 3057
and 3082 cm−1, and the in-plane bending vibration of C–H
at 1493 cm−1. The absorption peak at 835 cm−1 corresponds
to the out-plane vibration of C–H bonds on the 1,4-disubsti-
tuted benzene ring, indicating the presence of para-disubsti-
tuted structural units in aromatic compounds in the sample.
In addition, there are two strong absorption peaks at 2922
and 2850 cm−1, which belong to the stretching vibration of
C–H bonds in methylene groups, and the weaker absorption
peak at 1361 represents its in-plane bending vibration [37].
These classical absorption peaks mentioned above were
1 3

Polystyrene Nanometer-Sized Particles Supported Alkaline Imidazolium Ionic Liquids as…
Fig. 2 SEM images of PS-CH2-Cl NPs (a), nano-PS-CH2-[pIM][OH] (b) and micro-PS-CH2-[pIM][OH] (c), and size distribution of PS-CH2-
[pIM][B] particles (d), nano-PS-CH2-[pIM][OH] (e) and micro-nano-PS-CH2-[pIM][OH] (f) reused for eight times
reused nano-PS-CH2-[pIM][OH] (Fig. 2e) had no obvious
change, which attributed to its high cross-linked structure.
However, micro-PS-CH2-[pIM][OH] (Fig. 2f) was destroyed
after recycling even same amount cross-linked agent was
used, which affected its catalytic performance to a little bit
improvement in second reuse.
of Cl were lower than theoretical value, which indicated that
the actual monomer ratio in the polymer was little differ-
ent from monomer feed ratio. ILs section distributed on the
surface of PS from the increasing element content of N, it
demonstrated that ILs were immobilized on cross-linked
polymer particles and ILs loading ratio is over 85%.
EDS reflected the relative elements contents (N, Cl) on
the surface of PILs as shown in Table 1. Three places were
selected for the measurement. The relative elements contents
TG-DTA analysis was carried out to investigate the ther-
mal stability of PS-CH2-Cl and PS-CH2[pIM][B] catalysts,
as shown in Fig. 3. The weight loss was both observed
1 3

M. Ma et al.
Table 1 Element amount of
PS-CH2Cl and PS-CH2-[pIM]
[Cl]
PS-CH2Cl
VTheo
PS-CH2-[pIM][Cl]
Nano
Micro
VTheo
Nano
Micro
Cl (atom%)
N (atom%)
7.19
4.56
–
5.19
3.73
–
5.09
3.56
–
0
9.13
4.91
0
7.92
4.27
0
7.50
4.04
ILs loading amount
(mmol g−1
)
Fig. 3 TG-DTA patterns of PS-CH2-Cl and PS-CH2-[pIM][OH]
from PS-CH2-Cl and PS-CH2[pIM][B] below 200 °C, it
is attributed to the physically adsorbed solvent, and the
hydrophilic structure of ILs facilitated PS-CH2[pIM][B]
absorption of more solvent. The introduction of ILs led the
temperature of initial weight loss of the product rising up
to nearly 270 °C. The weight loss at this temperature is due
to the breakage of the bonds of imidazolium to the carrier.
When the temperature further increased up to higher than
350 °C, the weight loss was clearly observed in the curves
of both PS-CH2Cl and PS-CH2[pIM][B] which correspond
to the fracture and cracking of main polymer chains. Until
around 600 °C, there was no longer weight loss, remain-
ing nothing but residual carbon. Therefore, it suggested
that the catalyst can be used under 200 °C. There were no
obvious differences among PS-CH2[pIM][B] catalysts, but
a tiny disparity of second peak of DTA curve in Fig. 3b. It
showed that PS-CH2-[pIM][OH] lost weight at the lowest
temperature, while PS-CH2-[pIM][Ac] lost weight at the
highest temperature. It possibly owned to the interaction
between the anion and carrier.
Table 2 BET characterization data for PS-CH2Cl
Sample
BET SA
(m2 g−1
BJH Vp
BJH Dp (nm)
)
(cm3 g−1
)
Nano-PS-CH2Cl 21.72
Micro-PS-CH2Cl <1
0.092
–
2.59
–
Table 3 Akali amount of PS-CH2-[pIM][B]
Catalysts
Alkali amount
(mmol g−1
)
Nano-PS-CH2-[pIM][OH]
Nano-PS-CH2-[pIM][Ac]
Nano-PS-CH2-[pIM][Im]
Micro-PS-CH2-[pIM][OH]
1.1012
1.2010
1.2133
0.6811
The alkali amount of the PS-CH2-[pIM][B] was car-
ried out by the titration method and results were shown
in Table 3. Overall, nano-PS-CH2-[pIM][B] have a higher
alkali amount, more than 1 mmol g−1. It is known that the
polymer carriers are porous [36], and there exists a diffu-
sion process of basic anions during the ion exchange step.
The smaller the diffusion resistance is, the more conducive
to ion exchange is. Compared to micrometer-sized carrier,
BET reflected the pore structure of PS-CH2Cl as shown
in Table 2, nano-PS-CH2Cl with a specific surface area more
than 20 m2 g−1 and 2.59 nm of pore diameter which can
be considered as the channel of nanoparticles accumulation
rather than the pores of the particles themselves. The specific
surface area of micro-PS-CH2Cl is too small to measure.
1 3

Polystyrene Nanometer-Sized Particles Supported Alkaline Imidazolium Ionic Liquids as…
nanometer-sized carrier has a higher specific surface area,
and its diffusion resistance is greatly reduced or even neg-
ligible for its pores, so that more bases were supported.
Notably, the basic ion exchange is an important step in the
preparation process; moreover, multiplying the ion exchange
process plays a more significant role than extending time.
sites is conducive to the conversion of reactants. However,
the conversion tends to a balance, when the catalyst amount
exceeds a certain limit. Overall, these catalysts were efficient
in this reaction system.
Meanwhile, the reusability of PS-CH2-[pIM][B] catalysts
was evaluated by catalyzing Knoevenagel condensation in
methanol (Fig. 4a) and aqueous phase (Fig. 4b), respectively.
Catalysts were recovered by filtration through a 0.44 µm
organic nano-filtration membrane. It’s obvious that micro-
PILs is recovered quickly than nano-PILs, but both of them
could be recovered with negligible loss, which was reflected
by yield in recycling experiment. In methanol phase, the cat-
alytic performance of micro-PS-CH2-[pIM][OH] improved
in the second time, which mainly because it cracked into
smaller particles and decreased diffusion resistance. Eventu-
ally, its catalytic activity decreased 4.13%. Comparatively,
that of the nano-PS-CH2-[pIM][OH] decreased 9.36%. Even
so, the loss in catalytic activity reached 5.47% for nano-
PS-CH2-[pIM][Im] and 4.83% for nano-PS-CH2-[pIM]
[Ac] which exhibited an excellent stability and reusability
of nano-PS-CH2-[pIM][B].
3.2 Catalytic Performances of PS‑CH2‑[pIM][B]
Catalysts
To evaluate the PS-CH2-[pIM][B] described here, the cata-
lytic activities for Knoevenagel condensation were exam-
ined under the same reaction conditions as shown in Table 4.
Obviously, the carrier PS-CH2-Cl showed almost no catalytic
activity, because the yield was nearly similar to the sample
without catalyst. In addition, PS-CH2-[pIM][Cl] showed a
weak catalytic activity for the weak Lewis basic strength
of Cl−, while PS-CH2-[pIM][B] showed a better catalytic
activity. It means that the reaction was mainly catalyzed by
the loaded basic anions. The catalytic activity of catalysts
with different alkali anions was various. It was obvious that
nano-PS-CH2-[pIM][OH] had a higher catalytic efficiency
with 94% of yield, as well as the homogeneous catalyst
NaOH. The catalytic activity of nano-PS-CH2-[pIM][Ac]
was relatively lower, which is related to the base strength.
However, Im− showed a bit lower catalytic efficiency than
OH−, although its base strength is theoretically stronger than
that of OH−. It is probably due to the volume of Im−, which
is too large to hinder the diffusion of reactants. The catalytic
efficiency of micro-PS-CH2-[pIM][OH] was no better than
that of nano-PS-CH2-[pIM][OH], because of its greater dif-
fusion resistance. Additionally, it was found that a higher
catalyst loading corresponds to a higher catalytic efficiency
(Samples 4–7 in Table 4), and increasing catalytic active
Reactants and products are easily soluble in methanol
and the catalyst disperses well in methanol, so that they well
contacted with each other during the whole reaction process,
and the reaction achieved a relatively high yield. However,
the situation changes greatly when the solvent was replaced
by water. Firstly, the reactants are hardly soluble in aqueous
phase, which results in insufficient contact between the reac-
tants and the catalyst. Secondly, water as one of the products
in the condensation inhibits the forward course of the reac-
tion, hence, the yield turns out to be very low, nearly 30%
without catalyst.
Surprisingly, the prepared nanometer-sized alkaline cat-
alysts exhibited an efficient catalytic performance for the
Table 4 Activities of various
Samples
Catalysts
Catalyst loadings Yeild (%)
(%)a
Selectivity (%)
catalysts for Knoevenagel
condensation
1
2
3
4
5
6
7
8
Blank
Nano-PS-CH2-Cl
0
–
–
75.25
74.98
77.01
86.88
93.99
94.58
94.95
84.12
89.62
88.19
92.73
100
100
100
100
100
100
100
100
100
100
100
b
b
Nano-PS-CH2-[pIM][Cl]
Nano-PS-CH2-[pIM][OH]
Nano-PS-CH2-[pIM][OH]
Nano-PS-CH2-[pIM][OH]
Nano-PS-CH2-[pIM][OH]
Nano-PS-CH2-[pIM][Ac]
Nano-PS-CH2-[pIM][Im]
Micro-PS-CH2-[pIM][OH]
NaOH
0.1
0.2
0.4
0.6
0.2
0.2
0.2
0.2
9
10
11
Reaction conditions: nbenzaldehyde = nethyl cyanoacetate = 0.1 mol, 4 mL of methanol as solvent, 60 °C, 4 h
analkali:nbenzaldehyde
bThe same weight as catalyst of sample 5
1 3

M. Ma et al.
Fig. 4 Recycling of PS-CH2-[pIM][B] for Knoevenagel condensation in methanol (a) and aqueous phase (b)
Knoevenagel condensation in aqueous phase. As shown
in Fig. 4b, nano-PS-CH2-[pIM][B] provided more excel-
lent catalytic activities. Especially for nano-PS-CH2-[pIM]
[OH], its yield even reached 74%, and kept over 63% after
reusing 8 times, which was much higher than that for NaOH
and micro-PS-CH2-[pIM][OH]. Nano-PS-CH2-[pIM][Ac]
showed a relatively lower but still more stable catalytic effi-
ciency, nevertheless, its catalytic efficiency reduced more
in aqueous than in methanol. It may be due to the more
ions exchanging in water so that the more active ions lose,
or, the insoluble product covered the surface of the catalyst
as a solid, which led to the decrease of catalytic efficiency.
As shown in Fig. 5a, the prepared nano-PS-CH2-[pIM]
[OH] evenly dispersed in the system with water as a sol-
vent. Based on the results above, a catalytic mechanism for
Knoevenagel condensation over the prepared nano-PS-CH2-
[pIM][B] in aqueous phase is proposed in Scheme 3. The
small catalyst particles played a role of a surfactant, and
presented in the oil–water interface, which allowed more
Fig. 5 Dispersion of nano-PS-CH2-[pIM][OH] (a) and micro-PS-
CH2-[pIM][OH] (b) for Knoevenagel condensation in aqueous phase
Scheme 3 Catalytic mechanism of nano-PS-CH2-[pIM][OH] for Knoevenagel condensation
1 3

Polystyrene Nanometer-Sized Particles Supported Alkaline Imidazolium Ionic Liquids as…
uniform and sufficient contact with substances. Furthermore,
they can be regarded as solid extractants, because of the
enrichment of active sites. Reactants tend to enrich on their
surfaces and convert, and then be catalyzed greatly. Based
on this explanation, the smaller the catalyst particles were,
the better the catalytic performance was. It also explains why
nano-PS-CH2-[pIM][B] were more efficient than micro-PS-
CH2-[pIM][B] in aqueous phase.
to micro-PS-CH2-[pIM][B] prepared by the suspension
polymerization, the nanometer-sized catalysts performed
charming reusability and stability due to the advantages of
their nanometer size. We believe that nano-PS-CH2-[pIM]
[B] catalyst will have a significant potential application for
organic reaction in aqueous phase.
Acknowledgements We are very grateful to the National Natural Sci-
ence Foundation of China (NNSFC) for the support (No. 21576025).
In order to further explore the catalytic performance
of the prepared nano-PS-CH2-[pIM][OH] for the conden-
sation reaction, the experimental group of nano-PS-CH2-
[pIM][OH] catalyzed Knoevenagel condensation in different
solvents were carried out (Table 5). It was found that the
greater the polarity of the solvent was, the higher the yield
was. mainly because polar solvents benefit ILs, the polar
parts of catalysts, fully stretching to contact with reactants,
and facilitates the transformation of the reactant structure.
Unfortunately, any important relationship between the swell-
ing of the polystyrene particles [41] and its catalytic perfor-
mance in Knoevenagel condensation has not be found yet.
Compliance with Ethical Standards
Conflict of interest The authors declare that they have no conflict of
interest.
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Polystyrene nanometer-sized particles supported basic imi-
dazolium ionic liquids were synthesized and their perfor-
mances were evaluated by the Knoevenagel condensation
of benzaldehyde with ethyl cyanoacetate. The cross-linked
PS-CH2Cl NPs with narrow particle size distribution were
prepared by miniemulsion polymerization, the ionic liquids
were successfully loaded on these carriers by covalent bond,
and their alkali amounts were more than 1 mmol g−1. The
prepared nano-PS-CH2-[pIM][B] catalysts exhibited a good
thermal stability and an excellent catalytic activity for Kno-
evenagel condensation both in methanol and aqueous phase.
Notably, the yield for nano-PS-CH2-[pIM][OH] catalyzing
Knoevenagel condensation in aqueous phase reached 74%,
and kept over 63% after eight times reusing. Compared
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Table 5 Nano-PS-CH
2-[pIM][OH] for Knoevenagel condensation in
different solvents
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Samples
Solvent
Yeild (%)
Selectivity (%)
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1
2
3
4
5
6
7
8
9
DMSO
DMF
Methanol
Ethanol
Water
Tetrahydrofuran
Methylbenzene
Cyclohexane
No solvent
96.42
93.95
93.99
92.15
74.73
74.88
46.14
33.14
66.67
100
100
100
100
100
100
100
100
100
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Affiliations
Min Ma1 · Hansheng Li1 · Wang Yang1 · Qin Wu1 · Daxin Shi1 · Yun Zhao1 · Caihong Feng1 · Qingze Jiao1,2
1
2
School of Chemistry and Chemical Engineering, Beijing
Institute of Technology, Beijing 100081, China
School of Chemical Engineering and Materials Science,
Beijing Institute of Technology, Zhuhai, Zhuhai 519085,
China
1 3