Honeycomb-structured solid acid catalysts fabricated via the swelling-induced self-assembly of acidic poly(ionic liquid)s for highly efficient hydrolysis reactions

Article abstract of DOI:10.1016/S1872-2067(20)63658-0

The development of heterogeneous acid catalysts with higher activity than homogeneous acid catalysts is critical and still challenging. In this study, acidic poly(ionic liquid)s with swelling ability (SAPILs) were designed and synthesized via the free radical copolymerization of ionic liquid monomers, sodium p-styrenesulfonate, and crosslinkers, followed by acidification. The ³¹P nuclear magnetic resonance chemical shifts of adsorbed trimethylphosphine oxide indicated that the synthesized SAPILs presented moderate and single acid strength. The thermogravimetric analysis results in the temperature range of 300–345 °C revealed that the synthesized SAPILs were more stable than the commercial resin Amberlite IR-120(H) (245 °C). Cryogenic scanning electron microscopy testing demonstrated that SAPILs presented unique three-dimensional (3D) honeycomb structure in water, which was ascribed to the swelling-induced self-assembly of the molecules. Moreover, we used SAPILs with micron-sized honeycomb structure in water as catalysts for the hydrolysis of cyclohexyl acetate to cyclohexanol, and determined that their catalytic activity was much higher than that of homogeneous acid catalysts. The equilibrium concentrations of all reaction components inside and outside the synthesized SAPILs were quantitatively analyzed using a series of simulated reaction mixtures. Depending on the reaction mixture, the concentration of cyclohexyl acetate inside SAPIL-1 was 7.5–23.3 times higher than that outside of it, which suggested the high enrichment ability of SAPILs for cyclohexyl acetate. The excellent catalytic performance of SAPILs was attributed to their 3D honeycomb structure in water and high enrichment ability for cyclohexyl acetate, which opened up new avenues for designing highly efficient heterogeneous acid catalysts that could eventually replace conventional homogeneous acid catalysts.

Full text of DOI:10.1016/S1872-2067(20)63658-0

Chinese Journal of Catalysis 42 (2021) 297–309
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h n j c
Article
Honeycomb‐structured solid acid catalysts fabricated via the
swelling‐induced self‐assembly of acidic poly(ionic liquid)s for
highly efficient hydrolysis reactions
Bihua Chena, Tong Dinga, Xi Denga, Xin Wanga, Dawei Zhang a,b, Sanguan Maa, Yongya Zhanga,
Bing Nic, Guohua Gaoa,*
a Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of Chemistry and Molecular Engineering, East China Normal University,
Shanghai 200062, China
b Department of Chemistry, University of Cambridge, Cambridge CB2 1EW, U.K.
c School of Life Sciences, East China Normal University, Shanghai 200241, China
A R T I C L E I N F O
A B S T R A C T
Article history:
The development of heterogeneous acid catalysts with higher activity than homogeneous acid cata‐
lysts is critical and still challenging. In this study, acidic poly(ionic liquid)s with swelling ability
(SAPILs) were designed and synthesized via the free radical copolymerization of ionic liquid mon‐
omers, sodium p‐styrenesulfonate, and crosslinkers, followed by acidification. The 31P nuclear mag‐
netic resonance chemical shifts of adsorbed trimethylphosphine oxide indicated that the synthe‐
sized SAPILs presented moderate and single acid strength. The thermogravimetric analysis results
in the temperature range of 300–345 °C revealed that the synthesized SAPILs were more stable
than the commercial resin Amberlite IR‐120(H) (245 °C). Cryogenic scanning electron microscopy
testing demonstrated that SAPILs presented unique three‐dimensional (3D) honeycomb structure
in water, which was ascribed to the swelling‐induced self‐assembly of the molecules. Moreover, we
used SAPILs with micron‐sized honeycomb structure in water as catalysts for the hydrolysis of
cyclohexyl acetate to cyclohexanol, and determined that their catalytic activity was much higher
than that of homogeneous acid catalysts. The equilibrium concentrations of all reaction components
inside and outside the synthesized SAPILs were quantitatively analyzed using a series of simulated
reaction mixtures. Depending on the reaction mixture, the concentration of cyclohexyl acetate in‐
side SAPIL‐1 was 7.5–23.3 times higher than that outside of it, which suggested the high enrichment
ability of SAPILs for cyclohexyl acetate. The excellent catalytic performance of SAPILs was attribut‐
ed to their 3D honeycomb structure in water and high enrichment ability for cyclohexyl acetate,
which opened up new avenues for designing highly efficient heterogeneous acid catalysts that could
eventually replace conventional homogeneous acid catalysts.
Received 29 March 2020
Accepted 8 May 2020
Published 5 February 2021
Keywords:
Heterogeneous acid catalyst
Acidic poly(ionic liquid)
Swelling
3D honeycomb structure
Enrichment
Hydrolysis
Hydration
© 2021, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
1. Introduction
Acid catalysis is an important reaction that has been widely
used for the production of petrochemicals, conversion of bio‐
* Corresponding author. Tel/Fax: +86‐21‐62233323; E‐mail: ghgao@chem.ecnu.edu.cn
This work was supported by the National Natural Science Foundation of China (21773068, 21573072, 21811530273), the National Key Research and
Development Program of China (2017YFA0403102) and Shanghai Leading Academic Discipline Project (B409).
DOI: 10.1016/S1872‐2067(20)63658‐0 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 42, No. 2, February 2021

298
Bihua Chen et al. / Chinese Journal of Catalysis 42 (2021) 297–309
surfactants, and textile industries [29]. The efficient production
of cyclohexanol has attracted the attention of many research‐
ers. Three main cyclohexanol synthesis methods have been
developed, namely phenol hydrogenation, cyclohexane oxida‐
tion, and cyclohexene hydration [29,30]. Phenol hydrogenation
is disadvantageous owing to the high price of phenol, the feed‐
stock. The main problems associated with the cyclohexane
oxidation reaction are its low conversion and selectivity.
Therefore, the hydration of cyclohexene has been considered to
be the most promising cyclohexanol synthesis method owing to
its high atom economy and selectivity advantages. Many het‐
erogeneous acid catalysts, such as zeolites and sulfonic resins,
have been developed for the hydration of cyclohexene; howev‐
er, their catalytic activity has been typically lower than that of
homogeneous acid catalysts [30–33].
In this study, acidic poly(ionic liquid)s with swelling ability
(SAPILs) in water, which presented moderate and single acid
strength, adjustable acid concentration, and good stability,
were successfully prepared via copolymerization and acidifica‐
tion. The swelling of SAPILs in water spontaneously promoted
their self‐organization, and led to the formation of micron‐sized
three‐dimensional (3D) honeycomb structures. The prepared
SAPILs presented much better catalytic performance for the
hydrolysis of cyclohexyl carboxylate and direct hydration of
cyclohexene to cyclohexanol than homogeneous acid catalysts
did. The excellent catalytic performance of SAPILs was ana‐
lyzed in depth by studying the effect of the enrichment ability
and honeycomb structure of the SAPILs in swollen state.
mass, synthesis of fine chemicals, and other industrial process‐
es [1–4]. Conventional homogeneous acid catalysts, such as
H2SO4 and HF, have been used extensively for many ac‐
id‐catalyzed chemical processes owing to their high acid
strength, uniform distribution of acid sites, and excellent cata‐
lytic activity [1,5–7]. However, several shortcomings could be
associated with their use, such as significant environmental
pollution, equipment corrosion, and high energy consumption
[8,9]. To address these problems, much attention has been paid
to the replacement of conventional homogeneous acid catalysts
with heterogeneous ones [4,10]. To date, various heterogene‐
ous acid catalysts, such as zeolites, metal oxides, and ion ex‐
change resins, have been successfully used for many chemical
industrial processes [1,11,12]. However, heterogeneous acid
catalysts still present drawbacks, such as high mass transfer
resistance, facile leaching of active sites, low activity and selec‐
tivity, and poor stability [4,12,13].
Acidic poly(ionic liquid)s (APILs) [14–16], which combine
the characteristics of acids, polymers and ionic liquids (ILs), are
an emerging subclass of acidic ILs that have been successfully
used as heterogeneous acid catalysts [17–25] owing to their
advantages, including their structural designability, organic
frameworks, controllable acid strength, and adjustable acid
concentration [14]. However, similar to most heterogeneous
acid catalysts, APILs generally present lower catalytic activity
than homogeneous acid catalysts because of the lower degree
of exposure of the acid sites to the reactants [17]. Many efforts
have been made to increase the dispersibility of acid sites, and
consequently, to improve their catalytic activity, and an effec‐
tive approach was the synthesis of porous APILs [19–22]. The
presence of pores could substantially accelerate the mass
transfer and increase the accessibility of reactants to the acid
sites. Another useful method involved the preparation of APIL
microspheres and nanoparticles [23–25]. These micro‐ and
nanostructures could bring highly dispersed acid sites to the
reactants. Despite the significant advances in the synthesis of
highly efficient APIL catalysts, their activity has been typically
lower than that of homogeneous acid catalysts. Consequently,
the development of APILs with higher catalytic activity than
homogeneous acid catalysts is still a great challenge.
2. Experimental
2.1. Chemicals and materials
N2 (>99.9% purity) was supplied by Shanghai Tomoe Gases
Co., Ltd. Cyclohexene (99% purity), 1‐vinylimidazole (99%
purity), 1,3‐propane sultone (99% purity), 1,4‐butane sultone
(99% purity), and p‐toluenesulfonic acid monohydrate
(p‐TsOH·H2O, 99% purity) were purchased from J & K. Sodium
p‐styrenesulfonate (NaSS, >93% purity) was obtained from
TCI. Cyclohexyl propionate (98% purity), cyclohexyl butyrate
(98% purity), and phenyl acetate (98% purity) were supplied
by TCI. Chemically pure 2,2ʹ‐azobisisobutyronitrile (AIBN),
cyclohexanol, acetic acid, ethylene oxide (EO), and ethylene
glycol (EG) were purchased from Sinopharm. Analytical grade
H2SO4 and phenol were obtained from Sinopharm. Cyclohexyl
acetate (99% purity) was supplied by Ourchem. Reagent grade
Amberlite IR‐120(Na) was purchased from Adamas Reagent
Co., Ltd. Zeolite H‐ZSM‐5 (Si/Al ratio of 25) was obtained from
the Catalyst Plant of Nankai University. Trimethylphosphine
oxide (TMPO, 97% purity) was purchased from Macklin. In
Recently, we developed a series of carboxylate‐based
poly(ionic liquid)s (PILs) with high swelling abilities in eth‐
ylene carbonate [26–28]. These PILs were used as efficient
catalysts for the transformation of ethylene carbonate, and
their activity was comparable with that of the corresponding
homogeneous ILs. The high catalytic activity was attributed to
the sufficient exposure of the active sites to the reactants,
which was attributed to the swelling ability of the PILs in eth‐
ylene carbonate. These studies suggested that the swelling
ability of PILs was helpful for enhancing their catalytic activity.
Consequently, APILs with swelling ability in reactants could be
highly efficient heterogeneous acid catalysts that could be used
for many acid‐catalyzed reactions, such as hydrolysis, hydra‐
tion, esterification, and alkylation.
addition,
we
synthesized
1,8‐triethylene
gly‐
coldiyl‐3,3ʹ‐divinylimidazolium bromide ([EG3(VIm)2]Br2),
1,8‐dioctyl‐3,3ʹ‐ divinylimidazolium bromide ([O(VIm)2]Br2),
and 1‐vinyl‐3‐ (3‐sulfopropyl)imidazolium hydrogen sulfate
Cyclohexanol is a very important intermediate for the syn‐
thesis of adipic acid, hexamethylenediamine, cyclohexanone,
and ε‐caprolactam, and has been widely used in the coatings,
([VSIm]HSO4), an acidic IL (see Supporting Information). De‐
ionized water and other chemicals and materials were pur‐
chased from local suppliers and were used as received, without

Bihua Chen et al. / Chinese Journal of Catalysis 42 (2021) 297–309
299
further purification.
perature. CH2Cl2 was evacuated at 60 °C for 3 h. Then, the sam‐
ples that adsorbed TMPO were packed into a ZrO2 MAS rotor
inside a glovebox, for further analysis. The 31P solid‐state MAS
NMR spectra of the samples were recorded using a VNMRS‐400
WB (VARIAN, USA) spectrometer.
2.2. Characterization
Both 1H and 13C nuclear magnetic resonance (NMR) spectra
were recorded at the frequencies of 400 and 100 MHz using an
Ascend 400 (Bruker Daltonics Inc., USA) spectrometer with
tetramethylsilane as the internal standard. All 13C solid‐state
magic angle spinning (MAS) NMR spectra were acquired using
a VNMRS‐400 WB (VARIAN, USA) spectrometer under one
pulse conditions, at the frequency of 100 MHz, spinning rate of
9.0 kHz, and recycling delay of 4 s. Fourier‐transform infrared
(FT‐IR) spectra were obtained using a Nexus Spectrum 670
FT‐IR spectrometer (Thermo Nicolet, USA) using KBr pellets.
X‐ray photoelectron spectroscopy (XPS) experiments were
performed using an AXIS SUPRA (Shimadzu/Kratos, UK) pho‐
toelectron spectrometer with Al Kα radiation at θ = 90°, and its
binding energies were calibrated using the C 1s peak at 284.8
eV. Thermogravimetric analysis (TGA) was performed using a
STA449 F3 Jupiter (Netzsch, Germany) instrument at the heat‐
ing rate of 10 °C/min in the temperature range of 25−800 °C
under a stream of N2. The initial mass of each sample was 5–8
mg. N2 adsorption–desorption isotherms were obtained at
−196 °C using a BELSORP‐Max (MicrotracBEL, Japan) volumet‐
ric adsorption equipment. Prior to each measurement, the
samples were degassed at 150 °C under vacuum for 6 h. The
Brunauer–Emmett–Teller (BET) specific surface areas were
calculated utilizing the BET equation using five relative pres‐
sure points in the range of 0.05–0.30. X‐ray powder diffraction
(XRD) analysis was performed using an Ultima IV (Rigaku, Ja‐
pan) diffractometer with Cu Kα radiation. Scanning electron
microscopy (SEM) measurements were performed using a G2
Pro Desktop (Phenom, Netherlands) instrument. The internal
morphologies of the swollen samples were examined using an
S‐4800 (Hitachi, Japan) cryogenic scanning electron microsco‐
py apparatus equipped with an Alto‐2100 (Gatan, UK) refriger‐
ating system. After sufficient swelling for 24 h, the swollen
samples were separated, frozen at −160 °C, and placed inside
the sample holder. The top section of each frozen samples was
cut using a scraper. Then the samples were sublimated at −95
°C and imaged at 1.0 kV. The contact angles (CAs) of the sam‐
ples were obtained on a Theta (Biolin Scientific, Sweden) opti‐
cal contact angle measuring instrument. Before testing, the
samples were pressed into tablets using a 769YP‐24B (Tianjin
Keqi High & New Technology Corporation, China) tablet ma‐
chine. The acid concentrations of the samples were determined
via acid‐base titration using standard NaOH solution and phe‐
nolphthalein as the titrant and indicator, respectively [9,10,25].
The acid strengths of the samples were evaluated using the
31P‐TMPO NMR approach [34]. Before analysis, the samples
were treated as follows. Powder samples were added to sealed
glass flasks with stopcocks and were degassed at 100 °C for 24
h under vacuum to remove the adsorbed water. Then, an an‐
hydrous CH2Cl2 (distillated in CaH2) solution that contained the
TMPO probe molecule was added to the flask, which was
placed inside a glovebox. To ensure the sufficient adsorption of
TMPO, the loaded samples were stirred for 24 h at room tem‐
2.3. Preparation of SAPILs
2.3.1. Synthesis of IL monomers
First, 1,3‐propane sultone (12.21 g, 0.1 mol) was dissolved
in 20 mL THF in a 100 mL round‐bottom flask. Then, a solution
of 1‐vinylimidazole (9.41 g, 0.1 mol) in 5 mL THF was slowly
added to the flask. The mixture was stirred at room tempera‐
ture for 24 h. The obtained white solid was washed with diethyl
ether and dried under vacuum at 60 °C for 12 h. The desired
product, 1‐vinyl‐3‐(3‐sulfopropyl)imidazolium (VSIm), was
obtained at the yield of approximately 80% (17.25 g), and was
structurally characterized using NMR spectroscopy. 1H NMR
(400 MHz, DMSO‐d6, TMS) δ: 9.48 (s, 1H), 8.19 (s, 1H), 7.94 (s,
1H), 7.29 (dd, J = 15.6, 8.8 Hz, 1H), 5.95 (dd, J = 15.6, 2.4 Hz,
1H), 5.40 (dd, J = 8.8, 2.4 Hz, 1H), 4.34 (t, J = 7.0 Hz, 2H), 2.45 (t,
J = 7.0 Hz, 2H), 2.17−2.09 (m, 2H); 13C NMR (100 MHz,
DMSO‐d6, TMS) δ: 135.68, 128.96, 123.33, 119.08, 108.46,
48.23, 47.38, 25.92.
Similarly, the IL monomer 1‐vinyl‐3‐(4‐sulfobutyl)imidazo‐
lium (VSbIm) was synthesized via the reaction of
1‐vinylimidazole with 1,4‐butane sultone, and was structurally
characterized using NMR spectroscopy. 1H NMR (400 MHz,
D2O, TMS) δ: 9.08 (s, 1H), 7.79 (s, 1H), 7.61 (s, 1H), 7.14 (dd, J =
15.6, 8.8 Hz, 1H), 5.80 (dd, J = 15.6, 2.8 Hz, 1H), 5.43 (dd, J = 8.8,
2.8 Hz, 1H), 4.30 (t, J = 7.0 Hz, 2H), 2.96 (t, J = 7.6 Hz, 2H),
2.10−2.02 (m, 2H), 1.81−1.73 (m, 2H); 13C NMR (100 MHz, D2O,
TMS) δ: 134.52, 128.18, 122.72, 119.51, 109.28, 49.98, 49.23,
27.90, 20.87.
2.3.2. Synthesis of SAPILs
SAPILs were synthesized using a two‐step procedure that
consisted of free radical copolymerization and subsequent
acidification. The typical synthesis consisted of the following
steps. First, VSIm (1.05 g, 4.875 mmol), NaSS (1.00 g, 4.875
mmol), the [EG3(VIm)2]Br2 (0.12 g, 0.25 mmol) crosslinker, and
5 mL H2O were added to a 50 mL two‐necked round‐bottom
flask equipped with a magnetic stirrer. After stirring for 0.5 h at
room temperature, the mixture was gradually heated to 80 °C.
Then, AIBN (0.11 g, 5 wt%), the initiator, was added to the
flask, and the mixture was stirred at 80 °C for 24 h under N2
atmosphere. After copolymerization, the SAPIL precursor was
cooled to room temperature, washed with excess methanol and
water several times, and dried under vacuum at 60 °C for 12 h.
Then, the obtained SAPIL precursor and 100 mL of 1 M H2SO4
solution were added to a 250 mL beaker equipped with a mag‐
netic stirrer, and the mixture was stirred for 24 h at room tem‐
perature. The formed SAPIL was washed with excess water
several times until the pH of the washing liquid was neutral,
and dried under vacuum at 60 °C for 12 h. A canary yellow
SAPIL was obtained at the total yield of approximately 84%.
Other SAPILs featuring different types and contents of cross‐

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Bihua Chen et al. / Chinese Journal of Catalysis 42 (2021) 297–309
linkers and different IL monomer species were prepared using
similar processes.
autoclave was pressurized with N2 (1.0 MPa) and heated to 100
°C. After stirring for 0.5 h, the autoclave was cooled to room
temperature in an ice‐water bath, and then it was slowly vent‐
ed. After the SAPIL catalyst was separated, the reaction mixture
was analyzed via GC using n‐dodecane as the internal standard.
2.4. Measurement of the swelling ratio
During a typical measurement, dried SAPIL (50 mg) was
added to a sealed bottle that contained 10 mL solvent. The
swelling process was allowed to reach equilibrium for 24 h at
room temperature. Then the swollen SAPIL was removed from
the bottle, and the solvent on its surface was removed by wip‐
ing it with absorbent paper. The mass of the swollen SAPIL was
measured immediately afterward, and the swelling ratio (Q) of
the SAPIL was calculated using the following equation:
Q = (mswollen ‒ mdried)/mdried (g/g)
2.6. Quantitative analysis of the cyclohexyl acetate enrichment
ability of SAPILs in swollen state
Typically, a simulated reaction mixture of cyclohexyl acetate
(0.71 g, 5 mmol), water (4.41 g, 245 mmol), cyclohexanol (0.50
g, 5 mmol), and acetic acid (0.30 g, 5 mmol), which was the
equivalent of the reaction mixture used for the 50% conversion
of cyclohexyl acetate via hydrolysis, was prepared and named
Mixture‐50% (here 50% represents the conversion of cyclo‐
hexyl acetate in the simulated reaction mixture). Mixture‐50%
and SAPIL‐1 (0.102 g, equivalent to 0.2 mmol H+) were added
to a 25 mL sealed bottle equipped with a magnetic stirrer. After
stirring for 12 h at room temperature, the mixture was allowed
to sit for 4 h, and then, the aqueous phase, which contained
swollen SAPIL‐1, was separated. The swollen SAPIL‐1 was re‐
moved from the aqueous phase and was deswelled with ace‐
tone. The acetone solution was analyzed via GC using biphenyl
as the internal standard, and the concentrations of cyclohexyl
acetate, cyclohexanol, and acetic acid inside SAPIL‐1 were cal‐
culated. In addition, the concentrations of all components of the
aqueous phase (outside SAPIL‐1) were measured using a simi‐
lar method. The enrichment ability of SAPIL‐1 was determined
by comparing the concentrations of all components inside and
outside SAPIL‐1. The quantitative analysis of the enrichment
ability of SAPIL‐1 in Mixture‐10%, 30%, 70% and 90% (which
corresponded to the reaction mixtures used to achieve the hy‐
drolysis conversion of cyclohexyl acetate of 10%, 30%, 70%,
and 90%, respectively) was performed using the same proce‐
dure.
where mswollen and mdried represent the masses of the swollen
and dried SAPIL, respectively.
2.5. Catalytic tests
2.5.1. Hydrolysis of esters
The hydrolysis of cyclohexyl acetate was carried out in a 50
mL stainless‐steel Teflon‐lined YZPR‐50 (Yanzheng Experiment
Instrument, China) autoclave equipped with a magnetic stirrer
and an automatic temperature control system. During a typical
run, cyclohexyl acetate (1.42 g, 10 mmol), deionized water
(4.50 g, 250 mmol), and SAPIL (4 mol% catalyst relative to the
cyclohexyl acetate, equivalent to 0.4 mmol H+) were added to
the autoclave. The autoclave was pressurized with N2 (1 bar)
and heated to 100 °C. After stirring for 7 h, the autoclave was
cooled to room temperature in an ice‐water bath, and then it
was slowly vented. After the SAPIL catalyst was separated, the
reaction mixture was analyzed using a gas chromatography
(GC; GC‐2014, Shimadzu, Japan) instrument with a DM‐1701
(30 m  0.53 mm  1.0 μm) capillary column and flame ioniza‐
tion detector; biphenyl was used as the internal standard. The
hydrolysis of other esters was performed following the same
procedure.
For comparison, the concentrations of each component in
the aqueous phases of Mixture‐10%, 30%, 50%, 70%, and 90%
over the homogeneous acid catalyst p‐TsOH (0.102 g, equiva‐
lent to 0.2 mmol H+) were also obtained.
The SAPIL catalyst was recovered via deswelling with ace‐
tone, and then it was separated and washed with 5 mL acetone
three times. The recovered SAPIL was dried under vacuum at
60 °C for 12 h, and then it was reused for the next run.
3. Results and discussion
2.5.2. Direct hydration of cyclohexene
3.1. Synthesis and characterization of SAPILs
Typically, cyclohexene (1.64 g, 20 mmol), deionized water
(3.60 g, 200 mmol), and SAPIL (1 mol% catalyst relative to the
cyclohexene, equivalent to 0.2 mmol H+) were added to the
autoclave. The autoclave was pressurized with N2 (0.5 MPa)
and heated to 130 °C. After the reaction ended, the autoclave
was cooled to room temperature in an ice‐water bath, and then
it was slowly vented. After the separation of the SAPIL catalyst,
the reaction mixture was analyzed via GC using biphenyl as the
internal standard.
The SAPILs were synthesized using a two‐step procedure
(Scheme 1). First, SAPIL precursors were prepared via the free
radical copolymerization of the IL monomers (VSIm or VSbIm),
NaSS, and crosslinkers ([EG3(VIm)2]Br2 or [O(VIm)2]Br2) in the
presence of AIBN as the initiator at 80 °C for 24 h in H2O under
N2 atmosphere. Then, the target SAPILs were obtained via the
acidification of the SAPIL precursors in 1 M H2SO4 solution. Six
SAPIL samples were successfully synthesized, and the synthesis
conditions for each sample are listed in Table 1.
2.5.3. Hydration of EO
The structure of the synthesized SAPILs was characterized
using NMR spectroscopy. The 13C solid‐state MAS NMR spectra
of the SAPILs and 13C NMR spectra of the corresponding mon‐
omers, namely VSIm and NaSS, are illustrated in Fig. 1. The
Typically, EO (2.20 g, 50 mmol), deionized water (9.01 g,
500 mmol) and SAPIL (0.12 mol% catalyst relative to the EO,
equivalent to 0.06 mmol H+) were added to the autoclave. The

Bihua Chen et al. / Chinese Journal of Catalysis 42 (2021) 297–309
301
Copolymerization
Acidification
H+
＝
H O
2
Crosslinker
＝
＝
＝
＝
Scheme 1. Schematic illustration of the synthesis process of acidic poly(ionic liquid)s with swelling ability (SAPILs). Here AIBN, VSIm, VSbIm,
[EG3(Vim)2]Br2, and [O(Vim)2]Br2 denote 2,2ʹ‐azobisisobutyronitrile, 1‐vinyl‐3‐(3‐sulfopropyl)imidazolium, 1‐vinyl‐3‐(4‐sulfobutyl)imidazolium,
1,8‐triethylene glycoldiyl‐3,3ʹ‐divinylimidazolium bromide, and 1,8‐dioctyl‐3,3ʹ‐divinylimidazolium bromide, respectively.
signals at δ ≈ 115 and 108, which belonged to the vinyl groups
of NaSS and VSIm, respectively, were not observed in the spec‐
tra of SAPILs, which indicated the complete copolymerization
of the monomers. The new signals at δ ≈ 57 and 40 were as‐
signed to the C atoms of the newly formed polymer chains [26].
The signals at δ ≈ 68 in the spectra of SAPIL‐1–4 belonged to
the [EG3(VIm)2]Br2 crosslinker (Fig. S1(a)), and the intensity of
the signals increased as the molar fraction of [EG3(VIm)2]Br2 in
the SAPILs increased.
The successful copolymerization of the monomers was con‐
firmed using the FT‐IR spectra of the SAPILs (Figs. S2–S4). The
band at approximately 1630 cm‒1 which belonged to the
stretching vibrations of the C=C bonds of the vinyl groups of
NaSS and VSIm was not observed in the FT‐IR spectrum of
SAPIL‐1 (Fig. S2). The bands at approximately 1644, 1569, and
1553 cm‒1 were attributed to the stretching vibrations of the
C=N and C=C bonds of the imidazolium rings [25]. The bands at
approximately 1605, 1569, and 1498 cm‒1 were associated
with the stretching vibrations of the benzene rings. The band at
approximately 837 cm‒1 was assigned to the out‐of‐plane
bending vibration of the p‐disubstituted benzene rings. The
band at approximately 1037 cm‒1 was associated with the
presence of the C–S moieties on VSIm and NaSS [10]. The bands
New signals from
Signals belonging
polymer chains
to vinyl
groups disappeared
Signals from
Table 1
[EG3(VIm)2]Br2
Conditions for the synthesis of acidic poly(ionic liquid)s with swelling
ability (SAPILs) and their swelling ratios (Q) in water.c
e
ILa
(mmol)
NaSSb
(mmol)
Crosslinker
(mmol)
Q
(g/g)
d
c
b
a
Sample
SAPIL‐1
SAPIL‐2
SAPIL‐3
SAPIL‐4
SAPIL‐5d
4.875
4.750
4.500
4.000
4.875
4.875
4.875
4.750
4.500
4.000
4.875
4.875
0.25
0.50
1.00
2.00
0.25
24.5
14.3
6.5
1.6
14.4
27.4
Signals from DMSO-d6
NaSS
SAPIL‐6e
a
0.25
VSIm
b
c
Ionic liquid. Sodium p‐styrenesulfonate. SAPIL‐1–4: 1‐vinyl‐3‐
(3‐sulfopropyl)imidazolium (VSIm) and 1,8‐triethylene gly‐
coldiyl‐3,3ʹ‐divinylimidazolium bromide ([EG3(VIm)2]Br2) were used as
the IL and crosslinker, respectively. d SAPIL‐5: VSIm and 1,8‐dioctyl‐
3,3ʹ‐divinylimidazolium bromide ([O(VIm)2]Br2) were used as the IL
and crosslinker, respectively; e SAPIL‐6: 1‐vinyl‐3‐(4‐sulfobutyl)imidaz‐
olium (VSbIm) and [EG3(VIm)2]Br2 were used as the IL and crosslinker,
respectively.
200 180 160 140 120 100 80
60
40
20
0
Chemical Shift ()
Fig. 1. 13C solid‐state magic angle spin NMR spectra of SAPIL‐1 (a),
SAPIL‐2 (b), SAPIL‐3 (c), SAPIL‐4 (d), SAPIL‐5 (e), and 13C NMR spectra
of 1‐vinyl‐3‐(3‐sulfopropyl)imidazolium (VSIm) and sodium
p‐styrenesulfonate (NaSS) (dimethyl sulfoxide (DMSO)‐d6).

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Bihua Chen et al. / Chinese Journal of Catalysis 42 (2021) 297–309
at approximately 1175, 1126, and 1006 cm‒1 were assigned to
the asymmetric and symmetric stretching vibrations of the S=O
bonds of the sulfonic groups [10,25]. The FT‐IR spectra of
SAPIL‐2–5, which were synthesized using different types and
amounts of crosslinkers were similar (Fig. S3), and no signifi‐
cant differences were observed in the spectra obtained before
and after their acidification (Fig. S4).
The XPS spectra of SAPIL‐1 and its precursor are depicted in
Fig. 2(a), where the characteristic peaks of C, N, O, and S were
distinctly observed. The C 1s peaks at approximately 284.8 and
286.2 eV belonged to the C–C and C–S bonds (Fig. S5) [10]. The
S 2s and S 2p peaks at approximately 229.0, 165.3, and 164.3 eV
were associated with the S=O and S–O bonds (Fig. S6) [10].
These results suggested that VSIm and NaSS were successfully
copolymerized, and were in good agreement with the results of
the 13C solid‐state MAS NMR and FT‐IR spectroscopy analyses.
Furthermore, the characteristic peak of Na 1s at approximately
1071.8 eV was not observed in the spectrum of SAPIL‐1 (Fig.
2(a)), which indicated that SAPIL‐1 was completely acidified.
The XRD pattern of SAPIL‐1 (Fig. S7) revealed the amorphous
structure of the SAPILs. The BET specific surface areas of the
dried SAPILs were negligible, which suggested that the SAPILs
presented nonporous structure.
The thermal stabilities of the SAPILs were investigated us‐
ing TGA. The TGA curves of SAPILs and Amberlite IR‐120(H)
are illustrated in Fig. 2(b). All SAPIL samples presented mass
loss in the temperature range of 25–200 °C, which was as‐
signed to the desorption of water. The mass loss in the temper‐
ature range of 300–600 °C was associated with the decomposi‐
tion of the sulfonic groups and imidazolium rings and destruc‐
tion of the polymer networks. As the molar fraction of the
crosslinker increased from 2.5% to 20%, the initial decomposi‐
tion temperatures of SAPILs increased from 300 to 345 °C, re‐
spectively, which could be attributed to the increase in their
crosslinking degree [35]. Furthermore, the thermal stability of
the SAPILs was much better than that of the commercial resin
Amberlite IR‐120(H) (245 °C).
attributed to the interactions between TMPO and the Brönsted
acid sites of SAPIL‐1, and demonstrated the moderate and sin‐
gle acid strength of SAPIL‐1 [34,36–38]. Conversely, multiple
peaks at δ ≈ 89, 66, 60, and 51 were observed in the spectrum
of TMPO after it interacted with H‐ZSM‐5 (Fig. 2(c)). These
results indicated that H‐ZSM‐5 presented multiple acid
strengths, including super (δ > 85), strong (δ = 70–85), medium
(δ = 60–70), and weak (δ < 60) acidities [36]. Compared with
H‐ZSM‐5, SAPIL‐1 with single acid strength could present po‐
tential advantages for improving reaction selectivity.
3.2. Swelling abilities of SAPILs
The swelling ability of PILs originates from the electrostatic
repulsions between the charges on the polymer chains and
osmotic imbalance between the inside and outside areas of the
PILs [39]. The Q values were used to evaluate the swelling abil‐
ity of the PILs. The swelling abilities of the SAPILs in water
were measured (Fig. S8) and the results are listed in Table 1.
The Q value of SAPIL‐1 in water was high (24.5). As the molar
fraction of the [EG3(VIm)2]Br2 crosslinker increased from 2.5%
(SAPIL‐1) to 5% (SAPIL‐2), 10% (SAPIL‐3), and 20% (SAPIL‐4),
the swelling ratio significantly decreased from 24.5 to 14.3, 6.5,
and 1.6, respectively. This could be attributed to the increase in
the crosslinker concentration increasing the crosslinking den‐
sity, which led to the motion of the polymer chains being re‐
stricted, and thus, contributed to the decrease in the swelling
ability of the SAPILs [40]. The swelling ability of SAPIL‐5, which
was synthesized using
a more hydrophobic crosslinker
([O(VIm)2]Br2) than [EG3(VIm)2]Br2, in water (Q = 14.4) was
lower than that of SAPIL‐1. This was attributed to the increase
in the hydrophobicity of the crosslinker reducing the interac‐
tions between SAPILs and water molecules. The swelling ability
of SAPIL‐6, which was synthesized using VSbIm, a monomer
with longer alkyl chain than VSIm, in water (Q = 27.4), was
slightly higher than that of SAPIL‐1. This could be attributed to
the longer alkyl chain weakening the electrostatic attractions
between the cations and anions [40,41]. Typically, the swelling
ability of SAPILs could be adjusted by changing the types and
contents of crosslinkers and the types of IL monomers used to
synthesize them. In addition, the swelling abilities of the SAPIL
precursors in water were similar with those of the corre‐
The 31P‐TMPO NMR approach has been demonstrated to be
an informative tool for the analysis of the acid properties of
heterogeneous acid catalysts [36–38]. The 31P solid‐state MAS
NMR spectrum of TMPO after it interacted with SAPIL‐1 pre‐
sented a single narrow peak at δ ≈ 62 (Fig. 2(c)). This peak was
300
SAPIL-1
Na 1s
345
100
H-ZSM-5 (Si/Al=25)
SAPIL-1
(a)
(b)
(c)
SAPIL-2
SAPIL-3
SAPIL-4
SAPIL-5
C 1s
O 1s
43
43
51
33
60
80
245
66
Amberlite IR-120(H)
62
1080
1075
1070
1065
N 1s
60
40
20
0
S 2p
S 2s

SAPIL-1
Na 1s
89
NaKLL

SAPIL-1 precursor
1200
1000
800
600
400
200
0
100
200
300
400
500
600
700
800
140
120
100
80
60
40
20
0
Chemical shift ()
Temperature (oC)
Binding energy (eV)
Fig. 2. (a) XPS profiles of SAPIL‐1 precursor and SAPIL‐1; (b) TG curves of SAPIL‐1–5 and Amberlite IR‐120(H); (c) 31P solid‐state magic angle spin‐
ning NMR spectra of TMPO chemically adsorbed on SAPIL‐1 and H‐ZSM‐5 (Si/Al = 25). The asterisks denote spinning sidebands. The peaks at δ ≈ 43
and 33 belonged to the crystalline and physiosorbed TMPO, respectively [36].

Bihua Chen et al. / Chinese Journal of Catalysis 42 (2021) 297–309
303
sponding SAPILs (Table S1), which suggested that acidification
did not alter the swelling abilities of SAPILs in water.
versely, SAPIL‐4, which presented very poor swelling ability in
water (Q = 1.6) did not exhibit typical 3D honeycomb structure
owing to its high crosslinking density and the strong interac‐
tions between the polymer chains in its structure (Fig. S10(c)).
These results suggested that the good swelling ability in water
of the SAPILs played a significant role in the formation of these
unique micron‐sized 3D honeycomb structures.
The swelling abilities of SAPILs in acidic, salt, and alkaline
solutions were also investigated. As illustrated in Table S2, the
swelling abilities of SAPIL‐1 in 1M H2SO4, 1M NaCl, and 1M
NaOH solutions were low (Q = 5.3, 6.9, and 4.3, respectively),
which could be attributed to the high ionic strengths of these
solutions [42]. After the SAPIL‐1 samples were recovered from
the above solutions, their Q values in water were similar to
those of a fresh SAPIL‐1 sample. In addition, the Q value of
SAPIL‐1 in water was maintained at ~24 during five swelling
(water)‐deswelling (acetone) cycles (Fig. S9). These results
suggested that SAPILs presented good chemical stability and
reversible swelling‐deswelling ability, which were very favora‐
ble for catalyst recycling.
The SEM and cryo‐SEM images revealed that the structure
of the SAPILs spontaneously changed from nonporous to mi‐
cron‐sized 3D honeycomb when they swelled in water. To the
best of our knowledge, this is the first report of the swell‐
ing‐induced self‐assembly behavior of PILs to form 3D honey‐
comb structures. A possible 3D honeycomb structure formation
mechanism was proposed, and it is schematically depicted in
Fig. 4. When the SAPILs swelled in water (Fig. 4(a)), the inter‐
actions between the polymer chains and the osmotic balance
between the interior and exterior of the SAPILs were broken,
and then, water was absorbed into the SAPILs [39,43]. As the
number of water molecules inside the SAPILs increased, the
entangled polymer chains became gradually stretched (Fig.
4(b)). After the swelling process reached equilibrium, which
was driven by the electrostatic interactions between the poly‐
mer chains and also the polymer chains and water and hydro‐
gen bonding interactions between the polymers chains and
water, the disordered polymer chains self‐organized into the
most stable 3D honeycomb structures (Fig. 4(c)) [44,45].
3.3. Swelling‐induced self‐assembly of SAPILs in water
The microstructure of the dried SAPILs was investigated
using SEM. As displayed in Fig. 3(a), dry SAPIL‐1 presented
irregular micron‐sized particles with nonporous structure,
which was in good agreement with the BET specific surface
area results. Cryo‐SEM was used to study the internal mor‐
phologies of SAPILs in swollen state. As illustrated in Fig. 3(b),
the swollen SAPIL‐1 sample presented unique 3D honeycomb
structure, in which the diameters of the channels were ap‐
proximately 20–30 μm after sublimation for 5 min. The in‐
crease in the sublimation time from 5 to 30 min resulted in the
partial deformation of the honeycomb structure because more
amorphous ice from the water absorbed by SAPIL‐1 sublimated
(Fig. 3(c)). The magnified image in Fig. 3(d) revealed that the
thickness of the swollen SAPIL‐1 was approximately 0.6 μm.
Similarly, SAPIL‐2, 3, and 5, which presented good swelling
ability in water (Q = 14.3, 6.5, and 14.4, respectively), also pre‐
sented unique 3D honeycomb structure in water (Figs. S10(a),
(b), and (d), respectively). The diameters of the honeycomb
channels gradually decreased from 20–30 to 3–6 μm as Q de‐
creased from 24.5 to 6.5, which could be attributed to the in‐
crease in the crosslinking degree of the SAPIL samples. Con‐
3.4. Catalytic activity of SAPILs for hydrolysis and hydration
reactions
Catalysts with 3D honeycomb structure are expected to be
highly efficient, because their unique structure not only pro‐
vides maximum active sites exposure to the reactants, but also
facilitates the accessibility of the reactants to the active sites
and shortens their diffusion length [46–50]. Therefore, SAPILs
with micron‐sized 3D honeycomb structure in water could be
highly efficient solid acid catalysts for hydrolysis and hydration
reactions.
3.4.1. Indirect and direct hydration of cyclohexene to
cyclohexanol
(a)
(b)
Hydrolysis of cyclohexyl acetate to cyclohexanol. The indirect
hydration of cyclohexene is a two‐step process comprising the
esterification of cyclohexene with carboxylic acid to cyclohexyl
carboxylate and subsequent hydrolysis of cyclohexyl carbox‐
ylate to cyclohexanol [30]. The catalytic activity of SAPILs was
evaluated for the hydrolysis of cyclohexyl acetate to cyclohex‐
anol (second step of indirect hydration). Table 2 presents the
20 μm
20 μm
(c)
(d)
(a)
(c)
(b)
SAPILs
SAPILs
SAPILs
5 μm
20 μm
H2O
H2O
H2O
Fig. 3. (a) SEM image of dried SAPIL‐1; cryogenic SEM images of fully
swollen SAPIL‐1 in water sublimated for 5 min (b) and 30 min (c); (d)
magnified image of area marked in (c).
Fig. 4. Schematic illustration of swelling‐induced self‐assembly process
of SAPILs in water.

304
Bihua Chen et al. / Chinese Journal of Catalysis 42 (2021) 297–309
Table 2
Hydrolysis of cyclohexyl acetate catalyzed using various catalysts.a
O
OH
O
H2O
O
OH
Entry
1
2
3
4
5
6
7
8 e
9
10 f
11
Catalyst
—
SAPIL‐1
SAPIL‐2
SAPIL‐3
Type
—
Acid concentration b (mmol/g)
Conversion c (%)
Selectivity d (%)
100
—
1.95
1.59
1.12
1.99
2.03
4.60
0.86
6.23
20.39
5.25
1.4
81.9
70.0
62.9
72.9
81.5
28.5
49.3
47.1
46.2
69.4
Hetero
Hetero
Hetero
Hetero
Hetero
Hetero
Hetero
Homo
Homo
Homo
98.3
98.7
98.9
98.5
98.7
96.8
36.3
99.2
SAPIL‐5
SAPIL‐6
Amberlite IR‐120(H)
H‐ZSM‐5
[VSIm]HSO4
H2SO4
98.9
98.4
p‐TsOH
a Reaction conditions: cyclohexyl acetate (10 mmol), H2O (250 mmol), catalyst (0.4 mmol), 100 °C, 7 h, 1 bar N2. b The acid concentrations of various
catalysts were measured via acid‐base titration (Table S3). c Conversion of cyclohexyl acetate. d Selectivity of cyclohexanol. e The acid concentration of
f
H‐ZSM‐5 (Si/Al = 25) was determined using the temperature programmed desorption of NH3. The acid concentration of H2SO4 was theoretically
calculated.
catalytic data for various catalysts for the hydrolysis of cyclo‐
hexyl acetate under optimized reaction conditions, which are
summarized in Table S4. In the absence of a catalyst, the con‐
version of cyclohexyl acetate was only 1.4% (Table 2, entry 1).
The cyclohexyl acetate conversion over SAPIL‐1, which pre‐
sented high swelling ability in water (Q = 24.5), was 81.9%
(Table 2, entry 2). As the molar fraction of the [EG3(VIm)2]Br2
crosslinker increased from 2.5% to 5% and 10%, the swelling
ability of SAPILs decreased from Q = 24.5 to 14.3 and 6.5, re‐
spectively; moreover, the cyclohexyl acetate conversion de‐
creased from 81.9% to 70.0% and 62.9%, respectively (Table 2,
entries 3 and 4). This could be attributed to the low swelling
ability of the SAPILs hindering the exposure of the acid sites
and leading to the incomplete accessibility of the reactants to
the acid sites. The Q value and cyclohexyl acetate conversion
ability of SAPIL‐5, which was synthesized using [O(VIm)2]Br2, a
more hydrophobic crosslinker than [EG3(Vim)2]Br2, were 14.4
and 72.9% respectively (Table 2, entry 5). In addition, the Q
value and cyclohexyl acetate conversion ability of SAPIL‐6,
which was synthesized using VSbIm, an IL monomer with a
longer alkyl chain than VSIm, were 27.4 and 81.5%, respec‐
tively (Table 2, entry 6). Typically, the catalytic activity of the
SAPILs was approximately linearly correlated with their swell‐
ing ability in water (Fig. S11).
alytic kinetic investigations were performed and the results are
presented in Fig. 5 and Table S5. The catalytic rate of SAPIL‐1
was much higher than those of the other analyzed catalysts,
and allowed the reaction to reach equilibrium after 7 h. These
results further indicated that SAPILs were superior to other
acid catalysts for the hydrolysis of cyclohexyl acetate. The reac‐
tion rate of the process catalyzed by H‐ZSM‐5 was similar to
those of the reactions catalyzed by H2SO4 and [VSIm]HSO4, but
the selectivities of these three catalysts only ranged from
36.3% to 58.0%, which were much lower than those of the
other analyzed catalysts (Table S5).
In addition to their remarkable catalytic performance for
the hydrolysis of cyclohexyl acetate, SAPILs also exhibited ex‐
cellent catalytic activity for the hydrolysis of other esters. As
presented in Fig. 6 and Table S6, the catalytic activity of
SAPIL‐1 was much higher than those of p‐TsOH, H2SO4,
[VSIm]HSO4, and Amberlite IR‐120(H) for the hydrolysis of
cyclohexyl propionate, cyclohexyl butyrate, and phenol acetate
under the same reaction conditions. These results demon‐
strated that SAPILs were some of the best heterogeneous acid
100
SAPIL-1
p-TsOH
H-ZSM-5 (Si/Al=25)
80
H2SO4
For comparison, heterogeneous acid catalysts Amberlite
IR‐120(H) and H‐ZSM‐5, and homogeneous acid catalysts
H2SO4, [VSIm]HSO4, and p‐TsOH were used to catalyze the hy‐
drolysis of cyclohexane acetate to cyclohexanol. The catalytic
activities of these catalysts were much lower than that of
SAPIL‐1 (Table 2, entries 7–11). Particularly, the cyclohexanol
selectivity of H‐ZSM‐5 was only 36.3%, which was much lower
than those of the analyzed SAPILs (98.3%–98.9%). The low
selectivity of H‐ZSM‐5 could be attributed to the multiple acid
strengths of H‐ZSM‐5, as demonstrated by its 31P solid‐state
MAS NMR spectrum (Fig. 2(c)). These results demonstrated
that SAPILs with single acid strength presented advantages for
improving the selectivity.
[VSIm]HSO4
Amberlite IR-120(H)
60
40
20
0
1
3
5
7
9
Time (h)
Fig. 5. Catalytic kinetics of various catalysts for the hydrolysis of cyclo‐
hexyl acetate. Reaction conditions: 10 mmol cyclohexyl acetate, 250
mmol H2O, 0.4 mmol catalyst, 100 °C, 1 bar N2.
To further compare the efficiencies of various catalysts, cat‐

Bihua Chen et al. / Chinese Journal of Catalysis 42 (2021) 297–309
305
outside of SAPIL‐1 (Figs. S12 and 13). However, the concentra‐
tions of cyclohexyl acetate inside SAPIL‐1 were 7.5–23.3 times
higher than those outside of SAPIL‐1 (Figs. 7(a) and S13). For
each simulated reaction mixture, the ratio of the cyclohexyl
acetate concentrations inside and outside of SAPIL‐1 was much
higher than those of cyclohexanol and acetic acid (Fig. S13).
These results suggested that cyclohexyl acetate could be highly
concentrated by SAPIL‐1 during the entire catalytic process.
The high enrichment ability of SAPIL‐1 for cyclohexyl acetate
could be attributed to its unique chemical structure and mi‐
cron‐sized 3D honeycomb morphology it formed in water.
Furthermore, to analyze the enrichment ability of SAPIL‐1
intuitively, the cyclohexyl acetate‐to‐cyclohexanol molar ratio
inside and outside of SAPIL‐1 were calculated. As presented in
Fig. 7(b), the cyclohexyl acetate‐to‐cyclohexanol molar ratios
inside SAPIL‐1 were much higher than those outside of SAPIL‐1
for the same simulated reaction mixtures. For example, for
Mixture‐50%, the cyclohexyl acetate‐to‐cyclohexanol molar
ratio inside SAPIL‐1 was 0.332, which was 5.2 times higher
than that outside of SAPIL‐1 (0.064). Therefore, these results
also demonstrated the high enrichment of SAPIL‐1 for cyclo‐
hexyl acetate.
For comparison, we analyzed the catalysis of the simulated
reaction mixtures (Mixture‐10%, 30%, 50%, 70%, and 90%) in
the presence of the homogeneous acid catalyst p‐TsOH, and
measured the concentrations of cyclohexyl acetate, cyclohexa‐
nol, and acetic acid in the aqueous phase. The concentrations of
all components in the homogeneous system (p‐TsOH) were
very similar to those outside of SAPIL‐1 (Table S7). Thus, the
concentrations of cyclohexyl acetate in the homogeneous sys‐
tem (p‐TsOH) were much lower than those inside SAPIL‐1.
These results indicated that the density of cyclohexyl acetate in
the homogeneous system (p‐TsOH) was much lower than that
inside SAPIL‐1 during the hydrolysis process.
SAPIL-1
p-TsOH
H2SO4
99.2
100
80
60
40
20
0
92.9
85.9
85.3
79.6
[VSIm]HSO4
Amberlite IR-120(H)
53.0
44.0
27.0
26.4
18.7
18.4
14.2
10.3 10.4
6.9
O
O
O
O
O
O
Fig. 6. Catalytic activity of various catalysts for the hydrolysis of cyclo‐
hexyl propionate, cyclohexyl butyrate, and phenyl acetate. Reaction
conditions: 10 mmol ester, 250 mmol H2O, 0.4 mmol catalyst, 100 °C, 7
h, 1 bar N2.
catalysts for the hydrolysis of esters.
Enrichment ability of SAPILs in swollen state. The catalytic
activity of heterogeneous acid catalysts is typically much lower
than that of homogeneous acid catalysts [17]. However, SAPILs
exhibited much higher activities than homogeneous acid cata‐
lysts for the hydrolysis of esters. The excellent catalytic activity
of SAPILs was attributed to their unique micron‐sized 3D hon‐
eycomb structure and ester enrichment ability in swollen state.
To reveal the role of SAPILs during these catalytic processes,
the equilibrium concentrations of each component of a series of
simulated reaction mixtures (Mixture‐10%, 30%, 50%, 70%,
and 90%) inside and outside of SAPIL‐1 in aqueous phase were
analyzed using GC, and the concentrations of cyclohexyl ace‐
tate, cyclohexanol, and acetic acid inside and outside SAPIL‐1
were obtained (Table S7). The concentrations of acetic acid
inside and outside SAPIL‐1 were the same for all simulated
reaction mixtures, which indicated that SAPIL‐1 did not present
enrichment ability for acetic acid. The concentrations of cyclo‐
hexanol inside SAPIL‐1 were 1.3–2.6 times higher than those
CA tests indicated that the CA of cyclohexyl acetate on
SAPIL‐1 (21.3°) was lower than that of cyclohexanol (41.2°)
(Fig. 7(c)). These results indicated that the affinity of cyclohexyl
acetate for SAPIL‐1 was higher than that of cyclohexanol, which
resulted in the enrichment of cyclohexyl acetate inside SAPIL‐1.
250
200
150
100
50
3.0
2.5
2.0
1.5
1.0
0.5
0.0
(a)
(b)
(c)
Inside SAPIL-1
Outside SAPIL-1
Homogeneous system (p-TsOH)
Inside SAPIL-1
Outside SAPIL-1
Homogeneous system (p-TsOH)
2.645
204.1
i)
21.3°
41.2°
140.6
104.5
81.5
68.9
ii)
0.758
0.256
0.161
0.332
13.5
13.1
12.9
0.104
0.101
16.7
0.065
0.064
0.187
0.041
14.8
0.043
0.019
0.015
10.9
10.7
8.8
0.088
5.8
4.4
0
Mix.-50% Mix.-70% Mix.-90%
Mix.-10% Mix.-30%
Mix.-50% Mix.-70% Mix.-90%
Mix.-10% Mix.-30%
Simulated reaction mixture
Simulated reaction mixture
Fig. 7. (a) Concentration of cyclohexyl acetate in the aqueous phase of the simulated reaction mixtures. (b) Cyclohexyl acetate‐to‐cyclohexanol molar
ratio in the aqueous phase of the simulated reaction mixtures. (c) Contact angles of (i) cyclohexyl acetate and (ii) cyclohexanol on the surface of
SAPIL‐1. Mixture‐10%: cyclohexyl acetate (9 mmol), H2O (249 mmol), cyclohexanol (1 mmol), acetic acid (1 mmol); Mixture‐30%: cyclohexyl acetate
(7 mmol), H2O (247 mmol), cyclohexanol (3 mmol), acetic acid (3 mmol); Mixture‐50%: cyclohexyl acetate (5 mmol), H2O (245 mmol), cyclohexanol
(5 mmol), acetic acid (5 mmol); Mixture‐70%: cyclohexyl acetate (3 mmol), H2O (243 mmol), cyclohexanol (7 mmol), acetic acid (7 mmol); Mix‐
ture‐90%: cyclohexyl acetate (1 mmol), H2O (241 mmol), cyclohexanol (9 mmol), acetic acid (9 mmol); SAPIL‐1 (0.102 g, 0.2 mmol); p‐TsOH (0.038 g,
0.2 mmol).

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Bihua Chen et al. / Chinese Journal of Catalysis 42 (2021) 297–309
Consequently, during the hydrolysis of cyclohexyl acetate to
cyclohexanol, the density of cyclohexyl acetate inside SAPIL‐1,
which presented micron‐sized 3D honeycomb structure in wa‐
ter and high enrichment ability for cyclohexyl acetate, was high
during the entire catalytic process (Fig. S14), and thus, it led to
the great enhancement of the catalytic activity of SAPIL‐1. Ow‐
ing to their unique 3D honeycomb structure in water and the
high enrichment ability for esters, SAPILs presented excellent
catalytic activity for the hydrolysis of esters.
Reaction mechanism. Typically, acid‐catalyzed reactions in‐
volve nucleophile substitution (SN1 and SN2) or elimination
(E1) reaction mechanisms [7,18,19,51–53]. In this study, the
hydrolysis of cyclohexyl acetate was a typical nucleophile sub‐
stitution reaction (SN2 mechanism). The mechanism of the hy‐
drolysis of cyclohexyl acetate over SAPILs is presented in Fig.
S15. First, the carbonyl group of cyclohexyl acetate was proto‐
nated by the SAPIL catalyst, and thus, it became more electro‐
philic. Second, the protonated cyclohexyl acetate was subjected
to the nucleophilic attack of the O atom of H2O, which led to the
formation of the intermediate. Third, cyclohexanol, the product,
was obtained via the proton transfer from the intermediate.
Lastly, the SAPIL catalyst was regenerated after the release of
acetic acid.
Catalyst recycling. The reusability of the SAPILs was tested
for the hydrolysis of cyclohexyl acetate under optimized reac‐
tion conditions. The SAPIL‐1 catalyst was easily recovered via
deswelling (Fig. S16), and presented remarkable reusability
and stability after 10 cycles (Fig. 8). The conversion of cyclo‐
hexyl acetate and selectivity of cyclohexanol were maintained
at approximately 80% and 98%, respectively. In addition, the
acid concentration (1.91 mmol/g) and swelling ability in water
(Q = 23.0) of SAPIL‐1 were not changed after 10 cycles (Table
S8). No significant differences were observed in the FT‐IR
spectra of SAPIL‐1 before and after it was reused (Fig. S17).
The superior reusability of the SAPILs was ascribed to their
good thermal and chemical stability and reversible swell‐
ing–deswelling ability.
5
4
3
2
1
0
SAPIL-1
p-TsOH
0.30
0.25
0.20
0.15
0.10
0.05
0.00
Y1
Y2
1
3
6
9
12
24
Time (h)
1
3
5
7
9
11 13 15 17 19 21 23 25
Time (h)
Fig. 9. Direct hydration of cyclohexene to cyclohexanol catalyzed by
SAPIL‐1 and p‐TsOH. Reaction conditions: 20 mmol cyclohexene, 200
mmol H2O, 0.2 mmol (1 mol%) catalyst, 130 °C, 0.5 MPa N2, 1000 rpm.
hydration of cyclohexene under the same reaction conditions.
The catalytic activity of SAPIL‐1 was higher than that of the
homogeneous acid catalyst p‐TsOH. The cyclohexanol yields
obtained over SAPIL‐1 were 2%–31% higher than those ob‐
tained over p‐TsOH (Fig. 9). These results indicated that SAPILs
were highly efficient heterogeneous acid catalysts for the direct
hydration of cyclohexene to cyclohexanol.
3.4.2. Hydration of EO to EG
EG is an important chemical raw material for the manufac‐
ture of polyester, antifreeze, lubricants, and other chemicals
[54]. EG is most commonly synthesized via the catalytic hydra‐
tion of EO. Various types of catalysts such as H2SO4,
ion‐exchange resins, metal oxides, and zeolites, have been used
for the industrial hydration of EO to EG [55–57]. In this study,
we used APILs as catalysts for the hydration of EO to EG for the
first time. As presented in Fig. 10, the catalytic performance of
SAPILs for the hydration of EO to EG was good, and was much
Direct hydration of cyclohexene to cyclohexanol. The catalytic
performance of SAPILs was evaluated via the direct hydration
of cyclohexene to cyclohexanol. Fig. 9 and Table S9 illustrate
the catalytic activities of SAPIL‐1 and p‐TsOH for the direct
Conversion of EO
Selectivity of EG
100
80
60
40
20
0
100
80
60
40
20
0
Selectivity
Conversion
100
80
60
40
20
0
100
80
60
40
20
0
1
2
3
4
5
6
7
8
9
10
Fig. 10. Hydration of ethylene oxide (EO) to ethylene glycol (EG) over
various catalysts. Optimized reaction conditions (see Table S10): 50
mmol EO, 500 mmol H2O, 0.06 mmol (0.12 mol%) catalyst, 100 °C, 0.5 h
1.0 MPa N2.
Recycling run
Fig. 8. Catalytic reusability of SAPIL‐1 for the hydrolysis of cyclohexyl
acetate.

Bihua Chen et al. / Chinese Journal of Catalysis 42 (2021) 297–309
307
better than that of the commercial resin Amberlite IR‐120(H)
and H‐ZSM‐5. The catalytic activity of SAPIL‐1, which did not
present enrichment ability for the EO (Fig. S18), for this reac‐
tion was comparable with those of homogeneous acid catalysts.
That could be attributed to the high swelling ability of SAPIL‐1
in water facilitating the exposure of the acid sites to the reac‐
tants [26]. Therefore, SAPILs were highly efficient heterogene‐
ous acid catalysts for the hydration of EO to EG, and presented
potential for future industrial applications.
after 10 cycles. These features of SAPILs could provide a new
strategy for designing stable and highly efficient heterogeneous
acid catalysts.
Conflicts of interest
There are no conflicts to declare.
Appendix A. Supporting Information
4. Conclusions
Syntheses of [EG3(VIm)2]Br2, [O(VIm)2]Br2 and [VSIm]HSO4;
NMR spectra; FT‐IR spectra; XPS spectra; XRD pattern; swelling
abilities of SAPILs and SAPIL precursors; cryo‐SEM images;
catalytic data for hydrolysis of esters; catalytic data for hydra‐
tion of cyclohexene and ethylene oxide; details in enrichment
ability; titration data; recycling process; reaction mechanism.
SAPILs with moderate and single acid strength, adjustable
acid concentration, and good stability were successfully syn‐
thesized via the free radical copolymerization of IL monomers,
NaSS, and crosslinkers, followed by acidification. The synthe‐
sized SAPILs spontaneously self‐assembled into micron‐sized
3D honeycomb structures when swelling in water. The diame‐
ters of the honeycomb channels gradually increased from 3–6
to 20–30 μm as the swelling ability of SAPILs in water in‐
creased from Q = 6.5 to 24.5. SAPILs with honeycomb structure
in water were used as highly efficient solid acid catalysts for
hydrolysis and hydration reactions. The catalytic activity of the
synthesized SAPILs was much higher than those of homogene‐
ous acid catalysts, such as H2SO4, [VSIm]HSO4, and p‐TsOH, for
the hydrolysis of cyclohexyl acetate to cyclohexanol. The con‐
centrations of cyclohexyl acetate and cyclohexyl ace‐
tate‐to‐cyclohexanol molar ratios inside SAPIL‐1 for a series of
simulated reaction mixtures were 7.5–23.3 and 4.5–16.4 times
higher, respectively, than those outside of SAPIL‐1, which indi‐
cated the high enrichment ability of SAPILs for cyclohexyl ace‐
tate. The excellent catalytic activity of the synthesized SAPILs
was attributed to their micron‐sized 3D honeycomb structure
in water and high enrichment ability for the reactants. Moreo‐
ver, the SAPILs presented outstanding catalytic activity for the
hydration of cyclohexene and EO. The SAPILs were facilely
recovered via deswelling, and displayed remarkable reusability
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酸性聚离子液体溶胀诱导自组装形成类蜂窝状固体酸催化剂及其
高效催化水解反应
陈必华a, 丁 桐a, 邓 熹a, 王 鑫a, 张大卫a,b, 马三罐a, 张永亚a, 倪 兵c, 高国华a,*
a华东师范大学化学与分子工程学院, 上海市绿色化学与化工过程绿色化重点实验室, 上海200062, 中国
b剑桥大学化学系, 剑桥 CB2 1EW, 英国
c华东师范大学生命科学学院, 上海200241, 中国
摘要: 酸催化反应在化学工业中占据着十分重要的地位. 传统的液体酸催化剂催化性能优异, 但面临着能耗高, 腐蚀设备
和环境污染严重等问题. 相比于传统的液体酸催化剂, 固体酸催化剂, 如分子筛和磺酸树脂等大大缓解了经济和环境方面
的问题, 但也存在着催化活性差和易失活等缺陷. 酸性聚离子液体因其高密度的反应活性位点, 可设计调变的结构和酸性
以及可循环利用等特性而成为一种新型的高效多相酸催化剂, 引起了广泛的研究兴趣. 然而, 当酸性聚离子液体用作催化
剂时, 由于其酸中心不能充分地暴露在反应底物中, 使得它们的催化活性难以达到甚至超越均相催化剂的水平. 因此, 发
展一种催化活性高于均相的酸性聚离子液体催化剂仍是一个巨大的挑战. 我们研究组发现在反应底物中溶胀的聚离子液
体可作为一种准均相催化剂, 其催化活性与相应的均相离子液体相当, 这为提高多相催化剂的催化活性提供了一种新的策

Bihua Chen et al. / Chinese Journal of Catalysis 42 (2021) 297–309
309
略.
本文报道了一种在水中溶胀且自组装成类蜂窝状网络结构的酸性聚离子液体催化剂, 该催化剂在水解和水合反应中
表现出优异的催化性能, 其活性高于均相酸催化剂. 首先通过自由基聚合和酸化两步合成了一系列在水中高度溶胀的酸
性聚离子液体(SAPIL-1–6). 以三甲基磷氧(TMPO)为探针分子, 用31P魔角旋转核磁共振(31P-TMPO NMR)对SAPILs的酸性
进行了分析. 结果表明, SAPILs具有中等强度的单一酸性. 热重分析表明SAPILs拥有优异的热稳定性能(300–345 ºC), 显
著地优于商用磺酸树脂Amberlite IR-120(H) (245 ºC). 扫描电镜和冷冻电镜表明, 当SAPILs在水中溶胀时, 无孔的结构会自
发地形成微米级三维类蜂窝状网络结构. 这些类蜂窝状网络结构的酸性聚离子液体在催化乙酸环己酯水解制备环己醇中
表现出卓越的催化性能, 其催化活性明显高于多相酸催化剂(Amberlite IR-120(H)和H-ZSM-5)和均相酸催化剂(硫酸, 对甲
苯磺酸和均相酸性离子液体[VSIm]HSO4). 通过气相色谱定量分析了在一系列模拟的反应体系中溶胀SAPIL-1内部和外部
各组分的平衡浓度, 发现SAPIL-1内部乙酸环己酯的浓度和乙酸环己酯与环己醇的摩尔比分别是其外部的7.5–23.3倍和
4.5–16.4倍, 这表明在反应过程中乙酸环己酯被大量富集. 此外, SAPILs在环己烯直接水合制备环己醇以及环氧乙烷水合
制备乙二醇的反应中均表现出优异的催化性能. 值得说明的是, SAPILs具有很好的循环使用性能, 10次循环后催化活性无
明显改变. 这些具有蜂窝状结构和对反应底物高富集SAPILs的成功合成及应用为开发高效的多相酸催化剂提供了一种新
的思路.
关键词: 多相酸催化剂; 酸性聚离子液体; 溶胀; 三维类蜂窝状网络结构; 富集; 水解; 水合
收稿日期: 2020-03-29. 接受日期: 2020-05-08. 出版日期: 2021-02-05.
*通讯联系人. 电话/传真: (021)62233323; 电子信箱: ghgao@chem.ecnu.edu.cn
基金来源: 国家自然科学基金(21773068, 21573072, 21811530273); 国家重点研发计划(2017YFA0403102); 上海市重点学科建设
项目(B409).
本文的电子版全文由Elsevier出版社在ScienceDirect上出版(http://www.sciencedirect.com/science/journal/18722067).