Highly recyclable polymer supported ionic liquids as efficient heterogeneous catalysts for batch and flow conversion of CO2 to cyclic carbonates

Article abstract of DOI:10.1039/c6ra26780g

A series of cross-linked polymer supported ionic liquids based on quaternary ammonium salts have been prepared using a facile method of radical polymerization. Styrene-functionalized quaternary ammonium chloride salts, which are derived from imidazole, tr

Full text of DOI:10.1039/c6ra26780g

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Highly recyclable polymer supported ionic liquids
as efficient heterogeneous catalysts for batch and
flow conversion of CO2 to cyclic carbonates†
a
Cite this: RSC Adv., 2017, 7, 2836
Tao Wang,ac Wenlong Wang, Yuan Lyu, a Xingkun Chen,ac Cunyao Li,ac Yan Zhang,ac
*
a
ab
*
Xiangen Song and Yunjie Ding
A series of cross-linked polymer supported ionic liquids based on quaternary ammonium salts have been
prepared using a facile method of radical polymerization. Styrene-functionalized quaternary ammonium
chloride salts, which are derived from imidazole, trimethylamine, triethylamine and triethanolamine, were
copolymerized with divinyl benzene (DVB) under solvothermal conditions. These polymer supported
ionic liquids displayed high efficiency in catalytic conversion of CO2 to cyclic carbonates. Notably, to the
best of our knowledge, it was the first time that the polymeric ionic liquid catalyst was applied in
a continuous flow fixed-bed reactor. During the study period of 130 hours, the polymer supported ionic
liquids exhibited high stability.
Received 14th November 2016
Accepted 6th December 2016
DOI: 10.1039/c6ra26780g
www.rsc.org/advances
highly thermal stability and non-ammability, exhibited high
activity for this reaction.5 However, the homogeneous catalytic
Introduction
ionic liquids still have the problems of catalysts recovery and
product purication.6 So, the immobilization of ionic liquids,
including covalent immobilization on polymers or inorganic
supports, have been extensively evaluated.7 Particularly, the
polymer supported imidazole-based ionic liquids (PSIL) are
a series of attractive catalysts because of their high activity and
excellent stability.8 Nevertheless, the other types of ionic liquids
supported on polymers were rarely investigated. Besides,
applying this kind of low-cost and robust catalyst to a contin-
uous xed-bed reactor is also very attractive due to their
potential industrial applications.
In this contribution, rstly, we designed two different
convenient ways for the synthesis of imidazolium based poly-
mer supported ionic liquids, and then a series of new polymer
supported quaternary ammonium chloride salts, which was
derived from trimethylamine, triethylamine and triethanol-
amine, were afforded by using the optimized method. The
prepared supported ionic liquids were systematically investi-
gated as catalysts for the batch and ow cycloaddition of CO2
with epoxides.
Nowadays, the inuence of global climate change on the human
living environment has attracted more and more people's
attention. Carbon dioxide (CO2) is the main greenhouse gas
produced by human activities, on the other hand, it is also one
of the most abundant carbon sources.1 Conversion of CO2 into
valuable chemicals facilitates the carbon cycle in nature, which
is an important goal of catalysis.2 In the eld of organic
solvents, green reagents and engineering plastics, the widely
used organic carbonates, like dimethyl carbonate and diphenyl
carbonate, can be derived from cyclic carbonates, which has
made the cycloaddition of CO2 with epoxides a commonly used
strategy.3
Considering the thermodynamic and kinetic stability of CO2
and the high electronegativity of the oxygen atom in epoxides,
which is easy to be induced by Lewis acids or other oxophilic
substances (such as –OH, –COOH, etc.), a suitable catalyst for
the reaction is needed for the desired high efficiency. Various
catalysts have been developed for the reactions, including metal
oxides, alkali metal salts, transition-metal complexes and ionic
liquids.4 It's worthwhile to notice that the ionic liquids (such as
quaternary ammonium salts and quaternary phosphonium
salts), which displayed excellent properties of low volatility,
Results and discussion
The imidazolium based PSIL were afforded by two simple
methods (Scheme 1). The vinyl functionalized imidazolium
chloride slat, 1,3-bis(4-vinylbenzyl)imidazolium chloride, was
synthesized by the reaction of 1-(chloromethyl)-4-vinylbenzene
with imidazole in the method of Path A. Then the imidazo-
lium based PSIL was afforded by the solvothermal copolymeri-
zation of 1,3-bis(4-vinylbenzyl)imidazolium chloride and DVB,
aDalian National Laboratory for Clean Energy, Dalian, 116023, P. R. China. E-mail:
dyj@dicp.ac.cn; luyuan@dicp.ac.cn; Fax: +86-411-84379143; Tel: +86-411-84379143
bState Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese
Academy of Sciences, Dalian, 116023, P. R. China
cUniversity of Chinese Academy of Sciences, Beijing 100039, P. R. China
† Electronic supplementary information (ESI) available: SEM, TEM images and
13C NMR spectra. See DOI: 10.1039/c6ra26780g
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Scheme 1 Synthesis of polymer supported ionic liquids of PSIL(IMD)-1
and PSIL(IMD)-2.
Fig. 2 Structures of the polymer supported ionic liquids PSIL(TMA),
PSIL(TEA), PSIL(TOA), the element analysis of N contents and textural
parameters of the PSIL.
which was denoted as PSIL(IMD)-1 (Fig. 1). It was worth
mentioning that the two steps were completed in an autoclave
without any separation and purication steps of the interme-
moderately porous with surface areas from 75.8 m2 gꢀ1 to
98.9 m2 gꢀ1. The pore volumes of all the PSIL were no more than
0.15 cm3 gꢀ1. Since the PSIL essentially maintained the same
polarity of the ionic liquids monomer, the relatively low pore
volumes of the PSIL were probably due to the strong van der
Waals force between ionic liquids in the backbone of the PSIL.9
diate product. Path
B copolymerized 1-(chloromethyl)-4-
vinylbenzene with DVB to get chloromethylated polymer
rstly. Then the polymer was further cross-linked by imidazole
to form the imidazolium based PSIL, which was denoted as
PSIL(IMD)-2. PSIL containing quaternary ammonium chloride
salts were also prepared in a similar manner of Path A.
PSIL(IMD)-1 exhibited
a little higher pore volume than
PSIL(IMD)-2, which was due to the different synthetic routes. In
the Path B, since imidazole was post immobilized on the
chloromethylated polymer, a portion of porous space in the
polymer was likely to be occupied. The results can be further
conrmed by the morphology of the PSIL using scanning elec-
tron microscopy (SEM) and transmission electron microscopy
(TEM) (Fig. S1–S5†). SEM and TEM images showed that all the
PSIL exhibited dense structure. Interestingly, aggregated poly-
mer particles were observed in PSIL(TMA), which was thought
to be result from the existence of water (H2O) during the prep-
aration of PSIL(TMA) (Fig. S3†). The addition of the aqueous
solution of trimethylamine made the reaction changed from
solution polymerization to suspension polymerization. Because
of the existence of H2O, the PSIL(TMA) exhibited relatively
higher pore volume than the other PSIL.
Trimethylamine-based
(PSIL(TMA)),
triethylamine-based
(PSIL(TEA)) and triethanolamine-based (PSIL(TOA)) polymer
supported ionic liquids were obtained by replacing imidazole
with trimethylamine, triethylamine and triethanolamine,
respectively (Fig. 2).
The nitrogen (N) contents of these materials were deter-
mined by elemental analysis (Fig. 2). The two imidazolium
based PSIL, which were afforded from different preparation
methods, gave signicant different N contents. PSIL(IMD)-1
gave a nitrogen content of 3.17%, which was just a little lower
than the added amount of it for the synthesis of PSIL(IMD)-1
(3.90%). While the nitrogen content of PSIL(IMD)-2 was as
little as 0.91%, which indicated that Path A was more feasible
than the Path B for the preparation of PSIL. Their pore prop-
erties were analysed by N2 physisorption (Fig. 2). They were
The structural information of PSIL(IMD)-1 was further
characterized by 13C MAS NMR (Fig. 3A). Probably due to the
addition of divinyl benzene, all the signals were overlapping
and were not very informative. The signals around 0–50 ppm
were attributed to the terminal CH2 protons, CH moiety and
methylene units that linked the benzene and imidazolium ring.
The signals around 100–150 ppm were assigned to carbon
atoms of benzene and imidazolium ring. It should be
mentioned that there was no signicant difference between the
13C MAS NMR spectrum of PSIL(IMD)-1 and PSIL(IMD)-2
(Fig. S6†), which indicated the feasibility of the two strategies
for the synthesis of PISL. The 13C MAS NMR spectrum of other
PISLs was also presented for the conrmation of their structural
Fig. 1 Illustration of solvothermal copolymerization of PSIL(IMD)-1.
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carbonate.8a,10 The reaction temperature was investigated in the
range of 100–140 ꢁC at 2 MPa with a reaction time of 5 h (Fig. 5).
The reaction temperature was shown to have a positive inu-
ence on the catalytic activity. In the relatively low temperature
range, the yield of propylene carbonate was drastically
increased from 53.9% to 83.3% with temperature from 100 ꢁC to
110 ꢁC. The rising of propylene carbonate yield slowed down in
the high temperature range. And the yield of propylene
ꢁ
carbonate achieved 99.9% at 130 C.
Fig. 3 (A) 13C MAS NMR spectra and (B) the TG curves of (a) PSIL(IMD)-
1.
To evaluate the catalytic activities of different PSIL for the
cycloaddition of CO2 with propylene oxide, the reaction was
ꢁ
carried out at 130 C for 5 h under the CO2 pressure of 2 MPa
(Table 1, entries 1–5). The yield of propylene carbonate cata-
lysed by PSIL(IMD)-1 could reach as high as 99.9% (entry 1).
While, the catalyst of PSIL(IMD)-2 only gave the yield of 80.2%
(entry 2). Considering the similar pore structures of the two
PSIL, the high activity of PSIL(IMD)-1 can be assigned to its
abundant nitrogen content. It was conrmed by the elemental
analysis of nitrogen contents. The other three PSIL also
exhibited satisfactory results with the yield of product higher
than 95% (entries 3–5). It was worth noted that the catalytic
activity of the three PSIL decreased in the order PSIL(TMA) >
PSIL(TEA) > PSIL(TOA), which was exactly the opposite order of
the nitrogen contents in the three PSIL. These indicated that the
species of quaternary ammonium salts in the backbone of the
PSIL had different inuences on the reaction. In order to verify
the catalytic efficiency of the PSIL, we also polymerized the
divinyl benzene itself, which was denoted as poly(DVB). The
poly(DVB) did not show catalytic activity in the reaction (entry
6). It was also notable that when the amount of substrate was
increased from 1.0 g to 3.0 g, the PSIL(IMD)-1 system still gave
an excellent yield of 96.4% (entry 7). By evaluating various
epoxides under the optimized conditions, the PSIL(IMD)-1
exhibited high yields for the target cyclic carbonates with the
S/C (substrate/catalyst) ratio of 127 (entries 8–11). In the re-
ported work of Dyson et al., a catalyst of poly[bvbim]Cl was
synthesized by direct polymerization of 1,3-bis(4-vinylbenzyl)
information (Fig. S7–S9†). Excellent thermal stability is a crucial
requirement for PSIL used as catalyst. As a representative, the
thermal stability of PSIL(IMD)-1 was characterized by ther-
mogravimetric analysis (TG) under N2 (Fig. 2B). The curve
shows an ideal thermal stability with decomposition tempera-
ꢁ
ture at 350 C.
The cycloaddition reaction of CO2 with epoxides in the
presence of PSIL was rstly conducted in a 30 mL autoclave for
the optimization of catalysts and conditions. Then the stability
of the optimized catalyst was tested in a continuous xed-bed.
The reaction conditions for the cycloaddition of CO2 with
propylene oxide were evaluated on the catalyst of PSIL(IMD)-1.
The inuence of the pressure of CO2 on the cycloaddition
reaction was evaluated at 120 ꢁC with a reaction time of 5 h
(Fig. 4). An increase in CO2 pressure from 1 MPa to 2 MPa
resulted in a little increase in the yield of propylene carbonate
(from 92.4% to 96.0%). The concentration of CO2 in the
substrate of propylene oxide increased with the pressure. This
facilitated the reaction because CO2 was a reactant. Further
increase of CO2 pressure from 2 MPa to 6 MPa leaded to
a decrease of the yield of propylene carbonate (from 96.0% to
87.2%). This was probably due to the gas–liquid diffusion
between propylene oxide and CO2, which have an effect on the
reaction kinetics in the mass transfer. So there are optical
ranges of CO2 pressure for the maximum yield of propylene
Fig. 4 Cycloaddition of CO2 with propylene oxide using the PSI- Fig. 5 Cycloaddition of CO2 with propylene oxide using the PSI-
L(IMD)-1 catalyst under different pressures of CO2 (reaction condi- L(IMD)-1 catalyst at different temperatures (reaction conditions: 0.12 g
tions: 0.12 g catalyst, 1.0 g (17.2 mmol) propylene oxide, 120 ꢁC, 5 h). catalyst, 1.0 g (17.2 mmol) propylene oxide, 2 MPa CO2, 5 h).
2838 | RSC Adv., 2017, 7, 2836–2841
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Table 1 Cycloaddition of CO2 with epoxides using the PSIL(IMD)-1
catalyst
Entrya
Catalyst
Epoxides R
Yieldc
1
2
3
4
PSIL(IMD)-1
PSIL(IMD)-2
PSIL(TMA)
PSIL(TEA)
PSIL(TOA)
Poly(DVB)
PSIL(IMD)-1
PSIL(IMD)-1
PSIL(IMD)-1
PSIL(IMD)-1
PSIL(IMD)-1
Me
Me
Me
Me
Me
Me
Me
Bu
99.9
80.2
97.5
96.3
95.4
None
96.4
89.1
98.3
98.5
97.5
5
6
7b
8
Fig. 6 Stability test of PISL(IMD)-1 in a fixed-bed reactor (reaction
conditions: 0.30 g catalyst, 15 mL minꢀ1 propylene oxide, 130 ꢁC, 2 MPa
CO2, GHSV ¼ 4000 hꢀ1).
9
10
11
Ph
CH2Cl
CH2OCH2CH]CH2
a
Reaction conditions: 0.12 g catalyst, 17.2 mmol epoxides, 2 MPa CO2,
reactor (Scheme 2 and Fig. 6). 0.3 g of PSIL(IMD)-1 was used
in the reaction. The substrate of propylene oxide was continu-
ously fed (15 mL minꢀ1) without any solvents. The gas hourly
space velocity (GHSV) was 4000 hꢀ1 and the pressure of CO2 was
2 MPa. The PSIL(IMD)-1 maintained stable activity through the
study period of 130 hours with the yield of propylene carbonate
slightly ranging between 41% and 45%. This was because the
comonomers of 1,3-bis(4-vinylbenzyl)imidazolium chloride and
DVB both had two styrene arms, and the ionic liquids could be
rmly incorporated into the backbone of PSIL. Han and co-
workers reported a polymer supported ionic liquids (PSIL 4),
which was afforded by the copolymerization of single vinyl
functionalized imidazole salt (3-butyl-1-vinylimidazolium chlo-
ride) and DVB.8a The experiment of recyclability exhibited their
PSIL 4 catalyst was used ve runs with no considerable decrease
in the yield of propylene carbonate (from 97.4% to 95.8%), since
the ionic liquid was graed onto a highly cross-linked polymer
matrix with one arm. In addition, comparing with the metal
existed catalysis systems (metal oxides, alkali metal salts and
b
c
130 ꢁC, 5 h. 3.0 g (51.6 mmol) propylene oxide. The yield for
propylene carbonate was determined by GC, the yields for other cyclic
carbonates were determined by H NMR.
1
imidazolium chloride without the cross-linker DVB.11 And the
poly[bvbim]Cl catalyst gave the conversions of allyl glycidyl
ether and styrene oxide no more than 81% with the S/C ratio of
100, which were conducted at 140 ꢁC and 2 MPa with a reaction
time of 24 h. The catalyst of PSIL(IMD)-1 exhibited high
conversions of these two substrates with 98% (entry 9 and entry
11). The main reason was because the density of the ionic
liquids in poly[bvbim]Cl was much larger than that in the
PSIL(IMD)-1, which resulted in deeply imbedded and insuffi-
cient use of the active sites.
Since the catalytic stability and reusability was of primal
interest for the design of heterogeneous catalyst, the stability
examination of PSIL(IMD)-1 was conducted in a xed-bed
Scheme 2 The simple flow chart of the reaction equipment.
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transition-metal complexes, etc.), there was no trouble of metal was washed with methanol in a Soxhlet for 12 h. PSIL(IMD)-1
ꢁ
leaching in the PSIL catalysts. So the catalyst of PSIL(IMD)-1 was nally dried under reduced pressure at 60 C for 24 h.
could show excellent catalytic stability.
PSIL(IMD)-2 (using the method of Path B). In an autoclave,
1.50 g of 1-(chloromethyl)-4-vinylbenzene and 1.50 g of DVB
were dissolved in 30 mL of DMF, followed by the addition of
0.075 g of AIBN. The mixture was rstly stirred for 1 h at the
room temperature, and then heated at 100 ꢁC for 24 h. Then the
solvent was removed under vacuum at 120 ꢁC and an orange
solid was obtained. Then 2.50 g of the solid and 0.257 g of
imidazole were dissolved in 25 mL of DMF. The new mixture
Conclusions
Two simple strategies were presented for the synthesis of PSIL.
Through the comparison of the nitrogen contents and the
catalytic activity of PSIL(IMD)-1 and PSIL(IMD)-2, the Path A was
proved as a more useful way than Path B. Using the method of
Path A, a series of polymer supported quaternary ammonium
chloride ionic liquids were afforded by the radical copolymeri-
zation of 4-vinylbenzyl functionalized quaternary ammonium
chloride salts with DVB. The prepared PSIL exhibited high
catalytic activities in the cycloaddition of CO2 with epoxides.
Especially for the PSIL(IMD)-1 catalyst, which exhibited higher
catalytic activity and stability than some reported polymer
supported imidazole-based ionic liquids. Considering that the
ionic liquids was rmly embedded in the polymer backbone,
the PSIL(IMD)-1 catalyst was applied in a xed-bed reactor for
the rst time. As expected, the PSIL(IMD)-1 exhibited stable
performance in the cycloaddition of CO2 with propylene oxide
aer continuous ow reaction for 130 hours.
ꢁ
was stirred at 60 C for 24 h. The precipitate was washed with
methanol in a Soxhlet for 12 h. PSIL(IMD)-2 was nally dried
ꢁ
under reduced pressure at 60 C for 24 h.
PSIL(TMA). Using the method of Path A for preparing
PSIL(IMD)-1, we replaced imidazole with 0.549 g of trimethyl-
amine (33% in H2O) and PSIL(TMA) was obtained.
PSIL(TEA). Using the method of Path A for preparing
PSIL(IMD)-1, we replaced imidazole with 0.940 g of triethyl-
amine and PSIL(TEA) was obtained.
PSIL(TOA). Using the method of Path A for preparing
PSIL(IMD)-1, we replaced imidazole with 1.39 g of triethanol-
amine and PSIL(TMA) was obtained.
Typical procedure for the synthesis of propylene carbonate via
an autoclave
Experimental
As a typical run for the cycloaddition of CO2 and propylene
oxide, PISL(IMD)-1 (0.12 g) and propylene oxide (1.0 g, 17.2
mmol) were added into a 30 mL autoclave. The reactor was
purged 2 times with CO2 and heaꢁted from room temperature to
the reaction temperature of 130 C, stirred for 5 h. During the
reaction, the pressure in the reactor was held constant (2.0 MPa)
by the injection of more CO2 with a pressure regulator. The
catalyst was separated by centrifugation. The product was ana-
lysed by an Agilent 7890A gas chromatograph with a HP-5
column using a FID detector and 1-butanol as an internal
standard. It was worth noting that the yields for other cyclic
carbonates were determined by 1H NMR using 1,3,5-trime-
thoxybenzene as an internal standard.
CO2 was supplied by Zhonghao Guangming research & design
institute of chemical industry Corp. with a purity of 99.99%.
Imidazole (99%), 1-(chloromethyl)-4-vinylbenzene (90%) and
AIBN (99%) were obtained from J&K chemical. Divinylbenzene
(80%) was also provided by J&K chemical as a mixture of 1,2-,
1,3-, and 1,4-divinylbenzene. Trimethylamine (33% in H2O),
triethylamine (99%), triethanolamine and other reagents were
obtained from Tianjin Kemiou Chemical Reagent Co., Ltd and
used as received. The N element analysis was performed by
organic element analyzer of Flash 2000, Thermo scientic.
Nitrogen isotherms at 77.3 K were measured using Quantach-
rome Autosorb-1. The sample was outgassed at 120 ꢁC for 10 h.
Thermogravimetric analysis (TG) was measured using
NETZSCH STA 449F3, and the sampꢀle1s were heated from 40 ꢁC
Typical procedure for the synthesis of propylene carbonate via
a xed-bed reactor
ꢁ
ꢁ
to 1000 C at the rate of 10 C min under N2. Transmission
electron microscope (TEM) images were taken on a JEM-2100
with an accelerating voltage of 200 kV. Polymer morphologies
were investigated on a JSM-7800F scanning electron microscope
(SEM). 13C MAS NMR spectra were obtained on a VARIAN
innity plus 400 spectrometer under a magic angle spinning
rate of 6 kHz.
The input of CO2 and propylene oxide. As shown in Scheme
2, CO2 was delivered with a ow of 50 mL minꢀ1 via a mass ow
controller (Beijing Sevenstar Electronics Co., Ltd. D07-11A).
Propylene oxide was fed through a liquid pump (NS, NP-KX)
and mixed with CO2 before entering the reactor. The liquid
ow was 15 mL minꢀ1
.
The reaction area. The xed-bed reactor was 40 cm long with
an inner diameter of 6 mm. 0.30 g of PSIL(IMD)-1 was fully
Synthesis of PSIL
PSIL(IMD)-1 (using the method of Path A). In an autoclave, mixed with 1.0 g of quartz sand. The mixture was placed in the
1.50 g of 1-(chloromethyl)-4-vinylbenzene and 0.316 g of imid- middle of the reactor, and the other area of the reactor was lled
azole were dissolved in 30 mL of N,N0-dimethylformamide with quartz sand. The reactor was heated by using a tube
(DMF). The mixture was stirred at 60 ꢁC for 24 h. Then 1.50 g of furnace and the temperature was monitored by using
DVB and 0.075 g of 2,20-azoisobutyronitrile (AIBN) were added a thermocouple.
to the mixture. It was rstly stirred for 1 h at the room
The product collection and analysis. As the reaction pro-
temperature, and then heated at 100 ꢁC for 24 h. The precipitate ceeded, the liquid product was stored in a collection vessel. The
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gas passed through a back pressure regulator and was analysed
on-line by an Agilent 7890A gas chromatograph with a Porapak-
QS column (3 m length, 3 mm diameter) using an TCD detector.
The liquid product was decompressed to ambient pressure and
collected in a decompression collection vessel. And it was ana-
lysed off-line by an Agilent 7890A gas chromatograph with a HP-
5 column using an FID detector.
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Acknowledgements
This work was supported by the Strategic priority research
program of the Chinese Academy of Sciences, Grant No.
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