CO2 cycloaddition reactions catalyzed by an ionic liquid grafted onto a highly cross-linked polymer matrix

Article abstract of DOI:10.1002/anie.200701467

(Chemical Equation Presented) A support group for ionic liquids: 3-Butyl-1-vinylimidazolium chloride supported covalently on a polymer cross-linked with divinylbenzene gives rise to a very active, stable, and selective heterogeneous catalyst 1 for the add

Full text of DOI:10.1002/anie.200701467

Angewandte
Chemie
DOI: 10.1002/anie.200701467
Heterogeneous Catalysis
CO2 Cycloaddition Reactions Catalyzed by an Ionic Liquid Grafted
onto a Highly Cross-Linked Polymer Matrix**
Ye Xie, Zhaofu Zhang, Tao Jiang, Jinling He, Buxing Han,* Tianbin Wu, and Kunlun Ding
In recent years, room-temperature ionic liquids (ILs) have
attracted much attention owing to their unique properties
such as non-volatility, non-flammability, and recyclability.[1]
Moreover, ILs can be functionalized by designing different
cations and anions. Some functional ILs have been synthe-
sized, and their applications in different fields have been
reported.[2] Specifically, ILs have shown great potential as
catalysts for various reactions.[3]
Scheme S1 in the Supporting Information), and various
catalysts have been developed for the reactions, including
alkali-metal halides,[9] organic bases,[10] metal oxides,[11] zeo-
lite,[12] titanosilicates,[13] smectites,[14] and metal complexes.[15]
However, disadvantages still exist such as low activities, air
sensitivity, and requirement of high temperatures and co-
catalysts. Recently, it was demonstrated that some ILs are
very active catalysts for the cycloaddition of CO2 to epox-
ides,[16] and ILs supported on silica have been used as
catalyst.[17]
Although the cycloaddition of CO2 to epoxides to produce
cyclic carbonates has been studied extensively, the design of
highly effective heterogeneous catalysts is still desirable. In
this work, we copolymerized 3-butyl-1-vinylimidazolium
chloride ([VBIM]Cl) with the cross-linker divinylbenzene
(DVB) to prepare a highly cross-linked polymer-supported IL
(PSIL), in which [VBIM]Cl was covalently anchored on
DVB-cross-linked polymer matrix. The catalytic performance
of the PSIL for the cycloaddition of CO2 to epoxides was
investigated, and it was demonstrated that the catalyst was
very active, selective, and stable, and could be easily
separated from the products and reused. Synthesis of effective
and completely insoluble polymer-supported IL catalysts is an
interesting topic. The unique advantage of the highly cross-
linked polymeric catalyst is that it is not soluble in any
commonly used organic solvents. To the best of our knowl-
edge, this is the first demonstration of supporting an IL on a
highly cross-linked polymer matrix to prepare an active and
insoluble catalyst.
It is known that both homogeneous and heterogeneous
catalysts have their advantages and shortcomings. For exam-
ple, homogeneous catalytic systems usually show higher
activity, while heterogeneous catalysts are more easily
separated from products. To combine the advantages of
heterogeneous and homogenous catalysts has been a topic of
interest for years, and immobilization of catalysts on suitable
supports is an effective way to do so. Polymeric materials are
very attractive supports of catalysts as a result of some
unusual advantages.[4] In recent years, immobilization of ILs
onto polymers has been studied. For example, Carlin and
Fuller prepared a catalytic membrane for heterogeneous
hydrogenation which was composed of ILs and poly(vinyli-
dene fluoride)-hexafluoropropylene copolymers with the
incorporation of Pd on activated carbon.[5] Vankelecom and
co-workers[6] fabricated hydrogenation catalysts that showed
excellent activity by impregnation of transition metals and ILs
into poly(diallyldimethylammonium chloride). Chi and Kim
supported imidazolium salts on polystyrene (PS) resin
through covalent bonding. The polymer-supported ILs were
highly efficient catalysts for some substitution reactions. In
particular, PS[hmim][BF4](hmim = 1-n-hexyl-3-methylimi-
dazolium cation) showed much higher catalytic activity than
the free IL.[7] Kou and co-workers reported that rhodium
nanoparticles in ILs stabilized by ionic copolymers displayed
excellent lifetimes and activity for arene hydrogenation in
ILs.[8]
The route to synthesize the PSIL is shown in Scheme 1.
[VBIM]Cl (3) was first prepared from 1-vinylimidazole (1)
and 1-chlorobutane (2). The cross-linked polymer with
supported IL 4 was synthesized by radical polymerization of
[VBIM]Cl (3) and DVB using AIBN as the initiator. The
composition of the product was determined by elemental
analysis (N 2.18, C 82.73, H 7.81, Cl 3.55 %) and indicated
that the supporting amount of IL was about 1 mmol per 1 g
polymer.
Conversion of CO2 into valuable chemicals is of great
importance. One of the most successful examples is the
synthesis of cyclic carbonates from CO2 and epoxides (see
The morphology of the PSIL was observed using scanning
electron microscopy (SEM; see Figure S1 in the Supporting
[*] Y. Xie, Dr. Z. Zhang, Dr. T. Jiang, J. He, Prof. B. Han, Dr. T. Wu, K. Ding
Beijing National Laboratory for Molecular Sciences
Institute of Chemistry
Chinese Academy of Sciences
Beijing 100080 (China)
Fax: (+86)10-6256-2821
E-mail: hanbx@iccas.ac.cn
[**] The authors are grateful to the National Natural Science Foundation
of China (20533010) and the Ministry of Science and Technology of
China (2006CB202504).
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Scheme 1. Synthesis of the cross-linked-polymer-supported ionic liquid
4. AIBN: azobis(isobutyronitrile).
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Information). The size of the PSIL particles was on the
micrometer scale (Figure S1a in the Supporting Information),
and they had a rough surface (Figure S1b in the Supporting
Information).
The catalytic activity of the PSIL for the reaction of CO2
and propylene oxide (PO) was first studied. The influence of
temperature on the yield of propylene carbonate (PC) was
investigated at CO2 pressures of 6 and 10 MPa using the PSIL
as catalyst (Figure 1). The yield of PC strongly depended on
Figure 3. Propylene carbonate yields versus CO2 pressure. Reaction
conditions: PO (14.8 mmol), PSIL (0.100 g), 383 K, 6 h.
to that of the view cell was the same as that in the reaction
system. It was demonstrated that there were two phases in the
system under all the experimental conditions studied. The top
phase was a CO2-rich phase and the bottom phase was a PO-
rich phase. The concentration of CO2 in the bottom phase
increased with increasing pressure. This favored the reaction,
considering that CO2 was a reactant. On the other hand, the
concentration of PO in the bottom phase decreased as the
pressure was increased, a condition which was not favorable
to the reaction because PO was also a reactant. In addition,
more PO was extracted into the CO2-rich phase at higher
pressures which reduced the reaction rate.[2f] The competition
of these opposite factors resulted in a maximum in the
pressure versus yield curve.
Figure 1. Yields of propylene carbonate versus temperature. Reaction
&
conditions: PO (14.8 mmol), PSIL (0.100 g); 10 MPa CO2, 10 h;
*
6 MPa CO2, 7 h.
the reaction temperature at both pressures. In the lower
temperature range, the yield increased with increasing
temperature. However, the yield decreased slightly in the
high-temperature region. The main reason was that the higher
temperature also accelerated the side reactions. On the basis
of the preliminary results, other experiments were conducted
at 383 K. The dependence of the PC yield on reaction time at
383 K and 6 MPa CO2 is shown in Figure 2. It illustrates that
To evaluate the PSIL catalyst described here, the activities
of various catalysts for the cycloaddition of CO2 to PO were
examined under the same reaction conditions (Table 1). 1-
Table 1: Activities of various catalysts for the cycloadditon reaction of
CO2 and propylene oxide.[a]
Entry
Catalyst
Active sites [mmol]
Yield [%]
1
2
3
4
5
6
1-vinylimidazole
1-butyl chloride
[BMIM]Cl
0.200
0.200
0.200
0.200
0.200[c]
0.200[d]
0.75
0.12
92.8
75.3
45.6
97.4
[VBIM]Cl
PVBIMCl[b]
PSIL 4
[a] Reaction conditions: PO (29.6 mmol), 6MPa CO2, 383 K, 7 h;
[b] Synthesized by direct polymerization of [VBIM]Cl; [c] Calculated
using the amount of [VBIM]Cl; [d] Calculated using the amount of
[VBIM]Cl.
Figure 2. Dependence of the propylene carbonate yields on reaction
time. Reaction conditions: PO (14.8 mmol), PSIL (0.100 g), 6 MPa
CO2, 383 K.
Vinylimidazole and 1-chlorobutane showed negligible cata-
lytic activities(Table 1, entries 1 and 2, respectively). The
catalytic activity of the PSIL (Table 1, entry 6) was compa-
rable to or even better than those of the liquid catalysts 1-
butyl-3-methylimidazolium chloride ([BMIM]Cl) and
[VBIM]Cl, a monomer of the PSIL (Table 1, entries 3 and
4, respectively). The main reason for this was that miscibility
of the two liquid catalysts and the substrate was very poor,
and there was an IL phase in the reaction system at the
beginning which was observed in our experiments using the
view cell.[18] The inter-phase mass transfer reduced the
reaction rate, while the PSIL microparticles synthesized in
this work could be well dispersed in the reaction mixture
the yield of PC increased smoothly with the reaction time and
that nearly all the PO could be converted within 7 h. In all the
experiments depicted in Figure 2, the by-products were not
detectable.
Figure 3 shows the dependence of PC yield on pressure at
383 K with a reaction time of 6 h. An increase in pressure
resulted in an increase in the PC yield in the low-pressure
region and a decrease of the yield in the high-pressure region.
To obtain some evidence to explain this phenomenon, we
observed the phase behavior of the CO2-PO system by using
the view cell reported previously.[18] The volume ratio of PO
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Table 2: Catalytic activity of the reaction of various epoxides with CO2.[a]
under stirring. Moreover, another advantage of the solid PSIL
was that it was insoluble in the product, whereas the liquid
catalysts were soluble in the product. Table 1 also shows that
the PSIL was much more active than poly[VBIM]Cl
(PVBIMCl; Table 1, entry 5), which was synthesized by
direct polymerization of [VBIM]Cl without the cross-linker
DVB. This is easy to understand if it considered that the
density of the active sites in PVBIMCl was much larger than
that in the PSIL, which resulted in insufficient use of the
active sites. Note also that PVBIMCl was soluble in the
product which was unfavorable with respect to the easy
separation of the catalyst from the products. Therefore, the
cross-linker in the PSIL not only prevented the dissolution of
the catalyst but also enhanced the activity.
Experiments were also carried out to examine the
recyclability of the catalyst using PO as the substrate. In
each cycle, PSIL as a solid catalyst was recovered by filtration
and then rinsed with tetrahydrofuran, acetone, and methanol,
respectively. After drying, the catalyst was reused for the next
run. The yields of PC for the five repeated runs are shown in
Figure 4. There was no considerable decrease in the yield of
PC, indicating that the catalyst was not only insoluble in the
reaction mixture but was also very stable.
Entry
Epoxide
Carbonate
Reaction
time [h]
Yield [%]
95.8
1
2
3
3
96.4
3
4
7
79.1
93.1
72
[a] Reaction conditions: epoxide (14.8 mmol), catalyst PSIL (0.100 g),
6 MPa CO2, 383 K.
hampered the nucleophilic attack from the chloride and
therefore decreased the rate of ring opening.
In conclusion, [VBIM]Cl can be supported on a DVB
cross-linked polymer by covalent bonds simply by copoly-
merization of [VBIM]Cl and DVB. The polymer-supported
ionic liquid is very active, selective, and stable for the
cycloaddition of CO2 to epoxides. Also, it can be easily
separated from the products and reused. We believe that this
route can be used to support some other ILs on highly cross-
linked polymer matrixes to prepare polymer/functional IL
composites, which are stable, insoluble in organic solvents,
and have desired functions.
Figure 4. Reuse of the catalyst. Reaction conditions: PO (29.6 mmol),
PSIL (0.200 g), 6 MPa CO2, 383 K, 7 h.
Experimental Section
The SEM images of the catalyst after reuse five times are
also shown in Figure S1c,d in the Supporting Information.
The morphology of the PSIL was not changed considerably
after being reused five times. The thermogram of the PSIL is
shown in Figure S2 in the Supporting Information. It can be
observed that the PSIL was thermally stable up to 2438C,
which was much higher than the temperature used for the
cycloaddition of CO2 to epoxides. Both the SEM image and
the thermogram provide further evidence for the high
stability of the catalyst.
Using PSIL as the catalyst, cycloadditions of CO2 with
other epoxides were also examined at 383 K and 6 MPa
(Table 2). The results showed that the PSIL was active for all
the substrates used. Epichlorohydrin and glycidyl phenyl
ether were the most active substrates, and excellent yields of
carbonates were achieved in 3 h (Table 2, entries 1 and 2,
respectively). Styrene oxide (Table 2, entry 3) showed less
activity with only 79.1% yield obtained after 7 h, which may
have resulted from the low reactivity of its b-carbon center. A
much longer reaction time was needed to complete the
conversion of cyclohexene oxide (Table 2, entry 4), the reason
for which may be that its special cyclic molecular structure
CO2 was supplied by Beijing Analytical Instrument Factory with a
purity of 99.995%. 1-Chlorobutane, azobis(isobutyronitrile) (AIBN),
propylene oxide (PO), and epichlorohydrin were A.R. grade (Beijing
Chemical Plant). Divinylbenzene (DVB) was purchased from
FLUKA, and 1-vinylimidazole was provided by Aldrich. Other
epoxides were supplied by ACROS ORGANICS. Styrene epoxide
was purified by distillation and AIBN was recrystallized three times
before use. Other chemicals were used as received.
The procedures to prepare 3-butyl-1-vinylimidazolium chloride
([VBIM]Cl) were similar to that reported.[8] Thus, 1-chlorobutane
(18.50 g, 200 mmol) and 1-vinylimidazole (5.50 g, 58.3 mmol) were
added to a 25 mL two-necked flask equipped with a magnetic stirrer.
The mixture was heated at reflux for 24 h in an oil bath at 708C under
the protection of nitrogen with stirring. Then, the reaction mixture
was cooled down to room temperature. The top phase was poured
out, and the solid residue was washed three times with ethyl acetate
and then dried at 508C for 12 h under vacuum.
DVB-cross-linked polymers with the supported [VBIM]Cl were
prepared by radical copolymerization. In a typical experiment, DVB
(3.2 g, 24.6 mmol), [VBIM]Cl (0.5 g, 2.68 mmol), and AIBN (0.05 g)
were dissolved in chloroform (150 mL) in a three-necked flask under
nitrogen protection. The mixture was maintained at 708C at reflux for
48 h with stirring. The cross-linked-polymer-supported IL (PSIL)
formed was collected by filtration and washed separately with
tetrahydrofuran, acetone, and methanol. Then, the solid was dried
Angew. Chem. Int. Ed. 2007, 46, 7255 –7258
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Communications
under vacuum at 508C followed by grinding before use. The product
2004, 45, 569; e) S. G. Lee, Chem. Commun. 2006, 1049; f) P. B.
Webb, M. F. Sellin, T. E. Kunene, S. Williamson, A. M. Z.
Slawin, D. J. Cole-Hamilton, J. Am. Chem. Soc. 2003, 125, 15577.
[3]a) J. A. Boon, J. A. Levinsky, J. L. Pflug, J. S. Wilkes, J. Org.
Chem. 1986, 51, 480; b) V. V. Namboodiri, R. S. Varma, Chem.
Commun. 2002, 342; c) T. Welton, Coord. Chem. Rev. 2004, 248,
2459; d) J. M. Xu, B. K. Liu, W. B. Wu, C. Qian, Q. Wu, X. F. Lin,
J. Org. Chem. 2006, 71, 3991; e) S. Z. Luo, X. L. Mi, L. Zhang, S.
Liu, H. Xu, J. P. Cheng, Angew. Chem. 2006, 118, 3165; Angew.
Chem. Int. Ed. 2006, 45, 3093.
[4]a) C. W. Tsang, B. Baharloo, D. Riendl, M. Yam, D. P. Gates,
Angew. Chem. 2004, 116, 5800; Angew. Chem. Int. Ed. 2004, 43,
5682; b) R. Akiyama, S. Kobayashi, Angew. Chem. 2002, 114,
2714; Angew. Chem. Int. Ed. 2002, 41, 2602; c) X. Q. Yu, J. S.
Huang, W. Y. Yu, C. M. Che, J. Am. Chem. Soc. 2000, 122, 5337;
d) M. Takeuchi, R. Akiyama, S. Kobayashi, J. Am. Chem. Soc.
2005, 127, 13096; e) A. Michrowska, K. Mennecke, U. Kunz, A.
Kirschning, K. Grela, J. Am. Chem. Soc. 2006, 128, 13261.
[5]R. T. Carlin, J. Fuller, Chem. Commun. 1997, 1345.
[6]A. Wolfson, I. F. J. Vankelecom, P. A. Jacobs, Tetrahedron Lett.
2003, 44, 1195.
was characterized by thermogravimetric analysis (TGA, PERKIN-
ELMER7 Series Thermal Analysis System) at a heating rate of
208CminÀ1. The morphology of the PSIL was observed by scanning
electron microscope (JEOL JSM 6700F). The compositions of the
polymer were determined using a Flash EA1112 analyzer.
The cycloaddition reactions were carried out in a 10 mL stainless
steel reactor with magnetic stirrer with known amounts of PO and
PSIL charged into the reactor. The reactor was heated to desired
temperature, and CO2 was added to a suitable pressure under stirring.
The mixture was kept at this pressure during the reaction. After the
reaction, the reactor was cooled to 08C by using iced water and CO2
was released and passed through a cold trap with N,N-dimethylfor-
mamide as absorbant. After the catalyst was precipitated, the product
was analyzed by GC (Agilent 4890 D) with acetophenone as the
internal standard. The retention time of the products were compared
with available authentic standards. The purity and structure of the
product obtained at some typical experimental conditions were also
checked by NMR spectroscopy and GC-MS methods. The procedures
for other epoxides were similar, and the products were analyzed at
room temperature on a Bruker 400 MHz NMR spectrometer using
CDCl3 as solvent.
[7]D. W. Kim, D. Y. Chi, Angew. Chem. 2004, 116, 489, Angew.
Chem. Int. Ed. 2004, 43, 483.
[8]X. D. Mu, J. Q. Meng, Z. C. Li, Y. Kou, J. Am. Chem. Soc. 2005,
127, 9694.
Received: April 5, 2007
Published online: July 16, 2007
[9]N. Kihara, N. Hara, T. Endo, J. Org. Chem. 1993, 58, 6198.
[10]H. Kawanami, Y. Ikushima, Chem. Commun. 2000, 2089.
[11]H. Yasuda, L. N. He, T. Sakakura, J. Catal. 2002, 209, 547.
[12]M. Tu, R. J. Davis, J. Catal. 2001, 199, 85.
[13]R. Srivastava, D. Srinivas, P. Ratnasamy, Catal. Lett. 2003, 91,
133.
Keywords: cycloaddition · epoxides · ionic liquids · polymers ·
supported catalysts
.
[1]a) H. Zhao, S. V. Malhotra, Aldrichimica Acta 2002, 35, 75; b) R.
Sheldon, Chem. Commun. 2001, 2399; c) P. Wasserscheid, W.
Keim, Angew. Chem. 2000, 112, 3926; Angew. Chem. Int. Ed.
2000, 39, 3772; d) T. Welton, Chem. Rev. 1999, 99, 2071; e) C. C.
Tzschucke, C. Markert, W. Bannwarth, S. Roller, A. Hebel, R.
Haag, Angew. Chem. 2002, 114, 4136; Angew. Chem. Int. Ed.
2002, 41, 3964; f) J. Dupont, R. F. de Souza, P. A. Z. Suarez,
Chem. Rev. 2002, 102, 3667; g) P. Wasserscheid, T. Welton, Ionic
Liquids in Synthesis, Wiley-VCH, Weinheim, 2003; h) Ionic
Liquids: Industrial Applications to Green Chemistry (Eds.: R. D.
Rogers, K. R. Seddon), American Chemical Society, Washing-
ton, DC, 2002 (ACS Symposium Series 818).
[14]B. M. Bhanage, S. Fujita, Y. Ikushima, K. Torii, M. Arai, Green
Chem. 2003, 5, 71.
[15]a) R. L. Paddock, S. T. Nguyen, J. Am. Chem. Soc. 2001, 123,
11498; b) H. S. Kim, J. J. Kim, B. G. Lee, O. S. Jung, H. G. Jang,
S. O. Kang, Angew. Chem. 2000, 112, 4262; Angew. Chem. Int.
Ed. 2000, 39, 4096.
[16]a) Y. J. Kim, R. S. Varma, J. Org. Chem. 2005, 70, 7882; b) J. J.
Peng, Y. Q. Deng, New J. Chem. 2001, 25, 639; c) V. Calo, A.
Nacci, A. Monopoli, A. Fanizzi, Org. Lett. 2002, 4, 2561; d) H.
Kawanami, A. Sasaki, K. Matsuia, Y. Ikushima, Chem.
Commun. 2003, 896.
[17]J. Q. Wang, X. D. Yue, F. Cai, L. N. He, Catal. Commun. 2007, 8,
167.
[18]H. F. Zhang, B. X. Han, Z. S. Hou, Z. M. Liu, Fluid Phase
Equilib. 2001, 179, 131.
[2]a) K. Fukumoto, H. Ohno, Chem. Commun. 2006, 3081; b) D. M.
Li, F. Shi, S. Guo, Y. Q. Deng, Tetrahedron Lett. 2004, 45, 265;
c) G. H. Tao, L. He, N. Sun, Y. Kou, Chem. Commun. 2005, 3562;
d) S. Anjaiah, S. Chandrasekhar, R. Gree, Tetrahedron Lett.
7258
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