Catalytic fixation of CO2 to cyclic carbonates over biopolymer chitosan-grafted quarternary phosphonium ionic liquid as a recylable catalyst

Article abstract of DOI:10.1016/j.apcata.2014.07.008

We prepared chitosan-grafted quarternary phosphonium ionic liquid (1-butyl-triphenylphosphonium bromide) by a simple method and used it as catalyst for the synthesis of cyclic carbonates through CO₂ cycloaddition to epoxides in the absence of c

Full text of DOI:10.1016/j.apcata.2014.07.008

Applied Catalysis A: General 484 (2014) 26–32
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Catalytic fixation of CO2 to cyclic carbonates over biopolymer
chitosan-grafted quarternary phosphonium ionic liquid as a recylable
catalyst
Chen Jing-Xiana, Jin Bia, Dai Wei-Lia,∗, Deng Sen-Lina, Cao Liu-Rena, Cao Zong-Jiea,
Luo Sheng-Liana,∗, Luo Xu-Biaoa, Tu Xin-Mana, Au Chak-Tonga,b
a Key Laboratory of Jiangxi Province for Persistant Pollutants Control and Resources Recycle, Nanchang Hangkong University, Nanchang 330063, Jiangxi,
China
b Department of Chemistry, Hong Kong Baptist University, Kowloon Tong, Hong Kong, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 8 May 2014
Received in revised form 3 July 2014
Accepted 5 July 2014
Available online 12 July 2014
We prepared chitosan-grafted quarternary phosphonium ionic liquid (1-butyl-triphenylphosphonium
bromide) by a simple method and used it as catalyst for the synthesis of cyclic carbonates through CO2
cycloaddition to epoxides in the absence of co-catalyst and solvent. High yield of propylene carbonate
(96.3%) is obtained at 120 ◦C and 2.5 MPa in 4 h. We studied the roles of chitosan hydroxyl groups and ionic
liquid counter anions on catalytic activity. It is proposed that the facilitation of ring opening of epoxides is
a combined result of polarization (by hydrogen bonding due to hydroxyl groups), electronic interaction
(by [BuPh3P]+), and nucleophilic attack (by bromide anion). The catalyst can be easily recovered and
reused as demonstrated in a test of five runs without showing any significant loss of activity. From the
view point of commercial application, the catalyst is attractive because it is low-cost, ecology-safe, stable
and efficient. Furthermore, it is potentially applicable for fixed-bed continuous flow reactors.
© 2014 Elsevier B.V. All rights reserved.
Keywords:
Carbon dioxide fixation
Cycloaddition reaction
Chitosan
Quarternary phosphonium ionic liquid
Cyclic carbonate
1. Introduction
because of their polarity, structure adjustability, and acid-basic
property. Frequently they were immobilized on suitable mate-
Despite classified as a greenhouse gas, carbon dioxide is an ideal
C1 feedstock that is renewable, non-toxic, and low-cost [1–4]. With
the fast consumption of fossil fuels and the foreseeable “C1 cri-
sis”, the transformation of CO2 into valuable chemicals has received
much attention. One of the most attractive and feasible strategies
for CO2 utilization is the cycloaddition of CO2 into epoxides. The
desired products are cyclic carbonates that are used as polar apro-
tic solvents, electrolytes in lithium secondary batteries, as well as
precursors for the formation of polycarbonates and numerous fine
chemicals [2,4].
A wide range of homogeneous and heterogeneous catalysts
were developed, including alkali metal salts [5], organic bases
[6–8], metal oxides [9,10], modified molecular sieves [11,12],
organometallic complexes [13,14], and ionic liquids (ILs) [15–22].
The use of ILs for the reaction is studied extensively and intensely.
Besides being low-cost and simple to synthesize, ILs are used
rials (e.g., SiO2 [23], SBA-15 [24], MCM-41 [25], and polymers
[20,26–29]) for efficient catalyst separation and product purifica-
tion. However, the immobilization of ILs onto inorganic supports
always results in catalysts that are poor in reusability. On the other
hand, the use of polymers for the grafting of ILs is found promising.
For example, the grafting of imidazolium or quaternary phospho-
nium ILs on polymer nanoparticles [27], porous polymer beads
[20,26,28] and tubular polymer [29], as well as polymerizing imid-
azolium ILs themselves [17] or with cross-linker [22,30] generated
catalysts that were reported to be good in catalytic activity and
reusability. Nevertheless, the preparation procedures of catalysts
and/or supports are complicated, high-cost, and not environment-
benign. Hence, the development of an efficient, highly selective,
stable and reusable catalyst that can be fabricated cheaply and
simply for the cycloaddition reaction is still a challenge.
Chitosan (CS) which is a deacetylated derivative of chitin is a
natural, abundant, and low-cost biopolymer. The material exhibits
interesting properties such as nontoxicity, biocompatibility, and
controllable biodegradability [31–33]. Besides, CS can be easily
modified chemically or physically, making it a versatile support
material [31–33]. However, there are few reports of CS being
∗
Corresponding authors. Tel.: +86 791 83953373; fax: +86 791 83953373.
E-mail addresses: wldai81@126.com (D. Wei-Li), sllou@hnu.edu.cn
(L. Sheng-Lian).
http://dx.doi.org/10.1016/j.apcata.2014.07.008
0926-860X/© 2014 Elsevier B.V. All rights reserved.

C. Jing-Xian et al. / Applied Catalysis A: General 484 (2014) 26–32
27
used as catalyst support for the synthesis of cyclic carbonates
from CO2 and epoxides [34–37]. Xiao et al. [34] reported that CS-
ZnCl2 together with 1-butyl-3-methyl imidazolium IL showed good
catalytic activity for the cycloaddition reaction. However, the co-
catalyst 1-butyl-3-methyl imidazolium IL is hard to separate from
products, and there is the need of adding fresh 1-butyl-3-methyl
imidazolium IL in each reaction. Zhao et al. [35] and Tharun et al.
[36,37] developed CS-supported quarternary ammonium ILs which
showed certain catalytic activity in the cycloaddition reaction but
the harsh reaction conditions and poor reusability of catalysts limit
their application.
Recently, it was reported that task-specific ILs such as hydroxyl-
or carboxyl-functionalized imidazolium [18,20,24], guanidinium
[15], quarternary ammonium [18] and quarternary phosphonium
ILs [16] showed good activity for the cycloaddition reaction
due to hydrogen bonding between the functional groups and
epoxides. The CS active sites for bonding with ILs and to
facilitate hydrogen bonding are, respectively, the amino and
hydroxyl groups in hexosaminide. Recently, we reported the syn-
thesis of hydroxyl-functionalized quarternary phosphonium IL
([HOC2H4Ph3P]Br) that is much superior to the corresponding tra-
ditional IL ([C2H5Ph3P]Br) in terms of catalytic activity [16]. In this
paper, we reported for the first time the fabrication of a CS-grafted
quarternary phosphonium IL and its application in the synthesis of
cyclic carbonates. Similar to the case of [HOC2H4Ph3P]Br [16], the
hydroxyl groups of CS show synergetic effect that promotes the
reaction. The catalyst exhibits good catalytic activity and selectiv-
ity, even in the absence of co-catalyst and solvent. Furthermore, the
catalyst can be easily recovered by filtration and reused for up to 5
times without showing any significant loss of activity. It is envis-
aged that the catalyst is suitable for large-scale production of cyclic
carbonates.
Scheme 1. Synthesis procedure of CS-[BuPh3P]Br.
with ethanol for three times, and dried under vacuum at 80 ◦C for
12 h to give the CS-[BuPh3P]Br catalyst.
[BrBuPh3P]Br. 1H NMR (400 MHz, DMSO-d6): ı = 7.90-7.73 (m,
15H), 3.63–3.32 (m, 4H), 2.49–2.47 (m, 2H), 1.99–1.64 (m, 2H). 13
C
NMR (100.6 MHz, DMSO-d6): ı = 135.4, 130.8, 119.3, 34.2, 33.2, 20.8,
19.6. IR (neat): n = 3074, 3057, 2931, 2885, 2862, 1585, 1485, 1436,
750, 690 cm−1. HR-MS (QTOF): m/z = 399.0864, calcd. for C22H25PBr
(M+H): 399.0877.
2.3. Characterization
2. Experimental
Scanning electron microscopic (SEM) observations were carried
out over a Nova NanoSEM 450 microscope. Energy dispersive X-ray
spectroscopy (EDS) was performed using accessory (INCA 250) of
the Nova NanoSEM 450 instrument. X-ray diffraction (XRD) pat-
terns were obtained on a Rigaku D/Max 2200PC operating at 40 kV
2.1. Chemicals
Chitosan was purchased from Sinopharm Chemical Reagent
Co., Ltd. The degree of deacetylation was 90% and the average
molecular weight was 5 × 104. Propylene oxide, triphenylphos-
phine, 1-bromobutane, 1,4-dibromobutane were purchased from
Shanghai Jingchun Industry Co., Ltd. The other epoxides were pur-
chased from Alfa Aesar China Co., Ltd. All chemicals were used
as received. The CO2 (99.9% purity) purchased from Nanchang
Guoteng Gas Co. was used without any purification treatment.
˚
and 40 mA using Ni-filtered Cu K␣ radiation (ꢀ = 1.542 A). The XRD
patterns were recorded in the 2ꢁ range of 10◦ to 80◦. 1H and 13C
nuclear magnetic resonance (NMR) spectra were recorded on a
Bruker 400 MHz spectrometer using DMSO-d6 as solvent. High-
resolution mass spectra (MS-ESI) were recorded on a Waters Xevo
G2-S QTof whereas the FT-IR spectra were collected using a Bruker
vertex 70 FT-IR spectrophotometer. TG analysis was performed
on a SDT Q600 (TA Instruments-Waters LLC) at a heating rate of
15 ◦C/min in N2 flow.
2.2. Catalysts preparation
The preparation procedure of CS-grafted 1-butyl-triphenyl
phosphonium bromide (denoted hereinafter as CS-[BuPh3P]Br) is
illustrated in Scheme 1. First, a solution of triphenylphosphine
(10 mmol) in 10 mL toluene was dropped slowly into 10 mL toluene
solution containing 1,4-dibromobutane (10 mmol). The mixture
was stirred at 110 ◦C for 24 h under a nitrogen atmosphere. After
the reaction, the mixture was cooled down to room temperature
(RT), and the resultant crude solid was filtered out and washed
three times with diethyl ether, then dried under vacuum at 60 ◦C
for 12 h to give 4-bromobutyl-triphenyl phosphonium bromide
([BrBuPh3P]Br) as a white solid. Second, CS (2 g), 20 wt% NaOH
aqueous solution (2 g), [BrBuPh3P]Br (6 g) and isopropanol (20 mL)
were added into a 100 mL two-necked flask and the mixture was
stirred at 80 ◦C for 24 h under nitrogen. When the reaction was
completed, the mixture was neutralized by diluted hydrochloric
acid. With the addition of ethanol into the reaction solution, there
was the precipitation of product which was filtered out, washed
2.4. Cycloaddition reaction
The cycloaddition reactions were conducted in a 50 mL high-
pressure stainless-steel autoclave equipped with
a magnetic
stirring bar. In a typical run, the reactor was charged with epoxide
(35.7 mmol), catalyst (1.5 mol%, calculated according to the amount
of IL), and an appropriate amount of biphenyl (as internal standard
for GC analysis). After the reactor was fed with CO2 to a desired
pressure, the autoclave with its contents was heated to a selected
temperature and stirred for a designated period of time. Then the
reactor was cooled to 0 ◦C in an ice-water bath, and the residual CO2
was released. The resulting mixture was analyzed using a GC-mass
spectrometer. The products were quantitatively analyzed on a gas
chromatograph (Agilent 7890A) that was equipped with a FID and
a DB-wax capillary column (30 m × 0.53 mm × 1.0 ␮m).

28
C. Jing-Xian et al. / Applied Catalysis A: General 484 (2014) 26–32
Table 1
Catalytic performance of various catalysts a
Entry
Catalyst
Catalytic results
Yield (%)
Sel. (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
None
0
2.9
0
CSb
97.0
98.1
98.5
98.7
98.5
98.6
98.4
99.0
98.6
98.5
87.2
99.7
99.6
Ph3P
15.0
16.3
96.3
85.2
84.2
98.0
92.3
83.9
90.1
0.51
96.6
21.2
CS/Ph3P/BuBrc
CS-[BuPh3P]Br
[BuPh3P]Br
[BrBuPh3P]Br
[HOBuPh3P]Br
CS/[BuPh3P]Brd
CS-[BuPh3P]Bre
CS-[BuPh3P]Br/H2Oe
KBr
[BuPh3P]Br/KBrf
Ph3P/KBrf
a
Reaction conditions: PO 35.7 mmol, catalyst 1.5 mol%, CO2 pressure 2.5 MPa,
temp. 120 ◦C, time 4 h.
Fig. 1. FT-IR spectra of (a) [BuPPh3]Br, (b) CS-[BuPh3P]Br, and (c) CS.
b
CS 0.24 g.
c
CS 0.24 g, Ph3P 0.54 mmol, BuBr 0.54 mmol.
CS 0.24 g, [BuPh3P]Br 0.54 mmol.
CS-[BuPh3P]Br 0.9 mol%, H2O 0.32 mmol.
d
3. Results and discussion
e
f
Equal catalyst amount (0.54 mmol).
3.1. Catalyst characterization
To confirm the successful grafting of quarternary phosphonium
IL on CS, the FT-IR spectra of [BuPh3P]Br, CS-[BuPh3P]Br, and CS
were collected (Fig. 1). The spectra of both [BuPh3P]Br (Fig. 1a) and
repeating hexosaminide residues [42]. When CS is used for grafting
[BuPh3P]Br, there is the involvement of the NH2 sites at the C2 posi-
tion of pyranoid rings. The consequence is destruction of hydrogen
bonding and hence lowering of symmetry and regularity of the CS
chains.
The morphology of CS and CS-[BuPh3P]Br were studied by SEM.
As shown in Fig. 3, the smooth surface of CS is roughened with the
grafting of [BuPh3P]Br, which is ascribed to the loss of crystallinity,
in consistent with the results of XRD analysis. The EDS spectrum of
CS-[BuPPh3]Br (Fig. S1, Electronic Supplementary Material) shows
the presence of bromine and phosphine, and can be considered
as additional evidence for the successful grafting of quarternary
phosphonium ILs on CS. Based on the EDS results, the ILs loading
on CS is estimated to be 2.28 mmol/g.
The thermal stability of CS and CS-[BuPh3P]Br as investigated
by TG analysis. Weight loss attributable to the decomposition of
polysaccharide chain [36,43] is observed at ca. 250 ◦C over CS and
at ca. 210 ◦C over CS-[BuPh3P]Br (Fig. S2, Electronic Supplementary
Material). The poorer thermal stability of CS-[BuPh3P]Br is due the
breakdown of CS crystallinity upon [BuPh3P]Br grafting. It should be
noted that the reaction temperature for the cycloaddition reaction
is within the 90–140 ◦C range, far below the destruction tempera-
ture of the catalyst. Furthermore, the thermal stability results of CS
and CS-[BuPh3P]Br are in good agreement with those obtained in
XRD and SEM analysis.
CS-[BuPh3P]Br (Fig. 1b) exhibit bands at 1585, 1485 and 1433 cm−1
,
characteristic of the stretching vibrations of benzene skeleton.
The peaks at 890 and 1155 cm−1 that appear in the spectra of
both CS-[BuPh3P]Br (Fig. 1b) and CS (Fig. 1c) are characteristic
of saccharine structure [36,38]. The broad band in the region of
3200–3500 cm−1 observed in Fig. 1b and c is attributed to stretch-
ing vibrations of –NH and –OH with inter- as well as intra-molecular
hydrogen bonding [39,40]. It is noted that the 3200–3500 cm−1
band of CS-[BuPh3P]Br is weaker than that of CS, plausibly due to
the destruction of hydrogen bonding during the grafting process.
Compared with the spectrum of CS, there is the detection of the
1541 cm−1 band that is characteristic of C–N stretching vibration
in the spectrum of CS-[BuPh3P]Br. All the results indicate that there
is successful grafting of [BuPh3P]Br on the CS.
The XRD patterns of CS and CS-[BuPh3P]Br are shown in Fig. 2.
In the case of CS, there are CS peaks at 2ꢁ = 13◦ and 20◦ that can be
ascribed to the chitosan structure [41]. As for CS-[BuPh3P]Br, there
is only one broad peak centered at 2ꢁ = 22◦, indicating amorphous
structure. With CS, crystallinity is enhanced due to intramolecular
hydrogen bonding between the NH2 and OH groups within the
3.2. Catalytic performance
The catalytic activity of CS-[BuPh3P]Br for the cycloaddition of
CO2 to epoxides is compared with those of the other catalysts.
In this study, the reaction of PO with CO2 to produce propylene
carbonate (PC) was chosen as model reaction, and the results are
summarized in Table 1. In the absence of a catalyst, there is no
reaction whatsoever (entry 1). In the presence of CS, Ph3P, and
CS/Ph3P/BuBr (entries 2–4), PC yield is poor (<16.3%). It is only
over CS-[BuPh3P]Br that excellent performance can be achieved (PC
yield = 96.3%) (entry 5). It is noted that CS-[BuPh3P]Br is also supe-
rior to the homogeneous catalysts [BuPh3P]Br and [BrBuPh3P]Br
(entry 5 vs 6 and 7) in performance. We deduce that there is syn-
ergism between phosphonium IL and the hydroxyl groups of CS,
leading to the performance of CS-[BuPh3P]Br superior to that of
[BuPh3P]Br and [BrBuPh3P]Br. It is reminded that [BuPh3P]Br is
Fig. 2. XRD patterns of CS and CS-[BuPh3P]Br.

C. Jing-Xian et al. / Applied Catalysis A: General 484 (2014) 26–32
29
Fig. 3. SEM images of (a) CS and (b) CS-[BuPh3P]Br.
catalytically active itself while [BrBuPh3P]Br is the intermediate
adopted for the formation of CS-[BuPh3P]Br.
In order to study the synergism between phosphonium IL and
CS hydroxyl groups as well as to confirm the synergistic effect
on the cycloaddition reaction, catalysts with or without hydroxyl
groups were prepared and investigated. From Table 1, one can see
that over [BuPh3P]Br with no hydroxyl groups, PC yield is only
85.2%. While over the corresponding hydroxyl-functionalized cat-
alyst ([HOBuPh3P]Br), PC yield is 98.0%, much higher than that
over [BuPh3P]Br (entry 8 vs 7). It is noted that CS-[BuPh3P]Br is
slightly lower than [HOBuPh3P]Br (entry 5 vs 8), but much higher
than [BuPh3P]Br (entry 5 vs 6) in catalytic activity. Furthermore,
the mixing of CS with [BuPh3P]Br can also promote the catalytic
activity of [BuPh3P]Br (entry 6 vs 9). The above results illustrate
that the superior activity of CS-[BuPh3P]Br is likely to be due to
the CS hydroxyl groups that are present in abundance. In other
words, the hydroxyl groups of CS promote the ring opening of epox-
ide through efficient synergistic interaction involving the step of
hydrogen bonding [18,21,24,30,44]. We purposely added certain
amount of water into the reaction system and observed significant
enhancement of activity over CS-[BuPh3P]Br (entry 10 vs 11). The
result further proves that hydroxyl groups have a critical role in the
cycloaddition reaction.
It is well known that the nucleophilic attack of epoxides
by halide anions is also crucial for the opening of epoxy ring
[18,21,24,30,44]. Compared to [BuPh3P]Br, [BrBuPh3P]Br that has
one more bromine atom as substituent group shows activity sim-
ilar to that of [BuPh3P]Br (entry 6 vs 7). Nevertheless, despite KBr
by itself is almost inactive (entry 12), the addition of KBr results in
activity enhancement of [BuPh3P]Br (entry 6 vs 13) and Ph3P (entry
3 vs 14). The results clearly confirm the important role of bromide
anions in the cycloaddition reaction.
Fig. 4. Effect of catalyst loading on PC yield and selectivity. (Reaction conditions:
PO 35.7 mmol, CO2 pressure 2.5 MPa, 120 ◦C, 4 h.)
roughly at ca. 99%. With the change of reaction temperature from
90 to 120 ◦C, there is a rise of PC yield from ca. 63 to 95%. Above
120 ◦C, there is a slight decrease of PC yield and selectivity. An
increase of reaction temperature results in increase of reactivity
but decrease in solubility of gas-phase CO2 in the reaction system.
At temperature higher than 120 ◦C, there is decrease of propylene
3.3. Effect of reaction parameters
Fig. 4 shows the effect of CS-[BuPh3P]Br loading on PC yield and
selectivity. It is observed that PC selectivity stays at ca. 99% in the
catalyst loading range of 0.3–2.1 mol%. With increase of catalyst
loading from 0.3 to 1.5 mol%, there is sharp rise of PC yield from
63.8 to 96.3%. The increase of PC yield is not significant above cata-
lyst loading of 1.5 mol%, plausibly due to the poor dispersion of the
excess catalyst that hinders the mass transfer of reagents towards
the active sites [45].
The influence of reaction temperature on the activity of CS-
[BuPh3P]Br is shown in Fig. 5. It is found that PC selectivity is
insensitive to the change of temperature from 90 to 140 ◦C, staying
Fig. 5. Effect of reaction temperature on PC yield and selectivity. (Reaction condi-
tions: PO 35.7 mmol, catalyst 1.5 mol%, CO2 pressure 2.5 MPa, 4 h.)

30
C. Jing-Xian et al. / Applied Catalysis A: General 484 (2014) 26–32
Fig. 6. Effect of CO2 pressure on PC yield and selectivity. (Reaction conditions: PO
Fig. 7. Effect of reaction time on PC yield and selectivity. (Reaction conditions: PO
35.7 mmol, catalyst 1.5 mol%, 120 ◦C, 4 h.)
35.7 mmol, catalyst 1.5 mol%, CO2 pressure 2.5 MPa, 120 ◦C.)
oxide (PO) conversion due to the poor CO2 solubility [19]. Further-
more, a higher reaction temperature accelerates side reactions such
as isomerization to acetone, as well as hydrolysis to diol, causing
decline of selectivity to propylene carbonate (PC) as a result [30,44].
The results indicate that the temperature has a pronounce effect on
the reaction, and 120 ◦C is the most suitable for cycloaddtion.
We also studied the influence of CO2 pressure on the cycloaddi-
tion reaction. As shown in Fig. 6, an increase of CO2 pressure from
1.5 to 2.5 MPa results in a rapid rise of PC yield, but there is decline
of PC yield in the high-pressure region (2.5–3.5 MPa). It can be seen
that a moderate CO2 pressure of 2.5 MPa is the most suitable for the
reaction. Similar effects of CO2 pressure on catalytic activity were
observed in other catalytic systems [15,26,30,35,36,45–47]. The
phenomenon can be explained based on the concepts of reaction
equilibrium and mass transfer of reactants. For the cycloaddition
reaction, PC is in its liquid form under the adopted reaction condi-
tions, and the increased of CO2 pressure should be a positive factor
for PO conversion. It is regarded that there are two phases in the
reaction system. In the low-pressure region (1.5–2.5 MPa), a rise of
CO2 pressure leads to an increase of CO2 concentration in the liquid
phase, which is a favorable factor for the reaction. However, in the
high-pressure region (2.5–3.5 MPa), it is envisaged that too high a
CO2 pressure could decrease the concentration of PO as a result of
dilution by excessive amount of CO2, which was not favorable for
the reaction. In addition, it is known that acidic CO2 dissolves in
basic epoxide, forming CO2-epoxide complexes as a result [23,48].
Too high a CO2 pressure could retard the separation of CO2 from PO
in the duration of cycloaddition reaction, decreasing the PC yield as
a result [10,15,16,47]. Therefore, a higher CO2 pressure does not
benefit the cycloaddition reaction catalyzed by chitosan-grafted
phosphonium IL. It is noted that the PC selectivity (ca. 99%) is almost
independent of the rise of CO2 pressure.
and selectivity across the five consecutive runs. To further investi-
gate the stability of CS-[BuPh3P]Br, we characterized the recovered
catalyst (after five runs) by TG and FT-IR. It was found that sim-
ilar to the fresh catalyst, the used catalyst is stable at ca. 210 ◦C
(Fig. S3, Electronic Supplementary Material). Furthermore, the FT-
IR spectra of fresh and recycled (5 times) CS-[BuPh3P]Br are alike
(Fig. S4, Electronic Supplementary Material). The results show that
the CS-[BuPh3P]Br catalyst is thermally as well as chemically sta-
ble, indicating that the catalyst has high potential for commercial
application.
3.5. Catalytic activity towards different epoxides
We find that the CS-[BuPh3P]Br catalyst can be applied to other
epoxide substrates for the synthesis of cyclic carbonates (Table 2).
A variety of monosubstituted terminal epoxides (entries 1–5) can
be converted to the corresponding cyclic catbonates in high yield
and selectivity. It is well known that the epoxides with an electron-
withdrawing group are able to stabilize the intermediate structure
formed after ring-opening of epoxides, thus enhancing the reac-
tivity [49]. Hence, the epichlorohydrin (entry 2) shows the best
reactivity, and the corresponding carbonates are obtained almost
quantitatively in 4 h. On the other hand, the conversions of other
epoxides (i.e. 1,2-hexene oxide, styrene oxide, glycidyl phenyl
ether) were lower than those of other monosubstituted terminal
epoxides, probably because of the low reactivity of the ␤-carbon
The influence of reaction time on PC yield and selectivity was
investigated and the results are depicted in Fig. 7. It can be seen
that the yield of PC increases rapidly to reach ca. 96% within the
first 4 h while PC selectivity remains at ca. 99% throughout. Thus,
a reaction time of 4 h is appropriate for the cycloaddition reaction
under the adopted conditions.
3.4. Catalyst recovery
The stability and recyclability of the CS-[BuPh3P]Br catalyst was
tested in an experiment of five runs (Fig. 8). The catalyst was
recovered by simple filtration, drying in vacuum, and then reused
for the next cycle. It was found that the catalyst CS-[BuPh3P]Br
performs well without showing any significant loss of PC yield
Fig. 8. Recycle of catalyst for cycloaddition reaction. (Reaction conditions: PO
35.7 mmol, catalyst 1.5 mol%, CO2 pressure 2.5 MPa, 120 ◦C, 4 h.)

C. Jing-Xian et al. / Applied Catalysis A: General 484 (2014) 26–32
31
Table 2
Cycloaddition of CO2 to various epoxides.a
Entry
Epoxide
Product
Time (h)
Catalytic results
Yield (%)
Sel. (%)
O
O
O
O
1
4
96.3
98.7
CH3
CH3
O
O
2
3
4
6
97.3
97.1
98.6
97.6
O
O
Cl
Cl
O
O
O
O
O
CH3
( )3
CH3
( )3
O
O
O
4
5
6
96.9
97.4
O
O
O
O
O
6
95.0
74.3
96.4
98.9
Ph
O
Ph
O
O
O
6
24
O
a
Reaction conditions: PO 35.7 mmol, CS-[BuPh3P]Br 1.5 mol%, CO2 pressure 2.5 MPa, temp. 120 ◦C.
atom of those epoxides (entries 3–5) [36]. The disubstituted epox-
ide, cyclohexene oxide (entry 6), exhibits the lowest activity for the
production of the corresponding cyclic carbonate, probably due to
the high steric hindrance as a result of the rings.
the epoxy ring, which was activated by the hydroxyl groups
and phosphonium cation through hydrogen bonding as well as
electronic interaction. Then the haloalkoxy intermediate further
reacts with CO2 to form a linear halocarbonate that transforms
3.6. Possible mechanism
From the results displayed so far, one can see that both the
hydroxyl groups and bromide anions are essential for the excel-
lent performance of the CS-[BuPh3P]Br catalyst. It was reported
that when carboxyl- or hydroxyl-functionalized IL are used as cat-
alysts, there is polarization of C–O bond as a result of hydrogen
bonding between the hydrogen atom of carboxyl (or hydroxyl)
and the oxygen atom of epoxide, facilitating the opening of
epoxide ring [15,18,21,26,30,44,47]. In the use of imidazolium
[19] or guanidinium [50] IL as catalysts, it is regarded that
the opening of epoxy rings can be attributed to the coopera-
tion of nucleophilic attack by an anion and the activation by
an onium cation through electronic interaction. On the basis
of these findings, we consider that the synergism of polariza-
tion (due to hydrogen bonding) and electronic interaction (due
to phosphonium cation) as well as the nucleophilic attack by
a bromide anion account for the excellent activity of the cat-
alyst. A mechanism is proposed for the catalytic process as
shown in Scheme 2. The bromide anion of the catalyst opens
Scheme 2. Proposed mechanism for the cycloaddition reaction.

32
C. Jing-Xian et al. / Applied Catalysis A: General 484 (2014) 26–32
into cyclic carbonate through the intramolecular substitution of the
bromide anion.
[14] C.J. Whiteoak, N. Kielland, V. Laserna, E.C. Escudero-Adán, E. Martin, A.W. Kleij,
J. Am. Chem. Soc. 135 (2013) 1228–1231.
[15] W.L. Dai, B. Jin, S.L. Luo, X.B. Luo, X.M. Tu, C.T. Au, J. Mol. Catal. A: Chem. 378
(2013) 326–332.
4. Conclusion
[16] W.L. Dai, B. Jin, S.L. Luo, X.B. Luo, X.M. Tu, C.T. Au, Appl. Catal. A: Gen. 470 (2014)
183–188.
[17] S. Ghazali-Esfahani, H.B. Song, E. Pa˘unescu, F.D. Bobbink, H.Z. Liu, Z.F. Fei, G. Lau-
renczy, M. Bagherzadeh, N. Yan, P.J. Dyson, Green Chem. 15 (2013) 1584–1589.
[18] J. Sun, S.J. Zhang, W.G. Cheng, J.Y. Ren, Tetrahedron Lett. 49 (2008) 3588–3591.
[19] L. Han, H.J. Choi, S.J. Choi, B. Liu, D.W. Park, Green Chem. 13 (2011) 1023–1028.
[20] Y.Y. Zhang, S.F. Yin, S.L. Luo, C.T. Au, Ind. Eng. Chem. Res. 51 (2012) 3951–3957.
[21] R.A. Watile, K.M. Deshmukh, K.P. Dhake, B.M. Bhanage, Catal. Sci. Technol. 2
(2012) 1051–1055.
[22] Y. Xie, Z.F. Zhang, T. Jiang, J.L. He, B.X. Han, T.B. Wu, K.L. Ding, Angew. Chem.
Int. Ed. 46 (2007) 7255–7258.
[23] L.F. Xiao, F.W. Li, J.J. Peng, C.G. Xia, J. Mol. Catal. A: Chem. 253 (2006) 265–269.
[24] W.L. Dai, L. Chen, S.F. Yin, S.L. Luo, C.T. Au, Catal. Lett. 135 (2010) 295–304.
[25] S. Udayakumar, Y.S. Son, M.K. Lee, S.W. Park, D.W. Park, Appl. Catal. A: Gen. 347
(2008) 192–199.
[26] W.L. Dai, L. Chen, S.F. Yin, W.H. Li, Y.Y. Zhang, S.L. Luo, C.T. Au, Catal. Lett. 137
(2010) 74–80.
[27] Y.B. Xiong, H. Wang, R.M. Wang, Y.F. Yan, B. Zheng, Y.P. Wang, Chem. Commun.
46 (2010) 3399–3401.
[28] L.N. Han, H.J. Choi, D.K. Kim, S.W. Park, B.Y. Liu, D.W. Park, J. Mol. Catal. A: Chem.
338 (2011) 58–64.
By grafting quarternary phosphonium IL onto chitosan surface,
we synthesize the CS-[BuPh3P]Br catalyst that is efficient in the syn-
thesis of cyclic carbonates from CO2 and epoxides in the absence
of co-catalyst and solvent. We find that the vicinal hydroxyl groups
in CS play a key role in promoting the cycloaddition reaction. The
superior performance of the catalyst can be attributed to the open-
ing of epoxy ring as an outcome of the combined effect of C–O
polarization, electronic interaction as well as nucleophilic attack.
The catalyst can be separated easily by filtration and used effec-
tively in a test of 5 cycles without significant loss of activity. In
terms of efficiency, stability, simplicity, recyclability, environment-
friendliness, and cost effectiveness, the CS-[BuPh3P]Br catalyst is
potentially an industrial catalyst for large-scale production of cyclic
carbonates.
[29] H.C. Cho, H.S. Lee, J. Chun, S.M. Lee, H.J. Kim, S.U. Son, Chem. Commun. 47 (2011)
917–919.
[30] T.Y. Shi, J.Q. Wang, J. Sun, M.H. Wang, W.G. Cheng, S.J. Zhang, RSC Adv. 3 (2013)
3726–3732.
Acknowledgments
This work was financially supported by the National Natural Sci-
ence Foundation of China (grant nos. 51104089, 51238002, and
51272099); and Natural Science Foundation of Jiangxi Province
(grant nos. 20122BAB213013). CTA thanks NCHKU for an adjunct
professorship.
[31] J.H. Zhou, Z.P. Dong, H.L. Yang, Z.Q. Shi, X.C. Zhou, R. Li, Appl. Surf. Sci. 279 (2013)
360–366.
[32] Z.D. Wang, L.C. Zheng, C.C. Li, D. Zhang, Y.N. Xiao, G.H. Guan, W.X. Zhu, Carbo-
hydr. Polym. 94 (2013) 505–510.
[33] C.S. Catano, M. Caiado, J. Farinha, I.M. Fonseca, A.M. Ramos, J. Vital, J.E. Castan-
heiro, Chem. Eng. J. 230 (2013) 567–572.
[34] L.F. Xiao, F.W. Li, C.G. Xia, Appl. Catal. A: Gen. 279 (2005) 125–129.
[35] Y. Zhao, J.S. Tian, X.H. Qi, Z.N. Han, Y.Y. Zhuang, L.N. He, J. Mol. Catal. A: Chem.
271 (2007) 284–289.
[36] J. Tharun, Y. Hwang, R. Roshan, S. Ahn, A.C. Kathalikkattil, D.W. Park, Catal. Sci.
Technol. 2 (2012) 1674–1680.
[37] J. Tharun, D.W. Kim, R. Roshan, Y. Hwang, D.W. Park, Catal. Commun. 31 (2013)
62–65.
[38] Z. Guo, R. Xing, S. Liu, Z. Zhong, P. Li, Carbohydr. Polym. 73 (2008) 173–177.
[39] E.A. Stepnova, V.E. Tikhonov, T.A. Babushkina, T.P. Klimova, E.V. Vorontsov, V.G.
Babak, S.A. Lopatin, I.A. Yamskov, Eur. Polym. J. 43 (2007) 2414–2421.
[40] M. Ghiaci, N. Dorostkar, M.V. Martínez-Huerta, J.L.G. Fierro, P. Moshiri, J. Mol.
Catal. A: Chem. 379 (2013) 340–349.
Appendix A. Supplementary data
Supplementary material related to this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.apcata.
2014.07.008.
References
[41] S. Lu, X. Song, D. Cao, Y. Chen, K. Yao, J. Polym. Sci. 91 (2004) 3497–3503.
[42] I. Aranaz, M. Mengíbar, R. Harris, I. Panos, B. Miralles, N. Acosta, G. Galed, Á.
Heras, Curr. Chem. Biol. 3 (2009) 203–230.
[43] H. Xie, S. Zhang, S. Li, Green Chem. 8 (2006) 630–633.
[44] J. Sun, J.Q. Wang, W.G. Cheng, J.X. Zhang, X.H. Li, S.J. Zhang, Y.B. She, Green
Chem. 14 (2012) 654–660.
[45] J. Sun, W.G. Cheng, W. Fan, Y.H. Wang, Z.Y. Meng, S.J. Zhang, Catal. Today 148
(2009) 361–367.
[46] C.W. Tsang, B. Baharloo, D. Riendl, M. Yam, D.P. Gates, Angew. Chem. Int. Ed. 43
(2004) 5682–5685.
[47] W.L. Dai, B. Jin, S.L. Luo, X.B. Luo, X.M. Tu, C.T. Au, Catal. Sci. Technol. 4 (2014)
556–562.
[48] R. Nomura, M. Kimura, S. Teshima, A. Ninagawa, H. Matsuda, Bull. Chem. Soc.
Jpn. 55 (1982) 3200–3203.
[49] B. Ochiai, T. Iwamoto, T. Endo, Green Chem. 8 (2006) 138–140.
[50] H.B. Xie, H.F. Duan, S.H. Li, S.B. Zhang, New J. Chem. 29 (2005) 1199–1203.
[1] T. Sakakura, J.C. Choi, H. Yasuda, Chem. Rev. 107 (2007) 2365–2387.
[2] W.L. Dai, S.L. Luo, S.F. Yin, C.T. Au, Appl. Catal. A: Gen. 366 (2009) 2–12.
[3] T. Sakakura, K. Kohno, Chem. Commun. (2009) 1312–1330.
[4] M. North, R. Pasquale, C. Young, Green Chem. 12 (2010) 1514–1539.
[5] T. Sako, T. Fukai, R. Sahashi, Ind. Eng. Chem. Res. 41 (2002) 5353–5358.
[6] H. Kawanami, Y. Kushima, Chem. Commun. (2000) 2089–2090.
[7] A. Barbarini, R. Maggi, A. Mazzacani, G. Mori, G. Sartori, R. Sartorio, Tetrahedron
Lett. 44 (2003) 2931–2934.
[8] J.L. Jiang, R.M. Hua, Synth. Commun. 36 (2006) 3141–3148.
[9] B.M. Bhanage, S.I. Fujita, Y. Ikushima, M. Arai, Appl. Catal. A: Gen. 219 (2001)
259–266.
[10] W.L. Dai, S.F. Yin, R. Guo, S.L. Luo, X. Du, C.T. Au, Catal. Lett. 136 (2010) 35–44.
[11] E.J. Doskocil, J. Phys. Chem. B 109 (2005) 2315–2320.
[12] R. Srivastava, D. Srinivas, P. Ratnasamy, Catal. Lett. 91 (2003) 133–139.
[13] F. Ahmadi, S. Tangestaninejad, M. Moghadam, V. Mirkhani, I. Mohammadpoor-
Baltork, A.R. Khosropour, Polyhedron 32 (2012) 68–72.