A direct synthesis of styrene carbonate from styrene with the Au/SiO 2-ZnBr2/Bu4NBr catalyst system

Article abstract of DOI:10.1016/j.jcat.2004.12.015

This paper reports a direct route for the preparation of cyclic styrene carbonate from styrene, which avoids the preliminary synthesis and isolation of styrene oxide. A catalyst system consisting of Au/SiO₂, zinc bromide, and tetrabutylammonium bromide (Bu₄NBr) has been applied to the one-pot synthesis of styrene carbonate from styrene, organic peroxide, and CO₂. Au/SiO₂ is active for the epoxidation of styrene, and zinc bromide and Bu₄NBr cooperatively catalyze the subsequent CO₂ cycloaddition to epoxide. The influence of various reaction parameters, such as CO₂ pressure, reaction temperature, reaction time, catalyst amount, ratio of oxidant to styrene, and oxidant type, has been investigated in detail.

Full text of DOI:10.1016/j.jcat.2004.12.015

Journal of Catalysis 230 (2005) 398–405
www.elsevier.com/locate/jcat
A direct synthesis of styrene carbonate from styrene with
the Au/SiO –ZnBr /Bu NBr catalyst system
2
2
4
a
a
b
a
a,∗
Jianmin Sun , Shin-ichiro Fujita , Fengyu Zhao , Masashi Hasegawa , Masahiko Arai
a
Division of Materials Science and Engineering, Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Japan
b
Changchun Institute of Applied Chemistry, Chinese Academy of Science, Changchun, 130022, China
Received 27 October 2004; revised 10 December 2004; accepted 13 December 2004
Available online 25 January 2005
Abstract
This paper reports a direct route for the preparation of cyclic styrene carbonate from styrene, which avoids the preliminary synthesis
and isolation of styrene oxide. A catalyst system consisting of Au/SiO , zinc bromide, and tetrabutylammonium bromide (Bu NBr) has
2
4
been applied to the one-pot synthesis of styrene carbonate from styrene, organic peroxide, and CO . Au/SiO is active for the epoxidation
2
2
of styrene, and zinc bromide and Bu NBr cooperatively catalyze the subsequent CO cycloaddition to epoxide. The influence of various
4
2
reaction parameters, such as CO pressure, reaction temperature, reaction time, catalyst amount, ratio of oxidant to styrene, and oxidant type,
2
has been investigated in detail.

2004 Elsevier Inc. All rights reserved.
Keywords: Styrene carbonate; One-pot synthesis; Epoxidation; Carbon dioxide; Supported gold catalyst; Zinc bromide; Ammonium salt
1
. Introduction
carried out one-pot synthesis of styrene carbonate (SC) from
styrene, CO2, and molecular oxygen with homogeneous
rhodium complex catalysts [9,10]. They also used various
metal oxide catalysts [11], and the highest yield was ob-
tained with niobium oxide. The SC yield obtained was low
Chemical fixation of carbon dioxide (CO2) has received
much attention recently for its use in the preservation of
the environment and because CO2 is a cheap and abundant
raw material [1,2]. The formation of cyclic carbonates is
an effective CO2 chemical fixation route [3–6]. Cyclic car-
bonates can be used extensively in various fields [7,8] and
are currently manufactured from epoxides produced by the
epoxidation of olefins (Scheme 1). Thus one-pot synthesis of
cyclic carbonates from olefins and CO2 should be more in-
teresting and economical, because the reaction uses readily
available and low-priced chemicals olefins, and the prelimi-
nary synthesis of epoxides is avoided.
Only a few studies of the direct synthesis of cyclic car-
bonates from olefins have been reported so far. Aresta et al.
*
Scheme 1. Two-step (a) and one-step (b) synthesis of styrene carbonate
from styrene.
Corresponding author. Fax: +81-11-706-6594.
E-mail address: marai@eng.hokudai.ac.jp (M. Arai).
0021-9517/$ – see front matter  2004 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2004.12.015

J. Sun et al. / Journal of Catalysis 230 (2005) 398–405
399
because of low SC selectivity due to the formation of by-
products such as benzaldehyde and benzoic acid [10,11].
Srivastava et al. [12] used titanosilicate catalysts for the syn-
thesis of cyclic carbonate in a single reactor; initially they
conducted the epoxidation of olefin with H2O2 or tert-butyl
iodide (Bu4NI), styrene, aqueous tert-butyl hydroperoxide
(TBHP) (70 wt% in water), cumene hydroperoxide (CHP)
(80 wt% in cumene), and ethyl acetate were purchased from
Wako. Silica gel (Davisil grade 646) and anhydrous tert-
butyl hydroperoxide (5.0–6.0 M solution in decane) were
supplied by Aldrich.
◦
hydroperoxide (TBHP) at 60 C and then added CO2 and a
◦
cocatalyst with an organic base at a temperature of 120 C.
The yield was not high, and their catalyst system required
the toxic organic solvent CH2Cl2 and a long reaction time.
We have also reported the one-pot synthesis of SC from
styrene with TBHP as an oxidant in the presence of quar-
ternary ammonium halides or imidazolium salts [13,14].
The most effective catalyst was tetrabutylammonium bro-
mide (Bu4NBr) [14]. Although the SC yield was higher than
those reported previously, a long reaction time was required
and the homogeneous nature of Bu4NBr was a drawback in
separation and recycling. Thus, effective catalysts are still
being sought for the one-pot synthesis of cyclic carbonates
from olefins and CO2.
2.2. Preparation and characterization of Au/SiO2 catalysts
Several silica-supported gold catalysts with different gold
loadings were prepared according to methods described in
the literature [33]. Silica was suspended in distilled water
with vigorous stirring, and the required amount of an aque-
−2
−3
ous solution of tetrachloroauric acid (10 mol dm ) was
added dropwise to the suspension. During the addition, the
pH value of the suspension was adjusted to 8.5 with the am-
monia solution. Then, the suspension was further aged at
room temperature for 2 h with stirring while the pH was kept
constant with the addition of a diluted ammonia solution.
−2
−3
Recently we have found that the catalytic performance of
Bu4NBr in the coupling reaction of CO2 and styrene ox-
ide is greatly enhanced by the presence of zinc bromide
A solution of NaBH4 (10 mol dm ) was prepared in ice
water, and the appropriate amount of the solution was added
to the suspension with stirring. The total amount of NaBH4
added was equivalent to that of tetrachloroauric acid. On the
addition of the NaBH4 solution, the colour of the mixture
turned to dark purple from yellow, indicating that the re-
duction of the auric ion had occurred. A purple colour is
characteristic of metallic small gold particles [34]. After ag-
ing for 2 h, solids in the resulting suspension were filtered
off, washed thoroughly with distilled water, and dried in air
(ZnBr2) [15]. Styrene oxide is quantitatively transformed
to SC in a very short reaction time of 30 min at a low re-
◦
action temperature of 80 C. If this ZnBr2/Bu4NBr were to
be combined with another active species effective for epox-
idation, the resulting system would be a novel catalyst for
the one-pot synthesis of cyclic carbonates. Gold catalysts
have successfully been applied to many reactions, such as
low-temperature CO oxidation [16,17], gas-phase epoxida-
tion of propylene [18–20], water–gas-shift reaction [21,22],
and hydrogenation of unsaturated hydrocarbons [23,24]. In
addition, supported Au catalysts are also effective for liquid-
phase epoxidation [25–32]. Thus, it is interesting to apply
supported Au catalysts together with the catalyst system of
ZnBr2/Bu4NBr for the one-pot synthesis of cyclic carbon-
ates.
◦
at 110 C overnight. The filtrate was colourless, suggesting
that all of the gold species had been loaded onto the silica
support.
For comparison with the NaBH4 reduction, 1 wt% Au/
SiO2 samples were also prepared by reduction with flowing
◦
H2 at 400 C for 2 h after the loading of Au precursors in the
same manner as described above.
All of the supported gold catalysts we prepared were
characterized by X-ray diffraction (XRD) and UV–vis spec-
troscopy. XRD patterns of the catalysts were measured with
a JEOL JDX-8020 powder diffractometer operating at 30 kV
and 30 mA and Cu-Kα monochromatized radiation (λ =
0.154178 nm). We evaluated the size of supported gold crys-
tallites, D, from the full peak width at half-maximum by
using the Scherrer equation [35], D = 0.9λ/(β cosθ), where
λ is the wavelength of the X-ray radiation, and β is the
peak width at half-maximum. A correction for the instru-
mental broadening was made with silicon powder. Diffuse
reflectance UV–vis spectra were recorded with a Shimadzu
UV-3100 PC spectrophotometer.
In the present work, a catalyst system consisting of Au/
SiO2, ZnBr2, and Bu4NBr is applied to the one-pot syn-
thesis of SC from styrene and CO2. The effects of reaction
parameters, such as Au loading, CO2 pressure, temperature,
reaction time, catalyst amount, type of oxidant, and ammo-
nium salts, have been investigated in detail to achieve high
SC yields under optimal conditions. Good SC yields have
◦
been obtained under mild reaction conditions (at 80 C for
4
h with 1 MPa CO2).
2
. Experimental
2
.1. Materials
2.3. Catalytic reactions
Tetrachloroauric acid (HAuCl4 · 4H2O), 25 wt% aqueous
One-pot synthesis of SC was conducted in a 50-ml
stainless-steel autoclave. In a typical run, the reactor was
charged with Au/SiO2 (0.1 g), ZnBr2 (0.44 mmol), Bu4NBr
(0.88 mmol), styrene (17.3 mmol), and TBHP (25.4 mmol)
ammonia solution, sodium tetrahydroborate (NaBH4), zinc
bromide, tetrabutylammonium bromide (Bu4NBr), tetra-
butylammonium chloride (Bu4NCl), tetrabutylammonium

400
J. Sun et al. / Journal of Catalysis 230 (2005) 398–405
Fig. 2. UV–vis spectra of supported Au/SiO catalysts with different Au
2
loadings of (a) 1 wt%, (b) 3 wt%, (c) 5 wt%, and (d) 8 wt%.
Fig. 1. XRD patterns of supported Au/SiO catalysts with different Au load-
2
ings of (a) 1 wt%, (b) 3 wt%, (c) 5 wt%, and (d) 8 wt%.
tion depend on the crystallite size, and larger gold crystallites
show a broader band at a higher wavelength [38,39]. As a
source of complementary information on the chemical state
and particle size of gold particles, diffuse reflectance UV–
vis spectra of the Au/SiO2 samples were measured. Fig. 2
gives the UV–vis spectra obtained. Bands observed around
in decane and heated to a desired temperature. Then CO2
was fed into the reactor with an HPLC pump equipped with a
cooler. The pressure inside of the reactor was monitored and
controlled by a back-pressure regulator. After the reaction,
the reactor was cooled to room temperature and depressur-
ized to atmospheric pressure. Then the Au/SiO2 catalyst was
filtered off from the reaction mixture. After the filtration, the
organic phase was diluted to 25 ml with ethyl acetate and
analyzed with a gas chromatograph (Shimadzu 390B) with
a capillary column (GL Science TC-17). The quantities of
styrene substrate consumed and products formed were de-
termined from the results obtained from authentic standards.
The structure of the products was further confirmed with a
mass spectrometer (Shimadzu QP5050A). In some runs, ei-
ther an aqueous solution of TBHP or a cumene solution of
CHP was used instead of the TBHP decane solution.
5
10 nm indicate the existence of metallic gold over the cat-
alysts. With increasing gold loading, the absorption band
red-shifts slightly and the bandwidth increases. These results
show that the size of gold particles increases with the gold
loading, which is consistent with the results of XRD (Fig. 1).
3
.2. Catalytic activity
Effects of various catalyst preparation and reaction para-
meters on the one-pot synthesis of SC have been examined
in detail to maximize the SC yield. Under the present reac-
tion conditions, SC and styrene epoxide (SO) were formed
along with benzaldehyde (BA) and other by-products, which
were oligomers of the substrate and those of SO and/or SC.
The total styrene conversion and the product distribution de-
pend strongly on the conditions used, as described in the
following. The overall conversion of styrene was calculated
from moles of styrene consumed against the initial moles of
styrene used. The yield and selectivity of a product were de-
termined from moles of the product formed against the initial
moles of styrene used and against the moles of styrene con-
sumed, respectively:
3
. Results and discussion
3
.1. Catalyst characterization
XRD patterns of Au/SiO2 catalysts with various metal
loadings are presented in Fig. 1. The diffraction peaks as-
cribed to metallic gold [23] are observed along with a broad
◦
one for SiO2 at 2θ = 22 and tend to sharpen with increasing
gold loading. The average sizes of metallic gold crystallites
were determined from the most intense diffraction peaks at
◦
2
5
θ = 38.1 , which are 8.0, 9.6, 10.0, and 11.6 nm for 1, 3,
yield (%) = total conversion (%) × selectivity (%).
, and 8 wt% Au/SiO2 catalysts, respectively. The size of
gold crystallites determined increases with the gold loading,
and this is contrary to the trend observed for Au samples dis-
persed on a structured support of MCM-41 [36].
Because of their plasmon vibration arising from the col-
lective oscillations of the free conduction band electrons,
crystallites of metallic gold have a characteristic absorption
in the visible region of the electromagnetic spectrum at 500–
3.2.1. Influence of catalyst preparation conditions
Table 1 shows the influence of Au loading on the one-pot
synthesis of SC. It can be seen that the SC yield increases
slightly with the Au loading; however, the differences ob-
served in the yield and in the total conversion are small
under the conditions used. These results are discussed in
Section 3.3.
6
00 nm [17,37]. The position and the width of this absorp-

J. Sun et al. / Journal of Catalysis 230 (2005) 398–405
401
Table 1
pressure does not significantly affect the conversion and the
yields; however, both the conversion and the SC yield de-
crease at 15 MPa. According to visual observations of the
reaction mixture [14], it includes three phases of CO2-rich
gas, liquid (styrene, TBHP or CHP, ZnBr /Bu NBr), and
Influence of Au loading on the activity of Au/SiO catalyst in the one-pot
synthesis of SC
2
a
Entry
Au loading
wt%)
Conversion
(%)
Yield (%)
SC
(
SO
BA
2
4
1
2
3
4
1
3
5
8
83
81
88
81
31
33
35
37
5
7
4
4
22
16
16
20
solid (Au/SiO2) at lower pressures, whereas it exists in two
phases (fluid phase and catalyst solid phase) at a high pres-
sure of 15 MPa. This phase change would cause an increase
in the volume where the reaction proceeds, and, hence, the
concentration of the substrate would be low at this elevated
pressure, resulting in lowered styrene conversion and SC
yield. It was also found that much oligomer was produced
at such a high pressure. In addition to the change in the reac-
tion volume, differences in the properties between the liquid
and CO2 gas phases where the reacting species are dissolved
should also be significant in determining the reaction rates.
The results in Fig. 3 indicate that high CO2 pressures are not
necessary for the synthesis of SC, and so the influence of
other reaction conditions has been examined at a CO2 pres-
sure of 1 MPa.
a
Reaction conditions: Au/SiO , 0.1 g; ZnBr , 0.44 mmol; Bu NBr,
2
2
4
0.88 mmol; styrene, 17.3 mmol; TBHP (in decane), 25.4 mmol; CO pres-
2
◦
sure, 8 MPa; 80 C; 4 h.
Table 2 shows the influence of the preparation conditions
of 1 and 5 wt% Au/SiO2 catalysts on the one-pot synthe-
sis of SC. With an unreduced 1 wt% Au/SiO2 sample, a low
SC yield of 21% is obtained (entry 1). When this sample
is reduced with H2 or NaBH4, the SC yield increases to
more than 30% (entries 2 and 3). However, when the reduced
◦
catalyst is used after calcination at 400 C, the SC yield de-
creases to a value similar to that obtained with the unreduced
sample (entry 4). A decrease in the SC yield with calcina-
tion is also observed with a 5 wt% Au/SiO2 sample (entries
Quaternary ammonium salts with different counter-an-
ions have been examined. Table 3 shows that bromide
counter-ion is better than chloride and iodide anions. In the
cycloaddition of CO2 (the second step of SC synthesis), the
ring opening of epoxide occurs by nucleophilic attack of the
halide anion to a carbon of the epoxide group [40]. The nu-
5
and 6). XRD measurements showed that the size of gold
crystallites increased with calcinations, but this change was
not significant, suggesting that the decrease in the activity
is ascribable to the change in the state of gold crystallites.
Thus, both the metallic and cationic gold species are active,
but the former is more active than the latter.
−
−
−
cleophilic tendency decreases in the order I > Br > Cl ,
but the yield of SC observed is the highest with Bu4NBr. In
3
.2.2. Influence of CO2 pressure, quaternary ammonium
salts, and oxidants
Fig. 3 demonstrates the influence of CO2 pressure on
the styrene conversion and the yields of SC, SO, and BA.
At atmospheric pressure (0.1 MPa), the styrene conversion
is 83% and the yields of SC, SO, and BA are 21, 6, and
1
6%, respectively. When the pressure is raised to 1 MPa,
the conversion and the SC yield increase to 89 and 35%, re-
spectively. In the region between 1 and 12 MPa, the CO2
Table 2
Influence of preparation conditions on the activity of Au/SiO catalysts in
2
a
the one-pot synthesis of SC
Fig. 3. Effect of CO pressure on the conversion and product yields.
2
Styrene, 17.3 mmol; anhydrous TBHP, 25.4 mmol; 5 wt% Au/SiO , 0.1 g;
Entry Au loading Preparation conditions
wt%)
Conversion Yield (%)
2
◦
ZnBr , 0.44 mmol; Bu NBr, 0.88 mmol; 80 C; 4 h.
(
(%)
2
4
SC SO BA
1
2
3
4
1
1
1
1
No reduction
H reduction
79
73
83
78
21 4 14
35 7 23
31 5 22
26 5 14
Table 3
2
a
Influence of quaternary ammonium salts on the one-pot synthesis of SC
NaBH reduction
4
NaBH reduction and
4
Entry
Ammonium
salt
Conversion
(%)
Yield (%)
SC
b
calcination
SO
BA
5
6
5
5
NaBH reduction
88
84
35 4 16
28 4 25
4
NaBH reduction and
4
1
2
3
Bu4NCl
Bu4NBr
Bu NI
4
82
89
71
31
35
30
8
6
4
13
19
15
b
calcination
a
Reaction conditions: Au/SiO , 0.1 g; ZnBr , 0.44 mmol; Bu NBr,
2
2
4
a
0.88 mmol; styrene, 17.3 mmol; TBHP (in decane), 25.4 mmol; CO pres-
Reaction conditions: 5 wt% Au/SiO2, 0.1 g; ZnBr2, 0.44 mmol; Bu4NX
2
◦
sure, 8 MPa; 80 C; 4 h.
b
(X = Cl, Br, I), 0.88 mmol; styrene, 17.3 mmol; TBHP (in decane),
◦
25.4 mmol; CO pressure, 1 MPa; 80 C; 4 h.
2
◦
Calcination was carried out at 400 C for 2 h in air.

402
J. Sun et al. / Journal of Catalysis 230 (2005) 398–405
the case of Bu4NI, TBHP was consumed for its oxidation
in addition to the desired epoxidation reaction, thus lead-
ing to the decrease in SC and SO yields as observed. When
Bu4NI was added into the mixture of TBHP and styrene,
the color of the mixture rapidly changed to dark orange, in-
dicative of the formation of iodine by oxidation with TBHP.
Thus Bu4NBr is more suitable than the other two quaternary
ammonium salts examined.
the phase behavior and solubility should be measured in de-
tail.
3.2.3. Influence of catalyst amount, temperature, and time
Table 5 shows the effect of catalyst amount on the forma-
tion of SC. The carbonate yield increases when the amount
of Au/SiO is increased from 0.05 to 0.1 g (entries 1, 2).
2
When it is further increased, however, the SC yield does not
The influence of the kinds of oxidants and the molar ra-
tio of oxidant to styrene is shown in Table 4. When the
TBHP/styrene ratio is raised from 1 to 1.5, the styrene con-
version and the SC yield increase from 82 to 89% and from
increase (entry 4). It is also shown that even if the amounts
of ZnBr and Bu NBr are doubled while the amount of
2
4
Au/SiO is kept at 0.1 g, the SC yield is not improved (en-
2
try 5). As described below, Au/SiO catalyzes the epoxida-
2
2
4 to 35%, respectively (entries 1, 2); the selectivity of SC
tion of styrene only, and it is the rate-determining step of
increases from 29 to 39%. However, a further increase in
the ratio does not improve the styrene conversion and the
SC yield (entry 3). In the case of CHP as an oxidant, the
yield of SC is also markedly enhanced with an increase in
the CHP/styrene ratio from 1 to 1.5, but the yield at a ratio
of 2 is not so different from that at 1.5 (entries 4–6). Thus,
the optimal molar ratio of oxidant to styrene is 1.5 for both
TBHP and CHP.
the one-pot synthesis of SC. Thus the yield of SC is not
improved when the amounts of ZnBr and Bu NBr are in-
creased under the conditions used.
2
4
The reusability of Au/SiO catalyst was examined. Af-
2
ter a reaction run, the solid catalyst was removed from the
reaction system by filtration, washed with acetone, dried at
room temperature, and then subjected to the next catalytic
run, in which fresh ZnBr and Bu NBr were added. The re-
2
4
Table 4 also indicates that CHP is more effective for the
synthesis of SC than is TBHP. Previously CHP had been
found to show better performance than anhydrous TBHP in
the epoxidation of cyclohexene catalyzed by a mesoporous
titanosilicate catalyst [41]. When aqueous TBHP is used in-
stead of anhydrous TBHP, the SC yield decreases from 35
to 21% (entry 7 of Table 4). Thus the choice of oxidant is
very important, and CHP is a better oxidant for the one-pot
synthesis of SC.
The water contained in the oxidant is detrimental to the
carbonate synthesis. An important factor is the hydrolysis
of epoxide to phenyl glycol [42], which would decrease
the quantity of the starting material for the second step of
SC synthesis. Indeed, the formation of a small amount of
phenyl glycol was observed after the reaction when aqueous
TBHP was used, whereas it was not detected after the re-
action when anhydrous TBHP was used. The miscibility of
reacting species in organic and water phases would also be
important. It was observed that ZnBr2 and TBAB were solu-
ble in a mixture of TBHP and styrene. For further discussion,
sults with the used catalyst are similar to those obtained with
fresh catalyst (entry 3), indicating that Au/SiO is recyclable
2
without any loss of activity.
The influence of reaction temperature on the styrene con-
version and the product distribution is given in Table 6; for
◦
these the reaction temperature was varied from 60 C to
9
◦
0 C. As expected, the styrene conversion increases with
Table 5
a
Influence of the amounts of catalysts on the one-pot synthesis of SC
Entry Au/SiO b
ZnBr
(mmol)
Conversion
(%)
Yield (%)
SC SO BA
2
2
(g)
1
2
3
4
5
0.05
0.1
0.1
0.2
0.1
0.44
0.44
0.44
0.44
0.88
70
76
78
75
74
34
42
41
42
42
3
1
2
1
5
17
19
17
15
16
c
a
Reaction conditions: Bu NBr/ZnBr = 2 (molar ratio); styrene,
4
2
◦
17.3 mmol; CHP, 25.4 mmol; CO pressure, 1 MPa; 80 C; 4 h.
2
b
Au loading: 5 wt%.
With the used Au/SiO and fresh ZnBr and Bu NBr.
c
2
2
4
Table 4
Influence of the kinds of oxidants and the oxidant/styrene molar ratio on the
one-pot synthesis of SC
Table 6
Influence of reaction temperature and time on the one-pot synthesis of SC
a
a
Entry Oxidant
Oxidant/styrene Conversion
Yield (%)
Entry
Temperature
( C)
Time
(h)
Conversion
(%)
Yield (%)
◦
molar ratio
(%)
SC SO BA
SC
SO
BA
1
2
3
4
5
6
7
Anhydrous TBHP
Anhydrous TBHP 1.5
Anhydrous TBHP
CHP
CHP
1
82
89
90
75
76
79
85
24
35
36
24
42
45
21
3
6
8
2
1
3
6
16
19
21
10
19
21
17
1
2
3
4
5
6
7
60
70
80
90
80
80
80
4
4
4
4
1
3
6
54
58
76
77
53
66
75
18
27
42
40
18
40
43
2
2
1
3
3
3
2
9
10
19
15
8
2
1
1.5
2
1.5
CHP
Aqueous TBHP
13
15
a
a
Reaction conditions: 5 wt% Au/SiO , 0.1 g; ZnBr , 0.44 mmol;
Reaction conditions: 5 wt% Au/SiO , 0.1 g; ZnBr , 0.44 mmol;
2 2
2
2
◦
Bu NBr, 0.88 mmol; styrene, 17.3 mmol; CO pressure, 1 MPa; 80 C;
Bu NBr, 0.88 mmol; styrene, 17.3 mmol; CHP, 25.4 mmol; CO pressure,
4
2
4
2
4
h.
1 MPa.

J. Sun et al. / Journal of Catalysis 230 (2005) 398–405
403
temperature. The SC yield also increases with temperature,
Table 7
◦
Results of epoxidation of styrene (ST) and cycloaddition of CO to styrene
oxide (SO) using different combinations of catalysts
reaching a maximum value at 80 C. Further increase in the
2
a
◦
reaction temperature to 90 C causes a slight decrease in the
Entry Au/SiO2 Bu4NBr ZnBr2 Substrateb Conversion Yield (%)
SC yield. In contrast, the yield of SO increases a little, proba-
bly because of the decomposition of SC to SO [43,44]. Thus,
(
%)
SC SO BA
◦
a relatively low temperature of 80 C is sufficient for the one-
1
2
3
4
!
!
!
!
ST
ST
ST
ST
71
73
67
72
0
0
0
0
40 15
41 17
42 11
pot synthesis of SC.
!
!
!
c
Table 6 indicates that the styrene conversion and the SC
d
◦
35
9
yield are 53 and 18%, respectively, for 1 h at 80 C (entry 5),
which increase to 66 and 40% for a longer time of 3 h (en-
try 6). However, a further increase of the reaction time to 6 h
does not increase the conversion and the yield as much (en-
try 7), probably because of complete decomposition of the
oxidant. Table 6 also shows that the yield of BA is small, but
it increases with a reaction time similar to that of SC. It is
reported in the literature that the epoxidation of olefin pro-
ceeds in parallel with the formation of benzaldehyde through
the cleavage of the C=C bond [45]. Moreover, as described
in the next section, the cycloaddition of CO2 to SO proceeds
much faster than the epoxidation, and, hence, the yields of
SC and BA should increase similarly with time.
5
6
7
8
9
!
!
SO
SO
SO
SO
SO
0
0
100
28
0
100
–
–
–
–
–
–
–
–
–
–
!
!
100
28
e
e
e
!
!
0
!
100
a
Reaction conditions: 5 wt% Au/SiO , 0.1 g; ZnBr , 0.44 mmol;
2
2
Bu NBr, 0.88 mmol; styrene (ST) and SO, 17.3 mmol; TBHP (in decane),
4
◦
25.4 mmol; CO pressure, 8 MPa; 80 C; 4 h. The catalysts marked with !
2
were used.
b
ST and SO represent styrene and styrene oxide, respectively.
c
d
e
CHP was used as the oxidant instead of TBHP.
wt% Au/SiO (0.1 g) was used.
1
2
Reaction time was 1 h.
Table 8
Influence of CO pressure on epoxidation of styrene
3
.3. Reaction processes considerations
a
2
Entry
Pressure
Conversion
(%)
Yield (%)
SO
In the above sections, the influence of catalyst prepara-
(MPa)
BA
tion and reaction variables on the one-pot synthesis of SC
has been examined in detail to find optimum conditions.
Although the present catalyst system is a little complex com-
pared with our previous one, with Bu4NBr alone as catalyst
b
1
2
3
0
8
15
67
80
55
32
30
18
4
4
2
a
Reaction conditions: 8 wt% Au/SiO , 0.1 g; styrene, 17.3 mmol; TBHP
2
in decane) 25.4 mmol; 80 C; 4 h. SO: styrene oxide; BA: benzaldehyde.
[
14], the improved yield of the desired product of SC can
◦
(
be achieved in a shorter reaction time of 4 h and at a mild
b
Under ambient atmosphere.
◦
temperature of 80 C.
To obtain more information on the roles of catalyst com-
ponents, epoxidation of styrene (first step) and CO2 cycload-
dition to styrene oxide (second step) were conducted sepa-
rately (Table 7). It is shown that Au/SiO2 is active for the
epoxidation, and ZnBr2 and Bu4NBr have no effect on this
reaction (entries 1, 2). When CHP is used instead of TBHP,
the conversion decreases slightly and the SO yield does not
change; but the BA yield decreases (entry 3), and so the se-
lectivity of SO increases from 56 to 63%. The gold loading
has a marginal effect on the SO yield (entry 4). On the other
hand, Au/SiO2 has no activity for the CO2 cycloaddition,
and both ZnBr2 and Bu4NBr are required for this addition to
proceed smoothly (entries 5–8), although ZnBr2 has no ac-
tivity for the reaction. It is strongly suggested that Lewis acid
Scheme 2. Reaction pathways for the one-pot synthesis of SC from ST.
oxide and benzaldehyde are formed in parallel, and then the
styrene oxide formed is converted to SC. It can be seen from
a comparison of the styrene and styrene oxide conversion
2
(ZnBr2) and Lewis base (Bu4NBr) work together to open
the epoxy ring, as proposed in the literature [46–50]. In ad-
dition, the influence of CO2 pressure on the epoxidation of
styrene was examined in the presence of an Au/SiO2 catalyst
alone. The results obtained are given in Table 8, indicating
that the conversion at 8 MPa is slightly larger than that in
the absence of CO2, and the conversion decreases signifi-
cantly at a higher pressure of 15 MPa, similar to the trend
in the one-pot synthesis of SC, as shown in Fig. 3. The re-
action pathways are illustrated in Scheme 2. First styrene
levels and of the effects of CO pressure (Fig. 3, Table 8),
the CO cycloaddition proceeds much more rapidly than the
2
epoxidation. All of the styrene oxide used can be consumed
in a reaction time of 1 h. These results demonstrate that the
epoxidation reaction (the first step) is the rate-determining
step in the one-pot synthesis of SC.
A comparison of the results with 5 wt% and 1 wt% Au/
SiO catalysts given in Table 7 (entries 1, 4) shows that the
2
yield of SO is slightly larger with the former than with the

404
J. Sun et al. / Journal of Catalysis 230 (2005) 398–405
latter, although the conversion levels are almost the same; the
quantity of by-products, including BA, is smaller with the
former than with the latter. The influence of Au loading was
studied with Au/TiO2 for epoxidation of styrene [31]; the
total conversion and the SO selectivity did not change much
with the Au loading, except for at very small loading [31].
As illustrated in Scheme 2, the epoxidation of styrene and
the CO2 addition to the epoxide are catalyzed by Au/SiO2
and ZnBr2/Bu4NBr, respectively. There are three phases in
the reaction mixture below 15 MPa, which are the Au/SiO2
solid phase; the liquid phase containing styrene, the oxidant,
and ZnBr2/Bu4NBr; and the CO2-rich gas phase. Some CO2
can dissolve in the liquid phase. The epoxidation reaction
occurs at the interface between the solid and the liquid phase.
Then the styrene epoxide formed reacts with CO2 dissolved
in the liquid phase. This reaction could occur partly at the
interface between the liquid and the gas phases. When the
pressure was elevated to 15 MPa or above, the liquid and
gas phases change into a homogeneous fluid phase, and so
the cycloaddition of CO2 took place in the fluid phase at the
elevated pressures.
solvent, a high reaction temperature, a long reaction time,
or high CO2 pressure. Furthermore, the two sequential steps
(epoxidation and CO2 cycloaddition) can be conducted in
a single reactor under the same reaction conditions. Thus,
the one-pot synthesis of SC studied in this work has these
advantages, and it would make a simple carbonate synthe-
sis process with industrial potential, from environmental and
economic points of view.
Acknowledgments
The authors are grateful for financial support from the
Japan Society for the Promotion of Science (JSPS). Addi-
tional support from the Scientific Research Foundation for
Returned Overseas Chinese Scholars, State Education Min-
istry (SEM) of China, and Innovation Fund of Jilin Univer-
sity, China (202CX094), is appreciated.
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