Highly active and selective binary catalyst system for the coupling reaction of CO2 and hydrous epoxides

Article abstract of DOI:10.1016/j.molcata.2013.07.014

Although most of the zinc-containing catalysts show excellent catalytic activity for the coupling reaction of carbon dioxide (CO₂) with epoxides, generally, trace amounts of water in the reaction systems could cause a significant decrease of the catalytic activity. In this work, a nanoporous zinc-cobalt double metal cyanide complex (Zn-Co^III DMCC) with high surface area was synthesized via a modified hydrothermal process using tri-block copolymer EO₂₀PO₇₀EO₂₀ (P123) as the template. When cetyltrimethylammonium bromide (CTAB) was used as the co-catalyst, the Zn-Co^III DMCC/CTAB binary catalyst system could effectively catalyze the coupling reaction of CO₂ with hydrous propylene oxide (PO), producing propylene carbonate (PC) with nearly 100% productivity and 100% selectivity. This binary catalyst system could also catalyze the coupling reaction of other commercial epoxides and CO₂ without using any solvents with nearly 100% productivity and 100% selectivity. With such new catalyst system, hundreds of ppm water in the reaction system will no longer be the matter.

Full text of DOI:10.1016/j.molcata.2013.07.014

Journal of Molecular Catalysis A: Chemical 379 (2013) 38–45
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Highly active and selective binary catalyst system for the coupling
reaction of CO and hydrous epoxides
2
∗
Ren-Jian Wei, Xing-Hong Zhang , Bin-Yang Du, Zhi-Qiang Fan, Guo-Rong Qi
MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University,
Hangzhou 310027, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Although most of the zinc-containing catalysts show excellent catalytic activity for the coupling reaction
of carbon dioxide (CO2) with epoxides, generally, trace amounts of water in the reaction systems could
cause a significant decrease of the catalytic activity. In this work, a nanoporous zinc-cobalt double metal
Received 25 March 2013
Received in revised form 23 July 2013
Accepted 25 July 2013
III
cyanide complex (Zn–Co DMCC) with high surface area was synthesized via a modified hydrothermal
Available online 5 August 2013
process using tri-block copolymer EO20PO70EO20 (P123) as the template. When cetyltrimethylammo-
III
nium bromide (CTAB) was used as the co-catalyst, the Zn–Co DMCC/CTAB binary catalyst system could
Keywords:
Carbon dioxide
Epoxides
Coupling reaction
Double metal cyanide complex
effectively catalyze the coupling reaction of CO2 with hydrous propylene oxide (PO), producing propylene
carbonate (PC) with nearly 100% productivity and 100% selectivity. This binary catalyst system could also
catalyze the coupling reaction of other commercial epoxides and CO2 without using any solvents with
nearly 100% productivity and 100% selectivity. With such new catalyst system, hundreds of ppm water
in the reaction system will no longer be the matter.
©
2013 Elsevier B.V. All rights reserved.
1
. Introduction
the needs of co-solvents. The zinc halide salts with organic salts
as co-catalysts such as ZnBr /PPh PI [19] were highly active (the
2
4
Carbon dioxide is an important C1 feedstock because it is
maximum TON: 6272, mol PO/mol Zn) and selective (>99%) for
nontoxic, nonflammable, abundant and economical. The chemi-
cal fixation of CO2 into valuable organic chemicals has attracted
much attention for the goals of environmental protection and
sustainable development. The coupling reaction of CO2 with epox-
ides to produce cyclic carbonates is considered to be one of the
promising routes for CO2 utilization [1,2]. Cyclic carbonates can be
used as low-toxic aprotic polar solvents, monomers for producing
polycarbonates, pharmaceutical chemical intermediates, and many
biomedical applications [3].
the coupling reaction of CO -propylene oxide (PO). However, pro-
2
pylene carbonate (PC) productivity and selectivity of this catalyst
system were strongly influenced by the trace water in the system.
For example, the PO conversion decreased sharply from 89.6% to
8.1% with the presence of trace water, i.e. the molar ratio of 0.1 for
H O/ZnBr . The complete removal of trace water from hydrophilic
2
2
epoxides is clearly difficult and energy-consuming. It would be
much better if commercial epoxides, which generally contain sev-
eral hundreds of ppm water, could be directly used. Therefore,
developing a water-endurable catalyst for the coupling reaction
Numerous catalysts with significant advances have been dis-
covered for CO -epoxides coupling reaction in recent decades.
of CO -propylene oxide (PO) will be significant and promising to
2
2
Heterogeneous catalysts such as metal oxides [4–6], zeolite and its
the field because most of catalysts reported showed decrease in
activity with the presence of trace water. Fortunately, we have pre-
viously found that zinc–cobalt (III) double metal cyanide complex
supported catalysts [7–10] were utilized to catalyze CO -epoxides
2
coupling reaction. Although these catalysts were commonly easy
to be separated from the products and found to be reusable up
to several runs, their catalytic activities were rather low. Homo-
geneous catalysts such as amines and phosphines [11,12], alkali
metal halides [13], metal-salen complexes [14,15], metal por-
phyrins [16,17], and transition metal complexes [18–20] were also
extensively investigated. However, most of them suffered from low
catalytic activity, low catalyst stability, air/water sensitivity and
III
(Zn–Co DMCC) could improve the production of PC with the addi-
tion of a small amount of water during PO-CO2 copolymerization
III
[21], which suggested that such Zn–Co DMCC catalysts were sta-
ble in the presence of trace water and would be good candidates
for the coupling reaction of CO2 with hydrous epoxides.
A
typical
formula
of
DMCC
catalyst
is
etc.;
MIIu[M(CN)n]v·xM Xw·yL·zH O (M = Zn , Fe2+, Ni2+, Co
II
II
2+
2+
2
2+
3+
2+
3+
−
−
−
−
M = Co , Co , Fe , Fe , etc.; X = Cl , Br , I and OAc , etc.; L
is an electron-donating complex agent) [21–24]. For all of the
reported DMCCs, Zn–Co DMCC exhibited the most excellent
III
∗ Corresponding author. Tel.: +86 571 87953732; fax: +86 571 87953732.
E-mail addresses: xhzhang@zju.edu.cn, zxhhzz@gmail.com (X.-H. Zhang).
catalytic performance. DMCC catalyst was initially used as a
1
381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2013.07.014

R.-J. Wei et al. / Journal of Molecular Catalysis A: Chemical 379 (2013) 38–45
39
result (wt%): Co: 12.73, Zn: 28.01; C: 30.56; N: 15.23; Cl: 5.07; H:
3
.12.
2
.3. Coupling reaction of CO -epoxides
2
The general process of CO -PO coupling reaction was given as
2
Scheme
CH2 CHCH2OCH2
1. Coupling
reaction
of
epoxides
(R = CH3,
Ph-,
CH2Cl,
following: a 100 ml autoclave equipped with a mechanical stirrer
was pre-dried under vacuum at 80 C for 2 h. Zn–Co DMCC (10 mg)
and appropriate amounts of quaternary ammonium salts were
,
p(t-Bu)Ph-O-CH2
)
and CO2 catalyzed by the binary
◦
III
III
catalyst system of Zn–Co DMCC with various quaternary ammonium salts.
transferred into the autoclave, which was then dried for another
◦
2
h. After the autoclave was cooled down to 25 C, PO (30 ml) was
high-efficiency catalyst for ring-opening polymerization (ROP)
of epoxides [25–27], and later developed to be an active and
injected into the autoclave using a syringe under negative pressure.
The reactor was then heated and pressured to the desired temper-
ature and CO2 pressure under vigorous stirring (500 rpm). After
the reaction, the autoclave was cooled with ice-water bath and
the pressure was vented. A small amount of sample was removed
for H NMR measurement. The crude products were then trans-
ferred to a round-bottomed flask and dried under reduced pressure.
Pure propylene carbonate (PC, 4-methyl-1, 3-dioxolan-2-one) was
obtained. H NMR (CDCl , TMS, 400 MHz): ı = 1.5 (d, 3H), 4.0 (t, 1H),
selective catalyst for CO -epoxides copolymerization [21,24]. Very
2
III
recently, Zn–Co DMCC was also utilized for the copolymerization
of CS /epoxides [28], cyclic anhydrides/epoxides [29], as well
2
as CO /cyclic anhydrides/epoxides terpolymerization [30] with
2
1
satisfied catalytic activities. Generally, cyclic carbonate would be
produced if relative high reaction temperature was applied for the
III
copolymerization of CO -epoxides catalyzed by sole Zn–Co DMCC
[
2
1
III
21]. This intrigued us to develop a new Zn–Co DMCC system for
3
4
.5 (t, 1H), and 4.9–5.1 (m, 1H).
the PC production from CO -hydrous epoxide coupling reaction.
2
III
The reusability of the Zn–Co DMCC catalyst was tested as fol-
A novel highly active binary catalyst system with a nanoporous
III
lows: after the PO-CO2 coupling reaction, the crude product was
centrifuged at 15,000 rpm, and the non-solvable catalysts were
deposited at the bottom of centrifuge tube. The supernatant liq-
uid was then separated, and the remained catalyst was collected.
The collected catalyst was used for the next run in the same auto-
clave under the same reaction condition as that of the first run. The
catalysts were recycled for four runs.
zinc–cobalt double metal cyanide complex (Zn–Co DMCC) as the
main catalyst and quaternary ammonium salts as the co-catalyst
was successfully developed in the present work, which could
catalyze the coupling reaction of CO2 with commercial hydrous
epoxides to afford cyclic carbonates with nearly 100% productivity
and 100% selectivity under moderate conditions (Scheme 1).
The coupling reactions of CO2 with other commercial epox-
ides such as styrene oxide, allyl glycidyl ether, epichlorohydrin,
2
. Experimental
4
-tert-butylphenyl glycidyl ether with CO2 were performed using
2.1. Materials
1
the similar procedure described above. The H NMR results of the
corresponding cyclic carbonates were listed below:
K Co(CN) (Yixing City Lianyang Chemical Co., Ltd, China, 99%)
1
3
6
4
-Phenyl-1, 3-dioxolan-2-one: H NMR (CDCl , TMS, 400 MHz):
3
was recrystallized in de-ionized water before use. ZnCl2 and tert-
BuOH were analytical grade and used without further purification.
Propylene oxide (>99%) was reagent grade and used without purifi-
cation. Carbon dioxide with 99.995% purity was used as received.
Cetyltrimethylammonium bromide (CTAB), dodecyltrimethylam-
monum bromide (C10H21(CH ) NBr), decyltrimethylammonium
ı (ppm) = 4.35 (t, 1H), 4.81 (t, 1H), 5.69 (t, 1H), 7.38–7.43(m, 5H).
-(Allyloxymethyl)-1, 3-dioxolan-2-one: 1H NMR (CDCl , TMS,
4
3
4
00 MHz): ı (ppm) = 3.61–3.72 (m, 2H), 4.07(m, 2H), 4.41 (t, 1H),
4
.52 (t, 1H), 4.84 (t, 1H), 5.25(m, 2H), 5.87(m, 1H).
4
-(Chloromethyl)-1,3-dioxolan-2-one: 1H NMR (CDCl , TMS,
3
3
3
4
4
00 MHz): ı (ppm) = 3.72–3.83(m, 2H), 4.43 (t, 1H), 4.61 (t, 1H),
bromide (C12H25(CH ) NBr), tetraethylammonium bromide, tetra-
3
3
.99 (t, 1H).
butylammonium bromide, tetrabutylammonium chloride, tetra-
butylammonium iodide were all analytical grade and used without
further purification. Styrene oxide, allyl glycidyl ether, epichloro-
hydrin, 4-tert-butylphenyl glycidyl ether were industrial grade and
used as received. Pluronic copolymer P123 (EO20PO70EO20) was
kindly provided by BASF SE and used directly.
1
4
-((4-tert-butylphenoxy)methyl)-1, 3-dioxolan-2-one: H NMR
(
CDCl , TMS, 400 MHz): ı (ppm) = 1.30(s, 9H), 4.14(m, 1H), 4.24(m,
3
1
H), 4.53(m, 1H), 4.60(m, 1H), 5.01(m, 1H), 6.86 (d, 2H), 7.34 (t, 2H).
2.4. Characterization
Scanning electron microscopy (SEM) and transmission electron
2
.2. Synthesis of nanoporous Zn–CoIII DMCC
microscopy (TEM) were used to characterize the morphology of
III
the Zn–Co DMCC catalyst, respectively. The SEM observation was
The given amounts of ZnCl2 (8.0 g) were dissolved in 20 ml de-
performed on a JEOL JSM 840A SEM under vacuum after the sample
was sputter-coated with gold at 10 mA for 1 min. The TEM observa-
tion was performed on a JEOL JEM-1230 TEM with an acceleration
voltage of 80 kV. The specific surface area was determined from N2
adsorption–desorption isotherm (at liquid nitrogen temperature)
according to the BET method by using a Quantachrome automated
gas sorption system (AUTOSORB-1-C). The pore size distribution
was calculated by analyzing the adsorption/desorption branches
of the isotherm using the Barrett–Joyner–Halenda (BJH) method.
X-ray photo-electron spectroscopy (XPS) experiment was carried
out on an RBD PHI-5000C ESCA system (Perkin–Elmer) with Mg
K␣ radiation (hv = 1253.6 eV). 1H NMR spectra were recorded on
a Bruker Advance DMX 400-MHz spectrometer by using TMS as
the internal reference and CDCl3 as the solvent. Water content
in PO was determined by using a MaxTitra20Q moisture meter
ionized water. 1.8 g K Co(CN)6 and 1.5 g P123 were dissolved in
3
2
0 ml de-ionized water. The mixture solution of K Co(CN) and
3
6
◦
P123 was then drop-wisely added into ZnCl2 solution at 30 C for
3
0 min under vigorous mechanical stirring (1000 rpm). Afterward,
◦
the reaction solution was heated to 75 C and greatly agitated for
3
h, forming a uniform white suspension. This suspension was kept
◦
at the elevated temperature of 95–100 C for 24 h under N2 atmo-
sphere. The white precipitates were then separated by pressure
filtration, and washed by de-ionized water for at least three times.
The obtained semi-solid was then re-dispersed in tert-BuOH with
◦
magnetic stirring at 30 C for another 24 h. Finally, the precipitates
were isolated by pressure filtration, washed by tert-BuOH for sev-
◦
◦
eral times at 30 C, and dried under vacuum at 100 C for 24 h until
a constant weight (loose powder) was obtained. Elemental analysis

4
0
R.-J. Wei et al. / Journal of Molecular Catalysis A: Chemical 379 (2013) 38–45
Fig. 1. SEM (a), TEM (b) images, nitrogen adsorption–desorption isotherms and pore size distribution (c) and the proposed initiating site of the obtained Zn–CoIII DMCC
catalyst (d). CA in (d) represents the complex agent employed during the preparation of catalyst, e.g. tert-BuOH in this work).
(
Shanghai Tianmei Scientific Instruments Co., Ltd. China) according
Fig. 2 shows X-ray photo-electron spectroscopy (XPS) of the
obtained Zn–Co DMCC catalyst. The binding energy of Zn 2p3 in
III
to the Coulometric method. GC–MS analysis was carried out on a
Thermo Finnigan ThermoQuest (San Jose, CA, USA) gas chromato-
graph equipped with a split/splitless injector and coupled with a
mass detector.
III
Zn–Co DMCC catalyst was 1022.5 eV and lower than that of pure
ZnCl2 (the 2p3 XPS line at 1023.7 eV). This shift could be attributed
to the substitution of Cl for CN ligands and the coordination of
tert-BuOH to the unsaturated Zn metal centers. Furthermore, the
peak assigned to Co 2p3 shifted from 781.0 eV for K Co(CN)6
/2
3
3
. Results and discussion
[
22] to 782.0 eV for the catalyst, indicating the change of microen-
vironments for the corresponding Co metal elements due to the
3
.1. Synthesis and morphology of nanoporous Zn–CoIII DMCC
2+
3+
generated Zn –CN–Co complexes [32], as shown in Fig. 1d.
Structurally, it was a three-dimensional backbone in which zinc
and cobalt atoms were linked by cyanide bridges [28,30,32].
A nanoporous Zn–CoIII DMCC was successfully prepared in
water by a modified hydrothermal process with tri-block copoly-
mer EO20PO70EO20 (P123) as the template (see Section 2). P123
could form spherical micelles with diameters of tens nanometres
in aqueous solution [22,31]. Such P123 spherical micelles led to
III
The local initiating site of Zn–Co DMCC has been proved to be
zinc-hydroxyl (Zn OH) bond on its surface in our recent works
III
[
21,24]. Therefore, the Zn–Co DMCC with high surface area can
III
the formation of nanopores in the resultant Zn–Co DMCC cat-
alyst and hence the increase of surface area of Zn–CoIII DMCC
catalyst. Note that, water/tert-BuOH mixture could not be used
because P123 could completely dissolve into tert-BuOH. However,
tert-BuOH was used in the last slurry step in order to increase the
activity of the final catalyst and completely remove P123. The SEM
and TEM images show that the obtained Zn–CoIII DMCC catalyst had
spherical shape with average diameter of ∼50 nm and partial aggre-
VG ESCALAB MARK II
Zn2p3/2&1/2
Mg Ka:1253.6eV
CAE: 50eV
Step: 0.2eV, 0.5eV
C KLL
Co2p3/2&1/2
O1s
O KVV
Zn LMM
C1s
gation (Fig. 1a and b). Fig. 1c shows the N adsorption–desorption
2
isotherm of the obtained Zn–CoIII DMCC catalyst, which could be
attributed to the typical type IV isotherm. The pore size distribution
was rather broad ranging from several nm to ∼100 nm as shown
in the inset of Fig. 1c, suggesting that the pores were in disorder.
N1s
Cl2p
Zn3s
Zn3p
III
The calculated BET surface area of the obtained Zn–Co DMCC cat-
alyst was 580 m /g. To the best of our knowledge, this was the
first example of Zn–Co DMCC catalyst with nanopores and a high
surface area. Furthermore, the hydrothermal process at 95–100 C
might contribute to the formation of the nanostructure because the
control catalyst synthesized without hydrothermal process congre-
gated and showed very low surface area (∼40 m /g).
2
III
◦
0
100 200 300 400 500 600 700 800 900 1000
Binding Energy (eV)
2
Fig. 2. XPS spectra of the obtained Zn–CoIII DMCC catalyst.

R.-J. Wei et al. / Journal of Molecular Catalysis A: Chemical 379 (2013) 38–45
41
Table 1
Catalytic performances of Zn–Co DMCC catalyst for the coupling of PO-CO2 in the presence of various quaternary ammonium salts.a
III
Entry
1
Co-catalyst
CO2 [MPa]
Time [h]
Selectivityb [%]
Yieldc [%]
TONd
nBu4NCl
nBu4NCl
nBu4NBr
nBu4NBr
5
0.4
5
5
5
5
5
5
5
7
2
7
7
7
7
7
7
7
55.5
100
62.3
100
31.1
100
100
100
100
4.5
20.0
5.0
63.1
2.8
452
1990
497
6300
284
e
2
3
4
f
5
6
7
8
9
nBu4NI
C10H21(CH3)3NBr
C12H25(CH3)3NBr
C16H33(CH3)3NBr
C6H5CH2(CH3)3NCl
8.3
823
13.7
18.4
8.7
1368
1840
864
a
◦
Reaction conditions: PO 30 ml (0.429 mol), water content 260 ppm, catalyst 10 mg (0.043 mmol Zn), co-catalyst: 0.028 mmol, temp. 120 C.
b
c
d
e
f
Calculated from the 1H NMR spectra.
The conversion of PO to PC.
Moles of PO converted to PC per mole of Zn.
nBu4NCl 0.43 mmol, the initial CO2 was 0.4 MPa.
nBu4NBr 0.29 mmol.
accommodate more initiating sites than the unmodified one. The
Zn–Co DMCC with nanopores obtained here indeed exhibited
excellent activity for the coupling reaction of hydrous PO with CO2
by using quaternary ammonium salts as co-catalysts (see below).
strong leaving and nucleophilic abilities, which are favorable for
PO activation [34]. Moreover, the use of more quaternary ammo-
nium salts led to 100% selectivity of PC (entries 2 and 4, Table 1). The
data of Table 1 also indicated that the quaternary ammonium salts
with long alkyl chain showed relatively high TON in the order of
III
C16H33(CH ) > C12H (CH ) > C10H (CH ) (entries 6–8, Table 1),
3
.2. Catalytic performances of Zn–CoIII DMCC with various
3
3
25
3
3
21
3 3
which was consistent with the previous report [35]. The bigger
cations in quaternary ammonium salts might be helpful for enhanc-
quaternary ammonium salts
-
ing the leaving ability of Br .
It was well known that the quaternary ammonium salts could
A control experiment for CO -PO coupling reaction was also car-
catalyze the coupling reaction of CO -PO with 100% PC selectiv-
ity although they suffered from very low catalytic activity at the
2
2
ried out at a low catalyst concentration (10 mg catalyst for 30 ml
PO, entry 2 in Table 1) and a low CO pressure (0.4 MPa) for 2 h. The
same time [33]. Several quaternary ammonium salts were thus
2
selected as co-catalysts of Zn–CoIII DMCC for the coupling reac-
resulting TON could be even up to 1990, which were about twice of
III
the reported TON (958) by using a non-porous Zn–Co DMCC cata-
tion of CO -PO. Initially, the molar ratio of co-catalysts to zinc of
Zn–Co DMCC was set to 0.65. As expected, the addition of small
amounts of alkyl quaternary ammonium salts improved the pro-
ductivity and selectivity of PC, as seen from the entries of 1, 3, 5–9
in Table 1 (Note that the calculation of PC selectivity was given
in the caption of Fig. 3). The results indicated that the quaternary
ammonium salts with Br were better than those with Cl and I
for improving the productivity and selectivity of PC (entries 1, 3
and 5 in Table 1). It was thought that Br has a comprehensive
2
III
III
lyst [35]. A traditional Zn–Co DMCC catalyst [32], which exhibited
2
a non-porous structure and a low surface area of 230 m /g, was
also tested for the PO-CO2 coupling reaction under the same con-
dition as that of entry 8 of Table 1. A low TON of 595 was obtained.
III
These results suggested that the nanoporous Zn–Co DMCC cata-
-
-
-
lyst with high surface area would improve the catalytic activity for
the PO-CO2 coupling reaction.
-
3
.3. The effects of reaction parameters on the CO -PO coupling
2
reaction
d
O
b
O
c
From the above results, cetyltrimethyl-ammonium bromide
(CTAB) was then chosen as the optimum co-catalyst for Zn–Co
DMCC. This binary catalyst system was employed to catalyze
the coupling reaction of CO2 with reagent-grade PO containing
60 ppm water. Table 2 summarized the effects of CTAB/Zn molar
O
O
f
III
e
g
d
a
O
a
O
O
2
j
h
j
h i
ratios, reaction times, pressures and temperatures on the CO -PO
i
2
g
III
coupling reaction. When only Zn–Co DMCC was used as the cat-
b
c
alyst, the TON and PC selectivity were 150 and 17.6%, respectively
entry 1 of Table 2). In another word, 82.4% of the product was poly
H O
f
e
2
(
1
(ether-r-carbonate) [21]. However, when only CTAB was used as the
H O
catalyst, 100% selectivity was achieved for the coupling reaction of
CO2-PO with a rather low TON of 217 (entry 2, Table 2). By increas-
ing the CTAB loading to 0.43 mmol, a low TON of 512 was obtained
2
2
3
H O
2
III
(entry 3, Table 2). Interestingly, if 10 mg Zn–Co DMCC was used
as the main catalyst and CTAB was used as the co-catalyst, the TON
(and the yield of PC) increased significantly from 1840 (18.4%) to
6
5
4
3
2
1
0
9
1
934 (99.6%) when the CTAB/Zn molar ratio raised from 0.65 to
0 (entries 4–10, Table 2). It can be also seen that the increase of
chemical shift (ppm)
Fig. 3. Selected 1H NMR spectra of the crude products (from Table 1).
Line 1: Zn/nBu4NBr = 0.043 mmol/0.028 mmol (entry 3, Table 1), line 2:
Zn/nBu4NBr = 0.043 mmol/0.29 mmol (entry 4, Table 1), line 3: Zn–Co DMCC/CTAB
CTAB/Zn molar ratio from 6.28 to 10 only resulted in a slow increase
of the TON. Furthermore, the TON increased from 3457 to 6373
when the reaction times were prolonged from 3 h to 9 h (entries
III
catalyst system, entry
8 in Table 1. The selectivity of PC was calculated as
6
, 11–13, Table 2). Increasing the reaction pressure from 1.0 MPa
WPC = 102A1.5/[102(A5.0 + A4.2 − 2A4.6 + A1.5) + 58A3.5] (from line 1). The peaks noted
as ( ) were attributed to the quaternary ammonium salts in the catalyst system.
to 5.0 MPa could also significantly increased the TON from 4783 to

4
2
R.-J. Wei et al. / Journal of Molecular Catalysis A: Chemical 379 (2013) 38–45
Table 2
Effects of reaction parameters on the coupling reaction of CO2 and PO catalyzed by binary Zn–CoIII DMC/CTAB catalyst system.a
◦
Selectivity b [%]
Yieldc [%]
TONd
Entry
1
CTAB/Zn [mmol/mmol]
Time [h]
CO2 [MPa]
Temp. [ C]
0/0.043
0.27/0
0.43/0
0.65
1.91
3.26
4.15
6.28
9.53
10.0
3.26
3.26
3.26
6.28
6.28
6.28
6.28
6.28
6.28
10.0
7
7
7
7
7
7
7
7
7
7
3
5
9
7
7
7
7
7
7
7
5
5
5
5
5
5
5
5
5
5
5
5
5
1
3
4
4
4
4
4
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
140
100
80
17.6
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
1.5
13.6
32.1
18.4
32.0
61.6
75.1
92.2
97.3
99.6
34.7
44.5
63.9
47.9
81.7
89.7
93.4
64.2
47.6
52.7
150
217
512
e
2
3
f
4
5
6
7
8
9
1840
3190
6147
7492
9195
9704
9934
3457
4437
6373
4783
8155
8953
9318
6407
4749
5256
10
11
12
13
14
15
16
17
18
19
20
80
a
b
c
Reaction conditions: PO:30 ml (0.429 mol), water content: 260 ppm, catalyst: 10 mg (0.043 mmol Zn).
Calculated from the 1H NMR spectra, see Fig. 2.
The conversion of PO to PC.
d
e
Moles of PO converted per mole of Zn.
The TON was determined on the molar of CTAB, and the CTAB loadings were 0.27 mmol and 0.43 mmol, respectively, which were equal to the loadings of entry 8 and
entry 10.
f
The TON was determined on the molar of CTAB, and the CTAB loadings were 0.27 mmol and 0.43 mmol, respectively, which were equal to the loadings of entry 8 and
entry 10.
9
1
195 when the CTAB/Zn molar ratio was fixed to be 6.28 (entries 8,
4–16, Table 2).
works were reported to disclose the effects of water on the cou-
pling reaction of CO with epoxides [33,37]. In our previous work, it
2
The binary catalyst system of Zn–Co III DMCC/CTAB could
was found that more amounts of water in Zn–Co DMCC-catalyzed
III
efficiently catalyze the reaction in a wide range of reaction temper-
CO -PO copolymerization system caused the production of more
2
◦
◦
◦
ature from 80 C to 140 C. An increase of temperature from 80 C
amounts of propylene carbonate (PC) via a possible “backbiting”
◦
III
to 140 C resulted in a rapid increase of TON from 4749 to 9318
mechanism [21]. It was hence hypothesized that Zn–Co DMCC
under 4.0 MPa CO2 pressure within 7 h (entries 16–19, Table 2).
The productivity at 80 C was satisfied and the PC selectivity also
might be endurable to water in CO -PO coupling reaction system
due to its unique Zn OH bond for initiating the reaction. The effects
2
◦
reached 100% (entries 19–20, Table 2). At relative high temper-
of water on CO -PO coupling reaction catalyzed by the binary
Zn–Co DMCC/quaternary ammonium salt catalyst were then sys-
tematically studied.
2
◦
III
ature of 140 C, the PC selectivity still remained 100% (entry 17,
III
Table 2), indicating that the Zn–Co DMCC/CTAB binary catalyst
system could work well at such high temperature. Usually, the
selectivity and productivity of some zinc-containing catalysts often
decreased sharply due to the deactivation of the catalysts when the
As seen in Table 3, when CaH -dried PO with water content
2
of 70 ppm (entry 1, Table 3) was used for the coupling reaction,
the PC selectivity was 100% and the TON was 9970, which was
slightly higher than that of 9934 for the reaction with reagent-
grade PO (260 ppm water, entry 10, Table 2). When the quaternary
◦
reaction temperature exceeded 120 C [36].
The experimental results indicated that the Zn–CoIII
DMCC/CTAB binary catalyst system was efficient for the cou-
ammonium salts with long alkyl chain (C12H25(CH ) NBr and
3
3
pling reaction of PO and CO . The cooperative catalytic effects of
C10H21(CH ) NBr, entries 2–3, Table 3) were employed as co-
2
3 3
each component contributed to the high productivity and selec-
tivity for the PO-CO2 coupling reaction. With the same loading of
CTAB, TONs of entries 2 and 8 were 217 and 9195, respectively,
catalysts for the coupling reaction of CO2 with reagent-grade PO,
PC selectivity still kept 100%, while TONs were slightly lower than
that of entry 10 in Table 2. An industrial-grade PO with water
content of 800 ppm was directly tested (entry 4, Table 3), TON
III
which might suggested that Zn–Co DMCC was responsible for
1
the high productivity. Similar phenomena were observed for the
entries 3 and 10 in Table 2. On the other hand, the selectivity of
and PC selectivity were 9624 and 99% ( H NMR spectrum, 99.7%
from GC–MS, see Figure S1), respectively. Furthermore, PO with
quantitative amounts of deionized water was tested. Note that the
quantitative amounts of water were achieved by adding deionized
water into CaH -dried PO by microsyringe. For the CO -PO cou-
III
all the Zn–Co DMCC/CTAB binary catalyst systems in Table 2
remained 100% regardless of the reaction conditions. This strongly
indicated that CTAB was responsible for the 100% PC selectivity. It
2
2
III
was worthy to note that the dosage of such Zn–Co DMCC/CTAB
pling reaction with the PO/water molar ratio of 10, the TON and
PC selectivity were 8895 and 97.8%, respectively. Remarkably, even
binary catalysts used was rather low so that they could be eas-
ily dispersed in the reaction system, as evidenced by the clear
resultant crude product.
with the molar ratio of PO/water of 3 (i.e. 2.574 g H O in 24.88 g PO),
2
the TON and PC selectivity still remained high although they were
clearly declined to be 7684 and 95.2% respectively. The failure for
achieving 100% PC selectivity could be attributed to the generation
of 1,2-propylene glycol via the reaction of water and PO (see Fig. 4),
3
.4. The effect of water on the CO -PO coupling reaction
2
1
which was confirmed by H NMR and GC–MS results. Therefore, the
The water content in most of the reported catalyst systems
for the coupling reaction of CO2 with epoxides often negatively
affected the catalytic activity and selectivity. To date, only few
III
Zn–Co DMCC/CTAB binary catalyst system was active in hydrous
situations, rendering it a promising catalyst system in application.

R.-J. Wei et al. / Journal of Molecular Catalysis A: Chemical 379 (2013) 38–45
43
Table 3
The effect of water on the coupling of PO and CO2.a
b
Yieldc [%]
TONd
Entry
Co-catalyst
H2O [ppm]
PO/H2O [mol/mol]
Selectivity [%]
1
2
3
4
5
6
CTAB
70
260
260
800
30,100
93,750
4430:1
1190:1
1190:1
390:1
10:1
100
100
100
99.0
97.8
95.2
100
9970
9745
9704
9624
8895
7684
C12H25(CH3)3NBr
C10H21(CH3)3NBr
CTAB
CTAB
CTAB
97.7
97.3
96.5
89.2
77.0
3:1
a
◦
Reaction conditions: PO 30 ml (0.429 mol), catalyst 10 mg (0.043 mmol Zn), CTAB 0.43 mmol, reaction time 7 h, temp.: 120 C, and pressure 5.0 MPa.
b
c
Calculated from the 1H NMR spectra.
The conversion of PO to PC.
d
Moles of PO converted per mole of Zn.
Table 4
Coupling reaction of CO2 and various unpurified commercial epoxides catalyzed by binary Zn–Co DMCC/CTAB catalyst system.a
III
Entry
Epoxide
Cyclic carbonate
H2O [ppm]
Selectivityb [%]
Yieldc [%]
99.6
O
O
O
O
O
1
1350
100
C l
Cl
O
O
O
2
3
1390
100
100
99.0
O
O
O
O
O
460
640
99.7
99.2
O
O
O
O
O
O
4d
O
100
a
◦
Reaction conditions: epoxides 0.38 mol, catalyst 10.0 mg (0.043 mmol Zn), CTAB 0.43 mmol, reaction time 7 h, temperature 120 C, pressure 4.5 MPa.
b
c
Calculated from the 1H NMR spectra.
The conversion of epoxides.
d
The product was white solid at room temperature.
This Zn–CoIII DMCC/CTAB binary catalyst system was also found
acid site (Zn2+) associated with a Lewis base site (OH). When no
CTAB was used, the Zn OH initiated the copolymerization of PO
and CO2 via the routine (i) in Scheme 2 [21]. When CTAB was used,
a 100% selectivity of PC production was obtained because of a much
to be effectively applicable for a variety of other industrial-grade
terminal epoxides. As can be seen in Table 4, the CO2 and epoxide
coupling reactions were carried out for four different epoxides with
1
−
water contents from 460 to 1390 ppm. The 100% selectivity ( H
stronger nucleophilicity of Br . On the basis of the previous reports
NMR spectra, see Figure S2) and ≥99% yield were obtained for all
of the reactions shown in Table 4. With this new catalyst system,
hundreds of ppm water in the reaction system will no longer be the
matter.
[18,19,38,39], the synergistic effects between acidic site (Zn) and
basic site (Br in CTAB) was shown in the routine (ii) (Scheme 2).
−
Firstly, the PO monomer was coordinated with the Lewis acid site
−
Zn and the Br anion of CTAB then made a nucleophilic attack on
the less hindered carbon atom of the active PO to afford its ring-
opening, generating intermediate 1; Secondly, CO2 was inserted
into the zinc-alkoxide intermediate 1, producing the intermediate
3
.5. Catalyst recovery and reuse
The reusability of the Zn–CoIII DMCC catalyst was examined. As
shown in Table 5, the catalysts were recycled for four runs with
the same reaction condition. The yield and TONs were found to
be nearly the same in all cases, indicating that the Zn–Co DMCC
Table 5
Zn–Co DMCC catalyst recycles.a
III
III
Entry
Recycle No.
Selectivityb [%]
Yieldc [%]
TONd
catalyst was recyclable without significant loss of activity.
1
2
3
4
5
Fresh
100
100
100
100
100
99.6
99.4
99.4
99.3
99.3
9934
9920
9915
9910
9910
1
2
3
4
3
.6. Proposed mechanism of the CO -PO coupling reaction via the
2
III
catalysis of Zn–Co DMCC/CTAB catalyst system
Based on the above experimental results, a possible mechanism
a
Reaction conditions: PO 30 ml (0.429 mol), catalyst 10 mg (0.043 mmol Zn), CTAB
III
CO -PO coupling reaction catalyzed by the Zn–Co DMCC/CTAB
◦
2
0
.43 mmol, reaction time 7 h, temp.: 120 C, and pressure 5.0 MPa.
catalyst system was proposed (Scheme 2). In comparison with the
zinc halide salts, which is easily hydrolyzed, the Zn OH bond on
b
Calculated from the 1H NMR spectra.
The conversion of PO to PC.
Moles of PO converted per mole of Zn.
c
d
III
the surface of Zn–Co DMCC (Fig. 1d) [21,24] can represent a Lewis

4
4
R.-J. Wei et al. / Journal of Molecular Catalysis A: Chemical 379 (2013) 38–45
Scheme 2. Plausible mechanisms of the CO2-PO coupling reaction catalyzed by the Zn–CoIII DMCC/CTAB catalyst system. R4NBr represents CTAB, [Zn]-OH represents the
III
local active site of the Zn–Co DMCC catalyst.
O
a result, the intramolecular cyclic elimination would be produced
b c
f
more easily.
a
d
g
i
HO
O
O
4
. Conclusions
OH
a
i j
h
The nanoporous Zn–CoIII DMCC in conjunction with quaternary
g
j
ammonium salts were highly active and selective binary catalyst
systems for the coupling reaction between CO2 and hydrous epox-
ides. These nanoporous Zn–Co DMCC/quaternary ammonium salt
h
III
binary catalyst systems were tolerant to water and suitable for
various epoxides with solvent-free procedures. These unparalleled
characteristics rendered such catalyst systems promising applica-
tions for the chemical fixation of CO2.
f
a
H O
2
b c d
1
a
H O
2
Acknowledgments
2
The authors are grateful for the financial support by the National
Science Foundation of the People’s Republic of China (No. 21074106
and 21274123), and the Science and Technology Plan of Zhejiang
Province (No. 2010C31036).
6
5
4
3
2
1
0
chemical shift (ppm)
Fig. 4. 1H NMR spectra of the crude product of PO-CO2 coupling reaction catalyzed
by using the Zn–Co DMC/CTAB catalyst system with different water content (line 1:
Appendix A. Supplementary data
III
mol (PO/H2O) = 10:1, line 2: mol (PO/H2O) = 3:1, entries 5–6, Table 3). The selectivity
of PC in the product was calculated by the equation: WPC = 102A4.6/(102A4.6 + 76A3.6).
The peaks noted as ( ) were attributed to the quaternary ammonium salts in the
catalyst system.
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.molcata.
2013.07.014.
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