Synthesis of Cyclohexene Carbonate Catalyzed by Polymer-Supported Catalysts

Article abstract of DOI:10.1080/00397911.2014.979297

A series of polystyrene-divinylbenzene cross-linked resin (PS)-supported zinc chloride catalysts were prepared in this study. They can efficiently catalyze the solventless cycloaddition of cyclohexene oxide with carbon dioxide in the presence of tetrabutylammonium bromide (TBAB) as cocatalyst under relatively mild reaction conditions. The catalyst is composed of carrier, connecting arm, ligand, and active ingredient. The connecting arms of different lengths can significantly affect the catalytic activity. Among these catalysts, the one using diethylene glycol as connecting arm and 2-aminopyridine as ligand, named PS-DEG-2ap-ZnCl₂, showed the optimal catalytic performance. The yield of cyclohexene carbonate was 95.18% calculated by gas chromatographic analysis under the optimal conditions (393 K, 5 MPa, 6 h). Moreover, the catalyst showed good stability and reusability. From the viewpoint of industrial application, the catalyst is attractive because of its excellent catalytic efficiency on the synthesis of cyclohexene carbonate. GRAPHICAL ABSTRACT.

Full text of DOI:10.1080/00397911.2014.979297

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Synthesis of Cyclohexene Carbonate Catalyzed by
Polymer-Supported Catalysts
Ke Cuiab, Zhengyong Lianga, Jing Zhanga & Yadong Zhanga
a School of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou, China
b School of Physics and Chemistry, Henan Polytechnic University, Jiaozuo, China
Accepted author version posted online: 11 Nov 2014.
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To cite this article: Ke Cui, Zhengyong Liang, Jing Zhang & Yadong Zhang (2014): Synthesis of Cyclohexene Carbonate
Catalyzed by Polymer-Supported Catalysts, Synthetic Communications: An International Journal for Rapid Communication of
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Synthesis of Cyclohexene Carbonate Catalyzed by Polymer-Supported Catalysts
Ke Cui1,2, Zhengyong Liang1, Jing Zhang1, Yadong Zhang1
1School of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou, China,
2School of Physics and Chemistry, Henan Polytechnic University, Jiaozuo, China
Address correspondence to Yadong Zhang, School of Chemical Engineering and Energy,
Zhengzhou University, Zhengzhou, 450001, China. E-mail: zzuzhangyadong@126.com
Abstract
A series of polystyrene-divinylbenzene cross-linked resin (PS) supported zinc chloride
catalysts were prepared in this paper. They can efficiently catalyze the solventless
cycloaddition of cyclohexene oxide with carbon dioxide in the presence of
tetrabutylammonium bromide (TBAB) as co-catalyst under relatively mild reaction
conditions. The catalyst is composed of carrier, connecting arm, ligand and active
ingredient. The connecting arms of different lengths can significantly affect the catalytic
activity. Among these catalysts, the one using diethylene glycol as connecting arm and
2-aminopyridine as ligand, which named PS-DEG-2ap-ZnCl2, showed the optimal
catalytic performance. The yield of cyclohexene carbonate was 95.18% calculated by GC
analysis under the optimal conditions (393K, 5 MPa , 6 h ). Moreover, the catalyst
showed good stability and reusability. From the viewpoint of industrial application, the
catalyst is attractive because of its excellent catalytic efficiency on the synthesis of
cyclohexene carbonate.
1

KEYWORDS: Carbon dioxide; Cyclohexene carbonate; Chemical fixation;
Polymer-supported catalysts
INTRODUCTION
Recent researches show that carbon dioxide (CO2), a major greenhouse gas, is a cheap,
abundant, and environmentally benign C1 resource.[1–10] In this context, the effective
processes of CO2 utilization have drawn much attention since they could heavily reduce
the damage to environment and bring considerable economic benefits. One of the
advantageous ways in this area is the cycloaddition of CO2 with epoxide to afford cyclic
carbonate, which can be widely used for various purposes, such as polar solvents,
electrolytic materials and organic synthetic intermediates.[11–18] However, it still remains
a difficult target to incorporate CO2 into other organic substances because of its inert
nature. Therefore, it is urgent to find a cheap, effective, and environmentally benign
catalyst system, which can activate CO2 and make the cycloaddition reaction proceed
smoothly.
2

In the past years, a large number of catalysts have been developed for the cycloaddition
of CO2 with epoxide. These catalysts include alkali metal salts, [19–22] metal
complexes,[23–31] ionic liquids,[32–45] metal oxides,[46] zeolites,[47] and so on. However,
most cases suffer from low catalyst activity,[46,48] high costs, the need for co-solvents[49,50]
as well as the requirement for high pressure and high temperature. So, designing of
highly efficient, easily separated catalysts still remains a challenge, especially a certain
catalyst system for the cycloaddition reaction of CO2 with cyclohexene oxide, because of
the lower activity of cyclohexene oxide than that of other epoxides.
In our continuous study, we found that the polystyrene-divinylbenzene cross-linked resin
(PS) supported catalysts exhibited excellent catalytic performance for some chemical
reactions.[51–59] In this article, we developed a series of PS supported metal salt catalysts
and used them to catalyze the cycloaddition reaction of CO2 with cyclohexene oxide
(Scheme 1) to afford cyclohexene carbonate. And desirable results were achieved.
RESULTS AND DISCUSSION
Transition-metal ions are considered to be classical Lewis acids and they are often used
to catalyze some organic reactions. In this work, several transition metal salts were
selected to catalyze the cycloaddition of CO2 with cyclohexene oxide. And the results
were shown in Table 1.
3

As shown in Table 1, Zn(CH3COO)2 (entry 2, Table 1), Co(CH3COO)2 (entry 3, Table 1),
NiCl2 (entry 5, Table 1), FeCl3 (entry 6, Table 1) and CoCl2 (entry 7, Table 1) could well
catalyze the reaction, however, the yields of cyclohexene carbonate were not high, which
were 76.47%, 70.19%, 34.35%, 58.24% and 70.35%, respectively. In contrast, ZnCl2
(entry 1, Table 1) and ZnBr2 (entry 4, Table 1) showed highly activity among the tested
catalysts. The yields reached 88.24% and 88.07%, respectively.
In order to take advantage of relatively easy phase separation and catalyst recovery, a
serial of PS supported metal salt catalysts were prepared, and their catalysis activities
were investigated. The results were shown in Table 2.
As shown in Table 2, all the PS supported catalysts could well catalyze the reaction.
Moreover, there was no significant decrease in the catalytic activity compared with that
of the bare catalyst. That might be due to the finely dispersiveness of the metal salts on
PS ruslting in the adequate contacts of the active ingredients with substrates. Another
notable result was that the conversion and yield decreased significantly in absence of
TBAB (entry 8, Table 2). This suggested that TBAB play an important role in the
cycloaddition reaction. In a comprehensive consideration, ZnCl2 was selected as the
optimal metal salt for the following investigation.
The structure of catalytic materials has great influence on the activities of them. In order
to find the optimized catalyst, the catalysts with different connecting arms were examined
in the cycloaddition of CO2 with cyclohexene oxide. The results were listed in Table 3.
4

As shown in Table 3, it was worth noting that all these catalysts were active for the
cycloaddition reaction. The TOF was the highest when the reaction was catalyzed by
PS-DEG-2ap-ZnCl2. It was possibly due to the connecting arm with suitable length which
could provide desirable degree of freedom to improve the collision probability of active
ingredients and substrates. Another notable result was that the conversion of cyclohexene
oxide was the highest when PS-PEG600-2ap-ZnCl2 was used. It may suggest that the
hydroxyl groups on PEG600 could also catalyze the reaction.[60] The effects of the pore
structure on the conversion and yield were studied. Some small but noticeable changes
appeared in the pore volumes and pore diameters with the change of connecting arms.
And the effects had not shown regularly changes. It suggested that the connecting arms
are mainly grafted on the surface of PS. Overall, PS-DEG-2ap-ZnCl2 was selected as the
catalyst for the following investigation.
The dependence of cyclohexene carbonate yield on temperature was plotted in Figure 1.
It was evident from the graph that the yield of cyclohexene carbonate reached 90.08%
when the temperature was increased to 393 K, but a further increase in the temperature
caused a decrease in the yield. This was possibly due to the damage of surface structure
of the catalysts and the cycloaddition reaction being a exothermic reaction.[61]
Accordingly, 393K was chosen as the optimal temperature for the cycloaddition reaction
of cyclohexene oxide with CO2.
5

Figure 2 revealed the effect of reaction time on the yield of cyclohexene carbonate. It can
be noted that the yield reached maximum value (92.60%) after reacting for 6 h. Longer
reaction time may result in side reactions and decreased the yield. It suggested that the
optimum reaction time for this reaction was 6 h.
The effect of initial pressure was investigated at 393K over 6 h and the results were
shown in Figure 3. The yield of cyclohexene carbonate increased with the pressure
increasing from 2 MPa to 7 MPa. But the yield increased slightly (only from 95.18% to
96.47%) as the pressure was increased from 5MPa to 7 MPa. The reason may be that
higher pressure, which could improve the concentration of CO2 in the liquid phase,
enhanced mass transfer between the two gas-liquid phases. Therefore, the optimum initial
pressure was 5 MPa.
Experiments were carried out to examine the reusable performance of the catalyst. After
each cycle, the catalyst was recovered by filtration, washed, dried in vacuum and then
reused in the next batch. The results were shown in Figure 4. There is only a slight loss of
its original catalytic activity.
Based on the above experimental results and analysis, the optimum conditions of the
cycloaddition of cyclohexene oxide with CO2 were obtained as follows:
PS-DEG-2ap-ZnCl2 0.07mmol, cyclohexene oxide 200 mmol, TBAB 0.9mmol, 393K, 5
MPa and 6 h. Under these conditions, the yield of cyclohexene carbonate was 95.18%
calculated by GC analysis.
6

In order to demonstrate the general applicability of PS-DEG-2ap-ZnCl2, we next
investigated the cycloaddition of other epoxides with CO2. The results were summarized
in Table 4. As shown in Table 4, the catalyst was active for all the substrates used. The
conversion and selectivity of the reactions were all above 90%. Among them, propylene
oxide were the most active substrates, and the conversion and selectivity were 99.66%
and 99.37% respectively (entry 4, Table 4). All in all, the PS-DEG-2ap-ZnCl2 catalyst is
very active and selective for the coupling of CO2 onto a wide range of epoxides.
CONCLUSION
The PS supported zinc chloride catalysts were highly efficient for the solventless
cycloaddition of cyclohexene oxide with carbon dioxide in the presence of
tetrabutylammonium bromide (TBAB) as co-catalyst under relatively mild reaction
conditions. Moreover, the synthetic process of cyclohexene carbonate was optimized and
the yield reached 95.18% catalyzed by PS-DEG-2ap-ZnCl2. And the catalyst is stable,
environmentally benign, and reusable. Therefore, it is attractive for the synthesis of
cyclohexene carbonate from an industrial viewpoint. The present work also provides new
ideas for designing catalysts. The connecting arms with suitable length can provide
desirable degree of freedom for the active ingredients to improve the collision probability
with substrates, resulting in higher reaction yield. Furthermore, different active
ingredients can be coordinated by changing ligands in order to use the catalysts for
various reactions.
7

EXPERIMENTAL
All chemicals used in this study were commercially available. They were used without
further purification unless otherwise mentioned. CO2 (99.9%) was obtained from
Zhengzhou Keyi Gas Co., Ltd.,China. Chloromethylated polystyrene-divinylbenzene
crosslinked resin (PS) was supplied by Hebi Jiaxin Technology Co., Ltd.,China.
Cyclohexene oxide was purchased from Yueyang Changde Chemical Industrial Co.,
Ltd.,China. Tetrabutylammonium bromide (TBAB) was obtained from Aladdin
Industritrial Corporation.
1H and 13C NMR spectra were obtained using a Bruker AVⅡ-400 instrument. FT-IR
spectra were recorded with a Nicolet Fourier transform infrared spectrometer (IR-200).
Gas chromatography mass spectra (GC/MS) were obtained using a gas chromatograph
Shimadzu GC-2010 detector linked to a Shimadzu GCMS-QP 2010 mass. BET surface
areas and pore structure were measured by N2 adsorption at the temperature of liquid
nitrogen using a NOVA 4200e analyzer after the evacuation of the sample at 70℃ for 6
h. The morphology of the samples was observed by scanning electron microscopy (SEM)
(JEOL JSM-7500F). Contents of metal elements were determined by icp-aes (TJA IRIS
Advantage). Thermogravimetric-differential scanning calorimetry (TG-DSC) analysis
(from 20 to 650 °C) of samples was performed in N2 flow on a NETZSCH-STA-409 PC
equipment.
8

Catalysts Preparation And Structure Determination
The details of catalyst synthesis and characterization are presented in the Supplemental
materials. The preparation procedure is illustrated in Scheme S1 (Supplemental
materials). A series of modified PS were obtained after PS reacting with ethylene glycol,
diethylene glycol, triethylene glycol and polyethylene glycol (PEG200, PEG300,
PEG400, PEG600) in Step 1 (see Supplemental materials), named PS-EG, PS-DEG,
PS-TEG, PS-PEG200, PS-PEG300, PS-PEG400 and PS-PEG600, respectively. Then the
above modified PS further reacted with 2-aminopyridine to afford intermediates (Step 2,
Supplemental materials), named PS-EG-2ap, PS-DEG-2ap, PS-TEG-2ap,
PS-PEG200-2ap, PS-PEG300-2ap, PS-PEG400-2ap and PS-PEG600-2ap, respectively.
And the catalysts were prepared after metal salts being coordinated (Step 3,
Supplemental materials). FT-IR and surface properties studies were performed to
characterize PS and the above modified PS. The FT-IR spectra were shown in Figure S1,
Figure S2 (Supplemental materials). The BET surface areas and pore structure results
were given in Table S1 (Supplemental materials). The contents of metal elements were
determined by icp-aes and the results were given in Table S2 (Supplemental materials).
The morphology of the samples, observed by scanning electron microscopy (SEM), was
shown in Figure S3 (Supplemental materials). Figure S4 (Supplemental materials)
showed the thermogravimetric-differential scanning calorimetry (TG-DSC) analysis.
General Procedure Of Cycloaddition Reaction Of Epoxides With CO2
All the cycloaddition reactions were carried out in a 100 mL stainless steel autoclave
equipped with a magnetic stirrer. In a typical run, the reactor was charged with
9

cyclohexene oxide (200 mmol), catalyst (0.07 mmol, calculated by metal elements) and
the desired amount of TBAB. The reactor was pressurized with CO2 to 5 MPa and then
was heated to 120 °C and stirred for a designated period of time. Then the reactor was
cooled quickly to a low temperature by placing in an ice/water bath and pressure was
released slowly. The reaction mixture was filtered and the catalysts was washed by
dichloroethane for reusing. The remained liquid mixture was distilled under reduced
pressure to obtain purified cyclohexene carbonate. The product was characterized by IR,
1H NMR, 13C NMR and GC/MS (see Supplemental materials, Figure S5, Figure S6,
Figure S7). The yield was calculated by the GC peak area with dimethyl succinate as an
internal standard.
ACKNOWLEDGMENT
We thank the National Natural Science Foundation of China (81101748) and Henan
Outstanding Person Foundation (2010HASTIT037) for financial support.
SUPPORTING INFORMATION
Supplemental data for this article can be accessed on the publisher’s website.
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18

Table 1 Coupling of carbon dioxide and cyclohexene oxide catalyzed by different
catalystsa
Entry Catalysts
Conversion (%) Yield (%)b TOF (h-1)c
1
2
3
4
5
6
7
ZnCl2
95.85
88.24
76.47
70.19
88.07
34.35
58.24
70.35
504.2
436.8
401.1
503.3
196.3
332.8
402.0
Zn(CH3COO)2 80.26
Co(CH3COO)2 78.31
ZnBr2
NiCl2
FeCl3
CoCl2
95.13
43.67
68.54
87.91
aReaction conditions: cyclohexene oxide 200 mmol, catalyst 0.07 mmol (calculated by
metal elements), TBAB 0.9mmol, initial pressure 4MPa, 393K, 5h.
bGC yield.
cTOF: moles of cyclohexene carbonate produced per mole of a catalyst per hour.
19

Table 2 Coupling of carbon dioxide and cyclohexene oxide catalyzed by different PS
supported catalystsa
Entry Catalysts
Conversion (%) Yield (%)b
TOF (h-1)c
502.5
425.7
396.0
503.8
206.3
336.6
375.3
395.1
1
2
3
4
5
6
7
8d
PS-2ap-ZnCl2
95.69
77.45
76.31
94.03
41.98
67.52
87.77
78.26
87.93
74.50
69.30
88.16
36.10
58.90
65.68
69.15
PS-2ap-Zn(CH3COO)2
PS-2ap-Co(CH3COO)2
PS-2ap-ZnBr2
PS-2ap-NiCl2
PS-2ap-FeCl3
PS-2ap-CoCl2
PS-2ap-ZnCl2
aReaction conditions: cyclohexene oxide 200 mmol, catalyst 0.07 mmol (calculated by
metal elements), TBAB 0.9mmol, initial pressure 4MPa, 393K, 5h.
bGC yield.
cTOF: moles of cyclohexene carbonate produced per mole of a catalyst per hour.
dWith catalyst in absence of TBAB.
20

Table 3 Effect of catalysts with different connecting armsa
Entry Catalysts
Pore volume
(cm3.g-1)
0.2019
Pore diameter Conversion Selectivity TOF
(nm)
(%)
(%)
(h-1)b
502.5
481.2
513.0
457.1
469.0
489.6
505.1
509.1
1
2
3
4
5
6
7
8
PS-2ap-ZnCl2
10.65
13.82
14.35
13.91
12.69
12.21
12.17
11.68
95.69
96.41
97.71
95.62
96.92
98.38
98.07
98.43
91.89
87.35
91.88
83.65
84.69
87.09
90.13
90.51
PS-EG-2ap-ZnCl2
PS-DEG-2ap-ZnCl2
PS-TEG-2ap-ZnCl2
0.1564
0.1701
0.1726
PS-PEG200-2ap-ZnCl2 0.1788
PS-PEG300-2ap-ZnCl2 0.1769
PS-PEG400-2ap-ZnCl2 0.1961
PS-PEG600-2ap-ZnCl2 0.2014
aReaction conditions: cyclohexene oxide 200 mmol, catalyst 0.07 mmol (calculated by
zinc), TBAB 0.9mmol, initial pressure 4MPa, 393K, 5h.
bTOF: moles of cyclohexene carbonate produced per mole of a catalyst per hour.
21

a
Table 4 Catalytic activities of the reactions of various epoxides with CO2
Entry Epoxide Product Xb (%) Sc (%)
1
2
3
4
99.25 97.17
93.74 95.27
96.36 95.71
99.66 99.37
a Reaction conditions: epoxides 200 mmol, PS-DEG-2ap-ZnCl2 0.07 mmol (calculated
by zinc), TBAB 0.9mmol, initial pressure 5MPa, 393K, 6h.
b X: epoxides conversion.
c S: carbonates selectivity.
22

Scheme 1. Synthesis of Cyclohexene Carbonate Using CO2 and Cyclohexene Oxide and
the Structure of Catalysts
23

Figure 1. Effect of reaction temperature on cyclohexene carbonate yield. Reaction
conditions: PS-DEG-2ap-ZnCl2 0.07mmol (calculated by zinc), cyclohexene oxide 200
mmol, TBAB 0.9mmol, initial pressure 4MPa, 5h.
24

Figure 2. Effect of reaction time on cyclohexene carbonate yield. Reaction conditions:
PS-DEG-2ap-ZnCl2 0.07mmol (calculated by zinc), cyclohexene oxide 200 mmol, TBAB
0.9mmol, initial pressure 4MPa, 393K.
25

Figure 3. Effect of initial pressure on cyclohexene carbonate yield. Reaction conditions:
PS-DEG-2ap-ZnCl2 0.07mmol (calculated by zinc), cyclohexene oxide 200 mmol, TBAB
0.9mmol, 393K, 6h.
26

Figure 4. Recycling of PS-DEG-2ap-ZnCl2 in the cycloaddition of CO2. Reaction
conditions: PS-DEG-2ap-ZnCl2 0.07mmol (calculated by zinc), cyclohexene oxide 200
mmol, TBAB 0.9mmol, initial pressure 5MPa, 393K, 6h.
27