Cycloaddition of CO2 to Epoxides Catalyzed by Carboxyl

Article pubs.acs.org/IECR

Cycloaddition of CO2 to Epoxides Catalyzed by CarboxylFunctionalized Imidazolium-Based Ionic Liquid Grafted onto Cross-Linked Polymer Yuanyuan Zhang,† Shuangfeng Yin,*,† Shenglian Luo,† and Chak Tong Au†,‡ †

State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, Hunan, China ‡ Department of Chemistry, Hong Kong Baptist University, Kowloon Tong, Hong Kong, China ABSTRACT: Carboxyl-functionalized imidazolium-based ionic liquid was grafted onto cross-linked divinylbenzene polymer (PDVB-CEIMBr) for the first time. The catalyst was characterized and used for the cycloaddition of CO2 to epoxides under relatively mild reaction conditions without any use of cocatalyst and organic solvent. The effects of reaction conditions (temperature, initial CO2 pressure, and time) and water addition on the yield of cyclic carbonate were investigated systematically. Under optimal conditions (140 °C, initial CO2 pressure 2.0 MPa and 4 h), propylene carbonate selectivity and yield was 99.8% and 96.1%, respectively. Moreover, the catalyst shows good stability and reusability. A plausible reaction mechanism was proposed for the catalytic reaction. It is suggested that synergetic effect among Brønsted acid COOH, Lewis base Br− and the support facilitates the coupling reaction. From the viewpoint of industrial application, the catalyst is attractive because of its excellent catalytic efficiency.

1. INTRODUCTION It is well-known that CO2 is a greenhouse gas blamed for global warming. Meanwhile, CO2 is also the most inexpensive and abundant in the C1 family.1−3 Considering environmental protection and resource utilization, it is interesting to convert CO2 into valuable chemicals. An important reaction of CO2 fixation is the synthesis of five-membered cyclic carbonates via cycloaddition of CO2 to epoxides.4 Cyclic carbonates are chemical products that are used as polar aprotic solvents and electrolytic elements of lithium secondary batteries. Besides, cyclic carbonates can serve as precursors for the formation of polycarbonates as well as intermediates in the production of pharmaceuticals, fine chemicals and agricultural chemicals.1,3,4 A lot of homogeneous and heterogeneous catalysts have been explored for the cycloaddition of CO2 to epoxides.3,5 The common homogeneous catalysts are alkali metal salts, organophosphorous compounds, organic bases, and organometallic compounds.6−9 The homogeneous catalysts are efficient, but the inherit disadvantage is the high cost for product separation and/or catalyst fabrication. Polymeric nanoparticles,4 metal oxides,10,11 molecular sieves,12 supported catalysts13−15 are usually used as heterogeneous catalysts; however, their application is limited due to low activity, poor stability and/or reusability. Ionic liquids (ILs) have attracted much attention as a result of their unique properties, such as nonvolatility, nonflammability, and recyclability.2,16 Compared to traditional ILs, the functionalized ionic liquids (e.g., those with hydroxyl or carboxyl groups) show additional advantages such as alterable polarity, lower viscosity, higher solubility of inorganic salts, and immobilization potential. They were found to show much better catalytic efficiency than single ILs toward the synthesis of cyclic carbonates.17,18 For example, Sun et al. prepared a series of carboxyl IL catalysts containing Brønsted acidic sites and © 2012 American Chemical Society

Lewis basic sites, and this type of catalysts showed high catalytic activity with no use of a cocatalyst and organic solvent.19 As a strategy for the fabrication of stable and reusable heterogeneous catalysts, functional ILs were immobilized on SiO213,20 as well as on mesoporous materials such as MCM-41 and SBA-15,14,21,22 and polymer.15,23 However, catalyst reusability is not satisfactory. For example, we fabricated hydroxyl ionic liquid catalyst that was immobilized on SBA-15 molecular sieve; we observed good catalytic activity toward the coupling reaction but poor catalyst reusability.24 The use of polymer materials as support has been regarded as a good method to immobilize homogeneous catalysts by means of chemical grafting.25−27 In our previous research, we grafted hydroxyl ILs onto cross-linked divinylbenzene polymer and observed good catalytic efficiency in the synthesis of cyclic carbonates from CO2 and epoxides.28 To the best of our knowledge, however, there is still no report on the immobilization of carboxyl ILs on polymer. In the present work, we first grafted “3-(2-carboxyl-ethyl)-1(3-amino-propyl) imidazole bromide” (carboxyl-functionalized) onto cross-linked divinylbenzene polymer (denoted as PDVBCEIMBr). We characterized the PDVB-CEIMBr catalyst and evaluated it for the synthesis of cyclic carbonates without any use of organic solvent and cocatalyst. A systematic investigation was conducted on the effects of reaction parameters (temperature, initial CO2 pressure, time) and water addition on the yield of cyclic carbonate. Received: Revised: Accepted: Published: 3951

December 21, 2011 February 24, 2012 February 24, 2012 February 25, 2012 dx.doi.org/10.1021/ie203001u | Ind. Eng. Chem. Res. 2012, 51, 3951−3957

Industrial & Engineering Chemistry Research

Article

2. EXPERIMENTAL SECTION 2.1. Chemicals. The PDVB grafted with 1-(3-amino-propyl) imidazole (denoted as PDVB-IM; QuadraPure IMDAZ; 100− 400 μm particle size; extent of labeling: 1.5 mmol/g loading, 1% cross-linked with divinylbenzene) was supplied by Aldrich Chemical Co. 3-Bromopropionic acid and 1,2-butylene oxide were purchased from TCI. Propylene oxide (PO) was obtained from Sinopharm Chemical Reagent Co. Ltd. Other epoxides were bought from Acros Organics. Analytical grade ethanol, n-propionic acid, bromoethane, and 1,1,1-trichloroethane were purchased from Tianjin Damao Chemical Reagent Company. All the chemicals were used as received except that the epoxides were dried by distillation over CaH2 before use. CO2 (purity of 99.9%) provided by Changsha Gas Co. was dehydrated by 4A molecular sieve in a high pressure stainless-steel tube. 2.2. Preparation of IL Grafted onto Cross-Linked Polymer. The procedure of grafting IL (3-(2-carboxyl-ethyl)1-(3-amino-propyl) imidazolium bromide) onto PDVB is shown in Scheme 1. In a typical procedure, 20 mL of 1,1,

The liquid products were qualitatively analyzed on a GC/MS instrument (6890N/5973N) using an Agilent HP-5MS capillary column (30 m × 0.45 mm ×0.8 μm). For quantitative determination, the products were analyzed on an Agilent 7820A GC with flame ionization detection (FID) and Agilent AB-FFAP capillary column (30 m × 0.25 mm ×0.25 μm). The GC yield was obtained using internal standard substance of biphenyl.

3. RESULTS AND DISCUSSION 3.1. Characterization. Figure 1 shows the morphology of PDVB-IM and PDVB-CEIMBr. As shown in Figure 1(a, b), the

Scheme 1. Synthesis of Carboxyl-Functionalized Imidazolium-Based Ionic Liquid Grafted onto Cross-Linked Polymer (PDVB-CEIMBr)

Figure 1. SEM images of PDVM-IM (a, b) and PDVB-CEIMBr (c, d).

surface of PDVB-IM is smooth. However, with the grafting of IL, it becomes extremely rough (Figure 1 c, d). The EDS spectrum of PDVB-CEIMBr (Figure 2) shows the existence of

1-trichloroethane was added to the mixture of 3-bromopropionic acid (6.0 mmol) and PDVB-IM (2.0 g), followed by reflux for 24 h. Then, the as-obtained solid was filtered out, washed (three times with 1,1,1-trichloroethane) and dried in vacuum (60 °C for 4 h) successively. Finally, the solid PDVBCEIMBr catalyst (pale yellow in color) was obtained. The PDVB grafted with 3-ethyl-1-(3-amino-propyl) imidazolium bromide (denoted as PDVB-EIMBr) was prepared similarly, except that bromoethane was used rather than 3-bromopropionic acid. 2.3. Catalyst Characterization. The morphology of the samples was observed by scanning electron microscopy (SEM) (HITACHI S-4800 microscope). Energy dispersive X-ray spectroscopy (EDS) analysis was performed using an accessory (Horiba 7593-H) of the HITACHI S-4800 instrument. Thermogravimetric-differential scanning calorimetry (TGDSC) analysis (from 25 to 600 °C) of samples was performed in N2 flow (heating rate = 10 °C/min) on a NETZSCH-STA449C equipment. Elemental analysis was performed by using a Vario EL III analyzer. The FT-IR spectra were recorded using a Bruker vector 22 FT-IR spectrophotometer (KBr tablets). 2.4. Coupling Reactions. The syntheses of cyclic carbonates from CO2 and epoxides using PDVB-CEIMBr as catalyst were carried out in a 30 mL stainless steel autoclave equipped with a magnetic bar. In a typical experiment, catalyst (0.44 mmol%, calculated based on the amount of IL), an appropriate amount of biphenyl (as internal standard for gas chromatography (GC) analysis) and epoxide (28.6 mmol) were successively charged into the reactor. Then, the reactor was pressurized with an appropriate amount of CO2 and heated to a desired temperature in an oil bath. After a designated reaction time, the reactor was cooled to 0 °C in an ice−water bath, followed by slow venting of the remaining CO2.

Figure 2. EDS spectrum of PDVB-CEIMBr.

bromine, illustrating that the IL has been successfully grafted onto the polymer support. The TG and DSC curves of PDVB-IM and PDVB-CEIMBr are depicted in Figure 3. The results indicate that the PDVB-IM support is stable up to 300 °C, and PDVB-IM pyrolysis happens above 300 °C. On the other hand, PDVB-CEIMBr is stable up to 200 °C, and the slight weight loss may be due to the loss of physisorbed water. There are two endothermic peaks, one at 330 °C and the other at 420 °C (Figure 3b), attributable to the decomposition of IL and polymer support, 3952

dx.doi.org/10.1021/ie203001u | Ind. Eng. Chem. Res. 2012, 51, 3951−3957


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rise of temperature. The yield of PC increases quickly with temperature rise up to 140 °C; further rise of temperature shows almost no effect on PC yield. It is clear that the activity of the catalyst is extremely sensitive to reaction temperature. At 170 °C, PO is nearly completely converted into PC. It is noted that PC selectivity is always above 99.5%. In other words, PC selectivity is independent of temperature. Besides temperature, initial CO2 pressure usually has a significant effect on the reaction.15,18,19 As shown in Figure 5,

Figure 5. Effect of initial CO2 pressure on the catalytic performance of PDVB-CEIMBr (Reaction conditions: PO 28.6 mmol, catalyst 0.44 mol %, temperature 140 °C, time 4 h). Figure 3. TG and DSC curves of PDVM-IM (a) and PDVB-CEIMBr (b).

the PC yield is affected by CO2 initial pressure. When the initial CO2 pressure is 2.0 MPa, a yield of 96.1% is obtained under the reaction conditions of 140 °C and 4 h. However, when CO2 initial pressure is above 2.0 MPa, there is significant decline in PC yield. It is observed that PC selectivity is always above 99.0% and can be considered as being independent of CO2 initial pressure. Such an effect of CO2 pressure on catalytic activity has been observed in other catalytic systems.7,13,28 For example, when immobilized ionic liquid/ZnCl2 was used for the coupling of PO and CO2, the best catalyst activity appeared at a CO2 pressure of 1.5 MPa at 110 °C.13 The results are unexpected because PC is in its liquid form under the adopted reaction conditions and the increase of CO2 pressure should be a favorable factor for PO conversion. A possible explanation is that acidic CO2 dissolves in basic epoxide and liquefies as a result of CO2−epoxide complexing.7,13,28 The high CO2 pressure promotes this kind of CO2−PO interaction rather than enhancing the interaction between PO and catalyst, thus leading to the low catalytic activity. Figure 6 shows the influence of reaction time on PC yield at 140 °C and an initial CO2 pressure of 2 MPa. There is increase of PC yield from 19.5% to 97.4% when reaction time is extended from 1 to 6 h. The reaction proceeds rapidly within the first 3 h and reached a PC yield of 96.1% at the fourth hour. Further extension of reaction time shows little effect on PC yield. It is observed that PC selectivity is always above 99.0%. 3.2.2. Effect of Water. As reported previously, the presence of a small amount of water could influence the coupling reaction greatly.29 Our previous research demonstrated that water had a negative effect on catalytic performance.7 However, in some other catalytic systems, the addition of an appropriate amount of water can improve catalytic activity.24,29,30 In the present study, we made a systematic investigation on the effect

respectively. The results clearly demonstrate that the catalyst is stable in air and is thermally stable up to 200 °C. 3.2. Catalytic Performance. To evaluate the catalytic performance of PDVB-CEIMBr, we chose PO as a model substrate to fix CO2 and investigated the influence of reaction parameters (temperature, initial CO2 pressure, and time) on the yield of cyclic carbonates. Additionally, the catalytic activity of PDVB-CEIMBr in the presence of different amounts of water was investigated. 3.2.1. Effect of Reaction Parameters. Figure 4 illustrates the dependence of PC yield and selectivity on reaction temperature. On the whole, there is improvement in catalytic activity with

Figure 4. Effect of reaction temperature on the catalytic performance of PDVB-CEIMBr (reaction conditions: PO 28.6 mmol, catalyst 0.44 mol %, initial CO2 pressure 2.0 MPa, time 4 h). 3953



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and ILs.26,27 The results demonstrated that there was synergistic effect between supports and ILs. In the present study, we investigated the synergistic effects among the carboxyl group, Br−, and the PDVB-IM polymer support (Table 1). One can see that when PDVB-IM or Table 1. Comparison Experiments of Cycloaddition of CO2 to PO over Various Catalyst Systemsa catalytic results

Figure 6. Effect of reaction time on the catalytic performance of PDVBCEIMBr (reaction conditions: PO 28.6 mmol, catalyst 0.44 mol %, temperature 140 °C, initial CO2 pressure 2.0 MPa).

of water on the PDVB-CEIMBr catalyst. We used dehydrated PO and CO2 as starting materials. The operation of water addition was conducted using a microscale syringe under the protection of a flow of CO2. Figure 7 shows the effect of water

entry

catalysts

yield (%)

sel. (%)

1 2b 3c 4b 5 6c 7 8d 9e

PDVB-IM BrCH2CH2COOH PDVB-IM/CH3CH2COOH PDVB-IM/BrCH2CH2COOH PDVB-EIMBr PDVB-EIMBr/CH3CH2COOH PDVB-CEIMBr HBetBr PS-Thr

8.9 5.4 13.6 92.0 31.2 81.1 96.1 76.0 96.0

97.4 88.8 94.0 99.2 99.2 98.8 99.8  

a

Reaction conditions: PO 28.6 mmol, catalyst 0.44 mol %, temperature 140 °C, initial CO2 pressure 2.0 MPa, time 4 h. b BrCH2CH2COOH: 0.44%. cCH3CH2COOH: 0.44%. dReaction conditions: catalyst 2.5 mol %, temperature 140 °C, CO2 pressure 8 MPa, time 8 h.18 eReaction conditions: catalyst 0.6 mol %, temperature 130 °C, CO2 pressure 9 MPa, time 24 h.31

3-bromopropionic acid is selected as catalyst, the PC yield is extremely low (Table 1, entries 1 and 2). However, the activity of PDVB-CEIMBr obtained as a result of chemical reaction between PDVB-IM and 3-bromopropionic acid is much higher. It is interesting to observe that a mixture of PDVB-IM and 3-bromopropionic acid also shows high catalyst activity, but when n-propionic acid is used instead of 3-bromopropionic acid, the activity is extremely poor (Table 1, entries 3 and 4). In terms of PC yield and selectivity, the PDVB−CEIMBr catalyst, which contains a COOH group, is superior to PDVB−EIMBr without a COOH group (Table 1, entries 5, 7). Furthermore, when n-propionic acid was added to PDVB−EIMBr, the PC yield is 81.1%, much higher than that observed over PDVB−EIMBr alone (31.2%) but lower than that observed over PDVB-CEIMBr (Table 1, entries 6 and 7). It is observed that the activities of PDVB-IM/ BrCH2CH2COOH and PDVB-CEIMBr are much higher than those of the other catalysts (Table 1, entries 4 and 7). The phenomena can be attributed to synergistic effect of support (PDVB-IM) and functional groups (COOH and Br). The results of Table 1 also illustrate the synergistic effect of the two functional groups on the epoxide ring; that is, abduction polarization of the oxygen atom by hydrogen-bonding of the Brønsted-acidic center (COOH), and nucleophilic activation of the carbon atom by the Lewis-basic center (Br−). Consequently, there is easy opening of the epoxy ring, and effective promotion of the reaction (Table 1, entries 3−7). It is deduced that the high catalytic activity and PC selectivity can be attributed to such a synergistic effect. For comparison, the performances of some other catalysts are also listed in Table 1. Zhou et al.18 reported a PC yield of 76% over betaine hydrobromide (HBetBr) under the reaction conditions of 140 °C, CO2 pressure 8 MPa, and 8 h (Table 1, entry 8). Over polystyrene-supported threonine (PS-Thr), as reported by Qi et al.,31 a reaction time of 24 h (Table 1, entry 9) was required to achieve a PC yield comparable to that of the

Figure 7. Effect of the amount of water on the catalytic performance of PDVB-CEIMBr (reaction conditions: PO 28.6 mmol, catalyst 0.44 mol %, temperature 140 °C, initial CO2 pressure 2.0 MPa, time 3 h).

on PC yield and selectivity. When H2O/PO (mol/mol) is less than 0.6, there is only slight change in PC yield; however, when the H2O/PO ratio is raised to 1.0, PC yield decreases to 58.7%. The introduction of water promotes the hydrolysis of PO to byproduct 1,2-propylene glycol, resulting in decrease of PC selectivity. Therefore, the addition of a trace amount water has a negative effect on catalytic performance, similar to the case of ZnBr2−Ph4PI7 but different from those of hydroxyl IL catalysts.24 3.2.3. Comparison of Various Catalysts. In the investigations of synergistic effects between acidic and basic functional groups,17,19,24 researchers pointed out that an epoxide was activated by acid functional groups through the O atom of epoxide, whereas in the cases of basic functional groups an epoxide was activated through effective nucleophilic attack on the sterically less hindered β-carbon atom of the epoxide. The coordination of both acidic and basic functional groups hence promoted the reaction. Furthermore, other researchers investigated the relationship between supports 3954



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for the synthesis of cyclic carbonates using a fix-bed continuous flow reactor. 3.3. Cycloaddition of CO2 to Other Epoxides. The cycloaddition of CO2 to other epoxides over PDVB-CEIMBr was examined at 140 °C and 2 MPa (initial CO2 pressure). Table 3 shows that the catalyst is active for all the selected

present study (Table 1, entry 7). Compared with our case of PDVB-CEIMBr, the amount of catalysts used in the studies of Zhou et al. and Qi et al. is much larger and the reaction conditions are much more severe (higher pressure and longer reaction time). In other words, PDVB-CEIMBr is superior to HBetBr and PS-Thr in performance. 3.2.4. Catalyst Recycling. Experiments were carried out to examine the stability of activity in terms of catalyst recycling (reaction time = 4 h) using PO as substrate at 140 °C and an initial CO2 pressure of 2 MPa. After each cycle, the catalyst was recovered by simple filtration, and the solid catalyst was reused for the next run after it was washed with PO and dried in vacuum. The yields of PC for the five repeated runs are shown in Figure 8. There is no significant decrease in PC yield,

Table 3. Cycloaddition of CO2 to Epoxides Catalyzed by PDVB-CEIMBra catalytic results entry

epoxides

time (h)

yield (%)

sel. (%)

1 2 3 4 5

propylene oxide epichlorohydrin styrene oxide 1,2-butylene oxide cyclohexene oxide

4 4 4 4 24

96.1 98.8 93.8 95.0 70.4

99.8 99.3 98.7 99.9 98.4

a

Reaction conditions: PO 28.6 mmol, catalyst 0.44 mol %, temperature 140 °C, initial CO2 pressure 2.0 MPa.

substrates under the adopted conditions. The catalyst is the most active when epichlorohydrin is selected as substrate, achieving 98.8% yield within 4 h (Table 3, entry 2). The reactivity of styrene oxide and 1,2-butylene oxide are also good, only a little poorer than that of PO and epichlorohydrin (Table 3, entries 3 and 4). It is observed that much longer time is needed to complete the conversion of cyclohexene oxide (Table 3, entry 5), which could be explained by the fact that the two rings of cyclohexene oxide hinder nucleophilic attack of Br−, causing the rate of ring-opening to decrease. 3.4. Plausible Reaction Mechanism. As shown in Table 1, the grafted ILs with carboxyl functional groups exhibit better catalytic activities than those without functional groups. Based on the results of Zhou et al.,18 Sun et al.,19 and Han et al.,32 as well as those of the present study, we deduce that similar to the hydroxyl groups,17,24,25 carboxylic acid groups promote ring-opening of epoxides by forming hydrogen bonds during cycloaddition of CO2 to epoxides. The mechanism for the synthesis of cyclic carbonates over PDVB-CEIMBr is postulated in Scheme 2. First,

Figure 8. Recycling of PDVB-CEIMBr in the cycloaddition of CO2 (reaction conditions: PO 28.6 mmol, catalyst 0.44 mol %, temperature 140 °C, initial CO2 pressure 2.0 MPa, time 4 h).

indicating that the PDVB-CEIMBr catalyst is stable and reusable. Besides, the selectivity to PC is above 99.5%. Moreover, we characterized the recovered catalyst by FT-IR (Figure 9) and elemental analysis (Table 2) techniques. In both

Scheme 2. Plausible Mechanism on the Cycloaddition of CO2 to Epoxide over PDVB-CEIMBr Catalyst

Figure 9. FT-IR spectra of PDVB-CEIMBr: (a) fresh, (b) recovered.

Table 2. Elemental Analysis of PDVB-CEIMBr fresh recovered

C (%)

H (%)

N (%)

68.56 68.53

7.60 7.62

5.03 5.03

cases, the recovered catalyst showed results similar to those of the fresh one. It can be deduced that this catalyst has potential 3955



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(8) Jing, H. W.; Nguyen, S. T. SnCl4-organic base: Highly efficient catalyst system for coupling reaction of CO2 and epoxides. J. Mol. Catal. A: Chem. 2007, 261, 12. (9) Jutz, F.; Grunwaldt, J. D.; Baiker, A. Mn(III)(salen)-catalyzed synthesis of cyclic organic carbonates from propylene and styrene oxide in “supercritical” CO2. J. Mol. Catal. A: Chem. 2008, 279, 94. (10) Yasuda, H.; He, L. N.; Takahashi, T.; Sakakura, T. Non-halogen catalysts for propylene carbonate synthesis from CO2 under supercritical conditions. Appl. Catal., A 2006, 298, 177. (11) Dai, W. L.; Yin, S. F.; Guo, R.; Luo, S. L.; Du, X.; Au, C. T. Synthesis of propylene carbonate from carbon dioxide and propylene oxide using Zn-Mg-Al composite oxide as high-efficiency catalyst. Catal. Lett. 2010, 136, 35. (12) Srivastava, R.; Srinivas, D.; Ratnasamy, P. Synthesis of polycarbonate precursors over titanosilicate molecular sieves. Catal. Lett. 2003, 91, 133. (13) Xiao, L. F.; Li, F. W.; Peng, J. J.; Xia, C. G. Immobilized ionic liquid/zinc chloride: Heterogeneous catalyst for synthesis of cyclic carbonates from carbon dioxide and epoxides. J. Mol. Catal. A: Chem. 2006, 253, 265. (14) Udayakumar, S.; Lee, M. K.; Shim, H. L.; Park, S. W.; Park, D. W. Imidazolium derivatives functionalized MCM-41 for catalytic conversion of carbon dioxide to cyclic carbonate. Catal. Commun. 2009, 10, 659. (15) Xie, Y.; Zhang, Z. F.; Jiang, T.; He, J. L.; Han, B. X.; Wu, T. B.; Ding, K. L. CO2 cycloaddition reactions catalyzed by an ionic liquid grafted onto a highly cross-linked polymer matrix. Angew. Chem. 2007, 119, 7393. (16) Jutz, F.; Andanson, J. M.; Baiker, A. Ionic liquids and dense carbon dioxide: A beneficial biphasic system for catalysis. Chem. Rev. 2011, 111, 322. (17) Sun, J.; Zhang, S. J.; Cheng, W. G.; Ren, J. Y. Hydroxylfunctionalized ionic liquid: a novel efficient catalyst for chemical fixation of CO2 to cyclic carbonate. Tetrahedron Lett. 2008, 49, 3588. (18) Zhou, Y. X.; Hu, S. Q.; Ma, X. M.; Liang, S. G.; Jiang, T.; Han, B. X. Synthesis of cyclic carbonates from carbon dioxide and epoxides over betaine-based catalysts. J. Mol. Catal. A: Chem. 2008, 284, 52. (19) Sun, J.; Han, L. J.; Cheng, W. G.; Wang, J. Q.; Zhang, X. P.; Zhang, S. J. Efficient acid−base bifunctional catalysts for the fixation of CO2 with epoxides under metal- and solvent-free conditions. ChemSusChem 2011, 4, 502. (20) Motokura, K.; Itagaki, S.; Iwasawa, Y.; Miyaji, A.; Baba, T. Silicasupported aminopyridinium halides for catalytic transformations of epoxides to cyclic carbonates under atmospheric pressure of carbon dioxide. Green Chem. 2009, 11, 1876. (21) Udayakumar, S.; Lee, M. K.; Shim, H. L.; Park, D. W. Functionalization of organic ions on hybrid MCM-41 for cycloaddition reaction: The effective conversion of carbon dioxide. Appl. Catal., A 2009, 365, 88. (22) Dharman, M. M.; Choi, H. J.; Kim, D. W.; Park, D. W. Synthesis of cyclic carbonate through microwave irradiation using silicasupported ionic liquids: Effect of variation in the silica support. Catal. Today 2011, 164, 544. (23) Xiong, Y. B.; Wang, H.; Wang, R. M.; Yan, Y. F.; Zheng, B.; Wang, Y. P. A facile one-step synthesis to cross-linked polymeric nanoparticles as highly active and selective catalysts for cycloaddition of CO2 to epoxides. Chem. Commun. 2010, 46, 3399. (24) Dai, W. L.; Chen, L.; Yin, S. F.; Luo, S. L.; Au, C. T. 3-(2Hydroxyl-ethyl)-1-propylimidazolium bromide immobilized on SBA15 as efficient catalyst for the synthesis of cyclic carbonates via the coupling of carbon dioxide with epoxides. Catal. Lett. 2010, 135, 295. (25) Sun, J.; Cheng, W. G.; Fan, W.; Wang, Y. H.; Meng, Z. Y.; Zhang, S. J. Reusable and efficient polymer-supported task-specific ionic liquid catalyst for cycloaddition of epoxide with CO2. Catal. Today 2009, 148, 361. (26) Xie, Y.; Ding, K. L.; Liu, Z. M.; Li, J. J.; An, G. M.; Tao, R. T.; Sun, Z. Y.; Yang, Z. Z. The immobilization of glycidyl-groupcontaining ionic liquids and its application in CO2 cycloaddition reactions. Chem.Eur. J. 2010, 16, 6687.

a hydrogen bond is formed between the H atom of PDVB-CEIMBr carboxylic acid group and the O atom of epoxide, resulting in polarization of C−O bonds. Then, the Lewis base Br− makes a nucleophilic attack on the less sterically hindered β-carbon atom of the epoxide to open the epoxy ring. Afterward, there is insertion of CO2 into the intermediate for the formation of the corresponding cyclic carbonate, together with the simultaneous regeneration of the catalyst.

4. CONCLUSIONS In summary, we grafted carboxyl-functionalized imidazoliumbased ionic liquid onto cross-linked divinylbenzene polymer (PDVB-CEIMBr). The catalyst is active and selective for the cycloaddition of CO2 to epoxides without any use of cocatalyst and organic solvent. The excellent performance can be ascribed to the synergetic effects among COOH group, Br− and the support. As a heterogeneous catalyst, PDVB-CEIMBr can be easily separated from the products by simple filtration and reused for at least five times without significant decline of product yield. In view of the simplicity, activity, stability, and reusability of PDVB-CEIMBr, we believe that the catalyst has great potential in industrial applications for the synthesis of five-membered cyclic carbonates.

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AUTHOR INFORMATION

Corresponding Author

*Tel (Fax): 86-731-88821310. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by NSF of Hunan Province (10JJ1003), National 863 Program of China (2009AA05Z319), the National Natural Science Foundation of China (Grant Nos 20507005), the program for New Century Excellent Talents in Universities (NCET-10-0371), and the Fundamental Research Funds for the Central Universities. C.T.A. thanks the Hunan University for an adjunct professorship.

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REFERENCES

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Cycloaddition of CO2 to Epoxides Catalyzed by Carboxyl

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