Catal Lett (2010) 135:295–304 DOI 10.1007/s10562-010-0285-4
3-(2-Hydroxyl-Ethyl)-1-Propylimidazolium Bromide Immobilized on SBA-15 as Efficient Catalyst for the Synthesis of Cyclic Carbonates via the Coupling of Carbon Dioxide with Epoxides Wei-Li Dai • Lang Chen • Shuang-Feng Yin Sheng-Lian Luo • Chak-Tong Au
•
Received: 14 December 2009 / Accepted: 20 January 2010 / Published online: 9 February 2010 Ó Springer Science+Business Media, LLC 2010
Abstract 3-(2-hydroxyl-ethyl)-1-propylimidazolium bromide (HEPIMBr) was immobilized onto SBA-15, Al-SBA15 mesoporous molecular sieves and SiO2 by chemical grafting method. The materials were characterized and used as catalysts for solvent-free synthesis of cyclic carbonates from CO2 and epoxides under relatively mild conditions. Among them, the HEPIMBr immobilized on SBA-15 performs the best, showing carbonate selectivity and yield of ca. 100% (120 °C, 2.0 MPa, 2 h). The catalyst is thermally stable and shows good reusability. Based on the experimental results, a plausible reaction mechanism has been proposed for the catalytic reaction. Keywords SBA-15 Hydroxyl ionic liquids Carbon dioxide Epoxide Cyclic carbonate
1 Introduction Carbon dioxide is abundant, inexpensive, nontoxic, nonflammable, and highly functional, and its conversion to useful chemicals has received much attention. It has been used as C1 building block in organic synthesis,
W.-L. Dai L. Chen S.-F. Yin (&) S.-L. Luo (&) C.-T. Au College of Chemistry and Chemical Engineering, Hunan University, 410082 Changsha, Hunan, China e-mail:
[email protected];
[email protected] S.-L. Luo e-mail:
[email protected] C.-T. Au Department of Chemistry, Center for Surface Analysis and Research, Hong Kong Baptist University, Kowloon Tong, Hong Kong, China
especially as a phosgene substitute [1, 2]. Of the most interesting is the coupling of CO2 with high-energy epoxides for the generation of cyclic carbonates that are used as aprotic polar solvents, monomers, as well as raw materials and intermediates in the production of fine chemicals, engineering plastics, and pharmaceuticals [1, 2]. A wide range of catalysts have been explored for the generation of cyclic carbonates. The homogeneous catalysts are alkali metal salts [3, 4], organotin or organoantimony compounds [5, 6], ionic liquids or onium salts [7–20], and transition metal complexes [21–31]. For heterogeneous interactions, metal oxides [32–34], zeolites [35, 36], smectites [37, 38], and polyoxometalate [39, 40] are used. Generally, the homogeneous catalysts are superior to the heterogeneous ones in catalytic performance. However, in terms of catalyst stability and separation of catalyst from product, the use of heterogeneous catalysts is preferred. In the past 10 years, there is much interest in the use of ionic liquids as catalysts for the synthesis of cyclic carbonates. Using 1-octyl-3-methylimidazolium tetrafluoroborate (1.78 mol%) as a catalyst at high reaction pressure (14.0 MPa), Kawanami et al. [7] achieved 98% PC yield (PC selectivity = 100%) after a reaction period of 5 min. Zhang and coworkers reported that a hydroxyl-functionalized ionic liquid (HIL), viz. 3-(2-hydroxyl-ethyl)-1methylimidazolium bromide (HEMIMBr) showed good activity and selectivity for the coupling of epoxide and CO2 under mild reaction conditions, much higher than those of traditional ionic liquids. The catalyst shows good thermal stability and water-tolerance, and there is no need of using a co-catalyst and organic solvent. Nonetheless, steps have to be conducted to separate the product from the catalyst.
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One of the best protocols to facilitate the separation procedure is to have the catalyst immobilized on a support [41]. Xiao et al. [15] immobilized 3-n-butyl-1-propylimidazolium bromide on SiO2, and evaluated its catalytic activity for the coupling reaction. However, the use of zinc halides as co-catalysts (e.g., ZnCl2, ZnBr2) is required in this approach. Recently, Udayakumar et al. [42–46] and Zhang et al. [47] reported some ionic liquid (without hydroxyl groups) functionalized catalysts via immobilization of imidazolium or quaternary ammonium salts onto MCM-41 or SiO2. However, their catalytic activities were not satisfactory. We have made a series of attempts to develop high-efficiency catalysts and absorbents for CO2 utilization [2, 48–50]. As an extension of our researches, we immobilized a HIL, viz. 3-(2-hydroxyl-ethyl)-1-propylimidazolium bromide (HEPIMBr), on SBA-15, Al-SBA-15 mesoporous molecular sieves and SiO2. The SBA-15 materials were selected for their special pore structures, high surface area and abundant surface hydroxyl groups. The as-synthesized catalysts were characterized and evaluated for the coupling of CO2 and epoxides. The effects of CO2 pressure, reaction temperature, reaction time, and water addition on the target reaction were systematically investigated. Finally, a plausible reaction mechanism has been proposed.
2 Experimental 2.1 Chemicals The reagents and chemicals (analytic grade unless otherwise stated) were purchased from Aldrich Chemical Co., and were used without further purification except that the epoxides were dried by distillation over CaH2. Prior to use, the CO2 (99.99% purity) purchased from Changsha Gas Co. was dehydrated by 4A molecular sieve in a highpressure stainless-steel tube. Except in the case that involved water addition manipulation, the weighing and handling procedures were conducted in a glove box under an atmosphere of dry argon.
Scheme 1 Preparation procedure of SBA-15-HEPIMBr
SBA-15
SBA-15
OH OH
Si
2.2 Catalyst Synthesis 2.2.1 Preparation of SBA-15 and Al-SBA-15 The SBA-15 and Al-SBA-15 (Si/Al = 22) were prepared as described by Zhao et al. [51] and Srivastava et al. [52] using tetraethyl orthosilicate (TEOS, Aldrich Co.) as silica source, aluminum isopropoxide as Al source, Pluronic P123 as template (amphiphilic tri-block co-polymer, poly(ethylene glycol)-block-poly(propylene glycol)-blockpoly(ethylene glycol), EO20PO70EO20; average molecular weight = 5,800, Adrich Co.), and HCl to control the pH. Prior to catalyst immobilization, the SBA-15 and Al-SBA15 materials were calcined at 550 °C for 6 h and 500 °C for 4 h in air, respectively. 2.2.2 Immobilization of HEPIMBr on SBA-15 and Al-SBA15 [52–54] The immobilization of HEPIMBr on SBA-15 is illustrated in Scheme 1. In a typical procedure, 3 g of SBA-15 was boiled in 50 g of water for 2 h, and dried in air at 125 °C for 6 h. Then 3-chloropropyltriethoxysilane (10 mmol) in 80 mL of dry toluene was added to the support material and the mixture was refluxed under nitrogen for 12 h. The treated SBA-15 (grafted with chloropropyl group; denoted as SBA-15-Pr-Cl) was filtered out, washed twice with dry toluene and once with anhydrous diethyl ether, and dried at 60 °C for 4 h in vacuum. Then imidazole (10 mmol) in 50 mL of dry toluene was added to the SBA-15-Pr-Cl and the mixture was refluxed for 12 h. The resulting solid was filtered out, washed and dried in a similar manner to give the imidazole-grafted SBA-15 (SBA-15-Pr-imidazole). Finally, 2-bromoethanol (10 mmol) in 50 mL of dry toluene was added to the SBA-15-Pr-imidazole and the mixture was refluxed for 24 h. After similar steps of filtering, washing and drying, the SBA-15 with immobilized HEPIMBr was obtained (denoted as SBA-15-HEPIMBr hereinafter). Similarly, the Al-SBA-15 with immobilized HEPIMBr was also fabricated (denoted as Al-SBA-15HEPIMBr hereinafter).
Cl
+ (EtO) 3 Si
SBA-15
Si
Cl
(1)
OEt HN Cl +
N
SBA-15
Si
N
N
(2)
OEt
OEt
OH SBA-15
Si OEt
123
N
N + HO
Br
SBA-15
Si OEt
N
N
Br (3)
3-(2-Hydroxyl-Ethyl)-1-Propylimidazolium Bromide Immobilized on SBA-15
297
2.2.3 Immobilization of HEPIMBr on SiO2 [15]
2.4 Coupling Reactions
The procedure for the immobilization of HEPIMBr on SiO2 is shown in Scheme 2. In a typical procedure, 20 mL of TEOS, 6.27 mL of 3-chloropropyltriethoxysilane, 5 mL of ethanol and 6 mL of water were mixed. Upon the formation of a clean and homogeneous liquid mixture, 5 mL of HCl (36 wt. %) was added and there was the occurrence of gradual coagulation. After ageing at 60 °C for 12 h, the mixture was dried in vacuum at 80 °C for 5 h to obtain the SiO2 that was grafted with chloropropyl group (SiO2-PrCl). Then imidazole (26.5 mmol) in 30 mL of dry toluene was added and the mixture was refluxed for 12 h. The resulting imidazole-grafted SiO2 (SiO2-Pr-imidazole) was washed twice with dry toluene and once with anhydrous diethyl ether, and dried at 60 °C for 4 h in vacuum. Finally, 2-bromoethanol (26.5 mmol) in 30 mL of dry toluene was added to the SiO2-Pr-imidazole, and the mixture was refluxed for 24 h. After similar steps of filtering, washing with dry toluene and diethyl ether, and drying in vacuum, the SiO2-immobilized HEPIMBr was obtained (denoted as SiO2-HEPIMBr hereinafter).
The coupling reactions were carried out in 30 mL highpressure stainless-steel autoclaves equipped with a magnetic bar. In a typical reaction, the reactor was charged with an appropriate amount of catalyst, epoxide and biphenyl (as internal standard for GC analysis). After the reactor was fed with CO2 to a desired pressure, the autoclave with its contents was heated to a designated temperature and stirred for a designated period of time. Then the reactor was cooled to 0 °C in an ice-water bath, and the remaining CO2 was released using an aspirator and absorbed in an aqueous solution saturated with K2CO3. The resulting product mixture was defined by GC-MS over an Agilent 6890-5973 MSD GC-mass spectrometer and by 1 H NMR over a Bruck400 superconductive nuclear magnetic resonance spectrometer with TMS as internal standard. All the products were quantitatively analyzed by an Agilent 6820 gas chromatography equipped with a TCD and DB-wax capillary column (30 m 9 0.45 mm 9 0.85 lm).
2.3 Catalyst Characterization 3 Results and Discussion The powder X-ray diffraction (XRD) patterns were obtained on a Brucker D8 advance diffractometer with monochromatized Cu Ka radiation (k = 0.15406 nm) at a setting of 40 kV and 40 mA. Elemental analysis was performed by using a Vario EL III analyzer. Surface areas of the catalysts were determined by the BET method according to the nitrogen adsorption-desorption isotherms obtained over a Nova 1200 surface area analyzer; before each measurement, the sample was heated to 423 K and kept at this temperature for 3 h. Scanning electron microscopy (SEM) observations were carried out by means of a LEO-1530 microscope. The FT-IR spectra were recorded using a Bruker vector 22 FT-IR spectrophotometer (KBr tablets). TGA and DTA analyses were performed on a Perkin Elmer thermalgravimetric analyzer (TGA 6) and Perkin Elmer differential thermal analyzer (DTA 7), respectively; the sample was kept in a flow of air and heated at a rate of 10 °C/min.
Scheme 2 Preparation procedure of SiO2-HEPIMBr
3.1 Characterization We investigated the immobilization of HEPIMBr on SBA15 and Al-SBA-15 by XRD, SEM, elemental analysis, N2 physical adsorption, FT-IR, and TGA-DTA techniques, using the fabricated SBA-15 and Al-SBA-15 as references. As shown in Fig. 1, The XRD patterns of SBA-15 and Al-SBA-15 show peaks ascribable to SBA-15, and upon HEPIMBr immobilization, there is weakening of peak intensity, especially that of (110) and (200) reflections. The results suggest that the immobilization of HEPIMBr on SBA-15 and Al-SBA-15 has little effect on the crystal phase of SBA-15 and Al-SBA-15 but has disturbed the long-range order of mesopores. The SEM images of Fig. 2 reveal that the immobilization of HEPIMBr on SBA-15 and Al-SBA-15 causes little change in the morphology and particle size of the support materials.
Cl +TEOS
(OEt)3 Si SiO 2
Cl + HN
SiO2
N
N
N + HO
Br
SiO 2
Cl
SiO2
N SiO2
(1) (2)
N N
N Br
OH (3)
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298
Fig. 1 Small-angle XRD patterns of SBA-15, Al-SBA-15, SBA-15HEPIMBr, and Al-SBA-15-HEPIMBr
The results of elemental analysis and N2 physical adsorption are listed in Table 1. Since HEPIMBr is the only compound in the system that contains nitrogen, the loading amount of HIL can be estimated according to the result of N analysis. It was found that the amount of HEPIMBr in SBA-15-HEPIMBr and Al-SBA-15-HEPIMBr is 1.77 and 2.29 mmol/g respectively. Based on the C/N ratios, it is clear that HEPIMBr has been grafted on SBA-15 and Al-SBA-15 [47]. Moreover, it is evident that the immobilization of HEPIMBr causes significant change in specific surface area and pore structure of the supports. The BET surface area and pore volume of SBA-15 decrease from 755 to 187 m2/g and 1.07 to 0.37 mL/g, and
W. Dai et al.
those of Al-SBA-15 decrease from 932 to 127 m2/g and 1.21 to 0.25 mL/g, respectively. We estimate that the loading amount of HEPIMBr in SBA-15-HEPIMBr and Al-SBA-15-HEPIMBr are 9.45 9 10-3 and 18.03 9 10-3 mmol/m2, respectively. Figure 3 shows the FT-IR spectra of SBA-15, SBA-15HEPIMBr, Al-SBA-15, and Al-SBA-15-HEPIMBr. Weak peaks at ca. 3,129, 1,575, 1,500, 1,448, 739, and 626 cm-1 related to the imidazole group of HEPIMBr appears in the spectra of SBA-15-HEPIMBr and Al-SBA-15-HEPIMBr. These are in accordance with the available spectrum of imidazole [55]. The results indicate that HEPIMBr has been successfully immobilized or supported on the support materials. Depicted in Fig. 4 are the TGA and DTA curves of SBA15-HEPIMBr and Al-SBA-15-HEPIMBr. It is clear that the immobilized HEPIMBr are stable in air up to 250 °C. Upon heating to 700 °C, there is the pyrolysis and complete decomposition of HEPIMBr. The decomposition process can be divided into two steps showing exothermic peaks at 353 and 547 °C for SBA-15-HEPIMBr, and at 354 and 606 °C for Al-SBA-15-HEPIMBr. With reference to the results of the support materials, the percentages of weight loss originating from HEPIMBr are ca. 34 and 43% for SBA-15-HEPIMBr and Al-SBA-15-HEPIMBr, respectively, almost consistent with the results of element analysis (Table 1). The higher loading of HEPIMBr on Al-SBA-15 in comparison to that on SBA-15 may be attributed to the
Fig. 2 SEM images of a SBA-15, b SBA-15-HEPIMBr, c Al-SBA-15, and d Al-SBA-15-HEPIMBr
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3-(2-Hydroxyl-Ethyl)-1-Propylimidazolium Bromide Immobilized on SBA-15
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Table 1 Physicochemical properties of catalysts Catalyst
BET results Amount of HIL per unit Amount of surface area (mmol/m2) 9 10-3 HILa (mmol/g) 2 b 3 N (wt%) C (wt%) H (wt%) SBET (m /g) Pore volume (cm /g)
SBA-15
—
—
—
—
755
1.07
—
Al-SBA-15
—
—
—
—
932
1.21
—
1.77
187
0.37
2.29
127
0.25
18.03
13
0.01
—
Elemental analysis results
SBA-15-HEPIMBr
4.95
Al-SBA-15-HEPIMBr SiO2-HEPIMBr
6.41 —
11.97
1.85
14.96
2.62
—
—
a
Calculation based on nitrogen in HIL
b
Calculated by the BJH method
—
1448
1500 a
Transmittance, %
9.45
739 1575
b
626 3129
c d 4000
3500
3000
2500
2000
1500
1000
500
-1
Wavenumber, cm
Fig. 3 FT-IR spectra of a SBA-15, b SBA-15-HEPIMBr, c Al-SBA15, d Al-SBA-15-HEPIMBr
fact that Al-SBA-15 (932 m2/g) is higher than SBA-15 (755 m2/g) in specific surface area (Table 1). It is noted that more hydroxyl groups and hydrogen-bonded hydroxyl groups were formed on the (Al-) SBA-15 surface during the boiling treatment procedure than that during traditional procedure such as drying in air or under vacuum [53]. It was suggested that the presence of surface hydroxyl and hydrogen-bonded hydroxyl groups are suitable sites for the anchoring of aminosilane (e.g., 3-aminopropyltriethoxysilane, and N-(2-aminoethyl)-3-aminopropyltrimethoxy silane etc.) [53]. In a similar manner, the 3-chloropropyltriethoxysilane could also be anchored to the surface hydroxyl and hydrogen-bonded hydroxyl groups, enabling the immobilization of HEPIMBr in relatively larger quantity. 3.2 Catalytic Performance 3.2.1 Catalytic Performance of the As-Fabricated Catalysts The catalytic performance of the as-fabricated materials was evaluated for the synthesis of cyclic carbonates from
Fig. 4 TGA and DTA curves of a SBA-15, b Al-SBA-15, c SBA-15HEPIMBr, and d Al-SBA-15-HEPIMBr
propylene oxide (PO) and CO2. In other words, PO is used as a model substrate to fix CO2. As depicted in Table 2, the support materials viz. SiO2, SBA-15, and Al-SBA-15 are poor in catalytic performance, giving propylene carbonate (PC) yields of 2.7, 11.3 and 1.5%, respectively (Table 2, entries 1–3). With the immobilization of HEPIMBr, the materials become active and selective, giving PC yields of 74.5, 98.7 and 97.7% (Table 2, entries 4–6). Over SBA-15HEPIMBr and Al-SBA-15-HEPIMBr, PC can be generated
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Time (h)
Catalytic results
XPO(%) SPC(%) YPC(%)
1
SiO2
2
2.7
2
SBA-15
2
11.5
98.3
11.3
3
Al-SBA-15
2
3.1
47.8
1.5
4
SiO2-HEPIMBr
2
74.6
99.8
74.5
5
SBA-15-HEPIMBr
2
99.0
99.7
98.7
6
Al-SBA-15-HEPIMBr
2
97.9
99.8
97.7
7c 8c
Al-SBA-15-HEPIMBr HEMIMBr
2 2
93.0 50.7
99.5 99.2
92.5 50.3
9
SBA-15-HEPIMBr
1
78.1
99.1
77.4
10
SBA-15-HEPIMBr
4
99.5
99.7
99.2
d
100
2.7
11
SBA-15-HEPIMBr
1
90.5
99.6
90.1
12e
SBA-15HEPIMBr ? KBr
1
87.3
99.7
87.1
KBr
1
—
—
—
e,f
13 a
Reaction conditions: PO 2 mL (28.6 mmol), catalyst 0.2 g, initial CO2 pressure 2.5 MPa, temperature 120 °C
b
XPO: PO conversion, SPC: PC selectivity; YPC: PC yield
c
ILs amount equal to that of 0.2 g SBA-15-HEPIMBr
d
Initial CO2 pressure 2.0 MPa
e
Metal bromide: 50 mg
f
Almost no reaction
under mild conditions in quantitative amounts (Table 2, entries 5, 6). Similar to the preparation procedure reported by Xiao et al. [15], we fabricated a SiO2-immobilized HEPIMBr catalyst. Compared to SBA-15-HEPIMBr and Al-SBA-15-HEPIMBr, SiO2-HEPIMBr fabricated shows much lower catalytic activity (Table 2, entry 4), plausibly a result of being small in specific surface area and low in porosity. Srivastava et al. reported that the SBA-15 functionalized with both Al3? (Lewis acid) and adenine (Lewis base) was much superior to the SBA-15 functionalized with only adenine in catalytic performance [52]. However, in our study, although the loading of HEPIMBr on Al-SBA15 (2.29 mmol/g; 18.03 9 10-3 mmol/m2) is significantly higher than that on SBA-15 (1.77 mmol/g; 9.45 9 10-3 mmol/m2), the PC yields over SBA-15-HEPIMBr (0.2 g) and Al-SBA-15-HEPIMBr (0.2 g) are rather similar (98.7 and 97.7%, respectively; Table 2, entries 5 and 6). However, when the amount of HEPIMBr immobilized on Al-SBA-15 is regulated to be the same as that of SBA-15HEPIMBr, the PC yield becomes 92.5% (Table 2, entry 7). The decrease could be due to a decline in surface area. Furthermore, because the basicity of HEPIMBr is low (lower than that of adenine) and the OH sites of HEPIMBr can be regarded as acidic [19, 20], the inclusion of Al3? would result in a significant rise of acidity. The rise in acidity could be another reason for the decline of
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performance. It is noted that the unsupported HEMIMBr performs badly (Table 2, entry 8). In other words, the SBA-15 and Al-SBA-15 materials play an important role in enhancing the catalytic performance. Similar synergic effect between support and HIL has been reported by other researchers [8, 9, 56–58]. We observed that the PC yield increases with reaction time: with a change of reaction time from 1 to 2 h, the PC yield over SBA-15-HEPIMBr increases from 77.4 to 98.7% (Table 2, entries 9, 5); at 4 h, the PC yield is 99.2% (Table 2, entry 10). It is noted that the selectivity to PC is always above 99% over SiO2HEPIMBr, SBA-15-HEPIMBr and Al-SBA-15-HEPIMBr. 3.2.2 Effect of Reaction Parameters on Catalytic Performance of SBA-15-HEPIMBr Because of the good performance (Table 2) and thermal stability (Fig. 4) of SBA-15-HEPIMBr, the catalyst was adopted for further study. The effects of factors such as CO2 pressure, reaction temperature, and water addition on the target reaction were investigated. 3.2.2.1 CO2 Pressure and Reaction Temperature Almost all the reports on cyclic carbonate synthesis suggest that CO2 pressure and reaction temperature are important parameters affecting the coupling of CO2 onto epoxides [8–19, 22–29]. However, the CO2 pressure and reaction temperature for optimal performance is dependent on the catalyst employed. As shown in Table 2, the PC yield with initial CO2 pressure of 2.0 MPa is higher than that with initial CO2 pressure of 2.5 MPa (Table 2, entries 9 and 11; 1 h, 120 °C). It can be seen from Fig. 5 that the catalytic activity of SBA-15-HEPIMBr at 120 °C is the highest at a CO2 pressure of 2.0 MPa. Above CO2 pressure of 3.5 MPa, there is significant decline in PO conversion and PC selectivity. Such an effect of CO2 pressure on catalytic activity has been observed in other catalytic systems [14, 15]. For example, when immobilized ionic liquid/ZnCl2
100
100 80
98
Yield Sel.
60
96 40 94
20 0
Selectivity, %
Entry Catalyst
b
Yield, %
Table 2 Catalytic performance of various catalystsa
92 1
2
3
4
5
Initial CO 2 pressure, MPa Fig. 5 Effect of initial CO2 pressure on catalytic performance over SBA-15-HEPIMBr. Reaction conditions: PO, 28.6 mmol; catalyst, 0.2 g; temperature, 120 °C; time 1 h
3-(2-Hydroxyl-Ethyl)-1-Propylimidazolium Bromide Immobilized on SBA-15
3.2.2.2 Effect of Water In a number of occasions, reproducibility of performance of catalysts of similar nature is poor for the coupling of CO2 with epoxides [59]. We conducted a systematic investigation on the effect of water on the coupling reaction over ZnBr2–Ph4PI, and found that a small amount of water could have a negative effect on the reaction [49]. On the other hand, higher activity was reported over ionic liquid homogeneous catalysts (e.g. quaternary ammonium salts, imidazolium, and phosphonium) in the presence of a certain amount of water (as solvent) [19, 20]. Such discrepancy motivated us to investigate the effect of water presence on the target reaction over the SBA-15-HEPIMBr catalyst. We used dehydrated PO and CO2 as starting materials and added water deliberately to the autoclave by a microscale syringe under the protection of a flow of CO2. As
depicted in Fig. 7, the addition of a trace amount of water has positive effect on PO conversion. When the ratio of PO/H2O is 0.01, the PC yield reaches maximum of 91.4%. Further addition of water results in decline of PC yield: at a PO/H2O ratio of 0.2, PC yield becomes 69.3%. The addition of water has a negative effect on the selective to PC. With PO/H2O ratio increases from 0.01 to 0.2, PC selectivity declines from 99.1 to 95.5%. The present result is consistent with that of Sun et al. [20]. It is proposed that the OH groups of water play a role similar to that of HIL in the coupling reaction. 3.2.3 Catalyst Recycling Experiments were carried out to test the reusability of the catalyst and reproducibility of catalytic performance. In each cycle, SBA-15-HEPIMBr was recovered by filtration and then rinsed with PO. After drying, the catalyst was reused for the next run. We found that the PC selectivity is always above 99.0%. The yields of PC in the first seven consecutive runs are shown in Fig. 8. One can see that although there is a distinct decline in PC yield in the first three cycles, the catalytic activity of SBA-15-HEPIMBr remains almost unchanged after the 4th run. Hence despite the initial decline, the catalyst can be considered as reusable. The initial decline of PC yield could be due to HEPIMBr leaching. Poor reusability has been reported before over catalysts of similar kind [15, 47, 60]. 3.3 Coupling of CO2 with other Epoxides In order to demonstrate the general applicability of SBA15-HEPIMBr for CO2 chemical fixation, we examined the reactions of other epoxides with CO2. The results are summarized in Table 3. One can see that SBA-15-HEPIMBr is active, and almost 100% yield of carbonates is
100
95
Yield Sel.
90
100
100
60 96 40 94
20 0
92 60
80
100
120
140
160
180
200
Yield, %
98
Selectivity, %
Yield, %
85
Yield Sel.
80
99
98 80 97 75
Selectivity, %
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 [15]. 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 due to CO2-epoxide complexing [5, 15]. Thus, rather than promoting the interaction between PO and catalyst, the high CO2 pressure enhances this kind of CO2–PO interaction, consequently leading to the low catalytic activity. As shown in Fig. 6, PC yield increases form ca. 6–98% when reaction temperature is raised from 80 to 140 °C, and remains at this level within the 140–180 °C range. The selectivity to PC is above 99.5% within the 80–180 °C range. It is known that the coupling reaction is exothermic. Generally, when the reaction temperature is higher than 140 °C, PC selectivity slightly declines, plausibly a result of PC polymerization [26]. We gather that the optimal conditions for the target reaction are initial CO2 pressure of 2.0 MPa and reaction temperature of 140 °C.
301
96
70
95
65 0
0.05
0.1
0.15
0.2
Temperature, oC
H2O/PO, molar ratio
Fig. 6 Effect of reaction temperature on catalytic performance over HEPIMBr-SBA-15. Reaction conditions: PO, 28.6 mmol; catalyst, 0.2 g; initial CO2 pressure, 2.0 MPa; time 1 h
Fig. 7 Effect of water addition on catalytic performance over SBA15-HEPIMBr. Reaction conditions: PO, 28.6 mmol; catalyst, 0.2 g; initial CO2 pressure, 2.5 MPa; temperature, 120 °C; time 1 h
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Table 3 Coupling of CO2 with epoxides catalyzed by SBA-15HEPIMBra
Yield, %
80
Entry
Substrate
Catalytic resultsb
Product
X(%)
Y(%)
100
99.5
60
1
O
O
40
O
O
20
0 1
2
3
4
5
6
7
Number of recycle
2
O
O
Fig. 8 Catalytic activity of recycled SBA-15-HEPIMBr. Reaction conditions: PO, 28.6 mmol; catalyst, 0.2 g; initial CO2 pressure, 2.0 MPa; temperature, 120 °C; time 2 h
achieved under mild conditions when ethylene oxide, epichlorohydrin and styrene oxide are selected as substrates (Table 3, entries 1, 3, 4). When cyclohexene oxide is adopted as substrate, much longer time is needed for complete conversion (Table 3, entry 5), plausibly because of the hindrance originated from the two rings of cyclohexene oxide. Furthermore, for the adopted epoxide substrates, the selectivity to the corresponding carbonates is always C99.5%. Compared to most of the solid catalysts reported in literature [16–18, 21–28], the SBA-15-HEPIMBr catalyst is much more active and selective for the coupling of CO2 onto a wide range of epoxides.
O
3
97.4
99.6
99.2
99.8
99.6
31.3
31.2
O
O
O
Cl
97.6
O
O
Cl
4
O
O O
O
3.4 Reaction Mechanism There are several suggestions for the mechanism of CO2 and epoxides coupling. Generally, zinc halide was added to accelerate the catalytic activity of ionic liquids. Kim et al. found that zinc halide can coordinate with ionic liquid to form the ‘‘real’’ catalyst [23–25]. Xia and coworkers also hold the same view [12, 15]. As to the composite catalyst of zinc halide and quaternary ammonium (phosphonium) salts, many researchers suggested the dual requirement of Lewis acid and Lewis base sites for the activation of epoxide and CO2, respectively [11, 16, 25, 61]. The mechanism differs from that reported by others in which there is the initial coordination of epoxide with a Lewis acid site and then the opening of the epoxide ring by a nucleophilic reagent (e.g., halide ion) [10, 13, 18]. However, Zhang et al. regarded that at the initial stage, different parts of epoxide are attacked by the OH group and the Lewis basic site (bromine anion) of ionic liquid coordinately. In this study, we compared the catalytic performance of SBA-15-HEPIMBr, SBA-15-HEPIMBr ? KBr, and KBr to demonstrate the function of bromine anion (Table 2, entries 9, 12, 13). Obviously, the addition of bromide
123
5c
O
O O
O
a
Reaction conditions: epoxide 28.6 mmol, catalyst 0.2 g, initial CO2 pressure 2.0 MPa, 2 h, 120 °C b
X: conversion of epoxide; Y: carbonate yield
c
Reaction time: 6 h
markedly improves the catalytic performance of SBA-15HEPIMBr. Based on our results and those of previous reports, we propose a mechanism (Shown in Scheme 3) for the target reaction over SBA-15-HEPIMBr, a mechanism similar to that for the homogeneous HEPIMBr catalyst.
3-(2-Hydroxyl-Ethyl)-1-Propylimidazolium Bromide Immobilized on SBA-15
R
O O
O
O H
A
Br A
A Br R
H O
R
I H O
R
Br
H
O
II
CO 2
O O
O
OA
O
C O
R
Br III
Scheme 3 Mechanistic steps for CO2 coupling with epoxide over SBA-15-HEPIMBr
First, the coordination of the H atom of a hydroxyl group of HEPIMBr with the O atom of epoxide through hydrogen bonding results in the formation of intermediate (I). Due to the polarization of the C–O bonds, the halide anion makes a nucleophilic attack on the sterically less hindered b-carbon atom of the epoxide. At the same time, there is ring opening of epoxide and generation of an oxy anion (species II). The insertion of CO2 into the haloalkoxy species would result in the formation of a linear halocarbonate (III) that transforms into a cyclic carbonate through intramolecular substitution of the halide.
4 Conclusion In conclusion, immobilized 3-(2-hydroxyl-ethyl)-1-propylimidazolium bromide was reported for the first time as a recyclable and high-efficiency catalyst for the synthesis of cyclic carbonate from the coupling of CO2 with epoxides under mild reaction conditions in a solvent-free environment and in the absence of a co-catalyst. The excellent catalytic performance is ascribed to the existence of OH groups. It is noted that the immobilized HEPIMBr shows higher catalytic performance than the pure HIL (HEMIMBr), possibly a result of the synergic effect between support and HEPIMBr. Despite the reusability of the catalyst is not satisfactory, the catalyst exhibits good thermal stability, and can be easily recovered by simple filtration for recycling. Therefore, the catalyst is attractive for the synthesis of cyclic carbonates from an industrial viewpoint. The fabrication and application of SBA-15 and other solid materials for the immobilization of hydroxyl ionic liquids for better reusability of catalyst is being conducted in our laboratory. Acknowledgments This work was supported by National 863 Program of China (2009AA05Z319), National Natural Science Foundation
303
of China (Grant Nos 20873038 and 20507005), and the Outstanding Young Research Award of National Natural Science Foundation of China (Grant No E50725825). CTA thanks the Hunan University for an adjunct professorship and the Hong Kong Baptist University for financial support (FRG/08-09/II-09). The spectral database for organic compounds SDBS (http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced Industrial Science and Technology, date of access)) is gratefully acknowledged.
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