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Direct synthesis of propylene carbonate from propylene and carbon dioxide catalyzed by quaternary ammonium heteropolyphosphatotungstate–TBAB system
Journal of Energy Chemistry 24(2015)353–358
Direct synthesis of propylene carbonate from propylene and carbon dioxide catalyzed by quaternary ammonium heteropolyphosphatotungstate–TBAB system Gongda Zhaoa,b,c ,
Yi Zhanga , Hengyun Zhanga,
Jun Lia ,
Shuang Gaoa∗
a. Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian National Laboratory for Clean Energy, Dalian 116023, Liaoning, China; b. State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116012, Liaoning, China; c. University of Chinese Academy of Sciences, Beijing 100049, China [ Manuscript received December 7, 2014; revised January 14, 2015 ]
Abstract In this paper, we have developed a highly efficient method for the direct preparation of propylene carbonate from propylene and carbon dioxide (CO2 ) using quaternary ammonium heteropolyphosphatotungstate–quaternary ammonium halide catalytic system with anhydrous hydrogen peroxide as an oxidant through one-pot two-step process. The effects of the amount of tetrabutylammonium bromide (TBAB), the concentration of hydrogen peroxide and other reaction conditions were investigated. The catalyst system gave an optimum propylene oxide yield (91%) at 75 ◦ C in oxidation step and the highest propylene carbonate yield (99%) at 140 ◦ C and 3.0 MPa in cycloaddition step. Based on the results, a reaction mechanism has been proposed. Key words quaternary ammonium heteropolyphosphatotungstate; propylene carbonate; carbon dioxide; tetrabutylammonium bromide
1. Introduction Carbon dioxide (CO2 ) is not only one of the main greenhouse gases but also an easily available, nontoxic, nonflammable and naturally abundant carbon resource [1−5]. From the viewpoints of environment protection and resource needs, chemical transformation of CO2 into useful organic compounds has attracted much attention in recent years. One of the most promising route of CO2 chemical fixation is the cycloaddition reaction of CO2 with epoxides to product cyclic carbonates which are widely used for various purposes, such as monomers, aprotic polar solvents, organic synthetic intermediates, fuel additives and green reagents [6−10]. Although the synthesis route of cyclic carbonates from CO2 and epoxides is quite atom-efficient and has been used commercially [11−15], such a cycloaddition reaction usually requires the initial synthesis of the epoxide; an additional step which requires chemical separation and involves expensive or toxic reagents sometimes. Another approach of synthesis of cyclic carbonates starting from olefins and CO2 directly, a socalled one-pot “oxidative carboxylation” of olefin, seems to be simpler and even cheaper (Scheme 1). Although the above straightforward synthesis from olefins has been known since ∗
1962 [16], only a few works have been reported up to now [17−33].
Scheme 1. Direct synthesis of cyclic carbonates from alkenes and CO2
Arai and coworkers carried out an efficiently direct synthesis of styrene carbonate (SC) from styrene and CO2 using the quaternary ammonium halides or imidazolium salts as catalysts and tert-butyl hydroperoxide (TBHP) as oxidant [21,22]. Tetrabutylammonium bromide (TBAB) gave the best yield of SC at 39%. TBAB and 1,8-diazabicylco [5.4.0] undec-7-ene (DBU) combine with H2 O2 as oxidant in water were successfully used by Li et al. for the one-pot synthesis of carbonates from olefins and CO2 [23]. When the reaction was carried out using excess DBU with a CO2 pressure of 250−300 psi at 60 ◦ C, the water soluble sodium salt of 4-styrene sulfonic acid was converted into the corresponding
Corresponding author. Tel: +86-411-84379248; Fax: +86-411-84379248; E-mail: [email protected]
Copyright©2015, Science Press and Dalian Institute of Chemical Physics. All rights reserved. doi: 10.1016/S2095-4956(15)60322-9
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cyclic carbonate with 89% yield. Based on the above reaction mechanism, He et al. showed a binary catalyst comprising sodium phosphotungstate and TBAB for the direct synthesis of SC using styrene as substrate and 30% H2 O2 as an oxidant [24]. 65% yield of SC and 28% of phenacyl benzoate was obtained after a reaction time of 12 h under 50 ◦ C. A series of Au catalyst system with TBHP as an oxidant were explored by Arai and Sun as catalysts for the direct synthesis of cyclic carbonate [25−29], such as the Au/SiO2 -ZnBr2 /TBAB catalyst system which gave a 45% yield for SC using cumene hydroperoxide (CHP) as an oxidant. Qiao and Yokoyama used an ionic liquid ([BMIm]BF4 ) with the urea hydrogen peroxide as oxidant and methyltrioxorhenium (MTO) as oxidation catalyst for the direct synthesis of cyclic carbonates from olefins and CO2 [30]. The catalyst system gave SC in 83% yield at 110 ◦ C and 30 atmospheres pressure. Liu and co-workers proposed a titanosilicate-quaternary ammonium halide catalyst system for the direct synthesis of propylene carbonate (PC) using H2 O2 as an oxidant with a yield of 48% [31]. Bai and Jing reported a one-pot, highly selectively synthesis of SC using Ru(TPP)O2 /TBAI as catalyst system and O2 as an oxidant [32]. At 30 ◦ C and 1.1 MPa CO2 pressure in ethanol, this method can provide a high yield (89%) of SC in the presence of 4 mol% of catalyst. In 2011, Chen and co-workers showed the use of MoO2 (acac)2 and quaternary ammonium salt as catalyst for the synthesis of cyclic carbonates directly from olefins and CO2 [33]. Through a one-pot multistep process, 83% yield of cyclic carbonate was observed using TBHP as oxidant. All of the above catalysts have their advantages, and also have shortcomings which limited their uses in industry. Therefore, it is still greatly important to explore simple and highly efficient catalyst with an economical oxidant under mild reaction conditions for the direct synthesis of cyclic carbonates from CO2 and olefins. Herein we report a simple and efficient catalyst system consisting of quaternary ammonium heteropolyphosphatotungstate and TBAB for the direct synthesis of PC from propylene and CO2 using anhydrous H2 O2 as an oxidant. 2. Experimental 2.1. Materials Propylene oxide (98%) was purchased from Aldrich without further purification. Propylene (98%), toluene (99%), tributyl phosphate (99%) and 50% H2 O2 aqueous solution were purchased locally and used directly. Quaternary ammonium heteropolyphosphatotungstate was synthesized according to the published procedure of Catalyst C (Cat.C)’s synthesis [34]. Anhydrous H2 O2 in solution was synthesized from refluxing 50% H2 O2 aqueous solution and organic solvent in vacuum condition and H2 O2 concentrations were determined by iodometric titration.
2.2. Experimental procedures for the epoxidation and cycloaddition The catalytic reaction was conducted in a 200 mL highpressure stainless steel reactor. The appropriate amounts of quaternary ammonium heteropolyphatotungstate, anhydrous H2 O2 and propylene were charged into reactor and then the reactor was heated to the desired temperature using an oil bath. After the epoxidation reaction was finished, the reactor was cooled to about 0 ◦ C and depressed to an atmospheric pressure. The solid catalyst was removed and the liquid phase was analyzed on a gas chromatograph (Agilent 6820) packed with a packed column (PEG-20M) using a flame ionization detector. The reactor was charged with the appropriate amounts of TBAB and then was pressurized with CO2 to the desired pressure. The reactor was heated to the desired temperature and was cooled to room temperature after the reaction was over. The reactor was cooled to about 0 ◦ C and the remaining CO2 in the reactor was released slowly. The remaining propylene oxide was analyzed by a gas chromatograph mentioned above. The reaction products were analyzed by gas chromatograph (Agilent 7890) packed with a capillary column (PEG-20M) using a flame ionization detector. 2.3. Recycle of catalysts In the epoxidation reaction, the Cat.C will precipitate from the reaction system while H2 O2 was consumed. The solid catalyst can be separated easily and used in the second recycle with almost no loss of catalytic activity. By distilling all of the organic solvents, TBAB can be recycled while the cycloaddition reaction was over. 3. Results and discussion 3.1. Epoxidation of propylene and direct synthesis of PC through one-pot one-step progress The epoxidation of various olefins catalyzed by the complex composed of tungstate and phosphate with dilute H2 O2 solution as oxidant was reported by Venturello et al. in 1983 [35,36]. Ishii et al. carried out the system consisting of H3 PW12 O40 and cetylpyridinium chloride which can catalyze the epoxidation of different olefins with commercially available H2 O2 solution as oxidant in 1988 [37]. Xi et al. have proposed a series of quaternary ammonium heteropolyphosphatotungstate with a new concept of “reaction controlled phase transfer catalyst” since 2001 [34,38,39]. These reaction controlled phase transfer quaternary ammonium heteropolyphosphatotungstate catalysts solved the recycling problem of homogeneous catalyst and took on a prospect of industrial application for epoxidation of propylene using anhydrous H2 O2 as oxidant especially [40]. Therefore a kind of reaction-controlled phase-transfer quaternary ammonium heteropolyphosphatotungstate (Cat.C [34])–TBAB sys-
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tem was carried out for the direct synthesis of PC from propylene and CO2 . In order to obtain the information on the roles of catalyst components, the epoxidation of propylene and the direct synthesis of PC through one-pot one-step progress were inves-
tigated firstly. The results of Cat.C to catalyze the epoxidation of propylene with anhydrous H2 O2 in the solution of toluene (TOL) and tributyl phosphate (TBP) as oxidant were summarized in Table 1.
Table 1. Epoxidation of propylene and direct conversing of propylene into propylene carbonate using different cycloaddtion catalystsa Entry 1 2c 3c 4d 5 6 7 8
Solvent TOL/TBP TOL/TBP ethyl acetate TOL/PC TOL/TBP TOL/TBP TOL/TBP TOL/TBP
Co-catalyst − − − − − Zn(NO3 )2 TBAB ZnBr2
pCO2 (MPa, r.t.) − − − 3 3 3 3
H2 O2 Conversionb (%) 97 93 92 99 93 95 64 95
PO yieldb (%) 89 91 87 83 86 57 32 49
PC Yieldb (%) − − − − − − − <1
a Conditions: 0.2 mmol Cat.C, H O : Cat.C (mole ratio)=250 : 1, propylene : H O (mole ratio)=3−3.5 : 1, concentration of anhydrous H O in the 2 2 2 2 2 2 solution of TOL/TBP being 1.6 mmol/g, TOL : TBP (volume ratio)=2 : 5, 75 ◦ C, 3.5 h, unless otherwise noted; b Determined by GC; c Concentration of anhydrous H2 O2 in the solution being 0.8 mmol/g; d 0.2 mmol Cat.C, H2 O2 : Cat.C (mole ratio)=125 : 1, concentration of anhydrous H2 O2 in the solution being 0.8 mmol/g
Based on the results mentioned above, H2 O2 was almost converted (H2 O2 conversion of 97%) by Cat.C after 3.5 h at 70 ◦ C, affording an 89% yield of PO (Table 1, Entry 1) and Cat.C precipitated from the reaction system well when the epoxidation reaction was over. In contrast, pumping CO2 in this system gave a little lower yield of PO and Cat.C was inactive for the cycloaddition reaction under the same reaction conditions (Table 1, Entry 5). To understand the role of the solvent in the epoxidation, other solvents were also tested substituting for the TOL/TBP for the epoxidation of propylene. It could be seen from Table 1, that moderate PO conversion and moderate PC yield were achieved with the ethyl acetate or PC as solvent (Table 1, Entries 3−4). It is well known that a catalyst such as TBAB or zinc salts is often used in cyclic carbonate synthesis from epoxides and CO2 . While adding a catalyst amount of TBAB or zinc salts to the epoxidation system strongly prevented the epoxidation reaction proceeding and only trace PC was obtained (Table 1, Entries 6−8). This is maybe due to the rapid decomposition
of H2 O2 catalyzed by TBAB or zinc salts during the epoxidation reaction. Based on the results mentioned above, it is suggested strongly that the cycloaddiotion catalyst such as TBAB or zinc salts should not be introduced until the epoxidation reaction was over. 3.2. Cycloaddition of PO and CO2 catalyzed by TBAB in the solution of TOL/TBP The synthesis of PC from PO and CO2 catalyzed by TBAB in the solution of toluene-TBP under various reaction conditions was investigate, and the representative results were summarized in Table 2. In order to imitate the situation after the epoxidation reaction, the effect of the amount of water were investigated in the cycloaddition reaction. Although TBAB combined with ZnBr2 was an efficient cycloaddition catalyst, adding water to the TBAB/ZnBr2 system not only prevented the cycloaddition reaction proceeding markedly, but also caused the zinc salt
Table 2. Cycloaddition of CO2 to propylene oxide under various reaction conditionsa Entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
TBAB (mmol) 0.5 0.5 0.5 0.5 1.0 1.0 1.0 0.5 0.5 0.5 0.5 2.0 2.0 2.0 2.0
Co-catalystb ZnBr2 ZnBr2 ZnBr2 ZnBr2 − − − − − − − − − − −
H2 O (%) − 1.6 1.6 2.8 − 1.6 2.8 2.8 2.8 2.8 2.8 2.8 2.8 2.8 2.8
Temperature (◦ C) 70 70 70 70 140 140 140 70 100 120 140 130 140 140 140
Time (h) 4 4 8 4 4 4 4 4 4 4 4 4 1 3 4
PC yieldc (%) 85 46 72 23 42 50 53 7 15 29 56 68 37 78 85
a Conditions : TOL : TBP (volume ratio)=2 : 5, 60 mmol PO, pCO2 =3 MPa (r.t.), unless otherwise noted; b TBAB : ZnBr2 (mole ratio)=1.5 : 1; c Determined by GC
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precipitation after the cycloaddition reaction (Table 2, Entries 1−4). On the contrary, the activity of TBAB along was improved effectively with the presence of water (Table 2, Entries 5−7), though the yield of PC was very low compared to the TBAB/ZnBr2 system under the same cycloaddition reaction conditions (Table 2, Entry 8). As an important cycloaddition process of epoxide and CO2 catalyzed by TBAB [11−15], the yield of cyclic carbonate was sensitive to the reaction time and temperature. With the increasing temperature in the cycloaddition reaction from 70 to 140 ◦ C, the yield of PC increased from 7% to 56% smoothly (Table 2, Entries 8−11). With the shortening of the cycloaddition reaction time from 4 to 1 h, the yield of PC gradually decreased from 85% to 37% (Table 2, Entries 13−15). The highest yield of PC could be achieved by increasing the content of TBAB in the cycloaddition reaction catalyzed by TBAB along (Table 2, Entry 15). 3.3. Direct synthesis of PC through one-pot two-step progress catalyzed by Cat.C–TBAB system with anhydrous H 2 O2 as an oxidant Then based on the results mentioned above, the direct synthesis of PC through one-pot two-step progress from propylene and CO2 using anhydrous H2 O2 as an oxidant was carried out, and the representative results were illustrated in
Table 3. When a one-pot two-step synthesis of propylene carbonate without TBAB was tested, the catalyst system showed little conversing of PO (conversing only 5%) and no product of PC (Table 3, Entry 1). Further experiments showed that the concentration of H2 O2 could remarkablely affect the yield of PC. The higher yield of PC was obtained at a lower concentration of H2 O2 and this may be due to reduction of the side reaction between water and PO (Table 2, Entry 3). The amount of TBAB had such a marked influence on the catalytic reaction, that further increasing the amount of TBAB caused an increase in the PC yield and the completely consumption of PO (Table 3, Entries 3 and 5). With the depressing of the CO2 pressure in cycloaddition process from 3 to 1 MPa, the conversion of PO decreased from 99% to 89% and the yield of PC decreased from 99% to 85% gradually (Table 3, Entry 5−7). Other solvents instead of TOL/TBP were also examined in the one-pot two-step synthesis of PC. When ethyl acetate was used, the PC yield decreased slightly from 91% to 85% under the same conditions though the PO was converted completely (Table 3, Entry 8). Using product as solvent is a more economically viable process because of the easy product separation. When TOL-PC substituted the TOL-TBP as solvent, the one-pot two-step progress gave a disappointing result by 84% PO conversion (Table 3, Entry 9).
Table 3. The one-pot two-step synthesis of propylene carbonate catalyzed by the quaternary ammonium heteropolyphosphatotungstate/TBAB systema Entry 1 2 3 4d 5d 6d 7d 8d 9e
Concentration of H2 O2 (mmol/g) 1.6 1.6 1.6 0.8 0.8 0.8 0.8 0.8 0.8
Solution TOL/TBP TOL/TBP TOL/TBP TOL/TBP TOL/TBP TOL/TBP TOL/TBP ethyl acetate TOL/PC
p(CO2 ) (MPa, r.t.) 3 3 3 3 3 2 1 3 3
TBAB (mmol) − 2.0 2.7 1.3 1.6 1.6 1.6 1.3 1.3
PO conversionb (%) 5 83 > 99 93 > 99 96 89 > 99 84
PC yieldc (%) − 75 78 91 > 99 92 85 85 −
a
Epoxidation conditions: 0.19 mmol Cat.C, H2 O2 : Cat.C (mole ratio)=250 : 1, propylene : H2 O2 (mole ratio)=3−3.5 : 1, 75 ◦ C, 3.5 h; Cycloaddition conditions: 140 ◦ C, 4 h, unless otherwise noted; b Determined by GC; c Determined by GC, yield based on PO; d 0.1 mmol Cat.C; e 0.2 mmol Cat.C, H2 O2 : Cat.C (mole ratio)=125 : 1
3.4. Direct synthesis of PC through one-pot two-step progress catalyzed by Cat.C–various cycloaddition catalysts with anhydrous H 2 O2 as an oxidant Based on the above results, the catalyst system of Cat.C– TBAB showed high yield of PC and the complete conversing of PO for the direct one-pot two-step synthesis of PC from propylene and CO2 . To test the utility of the the catalyst system of Cat.C/anhydrous H2 O2 , various cycloaddtion catalysts substituting for TBAB, including other quaternary ammoniums, organic bases, metal oxide, were explored for the direct one-pot two-step synthesis of propylene carbonate. The representative results were listed in Table 4. It was worth mentioned that TBAI showed more effective catalytic performance than the TBAB under the same conditions (Table 4, Entry 2). This may be due to the more strong
nucleophilicity of the iodine anion according to the reports from Aria and Sun [25,26,41]. Based on the same principle, the more bulky [(n-C4 H9 )4 N]+ cation which making the halogen anion more nucleophilic afforded higher activity than the [(n-C16H33 )(CH3 )3 N]+ cation under the same reaction conditions (Table 4, Entry 6). The organic base and metal oxide, which were reported to be used in the cycloaddition reaction of epoxides and CO2 as active catalysts [11−15], had poor performance in both the yield of PC and the conversion of PO for the direct one-pot two-step synthesis of PC. 3.5. Possible reaction mechanism Apparently, the mechanism of the one-pot two-step synthesis of propylene carbonate catalyzed by quaternary ammonium heteropolyphosphatotungstate–TBAB consists of the
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epoxidation reaction mechanism among heteropolyphosphatotungstate, H2 O2 and propylene, and the cycloaddition reaction mechanism between TBAB and PO. Several groups have reported the epoxidation reaction mechanism of quaternary ammonium heteropolyphosphatotungstate [34−40,42]. The cycloaddition reaction mechanism of TBAB, including the situation with the presence of water in the cycloaddition reaction, has been studied by a few groups [43]. Based on the previous reports, a plausible catalytic cycle for the one-pot two-step progress is depicted in Scheme 2. Table 4. Influence of various cycloaddition catalysts on the one-pot two-step synthesis of PCa Entry 1 2 3 4 5 6 7 8 9 10 11 12
Cycloaddition catalyst TBAB TBAI DMAP pyridine DBU (n-C16 H33 )(CH3 )3 NBr MgO 4,4’-dipyridine ZnBr2 KBr triethylamine TEA
PO conversionb (%) 93 > 99 7 3 7 72 1 4 22 2 3 2
PC yieldc (%) 91 > 99 2 <1 <1 47 <1 <1 7 <1 <1 <1
a
Epoxidation conditions, 0.1 mmol Cat.C, H2 O2 : Cat.C (mole ratio)=250 : 1, propylene : H2 O2 (mole ratio)=3−3.5 : 1, 75 ◦ C, 3.5 h, p(CO2 ) =3 MPa (r.t.); Cycloaddition conditions, 140 ◦ C, 4 h, 1.3 mmol cycloaddition catalyst, unless otherwise noted; b Determined by GC; c Determined by GC, yield based on PO
generated from ring-opening, which leads to the formation of the propylene carbonate and regeneration of the TBAB. 4. Conclusions In summary, the system of quaternary ammonium heteropolyphosphatotungstate–TBAB is a highly efficient catalyst for the direct synthesis of PC from CO2 and propylene. The quaternary ammonium heteropolyphosphatotungstate is easy to prepare, commercially available, reusable and highly active for the epoxidation step. As a green oxidant, H2 O2 will be consumed after the epoxidation reaction with the only byproduct of water which can promote the cycloaddition reaction to a certain extent. Thus the one-pot two-steps synthesis of PC from propylene and CO2 have these advantages and would be of great industry potential for the utilization of CO2 from an economic point of view. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
Scheme 2. Proposed mechanism of the one-pot two-step synthesis of propylene carbonate catalyzed by Cat.C–TBAB system with anhydrous H2 O2 as an oxidant
As one of the epoxidation product from propylene and H2 O2 , water will promote the nucleophilic attack of TBAB to a small extent on the less sterically hinderer carbon atom of the second epoxidation product of propylene oxide, which leads to the opening of epoxide ring. Then the CO2 will be attacked to give a CO2 -adduct by the oxygen anion species
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[28] Wang Y L, Sun J H, Xiang D, Wang L, Sun J M, Xiao F S. Catal Lett, 2009, 129: 437 [29] Xiang D, Liu X F, Sun J S, Xiao F S, Sun J M. Catal Today, 2009, 148: 383 [30] Ono F, Qiao K, Tomida D, Yokoyama C. Appl Catal A, 2007, 333: 107 [31] Zhang J, Liu Y M, Li N N, Wu H H, Li X H, Xie W, Zhao Z L, Wu P, He M Y. Chin J Catal (Cuihua Xuebao), 2008, 29: 589 [32] Bai D S, Jing H W. Green Chem, 2010, 12: 39 [33] Chen F W, Dong T, Xu T G, Li X F, Hu C W. Green Chem, 2011, 13: 2518 [34] Li J, Gao S, Li M, Zhang R H, Xi Z W. J Mol Catal A, 2004, 218: 247
[35] Venturello C, Alneri E, Ricci M. J Org Chem, 1983, 48: 3831 [36] Venturello C, D’Aloisio R. J Org Chem, 1988, 53: 1553 [37] Ishii Y, Yamawaki K, Ura T, Yamada H, Yoshida T, Ogawa M. J Org Chem, 1988, 53: 3587 [38] Xi Z W, Zhou N, Sun Y, Li K L. Science, 2001, 292: 1139 [39] Zhou N, Xi Z W, Cao G Y, Gao S. Appl Catal A, 2003, 250: 239 [40] Oyama S T ed. Mechanisms in Homogeneous and Heterogeneous Epoxidation Catalysis. Elsevier, 2008 [41] Sun J M, Fujita S I, Zhao F Y, Arai M. Appl Catal A, 2005, 287: 221 [42] Noyori R, Aoki M, Sato K. Chem Common, 2003, 16: 1977 [43] Sun J, Ren J Y, Zhang S J, Cheng W G. Tetrahedron Lett, 2009, 50: 423
Direct synthesis of propylene carbonate from propylene and carbon dioxide catalyzed by quaternary ammonium heteropolyphosphatotungstate–TBAB system Gongda Zhaoa,b,c ,
Yi Zhanga , Hengyun Zhanga,
Jun Lia ,
Shuang Gaoa∗
a. Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian National Laboratory for Clean Energy, Dalian 116023, Liaoning, China; b. State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116012, Liaoning, China; c. University of Chinese Academy of Sciences, Beijing 100049, China [ Manuscript received December 7, 2014; revised January 14, 2015 ]
Abstract In this paper, we have developed a highly efficient method for the direct preparation of propylene carbonate from propylene and carbon dioxide (CO2 ) using quaternary ammonium heteropolyphosphatotungstate–quaternary ammonium halide catalytic system with anhydrous hydrogen peroxide as an oxidant through one-pot two-step process. The effects of the amount of tetrabutylammonium bromide (TBAB), the concentration of hydrogen peroxide and other reaction conditions were investigated. The catalyst system gave an optimum propylene oxide yield (91%) at 75 ◦ C in oxidation step and the highest propylene carbonate yield (99%) at 140 ◦ C and 3.0 MPa in cycloaddition step. Based on the results, a reaction mechanism has been proposed. Key words quaternary ammonium heteropolyphosphatotungstate; propylene carbonate; carbon dioxide; tetrabutylammonium bromide
1. Introduction Carbon dioxide (CO2 ) is not only one of the main greenhouse gases but also an easily available, nontoxic, nonflammable and naturally abundant carbon resource [1−5]. From the viewpoints of environment protection and resource needs, chemical transformation of CO2 into useful organic compounds has attracted much attention in recent years. One of the most promising route of CO2 chemical fixation is the cycloaddition reaction of CO2 with epoxides to product cyclic carbonates which are widely used for various purposes, such as monomers, aprotic polar solvents, organic synthetic intermediates, fuel additives and green reagents [6−10]. Although the synthesis route of cyclic carbonates from CO2 and epoxides is quite atom-efficient and has been used commercially [11−15], such a cycloaddition reaction usually requires the initial synthesis of the epoxide; an additional step which requires chemical separation and involves expensive or toxic reagents sometimes. Another approach of synthesis of cyclic carbonates starting from olefins and CO2 directly, a socalled one-pot “oxidative carboxylation” of olefin, seems to be simpler and even cheaper (Scheme 1). Although the above straightforward synthesis from olefins has been known since ∗
1962 [16], only a few works have been reported up to now [17−33].
Scheme 1. Direct synthesis of cyclic carbonates from alkenes and CO2
Arai and coworkers carried out an efficiently direct synthesis of styrene carbonate (SC) from styrene and CO2 using the quaternary ammonium halides or imidazolium salts as catalysts and tert-butyl hydroperoxide (TBHP) as oxidant [21,22]. Tetrabutylammonium bromide (TBAB) gave the best yield of SC at 39%. TBAB and 1,8-diazabicylco [5.4.0] undec-7-ene (DBU) combine with H2 O2 as oxidant in water were successfully used by Li et al. for the one-pot synthesis of carbonates from olefins and CO2 [23]. When the reaction was carried out using excess DBU with a CO2 pressure of 250−300 psi at 60 ◦ C, the water soluble sodium salt of 4-styrene sulfonic acid was converted into the corresponding
Corresponding author. Tel: +86-411-84379248; Fax: +86-411-84379248; E-mail: [email protected]
Copyright©2015, Science Press and Dalian Institute of Chemical Physics. All rights reserved. doi: 10.1016/S2095-4956(15)60322-9
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cyclic carbonate with 89% yield. Based on the above reaction mechanism, He et al. showed a binary catalyst comprising sodium phosphotungstate and TBAB for the direct synthesis of SC using styrene as substrate and 30% H2 O2 as an oxidant [24]. 65% yield of SC and 28% of phenacyl benzoate was obtained after a reaction time of 12 h under 50 ◦ C. A series of Au catalyst system with TBHP as an oxidant were explored by Arai and Sun as catalysts for the direct synthesis of cyclic carbonate [25−29], such as the Au/SiO2 -ZnBr2 /TBAB catalyst system which gave a 45% yield for SC using cumene hydroperoxide (CHP) as an oxidant. Qiao and Yokoyama used an ionic liquid ([BMIm]BF4 ) with the urea hydrogen peroxide as oxidant and methyltrioxorhenium (MTO) as oxidation catalyst for the direct synthesis of cyclic carbonates from olefins and CO2 [30]. The catalyst system gave SC in 83% yield at 110 ◦ C and 30 atmospheres pressure. Liu and co-workers proposed a titanosilicate-quaternary ammonium halide catalyst system for the direct synthesis of propylene carbonate (PC) using H2 O2 as an oxidant with a yield of 48% [31]. Bai and Jing reported a one-pot, highly selectively synthesis of SC using Ru(TPP)O2 /TBAI as catalyst system and O2 as an oxidant [32]. At 30 ◦ C and 1.1 MPa CO2 pressure in ethanol, this method can provide a high yield (89%) of SC in the presence of 4 mol% of catalyst. In 2011, Chen and co-workers showed the use of MoO2 (acac)2 and quaternary ammonium salt as catalyst for the synthesis of cyclic carbonates directly from olefins and CO2 [33]. Through a one-pot multistep process, 83% yield of cyclic carbonate was observed using TBHP as oxidant. All of the above catalysts have their advantages, and also have shortcomings which limited their uses in industry. Therefore, it is still greatly important to explore simple and highly efficient catalyst with an economical oxidant under mild reaction conditions for the direct synthesis of cyclic carbonates from CO2 and olefins. Herein we report a simple and efficient catalyst system consisting of quaternary ammonium heteropolyphosphatotungstate and TBAB for the direct synthesis of PC from propylene and CO2 using anhydrous H2 O2 as an oxidant. 2. Experimental 2.1. Materials Propylene oxide (98%) was purchased from Aldrich without further purification. Propylene (98%), toluene (99%), tributyl phosphate (99%) and 50% H2 O2 aqueous solution were purchased locally and used directly. Quaternary ammonium heteropolyphosphatotungstate was synthesized according to the published procedure of Catalyst C (Cat.C)’s synthesis [34]. Anhydrous H2 O2 in solution was synthesized from refluxing 50% H2 O2 aqueous solution and organic solvent in vacuum condition and H2 O2 concentrations were determined by iodometric titration.
2.2. Experimental procedures for the epoxidation and cycloaddition The catalytic reaction was conducted in a 200 mL highpressure stainless steel reactor. The appropriate amounts of quaternary ammonium heteropolyphatotungstate, anhydrous H2 O2 and propylene were charged into reactor and then the reactor was heated to the desired temperature using an oil bath. After the epoxidation reaction was finished, the reactor was cooled to about 0 ◦ C and depressed to an atmospheric pressure. The solid catalyst was removed and the liquid phase was analyzed on a gas chromatograph (Agilent 6820) packed with a packed column (PEG-20M) using a flame ionization detector. The reactor was charged with the appropriate amounts of TBAB and then was pressurized with CO2 to the desired pressure. The reactor was heated to the desired temperature and was cooled to room temperature after the reaction was over. The reactor was cooled to about 0 ◦ C and the remaining CO2 in the reactor was released slowly. The remaining propylene oxide was analyzed by a gas chromatograph mentioned above. The reaction products were analyzed by gas chromatograph (Agilent 7890) packed with a capillary column (PEG-20M) using a flame ionization detector. 2.3. Recycle of catalysts In the epoxidation reaction, the Cat.C will precipitate from the reaction system while H2 O2 was consumed. The solid catalyst can be separated easily and used in the second recycle with almost no loss of catalytic activity. By distilling all of the organic solvents, TBAB can be recycled while the cycloaddition reaction was over. 3. Results and discussion 3.1. Epoxidation of propylene and direct synthesis of PC through one-pot one-step progress The epoxidation of various olefins catalyzed by the complex composed of tungstate and phosphate with dilute H2 O2 solution as oxidant was reported by Venturello et al. in 1983 [35,36]. Ishii et al. carried out the system consisting of H3 PW12 O40 and cetylpyridinium chloride which can catalyze the epoxidation of different olefins with commercially available H2 O2 solution as oxidant in 1988 [37]. Xi et al. have proposed a series of quaternary ammonium heteropolyphosphatotungstate with a new concept of “reaction controlled phase transfer catalyst” since 2001 [34,38,39]. These reaction controlled phase transfer quaternary ammonium heteropolyphosphatotungstate catalysts solved the recycling problem of homogeneous catalyst and took on a prospect of industrial application for epoxidation of propylene using anhydrous H2 O2 as oxidant especially [40]. Therefore a kind of reaction-controlled phase-transfer quaternary ammonium heteropolyphosphatotungstate (Cat.C [34])–TBAB sys-
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tem was carried out for the direct synthesis of PC from propylene and CO2 . In order to obtain the information on the roles of catalyst components, the epoxidation of propylene and the direct synthesis of PC through one-pot one-step progress were inves-
tigated firstly. The results of Cat.C to catalyze the epoxidation of propylene with anhydrous H2 O2 in the solution of toluene (TOL) and tributyl phosphate (TBP) as oxidant were summarized in Table 1.
Table 1. Epoxidation of propylene and direct conversing of propylene into propylene carbonate using different cycloaddtion catalystsa Entry 1 2c 3c 4d 5 6 7 8
Solvent TOL/TBP TOL/TBP ethyl acetate TOL/PC TOL/TBP TOL/TBP TOL/TBP TOL/TBP
Co-catalyst − − − − − Zn(NO3 )2 TBAB ZnBr2
pCO2 (MPa, r.t.) − − − 3 3 3 3
H2 O2 Conversionb (%) 97 93 92 99 93 95 64 95
PO yieldb (%) 89 91 87 83 86 57 32 49
PC Yieldb (%) − − − − − − − <1
a Conditions: 0.2 mmol Cat.C, H O : Cat.C (mole ratio)=250 : 1, propylene : H O (mole ratio)=3−3.5 : 1, concentration of anhydrous H O in the 2 2 2 2 2 2 solution of TOL/TBP being 1.6 mmol/g, TOL : TBP (volume ratio)=2 : 5, 75 ◦ C, 3.5 h, unless otherwise noted; b Determined by GC; c Concentration of anhydrous H2 O2 in the solution being 0.8 mmol/g; d 0.2 mmol Cat.C, H2 O2 : Cat.C (mole ratio)=125 : 1, concentration of anhydrous H2 O2 in the solution being 0.8 mmol/g
Based on the results mentioned above, H2 O2 was almost converted (H2 O2 conversion of 97%) by Cat.C after 3.5 h at 70 ◦ C, affording an 89% yield of PO (Table 1, Entry 1) and Cat.C precipitated from the reaction system well when the epoxidation reaction was over. In contrast, pumping CO2 in this system gave a little lower yield of PO and Cat.C was inactive for the cycloaddition reaction under the same reaction conditions (Table 1, Entry 5). To understand the role of the solvent in the epoxidation, other solvents were also tested substituting for the TOL/TBP for the epoxidation of propylene. It could be seen from Table 1, that moderate PO conversion and moderate PC yield were achieved with the ethyl acetate or PC as solvent (Table 1, Entries 3−4). It is well known that a catalyst such as TBAB or zinc salts is often used in cyclic carbonate synthesis from epoxides and CO2 . While adding a catalyst amount of TBAB or zinc salts to the epoxidation system strongly prevented the epoxidation reaction proceeding and only trace PC was obtained (Table 1, Entries 6−8). This is maybe due to the rapid decomposition
of H2 O2 catalyzed by TBAB or zinc salts during the epoxidation reaction. Based on the results mentioned above, it is suggested strongly that the cycloaddiotion catalyst such as TBAB or zinc salts should not be introduced until the epoxidation reaction was over. 3.2. Cycloaddition of PO and CO2 catalyzed by TBAB in the solution of TOL/TBP The synthesis of PC from PO and CO2 catalyzed by TBAB in the solution of toluene-TBP under various reaction conditions was investigate, and the representative results were summarized in Table 2. In order to imitate the situation after the epoxidation reaction, the effect of the amount of water were investigated in the cycloaddition reaction. Although TBAB combined with ZnBr2 was an efficient cycloaddition catalyst, adding water to the TBAB/ZnBr2 system not only prevented the cycloaddition reaction proceeding markedly, but also caused the zinc salt
Table 2. Cycloaddition of CO2 to propylene oxide under various reaction conditionsa Entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
TBAB (mmol) 0.5 0.5 0.5 0.5 1.0 1.0 1.0 0.5 0.5 0.5 0.5 2.0 2.0 2.0 2.0
Co-catalystb ZnBr2 ZnBr2 ZnBr2 ZnBr2 − − − − − − − − − − −
H2 O (%) − 1.6 1.6 2.8 − 1.6 2.8 2.8 2.8 2.8 2.8 2.8 2.8 2.8 2.8
Temperature (◦ C) 70 70 70 70 140 140 140 70 100 120 140 130 140 140 140
Time (h) 4 4 8 4 4 4 4 4 4 4 4 4 1 3 4
PC yieldc (%) 85 46 72 23 42 50 53 7 15 29 56 68 37 78 85
a Conditions : TOL : TBP (volume ratio)=2 : 5, 60 mmol PO, pCO2 =3 MPa (r.t.), unless otherwise noted; b TBAB : ZnBr2 (mole ratio)=1.5 : 1; c Determined by GC
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precipitation after the cycloaddition reaction (Table 2, Entries 1−4). On the contrary, the activity of TBAB along was improved effectively with the presence of water (Table 2, Entries 5−7), though the yield of PC was very low compared to the TBAB/ZnBr2 system under the same cycloaddition reaction conditions (Table 2, Entry 8). As an important cycloaddition process of epoxide and CO2 catalyzed by TBAB [11−15], the yield of cyclic carbonate was sensitive to the reaction time and temperature. With the increasing temperature in the cycloaddition reaction from 70 to 140 ◦ C, the yield of PC increased from 7% to 56% smoothly (Table 2, Entries 8−11). With the shortening of the cycloaddition reaction time from 4 to 1 h, the yield of PC gradually decreased from 85% to 37% (Table 2, Entries 13−15). The highest yield of PC could be achieved by increasing the content of TBAB in the cycloaddition reaction catalyzed by TBAB along (Table 2, Entry 15). 3.3. Direct synthesis of PC through one-pot two-step progress catalyzed by Cat.C–TBAB system with anhydrous H 2 O2 as an oxidant Then based on the results mentioned above, the direct synthesis of PC through one-pot two-step progress from propylene and CO2 using anhydrous H2 O2 as an oxidant was carried out, and the representative results were illustrated in
Table 3. When a one-pot two-step synthesis of propylene carbonate without TBAB was tested, the catalyst system showed little conversing of PO (conversing only 5%) and no product of PC (Table 3, Entry 1). Further experiments showed that the concentration of H2 O2 could remarkablely affect the yield of PC. The higher yield of PC was obtained at a lower concentration of H2 O2 and this may be due to reduction of the side reaction between water and PO (Table 2, Entry 3). The amount of TBAB had such a marked influence on the catalytic reaction, that further increasing the amount of TBAB caused an increase in the PC yield and the completely consumption of PO (Table 3, Entries 3 and 5). With the depressing of the CO2 pressure in cycloaddition process from 3 to 1 MPa, the conversion of PO decreased from 99% to 89% and the yield of PC decreased from 99% to 85% gradually (Table 3, Entry 5−7). Other solvents instead of TOL/TBP were also examined in the one-pot two-step synthesis of PC. When ethyl acetate was used, the PC yield decreased slightly from 91% to 85% under the same conditions though the PO was converted completely (Table 3, Entry 8). Using product as solvent is a more economically viable process because of the easy product separation. When TOL-PC substituted the TOL-TBP as solvent, the one-pot two-step progress gave a disappointing result by 84% PO conversion (Table 3, Entry 9).
Table 3. The one-pot two-step synthesis of propylene carbonate catalyzed by the quaternary ammonium heteropolyphosphatotungstate/TBAB systema Entry 1 2 3 4d 5d 6d 7d 8d 9e
Concentration of H2 O2 (mmol/g) 1.6 1.6 1.6 0.8 0.8 0.8 0.8 0.8 0.8
Solution TOL/TBP TOL/TBP TOL/TBP TOL/TBP TOL/TBP TOL/TBP TOL/TBP ethyl acetate TOL/PC
p(CO2 ) (MPa, r.t.) 3 3 3 3 3 2 1 3 3
TBAB (mmol) − 2.0 2.7 1.3 1.6 1.6 1.6 1.3 1.3
PO conversionb (%) 5 83 > 99 93 > 99 96 89 > 99 84
PC yieldc (%) − 75 78 91 > 99 92 85 85 −
a
Epoxidation conditions: 0.19 mmol Cat.C, H2 O2 : Cat.C (mole ratio)=250 : 1, propylene : H2 O2 (mole ratio)=3−3.5 : 1, 75 ◦ C, 3.5 h; Cycloaddition conditions: 140 ◦ C, 4 h, unless otherwise noted; b Determined by GC; c Determined by GC, yield based on PO; d 0.1 mmol Cat.C; e 0.2 mmol Cat.C, H2 O2 : Cat.C (mole ratio)=125 : 1
3.4. Direct synthesis of PC through one-pot two-step progress catalyzed by Cat.C–various cycloaddition catalysts with anhydrous H 2 O2 as an oxidant Based on the above results, the catalyst system of Cat.C– TBAB showed high yield of PC and the complete conversing of PO for the direct one-pot two-step synthesis of PC from propylene and CO2 . To test the utility of the the catalyst system of Cat.C/anhydrous H2 O2 , various cycloaddtion catalysts substituting for TBAB, including other quaternary ammoniums, organic bases, metal oxide, were explored for the direct one-pot two-step synthesis of propylene carbonate. The representative results were listed in Table 4. It was worth mentioned that TBAI showed more effective catalytic performance than the TBAB under the same conditions (Table 4, Entry 2). This may be due to the more strong
nucleophilicity of the iodine anion according to the reports from Aria and Sun [25,26,41]. Based on the same principle, the more bulky [(n-C4 H9 )4 N]+ cation which making the halogen anion more nucleophilic afforded higher activity than the [(n-C16H33 )(CH3 )3 N]+ cation under the same reaction conditions (Table 4, Entry 6). The organic base and metal oxide, which were reported to be used in the cycloaddition reaction of epoxides and CO2 as active catalysts [11−15], had poor performance in both the yield of PC and the conversion of PO for the direct one-pot two-step synthesis of PC. 3.5. Possible reaction mechanism Apparently, the mechanism of the one-pot two-step synthesis of propylene carbonate catalyzed by quaternary ammonium heteropolyphosphatotungstate–TBAB consists of the
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epoxidation reaction mechanism among heteropolyphosphatotungstate, H2 O2 and propylene, and the cycloaddition reaction mechanism between TBAB and PO. Several groups have reported the epoxidation reaction mechanism of quaternary ammonium heteropolyphosphatotungstate [34−40,42]. The cycloaddition reaction mechanism of TBAB, including the situation with the presence of water in the cycloaddition reaction, has been studied by a few groups [43]. Based on the previous reports, a plausible catalytic cycle for the one-pot two-step progress is depicted in Scheme 2. Table 4. Influence of various cycloaddition catalysts on the one-pot two-step synthesis of PCa Entry 1 2 3 4 5 6 7 8 9 10 11 12
Cycloaddition catalyst TBAB TBAI DMAP pyridine DBU (n-C16 H33 )(CH3 )3 NBr MgO 4,4’-dipyridine ZnBr2 KBr triethylamine TEA
PO conversionb (%) 93 > 99 7 3 7 72 1 4 22 2 3 2
PC yieldc (%) 91 > 99 2 <1 <1 47 <1 <1 7 <1 <1 <1
a
Epoxidation conditions, 0.1 mmol Cat.C, H2 O2 : Cat.C (mole ratio)=250 : 1, propylene : H2 O2 (mole ratio)=3−3.5 : 1, 75 ◦ C, 3.5 h, p(CO2 ) =3 MPa (r.t.); Cycloaddition conditions, 140 ◦ C, 4 h, 1.3 mmol cycloaddition catalyst, unless otherwise noted; b Determined by GC; c Determined by GC, yield based on PO
generated from ring-opening, which leads to the formation of the propylene carbonate and regeneration of the TBAB. 4. Conclusions In summary, the system of quaternary ammonium heteropolyphosphatotungstate–TBAB is a highly efficient catalyst for the direct synthesis of PC from CO2 and propylene. The quaternary ammonium heteropolyphosphatotungstate is easy to prepare, commercially available, reusable and highly active for the epoxidation step. As a green oxidant, H2 O2 will be consumed after the epoxidation reaction with the only byproduct of water which can promote the cycloaddition reaction to a certain extent. Thus the one-pot two-steps synthesis of PC from propylene and CO2 have these advantages and would be of great industry potential for the utilization of CO2 from an economic point of view. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
Scheme 2. Proposed mechanism of the one-pot two-step synthesis of propylene carbonate catalyzed by Cat.C–TBAB system with anhydrous H2 O2 as an oxidant
As one of the epoxidation product from propylene and H2 O2 , water will promote the nucleophilic attack of TBAB to a small extent on the less sterically hinderer carbon atom of the second epoxidation product of propylene oxide, which leads to the opening of epoxide ring. Then the CO2 will be attacked to give a CO2 -adduct by the oxygen anion species
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