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Direct oxidative carboxylation of styrene to styrene carbonate in the presence of ionic liquids
Catalysis Communications 5 (2004) 83–87 www.elsevier.com/locate/catcom
Direct oxidative carboxylation of styrene to styrene carbonate in the presence of ionic liquids Jianmin Sun, Shin-ichiro Fujita, Bhalchandra M. Bhanage, Masahiko Arai
*
Division of Materials Science and Engineering, Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Japan Received 17 July 2003; received in revised form 25 November 2003; accepted 25 November 2003 Published online: 30 December 2003
Abstract Styrene carbonate is synthesized by a direct benign route from styrene and compressed CO2 in the presence of ionic liquid, in which two sequential reactions of epoxidation and cycloaddition are coupled for a one-pot synthesis. The structure of ionic liquids and the nucleophilicity of anions strongly affect the yield of styrene carbonate formed and tetrabutylammonium bromide (TBAB) is the most effective catalyst. Other reaction parameters such as CO2 pressure, reaction temperature, time, and TBAB concentration have also been investigated and a plausible reaction mechanism is proposed. Ó 2003 Elsevier B.V. All rights reserved. Keywords: Cyclic carbonate; One-pot synthesis; Styrene; Carbon dioxide; Tetrabutylammonium bromide; Ionic liquid; tert-Butyl hydroperoxide
1. Introduction Cyclic carbonates have gained increasing market demand because of their extensive use as monomers, aprotic polar solvents, pharmaceutical/fine chemical intermediates, and fuel additives [1,2]. Recently, conventional synthesis using phosgene as a starting material has been replaced by innovative technologies based on cycloaddition of carbon dioxide to epoxides, from the viewpoint of Green Chemistry [3,4] and chemical fixation of CO2 [5–7]. Epoxides are important intermediates for the manufacture of a range of important commercial products [8]. Olefin epoxidation is one of the main routes for the epoxide production [9]. The direct oxidative carboxylation of olefins utilizing CO2 as a building block seems to be a more interesting method for the synthesis of cyclic carbonates. This method couples two processes, epoxidation of olefins and CO2 cycloaddition to epoxides (Scheme 1). It is of practical importance that easily available, low-priced chemicals of olefins are used to produce valuable chemicals. Despite its usefulness, this method has not *
Corresponding author. Tel./fax: +81-11-706-6594. E-mail address: [email protected] (M. Arai).
1566-7367/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2003.11.016
well been studied so far. Only a few reports are found in the literature on the direct synthesis of carbonates using homogeneous catalysts and heterogeneous metal oxide catalysts [6,7], which, however, suffer from short lifetime and low yield of the desired carbonates. So, stable and effective catalysts with high selectivity to carbonates are being sought. More recently catalytic reactions using ionic liquids have attracted considerable interests because they possess unique advantages of negligible vapour pressure, excellent thermal stability, and interesting intrinsic physicochemical characteristics. Ionic liquids have been used as effective solvents for clean chemical reactions, namely as replacements for volatile organic and dipolar aprotic solvents. They may provide a medium for clean reactions with minimal waste and efficient product extraction. It has been found that catalytic activity and product selectivity are accelerated when they are used for some reactions such as hydrogenation, hydroformylation, Heck reactions and oxidations [10,11]. Moreover, ionic liquids themselves can also be used as prominent acid–base catalysts [12,13]. Ionic liquids have been reported to catalyse the cycloaddition of CO2 to epoxides [14–16], but high temperature and toxic polar solvents such as DMF and CH2 Cl2 are needed.
84
J. Sun et al. / Catalysis Communications 5 (2004) 83–87 O
O O
O
O
+ CO2 Scheme 1. Two reactions involved in a one-pot synthesis of styrene carbonate from styrene and CO2 .
On the basis of those previous studies, the present work has been undertaken to apply ionic liquids for a one-pot synthesis of styrene carbonate from styrene and CO2 . Effectiveness of several ionic liquids and effects of reaction parameters such as CO2 pressure, temperature, and time have been investigated. A plausible mechanism for this one-pot synthesis has also been proposed.
2. Experimental Commercial chemicals, styrene, tert-butyl hydroperoxide (70 wt.% in water, TBHP) as an oxidant, tetrabutylammonium bromide (TBAB), tetrabutylammonium chloride (TBACl), tetrabutylammonium iodide (TBAI), 1-ethyl-3-methylimidazolium tetrafluoroborate (Aldrich, [C2 -mim]þ BF 4 ), 1-butyl-3-methylimidazolium hexafluorophosphate ([C4 -mim]þ PF 6 ), and 1-butyl-3methylimidazolium tetrafluoroborate (Fluka, [C4 -mim]þ BF 4 ) were used as received. Hydrogen peroxide (30 wt.% in water) was also used as received as an oxidant. All compounds except for the methylimidazolium salts þ of [C2 -mim]þ BF 4 and [C4 -mim] BF4 were purchased from Wako Pure Chemical Ind. In a typical experiment, TBAB (2 mmol, 0.66 g) was added to styrene (17.3 mmol, 2 ml) and TBHP (25.4 mmol, 4 ml) in a 50 cm3 stainless steel autoclave and CO2 was charged to the reactor up to 3 MPa. The reactor was heated up to 80 °C and stirred for half an hour. Then liquid CO2 was further injected up to 15
MPa under stirring and the reaction mixture was stirred for 6 h. After the reaction, the reactor was cooled to 0 °C by ice water and depressurised. The reaction mixture included two or three liquid phases (aqueous phase, organic phase, and/or ionic liquid phase) depending on the kind of ionic liquids used. After the organic phase was extracted with ethyl acetate, the reaction products in the ethyl acetate phase were analysed by a gas chromatograph (Shimadzu 390B) with a capillary column (GL Science TC-17). The concentrations of styrene, styrene epoxide, benzaldehyde and styrene carbonate in the ethyl acetate solution were determined from results obtained with authentic standards. The structures of the products were confirmed by a gas chromatograph mass spectrometer (Shimadzu QP5050A) using the same capillary column. In some experiments, the concentrations of TBHP and TBAB in the aqueous and organic phases were determined by the gas chromatograph before the reaction. The phase behaviour in the reaction mixture has been visually examined under the reaction conditions with a reactor attached with transparent sapphire windows. When tetrabutylammonium halides were used, the starting reaction mixture included three phases, being the aqueous phase (TBHP and halides), the organic phase (styrene, TBHP and halides), and the CO2 -rich gas phase. When imidazolium ionic liquids were used, they were poorly soluble in the aqueous, organic, and the gas phases, and so there were four phases in the reaction mixture. The present reaction mixture was complicated in that the volume of these phases and the concentration of reacting species in them changed during the reaction. 3. Results and discussion 3.1. Influence of ionic liquids Table 1 shows results of the one-pot synthesis of styrene carbonate (SC) from styrene and CO2 using different ionic liquids. Under the reaction conditions used, SC and styrene epoxide (SO) were formed along
Table 1 Results on direct synthesis of styrene carbonate using various ionic liquids Entry
1 2 3 4 5 6
Ionic liquid Kind
Amount (mmol)
TBAB TBAI TBACl [C4 -mim]þ BF 4 [C4 -mim]þ PF 6 [C2 -mim]þ BF 4
2 2 2 10.8 10.6 13.0
Conversiona (%)
Yieldb (%) SC
SO
BA
74 72 84 57 17 36
33 16 8 0 0 0
2 7 3 0 0 0
5 4 8 6 2 2
Reaction conditions: styrene, 17.3 mmol; TBHP, 25.4 mmol; CO2 pressure, 15 MPa; temperature, 80 °C; time, 6 h. Total conversion of styrene. b Yield is moles of the product formed against initial moles of styrene used. SC: styrene carbonate; SO: styrene epoxide; BA: benzaldehyde. a
J. Sun et al. / Catalysis Communications 5 (2004) 83–87
with benzaldehyde (BA) and other byproducts, which were oligomers of SO and/or SC and some unidentified larger molecules. Table 1 indicates that types of ionic liquids used have great effect on both conversion of styrene and yield of SC and that the SC yield depends on the kinds of anions and cations involved. When imidazolium ions (entries 4–6) are used, they are unfortunately ineffective to the synthesis of SC. When ammonium ions are used instead (entries 1–3), however, SC can be produced and the bromide counter ion is better than chloride and iodide anions. Aresta et al. have reported that the role of catalyst is quite important in the second step of direct synthesis of cyclic carbonate [7]. The ring opening of epoxide may occur by nucleophilic attack of the halide anion to a carbon of the epoxide group [17]. The nucleophilic tendency decreases in the order I > Br > Cl , but the yield of SC observed is the highest with TBAB as seen in Table 1. In the case of TBAI, TBHP was consumed for its oxidation in addition to the desired carbonate synthesis, thus leading to the decrease of SC yield as observed. When TBAI was added into the mixture of TBHP and styrene, the colour of the mixture rapidly changed to dark orange, indicative of the formation of iodine by oxidation with TBHP. Possible reasons for the better performance of TBAB compared with the ionic liquids including the imidazolium cations could be ascribed to differences in the structure and in the solubility. The bulkiness of the tetrahedral ammonium ion [NBu4 ]þ forces the halogen anion away from the cation, thus making the anion more nucleophilic. On the contrary, the planar cation [bmim]þ binds the anion more tightly compared with [NBu4 ]þ . In addition, the imidazolium ions are not dissolved in both the aqueous phase including TBHP oxidant and the organic phase including the styrene and TBHP, and so there are four phases including CO2 -rich gas phase. In this case, the contact of the reacting species present in the different phases should not be good as compared with the case of TBAB, which can be soluble
85
both in the aqueous phase and organic phase and there are three phases. The quaternary ammonium salts are widely used as phase transfer catalysts for liquid–liquid or liquid–solid heterogeneous organic reactions [18]. 3.2. Influence of reaction parameters with TBAB One-pot synthesis of SC has been carried out using TBAB under different reaction conditions (Table 2). The total conversion and the product distribution depend strongly on the conditions used. The CO2 pressure significantly affected the product selectivity (entries 1, 7, 8). The SC yield increased from 25% to 33% with increasing pressure from 10 MPa to 15 MPa but it dropped drastically to 13% at 18 MPa. Such a high pressure might promote the formation of oligomers. A similar result that increasing CO2 pressure is not favourable to the carbonate synthesis has also been reported in CO2 addition to propylene oxide [19]. The influence of reaction temperature was also observed (entries 1,5,6). As the reaction temperature was raised, the styrene conversion increased and SC yield also increased from 5% at 60 °C to 33% at 80 °C but the SC yield did not change so much up to 90 °C. The results of entries 1 and 3 indicate that TBHP is more effective than H2 O2 although the two reaction systems with TBHP and H2 O2 are alike. When the oxidant was changed to H2 O2 (30% aqueous solution), the yield of SC dropped drastically and much more BA was produced. The effect of TBAB concentration was also examined (entries 1, 4). As TBAB concentration was tripled, the yield of SC decreased from 33% to 19%, while SO yield increased from 2% to 14%. Larger TBAB concentration would have a negative effect on the cycloaddition of CO2 to SO. Finally, when the reaction time was increased (entries 1, 2), the yield of SC decreased but that of BA increased and SO little changed. Probably this was due to the decomposition of SO and styrene oxidation for prolonging reaction time from 6 to 14 h.
Table 2 Effects of various reaction parameters in one-pot synthesis of styrene carbonate using TBAB Entry
TBAB (mmol)
Temperature (°C)
CO2 pressure (MPa)
Conversiona (%)
Yieldb (%) SC
SO
BA
1 2c 3d 4 5 6 7 8
2 2 2 6 2 2 2 2
80 80 80 80 90 60 80 80
15 15 15 15 15 15 18 10
74 96 62 86 94 57 83 93
33 18 4 19 31 5 13 25
2 3 2 14 5 5 1 3
5 11 21 4 5 3 4 4
Reaction conditions: styrene, 17.3 mmol; TBHP, 25.4 mmol; time, 6 h. Total conversion of styrene. b Yield is moles of the product against initial moles of styrene used. c Reaction time, 14 h. d H2 O2 (25.4 mmol) is used as oxidant. a
86
J. Sun et al. / Catalysis Communications 5 (2004) 83–87 O O -
Br
TBHP
O
O
-
Br
(4)
(1) tert-butanol OBr-
-
(3)
O Br
+ H2O
O
(2) H2O
OH
O
O-
(5) OH-
Br
OH-
Br
CO2
Scheme 2. A proposed mechanism for the direct synthesis of styrene carbonate from styrene and CO2 with TBHP in the presence of TBAB.
The yield of SC obtained in the present work is still not satisfactory for practical applications; however, it is much better compared with the previous works [6,7] in which BA is the main product and the SC yield is very low (only 4%). It should be noted, moreover, that organic solvents and high reaction temperatures are not needed in our reaction systems, which are beneficial from the environmental and energy points of view. In order to ascertain whether the epoxidation of styrene is really catalysed by TBAB, we have tried this reaction under the same conditions but in absence of CO2 (the first step reaction). It was observed that the total conversion of styrene was 88% and the yields of SO and BA were 16% and 3%, respectively. However, in the absence of TBAB, styrene conversion was 50% and the yields of SO and BA were only 3% and 3%, indicating that TBAB did catalyse the epoxidation of styrene. 3.3. Mechanism considerations It has been reported that epoxidation with H2 O2 can be promoted by bromide over WO2 4 catalysts on layered double hydroxides [20] and cycloaddition of CO2 to oxiranes can be catalysed by quaternary onium salts [16,21]. Based on the present and literature results, a reaction mechanism has been postulated, which involves the following steps: (1) catalytic oxidation of Br with TBHP leading to the formation of hypobromite OBr ; (2) bromination of the olefin in the presence of H2 O with the formation of bromohydrin; (3) base-promoted dehydrobromination of the bromohydrin leading to epoxide; (4) ring opening of the epoxide through nucleophilic attack by the bromide ion, producing an oxy anion species; (5) attack by CO2 forming the cyclic carbonate (Scheme 2). At the initial stage of reaction, there are three phases in the reaction mixture, which are the aqueous phase (TBAB and TBHP), the organic phase (styrene, TBAB and TBHP), and the CO2 -rich gas phase. The step (1) mainly takes place in the organic phase due to higher
concentrations of TBHP and TBAB in the organic phase than in the aqueous phase, as suggested from the concentrations of the reacting species in the different phases measured by gas chromatography. And the steps (2) and (3) should occur significantly at the interfacial layer between the aqueous and organic phases. The steps (4) and (5) should occur in the organic phase dissolving the oxy anion species and CO2 molecules, which may be possible due to high CO2 pressure used. After the reaction, the three phases were still observed but the volume of the aqueous phase decreased significantly, probably due to the consumption of TBHP in this phase. Thus, the phase behaviour changes and the phases in which the above steps occur also change during the reaction. The reaction system is so complicated and the reaction result will be influenced by several factors such as intrinsic reactivity of the reacting species, the phase behaviour, the contacting of the phases, and the distribution of the reacting species between those phases. The optimisation of the reaction conditions is worth further investigating for better one-pot carbonate synthesis.
4. Conclusions The direct synthesis of styrene carbonate from styrene and CO2 is attractive because starting materials are easily available and cheap, preliminary synthesis of styrene epoxide is avoided, and the work-up procedures are simple. A moderate yield of styrene carbonate can be obtained using a low-priced ionic liquid (TBAB) with TBHP in the absence of organic solvents. Further optimisation of the reaction conditions is needed to achieve better reaction results.
Acknowledgements The authors gratefully acknowledge the financial support from Japan Society for the Promotion of
J. Sun et al. / Catalysis Communications 5 (2004) 83–87
Science (JSPS), the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry (SEM) of China, and Innovative Foundation of Jilin University, China.
References [1] A.-A.G. Shaikh, S. Sivaram, Chem. Rev. 96 (1996) 951. [2] K. Weissermel, H. Arpe, Industrial Organic Chemistry, VCH, New York, 1997. [3] B.M. Trost, Science 254 (1991) 1471. [4] P.T. Anastas, J.C. Warner, Green Chemistry: Theory and Practice, Oxford University Press, Oxford, 1998. [5] S. Fujita, B.M. Bhanage, Y. Ikushima, M. Shirai, K. Torii, M. Arai, Catal. Lett. 79 (2002) 95, and references therein. [6] M. Aresta, A. Dibenedetto, J. Mol. Catal. A 182 (2002) 399, and references therein. [7] M. Aresta, A. Dibenedetto, I. Tommasi, Appl. Organomet. Chem. 14 (2000) 799.
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[8] K. Bauer, D. Garbe, H. Surburg, Common Fragrance and Flavour Materials, Wiley-VCH, New York, 1997. [9] G. Scienel, R. Rieth, K.T. Rowbottom, UllmannÕs Encyclopedia of Organic Chemicals, Wiley-VCH, Weinheim, 1999. [10] J.D. Holbrey, K.R. Seddon, Clean Prod. Proc. 1 (1999) 223. [11] P. Wasserscheid, W. Keim, Angew. Chem. Int. Ed. 39 (2000) 3772. [12] D.B. Zhao, M. Wu, Y. Kou, E.Z. Min, Catal. Today 74 (2002) 157. [13] T. Welton, Chem. Rev. 99 (1999) 2071. [14] J. Peng, Y. Deng, New J. Chem. 25 (2001) 639. [15] H. Yang, Y. Gu, Y. Deng, F. Shi, Chem. Commun. (2002) 274. [16] V. Calo, A. Nacci, A. Monopoli, A. Fanizzi, Org. Lett. 4 (2002) 2561. [17] T. Nishikubo, A. Kameyama, J. Yamashita, M. Tomoi, J. Polym. Sci. Poly. Chem. Ed. 31 (1993) 939. [18] M. Tomoi, W.T. Ford, Syntheses and Separations Using Functional Polymers, Wiley, New York, 1988, p. 181. [19] B.M. Bhanage, S. Fujita, Y. Ikushima, K. Torii, M. Arai, Green Chem. 5 (2003) 71. [20] B.F. Sels, D.E. De Vos, P.A. Jacobs, J. Am. Chem. Soc. 123 (2001) 8350. [21] A. Behr, Carbon Dioxide Activation by Metal Complexes, VCH, 1988, p. 91.
Direct oxidative carboxylation of styrene to styrene carbonate in the presence of ionic liquids Jianmin Sun, Shin-ichiro Fujita, Bhalchandra M. Bhanage, Masahiko Arai
*
Division of Materials Science and Engineering, Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Japan Received 17 July 2003; received in revised form 25 November 2003; accepted 25 November 2003 Published online: 30 December 2003
Abstract Styrene carbonate is synthesized by a direct benign route from styrene and compressed CO2 in the presence of ionic liquid, in which two sequential reactions of epoxidation and cycloaddition are coupled for a one-pot synthesis. The structure of ionic liquids and the nucleophilicity of anions strongly affect the yield of styrene carbonate formed and tetrabutylammonium bromide (TBAB) is the most effective catalyst. Other reaction parameters such as CO2 pressure, reaction temperature, time, and TBAB concentration have also been investigated and a plausible reaction mechanism is proposed. Ó 2003 Elsevier B.V. All rights reserved. Keywords: Cyclic carbonate; One-pot synthesis; Styrene; Carbon dioxide; Tetrabutylammonium bromide; Ionic liquid; tert-Butyl hydroperoxide
1. Introduction Cyclic carbonates have gained increasing market demand because of their extensive use as monomers, aprotic polar solvents, pharmaceutical/fine chemical intermediates, and fuel additives [1,2]. Recently, conventional synthesis using phosgene as a starting material has been replaced by innovative technologies based on cycloaddition of carbon dioxide to epoxides, from the viewpoint of Green Chemistry [3,4] and chemical fixation of CO2 [5–7]. Epoxides are important intermediates for the manufacture of a range of important commercial products [8]. Olefin epoxidation is one of the main routes for the epoxide production [9]. The direct oxidative carboxylation of olefins utilizing CO2 as a building block seems to be a more interesting method for the synthesis of cyclic carbonates. This method couples two processes, epoxidation of olefins and CO2 cycloaddition to epoxides (Scheme 1). It is of practical importance that easily available, low-priced chemicals of olefins are used to produce valuable chemicals. Despite its usefulness, this method has not *
Corresponding author. Tel./fax: +81-11-706-6594. E-mail address: [email protected] (M. Arai).
1566-7367/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2003.11.016
well been studied so far. Only a few reports are found in the literature on the direct synthesis of carbonates using homogeneous catalysts and heterogeneous metal oxide catalysts [6,7], which, however, suffer from short lifetime and low yield of the desired carbonates. So, stable and effective catalysts with high selectivity to carbonates are being sought. More recently catalytic reactions using ionic liquids have attracted considerable interests because they possess unique advantages of negligible vapour pressure, excellent thermal stability, and interesting intrinsic physicochemical characteristics. Ionic liquids have been used as effective solvents for clean chemical reactions, namely as replacements for volatile organic and dipolar aprotic solvents. They may provide a medium for clean reactions with minimal waste and efficient product extraction. It has been found that catalytic activity and product selectivity are accelerated when they are used for some reactions such as hydrogenation, hydroformylation, Heck reactions and oxidations [10,11]. Moreover, ionic liquids themselves can also be used as prominent acid–base catalysts [12,13]. Ionic liquids have been reported to catalyse the cycloaddition of CO2 to epoxides [14–16], but high temperature and toxic polar solvents such as DMF and CH2 Cl2 are needed.
84
J. Sun et al. / Catalysis Communications 5 (2004) 83–87 O
O O
O
O
+ CO2 Scheme 1. Two reactions involved in a one-pot synthesis of styrene carbonate from styrene and CO2 .
On the basis of those previous studies, the present work has been undertaken to apply ionic liquids for a one-pot synthesis of styrene carbonate from styrene and CO2 . Effectiveness of several ionic liquids and effects of reaction parameters such as CO2 pressure, temperature, and time have been investigated. A plausible mechanism for this one-pot synthesis has also been proposed.
2. Experimental Commercial chemicals, styrene, tert-butyl hydroperoxide (70 wt.% in water, TBHP) as an oxidant, tetrabutylammonium bromide (TBAB), tetrabutylammonium chloride (TBACl), tetrabutylammonium iodide (TBAI), 1-ethyl-3-methylimidazolium tetrafluoroborate (Aldrich, [C2 -mim]þ BF 4 ), 1-butyl-3-methylimidazolium hexafluorophosphate ([C4 -mim]þ PF 6 ), and 1-butyl-3methylimidazolium tetrafluoroborate (Fluka, [C4 -mim]þ BF 4 ) were used as received. Hydrogen peroxide (30 wt.% in water) was also used as received as an oxidant. All compounds except for the methylimidazolium salts þ of [C2 -mim]þ BF 4 and [C4 -mim] BF4 were purchased from Wako Pure Chemical Ind. In a typical experiment, TBAB (2 mmol, 0.66 g) was added to styrene (17.3 mmol, 2 ml) and TBHP (25.4 mmol, 4 ml) in a 50 cm3 stainless steel autoclave and CO2 was charged to the reactor up to 3 MPa. The reactor was heated up to 80 °C and stirred for half an hour. Then liquid CO2 was further injected up to 15
MPa under stirring and the reaction mixture was stirred for 6 h. After the reaction, the reactor was cooled to 0 °C by ice water and depressurised. The reaction mixture included two or three liquid phases (aqueous phase, organic phase, and/or ionic liquid phase) depending on the kind of ionic liquids used. After the organic phase was extracted with ethyl acetate, the reaction products in the ethyl acetate phase were analysed by a gas chromatograph (Shimadzu 390B) with a capillary column (GL Science TC-17). The concentrations of styrene, styrene epoxide, benzaldehyde and styrene carbonate in the ethyl acetate solution were determined from results obtained with authentic standards. The structures of the products were confirmed by a gas chromatograph mass spectrometer (Shimadzu QP5050A) using the same capillary column. In some experiments, the concentrations of TBHP and TBAB in the aqueous and organic phases were determined by the gas chromatograph before the reaction. The phase behaviour in the reaction mixture has been visually examined under the reaction conditions with a reactor attached with transparent sapphire windows. When tetrabutylammonium halides were used, the starting reaction mixture included three phases, being the aqueous phase (TBHP and halides), the organic phase (styrene, TBHP and halides), and the CO2 -rich gas phase. When imidazolium ionic liquids were used, they were poorly soluble in the aqueous, organic, and the gas phases, and so there were four phases in the reaction mixture. The present reaction mixture was complicated in that the volume of these phases and the concentration of reacting species in them changed during the reaction. 3. Results and discussion 3.1. Influence of ionic liquids Table 1 shows results of the one-pot synthesis of styrene carbonate (SC) from styrene and CO2 using different ionic liquids. Under the reaction conditions used, SC and styrene epoxide (SO) were formed along
Table 1 Results on direct synthesis of styrene carbonate using various ionic liquids Entry
1 2 3 4 5 6
Ionic liquid Kind
Amount (mmol)
TBAB TBAI TBACl [C4 -mim]þ BF 4 [C4 -mim]þ PF 6 [C2 -mim]þ BF 4
2 2 2 10.8 10.6 13.0
Conversiona (%)
Yieldb (%) SC
SO
BA
74 72 84 57 17 36
33 16 8 0 0 0
2 7 3 0 0 0
5 4 8 6 2 2
Reaction conditions: styrene, 17.3 mmol; TBHP, 25.4 mmol; CO2 pressure, 15 MPa; temperature, 80 °C; time, 6 h. Total conversion of styrene. b Yield is moles of the product formed against initial moles of styrene used. SC: styrene carbonate; SO: styrene epoxide; BA: benzaldehyde. a
J. Sun et al. / Catalysis Communications 5 (2004) 83–87
with benzaldehyde (BA) and other byproducts, which were oligomers of SO and/or SC and some unidentified larger molecules. Table 1 indicates that types of ionic liquids used have great effect on both conversion of styrene and yield of SC and that the SC yield depends on the kinds of anions and cations involved. When imidazolium ions (entries 4–6) are used, they are unfortunately ineffective to the synthesis of SC. When ammonium ions are used instead (entries 1–3), however, SC can be produced and the bromide counter ion is better than chloride and iodide anions. Aresta et al. have reported that the role of catalyst is quite important in the second step of direct synthesis of cyclic carbonate [7]. The ring opening of epoxide may occur by nucleophilic attack of the halide anion to a carbon of the epoxide group [17]. The nucleophilic tendency decreases in the order I > Br > Cl , but the yield of SC observed is the highest with TBAB as seen in Table 1. In the case of TBAI, TBHP was consumed for its oxidation in addition to the desired carbonate synthesis, thus leading to the decrease of SC yield as observed. When TBAI was added into the mixture of TBHP and styrene, the colour of the mixture rapidly changed to dark orange, indicative of the formation of iodine by oxidation with TBHP. Possible reasons for the better performance of TBAB compared with the ionic liquids including the imidazolium cations could be ascribed to differences in the structure and in the solubility. The bulkiness of the tetrahedral ammonium ion [NBu4 ]þ forces the halogen anion away from the cation, thus making the anion more nucleophilic. On the contrary, the planar cation [bmim]þ binds the anion more tightly compared with [NBu4 ]þ . In addition, the imidazolium ions are not dissolved in both the aqueous phase including TBHP oxidant and the organic phase including the styrene and TBHP, and so there are four phases including CO2 -rich gas phase. In this case, the contact of the reacting species present in the different phases should not be good as compared with the case of TBAB, which can be soluble
85
both in the aqueous phase and organic phase and there are three phases. The quaternary ammonium salts are widely used as phase transfer catalysts for liquid–liquid or liquid–solid heterogeneous organic reactions [18]. 3.2. Influence of reaction parameters with TBAB One-pot synthesis of SC has been carried out using TBAB under different reaction conditions (Table 2). The total conversion and the product distribution depend strongly on the conditions used. The CO2 pressure significantly affected the product selectivity (entries 1, 7, 8). The SC yield increased from 25% to 33% with increasing pressure from 10 MPa to 15 MPa but it dropped drastically to 13% at 18 MPa. Such a high pressure might promote the formation of oligomers. A similar result that increasing CO2 pressure is not favourable to the carbonate synthesis has also been reported in CO2 addition to propylene oxide [19]. The influence of reaction temperature was also observed (entries 1,5,6). As the reaction temperature was raised, the styrene conversion increased and SC yield also increased from 5% at 60 °C to 33% at 80 °C but the SC yield did not change so much up to 90 °C. The results of entries 1 and 3 indicate that TBHP is more effective than H2 O2 although the two reaction systems with TBHP and H2 O2 are alike. When the oxidant was changed to H2 O2 (30% aqueous solution), the yield of SC dropped drastically and much more BA was produced. The effect of TBAB concentration was also examined (entries 1, 4). As TBAB concentration was tripled, the yield of SC decreased from 33% to 19%, while SO yield increased from 2% to 14%. Larger TBAB concentration would have a negative effect on the cycloaddition of CO2 to SO. Finally, when the reaction time was increased (entries 1, 2), the yield of SC decreased but that of BA increased and SO little changed. Probably this was due to the decomposition of SO and styrene oxidation for prolonging reaction time from 6 to 14 h.
Table 2 Effects of various reaction parameters in one-pot synthesis of styrene carbonate using TBAB Entry
TBAB (mmol)
Temperature (°C)
CO2 pressure (MPa)
Conversiona (%)
Yieldb (%) SC
SO
BA
1 2c 3d 4 5 6 7 8
2 2 2 6 2 2 2 2
80 80 80 80 90 60 80 80
15 15 15 15 15 15 18 10
74 96 62 86 94 57 83 93
33 18 4 19 31 5 13 25
2 3 2 14 5 5 1 3
5 11 21 4 5 3 4 4
Reaction conditions: styrene, 17.3 mmol; TBHP, 25.4 mmol; time, 6 h. Total conversion of styrene. b Yield is moles of the product against initial moles of styrene used. c Reaction time, 14 h. d H2 O2 (25.4 mmol) is used as oxidant. a
86
J. Sun et al. / Catalysis Communications 5 (2004) 83–87 O O -
Br
TBHP
O
O
-
Br
(4)
(1) tert-butanol OBr-
-
(3)
O Br
+ H2O
O
(2) H2O
OH
O
O-
(5) OH-
Br
OH-
Br
CO2
Scheme 2. A proposed mechanism for the direct synthesis of styrene carbonate from styrene and CO2 with TBHP in the presence of TBAB.
The yield of SC obtained in the present work is still not satisfactory for practical applications; however, it is much better compared with the previous works [6,7] in which BA is the main product and the SC yield is very low (only 4%). It should be noted, moreover, that organic solvents and high reaction temperatures are not needed in our reaction systems, which are beneficial from the environmental and energy points of view. In order to ascertain whether the epoxidation of styrene is really catalysed by TBAB, we have tried this reaction under the same conditions but in absence of CO2 (the first step reaction). It was observed that the total conversion of styrene was 88% and the yields of SO and BA were 16% and 3%, respectively. However, in the absence of TBAB, styrene conversion was 50% and the yields of SO and BA were only 3% and 3%, indicating that TBAB did catalyse the epoxidation of styrene. 3.3. Mechanism considerations It has been reported that epoxidation with H2 O2 can be promoted by bromide over WO2 4 catalysts on layered double hydroxides [20] and cycloaddition of CO2 to oxiranes can be catalysed by quaternary onium salts [16,21]. Based on the present and literature results, a reaction mechanism has been postulated, which involves the following steps: (1) catalytic oxidation of Br with TBHP leading to the formation of hypobromite OBr ; (2) bromination of the olefin in the presence of H2 O with the formation of bromohydrin; (3) base-promoted dehydrobromination of the bromohydrin leading to epoxide; (4) ring opening of the epoxide through nucleophilic attack by the bromide ion, producing an oxy anion species; (5) attack by CO2 forming the cyclic carbonate (Scheme 2). At the initial stage of reaction, there are three phases in the reaction mixture, which are the aqueous phase (TBAB and TBHP), the organic phase (styrene, TBAB and TBHP), and the CO2 -rich gas phase. The step (1) mainly takes place in the organic phase due to higher
concentrations of TBHP and TBAB in the organic phase than in the aqueous phase, as suggested from the concentrations of the reacting species in the different phases measured by gas chromatography. And the steps (2) and (3) should occur significantly at the interfacial layer between the aqueous and organic phases. The steps (4) and (5) should occur in the organic phase dissolving the oxy anion species and CO2 molecules, which may be possible due to high CO2 pressure used. After the reaction, the three phases were still observed but the volume of the aqueous phase decreased significantly, probably due to the consumption of TBHP in this phase. Thus, the phase behaviour changes and the phases in which the above steps occur also change during the reaction. The reaction system is so complicated and the reaction result will be influenced by several factors such as intrinsic reactivity of the reacting species, the phase behaviour, the contacting of the phases, and the distribution of the reacting species between those phases. The optimisation of the reaction conditions is worth further investigating for better one-pot carbonate synthesis.
4. Conclusions The direct synthesis of styrene carbonate from styrene and CO2 is attractive because starting materials are easily available and cheap, preliminary synthesis of styrene epoxide is avoided, and the work-up procedures are simple. A moderate yield of styrene carbonate can be obtained using a low-priced ionic liquid (TBAB) with TBHP in the absence of organic solvents. Further optimisation of the reaction conditions is needed to achieve better reaction results.
Acknowledgements The authors gratefully acknowledge the financial support from Japan Society for the Promotion of
J. Sun et al. / Catalysis Communications 5 (2004) 83–87
Science (JSPS), the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry (SEM) of China, and Innovative Foundation of Jilin University, China.
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