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Non-halogen catalysts for propylene carbonate synthesis from CO2 under supercritical conditions
Applied Catalysis A: General 298 (2006) 177–180 www.elsevier.com/locate/apcata
Non-halogen catalysts for propylene carbonate synthesis from CO2 under supercritical conditions Hiroyuki Yasuda, Liang-Nian He 1, Toshikazu Takahashi, Toshiyasu Sakakura * National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan Received 21 July 2005; received in revised form 29 August 2005; accepted 28 September 2005 Available online 4 November 2005
Abstract Cesium–phosphorous–silicon mixed oxide (Cs–P–Si oxide), a typical acid–base bifunctional catalyst, efficiently catalyzes propylene carbonate synthesis from CO2 and propylene oxide under supercritical conditions (8–10 MPa). This catalyst contains no halogens and requires no additional solvents. A portion of the catalyst was eluted into the product solution during catalysis. It was also found that Cs3PO4, one of the species that may be eluted from the Cs–P–Si oxide, is an effective non-halogen homogeneous catalyst. # 2005 Elsevier B.V. All rights reserved. Keywords: Cyclic carbonate; Non-halogen; Supercritical carbon dioxide; Cesium–phosphorous–silicon mixed oxide; Cesium phosphate
1. Introduction Five-membered cyclic carbonates (e.g. ethylene carbonate and propylene carbonate) are commercially important compounds. They are used as electrolytes in lithium batteries, as aprotic polar solvents, and as intermediates for producing polycarbonates and fine chemicals [1,2]. Cyclic carbonates are currently synthesized by cycloaddition of carbon dioxide to epoxides using homogeneous catalysts, such as alkali halides or alkylammonium halides [3]. Numerous homogeneous catalytic systems including metal complexes [4–6], ionic liquids [7,8], and onium salts [9,10] have also been reported to be effective in the production of cyclic carbonates. However, most of the anionic components of these catalysts contain halides. Therefore, the development of non-halogen catalysts would help to decrease halogen impurities in the products. While several solid catalysts based on metal oxides have been proposed [11–15], the activity and selectivity of these catalysts are insufficient. In addition, combination with halides [11,12] and/or the use of
* Corresponding author. Tel.: +81 29 861 4719; fax: +81 29 861 4719. E-mail address: [email protected] (T. Sakakura). 1 Present address: Institute of Elemento-Organic Chemistry, Nankai University, Tianjin 300071, PR China. 0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2005.09.034
polar solvents, such as DMF (N,N-dimethylformamide) [12– 14], are needed to increase the yields. The solid catalysts are thought to catalyze the reaction via an acid–base bifunctional mechanism [12–15]. Alkali metal–phosphorous–silicon mixed oxides are acid–base bifunctional catalysts that are commercially used for the production of ethylenimine [16,17]. Cesium– phosphorous–silicon mixed oxide has also been reported to promote N-methylation of amino alcohols under supercritical methanol conditions via acid–base catalysis [18]. Thus, it would be interesting to investigate the utility of cesium– phosphorous–silicon mixed oxide as a non-halogen catalyst for cyclic carbonate synthesis. In this study, we have investigated the catalytic performance and stability of this mixed oxide for propylene carbonate synthesis from CO2 and propylene oxide under supercritical conditions. 2. Experimental 2.1. Catalyst preparation The cesium–phosphorous–silicon mixed oxide (abbreviated as Cs–P–Si oxide) was prepared by impregnating silica (Fuji Silysia Chemical Ltd., CARiACT Q-30) with a mixed aqueous solution of cesium nitrate and ammonium dihydrogenpho-
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sphate, followed by drying at 120 8C and calcining in air at 500 8C for 2 h according to previously reported methods [18]. The atomic ratio of Cs:P:Si in the catalyst was 1:0.8:5. The specific surface area of the Cs–P–Si oxide was 7 m2 g 1. Cesium–silicon mixed oxide (Cs:Si of 1:5, 57 m2 g 1, abbreviated as Cs–Si oxide) and phosphorous–silicon mixed oxide (P:Si of 0.8:5, 53 m2 g 1, abbreviated as P–Si oxide) were prepared in a similar manner. SmOCl was prepared as in [12]. Cs3PO4 was obtained by neutralizing an aqueous solution of cesium carbonate with an aqueous solution of phosphoric acid and was used after drying under a vacuum at 200 8C.
2.2. Reaction The cycloaddition of CO2 to propylene oxide was carried out using propylene oxide (4 cm3) and a catalyst (1 g) in a batch reactor as described previously [12]. Prior to the reaction, the catalyst was evacuated at 300 8C for 2 h. The cycloaddition reaction was also carried out in a fixed-bed continuous-flow reactor (1 cm inner diameter and 23 cm long), which was operated in the up-flow mode. The temperature of the catalyst bed was isothermally maintained by an electric furnace. The pressure in the reactor was controlled by a backpressure regulator. A mixture of propylene oxide and supercritical CO2 was continuously introduced into the reactor with an HPLC pump, passed through the catalyst bed and the backpressure regulator, and expanded to separate the liquid products from the CO2. The standard reaction conditions were 10 g of catalyst, a total pressure of 14 MPa, a reaction temperature of 200 8C, a flow rate of propylene oxide of 0.05 cm3 min 1, a flow rate of liquid CO2 of 0.2 cm3 min 1, and a weight hourly space velocity (WHSV) of 0.25 h 1. The volume ratio of CO2 to propylene oxide was 5. Prior to the run, the catalyst was treated in a He stream at 300 8C for 2 h. The liquid reaction products were periodically collected, analyzed by GC, and further characterized using GC–MS. 3. Results and discussion We first investigated the catalytic performance of Cs–P–Si oxide in a batch reactor. Fig. 1 shows the temperature dependence of the yield and selectivity of propylene carbonate formation. The reaction was performed under supercritical CO2 conditions (14 MPa), where CO2 could act not only as a reagent but also as a solvent. Catalytic activity appeared above 150 8C, and propylene carbonate was produced in high yield (81%) with excellent selectivity (99%) at 200 8C. A further temperature increase to 225 8C resulted in the formation of a large number of isomers and oligomers of propylene oxide as by-products, decreasing the selectivity to 35%. The Cs–P–Si oxide catalyst requires a relatively high temperature (200 8C) compared with other recently reported homogeneous catalysts that are effective at temperatures below 120 8C [4–10]. However, from an industrial standpoint, a temperature range of 150–200 8C is preferable because the cycloaddition reaction is exothermic and heat recovery is indispensable.
Fig. 1. Temperature dependence of the yield (*) and selectivity (&) of propylene carbonate formation using a Cs–P–Si oxide catalyst. Reaction conditions: catalyst, 1 g; propylene oxide, 4 cm3; 14 MPa; 8 h.
Fig. 2(a) shows the effects of CO2 pressure on the conversion of propylene oxide and the selectivity of the reaction catalyzed by Cs–P–Si oxide at 200 8C. Interestingly, conversion was inversely dependent on CO2 pressure. Upon increasing the CO2 pressure from 3 to 20 MPa, conversion decreased from 100 to 32%. In contrast, selectivity increased with CO2 pressure and reached nearly 100% at pressures above 10 MPa. Consequently, there is an optimal pressure range between 8 and 10 MPa for the production of propylene carbonate (Fig. 2(b)). This contrasts with the SmOCl catalyst, for which the yield increases with CO2 pressure up to 30 MPa [12]. Thus, for the Cs–P–Si oxide catalyst, supercritical conditions, especially near-critical pressures of CO2 (around 7.3 MPa), are preferred, while higher pressures (14 MPa) have negative effects. Although the reasons for such dependencies are currently unclear, similar pressure effects are often observed for various homogeneously catalyzed reactions in supercritical CO2, including Diels–Alder reactions, oxidation reactions, and carbonate synthesis [19–24]. Table 1 compares the activities and selectivities of the Cs–P– Si oxide catalyst and other oxide-based catalysts. Use of the Cs–P–Si oxide catalyst resulted in higher yield and selectivity than use of the previously reported SmOCl [12], MgO [13], or Mg–Al mixed oxide [14] catalysts (Table 1, entries 1–4). For these reported catalysts, the addition of DMF increased the yield [12]. For the Cs–P–Si oxide catalyst, the addition of DMF slightly decreased the yield and selectivity (Table 1, entry 5). The catalytic performance of Cs–P–Si oxide was also compared with the performance of Cs–Si oxide and P–Si oxide in order to elucidate the respective roles of Cs and P in the Cs–P–Si oxide catalyst. The Cs–Si oxide catalyst was active for the reaction (Table 1, entry 6). The loading of Cs on potassium-exchanged X-type zeolite or alumina also promotes the cycloaddition of CO2 to epoxides [15]. However, the yield of the Cs–Si oxide catalyst was much lower than the yield of the Cs–P–Si oxide catalyst. This suggests that P is important for accelerating the reaction, although addition of P slightly lowers the selectivity. P–Si oxide was an ineffective catalyst.
H. Yasuda et al. / Applied Catalysis A: General 298 (2006) 177–180
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Fig. 3. Time-course of the yield (*) and selectivity (&) of propylene carbonate synthesis using a Cs–P–Si oxide catalyst and the yield (*) and selectivity (&) using a SmOCl catalyst. Reaction conditions for Cs–P–Si oxide: F(propylene oxide), 0.05 cm3 min 1; F(CO2 as a liquid), 0.2 cm3 min 1; WHSV, 0.25 h 1; Cs–P–Si oxide, 10 g; 14 MPa; 200 8C. Reaction conditions for SmOCl: F(propylene oxide), 0.1 cm3 min 1; F(CO2 as a liquid), 0.4 cm3 min 1; WHSV, 0.5 h 1; SmOCl, 10 g; 14 MPa; 200 8C.
Fig. 2. Pressure dependence of (a) the conversion of propylene oxide (*) and the selectivity of propylene carbonate formation (&) and (b) the yield of propylene carbonate (*) using a Cs–P–Si oxide catalyst. Reaction conditions: catalyst, 1 g; propylene oxide, 4 cm3; 200 8C; 8 h.
Table 1 Propylene carbonate synthesis from carbon dioxide and propylene oxidea Entry
Catalyst
Pressure (MPa)
Yield (%)
Selectivity (%)
1 2b 3b 4b 5c 6 7 8d 9d,e
Cs–P–Si oxide SmOCl MgO Mg–Al oxide Cs–P–Si oxide Cs–Si oxide P–Si oxide Cs3PO4 Cs3PO4
8 14 14 14 8 8 8 8 8
94 58 23 24 92 43 <1 12 95
96 97 78 31 92 >99 <1 96 96
a b c d e
Reaction conditions: catalyst, 1 g; propylene oxide, 4 cm3; 200 8C; 8 h. Cited from Ref. [12]. DMF (5 cm3) was added. Cs3PO4 (0.28 g, 1 mol%) was used. Ethylene carbonate (5.2 g) was added as a solvent.
As Cs–P–Si oxide was found to effectively catalyze propylene carbonate synthesis in a batch reactor, we examined the catalytic stability of Cs–P–Si oxide using a continuous-flow reactor. Fig. 3 shows the time-course of yield and selectivity of propylene carbonate formation in the continuous-flow reaction. For the Cs– P–Si oxide catalyst, greater than 50% yield with excellent selectivity (>99%) was obtained in the initial period (3 h), whereas the yield gradually decreased with time on stream (t) to 18% at t = 16 h. For comparison, we include in Fig. 3 results obtained using SmOCl, which is an effective catalyst for this reaction [12]. Under these experimental conditions, the catalytic activity was maintained for 30 h, although the yield was not high. Elemental analysis of the product solution obtained in the run using the Cs–P–Si oxide catalyst demonstrated a Cs concentration of 50 ppm and a P concentration of 20 ppm at t = 16 h. This shows that the components of the catalyst were partially eluted into the product solution, thus decreasing the catalytic activity. Since Cs–P–Si oxide showed high catalytic activity in a batch reactor, it is possible that Cs- and/or P-containing species eluted from Cs–P–Si oxide function as efficient non-halogen catalysts. Thus, we examined the catalytic activity of Cs3PO4, one such possibly eluted species. As expected, Cs3PO4 catalyzed propylene carbonate synthesis (Table 1, entry 8), although the yield was considerably lower than in the case of Cs–P–Si oxide. The catalytic activity was greatly enhanced by the addition of ethylene carbonate (Table 1, entry 9), which probably increased the solubility of the catalyst. These results suggest that even phosphate species that are less nucleophilic than halides can promote the reaction at high temperatures under supercritical conditions. Therefore, it is conceivable that phosphate salts, including Cs3PO4, may be promising as non-halogen homogeneous catalysts for propylene carbonate synthesis.
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In summary, we have demonstrated that Cs–P–Si oxide, a typical acid–base bifunctional catalyst, is highly active but not stable enough to serve as a solid catalyst for propylene carbonate synthesis from CO2 and propylene oxide under supercritical conditions. We have also demonstrated that Cs3PO4 is an effective non-halogen homogeneous catalyst for this reaction. Acknowledgements We thank Mr. H. Tsuneki and Mr. T. Oku (Nippon Shokubai Co. Ltd.) for supplying Cs–P–Si oxide samples and for helpful discussions. We also thank Mr. S. Kitazume for helpful technical assistance. References [1] J.H. Clements, Ind. Eng. Chem. Res. 42 (2003) 663. [2] S. Fukuoka, M. Kawamura, K. Komiya, M. Tojo, H. Hachiya, K. Hasegawa, M. Aminaka, H. Okamoto, I. Fukawa, S. Konno, Green Chem. 5 (2003) 497. [3] D.J. Darensbourg, M.W. Holtcamp, Coord. Chem. Rev. 153 (1996) 155. [4] H.S. Kim, J.J. Kim, B.G. Lee, O.S. Jung, H.G. Jang, S.O. Kang, Angew. Chem. Int. Ed. 39 (2000) 4096.
[5] R.L. Paddock, S.T. Nguyen, J. Am. Chem. Soc. 123 (2001) 11498. [6] X.-B. Lu, B. Liang, Y.-J. Zhang, Y.-Z. Tian, Y.-M. Wang, C.-X. Bai, H. Wang, R. Zhang, J. Am. Chem. Soc. 126 (2004) 3732. [7] J. Peng, Y. Deng, New J. Chem. 25 (2001) 639. [8] H. Kawanami, A. Sasaki, K. Matsui, Y. Ikushima, Chem. Commun. (2003) 896. [9] V. Calo´, A. Nacci, A. Monopoli, A. Fanizzi, Org. Lett. 4 (2002) 2561. [10] L.-N. He, H. Yasuda, T. Sakakura, Green Chem. 5 (2003) 92. [11] M. Aresta, A. Dibenedetto, J. Mol. Catal. A 182/183 (2002) 399. [12] H. Yasuda, L.-N. He, T. Sakakura, J. Catal. 209 (2002) 547. [13] T. Yano, H. Matsui, T. Koike, H. Ishiguro, H. Fujihara, M. Yoshihara, T. Maeshima, Chem. Commun. (1997) 1129. [14] K. Yamaguchi, K. Ebitani, T. Yoshida, H. Yoshida, K. Kaneda, J. Am. Chem. Soc. 121 (1999) 4526. [15] M. Tu, R.J. Davis, J. Catal. 199 (2001) 85. [16] K. Tanabe, W.F. Ho¨lderich, Appl. Catal. A 181 (1999) 399. [17] H. Tsuneki, Appl. Catal. A 221 (2001) 209. [18] T. Oku, T. Ikariya, Angew. Chem. Int. Ed. 41 (2002) 3476. [19] Y. Ikushima, N. Saito, M. Arai, J. Phys. Chem. 96 (1992) 2293. [20] A.A. Clifford, K. Pople, W.J. Gaskill, K.D. Bartle, C.M. Rayner, Chem. Commun. (1997) 595. [21] X.-W. Wu, Y. Oshima, S. Koda, Chem. Lett. (1997) 1045. [22] T. Sakakura, Y. Saito, M. Okano, J.-C. Choi, T. Sako, J. Org. Chem. 63 (1998) 7095. [23] H. Kawanami, Y. Ikushima, Chem. Commun. (2000) 2089. [24] S. Fujita, B.M. Bhanage, Y. Ikushima, M. Arai, Green Chem. 3 (2001) 87.
Non-halogen catalysts for propylene carbonate synthesis from CO2 under supercritical conditions Hiroyuki Yasuda, Liang-Nian He 1, Toshikazu Takahashi, Toshiyasu Sakakura * National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan Received 21 July 2005; received in revised form 29 August 2005; accepted 28 September 2005 Available online 4 November 2005
Abstract Cesium–phosphorous–silicon mixed oxide (Cs–P–Si oxide), a typical acid–base bifunctional catalyst, efficiently catalyzes propylene carbonate synthesis from CO2 and propylene oxide under supercritical conditions (8–10 MPa). This catalyst contains no halogens and requires no additional solvents. A portion of the catalyst was eluted into the product solution during catalysis. It was also found that Cs3PO4, one of the species that may be eluted from the Cs–P–Si oxide, is an effective non-halogen homogeneous catalyst. # 2005 Elsevier B.V. All rights reserved. Keywords: Cyclic carbonate; Non-halogen; Supercritical carbon dioxide; Cesium–phosphorous–silicon mixed oxide; Cesium phosphate
1. Introduction Five-membered cyclic carbonates (e.g. ethylene carbonate and propylene carbonate) are commercially important compounds. They are used as electrolytes in lithium batteries, as aprotic polar solvents, and as intermediates for producing polycarbonates and fine chemicals [1,2]. Cyclic carbonates are currently synthesized by cycloaddition of carbon dioxide to epoxides using homogeneous catalysts, such as alkali halides or alkylammonium halides [3]. Numerous homogeneous catalytic systems including metal complexes [4–6], ionic liquids [7,8], and onium salts [9,10] have also been reported to be effective in the production of cyclic carbonates. However, most of the anionic components of these catalysts contain halides. Therefore, the development of non-halogen catalysts would help to decrease halogen impurities in the products. While several solid catalysts based on metal oxides have been proposed [11–15], the activity and selectivity of these catalysts are insufficient. In addition, combination with halides [11,12] and/or the use of
* Corresponding author. Tel.: +81 29 861 4719; fax: +81 29 861 4719. E-mail address: [email protected] (T. Sakakura). 1 Present address: Institute of Elemento-Organic Chemistry, Nankai University, Tianjin 300071, PR China. 0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2005.09.034
polar solvents, such as DMF (N,N-dimethylformamide) [12– 14], are needed to increase the yields. The solid catalysts are thought to catalyze the reaction via an acid–base bifunctional mechanism [12–15]. Alkali metal–phosphorous–silicon mixed oxides are acid–base bifunctional catalysts that are commercially used for the production of ethylenimine [16,17]. Cesium– phosphorous–silicon mixed oxide has also been reported to promote N-methylation of amino alcohols under supercritical methanol conditions via acid–base catalysis [18]. Thus, it would be interesting to investigate the utility of cesium– phosphorous–silicon mixed oxide as a non-halogen catalyst for cyclic carbonate synthesis. In this study, we have investigated the catalytic performance and stability of this mixed oxide for propylene carbonate synthesis from CO2 and propylene oxide under supercritical conditions. 2. Experimental 2.1. Catalyst preparation The cesium–phosphorous–silicon mixed oxide (abbreviated as Cs–P–Si oxide) was prepared by impregnating silica (Fuji Silysia Chemical Ltd., CARiACT Q-30) with a mixed aqueous solution of cesium nitrate and ammonium dihydrogenpho-
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sphate, followed by drying at 120 8C and calcining in air at 500 8C for 2 h according to previously reported methods [18]. The atomic ratio of Cs:P:Si in the catalyst was 1:0.8:5. The specific surface area of the Cs–P–Si oxide was 7 m2 g 1. Cesium–silicon mixed oxide (Cs:Si of 1:5, 57 m2 g 1, abbreviated as Cs–Si oxide) and phosphorous–silicon mixed oxide (P:Si of 0.8:5, 53 m2 g 1, abbreviated as P–Si oxide) were prepared in a similar manner. SmOCl was prepared as in [12]. Cs3PO4 was obtained by neutralizing an aqueous solution of cesium carbonate with an aqueous solution of phosphoric acid and was used after drying under a vacuum at 200 8C.
2.2. Reaction The cycloaddition of CO2 to propylene oxide was carried out using propylene oxide (4 cm3) and a catalyst (1 g) in a batch reactor as described previously [12]. Prior to the reaction, the catalyst was evacuated at 300 8C for 2 h. The cycloaddition reaction was also carried out in a fixed-bed continuous-flow reactor (1 cm inner diameter and 23 cm long), which was operated in the up-flow mode. The temperature of the catalyst bed was isothermally maintained by an electric furnace. The pressure in the reactor was controlled by a backpressure regulator. A mixture of propylene oxide and supercritical CO2 was continuously introduced into the reactor with an HPLC pump, passed through the catalyst bed and the backpressure regulator, and expanded to separate the liquid products from the CO2. The standard reaction conditions were 10 g of catalyst, a total pressure of 14 MPa, a reaction temperature of 200 8C, a flow rate of propylene oxide of 0.05 cm3 min 1, a flow rate of liquid CO2 of 0.2 cm3 min 1, and a weight hourly space velocity (WHSV) of 0.25 h 1. The volume ratio of CO2 to propylene oxide was 5. Prior to the run, the catalyst was treated in a He stream at 300 8C for 2 h. The liquid reaction products were periodically collected, analyzed by GC, and further characterized using GC–MS. 3. Results and discussion We first investigated the catalytic performance of Cs–P–Si oxide in a batch reactor. Fig. 1 shows the temperature dependence of the yield and selectivity of propylene carbonate formation. The reaction was performed under supercritical CO2 conditions (14 MPa), where CO2 could act not only as a reagent but also as a solvent. Catalytic activity appeared above 150 8C, and propylene carbonate was produced in high yield (81%) with excellent selectivity (99%) at 200 8C. A further temperature increase to 225 8C resulted in the formation of a large number of isomers and oligomers of propylene oxide as by-products, decreasing the selectivity to 35%. The Cs–P–Si oxide catalyst requires a relatively high temperature (200 8C) compared with other recently reported homogeneous catalysts that are effective at temperatures below 120 8C [4–10]. However, from an industrial standpoint, a temperature range of 150–200 8C is preferable because the cycloaddition reaction is exothermic and heat recovery is indispensable.
Fig. 1. Temperature dependence of the yield (*) and selectivity (&) of propylene carbonate formation using a Cs–P–Si oxide catalyst. Reaction conditions: catalyst, 1 g; propylene oxide, 4 cm3; 14 MPa; 8 h.
Fig. 2(a) shows the effects of CO2 pressure on the conversion of propylene oxide and the selectivity of the reaction catalyzed by Cs–P–Si oxide at 200 8C. Interestingly, conversion was inversely dependent on CO2 pressure. Upon increasing the CO2 pressure from 3 to 20 MPa, conversion decreased from 100 to 32%. In contrast, selectivity increased with CO2 pressure and reached nearly 100% at pressures above 10 MPa. Consequently, there is an optimal pressure range between 8 and 10 MPa for the production of propylene carbonate (Fig. 2(b)). This contrasts with the SmOCl catalyst, for which the yield increases with CO2 pressure up to 30 MPa [12]. Thus, for the Cs–P–Si oxide catalyst, supercritical conditions, especially near-critical pressures of CO2 (around 7.3 MPa), are preferred, while higher pressures (14 MPa) have negative effects. Although the reasons for such dependencies are currently unclear, similar pressure effects are often observed for various homogeneously catalyzed reactions in supercritical CO2, including Diels–Alder reactions, oxidation reactions, and carbonate synthesis [19–24]. Table 1 compares the activities and selectivities of the Cs–P– Si oxide catalyst and other oxide-based catalysts. Use of the Cs–P–Si oxide catalyst resulted in higher yield and selectivity than use of the previously reported SmOCl [12], MgO [13], or Mg–Al mixed oxide [14] catalysts (Table 1, entries 1–4). For these reported catalysts, the addition of DMF increased the yield [12]. For the Cs–P–Si oxide catalyst, the addition of DMF slightly decreased the yield and selectivity (Table 1, entry 5). The catalytic performance of Cs–P–Si oxide was also compared with the performance of Cs–Si oxide and P–Si oxide in order to elucidate the respective roles of Cs and P in the Cs–P–Si oxide catalyst. The Cs–Si oxide catalyst was active for the reaction (Table 1, entry 6). The loading of Cs on potassium-exchanged X-type zeolite or alumina also promotes the cycloaddition of CO2 to epoxides [15]. However, the yield of the Cs–Si oxide catalyst was much lower than the yield of the Cs–P–Si oxide catalyst. This suggests that P is important for accelerating the reaction, although addition of P slightly lowers the selectivity. P–Si oxide was an ineffective catalyst.
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179
Fig. 3. Time-course of the yield (*) and selectivity (&) of propylene carbonate synthesis using a Cs–P–Si oxide catalyst and the yield (*) and selectivity (&) using a SmOCl catalyst. Reaction conditions for Cs–P–Si oxide: F(propylene oxide), 0.05 cm3 min 1; F(CO2 as a liquid), 0.2 cm3 min 1; WHSV, 0.25 h 1; Cs–P–Si oxide, 10 g; 14 MPa; 200 8C. Reaction conditions for SmOCl: F(propylene oxide), 0.1 cm3 min 1; F(CO2 as a liquid), 0.4 cm3 min 1; WHSV, 0.5 h 1; SmOCl, 10 g; 14 MPa; 200 8C.
Fig. 2. Pressure dependence of (a) the conversion of propylene oxide (*) and the selectivity of propylene carbonate formation (&) and (b) the yield of propylene carbonate (*) using a Cs–P–Si oxide catalyst. Reaction conditions: catalyst, 1 g; propylene oxide, 4 cm3; 200 8C; 8 h.
Table 1 Propylene carbonate synthesis from carbon dioxide and propylene oxidea Entry
Catalyst
Pressure (MPa)
Yield (%)
Selectivity (%)
1 2b 3b 4b 5c 6 7 8d 9d,e
Cs–P–Si oxide SmOCl MgO Mg–Al oxide Cs–P–Si oxide Cs–Si oxide P–Si oxide Cs3PO4 Cs3PO4
8 14 14 14 8 8 8 8 8
94 58 23 24 92 43 <1 12 95
96 97 78 31 92 >99 <1 96 96
a b c d e
Reaction conditions: catalyst, 1 g; propylene oxide, 4 cm3; 200 8C; 8 h. Cited from Ref. [12]. DMF (5 cm3) was added. Cs3PO4 (0.28 g, 1 mol%) was used. Ethylene carbonate (5.2 g) was added as a solvent.
As Cs–P–Si oxide was found to effectively catalyze propylene carbonate synthesis in a batch reactor, we examined the catalytic stability of Cs–P–Si oxide using a continuous-flow reactor. Fig. 3 shows the time-course of yield and selectivity of propylene carbonate formation in the continuous-flow reaction. For the Cs– P–Si oxide catalyst, greater than 50% yield with excellent selectivity (>99%) was obtained in the initial period (3 h), whereas the yield gradually decreased with time on stream (t) to 18% at t = 16 h. For comparison, we include in Fig. 3 results obtained using SmOCl, which is an effective catalyst for this reaction [12]. Under these experimental conditions, the catalytic activity was maintained for 30 h, although the yield was not high. Elemental analysis of the product solution obtained in the run using the Cs–P–Si oxide catalyst demonstrated a Cs concentration of 50 ppm and a P concentration of 20 ppm at t = 16 h. This shows that the components of the catalyst were partially eluted into the product solution, thus decreasing the catalytic activity. Since Cs–P–Si oxide showed high catalytic activity in a batch reactor, it is possible that Cs- and/or P-containing species eluted from Cs–P–Si oxide function as efficient non-halogen catalysts. Thus, we examined the catalytic activity of Cs3PO4, one such possibly eluted species. As expected, Cs3PO4 catalyzed propylene carbonate synthesis (Table 1, entry 8), although the yield was considerably lower than in the case of Cs–P–Si oxide. The catalytic activity was greatly enhanced by the addition of ethylene carbonate (Table 1, entry 9), which probably increased the solubility of the catalyst. These results suggest that even phosphate species that are less nucleophilic than halides can promote the reaction at high temperatures under supercritical conditions. Therefore, it is conceivable that phosphate salts, including Cs3PO4, may be promising as non-halogen homogeneous catalysts for propylene carbonate synthesis.
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H. Yasuda et al. / Applied Catalysis A: General 298 (2006) 177–180
In summary, we have demonstrated that Cs–P–Si oxide, a typical acid–base bifunctional catalyst, is highly active but not stable enough to serve as a solid catalyst for propylene carbonate synthesis from CO2 and propylene oxide under supercritical conditions. We have also demonstrated that Cs3PO4 is an effective non-halogen homogeneous catalyst for this reaction. Acknowledgements We thank Mr. H. Tsuneki and Mr. T. Oku (Nippon Shokubai Co. Ltd.) for supplying Cs–P–Si oxide samples and for helpful discussions. We also thank Mr. S. Kitazume for helpful technical assistance. References [1] J.H. Clements, Ind. Eng. Chem. Res. 42 (2003) 663. [2] S. Fukuoka, M. Kawamura, K. Komiya, M. Tojo, H. Hachiya, K. Hasegawa, M. Aminaka, H. Okamoto, I. Fukawa, S. Konno, Green Chem. 5 (2003) 497. [3] D.J. Darensbourg, M.W. Holtcamp, Coord. Chem. Rev. 153 (1996) 155. [4] H.S. Kim, J.J. Kim, B.G. Lee, O.S. Jung, H.G. Jang, S.O. Kang, Angew. Chem. Int. Ed. 39 (2000) 4096.
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