Ind. Eng. Chem. Res. 2002, 41, 5353-5358
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Cycloaddition of Oxirane Group with Carbon Dioxide in the Supercritical Homogeneous State Takeshi Sako,* Toshiyuki Fukai, and Ryotaro Sahashi Department of Materials Science, Shizuoka University, 3-5-1 Johoku, Hamamatsu, Shizuoka 432-8561, Japan
Masato Sone Department of Organic and Polymer Materials Chemistry, Tokyo University of Agriculture & Technology, 2-24-16 Naka-cho, Koganei, Tokyo 184-8588, Japan
Mitsuo Matsuno Central Technical Research Laboratory, Nippon Mitsubishi Oil Co., Chidori-cho 8, Naka-ku, Yokohama, Kanagawa 231-0815, Japan
Cycloaddition of an oxirane group of 2-methyloxirane or 2,2-dimethyloxirane with carbon dioxide was carried out in both supercritical homogeneous and vapor-liquid coexisting regions to produce a cyclic carbonate compound of 4-methyl-1,3-dioxolan-2-one or 4,4-dimethyl-1,3-dioxolan-2-one. The yield of the cyclic carbonate exceeded 95%, when the reaction proceeded in the supercritical uniform condition. On the other hand, the yield was less than 5%, when the reaction occurred in the two-phase region. While almost all kinds of alkali metal halides exhibited the high catalytic activity in a polar liquid solvent, limited numbers of halides were found to have much activity in supercritical carbon dioxide. This might be due to differences in solvent properties such as polarity and dissolving power between the supercritical and liquid solvents. The selective separation of cyclic carbonate from the supercritical mixture of reactants and products was investigated by manipulating the temperature and pressure. When decreasing the temperature and pressure of the effluent from the reactor below the critical temperature and pressure of carbon dioxide, a condensate of cyclic carbonate with a purity of more than 99% was obtained from the outlet stream. Introduction Cyclic carbonates have many promising applications including environmentally friendly organic solvents, electrolyte solutions for lithium ion batteries, and monomers for polymeric materials.1-3 The synthesis from oxirane and carbon dioxide represents a typical example of successful utilization and chemical fixation of carbon dioxide. The reactions have so far been investigated in polar aprotic liquid solvents such as dimethylformamide and N-methylpyrrolidone. A variety of substances, from alkali metal salts to classical organometallic complexes, have been found to catalyze the formation of cyclic carbonates and polycarbonates. Koinuma et al. reported that a cyclic carbonate 4-methyl1,3-dioxolan-2-one was produced from the reaction of 2-methyloxirane, carbon dioxide, and hydrogen using a transition metal complex as catalyst in a liquid solvent under high-pressure conditions.4 Takeda et al. found that the reaction of 2-methyloxirane and carbon dioxide catalyzed by a porphyrin complex in liquid solvent gave 4-methyl-1,3-dioxolan-2-one under atmospheric pressure, where the polymerization of oxirane occurred simultaneously.5 Kihara et al. carried out a similar reaction to that of Takeda et al. using an alkali metal halide as the catalyst in liquid N-methylpyrrolidone at atmospheric pressure and obtained 4-phenoxymethyl1,3-dioxolan-2-one selectively.6 At the present time, * To whom correspondence should be addressed. Telephone: +81-53-478-1165.Fax: +81-53-478-1165.E-mail: ttsako@ipc. shizuoka.ac.jp.
some five-membered cyclic carbonates are being synthesized commercially from oxiranes and gaseous (not supercritical) carbon dioxide in a liquid solvent. The selectivity is more than 90% in many cases. However, the reaction is slow, and often more than 10 h are required to complete the reaction. Recently Lu et al. reported the synthesis of 4-methyl1,3-dioxolan-2-one from supercritical carbon dioxide and 2-methyloxirane with a complex catalyst.7 They showed that the reaction rate was high at first but decreased gradually as the reaction proceeded, owing to the precipitation of the liquid carbonate, which prevented contact between the catalyst and reactants. However, they did not discuss in detail the effect of the temperature, pressure, and catalyst on the reaction rate and product yield. In this paper, we report on a study of the optimum conditions for the formation and purification of cyclic carbonates using supercritical carbon dioxide as both reactant and solvent. This operation makes use of the excellent properties of supercritical carbon dioxide that may enhance the reaction rate by changing the reaction field from a coexisting vapor-liquid to a supercritical uniform phase and separate the carbonate selectively from the reaction mixture by adjusting the dissolving power. The effects of the operating variables such as the temperature, pressure, kind of catalyst, and phase conditions on the reaction rate and the desired product selectivity and yield were examined and discussed. Furthermore, the appropriate operating conditions of
10.1021/ie020164j CCC: $22.00 © 2002 American Chemical Society Published on Web 09/25/2002
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Ind. Eng. Chem. Res., Vol. 41, No. 22, 2002 Scheme 1
Figure 1. Experimental apparatus used for batch reaction, flow reaction, and selective separation of carbonate in supercritical condition.
temperature and pressure were investigated for cycloaddiition of the oxirane group and carbon dioxide and for the selective condensation of the carbonate from the reaction mixture in the supercritical state. Experimental Section Carbon dioxide with a minimum purity of 99.9% was purchased from Showa Tansan Co. Ltd. Two kinds of oxirane compounds, 2-methyloxirane and 2,2-dimethyloxirane, were provided by Wako Pure Chemicals Co. The minimum stated purities were 99 mol %. Alkali metal halides used as catalysts were commercially available. Lithium bromide with a purity of 99% was supplied by Junsei Co., and the others were supplied by Wako Pure Chemicals Co. All gases, catalysts, and chemicals were used without further purification. The experimental apparatus used for the batch and flow reactions under high-pressure conditions is illustrated in Figure 1. The reactor (4) was a stainless steel 316 vessel having a volume of 50 cm3 and sapphire view windows on both sides. It was placed in a temperature controlled air bath (5). The temperature variation of each run was observed to be less than 1.0 °C. The maximum working temperature and pressure were 200 °C and 40 MPa, respectively. The batch-mode operation was employed for the reaction analysis to investigate the effect of the operating variables including the temperature, pressure, and existing phase in the reaction field on the oxirane conversion and carbonate yield. The flow-mode operation was used for the selective separation of the carbonate from the supercritical reaction mixture and the solubility test of the catalyst in the supercritical condition. In the case of the batch reaction, the reactor was charged with given amounts of solid catalyst and oxirane compound. Carbon dioxide from the gas cylinder (1) was compressed and sent to the reactor below around 3 MPa of the reaction pressure using a high-pressure pump (3), and the stop valves V3 and V4 were closed. The reactor was then heated with the air bath to the reaction temperature. The second reactant, oxirane compound (2), was loaded using the high-pressure pump. The temperature of the reactor then decreased a little due to adding the second reactant without preheating. The temperature returned to the reaction temperature within a few minutes, and the pressure was finally adjusted to the reaction pressure by introducing carbon dioxide using the high-pressure pump. The starting time of the reaction was defined as the time of return to the reaction temperature. Phase behavior
in the reactor was observed through the sapphire windows in the reactor wall. After the reaction time passed, the reactor was cooled to 5 °C using water and the pressure of the carbon dioxide gas was reduced to atmospheric pressure. The reactor was subsequently opened, and the products and unreacted reactant were collected with water. The products and unreacted reactants were analyzed with a Shimadzu GC-17A gas chromatograph with an FID detector. A DB-5 capillary column of 0.25 mm in diameter × 30 m in length was used. The column temperature was 40-110 °C, and helium at 4 cm3/min was used as the carrier gas. In the case of the flow reaction, the same reactor as that used for the batch reaction was employed. About 3 g of the catalyst was loaded into the reactor. The reactor was heated to the reaction temperature in the air bath. Carbon dioxide and oxirane compound were pressurized and delivered into the reactor using the high-pressure pumps. The pressure was kept constant by the first back-pressure regulator (6). At the beginning, only supercritical carbon dioxide passed through the reactor. When the temperature, pressure, and flow rate reached a steady state, it was allowed to mix with preheated oxirane compound. When recovering a desired product from the supercritical reaction mixture, the product was condensed in a product separation vessel (8) with a volume of 200 cm3, whose temperature and pressure were controlled by a water bath (9) and the second backpressure regulator (7). All flow reactions were carried out in the supercritical phase. Results and Discussion For the synthesis of a cyclic carbonate from carbon dioxide and an oxirane compound in N-methylpyrrolidone under atmospheric pressure, Kihara et al. indicated that their reaction proceeded with high yield in the presence of an alkali metal halide as catalyst.6 In this work, the same catalyst was used for the cycloaddition of an oxirane compound with carbon dioxide in a supercritical condition. The product yield was defined as the molar ratio of the product to the fed oxirane compound, and the product selectivity was defined as the molar ratio of the product to the reacted oxirane compound. 2-Methyloxirane of 10 g (172 mmol) was reacted with carbon dioxide using 0.15 g of lithium bromide at various temperatures from 50 to 100 °C for pressures of 6, 8, and 10 MPa and a reaction time of 2 h in a batch reactor. Lithium bromide was the catalyst with the amount corresponding to 1 mol % of 2-methyloxirane (Scheme 1). Figure 2 shows the temperature dependence of the yield of the desired product 4-methyl-1,3-dioxolan-2-one at various pressure conditions. The product yield of each isobaric line increased steeply from less than 5% to more than 95% in a narrow temperature region. The rising temperature moved to a lower level when the pressure increased. The main reason for this phenomenon might be a change of the catalyst environment. As seen
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Figure 2. Temperature dependence of yield of 4-methyl-1,3dioxolan-2-one at various pressure conditions using lithium bromide catalyst.
Figure 3. Degree of reaction progress at various temperature and pressure conditions for cycloaddition of 2-methyloxirane with carbon dioxide.
through the view window, there was a dramatic change of phase surrounding the catalyst from a coexisting vapor-liquid to a supercritical homogeneous state, when the reaction temperature increased under a constant pressure condition: that is, a critical locus existed near the yield-rising temperature for each isobaric line. This result clearly shows that the supercritical homogeneous region could accelerate the reaction considerably. This might be due to the disappearance of the diffusion barrier between the vapor-liquid interface, the high diffusivity of the reactants in the supercritical phase, and the increase in collision probability of the reactants and catalyst. A similar acceleration of the reaction was observed for the hydrogenation of carbon dioxide in the supercritical state.8 Figure 3 shows the degree of reaction progress at various temperature and pressure conditions. The cycloaddition completed in the temperature and pressure region shown by the open squares, where the supercritical homogeneous phase was kept in the reactor during the whole reaction. The reaction did not occur in the region shown by the solid circles, where the vapor-liquid twophase region existed from the beginning. On the other hand, the reaction commenced but soon stopped in the region shown by the ×’s, where the supercritical homo-
Figure 4. Temperature dependence of yield of 4,4-dimethyl-1,3dioxolan-2-one at 12 MPa using lithium bromide catalyst.
Scheme 2
geneous phase existed initially but changed to a twophase region as the reaction proceeded. The boundary between the supercritical homogeneous phase and the vapor-liquid two-phase regions would exist near the dotted line. Judging from the results, the uniform reaction system was important in realizing high product selectivity as well as high reaction rates. The cycloaddition of 2,2-dimethyloxirane with carbon dioxide using lithium bromide was carried out at various temperature and pressure conditions in order to compare it with that of 2-methyloxirane and carbon dioxide (Scheme 2). Figure 4 shows the yield of 4,4-dimethyl-1,3-dioxolan2-one at various temperatures for a pressure of 12 MPa and a reaction time of 2 h. The carbonate yield increased sharply in the same manner as that of the reaction of 2-methyloxirane with carbon dioxide. However, there was a difference between the two reactions. The rising temperature shifted to the higher side of 100 °C for 2,2dimethyloxirane. This was because 2,2-dimethyloxirane with its larger molecular weight required a higher temperature than 2-methyloxirane to dissolve completely into the supercritical carbon dioxide. The phase change from a coexisting vapor-liquid to a supercritical uniform state was also observed as the temperature increased. As shown in Figure 5, the extent of reaction progress was classified into three groups in the same way as the cycloaddition of 2-methyloxirane with carbon dioxide. However, it should be noted that the partly completed reaction curve given by a broken line moved upward, compared with the corresponding curve of 2-methyloxirane. This occurred for the same reason as that for the shift of the rising temperature of the yield to the higher side. Catalytic activity among nine kinds of alkali metal halides was compared for the cycloaddition of 2-methyloxirane with supercritical carbon dioxide at 100 °C and 14 MPa for a reaction time of 2 h in a batch reactor.
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Figure 5. Degree of reaction progress at various temperature and pressure conditions for cycloaddition of 2,2-dimethyloxirane with carbon dioxide. Table 1. Comparison of Catalytic Activity for Reaction of 2-Methyloxirane with Carbon Dioxide at Supercritical Conditions (Reaction Conditions: 100 °C, 14 MPa, 2 h, 172 mmol of 2-methyloxirane, catalyst 1 mol % of 2-methyloxirane) metal halide
conversion (%)
metal halide
conversion (%)
LiCl LiBr LiI NaCl NaBr
0 97 94 0 0
NaI KCl KBr KI
63 0 0 11
The phase in the reactor remained homogeneous during the reaction, even when the reaction proceeded and the composition changed. The experimental results are given in Table 1. Kihara et al. reported that almost all alkali metal halides have high catalytic activity in a polar aprotic liquid solvent such as N,N-dimethylformamide or N-methylpyrrolidone. The order of catalytic activity was lithium salt > sodium salt > potassium salt for cations and chloride > bromide > iodide for halides. However, only four halides were found to perform highly at a supercritical condition with the order of catalytic activity being lithium bromide > lithium iodide > sodium iodide > potassium iodide, indicating that the alkali metal halides comprising a halogen anion with large ionic diameter and a metal cation with small ionic diameter would work well in the supercritical state. The difference in catalytic activity in the liquid and supercritical solvents might be due to the differences in solvent properties such as polarity and dissolving power of the catalyst in both solvents. The relationship between the yield of 4-methyl-1,3dioxolan-2-one and the amount of catalyst was investigated. Lithium bromide, which had the highest catalytic activity, was used in this experiment. The result is shown in Figure 6, where the catalyst concentration was the mol % of 2-methyloxirane. The yield decreased with increase in the concentration of catalyst. This was due to acceleration of side reactions of the catalyst. The optimum concentration of catalyst was found to be 1 mol % of the oxirane compound. From the viewpoint of both a mechanistic study and industrial application, it is important to know whether the catalyst dissolves in a supercritical reaction mixture. Lithium bromide, which was the most active catalyst,
Figure 6. Relationship between carbonate yield and catalyst concentration of lithium bromide at 100 °C and 8 MPa.
dissolved in the following order: liquid carbonate > liquid oxirane > supercritical carbon dioxide. Judging from observations of the inside of the reactor through the sapphire windows, the catalyst did not dissolve in the binary supercritical mixture of the reactants with a molar ratio of carbon dioxide to 2-methyloxirane of 20. Whether or not the catalyst would dissolve in the ternary supercritical mixture of the reactants (carbon dioxide and 2-methyloxirane) and the main product (4methyl-1,3-dioxolan-2-one) was checked, where the main product was polar and a good solvent for an electrolyte such as the catalyst used in this work. The reaction of 2-methyloxirane and carbon dioxide using lithium bromide was carried out at 80 °C and 10 MPa for 48 h using the flow reactor. The flow rates of carbon dioxide and 2-methyloxirane were 0.184 and 0.0121 g/min, which corresponded to 20 times the molar ratio of carbon dioxide to 2-methyloxirane. The residence time in the reactor was about 60 min. Bromide anions in the effluent were analyzed by ion chromatography. The average concentration of bromide anions for three runs was around 0.1% on the basis of the liquid mixture of 2-methyloxirane and carbonate. As the concentration of the catalyst in the effluent from the reactor was very low, the catalyst loss would be small. A mechanistic study of the reaction of 2-methyloxirane and carbon dioxide in a polar liquid solvent under atmospheric pressure was reported by Kihara et al.,6 Darensbourg et al.,9 and Yano et al.10 Typical reaction pathways are given as follows: There are two possible reaction pathways in the supercritical condition: Scheme 3 for the catalyst dissolving model and Scheme 4 for the solid catalyst model. Scheme 3 is established in a polar liquid solvent. The halide catalyst dissolves in the solvent and dissociates into ions. On the other hand, Scheme 4 is favorable to a catalyst which does not dissolve in the solvent. In the case of a supercritical solvent, it is difficult to determine which scheme is appropriate, because although the solubility of the halide catalyst is very low, it still dissolves in the supercritical mixture of the reactants and products. Further study is necessary to determine the real reaction pathway. The typical composition of the products for each catalyst is given in Table 2. As 1 mol of all kinds of products including byproducts is synthesized from 1 mol of 2-methyloxirane, the composition of each product is
Ind. Eng. Chem. Res., Vol. 41, No. 22, 2002 5357 Scheme 3
Scheme 4
Table 2. Compositions of the Product Mixture of Cycloaddition of 2-Methyloxirane with Carbon Dioxide in the Supercritical State (Reaction Conditions: 80 °C, 8 MPa, 2 h, 172 mmol of 2-methyloxirane, catalyst 1 mol % of 2-methyloxirane) compositions of product mixture (mol %) run no.
catalyst propanone
1 2 3 4
LiBr LiI NaI KI
1.01 0.96 1.40 1.00
propanal 0.12 0.06 0.06 1.21
1,2-propanediol 1-halo-2-propanol (A) 2-halo-1-propanol (B) 4-methyl-1,3-dioxolan-2-one 0.20 0.20 0.17 0.21
1.20 2.25 1.31 2.12
0.26 0.27 0.17 0.16
97.2 96.3 96.9 95.3
a (A): 1-halo-2-propanol is 1-bromo-2-propanol for run no. 1, and 1-iodo-2-propanol for run no. 2,3,4. (B): 2-halo-1-propanol is 2-bromo1-propanol for run no. 1, and 2-iodo-1-propanol for run no. 2,3,4.
Table 3. Selective Condensation of 4-Methyl-1,3-dioxolan-2-one from Reaction Mixture by Changing Temperature and Pressure reaction conditions run no. 1 2 3 4 5 6 7 8 9 a
separation conditions
composition of condensatesa (mol %)
P1 (MPa)
T1 (°C)
P2 (MPa)
T2 (°C)
A
B
C
20 20 20 20 20 20 20 20 20
100 100 100 100 100 100 100 100 100
0.1 2.0 2.0 2.0 2.0 4.0 4.0 4.0 4.0
20 20 40 60 80 20 40 60 80
57.3 50.2 22.4 4.9 0.6 7.4 0.5 0.2 0.1
0.4 0.4 1.0 0.3 1.1 0.3 0.2 0.6 0.5
42.3 49.4 76.6 94.8 98.3 92.3 99.3 99.2 99.4
A ) 2-methyloxirane; B ) byproducts; C ) 4-methyl-1,3-dioxolan-2-one.
equal to the selectivity for the corresponding product. The main product was the desired compound, 4-methyl1,3-dioxolan-2-one. The minor byproducts were propanone, propanal, 1,2-propanediol, 1-halo-2-propanol, and 2-halo-1-propanol with the total amount being less than 5 mol %. Although the catalytic activity represented by the 2-methyloxirane conversion was different for each of these catalysts, the product selectivities were almost the same. The selective separation of the main product 4-methyl-1,3-dioxolan-2-one from the supercritical mixture of the reactants and products was examined using the flow reactor equipped with a product separation vessel. The experimental apparatus shown in Figure 1 was used, which was the same as that used for the batch reaction. All stop valves in the line, indicated by V1-V4, were open for the flow reaction. The temperature and pressure of the product separation vessel were lower than those of the reactor in order to condense the desired product from the supercritical mixture. A proper condition must be selected to precipitate only the required component from the mixture. The experimental conditions for the reactor and product separation vessel were as follows: the reaction temperature and pressure were 100 °C and 20 MPa so that the supercritical homogeneous phase remained in the reactor; the separation temperature and pressure were changed from 20 to 80 °C and from 0.1 to 4 MPa, and the flow rates of carbon
dioxide and 2-methyloxirane were 1.61 and 0.106 g/min, respectively. As the volume of the reactor was 50 cm3, the residence time was estimated, by using the equation of state for carbon dioxode,11 to be 15 min. The composition of the condensate in the product separation vessel is summarized in Table 3. All components in the effluent from the reactor except carbon dioxide condensed under the separation conditions of run no. 1 in Table 3. Therefore, the composition of the condensate of run no. 1 was the same as the composition at the exit of the reactor on a carbon dioxide-free basis. When the temperature of the product separation vessel increased from 20 to 80 °C at a constant pressure of 2 MPa, the composition of the carbonate increased steeply from 49.4 to 98.3 mol %. The same increase in temperature at 4 MPa caused a further increase in composition of the carbonate from 92.3 to 99.4 mol %. This was due to the fact that the solubility of the carbonate with larger molecular weight was smaller than that of 2-methyloxirane with smaller molecular weight. As a result, the selective condensation of the target component 4-methyl-1,3-dioxolan-2-one was possible at temperatures of 40-80 °C for a pressure of 4 MPa. Conclusions Cycloaddition of the oxirane group with carbon dioxide was studied in both supercritical homogeneous and
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vapor-liquid coexisting conditions. It was found that the reaction proceeded completely and rapidly in the supercritical state, while it hardly occurred in the coexisting vapor-liquid state. Catalytic activity in the supercritical solvent was different from that in the liquid solvent. Among the alkali metal halides with excellent catalytic activity in a polar liquid solvent, some exhibited high catalytic activity even in the supercritical condition while others gave poor or no activity. The dissolving power of the supercritical solvent could be controlled easily by adjusting the temperature and pressure. When the temperature and pressure of the effluent from the reactor decreased to 40 °C and 4 MPa, the condensate of the cyclic carbonate with more than 99 mol % purity was deposited from the effluent. A simplified production process combining the reaction and separation processes can be realized by using supercritical carbon dioxide. Acknowledgment This work was supported by NEDO for the Leading Research Program of Advanced Utilization Technology of Supercritical Fluids in New Sunshine Program, AIST, from 1997-1999. Literature Cited (1) Peppel, W. J. Preparation and Properties of the Alkylene Carbonates. Ind. Eng. Chem. 1958, 50, 767. (2) Yamazaki, N.; Nakahama, S. Polymers Derived from Carbon Dioxide and Carbonates. Ind. Eng. Chem. Prod. Res. Dev. 1979, 18, 249.
(3) Rokicki, G.; Pawlicki, J.; Kuran, W. Poly(ether-Carbonate)s from Diphenolates, Cyclic Carbonates, and Dihalo Compounds. Polym. J. 1985, 17, 509. (4) Koinuma, H.; Kato, H.; Hirai, H. Synthesis of 1,2-Propanediol Formates from Carbon Dioxide, Hydrogen, and Methyloxirane Catalyzed by Transition Metal Complex. Chem. Lett. 1977, 517. (5) Takeda, N.; Inoue, S. Activation of Carbon Dioxide by Tetraphenylporphinato-aluminium Methoxide. Reaction with Epoxide. Bull. Chem. Soc. Jpn. 1978, 51, 3564. (6) Kihara, N.; Hara, N.; Endo, T. Catalytic Activity of Various Salts in the Reaction of 2,3-Epoxypropyl Phenyl Ether and Carbon Dioxide under Atmospheric Pressure. J. Org. Chem. 1993, 58, 6198. (7) Lu, X. B.; Pan, Y. Z.; Ji, D. F.; He, R. Catalytic Formation of Propylene Carbonate from Supercritical Carbon Dioxide/Propylene Oxide Mixture. Chin. Chem. Lett. 2000, 11, 589. (8) Jessop, P. G.; Hsiao, Y.; Ikariya, T.; Noyori, R. Homogeneous Catalysis in Supercritical Fluids: Hydrogenation of Supercritical Carbon Dioxide to Formic Acid, Alkyl Formates, and Formamides. J. Am. Chem. Soc. 1996, 118, 344. (9) Darensbourg, D. J.; Holtcamp, M. W. Catalysis for the Reaction of Epoxides and Carbon Dioxide. Coord. Chem. Rev. 1996, 153, 155. (10) Yano, T.; Matsui, H.; Koike, T.; Ishiguro, H.; Fujihara, H.; Yoshihara, M.; Maeshima, T. Magnesium Oxide-catalysed Reaction of Carbon Dioxide with an Epoxide with Retention of Stereochemistry. Chem. Commun. 1997, 1129. (11) Huang, F.-H.; Li, M.-H.; Lee, L. L.; Starling, K. E. An Accurate Equation of State for Carbon Dioxide. J. Chem. Eng. Jpn. 1985, 18, 490.
Received for review February 26, 2002 Revised manuscript received August 8, 2002 Accepted August 11, 2002 IE020164J
