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Synthesis of propylene carbonate from urea and 1,2-propanediol
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Chinese Chemical Letters 20 (2009) 131–135 www.elsevier.com/locate/cclet
Synthesis of propylene carbonate from urea and 1,2-propanediol Zhi Wen Gao a,b, Shou Feng Wang a, Chun Gu Xia a,* a
State Key Laboratory for Oxo Synthesis and Seletive Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 73000, China b Graduate School of the Chinese Academy of Sciences, Beijing 100039, China Received 11 June 2008
Abstract The production of propylene carbonate (PC) from urea and 1,2-propanediol (PG) was investigated in a batch process. The catalytic performances of zinc chloride and magnesium chloride were investigated for this reaction system. The influences of various operation conditions on the PC yield were explored. In this work, MgCl2 and ZnCl2 showed the excellent catalytic activity toward PC synthesis, and the yields of propylene carbonate reached 96.5% and 92.4%, respectively. The optimum reaction conditions were as follows: ethanol/urea molar ratio of 4, catalyst concentration of 1.5%, reaction temperature of 160 8C, reaction time of 3 h, respectively. The route from urea and 1,2-propanediol shows advantages, such as mild reaction condition and safe operation. The catalytic system is environmentally benign. # 2008 Chun Gu Xia. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. Keywords: Urea; 1,2-Propanediol; Propylene carbonate
Propylene carbonates are commercially important compounds and are used widely in the fields of organic synthesis, gas separation, electrochemistry, metal extraction, etc. In addition to their biodegradability and high solvency, they have high boiling and flash points, low odor levels and evaporation rate and low toxicities [1–4]. At present, PC is mostly used for the production of dimethyl carbonate (DMC) through transesterification with methanol, which promotes the development of a new route for synthesis of PC [5]. Conventionally, cyclic carbonates are mainly synthesized by the cycloaddition of carbon dioxide to cyclic oxides, which causes serious safety problems because propylene oxide is a dangerous chemical substance [6]. Compared with the traditional routes for synthesis of propylene carbonates, the route from urea and 1,2-propanediol shows advantages, such as cheap and easily available feedstock, mild reaction condition and safe operation. In addition, the byproduct ammonia can be recycled to the urea production unit. Even more important is, by the present route, that PG, as a byproduct in the transesterification process for the production of DMC according to Eqs. (1) and (2), can be reconverted into the raw material (Scheme 1) [7]. This would increase the efficiency of utilization of the raw material and greatly lower the cost for the production of DMC. Because of its bright prospects, the synthesis of PC from urea and PG has attracted much attention from both industrial and academic fields.
* Corresponding author. E-mail address: [email protected] (C.G. Xia). 1001-8417/$ – see front matter # 2008 Chun Gu Xia. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. doi:10.1016/j.cclet.2008.10.038
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Scheme 1. The mechanism of synthesis propylene carbonate and dimethyl carbonate.
At present, the achievement on alcoholysis of urea mainly comes from research on the catalyst. Su and Speranza [8] used urea and alkylene glycol to prepare the corresponding alkylene carbonates over catalyst or without any catalysts for the first time, conversion of the alkylene glycol was only 66% and there was a severe decomposition of urea. Zhao et al. [9] improved the yield of propylene carbonates up to 94% by using a catalyst of dehydrated zinc acetate which was the innovative work. Recently, Li et al. [10] succeeded in increasing the yield of PC by using a catalyst of zinc oxide. They considered that ZnO et al. were favorable to promote urea decomposition to form the isocyanate species, and the formation of isocyanate species was the key to urea alcoholysis. The catalytic activity of urea decomposition was consistency to the catalytic performance for synthesis of propylene carbonate. In our work, the MgCl2 and ZnCl2 were used to facilitate the production of PC from urea and PG. Several operation factors such as reaction temperature, the initial molar ratio of feed materials and catalyst concentration were investigated. 1. Experimental The reaction was performed in a 100 mL three-necked-flask, which was equipped with a magnetic stirrer, reflux condenser and thermocouple thermometer. Urea, 1,2-propanediol and catalyst were charged into the reactor according to the given amount. The reactor was firstly flushed with nitrogen to replace air within the reactor. Then the reactor was adjusted to the desired initial pressure and heated to the reaction temperature with stirring. The pressure of reactor was maintained at a definite value, a great deal of ammonia produced during the experiment was released from the reaction system passing through an absorption device. After the experiment, the reactor was cooled to room temperature, and the product mixture in the reactor was clarified and analyzed. The products were identified using Aglient-6890 equipped with a HP-1MS capillary column and Aglient-5973 mass selective detector by comparing retention times and fragmentation patters with authentic samples, and quantificationally analyzed by gas chromatography (Aglient-6820) equipped with a SE-54 (50 m 0.32 mm 0.5 mm) capillary column and a FID detector. The anisoin was used as internal standard to the quantitative analysis (Fig. 1). 2. Results and discussion The amount of catalyst is the main factor effecting the reaction of urea and PG to synthesize PC, so firstly we examined the effect of catalyst concentration on the reaction. The synthesis of PC without catalyst or with catalysts was conducted under the following conditions: reaction temperature, 170 8C; reaction time, 2 h; molar ratio of urea to PG is 4, zinc chloride (ZnCl2) and magnesium chloride (MgCl2) were used as the catalyst in the experiments individually, all of them were commercial reagent.
Fig. 1. GC spectrum obtained by Aglient-6820.
Z.W. Gao et al. / Chinese Chemical Letters 20 (2009) 131–135
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Fig. 2. Effect of the catalysts concentration on the yields of PC (n(urea):n(PG) = 1:4, 170 8C, 2 h).
As shown in Fig. 2 without catalyst only yielded 15.3% PC. The PC yields increase significantly with an increase of n(catalysts):n(PG) from naught to 0.5%. However, when the catalyst concentration was 1.5%, the PC yield reached the maximum value, and the PC yield declined with the increase of catalyst loading. The effect of catalysts concentration on PC yield shows that PC yield increases with the increase of catalyst loading. The reason is that the number of catalytic active sites in the reaction system was increased and the reaction rate was accelerated. The higher catalyst concentration may cause the consumption of PC by side reactions. The influence of the reaction temperature on the PC yield is shown in Fig. 2. The reaction conditions are the same as those in above-mentioned conditions but the catalysts concentration is 1.5% and the reaction temperature is a variant. The top limit of the reaction temperature is defined at 180 8C because the normal boiling point of PG is 188.2 8C. It can be seen from Fig. 3 that, with the elevation of the reaction temperature, the PC yields increase first and reaches their maximum at 160 8C, and then decrease because of the higher reaction temperature may accelerate the side reactions. Although the conversion of urea rose obviously, the yields of PC declined rapidly. Fig. 4 shows the influence of PG/urea initial molar ratio on PC yield. The reaction conditions are the same as the above-mentioned conditions except the temperature is 160 8C and the n(urea):n(PG) is a variant. It can be determined from this figure that PC yield increases with the increase of PG/urea molar ratio from 2 to 4. However, the reaction rate is low when the molar ratio of PG/urea is higher than 4. Because the concentration of urea is much lower, which results in a lower reaction rate and then in a lower PC yield. It is uneconomical for the PC synthesis. When the molar ratio of PG/urea is lower, urea concentration is higher in the reactor and the side reactions would take place actively in the
Fig. 3. Effect of the reaction temperature on the yields of PC (n(PG):n(urea):n(catalysts) = 4:1:0.015).
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Fig. 4. Effect of the molar ratio (PG/urea) on the yields of PC (n(PG):n(urea):n(catalysts) = 4:1:0.015, 160 8C, 2 h).
Fig. 5. Effect of the reaction time on the yields of PC (n(PG):n(urea):n(catalysts) = 4:1:0.015, 160 8C).
reaction temperature range. At the same time, a large amount of urea is vaporized and adhered to the surface walls of condenser. The mass loss of urea led to the lower PC yield, the optimum initial molar ratio of PG and urea was 4. The effect of reaction time on PC synthesis illustrated that the yield of PC increased with the reaction time (see Fig. 5). The reaction conditions are the same as the above-mentioned conditions except n(urea):n(PG) 1:4 and the reaction time is a variant. Because the initial concentration of urea is larger, the reaction proceeded rapidly within first 0.5 h, and then the yields of PC increased slightly. The PC yields reach the maximum value at 92.4% and 96.5% catalyzed by MgCl2 and ZnCl2 after 3 h individually, and then the PC yields gradually decreases with the prolonging of the reaction time. The reason is thought to be that, when the PC concentration in the reactor was comparatively higher, the side reactions such as the polymerization of PC will occur in accordance with the major reactions. The optimum reaction time was 3 h for the PC synthesis. 3. Conclusions A process was proposed for synthesis of PC from urea and PG has been made. The operation conditions have a significant influence on the PC yield. The optimal reaction conditions over zinc chloride and magnesium chloride are as follows: at a reaction temperature of 160 8C, reaction time of 3 h, n(PG):n(urea):n(catalysts) = 4:1:0.015. The highest yields of PC is 96.5% from urea and PG with magnesium chloride as catalyst.
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Acknowledgment This work was financially supported by the National Natural Science Fund for Distinguished Young Scholars of China (No. 20625308). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
X. Zhao, N. Sun, S. Wang, et al. Ind. Eng. Chem. Res. 47 (2008) 1365. M.S. Shaharun, H. Mukhtar, B.K. Dutt, Chem. Eng. Sci. 63 (2008) 3024. Y. Du, L.N. He, D.L. Kong, Catal. Commun. 9 (2008) 1754. L.F. Xiao, Q.F. Yue, C.G. Xia, et al. J. Mol. Catal. A: Chem. 279 (2008) 230. H.Y. Ju, M.D. Manju, K.H. Kim, et al. Korean J. Chem. Eng. 24 (2007) 917. I. Kim, M.J. Yi, K.J. Lee, et al. Catal. Today 111 (2006) 292. Y. Zhao, L.N. He, Y.Y. Zhuang, et al. Chin. Chem. Lett. 19 (2008) 286. W.Y. Su, G.P. Speranza, EP Patent 0443758A1 (1991). X. Zhao, Y. Zhang, Y. Wang, Ind. Eng. Chem. Res. 43 (2004) 4032. Q. Li, N. Zhao, W. Wei, et al. J. Mol. Catal. A: Chem. 270 (2007) 44.
Chinese Chemical Letters 20 (2009) 131–135 www.elsevier.com/locate/cclet
Synthesis of propylene carbonate from urea and 1,2-propanediol Zhi Wen Gao a,b, Shou Feng Wang a, Chun Gu Xia a,* a
State Key Laboratory for Oxo Synthesis and Seletive Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 73000, China b Graduate School of the Chinese Academy of Sciences, Beijing 100039, China Received 11 June 2008
Abstract The production of propylene carbonate (PC) from urea and 1,2-propanediol (PG) was investigated in a batch process. The catalytic performances of zinc chloride and magnesium chloride were investigated for this reaction system. The influences of various operation conditions on the PC yield were explored. In this work, MgCl2 and ZnCl2 showed the excellent catalytic activity toward PC synthesis, and the yields of propylene carbonate reached 96.5% and 92.4%, respectively. The optimum reaction conditions were as follows: ethanol/urea molar ratio of 4, catalyst concentration of 1.5%, reaction temperature of 160 8C, reaction time of 3 h, respectively. The route from urea and 1,2-propanediol shows advantages, such as mild reaction condition and safe operation. The catalytic system is environmentally benign. # 2008 Chun Gu Xia. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. Keywords: Urea; 1,2-Propanediol; Propylene carbonate
Propylene carbonates are commercially important compounds and are used widely in the fields of organic synthesis, gas separation, electrochemistry, metal extraction, etc. In addition to their biodegradability and high solvency, they have high boiling and flash points, low odor levels and evaporation rate and low toxicities [1–4]. At present, PC is mostly used for the production of dimethyl carbonate (DMC) through transesterification with methanol, which promotes the development of a new route for synthesis of PC [5]. Conventionally, cyclic carbonates are mainly synthesized by the cycloaddition of carbon dioxide to cyclic oxides, which causes serious safety problems because propylene oxide is a dangerous chemical substance [6]. Compared with the traditional routes for synthesis of propylene carbonates, the route from urea and 1,2-propanediol shows advantages, such as cheap and easily available feedstock, mild reaction condition and safe operation. In addition, the byproduct ammonia can be recycled to the urea production unit. Even more important is, by the present route, that PG, as a byproduct in the transesterification process for the production of DMC according to Eqs. (1) and (2), can be reconverted into the raw material (Scheme 1) [7]. This would increase the efficiency of utilization of the raw material and greatly lower the cost for the production of DMC. Because of its bright prospects, the synthesis of PC from urea and PG has attracted much attention from both industrial and academic fields.
* Corresponding author. E-mail address: [email protected] (C.G. Xia). 1001-8417/$ – see front matter # 2008 Chun Gu Xia. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. doi:10.1016/j.cclet.2008.10.038
132
Z.W. Gao et al. / Chinese Chemical Letters 20 (2009) 131–135
Scheme 1. The mechanism of synthesis propylene carbonate and dimethyl carbonate.
At present, the achievement on alcoholysis of urea mainly comes from research on the catalyst. Su and Speranza [8] used urea and alkylene glycol to prepare the corresponding alkylene carbonates over catalyst or without any catalysts for the first time, conversion of the alkylene glycol was only 66% and there was a severe decomposition of urea. Zhao et al. [9] improved the yield of propylene carbonates up to 94% by using a catalyst of dehydrated zinc acetate which was the innovative work. Recently, Li et al. [10] succeeded in increasing the yield of PC by using a catalyst of zinc oxide. They considered that ZnO et al. were favorable to promote urea decomposition to form the isocyanate species, and the formation of isocyanate species was the key to urea alcoholysis. The catalytic activity of urea decomposition was consistency to the catalytic performance for synthesis of propylene carbonate. In our work, the MgCl2 and ZnCl2 were used to facilitate the production of PC from urea and PG. Several operation factors such as reaction temperature, the initial molar ratio of feed materials and catalyst concentration were investigated. 1. Experimental The reaction was performed in a 100 mL three-necked-flask, which was equipped with a magnetic stirrer, reflux condenser and thermocouple thermometer. Urea, 1,2-propanediol and catalyst were charged into the reactor according to the given amount. The reactor was firstly flushed with nitrogen to replace air within the reactor. Then the reactor was adjusted to the desired initial pressure and heated to the reaction temperature with stirring. The pressure of reactor was maintained at a definite value, a great deal of ammonia produced during the experiment was released from the reaction system passing through an absorption device. After the experiment, the reactor was cooled to room temperature, and the product mixture in the reactor was clarified and analyzed. The products were identified using Aglient-6890 equipped with a HP-1MS capillary column and Aglient-5973 mass selective detector by comparing retention times and fragmentation patters with authentic samples, and quantificationally analyzed by gas chromatography (Aglient-6820) equipped with a SE-54 (50 m 0.32 mm 0.5 mm) capillary column and a FID detector. The anisoin was used as internal standard to the quantitative analysis (Fig. 1). 2. Results and discussion The amount of catalyst is the main factor effecting the reaction of urea and PG to synthesize PC, so firstly we examined the effect of catalyst concentration on the reaction. The synthesis of PC without catalyst or with catalysts was conducted under the following conditions: reaction temperature, 170 8C; reaction time, 2 h; molar ratio of urea to PG is 4, zinc chloride (ZnCl2) and magnesium chloride (MgCl2) were used as the catalyst in the experiments individually, all of them were commercial reagent.
Fig. 1. GC spectrum obtained by Aglient-6820.
Z.W. Gao et al. / Chinese Chemical Letters 20 (2009) 131–135
133
Fig. 2. Effect of the catalysts concentration on the yields of PC (n(urea):n(PG) = 1:4, 170 8C, 2 h).
As shown in Fig. 2 without catalyst only yielded 15.3% PC. The PC yields increase significantly with an increase of n(catalysts):n(PG) from naught to 0.5%. However, when the catalyst concentration was 1.5%, the PC yield reached the maximum value, and the PC yield declined with the increase of catalyst loading. The effect of catalysts concentration on PC yield shows that PC yield increases with the increase of catalyst loading. The reason is that the number of catalytic active sites in the reaction system was increased and the reaction rate was accelerated. The higher catalyst concentration may cause the consumption of PC by side reactions. The influence of the reaction temperature on the PC yield is shown in Fig. 2. The reaction conditions are the same as those in above-mentioned conditions but the catalysts concentration is 1.5% and the reaction temperature is a variant. The top limit of the reaction temperature is defined at 180 8C because the normal boiling point of PG is 188.2 8C. It can be seen from Fig. 3 that, with the elevation of the reaction temperature, the PC yields increase first and reaches their maximum at 160 8C, and then decrease because of the higher reaction temperature may accelerate the side reactions. Although the conversion of urea rose obviously, the yields of PC declined rapidly. Fig. 4 shows the influence of PG/urea initial molar ratio on PC yield. The reaction conditions are the same as the above-mentioned conditions except the temperature is 160 8C and the n(urea):n(PG) is a variant. It can be determined from this figure that PC yield increases with the increase of PG/urea molar ratio from 2 to 4. However, the reaction rate is low when the molar ratio of PG/urea is higher than 4. Because the concentration of urea is much lower, which results in a lower reaction rate and then in a lower PC yield. It is uneconomical for the PC synthesis. When the molar ratio of PG/urea is lower, urea concentration is higher in the reactor and the side reactions would take place actively in the
Fig. 3. Effect of the reaction temperature on the yields of PC (n(PG):n(urea):n(catalysts) = 4:1:0.015).
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Fig. 4. Effect of the molar ratio (PG/urea) on the yields of PC (n(PG):n(urea):n(catalysts) = 4:1:0.015, 160 8C, 2 h).
Fig. 5. Effect of the reaction time on the yields of PC (n(PG):n(urea):n(catalysts) = 4:1:0.015, 160 8C).
reaction temperature range. At the same time, a large amount of urea is vaporized and adhered to the surface walls of condenser. The mass loss of urea led to the lower PC yield, the optimum initial molar ratio of PG and urea was 4. The effect of reaction time on PC synthesis illustrated that the yield of PC increased with the reaction time (see Fig. 5). The reaction conditions are the same as the above-mentioned conditions except n(urea):n(PG) 1:4 and the reaction time is a variant. Because the initial concentration of urea is larger, the reaction proceeded rapidly within first 0.5 h, and then the yields of PC increased slightly. The PC yields reach the maximum value at 92.4% and 96.5% catalyzed by MgCl2 and ZnCl2 after 3 h individually, and then the PC yields gradually decreases with the prolonging of the reaction time. The reason is thought to be that, when the PC concentration in the reactor was comparatively higher, the side reactions such as the polymerization of PC will occur in accordance with the major reactions. The optimum reaction time was 3 h for the PC synthesis. 3. Conclusions A process was proposed for synthesis of PC from urea and PG has been made. The operation conditions have a significant influence on the PC yield. The optimal reaction conditions over zinc chloride and magnesium chloride are as follows: at a reaction temperature of 160 8C, reaction time of 3 h, n(PG):n(urea):n(catalysts) = 4:1:0.015. The highest yields of PC is 96.5% from urea and PG with magnesium chloride as catalyst.
Z.W. Gao et al. / Chinese Chemical Letters 20 (2009) 131–135
135
Acknowledgment This work was financially supported by the National Natural Science Fund for Distinguished Young Scholars of China (No. 20625308). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
X. Zhao, N. Sun, S. Wang, et al. Ind. Eng. Chem. Res. 47 (2008) 1365. M.S. Shaharun, H. Mukhtar, B.K. Dutt, Chem. Eng. Sci. 63 (2008) 3024. Y. Du, L.N. He, D.L. Kong, Catal. Commun. 9 (2008) 1754. L.F. Xiao, Q.F. Yue, C.G. Xia, et al. J. Mol. Catal. A: Chem. 279 (2008) 230. H.Y. Ju, M.D. Manju, K.H. Kim, et al. Korean J. Chem. Eng. 24 (2007) 917. I. Kim, M.J. Yi, K.J. Lee, et al. Catal. Today 111 (2006) 292. Y. Zhao, L.N. He, Y.Y. Zhuang, et al. Chin. Chem. Lett. 19 (2008) 286. W.Y. Su, G.P. Speranza, EP Patent 0443758A1 (1991). X. Zhao, Y. Zhang, Y. Wang, Ind. Eng. Chem. Res. 43 (2004) 4032. Q. Li, N. Zhao, W. Wei, et al. J. Mol. Catal. A: Chem. 270 (2007) 44.