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carbon solid base catalysts
Studies in Surface Science and Catalysis 153 S.-E. Park, J.-S. Chang and K.-W. Lee (Editors) ©2004 Published by ElsevierB.V.
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Synthesis of dimethyl carbonate by transesterification over CaO/carbon solid base catalysts Tong Wei, Mouhua Wang, Wei Wei, Yuhan Sun*, Bing Zhong State Key Laboratory of Coal Conversion, Institute of Coal Chemistry Chinese Academic of Sciences, Taiyuan, 030001 PR China Carbon supported CaO solid base catalysts, CaO/C and CaO-C were prepared by loading CaO on carbon through impregnation and pugging method, respectively. The catalytic performance of CaO-C prepared by pugging method was much higher than that of CaO/C prepared by impregnation method due to its larger effective pore diameter and smaller inner diffusion resistance. When the CaO-C composite was used in catalytic distillation reactor, PC conversion and DMC yield reached 100% and 98% at 337K with 0.3 h"1 of LHSV, respectively. 1. INTRODUCTION Dimethyl carbonate (DMC) is an important intermediate for polycarbonate resins as well as a useful carbonylation and methylation agent , and it is promising as a substitute for phosgene, dimethyl sulfate, or methyl iodide due to its negligible toxicity. DMC could be prepared by oxidative carbonylation of methanol, carbonylation of methyl nitrite or transesterification of cyclic carbonate with methanol [3'41. Because of the moderate reaction conditions and the avoidance of equipment corrosion, transesterification method (see Scheme 1) has attracted much attention in recent years. The transesterification of cyclic carbonate with methanol could be catalyzed by both acid and base, but basic catalysts were more effective . In our previous work, CaO prepared from the dissociation of CaCO3 at elevated temperature was found very effective for synthesis of DMC from methanol and propylene carbonate (PC) [6 '. However, ultra fine CaO prepared from CaCO3 were difficult to be filtered from the products and reused. Therefore, CaO were loaded on carbon by impregnation and pugging method to prepare carbon supported CaO-based solid bases in the present work, respectively, which were used for the reaction of propylene carbonate (PC) with methanol. The effects of preparation method on the structure of the solid base and subsequently their catalytic performance were investigated in detail.
*To whom correspondence should be addressed: E-mail: vhsun(a>,sxicc.ac.cn, weiwei(a),sxicc.ac.cn: Fax: +86-351-4041153 Tel: +86-351-4053801
42
2. EXPERIMENTAL 2.1 Preparation of catalyst CaO was prepared by heating CaC03 at 1173K for 1 h in N2 atmosphere. CaO/C catalyst was prepared by impregnating calcium acetate on active carbon (0.15-0.18mm) from its aqueous solution, followed by drying in air at 393K for 6 h and then calcined in N2 at 1073K for 1 h. The CaO content in CaO/C was 28.3 wt%. CaO-C composite made from linear phenolic resin 217 (provided by Tianjin Resin Company), hexamethylenetetramine and CaCO3 (weight ratio=5:l:5). After grounded and mixed homogeneously, the mixture was pumped at 20 MPa and solidified at 473K for 10 h in N2, and then broken into small particles ranging from 0.18 to 0.28 mm. The solidified mixture was activated at 1173K for 1 h inN 2 to prepare CaO-C composite catalyst. The CaO content in CaO-C composites was 50 wt%. For the comparison, phenolic carbon was prepared from linear phenolic resin and hexamethylenetetramine only with the ratio and procedure same as that of CaO-C. 2.2 Characterization XRD of samples was carried out in Rigaku D/max- y A using Cu target with Ni filtration. Pore distribution and surface area of the samples were determined with the BET method through Micromeritics ASAP-2000, and the BET surface area for CaO/C, CaO-C composite and phenolic carbon were 1012.0 m2/g, 288.8 m2/g and 207.7m2/g, respectively. CO2-TPD measurement was performed at a heat rate of 12K/min under He flow (50mL/min), and CO2 desorbed was detected by a BALZA Q-Mass spectrometer. 2.3 Evaluation of the catalyst The reaction was carried out in a batch reactor with the mole ratio of methanol to propylene of 4:1, and the different amount of catalyst samples (0.90 wt% (CaO), 2.80 wt% (CaO/C), 1.80 wt% (CaO-C)) were used to keep the same CaO content in the reactor. After the reaction proceeded for a certain time at expected temperature under strong stirring, the reactor was cooled down to room temperature. The reaction condition for catalytic distillation reactor was described in the caption of Fig. 7. The products were analyzed on a gas chromatograph with a TCD after centrifugal separation from the catalyst. 3. RESULTS AND DISCUSSION
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3.1 Characterization of the catalysts It is well known that carbon is an inert supporter, which hardly reacts with an active substance. Therefore, CaO was loaded on carbon by impregnation and pugging method to prepare solid base catalyst with high performance for DMC synthesis by transesterification, respectively. The XRD patterns of CaO, CaO/C and CaO-C are illustrated in Figure 1. It indicated that the loading of CaO on carbon hardly changes its crystal structure, although CaO could be dispersed more homogeneously on carbon when the catalyst was prepared by impregnation method. Since base strength and basicity were the main factors that influence the catalytic behavior of solid base for this reaction, the base strength and basicity of CaO, CaO/C and CaO-C was determined by CO2-TPD (see Fig. 2 and Table 1). CaO and carbon supported CaO showed the same CO2 desorption at 923K, implying that the base strength of CaO hardly changed whatever it was supported on carbon. In addition, the basicity of CaO/C was much higher than that of pure CaO and CaO-C due to the homogeneous dispersion of CaO on carbon (see Fig.l and Table 1). These indicated that carbon supported CaO solid base catalysts, which possessed the same base strength as pure CaO, could be prepared by both impregnation method and pugging method. Moreover, CaO could be dispersed homogeneously on carbon by impregnation method and subsequently the basicty of the catalyst was improved remarkably. The pore distribution of CaO/C and CaO-C catalysts is shown in Fig. 3. For CaO/C catalyst, both micropore and mesopore were present with the volume ratio of micropores (0.44cm /g ) to mesopores (0.26 cm /g) was 1.67. This indicated that most active CaO was loaded on the microporous surface. As far as CaO-C catalyst was considered, it was suggested that only mesopores were effective although both micropores and mesopors with diameter were present. Since only micropores existed in phenolic carbon, the mesopores in CaO-C composite might result from the addition of CaCO3 in the starting materials. This was reasonable if CaCO3 dissociated into CaO and CO2 at 1173K, and then the effluent of CO2 led to the formation of mesopores.
Fig. 1. XRD pattern of CaO, CaO-C and CaO/C catalyst
Fig. 2.
CO2-TPD of CaO, CaO/C and CaO-C catalyst * based on CaO
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Table 1 C0 2 uptake of CaO, CaO-C and CaO/C catalyst Catalyst
C0 2 uptake/ (mmol/g)*
CaO
0.32
CaO/C
0.95
CaO-C
0.34
Fig. 3. Pore structure of CaO/C and CaO-C catalyst 3.2 Catalytic performance of the catalysts The catalytic performance of CaO/C and CaO-C are illustrated in Fig. 4. As we reported in our previous work '7 , CaO exhibited excellent catalytic performance for the reaction. At 353K, DMC yield was 43% after 2 h with CaO as catalyst. The high performance hardly decreased when CaO was loaded on carbon by pugging method, but DMC yield was only 4.5% when CaO/C used as catalyst. As mentioned above, the crystal structure and base strength, which were the main factors that affect the catalytic behavior of CaO, hardly changed when CaO was loaded on carbon, but the basicity of CaO/C was far higher than hat of pure CaO and CaO-C. Therefore, the catalytic performance difference should come from the mass transfer. CaO used in the present work were ultra fine particles with diameter about 10~20nm (see Fig. 5) and the reaction mainly proceeded on the surface of the particles under strong mixing. Whereas, for supported catalysts CaO/C and CaO-C, the reaction mainly took place on the inner surface. The diameter of effective pores of CaO-C catalyst was about 40 nm, while the effective pores in CaO/C were far narrower than that of CaO-C, mainly micropores and a small quantity of mesopores with diameter only about 4 nm. This led to the increase of inner diffusion resistance, and consequently the total reaction rate decreased remarkably. Furthermore, the reaction was carried out in liquid phase, and the molecule movement was slow in pores due to the strong interaction between molecules via H-bonds. Therefore, the inner diffusion was the main factor with CaO/C as catalyst, which decreased the reaction rate. Detailed investigation of inner diffusion on the reaction rate was discussed in other paper [7 ]. From the results above, it can be seen that when CaO was loaded on carbon by pugging method, it still showed high performance for transesterification of propylene carbonate with
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methanol. To illustrate the reusability of the catalyst, CaO-C was reused two times (see Fig. 6). The catalytic activity hardly changed when CaO /C catalyst was used for three times. Thus, such a type of catalyst had the good stability. Transesterification of propylene carbonate with methanol was a reversible reaction, in which DMC yield is limited by thermal equilibrium. At 101.3kPa, azeotropic temperature of DMC and methanol was 337K. When the reaction was carried out on catalytic distillation reactor at this temperature, DMC could be removed from the catalyst as soon as it was produced, so the equilibrium could be pushed and consequently PC conversion and DMC yield were improved. It can be seen from Figure 7 that PC conversion and DMC yield reached 100% and 98 %, respectively, when PC LHSV was 0.3 h"1 at 337K. With the rise of PC LHSV, PC conversion and DMC yield decreased regularly due to the increase of feed rate. As a result, PC conversion was 78% at 1.2 h"1 of PCLHSV. By contrast, Jiang[8] et al used 12-tungstophosphoric acid supported carbon molecular sieves as catalyst in catalytic distillation apparatus. PC conversion was only 45% when PC LHSV was 0.01 h'1. This indicated that the CaO-C composite was an efficient and convenient heterogeneous catalyst for synthesis of DMC from PC and methanol.
Fig. 4. Catalytic performance of CaO, CaO/C and CaO-C catalyst. Temperature: 353K Time: 2h
Fig. 5. TEM of pure CaO catalyst
Fig. 6. Reusability of CaO-C Fig. 7. Effect of PC LHSV on PC conversion and DMC yield. Catalyst: CaO-C ( 50% CaO); catalyst catalyst. Temperature: 323K size: 0.9-2.0mm; reflux ratio: 4; reaction time: 12h;
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4. CONCLUSION In conclusion, carbon supported CaO solid base could be prepared by loading CaO on carbon via impregnation and pugging method, respectively. The catalysts had the same crystal structure and base strength as pure CaO. However, the catalytic performance of CaO-C prepared by pugging method was much more higher than that of CaO/C prepared by impregnation method due to its larger pore diameter and then smaller inner diffusion resistance. In addition, CaO-C catalyst could be reused with little deactivation. When the CaO-C composite was used in catalytic distillation reactor, PC conversion and DMC yield reached 100% and 98% at 337K with 0.3 h"1 of LHSV, respectively. REFERENCE 1. 2. 3. 4. 5. 6. 7. 8.
Y. Sato, T. Yamamoto, Y. Souma, Catal. Letts, 65 (2000) 123. M. A. Pacheco, C. L. Marshall, Energy & Fuels, 11(1991) 2. P. Tundo, Pure Appl.Chem., 73 (2001) 1117. S. Fujita, B. M. Bhanage, Y. Ikushima and M. Arai, Green Chemistry, 3(2001) 87. Y. Ikeda, T. Sakaihori, K. Tomishige, K. Fujimoto, Cata. Letters, 66(2000) 59. T. Wei, M. Wang, W. Wei, et al, ACS symposium, No. 852 (2003) 175 T. Wei, M. Wang, W. Wei, et al, Green Chemistry, 5(2003) 343. Z. Y.Jiang, W.Yong, Pertochemical Technology (China), 30(2001)173.
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Synthesis of dimethyl carbonate by transesterification over CaO/carbon solid base catalysts Tong Wei, Mouhua Wang, Wei Wei, Yuhan Sun*, Bing Zhong State Key Laboratory of Coal Conversion, Institute of Coal Chemistry Chinese Academic of Sciences, Taiyuan, 030001 PR China Carbon supported CaO solid base catalysts, CaO/C and CaO-C were prepared by loading CaO on carbon through impregnation and pugging method, respectively. The catalytic performance of CaO-C prepared by pugging method was much higher than that of CaO/C prepared by impregnation method due to its larger effective pore diameter and smaller inner diffusion resistance. When the CaO-C composite was used in catalytic distillation reactor, PC conversion and DMC yield reached 100% and 98% at 337K with 0.3 h"1 of LHSV, respectively. 1. INTRODUCTION Dimethyl carbonate (DMC) is an important intermediate for polycarbonate resins as well as a useful carbonylation and methylation agent , and it is promising as a substitute for phosgene, dimethyl sulfate, or methyl iodide due to its negligible toxicity. DMC could be prepared by oxidative carbonylation of methanol, carbonylation of methyl nitrite or transesterification of cyclic carbonate with methanol [3'41. Because of the moderate reaction conditions and the avoidance of equipment corrosion, transesterification method (see Scheme 1) has attracted much attention in recent years. The transesterification of cyclic carbonate with methanol could be catalyzed by both acid and base, but basic catalysts were more effective . In our previous work, CaO prepared from the dissociation of CaCO3 at elevated temperature was found very effective for synthesis of DMC from methanol and propylene carbonate (PC) [6 '. However, ultra fine CaO prepared from CaCO3 were difficult to be filtered from the products and reused. Therefore, CaO were loaded on carbon by impregnation and pugging method to prepare carbon supported CaO-based solid bases in the present work, respectively, which were used for the reaction of propylene carbonate (PC) with methanol. The effects of preparation method on the structure of the solid base and subsequently their catalytic performance were investigated in detail.
*To whom correspondence should be addressed: E-mail: vhsun(a>,sxicc.ac.cn, weiwei(a),sxicc.ac.cn: Fax: +86-351-4041153 Tel: +86-351-4053801
42
2. EXPERIMENTAL 2.1 Preparation of catalyst CaO was prepared by heating CaC03 at 1173K for 1 h in N2 atmosphere. CaO/C catalyst was prepared by impregnating calcium acetate on active carbon (0.15-0.18mm) from its aqueous solution, followed by drying in air at 393K for 6 h and then calcined in N2 at 1073K for 1 h. The CaO content in CaO/C was 28.3 wt%. CaO-C composite made from linear phenolic resin 217 (provided by Tianjin Resin Company), hexamethylenetetramine and CaCO3 (weight ratio=5:l:5). After grounded and mixed homogeneously, the mixture was pumped at 20 MPa and solidified at 473K for 10 h in N2, and then broken into small particles ranging from 0.18 to 0.28 mm. The solidified mixture was activated at 1173K for 1 h inN 2 to prepare CaO-C composite catalyst. The CaO content in CaO-C composites was 50 wt%. For the comparison, phenolic carbon was prepared from linear phenolic resin and hexamethylenetetramine only with the ratio and procedure same as that of CaO-C. 2.2 Characterization XRD of samples was carried out in Rigaku D/max- y A using Cu target with Ni filtration. Pore distribution and surface area of the samples were determined with the BET method through Micromeritics ASAP-2000, and the BET surface area for CaO/C, CaO-C composite and phenolic carbon were 1012.0 m2/g, 288.8 m2/g and 207.7m2/g, respectively. CO2-TPD measurement was performed at a heat rate of 12K/min under He flow (50mL/min), and CO2 desorbed was detected by a BALZA Q-Mass spectrometer. 2.3 Evaluation of the catalyst The reaction was carried out in a batch reactor with the mole ratio of methanol to propylene of 4:1, and the different amount of catalyst samples (0.90 wt% (CaO), 2.80 wt% (CaO/C), 1.80 wt% (CaO-C)) were used to keep the same CaO content in the reactor. After the reaction proceeded for a certain time at expected temperature under strong stirring, the reactor was cooled down to room temperature. The reaction condition for catalytic distillation reactor was described in the caption of Fig. 7. The products were analyzed on a gas chromatograph with a TCD after centrifugal separation from the catalyst. 3. RESULTS AND DISCUSSION
43
3.1 Characterization of the catalysts It is well known that carbon is an inert supporter, which hardly reacts with an active substance. Therefore, CaO was loaded on carbon by impregnation and pugging method to prepare solid base catalyst with high performance for DMC synthesis by transesterification, respectively. The XRD patterns of CaO, CaO/C and CaO-C are illustrated in Figure 1. It indicated that the loading of CaO on carbon hardly changes its crystal structure, although CaO could be dispersed more homogeneously on carbon when the catalyst was prepared by impregnation method. Since base strength and basicity were the main factors that influence the catalytic behavior of solid base for this reaction, the base strength and basicity of CaO, CaO/C and CaO-C was determined by CO2-TPD (see Fig. 2 and Table 1). CaO and carbon supported CaO showed the same CO2 desorption at 923K, implying that the base strength of CaO hardly changed whatever it was supported on carbon. In addition, the basicity of CaO/C was much higher than that of pure CaO and CaO-C due to the homogeneous dispersion of CaO on carbon (see Fig.l and Table 1). These indicated that carbon supported CaO solid base catalysts, which possessed the same base strength as pure CaO, could be prepared by both impregnation method and pugging method. Moreover, CaO could be dispersed homogeneously on carbon by impregnation method and subsequently the basicty of the catalyst was improved remarkably. The pore distribution of CaO/C and CaO-C catalysts is shown in Fig. 3. For CaO/C catalyst, both micropore and mesopore were present with the volume ratio of micropores (0.44cm /g ) to mesopores (0.26 cm /g) was 1.67. This indicated that most active CaO was loaded on the microporous surface. As far as CaO-C catalyst was considered, it was suggested that only mesopores were effective although both micropores and mesopors with diameter were present. Since only micropores existed in phenolic carbon, the mesopores in CaO-C composite might result from the addition of CaCO3 in the starting materials. This was reasonable if CaCO3 dissociated into CaO and CO2 at 1173K, and then the effluent of CO2 led to the formation of mesopores.
Fig. 1. XRD pattern of CaO, CaO-C and CaO/C catalyst
Fig. 2.
CO2-TPD of CaO, CaO/C and CaO-C catalyst * based on CaO
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Table 1 C0 2 uptake of CaO, CaO-C and CaO/C catalyst Catalyst
C0 2 uptake/ (mmol/g)*
CaO
0.32
CaO/C
0.95
CaO-C
0.34
Fig. 3. Pore structure of CaO/C and CaO-C catalyst 3.2 Catalytic performance of the catalysts The catalytic performance of CaO/C and CaO-C are illustrated in Fig. 4. As we reported in our previous work '7 , CaO exhibited excellent catalytic performance for the reaction. At 353K, DMC yield was 43% after 2 h with CaO as catalyst. The high performance hardly decreased when CaO was loaded on carbon by pugging method, but DMC yield was only 4.5% when CaO/C used as catalyst. As mentioned above, the crystal structure and base strength, which were the main factors that affect the catalytic behavior of CaO, hardly changed when CaO was loaded on carbon, but the basicity of CaO/C was far higher than hat of pure CaO and CaO-C. Therefore, the catalytic performance difference should come from the mass transfer. CaO used in the present work were ultra fine particles with diameter about 10~20nm (see Fig. 5) and the reaction mainly proceeded on the surface of the particles under strong mixing. Whereas, for supported catalysts CaO/C and CaO-C, the reaction mainly took place on the inner surface. The diameter of effective pores of CaO-C catalyst was about 40 nm, while the effective pores in CaO/C were far narrower than that of CaO-C, mainly micropores and a small quantity of mesopores with diameter only about 4 nm. This led to the increase of inner diffusion resistance, and consequently the total reaction rate decreased remarkably. Furthermore, the reaction was carried out in liquid phase, and the molecule movement was slow in pores due to the strong interaction between molecules via H-bonds. Therefore, the inner diffusion was the main factor with CaO/C as catalyst, which decreased the reaction rate. Detailed investigation of inner diffusion on the reaction rate was discussed in other paper [7 ]. From the results above, it can be seen that when CaO was loaded on carbon by pugging method, it still showed high performance for transesterification of propylene carbonate with
45
methanol. To illustrate the reusability of the catalyst, CaO-C was reused two times (see Fig. 6). The catalytic activity hardly changed when CaO /C catalyst was used for three times. Thus, such a type of catalyst had the good stability. Transesterification of propylene carbonate with methanol was a reversible reaction, in which DMC yield is limited by thermal equilibrium. At 101.3kPa, azeotropic temperature of DMC and methanol was 337K. When the reaction was carried out on catalytic distillation reactor at this temperature, DMC could be removed from the catalyst as soon as it was produced, so the equilibrium could be pushed and consequently PC conversion and DMC yield were improved. It can be seen from Figure 7 that PC conversion and DMC yield reached 100% and 98 %, respectively, when PC LHSV was 0.3 h"1 at 337K. With the rise of PC LHSV, PC conversion and DMC yield decreased regularly due to the increase of feed rate. As a result, PC conversion was 78% at 1.2 h"1 of PCLHSV. By contrast, Jiang[8] et al used 12-tungstophosphoric acid supported carbon molecular sieves as catalyst in catalytic distillation apparatus. PC conversion was only 45% when PC LHSV was 0.01 h'1. This indicated that the CaO-C composite was an efficient and convenient heterogeneous catalyst for synthesis of DMC from PC and methanol.
Fig. 4. Catalytic performance of CaO, CaO/C and CaO-C catalyst. Temperature: 353K Time: 2h
Fig. 5. TEM of pure CaO catalyst
Fig. 6. Reusability of CaO-C Fig. 7. Effect of PC LHSV on PC conversion and DMC yield. Catalyst: CaO-C ( 50% CaO); catalyst catalyst. Temperature: 323K size: 0.9-2.0mm; reflux ratio: 4; reaction time: 12h;
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4. CONCLUSION In conclusion, carbon supported CaO solid base could be prepared by loading CaO on carbon via impregnation and pugging method, respectively. The catalysts had the same crystal structure and base strength as pure CaO. However, the catalytic performance of CaO-C prepared by pugging method was much more higher than that of CaO/C prepared by impregnation method due to its larger pore diameter and then smaller inner diffusion resistance. In addition, CaO-C catalyst could be reused with little deactivation. When the CaO-C composite was used in catalytic distillation reactor, PC conversion and DMC yield reached 100% and 98% at 337K with 0.3 h"1 of LHSV, respectively. REFERENCE 1. 2. 3. 4. 5. 6. 7. 8.
Y. Sato, T. Yamamoto, Y. Souma, Catal. Letts, 65 (2000) 123. M. A. Pacheco, C. L. Marshall, Energy & Fuels, 11(1991) 2. P. Tundo, Pure Appl.Chem., 73 (2001) 1117. S. Fujita, B. M. Bhanage, Y. Ikushima and M. Arai, Green Chemistry, 3(2001) 87. Y. Ikeda, T. Sakaihori, K. Tomishige, K. Fujimoto, Cata. Letters, 66(2000) 59. T. Wei, M. Wang, W. Wei, et al, ACS symposium, No. 852 (2003) 175 T. Wei, M. Wang, W. Wei, et al, Green Chemistry, 5(2003) 343. Z. Y.Jiang, W.Yong, Pertochemical Technology (China), 30(2001)173.