Synthesis of Dimethyl Carbonate from Methanol and Carbon Dioxide Catalyzed by Potassium Hydroxide un

Chinese Chemical Letters Vol. 16, No. 9, pp 1267-1270, 20051267http://www.imm.ac.cn/joumal/ccl.htmlSynthesis of Dimethyl Carbonate from Methanol and Carbon DioxideCatalyzed by Potassium Hydroxide under Mild ConditionsHong WANG', Bin LU, Qing Hai CAI23, Feng wU2, Yong Kui SHAN*1 College of Chemical and Environmental Engineering, Harbin University of Science andTechnology, Harbin 1500802 Department of Chemistry, Harbin Normal university, Harbin 1500803 Department of Chemistry, East China Normal University, Shanghai 200062Abstract: The synthesis of dimethyl carbonate (DMC) from methanol and carbon dioxide usingpotassium hydroxide as catalyst in the presence of CH3I and the effect of ionic liquid on thereaction were investigated. The results showed that KOH is an effctive catalyst; the highselctivity and raised yield of DMC formation under mild conditions were achieved. However,the addition of the ionic liquid, 1-ethy1-3- methylimidazolium bromide (emimBr), can evidentlyaccelerate the conversion of methanol and yield of the product.Keywords: Dimethyl carbonate, methanol, carbon dioxide, methyl iodide, ionic liquid, 1-ethyl-3-methylimidazolium bromide.Dimethyl carbonate (DMC) is an important carbonylation and methylation agent,alternate of poisonous dimethyl sulfate and phosgene. Besides, DMC is also anintermediate for higher carbonates and carbamates' as well as a promising octaneenhancer. The widely used method of its preparation is the oxidative carbonylation ofmethanol by carbon monoxide and oxygen with copper (I) and/or palladium (I) ascatalyst'. Carbon monoxide is expensive and it may accompany a potential explosionhazard. Recently, the utilization of carbon dioxide, as a readily available, inexpensiveand environmentally acceptable starting material for DMC synthesis has been attempted.The possible organotin catalyzed formation of DMC from CO2 was first proposed byJapanese groups*.. Kizlink et al.have paid much attention to improving the catalyticactivities of this reaction, but a high turmover number (TON) is still not achieved' 8.The utilization of zirconia or modified zirconia, such as H3POy/ZrO2 or CeO2-ZrO2, tosynthesis of DMC from CO2 and methanol was investigated by Tomishige's group1.Although the selectivity of DMC over these catalysts was very high (ca. 100%),unfortunately, the methanol conversion is very low (less than 1%). Zhao et al.reported that metal acetate effectively catalyzed the formation of DMC from carbondioxide and methanol12; Fujimoto and Arai et al. reported the synthesis of DMC in thepresence of base K2CO3 and methyl iodide under中国煤化工hough theYHCNMHG*E- mail: qinghaic@ sina.com1268Hong WANG et al.methanol conversion was higher, the yield was still very low (less than or about 4%).Sakakura et al. applied an organotin catalyst to the synthesis of DMC from orthoester'and acetals', the yield of DMC reported in these systems was high, however, thesesystems have disadvantages of the high cost of the starting materials and the difficulty inthe catalyst-product separation due to the homogeneous nature of the catalyst. So thedirect synthesis of DMC from methanol and carbon dioxide is still far from satisfactorydue to the difficulty in the activation of carbon dioxide, deactivation of the catalysts dueto water formation in reaction process and the thermodynamic limitation. Under thesecircumstances, it is very important to investigate the reaction of DMC synthesis frommethanol and carbon dioxide so as to raise the yield and selectivity of DMC. In ourpresent investigation, we report the synthesis of dimethyl carbonate from methanol andcarbon dioxide using KOH as catalyst and the effect of ionic liquid on the reaction undermild conditions. The results showed that the high selectivity (ca. 100%) and raisedyield (11.0%) of DMC was achieved in the presence of ionic liquid emimBr.All experiments were carried out in a stainless steel reactor with inner volume of500 mL provided with a mechanical stirrer and an electric heater. Potassium hydroxide(0.02-0.12 mol) was added in the reactor with a certain amount of anhydrate methanol,and then CHzI (0.02-0.12 mol) was charged into it. After being purged three times withCO2, the reactor was pressured to a certain pressure and heated to the desired temperaturewith stirring. After the required time, the liquid phase was cooled, sampled andanalyzed by GC and GC-MS. The yield was calculated on the basis of methanol.When the reaction of CO2 and methanol was carried out in the presence of KOH andmethyl iodide, DMC was formed. The yield of DMC increased in proportion with theamount of KOH up to the maximum 8.5%, and then it dropped down to minimum value4.7% with continuous increase of KOH amount (Figure 1). This phenomenon may beascribed to the fact that the rate of direct reaction of KOH with CH3I rapidly increasedwhen the molar ratio of KOH to CH3OH was above 0.054/0.85, the catalyst KOH andCH3I were greatly consumed ". In all the experiments, no by-product was detected byGC and GC-MS. The catalyst showed high activity and selectivity for the formation ofDMC, the highest yield of DMC was 8.5% and the selectivity was 100%.Figure 1 The effect of KOH amount on the reaction9「500.020.04中国煤化工AmonutofKYHCNMHGSynthesis of Dimethyl Carbonate from Methanol and Carbon Dioxide 1269The experiment was also carried out with respect to methyl iodide. Withoutmethyl iodide in the reaction system, the formation of DMC was almost to zero. Theyield of DMC increased with increase of the amount of CH3I added in the reactionsystem, the yield reached maximum of 8.5%, while the amount of CH3I was 0.048 mol,and then the yield of DMC basically become constant, when the amount of CHgIincreased further. This implies that the addition of CH3I plays very vital role in thesynthesis of DMC catalyzed by basic catalysts. It was also obtained from theexperimental results that the maximum formation of DMC was 0.036 mol, when theamount of CH3I was 0.048 mol, its surplus quantity in the reaction mixture wasdetermined to be about 0.02 mol. This finding showed that the consumed amount ofCH3I was about 0.028 mol, which is less than the amount of DMC formation.Therefore, CH3I acts as a promoter in the reaction process.Figure 2 the effect of CO2 pressure on the reaction品653L4810Pressure(MPa)Figure 3 the effect of ionic liquid emimBr on the reaction12「702Amount of emimBr(g)Figure 2 illustrates the effect of CO2 pressure upon the yields of DMC in thepresence of KOH. As the pressure of CO2 is raised, the DMC formation shows twomaxima near 2.0 MPa and 7.3 MPa. It is a very similar to Arai's work, two maximanear 4.5 and 8.0 MPa with K2CO3 as catalyst. Figure 2 also indicates that high CO2pressures are not required for the DMC formatil中国煤化itions aredetrimental for the reaction. The high yield (8.5%MYH.CNMHGareactant,ichieved atabout 2.0 MPa. It has been reported that when C1270Hong WANG et al.the reaction rates are maximal near the critical pressure of CO2 (7.3 MPa) 12-14. And atlow pressure of CO2, liquid and gaseous CO2 coexists in the reactor. With the increaseof the CO2 pressure, the volume of liquid phase increased gradually. Probably, theincrease in the liquid volume should arise from absorption of CO2 into the liquid. Scthe increase in the yield of DMC observed up to 2.0 MPa should be ascribed to theabsorption of CO2. However, the volume increased slightly with increasing the pressureup to 6.0 MPa. The decrease of the yield of DMC observed between 2.0 MPa and 7.3MPa might result from the dilution effect 14.When ionic liquid emimBr was added in the reaction system, the conversion ofmethanol and yield of DMC evidently increased (Figure 3). The maximum value ofDMC yield reached at 11%, when 4 g of emimBr was added. Subsequently, the yield ofDMC formation basically kept constant with the continuous increase of emimBr amount.The reason was that the higher yield obtained in the presence of ionic liquid may beascribed to the strong polarity and electrostatic field of the ionic liquid, which maystabilize the charged intermediate 18.19. However, its promotion mechanism needs to befurther investigated.References0. Yoshio, Appl. Catal. A: General, 1997, 155, 133.A. Shaikh, S. Sivaram, Chem. Rev, 1996, 96, 951.3. A. Behr, Angew. Chem. Int. Ed. EngL, 1988, 27, 661.4. S. Sakai, T. Fujinami, S. Furusawa, Nippon Kagaku Kaishi, 1975, 1789.5. N. Yamazaki, S. Nakahama, F. Higashi, Rep. Asahi Glass Found. Ind. Technol, 1978, 33, 31.6. J. Kizlink, I. Pastucha, Collect. Czech. Chem. Commun, 1995, 60, 687.7. J. Kizlink, L. Pastucha, Collect. Czech. Chem. Commun., 1994, 59, 2116.8. J. Kizlink, Collect. Czech. Chem. Commun, 1993, 58, 1399.9. K. Tomishige, T. Sakaihori, Y. Ikeda, K. Fujimoto, Catal. Lett, 1999, 58, 225.Y. Ikeda, M. Asadullah, K. Fujimoto, K. Tomishige, J. Phys. Chem. B, 2001, 105, 106531. K. Tomishige, Y. Furusawa, Y. Ikeda, M. Asadullah, K. Fujimoto, Catal. Lett, 2001, 76, 71.12. T. Zhao, Y. Han, Y. Sun, Fuel Processing Tech., 2000, 62, 187.13. S. Fang, K. Fujimoto, Appl Catal. A: General, 1996, 142, L1. .14.s. Fujita, M. Bhalchandra, Y. lkushima, M. Arai, Green Chem., 2001,3, 87.15. T. Sakakura, Y. Saito, M. Okano, J. Choi, T. Sako, J. Org. Chem., 1998, 63, 7095.6. T. Sakakura, J. Choi, Y. Saito, T. Masuda, T. Sako, T. Oriyama, J. Org. Chem, 1999, 64,4506.Q. Y. Xing, R. Q. Xu, Z. Zhou, Ji Chu You Ji Hua Xue (Elementary Organic Chemistry, inChinese). The people's Education Publishers, Beijing, 1981, I, 281.8. K. Qiao, Y. Deng, J. Mol. Catal. A: Chem, 2001, 171, 81.19. M. J. Earle, P. B. McCormac, K. R. Seddon, Green Chem, 2000, 2, 261.Received 1 November, 2004中国煤化工MYHCNMHG