Catalytic Esterification of Carbon Dioxide and Methanol for the

Chapter 10 Catalytic Esterification of Carbon Dioxide and

Downloaded by UNIV OF GUELPH LIBRARY on September 13, 2012 | http://pubs.acs.org Publication Date: June 26, 2003 | doi: 10.1021/bk-2003-0852.ch010

Methanol for the Preparation of Dimethyl Carbonate Fa-hai Cao, Ding-ye Fang, Dian-hua Liu, and Wei-yong Ying Department of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China

Dimethyl carbonate (DMC) is an important methylating agent and potential additive for clean fuel production. Several processes have been developed for the manufacture of D M C . The direct synthesis of D M C from CO2 and methanol is especially attractive and important. In this work, the continuous synthesis of D M C from methanol and CO2 in the region near the critical point of CO2, with methyl iodide and potassium carbonate as the promoters, was investigated. The reactions were performed in a stainless steel autoclave with an inner volume of 500 mL equipped with a magnetic stirrer and an electric heater. The effects of the reaction temperature and pressure were determined first. It was shown that the yield of D M C increases with the increase of the reaction pressure. When the pressure approaches the critical pressure of CO2, the optimal yield of D M C is obtained. Then the reaction characteristic was discussed from the unique characteristics of supercritical CO2. Finally, a new process for the production of D M C has been proposed, for which water is used as an extractant for the separation of D M C from the reaction mixture.

© 2003 American Chemical Society

In Utilization of Greenhouse Gases; Liu, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

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Introduction Carbon dioxide is commonly regarded as a major greenhouse gas. It is mainly produced during the combustion of fossil fuels (coal, oil and natural gas). On the other hand, carbon dioxide can be converted into many useful organic compounds. Therefore, the utilization and recycling of carbon dioxide can be beneficial to the environment. We proposed a process in which dimethyl carbonate (DMC) is produced by the catalytic esterification of carbon dioxide and methanol according to the reaction shown below: 2CH OH + C 0 - C H O C ( 0 ) O C H + H 0 3

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D M C has attracted much attention as a non-toxic substitute for dimethyl sulfate and phosgene, which are toxic and corrosive methylating or carbonylating agents. In addition, D M C is considered to be an option for meeting the oxygenate specification for fuels. The conventional synthesis of D M C is via the reaction of methanol and phosgene. Owing to the high toxicity of the raw materials and severe corrosity, this method has been abandoned gradually. The other two widely used methods for synthesis of D M C are an ester exchange process (1-2) and the oxidative carbonylation of methanol (3-5). Recently, a more challenging method (6-9) is the direct synthesis from carbon dioxide and methanol. Although metallic magnesium powder, Sn(IV) and Tl(IV) alkoxides have been used as the catalysts, unfortunately, the yield of D M C was low even in the presence of chemical dehydrates mainly due to thermodynamic limits. On the other hand, regions near the critical point are considered to be very important because in these regions the supercritical characteristics have the greatest effect on the reactions. The supercritical conditions will also play a crucial role in the activation and conversion of carbon dioxide. In this work, we studied the process for the continuous synthesis of D M C near the critical regions of carbon dioxide. Carbon dioxide performed not only as a medium of the supercritical fluid but also as one of the reactants.

Experimental Chemicals. Methanol (99.5% purity), dimethyl carbonate (99.8% purity) and and K C 0 (99.8% purity) were all commercially obtained. Compressed carbon dioxide (>99.9% purity) was used. Apparatus and Analysis, The experimental configuration is shown in Figure 1. The reactor (4) was a stainless steel autoclave (FYX-5A, made in

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In Utilization of Greenhouse Gases; Liu, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003. 2

1 — Cylinder of feed gas ; 2—Cylinder of N ; 3—Pressure reducing valve; 4—Autoclave; 5—Heater; 6—Tapped hole; 7—Condenser; 8— Gas-liquid separator; 9—Gas chromatograph; 10-Vent; 11 —Bulb meter

Figure 1. The schematic diagram of the experimental apparatus


162 Dalian, China) with an inner volume of 500 mL, a magnetic stirrer and an electric heater (5). First given amounts of C H I and K C 0 were mixed with methanol (150 mL) in the autoclave, and then the autoclave was sealed. The autoclave was then flushed with nitrogen and then purged with C 0 + N until the desired pressure was reached at room temperature. The autoclave was then heated and stirred constantly at the desired temperature and pressure for a given period of time. The products, which were collected to a proper volume from the tapped hole (6), were cooled to ambient temperature. The samples so obtained were analyzed by gas chromatograph (GC-900B, made in Shanghai, China) using a thermal conductivity detector with a TDX-02 column (80-100 mesh). After condensation (7), gas-liquid separation (8) and depressurization, the products were continuously monitored on-line by GC. 3

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Results and Discussions In all experiments, the molar ratio of feed gases was C 0 : N = 3 : 5 . The rotation speed of the magnetic stirrer was controlled at 250 rpm. The preliminary tests showed that the optimal dosages of CH I and K C 0 were 10.0 mL and 8.0 g, respectively. If one of these two compounds was absent, the other compound had no activity for the catalytic reaction. x is defined as the mole fraction of D M C in the liquid-phase product. 2

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Effects of Reaction Pressure and Temperature Figures 2 and 3 show the effects of the reaction pressure and the temperature on x , respectively. Figure 2 shows that x increases initially with the increase of the reaction pressure. When the reaction pressure approaches about 7.3 MPa, x reaches the maximum. As pressure is further increased, x decreases. The change tendency of Figure 3 is similar to that of Figure 2, with the peak value of x obtained at 80-100 C. Obviously, near the critical regions of carbon dioxide, carbon dioxide can be converted effectively into D M C . D

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Supercritical Phenomena and Reaction Pathway (10-12) There is a conjugated double bond in a molecule of carbon dioxide. Near the critical regions of carbon dioxide, the reaction rate varies with the change of



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Catalytic Esterification of Carbon Dioxide and Methanol for the

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