Chapter 10

Catalytic Esterification of Carbon Dioxide and

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 159

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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 3 OH + C 0 2 - CH 3 OC(0)OCH 3 + H 2 0

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 CH3I and K 2 C 0 3 (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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Figu

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Dalian, China) with an inner volume of 500 mL, a magnetic stirrer and an electric heater (5). First given amounts of CH 3 I and K 2 C 0 3 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 C0 2 +N 2 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.

Results and Discussions

In all experiments, the molar ratio of feed gases was C0 2 :N 2 =3:5 . The rotation speed of the magnetic stirrer was controlled at 250 rpm. The preliminary tests showed that the optimal dosages of CH 3 I and K 2 C 0 3 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 D is defined as the mole fraction of D M C in the liquid-phase product.

Effects of Reaction Pressure and Temperature

Figures 2 and 3 show the effects of the reaction pressure and the temperature on x D , respectively. Figure 2 shows that x D increases initially with the increase of the reaction pressure. When the reaction pressure approaches about 7.3 MPa, x D reaches the maximum. As pressure is further increased, x D

decreases. The change tendency of Figure 3 is similar to that of Figure 2, with the peak value of x D obtained at 80-100eC. Obviously, near the critical regions of carbon dioxide, carbon dioxide can be converted effectively into D M C .

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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the electric polarization of the conjugated system of the molecule, so that carbon dioxide can take part in an addition reaction. Furthermore, carbon dioxide has a Lewis acid carbon atom and two weak Lewis base oxygen atoms. It is therefore not surprising that it can undergo various reactions, such as initially binding "abnormally" to form the intermediate. The presence of the basic material, K 2 C 0 3 , is useful for the activation of methanol to form CH 3 0". The nucleophilic attack of CH 3 0" upon C 0 2 leads to the formation of D M C . Methyl iodide is only involved in the catalytic cycle. Based on above analysis, near the critical point of carbon dioxide, x D reaches the maximum value. The possible reaction pathway is described as follows:

K 2 C 0 3

C H 3 O H > C H 3 C r + H +

j c o > [ C H 3 O Q O ) 0 ]

C H I — ^ C H 3 O 0 ( O ) O C H 3 + HI

| C H 3 O H

CH 3 I

A New Tentative Process for the Commercial Production of D M C

In the effluents, in addition to the products, D M C and water, there are large amounts of the feed gases and small amounts of by-products like dimethyl ether. In order to obtain a pure product of DMC, a two-step method can be utilized to separate dimethyl carbonate from the reaction products. The first step is pre-distillation. It uses the azeotropic feature of methanol with dimethyl carbonate. The azeotropic mixture of C H 3 O H and D M C can be separated from the by­products and the feed gases in a packed column (2). The second step is a refining step, for which an extractive distillation was adopted. Based on the features that D M C is insolvable in water, and that water does not react with D M C during distillation. We therefore selected water as the extraction reagent to extract and separate D M C from the azeotrope.

Figure 4 shows the schematic diagram of our tentative process for the commercial production of DMC. After pre-distillation, the gasified azeotrope is added into the extractive distillation column (3) and water is sprayed into the top of the column. After cooling (4), the distillate, which contains mostly dimethyl carbonate, flows into the separator (5) and is separated into two parts: one is the organic phase that contained 97% D M C , and the other is the aqueous phase that contained 87% water, 1.2% methanol and 11.8% D M C . The effluent, which contained 95% water, 5% methanol and less than 0.05% D M C , is discharged from the bottom of the extractive distillation column (3) and added into the fractionating tower (6). The aqueous phase is pumped back to the extractive distillation column (3). After fractionating (6), water was returned to the top of the extractive distillation column (3).

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. Th

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If it is possible to adopt one more extractive distillation column, we can obtain D M C in higher purity.

The drawback of this method for D M C synthesis from methanol and C 0 2 is that the conversion is limited by the reaction equilibrium. But this limitation is avoidable by the removal of water from the reaction system, for example, by introducing trimethyl orthoformate. If it is possible to develop a suitable method for the removal of water, the D M C yield will be improved significantly.

Conclusion

The continuous catalytic synthesis of dimethyl carbonate from carbon dioxide and methanol in the presence of CH 3 I and K 2 C 0 3 with reaction temperature and pressure near the critical point of carbon dioxide was studied in an agitated reactor. The optimal yield of dimethyl carbonate was obtained under a reaction pressure of 7.3 MPa, whereas the reaction temperature is 80-100 °C. It can be concluded that carbon dioxide is effectively activated in the supercritical state. A possible reaction pathway was presented for the supercritical synthesis of dimethyl carbonate. The results obtained will be useful for the esterification of carbon dioxide and methanol.

Acknowledgments

Supports from the Research Fund for the Doctoral Program of Higher Education of China (97025105) and the Basic Research Project of SINOPEC (598013) are very appreciated.

References

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