文献

Jointly published by React.Kinet.Catal.Lett. Akadémiai Kiadó, Budapest Vol. 89, No. 2, 303−309 and Springer, Dordrecht (2006)RKCL4976DIRECT SYNTHESIS OF DIMETHYL CARBONATE FROM CH3OH AND CO2 BY H3PW12O40/Ce x Ti1-x O2 CATALYSTKyung Won La and In Kyu Song*School of Chemical and Biological Engineering, Seoul National University,Shinlim-dong, Kwanak-ku, Seoul 151-744, South KoreaReceived April 13, 2006, in revised form June 22, 2006, accepted July 3, 2006AbstractCe x Ti1-x O2 and H3PW12O40/Ce x Ti1-x O2 catalysts were prepared using a sol-gelmethod, and applied to the direct synthesis of dimethyl carbonate from methanoland carbon dioxide. H3PW12O40/Ce x Ti1-x O2 showed a better catalytic performancethan the corresponding Ce x Ti1-x O2, due to the bifunctional catalysis of Brönstedacid sites (provided by H3PW12O40) and base sites (provided by Ce x Ti1-x O2).H3PW12O40/Ce0.1Ti0.9O2 showed the highest catalytic performance among theH3PW12O40/Ce x Ti1-x O2 catalysts.Keywords: Dimethyl carbonate, heteropolyacid, methanol, carbon dioxideINTRODUCTIONDimethyl carbonate (DMC) has been used as a green chemical and an alternative to corrosive and toxic reagents such as dimethyl sulfur and phosgene ___________________________*Corresponding author. Tel.: +82-2-880-9227; Fax: +82-2-888-7295E-mail: inksong@snu.ac.kr0133-1736/2006/US$ 20.00.© Akadémiai Kiadó, Budapest.All rights reserved.304 KYUNG WON LA, IN KYU SONG: DIMETHYL CARBONATE [1]. Several commercial processes have been developed for the production of DMC, including the methanolysis of phosgene [2], oxidative carbonylation of methanol catalyzed by cuprous chloride [3], and direct synthesis catalyzed by palladium and an alkyl nitrate promoter [4]. However, these processes involve the use of toxic, flammable, and corrosive gases such as phosgene, hydrogen chloride, and carbon monoxide. Therefore, the direct synthesis of DMC from methanol and carbon dioxide is an attractive method in an environmental point of view [5]. A wide variety of catalysts such as organometallic compounds [6], metal tetra-alkoxides [7], potassium carbonate [8], zirconia [9,10], H3PW12O40/ZrO2 [11], and H3PO4-V2O5 [12] have been used for the direct synthesis of DMC from methanol and carbon dioxide. However, the yield of DMC obtained from the one-step synthesis from methanol and carbon dioxide is still too low [13].In this study, H3PW12O40/Ce x Ti1-x O2 catalysts were prepared using a sol-gel method with the aim of providing both acid and base sites, and applied to the direct synthesis of DMC from methanol and carbon dioxide in a batch reactor. ZrO2 and H3PW12O40/ZrO2 were also prepared using a sol-gel method for comparison.EXPERIMENTALCe x Ti1-x O2 was prepared using a sol-gel method with Ce(NO3)3·6H2O (Aldrich) and Ti(OCH(CH3)2)4 (Aldrich) as precursors, with a variation in the cerium content from 0 to 1.0 (x=0, 0.1, 0.2, 0.4, 0.6, 0.8 and 1.0). Ti(OCH(CH3)2)4 was added to the ethanol solution containing Ce(NO3)3·6H2O. Ammonium hydroxide (29 wt.% NH3) was then added to the mixed solution containing cerium and titanium precursors. The mixed solution was stirred for 1 h at room temperature. The precipitate was filtered and dried in an air stream at 80o C for 24 h. The resulting solid was calcined at 300o C for 4 h to yield Ce x Ti1-x O2.The H3PW12O40/Ce x Ti1-x O2 catalyst was also prepared using a sol-gel method. H3PW12O40 was dissolved in ethanol and added to an ethanol solution containing Ce(NO3)3·6H2O. Ti(OCH(CH3)2)4 and ammonium hydroxide were then added to the solution containing H3PW12O40 and the cerium precursor. The mixed solution was stirred for 1 h at room temperature. The precipitate was filtered and dried in an air stream at 80o C for 24 h. The resulting solid was calcined at 300o C for 4 h to yield the H3PW12O40/Ce x Ti1-x O2. ZrO2 and H3PW12O40/ZrO2 were also prepared by a sol-gel method according to the method reported elsewhere for comparison [14]. The loading of H3PW12O40 on the metal oxides was fixed to 15 wt% in all cases.The direct synthesis of DMC from methanol and carbon dioxide was carried out in a 100 mL stainless steel autoclave. 6.4 g of methanol (200 m-mole,KYUNG WON LA, IN KYU SONG: DIMETHYL CARBONATE 305 Sigma) and 0.5 g of the catalyst were charged into the autoclave, and then the reactor was purged with carbon dioxide. After pressurizing the autoclave to 5 MPa with carbon dioxide, the reactor was heated to the reaction temperature with constant stirring. The catalytic reaction was carried out at 170o C for 12 h. The reaction products were periodically sampled and analyzed using gas chromatography (GC).Fig. 1. XRD patterns of ZrO2 and Ce x Ti1-x O2RESULTS AND DISCUSSIONFigure 1 shows the XRD patterns of ZrO2 and Ce x Ti1-x O2 prepared using the sol-gel method. The peaks at 2θ = 28.0o and 31.4o were assigned to the (11ī) and (111) planes for the monoclinic phase of ZrO2, respectively, and the peaks appearing at 2θ = 30.2o and 60.0o corresponded to the (111) plane of the tetragonal phase, which is in good agreement with a previous report [15]. Cubic cerianite phases of CeO2 oriented along the (111), (200), (220), and (311) planes were identified by the diffraction peaks appearing at 2θ = 28.7, 33.0, 47.4, and 56.3o, respectively [16]. The XRD peak for the tetragonal anatase phase of TiO2 appeared at 2θ = 25.3o, while the XRD peaks for the rutile phase of TiO2appeared at 2θ = 27.4, 36.0, 41.0 and 54.3o [17]. Although the characteristic XRD peaks of Ce x Ti1-x O2 (x=0.2-0.8) were shifted to higher angles with decreasing cerium content, the XRD pattern of Ce x Ti1-x O2 (x = 0.2-306 KYUNG WON LA, IN KYU SONG: DIMETHYL CARBONATE 0.8) was similar to that of CeO2 (x = 1.0). However, Ce0.1Ti0.9O2 showed a mixed XRD pattern of CeO2 and TiO2. This indicates that the addition of a small amount of cerium (x = 0.1) stabilizes the crystalline phase of Ce0.1Ti0.9O2[18].A series of preliminary experiments showed that the H3PW12O40/ZrO2 catalyst always had a better catalytic performance than ZrO2, suggesting that a catalyst should retain both acid and base sites for the efficient synthesis of DMC from methanol and carbon dioxide as previously reported [11]. It was also observed that the H3PW12O40/ZrO2 catalyst prepared using the sol-gel method had a better performance than that prepared using the impregnation method. Furthermore, it was found that the 15 wt.% H3PW12O40/ZrO2 catalyst showed the best catalytic performance among the H3PW12O40/ZrO2 catalysts with different H3PW12O40 loadings. Therefore, Ce x Ti1-x O2 and H3PW12O40/Ce x Ti1-x O2 catalysts were prepared using a sol-gel method, and the loading of H3PW12O40 on Ce x Ti1-x O2 was fixed to 15 wt.%.Fig. 2. Catalytic performance of ZrO2 and Ce x Ti1-x O2 in the direct synthesis ofDMC from methanol and carbon dioxide performed at 170o C for 12 hFigure 2 shows the catalytic performance of ZrO2 and Ce x Ti1-x O2 on the direct synthesis of DMC from methanol and carbon dioxide performed at 170o C for 12 h. The catalytic performance of pure metal oxides was decreased in the following order of ZrO2>CeO2>TiO2. However, Ce x Ti1-x O2 (x=0.1-0.8) showed a better catalytic performance than ZrO2. Ce0.1Ti0.9O2 showed the best catalyticKYUNG WON LA, IN KYU SONG: DIMETHYL CARBONATE 307 performance of all Ce x Ti1-x O2 catalysts examined. It is believed that the enhanced catalytic performance of Ce0.1Ti0.9O2 is closely related to the stabilized crystalline phase of Ce0.1Ti0.9O2, as shown in Fig. 1.Fig. 3. Catalytic performance of H3PW12O40/ZrO2, Ce x Ti1-x O2, andH3PW12O40/Ce x Ti1-x O2 in the direct synthesis of DMC from methanol and carbondioxide performed at 170o C for 12 hFigure 3 shows the catalytic performance of H3PW12O40/ZrO2, Ce x Ti1-x O2, and H3PW12O40/Ce x Ti1-x O2 in the direct synthesis of DMC from methanol and carbon dioxide at 170o C for 12 h. The catalysts examined in this work were highly selective for the formation of DMC without by-products. The amount of DMC produced increased with increasing reaction time but there was no significant increase after 10 h. Although the amount of DMC produced was quite low even on the most active catalyst, the catalytic performance of H3PW12O40/Ce x Ti1-x O2 was comparable to the reported value [11]. As expected, the H3PW12O40/Ce x Ti1-x O2 catalyst showed a better catalytic performance than the corresponding Ce x Ti1-x O2. This might be due to the bifunctional catalysis of Brönsted acid sites (provided by H3PW12O40) and base sites (provided by Ce x Ti1-x O2) [9,11]. The catalytic performance of H3PW12O40/Ce x Ti1-x O2 showed a volcano curve with respect to the cerium content. It should be noted that H3PW12O40/Ce0.1Ti0.9O2 showed the highest308 KYUNG WON LA, IN KYU SONG: DIMETHYL CARBONATE catalytic performance among the H3PW12O40/Ce x Ti1-x O2 catalysts, and a better catalytic performance than the H3PW12O40/ZrO2 catalyst.Fig. 4. SEM images of H3PW12O40/ZrO2 and H3PW12O40/Ce0.1Ti0.9O2:(a) H3PW12O40/ZrO2 (before reaction), (b) H3PW12O40/ZrO2 (after reaction),(c) H3PW12O40/Ce0.1Ti0.9O2 (before reaction), (d) H3PW12O40/Ce0.1Ti0.9O2 (afterreaction)Figure 4 shows SEM images of H3PW12O40/ZrO2 and H3PW12O40/Ce0.1Ti0.9O2 catalysts obtained before and after the reaction. In both cases, there was no significant difference in the catalyst morphology before and after the reaction. This suggests that these heterogeneous catalysts are stable during the catalytic reaction performed in the liquid phase.CONCLUSIONSIn the direct synthesis of dimethyl carbonate from methanol and carbon dioxide, it was found that the catalytic performance of metal oxides was in the following order: Ce0.1Ti0.9O2>Ce x Ti1-x O2 (x = 0.2-0.8)>ZrO2>CeO2>TiO2.KYUNG WON LA, IN KYU SONG: DIMETHYL CARBONATE 309 H3PW12O40/Ce x Ti1-x O2 showed a better catalytic performance than the corresponding Ce x Ti1-x O2, due to the bifunctional catalysis of Brönsted acid sites (provided by H3PW12O40) and base sites (provided by Ce x Ti1-x O2). The H3PW12O40/Ce0.1Ti0.9O2 showed the highest catalytic performance among the H3PW12O40/Ce x Ti1-x O2 catalysts, and a better catalytic performance than the H3PW12O40/ZrO2 catalyst.Acknowledgements. This work was supported by Small and Medium Business Administration, Seoul City, and Separation Design Tech Co.: 2005-017-7). REREFENCES1. A.A. Shaikh, S. Silvaram: Chem. Rev., 96, 951 (1996).2. H. Babad, A.G. Zeiler: Chem. Rev., 73, 75 (1973).3. D. Molzahn, M.E. Jones, G.E. Hartwell, J. Puga: US Patent, 5,387,708 (1995).4. D. Delledonne, F. Rivetti, U. Ramano: Appl. Catal. A, 221, 241 (2001).5. K. Tomishige, Y. Ikeda, T. Sakaihiro, K. Fujimoto: J. Catal., 192, 355 (2000).6. J. Kizlink: Collect. Czech. Chem. Comm., 58, 1399 (1993).7. J. Kizlink, I. Pastucha: Collect. Czech. Chem. Comm., 60, 687 (1995).8. S. Fang, K. Fujimoto: Appl. Catal. A, 142, L1 (1996).9. K. Tomishige, T. Sakaihiro, Y. Ikeda, K. Fujimoto: Catal. Lett., 58, 225 (1999).10. K.T. Jung, A. Bell: J. Catal., 204, 339 (2001).11. C. Jiang, Y. Guo, C. Wang, C. Hu, Y. Wu, E. Wang: Appl. Catal. A, 256, 203 (2003).12. X.L. Wu, M. Xiao, Y.Z. Meng, Y.X. Lu: J. Mol. Catal. A, 238, 158 (2005).13. T. Zhao, Y. Han, Y. Sun: Fuel Process Technol., 62, 187 (2000).14. M.D. Biju, F. Lefebvre, S.B. Halligudi: J. Catal., 231, 1 (2005).15. K.T. Jung, Y.G. Shul, A.T. Bell: Korean J. Chem. Eng., 18, 992 (2001).16. U.L. Stangar, U. Opara, B. Orel: J. Sol-Gel Sci. Technol., 8, 751 (1997).17. J.H. Kim, B.H. Noh, G. Lee, S. Hong: Korean J. Chem. Eng., 22, 370 (2005).18. T. López, F. Dojas, R. Alexander-Katz, F. Galindo, A. Balankin, A. Buljan: J. Solid StateChem., 177, 1873 (2004).。