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Preparation and Characterization of MgO-CeO2 Mixed Oxide Catalysts by Modified Coprecipitation Using Ionic Liquids for Dimethyl Carbonate Synthesis Haznan Abimanyu, Byoung Sung Ahn, Chang Soo Kim,* and Kye Sang Yoo* Clean Energy Research Center, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, South Korea
Mixed oxides of magnesium and cerium were prepared by a modified coprecipitation method using roomtemperature ionic liquids. The catalytic performance of these catalysts has been successfully investigated in the cogeneration of dimethyl carbonate and ethylene glycol via the transesterification process of ethylene carbonate with methanol. It was found that addition of an ionic liquid as a template material in the coprecipitaion method increased the surface area and decreased the particle size of the catalyst. Moreover, ionic liquids increased the surface basicity of the particles as well. Among the ionic liquids, it has been found that 1-butyl3-methylimidazolium tetrafluoroborate ([Bmim][BF4]) was the most effective ionic liquid to prepare nanoporous MgO-CeO2 catalyst, which showed the best catalytic performance. 1. Introduction The synthesis and application of dimethyl carbonate (DMC) are achieving increasing importance due to its low toxicity and versatile reactivity. DMC can be used as environmentally friendly intermediate and starting material for organic synthesis via carbonylation and it can replace methylating agents such as dimethyl sulfate and methyl halides.1-3 DMC is also being considered as a component for replacing methyl tert-butyl ether (MTBE) as an oxygen-containing additive for gasoline, owing to its high oxygen content, good blending octane, and quick biodegradation.4 Moreover, DMC can be used in lithium batteries as an electrolyte due to its high dielectric constant.5 With the exception of the phosgenation route, there are three main production technologies for DMC synthesis: methanol oxycarbonylation, carbonylation of methyl nitrite, and transesterification of ethylene carbonate (EC) or urea.2,5,6 The phosgenation route has been losing attraction recently due to the use of virulent phosgene.5 In transesterification process, DMC is cogenerated with ethylene glycol (EG). This reaction takes place in the presence of a catalyst at about 100-150 °C at moderate pressure.7 Numerous homogeneous and heterogeneous, acid or base catalysts have been reported as being useful for this reaction. However, the base-catalyzed reaction appears generally to be the most effective for the synthesis of DMC.8,9 Various heterogeneous catalysts such as alkali-treated zeolite,10-12 basic metal oxides,13,14 and hydrotalcite15 have been used for the transesterification. In our previous work, MgO-CeO2 catalyst systems, which were prepared via coprecipitation of magnesium chloride and cerium(III) nitrate, were found to have an attractive catalytic performance without decay of activity and had excellent selectivity to the sum of dimethyl carbonate at a temperature of 150 °C and pressure of 0.2 MPa.16 The modification presented in this study is to apply an ionic liquid as a templating material. Ionic liquids (ILs) are an exceptional type of solvent consisting virtually only of ions. They have practically no vapor pressure and possess tunable solvent properties.17,18 In this study, various ionic liquids were used as templates in coprecipitation methods to prepare nanoporous MgO-CeO2 mixed oxide particles. The * To whom correspondence should be addressed. E-mail:
[email protected] (C.S.K.);
[email protected] (K.S.Y.).
low vapor pressure of an ionic liquid could assist in reducing the shrinkage problem during aging and drying, which could prevent reduction of surface area. An anion part of the ionic liquid was a crucial factor due to its varying strength of hydrogen bonding with water during the precipitation of precursor. A cation part of the ionic liquid had an influence on determining the pore size and volume of particles.19 Here nanoporous MgOCeO2 particles with high surface area were successfully synthesized using room-temperature ionic liquids. The prepared catalysts were characterized using X-ray diffraction (XRD), N2 physisorption, and scanning electron microscopy (SEM). 2. Experimental Section 2.1. Catalyst Preparation. Mixtures of Mg(OH)2-Ce(OH)4 hydroxide gels were prepared from their salt solutions as precursors by coprecipitation method. A 6.853 g sample of MgCl2‚6H2O (98%, Kanto) and 3.659 g of Ce(NO3)3‚6H2O (98.5%, Kanto) at a molar ratio (Mg/Ce) of 4 were first dissolved with 100 mL of distilled water to get 2 g of dried catalyst. One of the ILs (C-TRY, Korea) presented in Table 1 was added to the mixed solution with an IL/mixed oxide molar ratio of 3 at room temperature. A 1 M NaOH aqueous solution was slowly added to the mixture until the pH value reached 10 with stirring. The precipitates were further aged at room temperature for 5 h in the mother liquid. After filtration and washing with distilled water, the excess IL was extracted using acetonitrile (CH3CN) and filtrated. The obtained solid was dried at 110 °C for 12 h and then calcined at 500 °C for 5 h in air. The catalyst “MC41” was used as the control sample for comparison with other catalysts. It was prepared with the same method described above, but without IL addition. 2.2. Catalyst Characterization. The specific surface area, pore volume, and pore size were measured by using the BET method on a Micromeritics ASAP 2020 instrument. X-ray powder diffraction (XRD) patterns of catalysts were recorded using an X-ray diffractometer (Shimadzu XRD-6000) operated at 40 kV and 30 mA, using Cu KR (λ ) 0.154 18 nm) radiation to determine the crystal structure and crystallinity of the catalyst particles. Observation with field-emission SEM (HITACHI S-4200) for the samples was carried out at an accelerating voltage of 15.0 kV. The surface basicity and the base strength distribution were respectively analyzed by retroaldolization of the diacetone
10.1021/ie070528d CCC: $37.00 © 2007 American Chemical Society Published on Web 10/12/2007
Ind. Eng. Chem. Res., Vol. 46, No. 24, 2007 7937 Table 1. List of Room-Temperature Ionic Liquids symbola
full name
BPF6 HPF6 OPF6 BCF3 BBF4
1-butyl-3-methylimidazolium hexafluorophosphate 1-hexyl-3-methylimidazolium hexafluorophosphate 1-octyl-3-methylimidazolium hexafluorophosphate 1-butyl-3-methylimidazolium trifluoromethanesulfonate 1-butyl-3-methylimidazolium tetrafluoroborate
a
Used in this paper.
Table 2. Hammett Indicators Used for Benzoic Acid Titration indicator
base strength (H-)
phenolphthalein thymolphthalein 2,4-dinitroaniline 4-chloro-2-nitroaniline 4-nitroaniline 4-chloroaniline
8.2-9.8 9.3-10.5 15.0 17.2 18.4 26.5
alcohol (DAA) method and benzoic acid titration method using Hammett indicators according to the procedure in the literature.20 The catalytic basicity measurements by retroaldolization of DAA were carried out in a magnetically stirred liquid-phase suspension of catalyst at 50 °C. DAA (99%, Fluka) was solved in 40 mL of cyclohexane, resulting in a 0.05 M DAA solution. Samples of the reaction mixture were taken and analyzed by gas chromatography with a 25 m HP-FFAP Polyethylene Glycol TPA capillary column linked to a flame ionization detector at an isothermal column temperature of 80 °C. The basic strength distribution of the solid bases (H-) was determined from color changes of Hammett indicators (see Table 2) by benzoic acid (0.02 mol/L anhydrous cyclohexane solution) titration. 2.3. Catalytic Activity Measurement. The catalytic activity was measured by the transesterification process of ethylene carbonate with methanol, which was carried out in a vertical tube reactor according to Abimanyu et. al.16,21 with 1 g of catalyst powder, feed molar ratio (MeOH/EC) ) 4.0, and flow rate of 0.08 mL/min, corresponding to a liquid hourly space velocity (LHSV) of 3 h-1. The reaction temperature was fixed at 150 °C, and the reaction pressure was maintained to be constant by a BPR (back-pressure regulator) at 3.5 psig (0.2 MPa). The output products were analyzed using a gas chromatographic system (HP 6890 series) equipped with an FID detector and a capillary column (200 µm × 25.0 m, HP-FFAP Polyethylene Glycol TPA). 3. Results and Discussion 3.1. Effect of Ionic Liquid on Structural Properties. Figure 1 demonstrates XRD patterns of MgO-CeO2 mixed oxide catalysts. Figure 1a shows the pattern of mixed oxide prepared by conventional coprecipitation, while Figure 1b-f are respectively XRD patterns of catalysts prepared by coprecipitation with addition of ionic liquids according to Table 1. In the XRD profile of the MgO-CeO2 catalyst prepared by the coprecipitation method, Figure 1a, individual phases of CeO2 and MgO were observed without producing new compounds. The intensive and sharp diffractions at 2θ ) 31.8°, 43.1°, 45.5°, and 62.4° can be respectively primarily attributed to MgO corresponding to the Miller indices of (004), (200), (111), and (220), while the diffraction peaks at 2θ ) 28.8°, 33.3°, 47.7°, and 56.6° could be assigned to CeO2 corresponding to the Miller indices of (111), (200), (220), and (311), respectively. With addition of IL in the preparation of mixed oxides, the MgO peaks in the XRD patterns faded out and some of them disappeared. The XRD pattern of mixed oxide (BPF6) prepared with [Bmim][PF6] shows a weakened MgO peak at 2θ ) 31.8°
Figure 1. XRD profiles of MgO-CeO2 mixed oxide catalysts prepared with various ionic liquids: (a) MC41, (b) BPF6, (c) HPF6, (d) OPF6, (e) BCF3, and (f) BBF4. 0, MgO; O, CeO2.
Figure 2. Crystallite size and pore size of various catalysts.
as depicted in Figure 1b. Meanwhile, mixed oxides prepared using ILs with longer chains of cation and containing same anion ([PF6]-) display more MgO peaks in XRD patterns as illustrated in Figure 1c,d. Moreover, in Figure 1e,f there are some weakened MgO peaks (2θ ) 31.8° and 43.1°). Especially the peak of MgO over BBF4 is reduced significantly. This means that MgO is in a fine particle slaved in ceria. The addition of IL in the catalyst preparation decreased the crystallinity of particles and also changed the structure of the catalyst. The absence of peaks of MgO for the catalyst prepared by modified coprecipitation is presumably due to suppression of crystal growth of ceria. This fact is supported by the suggestion of Saito et al.22 that the MgO component is in a fine particle state. Thus, fine MgO particles over some catalysts prepared with IL have been well dispersed in the mixed oxide. The width of peaks of MgO-CeO2 in the X-ray diffraction curve reflects the average grain size, and it can be calculated using Scherrer’s formula. The average crystallite sizes of the catalysts were calculated and are illustrated in Figure 2 together with pore size. Obviously, it can be seen that the particle sizes of the catalysts prepared with addition of IL except BPF6 are much smaller than those of the catalysts prepared without IL. This result is in good agreement with the SEM data shown in Figure 3. This means that IL can avoid crystallite growth during catalyst preparation. On the other hand, pore sizes of the catalysts increased dramatically with the addition of ILs. This proves that IL could assist in reducing the problem of the gel shrinkage of the particles during sol aging and gel drying. The catalysts prepared using IL containing the same anion [PF6]-
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Figure 3. SEM images of MgO-CeO2 catalysts prepared by coprecipitation with and without IL: (a) MC41, (b) BPF6, (c) HPF6, (d) OPF6, (e) BCF3, and (f) BBF4.
had a decreasing pore size along with the length of cation chain as shown in Figure 2. This means that the templating effect leading to the formation of porous structure over HPF6 and OPF6 is less than that of [Bmim]+-based IL. The SEM results also support the above discussion (see Figure 3). The structural properties of the magnesium-cerium mixed oxides are shown in Figure 4. The surface area and pore volume of mixed oxides prepared by a modified coprecipitation method are obviously greater than those of the catalysts prepared without IL. When the catalysts were prepared using ILs, the surface area was enhanced 2.5-6.0 times more than that of particles prepared by coprecipitation without IL. Meanwhile, the pore volume increased 5.5-10.7 times. The effect of the anion part in the ionic liquid on the surface area and pore volume is illustrated in Figure 4a. The catalyst prepared with ionic liquid containing [BF4]- showed the highest surface area (55.4 m2/g) and the biggest pore volume (0.43 cm3/g), while BPF6 particles possessed the lowest surface area and pore volume of 26.8 m2/g and 0.22 cm3/g, respectively. It is worth noting that the anion part in the ionic liquid has the effect of improving the surface area and the pore volume of the particles. This phenomenon could be happening probably due to the hydrogen bond-co-π-π stack mechanism based on the special molecular structure and
property of the IL.23 The [BF4]- anion can maintain well an interaction balance to enhance the template effect. Figure 4b shows the effect of the cation part in the ionic liquid on the surface area and the pore volume. The higher surface area and pore volume could be obtained with larger cation parts of the ionic liquid. This is mainly attributed to the formation of a bigger IL template by a longer chain. However, OPF6 shows a negative effect of cation on the formation of a desirable pore structure. Even though it possesses the highest surface area (64.6 m2/g) and pore volume (0.42 cm3/g), the particles formed agglomerate due to less template effect (see Figure 3d). Hence, it was found that BBF4 is the most effective template in this synthesis system. It can reduce the shrinkage problem during aging and drying of precipitates; therefore, a reduction of surface area, pore size, and volume can be avoided. Recently, investigation of the kinetic growth of small particles to form larger regular structures or even patterns has attracted considerable attention. Since diffusion-limited aggregation (DLA) mostly leads to disordered particles with no structural porosity, the present reaction is believed to proceed via reactionlimited aggregation (RLA), where only a small fraction of the collisions result in the two particles involved adhering to each other.24 The formation mechanism of the present magnesia-
Ind. Eng. Chem. Res., Vol. 46, No. 24, 2007 7939
Figure 5. Correlation of surface area of catalyst to EC conversion and DMC yield.
Figure 4. Effect of (a) anion parts in ionic liquid with [Bmim]+ and (b) cation parts in ionic liquid with [PF6]- on surface area and pore volume of particles.
Figure 6. Correlation of rate constant (kDAA) measured as surface basicity of catalyst to EC conversion and DMC yield.
ceria mixed oxides in ionic liquids is interesting. A possible mechanism for the synthesis process of MgO-CeO2 mixed oxides in ionic liquids consists of three steps. At the beginning of the reaction in step I, magnesia salts and ceria salts hydrolyze quickly and presumably form into an amorphous MgO-CeO2 sol from a mixture of ionic liquid and salts of magnesium and cerium in water. In preparation step II, the sol particles aggregate effectively in the ionic liquid into a sol-gel network. Concomitantly, the ionic liquid becomes entrapped in the growing covalent magnesia-ceria network, rather than being chemically bound to the inorganic matrix. The amorphous sol particle ripens gradually into the fine particle as the reaction time proceeds. The anions interact with the hydroxides of magnesium and cerium and form hydrogen bonds in the reaction system. The cations will be also aligned and arrayed along the sol-gel phase, driven by the Coulomb coupling force with the anions. The fluid state of ILs makes relocation of the molecules possible and makes a localized vacant area among the molecules. A highly structured frame from the extended hydrogen bond system is formed and acts as a template for the nanostructured particles. In the final step of preparation, where water and ionic liquid are removed, the ionic liquid provides an attractive method for achieving a longer aging time before extraction due to its nonvolatile properties. A long aging time before extraction further increases the stability of the aerogel network and prevents shrinkage.25 A negligible vapor pressure of ionic liquids can prevent gel shrinkage and reduction in surface area, compared to conventional solvents which evaporate quickly before formation of a stable sol-gel network during the aging process. The anion and the cation in IL play an important role and act synergistically with each other for building a template for the nanostructured materials. The high ionic strength of ILs
increases the rate of aggregation. It is believed that there is no effect on the pore structure, when only anion or cation ligand is used as solvent. To investigate the effect of a nitrogencontaining ligand on the particle structure, N-methylimidazole (Nmim) was used as solvent in the preparation of MgO-CeO2 mixed oxide. It was found that the surface area of about 52 m2/g of the mixed oxide prepared using Nmim was obviously higher than that prepared with MC41 particles but lower than for the mixed oxide prepared with BBF4. The crystallite size about 35 nm calculated using Scherrer’s equation based on XRD result was evidently greater than that for the particles of mixed oxides prepared using ILs. Moreover, the pore size and pore volume of the mixed oxide were 181.26 nm and 0.26 cm3/g, respectively. The pore size was much bigger and the pore volume was smaller than those of mixed oxides prepared using ILs. These indicate that the effect of Nmim on the structural properties of the mixed oxide was not similar to that of ILs. Nmim increased the surface area of the particles, but did not improve their pore structure. 3.2. Effect of Ionic Liquid on Catalytic Performance. The catalytic performance of the catalysts was examined by transesterification between EC and methanol in a vertical tube reactor. Figure 5 shows a correlation between the catalytic performance of the catalysts and the surface area. Among the catalysts, the catalyst BBF4 prepared with ionic liquid [Bmim][BF4] produced the highest EC conversion (65.3%) and the highest DMC yield (56.6%), as well. Although its surface area was the biggest, the catalyst OPF6 obtained the lowest EC conversion and DMC yield of 46.4% and 42.1%, respectively. In the group of catalysts prepared with IL containing the same anion part ([PF6]-), the catalytic performance was reduced with the surface area. BPF6 with a surface area 26.8 m2/g could
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Table 3. DMC Selectivity and Base Strength Distribution (H-) of MgO-CeO2 Mixed Oxide Catalysts base strength distribution (H-)a (%)
a
catalysts
DMC selectivity (%)
8.2-10.5
10.5-15.0
15.0-17.2
17.2-18.4
18.4+
MC41 BPF6 HPF6 OPF6 BCF3 BBF4
90.33 92.99 85.61 90.67 87.03 87.00
26.4 30.3 29.5 21.1 9.6 19.3
27.0 35.7 34.8 44.5 33.8 32.5
29.3 18.8 21.0 17.6 29.7 31.7
11.4 8.9 8.7 8.8 14.1 8.6
5.9 6.1 6.0 8.0 12.9 7.8
Measured from the color changes of Hammett indicators by benzoic acid titration.
Scheme 1
convert 52.8% of EC to generate 49.0% of DMC. Meanwhile, OPF6 had the biggest surface area, 64.6 m2/g, but it generated the lowest DMC and converted the lowest EC, as well. On the other hand, the catalysts prepared using ionic liquids with the same cation part ([Bmim]+) showed relative correlation of the surface area to the catalytic activity. The surface area tended to increase significantly from BPF6 (26.8 m2/g) to BCF3 (37.9 m2/g) and to BBF4 (55.4 m2/g), but the catalytic activity showed a slight increase with EC conversion of 52.8%, 58.0%, and 65.3%, respectively. Figure 6 illustrates a correlation between the catalytic performance of the catalysts and the surface basicity. The retroaldolization of DAA is mentioned solely catalyzed by basic sites according to Scheme 1. The catalytic activity for the formation of acetone from DAA can be used as a direct measure of basicity.20 The catalyst activity could be characterized by the first-order rate constant (kDAA) per unit surface area. The reaction rate was obtained by following the consumption of DAA with time. The catalysts prepared with IL containing the same anion ([PF6]-) produced a decrease of surface basicity with increasing length of a cation chain. Meanwhile, EC conversion and DMC yield decreased as well with a decrease of surface basicity. OPF6 with the lowest basicity of 9.83 × 10-3 m-2 s-1 obtained the lowest EC conversion and DMC yield of 46.4% and 42.1%, respectively. It is worth remarking that the larger the cation part of IL containing the same anion, the greater decrease of the basicity of the catalyst. This is mainly attributed to the fact that poor structure causes less surface basicity determining catalytic performance. Furthermore, the catalysts with ILs containing different anions show also a similar effect of the surface basicity on the catalytic activity. The surface basicity increased from 13.12 × 10-3 to 18.96 × 10-3 m-2 s-1 for BPF6 and BCF3, respectively, and the EC conversion increased significantly from 52.8% to 58.0%. However, both catalysts produced quite similar DMC yields of 49.0% and 50.4%, respectively. Quite the opposite, DMC yield increased from 50.4% to 56.6% with an increase of surface basicity respectively for BCF3 and BBF4, and EC conversion increased as well from 58.0% to 65.3%, respectively. It is important to note that BBF4 possesses the highest surface basicity of 25.6 × 10-3 m-2 s-1 and the highest catalytic activity, as well. In our previous work,16 it was observed that strong base increases the EC conversion but not the DMC yield, so it produces more byproducts in the reaction. Meanwhile, moderate base is responsible for producing more DMC in this reaction.15
Table 3 illustrates DMC selectivity and base strength distribution (H-) of the mixed oxides. Although its basicity is the lowest among the catalysts containing the anion [PF6]-, OPF6 has a higher DMC selectivity than HPF6. This is possibly caused by the highest moderate base strength (10.5 e H- e 15.0) dominating on the surface of OPF6 particles, while HPF6 has the lowest among the catalysts containing the anion [PF6]-. On the other hand, HPF6 has higher catalytic activity than OPF6, because the basicity of OPF6 is lower. Despite having the highest strong base strength (17.2 e H- e 26.5), BCF3 has DMC selectivity similar to BBF4 due to a higher moderate base strength dominating on the surface of BCF3 particles. However, BBF4 has a better activity than BCF3 with higher values of EC conversion and DMC yield because of a higher basicity. The base strength distribution contributes also in the catalytic activity, whereas strong base strength stimulates the EC conversion and moderate base strength has an influence on the DMC yield. 4. Conclusions The mixed oxides of magnesium and cerium have been prepared using room-temperature ionic liquids and characterized. Transesterification of ethylene carbonate with methanol over MgO-CeO2 mixed oxide catalysts has been successfully demonstrated. The addition of ILs in the coprecipitation method changed not only the structure of particles, but also the surface basicity and the base strength distribution. ILs increased considerably the surface area and pore volume of the catalysts. Meanwhile, the crystallite size of the catalysts was reduced significantly. The surface area affects slightly the catalytic activity. On the other hand, the basicity had a significant effect on the catalytic activity. The base strength distribution contributes also to the catalytic activity, whereas strong base strength stimulates the EC conversion and moderate base strength has an influence on DMC yield. Among the ionic liquids, it was found that catalyst prepared using 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF4]) displayed the best performance in terms of activity. Literature Cited (1) Ono, Y. Dimethyl carbonate for environmentally benign reactions. Catal. Today 1995, 35, 15. (2) Ono, Y. Catalysis in the production and reactions of dimethyl carbonate, an environmentally benign building block. Appl. Catal., A: Gen. 1997, 155, 133. (3) Tundo, P. New developments in dimethyl carbonate chemistry. Pure Appl. Chem. 2001, 73, 1117. (4) Pacheco, M. A.; Marshall, C. L. Review of dimethyl carbonate (DMC) manufacture and its characteristics as a fuel additive. Energy Fuels 1997, 11, 2. (5) Wei, T.; Wang, M.; Wei, W.; Sun, Y.; Zhong, B. Synthesis of dimethyl carbonate by transesterification over CaO/carbon composites. Green Chem. 2003, 5, 343.
Ind. Eng. Chem. Res., Vol. 46, No. 24, 2007 7941 (6) Delledonne, D.; Rivetti, F.; Romano, U. Developments in the production and application of dimethyl carbonate. Appl. Catal., A: Gen. 2001, 221, 241. (7) Bhanage, B. M.; Fujita, S.; He, Y.; Ikushima, Y.; Shirai, M.; Torii, K.; Arai, M. Concurrent synthesis of dimethyl carbonate and ethylene glycol via transesterification of ethylene carbonate and methanol using smectite catalysts containing Mg and/or Ni. Catal. Lett. 2002, 83, 137. (8) Knifton, J. F. (Texaco Inc.). U.S. Patent 4,661,609, 1987. (9) Knifton, J. F.; Duranleau, R. G. Ethylene glycol-dimethyl carbonate cogeneration. J. Mol. Catal. 1991, 67, 389. (10) Kondoh, T.; Okada, Y.; Tanaka, F.; Asaoka, S.; Yamamoto, S. (Chiyoda Corporation). U.S. Patent 5,436,362, 1995. (11) Tatsumi, T.; Watanabe, Y.; Koyano, K. A. Synthesis of dimethyl carbonate from ethylene carbonate and methanol using TS-1 as solid base catalyst. Chem. Commun. 1996, 19, 2281. (12) Feng, X. J.; Lu, X. B.; He, R. Tertiary amino group covalently bonded to MCM-41 silica as heterogeneous catalyst for the continuous synthesis of dimethyl carbonate from methanol and ethylene carbonate. Appl. Catal., A: Gen. 2004, 272, 347. (13) Urano, Y.; Kirishiki, M.; Onda, Y.; Tsuneki, H. H. (Nippon Shokubai Co. Ltd.). U.S. Patent 5,430,170, 1995. (14) Bhanage, B. M.; Fujita, S.; Ikushima, Y.; Arai, M. Synthesis of dimethyl carbonate and glycols from carbon dioxide, epoxide, and methanol using heterogeneous basic metal oxide catalysts with high activity and selectivity. Appl. Catal., A: Gen. 2001, 219, 259. (15) Watanabe, Y.; Tatsumi, T. Hydrotalcite-type materials as catalysts for the synthesis of dimethyl carbonate from ethylene carbonate and methanol. Microporous Mesoporous Mater. 1998, 22, 399. (16) Abimanyu, H.; Kim, C. S.; Ahn, B. S.; Yoo, K. S. Synthesis of dimethyl carbonate by transesterification with various MgO-CeO2 mixed oxide catalysts. Catal. Lett. 2007, 1-2, 30.
(17) Wasserscheid, P.; Keim, W. Ionic LiquidssNew “Solutions” for Transition Metal Catalysis. Angew. Chem., Int. Ed. 2000, 39, 3772. (18) Welton, T. Room-Temperature Ionic Liquids. Solvents for Synthesis and Catalysis. Chem. ReV. 1999, 99, 2071. (19) Yoo, K. S.; Lee, T. G.; Kim, J. S. Preparation and characterization of mesoporous TiO2 particles by modified sol-gel method using ionic liquids. Microporous Mesoporous Mater. 2005, 84, 211. (20) Ka¨ssner, P.; Baerns, M. Comparative characterization of basicity and acidity of metal oxide catalysts for the oxidative coupling of methane by different methods. Appl. Catal., A: Gen. 1996, 139, 107. (21) Abimanyu, H.; Ahn, B. S.; Yoo, K. S. Model to Predict Dimethyl Carbonate Synthesis Through Transesterification of Ethylene Carbonate with Methanol in Fixed-bed Reactor. J. Ind. Eng. Chem. 2005, 11, 502. (22) Saito, M.; Itoh, M.; Iwamoto, J.; Li, C. Y.; Machida, K. Synergistic Effect of MgO and CeO2 as a Support for Ruthenium Catalysts in Ammonia Synthesis. Catal. Lett. 2006, 106, 107. (23) Zhou, Y.; Schattka, J. H.; Antonietti, M. Room-Temperature Ionic Liquids as Template to Monolithic Mesoporous Silica with Wormlike Pores via a Sol-gel Nanocasting Technique. Nano Lett. 2004, 3, 477. (24) Family, F.; Landau, D.; Eds. Kinetics of Aggregation and Gelation; North-Holland: Amsterdam, 1984. (25) Dai, S.; Ju, Y. H.; Gao, H. J.; Lin, J. S.; Pennycook, S. J.; Barnes, C. E. Preparation of silica aerogel using ionic liquids as solvents. Chem. Commun. 2000, 243-244.
ReceiVed for reView April 16, 2007 ReVised manuscript receiVed August 10, 2007 Accepted August 29, 2007 IE070528D
