Ind. Eng. Chem. Res. 2010, 49, 9609–9617
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Synthesis of Dimethyl Carbonate from Methanol and Carbon Dioxide: Circumventing Thermodynamic Limitations Valerie Eta,† Pa¨ivi Ma¨ki-Arvela,† Anne-Riikka Leino,‡ Krisztia´n Korda´s,‡ Tapio Salmi,† Dmitry Yu. Murzin,*,† and Jyri-Pekka Mikkola†,§ Laboratory of Industrial Chemistry and Reaction Engineering, Process Chemistry Centre, Åbo Akademi UniVersity, Biskopsgatan 8, 20500 Turku/Åbo, Finland, Laboratory of Microelectronics and Materials Physics, EMPART Research Group of Infotech Oulu, UniVersity of Oulu, PL 4500, 90014 Oulu, and Technical Chemistry, Department of Chemistry, Chemical-Biological Center, Umeå UniVersity, SE-90187, Umeå
The synthesis of dimethyl carbonate from methanol and CO2 catalyzed by ZrO2 doped with KCl was investigated using chemical traps for water to circumvent thermodynamic limitations. The reaction, promoted by magnesium, occurred via the formation of carbonated magnesium methoxide (CMM) which adsorbed on the surface of ZrO2. The surface migration of the oxygen atom of ZrO2 to the surface methoxy groups of CMM resulted in the formation of dimethyl carbonate. The resulting MgO then reacted with methanol forming water and regenerating magnesium methoxide. The water formed reacted with the dehydrating agent, thus shifting the equilibrium toward a higher yield of DMC. The yield of 7.2 mol % DMC and 13.6 mol % conversion of methanol was obtained when methanol reacted with CO2 at 150 °C and 9.5 MPa for 8 h. The plausible reaction pathway is described. 1. Introduction The synthesis of renewable dimethyl carbonate (DMC) based on the utilization of carbon resources of anthropogenic origin is attracting much attention because of its potential industrial applications and ecological benefits.1 DMC exhibits versatile chemical reactivity and unique physical properties such as low toxicity and excellent biodegradability and thus is an environmentally benign building block for chemicals.2 It also finds applications in the synthesis of higher carbonates, carbamates, polyurethane, and a promising octane booster to meet the oxygenate specifications in gasoline. Furthermore, it can be used as a solvent, an alkylation agent, and a fuel additive to replace, for example, methyl or ethyl tert-butyl ether and a substitute for toxic carbonylating and methylating agents.3 Several routes exist for the synthesis of DMC such as phosgenation of methanol, oxidative carbonylation of methanol, urea methanolysis, transesterification of cyclic carbonates, and the direct reaction of CO2 with methanol. The direct synthesis of DMC from CO2 and methanol is the most promising and sustainable route eliminating toxic feedstocks like phosgene, carbon monoxide, and ethylene/propylene oxide used for phosgenation, oxidative carboxylation, and transesterification, respectively. Chemical utilization of CO2 as a renewable feedstock for the production of chemicals could form an integral part of carbon management and generating economic benefits. Although the direct synthetic method affords environmental benefits, the thermodynamic stability and inertness of CO2 limits its utilization for chemical synthesis.. Therefore an efficient energy source is required for the activation of CO2 prior to applications in organic synthesis. Efforts toward improving the yield of DMC from the direct route have mainly been focused on catalyst development and optimization of reaction conditions without addressing the * To whom correspondence should be addressed. E-mail: dmurzin@ abo.fi. † Åbo Akademi University. ‡ University of Oulu. § Umeå University.
limitations imposed by thermodynamics. Both homogeneous and heterogeneous catalysts have been reported as effective catalysts in the benign one-step synthesis of DMC. Examples of these comprise species such as organometallic compounds,4 zirconia or modified zirconia,5-7 potassium carbonate,8 metal tetraoxides,9 and modified copper-nickel catalyst10,11 with pressures ranging from 10 to 20 MPa for 10-15 h. Although the selectivity of DMC over these catalysts was high (>90%) under the reaction conditions reported, the yields of DMC obtained were nevertheless lower than 0.5 mol % relative to methanol even in the presence of additives like CaCl2 and molecular sieves. Orthoesters and acetals12-14 have been used as starting materials to improve the yield of DMC by employing homogeneous catalysts. The economy of the process and the difficulty of catalyst product separation render these synthetic routes a subject for further research. The transesterification reactions using ethylene oxide or ethylene carbonate over MgO15 or K2CO3/phosphonium halide functionalized polyethylene glycol16 offer another expectation for improved yields. DMC yield of 96 mol % was obtained relative to ethylene oxide over MgO with a selectivity of 28 mol % at 150 °C and 8 MPa. Other products included 31 mol % of ethylene carbonate and 26 mol % of ethylene glycol. The low selectivity and low methanol conversion achieved makes this reaction route unsatisfactory. The major difficulty associated with the synthesis of DMC in high yields from the direct reaction of carbon dioxide and methanol is the thermodynamic limitations embedded in the process. 2CH3OH + CO2 h (CH3O)2CO + H2O ∆G ) +26 kJ/mol (1) Other problems giving rise to the low yields of DMC include catalyst deactivation, hydrolysis of the DMC formed, the high bond energy of CO2, the reversible nature of the reaction, and the inability to utilize physical traps of water such as zeolites, CaCl2, and molecular sieves due to high operating temperatures and pressures. Since the reaction is nonspontaneous (eq 1) and the reaction equilibrium is quickly established (0.5 mmol DMC/
10.1021/ie1012147 2010 American Chemical Society Published on Web 09/24/2010
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Ind. Eng. Chem. Res., Vol. 49, No. 20, 2010
Scheme 1. Formation of DMC and Utilization of the Coproduced Water to Form Butylene Glycola
a Notation: 1, DMC; 2, butylene oxide; 3, butylene carbonate; 4, butylene glycol; 5, 1-methoxy-2-butanol; 6, 2-methoxybutanol.
mmol methanol at 150 °C),17 a chemical dehydrating agent can enhance the yield of DMC by disrupting the thermodynamic limitation, thereby shifting the equilibrium toward higher DMC yields (Scheme 1). Dehydrating additives like trimethyl orthoacetate, 2,2-dimethoxypropane acetonitrile, and a dehydrating tube packed with molecular sieves 3A18-21 have proven that the equilibrium yield can indeed be shifted thereby improving the yield of DMC. Herein, a concept of water removal via the addition of a chemical dehydrating agent in the direct reaction of methanol and CO2 catalyzed by ZrO2 doped with KCl, using magnesium turnings as a promoter, is presented. The effectiveness of butylene oxide, trimethyl orthoformate, and dimethoxymethane as effective traps for water are compared. The surface characteristics of the catalyst system for the formation of DMC are presented and discussed. 2. Experimental Section 2.1. Materials. Zirconyl chloride octahydrate, aqueous ammonium hydroxide (28%), methanol (99.8%), butylene oxide (99%), trimethyl orthoformate (99%), dimethoxymethane (99%), DMC (99%), magnesium methoxide (8 wt % solution in methanol), basic magnesium carbonate, potassium chloride, and magnesium turnings were purchased from Sigma Aldrich. CO2 (99.99%) was purchased from AGA. All chemicals were used as received. 2.2. Catalyst Preparation. Zirconia was prepared from zirconyl chloride octahydrate by modification of methods previously reported.22 A 0.5 M solution of zirconyl chloride was prepared at room temperature by dissolving ZrOCl2 in distilled water. The solution was precipitated by a dropwise addition of ammonium hydroxide solution under stirring and adjusting the pH to 9. The content was boiled in the mother liquor while maintaining the pH at 9 for 72 h. The product was washed several times with distilled water and filtered until a negative test for chloride ions was obtained using AgNO3. The cake was dried at 60 °C for 24 h and calcined for 5 h at 800 °C. Zirconia doped with potassium chloride (ZrO2-KCl) was prepared by addition of 20 wt % KCl to ZrO2 by the wet impregnation method. ZrO2-KCl-K2CO3 was also prepared by impregnating 18 wt % of KCl and 12 wt % of K2CO3 to
ZrO2. The slurries were dried, crushed, and calcined at 800 °C for 5 h. The catalysts were sieved through a 150 µm screen. 2.3. Catalytic Tests. The experiments were performed in an autoclave (Parr Inc.) with an inner volume of 300 mL equipped with a stirrer and an electric heater. In a typical reaction sequence, 1 g of catalyst and 0.5 g of magnesium turnings were introduced into the reactor followed by the addition of methanol (463 mmol) and butylene oxide (14.5 mmol). The content of the reactor was purged and pressurized to 5 MPa with CO2. The reactor was heated to 150 °C and kept at this temperature under stirring for 8 h. The product was cooled to 5 °C before venting out excess CO2. For the kinetic experiments, the amount of methanol (570 mmol) and dehydrating additive (19 mmol) were increased to allow for sampling. In addition, the reactor was slightly modified by including a sampling line made of stainless steel equipped with a system of valves at the inlet and outlet. The content trapped in the sampling line was cooled and the liquid collected slowly in a glass tube. The product in the liquid phase was analyzed by gas chromatography (Agilent Technologies, 6890N) using a 60 m × 320 µm × 0.5 µm capillary column (J&W 123-506 E DB5) equipped with a flame ionization detector (FID). The products were also identified by gas chromatography-mass spectrometry (GC-MS, Agilent Technologies, 6890N). The solid catalyst left in the reactor was dried and reduced over a stream of H2 in a tube furnace to obtain ZrO2-KC-Mg. On the account of the sensitivity of the solution to undergo oxidation and hydrolysis, all operations were carried out in the absence of air or moisture. The liquid phase was also analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES, PerkinElmer Optima 5300 DV) while the solid catalyst was dried at 60 °C and characterized. 2.4. Catalyst Characterization. 2.4.1. Surface Analysis of ZrO2-KCl and ZrO2-KCl-Mg. The structural properties of ZrO2-KCl and ZrO2-KCl-Mg (dried at 60 °C for 24 h) were carried out by X-ray diffraction (XRD, Siemens D5000), transmission electron microscopy (TEM, LEO 912 Omega), and electron diffraction. The details for the analytical procedures are described elsewhere.17 SEM/EDX and the quantitative spot analysis of the samples previously coated with carbon were measured using the SEM/EDX (Oxford Instruments) at 20 keV for 7 s with a takeoff angle of 36° and a dead time of 15.5 s. The specific surface area of the catalysts was studied by measuring the nitrogen adsorption-desorption isotherms (Sorptomatic 1900) in liquid nitrogen. Prior to measurements, the samples were outgassed for 3 h at 150 °C to a residual pressure below 0.01 Pa. The total surface area was calculated according to the BET equation. 2.4.2. Thermogravimetric Analysis. Thermogravimetric analysis was performed with a DSC-TGA (Q Series instrument) by subjecting 10.5 mg of ZrO2-KCl-Mg to 800 °C at 10 °C min-1 under a steady stream of nitrogen at 100 mL min-1. 2.4.3. Basicity and the Analysis of Evolved Gases. The basicity of the catalysts was studied by performing temperature programmed desorption of CO2 (CO2-TPD) (Micromeritics AutoChem 290) using a conventional flow-through reactor with CO2 as the probe molecule. A total of 100 mg of catalyst was flushed with a stream of helium (100 mL min-1) to clean the sample at 550 °C for 30 min before cooling to 50 °C. A stream of CO2 was allowed to saturate the solid particles for 30 min, and weakly bound CO2 and other physisorbed molecules were removed by a stream of pure He for 1 h. Chemisorbed CO2 was desorbed by heating the catalyst to 900 °C at a rate of 10 °C min-1 using He as the carrier gas. The effluent from the reactor was identified by the thermal conductivity detector. CO2
Ind. Eng. Chem. Res., Vol. 49, No. 20, 2010 Table 1. Synthesis of DMC from Methanol and CO2 Using Butylene Oxide as a Chemical Dehydrant at 9.5 MPa and 150 °C for 8 h mmol converted entry
catalyst
methanol
1 2 3 4 5 6 7 8 9 10 11
ZrO2-KCl ZrO2-KCl ZrO2-KCl-K2CO3 ZrO2-KCl-Mg ZrO2-KCl-Mg spent ZrO2-KCl-Mg Mg turnings filtrateb Mg(OCH3)2c Mg(OCH3)2 + ZrO2-KCl MgCO3
3 48 50 63 8 49 29 8 15 18 6
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a
mmol formed
butylene oxide
DMC
butylene carbonate
methoxybutanol
butylene glycol
2.6 2.8 2.1
6 7 6
6.8 7.3 7.6
3.3 1.1 1.2 1.3 1.1 4
8 4 8 6 4 2
6.2 10.4 0.2 9.6 9.2 8.0
0.4 11.4 10.8 16.6 0.5 8.6 9.2 0.7 2.8 4.1 0.3
14.1 14.4 14.2 14.4 14.2 14.2 14.3 11.7
a The input amounts in mmol were as follows: methanol ) 463, CO2 ) 490, butylene oxide ) 14.5. b The starting material was the filtrate obtained from the reaction with ZrO2-KCl + Mg (entry 4); results reported with respect to the original amount of methanol fed to the reactor. c The reaction was performed with 2 mL of solution of magnesium methoxide in methanol as a homogeneous catalyst.
Table 2. Conversions of Methanol and Butylene Oxide and the Selectivities of DMC and Butylene Glycola conversion (mol %) entry
catalyst
methanol
1 2 3 4 5 6 7 8 9 10 11
ZrO2-KCl ZrO2-KCl ZrO2-KCl-K2CO3 ZrO2-KCl-Mg ZrO2-KCl-Mgd spent ZrO2-KCl-Mg Mg turnings filtrated Mg(OCH3)2 Mg(OCH3)2 + ZrO2-KCl MgCO3
