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Synthesis of Dimethyl Carbonate from Urea and Methanol Over Synthesis of Dimethyl Carbonate from Urea and Methanol over Heterogeneous Catalysts Mouhua wang Tong Wei Wei Wei Jinhai Yang, Xiuzhi Wang, Yuhan Sun (State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, China, 030001) Introduction In recent years, much attention has been paid to dimethyl carbonate (DMC) as an environmentally benign building block [1]. DMC has a versatile chemical property and has been used mainly to be a methylating and methoxycarbonylating agent as a safe substitute for dimethyl sulfate or methyl halides that are toxic or corrosive, it can also be used as a solvent to replace halogenated solvents such as chlorobenzene [2]. In addition, DMC is believed to be a good additive of gasoline in the future due to its high octane number of gasoline and good volatile. Dimethyl carbonate was usually produced from methanol and phosgene in a concentrated sodium hydroxide solution [3]. Because of the use of phosgene for its production, DMC has been limited in industrial use. Then, a non-phosgene process for preparing DMC by oxidation carbonlating of methanol in liquid phase was put on stream in the EniChem-Ravenna factory using CuCl catalysts [4]. However, the shortcomings of this process were low production rate, high cost of the separation of products and reactants, high recycles requirements and the need for corrosion resistant reactor and process lines. Another non-phosgene process was the synthesis of DMC via transesterification method [5]. While the method had its advantages, the reaction rate of epoxides with carbon dioxide was slow and required high pressure, and the exchange reaction of the cyclic carbonate with methanol was limited by equilibrium. Moreover, the reactant in the process (such as epoxides) was expensive, and then affected its economic. In our laboratory, a novel method to synthesize DMC from urea and methanol over solid base catalysts was explored. At the first, products were carbamate and ammonia, and then carbamate and methanol were further converted into DMC and ammonia. Ammonia could be recycled to produce urea with CO2. Obviously, this was an economically benign process. However, the related detail study for the process hasn’t been seen until now. Only two U.S. patents [6] were published for producing DMC from urea and methanol in homogenous catalyst in 2000. Homogenous catalysts were usually some complexes of organotin with a high boiling electron donor compound acting as solvent, which included bidentate ligands, and its preparation was very difficult and very expensive. In addition, the separation of the products was also very complex. Thus, homogeneous catalysts should be disadvantageous to industrialization. Therefore, considering the reaction thermodynamic and the reaction mechanism over homogenous catalysts, solid-base catalysts (such as CaO and MgO and ZnO-K2O and CaO loaded carbon) were studied for synthesis of dimethyl carbonate from urea and methanol here. Results and discussion The preparation of the solid-base catalysts was as same as the previous work [7]. The reaction was carried out in a 250mL flask equipped with reflux condenser, water bath and magnetic stirring. The products were analyzed by Gas Chromatograph after centrifugal separation of solid catalyst from liquid. Table1 showed the performance of these catalysts for the synthesis of DMC with urea and methanol. The catalysts preformed very well. The conversions of urea were above 70%, and the selectivity of DMC, especially for ZnO-K2O and CaO/Carbon, exceeded 30%, which were much better than the homogenous catalysts. The by-product was only methyl carbamate. This indicated that the first step was easier than the second step. In addition, the catalysts showed the following different reaction performance for methyl carbamate to DMC: CaO<MgO<Zn-K2O<CaO/carbon. The characterization by XRD, BET, and CO2-TPD indicated that the base strength and base density as well as pore structure of the catalysts might be very important. The results preliminarily demonstrated that the methanol - + - and urea were respectively activated to CH3O and (NH2)2-C (O ) over the catalysts, and - + + - - then CH3O attacked the C of (NH2)2-C (O ), and formed carbamate, and the CH3O sequentially attacked the C+ of the activated carbamate, and finally led to dimethyl carbonate. Because of the activation of carbamate was more difficult than that of urea, the selectivity of carbamate was higher than that of DMC. In conclusion, the solid-base catalysts demonstrated the high performance for the synthesis of DMC with urea and methanol. The heterogeneous catalysts would put forward to the industrial of this process. Table 1. The reaction performance of the solid-base catalysts Catalyst CH OH/Urea Urea conversion DMC selectivity methyl carbamate Catalyst 3 (g) (mol ratio) % ％ Selectivity, % CaO 4 4:1 80 25.65 74.35 MgO 1 4:1 88 28.55 71.45 ZnO-K2O 1 3:1 82 38.77 61.23 CaO-C 2 4:1 85 33.47 66.53 References: [1]. Yoshio Ono. Applied Catalysis A: General. 155(1997)133. [2]. A.G.Shaikh, Chem. Rev., 96(1996)951. [3]. H.Murdock, J.Phys.Chem., 23(1918)508. [4]. U.Romono, R.Tesei, et al,Ind.Eng.Chem.Prod. Res.＆Dev., 19(3)(1980)396. [5]. J. Haggin, C&EN, 70(18)(1992)25; [6]. Ryu. U.S.5902894(1999); Ryu. U.S.6010976(2000). [7]. T. Wei, M.Wang, W. Wei, Preprints of Symposia, Division of Fuel Chemistry, 223rd ACS National Meeting, (2002) 312. .
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