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Synthesis of Dimethyl Carbonate from Urea and Methanol M

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Synthesis of Dimethyl Carbonate from Urea and Methanol M Reducing the Carbon Footprint of Fuels and Petrochemicals DGMK Conference October 8 – 10, 2012, Berlin, Germany Synthesis of Dimethyl Carbonate from Urea and Methanol M. Polyakov*, V.N. Kalevaru*, K. Müller**, W. Arlt**, J. Strautmann***, D. Kruse***, A. Martin* *Leibniz Institute for Catalysis at University of Rostock, Germany, **Friedrich-Alexander- University Erlangen-Nuremberg, Germany, ***Creavis Technologies and Innovation, Evonik Industries AG, Marl, Germany This project is funded by the German state of North Rhine-Westphalia (Ministry for innovation, science and research) and co-financed by the EU (European Regional Development Fund) as well as by Evonik Industries AG. Abstract Alcoholation of urea with methanol to produce dimethyl carbonate (DMC) is an interesting approach from both the ecological and economical points of view because the urea synthesis usually occurs by the direct use of carbon dioxide. Literature survey reveals that metal oxide catalysts for instance MgO, ZnO, etc. or polyphosphoric acids are mostly used as catalysts for this reaction. In this contribution, we describe the application of ZnO, MgO, CaO, TiO2, ZrO2 or Al2O3 catalysts for the above mentioned reaction. The catalytic activity of different metal oxides towards DMC synthesis was checked and additionally a comparison of achieved conversions with that of predictions made by thermodynamic calculations was also carried out. The achieved conversions are in good agreement with those of calculated ones. The test results reveal that the reaction pressure and temperature have a strong influence on the formation of DMC. Higher reaction pressure improved the yield of DMC. Among different catalysts investigated, ZnO displayed the best performance. The conversion of urea in most cases is close to 100 % and methyl carbamate MC is the major product of the reaction. A part of MC is subsequently converted to DMC, which however depends upon the reaction conditions applied and nature of catalyst used. From the best case, a DMC yield of ca. 8 % could be successfully achieved over ZnO catalyst. Introduction There is a great concern on the utilisation of CO2 due to its increased atmospheric concentration by anthropological emissions in the last few decades. Therefore, the fixation of inexpensive and abundantly available CO2 in valuable chemicals is gaining lot of interest in recent times. Despite this, there are only few industrial processes where CO2 is consumed directly. One such process is the synthesis of urea by the reaction of CO2 with NH3. About 70 Mt of CO2 p. a. are converted by this method worldwide [1]. Moreover, syntheses of various commercially important organic carbonates using CO2 are another suitable option of the future. Among different organic carbonates, the synthesis of dimethyl carbonate (DMC) in particular receives huge interest from scientific community because of the better utilization of zero cost CO2 for producing DMC with high commercial significance. DMC has various industrial applications. For example, it is a good substitute for phosgene in the polycarbonate synthesis [2]. Furthermore, DMC is also used as aprotic solvent and as a component of lithium ion batteries electrolytes. Until the 1980s, DMC was produced mainly by the reaction of methanol and phosgene [2]. Later on, this synthesis method was replaced by oxidative carbonylation of methanol (e.g. EniChem) [3]. Another synthesis method is the ethylene carbonate transesterification with methanol [4]. However these methods are still using toxic, corrosive und explosive substances. On the other hand, the alcoholation of urea with methanol to produce DMC is an interesting approach from both the environmental and economical view points [5]. Due to its DGMK-Tagungsbericht 2012-3, ISBN 978-3-941721-26-5 203 Reducing the Carbon Footprint of Fuels and Petrochemicals chemical fixation, certain amounts of CO2 exhausts from manufactures can be minimised. The urea synthesis runs normally at 180 °C and at high pressures. In this process, ammonium carbamate is formed first as an intermediate, which is later on dehydrated to urea. The urea then may undergo alcoholysis (e.g. by using methanol) to finally form the desired product DMC. The synthesis of DMC using methyl carbamate (MC) as an intermediate is shown below in Scheme 1 [6]. In this process, ammonia formed during the reaction can be recycled. Scheme 1: Urea alcoholysis by methanol to DMC combined with ammonia recycling. The synthesis of MC from urea runs without any catalyst [7]. For the whole reaction high temperatures and low pressures are favourable. The removal of ammonia can shift the reaction equilibrium to improve the yield of DMC formation. Wang et al. reported on those tests with metal oxide catalysts leading to 17 % DMC yield in an autoclave at autogenic pressure [5, 6]. During the reaction, ammonia was removed from the autoclave. DMC yields up to 70 % are also possible by using the catalytic distillation technique [8]. Polyphosphoric acid showed the best DMC yield in a batch process as reported by Sun et al. [9]. In the case of the reactive rectification, a DMC yield up to 92.2 % was also claimed [10]. The whole reaction network showing the formation of expected products during the process of urea alcoholysis with methanol is illustrated in Scheme 2. Furthermore, some side reactions can also take place and hence such possibility is included in the scheme. The formation of products (desired and undesired) however depends upon the reaction conditions applied and the nature of catalyst used. O O HO N NH2 urea O +HNCO +HNCO,-NH3, -H2O H N N NH -HNCO 2 H 2 H 2N NH2 NN biuret ammelide - OH NH + - 3 NH4 NCO H 3 N O OH + H 2 +NH3 HO N - -H2O -NH3 O NH NN HO N 2 HNC O H3C MeOH CO 2 isocyanic acid N NH cyanuric acid H 2 +MeOH OH NN N-methylurea - H2O O methyl ammeline carbamate NH2 H 2N OCH3 +NH3 O -H2O H C 3 MeOH CO2 NH N OCH H N N 2 H 3 2 O N-methyl methyl carbamate DMC NN OCH H 3CO 3 de c om pos itio n O CO2 melamine H3C CH3 DME NH2 Scheme 2. Possible side reactions and expected intermediate products during the process of urea alcoholysis by methanol to produce DMC. DGMK-Tagungsbericht 2012-3 204 Reducing the Carbon Footprint of Fuels and Petrochemicals In addition, the decomposition of DMC to dimethyl ether over MgO or Al2O3 catalysts is described [11]. Moreover, the reaction between urea and DMC to N-methylurea [12] and high temperature pyrolysis of urea to biuret were also reported elsewhere [13]. Scheme 2 and these above mentioned investigations point to a very complex reaction network. In this contribution, we describe the application of ZnO, MgO, CaO, TiO2, ZrO2 or Al2O3 catalysts for the synthesis of DMC from urea and methanol. The catalytic activity of these metal oxides towards DMC synthesis was checked and additionally a comparison of achieved conversions with that of predictions made by thermodynamic calculations was also carried out. Experimental The experiments were conducted in stainless steel 100 ml autoclaves (Roth). The autoclaves were heated in a heating block. The upper part of each autoclave was additionally heated with a heating jacket to the reaction temperature to prevent crystallisation of urea or methyl carbamate on the inner surface. The liquid phase was magnetically stirred at a stirring rate of 600 rpm. The pressure inside the reactor was continuously measured with a manometer. All used catalysts (ZnO, MgO, CaO, ZrO2, TiO2 or Al2O3) were used as purchased without further purification. In a typical experiment, urea (4.5 g), methanol and a catalyst (1 g) were placed into the autoclave. In general, the molar ratio of urea to methanol was 1: 8.2. The reactions were normally stopped after 4 h of reaction. The products were analysed in a systematic way using GC and NMR techniques according to the procedure described below. After the reaction, the product mixture was dissolved in methanol for GC analysis and 1H-NMR analysis. DMC and MC yield was measured with GC (Shimadzu, GC2014) equipped with FID and TCD, separation was carried out on a HP-5 column. However, it should be noted that the analysis of urea by GC is not possible because of its decomposition at temperatures above 175 °C. Therefore, the urea analysis and also DMC and MC yields (as a double check for GC results) as well as other by-products was estimated from 1H-NMR (AV 300 (Bruker), 7,0 Tesla, 300 MHz). The internal standard used during NMR analysis was 1,4-dichlorobenzene (110 mg), and the solvent was DMSO-D6. In addition, one should note that a reaction between NH3 (formed during the course of reaction) and DMC (the formed target product) is also expected; such reaction in turn leads to the formation of MC from DMC even at room temperature. In view of this, the samples collected were not stored longer than 2 days after their preparation for analysis. As an example, one such NMR spectrum showing all products of the reaction is illustrated in Fig. 1. H3COH O C H3CO OCH3 O Cl DMC C C H CO NH HC CH 3 2 O MC HC CH C C H COH H N NH 3 Cl O 2 2 1,4-Dichlorobenzene urea C H3CO NH2 MC DMSO-D6 99,5 % Fig. 1: NMR spectrum showing all the reaction and product components. DGMK-Tagungsbericht 2012-3 205 Reducing the Carbon Footprint of Fuels and Petrochemicals Results and Discussion Thermodynamic calculations At standard conditions, the whole reaction (from urea to DMC) is only slightly endergonic (Rg(25°C) ≈ +2.2 kJ/mol). Due to endothermicity (Rh(25°C) ≈ +61 kJ/mol) the thermodynamic driving force increases as temperature increases.
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