Reducing the Carbon Footprint of Fuels and Petrochemicals DGMK Conference October 8 – 10, 2012, Berlin, Germany

Synthesis of Dimethyl from and 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 (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 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 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 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 of methanol (e.g. EniChem) [3]. Another synthesis method is the 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 (MC) as an intermediate is shown below in Scheme 1 [6]. In this process, 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. Concerning MC as reaction intermediate product, the first reaction step is thermodynamically favourable (Rg(25°C) ≈ -13.2 kJ/mol), but the second reaction step of DMC formation is thermodynamically unfavourable (Rg(25°C) ≈ +15.4 kJ/mol). Therefore, the expected DMC yield without ammonia removal from the reaction mixture would be quite low.

0,03

125 °C 0,02 110 °C 100 °C [%] eq Y-DMC 0,01

0,00 0246810 pressure [bar]

Fig. 2: Dependence of DMC yield on the reaction pressure and temperature (Molar ratio of urea : methanol = 1: 13).

The dependence of DMC yield on the reaction pressure and the temperature is shown in Fig. 2. The reaction pressures and temperatures in this case are varied in the range from 4 to 10 bar and 100 to 125 °C, respectively. It is evident that for the parameter range the pressure increase exhibits a negative effect on the formation of DMC. At the same time, increase in temperature favours the formation of DMC; as a result the yield of DMC can be enhanced.

60

150 °C, 14% urea 150 °C, 11% urea 50 150 °C, 4,8% urea 170 °C, 14% urea 170 °C, 11% urea 40 170 °C, 4,8 % urea 190 °C, 14% urea 190 °C, 11% urea 30 190 °C, 4,8 % urea Y(DMC) [%] Y(DMC)

20

10

0 10 12 14 16 18 20 22 p [bar] Fig. 3: Dependence of DMC yield on the temperature and mol % of urea in methanol as a function of pressure.

DGMK-Tagungsbericht 2012-3 206 Reducing the Carbon Footprint of Fuels and Petrochemicals

The Influence of the initial urea concentration on the yield of DMC at different reaction temperatures is depicted in Fig. 3. It can be seen that increase in urea concentration has shown an adverse effect on the yield of DMC and vice versa. On the other hand, increase in temperature (under identical concentration of urea) has exhibited promotional effect on the yield of DMC. In the case of higher reaction pressures better DMC yields could also be possible. From the best case, over 50 % yield of DMC can be achieved under the following conditions, e.g. T = 190 °C, p = 19 bar, urea concentration = 4.8 %.

Experimental work It has been observed that the reaction of urea with methanol runs even without a catalyst but interestingly the reaction proceeds mainly to form MC as the only product. Nevertheless, a very little amount of DMC was also formed (Fig. 4).

80,00 80,00 80,00 72,4 68,8 68,8 70,00 70,00 70,00

DMC 59,7 DMC DMC 60,00 MC 60,00 MC 60,00 MC 55,2 54,0

49,5 50,00 50,00 50,00 44,7

40,00 40,00 40,00 35,7 Y [%] Y [%] Y [%] 32,7

30,00 30,00 30,00

20,00 20,00 20,00

10,00 10,00 7,8 10,00

1,6 1,4 0,3 0,6 0,4 0,5 1,0 0,2 0,1 0,00 0,00 0,00 no catalyst ZnO CaO MgO no catalyst ZnO CaO MgO no catalyst MgO

Fig. 4: Effect of reaction temperature on the yields of DMC and MC obtained over different metal oxide catalysts (150 °C (left), 170 °C (middle) und 190 °C (right)).

It is also evident from these results that MC was the main product of the reaction. Interestingly, this is in good agreement with the thermodynamic calculations, and also in line with the results reported by Yang et al. [7]. The tests revealed reasonably good catalytic activity of ZnO, CaO and MgO catalysts while TiO2, ZrO2 and Al2O3 were found to be more or less inactive for the reaction under the conditions applied and hence such results are not shown in Fig. 4.

The expected DMC yield at 150 °C and at 13 bar is 1.7 % that can be increased to 5.6 % at 170 °C and 17 bar (cf. Fig. 3). This result implies that both the reaction temperature and pressure can show significant impact on the catalytic performance of the solids. It is also obvious that in the case of ZnO after 4 h of reaction, the reaction equilibrium for the formation of DMC seems to be achieved. Moreover, it was shown that the DMC yield increases as the temperature increases, in contrast, it decreases as the pressure increase (Fig. 4). The conversion of urea in most cases is nearly 100 %. The main products of the reaction are MC and DMC. However, the formation of certain amounts of unknown products can not be excluded due to the possibility of other reactions that can take place under the conditions applied (see Scheme 2). In the present case, the sum of MC and DMC accounts to ca. 70 %; this gives indications that some additional not identified products might have formed besides the major products.

The urea conversion data obtained at constant reaction temperature (T = 150 °C) over different metal oxide catalysts are presented in Fig. 5.

DGMK-Tagungsbericht 2012-3 207 Reducing the Carbon Footprint of Fuels and Petrochemicals

100

90 MC DMC 80 urea 70 72 60 69 50 60 56 Y/X [mol%] 40 45

30 0,3 0,6 20 1,6 1,4 0,2 26 10 20 14 18 15 0 no catalyst ZnO CaO MgO TiO2

Fig. 5: Dependence of conversion of urea and the yields of MC and DMC on the nature of metal oxide (T = 150 °C).

The mass balance after the reaction was almost 100 % during the blank tests (i.e. after the reaction without a catalyst). However, in other cases, about 10-35 mol % of reagents used were missing after the catalytic tests. The reason for this missing seems to be an adsorption of urea on the solid catalyst surface or probably due to some further unidentified side reactions. The urea adsorption was strong on the surface of CaO solid as proven by CHNS analysis.

In order to minimise the NH3 amount in the system in the direction of achieving higher DMC yields without removing of ammonia, one separate experiment was carried out using MC as a reactant. However, in the case of ZnO the DMC yield decreased from 7.8 to 1.6 %. The reason might be in-situ reaction of ZnO with isocyanic acid (NHCO) to form Zn(NCO)2(NH3)2 complex [14]. This compound is much more active for the reaction than the parent ZnO solid. The formation of isocyanic acid is expected from the thermal decomposition of urea during the course of the reaction at temperatures applied [13]. The side reaction of DMC and urea was also studied additionally and found that the products formed are MC and N-methylurea, which is in good agreement with the recent results of Wang et al. [12].

Conclusions The molar concentration of urea in the reactant feed mixture, reaction temperature, and the reaction pressure has shown clear influence on the yields of DMC. Furthermore, nature of catalyst also plays a key role on the catalytic performance, ZnO seems to be the best suited solid. The catalytic tests revealed that the optimal reaction temperature for the synthesis of DMC via urea alcoholysis with methanol is 170 °C (for reaction pressures below 20 bar). In the case of a higher temperature, the high autogenic pressure is unfavourable for the DMC formation. The most active catalyst for the second step synthesis of DMC from MC was Zn(NCO)2(NH3)2 formed in situ from ZnO. Using this catalyst, the predicted equilibrium yield of DMC was achieved after 4 h of reaction. In order to achieve higher DMC yields, ammonia removal is crucial and also the performance of reaction at low pressures (e.g. distillation) is recommendable. In the case of high DMC yield, the undesired side reaction of DMC with urea is however possible. MC and N-methylurea can be formed in that case. On the whole, it can be stated that ZnO solid has given the best yield of DMC (ca. 8 % at 170 °C) among various other catalysts applied.

DGMK-Tagungsbericht 2012-3 208 Reducing the Carbon Footprint of Fuels and Petrochemicals

Literature [1] M. Aresta, A. Dibenedetto, Dalton Trans. 28, 2975, (2007). [2] T. Sakakura, J.-C. Choi, H. Yasuda, Chem. Rev. 107, 2365, (2007). [3] T. Sakakura, K. Kohno, Chem. Commun. 11, 1312, (2009). [4] Y. Yamamoto, T. Matsuzaki, S. Tanaka, K. Nibishira, K. Ohdan, A. Nakamuraa, Y. Okamotoc, J. Chem. Soc. Faraday Trans. 93, 3721, (1997). [5] M. Wang, H. Wang, N. Zhao, W. Wei, Y, Sun, Catal. Commun. 7, 6, (2006). [6] M. Wang, H. Wang, N. Zhao, W. Wei, Y. Sun, Ind. Eng. Chem. Res. 44, 7596, (2005). [7] B. Yang, D. Wang, H. Lin, J. Sun, X. Wang, Catal. Commun. 7, 472, (2006). [8] M. Wang, H. Wang, N. Zhao, W. Wei, Y. Sun, Ind. Eng. Chem. Res. 46, 2683, (2007). [9] J. Sun, B. Yang, X. Wang, D. Wang, H. Lin, J. Mol. Catal. A: Chem. 239, 82, (2005). [10] X. Wang, B. Yang, D. Wang, X. Zhai, Chem. Eng. J., 122, 15, (2006). [11] Y. Fu, H. Zhu, J. Shen Thermochim. Acta 434, 88, (2005). [12] D. Wang, X. Zhang, Y. Gai, F. Xiao, W. Wei, Y. Sun, Catal. Commun. 11, 430, (2010). [13] P. M. Schaber, J. Colson, S. Higgins, D. Thielen, B. Anspach, J. Brauer, Therm. Acta 424, 131, (2004). [14] W. Zhao, W. Peng, D. Wang, N. Zhao, J. Li, F. Xiao, Catal. Commun. 10, 655, (2009).

DGMK-Tagungsbericht 2012-3 209