Transesterification of ethylene carbonate with methanol over Zn-La mixed oxide catalysts

JOURNAL OF FUEL CHEMISTRY AND TECHNOLOGY Volume 46, Issue 8, Aug 2018 Online English edition of the Chinese language journal Cite this article as: J Fuel Chem Technol, 2018, 46(8), 977984

RESEARCH PAPER

Transesterification of ethylene carbonate with methanol over Zn-La mixed oxide catalysts LI Hong-guang1,2, ZHANG Guo-quan1,2,*, WANG Yi1,2, ZHANG Sheng-jun1,2 1

Shaanxi Coal and Chemical Technology Institute Co., Ltd., Xi’an, 710065, China;

2

State Energy Key Lab of Clean Coal Grading Conversion, Xi’an, 710065, China

Abstract:

The transesterification of ethylene carbonate (EC) with methanol to synthesis dimethyl carbonate (DMC) and ethylene

glycol (EG) over ZnO, La2O3 and Zn-La mixed oxides were explored. The catalysts were prepared by co-precipitation method and characterized by BET, XRD, TG-DSC, CO2-TPD and Hammett titration. The influence of Zn/La atomic ratio, calcination temperature and reaction parameters (reaction temperature, reaction time, catalyst amount) on the catalytic activity were investigated. The results indicated that the binary Zn-La mixed oxide with Zn/La atomic ratio of 2:1 calcined at 500C showed the highest catalytic performance for the title reaction due to its strongest basicity, and the amount of strong basic sites of the catalyst should be responsible for the high transesterification activity. Key words:

transesterification; dimethyl carbonate; ethylene glycol

Dimethyl carbonate (DMC) has attracted great interests as an important green chemical, because of its negligible ecotoxicity and low bioaccumulation and persistence, which can be used to replace phosgene, dimethyl sulfate and methyl chloroformate in many chemical reactions, such as methylation, carbonylation and ester interchange reaction [1,2]. DMC can also be used as green solvents and electrolyte solutions for lithium ion battery[3]. Additionally, its high oxygen content makes it a promising candidate to be used as an oxygenated fuel additive, for instance to replace methyl-tert-butyl ether (MTBE)[4]. The oxidative carbonylation or phosgenation of methanol is currently used for the synthesis of DMC. Although these processes are fully developed and highly efficient, the use of toxic and corrosive reagents and the generation of wastes become important setbacks, requiring alternative techniques that are environmentally friendly[5,6]. As a result, many novel and green synthesis routes have been reported, including indirect catalytic transesterification with carbonates or urea as well as direct carbonylation of methanol with CO2[3,7–9]. However, the carbonylation of methanol with CO2 suffers from a low production rate, need for dehydrating agent and high reaction pressure. Therefore, the method of transesterification with carbonates or urea for the synthesis of DMC has gained a lot of research attention. Many efforts have

been made to improve the conversion and selectivity of the route of DMC synthesis by transesterification of methanol with ethylene carbonate (EC), and most of them have been focused on the catalyst development[4,8,10–15]. Although many of the reported homogeneous catalysts offered good conversion and selectivity for the synthesis of DMC, the difficulty of separating catalysts from reactants/products restrict the wide industrial use. Many kinds of mixed metal oxides, such as Mg-Ce, Cu-Zn-Al, Ce-La, Ca-M-Al (M=Mg, La, Ce, Y), Ca-Zn, Mg-Al-La, Ca-Zr, Zn-Y and CeO2, have been explored for the synthesis of DMC by transesterification[8,11–13,16–21]. However, most catalysts suffered from low catalytic activity, high reaction temperature or involvement of noble metals[20]. Hence there is need to develop an effective and suitable heterogeneous catalyst for the synthesis of DMC via transesterification of cyclic carbonate with methanol. The results of previous reports have indicated that ZnO has strong ability for the activation of methanol[3,22] and the activity of transesterification reaction relates with the basicity of catalysts[13]. In addition, the introduction of lanthanum could improve the basicity[13,23–25]. Therefore, in the present work, binary zinc-lanthanum mixed oxides were prepared and used for the DMC synthesis via transesterification of EC and methanol.

Received: 08-May-2018; Revised: 06-Jul-2018. Foundation items: Supported by the Science and Technology Research and Development Program of Shaanxi Province, China (2015GY085). Corresponding author. Tel: +86-29-88749702, Fax: +86-29-88749702, E-mail: [email protected]. Copyright  2018, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.

LI Hong-guang et al / Journal of Fuel Chemistry and Technology, 2018, 46(8): 977984

Fig. 1

XRD patterns (a) and CO2-TPD (b) profiles of the catalysts with different Zn/La atomic ratios ; ZnO; : La2O3; : La(OH)3

Table 1 BET specific surface area of various samples Catalyst

ZnO

La2O3

ZL-2-700

ZL-4-700

ZL-6-700

ZL-2-500

ZL-2-uncalcined

BET area A/(m2g1)

1.87

6.67

4.46

3.08

2.14

5.61

5.31

The influence of Zn/La atomic ratio, calcination temperature and reaction parameters on the catalytic activity were investigated. The relationship between the characters of catalysts and the catalytic activity was explored based on the characterization results.

1 Experimental 1.1

Materials

Methanol with ethylene carbonate, used for the transesterification reaction, were supplied by Sinopharm Chemical Reagent Co. Ltd. Nitrate, sodium carbonate, sodium hydroxide, ZnO and La2O3 were also supplied by Sinopharm Chemical Reagent Co. Ltd. 1.2

Catalyst preparation

Zn-La mixed oxides with different atomic ratios were prepared by co-precipitation method. Typically, a salt solution of Zn(NO3)2·6H2O and La(NO3)3·5H2O and a solution of 2 mol/L NaOH were prepared and added dropwise to 50 mL of deionized water under vigorous stirring with a constant pH of 11. Then, the precipitate was aged at room temperature for 3 h under vigorous stirring, followed by filtration and thorough washing with deionized water to remove the residual sodium ions. Finally, the precipitates were dried at 80°C for 12 h and calcined at different temperatures for 5 h in air. The catalysts were denoted as ZL-x-t, where x and t represent the atomic ratio of Zn/La and calcination temperature, respectively. ZnO and La2O3 were analytical grade commercial reagents. 1.3

Catalyst characterization

The surface area was determined with the Brunauer-Emmett-Teller (BET) method by N2 adsorption-desorption at liquid nitrogen using a Micromeritics Tristar II instrument. X-ray diffraction (XRD) patterns were recorded on a Panalytical X’Pert Pro X-ray diffractometer with Cu K radiation over the range of 5–90. Thermogravimetric analysis (TG-DSC) was performed on a simultaneous thermal analyzer (STA449F3, Netzsch) with 60 mL/min of air flow from 25 to 800°C at a heating rate of 10C/min. Hammett indicator method was used to determine the basic strength of the catalyst. Basicity was measured by the method of Hammett indicator-benzene carboxylic acid (0.01 mol/L anhydrous methanol solution) titration[20,26]. In addition, the surface basicity of the catalysts were also measured by temperature programmed desorption of CO2 (CO2-TPD) on AutoChem II 2920 chemisorption analyzer (Micromeritics). Each sample (0.15 g) was pretreated in Ar flow at calcined temperatures. The adsorption of CO2 was performed at 55°C and then followed by an Ar purge to remove the physisorbed CO2. The desorption process was performed with a heating rate of 10°C/min from 55°C to calcined temperatures. 1.4

Catalytic testing

(1) In a typical reaction, 8 g EC (90 mmol), 23 g methanol (720 mmol) and 0.6 g catalyst (7.5% weight percent respect to the amount of EC) were added into a 50 mL flask.

LI Hong-guang et al / Journal of Fuel Chemistry and Technology, 2018, 46(8): 977984 Table 2 Base strength (H_) and Basicity obtained from Hammett indicator measurements

Fig. 2

Catalyst

Base strength

Total basicity w/(mmolg1catal.)

ZL-2-uncalcined

9.3 < H_ < 15

13.3

ZL-2-500

9.3 < H_ < 15

27.6

ZL-2-700

9.3 < H_ < 15

20.7

ZL-4-700

9.3 < H_ < 15

19.3

ZL-6-700

9.3 < H_ < 15

16.2

Catalytic performances of ZnO, La2O3 and Zn-La catalysts on the transesterification of EC with methanol

reaction conditions: EC 8.0 g, MeOH 23.0 g, catalyst amount 0.62 g, reaction temperature 40°C, reaction time 2 h

The reaction mixture was heated up to 40°C and the reaction was conducted at atmospheric pressure. After the completion of reaction, the quantitative analysis of the product was determined by Agilent 7890 GC (FID detector) on a capillary column HP-INNOWAX (30 m  0.25 mm).

2 2.1

Results and discussion Effect of Zn/La atomic ratio

The XRD patterns of the samples with different atomic ratios of Zn/La calcined at 700°C are shown in Figure 1(a). It can be found that the typical patterns of ZnO and La 2O3 are observed for all the catalysts. In addition, the peak intensity of La2O3 increases with the decreasing of Zn/La atomic ratio. The increase of the lanthanum content of the catalyst may enhance its basic sites, and thus improves the catalytic performance. Moreover, the crystal of La(OH)3 is found on ZL-2-700 and ZL-4-700 samples. The formation of La(OH)3 may be ascribed to the deliquesce of La2O3 through adsorbing some moisture when exposed to air. The BET surface areas of the catalysts are given in Table 1. It can be found that ZnO has the smallest surface area of 1.87 m2/g, while La2O3 has the largest surface area. The surface area of the Zn-La catalyst increases with the decreasing of

Zn/La atomic ratio. Compared with the other Zn-La catalysts, ZL-2 shows the largest surface area. The results indicate that the introduction of lanthanum improves the surface area. And the increase of surface area may help the catalyst to exposure more active sites to improve the catalytic performance. The base strength (H_) and total basicity of the catalysts obtained from Hammett indicator measurements are shown in Table 2. The mixed Zn-La catalysts show strong base strength, and the H_ alkaline indexes for all the catalysts are between 9.3 and 15. Among the samples with different Zn/La atomic ratios and calcined at 700°C, ZL-2-700 possesses more basic sites, simultaneously with the amount of total basicity up to 20.7 mmol/gcatal.. In addition, the amount of total basicity increases with the decreasing of Zn/La atomic ratio, which suggests that the increase of lanthanum content favors to improve the catalyst basicity[13,23,26]. Moreover, the larger surface area of ZL-2-700 may also help to exposure more basic sites. The results of CO2-TPD are showed in Figure 1(b), which indicates that the amount of basic sites increase with the increasing of lanthanum content, especially for the second CO2 desorption peak appeared around 570°C. The phenomenon shows that lanthanum content has a significant influence on the catalyst basicity, which is in accordance with the results of Hammett titration. The strongest CO2 desorption peak appears on the ZL-2-700 sample, indicating that the catalyst basicity increases with the increasing amount of lanthanum. Moreover, it can be found that the introduction of lanthanum has a negligible influence on the first CO2 desorption peak appeared around 130°C. The above results indicate that the introduction of lanthanum improves the amount of moderately basic sites, which can be ascribed to the strong base of La oxide itself[13,23,25], and then help to improve the catalytic performance. The transesterification of EC and methanol for DMC synthesis was investigated over various catalysts (Figure 2). For comparison, the catalytic activity of ZnO and La2O3 were also explored, and the results suggest that pure ZnO and La2O3 show almost no catalytic activity for the reaction. It can be obviously found that the binary Zn-La mixed catalysts show much higher catalytic activity than pure metal oxides (ZnO and La2O3).

LI Hong-guang et al / Journal of Fuel Chemistry and Technology, 2018, 46(8): 977984

Fig. 3

Dependence of the EC conversion on the total basicity of Zn-La catalysts (the value of total basicity were obtained from Table 2)

Fig. 4

TG-DSC analysis of the ZL-2-uncalcined sample and XRD patterns of ZL-2 catalysts calcined at different temperatures ; ZnO; : La2O3; : La(OH)3; : La2O2CO3

Besides, EC conversion and DMC yield increase with the increasing of lanthanum content for all the Zn-La catalysts, which indicate that the catalytic activity of the Zn-La catalyst improves with the decreasing of Zn/La atomic ratio. Among all the examined catalysts, ZL-2-700 shows the highest EC conversion of 76.1%, with DMC yield of 76.1% and EG yield of 75.0%. The significant improvement of binary Zn-La catalysts’ catalytic performance can be related with the characterization results of the catalysts. Firstly, the increase of surface area with the introduction of lanthanum favors the catalysts to exposure more active sites, which then improve the catalytic performance. In addition, the results of previous reports indicated that the catalytic activity for the transesterification reaction strongly depended on the basicity of the catalysts[13,27,28]. The results of Table 2 and Figure 1 show that the introduction of lanthanum improves the catalyst basicity, which then favors to get high yield of DMC. It can be found that the catalytic activity follows the order of the amount of basicity: ZL-2-700 > ZL-4-700 > ZL-6-700. Figure 3(a) shows the relationship between EC conversion and total basicity of Zn/La catalysts, and the results suggest that the catalytic activity increases linearly with the increasing of basic sites amount. The above results keep in accordance with the

obtained conclusions by previous reports[10,18,28]. ZnO has strong ability to activate methanol and the introduction of lanthanum improves the surface basicity. The synergistic effect between ZnO and lanthanum oxides improves the catalytic performance. Therefore, the increase of surface area, the amount of basicity with the introduction of lanthanum and the synergistic effect between ZnO and lanthanum oxides are responsible for the improvement of activity. 2.2

Effect of calcination temperature

It is well known that calcination temperature has significant influence on catalyst composition, surface area, and basicity[29]. TG-DSC analysis was implemented for the ZL-2-uncalcined sample, as shown in Figure 4(a). It can be seen that there are two major weight loss processes, which represent the decomposition of lanthanum carbonate to form La 2O2CO3 at 380–480°C, and the decomposition of La2O2CO3 to form La2O3 at 600–740°C, respectively[30,31]. According to the XRD patterns of ZL-2 catalyst calcined at different temperatures, as shown in Figure 4(b), La2O2CO3 is the main phase after calcined at 500°C for ZL-2-500, while La2O3 is the main composition after treatment at 700°C for ZL-2-700, which are consistent with the results of TG-DSC analysis.

LI Hong-guang et al / Journal of Fuel Chemistry and Technology, 2018, 46(8): 977984

Fig. 5

CO2-TPD profiles and catalytic performance of ZL-2 catalysts calcined at different temperatures for the synthesis of DMC reaction conditions: EC 8.0 g, MeOH 23.0 g, catalyst amount 0.62 g, reaction temperature 40°C, reaction time 2 h

Fig. 6

Effect of reaction temperature (a), reaction time (b) and catalyst amount (c) on the activity of transesterification reaction conditions: EC 8.0 g, MeOH 23.0 g

In addition, the surface area increases from 5.31 m2/g (ZL-2-uncalcined) to 5.61 m2/g (ZL-2-500), and then decreases to 4.46 m2/g (ZL-2-700) with the increase of calcination temperature. The larger surface area of ZL-2-500 may favor the catalyst to expose more basic sites. Moreover, it can be found that the amount of basicity changes with the crystal transformation of lanthanum. Compared with ZL-2-700 and ZL-2-uncalcined, ZL-2-500 has larger amount of basicity, which could be ascribed to the formation of La2O2CO3[23,30]. The CO2-TPD profiles in Figure 5(a) indicate that the transformation of La2O2CO3 to La2O3 at high temperature leads to the decrease of catalyst basicity and base

strength, as verified by the second CO2 desorption peak shifting to lower temperature for ZL-2-500 and the results of Hammett titration in Table 2. Therefore, the formation of La2O2CO3 for the catalyst calcined at 500°C could offer more basic sites than La2O3, which can be related to the more lattice oxygen formed on the surface of La2O2CO3, accordance with the previous report[23]. The transesterification of EC and methanol for DMC synthesis was investigated over ZL-2 catalysts calcined at different temperatures, and the results are shown in Figure 5(b). The ZL-2-uncalcined catalyst shows almost no activity, which may be ascribed to the existence of crystal water on the

LI Hong-guang et al / Journal of Fuel Chemistry and Technology, 2018, 46(8): 977984

catalyst. The activity of ZL-2 catalysts increase with the increasing of calcination temperature, and reach the maximum value for the catalyst calcined at 500°C, but then decrease with further increasing of calcination temperature. For the ZL-2-500 catalyst, EC conversion, DMC yield, and EG yield are up to 82%, 82%, and 81% respectively. The high catalytic activity of ZL-2-500 can be related with its abundant basic sites (Table 2) and strong base strength, as evidenced by previous reports[13,32]. The results of Figure 3(b) suggest that the catalytic activity depends on the total basicity of the catalysts. ZL-2-500 has the most basicity, therefore shows the best activity.

increases with the increasing of the amount of basicity. The good performance of Zn-La mixed oxides can be related to the synergistic effect between ZnO and lanthanum oxides. The best catalytic activity is found for the catalyst of Zn/La with atomic ratio of 2:1 calcined at 500°C, and DMC yield could reach 82% on the optimal reaction conditions.

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2.3

Effect of reaction parameters

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3

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