Microwave assisted synthesis of glycerol carbonate from glycerol and urea

Introduction

Glycerol is regarded as one of the most important bio-platform molecules. As a by-product from biodiesel production, glycerol is largely available as a result of the increase in the production of biodiesel. Effective and efficient conversion of glycerol to value-added products is an interesting topic. Glycerol is polyhydric, allowing some value-added products to be produced from it by different reactions, such as esterification [1], dehydration [2], oxidation [3], and hydrogenolysis [4].

Glycerol carbonate can be used as an intermediate in chemical reactions, especially as a new versatile building block for producing many chemicals [5, 6]. Glycerol carbonate can be synthesized by using different reactants, such as glycerol and glycidol [7]. In recent years, synthesis of glycerol carbonate from glycerol has received much attention. Glycerol can react with carbonyl chloride, carbon monoxide [8], carbon dioxide [9], dimethyl carbonate [10, 11], diethyl carbonate [12], cyclic carbonate [13] and urea to produce glycerol carbonate. The glycerolysis of urea will release ammonia as by-product (as shown in Scheme 1), making it very easy to separate product mixtures. Hydrotalcite-like solid catalysts [14], Zn/Al mixed oxide [15], metal monoglycerolates [16], lanthanum oxide [17], zinc chloride and zinc sulfate [18] have been used as catalysts for the synthesis of glycerol carbonate from glycerol and urea, but the reaction usually need long reaction time and/or excess of one reagent.

Scheme 1:

Microwave assisted synthesis of glycerol carbonate from glycerol and urea.

Microwave irradiation have been used in various processes, such as chemical reaction [19, 20], material synthesis [21], nanotechnology [22] and biochemical processes [23, 24]. It has been reported that microwave irradiation could raise the yield and reduced the reaction time in the synthesis of carbonates from CO₂ and epoxide [25], NaHCO₃ and olefins [26], CO and diols [27], and polycarbonates from open-ring polymerization of trimethylene carbonate [28]. In this work, we studied the effect of microwave irradiation on glycerol carbonate synthesis from glycerol and urea, hoping to get similar results. It was demonstrated that microwave irradiation could enhance the efficiency of the reaction effectively.

Materials and methods

Results and discussion

The influence of the concentration of catalyst, temperature, reaction time, microwave power and molar ratio on the synthesis of glycerol carbonate from glycerol and urea were studied in this work. Several commonly used catalysts [18, 29] for urea alcoholysis, including ZnCl₂, ZnSO₄, CuSO₄, MgSO₄ were tested, and the results are shown in Table 1. All the catalysts were effective for the reaction, and ZnSO₄ showed the best performance. In the following research, ZnSO₄ was chosen as catalyst for further optimization of the reaction conditions.

The effect of ZnSO₄ amount on the conversion and yield of glycerol carbonate under microwave irradiation is presented in Fig. 1. The reaction could take place under catalyst free conditions, but the catalyst could accelerate reaction rate effectively. The yield of glycerol carbonate reached 90.0% when 2.8 mol% ZnSO₄ was added, and the change of the yield was not obvious with further increasing the amount of the catalyst.

Fig. 1:

Effect of the concentration of catalyst (ZnSO₄) on the conversion of glycerol and the yield of glycerol carbonate. Reaction conditions: temperature 150°C, reaction time 100 min, 600 W microwave irradiation, molar ratio of urea to glycerol =1:1, pressure 15 KPa.

The effect of temperature on the conversion of glycerol and yield of glycerol carbonate under microwave irradiation is shown in Fig. 2. Under microwave irradiation, glycerol and urea could react at lower temperature, 50.9% of glycerol was transformed into glycerol carbonate at 125°C in 100 min. The yield of glycerol carbonate increased with temperature, and reached 90.0% at 150°C. With further increase of temperature, the yield of glycerol carbonate reduced, this may come from too fast decomposition of urea and some lost before reaction result from instability around 160°C.

Fig. 2:

Effect of temperature on the conversion of glycerol and the yield of glycerol carbonate. Reaction conditions: concentration of catalyst 2.8 mol% to glycerol, reaction time 100 min, 600 W microwave irradiation, molar ratio of urea to glycerol=1:1, pressure 15 KPa.

The dependence of conversion of glycerol and yield of glycerol carbonate on reaction time is illustrated in Fig. 3. The yield increased rapidly with time during the initial period, and reached 80.6% in 40 min under microwave (M) irradiation. Then the yield increased slowly with further extension of reaction time to 90.0% at 100 min. For comparison, the reaction was also carried out at the same temperature without (C) microwave and the results are also shown in Fig. 3. The yield of glycerol carbonate was 39.8% at 40 min and 61.0% at 100 min. The results indicate that microwave irradiation could enhance the yield of glycerol carbonate effectively when other reaction conditions were the same.

Fig. 3:

Effect of reaction time on the conversion of glycerol and the yield of glycerol carbonate with (M) and without (C) microwave irradiation (600 W). Reaction conditions: concentration of catalyst 2.8 mol% to glycerol, temperature 150°C, molar ratio of urea to glycerol=1:1, pressure 15 KPa.

The effect of microwave power on the conversion of glycerol and yield of glycerol carbonate is illustrated in Fig. 4. It can be seen from the figure that the conversion and yield increased with increasing power in lower power range. However, the conversion and yield became independent of the power when the power exceeded 600 W.

Fig. 4:

Effect of microwave power on the conversion of glycerol and the yield of glycerol carbonate. Reaction conditions: concentration of catalyst 2.8 mol% to glycerol, temperature 150°C, reaction time 100 min, molar ratio of urea to glycerol =1:1, pressure 15 KPa.

We also conducted the reaction at different urea to glycerol molar ratios (Ru), and the results are given in Fig. 5. It can be known from the figure that as the increase of the molar ratio of urea to glycerol, the conversion of glycerol increased synchronously, nearly all the glycerol could be converted (conversion 97.6%), while the yield of glycerol carbonate grew to 93.7% at 1.3:1 Ru and then started to decline slightly.

Fig. 5:

Effect of urea to glycerol molar ratio (Ru) on the reaction. Reaction conditions: concentration of catalyst 2.8 mol% to glycerol, temperature 150°C, reaction time 100 min, 600 W microwave irradiation, pressure 15 KPa.

Conclusions

The reaction of glycerol and urea to produce glycerol carbonate has been studied with and without microwave irradiation. It is shown that microwave can enhance the reaction effectively when ZnSO₄ is used as the catalyst. The conversion of glycerol and yield of the desired product depend on reaction conditions, such as amount of catalyst, reaction time, temperature, power of microwave, molar ratio of urea to glycerol. At optimized condition, the yield of the product can be as high as 93.7%. We believe that this route has promising potential of application in the synthesis of glycerol carbonate from glycerol and urea.