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Vietnam Journal of Science and Technology 56 (3B) (2018) 235-243 SYNTHESIS OF SOLID ACID-BASE CATALYSTS BASED ON Zn-Sn AND Al-Sn USED FOR CARBONYLATION OF GLYCEROL WITH UREA INTO GLYCEROL CARBONATE Ba Khiem Nguyen1, 2, *, T-Que Phuong Phan3, Huu Thien Pham3, Dinh Thanh Nguyen2, 3 1 College of Construction No2, No 190 Vo Van Ngan, Binh Tho Ward, Thu Duc District, Ho Chi Minh City 2 Graduate University of Science and Technology, No 18 Hoang Quoc Viet Street, Nghia Do Ward , Cau Giay District, Ha Noi 3 Institute of Applied Materials Science - VAST, No 1A TL 29 Street, Thanh Loc Ward, District 12, Ho Chi Minh City * Email: nguyenbakhiem789@gmail.com Received: 24 July 2018; Accepted for publication: 8 September 2018 ABSTRACT In this study, we successfully synthesized Zn-Sn and Al-Sn catalysts by the combination of co-precipitation and hydrothermal methods. These catalysts were prepared by hydrothermal method using ZnCl2, SnCl4 and AlCl3 precursors. Structure and physical properties of catalysts were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). The products of glycerol conversion product was also tested by gas chromatography. The catalytic activity of Zn-Sn and Al-Sn depends on reaction conditions (temperature and catalysts). The main product formed after the carbonylation reaction was glycerol carbonate. The selectivity of the main product was highest when using the Zn-Sn catalyst 5 % at a 145 °C during 5 hours, that the yield was 77.0 %, the conversion degree was 88.5 % and the selectivity was 86.9 %. For AlSn catalyst under the same reaction conditions, the efficiency was 61.3 % and the product selectivity achieved up to 74.1 %. Keywords: catalysts, co-precipitation method, glycerol, glycerol carbonate. 1. INTRODUCTION In biodiesel production, the amount of glycerol accounts for about 10% of the products, along with the demand for biodiesel, a large amount of glycerol is produced, but the value is relatively low. Thus derivatives of glycerol with potential values are a great interest of scientists [1, 2]. Glycerol carbonate is one of the most common glycerol derivatives [3]. Glycerol carbonate is a relatively new product in the chemical industry with great potential applications thanks to its low toxicity, good biodegradability and high boiling point. Glycerol carbonate is the most Ba Khiem Nguyen, T-Que Phuong Phan, Huu Thien Pham, Dinh Thanh Nguyen important intermediates in the production of useful substances in the polymer industry and other industries. Glycerol carbonate can be used to replace important compounds such as ethylene carbonate or propylene carbonate with various applications in many industries. We are interested in application of glycerol carbonate in the production of glycidol, a precursor widely used in organic synthesis, textiles, plastics, pharmaceuticals, cosmetics and other industries [4-9]. The recent study of Judith Granados-Reyes et al. published in 2016 [10], indicate that hydrocalumite (CaAl-LDHs) catalysts were tested in the reaction between glycerol and dimethyl carbonate to produce glycerol carbonate. After 3 hours of reaction, the glycerol conversion efficiency was as high as of 70-84 %, selectivity of glycerol carbonate was in the average range of 52-65 %, and a small amount of glycidol (7-15 %) was formed. The continuous increase in reaction time to 24 hours results in an increased selectivity of glycerol carbonate to 65-75 %. The multiple uses of catalysts can increase the glycidol content (30 %) while do not reduce the selectivity of glycerol carbonate (~70 %). The research team of Maria J. Climent [11] studied the use of oxides MgO and ZnO, and HT (Fe/Mg, Zn/Li) catalysts in converting glycerol to glycerol carbonate at 145 oC with the amount of catalyst used of 5 % wt, and reaction time for 5 hours. Under this reaction condition, the average yield was over 50 % and glycerol conversion was over 80 %. In 2016, Zati Ismah Ishak and his colleagues used liquid ion (LL) to convert glycerol to glycerol carbonate. Catalysts such as MA [NO3], EA [NO3] [Cl] gave the reaction yield of under 10 %, which is greater than that of the non-catalytic reaction, only 5 %. In addition, the authors continue to use LL catalyst system such as HEA [Fmt], emim [DMP], bmim [Dca], with the average conversion and efficiency of over 50 %, even the highest yield of 93 % with emim [Ac] [12]. Therefore, the choice of a catalyst suitable for both reactions and high glycerol conversion efficiency is our current interest. Among the catalysts, layered double hydroxide is commonly referred to hydrotalcite (HT) based on the name of a natural mineral Mg6Al2(OH)16CO3.4H2O. The general formula of HT is [MII1–xMIIIx(OH)2]x+[An–]x/n.mH2O. With such structure, the HT has the properties of absorption and ion exchange. Another special feature of HT is that the product after heating is able to memorize its layer structure when being brought back to the solution. Besides, by adding different M2+ and M3+ metals, different types of HT can be created flexibly depending on the function and purpose of use. For now, the other authors use co-precipitation method for preparing the catalysts. In our research, we used a combination of co-precipitation and hydrothermal method to synthesize ZnSn and Al-Sn. This is the first time the Al-Sn catalyst has been synthesized by this method with the purpose of choosing the suitable acid-base sites balance catalysts for the carbonylation of glycerol and urea. The combination of these two methods is a better way of aging for crystals. We also investigated the effect of synthesis conditions on the physical properties of the Zn-Sn and Al-Sn, and tested the activity of the catalysts particles as catalysts in synthesis of glycerol carbonate. 2. CHEMICALS AND METHOD 2.1. Chemicals and method The catalysts were synthesized by co-precipitation using zinc clorua (ZnCl2, 99 %, Merck), tin chloride (SnCl2.5H2O, 98 %, China) and aluminium chloride AlCl3 with sodas (Na2CO3, 99 %, China) acting as a precipitant. Glycerol (C3H8O3, > 99.5 %, Merck) and urea (CO(NH2)2, 99.5 %, Merck) were used for cabonylation reactions. The catalysts were synthesized by chemical co-precipitation method using zinc chloride (ZnCl2) and tin chloride (SnCl4.5H2O) with Na2CO3 as a co-precipitant. The precipitate was stirred for 2 hrs at room temperature, then 236 Synthesis of solid acid-base catalysts based on Zn-Sn and Al-Sn used for carbonylation … transferred into autoclave at 180 oC for 20 hrs via hydrothermal route. After that sthe ample removed from autoclave was filtered and dried at 80 °C for 10 hrs. 2.2. Physical characterization X-ray diffraction (XRD) patterns of catalysts were recorded with SIEMENS-D5000 (Ho Chi Minh City University of Technology) diffractometer using monochromatic high intensity Cu Kα radiation (λ = 0.15418 nm) at the scanning rate of 0.03o/s and in the scanning angle from 10 to 80o. Scanning Electron Microscope (SEM) was conducted using JSM-6500F, JEOL, whose images are available at the National Institute of Hygiene and Epidemiology. 2.3. Performance evaluation The products were analyzed by gas chromatography, FID detector (Perkin Elmer Claus 680) and FFAP capillary column (30 m in length, 0.25 mm of the diameter). The temperature of the system was programmed as follows: hold for 5 mins at 35 °C, then heated by 10 °C/min rate from 35 °C to 60 °C and kept for 1 min at 60 °C, then heated with 15 °C/min from 60 °C to 230 °C and hold for 10 mins at 230 °C. Conversion (%) = – × 100 Yield (%) = × 100 Selectivity (%) = . (1) (2) (3) 3. RESULTS AND DISCUSSION Figure. 1 XRD pattern of Zn-Sn catalyst. The XRD diagrams of catalyst samples prepared by hydrothermal method, recorded for 2 = 10-80o are showed in Figs 1, 2. The results of X-ray diffraction analysis of Zn-Sn samples 237 Ba Khiem Nguyen, T-Que Phuong Phan, Huu Thien Pham, Dinh Thanh Nguyen compared with standard Zn-Sn spectra (JCPDS No. 20-1455) [13] showed that the diffraction peaks appeared at 2 = 22.6, 32.5, 40.1, 46.7, 52.4, 57.8o are ascribed to (200), (220), (222), (400), (420), and (422) crystal planes of the double layer structure of Zn-Sn hydrotalcite, respectively. The peaks were not much sharp, showing the presence of a small amount of impurities in the form of crystals. This diagram is similar to the XRD spectrum of ZnSn(OH)6 synthesized by Swetha Sandesh [14]. Figure 2. XRD pattern of Al-Sn catalyst. Figure 2 shows the X-ray diffraction pattern of Al-Sn samples includes peaks of the crystalline phase of the standard Al-Sn phases, SnO phase and AlO(OH) phase corresponding to 2 = 22.6, 29.9, 32.3, 38.9, 42.5, 43.9 and 56.2 o. These peaks have high width, low intensity as the signal of the baseline. The diffraction baseline of the Al-Sn samples is generally rough, indicating low crystallization and the presence of impurities. The crystal sizes of Zn-Sn and AlSn are 61.5 nm and 23.9 nm as estimated based on the Scherrer equation [15]: dXRD = (4) where: dXRD is the mean size of the ordered (crystalline) domains; K is a dimensionless shape factor ~ 0.9; λ is the X-ray wavelength; β is the line broadening at half the maximum intensity; θ is the Bragg angle. The overall morphology of the Zn-Sn particles was monitored by SEM. As can be seen in Figure 3a, a large amount of Zn-Sn particles have the cubic shape with a length of 80 to 100 nm, of relative uniformity and high dispersion. The comparison with the studies of Wang et al. [16] shows the similarities in material morphology [16]. The SEM image of the Al-Sn samples showed that the particles crystallittes were small in size, fairly uniform, dispersed, and of highly homogeneous (Fig. 3b). In addition, Al-Sn has cylinder crystals with length of 60-80 nm, stacked and look like columnar structure by the SEM image of Al-Sn-N (structure of sputterdeposited Al-Sn-N thin film) in Lewin et al. studies [17]. The SEM image also shows the formation of outer cavities, with outer capillary channels formed from a combination of primary particles. 238 Synthesis of solid acid-base catalysts based on Zn-Sn and Al-Sn used for carbonylation … (a) (b) Figure 3. SEM image of catalysts Zn-Sn (a) and Al-Sn (b). Figure 4. Effect of catalysts on glycerol conversion and glycerol carbonate selectivity. The reactions were performed in the absence of solvent, under inert atmosphere with a molar glycerol/urea ratio of 1/1, at 145 oC and 5 wt % of Zn-Sn and Al-Sn catalyst. The results obtained are summarized in Table 1 and Figure 4. The study determined the best conditions as follows: 145 oC, in 5 hours, 5% catalyst for the conversion reaction from glycerol to glycerol carbonate. Using the Zn-Sn and Al-Sn catalysts: the conversion degree was 88.5 % and 78.2 %; the selectivity of glycerol carbonate was 86.9 % and 74.1 %; and the yield of carbonylation was 77 % and 58.4 %, respectively. The Zn-Sn gave performance values higher than the Al-Sn. The reaction of Zn-Sn and Al-Sn with urea released Zn2+ and Al3+, respectively in the liquid environment. This complex acts as the mediator in reaction conversion of glycerol to glycerol carbonate with urea. To explain the difference in activity of two catalysts, in our opinion the ZnSn catalyst has amount of active sites more than Al-Sn, so that Zn-Sn catalyst has high catalytic yield and glycerol carbonate selectivity. As seen via the results shown in Table 1, the selectivity and yield of our catalysts were not as good like the results by Pandian et al. [18]. This can be explained as follows, in [18] the authors used amount of 10 % wt catalyst and the catalyst is calcined at 600 ° C. In the present study, we used 5 % catalysts and our catalyst is heated in hydrothemal at 200 °C without calcining. The choice of 5 % wt catalysts used for carbonylation 239 Ba Khiem Nguyen, T-Que Phuong Phan, Huu Thien Pham, Dinh Thanh Nguyen has also been studied in a variety of other studies [17-20]. In the next time, we will study the effect of catalysts mass on the efficiency of the reaction as well as optimizing of the reaction conditions. Carbonylation of glycerol with urea into glycerol carbonate is based on the acid-base catalyst system, so the presence of Sn in the catalyst serves supply as the amount of acid sites for the Zn-Sn and Al-Sn catalysts. The balance between two acid sites (Sn) and base sites (Zn or Al) keep important role in the glycerolysis and urea to glycerol carbonate. Table 1. Results of carbonylation of glycerol and urea in the presence of solid catalysts. Entry Catalyst Glycerol conversion (%) Glycerol Carbonate yield Glycerol Carbonate selectivity (%) (%) 1 Blank 15.6 16.6 10.8 2 Zn-Sn 88.5 77.0 86.9 3 Al-Sn 78.8 58.4 74.1 4 ZnO* 66.4 65.1 98.1 5 SnO2* 39.6 39.2 99.0 6 Zn2Sn-600* 80.0 79.2 99.0 Reaction conditions: Glycerol/urea molar ratio = 1/1, 145 oC, 5 wt% catalysts at 5 h of reaction time. *Reaction conditions: Glycerol/ urea = 1/1.10 wt% catalysts, temperature = 155 oC, time = 4 h [18]. Figure 5. Effect of reaction temperature on glycerol conversion, yield and glycerol carbonate selectivity. The results showed that glycerol conversion to glycerol carbonate on Al-Sn catalysts was negligible at 175 oC. At 145 oC, 78.8 % of glycerol was converted and glycerol carbonate accounted for 58.4 % of the products. The selectivity of glycerol carbonate was very high, up to 74.1 % at 145°C. There was no significant difference in glycerol conversion at 145 oC and 175 o C on Al-Sn catalysts. However, the selectivity of glycerol carbonate was higher at 145 oC with the yield of 77%. When the temperature rose to 175oC, the efficiency was only 68.4 %. It can be 240 Synthesis of solid acid-base catalysts based on Zn-Sn and Al-Sn used for carbonylation … explained that when the temperature increases from 145 to 175 oC, amount of glycerol carbonate is converted to another, which leads to a decrease in the selection of glycerol carbonate by temperature. Therefore, we used 145oC for the conversion of glycerol to glycerol carbonate. Table 1 and 2 show that, with other synthetic methods and different reaction conditions compared with those reported in [14, 17-20], the results in this work are quite good. In the next time, we will improve the conditions of carbonylation reaction to get better results. Table 2. Results of carbonylation of glycerol and urea in the presence of solid catalysts at different temperature. Entry Reaction Temp (oC) Glycerol conversion (%) Glycerol Carbonate yield Glycerol Carbonate selectivity (%) (%) Zn-Sn 145 88.5 77.0 86.9 This work 175 89.1 68.4 61.0 Al-Sn 145 78.8 58.4 74.1 This work 175 85.9 27.0 23.1 Sn-beta [11] 145 70.0 25.9 37.0 Zn1-TPA [19] 140 49.5 42.3 85.4 4. CONCLUSIONS All in all, the solid acid-base catalysts for carbonylation reaction from glycerol to glycerol carbonate was employed in this study. The structure of the products was studied using XRD, SEM. The products of glycerol conversion product was also tested by gas chromatography. As the results, the Zn-Sn catalyst for conversion, selectivity and efficiency were significantly higher than the Al-Sn catalyst in reaction conditions at 145 oC, 5 hours and 5 % wt catalyst. The selectivity of the main product was highest when using the Zn-Sn catalyst of 5 % at 145 °C during 5 hours. 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