Conversion of as By-Product from Biodiesel Production to Value-Added Zul Ilham and Shiro Saka Abstract Current environmental issues, fluctuating fossil fuel price and energy security have led to an increase in worldwide biodiesel production, thus, creating a large surplus of glycerol formed as a by-product during the transesterification reaction. In this study, the conversion of glycerol from three different biodiesel production technologies (two-step supercritical dimethyl carbonate, supercritical methanol and alkali-catalyzed method) to glycerol carbonate by supercritical dimethyl carbonate was investigated. It was found that after treatment at 300 C/20MPa/20min in supercritical dimethyl carbonate, the yield of glycerol carbonate could reach 98wt% of the theoretical maximum when pure glycerol from two-step supercritical dimethyl carbonate or supercritical methanol were used. However, glycerol from alkali-catalyzed method which contains impurities such as sodium salts and water led to the decomposition of glycerol carbonate to glycidol. In addition, the results presented in this study showed the importance to monitor the reaction pressure in supercritical dimethyl carbonate to prevent decomposition and to ensure satisfying yield of glycerol carbonate. The non-catalytic supercritical dimethyl carbonate is, therefore, a potential method to convert glycerol to value-added product, glycerol carbonate. 1 Introduction The growing biodiesel production in recent years has generated a surplus of glycerol as a by-product. Without new applications, the overproduction will create a glut in glycerol market. Therefore, biodiesel production methods which are related to the manufacturing of high value-added chemicals are desired and needed. In line with this, one-step and two-step supercritical dimethyl carbonate method [1-3] has been established as new methods for biodiesel production without producing glycerol. From these processes, glycerol carbonate could be obtained instead of glycerol. In this study, the route for glycerol carbonate formation has Z. Ilham* and S. Saka ( ) Graduate School of Energy Science, Kyoto University, Kyoto, 606-8501, Japan e-mail: saka@energy.kyoto-u.ac.jp * PhD Student (AUN-SEED Net JICA) in Graduate School of Energy Science, Kyoto University from University of Malaya, Kuala Lumpur, 50603, Malaysia
been further elucidated to discuss the optimized reaction condition, utilization of glycerol from other production methods and the decomposition of glycerol carbonate to glycidol. 2 Experimental Various authentic compounds for standards and chemicals were obtained from Nacalai Tesque Inc., Japan, all of which are of highest purity available. from the two-step supercritical dimethyl carbonate method (hereinafter described as glycerol from the two-step method), glycerol from supercritical methanol method and unpurified glycerol from alkali-catalyzed method (hereinafter described as alkali-catalyzed method glycerol) were prepared and used in this study. Its properties are presented in Table 1. Those were, then, treated in the batch-type supercritical biomass conversion system [4] to have a reaction at supercritical conditions with dimethyl carbonate (Tc:274.9 C/Pc:4.63MPa) in a molar ratio of 1:10. All the experiments and analysis were conducted in compliance with the procedures described in our previous papers [1-5] and the reaction temperature and pressure were monitored by thermocouple and pressure gauge, respectively. Products were analyzed by High Performance Liquid Chromatography (HPLC) (Column: Ultrahydrogel 120, oven temperature: 40ºC, flow: 1mL/min, mobile phase: water, detector: RID 10A). Water content was detected by Karl-Fischer method and sodium salts was determined by titration [2]. Table 1 Properties of glycerol from various biodiesel production technologies Method purity Water Sodium Salts (wt%) (wt%) (wt%) Alkali-catalyzed 70 10 20 Supercritical methanol >99 n.d. n.d. Supercritical dimethyl >98 ~2 n.d. carbonate n.d. not detected
3 Results and Discussion 3.1 Optimization of Formation in Supercritical Dimethyl Fig. 1 shows the yield of glycerol from the two-step method as treated in supercritical dimethyl carbonate at 300 C with different reaction pressures. It could be seen from the graph that the trend shows higher formation of glycerol carbonate at higher reaction pressure. This is in agreement with our previous finding of supercritical fluid behavior where higher reaction pressure always leads to higher yield [5]. When the reaction condition was maintained at 300ºC/20MPa, the highest yield of glycerol carbonate based on the theoretical value at 98wt% could be achieved after 20 min reaction. However, when treated at 300ºC/5MPa and 300 º C/10MPa, yield of glycerol carbonate are low, indicating possible decomposition of glycerol carbonate under low reaction pressures. The reaction scheme for the conversion could be expected to proceed in a non-catalytic manner as depicted in Fig. 2. undergoes esterification with dimethyl carbonate, leading to the formation of thermodynamically stable five-member cyclic glycerol carbonate. Methanol, formed from this reaction, was removed by evaporation. The scheme is in agreement with several previous studies describing a similar method in a catalytic manner [6, 7]. (wt%) 100 80 60 40 20 300 C/40MPa 300 C/20MPa 300 C/15MPa 300 C/10MPa 300 C/5MPa 0 0 10 20 30 40 50 60 Reaction Time (min) Fig. 1 Yield of glycerol carbonate as glycerol from the two-step method was treated in supercritical dimethyl carbonate at 300 C with different reaction pressure
CHOH + CH 3 OCOOCH 3 CH 2 -O CH - O C - O + 2 CH 3 OH Dimethyl Methanol Fig. 2 Reaction scheme for glycerol carbonate formation in supercritical dimethyl carbonate 3.2 Yield of from Produced by Different Production Technologies Based on the optimized condition presented beforehand, glycerol from other biodiesel production methods (glycerol from supercritical methanol and alkali-catalyzed method glycerol [4]) were also treated in supercritical dimethyl carbonate at 300 C/20MPa to check for the possible conversion of glycerol to glycerol carbonate. Comparison of the yield is presented in Fig. 3. Pure glycerol produced by supercritical methods (two-step supercritical dimethyl carbonate method and supercritical methanol method) showed higher conversion to glycerol carbonate while alkali-catalyzed method glycerol showed significantly lower yield. This is possibly contributed by the amount of impurities available in the alkali-catalyzed method glycerol as shown in Table 1. 100 (wt%) 80 60 40 20 from Two-step Supercritical Dimethyl Method from Supercritical Methanol Method from Alkali-catalyzed Method 0 0 10 20 30 40 50 60 Retention Time (min) Fig. 3 Yield of glycerol carbonate from glycerol obtained by different production technologies
3.3 Decomposition of to Glycidol To confirm the effect of impurities in alkali-catalyzed method glycerol in reducing the yield of glycerol carbonate, a thorough analysis on the HPLC chromatogram of products obtained from alkali-catalyzed method glycerol was done. Interestingly, as shown in Fig. 4, the chromatogram showed an additional peak at 8.55 min, apart from only glycerol carbonate peak at 16.31 min found in products obtained from pure glycerol produced by supercritical methods (two-step supercritical dimethyl carbonate method and supercritical methanol method). When analyzed and compared with several authentic compounds, this was determined to be glycidol. Based on these findings, decomposition was expected to occur in the presence of water and sodium salts (impurities in alkali-catalyzed glycerol) to partly decompose glycerol carbonate into glycidol. The proposed scheme for this decomposition pathway is presented in Fig. 5. As discussed beforehand (Fig. 1), low reaction pressures in supercritical condition may also lead to the decomposition, following similar pathway as described. Glycidol Authentic Standards Obtained Products 0 10 20 30 40 Retention Time (min) Fig. 4 HPLC chromatograms of obtained products after glycerol from alkali-catalyzed method treated in supercritical dimethyl carbonate at 300 C/20MPa/20min. Standards of glycerol carbonate and glycidol are shown as authentic compounds
Main Reaction CHOH + CH 3 OCOOCH 3 CH 2 -O CH - O C - O + 2 CH 3 OH Dimethyl Methanol Decomposition H 2 O Sodium Salts CH 2 CH Glycidol O + CO 2 Carbon Dioxide Fig. 5 Reaction scheme for decomposition pathway of glycerol carbonate to form glycidol when glycerol with impurities treated in supercritical dimethyl carbonate 3.4 and Glycidol from Non-catalytic Supercritical Dimethyl Based on the results presented beforehand, it was found that the supercritical dimethyl carbonate could esterified glycerol to glycerol carbonate in high yield without any catalyst applied and could also form glycidol when decomposed. carbonate is a stable, colorless liquid currently used industrially as solvent and surfactant [7] while glycidol, is important in the production of epoxy resins and polyurethanes. Its high functionality, together with the versatile and well-investigated reactivity of its hydroxyl functions could supply as a basis for a variety of derivatives [8]. 4. Conclusions The results presented in this study showed that glycerol could be converted to value-added glycerol carbonate in supercritical dimethyl carbonate (300ºC/20MPa/20min) without any catalyst applied to have 98wt% yield. In addition, when glycerol with impurities such as water and sodium salts was used, glycerol carbonate would decompose to glycidol via the decomposition pathway. The reaction pressure must also be observed in supercritical dimethyl carbonate process as low reaction pressure could also lead to the decomposition. The formation of value-added chemicals, glycerol carbonate as well as the formation of glycidol from the non-catalytic supercritical dimethyl carbonate method
showed the potential of this method to be an alternative way to reduce glycerol glut from biodiesel production. Acknowledgement This study is partly funded by Japan Science and Technology Agency (JST) and Global-COE Program Energy Science in the Age of Global Warming, Kyoto University, supported by Ministry of Education, Culture, Sports, Science and Technology-Japan, for all of which the authors highly acknowledge. References 1. Ilham Z, Saka S (2009) Dimethyl carbonate as potential reactant in non-catalytic biodiesel production by supercritical method. Bioresour Technol 100:1793-1796 2. Ilham Z, Saka S (2010) Two-step supercritical dimethyl carbonate method for biodiesel production from Jatropha curcas oil. Bioresour Technol 101:2735-2740 3. Ilham Z, Saka S (2011) Production of biodiesel with glycerol carbonate by non-catalytic supercritical dimethyl carbonate. Lipid Technol 23:10-13 4. Kusdiana D, Saka S (2004) Effects of water on biodiesel fuel production by supercritical methanol treatment. Bioresour Technol 91:289-295 5. Warabi Y, Kusdiana D, Saka S (2004) Biodiesel fuel from vegetable oil by various supercritical alcohols. Appl Biochem Biotech 113-116:793-801 6. Ochoa-Gomez JR, Gomez-Jimenez-Aberasturi O, Maestro-Madurga B, Pesquera-Rodriguez A, Ramirez-Lopez C, Lorenzo-Ibaretta L, Torrecilla-Soria J, Villaran-Velasco MC (2009) Synthesis of glycerol carbonate from glycerol and dimethyl carbonate transesterification: Catalyst screening and reaction optimization. App Catal A: Gen 366:315-324 7. Herseczki Z, Varga T, Marton G (2009) Synthesis of glycerol carbonate from glycerol, a by-product of biodiesel production. Int J Chem Reactor Eng 7:1-14 8. Pagliaro M, Ciriminna R, Kimura H, Rossi M, DellaPina C (2007) From glycerol to value-added products. Angew Chem Int Ed 46:4434-4440