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Microwave-Assisted Catalyst-Free Transesterification of Triglycerides with 1-Butanol under Supercritical Conditions Jeroen Geuens,† Jennifer M. Kremsner,‡ Bernd A. Nebel,‡ Sigurd Schober,‡ Roger A. Dommisse,§ Martin Mittelbach,‡ Serge Tavernier,† C. Oliver Kappe,‡ and Bert U. W. Maes*,§ Department of Industrial Sciences, Karel De Grote-Hogeschool (AUHA), Salesianenlaan 30, B-2660 Hoboken, Belgium; Christian Doppler Laboratory for MicrowaVe Chemistry (CDLMC) and Institute of Chemistry, Karl-Franzens-UniVersity Graz, Heinrichstrasse 28, A-8010 Graz, Austria; and Department of Chemistry, UniVersity of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium ReceiVed October 18, 2007. ReVised Manuscript ReceiVed NoVember 14, 2007
A proof-of-concept study has been made to convert rapeseed oil into fatty acid butyl esters by means of microwave irradiation without using a catalyst. High conversions can be reached when the transesterification of triglycerides with 1-butanol was performed under near-critical or supercritical conditions. Microwave heating is an attractive method to perform high-temperature chemistry since the reaction mixture can be heated up considerably faster under carefully controlled conditions.
Introduction The importance of chemicals obtained from renewable resources is growing due to the decrease of mineral oil resources. At the moment the production of fatty acid methyl esters (FAME) in Europe is rising due to European legislations concerning the substitution of mineral diesel by so-called biodiesel. When the word “biodiesel” is used, one means FAME according to European standard EN 14214:2003. Research already proved the possibility to use vegetable oil for biodiesel production.1–6 Even waste fats and oils can be transformed into FAME7–11 for biodiesel use, but there are some disadvantages: FAME have a very short polar “head” (methyl carboxylate) which turns these esters into rather aggressive fluids. This was demonstrated by experiments with FAME in diesel engines:12–15 FAME caused swelling of the elastomeric material, even to such * Corresponding author: Tel +3232653205, Fax +3232653233, e-mail
[email protected]. † Karel De Grote-Hogeschool (AUHA). ‡ Karl-Franzens-University Graz. § University of Antwerp. (1) Meher, L. C.; Vidya Sagar, D.; Naik, S. N. Renewable Sustainable Energy ReV. 2006, 10, 248–268. (2) Schuchardt, U.; Sercheli, R.; Vargas, R. M. J. Braz. Chem. Soc. 1998, 9, 199–210. (3) Ma, F.; Hanna, M. A. Bioresour. Technol. 1999, 70, 1–15. (4) Mittelbach, M.; Wörgetter, M.; Pernkopf, J.; Junek, H. Energy Agric. 1983, 2, 369–384. (5) Mittelbach, M.; Trathnigg, B. Fat Sci. Technol. 1990, 92, 145–148. (6) Mittelbach, M. Österr. Chem. Ztg. 1989, 90, 147–151. (7) Mittelbach, M.; Pokits, B.; Silberholz, A. Proc. Altern. Energy Conf., NashVille 1992, 74–78. (8) Ahn, E.; Mittelbach, M. Food Safety Assur. Pre-harVest Phase 2002, 1, 342–345. (9) Zhang, Y.; Dubé, M. A.; McLean, D. D.; Kates, M. Bioresour. Technol. 2003, 89, 1–16. (10) Zhang, Y.; Dubé, M. A.; McLean, D. D.; Kates, M. Bioresour. Technol. 2003, 90, 229–240. (11) Dorado, M. P.; Ballesteros, E.; de Almeida, J. A.; Schellert, C.; Löhrlein, H. P.; Krause, R. Trans. ASAE 2002, 45, 525–529. (12) Schäfer, A. Elaeis 1995 (Special Issue), 61-78. (13) Romig, C. Elaeis 1995 (Special Issue), 121-135. (14) National biodiesel board, http://www.biodiesel.org/pdf_files/ fuelfactsheets/bdusage.pdf.
an extent that the engine started leaking. In addition, FAME dissolves certain kinds of paints used in the automotive industry. By producing fatty acid alkyl esters of higher alcohols, the “head” of the ester molecule becomes less polar, which results in less aggressive liquids. A second disadvantage of FAME is their rather high cloud and pour point.16–18 For fatty acid alkyl esters of higher alcohols these parameters are much lower.16–18 Unfortunately, hitherto the production of these fatty acid alkyl esters of higher alcohols is more expensive than the production of FAME since the higher alcohols are more expensive than methanol. However, 1-butanol could become a cheap higher alcohol in the near future, taking into account the research performed to produce it via a new bacterial process.19 Besides the use as a fuel, fatty acid butyl esters can readily be used for other applications such as cleaning agents, hydraulic fluids, lubricants, etc. An advantage of 1-butanol in comparison with methanol is its higher boiling point. This means that transesterification reactions can be performed at a higher temperature without reaching extremely high autogenic pressures when the reactions are performed in a closed vessel. On the other hand, a higher temperature necessarily means more energy input to heat the reaction mixture. When conventional heating is used, it will take a substantially longer time for the reaction mixture to reach the desired temperature. This problem can be addressed by using microwave irradiation to heat up the reaction mixture. In addition, microwave irradiation allows to perform the reactions at a very high temperature and autogenic pressure (near-critical or supercritical) when using dedicated commercially available equipment (Synthos 3000). Under such reaction conditions, the (15) Graboski, M. S.; McCormick, R. L. Prog. Energy Combust. Sci. 1998, 24, 125–164. (16) Knothe, G. Fuel Process. Technol. 2005, 86, 1059–1070. (17) Lang, X.; Dalai, A. K.; Bakhshi, N. N.; Reaney, M. J.; Hertz, P. B. Bioresour. Technol. 2001, 80, 53–62. (18) Lee, I.; Johnson, L. A.; Hammond, E. G. J. Am. Oil Chem. Soc. 1995, 72, 1155–1160. (19) Ezeji, T. C.; Qureshi, N.; Blaschek, H. P. Curr. Opin. Biotechnol. 2007, 18, 220–227.
10.1021/ef700617q CCC: $40.75 2008 American Chemical Society Published on Web 12/11/2007
644 Energy & Fuels, Vol. 22, No. 1, 2008 Scheme 1. Transesterification of a Triglyceride with 1-Butanol without Catalyst
alcohol itself might start acting as a catalyst, as has been reported for water, avoiding the use of a catalyst.20 Several articles have been published on the production of biodiesel (FAME) under microwave irradiation,21,23–25 but only in one of them the use of higher alcohols is mentioned22 and all reports use a catalyst. At the end of the reaction, two layers are formed: an ester layer and a glycerol layer. The main part of the catalyst will be present in the glycerol layer, and a part will be found in the ester layer. This induces two disadvantages: the glycerol needs to be purified for further use, and the catalyst has to be washed out of the ester layer, requiring a large amount of water which afterward has to be treated in a water purification plant. In the articles dealing with the transesterification of triglycerides with classical heating, a catalyst is used unless the reaction is performed under supercritical conditions.26,27 Transesterification reactions without a catalyst in near-critical or supercritical conditions using microwave irradiation have not been reported so far. In this proof-of-concept study we therefore used refined rapeseed oil and 1-butanol as reactants to investigate the feasibility of microwave-assisted catalyst-free transesterifications (Scheme 1). Experimental Section Refined rapeseed oil was obtained from Proviron, and analytical grade 1-butanol was used. It is important that the oil and the 1-butanol have a low water content since the presence of water will cause an increase in pressure (1-butanol and water form an azeotrope with a boiling point of 93 °C). The presence of water will also cause the hydrolysis of the formed ester, giving the fatty acid as a byproduct. The reactions were performed on a small scale (10 mL of rapeseed oil) in different microwave reactors: the MARS (multimode, CEM Corp.), the Discover (single-mode, CEM Corp.), and the Synthos 3000 (multimode, Anton Paar GmbH). An autoclave was used to make a comparison between the heating speed of microwave heating vs the heating speed when using conventional heating. The reaction mixtures were analyzed by means of gel permeation size exclusion chromatography (GPSEC), using isocratic elution with THF and refractive index detection. The GPSEC technique allows us to separate the tri-, di-, and monoglycerides (TG, DG, and MG) and the fatty acid butyl esters (FABE), and therefore one can deduce the degree of conversion directly from the GPSEC chromatogram.
Results and Discussion Low-Temperature Regime. Since there are not so much data available on the transesterification of oil with 1-butanol, some reference experiments were carried out with the use of 1% KOH (20) Kremsner, J. M.; Kappe, C. O. Eur. J. Org. Chem. 2005, 367, 2– 3679, and references cited therein. (21) Barnard, T. M.; Leadbeater, N. E.; Boucher, M. B.; Stencel, L. M.; Wilhite, B. A. Energy Fuels 2007, 21, 1777–1781. (22) Amore, K. M.; Leadbeater, N. E. Macromol. Rapid Commun. 2007, 28, 473–477. (23) Mazzocchia, C.; Modica, G.; Kaddouri, A.; Nannicini, R. C. R. Chimie 2004, 7, 601–605. (24) Leadbeater, N. E.; Stencel, L. M. Energy Fuels 2006, 20, 2281– 2283. (25) Hernando, J.; Leton, P.; Matia, M. P.; Novella, J. L.; Alvarez-Builla, J. Fuel 2007, 86, 1641–1644. (26) Warabi, Y.; Kusdiana, D.; Saka, S. Bioresour. Technol. 2004, 91, 283–287. (27) Demirbas, A. Energy ConVers. Manage. 2006, 47, 2271–2282.
Geuens et al.
as a catalyst. The reference experiments were performed at reflux temperature (117 °C) for 30 min in a round-bottomed flask in a Discover single-mode microwave reactor. Two reactions were carried out, the first one having a volumetric 1-butanol:oil ratio of 4.0:1.0 and the second one 2.5:1.0. GPSEC analysis revealed that all of the triglycerides were converted to butyl esters; the conversion was in both cases quantitative. Medium-Temperature Regime. The first test reactions in closed vessels were performed in a multimode instrument (MARS). Two reactions were carried out with 1% KOH at 150 °C: one with a 1-butanol:oil ratio of 4.0:1.0 and one with a ratio of 2.5:1.0. After 30 min, a conversion of 100% was obtained in both cases. When repeating these reactions without catalyst, no conversion was obtained, even if the reaction time was extended to 150 min. The absence of any conversion is due to the fact that the temperature and autogenic pressure parameter are substantially lower than required for critical conditions (the supercritical pressure for 1-butanol is 49 bar and the supercritical temperature is 287 °C). The maximum pressure and temperature for the employed reaction vessels (Greenchem) are limited to 14 bar and 200 °C, which simply does not allow to reach near-critical conditions of 1-butanol. High-Temperature Regime. Becaused of its unique high limits in pressure and temperature, the subsequent transesterifications were performed in the Synthos 3000.20 This multimode microwave autoclave provides access to ca. 300 °C internal reaction temperature and 80 bar of pressure. Nevertheless, it proved to be difficult to reach these high temperatures since most organic solvents (like 1-butanol) become increasingly microwave transparent with higher temperature.20 In an earlier publication, we have shown that silicon carbide (SiC) passive heating elements can significantly improve the heating rates of low absorbing solvents under microwave irradiation.28 Preliminary experiments employing up to three SiC elements (cylinders) placed within the 80 mL quartz reaction vessels demonstrated that upon using a temperature range of ca. 250–300 °C (i.e., in the near- and supercritical region of 1-butanol) conversion of the triglycerides to FABE was indeed observed. After 90 min reaction time typically a significant amount of FABE was detected by GPSEC (20–80%), along with the intermediate di- and monoglycerides. When using the SiC inserts, it was not possible to stir the reaction mixture with a magnetic stir bar. Therefore, a technical modification of the experimental setup was developed. When a special glass support, which is mounted in the Teflon caps of the quartz reaction vessels, was used, six SiC cylinders could be placed in the vessel at a certain height. This new setup enabled stirring of the reaction mixture (Figure 1). This assembly allowed us—within less than 10 min—to heat the reaction mixture to ca. 310 °C, corresponding to an autogenic pressure of 80 bar, and to maintain this temperature for 4 h (Figure 2). It has to be mentioned that measurement of internal reaction temperatures was, however, not possible. Temperature measurement was performed by an external IR sensor, registering the surface temperature of the quartz reaction vessel.29 Gratifyingly, the experiment described in Figure 2 (1-butanol: oil ratio of 2.5:1.0) performed at ca. 310 °C for 4 h led to almost complete conversion of rapeseed oil to FABE, despite the fact (28) Kremsner, J. M.; Kappe, C. O. J. Org. Chem. 2006, 71, 4651– 4658.
Transesterification of Triglycerides with 1-Butanol
Figure 1. (a) Schematic graph and (b) picture of the quartz microwave reaction vessel containing the reaction mixture, a stirring bar, and the six SiC cylinders which are kept in a certain height employing a special glass support mounted in the cap.
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Warabi describing transeserification of rapeseed oil in supercritical 1-butanol using conventional heating.26 Autoclave. In order to demonstrate that the reaction mixture can indeed be heated much faster when using microwave heating, we carried out the same reaction in an autoclave. The maximum attainable temperature of the autoclave used was 280 °C. The reaction mixture was heated (as fast as allowed) to this temperature. Interestingly, the autoclave needed 45 min to reach this temperature, whereas the microwave reactor easily reached 310 °C within only 8 min (Figure 2). Although it was not our primary aim to compare both heating techniques from the point of view of conversion, we also determined the conversion for the autoclave experiment. GPSEC revealed that it had a substantially lower conversion (65% FABE vs 91% FABE for microwave heating). This is perfectly normal as the reaction in the autoclave took place at a considerable lower temperature (280 °C vs 310 °C). Moreover, even a comparison of conversion at the same reaction temperature would be difficult as the cooling periods for microwave and conventional heating are also significantly different. This is due to the fact that in the autoclave the temperature decreases very slowly after switching off the heater while in the microwave reactor a built in air-flow fan system can cool the reaction mixture down to 50 °C within only 20 min. Conclusions
Figure 2. Temperature (T, external IR), pressure (p), and power (P) profiles for the reaction of 10 mL of rapeseed oil with 25 mL of 1-butanol (Synthos 3000 multimode microwave reactor, 1400 W maximum power, six SiC cylinders (Figure 1), magnetic stirring, calculated internal temperature ca. 310 °C).
that no catalyst was used. According to GPSEC analysis of the reaction mixture, 91% fatty acid butyl esters were present as well as small amounts of monoglycerides, diglycerides, and the free fatty acid. On the basis of the calculated internal reaction temperature (310 °C), it can be concluded that the experiment shown in Figure 2 is indeed performed under supercritical conditions.30 This is in agreement with the results reported by (29) In principle, an internal gas balloon thermometer for measuring the inner temperature of the reaction mixture in addition to the surface IR temperature of the reaction vessel can be employed with the Synthos 3000 microwave reactor. However, in this particular case, because of the use of the glass support, it was not possible to use the gas balloon thermometer. Therefore, in all the experiments the surface temperature of the reaction vessels was measured by an external IR sensor. Comparative measurements with an internal gas balloon thermometer confirmed that the IR values were always a constant ca. 30 °C lower than the reaction temperature monitored inside the vessel. All microwave autoclave reaction temperatures given in this article therefore refer to internal temperatures calculated from the measured vessel surface temperature using a calibration value of 30 °C. (30) Weast R. C.; Astle M. J.; Beyer W. H. CRC Handbook of Chemistry and Physics, 64th ed.; CRC Press: Boca Raton, FL, 1984.
Triglycerides can be converted to fatty acid butyl esters (FABE) without the use of a catalyst at near-critical or supercritical conditions of the alcohol using microwave irradiation as exemplified for rapeseed oil. Since no catalyst is used, the byproduct (glycerol) is free of salts and can be used without the need of purification. The excess of 1-butanol, which is used in the reaction, can easily be flashed off and reused. This makes the process a no-waste process. (All of the products can be used or reused, and no purification steps, for example washing steps, are needed.) Best results were obtained employing supercritical conditions (310 °C, ca. 80 bar) using a microwave autoclave. Since 1-butanol becomes very low microwave absorbing at the high temperatures required, passive heating elements made from silicon carbide were used to facilitate microwave heating of the reaction mixture. Microwave heating is an attractive method to perform these high-temperature transesterifications taking into account that the reaction mixture can be heated up much faster. For scale-up, one could for example consider to construct a continuous SiC-coated tubular reactor heated by means of microwave irradiation. In this way, the reaction mixture can be heated very quickly, and when working with dedicated materials (microwave transparent composites), it might be possible to reach the required high pressures and temperatures. Acknowledgment. The authors thank the University of Antwerp (BOF) and the Karel-De Grote Hogeschool for financial support. We also thank Anton Paar GmbH (Graz, Austria) for providing microwave instrumentation (Synthos 3000) and passive heating elements. Supporting Information Available: More details about the experiments. This information is available free of charge via the Internet at http://pubs.acs.org. EF700617Q
