Ind. Eng. Chem. Res. 2010, 49, 12135–12139
12135
Solid-Acid-Catalyzed Esterification of Oleic Acid Assisted by Microwave Heating Carlos A. R. Melo Ju´nior,† Carlos E. R. Albuquerque,† Juliana S. A. Carneiro,† Claudio Dariva,† Montserrat Fortuny,† Alexandre F. Santos,† Silvia M. S. Egues,† and Andre´ L. D. Ramos*,‡ Instituto de Tecnologia e Pesquisa - ITP, PEP/UniVersidade Tiradentes, AV. Murilo Dantas, 300, Farolaˆndia, Aracaju, SE, 49032-490, Brazil, and Departamento de Engenharia Quı´mica, UniVersidade Federal de Sergipe, Cidade UniVersita´ria Prof. Jose´ Aloı´sio de Campos, AV. Marechal Rondon S/N, Sa˜o Cristo´Va˜o, SE, 49100-000, Brazil
This work aims to evaluate the activity and stability of solid acid catalysts (niobium oxide and sulfated zirconia) in the methyl esterification of oleic acid assisted by microwave heating, an alternative route to the basecatalyzed transesterification reaction using conventional heating. Both catalysts have yielded similar conversions (68% for Nb2O5 and 68.7% for sulfated zirconia at 200 °C, t ) 20 min, methanol-to-fatty acid molar ratio ) 10, 5% w/w catalyst), much higher than the one obtained without catalyst (39.3%), besides a small deactivation after the fourth use (3% for Nb2O5, 11% for sulfated zirconia, compared to the first use). The results have not indicated a pronounced increase on reaction conversions by using microwave heating when compared to conventional data found in the literature. However, microwave irradiation remains attractive for heating esterification systems, since it allows for achieving high temperatures at reduced times. 1. Introduction Due to the increase in the price of petroleum and the environmental concerns about pollution, biodiesel production has been largely increased worldwide.1 In the Brazilian context, biodiesel production has been officially encouraged by the government since 2004, and the use of B5 (5% w/w biodiesel in diesel) is mandatory since January 2010, generating a demand of about 2.3 billion liters of biodiesel. The reasons for this growth are the political, economical, social, and environmental impacts of this biofuel. For instance, the higher cetane number of biodiesel results in shorter ignition delay and longer combustion duration, resulting in low particulate emissions and minimum carbon deposits on injector nozzles.2 Homogeneous base-catalyzed transesterification of triglycerides is the most commercially employed method for the biodiesel production.3 Despite all its advantages, including its low cost, there is a series of inconvenient aspects to this technology, such as the rigorous control of raw materials to present very limited content of water and free fatty acids (FFA) in order to avoid saponification,4 as soap formation decreases selectivity toward biodiesel, makes difficult the separation of the alkyl esters and glycerol, and contributes to emulsion formation during washing step.5,6 An alternative route for biodiesel production, the focus of the present work, is the esterification of fatty acids using acid catalysts. This reaction may be employed after the hydrolysis step of triglycerides7,8 or conjugated with the transesterification reaction.9,10 Since the free fatty acid is the reactant of the esterification reaction, there is no limitation in the free fatty acid content in the raw material, making it possible to use acid raw materials like waste cooking oil and beef tallow, without the need of a neutralization step. Soap is not formed, since the catalyst is not a base. An even more attractive route is the use of heterogeneous solid catalysts for the esterification reaction, decreasing the separation unit operations, making possible the reutilization of the catalyst, and producing high-purity glycerol (free of salts). * Corresponding author. Phone: + 55 79 2105 6687. E-mail:
[email protected]. † PEP/Universidade Tiradentes. ‡ Universidade Federal de Sergipe.
Concerning the catalysts used in the esterification reaction, solid acid catalysts are the most studied in the literature, like sulfonic exchange acid resins;10-13 sulfonated carbon composite;14 organosulfonic acid-functionalized mesoporous silicas;15 zeolites;16,17 niobium oxide;17-20 sulfated zirconia, tin oxide, or titania;20-23 tungstated zirconia;9,24 and Fe-Zn double-metal cyanide complexes.25 Some authors have presented important reviews about the use of solid acid catalysts for biodiesel production.3,26 On the other hand, all mentioned works use conventional heating. Microwave radiation has been widely used in organic synthesis with some advantages compared to convective heating, like energy savings, very low processing times, and homogeneous and fast heating.27,28 The heating of liquids using microwaves can be explained by the interaction of matter with the electromagnetic field of the incident radiation causing the movement of ions and rotation of molecule with induced and permanent dipoles.29 The ability of a specific sample to convert electromagnetic energy into heat at a given frequency and temperature strongly depends on the dielectric properties and the relaxation times of the mixture. As a consequence, the use of microwave irradiation provides a selective heating of the materials and the formation of “hot spots” into the sample, which favorably affect reaction rates, the so-called thermal effects.30 Moreover, some authors consider that microwaves can provide specific effects (not purely thermal), generally connected to the selective absorption of microwave energy by polar molecules, yielding modifications to the thermodynamic properties of the reaction systems, to the orientation of the molecular collisions, and to the activation energy.31 Concerning the use of microwave radiation for the biodiesel production, some authors have studied noncatalytic and catalytic transesterification of triglycerides, reporting a clear promoting effect of the microwave heating.32-34 However, few works deal with the esterification of fatty acids assisted by microwave radiation for biodiesel production. Some studies considered short chain fatty acids, like 2,4,6-trimethylbenzoic acid,35 acetic acid36 and propionic acid,37 or used other alcohols, like 2-ethylhexanol38 and butanols.39 We have previously reported the noncatalytic esterification of oleic acid assisted by microwave radiation.40 It should be
10.1021/ie100501d 2010 American Chemical Society Published on Web 10/22/2010
12136
Ind. Eng. Chem. Res., Vol. 49, No. 23, 2010
Table 1. Summary of the Variable Levels Investigated in the Reactions catalyst temperature (°C) catalyst concentration (%) molar ratio
none 150 0 5
Nb2O5 175 0.1 10
S-ZrO2 200 1 20
3
5
emphasized that the literature lacks information on the use of microwave heating during the esterification of long chain fatty acids. The aim of the current work was not to present a kinetic study of the esterification reaction but to investigate the activity and reuse of solid acid catalysts (niobium oxide and sulfated zirconia) in the methyl esterification of oleic acid assisted by microwave heating, evaluating the effects of operational variables, such as temperature, molar ratio of reactants and catalyst concentration, and checking differences between noncatalytic and catalytic reactions.
Figure 1. X-ray diffraction pattern of sulfated zirconia used as catalyst. Table 2. Textural Properties of Catalysts
2. Experimental Section 2.1. Materials and Catalysts. The chemicals oleic acid (C18H34O2, VETEC, 80%) and methanol (VETEC, 99.9%) were used as received. The impurities in oleic acid are other fatty acids (most of them C18 fatty acids, like stearic, linoleic, and linolenic acids), which are also reactants of the reaction. Niobium oxide (Nb2O5) was obtained in a hydrated form (HY340, CBMM) and was calcinated at 300 °C for 3 h before use. Sulfated zirconia (S-ZrO2) was prepared by mixing zirconium(IV) hydroxide (Aldrich, 97%) as precursor and ammonium sulfate (Aldrich, 99%) as sulfating agent, in an aqueous solution, for 2 h. The suspension was then dried at 120 °C overnight and calcinated at 500 °C for 5 h. 2.2. Catalysts Characterization. Textural properties of the catalyst were measured with a ASAP 2000 MICROMERITICS apparatus, using a high vacuum (10-5 Torr) system for degassing. After a drying step at 200 °C for 24 h, the sample was cooled down to -196 °C, and N2 adsorption isotherms were collected, from p/p0 ) 10-6 up to p/p0 ) 1. X-ray diffraction patterns were recorded on a MINIFLEX RIGAKU spectrometer using Cu KR (30 kV and 15 mA) radiation. Samples were not reduced for the analyses. Acidity was inferred using the method developed by Lo´pez et al.,41 which consists of an ion-exchange step, addition of the catalyst sample to a NaCl solution for 30 h at 28 °C, filtering, and titration with a NaOH solution. 2.3. Reaction Procedure. The experimental apparatus and procedure have been described in detail elsewhere.40 Briefly, the experimental unit consists of a microwave batch reactor (Synthos 3000, Anton-Paar), equipped with two magnetrons (1400 W, 2.45 GHz), a rotor system with the capacity of eight quartz vessels (80 mL each vessel), a magnetic stirrer in each vessel (up to 600 rpm), an infrared sensor (placed on the rotor bottom), an immersing gas bulb thermosensor (placed at one reference vessel), and a built-in forced-air cooling system. The investigated variables were type and concentration of catalyst (Nb2O5 and S-ZrO2), temperature (from 150 to 200 °C), reaction time (from 5 to 50 min), molar ratio (MR) of alcohol to acid (from 5 to 20), and reuse of catalysts. Table 1 displays a summary of the reaction conditions employed. The experimental procedure consisted of charging four reaction flasks with the reaction mixture and the catalyst, the concentration (w/w) of which is related to the mass of oleic acid. The reaction flasks were then pressurized with nitrogen at 20 bar in order to decrease the amount of methanol in the gas phase, coupled to the rotor, and placed into the equipment. The experimental variables (temperature and reaction time) were selected for the equipment, and after the experimental run was
catalyst
SBET (m2/g)
Vp (cm3/g)
Nb2O5 S-ZrO2
150 34.8
0.5 0.034
Table 3. Acidity of Catalysts by Ion-Exchange and Titration catalyst
acidity (µmol/g)
Nb2O5 S-ZrO2
144 ( 7 2.6 ( 0.3
completed, the system was cooled down up to cell temperatures around 40 °C. The vessels were removed from the equipment, the suspension was filtered under vacuum to recover the catalyst, the liquid phase was heated at 130 °C to eliminate any remaining methanol/water, and the acidity was measured using 0.25 N NaOH as titration agent. The reaction conversion, a mean value of the four reaction vessels, was then determined on the basis of the characterization of the acid index (AI) before and after the reaction.40 3. Results and Discussion 3.1. Crystalline, Textural, and Acid Properties of Catalysts. Niobium oxide sample was submitted to X-ray diffraction, but peaks were not observed, with a high signal-noise ratio. This observation confirms the information of the supplier, who claims that the catalyst sample is an amorphous material, without a crystalline pattern. Figure 1 displays the X-ray pattern of the sulfated zirconia. Peaks that are ascribed to the tetragonal crystalline phase of zirconia were detected. The results indicated that the sulfatation of zirconia apparently has not altered the crystallinity of the material, as the sulfated zirconia pattern found is very similar to the one of pure zirconia. Table 2 presents the textural properties of the catalysts. Niobium oxide presented a much higher surface area than sulfated zirconia, which is reflected in the lower pore volume of the latter. Table 3 shows the acidity of the catalysts measured by ion-exchange with NaCl and titration with NaOH. Niobium oxide exhibits higher acidity, with may represent an increase in the activity for the esterification reaction. 3.2. Reaction Results. 3.2.1. Catalysts Comparison. Gas chromatographic analyses were accomplished in order to evaluate the possible formation of byproduct during the reaction, and the results indicated that just methyl esters were presented as reaction products, representing a selectivity of 100% toward biodiesel. Table 4 presents the conversions obtained for the noncatalytic and catalytic reactions under the same reaction conditions. The results indicated that the reaction occurs even
Ind. Eng. Chem. Res., Vol. 49, No. 23, 2010
12137
Table 4. Comparison between Catalysts in the Esterification of Oleic Acid Assisted by Microwave Radiationa
a
catalyst
conversion (%)
noncatalytic Nb2O5 S-ZrO2
39.3 68.0 68.7
T ) 200 °C, t ) 20 min, MR ) 10, 5% w/w catalyst.
in the absence of catalysts;40 on the other hand, conversions almost double in the presence of solid catalysts, evidencing the promoting effect of these solid acid catalysts in the esterification reaction.17-23 Conversions obtained for Nb2O5 and S-ZrO2 were similar, indicating that the activity is not directly related to the surface area, since Nb2O5 has presented a surface area 4-fold higher than S-ZrO2. In this sense, the activity could be related to the type and number of acid sites presented in the materials. The comparison of the results obtained in this work using microwave radiation with those of conventional heating17-23 is not a direct task, as the operational conditions (temperature, catalyst concentration, molar ratio, reaction time, alcohol, and fatty acids used) are not the same. Aranda et al.19 studied the use of Nb2O5 for the esterification of palm fatty acids and obtained a conversion around 45% at 160 °C, t ) 20 min, MR ) 3, 6% w/w catalyst. Although the reaction conditions are not the same, conversions obtained in the current work are on the same order of magnitude as those obtained by Aranda et al.,19 which does not indicate a significant increase of the reaction conversion upon using microwave heating compared to conventional heating mode. In spite of that, it should be pointed out that the microwave application provided very fast heating rates, requiring around 4 min to reach temperatures of 150-200 °C. These heating rates agree with those obtained in a previous work40 for which the noncatalytic esterification of methanol and oleic acid was studied. These results suggest that both catalytic materials used here may not introduce significant changes to the dielectric properties of the medium. Figure 2a shows the results of the reuse of niobium oxide in successive cycles of esterification batches, while Figure 2b displays the results for the sulfated zirconia. It must be highlighted that niobium oxide keeps its activity, with a very low decrease in conversion (less than 2% after the third use), while surface zirconia loses its activity faster (11% after the third use). Aranda et al.19 have already shown the small deactivation of niobium oxide in the esterification reaction in a continuous fixed bed reactor, which favors its industrial use. Concerning the use of sulfated zirconia, Lo´pez et al.42 has observed some deactivation (41% after the fifth use) in the esterification of caprylic acid with ethanol at 75 °C, and this result was attributed to sulfur leaching, poisoning (e.g., by water), pore filling, or a combination of them. On the other hand, Garcia et al.43 have shown a pronounced deactivation (70% after the third use) of sulfated zirconia in the transesterification of soybean oil, which was ascribed to two possible reasons: sulfate loss or hydrolysis of sulfated zirconia, the first one being accelerated by the water present in the reaction mixture. Since water is a product of the esterification reaction, the sulfate loss could be the cause of the deactivation in the present work. However, it must be emphasized that the deactivation observed here is much less pronounced than that reported by Garcia et al.,43 which may be related to the different zirconium precursor usedsZrOCl2 by Garcia et al. and Zr(OH)4 in this work. It should be pointed out that each test in this work is an independent run, involving the filtering of the catalysts, drying, washing, and reintroduction
Figure 2. Evaluation of the reuse of the solid catalysts in the esterification of oleic acid assisted by microwave radiation: (a) niobium oxide at 200 °C, 30 min of reaction time, molar ration of 10, and catalyst concentration of 3% (w/w) and (b) sulfated zirconia at 200 °C, 20 min of reaction time, molar ration of 10, and catalyst concentration of 5% (w/w).
into the reactor with fresh reactants. In this sense, the results obtained here are strong evidence of the stability of Nb2O5. 3.2.2. Effect of Reaction Conditions. The activity of niobium oxide and sulfated zirconia under the same reaction conditions were very similar (Table 4), but the stability of the niobium oxide after some reuse cycles is higher than that of the sulfated zirconia, where a very small deactivation was observed (Figure 2). Considering these aspects, the influence of reaction conditions was evaluated using niobium oxide as catalyst. Figure 3 displays kinetic curves of noncatalytic and niobium oxide catalyzed esterification reactions at fixed temperature of 200 °C, from where is the promoting effect of the catalyst is clear. After 45 min of reaction, the noncatalytic reaction yields a conversion around 58%, while 5% w/w niobium oxide promotes a conversion of 84%. The conversion enhancement is proportional to the catalyst concentration, as can be visualized in Figure 4. An important feature that should be noted is that a small catalyst concentration (0.1% w/w) has already promoted an increase around 9% in the reaction conversion. Reaction temperature is another critical variable, as seen in Figure 5. Figure 6 presents the effect of reactants molar ratio in the reaction conversion. The conversion is kept approximately constant when the molar ratio increases from 5 to 20. This result agrees with the kinetic behavior expected for an acidic catalytic esterification reaction, which is generally described as a firstorder reaction with respect to the fatty acid and as zero-order related to the alcohol (when this one is used in excess).44,45
12138
Ind. Eng. Chem. Res., Vol. 49, No. 23, 2010
Figure 3. Kinetic curves in the esterification of oleic acid assisted by microwave radiation: (9) noncatalytic and (b) 5% w/w Nb2O5. T ) 200 °C, MR ) 10.
Figure 6. Effect of molar ratio on niobium oxide catalyzed esterification of oleic acid assisted by microwave radiation. T ) 200 °C, t ) 20 min, MR ) 10, 5% w/w catalyst.
which means that the microwave effect observed for these systems and in the experimental conditions evaluated is essentially thermal. Although microwave radiation did not promote enhancements in the esterification reaction with the systems used in this work, the technology has some important advantages, specially the minor energy requirements. So it is important to perform some studies concerning the scale up of the unit and process integration of the esterification of fatty acids assisted by microwave radiation using solid acid catalysts, like the ones presented by Kiss46 related to the reactive absorption technology and by Dimian et al.47 regarding the dual reactive distillation. Figure 4. Effect of catalyst concentration on niobium oxide catalyzed esterification of oleic acid assisted by microwave radiation. T ) 200 °C, t ) 30 min, MR ) 10.
Figure 5. Effect of temperature on niobium oxide catalyzed esterification of oleic acid assisted by microwave radiation. t ) 10 min, MR ) 10, 5% w/w catalyst.
For these systems, the conversion can be assumed to be independent of the initial concentration of the fatty acid, which means that the conversion is expected to be constant when varying MR and that the surface is saturated with methanol, independent of methanol concentration in the liquid phase in the range studied. The results obtained in this work suggest that the use of microwaves may not cause significant changes in the kinetic behavior of the methyl esterification of oleic acid based on the use of catalysts such as niobium oxide and sulfated zirconia (at least under the conditions employed in this work),
4. Conclusions This work aimed at the utilization of heterogeneous solid catalysts (niobium oxide and sulfated zirconia) in the esterification of oleic acid assisted by microwave radiation. To the best of our knowledge, the use of microwaves in the solid acid catalytic esterification of fatty acids based on the use of niobium oxide and sulfated zirconia to produce biodiesel has not been reported in the open literature. It was shown that both catalysts were active in the reaction, but the results did not indicate a significant increase in the reaction conversion upon using the microwave heating compared to conventional heating. Catalysts have similar activities at similar reaction conditions, but niobium oxide was more stable than the sulfated zirconia. Catalyst concentration and temperature were critical variables in relation to the reaction conversion. The molar ratio of reactants did not affect the conversion significantly, which agrees with the standard kinetic assumptions generally applied for the acidcatalyzed esterification mechanism. All of these results suggest that the microwave effect in the solid acid catalytic esterification of oleic acid using niobium oxide and sulfated zirconia is essentially thermal. Further studies are developing oxide-based catalysts that favor the interaction of the esterification reaction media with the microwaves. Acknowledgment The authors gratefully acknowledge financial support from CNPq, CAPES, FINEP, Petrobras, and FAPITEC/SE. Literature Cited (1) Marchetti, J. M.; Miguel, V. U.; Errazu, A. F. Possible methods for biodiesel production. Renewable Sustainable Energy ReV. 2007, 11, 1300.
Ind. Eng. Chem. Res., Vol. 49, No. 23, 2010 (2) Barnwal, B. K.; Sharma, M. P. Prospects of biodiesel production from vegetable oils in India. Renewable Sustainable Energy ReV. 2005, 9, 363. (3) Zabeti, M.; Daud, W. M. A. W.; Aroua, M. K. Activity of solid catalysts for biodiesel production: A review. Fuel Process. Technol. 2009, 90, 770. (4) Ma, F.; Hanna, M. A. Biodiesel production: A review. Bioresour. Technol. 1999, 70, 1. (5) Di Serio, M.; Tesser, R.; Dimiccoli, M.; Cammarota, F.; Nastasi, M.; Santacesaria, E. Synthesis of biodiesel via homogeneous Lewis acid catalyst. J. Mol. Catal. A: Chem. 2005, 239, 111. (6) Van Gerpen, J. Biodiesel processing and production. Fuel Process. Technol. 2005, 86, 1097. (7) Kusdiana, D.; Saka, S. Two-step preparation for catalyst-free biodiesel fuel production. Appl. Biochem. Biotechnol. 2004, 115, 781. (8) Minami, E.; Saka, S. Kinetics of hydrolysis and methyl esterification for biodiesel production in two-step supercritical methanol process. Fuel 2006, 85, 2479. (9) Suwannakarn, K.; Lotero, E.; Ngaosuwan, K.; Goodwin, J. G., Jr. Simultaneous free fatty acid esterification and triglyceride transesterification using a solid acid catalyst with in situ removal of water and unreacted methanol. Ind. Eng. Chem. Res. 2009, 48, 2810. (10) Liu, Y.; Wang, L.; Yan, Y. Biodiesel synthesis combining preesterification with alkali catalyzed process from rapeseed oil deodorizer distillate. Fuel Proc. Technol. 2009, 90, 857. (11) Yan, S.; Salley, S. O.; Ng, K. Y. S. Simultaneous transesterification and esterification of unrefined or waste oils over ZnO-La2O3 catalysts. Appl. Catal., A 2009, 353, 203. (12) Santacesaria, E.; Tesser, R.; Di Serio, M.; Guida, M.; Gaetano, D.; Agreda, A. G.; Cammarota, F. Comparison of different reactor configurations for the reduction of free acidity in raw materials for biodiesel production. Ind. Eng. Chem. Res. 2007, 46, 8355. (13) Ozbay, N.; Oktar, N.; Tapan, N. A. Esterification of free fatty acids in waste cooking oils (WCO): Role of ion-exchange resins. Fuel 2008, 87, 1789. (14) Mo, X.; Lotero, E.; Lu, C.; Liu, Y.; Goodwin, J. G. A novel sulfonated carbon composite solid acid catalyst for biodiesel synthesis. Catal. Lett. 2008, 123, 1. (15) Mbaraka, I. K.; Radu, D. R.; Lin, V.-Y.; Shanks, B. H. Organosulfonic acid-functionalized mesoporous silicas for the esterification of fatty acid. J. Catal. 2003, 219, 329. (16) Marchetti, J. M.; Errazu, A. F. Comparison of different heterogeneous catalysts and different alcohols for the esterification reaction of oleic acid. Fuel 2008, 87, 3477. (17) Souza, M. S.; Aguieiras, E. C. G.; Silva, M. A. P.; Langone, M. A. P. Biodiesel synthesis via esterification of feedstock with high content of free fatty acids. Appl. Biochem. Biotechnol. 2009, 154, 253. (18) Srilatha, K.; Lingaiah, N.; Prasad, S. S.; Devi, B. L. A. P.; Prasad, R. B. N.; Venkateswar, S. Influence of carbon chain length and unsaturation on the esterification activity of fatty acids on Nb2O5 catalyst. Ind. Eng. Chem. Res. 2009, 48, 10816. (19) Aranda, D. A. G.; Gonc¸alves, J. A.; Peres, J. S.; Ramos, A. L. D.; Melo, C. A. R., Jr.; Antunes, O. A. C.; Furtado, N. C.; Taft, C. A. The use of acids, niobium oxide, and zeolite catalysts for esterification reactions. J. Phys. Org. Chem. 2009, 22, 709. (20) Kiss, A. A.; Dimian, A. C.; Rothenberg, G. Biodiesel by catalytic reactive distillation powered by metal oxides. Energy Fuels 2008, 22, 598. (21) Kiss, A. A.; Dimian, A. C.; Rothenberg, G. Solid acid catalysts for biodiesel productionsTowards sustainable energy. AdV. Synth. Catal. 2006, 348, 75. (22) Kiss, A. A.; Omota, F.; Dimian, A. C.; Rothenberg, G. The heterogeneous advantage: Biodiesel by catalytic reactive distillation. Top. Catal. 2006, 40, 141. (23) Yu, G. X.; Zhou, X. L.; Li, C. L.; Chen, L. F.; Wang, J. A. Esterification over rare earth oxide and alumina promoted SO42-/ZrO2. Catal. Today 2009, 148, 169. (24) Ngaosuwan, K.; Lotero, E.; Suwannakarn, K.; Goodwin, J. G., Jr.; Praserthdam, P. Hydrolysis of triglycerides using solid acid catalysts. Ind. Eng. Chem. Res. 2009, 48, 4757.
12139
(25) Sreeprasanth, P. S.; Srivastava, R.; Srinivas, D.; Ratnasamy, P. Hydrophobic, solid acid catalysts for production of biofuels and lubricants. Appl. Catal., A 2006, 314, 148. (26) Lotero, E.; Liu, Y.; Lopez, D. E.; Suwannakarn, K.; Bruce, D. A.; Goodwin, J. G., Jr. Synthesis of biodiesel via acid catalysis. Ind. Eng. Chem. Res. 2005, 44, 5353. (27) Leadbeater, N. E.; Stencel, L. M. Fast, easy preparation of biodiesel using microwave heating. Energy Fuels 2006, 20, 2281. (28) Fortuny, M.; Ramos, A. L. D.; Dariva, C.; Egues, S. M. S.; Santos, A. F.; Nele, M.; Coutinho, R. C. C. Principais aplicac¸o˜es das microondas na produc¸a˜o e refino de petro´leo. Quim. NoVa 2008, 31, 1553. (29) Meredith, R. Engineers’ Handbook of Industrial MicrowaVe Heating; The Institution of Electrical Engineers: London, 1998. (30) Berlan, J. Microwaves in chemistry: Another way of heating reaction mixtures. Radiat. Phys. Chem. 1995, 45, 581. (31) Loupy, A. MicrowaVes in Organic Synthesis; Wiley-VCH Verlag GmbH & Co.: Weinheim, 2002. (32) Azcan, N.; Danisman, A. Microwave assisted transesterification of rapeseed oil. Fuel 2008, 87, 1781. ´ lvaro, G.; Westphal, E.; Viana, L. H.; Jacob, R. G.; (33) Perin, G.; A Lenarda˜o, E. J.; D’Oca, M. G. M. Transesterification of castor oil assisted by microwave irradiation. Fuel 2008, 87, 2838. (34) Zhang, S.; Zu, Y. G.; Fu, Y. J.; Luo, M.; Zhang, D. Y.; Efferth, T. Rapid microwave-assisted transesterification of yellow horn oil to biodiesel using a heteropolyacid solid catalyst. Bioresour. Technol. 2010, 101, 931. (35) Pipus, G.; Plazl, I.; Koloini, T. Esterification of benzoic acid with 2-ethylhexanol in a microwave stirred-tank reactor. Ind. Eng. Chem. Res. 2002, 41, 1129. (36) Kabza, K. G.; Chapados, B. R.; Gestwicki, J. E.; McGrath, J. L. Microwave-induced esterification using heterogeneous acid catalyst in a low dielectric constant medium. J. Org. Chem. 2000, 65, 1210. (37) Toukoniitty, B.; Mikkola, J. P.; Eranen, K.; Salmi, T.; Murzin, D. Y. Esterification of propionic acid under microwave irradiation over an ionexchange resin. Catal. Today 2005, 100, 431. (38) Raner, K. D.; Strauss, C. R. Influence of microwaves on the rate of esterification of 2,4,6-trimethylbenzoic acid with 2-propanol. J. Org. Chem. 1992, 57, 6231. (39) Chemat, F.; Poux, M.; Galema, S. A. Esterification of stearic acid by isomeric forms of butanol in a microwave over under homogeneous and heterogeneous reaction conditions. J. Chem. Soc., Perkin Trans. 1997, 2, 2371. (40) Melo Ju´nior, C. A. R.; Albuquerque, C. E. R.; Fortuny, M.; Dariva, C.; Egues, S.; Santos, A. F.; Ramos, A. L. D. Use of microwave irradiation in the noncatalytic esterification of C18 fatty acids. Energy Fuels 2009, 23, 580. (41) Lo´pez, D. E.; Goodwin, J. G., Jr.; Bruce, D. A. Transesterification of triacetin with methanol on Nafion acid resins. J. Catal. 2007, 245, 381. (42) Lo´pez, D. E.; Goodwin, J. G., Jr.; Bruce, D. A.; Furuta, S. Esterification and transesterification using modified-zirconia catalysts. Appl. Catal., A 2008, 339, 76. (43) Garcia, C. M.; Teixeira, S.; Marciniuk, L. L.; Schuchardt, U. Transesterification of soybean oil catalyzed by sulfated zirconia. Bioresour. Technol. 2008, 99, 6608. (44) Aranda, D. A. G.; Santos, R. T. P.; Tapanes, N. C. O.; Ramos, A. L. D.; Antunes, O. A. C. Acid-catalyzed homogeneous esterification reaction for biodiesel production from palm fatty acids. Catal. Lett. 2008, 122, 20. (45) Cardoso, A. L.; Neves, S. C. G.; Silva, M. J. Kinetic study of alcoholysis of the fatty acids catalyzed by tin chloride (II): An alternative catalyst for biodiesel production. Energy Fuels 2009, 23, 1718. (46) Kiss, A. A. Novel process for biodiesel by reactive absorption. Sep. Purif. Technol. 2009, 69, 280. (47) Dimian, A. C.; Bildea, C. S.; Omota, F.; Kiss, A. A. Innovative process for fatty acid esters by dual reactive distillation. Comput. Chem. Eng. 2009, 33, 743.
ReceiVed for reView March 5, 2010 ReVised manuscript receiVed October 1, 2010 Accepted October 10, 2010 IE100501D
