- Email: [email protected]
Transesterification of rapeseed oil for the production of biodiesel using homogeneous and heterogeneous catalysis
Fuel Processing Technology 90 (2009) 1016–1022
Contents lists available at ScienceDirect
Fuel Processing Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / f u p r o c
Transesterification of rapeseed oil for the production of biodiesel using homogeneous and heterogeneous catalysis K.G. Georgogianni a, A.K. Katsoulidis a, P.J. Pomonis a, G. Manos b, M.G. Kontominas a,⁎ a b
Section of Industrial and Food Chemistry, Department of Chemistry, University of Ioannina 45110-Ioannina, Greece Technological Educational Institute (TEI) of Epirus, Faculty of Agricultural Technology, 47100 Arta, Greece
a r t i c l e
i n f o
Article history: Received 15 January 2009 Received in revised form 6 March 2009 Accepted 9 March 2009 Keywords: Biodiesel Rapeseed Transesterification Ultrasonication Heterogeneous/homogeneous catalysis
a b s t r a c t In the present work, the transesterification reaction of rapeseed oil with methanol, in the presence of alkaline catalysts, either homogeneous (NaOH) or heterogeneous (Mg MCM-41, Mg–Al Hydrotalcite, and K+ impregnated zirconia), using low frequency ultrasonication (24 kHz) and mechanical stirring (600 rpm) for the production of biodiesel fuel was studied. Selection of heterogeneous catalysts was based on a combination of their porosity and surface basicity. Their characterization was carried out using X-ray diffraction (XRD), Nitrogen adsorption–desorption porosimetry and scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS). The activities of the catalysts were related to their basic strength. Mg–Al hydrotalcite showed particularly the highest activity with conversion reaching 97%). The activity of ZrO2 in the transesterification reaction increased as the catalyst was doped with more potassium cations, becoming thus more basic. Use of ultrasonication significantly accelerated the transesterification reaction compared to the use of mechanical stirring (5 h vs. 24 h). Given the differences in experimental design, it can be concluded that the homogeneous catalyst accelerated significantly the transesterification reaction, as compared to all heterogeneous catalysts, using both mechanical stirring (15 min vs. 24 h) and ultrasonication (10 min vs. 5 h). However, the use of homogeneous base catalysts requires neutralization and separation from the reaction mixture leading to a series of environmental problems related to the use of high amounts of solvents and energy. Heterogeneous solid base catalysts can be easily separated from the reaction mixture by simple filtration, they are easily regenerated and bear a less corrosive nature, leading to safer, cheaper and more environment-friendly operations. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Biodiesel is a processed fuel derived from the esterification and transesterification of free fatty acids (FFAs) and triglycerides respectively that occur naturally in renewable biological sources such as plant oils and animal fats. Three main synthetic approaches have been used for biodiesel production that include alkali catalyzed transesterification, acid-catalyzed transesterification (with simultaneous esterification of free fatty acids) and noncatalytic conversion via transesterification and esterification under supercritical alcohol conditions [1–4]. The primary commercial process used today for biodiesel production is alkali catalyzed transesterification [5]. This chemical process converts the triglycerides (TGs) present in vegetable oils and animal fats into fatty acid methyl esters (FAMEs) in a multistep synthesis with glycerol being liberated as a by-product [2,6]. The search for new catalysts has been intensely pursued by researchers. Additionally, the catalyst employed has a direct impact on the purity of the feedstock required, the kinetics of the reaction, and the extent of post-reaction processing required. Unfortunately, ⁎ Corresponding author. Tel.: +30 2651098342; fax: +30 2651098795. E-mail address: [email protected] (M.G. Kontominas). 0378-3820/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2009.03.002
the prevailing commercial process (alkali catalyzed batch reactor) has serious limitations and inefficiencies [7] and is prone to undesirable by-product formation, especially when free fatty acids are present in the lipid feedstock. Furthermore, in both the acid and alkali catalyzed approaches, conversion efficiency is highly dependent upon the water and free fatty acid content of the feedstocks [8]. More recently, it has been demonstrated that supercritical alcohol [9] can esterify free fatty acids and transesterify triglycerides simultaneously with virtually no sensitivity to water content. A major advantage of this process is that simultaneous esterification and transesterification makes the use of cheaper feedstocks, which often contain increased levels of free fatty acids [9,10]. Transesterification reactions can be performed using acid catalysts, such as sulfuric, sulfonic, phosphoric and hydrochloric acids [5,11] or basic catalysts, such as metal hydroxides [12], metal alkoxides [13], alkaline-earth oxides [14] or hydrotalcites [15]. Basic catalysts are usually preferred to acid catalysts because of the higher activity and the lower process temperatures required as compared to acid-catalyzed transesterification [12]. Nowadays, most industrial applications are performed in batch or continuous stirred tank reactors at temperatures ranging from 60 to 200 °C using homogeneous basic catalysts, such as alkaline hydroxides or metal alkoxides [5,16]. However, the use of
K.G. Georgogianni et al. / Fuel Processing Technology 90 (2009) 1016–1022
homogeneous base catalysts requires neutralization and difficult separation from the final reaction mixture leading to a series of environmental problems related to the use of high amounts of solvents and energy. Heterogeneous basic catalysts, able to catalyze the transesterification of alkyl esters could solve such problems; they can be easily separated from the reaction mixture by simple filtration, without the use of solvent, they are easily regenerated and have a less corrosive character, leading to safer, cheaper and more environmentfriendly operations. Therefore, it is of interest to investigate the possibility of replacing the homogeneous base catalysts by solid base catalysts in transesterification reactions, and in addition, to study the kinetics of the heterogeneously base-catalyzed process in order to evaluate its potential industrial applicability [17]. The predominant basic catalyzed production processes require the use of high quality, waste oils. The chemistry of the basic transesterification reaction limits feedstock flexibility due to undesirable side reactions (neutralization reactions) [6,11]. Furthermore the currently employed catalysts are not reused so they must be neutralized and discarded as an aqueous salt waste stream. Besides, the predominant production mode is a batch or semi-continuous process where reactants are added continuously to a flow reactor. Operational problems in the conventional production process are associated with the catalyst (e.g. potassium and sodium hydroxide) because they are hazardous, caustic and hygroscopic. In the past two decades, sonochemistry, i.e. chemical reaction aided by ultrasound irradiation, has developed into an expanding research area. Ultrasound energy is known to produce chemical and physical effects that arise from local increases of pressure and temperature leading to the formation of cavitation bubbles. The collapse of these cavitation bubbles disrupts the phase boundary in a two phase liquid system and causes emulsification by ultrasonic jets that impinge one liquid in to the other leading to very effective micro-mixing [10]. This effect can be employed for biodiesel production [16–21]. 2. Materials and methods 2.1. Reagents and materials Sodium hydroxide (N96%) was purchased from Merck (DarmstadtGermany) and used after milling, to facilitate the dilution in methanol. Methanol of purity N95%, petroleum ether (pro analysis), Mg(NO3)2.6H2O, Al(NO3)3.9H2O, (NH4)2CO3, n-hexadecyltrimethylammonium bromide (CTAB), ZrOCl2·8H2O, KNO3 were purchased from Aldrich, Steinheim, Germany. Aqueous ammonia solution (33%), tetraethoxysilane (98%) was purchased by Merck, KGaA, Darmstadt, Germany. The rapeseeds (4.7% moisture content and 43% oil content) were provided by the Agricultural experiment station of the Technological Educational Institute of Epirus in Arta, Greece.
1017
The pH was adjusted to 7.5 by addition of concentrated ammonia solution. The mixture was aged for 3 h at 65 °C and the precipitate was filtered, dried and calcined at 500 °C for 3 h. 2.2.1.3. Mesoporous K/ZrO2. Mesoporous zirconia was synthesized according to the methodology proposed by Hudson and Knowles [24]. One hundred mL aqueous solution of ZrOCl2·8H2O 0.1 M was added to an 100 mL aqueous solution of CTAB 12.5 wt.% leading to a metal: surfactant ratio of 1:5. The mixture was stirred for 15 min and then NaOH 1 M was added until the pH reached 11.5. The mixture was stirred at room temperature for 60 min in a closed vessel and then placed at 90 °C for 4 days. The mixture was then allowed to cool, the solid was filtered, washed with water and dried at 100 °C. The powder was calcined at 500 °C for 5 h. Following that, the zirconium oxide was impregnated by grinding with different quantities of KNO3 (10% and 20% wt) with the addition of 1–2 mL H2O. The final solids were dried at 100 °C and calcined at 500 °C for 3 h. The samples were designated as 10K/ZrO2 and 20K/ZrO2. 2.2.2. Characterization of catalysts Characterization of the catalysts was carried out using the following techniques: X-ray diffraction: the crystalline structure of the catalysts was characterized by XRD in a Bruker Advance P8 system using Cu Kα radiation (λ = 1.5418 Å). Nitrogen adsorption–desorption porosimetry: the catalysts were tested for their surface area and porosity by N2 adsorption– desorption porosimetry (77 K) in a Sorptomatic 1990 Fisons porosimeter. Prior to each measurement the sample was degassed at 150 °C and 10− 3 mbar for 6 h. SEM–EDS: the solids were photographed by scanning electron microscopy (SEM) in a Jeol JSM 5600 system operating at 20 kV as to estimate the particle size and the shape of each catalyst. The ratio between cations for each sample was calculated by measurements of energy dispersive spectra (EDS), obtained by the ISIS Oxford Microanalysis facility of the system [25]. 2.3. Procedure of rapeseed oil transesterification 2.3.1. Solvent extraction Whole rapeseeds (20 g) were conditioned by heating in an oven at 85 °C for 1 h, macerated in petroleum ether (50–100 ml) in a home type blender, to give a product with a particle size of coarse sand. The solid material was transferred quantitatively to a Soxhlet thimble and extracted with petroleum ether for various periods of time. Solvent was removed from both the extracted oil and the solid residue using a vacuum rotary evaporator at 40 °C [26].
2.2. Procedures 2.2.1. Catalyst preparation The heterogeneous catalysts studied were prepared according to established methods that produce materials with increased porosity or basicity [22–24] as follows. 2.2.1.1. Mg/MCM-41. Spheres of MCM-41 material were synthesized by the method of Grun et al. [22]. One gram of MCM-41 was impregnated with an aqueous solution of Mg(NO3)2·6H2O, leading to ca. 11% MgO loading, dried at 100 °C for 12 h and calcined at 600 °C for 2 h. 2.2.1.2. Mg–Al hydrotalcite. Mg–Al hydrotalcite was synthesized by the route proposed by Cantrell et al. [23]. One hundred mL of an aqueous mixture of 1.125 M Mg(NO3)2·6H2O and 0.375 M Al (NO3)3·9H2O (Mg:Al = 3) were slowly mixed over a period of 1 h along with 100 mL of an aqueous solution of 2 M (NH4)2CO3 at 65 °C.
2.3.2. Conventional oil transesterification using homogeneous catalyst Rapeseed oil (80 g, 0.102 mol) produced as described above, methanol (30 ml/0.75 mol) corresponding to a 7:1 molar ratio of alcohol to oil and NaOH in various concentrations (1.0%, 1.5%, 2.0% wt/wt) were refluxed together in a 500 ml glass reactor equipped with a glass anchorshaped mechanical stirrer, a water condenser and funnel. Heating was achieved by means of a heating mantle controlled by a proportional integral derivative (PID) temperature controller. The temperature was raised to 60 °C and the mixture was stirred either using a mechanical stirrer (600 rpm) or a low frequency ultrasonicator (24 kHz, 200 W, UP 200 S, IKA, England). The horn of the ultrasonicator was placed at half depth of the reaction mixture. Ten ml samples were taken from the reaction mixture at predetermined time intervals, neutralized and analyzed by thin layer chromatography (TLC). TLC analysis was performed on glass plates coated with Silica Cel G (Merck), and developed in a solvent system of petroleum ether/
1018
K.G. Georgogianni et al. / Fuel Processing Technology 90 (2009) 1016–1022
diethyl ether/hydrochloric acid (8:2:0.1). Spots were developed by iodine vapor stain [18–22,26–28]. After the complete conversion of the vegetable oil, the reaction was stopped and the mixture was allowed to stand for phase separation: the ester mixture formed the upper layer and glycerin formed the lower layer [18,26]. The residual catalyst and non-reacted methanol were distributed between the two phases. After phase separation, in a separatory funnel, the ester mixture was purified further by dissolving in petroleum ether adding water and phosphoric acid to adjust pH to ca.7 and washing three times with water. Esters were then dried over anhydrous sodium sulfate and analyzed by Gas Chromatography. 2.3.3. Conventional oil transesterification using heterogeneous catalyst The general procedure of transesterification reaction was typically as follows: rapeseed oils (5 g,), methanol (65 mL) and 0.5 g of catalyst were refluxed together in a 100 mL glass reactor equipped with a glass anchor-shaped mechanical stirrer, a water condenser and funnel. Heating was achieved by means of a heating mantle controlled by a proportional integral derivative (PID) temperature controller. The temperature was raised to 60 °C and the mixture was stirred either via the mechanical stirrer (600 rpm) for 24 h or via ultrasonicator (24 kHz, 70% energy efficiency, 200 W, UP 200 S, IKA, England) for 5 h. The horn of the ultrasonicator was placed at half depth of the reaction mixture. Then the reaction mixture was neutralized with 4% methanol solution of citric acid and was allowed to stand for phase separation: the ester mixture formed the upper layer and glycerin formed the lower layer [29]. The residual catalyst was separated by filtration. The liquid phase was separated using a separatory funnel, the ester mixture was evaporated so as to contain no methanol at all, dried over anhydrous sodium sulfate and the biodiesel phase was analyzed by gas chromatography. 2.4. Sampling and analysis The fatty acid composition of rapeseed oil was determined by a well-established gas chromatographic procedure [30], and is shown in Table 1. In brief, 0.1 mL of the transesterified oil sample was dissolved in 5 mL of petroleum ether, and 3 μL of this solution were injected into a Varian 3700 GC for identification and quantification purposes. The standard mixture of the fatty acid methyl esters used was purchased from Sigma (stock no. 189-3). The analysis of biodiesel by GC was carried out as described by Georgogianni et al. [31,32]. 2.5. Determination of total contamination and sulphur content The total contamination of biodiesel and the sulphur content were determined according to EN 12662 and EN-ISO 20846:2004 respectively.
Table 2 Surface areas (Sp) and the content of K or Mg in the catalysts. Catalyst
Surface area, Sp (m2 g− 1) (BET)
Content of K2O or MgO (wt.%)
Mg/MCM-41 ΜgAl Ηydrotalcite 10 Κ/ZrO2 20 Κ/ZrO2
1289 82 272 192
4 25 4 7.8
basicity of the catalysts, is expressed as % wt. of K2O and MgO in total mass of the solids. 3.1.1. Mg/MCM-41 The Mg/MCM-41 material displays slightly disordered hexagonal mesostructure as shown in the XRD pattern (Fig. 1 top left). The N2 adsorption–desorption isotherm (Fig.1 top right) is typical for organized mesoporous materials with a knee of adsorption at P/P0 = 0.25. The specific surface area is 1289 m2/g (BET) and the mean pore diameter is 21 Å as calculated by the Horwath–Kawazoe method (Table 2). SEM images are presented in Fig. 1 (bottom) in which spherical particles of silica of diameter 300–500 nm are observed. The atomic ratio of Si/Mg was 16/1 (EDS), which corresponds to 4% wt MgO in silica. 3.1.2. Mg–Al hydrotalcite The crystal structure of the calcinated Mg–Al Hydrotalcite calcined sample corresponds to MgO as shown in the XRD pattern (Fig. 2 top left). The atomic ratio Mg/Al = 2.3, in the final solid, was lower than nominal value in preparation bath (Mg/Al = 3) since Mg did not completely precipitate during preparation and the content of magnesia in the sample is 25% wt. The specific surface area calculated was 81 m2/g (BET) (Table 2) and the pores were slit-shaped as demonstrated from the H3 hysteresis loop of the adsorption– desorption isotherm (Fig. 2 top right). In the SEM image of calcined Mg–Al Hydrotalcite irregular particles greater than 100 μm were observed with a rough surface (Fig. 2 bottom). 3.1.3. Mesoporous K/ZrO2 The XRD patterns of pure and potassium impregnated zirconia (Fig. 3 top left) exhibited a very diffuse diffraction peak, a fact indicating that the samples are not crystalline but more or less amorphous. The N2 isotherm of pure ZrO2 (Fig. 3 top right), with the characteristic H2 loop, corresponds to a mesoporous material with random porous network. The incorporation of K+ in ZrO2 partially blocks the porosity, and the specific surface area decreases in the following order: ZrO2 346 m2/g, 10K/ZrO2 272 m2/g and 20K/ZrO2 192 m2/g (Table 2). The atomic ratio Zr/K was 8.9 for 10K/ZrO2 and 4.6 for 20K/ZrO2 from which the percentage of K2O in each catalyst was calculated as 4% wt and 7.8% wt respectively. Aggregates higher than 10 μm were observed in the SEM images of potassium impregnated zirconia catalysts (Fig. 3 bottom).
3. Results and discussion 3.2. Transesterification reaction using homogeneous catalysis 3.1. Catalyst characterization The main characteristics of the catalysts are presented in the Table 2. The surface area was calculated by BET methodology in the region 0.05 b P/P0 b 0.3. The content of K and Mg, which is responsible for the
Table 1 Principal fatty acid composition and molecular weight (Mr) for rapeseed oil. Fatty acid
Mr
Content, %
Palmitic (16:0) Stearic (18:0) Oleic (18:1) Linoleic (18:2)
256 284 282 280
4 2 62 32
3.2.1. Fatty acid composition of rapeseed oil Fatty acid composition of rapeseed oil is shown in Table 1. Given that its acid value was determined to be 0.9%, that is less than 1.0%, alkaline transesterification was correctly chosen for experimental biodiesel production. High free fatty acid and water content are known to produce large amounts of soap which in turn lower the yield of esters while making the separation of ester and glycerol difficult [33]. 3.3. Conventional versus ultrasound assisted transesterification reaction: effect of stirring method and catalyst concentration A set of experiments was carried out to determine the effect of ultrasonication versus mechanical stirring on the transesterification
K.G. Georgogianni et al. / Fuel Processing Technology 90 (2009) 1016–1022
1019
Fig. 1. Mg/MCM-41 catalyst: XRD pattern (top left), N2 adsorption–desorption isotherm (top right) and SEM images (bottom).
reaction. Yields of methyl esters isolated by conventional transesterification using mechanical stirring and ultrasonication as a function of time and NaOH concentration are given in Table 3. Results in Table 3 showed that the highest yields were obtained when the catalyst was used at higher concentrations. To be more specific, by using 2.0% NaOH wt/wt of oil and mechanical stirring the transesterification reaction was close to being completed in 20 min. On the other hand, by using lower concentrations of NaOH (1.0 or 1.5% wt/wt) the reaction was not completed even after 40 min. In conclusion, by increasing the amount of homogeneous catalyst the yields
of methyl esters also increased. As stated previously, high concentrations of alkaline catalyst form soaps in the presence of large residues of fatty acids resulting in emulsion formation between soaps and water molecules. Thus, the yields of isolated esters are very low. In the present study, this phenomenon did not occur and the increase of catalyst's concentration caused an increase in the yield of the isolated fatty acid methyl esters. Part of the explanation for such a finding is the fact that the rapeseed oil used in the present study was crude rather than refined thus the transesterification reaction requires larger quantities of alcohol [34]. However, a further increase in
Fig. 2. Mg–Al Hydrotalcite catalyst: XRD pattern (top left), N2 adsorption–desorption isotherm (top right) and SEM images (bottom).
1020
K.G. Georgogianni et al. / Fuel Processing Technology 90 (2009) 1016–1022
Fig. 3. Zirconia catalysts: XRD pattern (top left), N2 adsorption–desorption isotherm (top right), SEM images of 10K/ZrO2 (bottom left) and 20K/ZrO2 (bottom right).
catalyst concentration (i.e. 2.5% NaOH) led to extensive soap formation and thus to a decrease in yields of fatty acid methyl esters. This finding supports the above postulation. With regard to NaOH concentration used, Ataya et al. [35] similarly reported an increased triglyceride conversion with increasing NaOH concentration between 1.0 and 3.0% w/w for canola oil in a two phase agitated system. As mentioned previously the molar ratio of methanol to oil was chosen to be 7:1 based on literature reports according to which rapeseed, peanut, sunflower seed and soybean oils behaved similarly and that the highest conversion to ester (93–98%) was observed at a ratio close to 6:1 [12]. Yields of methyl esters under present experimental conditions (i.e. 95%) are higher than those reported by Freedman et al. [12] for rapeseed oil (i.e. 90%) after 50 min of reaction time using a molar ratio of 6:1 and 0.5% of sodium methoxide as catalyst.
Table 3 Yields and rate constants of isolated methyl esters of rapeseed oil with mechanical stirring and ultrasonication. Time (min) Mechanical stirring (600 rpm)
Ultrasonication (24 kHz)
3 5 8 10 15 20 30 40 60 3 5 8 10 15 20 30 40 60
1.0% w/w NaOH
1.5% w/w NaOH
2.0% w/w NaOH
Yield (%)
Yield (%)
Yield (%)
16 29 31 35 50 54 75 84 93 21 33 39 45 56 67 73 82 91
22 39 47 53 62 68 83 91 96 29 50 66 78 87 93 96 96 96
35 63 74 83 96 96 96 96 96 58 73 81 86 95 96 96 96 96
Similar results were obtained by using ultrasonication instead of mechanical stirring. To be more specific, by using 2.0% NaOH wt/wt of oil the transesterification reaction was completed in 20 min. On the other hand, by using lower concentrations of NaOH (1.5% wt/wt) the reaction was completed in 30 min and by using 1.0% NaOH the reaction was not completed even after 40 min. In conclusion, by increasing the amount of catalyst the yields of methyl esters also increase. For a given time the yields of isolated products with ultrasonication are generally higher than those with mechanical stirring, probably due to less soap formation through the use of ultrasonication. Indeed, after less than 1 min of mixing with sonication the mixture became homogeneous. Despite this general observation a 96% conversion to esters was achieved in the same amount of time (i.e. 20 min) using both mechanical stirring and ultrasonication. Present conversion to esters' values are comparable to those of Colucci et al. [21] who reported a 95% conversion of refined soybean oil to methyl esters using a molar ratio of methanol to oil 6:1 after 10 min at 60 °C, using KOH catalyst at a concentration of 2.2% w/w. Present results differ however than those of Stavarache et al. [18] who reported that conversion of vegetable oil (no further information on the nature of the oil was provided) to methyl esters was the highest for a 1.0% (w/w) NaOH concentration (i.e. 95% after 10 min at room temperature using ultrasonication (28 kHz) versus 91% after 10 min using mechanical stirring (1800 rpm). In this case ultrasonication increased conversion of oil to methyl esters. Lifka and Ondruschka [20] studied the effect ultrasonication versus mechanical stirring on the alkaline transesterification of rapeseed oil using NaOH at a concentration of 0.5% w/w at 45 °C. A conversion of 80–85% was obtained for both ultrasonicated and mechanically stirred reactions after 30 min. Finally, Siatis et al. [36] reported a 95% conversion of Cynara seed oil to fatty acid methyl esters using an alkali catalyzed ultrasonically assisted transesterification reaction in approximately 30 min. 3.4. Transesterification reaction using heterogeneous catalysis Resent research on oil alcoholysis has been focused on the use of heterogeneous catalysts [37–40]. Corma et al. [15] evaluated alcoholysis of triglycerides using basic solid catalysts such as Cs-MCM-41,
K.G. Georgogianni et al. / Fuel Processing Technology 90 (2009) 1016–1022
Cs-sepiolite and hydrotalcites. The reaction was carried out at 240 °C for 5 h. Hydrotalcite gave a good conversion of 97% followed by Cs-sepiolite (45%) and Cs-MCM-41 (26%). These results are in agreement with respective results of the present work as hydrotalcite gave a good conversion of 97%. Also in the present work Mg-MCM-41 was used and gave a better conversion than Cs-MCM-41 (89% vs. 26%). Leclercq et al. [41] studied the alcoholysis of rapeseed oil in the presence of Cs-exchanged NaX faujasites and commercial hydrotalcite (KW2200) catalysts. At a high methanol to oil ratio of 275 and a reaction time of 22 h under methanol reflux, the cesium exchanged NaX faujasites gave a conversion of 70% whereas a 34% conversion was obtained by using hydrotalcite. It is obvious that under such conditions hydrotalcite did not accelerate the transesterification reaction leading to a small yield of methyl esters. Baynese et al. [38] patented the use of ETS-4, and ETS-10 catalysts to provide conversions of 85.7 and 52.6% respectively, at 220 °C and a reaction time of 1.5 h. Suppes et al. [37] achieved conversions of 78% at 240 °C and N95% at 260 °C for ethyl esters with a residence time of 18 min using calcium carbonate as catalyst. All above studies required temperatures in excess of 200 °C to achieve N90% conversion within the timescale of the experiments. Zeolites that have been modified by ion exchange of alkali cations or by decomposition of an occluded alkali metal salt emerge as interesting solid bases [42–45]. They are known to catalyze reactions that require a basic surface site. The base strength of the alkali ionexchanged zeolite increases with increasing electropositivity of the exchange cation. The occlusion of alkali metal oxide clusters in zeolite cages via decomposition of impregnated alkali metal salts results to a further increase in the basicity of these materials. This statement is in agreement with the results of the present work, as the basicity of catalyst led also to higher yields of methyl esters. In this work, four different heterogeneous catalysts (Mg/MCM-41, Mg–Al Hydrotalcite, 10K/ZrO2 and 20K/ZrO2) were prepared and used in the transesterification reaction for the production of biodiesel. Their activities depend on their different basic strength towards transesterification. Table 4 presents the conversions of the triglycerides of the rapeseed oil into methyl esters using mechanical stirring and ultrasonication respectively. As shown in this table, the conversions to methyl esters for a given reaction time were significantly higher using ultrasonication as compared to mechanical stirring. Thus ultrasounds significantly accelerate the transesterification reaction compared to mechanical stirring (5 h vs. 24 h). Also the activities of the catalysts were related to their basic strength. Hydrotalcite, as the most basic catalyst among the four catalysts used in the present study, showed the highest activity (conversion 97%). Xie et al. [46] reported that when the Mg/Al molar ratio exceeds 3.0, the catalytic activity decreases due to the formation of new weaker basic sites. MCM-41 gave also high yields of methyl esters in the transesterification reaction (conversion 87%) as it was
Table 4 Conversion of rapeseed oil in the presence of different catalysts using both mechanical stirring and ultrasonication. Time (h)
Mechanical stirring
Ultrasonication
5 10 15 20 24 1 2 3 4 5
Yield (%) Type of catalyst Mg/MCM-41
MgAl hydrotalcite
10K/ZrO2
20K/ZrO2
21 43 56 73 85 25 48 59 75 89
28 54 73 89 97 32 58 76 89 96
30 37 49 61 67 29 39 52 63 70
38 42 56 69 89 35 48 59 73 83
1021
Table 5 Turn over frequencies (TOFs) for the transesterification reaction estimated as the moles of rapeseed oil reacted per min and mole of catalysts. Catalysts
Type of stirring
Turn over Time Yield Initial Equivalents of frequency (min) % moles of base in the rapeseed oil (TOF)b reacting mixture
NaOH 1.0% NaOH 1.5% NaOH 2.0% NaOH 1.0% NaOH 1.5% NaOH 2.0% Mg/MCM-41 MgAl Hydrotal 10K/ZrO2 20K/ZrO2 Mg/MCM-41 MgAl Hydrotal 10K/ZrO2 20K/ZrO2
Mechanical Mechanical Mechanical Ultason/tion Ultason/tion Ultason/tion Mechanical Mechanical Mechanical Mechanical Ultason/tion Ultason/tion Ultason/tion Ultason/tion
0.00750a 0.01125a 0.01500a 0.00750a 0.01125a 0.01500a 0.00050 0.00312 0.00021 0.00041 0.00050 0.00312 0.00021 0.00041
a b
60 60 60 60 60 60 1440 1440 1440 1440 300 300 300 300
93 96 96 91 96 96 85 97 67 89 89 96 70 83
0.102000 0.102000 0.102000 0.102000 0.102000 0.102000 0.006375 0.006375 0.006375 0.006375 0.006375 0.006375 0.006375 0.006375
0.2108 0.1451 0.1088 0.2062 0.1451 0.1088 0.0075 0.0014 0.0141 0.0096 0.0378 0.0065 0.0701 0.0518
Estimated considering that x% (1, 1.5, 2) are gr of NaOH per 100 ml of MeOH. TOF = moles of rapeseed oil reacted per min and mole of catalysts.
enriched with 4% w/w MgO becoming a substantially basic catalyst. It is important to mention that the catalyst activity of 20K/ZrO2 in the transesterification reaction increased compared to 10K/ZrO2 as it was enriched with more potassium cations and the catalyst became more basic. The yield increases sharply with addition of alkali up to 5%. Above this point the increase is less sharp indicating diffusion or other kind of limitations in the final yield. These limitations are of similar nature for both the long term mechanical stirring (24 h) and the short term ultrasonication (5 h) experiment. 3.5. Comparison of homogeneous and heterogeneous catalysis As data in Table 5 shows, the moles of rapeseed oil reacted per mole of catalyst (TOF) and per min are 1–2 orders of magnitude higher for the homogeneous catalysts compared to heterogeneous ones. The TOF for homogeneous catalysts was ca. 0.1–0.2 while the use of ultasonication marginally affected the results. On the contrary for heterogeneous catalysts the TOF was in the range 0.015–0.010 using mechanical stirring but increased to 0.075–0.05 using ultrasonication. The difference is due to diffusion limitations of the bulky rapeseed molecules into the relatively small size pores of the solids. It may be suggested that the use of basic catalysts supported on solids bearing wide macropores, such as those of ceramic froths, may combine the activity of homogeneous systems with the benefits of heterogeneous ones without the diffusion constrains of the latter. Given the differences in experimental design, it can be concluded that homogeneous catalyst significantly accelerated the transesterification reaction, as compared to all heterogeneous catalysts, using both mechanical stirring (15 min vs. 24 h) and ultrasonication (10 min vs. 5 h). However, the use of homogeneous base catalysts requires neutralization and separation from the reaction mixture leading to a series of environmental problems related to the use of high amounts of solvents and energy. Heterogeneous solid base catalysts can be easily separated from the reaction mixture without the use of water, they are easily regenerated and have a less corrosive nature, leading to safer, cheaper and more environment-friendly operations. The small amount of biodiesel produced using the present experimental set up did not allow evaluation of quality of biodiesel produced. Nevertheless, the total contamination of biodiesel and the sulphur content were found to be 20 mg/kg and 8 mg/kg respectively. 4. Conclusions In the present study the transesterification reaction of rapeseed oil in the presence of alkaline catalysts, either homogeneous (NaOH) or
1022
K.G. Georgogianni et al. / Fuel Processing Technology 90 (2009) 1016–1022
heterogeneous (Mg MCM-41, Mg–Al Hydrotalcite, and K+ impregnated zirconia), using low frequency ultrasonication (24 kHz) and mechanical stirring (600 rpm) for the production of biodiesel fuel was studied. Four different heterogeneous catalysts were prepared according to known methods that produce materials with increased porosity and basicity. Mg–Al hydrotalcite, the most basic catalyst used in the present study, showed the highest activity (conversion 97%). MCM-41 gave high yields of methyl esters in the transesterification reaction (conversion 87%) as it was enriched with 20% Mg2+ becoming substantially basic catalyst. The catalyst activity of ZrO2 in the transesterification reaction increased as it was enriched with more potassium anions as the catalyst became more basic. Also ultrasounds significantly accelerated the transesterification reaction compared to mechanical stirring (5 h vs. 24 h). Given the differences in experimental design, it can be concluded that homogeneous catalyst significantly accelerated the transesterification reaction, as compared to all heterogeneous catalysts, using both mechanical stirring (15 min vs. 24 h) and ultrasonication (10 min vs. 5 h). Acknowledgements We acknowledge the technical support from the Network of Laboratory Units and Centers of the University of Ioannina for the SEM–EDS and XRD measurements. References [1] H.F. Gerçel, A.E.Pütün, E. Pütün. Hydropyrolysis of extracted Euphorbia rigida in a well-swept fixed-bed tubular reactor, Energy Sources 24 (2002) 423–430. [2] R. Ma, M.A. Hanna, Biodiesel production: a review, Bioresour. Technol. 70 (1999) 1. [3] D. Kusdiana, S. Saka, Kinetics of transesterification in rape seed oil to biodiesel fuel as treated in supercritical methanol, Fuel 80 (2001) 693–698. [4] D. Kusdiana, S. Saka, Methyl esterification of free fatty acids of rape seed oil as treated in supercritical methanol, J. Chem. Eng. Jpn. 34 (2001) 383–387. [5] J. Otera, Trans-esterification, Chem. Rev. 93 (1993) 1449–1470. [6] E. Lotero, Y. Liu, D.E. Lopez, K. Suwannakarn, D.A. Bruce, J.G. Goodwin, Synthesis of biodiesel via acid catalysis, Ind. Eng. Chem. Res. 44 (2005) 5353–5363. [7] D.E. Lopez, J.G. Goodwin, D.A. Bruce, E. Lotero, Transesterification of triacetin with methanol on acid and base catalysts, Appl. Catal. A: Gen. 295 (2005) 97–105. [8] D. Kusdiana, S. Saka, Effects of water on biodiesel fuel production by supercritical methanol treatment, Bioresour. Technol. 92 (2004) 289. [9] J.M.N. Van Kasteren, A.P. Nisworo, A process model to estimate the cost of industrial scale biodiesel production from waste cooking oil by supercritical transesterification, Resour. Conserv. Recycling 50 (2007) 442–458. [10] K. Bunyakiat, S. Makmee, S. Sawangkeaw, S. Ngamprasertsith, Continuous production of biodiesel via transesterification from vegetable oils in supercritical methanol, Energy Fuels 20 (2006) 812–817. [11] B. Davies, G.V. Jeffreys, Continuous transesterification of ethyl alcohol and buthyl acetate in a sieve plate column: II. Batch reaction kinetics studies, Trans. Inst. Chem. Eng. 51 (1973) 271–274. [12] B. Freedman, R.O. Butterfield, E.H. Pryde, Transesterification kinetics of soybean oil, J. Am. Oil. Chem. Soc. 63 (1986) 1375–1380. [13] J. Schmidt, D. Reusch, K. Elgeti, R. Schomacker, Kinetics of transesterification of ethanol and butyl acetate—a model system for reactive rectification, Chem. Ing. Tech. 71 (1999) 704–708. [14] S. Gryglewicz, Alkaline–earth metal compounds as alcoholysis catalysts for ester oils, Synthesis. Appl. Catal. A 192 (2000) 23–28. [15] A. Corma, S. Iborra, S. Miquel, J. Primo, Catalysts for the production of fine chemicals, J. Catal. 173 (1998) 315–321. [16] D. Darnoko, M. Cheryon, Kinetics of palm oil transesterification in a batch reactor, J. Am. Oil. Chem. Soc. 77 (2000) 1263–1267. [17] T.F. Dossin, M. Reyniers, G.B. Marin, Kinetics of heterogeneously MgO-catalyzed transesterification, Applied Catal. B: Environ. 62 (2006) 35–45.
[18] C. Stavarache, M. Vinatoru, R. Nishimura, Y. Maeda, Fatty acids methyl esters from vegetable oils by means of ultrasonic energy, Ultrasonics Sonochem. 12 (2005) 367–372. [19] C. Stavarache, M. Vinatoru, B. Yim, Y. Maeda, Quick method for transesterification of vegetable oils, Chem. Lett. 32 (2003) 716–717. [20] J. Lifka, B. Ondruschka, Influence of mass transfer on the production of biodiesel, Chem. Engin. Technol. 27 (2004) 1156–1159. [21] J.A. Colucci, E.E. Borrero, F. Alape, Biodiesel from an alkaline transesterification reaction of soybean oil using ultrasonic mixing, J. Am. Oil. Chem. Soc. 82 (2005) 525–530. [22] M. Grun, K.K. Unger, A. Matsumoto, K. Tsutsumi, Novel pathways for the preparation of mesoporous MCM-41 materials: control of porosity and morphology, Microporous Mesoporous Mater. 27 (1999) 207–216. [23] D. Cantrell, L.J. Gillie, A.F. Lee, K. Wilson, Structure-reactivity correlations in MgAl hydrotalcite catalysts for biodiesel synthesis, Applied Catal. A 287 (2005) 183–190. [24] M.J. Hudson, J.A. Knowles, Preparation and characterisation of mesoporous, highsurface-area zirconium(IV) oxide, J. Mater. Chem. 6 (1996) 89–95. [25] W. Xie, X. Huang, H. Li, Soybean oil methyl esters preparation using NaX zeolites loaded with KOH as a heterogeneous catalyst, Bioresource Technol. 98 (2007) 936–939. [26] http://ec.europa.eu/agriculture/markets/cotton/reports/rep_en.pdf, Working Paper of the Directorate-General for Agriculture, the Cotton Sector. [27] G. Kildiran, S. Yucel, O. Turkay, In-situ alcoholysis of soybean oil, J. Americ. Oil. Chem. Soc. 73 (1996) 225–228. [28] T.J. Mason, J.P. Lorimer, The uses of ultrasound in chemistry and processing, Applied Sonochemistry Weinheim, Wiley-VCH, 2002. [29] W. Kemp, Practical Organic Chemistry, McGraw-Hill, London, 1967. [30] R. Alcantara, J. Amores, L. Canoira, E. Fidalgo, M.J. Franco, A. Navarro, Catalytic production of biodiesel from soybean oil, used frying oil and tallow, Biomass Bioenergy 18 (2000) 515–527. [31] K.G. Georgogianni, M.G. Kontominas, E. Tegou, D. Avlonitis, V. Gergis, Biodiesel production: reaction and process parameters of alkali-catalyzed transesterification of waste frying oils, Energy Fuels 21 (2007) 3023–3027. [32] K.G. Georgogianni, M.G. Kontominas, P.J. Pomonis, D. Avlonitis, V. Gergis, Conventional and in situ transesterification of sunflower seed oil for the production of biodiesel, Fuel Process. Technol. 89 (2008) 503–509. [33] A. Demirbas, Biodiesel fuels from vegetable oils via catalytic and non-catalytic supercritical alcohol transesterifications and other methods: a survey, Energy Convers. Manag. 44 (2003) 2093–2109. [34] G.B. Bradshaw, W.C. Meuly, US Patent 2.360.844 (1944). [35] F. Ataya, M.A. Dube, M. Ternan, Single-phase and two-phase base-catalyzed transesterification of canola oil to fatty acid methyl esters at ambient conditions, Ind. Eng. Chem. Res. 45 (2006) 5411–5417. [36] N.G. Siatis, A.C. Kimbaris, C.S. Pappas, P.A. Tarantilis, M.G. Polissiou, Improvement of biodiesel production based on the application of ultrasound: monitoring of the procedure by FTIR spectroscopy, J. Americ. Oil. Chem. Soc. 83 (2006) 53–57. [37] G.J. Suppes, K. Bockwinkel, S. Lucas, J.B. Botts, M.H. Mason, A.J. Heppert, J. calcium carbonate catalyzed alcoholysis of fats and oils, J. Am. Oil Chem. Soc. 78 (2001) 139–149. [38] R. Baynese, Cornelis, Hinnekens, Herve, Martens, Julien, US Patent 508 (1996), 457. [39] M.J. Ramos, A. Casas, L. Rodríguez, R. Romero, Á. Pérez, Transesterification of sunflower oil over zeolites using different metal loading: a case of leaching and agglomeration studies, Applied Catal. A 346 (2008) 79–85. [40] C.V. McNeff, L.C. McNeff, B. Yan, D.T. Nowlan, M. Rasmussen, A.E. Gyberg, B.J. Krohn, R.L. Fedie, T.R. Hoye, A continuous catalytic system for biodiesel production, Applied Catal. A 343 (2008) 39–48. [41] E. Leclercq, A. Finiels, C. Moreau, Transesterification of rapeseed oil in the presence of basic zeolites and related solid catalysts, J. Am. Oil Chem. Soc. 78 (2001) 1161–1165. [42] A. Philippou, N.W. Anderson, Aldol-type reactions over basic microporous titanosilicate ETS-10. Type catalysts, J. Catal. 189 (2000) 395–400. [43] A. Philippou, J. Rocha, M.W. Anderson, The strong basicity of the microporous titanosilicate ETS-10, Catal. Lett. 57 (1999) 151–153. [44] R.J. Davis, E.J. Doskocil, S. Bordawekar, Structure/function relationships for basic zeolite catalysts containing occluded alkali species, Catal. Today 62 (2000) 241–247. [45] H. Hattori, Heterogeneous basic catalysis, Chem. Rev. 95 (1995) 537–558. [46] W. Xie, H. Peng, L. Chen, Calcined Mg–Al hydrotalcites as solid base catalysts for methanolysis of soybean oil, J. Mol. Catal. A Chem. 246 (2005) 24–32.
Contents lists available at ScienceDirect
Fuel Processing Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / f u p r o c
Transesterification of rapeseed oil for the production of biodiesel using homogeneous and heterogeneous catalysis K.G. Georgogianni a, A.K. Katsoulidis a, P.J. Pomonis a, G. Manos b, M.G. Kontominas a,⁎ a b
Section of Industrial and Food Chemistry, Department of Chemistry, University of Ioannina 45110-Ioannina, Greece Technological Educational Institute (TEI) of Epirus, Faculty of Agricultural Technology, 47100 Arta, Greece
a r t i c l e
i n f o
Article history: Received 15 January 2009 Received in revised form 6 March 2009 Accepted 9 March 2009 Keywords: Biodiesel Rapeseed Transesterification Ultrasonication Heterogeneous/homogeneous catalysis
a b s t r a c t In the present work, the transesterification reaction of rapeseed oil with methanol, in the presence of alkaline catalysts, either homogeneous (NaOH) or heterogeneous (Mg MCM-41, Mg–Al Hydrotalcite, and K+ impregnated zirconia), using low frequency ultrasonication (24 kHz) and mechanical stirring (600 rpm) for the production of biodiesel fuel was studied. Selection of heterogeneous catalysts was based on a combination of their porosity and surface basicity. Their characterization was carried out using X-ray diffraction (XRD), Nitrogen adsorption–desorption porosimetry and scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS). The activities of the catalysts were related to their basic strength. Mg–Al hydrotalcite showed particularly the highest activity with conversion reaching 97%). The activity of ZrO2 in the transesterification reaction increased as the catalyst was doped with more potassium cations, becoming thus more basic. Use of ultrasonication significantly accelerated the transesterification reaction compared to the use of mechanical stirring (5 h vs. 24 h). Given the differences in experimental design, it can be concluded that the homogeneous catalyst accelerated significantly the transesterification reaction, as compared to all heterogeneous catalysts, using both mechanical stirring (15 min vs. 24 h) and ultrasonication (10 min vs. 5 h). However, the use of homogeneous base catalysts requires neutralization and separation from the reaction mixture leading to a series of environmental problems related to the use of high amounts of solvents and energy. Heterogeneous solid base catalysts can be easily separated from the reaction mixture by simple filtration, they are easily regenerated and bear a less corrosive nature, leading to safer, cheaper and more environment-friendly operations. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Biodiesel is a processed fuel derived from the esterification and transesterification of free fatty acids (FFAs) and triglycerides respectively that occur naturally in renewable biological sources such as plant oils and animal fats. Three main synthetic approaches have been used for biodiesel production that include alkali catalyzed transesterification, acid-catalyzed transesterification (with simultaneous esterification of free fatty acids) and noncatalytic conversion via transesterification and esterification under supercritical alcohol conditions [1–4]. The primary commercial process used today for biodiesel production is alkali catalyzed transesterification [5]. This chemical process converts the triglycerides (TGs) present in vegetable oils and animal fats into fatty acid methyl esters (FAMEs) in a multistep synthesis with glycerol being liberated as a by-product [2,6]. The search for new catalysts has been intensely pursued by researchers. Additionally, the catalyst employed has a direct impact on the purity of the feedstock required, the kinetics of the reaction, and the extent of post-reaction processing required. Unfortunately, ⁎ Corresponding author. Tel.: +30 2651098342; fax: +30 2651098795. E-mail address: [email protected] (M.G. Kontominas). 0378-3820/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2009.03.002
the prevailing commercial process (alkali catalyzed batch reactor) has serious limitations and inefficiencies [7] and is prone to undesirable by-product formation, especially when free fatty acids are present in the lipid feedstock. Furthermore, in both the acid and alkali catalyzed approaches, conversion efficiency is highly dependent upon the water and free fatty acid content of the feedstocks [8]. More recently, it has been demonstrated that supercritical alcohol [9] can esterify free fatty acids and transesterify triglycerides simultaneously with virtually no sensitivity to water content. A major advantage of this process is that simultaneous esterification and transesterification makes the use of cheaper feedstocks, which often contain increased levels of free fatty acids [9,10]. Transesterification reactions can be performed using acid catalysts, such as sulfuric, sulfonic, phosphoric and hydrochloric acids [5,11] or basic catalysts, such as metal hydroxides [12], metal alkoxides [13], alkaline-earth oxides [14] or hydrotalcites [15]. Basic catalysts are usually preferred to acid catalysts because of the higher activity and the lower process temperatures required as compared to acid-catalyzed transesterification [12]. Nowadays, most industrial applications are performed in batch or continuous stirred tank reactors at temperatures ranging from 60 to 200 °C using homogeneous basic catalysts, such as alkaline hydroxides or metal alkoxides [5,16]. However, the use of
K.G. Georgogianni et al. / Fuel Processing Technology 90 (2009) 1016–1022
homogeneous base catalysts requires neutralization and difficult separation from the final reaction mixture leading to a series of environmental problems related to the use of high amounts of solvents and energy. Heterogeneous basic catalysts, able to catalyze the transesterification of alkyl esters could solve such problems; they can be easily separated from the reaction mixture by simple filtration, without the use of solvent, they are easily regenerated and have a less corrosive character, leading to safer, cheaper and more environmentfriendly operations. Therefore, it is of interest to investigate the possibility of replacing the homogeneous base catalysts by solid base catalysts in transesterification reactions, and in addition, to study the kinetics of the heterogeneously base-catalyzed process in order to evaluate its potential industrial applicability [17]. The predominant basic catalyzed production processes require the use of high quality, waste oils. The chemistry of the basic transesterification reaction limits feedstock flexibility due to undesirable side reactions (neutralization reactions) [6,11]. Furthermore the currently employed catalysts are not reused so they must be neutralized and discarded as an aqueous salt waste stream. Besides, the predominant production mode is a batch or semi-continuous process where reactants are added continuously to a flow reactor. Operational problems in the conventional production process are associated with the catalyst (e.g. potassium and sodium hydroxide) because they are hazardous, caustic and hygroscopic. In the past two decades, sonochemistry, i.e. chemical reaction aided by ultrasound irradiation, has developed into an expanding research area. Ultrasound energy is known to produce chemical and physical effects that arise from local increases of pressure and temperature leading to the formation of cavitation bubbles. The collapse of these cavitation bubbles disrupts the phase boundary in a two phase liquid system and causes emulsification by ultrasonic jets that impinge one liquid in to the other leading to very effective micro-mixing [10]. This effect can be employed for biodiesel production [16–21]. 2. Materials and methods 2.1. Reagents and materials Sodium hydroxide (N96%) was purchased from Merck (DarmstadtGermany) and used after milling, to facilitate the dilution in methanol. Methanol of purity N95%, petroleum ether (pro analysis), Mg(NO3)2.6H2O, Al(NO3)3.9H2O, (NH4)2CO3, n-hexadecyltrimethylammonium bromide (CTAB), ZrOCl2·8H2O, KNO3 were purchased from Aldrich, Steinheim, Germany. Aqueous ammonia solution (33%), tetraethoxysilane (98%) was purchased by Merck, KGaA, Darmstadt, Germany. The rapeseeds (4.7% moisture content and 43% oil content) were provided by the Agricultural experiment station of the Technological Educational Institute of Epirus in Arta, Greece.
1017
The pH was adjusted to 7.5 by addition of concentrated ammonia solution. The mixture was aged for 3 h at 65 °C and the precipitate was filtered, dried and calcined at 500 °C for 3 h. 2.2.1.3. Mesoporous K/ZrO2. Mesoporous zirconia was synthesized according to the methodology proposed by Hudson and Knowles [24]. One hundred mL aqueous solution of ZrOCl2·8H2O 0.1 M was added to an 100 mL aqueous solution of CTAB 12.5 wt.% leading to a metal: surfactant ratio of 1:5. The mixture was stirred for 15 min and then NaOH 1 M was added until the pH reached 11.5. The mixture was stirred at room temperature for 60 min in a closed vessel and then placed at 90 °C for 4 days. The mixture was then allowed to cool, the solid was filtered, washed with water and dried at 100 °C. The powder was calcined at 500 °C for 5 h. Following that, the zirconium oxide was impregnated by grinding with different quantities of KNO3 (10% and 20% wt) with the addition of 1–2 mL H2O. The final solids were dried at 100 °C and calcined at 500 °C for 3 h. The samples were designated as 10K/ZrO2 and 20K/ZrO2. 2.2.2. Characterization of catalysts Characterization of the catalysts was carried out using the following techniques: X-ray diffraction: the crystalline structure of the catalysts was characterized by XRD in a Bruker Advance P8 system using Cu Kα radiation (λ = 1.5418 Å). Nitrogen adsorption–desorption porosimetry: the catalysts were tested for their surface area and porosity by N2 adsorption– desorption porosimetry (77 K) in a Sorptomatic 1990 Fisons porosimeter. Prior to each measurement the sample was degassed at 150 °C and 10− 3 mbar for 6 h. SEM–EDS: the solids were photographed by scanning electron microscopy (SEM) in a Jeol JSM 5600 system operating at 20 kV as to estimate the particle size and the shape of each catalyst. The ratio between cations for each sample was calculated by measurements of energy dispersive spectra (EDS), obtained by the ISIS Oxford Microanalysis facility of the system [25]. 2.3. Procedure of rapeseed oil transesterification 2.3.1. Solvent extraction Whole rapeseeds (20 g) were conditioned by heating in an oven at 85 °C for 1 h, macerated in petroleum ether (50–100 ml) in a home type blender, to give a product with a particle size of coarse sand. The solid material was transferred quantitatively to a Soxhlet thimble and extracted with petroleum ether for various periods of time. Solvent was removed from both the extracted oil and the solid residue using a vacuum rotary evaporator at 40 °C [26].
2.2. Procedures 2.2.1. Catalyst preparation The heterogeneous catalysts studied were prepared according to established methods that produce materials with increased porosity or basicity [22–24] as follows. 2.2.1.1. Mg/MCM-41. Spheres of MCM-41 material were synthesized by the method of Grun et al. [22]. One gram of MCM-41 was impregnated with an aqueous solution of Mg(NO3)2·6H2O, leading to ca. 11% MgO loading, dried at 100 °C for 12 h and calcined at 600 °C for 2 h. 2.2.1.2. Mg–Al hydrotalcite. Mg–Al hydrotalcite was synthesized by the route proposed by Cantrell et al. [23]. One hundred mL of an aqueous mixture of 1.125 M Mg(NO3)2·6H2O and 0.375 M Al (NO3)3·9H2O (Mg:Al = 3) were slowly mixed over a period of 1 h along with 100 mL of an aqueous solution of 2 M (NH4)2CO3 at 65 °C.
2.3.2. Conventional oil transesterification using homogeneous catalyst Rapeseed oil (80 g, 0.102 mol) produced as described above, methanol (30 ml/0.75 mol) corresponding to a 7:1 molar ratio of alcohol to oil and NaOH in various concentrations (1.0%, 1.5%, 2.0% wt/wt) were refluxed together in a 500 ml glass reactor equipped with a glass anchorshaped mechanical stirrer, a water condenser and funnel. Heating was achieved by means of a heating mantle controlled by a proportional integral derivative (PID) temperature controller. The temperature was raised to 60 °C and the mixture was stirred either using a mechanical stirrer (600 rpm) or a low frequency ultrasonicator (24 kHz, 200 W, UP 200 S, IKA, England). The horn of the ultrasonicator was placed at half depth of the reaction mixture. Ten ml samples were taken from the reaction mixture at predetermined time intervals, neutralized and analyzed by thin layer chromatography (TLC). TLC analysis was performed on glass plates coated with Silica Cel G (Merck), and developed in a solvent system of petroleum ether/
1018
K.G. Georgogianni et al. / Fuel Processing Technology 90 (2009) 1016–1022
diethyl ether/hydrochloric acid (8:2:0.1). Spots were developed by iodine vapor stain [18–22,26–28]. After the complete conversion of the vegetable oil, the reaction was stopped and the mixture was allowed to stand for phase separation: the ester mixture formed the upper layer and glycerin formed the lower layer [18,26]. The residual catalyst and non-reacted methanol were distributed between the two phases. After phase separation, in a separatory funnel, the ester mixture was purified further by dissolving in petroleum ether adding water and phosphoric acid to adjust pH to ca.7 and washing three times with water. Esters were then dried over anhydrous sodium sulfate and analyzed by Gas Chromatography. 2.3.3. Conventional oil transesterification using heterogeneous catalyst The general procedure of transesterification reaction was typically as follows: rapeseed oils (5 g,), methanol (65 mL) and 0.5 g of catalyst were refluxed together in a 100 mL glass reactor equipped with a glass anchor-shaped mechanical stirrer, a water condenser and funnel. Heating was achieved by means of a heating mantle controlled by a proportional integral derivative (PID) temperature controller. The temperature was raised to 60 °C and the mixture was stirred either via the mechanical stirrer (600 rpm) for 24 h or via ultrasonicator (24 kHz, 70% energy efficiency, 200 W, UP 200 S, IKA, England) for 5 h. The horn of the ultrasonicator was placed at half depth of the reaction mixture. Then the reaction mixture was neutralized with 4% methanol solution of citric acid and was allowed to stand for phase separation: the ester mixture formed the upper layer and glycerin formed the lower layer [29]. The residual catalyst was separated by filtration. The liquid phase was separated using a separatory funnel, the ester mixture was evaporated so as to contain no methanol at all, dried over anhydrous sodium sulfate and the biodiesel phase was analyzed by gas chromatography. 2.4. Sampling and analysis The fatty acid composition of rapeseed oil was determined by a well-established gas chromatographic procedure [30], and is shown in Table 1. In brief, 0.1 mL of the transesterified oil sample was dissolved in 5 mL of petroleum ether, and 3 μL of this solution were injected into a Varian 3700 GC for identification and quantification purposes. The standard mixture of the fatty acid methyl esters used was purchased from Sigma (stock no. 189-3). The analysis of biodiesel by GC was carried out as described by Georgogianni et al. [31,32]. 2.5. Determination of total contamination and sulphur content The total contamination of biodiesel and the sulphur content were determined according to EN 12662 and EN-ISO 20846:2004 respectively.
Table 2 Surface areas (Sp) and the content of K or Mg in the catalysts. Catalyst
Surface area, Sp (m2 g− 1) (BET)
Content of K2O or MgO (wt.%)
Mg/MCM-41 ΜgAl Ηydrotalcite 10 Κ/ZrO2 20 Κ/ZrO2
1289 82 272 192
4 25 4 7.8
basicity of the catalysts, is expressed as % wt. of K2O and MgO in total mass of the solids. 3.1.1. Mg/MCM-41 The Mg/MCM-41 material displays slightly disordered hexagonal mesostructure as shown in the XRD pattern (Fig. 1 top left). The N2 adsorption–desorption isotherm (Fig.1 top right) is typical for organized mesoporous materials with a knee of adsorption at P/P0 = 0.25. The specific surface area is 1289 m2/g (BET) and the mean pore diameter is 21 Å as calculated by the Horwath–Kawazoe method (Table 2). SEM images are presented in Fig. 1 (bottom) in which spherical particles of silica of diameter 300–500 nm are observed. The atomic ratio of Si/Mg was 16/1 (EDS), which corresponds to 4% wt MgO in silica. 3.1.2. Mg–Al hydrotalcite The crystal structure of the calcinated Mg–Al Hydrotalcite calcined sample corresponds to MgO as shown in the XRD pattern (Fig. 2 top left). The atomic ratio Mg/Al = 2.3, in the final solid, was lower than nominal value in preparation bath (Mg/Al = 3) since Mg did not completely precipitate during preparation and the content of magnesia in the sample is 25% wt. The specific surface area calculated was 81 m2/g (BET) (Table 2) and the pores were slit-shaped as demonstrated from the H3 hysteresis loop of the adsorption– desorption isotherm (Fig. 2 top right). In the SEM image of calcined Mg–Al Hydrotalcite irregular particles greater than 100 μm were observed with a rough surface (Fig. 2 bottom). 3.1.3. Mesoporous K/ZrO2 The XRD patterns of pure and potassium impregnated zirconia (Fig. 3 top left) exhibited a very diffuse diffraction peak, a fact indicating that the samples are not crystalline but more or less amorphous. The N2 isotherm of pure ZrO2 (Fig. 3 top right), with the characteristic H2 loop, corresponds to a mesoporous material with random porous network. The incorporation of K+ in ZrO2 partially blocks the porosity, and the specific surface area decreases in the following order: ZrO2 346 m2/g, 10K/ZrO2 272 m2/g and 20K/ZrO2 192 m2/g (Table 2). The atomic ratio Zr/K was 8.9 for 10K/ZrO2 and 4.6 for 20K/ZrO2 from which the percentage of K2O in each catalyst was calculated as 4% wt and 7.8% wt respectively. Aggregates higher than 10 μm were observed in the SEM images of potassium impregnated zirconia catalysts (Fig. 3 bottom).
3. Results and discussion 3.2. Transesterification reaction using homogeneous catalysis 3.1. Catalyst characterization The main characteristics of the catalysts are presented in the Table 2. The surface area was calculated by BET methodology in the region 0.05 b P/P0 b 0.3. The content of K and Mg, which is responsible for the
Table 1 Principal fatty acid composition and molecular weight (Mr) for rapeseed oil. Fatty acid
Mr
Content, %
Palmitic (16:0) Stearic (18:0) Oleic (18:1) Linoleic (18:2)
256 284 282 280
4 2 62 32
3.2.1. Fatty acid composition of rapeseed oil Fatty acid composition of rapeseed oil is shown in Table 1. Given that its acid value was determined to be 0.9%, that is less than 1.0%, alkaline transesterification was correctly chosen for experimental biodiesel production. High free fatty acid and water content are known to produce large amounts of soap which in turn lower the yield of esters while making the separation of ester and glycerol difficult [33]. 3.3. Conventional versus ultrasound assisted transesterification reaction: effect of stirring method and catalyst concentration A set of experiments was carried out to determine the effect of ultrasonication versus mechanical stirring on the transesterification
K.G. Georgogianni et al. / Fuel Processing Technology 90 (2009) 1016–1022
1019
Fig. 1. Mg/MCM-41 catalyst: XRD pattern (top left), N2 adsorption–desorption isotherm (top right) and SEM images (bottom).
reaction. Yields of methyl esters isolated by conventional transesterification using mechanical stirring and ultrasonication as a function of time and NaOH concentration are given in Table 3. Results in Table 3 showed that the highest yields were obtained when the catalyst was used at higher concentrations. To be more specific, by using 2.0% NaOH wt/wt of oil and mechanical stirring the transesterification reaction was close to being completed in 20 min. On the other hand, by using lower concentrations of NaOH (1.0 or 1.5% wt/wt) the reaction was not completed even after 40 min. In conclusion, by increasing the amount of homogeneous catalyst the yields
of methyl esters also increased. As stated previously, high concentrations of alkaline catalyst form soaps in the presence of large residues of fatty acids resulting in emulsion formation between soaps and water molecules. Thus, the yields of isolated esters are very low. In the present study, this phenomenon did not occur and the increase of catalyst's concentration caused an increase in the yield of the isolated fatty acid methyl esters. Part of the explanation for such a finding is the fact that the rapeseed oil used in the present study was crude rather than refined thus the transesterification reaction requires larger quantities of alcohol [34]. However, a further increase in
Fig. 2. Mg–Al Hydrotalcite catalyst: XRD pattern (top left), N2 adsorption–desorption isotherm (top right) and SEM images (bottom).
1020
K.G. Georgogianni et al. / Fuel Processing Technology 90 (2009) 1016–1022
Fig. 3. Zirconia catalysts: XRD pattern (top left), N2 adsorption–desorption isotherm (top right), SEM images of 10K/ZrO2 (bottom left) and 20K/ZrO2 (bottom right).
catalyst concentration (i.e. 2.5% NaOH) led to extensive soap formation and thus to a decrease in yields of fatty acid methyl esters. This finding supports the above postulation. With regard to NaOH concentration used, Ataya et al. [35] similarly reported an increased triglyceride conversion with increasing NaOH concentration between 1.0 and 3.0% w/w for canola oil in a two phase agitated system. As mentioned previously the molar ratio of methanol to oil was chosen to be 7:1 based on literature reports according to which rapeseed, peanut, sunflower seed and soybean oils behaved similarly and that the highest conversion to ester (93–98%) was observed at a ratio close to 6:1 [12]. Yields of methyl esters under present experimental conditions (i.e. 95%) are higher than those reported by Freedman et al. [12] for rapeseed oil (i.e. 90%) after 50 min of reaction time using a molar ratio of 6:1 and 0.5% of sodium methoxide as catalyst.
Table 3 Yields and rate constants of isolated methyl esters of rapeseed oil with mechanical stirring and ultrasonication. Time (min) Mechanical stirring (600 rpm)
Ultrasonication (24 kHz)
3 5 8 10 15 20 30 40 60 3 5 8 10 15 20 30 40 60
1.0% w/w NaOH
1.5% w/w NaOH
2.0% w/w NaOH
Yield (%)
Yield (%)
Yield (%)
16 29 31 35 50 54 75 84 93 21 33 39 45 56 67 73 82 91
22 39 47 53 62 68 83 91 96 29 50 66 78 87 93 96 96 96
35 63 74 83 96 96 96 96 96 58 73 81 86 95 96 96 96 96
Similar results were obtained by using ultrasonication instead of mechanical stirring. To be more specific, by using 2.0% NaOH wt/wt of oil the transesterification reaction was completed in 20 min. On the other hand, by using lower concentrations of NaOH (1.5% wt/wt) the reaction was completed in 30 min and by using 1.0% NaOH the reaction was not completed even after 40 min. In conclusion, by increasing the amount of catalyst the yields of methyl esters also increase. For a given time the yields of isolated products with ultrasonication are generally higher than those with mechanical stirring, probably due to less soap formation through the use of ultrasonication. Indeed, after less than 1 min of mixing with sonication the mixture became homogeneous. Despite this general observation a 96% conversion to esters was achieved in the same amount of time (i.e. 20 min) using both mechanical stirring and ultrasonication. Present conversion to esters' values are comparable to those of Colucci et al. [21] who reported a 95% conversion of refined soybean oil to methyl esters using a molar ratio of methanol to oil 6:1 after 10 min at 60 °C, using KOH catalyst at a concentration of 2.2% w/w. Present results differ however than those of Stavarache et al. [18] who reported that conversion of vegetable oil (no further information on the nature of the oil was provided) to methyl esters was the highest for a 1.0% (w/w) NaOH concentration (i.e. 95% after 10 min at room temperature using ultrasonication (28 kHz) versus 91% after 10 min using mechanical stirring (1800 rpm). In this case ultrasonication increased conversion of oil to methyl esters. Lifka and Ondruschka [20] studied the effect ultrasonication versus mechanical stirring on the alkaline transesterification of rapeseed oil using NaOH at a concentration of 0.5% w/w at 45 °C. A conversion of 80–85% was obtained for both ultrasonicated and mechanically stirred reactions after 30 min. Finally, Siatis et al. [36] reported a 95% conversion of Cynara seed oil to fatty acid methyl esters using an alkali catalyzed ultrasonically assisted transesterification reaction in approximately 30 min. 3.4. Transesterification reaction using heterogeneous catalysis Resent research on oil alcoholysis has been focused on the use of heterogeneous catalysts [37–40]. Corma et al. [15] evaluated alcoholysis of triglycerides using basic solid catalysts such as Cs-MCM-41,
K.G. Georgogianni et al. / Fuel Processing Technology 90 (2009) 1016–1022
Cs-sepiolite and hydrotalcites. The reaction was carried out at 240 °C for 5 h. Hydrotalcite gave a good conversion of 97% followed by Cs-sepiolite (45%) and Cs-MCM-41 (26%). These results are in agreement with respective results of the present work as hydrotalcite gave a good conversion of 97%. Also in the present work Mg-MCM-41 was used and gave a better conversion than Cs-MCM-41 (89% vs. 26%). Leclercq et al. [41] studied the alcoholysis of rapeseed oil in the presence of Cs-exchanged NaX faujasites and commercial hydrotalcite (KW2200) catalysts. At a high methanol to oil ratio of 275 and a reaction time of 22 h under methanol reflux, the cesium exchanged NaX faujasites gave a conversion of 70% whereas a 34% conversion was obtained by using hydrotalcite. It is obvious that under such conditions hydrotalcite did not accelerate the transesterification reaction leading to a small yield of methyl esters. Baynese et al. [38] patented the use of ETS-4, and ETS-10 catalysts to provide conversions of 85.7 and 52.6% respectively, at 220 °C and a reaction time of 1.5 h. Suppes et al. [37] achieved conversions of 78% at 240 °C and N95% at 260 °C for ethyl esters with a residence time of 18 min using calcium carbonate as catalyst. All above studies required temperatures in excess of 200 °C to achieve N90% conversion within the timescale of the experiments. Zeolites that have been modified by ion exchange of alkali cations or by decomposition of an occluded alkali metal salt emerge as interesting solid bases [42–45]. They are known to catalyze reactions that require a basic surface site. The base strength of the alkali ionexchanged zeolite increases with increasing electropositivity of the exchange cation. The occlusion of alkali metal oxide clusters in zeolite cages via decomposition of impregnated alkali metal salts results to a further increase in the basicity of these materials. This statement is in agreement with the results of the present work, as the basicity of catalyst led also to higher yields of methyl esters. In this work, four different heterogeneous catalysts (Mg/MCM-41, Mg–Al Hydrotalcite, 10K/ZrO2 and 20K/ZrO2) were prepared and used in the transesterification reaction for the production of biodiesel. Their activities depend on their different basic strength towards transesterification. Table 4 presents the conversions of the triglycerides of the rapeseed oil into methyl esters using mechanical stirring and ultrasonication respectively. As shown in this table, the conversions to methyl esters for a given reaction time were significantly higher using ultrasonication as compared to mechanical stirring. Thus ultrasounds significantly accelerate the transesterification reaction compared to mechanical stirring (5 h vs. 24 h). Also the activities of the catalysts were related to their basic strength. Hydrotalcite, as the most basic catalyst among the four catalysts used in the present study, showed the highest activity (conversion 97%). Xie et al. [46] reported that when the Mg/Al molar ratio exceeds 3.0, the catalytic activity decreases due to the formation of new weaker basic sites. MCM-41 gave also high yields of methyl esters in the transesterification reaction (conversion 87%) as it was
Table 4 Conversion of rapeseed oil in the presence of different catalysts using both mechanical stirring and ultrasonication. Time (h)
Mechanical stirring
Ultrasonication
5 10 15 20 24 1 2 3 4 5
Yield (%) Type of catalyst Mg/MCM-41
MgAl hydrotalcite
10K/ZrO2
20K/ZrO2
21 43 56 73 85 25 48 59 75 89
28 54 73 89 97 32 58 76 89 96
30 37 49 61 67 29 39 52 63 70
38 42 56 69 89 35 48 59 73 83
1021
Table 5 Turn over frequencies (TOFs) for the transesterification reaction estimated as the moles of rapeseed oil reacted per min and mole of catalysts. Catalysts
Type of stirring
Turn over Time Yield Initial Equivalents of frequency (min) % moles of base in the rapeseed oil (TOF)b reacting mixture
NaOH 1.0% NaOH 1.5% NaOH 2.0% NaOH 1.0% NaOH 1.5% NaOH 2.0% Mg/MCM-41 MgAl Hydrotal 10K/ZrO2 20K/ZrO2 Mg/MCM-41 MgAl Hydrotal 10K/ZrO2 20K/ZrO2
Mechanical Mechanical Mechanical Ultason/tion Ultason/tion Ultason/tion Mechanical Mechanical Mechanical Mechanical Ultason/tion Ultason/tion Ultason/tion Ultason/tion
0.00750a 0.01125a 0.01500a 0.00750a 0.01125a 0.01500a 0.00050 0.00312 0.00021 0.00041 0.00050 0.00312 0.00021 0.00041
a b
60 60 60 60 60 60 1440 1440 1440 1440 300 300 300 300
93 96 96 91 96 96 85 97 67 89 89 96 70 83
0.102000 0.102000 0.102000 0.102000 0.102000 0.102000 0.006375 0.006375 0.006375 0.006375 0.006375 0.006375 0.006375 0.006375
0.2108 0.1451 0.1088 0.2062 0.1451 0.1088 0.0075 0.0014 0.0141 0.0096 0.0378 0.0065 0.0701 0.0518
Estimated considering that x% (1, 1.5, 2) are gr of NaOH per 100 ml of MeOH. TOF = moles of rapeseed oil reacted per min and mole of catalysts.
enriched with 4% w/w MgO becoming a substantially basic catalyst. It is important to mention that the catalyst activity of 20K/ZrO2 in the transesterification reaction increased compared to 10K/ZrO2 as it was enriched with more potassium cations and the catalyst became more basic. The yield increases sharply with addition of alkali up to 5%. Above this point the increase is less sharp indicating diffusion or other kind of limitations in the final yield. These limitations are of similar nature for both the long term mechanical stirring (24 h) and the short term ultrasonication (5 h) experiment. 3.5. Comparison of homogeneous and heterogeneous catalysis As data in Table 5 shows, the moles of rapeseed oil reacted per mole of catalyst (TOF) and per min are 1–2 orders of magnitude higher for the homogeneous catalysts compared to heterogeneous ones. The TOF for homogeneous catalysts was ca. 0.1–0.2 while the use of ultasonication marginally affected the results. On the contrary for heterogeneous catalysts the TOF was in the range 0.015–0.010 using mechanical stirring but increased to 0.075–0.05 using ultrasonication. The difference is due to diffusion limitations of the bulky rapeseed molecules into the relatively small size pores of the solids. It may be suggested that the use of basic catalysts supported on solids bearing wide macropores, such as those of ceramic froths, may combine the activity of homogeneous systems with the benefits of heterogeneous ones without the diffusion constrains of the latter. Given the differences in experimental design, it can be concluded that homogeneous catalyst significantly accelerated the transesterification reaction, as compared to all heterogeneous catalysts, using both mechanical stirring (15 min vs. 24 h) and ultrasonication (10 min vs. 5 h). However, the use of homogeneous base catalysts requires neutralization and separation from the reaction mixture leading to a series of environmental problems related to the use of high amounts of solvents and energy. Heterogeneous solid base catalysts can be easily separated from the reaction mixture without the use of water, they are easily regenerated and have a less corrosive nature, leading to safer, cheaper and more environment-friendly operations. The small amount of biodiesel produced using the present experimental set up did not allow evaluation of quality of biodiesel produced. Nevertheless, the total contamination of biodiesel and the sulphur content were found to be 20 mg/kg and 8 mg/kg respectively. 4. Conclusions In the present study the transesterification reaction of rapeseed oil in the presence of alkaline catalysts, either homogeneous (NaOH) or
1022
K.G. Georgogianni et al. / Fuel Processing Technology 90 (2009) 1016–1022
heterogeneous (Mg MCM-41, Mg–Al Hydrotalcite, and K+ impregnated zirconia), using low frequency ultrasonication (24 kHz) and mechanical stirring (600 rpm) for the production of biodiesel fuel was studied. Four different heterogeneous catalysts were prepared according to known methods that produce materials with increased porosity and basicity. Mg–Al hydrotalcite, the most basic catalyst used in the present study, showed the highest activity (conversion 97%). MCM-41 gave high yields of methyl esters in the transesterification reaction (conversion 87%) as it was enriched with 20% Mg2+ becoming substantially basic catalyst. The catalyst activity of ZrO2 in the transesterification reaction increased as it was enriched with more potassium anions as the catalyst became more basic. Also ultrasounds significantly accelerated the transesterification reaction compared to mechanical stirring (5 h vs. 24 h). Given the differences in experimental design, it can be concluded that homogeneous catalyst significantly accelerated the transesterification reaction, as compared to all heterogeneous catalysts, using both mechanical stirring (15 min vs. 24 h) and ultrasonication (10 min vs. 5 h). Acknowledgements We acknowledge the technical support from the Network of Laboratory Units and Centers of the University of Ioannina for the SEM–EDS and XRD measurements. References [1] H.F. Gerçel, A.E.Pütün, E. Pütün. Hydropyrolysis of extracted Euphorbia rigida in a well-swept fixed-bed tubular reactor, Energy Sources 24 (2002) 423–430. [2] R. Ma, M.A. Hanna, Biodiesel production: a review, Bioresour. Technol. 70 (1999) 1. [3] D. Kusdiana, S. Saka, Kinetics of transesterification in rape seed oil to biodiesel fuel as treated in supercritical methanol, Fuel 80 (2001) 693–698. [4] D. Kusdiana, S. Saka, Methyl esterification of free fatty acids of rape seed oil as treated in supercritical methanol, J. Chem. Eng. Jpn. 34 (2001) 383–387. [5] J. Otera, Trans-esterification, Chem. Rev. 93 (1993) 1449–1470. [6] E. Lotero, Y. Liu, D.E. Lopez, K. Suwannakarn, D.A. Bruce, J.G. Goodwin, Synthesis of biodiesel via acid catalysis, Ind. Eng. Chem. Res. 44 (2005) 5353–5363. [7] D.E. Lopez, J.G. Goodwin, D.A. Bruce, E. Lotero, Transesterification of triacetin with methanol on acid and base catalysts, Appl. Catal. A: Gen. 295 (2005) 97–105. [8] D. Kusdiana, S. Saka, Effects of water on biodiesel fuel production by supercritical methanol treatment, Bioresour. Technol. 92 (2004) 289. [9] J.M.N. Van Kasteren, A.P. Nisworo, A process model to estimate the cost of industrial scale biodiesel production from waste cooking oil by supercritical transesterification, Resour. Conserv. Recycling 50 (2007) 442–458. [10] K. Bunyakiat, S. Makmee, S. Sawangkeaw, S. Ngamprasertsith, Continuous production of biodiesel via transesterification from vegetable oils in supercritical methanol, Energy Fuels 20 (2006) 812–817. [11] B. Davies, G.V. Jeffreys, Continuous transesterification of ethyl alcohol and buthyl acetate in a sieve plate column: II. Batch reaction kinetics studies, Trans. Inst. Chem. Eng. 51 (1973) 271–274. [12] B. Freedman, R.O. Butterfield, E.H. Pryde, Transesterification kinetics of soybean oil, J. Am. Oil. Chem. Soc. 63 (1986) 1375–1380. [13] J. Schmidt, D. Reusch, K. Elgeti, R. Schomacker, Kinetics of transesterification of ethanol and butyl acetate—a model system for reactive rectification, Chem. Ing. Tech. 71 (1999) 704–708. [14] S. Gryglewicz, Alkaline–earth metal compounds as alcoholysis catalysts for ester oils, Synthesis. Appl. Catal. A 192 (2000) 23–28. [15] A. Corma, S. Iborra, S. Miquel, J. Primo, Catalysts for the production of fine chemicals, J. Catal. 173 (1998) 315–321. [16] D. Darnoko, M. Cheryon, Kinetics of palm oil transesterification in a batch reactor, J. Am. Oil. Chem. Soc. 77 (2000) 1263–1267. [17] T.F. Dossin, M. Reyniers, G.B. Marin, Kinetics of heterogeneously MgO-catalyzed transesterification, Applied Catal. B: Environ. 62 (2006) 35–45.
[18] C. Stavarache, M. Vinatoru, R. Nishimura, Y. Maeda, Fatty acids methyl esters from vegetable oils by means of ultrasonic energy, Ultrasonics Sonochem. 12 (2005) 367–372. [19] C. Stavarache, M. Vinatoru, B. Yim, Y. Maeda, Quick method for transesterification of vegetable oils, Chem. Lett. 32 (2003) 716–717. [20] J. Lifka, B. Ondruschka, Influence of mass transfer on the production of biodiesel, Chem. Engin. Technol. 27 (2004) 1156–1159. [21] J.A. Colucci, E.E. Borrero, F. Alape, Biodiesel from an alkaline transesterification reaction of soybean oil using ultrasonic mixing, J. Am. Oil. Chem. Soc. 82 (2005) 525–530. [22] M. Grun, K.K. Unger, A. Matsumoto, K. Tsutsumi, Novel pathways for the preparation of mesoporous MCM-41 materials: control of porosity and morphology, Microporous Mesoporous Mater. 27 (1999) 207–216. [23] D. Cantrell, L.J. Gillie, A.F. Lee, K. Wilson, Structure-reactivity correlations in MgAl hydrotalcite catalysts for biodiesel synthesis, Applied Catal. A 287 (2005) 183–190. [24] M.J. Hudson, J.A. Knowles, Preparation and characterisation of mesoporous, highsurface-area zirconium(IV) oxide, J. Mater. Chem. 6 (1996) 89–95. [25] W. Xie, X. Huang, H. Li, Soybean oil methyl esters preparation using NaX zeolites loaded with KOH as a heterogeneous catalyst, Bioresource Technol. 98 (2007) 936–939. [26] http://ec.europa.eu/agriculture/markets/cotton/reports/rep_en.pdf, Working Paper of the Directorate-General for Agriculture, the Cotton Sector. [27] G. Kildiran, S. Yucel, O. Turkay, In-situ alcoholysis of soybean oil, J. Americ. Oil. Chem. Soc. 73 (1996) 225–228. [28] T.J. Mason, J.P. Lorimer, The uses of ultrasound in chemistry and processing, Applied Sonochemistry Weinheim, Wiley-VCH, 2002. [29] W. Kemp, Practical Organic Chemistry, McGraw-Hill, London, 1967. [30] R. Alcantara, J. Amores, L. Canoira, E. Fidalgo, M.J. Franco, A. Navarro, Catalytic production of biodiesel from soybean oil, used frying oil and tallow, Biomass Bioenergy 18 (2000) 515–527. [31] K.G. Georgogianni, M.G. Kontominas, E. Tegou, D. Avlonitis, V. Gergis, Biodiesel production: reaction and process parameters of alkali-catalyzed transesterification of waste frying oils, Energy Fuels 21 (2007) 3023–3027. [32] K.G. Georgogianni, M.G. Kontominas, P.J. Pomonis, D. Avlonitis, V. Gergis, Conventional and in situ transesterification of sunflower seed oil for the production of biodiesel, Fuel Process. Technol. 89 (2008) 503–509. [33] A. Demirbas, Biodiesel fuels from vegetable oils via catalytic and non-catalytic supercritical alcohol transesterifications and other methods: a survey, Energy Convers. Manag. 44 (2003) 2093–2109. [34] G.B. Bradshaw, W.C. Meuly, US Patent 2.360.844 (1944). [35] F. Ataya, M.A. Dube, M. Ternan, Single-phase and two-phase base-catalyzed transesterification of canola oil to fatty acid methyl esters at ambient conditions, Ind. Eng. Chem. Res. 45 (2006) 5411–5417. [36] N.G. Siatis, A.C. Kimbaris, C.S. Pappas, P.A. Tarantilis, M.G. Polissiou, Improvement of biodiesel production based on the application of ultrasound: monitoring of the procedure by FTIR spectroscopy, J. Americ. Oil. Chem. Soc. 83 (2006) 53–57. [37] G.J. Suppes, K. Bockwinkel, S. Lucas, J.B. Botts, M.H. Mason, A.J. Heppert, J. calcium carbonate catalyzed alcoholysis of fats and oils, J. Am. Oil Chem. Soc. 78 (2001) 139–149. [38] R. Baynese, Cornelis, Hinnekens, Herve, Martens, Julien, US Patent 508 (1996), 457. [39] M.J. Ramos, A. Casas, L. Rodríguez, R. Romero, Á. Pérez, Transesterification of sunflower oil over zeolites using different metal loading: a case of leaching and agglomeration studies, Applied Catal. A 346 (2008) 79–85. [40] C.V. McNeff, L.C. McNeff, B. Yan, D.T. Nowlan, M. Rasmussen, A.E. Gyberg, B.J. Krohn, R.L. Fedie, T.R. Hoye, A continuous catalytic system for biodiesel production, Applied Catal. A 343 (2008) 39–48. [41] E. Leclercq, A. Finiels, C. Moreau, Transesterification of rapeseed oil in the presence of basic zeolites and related solid catalysts, J. Am. Oil Chem. Soc. 78 (2001) 1161–1165. [42] A. Philippou, N.W. Anderson, Aldol-type reactions over basic microporous titanosilicate ETS-10. Type catalysts, J. Catal. 189 (2000) 395–400. [43] A. Philippou, J. Rocha, M.W. Anderson, The strong basicity of the microporous titanosilicate ETS-10, Catal. Lett. 57 (1999) 151–153. [44] R.J. Davis, E.J. Doskocil, S. Bordawekar, Structure/function relationships for basic zeolite catalysts containing occluded alkali species, Catal. Today 62 (2000) 241–247. [45] H. Hattori, Heterogeneous basic catalysis, Chem. Rev. 95 (1995) 537–558. [46] W. Xie, H. Peng, L. Chen, Calcined Mg–Al hydrotalcites as solid base catalysts for methanolysis of soybean oil, J. Mol. Catal. A Chem. 246 (2005) 24–32.