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Synthesis of biodiesel from soybean oil and methanol catalyzed by zeolite beta modified with La3+
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Catalysis Communications 8 (2007) 2159–2165 www.elsevier.com/locate/catcom
Synthesis of biodiesel from soybean oil and methanol catalyzed by zeolite beta modified with La3+ Qing Shu, Bolun Yang *, Hong Yuan, Song Qing, Gangli Zhu Department of Chemical Engineering, State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an Shaanxi 710049, PR China Received 4 December 2006; received in revised form 13 March 2007; accepted 3 April 2007 Available online 6 May 2007
Abstract La/zeolite beta was prepared by an ion exchange method and used to synthesize the biodiesel (fatty acid methyl esters, FAME). The La(NO3)3 was applied as the ion exchange precursor to incorporate La ion into zeolite beta. The composition of the zeolite beta before and after ion exchange was analyzed by the SEM microphotographs and EDS spectrograms, the Brønsted and Lewis acid sites were investigated by FTIR imaging. The transesterification was carried out in a batch reactor and the composition of the FAME product was determined by a potassium hydroxide saponification method. The syntheses conditions with respect to catalytic activities have been optimized individually. Results of the experiment showed that La/zeolite beta shows higher conversion and stability than zeolite beta for the production of biodiesel, which may be correlated to the higher quantity of external Brønsted acid sites available for the reactants. The product consists of a mixture of monoalkyl esters primarily, and when the methanol/ soybean oil molar ratio was 14.5, reaction temperature at 333 K, reaction time 4 h and catalyst/soybean oil mass ratio of 0.011, the conversion of triglyceride 48.9 wt% was obtained from this optimal reaction condition. 2007 Published by Elsevier B.V. Keywords: Biodiesel; Soybean oil; La/zeolite beta; Transesterification; Solid acid
1. Introduction Biodiesel is receiving increasing attention as an alternative, non-toxic, biodegradable and a renewable diesel fuel [1,2]. Compared with petroleum diesel in diesel engines, biodiesel greatly decreases the emission of carbon monoxide, sulfur, hydrocarbons, particle matter, polyaromatics and smoke during the combustion process. Furthermore, burning biodiesel has no net addition to atmospheric carbon (CO2) levels, because it is made from agricultural materials produced via photosynthetic carbon fixation [3,4]. Biodiesel is produced by the transesterification of renewable materials composed of C14–C20 fatty acid triglycerides with short-chain alcohol such as methanol or ethanol *
Corresponding author. Tel.: +86 29 82663189; fax: +86 29 82668789. E-mail address: [email protected] (B. Yang).
1566-7367/$ - see front matter 2007 Published by Elsevier B.V. doi:10.1016/j.catcom.2007.04.028
under the presence of a catalyst. As alcohol, methanol is the least expensive and readily available from syngas. Therefore, biodiesel is often referred to as fatty acids methyl esters (FAME), and glycerol is also produced as a byproduct. There are several comprehensive studies of the base-catalyzed transesterification to produce FAME [5,6] and a higher conversion of vegetable oil has been reported by using the strong base solutions such as sodium hydroxide (NaOH) and potassium hydroxide (KOH), and the basic catalysts (for example KOH) can get yields of 97–99% in 0.5–1 h [5,6]. However, the base-catalyzed process still has two limitations: (1) The basic catalyst is sensitive to the purity of reactants, especially in water and free fatty acids (FFA), so the raw materials must be refined (moisture level no more than 0.06 wt%, and FFA no more than 0.5 wt%). A minor amount of moisture can initiate oil hydrolyzation to form FFA and glycerol. FFA will react
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with the base catalyst to form soaps, and these soaps can bring about the emulsification between FAME and glycerol, which will make the separation of FAME–glycerol mixture become more difficult when the water washing method is used. (2) If the FFA content of vegetable oil is more than 0.5 wt%, an acidic catalyst must be firstly applied to reduce the FFA content by esterifying it, and such a pretreatment process adds a neutralization step as well as separation steps to the overall process. For these reasons, highly refined vegetable oils are required to progress such a reaction smoothly. However, biodiesel made from low-grade vegetable oils with a higher concentration of FFA is more commercially competitive than petroleum-based products in the aspect of cost reduction, and an acid catalyst can be used as a possible solution for the production of biodiesel [7], and the saponification reaction can be effectively avoided. Otherwise the acid catalyst can simultaneously catalyze both esterification and transesterification. Thus, biodiesel can be produced directly from low-cost lipid feedstocks, generally associated with high FFA concentrations (such as used cooking oil and greases, commonly have FFA levels of more than 6%). The catalyst and the alcohol are not necessary to comply with rigorous specifications (total water content must be 0.1–0.3 wt% or less). For acid-catalyzed systems, sulfuric acid has been the most investigated catalyst. However, the higher temperatures (373–473 K) and higher methanol/oil molar ratio (20–35:1) are needed, the corrosion and pollution are also should be considered in this case. For these reasons, the solid acid catalysts like zeolites are being sought as a kind of substitute for liquid acids [8]. Applying zeolite in catalytic process can provide a number of advantages such as the ease of separation from the liquid products, regenerability, and no toxicity, corrosion or environmental pollution. These properties provided a strong stimulus for substituting the base catalysts with zeolite in a number of catalytic processes. Zeolite beta is a high silica zeolite, containing an intersecting three-dimensional structure of 12-membered ring channels. Due to this relatively voluminous channel structure, it is possible to carry out numerous acid-catalyzed reactions effectively [9]. Furthermore, zeolite beta also has adjustable acidity in the protonic form, and this is another factor affecting the efficiency of reaction. Zeolite beta has been found to be a suitable transesterification catalyst to synthesize a variety of products [10]. It is clear that the transesterification proceeds on the Brønsted acidic sites of the zeolite beta, and the Brønsted acid sites of the zeolite beta also can be subtly adjusted by modification with metal cations, bringing about modified catalysts with suitable acidity to fit different transesterifications [11–15]. La3+ exchanged zeolite beta exhibits high catalytic activities when applied in a number of catalytic processes [16–19]. However, applying such catalyst to the transesterification of triglycerides with methanol to produce biodiesel is relatively less researched in current literatures.
Based on the considerations indicated above, this study was aimed to apply La/zeolite beta catalyst in the transesterification of triglycerides with methanol, and the syntheses conditions with respect to catalytic activities have been optimized individually by changing the experimental conditions such as reaction temperature, reaction time, amount of catalyst, catalyst/soybean oil mass ratio, and methanol/soybean oil molar ratio. To prove that the La/zeolite beta has higher catalytic activity than zeolite beta, the catalytic activity of each was compared when applied there to catalytic transesterification process. 2. Experimental 2.1. Catalyst preparation La/zeolite beta was prepared as follows: 10 g of zeolite beta (supplied by Nankai University Catalyst Manufacturing, China) was suspended in 250 mL of Lanthanum nitrate aqueous solution (0.1 mol/L) at first, and then the mixtures were vigorously stirred at room temperature for 3 h. The La/zeolite beta was thoroughly washed with deionized water, then filtered and dehydrated at 393 K for 2 h. This process was repeated twice to maximize the ion exchange level. Finally the samples were ripened at 373 K for 24 h and calcined at 523 K for 4 h in an electrical furnace. The lanthanum content loaded on zeolite beta could be investigated by EDTA (ethylene diamine tetraacetic acid) titration method by using C37H41O13N2S (methylthymol blue) as an indicator at pH 6. The titration was carried out in a 50 mL conical flask, and a drop of C37H41O13N2S was added into the solution at first, then titrated by EDTA, and 10 mL N4(CH2)6(hexamethylene tetramine) was used as a buffer. The equilibrium phase was reached when the color of solution changed from blue to yellow. The concentration of exchanged cations C (mol g1) could be calculated from the Eq. (1), where ni is the initial number of moles of cation in solution, ns is the number of moles of cations in solution which has been ion exchanged, and m is the mass of the exchanger used C¼
ð ni ns Þ m
ð1Þ
2.2. Catalyst characterization The chemical compositions of the zeolite beta before and after ion exchange were analyzed by using a scanning electron microscope connected with energy dispersive spectroscopy (SEM/EDS). The SEM microphotographs were taken on a VEGA Ts 2136MX-model scanning electron microscope, and the operating conditions were: 20 kV, 80 mA beam current. The samples were carefully ground prior to the measurements and suspended in alcohol at first, and then a drop of this mixture was placed on a carbon film deposited on a copper grid. Finally
Q. Shu et al. / Catalysis Communications 8 (2007) 2159–2165
the sample-loaded copper grid was put into the SEM after evaporation of the alcohol. The chemical compositions of the samples (O, Al, Si and La) were investigated by an EDS analysis. The FTIR spectroscopy of Brønsted and Lewis acid sites in the zeolite beta before and after ion exchange was recorded with a FTIR-spectrometer (Nicolet 360). The FTIR measurement was performed as follows. First, the sample was crushed into a powder and outgassed under a high vacuum condition for 4 h at 473 K. Second, the samples were cooled to ambient temperature to introduce into the vapor flow of saturated pyridine during 2 h. Third, the pyridine was removed by heating the sample at 473 K for 1 h. And finally, the samples were compressed into self-supporting membranes to performance FTIR measurement. 2.3. Transesterification reaction procedure and product analysis Transesterifications were performed in a 250 mL fournecked round-bottomed flask, equipped with a reflux condenser, a thermometer and a mechanical stirrer. The flask was immersed into a water bath with the temperature controlled in the range of 318–338 K under atmospheric pressure. The reaction mixtures consisted of 92.5 g soybean oil (100 mL), 27.8–63.4 g methanol (35–80 mL) and 0.5–1.5 g catalyst. For a typical run, the soybean oil and La/zeolite beta were heated to the required temperature in the water bath while stirring at 500 rpm, and then the methanol was added into the reactor to start the reaction. The transesterification was stopped after 4–8 h, and the reaction mixture was allowed to phase separate. The reaction mixture was distillated to separate the reactants and product, and the FAME–glycerol mixture will be formed because they have near boiling point, which is much higher than methanol or triglyceride. Finally, the water washing method was used to separate the glycerol from the FAME–glycerol mixture and to obtain purified FAME. In order to confirm the composition of purified product, the saponification reactions were carried out and the saponification value of FAME (mgKOH/g) was determined according to ISO 3657-1988, 185-195. After the saponification reaction, the residual potassium hydroxide was titrated by the 1.16 mol/L hydrochloric acid and was used the phenolphthalein as an indicator. The titration was stopped when the color of solution changed from red to yellow. The process of the saponification reaction is shown in the following equation: O
O R1
C
OCH3 + KOH
R1
C
OK
+ CH3 OH
ð2Þ
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The FAME was formed from the conversion of triglycerides. Thus it can be determined from the analysis of the saponification reaction. 2.4. Determination the conversion of triglyceride The mean molecular weight of the soybean oil was calculated by averaging the molecular weights of each triglyceride in it. Soybean oil mainly consists of five kinds of triglyceride, including oleic, palmitic, linoleic, linolenic and stearic acid triglyceride, giving a mean molecular weight of soybean oil of 871.4 and FAME of 291.8. The stoichiometric ratio for the transesterification reaction requires three moles of alcohol and one mole of triglyceride to yield three moles of FAME and one mole of glycerol. The theoretic conversion value of triglyceride v can be calculated from Eq. (3) based on the assumption that purified product MFAME only consists of FAME, and Msoybean oil refers to the initial amount of soybean oil. 1 M FAME =291:8 v¼ 3 M soybean oil =871:4
ð3Þ
Since final products have been distillate to obtain the purified FAME, little of diglyceride and monoglyceride have neglectable effect on the meaning of Eq. (3). The experiment conversion value of triglyceride can be obtained from the analysis of saponification result, and the saponification result is used to calculate the conversion of triglyceride. 3. Results and discussion 3.1. SEM/EDS analysis The SEM microphotographs that depicted the structures of unmodified and ion exchanged zeolite beta are shown in Fig. 1. From these photographs, we can notice that the crystal structure of La/zeolite beta became considerably less agglomerated than zeolite beta and the reason may be explained by the modification result of La3+. As we know, the framework structure of zeolite beta mainly consists of octahedral coordinated aluminum (non-framework aluminum) and tetrahedral coordinated aluminum (framework aluminum). Octahedral coordinated aluminum can be reverted to tetrahedral coordinated aluminum during the La3+ exchanging [20], and the ratio of them both will be varied. The reason for this phenomenon may be related to the structure of zeolite beta itself, because the framework of zeolite beta is deformable and can be influenced by the nature of compensating cation. The compensating Na+ ion that balances the negative charge of the framework aluminum can be replaced by La3+, and this cation replacement will influence the ratio of non-framework aluminum and framework aluminum. The reason can be explained as
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Fig. 1. SEM microphotographs of (a) unmodified zeolite beta; (b) La/zeolite beta after ion exchanged.
follows: the Al–O bond in the framework is easy to break when the Na+ is acted as compensating cation, thus leading to the removal of aluminum from the framework, this phenomenon can be related to the relatively high electron affinity of Na+. After Na+ is exchanged with La3+, the instable framework aluminum will be stabilized, owing to the low electron affinity of La3+, and the non-framework aluminum will be also reinserted back into the framework. These processes could influence the morphology of La/zeolite beta and thus the agglomeration phenomenon of the La/ zeolite beta can be greatly ameliorated. The chemical structure of the zeolite beta may be greatly affected since the La3+ could be incorporated into its framework structure. Therefore, EDS was used to analyze the chemical compositions of the unmodified and ion exchanged zeolite beta, and the spectrograms of them are shown in Fig. 2. Fig. 2a indicates that the content of Na, Al and Si are 0.62, 6.13 and 27.83% (w/w) in the unmodified zeolite beta and Fig. 2b indicates that the content of Na, Al, Si and La are 0, 5.73, 23.91 and 3.40% (w/w) in the La/zeolite beta respectively. The EDS measurements might be not reliable
by unsuitable operation which can not represent the bulk composition. To make this measurement of La exchange extent more precise, an EDTA titration was performed. From the EDTA titration result, the concentration of exchanged cations La can been known as 0.3 · 103 mol/ g. It agreed well with the result of the EDS analysis. The Si/Al of the zeolite beta and the La/zeolite beta is 3.90 and 4.17 respectively. The higher Si/Al can be related to higher structure stability of the tetrahedral framework aluminum. Since the La3+ can balance the lattice charge of zeolite beta, incorporation of La3+ in zeolite beta is also helpful for the structural stability of the La/zeolite beta. Furthermore, this stable tetrahedral framework structure can prevent the formation of Lewis acid sites, making the number of Brønsted acidic sites that can accelerate the transesterification to maintain stability during the entire reaction process. 3.2. FTIR spectroscopy of Brønsted and Lewis acid sites The relative concentration of Brønsted and Lewis acid sites was assessed by the FTIR spectra of absorbed Pyri-
Fig. 2. EDS spectrogram of (a) unmodified zeolite beta; (b) La/zeolite beta after ion exchanged.
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39
A
38 B
Conversion (wt%)
Transmittance
37 36 35 34 33 32 31
1000
1200
1400
1600
1800
2000
315
320
325
-1
dine, and the results are presented in Fig. 3. Pyridinium ion signal (Pyridine on Brønsted acid sites) appeared at wave numbers of 1620 cm1 and pyridine on Lewis acid sites appeared at 1455 cm1 [17]. From this spectroscopy, we can notice that the intensity of the Lewis acid sites does not vary obviously, and the intensity of the Brønsted acid sites of zeolite beta is weaker than that of La3+ exchanged zeolite beta. The increment of the intensity of the Brønsted acid sites can be attributed to the presence of Si–OH–La groups and La–OH groups in the La/zeolite beta after ion exchange. A lanthanum atom could replace the aluminium atom of the bridged hydroxyl groups, and this isomorphous substitution reaction will result in the formation of new acid sites similar to Brønsted acid sites. Moreover, as shown in Eqs. (4) and (5), ion exchange between La3+ and zeolite beta can 3þ result in the formation of metal hydrate cation LaðH2 OÞn due to the electrosatic field of metal cations. The splitting of water molecule will occur to form LaðOHÞþ n , and new Brønsted acid sites are also formed from this process [17] 3þ LaðH2 OÞn
!
þ LaðOHÞn
335
340
345
3þ
ð4Þ
þ 2Hþ
ð5Þ
It can be concluded that the concentration of Brønsted sites was increased. These new Bronsted sites should be related to the weak Brønsted sites, according to the study of Guzman et al. [21]. Thus, the strength of the acid sites would decrease by the combination of these two considerations. 3.3. Effect of reaction temperature In order to study the influence of the reaction temperature on the conversion of triglyceride, experiments using La/zeolite beta catalyst were conducted at 318, 323, 328, 333, 338 and 343 K. As shown in Fig. 4, highest conversion of triglyceride is 38.9 wt% at 333 K. The explanation for above observations is that the higher temperatures give higher reaction rate to produce more FAME. However, it
Fig. 4. Influence of the reaction temperature on the conversion of triglyceride. Reaction time: 4 h; Methanol/soybean oil volume ratio: 0.5 and the mass of catalyst: 1 g.
42 40 38
Conversion (wt%)
Fig. 3. FTIR spectra of unmodified and ion exchanged zeolite beta samples after pyridine chemisorption: (A) unmodified zeolite beta; (B) La/ zeolite beta after ion exchanged.
La3þ þ H2 O ! LaðH2 OÞn
330
Temperature (K)
Wavenumbers (cm )
36 34 32 30 28 0.4
0.6
0.8
1.0
1.2
1.4
1.6
Catalyst (g)
Fig. 5. Influence of the catalyst mass on the conversion of triglyceride. Reaction time: 4 h; Methanol/soybean oil volume ratio: 0.5 and the reaction temperature: 333 K.
also forms more byproduct glycerol to enhance the solubility of glycerol in the fatty phase, and intensify the glycerolysis of FAME. It should be noted that the mass transfer between the two liquid phases is the rate determining step, and the temperature has small effect on it. Therefore, a suitable temperature for the synthesis of FAME is present and it is 333 K in this experimental system. 3.4. Effect of catalyst loading The influence of mass of La/zeolite beta on the conversion of triglyceride was investigated, and experiments were performed with different loadings of the La/zeolite beta respectively. The results are shown in Fig. 5. From this figure, it can be noted that when the catalyst loading is no more than 1.0 g, an increase in the mass of catalyst loading from 0.5 to 1.0 g causes a marked increase of conversion of triglyceride from 29.6 to 38.9 wt%. Further increase in the mass of catalyst loading from 1.0 to 1.5 g causes a negligible increase of conversion of triglyceride from 38.9 to 40.6 wt%.
Q. Shu et al. / Catalysis Communications 8 (2007) 2159–2165
This result can be explained as follows: the solubility of methanol in soybean oil is limited, and thus the transesterification can be only carried out in the interface of these two-phases in the less loading of catalyst; the reaction will be enhanced by increasing catalyst loading to increase the proton concentration in the interface, and more FAME thus will be formed. Since FAME will act as a mutual solvent, the reactant will become single-phase; in this case, the effect of increasing catalyst continuously will not be notable. Thus, the 1.0 g is considered as a suitable loading of catalyst in this experimental system.
50 45
Conversion (wt%)
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40 35 30 25 20 15 0.3
0.4
0.5
0.6
0.7
0.8
Methanol/Soybean oil (v/v)
3.5. Effect of reaction time In order to study the influence of reaction time on the conversion of triglyceride, experiments using the same mass La/zeolite beta catalyst were conducted at 1, 2, 4, 6 and 8 h respectively. The results are shown in Fig. 6. From this figure, one can notice that when the reaction time is no more than 4 h, an increase in the reaction time causes a marked increase of conversion of triglyceride from 18.5 to 38.9 wt%. Further increase in the reaction time from 4 to 8 h, causes a less effect on the conversion of triglyceride from 38.9 to 42.1 wt%. Thus the 4 h can be chosen as a suitable reaction time for the synthesis of FAME. 3.6. Effect of molar ratio of methanol/soybean oil Since the transesterification is a reversible reaction, in the reaction mixture, the mole of methanol must be excess enough to force the reaction towards the formation of the FAME. The effect of the molar ratio of methanol to triglyceride on the yields of FAME was examined by varying the amount of methanol with a fixed amount of soybean oil of 100 mL in reactions at 333 K. The initial amount of methanol was set at 35, 45, 55, 75, 80 mL respectively. The results are shown in Fig. 7. The conversion of triglyceride increased with increasing methanol addition. When the amount of methanol was 80 mL, corresponding to the
Fig. 7. Influence of the volume ratio of methanol/ soybean oil on the conversion of triglyceride. Reaction time: 4 h; the mass of catalyst: 1 g and the reaction temperature: 333 K.
molar ratio of methanol/soybean oil 14.5, the conversion of triglyceride reached in 48.9 wt%. The excess of methanol is favorable to the conversion of triglycerides into monoglycerides, but there is also a slight recombination of FAME and glycerol to form monoglycerides. The FAME and the byproduct glycerol are nearly immiscible, but monoglyceride can significantly effect on glycerol solubility in FAME, so the glycerolysis of FAME will occur, and then decreased the triglycerides conversion. From above analysis, the molar ratio of methanol to triglyceride cannot be too high. In the case of base catalyst being used, methoxide ion forms by dissociation of methoxide salt that is from the reaction between methanol and hydroxyl ion. The methoxide ion is stronger nucleophiles that can facilitate the transesterification, so the methanol/oil molar ratio is much lower than the case of acid catalyst being used. 3.7. Comparison the catalytic activity The catalytic activities of zeolite beta and La/zeolite beta are shown in Fig. 8. It should be noted that the access of triglyceride to sites of the zeolite beta interior may be 50
zeolite beta La/zeolite beta
45
50
40
Conversion (wt%)
Conversion (wt%)
45 40 35 30
35 30 25 20 15 10 5
25
0 20
A
B
Catalyst 0
1
2
3
4
5
6
7
8
9
Time (hours)
Fig. 6. Influence of the reaction time on the conversion of triglyceride. The mass of catalyst: 1 g; Methanol/soybean oil volume ratio: 0.5 and the reaction temperature: 333 K.
Fig. 8. Catalytic activities of zeolite beta and La/zeolite beta related to different conversion of triglyceride. (A) Reaction time: 4 h; Methanol/ soybean oil volume ratio: 0.5; the mass of catalyst: 1 g and the reaction temperature: 333 K. (B) Reaction time: 4 h; Methanol/soybean oil volume ratio: 0.8; the mass of catalyst: 1 g and the reaction temperature: 333 K.
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restricted due to its sizes, thus, the catalytic activity mainly comes from sites on the zeolite beta exterior surfaces. Since the La ion exchange is supposed to occur on the outer surface of the zeolite beta due to the large size of aqueous Lanthanum species, it can greatly increase the sites on the zeolite beta exterior surfaces. Otherwise, this modification will lead to a certain disintegration of agglomerates of the zeolite beat. Thus, it would be implied that the La/zeolite beta has higher quantity of external Brønsted acid sites available for the reactant. The conversion of triglyceride is increased by 10 wt% compared with zeolite catalyst. Vicente et al. reported similar experiment in 1998 [22]. In his work, the catalyst was strong cation exchange resin (Amberlyst-15), the methanol/sunflower oil molar ratio was 6, reaction temperature was 333 K, reaction time 8 h and the catalyst/sunflower oil mass ratio was 0.01, the conversion of sunflower oil was only 0.7%. The La/zeolite beta thus showed a superior catalytic activity relative to the Amberlyst-15 catalyst. 4. Conclusions The La/zeolite beta catalyst with higher quantity of external Brønsted acid sites was obtained by the lanthanum ion exchange on zeolite beta. The transesterification can proceed efficiently at 333 K, the methanol/ soybean oil molar ratio 14.5, and catalyst/soybean oil mass ratio of 0.011, the conversion of triglyceride 48.9 wt% was obtained in 4 h from these reaction conditions. The La/zeolite beta catalyst can effectively solve the two limitations of the base-catalyzed process. Moreover, compared with the liquid acid-catalyzed process, this catalyst can be separated easily by simple filtration and without
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the corrosion and pollution problems. It opens up a possible way of synthesizing the biodiesel from recoverable porous solid catalysts in the near future. References [1] M.A. Kalam, H.H. Masjuki, Biomass. Bioenerg. 23 (2002) 471. [2] F. Ma, M.A. Hanna, Bioresour. Technol. 70 (1999) 1. [3] G. Antolı´n, F.V. Tinaut, Y. Bricen´o, V. Castano, Bioresour. Technol. 83 (2002) 111. [4] F.R. Abreu, M.B. Alves, C.C.S. Macedo, J. Mol. Catal. A: Chem. 227 (2005) 263. [5] G. Vicente, M. Martinez, J. Aracil, Bioresour. Technol. 92 (2004) 297. [6] J.M. Encinar, J.F. Gonza´lez, A.R. Reinares, Ind. Eng. Chem. Res. 44 (2005) 5491. [7] E. Lotero, Y.J. Liu, Ind. Eng. Chem. Res. 44 (2005) 5353. [8] J.W. Kim, D.J. Kim, J.U. Han, Catal. Today 87 (2003) 195. [9] J.C. Jansen, E.J. Creyghton, Catal. Today 38 (1997) 205. [10] M. Sasidharan, R. Kumar, J. Mol. Catal. A: Chem. 210 (2004) 93. [11] O. Bortnovsky, Z. Sobalı´k, B. Wichterlova´, Mater. Micropor. Mesopor. 46 (2001) 265. [12] Y.J. Wang, X.Q. Zhao, B.G. Yuan, Chin. J. Catal. 18 (1997) 331. [13] J. Penzien, A. Abraham, J.A.V. Bokhoven, J. Phys. Chem. 108 (2004) 4116. [14] J. Valverde, A.D. Lucas, M.G.J. Lez, Chem. Eng. Data 46 (2001) 1404. [15] S. Samanta, S. Giri, P.U. Sastry, Ind. Eng. Chem. Res. 42 (2003) 3012. [16] K. Kili, F.L. Normand, J. Mol. Catal. A: Chem. 140 (1999) 267. [17] P. Tynja¨la¨, T.T. Pakkanen, J. Mol. Catal. A: Chem. 110 (1996) 153. [18] C. Jia, P. Beaunier, P. Massiani, Micropor. Mesopor. 24 (1998) 69. [19] M. Bjørgen, F. Bonino, S.J. Kolboe, J. Am. Chem. Soc. 125 (2003) 15863. [20] C. Yang, Q.H. Xu, Zeolites 19 (1997) 404. [21] A. Guzman, I. Zuazo, A. Feller, R. Olindo, Micropor. Mesopor. 83 (2005) 309. [22] G. Vicente, A. Coteron, M. Martinez, Aracil, J. Ind. Crop. Prod. 8 (1998) 29.
Catalysis Communications 8 (2007) 2159–2165 www.elsevier.com/locate/catcom
Synthesis of biodiesel from soybean oil and methanol catalyzed by zeolite beta modified with La3+ Qing Shu, Bolun Yang *, Hong Yuan, Song Qing, Gangli Zhu Department of Chemical Engineering, State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an Shaanxi 710049, PR China Received 4 December 2006; received in revised form 13 March 2007; accepted 3 April 2007 Available online 6 May 2007
Abstract La/zeolite beta was prepared by an ion exchange method and used to synthesize the biodiesel (fatty acid methyl esters, FAME). The La(NO3)3 was applied as the ion exchange precursor to incorporate La ion into zeolite beta. The composition of the zeolite beta before and after ion exchange was analyzed by the SEM microphotographs and EDS spectrograms, the Brønsted and Lewis acid sites were investigated by FTIR imaging. The transesterification was carried out in a batch reactor and the composition of the FAME product was determined by a potassium hydroxide saponification method. The syntheses conditions with respect to catalytic activities have been optimized individually. Results of the experiment showed that La/zeolite beta shows higher conversion and stability than zeolite beta for the production of biodiesel, which may be correlated to the higher quantity of external Brønsted acid sites available for the reactants. The product consists of a mixture of monoalkyl esters primarily, and when the methanol/ soybean oil molar ratio was 14.5, reaction temperature at 333 K, reaction time 4 h and catalyst/soybean oil mass ratio of 0.011, the conversion of triglyceride 48.9 wt% was obtained from this optimal reaction condition. 2007 Published by Elsevier B.V. Keywords: Biodiesel; Soybean oil; La/zeolite beta; Transesterification; Solid acid
1. Introduction Biodiesel is receiving increasing attention as an alternative, non-toxic, biodegradable and a renewable diesel fuel [1,2]. Compared with petroleum diesel in diesel engines, biodiesel greatly decreases the emission of carbon monoxide, sulfur, hydrocarbons, particle matter, polyaromatics and smoke during the combustion process. Furthermore, burning biodiesel has no net addition to atmospheric carbon (CO2) levels, because it is made from agricultural materials produced via photosynthetic carbon fixation [3,4]. Biodiesel is produced by the transesterification of renewable materials composed of C14–C20 fatty acid triglycerides with short-chain alcohol such as methanol or ethanol *
Corresponding author. Tel.: +86 29 82663189; fax: +86 29 82668789. E-mail address: [email protected] (B. Yang).
1566-7367/$ - see front matter 2007 Published by Elsevier B.V. doi:10.1016/j.catcom.2007.04.028
under the presence of a catalyst. As alcohol, methanol is the least expensive and readily available from syngas. Therefore, biodiesel is often referred to as fatty acids methyl esters (FAME), and glycerol is also produced as a byproduct. There are several comprehensive studies of the base-catalyzed transesterification to produce FAME [5,6] and a higher conversion of vegetable oil has been reported by using the strong base solutions such as sodium hydroxide (NaOH) and potassium hydroxide (KOH), and the basic catalysts (for example KOH) can get yields of 97–99% in 0.5–1 h [5,6]. However, the base-catalyzed process still has two limitations: (1) The basic catalyst is sensitive to the purity of reactants, especially in water and free fatty acids (FFA), so the raw materials must be refined (moisture level no more than 0.06 wt%, and FFA no more than 0.5 wt%). A minor amount of moisture can initiate oil hydrolyzation to form FFA and glycerol. FFA will react
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with the base catalyst to form soaps, and these soaps can bring about the emulsification between FAME and glycerol, which will make the separation of FAME–glycerol mixture become more difficult when the water washing method is used. (2) If the FFA content of vegetable oil is more than 0.5 wt%, an acidic catalyst must be firstly applied to reduce the FFA content by esterifying it, and such a pretreatment process adds a neutralization step as well as separation steps to the overall process. For these reasons, highly refined vegetable oils are required to progress such a reaction smoothly. However, biodiesel made from low-grade vegetable oils with a higher concentration of FFA is more commercially competitive than petroleum-based products in the aspect of cost reduction, and an acid catalyst can be used as a possible solution for the production of biodiesel [7], and the saponification reaction can be effectively avoided. Otherwise the acid catalyst can simultaneously catalyze both esterification and transesterification. Thus, biodiesel can be produced directly from low-cost lipid feedstocks, generally associated with high FFA concentrations (such as used cooking oil and greases, commonly have FFA levels of more than 6%). The catalyst and the alcohol are not necessary to comply with rigorous specifications (total water content must be 0.1–0.3 wt% or less). For acid-catalyzed systems, sulfuric acid has been the most investigated catalyst. However, the higher temperatures (373–473 K) and higher methanol/oil molar ratio (20–35:1) are needed, the corrosion and pollution are also should be considered in this case. For these reasons, the solid acid catalysts like zeolites are being sought as a kind of substitute for liquid acids [8]. Applying zeolite in catalytic process can provide a number of advantages such as the ease of separation from the liquid products, regenerability, and no toxicity, corrosion or environmental pollution. These properties provided a strong stimulus for substituting the base catalysts with zeolite in a number of catalytic processes. Zeolite beta is a high silica zeolite, containing an intersecting three-dimensional structure of 12-membered ring channels. Due to this relatively voluminous channel structure, it is possible to carry out numerous acid-catalyzed reactions effectively [9]. Furthermore, zeolite beta also has adjustable acidity in the protonic form, and this is another factor affecting the efficiency of reaction. Zeolite beta has been found to be a suitable transesterification catalyst to synthesize a variety of products [10]. It is clear that the transesterification proceeds on the Brønsted acidic sites of the zeolite beta, and the Brønsted acid sites of the zeolite beta also can be subtly adjusted by modification with metal cations, bringing about modified catalysts with suitable acidity to fit different transesterifications [11–15]. La3+ exchanged zeolite beta exhibits high catalytic activities when applied in a number of catalytic processes [16–19]. However, applying such catalyst to the transesterification of triglycerides with methanol to produce biodiesel is relatively less researched in current literatures.
Based on the considerations indicated above, this study was aimed to apply La/zeolite beta catalyst in the transesterification of triglycerides with methanol, and the syntheses conditions with respect to catalytic activities have been optimized individually by changing the experimental conditions such as reaction temperature, reaction time, amount of catalyst, catalyst/soybean oil mass ratio, and methanol/soybean oil molar ratio. To prove that the La/zeolite beta has higher catalytic activity than zeolite beta, the catalytic activity of each was compared when applied there to catalytic transesterification process. 2. Experimental 2.1. Catalyst preparation La/zeolite beta was prepared as follows: 10 g of zeolite beta (supplied by Nankai University Catalyst Manufacturing, China) was suspended in 250 mL of Lanthanum nitrate aqueous solution (0.1 mol/L) at first, and then the mixtures were vigorously stirred at room temperature for 3 h. The La/zeolite beta was thoroughly washed with deionized water, then filtered and dehydrated at 393 K for 2 h. This process was repeated twice to maximize the ion exchange level. Finally the samples were ripened at 373 K for 24 h and calcined at 523 K for 4 h in an electrical furnace. The lanthanum content loaded on zeolite beta could be investigated by EDTA (ethylene diamine tetraacetic acid) titration method by using C37H41O13N2S (methylthymol blue) as an indicator at pH 6. The titration was carried out in a 50 mL conical flask, and a drop of C37H41O13N2S was added into the solution at first, then titrated by EDTA, and 10 mL N4(CH2)6(hexamethylene tetramine) was used as a buffer. The equilibrium phase was reached when the color of solution changed from blue to yellow. The concentration of exchanged cations C (mol g1) could be calculated from the Eq. (1), where ni is the initial number of moles of cation in solution, ns is the number of moles of cations in solution which has been ion exchanged, and m is the mass of the exchanger used C¼
ð ni ns Þ m
ð1Þ
2.2. Catalyst characterization The chemical compositions of the zeolite beta before and after ion exchange were analyzed by using a scanning electron microscope connected with energy dispersive spectroscopy (SEM/EDS). The SEM microphotographs were taken on a VEGA Ts 2136MX-model scanning electron microscope, and the operating conditions were: 20 kV, 80 mA beam current. The samples were carefully ground prior to the measurements and suspended in alcohol at first, and then a drop of this mixture was placed on a carbon film deposited on a copper grid. Finally
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the sample-loaded copper grid was put into the SEM after evaporation of the alcohol. The chemical compositions of the samples (O, Al, Si and La) were investigated by an EDS analysis. The FTIR spectroscopy of Brønsted and Lewis acid sites in the zeolite beta before and after ion exchange was recorded with a FTIR-spectrometer (Nicolet 360). The FTIR measurement was performed as follows. First, the sample was crushed into a powder and outgassed under a high vacuum condition for 4 h at 473 K. Second, the samples were cooled to ambient temperature to introduce into the vapor flow of saturated pyridine during 2 h. Third, the pyridine was removed by heating the sample at 473 K for 1 h. And finally, the samples were compressed into self-supporting membranes to performance FTIR measurement. 2.3. Transesterification reaction procedure and product analysis Transesterifications were performed in a 250 mL fournecked round-bottomed flask, equipped with a reflux condenser, a thermometer and a mechanical stirrer. The flask was immersed into a water bath with the temperature controlled in the range of 318–338 K under atmospheric pressure. The reaction mixtures consisted of 92.5 g soybean oil (100 mL), 27.8–63.4 g methanol (35–80 mL) and 0.5–1.5 g catalyst. For a typical run, the soybean oil and La/zeolite beta were heated to the required temperature in the water bath while stirring at 500 rpm, and then the methanol was added into the reactor to start the reaction. The transesterification was stopped after 4–8 h, and the reaction mixture was allowed to phase separate. The reaction mixture was distillated to separate the reactants and product, and the FAME–glycerol mixture will be formed because they have near boiling point, which is much higher than methanol or triglyceride. Finally, the water washing method was used to separate the glycerol from the FAME–glycerol mixture and to obtain purified FAME. In order to confirm the composition of purified product, the saponification reactions were carried out and the saponification value of FAME (mgKOH/g) was determined according to ISO 3657-1988, 185-195. After the saponification reaction, the residual potassium hydroxide was titrated by the 1.16 mol/L hydrochloric acid and was used the phenolphthalein as an indicator. The titration was stopped when the color of solution changed from red to yellow. The process of the saponification reaction is shown in the following equation: O
O R1
C
OCH3 + KOH
R1
C
OK
+ CH3 OH
ð2Þ
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The FAME was formed from the conversion of triglycerides. Thus it can be determined from the analysis of the saponification reaction. 2.4. Determination the conversion of triglyceride The mean molecular weight of the soybean oil was calculated by averaging the molecular weights of each triglyceride in it. Soybean oil mainly consists of five kinds of triglyceride, including oleic, palmitic, linoleic, linolenic and stearic acid triglyceride, giving a mean molecular weight of soybean oil of 871.4 and FAME of 291.8. The stoichiometric ratio for the transesterification reaction requires three moles of alcohol and one mole of triglyceride to yield three moles of FAME and one mole of glycerol. The theoretic conversion value of triglyceride v can be calculated from Eq. (3) based on the assumption that purified product MFAME only consists of FAME, and Msoybean oil refers to the initial amount of soybean oil. 1 M FAME =291:8 v¼ 3 M soybean oil =871:4
ð3Þ
Since final products have been distillate to obtain the purified FAME, little of diglyceride and monoglyceride have neglectable effect on the meaning of Eq. (3). The experiment conversion value of triglyceride can be obtained from the analysis of saponification result, and the saponification result is used to calculate the conversion of triglyceride. 3. Results and discussion 3.1. SEM/EDS analysis The SEM microphotographs that depicted the structures of unmodified and ion exchanged zeolite beta are shown in Fig. 1. From these photographs, we can notice that the crystal structure of La/zeolite beta became considerably less agglomerated than zeolite beta and the reason may be explained by the modification result of La3+. As we know, the framework structure of zeolite beta mainly consists of octahedral coordinated aluminum (non-framework aluminum) and tetrahedral coordinated aluminum (framework aluminum). Octahedral coordinated aluminum can be reverted to tetrahedral coordinated aluminum during the La3+ exchanging [20], and the ratio of them both will be varied. The reason for this phenomenon may be related to the structure of zeolite beta itself, because the framework of zeolite beta is deformable and can be influenced by the nature of compensating cation. The compensating Na+ ion that balances the negative charge of the framework aluminum can be replaced by La3+, and this cation replacement will influence the ratio of non-framework aluminum and framework aluminum. The reason can be explained as
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Fig. 1. SEM microphotographs of (a) unmodified zeolite beta; (b) La/zeolite beta after ion exchanged.
follows: the Al–O bond in the framework is easy to break when the Na+ is acted as compensating cation, thus leading to the removal of aluminum from the framework, this phenomenon can be related to the relatively high electron affinity of Na+. After Na+ is exchanged with La3+, the instable framework aluminum will be stabilized, owing to the low electron affinity of La3+, and the non-framework aluminum will be also reinserted back into the framework. These processes could influence the morphology of La/zeolite beta and thus the agglomeration phenomenon of the La/ zeolite beta can be greatly ameliorated. The chemical structure of the zeolite beta may be greatly affected since the La3+ could be incorporated into its framework structure. Therefore, EDS was used to analyze the chemical compositions of the unmodified and ion exchanged zeolite beta, and the spectrograms of them are shown in Fig. 2. Fig. 2a indicates that the content of Na, Al and Si are 0.62, 6.13 and 27.83% (w/w) in the unmodified zeolite beta and Fig. 2b indicates that the content of Na, Al, Si and La are 0, 5.73, 23.91 and 3.40% (w/w) in the La/zeolite beta respectively. The EDS measurements might be not reliable
by unsuitable operation which can not represent the bulk composition. To make this measurement of La exchange extent more precise, an EDTA titration was performed. From the EDTA titration result, the concentration of exchanged cations La can been known as 0.3 · 103 mol/ g. It agreed well with the result of the EDS analysis. The Si/Al of the zeolite beta and the La/zeolite beta is 3.90 and 4.17 respectively. The higher Si/Al can be related to higher structure stability of the tetrahedral framework aluminum. Since the La3+ can balance the lattice charge of zeolite beta, incorporation of La3+ in zeolite beta is also helpful for the structural stability of the La/zeolite beta. Furthermore, this stable tetrahedral framework structure can prevent the formation of Lewis acid sites, making the number of Brønsted acidic sites that can accelerate the transesterification to maintain stability during the entire reaction process. 3.2. FTIR spectroscopy of Brønsted and Lewis acid sites The relative concentration of Brønsted and Lewis acid sites was assessed by the FTIR spectra of absorbed Pyri-
Fig. 2. EDS spectrogram of (a) unmodified zeolite beta; (b) La/zeolite beta after ion exchanged.
Q. Shu et al. / Catalysis Communications 8 (2007) 2159–2165
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39
A
38 B
Conversion (wt%)
Transmittance
37 36 35 34 33 32 31
1000
1200
1400
1600
1800
2000
315
320
325
-1
dine, and the results are presented in Fig. 3. Pyridinium ion signal (Pyridine on Brønsted acid sites) appeared at wave numbers of 1620 cm1 and pyridine on Lewis acid sites appeared at 1455 cm1 [17]. From this spectroscopy, we can notice that the intensity of the Lewis acid sites does not vary obviously, and the intensity of the Brønsted acid sites of zeolite beta is weaker than that of La3+ exchanged zeolite beta. The increment of the intensity of the Brønsted acid sites can be attributed to the presence of Si–OH–La groups and La–OH groups in the La/zeolite beta after ion exchange. A lanthanum atom could replace the aluminium atom of the bridged hydroxyl groups, and this isomorphous substitution reaction will result in the formation of new acid sites similar to Brønsted acid sites. Moreover, as shown in Eqs. (4) and (5), ion exchange between La3+ and zeolite beta can 3þ result in the formation of metal hydrate cation LaðH2 OÞn due to the electrosatic field of metal cations. The splitting of water molecule will occur to form LaðOHÞþ n , and new Brønsted acid sites are also formed from this process [17] 3þ LaðH2 OÞn
!
þ LaðOHÞn
335
340
345
3þ
ð4Þ
þ 2Hþ
ð5Þ
It can be concluded that the concentration of Brønsted sites was increased. These new Bronsted sites should be related to the weak Brønsted sites, according to the study of Guzman et al. [21]. Thus, the strength of the acid sites would decrease by the combination of these two considerations. 3.3. Effect of reaction temperature In order to study the influence of the reaction temperature on the conversion of triglyceride, experiments using La/zeolite beta catalyst were conducted at 318, 323, 328, 333, 338 and 343 K. As shown in Fig. 4, highest conversion of triglyceride is 38.9 wt% at 333 K. The explanation for above observations is that the higher temperatures give higher reaction rate to produce more FAME. However, it
Fig. 4. Influence of the reaction temperature on the conversion of triglyceride. Reaction time: 4 h; Methanol/soybean oil volume ratio: 0.5 and the mass of catalyst: 1 g.
42 40 38
Conversion (wt%)
Fig. 3. FTIR spectra of unmodified and ion exchanged zeolite beta samples after pyridine chemisorption: (A) unmodified zeolite beta; (B) La/ zeolite beta after ion exchanged.
La3þ þ H2 O ! LaðH2 OÞn
330
Temperature (K)
Wavenumbers (cm )
36 34 32 30 28 0.4
0.6
0.8
1.0
1.2
1.4
1.6
Catalyst (g)
Fig. 5. Influence of the catalyst mass on the conversion of triglyceride. Reaction time: 4 h; Methanol/soybean oil volume ratio: 0.5 and the reaction temperature: 333 K.
also forms more byproduct glycerol to enhance the solubility of glycerol in the fatty phase, and intensify the glycerolysis of FAME. It should be noted that the mass transfer between the two liquid phases is the rate determining step, and the temperature has small effect on it. Therefore, a suitable temperature for the synthesis of FAME is present and it is 333 K in this experimental system. 3.4. Effect of catalyst loading The influence of mass of La/zeolite beta on the conversion of triglyceride was investigated, and experiments were performed with different loadings of the La/zeolite beta respectively. The results are shown in Fig. 5. From this figure, it can be noted that when the catalyst loading is no more than 1.0 g, an increase in the mass of catalyst loading from 0.5 to 1.0 g causes a marked increase of conversion of triglyceride from 29.6 to 38.9 wt%. Further increase in the mass of catalyst loading from 1.0 to 1.5 g causes a negligible increase of conversion of triglyceride from 38.9 to 40.6 wt%.
Q. Shu et al. / Catalysis Communications 8 (2007) 2159–2165
This result can be explained as follows: the solubility of methanol in soybean oil is limited, and thus the transesterification can be only carried out in the interface of these two-phases in the less loading of catalyst; the reaction will be enhanced by increasing catalyst loading to increase the proton concentration in the interface, and more FAME thus will be formed. Since FAME will act as a mutual solvent, the reactant will become single-phase; in this case, the effect of increasing catalyst continuously will not be notable. Thus, the 1.0 g is considered as a suitable loading of catalyst in this experimental system.
50 45
Conversion (wt%)
2164
40 35 30 25 20 15 0.3
0.4
0.5
0.6
0.7
0.8
Methanol/Soybean oil (v/v)
3.5. Effect of reaction time In order to study the influence of reaction time on the conversion of triglyceride, experiments using the same mass La/zeolite beta catalyst were conducted at 1, 2, 4, 6 and 8 h respectively. The results are shown in Fig. 6. From this figure, one can notice that when the reaction time is no more than 4 h, an increase in the reaction time causes a marked increase of conversion of triglyceride from 18.5 to 38.9 wt%. Further increase in the reaction time from 4 to 8 h, causes a less effect on the conversion of triglyceride from 38.9 to 42.1 wt%. Thus the 4 h can be chosen as a suitable reaction time for the synthesis of FAME. 3.6. Effect of molar ratio of methanol/soybean oil Since the transesterification is a reversible reaction, in the reaction mixture, the mole of methanol must be excess enough to force the reaction towards the formation of the FAME. The effect of the molar ratio of methanol to triglyceride on the yields of FAME was examined by varying the amount of methanol with a fixed amount of soybean oil of 100 mL in reactions at 333 K. The initial amount of methanol was set at 35, 45, 55, 75, 80 mL respectively. The results are shown in Fig. 7. The conversion of triglyceride increased with increasing methanol addition. When the amount of methanol was 80 mL, corresponding to the
Fig. 7. Influence of the volume ratio of methanol/ soybean oil on the conversion of triglyceride. Reaction time: 4 h; the mass of catalyst: 1 g and the reaction temperature: 333 K.
molar ratio of methanol/soybean oil 14.5, the conversion of triglyceride reached in 48.9 wt%. The excess of methanol is favorable to the conversion of triglycerides into monoglycerides, but there is also a slight recombination of FAME and glycerol to form monoglycerides. The FAME and the byproduct glycerol are nearly immiscible, but monoglyceride can significantly effect on glycerol solubility in FAME, so the glycerolysis of FAME will occur, and then decreased the triglycerides conversion. From above analysis, the molar ratio of methanol to triglyceride cannot be too high. In the case of base catalyst being used, methoxide ion forms by dissociation of methoxide salt that is from the reaction between methanol and hydroxyl ion. The methoxide ion is stronger nucleophiles that can facilitate the transesterification, so the methanol/oil molar ratio is much lower than the case of acid catalyst being used. 3.7. Comparison the catalytic activity The catalytic activities of zeolite beta and La/zeolite beta are shown in Fig. 8. It should be noted that the access of triglyceride to sites of the zeolite beta interior may be 50
zeolite beta La/zeolite beta
45
50
40
Conversion (wt%)
Conversion (wt%)
45 40 35 30
35 30 25 20 15 10 5
25
0 20
A
B
Catalyst 0
1
2
3
4
5
6
7
8
9
Time (hours)
Fig. 6. Influence of the reaction time on the conversion of triglyceride. The mass of catalyst: 1 g; Methanol/soybean oil volume ratio: 0.5 and the reaction temperature: 333 K.
Fig. 8. Catalytic activities of zeolite beta and La/zeolite beta related to different conversion of triglyceride. (A) Reaction time: 4 h; Methanol/ soybean oil volume ratio: 0.5; the mass of catalyst: 1 g and the reaction temperature: 333 K. (B) Reaction time: 4 h; Methanol/soybean oil volume ratio: 0.8; the mass of catalyst: 1 g and the reaction temperature: 333 K.
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restricted due to its sizes, thus, the catalytic activity mainly comes from sites on the zeolite beta exterior surfaces. Since the La ion exchange is supposed to occur on the outer surface of the zeolite beta due to the large size of aqueous Lanthanum species, it can greatly increase the sites on the zeolite beta exterior surfaces. Otherwise, this modification will lead to a certain disintegration of agglomerates of the zeolite beat. Thus, it would be implied that the La/zeolite beta has higher quantity of external Brønsted acid sites available for the reactant. The conversion of triglyceride is increased by 10 wt% compared with zeolite catalyst. Vicente et al. reported similar experiment in 1998 [22]. In his work, the catalyst was strong cation exchange resin (Amberlyst-15), the methanol/sunflower oil molar ratio was 6, reaction temperature was 333 K, reaction time 8 h and the catalyst/sunflower oil mass ratio was 0.01, the conversion of sunflower oil was only 0.7%. The La/zeolite beta thus showed a superior catalytic activity relative to the Amberlyst-15 catalyst. 4. Conclusions The La/zeolite beta catalyst with higher quantity of external Brønsted acid sites was obtained by the lanthanum ion exchange on zeolite beta. The transesterification can proceed efficiently at 333 K, the methanol/ soybean oil molar ratio 14.5, and catalyst/soybean oil mass ratio of 0.011, the conversion of triglyceride 48.9 wt% was obtained in 4 h from these reaction conditions. The La/zeolite beta catalyst can effectively solve the two limitations of the base-catalyzed process. Moreover, compared with the liquid acid-catalyzed process, this catalyst can be separated easily by simple filtration and without
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the corrosion and pollution problems. It opens up a possible way of synthesizing the biodiesel from recoverable porous solid catalysts in the near future. References [1] M.A. Kalam, H.H. Masjuki, Biomass. Bioenerg. 23 (2002) 471. [2] F. Ma, M.A. Hanna, Bioresour. Technol. 70 (1999) 1. [3] G. Antolı´n, F.V. Tinaut, Y. Bricen´o, V. Castano, Bioresour. Technol. 83 (2002) 111. [4] F.R. Abreu, M.B. Alves, C.C.S. Macedo, J. Mol. Catal. A: Chem. 227 (2005) 263. [5] G. Vicente, M. Martinez, J. Aracil, Bioresour. Technol. 92 (2004) 297. [6] J.M. Encinar, J.F. Gonza´lez, A.R. Reinares, Ind. Eng. Chem. Res. 44 (2005) 5491. [7] E. Lotero, Y.J. Liu, Ind. Eng. Chem. Res. 44 (2005) 5353. [8] J.W. Kim, D.J. Kim, J.U. Han, Catal. Today 87 (2003) 195. [9] J.C. Jansen, E.J. Creyghton, Catal. Today 38 (1997) 205. [10] M. Sasidharan, R. Kumar, J. Mol. Catal. A: Chem. 210 (2004) 93. [11] O. Bortnovsky, Z. Sobalı´k, B. Wichterlova´, Mater. Micropor. Mesopor. 46 (2001) 265. [12] Y.J. Wang, X.Q. Zhao, B.G. Yuan, Chin. J. Catal. 18 (1997) 331. [13] J. Penzien, A. Abraham, J.A.V. Bokhoven, J. Phys. Chem. 108 (2004) 4116. [14] J. Valverde, A.D. Lucas, M.G.J. Lez, Chem. Eng. Data 46 (2001) 1404. [15] S. Samanta, S. Giri, P.U. Sastry, Ind. Eng. Chem. Res. 42 (2003) 3012. [16] K. Kili, F.L. Normand, J. Mol. Catal. A: Chem. 140 (1999) 267. [17] P. Tynja¨la¨, T.T. Pakkanen, J. Mol. Catal. A: Chem. 110 (1996) 153. [18] C. Jia, P. Beaunier, P. Massiani, Micropor. Mesopor. 24 (1998) 69. [19] M. Bjørgen, F. Bonino, S.J. Kolboe, J. Am. Chem. Soc. 125 (2003) 15863. [20] C. Yang, Q.H. Xu, Zeolites 19 (1997) 404. [21] A. Guzman, I. Zuazo, A. Feller, R. Olindo, Micropor. Mesopor. 83 (2005) 309. [22] G. Vicente, A. Coteron, M. Martinez, Aracil, J. Ind. Crop. Prod. 8 (1998) 29.