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Selective transesterification of triolein with methanol to methyl oleate and glycerol using alumina loaded with alkali metal salt as a solid-base catalyst
Applied Catalysis A: General 283 (2005) 111–116 www.elsevier.com/locate/apcata
Selective transesterification of triolein with methanol to methyl oleate and glycerol using alumina loaded with alkali metal salt as a solid-base catalyst Takahiro Ebiura, Tsuneo Echizen, Akio Ishikawa, Kazuhito Murai, Toshihide Baba* Department of Chemistry and Material Engineering, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan Received 11 September 2004; received in revised form 19 December 2004; accepted 30 December 2004 Available online 21 January 2005
Abstract Selective transesterification of triolein (trioleoyl glycerol) with methanol to methyl oleate and glycerol could be achieved at around 333 K using alumina loaded with alkali metal salt as a solid-base catalyst. The catalytic activities are shown to be relatively insensitive to the presence of water. A K2CO3-loaded alumina catalyst prepared by evacuation at 823 K gives methyl oleate and glycerol in the highest yields of 94 and 89%, respectively, at 333 K in 1 h. This catalyst also effectively catalyzes the glycerolysis of triolein with glycerol to give dioleoyl glycerol in 71% yield at 453 K in 5 h. # 2005 Elsevier B.V. All rights reserved. Keywords: Transesterification; Triolein; Glycerol; Methanol; Biodiesel
1. Introduction Biodiesel material, mono-alkyl esters of fatty acids derived from vegetable oils or animal fats, is a prime candidate as an alternative fuel for compression–ignition diesel engines [1]. Biodiesel has superior cetane number and lubricity characteristics compared to petroleum middle distillates, with comparable heat of combustion and kinematic viscosity values, and is also non-flammable, making it safer to store and handle. Mono-alkyl esters, particularly mono-methyl esters, are blended with gas oil for use as fuel in diesel engines [2–4]. Mono-alkyl esters are usually produced by the transesterification of vegetable oils or animal fats with mono-alkyl alcohols, such as methanol. The reaction is commonly carried out in the presence of heterogeneous base or acid catalysts, but the reaction also proceeds under supercritical conditions of methanol without * Corresponding author. Present address: Department of Environmental Chemistry and Engineering, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midoriku, Yokohama 226-8502, Japan. Tel.: +81 45 924 5480; fax: +81 45 924 5480. E-mail address: [email protected] (T. Baba). 0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.12.041
using a catalyst [5]. Base catalysts include NaOH, KOH, their carbonates, and sodium and potassium alkoxides such as NaOCH3 [6–9]. Sulfonic acid and hydrochloric acid are often used as acid catalysts [9,10], and enzymes such as lipase are also used as biocatalysts [11]. The catalytic activity of a base is higher than that of an acid or enzyme for transesterification. However, a large amount of water is required to transfer the catalyst from the organic phase (including mono-alkyl esters) to a water phase when homogeneous base catalysts such as KOH are used. In this case, it is also difficult to separate the homogeneous catalyst from the reaction solution. Our group has already reported alumina loaded with alkali metal salt as an effective solid-base catalyst for a number of reactions, including C–C bond formation [12– 14]. In this work, transesterification of triolein with methanol and glycerolysis of triolein with glycerol are performed using this alkali metal salt-loaded alumina catalyst. The effects of reaction variables such as the molar ratio of methanol to triolein on the yields of the reaction products; methyl oleate and glycerol, are examined. Further, the effect of water on the catalytic activity of the catalyst is also investigated, as vegetable oils and animal fats usually
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contain water [15] and this material may affect the catalytic activity of this system.
2. Experimental 2.1. Catalyst preparation The alumina used as a support had a surface area of 131 m2/g. Alkali metal salts such as K2CO3 were loaded onto alumina by an impregnation method from aqueous solution followed by drying in air at 393 K for 12 h. Prior to reaction, the catalysts were evacuated under 103 Pa at a prescribed temperature for 2 h. The K2CO3-loaded catalyst was evacuated at 823 K, while the catalysts prepared from other alkali metal salts were evacuated at 673 K. Unless otherwise noted, the alkali metal salts were loaded at a dose of 2.6 mmol/g-Al2O3 as metal. 2.2. Reaction procedures Trioleoyl glycerol (triolein; purity 99%) was obtained from Sigma Chemical Co. and was used without further purification. Methanol and tetrahydrofuran (THF) were refluxed under CaH2 and Na metal, respectively, for 3 h before distillation. The catalytic reactions were carried out under a nitrogen atmosphere in a 50 cm3 autoclave, which was charged with glycerol, methanol, THF and the catalyst. The autoclave was then set in an oil bath heated to the reaction temperature. At the end of the reaction, the autoclave was cooled to room temperature and the catalyst was separated from the reaction solution by filtration. The reaction products were analyzed by high-performance liquid chromatography (HPLC) using a Crest Pack C18s column and an ultraviolet detector [16]. Glycerol, methyl oleate and methanol were analyzed by gas chromatography. In both analytical methods, ethyl decanoate was used as the internal standard to determine the amounts of products and unreacted reactants. The yields of dioleoyl glycerol (diolein) and monooleoyl glycerol (monoolein) with respect to the triolein charge amount are expressed as
Scheme 1.
3. Results and discussion 3.1. Reaction of triolein with methanol without a catalyst
Amount of dioleinðor monooleinÞ½mol 100½%: Charge amount of triolein½mol
The transesterification of triolein with methanol proceeded in the absence of a catalyst at around 400 K. The results are shown in Table 1. The conversion of triolein was very low at 383 K, but increased with rising reaction temperature to 93% in 5 h at 423 K. The yields of methyl oleate and glycerol at 423 K were 46 and 38%, respectively; the material balance was not maintained, resulting in the possible decomposition of triolein and reaction products such as methyl oleate. This suggests that catalysts such as solid-base catalysts should be used in order to allow transesterification to proceed selectively.
The yield of methyl oleate is expressed as
3.2. Solid-base-catalyzed transesterification
Amount of methyl oleate½mol 100½%: 3 Charge amount of triolein½mol The yield of glycerol is defined as Amount of glycerol½mol 100½%: Charge amount of triolein½mol The reaction is shown in Scheme 1.
3.2.1. Catalytic activity of alkali metal salt-loaded alumina The transesterification of triolein with methanol proceeded over a solid-base catalyst at a much lower reaction temperature than 423 K, at which the conversion of triolein was 93% without a catalyst. Reaction at 333 K using the alkali metal salt-loaded alumina gave the methyl oleate and glycerol yields shown in Table 2. Non-loaded alumina did
T. Ebiura et al. / Applied Catalysis A: General 283 (2005) 111–116 Table 1 Effect of temperature on the conversion of triolein and yields of the reaction products without using a catalyst Reaction temp. (K)
TOL conv. (mol%) Yield (mol%) DIO Mono GLY MEO
423
503
523
37
69
93
29 4 2 13
30 21 12 27
16 34 31 42
TOL: triolein; MEO: methyl oleate; DIO: diolein; Mono: monoolein; GLY: glycerol. Reaction time: 5 h; triolein 1.0 mmol; CH3OH 24.8 mmol; THF 5 ml.
not yield methyl oleate or glycerol within 1 h, and yielded only 7% methyl oleate over 12 h at 423 K without the production of glycerol. Thus, the catalytic activity of this material is generated by loading the alumina with alkali metal salts followed by evacuation of the samples at 673 or 823 K. Among the catalysts, alumina loaded with K2CO3, KF, LiNO3 and NaOH gave methyl oleate and glycerol in high yield over 1 h at 333 K. For example, K2CO3/Al2O3 afforded a 93% yield of methyl oleate and an 87% yield of glycerol. However, KOH/Al2O3 exhibited low catalytic activity as mentioned below. Our group has reported that the double-bond isomerization of 2,3-dimethyl-1-butene is a superior test reaction for estimation of the basic strength of solid-base catalysts, as calculated by comparing the rates of isomerization [17]. The basic strength represents the ability of the catalyst to abstract protons from the reactant. The yields of 2,3-dimethyl-2butene over solid-base catalysts are shown in Table 2 to Table 2 Catalytic activity of alumina loaded with alkali metal salt Yield (mol%)
Conv. (mol%)b, 2,3-DB-1
Alkali metal salta/(alumina)
MEO
GLY
K2CO3 K2CO3c KF
92 94 91
87 89 89
4.2 – 0
LiNO3 LiNO3c
85 91
76 88
0 –
NaOH NaOHc KOH
80 91 78
69 82 63
0 34.1
KNO3 NaNO3 RbNO3
11 5 3
2 1 0
49.4 – –
Al2O3d
0
0
0
TOL: triolein; MEO: methyl oleate; DIO: diolein; Mono: monoolein; GLY: glycerol. Catalyst 0.05 g; reaction time 1 h; reaction temperature 333 K; triolein 1.0 mmol; CH3OH 24.8 mmol; THF 5 ml. a The amount of alkali metal salt was 2.6 mmol/g-Al2O3 as a metal. b Catalyst 0.25 g; 2,3-dimethylbut-1-ene (2,3-DB-1) 24 mmol; reaction time 1 h; reaction temperature 201 K. c Catalyst 0.3 g. d Al2O3 0.3 g.
113
demonstrate this relationship between catalytic activity for transesterification and basic strength. The catalytic activities for transesterification do not correlate with those for the isomerization of 2,3-dimethyl-1-butene, suggesting that the basic strength of the catalyst is not always important in transesterification, although the basic strength of alkali metal salt-loaded alumina is higher than that of pure alumina. The solid-base catalysts also activate the carbonyl groups in carbonyl compounds, such as benzaldehyde, without proton abstraction [13,14]. KF/Al2O3 is effective for this type of reaction, but not for the isomerization of 2,3dimethyl-1-butene, the reaction of which proceeds via a pallyl carboanion intermediate by proton abstraction from 2,3-dimethyl-1-butene. As shown in Table 2, KF/Al2O3 exhibits higher catalytic activity for transesterification than KNO3/Al2O3, which displayed much higher catalytic activity for the isomerization of 2,3-dimethyl-1-butene. These results suggest that the activation of carbonyl groups in esters, such as triolein, is more important than the abstraction of protons from methanol to form CH3O anions. To compare the catalytic activity of K2CO3/Al2O3 with that of KOH as a heterogeneous catalyst, we carried out transesterification at 273 K for 1 h. The results are shown in Table 3. K2CO3/Al2O3 (0.05 g, K2CO3 loading of 0.13 mmol) exhibited nearly the same catalytic activity as 0.023 mmol of KOH. 3.2.2. Effect of molar ratio of methanol/triolein Since the transesterification is a reversible reaction, the effect of the molar ratio of methanol to triolein on the yields of esters such as methyl oleate and glycerol was examined by varying the amount of methanol in reactions at 333 K. The initial amount of triolein was set at 1.0 mmol, and 5.0 ml of THF was used as a solvent. The conversion of triolein and the yields of 1,2-diolein, 1,3-diolein, and methyl oleate are plotted against the initial amount of methanol in Fig. 1. The conversion of triolein and the yields of methyl oleate increased with increasing methanol addition. The yield of methyl oleate reached ca. 100% when more than 25 mmol of methanol was present Table 3 Catalytic activities of KOH and alumina loaded with alkali metal salt Catalyst
Amount
Conv. (mol%), TOL
Yield (mol%) DIO
Mono
GLY
MEO
KOH KOH KOH
0.012 mmol 0.023 mmol 0.073 mmol
34 60 89
24 29 2
5 34 26
0 3 55
12 31 64
K2CO3/Al2O3a KF/Al2O3a LiNO3/Al2O3a
0.05 g 0.05 g 0.05 g
70 54 43
45 37 31
16 9 7
6 0 0
30 9 7
TOL: triolein; MEO: methyl oleate; DIO: diolein; Mono: monoolein; GLY: glycerol. Catalyst 0.05 g; reaction time 1 h; reaction temperature 273 K; triolein 1.0 mmol; CH3OH 24.8 mmol; THF 5 ml. a 2.6 mmol/g-Al2O3 as metal loaded on Al2O3.
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Fig. 1. Effect of molar ratio of methanol to triolein on conversion of triolein and yields of esters. Reaction temperature: 333 K; reaction time: 1 h; triolein: 1.0 mmol; THF: 5.0 ml; K2CO3/Al2O3: 0.05 g (2.6 mmol/gAl2O3 as K metal loaded on alumina). The catalyst was evacuated at 823 K for 2 h prior to reaction. Graph shows triolein conversion (*), methyl oleate yield (*), glycerol yield (&), total diolein yield (~), and total monoolein yield (!).
(MeOH/triolein > 25). The yields of 1,3- and 1,2-dioleion, and a- and b-monoolein exhibited a maximum with increasing methanol content, and decreased at higher rates of methanol addition. The formation of glycerol was not observed when small amounts of methanol were added, but the yield increased at higher rates of methanol addition. Therefore, the optimum molar ratio of MeOH/triolein to produce methyl oleate and glycerol is approximately 25.
Fig. 3. Selectivities for transesterification of diolein with respect to triolein conversion. Reaction temperature: 363 K; triolein: 1.0 mmol; CH3OH: 24.8 mmol; THF: 5.0 ml. Conversion was varied by changing the reaction time. Graph shows the yield of 1,2-diolein yield (*) and 1,3-diolein (&).
3.2.3. Effect of reaction temperature The effects of reaction temperature on the conversion of triolein and yields of esters were examined using K2CO3/ Al2O3 as a catalyst. The results are shown in Fig. 2. The transesterification proceeded at 273 K, at which the conversion of triolein was 70%. The conversion increased with reaction temperature to nearly 100% at 333 K. The yield of diolein decreased with rising reaction temperature, whereas the yield of monoolein reached a maximum of 43% at 296 K. The yield of glycerol was low at temperatures below 296 K, reaching a maximum of 89% at 363 K. The yield of methyl oleate was ca. 93% at Table 4 Effect of solid-base catalyst on the selectivity to diolein formation Catalyst
1,2-DIO (mol)
1,3-DIO (mol)
Without catalyst LiNO3/Al2O3 KSCN/Al2O3 KNO3/Al2O3
93 29 34 31
7 71 66 69
The values show the amount of diolein (DIO) formation, when 100 mol triolein (TOL) was converted.
Table 5 Effect of water on the catalytic activity
Fig. 2. Effect of reaction temperature on conversion of triolein and yields of esters. Reaction time: 1 h; triolein: 1.0 mmol; CH3OH: 24.8 mmol; THF: 5.0 ml; K2CO3/Al2O3: 0.05 g (2.6 mmol/g-Al2O3 as K metal loaded on alumina). The catalyst was evacuated at 823 K for 2 h prior to reaction. Graph shows triolein conversion (*), methyl oleate yield (*), glycerol yield (&), total diolein yield (~), and total monoolein yield (!).
Catalyst
H2O (mmol)
Yield MEO (%)
GLY (%)
K2CO3/Al2O3
0 0.5
91 92
89 86
KF/Al2O3
0 0.5
91 87
89 82
MEO: methyl oleate; GLY: glycerol. Catalyst 0.05 g; reaction time 1 h; reaction temperature 333 K; triolein 1.0 mmol; CH3OH 24.8 mmol; THF 5 ml. Conversion of triolein was 100% in all cases.
T. Ebiura et al. / Applied Catalysis A: General 283 (2005) 111–116
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Table 6 Glycerolysis of TOL with GLY Catalyst
Weight (g)
GLY/TOL
Temp. (K)
Time (h)
Yield (%) a-Mono
b-Mono
1,2-DIO
1,3-DIO
K2CO3/Al2O3 KF/Al2O3 MgOa
0.20 0.20
6.5 6.5 12
453 453 519
5 10 5
63 55 65
9 8 8
8 6 9
20 12 15
TOL: triolein; MEO: methyl oleate; DIO: diolein; Mono: monoolein; GLY: glycerol. Glycerol 6.5 mmol; triolein 1.0 mmol; solvent: 1,4-dioxane 5 ml; K2CO3/ Al2O3 evacuated at 823 K for 1 h. KF/Al2O3 evacuated at 673 K for 1 h. a A. Corma, et al., J. Catal. 173 (1998) 315.
reaction temperatures higher than 333 K. On the basis of these results, the optimum reaction temperature for the synthesis of methyl oleate and glycerol is considered to be around 333 K. 3.2.4. Selectivity for dioleoyl glycerol In the transesterification of triolein with methanol, the primary products are 1,2- and 1,3-diolein, although methyl oleate is formed simultaneously with the production of these esters. The selectivity of transesterification for 1,2- and 1,3diolein in the absence of a catalyst was examined at 363 K as a function of triolein conversion; the results are shown in Fig. 3. Extrapolation of the selectivity to zero conversion of triolein gives the product selectivity in the primary reaction. The results are summarized in Table 4. Similar plots were also obtained for transesterification over the alkali metal salt-loaded alumina catalyst at 363 K. The results are included in Table 4. The selectivities were found not to depend on the type of base catalyst, suggesting that basic strength does not influence the selectivity or the rate of diolein formation. However, the selectivities for diolein over the solid-base catalyst were very different from those in the absence of the catalyst, indicating that the reaction mechanism for transesterification in the absence of a solid-base catalyst is likely to differ from that in the presence of the catalyst. 3.2.5. Effect of water on catalytic activity Vegetable oils and animal fats usually contain water [15]. To examine the effect of water on the transesterification of triolein with methanol, water was added directly to the reaction solution. The reaction was performed at 333 K over K2CO3/Al2O3 and LiNO3/Al2O3 catalysts. The reaction solution was prepared with triolein (1.0 mmol), MeOH (24.8 mmol) and THF (5.0 ml), and was spiked with 0.5 mmol of water prior to reaction. The conversion of triolein and the yields of methyl oleate and glycerol are summarized in Table 5. The conversion of triolein reached 100% using the catalysts, whereas the yields of methyl oleate and glycerol were slightly lowered by the addition of water. Thus, the catalytic activities of these solid-base catalysts are not significantly affected by water, meaning that transesterification can proceed even in the presence of water.
3.3. Glycerolysis of triolein with glycerol over K2CO3/Al2O3 K2CO3/Al2O3 and KF/Al2O3 also catalyzed the glycerolysis of triolein with glycerol to form monoolein. A reaction promoting this glycerolysis was carried out at 453 K, using a reaction mixture with a glycerol/triolein molar ratio of 6.5 and using 1,4-dioxane as a solvent. The results are shown in Table 6. K2CO3/Al2O3 gave bmonoolein in 63% yield and b-monoolein in 9% yield (total monoolein yield: 71%) in 5 h at 453 K, with accompanying yields of 20% 1,3-diolein and 8% 1,2diolein. The catalytic activity was higher than over KF/ Al2O3. K2CO3/Al2O3, which exhibited the highest catalytic activity for the transesterification of triolein with methanol, was thus also effective for the glycerolysis of triolein. TOL þ 3GLY ! 3Mono Corma et al. reported that the glycerolysis of triolein with glycerol is catalyzed by solid-base catalysts such as Csloaded MCM-41 and MgO at 513 K [18]. These results are also shown in Table 6 for comparison. It can be seen that K2CO3/Al2O3 can be used at lower reaction temperatures than MgO, which reached its highest catalytic activity at 513 K.
4. Conclusion Alumina loaded with alkali metal salts was demonstrated as a solid-base catalyst for the transesterification of triolein with methanol. Transesterification over these catalysts proceed efficiently at 333 K, lower than the boiling point of methanol. This demonstration shows that it is possible to perform this reaction at atmospheric pressure using a heterogeneous catalyst. The effective transformation of triolein to methyl oleate over solid-base catalysts represents a convenient route for biodiesel and glycerol production.
References [1] R.O. Dunn, G. Knothe, J. Oleo. Sci. 50 (2001) 415. [2] W. Korbitz, Renewable Energy 16 (1999) 1078.
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G. Knothe, R.O. Dunn, M.O. Bagby, ACS. Symp. Ser. 666 (1997) 172. G. Knothe, M.O. Bagby, Recent Res. Dev. Oil Chem. 1 (1997) 31. D. Kusdiana, S. Suka, J. Chem. Eng. Jpn. 34 (2001) 383. R.O. Feuge, A.T. Gros, J. Am. Oil Chem. Soc. 26 (1949) 97. F. Ma, M.A. Hanna, Bioresour. Technol. 70 (1999) 1. K. Krisnangkura, R. Simamaharnoop, J. Am. Oil Chem. Soc. 69 (1992) 166. [9] A. Cotevon, G. Vicente, M. Martinez, J. Aracil, Recent Res. Dev. Oil Chem. 1 (1997) 109. [10] Y. Shimada, Y. Watanabe, T. Samukawa, A. Sugihara, H. Noda, H. Fukuda, Y. Tominaga, J. Am. Oil Chem. Soc. 76 (1999) 789.
[11] [12] [13] [14] [15]
M.W. Formo, J. Am. Oil Chem. Soc. 31 (1954) 548. Y. Ono, T. Baba, Catal. Today 38 (1997) 321. Y. Ono, T. Baba, Catalysis 15 (2000) 1. T. Baba, Catal. Surv. Jpn. 4 (2000) 17. K.D. Green, M. Nakajima, S. Ichikawa, M. Mogi, Food Sci. Technol. Tokyo 3 (1997) 357. [16] M. Holcapec, P. Jandera, J. Fischer, B. Prokes, J. Chromatogr. A 858 (1999) 13. [17] H. Hawg, Y. Yu, T. Baba, Y. Ono, Catal. Lett. 59 (1999) 195. [18] A. Corma, S. Iborra, S. Miquel, J. Primo, J. Catal. 173 (1998) 315.
Selective transesterification of triolein with methanol to methyl oleate and glycerol using alumina loaded with alkali metal salt as a solid-base catalyst Takahiro Ebiura, Tsuneo Echizen, Akio Ishikawa, Kazuhito Murai, Toshihide Baba* Department of Chemistry and Material Engineering, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan Received 11 September 2004; received in revised form 19 December 2004; accepted 30 December 2004 Available online 21 January 2005
Abstract Selective transesterification of triolein (trioleoyl glycerol) with methanol to methyl oleate and glycerol could be achieved at around 333 K using alumina loaded with alkali metal salt as a solid-base catalyst. The catalytic activities are shown to be relatively insensitive to the presence of water. A K2CO3-loaded alumina catalyst prepared by evacuation at 823 K gives methyl oleate and glycerol in the highest yields of 94 and 89%, respectively, at 333 K in 1 h. This catalyst also effectively catalyzes the glycerolysis of triolein with glycerol to give dioleoyl glycerol in 71% yield at 453 K in 5 h. # 2005 Elsevier B.V. All rights reserved. Keywords: Transesterification; Triolein; Glycerol; Methanol; Biodiesel
1. Introduction Biodiesel material, mono-alkyl esters of fatty acids derived from vegetable oils or animal fats, is a prime candidate as an alternative fuel for compression–ignition diesel engines [1]. Biodiesel has superior cetane number and lubricity characteristics compared to petroleum middle distillates, with comparable heat of combustion and kinematic viscosity values, and is also non-flammable, making it safer to store and handle. Mono-alkyl esters, particularly mono-methyl esters, are blended with gas oil for use as fuel in diesel engines [2–4]. Mono-alkyl esters are usually produced by the transesterification of vegetable oils or animal fats with mono-alkyl alcohols, such as methanol. The reaction is commonly carried out in the presence of heterogeneous base or acid catalysts, but the reaction also proceeds under supercritical conditions of methanol without * Corresponding author. Present address: Department of Environmental Chemistry and Engineering, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midoriku, Yokohama 226-8502, Japan. Tel.: +81 45 924 5480; fax: +81 45 924 5480. E-mail address: [email protected] (T. Baba). 0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.12.041
using a catalyst [5]. Base catalysts include NaOH, KOH, their carbonates, and sodium and potassium alkoxides such as NaOCH3 [6–9]. Sulfonic acid and hydrochloric acid are often used as acid catalysts [9,10], and enzymes such as lipase are also used as biocatalysts [11]. The catalytic activity of a base is higher than that of an acid or enzyme for transesterification. However, a large amount of water is required to transfer the catalyst from the organic phase (including mono-alkyl esters) to a water phase when homogeneous base catalysts such as KOH are used. In this case, it is also difficult to separate the homogeneous catalyst from the reaction solution. Our group has already reported alumina loaded with alkali metal salt as an effective solid-base catalyst for a number of reactions, including C–C bond formation [12– 14]. In this work, transesterification of triolein with methanol and glycerolysis of triolein with glycerol are performed using this alkali metal salt-loaded alumina catalyst. The effects of reaction variables such as the molar ratio of methanol to triolein on the yields of the reaction products; methyl oleate and glycerol, are examined. Further, the effect of water on the catalytic activity of the catalyst is also investigated, as vegetable oils and animal fats usually
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contain water [15] and this material may affect the catalytic activity of this system.
2. Experimental 2.1. Catalyst preparation The alumina used as a support had a surface area of 131 m2/g. Alkali metal salts such as K2CO3 were loaded onto alumina by an impregnation method from aqueous solution followed by drying in air at 393 K for 12 h. Prior to reaction, the catalysts were evacuated under 103 Pa at a prescribed temperature for 2 h. The K2CO3-loaded catalyst was evacuated at 823 K, while the catalysts prepared from other alkali metal salts were evacuated at 673 K. Unless otherwise noted, the alkali metal salts were loaded at a dose of 2.6 mmol/g-Al2O3 as metal. 2.2. Reaction procedures Trioleoyl glycerol (triolein; purity 99%) was obtained from Sigma Chemical Co. and was used without further purification. Methanol and tetrahydrofuran (THF) were refluxed under CaH2 and Na metal, respectively, for 3 h before distillation. The catalytic reactions were carried out under a nitrogen atmosphere in a 50 cm3 autoclave, which was charged with glycerol, methanol, THF and the catalyst. The autoclave was then set in an oil bath heated to the reaction temperature. At the end of the reaction, the autoclave was cooled to room temperature and the catalyst was separated from the reaction solution by filtration. The reaction products were analyzed by high-performance liquid chromatography (HPLC) using a Crest Pack C18s column and an ultraviolet detector [16]. Glycerol, methyl oleate and methanol were analyzed by gas chromatography. In both analytical methods, ethyl decanoate was used as the internal standard to determine the amounts of products and unreacted reactants. The yields of dioleoyl glycerol (diolein) and monooleoyl glycerol (monoolein) with respect to the triolein charge amount are expressed as
Scheme 1.
3. Results and discussion 3.1. Reaction of triolein with methanol without a catalyst
Amount of dioleinðor monooleinÞ½mol 100½%: Charge amount of triolein½mol
The transesterification of triolein with methanol proceeded in the absence of a catalyst at around 400 K. The results are shown in Table 1. The conversion of triolein was very low at 383 K, but increased with rising reaction temperature to 93% in 5 h at 423 K. The yields of methyl oleate and glycerol at 423 K were 46 and 38%, respectively; the material balance was not maintained, resulting in the possible decomposition of triolein and reaction products such as methyl oleate. This suggests that catalysts such as solid-base catalysts should be used in order to allow transesterification to proceed selectively.
The yield of methyl oleate is expressed as
3.2. Solid-base-catalyzed transesterification
Amount of methyl oleate½mol 100½%: 3 Charge amount of triolein½mol The yield of glycerol is defined as Amount of glycerol½mol 100½%: Charge amount of triolein½mol The reaction is shown in Scheme 1.
3.2.1. Catalytic activity of alkali metal salt-loaded alumina The transesterification of triolein with methanol proceeded over a solid-base catalyst at a much lower reaction temperature than 423 K, at which the conversion of triolein was 93% without a catalyst. Reaction at 333 K using the alkali metal salt-loaded alumina gave the methyl oleate and glycerol yields shown in Table 2. Non-loaded alumina did
T. Ebiura et al. / Applied Catalysis A: General 283 (2005) 111–116 Table 1 Effect of temperature on the conversion of triolein and yields of the reaction products without using a catalyst Reaction temp. (K)
TOL conv. (mol%) Yield (mol%) DIO Mono GLY MEO
423
503
523
37
69
93
29 4 2 13
30 21 12 27
16 34 31 42
TOL: triolein; MEO: methyl oleate; DIO: diolein; Mono: monoolein; GLY: glycerol. Reaction time: 5 h; triolein 1.0 mmol; CH3OH 24.8 mmol; THF 5 ml.
not yield methyl oleate or glycerol within 1 h, and yielded only 7% methyl oleate over 12 h at 423 K without the production of glycerol. Thus, the catalytic activity of this material is generated by loading the alumina with alkali metal salts followed by evacuation of the samples at 673 or 823 K. Among the catalysts, alumina loaded with K2CO3, KF, LiNO3 and NaOH gave methyl oleate and glycerol in high yield over 1 h at 333 K. For example, K2CO3/Al2O3 afforded a 93% yield of methyl oleate and an 87% yield of glycerol. However, KOH/Al2O3 exhibited low catalytic activity as mentioned below. Our group has reported that the double-bond isomerization of 2,3-dimethyl-1-butene is a superior test reaction for estimation of the basic strength of solid-base catalysts, as calculated by comparing the rates of isomerization [17]. The basic strength represents the ability of the catalyst to abstract protons from the reactant. The yields of 2,3-dimethyl-2butene over solid-base catalysts are shown in Table 2 to Table 2 Catalytic activity of alumina loaded with alkali metal salt Yield (mol%)
Conv. (mol%)b, 2,3-DB-1
Alkali metal salta/(alumina)
MEO
GLY
K2CO3 K2CO3c KF
92 94 91
87 89 89
4.2 – 0
LiNO3 LiNO3c
85 91
76 88
0 –
NaOH NaOHc KOH
80 91 78
69 82 63
0 34.1
KNO3 NaNO3 RbNO3
11 5 3
2 1 0
49.4 – –
Al2O3d
0
0
0
TOL: triolein; MEO: methyl oleate; DIO: diolein; Mono: monoolein; GLY: glycerol. Catalyst 0.05 g; reaction time 1 h; reaction temperature 333 K; triolein 1.0 mmol; CH3OH 24.8 mmol; THF 5 ml. a The amount of alkali metal salt was 2.6 mmol/g-Al2O3 as a metal. b Catalyst 0.25 g; 2,3-dimethylbut-1-ene (2,3-DB-1) 24 mmol; reaction time 1 h; reaction temperature 201 K. c Catalyst 0.3 g. d Al2O3 0.3 g.
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demonstrate this relationship between catalytic activity for transesterification and basic strength. The catalytic activities for transesterification do not correlate with those for the isomerization of 2,3-dimethyl-1-butene, suggesting that the basic strength of the catalyst is not always important in transesterification, although the basic strength of alkali metal salt-loaded alumina is higher than that of pure alumina. The solid-base catalysts also activate the carbonyl groups in carbonyl compounds, such as benzaldehyde, without proton abstraction [13,14]. KF/Al2O3 is effective for this type of reaction, but not for the isomerization of 2,3dimethyl-1-butene, the reaction of which proceeds via a pallyl carboanion intermediate by proton abstraction from 2,3-dimethyl-1-butene. As shown in Table 2, KF/Al2O3 exhibits higher catalytic activity for transesterification than KNO3/Al2O3, which displayed much higher catalytic activity for the isomerization of 2,3-dimethyl-1-butene. These results suggest that the activation of carbonyl groups in esters, such as triolein, is more important than the abstraction of protons from methanol to form CH3O anions. To compare the catalytic activity of K2CO3/Al2O3 with that of KOH as a heterogeneous catalyst, we carried out transesterification at 273 K for 1 h. The results are shown in Table 3. K2CO3/Al2O3 (0.05 g, K2CO3 loading of 0.13 mmol) exhibited nearly the same catalytic activity as 0.023 mmol of KOH. 3.2.2. Effect of molar ratio of methanol/triolein Since the transesterification is a reversible reaction, the effect of the molar ratio of methanol to triolein on the yields of esters such as methyl oleate and glycerol was examined by varying the amount of methanol in reactions at 333 K. The initial amount of triolein was set at 1.0 mmol, and 5.0 ml of THF was used as a solvent. The conversion of triolein and the yields of 1,2-diolein, 1,3-diolein, and methyl oleate are plotted against the initial amount of methanol in Fig. 1. The conversion of triolein and the yields of methyl oleate increased with increasing methanol addition. The yield of methyl oleate reached ca. 100% when more than 25 mmol of methanol was present Table 3 Catalytic activities of KOH and alumina loaded with alkali metal salt Catalyst
Amount
Conv. (mol%), TOL
Yield (mol%) DIO
Mono
GLY
MEO
KOH KOH KOH
0.012 mmol 0.023 mmol 0.073 mmol
34 60 89
24 29 2
5 34 26
0 3 55
12 31 64
K2CO3/Al2O3a KF/Al2O3a LiNO3/Al2O3a
0.05 g 0.05 g 0.05 g
70 54 43
45 37 31
16 9 7
6 0 0
30 9 7
TOL: triolein; MEO: methyl oleate; DIO: diolein; Mono: monoolein; GLY: glycerol. Catalyst 0.05 g; reaction time 1 h; reaction temperature 273 K; triolein 1.0 mmol; CH3OH 24.8 mmol; THF 5 ml. a 2.6 mmol/g-Al2O3 as metal loaded on Al2O3.
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Fig. 1. Effect of molar ratio of methanol to triolein on conversion of triolein and yields of esters. Reaction temperature: 333 K; reaction time: 1 h; triolein: 1.0 mmol; THF: 5.0 ml; K2CO3/Al2O3: 0.05 g (2.6 mmol/gAl2O3 as K metal loaded on alumina). The catalyst was evacuated at 823 K for 2 h prior to reaction. Graph shows triolein conversion (*), methyl oleate yield (*), glycerol yield (&), total diolein yield (~), and total monoolein yield (!).
(MeOH/triolein > 25). The yields of 1,3- and 1,2-dioleion, and a- and b-monoolein exhibited a maximum with increasing methanol content, and decreased at higher rates of methanol addition. The formation of glycerol was not observed when small amounts of methanol were added, but the yield increased at higher rates of methanol addition. Therefore, the optimum molar ratio of MeOH/triolein to produce methyl oleate and glycerol is approximately 25.
Fig. 3. Selectivities for transesterification of diolein with respect to triolein conversion. Reaction temperature: 363 K; triolein: 1.0 mmol; CH3OH: 24.8 mmol; THF: 5.0 ml. Conversion was varied by changing the reaction time. Graph shows the yield of 1,2-diolein yield (*) and 1,3-diolein (&).
3.2.3. Effect of reaction temperature The effects of reaction temperature on the conversion of triolein and yields of esters were examined using K2CO3/ Al2O3 as a catalyst. The results are shown in Fig. 2. The transesterification proceeded at 273 K, at which the conversion of triolein was 70%. The conversion increased with reaction temperature to nearly 100% at 333 K. The yield of diolein decreased with rising reaction temperature, whereas the yield of monoolein reached a maximum of 43% at 296 K. The yield of glycerol was low at temperatures below 296 K, reaching a maximum of 89% at 363 K. The yield of methyl oleate was ca. 93% at Table 4 Effect of solid-base catalyst on the selectivity to diolein formation Catalyst
1,2-DIO (mol)
1,3-DIO (mol)
Without catalyst LiNO3/Al2O3 KSCN/Al2O3 KNO3/Al2O3
93 29 34 31
7 71 66 69
The values show the amount of diolein (DIO) formation, when 100 mol triolein (TOL) was converted.
Table 5 Effect of water on the catalytic activity
Fig. 2. Effect of reaction temperature on conversion of triolein and yields of esters. Reaction time: 1 h; triolein: 1.0 mmol; CH3OH: 24.8 mmol; THF: 5.0 ml; K2CO3/Al2O3: 0.05 g (2.6 mmol/g-Al2O3 as K metal loaded on alumina). The catalyst was evacuated at 823 K for 2 h prior to reaction. Graph shows triolein conversion (*), methyl oleate yield (*), glycerol yield (&), total diolein yield (~), and total monoolein yield (!).
Catalyst
H2O (mmol)
Yield MEO (%)
GLY (%)
K2CO3/Al2O3
0 0.5
91 92
89 86
KF/Al2O3
0 0.5
91 87
89 82
MEO: methyl oleate; GLY: glycerol. Catalyst 0.05 g; reaction time 1 h; reaction temperature 333 K; triolein 1.0 mmol; CH3OH 24.8 mmol; THF 5 ml. Conversion of triolein was 100% in all cases.
T. Ebiura et al. / Applied Catalysis A: General 283 (2005) 111–116
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Table 6 Glycerolysis of TOL with GLY Catalyst
Weight (g)
GLY/TOL
Temp. (K)
Time (h)
Yield (%) a-Mono
b-Mono
1,2-DIO
1,3-DIO
K2CO3/Al2O3 KF/Al2O3 MgOa
0.20 0.20
6.5 6.5 12
453 453 519
5 10 5
63 55 65
9 8 8
8 6 9
20 12 15
TOL: triolein; MEO: methyl oleate; DIO: diolein; Mono: monoolein; GLY: glycerol. Glycerol 6.5 mmol; triolein 1.0 mmol; solvent: 1,4-dioxane 5 ml; K2CO3/ Al2O3 evacuated at 823 K for 1 h. KF/Al2O3 evacuated at 673 K for 1 h. a A. Corma, et al., J. Catal. 173 (1998) 315.
reaction temperatures higher than 333 K. On the basis of these results, the optimum reaction temperature for the synthesis of methyl oleate and glycerol is considered to be around 333 K. 3.2.4. Selectivity for dioleoyl glycerol In the transesterification of triolein with methanol, the primary products are 1,2- and 1,3-diolein, although methyl oleate is formed simultaneously with the production of these esters. The selectivity of transesterification for 1,2- and 1,3diolein in the absence of a catalyst was examined at 363 K as a function of triolein conversion; the results are shown in Fig. 3. Extrapolation of the selectivity to zero conversion of triolein gives the product selectivity in the primary reaction. The results are summarized in Table 4. Similar plots were also obtained for transesterification over the alkali metal salt-loaded alumina catalyst at 363 K. The results are included in Table 4. The selectivities were found not to depend on the type of base catalyst, suggesting that basic strength does not influence the selectivity or the rate of diolein formation. However, the selectivities for diolein over the solid-base catalyst were very different from those in the absence of the catalyst, indicating that the reaction mechanism for transesterification in the absence of a solid-base catalyst is likely to differ from that in the presence of the catalyst. 3.2.5. Effect of water on catalytic activity Vegetable oils and animal fats usually contain water [15]. To examine the effect of water on the transesterification of triolein with methanol, water was added directly to the reaction solution. The reaction was performed at 333 K over K2CO3/Al2O3 and LiNO3/Al2O3 catalysts. The reaction solution was prepared with triolein (1.0 mmol), MeOH (24.8 mmol) and THF (5.0 ml), and was spiked with 0.5 mmol of water prior to reaction. The conversion of triolein and the yields of methyl oleate and glycerol are summarized in Table 5. The conversion of triolein reached 100% using the catalysts, whereas the yields of methyl oleate and glycerol were slightly lowered by the addition of water. Thus, the catalytic activities of these solid-base catalysts are not significantly affected by water, meaning that transesterification can proceed even in the presence of water.
3.3. Glycerolysis of triolein with glycerol over K2CO3/Al2O3 K2CO3/Al2O3 and KF/Al2O3 also catalyzed the glycerolysis of triolein with glycerol to form monoolein. A reaction promoting this glycerolysis was carried out at 453 K, using a reaction mixture with a glycerol/triolein molar ratio of 6.5 and using 1,4-dioxane as a solvent. The results are shown in Table 6. K2CO3/Al2O3 gave bmonoolein in 63% yield and b-monoolein in 9% yield (total monoolein yield: 71%) in 5 h at 453 K, with accompanying yields of 20% 1,3-diolein and 8% 1,2diolein. The catalytic activity was higher than over KF/ Al2O3. K2CO3/Al2O3, which exhibited the highest catalytic activity for the transesterification of triolein with methanol, was thus also effective for the glycerolysis of triolein. TOL þ 3GLY ! 3Mono Corma et al. reported that the glycerolysis of triolein with glycerol is catalyzed by solid-base catalysts such as Csloaded MCM-41 and MgO at 513 K [18]. These results are also shown in Table 6 for comparison. It can be seen that K2CO3/Al2O3 can be used at lower reaction temperatures than MgO, which reached its highest catalytic activity at 513 K.
4. Conclusion Alumina loaded with alkali metal salts was demonstrated as a solid-base catalyst for the transesterification of triolein with methanol. Transesterification over these catalysts proceed efficiently at 333 K, lower than the boiling point of methanol. This demonstration shows that it is possible to perform this reaction at atmospheric pressure using a heterogeneous catalyst. The effective transformation of triolein to methyl oleate over solid-base catalysts represents a convenient route for biodiesel and glycerol production.
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