3594
Energy & Fuels 2008, 22, 3594–3599
Transesterification of Glycerides Using a Heterogeneous Resin Catalyst Combined with a Homogeneous Catalyst M. Kim,†,‡ S. O. Salley,†,§ and K. Y. S. Ng*,†,‡,§ National Biofuel Energy Laboratory, NextEnergy, 461 Burroughs Street, Detroit, Michigan 48202, and AlternatiVe Energy Technology Program, and Department of Chemical Engineering, Wayne State UniVersity, 5050 Anthony Wayne DriVe, Detroit, Michigan 48202 ReceiVed June 9, 2008. ReVised Manuscript ReceiVed September 4, 2008
Biodiesel, produced by the transesterification of vegetable oils or animal fats with methanol, is a promising alternative diesel fuel. In this study, a series of strong base anion resins with different basicity, pore structure, degree of cross-linking, and particle size were investigated as heterogeneous catalysts for the transesterification of soybean oil with methanol. A trace amount of CH3ONa, functioning as a homogeneous catalyst, was found to exhibit synergetic effects with the resin catalysts. By using this combined method, the transesterification reaction approached completion after 6 h at 328 K. The reason for this synergetic effect can be attributed to the regeneration of active catalytic sites in a basic environment.
1. Introduction Because traditional fossil energy resources are limited, research is now being directed toward the identification and production of alternative renewable fuels. One of the more promising approaches is the conversion of vegetable oils and animal fats into biodiesel. Vegetable oils and animal fats are comprised of complex mixtures of triglycerides (TGs) and other minor components, such as free fatty acids (FFAs), gums, waxes, etc. Biodiesel is usually made through a chemical process called transestrification, whereby TGs react with methanol in the presence of a catalyst to produce a mixture of fatty acid alkyl esters and glycerol. Most of the commercial biodiesel that is currently produced in the U.S. comes from the transesterification of soybean oil using a homogeneous base (NaOH or KOH) catalyzed process. Alkali cations are removed after the transesterification reaction as soaps in the glycerol phase. An acidic neutralization step with aqueous acid is required to neutralize these salts. Even though homogeneous catalyzed biodiesel production processes are relatively fast and show high conversions, the removal of the homogeneous catalyst after the reaction is a main problem, because aqueous quenching resulting in the formation of stable emulsion and saponification makes separation of methyl esters difficult and a large amount of wastewater was produced to separate and clean the catalyst and products.1 To minimize problems associated with the homogeneous catalytic process, attempts have been made to develop heterogeneous catalyst systems. Solid base catalysts are promising to replace alkaline homogeneous catalysts, to minimize soap formation, separation, corrosion, and environmental problems.2-4 At the laboratory scale, many different heterogeneous catalysts * To whom correspondence should be addressed. Fax: 1-313-577-3810. E-mail:
[email protected]. † National Biofuel Energy Laboratory, NextEnergy. ‡ Alternative Energy Technology Program, Wayne State University. § Department of Chemical Engineering, Wayne State University. (1) Lopez, D. E.; Goodwin, J. G.; Bruce, D. A.; Lotero, E. Transesterification of triacetin with methanol on solid acid and base catalysts. Appl. Catal., A 2005, 295 (2), 97–105. (2) Pinto, A. C.; Guarieiro, L. L. N.; Rezende, M. J. C.; Ribeiro, N. M.; Torres, E. A.; Lopes, W. A.; Pereira, P. A. D.; de Andrade, J. B. Biodiesel: An overview. J. Braz. Chem. Soc. 2005, 16 (6B), 1313–1330.
have been reported, including MgO,3 hydrotalcites,3-5 zeolites loaded with sodium oxide,6,7 Li/CaO,8 KF/ZnO,9 mixed metal oxides (Al2O3-SnO and Al2O3-ZnO),10 Zn/I2,11 mixed oxides of zinc and aluminum,12 and potassium-loaded alumina.13 Catalytic activities of the heterogeneous base catalysts in the transesterification of soybean oil show a striking correlation to their corresponding basic strengths.4,8,13,14 Although alkali metalcontaining catalysts show strong basicities, some of the alkali metal ions are easily dissolved in the reaction media. Thus, the
(3) Di Serio, M.; Ledda, M.; Cozzolino, M.; Minutillo, G.; Tesser, R.; Santacesaria, E. Transesterification of soybean oil to biodiesel by using heterogeneous basic catalysts. Ind. Eng. Chem. Res. 2006, 45 (9), 3009–3014. (4) Xie, W. L.; Peng, H.; Chen, L. G. Calcined Mg-Al hydrotalcites as solid base catalysts for methanolysis of soybean oil. J. Mol. Catal. A: Chem. 2006, 246 (1-2), 24–32. (5) Corma, A.; Iborra, S.; Miquel, S.; Primo, J. Catalysts for the production of fine chemicals: Production of food emulsifiers, monoglycerides, by glycerolysis of fats with solid base catalysts. J. Catal. 1998, 173 (2), 315–321. (6) Suppes, G. J.; Dasari, M. A.; Doskocil, E. J.; Mankidy, P. J.; Goff, M. J. Transesterification of soybean oil with zeolite and metal catalysts. Appl. Catal., A 2004, 257 (2), 213–223. (7) Kim, H. J.; Kang, B. S.; Kim, M. J.; Park, Y. M.; Kim, D. K.; Lee, J. S.; Lee, K. Y. Transesterification of vegetable oil to biodiesel using heterogeneous base catalyst. Catal. Today 2004, 93-95, 315–320. (8) Watkins, R. S.; Lee, A. F.; Wilson, K. Li-CaO catalysed tri-glyceride transesterification for biodiesel applications. Green Chem. 2004, 6 (7), 335– 340. (9) Xie, W. L.; Huang, X. M. Synthesis of biodiesel from soybean oil using heterogeneous KF/ZnO catalyst. Catal. Lett. 2006, 107 (1-2), 53–59. (10) Macedo, C. C. S.; Abreu, F. R.; Tavares, A. P.; Alves, M. B.; Zara, L. F.; Rubim, J. C.; Suarez, P. A. Z. New heterogeneous metal-oxides based catalyst for vegetable oil trans-esterification. J. Braz. Chem. Soc. 2006, 7 (7), 1291–1296. (11) Li, H. T.; Xie, W. L. Transesterification of soybean oil to biodiesel with Zn/I-2 catalyst. Catal. Lett. 2006, 107 (1-2), 25–30. (12) Bournay, L.; Casanave, D.; Delfort, B.; Hillion, G.; Chodorge, J. A. New heterogeneous process for biodiesel production: A way to improve the quality and the value of the crude glycerin produced by biodiesel plants. Catal. Today 2005, 106 (1-4), 190–192. (13) Xie, W. L.; Peng, H.; Chen, L. G. Transesterification of soybean oil catalyzed by potassium loaded on alumina as a solid-base catalyst. Appl. Catal., A 2006, 300 (1), 67–74. (14) Gryglewicz, S. Rapeseed oil methyl esters preparation using heterogeneous catalysts. Bioresour. Technol. 1999, 70 (3), 249–253.
10.1021/ef800443x CCC: $40.75 2008 American Chemical Society Published on Web 10/16/2008
Transesterification of Glycerides
reaction proceeds according to homogeneous mechanisms.15 Other solid metal oxides, such as those of tin, magnesium, and zinc, are known heterogeneous catalysts but again function according to a homogeneous mechanism, leading to metal soaps or metal glycerates.12 Much work has focused on the preparation of solid catalysts possessing strong base sites.3,4,8-10,13,15 Strong basic sites are generated by removal of water or acidic gas molecules by pretreatment at high temperatures. These basic sites are fragile and can be easily contaminated by moisture, oxygen, carbon dioxide, and other gaseous substances when exposed to air. As a result, the exposed surface does not exhibit their intrinsic catalytic activities.13,16 Up to now, conversions for most heterogeneous catalysts are not high enough to be used for industrial-scale biodiesel production3,4,9,10,13,17 In comparison to homogeneous catalysts, relatively prolonged reaction periods are required in a heterogeneous catalytic process.11,17 The Esterfif-H is one of the few known processes that claim to have comparable performance as the homogeneous catalytic process.12 Recently, strong anion-exchange resins have been demonstrated to be a promising base catalyst for biodiesel synthesis.18 Strong anion-exchange resins can be classified as types 1 and 2. Type 1 is a quaternized amine product made by the reaction of trimethylamine with the copolymer of styrene and divinylbenzene and is the strongest basic functional group available. Type 2 functionality is obtained by the reaction of the styrene-divinylbenzene copolymer with dimethylethanolamine. The type 2 resin has a lower basicity than type 1, yet the efficiency of regeneration of the type 2 resin to the hydroxide form is somewhat higher than that of type 1 resin. Type 1 resins have better chemical stability than type 2 and are favored for relatively high-temperature application. The type 1 resins can be classified as gel type (Marathon A and Monosphere 550A) and macroporous type (Marathon MSA and Amberite 900) according to the structure and porosity. Gel-type structures exhibit microporosity with pore sizes typically up to 10 or 15 Å, while macroporous-type structures have considerably larger pore diameters up to several hundred angstroms, with a surface area reaching 500 m2/g or higher.19 Aracil et al.20 used anion- and cation-exchange resins in the transesterification reaction of sunflower oil to biodiesel, but the conversion was less than 1% after an 8 h reaction at 333 K. Recently, Yonemoto et al. reported that a batch transesterification reaction of triolein with ethanol was conducted using various ion-exchange resins produced by Mitsubishi Chemical Company. The best catalytic performance was obtained from the resin having a porous structure, the lowest cross-linking density, and the smallest particle size. Over 80% conversion to ethyloleate was obtained after a 3 h reaction at 323 K.18 Goodwin et al. tested a cation ion-exchange resin, Amberlyst15, as an acid catalyst for the transesterification of triacetin with (15) Granados, M. L.; Poves, M. D. Z.; Alonso, D. M.; Mariscal, R.; Galisteo, F. C.; Moreno-Tost, R.; Santamaria, J.; Fierro, J. L. G. Biodiesel from sunflower oil by using activated calcium oxide. Appl. Catal., B 2007, 73 (3-4), 317–326. (16) Hattori, H. Solid base catalysts: Generation, characterization, and catalytic behavior of basic sites. J. Jpn. Pet. Inst. 2004, 47 (2), 67–81. (17) Yan, S. L.; Du, Z. X. Load type calcium oxide catalyst, its preparation method and uses. Chinese National Invention Patent 1836772, 2006. (18) Shibasaki-Kitakawa, N.; Honda, H.; Kuribayashi, H.; Toda, T.; Fukumura, T.; Yonemoto, T. Biodiesel production using anionic ionexchange resin as heterogeneous catalyst. Bioresour. Technol. 2007, 98 (2), 416–421. (19) Wheaton, R. M. Dow Liquid Separation, Form 177-01751602XQRP, 2007. (20) Vicente, G. C.; Martinez, M.; Aracil, J. Application of the factorial design of experiments and response surface methology to optimize biodiesel production. Ind. Corps Prod. 1988, 8, 29–35.
Energy & Fuels, Vol. 22, No. 6, 2008 3595 Table 1. Properties of Resins (Source: Dowex Datasheet)
resin
matrix
Marathon A Monosphere 550 A Marathon MSA Amberite 900 DOWEX 1 × 4 DOWEX 1 × 2 DOWEX 1 × 2
gel gel macroporous macroporous gel gel gel
exchange particle cross-linking capacity size (µm) (%) (mEq/mL) 610 ( 50 590 ( 50 640 ( 50 300-1200 75-150 150-300 75-150
4 2 2
1.2 1.1 1.0 1.0 1.0 0.7 0.6
methanol. A conversion of 50% of triacetin was obtained after 150 min at 333 K.1 Gelbard and Vielfaure-Joly reported that immobilized guanidines and biguanides exhibited excellent catalytic properties on the transesterification of triglycerides.21 The efficiency of the recycled polymer-bound catalysts remained unaffected by more than 10 cycles after which alteration began to appear.22,23 In this work, a new approach is proposed to combine the merits of homo- and heterogeneous catalytic processes on transesterification. The effects of using a strong base anionexchanged resin as a heterogeneous catalyst and a trace level of sodium methoxide as a homogeneous catalyst simultaneously were investigated. In addition, the effects of pore structure, degree of cross-linking, particle size, and catalyst loading on the reaction rate were studied. 2. Experimental Section 2.1. Reagents. Anhydrous methyl alcohol (99.8%) and sodium hydroxide (99%) were obtained from Mallinckrodt Chemicals (Phillipsburg, NJ). Commercial edible-grade soybean oil [total acid number (TAN) ) 0.046 mg of KOH/g] was obtained from a retail source (COSTCO) and evacuated in a vacuum (5 × 10-2 torr) at 23 °C to remove water and gases dissolved in the oil phase. The titrant (0.1 N KOH in isopropanol) for the TAN measurement was purchased from LabChem, Inc. (Pittsburgh, PA). Strong basic anion-exchange resins were purchased from SigmaAldrich (St. Louis, MO). Marathon A and Monosphere 550A were purchased in hydroxide form, and the other resins were in chloride form. Table 1 summarizes the published physical properties and exchange capacity of the resins. The gel-type resins (DOWEX 1 × 2 and 1 × 4), with different cross-linking degree and different particle size, were also purchased from Sigma-Aldrich. 2.2. Resin Catalyst Preparation. A total of 100 g of Marathon MSA, Amberite 900, Dowex 1 × 2, and Dowex 1 × 4 (received as chloride form) resins was ion-exchanged in 1.0 M NaOH solution (250 mL) in methanol for 12 h. This ion-exchange process was repeated 4 times because the chloride ion has a higher selectivity (22 times) to the resin than the hydroxide ion.24 Marathon A and Marathon MSA resins (received as hydroxide form) were activated in 1.0 M NaOH solution in methanol for 12 h. The same activation process was used to regenerate the basic sites of the used catalysts. All of the ion-exchanged, activated, and regenerated resins (10 g) were then leached in pure methanol for 12 h and then filtered to remove physically absorbed NaOH. Moreover, all of the leached resins were washed again with methanol, and the pH of the washed methanol remained at 7.0. The density of the base sites of the resin was determined by the following titration method: 5.0 g of resin (21) Gelbard, G.; Vielfaure-Joly, F. Polynitrogen strong bases as immobilized catalysts for the transesterification of vegetable oils. C. R. Acad. Sci., Ser. IIc: Chim. 2000, 3 (7), 563–567. (22) Gelbard, G.; Vielfaure-Joly, F. Polynitrogen strong bases as immobilized catalysts. React. Funct. Polym. 2001, 48 (1-3), 65–74. (23) Schuchardt, U.; Vargas, R. M.; Gelbard, G. Transesterification of soybean oil catalyzed by alkylguanidines heterogenized on different substituted polystyrenes. J. Mol. Catal. A: Chem. 1996, 109 (1), 37–44. (24) AG 1, AG MP-1 and AG 2 strong anion exchange resin instruction manual. LIT212 Rev. C, Bio-Rad, Hercules, CA, 1997.
3596 Energy & Fuels, Vol. 22, No. 6, 2008 (swelled and wetted with methanol) was added into a flask containing 40 mL of deionized water, and then 1.000 mL of hydrochloric acid (12.1 N) was added to the flask. The flask was put aside for 12 h at room temperature to neutralize the hydroxide form of the resin. After neutralization, the resin was filtered out from the solution. The hydrochloric acid remaining in the solution was titrated with 1.0 M NaOH solution. 2.3. Transesterification Procedure and Analysis Methods. Erlenmeyer flasks (125 mL) containing soybean oil, methanol, and resin catalysts were used as batch reactors. In most studies, a molar ratio of 10:1 methanol/soybean oil and a mass ratio of 1:3 catalyst/ soybean oil were used. Flasks containing 30.0 g of soybean oil and 10.0 g of resin catalyst were heated in a shaking bath maintained at 328 K. A total of 6.8 g of methanol and 4.0 g of sodium methoxide solution (0.02 M) were sequentially added to the flasks. The flasks containing reaction mixtures were capped with glass stoppers and sealed with vacuum grease. The flasks were kept in the incubator (Series 25 incubator, New Brunswick Scientific Co.), with a shaking speed of 350 rpm. Although shaking speed can influence the rate of transesterification, the shaking speed was fixed at 350 rpm, which is comparable to other studies.7 At fixed time intervals, 1.0 mL of liquid was withdrawn from the upper layer of the resin catalyst by using syringes equipped with a filter (pore size of 0.45 µm). The sample solutions collected in small vials were first dried in a hood to remove dissolved methanol. The upper portion of the sample (methyl ester phase) was taken for GC analysis. Because triglyceride saponification can be negligible because a very small amount of sodium methoxide (0.018 wt %) and heterogeneous catalysts were used as catalysts and soap was not observed from the product, the yield (%) can be defined as the total weight percent of fatty acid methyl esters from the weight of the product after removing methanol and free glycerol. Fatty acid methyl esters in the samples were quantified using gas chromatography-mass spectrometry (GC-MS, Clarus 500 GC-MS, Perkin-Elmer) with a capillary column (Rtx-WAX catalog number 12426). Ethyl arachidate (Nu-Chek Prep, Inc., Elysian, MN) was used as an internal standard. The level of total glycerol in the prepared biodiesel was measured with a gas chromatography-flame ionization detector (GC-FID, Clarus 500, Perkin-Elmer) equipped with the Elite-5ht capillary column (5% diphenyl dimethylpolysiloxane) according to the American Society for Testing and Materials (ASTM) D6584 method. The total acid number of the soybean oil was determined according to ASTM D 664 using Brinkmann/Metrohm 809 titrando (Westbury, NY).
3. Results and Discussion The ASTM specification for biodiesel requires that the total glycerol be less than 0.24%.25 Hence, the degree of completion of the transesterification reaction is critical. If the reaction is incomplete, then there will be tri-, di-, and monoglycerides left in the reaction mixture, each containing a glycerol molecule that has not been released. Complete transesterification can be met by using a homogeneous catalyst, such as NaOH. In this conventional method, about 1% sodium hydroxide of soybean oil weight is used when the free fatty acids level is less than 1%, and when the free fatty acids level is above 1%, a higher weight percent of alkali catalyst is needed to neutralize the free fatty acids.25 Consequently, a large amount of water is required to separate and clean the catalyst and product.4 Moreover, the amount of soap formed increases with an increasing alkali catalyst.25 Thus, it is desirable to minimize the amount of alkali catalyst used. A heterogeneous catalyst does not produce soap through free fatty acid neutralization or triglyceride saponification.2 However, the performance of heterogeneous catalysts is still unfavorable compared to the alkali homogeneous catalysts. 3.1. Density of the Base Sites of the Resins. Resin ion exchange is usually carried out in aqueous solution because
Kim et al. Table 2. Densities of the Base Site of the Resins Ion-Exchanged in 1.0 M NaOH in Methanol resin catalysts
before reaction (mmol/g)
after reaction (mmol/g)
Marathon A Monosphere 550A Marathon MSA Amberite 900 Dowex 1 × 4 (75-150 µm) Dowex 1 × 2 (150-300 µm) Dowex 1 × 2 (75-150 µm)
1.70 1.47 0.50 0.54 0.60 0.62 0.66
1.57 1.40 0.43 0.43 0.53 0.57 0.58
resins are ionic compounds. However, the resin can absorb water molecules, which interfere with the transesterification reaction and thus result in poor yields and high levels of soap, free fatty acids, and triglycerides in the final product.25 To minimize the negative effects of water, 1.0 M NaOH in methanol instead of 1.0 M NaOH aqueous solution was used in the ion-exchange process. The base site strengths of the resin tested in this study can be regarded as the same because the basic sites are composed of the same functional group (trimethylammonium hydroxide).19 Although there is small differences in ionexchange capacities between the gel-phase (Monosphere 550A and Marathon A) and macroporous-phase (Marathon MSA and Amberite 900) resins (Table 1), large differences in the amounts of base sites between the gel- and macroporous-phase resins are observed (Table 2). The apparent inconsistency between the exchange capacity and the amount of base sites can mainly be attributed to the level of regeneration and treatment conditions. This is especially true for the macroporous resins, which are received in chloride form. The level of regeneration depends upon the resin structure and is low for the macroporous resin.19The amounts of the base sites of the Marathon A and Monosphere 550A are about 3 times higher than that of macroporous resins. Macroporous or cross-linked resins are known to have lower base site densities.19 After a batch reaction, the densities of the base sites are decreased by 5-20% from their initial values. The percentage loss of base sites in macroporous resins (14% for Marathon MSA and 20% for Amberite 900) are larger than those in the gel-phase resins (7% for Marathon A and 5% Monosphere 550A). The higher loss of hydroxide on macroporous resin can be attributed to two possible causes: (1) thermal decomposition of the ammonium functional group of the base site during the catalytic reaction,26 or (2) ion exchange with organic ions, such as oleate, linoleate, and linolenate, evolved during the reaction.25 3.2. Catalytic Activities of the Resins. Figure 1 shows heterogeneous catalytic activities of the resin catalysts. The internal structure of resin gave a great influence on catalytic activity. High initial catalytic activities were observed for the macroporous resins, such as Amberite 900 and Marathon MSA, possibly because of the large pore size and effective surface areas (500 m2/g) of the macroporous structure. Moreover, Dowex 1 × 2 also showed high catalytic activity because it has a low degree of cross-linking, large pore aperture, and small particle size. However, the gel-type resins (Marathon A and Monosphere 550A) showed lower activities, even though they have 3 times higher base site density than the macroporous structure resins. The key parameter to the final conversion was the accessible base site density. This suggests the gel-type resin has negligible amounts of effective pores where triglyceride can (25) Van Gerpen, J.; Shanks, B.; Pruszko, R.; Clements, D.; Knothe, G. Biodiesel Production Technology, 2004. (26) McGarvey, F. X.; Hauser, E. W.; Bachs, B.; Stellitano, J. Thermal degredation of strongly basic anion exchange resins in caustic regenerants. Proceedings of International Water Conference, 1987; IWC-87-9; pp 93-102.
Transesterification of Glycerides
Figure 1. Heterogeneous catalytic activities of resins in a neutral or acidic environment. Reaction conditions: acidic soybean oil, 30 g; methanol/oil molar ratio, 10:1; reaction temperature, 326 K; and shaking speed, 350 rpm.
Figure 2. Synergy effect of the hetero- and homogeneous catalyst on the transesterification of soybean oil with methanol. Reaction conditions: soybean oil, 30 g; 5.1 mL of 0.020 M CH3ONa solution in methanol was added in the case of using CH3ONa; methanol/oil molar ratio, 10: 1; reaction temperature, 326 K; and shaking speed, 350 rpm.
diffuse. Resin catalysts, including Dowex catalysts, swelled significantly in polar solvents, such as water or methanol. Thus, the surface area and pore size of the resins under the reaction condition cannot be easily measured. To enhance the reaction rates, two different sets of heterogeneous catalyst batch experiments, with and without the addition of homogeneous CH3ONa catalyst, were investigated. In the first batch, 4.0 g (5.1 mL) of 0.02 M CH3ONa solution in methanol and 6.0 g of methanol were added to the mixture of 30.0 g of soybean oil (acid number: 0.046 mg of KOH/g) and 10.0 g of resin catalyst. In the other batch, 10.8 g of methanol was added to the mixture of 30.0 g of soybean oil and 10.0 g of resin. Both protocols yield molar ratios of methanol/soybean oil of 10:1. In the first batch, free fatty acid contained in soybean oil reacts with some of the CH3ONa, yielding CH3OH and organic sodium salt. Given that the amount of CH3ONa added to the reaction mixture was 0.018 wt % soybean oil, only 0.003 wt % is consumed during neutralization. Thus, the amount of CH3ONa added corresponded to 0.015 wt % NaOH, compared to 1.0 wt % sodium hydroxide used in the conventional homogeneous method. However, as shown in Figure 2, the activities of resin were significantly increased and the yields approached 100% after 6 h at 328 K when both resin catalyst and 0.018 wt % CH3ONa were used. Typical glycerol concentrations in the prepared biodiesel were as follows: free glycerol, 0.164%; monoglyceride, 0.018%; diglyceride, 0.006%;
Energy & Fuels, Vol. 22, No. 6, 2008 3597
and triglyceride, 0.000%. It should be noted that with 0.018 wt % homogeneous catalyst (CH3ONa) alone (Figure 2), the reaction could not go to completion and leveled off at 39% yield. The enhanced activity observed cannot be attributed to a simple addition of CH3ONa and resin activities. It is postulated that CH3ONa functions as a co-catalyst, maintaining the basic environment needed to enhance the activity of the heterogeneous catalyst. The active sites of the heterogeneous catalyst are insoluble. Marathon A (gel-type resin) exhibited a very low activity for the entire reaction period. On the other hand, Marathon MSA (macroporous resin) showed a high initial yield without added CH3ONa and then leveled off. The yield from the Marathon A at a 3 h reaction was 5.6%. The yield of the homogeneous catalyst (CH3ONa, 0.018%) at 3 h was 22%. However, 54% yield was observed from the combined catalysts. This result can be interpreted as follow: although resin catalysts have numerous base sites, only a limited fraction of the sites can be accessed by the relatively large triglyceride molecules.19 This is especially true for gel-phase resins because their catalytic activities were negligible in a neutral or acidic environment as shown in Figure 1. Major deactivating agents are water and free fatty acids,18,27 which are included in oil or evolved during transesterification. First of all, some strong base sites can be neutralized by free fatty acids contained in the feed reactant. In addition, some of the active base sites form hydrogen bonds with water molecules contained in the feedstock and then become inactive. Finally, a certain fraction of the base sites could be ion-exchanged with organic anions generated during the catalytic transesterification reaction18 as shown in Figure 3b. Especially, the long chain of the organic anion can be exchanged with hydroxides located on the outer surface of the resin. This long organic chain anchored on the resin surface blocks the pore openings of the resin and deactivates the resin catalyst. However, with a trace level of the homogeneous catalyst (CH3ONa), the deactivated surface hydroxide groups can be regenerated as a result of the reaction of hydrogen-bonded water molecules with CH3ONa. Moreover, the free hydroxide ion generated assists methanol to react with the bonded organic anion or a triglyceride to form a methyl ester. The free hydroxide replaces the organic anion and thus regenerates the site as shown in Figure 3c. The removed organic anion is saponified by combining with the sodium ion. Thus, the homogeneous catalyst provides a synergetic effect by providing a basic environment and enhancing the availability and mobility of the hydroxide anion. As shown in Figure 2, the initial yield of the macroporous resin (Marathon MSA) was higher than that of the gel-phase resin (Marathon A); however, the final yields were similar. This may be attributed to the higher base site densities of the geltype resins. This suggests that the gel-type resin could provide a larger number of hydroxide ions to the reaction medium in a basic environment than the macroporous-type resin. Another possible explanation for the high activity of the combined catalysts observed can be attributed to the “purifying” effect of anion-exchange resins. Generally, ion-exchange resins have been used for separation and purifying purposes by adsorbing selective anions.19 Deactivating agents, such as water and free fatty acids, can penetrate into the pores of anionexchange resins to reach sites that are inaccessible to the much larger triglycerides. The normally unused sites located inside the anion-exchange resin may purify the reactants and allow (27) Kharchafi, F.; Jerome, F.; Adam, I.; Pouilloux, Y.; Barrault, J. Design of well balanced hydrophilic-lipophilic catalytic surfaces for the direct and selective monoesterification of various polyols. New J. Chem. 2005, 29 (7), 928–934.
3598 Energy & Fuels, Vol. 22, No. 6, 2008
Kim et al.
Figure 3. Schematic diagram of the activity of the base resin catalyst in different mediums.
for the homogeneous catalyst as well as the accessible base sites on the resins to function more effectively. However, in comparison to the micropores (10-15 Å) of the resins, the molecular size of the most deactivating organic anions (e.g., linoleic acid, C18:2) is too large to diffuse into the micropores of the resins. Therefore, this purification effect may not be the main reason for the high catalytic activity observed for the combined catalysts. In addition, the amount of the homogeneous catalyst added was very small (0.018%); thus, the observed high catalytic activity cannot be attributed to the homogeneous catalyst alone (Figure 2), even with certain purification effects from the resins to remove deactivating agents. After 6 h, the yield values of the Marathon MSA and Marathon A (with CH3ONa) approached completion. Figure 4 shows the influence of cross-linking and particle size of the resins on the catalytic activity. Dowex 1 × 2 and 1 × 4 resins have 2 and 4% cross-linked structures, respectively. With the same range of particle sizes (75-150 µm), Dowex 1 × 2 had a higher yield than Dowex 1 × 4 resin. This suggests that the resin that has a lower cross-linking has larger pores and a higher base site density. This result agrees with the findings of ShibasakiKitakawa et al. that the lowest cross-linking density and the smallest particle size gave the highest reaction rate.18 Figure 5 presents the effect of the methanol/oil ratio on the yield. Basically, the miscibility of oil to methanol is poor regardless of
Figure 4. Influences of the degree of cross-linking and particle size on the catalytic activity. Reaction conditions: acidic soybean oil, 30 g; resin catalyst, 10.0 g; methanol/oil molar ratio, 10:1; reaction temperature, 326 K; and shaking speed, 350 rpm.
composition at low temperature and pressure.28 The stoichiometric molar ratio of methanol/soybean oil in the transesterification of soybean oil with methanol is 3:1. However, in practice, a higher molar ratio is employed to speed up the reaction rate and produce more methyl esters. Xie et al. reported that a maximum conversion of 67% was obtained with the molar ratio of 15:1 and a reaction time of 9 h on the 7.5 wt % potassium-loaded alumina catalyst at
Transesterification of Glycerides
Figure 5. Effect of the methanol/oil molar ratio on the conversion of soybean oil with methanol to biodiesel. Reaction conditions: soybean oil, 30 g; 5.1 mL of 0.02 M CH3ONa solution in methanol was added; resin catalyst, 10.0 g; reaction temperature, 326 K; shaking speed, 350 rpm. (a) Marathon MSA and (b) Monosphere 550A.
65 °C.13 Another study reported that the conversion reached a maximum value of 87% on a solid base catalyst ZnO loaded with KF when the methanol/oil ratio was close to 10:1.9 ShibasakiKitakawa et al. reported that the maximum conversion was obtained at the molar ratio (methanol/oil) of 20:1, and almost the same conversion was attained at the molar ratio of 10:1, with a reaction time of 9 h at 50 °C.18 In this work, the initial reaction rates were the highest at the molar ratio of 7:1 (parts a and b of Figure 5). The molar ratio of 10:1 had a higher yield at a 6 h reaction than the others with Marathon MSA catalyst as shown in Figure 5a. The mixture of the molar ratio 13:1 showed the lowest reaction rate and yield for the entire reaction period. The lower initial reaction rate observed at a higher methanol/oil ratio can be attributed to a lower collision frequency between reactants and catalyst. Figure 6 presents the deactivation and regeneration behavior of resin catalysts. The yield of the used Marathon MSA was about half that of the fresh catalyst. The regenerated Marathon MSA and Monosphere 550A showed lower catalytic activities than the fresh ones as shown in parts a and b of Figure 6. It was found that macroporous resin (Marathon MSA, Figure 6a) showed poorer regeneration efficiency and higher regeneration cost than the gel-type resin (Monosphere 550A, Figure 6b). This difference may be attributed to both thermal decomposition of ammonium sites of the resin26 and the resin ion-exchanged with organic anion.24 The regenerated Marathon A showed nearly the same activity with the fresh one. 4. Conclusions The following conclusions can be drawn from this study: Strongly basic anion-exchange resins can be used as a heterogeneous catalyst for the transesterification reaction of soybean
Energy & Fuels, Vol. 22, No. 6, 2008 3599
Figure 6. Catalytic activities of the new (ion-exchanged), used, and regenerated resin catalyst (ion-exchanged after used). Reaction conditions: soybean oil, 30 g; 5.1 mL of 0.020 M CH3ONa solution in methanol was added; resin catalyst, 10 g; methanol/oil molar ratio, 10: 1; reaction temperature, 328 K; shaking speed, 350 rpm. (a) Marathon MSA and (b) Monosphere 550A.
oil with methanol. Gel types of resins have higher base site densities than macroporous resins. However, high catalytic activities were observed in the macroporous resins because of macroporous structures. The base site catalytic activities of the anion-exchanged resins are also dependent upon the pH value of the reaction medium. The catalytic activities of the resins were highly restricted or deactivated in an acidic or neutral medium because of neutralization of the base sites and adsorbed organic anions on the surface of the catalyst. The catalytic activities were dramatically increased in the basic environment because the deactivated sites in an acidic or neutral solution became activated by a trace amount of homogeneous base. Catalytic activity of the resin was related to the degree of cross-linking or particle size of the resin. A lower cross-linking degree gives a larger pore aperture and larger number of active sites. A smaller particle size gives a larger catalytic surface area. The resin that has a lower degree of cross-linking and smaller particle size gives better activity. The optimum methanol/oil ratio was found to be 7:1. The used Monosphere 550A catalyst lost 5% of the yield and regenerated. Acknowledgment. Financial support from the Department of Energy (Grant DE-FG36-05GO85005) and Michigan’s 21st Century Job Fund is gratefully acknowledged. EF800443X (28) Tang, Z.; Do, Z. X.; Min, E. Z.; Gao, L.; Jiang, T.; Han, B. X. Phase equilibria of methanol--triolein system at elevated temperature and pressure. Fluid Phase Equilib. 2006, 239 (1), 8–11.
