Energy & Fuels 2008, 22, 2067–2069
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Transesterification of Soybean Oil Using Heterogeneous Catalysts Alok Kumar Singh and Sandun D. Fernando* Department of Agricultural and Biological Engineering, Mississippi State UniVersity, Mississippi State, Mississippi 39762 ReceiVed January 31, 2008. ReVised Manuscript ReceiVed March 17, 2008
Biodiesel is a biodegradable, nontoxic, and clean-burning fuel that can be made from biorenewable fats and oils. At present, biodiesel is primarily produced in batch reactors, where the separation of catalysts, glycerol, and biodiesel from the reactor is onerous. Solid catalysts can be used to allay this separation problem. In the present study, seven different solid catalysts (metal oxides), MgO, CaO, PbO, PbO2, Pb3O4, Tl2O3, and ZnO, with different Brunauer-Emmett-Teller (BET) surface area, acidity/basicity, and the acid/base site strength were selected for the transesterification. Biodiesel (fatty acid methyl esters, FAMEs) yields were determined for these catalysts at three different temperatures (75, 150, and 225 °C) and high pressure. It was observed that more than 89% yield of biodiesel was achieved with PbO and PbO2 solid catalysts.
1. Introduction Biodiesel is a fuel comprised of monoalkyl esters of long chain fatty acids derived from vegetable oils or animal fats, designated B100, and meeting the requirements of the American Society for Testing and Materials (ASTM) D 6751.2 It is a cleanburning fuel, which is nontoxic, biodegradable, and considered as the fuel of the future. It can be used neat or mixed with petroleum diesel to produce a biodiesel blend that can be used in compression ignition engines under a variety of operating conditions. Pure biodiesel fuel contains no petroleum fuels and emits virtually no sulfur, aromatics, particulates, or carcinogenic compounds and is thus a safer alternative to petroleum diesel. Biodiesel can be used in all conventional diesel engines, delivers similar performance and engine durability to petroleum diesel, and requires virtually no modifications in fuel handling and delivery systems. The most common method for producing biodiesel is transesterification1 (Figure 1), in which, according to stoichiometry, 1 mol of triglyceride reacts with 3 mol of alcohol (primarily methanol) in the presence of a strong catalyst (acid, base, or enzymatic), producing a mixture of fatty acid alkyl esters (biodiesel) and glycerol.1–3 Because the transesterification is a reversible reaction, a stoichiometric ratio of 6:1 is practically used2 to increase the concentration of the product (biodiesel) and to precede the equilibrium toward the product side.4 Catalyst selection for transesterification is based on the free fatty acid (FFA) content of the oil. If the FFA content is high, acid-catalyzed esterification followed by transesterification is followed. However, the rate is relatively slow, and higher molar * To whom correspondence should be addressed. Telephone: +1-662325-3282. Fax: +1-662-325-3853. E- mail:
[email protected]. (1) Singh, A. K.; Fernando, S. D.; Hernandez, R. Base-catalyzed fast transesterification of soybean oil using ultrasonication. Energy Fuels 2007, 21, 1161–1164. (2) Ma, F.; Hanna, M. A. Biodiesel production: A review. Bioresour. Technol. 1999, 70, 1–15. (3) Serio, M. D.; 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, 3009–3014. (4) Khan, A. K. Research into biodiesel kinetics and catalyst development. The University of Queensland: Brisbane, Queensland, Australia, 2002.
Figure 1. Transesterification of triglyceride and oil.
ratios of oil/methanol are required to tend the reaction to the left.2 If the FFA content is low, the base-catalyzed transesterification is most desirable and is relatively faster than acidcatalyzed transesterification.5 Homogeneous catalysts (such as NaOH, KOH, and NaOCH3, etc.) are generally used in the basecatalyzed transesterification. Despite industrial applicability, homogeneous catalysts have their own share of limitations: The catalyst dissolves fully in the glycerin layer and partially in the fatty acid methyl ester (FAME) layer, which makes the product separation arduous. As a result, biodiesel should be cleaned through a slow, tedious, and environmentally unfriendly waterwashing process to remove excess catalyst. Catalyst-contaminated glycerin has little value in today’s market and is increasingly becoming a disposal issue. Another negative aspect of the homogeneously catalyzed process is that the catalysts are non-reusable. Heterogeneous catalysts, on the other hand, make product separation easier and catalysts reusable. With the use of solid catalysts, the refining steps in the purification process can be reduced. Also, heterogeneous catalysts have the potential to simplify the production process by enabling usage of continuous packed bed reactors. 1.1. Biodiesel Production Using Solid Catalyst. As compared to the homogenously catalyzed process, the transesterification with solid catalyst occurs at harsher reaction conditions, i.e., at higher temperatures and pressures. This is because of the fact that the solid catalyzed process is a three-phase system (5) Schuchardta, U.; Serchelia, R.; Vargas, R. M. Tranesterification of vegetable oils: A review. J. Braz. Chem. Soc. 1998, 9 (1), 199–210.
10.1021/ef800072z CCC: $40.75 2008 American Chemical Society Published on Web 05/02/2008
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(oil, methanol, and catalyst) and, for mass-transfer reasons, it protracts the transesterification.6 In one study, supported CaO/MgO was used7 for the transesterification of rapeseed oil at a relatively low temperature of 65 °C by impregnating on a MgO carrier, followed by the calcination at 700 °C in Ca(Ac)2 solution. The catalyst showed higher activity with a glycerol yield of more than 80% purity. In other work,3 soybean oil was transesterified at 100 °C with methanol using MgO and calcined hydrotalcites (CHT) as catalysts. Four different basic sites were individuated at MgO and the calcined hydrotalcites for the transesterification, and the strongest basic site was able to perform the transesterification reaction at a temperature below 100 °C. More than 45% of biodiesel yield was observed in the case of MgO, and more than 75% yield was observed with CHT. At a higher temperature of 200 °C, more than 95% of FAME yield was observed for MgO and CHT. Biodiesel production with high surface area (HSA) nanocrystalline metal oxides on TiO2, MgO, and CaO supports were investigated.8 M-Acetylacetonate (AcAc) was supported on the HSA support, where M was Na, K, Ca, Li, V, Fe, N, and Al. The best catalysts tested were CaO and AcAc supported on MgO and TiO2. In our earlier work of catalyst screening and their reaction kinetics determination,6 five different solid catalysts (MgO, CaO, BaO, PbO, and MnO2) were used for the transesterification of soybean oil at 215 °C. Biodiesel yield of 85% in 14 min with BaO and a yield of 84% in 85 min with PbO were achieved. In a different work, biodiesel production of jatropha curcas9 oil with a solid catalyst CaO dipped in ammonium nitrate followed by calcination at 900 °C showed an oil conversion of 93% at 70 °C for 3.5 h of transesterification. The catalyst dosages and the oil/methanol ratio used in the study were 1.5% and 9:1, respectively. In a different work of soybean oil transesterification10 with SrO as a heterogeneous catalyst, a yield in excess of 95% was observed below 70 °C within 30 min. A long catalyst lifetime of SrO was also investigated because it sustained the activity after repeated used for 10 cycles. The present work was targeted on comparing the most effective catalysts based on the literature and our own preliminary studies under identical conditions. The present work deals with the transesterification of soybean oil with seven heterogeneous metal oxide catalysts (MgO, CaO, PbO, PbO2, Pb3O4, ZnO, and Tl2O3) at three different temperatures of 75, 150, and 225 °C. Catalyst characterization on the basis of their surface area, acidic/basic site strength, and acidity/basicity was performed. 2. Experimental Section 2.1. Reagents and Materials. Solvent-extracted degummed soybean oil donated by Bungi Corporation Marks, MS, was used as the triglyceride. The solid catalysts (PbO, PbO2, Pb3O4, MgO, ZnO, CaO, and Tl2O3) and methanol (99.9%) used in the study were purchased from Sigma Aldrich, St. Louis, MO. (6) Singh, A. K.; Fernando, S. D. Reaction kinetics of soybean oil transesterification using heterogeneous metal oxide catalysts. Chem. Eng. Technol. 2007, 30 (12), 1–6. (7) Wang, G.; Yan, S.; Zhou, C.; Liang, B. Preparation and activity measurement of CaO/MgO catalyst for biodiesel fuel production. Zhongguo Youzhi 2005, 30 (10), 66–69. (8) Dean, M.; Yoder, S.; Doskocil, E. J. Catalysis and reaction engineering. In Nanocrystalline Metal Oxide-Based Catalysts for Biodiesel Production from Soybean Oil; 2006; p 1. (9) Zhu, H.; Wu, Z.; Chen, Y.; Zhang, P.; Duan, S.; Liu, X.; Mao, Z. Preparation of biodiesel catalyzed by solid super base of calcium oxide and its refining process. Chin. J. Catal. 2006, 27 (5), 391–396. (10) Liu, X.; He, H.; Wang, Y.; Zhu, S. Transesterification of soybean oil to biodiesel using SrO as a solid base catalyst. Catal. Commun. 2007, 8 (7), 1107–1111.
Singh and Fernando 2.2. Equipment. The transesterification was carried in a fully automated high-pressure high-temperature batch reactor (PARR Instrument, 4843). The equipment consists of a high-pressure cylindrical chamber, a heater, a water line (to control the temperature) and a stirrer. 2.3. Transesterification of Soybean Oil. A mixture of 30 mL of methanol and 100 mL of soybean oil (equivalent to 7:1 molar ratio) was prepared using a magnetic stirrer, and then 2 g of solid catalyst was added into the high-pressure reaction vessel. Three different temperatures of 75, 150, and 225 °C were selected for the comparison of the biodiesel (FAME) yield. The transesterification was performed at the selected temperature for 2 h, and then the products were separated, frozen, and sent for gas chromatography (GC) analysis. The products were frozen to terminate the transesterification reaction. 2.4. Gas Chromatography Analysis. The top layer of each sample, after stabilization, was analyzed for FAME composition at the Mississippi State Chemical Laboratory, Mississippi State University, with gas chromatography. 2.5. Determination of the Surface Area of the Catalysts. Surface area of the metal oxides was measured with the multipoint Brunauer-Emmett-Teller (BET) method from Quantachrome Instruments (Autosorb-1-C/TCD). This was performed using nitrogen adsorption/desorption isotherms at liquid nitrogen temperature and relative pressures (P/P0) ranging from 0.04-0.4, where a linear relationship was maintained. 2.6. Determination of Acid/Base Strength. Site strength refers to the relative tendency of an acid or base to donate or accept a proton. The strength of acid and bases can be compared by their reaction with water. Acidic and basic site strengths of each of the metal oxides were determined by basic and acidic Hammett indicators, respectively.11 Approximately 50 mg of sample was shaken with 1 mL of a solution of Hammett indicator diluted in benzene and methanol for basic and acidic tests, respectively, and left to equilibrate for 2 h. The color of the catalyst was then noted. The basic Hammett indicator (for acid site strength) used were Neutral red (pKa ) 6.8), Methyl red (pKa ) 4.8), P-dimethylaminoazobenzene (pKa ) 3.3), and Crystal violet (pKa ) 0.8). The acidic Hammett indicator (for base site strength) used were Phenolphthalein (pKBH+ ) 8.2), Nile blue (pKBH+ ) 10.1), Tropaeolin (pKBH+ ) 11), 2,4-dinitroaniline (pKBH+ ) 15), 4-chloro-2-nitroaniline (pKBH+ ) 18.2), and 4-chloroaniline (pKBH+ ) 26.5). The H0 value of a sample at acid site was determined by the smallest H0 value among the Hammett indicators, which has been subjected to a color change and has a H0 value less than 7.0. In addition, the H0 value of a sample at the base site was determined by the greatest H0 value among the Hammett indicators, which had been subjected to a color change and having a H0 value more than 7.0. 2.7. Determination of Acidity/Basicity. A common method for evaluating the basicity of a base is to report the acidity of the conjugate acid and vice versa for the acidity, and in our case, the method of titration was used to determine the acidity/basicity of the catalysts. For basicity, the basic catalyst was mixed with a known concentration of HCl. The basic catalyst will neutralize HCl by an equivalent amount to its basicity. As a result, the original concentration of HCl will be reduced. The resultant concentration of HCl was determined by titration with NaOH, and finally, the adsorbed amount of HCl on the catalyst was determined. In retrospect, for acidity determination, an acidic catalyst was mixed with a known concentration of NaOH and the amount of NaOH adsorbed to the catalysts was determined via titration with HCl. For amphoteric catalysts, both acidity as well as basicity were determined.
3. Results and Discussion The FAME yield after transesterification varied significantly among the catalysts tested. Also, there was a diverse response (11) Xie, W.; Huang, X. Synthesis of biodiesel from soybean oil using heterogeneous KF/ZnO catalyst. Catal. Lett. 2006, 107 (1-2), 53–59.
Transesterification of Soybean Oil
Energy & Fuels, Vol. 22, No. 3, 2008 2069 Table 2. Site Strength of the Metal Oxides and Their Respective Acidity/Basicity Value
catalyst
type
acid/base site strength, (H_)
MgO Cao ZnO PbO PbO2 Pb3O4 Tl2O3
basic basic amphoteric amphoteric amphoteric basic basic
11 < (H_) < 15 10.1 < (H_) < 11 6.8 < (H_) < 8.2 6.8 < (H_) < 8.2 6.8 < (H_) < 8.2 6.8 < (H_) < 8.2 10.1 < (H_) < 11
acidity (mmol basicity (mmol of NaOH/g of of HCl/g of catalyst) catalyst) 46.05 16.24 12.25 5.747 17.86
32.353 7.58 7.00 14.545 15.93
Figure 2. Biodiesel yield with different solid catalysts. Table 3. Leaching of Metals from Their Respective Metal Oxide in Biodiesel and Glycerol Samples
Table 1. Surface Area of the Metal Oxides catalyst
MgO
CaO
Tl2O3
ZnO
Pb3O4
PbO
PbO2
area (m2/g)
157.4
61.39
6.17
4.04
0.98
0.55
0.38
to temperature variations among different catalysts. Figure 2 depicts the biodiesel yield (FAME) for all of the catalysts (MgO, CaO, PbO, PbO2, Pb3O4, ZnO, and Tl2O3) at three different temperatures of 75, 150, and 225 °C. MgO and Pb3O4 showed an increasing trend. Initially, at 75 °C, both of them had a very little effect (less than 5%) on the transesterification, but at higher temperatures (at 215 °C), they gave an yield of 74 and 89%, respectively, in terms of biodiesel yield. MgO has been used in a different work,3 and similar behavior was found in our case. The FAME yield of Tl2O3 and ZnO peaked around 150 °C and showed a sharp decline at 225 °C. This may be attributed to cracking of esters at higher temperatures. This observation is further reinforced by the fact that the resultant product looked much darker than in color than samples that had higher FAME yields. PbO and PbO2 showed almost an identical trend at all of the three temperatures tested. A maximum FAME yield of 89% was observed for both the catalysts at 150 °C. The only difference in Pb3O4 was that the increasing FAME yield trend sustained even beyond 225 °C. Lead oxides by far were the most potent for transesterification from all of the oxide catalysts tested. It was interesting to note that CaO has displayed a different trend to the other catalysts. CaO was selective toward transesterification at all of the temperatures tested and gave FAME yields of 46, 81, and 67% at 75, 150, and 225 °C, respectively. 3.1. Surface Area of the Catalysts. Table 1 shows the surface area of the catalysts based on the nitrogen adsorption/ desorption (multipoint BET) method. It was found that MgO had the largest area of 157.4 m2/g, whereas the PbO had the minimum of 0.55 m2/g. All of the three lead catalyst were found to have a very small surface area (0.3-1.0 m2/g). However, it was interesting to note that despite the comparatively lower surface area, lead oxides gave the highest FAME yields. 3.2. Acidity/Basicity of the Catalysts. On the basis of the method described in sections 2.6 and 2.7 using Hammett indicators followed by the titration, acid/base site strength and acidity/basicity was determined. Table 2 depicts the type of catalyst with their site strength and acidity/basicity values. MgO was found to be highly basic, with a basicity of 46.05 mmol of HCl/g of MgO, and had a positive effect on the transesterification. It should be noted that, even at higher temperatures, MgO-catalyzed samples did not show any sign of cracking. ZnO, PbO, and PbO2 was found to be amphoteric with a site strength (H_) in the range of 6.8 and 8.2. The remaining catalysts were found to be basic.
catalyst
leaching in glycerol (mg/kg of glycerol)
leaching in biodiesel (mg/kg of biodiesel)
PbO ZnO CaO MgO PbO2 Tl2O3 Pb3O4
2100 45 1500 460 4400 35 000 8100
13 000 110 6800 8200 710 19 000 760
3.3. Leaching Analysis. To obtain an idea of solid catalysts separation, FAME fraction and glycerol fraction of each of the samples were sent for elemental analysis via flame atomic absorption analysis (FLAA). The analysis was performed at the Mississippi State Chemical Laboratory. It should be noted that all samples were centrifuged at 400 rpm for 20 min, and the liquid fraction was sent for the analysis. Table 3 shows the amount of the metal fraction that remained intact in the biodiesel and glycerol fractions. We would like to point out that these are not necessarily soluble leachates but are a good representation of the elemental composition of products subsequent to centrifugal separation. It is apparent that thallium oxide had the largest residual elements in both biodiesel and glycerol samples, whereas zinc oxide had the minimum. Pb3O4, despite having the lowest surface area and comparatively low leaching tendency, rendered one of the best FAME yields. 4. Conclusions The solid catalysts used in this study (MgO, CaO, PbO, PbO2, Pb3O4, ZnO, and Tl2O3) had varying selectivity toward the transesterification reaction depending upon their acidity/basicity, acid/base site strength, surface area, and leaching tendencies. Lead oxide catalysts were found to be most favorable toward the transesterification and resulted in more than 89% of the biodiesel yield. MgO and Pb3O4 showed an increasing FAME yield trend from 75 to 225 °C, which appeared to continue beyond tested temperatures. Moreover, in this screening study, MgO was found to be the catalyst with the highest surface area, highest Lewis basicity, and relatively low leaching. All of the other catalysts tested displayed a potential tendency to crack transesterification products at higher temperatures. Tl2O3 and ZnO, despite their opposite leaching behavior, displayed similar trends toward transesterification, which could be attributed to their approximate similarity in surface area. Although CaO did not push toward completion, it catalyzed the transesterification reaction in all of the three temperatures tested. EF800072Z
