Energy & Fuels 2009, 23, 1089–1092
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Application of Sodium Aluminate As a Heterogeneous Base Catalyst for Biodiesel Production from Soybean Oil Tao Wan, Ping Yu, Shenggang Wang, and Yunbai Luo* College of Chemistry and Molecular Sciences, Wuhan UniVersity, Wuhan 430072, P.R. China ReceiVed October 19, 2008. ReVised Manuscript ReceiVed January 4, 2009
In this study, the production of biodiesel from soybean oil by transesterification was carried out over sodium aluminate as a heterogeneous catalyst. The solid base showed high catalytic activity for methanolysis reaching a 93.9% yield under optimal reaction conditions (reflux temperature, 1.5 wt% of catalyst, 12:1 molar ratio of methanol/oil, and 50 min). The catalyst treated at different temperatures was characterized by inductively coupled plasma-optical emission spectroscopy (ICP-OES), X-ray diffraction (XRD), thermogravimetry/ differential thermogravimetry (TG/DTG), and Fourier transform infrared spectroscopy (FT-IR). The reaction contained homogeneous and heterogeneous contributions at the same time. Removing water and carbon dioxide was an effective way to elevate the catalyst stability in methanol.
1. Introduction Nowadays, biodiesel has become very attractive because of its environmental benefits and because it is made from renewable resources. Biodiesel has the advantages of lowering the combustion emission of carbon monoxide, particulate matter, and sulfur compounds, as well as limiting greenhouse emissions due to its closed carbon dioxide cycle.1,2 Transesterification is the process used to make biodiesel fuel as defined in Europe and the U.S.A.3 Most biodiesel today is produced in the presence of homogeneous catalysts such as sodium methoxide, sodium or potassium hydroxide.4 However, the formation of soap lowers biodiesel yield and requires the separation of ester and glycerol, making the washing process difficult. To address this problem, more and more research at present has focused on the use of heterogeneous catalysts. Heterogeneous catalysts have several advantages including easier operational procedures, catalyst separation, and reduction of environment pollutants, among others. As the catalytic activity of basic catalysts is higher than that of acid solids, they have been preferably studied.5 However, although several basic solids have shown promising activities such as alkali earth oxides CaO,6-8 MgO,9 SrO,10 and so on; alkali-doped metal oxides * To whom correspondence should be addressed. E-mail: ybai@ whu.edu.cn. Phone: +86(27)6877-2263. Fax: +86(27)68754067. (1) Graboski, M. S.; McCormick, R. L. Prog. Energy Combust. Sci. 1998, 24, 125–164. (2) Karmee, S. K.; Chadha, A. Bioresour. Technol. 2005, 96, 1425– 1429. (3) Ma, F.; Hanna, M. A. Bioresour. Technol. 1999, 70, 1–15. (4) Bak, Y. C.; Choi, J. H.; Kim, S. B.; Kang, D. W. Korean J. Chem. Eng. 1996, 13, 242–245. (5) Serio, M. D.; Tesser, R.; Pengmei, L.; Santacesaria, E. Energy Fuels 2008, 22, 207–217. (6) Gryglewicz, S. Bioresour. Technol. 1999, 70, 249–253. (7) Demirbas, A. Energy ConVers. Manage. 2007, 48, 937–941. (8) Granados, M. L.; Zafra Poves, M. D.; Martı´n Alonso, D.; Mariscal, R.; Cabello Galisteo, F.; Moreno-Tost, R.; Santamaría, J.; Fierro, J. L. G. Appl. Catal. B. 2007, 73, 317-326. (9) Dossin, T. F.; Reyniers, M.-F.; Berger, R. J.; Marin., G. B. Appl. Catal., B 2006, 67, 136–148. (10) Xuejun, L.; Huayang, H.; Yujun, W.; Shenlin, Z. Catal. Commun. 2007, 8, 1107–1111.
KNO3/Al2O3,11 K2CO3/Al2O3,12 KF/Al2O3,13 Li/CaO,14 KF/ ZnO,15 and so on; basic hydrotalcites Mg-Al,16 Li-Al,17 and so on; anion-exchange resins;18 basic zeolites;19 A-B-O type metal oxides, where A is an alkaline-earth metal, alkaline metal, or rare earth metal and B is a transition metal;20 and solid Brønsted bases,21 the basic heterogeneous catalysts were not satisfactory in the industrial field to date. Because of the versatility of its technological applications, sodium aluminate (SA) is an important commercial inorganic chemical. It is formulated as NaAlO2, Na2O · Al2O3, or Na2Al2O4. SA is mainly in demand for use in effective water treatment systems. It is also used by producers of paper, paint pigments, alumina-containing catalysts, dishwasher detergents, ingot molds, and molecular sieves, concrete, and so on.22 It is generally believed that NaAlO2 is water soluble and shows strong basicity in water, insoluble in alcohol. To the best of our knowledge, there has been no published study yet using NaAlO2 as a heterogeneous catalyst to produce biodiesel from vegetable or animal oils. In this study, we (11) Wenlei, X.; Hong, P.; Ligong, C. Appl. Catal., A 2006, 300, 67– 74. (12) Alonso, D. M.; Mariscal, R.; Moreno-Tost, R.; Zafra Poves, M. D.; Granados, M. L. Catal. Commun. 2007, 8, 2080–2086. (13) Bo, X.; Guomin, X.; Lingfeng, C.; Ruiping, W.; Lijing, G. Energy Fuels 2007, 21, 3109–3112. (14) Watkins, R. S.; Lee, A. F.; Wilson, K. Green Chem. 2004, 6, 335– 340. (15) Wenlei, X.; Xiaoming, H. Catal. Lett. 2006, 107, 53–59. (16) Cantrell, D. G.; Gillie, L. J.; Lee, A. F.; Wilson, K. Appl. Catal., A 2005, 287, 183–190. (17) Corma, A.; Abd Hamid, S. B.; Iborra, S.; Velty, A. J. Catal. 2005, 234, 340–347. (18) Shibasaki-Kitakawa, N.; Honda, H.; Kuribayashi, H.; Toda, T.; Fukumura, T.; Yonemoto, T. Bioresour. Technol. 2007, 98 (2007)), 416– 421. (19) Leclercq, E.; Finiels, A; Moreau, C. JAOCS 2001, 78, 1161–1165. (20) Kawashima, A.; Matsubara, K.; Honda, K. Bioresour. Technol. 2008, 99, 3439–3443. (21) Yijun, L.; Loterob, E.; Goodwin, J. G., Jr,; Changqing, L. J. Catal. 2007, 246, 428–433. (22) Rayzman, V.; Filipovich, I.; Nisse, L.; Vlasenko, Y. JOM 1998, 50, 32–37.
10.1021/ef800904b CCC: $40.75 2009 American Chemical Society Published on Web 02/04/2009
1090 Energy & Fuels, Vol. 23, 2009
Wan et al.
Table 1. Operation Parameters for Spectro Genesis EOP ICP-OES RF generator power (W) frequency of RF generator (MHz) nebulizer type carrier gas flow rate (L min -1) coolant gas flow rate (L min -1) auxiliary gas flow rate (L min -1) netto time (sec) analytical wavelength (nm)
1400 27.12 crossflow 1 14 1 12 Na 589.592
investigated its catalytic activity and the durability of soybean oil to fatty acid methyl ester. 2. Experimental Section 2.1. Chemicals. Commercial edible-grade soybean oil was purchased from the supermarket and used without further purification. SA, methanol and n-hexane were obtained from Sinopharm Chemical Reagent Co., Ltd., (Shanghai, China). Lauric acid methyl ester, used as an internal standard, was purchased from SigmaAldrich Corporation (Missouri, U.S.A.). 2.2. Reaction Procedures. The reaction was conducted in a 250 mL one-neck flask equipped with a water-cooled condenser and a magnetic stirrer charged with 15 g of soybean oil, different volumes of methanol, and varied amounts of catalysts freshly prepared in different conditions. Each reaction was carried out at a methanol reflux temperature with vigorous stirring for the required time. We performed catalyst durability tests by repeating the transesterification reaction several times with used catalysts. Catalysts were separated from the previous reaction mixture by centrifugation and calcined at 600 °C. 2.3. Catalyst Characterization. The catalysts were characterized using several techniques of the Hammett indicator, inductively coupled plasma-optical emission spectroscopy (ICP-OES), X-ray diffraction (XRD), thermogravimetry/differential thermogravimetry (TG/DTG), and Fourier transform infrared spectroscopy (FT-IR). The Hammett indicator is used to determine the basic strength of the solid bases (H_). About 200 mg of sample was shaken with 10 mL methanol solution of the Hammett indicator and left for 1 h to achieve equilibration. The concentrations of 0.02 mol/L bromthymol blue (H_ ) 7.2), phenolphthalein (H_ ) 9.8), 2, 4-dinitroaniline (H_ ) 15.0), and 4-nitroaniline (H_ ) 18.4) were used for Hammett indicators. Methanol was used as a solvent. Sodium solubility in methanol was determined by a Spectro Genesis EOP ICP-OES (Keleve, Germany). The ICP-OESSpectrometer settings are shown in Table 1. The XRD measurements were performed on a XRD-6000 powder diffraction using Cu-KR radiation over a 2θ range of 10-80° with a step size of 0.02° at a scanning speed of 4°/min. The data were analyzed with the DiffracPlus software, and phases were identified according to the Powder Diffraction (PDF) database (JCPDS, International Centre for Diffraction Date). Thermal analysis of NaAlO2 was carried out on an AETARAM SETSYS 16TG/DTA/DSC apparatus operating under a flow of air at a 10 K/min heating rate up to 800 °C. Avatar 360 FT-IR was used for the infrared analysis of the solids with the KBr pellet technique. The range of scanning was from 400 cm-1 to 4000 cm-1. 2.4. Method of Analysis. The samples were analyzed with Varian-3900 gas chromatography with an automatic injection system. Data collection and analysis were performed with a Varian3900 workstation. A capillary column (HP5, 30m × 0.32 mm × 0.25 µm) was used for separation. Nitrogen was used for the carrier gas, and the split ratio was 10:1. Samples were prepared by dissolving about 15 mg of biodiesel sample into a 10 mL of n-hexane. About 5 mg of lauric acid methyl ester was added as a reference of crude biodiesel.23 Samples were placed in a gas (23) Yong, W.; Shiyi, O.; Pengzhan, L.; Feng, X.; Shuze, T. J. Mol. Catal. A. 2006, 252, 107–112.
Table 2. Basic Strengths, Solubility, and Catalytic Activities with Different Calcination Temperatures of NaAlO2 thermal treatmenta no drying under infrared light 200 °C 400 °C 600 °C 800 °C 600 °C used and recalcined at 600 °C
basic strength (H_)
solubilityb (µg · mL-1)
yieldc (%)
18 < H_ 15 < H_
