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Energy & Fuels 2009, 23, 1786–1789
Transesterification of Canola Oil to Biodiesel Using MgO Loaded with KOH as a Heterogeneous Catalyst† Oguzhan Ilgen and A. Nilgun Akin* Chemical Engineering Department, and AlternatiVe Fuels Research and DeVelopment Center, Kocaeli UniVersity, 41040, Kocaeli, Turkey ReceiVed May 13, 2008. ReVised Manuscript ReceiVed July 3, 2008
In this study, transesterification of canola oil with methanol has been studied in a heterogeneous system, using MgO-supported KOH catalysts. All of the catalysts were prepared by incipient-wetness impregnation of an aqueous solution of KOH on MgO support. Effects of the methanol/canola oil molar ratio, reaction temperature, loadings of KOH, and reaction time in biodiesel production were investigated. The catalyst with 20 wt % KOH loaded on MgO gave the highest basicity and the best catalytic activity for this reaction. The highest conversion of canola oil of 99.36% and the highest FAME yield of 95.05% were obtained.
1. Introduction Biodiesel is defined by the European Parliament in Directive 2003/30/EC as a “methyl-ester produced from vegetable or animal oil, of diesel quality, to be used as biofuel”. Biodiesel has been introduced as an alternative fuel of wide acceptance because of its higher cetane number and lubricity and its lower environmental impact as compared to diesel petroleum fuels. Replacing petroleum-derived diesel fuel with biodiesel reduces the life-cycle of global warming gas emissions by 45%.1,2 However, the cost of producing biodiesel is greater than the cost of producing petroleum-derived diesel. Biodiesel is usually produced by the transesterification of vegetable oils or animal fats with methanol or ethanol in the presence of a homogeneous catalyst, which is NaOH or KOH dissolved in methanol, a corrosive liquid.3 The overall reaction can be represented as
Because each vegetable oil has a specific composition, its properties differ as well, resulting in different biodiesel characteristics. Canola oil is mainly made of oleic acid, which contains only one unsaturation in its 18 carbon chain. Therefore, it is a feedstock that results in a biodiesel that has excellent cold-flow properties.4 This is the main advantage of canola for † From the Conference on Fuels and Combustion in Engines. * To whom correspondence should be addressed. Telephone: +90-2623350123. Fax: +90-262-3355241. E-mail:
[email protected]. (1) MacLeod, C. S.; Harvey, A. P.; Lee, A. F.; Wilson, K. Chem. Eng. J. 2008, 135, 63–70. (2) Ma, F.; Hanna, M. A. Bioresour. Technol. 1999, 70, 1–15. (3) Huaping, Z.; Zongbin, W.; Yuanxiong, C.; Ping, Z.; Shijie, D.; Xiaohua, L.; Zongqiang, M. Chin. J. Catal. 2006, 27 (5), 391–396. (4) Dossin, T. Kinetics and reactor modelling of MgO-catalysed transesterification for sustainable development. Ph.D. Dissertation, Vrije Universiteit, Brussel, Belgium, 2006; pp 13-14.
biodiesel production. Also, canola is widely grown in Europe and is the most common existing vegetable oil feedstock, with 42.5% average oil content, and it offers superior fuel properties.2 Homogeneous catalytic systems have many drawbacks. Removal of these catalysts to purify biodiesel fuel and glycerol as a byproduct is difficult and requires a large amount of water. Consequently, a considerable amount of wastewater is inevitably produced. To overcome these problems, the transesterification over environmentally benign solid catalysts is a promising route. Heterogeneous catalysts could be easily separated from the reaction mixture by filtration and then reused. Also, they are less corrosive, leading to safer, cheaper, and more environmentfriendly operations. However, biodiesel yields obtained using heterogeneous catalysts need to be improved. Therefore, new catalytic materials are extremely desirable for the development of an environmentally benign process as well as for simplifications in the existing processes associated with homogeneous catalysts.5,6 For this reason, many heterogeneous basic catalysts, such as KNO3/Al2O3,7 Na/NaOH/γ-Al2O3,8 calcined Mg-Al hydrotalcites,9,10 zeolite and metal catalysts,11,12 Eu2O3Al2O3,13 and Li/CaO14,15 have been developed to promote the transesterification of vegetable oils. (5) Jitputti, J.; Kitiyanan, B.; Rangsunvigit, P.; Bunyakiat, K.; Attanatho, L.; Jenvanitpanjakul, P. Chem. Eng. J. 2006, 116, 61–66. (6) Pinto, A. C.; Guarieiro, L. L. N.; Rezendea, M. J.C.; Ribeiroa, N. M.; Torresb, E. A.; Lopesc, W. A.; Pereirac, P. A. P.; Andrade, J. B. J. Braz. Chem. Soc. 2005, 16, 1313–1330. (7) Xie, W.; Peng, H.; Chen, L. Appl. Catal., A 2006, 300, 67–74. (8) Kim, H.; Kang, B.; Kim, M.; Park, Y.; Kim, D.; Lee, J.; Lee, K. Catal. Today 2004, 93, 315–320. (9) Xie, W.; Peng, H.; Chen, L. J. Mol. Catal. A: Chem. 2005, 246, 24–32. (10) Ilgen, O.; Dincer, I.; Yildiz, M.; Alptekin, E.; Boz, N.; Canakci, M.; Akin, A. N. Turk. J. Chem. 2007, 31, 509–514. (11) Suppes, G. J.; Dasari, M. A.; Doskocil, E. J.; Mankidy, P. J.; Goff, M. J. Appl. Catal., A 2004, 257, 213–223. (12) Xie, W.; Huang, X.; Li, H. Bioresour. Technol. 2007, 98, 936– 939. (13) Li, X.; Lu, G.; Guo, Y.; Guo, Y.; Wang, Y.; Zhang, Z.; Liu, X.; Wang, Y. Catal. Commun. 2007, 8, 1969–1972. (14) Watkins, R. S.; Lee, A. F.; Wilson, K. Green Chem. 2004, 6, 335– 340. (15) Meher, L. C.; Kulkarni., M. G.; Dalai., A. K.; Naik., S. N. Eur. J. Lipid Sci. Technol. 2006, 108, 389–397.
10.1021/ef800345u CCC: $40.75 2009 American Chemical Society Published on Web 08/08/2008
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In this study, transesterification of canola oil with methanol has been studied in a heterogeneous system, using MgOsupported KOH catalysts. Effects of the methanol/canola oil molar ratio, reaction temperature, loadings of KOH, and reaction time in biodiesel production were investigated. The catalysts were also characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), and Hammett titration techniques. 2. Experimental Section 2.1. Catalyst Preparation and Characterization. The catalysts were prepared by incipient wetness impregnation of magnesia (Merck, 105862) with an aqueous solution of KOH (Merck, 105012). The catalysts were dried at 393 K for 16 h and calcined at 773 K for 5 h. The basic strength of the solid bases (H_) was determined using Hammett indicators. About 50 mg of sample was shaken with an appropriate volume of a methanol solution of Hammett indicator and left to equilibrate for 2 h, after which no further color changes were observed. In these experiments, the following Hammett indicators were used: dimethylaminoazobenzene (H_ ) 3.3), phenolphthalein (H_ ) 8.2), 2,4-dinitroaniline (H_ ) 15), and nitroaniline (H_ ) 18.4). To measure the basicity of the catalysts, the method of Hammett indicator-benzene carboxylic acid titration was used.16 X-ray powder diffraction patterns were obtained from calcined catalyst samples with a Rigaku D/MAXUltima+/PC X-ray diffractometer using Cu KR radiation at 40 kV and 40 mA and a scan speed of 2 °C/min. The surface morphology of the prepared catalysts was investigated by a scanning electron microscope (JEOL 6335F SEM). 2.2. Transesterification Reaction. Commercial edible-grade canola oil was obtained from the market. A 250 cm3 three-necked glass flask with a water-cooled condenser was charged with 50 g of canola oil, different volumes of anhydrous alcohol, and varied amounts of catalyst. The mixture was refluxed for the required temperature and 9 h of reaction time under stirring at 1000 rpm. After the reaction, the solid catalyst was separated by filtration. The liquid was put into a separating funnel and kept at ambient temperature for 24 h, after which two liquid phases appeared. The upper layer was biodiesel, and the lower layer was glycerol. The conversion of canola oil was calculated from the quantity of total glycerol in the product. The procedure was carried out according to the American Oil Chemists’ Society (AOCS) Official Method Ca 14-56. The content in fatty acid methyl esters (FAME) of the upper layer was determined by following the European regulated procedure EN 14103. Basically, 250 mg of the organic layer was added to 5 mL of a heptane solution of the internal standard methylheptadecanoate. The weight content in FAME of the organic layer was considered to represent the wt % yield in FAME of the catalytic reaction. The FAME content (wt %) was calculated from the formula
wt % )
(∑ A - A ) ⁄ (A i
MH
MH)((CMHVMH100)/W)
where ∑Ai is the total peak area from methyl ester, from methyl miristate (C14) to methyl nervonate (C24:1), AMH the area of methyl heptadecanoate, which response factor is equal to those of FAME, CMH the concentration in mg/mL of the methyl heptadecanoate (10 mg/mL), VMH is the volume in milliliters of the methyl heptadecanoate solution (5 mL), and W is the weigh in milligrams of the sample (250 mg). This solution was analyzed in an Agilent 6890 GC with a HP INNOwax capillary column.
3. Results and Discussion 3.1. Catalyst Characterization. The base strengths of all of the catalysts were measured by using Hammett indicators. (16) Xie, W.; Huang, X. Catal. Lett. 2006, 107, 53–59.
Figure 1. Basicity of KOH/MgO samples with different loadings of KOH.
Figure 2. XRD patterns of the prepared catalysts.
The addition of KOH to MgO having a base strength of H ) 7.2-9.8 increased basicity of all catalysts. As shown in Figure 1, catalyst basicity (total basicity) was increased and came up to a maximum value at a loading of 20 wt % KOH by increasing the amount of KOH from 10 to 20 wt %. The diffraction spectra of the catalysts prepared are shown in Figure 2. From Figure 2, it can be seen that there are typical characteristic peaks both at 37.0°, 43.0°, and 62.3°, which can be signed to the characteristic peaks of MgO crystalline.17 The XRD analysis of the 10% KOH/MgO sample shows the crystalline phase corresponding to magnesia. However, the presence of K2CO3 as well as magnesia was observed for 20 and 30 wt % KOH/MgO catalysts. The presence of K2CO3 shows an interaction between KOH and MgO. As reported in the literature,18,19 K addition generates an interaction between K and Mg in MgO; this interaction may weaken the Mg-O bonds, thus facilitating the migration of the O2- species that may be able to react with CO2 in air during the calcination step. (17) Duan, G.; Yang, X.; Chen, J.; Huang, G.; Lu, L.; Wang, X. Powder Technol. 2007, 172, 27–29. (18) Jimenez, R.; Garcia, X.; Cellier, C.; Ruiz, P.; Gordon, A. L. Appl. Catal., A 2006, 297, 125–134. (19) Jimenez, R.; Garcia, X.; Cellier, C.; Ruiz, P.; Gordon, A. L. Appl. Catal., A 2006, 314, 81–88.
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Figure 4. Conversion of canola oil and FAME yield as a function of the methanol/canola oil molar ratio. Reaction conditions: reaction time, 9 h; catalyst amount, 3 wt %; temperature, 333 K.
Figure 3. SEM images of catalysts.
This interaction is the main difference between the homogeneous KOH catalyst and heterogeneous KOH/MgO catalyst. Small crystallite of KOH was also observed at 30 wt % KOH/MgO catalyst. Figure 3 shows the SEM images of the catalysts prepared. As seen in the figure, at higher KOH loadings, dispersion of K2CO3 species evidenced by XRD on magnesia support was much higher. However, agglomerates of amorphous potassium species at small amounts were observed at 10% KOH/MgO catalyst. 3.2. Transesterification Reaction. Effects of the methanol/ canola oil molar ratio, reaction temperature, loadings of KOH, reaction time, and amount of catalyst in biodiesel production were investigated. The molar ratio of methanol to canola oil is one of the important factors that affect the conversion to methyl esters. Stoichiometrically, 3 mol of methanol is required for each mole of triglyceride, but in practice, a higher molar ratio is employed to drive the reaction toward completion and produce more methyl esters as products. The dependence of conversions of canola oil and yields of esters on the methanol/canola oil molar ratio was studied at four different ratios higher than the stoichiometrical ratio in the presence of 10 wt % KOH/MgO catalyst. Figure 4 reflects the effect of methanol/oil molar ratios on the conversion and FAME yield. As shown in this figure, the highest FAME yield was obtained in the reaction carried out at a 15:1 methanol/canola oil molar ratio at 333 K and the highest conversion of canola oil was obtained in the reaction carried out at a 12:1 methanol/canola oil molar ratio at the same temperature. Similar results were observed in the literature for different heterogeneous catalysts. Xie et al.7 studied KNO3/Al2O3 as a solid base catalyst to catalyze the transesterification of soybean oil to biodiesel and found that the maximum conversion was obtained when the molar ratio was very close to 15:1. Yan et
Figure 5. Conversion of canola oil and FAME yield as a function of the reaction temperature. Reaction conditions: methanol/oil molar ratio, 6:1; catalyst amount, 3 wt %; reaction time, 9 h.
al.20 studied CaO/MgO as a solid base catalyst to catalyze the transesterification of rapeseed oil to biodiesel and found that both molar ratios of 3:1 and 6:1 gave poor conversions in heterogeneous catalysis and a higher molar ratio of methanol/ oil resulted in conversions higher than 90%. It was reported that excess methanol can promote the transesterification reaction forward and also extract products, such as glycerin and methyl esters, from the system to renew the surface of the catalyst. To remove excess methanol usage because of environmental and economical reasons, the minimum possible amount of the methanol/oil ratio (6:1) was used in the rest of this study to obtain higher FAME yields close to the ones obtained at a 15:1 alcohol/oil ratio. The reaction temperature can influence the reaction rate and the biodiesel yield because the intrinsic rate constants are strong functions of temperature.21 Commonly, the methanolysis is conducted close to the boiling point of methanol at atmospheric pressure. In this study, the effects of the reaction temperature on the conversions of canola oil and yields of esters were investigated at three different reaction temperatures, such as room temperature, boiling point temperature of methanol, and 333 K (slightly less than boiling temperature). The catalyst used in this experiment was 20 wt % KOH/MgO. The results are shown in Figure 5. The conversion of canola oil and FAME yield increased with the rise of the reaction temperature from room temperature to the boiling point (338 K) of methanol. The conversion of canola oil and FAME yield at 338 K was obtained as 96.18 and 95.05%, respectively. To study effect of KOH loadings of catalysts on their catalytic activities, a series of KOH/MgO samples with loadings of KOH (20) Yan, S.; Lu, H.; Liang, B. Energy Fuels 2008, 22, 646–651. (21) Liu, X.; Piao, X.; Wang, Y.; Zhu, S. Energy Fuels 2008, 22, 1313– 1317.
Transesterification of Canola Oil to Biodiesel
Figure 6. Influence of loadings of KOH on the conversion and FAME yield. Reaction conditions: methanol/oil molar ratio, 6:1; catalyst amount, 3 wt %; reaction time, 9 h; temperature, 333 K.
Figure 7. Conversion of canola oil and FAME yield as a function of the reaction time. Reaction conditions: methanol/oil molar ratio, 6:1; catalyst amount, 3 wt %; temperature, 338 K.
ranging from 10 to 30 wt % were prepared and tested in the transesterification reaction. The results are shown in Figure 6. As loadings of KOH were raised from 10 to 20 wt %, the conversion of canola oil was increased. However, by increasing the amount of KOH from 20 to 30 wt %, a significant change in conversion was not observed. The highest FAME yield of 94.32% was obtained at loadings of KOH of 30 wt % on MgO. It is obvious that the change in the activity of the catalysts was well-correlated to the change of their basicity as shown in Figure 1. This result is also in agreement with the results obtained by D’Cruz et al.22 In their study, 1.25 wt % K/MgO catalyst was used for the transesterification of canola oil with methanol and they found a very poor ester yield with this catalyst containing a very low K content compared to the catalyst in this study. The dependence of conversions of canola oil and yields of esters on the reaction time was studied in the presence of 20 wt % KOH/MgO catalyst at reflux of methanol. The reaction time was varied in the range of a 1-9 h period, including five different reaction times performed at different batches. Figure 7 reflects the effect of the reaction time on the conversion and FAME yield. As shown in this figure, conversion increased in the reaction time range between 1 and 7 h. However, an increasing reaction time to 9 h caused a slight decrease in conversion. Although the FAME yield decreased in the reaction time range between 3 and 5 h, after 5 h of reaction time, the FAME yield of reaction was increased. The transesterification process consists of a sequence of three consecutive and reversible reactions transforming the triglyceride into a diglyceride, following a monoglyceride, and finally a glycerin, where FAME is produced from each step. These results indicate that monoglycerides are poorly stable intermediates under these (22) D’Cruz, A.; Kulkarni, M. G.; Meher, L. C.; Dalai, A. K. J. Am. Oil Chem. Soc. 2007, 84, 937–943.
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Figure 8. Influence of the catalyst amount on the FAME yield. Reaction conditions: methanol/oil molar ratio, 6:1; reaction time, 9 h; temperature, 338 K.
reaction conditions and, once formed, are not easily converted to glycerol and methyl ester. The highest conversion of canola oil of 99.36% was obtained after 7 h of reaction time, and the highest FAME yield of 95.05% was obtained after 9 h of reaction time. The effect of the amount of 20 wt % KOH/MgO catalyst on the FAME yield was investigated at a 6:1 molar ratio of methanol/canola oil at reflux of methanol for 9 h. The catalyst amount was varied in the range of 1.5-6 wt %, which was referenced to the starting oil weight. As shown in Figure 8, the FAME yield was increased first with the increase of the catalyst amount from 1.5 to 3%. However, with a further increase in the catalyst amount, the FAME yield was decreased, which was possibly due to a mixing problem involving reactants, products, and solid catalyst. When the amount of catalyst was increased, the slurry (mixture of catalyst and reactants) became too viscous, giving rise to a problem of mixing. In addition, when the catalyst loading amount was not enough, the maximum production yield could not be reached. To avoid this kind of problem, an optimum amount of catalyst loading had to be investigated. This phenomenon has also been found by some other researchers.7-9 The reuse of the catalyst was also investigated for 20 wt % KOH/MgO catalyst, which gives the highest FAME yield of 95.05% with the optimum reaction conditions. The catalyst was separated by filtration after the reaction and then reused in the following reaction. It was shown that the reaction catalyzed by a used catalyst provided only 26.45% FAME yield, which was much lower than the FAME yield over the fresh catalyst. This indicated that the spent catalyst was significantly deactivated. This low activity may be explained by dissolution of K species in the heterogeneous KOH/MgO catalyst.23 4. Conclusions The catalyst with 20 wt % KOH loaded on MgO was found to be the optimum catalyst, which gave the highest basicity and the best catalytic activity for the reaction conditions studied. When the reaction was carried out at reflux of methanol, with a molar ratio of methanol/canola oil of 6:1, and a catalyst amount 3%, the highest conversion of canola oil of 99.36% was obtained after 7 h of reaction time and the highest FAME yield of 95.05% was obtained after 9 h of reaction time. Acknowledgment. This study has been supported by The Turkish Scientific and Research Council (TUBITAK) through project 106M041 and Kocaeli University through project BAP-2007/54. EF800345U (23) Arzamendi, G.; Arguinarena, E.; Campo, I.; Zabala, S.; Gandia, L. M. Catal. Today 2008, 133 (135), 305–313.
