3740
Energy & Fuels 2007, 21, 3740–3743
Transesterification of Cottonseed Oil to Biodiesel by Using Heterogeneous Solid Basic Catalysts Cui Lingfeng, Xiao Guomin,* Xu Bo, and Teng Guangyuan School of Chemistry and Chemical Engineering, Southeast UniVersity, 211189 Nanjing, China ReceiVed July 15, 2007. ReVised Manuscript ReceiVed August 31, 2007
Alkyl esters of long chain fatty acid are called biodiesel. These esters can be obtained from the transesterification of triglycerides with methanol/ethanol. This work investigated the possibility of using KF/ γ-Al2O3 as heterogeneous catalysts for the transesterification of cottonseed oil with methanol. The operation variables used were methanol/oil molar ratio (6:1–18:1), catalyst concentration (1–5 wt %), temperature (50–68 °C), and catalyst type. The biodiesel with the best properties was obtained using a methanol/oil molar ratio of 12:1, catalyst (4%), and 65 °C temperature with the catalyst KF/γ-Al2O3. The strongest basic sites (superbasic) promote the transesterification reaction also at very low temperature (65 °C), while the basic sites of medium strength require higher temperatures to promote the same reaction.
1. Introduction The limited (and fast diminishing) resources of fossil fuels, increasing prices of crude oil, and environmental concerns have been the diverse reasons for exploring the use of vegetable oils as alternative fuels. However, their direct use has not been satisfactory because of their viscous nature and low ignition quality.1 Methyl/ethyl esters of fatty acids present in such oils have proved promising enough to be called biodiesel. Derived from renewable sources, they can be used without any modification in engine design. Also, they produce much lower levels of most of the pollutants and “potential or probable carcinogens”. Biodiesel is a nonpetroleum-based fuel that generally consists of fatty acid methyl esters (FAME) or fatty acid ethyl esters (FAEE), derived from the transesterification of triglycerides (TG) with methanol or ethanol, respectively.2 Complete conversion of the triglycerides involves three consecutive reactions with monoglyceride and diglyceride as intermediates.3 In the process of transesterification, two liquid phases are formed. The lower phase mainly consists of glycerol and some catalyst, * Corresponding author: Tel 13605197225; fax 025-52090612; e-mail
[email protected]. (1) Komers, K.; Stloukal, R.; Macheck, J.; Skopal, F.; Komersova, A. Biodiesel fuel from rapeseed oil, methanol, and KOH. Analytical methods in research and production. Fett/Lipid 1998, 100, 507. (2) Vicente, G.; Martinez, M.; Aracil, J. A Comparative Study of Vegetable Oils for Biodiesel Production in Spain. Energy Fuels 2006, 20, 394. (3) Karaosmanoglu, F.; Cigizoglu, K. B.; Tuter, M.; Ertekin, S. Investigation of the refining step of biodiesel production. Energy Fuels 1996, 10, 890. (4) Mariod, A.; Klupsch, S.; Hussein, I. H.; Ondruschka, B. Synthesis of Alkyl Esters from Three Unconventional Sudanese Oils for Their Use as Biodiesel. Energy Fuels 2006, 20, 2249. (5) Encinar, J.; Gonzalez, J. F.; Rodriguez-Reinares, A. Biodiesel from used frying oil. Variables affecting the yields and characteristics of the biodiesel. Ind. Eng. Chem. Res. 2005, 44, 5491. (6) Cerce, T.; Peter, S.; Weidner, E. Biodiesel-transesterification of Biological Oils with Liquid catalysts:thermodynamic properties of oilmethanol-amine mixtures. Ind. Eng. Chem. Res. 2005, 44, 9535. (7) Shah, S.; Sharma, S.; Gupta, M. N. Biodiesel preparation by lipasecatalyzed transesterification of jatropha oil. Energy Fuels 2004, 18, 154. (8) Leadbeater, N. E.; Stencel, L. M. Fast, easy preparation of biodiesel using microwave heating. Energy Fuels 2006, 20, 2281.
intermediate products, and may contain water and soap (from residual free fatty acids in the oil). Glycerol as a byproduct of the transesterification reaction has a number of applications in the pharmaceutical, cosmetics, food, and plastics industries but requires extensive washing and purification from the trace compounds. The upper phase mainly contains methyl ester, which after removing an excess of methanol and washing with water is used as biodiesel.4–8 The most commonly used technology for TG transesterification is based on the use of batch plants, in which a basic homogeneous catalyst is used (NaOH or NaOCH3), and at the end of the reaction, the catalyst is neutralized with acetic acid or a mineral acid. A continuous transesterification process would be a good opportunity for reducing the production costs.2 The use of homogeneous catalysts requires extensive conditioning and purification steps for the reaction products (methyl ester and glycerol) to separate the catalysts. In contrast, heterogeneous catalysts are easily removed from the reaction mixture, making the purification step easier.9 The cost of biodiesel could certainly be reduced through the use of a heterogeneous catalyst, instead of a homogeneous one, providing for higher quality esters and glycerol, which are more easily separated, and there is no need for further expensive refining operations.3 Many heterogeneous catalysts that are based on acid11 and basic9,10 solids have recently been proposed in the literature. In particular, the catalyst KF/γ-Al2O3 is a solid with interesting basic properties12 that has shown good activity in transesterification reactions. (9) Venkat Reddy, C. R.; Oshel, R.; verkade, J. G. Room-temperature conversion of soybean oil and poultry fat to biodiesel catalyzed by nanocrystalline calcium oxides. Energy Fuels 2006, 20, 1310. (10) 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, 3009. (11) Morterra, C.; Cerrato, G.; Pinna, F. Crystal Phase, Spectra Features, and Catalytic Activity of Sulfate-doped Zirconia Systems. J. Catal. 1995, 157, 109. (12) Ando, T.; Clark, J. H.; Cork, D. G. Fluoride-alumina reagents: the active species. Tetrahedron Lett. 1987, 28, 1421.
10.1021/ef700405e CCC: $37.00 2007 American Chemical Society Published on Web 10/20/2007
Transesterification of Cottonseed Oil to Biodiesel
Energy & Fuels, Vol. 21, No. 6, 2007 3741
Table 1. Fatty Acids Composition of the Used Cottonseed Oil Determined by Gas Chromatography (GC) Analysis fatty acid
content (%)
palmitic stearic oleic linoleic
22 2 14 62
In this paper, a study of the performances of KF/γ-Al2O3 in the transesterification of cottonseed oil with methanol is reported. 2. Experimental Section 2.1. Materials. A calcined KF/γ-Al2O3, with an KF/(γ-Al2O3) mass ratio of 0.20–0.50. The heterogeneous catalysts were prepared mixing two materials, A and solution B: A contained spherical γ-Al2O3, 50 g; solution B was prepared by dissolving KF, 50 mL. Spherical γ-Al2O3 was activated at 400 °C for 1 h, and then a prepared solution KF was added. The obtained mixture was aged for 24 h. After filtrating washing and drying at 100 °C for 8 h, the catalysts were obtained by calcination at 600 °C in air for 3 h. We have prepared five types of heterogeneous catalysts with different concentrations of KF (2, 4, 6, 8, and 10 M). The spherical γ-Al2O3 was obtained from Nanjing Chemical Industry Corp. Chemical Plant. The reagent KF was obtained from Guangdong Guanghua Chemical Factory Co., Ltd. Cottonseed oil was purchased in a food store in Henan province (the fatty acids composition of the cottonseed oil, as determined by gas chromatography (GC) analysis, is reported in Table 1). The acid number of cottonseed oil is 1.94 mg KOH/g. All other used reagents (when not specified) were supplied by Aldrich and used as received, without further purification. 2.2. Catalyst Characterization. The mass ratio of KF attached on spherical γ-Al2O3 has been measured. X-ray investigation of the solids was performed using a Japan D/max-5A mode diffractometer. The patterns were obtained using Cu KR radiation (λ ) 1.5418 Å) at 50 kV and 30 mA; the diffraction angle 2θ was scanned at a rate of 2°/min. The catalyst also has been tested by DSC-TGA. The samples were heated from 50 to 900 °C at a scanning rate of 15 °C/min under a nitrogen atmosphere with a flow rate of 15 mL/min. 2.3. Transesterification. The reaction of transesterification was carried out in a 250 mL spherical reactor, provided with thermostat, mechanical stirring, sampling outlet, and condensation systems. This installation was consistent with that described in the literature.13 The reactor was preheated to 65 °C, to eliminate moisture, and then 100 g of used cottonseed oil was added. When the reactor reached 65 °C again, the methanol and the catalyst were added, in the amounts established for each experiment, and the stirring system was connected, taking this moment as time zero of the reaction. Each experiment was prolonged for setting time, and thus the conversion to esters was practically complete. After cooling and filtrating, two phases were formed. The upper phase consisted of methyl esters, and the lower phase contained the glycerol and the excess methanol. The filtrated heterogeneous catalyst also can be used next time. After separation of the two layers by sedimentation, the methyl esters were purified by distilling the residual methanol at 65 °C. The resulting mixture was subjected to a distillation at 65 °C under a moderate vacuum (absolute pressure of 150 mmHg) to recover the excess methanol. 2.4. Analysis. The methyl ester content was assayed by gas chromatography in an HP 5890 chromatograph provided with a flame ionization detector, employing a silica capillary column of (13) Ma, F.; Clements, L. D.; Hanna, M. A. Biodiesel fuel from animal fat. Ancillary studies on transesterification of beef tallow. Ind. Eng. Chem. Res. 1998, 37, 3768.
Figure 1. XRD spectrum of catalyst KF/Al2O3: a, K3AlF6; b, Al2O3.
Figure 2. XRD spectrum of catalysts with different KF addition: 1, m(KF):m(Al2O3) ) 0.5036; 2, m(KF):m(Al2O3) ) 0.3714; a, K3AlF6; b, Al2O3.
15 m length and 0.32 mm inside diameter (phase: 5% phenylpolydimethylsiloxane).14 The following properties of the final best biodiesel product were determined: flash point, kinematic viscosity, acid number, cetane index, free glycerin, cloud point, total glycerin. The determined method is according to ASTM D6751-07a.15
3. Results and Discussion 3.1. Catalyst Characterization. In the X-ray diffraction (XRD) patterns of the catalyst only the presence of a K3AlF6like phase has been observed. As can be seen from Figure 1, the peak value of the Al2O3 phase is lower under calcination. Calcining at high temperature, KF and Al2O3 have had an effect and formed a new brilliant look.12 That is to say, calcining at high temperature has an important influence on the activation of the catalyst. Figure 2 shows the XRD spectrum of catalysts with different mass ratio. For higher mass ratio, the peak values of K3AlF6 and Al2O3 are higher. These differences in textural properties strongly affect the catalytic performances as will be seen in the next section. The DSC scans of Al2O3 and KF/Al2O3 are shown in Figures 3 and 4. In the DSC investigation of Al2O3, the weight losses are concentrated to temperature range of 100–800 °C. However, in the DSC investigation of KF/Al2O3, the weight losses are concentrated to three ranges (50–120, 230–300, and 500–580 (14) UNI 10946, 2001. (15) ASTM D6751, 2007.
3742 Energy & Fuels, Vol. 21, No. 6, 2007
Lingfeng et al.
Figure 5. Influence of mass ratio KF to Al2O3. Reaction conditions: T ) 65 °C; catalyst concentration ) 3%; contact time ) 3 h; methanol/ oil ) 12:1.
Figure 3. DSC curves for Al2O3.
Figure 4. DSC curves for KF/Al2O3. Table 2. Mass Ratio of KF to Al2O3 concentration of solution KF (mol/L)
mass ratio of KF to Al2O3 (%)
2 4 6 8 10
20.21 25.71 48.01 50.36 37.14
°C). At the temperature of 800 °C, phase transformation had taken place as can be seen in the TGA investigation in Figure 4. Table 2 shows the influence of concentration of solution KF on mass ratio of KF to Al2O3. As the concentration increases, the mass ratio is improved. However, the best result was obtained with a concentration of 8 M. For higher concentration the mass ratio is lower, as can be seen in Table 2. We also have to consider the influence of porosity. As the concentration of solution KF increases, the porosity might have been damaged and, therefore, makes the mass ratio lower. 3.2. Transesterification Reaction. Figure 5 shows the influence of molar ratio of KF to Al2O3 on ester yields. As the mass ratio increased, the ester yields also increased. As can be seen from the figure, the higher the mass ratio, the better the result is. The activity of the catalyst may concen about the mass ratio. As the mass ratio increases, the activity of catalyst increases. It is interesting to observe that the yield increases as the mass ratio increases, in agreement with previous XRD spectrum results. Figure 6 shows the influence of methanol/oil molar ratio on ester yields. The methanol/oil molar ratio is one of the most
Figure 6. Influence of methanol/oil molar ratio. Reaction conditions: T ) 65 °C; catalyst concentration ) 3%; contact time ) 3 h; catalyst mass ratio ) 50.36%.
important variables affecting the ester yield. The stoichiometric ratio for transesterification requires 3 mol of methanol and 1 mol of triglyceride. Since this is an equilibrium reaction, an excess of methanol will increase the ester conversion by shifting this equilibrium to the right. The ester yields increased as the percentage of methanol increased, with the best results being for a molar ratio of 12:1. For methanol/oil molar ratio less than 12:1 the reaction was incomplete, and at 15:1 methanol/oil molar ratio the separation of glycerol was difficult, since the excess methanol hindered the decantation by gravity so that the apparent yield of esters decreased because part of the glycerol remained in the biodiesel phase. Hence, the best results were obtained for an intermediate methanol/oil molar ratio of 12:1. In Figure 7, the percentages of esters obtained are plotted versus concentration for these five experiments. As can be observed, the best results were reached with a concentration of 4.0%. For higher values the yields were lower. This fact, as has been indicated, seems to be related to the free acidity of the oil. When there is a large free fatty acid content, the addition of more catalyst compensates for this acidity and avoids catalyst deactivation. The addition of an excessive amount of catalyst, however, gives rise to the formation of an emulsion, which increases the viscosity and leads to the formation of gels. These hinder the glycerol separation and, hence, reduce the apparent ester yield. The result of these two opposing effects is an optimal catalyst concentration that, in this case, is 4.0% KF/γ-Al2O3.
Transesterification of Cottonseed Oil to Biodiesel
Figure 7. Influence of catalyst concentration. Reaction conditions: T ) 65 °C; methanol/oil ) 12:1; contact time ) 3 h; catalyst mass ratio ) 50.36%.
Energy & Fuels, Vol. 21, No. 6, 2007 3743
Figure 9. Influence of contact time. Reaction conditions: T ) 65 °C; methanol/oil ) 12:1; catalyst concentration ) 3%; catalyst mass ratio ) 50.36%. Table 3. Comparison of the Biodiesel (T ) 65 °C; Catalyst Concentration ) 3%; Contact Time ) 3 h; Catalyst Mass Ratio ) 50.36%) and ASTM D6751-07a
Figure 8. Influence of temperature. Reaction conditions: methanol/oil ) 12:1; contact time ) 3 h; catalyst concentration ) 3%; catalyst mass ratio ) 50.36%.
As has been indicated, temperature was varied between 50 and 68 °C. For the same final reaction time, the percentage of esters increased with temperature. Figure 8 shows the influence of temperature on ester conversion. Hence, there was an initial period during which the reaction was very fast and then a second period, much longer than the first, in which the composition evolved slowly toward equilibrium. Therefore, the rate of reaction was strongly influenced by the reaction temperature. The equilibrium concentration was strongly conditioned by the temperature; that is, the equilibrium concentration increased as the temperature increased. In Figure 9, the influence of contact time has also been studied. As can be seen from the figure, 3 h is the proper contact time. For longer time the yields increase slowly. Given enough time, the reaction can be complete, but it is not advisable. Table 3 lists the biodiesel parameters obtained in run 4 and the ASTM D6751-07a. As can be observed, there is a good approximation of the biodiesel we have obtained to the
parameter
this work
biodiesel standard
units
flash point water kinematic viscosity, 40 °C cetane number free glycerin total glycerin acid number cloud point sulfur
131 0.04 4.34 48 0.13 0.18 0.30 -5 0.00012
93 0.05 1.9–6.0 47 0.2 0.24 0.50 -19 0.05
°C min % vol max mm2/s min % mass max % mass max mg KOH/g max °C % mass max
requirements of standard. The greatest differences are in the flash and cloud points, which were lower in standard. By contrast, the cetane number is higher than the standard. As can be seen from Table 3, some characteristics, such as the flash point and the lower sulfur content, are very much in accord with the standard from the viewpoint of the handling safety and the emission of contaminant gases during combustion. 4. Conclusions In the prepared catalyst KF/γ-Al2O3, a new K3AlF6-like phase had been observed. The biodiesel with the best properties was obtained using a methanol/oil molar ratio of 12:1, catalyst concentration (4%), contact time (3 h), and 65 °C temperature with the catalyst KF/ γ-Al2O3 (mass ratio 50.36%). Nevertheless, the transesterification progress could also be carried out in a fixed bed, which could be very interesting for industrial-scale production to achieve a continuous one. Acknowledgment. This work was supported by the Program for Hi-Tech Research of Jiangsu Province (No. BG 2006034). EF700405E
