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Ind. Eng. Chem. Res. 2009, 48, 9350–9353
Solid Acid Catalyst for Biodiesel Production from Waste Used Cooking Oils Cholada Komintarachat*,† and Sathaporn Chuepeng‡ Fundamental Science and Physical Education, Faculty of Resources and EnVironment, Kasetsart UniVersity, Chonburi 20230, Thailand, and Mechanical Engineering, Faculty of Engineering at Si Racha, Kasetsart UniVersity, Chonburi 20230, Thailand
This paper presents a study of the methanol based transesterification of waste used cooking oil (WCO, 15% free fatty acid content) with the synthesized solid acid catalyst at different temperatures and reaction times. The optimized conditions for fatty acid methyl ester (FAME) yield were investigated using methanol/WCO weight ratios of 0.15-0.35 and catalyst/WCO ratios of 0.25-1.25%. The results show that the methanol/ WCO ratio of 0.3 and the catalyst/WCO ratio of 1.0% give the highest yield of 97.5% FAME at 383 K in 2 h. In addition, the capacity of porous supports for WOx synthesized catalysts, based on FAME yield was in the following order: Al2O3 > SiO2 > SnO2 > ZnO. The WOx/Al2O3 catalyst was found to be readily reusable and exhibited consistency in activity upon reuse for three times without loss of selectivity. 1. Introduction Waste used cooking oils (WCO) and fats have posed drastic problems related to disposal for decades. However, they have been found to be unsuitable for feeding animals as they may be harmful for animal products consumed by humans. A possibility for the disposal of these products is transforming them into fuels for transport or other usages. Conversion of WCO and fat to esters, commonly known as biodiesel, has many environmental advantages over petroleum based diesel fuel. Fermentation,1 transesterification,2 and pyrolysis3 of biomass,4 in the forms of industrial and domestic wastes, have been proposed as alternative solutions for increased energy demand and environmental awareness. Among these approaches, transesterification seems to be a readily practical and efficient method for fuel production. Transesterification is the chemical process between triglycerides and short-chain alcohol in the presence of basic or acidic catalyst to produce monoester5 as depicted in Scheme 1. The long- and branched-chain triglyceride molecules are transformed to monoesters and glycerol. Commonly used short-chain alcohols are methanol, ethanol, propanol, and butanol.6 Methanol is commercially used due to its availability and cost effectiveness.7 The subsequent useful product is fatty acid methyl ester (FAME) commonly known as biodiesel. WCO is considered to be advantageous in cost but it contains high free fatty acid (FFA) and water contents. The use of base type catalyst will cause a saponification reaction resulting in unwanted soap problems on removal (see Scheme 2).8 In addition, the maximum content of 0.05% water in the fuel is imposed in the FAME standard (e.g., EN 14214-2003) as it can lead to engine corrosion9,10 and the hydrolysis of the esters.11 Also, FFA must be converted into their corresponding esters8 as in the reaction shown in Scheme 3. Alternatively, the application of acid catalyst in transesterification is a two-step process, i.e. (1) acid esterification to reduce FFA and (2) alkali transesterification.12 To eliminate the corrosion, environmental problems, and time saving for multiple reactions, solid acid * To whom correspondence should be addressed. E-mail: k-cholada@ src.ku.ac.th. Tel.: +6638 354587 ext. 2754. Fax: +6638 354587. † Fundamental Science and Physical Education, Faculty of Resources and Environment, Kasetsart University. ‡ Mechanical Engineering, Faculty of Engineering at Si Racha, Kasetsart University.
catalysts have recently replaced liquid acids for biodiesel production by simultaneous esterification and transesterification of high FFA-containing oil on a supported heteropolyacid catalyst system.13 The main aim of this research work is to compare activities of the synthesized catalysts over esterification and transesterification of WCO for biodiesel preparation. The development presented in this paper associates synthesizing solid acid catalyst in order to enhance acidity of the catalyst support, using tungsten solution. This has been accomplished for a retrofit of the waste used cooking oils at given FFA levels for biodiesel production. The parameters affected such as weight WCO to methanol ratio, catalyst amount, reaction temperature, time, and reusability of the synthesized catalyst will also be investigated. 2. Materials and Methods 2.1. Preparation of the Catalysts.14,15 In the synthesis of the catalysts, the following reagents were used: silica (SiO2), zinc oxide (ZnO), tin oxide (SnO2), hydrochloric acid (HCl), and dry sodium sulfate (Na2SO4) (>99%). All were supplied by Fluka. The aluminum oxide (γ-Al2O3) and ammonium metatungstate were obtained from Sigma-Aldrich. Catalysts, based on tungsten impregnation in dry porous support material, were prepared by pore filling in the presence of a solution excess (wet impregnation). Simple wet impregnation was carried out by dissolving a certain amount of ammonium metatungstate 25% w/v in 20 mL distilled water in a round-bottomed flask, followed by the addition of the porous support material (γ-Al2O3, SiO2, Scheme 1. Transesterification of Triglyceride with Alkyl Alcohol
Scheme 2. Esterification of Free Fatty Acid
10.1021/ie901175d CCC: $40.75 2009 American Chemical Society Published on Web 09/18/2009
Ind. Eng. Chem. Res., Vol. 48, No. 20, 2009 Scheme 3. Saponification of Free Fatty Acid Alkyl Ester
Table 2. Effect of Porous Supporters of Catalyst
Table 1. Properties of Porous Materials entry
porous support
pore volume (cm3/g)
surface area (m2/g)
1 2 3 4
Al2O3 SiO2 ZnO SnO2
0.95 0.75 0.2 0.3
260 250 40 112
ZnO, and SnO2). Characteristics of porous support materials such as surface area and pore volume are presented in Table 1. The pH value of the contents was maintained at 2 by HCl addition. The resulting porous support materials were filtered, and the catalyst samples were dried in ambient air at 393 K overnight and treated in ambient air at 723 K for 4 h. 2.2. Pretreatment of the Waste Used Cooking Oil. WCO were collected from the author’s university canteen. The sampled WCO, comprised of free water and other impurities, were prepared by heating at 378 K to remove water and dried over Na2SO4. The subsequent solution was filtered under vacuum to remove traces of any suspended matter and Na2SO4 crystals. Its FFA content was determined by a standard titrimetry method instead of gas chromatography analysis.16 The treated WCO presented an FFA level of 15% w/w. 2.3. Procedure for the Biodiesel Synthesis. To synthesis biodiesel through the transesterification: the treated WCO, methanol (chromatographic grade, 99.5%), and the catalyst were mixed together in a two-necked round-bottom flask equipped with a magnetic stirrer and a thermometer. The mixture was heated to the desired temperature for a certain period. Upon completion, the excess methanol was distilled under vacuum. After the mixture was centrifuged, it formed three phases, i.e. FAME in the upper layer, glycerol in the middle layer, and traces of catalyst and glycerol in the lower layer. The catalyst entrained in the lower layer can be recovered by filtering and washing with petroleum ether and drying at 333 K for 4 h. 2.4. Purification of the FAME Phase. The glycerol-rich phase was separated from the FAME layer in a separating funnel. The latter phase was washed with warm water three times and with water to provide a purified FAME, then it was dried over Na2SO4 and filtered under vacuum. The quantitative analysis of the FAME was carried out using a temperature programmed JASCO (GC, polyethylene glycol) gas chromatograph. One microliter of the upper oil layer was dissolved in 10-mL n-hexane and 1-µL internal standard solution (heptadecanoic acid methyl ester) for GC analysis. On each run, the 1-µL sample was injected into the GC at the temperature of 140 °C. After holding for 5 min, the GC’s oven was heated at 10 °C/min rate to 250 °C. Helium was a carrier gas at 10 bar of pressure and a 20 mL/min flow rate. The injector and the flame ionization detector (FID) were set up to the operating temperature of 250 °C. The GC determined the mass concentration of the FAME, and the FAME yield was calculated by the following expression.17 FAME yield )
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mactual CesternVester × 100 ≈ × 100 mtheoretical moil (1)
where mactual and mtheoretical are the actual and theoretical masses of FAME in grams; moil is the mass of WCO in grams; Cester in
entry
catalyst
1 2 3 4
WOx/Al2O3 (WAl) WOx/SiO2 (WS) WOx/ZnO (WZn) WOx/SnO2 (WSn)
FAME yields (wt %)a acid value (1/mg KOH)b 98 92 84 91
4.7 5.6 4.4 5.8
a Reaction condition: WCO 10 g, 0.3 methanol/WCO weight ratio, catalyst 1.0% w/w, 383 K, 2 h. b Tritration the FAME at the end of 2 h, reaction time with 0.02 N of KOH.
Figure 1. Effect of methanol amount on the reaction at the following conditions: WCO 10 g, WAl 1.0% w/w, 383 K, and 2 h.
grams per milliliter is the FAME concentration acquired by GC; n is the dilution multiplication of FAME; and Vester is the volume of the FAME layer in milliliters. 3. Results and Discussion 3.1. Effect of Different Porous Supports of the Catalyst. The prepared catalysts were investigated for synthesizing biodiesel from the WCO under reaction conditions of 383 K temperature, 100 rpm stirring speed, 0.3 methanol/WCO weight ratio, and 1.0% w/w catalyst for WCO. On the basis of the yield of FAME and the acid value at the end of 2 h of reaction time, the best catalyst was WOx/Al2O3 (WAl) and the rest were in the following order: SiO2 > SnO2 > ZnO; the results are shown in Table 2. This is due to the fact that the γ-Al2O3 used as a porous support had a higher surface area (260 m2/g) and greater volume (0.95 cm3/g) compared to the others in Table 1. Large pores can easily accommodate a bulky triglyceride molecule,18 giving WAl large active sites and surface area, resulting in the highest activities (esterification and transesterification). The advantages of the pore filling method are mainly a short impregnation time and a smaller volume of impregnating solutions.14,19 Therefore, the WAl was selected to further study the effect of various reaction parameters on FAME yield. 3.2. Effect of Methanol to WCO Weight Ratio. The methanol to oil weight ratio is one of the important parameters that affect the yield of FAME. Theoretically, the transesterification of vegetable oil requires 3 mol of methanol per mole of triglyceride.16 Since transesterification of triglyceride is a reversible reaction, the excess methanol shifts the equilibrium toward the direction of ester formation.20 In the present work, with preoptimized reaction parameters, the methanol to WCO weight ratio was varied in the range of 0.15-0.35 and its influence on the FAME yield was investigated. Figure 1 shows the effect of the methanol to oil weight ratio on the FAME yield at the end of 2 h of reaction time at 383 K. It can be clearly seen from the activity profile that, when the weight ratio
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Figure 2. Effect of catalyst amount. Reaction conditions: WCO 10 g, WAl as catalyst, 0.3 methanol/WCO weight ratio, 383 K, and 2 h.
Figure 3. Effect of Reaction time. Reaction conditions: WCO 10 g, WAl 1.0% w/w, 0.3 methanol/WCO weight ratio, and 383 K.
increased from 0.15 to 0.35, the FAME yield was found to increase from 88% to 97.5%. The highest conversion of 97.5% was registered at the weight ratio of 0.3. Further increase in the methanol amount has not shown any significant improvement in the FAME yield. 3.3. Effect of Catalyst Amount. The catalyst amount is also important for the reaction that needs to be optimized in order increase the FAME yield. The effect of the WAl amount (0.25-1.25% w/w of WCO) on the reaction was studied, and the results are shown in Figure 2. At low catalyst amounts, there were not enough active sites for the reaction while the reverse reaction may take place when excess catalyst was employed. The optimal amount of the catalyst was found to be 1.0% w/w. 3.4. Effect of Reaction Time. The effect of reaction time on the yield was investigated, and the results are shown in Figure 3. Increasing the reaction time up to 2 h enhanced the FAME yield. Further increase in the reaction time did not show any increase in the FAME yield. This may be due to catalyst deactivation from the reaction in longer time that the change in catalyst structure and state occurs. These lead to the loss of active sites on the catalyst’s surface, resulting in the reduction of catalyst activity. Therefore, the 2-h reaction time was chosen for the optimum for the WAl catalyst. 3.5. Effect of Reaction Temperature. The rate of reaction is strongly influenced by reaction temperature. The effect of temperature on FAME production from WCO using WAl was experimentally studied at different temperatures, e.g. 363, 373, 383, and 393 K with initial WCO of 10 g, WAl concentration of 1.0% w/w, 0.3 methanol/WCO weight ratio, and 2-h reaction time. The FAME yields obtained from specified temperatures are shown in Figure 4. The maximum FAME yield of 97.5%
Figure 4. Effect of reaction temperature on the FAME yield. Conditions: WCO 10 g, WAl 1.0% w/w, 0.3 methanol/WCO weight ratio, and 2 h.
Figure 5. Reusability of WAl in FAME yield. Conditions: WCO 10 g, WAl 1.0% w/w, 0.3 methanol/WCO weight ratio, 383 K, and 2 h. Table 3. Catalyst Comparison entry
catalyst
FAME yields (wt %)a
1 2 3 4
WAl K2CO3 H2SO4 KOH
97.5 91.5 95.0 97.5
a
Reaction conditions: WCO 10 g, WAl 1.0% w/w, 0.3 methanol/ WCO weight ratio, 383 K, and 2 h.
was obtained at 383 K. The results show that during the course of transesterification, the FAME yield continuously increased up to 383 K while stabilizing afterward. 3.6. Catalyst Stability and Reusability. Reusability of the catalyst was also investigated as the cost of the process can be reduced. After the reaction completion, the solid acid catalyst was recovered by filtering, washing with petroleum ether, and drying at 333 K (petroleum ether’s boiling point) for 4 h. The stability studies were carried out at a reaction temperature of 383 K, 10 g WCO, 0.3 methanol/WCO weight ratio, and 1.0% w/w catalyst. The results shown in Figure 5 reveal that the FAME yield decreased was subtle even at the third reuse. However, significant losses in activity were observed at the fourth reuse and onward. 3.7. Catalyst Comparison. Catalytic activities of conventional catalysts, i.e. potassium hydroxide (KOH), potassium carbonate (K2CO3), and sulfuric acid (H2SO4), were compared with the WAl catalyst under the same condition; the data are shown in Table 3. It can be seen that the WAl and KOH catalysts show highest activity compared to the rest by yielding the maximum FAME. However, the base KOH catalyst leads to promoting soap formation
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which generates separating problems. Therefore, the WAl catalyst can be considered to be one of the best choices as being economically convenient, user- and environmentally friendly, and advantageous for industrial investment. 4. Conclusion A study of solid acid catalysts for biodiesel production based on methanol transesterification is presented in this paper. The synthesized tungsten on alumina supported catalyst (WAl) has been found to be suitable for the synthesis of biodiesel from waste used cooking oil with high FFA (15% w/w). Such operating parameters, i.e. methanol to WCO weight ratio, catalyst amount, reaction time, and temperature, were investigated as they are found to be affected the FAME yields. The catalyst was very efficient for the reaction with the maximum FAME yield of 97.5%. The catalyst stability was found to be reusable with consistency in activity up to three times. This catalyst can be applied in the academic and industrial scale production of biodiesel. Acknowledgment The authors acknowledge Kasetsart University for financial support and thank the technical staff in the Laboratory of Organic Chemistry of the Faculty of Resources and Environment for their experimental assistance. Literature Cited (1) Akoh, C. C.; Chang, S. W.; Lee, G. C.; Shaw, J. F. Enzymatic Approach to Biodiesel Production. J. Agric. Food Chem. 2007, 55, 8995. (2) Al-Widyan, M. I.; Al-Shyoukh, A. O. Experimental evaluation of the transesterification of waste palm oil into biodiesel. Bioresour. Technol. 2002, 85, 253. (3) Lima, D. G.; Soares, V. C. D.; Ribeiro, E. B.; Carvalho, D. A.; Cardoso, E. C. V.; Rassi, F. C. Diesel-like fuel obtained by pyrolysisof vegetable oils. J. Anal. Appl. Pyrolysis. 2004, 71, 987. (4) Mondala, A.; Liang, K.; Toghiani, H.; Hernandez, R.; French, T. Biodiesel production by in situ transesterification of municipal primary and secondary sludges. Bioresour. Technol. 2009, 100, 1203. (5) Watkins, R. S.; Lee, A. F.; Wilson, K. Li-CaO catalysed tri-glyceride transesterification for biodiesel applications. Green Chem. 2004, 6, 335. (6) Hanh, H. D.; Dong, N. T.; Okitsu, K.; Nishimura, R.; Maeda, Y. Biodiesel production through transesterification of triolein with various alcohols in an ultrasonic field. Renewable Energy 2009, 34, 766.
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ReceiVed for reView July 23, 2009 ReVised manuscript receiVed September 11, 2009 Accepted September 13, 2009 IE901175D
