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Methyl Ester Production from Sunflower and Waste Cooking Oils Using Alkali-Doped Metal Oxide Catalysts Georgios Karavalakis,*,† Georgios Anastopoulos, and Stamos Stournas Laboratory of Fuels Technology and Lubricants, School of Chemical Engineering, National Technical UniVersity of Athens, Zografou Campus, 9 Iroon Polytechniou Street, 157 80 Athens, Greece
The purpose of this study was to investigate a series of alkali-doped metal oxide catalysts for their activity in the transesterification of sunflower oil and waste cooking oil with methanol. The metal oxides used as supports were Al2O3 and ZnO loaded with KHCO3 and KNO3, respectively, at various concentrations and calcined at different temperatures. These catalysts appeared to be promising candidates to replace homogeneous catalysts for biodiesel production as the reaction times and catalyst amount are low enough in order to achieve high ester yields. In most cases, the increase in loading concentration favorably influenced oil conversion. On the other hand, at calcination temperatures above 750 °C, a noticeable drop in biodiesel yield was observed which may be ascribed to decomposition effects of the active sites. In general, the use of sunflower oil resulted in higher ester yields when compared to waste cooking oil which was characterized by a high amount of free fatty acids and moisture. Biodiesel ester content was also strongly related with catalyst amount, methanol to oil molar ratio, reaction time, and catalyst reusability. Introduction Biodiesel, monoalkyl esters of fatty acids derived from vegetable oils, animal fats, and waste cooking oils, is known as an alternative and renewable fuel which provides less harmful emissions when compared to petroleum diesel fuel. Biodiesel is also nontoxic, free of sulfur and aromatics, and readily biodegradable and reduces reliance on oil imports. Most biodiesel fuels are currently synthesized with transesterification of lipids with a short-chain alcohol (methanol or ethanol) in the presence of an alkaline liquid catalyst, usually sodium or potassium hydroxides. These catalysts dissolve in the polar reactants and facilitate the transesterification, thereby acting as homogeneous catalysts.1-4 The major drawback of this process is the formation of soaps due to the reaction of the alkaline catalyst with the free fatty acids (FFA), or due to the saponification of the triglycerides and biodiesel.5 This phenomenon becomes even stronger with the use of low-quality feedstocks containing a high amount of FFA and water. These side reactions consume the catalyst and hinder phase separation of the biodiesel product from the glycerol byproduct. A further disadvantage is that the glycerol byproduct contains salts and catalytic species from the neutralization of the catalyst.6,7 In addition, removal of the catalyst to purify the biodiesel product is an energy consuming process and requires a large amount of water. To overcome these problems, the use of heterogeneous catalysts can provide an alternative way of biodiesel processing. Biodiesel production by heterogeneous catalysis would improve the economics and the environmental profile of the process. In principle, heterogeneous catalysis largely eliminates the formation of metal salts in the ester and glycerol products, thereby simplifying downstream separation steps.8,9 Consequently, production costs and wastewater streams should be reduced. The promise of a cheaper, cleaner, and simpler process * To whom correspondence should be addressed. Tel.: +01-951781-5799. Fax: +01-951-781-5790. E-mail:
[email protected]. † Present address: Bourns College of Engineering, Center for Environmental Research and Technology (CE-CERT), University of CaliforniasRiverside, 1084 Columbia Ave., Riverside, CA 92507, USA.
has resulted in a large body of research into heterogeneous catalysts for biodiesel.10-12 It has been reported that solid catalysts promote the transesterification of vegetable oils, as well as the esterification of FFAs, which are present to some extent in vegetable oils and in to a larger extent in waste cooking oils.13 More specifically, metal hydroxides,14 metal complexes,15 metal oxides such as calcium oxide,16 magnesium oxide,17 zirconium oxide,18 and supported catalysts19 have been investigated as solid catalysts. The catalysts are not consumed or dissolved in the reaction and therefore can be easily separated from the products. As a result, the products do not contain impurities of the catalyst, and the cost of final separation could be reduced. Nevertheless, one of the major problems associated with heterogeneous catalysts is the formation of three phases with alcohol and oil which leads to diffusion limitations thus lowering the rate of the reaction.20 One way of overcoming the mass-transfer problem in heterogeneous catalysis is using a certain amount of cosolvent to promote miscibility of oil and methanol and accordingly accelerate the reaction rate. Tetrahydrofuran (THF), dimethyl sulfoxide (DMSO), n-hexane, and ethanol are the most frequent cosolvents in transesterification of vegetable oils with methanol and solid catalysts. Gryglewicz21 used CaO as a solid base catalyst for transesterification of rapeseed oil with methanol, and after 170 min of reaction time methyl ester yields of 93% were obtained. However, by adding a certain amount of THF into the rapeseed oil/methanol mixture, the same yields of 93% were observed after 120 min of reaction time. Another way to promote mass-transfer problems associated with heterogeneous catalysts is using structure promoters or catalyst supports which can provide more specific surface area and pores for active species where they can anchor and react with large triglyceride molecules. In this study, metal oxides doped with potassium salts at different concentrations and calcined at various temperatures were used as catalysts for the transesterification of sunflower and waste cooking oils. Effects of calcination temperature, loading concentration, catalyst amount, reaction time, and methanol to oil molar ratio were investigated in order to define the optimal reaction condition
10.1021/ie101270e 2010 American Chemical Society Published on Web 10/12/2010
Ind. Eng. Chem. Res., Vol. 49, No. 23, 2010 Table 1. Main Physicochemical Properties and Fatty Acid Composition of Sunflower Oil and Waste Cooking Oil property
sunflower oil
waste cooking oil
viscosity (40 °C, mm2/s) density (15 °C, g/cm3) water content (mg/kg) pour point (°C) acid value (mgKOH/g) iodine number C16:0 C18:0 C18:1 C18:2 C18:3
32.58 0.9217 347 -10 0.6 121 5.62 2.14 25.14 66.2 0.1
32.6 0.9258 1800 +2 5.78 100 15.65 3.10 27.57 33.53 1.04
test method EN ISO 3104 EN ISO 3675 EN ISO 12937 ASTM D 97 EN 14104 EN 14111
necessary for a heterogeneously catalyzed system. The reusability of certain catalysts has also been studied.
Table 2. Sunflower Oil and Waste Cooking Oil Conversions with the Use of Al2O3-KHCO3 as Catalyst (Reaction Conditions: Catalyst Amount ) 1.5 wt % of the Oil, Methanol to Oil Molar Ratio ) 6:1, Reaction Time ) 90 min, Reaction Temperature ∼ 65 °C, and Atmospheric Pressure) ester content for given loading concentration, %
calcination temperature, °C
ester source
15%
20%
30%
35%
550
SUN WCO SUN WCO SUN WCO
78.3 56.3 88.2 65.7 97.5 86.8
82.4 69.2 94.2 73.1 94.2 82.9
90.2 77.8 97.3 81.9 95.8 75.6
95.3 84.4 98.2 89.5 93.6 70.3
750 950
Table 3. Sunflower Oil and Waste Cooking Oil Conversions with the Use of ZnO-KNO3 as Catalyst (Reaction Conditions: Catalyst Amount ) 1.5 wt % of the Oil, Methanol to Oil Molar Ratio ) 6:1, Reaction Time ) 90 min, Reaction Temperature ∼ 65 °C, and Atmospheric Pressure) ester content for given loading concentration, %
Experimental Section Catalyst Preparation. Supported metal oxide catalysts were prepared according to the conventional incipient-wetness impregnation of aqueous solutions of the corresponding metal salt precursors on Al2O3 and ZnO supports. The metal precursors applied to the study were of analytical grade potassium salts (KHCO3 and KNO3), purchased from Sigma-Aldrich. The KHCO3 precursor was loaded in Al2O3, while KNO3 was loaded in ZnO at concentrations of 15, 20, 30, and 35% by weight. The paste mixture (support and metal salt solution in distilled water) was dried in an oven at 120 °C overnight. The dried solid was then calcined at various temperatures (550, 750, and 950 °C) for 4 h. Transesterification Reaction Procedure. Refined sunflower oil (SUN) and waste cooking oil (WCO) were supplied by Elin Biofuels SA. The fatty acid composition and the main quality properties of both oils are given in Table 1. The oil feedstocks were transesterified with methanol in a 200 mL round-bottom flask equipped with a reflux condenser and a magnetic stirrer. The reaction temperature was kept approximately to 65 °C. Typically, for every batch, the concentration of the calcined catalyst was 1.5 wt % of the oil, the reaction time was 90 min, and the methanol to oil molar ratio was 6:1. Different experiments were performed with varied molar ratios between 3:1 and 15:1 and varied catalyst concentrations (0.5, 1.0, 1.5, 2.0, 2.5, 3.0, and 3.5 wt % of the oil). To establish the extent of conversion of the reaction from the start to the target temperature, several batches were run at different time intervals and corresponding biodiesel yield were obtained. At the end of each experiment the reaction mixture was cooled to room temperature. After cooling, the catalyst was separated by filtration and the mixture was centrifuged to ensure product purification. The mixture was allowed to settle, and two phases were formed, with biodiesel at the top layer and glycerol at the bottom layer. The methyl ester was analyzed by gas chromatography, according to the EN 14103 test method. Reusability Tests. The option of reusing the catalyst was evaluated in this study. The catalyst was recovered after the first run and was washed thoroughly with methanol and n-hexane to remove methyl ester species adhered to its surface. Prior to further experiments, the catalyst was calcined at 700 °C for about 3 h. The same amounts of feedstocks and methanol were added to the recycled catalyst each time to react under
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calcination temperature, °C
ester source
15%
20%
30%
35%
550
SUN WCO SUN WCO SUN WCO
77.4 68.4 89.2 82.7 96.2 89.8
83.7 77.9 94.2 89.6 98.4 94.5
92.2 86.3 97.3 93.1 98.2 94
95.8 91.2 98.2 94.7 96.7 93.1
750 950
the same conditions. The repeated tests were conducted for four reaction cycles. Results and Discussion Screening of the Catalysts. Tables 2 and 3 show the ester contents obtained for Al2O3 loaded with KHCO3 and ZnO loaded with KNO3 catalysts, respectively, after calcination at various temperatures. For comparison purposes, the same reaction conditions were used for each catalyst in all experimental runs. These reaction conditions did not represent the optimal conditions necessary to obtain the highest reaction yield; however, they provided the means of comparing catalytic activities among the catalysts. Reactions were carried out at 65 °C, a molar ratio of methanol to oil of 6:1, reaction time ) 90 min, and a catalyst amount of 1.5 wt % of the oil. As it can be seen from Table 2, all of the employed catalysts provided a strong catalytic activity toward sunflower and waste cooking oil transesterification. An important effect of the KHCO3 loading on the oil conversion was found. A 15 wt % Al2O3/KHCO3 showed the lowest oil conversion for both feedstocks. The conversion increased gradually with increasing KHCO3 loading, reaching a maximum of 98.2% for sunflower oil methyl ester. The same observation applies for ZnO loaded with KNO3. The higher oil conversion with the increase of potassium precursors may be associated with the high number of the active catalytic sites.12,14 Another parameter that affects oil conversion by controlling the generation of the catalytic active sites is the calcination temperature. It is clearly evident that, with increasing calcination temperature, higher oil conversions were achieved. It appeared that for all catalysts the optimal calcination temperature was found at the region of 750 °C. Further increase in the calcination temperature resulted in a drop of oil conversions, which can be attributed to the different KHCO3 and KNO3 decomposition extents and distribution of potassium on the Al2O3 and ZnO supports.22,23 It should be stressed that, at a calcination temperature of 950 °C, the higher catalytic efficiency was found with the use of 15 wt % of
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the potassium precursors. An increase of KHCO3 and KNO3 loading resulted in lower oil conversions at 950 °C, which might be associated with the increasing coverage of accessible basic site by inactive KHCO3 and KNO3. It is possible that when Al2O3 and ZnO are loaded too much with KHCO3 and KNO3, these precursors cannot be well-dispersed, and, consequently, a part of these compounds could be decomposed at high calcination temperatures. Concerning the waste cooking oil transesterification, the reaction efficiency of all catalysts was rather low as compared to sunflower oil transesterification (Table 3). Waste oils generally contain a high amount of FFA and water. Thus, the lower conversions with this type of oil may be ascribed to undesirable saponification and hydrolysis side-reactions, which lowered the yield of biodiesel, since a part of the catalyst was consumed in the esterification of FFA, leaving an insufficient catalyst amount to complete the transestification of waste cooking oil.24 It is also possible that oxidation products, such as hydroperoxides, aldehydes, ketones, and carboxylic acids which are present in the parent oil due to its thermal stressing,25 played a negative role on the effectiveness of the solid catalysts. It is hypothesized that the basic sites were poisoned by adsorption of the abovementioned polar compounds, and especially FFAs, on the surface of the catalysts. Influence of Reaction Parameters. There are several contributing factors affecting the yield of biodiesel during transesterification, including catalyst amount, methanol to oil molar ratio, and reaction time. During the screening process of the various catalysts, two catalysts were selected to study the influences of reaction conditions in the transesterification of both oils. These catalysts were Al2O3 loaded with 20 wt % of KHCO3 and calcined at 750 °C, and ZnO loaded with 20 wt % of KNO3 and calcined at 750 °C. Catalyst Amount. The catalyst amount was varied in the range of 0.5-3.0%. These percentages were weight fractions of the oil supplied for the reaction. Parts a and b of Figure 1 show the conversion of both oils with the use of Al2O320%KHCO3/750 and ZnO-20%KNO3/750 catalysts, respectively. It can be clearly seen that when the amount of catalysts was increased, conversion of the oils was found to increase. It is also evident from the results that ZnO loaded with 20% KNO3 catalyst, was found to be more effective in the transesterification of both oils than Al2O3-20%KHCO3/750. The highest conversions were obtained with a catalyst amount of 3.0 wt %, which was also the optimal catalyst amount for a complete reaction. Below that point, it resulted an incomplete conversion of triglycerides into the fatty acid esters. It should be stressed that the ester content of WCO remained well below the minimum specification limit of 96.5% over both catalysts, which can be attributed to the presence of FFA and other impurities in the parent oil. The highest yields obtained for WCO were 82.7 and 94.9% over Al2O3-20%KHCO3/750 and ZnO-20%KNO3/750 catalysts, respectively. On the other hand, the highest yields for sunflower oil were 96.8 and 97.6%. However, complete reaction of sunflower oil in terms of ester content was also achieved with lower catalyst amount (2.5 wt %). Methanol to Oil Molar Ratio. One of the main factors affecting the yield of biodiesel is the molar ratio of methanol to oil. Theoretically, the transesterification of oil requires 3 mol of methanol/(mol of triglyceride). In practice, excess methanol is typically applied to increase oil conversion by shifting the reaction equilibrium toward the direction of ester formation.2 Specifically, heterogeneous catalyzed reactions are known for their slow reaction rates, mainly due to the presence of a three-phase system, oilmethanol-catalyst. On this basis, when the mass transfer is limited,
Figure 1. Influence of catalyst amount on sunflower oil and waste cooking oil conversion: (a) Al2O3 loading with 20% of KHCO3, calcined at 750 °C, and (b) ZnO loading with 20% of KNO3, calcined at 750 °C. Reaction conditions: methanol to oil molar ratio ) 6:1; reaction time ) 90 min.
the rate of mass transfer seems to be much slower than the reaction rate.26 Therefore, the use of excess methanol ensures the improvement of reaction rates. Parts a and b of Figure 2 show the effect of methanol to oil molar ratio on the conversion of both oils in the presence of Al2O3 and ZnO loading with KHCO3 and KNO3 catalysts, respectively. It can be clearly seen that the stoichiometric molar ratio of 3:1 resulted in low ester conversions when the catalyst amounts and reaction time were 1.5 wt % and 90 min, respectively. When the molar ratio was increased, the conversion was found to increase, reaching a maximum value at a molar ratio of 9:1. Further increase of the methanol amount (12:1 and 15:1) beyond the optimal ratio led to a noticeable drop in methyl ester conversion. Probably, higher concentration of methanol interferes with the separation of glycerol because of its increased solubility. It is also possible that the presence of glycerol in the solution might drive the equilibrium in the backward direction.12,14 Reaction Time. The optimum reaction time for the production of biodiesel was determined by performing experimental
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Figure 2. Influence of methanol to oil molar ratio on sunflower oil and waste cooking oil conversion: (a) Al2O3 loading with 20% of KHCO3, calcined at 750 °C, and (b) ZnO loading with 20% of KNO3, calcined at 750 °C. Reaction conditions: catalyst amount ) 1.5 wt %; reaction time ) 90 min.
runs at varying reaction time in the range of 15-150 min, under an identical set of reaction conditions (catalyst amount ) 1.5 wt %; methanol to oil molar ratio ) 6:1). In Figure 3a,b, the correlation between both oils conversion and reaction time is presented for the various catalysts. It was found that the conversion of both oils increased sharply in the reaction time range between 15 and 90 min. Thereafter, the conversion followed a slower rate, as a result of a nearly equilibrium conversion, and reached a maximum conversion after 150 min of reaction time. It was observed that the application of ZnO loaded with KNO3 catalyst resulted in higher conversions than the Al2O3 supporter catalyst for the same time intervals. The maximum ester contents for sunflower and waste cooking oils over Al2O3-20%KHCO3/750 were 97.4 and 87.3%, respectively, whereas over ZnO-20%KNO3/750 were 96.9 and 95.1%. Catalyst Reusability. Reusability is one of the important parameters required to determine the economical application
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Figure 3. Influence of reaction time on sunflower oil and waste cooking oil conversion: (a) Al2O3 loading with 20% of KHCO3, calcined at 750 °C, and (b) ZnO loading with 20% of KNO3, calcined at 750 °C. Reaction conditions: catalyst amount ) 1.5 wt %; methanol to oil molar ratio ) 6:1.
of heterogeneous catalysts for biodiesel production. The catalysts of Al2O3-30%KHCO3/950 and ZnO-30%KNO3/900 were investigated by carrying out four consecutive runs at methanol to oil ratio of 6:1, reaction time of 90 min, and catalyst amount of 1.5 wt % of the oil. The reusability tests were only performed with the use of sunflower oil as feedstock. As shown in Figure 4, the experimental results presented a declining trend of ester conversion, which is attributed to the partial loss of catalytic activity after regeneration. It is possible that the loss of activity observed during the reaction cycles may be associated with deactivation of strong basic sites or by the deposition of reactants and products on the active sites.27 Conclusions The present study indicated that the metal oxides of Al2O3 and ZnO loaded with various concentrations of KHCO3 and KNO3, respectively, and calcined at different temperatures can
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Figure 4. Reusability study after four reaction cycles for Al2O3-30% KHCO3/950 and ZnO-30%KNO3/950 catalysts. Reaction conditions: methanol to oil molar ratio ) 6:1; catalyst amount ) 1.5 wt %; reaction time ) 90 min.
be used as potential catalysts for the transesterification of sunflower oil and waste cooking oil. A major advantage of this process is that the application of solid catalysts significantly simplifies the refining steps of biodiesel, resulting in lower amounts of waste streams. Moreover the catalysts can be reused, providing satisfactory oil conversions. It was found that the catalytic effectiveness was improved with the increase in loading concentration and calcination temperature. Catalysts calcined at 750 °C were found to exhibit the best catalytic activity. Further increase in the calcination temperature resulted in a drop of oil conversions, which can be attributed to the different KHCO3 and KNO3 decomposition extents. The application of waste cooking oil in the reaction system resulted in significantly lower conversion when compared to those of sunflower oil. This phenomenon may be ascribed to the presence of FFAs, impurities, and water, which favored side reactions of saponification and hydrolysis. The optimum catalyst amount for a complete reaction ranged between 2.5 and 3.0 wt % of the oil. In most cases, maximum conversion was achieved using a 9:1 methanol to oil molar ratio. Acknowledgment This paper is dedicated to the memory of Professor Stamos Stournas. Literature Cited (1) Knothe, G. Biodiesel and renewable diesel: A comparison. Prog. Energy Combust. Sci. 2010, 36, 364–373. (2) Leung, D. Y. C.; Wu, X.; Leung, M. K. H. A review on biodiesel production using catalyzed transesterification. Appl. Energy 2010, 87, 1083– 1095. (3) Wang, Y.; Ou, S.; Liu, P.; Zhang, Z. Preparation of biodiesel from waste cooking oil via two-step catalyzed process. Energy ConVers. Manage. 2007, 48, 184–188. (4) Schinas, P.; Karavalakis, G.; Davaris, C.; Anastopoulos, G.; Karonis, D.; Zannikos, F.; Stournas, S.; Lois, E. Pumpkin (Cucurbita pepo L.) seed oil as an alternative feedstock for the production of biodiesel in Greece. Biomass Bioenergy 2009, 33, 44–49.
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ReceiVed for reView June 11, 2010 ReVised manuscript receiVed September 9, 2010 Accepted September 28, 2010 IE101270E
