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Study of the microwave lipid extraction from microalgae for biodiesel production
Chemical Engineering Journal 250 (2014) 267–273
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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Study of the microwave lipid extraction from microalgae for biodiesel production Yong-Ming Dai a, Kung-Tung Chen b, Chiing-Chang Chen a,⇑ a b
Department of Science Application and Dissemination, National Taichung University of Education, Taichung 403, Taiwan, ROC Department of Chemical and Materials Engineering, Minghsin University of Science and Technology, Hsinchu 30401, Taiwan, ROC
h i g h l i g h t s Microalgae extracted by microwave using the solvent have the largest extracted of lipid, 30 wt.%. Li4SiO4 is also successfully tested in the transesterification reaction of microalgae oil. The transesterification efficiency by Li4SiO4 could approach 73.5% by the optimal conditions.
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Article history: Received 4 December 2013 Received in revised form 7 April 2014 Accepted 10 April 2014 Available online 24 April 2014 Keywords: Microalgae oil Microwave Transformation Solid catalysts
a b s t r a c t Biomass energy is considered as the most potential petroleum substitute in a shorter period of time, for its renewable ability and lower pollution. This research tends to extract algae oil from microalgae with microwave fragmentation technology. This process can reduce the production costs of microalgae biodiesel. The catalysts prepared in different conditions are characterized by BET, XRD and the conversion from the transesterification catalyzed by each catalyst which was determined using GC. Microwave is used for assisting in the lipid extraction of microalgae by solvents in this study. Microwave assists in lipid extraction under various solvents, and the extracting time and power are compared. The experimental results show that microalgae extracted using the solvent has the largest extracted quantity of microalgae lipid, 30 wt.%, and the heating performance for transesterification shows that the best conversion is 76.2% under 68 °C with the Li4SiO4 amount 3 wt.% and the oil/methanol molar ratio 1:18 for 4 h. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Biodiesel produced from rapeseeds, corns, soybeans, and palm oil has been rapidly developing in recent years [1–3]. The supply of oil, such as cooking oil, is limited and vast land is required for cultivating such supply. Planting abundant energy crops could result in land competing with food provisions and forest resources being destroyed. Therefore, second-generation biodiesel must be actively developed [4–6]. Algae present the advantages of rapid growth, high oil yield per unit area, and being cultivated on land that they are unsuitable for cultivation. Producing biodiesel using algae has been emphasized in various countries [7,8]. Microalgae contain rich oil, are widely distributed, and present high photosynthesis efficiency and favorable environment adaptability as opposed to oil crops and animal fat. Microalgae use sunlight for reducing CO2 to biofuels, foods, fertilizers, and valuable products. ⇑ Corresponding author. Tel.: +886 4 22183406; fax: +886 4 22183560. E-mail addresses: (C.-C. Chen).
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http://dx.doi.org/10.1016/j.cej.2014.04.031 1385-8947/Ó 2014 Elsevier B.V. All rights reserved.
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In addition, some of the microalgae strains are able to accumulate significant amount of lipid within their cells, in which the lipid can be converted to biodiesel through transesterification reaction [9,10]. One major difference between algal methyl esters and most common vegetable oil derived FAME is that some algal lipids contain a substantial quantity of long chain polyunsaturated fatty acids. The chain polyunsaturated fatty acids are better than fatty acid methyl esters in terms of fuel properties, including cetane number, oxidation stability and cold flow properties [11,12]. Various methods are applied for extracting microalgae, including supercritical extraction, ultrasonic extraction, microwave extraction, high-pressure homogenizer extraction, hydrothermal liquefaction and solvent extraction [10–19]. Nevertheless, a large amount of solvent is necessary for traditional oil extraction, which causes environmental pollution increases costs, and consumes much energy in the extraction process [13,14]. Microwaves reveal characteristics of even and rapid heating, little consumption of solvents, and short extraction time. Comparably, ultrasound can enhance the molecular energy of solvents in the liquid as well as strong penetration to destroy the cell wall so that the solvent can
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efficiently extract oil [20,21]. The production of biodiesel by using algae currently requires high production costs; therefore, biodiesel is expected to become the primary form of biomass energy only when an efficient, low-cost microalgal extraction technology has been developed. The transformation of fatty acids in vegetable oil (e.g., rapeseed, sunflower, and soybean oil) containing a low-molecularweight alcohol (methanol) to alkyl ester of fatty acids (fatty acid methyl ester, FAME) is useful in biodiesel fuel applications [20–23]. The overall process consists of three consecutive and reversible reactions in which di- and mono-glycerides form as the intermediates and alkyl ester as the final product. This stoichiometric reaction requires 1 mol of triglyceride and 3 mol of methanol [24]. However, an excess of methanol is used for increasing the yield of alkyl ester and allowing the phase separation from the formed glycerol. Industrially, biodiesel is produced using a homogeneous catalyst in the presence of basic species (such as sodium hydroxide and potassium hydroxide) [25,26]. The following two research papers on catalyzed biodiesel production of the same commonly used KOH as the catalyst. Reddy et al. revealed in the experimental results that a maximum yield (67%) of fatty acid ethyl ester was obtained at 265 °C, 20 min of reaction time and 1:9 dry algae to ethanol (wt./vol.) ratio [11]. Patil et al. proved that process optimization using the response surface methodology design was a valuable tool for evaluating the effects of the process variables such as dry algae to methanol (wt.%/ vol.%) ratio, catalyst (KOH) concentration and reaction time on the FAMEs yield [27]. However, several difficulties in this reaction are posed. The catalysts dissolve in methanol but cannot be reused. This results in the loss of useful materials and produces large quantity of water, because the purification of the ester phase is subsequently necessary [28,29]. By contrast, heterogeneous catalysis in the separation process is easier than in homogeneous catalysis. In addition, the heterogeneous method exhibits the advantage of eliminating the formation of soap, thus omitting the requirement of washing water and allowing the reusability of the catalyst. The use of a solid-base heterogeneous catalyst in the process provides several advantages, including the elimination of a quenching step to separate water and the ester phase as well as the elimination of soap formation [30–32]. Nevertheless, the use of heterogeneous catalysts generally requires more severe reaction conditions to obtain higher conversion. For example, high temperature or long reaction time, and even high pressure, are better needed for the reaction than in the case of homogeneous catalysts. Furthermore, there are some problems in low reaction rate, easy deactivation and high viscosity increasing the mass transfer resistance. In this paper, a simple microwave fragmentation technology is developed to extract algae oil from microalgae. Some parameters such as the effect of microwave power, using various solvents and reaction time of the extract algae oil from microalgae have been studied. Additionally, solid catalysts used in biodiesel production are also investigated to examine the optimum conditions of the methanol/oil ratio, catalyst amounts, reaction time and reaction temperature. 2. Experiment Dry microalgae powder (Far East Bio-Tec Co., Taiwan), Methanol, n-hexane, iso-propanol (ACS grade, ECHO Chemical Co., Miaoli, Taiwan) and reagent grade Li2CO3 (Shimakyu’s Pure Chemicals, Osaka, Japan) were used as received. Having extracted microalgae oil from microalgae by using the three methods of heating, ultrasound, and microwave, 5 g microalgae powder was proceeded Conventional heating, ultrasound, and microwave extraction using n-hexane, iso-propanol,
n-hexane/iso-propanol (1:1), and n-hexane/iso-propanol (1:2) solvents to extract the algae oil. 2.1. Conventional heating-assisted lipid extraction The extracted microalgae oil was performed in a 250 mL flatbottomed flask, equipped with a reflux condenser and a magnetic stirrer. The reactor was initially filled with 5 g microalgae powder in solvent, which was heated to 100 °C for 3 h while stirring at 300 rpm. 2.2. Ultrasound-assisted lipid extraction Microalgae powder and the solvent were loaded into a flat-bottomed flask. The flask was then rapidly placed into a 40 kHz ultrasonic bath, Crest Ultrasonics. The extraction was performed at room temperature for 30 min. 2.3. Microwave-assisted lipid extraction The experiment was carried out using a microwave extractor, MARS, CEM Corp. (Mathews, NC, USA). Microalgae powder and the solvent were loaded into a flat-bottomed flask. The effects of microwave time and power on the amount of lipid extracted from microalgae. The n-heptane/iso-propanol was removed under vacuum in a rotary evaporator to eliminate the organic solvent. The transesterification reaction involved three types of catalysts, namely Li4SiO4, Li2SiO3, and CaO. Li4SiO4 and Li2SiO3 were prepared using a solid-state reaction. A 0.133-mol amorphous SiO2 powder was added (0.12 lm, Shimakyu’s Pure Chemicals, Osaka, Japan) to an aqueous solution containing 0.266-mol LiCO3 (Katayama Chemical Co., Japan). The as-prepared solution was dried at 120 °C for 24 h. Finally, the SiO2 and Li2CO3 mixing powder was well ground and calcined at 900 °C in air for 4 h, and CaO (Shimakyu’s Pure Chemicals) was used as received. The conversion of microalgae oil to biodiesel was performed in a 250-mL flat-bottomed flask equipped with a reflux condenser and a magnetic stirrer. The reactor was initially filled with microalgae oil, which was heated to 65 °C for 3 h while stirring at 300 rpm. The reactant was stirred evenly to avoid splashing in the flask. The timing of the reaction was initiated once the mixture of methanol and the catalyst were added to the reactor. The effects of the molar ratio of methanol to oil (3:1–24:1), and the catalyst to oil weight ratio (1–7 wt.%), on the conversion of triglycerides to biodiesel were investigated. All of the experiments were performed at atmospheric pressure. After the transesterification reaction, DI water was added into the reaction mixture to stop the reaction. The biodiesel and glycerol layers were easily separated because of the varying densities of 0.86 and 1.126 g/cm3. A supernatant was filtered and excess methanol and water were evaporated before fatty acid methyl ester (FAME) analysis. Fig. 1 shows the chromatogram obtained from GC-FID analysis of FAME. The peaks were C16:0, C17:0, C18:0, C18:1, C18:2 and C18:3, respectively. These six peaks were the most common peaks in analyzing methyl esters from gas chromatograph. The base strength of the as-prepared catalyst (H_) was determined using Hammett indicators. Approximately 50 mg of the sample was shaken with a 1-mL methanol solution of the Hammett indicator. Two hours were allowed to elapse to reach equilibrium, after which no additional color change occurred. The basic strength was defined as being stronger than the weakest indicator that exhibited a color change, and weaker than the strongest indicator that produced no color change. Bromthymol blue (H_ = 7.2), phenolphthalein (H_ = 9.8), 2,4-dinitroaniline (H_ = 15.0), and 4-nitroaniline (H_ = 18.4), at a concentration of 0.02 mol/L were obtained
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The oven temperature program started at 120 °C (hold 1 min), increased at a rate of 30 °C/min 1 to 220 °C (hold 1 min), and then increased at a rate of 10 °C/min 1 to 250 °C (hold 1 min). The temperature of the programmed temperature injector was 90 °C for 0.05 min, programmed to 260 °C, at a rate of 10 °C/min 1. Nitrogen was used as a carrier gas exhibiting a flow-rate of 2 mL/ min 1. The amount of FAME was calculated using the internal standard (methyl heptadecanoate) method, according to method CNS 15051. 4. Results and discussion 4.1. Lipid extraction of microalgae
Fig. 1. Chromatogram obtained from GC-FID analysis of FAME.
from Sigma–Aldrich (St. Louis, MO, USA) and used as Hammett indicators. The characterization of the as-prepared catalysts was performed using a powder X-ray diffractometer (XRD, MAC MXP18, Tokyo, Japan), with Cu Ka radiation, over a 2h range of 20–80° exhibiting a step size of 0.04° and at a scanning speed of 3°/min 1. The microstructures of the as-prepared catalysts were observed using a field emission scanning electron microscope (FE-SEM, JEOL JSM-7401F, Tokyo, Japan). FAME concentrations, expressed as the biodiesel purity of the product, were determined using a gas chromatography system (Thermo trace GC ultra, Thermo Co., Austin, TX, USA) equipped with a flame ionization detector, a capillary column (Tr-biodiesel (F), Thermo Co., 30 m in length, 0.25 mm i.d., and 0.25 lm in film thickness), programmed column oven, and programmed temperature injector.
Various microalgae extractions were analyzed using FE-SEM, whereby different extractions appear as distinct destructions on microalgae. Microwave treatment is commonly used for organic synthesis and extraction because it presents favorable heating effects on polar molecules. During the same extraction time, the destruction of microalgae using traditional ultrasonic heating was limited (Fig. 2(b and c)), affecting the extraction effect. Conversely, the optimal effect was acquired using microwave extraction, because the microalgae frond absorbed microwave energy to accumulate the energy and instantly increased the temperature that caused severe breaks because of the bearable pressure being exceeded (Fig. 2(d)). Having extracted algae oil from microalgae by using the three methods, including heating, ultrasound, and microwave, the yield analysis was further proceeded (Fig. 3). The extracted crude oil contents from heating, ultrasound, and microwave, exhibited 14 wt.%, 5 wt.%, and 18 wt.%, respectively. The extracting microalgae using microwave energy acquired the highest oil content when
Fig. 2. FE-SEM image results of microalgae oil extracted using various treatment methods. (a) Raw microalgae, (b) heat, (c) ultrasonic and (d) microwave.
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Fig. 3. The results of microalgae oil extracted using various treatment methods.
Fig. 4. Microwave-assisted lipid extraction using various solvents.
the time reached 40 min, which was approximately three times more than the crude microalgae oil content extracted using ultrasound, and 4% more than that using heating. The microalgae frond is presumed to absorb microwave energy and instantly increase the temperature, causing the cell to break because of the internal pressure exceeding the bearable pressure of the cell wall. The extraction with heating and ultrasound only enhanced the cracking of the cell wall and did not instantly provide much energy that caused distinct extraction effects. These results are consistent with the observed results derived from FE-SEM. Therefore, microalgae extraction using microwave energy appears to exhibit the highest oil content. Microalgae oil was further extracted from algae oil using n-hexane, iso-propanol, and n-heptane/iso-propanol mixing solvent (Fig. 4). The experimental results indicated that the oil contents contained a n-heptane/iso-propanol mixing solvent > n-hexane > iso-propanol, possibly because n-hexane is a nonpolar solvent that, as an oil-soluble solvent, can dissolve in oil, presenting an optimal effect. Furthermore, iso-propanol in the mixing solvent absorbs microwave energy, enabling microalgae to absorb more microwave energy. Microalgae were proceeded microwave extraction using n-hexane, iso-propanol, n-hexane/iso-propanol (1:1), and n-hexane/isopropanol (1:2) solvents to extract the algae oil. As shown in Fig. 5, in the reaction of the n-hexane/iso-propanol (2:1) mixing solvent used in microwave extraction, the highest oil content, 28 wt.%, was acquired when the extraction time reached 40 min. Under the same condition, the oil content was 12 wt.% for
n-hexane when the extraction time reached 40 min (Fig. 4). Accordingly, the n-hexane used in the mixing solvent is a nonpolar solvent that, as an oil-soluble solvent, can dissolve in oil, whereas iso-propanol is a polar solvent, which can absorb microwave energy. During the extraction process of microalgae oil, the oil extraction yield of the mixing solvent increased with the reaction time. Both n-hexane and iso-propanol were used as the solvents in this experiment to enhance the total surface area between the two and reduce the surface tension to increase the oil extraction yield. Nonetheless, a higher oil yield during the reaction time (40 min) was observed when using the mixing solvent, which was stable and did not rise as the reaction increased to increase the oil extraction yield. A distinct solvent was used for microalgae extraction using microwave energy, and the yield analysis proceeded. Using the n-hexane, iso-propanol, n-hexane/iso-propanol (1:1), and n-hexane/iso-propanol (1:2) solvents to extract oil, the microwave energy was 400, 800, and 1000 W, respectively (Fig. 6). The microalgae extraction using microwave energy of 1000 W obtained the highest oil yield when the crude oil contents containing n-hexane, iso-propanol, n-hexane/iso-propanol (1:1), and n-hexane/iso-propanol (2:1) solvents exhibited 18 wt.%, 7 wt.%, 14 wt.%, and 28 wt.%, respectively. Thus, the highest oil content was obtained using the microwave extraction with the n-hexane/iso-propanol (2:1) solvent, which is approximately one time higher than that using n-hexane and approximately 20% more than that obtained using iso-propanol. The n-hexane/iso-propanol (2:1) mixing solvent was presumed to present high microwave absorption and extraction abilities, whereas the n-hexane exhibited high oil-solubility, and iso-propanol exhibited microwave energy absorption. Moreover, the microalgae frond absorbed microwave energy when extracting oil using microwave, instantly increasing the temperature and causing the cell wall to break because of the internal pressure exceeding the bearable pressure. In addition to rapidly cracking the cell wall, it enhanced the mass transfer among the frond and reduced the extraction time. Adding the polar solvent (iso-propanol) into the nonpolar solvent (n-hexane) to extract the crude oil], enabled a high microalgae oil extraction content.
4.2. Optimal conditions for transesterification reaction Fig. 7 shows the XRD results of the Li4SiO4 (JCPDS 74-2145), Li2CO3 (JCPDS 87-0728), and CaO (JCPDS 87-0728) catalysts. The crystalline phase of the SiO2 was transformed to the Li4SiO4 and Li2SiO3 phase after the solid state synthesis using Li2CO3. XRD
Fig. 5. Comparison of microwave-assisted lipid extraction at various extracting times.
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analyses did not indicate the presence of impurities (Li2CO3 and SiO2), which might have formed after the experiment from a decomposition reaction of Li4SiO4 with moisture and CO2 in the air [33]. Fig. 8 shows the experimental results of the microalgae oil catalyzed using Li4SiO4, Li2SiO3, CaO, and NaOH catalysts. The results indicated that the Li4SiO4 catalysts were more active than were the CaO and NaOH in the production of biodiesel from the microalgae oil. We determined that this activity was not directly related to the surface area, but rather the basic strength generated during solid-state synthesis. Table 1 shows the basic strength of the Li4SiO4, Li2SiO3, and CaO catalysts. CaO appeared to exhibit greater initial basic strength (15.0 < H_ < 18.4) than the Li4SiO4 (12.2 < H_ < 15.0) did. However, the basic strength of the CaO deactivated following the exposure of the catalysts to air (7.2 < H_ < 9.8), thus influencing the conversion to FAME [34]. This severely limited its practical application because of the need for inert atmosphere during handling, storage, reactor loading, and use. As shown in Table 1, the experimental results indicated that Li4SiO4 demonstrated catalytic activity and stability because of its basic strength being greater than 15 and the stability in transesterification reactions.
Fig. 6. Comparison of microwave-assisted lipid extraction at various microwave powers.
Fig. 7. XRD pattern of the catalyst.
Fig. 8. The conversion of Li4SiO4, Li2SiO3, CaO, and NaOH.
Table 1 Physical and chemical properties of the prepared catalyst. Catalyst
Surface area (m2/g)
Basic strength
Conversion (%)
CaO Li2SiO3 Li4SiO4
74.2 2.3 2.1
9.8 < H_ < 15.0 7.2 < H_ < 9.80 9.8 < H_ < 15.0
52 69 74
This study also investigated the effects of the catalyst amount on conversion. The catalyst amount varied from 1% to 8% (catalyst/oil weight ratio). As shown in Fig. 8, the conversion increased as the catalyst amount increased from 1% to 4%. Additional catalysts increased the contact opportunity of the catalyst and the reactant, which directly influenced the reaction speed and the conversion. In general, an increase in catalyst amount would increase the number of active sites available for the adsorption of the reactants to result in a more rapid increase in the number of sites of interaction between the reactants. However, with a further increase of the loading amount, the conversion had no significant difference. The rational reason was due to the rise of mixing problem (oil/MeOH/catalyst) and the resistance of mass transfer [35]. Therefore, a molar ratio higher than the stoichiometric molar ratio of methanol was required to shift the equilibrium for the reaction [36]. As shown in Fig. 9, conversion increased considerably as the methanol-loading amount increased. The maximal
Fig. 9. Influence of catalyst amount and methanol/oil molar ratio on the conversion.
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Fig. 10. Influence of reaction time and reaction temperature on the conversion.
conversion ratio was 76.2% at the methanol/oil molar ratio of 18:1. The increase in conversion was due to the shift in equilibrium toward the formation of FAME. Further increasing the alcohol to oil ratio caused a decrease in the oil concentration and accordingly decreased the reaction rate. Therefore, 18:1 was the appropriate methanol/oil molar ratio for this reaction. The excess methanol could be collected and recycled. Fig. 10 shows the effects of the reaction time and the reaction temperature on the conversion. More than 76.6% of the conversion occurred within a 1-h reaction time, and thereafter remained nearly constant because of a near equilibrium conversion. Four temperatures were used for the transesterification of refined soybean oil with methanol (18:1) using a 6-wt.% catalyst. After 4 h, the conversions were 76.2%, 75.1%, and 57.6% for 70, 65, and 50 °C, respectively. Temperature clearly influenced the reaction rate and the biodiesel purity.
5. Conclusion Microwave assists in lipid extraction under various solvents, and the extracting time and power are compared. The experimental results show that microalgae extracted by microwave using the solvent mixture of n-heptane and iso-propanol have the largest extracted quantity of microalgae lipid, 30 wt.%. The microalgae oil transesterification under various catalysts. Li4SiO4 catalyst is also successfully tested in the transesterification reaction of microalgae oil. Based on the results of this study, 3% of catalyst, reaction time of 3 h and 18:1 methanol to mole ratio are found to produce the highest yield of biodiesel. Li4SiO4 demonstrates catalytic activity and stable catalytic activity in transesterification reactions that it has the potential to provide energy-efficiency for algal biodiesel production.
Acknowledgment The authors thank NSC Taiwan for financially supporting this study under Grant NSC100-2622-M-42-001-CC1.
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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Study of the microwave lipid extraction from microalgae for biodiesel production Yong-Ming Dai a, Kung-Tung Chen b, Chiing-Chang Chen a,⇑ a b
Department of Science Application and Dissemination, National Taichung University of Education, Taichung 403, Taiwan, ROC Department of Chemical and Materials Engineering, Minghsin University of Science and Technology, Hsinchu 30401, Taiwan, ROC
h i g h l i g h t s Microalgae extracted by microwave using the solvent have the largest extracted of lipid, 30 wt.%. Li4SiO4 is also successfully tested in the transesterification reaction of microalgae oil. The transesterification efficiency by Li4SiO4 could approach 73.5% by the optimal conditions.
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Article history: Received 4 December 2013 Received in revised form 7 April 2014 Accepted 10 April 2014 Available online 24 April 2014 Keywords: Microalgae oil Microwave Transformation Solid catalysts
a b s t r a c t Biomass energy is considered as the most potential petroleum substitute in a shorter period of time, for its renewable ability and lower pollution. This research tends to extract algae oil from microalgae with microwave fragmentation technology. This process can reduce the production costs of microalgae biodiesel. The catalysts prepared in different conditions are characterized by BET, XRD and the conversion from the transesterification catalyzed by each catalyst which was determined using GC. Microwave is used for assisting in the lipid extraction of microalgae by solvents in this study. Microwave assists in lipid extraction under various solvents, and the extracting time and power are compared. The experimental results show that microalgae extracted using the solvent has the largest extracted quantity of microalgae lipid, 30 wt.%, and the heating performance for transesterification shows that the best conversion is 76.2% under 68 °C with the Li4SiO4 amount 3 wt.% and the oil/methanol molar ratio 1:18 for 4 h. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Biodiesel produced from rapeseeds, corns, soybeans, and palm oil has been rapidly developing in recent years [1–3]. The supply of oil, such as cooking oil, is limited and vast land is required for cultivating such supply. Planting abundant energy crops could result in land competing with food provisions and forest resources being destroyed. Therefore, second-generation biodiesel must be actively developed [4–6]. Algae present the advantages of rapid growth, high oil yield per unit area, and being cultivated on land that they are unsuitable for cultivation. Producing biodiesel using algae has been emphasized in various countries [7,8]. Microalgae contain rich oil, are widely distributed, and present high photosynthesis efficiency and favorable environment adaptability as opposed to oil crops and animal fat. Microalgae use sunlight for reducing CO2 to biofuels, foods, fertilizers, and valuable products. ⇑ Corresponding author. Tel.: +886 4 22183406; fax: +886 4 22183560. E-mail addresses: (C.-C. Chen).
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http://dx.doi.org/10.1016/j.cej.2014.04.031 1385-8947/Ó 2014 Elsevier B.V. All rights reserved.
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In addition, some of the microalgae strains are able to accumulate significant amount of lipid within their cells, in which the lipid can be converted to biodiesel through transesterification reaction [9,10]. One major difference between algal methyl esters and most common vegetable oil derived FAME is that some algal lipids contain a substantial quantity of long chain polyunsaturated fatty acids. The chain polyunsaturated fatty acids are better than fatty acid methyl esters in terms of fuel properties, including cetane number, oxidation stability and cold flow properties [11,12]. Various methods are applied for extracting microalgae, including supercritical extraction, ultrasonic extraction, microwave extraction, high-pressure homogenizer extraction, hydrothermal liquefaction and solvent extraction [10–19]. Nevertheless, a large amount of solvent is necessary for traditional oil extraction, which causes environmental pollution increases costs, and consumes much energy in the extraction process [13,14]. Microwaves reveal characteristics of even and rapid heating, little consumption of solvents, and short extraction time. Comparably, ultrasound can enhance the molecular energy of solvents in the liquid as well as strong penetration to destroy the cell wall so that the solvent can
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efficiently extract oil [20,21]. The production of biodiesel by using algae currently requires high production costs; therefore, biodiesel is expected to become the primary form of biomass energy only when an efficient, low-cost microalgal extraction technology has been developed. The transformation of fatty acids in vegetable oil (e.g., rapeseed, sunflower, and soybean oil) containing a low-molecularweight alcohol (methanol) to alkyl ester of fatty acids (fatty acid methyl ester, FAME) is useful in biodiesel fuel applications [20–23]. The overall process consists of three consecutive and reversible reactions in which di- and mono-glycerides form as the intermediates and alkyl ester as the final product. This stoichiometric reaction requires 1 mol of triglyceride and 3 mol of methanol [24]. However, an excess of methanol is used for increasing the yield of alkyl ester and allowing the phase separation from the formed glycerol. Industrially, biodiesel is produced using a homogeneous catalyst in the presence of basic species (such as sodium hydroxide and potassium hydroxide) [25,26]. The following two research papers on catalyzed biodiesel production of the same commonly used KOH as the catalyst. Reddy et al. revealed in the experimental results that a maximum yield (67%) of fatty acid ethyl ester was obtained at 265 °C, 20 min of reaction time and 1:9 dry algae to ethanol (wt./vol.) ratio [11]. Patil et al. proved that process optimization using the response surface methodology design was a valuable tool for evaluating the effects of the process variables such as dry algae to methanol (wt.%/ vol.%) ratio, catalyst (KOH) concentration and reaction time on the FAMEs yield [27]. However, several difficulties in this reaction are posed. The catalysts dissolve in methanol but cannot be reused. This results in the loss of useful materials and produces large quantity of water, because the purification of the ester phase is subsequently necessary [28,29]. By contrast, heterogeneous catalysis in the separation process is easier than in homogeneous catalysis. In addition, the heterogeneous method exhibits the advantage of eliminating the formation of soap, thus omitting the requirement of washing water and allowing the reusability of the catalyst. The use of a solid-base heterogeneous catalyst in the process provides several advantages, including the elimination of a quenching step to separate water and the ester phase as well as the elimination of soap formation [30–32]. Nevertheless, the use of heterogeneous catalysts generally requires more severe reaction conditions to obtain higher conversion. For example, high temperature or long reaction time, and even high pressure, are better needed for the reaction than in the case of homogeneous catalysts. Furthermore, there are some problems in low reaction rate, easy deactivation and high viscosity increasing the mass transfer resistance. In this paper, a simple microwave fragmentation technology is developed to extract algae oil from microalgae. Some parameters such as the effect of microwave power, using various solvents and reaction time of the extract algae oil from microalgae have been studied. Additionally, solid catalysts used in biodiesel production are also investigated to examine the optimum conditions of the methanol/oil ratio, catalyst amounts, reaction time and reaction temperature. 2. Experiment Dry microalgae powder (Far East Bio-Tec Co., Taiwan), Methanol, n-hexane, iso-propanol (ACS grade, ECHO Chemical Co., Miaoli, Taiwan) and reagent grade Li2CO3 (Shimakyu’s Pure Chemicals, Osaka, Japan) were used as received. Having extracted microalgae oil from microalgae by using the three methods of heating, ultrasound, and microwave, 5 g microalgae powder was proceeded Conventional heating, ultrasound, and microwave extraction using n-hexane, iso-propanol,
n-hexane/iso-propanol (1:1), and n-hexane/iso-propanol (1:2) solvents to extract the algae oil. 2.1. Conventional heating-assisted lipid extraction The extracted microalgae oil was performed in a 250 mL flatbottomed flask, equipped with a reflux condenser and a magnetic stirrer. The reactor was initially filled with 5 g microalgae powder in solvent, which was heated to 100 °C for 3 h while stirring at 300 rpm. 2.2. Ultrasound-assisted lipid extraction Microalgae powder and the solvent were loaded into a flat-bottomed flask. The flask was then rapidly placed into a 40 kHz ultrasonic bath, Crest Ultrasonics. The extraction was performed at room temperature for 30 min. 2.3. Microwave-assisted lipid extraction The experiment was carried out using a microwave extractor, MARS, CEM Corp. (Mathews, NC, USA). Microalgae powder and the solvent were loaded into a flat-bottomed flask. The effects of microwave time and power on the amount of lipid extracted from microalgae. The n-heptane/iso-propanol was removed under vacuum in a rotary evaporator to eliminate the organic solvent. The transesterification reaction involved three types of catalysts, namely Li4SiO4, Li2SiO3, and CaO. Li4SiO4 and Li2SiO3 were prepared using a solid-state reaction. A 0.133-mol amorphous SiO2 powder was added (0.12 lm, Shimakyu’s Pure Chemicals, Osaka, Japan) to an aqueous solution containing 0.266-mol LiCO3 (Katayama Chemical Co., Japan). The as-prepared solution was dried at 120 °C for 24 h. Finally, the SiO2 and Li2CO3 mixing powder was well ground and calcined at 900 °C in air for 4 h, and CaO (Shimakyu’s Pure Chemicals) was used as received. The conversion of microalgae oil to biodiesel was performed in a 250-mL flat-bottomed flask equipped with a reflux condenser and a magnetic stirrer. The reactor was initially filled with microalgae oil, which was heated to 65 °C for 3 h while stirring at 300 rpm. The reactant was stirred evenly to avoid splashing in the flask. The timing of the reaction was initiated once the mixture of methanol and the catalyst were added to the reactor. The effects of the molar ratio of methanol to oil (3:1–24:1), and the catalyst to oil weight ratio (1–7 wt.%), on the conversion of triglycerides to biodiesel were investigated. All of the experiments were performed at atmospheric pressure. After the transesterification reaction, DI water was added into the reaction mixture to stop the reaction. The biodiesel and glycerol layers were easily separated because of the varying densities of 0.86 and 1.126 g/cm3. A supernatant was filtered and excess methanol and water were evaporated before fatty acid methyl ester (FAME) analysis. Fig. 1 shows the chromatogram obtained from GC-FID analysis of FAME. The peaks were C16:0, C17:0, C18:0, C18:1, C18:2 and C18:3, respectively. These six peaks were the most common peaks in analyzing methyl esters from gas chromatograph. The base strength of the as-prepared catalyst (H_) was determined using Hammett indicators. Approximately 50 mg of the sample was shaken with a 1-mL methanol solution of the Hammett indicator. Two hours were allowed to elapse to reach equilibrium, after which no additional color change occurred. The basic strength was defined as being stronger than the weakest indicator that exhibited a color change, and weaker than the strongest indicator that produced no color change. Bromthymol blue (H_ = 7.2), phenolphthalein (H_ = 9.8), 2,4-dinitroaniline (H_ = 15.0), and 4-nitroaniline (H_ = 18.4), at a concentration of 0.02 mol/L were obtained
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The oven temperature program started at 120 °C (hold 1 min), increased at a rate of 30 °C/min 1 to 220 °C (hold 1 min), and then increased at a rate of 10 °C/min 1 to 250 °C (hold 1 min). The temperature of the programmed temperature injector was 90 °C for 0.05 min, programmed to 260 °C, at a rate of 10 °C/min 1. Nitrogen was used as a carrier gas exhibiting a flow-rate of 2 mL/ min 1. The amount of FAME was calculated using the internal standard (methyl heptadecanoate) method, according to method CNS 15051. 4. Results and discussion 4.1. Lipid extraction of microalgae
Fig. 1. Chromatogram obtained from GC-FID analysis of FAME.
from Sigma–Aldrich (St. Louis, MO, USA) and used as Hammett indicators. The characterization of the as-prepared catalysts was performed using a powder X-ray diffractometer (XRD, MAC MXP18, Tokyo, Japan), with Cu Ka radiation, over a 2h range of 20–80° exhibiting a step size of 0.04° and at a scanning speed of 3°/min 1. The microstructures of the as-prepared catalysts were observed using a field emission scanning electron microscope (FE-SEM, JEOL JSM-7401F, Tokyo, Japan). FAME concentrations, expressed as the biodiesel purity of the product, were determined using a gas chromatography system (Thermo trace GC ultra, Thermo Co., Austin, TX, USA) equipped with a flame ionization detector, a capillary column (Tr-biodiesel (F), Thermo Co., 30 m in length, 0.25 mm i.d., and 0.25 lm in film thickness), programmed column oven, and programmed temperature injector.
Various microalgae extractions were analyzed using FE-SEM, whereby different extractions appear as distinct destructions on microalgae. Microwave treatment is commonly used for organic synthesis and extraction because it presents favorable heating effects on polar molecules. During the same extraction time, the destruction of microalgae using traditional ultrasonic heating was limited (Fig. 2(b and c)), affecting the extraction effect. Conversely, the optimal effect was acquired using microwave extraction, because the microalgae frond absorbed microwave energy to accumulate the energy and instantly increased the temperature that caused severe breaks because of the bearable pressure being exceeded (Fig. 2(d)). Having extracted algae oil from microalgae by using the three methods, including heating, ultrasound, and microwave, the yield analysis was further proceeded (Fig. 3). The extracted crude oil contents from heating, ultrasound, and microwave, exhibited 14 wt.%, 5 wt.%, and 18 wt.%, respectively. The extracting microalgae using microwave energy acquired the highest oil content when
Fig. 2. FE-SEM image results of microalgae oil extracted using various treatment methods. (a) Raw microalgae, (b) heat, (c) ultrasonic and (d) microwave.
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Fig. 3. The results of microalgae oil extracted using various treatment methods.
Fig. 4. Microwave-assisted lipid extraction using various solvents.
the time reached 40 min, which was approximately three times more than the crude microalgae oil content extracted using ultrasound, and 4% more than that using heating. The microalgae frond is presumed to absorb microwave energy and instantly increase the temperature, causing the cell to break because of the internal pressure exceeding the bearable pressure of the cell wall. The extraction with heating and ultrasound only enhanced the cracking of the cell wall and did not instantly provide much energy that caused distinct extraction effects. These results are consistent with the observed results derived from FE-SEM. Therefore, microalgae extraction using microwave energy appears to exhibit the highest oil content. Microalgae oil was further extracted from algae oil using n-hexane, iso-propanol, and n-heptane/iso-propanol mixing solvent (Fig. 4). The experimental results indicated that the oil contents contained a n-heptane/iso-propanol mixing solvent > n-hexane > iso-propanol, possibly because n-hexane is a nonpolar solvent that, as an oil-soluble solvent, can dissolve in oil, presenting an optimal effect. Furthermore, iso-propanol in the mixing solvent absorbs microwave energy, enabling microalgae to absorb more microwave energy. Microalgae were proceeded microwave extraction using n-hexane, iso-propanol, n-hexane/iso-propanol (1:1), and n-hexane/isopropanol (1:2) solvents to extract the algae oil. As shown in Fig. 5, in the reaction of the n-hexane/iso-propanol (2:1) mixing solvent used in microwave extraction, the highest oil content, 28 wt.%, was acquired when the extraction time reached 40 min. Under the same condition, the oil content was 12 wt.% for
n-hexane when the extraction time reached 40 min (Fig. 4). Accordingly, the n-hexane used in the mixing solvent is a nonpolar solvent that, as an oil-soluble solvent, can dissolve in oil, whereas iso-propanol is a polar solvent, which can absorb microwave energy. During the extraction process of microalgae oil, the oil extraction yield of the mixing solvent increased with the reaction time. Both n-hexane and iso-propanol were used as the solvents in this experiment to enhance the total surface area between the two and reduce the surface tension to increase the oil extraction yield. Nonetheless, a higher oil yield during the reaction time (40 min) was observed when using the mixing solvent, which was stable and did not rise as the reaction increased to increase the oil extraction yield. A distinct solvent was used for microalgae extraction using microwave energy, and the yield analysis proceeded. Using the n-hexane, iso-propanol, n-hexane/iso-propanol (1:1), and n-hexane/iso-propanol (1:2) solvents to extract oil, the microwave energy was 400, 800, and 1000 W, respectively (Fig. 6). The microalgae extraction using microwave energy of 1000 W obtained the highest oil yield when the crude oil contents containing n-hexane, iso-propanol, n-hexane/iso-propanol (1:1), and n-hexane/iso-propanol (2:1) solvents exhibited 18 wt.%, 7 wt.%, 14 wt.%, and 28 wt.%, respectively. Thus, the highest oil content was obtained using the microwave extraction with the n-hexane/iso-propanol (2:1) solvent, which is approximately one time higher than that using n-hexane and approximately 20% more than that obtained using iso-propanol. The n-hexane/iso-propanol (2:1) mixing solvent was presumed to present high microwave absorption and extraction abilities, whereas the n-hexane exhibited high oil-solubility, and iso-propanol exhibited microwave energy absorption. Moreover, the microalgae frond absorbed microwave energy when extracting oil using microwave, instantly increasing the temperature and causing the cell wall to break because of the internal pressure exceeding the bearable pressure. In addition to rapidly cracking the cell wall, it enhanced the mass transfer among the frond and reduced the extraction time. Adding the polar solvent (iso-propanol) into the nonpolar solvent (n-hexane) to extract the crude oil], enabled a high microalgae oil extraction content.
4.2. Optimal conditions for transesterification reaction Fig. 7 shows the XRD results of the Li4SiO4 (JCPDS 74-2145), Li2CO3 (JCPDS 87-0728), and CaO (JCPDS 87-0728) catalysts. The crystalline phase of the SiO2 was transformed to the Li4SiO4 and Li2SiO3 phase after the solid state synthesis using Li2CO3. XRD
Fig. 5. Comparison of microwave-assisted lipid extraction at various extracting times.
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analyses did not indicate the presence of impurities (Li2CO3 and SiO2), which might have formed after the experiment from a decomposition reaction of Li4SiO4 with moisture and CO2 in the air [33]. Fig. 8 shows the experimental results of the microalgae oil catalyzed using Li4SiO4, Li2SiO3, CaO, and NaOH catalysts. The results indicated that the Li4SiO4 catalysts were more active than were the CaO and NaOH in the production of biodiesel from the microalgae oil. We determined that this activity was not directly related to the surface area, but rather the basic strength generated during solid-state synthesis. Table 1 shows the basic strength of the Li4SiO4, Li2SiO3, and CaO catalysts. CaO appeared to exhibit greater initial basic strength (15.0 < H_ < 18.4) than the Li4SiO4 (12.2 < H_ < 15.0) did. However, the basic strength of the CaO deactivated following the exposure of the catalysts to air (7.2 < H_ < 9.8), thus influencing the conversion to FAME [34]. This severely limited its practical application because of the need for inert atmosphere during handling, storage, reactor loading, and use. As shown in Table 1, the experimental results indicated that Li4SiO4 demonstrated catalytic activity and stability because of its basic strength being greater than 15 and the stability in transesterification reactions.
Fig. 6. Comparison of microwave-assisted lipid extraction at various microwave powers.
Fig. 7. XRD pattern of the catalyst.
Fig. 8. The conversion of Li4SiO4, Li2SiO3, CaO, and NaOH.
Table 1 Physical and chemical properties of the prepared catalyst. Catalyst
Surface area (m2/g)
Basic strength
Conversion (%)
CaO Li2SiO3 Li4SiO4
74.2 2.3 2.1
9.8 < H_ < 15.0 7.2 < H_ < 9.80 9.8 < H_ < 15.0
52 69 74
This study also investigated the effects of the catalyst amount on conversion. The catalyst amount varied from 1% to 8% (catalyst/oil weight ratio). As shown in Fig. 8, the conversion increased as the catalyst amount increased from 1% to 4%. Additional catalysts increased the contact opportunity of the catalyst and the reactant, which directly influenced the reaction speed and the conversion. In general, an increase in catalyst amount would increase the number of active sites available for the adsorption of the reactants to result in a more rapid increase in the number of sites of interaction between the reactants. However, with a further increase of the loading amount, the conversion had no significant difference. The rational reason was due to the rise of mixing problem (oil/MeOH/catalyst) and the resistance of mass transfer [35]. Therefore, a molar ratio higher than the stoichiometric molar ratio of methanol was required to shift the equilibrium for the reaction [36]. As shown in Fig. 9, conversion increased considerably as the methanol-loading amount increased. The maximal
Fig. 9. Influence of catalyst amount and methanol/oil molar ratio on the conversion.
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Fig. 10. Influence of reaction time and reaction temperature on the conversion.
conversion ratio was 76.2% at the methanol/oil molar ratio of 18:1. The increase in conversion was due to the shift in equilibrium toward the formation of FAME. Further increasing the alcohol to oil ratio caused a decrease in the oil concentration and accordingly decreased the reaction rate. Therefore, 18:1 was the appropriate methanol/oil molar ratio for this reaction. The excess methanol could be collected and recycled. Fig. 10 shows the effects of the reaction time and the reaction temperature on the conversion. More than 76.6% of the conversion occurred within a 1-h reaction time, and thereafter remained nearly constant because of a near equilibrium conversion. Four temperatures were used for the transesterification of refined soybean oil with methanol (18:1) using a 6-wt.% catalyst. After 4 h, the conversions were 76.2%, 75.1%, and 57.6% for 70, 65, and 50 °C, respectively. Temperature clearly influenced the reaction rate and the biodiesel purity.
5. Conclusion Microwave assists in lipid extraction under various solvents, and the extracting time and power are compared. The experimental results show that microalgae extracted by microwave using the solvent mixture of n-heptane and iso-propanol have the largest extracted quantity of microalgae lipid, 30 wt.%. The microalgae oil transesterification under various catalysts. Li4SiO4 catalyst is also successfully tested in the transesterification reaction of microalgae oil. Based on the results of this study, 3% of catalyst, reaction time of 3 h and 18:1 methanol to mole ratio are found to produce the highest yield of biodiesel. Li4SiO4 demonstrates catalytic activity and stable catalytic activity in transesterification reactions that it has the potential to provide energy-efficiency for algal biodiesel production.
Acknowledgment The authors thank NSC Taiwan for financially supporting this study under Grant NSC100-2622-M-42-001-CC1.
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