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Low-cost and High-efficient Extraction of Lipids from Chlorella by using Industrial Ionic Liquids
Available online at www.sciencedirect.com
ScienceDirect Energy Procedia 105 (2017) 927 – 932
The 8th International Conference on Applied Energy – ICAE2016
Low-cost and high-efficient extraction of lipids from chlorella by using industrial ionic liquids Haitao Lua, Xinhai Yua,b,*, Shan-Tung Tu a
Key Laboratory of Pressure Systems and Safety, Ministry of Education, School of Mechanical and Power Engineering, East China University of Science and Technology, Shanghai, China b State Key Laboratory of Bioreactor Engineering, East China University of Science and Technology, Shanghai, China
Abstract
Algae-sourced feedstocks remain confined to commercialization because of the high cost and energy consumption of biomass cultivation and feedstock extraction. In this study, to reduce the cost required for algae extraction, experiments with Chlorella vulgais extraction by ionic liquids (ILs) synthesized using industrial raw materials. The total fabrication cost of the synthesized [BMIM]Cl was 1.63% of the price of commercial reagent ILs. The average of the lipids yield was 75.4%, which is lower than that of 85.4% using commercial reagent grade [BMIM]Cl. The initial cost of the synthesized [BMIM]Cl for the extraction was 1.85% that of commercial reagent grade one. The IL loss due to [BMIM]Cl purification using alumina column chromatography was 88.6%. The impurities contained in the synthesized [BMIM]Cl was gradually removed with the recirculation of [BMIM]Cl.
© 2017 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2016 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or peer-review under responsibility of ICAE Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy.
Keywords: lipids extraction; chlorella;industrial ionic liquid;
1. Introduction Biodiesel has attracted considerable attention in recent years, as it is a biodegradable, renewable and non-toxic fuel. Compared with the conventional energy crops, chlorella can be used as the feedstock of biodiesel because of its higher photosynthetic efficiency, faster growth and higher biomass production
* Corresponding author. Tel.: +86-(0)21-6425-3513; fax: +86-(0)21-6425-3513. E-mail address: [email protected]. Xinhai Yu.
1876-6102 © 2017 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy. doi:10.1016/j.egypro.2017.03.419
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Haitao Lu et al. / Energy Procedia 105 (2017) 927 – 932
(55% in heterotrophic Chlorella protothecoides) [1]. Meanwhile, chlorella also has plenty of available biomass, such as protein, saccharides and carbohydrate. But its integrated cell wall structure is the maintain obstacle to efficiently extracting lipids and other biomass from chlorella. Because of high toughness of cell wall (9 kJ kg-1) [2], the cost of lipids extraction from microalgae, which accounts 3050% of the biodiesel production cost, is 6-10 times higher than that of petroleum diesel oil [3-5]. So the proper breaking-wall method is the key of energy-efficient and low-cost extraction. Comparing with traditional organic solvents and acid/base water solutions, ILs shows its heat and chemical stability, environmental friendliness and better product selectivity in cell wall hydrolysis in contrast to traditional acid/base water solutions and organic solvents for cell hydrolysis and dissolution [6-8]. Due to the difference of chemical polarity and anion, the lipids extraction effect of ILs showed great difference. And the blending of two ionic liquids, which have low lipids extraction yields, enhanced the yields because of the synergy effects of two different anions. However, the high price of ILs is the bottleneck for the commercialization and industrialization in microalgae lipids extraction procedure. The complex and cockamamie separation and purification process of ILs is the principal factor which increases the cost of production. To reduce the cost of IL for the extraction of lipids from chlorella, in this study, industrial grade chemical raw materials (1-chlorobutane and N-methylimidazole) were used to synthetize [BMIM]Cl (1Butyl-3-methylimidazolium Chloride). The separation and purification process were simplified. The performance of the synthesized [BMIM]Cl was compared with that of commercial reagent grade one. The synthesized [BMIM]Cl was recirculated for the Chlorella vulgais extraction and the molecular structure of the recirculated [BMIM]Cl was characterized. 2. Experimental 2.1. Synthesis industrial [BMIM]Cl The Chlorella vulgais was chosen as the experiment material, which was supplied by Jiaxing Zeyuan biological Co., Ltd. The reagent grade [BMIM]Cl was supplied by Shanghai Dibo Chemical Technology Co., Ltd. 1-chlorobutane and N-methylimidazole were supplied by Shanghai Kangtuo chemical Co., Ltd. The experimental schematic is illustrated in Fig.1. 1-chlorobutane and N-methylimidazole was used to synthesis [BMIIM]Cl according to the Roya’s steps [9]. The mole ratio of 1-chlorobutane and Nmethylimidazole was 1.2: 1. The reaction temperature and time were 353 K and 48 h, respectively. During the reaction, the reactants were mixed with a stirring speed of 300~500 rpm. The mixture in the flask stood for layering after the reaction completed. The upper and lower liquids were 1-chlorobutane and [BMIM]Cl, respectively. The reaction product was evaporated to remove the unreacted 1chlorobutane. The electricity consumption on the [BMIM]Cl synthesis was measured using electric meter by comparing the values with and without synthesis reaction. 2.2. Lipids extraction cycle experiment The C. vulgaris was cultivated in an open pond. The harvested C. vulgaris cells were dewatered by centrifugation following the removal of the supernatant, thus achieving a dry mass weight of 10 wt.%. The wet C. vulgaris was mixed with ILs (volume ratio of wet C. vulgaris to IL is 1:1) at room temperature in a flask, and the mixture was agitated using a magnetic stirrer at 383~393 K for 1 h. During heating, the C. vulgaris cells were hydrolysed. Then the flask was quickly immersed in ice water to quench the reaction. When the flask was cooled to room temperature, the ethanol and n-hexane were added into the flask to extract algae lipids for 30 min. Them, the extraction liquid was heated at 343 K. to remove
Haitao Lu et al. / Energy Procedia 105 (2017) 927 – 932
ethanol and n-hexane. Lipid esterification to fatty acid methyl ester (FAME) is as follows: lipids sample was mixed with 6 ml of CH3OH solution with a KOH concentration of 0.4 mol L-1. This solution was heated in a water bath at 343 K for 30 min. Then, 6 ml of CH3OH solution with a H2SO4 concentration of 0.3 mol L-1 and 3 ml of CH3OH solution with a BF3 concentration of 14 wt.% were added into the mixture. The mixture was heated at 343 K for 30 min. The contents of fatty acid methyl ester (FAME), were analyzed by a gas chromatograph (9790 ċ, Agilent Technologies Inc.). Methyl nonadecanoate was used as internal standard substance. For the recovery of [BMIM]Cl, the mixture of [BMIM]Cl and algae residue was centrifuged at 5000 rpm. The recovered [BMIM]Cl was used for the next turn of extraction as mentioned above. After five turns of extraction, the recovered [BMIM]Cl was purified by the alumina column chromatography (ACC) [10].
Fig. 1. Experimental schematic. a. [BMIM]Cl synthesized by industrial raw chemical material (1-chlorobutane and Nmethylimidazole) (T=353K, 48h); b. Wet C. vulgaris and [BMIM]Cl were mixed in flask; c. C. vulgaris cell wall hydrolysis (T=383K, 1h); d. Lipids were extracted from lysis by n-hexane and ethyl alcohol; e. Lipids esterification was taken to form FAME determined by GC; f. [BMIM]Cl was recovered and purified by alumina column chromatography (AAC).
3. Results and discussion 3.1. Lipids extraction experiment The raw material and electricity costs for the synthesis of [BMIM]Cl are shown in Table 1. The total cost of the synthesized [BMIM]Cl was 7851 U.S. $ per ton [BMIM]Cl, which is 1.63% of that of the reagent grade [BMIM]Cl. The cost reduction is mainly attributed to the simplification on the purification of [BMIM]Cl. The lipids yield by the synthesized [BMIM]Cl at various turns are shown in Fig. 2. The average of the lipids yield was 75.4%, which is lower than that of 85.4% using commercial reagent grade [BMIM]Cl. This is because the synthesized [BMIM]Cl contained a variety of impurities including the unreacted reactants and by-products considering that the purification treatment was simplified. These impurities most likely hindered the hydrolysis ability of [BMIM]Cl. Given the same biodiesel production using the lipids from C. vulgaris, the initial cost of the synthesized [BMIM]Cl for the extraction is 1.85% that of commercial reagent grade one. This result provides the feasibility of [BMIM]Cl for the algae extraction because the commercial IL is too expensive to be used in the real application.
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Table 1. The cost accounting of industrial IL
Cost (U.S. $ per ton of [BMIM]Cl)
1chlorobutane
N-methyl imidazole
ethyl acetate
acetonitrile
electricity cost
Total
1820
4241
1008
572
211
7851
Fig. 2. Lipids yields by synthesized [BMIM]Cl at various turns.
3.2. Ionic liquid recycling It was observed that the viscosity of recovered [BMIM]Cl increased with the rise in extraction turns. The recovered [BMIM]Cl was characterized by Nuclear Magnetic Resonance (AVANCE ċ, Bruker Corporation) to identify its molecular structure. As shown in Fig. 3, The chemical shift of 1H NMR of standard [BMIM]Cl is as follows: δ 8.58[1H, s], 7.34[1H, m], 7.28[1H, m], 4.06[2H, t], 3.75[3H, s], 1.71[2H, m], 1.16[2H, m], 0.78[3H, t]. This chemical shift of 1H NMR can be observed in the synthesized [BMIM]Cl. In addition, for the fresh synthesized [BMIM]Cl, there were new peaks appearing at 7.49, 6.98, 6.85 and 1.94 ppm, which correspond to impurities (the detailed structure needs further investigation in the future study). After five extraction turns, these peaks ascribed to impurities disappeared, indicating that the separation by centrifugation gradually removed the impurities. After five extraction turns, the [BMIM]Cl was purified by AAC separation to get rid of the hydrolysate dissolved in [BMIM]Cl. The result showed almost 88.6% of [BMIM]Cl could be recovered. The loss of the [BMIM]Cl using AAC separation is too high and unacceptable for the commercialisation of the algae
Haitao Lu et al. / Energy Procedia 105 (2017) 927 – 932
extraction using synthesized [BMIM]Cl. In the following study, we plan to promote the performance of AAC separation, increase the dry mass concentration in wet algae, and increase the turns of extraction so that the [BMIM]Cl loss per ton obtained lipids can be significantly reduced.
Fig. 3. 1H NMR of recycling [BMIM]Cl. a. fresh [BMIM]Cl synthesised by industrial raw chemical material; b. [BMIM]Cl after five turns of lipids extraction; c. fresh [BMIM]Cl supplied by Shanghai Dibo Chemical Technology Co., Ltd.
4. Conclusions The total fabrication cost of the synthesized [BMIM]Cl was 1.63% of the price of commercial reagent ILs. The average of the lipids yield was 75.4%, which is lower than that of 85.4% using commercial reagent grade [BMIM]Cl. The initial cost of the synthesized [BMIM]Cl for the extraction was 1.85% that of commercial reagent grade one. The IL loss due to [BMIM]Cl purification using alumina column chromatography was 88.6%. The impurities contained in the synthesized [BMIM]Cl was gradually removed with the recirculation of [BMIM]Cl. Acknowledgements This study was financially supported by the China Natural Science Foundation (Contract No. 21176069, 21476073), Program for New Century Excellent Talents in University (NCET-10-0380) and the Fundamental Research Funds for the Central Universities (WG1213011).
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References [1] Miao X, Wu Q. Biodiesel production from heterotrophic microalgal oil. J Bioresour. Technol. 2006;97:841-6. [2] Teixeira RE. Energy-efficient extraction of fuel and chemical feedstocks from algae. J Green Chem. 2012;14:419-27. [3] Wijffels RH, Barbosa MJ. An Outlook on Microalgal Biofuels. J Science. 2010;329:796-9. [4] Chisti Y. Biodiesel from microalgae. J Biotechnol. Adv. 2007;25:294-306. [5] Pienkos PT, Darzins A. The promise and challenges of microalgal-derived biofuels. J Biofpr. 2009;3:431-40. [6] Xu W, Dolbier WR, Salazar J. Ionic liquid, surrogate hydrogen bromide reagent for ring opening of cyclopropyl ketones. J. Org. Chem. 2008;73:3535-8. [7] Binder JB, Raines RT. Simple Chemical Transformation of Lignocellulosic Biomass into Furans for Fuels and Chemicals. J. Am. Chem. Soc. 2009;131:1979-85. [8] Yang Z, Pan WB. Ionic liquids: Green solvents for nonaqueous biocatalysis. J Enzyme Microb. Technol. 2005;37:19-28. [9] Jahanshahi R, Akhlaghinia B. Expanded perlite: an inexpensive natural efficient heterogeneous catalyst for the green and highly accelerated solvent-free synthesis of 5-substituted-1H-tetrazoles using [bmim]N3 and nitriles. J RSC Adv. 2015;5:104087-94. [10] Feng D, Li L, Yang F, Tan W, Zhao G, Zou H, et al. Separation of ionic liquid [Mmim][DMP] and glucose from enzymatic hydrolysis mixture of cellulose using alumina column chromatography. J Appl. Microbiol. Biotechnol. 2011;91:399-405.
ScienceDirect Energy Procedia 105 (2017) 927 – 932
The 8th International Conference on Applied Energy – ICAE2016
Low-cost and high-efficient extraction of lipids from chlorella by using industrial ionic liquids Haitao Lua, Xinhai Yua,b,*, Shan-Tung Tu a
Key Laboratory of Pressure Systems and Safety, Ministry of Education, School of Mechanical and Power Engineering, East China University of Science and Technology, Shanghai, China b State Key Laboratory of Bioreactor Engineering, East China University of Science and Technology, Shanghai, China
Abstract
Algae-sourced feedstocks remain confined to commercialization because of the high cost and energy consumption of biomass cultivation and feedstock extraction. In this study, to reduce the cost required for algae extraction, experiments with Chlorella vulgais extraction by ionic liquids (ILs) synthesized using industrial raw materials. The total fabrication cost of the synthesized [BMIM]Cl was 1.63% of the price of commercial reagent ILs. The average of the lipids yield was 75.4%, which is lower than that of 85.4% using commercial reagent grade [BMIM]Cl. The initial cost of the synthesized [BMIM]Cl for the extraction was 1.85% that of commercial reagent grade one. The IL loss due to [BMIM]Cl purification using alumina column chromatography was 88.6%. The impurities contained in the synthesized [BMIM]Cl was gradually removed with the recirculation of [BMIM]Cl.
© 2017 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2016 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or peer-review under responsibility of ICAE Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy.
Keywords: lipids extraction; chlorella;industrial ionic liquid;
1. Introduction Biodiesel has attracted considerable attention in recent years, as it is a biodegradable, renewable and non-toxic fuel. Compared with the conventional energy crops, chlorella can be used as the feedstock of biodiesel because of its higher photosynthetic efficiency, faster growth and higher biomass production
* Corresponding author. Tel.: +86-(0)21-6425-3513; fax: +86-(0)21-6425-3513. E-mail address: [email protected]. Xinhai Yu.
1876-6102 © 2017 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy. doi:10.1016/j.egypro.2017.03.419
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Haitao Lu et al. / Energy Procedia 105 (2017) 927 – 932
(55% in heterotrophic Chlorella protothecoides) [1]. Meanwhile, chlorella also has plenty of available biomass, such as protein, saccharides and carbohydrate. But its integrated cell wall structure is the maintain obstacle to efficiently extracting lipids and other biomass from chlorella. Because of high toughness of cell wall (9 kJ kg-1) [2], the cost of lipids extraction from microalgae, which accounts 3050% of the biodiesel production cost, is 6-10 times higher than that of petroleum diesel oil [3-5]. So the proper breaking-wall method is the key of energy-efficient and low-cost extraction. Comparing with traditional organic solvents and acid/base water solutions, ILs shows its heat and chemical stability, environmental friendliness and better product selectivity in cell wall hydrolysis in contrast to traditional acid/base water solutions and organic solvents for cell hydrolysis and dissolution [6-8]. Due to the difference of chemical polarity and anion, the lipids extraction effect of ILs showed great difference. And the blending of two ionic liquids, which have low lipids extraction yields, enhanced the yields because of the synergy effects of two different anions. However, the high price of ILs is the bottleneck for the commercialization and industrialization in microalgae lipids extraction procedure. The complex and cockamamie separation and purification process of ILs is the principal factor which increases the cost of production. To reduce the cost of IL for the extraction of lipids from chlorella, in this study, industrial grade chemical raw materials (1-chlorobutane and N-methylimidazole) were used to synthetize [BMIM]Cl (1Butyl-3-methylimidazolium Chloride). The separation and purification process were simplified. The performance of the synthesized [BMIM]Cl was compared with that of commercial reagent grade one. The synthesized [BMIM]Cl was recirculated for the Chlorella vulgais extraction and the molecular structure of the recirculated [BMIM]Cl was characterized. 2. Experimental 2.1. Synthesis industrial [BMIM]Cl The Chlorella vulgais was chosen as the experiment material, which was supplied by Jiaxing Zeyuan biological Co., Ltd. The reagent grade [BMIM]Cl was supplied by Shanghai Dibo Chemical Technology Co., Ltd. 1-chlorobutane and N-methylimidazole were supplied by Shanghai Kangtuo chemical Co., Ltd. The experimental schematic is illustrated in Fig.1. 1-chlorobutane and N-methylimidazole was used to synthesis [BMIIM]Cl according to the Roya’s steps [9]. The mole ratio of 1-chlorobutane and Nmethylimidazole was 1.2: 1. The reaction temperature and time were 353 K and 48 h, respectively. During the reaction, the reactants were mixed with a stirring speed of 300~500 rpm. The mixture in the flask stood for layering after the reaction completed. The upper and lower liquids were 1-chlorobutane and [BMIM]Cl, respectively. The reaction product was evaporated to remove the unreacted 1chlorobutane. The electricity consumption on the [BMIM]Cl synthesis was measured using electric meter by comparing the values with and without synthesis reaction. 2.2. Lipids extraction cycle experiment The C. vulgaris was cultivated in an open pond. The harvested C. vulgaris cells were dewatered by centrifugation following the removal of the supernatant, thus achieving a dry mass weight of 10 wt.%. The wet C. vulgaris was mixed with ILs (volume ratio of wet C. vulgaris to IL is 1:1) at room temperature in a flask, and the mixture was agitated using a magnetic stirrer at 383~393 K for 1 h. During heating, the C. vulgaris cells were hydrolysed. Then the flask was quickly immersed in ice water to quench the reaction. When the flask was cooled to room temperature, the ethanol and n-hexane were added into the flask to extract algae lipids for 30 min. Them, the extraction liquid was heated at 343 K. to remove
Haitao Lu et al. / Energy Procedia 105 (2017) 927 – 932
ethanol and n-hexane. Lipid esterification to fatty acid methyl ester (FAME) is as follows: lipids sample was mixed with 6 ml of CH3OH solution with a KOH concentration of 0.4 mol L-1. This solution was heated in a water bath at 343 K for 30 min. Then, 6 ml of CH3OH solution with a H2SO4 concentration of 0.3 mol L-1 and 3 ml of CH3OH solution with a BF3 concentration of 14 wt.% were added into the mixture. The mixture was heated at 343 K for 30 min. The contents of fatty acid methyl ester (FAME), were analyzed by a gas chromatograph (9790 ċ, Agilent Technologies Inc.). Methyl nonadecanoate was used as internal standard substance. For the recovery of [BMIM]Cl, the mixture of [BMIM]Cl and algae residue was centrifuged at 5000 rpm. The recovered [BMIM]Cl was used for the next turn of extraction as mentioned above. After five turns of extraction, the recovered [BMIM]Cl was purified by the alumina column chromatography (ACC) [10].
Fig. 1. Experimental schematic. a. [BMIM]Cl synthesized by industrial raw chemical material (1-chlorobutane and Nmethylimidazole) (T=353K, 48h); b. Wet C. vulgaris and [BMIM]Cl were mixed in flask; c. C. vulgaris cell wall hydrolysis (T=383K, 1h); d. Lipids were extracted from lysis by n-hexane and ethyl alcohol; e. Lipids esterification was taken to form FAME determined by GC; f. [BMIM]Cl was recovered and purified by alumina column chromatography (AAC).
3. Results and discussion 3.1. Lipids extraction experiment The raw material and electricity costs for the synthesis of [BMIM]Cl are shown in Table 1. The total cost of the synthesized [BMIM]Cl was 7851 U.S. $ per ton [BMIM]Cl, which is 1.63% of that of the reagent grade [BMIM]Cl. The cost reduction is mainly attributed to the simplification on the purification of [BMIM]Cl. The lipids yield by the synthesized [BMIM]Cl at various turns are shown in Fig. 2. The average of the lipids yield was 75.4%, which is lower than that of 85.4% using commercial reagent grade [BMIM]Cl. This is because the synthesized [BMIM]Cl contained a variety of impurities including the unreacted reactants and by-products considering that the purification treatment was simplified. These impurities most likely hindered the hydrolysis ability of [BMIM]Cl. Given the same biodiesel production using the lipids from C. vulgaris, the initial cost of the synthesized [BMIM]Cl for the extraction is 1.85% that of commercial reagent grade one. This result provides the feasibility of [BMIM]Cl for the algae extraction because the commercial IL is too expensive to be used in the real application.
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Table 1. The cost accounting of industrial IL
Cost (U.S. $ per ton of [BMIM]Cl)
1chlorobutane
N-methyl imidazole
ethyl acetate
acetonitrile
electricity cost
Total
1820
4241
1008
572
211
7851
Fig. 2. Lipids yields by synthesized [BMIM]Cl at various turns.
3.2. Ionic liquid recycling It was observed that the viscosity of recovered [BMIM]Cl increased with the rise in extraction turns. The recovered [BMIM]Cl was characterized by Nuclear Magnetic Resonance (AVANCE ċ, Bruker Corporation) to identify its molecular structure. As shown in Fig. 3, The chemical shift of 1H NMR of standard [BMIM]Cl is as follows: δ 8.58[1H, s], 7.34[1H, m], 7.28[1H, m], 4.06[2H, t], 3.75[3H, s], 1.71[2H, m], 1.16[2H, m], 0.78[3H, t]. This chemical shift of 1H NMR can be observed in the synthesized [BMIM]Cl. In addition, for the fresh synthesized [BMIM]Cl, there were new peaks appearing at 7.49, 6.98, 6.85 and 1.94 ppm, which correspond to impurities (the detailed structure needs further investigation in the future study). After five extraction turns, these peaks ascribed to impurities disappeared, indicating that the separation by centrifugation gradually removed the impurities. After five extraction turns, the [BMIM]Cl was purified by AAC separation to get rid of the hydrolysate dissolved in [BMIM]Cl. The result showed almost 88.6% of [BMIM]Cl could be recovered. The loss of the [BMIM]Cl using AAC separation is too high and unacceptable for the commercialisation of the algae
Haitao Lu et al. / Energy Procedia 105 (2017) 927 – 932
extraction using synthesized [BMIM]Cl. In the following study, we plan to promote the performance of AAC separation, increase the dry mass concentration in wet algae, and increase the turns of extraction so that the [BMIM]Cl loss per ton obtained lipids can be significantly reduced.
Fig. 3. 1H NMR of recycling [BMIM]Cl. a. fresh [BMIM]Cl synthesised by industrial raw chemical material; b. [BMIM]Cl after five turns of lipids extraction; c. fresh [BMIM]Cl supplied by Shanghai Dibo Chemical Technology Co., Ltd.
4. Conclusions The total fabrication cost of the synthesized [BMIM]Cl was 1.63% of the price of commercial reagent ILs. The average of the lipids yield was 75.4%, which is lower than that of 85.4% using commercial reagent grade [BMIM]Cl. The initial cost of the synthesized [BMIM]Cl for the extraction was 1.85% that of commercial reagent grade one. The IL loss due to [BMIM]Cl purification using alumina column chromatography was 88.6%. The impurities contained in the synthesized [BMIM]Cl was gradually removed with the recirculation of [BMIM]Cl. Acknowledgements This study was financially supported by the China Natural Science Foundation (Contract No. 21176069, 21476073), Program for New Century Excellent Talents in University (NCET-10-0380) and the Fundamental Research Funds for the Central Universities (WG1213011).
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References [1] Miao X, Wu Q. Biodiesel production from heterotrophic microalgal oil. J Bioresour. Technol. 2006;97:841-6. [2] Teixeira RE. Energy-efficient extraction of fuel and chemical feedstocks from algae. J Green Chem. 2012;14:419-27. [3] Wijffels RH, Barbosa MJ. An Outlook on Microalgal Biofuels. J Science. 2010;329:796-9. [4] Chisti Y. Biodiesel from microalgae. J Biotechnol. Adv. 2007;25:294-306. [5] Pienkos PT, Darzins A. The promise and challenges of microalgal-derived biofuels. J Biofpr. 2009;3:431-40. [6] Xu W, Dolbier WR, Salazar J. Ionic liquid, surrogate hydrogen bromide reagent for ring opening of cyclopropyl ketones. J. Org. Chem. 2008;73:3535-8. [7] Binder JB, Raines RT. Simple Chemical Transformation of Lignocellulosic Biomass into Furans for Fuels and Chemicals. J. Am. Chem. Soc. 2009;131:1979-85. [8] Yang Z, Pan WB. Ionic liquids: Green solvents for nonaqueous biocatalysis. J Enzyme Microb. Technol. 2005;37:19-28. [9] Jahanshahi R, Akhlaghinia B. Expanded perlite: an inexpensive natural efficient heterogeneous catalyst for the green and highly accelerated solvent-free synthesis of 5-substituted-1H-tetrazoles using [bmim]N3 and nitriles. J RSC Adv. 2015;5:104087-94. [10] Feng D, Li L, Yang F, Tan W, Zhao G, Zou H, et al. Separation of ionic liquid [Mmim][DMP] and glucose from enzymatic hydrolysis mixture of cellulose using alumina column chromatography. J Appl. Microbiol. Biotechnol. 2011;91:399-405.