- Email: [email protected]
A microalgae residue based carbon solid acid catalyst for biodiesel production
Bioresource Technology 146 (2013) 767–770
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Short Communication
A microalgae residue based carbon solid acid catalyst for biodiesel production Xiaobo Fu a,1, Dianhong Li a,1, Jie Chen a, Yuanming Zhang a,⇑, Weiya Huang b, Yi Zhu a, Jun Yang a, Chengwu Zhang b a b
Department of Chemistry, Jinan University, Guangzhou 510632, China Research Center of Hydrobiology, Jinan University, Guangzhou 510632, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A microalgae residue based carbon
solid acid catalyst was successfully prepared. The catalyst was in amorphous nature and had high density of functional groups. The catalyst showed high catalytic activity for biodiesel production. The catalyst can be regenerated and maintain high activity after five cycles.
a r t i c l e
i n f o
Article history: Received 29 May 2013 Received in revised form 21 July 2013 Accepted 25 July 2013 Available online 29 July 2013 Keywords: Biodiesel Solid acid Esterification Transesterification Microalgae residue
Microalage
Microalage residue
Oil extracted
Biodiesel
In situ partially hydrothermal carbonnization with H2SO4
a b s t r a c t Biodiesel production from microalgae is recognized as one of the best solutions to deal with the energy crisis issues. However, after the oil extraction from the microalgae, the microalgae residue was generally discarded or burned. Here a novel carbon-based solid acid catalyst derived from microalgae residue by in situ hydrothermal partially carbonization were synthesized. The obtained catalyst was characterized and subjected to both the esterification of oleic acid and transesterification of triglyceride to produce biodiesel. The catalyst showed high catalytic activity and can be regenerated while its activity can be well maintained after five cycles. Ó 2013 Elsevier Ltd. All rights reserved.
As traditional fossil fuel is not renewable and the exhaustion of fossil fuel is foreseen in the next several decades, finding alternative fuels have become increasingly attractive. Biodiesel, which consisting of fatty acid methyl ester, is renewable, biodegradable, non-toxic while it produces less sulfur dioxide and unburned gases than fossil fuel. Great effort has been devoted to produce biodiesel recently. Generally, Biodiesel is produced from edible vegetable
⇑ Corresponding author. Fax: +86 2085225036. 1
Solid acid catalyst
E-mail address: [email protected] (Y. Zhang). These authors contributed equally to this article.
0960-8524/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2013.07.117
and animal oils, which raise the pricing problem and crucial sustainability issues. The bottleneck are known as the cost related to the limited available of fat or oil resources and the catalyst chose (de Lima da Silva et al., 2010). In a broad sense, the edible vegetable oils are extracted from the crops. And the increased demand for these crops will heighten their price in food market, place additional requirements on the often strained agriculture system and impose a negative environment impact through the additional energy requirements and eutrophication which caused by intensive agricultural processes (Griffiths and Harrison, 2009). Therefore, finding an inexpensive source of feedstock lipids that does not
768
X. Fu et al. / Bioresource Technology 146 (2013) 767–770
use farmland is indispensable for biodiesel to compete economically with traditional fossil fuel. Simultaneously, finding a profitability of the catalytic process is also expected (Marchetti et al., 2008). Microalgae, a single-celled algae, has been recognized as the most promising candidate to convert solar energy into fuel through photosynthesis, mainly because of their potential for reducing the amount of land and water surface needed to produce fuels (Hunter, 2010) and converting solar energy more efficiently. Moreover, it has less negative environmental impact than the alternatives, especially biofuel crops such as corn and soy. In addition to its high biomass productivity, it is reported that microalgae can be comprised of 1–85% lipids by dry weight. Microalgae’s high growth rate and lipid content would make it possible to produce enough biodiesel to satisfy the enormous demand for fuels in a relatively small area (Krohn et al., 2011). Thus, to deal with the cost issue of biodiesel production, utilizing the oils extracted from the microalgae as the feedstock has been recognized as the one of the best solutions. To the catalytic process, there have been two kinds of catalysts existed, such as base catalyst and acid catalyst. Since the content of free fatty acid is high in many cheap feedstocks or not refined natural oil, the soap formation cannot be avoided if the amount of FFAs in the feedstock exceeded 1% during the base-catalyzed process, which increases the difficulty of separation of the mixtures (fatty acid methyl ester–glycerol) (Marchetti et al., 2007). A homogeneous acid such as H2SO4 can simultaneously catalyze esterification and transesterification, so it performs well with oils containing high FFAs than base catalysts. However it also suffers from a series of drawbacks, such as lower reaction rates, corrosiveness and the high methanol to oil molar ratio requirements (Soriano et al., 2009). The most popular method for addressing this issue is by acid-catalyzed esterification, followed by base-catalyzed transesterification. The first step involved the esterification reaction between methanol and FFA to produce the corresponding fatty acid methyl ester (FAME) using the homogeneous acid catalyst. However, long reaction time, no recovery of catalyst and high cost of reaction equipment were disadvantages of this process. To reduce energy consumption and processing costs, heterogeneous catalysts have been considered a viable alternative because it can eliminate separation, corrosion, toxicity and environmental problems (West et al., 2008). Recently, among the solid acid catalysts reported, a new class of sulfonated carbon-based solid acid catalyst was demonstrated as promising catalysts for the esterification reaction for the production of biodiesel. These catalysts can be cheaply and easily produced by incomplete carbonization of sulfopolycyclic aromatic hydrocarbons (such as the sulfonate derivative produced of naphthalene with concentrated H2SO4) or sulfonation of incompletely carbonized inorganic/organic compounds (Hao et al., 2008; Nakajima et al., 2007). In general, carbon-based solid acid catalysts prepared from the sulfonation of carbonized compounds, such as starch, glucose, can form a rigid carbon material composed of small polycyclic aromatic carbon sheets (a three dimensional sp3-bonded structure) by carbonization (Zong et al., 2007), through sulfonating of such carbon materials can be very likely to form a highly stable solid with a high density of active sites. These carbon solid acid catalysts also showed high catalytic activity and stability. To overcome some of the above problems, in the present communication, the synthesis of a carbon based solid acid catalyst with high densities of sulfonic acid groups that is based on the relatively cheap raw material–microalgae residue were reported. The material–microalgae residue is obtained as a waste during the biodiesel production from microalgae. The microalgae-residue based catalyst (MRC) was obtained by in situ partial carbonization and sulfonation of microalgae-residue with sulfuric acid, in a ratio of 1:12
(wt/wt) in an autoclave within 2 h. The catalyst was subjected to esterification of oleic acid and transesterification of triolein to test its catalytic activities. The obtained catalyst was thoroughly characterized to establish its physical and chemical characteristics. The structure of the catalyst was first determined by X-ray diffraction (XRD), as shown in Fig. S1a (Supporting information). The catalyst exhibited one broad diffraction peak at 2h angle of 10–30°, which is attributable to amorphous carbon composed of aromatic carbon sheets oriented in a considerably random form (Okamura et al., 2006; Tsubouchi et al., 2003). This structure is far from crystal and is reported to be favorable for anchoring the –SO3H groups. A scanning electron microscopy image (SEM) (Fig. S1b) showed the morphology of irregular particles, with a grain size larger than 10 lm, which is similar to the carbon catalyst prepared from glycerol reported by (Devi et al., 2009). Furthermore, the morphology showed nonporous characteristics, which was supported by its low BET surface areas less than 1 m2 g 1 determined by nitrogen adsorption measurement. Fig. S2 shows a Fourier transform-infrared (FT-IR) absorption spectrum for the catalyst. The spectrum for the sample have vibration bands at 1167 and 1027 cm 1, which are identified as O=S=O symmetric stretching and —SO23 stretching modes in –SO3H group, respectively. These groups were the active sites of the catalyst. The band at 3424 and 3429 cm 1 were assigned to the O–H stretching modes of the –COOH and phenolic –OH groups. The band at 1713 cm 1 was assigned to the –C=O stretching mode of the – COOH group. The broad band at 2300–2700 cm 1 was assigned to an overtone of the bending mode of –OH O= linked by a strong hydrogen bond, suggesting that some –SO3H groups are in close proximity to each other, which was accordant with the results reported by Hara (2010). X-ray photoelectroscopy (XPS) measurement of the catalyst was conducted to determine its chemical composition, seeing Fig. S3a. The single O 1s, C 1s and S 2p peak was observed respectively at 531.64 eV, 284.64 eV and 168.64 eV, which indicated no other impurities existed in the catalyst. The XPS results confirmed that amount of –COOH groups formed in the one-step hydrothermal carbonization process which led to higher acid site density. The single S 2p peak centered at 168.64 eV meant that essentially all S atoms in catalyst can be assigned to –SO3H groups. The acidity of the catalyst was determined through the neutralization titration (Chen and Fang, 2011), which was about 4.25 mmol/g. The –SO3H density is determined through XPS characterization (Fu et al., 2012). The results showed that the content of the sulfur is about 0.90 mmol/g. According to the results from the elemental analysis, the MBC catalyst was found to have the elemental composition of CH0.55S0.02O0.56. A Raman spectrum (Fig. S3b) of the catalyst showed two broad peaks at about 1330 and 1580 cm 1 corresponding to the G (graphitic) and D (disorder) modes, respectively, indicating that the MBC is partially crystalline in nature. Moreover, the intensity ratio of the D band to the G band for the MBC catalyst is about 0.81, indicating that the average grapheme size in the amorphous carbon is ca. 1 nm. These characterization results indicate that the minimum unit in the catalyst is a nano-graphene sheets comprised of about 10–20 six membered rings. The graphene sheets have a high density of –SO3H, –OH and –COOH groups. This is distinct from conventional solid acids with single functional groups. The results in this short communication were similar to the carbon solid acid catalyst derived from cellulose powder reported by Hara (2010). Considering that the main components of the microalgae residue was the cellulose, it is reasonable that the similar carbon solid structure with high density of functional groups were obtained. In a word, a carbon solid acid catalyst with high density of functional groups can be effectively prepared from microalgae residue.
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100
(a)
Conv.(%)
80 MBC catalyst Amberlyst-15
60 40 20 0
0
2
4
6
8
10
12
Reaction time (h)
80 20 60 40
10
20 0
1
2
3
4
5
6
7
Yield of methyl ester %
30
100
Conv.%
The catalyst was firstly evaluated in the esterification of oleic acid with methanol and then in the transesterification of triolein with methanol for the production of biodiesel at 80 °C in the presence of 5 wt% of catalyst. As shown in Fig. 1a, about 98% conversion of the esterification reaction and 24% yield of transesterification reaction in 12 h were achieved. Fig. 1a shows the catalytic results of the MBC catalyst in comparison to a typical solid acid catalyst, amberlyst-15. The MBC catalyst showed a much higher esterification activity. As shown in Fig. 1b, the MBC catalyst showed higher yield of methyl ester in the transesterification of triolein, confirming the high catalytic activity of the MBC catalyst. It is well known that the amberlyst-15 was an ion exchange resin bearing macropores. Although it has high acid density (about 5.3 mmol/g), most of acid sites resided in a poorly swelling network can only be penetrated by small ions (H+, Na+) instead of large molecules such as oleic acid (Song et al., 2012). With respect to the MBC catalyst, the high catalytic activity cannot be explained simply by the high density of strongly acidic –SO3H groups and good access of reactants to the –SO3H groups in the carbon material. It is reported that in addition to the high density of acidic –SO3H groups, the hydrophilic –COOH and –OH groups in the carbon catalyst also played an important role in promoting the activity of the catalyst (Hara, 2010). One possible explanation for the high catalytic activity of the solid acid carbon catalyst bearing –SO3H, –COOH and phenolic –OH groups may be the strong affinity between the hydrophilic parts of the reactants and the almost neutral –OH groups bonded to carbon catalyst matrix (Geng et al., 2011). Thus, the strong affinity favored the dispersion of the catalyst in methanol, resulting in its high catalytic activity.
0
Cycle number Fig. 2. Recycling performance of the MBC catalyst in methyl oleate formation by esterification of oleic acid with methanol (red pillar) and by transesterification of olein with methanol (black pillar). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
During the reusable process, the MBC catalyst lost its activity gradually in the esterification reaction for the first five cycles by washing with methanol, but it can be recovered by filtration after the batch experiment with concentrated sulfuric acid and then washing with the hot water. The deactivation of the catalyst was mainly caused by the extensive washing with methanol, which can react with the sulfonated groups leading to the leaching of the sulfonated groups during the washing process. The results were coincided with results of Chen and Fang (2011). After the regeneration, it was found that the catalytic activity of the esterification can be still maintained with over 95% conversion after five runs, as shown in Fig. 2. Moreover, the yield of methyl oleate from the transesterification can be maintained with over 22%, demonstrating the stability of the catalyst. The mechanism of catalyst regeneration by concentrated sulfuric acid has to be further researched. In conclusion, a novel carbon solid acid catalyst has been successfully developed via an in situ hydrothermal partially carbonization using microalgae residue as the starting materials. The obtained MBC catalyst has shown highly catalytic activity in esterification of oleic acid and transesterification of triolein with methanol. This result indicates that the new route of synthesis has provided a sustainable way for the biodiesel production. In addition to biodiesel production, such an effective, inexpensive and reusable polycyclic aromatic carbon acid catalyst has great potential for a wide range of other acid-catalyzed reactions.
25
Yield of methyl ester %
(b) 20
Acknowledgements
15
MBC catalyst Amberlyst-15
10 5 0
0
2
4
6
8
10
The authors would like to thank for the supports from Natural Science Foundation of Guangdong (No. S2011040001667), National High Technology Research and Development Program 863 (Nos. 2009AA064401, 2013AA065805), Major State Basic Research Development Program of China (No. 2011CB2009001) Natural Science Foundation of China (No. 21276104) and the Fundamental Research Funds for the Central Universities (No. 11612328).
12
Reaction time (h) Fig. 1. (a) Esterification of oleic acid with methanol by the MBC catalyst and the amberlyst-15 catalyst and (b) transesterification of triolein with methanol by the MBC catalyst and the amberlyst-15 catalyst.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.biortech.2013. 07.117.
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References Chen, G., Fang, B., 2011. Preparation of solid acid catalyst from glucose–starch mixture for biodiesel production. Bioresource Technology 102 (3), 2635–2640. de Lima da Silva, N., Santander, C., Batistella, C., Filho, R., Maciel, M., 2010. Biodiesel production from integration between reaction and separation system: reactive distillation process. Applied Biochemistry and Biotechnology 161 (1–8), 245– 254. Devi, B., Gangadhar, K.N., Prasad, P.S.S., Jagannadh, B., Prasad, R.B.N., 2009. A glycerol-based carbon catalyst for the preparation of biodiesel. ChemSusChem 2 (7), 617–620. Fu, Z., Wan, H., Hu, X., Cui, Q., Guan, G., 2012. Preparation and catalytic performance of a carbon-based solid acid catalyst with high specific surface area. Reaction Kinetics, Mechanisms and Catalysis 107 (1), 203–213. Geng, L., Wang, Y., Yu, G., Zhu, Y., 2011. Efficient carbon-based solid acid catalysts for the esterification of oleic acid. Catalysis Communications 13 (1), 26–30. Griffiths, M., Harrison, S.L., 2009. Lipid productivity as a key characteristic for choosing algal species for biodiesel production. Journal of Applied Phycology 21 (5), 493–507. Hao, S., Takai, K., Feiyu, K., Enoki, T., 2008. Electronic and magnetic properties of acid-adsorbed nanoporous activated carbon fibers. Carbon 46 (1), 110–116. Hara, M., 2010. Biodiesel production by amorphous carbon bearing SO3H, COOH and phenolic OH groups, a solid Brønsted acid catalyst. Topics in Catalysis 53 (11– 12), 805–810. Hunter, P., 2010. The tide turns towards microalgae. Current research aims to produce traditional biofuels from algae, but their potential to generate sustainable energy might be even greater and more ‘natural’. EMBO Reports 11 (8), 583–586.
Krohn, B.J., McNeff, C.V., Yan, B., Nowlan, D., 2011. Production of algae-based biodiesel using the continuous catalytic McgyanÒ process. Bioresource Technology 102 (1), 94–100. Marchetti, J., Miguel, V., Errazu, A., 2007. Heterogeneous esterification of oil with high amount of free fatty acids. Fuel 86 (5), 906–910. Marchetti, J.M., Miguel, V.U., Errazu, A.F., 2008. Techno-economic study of different alternatives for biodiesel production. Fuel Processing Technology 89 (8), 740– 748. Nakajima, K., Hara, M., Hayashi, S., 2007. Environmentally benign production of chemicals and energy using a carbon-based strong solid acid. Journal of the American Ceramic Society 90 (12), 3725–3734. Okamura, M., Takagaki, A., Toda, M., Kondo, J.N., Domen, K., Tatsumi, T., Hara, M., Hayashi, S., 2006. Acid-catalyzed reactions on flexible polycyclic aromatic carbon in amorphous carbon. Chemistry of Materials 18 (13), 3039–3045. Song, X.L., Fu, X.B., Zhang, C.W., Huang, W.Y., Zhu, Y., Yang, J., Zhang, Y.M., 2012. Preparation of a novel carbon based solid acid catalyst for biodiesel production via a sustainable route. Catalysis Letters 142 (7), 869–874. Soriano, N.U., Venditti, R., Argyropoulos, D.S., 2009. Biodiesel synthesis via homogeneous Lewis acid-catalyzed transesterification. Fuel 88 (3), 560–565. Tsubouchi, N., Xu, C., Ohtsuka, Y., 2003. Carbon crystallization during hightemperature pyrolysis of coals and the enhancement by calcium. Energy & Fuels 17 (5), 1119–1125. West, A.H., Posarac, D., Ellis, N., 2008. Assessment of four biodiesel production processes using HYSYS.Plant. Bioresource Technology 99 (14), 6587–6601. Zong, M.H., Duan, Z.Q., Lou, W.Y., Smith, T.J., Wu, H., 2007. Preparation of a sugar catalyst and its use for highly efficient production of biodiesel. Green Chemistry 9 (5), 434–437.
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Short Communication
A microalgae residue based carbon solid acid catalyst for biodiesel production Xiaobo Fu a,1, Dianhong Li a,1, Jie Chen a, Yuanming Zhang a,⇑, Weiya Huang b, Yi Zhu a, Jun Yang a, Chengwu Zhang b a b
Department of Chemistry, Jinan University, Guangzhou 510632, China Research Center of Hydrobiology, Jinan University, Guangzhou 510632, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A microalgae residue based carbon
solid acid catalyst was successfully prepared. The catalyst was in amorphous nature and had high density of functional groups. The catalyst showed high catalytic activity for biodiesel production. The catalyst can be regenerated and maintain high activity after five cycles.
a r t i c l e
i n f o
Article history: Received 29 May 2013 Received in revised form 21 July 2013 Accepted 25 July 2013 Available online 29 July 2013 Keywords: Biodiesel Solid acid Esterification Transesterification Microalgae residue
Microalage
Microalage residue
Oil extracted
Biodiesel
In situ partially hydrothermal carbonnization with H2SO4
a b s t r a c t Biodiesel production from microalgae is recognized as one of the best solutions to deal with the energy crisis issues. However, after the oil extraction from the microalgae, the microalgae residue was generally discarded or burned. Here a novel carbon-based solid acid catalyst derived from microalgae residue by in situ hydrothermal partially carbonization were synthesized. The obtained catalyst was characterized and subjected to both the esterification of oleic acid and transesterification of triglyceride to produce biodiesel. The catalyst showed high catalytic activity and can be regenerated while its activity can be well maintained after five cycles. Ó 2013 Elsevier Ltd. All rights reserved.
As traditional fossil fuel is not renewable and the exhaustion of fossil fuel is foreseen in the next several decades, finding alternative fuels have become increasingly attractive. Biodiesel, which consisting of fatty acid methyl ester, is renewable, biodegradable, non-toxic while it produces less sulfur dioxide and unburned gases than fossil fuel. Great effort has been devoted to produce biodiesel recently. Generally, Biodiesel is produced from edible vegetable
⇑ Corresponding author. Fax: +86 2085225036. 1
Solid acid catalyst
E-mail address: [email protected] (Y. Zhang). These authors contributed equally to this article.
0960-8524/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2013.07.117
and animal oils, which raise the pricing problem and crucial sustainability issues. The bottleneck are known as the cost related to the limited available of fat or oil resources and the catalyst chose (de Lima da Silva et al., 2010). In a broad sense, the edible vegetable oils are extracted from the crops. And the increased demand for these crops will heighten their price in food market, place additional requirements on the often strained agriculture system and impose a negative environment impact through the additional energy requirements and eutrophication which caused by intensive agricultural processes (Griffiths and Harrison, 2009). Therefore, finding an inexpensive source of feedstock lipids that does not
768
X. Fu et al. / Bioresource Technology 146 (2013) 767–770
use farmland is indispensable for biodiesel to compete economically with traditional fossil fuel. Simultaneously, finding a profitability of the catalytic process is also expected (Marchetti et al., 2008). Microalgae, a single-celled algae, has been recognized as the most promising candidate to convert solar energy into fuel through photosynthesis, mainly because of their potential for reducing the amount of land and water surface needed to produce fuels (Hunter, 2010) and converting solar energy more efficiently. Moreover, it has less negative environmental impact than the alternatives, especially biofuel crops such as corn and soy. In addition to its high biomass productivity, it is reported that microalgae can be comprised of 1–85% lipids by dry weight. Microalgae’s high growth rate and lipid content would make it possible to produce enough biodiesel to satisfy the enormous demand for fuels in a relatively small area (Krohn et al., 2011). Thus, to deal with the cost issue of biodiesel production, utilizing the oils extracted from the microalgae as the feedstock has been recognized as the one of the best solutions. To the catalytic process, there have been two kinds of catalysts existed, such as base catalyst and acid catalyst. Since the content of free fatty acid is high in many cheap feedstocks or not refined natural oil, the soap formation cannot be avoided if the amount of FFAs in the feedstock exceeded 1% during the base-catalyzed process, which increases the difficulty of separation of the mixtures (fatty acid methyl ester–glycerol) (Marchetti et al., 2007). A homogeneous acid such as H2SO4 can simultaneously catalyze esterification and transesterification, so it performs well with oils containing high FFAs than base catalysts. However it also suffers from a series of drawbacks, such as lower reaction rates, corrosiveness and the high methanol to oil molar ratio requirements (Soriano et al., 2009). The most popular method for addressing this issue is by acid-catalyzed esterification, followed by base-catalyzed transesterification. The first step involved the esterification reaction between methanol and FFA to produce the corresponding fatty acid methyl ester (FAME) using the homogeneous acid catalyst. However, long reaction time, no recovery of catalyst and high cost of reaction equipment were disadvantages of this process. To reduce energy consumption and processing costs, heterogeneous catalysts have been considered a viable alternative because it can eliminate separation, corrosion, toxicity and environmental problems (West et al., 2008). Recently, among the solid acid catalysts reported, a new class of sulfonated carbon-based solid acid catalyst was demonstrated as promising catalysts for the esterification reaction for the production of biodiesel. These catalysts can be cheaply and easily produced by incomplete carbonization of sulfopolycyclic aromatic hydrocarbons (such as the sulfonate derivative produced of naphthalene with concentrated H2SO4) or sulfonation of incompletely carbonized inorganic/organic compounds (Hao et al., 2008; Nakajima et al., 2007). In general, carbon-based solid acid catalysts prepared from the sulfonation of carbonized compounds, such as starch, glucose, can form a rigid carbon material composed of small polycyclic aromatic carbon sheets (a three dimensional sp3-bonded structure) by carbonization (Zong et al., 2007), through sulfonating of such carbon materials can be very likely to form a highly stable solid with a high density of active sites. These carbon solid acid catalysts also showed high catalytic activity and stability. To overcome some of the above problems, in the present communication, the synthesis of a carbon based solid acid catalyst with high densities of sulfonic acid groups that is based on the relatively cheap raw material–microalgae residue were reported. The material–microalgae residue is obtained as a waste during the biodiesel production from microalgae. The microalgae-residue based catalyst (MRC) was obtained by in situ partial carbonization and sulfonation of microalgae-residue with sulfuric acid, in a ratio of 1:12
(wt/wt) in an autoclave within 2 h. The catalyst was subjected to esterification of oleic acid and transesterification of triolein to test its catalytic activities. The obtained catalyst was thoroughly characterized to establish its physical and chemical characteristics. The structure of the catalyst was first determined by X-ray diffraction (XRD), as shown in Fig. S1a (Supporting information). The catalyst exhibited one broad diffraction peak at 2h angle of 10–30°, which is attributable to amorphous carbon composed of aromatic carbon sheets oriented in a considerably random form (Okamura et al., 2006; Tsubouchi et al., 2003). This structure is far from crystal and is reported to be favorable for anchoring the –SO3H groups. A scanning electron microscopy image (SEM) (Fig. S1b) showed the morphology of irregular particles, with a grain size larger than 10 lm, which is similar to the carbon catalyst prepared from glycerol reported by (Devi et al., 2009). Furthermore, the morphology showed nonporous characteristics, which was supported by its low BET surface areas less than 1 m2 g 1 determined by nitrogen adsorption measurement. Fig. S2 shows a Fourier transform-infrared (FT-IR) absorption spectrum for the catalyst. The spectrum for the sample have vibration bands at 1167 and 1027 cm 1, which are identified as O=S=O symmetric stretching and —SO23 stretching modes in –SO3H group, respectively. These groups were the active sites of the catalyst. The band at 3424 and 3429 cm 1 were assigned to the O–H stretching modes of the –COOH and phenolic –OH groups. The band at 1713 cm 1 was assigned to the –C=O stretching mode of the – COOH group. The broad band at 2300–2700 cm 1 was assigned to an overtone of the bending mode of –OH O= linked by a strong hydrogen bond, suggesting that some –SO3H groups are in close proximity to each other, which was accordant with the results reported by Hara (2010). X-ray photoelectroscopy (XPS) measurement of the catalyst was conducted to determine its chemical composition, seeing Fig. S3a. The single O 1s, C 1s and S 2p peak was observed respectively at 531.64 eV, 284.64 eV and 168.64 eV, which indicated no other impurities existed in the catalyst. The XPS results confirmed that amount of –COOH groups formed in the one-step hydrothermal carbonization process which led to higher acid site density. The single S 2p peak centered at 168.64 eV meant that essentially all S atoms in catalyst can be assigned to –SO3H groups. The acidity of the catalyst was determined through the neutralization titration (Chen and Fang, 2011), which was about 4.25 mmol/g. The –SO3H density is determined through XPS characterization (Fu et al., 2012). The results showed that the content of the sulfur is about 0.90 mmol/g. According to the results from the elemental analysis, the MBC catalyst was found to have the elemental composition of CH0.55S0.02O0.56. A Raman spectrum (Fig. S3b) of the catalyst showed two broad peaks at about 1330 and 1580 cm 1 corresponding to the G (graphitic) and D (disorder) modes, respectively, indicating that the MBC is partially crystalline in nature. Moreover, the intensity ratio of the D band to the G band for the MBC catalyst is about 0.81, indicating that the average grapheme size in the amorphous carbon is ca. 1 nm. These characterization results indicate that the minimum unit in the catalyst is a nano-graphene sheets comprised of about 10–20 six membered rings. The graphene sheets have a high density of –SO3H, –OH and –COOH groups. This is distinct from conventional solid acids with single functional groups. The results in this short communication were similar to the carbon solid acid catalyst derived from cellulose powder reported by Hara (2010). Considering that the main components of the microalgae residue was the cellulose, it is reasonable that the similar carbon solid structure with high density of functional groups were obtained. In a word, a carbon solid acid catalyst with high density of functional groups can be effectively prepared from microalgae residue.
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100
(a)
Conv.(%)
80 MBC catalyst Amberlyst-15
60 40 20 0
0
2
4
6
8
10
12
Reaction time (h)
80 20 60 40
10
20 0
1
2
3
4
5
6
7
Yield of methyl ester %
30
100
Conv.%
The catalyst was firstly evaluated in the esterification of oleic acid with methanol and then in the transesterification of triolein with methanol for the production of biodiesel at 80 °C in the presence of 5 wt% of catalyst. As shown in Fig. 1a, about 98% conversion of the esterification reaction and 24% yield of transesterification reaction in 12 h were achieved. Fig. 1a shows the catalytic results of the MBC catalyst in comparison to a typical solid acid catalyst, amberlyst-15. The MBC catalyst showed a much higher esterification activity. As shown in Fig. 1b, the MBC catalyst showed higher yield of methyl ester in the transesterification of triolein, confirming the high catalytic activity of the MBC catalyst. It is well known that the amberlyst-15 was an ion exchange resin bearing macropores. Although it has high acid density (about 5.3 mmol/g), most of acid sites resided in a poorly swelling network can only be penetrated by small ions (H+, Na+) instead of large molecules such as oleic acid (Song et al., 2012). With respect to the MBC catalyst, the high catalytic activity cannot be explained simply by the high density of strongly acidic –SO3H groups and good access of reactants to the –SO3H groups in the carbon material. It is reported that in addition to the high density of acidic –SO3H groups, the hydrophilic –COOH and –OH groups in the carbon catalyst also played an important role in promoting the activity of the catalyst (Hara, 2010). One possible explanation for the high catalytic activity of the solid acid carbon catalyst bearing –SO3H, –COOH and phenolic –OH groups may be the strong affinity between the hydrophilic parts of the reactants and the almost neutral –OH groups bonded to carbon catalyst matrix (Geng et al., 2011). Thus, the strong affinity favored the dispersion of the catalyst in methanol, resulting in its high catalytic activity.
0
Cycle number Fig. 2. Recycling performance of the MBC catalyst in methyl oleate formation by esterification of oleic acid with methanol (red pillar) and by transesterification of olein with methanol (black pillar). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
During the reusable process, the MBC catalyst lost its activity gradually in the esterification reaction for the first five cycles by washing with methanol, but it can be recovered by filtration after the batch experiment with concentrated sulfuric acid and then washing with the hot water. The deactivation of the catalyst was mainly caused by the extensive washing with methanol, which can react with the sulfonated groups leading to the leaching of the sulfonated groups during the washing process. The results were coincided with results of Chen and Fang (2011). After the regeneration, it was found that the catalytic activity of the esterification can be still maintained with over 95% conversion after five runs, as shown in Fig. 2. Moreover, the yield of methyl oleate from the transesterification can be maintained with over 22%, demonstrating the stability of the catalyst. The mechanism of catalyst regeneration by concentrated sulfuric acid has to be further researched. In conclusion, a novel carbon solid acid catalyst has been successfully developed via an in situ hydrothermal partially carbonization using microalgae residue as the starting materials. The obtained MBC catalyst has shown highly catalytic activity in esterification of oleic acid and transesterification of triolein with methanol. This result indicates that the new route of synthesis has provided a sustainable way for the biodiesel production. In addition to biodiesel production, such an effective, inexpensive and reusable polycyclic aromatic carbon acid catalyst has great potential for a wide range of other acid-catalyzed reactions.
25
Yield of methyl ester %
(b) 20
Acknowledgements
15
MBC catalyst Amberlyst-15
10 5 0
0
2
4
6
8
10
The authors would like to thank for the supports from Natural Science Foundation of Guangdong (No. S2011040001667), National High Technology Research and Development Program 863 (Nos. 2009AA064401, 2013AA065805), Major State Basic Research Development Program of China (No. 2011CB2009001) Natural Science Foundation of China (No. 21276104) and the Fundamental Research Funds for the Central Universities (No. 11612328).
12
Reaction time (h) Fig. 1. (a) Esterification of oleic acid with methanol by the MBC catalyst and the amberlyst-15 catalyst and (b) transesterification of triolein with methanol by the MBC catalyst and the amberlyst-15 catalyst.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.biortech.2013. 07.117.
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X. Fu et al. / Bioresource Technology 146 (2013) 767–770
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