Bioresource Technology 154 (2014) 345–348
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Short Communication
Biodiesel production from waste cooking oil using a heterogeneous catalyst from pyrolyzed rice husk Ming Li, Yan Zheng, Yixin Chen, Xifeng Zhu ⇑ Key Laboratory for Biomass Clean Energy of Anhui Province, University of Science and Technology of China, Hefei 230026, China
h i g h l i g h t s A solid acid catalyst was prepared from pyrolyzed rice husk. The effective acid sites of the prepared catalyst have favorable thermal stability. The catalyst contains both sulfonic acid groups and phenolic hydroxyl groups. The catalyst is potential to catalyze waste cooking oil to produce biodiesel.
a r t i c l e
i n f o
Article history: Received 5 November 2013 Received in revised form 12 December 2013 Accepted 14 December 2013 Available online 22 December 2013 Keywords: Rice husk char Solid acid catalyst Biodiesel Waste cooking oil
a b s t r a c t A solid acid catalyst was prepared by sulfonating pyrolyzed rice husk with concentrated sulfuric acid, and the physical and chemical properties of the catalyst were characterized in detail. The catalyst was then used to simultaneously catalyze esterification and transesterification to produce biodiesel from waste cooking oil (WCO). In the presence of the as-prepared catalyst, the free fatty acid (FFA) conversion reached 98.17% after 3 h, and the fatty acid methyl ester (FAME) yield reached 87.57% after 15 h. By contrast, the typical solid acid catalyst Amberlyst-15 obtained only 95.25% and 45.17% FFA conversion and FAME yield, respectively. Thus, the prepared catalyst had a high catalytic activity for simultaneous esterification and transesterification. In addition, the catalyst had excellent stability, thereby having potential use as a heterogeneous catalyst for biodiesel production from WCO with a high FFA content. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Given the potential exhaustion of traditional fossil fuels, the increasing price of petroleum, and the environment concerns, the search for alternative renewable fuels is gaining considerable attention. Biodiesel has the same properties as conventional diesel in terms of viscosity, flash point, cetane number, and many more (Noiroj et al., 2009), which make this green fuel one of the most promising among new energy resources. However, biodiesel still has not been commercialized globally. The high cost of feedstocks is a major restricting factor in the development of biodiesel because the most common commercial process for biodiesel production is homogeneous alkali-catalyzed transesterification using refined vegetable oils as raw materials (Shu et al., 2010). Synthesis of biodiesel from waste cooking oil (WCO) can effectively use biomass waste and will lower the production cost of biodiesel. However, a considerable amount of free fatty acids (FFAs) is present in WCO, which will be saponified by the alkali catalyst, so resulting in difficulty in product separation and causing a low biodiesel yield ⇑ Corresponding author. Tel./fax: +86 551 63600040. E-mail address: [email protected] (X. Zhu). 0960-8524/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2013.12.070
(Shu et al., 2010). Concentrated sulfuric acid shows a better performance with WCO because the acid can simultaneously catalyze esterification and transesterification. However, sulfuric acid has several drawbacks, such as equipment corrosion and the discharge of waste water. Use of the heterogeneous acid catalyst can eliminate the problems associated with homogeneous acid catalysts. Zeolite (Chung et al., 2008), ion-exchange resin (Ngaosuwan et al., 2009), inorganic-oxide solid acid (Lou et al., 2008), and supported noble-metal oxide (Lou et al., 2008) are used as heterogeneous catalysts for the biodiesel production. However, these catalysts are commonly hydrophilic, and their activity decreases because of the water produced from FFA esterification. Besides, they have low density of effective acid sites, microporous structure, bad stability, and high cost. These drawbacks have limited their practical use in biodiesel production. Hara et al. (2004) prepared a new type of sulfonated carbonbased solid acid catalyst from aromatic compounds. Along with the research thorough, carbon-based solid acid catalysts obtained from inexpensive materials including saccharides (D-glucose, sucrose, starch, etc.) (Lou et al., 2008) and biomasses (corn straw, peanut hull, wood char, etc.) (Dehkhoda et al., 2010; Kastner et al., 2012; Liu et al., 2013; Yu et al., 2011) are widely investigated.
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These catalysts had excellent catalytic activity for esterification of FFAs and transesterification of triolein (Hara, 2010), and are very promising as green catalysts for efficient biodiesel production (Hara, 2009). However, the utilization of carbon-based solid acid catalysts currently focuses more on the esterification of FFA than the preparation of biodiesel from WCO. Biomass pyrolysis is an efficient process of biomass conversion with high yield of liquid fuel (bio-oil), some non-condensable mixture gases, and 15–30% of solid char byproduct. The effective and high-value use of biochar can significantly improve the bio-oil economy. Since the biochar has been used for synthesis of silica and active carbon (Azargohar and Dalai, 2006; Li et al., 2011), it is of great interest to extend its application as a possible carbon precursor to prepare solid acid catalyst. The main objective of this work is to prepare a solid acid catalyst (RHC–SO3H) by sulfonating rice husk char (RHC) with concentrated sulfuric acid and to investigate the potential application of the catalyst for biodiesel production from WCO. The simultaneous catalytic activity for esterification of FFA and transesterification of triglyceride (TG) in WCO was discussed. 2. Experimental 2.1. Materials RHC was the byproduct of fast-pyrolyzing rice husk for bio-oil in our laboratory. Pyrolysis conditions of rice husk are as following: 480 °C/s of heating rate, 510 °C of pyrolysis temperature and 4 s of solid phase residence time. Proximate analysis of RHC was performed according to ASTM D3173 (moisture), ASTM D3174 (ash), and ASTM D3175 (volatile matters) methods. WCO was derived from the canteen of Institute of Plasma Physics Chinese Academy of Sciences. Its moisture content (wt%) was determined by Karl– Fischer moisture meter (ZDJ–3S, China), acid value (mg KOHg–1) and saponification value (mg KOHg–1) were measured according to ASTM D664 and ASTM D94, respectively. The molecular weight (M) was calculated from its acid value (AV) and saponification value (SV) (Shu et al., 2009). The results were shown in Table S1 (Supplementary information). 2.2. Catalyst preparation RHC was ground to powder (particle size 6 0.28 nm) and a 6 g sample was sulfonated using 60 mL concentrated sulfuric acid (95–98%) for 0.5 h in a 250 mL flask controlled at 90 °C in an oil bath. After cooling to ambient temperature, the mixture was added to distilled water, stirred, and filtrated. Then the precipitate was washed repeatedly with hot distilled water until the filtrate was free from sulfate ions. Following filtration, the sample was dried at 80 °C for 24 h in an air-drying oven. 2.3. Catalyst characterization The morphology was characterized by scanning electron microscopy (SEM, Sirion200, FEI, USA). The textural properties were assayed on an automatic surface area and pore analyzer (Quandasorb SI, Quantachrome, USA). The specific surface area was calculated according to Brunauer–Emmett–Teller (BET) equation at a relative pressure of 0.05–0.2. The framework vibration was examined by Fourier transform infrared spectroscopy (FTIR, Nicolet 6700, Thermo Scientific Instrument, USA). The sulfur-containing species were determined using X-ray photoelectron spectroscopy (XPS, ESCALAB, Thermo-VG Scientific 250, USA). The ratio of element S was determined using elemental analyzer (Vario MICRO, Elementar, Germany). The thermal stability was examined
by a thermogravimetric analyzer (TGA, Pyris 1, Perkinelmer, USA) coupling with a mass spectrometer (MS, Clarus SQ 8, Perkinelmer, USA). The strong and total acid sites were determined by cationexchange analysis.
2.4. Catalyst activity A continuous reaction apparatus with a rectifying column (Wu et al., 2009) was used. The re-boiling and reaction temperatures were 70 and 110 °C, respectively. At specific time intervals during reaction, samples (5 mL) were withdrawn from the reaction mixture for analysis. The conversion of FFAs was calculated by measuring acid value of initial and final samples (Wu et al., 2009). The weight percentage of fatty acid methyl ester (FAME) was measured by a TGA (Q500, TA instruments, USA) (Sousa et al., 2013). Meanwhile, a commercial widely used solid acid catalyst, Amberlyst15 was used as the reference catalyst. The experiments were all conducted three times.
3. Results and discussion 3.1. Catalyst characterization The SEM image showed that the catalyst was comprised of irregular particles with grain size of micrometer scale and few discrete pores on the surface. The under-developed porous structure was verified by its textural property, as measured by N2 adsorption isotherm. The surface area of RHC–SO3H was only 4 m2/g, and the average pore size was 7.7 nm, demonstrating the existence of some mesopores, These mesopores are favorable to macromolecules, such as oleic acid, triolein, and methyl oleate, diffusing in and out the interior of the catalyst (Shu et al., 2010), and thus improving catalytic activity. However, it is worth noting that the pretreatment on RHC by KOH solution (Yu et al., 2011) almost had no effect on the surface area of the catalyst. This was probably because silicon dioxide, the principal component reacting with KOH in RHC, was enclosed with carbon substrate. Therefore, most of the ash content was not removed, and pores failed to develop in the preprocessing. Further research on RHC–SO3H with high porosity is ongoing in our laboratory. The FTIR spectrum of RHC–SO3H showed a band at 1712 cm 1, which corresponds to the stretching mode of ASO3H (Dehkhoda et al., 2010). The broad band at 3400 cm 1 is attributed to the OAH stretching modes of the phenolic AOH and ACOOH, providing the presence of AOH in RHC–SO3H. The band assigned to the ACOOH at around 1700 cm 1 was not observed as it might have been overlapped because of the presence of the ASO3H band (Yu et al., 2011). The XPS analysis was conducted to determine the surface species bonded to the carbon matrix (Fig. S1(a), Supplementary information). The C 1s spectrum (Fig. S1(b)) showed an intense peak at 284.8 eV assigned to the carbon substrate of the catalyst (Dehkhoda et al., 2010), whereas the peak at around 289.0 eV assigned to the ACOOH (Dehkhoda et al., 2010) did not appear. Hence, combined with the FTIR and XPS results, no carboxylic groups were present in RHC–SO3H. A single peak occurred at 168.2 eV in the S 2p spectrum (Fig. S1(c)), which was assigned to the sulfur in ASO3H (Okamura et al., 2006). This meant that all the S atoms were contained in ASO3H. The density of 0.92 mmol/ g was thus calculated from the S content (2.83%). The density of the strong acid sites determined by titration was 0.88 mmol/g and almost identical with that of ASO3H, which further illustrates the prepared catalyst has no ACOOH. Moreover, the total acid density measured by back titration was 2.46 mmol/g, so the density of the phenolic AOH was approximately 1.6 mmol/g.
M. Li et al. / Bioresource Technology 154 (2014) 345–348
The thermal stabilities of RHC and RHC–SO3H were investigated via thermogravimetry–mass spectrometry (TG–MS) under a flow of helium (Fig. S2, Supplementary information). TG curves showed that the weight losses for both RHC and RHC–SO3H were about 1.4% in the range from room temperature to 120 °C, which is mainly due to the physically absorbed water. However, the weight loss of RHC–SO3H occurred at a higher temperature because ASO3H is hydrophilic and favors the adsorption of water (Fu et al., 2012). As a result, water molecules in RHC–SO3H are absorbed more firmly than RHC. In the temperature range of 180–680 °C, RHC–SO3H lost 5.8% of its weight more than that of RHC, which is in acceptable agreement with the content of ASO3H (5.7%). Moreover, the evolution profiles of SO2 (m/z = 64) and SO3 (m/z = 80) showed that ASO3H groups in the catalyst were mainly decomposed to SO2. These results indicate that RHC–SO3H has a favorable thermal stability. 3.2. Catalytic activity in the biodiesel production As transesterification is reversible and endothermic, excess methanol and a higher reaction temperature are necessary to improve the reaction rate and favor forward reaction, and thus, all experiments were conducted at a molar ratio of methanol to WCO of 20:1 and 110 °C. Besides, water is produced in esterification, and its instantaneous removal is favorable to the conversion of FFAs, so a continuous reaction apparatus was used, and this is
Fig. 1. (a) Esterification activity of RHC–SO3H and Amberlyst-15; (b) FAME yield from esterification and transesterification catalyzed by RHC–SO3H and Amberlyst15.
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another reason for the choice of 110 °C. Furthermore, a modest stirring rate of 350 r/min and a moderate catalyst amount of 5 wt% were applied, respectively. Fig. 1(a) shows the catalytic esterification performance of the prepared catalyst and Amberlyst-15. As can be seen, RHC–SO3H exhibited a much higher esterification activity than the latter. The esterification approached equilibrium for RHC–SO3H after 3 h, at which the FFA conversion was 98.17%, and by contrast, the FFA conversion for Amberlyst-15 was 95.64%. The prepared catalyst also resulted in a higher FAME yield, as showed in Fig. 1(b). Although RHC–SO3H has a lower ASO3H density (5.1 mmol/L for Amberlyst-15), its FAME yield was still higher than that of Amberlyst-15 within the same reaction time. For example, the FAME yields in the presence of the Amberlyst-15 and RHC– SO3H were 45.17% and 87.57% after 15 h, respectively. Compared to other reported solid acid catalysts, RHC–SO3H also showed a favorable activity in biodiesel production from WCO (Table S2, Supplementary information). Besides, it can be concluded from Fig. 1(a and b) that the esterification activity of the catalyst is much higher than its transesterification activity. Thus, feedstocks with a higher content of FFAs are better suited for biodiesel production using RHC–SO3H as a solid acid catalyst. As for the high catalytic performance of RHC–SO3H, it might be due to the following three reasons. First, RHC–SO3H contains a high density of strongly acidic ASO3H groups. Second, owing to the hydrophilicity of ASO3H, the catalyst can incorporate large amounts of water into the carbon bulk, so the effective surface area of the catalyst is much larger than that determined by BET method after dehydration. The RHC and RHC–SO3H samples were heated in a TGA from room temperature to 120 °C (2 °C/min) after being exposed to a wet environment with 80% relative humidity at 20 °C for 24 h. The results showed that the weight losses due to desorption of water were 4.7% and 11.8%, respectively, which implies that RHC–SO3H has a strong water–adsorption capacity. Thus, the incorporation provides an efficient solution for reactants to ASO3H and improves its catalytic performance (Okamura et al., 2006). Finally and most importantly, unlike Amberlyst-15, the prepared catalyst contains about 1.6 mmol/g of AOH that can adsorb the hydrophilic parts of the reactants by hydrogen bond (Hara, 2010). As a result, the strong affinity between methanol and the catalyst contributes to its high catalytic activity in the biodiesel production from WCO.
Fig. 2. Stability of RHC–SO3H. Reaction conditions: 20:1 methanol to WCO molar ratio, 5 wt% catalyst amount, 110 °C, 15 h.
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3.3. Catalyst stability
References
The stability of RHC–SO3H was investigated according to the method described by Park et al. (2010) under the presented reaction conditions in Section 3.2 for 15 h. In each run, the FFA conversion was determined at 3 h, and the FAME yield was measured after reaction. It was found that the FFA conversion and the FAME yield were still maintained 95.75% and 80.20% after five cycles (Fig. 2), indicating the excellent stability of the catalyst. The recovered catalyst after five cycles was characterized by ultimate analysis. The results showed that the S content decreased by 19.32% compared with that of fresh catalyst, so it can be concluded that the slight decline of the activity is due to the leaching of some ASO3H groups (Fu et al., 2012).
Azargohar, R., Dalai, A.K., 2006. Biochar as a precursor of activated carbon. Appl. Biochem. Biotechnol. 131 (1–3), 762–773. Chung, K.H., Chang, D.R., Park, B.G., 2008. Removal of free fatty acid in waste frying oil by esterification with methanol on zeolite catalysts. Bioresour. Technol. 99 (16), 7438–7443. Dehkhoda, A.M., West, A.H., Ellis, N., 2010. Biochar based solid acid catalyst for biodiesel production. Appl. Catal., A 382 (2), 197–204. Fu, Z.W., Wan, H., Hu, X.S., Cui, Q., Guan, G.F., 2012. Preparation and catalytic performance of a carbon-based solid acid catalyst with high specific surface area. React. Kinet. Mech. Catal. 107 (1), 203–213. Hara, M., 2009. Environmentally benign production of biodiesel using heterogeneous catalysts. Chemsuschem 2 (2), 129–135. Hara, M., 2010. Biodiesel production by amorphous carbon bearing SO3H, COOH and phenolic OH groups, a solid bronsted acid catalyst. Top. Catal. 53 (11–12), 805–810. Hara, M., Yoshida, T., Takagaki, A., Takata, T., Kondo, J.N., Hayashi, S., Domen, K., 2004. A carbon material as a strong protonic acid. Angew. Chem. Int. Ed. 43 (22), 2955–2958. Kastner, J.R., Miller, J., Geller, D.P., Locklin, J., Keith, L.H., Johnson, T., 2012. Catalytic esterification of fatty acids using solid acid catalysts generated from biochar and activated carbon. Catal. Today 190 (1), 122–132. Li, D.W., Chen, D.Y., Zhu, X.F., 2011. Reduction in time required for synthesis of high specific surface area silica from pyrolyzed rice husk by precipitation at low pH. Bioresour. Technol. 102 (13), 7001–7003. Liu, T.T., Li, Z.L., Li, W., Shi, C.J., Wang, Y., 2013. Preparation and characterization of biomass carbon-based solid acid catalyst for the esterification of oleic acid with methanol. Bioresour. Technol. 133, 618–621. Lou, W.Y., Zong, M.H., Duan, Z.Q., 2008. Efficient production of biodiesel from high free fatty acid-containing waste oils using various carbohydrate-derived solid acid catalysts. Bioresour. Technol. 99 (18), 8752–8758. Ngaosuwan, K., Lotero, E., Suwannakarn, K., Goodwin, J.G., Praserthdam, P., 2009. Hydrolysis of triglycerides using solid acid catalysts. Ind. Eng. Chem. Res. 48 (10), 4757–4767. Noiroj, K., Intarapong, P., Luengnaruemitchai, A., Jai-In, S., 2009. A comparative study of KOH/Al2O3 and KOH/NaY catalysts for biodiesel production via transesterification from palm oil. Renewable Energy 34 (4), 1145–1150. 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. Chem. Mater. 18 (13), 3039–3045. Park, J.Y., Kim, D.K., Lee, J.S., 2010. Esterification of free fatty acids using water-tolerable Amberlyst as a heterogeneous catalyst. Bioresour. Technol. 101, S62–S65. Shu, Q., Zhang, Q., Xu, G.H., Nawaz, Z., Wang, D.Z., Wang, J.F., 2009. Synthesis of biodiesel from cottonseed oil and methanol using a carbon-based solid acid catalyst. Fuel Process. Technol. 90 (7–8), 1002–1008. Shu, Q., Nawaz, Z., Gao, J.X., Liao, Y.H., Zhang, Q., Wang, D.Z., Wang, J.F., 2010. Synthesis of biodiesel from a model waste oil feedstock using a carbon-based solid acid catalyst: Reaction and separation. Bioresour. Technol. 101 (14), 5374–5384. Sousa, F.P., Luciano, M.A., Pasa, V.M.D., 2013. Thermogravimetry and viscometry for assessing the ester content (FAME and FAEE). Fuel Process. Technol. 109, 133–140. Wu, R.N., Wang, T.H., Xiu, Z.L., Guo, F., Pan, Y.Q., Yin, J.Z., 2009. Preparation of a biomass carbon-based solid acid catalyst. Chin. J. Catal. 30 (12), 1203–1208. Yu, J.T., Dehkhoda, A.M., Ellis, N., 2011. Development of biochar-based catalyst for transesterification of canola oil. Energy Fuels 25, 337–344.
4. Conclusions The solid acid catalyst derived from rice husk char showed excellent catalytic activity and stability in biodiesel production from WCO. This catalyst can efficiently and simultaneously catalyze esterification of FFA and transesterification of TG. In the presence of the catalyst, the FFA conversion was more than 98% after 3 h, and the FAME yield was nearly 90% after 15 h. Thus, this catalyst is potentially useful in biodiesel production, especially in converting feedstocks with high FFA content such as WCO, soapstock and inedible oil to biodiesel.
Acknowledgements The authors acknowledge the financial supports provided by the National High Technology Research and Development Program of China (2012AA051803), the Key Research Program of the Chinese Academy of Sciences (KGZD-EW-304-3), and the National Natural Science Foundation of China (50930006).
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. 12.070.
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Short Communication
Biodiesel production from waste cooking oil using a heterogeneous catalyst from pyrolyzed rice husk Ming Li, Yan Zheng, Yixin Chen, Xifeng Zhu ⇑ Key Laboratory for Biomass Clean Energy of Anhui Province, University of Science and Technology of China, Hefei 230026, China
h i g h l i g h t s A solid acid catalyst was prepared from pyrolyzed rice husk. The effective acid sites of the prepared catalyst have favorable thermal stability. The catalyst contains both sulfonic acid groups and phenolic hydroxyl groups. The catalyst is potential to catalyze waste cooking oil to produce biodiesel.
a r t i c l e
i n f o
Article history: Received 5 November 2013 Received in revised form 12 December 2013 Accepted 14 December 2013 Available online 22 December 2013 Keywords: Rice husk char Solid acid catalyst Biodiesel Waste cooking oil
a b s t r a c t A solid acid catalyst was prepared by sulfonating pyrolyzed rice husk with concentrated sulfuric acid, and the physical and chemical properties of the catalyst were characterized in detail. The catalyst was then used to simultaneously catalyze esterification and transesterification to produce biodiesel from waste cooking oil (WCO). In the presence of the as-prepared catalyst, the free fatty acid (FFA) conversion reached 98.17% after 3 h, and the fatty acid methyl ester (FAME) yield reached 87.57% after 15 h. By contrast, the typical solid acid catalyst Amberlyst-15 obtained only 95.25% and 45.17% FFA conversion and FAME yield, respectively. Thus, the prepared catalyst had a high catalytic activity for simultaneous esterification and transesterification. In addition, the catalyst had excellent stability, thereby having potential use as a heterogeneous catalyst for biodiesel production from WCO with a high FFA content. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Given the potential exhaustion of traditional fossil fuels, the increasing price of petroleum, and the environment concerns, the search for alternative renewable fuels is gaining considerable attention. Biodiesel has the same properties as conventional diesel in terms of viscosity, flash point, cetane number, and many more (Noiroj et al., 2009), which make this green fuel one of the most promising among new energy resources. However, biodiesel still has not been commercialized globally. The high cost of feedstocks is a major restricting factor in the development of biodiesel because the most common commercial process for biodiesel production is homogeneous alkali-catalyzed transesterification using refined vegetable oils as raw materials (Shu et al., 2010). Synthesis of biodiesel from waste cooking oil (WCO) can effectively use biomass waste and will lower the production cost of biodiesel. However, a considerable amount of free fatty acids (FFAs) is present in WCO, which will be saponified by the alkali catalyst, so resulting in difficulty in product separation and causing a low biodiesel yield ⇑ Corresponding author. Tel./fax: +86 551 63600040. E-mail address: [email protected] (X. Zhu). 0960-8524/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2013.12.070
(Shu et al., 2010). Concentrated sulfuric acid shows a better performance with WCO because the acid can simultaneously catalyze esterification and transesterification. However, sulfuric acid has several drawbacks, such as equipment corrosion and the discharge of waste water. Use of the heterogeneous acid catalyst can eliminate the problems associated with homogeneous acid catalysts. Zeolite (Chung et al., 2008), ion-exchange resin (Ngaosuwan et al., 2009), inorganic-oxide solid acid (Lou et al., 2008), and supported noble-metal oxide (Lou et al., 2008) are used as heterogeneous catalysts for the biodiesel production. However, these catalysts are commonly hydrophilic, and their activity decreases because of the water produced from FFA esterification. Besides, they have low density of effective acid sites, microporous structure, bad stability, and high cost. These drawbacks have limited their practical use in biodiesel production. Hara et al. (2004) prepared a new type of sulfonated carbonbased solid acid catalyst from aromatic compounds. Along with the research thorough, carbon-based solid acid catalysts obtained from inexpensive materials including saccharides (D-glucose, sucrose, starch, etc.) (Lou et al., 2008) and biomasses (corn straw, peanut hull, wood char, etc.) (Dehkhoda et al., 2010; Kastner et al., 2012; Liu et al., 2013; Yu et al., 2011) are widely investigated.
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These catalysts had excellent catalytic activity for esterification of FFAs and transesterification of triolein (Hara, 2010), and are very promising as green catalysts for efficient biodiesel production (Hara, 2009). However, the utilization of carbon-based solid acid catalysts currently focuses more on the esterification of FFA than the preparation of biodiesel from WCO. Biomass pyrolysis is an efficient process of biomass conversion with high yield of liquid fuel (bio-oil), some non-condensable mixture gases, and 15–30% of solid char byproduct. The effective and high-value use of biochar can significantly improve the bio-oil economy. Since the biochar has been used for synthesis of silica and active carbon (Azargohar and Dalai, 2006; Li et al., 2011), it is of great interest to extend its application as a possible carbon precursor to prepare solid acid catalyst. The main objective of this work is to prepare a solid acid catalyst (RHC–SO3H) by sulfonating rice husk char (RHC) with concentrated sulfuric acid and to investigate the potential application of the catalyst for biodiesel production from WCO. The simultaneous catalytic activity for esterification of FFA and transesterification of triglyceride (TG) in WCO was discussed. 2. Experimental 2.1. Materials RHC was the byproduct of fast-pyrolyzing rice husk for bio-oil in our laboratory. Pyrolysis conditions of rice husk are as following: 480 °C/s of heating rate, 510 °C of pyrolysis temperature and 4 s of solid phase residence time. Proximate analysis of RHC was performed according to ASTM D3173 (moisture), ASTM D3174 (ash), and ASTM D3175 (volatile matters) methods. WCO was derived from the canteen of Institute of Plasma Physics Chinese Academy of Sciences. Its moisture content (wt%) was determined by Karl– Fischer moisture meter (ZDJ–3S, China), acid value (mg KOHg–1) and saponification value (mg KOHg–1) were measured according to ASTM D664 and ASTM D94, respectively. The molecular weight (M) was calculated from its acid value (AV) and saponification value (SV) (Shu et al., 2009). The results were shown in Table S1 (Supplementary information). 2.2. Catalyst preparation RHC was ground to powder (particle size 6 0.28 nm) and a 6 g sample was sulfonated using 60 mL concentrated sulfuric acid (95–98%) for 0.5 h in a 250 mL flask controlled at 90 °C in an oil bath. After cooling to ambient temperature, the mixture was added to distilled water, stirred, and filtrated. Then the precipitate was washed repeatedly with hot distilled water until the filtrate was free from sulfate ions. Following filtration, the sample was dried at 80 °C for 24 h in an air-drying oven. 2.3. Catalyst characterization The morphology was characterized by scanning electron microscopy (SEM, Sirion200, FEI, USA). The textural properties were assayed on an automatic surface area and pore analyzer (Quandasorb SI, Quantachrome, USA). The specific surface area was calculated according to Brunauer–Emmett–Teller (BET) equation at a relative pressure of 0.05–0.2. The framework vibration was examined by Fourier transform infrared spectroscopy (FTIR, Nicolet 6700, Thermo Scientific Instrument, USA). The sulfur-containing species were determined using X-ray photoelectron spectroscopy (XPS, ESCALAB, Thermo-VG Scientific 250, USA). The ratio of element S was determined using elemental analyzer (Vario MICRO, Elementar, Germany). The thermal stability was examined
by a thermogravimetric analyzer (TGA, Pyris 1, Perkinelmer, USA) coupling with a mass spectrometer (MS, Clarus SQ 8, Perkinelmer, USA). The strong and total acid sites were determined by cationexchange analysis.
2.4. Catalyst activity A continuous reaction apparatus with a rectifying column (Wu et al., 2009) was used. The re-boiling and reaction temperatures were 70 and 110 °C, respectively. At specific time intervals during reaction, samples (5 mL) were withdrawn from the reaction mixture for analysis. The conversion of FFAs was calculated by measuring acid value of initial and final samples (Wu et al., 2009). The weight percentage of fatty acid methyl ester (FAME) was measured by a TGA (Q500, TA instruments, USA) (Sousa et al., 2013). Meanwhile, a commercial widely used solid acid catalyst, Amberlyst15 was used as the reference catalyst. The experiments were all conducted three times.
3. Results and discussion 3.1. Catalyst characterization The SEM image showed that the catalyst was comprised of irregular particles with grain size of micrometer scale and few discrete pores on the surface. The under-developed porous structure was verified by its textural property, as measured by N2 adsorption isotherm. The surface area of RHC–SO3H was only 4 m2/g, and the average pore size was 7.7 nm, demonstrating the existence of some mesopores, These mesopores are favorable to macromolecules, such as oleic acid, triolein, and methyl oleate, diffusing in and out the interior of the catalyst (Shu et al., 2010), and thus improving catalytic activity. However, it is worth noting that the pretreatment on RHC by KOH solution (Yu et al., 2011) almost had no effect on the surface area of the catalyst. This was probably because silicon dioxide, the principal component reacting with KOH in RHC, was enclosed with carbon substrate. Therefore, most of the ash content was not removed, and pores failed to develop in the preprocessing. Further research on RHC–SO3H with high porosity is ongoing in our laboratory. The FTIR spectrum of RHC–SO3H showed a band at 1712 cm 1, which corresponds to the stretching mode of ASO3H (Dehkhoda et al., 2010). The broad band at 3400 cm 1 is attributed to the OAH stretching modes of the phenolic AOH and ACOOH, providing the presence of AOH in RHC–SO3H. The band assigned to the ACOOH at around 1700 cm 1 was not observed as it might have been overlapped because of the presence of the ASO3H band (Yu et al., 2011). The XPS analysis was conducted to determine the surface species bonded to the carbon matrix (Fig. S1(a), Supplementary information). The C 1s spectrum (Fig. S1(b)) showed an intense peak at 284.8 eV assigned to the carbon substrate of the catalyst (Dehkhoda et al., 2010), whereas the peak at around 289.0 eV assigned to the ACOOH (Dehkhoda et al., 2010) did not appear. Hence, combined with the FTIR and XPS results, no carboxylic groups were present in RHC–SO3H. A single peak occurred at 168.2 eV in the S 2p spectrum (Fig. S1(c)), which was assigned to the sulfur in ASO3H (Okamura et al., 2006). This meant that all the S atoms were contained in ASO3H. The density of 0.92 mmol/ g was thus calculated from the S content (2.83%). The density of the strong acid sites determined by titration was 0.88 mmol/g and almost identical with that of ASO3H, which further illustrates the prepared catalyst has no ACOOH. Moreover, the total acid density measured by back titration was 2.46 mmol/g, so the density of the phenolic AOH was approximately 1.6 mmol/g.
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The thermal stabilities of RHC and RHC–SO3H were investigated via thermogravimetry–mass spectrometry (TG–MS) under a flow of helium (Fig. S2, Supplementary information). TG curves showed that the weight losses for both RHC and RHC–SO3H were about 1.4% in the range from room temperature to 120 °C, which is mainly due to the physically absorbed water. However, the weight loss of RHC–SO3H occurred at a higher temperature because ASO3H is hydrophilic and favors the adsorption of water (Fu et al., 2012). As a result, water molecules in RHC–SO3H are absorbed more firmly than RHC. In the temperature range of 180–680 °C, RHC–SO3H lost 5.8% of its weight more than that of RHC, which is in acceptable agreement with the content of ASO3H (5.7%). Moreover, the evolution profiles of SO2 (m/z = 64) and SO3 (m/z = 80) showed that ASO3H groups in the catalyst were mainly decomposed to SO2. These results indicate that RHC–SO3H has a favorable thermal stability. 3.2. Catalytic activity in the biodiesel production As transesterification is reversible and endothermic, excess methanol and a higher reaction temperature are necessary to improve the reaction rate and favor forward reaction, and thus, all experiments were conducted at a molar ratio of methanol to WCO of 20:1 and 110 °C. Besides, water is produced in esterification, and its instantaneous removal is favorable to the conversion of FFAs, so a continuous reaction apparatus was used, and this is
Fig. 1. (a) Esterification activity of RHC–SO3H and Amberlyst-15; (b) FAME yield from esterification and transesterification catalyzed by RHC–SO3H and Amberlyst15.
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another reason for the choice of 110 °C. Furthermore, a modest stirring rate of 350 r/min and a moderate catalyst amount of 5 wt% were applied, respectively. Fig. 1(a) shows the catalytic esterification performance of the prepared catalyst and Amberlyst-15. As can be seen, RHC–SO3H exhibited a much higher esterification activity than the latter. The esterification approached equilibrium for RHC–SO3H after 3 h, at which the FFA conversion was 98.17%, and by contrast, the FFA conversion for Amberlyst-15 was 95.64%. The prepared catalyst also resulted in a higher FAME yield, as showed in Fig. 1(b). Although RHC–SO3H has a lower ASO3H density (5.1 mmol/L for Amberlyst-15), its FAME yield was still higher than that of Amberlyst-15 within the same reaction time. For example, the FAME yields in the presence of the Amberlyst-15 and RHC– SO3H were 45.17% and 87.57% after 15 h, respectively. Compared to other reported solid acid catalysts, RHC–SO3H also showed a favorable activity in biodiesel production from WCO (Table S2, Supplementary information). Besides, it can be concluded from Fig. 1(a and b) that the esterification activity of the catalyst is much higher than its transesterification activity. Thus, feedstocks with a higher content of FFAs are better suited for biodiesel production using RHC–SO3H as a solid acid catalyst. As for the high catalytic performance of RHC–SO3H, it might be due to the following three reasons. First, RHC–SO3H contains a high density of strongly acidic ASO3H groups. Second, owing to the hydrophilicity of ASO3H, the catalyst can incorporate large amounts of water into the carbon bulk, so the effective surface area of the catalyst is much larger than that determined by BET method after dehydration. The RHC and RHC–SO3H samples were heated in a TGA from room temperature to 120 °C (2 °C/min) after being exposed to a wet environment with 80% relative humidity at 20 °C for 24 h. The results showed that the weight losses due to desorption of water were 4.7% and 11.8%, respectively, which implies that RHC–SO3H has a strong water–adsorption capacity. Thus, the incorporation provides an efficient solution for reactants to ASO3H and improves its catalytic performance (Okamura et al., 2006). Finally and most importantly, unlike Amberlyst-15, the prepared catalyst contains about 1.6 mmol/g of AOH that can adsorb the hydrophilic parts of the reactants by hydrogen bond (Hara, 2010). As a result, the strong affinity between methanol and the catalyst contributes to its high catalytic activity in the biodiesel production from WCO.
Fig. 2. Stability of RHC–SO3H. Reaction conditions: 20:1 methanol to WCO molar ratio, 5 wt% catalyst amount, 110 °C, 15 h.
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3.3. Catalyst stability
References
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4. Conclusions The solid acid catalyst derived from rice husk char showed excellent catalytic activity and stability in biodiesel production from WCO. This catalyst can efficiently and simultaneously catalyze esterification of FFA and transesterification of TG. In the presence of the catalyst, the FFA conversion was more than 98% after 3 h, and the FAME yield was nearly 90% after 15 h. Thus, this catalyst is potentially useful in biodiesel production, especially in converting feedstocks with high FFA content such as WCO, soapstock and inedible oil to biodiesel.
Acknowledgements The authors acknowledge the financial supports provided by the National High Technology Research and Development Program of China (2012AA051803), the Key Research Program of the Chinese Academy of Sciences (KGZD-EW-304-3), and the National Natural Science Foundation of China (50930006).
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. 12.070.
