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Synthesis porous carbon-based solid acid from rice husk for esterification of fatty acids
Microporous and Mesoporous Materials 219 (2016) 54e58
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Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Synthesis porous carbon-based solid acid from rice husk for esterification of fatty acids Danlin Zeng*, Qi Zhang, Shiyuan Chen, Shenglan Liu, Guanghui Wang The State Key Laboratory of Refractories and Metallurgy, Hubei Key Laboratory of Coal Conversion and New Carbon Material, College of Chemical Engineering and Technology, Wuhan University of Science and Technology, Wuhan 430081, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 May 2015 Received in revised form 16 July 2015 Accepted 26 July 2015 Available online 31 July 2015
A porous carbon solid acid was synthesized from biomass rice husk by incompletely carbonization, sodium hydroxide leaching and concentrated H2SO4 sulfonation. The solid acid was characterized by XRD, FT-IR, N2 adsorption-desorption and solid-state NMR spectroscopy. The characterization results reveal that the carbon solid acid shows ultra high surface area of 1233 m2/g and stronger acid strength than that of HZSM-5(Si/Al ¼ 38) zeolite. The catalytic performance was tested by the esterification of oleic acid with methanol. The results indicate that this solid acid catalyst is an excellent catalyst compared with other conventional solid acid. © 2015 Elsevier Inc. All rights reserved.
Keywords: Solid acid Rice husk Acidity Esterification
1. Introduction Currently industrial esterification processes are carried out by the catalysis of homogeneous Brønsted acids such as sulfuric acid. However, these homogeneous acid catalysts are difficult to be separated, and also cause serious environmental and corrosion problems. Recently, the application of solid catalysts instead of homogeneous liquid catalysts has been paid much attention in view of their convenience of separation and lack of corrosion or toxicity problems [1e5]. Due to the low densities of effective acid sites, inorganic-oxide solid acids such as zeolite or composite oxide cannot satisfy adequate requirement in esterification reactions [6,7]. Although strong acidic ion-exchange resins such as Nafion contain abundant sulfonic acid groups (eSO3H), that function as strong acid sites, their catalytic activities are generally much lower for their very low surface area [8]. These limitations have restricted the practical utility of acidic cation-exchangeable resins. Recently, a new type of sulfonated carbons derived from incomplete carbonization of simple natural product such as sugar, starch or cellulose, has been reported to show better catalytic performance for esterification of fatty acids, and higher stability than sulfonated mesoporous silica
* Corresponding author. Tel./fax: þ86 27 6886 2181. E-mail address: [email protected] (D. Zeng). http://dx.doi.org/10.1016/j.micromeso.2015.07.028 1387-1811/© 2015 Elsevier Inc. All rights reserved.
[5,9,10]. However, such materials were nonporous and exhibited low surface area, which may limit the accessibility to the active sites. The carbon solid acid with unique porous properties is synthesized here as an ideal candidate for the development of the high active catalyst. Every year three million tons of rice husk (RH) are produced in China. So far, such a resource is mainly considered as a waste, and consequently burnt without any profit, except in a few cases of domestic uses for cooking and heating [11]. The main objective of the present work was to prepare carbon based solid acid from rice husk by incomplete carbonization, leaching and sulfonation. The solid acid catalyst was also characterized by X-ray diffraction (XRD), Fourier-transform infrared spectra (FT-IR), scan electron microscope (SEM) and solid state nuclear magnetic resonance (NMR) spectroscopy. The research results will be helpful to obtain the fundamental information of the roles of surface functional groups in the solid acid, which is crucial in the design of a novel carbonbased solid acid for industrial application. 2. Experimental 2.1. Sample preparation Rice husk from a grain depot in Wuhan was used as raw material. Rice husk was calcined at 450 C for 15 h under a N2 flow, and then followed by grinding and leaching with 1 M NaOH at 100 C K
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for 5 h at the ratio of solid to liquid of 1 g: 10 ml to remove silica in the rice husk. After carefully washing with deionized water, the dried rice husk carbon was sulfonated with concentrated H2SO4 (98wt%), at 150 C under a N2 flow for 8 h at the ratio of solid to liquid of 1 g: 10 ml. At last, the mixture was diluted with deionized water, filtered, washed thoroughly, and dried at 120 C for 12 h to obtain the carbon solid acid catalyst. The elemental analysis of the rice husk is listed in Table 1. Recycling experiments were performed to determine the catalytic stability of the solid acid catalysts. At the end of each esterification cycle, the catalyst was centrifuged, washed with ethanol and dried at 105 C for 2 h before reusing. 2.2. Sample characterization The concentration of acid sites on the catalysts was determined by titration method in aqueous solution. One gram of the sample was placed in 50 ml of 0.05 M NaOH solution. The vials were sealed and shaken for 24 h and then 5 ml of the filtrate was pipetted and the excess of base was titrated with HCl. The numbers of acidic sites were calculated from the amount of NaOH that reacted with the catalyst. Surface area and porosity properties of samples were evaluated by N2 adsorption/desorption isotherms carried out on a Micromeritics ASAP 2020 sorption analyzer. Prior to the adsorptionedesorption measurements, all the samples were degassed at 150 C in N2 flow for 12 h. X-ray diffraction (XRD) was performed with a Philips X'PERTPro-MPD diffractometer, operating with Cu Ka radiation (40 kV, 30 mA) and Ni filter. All the NMR experiments were carried out at 9.4 T on a Varian Infinityplus-400 spectrometer with resonance frequencies of 400.12, 100.4 MHz for 1H, 13C, respectively. The 90 pulse widths for 1 H, 13C were measured to be 3.7, 4.4 ms, respectively. The chemical shifts were referenced to tetramethylsilane (TMS) for 1H, to hexamethylbenzene (HMB) for 13C, respectively. The magic angle spinning rate was 5 kHz. For the adsorption of probe molecules 2-13C-acetone, the samples were kept at 400 C under the vacuum less than 1 103 Pa for at least 8 h. The adsorption of 2-13C-acetone was performed at room temperature with a loading of ca. 0.1 mmol per gram catalyst. The various solid acids were employed as catalysts for the esterification of oleic acid with methanol. The SO2 4 /ZrO2 catalyst in this case was prepared using the classic two-step method [12]. The HZSM-5 zeolite (Si/Al ¼ 38) and Nafion NR 50 were pursed from Shanghai Guoyao Corporation. Prior to the reaction, all the catalysts were dried at 120 C for 5 h. The experiments were carried out by mixing oleic acid with methanol in a flask equipped with a reflux condenser, an oil bath and a magnetic stirrer. Once the mixture had reached the reaction temperature, the catalyst was added. The mixtures were withdrawn and centrifuged to separate the solution from the catalyst. Analysis of the reaction mixtures was carried out in an HP 6890 series gas chromatograph equipped with a flame ionization detector (FID).
Fig. 1. XRD patterns of (a) rice husk; (b) rice husk carbon and (c) the carbon solid acid.
2q ¼ 25 is corresponded to the diffraction of C (002) [13]. Compared with that of rice husk, the peak of the impurities (2q ¼ 18 , silica) was disappeared in the spectrum of rice husk carbon and the solid acid. According to Dahn's conclusion [14], it indicates that the solid acid and rice husk carbon consist of a single layer of polyhexagonal carbon atoms after leaching, implying that the BET surface area of the solid acid may be dramatically high. FT-IR spectroscopy was employed to explore the changes in functional groups induced by preparation process. It shows that the difference of the spectroscopy is mainly the bands of oxygen functionalities in the samples (Fig. 2). Two bands at 1039 and 1182 cm1 in the solid acid can be assigned to the SO2 asymmetric and symmetric stretching modes, respectively [5]. It indicates that sulfonic acid group was found on the surface of the sulfonated solid acid. The band at 1700 cm1 can be attributed to the C]O stretching mode of the eCOOH groups, while the broad band centered at 3429 cm1 was assigned to the eOH stretching mode [5]. The band at 1647 cm1 can be attributed to the C]C stretching mode of the samples. Therefore, eCOOH, eSO3H and eOH were found as the functional groups on the solid acid.
3. Results and discussion Fig. 1 illustrates the XRD patterns of rice husk, rice husk carbon and the carbon solid acid. The diffraction peak arising at around Table 1 Element analysis of rice husk (wt%). Sample
C
H
O
N
S
Rice husk
88.15
5.20
3.55
1.84
1.26
Fig. 2. FT-IR spectra of (a) rice husk, (b) rice husk carbon and (c) the carbon solid acid.
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Fig. 3 shows the 13C CP/MAS NMR spectra of rice husk carbon and the solid acid. The peaks at 128 and 153 ppm are due to polycyclic aromatic carbon atoms and aromatic carbon bonded to phenolic OH, respectively [15]. Compared with the spectra of rice husk carbon, the signals at 139 ppm (due to aromatic carbon bonded to the SO3H group, CeSO3H) can be clearly observed in the samples after sulfonation [16]. In addition, the existence of SO3H groups was confirmed in by FT-IR characterization. Thus, it can be concluded that the SO3H groups are successfully formed on the solid acid. N2 adsorption-desorption measurements were also performed to obtain more insights into the rice husk materials. The N2 adsorption-desorption isotherm of rice husk carbon and the carbon solid acid (Fig. 4) shows that both samples exhibit type II patterns, which are typical macropores materials. Furthermore, the hysteresis loop of both samples showed H3 categories, indicating many slit-like pores exist in the samples [17]. These results also prove that the textural properties of the solid acid were substantially maintained during the sulfonation process. The characterization results of the pore size distribution evaluated from adsorption data are also shown in Fig. 4b, implying that the average pore sizes of the two samples are in nanometers. The type and strength of acid sites are the fundamental properties of solid acid. As a sensitive and reliable technique [18], 13C MAS NMR spectra of 2-13C-acetone adsorbed on the catalysts are used to characterize the acidity of the solid acid derived from rice husk. The stronger Brønsted acidity will result in stronger hydrogen bonding between the carbonyl carbon and the acidic proton, and consequently, the more downfield of the 13C isotropic chemical shift. As shown in Fig. 5, the resonance at 113 ppm is due to the background of the NMR probe. The strong signal (at 219 ppm) is ascribed to acetone adsorbed on the weakly acidic OH groups present on the catalyst [19] (Fig. 5a). For the carbon solid acid (Fig. 5b), three clear resonances at 219 ppm (due to acetone adsorbed on the weak acidic eOH groups), 229 and 242 ppm are observed in the 13C MAS NMR spectra. Since the FT-IR characterization has confirmed the existence of eCOOH and eSO3H functional groups, the resonances at 229 and 242 can be assigned to
Fig. 4. The N2 adsorptionedesorption isotherm (a), and pore size distribution (b) of the rice husk carbon and carbon solid acid.
Fig. 3. 13C CP/MAS NMR spectrum of (a) rice husk carbon and (b)the carbon solid acid. The asterisk denotes spinning sidebands.
2-13C-acetone adsorbed on the Brønsted acid sites eCOOH and eSO3H groups, respectively [5]. The resonance at 229 ppm (due toCOOH groups) indicates that acid strength of eCOOH acid site on the catalyst is stronger than that of the bridging OH group in HZSM5 zeolite, while the 242 ppm signal corresponding to Brønsted sites of SO3H groups exhibits its acid strength is weaker than that of 100% H2SO4 (shown a 31C chemical shift of 245 ppm) [20]. The morphology of rice husk, rice husk carbon and carbon solid acid catalyst is shown in Fig. 6. The rice husk exhibited a typical network structure of the biological tissue (Fig. 6a). While after calcinations and leaching process lots of pores with sizes of micrometers can be clearly found in rice husk carbon. Fig. 6c shows that after the sulfonation treatment the porous structure still exists in the carbon solid acid, implying that the prepared carbon catalyst has a high surface area and abundant porous structure, which is in favor of catalysis reaction. In order to evaluate the activity of the solid acid in the reaction, a comparative study was made between the carbon solid acid, sulfated zirconia, HZSM-5(Si/Al ¼ 38) and Nafion NR 50 (Fig. 7). The reaction used was the esterification of oleic acid (20 mmol) with methanol (100 mmol) at 80 C for 9 h. The same amount (0.015 g) of all the catalysts was used in the reactions. A remarkable enhancement in the reactivity and the yield was observed with carbon catalyst as compared with the other solid acid catalysts examined.
D. Zeng et al. / Microporous and Mesoporous Materials 219 (2016) 54e58
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Fig. 7. Comparison of catalytic activity for conversion of oleic acid over the carbon solid acid and other typical solid acid. Fig. 5. 13C CP/MAS NMR spectra of 2-13C-acetone adsorbed on (a) rice husk carbon and (b) the carbon solid acid. The asterisk denotes spinning sidebands. Table 2 Textural properties and the catalytic performance of the various catalysts.
It is known that strong acid strength of solid acid leads to high esterification reaction activity. In this case, compared the 2-13Cacetone NMR characterization results of the carbon solid acid (the 13 C chemical shift is 242 ppm) with sulfated zirconia ([21], the 13C chemical shift is 234 ppm from the reference), H-ZSM-5 ([22], the 13 C chemical shift is 223 ppm) and Nafion NR50 [23], it can be concluded that the carbon solid acid shows the stronger acid strength than other three solid acids, consequently, it will exhibit higher activity in the esterification reaction, which is also consistent with the catalytic performance results in Table 2. The rice huskderived carbon solid acid catalyst afforded a high yield of above 90%. Therefore, it can be inferred that high reactivity in esterification reaction is due to the strong acid strength and high surface area of the carbon solid acid. To test the reusability of the catalyst, the carbon solid acid was reused for ten cycles for the esterification of oleic acid. After the esterification was completed, the used catalyst was separated from the reaction mixture by filtration, and then it was washed with ethanol to remove the residual reactants off the catalyst surface. Finally, the catalyst was dried at 105 C for 2 h before reusing. The results are presented in Fig. 8. Compared with those of the typical solid acid sulfated zirconia, the yields of the carbon solid acid gradually decreased after the first two cycles and then remained almost constant for the later cycles. The carbon solid acid catalyst
Catalysts
SBET (m2/g)
Vtot (cm3/g)
D (nm)
Total acid density (mmol/g)
Yield (%)
Solid acid SO2 4 /ZrO2 HZSM-5 Nafion NR50
1233 151 370 0.018
0.744 0.232 0.122 e
38.96 5.23 0.59 e
5.25 4.41 1.13 0.92
91 65 30 18
SBET, specific surface area from BET method; Vtot, total pore volume; D, average pore diameter. Reaction condition: 20 mmol oleic acid; 100 mmol methanol; 0.015 g catalyst; 80 C; 9 h.
shows good reusability for the esterification of oleic acid with methanol, which can be attributed to its ultra high surface area and the tight attachment of strong acid groups to the framework. Consequently, this kind of solid acid from rice husk is a promising green catalyst for esterification. 4. Conclusions In summary, a novel carbon solid acid was prepared from biomass rice husk by incompletely carbonization, sodium hydroxide leaching and concentrated H2SO4 sulfonation. The solid acid was characterized by XRD, FT-IR, N2 adsorption-desorption and solid-state NMR. The characterization results show that the carbon
Fig. 6. SEM of (a) rice husk; (b) rice husk carbon and (c)the carbon solid acid.
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Provincial Department of Education (B2014094) and the Open Research Fund of Hubei Province Key Laboratory of Coal Conversion and New Carbon Material (WKDM2013010).
References
Fig. 8. Stability test of the carbon solid acid and the typical solid acid SO2 4 /ZrO2.
solid acid exhibits ultra high surface area of 1233 m2/g and stronger acid strength than that of HZSM-5 zeolite. The catalytic performance was tested by the esterification of oleic acid with methanol. The results indicate that this solid acid catalyst is an excellent catalyst compared with other conventional solid acid. In addition, it is possible that this green and active porous carbon catalyst may find wide applications in reactions catalyzed by liquid acids. Acknowledgments We acknowledge the financial supports from the National Natural Science Foundation of China (21473126), the Fund of Hubei
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Contents lists available at ScienceDirect
Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Synthesis porous carbon-based solid acid from rice husk for esterification of fatty acids Danlin Zeng*, Qi Zhang, Shiyuan Chen, Shenglan Liu, Guanghui Wang The State Key Laboratory of Refractories and Metallurgy, Hubei Key Laboratory of Coal Conversion and New Carbon Material, College of Chemical Engineering and Technology, Wuhan University of Science and Technology, Wuhan 430081, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 May 2015 Received in revised form 16 July 2015 Accepted 26 July 2015 Available online 31 July 2015
A porous carbon solid acid was synthesized from biomass rice husk by incompletely carbonization, sodium hydroxide leaching and concentrated H2SO4 sulfonation. The solid acid was characterized by XRD, FT-IR, N2 adsorption-desorption and solid-state NMR spectroscopy. The characterization results reveal that the carbon solid acid shows ultra high surface area of 1233 m2/g and stronger acid strength than that of HZSM-5(Si/Al ¼ 38) zeolite. The catalytic performance was tested by the esterification of oleic acid with methanol. The results indicate that this solid acid catalyst is an excellent catalyst compared with other conventional solid acid. © 2015 Elsevier Inc. All rights reserved.
Keywords: Solid acid Rice husk Acidity Esterification
1. Introduction Currently industrial esterification processes are carried out by the catalysis of homogeneous Brønsted acids such as sulfuric acid. However, these homogeneous acid catalysts are difficult to be separated, and also cause serious environmental and corrosion problems. Recently, the application of solid catalysts instead of homogeneous liquid catalysts has been paid much attention in view of their convenience of separation and lack of corrosion or toxicity problems [1e5]. Due to the low densities of effective acid sites, inorganic-oxide solid acids such as zeolite or composite oxide cannot satisfy adequate requirement in esterification reactions [6,7]. Although strong acidic ion-exchange resins such as Nafion contain abundant sulfonic acid groups (eSO3H), that function as strong acid sites, their catalytic activities are generally much lower for their very low surface area [8]. These limitations have restricted the practical utility of acidic cation-exchangeable resins. Recently, a new type of sulfonated carbons derived from incomplete carbonization of simple natural product such as sugar, starch or cellulose, has been reported to show better catalytic performance for esterification of fatty acids, and higher stability than sulfonated mesoporous silica
* Corresponding author. Tel./fax: þ86 27 6886 2181. E-mail address: [email protected] (D. Zeng). http://dx.doi.org/10.1016/j.micromeso.2015.07.028 1387-1811/© 2015 Elsevier Inc. All rights reserved.
[5,9,10]. However, such materials were nonporous and exhibited low surface area, which may limit the accessibility to the active sites. The carbon solid acid with unique porous properties is synthesized here as an ideal candidate for the development of the high active catalyst. Every year three million tons of rice husk (RH) are produced in China. So far, such a resource is mainly considered as a waste, and consequently burnt without any profit, except in a few cases of domestic uses for cooking and heating [11]. The main objective of the present work was to prepare carbon based solid acid from rice husk by incomplete carbonization, leaching and sulfonation. The solid acid catalyst was also characterized by X-ray diffraction (XRD), Fourier-transform infrared spectra (FT-IR), scan electron microscope (SEM) and solid state nuclear magnetic resonance (NMR) spectroscopy. The research results will be helpful to obtain the fundamental information of the roles of surface functional groups in the solid acid, which is crucial in the design of a novel carbonbased solid acid for industrial application. 2. Experimental 2.1. Sample preparation Rice husk from a grain depot in Wuhan was used as raw material. Rice husk was calcined at 450 C for 15 h under a N2 flow, and then followed by grinding and leaching with 1 M NaOH at 100 C K
D. Zeng et al. / Microporous and Mesoporous Materials 219 (2016) 54e58
55
for 5 h at the ratio of solid to liquid of 1 g: 10 ml to remove silica in the rice husk. After carefully washing with deionized water, the dried rice husk carbon was sulfonated with concentrated H2SO4 (98wt%), at 150 C under a N2 flow for 8 h at the ratio of solid to liquid of 1 g: 10 ml. At last, the mixture was diluted with deionized water, filtered, washed thoroughly, and dried at 120 C for 12 h to obtain the carbon solid acid catalyst. The elemental analysis of the rice husk is listed in Table 1. Recycling experiments were performed to determine the catalytic stability of the solid acid catalysts. At the end of each esterification cycle, the catalyst was centrifuged, washed with ethanol and dried at 105 C for 2 h before reusing. 2.2. Sample characterization The concentration of acid sites on the catalysts was determined by titration method in aqueous solution. One gram of the sample was placed in 50 ml of 0.05 M NaOH solution. The vials were sealed and shaken for 24 h and then 5 ml of the filtrate was pipetted and the excess of base was titrated with HCl. The numbers of acidic sites were calculated from the amount of NaOH that reacted with the catalyst. Surface area and porosity properties of samples were evaluated by N2 adsorption/desorption isotherms carried out on a Micromeritics ASAP 2020 sorption analyzer. Prior to the adsorptionedesorption measurements, all the samples were degassed at 150 C in N2 flow for 12 h. X-ray diffraction (XRD) was performed with a Philips X'PERTPro-MPD diffractometer, operating with Cu Ka radiation (40 kV, 30 mA) and Ni filter. All the NMR experiments were carried out at 9.4 T on a Varian Infinityplus-400 spectrometer with resonance frequencies of 400.12, 100.4 MHz for 1H, 13C, respectively. The 90 pulse widths for 1 H, 13C were measured to be 3.7, 4.4 ms, respectively. The chemical shifts were referenced to tetramethylsilane (TMS) for 1H, to hexamethylbenzene (HMB) for 13C, respectively. The magic angle spinning rate was 5 kHz. For the adsorption of probe molecules 2-13C-acetone, the samples were kept at 400 C under the vacuum less than 1 103 Pa for at least 8 h. The adsorption of 2-13C-acetone was performed at room temperature with a loading of ca. 0.1 mmol per gram catalyst. The various solid acids were employed as catalysts for the esterification of oleic acid with methanol. The SO2 4 /ZrO2 catalyst in this case was prepared using the classic two-step method [12]. The HZSM-5 zeolite (Si/Al ¼ 38) and Nafion NR 50 were pursed from Shanghai Guoyao Corporation. Prior to the reaction, all the catalysts were dried at 120 C for 5 h. The experiments were carried out by mixing oleic acid with methanol in a flask equipped with a reflux condenser, an oil bath and a magnetic stirrer. Once the mixture had reached the reaction temperature, the catalyst was added. The mixtures were withdrawn and centrifuged to separate the solution from the catalyst. Analysis of the reaction mixtures was carried out in an HP 6890 series gas chromatograph equipped with a flame ionization detector (FID).
Fig. 1. XRD patterns of (a) rice husk; (b) rice husk carbon and (c) the carbon solid acid.
2q ¼ 25 is corresponded to the diffraction of C (002) [13]. Compared with that of rice husk, the peak of the impurities (2q ¼ 18 , silica) was disappeared in the spectrum of rice husk carbon and the solid acid. According to Dahn's conclusion [14], it indicates that the solid acid and rice husk carbon consist of a single layer of polyhexagonal carbon atoms after leaching, implying that the BET surface area of the solid acid may be dramatically high. FT-IR spectroscopy was employed to explore the changes in functional groups induced by preparation process. It shows that the difference of the spectroscopy is mainly the bands of oxygen functionalities in the samples (Fig. 2). Two bands at 1039 and 1182 cm1 in the solid acid can be assigned to the SO2 asymmetric and symmetric stretching modes, respectively [5]. It indicates that sulfonic acid group was found on the surface of the sulfonated solid acid. The band at 1700 cm1 can be attributed to the C]O stretching mode of the eCOOH groups, while the broad band centered at 3429 cm1 was assigned to the eOH stretching mode [5]. The band at 1647 cm1 can be attributed to the C]C stretching mode of the samples. Therefore, eCOOH, eSO3H and eOH were found as the functional groups on the solid acid.
3. Results and discussion Fig. 1 illustrates the XRD patterns of rice husk, rice husk carbon and the carbon solid acid. The diffraction peak arising at around Table 1 Element analysis of rice husk (wt%). Sample
C
H
O
N
S
Rice husk
88.15
5.20
3.55
1.84
1.26
Fig. 2. FT-IR spectra of (a) rice husk, (b) rice husk carbon and (c) the carbon solid acid.
56
D. Zeng et al. / Microporous and Mesoporous Materials 219 (2016) 54e58
Fig. 3 shows the 13C CP/MAS NMR spectra of rice husk carbon and the solid acid. The peaks at 128 and 153 ppm are due to polycyclic aromatic carbon atoms and aromatic carbon bonded to phenolic OH, respectively [15]. Compared with the spectra of rice husk carbon, the signals at 139 ppm (due to aromatic carbon bonded to the SO3H group, CeSO3H) can be clearly observed in the samples after sulfonation [16]. In addition, the existence of SO3H groups was confirmed in by FT-IR characterization. Thus, it can be concluded that the SO3H groups are successfully formed on the solid acid. N2 adsorption-desorption measurements were also performed to obtain more insights into the rice husk materials. The N2 adsorption-desorption isotherm of rice husk carbon and the carbon solid acid (Fig. 4) shows that both samples exhibit type II patterns, which are typical macropores materials. Furthermore, the hysteresis loop of both samples showed H3 categories, indicating many slit-like pores exist in the samples [17]. These results also prove that the textural properties of the solid acid were substantially maintained during the sulfonation process. The characterization results of the pore size distribution evaluated from adsorption data are also shown in Fig. 4b, implying that the average pore sizes of the two samples are in nanometers. The type and strength of acid sites are the fundamental properties of solid acid. As a sensitive and reliable technique [18], 13C MAS NMR spectra of 2-13C-acetone adsorbed on the catalysts are used to characterize the acidity of the solid acid derived from rice husk. The stronger Brønsted acidity will result in stronger hydrogen bonding between the carbonyl carbon and the acidic proton, and consequently, the more downfield of the 13C isotropic chemical shift. As shown in Fig. 5, the resonance at 113 ppm is due to the background of the NMR probe. The strong signal (at 219 ppm) is ascribed to acetone adsorbed on the weakly acidic OH groups present on the catalyst [19] (Fig. 5a). For the carbon solid acid (Fig. 5b), three clear resonances at 219 ppm (due to acetone adsorbed on the weak acidic eOH groups), 229 and 242 ppm are observed in the 13C MAS NMR spectra. Since the FT-IR characterization has confirmed the existence of eCOOH and eSO3H functional groups, the resonances at 229 and 242 can be assigned to
Fig. 4. The N2 adsorptionedesorption isotherm (a), and pore size distribution (b) of the rice husk carbon and carbon solid acid.
Fig. 3. 13C CP/MAS NMR spectrum of (a) rice husk carbon and (b)the carbon solid acid. The asterisk denotes spinning sidebands.
2-13C-acetone adsorbed on the Brønsted acid sites eCOOH and eSO3H groups, respectively [5]. The resonance at 229 ppm (due toCOOH groups) indicates that acid strength of eCOOH acid site on the catalyst is stronger than that of the bridging OH group in HZSM5 zeolite, while the 242 ppm signal corresponding to Brønsted sites of SO3H groups exhibits its acid strength is weaker than that of 100% H2SO4 (shown a 31C chemical shift of 245 ppm) [20]. The morphology of rice husk, rice husk carbon and carbon solid acid catalyst is shown in Fig. 6. The rice husk exhibited a typical network structure of the biological tissue (Fig. 6a). While after calcinations and leaching process lots of pores with sizes of micrometers can be clearly found in rice husk carbon. Fig. 6c shows that after the sulfonation treatment the porous structure still exists in the carbon solid acid, implying that the prepared carbon catalyst has a high surface area and abundant porous structure, which is in favor of catalysis reaction. In order to evaluate the activity of the solid acid in the reaction, a comparative study was made between the carbon solid acid, sulfated zirconia, HZSM-5(Si/Al ¼ 38) and Nafion NR 50 (Fig. 7). The reaction used was the esterification of oleic acid (20 mmol) with methanol (100 mmol) at 80 C for 9 h. The same amount (0.015 g) of all the catalysts was used in the reactions. A remarkable enhancement in the reactivity and the yield was observed with carbon catalyst as compared with the other solid acid catalysts examined.
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Fig. 7. Comparison of catalytic activity for conversion of oleic acid over the carbon solid acid and other typical solid acid. Fig. 5. 13C CP/MAS NMR spectra of 2-13C-acetone adsorbed on (a) rice husk carbon and (b) the carbon solid acid. The asterisk denotes spinning sidebands. Table 2 Textural properties and the catalytic performance of the various catalysts.
It is known that strong acid strength of solid acid leads to high esterification reaction activity. In this case, compared the 2-13Cacetone NMR characterization results of the carbon solid acid (the 13 C chemical shift is 242 ppm) with sulfated zirconia ([21], the 13C chemical shift is 234 ppm from the reference), H-ZSM-5 ([22], the 13 C chemical shift is 223 ppm) and Nafion NR50 [23], it can be concluded that the carbon solid acid shows the stronger acid strength than other three solid acids, consequently, it will exhibit higher activity in the esterification reaction, which is also consistent with the catalytic performance results in Table 2. The rice huskderived carbon solid acid catalyst afforded a high yield of above 90%. Therefore, it can be inferred that high reactivity in esterification reaction is due to the strong acid strength and high surface area of the carbon solid acid. To test the reusability of the catalyst, the carbon solid acid was reused for ten cycles for the esterification of oleic acid. After the esterification was completed, the used catalyst was separated from the reaction mixture by filtration, and then it was washed with ethanol to remove the residual reactants off the catalyst surface. Finally, the catalyst was dried at 105 C for 2 h before reusing. The results are presented in Fig. 8. Compared with those of the typical solid acid sulfated zirconia, the yields of the carbon solid acid gradually decreased after the first two cycles and then remained almost constant for the later cycles. The carbon solid acid catalyst
Catalysts
SBET (m2/g)
Vtot (cm3/g)
D (nm)
Total acid density (mmol/g)
Yield (%)
Solid acid SO2 4 /ZrO2 HZSM-5 Nafion NR50
1233 151 370 0.018
0.744 0.232 0.122 e
38.96 5.23 0.59 e
5.25 4.41 1.13 0.92
91 65 30 18
SBET, specific surface area from BET method; Vtot, total pore volume; D, average pore diameter. Reaction condition: 20 mmol oleic acid; 100 mmol methanol; 0.015 g catalyst; 80 C; 9 h.
shows good reusability for the esterification of oleic acid with methanol, which can be attributed to its ultra high surface area and the tight attachment of strong acid groups to the framework. Consequently, this kind of solid acid from rice husk is a promising green catalyst for esterification. 4. Conclusions In summary, a novel carbon solid acid was prepared from biomass rice husk by incompletely carbonization, sodium hydroxide leaching and concentrated H2SO4 sulfonation. The solid acid was characterized by XRD, FT-IR, N2 adsorption-desorption and solid-state NMR. The characterization results show that the carbon
Fig. 6. SEM of (a) rice husk; (b) rice husk carbon and (c)the carbon solid acid.
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D. Zeng et al. / Microporous and Mesoporous Materials 219 (2016) 54e58
Provincial Department of Education (B2014094) and the Open Research Fund of Hubei Province Key Laboratory of Coal Conversion and New Carbon Material (WKDM2013010).
References
Fig. 8. Stability test of the carbon solid acid and the typical solid acid SO2 4 /ZrO2.
solid acid exhibits ultra high surface area of 1233 m2/g and stronger acid strength than that of HZSM-5 zeolite. The catalytic performance was tested by the esterification of oleic acid with methanol. The results indicate that this solid acid catalyst is an excellent catalyst compared with other conventional solid acid. In addition, it is possible that this green and active porous carbon catalyst may find wide applications in reactions catalyzed by liquid acids. Acknowledgments We acknowledge the financial supports from the National Natural Science Foundation of China (21473126), the Fund of Hubei
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