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Efficient carbon-based solid acid catalysts for the esterification of oleic acid
Catalysis Communications 13 (2011) 26–30
Contents lists available at ScienceDirect
Catalysis Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c a t c o m
Short Communication
Efficient carbon-based solid acid catalysts for the esterification of oleic acid Liang Geng, Yu Wang, Gang Yu, Yuexiang Zhu ⁎ Beijing National Laboratory for Molecular Sciences, State Key Laboratory for Structural Chemistry of Unstable and Stable Species, College of Chemistry and Molecular Engineering, Peking University, Beijing 10087, China
a r t i c l e
i n f o
Article history: Received 28 April 2011 Received in revised form 9 June 2011 Accepted 10 June 2011 Available online 17 June 2011 Keywords: Carbon-based solid acid Dispersion Esterification Biodiesel
a b s t r a c t A series of carbon-based solid acid catalysts was prepared by the sulfonation of mesoporous carbon substrates with thin pore walls, and catalytic activity for the esterification of oleic acid with methanol was tested. The highest turnover frequency (TOF) observed was 78 h− 1, five times that of Amberlyst-15. The high catalytic activity may be attributed to the good dispersion of the catalysts in methanol. Catalysts with improved dispersion were obtained by a modified preparation, after which the highest TOF observed was 109 h− 1, seven times that of Amberlyst-15. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Biodiesel, a nonpetroleum-based fuel, is usually produced through the transesterification of triglycerides [1,2]. In most cases, the feedstock oil contains considerable amounts of undesirable free fatty acid (FFA), which must be converted into esters through preesterification. Conventionally, pre-esterification is catalyzed by liquid H2SO4, which involves costly neutralization and separation. Therefore, much attention has been paid to easily recycled solid acid catalysts as replacements for liquid acids [3]. Carbon-based solid acids are considered ideal catalysts for many reactions due to their chemical inertness and superior mechanical and thermal stability. Sulfonated carbon is among the most promising of the solid acids developed for FFA esterification [4]. Such catalysts are generally produced by the sulfonation of incompletely pyrolyzed biomass, such as sucrose [5], glucose [6], starch [7,8] or biochar [9]. Carbon precursors are directly pyrolyzed and sulfonated in concentrated H2SO4[5–11]. This simple process, however, usually produces sulfonated carbon with low surface area, low acid density and poor dispersion in liquid-phase reaction mixture. Recently, a sulfonated ordered mesoporous carbon (OMC) was used as a solid acid [12,13]. Using ordered mesoporous silica, usually SBA-15, as a template, OMC catalysts with high BET area and large
⁎ Corresponding author at: College of Chemistry and Molecular Engineering, Peking University, Beijing 10087, China. Tel.: + 86 10 62751703; fax: + 86 10 62751708. E-mail address: [email protected] (Y. Zhu). 1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2011.06.014
pores were synthesized. These characteristics favor the diffusion of long-chain FFA molecules to the catalytically active sites. It is reported that a uniformly carbon-coated alumina (CCA) was synthesized through repetition of carbon precursor adsorptionpyrolysis cycles [14–16]. From such CCAs, we prepared a series of disordered mesoporous carbon with high BET area, large pores and thin pore walls [17,18]. In this work, we grafted benzenesulfonic acid groups onto those carbon materials with 4-benzene-diazoniumsulfonate (4-BDS) to yield solid acids, for the sulfonation could be more efficient with 4-BDS instead of H2SO4[13]. Such catalysts, composed of small and thin carbon sheets, were readily dispersed in methanol, offering high catalytic efficiency in the esterification of oleic acid with methanol. To further improve the dispersion, grafting was carried out on CCAs, followed by the removal of the alumina. The resulting catalysts exhibited higher efficiency in the esterification of oleic acid. The highest TOF observed was 109 h − 1, seven times that of Amberlyst-15. 2. Experimental 2.1. Preparation of CCA CCA was prepared according to a previously reported procedure [15–18]. γ-Al2O3 (SBA150, Engelhard) was impregnated with aqueous solutions of sucrose. The weight ratio of sucrose and Al2O3 was kept at the monolayer dispersion threshold value. The sucrose-Al2O3 composite was carbonized at 600 °C under N2 gas to obtain CCA1. The impregnation and carbonization process were repeated one to three times to obtain CCA2, CCA3 and CCA4. The numbers indicate the number of repetitions of the impregnation and carbonization process and higher carbon content.
L. Geng et al. / Catalysis Communications 13 (2011) 26–30
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2.2. Preparation of sulfonated carbon catalysts CCAs were immersed in a 24% hydrofluoric acid (HF) solution at room temperature for 3 h to dissolve the alumina. The remaining carbon-based materials (C-CCAs) were washed repeatedly with water and dried at 110 °C. The sulfonation of C-CCAs was achieved by mixing C-CCA (0.4 g) and an aqueous solution of 0.22 g 4-BDS in 0.05 M HCl and holding the suspension at 5 °C for 3 h. After filtration and thorough washing with water, dimethylformamide (DMF) and acetone, the sulfonated carbon catalysts were obtained and are denoted SC-CCA1, SC-CCA2, SC-CCA3 and SC-CCA4. Activated carbon powder (Beijing Dali Fine Chemical Plant, China) was sulfonated by the same procedure to obtain sulfonated activated carbon (S-AC). To improve catalyst dispersion in solution, another series of sulfonated carbon was synthesized by first sulfonating CCAs, then removing the alumina. The catalysts prepared by this route are denoted SC*-CCA1, SC*-CCA2, SC*-CCA3 and SC*-CCA4. All procedures above are illustrated in a diagram (Fig. S1). Amberlyst-15 (Acros Organics) was ground into a powder for use as a control catalyst and was degassed at 100 °C for 7 h before catalytic tests.
2.3. Catalyst characterization Transmission electron microscopy (TEM) images were recorded on a Hitachi H-9000NAR high-resolution microscope. The N2 sorption analysis was performed on a Micromeritics ASAP 2010 volumetric adsorption system at 77 K. All samples were degassed at 150 °C prior to measurements. The specific surface area was determined using the BET method based on the adsorption data in the relative pressure (P/P0) range 0.05 to 0.20. Pore size distribution (PSD) was evaluated by the Barrett–Joyner–Halenda method from the adsorption branch of the isotherm. The total pore volume was estimated from the amount of N2 adsorbed at a relative pressure of 0.99. XPS Al2p signals were recorded on a Kratos Axis Ultra System. Fourier transform infrared (FT-IR) absorbance spectra were recorded on a Nicolet iN10 FT-IR Microscope using pure samples. Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku D/MAX0200 powder diffractometer. Strong acid density was determined by sulfur elemental analysis (Elementar Vario Micro Cube, Germany) and total acid density by potentiometric titration with 0.001 M NaOH.
2.4. Catalytic tests Esterification was performed in a stirred 20-mL autoclave at 65 °C. 50 mg of solid acid catalyst was added to 1 g of oleic acid (99%, TCI) and 8 mL of methanol. At selected reaction times, the autoclave was cooled down rapidly in a water bath. The catalyst was then removed by filtration, and the product was extracted and diluted with nhexane to 100 mL. The yield of methyl ester was analyzed by GC, where methyl heptadecanoate (AccuStandard, 10.0 mg/L in hexane) was used as the internal standard. For kinetic studies, the conversion results were fitted to a pseudo-first order rate equation to determine the rate constant k. Turnover frequency (TOF) was calculated according to the equation below, using the initial amount of oleic acid, n0,OA, and the number of strong acid sites on the catalyst, nH+: TOF = kn0;OA = nH
+
Fig. 1. N2 sorption isotherms and pore size distribution plots (inset) of C-CCAs, SC-CCAs and S-AC.
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Table 1 Textural properties of carbon materials and sulfonated carbons. Sample
BET area (m2/g)
Da (nm)
Vb (cm3/g)
C-CCA1 C-CCA2 C-CCA3 C-CCA4 AC SC-CCA1 SC-CCA2 SC-CCA3 SC-CCA4 S-AC Amberlyst-15
570 1035 1255 1231 1297 39 354 623 805 318 43
2.51 3.20 5.35 8.36 2.36 – 2.83 3.64 6.87 – 29.1
0.36 0.83 1.68 2.57 0.76 0.04 0.25 0.57 1.38 0.24 0.31
a b
Average pore diameter. Pore volume.
3. Results and discussion 3.1. Solid acids prepared by sulfonating thin-wall mesoporous carbons 3.1.1. Preparation and characterization of SC-CCAs Previously, we found that sucrose molecules could readily monolayer-disperse on γ-alumina surface [16]. After pyrolysis, sucrose then formed small and thin carbon sheets covering alumina surface [18]. Therefore, by carefully controlling sucrose loading and repetition of the impregnation and carbonization process, the alumina surface of CCAs was uniformly covered by carbon sheets as thin as 1–2 graphene layers (Fig. S2) [18]. After dissolving the alumina in HF (XPS analysis showed that alumina was removed completely, Fig. S3), carbon materials (C-CCAs) with different textures were obtained. Fig. 1 shows that all C-CCAs exhibited typical IV adsorption isotherms with a hysteresis loop and peaks in the mesopore range in PSD curves. Table 1 indicates that C-CCA1 possessed a denser structure than the other three, with a lower BET area, pore size and pore volume. Because carbon sheets in CCA1 were discrete and small, they aggregated into a denser structure once the alumina was removed. As carbon content increased, the carbon sheets grew larger and thicker and joined each other to form a continuous framework, which is stable enough to support itself and afford larger pores. The sulfonation of C-CCAs was achieved through covalent attachment of benzenesulfonic acid groups by reacting C-CCAs with
Fig. 3. XRD patterns a) and FT-IR spectra b) of C-CCA1, SC-CCA1 and SC*-CCA1.
4-BDS. As shown in Fig. 2, the morphology of sulfonated carbon SCCCAs was closely related to the corresponding C-CCAs. As the carbon content in CCA increased, larger pores emerged. XRD patterns (Fig. 3a) indicated that both C-CCAs and SC-CCAs possessed low crystallinity.
Fig. 2. TEM images of a) C-CCA1, b) C-CCA2, c) C-CCA3, d) C-CCA4, e) SC-CCA1, f) SC-CCA2, g) SC-CCA3 and h) SC-CCA4.
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Table 2 Acid densities and catalytic activities of solid acids. Sample
Strong acid density (mmol H+/g)
Total acid density (mmol H+/g)
k (h− 1)
TOF (h− 1)
SC-CCA1 SC-CCA2 SC-CCA3 SC-CCA4 SC*-CCA1 SC*-CCA2 SC*-CCA3 SC*-CCA4 S-AC Amberlyst-15
1.72 1.49 1.63 1.66 1.42 1.49 1.70 1.42 1.42 5.03
2.33 2.41 2.60 1.97 2.62 2.55 2.64 2.64 1.74 5.31
1.90 1.61 1.61 1.15 2.17 2.30 2.46 1.68 0.89 1.07
78 67 70 49 108 109 102 84 44 15
The presence of benzenesulfonic acid groups on C-CCAs was confirmed by FT-IR spectra (Fig. 3b). The bands at 1035 cm − 1 and 1008 cm − 1 (S=O stretching) and 1125 cm − 1, 1171 cm − 1and 1220 cm − 1 (stretching in SO3H) are consistent with SO3H groups [5,6]. Comparison of the textural properties of SC-CCAs and C-CCAs (Fig. 1, Table 1) showed that the BET area, pore size and pore volume of C-CCAs decreased dramatically after sulfonation. Acid densities in SC-CCAs were determined by both sulfur elemental analysis (strong acid density, the amount of SO3H) and potentiometric titration (total acid density, the sum of SO3H, COOH and OH) (Table 2). The difference between the two types of acid density is attributed to weak acid groups (COOH and OH) from the incomplete carbonization of sucrose. These groups contribute very little to catalytic esterification due to their insufficient acidity, but their hydrophilicity should favor the dispersion of catalysts [10]. With SO3H groups attached to carbon sheets, SC-CCAs could readily disperse in polar solvents. As shown in Fig. 4a, C-CCA1 precipitated in methanol in a few minutes, whereas SC-CCA1 formed a stable dispersion. In polar solvents, SO3H groups afford hydrophilicity and electrostatic repulsion to counteract the π–π interactions between carbon sheets. However, the size of carbon sheets was found to play a more important role in dispersion. Carbon sheets in SC-CCA1 were thin and small enough to be driven apart by such electrostatic repulsion and dispersed in methanol. As the carbon content in CCA increased, carbon sheets grew larger and gradually became insoluble.
3.1.2. Catalytic performance of SC-CCAs The catalytic performance of SC-CCAs in the esterification of oleic acid with methanol is shown in Fig. 5a. Because the amount of methanol was 66 times that of oleic acid, the reaction follows pseudofirst order kinetics (with plots fitted in Fig. 5a). The rate constants and TOF were determined and are shown in Table 2. Compared with commercial catalyst Amberlyst-15, SC-CCAs exhibited higher activities, even though the strong acid density of SC-CCAs was much lower.
Fig. 4. Photograph of a) C-CCA1 (left) and SC-CCA1(right) dispersed in methanol after 20 min and b) S-AC (left) and SC*-CCA1(right) dispersed in methanol after 5 h.
Fig. 5. Esterification of oleic acid with methanol over a) SC-CCAs and b) SC*-CCAs. Kinetic plots were fitted to first order reaction kinetics.
The highest TOF (SC-CCA1, 78 h − 1) was five times that of Amberlyst15 (15 h − 1). The high catalytic activity of SC-CCAs is closely related to catalyst dispersion in the liquid-phase reaction mixture. SC-CCA1 displayed the best dispersion among the four catalysts (Fig. 4a), which dramatically promoted oleic acid access to active sites and yielded the highest TOF. For SC-CCA2 and SC-CCA3 (TOF is 67 h − 1 and 70 h − 1, respectively), the carbon sheets grew larger and became less soluble, so they were not as efficient as SC-CCA1. Larger carbon sheets formed a rigid framework, and the corresponding sulfonated carbon SC-CCA4 showed the lowest TOF (49 h − 1). With respect to a solid acid catalyst, high BET area and large pores should favor the diffusion of the reactants, thereby leading to high activity. However, we found that the catalysts with a higher BET area offered lower TOFs (Tables 1 and 2). The catalytic performance is not related to the catalyst texture measured in a dry state. In other words, the catalyst dispersion outweighed texture and stood out as the key factor in determining catalytic performance. Amberlyst-15 is an ion exchange resin bearing macropores. Its low BET area (43 m 2/g) and high acid density (5.03 mmol/g) reveal that most of its acid sites reside in a poorly swelling network, which can only be penetrated by small ions (H +, Na +) instead of large molecules like oleic acid. Therefore, Amberlyst-15 showed much lower TOF in the esterification of oleic acid than SC-CCAs.
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3.2. Solid acids prepared by sulfonating CCA followed by the removal of alumina 3.2.1. Preparation and characterization of SC*-CCAs Given that catalyst dispersion is critical to the catalytic activity, another series of carbon catalysts SC*-CCAs with improved dispersion was prepared. These catalysts were synthesized by sulfonating CCAs prior to alumina removal. XRD patterns (Fig. 3a) and the data in Table 2 showed that this series of carbon catalysts possessed similar crystallinity and acid density to SC-CCAs. The successful grafting of benzenesulfonic acid groups onto CCAs was confirmed by FT-IR spectra (Fig. 3b) and by texture comparison of CCAs and sulfonated CCAs (Fig. S4). Compared with SC-CCAs, SC*-CCAs were better able to disperse in methanol. As shown in Fig. 4b, SC*-CCA1 could be well dispersed in methanol to give a uniform black solution. This solution is so stable that no deposition was detected even after three months. To investigate the effect of catalyst dispersion, sulfonated activated carbon (S-AC) was prepared as a control sample. AC and S-AC possessed micropore-dominated structures (Fig. 1 and Table 1), and the acid density of S-AC is much lower than that of SC*-CCAs (Table 2), indicating that the carbon particles of S-AC bear fewer SO3H, COOH and OH groups, and would disperse poorly in methanol (Fig. 4b). 3.2.2. Catalytic performance of SC*-CCAs The catalytic performance of SC*-CCAs in the esterification of oleic acid is shown in Fig. 5b. The rate constants and TOF are displayed in Table 2. SC*-CCAs exhibited higher activities than Amberlyst-15. The highest TOF (SC*-CCA2) exhibited a TOF of 109 h − 1, seven times that of Amberlyst-15 (15 h − 1). Furthermore, when compared to the poorly dispersing carbon catalyst S-AC, SC*-CCAs were more active and efficient, with rate constants and TOFs more than twice those of SAC. This confirms that catalysts dispersion is a key factor underlying the high catalytic efficiency of SC*-CCAs. As seen in Table 2, the rate constants and TOFs of SC*-CCAs are clearly higher than that of SC-CCAs. This demonstrates once again that the catalysts dispersion is essential for catalytic efficiency. SC*-CCAs were synthesized by sulfonation of CCAs prior to alumina removal, and the stacking of carbon sheets through π–π interactions after alumina was removed was hindered by the presence of SO3H functional groups. Consequently, the dispersion of SC*-CCAs in methanol is expected to be better than SC-CCAs, leading to higher catalytic activity. Similar to SC-CCA4, SC*-CCA4 exhibited the lowest TOF of the SC*CCAs. This could be attributed to the rigid carbon framework formed in CCA4, which made the corresponding sulfonated carbon SC*-CCA4 more difficult to disperse into small sheets than the three other SC*-CCAs.
4. Conclusions Sulfonated carbon catalysts were synthesized by the sulfonation of thin-wall mesoporous carbon substrates with 4-BDS. Such catalysts were composed of thin and small carbon sheets which facilitated their dispersion in polar solvent. Good dispersion of the catalysts leads to a high efficiency for the esterification of oleic acid with methanol. As the carbon content in CCA increased, catalysts dispersion became poor and yielded lower TOF. Modified preparation of catalysts further improved dispersion and catalytic efficiency.
Acknowledgments The authors are grateful for the financial support of the National Science Foundation of China (20773004) and the Major State Basic Research Development Program (grant no. 2011CB808702).
Appendix A. Supplementary data Supplementary data to this article can be found online at doi:10. 1016/j.catcom.2011.06.014.
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Contents lists available at ScienceDirect
Catalysis Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c a t c o m
Short Communication
Efficient carbon-based solid acid catalysts for the esterification of oleic acid Liang Geng, Yu Wang, Gang Yu, Yuexiang Zhu ⁎ Beijing National Laboratory for Molecular Sciences, State Key Laboratory for Structural Chemistry of Unstable and Stable Species, College of Chemistry and Molecular Engineering, Peking University, Beijing 10087, China
a r t i c l e
i n f o
Article history: Received 28 April 2011 Received in revised form 9 June 2011 Accepted 10 June 2011 Available online 17 June 2011 Keywords: Carbon-based solid acid Dispersion Esterification Biodiesel
a b s t r a c t A series of carbon-based solid acid catalysts was prepared by the sulfonation of mesoporous carbon substrates with thin pore walls, and catalytic activity for the esterification of oleic acid with methanol was tested. The highest turnover frequency (TOF) observed was 78 h− 1, five times that of Amberlyst-15. The high catalytic activity may be attributed to the good dispersion of the catalysts in methanol. Catalysts with improved dispersion were obtained by a modified preparation, after which the highest TOF observed was 109 h− 1, seven times that of Amberlyst-15. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Biodiesel, a nonpetroleum-based fuel, is usually produced through the transesterification of triglycerides [1,2]. In most cases, the feedstock oil contains considerable amounts of undesirable free fatty acid (FFA), which must be converted into esters through preesterification. Conventionally, pre-esterification is catalyzed by liquid H2SO4, which involves costly neutralization and separation. Therefore, much attention has been paid to easily recycled solid acid catalysts as replacements for liquid acids [3]. Carbon-based solid acids are considered ideal catalysts for many reactions due to their chemical inertness and superior mechanical and thermal stability. Sulfonated carbon is among the most promising of the solid acids developed for FFA esterification [4]. Such catalysts are generally produced by the sulfonation of incompletely pyrolyzed biomass, such as sucrose [5], glucose [6], starch [7,8] or biochar [9]. Carbon precursors are directly pyrolyzed and sulfonated in concentrated H2SO4[5–11]. This simple process, however, usually produces sulfonated carbon with low surface area, low acid density and poor dispersion in liquid-phase reaction mixture. Recently, a sulfonated ordered mesoporous carbon (OMC) was used as a solid acid [12,13]. Using ordered mesoporous silica, usually SBA-15, as a template, OMC catalysts with high BET area and large
⁎ Corresponding author at: College of Chemistry and Molecular Engineering, Peking University, Beijing 10087, China. Tel.: + 86 10 62751703; fax: + 86 10 62751708. E-mail address: [email protected] (Y. Zhu). 1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2011.06.014
pores were synthesized. These characteristics favor the diffusion of long-chain FFA molecules to the catalytically active sites. It is reported that a uniformly carbon-coated alumina (CCA) was synthesized through repetition of carbon precursor adsorptionpyrolysis cycles [14–16]. From such CCAs, we prepared a series of disordered mesoporous carbon with high BET area, large pores and thin pore walls [17,18]. In this work, we grafted benzenesulfonic acid groups onto those carbon materials with 4-benzene-diazoniumsulfonate (4-BDS) to yield solid acids, for the sulfonation could be more efficient with 4-BDS instead of H2SO4[13]. Such catalysts, composed of small and thin carbon sheets, were readily dispersed in methanol, offering high catalytic efficiency in the esterification of oleic acid with methanol. To further improve the dispersion, grafting was carried out on CCAs, followed by the removal of the alumina. The resulting catalysts exhibited higher efficiency in the esterification of oleic acid. The highest TOF observed was 109 h − 1, seven times that of Amberlyst-15. 2. Experimental 2.1. Preparation of CCA CCA was prepared according to a previously reported procedure [15–18]. γ-Al2O3 (SBA150, Engelhard) was impregnated with aqueous solutions of sucrose. The weight ratio of sucrose and Al2O3 was kept at the monolayer dispersion threshold value. The sucrose-Al2O3 composite was carbonized at 600 °C under N2 gas to obtain CCA1. The impregnation and carbonization process were repeated one to three times to obtain CCA2, CCA3 and CCA4. The numbers indicate the number of repetitions of the impregnation and carbonization process and higher carbon content.
L. Geng et al. / Catalysis Communications 13 (2011) 26–30
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2.2. Preparation of sulfonated carbon catalysts CCAs were immersed in a 24% hydrofluoric acid (HF) solution at room temperature for 3 h to dissolve the alumina. The remaining carbon-based materials (C-CCAs) were washed repeatedly with water and dried at 110 °C. The sulfonation of C-CCAs was achieved by mixing C-CCA (0.4 g) and an aqueous solution of 0.22 g 4-BDS in 0.05 M HCl and holding the suspension at 5 °C for 3 h. After filtration and thorough washing with water, dimethylformamide (DMF) and acetone, the sulfonated carbon catalysts were obtained and are denoted SC-CCA1, SC-CCA2, SC-CCA3 and SC-CCA4. Activated carbon powder (Beijing Dali Fine Chemical Plant, China) was sulfonated by the same procedure to obtain sulfonated activated carbon (S-AC). To improve catalyst dispersion in solution, another series of sulfonated carbon was synthesized by first sulfonating CCAs, then removing the alumina. The catalysts prepared by this route are denoted SC*-CCA1, SC*-CCA2, SC*-CCA3 and SC*-CCA4. All procedures above are illustrated in a diagram (Fig. S1). Amberlyst-15 (Acros Organics) was ground into a powder for use as a control catalyst and was degassed at 100 °C for 7 h before catalytic tests.
2.3. Catalyst characterization Transmission electron microscopy (TEM) images were recorded on a Hitachi H-9000NAR high-resolution microscope. The N2 sorption analysis was performed on a Micromeritics ASAP 2010 volumetric adsorption system at 77 K. All samples were degassed at 150 °C prior to measurements. The specific surface area was determined using the BET method based on the adsorption data in the relative pressure (P/P0) range 0.05 to 0.20. Pore size distribution (PSD) was evaluated by the Barrett–Joyner–Halenda method from the adsorption branch of the isotherm. The total pore volume was estimated from the amount of N2 adsorbed at a relative pressure of 0.99. XPS Al2p signals were recorded on a Kratos Axis Ultra System. Fourier transform infrared (FT-IR) absorbance spectra were recorded on a Nicolet iN10 FT-IR Microscope using pure samples. Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku D/MAX0200 powder diffractometer. Strong acid density was determined by sulfur elemental analysis (Elementar Vario Micro Cube, Germany) and total acid density by potentiometric titration with 0.001 M NaOH.
2.4. Catalytic tests Esterification was performed in a stirred 20-mL autoclave at 65 °C. 50 mg of solid acid catalyst was added to 1 g of oleic acid (99%, TCI) and 8 mL of methanol. At selected reaction times, the autoclave was cooled down rapidly in a water bath. The catalyst was then removed by filtration, and the product was extracted and diluted with nhexane to 100 mL. The yield of methyl ester was analyzed by GC, where methyl heptadecanoate (AccuStandard, 10.0 mg/L in hexane) was used as the internal standard. For kinetic studies, the conversion results were fitted to a pseudo-first order rate equation to determine the rate constant k. Turnover frequency (TOF) was calculated according to the equation below, using the initial amount of oleic acid, n0,OA, and the number of strong acid sites on the catalyst, nH+: TOF = kn0;OA = nH
+
Fig. 1. N2 sorption isotherms and pore size distribution plots (inset) of C-CCAs, SC-CCAs and S-AC.
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L. Geng et al. / Catalysis Communications 13 (2011) 26–30
Table 1 Textural properties of carbon materials and sulfonated carbons. Sample
BET area (m2/g)
Da (nm)
Vb (cm3/g)
C-CCA1 C-CCA2 C-CCA3 C-CCA4 AC SC-CCA1 SC-CCA2 SC-CCA3 SC-CCA4 S-AC Amberlyst-15
570 1035 1255 1231 1297 39 354 623 805 318 43
2.51 3.20 5.35 8.36 2.36 – 2.83 3.64 6.87 – 29.1
0.36 0.83 1.68 2.57 0.76 0.04 0.25 0.57 1.38 0.24 0.31
a b
Average pore diameter. Pore volume.
3. Results and discussion 3.1. Solid acids prepared by sulfonating thin-wall mesoporous carbons 3.1.1. Preparation and characterization of SC-CCAs Previously, we found that sucrose molecules could readily monolayer-disperse on γ-alumina surface [16]. After pyrolysis, sucrose then formed small and thin carbon sheets covering alumina surface [18]. Therefore, by carefully controlling sucrose loading and repetition of the impregnation and carbonization process, the alumina surface of CCAs was uniformly covered by carbon sheets as thin as 1–2 graphene layers (Fig. S2) [18]. After dissolving the alumina in HF (XPS analysis showed that alumina was removed completely, Fig. S3), carbon materials (C-CCAs) with different textures were obtained. Fig. 1 shows that all C-CCAs exhibited typical IV adsorption isotherms with a hysteresis loop and peaks in the mesopore range in PSD curves. Table 1 indicates that C-CCA1 possessed a denser structure than the other three, with a lower BET area, pore size and pore volume. Because carbon sheets in CCA1 were discrete and small, they aggregated into a denser structure once the alumina was removed. As carbon content increased, the carbon sheets grew larger and thicker and joined each other to form a continuous framework, which is stable enough to support itself and afford larger pores. The sulfonation of C-CCAs was achieved through covalent attachment of benzenesulfonic acid groups by reacting C-CCAs with
Fig. 3. XRD patterns a) and FT-IR spectra b) of C-CCA1, SC-CCA1 and SC*-CCA1.
4-BDS. As shown in Fig. 2, the morphology of sulfonated carbon SCCCAs was closely related to the corresponding C-CCAs. As the carbon content in CCA increased, larger pores emerged. XRD patterns (Fig. 3a) indicated that both C-CCAs and SC-CCAs possessed low crystallinity.
Fig. 2. TEM images of a) C-CCA1, b) C-CCA2, c) C-CCA3, d) C-CCA4, e) SC-CCA1, f) SC-CCA2, g) SC-CCA3 and h) SC-CCA4.
L. Geng et al. / Catalysis Communications 13 (2011) 26–30
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Table 2 Acid densities and catalytic activities of solid acids. Sample
Strong acid density (mmol H+/g)
Total acid density (mmol H+/g)
k (h− 1)
TOF (h− 1)
SC-CCA1 SC-CCA2 SC-CCA3 SC-CCA4 SC*-CCA1 SC*-CCA2 SC*-CCA3 SC*-CCA4 S-AC Amberlyst-15
1.72 1.49 1.63 1.66 1.42 1.49 1.70 1.42 1.42 5.03
2.33 2.41 2.60 1.97 2.62 2.55 2.64 2.64 1.74 5.31
1.90 1.61 1.61 1.15 2.17 2.30 2.46 1.68 0.89 1.07
78 67 70 49 108 109 102 84 44 15
The presence of benzenesulfonic acid groups on C-CCAs was confirmed by FT-IR spectra (Fig. 3b). The bands at 1035 cm − 1 and 1008 cm − 1 (S=O stretching) and 1125 cm − 1, 1171 cm − 1and 1220 cm − 1 (stretching in SO3H) are consistent with SO3H groups [5,6]. Comparison of the textural properties of SC-CCAs and C-CCAs (Fig. 1, Table 1) showed that the BET area, pore size and pore volume of C-CCAs decreased dramatically after sulfonation. Acid densities in SC-CCAs were determined by both sulfur elemental analysis (strong acid density, the amount of SO3H) and potentiometric titration (total acid density, the sum of SO3H, COOH and OH) (Table 2). The difference between the two types of acid density is attributed to weak acid groups (COOH and OH) from the incomplete carbonization of sucrose. These groups contribute very little to catalytic esterification due to their insufficient acidity, but their hydrophilicity should favor the dispersion of catalysts [10]. With SO3H groups attached to carbon sheets, SC-CCAs could readily disperse in polar solvents. As shown in Fig. 4a, C-CCA1 precipitated in methanol in a few minutes, whereas SC-CCA1 formed a stable dispersion. In polar solvents, SO3H groups afford hydrophilicity and electrostatic repulsion to counteract the π–π interactions between carbon sheets. However, the size of carbon sheets was found to play a more important role in dispersion. Carbon sheets in SC-CCA1 were thin and small enough to be driven apart by such electrostatic repulsion and dispersed in methanol. As the carbon content in CCA increased, carbon sheets grew larger and gradually became insoluble.
3.1.2. Catalytic performance of SC-CCAs The catalytic performance of SC-CCAs in the esterification of oleic acid with methanol is shown in Fig. 5a. Because the amount of methanol was 66 times that of oleic acid, the reaction follows pseudofirst order kinetics (with plots fitted in Fig. 5a). The rate constants and TOF were determined and are shown in Table 2. Compared with commercial catalyst Amberlyst-15, SC-CCAs exhibited higher activities, even though the strong acid density of SC-CCAs was much lower.
Fig. 4. Photograph of a) C-CCA1 (left) and SC-CCA1(right) dispersed in methanol after 20 min and b) S-AC (left) and SC*-CCA1(right) dispersed in methanol after 5 h.
Fig. 5. Esterification of oleic acid with methanol over a) SC-CCAs and b) SC*-CCAs. Kinetic plots were fitted to first order reaction kinetics.
The highest TOF (SC-CCA1, 78 h − 1) was five times that of Amberlyst15 (15 h − 1). The high catalytic activity of SC-CCAs is closely related to catalyst dispersion in the liquid-phase reaction mixture. SC-CCA1 displayed the best dispersion among the four catalysts (Fig. 4a), which dramatically promoted oleic acid access to active sites and yielded the highest TOF. For SC-CCA2 and SC-CCA3 (TOF is 67 h − 1 and 70 h − 1, respectively), the carbon sheets grew larger and became less soluble, so they were not as efficient as SC-CCA1. Larger carbon sheets formed a rigid framework, and the corresponding sulfonated carbon SC-CCA4 showed the lowest TOF (49 h − 1). With respect to a solid acid catalyst, high BET area and large pores should favor the diffusion of the reactants, thereby leading to high activity. However, we found that the catalysts with a higher BET area offered lower TOFs (Tables 1 and 2). The catalytic performance is not related to the catalyst texture measured in a dry state. In other words, the catalyst dispersion outweighed texture and stood out as the key factor in determining catalytic performance. Amberlyst-15 is an ion exchange resin bearing macropores. Its low BET area (43 m 2/g) and high acid density (5.03 mmol/g) reveal that most of its acid sites reside in a poorly swelling network, which can only be penetrated by small ions (H +, Na +) instead of large molecules like oleic acid. Therefore, Amberlyst-15 showed much lower TOF in the esterification of oleic acid than SC-CCAs.
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L. Geng et al. / Catalysis Communications 13 (2011) 26–30
3.2. Solid acids prepared by sulfonating CCA followed by the removal of alumina 3.2.1. Preparation and characterization of SC*-CCAs Given that catalyst dispersion is critical to the catalytic activity, another series of carbon catalysts SC*-CCAs with improved dispersion was prepared. These catalysts were synthesized by sulfonating CCAs prior to alumina removal. XRD patterns (Fig. 3a) and the data in Table 2 showed that this series of carbon catalysts possessed similar crystallinity and acid density to SC-CCAs. The successful grafting of benzenesulfonic acid groups onto CCAs was confirmed by FT-IR spectra (Fig. 3b) and by texture comparison of CCAs and sulfonated CCAs (Fig. S4). Compared with SC-CCAs, SC*-CCAs were better able to disperse in methanol. As shown in Fig. 4b, SC*-CCA1 could be well dispersed in methanol to give a uniform black solution. This solution is so stable that no deposition was detected even after three months. To investigate the effect of catalyst dispersion, sulfonated activated carbon (S-AC) was prepared as a control sample. AC and S-AC possessed micropore-dominated structures (Fig. 1 and Table 1), and the acid density of S-AC is much lower than that of SC*-CCAs (Table 2), indicating that the carbon particles of S-AC bear fewer SO3H, COOH and OH groups, and would disperse poorly in methanol (Fig. 4b). 3.2.2. Catalytic performance of SC*-CCAs The catalytic performance of SC*-CCAs in the esterification of oleic acid is shown in Fig. 5b. The rate constants and TOF are displayed in Table 2. SC*-CCAs exhibited higher activities than Amberlyst-15. The highest TOF (SC*-CCA2) exhibited a TOF of 109 h − 1, seven times that of Amberlyst-15 (15 h − 1). Furthermore, when compared to the poorly dispersing carbon catalyst S-AC, SC*-CCAs were more active and efficient, with rate constants and TOFs more than twice those of SAC. This confirms that catalysts dispersion is a key factor underlying the high catalytic efficiency of SC*-CCAs. As seen in Table 2, the rate constants and TOFs of SC*-CCAs are clearly higher than that of SC-CCAs. This demonstrates once again that the catalysts dispersion is essential for catalytic efficiency. SC*-CCAs were synthesized by sulfonation of CCAs prior to alumina removal, and the stacking of carbon sheets through π–π interactions after alumina was removed was hindered by the presence of SO3H functional groups. Consequently, the dispersion of SC*-CCAs in methanol is expected to be better than SC-CCAs, leading to higher catalytic activity. Similar to SC-CCA4, SC*-CCA4 exhibited the lowest TOF of the SC*CCAs. This could be attributed to the rigid carbon framework formed in CCA4, which made the corresponding sulfonated carbon SC*-CCA4 more difficult to disperse into small sheets than the three other SC*-CCAs.
4. Conclusions Sulfonated carbon catalysts were synthesized by the sulfonation of thin-wall mesoporous carbon substrates with 4-BDS. Such catalysts were composed of thin and small carbon sheets which facilitated their dispersion in polar solvent. Good dispersion of the catalysts leads to a high efficiency for the esterification of oleic acid with methanol. As the carbon content in CCA increased, catalysts dispersion became poor and yielded lower TOF. Modified preparation of catalysts further improved dispersion and catalytic efficiency.
Acknowledgments The authors are grateful for the financial support of the National Science Foundation of China (20773004) and the Major State Basic Research Development Program (grant no. 2011CB808702).
Appendix A. Supplementary data Supplementary data to this article can be found online at doi:10. 1016/j.catcom.2011.06.014.
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