Multifunctionalized Ordered Mesoporous Carbon as an Efficient and

Mar 1, 2013 - ABSTRACT: The hydrophobic surface of ordered mesoporous carbon. (OMC) via a hard-template route is difficult to sulfonate, which limits ...
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Multifunctionalized Ordered Mesoporous Carbon as an Efficient and Stable Solid Acid Catalyst for Biodiesel Preparation Binbin Chang, Jie Fu, Yanlong Tian, and Xiaoping Dong* Department of Chemistry, School of Sciences, Zhejiang Sci-Tech University, 928 Second Avenue, Xiasha Higher Education Zone, Hangzhou 310018, China ABSTRACT: The hydrophobic surface of ordered mesoporous carbon (OMC) via a hard-template route is difficult to sulfonate, which limits its practical applications in solid acid catalysis. Here, we reported an OMC solid acid catalyst with large surface area and ordered mesostructure, as well as high acidity. The pretreatment of hard-template prepared OMC with H2O2 brought numerous hydrophilic groups onto the carbon surface, which are favorable for the modification of −SO3H groups by a followed sulfonation treating with sulfuric acid. The results of X-ray diffraction, N2 adsorption− desorption, and transmission electron microscopy demonstrated the preservation of large surface area and ordered mesostructure. Infrared spectra indicated the successful modification of −SO3H groups and in the meantime suggested the existence of hydrophilic groups (−COOH and −OH). The acidity of catalyst estimated by an indirect titration method and the modified amount of −SO3H groups examined by energy dispersive spectra were 2.09 mmol H+ g−1 and 1.86 mmol −SO3H g−1, respectively. Such multifunctionalized OMC material with hydrophilic groups (−SO3H, −COOH, −OH) and the hydrophobic framework (polycyclic aromatic carbon) showed evidently improved catalytic activities for biodiesel production. Furthermore, its excellent stability and recycling property were demonstrated by five consecutive cycles.

1. INTRODUCTION With the reduction of oil resources and the increasing environmental concerns of conventional fossil fuels, biodiesel, as a renewable and environmentally friendly biofuel, has attracted great interest as a substitute for traditional fossil fuels.1−6 Traditionally, homogeneous base, homogeneous acid, and immobilized lipase were usually used as catalysts for the preparation of biodiesel.7−9 However, industrial applications of these traditional catalysts suffer from serious limitations, such as saponification which creates the serious problem of product separation,10 a corrosion problem,11 the high cost, and the long reaction time.12 The development and research of novel and efficient heterogeneous catalysts for production of biodiesel are an important industrial challenge. Mesoporous solid catalysts, including solid acid and solid basic catalysts, have received considerable attention due to their large surface area, highly ordered mesoporous structure, noncorrosion, nontoxicity, and easy separation for recycling.13−16 In comparison with mesoporous solid basic catalysts, mesoporous solid acid catalysts hold the important advantage of catalyzing transesterification and esterification simultaneously without soap formation. As the typical solid acids, sulfated oxides have been loaded into mesoporous solids during various methods and indeed exhibited good activities for biodiesel production.17,18 However, the limitation of loading amount obstructs the further improvement of catalytic performance. Meanwhile, the leaching of sulfate groups during the process of reaction results in the poor reusability of supported mesoporous solid acid catalysts.3,19 © XXXX American Chemical Society

In recent years, sulfonic acid functionalized carbon-based solid acid as a stable and highly active protonic acid catalyst has been paid much attention by researchers. Carbon-based solid acids can be readily prepared by incomplete carbonization of sulfopolycyclic aromatic compounds in sulfuric acid20 or sulfonation of incompletely carbonized natural organic matter, such as sugar21,22 and cellulosic materials.23 It has been confirmed that carbon-based solid acids bearing −SO3H groups possess high catalytic activity for the production of biodiesel. However, these carbon-based acid catalysts prepared from sugars possess low surface area (353 K) until the sulfate ions were no longer detected in the wash water (BaCl2 precipitation test) and were dried at 373 K for 6 h. The resulting materials were designated as OMC−SO3H and OMC−H2O2−SO3H, respectively. 2.2. Characterizations. The X-ray diffraction (XRD) patterns of powder samples were taken by a Bruker D8 Advance diffractometer using Cu Kα radiation (λ = 0.15418 nm) as an X-ray source. Nitrogen adsorption−desorption isotherms were carried out at 77 K using a Micromeritics ASAP 2020 analyzer. Before adsorption, the samples were outgassed at 473 K for 6 h. The specific surface area (SBET) was evaluated using the Brunauer−Emmett−Teller (BET) method, while the pore volume and pore size were calculated according to the Barrett−Joyner−Halenda (BJH) formula applied to the adsorption branch. Fourier transform infrared spectroscopy (FTIR) spectra of a sample in KBr pellet were recorded on a Nicolet Avatar 370 spectrometer. The pore structure was observed from a JEOL JEM-2100 transmission electron microscope (TEM) with an accelerating voltage of 200 KV. The number of acid sites was estimated by using an indirect titration method,32,33 which involves an aqueous ion-exchange step of the catalyst H+ ions with base of NaHCO3, followed by titration of the resulting solution with HCl aqueous solution (0.1 M). In a typical experiment, 30 mg of the catalyst was dispersed in 50 mL of 5 × 10−3 mol L−1 NaHCO3 solution, which was stirred for 24 h and separated by filtration. Then 5 mL of filtrate was taken out for titration with 0.1 M of HCl aqueous solution. Titration was performed three times, and the average number was reported. The amount of acid groups in the solid acid catalysts was estimated by the NaHCO3 consumed. 2.3. Catalytic Testing. Catalytic esterification of oleic acid, palmitic acid, and stearic acid with methanol or ethanol was performed in a 100 mL three-necked round-bottomed flask equipped with a reflux condenser, magnetic stirrer, and a water bath maintained at a specified temperature. In a typical experiment, 0.05 mol of oleic acid (OA) was mixed in anhydrous methanol (MeOH) in the round-bottom flask, and the required quantity of solid acid catalyst (0.1 g) was added. At regular time intervals, 4 mL of samples was extracted to check the progress of the esterification reaction. The samples drawn from the reaction mixture were centrifuged to separate the catalyst powder, and then the methanol and water were evaporated out of the samples. Then the product was analyzed for acid value (AV) by titration.20,34 The esterification reactions of palmitic acid and stearic acid were the same as the esterification of oleic acid. The conversion of aliphatic acid was calculated using the following formula

temperature carbonization, which seriously restricts the modification of −SO3H groups, subsequently resulting in the low acidity and low activity. Though reducing the carbonized temperature is helpful for increasing the amount of modified −SO3H groups, the mesoporosity of the structure becomes lower and lower with the decrease of carbonized temperature.28 Hence, it is still a challenge and a significant research interest to obtain SO3H-functionlized OMC solid acids with high −SO3H density and high acidity. The reason why those incompletely carbonized catalysts bear much more −SO3H groups and exhibited much better catalytic activities than carbon-based catalysts carbonized at high temperature is that incompletely carbonized materials possess numerous hydrophilic groups, such as −COOH and −OH groups, which are favorable for the immobilization of −SO3H groups on the carbon surface via forming covalent bonds or hydrogen bonds.6,29 In addition, the hydrophilic surface can easily adsorb hydrophilic reactants from solution, which provides good access for these reactants to −SO3H groups in the carbon materials and brings high catalytic activity.30 Herein, we reported a multifunctionalized OMC solid acid catalyst, which not only possessed high surface area, large pore volume, and narrow pore size distribution but also exhibited high density of −SO3H groups and high acidity. A H2O2 pretreatment of hard-template prepared OMC material introduced a high content of hydrophilic groups on the carbon surface, such as −COOH and −OH groups, which subsequently resulted in a high level of modification of −SO3H groups. In addition, this catalyst can simultaneously adsorb alcohol and aliphatic acid molecules in the esterification reaction for biodiesel production because of the coexistence of hydrophilic groups (−SO3H, −COOH, −OH) and the hydrophobic framework (polycyclic aromatic carbon). Such excellent structure and surface properties and high acidity made this multifunctionalized OMC material an efficient solid acid catalyst for the practical application of biodiesel production.

2. EXPERIMENTAL SECTION 2.1. Preparation of Catalyst Materials. Mesoporous silica SBA-15 was synthesized using an amphiphilic triblock copolymer, Pluronic P123 (EO20PO70EO20), as a structure directing agent.31 Ordered mesoporous carbon (OMC) was synthesized using SBA-15 as the hard template and furfuryl alcohol as the carbon source. Typically, 1.5 mL of furfuryl alcohol and a small quantity of oxalic acid were dissolved in 10 mL of alcohol. This solution was incorporated into 1 g of SBA15 by the wetness impregnation technique. After evaporating the ethanol and polymerizing furfuryl alcohol at 343 K, the residual unpolymerized furfuryl alcohol was evaporated at 423 K. Then, the polyfurfuryl alcohol/SiO2 composite was obtained, and the composite was thermal-treated in N2 at 1073 K to carbonize the polyfurfuryl alcohol. The silica template in the composite was removed by repeatedly washing with heated NaOH (2 M) solution. The OMC materials were pretreated by H2O2 oxidation to enhance their hydrophilic properties. In detail, 0.15 g of OMC was added into 8.0 g of 30 wt % H2O2 solution at room temperature, and then the reaction mixture was kept at 333 K for 1 h. The materials were separated from the solution, followed by washing with ethanol and drying at 373 K in an oven. The resulting materials were denoted as OMC−H2O2. Subsequently, these OMC and OMC−H2O2 materials were sulfonated using concentrated sulfuric acid at 423 K for 10 h in

conversion % = (1 − AVx /AV0) × 100%

where AVx is the instant acid value of samples drawn from the reaction mixture and AV0 is the initial acid value.

3. RESULTS AND DISCUSSION 3.1. Characteristics of Catalysts. Figure 1 shows the lowangle XRD patterns of SBA-15, OMC, OMC−H2O2, OMC− SO3H, and OMC−H2O2−SO3H samples. The sample of SBA15 shows three well-resolved diffraction peaks that can be indexed as (100), (110), and (200) reflections associated with two-dimensional hexagonal symmetry.35 These characteristic peaks can be observed in mesoporous carbon materials, which B

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were clearly observed in OMC and OMC−H2O2 samples. After sulfonation, the channel structures of OMC−SO3H and OMC−H2O2−SO3H have no significant difference with that of OMC, which indicates that the process of sulfonation does not influence the order of structure. FTIR spectra (Figure 4) are used to characterize the functional groups. In all samples, the absorption band at ∼3420 cm−1 could be assigned to the stretching vibration of the −OH group, which is favorable for the modification of −SO3H groups. In comparison with the OMC sample, after the oxidative treatment by H2O2, the absorption intensity of the O−H vibration becomes strong, and a new absorption peak at ∼1710 cm−1 is found in the OMC−H2O2 sample. This band should be attributed to the CO vibrational stretching of the −COOH groups.37−39 These results clearly indicate that functional groups are created in the carbon frameworks by the oxidative treatment. After sulfonation treatment, an additional absorption peak at ∼1040 cm−1 is found in the OMC−SO3H and OMC−H2O2−SO3H samples, and this absorption band can be ascribed to the SO symmetric stretching vibrations.40,41 This result demonstrates that −SO3H groups have been successfully modified on the surface of carbon frameworks. Additionally, the absorption bank at ∼1710 cm−1 assigned to the −COOH group vibration in the OMC− H2O2−SO3H sample becomes weak, which could be due to −COOH group partial oxidation by sulfuric acid.42 In addition, the peak at ∼1620 cm−1 in all samples is raised from the CC stretching vibration, suggesting the presence of polycyclic aromatic rings, which are considered as the product of the carbonization of organic substances.43,44 3.2. Catalytic Performances for Biodiesel Preparation. The large surface area of multifunctionalized OMC solid acid catalyst provides much more active sites, and its opened mesopores are favorable for the mass transfer in catalysis. Additionally, the coexistence of hydrophilic groups (−SO3H, −COOH, −OH) and the hydrophobic framework (polycyclic aromatic carbon) allows this material to easily adsorb methanol and aliphatic acid molecules at the same time. This is also one of the reasons why SO3H-bearing carbon-based solid acid catalysts perform higher activities than some other solid acids.45 In the esterification reaction, the active sites of −SO3H groups play the role of Bronsted acid as proton donor. Scheme 1 shows the mechanistic steps during the esterification reaction catalyzed by SO3H-bearing solid acid.6 In step 1, the proton of −SO3H is transferred to the carbonyl of aliphatic acid to form a positive carbon ion, which is subsequently transformed to a tetrahedral intermediate (a) by the nucleophilic attack of methanol in step 2. Meantime, the proton of methanol

Figure 1. Low-angle XRD patterns of SBA-15, OMC, OMC−H2O2, OMC−SO3H, and OMC−H2O2−SO3H.

indicate that these carbon materials completely replicate the ordered mesostructure of the mesoporous silica template SBA15.36 Meanwhile, this result also demonstrates that the process of oxidation with H2O2 and sulfonation with sulfuric acid did not destroy the ordered mesostructure of mesoporous carbon. In addition, the intensities of diffraction peaks (110) and (200) of OMC−SO3H and OMC−H2O2−SO3H become low and even difficult to discern, which can be ascribed to the decreased scattering contrast between walls and channels after sulfonation.27 Figure 2a shows N2 adsorption−desorption isotherms of OMC, OMC−H2O2, OMC−SO3H, and OMC−H2O2−SO3H catalysts. All these samples show typical type IV curves with a clear hysteresis loop at relative pressure from 0.40 to 0.65, which indicates that the uniform mesoporous channels are retained. The pore structure parameters are listed in Table 1. The specific surface area and the pore volume reduce after the sulfonation treatment with sulfuric acid, which may be attributed to the presence of a large number of −SO3H groups. Besides, the decrement in surface area of OMC−H2O2−SO3H is much more than that of OMC−SO3H, and similar results can be detected in pore volume and pore size (Table 1), which suggest the much higher density of −SO3H groups modified on the surface of OMC−H2O2−SO3H than that of OMC−SO3H. Consequently, OMC−H2O2−SO3H shows higher acidity than that of OMC−SO 3 H (Table 1). The pore diameter distributions of OMC, OMC−H2O2, OMC−SO3H, and OMC−H2O2−SO3H are shown in Figure 2b. The decrease in pore diameter after sulfonation demonstrates that −SO3H groups have successfully grafted on the pore channels of OMC and OMC−H2O2. To reveal the morphology and structure of materials, TEM images of OMC, OMC−H2O2, OMC−SO3H, and OMC− H 2 O 2 −SO 3 H are shown in Figure 3. The tube-type morphology and the highly ordered mesoporous structure

Figure 2. (a) N2 adsorption−desorption isotherms and (b) pore size distributions of OMC, OMC−H2O2, OMC−SO3H, and OMC−H2O2−SO3H samples. C

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Table 1. Textural Parameters and Chemical Properties of Catalysts SBETa −1

Vp b

Dpc

3 −1

d(100)d

−SO3Hf

aciditye

sample

(m g )

(cm g )

(nm)

(nm)

(mmol H g )

(mmol g−1)

SBA-15 OMC OMC−H2O2 OMC−SO3H OMC−H2O2−SO3H

746 1016 1070 749 475

1.03 0.91 1.17 0.83 0.57

8.3 3.7 3.8 3.6 3.5

9.02 9.14 8.98 9.22 9.08

− − − 1.06 2.09

− − − 0.98 1.86

2

+

−1

a Specific surface area estimated using the BET method. bPore volume estimated from BJH formula. cPore diameter of peak value in Figure 3b. ddspacing of 100 diffraction. eMeasured by acid−base titration. fMeasured by EDS data.

Scheme 1. Mechanism of SO3H-Bearing Solid Acid Catalyzed Esterification of Aliphatic Acid

Figure 3. TEM images of OMC, OMC−H2O2, OMC−SO3H, and OMC−H2O2−SO3H samples.

Figure 5. Comparison of catalytic activities on esterification with OMC−SO3H and OMC−H2O2−SO3H catalysts (molar ratio = 20:1; 2 h; 343 K; 0.1 g of catalyst).

Figure 4. FTIR spectra of OMC, OMC−H2O2, OMC−SO3H, and OMC−H2O2−SO3H samples.

the literature46,47 (Table 2). This result should be attributed to the higher acidity of OMC−H2O2−SO3H (Table 1) and abundant hydrophilic groups (−COOH and −OH) after oxidative treatment by H2O2. Though −COOH and −OH groups contribute very little to catalytic esterification due to their insufficient acidity, their hydrophilicity should favor the dispersion of catalysts.48 As shown in Figure 6, OMC−SO3H precipitates in methanol in a few minutes, whereas OMC− H2O2−SO3H forms a stable dispersion in methanol. In polar solvents, −SO3H, −COOH, and −OH groups afford the hydrophilicity and electrostatic repulsion to counteract the π−π interaction between carbon sheets.49 To further estimate the catalytic activities of OMC−SO3H and OMC−H2O2−SO3H, we investigated the influences of reaction parameters on esterification of oleic acid with

hydroxyl is rearranged and transferred to the hydroxyl oxygen of aliphatic acid (step 3). The tetrahedral intermediate (b) is dehydrated (step 4) and then transfers the proton of the hydroxyl group to −SO3− (step 5), finally resulting in the formation of biodiesel products of methyl esters and the recovering of active sites of −SO3H. Figure 5 compares the catalytic performance of two OMC solid acid catalysts, OMC−SO3H and OMC−H2O2−SO3H, in esterification of aliphatic acid (oleic acid, palmitic acid, and stearic acid) with methanol or ethanol at 343 K for 2 h with a 20:1 molar ratio. It clearly displays that the multifunctionalized OMC solid acid of OMC−H2O2−SO3H shows a much higher catalytic activity, which is almost twice as high as that of OMC−SO3H. Moreover, its catalytic activity is also much higher than those of many other solid acid catalysts reported in D

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forward reaction. Consequently, the conversion gradually increases with the increase of reaction temperature. 3.2.2. Effect of MeOH/OA Molar Ratio. Since esterification is a reversible reaction, the amount of methanol is usually excessive to force the equilibrium toward the direction of ester formation.50,51 Figure 8 depicts the OA conversion versus

Table 2. Comparisons of Conversion of Oleic Acid with Methanol Over OMC−H2O2−SO3H and Other Catalysts Reported in the Literature catalyst

conversion (%)

OMC−H2O2−SO3H OMC−150−SO3Ha CMK-3−873−SO3Hb SBA-15−Ph−SO3Hc SZ/MSd MF9S4e

86 7327 5546 5027 6534 5847

a

Reaction conditions: 50 mg of catalyst, molar ratio = 10:1, 10 h, 353 K. bReaction conditions: 0.1 catalyst, molar ratio = 10:1, 6 h, 353 K. c 50 mg of catalyst, molar ratio = 10:1, 10 h, 353 K. d0.1 g of catalyst, molar ratio = 20:1, 4 h, 343 K. e5% of acid weight catalyst, molar ratio = 60:1, 2 h, 403 K. Figure 8. Effect of MeOH/OA molar ratio on the conversion over OMC−SO3H and OMC−H2O2−SO3H (343 K; 2 h; 0.1 g of catalyst).

MeOH/OA molar ratio from 1:1 to 30:1 at 343 K for 5 h. OMC−SO3H and OMC−H2O2−SO3H show the same trend, and the OA conversion significantly increases with the increase of methanol. The extremely low OA conversions of ∼17% and ∼30% are obtained with a MeOH/OA molar ratio of 1:1 catalyzed by OMC−SO3H and OMC−H2O2−SO3H, respectively. However, using a 30:1 molar ratio, the OA conversions reach above 55% and 90%, respectively. This behavior demonstrates that the increased MeOH/OA molar ratio plays an important role in driving the equilibrium to the product side. When the MeOH/OA molar ratio is at a low value, a large number of methanol molecules are vaporized into the reflux condenser. Consequently, only little methanol exists in the reaction solution, which leads to the concentration of methanol decreasing in a great degree and then obtaining a low OA conversion. On the other hand, with enhancement of the methanol concentration, the rate of forward reaction is sharply raised; in the meantime, the rate of the reverse reaction is restricted. As a result, the enhancement of the MeOH/OA molar ratio contributes to drive the equilibrium to the product side and bring a higher conversion. 3.3. Reusability of Catalyst. The catalyst reusability is extremely vital to estimate the efficiency of solid acid catalysts, which contributes to reduce the cost of the practical applications process. In the case of carbon-based solid acid catalysts, the deactivation of catalytic activity is worthy of notice.52,53 To evaluate the reusability of catalyst, after each catalytic reaction the solid acid catalyst was separated by centrifugation and washed repeatedly with ethanol and distilled water. The washed catalyst was dried at 373 K and used for the next experiment. Figure 9 shows the reusability of the OMC− H2O2−SO3H catalyst through five consecutive cycles with a MeOH/OA molar ratio of 20:1 at 343 K for 2 h. It was noteworthy to mention that the catalyst was reusable without any appreciable decrease in catalytic activity. The OMC− H2O2−SO3H catalyst possesses a high density of −SO3H and −COOH groups, and electron-withdrawing −COOH groups increase the electron density between the carbon and sulfur atoms, which may result in the good stability.54 Furthermore, for checking the stability of −SO3H groups, the resulting mixture after catalytic reaction was washed with water to extract sulfate ions possibly leached from the catalyst. The BaCl2 precipitation test demonstrates that no sulfate ions exist in the

Figure 6. Photograph of OMC−SO3H (A) and OMC−H2O2−SO3H (B) dispersed in methanol after 40 min.

methanol, including reaction temperature and molar ratio of MeOH/OA. 3.2.1. Effect of Reaction Temperature. The effect of reaction temperature on esterification was investigated at different temperatures (333, 343, and 353 K) for 5 h with a 20:1 molar ratio of MeOH/OA (Figure 7). Though the OA

Figure 7. Effect of reaction temperature on the conversion over OMC−SO3H and OMC−H2O2−SO3H (MeOH/OA = 20:1; 0.1 g of catalyst).

conversions at various temperature catalyzed by OMC−SO3H are much lower than that at 333 K catalyzed by OMC−H2O2− SO3H, the two catalysts exhibit the same trend in catalytic performance, whereby increasing the reaction temperature enhances the conversion. On the one hand, the higher temperature endues reactants and products with more energy, subsequently accelerating the mass transfer and therefore increasing the reaction rate. On the other hand, because the esterification reaction is a reversible endothermic process, appropriately increasing temperature is favorable for the E

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washed water, suggesting the stability of −SO3H groups on the surface of the catalyst. Meanwhile, there is no decrease in the acidity of OMC−H2O2−SO3H after five cycles (2.09 mmol H+ g−1), which also indicates the stability of −SO3H groups on the surface of the catalyst. Consequently, the material presents great potential to be a stable and highly active solid acid catalyst for preparation of biodiesel.

4. CONCLUSION In summary, a multifunctionalized OMC material with hydrophilic groups and the hydrophobic framework has been successfully prepared via a simple oxidation treatment. After the pretreatment of hard-template prepared OMC with H2O2, a large number of hydrophilic groups can be created in a mesoporous carbon framework without destroying the ordered mesoporous structure, resulting in the material possessing multifunctionalized groups (−SO3H, −OH, −COOH) and a high acidity. Meanwhile, the material exhibits a high surface area, uniform and connected pore structure, and stable and remarkable performance on esterification. Importantly, deactivation of the solid acid catalyst in the reaction system is inhibited, and the catalyst can retain its initial activity after at least five consecutive catalytic cycles. Consequently, this material has great potential in industry applications used as the solid acid catalyst due to its excellent catalytic activity and stability. Besides, such a modifying method proposes a new strategy for preparing the hydrophilic solid acid material with high acidity and catalytic activity. AUTHOR INFORMATION

Corresponding Author

*Fax: +86 571 86843653. Tel.: +86 571 86843228. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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Figure 9. Recyclability of the OMC−H2O2−SO3H catalyst for the esterification of OA with MeOH (MeOH/OA = 20:1; 2 h; 343 K; 0.1 g of catalyst).

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ACKNOWLEDGMENTS

The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (21001093), the Qianjiang talent project of Zhejiang Province of China (2011R10048), the project sponsored by the Scientific Research Foundation (SRF) for the Returned Overseas Chinese Scholars (ROCS), State Education Ministry (SEM), and the Science Foundation of Zhejiang Sci-Tech University (0913840-Y). F

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp312820g | J. Phys. Chem. C XXXX, XXX, XXX−XXX