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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 foresterification of fatty acids

Danlin Zeng*, Qi Zhang, Shiyuan Chen, Shenglan Liu, Guanghui WangThe State Key Laboratory of Refractories and Metallurgy, Hubei Key Laboratory of Coal Conversion and New Carbon Material, College of ChemicalEngineering and Technology, Wuhan University of Science and Technology, Wuhan 430081, China

a r t i c l e i n f o

Article history:Received 5 May 2015Received in revised form16 July 2015Accepted 26 July 2015Available online 31 July 2015

Keywords:Solid acidRice huskAcidityEsterification

* Corresponding author. Tel./fax: þ86 27 6886 2181E-mail address: zdanly@163.com (D. Zeng).

http://dx.doi.org/10.1016/j.micromeso.2015.07.0281387-1811/© 2015 Elsevier Inc. All rights reserved.

a b s t r a c t

A porous carbon solid acid was synthesized from biomass rice husk by incompletely carbonization, so-dium 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 revealthat the carbon solid acid shows ultra high surface area of 1233 m2/g and stronger acid strength than thatof HZSM-5(Si/Al ¼ 38) zeolite. The catalytic performance was tested by the esterification of oleic acidwith methanol. The results indicate that this solid acid catalyst is an excellent catalyst compared withother conventional solid acid.

© 2015 Elsevier Inc. All rights reserved.

1. Introduction

Currently industrial esterification processes are carried out bythe catalysis of homogeneous Brønsted acids such as sulfuric acid.However, these homogeneous acid catalysts are difficult to beseparated, and also cause serious environmental and corrosionproblems. Recently, the application of solid catalysts instead ofhomogeneous liquid catalysts has been paid much attention inview of their convenience of separation and lack of corrosion ortoxicity problems [1e5].

Due to the low densities of effective acid sites, inorganic-oxidesolid acids such as zeolite or composite oxide cannot satisfyadequate requirement in esterification reactions [6,7]. Althoughstrong acidic ion-exchange resins such as Nafion contain abundantsulfonic acid groups (eSO3H), that function as strong acid sites,their catalytic activities are generally much lower for their very lowsurface area [8]. These limitations have restricted the practicalutility of acidic cation-exchangeable resins. Recently, a new type ofsulfonated carbons derived from incomplete carbonization ofsimple natural product such as sugar, starch or cellulose, has beenreported to show better catalytic performance for esterification offatty acids, and higher stability than sulfonated mesoporous silica

.

[5,9,10]. However, such materials were nonporous and exhibitedlow surface area, which may limit the accessibility to the activesites. The carbon solid acid with unique porous properties is syn-thesized here as an ideal candidate for the development of the highactive catalyst.

Every year three million tons of rice husk (RH) are produced inChina. So far, such a resource is mainly considered as a waste, andconsequently burnt without any profit, except in a few cases ofdomestic uses for cooking and heating [11]. The main objective ofthe present work was to prepare carbon based solid acid from ricehusk by incomplete carbonization, leaching and sulfonation. Thesolid acid catalyst was also characterized by X-ray diffraction (XRD),Fourier-transform infrared spectra (FT-IR), scan electron micro-scope (SEM) and solid state nuclear magnetic resonance (NMR)spectroscopy. The research results will be helpful to obtain thefundamental information of the roles of surface functional groupsin the solid acid, which is crucial in the design of a novel carbon-based solid acid for industrial application.

2. Experimental

2.1. Sample preparation

Rice husk from a grain depot in Wuhan was used as raw mate-rial. Rice husk was calcined at 450 �C for 15 h under a N2 flow, andthen followed by grinding and leaching with 1 M NaOH at 100 �C K

Fig. 1. XRD patterns of (a) rice husk; (b) rice husk carbon and (c) the carbon solid acid.

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 inthe rice husk. After carefully washing with deionized water, thedried rice husk carbon was sulfonated with concentrated H2SO4(98wt%), at 150 �C under a N2 flow for 8 h at the ratio of solid toliquid of 1 g: 10 ml. At last, the mixture was diluted with deionizedwater, filtered, washed thoroughly, and dried at 120 �C for 12 h toobtain the carbon solid acid catalyst.

The elemental analysis of the rice husk is listed in Table 1.Recycling experiments were performed to determine the cata-

lytic stability of the solid acid catalysts. At the end of each esteri-fication cycle, the catalyst was centrifuged, washed with ethanoland dried at 105 �C for 2 h before reusing.

2.2. Sample characterization

The concentration of acid sites on the catalysts was determinedby titration method in aqueous solution. One gram of the samplewas placed in 50 ml of 0.05 M NaOH solution. The vials were sealedand shaken for 24 h and then 5 ml of the filtrate was pipetted andthe excess of base was titrated with HCl. The numbers of acidic siteswere calculated from the amount of NaOH that reacted with thecatalyst.

Surface area and porosity properties of samples were evaluatedby N2 adsorption/desorption isotherms carried out on a Micro-meritics ASAP 2020 sorption analyzer. Prior to the adsorp-tionedesorption measurements, all the samples were degassed at150 �C in N2 flow for 12 h.

X-ray diffraction (XRD) was performed with a Philips X'PERT-Pro-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 VarianInfinityplus-400 spectrometer with resonance frequencies of400.12, 100.4 MHz for 1H, 13C, respectively. The 90� pulse widths for1H, 13C were measured to be 3.7, 4.4 ms, respectively. The chemicalshifts were referenced to tetramethylsilane (TMS) for 1H, to hex-amethylbenzene (HMB) for 13C, respectively. The magic anglespinning rate was 5 kHz.

For the adsorption of probe molecules 2-13C-acetone, the sam-ples were kept at 400 �C under the vacuum less than 1� 10�3 Pa forat least 8 h. The adsorption of 2-13C-acetone was performed atroom temperature with a loading of ca. 0.1 mmol per gram catalyst.

The various solid acids were employed as catalysts for theesterification of oleic acid with methanol. The SO4

2�/ZrO2 catalyst inthis case was prepared using the classic two-step method [12]. TheHZSM-5 zeolite (Si/Al ¼ 38) and Nafion NR 50 were pursed fromShanghai Guoyao Corporation. Prior to the reaction, all the catalystswere dried at 120 �C for 5 h. The experiments were carried out bymixing oleic acid with methanol in a flask equipped with a refluxcondenser, an oil bath and a magnetic stirrer. Once the mixture hadreached the reaction temperature, the catalyst was added. Themixtures were withdrawn and centrifuged to separate the solutionfrom the catalyst. Analysis of the reaction mixtures was carried outin an HP 6890 series gas chromatograph equipped with a flameionization detector (FID).

3. Results and discussion

Fig. 1 illustrates the XRD patterns of rice husk, rice husk carbonand the carbon solid acid. The diffraction peak arising at around

Table 1Element analysis of rice husk (wt%).

Sample C H O N S

Rice husk 88.15 5.20 3.55 1.84 1.26

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 huskcarbon and the solid acid. According to Dahn's conclusion [14], itindicates that the solid acid and rice husk carbon consist of a singlelayer of polyhexagonal carbon atoms after leaching, implying thatthe BET surface area of the solid acid may be dramatically high.

FT-IR spectroscopy was employed to explore the changes infunctional groups induced by preparation process. It shows that thedifference of the spectroscopy is mainly the bands of oxygenfunctionalities in the samples (Fig. 2). Two bands at 1039 and1182 cm�1 in the solid acid can be assigned to the SO2 asymmetricand symmetric stretching modes, respectively [5]. It indicates thatsulfonic acid group was found on the surface of the sulfonated solidacid. The band at 1700 cm�1 can be attributed to the C]Ostretching mode of the eCOOH groups, while the broad bandcentered at 3429 cm�1 was assigned to the eOH stretching mode[5]. The band at 1647 cm�1 can be attributed to the C]C stretchingmode of the samples. Therefore, eCOOH, eSO3H and eOH werefound as the functional groups on the solid acid.

Fig. 2. FT-IR spectra of (a) rice husk, (b) rice husk carbon and (c) the carbon solid acid.

Fig. 4. The N2 adsorptionedesorption isotherm (a), and pore size distribution (b) ofthe rice husk carbon and carbon solid acid.

D. Zeng et al. / Microporous and Mesoporous Materials 219 (2016) 54e5856

Fig. 3 shows the 13C CP/MAS NMR spectra of rice husk carbonand the solid acid. The peaks at 128 and 153 ppm are due topolycyclic aromatic carbon atoms and aromatic carbon bonded tophenolic OH, respectively [15]. Compared with the spectra of ricehusk carbon, the signals at 139 ppm (due to aromatic carbonbonded to the SO3H group, CeSO3H) can be clearly observed in thesamples after sulfonation [16]. In addition, the existence of SO3Hgroups was confirmed in by FT-IR characterization. Thus, it can beconcluded that the SO3H groups are successfully formed on thesolid acid.

N2 adsorption-desorption measurements were also performedto obtain more insights into the rice husk materials. The N2adsorption-desorption isotherm of rice husk carbon and the carbonsolid acid (Fig. 4) shows that both samples exhibit type II patterns,which are typical macropores materials. Furthermore, the hyster-esis loop of both samples showed H3 categories, indicating manyslit-like pores exist in the samples [17]. These results also prove thatthe textural properties of the solid acid were substantially main-tained during the sulfonation process. The characterization resultsof the pore size distribution evaluated from adsorption data are alsoshown in Fig. 4b, implying that the average pore sizes of the twosamples are in nanometers.

The type and strength of acid sites are the fundamental prop-erties of solid acid. As a sensitive and reliable technique [18], 13CMAS NMR spectra of 2-13C-acetone adsorbed on the catalysts areused to characterize the acidity of the solid acid derived from ricehusk. The stronger Brønsted acidity will result in stronger hydrogenbonding between the carbonyl carbon and the acidic proton, andconsequently, the more downfield of the 13C isotropic chemicalshift. As shown in Fig. 5, the resonance at 113 ppm is due to thebackground of the NMR probe. The strong signal (at 219 ppm) isascribed to acetone adsorbed on the weakly acidic OH groupspresent on the catalyst [19] (Fig. 5a). For the carbon solid acid(Fig. 5b), three clear resonances at 219 ppm (due to acetoneadsorbed on the weak acidic eOH groups), 229 and 242 ppm areobserved in the 13C MAS NMR spectra. Since the FT-IR character-ization has confirmed the existence of eCOOH and eSO3H func-tional groups, the resonances at 229 and 242 can be assigned to

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 andeSO3H groups, respectively [5]. The resonance at 229 ppm (due to-COOH groups) indicates that acid strength of eCOOH acid site onthe catalyst is stronger than that of the bridging OH group in HZSM-5 zeolite, while the 242 ppm signal corresponding to Brønsted sitesof SO3H groups exhibits its acid strength is weaker than that of100% H2SO4 (shown a 31C chemical shift of 245 ppm) [20].

The morphology of rice husk, rice husk carbon and carbon solidacid catalyst is shown in Fig. 6. The rice husk exhibited a typicalnetwork structure of the biological tissue (Fig. 6a). While aftercalcinations and leaching process lots of pores with sizes of mi-crometers can be clearly found in rice husk carbon. Fig. 6c showsthat after the sulfonation treatment the porous structure still existsin the carbon solid acid, implying that the prepared carbon catalysthas a high surface area and abundant porous structure, which is infavor of catalysis reaction.

In order to evaluate the activity of the solid acid in the reaction, acomparative study was made between the carbon solid acid,sulfated zirconia, HZSM-5(Si/Al¼ 38) and Nafion NR 50 (Fig. 7). Thereaction used was the esterification of oleic acid (20 mmol) withmethanol (100mmol) at 80 �C for 9 h. The same amount (0.015 g) ofall the catalysts was used in the reactions. A remarkable enhance-ment in the reactivity and the yield was observed with carboncatalyst as compared with the other solid acid catalysts examined.

Fig. 7. Comparison of catalytic activity for conversion of oleic acid over the carbonsolid acid and other typical solid acid.

Table 2Textural properties and the catalytic performance of the various catalysts.

Catalysts SBET(m2/g)

Vtot

(cm3/g)D(nm)

Total acid density(mmol/g)

Yield(%)

Solid acid 1233 0.744 38.96 5.25 91SO4

2�/ZrO2 151 0.232 5.23 4.41 65HZSM-5 370 0.122 0.59 1.13 30Nafion NR50 0.018 e e 0.92 18

SBET, specific surface area from BET method; Vtot, total pore volume; D, average porediameter.Reaction condition: 20 mmol oleic acid; 100 mmol methanol; 0.015 g catalyst;80 �C; 9 h.

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.

D. Zeng et al. / Microporous and Mesoporous Materials 219 (2016) 54e58 57

It is known that strong acid strength of solid acid leads to highesterification reaction activity. In this case, compared the 2-13C-acetone NMR characterization results of the carbon solid acid (the13C chemical shift is 242 ppm) with sulfated zirconia ([21], the 13Cchemical shift is 234 ppm from the reference), H-ZSM-5 ([22], the13C chemical shift is 223 ppm) and Nafion NR50 [23], it can beconcluded that the carbon solid acid shows the stronger acidstrength than other three solid acids, consequently, it will exhibithigher activity in the esterification reaction, which is also consis-tentwith the catalytic performance results in Table 2. The rice husk-derived carbon solid acid catalyst afforded a high yield of above90%. Therefore, it can be inferred that high reactivity in esterifica-tion reaction is due to the strong acid strength and high surface areaof the carbon solid acid.

To test the reusability of the catalyst, the carbon solid acid wasreused for ten cycles for the esterification of oleic acid. After theesterification was completed, the used catalyst was separated fromthe reaction mixture by filtration, and then it was washed withethanol to remove the residual reactants off the catalyst surface.Finally, the catalyst was dried at 105 �C for 2 h before reusing. Theresults are presented in Fig. 8. Compared with those of the typicalsolid acid sulfated zirconia, the yields of the carbon solid acidgradually decreased after the first two cycles and then remainedalmost constant for the later cycles. The carbon solid acid catalyst

Fig. 6. SEM of (a) rice husk; (b) rice husk

shows good reusability for the esterification of oleic acid withmethanol, which can be attributed to its ultra high surface area andthe tight attachment of strong acid groups to the framework.Consequently, this kind of solid acid from rice husk is a promisinggreen catalyst for esterification.

4. Conclusions

In summary, a novel carbon solid acid was prepared frombiomass rice husk by incompletely carbonization, sodium hydrox-ide leaching and concentrated H2SO4 sulfonation. The solid acidwas characterized by XRD, FT-IR, N2 adsorption-desorption andsolid-state NMR. The characterization results show that the carbon

carbon and (c)the carbon solid acid.

Fig. 8. Stability test of the carbon solid acid and the typical solid acid SO42�/ZrO2.

D. Zeng et al. / Microporous and Mesoporous Materials 219 (2016) 54e5858

solid acid exhibits ultra high surface area of 1233m2/g and strongeracid strength than that of HZSM-5 zeolite. The catalytic perfor-mance was tested by the esterification of oleic acid with methanol.The results indicate that this solid acid catalyst is an excellentcatalyst comparedwith other conventional solid acid. In addition, itis possible that this green and active porous carbon catalyst mayfind wide applications in reactions catalyzed by liquid acids.

Acknowledgments

We acknowledge the financial supports from the National Nat-ural Science Foundation of China (21473126), the Fund of Hubei

Provincial Department of Education (B2014094) and the OpenResearch Fund of Hubei Province Key Laboratory of Coal Conversionand New Carbon Material (WKDM2013010).

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