Hydrothermal Preparation and Characterization of Novel Corncob

May 12, 2014 - Novel corncob-derived solid acid catalysts were successfully synthesized for the first time by the hydrothermal method. The influences ...
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Hydrothermal Preparation and Characterization of Novel CorncobDerived Solid Acid Catalysts Huan Ma,†,‡ Jiabao Li,†,‡ Weiwei Liu,§,∥ Beijiu Cheng,‡ Xiaoyan Cao,⊥ Jingdong Mao,⊥ and Suwen Zhu*,‡ ‡

School of Life Sciences, and §School of Engineering, Anhui Agricultural University, 130 Changjiang West Road, Hefei, Anhui 230036, People’s Republic of China ∥ Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, Anhui 230031, People’s Republic of China ⊥ Department of Chemistry and Biochemistry, Old Dominion University, 4541 Hampton Boulevard, Norfolk, Virginia 23529, United States ABSTRACT: Novel corncob-derived solid acid catalysts were successfully synthesized for the first time by the hydrothermal method. The influences of different preparation conditions were investigated, and the structure−function relationships of the resulting catalysts were also discussed on the basis of the analysis of structure and composition. In comparison to conventional solid acid catalysts, the corncob-derived catalyst synthesized under optimized conditions exhibited higher catalytic activity in esterification reactions, yielding nearly 90% methyl oleate in only 2 h. The catalyst retained satisfactory catalytic activity for esterification, even after 8 reaction cycles. Solid-state magic angle spinning (MAS) 13C nuclear magnetic resonance (NMR) investigations further indicated that the catalyst was composed of polycyclic aromatic carbon sheets bearing −SO3H, −COOH, and −OH groups in adequate amounts and with proper proportions, contributing to its excellent catalytic activity. This work provides a green method to synthesize solid acid catalysts from biomass wastes and may contribute to a holistic approach for biomass conversion. KEYWORDS: hydrothermal preparation, solid acid, corncob, esterification, high catalytic activity

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reactions.13−15 To date, numerous HTC carbonaceous materials with specific properties and applications in soil enrichment, catalysis, water purification, energy storage, and CO2 sequestration have been reported.16,17 However, most studies have focused mainly on a limited number of feedstocks, especially pure carbohydrates, such as glucose, starch, and cellulose.18−21 Relatively, little attention has been paid to the use of natural lignocellulosic biomass as carbon precursors in hydrothermal synthesis, although lignocellulosic biomass is known to be the most abundant renewable resource on Earth.22 More surprisingly, to the best of our knowledge, no one has thus far employed HTC to synthesize carbon-based solid acid catalysts using waste biomass as the starting material. Results from previous studies indicate that the catalytic performance of a carbon-based solid acid is highly dependent upon the starting materials. Therefore, it is of great interest to synthesize biomass-derived solid acids by HTC reaction as well as to investigate their structure and performance. In the present work, we developed a simple and effective lowtemperature hydrothermal method for the synthesis of a biomass-derived solid acid using corncob as a precursor. The effects of different preparation parameters on the catalytic activity and the reusability of the corncob-derived solid acid were investigated using the esterification reaction of oleic acid with methanol. The physical and chemical properties of the corncob-derived catalyst were also characterized in detail, and

INTRODUCTION With the global energy crisis and rise in environmental pollution, the significant advantages of solid acid in biomass conversion have attracted considerable attention in recent years.1−3 Solid acid has served as an environmentally benign catalyst in biodiesel production and the saccharification of cellulosic biomass because of its easy separation, reusability, less corrosiveness, and non-toxictiy.4,5 However, a major obstacle to these processes is the lack of solid acid catalyst that is as active, stable, and inexpensive as sulfuric acid. Recently, the group of Hara has developed a novel carbon-based solid acid that performs much better in esterification reactions than conventional solid acids, such as zeolites, niobic acid, and Amberlyst15.6−9 This novel solid acid catalyst was synthesized by sulfonation of incompletely carbonized organic matter. Nonetheless, both the carbonization and sulfonation processes were carried out in harsh conditions with high temperature in an inert atmosphere (≥673 K carbonization and 423 K sulfonation under nitrogen flow). In addition, numerous harmful wastes were formed in these processes, which makes the synthesis energy-intensive and non-environmentally friendly.10,11 Therefore, it is highly desirable to develop green and energy-efficient methods to synthesize a carbon-based solid acid with stable and efficient catalytic performance.12 Low-temperature hydrothermal carbonization (HTC) provides an eco-friendly approach to obtain various carbonaceous materials using mild conditions (up to 523 K), self-generated pressures, and water as the carbonization medium.13 It can proceed with the same level of conversion efficiency as higher temperature processes because hydrolysis exhibits a lower activation energy than many dry thermochemical conversion © 2014 American Chemical Society

Received: Revised: Accepted: Published: 5345

January 28, 2014 May 7, 2014 May 12, 2014 May 12, 2014 dx.doi.org/10.1021/jf500490m | J. Agric. Food Chem. 2014, 62, 5345−5353

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Figure 1. (a) Effect of the carbonization temperature on esterification activity of the corncob-derived solid acid. Preparation conditions: HTC at 393−513 K for 10 h, followed by sulfonation at 363 K for 10 h. (b) Effect of the carbonization time on esterification activity of the corncob-derived solid acid. Preparation conditions: HTC at 453 K for 1−15 h, followed by sulfonation at 363 K for 10 h. (c) Effect of the sulfonation temperature on esterification activity of the corncob-derived solid acid. Preparation conditions: HTC at 453 K for 10 h, followed by sulfonation at 333−423 K for 10 h. (d) Effect of the sulfonation time on esterification activity of the corncob-derived solid acid. Preparation conditions: HTC at 453 K for 10 h, followed by sulfonation at 363 K for 1−15 h. Esterification reaction conditions: ratio of methanol/oleic acid of 10 (for a typical experiment, 10 g of oleic acid and 11.34 g of methanol were added), 10% catalyst (on the basis of the mass of oleic acid), 353 K, 300 rpm, and 2 h. temperature and the black precipitate was filtered and washed repeatedly with hot distilled water (>353 K) until the washing liquid was neutral. Finally, the black solid was oven-dried at 378 K for 24 h. Catalytic Activity of the Catalysts. The esterification reaction system contained 10 g of oleic acid and 11.34 g of methanol, with the ratio of methanol/oleic acid of 10 and 10 wt % catalyst loading based on the mass of oleic acid. The reaction system was conducted at 353 K and 300 rpm for 2 h. The yield of the esterification was determined according to the changes of the acid value in the oil phase and calculated as follows:

its structure−function relationship was investigated with scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), elemental analysis (EA), and 13C solid-state nuclear magnetic resonance (NMR).

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MATERIALS AND METHODS

Material. Corncob was provided by the maize base of Anhui Agricultural University, Hefei, China, milled into powder, and dried at 378 K for 24 h. All of the reagents and chemicals were purchased from Sinopharm Chemical Reagent Co., and the acid value of oleic acid used in the esterification was 196 mg of KOH/g. Preparation of the Novel Carbon-Based Solid Acid. The catalysts were prepared by a low-temperature hydrothermal method under a range of conditions. In a typical preparation, the corncob powder was pretreated at first by ultrasonic wave and dilute alkali and then 5 g of corncob powder and 40 mL of water were maintained in a sealed, Teflon-lined autoclave at a specific temperature (393−513 K) for 1−15 h. After cooling to room temperature, the resulting material was washed with pure water several times and oven-dried at 378 K for 10 h. Then, 5 g of the above dried powder was heated in 25 mL of concentrated H2SO4 to introduce SO3H groups under hydrothermal conditions. After sulfonation of the mixture for 1−15 h at a specific temperature (333−423 K), the mixture was cooled to room

ester yield (%) = (AV0 − AV1/AV0) × 100%

(1)

where AV0 and AV1 are the acid values of the oleic acid and products, respectively. Analysis Methods. The morphology of the solid acid was analyzed using SEM (Sirion 200, FEI Electron Optics Company, Hillsboro, OR) coupled with energy-dispersive X-ray (EDX, INCA Energy, U.K.). FTIR spectra were recorded on a Bruker Vector 33 FTIR spectrometer in the range of 4000−500 cm−1 with a resolution of 4 cm−1. The XRD was performed using a Rigaku D/MAX β power X-ray diffractometer using Cu Kα radiation (λ = 0.184 15 nm) at 30 kV and 30 mA in the scanning angle of 3−60° at a scanning speed of 10°/min. EA was performed on an Elementar Vario Micro Cube. 13C crosspolarization/total sideband suppression (CP/TOSS) NMR spectra were measured at room temperature using a Bruker Avance III 400 5346

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spectrometer at a 13C Larmor frequency of 100 MHz. A Bruker magic angle spinning (MAS) probe head was used with a 4 mm zirconia rotor. The spinning rate of the sample was 5 kHz. Sub-spectra for nonprotonated and mobile carbon groups were obtained by combining the 13 C CP/TOSS sequence with a 40 μs dipolar dephasing. Reusability of the Corncob-Derived Solid Acid Catalyst. To assess the catalytic stability of the corncob-derived solid acid, the reusability of the catalyst was investigated for the esterification of oleic acid and methanol. The reaction was conducted under the same conditions mentioned above for 8 cycles (2 h of reaction time per cycle). After each reaction cycle, the catalyst was recovered by filtration and washed with methanol or n-hexane until no methyl oleate was detected in the washing. The catalyst was then dried for the removal of the organic solvent and water under vacuum at 378 K for 2 h before reuse.

differences in the catalysts prepared under different conditions will be discussed. Figure 2 illustrates the morphologies of the corncob powders, the pretreated corncob, and the hydrothermal carbon

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RESULTS AND DISCUSSION The catalytic activities of the resulting corncob-derived solid acids were investigated by the esterification of oleic acid with methanol. Different preparation variables have different influences on the esterification activity of the synthesized catalysts (Figure 1). Within the tested range of 393−513 K, the HTC temperature greatly influenced the activity of the catalysts (Figure 1a). In comparison to the samples carbonized below 453 K, the resulting solid acids prepared at higher temperatures (453−513 K) were more active, with greater methyl oleate yields of nearly 90%. Similarly, increasing the carbonization time to 10 h resulted in an obvious increase in the catalytic activity of corncob-derived catalysts (Figure 1b). This may be attributed to the higher OH density in the samples carbonized at lower temperatures or in shorter times; these conditions allow for more water from esterification to be absorbed on the surface of the resulting catalyst, which can poison the acid sides and hinder the relatively hydrophobic reactants from accessing the catalyst.23,24 In addition, samples carbonized at lower temperatures or in shorter times are less rigid; their aromatic and SO3H groups will be leached out at high reaction temperatures or when higher fatty acids are used as reactants, resulting in a rapid deactivation of the catalyst. Therefore, taking both energy efficiency and time into consideration, 453 K and 10 h were selected as the suitable carbonization conditions for subsequent experiments. Panels c and d of Figure 1 show the influences of the sulfonation temperature and time on the performance of the catalyst, respectively. After carbonization at 453 K for 10 h, the catalytic activity of the solid acid is largely dependent upon the sulfonation temperature. The catalyst sulfonated at 363 K displayed the highest esterification activity; the catalytic activity then dropped markedly with increasing temperature from 363 to 423 K (Figure 1c). This may be because the further carbonization by H2SO4 during sulfonation at higher temperatures generated a more rigid catalyst with a lower SO3H density, which was confirmed in the structural characterization and elemental analysis. As shown in Figure 1d, 10 h was determined to be the optimal sulfonation time for the prepared catalyst. At this sulfonation time, the density of SO3H groups at the suitable sulfonation temperature of 363 K reached a plateau, which is consistent with previous reports.25 Within the tested range, the optimal preparation conditions for the most effective catalyst are found to be HTC at 453 K for 10 h, followed by hydrothermal sulfonation at 363 K for 10 h. Under these conditions, a relatively high yield of methyl oleate (nearly 90%) was reached by catalysis with the corncob-derived solid acid after only 2 h of esterification. The structural

Figure 2. Field emission scanning electron microscopy (FESEM) images of the corncob powder (A) before and (B) after ultrasonicassisted alkali pretreatment, (C and D) FESEM images of hydrothermal carbon materials obtained from carbonization at 453 K for 10 h, (E) FESEM images of the solid acid obtained from carbonization at 453 K for 10 h, followed by sulfonation at 363 K for 10 h. (F and G) Corresponding EDS spectra of the hydrothermal carbon material before and after sulfonation.

samples before and after sulfonation. In comparison to the raw material (Figure 2A), the pretreated corncob had a relatively rough surface and a distinctly looser structure with more pores, attributed to the swelling effect of the alkaline solution. A remarkable morphological transformation of corncob can be observed in panels C and D of Figure 2. When hydrothermally treated at 453 K, the fibrous corncob network was disrupted and carbon spheres with an average diameter of 400 nm started forming, indicating that the carbonization of corncob indeed occurred in the hydrothermal process. After sulfonation, the solid acid derived from corncob showed a completely different morphology (Figure 2E). By the further oxidation effect of H2SO4 and the intercalation of SO3H groups, the spherical particles disappeared and aggregated to form irregular particles with highly granular surfaces. The S of the SO3H groups can be observed in the EDX spectra after sulfonation (panels F and G of Figure 2). Figure 3 shows the FTIR spectra of the carbon precursors derived from corncob and the catalysts prepared under different conditions. According to the spectra shown in Figure 3a, the carbonized samples produced at 393−513 K (Figure 3a) contained both aromatic and aliphatic hydrocarbons, as indicated by the peaks at approximately 2899 cm−1 (aliphatic C−H stretching) and 894 cm−1 (aromatic C−H bending).18,19 The peaks assigned to OH and C−O near 3314 and 1037 5347

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Figure 3. FTIR spectra of the carbon precursors and the catalysts prepared under various conditions. (a) Spectra of the carbon precursors hydrothermally carbonized at different temperatures (393−513 K) for 10 h (corncob and pretreated corncob are also shown as controls). (b) Spectra of the catalysts prepared at different carbonization temperatures (393−513 K) for 10 h, followed by sulfonation at 363 K for 10 h. (c) Spectra of the catalysts prepared under carbonization at 453 K for 1−15 h, followed by sulfonation at 363 K for 10 h. (d) Spectra of the catalysts prepared under carbonization at 453 K for 10 h, followed by sulfonation at different temperatures (333−423 K) for 10 h.

cm−1,26 respectively, became weaker as the carbonization temperature increased, while the CO peak at approximately 1698 cm−1 was enhanced. This suggests that incomplete carbonization reactions occurred at hydrothermal conditions, resulting in structural changes in the carbon precursors. The structural changes could also be clearly observed in the SEM images (panels c and d of Figure 3). This result is consistent with previous studies of the HTC of cellulose and waste biomass.21 In comparison to the carbonized samples, the most obvious changes after sulfonation were the appearance of peaks at approximately 1030 and 1153 cm−1, which are identified as the OSO symmetric stretching and SO3 stretching modes in SO3H, respectively (panels b−d of Figure 3). This can be further confirmed by EA, detailed below. In addition, the CC peak intensity at approximately 1595 cm−1 was significantly enhanced, and the aliphatic C−H peak (2899 cm−1) diminished

after sulfonation. All of these observations indicate that the sulfonation process not only introduced SO3H groups into the solid acid but also led to the further carbonization of the carbon precursors, which was also demonstrated by the color change of the samples from brown to black after sulfonation. The peak intensities because of CO and SO3H were enhanced, while that for the OH group was significantly decreased, when the hydrothermal temperature was above 393 K (Figure 3b). Similarly, catalysts prepared at lower carbonization times (below 10 h) contained more OH groups (Figure 3c). This could partially explain why the catalysts carbonized at 393 K or in shorter times (below 10 h) had much lower catalytic activities, because greater water absorption by the OH groups during esterification can easily poison the acid sides of the catalysts and reduce the catalyst activity.23,27 The CO and SO3H peak intensities were strongly dependent upon the 5348

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Figure 4. XRD patterns of the catalysts prepared under various conditions. (a) Catalysts prepared with carbonization at different temperatures (393−513 K) for 10 h, followed by sulfonation at 363 K for 10 h (corncob and pretreated corncob are also shown). (b) Catalysts prepared with carbonization at 453 K for different times (1−15 h), followed by sulfonation at 363 K for 10 h. (c) Catalysts prepared with carbonization at 453 K for 10 h, followed by sulfonation at different temperatures (333−423 K) for 10 h.

composed of large carbon sheets. Simultaneously, the weak diffraction peak (2θ = 40−45°) became more visible as well. This suggests that further carbonization during sulfonation yields a graphite-like carbon structure in the catalysts. Noticeably, when the carbonized sample was sulfonated at 423 K, the broad diffraction peak at 2θ = −30° became sharp and moved to a higher angle because of the formation of a more carbonized and rigid structure, which would prevent large molecular reactants from coming into contact with the SO3H groups.28 This also contributed to a lower SO3H content in the catalyst, which is consistent with the result of EA and the limited esterification activity of the catalyst. EA was employed to estimate the S content of the catalysts prepared under different conditions. The SO3H density was calculated from the S content because all S atoms were present as SO3H groups. The S contents in the raw material and pretreated corncob are not shown in Table 1 because both of them contained little S, as illustrated by EDX (Figure 2). As seen in Table 1, the density of SO3H contained in various

sulfonation temperature (Figure 3d). The FTIR spectra of catalysts sulfonated for different times showed only slight differences (data not shown), indicating that the sulfonation time was less important in determining catalytic activity than the other variables. This result is in agreement with the esterification results (Figure 1d). Figure 4 illustrates the XRD patterns of the raw material, pretreated corncob, and catalysts prepared under different conditions. In comparison to the raw material, the characteristic peaks of pretreated corncob became more obvious, indicating that pretreatment removed impurities without destroying the “core” structure of corncob (Figure 4a). All of the XRD patterns of the corncob-derived catalysts exhibited a broad diffraction peak (2θ = 15−30°) attributable to amorphous carbon composed of polycyclic aromatic carbon sheets oriented in a considerably random fashion.8,28 With an increasing carbonization temperature, time, and sulfonation temperature, the amorphous carbon diffraction peak became more obvious and shifted to higher angles (panels b−d of Figure 4), indicating the production of a more carbonized structure 5349

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protonated carbons and carbons of mobile groups, such as CCH3 groups. In Figure 5a, the spectrum of raw corncob was shown as a reference to evaluate the structural changes of the carbon precursor carbonized at different hydrothermal temperatures (393−513 K). The 13C CP/TOSS spectrum of raw corncob is dominated by cellulose signals from 60 to 110 ppm. Signals in much lower abundances are seen for lignin (110−160 ppm) and hemicellulose [−OC(O)−CH3 at approximately 20 ppm and approximately 175 ppm]. Most of these carbon signals arose from protonated carbons and were selectively dephased in the DD spectrum (Figure 5a). The carbon precursor obtained at 393 K had a spectrum similar to that of raw corncob, indicating that nearly no structural changes occurred at the low temperature. Note that the signals associated with hemicellulose were less visible. Major spectral changes occurred as the hydrothermal temperature increased from 453 to 513 K. The signals for aliphatic groups (0−60 ppm) and aromatic groups (110−180 ppm) were enhanced in 13 C CP/TOSS spectra. At 513 K, the signals associated with cellulose disappeared; the spectrum was composed of the signals from mostly non-protonated polycyclic aromatic carbons (130 ppm), phenolic OH/aromatic C−O−C (153 ppm), and nonpolar alkyl carbons (0−60 ppm). Carbons of mobile groups, such as CCH3 (22 ppm) and OCH3 (57 ppm), also became more abundant. This indicates the initial formation of aromatic carbon sheets and other altered lignin residues at 453 K, as observed with FTIR and SEM. Panels b−d of Figure 5 show the spectra of the solid acid catalysts prepared by the hydrothermal sulfonation of carbon precursors under different conditions: carbonization temperature from 393 to 513 K (Figure 5b), carbonization time from 1 to 15 h (Figure 5c), and sulfonation temperature from 333 to 423 K (Figure 5d). In comparison to the spectra of carbon precursors before sulfonation (Figure 5a), those of solid acids produced by hydrothermal sulfonation lost cellulose signals and were composed primarily of signals from mostly nonprotonated aromatic carbons (130 ppm), phenolic OH/ aromatic C−O−C (153 ppm), and nonpolar alkyl carbons (0−60 ppm). Signals from COOR carbons (178 ppm) and ketone carbons (approximately 200 ppm) were also present.25,28,31 The aromatic C−O signals became gradually less abundant in the solid acids prepared from higher temperature carbon precursors. Furthermore, the aliphatic carbon peaks of the solid acid (0−60 ppm) were markedly reduced in intensity compared to those of the carbon precursor carbonized at 513 K (Figure 5a). All of these results revealed that the structure of the carbon precursor was further modified by H2SO4 during sulfonation, possibly leading to the development of polycyclic aromatic sheets in the solid acid.8,25,29,30 According to panels b−d of Figure 5, the strong suppression of the aliphatic C signals after DD indicated that most of the sp3hybridized carbons were protonated, except of mobile carbons of CCH3 and OCH3. In addition, the peak because of aromatic carbon-bearing SO3H groups could not be distinguished in the spectra of the sulfonated samples because the broad aromatic carbon (130 ppm) and aromatic C−O (150 ppm) peaks obscure the Ar−SO3H (140 ppm) peak.25,29,30 A remarkable reduction in the aromatic C−O peak intensity and an enhancement of the aromatic carbon peak with an increasing carbonization temperature from 393 to 513 K were observed in the CP/TOSS and DD spectra of the solid acids (Figure 5b). This might result from enhanced cross-linking between the carbon sheets at high temperatures, which would

Table 1. S Content and SO3H Density of Catalysts Prepared under Various Conditions samplea Ca-393 Ca-423 Ca-453 Ca-483 Ca-513 Ca-453 Ca-453 Ca-453 Ca-453 Ca-453 Ca-453 Ca-453 Ca-453 Ca-453

K, K, K, K, K, K, K, K, K, K, K, K, K, K,

10 h−Sul-363 K, 10 h 10 h−Sul-363 K, 10 h 10 h−Sul-363 K, 10 h 10 h−Sul-363 K, 10 h 10 h−Sul-363 K, 10 h 1 h−Sul-363 K, 10 h 5 h−Sul-363 K, 10 h 15 h−Sul-363 K, 10 h 10 h−Sul-333 K, 10 h 10 h−Sul-393 K, 10 h 10 h−Sul-423 K, 10 h 10 h−Sul-363 K, 1 h 10 h−Sul-363 K, 5 h 10 h−Sul-363 K, 15 h

S contentb (wt %)

SO3H densityc (mmol/g)

2.26 3.81 4.93 4.63 4.67 3.56 3.99 4.33 3.96 3.56 2.21 4.35 4.45 4.83

0.70 1.19 1.54 1.45 1.46 1.11 1.25 1.35 1.24 1.11 0.69 1.36 1.39 1.51

a

The solid acid samples prepared at various conditions are denoted with different abbreviations. For example, Ca-393 K,10 h−Sul-363 K,10 h indicates that the catalyst was prepared by carbonization at 393 K for 10 h, followed by sulfonation at 363 K for 10 h. bThe S content was estimated by EA. cThe density of SO3H groups was calculated from the S content.

catalysts ranged from 0.69 to 1.54 mmol/g and largely depended upon the preparation conditions. The SO3H density was dramatically enhanced (from 0.7 to 1.54 mmol/g) as the HTC temperature increased from 393 to 453 K, whereas it showed a slight decline (from 1.54 to 1.46 mmol/g) when the temperature exceeded 453 K. This may be because the sulfonation of the sample carbonized at 453 K resulted in amorphous carbon composed of small aromatic carbon sheets, to which the SO3H group can be easily bonded. In contrast, the carbon precursors carbonized at low temperatures (below 453 K) had higher densities of aliphatic C−H and lower densities of cyclic aromatic hydrocarbons, which would be oxidized by the consumption of H2SO4 in the sulfonation process and, thus, reduce the SO3H density in the catalyst. On the other hand, the sulfonation of the samples carbonized at high temperatures (above 453 K) resulted in increased sp2-hybridized cross-linking among the carbon sheets, which can be shown by the decreasing amounts of phenolic OH and COOH groups in NMR spectra (Figure 5a).25,29,30 As a result, this would prevent the intercalation of SO3H groups into the carbon sheets and limit reactant access to SO3H groups. Similar effects on SO3H density were observed for carbonization time and sulfonation temperature. Although they had the same SO3H density of 1.11 mmol/g, the catalyst Ca-453 K, 10 h-Sul-393 K, 10 h exhibited a much higher esterification activity than Ca-453 K, 1 h-Sul-363 K, 10 h (panels b and c of Figure 1). This could be explained by the distinct difference in the OH density of these two catalysts, which can be observed in the NMR spectra (panels b and c of Figure 5), because a higher water absorption by the OH group in the esterification reaction can easily poison the acid sides of the catalysts and, thus, reduce the catalyst activity. Figure 5 displays the spectra of 13C CP/TOSS and 13C CP/ TOSS with dipolar dephasing (DD) of the raw corncob, with its carbon precursors (Figure 5a) and the catalysts (panels b−d of Figure 5) prepared under different conditions. 13C CP/ TOSS spectra provided semi-quantitative structural information for all carbon sites, and DD spectra selected the signals of non5350

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Figure 5. Spectra of 13C CP/TOSS and corresponding 13C CP/TOSS with DD. (a) Spectra of raw corncob and carbon precursors hydrothermally carbonized at different temperatures (393−513 K). (b) Spectra of corncob-derived solid acids prepared at different carbonization temperatures (393−513 K). (c) Spectra of corncob-derived solid acids prepared with different carbonization times (1−15 h). (d) Spectra of corncob-derived solid acids prepared at different sulfonation temperatures (333−423 K).

in Figure 5b, as a result of the varying carbonization time from 1 to 15 h and the sulfonation temperature from 333 to 423 K, respectively. When the sulfonation temperature was above 363 K, however, the resonances of aliphatic carbon and COO groups significantly decreased (Figure 5d). At the same time, the COO peak shifted to 170 ppm because of the reduction in aliphatic carbon intensity. This result reveals that the concentrated H2SO4 acted as not only a sulfonating reagent but also a strong oxidant during sulfonation at high temperatures (above 363 K). This probably generated a prominent reduction in catalytic activity by contributing to a rigid catalyst structure with fewer SO3H groups. In addition, it should be noted that the catalytic activity of the solid acids may

prevent the incorporation of reactants in the carbon bulk and hinder reactant access to SO3H groups.5,25 As a result, the aggregation of small carbon sheets with high cross-linking would decrease the reactivity of SO3H groups, partially explaining the relatively low esterification activity of solid acids carbonized above 453 K. Figure 5b also shows that the catalyst prepared at 393 K had the highest content of aromatic C−O, such as phenolic OH groups, which caused its inefficiency in the esterification reaction, as mentioned before. Therefore, the sample carbonized at 453 K had the highest catalytic activity. As expected, the changes in peaks corresponding to the aromatic C−O groups and aromatic carbons in panels c and d of Figure 5 were similar to those seen 5351

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activity was a little lower than that of concentrated H2SO4 (95.4% for 2 h), the corncob-derived solid acid can be easily recovered and reused at the end of each step. Additionally, the corncob-derived solid acid can prevent corrosion, which normally occurs in the presence of H2SO4, and can be successfully prepared from biomass by hydrothermal methods, providing a holistic and green approach to biomass conversion. In this regard, the corncob-derived solid acid prepared by the hydrothermal method is more promising and competitive. The recycling performance of the corncob-derived solid acid catalyst was also investigated using the esterification reaction of oleic acid with methanol. Between each cycle of the reaction, methanol and n-hexane were employed as washing solvents to recovery the catalyst. As shown in Figure 6, the catalytic activity of the corncob-derived solid acid was maintained with a 66% conversion for methanol washing and a 75% conversion with nhexane washing after 8 cycles of reuse. As reported by Mo et al.,32 sulfonated catalysts were deactivated during the washing procedure mainly because of the leaching of polycyclic aromatic hydrocarbons containing −SO3H groups because these hydrocarbons are not completely insoluble in common solvents. Therefore, the different recycling performances of the corncobderived solid acids in methanol and n-hexane might result from the varying solubilities of the polycyclic aromatic hydrocarbons in the different washing solvents. Although the deactivation of the catalytic activity was noticed in the case of carbon-based solid acid,33−35 the corncob-derived catalyst prepared in this work still gave a satisfactory yield of methyl oleate after 8 cycles, indicating the high stability of the catalyst. To the best of our knowledge, this is the first report of the synthesis of carbon-based solid acids from lignocellulosic biomass employing a hydrothermal method. The high catalytic activity of the corn-derived catalyst obtained under optimal hydrothermal conditions (453 K carbonization for 10 h and 363 K sulfonation for 10 h) can be attributed to the synergetic combination of the specific structure (polycyclic aromatic carbon sheets) and three functional groups (SO3H, COOH, and phenolic OH groups) in adequate amounts. This work

not simply depend upon the synergetic combination of their structure and their functional groups, as suggested by previous studies.1,31 The catalytic activity also depends upon the functional groups (such as SO3H, COOH, and phenolic OH) being bonded to the carbon sheet in adequate amounts and, more importantly, in proper proportions. To evaluate the catalytic performance of the new catalyst, a comparative study was performed using different catalysts for the esterification of oleic acid with methanol at 353 K to form methyl oleate. The same amount of catalyst was used in all cases. The catalysts tested were concentrated H2 SO4 , Amberlyst-15, niobic acid, and corncob-derived solid acid. As shown in Table 2, no methyl oleate was detected when the Table 2. Comparison of the Esterification Yield of Oleic Acid with Methanol Catalyzed by Various Catalystsa catalyst

reaction time (h)

yield of methyl oleate (%)

control (no catalyst) corncob-based solid acid Amberlyst-15 niobic acid concentrated H2SO4

24 2 2 2 2

89.2 68.8 29.6 95.4

a

Reaction conditions: ratio of methanol/oleic acid of 10 (for a typical experiment, 10 g of oleic acid and 11.34 g of methanol were added), 353 K, 2 h, 300 rpm, and 10 wt % catalyst loading based on the mass of oleic acid.

reaction was conducted for 24 h in the absence of catalysts, implying the critical role of the catalyst in this esterification reaction. The corncob-based solid acid possessed a high catalytic activity, with an 89.2% esterification yield after only 2 h. The yields of Amberlyst-15 and niobic acid after 2 h were 68.8 and 29.6%, respectively. The synergetic effect of three different functional groups (SO3H, COOH, and phenolic OH groups) in the corncob-derived solid acid may be the main reason for its high catalytic activity compared to the conventional catalysts (niobic acid and Amberlyst-15), which contain only one functional group.28,31 Although its catalytic

Figure 6. Reusability of the novel corncob-derived solid acid (prepared under the optimized conditions). Reaction conditions for each cycle: molar ratio of methanol/oleic acid of 10 (10 g of oleic acid and 11.34 g of methanol), 353 K, 300 rpm, and 2 h of reaction time per cycle. 5352

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Journal of Agricultural and Food Chemistry

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provides a simple and energy-efficient method to synthesize carbon-based catalysts from lignocellulosic biomass and develops a holistic and green approach to biomass conversion and environmental protection.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-551-65786021. Fax: +86-551-65786021. Email: [email protected]. Funding

This work was financially supported by the Natural Science Foundation of Higher Education Institutes of Anhui Province, China (Grant KJ2014A073). Notes

The authors declare no competing financial interest. † Huan Ma and Jiabao Li contributed equally to this work.

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ACKNOWLEDGMENTS The authors thank Prof. Hong Wang at the Chinese Academy of Agriculture Sciences for help with the NMR test. The authors also thank Dr. Peirong Chen for providing the facilities for FTIR.

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REFERENCES

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dx.doi.org/10.1021/jf500490m | J. Agric. Food Chem. 2014, 62, 5345−5353