Abstract
A carbon-based solid acid catalyst (CHACS) derived corncob residual was prepared by incomplete hydrothermal carbonization followed by activation with phosphoric acid impregnation and sulfuric acid sulfonation. The structure of the solid acid catalyst was characterized using Fourier transform infrared spectra (FTIR), thermogravimetric analyzer (TGA), X-ray diffraction (XRD), scanning electron microscope (SEM), specific surface area (SSA), and elemental analysis. The -SO3H, -COOH, and phenolic -OH functional groups were successfully introduced. Phosphoric acid activation facilitated formation of the porous structure in the solid acid catalyst. The specific surface area and acid density were 1569 m2/g and 1.030 mmol/g, respectively. The CHACS exhibited better catalytic activity for hydrolysis conversion of corn stalk in ionic liquid with water as solvent. A total reducing sugars (TRS) yield of 68.3% was obtained in water and a TRS yield of 52.5% was obtained in an ionic liquid of [BMIM][Cl] at 140 °C for 120 min. The CHACS expressed good catalytic activity in each of 4 separate instances of reuse.
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Preparation of Carbon-based Solid Acid from Corncob Residual and its Performance for Acid-Catalyzed Hydrolysis
Rui Li, Guihua Yang,* Qiang Wang, Jiachuan Chen,* Jiran Dong, and Xinhan Zhang
A carbon-based solid acid catalyst (CHACS) derived corncob residual was prepared by incomplete hydrothermal carbonization followed by activation with phosphoric acid impregnation and sulfuric acid sulfonation. The structure of the solid acid catalyst was characterized using Fourier transform infrared spectra (FTIR), thermogravimetric analyzer (TGA), X-ray diffraction (XRD), scanning electron microscope (SEM), specific surface area (SSA), and elemental analysis. The -SO3H, -COOH, and phenolic -OH functional groups were successfully introduced. Phosphoric acid activation facilitated formation of the porous structure in the solid acid catalyst. The specific surface area and acid density were 1569 m2/g and 1.030 mmol/g, respectively. The CHACS exhibited better catalytic activity for hydrolysis conversion of corn stalk in ionic liquid with water as solvent. A total reducing sugars (TRS) yield of 68.3% was obtained in water and a TRS yield of 52.5% was obtained in an ionic liquid of [BMIM][Cl] at 140 °C for 120 min. The CHACS expressed good catalytic activity in each of 4 separate instances of reuse.
Keywords: Corn cob residual; Solid acid catalyst; Corn stalk; Total reducing sugar; Hydrolysis
Contact information: Key Lab of Pulp and Paper Science & Technology of the Ministry of Education, Qilu University of Technology, Jinan, Shandong, PR China, 250353;
* Corresponding authors: ygh2626@126.com; chenjc@qlu.edu.cn
INTRODUCTION
With the rapid consumption of non-renewable fossil resources and the increasing concern for environmental protection, the conversion of lignocellulosic biomass to various chemicals and biofuels has received more attention (Nanda et al. 2014). Corn stalk is a lignocellulosic biomass and agricultural stalk resource that is often burnt or discarded in the field without any economic benefit (Sun et al. 2013). Conversion of this abundant crop residual to value-added products would improve its use and value as well as broaden its scope of application.
Many efforts have been devoted to the conversion of corn stalk with the use of mineral acids (Mok et al. 1992; Torget et al. 2000) and enzymes (Ramakrishnan et al. 2010; Das et al.2011; Nemestóthy et al. 2017). However, there are many drawbacks as follows: inorganic acid catalysts such as sulfuric acid and hydrochloric acid are cheap and highly effective, but they cause environmental pollution and equipment corrosion, which hinders their widespread application. In addition, inorganic acid is difficult to separate from reaction systems, which results in large amounts of chemical waste. Enzyme treatment is very effective yet has many disadvantages such as high cost, long reaction time, and strict reaction conditions. Increasing interest in environmental protection and green chemistry is a major factor driving the exploration of recyclable solid acids with high efficiency. Various solid acids such as zeolites have been used to treat lignocellulosic biomass (Yang et al. 2015). Heteropoly compounds have low acid densities and thus cannot meet the catalytic demand in many catalytic reactions (Nakajima et al. 2007).
Carbon-based solid acid catalysts are attractive for their efficiency, non-corrosiveness, reduced reaction time, environmentally friendly qualities, and recyclability. Many studies have investigated the preparation of carbon-based solid acids obtained by incomplete carbonization and sulfonation (Suganuma et al. 2010; Guo et al. 2012; Bai et al. 2014). However, these carbon-based solid acids have lower specific surface area with inferior catalytic activities for biomass conversion. Therefore, the preparation of solid acids with high specific surface areas is necessary to meet the catalytic demand. Kitano et al. (2009) prepared a solid acid with a high specific surface area by incomplete carbonization of wood powder activated by ZnCl2 and impregnated by sulfuric acid sulfonation. The ZnCl2 activation method is rarely used due to its high cost and intractable pollution. H3PO4 was selected as an activating agent due to its powerful ability to confer catalytic properties to solid acids. Fu et al. (2012) and Teng et al. (1998) reported a solid acid catalyst prepared by H3PO4 impregnation that has lower specific surface area (113 m2• g-1, 854 m2• g-1); however, the catalytic performance was not sufficient for the present goals. Therefore, it is necessary to synthesize a solid acid catalyst with both of high specific surface area and catalytic activity.
In this work, a solid acid catalyst was prepared from corncob residual (CCR) as feedstock using the methods of hydrothermal carbonization, H3PO4 impregnated activation, and H2SO4sulfonation. The solid acid was analyzed and characterized by Fourier transform infrared spectra (FTIR), X-ray diffraction (XRD), thermogravimetric analyzer (TGA), scanning electron microscope (SEM), specific surface area (SSA), and elemental analysis, and the solid acid was evaluated on its performance for hydrolysis of corn stalk in ionic liquid and water.
EXPERIMENTAL
Materials
Corncob residual (CCR) was supplied by a furfural production mill in Dezhou, China and ground into particles passing through a 100-mesh sieve. CCR was used to synthesize the solid acid in this work. The chemical composition (based on mass) of the CCR was as follows: 72.3% cellulose according to TAPPI T429 cm-84 (1984), 7.2% hemicellulose according to TAPPI T223 cm-84 (1984), 18.9% acid-insoluble lignin according to TAPPI T222 om-88 (1988), and 6.9% ash according to TAPPI T211 om-93 (1993).
Corn stalk was obtained from a farm in Jinan and was hydrolyzed by solid acid. The chemical composition (based on mass) of corn stalk was as follows: 39.1% cellulose according to TAPPI T429 cm-84 (1984), 29.5% hemicellulose according to TAPPI T223 cm-84 (1984), 16.2% acid-insoluble lignin according to TAPPI T222 om-88 (1988), and 4.2% ash according to TAPPI T211 om-93 (1993). The content of sugar components was as follows: 1.15% arabinose, 0.45% galactose, 0.04% rhamnose, 37.56% glucose, 14.89% xylose, and 0.12% mannose.
All organic chemicals were commercial products of the highest purity available (> 98%) and used without further purification. 1-Butyl-3-methylimidazolium chloride ([BMIM][Cl]) (purity of 99%) was purchased from the Lanzhou Institute of Chemical Physics (Lanzhou, China) and used as the ionic liquid. Ethanol (99.7 vol%), phosphoric acid (85 wt.%), and glacial acetic acid (99.5 wt.%) were purchased from Tianjin Fuyu Fine Chemical Co., Ltd. (Tianjin, China).Sulfuric acid (95 to 98 wt.%) and hydrochloric acid (36 to 38 wt.%) were purchased from Laiyang Chemical Co., Ltd. (Laiyang, China). Sodium hydroxide (96 wt.%) was purchased from Tianjin Damao Chemical Co. (Tianjin, China). Silver nitrate (99.8 wt.%) was purchased from Shanghai Reagent Factory (Shanghai, China).
First, 5 g oven dried CCR and 50 mL deionized water were added into a 250 mL PARR stirred pressurized reactor while nitrogen gas was used as a purge (Zhang et al. 2015a). The reaction was conducted at 250 C with a stirring rate of 150 rpm for 4 h, followed by cooling to room temperature (Zhang et al. 2015b). The CCR-hydrochar (denoted as CH) was obtained by filtration and washed with ethanol and deionized water until colorless. The washed CH was dried at 105 C for 24 h prior to use.
A total of 1 g of CH was impregnated with 3 g of 40 wt.% H3PO4 aqueous solution. The CH was placed for 1 h at room temperature and dried at 90 C for 12 h. The sample was transferred to a tube furnace and heated to 400 C for 1 h under N2. The heating rate was 4 C/min. After activation, the porous carbon was cooled to room temperature under nitrogen. The sample was treated with boiling 1 M HCl solution for 1 h. The ratio of HCl to activated carbon was 200 mL:1 g. Finally, the sample was washed with hot deionized water (> 80 C) until the filtrate was neutral. The CH-active carbon (denoted as CHAC) was dried at 105 C for 12 h.
Next, 1 g CHAC and 30 mL 98% H2SO4 were added into a 100 mL PARR reactor lined with Teflon at 150 C for 15 h. The mixture was diluted with deionized water, filtered, washed until no SO42- ions were detected. When there was SO42- ion in the filtrate, the BaSO4 precipitation was produced by adding BaCl2 solution. If there was no SO42- ion, no precipitate was produced. The solid acid was completely washed, then vacuum-dried at 80 C for 12 h to obtain the solid acid catalyst (denoted as CHACS).
The CHACS was used to catalyze the hydrolysis of corn stalk. A reaction mixture was prepared with [BMIM][Cl], corn stalk, water, and CHACS. The dosages of corn stalk, water, and CHACS based 2g [BMIM][Cl] were 5%, 1%, and 5%, respectively (Bai et al. 2016). The mixing sequence began with the addition of corn stalk into [BMIM][Cl] followed by the addition of water and the CHACS catalyst. The reaction was conducted in an oil bath bottle with a pressure at 140 C for 30 min, 60 min, 90 min, 120 min, 150 min, and 180 min.
The solid was separated by filtration, and the liquid sample was diluted with deionized water. In addition, the dosages of corn stalk and CHACS based 5 mL distilled water mass, which were both 2%, were mixed with distilled water in a reaction kettle lined with Teflon (Tian et al. 2010). The reaction kettle was kept at 140 C and a stirring rate of 100 rpm for 30 min, 60 min, 90 min, 120 min, 150 min, and 180 min. After the reaction, the liquid sample was diluted with deionized water and analyzed by using 3, 5-dinitrosalicylic acid (DNS) to determine the produced total reducing sugars (TRS). Total reducing sugars included xylose, glucose, arabinose, galactose and mannose.
Catalysis Analyses
The density of acid on the CHACS catalyst was determined by titration in an aqueous solution. A total of 0.1 g of the CHACS was added into 20 mL of a 2 M NaCl solution. The mixed liquor was ultrasonically treated for 1 h and then filtered, transferred to a 50 mL volumetric flask, and diluted with deionized water to a specified volume. Next, 10 mL of solution was withdrawn and titrated with 5 mmol standard sodium hydroxide solution. The acid density was calculated by the amount of standard sodium hydroxide solution that stoichiometrically reacted with the CHACS catalyst (Zeng et al. 2016).
Fourier transform infrared spectra were collected on an FTIR spectrometer (IRPrestige-21, Shimadzu Co., Shimane prefecture, Japan). A dried sample was ground and pelletized with KBr powder. All spectra were recorded from 500 cm-1 to 4000 cm-1.
The crystallinities of the CCR, CH, CHAC, and CHACS were determined by X-ray diffraction (XRD, D8-ADVANCE, Bruker, Karlsruhe, Germany) at a scanning angle between 10° and 90 and a scan rate of 2 /min.
The thermal stability of the CHACS was determined using a thermogravimetric analyzer (TGA Q50, USA). A 10 mg quantity of CHACS was placed in a sample tray and heated from room temperature to 700 C at a rate of 10 C min-1. The heating process was carried out under a nitrogen blanket at a flow rate of 30 mL min-1.
An environmental scanning electron microscope (SEM) (QUANTA 200, Holland FEI Co., Ltd., Eindhoven, Holland) was used to observe the morphology of CCR, CH, CHAC, and CHACS. All samples were coated with a thin layer of gold (SCD 005, Switzerland BAL-TEC Corporation (Los Angeles, CA, USA) at an accelerating voltage of 15 kV.
The specific surface area of the CH, CHAC, and CHACS and the N2 adsorption–desorption isotherm of CHACS were evaluated by particle size analyzer (V-sorb 2800p, Beijing Jinaipu Technology Co., Ltd., Beijing, China).
The element content (wt.%) of carbon (C), hydrogen (H), sulfur (S), oxygen (O), and nitrogen (N) in the samples were determined using a Vario EL III Element Analyzer (Elementar, European Virtual Institute for Speciation Analysis, Fulda, Germany).
RESULTS AND DISCUSSION
Functional Group Analysis
An analysis of the functional groups in CCR, CH, CHAC, and CHACS is shown in Fig. 1. The intensity of the 2920 cm-1 band was reduced for the -CH spectrum after activation and sulfonation, which indicated that the CHACS sample became more dehydrated during the treatments. Two bands at 1015 cm-1and 1230 cm-1 in the CHACS spectrum were assigned to -SO3- and O=S=O stretching modes in the –SO3H group, respectively (Suganuma et al. 2008). Therefore, the sulfonic acid group was introduced onto the surface of CHACS. The peak at 1540 cm-1 was attributed to the C=C stretching mode of the samples.
The presence of carboxylic groups was supported by the C=O characteristic stretching vibration band at 1730 cm-1 and the stretching vibration bands of -OH at 3440 cm-1 (Liang and Yang 2009; Xiao et al. 2010). Therefore, -SO3H and -COOH were the principal functional groups introduced onto the surface of the CHACS, which was mainly achieved by the activation and sulfonation treatments.
Fig. 1. FTIR spectrum of (a) CCR, (b) CH, (c) CHAC, and (d) CHACS
Crystallinity Analysis
Figure 2 shows the XRD spectra of CCR, CH, CHAC, and CHACS. The diffraction peak arising at 2θ of 23° corresponded to the diffraction of C (002) (Zeng et al. 2013). These two diffraction peaks (2θ = 16° and 2θ = 22°) of CCR, assigned to the diffraction peaks of cellulose, disappeared. However, for the CH, CHAC, and CHACS, the two diffraction peaks were not detected. The results indicated that the crystalline structure of CH, CHAC, and CHACS was disrupted primarily due to the degradation of cellulose. The XRD spectrum of CHACS exhibited broad diffraction peaks at 2 of 10 to 30 centered at 25, which corresponded to the (002) plane of the micrographites oriented in a random manner (Okamura et al. 2006; Lin et al. 2010).
Fig. 2. XRD spectrum of (a) CCR, (b) CH, (c) CHAC, and (d) CHACS
Thermal Stability Analysis
The thermal stability of the CHACS was investigated by TGA, as shown in Fig. 3. With the increase of temperature from 25 °C to 200 °C, a weight loss of 8.15% of CHACS was observed, which was mainly attributed to the evaporation of absorption moisture in an open environment (Okamura et al. 2006; Suganuma et al. 2008). When the temperature was further increased over 200 °C, a continuous weight decrease was observed, which indicated its gradual decomposition. More weight loss was detected within the range 200 °C to 400 °C, which could be ascribed to oxidization of the non-graphitic and graphitic carbons (Shang et al. 2015). Lastly, a rapid weight loss was detected with the continued increase of temperature, which could be attributed to the continuing pyrolysis of incompletely carbonized CCR with high oxygen content (Ibrahim et al. 2015). Essentially, the CHACS catalyst was very stable at relatively high temperatures.
Fig. 3. TG and DTG curves of the CHACS
Fig. 4. SEM images of (a) CCR, (b) CH, (c) CHAC, and (d) CHACS
Surface Morphology Analysis
The morphologies of CCR, CH, CHAC, and CHACS analyzed by SEM are shown in Fig. 4. The microstructure of CCR was similar to plant tissues with some holes on the surface. The surface of CH was smooth with a low number of pores, which may have been caused by the release of volatile components from the CH. The CHAC activated by phosphoric acid was transformed into a sponge-like structure with a high specific surface area. The surface of CHACS was structurally amorphous, which was evidenced by the large number of cracks and holes. In addition, the surface of CHACS showed many slit-shaped pores. Typically, increased specific surface area is beneficial to catalytic performance.
Elemental Analysis
Table 1 shows the composition of the samples at different stages. Compared with CCR, the content of hydrogen and oxygen both decreased after hydrothermal treatment and phosphoric acid activation. In contrast, the ratios of C/H, C/O, and O/H were enhanced greatly, which indicated that dehydration occurred during hydrothermal treatment and phosphoric acid activation. After sulfonation, oxygen content increased greatly, yet the hydrogen content decreased in the CHACS, possibly due to the introduction of -SO3H and -COOH. In addition, dehydration occurred in the residual -OH under sulfonation, which can result in a decrease in the hydrogen content and an increase in the ratios of C/H and O/H in the CHACS. Thus, the presence of -OH, -SO3H, and -COOH in CHACS was further evidenced.
Table 1. Elemental Analysis, Specific Surface Area, and Acid Density of CCR, CH, CHAC, and CHACS
Specific Surface Area Analysis
The N2 adsorption–desorption isotherm of the CHACS is shown in Fig. 5. The isotherm belongs to a mixed-type according to IUPAC classifications. At relatively low pressures, the isotherm exhibited a type I spectrum with a fast increase in N2 adsorption corresponding to a predominantly microporous structure for the CHACS. Furthermore, at intermediate and high relative pressures, the isotherm was type II with a type H4 hysteresis loop, which indicated many slit-shaped pores existing in the CHACS (Kruk et al. 2001; Puziy et al. 2007). These results indicated that H3PO4 could be used as an effective activating agent for the promotion of the expansion of the CHACS structure corresponding directly to the development of porosity.
The specific surface areas of CH, CHAC, and CHACS were determined as 5.69 m2• g-1, 2124 m2• g-1, and 1569 m2• g-1, respectively (Table 1). Fu et al. (2012) suggested that the larger surface area of CHACS may be attributed to an impregnation activation step. Moreover, compared with CHAC, CHACS had a smaller specific surface area. These reductions were due to partial destruction of the open porous structure during H2SO4 sulfonation (Fu et al. 2012).
The acid density of CHACS was 1.0305 mmol/g. The solid acid catalyst prepared by H3PO4 had the similar acid density of 1.1 mmol/g (Fu et al. 2012).
Fig. 5. N2 adsorption–desorption isotherm of the CHACS
Catalytic Activities
The TRS yield of the hydrolysis of corn stalk treated by CHACS is shown in Fig. 6. The yield of TRS first increased with the extension of time and then decreased after reaching its maximum. When water was used as a solvent and the treatment reaction was carried out for a relatively short time (30 min to 60 min), TRS yield was lower. An optimal TRS yield of 68.32% was achieved at 120 min. However, further extension of time led to a decrease. This result was mainly due to the excessive reaction time promoting the degradation of holocellulose and reducing sugar content. When [BMIM][Cl] was used as solvent, the trend was similar to that of water as solvent, and the maximum TRS yield reached 52.52%, which was lower than the maximum TRS yield with water used as a solvent.
Therefore, the results suggest that the acid-catalyzed hydrolysis of corn stalk with CHACS in water would be the optimal procedure. When water was used as solvent, total reducing sugars included 51.9% xylose, 36.7% glucose, 5.8% arabinose, 2.7% galactose, and 2.9% mannose at 120 min. When [BMIM][Cl] was used as solvent, total reducing sugars included 55.8% xylose, 33.6% glucose, 4.9% arabinose, 3.6% galactose, and 2.1% mannose at 120 min.
In order to measure the amount of leaching of the CHACS, the following experiment was carried out: the CHACS was added to the water and [BMIM][Cl], and the test was run under the optimal reaction conditions with no corn stalk present. Then the solution was filtered, and that solution used to process corn stalk with no catalyst present. The yields of reducing sugar were 3.4% and 2.6% in water and [BMIM][Cl]. This indicated that essentially no solid acid was leached into the water and [BMIM][Cl].
Fig. 6. The effect of CHACS treatment time on TRS of corn stalk hydrolysis in water and [BMIM][Cl]
Table 2. Different Performances of Catalysts for Treatment of Corn Stalk
Reuse of the CHACS Catalyst
The effect of the amount of times CHACS was reused on the TRS yield of the hydrolysis of corn stalk is shown in Fig. 7. The reuses of CHACS were carried out under the optimal conditions of [BMIM][Cl] and water. The reclamation process of CHACS was that the used CHACS was separated from the treating system, washed with distilled water, and then vacuum-dried. The dried CHACS was then reused in the next hydrolysis process of corn stalk. The results in Fig. 7 show that the reused CHACS catalysts had good performances. The TRS yield gradually decreased as the number of times CHACS was reused increased. The TRS yield decreased from 68.3% to 45.4% when CHACS was reused for the fourth time in water. In [BMIM][Cl], the TRS yield decreased from 52.5% to 33.0% when CHACS was reused for the fourth time. The TRS yields decreased to the minimums of 40.3% and 30.9% at the fifth instance of reuse in water and [BMIM][Cl], respectively. The reason for these results may be the partially loss of the CHACS catalyst activity in the processes of reclamation and reuse. Bai et al. (2014) reported that glucose carbon-based solid acid catalyst recyclability was that the TRS decreased from 63.2% (3rd run) to 39.7% (2nd run) and 23.9% (3rd run). Fu et al. (2012) reported that the conversion of methyl acetate decreased notably from 54.0% to 34.6% during the first 4 reaction cycles. The recyclability of CHACS was no worse than that of the two catalysts mentioned above. The CHACS catalyst showed relatively good catalytic performance for the hydrolysis of corn stalk.
Fig. 7. Effect of reused times of CHACS catalyst on TRS yield of corn stalk
CONCLUSIONS
- A carbon-based solid acid catalyst (CHACS) derived from corncob residual was successfully prepared by hydrothermal carbonization followed by H3PO4 impregnated activation and H2SO4 sulfonation. The solid acid was functionalized with -SO3H, -COOH, and phenolic -OH groups. H3PO4-impregnated activation promoted porosity formation in the solid acid catalyst.
- The CHACS was efficient for the hydrolysis of corn stalk in both [BMIM][Cl] and water reaction systems. A TRS yield of 68.32% was obtained in water at 140 °C for 120 min, and a TRS yield of 52.52% was obtained in [BMIM][Cl] under the same conditions. The effect of CHACS was better in water than in the ionic liquid of [BMIM][Cl].
- The solid acid catalyst was easily reclaimed and reused in the hydrolysis of corn stalk and expressed good performance and stability in the reclamation and reuse processes.
ACKNOWLEDGMENTS
The authors are grateful for the financial support received from the National Key R&D Program of China (2017YFB0307900), National Natural Science Foundation of China (Grant No.31470602) and the Taishan Scholars Program.
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Article submitted: June 24, 2017; Peer review completed: August 4, 2017; Revised version received and accepted: August 12, 2017; Published: August 23, 2017.
DOI: 10.15376/biores.12.4.7439-7451