Accepted Manuscript Title: Physicochemical properties of corn stalk after treatment using steam explosion coupled with acid or alkali Author: Yong-Gang Sun Yu-Long Ma Li-Qiong Wang Feng-Zhi Wang Qian-Qian Wu Guan-Yu Pan PII: DOI: Reference:
S0144-8617(14)00976-X http://dx.doi.org/doi:10.1016/j.carbpol.2014.09.066 CARP 9324
To appear in: Received date: Revised date: Accepted date:
19-10-2013 9-9-2014 18-9-2014
Please cite this article as: Sun, Y.-G., Ma, Y.-L., Wang, L.-Q., Wang, F.-Z., Wu, Q.-Q., and Pan, G.-Y.,Physicochemical properties of corn stalk after treatment using steam explosion coupled with acid or alkali, Carbohydrate Polymers (2014), http://dx.doi.org/10.1016/j.carbpol.2014.09.066 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Physicochemical properties of corn stalk after treatment using steam
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explosion coupled with acid or alkali
3 Yong-Gang Suna,b, Yu-Long Maa,b,*, Li-Qiong Wanga, Feng-Zhi Wangb, Qian-Qian Wub,
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Guan-Yu Panb
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a
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University, Yinchuan 750021, China
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b
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State Key Laboratory Cultivation Base of Energy Sources and Chemical Engineering, Ningxia
College of Chemistry and Chemical Engineering, Ningxia University, Yinchuan 750021, China
*Corresponding author:
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Yu-Long Ma
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State Key Laboratory Cultivation Base of Energy Sources and Chemical Engineering
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Ningxia University
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Helanshan Rd. 539
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Yinchuan 750021
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China
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Tel: +86 951 2062380; Fax: +86 951 2062323
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E-mail: [email protected] (Yu-Long Ma)
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E-mail: [email protected] (Yong-Gang Sun); [email protected] (Li-Qiong
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Wang); [email protected] (Feng-Zhi Wang); [email protected] (Qian-Qian Wu);
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[email protected] (Guan-Yu Pan).
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Highlights:
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Steam explosion coupled with alkali was a better method for corn stalk pretreatment.
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This method could remove most of hemicellulose and lignin in corn stalk.
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This method changed the bonding distribution and structural feature of corn stalk.
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This method increased the crystallinity and thermal stability of corn stalk.
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31 Abstract
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The aim of this study was to evaluate comparatively the effects of different pretreatments
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including steam explosion, acid, and alkali, alone or in combination, on the structural properties
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and thermal stability of corn stalk. All of the treated treatments decreased the contents of
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hemicellulose and lignin and thereby increased the content of cellulose in corn stalks. But the
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combined treatments with alkali and steam explosion under 0.4-0.6 MPa were better as
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compared with other treatments based on the removals of hemicellulose and lignin, and about
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71.58-79.59% of hemicellulose and 64.32-71.83% of lignin were removed. Treatment with steam
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explosion coupled with acid or alkali changed the bonding distribution and surface morphology
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and increased the crystallinity and thermal stability of corn stalks, and the degradation
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temperature reached over 350 °C. These results suggest that steam explosion coupled with alkali
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is a better method for the depolymerization of corn stalk polymer.
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Keywords: Corn stalk; Depolymerization; Cellulose; Thermal stability; Crystalline
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1. Introduction Lignocellulose mainly consists of cellulose, hemicelluloses and lignin which are bonded together by covalent bonding, various intermolecular bridges, and van der Waals forces forming
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a complex polymer (Bian, Peng, Xu, & Sun, 2010; Egüés, Sanchez, Mondragon, & Labidi, 2012).
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Therefore, for effective utilization of cellulose for conversion into high valued products,
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lignocellulose must be depolymerized to change or destroy the connection between constitutional
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units and thereby to increase the rate and conversion effect of enzymatic hydrolysis for
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subsequent preparation of cellulosic ethanol and other chemicals (Gullón, Romaní, Vila, Garrote,
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& Parajó, 2012; Ruiz, Rodríguez-Jasso, Fernandes, Vicente, & Teixeira, 2013). Recently, many
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pretreatment methods such as acid, alkali, steam explosion, liquid hot water, ammonia fiber
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explosion, and biological processes have been used for depolymerization of lignocellulose
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(Kaushik, & Singh, 2011; Karagöz, Rocha, Özkan, & Angelidaki, 2012). But no single
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pretreatment method was found to meet these requirements. Among these pretreatment methods,
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steam explosion, dilute acid, dilute alkali, and oxidative agent have been widely employed
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(McIntosh, & Vancov, 2011; Peng F, Peng P, Xu, & Sun, 2012). Steam explosion is characterized
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by little to no use of chemical reagents, efficient separation of cellulose, reduced environmental
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pollution, and relatively high energy consumption (Ranjan, & Moholkar, 2013; Oliveira et al.,
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2013). Acid pretreatment is more efficient in hemicellulose solubilization but high temperature,
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equipment corrosion, and formation of many inhibiting by-products, whereas alkaline
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pretreatment is more effective in removal of lignin but high cost of alkali and environmental
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pollution (Talebnia, Karakashev, & Angelidaki, 2010). Based on the advantages and drawbacks
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of each pretreatment method, the use of two or more methods together may be a more effective
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alternative. In this work, two different schemes were conducted to process corn stalks. (1) Corn stalks were pretreated by acid or alkali. (2) Corn stalks were pretreated by steam explosion under
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different pressure followed by acid or alkali. The change of structural feature and thermal
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stability of corn stalks before and after pretreatment were comparatively investigated using
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Fourier-transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), Scanning electron
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microscope (SEM), and Derivative thermo-gravimetric (DTG) analysis techniques. The data
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from this study may be used as the foundation to further optimize the method for straw
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pretreatment and to improve the efficiency of enzymatic hydrolysis.
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2. Materials and methods
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2.1. Materials
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The corn stalk sample used in this study was obtained from Yinchuan, China. The sample was
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harvested at the dough stage of maturity. The plant was separated into ears, leaves and stalks.
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The stalks were dried at 105 °C for 16 h, ground in a rotary mill and passed through of 80
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meshes in size which was used as the raw material in this study.
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2.2. Pretreatment
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2.2.1. Steam explosion.
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The steam explosion was carried out in a zipper-clave autoclave. For pretreatment, 10 g of
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corn stalk with soaking in water at solid-liquid ratio of 1:20 (wt/vol) was introduced, which was
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then heated to targeted pressure (0.4 MPa, 0.6 MPa, and 0.8 MPa) using steam, respectively.
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Pretreatment time (1 h) started to count upon the desired pressure inside the zipper-clave
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autoclave was reached. After the pretreatment was completed, the steam was release suddenly, 5
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the pretreated sample was collected and cool down, it was dried at 105 oC for 16 h and the
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samples were screened into powders 80 mesh in size for further experiments (Chundawat,
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Venkatesh, & Dale, 2007; Pedersen, & Meyer, 2009).
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2.2.2. Acid and alkali
The raw material was treated with 5% (wt/wt) NaOH or 5% (wt/wt) H2SO4, The
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concentrations of NaOH and H2SO4 were determined according to the results from our previous
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study. (Sun, Ma, Ma, & Chang, 2013; Zhou, Ma, Xie, & Cai, 2011). The assays were performed
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on a rotary thermostat heating shaker series DHSZ-300 with an agitation of 150 rpm at 60 °C for
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24 h. The solid-liquid ratio was 1:20 (wt/vol). After treatment, the mixture was filtrated by
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Buchner funnel and the solid product was washed to neutral by distilled water, and then stored in
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sealed plastic bags at 4 °C. The total sugar yield (TSY) in the liquid phase fraction was
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quantified by ultraviolet spectrophotometer (UVS) according to 3, 5-dinitrosalicylic acid (DNS)
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colorimetric method (Qi, Gou, Han, & Yan, 2004).
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The amounts of glucose, xylose, and arabinose in the liquid fraction were determined using a
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HPLC method with a refraction index detector (Shimadzu, Japan) and an Aminex® HPX-87H
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lon Exclusion column at 65 °C. Sulfuric acid (0.005 M) at flow rate of 0.6 mL/min was used as
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mobile phase, all samples were filtered through 0.22 micron filters prior to analysis, and then 20
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µL of sample was injected. The complete sample elution was accomplished within 20 min. The
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concentrations of oligosaccharides were also obtained by HPLC determination of
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monosaccharides present in liquors previously subjected to quantitative post-hydrolysis. The
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increase in the concentrations of monosaccharides caused by post-hydrolysis provided a measure
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of the oligosaccharides concentration (Vegas, Alonso, Domínguez, & Parajó, 2004). The contents 6
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of degradation products [mainly furfural and 5-hydroxymethyl furfural (5-HMF)] and
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by-products of phenolic acid [mainly ferulic acid (FA) and p-coumaric acid (P-CA)] were
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determined by a HPLC method using a diode array detector (Shimadzu, Japan) and an
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InertSustain® C18 column at 40 oC. Acetic acid (0.5 wt%) and methanol ratio of 9:1 (vol/vol) at
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flow rate of 1 mL/min was used as mobile phase, the detection wavelength was 280 nm and the
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injection volume was 20 µL.
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2.2.3. Combination of steam explosion and acid or alkali
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The corn stalk sample was first treated with steam explosion and followed by 5% (wt/wt)
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NaOH or 5% (wt/wt) H2SO4. The treatment condition was the same as described above.
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2.3 Enzymatic hydrolysis
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The residue of pretreated corn stalk was enzymatically hydrolyzed as follows. The reaction conditions were 50 oC, pH 4.8 (0.05 M sodium citrate buffer), 150 rpm in a shake flask. The
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solid-to-liquid ratio is 1:10. The commercial enzyme Celluclast 1.5 L with a filter paper activity
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of 74 FPU/mL and Novozym 188 with a β-glucosidase activity of 154 IU/g, were employed for
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the enzymatic hydrolysis experiments（Adney, & Baker, 1996）. The cellulase loading was set at
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20 FPU/g dry matter of the Celluclast 1.5 L, which was supplemented with 15 IU/g of Novozym
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188 β-glucosidase, the values were determined according to the results from the fundamental
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research. An aliquot of 0.5 mL hydrolyzate was taken at different time intervals (6 h, 12 h, 24 h,
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36 h, 48 h, 60 h, 72 h) before the enzymatic hydrolysis test was completed. Upon completion, the
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entire enzymatic hydrolysis sample was ended by boiling the hydrolyzate for 10 min. After
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cooling with cold water the hydrolyzate was filtered through a 0.22 μm filter. The HPLC method
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was used for monosaccharide and oligomers determination. The 3, 5-dinitrosalicylic acid (DNS)
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colorimetric method was used for total sugar determination.
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2.4. Composition analysis of corn stalk
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All the treated and untreated samples were milled into 0.2 mm (80 meshes) particles using the DFT-40 cutting mill. The contents of cellulose, hemicellulose, and lignin in the treated and
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untreated corn stalks were analyzed according to the procedures of the Van Soest method
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(Goering, & Van Soest, 1970). The optimum pretreatment was determined based on the relatively
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high ratio of cellulose and lignin contents.
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2.5. Derivative thermo-gravimetric analysis
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The pyrolysis of biomass components was carried out in a Derivative Thermal Gravimetric (DTG) Analysis (Setaram, France). To mitigate the difference of heat and mass transfer, the
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sample weight was kept at ≈ 8 mg. The tested corn stalk was heated up to 600 °C at a constant
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heating rate of 5 °C/min. Purified nitrogen at a flow rate of 20 mL/min was used as the carrier
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gas to provide an inert atmosphere for pyrolysis.
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2.6. FT-IR analysis
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The corn stalk powder samples were prepared with potassium bromide (KBr) pellet method.
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Infrared spectra were measured with a FT-IR 8400S at room temperature. Each spectrum was
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collected with a resolution of 4 cm−1 with 32 scans from the 4000 to 500 cm−1 region. Three
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replicated spectra were collected for every sample. The background spectrum was obtained
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against the air.
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2.7. X-ray diffraction and crystallinity measurement
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The crystallinity of the untreated and treated samples was examined by XRD technique using CuKα radiation (λ = 1.54 Å), equipped with computerized data collection and analytical tools. 8
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The scattering angle range was 5-60◦ and a rate of 4◦/min.
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2.8. Surface morphology observation Morphological characterization of corn stalk particles was performed on images acquired
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using a SEM method. The samples were coated with platinum or gold of 10 nm thicknesses to
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make the samples conductive.
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3. Results and discussion
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3.1. FT-IR spectroscopic analysis
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The distributions of functional group of corn stalk after treatment with steam explosion, acid,
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and alkaline, alone or in combination, are showed in Fig.1. Absorption bands at 3360 cm-1 and
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2890 cm-1 were attributed to -OH stretching of hydrogen bonds and C-H stretching within the
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methylene of cellulose, respectively. These absorption peaks in the untreated corn stalk were
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similar to that in the treated sample, indicating that most of the crystalline cellulose in corn stalk
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was not disrupted by the tested pretreatment. These results were in agreement with the
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investigations of Kumar and Wyman (2009) who reported that the crystalline cellulose in corn
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stover was not disrupted by acid hydrolysis. Furthermore, the prominent bands at 1250 - 900
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cm-1 were typically related to the structural features of cellulose and hemicelluloses. The
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vibrations of these bands overlapped the C-O stretching vibrations at 1254 cm-1, C-O-H
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stretching of primary and secondary alcohols at 1056 cm-1, C-O-C glycosidic bond stretching and
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C-O-C ring skeletal vibration at 1160-1100 cm-1 (Moharram & Mahmoud, 2008), the adsorption
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peaks of these bands were enhanced after treatment with steam explosion, acid, and alkaline,
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alone or in combination. This suggested that the cellulose content in pretreated solid residue
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increased because the hemicelluloses fraction in corn stalk was removed by hydrolytic reaction.
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These results were in agreement with the changes of chemical composition of corn stalk after
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treatment (see Table 1). This was further supported by the fact that the adsorption peaks at 1730
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and 1250 cm-1 were detectable in the untreated corn stalk and these have been proposed to be
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associated with the alkyl ester of the acetyl group in hemicelluloses. Furthermore, the ester
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linkage C=O with an absorption peak at 1720 cm-1 was usually defined as the acetyl group in
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hemicelluloses structure and/or the linkage between hemicelluloses and lignin (Chundawat,
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Venkatesh, & Dale, 2006). Both of these bands were absent in pretreated solid residue as a result
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of the removal of hemicelluloses from corn stalk. Interestingly, the band at 895 - 830 cm-1 was
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dominated by the β-(1 → 4) - glycosidic bond (C1-O-C4), which was cleaved by treatment with a
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combination of steam explosion and dilute acid or alkali-catalyzed reaction.
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The distribution of lignin-associated bands was also investigated. The bands at 1600-1510 cm-1 belonged to aromatic skeletal stretching. The bands at 2852, 1460 and 1425 cm-1 were
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attributed to absorption of -C-H deformation within the methoxyl groups of lignin (Hsu, Guo,
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Chen, & Hwang, 2010). These peaks were disappeared in FT-IR of corn stalks after treatment
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with the combined process of steam explosion and acid or alkaline (see Figs.1B and1C). But
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based on the efficient removal of lignin, the depolymerization effect of steam explosion and
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alkali was better than that of steam explosion and acid, suggesting that the release of acid-soluble
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lignin in pretreated solid residue occurred. Moreover, the adsorption peak due to aromatic
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hydroxyl groups was observed at 1300 cm-1. This peak might be generated by cleavage of ether
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bonds within the lignin. These results implied that the partial lignin was removed.
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A
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a
B a
80
120
b
100
c
60
Transmittance/%
Transmittance/%
b
40
c 80
d
60
40
20
20
3386
4000
3500
1730 15051254
2890 3000
2500
2000
1500
831 1000
0 4000
3413 3500
1255 17251510 1056 895
2907 3000
500
2500
2000
1500 -1
Wavenumber /cm
-1
Wavenumber /cm
a
an
b
80
c
60
d 40
M
Transmittance/%
us
205 C
20
3395 4000
3500
1500 1690 12431056 831
2902 3000
2500
2000
1500 -1
Wavenumber /cm
1000
500
d
0
500
(a) acid; (b) 0.4 MPa steam explosion + acid; (c) 0.6 MPa steam explosion + acid; (d) 0.8 MPa steam explosion + acid
(a) untreated; (b) acid; (c) alkali
100
1000
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100
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te
(a) alkali; (b) 0.4 MPa steam explosion + alkali; (c) 0.6 MPa steam explosion + alkali; (d) 0.8 MPa steam explosion + alkali
Fig.1. Infrared spectra of corn stalk before and after treatment with different methods. (A) Acid and alkali; (B)
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steam explosion + acid; (C) steam explosion + alkali.
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3.2. Characterization of the pretreated solid and liquid fraction
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The chemical compositions in corn stalk before and after treatment with the different methods
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is given in Table 1. The untreated corn stalks were composed of 31.54% cellulose, 29.10%
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hemicellulose, and 38.12% lignin. It was found that the cellulose yields of 60.51% and 76.15%
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were achieved when corn stalks were pretreated with H2SO4 and NaOH at 60 °C for 24 h,
214
respectively. It may be attributed to the removal of lignin, hemicellulose and other soluble
215
substance because the contents of hemicellulose and lignin in the treated stalk were lower than 11
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those in raw material. These results were in agreement with the investigation from FT-IR (see
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Fig.1).
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Table 1 The solid-liquid phase compositions in corn stalk before and after treatment with different methods.
5%
5%
stalk
H2SO4
NaOH
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Component content
Steam explosion + 5%
Steam explosion + 5%
Corn
H2SO4 (MPa)
NaOH (MPa)
0.4
0.6
0.8
65.16
31.54
60.51
76.15
50.16
54.51
Hemicellulose
29.10
7.02
6.86
14.61
11.13
11.15
Lignin
38.12
32.21
16.52
34.57
33.79
23.01
80.41
an
0.8
79.84
70.91
8.27
5.94
4.61
10.74
13.6
23.87
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Cellulose
Liquid phase fraction (mg/mL)
0.6
cr
Solid phase fraction (%)
0.4
/
6.83
1.86
5.78
5.42
4.96
2.14
1.89
2.19
Glucose
/
0.73
0.36
0.16
0.08
0.11
0.65
0.55
0.73
Xylose
/
3.79
0.98
3.49
3.15
3.27
1.19
1.05
1.23
Arabinose
/
0.70
0.09
0.64
0.47
0.43
0.12
0.10
0.06
/
1.54
Acetic acid
/
1.14
Furfural
/
c
/
0.38
1.41
1.67
1.10
0.14
0.15
0.11
0.51
1.12
1.08
0.81
0.86
1.37
0.72
0.069
0.077
0.059
0.065
0.047
0.120
0.086
0.054
0.013
0.026
0.0028
0.0041
0.0016
0.044
0.031
0.011
0.0612
0.428
0.051
0.088
0.068
0.863
0.357
0.245
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5-HMF
d
(XOS+GOS+AOS) b
te
Oligosaccharides
Phenolic acid
(FA + P-CA) d
/
M
TSY a
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a
TSY: Total sugar yield; b XOS: Xylooligosaccharides, GOS: Glucooligosaccharides, AOS:
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arabinooligosaccharides; c 5-HMF: 5-hydroxymethyl furfural; d FA: Ferulic acid, P-CA: P-coumaric acid.
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Table 1 showed that most of hemicellulose and lignin could be removed while treating corn
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stalk with a combination of steam explosion and acid or alkali. For example, 84.16% of
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hemicellulose and 71.83% of lignin in corn stalk were removed after treatment with 0.8 MPa
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steam explosion + alkali and 0.4 MPa steam explosion + alkali, respectively. Similarly, the
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collaborative approach of steam explosion and H2SO4 could also remove hemicellulose and 12
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lignin and thereby resulted in the increase of cellulose content in corn stalk. These results suggest
227
that steam explosion coupled with acid or alkali resulted in a certain depolymerization to the
228
molecular structure of corn stalk, but considering the removal of hemicellulose and lignin, the
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depolymerization effect of steam explosion and alkali is better than that of steam explosion and
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acid. Zhang, Fu, & Chen, (2012) reported that treatment corn stalk with steam explosion
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improved the conversion efficiency of lignocellulose to xylose. Generally, an increase in
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cellulose content and/or a decrease in hemicellulose and lignin contents could improve the
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catalytic efficiency of cellulase for lignocellulosic materials. Additionally, during pretreatment,
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corn stalk degrades into sugars and other chemicals (mainly acetic acid, furfural, 5-HMF, and
235
phenolic acid). Acetic acid was produced during degradation of hemicellulose and lignin (Qin,
236
Liu, Li, Dale, & Yuan, 2012), it was generated in the hydrolysis of acetyl groups in hemicellulose.
237
The partial glucose could be transformed into 5-HMF under acidic or alkaline condition.
238
However, the amounts of 5-HMF were very low (0.001-0.04 mg/mL) in liquid phase fraction in
239
this research. Two pentoses (xylose and arabinose) could be transformed into furfural. Table 1
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showed that the maximum concentration of furfural was 0.12 mg/mL. The by-products of
241
phenolic acid (mainly FA and P-AC) were detected in the liquid phase fraction of corn stalk after
242
treatment using steam explosion coupled with acid or alkali. These organic compounds were
243
mainly degraded from the lignin-carbohydrate-complex (LCC) and lignin. Therefore,
244
pretreatment influence can be partly evaluated by composition of the liquid phase fraction,
245
mainly composed of glucose, xylose, arabinose, degradation production and by-products. As
246
listed in Table 1, monosaccharide took the main part of recovered sugars in the liquid phase
247
fraction. With the increase of treating bursting pressure, the pentose (mainly xylose and
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arabinose) and acetic acid, normally generated from hemicellulose degradation, were detected.
249
The result showed that total sugar content changed from 1.86 mg/mL to 6.83 mg/mL with
250
different pretreatment methods. This offers a good account for the removal of hemicellulose with
251
the combination of steam explosion and acid/alkali.
The concentration of the total solubilized sugars (both oligomers and monomers) from
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biomass is an important indicator of the suitability of a pretreatment process. Table 1 depicts the
254
comparison of steam explosion coupled with acid or alkali hydrolysis of corn stalk for
255
production of the total solubilized sugars, it shows data concerning the conversion of
256
lignocellulose into their respective monomers and oligomers (mainly xylooligosaccharides
257
(XOS), glucooligosaccharides (GOS) and arabinooligosaccharides (AOS). It was found that a
258
difference in amount of oligomers and monomers produced through steam explosion coupled
259
with acid or alkali pretreatment was described comparatively. The result indicates the effect of
260
steam explosion + acid pretreatment in contrast to steam explosion + alkali under the same
261
experiment conditions (0.4-0.8 MPa). As observed, there were obtained the higher quantities of
262
monomers (1.4-5.2 mg/mL) than oligomers (0.1-1.6 mg/mL) in steam explosion coupled with
263
acid or alkali pretreatment process, however, the difference was much bigger in steam explosion
264
+ acid pretreatment, which was obtained the higher concentration of oligomers (1.1-1.6 mg/mL)
265
than that of steam explosion + acid pretreatment (0.1-0.3 mg/mL). The combined methods with
266
steam explosion + acid are more effective than steam explosion + alkali hydrolysis for
267
production of the total solubilized sugars.
268
3.3 Effect of pretreatment on enzyme hydrolysis
269
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The enzyme hydrolysis of the treated sample under different conditions is summarized in 14
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Table 2. The result showed that enzymatic hydrolysis sugar contents were improved by steam
271
explosion coupled with acid or alkali pretreatments. The higher total sugar contents (16 - 40
272
mg/mL) was obtained in the alkali or steam explosion + alkali treatment samples compared with
273
the acid treatment samples for the enzymatic hydrolysis within 72 h. The lower total sugar
274
contents of the treated sample by H2SO4 or steam explosion + acid can be observed from 7
275
mg/mL to 17 mg/mL with the enzymatic hydrolysis time from 6 h to 72 h., It is because proper
276
hemicellulose and lignin were removed under alkali/acid pretreatment condition, and also the
277
physicochemical properties of corn stalk was interrupted. Additionally, increasing the
278
pretreatment pressure resulted in a higher removal rate of hemicellulose and lignin since this
279
released more sugar from the conversion of hemicellulose and cellulose.
280
Table 2 The enzyme hydrolysis of the treated sample under different conditions.
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270
d
Total sugar and glucose contents in the enzymatic hydrolysis a (mg/mL) 6h
5% H2SO4
8.69 (2.34) 16.64 (5.28) 7.29 (2.32) 8.12 (2.11) 7.05 (1.90) 16.54 (5.69) 17.29 (5.09) 16.71 (5.42)
12 h
24 h
36 h
48 h
60 h
72 h
10.21 (3.12) 24.14 (7.02) 9.92 (3.31) 9.30 (3.10) 9.14 (2.72) 22.94 (7.54) 16.43 (6.93) 19.85 (6.80)
13.00 (3.34) 28.66 (7.66) 11.60 (3.57) 11.85 (3.64) 10.21 (3.12) 28.17 (8.31) 28.00 (7.87) 27.67 (7.21)
15.07 (4.19) 31.38 (8.15) 17.37 (3.86) 14.65 (3.65) 16.88 (3.50) 32.45 (9.76) 35.83 (9.56) 35.83 (9.13)
16.44 (4.09) 41.11 (9.77) 17.46 (4.35) 17.21 (4.22) 19.93 (3.78) 42.67 (10.22) 40.20 (10.82) 39.44 (9.83)
17.44 (4.28) 38.49 (7.77) 19.60 (4.61) 17.78 (4.14) 18.11 (3.85) 39.52 (10.31) 37.25 (9.58) 37.66 (3.79)
17.89 (3.96) 39.27 (9.14) 17.33 (4.65) 15.82 (3.54) 16.31 (3.82) 39.31 (9.39) 38.37 (9.46) 39.45 (8.72)
Ac ce p
te
Treatment method
5% NaOH
0.4 MPa Steam explosion + 5% H2SO4 0.6 MPa Steam explosion + 5% H2SO4 0.8 MPa Steam explosion + 5% H2SO4 0.4 MPa Steam explosion + 5% NaOH 0.6 MPa Steam explosion + 5% NaOH 0.8 MPa Steam explosion + 5% NaOH
281
a
The value in brackets express glucose content in enzymatic hydrolysis process (mg/mL). 15
Page 15 of 26
282
3.4. X-ray diffraction analysis The X-ray diffraction patterns of the treated and untreated corn stalks are depicted in Fig.2.
284
The intensities of the amorphous region at 2θ = 18° and the crystalline region at 2θ = 22° were
285
used to calculate the crystallinity index (CrI) (Kim, & Holtzapple, 2006).
286
CrI = [intensity at 2θ = 22° intensity at 2θ =18°] / [intensity at 2θ=22°]
ip t
283
cr
(1)
As shown in Fig.2, the diffraction peak of the untreated corn stalk at (002) was wide and
288
round. However, this peak became sharper and narrower when the tested material was treated
289
with acid or alkaline. In addition, a shoulder peak at (101) and a weak peak at (040) appeared
290
when corn stalks were treated with acid and alkaline. These peaks are assigned to the cellulose
291
phase, suggesting the removal of most of lignin and hemicellulose from corn stalk. By a
292
calculation of Eq. (1), CrI values of corn stalk after treatment with H2SO4 and NaOH increased
293
to 63.5% and 67.0% as compared with 51.8% of the untreated stalk, respectively. The reason
294
might be that the amorphous hemicellulose was hydrolyzed while crystalline structure was
295
change thus increasing of crystallinity index in biomass. Moreover, the swelling ability of alkali
296
to the amorphous region of lignocellulose was relatively strong as compared to that of acid and
297
resulted in the increase of the dissolution rate of lignin and hemicellulose. Zheng, et al. (2012)
298
reported that the crystallinity, porosity and inner surface area of corn stalk were increased when
299
the ordered rearrangement and the secondary crystallization were proceeding in the
300
non-crystallized portion and the imperfect part of cellulose crystal structure.
Ac ce p
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d
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us
287
16
Page 16 of 26
Intensity /a.u.
(002)
I002
Iam
ip t
(101)
(040)
c
cr
b a
301
20
30
40
50
60
70
us
10
2-theta (degree)
Fig.2. X-ray diffraction pattern of untreated and treated corn stalk: (a) corn stalk; (b) acid; (c) alkali.
303
3.5. The thermal stability analysis
an
302
DTG curve was usually used to reflect the degradation of biomass. Generally, the
305
decomposition reaction of lignocellulose is divided into three processes including (1)
306
dehydration and drying stage in 25-250 °C, (2) the pyrolysis stage in 250-500 °C, and (3) the
307
depth of the pyrolysis stage in 500-600 °C. In this paper, the pyrolysis characteristics and DTG
308
curves of corn stalk before and after treatment with the tested methods are given in Table 3 and
309
Fig.3, respectively. As can be noted, the DTG curves contain a strong peak, which is attributed to
310
the primary pyrolysis. This result is consistent with that of other studies on biomass materials
311
(Jiang, Nowakowski, & Bridgwater, 2010). Physical and chemical pretreatment had little effect
312
on the shape of DTG curves. The maximum weight loss rate and pyrolysis temperature of corn
313
stalks after treatment with steam explosion, acid, and alkaline, alone or in combination, were
314
higher than those of the untreated material.
Ac ce p
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304
315
When temperature was lower than 250 °C, the weight loss of corn stalk was mainly attributed
316
to the removal of absorbed water and crystal water, the dehydration peak and drying curves were 17
Page 17 of 26
disappeared with the combined pretreatment using acid or alkali and steam explosion (Figs. 3B
318
and 3C). This was also supported by the fact that the heat absorption and weight loss rate of
319
dehydration and drying stage were changed in untreated and treated corn stalks (see Table 3).
320
With the increase of temperature (>250 °C), the DTG profiles of the treated and untreated
ip t
317
displayed a significant difference. Although the shapes of DTG curve did not exhibited a
322
significant change under different pretreatment condition, the peaks of weight loss shifted
323
towards the high temperature region. Because treatment corn stalk with acid or alkali could
324
remove most of hemicelluloses and some lignin, the temperature of the maximum degradation
325
rate shifted from 332 °C to 365 and 357 °C at 5 °C/min and the maximum weight loss rate from
326
41.5% to 81.8% and 81.0%, respectively. These results on the lateral shift to higher temperatures
327
for the maximum region of weight loss are in agreement with those from other investigators on
328
polypropylene, oil shale and their mixture (Deng, Zhang, & Wang, 2008). It also could be
329
concluded that lower weight loss rate of dehydration stage and lower heat absorption of pyrolysis
330
stage were obtained when corn stalks were treated with steam explosion coupled with acid or
331
alkali as compared to those of acid or alkaline treatment alone. The main reason of this was
332
related to partial removal of hemicelluloses and lignin from the corn stalk and higher crystallinity
333
of cellulose. These were consistent with the results obtained from XRD and FT-IR measurements
334
(see Figs.1 and 2).
Ac ce p
te
d
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an
us
cr
321
335 336 337 338 18
Page 18 of 26
Table 3 Pyrolysis characteristics of corn stalk before and after treatment with different methods. dehydration and drying stage
Pyrolysis stage
Treatment method
Heat absorption (J/g)
Weight loss rate (%)
Heat absorption (J/g)
Weight loss rate of stage 1 (%)
Weight loss rate of stage 2 (%)
Untreated
113.37
29.62
241.94
35.11
6.44
332.11
5% H2SO4
53.26
0.21
195.90
71.80
10.06
365.56
5% NaOH
92.18
3.14
205.69
70.86
10.24
357.05
52.16
1.63
118.94
50.66
2.16
161.85
0.8 MPa Steam explosion + 5% H2SO4
55.41
0.74
87.47
0.4 MPa Steam explosion + 5% NaOH
75.56
2.08
0.6 MPa Steam explosion + 5% NaOH
62.63
2.15
0.8 MPa Steam explosion + 5% NaOH
72.17
1.58
us
cr
ip t
Pyrolysis temperature (°C)
—
348.82
69.21
8.18
354.35
69.28
—
339.14
225.09
89.31
—
356.08
192.51
66.62
10.58
357.89
114.79
69.99
—
354.59
M
an
73.55
In thermal analysis, the pyrolysis of hemicelluloses and cellulose in the untreated and treated
Ac ce p
340
d
0.4 MPa Steam explosion + 5% H2SO4 0.6 MPa Steam explosion + 5% H2SO4
te
339
341
corn stalks occurred quickly at about 225-325 °C, which is attributed to thermal
342
depolymerisation of hemicelluloses or pectin. The major decomposition peak at about 300-430
343
°C was attributed to cellulose decomposition. However, lignin was more difficult to decompose,
344
as its weight loss happened in a wide temperature range from 250-500 °C (Chen, Yu, Zhang, &
345
Lu, 2011; Kaushik, & Singh, 2011). Additionally, Fig.3 showed that the pyrolysis temperature
346
and thermal stability of corn stalk were enhanced after treatment with the combined method of
347
steam explosion and acid or alkaline due to the removal of most of hemicellulose and lignin.
348
These results suggest that dilute acid and dilute alkali lead to a certain depolymerization to the 19
Page 19 of 26
349
molecular of corn stalk, but the depolymerization effect of dilute alkali is better than that of
350
dilute acid in terms of delignification.
0.1
A
0.0
B
0.0
c
ip t
0.1
-0.1
-0.4
cr
a -0.3
-0.2 -0.3
c
-0.4
b
-0.5
us
-0.2
a
-0.5
-0.6
128 100
314 352 200
300
-0.7
330 359
100
400
500
600
o
temperature / C
200
an
-0.6
300
400
500
600
o
temperature / C
(a) 0.4 MPa steam explosion + acid; (b) 0.6 MPa steam explosion +acid; (c) 0.8 MPa steam explosion + acid; (d) acid
(a) alkali; (b) acid; (c) untreated
M
351 0.2
d
b
DTG /mg·min-1
DTG /mg·min-1
-0.1
C
d
0.0
d
-0.6
a
Ac ce p
DTG /mg·min-1
b -0.4
te
-0.2
-0.8
c
-1.0
352
-1.2
100
200
300
400
500
600
o
temperature / C
(a) 0.4 MPa steam explosion + alkali; (b) 0.6 MPa steam explosion + alkali; (c) 0.8 MPa steam explosion + alkali; (d) alkali
352 353
Fig.3. DTG curves of corn stalk before and after treatment with different methods. (A) Acid and alkali; (B)
354
steam explosion + acid; (C) steam explosion + alkali.
355
3.6. Scanning electron microscopy
356
20
Page 20 of 26
C
D
E
F
G
H
an
us
357
ip t
B
cr
A
358
te
d
M
I
359
Fig.4. SEM images of corn stalk before and after treatment with different methods. (A) Untreated; (B) acid; (C)
361
Alkali; (D) 0.4 MPa steam explosion + acid; (E) 0.6 MPa steam explosion + acid; (F) 0.8 MPa steam explosion
362
+ acid; (G) 0.4 MPa steam explosion + alkali; (H) 0.6 MPa steam explosion + alkali; (I) 0.8 MPa steam
363
explosion + alkali.
Ac ce p
360
364
SEM micrographs of the untreated and treated rice straw fibers were shown in Fig.4. The
365
untreated sample showed an even and smooth flat surface, indicating a rigid and highly ordered
366
surface structure (see Fig. 4A). However, the nature surface was broken into some large
367
fragments and appeared porous structure and rift after treatment with alkaline or acid (see Figs.
368
4B and 4C). When corn stalks were treated with acid and steam explosion under different 21
Page 21 of 26
pressure, the treated sample had a rugged, rough, loose, and broken surface (see Figs. 4D - 4F). It
370
was clearly found that morphologies of the treated corn stalks under different conditions were
371
also obviously different. For example, the surface of raw samples by 0.8 MPa steam explosion +
372
alkali looked more roughness than that by 0.4 MPa or 0.6 MPa steam explosion + alkali (see
373
Figs.4G - 4I). The reason is that lignin and hemicellulose of interwoven wrapped cellulose
374
crystalline structure was broken and weakened the mutually binding force between holocellulose
375
and lignin by the pretreatment with different burst pressure in acid-alkali. This would obviously
376
favor the catalyst contacting the inner linkage, hence accelerating the biodegradation process.
377
4. Conclusions
cr
us
an
Treatment with steam explosion and alkali could remove most of hemicellulose (84.16%) and
M
378
ip t
369
lignin (71.83%) in corn stalk as compared to those of the untreated stalk. The surface
380
morphology become loose and the thermal stability increased when corn stalks were
381
depolymerized with steam explosion and acid or alkali. These results suggested that steam
382
explosion coupled with acid or alkali could lead to a depolymerization of the molecular structure
383
of corn stalk, but in consideration of removal of hemicellulose and lignin, the depolymerization
384
effect of steam explosion and alkali was better than that of steam explosion and acid.
385
Acknowledgement
te
Ac ce p
386
d
379
The authors would like to thank the National Natural Science Foundation of China (NSFC) for
387
financial support (NO. 21266023).
388
References
389
Adney, B., Baker, J. (1996). Measurement of cellulase activities. In: NREL Analytical Procedure,
390 391
National Renewable Energy Laboratory Golden, Co., NREL/TP-510-42628. Bian, J., Peng, F., Xu, F., Sun, R.C., Kennedy, J.F. (2010). Fractional isolation and structural 22
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characterization of hemicelluloses from Caragana korshinskii. Carbohydrate Polymers, 80,
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753-760. Chen, X.L., Yu, J., Zhang, Z.B., Lu C.H. (2011). Study on structure and thermal stability
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properties of cellulose fibers from rice straw. Carbohydrate Polymers, 85, 245-250.
ip t
394
Chundawat, S.P.S., Venkatesh, B., Dale B.E. (2007). Effect of particle size based separation of
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milled corn stover on AFEX pretreatment and enzymatic digestibility. Biotechnology and
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Bioengineering, 96, 219-231.
Deng, N., Zhang, Y.F, Wang, Y. (2008). Thermogravimetric analysis and kinetic study on
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pyrolysis of representative medical waste composition. Waste Management, 28, 1572-1580.
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Egüés, I., Sanchez, C., Mondragon, I., Labidi, J. (2012). Effect of alkaline and autohydrolysis
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processes on the purity of obtained hemicelluloses from corn stalks. Bioresource Technology,
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treatments in lignocellulose biorefineries. Biofuels, Bioproducts and Biorefining, 6, 219-232. Goering, H.K., Van Soest, P.J. (1970). Forage Fiber Analysis; USDA-ARS Agriculture
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Gullón, P., Romaní, A., Vila, C., Garrote, G., Parajó, J.C. (2012). Potential of hydrothermal
Handbook 379; U.S. Government Printing Office: Washington, DC, 12-20.
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Hsu, T.C., Guo, G.L., Chen, W.H., Hwang, W.S. (2010). Effect of dilute acid pretreatment of rice
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straw on structural properties and enzymatic hydrolysis. Bioresource Technology. 101,
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4907-4913.
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Jiang, G.Z, Nowakowski, D.J., Bridgwater, A.V. (2010). A systematic study of the kinetics of lignin pyrolysis. Thermochimica Acta, 498, 61-66.
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Karagöz, P., Rocha, I.V., Özkan, M., Angelidaki, I. (2012). Alkaline peroxide pretreatment of
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rapeseed straw for enhancing bioethanol production by Same Vessel Saccharification and
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Co-Fermentation, Bioresource Technology. 104, 349-357.
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Kaushik, A., Singh, M. (2011). Isolation and characterization of cellulose nanofibrils from wheat 23
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straw using steam explosion coupled with high shear homogenization. Carbohydrate Research,
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346, 76-85. Kumar, R., Wyman, C.E. (2009). Cellulase adsorption and relationship to features of corn stover
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solids produced by leading pretreatments. Biotechnology and Bioengineering, 103, 252-267.
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stover. Bioresource Technology, 97, 583-591.
cr
422
Kim, S., Holtzapple, M.T. (2006). Effect of structural features on enzyme digestibility of corn
McIntosh, S., Vancov, T. (2011). Optimisation of dilute alkaline pretreatment for enzymatic saccharification of wheat straw. Biomass & Bioenergy, 35, 3094-3103.
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Moharram, M.A., Mahmoud, O.M. (2008). FTIR spectroscopic study of the effect of microwave
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heating on the transformation of cellulose I into cellulose II during mercerization. Journal of
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Applied Polymer Science, 107, 30-36.
an
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Oliveira, F.M.V., Pinheiro I.O., Souto-Maior A.M., Martin C., Goncalves A.R., Rocha G.J.M.
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(2013). Industrial-scale steam explosion pretreatment of sugarcane straw for enzymatic
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hydrolysis of cellulose for production of second generation ethanol and value-added products.
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Bioresource Technology. 130, 168-173.
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Peng, F., Peng, P., Xu, F., Sun, R.C. (2012). Fractional purification and bioconversion of
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hemicelluloses. Biotechnology Advances, 30, 879-903. Pedersen, M., Meyer, A.S. (2009). Influence of substrate particle size and wet oxidation on
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physical surface structures and enzymatic hydrolysis of wheat straw. Biotechnology Progress,
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25(2), 399-408.
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Qin, L., Liu, Z.H., Li, B.Z., Dale, B.E., Yuan, Y.J. (2012). Mass balance and transformation of
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corn stover by pretreatment with different dilute organic acids. Bioresource Technology, 112,
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319-326.
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Qi, X.J., Gou, J.X., Han, X.J., Yan, B. 2004. Study on measuring reducing sugar by DNS reagent. Journal of Cellulose Science Technology, 12, 17-19 (In Chinese). 24
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Ranjan, A., Moholkar V.S. (2013). Comparative study of various pretreatment techniques for rice straw saccharification for the production of alcoholic biofuels. Fuel, 112, 567-571. Ruiz, H.A., Rodríguez-Jasso, M.A., Fernandes, B.D., Vicente, A.A., Teixeira, J.A. (2013). Hydrothermal processing, as an alternative for upgrading agriculture residues and marine
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biomass according to the biorefinery concept: A review. Renewable and Sustainable Energy
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Reviews, 21, 35-51.
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Sun, Y.G., Ma, Y.L., Ma, X.X., Chang, X. (2013). Experimental study on the molecular structure of corn stalk after depolymerization with acid or alkali. Chemistry, 76(7), 645-649 (In
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Chinese).
us
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Talebnia, F., Karakashev, D., Angelidaki, I. (2010). Production of bioethanol from wheat straw:
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An overview on pretreatment, hydrolysis and fermentation. Bioresource. Technology, 101,
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4744-4753.
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Vegas, R., Alonso, J.L., Domínguez, H., Parajó, J.C. (2004). Processing of rice husk autohydrolysis liquors for obtaining food ingredients. Journal of Agricultural and Food
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Chemistry, 52, 7311-7317.
te
Zhang, Y.Z., Fu, X.G., Chen, H.G. (2012). Pretreatment based on two-step steam explosion
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d
455
458
combined with an intermediate separation of fiber cells-optimization of fermentation of corn
459
straw hydrolysates. Bioresource Technology, 121, 100-104.
460
Zheng, M.X., Li, L.Q., Zheng, M.Y., Wang, X., Ma, H.L., et al. (2012). Effect of alkali
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pretreatment on cellulosic structural changes of corn stover. Environmental Science
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Technology, 35, 27-31.
463
Zhou, D.F., Ma, Y.L., Xie, L., Cai, Y. (2011). Experimental study on pretreatment of maize stalk
464
cellulose with H2O2 and alkaline. Renewable Energy Resources, 29(1), 19-22 (In Chinese).
25
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465
Figure captions
466
Figure 1.
471 472
ip t
470
X-ray diffraction pattern of untreated and treated corn stalk: (a) Corn stalk; (b) acid;
(c) alkali.
Figure 3. DTG curves of corn stalk before and after treatment with different methods. (A) Acid
cr
469
Figure 2.
and alkali; (B) steam explosion + acid; (C) steam explosion + alkali. Figure 4.
us
468
Acid and alkali; (B) steam explosion + acid; (C) steam explosion + alkali.
SEM images of corn stalk before and after treatment with different methods. (A)
an
467
Infrared spectra of corn stalk before and after treatment with different methods. (A)
Untreated; (B) acid; (C) Alkali; (D) 0.4 MPa steam explosion + acid; (E) 0.6 MPa steam
474
explosion + acid; (F) 0.8 MPa steam explosion + acid; (G) 0.4 MPa steam explosion +
475
alkali; (H) 0.6 MPa steam explosion + alkali; (I) 0.8 MPa steam explosion + alkali.
Ac ce p
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S0144-8617(14)00976-X http://dx.doi.org/doi:10.1016/j.carbpol.2014.09.066 CARP 9324
To appear in: Received date: Revised date: Accepted date:
19-10-2013 9-9-2014 18-9-2014
Please cite this article as: Sun, Y.-G., Ma, Y.-L., Wang, L.-Q., Wang, F.-Z., Wu, Q.-Q., and Pan, G.-Y.,Physicochemical properties of corn stalk after treatment using steam explosion coupled with acid or alkali, Carbohydrate Polymers (2014), http://dx.doi.org/10.1016/j.carbpol.2014.09.066 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1
Physicochemical properties of corn stalk after treatment using steam
2
explosion coupled with acid or alkali
3 Yong-Gang Suna,b, Yu-Long Maa,b,*, Li-Qiong Wanga, Feng-Zhi Wangb, Qian-Qian Wub,
5
Guan-Yu Panb
cr
ip t
4
6 7
a
8
University, Yinchuan 750021, China
9
b
an
us
State Key Laboratory Cultivation Base of Energy Sources and Chemical Engineering, Ningxia
College of Chemistry and Chemical Engineering, Ningxia University, Yinchuan 750021, China
*Corresponding author:
11
Yu-Long Ma
12
State Key Laboratory Cultivation Base of Energy Sources and Chemical Engineering
13
Ningxia University
14
Helanshan Rd. 539
15
Yinchuan 750021
16
China
17
Tel: +86 951 2062380; Fax: +86 951 2062323
18
E-mail: [email protected] (Yu-Long Ma)
19
E-mail: [email protected] (Yong-Gang Sun); [email protected] (Li-Qiong
20
Wang); [email protected] (Feng-Zhi Wang); [email protected] (Qian-Qian Wu);
21
[email protected] (Guan-Yu Pan).
Ac ce p
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1
Page 1 of 26
22 23 24
Highlights:
26
Steam explosion coupled with alkali was a better method for corn stalk pretreatment.
27
This method could remove most of hemicellulose and lignin in corn stalk.
28
This method changed the bonding distribution and structural feature of corn stalk.
29
This method increased the crystallinity and thermal stability of corn stalk.
an
us
cr
ip t
25
30
M
31 Abstract
33
The aim of this study was to evaluate comparatively the effects of different pretreatments
34
including steam explosion, acid, and alkali, alone or in combination, on the structural properties
35
and thermal stability of corn stalk. All of the treated treatments decreased the contents of
36
hemicellulose and lignin and thereby increased the content of cellulose in corn stalks. But the
37
combined treatments with alkali and steam explosion under 0.4-0.6 MPa were better as
38
compared with other treatments based on the removals of hemicellulose and lignin, and about
39
71.58-79.59% of hemicellulose and 64.32-71.83% of lignin were removed. Treatment with steam
40
explosion coupled with acid or alkali changed the bonding distribution and surface morphology
41
and increased the crystallinity and thermal stability of corn stalks, and the degradation
42
temperature reached over 350 °C. These results suggest that steam explosion coupled with alkali
43
is a better method for the depolymerization of corn stalk polymer.
Ac ce p
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32
2
Page 2 of 26
44
Keywords: Corn stalk; Depolymerization; Cellulose; Thermal stability; Crystalline
45 46
ip t
47
cr
48 49
us
50
an
51
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3
Page 3 of 26
53 54
1. Introduction Lignocellulose mainly consists of cellulose, hemicelluloses and lignin which are bonded together by covalent bonding, various intermolecular bridges, and van der Waals forces forming
56
a complex polymer (Bian, Peng, Xu, & Sun, 2010; Egüés, Sanchez, Mondragon, & Labidi, 2012).
57
Therefore, for effective utilization of cellulose for conversion into high valued products,
58
lignocellulose must be depolymerized to change or destroy the connection between constitutional
59
units and thereby to increase the rate and conversion effect of enzymatic hydrolysis for
60
subsequent preparation of cellulosic ethanol and other chemicals (Gullón, Romaní, Vila, Garrote,
61
& Parajó, 2012; Ruiz, Rodríguez-Jasso, Fernandes, Vicente, & Teixeira, 2013). Recently, many
62
pretreatment methods such as acid, alkali, steam explosion, liquid hot water, ammonia fiber
63
explosion, and biological processes have been used for depolymerization of lignocellulose
64
(Kaushik, & Singh, 2011; Karagöz, Rocha, Özkan, & Angelidaki, 2012). But no single
65
pretreatment method was found to meet these requirements. Among these pretreatment methods,
66
steam explosion, dilute acid, dilute alkali, and oxidative agent have been widely employed
67
(McIntosh, & Vancov, 2011; Peng F, Peng P, Xu, & Sun, 2012). Steam explosion is characterized
68
by little to no use of chemical reagents, efficient separation of cellulose, reduced environmental
69
pollution, and relatively high energy consumption (Ranjan, & Moholkar, 2013; Oliveira et al.,
70
2013). Acid pretreatment is more efficient in hemicellulose solubilization but high temperature,
71
equipment corrosion, and formation of many inhibiting by-products, whereas alkaline
72
pretreatment is more effective in removal of lignin but high cost of alkali and environmental
73
pollution (Talebnia, Karakashev, & Angelidaki, 2010). Based on the advantages and drawbacks
74
of each pretreatment method, the use of two or more methods together may be a more effective
Ac ce p
te
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55
4
Page 4 of 26
75 76
alternative. In this work, two different schemes were conducted to process corn stalks. (1) Corn stalks were pretreated by acid or alkali. (2) Corn stalks were pretreated by steam explosion under
78
different pressure followed by acid or alkali. The change of structural feature and thermal
79
stability of corn stalks before and after pretreatment were comparatively investigated using
80
Fourier-transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), Scanning electron
81
microscope (SEM), and Derivative thermo-gravimetric (DTG) analysis techniques. The data
82
from this study may be used as the foundation to further optimize the method for straw
83
pretreatment and to improve the efficiency of enzymatic hydrolysis.
84
2. Materials and methods
85
2.1. Materials
cr
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an
M
The corn stalk sample used in this study was obtained from Yinchuan, China. The sample was
d
86
ip t
77
harvested at the dough stage of maturity. The plant was separated into ears, leaves and stalks.
88
The stalks were dried at 105 °C for 16 h, ground in a rotary mill and passed through of 80
89
meshes in size which was used as the raw material in this study.
90
2.2. Pretreatment
91
2.2.1. Steam explosion.
Ac ce p
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87
92
The steam explosion was carried out in a zipper-clave autoclave. For pretreatment, 10 g of
93
corn stalk with soaking in water at solid-liquid ratio of 1:20 (wt/vol) was introduced, which was
94
then heated to targeted pressure (0.4 MPa, 0.6 MPa, and 0.8 MPa) using steam, respectively.
95
Pretreatment time (1 h) started to count upon the desired pressure inside the zipper-clave
96
autoclave was reached. After the pretreatment was completed, the steam was release suddenly, 5
Page 5 of 26
97
the pretreated sample was collected and cool down, it was dried at 105 oC for 16 h and the
98
samples were screened into powders 80 mesh in size for further experiments (Chundawat,
99
Venkatesh, & Dale, 2007; Pedersen, & Meyer, 2009).
ip t
101
2.2.2. Acid and alkali
The raw material was treated with 5% (wt/wt) NaOH or 5% (wt/wt) H2SO4, The
cr
100
concentrations of NaOH and H2SO4 were determined according to the results from our previous
103
study. (Sun, Ma, Ma, & Chang, 2013; Zhou, Ma, Xie, & Cai, 2011). The assays were performed
104
on a rotary thermostat heating shaker series DHSZ-300 with an agitation of 150 rpm at 60 °C for
105
24 h. The solid-liquid ratio was 1:20 (wt/vol). After treatment, the mixture was filtrated by
106
Buchner funnel and the solid product was washed to neutral by distilled water, and then stored in
107
sealed plastic bags at 4 °C. The total sugar yield (TSY) in the liquid phase fraction was
108
quantified by ultraviolet spectrophotometer (UVS) according to 3, 5-dinitrosalicylic acid (DNS)
109
colorimetric method (Qi, Gou, Han, & Yan, 2004).
an
M
d
te
The amounts of glucose, xylose, and arabinose in the liquid fraction were determined using a
Ac ce p
110
us
102
111
HPLC method with a refraction index detector (Shimadzu, Japan) and an Aminex® HPX-87H
112
lon Exclusion column at 65 °C. Sulfuric acid (0.005 M) at flow rate of 0.6 mL/min was used as
113
mobile phase, all samples were filtered through 0.22 micron filters prior to analysis, and then 20
114
µL of sample was injected. The complete sample elution was accomplished within 20 min. The
115
concentrations of oligosaccharides were also obtained by HPLC determination of
116
monosaccharides present in liquors previously subjected to quantitative post-hydrolysis. The
117
increase in the concentrations of monosaccharides caused by post-hydrolysis provided a measure
118
of the oligosaccharides concentration (Vegas, Alonso, Domínguez, & Parajó, 2004). The contents 6
Page 6 of 26
of degradation products [mainly furfural and 5-hydroxymethyl furfural (5-HMF)] and
120
by-products of phenolic acid [mainly ferulic acid (FA) and p-coumaric acid (P-CA)] were
121
determined by a HPLC method using a diode array detector (Shimadzu, Japan) and an
122
InertSustain® C18 column at 40 oC. Acetic acid (0.5 wt%) and methanol ratio of 9:1 (vol/vol) at
123
flow rate of 1 mL/min was used as mobile phase, the detection wavelength was 280 nm and the
124
injection volume was 20 µL.
125
2.2.3. Combination of steam explosion and acid or alkali
cr us
The corn stalk sample was first treated with steam explosion and followed by 5% (wt/wt)
an
126
ip t
119
NaOH or 5% (wt/wt) H2SO4. The treatment condition was the same as described above.
128
2.3 Enzymatic hydrolysis
129
M
127
The residue of pretreated corn stalk was enzymatically hydrolyzed as follows. The reaction conditions were 50 oC, pH 4.8 (0.05 M sodium citrate buffer), 150 rpm in a shake flask. The
131
solid-to-liquid ratio is 1:10. The commercial enzyme Celluclast 1.5 L with a filter paper activity
132
of 74 FPU/mL and Novozym 188 with a β-glucosidase activity of 154 IU/g, were employed for
133
the enzymatic hydrolysis experiments（Adney, & Baker, 1996）. The cellulase loading was set at
134
20 FPU/g dry matter of the Celluclast 1.5 L, which was supplemented with 15 IU/g of Novozym
135
188 β-glucosidase, the values were determined according to the results from the fundamental
136
research. An aliquot of 0.5 mL hydrolyzate was taken at different time intervals (6 h, 12 h, 24 h,
137
36 h, 48 h, 60 h, 72 h) before the enzymatic hydrolysis test was completed. Upon completion, the
138
entire enzymatic hydrolysis sample was ended by boiling the hydrolyzate for 10 min. After
139
cooling with cold water the hydrolyzate was filtered through a 0.22 μm filter. The HPLC method
140
was used for monosaccharide and oligomers determination. The 3, 5-dinitrosalicylic acid (DNS)
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130
7
Page 7 of 26
141
colorimetric method was used for total sugar determination.
142
2.4. Composition analysis of corn stalk
143
All the treated and untreated samples were milled into 0.2 mm (80 meshes) particles using the DFT-40 cutting mill. The contents of cellulose, hemicellulose, and lignin in the treated and
145
untreated corn stalks were analyzed according to the procedures of the Van Soest method
146
(Goering, & Van Soest, 1970). The optimum pretreatment was determined based on the relatively
147
high ratio of cellulose and lignin contents.
148
2.5. Derivative thermo-gravimetric analysis
cr
us an
149
ip t
144
The pyrolysis of biomass components was carried out in a Derivative Thermal Gravimetric (DTG) Analysis (Setaram, France). To mitigate the difference of heat and mass transfer, the
151
sample weight was kept at ≈ 8 mg. The tested corn stalk was heated up to 600 °C at a constant
152
heating rate of 5 °C/min. Purified nitrogen at a flow rate of 20 mL/min was used as the carrier
153
gas to provide an inert atmosphere for pyrolysis.
154
2.6. FT-IR analysis
d
te
Ac ce p
155
M
150
The corn stalk powder samples were prepared with potassium bromide (KBr) pellet method.
156
Infrared spectra were measured with a FT-IR 8400S at room temperature. Each spectrum was
157
collected with a resolution of 4 cm−1 with 32 scans from the 4000 to 500 cm−1 region. Three
158
replicated spectra were collected for every sample. The background spectrum was obtained
159
against the air.
160
2.7. X-ray diffraction and crystallinity measurement
161 162
The crystallinity of the untreated and treated samples was examined by XRD technique using CuKα radiation (λ = 1.54 Å), equipped with computerized data collection and analytical tools. 8
Page 8 of 26
163
The scattering angle range was 5-60◦ and a rate of 4◦/min.
164
2.8. Surface morphology observation Morphological characterization of corn stalk particles was performed on images acquired
166
using a SEM method. The samples were coated with platinum or gold of 10 nm thicknesses to
167
make the samples conductive.
168
3. Results and discussion
169
3.1. FT-IR spectroscopic analysis
cr us
The distributions of functional group of corn stalk after treatment with steam explosion, acid,
an
170
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165
and alkaline, alone or in combination, are showed in Fig.1. Absorption bands at 3360 cm-1 and
172
2890 cm-1 were attributed to -OH stretching of hydrogen bonds and C-H stretching within the
173
methylene of cellulose, respectively. These absorption peaks in the untreated corn stalk were
174
similar to that in the treated sample, indicating that most of the crystalline cellulose in corn stalk
175
was not disrupted by the tested pretreatment. These results were in agreement with the
176
investigations of Kumar and Wyman (2009) who reported that the crystalline cellulose in corn
177
stover was not disrupted by acid hydrolysis. Furthermore, the prominent bands at 1250 - 900
178
cm-1 were typically related to the structural features of cellulose and hemicelluloses. The
179
vibrations of these bands overlapped the C-O stretching vibrations at 1254 cm-1, C-O-H
180
stretching of primary and secondary alcohols at 1056 cm-1, C-O-C glycosidic bond stretching and
181
C-O-C ring skeletal vibration at 1160-1100 cm-1 (Moharram & Mahmoud, 2008), the adsorption
182
peaks of these bands were enhanced after treatment with steam explosion, acid, and alkaline,
183
alone or in combination. This suggested that the cellulose content in pretreated solid residue
184
increased because the hemicelluloses fraction in corn stalk was removed by hydrolytic reaction.
Ac ce p
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9
Page 9 of 26
These results were in agreement with the changes of chemical composition of corn stalk after
186
treatment (see Table 1). This was further supported by the fact that the adsorption peaks at 1730
187
and 1250 cm-1 were detectable in the untreated corn stalk and these have been proposed to be
188
associated with the alkyl ester of the acetyl group in hemicelluloses. Furthermore, the ester
189
linkage C=O with an absorption peak at 1720 cm-1 was usually defined as the acetyl group in
190
hemicelluloses structure and/or the linkage between hemicelluloses and lignin (Chundawat,
191
Venkatesh, & Dale, 2006). Both of these bands were absent in pretreated solid residue as a result
192
of the removal of hemicelluloses from corn stalk. Interestingly, the band at 895 - 830 cm-1 was
193
dominated by the β-(1 → 4) - glycosidic bond (C1-O-C4), which was cleaved by treatment with a
194
combination of steam explosion and dilute acid or alkali-catalyzed reaction.
cr
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195
ip t
185
The distribution of lignin-associated bands was also investigated. The bands at 1600-1510 cm-1 belonged to aromatic skeletal stretching. The bands at 2852, 1460 and 1425 cm-1 were
197
attributed to absorption of -C-H deformation within the methoxyl groups of lignin (Hsu, Guo,
198
Chen, & Hwang, 2010). These peaks were disappeared in FT-IR of corn stalks after treatment
199
with the combined process of steam explosion and acid or alkaline (see Figs.1B and1C). But
200
based on the efficient removal of lignin, the depolymerization effect of steam explosion and
201
alkali was better than that of steam explosion and acid, suggesting that the release of acid-soluble
202
lignin in pretreated solid residue occurred. Moreover, the adsorption peak due to aromatic
203
hydroxyl groups was observed at 1300 cm-1. This peak might be generated by cleavage of ether
204
bonds within the lignin. These results implied that the partial lignin was removed.
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196
10
Page 10 of 26
A
140
a
B a
80
120
b
100
c
60
Transmittance/%
Transmittance/%
b
40
c 80
d
60
40
20
20
3386
4000
3500
1730 15051254
2890 3000
2500
2000
1500
831 1000
0 4000
3413 3500
1255 17251510 1056 895
2907 3000
500
2500
2000
1500 -1
Wavenumber /cm
-1
Wavenumber /cm
a
an
b
80
c
60
d 40
M
Transmittance/%
us
205 C
20
3395 4000
3500
1500 1690 12431056 831
2902 3000
2500
2000
1500 -1
Wavenumber /cm
1000
500
d
0
500
(a) acid; (b) 0.4 MPa steam explosion + acid; (c) 0.6 MPa steam explosion + acid; (d) 0.8 MPa steam explosion + acid
(a) untreated; (b) acid; (c) alkali
100
1000
cr
0
ip t
100
206
te
(a) alkali; (b) 0.4 MPa steam explosion + alkali; (c) 0.6 MPa steam explosion + alkali; (d) 0.8 MPa steam explosion + alkali
Fig.1. Infrared spectra of corn stalk before and after treatment with different methods. (A) Acid and alkali; (B)
208
steam explosion + acid; (C) steam explosion + alkali.
209
3.2. Characterization of the pretreated solid and liquid fraction
210
Ac ce p
207
The chemical compositions in corn stalk before and after treatment with the different methods
211
is given in Table 1. The untreated corn stalks were composed of 31.54% cellulose, 29.10%
212
hemicellulose, and 38.12% lignin. It was found that the cellulose yields of 60.51% and 76.15%
213
were achieved when corn stalks were pretreated with H2SO4 and NaOH at 60 °C for 24 h,
214
respectively. It may be attributed to the removal of lignin, hemicellulose and other soluble
215
substance because the contents of hemicellulose and lignin in the treated stalk were lower than 11
Page 11 of 26
216
those in raw material. These results were in agreement with the investigation from FT-IR (see
217
Fig.1).
218
Table 1 The solid-liquid phase compositions in corn stalk before and after treatment with different methods.
5%
5%
stalk
H2SO4
NaOH
ip t
Component content
Steam explosion + 5%
Steam explosion + 5%
Corn
H2SO4 (MPa)
NaOH (MPa)
0.4
0.6
0.8
65.16
31.54
60.51
76.15
50.16
54.51
Hemicellulose
29.10
7.02
6.86
14.61
11.13
11.15
Lignin
38.12
32.21
16.52
34.57
33.79
23.01
80.41
an
0.8
79.84
70.91
8.27
5.94
4.61
10.74
13.6
23.87
us
Cellulose
Liquid phase fraction (mg/mL)
0.6
cr
Solid phase fraction (%)
0.4
/
6.83
1.86
5.78
5.42
4.96
2.14
1.89
2.19
Glucose
/
0.73
0.36
0.16
0.08
0.11
0.65
0.55
0.73
Xylose
/
3.79
0.98
3.49
3.15
3.27
1.19
1.05
1.23
Arabinose
/
0.70
0.09
0.64
0.47
0.43
0.12
0.10
0.06
/
1.54
Acetic acid
/
1.14
Furfural
/
c
/
0.38
1.41
1.67
1.10
0.14
0.15
0.11
0.51
1.12
1.08
0.81
0.86
1.37
0.72
0.069
0.077
0.059
0.065
0.047
0.120
0.086
0.054
0.013
0.026
0.0028
0.0041
0.0016
0.044
0.031
0.011
0.0612
0.428
0.051
0.088
0.068
0.863
0.357
0.245
Ac ce p
5-HMF
d
(XOS+GOS+AOS) b
te
Oligosaccharides
Phenolic acid
(FA + P-CA) d
/
M
TSY a
219
a
TSY: Total sugar yield; b XOS: Xylooligosaccharides, GOS: Glucooligosaccharides, AOS:
220
arabinooligosaccharides; c 5-HMF: 5-hydroxymethyl furfural; d FA: Ferulic acid, P-CA: P-coumaric acid.
221
Table 1 showed that most of hemicellulose and lignin could be removed while treating corn
222
stalk with a combination of steam explosion and acid or alkali. For example, 84.16% of
223
hemicellulose and 71.83% of lignin in corn stalk were removed after treatment with 0.8 MPa
224
steam explosion + alkali and 0.4 MPa steam explosion + alkali, respectively. Similarly, the
225
collaborative approach of steam explosion and H2SO4 could also remove hemicellulose and 12
Page 12 of 26
lignin and thereby resulted in the increase of cellulose content in corn stalk. These results suggest
227
that steam explosion coupled with acid or alkali resulted in a certain depolymerization to the
228
molecular structure of corn stalk, but considering the removal of hemicellulose and lignin, the
229
depolymerization effect of steam explosion and alkali is better than that of steam explosion and
230
acid. Zhang, Fu, & Chen, (2012) reported that treatment corn stalk with steam explosion
231
improved the conversion efficiency of lignocellulose to xylose. Generally, an increase in
232
cellulose content and/or a decrease in hemicellulose and lignin contents could improve the
233
catalytic efficiency of cellulase for lignocellulosic materials. Additionally, during pretreatment,
234
corn stalk degrades into sugars and other chemicals (mainly acetic acid, furfural, 5-HMF, and
235
phenolic acid). Acetic acid was produced during degradation of hemicellulose and lignin (Qin,
236
Liu, Li, Dale, & Yuan, 2012), it was generated in the hydrolysis of acetyl groups in hemicellulose.
237
The partial glucose could be transformed into 5-HMF under acidic or alkaline condition.
238
However, the amounts of 5-HMF were very low (0.001-0.04 mg/mL) in liquid phase fraction in
239
this research. Two pentoses (xylose and arabinose) could be transformed into furfural. Table 1
240
showed that the maximum concentration of furfural was 0.12 mg/mL. The by-products of
241
phenolic acid (mainly FA and P-AC) were detected in the liquid phase fraction of corn stalk after
242
treatment using steam explosion coupled with acid or alkali. These organic compounds were
243
mainly degraded from the lignin-carbohydrate-complex (LCC) and lignin. Therefore,
244
pretreatment influence can be partly evaluated by composition of the liquid phase fraction,
245
mainly composed of glucose, xylose, arabinose, degradation production and by-products. As
246
listed in Table 1, monosaccharide took the main part of recovered sugars in the liquid phase
247
fraction. With the increase of treating bursting pressure, the pentose (mainly xylose and
Ac ce p
te
d
M
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cr
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226
13
Page 13 of 26
arabinose) and acetic acid, normally generated from hemicellulose degradation, were detected.
249
The result showed that total sugar content changed from 1.86 mg/mL to 6.83 mg/mL with
250
different pretreatment methods. This offers a good account for the removal of hemicellulose with
251
the combination of steam explosion and acid/alkali.
The concentration of the total solubilized sugars (both oligomers and monomers) from
cr
252
ip t
248
biomass is an important indicator of the suitability of a pretreatment process. Table 1 depicts the
254
comparison of steam explosion coupled with acid or alkali hydrolysis of corn stalk for
255
production of the total solubilized sugars, it shows data concerning the conversion of
256
lignocellulose into their respective monomers and oligomers (mainly xylooligosaccharides
257
(XOS), glucooligosaccharides (GOS) and arabinooligosaccharides (AOS). It was found that a
258
difference in amount of oligomers and monomers produced through steam explosion coupled
259
with acid or alkali pretreatment was described comparatively. The result indicates the effect of
260
steam explosion + acid pretreatment in contrast to steam explosion + alkali under the same
261
experiment conditions (0.4-0.8 MPa). As observed, there were obtained the higher quantities of
262
monomers (1.4-5.2 mg/mL) than oligomers (0.1-1.6 mg/mL) in steam explosion coupled with
263
acid or alkali pretreatment process, however, the difference was much bigger in steam explosion
264
+ acid pretreatment, which was obtained the higher concentration of oligomers (1.1-1.6 mg/mL)
265
than that of steam explosion + acid pretreatment (0.1-0.3 mg/mL). The combined methods with
266
steam explosion + acid are more effective than steam explosion + alkali hydrolysis for
267
production of the total solubilized sugars.
268
3.3 Effect of pretreatment on enzyme hydrolysis
269
Ac ce p
te
d
M
an
us
253
The enzyme hydrolysis of the treated sample under different conditions is summarized in 14
Page 14 of 26
Table 2. The result showed that enzymatic hydrolysis sugar contents were improved by steam
271
explosion coupled with acid or alkali pretreatments. The higher total sugar contents (16 - 40
272
mg/mL) was obtained in the alkali or steam explosion + alkali treatment samples compared with
273
the acid treatment samples for the enzymatic hydrolysis within 72 h. The lower total sugar
274
contents of the treated sample by H2SO4 or steam explosion + acid can be observed from 7
275
mg/mL to 17 mg/mL with the enzymatic hydrolysis time from 6 h to 72 h., It is because proper
276
hemicellulose and lignin were removed under alkali/acid pretreatment condition, and also the
277
physicochemical properties of corn stalk was interrupted. Additionally, increasing the
278
pretreatment pressure resulted in a higher removal rate of hemicellulose and lignin since this
279
released more sugar from the conversion of hemicellulose and cellulose.
280
Table 2 The enzyme hydrolysis of the treated sample under different conditions.
M
an
us
cr
ip t
270
d
Total sugar and glucose contents in the enzymatic hydrolysis a (mg/mL) 6h
5% H2SO4
8.69 (2.34) 16.64 (5.28) 7.29 (2.32) 8.12 (2.11) 7.05 (1.90) 16.54 (5.69) 17.29 (5.09) 16.71 (5.42)
12 h
24 h
36 h
48 h
60 h
72 h
10.21 (3.12) 24.14 (7.02) 9.92 (3.31) 9.30 (3.10) 9.14 (2.72) 22.94 (7.54) 16.43 (6.93) 19.85 (6.80)
13.00 (3.34) 28.66 (7.66) 11.60 (3.57) 11.85 (3.64) 10.21 (3.12) 28.17 (8.31) 28.00 (7.87) 27.67 (7.21)
15.07 (4.19) 31.38 (8.15) 17.37 (3.86) 14.65 (3.65) 16.88 (3.50) 32.45 (9.76) 35.83 (9.56) 35.83 (9.13)
16.44 (4.09) 41.11 (9.77) 17.46 (4.35) 17.21 (4.22) 19.93 (3.78) 42.67 (10.22) 40.20 (10.82) 39.44 (9.83)
17.44 (4.28) 38.49 (7.77) 19.60 (4.61) 17.78 (4.14) 18.11 (3.85) 39.52 (10.31) 37.25 (9.58) 37.66 (3.79)
17.89 (3.96) 39.27 (9.14) 17.33 (4.65) 15.82 (3.54) 16.31 (3.82) 39.31 (9.39) 38.37 (9.46) 39.45 (8.72)
Ac ce p
te
Treatment method
5% NaOH
0.4 MPa Steam explosion + 5% H2SO4 0.6 MPa Steam explosion + 5% H2SO4 0.8 MPa Steam explosion + 5% H2SO4 0.4 MPa Steam explosion + 5% NaOH 0.6 MPa Steam explosion + 5% NaOH 0.8 MPa Steam explosion + 5% NaOH
281
a
The value in brackets express glucose content in enzymatic hydrolysis process (mg/mL). 15
Page 15 of 26
282
3.4. X-ray diffraction analysis The X-ray diffraction patterns of the treated and untreated corn stalks are depicted in Fig.2.
284
The intensities of the amorphous region at 2θ = 18° and the crystalline region at 2θ = 22° were
285
used to calculate the crystallinity index (CrI) (Kim, & Holtzapple, 2006).
286
CrI = [intensity at 2θ = 22° intensity at 2θ =18°] / [intensity at 2θ=22°]
ip t
283
cr
(1)
As shown in Fig.2, the diffraction peak of the untreated corn stalk at (002) was wide and
288
round. However, this peak became sharper and narrower when the tested material was treated
289
with acid or alkaline. In addition, a shoulder peak at (101) and a weak peak at (040) appeared
290
when corn stalks were treated with acid and alkaline. These peaks are assigned to the cellulose
291
phase, suggesting the removal of most of lignin and hemicellulose from corn stalk. By a
292
calculation of Eq. (1), CrI values of corn stalk after treatment with H2SO4 and NaOH increased
293
to 63.5% and 67.0% as compared with 51.8% of the untreated stalk, respectively. The reason
294
might be that the amorphous hemicellulose was hydrolyzed while crystalline structure was
295
change thus increasing of crystallinity index in biomass. Moreover, the swelling ability of alkali
296
to the amorphous region of lignocellulose was relatively strong as compared to that of acid and
297
resulted in the increase of the dissolution rate of lignin and hemicellulose. Zheng, et al. (2012)
298
reported that the crystallinity, porosity and inner surface area of corn stalk were increased when
299
the ordered rearrangement and the secondary crystallization were proceeding in the
300
non-crystallized portion and the imperfect part of cellulose crystal structure.
Ac ce p
te
d
M
an
us
287
16
Page 16 of 26
Intensity /a.u.
(002)
I002
Iam
ip t
(101)
(040)
c
cr
b a
301
20
30
40
50
60
70
us
10
2-theta (degree)
Fig.2. X-ray diffraction pattern of untreated and treated corn stalk: (a) corn stalk; (b) acid; (c) alkali.
303
3.5. The thermal stability analysis
an
302
DTG curve was usually used to reflect the degradation of biomass. Generally, the
305
decomposition reaction of lignocellulose is divided into three processes including (1)
306
dehydration and drying stage in 25-250 °C, (2) the pyrolysis stage in 250-500 °C, and (3) the
307
depth of the pyrolysis stage in 500-600 °C. In this paper, the pyrolysis characteristics and DTG
308
curves of corn stalk before and after treatment with the tested methods are given in Table 3 and
309
Fig.3, respectively. As can be noted, the DTG curves contain a strong peak, which is attributed to
310
the primary pyrolysis. This result is consistent with that of other studies on biomass materials
311
(Jiang, Nowakowski, & Bridgwater, 2010). Physical and chemical pretreatment had little effect
312
on the shape of DTG curves. The maximum weight loss rate and pyrolysis temperature of corn
313
stalks after treatment with steam explosion, acid, and alkaline, alone or in combination, were
314
higher than those of the untreated material.
Ac ce p
te
d
M
304
315
When temperature was lower than 250 °C, the weight loss of corn stalk was mainly attributed
316
to the removal of absorbed water and crystal water, the dehydration peak and drying curves were 17
Page 17 of 26
disappeared with the combined pretreatment using acid or alkali and steam explosion (Figs. 3B
318
and 3C). This was also supported by the fact that the heat absorption and weight loss rate of
319
dehydration and drying stage were changed in untreated and treated corn stalks (see Table 3).
320
With the increase of temperature (>250 °C), the DTG profiles of the treated and untreated
ip t
317
displayed a significant difference. Although the shapes of DTG curve did not exhibited a
322
significant change under different pretreatment condition, the peaks of weight loss shifted
323
towards the high temperature region. Because treatment corn stalk with acid or alkali could
324
remove most of hemicelluloses and some lignin, the temperature of the maximum degradation
325
rate shifted from 332 °C to 365 and 357 °C at 5 °C/min and the maximum weight loss rate from
326
41.5% to 81.8% and 81.0%, respectively. These results on the lateral shift to higher temperatures
327
for the maximum region of weight loss are in agreement with those from other investigators on
328
polypropylene, oil shale and their mixture (Deng, Zhang, & Wang, 2008). It also could be
329
concluded that lower weight loss rate of dehydration stage and lower heat absorption of pyrolysis
330
stage were obtained when corn stalks were treated with steam explosion coupled with acid or
331
alkali as compared to those of acid or alkaline treatment alone. The main reason of this was
332
related to partial removal of hemicelluloses and lignin from the corn stalk and higher crystallinity
333
of cellulose. These were consistent with the results obtained from XRD and FT-IR measurements
334
(see Figs.1 and 2).
Ac ce p
te
d
M
an
us
cr
321
335 336 337 338 18
Page 18 of 26
Table 3 Pyrolysis characteristics of corn stalk before and after treatment with different methods. dehydration and drying stage
Pyrolysis stage
Treatment method
Heat absorption (J/g)
Weight loss rate (%)
Heat absorption (J/g)
Weight loss rate of stage 1 (%)
Weight loss rate of stage 2 (%)
Untreated
113.37
29.62
241.94
35.11
6.44
332.11
5% H2SO4
53.26
0.21
195.90
71.80
10.06
365.56
5% NaOH
92.18
3.14
205.69
70.86
10.24
357.05
52.16
1.63
118.94
50.66
2.16
161.85
0.8 MPa Steam explosion + 5% H2SO4
55.41
0.74
87.47
0.4 MPa Steam explosion + 5% NaOH
75.56
2.08
0.6 MPa Steam explosion + 5% NaOH
62.63
2.15
0.8 MPa Steam explosion + 5% NaOH
72.17
1.58
us
cr
ip t
Pyrolysis temperature (°C)
—
348.82
69.21
8.18
354.35
69.28
—
339.14
225.09
89.31
—
356.08
192.51
66.62
10.58
357.89
114.79
69.99
—
354.59
M
an
73.55
In thermal analysis, the pyrolysis of hemicelluloses and cellulose in the untreated and treated
Ac ce p
340
d
0.4 MPa Steam explosion + 5% H2SO4 0.6 MPa Steam explosion + 5% H2SO4
te
339
341
corn stalks occurred quickly at about 225-325 °C, which is attributed to thermal
342
depolymerisation of hemicelluloses or pectin. The major decomposition peak at about 300-430
343
°C was attributed to cellulose decomposition. However, lignin was more difficult to decompose,
344
as its weight loss happened in a wide temperature range from 250-500 °C (Chen, Yu, Zhang, &
345
Lu, 2011; Kaushik, & Singh, 2011). Additionally, Fig.3 showed that the pyrolysis temperature
346
and thermal stability of corn stalk were enhanced after treatment with the combined method of
347
steam explosion and acid or alkaline due to the removal of most of hemicellulose and lignin.
348
These results suggest that dilute acid and dilute alkali lead to a certain depolymerization to the 19
Page 19 of 26
349
molecular of corn stalk, but the depolymerization effect of dilute alkali is better than that of
350
dilute acid in terms of delignification.
0.1
A
0.0
B
0.0
c
ip t
0.1
-0.1
-0.4
cr
a -0.3
-0.2 -0.3
c
-0.4
b
-0.5
us
-0.2
a
-0.5
-0.6
128 100
314 352 200
300
-0.7
330 359
100
400
500
600
o
temperature / C
200
an
-0.6
300
400
500
600
o
temperature / C
(a) 0.4 MPa steam explosion + acid; (b) 0.6 MPa steam explosion +acid; (c) 0.8 MPa steam explosion + acid; (d) acid
(a) alkali; (b) acid; (c) untreated
M
351 0.2
d
b
DTG /mg·min-1
DTG /mg·min-1
-0.1
C
d
0.0
d
-0.6
a
Ac ce p
DTG /mg·min-1
b -0.4
te
-0.2
-0.8
c
-1.0
352
-1.2
100
200
300
400
500
600
o
temperature / C
(a) 0.4 MPa steam explosion + alkali; (b) 0.6 MPa steam explosion + alkali; (c) 0.8 MPa steam explosion + alkali; (d) alkali
352 353
Fig.3. DTG curves of corn stalk before and after treatment with different methods. (A) Acid and alkali; (B)
354
steam explosion + acid; (C) steam explosion + alkali.
355
3.6. Scanning electron microscopy
356
20
Page 20 of 26
C
D
E
F
G
H
an
us
357
ip t
B
cr
A
358
te
d
M
I
359
Fig.4. SEM images of corn stalk before and after treatment with different methods. (A) Untreated; (B) acid; (C)
361
Alkali; (D) 0.4 MPa steam explosion + acid; (E) 0.6 MPa steam explosion + acid; (F) 0.8 MPa steam explosion
362
+ acid; (G) 0.4 MPa steam explosion + alkali; (H) 0.6 MPa steam explosion + alkali; (I) 0.8 MPa steam
363
explosion + alkali.
Ac ce p
360
364
SEM micrographs of the untreated and treated rice straw fibers were shown in Fig.4. The
365
untreated sample showed an even and smooth flat surface, indicating a rigid and highly ordered
366
surface structure (see Fig. 4A). However, the nature surface was broken into some large
367
fragments and appeared porous structure and rift after treatment with alkaline or acid (see Figs.
368
4B and 4C). When corn stalks were treated with acid and steam explosion under different 21
Page 21 of 26
pressure, the treated sample had a rugged, rough, loose, and broken surface (see Figs. 4D - 4F). It
370
was clearly found that morphologies of the treated corn stalks under different conditions were
371
also obviously different. For example, the surface of raw samples by 0.8 MPa steam explosion +
372
alkali looked more roughness than that by 0.4 MPa or 0.6 MPa steam explosion + alkali (see
373
Figs.4G - 4I). The reason is that lignin and hemicellulose of interwoven wrapped cellulose
374
crystalline structure was broken and weakened the mutually binding force between holocellulose
375
and lignin by the pretreatment with different burst pressure in acid-alkali. This would obviously
376
favor the catalyst contacting the inner linkage, hence accelerating the biodegradation process.
377
4. Conclusions
cr
us
an
Treatment with steam explosion and alkali could remove most of hemicellulose (84.16%) and
M
378
ip t
369
lignin (71.83%) in corn stalk as compared to those of the untreated stalk. The surface
380
morphology become loose and the thermal stability increased when corn stalks were
381
depolymerized with steam explosion and acid or alkali. These results suggested that steam
382
explosion coupled with acid or alkali could lead to a depolymerization of the molecular structure
383
of corn stalk, but in consideration of removal of hemicellulose and lignin, the depolymerization
384
effect of steam explosion and alkali was better than that of steam explosion and acid.
385
Acknowledgement
te
Ac ce p
386
d
379
The authors would like to thank the National Natural Science Foundation of China (NSFC) for
387
financial support (NO. 21266023).
388
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389
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Zhang, Y.Z., Fu, X.G., Chen, H.G. (2012). Pretreatment based on two-step steam explosion
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Zheng, M.X., Li, L.Q., Zheng, M.Y., Wang, X., Ma, H.L., et al. (2012). Effect of alkali
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Zhou, D.F., Ma, Y.L., Xie, L., Cai, Y. (2011). Experimental study on pretreatment of maize stalk
464
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465
Figure captions
466
Figure 1.
471 472
ip t
470
X-ray diffraction pattern of untreated and treated corn stalk: (a) Corn stalk; (b) acid;
(c) alkali.
Figure 3. DTG curves of corn stalk before and after treatment with different methods. (A) Acid
cr
469
Figure 2.
and alkali; (B) steam explosion + acid; (C) steam explosion + alkali. Figure 4.
us
468
Acid and alkali; (B) steam explosion + acid; (C) steam explosion + alkali.
SEM images of corn stalk before and after treatment with different methods. (A)
an
467
Infrared spectra of corn stalk before and after treatment with different methods. (A)
Untreated; (B) acid; (C) Alkali; (D) 0.4 MPa steam explosion + acid; (E) 0.6 MPa steam
474
explosion + acid; (F) 0.8 MPa steam explosion + acid; (G) 0.4 MPa steam explosion +
475
alkali; (H) 0.6 MPa steam explosion + alkali; (I) 0.8 MPa steam explosion + alkali.
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te
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M
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26
Page 26 of 26
