Article pubs.acs.org/EF
Pretreatment of Corn Stover with the Modified Hydrotropic Method To Enhance Enzymatic Hydrolysis Hongyan Mou,*,† Bin Li,‡ and Pedro Fardim*,†,§ †
Laboratory of Fiber and Cellulose Technology, Åbo Akademi University, Porthaninkatu 3, FI-20500 Turku, Finland CAS Key Laboratory of Biobased Materials, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences (CAS), Qingdao, Shandong 266101, People’s Republic of China § Center of Excellence for Advanced Materials Research (CEAMR), King Abdulaziz University Jeddah 21589, Saudi Arabia ‡
S Supporting Information *
ABSTRACT: Hydrotropic pretreatment using sodium xylene sulfonate (SXS) could remove lignin and xylan from corn stover to enhance enzymatic saccharification. Peracetic acid (PAA) treatment prior to the hydrotropic process [so-called modified hydrotropic pretreatment (MHP)] could double the delignification efficiency and remarkably increase glucan conversion. After pretreatment, samples were treated by PFI refining for comparison. With the supplement of PFI refining before enzymatic hydrolysis of the MHP-treated corn stover, 87.6% of the glucan yield could be achieved and the corresponding xylan yield was 43.7%. In addition, the pretreated corn stover was analyzed by Fourier transform infrared spectroscopy (FTIR), X-ray diffractometer (XRD), and scanning electron microscopy (SEM). The lignin precipitate from the spend liquor was also investigated by FTIR. The cleavage of the lignin structure could be observed from FTIR results. The crystallinity index (CrI) of corn stover after MHP was increased according to XRD analysis, while the reduction of total CrI of cellulose between pretreatment samples was analyzed by FTIR. SEM analysis demonstrated that PAA treatment affected the morphology of corn stover fiber by generating pores and allowing for better contact of the enzyme to polysaccharides.
1. INTRODUCTION Nowadays, corn stover is an agriculture waste that has been widely used for bioethanol production. Extensive work has been performed on different types of pretreatment of corn stover, such as dilute acid/alkaline, steam explosion, ammonia, ionic liquid, organic solvent extraction, and fungi methods, aiming to remove hemicellulose or lignin and finally improve the enzymatic digestibility.1 The composition of corn stover mainly consists of cellulose, hemicellulose, and lignin.2 To obtain good efficiency of bioconversion in the hydrolysis process, lignin as the main barrier needs to be removed.3 The hydrotropic method by the use of sodium xylene sulfonate (SXS) was able to be used for delignification, which had been introduced by Andelin.4 Recently, Mou et al. reported that hydrotropic pretreatment could remove more lignin from birch wood compared to the room-temperature ionic liquid method and hydrothermal pretreatment. After hydrotropic pretreatment and enzymatic hydrolysis, the glucose yield of birch could reach 84%, but this pretreatment method was ineffective for pine wood because of its high content of guaiacyl lignin.5,6 Hydrotropic pretreatment is suitable for non-woody biomass as well. The glucan yield obtained from the common reed after hydrotropic pretreatment could be about 4.5% higher than that from diluted alkaline pretreatment.7 Additionally, the hydrotropic agent could be recovered from the spent solution or reused directly for the further pretreatment process.6 This advantage was beneficial for controlling the process cost and heat savings. It has been suggested that the proper amount of SXS for lignin removal was at least 30% (w/w).6 To maintain the delignification efficiency with a low dosage of hydrotropic © 2014 American Chemical Society
agent, the process of lignin modification has to be introduced prior to the hydrotropic pretreatment. As reported, peracetic acid (PAA) is one kind of strong oxidation agent, which has been used to remove lignin from woody biomass.8 PAA pretreatment conditions on sugar cane bagasse were evaluated by Teixeira et al., and they demonstrated that good enzymatic digestibility of sugar cane bagasse could be obtained, especially when the loading of PAA was 21% based on dry mass.9 Using sodium hydroxide prior to PAA treatment is able to not only increase the sugar yield but also decrease the PAA loading in the pretreatment process.9 Recently, Kumar et al. has researched the PAA pretreatment of poplar, corn stover, and pine sawdust.10 They reported that, at certain conditions, over 90% of lignin was removed after PAA pretreatment, with the PAA loading of 5 g/g of biomass. The selectivity of PAA on lignin was dependent upon material type, but PAA has little impact on cellulose structure. This was also demonstrated by Zhao et al.1 Most recently, it was reported that PAA followed by ionic liquid pretreatment of pine biomass could increase biomass loading from 5 to 15% without a significant decrease in cellulose conversion.11 Thus, it can be concluded that PAA has good selectivity on lignin removal without degradation of cellulose at controlled conditions when it was used as a pre-pretreatment agent. Furthermore, PAA is a cost-effective and environmentally benign agent. Special Issue: International Biorefinery Conference Received: January 17, 2014 Revised: March 14, 2014 Published: March 18, 2014 4288
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Table 1. Chemical Composition of Corn Stover after Pretreatment and the Effects of Different Pretreatmenta pretreatment raw corn stover H2O SXS MHP (PAA + SXS)
glucan 31.22 39.46 48.83 64.22
(0.53) (0.14) (0.06) (0.14)
xylan 17.66 22.70 19.23 15.83
(0.09) (0.06) (0.08) (0.15)
arabinan 1.91 2.09 2.06 0.56
(0.02) (0.03) (0.01) (0.00)
AIL 14.20 20.29 15.61 9.39
ASL
(0.23) (0.21) (0.46) (0.19)
0.85 1.13 0.85 0.81
(0.01) (0.01) (0.02) (0.00)
Rdelignification (%)
Rglucan (%)
Rxylan (%)
Ysolid (%)
NDb ND 33.25 (1.79) 65.58 (0.64)
100 100.46 (0.38) 95.48 (0.11) 99.45 (0.23)
100 100.26 (0.31) 66.46 (0.27) 45.52 (0.43)
100 83.43 61.04 50.78
a
H2O, H2O pretreatment; SXS, hydrotropic pretreatment; MHP (PAA + SXS), modified hydrotropic pretreatment. Standard deviation was given in parentheses. The extractives and ash contents of raw corn stover are 22.61 and 6.89%, respectively. bND = not detected. liquor, and the mixture was centrifuged at 8237g for 5 min to collect the precipitated lignin. 2.2.1. Hydrotropic Pretreatment. The hydrotropic pretreatment (i.e., SXS pretreatment) was conducted under the same conditions as H2O pretreatment. The dosage of SXS was 10% (w/w), together with 0.5% (w/w) formic acid (on the basis of the dry weight of corn stover). After pretreatment, samples were put into a cloth bag with the mesh of 300 and the spent liquor was collected. Subsequently, the samples were washed with NaOH solution [10% (w/w)] for 5 min to dissolve lignin, followed with tap water until neutrality. The subsequent handling for the washed corn stover and the collection of precipitated lignin were the same as mentioned above. 2.2.2. Modified Hydrotropic Pretreatment. Modified hydrotropic pretreatment (MHP) is the PAA treatment followed by hydrotropic pretreatment. PAA treatment of corn stover was performed at 30 ± 2 °C for 4 h with the solid loading of 5% (w/w). The dosage of PAA was 5.5 g/g of dry corn stover. After reaction, the stock was washed by the approach mentioned above, and the washed samples were stored in plastic bags for balance of moisture. Subsequently, the PAA-treated samples were cooked by SXS pretreatment under the same conditions as described previously. After pretreatment, the handling of pretreated samples and spent liquor was the same as mentioned above. 2.2.3. PFI Refining. To further increase enzymatic digestibility, the pretreated and washed corn stover was modified by a PFI beating machine (mode PL11-00, Xianyang TEST Equipment Co., Ltd., China). The mechanical beating was carried out under atmospheric conditions, with the beating gap of 0.24 mm and the rotational speed of 1400 rpm. The beating consistency and the revolutions were 10% (w/w) and 4000, respectively. After beating, the substrates were stored at 4 °C for characterization and enzymatic hydrolysis. 2.2.4. Enzymatic Hydrolysis. Enzymatic hydrolysis of pretreated and beat corn stover was carried out in duplicate with a substrate consistency of 2% (w/v). A mixture of cellulase (20 FPU/g of substrate) and β-glucosidase (5 IU/g of dry biomass) was added together with 0.05 M sodium citrate buffer (pH 4.8), and hydrolysis was run at 50 °C for a certain time in a serum bottle (25 mL) placed in an incubator shaker at 90 rpm. To each bottle, 200 μL of a 2% sodium azide solution was added to prevent the growth of organisms during hydrolysis. Upon completion, the supernatant was filtered through a 0.22 μm membrane and stored at −4 °C for further analysis. 2.2.5. Analysis Methods and Characterization. The chemical analysis of untreated and pretreated corn stover was conducted according to the National Renewable Energy Laboratory (NREL) analytical procedure. Acid and enzymatic hydralyzates (0.22 μm filtered) were analyzed by a high-performance liquid chromatography (HPLC) system (model 1200, Agilent, Santa Clara, CA) equipped with a refractive index detector and Bio-Rad Aminex HPX-87H column (1.300 × 7.8 mm). The column was used at 55 °C with 0.005 M H2SO4 as a mobile phase (0.5 mL/min). All of the pretreatments, enzymatic hydrolysis, and component analysis were carried out in duplicate. The effectiveness of pretreatments was evaluated by the solid yield (Ysolid), recovery rate of glucan or xylan (Rglucan/xylan), delignification rate (Rdelignification), and sugar yields. The calculation equations were listed as follows:
In this study, the hydrotropic method at low concentrations of SXS was first employed to pretreat corn stover. To improve the pretreatment effectiveness, PAA treatment before hydrotropic pretreatment was conducted and post-mechanical refining was also carried out for comparison. In addition, the treated corn stover samples were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffractometer (XRD), and scanning electron microscopy (SEM), and the lignin samples collected from spent liquor of pretreatment were investigated by FTIR. It will be expected that the modified hydrotropic method by PAA treatment could enhance the enzymatic hydrolysis of corn stover.
2. MATERIALS AND METHODS 2.1. Materials. Corn stover, harvested in autumn in 2012 from Qingdao, Shandong, China, was cut into 3−5 cm pieces in length. The corn stover was air-dried and then milled and screened to obtain the particle size between 0.425 and 8 mm. The screened corn stover was stored in a ziplock bag for component analysis and pretreatment experiments after moisture balance. The chemical composition of the raw corn stover is shown in Table 1. Acetic acid (AA) was sourced from Fuyu Chemicals (China). Other chemicals used, such as SXS and hydrogen peroxide [HP, 30% (w/ w)], were obtained from Sinopharm Chemical Reagent Co., Ltd. All chemicals were of analytical grade and used as received, except for PAA, which was made on the basis of the previous report.12 Briefly, certain volumes of AA and sulfuric acid were put in a stoppered flask and mixed homogeneously. Subsequently, HP was added, and the mixture was magnetically stirred at room temperature for 24 h. The initial ratio of AA/HP was 3:2 (v/v). The amount of H2SO4 as a catalyst was 0.1% (v/v) based on the total volume of AA and HP used. The concentration of synthesized PAA is about 20% (w/w). Commercial enzymes of Celluclast 1.5L (cellulase) and Novozyme (β-glucosidase) were purchased from Sigma-Aldrich China, Inc. and used without any purification. The activities of cellulase and βglucosidase used were 121 filter paper units (FPU)/mL and 741 international units (IU)/mL, respectively, as measured according to International Union of Pure and Applied Chemistry (IUPAC) standard methods.13 2.2. Methods. For comparison, H2O pretreatment was performed. The pretreatment of corn stover was conducted in a cooking reactor (model PL1-00, Xianyang TEST Equipment Co., Ltd., China). The parallel experiments were carried out simultaneously. For each test, 40 g of screened corn stover (oven dry basis) was treated with the liquid/ solid ratio of 8:1. The pretreatment was performed on the scheduled program (30 min of heating and 30 min of heat preservation at the maximum temperature of 140 °C). During pretreatment, the reactor was rotated at 1 rpm. After pretreatment, the bombs were cooled immediately to room temperature with cold tap water. Subsequently, samples were taken out and put in a Nylon bag (with the mesh of 300) to collect spent liquor for pH analysis. After that, the samples were washed using tap water to neutralize the pH value. Finally, the washed samples were completely transferred to a preweighed plastic bag, weighed, sealed, and stored at 4 °C for the next step of analysis. In addition, 100 mL of distilled water was added in 100 mL of spent
Ysolid (%) = 4289
pretreated biomass (g) × 100 original biomass (g)
(1)
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Energy & Fuels R glucan/xylan (%) =
Article
YsolidCglucan/xylan of pretreated biomass Cglucan/xylan of original biomass
R delignification (%) = 1 −
In addition, lower xylan and arabinan contents were obtained after MHP compared to that after SXS pretreatment. This was probably because the more removal of lignin led to more degradation of hemicellulose during SXS pretreatment after PAA treatment. This was in agreement with the fact that SXS can remove both lignin and hemicellulose in pretreatment.7 In addition, it is seen from Table 1 that, after hydrotropic pretreatment, more than 33% lignin was removed from feedstock, while the delignification was doubled after MHP. Meanwhile, about half of the amount of xylan was removed without glucan degradation. Thus, it can be concluded that PAA treatment of corn stover before hydrotropic pretreatment could significantly promote the delignification and MHP showed better selectivity upon reducing lignin and hemicelluloses than hydrotropic pretreatment. This will be more beneficial for the downstream enzymatic saccharification. In addition, please note that, after hydrotropic pretreatment, caution should be taken to prevent the lignin redeposition on samples, as mentioned previously.5 3.2. FTIR Characterization of Corn Stover and Lignin. The pretreated corn stover samples and the lignin samples precipitated from the spent liquor after pretreatment processes were investigated. The chemical bands were assigned and summarized in Table 2 and Supplementary Figures 1 and 2 of the Supporting Information.
(2)
YsolidC lignin of pretreated biomass C lignin of original biomass
(3)
The sugar yields (%) were expressed as the percentage of the sugar (glucan or xylan) in enzymatic hydrolyzate divided by the sugar (glucan or xylan) in the corresponding original biomass. 2.2.6. FTIR Analysis. The spectra of pretreated corn stover and lignin were collected on Nicolet 6700 Fourier transform infrared spectroscopy (FTIR) (Thermo Fisher, Waltham, MA). The dried samples were prepared through KBr pellet, and the weight ratio of KBr/sample was 100:1. Spectra were collected at a resolution of 4 cm−1 in the range of 400−4000 cm−1, and 32 scans per sample were conducted. 2.2.7. XRD Analysis. Crystallinity of untreated and pretreated corn stover was analyzed by an X-ray diffractometer (XRD, D8 AVANCE, Bruker, Germany) equipped with Ni-filtered Cu Kα radiation generated at 40 kV and 40 mA. The scattering angle (2θ) ranges from 5° to 60° with a scan rate of 4° min−1. The crystallinity index (CrI) was calculated according to the empirical method developed by Segal et al.14 using the following equation:
CrI =
I002 − Iamorph I002
where I002 is the maximum intensity of the (002) lattice diffraction and Iamorph is the minimum intensity diffraction between the 002 and the 101 peaks. 2.2.8. SEM Analysis. The imaging of corn stover before and after treatment was carried out by the use of SEM (Hitachi S-4800, Japan) at a beam with accelerating voltages from 3.0 kV. Before analysis, the samples were oven-dried at 65 °C for 24 h. Subsequently, the dried samples were pasted on the specimen stub using carbon tape and coated with platinum.
Table 2. Summary of Chemical Band Assignment by FTIR19−23 band position (cm−1) 3400 2919−2930 1738−1734 1649 1392−1390 1327 1190 1040 897 796 693
3. RESULTS AND DISCUSSION 3.1. Chemical Composition of Corn Stover after Pretreatment. The goal of pretreatment is to reduce lignin or hemicellulose, increase specific surface area, and decrease the crystallinity of cellulose, thereby improving the enzymatic digestibility of lignocelluloses.15 The chemical composition changes of corn stover after pretreatments are given in Table 1. As seen, the glucan content of raw corn stover is about 31%, which is lower than the typical glucan content of corn stover (35−40%).15,16 However, the extractive content (about 23%) of raw corn stover was much higher than the typical extractive content of corn stover (5−12%) reported in the literature.17 This was possibly because the raw corn stover was collected from saline land in a location beside the ocean. Table 1 also presents that, after pretreatment, the glucan content increased significantly. For instance, the glucan content was about 64% after the MHP, which was 39.5 and 48.8% higher compared to H2O pretreatment and SXS pretreatment, respectively. This was possibly because the structure of lignin is changed after PAA treatment, thus facilitating the lignin removal in the subsequent hydrotropic pretreatment. Previous studies also reported that PAA could promote delignification.10,11 The increase of xylan and lignin after H2O pretreatment was mainly due to the removal of extractives and ash. Little degradation of hemicellulose may be associated with the relatively low temperature used for H2O pretreatment. The temperature of hydrothermal pretreatment is generally between 130 and 220 °C,18 but in this study, all pretreatment experiments were conducted at the same temperature (140 °C) for comparison.
assignment O−H stretching (polysaccharides/lignin/wax) C−H (CH3 and CH2) stretching (polysaccharides/lignin) CO conjugates (xylans) CO (lignin) phenolic O−H (lignin) C−C (C−O) of syringyl rings of lignin sulfonic group on lignin C−O−C stretching polysaccharides β-glucosidic (polysaccharides) skeletal deformation of aromatic rings β-glycosidic ethyl linkages of sugar units
The peaks near 3400 cm−1 were presented as OH stretching, which exists in lignin and carbohydrates (see Supplementary Figure 1 of the Supporting Information).19 The band at 2900 cm−1 was attributed to C−H stretching vibration. The band at 1649−1640 cm−1 was assigned to CO from lignin, and its intensity were reduced after pretreatment, particularly for the MHP-treated samples (see Supplementary Figure 2 of the Supporting Information). The intensity of the peak at 1392− 1390 cm−1 (assignment of phenolic lignin) also decreased obviously after pretreatment.20 The band at 1327 cm−1 was attributed to C−O of syringyl rings in lignin.21 Skeletal deformations of aromatic rings appeared at 796 cm−1. The decrease of intensity of these peaks confirmed the removal of lignin after MHP. It also proved that the lignin after modification by PAA became easier to be removed by MHP. The band at around 1738−1734 cm−1 was characteristic of C O conjugates in hemicelluloses,22 and its reduction was due to the removal of hemicellulose after pretreatment (see panels c−e of Supplementary Figure 1 of the Supporting Information). In 4290
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addition, the peak at 897 cm−1 was associated to the βglucosidic bonds of cellulose, and its increase after pretreatment was also because of the removal of lignin and hemicelluloses. The main differences in the FTIR spectra of lignin samples could be found from the bands at 1190 cm−1 (attributed to the absorption of the sulfonic group), which did not appear in the spectra of the lignin samples collected from H2O pretreatment and SXS pretreatment (see Supplement Figure 2 of the Supporting Information). Also, the weak shoulder shown at 1040 cm−1 was assigned as C−O−C stretching of residue hemicelluloses,23 which indicated that the degradation of hemicelluloses was relatively stronger in MHP (PAA + SXS pretreatment). Therefore, the lignin fraction after MHP contained a higher hemicellulose residue compared to SXS pretreatment. The peak at 693 cm−1 was the β-glycosidic ethyl a linkage of sugar units. The phenolic units and syringyl unit of MHP lignin shown at 1390 and 1327 cm−1 were weaker than those of SXS lignin. It was because the PAA as a strong oxidation agent could break down the lignin structure.1 Phenolic hydroxyl groups of lignin were demonstrated to have more critical inhibitory effects than physical barrier and non-specific adsorption on enzymatic hydrolysis of cellulose.24 The elimination of phenolic units of lignin by MHP were a clear benefit for enhancing the enzymatic hydrolysis. 3.3. Crystallinity of Pretreated Corn Stover. Besides lignin hindrance, crystallinity of cellulose is another important factor to affect enzymatic saccharification.25 XRD was the most common technology for crystallinity determination. For the conventional X-ray methods, the crystallinity was of the entire material, including the hemicellulose and lignin, in addition to amorphous cellulose.25 FTIR analysis can also be applied for the characterization of cellulose crystallinity. Two infrared ratios related to the cellulose structure were calculated: (1) α 1423 cm−1/α 897 cm−1, the ratio of peak areas at 1423 and 897 cm−1, which is referred to as the lateral order index (LOI),26 and (2) α 1372 cm−1/α 2900 cm−1, the ratio of peak areas at 1372 and 2900 cm−1, which is known as the total crystallinity index (TCI).27 The XRD patterns of untreated and pretreated corn stover are shown in Supplementary Figure 3 of the Supporting Information, and the corresponding CrI is given in Table 3.
the Supporting Information). The relative cellulose crystallinity value was calculated by FTIR, and the results are listed in Table 3 as well. It is seen that there is a slight decrease of total CrI after pretreatment, which is associated with the destruction of cellulose. The hydrotropic pretreatment could obviously reduce the LOI and TCI, while the MHP by introducing PAA prior to hydrotropic pretreatment had less impact on LOI and TCI (Table 3) than the hydrotropic pretreatment. 3.4. Morphology of Pretreated Corn Stover. The samples were modified after pretreatments, and the morphology of the fiber supplies evidence on the influence of pretreatment on the fibers (Figure 1). The pores particularly
Figure 1. SEM morphology of corn stover after pretreatments (a, raw corn stover; b, PAA-treated samples; c, MHP pretreated corn stover; and d, MHP pretreated corn stover after PFI beating).
in parenchyma cells were created by PAA treatment (Figure 1b), and similar results were also found by Zeng et al.30 Because of the pore generation, SXS solution penetration into the corn stover and the effective degradation of lignin and hemicelluloses could be further strengthened. As a consequence, the rind was dramaticly broken after MHP (Figure 1c). It was reported that lignin content, composition, and distribution as well as cell wall thickness influence the enzymatic hydrolysis of cellulose of stover.15 Beating performance could improve the surface contact area of pretreated corn stover for enzyme. Thus, corn stover was changed to be more favorable for enzyme accessibility after pretreatment and post-PFI refining. 3.5. Glucan and Xylan Yields of Pretreated Corn Stover. After pretreatment, the physical hindrance lignin was reduced and all of the samples were hydrolyzed. The gucan yield was significantly increased by hydrotropic pretreatment compared to the samples only treated by water (Figure 2), and the order of the glucan yield observed from pretreatments was MHP > SXS > H2O. This is in agreement with the delignification results shown in Table 1. Figure 2 also exhibits that PAA treatment could improve the hydrotropic pretreatment efficiency and the MHP pretreated corn stover was more accessible to enzyme. This was probably because, after PAA treatment, the changes of fiber morphology (Figure 1) and component chemical structure (see Supplementary Figures 1 and 2 of the Supporting Information) could enhance the efficiency of hydrotropic treatment. The opening of the cell wall by MHP treatment was caused by the removal of lignin and
Table 3. Crystallinity of Corn Stover by XRD and FTIR after Variety Pretreatment
a
components
CrI
LOI
TCI
raw H2O SXS MHP MHP*a
0.48 0.44 0.53 0.59 0.53
1.32 1.34 1.25 1.35 1.31
1.11 1.01 0.98 1.06 1.07
MHP* was the MHP treated sample after PFI refining.
The CrI was influenced by the compositions of the sample.28 The increase of CrI after pretreatment (particularly for SXS pretreatment and MHP) was due to the removal of the amorphous substances, such as lignin and hemicelluloses.10,28 For example, the CrI increased about 23% after MHP compared to the raw corn stover. In addition, there was slight decrease of CrI after PFI refining, which is because of the modification of the crystalline regions of cellulose.29 Correspondingly, the intensity of 101 and 002 peaks for the sample after beating was clearly decreased (Supplementary Figure 3 of 4291
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lignin and hemicellulose was enhanced by applying PAA treatment prior to hydrotropic pretreatment. Consequently, the crystallinity of pretreated corn stover increased, while the total crystallinity of cellulose slightly decreased after MHP. Also, the pores of fibers created by PAA treatment could facilitate enzymatic hydrolysis and improve the glucan yield. The supplement of post-refining could gain about 10% increase of glucan yield after enzymatic hydrolysis. The modified hydrotropic method could be a promising fractionation technology of lignocellulosic biomass because of the advantages, such as good selectivity, on removal of lignin and hemicellulose while preserving the cellulose and lowering the consumption of chemicals.
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ASSOCIATED CONTENT
S Supporting Information *
FTIR spectra of raw and treated corn stover (Supplementary Figure 1), FTIR spectra of lignin collected from spent liquor after different pretreatments (Supplementary Figure 2), and XRD pattern of untreated and pretreated corn stover (Supplementary Figure 3). This material is available free of charge via the Internet at http://pubs.acs.org.
Figure 2. Glucan yield of pretreated corn stover after enzyme hydrolysis.
xylan (Table 1), and this could help enzyme access into fibers, thus enhancing the hydrolysis of cellulose to glucose. In addition, beating could further improve the enzyme hydrolysis performance because of the increase of specific surface area (Figure 1).28 As a result of all positive changes of the MHP sample, the glucan yield increased to about 90% after 48 h of enzymatic hydrolysis and post-refining, and this is about a 20% improvement compared to hydrotropic pretreatment under the same conditions. During the enzyme hydrolysis procedure, xylan was hydrolyzed as well. The maximum yield of xylan was obtained from SXS samples with beating treatment (Figure 3). Even after 72
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AUTHOR INFORMATION
Corresponding Authors
*Telephone: +358-44-2077304. E-mail: hmou@abo.fi. *Telephone: +358-50-4096424. E-mail: pfardim@abo.fi. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank the BIOREGS graduate school for the financial support of this work. The authors are grateful to the kind support from the Committee of the 4th International Conference on Biorefinerytowards Bioenergy (ICBB 2013) in Xiamen, China.
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Figure 3. Xylan yield of pretreated corn stover after enzymatic hydrolysis.
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h, the xylan hydrolysis yield was still in increasing tendency. PAA treatment was helpful for improving the glucan yield but did not favor the xylan yield increasing. This may be due to the fact that the hydrotropic pretreatment can remove both lignin and hemicellulose.7 The removal of xylan after SXS pretreatment or MHP leads to low xylan yields, and the removal of hemicellulose could also facilitate the glucan digestibility.28
NOMENCLATURE SXS = sodium xylene sulfonate MHP = modified hydrotropic pretreatment PAA = peracidic acid AA = acetic acid FTIR = Fourier transform infrared spectroscopy XRD = X-ray diffractometer SEM = scanning electron microscopy CrI = crystallinity index LOI = lateral order index TCI = total crystallinity index AIL = acid-insoluble lignin ASL = acid-soluble lignin REFERENCES
(1) Zhao, X.; Cheng, K.; Liu, D. Appl. Microbiol. Biotechnol. 2009, 82, 815−827. (2) Zhang, Q.; Jang, L.; Lu, J.; Hou, L.; Jin, H.; Pu, J. Sci. Technol. Food Ind. 2006, 10, 198−201. (3) Lee, S. H.; Doherty, T. V.; Linhardt, R. J.; Dordick, J. S. Biotechnol. Bioeng. 2009, 102, 1368−1373. (4) Andelin, J. Background Paper (OTA-BP-O-54); Office of Technology Assessment, U.S. Congress: Washington, D.C., 1989; p 74. (5) Mou, H. Y.; Orblin, E.; Kruus, K.; Fardim, P. Bioresour. Technol. 2013, 42, 540−545.
4. CONCLUSION In this work, the MHP was conducted to treat corn stover for the enhancement of enzymatic hydrolysis. The removal of 4292
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(6) Korpinen, R.; Fardim, P. O Papel 2009, 70, 69−82. (7) Mou, H. Y.; Orblin, E.; Fardim, P. Bioresour. Technol. 2013, 150, 36−41. (8) Lai, Y. Z.; Sarkanen, K. V. Tappi J. 1968, 51, 449−453. (9) Teixeira, L. C.; Linden, J. C.; Schroeder, H. A. Renewable Energy 1999, 16, 1070−1073. (10) Kumar, R.; Hu, F.; Hubbell, C. A.; Ragauskas, A. J.; Wyman, C E. Bioresour. Technol. 2013, 130, 372−381. (11) Uju; Abe, K.; Uemura, N.; Oshima, T.; Goto, M.; Kamiya, N. Bioresour. Technol. 2013, 138, 87−94. (12) Zhao, X.; Zhang, T.; Zhou, Y.; Liu, D. J. Mol. Catal. A: Chem. 2007, 271, 246−252. (13) Ghose, T. K. Pure Appl. Chem. 1987, 59, 257−268. (14) Segal, L.; Creely, J.; Martin, A.; Conrad, C. Text. Res. J. 1959, 29, 786−794. (15) Mosier, N.; Wyman, C.; Dale, B.; Elander, R.; Lee, Y. Y.; Holtzapple, M.; Ladisch, M. Bioresour. Technol. 2005, 96, 673−686. (16) Menon, V.; Rao, M. Prog. Energy Combust. 2012, 38, 522−550. (17) Zhu, J. Y.; Pan, X. J. Bioresour. Technol. 2010, 101, 4992−5002. (18) Nitsos, C. K.; Matis, K. A.; Triantafyllidis, K. S. ChemSusChem. 2013, 6, 110−122. (19) Sun, R.; Lawther, J. M.; Banks, W. B. Ind. Crops Prod. 1995, 54, 127−145. (20) Suëtaka, W. Plenum Press: New York, 1995; p 142. (21) Derkacheva, O.; Sukhov, D. Macromol. Symp. 2008, 265, 61−68. (22) Lima, M. A.; Lavorente, G. B.; da Silva, H. K. P.; Bragatto, J.; Rezende, C. A.; Bernardinelli, O. D.; deAzevedo, E. R.; Gomez, L. D.; McQueen-Mason, S. J.; Labate, C. A.; Polikarpov, I. Biotechnol. Biofuels 2013, 6, 1−17. (23) Adjei, T. Master’s Thesis, Virginia Polytechnic Institute and State University, Blacksburg, VA, Dec 6, 2007; p 86. (24) Pan, X. J. Biobased Mater. Bioenergy 2008, 2, 25−32. (25) Zhu, L.; O’Dwyer, J. P.; Chang, V. S.; Granda, C. B.; Holtzapple, M. T. Bioresour. Technol. 2008, 99, 3817−3828. (26) Hurtubise, F. G.; Krassig, H. Anal. Chem. 1960, 32, 177−181. (27) Nelson, M. L.; O’Connor, R. T. J. Appl. Polym. Sci. 1964, 8, 1325−1341. (28) Kim, T. H.; Kim, J. S.; Sunwoo, C.; Lee, Y. Y. Bioresour. Technol. 2003, 90, 39−47. (29) Leu, S. Y.; Zhu, J. Y. Bioenergy Res. 2013, 6, 405−415. (30) Zeng, M.; Ximenes, E.; Ladisch, M. R.; Mosier, N. S.; Vermerris, W.; Huang, C. P.; Sherman, D. M. Biotechnol. Bioeng. 2012, 109, 398− 404.
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dx.doi.org/10.1021/ef5001634 | Energy Fuels 2014, 28, 4288−4293
