Comparison of Hydrogen Peroxide and Ammonia Pretreatment of

Sep 5, 2014 - Three pretreatments, hydrogen peroxide pretreatment (HP), ammonia fiber expansion pretreatment (AFEX), and hydrogen peroxide presoaking ...
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Comparison of Hydrogen Peroxide and Ammonia Pretreatment of Corn Stover: Solid Recovery, Composition Changes, and Enzymatic Hydrolysis Chao Zhao,*,†,‡ Qianjun Shao,‡,§ Bin Li,‡ and Weimin Ding*,† †

College of Engineering, Nanjing Agricultural University, Nanjing, Jiangsu 210031, China National Engineering Research Center for Wood-based Resource Utilization, School of Engineering, Zhejiang A&F University, Lin’an, Zhejiang 311300, China § School of Mechanical Engineering, Ningbo University, Ningbo, Zhejiang 315211, China ‡

ABSTRACT: Three pretreatments, hydrogen peroxide pretreatment (HP), ammonia fiber expansion pretreatment (AFEX), and hydrogen peroxide presoaking prior to AFEX (H-AFEX), were employed to improve enzymatic digestibility of corn stover. Effects of varied chemical loadings on solid recovery, composition changes, and enzymatic hydrolysis by different methods were investigated and compared. The influences of the lignin solubilization and xylan removal from treated corn stover on subsequent enzymatic hydrolysis were assessed. The highest fermentable sugar yields of HP, AFEX, and H-AFEX processes following enzymatic hydrolysis at 15 FPU/g glucan for 72 h were 370.9, 497.8, and 527.6 g monosaccharides per kilogram of untreated dry biomass, respectively. For AFEX and H-AFEX, solubilization of 15−20% of the lignin and removal of 5−10% of the xylan resulted in close to optimal enzymatic digestion. As a result, a certain amount of lignin solubilization and xylan removal were necessary for an effective pretreatment for bioethanol production. solubilization and cellulose availability.20 The ammonia and hydrogen peroxide combined pretreatment has been used for bamboo (presoaking in hydrogen peroxide solution prior to AFEX process), which led to high enzymatic digestibility.4 In the present study, hydrogen peroxide pretreatment (HP), AFEX, and hydrogen peroxide presoaking prior to AFEX (HAFEX) were employed to improve enzymatic digestibility of CS. The effects of the three different methods on solid recovery, composition changes, and enzymatic hydrolysis were comparatively evaluated using the same CS material, the same cellulase enzyme, and identical laboratory analytical procedures. We focused on the differences of the three pretreatment methods. We also determined the extent of lignin solubilization and xylan removal caused by the different pretreatments and how these factors affected the total sugar yield.

1. INTRODUCTION Agricultural residue is considered one of the most strategically important sustainable energy sources due to its large quantities and various forms, and fuel ethanol is considered one of the best alternative or supplementary fuels to gasoline with the potential to alleviate rising carbon dioxide levels. Thus, cellulosic ethanol produced from agricultural residue has become a hot topic in the energy field and has been widely studied in recent years.1−3 Pretreatment, enzymatic hydrolysis, and fermentation are three main steps in the bioconversion of biomass to bioethanol.4 Efficient enzymatic hydrolysis requires pretreatment to alter or remove structural and compositional impediments to hydrolysis.5 This is known as biomass recalcitrance which is partially influenced by the presence of lignin and its complex interactions with carbohydrates.6,7 Consequently, the development of effective pretreatments to overcome biomass recalcitrance is one of the major technological challenges for bioconversion process commercialization.5−8 Corn stover (CS) is one of the most abundant crop residues in the world and a potential feedstock for biofuel production.9,10 The ammonia-based pretreatments such as ammonia fiber expansion pretreatment (AFEX), low moisture anhydrous ammonia pretreatment (LMAA), and ammonia recycle percolation (ARP) are very effective for enhancing enzymatic hydrolysis of CS.11−13 Some research shows that lignin and hemicellulose are the main factors that influence enzymatic digestibility.14,15 Lignin removal is an oft-cited goal for pretreatment, but anhydrous ammonia processes (LMAA, AFEX) are not effective at removing lignin and hemicelluloses.16−19 Hydrogen peroxide is a delignifying reagent and can enhance enzymatic conversion by increasing lignin © 2014 American Chemical Society

2. METHODS 2.1. Materials and Chemicals. Corn stover, harvested (about 95 d from sowing to harvesting) in July 2012, was kindly provided by Pingshan farm in Lin’an City (30.23, 119.72), Zhejiang Province, China. The entire above ground plant was used, including corn stalk, leaves, husks, and ear. Air-dried samples were cut to 3−5 cm and then further dried using a 40 °C oven until the CS moisture content was reduced to 5−15% (dry basis). Before treatment, the sample was milled using a laboratory mill and screened to obtain the particle size less than 1.0 mm with a sieve shaker. The milled CS was collected in sealed plastic bags kept at −20 °C for all tests. The composition of raw material was measured as described in Section 2.4. Received: June 21, 2014 Revised: September 1, 2014 Published: September 5, 2014 6392

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Table 1. Solid Recovery and Composition Changes of Treated Corn Stover from Different Pretreatments methods untreated HPa

AFEXb

H-AFEXb

ammonia loading (g/g dry biomass)

H2O2 loadingc (g/g dry biomass)

solid recovery (%)

glucand (%)

xylan (%)

lignin (%)

glucan removal (%)

xylan removal (%)

lignin removal (%)

0 0 0 0 0 0.5 1.0 1.5 2.0 2.5 1.0 1.0 1.0 1.0 1.0

0.5 0.7 0.9 1.4 2.0 0 0 0 0 0 0.1 0.3 0.5 0.7 0.9

100 96.4 95.5 92.4 90.1 87.0 97.7 97.6 97.6 97.2 96.2 96.4 96.0 94.4 91.7 90.4

34.7 32.1 32.1 30.5 28.7 29.1 32.3 30.8 30.3 28.5 27.6 33.4 33.0 31.5 30.7 28.8

21.4 19.7 19.6 18.6 17.1 15.0 20.8 20.5 20.1 19.5 18.4 20.4 20.0 19.9 19.4 18.3

20.6 17.6 16.4 14.7 12.9 12.1 18.5 17.4 17.1 16.6 15.3 18.7 17.5 17.0 16.1 15.4

7.4 7.5 12.0 17.1 15.9 6.9 11.0 10.9 17.8 20.4 3.7 4.9 9.3 11.5 17.0

7.7 8.3 13.3 20.2 30.0 2.8 4.1 6.2 8.9 14.0 4.7 6.5 7.2 9.5 14.4

14.7 20.5 28.5 37.3 41.3 10.0 15.7 17.2 19.5 21.8 9.2 15.0 17.4 21.6 25.2

HP pretreatment conditions: 60 °C, 1 h residence time, and 0.11 water loading. bAFEX and H-AFEX pretreatment conditions: 130 °C, 10 min residence time, and 0.7 water loading. cThe H2O2 loading was g 30 wt % H2O2 solution/g of dry biomass. dHere, the glucan included other glucosebased monosaccharides and oligosaccharides present in corn stover.

a

All chemicals were purchased from Shanghai Sigma-Aldrich Chemical Co. Ltd. (China), unless otherwise noted. Sulfuric acid (98 wt %, Minxing Chemical Co. Ltd. Zhejiang, China) was used in acid hydrolysis for composition analysis. Ammonia (99 wt %, Longsan Chemical Co. Ltd., Zhejiang, China) and hydrogen peroxide solutions (30 wt %, Tongsheng Chemical Co. Ltd., Jiangsu, China) were used in the pretreatment process. The cellulase from Trichoderma reesei (Novozyme 50013) and β-glucosidase from Aspergillus niger (Novozyme 50010) were provided by Novozymes (China) Investment Co. Ltd. Xylanase was provided by Shandong Zesheng Bioengineering Technology Co. Ltd. (China). 2.2. Pretreatments. 2.2.1. Hydrogen Peroxide Pretreatment (HP). Prior to pretreatment, CS (10 g dry biomass) was saturated by different loadings of 30 wt % hydrogen peroxide solution. After equilibrating for 10 min, the prewetted biomass was put into a 200 mL screw-cap flask. HP pretreatment was performed in a water bath at 60 °C for 1 h and then cooled down to room temperature. The treated CS was taken out from the flask and oven-dried for about 24 h at 40 °C, then stored in sealed plastic bags, and kept at −20 °C until undergoing composition analysis or enzymatic hydrolysis. The pretreatment variable was hydrogen peroxide loading which varied from 0.5 to 2.0 g 30 wt % hydrogen peroxide solution per g dry biomass. 2.2.2. Ammonia Fiber Expansion Pretreatment (AFEX). To carry out AFEX pretreatment, the biomass (20 g dry biomass) was saturated by deionized water until 0.7 water loading (g water per g dry biomass) was obtained. After penetrating for 10 min, the prewetted biomass was put into a high pressure reactor and sealed. The required weight of liquid ammonia was charged into the reactor containing biomass through a pressure cylinder, and the temperature of the reactor was raised rapidly to 130 °C and held for 10 min. Treatment was ended by opening the exhaust valve to release ammonia, and then, the treated CS was taken out from the reactor. The drying and storage of treated CS were identical to the HP process. Ammonia loading was the only pretreatment variable that was investigated and varied from 0.5 to 2.5 g ammonia per g dry biomass. 2.2.3. Ammonia and Hydrogen Peroxide Combined Pretreatment (H-AFEX). The H-AFEX pretreatment process was similar to the AFEX process but added a presoaking step using 30 wt % hydrogen peroxide solution. After saturation of hydrogen peroxide and water into the biomass, it was put into the reactor for AFEX treatment at 130 °C for 10 min at 1.0 ammonia loading (g ammonia per g dry biomass). The collection and drying of treated biomass were in accordance with the AFEX pretreatment mentioned above. The pretreatment variable

was hydrogen peroxide loading, and its range was from 0.1 to 2.0 g 30 wt % hydrogen peroxide solution per g dry biomass. 2.3. Enzymatic Digestibility. Without water washing, enzymatic hydrolysis of treated substrates was conducted on the basis of the NREL protocol at a total volume of 15 mL in 20 mL screw-cap vials.21 All samples were diluted to 1% glucan concentrations in sodium citrate buffer (0.05 M). Tetracycline and cycloheximide were added at 40 and 30 μg/mL to inhibit microbial growth. After equilibrating in a shaking incubator at 50 °C and 150 rpm for 1 h, Novozyme 50013(15 FPU/g glucan), Novozyme 50010 (64 CGU/g glucan), and xylanase (1000 IU/g glucan) were added, which marked the start (0 h) of enzymatic hydrolysis. One mL of supernatant was taken at 72 h, after which the sample was heated at 100 °C for 20 min, chilled at −20 °C for 5 min, and centrifuged at 15 000 rpm for 5 min. The hydrolyzate was frozen at −20 °C until undergoing HPLC analysis. Each sample was run in duplicate with the mean value reported. 2.4. Analytical Methods. 2.4.1. Composition Analysis. The moisture contents of untreated and treated biomass were measured by a moisture analyzer (Sartorius, Model MA35), and the composition analysis was performed on the basis of the standard LAP (Laboratory Analytical Procedures) from NREL.21 The average of duplicate runs was used in reporting. 2.4.2. HPLC Analysis. The monosaccharide sugars in hydrolyzates from composition analysis and enzymatic hydrolysis samples were quantified by an Agilent 1200 Series HPLC system equipped with a refractive index (RI) detector. A Bio-Rad Aminex HPX-87H column was employed for separating monosaccharides. The column was operated at 50 °C, and the mobile phase was sulfuric acid solution (0.005 M, 0.22 μm filtered, and degassed) at a flow rate of 0.5 mL/ min. The concentration of monosaccharide was determined using a calibration curve of standard sugars. The samples were run in duplicate to verify reproducibility. 2.4.3. Calculations. The solid recovery of treated biomass was defined as follows:

solid recovery(%) =

g of dry treated biomass × 100% g of dry untreated biomass

(1)

In order to compare and track the composition changes, the percent contents of glucan, xylan, and lignin in the treated solids were multiplied by the solid recovery to convert to untreated CS basis. Glucose/xylose contents were converted to glucan/xylan for glucan/xylan conversions during enzymatic hydrolysis. The glucan/ xylan conversions were defined as follows: 6393

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g of total released glucose × 0.9 × 100% g of initial glucan added

insoluble lignin increased from 10.0% to 21.8% as ammonia loading increased from 0.5 to 2.5. Compared with AFEX pretreatment, the lignin removal of HP pretreatment was greater, especially under high chemical loading. Greater delignification was observed with 0.5 hydrogen peroxide loading at 130 °C in the H-AFEX process (17.4%) than the 0.5 dose in the HP pretreatment at 60 °C (14.7%). There were two aspects contributing to this phenomenon. First, the rate and extent of chemical reaction could be intensified under high temperature, which was accompanied by enhancement of delignification potential.4 Furthermore, when pH is above 8.5, the hydrogen peroxide is accelerated and readily decomposed to perhydroxy anion and active hydroxyl radicals.26 Gould studied alkaline peroxide pretreatment of crop residues and reported that the optimum pH for delignification is 11.5− 11.6.25 The copresence of hydrogen peroxide and ammonia promotes the degradation and oxidation of lignin. 3.3. Comparison of Glucan/Xylan Conversion by Different Pretreatments. The effects of various chemical loadings on glucan/xylan conversions and lignin/xylan removal by different pretreatments are given in Figure 1. For the purpose of comparison, enzymatic hydrolysis was also carried out on raw material not subjected to any pretreatment.

(2)

0.9 is the conversion factor of glucose to equivalent glucan. xylan conversion(%) =

g of total released xylose × 0.88 × 100% g of initial xylan added

(3)

0.88 is the conversion factor of xylose to equivalent xylan. The total sugar yield (g) was calculated as the total mass of glucose and xylose released per kilogram of untreated dry biomass.

3. RESULTS AND DISCUSSION 3.1. Comparison of Solid Recovery by Different Pretreatments. The raw material used in this study contained about 34.7% glucan, 21.4% xylan, 4.3% arabinan, and 20.6% Klason lignin. Table 1 shows the effect of varied chemical loadings on solid recovery by the different pretreatments. The solid recovery decreased with increasing chemical loading irrespective of the pretreatment method. However, the solid recovery of AFEX-treated CS (96.2−97.7%) was much higher than that of HP-treated CS (87.0−96.4%) and H-AFEX-treated CS (76.5−96.4%) under the described conditions. Compared with ammonia, the hydrogen peroxide had a significant effect on weight loss (especially under high loading), and the interaction of ammonia and hydrogen peroxide led to the lowest solid yield. Several studies suggested that the AFEX pretreatment significantly modified the secondary cell walls (S1) and the middle lamella regions, and the clear changes in biomass during AFEX were cleaving lignin−carbohydrate ester linkages rather than removing any of the lignin or hemicelluloses.22,23 There was very little solid loss (less than 3.8%) with AFEX pretreatment. However, lignin reacts readily with hydrogen peroxide and can produce low molecular weight degradation products.24 During H-AFEX pretreatment, the alkaline peroxide reaction not only attacks the major sites of lignin but also can cause the changes in physical, microscopic, and morphological properties of cellulose.25 3.2. Comparison of Composition Changes by Different Pretreatments. The composition changes for the three different pretreatments are presented in Table 1. The glucan and xylan removals of HP pretreatment were 15.9% and 30.0%, respectively, at 2.0 hydrogen peroxide loading, and for AFEX pretreatment, they were 17.8% and 8.9%, respectively, at 2.0 ammonia loading (20.4% and 14.0% at 2.5 ammonia loading). The data imply that high hydrogen peroxide loading mainly caused xylan removal, while high ammonia loading mainly caused glucan removal (including other glucose-based monosaccharides and oligosaccharides). The glucan and xylan removals of the H-AFEX process at 0.9 hydrogen peroxide loading (and 1.0 ammonia loading) were 17.0% and 14.4%, respectively, while the corresponding parts of AFEX pretreatment (1.0 ammonia loading) and HP pretreatment (0.9 hydrogen peroxide loading) were 11.0% and 4.1%, 12.0% and 13.3%, respectively. H-AFEX pretreatment was more likely to cause polysaccharide loss. In the range of experimental conditions, the polysaccharide loss of the H-AFEX process at 1.0 ammonia loading and 0.9 hydrogen peroxide loading was approximate to AFEX at 2.5 ammonia loading. This indicates that the copresence of hydrogen peroxide and ammonia caused more polysaccharide loss. The acid-insoluble lignin removal by HP pretreatment increased from 14.7% to 41.3% as hydrogen peroxide loading increased from 0.5 to 2.0. For AFEX pretreatment, acid-

Figure 1. Glucan/xylan conversions following enzymatic hydrolysis of untreated and pretreated corn stover and pretreatment chemical loading impact on increased lignin/xylan removal following pretreatment. UN represents the untreated biomass; Gc represents the glucan conversion; Xc represents the xylan conversion. The enzymatic hydrolysis was run in duplicate with the mean value reported. The error bar shows the standard deviation.

For HP pretreatment, the glucan conversion increased significantly with increasing hydrogen peroxide loading from 0.5 to 1.4, and then, it dropped, while the xylan conversion remained relatively constant at around 50% over the entire range. The maximum glucan conversion (73.4%) peaked at 1.4 hydrogen peroxide loading, and the xylan conversion was 50.0% under this condition. This could be regarded as the optimal HP pretreatment condition. Compared with raw material, the glucan conversion increased due to the presence of hydrogen peroxide, whereas the xylan conversion was insensitive to hydrogen peroxide. Increased cellulose availability and lignin 6394

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lignin in the inner cell wall, and a striking increase in the cellulose polymer’s hydration (pH 11.5 with 1% H2O2-treated straw increased the water absorbency by 300%).25,26 3.4. Effect of Lignin Removal on Total Sugar Yield. Figure 2 illustrates the effect of lignin removal on enzymatic

solubilization are also observed during the peroxide pretreatment of wheat straw and cotton stalk.20,27 For AFEX pretreatment, it was obvious that the glucan and xylan conversions increased synchronously in the presence of ammonia. However, the glucan conversion (75.2−82.1%) and xylan conversion (75.2−79.5%) had no significant changes over the entire ammonia loading range. This means that AFEXtreated CS was relatively susceptible to hydrolyze even at low ammonia loading. The maximum glucan conversion (82.1%) was obtained at 1.0 ammonia loading, and the xylan conversion was also fairly high (79.5%). This reaction condition could be regarded as an optimum AFEX pretreatment condition. Compared with polysaccharide conversions of the HP process under the optimal condition, the glucan and xylan conversions of AFEX were 11.9% and 50.8% higher, respectively. Therefore, the ammonia treatment had a significant effect on polysaccharide conversion of CS (especially the xylan conversion) compared with hydrogen peroxide treatment. The investigation by Chundawat and his colleagues elaborates the mechanism of AFEX-treatment: ammonia fiber expansion dissolves and extracts cell wall decomposition products including arabinoxylan oligomers, lignin-based phenolics, and amides and deposits them on the outer cell wall as the ammonia evaporates. A highly porous structure is formed during AFEX treatment and significantly enhances the accessibility of cellulase to embedded cellulosic microfibrils.22,28 Furthermore, AFEX leads to a decrease in intrachain hydrogen bonds of cellulose fibrils, which mostly causes the inherent recalcitrance of crystalline cellulose. The disruption of the hydrogen bond network increases the number of solvent-exposed glucan chain hydrogen bonds with water and hence increases the glycosidic bond accessibility.29 For H-AFEX pretreatment, the highest glucan and xylan conversions, 88.1% and 90.6%, respectively, were both at 0.5 hydrogen peroxide loading. Compared with the polysaccharide conversions of AFEX and HP under their optimal conditions, the glucan and xylan conversions of H-AFEX were 5.7% and 11.2% higher than those of AFEX pretreatment and 14.4% and 40.6% higher than those of HP pretreatment. This implies that the interaction of ammonia and hydrogen peroxide was more effective toward improving polysaccharide conversions of CS than either liquid ammonia or hydrogen peroxide alone. This is consistent with earlier findings: hydrogen peroxide pretreatment under alkaline conditions has a significant effect on improving polysaccharide conversions compared with hydrogen peroxide alone.4,30 Investigation by Gould elucidates the mechanism of alkaline peroxide pretreatment of crop residues. The decomposition of H2O2 from H-AFEX pretreatment can be expressed by reactions 4 and 5.25 H 2O2 → H+ + HOO−

(4)

H 2O2 + HOO− → ·OH + O2− · + H 2O

(5)

Figure 2. Sugar yields following enzymatic hydrolysis of pretreated corn stover at different levels of lignin removal during pretreatment. Sugar represents the total sugar yield from enzymatic hydrolysis. Glu represents the glucose yield from enzymatic hydrolysis. Xyl represents the xylose yield from enzymatic hydrolysis. The enzymatic hydrolysis was run in duplicate with the mean value reported. The error bar shows the standard deviation.

hydrolysis sugar yields for different pretreatments. For AFEX and H-AFEX pretreatments, when the lignin removal was less than 17%, the sugar yield increased with increasing lignin removal, and after which it deceased. The total sugar yield reached relatively high values when the lignin removal ranged from 15% to 20%. However, the sugar yield of the HP process was insensitive to lignin removal when it ranged from 14.7% to 37.3%, while a significant drop caused by decreasing glucose yield was observed when the lignin removal was more than 37.3%. Irrespective of the pretreatment method, the sample having the highest lignin removal (HP: 41.3%; AFEX: 21.8%; H-AFEX: 25.2%) did not give the highest sugar yield. The maximum sugar yield occurred at 20.5%, 15.7%, and 17.4% lignin removal for the HP, AFEX, and H-AFEX processes, respectively. These results suggested that a certain lignin solubilization or remobilization was crucial for an effective pretreatment for bioethanol production. This is consistent with CS treated by green liquor, and this trend is similar to results obtained from pretreatment of sugar cane bagasse, softwoods, and wheat straw.31−34 The removal of lignin appears to involve changes in physical, microscopic, and morphological properties of the cellulose fibers and a more exposed accessible area.33 3.5. Effect of Xylan Removal on Total Sugar Yield. Figure 3 shows the effect of xylan removal on the total sugar yield in different pretreatments. For the AFEX and H-AFEX

The hydroxyl radical is a powerful lignin oxidant which could delignify lignocellulose by degradation and oxidation, while the perhydroxy anion cleaves diferulate linkages which cross-link polysaccharides resulting in depolymerization of carbohydrates.26 During alkaline peroxide pretreatment, the (5) reaction could represent the primary way leading to the structure changes and composition characteristic of CS.25 Therefore, the mechanism of high enzymatic conversion caused by the H-AFEX process involves the release, relocalization of 6395

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Figure 4. Correlation between the glucose yield and xylose yield following enzymatic hydrolysis of AFEX-treated and H-AFEX-treated corn stover. The enzymatic hydrolysis was run in duplicate with the mean value reported. The error bar shows the standard deviation.

high hydrogen peroxide loading mainly caused xylan loss. Ammonia had a significant effect on polysaccharide conversions, while hydrogen peroxide only improved glucan conversion. The weight loss, polysaccharide removal, and lignin solubilization could be promoted in the co-occurrence of ammonia and hydrogen peroxide. The pretreatments reached relatively high sugar yields at 15−20% lignin solubilization and 5−10% xylan removal. Therefore, a high retention of carbohydrates with a certain lignin solubilization and xylan removal rate is the key for an effective pretreatment for cellulosic ethanol production.

Figure 3. Sugar yields following enzymatic hydrolysis of pretreated corn stover at different levels of xylan removal during pretreatment. Sugar represents the total sugar yield from enzymatic hydrolysis. Glu represents the glucose yield from enzymatic hydrolysis. Xyl represents the xylose yield from enzymatic hydrolysis. The enzymatic hydrolysis was run in duplicate with the mean value reported. The error bar shows the standard deviation.

processes, when the xylan removal was less than 8%, the sugar yield increased as xylan removal increased, while it showed almost a negative linear correlation with xylan removal when it was more than 8%. Similarly to lignin removal, the sugar yield of the HP process was insensitive to xylan removal (7.7% to 30.0%) under experimental conditions. The maximum sugar yields of the HP, AFEX, and H-AFEX processes were obtained at 8.2%, 6.2%, and 7.2% xylan removal, respectively. Therefore, 5−10% xylan removal was within the region of high enzymatic hydrolysis. The removal of xylan from the biomass enhanced enzymatic digestion owing to the improvement of the accessibility of cellulose or xylan to the enzyme. However, high xylan removal would be related to the release of xylan products such as xylo-oligomers that may inhibit the cellulases.35 Furthermore, higher xylan removal was accompanied by greater loss of solids and glucan, which could influence the sugar recovery.36 A significant positive linear correlation is shown in Figure 4 between glucose yield and xylose yield during AFEX and HAFEX pretreatment of CS following enzymatic hydrolysis. This result is consistent with that observed for switchgrass treated by AFEX.23,35 Thus, increasing the recovery of xylan was more important than removal to improve glucose yield. Therefore, xylan removal is not always necessary for high hydrolysis yields during alkaline-based pretreatments.37 However, there is a close relationship between enzymatic digestibility with hemicellulose removal for sugar cane bagasse treated by hot water.38

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86 574-8760-0872. E-mail: [email protected]. *Tel.: +86 25-5860-6502. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by funds from Preresearch Project of Research Center of Biomass Resource Utilization, Zhejiang A&F University (No. 2013SWZ03), and China National Science & Technology Pillar Program (No. 2013BAD08B04).

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REFERENCES

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4. CONCLUSION Hydrogen peroxide had a significant effect on weight loss and lignin solubilization compared with ammonia. High ammonia loading mainly caused glucan and soluble sugar losses, while 6396

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dx.doi.org/10.1021/ef5013837 | Energy Fuels 2014, 28, 6392−6397