Phase-Exchange Solvent Pretreatment Improves the Enzymatic

Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

pubs.acs.org/journal/ascecg

Phase-Exchange Solvent Pretreatment Improves the Enzymatic Digestibility of Cellulose and Total Sugar Recovery from Energy Sorghum Qiang Yu,† Yunxuan Wang,‡ Wei Qi,† Wen Wang,† Qiong Wang,*,† Shixiang Bian,§ Yinping Zhu,§ Xinshu Zhuang,*,† Zhongming Wang,† and Zhenhong Yuan† †

Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, CAS Key Laboratory of Renewable Energy, Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, No. 2, Nengyuan Rd., Wushan, Guangzhou 510640, China ‡ School of Chemistry and Chemical Engineering, Guangzhou University, 230 University City Outer Ring Road, Guangzhou 510006, China § University of Science and Technology of China, No. 96, JinZhai Rd., Baohe, Hefei 230026, China S Supporting Information *

ABSTRACT: Traditional liquid hot water pretreatment (LHWP) has a high water consumption, a high reaction temperature, and low lignin removal, making it unsuitable for industrial applications of biomass conversion. In this study, we developed a new phase-exchange solvent pretreatment (PESP) based on the varied phase composition of furfural (FF)-water at different temperatures. Substitution of water with FF had no significant influence on xylan hydrolysis, but it improved the mass transfer performance. At the optimum conditions of 180 °C, an FF:water ratio of 30:70 (v/v), a solid:liquid ratio of 1:8 (w/v), and 0.2 wt % sulfuric acid for 30 min, the PESP of energy sorghum achieved a 74.98% total xylose yield and removed 85.08% of the lignin, while there was a selective distribution of sugar and lignin in the aqueous and organic phases, respectively. Moreover, 99.58% of the cellulase enzyme digestibility and 94.02% of the total sugar recovery were achieved after 72 h. This unusually high enzymatic digestibility could be attributed to the physicochemical changes in the substrate after the pretreatment. When compared with a traditional LHWP (5% solid loading), the water consumption decreased by approximately 72% and the lignin removal increased by 60.74%. These results demonstrate that PESP is a promising technology for biorefining lignocellulosic biomass with high efficiency and low energy consumption. KEYWORDS: Biomass, Enzymatic hydrolysis, Pretreatment, Sugar recovery

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INTRODUCTION

energy consumption, and (6) minimize the use of chemicals or make them recyclable. Liquid hot water pretreatment4,5 (LHWP, at 160−240 °C) is a green approach because it does not require the addition of chemicals such as acids, alkalis, or organic solvents. It has been reported that LHWP causes a high dissolution of hemicellulose (>80%), and the substrates can be digested and fermented directly without a detoxification operation.6−8 However, LHWP requires high water consumption to maintain a high concentration of hydronium ions and a good heat/mass transfer performance.9,10 Additionally, similar to other low pH pretreatments,11 most of the lignin remains as solids

The native three-dimensional structure of lignocellulosic biomass, which is composed of cellulose, hemicellulose, and lignin, protects it from microbial and enzymatic deconstruction.1 Owing to this characteristic recalcitrance of lignocellulosic biomass, the cost of bioethanol production is high. To improve the efficiency of the biochemical conversion of lignocellulosic biomass and overcome its recalcitrance to degradation, pretreatment is necessary.2,3 An effective pretreatment process should (1) retain most of the cellulose, while removing the steric hindrance from the lignin and hemicellulose, (2) maintain a high sugar concentration and yield during the pretreatment and enzymatic hydrolysis, (3) reduce the formation of inhibitors of enzymes and microorganisms, (4) recover and reuse most of the lignin, (5) have low water and © XXXX American Chemical Society

Received: August 29, 2017 Revised: November 21, 2017

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DOI: 10.1021/acssuschemeng.7b02995 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. Simplified block flow diagram of the proposed PESP process.

temperatures of 160−240 °C. Additionally, the different products obtained were distributed selectively into the aqueous and organic phases when the system was cooled. FF, which is one of the renewable chemicals obtained from biomass, similar to GVL and THF, was selected as the solvent owing to its varied phase composition with water at different temperatures. It is only slightly soluble in water at room temperature, and it completely dissolves at a critical temperature of 122.7 °C. As a result, a PESP based on FF and water not only maintains the competitive edge of a monophasic reaction, but also facilitates the separation of products. Most of the sugars are expected to be recovered in the aqueous phase after FF−water pretreatment, and FF can be recycled after distillation. Similar to the THF pretreatment process,25 the dissolved lignin product precipitates as a solid after separating and recovering the FF (Figure 1). Additionally, these lignin solids can be further refined as highly valuable chemicals and fuels. Herein, we subjected energy sorghum to a PESP and conducted a systematic investigation of the released sugars. Moreover, we employed acid catalysts to enhance the total xylose recovery and lignin removal, and we discuss the possible mechanisms for the enhancement of enzymatic digestibility and total sugar recovery following PESP. The results obtained in this study provide useful insights into the industrial applications of PESP technology, and they can aid the development of novel pretreatment processes with high efficiency and low energy consumption.

following LHWP. Owing to these disadvantages, the costs associated with energy input and cellulase loading are high, making LHWP unsuitable for industrial applications. Although some two-stage methods12,13 or combined pretreatments9,14 have been developed, most of them have complex processes or produce unsatisfactory results. In contrast to LHWP, organic solvent pretreatment15,16 using an alcohol,17 organic acid,18 or acetone,19 not only minimizes water consumption, but also achieves a high fractionation of lignin (>70%).20 However, alcohols with low boiling points are flammable and explosive, while polylols with high boiling points have high recycling costs. Moreover, formylation of cellulose during formic acid pretreatment decreases the enzymatic digestibility of cellulose.21 Recently, pretreatment with some biomass-derived solvents such as γ-valerolactone (GVL)22,23 and tetrahydrofuran (THF)24,25 has been demonstrated to achieve high lignin removal and saccharification with easy solvent recyclability. However, all these pretreatments are conducted in a monophasic system, and the separation of the products in the liquid fraction is still a major issue.26 A biphasic reaction system is usually used to dehydrate sugars into furfural (FF),27 5-hydroxymethylfurfural (HMF),28 and levulinic acid. These products are formed in the aqueous phase and can be extracted simultaneously into an organic phase to prevent further degradation.29 Unlike the traditional functions of a biphasic reaction system, during the pretreatment of lignocellulosic biomass, the hydrolysis of hemicellulose and the dissolution of lignin are expected to occur in the aqueous and organic phases for the easy recovery of sugars and lignin from the aqueous and organic phases, respectively. In the present study, we developed a phase-exchange solvent pretreatment (PESP), which comprises a monophasic system in which hemicellulose hydrolysis and lignin dissolution occur at reaction

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RESULTS AND DISCUSSION Comparison of Different Aqueous−Organic Solution Pretreatments. The optimum reaction temperature of LHWP for energy sorghum is 180 °C,30 and here, 50% of the water was replaced with three biomass-derived solvents, GVL, THF, and B


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Figure 2. Total xylose yield and concentration (a) and chemical composition changes (b) after different pretreatments of energy sorghum (180 °C, an organic solvent:water ratio of 50:50 (v/v) and a solid:liquid ratio of 1:8 (w/v) for 30 min; for LHWP, 100% water was used).

Figure 3. Effect of reaction time (a), temperature (b), FF:water ratio (c), and solid:liquid ratio (d) on the concentrations of total xylose and xylooligomers and the total xylose yield.

FF, to evaluate their performance in terms of sugar release and lignin dissolution (conducted at 180 °C, an organic solvent:water ratio of 50:50 (v/v) and a solid:liquid ratio of 1:8 (w/v) for 30 min). As shown in Figure 2a, most of the xylose existed in the form of oligomers (>80%) in all four pretreatments, and the highest total xylose concentration (30 g/L) was obtained following the FF−water pretreatment because of the selective distribution of sugars into the aqueous phase. The total xylose

yields were 50.24%, 32.87%, 48.91%, and 51.08% for the THF− water, GVL−water, FF−water, and LHWP, respectively. These four pretreatments (as listed above) removed 54.63%, 39.41%, 70.2%, and 66.69%, respectively, of the xylan; 65.09%, 69.99%, 74.48%, and 35.71%, respectively, of the lignin; and the amounts of the remaining glucans were 97.29%, 89.51%, 92.89%, and 93.19%, respectively (Figure 2b). It is obvious that the pretreatment conditions used above may not be optimum C


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Figure 4. Effect of the addition of different acid catalysts on sugar recovery (a), degradation (b), and chemical composition of the substrate (c) (180 °C, an FF:water ratio of 30:70 (v/v), a solid:liquid ratio of 1:8 (w/v), a 0.5 wt % acid catalyst loading, and a 30 min reaction time).

reaction rate constant, k2, changed only slightly with or without FF in the LHW. Moreover, as discussed earlier, the proportion of xylooligomers in the total xylose content increased to 84.34% in the FF−water system (50:50 (v/v)), while it was 80.39% in the LHW system. Nevertheless, no significant improvement in the total xylose recovery was noted when 50% of the water was replaced with FF (Figure 2a). In general, while the addition of FF had no direct influence on the activation energy of the reaction, it might improve the heat and mass transfer performance. Although the quantity of hydronium ions decreased at a low water loading, autohydrolysis did not decrease in the FF−water system. Optimization of PESP To Maximize Total Xylose Recovery. Energy sorghum was pretreated with FF−water at 180 °C, a 50:50 (v/v) organic solvent:water ratio, and a 1:8 (w/v) solid:liquid ratio for 10−70 min. The peak of total xylose yield appeared at 30 min and then decreased with increasing reaction times (Figure 3a). In contrast, the xylan removal ratio increased with increasing reaction times, and it reached 41.73%, 70.20%, and 82.46% after 10, 30, and 70 min, respectively (Table S3). Similarly, despite the linear increase in xylan removal (36.32% at 160 °C for 30 min, reaching 94.76% at 200 °C for 30 min), the total xylose yield decreased to less than 8% of that obtained at a high reaction temperature of 200 °C for 30 min, which resulted from further degradation (Figure 3b). Furthermore, the influence of the FF:water ratio on hemicellulose hydrolysis was investigated. As presented in Figure 3c, the total xylose yield decreased from 57.04% at an FF loading

for maximum sugar release. However, the data indicated that, similar to popular biomass-derived solvents (like GVL and THF), FF addition exhibited remarkable performance in terms of lignin removal and sugar yield. Furthermore, control experiments were designed to detect the stability of FF when heated to 180 °C for 30 min in the presence of an acid, and it was found that more than 97% of the initial FF was retained (Figure S1 and Table S1). To understand the effect of FF on sugar recovery, the xylose monomer consumption in the control group and oligosaccharide formation in the experimental group were calculated and compared with those obtained following LHWP. Previous studies have reported that hemicellulose can be autohydrolyzed in an acidic environment resulting from the hydronium ions in LHW and acetyl groups in biomass.31 A monophasic hemicellulose hydrolysis model is usually used to describe this process (eq 1),32 and the breakdown of oligomers to monomers is much slower than their formation, which differs completely from the process of dilute acid hydrolysis.33 kf

k1

k2

Xylan → Xylooligomers → Xylose → Furfural

(1)

In the present study, the concentration of xylose in the aqueous phase of the controlled FF−water system decreased with increasing reaction times, which indicated that degradation occurred. However, approximately 28.27% of the xylose was degraded in 30 min, which was close to that noted in the LHW system (27.02%) (Table S2), thus suggesting that the chemical D


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Figure 5. Effect of FA (a, b) and SA (c, d) addition on sugar recovery and degradation (180 °C, an FF:water ratio of 30:70 (v/v), a solid:liquid ratio of 1:8 (w/v), and a 30 min reaction time).

especially SA (92%) and MA (72.87%), was added. Moreover, lignin removal increased to 77.43%, 74.56%, and 78.83% in the FF−water−SA, FF−water−FA, and FF−water−MA systems, respectively, whereas it was only 36.47%, 32.06%, and 37.13% in the corresponding acid−water systems, respectively, and 61.39% in the FF−water system (Figure 4c). Figure 5 illustrates the correlation between the addition of an acid catalyst and the product distribution in aqueous phase. The concentrations of xylose and glucose increased as the FA loading increased from 1.89 and 0.18 g/L at 0.1 wt % to 8.45 and 1.16 g/L at 1.5 wt %, respectively (Figure 5a). However, the concentration and yield of total xylose first increased to 28.63 g/L and 66.45% at a 0.3 wt % FA loading and then decreased to 18.33 g/L and 42.55% at a 1.0 wt % FA loading, respectively. Although there were only slight variations in the total glucose recovery (approximately 7%), the total glucose yield presented a positive trend with increasing SA loading (Figure 5c). A high total glucose recovery of 29.34% was obtained at a 1.0 wt % SA loading, while the total xylose yield decreased significantly (only 20.93%). The concentrations of byproducts such as glucuronic acid, glycolic acid, acetic acid, and 5-HMF after the MA and SA catalysis (Figure 5b, d) were well correlated with the results of the total sugar recovery. It must be noted that the hydrolysis of hemicellulose improves with the increasing production of these acids.34 Moreover, there was no levulinic acid production; information about the changes in the mass of FF during the PESP process is presented in Table S2. Thus, taken together, the results

of 10% to 23.76% at an FF loading of 70%, and the optimum FF loading was 30% with a total xylose concentration and yield of 23.64 g/L and 54.86%, respectively. We speculated that the autohydrolysis of the aqueous phase contributed more to the dissolution of xylan. As indicated in Figure 3d, the PESP system presented good heat and mass transfer performances even at a high solid:liquid ratio of 1:8 (w/v). The total xylose yield remained relatively constant, ranging from 46.34% to 54.86%, with an increase in the solid:liquid ratio from 1:14 (w/v) to 1:8 (w/v). However, compared with the latter condition, the 75% increase in FF loading per gram of biomass at the solid:liquid ratio of 1:14 (w/v) might slightly reduce the yield because of the negative relationship between FF loading and sugar recovery. Acid Catalysis in PESP Enhances Total Xylose Recovery and Lignin Removal. Inorganic or organic acids are usually employed to enhance hemicellulose and lignin dissolution during organosolvent pretreatment.29 In the present study, under the optimum PESP conditions (an FF:water ratio of 30:70 (v/v), 180 °C, a solid:liquid ratio of 1:8 (w/v), and 30 min), sulfuric acid (SA), formic acid (FA), and maleic acid (MA) with 0.5 wt % loadings were used to improve the total xylose recovery. As shown in Figure 4a, the total xylose yields were 53.08%, 59.52%, and 45.97% in the FF−water-SA, FF− water−FA, and FF−water−MA systems, respectively. Although there was no significant improvement in total xylose recovery, the proportion of xylose monomers in the aqueous phase increased significantly when an acid catalyst (Figure 4b), E


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Figure 6. Enzymatic digestibility of the energy sorghum samples after different pretreatments: (a) 180 °C, an FF:water ratio of 50:50 (v/v) and a solid:liquid ratio of 1:8 (w/v) for 10−70 min, (b) 160−200 °C, an FF:water ratio of 50:50 (v/v) and a solid:liquid ratio of 1:8 (w/v) for 30 min. (c) A: 180 °C, 100% water, and a solid:liquid ratio of 1:8 (w/v) for 30 min; B: 180 °C, an FF:water ratio of 50:50 (v/v) and a solid:liquid ratio of 1:8 (w/v) for 30 min; C: 180 °C, an FF:water ratio of 30:70 (v/v) and a solid:liquid ratio of 1:8 (w/v) for 30 min; D: 180 °C, an FF:water ratio of 30:70 (v/v), a solid:liquid ratio of 1:8 (w/v), and 0.3 wt % FA for 30 min; E: 180 °C, an FF:water ratio of 30:70 (v/v), a solid:liquid ratio of 1:8 (w/v) and 0.2 wt % SA for 30 min; F: THF−water−SA system;25 G: GVL−water−SA system23).

only 48.17% (Figure 6b), which was close to that of the raw material (40.98%). However, after pretreatment at 200 °C for 30 min, the 72-h enzymatic digestibility of the treated samples increased to 99.98% because of the high xylan (94.76%) and lignin (92.09%) removal. Considering the total xylose and glucose recovered in the aqueous phase and the glucose produced during enzymatic hydrolysis, the total sugar recovery was 47.77%, 77.51%, and 79.25%, respectively, after noncatalyzed treatment at 160, 180, and 200 °C for 30 min, respectively (Table S3). As mentioned earlier, the loading of an acid catalyst led to the removal of more lignin and xylan. Accordingly, the 72-h enzymatic digestibility of the samples and the total sugar recovery of the different pretreatments with or without catalyst addition were compared (Figure 6c). The 72-h enzymatic digestibility of the LHW-treated samples (180 °C, a 30 min reaction time, and a solid:liquid ratio of 1:8 (w/v)) was only 61.62% with a total sugar recovery of 66.23%, and the values increased when a portion of the water was replaced with FF. At an FF:water ratio of 30:70 (v/v) (180 °C, a 30 min reaction time, a solid:liquid ratio of 1:8 (w/v)), the 72-h enzymatic digestibility of the samples and the total sugar recovery reached 91.2% and 80.04%, respectively. Interestingly, with the addition of 0.2 wt % SA, the 72-h enzymatic digestibility of the samples

demonstrate that the optimum catalyst loading was 0.2 and 0.3 wt % for the FF−water−SA and FF−water−FA systems, respectively. When compared with the FF−water pretreatment without an acid catalyst (Table S3), the total xylose recovery and lignin removal reached 74.98% (a 36.65% increase) and 85.08% (a 38.59% increase), respectively, when 0.2 wt % SA was loaded. Moreover, 89.65% of the initial glucans remained in the solid fraction, and high xylan and lignin removal can improve the accessibility of glucans to enzymes. Effect of PESP on the Enzymatic Digestibility of Cellulose. All the samples were tested for enzymatic digestibility to assess the effects of PESP. As shown in Figure 6a, the samples subjected to PESP showed significantly improved enzymatic digestibility of cellulose, and the 72-h enzymatic digestibility of the samples treated at 180 °C for 10− 70 min was 60.11%, 80.78%, 80.64%, and 77.40%, respectively. These results are in accordance with those of lignin removal (59.85%, 74.48%, 80.87%, and 76.25%, respectively, after 10− 70 min of pretreatment). It must be noted that the removal of lignin might remove the cellulose shield and enhance the rate and extent of enzymatic hydrolysis.35 Moreover, the improvement in enzymatic digestibility appeared to be more sensitive to the reaction temperature than time. The 72-h enzymatic digestibility of the treated samples at 160 °C for 30 min was F


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Figure 7. Scanning electron microscopic images of the samples after different pretreatments (200× ): (a) untreated; (b) treated at 180 °C, an FF:water ratio of 30:70 (v/v), a solid:liquid ratio of 1:8 (w/v), and 0.2 wt % SA for 30 min; (c) treated at 180 °C, an FF:water ratio of 30:70 (v/v), a solid:liquid ratio of 1:8 (w/v), and 0.3 wt % FA for 30 min; (d) treated at 180 °C, 100% water, and a solid:liquid ratio of 1:8 (w/v) for 30 min.

different pretreated substrates (65.80% for FF−water−SA, 64.46% for FF−water−FA, 62.17% for FF−water, 63.18% for LHWP, and 38.07% for the untreated biomass).

and the total sugar recovery reached 99.58% and 94.02%, respectively. A Fourier-transform infrared spectroscopy analysis of different residual solids (Figure S2), showed that the lignin residue became more hydrophobic after PESP36 because of the decrease in hydroxyl groups at 1040 cm−1 and carboxyl groups at 1740 cm−1. Additionally, hydrophobic interactions used to be considered to be one of the main factors involved in the nonproductive binding of cellulosic enzymes to lignin.37 When compared with the GVL−water−SA system23 (170 °C, a 60 min reaction time, a solid:liquid ratio of 1:15 (w/v), a GVL:water ratio of 80:20 (v/v), and 0.1 wt % SA) and the THF−water−SA system25 (150 °C, a 25 min reaction time, a solid:liquid ratio of 1:20 (w/v), a THF:water ratio of 50:50 (v/ v), and 0.5 wt % SA), the organic solvent usage in the FF− water−SA system was 1.6−2.7 times lower. Although more work should be done in the near future to investigate the effect of reduced enzyme loadings on total sugar yields, the proposed PESP process has the potential to effectively solve the bottleneck problems of traditional LHWP, such as low lignin removal and high water consumption.38 The possible mechanisms responsible for the enhancement of the enzymatic digestibility of the substrate during PESP were investigated based on an analysis of morphological changes in the substrate. As shown in Figure 7a, untreated sorghum consisted of rigid and highly ordered fibrils, whereas the fibers of the treated sorghum samples were separated from the initial connected structure and fully exposed (Figure 7b−d), especially after the FF−water−SA pretreatment. These morphological changes in the substrate may result from the different chemical compositions and the varied dissolution of chemicals during the four pretreatment processes.39 Furthermore, the microstructure characteristics of the substrate could directly affect the accessibility of the substrate to enzymes.40,41 In contrast, consistent with the previous results of the LHWP, the crystallinity index (CrI) of the substrate might not have a direct relationship with the enzymatic hydrolysis of biomass.30 There were no significant differences in the CrI value for the

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CONCLUSIONS The present study demonstrated a novel method of PESP of energy sorghum to improve the total xylose concentration (up to 32.30 g/L), enzymatic digestibility (up to 99.58%), and total sugar recovery (up to 94.02%) at the optimal conditions of 180 °C, an FF:water ratio of 30:70 (v/v), a solid:liquid ratio of 1:8 (w/v), 0.2 wt % sulfuric acid, and a 30 min reaction time. When compared with the traditional LHWP (5% solid loading), water consumption was reduced by approximately 72% and lignin removal was increased by 60.74%. The substitution of water with FF enhanced the changes in the biomass microstructure and improved the accessibility of cellulose to enzymes. The findings of this study may contribute to a breakthrough in realizing the high efficiency potential of biorefineries.

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MATERIALS AND METHODS

Materials. Energy sorghum was provided kindly by the National Energy R&D Centre for Nonfood Biomass (China Agricultural University, Beijing, China). The obtained sorghum material was milled by an electric grinder (DXF-10C, Guangzhou Daxiang Machine Technology Co. Ltd., Guangzhou, China) and sieved to approximately 1−2 mm. Its chemical composition (on a dry weight basis) was determined based on the National Renewable Energy Laboratory procedure42 as follows: 35.76% glucan, 21.23% xylan, 29.50% K-lignin, 8.24% water−ethanol extractives, and 5.27% other compounds, including ash. Analytical reagents, including FF (99%), GVL (98%), THF (99%), FA (98%), and other chemicals, were purchased from Aladdin (Shanghai, China). Cellulase was obtained from the Imperial Jade Biotechnology Co. Ltd. (Ningxia, China), and its activity was 113.2 filter paper units (FPU)/g with 306.9 mg protein/g. The xylanase activity in this commercial cellulase was 85 × 103 IU/g. Pretreatment and Enzymatic Hydrolysis. Pretreatment was performed in a 316-l stainless steel autoclave reactor with a capacity of 100 mL (MS-100-C276, Anhui Kemi Machinery Technology Co. Ltd., Hefei, China) with the ability to withstand a pressure of 10 MPa and a G



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temperature of 300 °C. The autoclave was placed in an electric heating furnace (600 W), and the reaction temperature (RT) was controlled by a programmable intelligent temperature controller, with the reactant temperature being measured directly by a thermocouple. The initial heating period from room temperature to one-half of the RT was 15 min, while the heating period from 1/2 of the RT to the RT − 20 °C was 20 min, and the last heating period from RT − 20 °C to the RT was 15 min. The reactor was filled with energy sorghum and phase-exchange solvent (FF and water) at solid:liquid ratios of 1:5, 1:8, 1:11, and 1:14 and then sealed and heated to RTs of 160, 180, and 200 °C with a magnetic agitator operating at 500 rpm for 10, 30, 50, and 70 min, respectively. Subsequently, the reactor was removed from the heating furnace and cooled in a cold-water bath (approximately 30 min to room temperature). The aqueous−organic phase could be observed clearly after centrifugation at 10,000 rpm for 10 min. Liquid samples were collected from the aqueous phase, and the quantities of released sugar and degradation products were measured by an e2695 high-performance liquid chromatograph (Waters, Milford, MA, USA) using a Shodex SH1011 column (Showa Denko K. K., Tokyo, Japan) coupled with a refractive index and ultraviolet detector. Total sugars were determined based on secondary hydrolysis into monomers with 4% SA,43 and the oligosaccharides were calculated by subtracting the initial monosaccharides from the total sugars. To evaluate the changes in their chemical composition, solid samples were collected and washed with acetone/water approximately five times until they became colorless, and lignin content here referred in particular to K-lignin. Similar to the operation of FF−water pretreatment, the THF and GVL pretreatments were conducted at 180 °C, with an organic solvent:water ratio of 50:50 (v/v) and a solid:liquid ratio of 1:8 (w/v) for 30 min. The details of the LHWP44 and enzymatic hydrolysis have been described elsewhere.32,45 The washed solids were further hydrolyzed at 50 °C, pH 4.8, and 5% (w/v) substrate, with a cellulase-loading of 40 FPU per g dry solid for 72 h. All experiments were performed in duplicate. The formulas used were as follows:

*E-mail: [email protected] (Q. Wang). *E-mail: [email protected] (X. Zhuang). ORCID

Qiang Yu: 0000-0003-1461-1497 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported financially by the Pearl River S&T Nova Program of Guangzhou, China (Grant No. 201610010110), the Young Top-notch Talent of Guangdong Province, China (Grant No. 2016TQ03N647), the National Natural Science Foundation of China (Grants No. 21476233, No. 21506216, No. 51506207, and No. 51561145015), the Key Project of the Natural Science Foundation of Guangdong Province (Grant No. 2015A030311022), the Natural Science Foundation for the Research Team of Guangdong Province (Grant No. 2016A030312007), the Guangdong Key Laboratory of New and Renewable Energy Research and Development (Grant No. Y709ji1001), and the Youth Innovation Promotion Association, CAS (Grant No. 2015289).

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REFERENCES

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total xylose (glucose) recovered in aqueous phase × 100 potential xylose (glucose) in the substrate

Xylan/glucan/lignin removal % initial xylan/glucan/lignin − residual xylan/glucan/lignin in solid = initial xylan/glucan/lignin × 100

and

Enzymatic digestibility % glucose in the liquid fraction = × 100 potential glucose in the substrate Characterization of the Biomass. The morphology of the untreated and treated energy sorghum samples were observed under a scanning electron microscope (S4800, Hitachi, Tokyo, Japan). The samples were characterized at a beam accelerating voltage of 2.0 kV, and the images were obtained at magnifications of 200× , as described in previous studies.46 To determine the CrI values, the samples were analyzed by X-ray diffraction using an X’Pert Pro MPD generator (PW3040/60, Philips, Amsterdam, The Netherlands), and the CrI values were calculated according to the intensity of the crystalline (2θ = 22.5°) and amorphous (2θ = 18.7°) regions.12

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AUTHOR INFORMATION

Corresponding Authors

Total xylose (glucose) yield% =

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02995. Control experiments for the PESP process and the mass balance of different PESP pretreatments. (PDF) H


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