Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Kinetic resolution wikipedia , lookup
Hofmann–Löffler reaction wikipedia , lookup
Strychnine total synthesis wikipedia , lookup
Baylis–Hillman reaction wikipedia , lookup
Ring-closing metathesis wikipedia , lookup
Physical organic chemistry wikipedia , lookup
Petasis reaction wikipedia , lookup
Research Article pubs.acs.org/journal/ascecg Coproduction of Furfural and Easily Hydrolyzable Residue from Sugar Cane Bagasse in the MTHF/Aqueous Biphasic System: Influence of Acid Species, NaCl Addition, and MTHF Xing-kang Li,†,§ Zhen Fang,*,‡,† Jia Luo,† and Tong-chao Su† † Key Laboratory of Tropical Plant Resources and Sustainable Use, Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences, 88 Xuefulu, Kunming, Yunnan Province 650223, China ‡ Biomass Group, College of Engineering, Nanjing Agricultural University, 40 Dianjiangtai Road, Nanjing, Jiangsu 210031, China § University of Chinese Academy of Sciences, 19A Yuquan Road, Beijing 100049, China ABSTRACT: In order to develop a process for the simultaneous production of furfural and easily hydrolyzable cellulose, the degradation of sugar cane bagasse in a single aqueous system and in a 2methyltetrahydrofuran (MTHF)/aqueous AlCl3 biphasic system was studied. In single aqueous system, the influence of acid species (FeCl3, HCl, and AlCl3) on furfural production and cellulose degradation was investigated at 150 °C. FeCl3 and HCl promoted furfural production from hemicellulose but with severe cellulose degradation. AlCl3 decreased cellulose degradation with considerable furfural yield and high glucan content in solid residues. The role of NaCl in furfural production and cellulose decomposition was also investigated in the single aqueous system using different acids as catalysts. Addition of NaCl significantly promoted furfural yield but also accelerated cellulose decomposition when FeCl3 or HCl was used as catalyst. In the AlCl3-catalyzed system, NaCl had less influence on residue yield and its composition, although NaCl also promoted furfural production. The influence of MTHF on furfural yield, residue composition, and enzymatic hydrolysis of residue was also studied. Under the best conditions (0.45 g of bagasse, 9 mL of MTHF, 9 mL of water, 0.1 M AlCl3, 150 °C, 45 min, and 10 wt % NaCl), 58.6% furfural was obtained while more than 90% of cellulose remained in the residue. The organic phase was separated from the aqueous phase directly by decantation. After reuse of organic phase for 3 cycles, 11.5 g/L furfural was obtained. The catalyst-containing aqueous phase could be reused directly after decantation of the organic phase without loss of activity. The obtained residue was easy to hydrolyze and produced 89.3% glucose yield after 96-h enzymatic hydrolysis at low cellulase loading (30 FPU of cellulase/g glucan). KEYWORDS: Furfural, Glucose, Bagasse, Hydrolysis, 2-Methyltetrahydrofuran/aqueous, AlCl3, NaCl ■ biorefinery process.7 However, few studies have focused on the coproduction of furfural and cellulose-enriched residue. Nowadays, most industrial production of furfural is adapted from the Quaker Oats process started in 19218 that uses strong mineral acids such as H2SO4 as catalyst and is operated at high temperature (>150 °C) causing complete degradation of cellulose.4 Some metal chlorides (e.g., AlCl3 and CrCl3) have both moderate Lewis and Bronsted acidity in water. Lewis acidity catalyzes the isomerization of xylose to xylulose, and Bronsted acidity catalyzes the dehydration of xylulose into furfural.9,10 The combination of Lewis and Bronsted acidity enables furfural formation under mild conditions that may avoid severe decomposition of cellulose. It is found that AlCl3 INTRODUCTION The dwindling reserve of fossil resources and environmental concern associated with fossil fuel application spur research on the conversion of lignocellulosic biomass to renewable fuels and chemicals.1,2 Furfural is a platform chemical that has potential to produce biofuels and attracts much attention from academia and industry.3 It is produced from pentose (e.g., xylose and arabinose) in hemicellulose that occupies 20−30 wt % of typical terrestrial biomass such as wood. Cellulose, accounting for another 30−50 wt % biomass, is normally hydrolyzed to fermentable sugars for bioethanol.4,5 In lignocellulosic biomass, cellulose is surrounded by hemicellulose and protected by lignin. Pretreatment of lignocellulosic biomass is required to remove hemicellulose and lignin and make cellulose highly accessible to catalysts or cellulases.6 A chemical method that could efficiently convert hemicellulose to furfural and simultaneously produce cellulose-enriched residue is encouraged as it remarkably promotes the economics of the © 2016 American Chemical Society Received: August 3, 2016 Revised: August 25, 2016 Published: August 30, 2016 5804 DOI: 10.1021/acssuschemeng.6b01847 ACS Sustainable Chem. Eng. 2016, 4, 5804−5813 Research Article ACS Sustainable Chemistry & Engineering Scheme 1. Furfural and Cellulosic Residue Production from Xylan Catalyzed by Lewis Acid/Bronsted Acid similar enzymatic digestibility to Avicel cellulose.26 In this process, lignin was extracted into the organic phase, and soluble sugars were reserved in the aqueous phase (mainly from hemicellulose) as byproducts simultaneously. In this study, sugar cane bagasse, one of the commonly used biomasses in the furfural industry,7 was chosen as raw material for one-pot production of furfural and cellulose-enriched residue using AlCl3, FeCl3, and HCl as catalysts. The influences of AlCl3, NaCl, and MTHF on furfural yield and enzymatic accessibility to residue were studied (Scheme 1). can reduce the active energy of xylan degradation in the organosolv pulping process.11 Normally, furfural is produced with very low yield from the reactions of lignocelluloses in the single aqueous system.12 Adding salts such as NaCl can significantly influence Bronsted acid-catalyzed furfural production from xylose in both the single aqueous system13,14 and the organic/aqueous biphasic system.15 However, the role of NaCl in Lewis-acid-catalyzed furfural production from actual biomass has not been studied. Adding NaCl also improves 5-hydroxymethylfurfural (HMF) production from glucose over AlCl3 catalyst,16 and accelerates cellulose degradation in aqueous organic acid solution17 and subcritical water.18 Therefore, new methods that avoid the degradation of cellulose during furfural production over Lewis acid catalyst in NaCl solutions are required to develop. Adding organic solvent considerably promotes furfural yield by reactive extraction, while the optional organic solvents could be ethanol, methanol, γ-valerolactone, tetrahydrofuran, 2methyltetrahydrofuran (MTHF), and alkylphenols.15,19,20 Among these solvents, MTHF deserves particular attention due to its high immiscibility with water, stability under acidic environment, and easy recycle (boiling point about 80 °C).21 FeCl322 and SnCl423 catalyzed furfural production in the MTHF/water biphasic system, and the reaction with the assistance of NaCl was studied. Furfural was efficiently extracted into MTHF, with its yield promoted from 34−50% to 54−78%. After reaction, the MTHF phase rich in furfural was easily separated from the aqueous phase containing catalyst by decantation. Furfural could be purified after distillation of MTHF, and the aqueous phase was reused without the loss of activity.22 However, these studies use pure xylose and xylan or hemicellulose hydrolysate as substrate rather than actual lignocelluloses that may have different furfural yields.24 MTHF also effectively removes lignin, the third abundant component in raw lignocelluloses that greatly restrains the activity of cellulases by competitive adsorption during cellulose saccharification.25 One-step fractionation of biomass in aqueous oxalic acid/MTHF biphasic system provided cellulose with ■ EXPERIMENTAL SECTION Materials. Bagasse was purchased from Dehong Liangheliliang Biological Food Ltd. (Yunnan). It was air-dried, milled, and passed through a 20-mesh sieve (<0.85 mm). It was air-dried in an oven (WFO-710, EYELA, Tokyo Rikakikai Co., Ltd.) again at 105 °C for 4 h before use. Its composition was determined with two repetitions according to the National Renewable Energy Laboratory (NREL) procedure27 with (wt %) 38.3 ± 1.0 glucan, 20.4 ± 1.5 xylan, 3.1 ± 0.4 arabinan, 3.0 ± 0.2 manan, 22.5 ± 0.9 lignin, 2.1 ± 0.2 ash, and 8.4 ± 0.3 extractives. A detailed analytical procedure was described in previous work.28 Microcrystalline cellulose Avicel PH101 was purchased from Sigma-Aldrich (Shanghai). MTHF (>99%), HMF (>99%), furfural (≥99.5%), cellubiose (≥99%), D-(+)-glucose (≥99.5%), D-(+)-xylose (99%), L-(+)-arabinose (≥99%), and Dmannose (≥99%) were bought from Aladdin Factory Co., Ltd. (Shanghai). AlCl3·6H2O (≥99%) and FeCl3·6H2O (>99%) were from the Xilong Chemical Factory Co., Ltd. (Shantou, Guangdong). Concentrated HCl (36−38%) was bought from Shandian Medicine Co., Ltd. (Kunming, Yunnan). Cellulase (Celluclast 1.5L from Trichoderma reesei ATCC 26921, ≥700 Ug−1) and β-glucosidase (Novozyme 188 from Aspergillus niger) were purchased from SigmaAldrich (Shanghai). The activities of cellulase and β-glucosidase were 88.8 FPU/mL and 514 cellobiase unit (CBU)/mL, respectively, determined by the method established by Ghose et al.29 Acid-Catalyzed Reactions and Analysis of the Filtrate. Acidcatalyzed reactions of bagasse were carried out in a microwave reactor (Monowave 300, Anton Paar, Graz, Austria). For a typical reaction, 0.3 g dried bagasse, appropriate amount of chlorides [e.g., 0.217 g of AlCl3·6H2O (0.1 M) and 0.9 g of NaCl (10 wt %)], 9 mL aqueous 5805 DOI: 10.1021/acssuschemeng.6b01847 ACS Sustainable Chem. Eng. 2016, 4, 5804−5813 Research Article ACS Sustainable Chemistry & Engineering solution and 9 mL MTHF were added into a 30 mL quartz tube with a magnetic stir bar stirring at 600 rpm. The tube with sample was heated by microwave irradiation to 130−170 °C (5.6−13.1 bar) within 3 min for 30 min reaction. After reaction, the tube was cooled down to 40 °C within 5−10 min by pressurized air. The product mixture was taken out from the tube and filtered through a 0.22 μm membrane. Deionized water was used to wash the residue and dilute the filtrate to 150 mL in a homogeneous phase (MTHF and water were miscible). The filtrate was analyzed by HPLC (LC-20A, Shimadzu) equipped with Bio-Rad Aminex HPX-87H column, and 50 mM H2SO4 was used as mobile phase at flow rate of 0.6 mL min−1. Oven temperature was set at 60 °C. Furfural and HMF were determined by UV detector at 284 nm, while pentose and hexose in aqueous product were determined by refractive index (RI) detector. Standard calibrated curves for these products were determined with five points (0.05, 0.1, 0.2, 0.4, and 0.8 mg/mL, R2 > 0.999).28 The yields of furfural, HMF, glucose, and xylose were defined as their weights in the filtrate divided by their weight equivalents contained in bagasse:19 partition coefficient = (product concentration in organic phase) /(product concentration in aqueous phase) (7) Haan et al. reported separation of furfural from a liquid aqueous phase comprising furfural and organic acids.31 However, in this study, after the removal of MTHF by vacuum rotary evaporator (RE-52AA, Yarong biological and chemical instrument factory, Shanghai) at 30 °C for about 30 min, products in the MTHF phase from the R−Al−Na− MTHF reaction system (MTHF/AlCl3−NaCl aqueous solution at 150 °C for 45 min) were dissolved in CDCl3 for 13C NMR analysis on Avance III 600 spectrometer (600 MHz, Bruker BioSpin GmbH, Karlsruher, Germany) at 22 °C. The solid residue was washed with 30 mL of distilled water 3 times and dried in a freeze dryer (PDU-1200, EYELA, Tokyo Rikakikai Co., Ltd.) for 48 h for weight (residue yield) and composition determination (the same method for raw bagasse), and subsequent enzymatic hydrolysis. Characterization of Bagasse and Residues. SEM (ZEISS EVO LS10, Cambridge, U.K.) operated at 10 keV was used to observe the morphology of the solid samples. The FT-IR measurement was performed on a Nicolet is10 spectrometer (Madison, WI) with a resolution of 4 cm−1 at 500−4000 cm−1. Enzymatic Hydrolysis of Residue. Enzymatic hydrolysis was carried out in 20 mL glass vials containing 2500 μL of 0.1 M citrate buffer, 50 μL of 2% sodium azide, 75 mg of solid residue, 5−30 FPU of cellulase, and 20 CBU of β-glucosidase. Deionized water was supplemented to bring the total volume of reaction solution to 5000 μL in the vials. Temperature was kept at 50 °C with rotation rate at 150 rpm for hydrolysis in a thermostat incubator (SPH-100B, Shanghai Shiping Laboratory Equipment Co., Ltd.). During hydrolysis, a 200 μL sample was taken out of the vials by a 1 mL pipettor at 3, 6, 12, 24, and 96 h for glucose and cellubiose analysis by LC-20A HPLC equipped with RI detector, using the same conditions as the above analysis for the sugars. Glucose yield (wt %) was defined as follows: furfural yield (wt %) = (furfural weight in filtrate)/(pentosan weight in bagasse × 1.375) × 100% (1) HMF yield (wt %) = (HMF weight in filtrate) /(hexosan weight in bagasse × 1.286) × 100% (2) glucose yield (wt %) = (glucose weight in filtrate)/(glucan weight in bagasse × 0.90) (3) × 100% xylose yield (wt %) = (xylose weight in filtrate) /(xylan weight in bagasse × 0.88) × 100% (4) glucose yield (wt %) = 0.90 × (glucose weight in hydrolysate) residue yield (wt %) = (weight of recovered residue) /(weight of bagasse) × 100% ■ (5) Under these definitions, theoretical yield of all products were 100%. CSF (combined severity factor) used in this study is defined as CSF = log{t exp[(TH − 100)/14.75]} − pH /(glucan weight in residue) × 100% (8) RESULTS AND DISCUSSION Furfural Production from Bagasse Using Different Acids as Catalysts. Time evolution of furfural production from bagasse (about 0.3 g) was studied in 9 mL of aqueous phase at 150 °C using different chloride salts as catalyst (Figure 1). Fixed moles of AlCl3 (0.1 M, pH = 3.4), FeCl3 (0.1 M, pH = 1.6), and HCl (0.3 M, pH = 0.52) were used to keep the concentration of chloride ions the same for different reactions. Furfural yields of 49.2% and 54.8% were obtained at 90 min with FeCl3 and at 45 min with HCl, respectively. Surprisingly, AlCl3 gave furfural yield of 35.5% at 30 min, much lower than those with FeCl3 and HCl. However, severe degradation of cellulose was observed using FeCl3 or HCl as catalyst, with glucose yield of more than 20%. This was attributed to their strong Bronsted acidity for a long reaction time. The CSF used for furfural production was much higher than that for hemicellulose removal and lignocellulose pretreatment, so when the furfural yield reached the peak value the hemicellulose removal procedure has already finished. Thus, furfural production from lignocellulose in one pot often led to severe decomposition of cellulose. To obtain a high yield of furfural and high recovery of cellulose in residue simultaneously, both furfural yield and degradation products of cellulose such as glucose and HMF should be monitored. In the FeCl3-catalyzed system, the yield of furfural increased from (6) where t is the reaction time of treatment in minutes, TH is the reaction temperature in °C, and pH is the acidity of the aqueous solution before reaction.30 All of the experiments were repeated 2 times unless otherwise mentioned, and the errors were 0.1−4.3% for furfural, 0.8−8.0% for HMF, 0−6.3% for glucose, 0.3−5.8% for xylose, and 0.5−3.4% for residue, respectively. Solvent Separation and Recycle. MTHF was insoluble in water; two phases were formed after MTHF was added to aqueous AlCl3. Adding NaCl further decreased MTHF solubility in the aqueous phase. So, the liquid product was centrifuged at 3000 rpm (relative centrifugal force of 2013) for 5 min in a refrigerated centrifuge (TG18G-II, Kaida Scientific Instrument Co., Ltd., Hunan). Then, the MTHF phase (about 8.5 mL) was drawn out directly with a 5 mL syringe; the salt-containing aqueous phase was separated from the residue by filtration with a 0.22 μm filter membrane and can be reused directly. Several runs were done to obtain enough used organic/ aqueous phase. To examine the reusability of the organic phase, 9 mL of used MTHF was added to 9 mL of fresh aqueous phase. Similarly, 9 mL of used aqueous phase was added to 9 mL of fresh MTHF to test the reusability of the aqueous phase. Also, 5 mL of used organic/ aqueous phase after each recycle was diluted to 150 mL using deionized water to determine product concentration in both phases and calculate the partition efficiency as follows: 5806 DOI: 10.1021/acssuschemeng.6b01847 ACS Sustainable Chem. Eng. 2016, 4, 5804−5813 Research Article ACS Sustainable Chemistry & Engineering 0.58 for FeCl3 and 0.39 for HCl at 45 min); (ii) the shortest time for the completion of furfural production (30 min vs 90 min for FeCl3 and 45 min for HCl); and (iii) moderate pH (3.4) as compared to that for FeCl3 (1.6) and HCl (0.52). Role of NaCl in Furfural Production in the AlCl3 System. NaCl was found to significantly increase furfural yield from xylose in Bronsted-acid-catalyzed reactions;14 however, the influence of NaCl on furfural production with Lewis acid has not been studied yet. The effect of NaCl on furfural production from bagasse with different catalysts was studied in 9 mL of aqueous phase at 150 °C (Figure 2). In the Figure 1. Furfural production vs time from raw bagasse in aqueous solution with (a) 0.1 M FeCl3, (b) 0.3 M HCl, and (c) 0.1 M AlCl3 (0.3 g bagasse, 9 mL aqueous solution and magnetic stirring rate of 600 rpm at 150 °C; ◊ xylose; □ glucose; △ furfural; ○ HMF). 26.3% to 49.2% as time was prolonged from 45 to 90 min, and changed slightly within 47.2−49.2% as time rose further to 180 min (Figure 1a). The yield of xylose was 53.9% at 45 min, but gradually dipped to 10.1% at 180 min. The yields of glucose and HMF at 90 min were 19.9% and 1.0%, respectively, and rose to 25.5% and 1.6% at 120 min. In the HCl-catalyzed system, the yield of furfural increased from 18.3% to 54.8% as time was prolonged from 5 to 45 min, but remarkably decreased to 44.5% at 120 min (Figure 1b). The yield of xylose sharply jumped from 73.3% to 6.5% as time increased from 5 to 75 min, while the yields of glucose and HMF were 21.3% and 1.3% at 45 min, and 29.8% and 1.8% at 90 min, respectively. In the AlCl3-catalyzed system, the yield of furfural increased from 13.0% to 35.5% as time was prolonged from 5 to 30 min, and slightly declined to 31.6% at 75 min (Figure 1c). Simultaneously, the yield of xylose remarkably decreased from 38.9% to 5.1% as time increased from 5 to 30 min, and decreased further to 1.2% at 75 min. The yields of glucose and HMF at 30 min were 0.7% and 5.6%, respectively. AlCl3 seriously catalyzed the degradation of glucose to HMF, which made the glucose yield much lower than the HMF yield. When the highest furfural yield was obtained, CSF values were 1.8, 2.6, and −0.45, respectively, for FeCl3-, HCl-, and AlCl3-catalyzed reaction systems. In comparison, the CSF value applied for acid-assisted lignocellulose pretreatment varied from 1.0 to 2.0.30,32 Although CSF for FeCl3-catalzyed furfural production was similar to that for traditional acid-assisted pretreatment, it did cause severe degradation of cellulose, suggesting that appropriate CSF may be different when different acid species were used as catalysts.33 When HCl was used as catalyst, CSF was much higher than that for pretreatment and also caused severe degradation of cellulose, in agreement with phenomena found in industrial production of furfural. A very interesting result was found when AlCl3 was used as catalyst. AlCl3 facilitated hemicellulose removal and furfural formation even at a low CSF value of −0.45. In summary, AlCl3 was more suitable to coproduce furfural and cellulosic residue from bagasse, because it had three advantages: (i) considerable furfural yield but the least degradation of cellulose (yield ratio of glucose to furfural was 0.02 for AlCl3, Figure 2. Influence of NaCl on the degradation of raw bagasse in the aqueous phase with (a) 0.1 M FeCl3, (b) 0.3 M HCl, and (c) 0.1 M AlCl3 (0.3 g bagasse, 9 mL aqueous solution and magnetic stirring rate of 600 rpm at 150 °C; ◊ xylose; □ glucose; △ furfural; ○ HMF). FeCl3-catalyzed reaction at 150 °C for 90 min (Figure 2a), furfural yield slightly increased from 49.2% to 52.5% as NaCl increased from 0 to 10 wt %, but decreased to 46.2−47.2% when NaCl exceeded 10 wt %. However, the yield of xylose substantially decreased from 25.6% to 5.1% as NaCl increased from 0 to 20 wt %. Cellulose degradation was also promoted when NaCl increased to 20 wt %, with the yields of glucose and HMF increasing from 19.9% and 0.9% to 29.4% and 3.0%, respectively. In HCl-catalyzed reaction at 150 °C for 45 min, NaCl with less than 10 wt % showed a slight influence on furfural production with furfural yield of 54.8−56.6% (Figure 2b). However, when NaCl increased from 10 to 20 wt %, furfural yield remarkably decreased from 55.9% to 35.6%. Simultaneously, the degradation of xylose was substantially accelerated. Xylose yield decreased from 24.0% to 7.8% when NaCl increased from 0 to 5 wt %. Marcotullio et al.13 proposed that the Cl− anion could promote the formation of 1,2-enediol, an intermediate from xylose dehydration. The yields of glucose and HMF were also promoted from 21.3% and 1.3% to 30.1% and 2.4%, respectively, when NaCl increased to 10 wt %. In the AlCl3-catalyzed system at 150 °C for 30 min, with NaCl increasing from 0 to 20 wt %, furfural yield almost linearly rose from 35.5% to 56.3%, while xylose yield slightly declined from 5.1% to 3.1% (Figure 2c). It demonstrated that adding NaCl significantly improved furfural production from xylose but relieved the degradation of furfural. When 20 wt % NaCl was added, furfural yield of 56.3% was achieved, which was comparable to the optimized furfural yields obtained using 5807 DOI: 10.1021/acssuschemeng.6b01847 ACS Sustainable Chem. Eng. 2016, 4, 5804−5813 Research Article ACS Sustainable Chemistry & Engineering almost constant until 170 °C (Figure 3a). Cellulose in lignocellulosic materials contains both crystalline and amorphous parts. Hydrolysis of amorphous cellulose could be easily achieved at low temperature of 150 °C, but the degradation of crystalline cellulose needs much more severe conditions (e.g., >160 °C).17,34 So, the increase of HMF yield with temperature was mainly from the degradation of amorphous cellulose. On the other hand, the yield of solid residue continuously decreased from 62.7% to 39.9% with temperature growing from 130 to 170 °C. The selected temperature was 150 °C. Under the fixed conditions (150 °C and 0.1 M AlCl3 without NaCl), prolonging the reaction time from 15 to 45 min significantly promoted the yields of both furfural and HMF from 25.7% and 3.2% to 46.0% and 6.3%, respectively, and changed little until 75 min (Figure 3b). The yield of solid residue decreased from 64.3% to 53.8% with time increasing from 15 to 45 min, and changed slightly until 75 min. The selected time was 45 min. Under the fixed conditions (150 °C and 45 min without NaCl), AlCl3 concentration increasing from 0.03 to 0.1 M significantly promoted furfural yield from 31.9% to 46.0%, but excessive AlCl3 slightly influenced furfural yield (Figure 3c). The yields of HMF and solid residue were slightly affected by the variation of AlCl3 concentration. So, 0.1 M AlCl3 was selected as the optimized concentration. The influence of NaCl on furfural yield was optimized at 150 °C with 0.1 M AlCl3 for 45 min (Figure 3d). NaCl increasing from 5 to 10 wt % promoted furfural yield greatly by 24% (from 48.2% to 60.0%), while NaCl concentration of <5 wt % or >10 wt % had a slight influence on furfural yield. The highest furfural yield (62.2%) was obtained at 20 wt % NaCl, which was comparable to the results (furfural yield of 55−66%) obtained from different lignocellulosic biomass in a microwave-assisted tetrahydrofuran/aqueous saturated NaCl system at 160 °C for 60 min in previous studies.9 High NaCl concentration in aqueous solution had slight effects on the variation of HMF and residue yields. Concentration of 10 wt % NaCl was selected as the best value. The influence of MTHF and NaCl was studied at 150 °C for 45 min using 0.1 M AlCl3 (Table 1). The total volume of reaction liquor (including aqueous and organic phases) was adjusted to 18 mL to keep the liquid/biomass ratio fixed (0.45 g of bagasse). Table 1 lists different systems with results and reaction conditions: R-Al (18 mL of 0.1 M AlCl3 aqueous solution at 150 °C for 45 min, where R means reaction), R-AlNa (18 mL of water with 0.1 M AlCl3 and 10 wt % NaCl), RAl-MTHF (9 mL of MTHF and 9 mL of water with 0.1 M AlCl3), and R-Al-Na-MTHF (9 mL of MTHF and 9 mL of water with 0.1 M AlCl3 and 10 wt % NaCl). In the AlCl3catalyzed reaction without NaCl, adding MTHF significantly FeCl3 (52.5% at 150 °C for 90 min in aqueous solution containing 0.1 M FeCl3 and 10 wt % NaCl) or HCl (56.6% at 150 °C for 45 min in aqueous solution containing 0.3 M HCl and 10 wt % NaCl). More importantly, both glucose and HMF yields from the degradation of cellulose were not stimulated by NaCl. In summary, furfural yields in the FeCl3- or HCl-catalyzed system at 150 °C declined remarkably as NaCl exceeded 10 wt %, but the degradation of cellulose was promoted. However, in the AlCl3-catalyzed system, furfural production was significantly enhanced by adding NaCl without serious degradation of cellulose as both glucose and HMF yields were still low (<2% glucose yield and about 7% HMF yield). Role of MTHF and Furfural Production in MTHF/ Aqueous AlCl3 Biphasic System. Addition of organic solvent could benefit furfural production20 by reactive extraction, while MTHF efficiently promoted furfural yield catalyzed by FeCl322 or SnCl4.23 Production of furfural from bagasse was studied in an MTHF/aqueous AlCl3 biphasic system (volume ratio of 1:1) at 130−170 °C (Scheme 1). The influence of temperature, time, AlCl3 concentration, and NaCl usage on the yields of furfural, HMF, and residue in MTHF/aqueous AlCl3 biphasic solution was studied by single-factor experiment (Figure 3). Figure 3. Furfural, HMF, and residue yields changed with (a) temperature, (b) time, (c) AlCl3 concentration, and (d) NaCl concentration (0.3 g of bagasse, 9 mL of MTHF, 9 mL of aqueous solution, magnetic stirring rate of 600 rpm; × residue; △ furfural; ○ HMF). Under the fixed conditions (60 min and 0.1 M AlCl3 without NaCl), temperature increasing from 130 to 150 °C significantly promoted the yields of both furfural and HMF from 8.7% and 1.1% to 45.8% and 6.6%, respectively, while both yields stayed Table 1. Influence of NaCl and MTHF on the Yields of Furfural and HMF reaction conditiona a reaction syst AlCl3 R-Al R-Al-Na R-Al-MTHF R-Al-Na-MTHF 18 mL 0.1 M 18 mL 0.1 M 9 mL 0.1 M 9 mL 0.1 M NaCl MTHF 10 wt % 10 wt % partition coefficientb product yield (%) 9 mL 9 mL HMF 4.3 7.0 6.1 7.9 ± ± ± ± 0.2 0.3 0.1 0.1 furfural org(furfural)/aq(furfural) org(HMF)/aq(HMF) ± ± ± ± 6.3 ± 0.4 8.7 ± 0.3 1.4 ± 0.1 2.2 ± 0.1 30.6 50.0 43.6 58.6 0.3 0.6 0.3 0.5 a R-Al, AlCl3 aqueous solution; R-Al-Na, AlCl3 and NaCl; R-Al-MTHF, MTHF and water with AlCl3; R-Al-Na-MTHF, MTHF and water with AlCl3 and NaCl (0.45 g of bagasse, 150 °C, 45 min and stirring rate of 600 rpm). bPartition coefficient was defined as product concentration in the organic phase divided by that in the aqueous phase. 5808 DOI: 10.1021/acssuschemeng.6b01847 ACS Sustainable Chem. Eng. 2016, 4, 5804−5813 Research Article ACS Sustainable Chemistry & Engineering Table 3. Recycle of Organic and Aqueous Phasesa in MTHF/Aqueous AlCl3 System promoted furfural yield from 30.6% to 43.6% (Table 1, R-Al vs R-Al-MTHF) because the reactive extraction of furfural into the MTHF phase reduced the degradation of furfural in the aqueous phase. The concentration of furfural in MTHF phase was 6.3 times that in the aqueous phase. Similarly, in the AlCl3catalyzed reaction with 10 wt % NaCl, furfural yield grew from 50.0% to 58.6% as MTHF was added (Table 1, R-Al-Na vs RAl-Na-MTHF). Simultaneously, the yield of solid residue decreased from 62.6−63.0% to 54.7−54.9% with a slight growth in HMF yield. The results of NaCl influence in Table 1 were in accordance with the results in Figure 2c. Adding 10 wt % NaCl in aqueous AlCl3 solution resulted in the furfural yield greatly increasing from 30.6% to 50.0%, while the yield of residue changed little (Table 1, R-Al vs R−Al-Na). In MTHF/aqueous AlCl3 solution (Table 1, R-Al-MTHF vs R−Al-Na-MTHF), NaCl improved furfural yield from 43.6% to 58.6% with almost constant residue yield of 54.7% because of the enhanced partition coefficient of furfural in organic phase (Table 1, R-Al-MTHF vs R−Al-NaMTHF).4 On the other hand, HMF formation was also promoted by NaCl, especially in aqueous solution. In summary, to simultaneously achieve high furfural and cellulosic residue yields, the suggested conditions for bagasse conversion in MTHF/aqueous AlCl3 biphasic system were 150 °C, 45 min, 0.1 M AlCl3, and 10 wt % NaCl with furfural yield of 60.0% and residue yield of 53.9%.These conditions were further used for the following optimization of solid loading. Influence of Solid Loading and Solvent Recycle on Furfural Production. In a practical process, a relatively high ratio of solid biomass to reaction solvent was always required to achieve better economics, although it might affect the efficiency of mass and heat transfer. Here, the influence of solid loading on furfural yield was tested in MTHF/AlCl3 aqueous solution at 150 °C for 45 min (Table 2). Other conditions were 9 mL of product yield in MTHF phase (%) HMF fresh MTHF recycle 1 recycle 2 fresh aqueous recycle 1 recycle 2 furfural residue 0.3 0.45 0.6 8.6 ± 0.3 7.9 ± 0.1 7.4 ± 0.5 60.0 ± 0.4 58.6 ± 0.5 54.0 ± 0.8 53.9 ± 0.4 54.7 ± 2.5 57.2 ± 1.0 0.1 0.3 0.3 0.1 0.6 0.4 58.6 55.6 53.8 58.6 60.8 61.2 ± ± ± ± ± ± 0.5 0.5 0.7 0.5 0.8 1.5 4.4 ± 0.2 8.0 ± 0.1 11.5 ± 0.4 Spent MTHF containing a high concentration of furfural from the latest reaction was directly used in the next recycle without any pretreatment. Spent aqueous solution containing residual sugars from the latest reaction was directly used in the next reaction without supplement of AlCl3 and NaCl, but another 0.45 g of bagasse was added (0.45 g of bagasse, 9 mL of MTHF, 9 mL of aqueous solution, 150 °C, 45 min, magnetic stirring rate of 600 rpm, 0.1 M AlCl3, and 10 wt % NaCl). products. The recycle of the aqueous phase containing residual sugars from the latest reaction slightly promoted the furfural yield in the next reaction. Furfural yields were enhanced from 58.6% to 60.8% and 61.2% with the first and second recycle of the aqueous phase. HMF yield remarkably increased from 7.9% to 9.9% and 11.1%, respectively, for the first and second recycle of aqueous solution. Influence of MTHF and NaCl on the Composition and Structure of Residues. Besides a high furfural yield, it is important to obtain a high cellulose yield that has good structure for easily enzymatic hydrolysis. Table 4 showed the glucan and xylan contents in solid residues as well as lignin content and residue yields after different reactions under the same conditions in Table 1 (150 °C, 45 min, 0.1 M AlCl3), while the conditions for MTHF extraction were 0.45 g of bagasse, 18 mL of MTHF, 150 °C, and 45 min. After reaction in aqueous AlCl3 or aqueous AlCl3−NaCl solution, xylan content in residues from R-Al and R-Al-Na systems was sharply reduced from 20.4% to 3.5% and 2.8%, respectively. Glucan content remarkably increased from 38.3% to 52.1−53.3% because hemicellulose and extractives were removed from solid residues. Adding MTHF did not influence the degradation and removal of hemicellulose. However, glucan content in residues from R-Al-MTHF and R-Al-Na-MTHF rose further to 63.2− 64.1%, indicating that other chemical compositions such as lignin in raw bagasse were removed by MTHF. Nuclear magnetic resonance (NMR) analysis of the organic phase after reaction in MTHF/aqueous AlCl3−NaCl solution (R-Al-NaMTHF) demonstrated that some aromatic products were indeed extracted by MTHF (Figure 4). For subsequent enzymatic hydrolysis, the removal of lignin and other impurities in bagasse might be more important than the reduction of cellulose crystallinity. The crystalline structure of cellulose only hindered the contact of cellulosic substrate with enzyme molecules, and retarded the efficiency of enzymatic hydrolysis. However, competitive adsorption by lignin and other impurities might result in irreversible damage of enzymes.25 Extraction of bagasse by MTHF without the assistance of AlCl3 could hardly remove hemicellulose or lignin from bagasse to increase cellulose concentration in the residues. So, the combination of MTHF and aqueous AlCl3 corporately removed hemicellulose and lignin to yield a cellulose-enriched residue. On the product yield (%)a HMF ± ± ± ± ± ± furfural concentration in organic phase (g L−1) a Table 2. Influence of Solid Loading on Furfural Yield in MTHF/Aqueous AlCl3 System bagasse loading (g) 7.9 7.3 6.5 7.9 9.9 11.1 furfural 9 mL of MTHF, 9 mL of aqueous solution, 150 °C, 45 min, magnetic stirring rate of 600 rpm, 0.1 M AlCl3, and 10 wt % NaCl. a MTHF, 9 mL of aqueous solution, magnetic stirring rate of 600 rpm, 0.1 M AlCl3, and 10 wt % NaCl. When bagasse loading increased from 0.3 to 0.6 g, the yield of furfural remarkably decreased from 60.0% to 54.0%, while residue yield increased from 53.9% to 57.2%. Increased bagasse loading led to higher furfural concentration both in the organic phase and in the acidic aqueous phase. Exposure of furfural to acidic aqueous phase accelerated furfural degradation to humins. Polymeric soluble lignin also greatly affected furfural yield.35 Organic and aqueous phases were recycled 2 times (Table 3). Increasing concentration of furfural in the spent MTHF decreased the efficiency of furfural production in the next recycle. Furfural yield decreased from 58.6% to 55.6% and 53.8% for the first and second recycle. However, high furfural concentration of 11.5 g L−1 (>1 wt %, based on the mass of organic solvent) was realized in the MTHF phase after 2 recycles, which benefited the post-treatment of organic 5809 DOI: 10.1021/acssuschemeng.6b01847 ACS Sustainable Chem. Eng. 2016, 4, 5804−5813 Research Article ACS Sustainable Chemistry & Engineering Table 4. Residue Yields and Glucan Contents in Residues after Reactions residue composition (wt %) reaction systems raw bagasse MTHF extraction R-Al R-Al-Na R-Al-MTHF R-Al-Na-MTHF reaction conditions glucan 0.45 g bagasse, 18 mL MTHF, 150 °C, 45 min 0.45 g bagasse, 18 mL water, 0.1 M AlCl3, 150 °C, 45 min 0.45 g bagasse, 18 mL water, 0.1 M AlCl3, 10 wt % NaCl, 150 °C, 45 min 0.45 g bagasse, 9 mL MTHF, 9 mL water, 0.1 M AlCl3, 150 °C, 45 min 0.45 g bagasse, 9 mL MTHF, 9 mL water, 0.1 M AlCl3, 10 wt % NaCl, 150 °C, 45 min 38.3 40.4 53.3 52.1 62.5 63.7 ± ± ± ± ± ± 1.0 0.8 0.5 0.7 0.7 0.4 xylan 20.4 21.2 3.45 2.8 2.6 3.2 ± ± ± ± ± ± 1.5 2.0 0.3 0.2 0.2 0.2 lignin 22.5 23.4 35.2 36.1 25.2 26.7 ± ± ± ± ± ± 0.9 1.2 0.6 1.0 1.4 0.7 residue yield (wt %) 94.8 63.0 62.6 54.9 54.7 ± ± ± ± ± 0.2 1.0 0.9 0.7 2.5 Figure 4. 13C NMR spectra of organic phase after reaction from R-Al-Na-MTHF system (600 MHz, CDCl3, and 22 °C). polysaccharides.36 It could be concluded that after reaction in aqueous AlCl3 or MTHF/aqueous AlCl3 solution, the carbonyl and carboxyl stretching at 1715 cm−1 for the solid residues became weaker, proving the removal of hemicellulose (mainly containing acetyl group).38 Simultaneously, absorptions at 1370−1205 and 1160−1030 cm−1 were remarkably strengthened, indicating the substantial increase of polysaccharides in the solid residues after reactions.38 Enzymatic Digestibility of Residues. Enzymatic hydrolysis of bagasse and residues after different reactions was carried out with low cellulase loading of 10 FPU (filter paper unit) (Figure 7). Cellulose in raw bagasse was hard to hydrolyze, with low glucose yield of 16.2% obtained after 96-h reaction, similar to previous studies (8.6−19.3% for 72-h enzymatic hydrolysis).39,40 At the initial stage (<6 h), glucose was released rapidly from all substrates, and significantly slowed down until 96 h passed.40 After reaction in AlCl3 aqueous solution (R-Al system), the residue yielded a high glucose yield of 48.1% after 96-h hydrolysis, mainly due to the removal of hemicellulose. Adding NaCl in the R-Al system suppressed glucose yield to 43.6% after 96 h. The negative effect of NaCl on enzymatic digestibility of residues was caused by (i) acceleration of pseudolignin formation via the condensation of furfural with high concentration,41,42 and (ii) more recalcitrant structure of other hand, 10 wt % NaCl in aqueous AlCl3 or MTHF/aqueous AlCl3 solution showed less influence on the variation of residue composition (Table 4), although the furfural production was promoted (50.0−58.6% with NaCl vs 30.6−43.6% without NaCl in Table 4). In conclusion, reaction of bagasse in MTHF/ AlCl3−NaCl aqueous solution produced not only high furfural yield of 58.6%, but also led to a high glucan content of 63.7% in the residue (residue yield 54.7%). Scanning electron microscopy (SEM) images (Figure 5) showed that, after treatment with aqueous AlCl3 or MTHF/ aqueous AlCl3 solution, the surface of residues became neater with fewer deposits on the parallel cellulosic microfibers, because of the removal of hemicellulose and lignin. Fourier transform infrared spectroscopy (FT-IR, Figure 6) illustrated that all samples had absorptions at 3410−3460 and 2896−2905 cm−1 corresponding to the stretching of the −OH group and C−H bonds of alkyl group, respectively.36 On the basis of ref 37, the band around 1715 cm−1 was from carbonyl and carboxyl stretching. The bands at 1605 and 1512 cm−1 were from the skeletal and stretching vibration of benzene rings. Absorptions at 1370−1205 cm−1 were mainly from the bending of C−H or O−H bonding in polysaccharides, including cellulose and hemicellulose; those absorptions at 1160−1030 cm−1 were from the bending of C−O or C−O−C linkages in 5810 DOI: 10.1021/acssuschemeng.6b01847 ACS Sustainable Chem. Eng. 2016, 4, 5804−5813 Research Article ACS Sustainable Chemistry & Engineering Figure 5. SEM images of original bagasse and residues after reactions. Figure 6. FT-IR spectra of bagasse and residues after reactions (a, wavenumber at 400−4000 cm−1; b, wavenumber at 800−2000 cm−1).36 (58.4% vs 81.0%) than that from enzymatic hydrolysis of Avicel PH101 cellulose, which might be attributed to high lignin content in residues that inhibited enzymatic hydrolysis of cellulose. Adsorption of cellulases by lignin decelerated cellulose degradation.45 As compared with other studies where bagasse was pretreated with the organosolv method, this glucose yield was also lower. Agnihotri et al.46 reported almost complete conversion of cellulose for bagasse pretreated at 210 °C for 90 min in 50:50 (v/v)% ethanol/water media lignin in the raw material formed during the chemical degradation reaction.43,44 Adding MTHF to the AlCl3−NaClcatalyzed system further improved the enzymatic hydrolysis of residue, with glucose yield enhanced to 58.4% after 96 h. MTHF effectively removed lignin, which relieved the physical barrier and nonproductive bonding of lignin to cellulases, and increased glucan content in the pretreated residue. Lignin content in residue decreased from 36.1% to 26.7% when MTHF was added. However, this glucose yield was still lower 5811 DOI: 10.1021/acssuschemeng.6b01847 ACS Sustainable Chem. Eng. 2016, 4, 5804−5813 Research Article ACS Sustainable Chemistry & Engineering 53.8% for the second recycle of organic phase and 61.2% for the second recycle of aqueous phases. Enzymatic hydrolysis of cellulosic residue was also remarkably enhanced with the highest glucose yield of 58.4% at 50 °C for 96 h [10 FPU of cellulase and 20 CBU (cellubiase unit) of β-glucosidase], 3.6 times that from raw bagasse. After cycles, the organic phase was decanted and distilled to separate furfural and lignin and obtain purified MTHF. Lignin and furfural concentrations in the aqueous phase were low; they tended to solubilize in the organic phase. After many cycles, the aqueous phase could be purified by extraction with the organic phase. ■ Figure 7. Enzymatic hydrolysis of cellulosic residues (75 mg solid residue, 10 FPU cellulase, 20 CBU β-glucosidase, 50 °C, and rotation rate of 150 rpm). AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Website: http://biomassgroup.njau.edu.cn/. using pH 3.5 formic acid as catalyst. Considering the higher enzyme loading applied in their study (30 FPU of cellulase, 32 pNPGU of β-glucosidase per gram of cellulose), we investigated the influence of different cellulase dosage on the enzymatic hydrolysis of residue. Increasing cellulase loading often improved enzymatic hydrolysis rate.47 When cellulase loading increased from 5 to 10, 15, and 30 FPU, glucose yield sharply increased from 45.4% to 58.4%, 68.8%, and 89.3%, respectively, after 96-h digestion (Figure 8), indicating that high Notes The authors declare no competing financial interest. ■ ACKNOWLEDGMENTS The authors wish to acknowledge financial support from Nanjing Agricultural University (68Q-0603), and the Yunnan Provincial Government (Baiming Haiwai Gaocengci Rencai Jihua). ■ REFERENCES (1) Kunkes, E. L.; Simonetti, D. A.; West, R. M.; Serrano-Ruiz, J. C.; Gartner, C. A.; Dumesic, J. A. Catalytic conversion of biomass to monofunctional hydrocarbons and targeted liquid-fuel classes. Science 2008, 322 (5900), 417−421. (2) Diercks, R.; Arndt, J.-D.; Freyer, S.; Geier, R.; Machhammer, O.; Schwartze, J.; Volland, M. Raw material changes in the chemical industry. Chem. Eng. Technol. 2008, 31 (5), 631−637. (3) Lange, J.-P.; van der Heide, E.; van Buijtenen, J.; Price, R. Furfural - A promising platform for lignocellulosic biofuels. ChemSusChem 2012, 5 (1), 150−166. (4) Luterbacher, J. S.; Martin Alonso, D.; Dumesic, J. A. Targeted chemical upgrading of lignocellulosic biomass to platform molecules. Green Chem. 2014, 16 (12), 4816−4838. (5) Bornscheuer, U.; Buchholz, K.; Seibel, J. Enzymatic degradation of (ligno)cellulose. Angew. Chem., Int. Ed. 2014, 53 (41), 10876− 10893. (6) Mosier, N.; Wyman, C.; Dale, B.; Elander, R.; Lee, Y. Y.; Holtzapple, M.; Ladisch, M. Features of promising technologies for pretreatment of lignocellulosic biomass. Bioresour. Technol. 2005, 96 (6), 673−686. (7) Cai, C. M.; Zhang, T.; Kumar, R.; Wyman, C. E. Integrated furfural production as a renewable fuel and chemical platform from lignocellulosic biomass. J. Chem. Technol. Biotechnol. 2014, 89 (1), 2− 10. (8) Zeitsch, K. J. In Chemistry and Technology of Furfural and Its Many By-Products, 1st ed.; Elsevier: Amsterdam, The Netherlands, 2000; Chapter Introduction, pp 1−2. (9) Yang, Y.; Hu, C. W.; Abu-Omar, M. M. Synthesis of furfural from xylose, xylan, and biomass using AlCl3·6H2O in biphasic media via xylose isomerization to xylulose. ChemSusChem 2012, 5 (2), 405−410. (10) Choudhary, V.; Sandler, S. I.; Vlachos, D. G. Conversion of Xylose to Furfural Using Lewis and Bronsted Acid Catalysts in Aqueous Media. ACS Catal. 2012, 2 (9), 2022−2028. (11) Schwiderski, M.; Kruse, A.; Grandl, R.; Dockendorf, D. Comparison of the influence of a Lewis acid AlCl3 and a Bronsted acid HCl on the organosolv pulping of beech wood. Green Chem. 2014, 16 (3), 1569−1578. (12) Mamman, A. S.; Lee, J.-M.; Kim, Y.-C.; Hwang, I. T.; Park, N.-J.; Hwang, Y. K.; Chang, J.-S.; Hwang, J.-S. Furfural: Hemicellulose/ Figure 8. Influence of cellulase loading on glucose yield from residue R-Al-Na-MTHF (75 mg solid residue, 20 CBU β-glucosidase, 50 °C, and rotation rate of 150 rpm). cellulase dosage provided more active sites for the enzymatic hydrolysis of cellulosic substrates. Pan et al.48 obtained about 90% glucose yield for soft wood residue with similar residual lignin (27.4%), although enzyme loading was higher (40 FPU/ g glucan) in their study. ■ CONCLUSION Conversion of bagasse in aqueous solution with FeCl3 and HCl benefited furfural production from hemicellulose but degraded cellulose seriously. Using AlCl3, replacing FeCl3 and HCl achieved lower furfural yield of 35.5% with less cellulose decomposition (glucose yield of 0.7% and HMF yield of 5.5%). Adding NaCl remarkably promoted furfural production by 34.4−63.4% in the AlCl3-catalyzed system with less influence on the variation of residue composition. The introduction of MTHF increased furfural yield by 17.2−42.5% and glucan content in residue to 63.7%. Under the best conditions (9 mL MTHF, 9 mL water, 0.1 M AlCl3, 150 °C, 45 min, and 10 wt % NaCl), furfural yield of 58.6% was obtained while more than 90% of glucan was maintained in the residue. Both organic and aqueous phases were recycled 2 times, with furfural yield of 5812 DOI: 10.1021/acssuschemeng.6b01847 ACS Sustainable Chem. Eng. 2016, 4, 5804−5813 Research Article ACS Sustainable Chemistry & Engineering xylosederived biochemical. Biofuels, Bioprod. Biorefin. 2008, 2 (5), 438−454. (13) Marcotullio, G.; De Jong, W. Chloride ions enhance furfural formation from D-xylose in dilute aqueous acidic solutions. Green Chem. 2010, 12 (10), 1739−1746. (14) Enslow, K. R.; Bell, A. T. The Role of Metal Halides in Enhancing the Dehydration of Xylose to Furfural. ChemCatChem 2015, 7 (3), 479−489. (15) Guerbuez, E. I.; Wettstein, S. G.; Dumesic, J. A. Conversion of Hemicellulose to Furfural and Levulinic Acid using Biphasic Reactors with Alkylphenol Solvents. ChemSusChem 2012, 5 (2), 383−387. (16) Wrigstedt, P.; Keskivali, J.; Leskela, M.; Repo, T. The Role of Salts and Bronsted Acids in Lewis Acid-Catalyzed Aqueous-Phase Glucose Dehydration to 5-Hydroxymethylfurfural. ChemCatChem 2015, 7 (3), 501−507. (17) vom Stein, T.; Grande, P.; Sibilla, F.; Commandeur, U.; Fischer, R.; Leitner, W.; de Maria, P. D. Salt-assisted organic-acid-catalyzed depolymerization of cellulose. Green Chem. 2010, 12 (10), 1844− 1849. (18) Jiang, Z. C.; Yi, J.; Li, J. M.; He, T.; Hu, C. W. Promoting effect of sodium chloride on the solubilization and depolymerization of cellulose from raw biomass materials in water. ChemSusChem 2015, 8 (11), 1901−1907. (19) Cai, C. M.; Zhang, T.; Kumar, R.; Wyman, C. E. THF cosolvent enhances hydrocarbon fuel precursor yields from lignocellulosic biomass. Green Chem. 2013, 15 (11), 3140−3145. (20) Rivas, S.; Gonzalez-Munoz, M. J.; Santos, V.; Parajo, J. C. Production of furans from hemicellulosic saccharides in biphasic reaction systems. Holzforschung 2013, 67 (8), 923−929. (21) Pace, V.; Hoyos, P.; Castoldi, L.; Dominguez de Maria, P.; Alcantara, A. R. 2-Methyltetrahydrofuran (2-MeTHF): A biomassderived solvent with broad application in organic chemistry. ChemSusChem 2012, 5 (8), 1369−1379. (22) vom Stein, T.; Grande, P. M.; Leitner, W.; de Maria, P. D. Ironcatalyzed furfural production in biobased biphasic systems: From pure sugars to direct use of crude xylose effluents as feedstock. ChemSusChem 2011, 4 (11), 1592−1594. (23) Zhou, L.; Yang, X.; Xu, J.; Shi, M.; Wang, F.; Chen, C.; Xu, J. Depolymerization of cellulose to glucose by oxidation-hydrolysis. Green Chem. 2015, 17 (3), 1519−1524. (24) Kim, E. S.; Liu, S.; Abu-Omar, M. M.; Mosier, N. S. Selective Conversion of Biomass Hemicellulose to Furfural Using Maleic Acid with Microwave Heating. Energy Fuels 2012, 26 (2), 1298−1304. (25) Yu, Z. Y.; Jameel, H.; Chang, H. M.; Park, S. The effect of delignification of forest biomass on enzymatic hydrolysis. Bioresour. Technol. 2011, 102 (19), 9083−9089. (26) vom Stein, T.; Grande, P. M.; Kayser, H.; Sibilla, F.; Leitner, W.; Dominguez de Maria, P. From biomass to feedstock: One-step fractionation of lignocellulose components by the selective organic acid-catalyzed depolymerization of hemicellulose in a biphasic system. Green Chem. 2011, 13 (7), 1772−1777. (27) Sluiter, A.; Hames, B.; Ruiz, R.; Scarlata, C.; Sluiter, B. Determination of Structural Carbohydrates and Lignin in Biomass; Document NREL/TP-510-42618; National Renewable Energy Laboratory: Golden Colorado, 2008. (28) Su, T. C.; Fang, Z.; Zhang, F.; Luo, J.; Li, X. K. Hydrolysis of Selected Tropical Plant Wastes Catalyzed by a Magnetic Carbonaceous Acid with Microwave. Sci. Rep. 2015, 5, 17538. (29) Ghose, T. K. Measurement of cellulase activities. Pure Appl. Chem. 1987, 59 (2), 257−268. (30) Lee, J. W.; Jeffries, T. W. Efficiencies of acid catalysts in the hydrolysis of lignocellulosic biomass over a range of combined severity factors. Bioresour. Technol. 2011, 102 (10), 5884−5890. (31) Haan, J. P. Process for separating furfural from a liquid aqueous phase comprising furfural and one or more organic acids. Patent WO2011161141A1, 20111229, 2011. (32) Pedersen, M.; Vikso-Nielsen, A.; Meyer, A. S. Monosaccharide yields and lignin removal from wheat straw in response to catalyst type and pH during mild thermal pretreatment. Process Biochem. 2010, 45 (7), 1181−1186. (33) Pedersen, M.; Meyer, A. S. Lignocellulose pretreatment severity - relating pH to biomatrix opening. New Biotechnol. 2010, 27 (6), 739−750. (34) Lai, D. M.; Deng, L.; Li, J.; Liao, B.; Guo, Q. X.; Fu, Y. Hydrolysis of cellulose into glucose by magnetic solid acid. ChemSusChem 2011, 4 (1), 55−58. (35) Dussan, K.; Girisuta, B.; Lopes, M.; Leahy, J. J.; Hayes, M. H. B. Effects of Soluble Lignin on the Formic Acid-Catalyzed Formation of Furfural: A Case Study for the Upgrading of Hemicellulose. ChemSusChem 2016, 9 (5), 492−504. (36) Bu, L. X.; Xing, Y.; Yu, H. L.; Gao, Y. X.; Jiang, J. X. Comparative study of sulfite pretreatments for robust enzymatic saccharification of corn cob residue. Biotechnol. Biofuels 2012, 5, 87. (37) Kaparaju, P.; Felby, C. Characterization of lignin during oxidative and hydrothermal pretreatment processes of wheat straw and corn stover. Bioresour. Technol. 2010, 101 (9), 3175−3181. (38) Nadji, H.; Diouf, P. N.; Benaboura, A.; Bedard, Y.; Riedl, B.; Stevanovic, T. Comparative study of lignins isolated from Alfa grass (Stipa tenacissima L.). Bioresour. Technol. 2009, 100 (14), 3585−3592. (39) Jiang, L. Q.; Fang, Z.; Li, X. K.; Luo, J.; Fan, S. P. Combination of dilute acid and ionic liquid pretreatments of sugarcane bagasse for glucose by enzymatic hydrolysis. Process Biochem. 2013, 48 (12), 1942−1946. (40) Zhang, Z. Y.; Wong, H. H.; Albertson, P. L.; Harrison, M. D.; Doherty, W. O. S.; O’Hara, I. M. Effects of glycerol on enzymatic hydrolysis and ethanol production using sugarcane bagasse pretreated by acidified glycerol solution. Bioresour. Technol. 2015, 192, 367−373. (41) Hu, F.; Jung, S.; Ragauskas, A. Pseudo-lignin formation and its impact on enzymatic hydrolysis. Bioresour. Technol. 2012, 117, 7−12. (42) Hu, F.; Ragauskas, A. Suppression of pseudo-lignin formation under dilute acid pretreatment conditions. RSC Adv. 2014, 4 (9), 4317−4323. (43) Pu, Y. Q.; Hu, F.; Huang, F.; Davison, B. H.; Ragauskas, A. J. Assessing the molecular structure basis for biomass recalcitrance during dilute acid and hydrothermal pretreatments. Biotechnol. Biofuels 2013, 6 (1), 15. (44) Zeng, Y. N.; Zhao, S.; Yang, S. H.; Ding, S. Y. Lignin plays a negative role in the biochemical process for producing lignocellulosic biofuels. Curr. Opin. Biotechnol. 2014, 27, 38−45. (45) Lee, S. H.; Doherty, T. V.; Linhardt, R. J.; Dordick, J. S. Ionic liquid-mediated selective extraction of lignin from wood leading to enhanced enzymatic cellulose hydrolysis. Biotechnol. Bioeng. 2009, 102 (5), 1368−1376. (46) Agnihotri, S.; Johnsen, I. A.; Boe, M. S.; Oyaas, K.; Moe, S. Ethanol organosolv pretreatment of softwood (Picea abies) and sugarcane bagasse for biofuel and biorefinery applications. Wood Sci. Technol. 2015, 49 (5), 881−896. (47) Jin, S. G.; Zhang, G. M.; Zhang, P. Y.; Li, F.; Fan, S. Y.; Li, J. Thermo-chemical pretreatment and enzymatic hydrolysis for enhancing saccharification of catalpa sawdust. Bioresour. Technol. 2016, 205, 34−39. (48) Pan, X. J.; Arato, C.; Gilkes, N.; Gregg, D.; Mabee, W.; Pye, K.; Xiao, Z. Z.; Zhang, X.; Saddler, J. Biorefining of softwoods using ethanol organosolv pulping: Preliminary evaluation of process streams for manufacture of fuel-grade ethanol and co-products. Biotechnol. Bioeng. 2005, 90 (4), 473−481. 5813 DOI: 10.1021/acssuschemeng.6b01847 ACS Sustainable Chem. Eng. 2016, 4, 5804−5813