A Simple Two-Step Method for the Selective Conversion of

Jul 17, 2017 - In order to obtain high furfural yield, FL was subjected to further ...... for the Doctoral Program of Higher Education of China (20120...
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Research Article pubs.acs.org/journal/ascecg

A Simple Two-Step Method for the Selective Conversion of Hemicellulose in Pubescens to Furfural Yiping Luo,† Zheng Li,† Yini Zuo,† Zhishan Su,† and Changwei Hu*,† †

Key Laboratory of Green Chemistry and Technology, Ministry of Education, College of Chemistry, Sichuan University, No. 29 Wangjiang Road, Chengdu, Sichuan 610064, China S Supporting Information *

ABSTRACT: A two-step method was adopted to produce furfural from the selective dissolution and conversion of hemicellulose in pubescens. First, in GVL(γ-valerolactone)-H2O co-solvent at 160 °C, H2O promoted the cleavage of chemical bonds linking hemicellulose, lignin, and cellulose, and GVL further helped the co-dissolution of hemicellulose (93.6 wt %) and lignin derivatives (80.2 wt %), leaving a high purity cellulose (83.3 wt %). Heating to 200 °C, the liquid system obtained with NaCl and THF added, achieved the maximum yield of 76.9 mol % with 82.2% selectivity to furfural based on the moles of converted hemicellulose using a 5 wt % pubescens to solvent ratio. It was demonstrated that NaCl with GVL promoted the depolymerization of oligomers to small molecular products (Mw < 150 Da), while the co-contribution of NaCl and co-solvent improved the selectivity to furfural. Cl− could interact strongly with C-OH-2,3,4 of the xylose unit, and the dehydration of xylose to form furfural might first occur on C-OH-4 of xylose, then on C-OH-2,3 of xylose, which enhanced the dehydration and ring open reaction via the cleavage of C1− O6 bonds, then promoted the formation of furfural by inhibiting the retro-aldol reaction to form lactic acid. The co-contribution of NaCl and co-solvent was benefical not only for the selective conversion of the mixture containing hemicellulose-derived monomers and oligomers to furfural but also for obtaining a lower molecular weight lignin derivatives (150−500 Da) that could be further used. KEYWORDS: Biomass, Furfural, Hemicellulose, Solvent effects, NaCl

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INTRODUCTION Increased energy consumption and environmental concerns have driven many efforts toward the development of alternative lignocellulosic biomass as feedstocks to produce liquid fuels and chemicals.1−3 Lignocellulosic biomass largely comprises three components, hemicellulose, cellulose, and lignin, with the associated highly complex physical structure.4 The effective use of the three components in raw biomass to the fullest is promising for the development of lignocellulose-based biorefineries. However, the complex composition makes the utilization of raw biomass to selectively produce chemicals challenging, ascribing to the fact that the physical and chemical properties of the three main components in biomass are quite different. Furfural, identified by the U.S. Department of Energy (DOE) as one of the top 12 value-added products, is widely used in oil refining, plastics, and the pharmaceutical and agrochemical industries.5 It could be derived from raw biomass, generally from C5 sugars, mainly xylose, the monomer unit contained in hemicellulose of lignocellulosic materials.6,7 Therefore, many researchers used pure xylose or pure hemicellulose as the starting material to produce furfural with high yield.8−15 However, the preparation of pure hemicellulose or xylose was © 2017 American Chemical Society

not easy, and their state and structure were quite different from that in actual lignocellulosic biomass. Therefore, the direct utilization of raw biomass as the starting material to produce furfural is promising. Industrially, furfural was produced from hemicellulose in lignocellulosic materials using homogeneous mineral acid catalysts in aqueous solution, with a yield of about 40%−50%.5,11,16−19 Considerable efforts have been made to produce furfural from raw biomass with the addition of HCl or H2SO4.17,20 However, mineral acid-catalyzed processes pose the following problems: corrosion of equipment, difficult recovery of the catalyst from the reaction mixture, environmental pollution, and health risks. Solid acid catalysts, with easy separation and recovery, would be beneficial in view of the principles of green chemistry.16 High yield of furfural from biomass has been achieved over solid acid catalysts.21,22 However, solid acid catalysts face the problem of easy deactivation in aqueous solution or biphasic system containing salt.11 We discovered previously a two-step process for the selective dissolution and conversion of hemicellulose in Received: June 3, 2017 Revised: July 13, 2017 Published: July 17, 2017 8137

DOI: 10.1021/acssuschemeng.7b01766 ACS Sustainable Chem. Eng. 2017, 5, 8137−8147

Research Article

ACS Sustainable Chemistry & Engineering

autoclave. In a typical run, 0.5 g of pubescens with 10 mL solvent was loaded in the reactor. Then, the reactor was sealed, and the inner air was replaced by nitrogen. The initial pressure was added to 1.0 MPa with nitrogen. The reactor was heated from room temperature to the desired temperature quickly and then kept at the desired temperature for a different time. After the reaction, the reactor was fetched from the electric furnace, and cooled to room temperature naturally. The content in the autoclave, a mixture of reaction solvent, liquid products, and solid residue, was collected. The mixture collected was filtered through a preweighed filter paper, and the solid residue obtained was dried at 110 °C in an oven for conversion calculation and further characterization. The obtained liquid was noted as filtrate liquid (FL). Enhancement of Furfural Formation. In order to obtain high furfural yield, FL was subjected to further reaction at higher temperature with the addition of NaCl and THF. The yield (wt %) of liquid products is defined as follows: Yield (wt %) = liquid products amount (g)/amount of hemicellulose contained in pubescens (g) × 100%. According to the literature,21 hemicellulose in biomass is mainly composed of C5 sugar units, accompanied by no more than 15% C6 sugars. So, for simplicity of the calculations, the molecular weight of the hemicellulose unit is considered to be 132. The molecular weight of furfural is 96. Thus, from 132 g of hemicellulose, 96 g of furfural formation is possible (considering 100% conversion and 100% selectivity). Then, the yield (mol %) and selectivity to furfural are calculated as follows: Yield (mol %) = furfural yield (mol)/theoretical yield of furfural (mol) × 100%, Selectivity (%) = furfural yield (mol)/ moles of converted hemicellulose (mol).21 Characterization of Solid Residue. The contents of the three components (i.e., hemicellulose, cellulose, and lignin) in pubescens and in reaction residues were determined through a typical chemical titration method.30,31The details for chemical titration method are given in the Supporting Information. The average deviation of titration was less than ±0.5 wt %. Analysis of Liquid Products. Liquid products were quantitatively measured by Dionex U-3000 high performance liquid chromatography (HPLC) equipped with a Dionex PG-3000 pump, an Aminex HPX-87 column (Bio-Rad), and a Shodex 101 refractive index detector (RID). The temperature of the column oven and detector were 50 and 35 °C, respectively, and the mobile phase was 0.005 M H2SO4 solution at a flow rate of 0.6 mL/min. The contents of the liquid products were quantified by an external standard method. The molecular weight distribution of liquid products was determined by GPC (Agilent 1260) equipped with a gel permeation chromatography column (Agilent PL aquagel−OH 20) and refractive index detector. The injection volume was 5 μL, and pure water was used as the mobile phase at a flow rate of 1.0 mL/min. Polysaccharide with molecular weight from 150 to 642,000 Da was used as the standard for the molecular weight calibration. The liquid products derived from pubescens were also characterized by ESI-MS (Shimadzu) and FT-IR (Nicolet 6700). In Situ ATR-IR Analysis. In situ ATR-IR spectra of the xylose solution and hydrolysate from pubescens in the presence or absence of NaCl or THF with increasing temperature were collected by a ReactIR iC10 system (Mettler Toledo) with a liquid nitrogen-cooled mercury cadmium telluride (MCT) detector. Measurements were taken optically using a DiComp immersion probe and ZnSe as an ATR crystal. During the entire process, high purity nitrogen at a flow rate of 2.5 L·min−1 was supplied continuously to ensure a light path clean, and each spectrum represented 256 co-added scans measured every 1 min at a resolution of 8 cm−1 in the range of 4000−650 cm−1 with the Happ−Genzel apodization method.32 13 C and 1H NMR Analysis. To investigate the mechanism of the interaction of NaCl and THF with hemicellulose, D(+)-xylose was chosen as a model compound, and the NMR spectra of xylose were measured by using a Bruker Advance 400 MHz spectrometer. Xylose (55 mg) and 5 wt % NaCl were dissolved in DMSO-d6 (0.55 mL) and D2O (0.55 mL), respectively, for NMR measurements. Theoretical Calculation. All the geometry optimizations in aqueous solution at room temperature were performed at the M06-

pubescens with AlCl3 and SiO2 as catalysts, where 33.1 wt % furfural (based on the weight of hemicellulose in pubescens) was obtained.23 Recently, Morais et al. developed a green and efficient approach to selectively dissolve hemicellulose in biomass using high-pressure CO2 as a sustainable catalyst, obtaining 43 mol % of furfural.24 Li et al. obtained a high yield of furfural (57.80 mol %) from corncob via a two-step process.25 However, the yield of furfural still needs to be improved. Thus, the development of new technologies to produce furfural with high yield and selectivity directly from raw biomass in an inexpensive and environmentally sustainable manner deserves further study. The understanding of the mechnism for the production of furfural from hemicellulose in raw biomass is important, which facilitates the design of effective catalytic pathways for the production of furfural. In biomass conversion, NaCl was generally added to a biphasic system to improve the yield and selectivity of value-added chemicals due to its salt effect.15,17,26 Li et al. reported that NaCl could increase the partiton coefficient of unstable furfural, which improved the yield of furfural from bassage in the H2O/THF system.17 However, the function of NaCl beyond its salt effect was ignored for the conversion of hemicellulose in biomass to furfural. A few reports used NaCl as an additive in the conversion of cellulose in biomass to value-added chemicals, and significant promotion of the solubilization and depolymerization of cellulose was observed.27,28 Also very important is the role played by NaCl, which was reported to increase the reaction rate for the conversion of xylose to furfural in aqueous acidic solution besides the salt effect.29 These encouraged us to investigate the applicability of NaCl in the depolymerization and conversion of hemicellulose in raw biomass for the production of furfural. Herein, we report a simple two-step method for selective conversion of hemicellulose in pubescens to furfural. The simultaneous dissolution of hemicellulose and lignin in pubescens in a GVL/H2O co-solvent was achieved, while a high purity cellulose was obtained. Without the addition of mineral acid or heterogeneous catalyst in the system, the use of only NaCl as promoter in the co-solvent thermal process realized the selective conversion of hemicellulose to furfural, while lignin with lower molecular weight was obtained that could be further used as feedstocks. It provides not only a way for the utilization of hemicellulose in pubescens to furfural but also guidance to use raw biomass effectively to its fullest. The function of NaCl beyond its salt effect was further studied, which helped understand the mechanism for the interaction of NaCl with hemicellulose-derived oligomers and monomers in biomass in a co-solvent system and gave a valuable idea for obtaining furfural with high yield and selectivity from hemicellulose in raw biomass.

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MATERIALS AND METHODS

Materials. A pubescens sample (80 meshes, Anji county of Zhejiang Province in China) was washed three times with distilled water and dried at 110 °C in an oven overnight before use. The main components of dried pubescens were 46.5 wt % cellulose, 17.9 wt % hemicellulose, and 25.4 wt % lignin.23 Sodium chloride (NaCl, AR, Chengdu Kelong Chemical Regent Factory in China), tetrahydrofuran (THF, AR, Chengdu Kelong Chemical Regent Factory in China), and γ-valerolactone (GVL, 99%, J&K Scientific, Ltd.) were purchased commercially and used without further purification. Optimization of Reaction Conditions for Dissolution of Hemicellulose and Lignin. Solvent-thermal dissolution and conversion of pubescens was conducted in a stainless steel sealed 8138

DOI: 10.1021/acssuschemeng.7b01766 ACS Sustainable Chem. Eng. 2017, 5, 8137−8147

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Figure 1. (A) Effect of GVL concentration on the conversion of pubescens feedstock and the three components in pubescens at 160 °C for 4 h. (B) Effect of GVL concentration on the distribution of small molecular products derived from hemicellulose in pubescens at 160 °C for 4 h. 2X/6-311++G(d, p) level33 using the Gaussian 09 program34 and characterized by frequency analysis. The self-consistent reaction field (SCRF) method based on the universal solvation model SMD35 was adopted to evaluate the effect of solvent. Results and Discussion. Co-Dissolution of Hemicellulose and Lignin in GVL/H2O System. GVL is a green solvent that could be derived from biomass.36−39 The sustainability of the biomass conversion process by using GVL solvent had been reported by Alonso et al.,39 elaborating the alleviation of the need to purchase and transport petroleum-derived solvents for biomass conversion.40 Gurbuz et al. obtianed a high yield of 80 mol % furfural from xylose in the presence of H-mordenite catalyst using GVL with 10 wt % water as the solvent.11 Luterbacher et al. achieved the complete solubilization of biomass (corn stover, hardwood, and softwood) using a solvent mixture of biomass-derived GVL, water, and dilute sulfuric acid.20 Stimulated by these studies, we began our study by using GVL/H2O co-solvent to convert pubescens under different reaction conditions. Most hemicellulose (93.5 wt %) and lignin (87.6 wt %) in pubescens can be removed at 160 °C for 4 h to obtain a high purity cellulose (84.2 wt %) as a solid residue by the GVL/H2O (1:1) solvent (Figure S1). Fang et al. reported that 79.6 wt % hemicellulose and 79.4 wt % lignin could be removed from birch sawdust in GVL/H2O (1:1) at 170 °C for 2 h in a microwave reactor, while 79.2 wt % cellulose was not converted.41 Compared with their work, more hemicellulose and lignin were converted to obtain a higher cellulose purity in the present work. This might be caused by the nature of the raw material and convention method used. We also found that the conversion of pubescens feedstock and the three components in pubescens depended more on temperature than on reaction time (Figure S2). Water content in the GVL−water mixture is a key factor for the fractionation of biomass.41 It is shown in Figure 1(A) that a lower proportion of cellulose was dissolved in the entire solvent system due to a lower reaction temperature being used (160 °C). In a pure H2O system, 96.5 wt % hemicellulose and 62.7 wt % liginin were converted. In a pure GVL system, as shown in Figure 1(A), the dissolution of hemicellulose and lignin were, respectively, 11.4 and 35.0 wt %. The conversion of the three components in pubescens was quite low due to the low conversion of pubescens in the pure GVL system. Pure H2O promoted the dissolution of hemicellulose while GVL did not. In the GVL/H2O co-solvent system, more than 90.0 wt % hemicellulose was dissolved when the concentration of GVL was 25% or 50%. However, only 54.1 wt % hemicellulose was dissolved when the GVL concentration increased to 75% in the GVL/H2O co-solvent system, suggesting that a higher concentration of GVL led to a lower dissolution of hemicellulose. Lê et al. reported that the removal of hemicelluloses increased with increasing water content in the fractionation liquor due to enhanced hydrolytic degradation.42 Therefore, H2O was the main contributor to hemicellulose dissolution compared to GVL. Xue et al. reported that GVL/D2O could offer more hydrogen bond acceptors and donors to interact with lignin, so more lignin solubility could be obtained in the GVL/D2O system.43 That was why low dissolution of lignin was observed in pure H2O or the pure GVL system. In the GVL/H2O co-solvent with different

ratios, about 80 wt % lignin was dissolved, and delignification reached the maximum in the 50% GVL/H2O system. The co-action of H2O and GVL facilitated the selective co-dissolution of hemicellulose and lignin from pubescens. Efficient simultaneous dissolution of hemicellulose (93.6 wt %) and lignin (80.2 wt %) in pubescens could be achieved without significant degradation of cellulose when only 25% GVL was used. We speculated that pure H2O solvent could promote the cleavage of chemical bonds linking hemicellulose, lignin, and cellulose, while the GVL solvent helped further dissolve the hemicellulose and lignin derivatives. GPC analyses of liquid products in the GVL/H2O system with different ratios at 160 °C for 4 h are shown in Table S3. In the pure H2O system, a higher weight-average molecular product (Mw = 18794 Da) was obtained. It was indicated that oligomers derived from hemicellulose and lignin with high molecular weight existed after being treated by H2O. After the pubescens samples were treated by pure GVL, the value of Mw was 3070 Da, which was much lower than that in the pure H2O system. Because the dissolution of the three components in pubescens was quite low, it was difficult to say if GVL exhibited better performance on the depolymerization of oligomers than H2O. After the pubescens samples were treated in the GVL/H2O co-solvent system, it was observed that the value of Mw fell into the range of 1500−3500 Da. The results suggested that the co-existence of GVL with H2O significantly promoted the depolymerization of oligomers derived from hemicellulose and lignin to lower molecular weight oligomers. Therefore, the results suggested that oligomers existed after being treated by the entire solvent system. Although the lignin component also dissolved, actually, the concentration of monophenols derived from lignin was low (70%) in GVL for a variety of acid catalysts containing Brønsted sites with 2 wt % hemicellulose as starting materials.11 Ramakanta et al. obtained 56 mol % furfural in water/p-xylene (60 g) with 0.3 g HUSY catalyst when 0.6 g of bagasse was used as the starting material (1% bagasse).21 Therefore, the concentration of the starting material has a significant effect on the production of furfural from hemicellulose in biomass. Some literature reported that xylose will lose three H2O to form furfural via the dehydration reaction, and the process is easily carried out under acidic conditions.8,44,46 However, without extra addition of acid, this work provided a two-step method for the selective conversion of hemicellulose in pubescens with the contribution of NaCl and co-solvent. In fact, high temperature water would play the roles of acid and base catalysts.50 Jing Qi et al. reported the production of furfural from D-xylose without any acid catalyst in high temperature liquid water.51 In addition, some organic acid such as formic acid, acetic acid, and lactic acid were also detected in the products of the first-step or second-step reaction in the present work. We also detected the pH value of liquid products in the first-step and second-step reactions, as shown in Table S8. The results showed that the reaction system was under acidic conditions, although no extra acid was added. Therefore, the high temperature water and organic acid produced during the reaction created an acidic environment, which made the production of furfural from pubescens without extra addition of acid in GVL/H2O possible. To investigate the molecular weight distribution of the liquid products in the second-step reaction, the liquid products were analyzed by GPC (Figure 2 and Table S9). For the liquid products in FL obtained from the 25% GVL/H2O system for the first step, the values of Mn and Mw were, respectively, 163 and 1557 Da, while the polydispersity of liquid products was 9.55 (Table S3). As shown in Figure 2(A), the percentage of the molecular weight of oligomers greater than 1000 Da was 36%, while that in the range of 150−500 Da was 30%, and only 13% of liquid products in FL was below 150 Da. The results indicated that oligomers existed in FL after the first-step reaction, and the molecular weight distribution of oligomers was disperse. When FL was further heated at 200 °C for 2 h, it is observed from Figure 2(B) that the molecular weight of oligomers fell into the range of 150−500 Da, and the percentage for the molecular weight of oligomers below 150 Da increased to 36%. We also observed that the 8141

DOI: 10.1021/acssuschemeng.7b01766 ACS Sustainable Chem. Eng. 2017, 5, 8137−8147

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ACS Sustainable Chemistry & Engineering

Figure 3. (A) Structure of α-D-xylose and β-D-xylose. (B) 1H NMR spectra of xylose with and without NaCl in DMSO-d6. (C) 1H NMR spectra of xylose with and without NaCl in DMSO-d6 with chemical shifts from 4.4 to 5.2. (D) 1H NMR spectra of xylose with and without NaCl in DMSO-d6 with chemical shifts from 2.4 to 3.8. The assignment of peaks is based on the literature.57,58 value of Mw and polydispersity significantly decreased to 322 Da and 2.08, respectively. This indicated that GVL contained in FL promoted further conversion of oligomers, which was in accordance with HPLC results. The addition of THF showed similar molecular weight distribution of liquid products as that obtained when FL was further heated at 200 °C for 2 h (Figure 2(C)). Therefore, the addition of only THF did not add a significant effect on promotion of the depolymerization of oligomers compared with the GVL solvent. As shown in Figure 2(D), the percentage of the molecular weight of oligomers greater than 1000 significantly decreased from 36% in FL for the first step to 1% with the addition of 5%NaCl, while the liquid products were mainly in the form of small molecular weight products (below 150 Da). A lower polydispersity of 1.19 was also obtained with the addition of NaCl. With the addition of NaCl and THF, the molecular weight of liquid products was mainly below 500 Da, while the molecular weight of liquid products greater than 1000 obtained was only 2%. Therefore, based on the GPC and HPLC results, a small amount of GVL in FL with NaCl promotes the depolymerization of oligomers to small molecular products, while the co-contribution of NaCl and co-solvent help change the distribution of small molecular products and then improve the selectivity to furfural. Here, 97.9 wt % of small molecular products mainly from hemicellulose in pubescens in the NaCl-FL/THF system was obtained. It indicated that the dissolved hemicellulose derivatives were mainly converted to small molecular products (molecular weight below 150 Da). However, the products derived from lignin existed also in the liquid products. Although the values of Mn and Mw were, respectively, only 143 and 192 Da in the NaCl-FL/THF system, nearly half of the liquid products within the range of 150−500 Da and a small part of liquid products greater than 500 Da were also obtained. Therefore, the molecular weight distribution of the liquid products in the range of 150−500 Da represented the molecular weight of oligomers mainly from dissolved lignin derivatives. ESI-MS results confirmed that mono units such as 4ethylphenol, guaiacol, syringaldehyde, vanillic, etc. from lignin made up oligomers presented in the NaCl-FL/THF system (Figure S5). The highly complex molecular structure of lignin, combined with its highly recalcitrant chemical nature, has so far hampered many efforts to increase its value. The low molecular weight oligomers from lignin provided a way for the further use of lignin, such as the production of cyclohexanol.52,53 Furfural can be almost removed completely with

distillation in the GVL/H2O co-solvent system, then further used to produce value-added chemicals (such as GVL); it could also be kept in the reactor and subsequently processed to produce GVL.17,38,39,54 Our work showed that 84.9% furfural could be removed through vacuum distillation at 50 °C. IR (Figure S6) and ESI-MS (Figure S7) results demonstrated that the remaining liquid products after distillation were mainly from lignin, which could be further upgraded to biofuels or recovered through precipitation with the addition of water and then further used as starting materials.39,41,42,53,55,56 Although GVL is an expensive solvent and this increases the process cost, it can be recycled.39 THF has a low boiling point (66 °C)40 and can be easily removed from furfural. The realization of the recyclability of the solvent would prevent the formation of waste. Therefore, this system not only can be considered as a sustainable conversion process but also as a guiding system to use all three components of the actual biomass to the fullest. Roles of NaCl with Co-Solvent for Conversion of Hemicellulose Derivatives to Furfural. To investigate the formation mechanism of furfural by the interaction of NaCl with hemicellulose, xylose, which is the monomer unit contained in hemicellulose of lignocellulosic materials, was used as the model compound. 13C NMR and 1H NMR spectra of xylose with and without 5% NaCl in DMSOd6 were then measured (Figure 3 and Figure S9). It is shown from Figure 3(B) that the 1H NMR signal of α-OH-1 was not affected by the addition of NaCl, suggesting that there is no interaction between the hydrogen atoms of α-OH-1 and Cl−, while the hydroxyl hydrogen signals of α, β-OH-2,3,4 moved downfield and broadened. Qrtiz et al. and Fernández-Bertrán et al. reported the occurrence of hydrogen bonding OH···F− when KF was added into the xylose solution, which broadened the hydroxylic hydrogen NMR signals.57,58 Cl− is a good hydrogen acceptor, and it favored the attack of hydrogen in the moreactivated hydroxyl group in xylose. Therefore, these results indicated that Cl− could form hydrogen bonds with hydroxyl hydrogen of α-, βOH-2,3,4 but not that of α-OH-1. As shown in Figure 3(C) and (D), 1 H NMR signals of all the ring hydrogens changed significantly with the addition of NaCl. This indicated that the ring hydrogens might be affected after the formation of hydrogen bonding OH···Cl−. However, the 13C NMR signals (Figure S9) of xylose showed a slight change when NaCl was added, which implied that the interaction of xylose 8142

DOI: 10.1021/acssuschemeng.7b01766 ACS Sustainable Chem. Eng. 2017, 5, 8137−8147

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ACS Sustainable Chemistry & Engineering with Cl− was not as strong as that of a chemical bond such as covalent bond. To further reveal the possible interaction between NaCl and hemicellulose in a co-solvent system, we performed in situ ATR-IR experiments of the xylose solution in the presence or absence of NaCl or THF at room temperature. As shown in Figure 4, the aqueous

broadened, indicating the synergistic effect of THF and NaCl on C1−O6 bonds. The increased intensity of a new peak at 1068 cm−1 was observed with the addition of THF in the xylose-H2O/THF system or xylose-NaCl-H2O/THF system. It seems that THF promoted the blue shift of peaks assigned to C1−O6 bonds. The peak at 1151 cm−1 assigned to the interaction of C1−H and C1−OH gave a blue shift when NaCl was added. The peak at 940 and 900 cm−1 assigned to the stretching vibration of skeleton C−C bonds also showed changes with the addition of NaCl and THF. It can be seen that the interaction between xylose with NaCl and THF made the skeleton C−C bonds and C−H bonds change. The peaks at 1120, 1090, and 980 cm−1, respectively, assigned to the stretching vibration of C5−O6 or C2−O and C4−O, C3−OH, and C4−O or the interaction of C5−H5a with C5−H5b showed only a slight change in the presence of NaCl or THF. This indicated that the addition of NaCl or THF had a slight effect on C5−O6, C2−O, C4−O, C3−OH, C5−H5a, and C5−H5b. We also studied the effect of NaCl or THF on the xylose solution by AIR-IR spectra in the presence of GVL (Figure S15). It showed a similar trend in the presence of GVL as that in the absence of GVL. The 3600− 3100 cm−1 region comprises the OH stretching frequencies of the hydroxyl groups of the sugar,58 which are shown in Figure S16. It was observed that there is a slight change in OH peaks with the addition of THF in the presence of GVL, while NaCl made the peaks broadened. Notably, the OH peaks became more broadened with the addition of NaCl and THF. Mathlauthi et al. reported that when the hydroxylic hydrogen engages in hydrogen bonding interactions with a basic atom like the fluoride anion, the band lowers its frequency, increases its intensity, and broadens its line width.59 Therefore, the observed effect may be due to hydrogen bonding of the hydroxyl hydrogen and Cl− with the help of THF in the presence of GVL. A theoretical calculation was also used to study the interaction of xylose with H2O, Cl−, or THF (Figure S10). The results confirmed that Cl− had interaction with COH-2, C-OH-3, and C-OH-4 (Figure S10(C)). We also found that both H2O and THF could interact with C-OH-2 and C-OH-4 (Figure S10(B) and S10(D)). Furthermore, the length of C1−O6 in xylose increased from 1.417 to 1.420 Å after Cl− interacted with hydroxyl hydrogen of C-OH-2,3,4 in xylose, while the length of C1−O6

Figure 4. ATR-IR spectra of xylose solution in the presence or absence of NaCl or THF at room temperature. (a) xylose and H2O, (b) xylose and H2O/THF(1:4), (c) xylose, H2O, and 5% NaCl, (d) xylose and H2O/THF(1:4) in the presence of 5% NaCl. xylose solution showed eight IR peaks over the range of 900−1200 cm−1. Since the assignment of IR peaks for xylose in the literature is vague, a theoretical calculation was used for the assignment of IR peaks in xylose (Figure S10(A) and Table S2). According to the theoretical value, we assigned the maximum peak at 1048 cm−1 in the aqueous xylose solution to the C1−O6 stretching vibration (Figure 4). Thus, the red shift of the peak at 1048 cm−1 with the addition of THF suggested the action of THF on C1−O6 bonds. It is also observed that the peak at 1048 cm−1 was broadened with the addition of NaCl, which thus indicated NaCl will also affect C1−O6 bonds. When THF and NaCl were added, the peak at 1048 cm−1 became more

Figure 5. ATR-IR spectra of xylose solution and FL in the presence of NaCl and THF with increasing temperature: (A) 3D image of ATR-IR spectra in xylose-NaCl-H2O/THF system. (B) ATR-IR spectra of xylose solution at different temperatures in xylose-NaCl-H2O/THF system: (a) RT, (b) 30 °C, (c) 40 °C, (d) 60 °C, (e) 80 °C, (f) 100 °C. (C) 3D image of ATR-IR spectra in NaCl-FL/THF system. (D) ATR-IR spectra of FL at different temperatures in NaCl-FL/THF system: (a) FL, RT, (b) FL and THF, RT, (c) FL, THF, and NaCl, RT, (d) FL, THF, and NaCl, 60 °C, (e) FL, THF, and NaCl, 80 °C, (f) FL, THF, and NaCl, 100 °C. 8143

DOI: 10.1021/acssuschemeng.7b01766 ACS Sustainable Chem. Eng. 2017, 5, 8137−8147

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ACS Sustainable Chemistry & Engineering

Scheme 1. (A) Conversion of Pubescens to Furfural with High Yield and Lower Molecular Weight Lignin Derivatives in H2O/ GVL/THF System. (B) Proposed Reaction Network for NaCl-Promoted Depolymerization and Conversion of Hemicellulose in Pubescens to Furfural with High Yield in H2O/GVL/THF Systema

a

Dimer is a representative for oligomers. xylose-NaCl-H2O/GVL/THF system at 100 °C. Therefore, based on NMR, in situ ATR-IR, and theoretical calculation results, we speculated that hydrogen bonding OH···Cl− could be formed with the help of H2O, THF, and GVL, which may promote the dehydration reaction and the cleavage of C1−O6 bonds in xylose to form furfural. The same in situ ATR-IR analysis was also applied in FL from the 25% GVL/H2O system for the first step. The peak at 1236 cm−1 was assigned to the stretching vibration of C = O in xylan, while the peaks at 1200−1000 cm−1 are dominated by ring vibrations overlapped with stretching vibrations of (C−OH) side groups and the (C−O−C) glycosidic bond vibration.60,61 Similarly, we also observed the changes in the region of 1000−1236 cm−1 with the addition of NaCl or THF (Figures S26 and S28). The peak in the region of 4000−2600 cm−1, which is assigned to the OH stretching frequencies of the hydroxyl groups, increased its intensity and broadened with increasing temperature in the presence of NaCl (Figure S27). These changes were especially significant when NaCl and THF were added together in FL (Figure 5(C) and (D)) because it led to an obvious shift of IR spectra. The specific co-contribution of NaCl with co-solvent in promoting the formation of furfural was proved by NMR, theoretical calculation, and in situ ATR-IR analysis. Thus, xylose was used as the starting material to perform the reaction in the NaCl-H2O/GVL/THF system at 200 °C for 2 h, and the results are shown in Tables S10 and S11. When xylose was heated in water at 200 °C for 2 h, 50.41 mol % furfural was obtained, while the yield of furfural increased to 56.46 mol % with the addition of 5% NaCl under the same reaction conditions. We also observed that the yield of furfural increased, respectively, from 50.41 to 55.98 mol % and 56.46 to 61.47 mol % when 25% GVL was added in the xylose/H2O and 5%NaCl-xylose/H2O system. The activation energy barrier for xylose dehydration is decreased in GVL, whereas the barrier for furfural degradation is higher in GVL.44 Therefore, a small amount of GVL can make a contribution for the production of furfural. Interestingly, more than half of xylose was not reacted, and a quite low yield of furfural was obtained with the

increased to 1.428 and 1.424 Å after THF and H2O interacted with the hydroxyl hydrogen of C-OH-2,4 in xylose, respectively. So, the cleavage of C1−O6 will be promoted after the formation of hydrogen bonds between Cl− and hydroxyl hydrogen of xylose. The reactivity index analysis of free D-xylose was studied (Figure S17). The results showed that the global nucleophilic index of free D-xylose is 1.86 eV and that localized on an O4 atom was 0.11 eV, while that for O1 was only 0.01 eV. This indicated the highest capacity for the O4 atom to accept an H proton, while the O1 atom has a weak capacity to accept an H proton. These results further confirmed that the dehydration of xylose to form furfural does not easily occur on C-OH-1 of xylose. Because the O4 atom has a higher nucleophilic index compared with the O2 and O3 atoms, we speculated that the dehydration of xylose to form furfural first occurred on C-OH-4 of xylose and then on C-OH2,3 of xylose. These results matched well with NMR results and in situ ATR-IR results. Without the addition of GVL, the influence of increasing temperature on the xylose solution in the presence or absence of NaCl or THF was further studied using in situ ATR-IR. It is shown that the peak at 1048 cm−1 in the NaCl and THF-free aqueous xylose solution broadened gradually with increasing temperature (Figure S18). The strong vibration shift and broadening of the peak at 1048 cm−1 with the addition of NaCl was observed when temperature increased to 100 °C (Figure S19). A new peak at 1064 cm−1 was always seen in the ATR-IR spectra of the aqueous xylose solution containing THF (Figure S20). Besides, we observed a significant blue shift of the new peak at 1068 cm−1, and the intensity of the peak gradually increased above 40 °C, as shown in Figure 5(A) and (B), with the addition of NaCl and THF. The other peaks like 1151, 1090, 980, and 940 cm−1 assigned to skeleton C−C, C−H, C−O bonds also changed a lot in chemical shifts. In the presence of GVL, the peaks above showed significant changes with the addition of NaCl or THF than those in the absence of GVL, suggesting that GVL also makes a contribution to the interaction between xylose with NaCl and THF (Figures S21, S22, S23, and S24). Meanwhile, we also detected the formation of furfural by HPLC in the 8144

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addition of THF in the absence or presence of GVL. Therefore, a lower yield of furfural from xylose was obtained in the NaCl-H2O/ THF system (49.55 mol %) and NaCl-H2O/THF/GVL system (50.26 mol %). However, 76.9 mol % furfural was obtained directly from raw biomass, which was higher than the yield obtained from xylose. ESIMS results showed xylose derivatives ([2xylose-H2O], [xylose+H2O], [2xylose+FA+H-2H2O]+, etc.) and intermediate species ([xylose+H− H2O]+, [xylose+H]+, etc.) existed in FL (Figure S29), which is favorable for the production of furfural. Thus, it suggested that NaClH2O/GVL/THF is a favorable system for the conversion of a mixture containing hemicellulose-derived monomers and oligomers from actual biomass to produce furfural with high yield, while it is not very efficient for the conversion of model compound xylose. According to our findings above, we tentatively proposed a reaction network for NaCl-promoted depolymerization and conversion of hemicellulose in pubescens to furfural with high yield in the GVL/ H2O/THF system (Scheme 1). In the first step, H2O promoted the cleavage of chemical bonds linking hemicellulose, lignin, and cellulose, while GVL could further help dissolve the hemicellulose and lignin derivatives, obtaining a high purity cellulose. After the first-step reaction, the extracted hemicellulose derivatives could be divided mainly into two parts, that is, small molecular products (such as xylose, furfural, and acetic acid) and oligomers. In the second-step reaction of FL, Cl− could form intermolecular hydrogen bonds with hydroxyl hydrogen of C-OH-2,3,4 in xylose with the help of H2O, GVL, and THF, which would promote the dehydration reaction, while the formation of intramolecular hydrogen bonds was inhibited to perform the retro-aldol reaction for the formation of lactic acid. The significant interaction between Cl− and −OH will also promote the cleavage of C1−O6 bonds and benefit the ring open reaction to produce furfural. The dehydration of xylose to form furfural first occurred on C-OH-4 of xylose and then on C-OH-2,3 of xylose. The production of furfural from xylose generally proceeded via intermediate xylulose in an isomerization reaction.62,63 We observed xylulose in FL, but it completely dissappeared in the NaCl-FL/GVL/THF system in the second step. So, the isomerization reaction also existed. The production of furfural may also occur directly from hemicellulosederived oligomers; however, the mechanism for the production of furfural directly from oligomers in this actual biomass conversion system requires further study.

Research Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01766. Chemical titration details and results, HPLC data, 1H and 13C NMR spectra, in situ ATR-IR spectra, and computational details and results. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Changwei Hu: 0000-0002-4094-6605 Author Contributions

Y. P. Luo carried out the majority of the experiment and wrote the paper. Z. Li helped with the analysis of ESI-MS and GPC. Y. N. Zuo and Z. S. Su carried out the theoretical calculation. C. W. Hu supervised the project and revised the paper. All authors have given approval to the final version of the manuscript and contributed to the scientific discussion. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the National Basic Research Program of China (973 Program, 2013CB228103) and Special Research Fund for the Doctoral Program of Higher Education of China (20120181130014). The characterization of solid residues and liquid products from the Analytical and Testing Center of Sichuan University and comprehensive training platform of the specialized laboratory, College of Chemistry, Sichuan University, are greatly appreciated.

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REFERENCES

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CONCLUSIONS In summary, we have demonstrated one strategy of complete utilization of pubescens, where the production of furfural with high yield and selectivity from a mixture containing hemicellulose derived monomers and oligomers in pubescens was realized via a two-step method. It provides ways to improve the processes in chemistry or the chemical industry by using renewable biomass as starting materials. NaCl with a co-solvent was helpful for the depolymerization of oligomers and the conversion of monomers and oligomers to small molecular products, especially for the formation of furfural. Cl− can interact strongly with C-OH-2,3,4 of the xylose unit in hemicellulose with the help of GVL, H2O, and THF, and the dehydration of xylose to form furfural might first occur on COH-4 of xylose, then on C-OH-2, 3 of xylose, which enhanced the formation of furfural by promoting the dehydration reaction. Besides, Cl− in the co-solvent system had a significant effect on the cleavage of C1−O6 bonds after the formation of hydrogen bonds with hydroxyl hydrogen, which was helpful for the ring open reaction to produce furfural. The present finding is interesting in sustainable chemistry, where NaCl was used as a promoter beyond its salt effect in the co-solvent thermal process for the utilization of hemicellulose to produce furfural with high yield and selectivity. 8145

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