Highly Efficient Conversion of Xylose Residues to ... - ACS Publications

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

Highly Efficient Conversion of Xylose Residues to Levulinic Acid over FeCl3 Catalyst in Green Salt Solutions Chao Wang,† Qilin Zhang,† Yanglei Chen,† Xueming Zhang,† and Feng Xu*,†,‡ †

Beijing Key Laboratory of Lignocellulosic Chemistry, Beijing Forestry University, Beijing, 100083, China Shandong Key Laboratory of Pulping and Papermaking Engineering, Qilu University of Technology, Jinan, 250353, China

‡

S Supporting Information *

ABSTRACT: The economically viable synthesis of levulinic acid (LA), a promising and valuable renewable biomass-derived platform for bioproducts, with high carbon efficiency is a challenge. A direct and highly effective catalytic system for conversion of xylose residues (XRs) into LA under mild conditions by using FeCl3 as catalyst and cheaply available NaCl as promoter has been developed. The NaCl solution exhibits high carbon efficiency in LA (68.0 mol %) when compared with the non-NaCl systems (48.5 mol %) due to the moderate increase of the acidity and the higher viscosity of the NaCl system than water. The experimental results demonstrated that the presence of NaCl caused no distinctive changes on reaction pathways but increased the dissolution rate and the hydrolysis rate of XRs cellulose. Moreover, further integration of our degradation process with a reactive extraction step makes energy-efficient separation of LA. The NaCl solutions easily and efficiently extracted LA into LA-derived solvent 2methyltetrahydrofuran from aqueous solutions. The efficiency and integration of the reaction process presented a great potential for LA production from renewable biomass with the aid of concentrated seawater. KEYWORDS: Levulinic acid, Xylose residues, FeCl3, NaCl, 2-Methyltetrahydrofuran

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INTRODUCTION Driven by increasing environmental concerns about the negative impact of the utilization of fossil resources, noteworthy efforts have been currently underway to develop approaches to fuels and chemicals based on renewable alternatives. Biomass with wide availability and enormous potential has been acknowledged as a significant feeds for manufacturing useable transport fuels and highly valuable chemicals.1 Corncob, one type of agricultural waste, is an economic biomass, and more than 40 million tons of corncob were produced annually in China.2 Current processes for the fractionation of corncob focused on the production of hemicellulose-based products, such as xylose, arabinose, and furfural. The residual fractions that constituted a major part of corncob were unfortunately treated as waste or used for energy production with low value, which needed to be changed urgently. It is an acknowledgment that xylose residues (XRs) from corncob still contained a high amount of cellulose after the production of xylose utilizing the hemicellulose.3,4 The XRs cellulose can be transformed into promising green intermediate molecules, such as 5-hydroxymethylfurfural (HMF),5,6 levulinic acid (LA),7 which can be considered as the starting materials to generate different kinds of fuels and fuel additives.8 LA is a promising and useful building block in further production of numerous LA-derived chemicals. It is also one of the most attractive 12 platform chemicals derived from lignocellulosic biomass.9 In the past years, a number of © XXXX American Chemical Society

significant applications have been developed to facilitate producing LA, for example, animal feed, renewable solvent, and all kinds of materials for different industries. 10 Furthermore, it is catalytically transformed to fuel and fuel additives, such as 2-methyl-tetrahydrofuran (2-MeTHF)11 and levulinate esters.12 LA is also hydrogenated to an organic intermediate, gamma-valerolactone (GVL).13 According to the previous studies,14,15 LA was obtained from catalytic hydrolysis of starch waste and lignocellulosic biomass by mineral acids (HCl and H2SO4). High yields of 70 mol % were obtained by the Biofine technology7 beyond 200 °C. However, corrosive liquid acids catalytic process needs expensive corrosion-resistant equipment and causes serious environmental pollution. Nevertheless, there were few efficient solid catalysts16,17 with excellent catalytic performance, since catalytic rates of solid catalyst were inhibited. Most of solid acid catalysts suffered from other problems such as separation of solid acid catalyst from the reaction systems. Economical metal chlorides were widely investigated for selective conversion of xylose to furfural18 and glucose to HMF.19 The acid-catalyzed isomerization of aldoses-to-ketoses was investigated by metal cations (Al3+, Cr3+, Cu2+, Zn2+, and In3+). Metal cations (e.g., Al3+, Fe3+, and Cr3+) in aqueous solutions formed hydrolyzed Received: September 8, 2017 Revised: January 21, 2018 Published: February 13, 2018 A

DOI: 10.1021/acssuschemeng.7b03183 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 1. Composition of original XRs and residues (a), influence of reaction time on the liquid products (b). Reaction conditions: XRs (1.2 g), FeCl3 (0.6 g), 30 mL of water, 180 °C. rpm. In the case where higher temperature was used (>180 °C), a 100 mL 316L stainless steel reactor was used in place of a glass reactor. After finishing the reaction, the glass reactor was cooled first by fan and then immersed into an ice bath to room temperature, while the stainless steel reactors were immediately plunged into an ice bath to rapidly cool to room temperature. Analytic Methods. The remanent contents, including water, liquid products, and solid residues, were fully collected. The collected mixture was first filtered, and then the filter residues were washed three times. Finally the filter residues were oven-dried at 70 °C for 48 h and weighed. The liquid products including glucose, HMF, FA, and LA were analyzed using an Agilent 1260 system equipped with an organic acid column (Bio-Rad Aminex HPX-87H) and a refractive index detector. Degassed 5 mM H2SO4 solution was the mobile phase with flow rate of 0.6 mL/min and the column temperature was 50 °C. The yield and selectivity of liquid products are defined as follows:

metal species and H3O+, which increased the Brønsted acidity and the Lewis acidity. Recently, Li et al.20 presented that a LA yield of 46.8 mol % was obtained from corncob residues by AlCl3 catalysts. It was demonstrated that metal chloride was efficient for the production of LA from biomass. However, the LA yields from lignocellulose urgently need to be increased. vom Stein et al. reported the NaCl assisted dicarboxylic acids catalyzed decomposition of different cellulose to oligomers and glucose. The experiments investigated that various effects greatly controlled the decomposition processing.21 In addition, Luo et al. investigated that the NaCl not only improved the furfural selectivity in THF/H2O systems but also assisted the ring-open reaction of xylose.22 However, the function of NaCl in LA production from complex lignocellulose still needs to investigate in detail. Searching a novel technology and understanding the mechanism of salts-assisted conversion of lignocellulose to LA are urgent and significant. Herein, this study focuses on selective converting the XRs cellulose to LA using cheaply available FeCl3 as the catalyst in salts solutions under hydrothermal conditions. The object of the present work was to obtain high yield of LA directly from biomass waste, simplifying the LA recovery, and finally facilitating LA upgrading and valorization. Meanwhile, several salts were tested into the solutions, and the effect of NaCl was further studied in detail.

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XRs conversion (%) =

Liquid product amount (g) × 100 XRs amount (g)

Cellulose conversion (%) =

(1)

Cellulose converted (g) × 100 Cellulose in XRs (g) (2)

Glucose yield (mol%) =

HMF yield (mol%) =

EXPERIMENTAL SECTION

Materials. XRs was obtained from Shandong Longlive Biotechnology Co., Ltd. (Shandong, China), which was produced from acidcatalytic hydrolysis of corncob by H2SO4 to obtain xylose product. For experimental use, it was first washed with distilled water, subsequently crushed into fine powder (>80 mesh), and oven-dried at 50 °C for 48 h. The compositional analysis of XRs was conducted by the protocols NREL/TP-510−42618.23 On a dry weight basis, the XRs mainly contained approximately 61.0% glucan, 6.5% xylan, 21.0% Klason lignin, and 1.0% acid-soluble lignin. FeCl3, NaCl and other inorganic chemicals were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai China). LA (98.0%), formic acid (FA) (>98.0%), glycol (98%), 2-methyltetrahydrofuran (2-MeTHF) (99.0%), tetrahydrofuran (THF) (99.0%), and methyl isobutyl ketone (MIBK) (>99.0%) were purchased from Aladdin Industrial Corporation (Shanghai, China). Catalytic Tests. The reactions were carried out in 75 mL thickwalled glass reactors. Typically, 1.2 g of XRs was first loaded into reactor with 30 mL of distilled water, where certain quantity of FeCl3 and different amounts of salts (wt %, based on the aqueous solutions) were aggregated. The reactors were flushed with N2 for three times. Then, the reactors were transferred immediately into an oil bath heated to a designed temperature and kept for designed reaction time. The mixture was stirred by a magnetic stir bar in the reactor at 500

Glucose amount (mol) × 100 Total glucan in XRs (mol)

HMF amount (mol) × 100 Total glucan in XRs (mol)

(3)

(4)

LA yield (mol%) =

LA amount (mol) × 100 Total glucan in XRs (mol)

(5)

FA yield (mol%) =

FA amount (mol) × 100 Total amount (mol)

(6)

Characterization of the Solid Residues after Degradations. The components in XRs and the unconverted solid residues after reaction were determined according to NREL protocols NREL/TP510−42618.23 The crystalline forms of XRs and unconverted solid residues during different reaction conditions were examined by XRD measurement on D8 Advance instrument (Bruker AXS, Germany) with Ni-filtered Cu Kα radiation (wavelength = 0.154 nm) at 40 kV and 30 mA. The crystallinity index (CrI) was computed according to the following equation:

CrI =

I002 − IAM I002

(7)

where I002 was the signal for the crystalline portion of XRs corresponding to the (002) (2θ = 22.2°) and IAM was the intensity for the amorphous portion at 2θ = 18.2°. The different morphology of raw and pretreated XRs were performed by S-3400 N scanning B


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ACS Sustainable Chemistry & Engineering electron microscope (HITACHI, Japan). FTIR spectra of samples were obtained on a Thermo Scientific Nicolet iN 10 FTIR Microscope (Thermo Nicolet Corp., USA) equipped with an MCT detector cooled by liquid nitrogen. Analytical pyrolysis was conducted with residues after reactions by a CDS 5200 pyrolyzer and a 7890A/5975 GC/MS (Agilent Technologies, USA) (Py-GC/MS). Typically, approximately 0.1 mg of materials were pyrolyzed to 600 °C for 15 s. The injection temperature was 270 °C, and the split ratio was 50:1. The analytic program was performed: the starting temperature of 40 °C for 5 min, from 40 to 270 °C (10 °C/min) for 1 min. The ionization energy was 70 eV, and the mass scanning range of 45−450 m/z was set.

insoluble lignin was slightly decreased but possibly quickly increased over time because of the formation of some humins and pseudolignin.25 The liquid products of XRs were non-negligible under hydrothermal condition, but the yield and selectivity were relatively low without FeCl3. These liquid products included oligomers, glucose, LA, FA, and so on. When FeCl3 was selected as acidic catalyst, a large quantity of glucose was detected after 0.5 h. With reaction time increasing, the glucose and HMF were consumed because of their further conversion. The consumption of HMF as reaction time growing was mainly attributed to HMF further rehydration. The cellulose conversion to LA was carried out tandem reactions, including the cellulose hydrolysis to glucose, the glucose dehydration to HMF, followed by HMF rehydration. Large quantities of LA and FA were obtained after the reaction time of 2.0 h. An interesting finding in this study is the nonstoichiometric production of FA and LA (Figure 1b). This observation was explained that there were conceivable reactions to form the superfluous FA, which included some pathways from the intermediates of hexose carbohydrates.26 FA was possibly formed by the conversion of formylated xylan in remaining hemicellulose.27 FA also was produced with the transformation of acetic acid.28 Thus, the formation of FA was via multiple approaches. When the reaction finished, the solid residues were washed with ethanol and following dried in the oven for overnight. Some pale brown powers were produced with a little amount of sugars and ashes. To help us understand reaction mechanisms that could account for the surprising catalytic reactivity of FeCl3, SEM images about the microstructure of XRs and residues after reaction was presented in Figure S1. As for the heterogeneous distribution of raw XRs, particles with common surface features were imaged for comparison. The outer structure of raw XRs appeared some pores at the edge of particles, which was like an open vascular network (Figure S1b). As the reaction time increased, the macrostructure of the residues was divided into small particles. Although most of the superstructures is collapsed, the significant amount of lignin still presented in residues and united following FeCl3 catalysis (Figure S1e). XRs cellulose exhibited two characteristic peaks at 2θ of 18.2 and 22.2°, which are typical peaks of cellulose (Figure S2). After the reaction time of 0.5 h, the CrI exhibited an increased trend, indicating that the amorphous cellulosic structure was first converted. With the reaction time increasing, the CrI value was decreased, which demonstrated that microcrystalline structure was destroyed until disappeared. These observations confirmed that the decomposition of the cellulose in XRs took place using FeCl3 catalysis. FTIR spectroscopy was employed to characterize the raw material and the solid residues after FeCl3 catalysis. The distinctive regions in the spectra of the three main components of biomass ranged from 1750 to 750 cm−1 (Figure S3). The weak absorption peak at 1630 cm−1, which is ascribed to the asymmetric stretching of CO bands in glucuronic acid or glucuronic acid carboxylatethe, disappeared after treatment time of 1.0 h at 180 °C in the FeCl3 catalytic reaction system. The disappearance of the peak suggests that hemicellulose in XRs was fully converted. The characteristic absorption peaks of cellulose are appeared at 1370, 1320, 1163, 1057, and 898 cm−1 (shown in Figure S3 and Table S1). All of these bands decreased gradually before they nearly disappeared as the

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RESULTS AND DISCUSSION Selective Conversion of XRs Cellulose. XRs were selected as representative biomass waste because of the removal of most hemicellulose without consideration of furfural, which decreased the following separation cost in the whole processes. An analysis of the XRs is shown in Figure 1. The XRs contained 61.0 wt % cellulose, 6.5 wt % hemicellulose, and 22.0 wt % lignin. First, the transformation of XRs cellulose to LA in the proposed reaction system was investigated. Different metal halides, including AlCl3·6H2O, CrCl3·6H2O, FeCl3 and CuCl2· 2H2O, were tested and compared. The reaction conditions were 180 °C, 0.12 M metal halides, and 2 h. Table 1 Table 1. LA Yields from XRs Conversion Reactions with Different Metal Halide Catalysts under Hydrothermal Conditiona entry 1 2 3 4 5

catalyst

pH

LA yield (mol %)

AlCl3·6H2O CrCl3·6H2O FeCl3 CuCl2·2H2O

6.70 2.96 3.15 1.60 2.81

26.0 25.0 48.5 34.0

Reaction condition: 180 °C, 2h, 1.2 g XRs, 0.12 M metal chlorides in 30 mL solutions.

a

summarized the metal halides used in the present study, their corresponding pH and relevant analytical data. The corresponding pH values are mainly connected with the first equilibrium constants of the different metal cations hydrolysis, where the Fe cations were proved to be the strongest. As Brønsted acidity typically dominates cellulose hydrolysis kinetics and following sugar degradation kinetics. As shown in Table 1, the yield of products was significantly increased with the addition of metal chlorides. The corresponding acidity of metal chlorides was in accord with the conversion performances of cellulose to LA.24 Under the same reaction conditions (180 °C and 2 h) and metal halides concentration (0.12 M), the LA yield obtained in the FeCl3-catalyzed system was much higher than those of the other systems. Herein, FeCl3 was selected as the acid catalyst in the following experiments. Effect of Reaction Time on the Liquid Product. The FeCl3-catalytic hydrolysis of XRs to LA was first conducted in this section. The composition data of remanent residues (based on 100g XRs) was illustrated in Figure 1. Over 40.0% of the cellulose initially present in XRs was converted to LA and other products in 0.5 h. When the reaction time increased to 2 h, the XRs cellulose was nearly fully converted in this reaction system. The results demonstrated that FeCl3 remarkably improved the conversion of XRs cellulose at mild temperature. But the composition of the remaining solids from reactions showed that C


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the conversion of lignocellulosic biomass to LA with a high yield.30 To improve the yield and selectively catalytic conversion to LA, NaCl was first introduced to the reaction solutions. Figure 3 demonstrated that the LA yields were

reaction time increased, which thus indicates the complete conversion of cellulose. In particular, the band at 898 cm−1, which represents the crystalline structure of cellulose, was disappeared as the reaction time 1.5 h, which further supports the idea that crystalline cellulose was also converted. The characteristic absorption peaks of lignin in XRs occur at 1605, 1513, 1458, 1428, 1264, and 1030 cm−1. After the removal of hemicellulose and cellulose, the corresponding absorb peaks of lignin in the residue were highly enriched, whereas the interaction of different chemical groups in hemicellulose and cellulose with those in lignin disappeared. After FeCl3 catalysis reaction, the increasing bond at 1700 cm−1 which represents the stretching vibration of the CO stretch from acids, aldehydes, and ketones29 demonstrated that some humins and pseudolignin were formed in the FeCl3 catalysis reaction. Effect of FeCl3 Amount on the Liquid Product Yield from Cellulose in XRs. The amount of FeCl3 remarkably affected the yield of liquid products from cellulose degradation (Figure 2). The LA yield increased when the amount of FeCl3

Figure 3. Effects of NaCl dosage on the XRs conversion to LA. Reaction conditions: XRs (1.2 g), FeCl3 (0.6 g), 30 mL of water, 180 °C, 2.0 h.

significantly increased as the NaCl dosage increasing. The yield of FA was increased with increasing amount of NaCl. With the NaCl amounts increasing to 20 wt %, the LA selectivity for the system was improved from 48.5 to 65.0 mol %. The highest yield of LA was 68.0 mol % in the 40 wt % NaCl solutions. This result was consistent with that in a previous report,30 that higher NaCl concentrations promoted the conversion of lignocellulose to LA. When NaCl was increased to 40 wt %, quick increasing behavior of the LA yield was not observed. The pH value was almost kept constant (from 1.59 to 1.50) when the NaCl additions was increased from 20 to 40 wt %. This result confirms that the pH change was not the only dominant function of NaCl in this catalytic system. The obtained LA yield of 68.0 mol % was comparable to the 70 mol % yield achieved when catalyzed by mineral acids. 36 Furthermore, the XRs lignin was converted slightly, which was beneficial to the further applications. The mass balance of the optimal reaction was shown in Figure S4. The highest LA yield of 68.0 mol % was equated to the production of 283 kg of LA from 1 dry ton of CAHR. Py-GC/MS spectra of residue was shown in Figure S5. The chromatogram for residue after the optimal reaction showed the presence of various furanics (e.g., 2-methylfuran, 2,5-dimethylfuran, and 5-methylfurfural), which obviously indicated the presence of furanic groups of humins in the residue. This is consistent with that of glucose-humins.37 The performance of the designed catalytic system was compared with other analogous catalytic systems focusing on the chemical conversion of cellulose or lignocellulosic biomass to LA that reported in the literatures. Table S2 presented these results of degradation processes for cellulose or biomass by using homogeneous catalysts and heterogonous catalysts. For homogeneous catalytic processes, the inorganic acid catalyst is effective for the conversion of lignocellulose. Li et al. reported that the addition of NaCl into GVL/H2O system with HCl improved the LA yield from corn stover.38 However, the inorganic acid catalyst was more corrosive than metal halides

Figure 2. Effects of FeCl3 amount on the yields of HMF, LA, and FA. Reaction conditions: XRs (1.2 g) in 30 mL of water, 180 °C, 2.0 h.

increased to 0.6 g. However, upon further increase in the amount of FeCl3 to 1.2 g, the LA yield did not result in a significant increase. The formation trend of FA exhibited similar to that of LA. Li et al. reported the formation of lactic acid was observed from the AlCl3-catalyzed conversion of corncob residue.20 However, the lactic acid was not observed, whereas more LA and FA were produced in the present work. This may be attributed to the nature of FeCl3 catalysts, in agreement with the previous literature.30 Functional of NaCl in Improvement of the Selectivity to LA. Inexpensive NaCl has been widely employed to increase HMF and furfural yields in biphasic systems.31,32 In particular, Gomes et al. applied the biphasic systems (THF/H2O-NaCl), for the conversion of the relatively simple sugar cane molasses, a mixture of disaccharides and monosaccharides, to HMF with the yield of 50−65%.33 Qin et al. obtained a high yield of LA (61.8%) from cellulose using 1.5 M H3PO4 and high concentrations of KCl, while achieved a low LA yield of 30% from complex bagasse and paper.34 Therefore, the differences of the starting feeds greatly determined the efficiency of the conversion. The strong recalcitrance of lignocellulosic biomass hampers deconstruction of highly complex cell walls35 and subsequent high-value utilization in the current biorefinery. However, few reports demonstrated that NaCl salt promoted D


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into the water with 0.6 g of FeCl3 before the reaction, some solids were formed, and the pH value tended to neutral. Due to the possible formation of [FeFM](M − 3)−,40 KF was not suitable with the FeCl3 catalyst for the conversion of XRs to LA with evidence of neglectable yield of LA (Figure 4), so these results obtained demonstrated that Cl− was the most suitable ion for significant facilitating LA formation and destroying the hydrogen bonding. In order to evaluate how the Cl ions weaken the hydrogen bond network of cellulose, some glycol was added in the reaction system to observe the effect of it on the yield. The LA yield actually declined as the amount of glycol added into the reaction system (Table S3). Effects of NaCl on the Dissolution and Hydrolysis of XRs Cellulose. From results obtained above, chloride ions played significant roles to the conversion of XRs to LA. In order to thoroughly define the ability of Cl− in the cellulose utilization, a series of experiments were designed without FeCl3 in NaCl solutions (Figure 5a). NaCl obviously improved cellulose conversion from 32.9 to 44.5% at 180 °C. To reach efficient hydrolysis performance with cellulose, high temperature (>220 °C) is needed without acid catalysts. With the reaction temperature increasing to 200 °C, cellulose conversion was improved from 44.5 to 91.2%, so it was concluded that NaCl also significantly accelerated the cellulose conversion under relatively mild reaction conditions. Attributed to the robust interaction between Cl− and hydroxyls of cellulose, relative low temperature would facilitate the cellulose depolymerization by activating the Cl−. However, large amounts of monomolecular chemicals were hardly observed during the experiments. Some detected liquid products in small amounts were shown in Figure 5b. A very low amount of LA was detected at 180 °C from noncatalyzed hydrothermal reaction, which was attributed to incomplete, slow, and nonselective conversion of cellulose to LA in NaCl solutions. From the results shown in Figure 5b, the yield of glucose from cellulose with NaCl was at least 2 times higher than that without NaCl at 180 °C, which is attributed to the fact that NaCl promoted the depolymerization of cellulose to produce soluble oligomers with low molecular weights and glucose. Meanwhile, vom Stein and co-workers pointed that the aqueous solutions with high concentrations of NaCl playing a similar role of ionic liquids greatly disrupted the major hydrogen bonds between the cellulose chains.21 To further demonstrate that the NaCl affected the hydrolysis of cellulose, FTIR spectroscopy

and not easy to handle in the process. For the heterogonous catalytic processes, the preparation of these catalysts was complex and consumed a lot of water or organic solvents. Compared with the catalysts above, the FeCl3−NaCl system in this work could achieve comparative yield (68.0 mol %) under mild temperature. From viewpoint of reaction conditions and the cost of raw materials, promoter, and solvents, the FeCl3− NaCl system had many obvious advantages over those catalytic systems (Table S2). To further investigate salts effect on the dehydration of XRs to LA, different salts were applied (Figure 4). With the Na2SO4,

Figure 4. Effects of different salts species on the LA yield. Reaction conditions: XRs (1.2 g), FeCl3 (0.6 g), NaCl (40 wt %), 30 mL of water, 180 °C, 2 h.

NaNO3, and Na2CO3 addition, the reaction system with these salts mentioned above showed negative to the formation of LA. Nearly negligible LA yield was obtained. This results evidenced that anions mainly influenced the yield of LA. LiCl, LiBr, and KCl exhibited similar effect concerning the promotion of LA yield. This is consistent with previous concepts that LiBr or LiCl has been used to enhance the dissolution of polymers that having strong H-bonding.39 The performance of LiCl was a little better than that of NaCl (Figure 4), whereas the price of LiCl is 25 times higher than that of NaCl in China. In addition, cations presented an increase trend of LA yield as the decrease of ion radius. Furthermore, the positive effect of KBr on the LA formation was less than that of KCl. When the KF was added

Figure 5. Effects of NaCl addition on the cellulose conversion (a) and the distribution of liquid products (b) without FeCl3. Reaction conditions: XRs (1.2 g), 30 mL of H2O after the reaction time of 2.0 h, 180, 200, and 220 °C. E


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series of texts about catalyst reusability were carried out with the FeCl3 to evaluate the reaction efficiency in XRs conversion to LA. After each reaction, the mixture was first filtered, following by the extraction (2×) with 2-MeTHF before the next recycle. About 3.0% LA remained in the aqueous solutions. When 3.0% (10.5 mg) LA was added in the new reaction system, the little existing LA did not inhibit the formation of the new LA (Table S4). The yield of LA gradually decreased to 61.0, 57.0, 55.0, and 52.5 mol %, respectively (Figure 6). The

was employed to characterize the raw material and the solid residues after hydrothermal reaction with NaCl (Figure S3). The absorbance of the −OH stretching vibration at 3450−3300 cm−1 decreased and shifted to a higher wavenumber when the NaCl was added in the reaction systems. The NaCl solutions with high NaCl amounts not only break the hydrogen bonds between cellulose chains, but also slightly adjust the pH values of the medium. At 200 and 220 °C, the LA yields in 20 wt % NaCl solutions were 3 times higher than that in pure water. Qin et al. reported 20% of LA was obtained from cellulose with KCl at 200 °C.34 Herein, it is demonstrated that high amounts of NaCl facilitate cellulose conversion to glucose to readily produce the desired product of LA. Therefore, the FeCl3 and NaCl synergistically promote XRs degradation to LA under mild conditions. Catalyst Reusability. In order to further exploit its drop-in products such as 2-MeTHF, GVL, and 5-ALA, it is generally essential to separate LA from aqueous reaction solutions. However, only a few works reporting on LA production have explored the separation of LA. Currently, extraction has been considered as a favorable approach to separate LA, and the formed LA can be recovered by extraction from aqueous solutions into extraction solvents by addition of NaCl. Dumesic et al.41 reported that GVL extracted 75.0% of LA of aqueous solutions for further application. However, certain amounts of HCl catalysts were extracted to the GVL agent, which would inhibit the catalyst reusability. In this study, three solvents were investigated for the extraction performance of LA (Table 2).

Figure 6. Reusability of the aqueous phase containing catalysts and NaCl for XRs degradation to LA. Reaction conditions: Fresh XRs (1.2 g) were added into aqueous solutions at 180 °C for 2.0 h.

results demonstrate that the LA yield slightly decreased after 4 recycling steps. Partial loss of catalytic activity or the decrease in LA yield was possibly attributed to the slightly flowing away of FeCl3 catalysts and the existence of some soluble humins in the next reaction medium. Proposed Reaction Pathway for Cellulose Depolymerization. On the basis of the obtained products distribution in this work, a reaction pathway for XRs cellulose depolymerization in the NaCl medium over FeCl3 catalyst was proposed and summarized in Scheme S1. XRs Cellulose was first dissolved and successively hydrolyzed into oligomers or glucose as a result of the cellulose swelling and dissolving capacity on the aid of the NaCl and FeCl3. Then, the glucose intermediate was further dehydrated into HMF over the Lewis acid sites or Brønsted acid sites. Next, the formed HMF intermediate rehydrated into LA with two molecules water over the Brønsted acid sites in the reaction systems. Unfortunately, sugar and HMF will be converted into insoluble humins and mixed or deposited on the surface of the lignin. To sum up, it was concluded that the depolymerization of XRs cellulose in the NaCl medium with FeCl3 catalyst was a relatively complicated reaction system which mainly included swelling, dissolution, hydrolysis, dehydration, rehydration, and polymerization, and was affected not only by the acid catalyst but also the reaction medium.

Table 2. Extraction of LA from Aqueous Reaction System Using Different Solventsa entry

extraction solvent

volume ratiob

extraction rate (%)

1 2 3 4 5 6 7

THF THF 2-MeTHF 2-MeTHF THF+ 2-MeTHF MIBK MIBK

1:1 2:1 1:1 2:1 (1 + 1):1 1:1 2:1

88.0 92.5 81.0 88.0 89.0 59.0 75.0

a

A high yield (68.0 mol %) of LA was obtained in aqueous reaction system with 0.6 g of FeCl3 and 40 wt % NaCl. bVolume ratio: volume of extracting solvent/volume of aqueous solution with FeCl3 and NaCl.

When the experiments were conducted with 0.12 M FeCl3 in 40 wt % NaCl solutions, the aqueous solution with LA yield of 68.0 mol % was regarded as the extraction raffinate, while MIBK was considered the extractant. After the filtration process, only 59.0 mol % LA was isolated to the MIBK phase attributing to the salt-out effect. When THF was selected as extraction solvent, 88 mol % LA was extracted to the upper phase. However, THF was obtained from fossil energies, and some FeCl3 catalyst was extracted to the THF phase. When 2MeTHF was selected as extraction solvent, 81% LA was successfully extracted to the organic phase. 2-MeTHF was obtained from renewable biomass, and it possesses many promising properties, such as low formation probability of peroxides, more stability, and higher immiscibility than THF. The effect of salt existed in aqueous solution on the miscibility of 2-MTHF, and water has been determined and was detected to only have a minor influence.42 It is highlighted that 2MeTHF is a promising extracting solvent from lignocellulose. A

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CONCLUSIONS Herein, combining FeCl3 with NaCl effectively promoted the XRs conversion to LA with the maximum yield of 68.0 mol %. More than 80% of the obtained LA is efficiently extracted into a biomass-based 2-MeTHF phase due to the salt-out effect. The reaction system accords with in the concept of the sustainable and green chemistry. The effective integrated system included the LA production directly from biomass and the catalytic conversion of LA to drop-in chemicals and materials. The results showed that the analogous concentrated seawater could F


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be a green and promising alternative to freshwater or ionic liquid for future lignocellulose biorefineries.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b03183. Characterizations of XRs and residues after reactions and mass balance of the optimal reaction system, and literature overview of the acid-catalyzed conversion of lignocellulose to LA (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel/Fax: +86-10-62336387. Email address: [email protected]. cn. ORCID

Xueming Zhang: 0000-0001-5352-8616 Feng Xu: 0000-0003-2184-1872 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the grant from the National Key R&D Program of China (2016YFD0600803). REFERENCES

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