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ethylene glycol binary solvent
Industrial Crops & Products 144 (2020) 112038
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Highly efficient and selectivefractionation strategy for lignocellulosic biomass with recyclable dioxane/ethylene glycol binary solvent
T
Yongjian Zhang, Junfeng Feng, Zhanping Xiao, Yingming Liu, Haoyang Ma, Zining Wang, Hui Pan* Jiangsu Co-Innovation Center for Efficient Processing and Utilization of Forest Resources, College of Chemical Engineering, Nanjing Forestry University, 159# Longpan Road, Nanjing 210037, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Poplar Biorefinery Organosolv Microwave heating Lignin Cellulose
Poplar is among the short-rotation and dedicated energy crops which serve as lignocellulosic feedstock for future bio-refinery. In this study, a promising bio-refinery strategy was developed to efficiently fractionate poplar wood into its three components with dioxane/ethylene glycol as reaction medium and H2SO4 as catalyst under microwave irradiation. Up to 94.5% lignin and 95.1% hemicellulose can be selectively separated from raw poplar while achieve 78.5% cellulose retention at 120 °C with 1.5wt% H2SO4 for 10min. The characterization of recovered lignin implied that it has relatively low molecular weight as well as less condensed structures, which enable its easy dissolving in common solvents and facilitate its further applications. In addition, pure cellulose comparable to commercial α-cellulose can be obtained by slightly increasing fractionation time and temperature. The high quality fractions, easy-handle operation process and the possible solvent recovery entitle the proposed fractionation approach have great potential to scale up for industrial implementation.
1. Introduction The depletion of fossil oil and the environmental problems drive the urgent seeking for alternative resources that is sustainable and renewable (Ragauskas et al., 2006). Lignocellulosic biomass is an abundant bio-renewable carbon source and is considered as a promising feedstock to replace fossil oil for biofuels and valuable chemicals (Huber et al., 2006; Wang et al., 2013). Poplar are commonly classified as short-rotation woody crops and have wide distribution in northern Europe, North America, and China (Sannigrahi et al., 2016; Yang et al., 2011; Gabrielle et al., 2013). They can be grown on economically marginal crop land and have high yield excess of 7 tons per acre per year with desirable qualities for biofuels, biochemical or other bio-based products production (Sannigrahi et al., 2016; Department of Energy, 2011). Similar to most lignocellulose biomass, this material has a recalcitrant “concrete” structure formed by three main components, namely, cellulose (40–60%), hemicellulose (10–40%) and lignin (15–30%), which hinders the direct and efficient utilization of lignocellulosic biomass (Sannigrahi and Ragauskas, 2013). Fractionation of these three components with high selectivity and thus reducing the heterogeneity of lignocelluloses is one of the key steps to further realizing the shift from fossil energy to biomass resources (Ragauskas et al., 2006).
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Many approaches have been developed for lignocellulose fractionation, such as diluted acid treatment (Tian et al., 2011), alkaline treatment (Zhao et al., 2008; Zhu et al., 2015), ionosolv (Brandt et al., 2011) and organosolv pulping (Borand and Karaosmanoǧlu, 2018; Lancefield et al., 2017). Diluted acids treatment can reduce the recalcitrance of biomass by partially removal of hemicellulose and lignin yet rendering the alteration in lignin structure and even the formation of pseudo-lignin that attaches to the surface of cellulose fiber which leads to limited delignification (Pu et al., 2013; Sannigrahi et al., 2011). Alkaline treatment features its high selectivity of delignification, but the harsh alkaline environment induces severe lignin condensation and complicates its downstream valorization (Azadi et al., 2013; Rinaldi et al., 2016). Ionosolv pulping can selectively dissociate lignin and hemicellulose at lower temperatures (< 160 °C). However, the cost and toxicity associated with ionic liquid prohibits its industrial implementation (Brandt et al., 2013). Organosolv pulping is considered one of the most effective technologies for lignocellulosic biomass fractionation for its ability to efficiently fractionate the biomass into cellulose pulp, aqueous hemicellulose-derived stream and lignin without severe modification and deterioration of the fractions (Zhao et al., 2009; Alvira et al., 2010; Shuai and Luterbacher, 2016). Generally a mixed solvent of water and
Corresponding author. E-mail address: [email protected] (H. Pan).
https://doi.org/10.1016/j.indcrop.2019.112038 Received 30 March 2019; Received in revised form 1 September 2019; Accepted 9 December 2019 0926-6690/ © 2019 Elsevier B.V. All rights reserved.
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an organic solvent, including alcohols (Pu et al., 2013; Huijgen et al., 2014), cyclic ether (Quesada-Medina et al., 2010; Grande et al., 2015) and polyols (Alriols et al., 2009; Novo et al., 2011) was applied in organosolv process to achieve satisfied fractionation results. For instance, ethanol aqueous has been widely studied for pretreatment purposes of various raw biomass feedstocks and gets a delignification rate not more than 80% (Huijgen et al., 2014; Obama et al., 2012). It has also been reported that enzyme digestible cellulose pulp as well as high quality lignin could be obtained with n-butanol and water because that alcoholic solvent can reduce the lignin repolymerization by trapping the reactive cationic intermediate released during fractionation, thereby increasing lignin removal rate and improving the quality of isolated lignin (Lancefield et al., 2017). The study of Quesada-Medina et al. (2010) suggest that the optimum delignification of almond shells could be achieved by using the mixture of Diox/water (75/25, v/v), which is believed due to the well matched Hildebrand δ-value of the mixed solvent with lignin. Microwave heating has been widely used in biomass thermochemical conversion process (Motasemi and Afzal, 2013; Sturm et al., 2014). The conversion of raw biomass materials could be reached in minutes due to the high heating rate, selective and volumetric heating of microwave irradiation (Xie et al., 2016a; Kržan and Kunaver, 2006). Ethylene glycol (EG) has excellent thermal response to microwave radiation due to its unique dielectric property (Kappe, 2004). It could serve as fractionation medium as well as microwave absorber during the microwave-assisted biomass fractionation process. Meanwhile, it could stabilize the active terminal groups released during lignin degrading and reduce the condensation reaction (Deuss et al., 2017). Given the high viscosity of EG, water was usually employed with EG in biomass conversion to reduce both the viscosity and cost of mixed solvent (Zhang et al., 2016; Zheng et al., 2018). However, the addition of water is bound to cause a decrease in the lignin solubility in EG due to the hydrophobicity of lignin. Dioxane (Diox) was demonstrated has excellent lignin solubility and the ability of promoting catalysis action of mineral acid (Mellmer et al., 2018; Schuerch, 1952). Mixing Diox instead of water with EG is expected to have synergistic effect on lignin solubility while decrease the viscosity of using EG along during biomass fractionation. In this study, a facile one-step fractionation process for lignocellulosic biomass was first reported, to the best of our knowledge, by using a mixture of Diox and EG as the reaction medium and sulfuric acid as the catalyst under microwave irradiation. Raw poplar wood was fractionated into highly pure cellulose, high quality lignin and pentose rich solution under mild conditions. The effect of fractionation conditions on the properties of each fraction was studied by characterization of FT-IR, SEM, 2D-NMR, XRD and GPC.
Table 2 Fractionation conditions and labelling of the obtained products. Experiments
Temp. (℃)
Catalyst (Wt%)
Time (min)
Crude cellulose label
Lignin precipitate label
1 2 3 4 5 6 7 8 9
100 120 140 160 120 120 120 120 120
1.5 1.5 1.5 1.5 0.0 0.5 1.0 1.5 1.5
20 20 20 20 10 10 10 10 30
C1 C2 C3 C4 C5 C6 C7 C8 C9
L1 L2 L3 L4 L5 L6 L7 L8 L9
lignin was purchased from TCI (Shanghai Co., Lt.). All chemicals used in this study were of analytical grade, commercially available, and ready for use. 2.2. Fractionation process Fractionation of poplar particle with Diox/EG (2/1, v/v) was carried out on a Microwave Extraction System (Milestone Ethos Ex, Italy) equipped with a fiber optic temperature probe (ATC-400FO, Milestone, Italy). Detailed fractionation conditions and labeling of the obtained fractions were listed in Table 2. The schematic diagram of the fractionation process was shown in Fig. 1. In brief, a mixture of 2 g of poplar particle, 13.3 ml of Diox, 6.7 ml of EG, and 0.3 g sulfuric acid (98%) were placed in an 100 ml Teflon vessel with magnetic stir. For each run, one vessel was placed in the microwave chamber, and the maximum microwave irradiation output power was set at 300 W. A preheating time of 3 min was programmed together with the desired reaction time for each run. After the completing of the reaction, the vessel was cooled to room temperature before opening. The reaction mixtures were first vacuumfiltered to separate the solid residue from the filtrate. The solid residue, which was designated as crude cellulose, was washed and oven dried at 105 °C for 24 h and preserved in a desiccator for further analysis. The filtrate was evaporated under reduced pressure at 40 °C to remove and recycle Diox. Subsequently, the lignin fraction was precipitated by adding excessive deionized water to the filtrate and collected by centrifugation (12,500 r/min, 5 min). After freeze-drying for 24 h, collected lignin precipitate was weighed and sealed in a plastic bag for further analysis. The supernatant from centrifugation was concentrated under reduced pressure at 60 °C and stored at −20 °C for further analysis. The crude cellulose yield and lignin recovery rate were calculated by Eqs. (1) and (2), respectively.
2. Experimental section
Crude Cellulose yield (wt%) = (MCC/MOP) × 100%
(1)
2.1. Materials
Lignin recovery rate (wt%) = (MLP/ML) × 100%
(2)
Where MCC and MLP are the mass (g) of obtained crude cellulose and
Raw poplar shavings were purchased from Caoxiansenyi Wood Industry Co., Ltd, Heze, Shandong Province, China. The raw poplar shavings were pulverized into particles capable of passing through a 40mesh sieve and then oven dried at 105 °C for 24 h. Dried poplar particles were sealed in plastic bags and stored in a desiccator before use. The chemical composition of the raw poplar is analyzed according to literature (Si et al., 2017) and the results are shown in Table 1. Alkali Table 1 Main chemical composition of raw poplar. Materials
Cellulose
Hemicellulose
Acid insoluble lignin
Acid soluble lignin
Ash
poplar
49.2 %
19.3%
23.1%
1.4%
0.5%
Fig. 1. Schematic diagram of the fractionation process. 2
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Fig. 2. Effects of (a) fractionation temperature, (b) sulfuric acid concentration, and (c) fractionation time on the fractionation efficiency ((a) Fractionation time 20 min; H2SO4 1.5 wt%; (b) fractionation time 10 min; temperature 120 °C; (c) H2SO4 1.5 wt%; temperature 120 °C).
2.4. Characterizations of crude cellulose and lignin fractions
lignin precipitate respectively; ML is the mass of lignin in raw poplar; MOP is the mass of original poplar.
The crude cellulose was analyzed by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR) and X-ray diffraction (XRD). The SEM analysis was performed using a Quanta 200 microscope (FEI, USA). FT-IR analysis, which employed the potassium bromide pelleting method, was performed using a Nicolet 380 spectrometer (Thermo, USA) at room temperature. At least 32 scans were taken with a resolution of 2 cm−1 between the regions of 400 and 4000 cm−1. The particle size distribution (PSD) was analyzed using the Particle Sizing system (AccuSizer 780SIS, Santa Barbara, CA, USA). In each test, 0.05 mg of the sample in powder form was dispersed in 250 ml of deionized water and vortexed for 1 min. XRD patterns were recorded using an Ultima IV (Rigaku, Japan) system equipped with a D/ teX-Ultra diffractometer at a scan rate of 5° min−1 from 5 to 40°. The crystallinity index (CrI) was determined using the following equation:
2.3. Chemical composition analysis The composition of crude cellulose was analyzed using analytical procedures described in previous work (Si et al., 2017). In brief, around 0.3 g dried sample was Soxhlet extracted with ethanol-benzene mixture (v/v, 1/2) for 10 h. Thereafter, the sample was washed with ethanol and oven-dried to a constant weight. Subsequently, the dried and extractive-free sample was hydrolyzed using 72 wt% H2SO4 (3 mL) under mechanical agitation at 30 °C for 1 h. The hydrolyzed suspension was diluted to 4 wt% H2SO4 concentrations and refluxed for 5 h. After filtration, the amounts of glucose and xylose in the filtrate were analyzed on an Agilent 1200 series high performance liquid chromatography system (HPLC) to estimate cellulose and hemicellulose. The analysis was conducted by using a Bio-Rad Aminex HPX-87H column operating at 60 °C with a refractive index detector and 5 mM H2SO4 aqueous solution as the mobile phase at a flow rate of 0.6 mL min−1. The amount of acid-soluble lignin (LAS) in the filtrate was analyzed using an ultraviolet spectrometer (UV-2550, Shimadzu, Japan) in the range from 200 to 400 nm at room temperature. The filtration cake, namely, the acid-insoluble lignin (LIS), was washed with excessive deionized water and then oven dried at 105 °C for 24 h. The percentages of cellulose retention and delignification as well as the removal rate of hemicellulose were calculated by the following equations: Cellulose retention (%) = (mC/MC) × 100%
(3)
Delignification (%) = (1-rL/RL) × 100%
(4)
Removal rate of hemicellulose (%) = (1-rH/RH) × 100%
(5)
CrI (%)=((I002-IAM)/I002) ×100%
(6)
Where I002 is the scattered intensity of crystalline cellulose I at about 2θ = 22.4° and IAM is the scattered intensity of amorphous cellulose at about 2θ = 15.5°. The solubility of the lignin precipitates in Dixo, acetone, N,N-dimethylformamide (DMF), 2-methoxyethanol (MC), methanol and EG were studied by directly dissolving the lignin precipitates in above solvents at a concentration of 5 mg/ml and sonicated for 5 min, followed by centrifugation for 5 min at 3000 rpm. The number average (Mn) and weight average (Mw) molecular weights of the lignin precipitate were analyzed using an Agilent GPC/ HPLC 1200 system equipped with three Jordi Gel DVB Mixed Bed HPLC columns (250 × 10 mm) connected in series at 30 °C. Tetrahydrofuran (THF) was used as the mobile phase at a flow rate of 1.0 mL/min. The inter-unit linkages and the aromatic and aliphatic chemical moieties of the lignin precipitate were analyzed using two-dimensional 13C–1H heteronuclear single quantum correlation (2D HSQC) NMR equipped with an Avance-400 spectrometer at 20 °C. In general, 30 mg of lignin sample was dissolved in 0.6 ml of DMSO-d6. The standard Bruker pulse sequence hsqcetgpsisp2.2 was used with a pulse width of 11.27 ppm, an acquisition time of 106.8 ms and a relaxation delay time of 2 s.
Where mC and MC are the masses of cellulose in crude cellulose and original poplar particle, respectively; rL and RL are the percentages of lignin in crude cellulose and original poplar particle, respectively; rH and RH are the percentage of hemicellulose in crude cellulose and original poplar particle, respectively. 3
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3. Results and discussion
more severe, the size of crude cellulose sample further reduced and mostly single wood cell wall fragments were observed (i.e., sample C3 in Fig. 3f). The results of PSD analysis are consistent with those of morphological structures of the samples observed in SEM. This result is mainly due to the significant removal of hemicellulose and lignin located at the middle lamella of wood cell wall and therefore causes the dissociation of wood cell bundles. In addition, the size reduction of crude cellulose sample obtained under more severe condition may be also contributed to the degradation of amorphous cellulose since the cellulose retention of C3 is only 51.3% (Table S1). It is worth noting that no long fibers were found in the crude cellulose samples, which is not in agreement with previous studies (Schutyser et al., 2015). The main reason is because that the raw poplar used in this study was wood shavings, which may have reduced fiber length than wood chips used in other studies. In addition, poplar is inherently shorter in fiber length (Tower, 2013). The XRD patterns and the crystallinity index (CrI) results of raw poplar and crude cellulose products C8 and C3 are shown in Fig. 4. They all display a typical cellulose I pattern (2θ = 15.5, 22.4 and 34.4° corresponds to 101, 002 and 040 planes respectively) (Park et al., 2010). Compared to raw poplar, the crystallinity of crude cellulose samples significantly increased, which should be mainly due to the removal of hemicellulose and lignin (Ouyang et al., 2018). The crystallinity index of sample C3 (76%) is comparable with that of commercial α-cellulose (78%), indicating the removal of most amorphous cellulose as well under harsher treatment condition (Park et al., 2010). The FT-IR spectra of raw poplar and crude cellulose samples C8 and C3 are shown in Fig. 5. The peak at 1735 cm−1 in the spectrum of raw poplar is assigned to the stretching vibration of the carboxyl and acetyl groups in hemicellulose (Xie et al., 2016b). The intensity of this peak significantly reduced in the spectrum of C8 and almost disappeared in the spectrum of C3, indicating the removal of hemicellulose by the cleavage of the linkages between ferulic acid or (p-) hydroxyl cinnamic acids with lignin (Sun and Chen, 2007). The characteristic peaks of lignin at 1594 and 1505 cm−1 assigned to the vibration of the aromatic skeleton and the peaks at 1456 and 1230 cm-1 corresponding to the deformation of the C–H aromatic ring on lignin methoxy groups is clearly presented in the spectrum of raw poplar. Similar to the circumstance of hemicellulose characteristic peak, the first two peaks almost disappeared while the intensities of the latter two peaks decreased in both spectra of C8 and C3, indicated that lignin was significantly removed from the raw poplar after fractionation treatment. In addition, the FT-IR spectra of both C8 and C3 display very similar characteristic peaks as those of microcrystalline cellulose, suggesting that the crude cellulose obtained from this fractionation process is highly pure cellulose. Moreover, these results also indicate that the fractionation conditions did not alter the chemical properties of the crude cellulose products. It has great potential to be used as pure crystalline cellulose feedstocks to prepare high value-added platform chemicals or nanocellulose materials without extra purification step (Xie et al., 2016a).
3.1. Fractionation process The effects of the acid concentration, fractionation temperature and time on the fractionation efficiency were evaluated by cellulose retention, delignification and the removal rate of hemicellulose (Fig. 2). It can be seen from Fig. 2a that significant increase in removal of both lignin and hemicellulose were observed as the temperature increased from 100 to 120 °C, and they both begun to level off while the temperature further increased to even 160 °C. Meanwhile, cellulose retention exhibited a continuous sharp decrease along with the increasing temperature. Virtually all the raw materials degraded at 160 °C. Fig. 2b shows the effect of the sulfuric acid concentration on the fractionation efficiency. It can be seen that the presence of sulfuric acid had a significant effect on the fractionation of poplar wood. Either lignin or hemicellulose can hardly be removed in the absence of sulfuric acid, whereas a low concentration of sulfuric acid (0.5 wt%) significantly increase the delignification rate from around 10% to 80.9%. When the acid concentration further increased to 1.5 wt% the delignification rate reaches 94.5%. The removal of hemicellulose showed similar trend as that of lignin with the variation of acid concentration. Cellulose was not very sensitive to acid concentration as compared to hemicellulose and lignin. The cellulose content maintained at a high level (78.4%) even in the presence of a high concentration of acid (1.5 wt%). This result should be attributed to the more compact structure of cellulose molecule than those of hemicellulose and lignin, which would lead to less accessibility of cellulose (Fengel and Wegener, 1989; Rowell, 2016). It can be seen from Fig. 2c that poplar wood quickly reached high fractionation rate within 10 min and then kept at an almost equilibrium stage as the fractionation time increased to 30 min, indicating that microwave-assisted fractionation of lignocellosic biomass could increase the reaction rate and therefore prevent the formation of condensation byproducts during prolonged reaction time (Ma et al., 2015). It is worth noting that fractionation conditions showed similar effects on the removal of lignin and hemicellulose. This is mainly because that, structurally, lignin associates closely with hemicellulose in the middle lamella and primary wall of plant cell (Fengel and Wegener, 1989). It was reported that the correlation factor of degree of delignification with hemicellulose dissolution is up to 0.96 (Zhao et al., 2014). The optimal condition was found to be at 120 °C in the presence of 1.5 wt% H2SO4 for 10 min, which could reach a crude cellulose yield of 42.7%, lignin recovery rate of 56.9%, and delignification rate of 94.5%. 3.2. Characterizations of crude cellulose Two representative crude cellulose samples (C8 and C3) were characterized in detail. C8 was obtained under the optimum fractionation condition. While C3 has the lowest lignin content among all crude cellulose samples obtained in the study. The analysis and comparison of these two samples are expected to provide insight into the fractionation mechanism as well as the information for the future industrial implementation of this fraction strategy. Fig. 3 presents the visual appearance and morphologic characteristics of the raw poplar wood and two typical crude cellulose samples (i.e., C8 and C3, see Table 1 for label). Raw poplar wood and crude cellulose exhibit a trend of lightening in colors as their lignin contents decreased (Table S1). The color of C3 (Fig. 3c) is almost pure white owning to its ultra-low lignin content (0.2%). This result is quite straightforward because lignin is the main chromogenic component in wood tissue. The SEM images clearly display a decrease in particle diameters from raw poplar to crude cellulose samples. Bundles of several wood cells with average diameters around 174 μm were observed in SEM image of raw poplar (Fig. 3d). After fractionation process at relatively mild condition, the wood cell bundles dissociated to some extent with fewer wood cells attached together (i.e., sample C8 in Fig. 3e). As the fractionation condition turned
3.3. Recovery and characterization of lignin precipitates The effects of fractionation conditions on the lignin recovery rate are shown in Fig. 6. The average lignin recovery rate was around 55% under most of the fractionation conditions described herein, which is close to the results in literature (Schutyser et al., 2015). It should be noted that no lignin precipitate was recovered when sulfuric acid was absent during the fractionation process. Acidic conditions are known to promote cleavage of ether bonds in the carbohydrate polymers and β-O4 ether bonds in lignin (Rinesch et al., 2017; Luterbacher et al., 2014; Li et al., 2015), especially under mild reaction conditions (i.e., at 120 °C for 10 min;). However, lignin recovery did not exhibit dramatic changes within the acid contents between 0.5–1.5%. It is noted that the delignification rate increased slightly with increasing catalyst contents (Fig. 2b). Therefore more water soluble lignin-derived fraction was 4
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Fig. 3. The visual appearance, SEM images and PSD results of raw poplar and crude cellulose C3 and C8. (a, b, c are visual appearances of raw poplar particles, crude cellulose C8 and C3, respectively; d, e, f are the SEM images of raw poplar particles, crude cellulose C8 and C3, respectively; g, h, k are PSD of raw poplar particles, crude cellulose C8 and C3, respectively).
Fig. 4. XRD patterns and crystallinity of raw poplar and crude cellulose C3 and C8.
Fig. 5. FTIR spectra of raw poplar and crude cellulose.
high lignin recovery at high fractionation temperatures (i.e., 160 °C) could be attributed to the formation of insoluble humins. High temperature will cause the massive degradation of lignin and hemicellulose as well as cellulose (Fig. 2a). It is reported that the degradation intermediates of carbohydrate could further repolymerize into insoluble
produced at higher acidic fractionate conditions. Similar result was reported in other study (Zheng et al., 2018). Fractionation temperature also had a significant impact on lignin recovery. As the temperature increased from 100 to 160 °C, so did the lignin recovery rate, which increased from 44.5% to 93.5% (Table S2). It is worth noting that the 5
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Signals from the typical substructures, including β-aryl ether (β-O-4, A and A’), resinol (β-β, B) and phenylcoumaran (β-5, C), were identified in the side chain regions of lignin (Chen et al., 2017; Si et al., 2017). The 1H-13C correlation signals of resinols are show at 85.4/4.65 (Bα), 54.0/3.06 (Bβ) and 71.6/3.80–4.19 (Bγ). Moreover, phenylcoumarans were assigned by cross-signals at 87.6/5.50 (Cα) and 53.1/3.46 (Cβ). It can be seen from Fig. 8a that lignin sample L1 contains predominantly β-aryl ethers (both Aβ-G and Aβ-S) with a small amount of resinols and phenylcoumarans. That is very similar to the native lignin in poplar given in the literature (Chen et al., 2017), indicating that mild fractionate condition would prevent the alteration in structures of resulted lignin. By contrast, the signals of Aβ, Aβ and A’α in the side-chain region of L2 significantly decreased (Fig. 8b) and even disappeared in that of L3 (Fig. 8c) and L4 (Fig. 8d), suggesting profound cleavage of β-O-4 linkages occurred under higher fractionation temperature. Partially, this is also the reason of reduced molecular weight of lignin obtained with increased fractionation temperature (Table 3). In the aromatic regions of lignin samples, signals from syringyl (S), guaiacyl (G), p-hydroxybenzoates (PB) as well as some condensed units were assigned according to literature (Si et al., 2017; Chen et al., 2017; Wen et al., 2013). Namely, G-units were assigned by three different cross-signals at δC/δH 111.5/6.90 (G2), 114.8/6.71 (G5) and 119.3/6.70 (G6). And S and PB-units show a prominent signal for the C2,6-H2,6 correlations at δC/δH 104.9/6.67 and 131.5/7.64, respectively. In addition, condensed Scondensed and Gcondensed, were observed at δC/δH 105.7/6.54 and 113.4/6.68, respectively. As shown in Fig. 8e, the aromatic units in L1 include all S, G and PB-units with little condensed units. As a comparison of L1, the peak area of G2 and G6 in the aromatic region of L2 and L3 were significantly reduced (Fig. 8f, 8 g), probably caused by lignin condensation at 2- and 6-positions (Chen et al., 2017). This result can also be demonstrated by the enhancement of the signals of Scondensed and Gcondensed units in the aromatic region of L2 and L3. This also indicates that lignin degradation and condensation occur simultaneously during fractionation (Chen et al., 2017). Interestingly, the increasing condensed units in L2 and L3 did not increase its molecular weight (Table 3) as a comparison of L1, suggesting that the degradation rate of lignin during fractionation might be greater than that of condensation. Similar conclusion was also reported in other study (Wu et al., 2015). It is also worth noting that the intensity of G-units in the aromatic region of L1 and L2 decreases more obviously than that of Sunits, indicating a preferentially degradation of G-units during fractionation (Chen et al., 2017). Notably, all of the 1H-13C cross-signals in the aromatic regions of L4 are significantly weakened. That probably caused by the mass presence of humins in L4, which has a low H/C ratio and would be invisible in the 1H-13C correlation spectra (Chen et al., 2017). In addition, the cross-signals of polysaccharides, such as β-Dxylopyranoside, 2-O-acetyl-β-D-xylopyranoside and 3-O-acetyl-β-D-xylopyranoside, mentioned in literature (Chen et al., 2017) were not detected in all of the four lignin samples, suggesting that the fractionation process proposed herein can efficiently extract lignin with high purity.
Fig. 6. Recovery rate of lignin samples obtained under different fractionation conditions.
humins under harsh treatment conditions (Sannigrahi et al., 2011). The dissolution properties of recovered lignin were evaluated with common solvents and compared with that of commercial alkali lignin. The colors of recovered lignin in organic solvents varied from pale brown to dark brown as the fractionation temperature increased (Fig. 7). And they all significantly lighter than alkali lignin. Giving the very similar FT-IR spectra of all recovered lignin samples (Fig. S1), the increase in color with increased fractionation temperature may be related to the association of humins formed at high temperature. It can be seen from Fig. 7 that all recovered lignin samples are more soluble in Diox, DMF, 2-methoxyethanol, and EG than in methanol and acetone. The dissolution of lignin in organic solvents involves complicated mechanism. Although an early study suggested that the ability of solvents to dissolve lignin increases as the hydrogen-bonding capacities of the solvents increase and as their solubility parameters approach a value of lignin itself (Schuerch, 1952), no universal rules have been found for organic solvents in lignin dissolution (Sameni et al., 2017; Zhao et al., 2017). It is generally believed that the dissolution of in organic solvents involves the types and the molecular weight of lignin as well as the hydrogen-bonding capacity or basic strength of the solvent. Nevertheless, the relatively lower lignin dissolution capacity of acetone and methanol compared to DMF or Diox has been reported in literature (Quesada-Medina et al., 2010; Duval et al., 2016; Melro et al., 2018). EG is considered an excellent solvent for a wide variety of lignin samples including alkali lignin (Schuerch, 1952), while the relatively poor solubility of L3 and L4 in EG might be caused by the presence of the aforementioned insoluble humin. The solubility of lignin precipitates obtained from this method is much better than that of alkali lignin, which would facilitate their applications in various processes. Table 3 lists the molecular weight and molecular weight distribution of four typical lignin precipitate samples. All of these lignin samples have a relatively narrow molecular weight distribution (PDI ≤ 1.7) and lower average molecular weight than alkali lignin (Mw ≥ 2330) and other organoslov lignin (Mw ≥ 2170) reported in literature (Yuan et al., 2013; Tolvert et al., 2015). The smaller molecular weight of lignin obtained from this method may also be one of the reasons for their good solubility in common organic solvents. Fractionation temperature is one of the reaction variables that has the greatest influence on the structure and properties of recovered lignin. Therefore, the lignin samples obtained at 4 different fractionation temperatures were selected for characterization to further understand their detailed chemical structures by 2D HSQC NMR. The NMR spectra depicting the side chain (δC/δH 50–90/3-5.5) and aromatic (δC/ δH 100–135/5.5–8.5) regions were shown in Fig. 8. The detailed 1H-13C cross-signals assignments were listed in Table S3 and the identified chemical structures were shown in Fig. S2.
4. Conclusions In this study, poplar wood particles were efficiently fractionated with a binary solvent of Diox and EG by a one-step process under mild treatment conditions. The fractionation conditions were optimized and its effects on the fractions properties were studied. The results showed that fractionation temperature is the most critical factor in the fractionation process and has significant effect on cellulose retention and lignin precipitates recovery rate and its physical chemical properties. Up to 94.5% lignin and 95.1% hemicellulose can be selectively separated from raw poplar while achieve 78.5% cellulose retention at 120 °C with 1.5 wt% H2SO4 for 10 min. The lignin precipitates from this method have light colors with low molecular weight, less condensed structure and good solubility in common solvent. The remaining hemicellulose-derived stream together with EG can be used as polyols 6
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Fig. 7. The visual appearance and dissolution property of lignin precipitates in common solvents (L1-L4 were obtained under 100, 120, 140 and 160 °C, respectively with 1.5 wt% H2SO4 for 10 min).
lignin fractions, mild fractionation conditions, easy solvent recovery and easy-handle operation process entitle the proposed fractionation approach a great potential to scale up for industrial implementation.
Table 3 Molecular weight and molecular weight distribution of lignin precipitates. Sample label
Mn
Mw
PDI
L1 L2 L3 L4
938 874 718 714
1553 1446 1189 1222
1.66 1.65 1.67 1.70
Acknowledgement The authors are grateful for the financial support by National Natural Science Foundation of China (No. 31770631), the project of the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institution, China and Top-notch Academic Programs Project of Jiangsu Higher Education Institutions (TAPP) of China (PPZY2015C221), Nanjing Forestry University scientific research startup funds (GXL2018037).
for bio-based materials. By increasing the fractionation time to 20 min and temperature to 140 °C, almost pure cellulose with high crystallinity index (76%) comparable to that of commercial α-cellulose (78%) could be obtained. Diox can be recycled with high recovery rate (80.6%) and high purity by reduced pressure distillation (Fig. S3) to decrease the process cost. In summary, the high quality of resulting cellulose and 7
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Fig. 8. Side chain (a–d) and aromatic regions (e–h) in 2D HSQC NMR spectra of four lignin samples.
Appendix A. Supplementary data
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Highly efficient and selectivefractionation strategy for lignocellulosic biomass with recyclable dioxane/ethylene glycol binary solvent
T
Yongjian Zhang, Junfeng Feng, Zhanping Xiao, Yingming Liu, Haoyang Ma, Zining Wang, Hui Pan* Jiangsu Co-Innovation Center for Efficient Processing and Utilization of Forest Resources, College of Chemical Engineering, Nanjing Forestry University, 159# Longpan Road, Nanjing 210037, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Poplar Biorefinery Organosolv Microwave heating Lignin Cellulose
Poplar is among the short-rotation and dedicated energy crops which serve as lignocellulosic feedstock for future bio-refinery. In this study, a promising bio-refinery strategy was developed to efficiently fractionate poplar wood into its three components with dioxane/ethylene glycol as reaction medium and H2SO4 as catalyst under microwave irradiation. Up to 94.5% lignin and 95.1% hemicellulose can be selectively separated from raw poplar while achieve 78.5% cellulose retention at 120 °C with 1.5wt% H2SO4 for 10min. The characterization of recovered lignin implied that it has relatively low molecular weight as well as less condensed structures, which enable its easy dissolving in common solvents and facilitate its further applications. In addition, pure cellulose comparable to commercial α-cellulose can be obtained by slightly increasing fractionation time and temperature. The high quality fractions, easy-handle operation process and the possible solvent recovery entitle the proposed fractionation approach have great potential to scale up for industrial implementation.
1. Introduction The depletion of fossil oil and the environmental problems drive the urgent seeking for alternative resources that is sustainable and renewable (Ragauskas et al., 2006). Lignocellulosic biomass is an abundant bio-renewable carbon source and is considered as a promising feedstock to replace fossil oil for biofuels and valuable chemicals (Huber et al., 2006; Wang et al., 2013). Poplar are commonly classified as short-rotation woody crops and have wide distribution in northern Europe, North America, and China (Sannigrahi et al., 2016; Yang et al., 2011; Gabrielle et al., 2013). They can be grown on economically marginal crop land and have high yield excess of 7 tons per acre per year with desirable qualities for biofuels, biochemical or other bio-based products production (Sannigrahi et al., 2016; Department of Energy, 2011). Similar to most lignocellulose biomass, this material has a recalcitrant “concrete” structure formed by three main components, namely, cellulose (40–60%), hemicellulose (10–40%) and lignin (15–30%), which hinders the direct and efficient utilization of lignocellulosic biomass (Sannigrahi and Ragauskas, 2013). Fractionation of these three components with high selectivity and thus reducing the heterogeneity of lignocelluloses is one of the key steps to further realizing the shift from fossil energy to biomass resources (Ragauskas et al., 2006).
⁎
Many approaches have been developed for lignocellulose fractionation, such as diluted acid treatment (Tian et al., 2011), alkaline treatment (Zhao et al., 2008; Zhu et al., 2015), ionosolv (Brandt et al., 2011) and organosolv pulping (Borand and Karaosmanoǧlu, 2018; Lancefield et al., 2017). Diluted acids treatment can reduce the recalcitrance of biomass by partially removal of hemicellulose and lignin yet rendering the alteration in lignin structure and even the formation of pseudo-lignin that attaches to the surface of cellulose fiber which leads to limited delignification (Pu et al., 2013; Sannigrahi et al., 2011). Alkaline treatment features its high selectivity of delignification, but the harsh alkaline environment induces severe lignin condensation and complicates its downstream valorization (Azadi et al., 2013; Rinaldi et al., 2016). Ionosolv pulping can selectively dissociate lignin and hemicellulose at lower temperatures (< 160 °C). However, the cost and toxicity associated with ionic liquid prohibits its industrial implementation (Brandt et al., 2013). Organosolv pulping is considered one of the most effective technologies for lignocellulosic biomass fractionation for its ability to efficiently fractionate the biomass into cellulose pulp, aqueous hemicellulose-derived stream and lignin without severe modification and deterioration of the fractions (Zhao et al., 2009; Alvira et al., 2010; Shuai and Luterbacher, 2016). Generally a mixed solvent of water and
Corresponding author. E-mail address: [email protected] (H. Pan).
https://doi.org/10.1016/j.indcrop.2019.112038 Received 30 March 2019; Received in revised form 1 September 2019; Accepted 9 December 2019 0926-6690/ © 2019 Elsevier B.V. All rights reserved.
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an organic solvent, including alcohols (Pu et al., 2013; Huijgen et al., 2014), cyclic ether (Quesada-Medina et al., 2010; Grande et al., 2015) and polyols (Alriols et al., 2009; Novo et al., 2011) was applied in organosolv process to achieve satisfied fractionation results. For instance, ethanol aqueous has been widely studied for pretreatment purposes of various raw biomass feedstocks and gets a delignification rate not more than 80% (Huijgen et al., 2014; Obama et al., 2012). It has also been reported that enzyme digestible cellulose pulp as well as high quality lignin could be obtained with n-butanol and water because that alcoholic solvent can reduce the lignin repolymerization by trapping the reactive cationic intermediate released during fractionation, thereby increasing lignin removal rate and improving the quality of isolated lignin (Lancefield et al., 2017). The study of Quesada-Medina et al. (2010) suggest that the optimum delignification of almond shells could be achieved by using the mixture of Diox/water (75/25, v/v), which is believed due to the well matched Hildebrand δ-value of the mixed solvent with lignin. Microwave heating has been widely used in biomass thermochemical conversion process (Motasemi and Afzal, 2013; Sturm et al., 2014). The conversion of raw biomass materials could be reached in minutes due to the high heating rate, selective and volumetric heating of microwave irradiation (Xie et al., 2016a; Kržan and Kunaver, 2006). Ethylene glycol (EG) has excellent thermal response to microwave radiation due to its unique dielectric property (Kappe, 2004). It could serve as fractionation medium as well as microwave absorber during the microwave-assisted biomass fractionation process. Meanwhile, it could stabilize the active terminal groups released during lignin degrading and reduce the condensation reaction (Deuss et al., 2017). Given the high viscosity of EG, water was usually employed with EG in biomass conversion to reduce both the viscosity and cost of mixed solvent (Zhang et al., 2016; Zheng et al., 2018). However, the addition of water is bound to cause a decrease in the lignin solubility in EG due to the hydrophobicity of lignin. Dioxane (Diox) was demonstrated has excellent lignin solubility and the ability of promoting catalysis action of mineral acid (Mellmer et al., 2018; Schuerch, 1952). Mixing Diox instead of water with EG is expected to have synergistic effect on lignin solubility while decrease the viscosity of using EG along during biomass fractionation. In this study, a facile one-step fractionation process for lignocellulosic biomass was first reported, to the best of our knowledge, by using a mixture of Diox and EG as the reaction medium and sulfuric acid as the catalyst under microwave irradiation. Raw poplar wood was fractionated into highly pure cellulose, high quality lignin and pentose rich solution under mild conditions. The effect of fractionation conditions on the properties of each fraction was studied by characterization of FT-IR, SEM, 2D-NMR, XRD and GPC.
Table 2 Fractionation conditions and labelling of the obtained products. Experiments
Temp. (℃)
Catalyst (Wt%)
Time (min)
Crude cellulose label
Lignin precipitate label
1 2 3 4 5 6 7 8 9
100 120 140 160 120 120 120 120 120
1.5 1.5 1.5 1.5 0.0 0.5 1.0 1.5 1.5
20 20 20 20 10 10 10 10 30
C1 C2 C3 C4 C5 C6 C7 C8 C9
L1 L2 L3 L4 L5 L6 L7 L8 L9
lignin was purchased from TCI (Shanghai Co., Lt.). All chemicals used in this study were of analytical grade, commercially available, and ready for use. 2.2. Fractionation process Fractionation of poplar particle with Diox/EG (2/1, v/v) was carried out on a Microwave Extraction System (Milestone Ethos Ex, Italy) equipped with a fiber optic temperature probe (ATC-400FO, Milestone, Italy). Detailed fractionation conditions and labeling of the obtained fractions were listed in Table 2. The schematic diagram of the fractionation process was shown in Fig. 1. In brief, a mixture of 2 g of poplar particle, 13.3 ml of Diox, 6.7 ml of EG, and 0.3 g sulfuric acid (98%) were placed in an 100 ml Teflon vessel with magnetic stir. For each run, one vessel was placed in the microwave chamber, and the maximum microwave irradiation output power was set at 300 W. A preheating time of 3 min was programmed together with the desired reaction time for each run. After the completing of the reaction, the vessel was cooled to room temperature before opening. The reaction mixtures were first vacuumfiltered to separate the solid residue from the filtrate. The solid residue, which was designated as crude cellulose, was washed and oven dried at 105 °C for 24 h and preserved in a desiccator for further analysis. The filtrate was evaporated under reduced pressure at 40 °C to remove and recycle Diox. Subsequently, the lignin fraction was precipitated by adding excessive deionized water to the filtrate and collected by centrifugation (12,500 r/min, 5 min). After freeze-drying for 24 h, collected lignin precipitate was weighed and sealed in a plastic bag for further analysis. The supernatant from centrifugation was concentrated under reduced pressure at 60 °C and stored at −20 °C for further analysis. The crude cellulose yield and lignin recovery rate were calculated by Eqs. (1) and (2), respectively.
2. Experimental section
Crude Cellulose yield (wt%) = (MCC/MOP) × 100%
(1)
2.1. Materials
Lignin recovery rate (wt%) = (MLP/ML) × 100%
(2)
Where MCC and MLP are the mass (g) of obtained crude cellulose and
Raw poplar shavings were purchased from Caoxiansenyi Wood Industry Co., Ltd, Heze, Shandong Province, China. The raw poplar shavings were pulverized into particles capable of passing through a 40mesh sieve and then oven dried at 105 °C for 24 h. Dried poplar particles were sealed in plastic bags and stored in a desiccator before use. The chemical composition of the raw poplar is analyzed according to literature (Si et al., 2017) and the results are shown in Table 1. Alkali Table 1 Main chemical composition of raw poplar. Materials
Cellulose
Hemicellulose
Acid insoluble lignin
Acid soluble lignin
Ash
poplar
49.2 %
19.3%
23.1%
1.4%
0.5%
Fig. 1. Schematic diagram of the fractionation process. 2
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Fig. 2. Effects of (a) fractionation temperature, (b) sulfuric acid concentration, and (c) fractionation time on the fractionation efficiency ((a) Fractionation time 20 min; H2SO4 1.5 wt%; (b) fractionation time 10 min; temperature 120 °C; (c) H2SO4 1.5 wt%; temperature 120 °C).
2.4. Characterizations of crude cellulose and lignin fractions
lignin precipitate respectively; ML is the mass of lignin in raw poplar; MOP is the mass of original poplar.
The crude cellulose was analyzed by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR) and X-ray diffraction (XRD). The SEM analysis was performed using a Quanta 200 microscope (FEI, USA). FT-IR analysis, which employed the potassium bromide pelleting method, was performed using a Nicolet 380 spectrometer (Thermo, USA) at room temperature. At least 32 scans were taken with a resolution of 2 cm−1 between the regions of 400 and 4000 cm−1. The particle size distribution (PSD) was analyzed using the Particle Sizing system (AccuSizer 780SIS, Santa Barbara, CA, USA). In each test, 0.05 mg of the sample in powder form was dispersed in 250 ml of deionized water and vortexed for 1 min. XRD patterns were recorded using an Ultima IV (Rigaku, Japan) system equipped with a D/ teX-Ultra diffractometer at a scan rate of 5° min−1 from 5 to 40°. The crystallinity index (CrI) was determined using the following equation:
2.3. Chemical composition analysis The composition of crude cellulose was analyzed using analytical procedures described in previous work (Si et al., 2017). In brief, around 0.3 g dried sample was Soxhlet extracted with ethanol-benzene mixture (v/v, 1/2) for 10 h. Thereafter, the sample was washed with ethanol and oven-dried to a constant weight. Subsequently, the dried and extractive-free sample was hydrolyzed using 72 wt% H2SO4 (3 mL) under mechanical agitation at 30 °C for 1 h. The hydrolyzed suspension was diluted to 4 wt% H2SO4 concentrations and refluxed for 5 h. After filtration, the amounts of glucose and xylose in the filtrate were analyzed on an Agilent 1200 series high performance liquid chromatography system (HPLC) to estimate cellulose and hemicellulose. The analysis was conducted by using a Bio-Rad Aminex HPX-87H column operating at 60 °C with a refractive index detector and 5 mM H2SO4 aqueous solution as the mobile phase at a flow rate of 0.6 mL min−1. The amount of acid-soluble lignin (LAS) in the filtrate was analyzed using an ultraviolet spectrometer (UV-2550, Shimadzu, Japan) in the range from 200 to 400 nm at room temperature. The filtration cake, namely, the acid-insoluble lignin (LIS), was washed with excessive deionized water and then oven dried at 105 °C for 24 h. The percentages of cellulose retention and delignification as well as the removal rate of hemicellulose were calculated by the following equations: Cellulose retention (%) = (mC/MC) × 100%
(3)
Delignification (%) = (1-rL/RL) × 100%
(4)
Removal rate of hemicellulose (%) = (1-rH/RH) × 100%
(5)
CrI (%)=((I002-IAM)/I002) ×100%
(6)
Where I002 is the scattered intensity of crystalline cellulose I at about 2θ = 22.4° and IAM is the scattered intensity of amorphous cellulose at about 2θ = 15.5°. The solubility of the lignin precipitates in Dixo, acetone, N,N-dimethylformamide (DMF), 2-methoxyethanol (MC), methanol and EG were studied by directly dissolving the lignin precipitates in above solvents at a concentration of 5 mg/ml and sonicated for 5 min, followed by centrifugation for 5 min at 3000 rpm. The number average (Mn) and weight average (Mw) molecular weights of the lignin precipitate were analyzed using an Agilent GPC/ HPLC 1200 system equipped with three Jordi Gel DVB Mixed Bed HPLC columns (250 × 10 mm) connected in series at 30 °C. Tetrahydrofuran (THF) was used as the mobile phase at a flow rate of 1.0 mL/min. The inter-unit linkages and the aromatic and aliphatic chemical moieties of the lignin precipitate were analyzed using two-dimensional 13C–1H heteronuclear single quantum correlation (2D HSQC) NMR equipped with an Avance-400 spectrometer at 20 °C. In general, 30 mg of lignin sample was dissolved in 0.6 ml of DMSO-d6. The standard Bruker pulse sequence hsqcetgpsisp2.2 was used with a pulse width of 11.27 ppm, an acquisition time of 106.8 ms and a relaxation delay time of 2 s.
Where mC and MC are the masses of cellulose in crude cellulose and original poplar particle, respectively; rL and RL are the percentages of lignin in crude cellulose and original poplar particle, respectively; rH and RH are the percentage of hemicellulose in crude cellulose and original poplar particle, respectively. 3
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3. Results and discussion
more severe, the size of crude cellulose sample further reduced and mostly single wood cell wall fragments were observed (i.e., sample C3 in Fig. 3f). The results of PSD analysis are consistent with those of morphological structures of the samples observed in SEM. This result is mainly due to the significant removal of hemicellulose and lignin located at the middle lamella of wood cell wall and therefore causes the dissociation of wood cell bundles. In addition, the size reduction of crude cellulose sample obtained under more severe condition may be also contributed to the degradation of amorphous cellulose since the cellulose retention of C3 is only 51.3% (Table S1). It is worth noting that no long fibers were found in the crude cellulose samples, which is not in agreement with previous studies (Schutyser et al., 2015). The main reason is because that the raw poplar used in this study was wood shavings, which may have reduced fiber length than wood chips used in other studies. In addition, poplar is inherently shorter in fiber length (Tower, 2013). The XRD patterns and the crystallinity index (CrI) results of raw poplar and crude cellulose products C8 and C3 are shown in Fig. 4. They all display a typical cellulose I pattern (2θ = 15.5, 22.4 and 34.4° corresponds to 101, 002 and 040 planes respectively) (Park et al., 2010). Compared to raw poplar, the crystallinity of crude cellulose samples significantly increased, which should be mainly due to the removal of hemicellulose and lignin (Ouyang et al., 2018). The crystallinity index of sample C3 (76%) is comparable with that of commercial α-cellulose (78%), indicating the removal of most amorphous cellulose as well under harsher treatment condition (Park et al., 2010). The FT-IR spectra of raw poplar and crude cellulose samples C8 and C3 are shown in Fig. 5. The peak at 1735 cm−1 in the spectrum of raw poplar is assigned to the stretching vibration of the carboxyl and acetyl groups in hemicellulose (Xie et al., 2016b). The intensity of this peak significantly reduced in the spectrum of C8 and almost disappeared in the spectrum of C3, indicating the removal of hemicellulose by the cleavage of the linkages between ferulic acid or (p-) hydroxyl cinnamic acids with lignin (Sun and Chen, 2007). The characteristic peaks of lignin at 1594 and 1505 cm−1 assigned to the vibration of the aromatic skeleton and the peaks at 1456 and 1230 cm-1 corresponding to the deformation of the C–H aromatic ring on lignin methoxy groups is clearly presented in the spectrum of raw poplar. Similar to the circumstance of hemicellulose characteristic peak, the first two peaks almost disappeared while the intensities of the latter two peaks decreased in both spectra of C8 and C3, indicated that lignin was significantly removed from the raw poplar after fractionation treatment. In addition, the FT-IR spectra of both C8 and C3 display very similar characteristic peaks as those of microcrystalline cellulose, suggesting that the crude cellulose obtained from this fractionation process is highly pure cellulose. Moreover, these results also indicate that the fractionation conditions did not alter the chemical properties of the crude cellulose products. It has great potential to be used as pure crystalline cellulose feedstocks to prepare high value-added platform chemicals or nanocellulose materials without extra purification step (Xie et al., 2016a).
3.1. Fractionation process The effects of the acid concentration, fractionation temperature and time on the fractionation efficiency were evaluated by cellulose retention, delignification and the removal rate of hemicellulose (Fig. 2). It can be seen from Fig. 2a that significant increase in removal of both lignin and hemicellulose were observed as the temperature increased from 100 to 120 °C, and they both begun to level off while the temperature further increased to even 160 °C. Meanwhile, cellulose retention exhibited a continuous sharp decrease along with the increasing temperature. Virtually all the raw materials degraded at 160 °C. Fig. 2b shows the effect of the sulfuric acid concentration on the fractionation efficiency. It can be seen that the presence of sulfuric acid had a significant effect on the fractionation of poplar wood. Either lignin or hemicellulose can hardly be removed in the absence of sulfuric acid, whereas a low concentration of sulfuric acid (0.5 wt%) significantly increase the delignification rate from around 10% to 80.9%. When the acid concentration further increased to 1.5 wt% the delignification rate reaches 94.5%. The removal of hemicellulose showed similar trend as that of lignin with the variation of acid concentration. Cellulose was not very sensitive to acid concentration as compared to hemicellulose and lignin. The cellulose content maintained at a high level (78.4%) even in the presence of a high concentration of acid (1.5 wt%). This result should be attributed to the more compact structure of cellulose molecule than those of hemicellulose and lignin, which would lead to less accessibility of cellulose (Fengel and Wegener, 1989; Rowell, 2016). It can be seen from Fig. 2c that poplar wood quickly reached high fractionation rate within 10 min and then kept at an almost equilibrium stage as the fractionation time increased to 30 min, indicating that microwave-assisted fractionation of lignocellosic biomass could increase the reaction rate and therefore prevent the formation of condensation byproducts during prolonged reaction time (Ma et al., 2015). It is worth noting that fractionation conditions showed similar effects on the removal of lignin and hemicellulose. This is mainly because that, structurally, lignin associates closely with hemicellulose in the middle lamella and primary wall of plant cell (Fengel and Wegener, 1989). It was reported that the correlation factor of degree of delignification with hemicellulose dissolution is up to 0.96 (Zhao et al., 2014). The optimal condition was found to be at 120 °C in the presence of 1.5 wt% H2SO4 for 10 min, which could reach a crude cellulose yield of 42.7%, lignin recovery rate of 56.9%, and delignification rate of 94.5%. 3.2. Characterizations of crude cellulose Two representative crude cellulose samples (C8 and C3) were characterized in detail. C8 was obtained under the optimum fractionation condition. While C3 has the lowest lignin content among all crude cellulose samples obtained in the study. The analysis and comparison of these two samples are expected to provide insight into the fractionation mechanism as well as the information for the future industrial implementation of this fraction strategy. Fig. 3 presents the visual appearance and morphologic characteristics of the raw poplar wood and two typical crude cellulose samples (i.e., C8 and C3, see Table 1 for label). Raw poplar wood and crude cellulose exhibit a trend of lightening in colors as their lignin contents decreased (Table S1). The color of C3 (Fig. 3c) is almost pure white owning to its ultra-low lignin content (0.2%). This result is quite straightforward because lignin is the main chromogenic component in wood tissue. The SEM images clearly display a decrease in particle diameters from raw poplar to crude cellulose samples. Bundles of several wood cells with average diameters around 174 μm were observed in SEM image of raw poplar (Fig. 3d). After fractionation process at relatively mild condition, the wood cell bundles dissociated to some extent with fewer wood cells attached together (i.e., sample C8 in Fig. 3e). As the fractionation condition turned
3.3. Recovery and characterization of lignin precipitates The effects of fractionation conditions on the lignin recovery rate are shown in Fig. 6. The average lignin recovery rate was around 55% under most of the fractionation conditions described herein, which is close to the results in literature (Schutyser et al., 2015). It should be noted that no lignin precipitate was recovered when sulfuric acid was absent during the fractionation process. Acidic conditions are known to promote cleavage of ether bonds in the carbohydrate polymers and β-O4 ether bonds in lignin (Rinesch et al., 2017; Luterbacher et al., 2014; Li et al., 2015), especially under mild reaction conditions (i.e., at 120 °C for 10 min;). However, lignin recovery did not exhibit dramatic changes within the acid contents between 0.5–1.5%. It is noted that the delignification rate increased slightly with increasing catalyst contents (Fig. 2b). Therefore more water soluble lignin-derived fraction was 4
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Fig. 3. The visual appearance, SEM images and PSD results of raw poplar and crude cellulose C3 and C8. (a, b, c are visual appearances of raw poplar particles, crude cellulose C8 and C3, respectively; d, e, f are the SEM images of raw poplar particles, crude cellulose C8 and C3, respectively; g, h, k are PSD of raw poplar particles, crude cellulose C8 and C3, respectively).
Fig. 4. XRD patterns and crystallinity of raw poplar and crude cellulose C3 and C8.
Fig. 5. FTIR spectra of raw poplar and crude cellulose.
high lignin recovery at high fractionation temperatures (i.e., 160 °C) could be attributed to the formation of insoluble humins. High temperature will cause the massive degradation of lignin and hemicellulose as well as cellulose (Fig. 2a). It is reported that the degradation intermediates of carbohydrate could further repolymerize into insoluble
produced at higher acidic fractionate conditions. Similar result was reported in other study (Zheng et al., 2018). Fractionation temperature also had a significant impact on lignin recovery. As the temperature increased from 100 to 160 °C, so did the lignin recovery rate, which increased from 44.5% to 93.5% (Table S2). It is worth noting that the 5
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Signals from the typical substructures, including β-aryl ether (β-O-4, A and A’), resinol (β-β, B) and phenylcoumaran (β-5, C), were identified in the side chain regions of lignin (Chen et al., 2017; Si et al., 2017). The 1H-13C correlation signals of resinols are show at 85.4/4.65 (Bα), 54.0/3.06 (Bβ) and 71.6/3.80–4.19 (Bγ). Moreover, phenylcoumarans were assigned by cross-signals at 87.6/5.50 (Cα) and 53.1/3.46 (Cβ). It can be seen from Fig. 8a that lignin sample L1 contains predominantly β-aryl ethers (both Aβ-G and Aβ-S) with a small amount of resinols and phenylcoumarans. That is very similar to the native lignin in poplar given in the literature (Chen et al., 2017), indicating that mild fractionate condition would prevent the alteration in structures of resulted lignin. By contrast, the signals of Aβ, Aβ and A’α in the side-chain region of L2 significantly decreased (Fig. 8b) and even disappeared in that of L3 (Fig. 8c) and L4 (Fig. 8d), suggesting profound cleavage of β-O-4 linkages occurred under higher fractionation temperature. Partially, this is also the reason of reduced molecular weight of lignin obtained with increased fractionation temperature (Table 3). In the aromatic regions of lignin samples, signals from syringyl (S), guaiacyl (G), p-hydroxybenzoates (PB) as well as some condensed units were assigned according to literature (Si et al., 2017; Chen et al., 2017; Wen et al., 2013). Namely, G-units were assigned by three different cross-signals at δC/δH 111.5/6.90 (G2), 114.8/6.71 (G5) and 119.3/6.70 (G6). And S and PB-units show a prominent signal for the C2,6-H2,6 correlations at δC/δH 104.9/6.67 and 131.5/7.64, respectively. In addition, condensed Scondensed and Gcondensed, were observed at δC/δH 105.7/6.54 and 113.4/6.68, respectively. As shown in Fig. 8e, the aromatic units in L1 include all S, G and PB-units with little condensed units. As a comparison of L1, the peak area of G2 and G6 in the aromatic region of L2 and L3 were significantly reduced (Fig. 8f, 8 g), probably caused by lignin condensation at 2- and 6-positions (Chen et al., 2017). This result can also be demonstrated by the enhancement of the signals of Scondensed and Gcondensed units in the aromatic region of L2 and L3. This also indicates that lignin degradation and condensation occur simultaneously during fractionation (Chen et al., 2017). Interestingly, the increasing condensed units in L2 and L3 did not increase its molecular weight (Table 3) as a comparison of L1, suggesting that the degradation rate of lignin during fractionation might be greater than that of condensation. Similar conclusion was also reported in other study (Wu et al., 2015). It is also worth noting that the intensity of G-units in the aromatic region of L1 and L2 decreases more obviously than that of Sunits, indicating a preferentially degradation of G-units during fractionation (Chen et al., 2017). Notably, all of the 1H-13C cross-signals in the aromatic regions of L4 are significantly weakened. That probably caused by the mass presence of humins in L4, which has a low H/C ratio and would be invisible in the 1H-13C correlation spectra (Chen et al., 2017). In addition, the cross-signals of polysaccharides, such as β-Dxylopyranoside, 2-O-acetyl-β-D-xylopyranoside and 3-O-acetyl-β-D-xylopyranoside, mentioned in literature (Chen et al., 2017) were not detected in all of the four lignin samples, suggesting that the fractionation process proposed herein can efficiently extract lignin with high purity.
Fig. 6. Recovery rate of lignin samples obtained under different fractionation conditions.
humins under harsh treatment conditions (Sannigrahi et al., 2011). The dissolution properties of recovered lignin were evaluated with common solvents and compared with that of commercial alkali lignin. The colors of recovered lignin in organic solvents varied from pale brown to dark brown as the fractionation temperature increased (Fig. 7). And they all significantly lighter than alkali lignin. Giving the very similar FT-IR spectra of all recovered lignin samples (Fig. S1), the increase in color with increased fractionation temperature may be related to the association of humins formed at high temperature. It can be seen from Fig. 7 that all recovered lignin samples are more soluble in Diox, DMF, 2-methoxyethanol, and EG than in methanol and acetone. The dissolution of lignin in organic solvents involves complicated mechanism. Although an early study suggested that the ability of solvents to dissolve lignin increases as the hydrogen-bonding capacities of the solvents increase and as their solubility parameters approach a value of lignin itself (Schuerch, 1952), no universal rules have been found for organic solvents in lignin dissolution (Sameni et al., 2017; Zhao et al., 2017). It is generally believed that the dissolution of in organic solvents involves the types and the molecular weight of lignin as well as the hydrogen-bonding capacity or basic strength of the solvent. Nevertheless, the relatively lower lignin dissolution capacity of acetone and methanol compared to DMF or Diox has been reported in literature (Quesada-Medina et al., 2010; Duval et al., 2016; Melro et al., 2018). EG is considered an excellent solvent for a wide variety of lignin samples including alkali lignin (Schuerch, 1952), while the relatively poor solubility of L3 and L4 in EG might be caused by the presence of the aforementioned insoluble humin. The solubility of lignin precipitates obtained from this method is much better than that of alkali lignin, which would facilitate their applications in various processes. Table 3 lists the molecular weight and molecular weight distribution of four typical lignin precipitate samples. All of these lignin samples have a relatively narrow molecular weight distribution (PDI ≤ 1.7) and lower average molecular weight than alkali lignin (Mw ≥ 2330) and other organoslov lignin (Mw ≥ 2170) reported in literature (Yuan et al., 2013; Tolvert et al., 2015). The smaller molecular weight of lignin obtained from this method may also be one of the reasons for their good solubility in common organic solvents. Fractionation temperature is one of the reaction variables that has the greatest influence on the structure and properties of recovered lignin. Therefore, the lignin samples obtained at 4 different fractionation temperatures were selected for characterization to further understand their detailed chemical structures by 2D HSQC NMR. The NMR spectra depicting the side chain (δC/δH 50–90/3-5.5) and aromatic (δC/ δH 100–135/5.5–8.5) regions were shown in Fig. 8. The detailed 1H-13C cross-signals assignments were listed in Table S3 and the identified chemical structures were shown in Fig. S2.
4. Conclusions In this study, poplar wood particles were efficiently fractionated with a binary solvent of Diox and EG by a one-step process under mild treatment conditions. The fractionation conditions were optimized and its effects on the fractions properties were studied. The results showed that fractionation temperature is the most critical factor in the fractionation process and has significant effect on cellulose retention and lignin precipitates recovery rate and its physical chemical properties. Up to 94.5% lignin and 95.1% hemicellulose can be selectively separated from raw poplar while achieve 78.5% cellulose retention at 120 °C with 1.5 wt% H2SO4 for 10 min. The lignin precipitates from this method have light colors with low molecular weight, less condensed structure and good solubility in common solvent. The remaining hemicellulose-derived stream together with EG can be used as polyols 6
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Fig. 7. The visual appearance and dissolution property of lignin precipitates in common solvents (L1-L4 were obtained under 100, 120, 140 and 160 °C, respectively with 1.5 wt% H2SO4 for 10 min).
lignin fractions, mild fractionation conditions, easy solvent recovery and easy-handle operation process entitle the proposed fractionation approach a great potential to scale up for industrial implementation.
Table 3 Molecular weight and molecular weight distribution of lignin precipitates. Sample label
Mn
Mw
PDI
L1 L2 L3 L4
938 874 718 714
1553 1446 1189 1222
1.66 1.65 1.67 1.70
Acknowledgement The authors are grateful for the financial support by National Natural Science Foundation of China (No. 31770631), the project of the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institution, China and Top-notch Academic Programs Project of Jiangsu Higher Education Institutions (TAPP) of China (PPZY2015C221), Nanjing Forestry University scientific research startup funds (GXL2018037).
for bio-based materials. By increasing the fractionation time to 20 min and temperature to 140 °C, almost pure cellulose with high crystallinity index (76%) comparable to that of commercial α-cellulose (78%) could be obtained. Diox can be recycled with high recovery rate (80.6%) and high purity by reduced pressure distillation (Fig. S3) to decrease the process cost. In summary, the high quality of resulting cellulose and 7
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Fig. 8. Side chain (a–d) and aromatic regions (e–h) in 2D HSQC NMR spectra of four lignin samples.
Appendix A. Supplementary data
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