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Characterization of solvent-fractionated lignins from woody biomass treated via supercritical water oxidation
Accepted Manuscript Characterization of solvent-fractionated lignins from woody biomass treated via supercritical water oxidation Jin Ho Seo, Hanseob Jeong, Hyung Woon Lee, Cheol Soon Choi, Jin Ho Bae, Soo Min Lee, Yong Sik Kim PII: DOI: Reference:
S0960-8524(18)31747-4 https://doi.org/10.1016/j.biortech.2018.12.076 BITE 20833
To appear in:
Bioresource Technology
Received Date: Revised Date: Accepted Date:
15 November 2018 21 December 2018 22 December 2018
Please cite this article as: Ho Seo, J., Jeong, H., Lee, H.W., Choi, C.S., Bae, J.H., Lee, S.M., Kim, Y.S., Characterization of solvent-fractionated lignins from woody biomass treated via supercritical water oxidation, Bioresource Technology (2018), doi: https://doi.org/10.1016/j.biortech.2018.12.076
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Characterization of solvent-fractionated lignins from woody biomass treated via supercritical water oxidation
Jin Ho Seoa, Hanseob Jeongb, Hyung Woon Leeb, Cheol Soon Choic, Jin Ho Baec, Soo Min Leeb, * and Yong Sik Kima,c, *
a
The Institute of Forest Science, College of Forest and Environmental Sciences, Kangwon
University, Chuncheon, 24341, Republic of Korea b
Wood Chemistry Division, National Institute of Forest Science, Seoul, 02455, Republic of
Korea c
Div. of Forest Material Science & Engineering, College of Forest and Environmental
Sciences, Kangwon University, Chuncheon, 24341, Republic of Korea
* Co-corresponding author at: Wood Chemistry Division, National Institute of Forest Science. E-mail address: [email protected] (S.M. Lee). * Corresponding author at: Div. of Forest Material Science & Engineering, College of Forest and Environmental Sciences, Kangwon Nation University. E-mail address: [email protected] (Y.S. Kim).
Abstract Crude supercritical lignin (SCL) extracted from hardwood (Quercus mongolica) treated via supercritical water (SCW) oxidation was subjected to sequential fractionation with four organic solvents; five lignin fractions (F1−F4 and FIN) were thus obtained. The molecular weight (MW) of the fractionated lignins gradually increased as fractionation proceeded. However, the content of methoxyl groups and phenolic hydroxyl groups tended to decrease with increasing molecular weight of the lignins. The functional groups of SCL and the fractionated lignins were very similar based on Fourier-transform infrared analysis. The syringyl/guaiacyl ratio (S/G ratio) of the fractionated lignins increased with an increase in the MW. The thermal stability decreased with decreasing MW of the fractionated lignins, and all fractions except for F1 had a maximum degradation temperature of around 360℃. The glass transition temperature (Tg) of the fractions increased from 83℃ to 137℃ with increasing MW.
Keywords: supercritical lignin, fractionation, characterization, molecular weight distribution
1. Introduction Lignin is a component of plants that is a sustainable resource and widely available biopolymer. Lignin is composed of phenylpropanol units, i.e., sinapyl, coniferyl, and pcoumaryl alcohol, wherein each monomer unit forms an amorphous structure with bonds such as 5-5, β-β, α-O-4, and β-O-4 linkages (Jiang et al., 2016). Approximately 70 million tons of lignin is generated from Kraft pulp production worldwide (Orbisresearch, 2017). However, most of the lignin is burned for energy production or disposed in land-fills (Orbisresearch, 2017), and a small amount is isolated from the black liquor for other purposes. Only a small portion of lignin is used for select applications such as materials, fuels, chemicals, dispersants, carbon fibres, and resins (Jiang et al., 2016; Yang et al., 2007; Ragauskas et al., 2014). In recent years, much research has been focused on converting lignin into value-added products due to the sustainability and potential of lignin as a natural resource. However, various applications of lignin have been restricted by its complex and heterogeneous features, such as its broad molecular weight distribution, heterogeneous reactivity, low solubility, amorphous structure, etc. In addition, the structure of lignin may vary considerably based on the pretreatment applied, and the physical and chemical properties may vary depending on the source of the feedstock and process conditions (Jiang et al., 2016). Generally, lignin can be obtained as a co-product from various pretreatment methods such as chemical pulping, biological pretreatment, steam explosion, ionic liquid (IL) and deep-eutectic solvent (DES) processing, and supercritical water (SCW) oxidation (Kumar et al., 2017). Kraft pulping is the most common pretreatment method that generates lignin as a co-product (Wells et al., 2016). Of the other pretreatment methods, biological pretreatment is regarded as a low-energy, efficient, and environmentally safe method compared with conventional chemical and physical methods (Kumar et al., 2017). Steam explosion is the
most commonly used physicochemical pretreatment method (Kumar et al., 2017), whereas the pretreatment methods using ILs and DES are comparatively new techniques. Various studies have been conducted to achieve efficient utilization of lignins extracted via different pretreatments based on the resulting characteristics. The low impurity content of organosolv lignin has spurred its use as a precursor for carbon fibre (Dallmeyer et al., 2010). However, lignin-based carbon fibre, produced by conventional spinning methods, has comparatively inferior mechanical properties and there is still the need to improve fibre production and treatment techniques (Ding et al., 2016). Kraft lignin has been applied in adhesives and various composites, and many studies have used lignosulfonate as dispersants, surfactants, and as a partial substitute for phenol, etc. (Olivares et al., 1988; Faruk et al., 2015; Xu et al., 2017; Alonso et al., 2004). Although technical lignins such as Kraft lignin and lignosulfonate have been explored for various applications, high value-added application of these lignins is limited due to the unique chemical reactivity, heterogeneous structure, and impurities of these lignins (Vishtal et al., 2011). Furthermore, there are still difficulties in the recovery and separation of lignins (Vishtal et al., 2011). The SCW treatment used in this study is very suitable for the saccharification of lignocellulosic biomass because SCW has low density and viscosity, close to those of gases, and an even higher diffusion coefficient than those of liquids (Lachance, 1995). Furthermore, the carbohydrate of lignocellulosic biomass can be saccharified by supercritical reaction over a period of 30−60 s, which is economical. Therefore, it is essential to characterize lignin derived from the SCW oxidation process for efficient utilization of lignin. During the SCW process, lignin is degraded via cleavage of the aryl-ether linkage and propane side chains (Moon et al., 2011). According to Ehara et al. (2002), during the SCW process, lignin with the biphenyl structure is more stable than that with the β-O-4 structure. Jiang et al. (2016) reported that lignin residues from the SCW process can be used as a source of aromatics
production. Varman et al. (2010) characterized the water-soluble and methanol-soluble and insoluble portions derived from oil palm treated by SCW, and Pińkowska et al. (2012) investigated the hydrothermal decomposition characteristics of alkali lignin via SCW. However, most researchers have studied the characteristics of lignin obtained as a co-product without solvent fractionation. Therefore, it is thought that fractionation of the lignin residue obtained from the SCW reaction using various organic solvents is necessary to clarify the characteristics of lignin and improve its homogeneity. Fractionation using organic solvents is an effective method of obtaining a more uniform type of specific lignin. Fractionation with various solvents was previously performed to produce fractions having a specific MW range, branching, or chemical composition (Francuskiewicz, 1994). For instance, Mörck et al. (1986) developed sequential fractionation using dichloromethane, isopropanol, methanol, and a mixture of methanol-dichloromethane as solvents and reported that the low MW fractions showed narrow polydispersity, whereas the high MW fraction was more polydisperse. Birch organosolv lignin, commercial Alcell, and steam-exploded eucalypt and pine lignins were fractionated using ether and acetone as organic solvents (Van de Pas et al. (2011). Thring et al. (1996) reported that Alcell lignins were successively fractionated by ether and methanol. The low MW fraction was more homogenous and the amount of guaiacyl lignin was higher (Thring et al., 1996). The fractionated soda-anthraquinone lignin extracted via ash-AQ pulping of oil palm empty fruit bunch fibre was separated into four fractions by using extraction solvents such as methylene chloride, n-propanol, and methanol/methylene chloride (Sun et al., 2000). It was found that the thermal stability increased as the MW increased, whereas the carbonyl group content of the lignin fractions decreased. Domínguez-Robles et al. (2018) studied a green fractionation method using different ratios of acetone in water, where the obtained fractions were characterized by a specific MW range and functional groups. The proposed acetone
fractionation process definitively generated pure and uniform lignin. Yuan et al. (2009) clearly characterized degraded lignins from Eucalyptus pellita KP-AQ pulping using solvent fractionation, and Li et al. (2012) used five solvents to generate heterogeneous bamboo organosolv lignin. Recently, there has been a growing interest in research on biorefinery using lignocellulosic biomass, and numerous technical lignins generated from various pretreatment methods have been obtained with high utilization of the wood components. Therefore, fractionation into more homogeneous lignin is required for the development of lignin-based biomaterials and/or biochemicals, as technical lignins have different physical and chemical properties based on the pretreatment methods. Thus, separating lignin into fractions with a homogenous molecular weight and size based on their solubility in an organic solvent may be a promising alternative for the effective utilization of technical lignin. In this study, we focus on the fractionation of supercritical lignin (SCL) extracted from Mongolian oak (Quercus mongolica) that was treated via SCW by using a sequence of organic solvents and characterization of the fractionated lignins to provide basic information for efficient utilization of the lignin fractions. The various fractions and SCL are subjected to thermal analyses, including thermogravimetry (TG), differential thermogravimetry (DTG), and differential scanning calorimetry (DSC), to evaluate their thermal decomposition behaviour and stability. Structural and chemical analyses are also performed to determine the S/G ratio, molecular weight distribution, and functional groups of the fractionated lignins.
2. Materials and methods 2.1. Materials Crude SCL was kindly provided by the National Institute of Forest Science. SCL is a hardwood (Quercus mongolica) lignin produced by a continuous pilot-scale SCW system (Hanwha Chemical, Seoul, Republic of Korea). A 50 kg sample of milled biomass (Quercus mongolica) was hydrolyzed each day via SCW treatment using a total flow rate of 130±15 kg h−1 and constant temperature (380±5℃) and pressure (230±5 bar). The solid residue was separated from the hydrolysates by filtration using a 3-micron filter (#BPONG3P1S, Filter Specialists, Inc., Seoul, Republic of Korea) and was freeze-dried for one week using a lyophilizer (FDU-2100, Eyela, Tokyo, Japan) (Jeong et al., 2017). The resulting solid residue was re-dissolved in excess 0.5 M NaOH. The mixture was then filtered to remove any insoluble material. The filtrate was adjusted to ~pH 2.0 using conc. HCl. The precipitated crude SCL was then isolated by filtration, repeatedly washed with fluent deionized water, and dried in a 60℃ oven drier for two weeks. All other chemicals were purchased from either Sigma-Aldrich or Fisher Scientific (USA) and used as received.
2.2. Fractionation of lignin Crude SCL was sequentially fractionated with ethyl acetate, chloroform, acetone, and 96% dioxane. The solvent selection was based on the polarity and the Hildebrand solubility parameter. In this study, the properties of the solvents are as follows: chloroform (P = 2.70, δ = 9.21), ethyl acetate (P = 4.40, δ = 9.10), acetone (P = 5.10, δ = 9.77), and 96% dioxane (P = 5.27, δ = 10.00). Here, P is the polarity index and δ is the Hildebrand solubility parameter. Crude SCL (10 g) was dissolved in 250−400 mL of the various solvents and stirred for 6 h with a magnetic bar at room temperature. The solution was filtered through a Nylon membrane filter (0.45 μm, 45 mm), and the filtrate was evaporated by using a rotary
evaporator. The dried filtrate was dissolved in a small quantity of dioxane and precipitated with cold deionized water. Thereafter, this precipitate was freeze-dried using a freeze-dryer (FDCF-12006, OPERON) to obtain the fractionated lignin powder. The solid residue remaining after filtering was dissolved in the next solvent, and the final insoluble fractions were dried in a 60℃ oven-drier for 2 weeks prior to measuring the dry weight. The respective lignin fractions obtained from solvent fractionation are designated as F1 to F4 and the final solid residue after fractionation with 96% dioxane is designated as FIN. The fractionation process is illustrated in Scheme 1; the recovery factors for all lignin fractions recovered from 10 g of crude SCL were in the 80−85% range.
2.3. Acetylation of lignin SCL and the respective lignin fractions were acetylated for chemical analysis by gel permeation chromatography (GPC) and proton nuclear magnetic resonance (1H-NMR) spectroscopy. About 100 mg of lignin was dissolved in 4 mL of pyridine/acetic anhydride (1:1, v/v) and stirred at room temperature for 24 h. The solution was added dropwise to 200 mL of ice-distilled water with stirring. The precipitated lignin was filtered through a Nylon membrane filter (0.45 μm, 47 mm), washed with flowing cold deionized water, and then freeze-dried using a freeze-dryer (FDCF-12006, OPERON).
2.4. Molecular weight distribution The molecular weight distribution of the lignin samples was determined by gel permeation chromatography (GPC; Shimadzu, UV and RI detectors) after acetylation of each fraction. The acetylated lignins were dissolved in tetrahydrofuran (THF) and analyzed using GPC with PLgel columns (PLgel 5 μm mixed-C, -D, and PLgel 3 μm mixed-E) at 40℃ with UV detection at 280 nm. The mobile phase comprised tetrahydrofuran (THF; 1 mL min−1), and
100 μL of lignin (1 mg mL−1) was injected. Calibration of the molecular weight was performed using polystyrene standards ranging between 1,480 and 1,233,000 g mol−1.
2.5. Analysis of methoxyl content The methoxyl content was analyzed by gas-chromatography/flame ionization detection according to the literature (Park et al., 2018). Lignin (30 mg) was placed in a vial with 4 mL of hydriodic acid and reacted in an oil bath at 130℃ for 30 min with shaking. The solution was cooled in ice-water. An internal standard (200 μL ethyl iodide in pentane, 300 mg mL−1) was added to the solution, followed by 3 mL of pentane. The mixture was shaken and cooled in ice-water. An aliquot of the pentane phase was separated into a GC vial. The methyl iodide was analyzed using gas chromatography / flame ionization detector (GC/FID, 7890B, Agilent) with DB-624 column (30 m × 0.25 mm i.d.). The injector and detector temperature were 200 and 230℃. The oven temperature was held at 40℃ for 5 min and raised to 180℃ at 10 ℃ min−1. The flow rate was 1.1 mL min−1 and the split ratio was 1/120.
2.6. Structural analysis Fourier transform infrared (FT-IR) analysis of the lignins was performed on a PerkinElmer Frontier instrument with an attenuated total reflectance (ATR) attachment. A total of 256 scans per sample was performed from 4000 to 500 cm−1 at a resolution of 4.0 cm−1. FT-IR absorbance spectra were directly obtained using lignin samples fully oven-dried at 40℃. Quantitative 1H NMR analysis was performed on a FT-NMR 600 MHz spectrometer (Bruker Avance Ⅱ600) to measure the amount of both aliphatic and phenolic hydroxyl groups. Acetylated lignin (20 mg) was dissolved in 1 mL of CDCl3. A total of 128 scans per sample was performed at 300 K with a 1.3 s acquisition time, a 90° pulse, and a 7 s relaxation delay. The relative integral of the hydroxyl group signal was analyzed by setting the integral of the
methoxyl region (4.0−3.5 ppm) to 3.0. Then, the hydroxyl content (mmol g−1) was calculated using a following Eq. (1). (1) Where A was the methoxyl content (mmol g−1) determined in analysis of methoxy content by gas-chromatography. B and C were the integral values of hydroxyl group and methoxyl group, respectively, as determined by quantitative 1H NMR analysis.
2.7. Analysis of pyrolysis products The lignin types in SCL and in each fraction and S/G ratio were determined by pyrolysis gas chromatography/mass spectrometry (Py-GC/MS). A pyrolyzer (Py-2020iD, Frontier lab)−GC (5975C, Agilent Technologies)/MS (7890A, Agilent Technologies) instrument equipped with a DB-5MS (30 m × 0.25 mm × 0.25 μm) column was used to analyze the pyrolysis products of the lignins. The S/G ratio was calculated by dividing the sum of the decomposition products corresponding to the units of guaiacyl (G) and syringyl (S) in the pyrolysis products. A 0.5 mg sample was placed in a deactivated SUS cup and was allowed to free-fall into a furnace preheated to 550oC. The pyrolysis vapour was introduced into the separation column after the GC inlet at a ratio of 1/70 (column flow: 0.7 mL min−1). During GC separation, the GC oven temperature was held at 40℃ for 5 min and raised to 310℃ at 10 ℃ min−1, and held at 310℃ for 10 min; the separated pyrolysis products were detected by a MS detector. The pyrolysis compounds were identified using the National Institute of Standards and Technology (NIST) mass spectral library.
2.8. Thermal analysis The thermal characteristics of the lignins were analyzed by thermogravimetric analysis (TGA 8000, Perkin Elmer). The temperature was raised to 650℃ in increments of 20 ℃ min−1; the
sample carrier gas was He. DSC analysis was performed with a DSC Q2000 (TA Instrument, UK) apparatus. The first scan was performed from 20 to 105℃ at 10 ℃ min−1, and the temperature was held for 10 min, followed by cooling to 20℃ at 10 ℃ min−1. The second and third scans were performed from 20 to 180℃ at 5 ℃ min−1. The glass transition temperature (Tg) was determined from the second scan.
3. Results and Discussion 3.1. Yields and molecular weight distributions of SCL and fractionated lignins The yields and MW of the fractionated lignins are shown in Table 1. The yields of the F3 and FIN fractions were relatively higher than those of the other fractions, and the yields of each fraction varied. It is assumed that the solubility of the lignins of different structures differs in the organic solvents (Park et al., 2018). In order to analyze the molecular weight distributions of the SCL and fractionated lignins, each lignin sample was acetylated before GPC analysis. The MW of each fraction gradually increased from F1 to F4 (1420 to 18840 g mol−1). The MW of SCL was 5400 g mol−1 based on the weight average molecular weight (Mw). The polydispersity indices (PDIs) of SCL, F1, F2, F3, and F4 were 2.4, 1.3, 1.3, 1.9, and 1.9, respectively, indicating that the respective fractions were more homogenous than SCL (Table 1). These results suggests that the MW distribution of lignin can be more homogenized by separating lignin having a large MW distribution into specific MW ranges via solubility-based fractionation using various organic solvents; similar results were reported in studies by Park et al. (2018) and Kim et al. (2017). However, it was impossible to analyze the molecular weight distribution of FIN due to its insolubility in any organic solvent. It is assumed that crude SCL contained a small amount of unreacted wood powder. Thus, GPC analysis of the acetylated SCL was carried out by filtering out the solid component after acetylation of SCL.
3.2. Analysis of chemical structure of SCL and fractionated lignins Analysis of the FT-IR spectra was based on the assignments given by Lisperguer et al. (2009) and Boeriu et al. (2004). All fractions showed a broad band at 3600−3000 cm−1, attributed to the hydroxyl groups of the aliphatic and phenolic structures. The bands between 2850 and 2830 cm−1 are predominantly attributed to CH stretching in the methylene and methyl groups. The peaks below 2000 cm−1 are derived from multiple functional groups; therefore, analysis of the region below 2000 was very difficult. Weak and medium bands were found at 1720−1660 cm−1, derived from the unconjugated C=O of the carbonyl groups. The profile of the aromatic ring bands at 1510−1500 cm−1 was similar for SCL and the F1−F4 fractions, whereas that of FIN differed. The CH bending signals were observed between 1460 and 1450 cm−1. A strong band was observed at 1120−1110 cm−1, associated with the phenolic hydroxyl group. Weak bands were observed in the 1030−1020 cm−1 spectral region for SCL and the F1−F4 fractions, but not for FIN, due to the C-O deformation in the methoxyl groups. The spectrum of FIN appeared similar to that of cellulose (Auta et al., 2017). Thus, the final insoluble fraction, FIN, was considered to be a carbohydrate comprising wood powder that remained unreacted during the SCW process. These results suggest that some of the unreacted solids could not be completely removed during the extraction of lignin from the solid residues obtained after SCW process of the lignocellulosic biomass. Therefore, for analysis of the chemical content and for acquisition of the FT-IR spectrum of SCL, crude SCL was dissolved in 96% dioxane to remove the unreacted wood powder and the purified SCL sample was analyzed.
3.3 Determination of functional groups of SCL and fractionated lignins The methoxyl group and both phenolic and aliphatic hydroxyl groups are the specific
functional groups in lignin. The methoxyl groups present in each fraction were quantified using gas chromatography according to a reported method (Park et al., 2018). As fractionation proceeded, the content of methoxyl groups gradually decreased from F1 (5.0 mmol g−1) to F4 (3.5 mmol g−1) (Table 2). The decrease in the content of methoxyl groups is considered to be closely related to the greater insolubility of the S unit in organic solvents (Thring et al., 1996). However, FIN mostly comprised unreacted wood powder, as identified by FT-IR analysis, resulting in a lower methoxyl content (Table 2). The hydroxyl group of lignin is known to be the most important functional group for lignin applications (Li et al., 2012). The phenolic and aliphatic hydroxyl groups in the SCL and fractionated lignins were determined by quantitative 1H NMR, as shown in Table 2. The total content of hydroxyl groups in each fraction followed a trend similar to that of the methoxyl groups, showing a decrease from F1 (5.1 mmol g−1) to F4 (3.3 mmol g−1). However, the content of phenolic hydroxyl groups and aliphatic hydroxyl groups followed opposite trends. The content of phenolic hydroxyl groups tended to decrease from F1 (3.2 mmol g−1) to F4 (0.8 mmol g−1) in proportion to the total hydroxyl content, whereas the content of aliphatic hydroxyl groups tended to increase slightly from F1 (1.9 mmol g−1) to F4 (2.5 mmol g−1). These results are similar to those of other studies, where when the content of phenolic hydroxyl groups was high, lignin became more soluble in organic solvents (Sameni et al., 2017). In addition, the greater cleavage of the β-O-4 linkages in the low molecular weight fraction resulted in a higher content of phenolic hydroxyl groups as terminal groups (Ehara et al., 2002). Generally, more hydroxyl groups facilitate the interaction of lignin with various polymers (Li et al., 2012). Therefore, a lower MW fraction with a higher phenolic hydroxyl group content is more suitable for applications in various fields. Qin et al. (2015) reported that grafted sulfonated alkali lignin was better than naphthalene sulfonate for reducing the
viscosity of coal-water slurry as a dispersant. Shweta et al. (2015) used lignin for nickel removal via physical adsorption through the oxygen-containing groups in lignin. Lignin was also incorporated into polybutylene succinate by Ferry et al. (2015), and this modified lignin slightly improved the flammability parameters. Moreover, the methoxyl group and hydroxyl groups are known to contribute to the antioxidant ability of lignin, and a higher methoxyl content in the lignin was favourable for stabilizing phenoxyl radicals (Li et al., 2012). Moreover, Gregorova et al. (2007) reported that a high content of methoxyl groups in low MW lignin induced an increase in the antiradical and antioxidant efficiency. Therefore, it is considered that the lignin fraction with a low MW and higher methoxyl content may be potentially applicable in lignin-based biomaterials or as an antioxidant chemical.
3.4. Pyrolysis products of SCL and fractionated lignins Table 3 shows the pyrolysis products of SCL and each fraction. The main pyrolysis products of each fraction (except FIN) were guaiacol, 4-methylguaiacol, 4-ethylguaiacol, 4vinylguaiacol, isoeugenol, acetoguaiacone, vanillin, and 4-hydroxy, 3-methoxy, and benzoic acid groups for the G type, and 3,4-dimethoxyphenol, 4-allysyringol, syringylaldehyde, acetosyringone, and sinapinaldehyde for the S type. FIN was mainly composed of carbohydrate degradation products such as hydroxyacetone, 1,2-cyclopentanedione, and levoglucosan. Thus, this result supports the FT-IR analysis of FIN presented above, indicating that FIN contained cellulose. The S/G ratio of each fraction from the pyrolysis products was investigated by Py-GC/MS analysis. The S/G ratio of each fraction ranged from 0.8 to 1.05, whereas the S/G ratio of SCL was relatively high at 1.25 (Table 3). ). Among the G units, vanillic acid was the major pyrolysis product, whereas 3,4-dimethoxyphenol and 4-allylsyringol were detected as the major pyrolysis products in the S units. Overall, the components of the G units varied more
than those of the S units in the lignin pyrolysis products. In the fractions with lower MW, peak area of the G units outweighed that of the S units, but there were larger peak area of the S units in the fractions with higher MW. Further, as fractionation proceeded, both peak area of the G and S units increased; however, the S/G ratio increased slightly because peak area of the S units was slightly larger than that of G units. It was assumed that smaller amounts of S units dissolved in the organic solvent (which had relatively low polarity among the organic solvents used in this study) due to the insolubility of the S units, and organic solvents with a higher polarity could better dissolve the S units (Tring et al., 1996).
3.5. Thermal analysis of SCL and fractionated lignins Thermal decomposition of lignin is known to occur by the cleavage of various bonds at different temperatures, depending on the bond energies (Jakab et al., 1997). For this reason, TG analysis has been widely used to investigate the thermal decomposition behavior and thermal stability of lignin at high temperature. In addition, characterization of the thermal decomposition of lignin is critical for its application in carbon fibre, lignin-polymer composites, and etc. Fig. 1(a) shows the thermograms of the SCL and fractionated lignins. As fractionation proceeded, the char residue of the respective fractions increased (i.e., from F1 (28%) to F4 (36%)), and the thermal stability of the lignin increased with an increase in the MW. The thermal stability may be influenced by the structure and the degree of condensation and branching (Yoshida et al., 1987). Jakab et al. (1997) reported that lignin with a low methoxyl content produced the highest char yield and hardwood lignin had the lowest char yield, where the latter has a high methoxyl content. Furthermore, Wang et al. (2015) investigated the pyrolysis behaviour of various lignins and reported that the final char residue decreased with an increase in the methoxyl content. Therefore, it is considered that lignin fractions such as
F3 and F4 have higher thermal stability because of their high MW and low content of functional groups. However, FIN generated 5.9% char residue, and based on the Py-GC/MS and FT-IR data, most of the carbohydrate-derived material was thermally decomposed, leading to low residue. DTG curves of the lignins in Fig. 1(b) provided the corresponding rate of weight loss. The maximum degradation temperatures (TM) and glass transition temperatures (Tg) of the SCL and fractionated lignins are listed in Table 4. All lignins began to deteriorate around 180℃, and the TM of the lignins, except for F1, was observed around 360℃, related to cleavage of the methyl-aryl ether bonds (Sun et al., 2000). The thermogram of F1 showed a TM of about 386℃ with a shoulder near 360℃. According to Park et al. (2018), carbon-carbon linkages are cleaved at 370−400℃. The shift of the TM to high temperature may be attributed to a more stable condensed structure. Therefore, it is assumed that F1 has a relatively larger percentage of condensed structures. Although the β-O-4 structure is the most abundant linkage, the number of C-C bonds is very important in the structure of lignin. In addition, the C-C linkages, which involve the C5 in the aromatic ring, are the most abundant among the C-C bonds (Tejado et al., 2007). In the case of the G units, it is possible to form this C-C linkage; however, this linkage is rarely present in the S units because of the methoxyl group at the C5 position (Tejado et al., 2007). Therefore, it is considered that the F1 fraction with a lower MW had a higher TM, as a relatively larger amount of the G units formed condensed C-C linkages, corresponding to the Py-GC/MS data. DSC is the method most widely used to determine the Tg of lignin. Generally, the Tg is affected by various factors, including the MW, water content, and crystalline phase (Gregorova, 2013). Fig. 2 and Table 4 show the DSC thermograms and Tg of the SCL and fractionated lignins. The Tg of F1 was observed around 83℃, whereas the Tg of the other fractions was higher than 120℃. Thus, the Tg increased with increasing MW. Gregorova
(2013) reported that polymers with low MW may have a lower Tg due to the increase in the polymer free volume. In the polymer, the interior units within the chain are restrained by two covalent bonds, whereas the chain ends are tied down on one end (Bair et al., 2014). Thus, the chain ends have more free volume than the covalently bonded units within the chain because of their higher mobility (Bair et al., 2014). This means that a polymer with lower MW, which has more chain ends, has greater free volume and a lower Tg. Therefore, it is considered that F1 had the lowest Tg because the polymer free volume increased due to the lower MW.
4. Conclusions In this study, crude SCL extracted from the residue generated after SCW pretreatment of hardwood was fractionated using organic solvents and characterized. The functional group (methoxyl and phenolic hydroxyl groups) content and S/G ratio differed for each fraction, indicative of different chemical compositions. The fraction with a higher molecular weight exhibited improved thermal stability, associated with the lignin structure in the narrow MW range. Therefore, it can be proposed that more homogeneous lignin has great potential for application in lignin-based materials with specific chemical and physical properties.
E-supplementary data for this work can be found in e-version of this paper online.
Acknowledgements This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No. 2018R1A6A1A0325582).
References
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of coal water slurry. Energy Convers. Manag. 48, 2433-2438. [43] Yoshida, H., Mörck, R., Kringstad, K. P., Hatakeyama, H., 1987. Fractionation of kraft lignin by successive extraction with organic solvents. II. Thermal properties of kraft lignin fractions. Holzforsch. 41, 171-176. [44] Yuan, T.Q., He, J., Xu, F., Sun, R.C., 2009. Fractionation and physico-chemical analysis of
degraded lignins from the black liquor of Eucalyptus pellita KP-AQ pulping. Polym. Degrad. Stab. 94, 1142-1150.
List of captions Scheme 1. Schematic diagram of sequential solvent fractionation of crude SCL. Figure 1. TG (a) and DTG (b) thermograms of SCL and fractionated lignins. Figure 2. DSC thermograms of SCL and fractionated lignins.
Table 1. Yield and molecular weight distribution of SCL and fractionated lignins Table 2. Content of methoxyl and hydroxyl groups in SCL and fractionated lignins Table 3. Pyrolysis products of SCL and fractionated lignins Table 4. Maximum degradation temperature and glass transition temperature of SCL and fractionated lignins
Scheme 1.
Figure 1.
Figure 2.
Table 1. Samples
Yield (%)
Mn (g mol-1)
Mw (g mol-1)
PDI (Mw/Mn)
SCL
-
2280
5400
2.4
F1
3.7
1090
1420
1.3
F2
2.0
1780
2380
1.3
F3
41.7
3480
6750
1.9
F4
4.3
9740
18840
1.9
FIN a
Not available
30.6
a
N.A
a
N.A
N.Aa
Table 2. Samples
Methoxyl groups
Phenolic OH
-1
Total OH
(mmol g )
(mmol g )
(mmol g-1)
SCL
3.1
1.1
2.4
3.5
F1
5.0
3.2
1.9
5.1
F2
4.5
2.9
1.9
4.8
F3
4.0
1.3
2.7
4.0
F4
3.5
0.8
2.5
3.3
FIN
0.6
N.Aa
N.Aa
N.Aa
Not available
-1
Aliphatic OH
(mmol g )
a
-1
Table 3. Types
Compounds
Distribution of pyrolysis products (peak area, × 106) SCL
Hydroxyacetone
26.3
F1
F2
F3
F4
FIN
-
-
-
-
109.0
Furfural
-
-
-
-
-
23.7
1,2-Cyclopentanedione
-
-
-
-
-
102.3
-
-
-
-
-
23.8
375.8
-
-
-
-
946.6
-
-
-
-
-
1,2-Cyclopentanedione, 3methylLevoglucosan H-lignin Phenol
8.0
Guaiacol
103.0
81.0
81.7
72.0
74.9
46.5
4-Methylguaiacol
118.7
234.7
248.4
201.9
182.6
13.6
4-Ethylguaiacol
51.9
92.8
93.7
78.2
85.9
4-Vinylguaiacol
112.9
128.5
147.8
116.4
98.3
Orcinol, monomethyl ether
-
-
-
136.2
143.0
-
-
21.0
-
25.4 -
G-lignin Isoeugenol 4-Propylguaiacol
S-lignin
79.9 -
120.2 25.1
132.6 -
15.3 -
Acetoguaiacone
34.1
45.1
44.9
38.5
29.7
-
Vanillin
58.4
66.9
66.6
54.4
55.2
-
Vanillic acid
248.1
388.5
415.6
448.2
424.2
26.5
3,4-Dimethoxyphenol
364.3
330.4
346.3
364.3
361.0
48.9
4-Allylsyringol
332.2
321.8
351.0
461.1
383.1
44.5
Syringylaldehyde
145.7
151.2
159.3
173.1
189.8
83.0
112.2
117.8
124.4
112.3
15.3
-
-
-
-
11.7
Acetosyringone Benzeneacetic acid
-
Sinapinaldehyde
59.3
Phenol, 4,4'-methylenebis [2,6-dimethoxyC-lignin Pyrocatechol
17.6
27.1
77.2
-
94.0
-
26.2
-
-
-
-
-
93.7
107.9
118.9
108.0
110.4
-
-
-
-
Sum(H type)
-
Sum(G type)
807.0
1173.7
1241.7
1175.9
1083.4
127.3
Sum(S type)
1010.7
933.2
1001.5
1200.1
1140.2
120.4
Total
2313.5
2214.8
2370.1
2484.0
2334.0
1453.1
1.25
0.80
0.81
1.02
1.05
0.95
S/G ratio
-
8.0
Table 4. Samples
Max. degradation temp. (℃)
Glass transition temp. (℃)
SCL
367
119
F1
386
83
F2
368
123
F3
361
123
F4
363
137
FIN
373
N.Da
a
Not detected
Highlights Supercritical lignin was sequenced into five lignin fractions using four organic solvents. Each fraction represent different chemical compositions. The fraction with a higher MW exhibited improved thermal stability.
S0960-8524(18)31747-4 https://doi.org/10.1016/j.biortech.2018.12.076 BITE 20833
To appear in:
Bioresource Technology
Received Date: Revised Date: Accepted Date:
15 November 2018 21 December 2018 22 December 2018
Please cite this article as: Ho Seo, J., Jeong, H., Lee, H.W., Choi, C.S., Bae, J.H., Lee, S.M., Kim, Y.S., Characterization of solvent-fractionated lignins from woody biomass treated via supercritical water oxidation, Bioresource Technology (2018), doi: https://doi.org/10.1016/j.biortech.2018.12.076
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Characterization of solvent-fractionated lignins from woody biomass treated via supercritical water oxidation
Jin Ho Seoa, Hanseob Jeongb, Hyung Woon Leeb, Cheol Soon Choic, Jin Ho Baec, Soo Min Leeb, * and Yong Sik Kima,c, *
a
The Institute of Forest Science, College of Forest and Environmental Sciences, Kangwon
University, Chuncheon, 24341, Republic of Korea b
Wood Chemistry Division, National Institute of Forest Science, Seoul, 02455, Republic of
Korea c
Div. of Forest Material Science & Engineering, College of Forest and Environmental
Sciences, Kangwon University, Chuncheon, 24341, Republic of Korea
* Co-corresponding author at: Wood Chemistry Division, National Institute of Forest Science. E-mail address: [email protected] (S.M. Lee). * Corresponding author at: Div. of Forest Material Science & Engineering, College of Forest and Environmental Sciences, Kangwon Nation University. E-mail address: [email protected] (Y.S. Kim).
Abstract Crude supercritical lignin (SCL) extracted from hardwood (Quercus mongolica) treated via supercritical water (SCW) oxidation was subjected to sequential fractionation with four organic solvents; five lignin fractions (F1−F4 and FIN) were thus obtained. The molecular weight (MW) of the fractionated lignins gradually increased as fractionation proceeded. However, the content of methoxyl groups and phenolic hydroxyl groups tended to decrease with increasing molecular weight of the lignins. The functional groups of SCL and the fractionated lignins were very similar based on Fourier-transform infrared analysis. The syringyl/guaiacyl ratio (S/G ratio) of the fractionated lignins increased with an increase in the MW. The thermal stability decreased with decreasing MW of the fractionated lignins, and all fractions except for F1 had a maximum degradation temperature of around 360℃. The glass transition temperature (Tg) of the fractions increased from 83℃ to 137℃ with increasing MW.
Keywords: supercritical lignin, fractionation, characterization, molecular weight distribution
1. Introduction Lignin is a component of plants that is a sustainable resource and widely available biopolymer. Lignin is composed of phenylpropanol units, i.e., sinapyl, coniferyl, and pcoumaryl alcohol, wherein each monomer unit forms an amorphous structure with bonds such as 5-5, β-β, α-O-4, and β-O-4 linkages (Jiang et al., 2016). Approximately 70 million tons of lignin is generated from Kraft pulp production worldwide (Orbisresearch, 2017). However, most of the lignin is burned for energy production or disposed in land-fills (Orbisresearch, 2017), and a small amount is isolated from the black liquor for other purposes. Only a small portion of lignin is used for select applications such as materials, fuels, chemicals, dispersants, carbon fibres, and resins (Jiang et al., 2016; Yang et al., 2007; Ragauskas et al., 2014). In recent years, much research has been focused on converting lignin into value-added products due to the sustainability and potential of lignin as a natural resource. However, various applications of lignin have been restricted by its complex and heterogeneous features, such as its broad molecular weight distribution, heterogeneous reactivity, low solubility, amorphous structure, etc. In addition, the structure of lignin may vary considerably based on the pretreatment applied, and the physical and chemical properties may vary depending on the source of the feedstock and process conditions (Jiang et al., 2016). Generally, lignin can be obtained as a co-product from various pretreatment methods such as chemical pulping, biological pretreatment, steam explosion, ionic liquid (IL) and deep-eutectic solvent (DES) processing, and supercritical water (SCW) oxidation (Kumar et al., 2017). Kraft pulping is the most common pretreatment method that generates lignin as a co-product (Wells et al., 2016). Of the other pretreatment methods, biological pretreatment is regarded as a low-energy, efficient, and environmentally safe method compared with conventional chemical and physical methods (Kumar et al., 2017). Steam explosion is the
most commonly used physicochemical pretreatment method (Kumar et al., 2017), whereas the pretreatment methods using ILs and DES are comparatively new techniques. Various studies have been conducted to achieve efficient utilization of lignins extracted via different pretreatments based on the resulting characteristics. The low impurity content of organosolv lignin has spurred its use as a precursor for carbon fibre (Dallmeyer et al., 2010). However, lignin-based carbon fibre, produced by conventional spinning methods, has comparatively inferior mechanical properties and there is still the need to improve fibre production and treatment techniques (Ding et al., 2016). Kraft lignin has been applied in adhesives and various composites, and many studies have used lignosulfonate as dispersants, surfactants, and as a partial substitute for phenol, etc. (Olivares et al., 1988; Faruk et al., 2015; Xu et al., 2017; Alonso et al., 2004). Although technical lignins such as Kraft lignin and lignosulfonate have been explored for various applications, high value-added application of these lignins is limited due to the unique chemical reactivity, heterogeneous structure, and impurities of these lignins (Vishtal et al., 2011). Furthermore, there are still difficulties in the recovery and separation of lignins (Vishtal et al., 2011). The SCW treatment used in this study is very suitable for the saccharification of lignocellulosic biomass because SCW has low density and viscosity, close to those of gases, and an even higher diffusion coefficient than those of liquids (Lachance, 1995). Furthermore, the carbohydrate of lignocellulosic biomass can be saccharified by supercritical reaction over a period of 30−60 s, which is economical. Therefore, it is essential to characterize lignin derived from the SCW oxidation process for efficient utilization of lignin. During the SCW process, lignin is degraded via cleavage of the aryl-ether linkage and propane side chains (Moon et al., 2011). According to Ehara et al. (2002), during the SCW process, lignin with the biphenyl structure is more stable than that with the β-O-4 structure. Jiang et al. (2016) reported that lignin residues from the SCW process can be used as a source of aromatics
production. Varman et al. (2010) characterized the water-soluble and methanol-soluble and insoluble portions derived from oil palm treated by SCW, and Pińkowska et al. (2012) investigated the hydrothermal decomposition characteristics of alkali lignin via SCW. However, most researchers have studied the characteristics of lignin obtained as a co-product without solvent fractionation. Therefore, it is thought that fractionation of the lignin residue obtained from the SCW reaction using various organic solvents is necessary to clarify the characteristics of lignin and improve its homogeneity. Fractionation using organic solvents is an effective method of obtaining a more uniform type of specific lignin. Fractionation with various solvents was previously performed to produce fractions having a specific MW range, branching, or chemical composition (Francuskiewicz, 1994). For instance, Mörck et al. (1986) developed sequential fractionation using dichloromethane, isopropanol, methanol, and a mixture of methanol-dichloromethane as solvents and reported that the low MW fractions showed narrow polydispersity, whereas the high MW fraction was more polydisperse. Birch organosolv lignin, commercial Alcell, and steam-exploded eucalypt and pine lignins were fractionated using ether and acetone as organic solvents (Van de Pas et al. (2011). Thring et al. (1996) reported that Alcell lignins were successively fractionated by ether and methanol. The low MW fraction was more homogenous and the amount of guaiacyl lignin was higher (Thring et al., 1996). The fractionated soda-anthraquinone lignin extracted via ash-AQ pulping of oil palm empty fruit bunch fibre was separated into four fractions by using extraction solvents such as methylene chloride, n-propanol, and methanol/methylene chloride (Sun et al., 2000). It was found that the thermal stability increased as the MW increased, whereas the carbonyl group content of the lignin fractions decreased. Domínguez-Robles et al. (2018) studied a green fractionation method using different ratios of acetone in water, where the obtained fractions were characterized by a specific MW range and functional groups. The proposed acetone
fractionation process definitively generated pure and uniform lignin. Yuan et al. (2009) clearly characterized degraded lignins from Eucalyptus pellita KP-AQ pulping using solvent fractionation, and Li et al. (2012) used five solvents to generate heterogeneous bamboo organosolv lignin. Recently, there has been a growing interest in research on biorefinery using lignocellulosic biomass, and numerous technical lignins generated from various pretreatment methods have been obtained with high utilization of the wood components. Therefore, fractionation into more homogeneous lignin is required for the development of lignin-based biomaterials and/or biochemicals, as technical lignins have different physical and chemical properties based on the pretreatment methods. Thus, separating lignin into fractions with a homogenous molecular weight and size based on their solubility in an organic solvent may be a promising alternative for the effective utilization of technical lignin. In this study, we focus on the fractionation of supercritical lignin (SCL) extracted from Mongolian oak (Quercus mongolica) that was treated via SCW by using a sequence of organic solvents and characterization of the fractionated lignins to provide basic information for efficient utilization of the lignin fractions. The various fractions and SCL are subjected to thermal analyses, including thermogravimetry (TG), differential thermogravimetry (DTG), and differential scanning calorimetry (DSC), to evaluate their thermal decomposition behaviour and stability. Structural and chemical analyses are also performed to determine the S/G ratio, molecular weight distribution, and functional groups of the fractionated lignins.
2. Materials and methods 2.1. Materials Crude SCL was kindly provided by the National Institute of Forest Science. SCL is a hardwood (Quercus mongolica) lignin produced by a continuous pilot-scale SCW system (Hanwha Chemical, Seoul, Republic of Korea). A 50 kg sample of milled biomass (Quercus mongolica) was hydrolyzed each day via SCW treatment using a total flow rate of 130±15 kg h−1 and constant temperature (380±5℃) and pressure (230±5 bar). The solid residue was separated from the hydrolysates by filtration using a 3-micron filter (#BPONG3P1S, Filter Specialists, Inc., Seoul, Republic of Korea) and was freeze-dried for one week using a lyophilizer (FDU-2100, Eyela, Tokyo, Japan) (Jeong et al., 2017). The resulting solid residue was re-dissolved in excess 0.5 M NaOH. The mixture was then filtered to remove any insoluble material. The filtrate was adjusted to ~pH 2.0 using conc. HCl. The precipitated crude SCL was then isolated by filtration, repeatedly washed with fluent deionized water, and dried in a 60℃ oven drier for two weeks. All other chemicals were purchased from either Sigma-Aldrich or Fisher Scientific (USA) and used as received.
2.2. Fractionation of lignin Crude SCL was sequentially fractionated with ethyl acetate, chloroform, acetone, and 96% dioxane. The solvent selection was based on the polarity and the Hildebrand solubility parameter. In this study, the properties of the solvents are as follows: chloroform (P = 2.70, δ = 9.21), ethyl acetate (P = 4.40, δ = 9.10), acetone (P = 5.10, δ = 9.77), and 96% dioxane (P = 5.27, δ = 10.00). Here, P is the polarity index and δ is the Hildebrand solubility parameter. Crude SCL (10 g) was dissolved in 250−400 mL of the various solvents and stirred for 6 h with a magnetic bar at room temperature. The solution was filtered through a Nylon membrane filter (0.45 μm, 45 mm), and the filtrate was evaporated by using a rotary
evaporator. The dried filtrate was dissolved in a small quantity of dioxane and precipitated with cold deionized water. Thereafter, this precipitate was freeze-dried using a freeze-dryer (FDCF-12006, OPERON) to obtain the fractionated lignin powder. The solid residue remaining after filtering was dissolved in the next solvent, and the final insoluble fractions were dried in a 60℃ oven-drier for 2 weeks prior to measuring the dry weight. The respective lignin fractions obtained from solvent fractionation are designated as F1 to F4 and the final solid residue after fractionation with 96% dioxane is designated as FIN. The fractionation process is illustrated in Scheme 1; the recovery factors for all lignin fractions recovered from 10 g of crude SCL were in the 80−85% range.
2.3. Acetylation of lignin SCL and the respective lignin fractions were acetylated for chemical analysis by gel permeation chromatography (GPC) and proton nuclear magnetic resonance (1H-NMR) spectroscopy. About 100 mg of lignin was dissolved in 4 mL of pyridine/acetic anhydride (1:1, v/v) and stirred at room temperature for 24 h. The solution was added dropwise to 200 mL of ice-distilled water with stirring. The precipitated lignin was filtered through a Nylon membrane filter (0.45 μm, 47 mm), washed with flowing cold deionized water, and then freeze-dried using a freeze-dryer (FDCF-12006, OPERON).
2.4. Molecular weight distribution The molecular weight distribution of the lignin samples was determined by gel permeation chromatography (GPC; Shimadzu, UV and RI detectors) after acetylation of each fraction. The acetylated lignins were dissolved in tetrahydrofuran (THF) and analyzed using GPC with PLgel columns (PLgel 5 μm mixed-C, -D, and PLgel 3 μm mixed-E) at 40℃ with UV detection at 280 nm. The mobile phase comprised tetrahydrofuran (THF; 1 mL min−1), and
100 μL of lignin (1 mg mL−1) was injected. Calibration of the molecular weight was performed using polystyrene standards ranging between 1,480 and 1,233,000 g mol−1.
2.5. Analysis of methoxyl content The methoxyl content was analyzed by gas-chromatography/flame ionization detection according to the literature (Park et al., 2018). Lignin (30 mg) was placed in a vial with 4 mL of hydriodic acid and reacted in an oil bath at 130℃ for 30 min with shaking. The solution was cooled in ice-water. An internal standard (200 μL ethyl iodide in pentane, 300 mg mL−1) was added to the solution, followed by 3 mL of pentane. The mixture was shaken and cooled in ice-water. An aliquot of the pentane phase was separated into a GC vial. The methyl iodide was analyzed using gas chromatography / flame ionization detector (GC/FID, 7890B, Agilent) with DB-624 column (30 m × 0.25 mm i.d.). The injector and detector temperature were 200 and 230℃. The oven temperature was held at 40℃ for 5 min and raised to 180℃ at 10 ℃ min−1. The flow rate was 1.1 mL min−1 and the split ratio was 1/120.
2.6. Structural analysis Fourier transform infrared (FT-IR) analysis of the lignins was performed on a PerkinElmer Frontier instrument with an attenuated total reflectance (ATR) attachment. A total of 256 scans per sample was performed from 4000 to 500 cm−1 at a resolution of 4.0 cm−1. FT-IR absorbance spectra were directly obtained using lignin samples fully oven-dried at 40℃. Quantitative 1H NMR analysis was performed on a FT-NMR 600 MHz spectrometer (Bruker Avance Ⅱ600) to measure the amount of both aliphatic and phenolic hydroxyl groups. Acetylated lignin (20 mg) was dissolved in 1 mL of CDCl3. A total of 128 scans per sample was performed at 300 K with a 1.3 s acquisition time, a 90° pulse, and a 7 s relaxation delay. The relative integral of the hydroxyl group signal was analyzed by setting the integral of the
methoxyl region (4.0−3.5 ppm) to 3.0. Then, the hydroxyl content (mmol g−1) was calculated using a following Eq. (1). (1) Where A was the methoxyl content (mmol g−1) determined in analysis of methoxy content by gas-chromatography. B and C were the integral values of hydroxyl group and methoxyl group, respectively, as determined by quantitative 1H NMR analysis.
2.7. Analysis of pyrolysis products The lignin types in SCL and in each fraction and S/G ratio were determined by pyrolysis gas chromatography/mass spectrometry (Py-GC/MS). A pyrolyzer (Py-2020iD, Frontier lab)−GC (5975C, Agilent Technologies)/MS (7890A, Agilent Technologies) instrument equipped with a DB-5MS (30 m × 0.25 mm × 0.25 μm) column was used to analyze the pyrolysis products of the lignins. The S/G ratio was calculated by dividing the sum of the decomposition products corresponding to the units of guaiacyl (G) and syringyl (S) in the pyrolysis products. A 0.5 mg sample was placed in a deactivated SUS cup and was allowed to free-fall into a furnace preheated to 550oC. The pyrolysis vapour was introduced into the separation column after the GC inlet at a ratio of 1/70 (column flow: 0.7 mL min−1). During GC separation, the GC oven temperature was held at 40℃ for 5 min and raised to 310℃ at 10 ℃ min−1, and held at 310℃ for 10 min; the separated pyrolysis products were detected by a MS detector. The pyrolysis compounds were identified using the National Institute of Standards and Technology (NIST) mass spectral library.
2.8. Thermal analysis The thermal characteristics of the lignins were analyzed by thermogravimetric analysis (TGA 8000, Perkin Elmer). The temperature was raised to 650℃ in increments of 20 ℃ min−1; the
sample carrier gas was He. DSC analysis was performed with a DSC Q2000 (TA Instrument, UK) apparatus. The first scan was performed from 20 to 105℃ at 10 ℃ min−1, and the temperature was held for 10 min, followed by cooling to 20℃ at 10 ℃ min−1. The second and third scans were performed from 20 to 180℃ at 5 ℃ min−1. The glass transition temperature (Tg) was determined from the second scan.
3. Results and Discussion 3.1. Yields and molecular weight distributions of SCL and fractionated lignins The yields and MW of the fractionated lignins are shown in Table 1. The yields of the F3 and FIN fractions were relatively higher than those of the other fractions, and the yields of each fraction varied. It is assumed that the solubility of the lignins of different structures differs in the organic solvents (Park et al., 2018). In order to analyze the molecular weight distributions of the SCL and fractionated lignins, each lignin sample was acetylated before GPC analysis. The MW of each fraction gradually increased from F1 to F4 (1420 to 18840 g mol−1). The MW of SCL was 5400 g mol−1 based on the weight average molecular weight (Mw). The polydispersity indices (PDIs) of SCL, F1, F2, F3, and F4 were 2.4, 1.3, 1.3, 1.9, and 1.9, respectively, indicating that the respective fractions were more homogenous than SCL (Table 1). These results suggests that the MW distribution of lignin can be more homogenized by separating lignin having a large MW distribution into specific MW ranges via solubility-based fractionation using various organic solvents; similar results were reported in studies by Park et al. (2018) and Kim et al. (2017). However, it was impossible to analyze the molecular weight distribution of FIN due to its insolubility in any organic solvent. It is assumed that crude SCL contained a small amount of unreacted wood powder. Thus, GPC analysis of the acetylated SCL was carried out by filtering out the solid component after acetylation of SCL.
3.2. Analysis of chemical structure of SCL and fractionated lignins Analysis of the FT-IR spectra was based on the assignments given by Lisperguer et al. (2009) and Boeriu et al. (2004). All fractions showed a broad band at 3600−3000 cm−1, attributed to the hydroxyl groups of the aliphatic and phenolic structures. The bands between 2850 and 2830 cm−1 are predominantly attributed to CH stretching in the methylene and methyl groups. The peaks below 2000 cm−1 are derived from multiple functional groups; therefore, analysis of the region below 2000 was very difficult. Weak and medium bands were found at 1720−1660 cm−1, derived from the unconjugated C=O of the carbonyl groups. The profile of the aromatic ring bands at 1510−1500 cm−1 was similar for SCL and the F1−F4 fractions, whereas that of FIN differed. The CH bending signals were observed between 1460 and 1450 cm−1. A strong band was observed at 1120−1110 cm−1, associated with the phenolic hydroxyl group. Weak bands were observed in the 1030−1020 cm−1 spectral region for SCL and the F1−F4 fractions, but not for FIN, due to the C-O deformation in the methoxyl groups. The spectrum of FIN appeared similar to that of cellulose (Auta et al., 2017). Thus, the final insoluble fraction, FIN, was considered to be a carbohydrate comprising wood powder that remained unreacted during the SCW process. These results suggest that some of the unreacted solids could not be completely removed during the extraction of lignin from the solid residues obtained after SCW process of the lignocellulosic biomass. Therefore, for analysis of the chemical content and for acquisition of the FT-IR spectrum of SCL, crude SCL was dissolved in 96% dioxane to remove the unreacted wood powder and the purified SCL sample was analyzed.
3.3 Determination of functional groups of SCL and fractionated lignins The methoxyl group and both phenolic and aliphatic hydroxyl groups are the specific
functional groups in lignin. The methoxyl groups present in each fraction were quantified using gas chromatography according to a reported method (Park et al., 2018). As fractionation proceeded, the content of methoxyl groups gradually decreased from F1 (5.0 mmol g−1) to F4 (3.5 mmol g−1) (Table 2). The decrease in the content of methoxyl groups is considered to be closely related to the greater insolubility of the S unit in organic solvents (Thring et al., 1996). However, FIN mostly comprised unreacted wood powder, as identified by FT-IR analysis, resulting in a lower methoxyl content (Table 2). The hydroxyl group of lignin is known to be the most important functional group for lignin applications (Li et al., 2012). The phenolic and aliphatic hydroxyl groups in the SCL and fractionated lignins were determined by quantitative 1H NMR, as shown in Table 2. The total content of hydroxyl groups in each fraction followed a trend similar to that of the methoxyl groups, showing a decrease from F1 (5.1 mmol g−1) to F4 (3.3 mmol g−1). However, the content of phenolic hydroxyl groups and aliphatic hydroxyl groups followed opposite trends. The content of phenolic hydroxyl groups tended to decrease from F1 (3.2 mmol g−1) to F4 (0.8 mmol g−1) in proportion to the total hydroxyl content, whereas the content of aliphatic hydroxyl groups tended to increase slightly from F1 (1.9 mmol g−1) to F4 (2.5 mmol g−1). These results are similar to those of other studies, where when the content of phenolic hydroxyl groups was high, lignin became more soluble in organic solvents (Sameni et al., 2017). In addition, the greater cleavage of the β-O-4 linkages in the low molecular weight fraction resulted in a higher content of phenolic hydroxyl groups as terminal groups (Ehara et al., 2002). Generally, more hydroxyl groups facilitate the interaction of lignin with various polymers (Li et al., 2012). Therefore, a lower MW fraction with a higher phenolic hydroxyl group content is more suitable for applications in various fields. Qin et al. (2015) reported that grafted sulfonated alkali lignin was better than naphthalene sulfonate for reducing the
viscosity of coal-water slurry as a dispersant. Shweta et al. (2015) used lignin for nickel removal via physical adsorption through the oxygen-containing groups in lignin. Lignin was also incorporated into polybutylene succinate by Ferry et al. (2015), and this modified lignin slightly improved the flammability parameters. Moreover, the methoxyl group and hydroxyl groups are known to contribute to the antioxidant ability of lignin, and a higher methoxyl content in the lignin was favourable for stabilizing phenoxyl radicals (Li et al., 2012). Moreover, Gregorova et al. (2007) reported that a high content of methoxyl groups in low MW lignin induced an increase in the antiradical and antioxidant efficiency. Therefore, it is considered that the lignin fraction with a low MW and higher methoxyl content may be potentially applicable in lignin-based biomaterials or as an antioxidant chemical.
3.4. Pyrolysis products of SCL and fractionated lignins Table 3 shows the pyrolysis products of SCL and each fraction. The main pyrolysis products of each fraction (except FIN) were guaiacol, 4-methylguaiacol, 4-ethylguaiacol, 4vinylguaiacol, isoeugenol, acetoguaiacone, vanillin, and 4-hydroxy, 3-methoxy, and benzoic acid groups for the G type, and 3,4-dimethoxyphenol, 4-allysyringol, syringylaldehyde, acetosyringone, and sinapinaldehyde for the S type. FIN was mainly composed of carbohydrate degradation products such as hydroxyacetone, 1,2-cyclopentanedione, and levoglucosan. Thus, this result supports the FT-IR analysis of FIN presented above, indicating that FIN contained cellulose. The S/G ratio of each fraction from the pyrolysis products was investigated by Py-GC/MS analysis. The S/G ratio of each fraction ranged from 0.8 to 1.05, whereas the S/G ratio of SCL was relatively high at 1.25 (Table 3). ). Among the G units, vanillic acid was the major pyrolysis product, whereas 3,4-dimethoxyphenol and 4-allylsyringol were detected as the major pyrolysis products in the S units. Overall, the components of the G units varied more
than those of the S units in the lignin pyrolysis products. In the fractions with lower MW, peak area of the G units outweighed that of the S units, but there were larger peak area of the S units in the fractions with higher MW. Further, as fractionation proceeded, both peak area of the G and S units increased; however, the S/G ratio increased slightly because peak area of the S units was slightly larger than that of G units. It was assumed that smaller amounts of S units dissolved in the organic solvent (which had relatively low polarity among the organic solvents used in this study) due to the insolubility of the S units, and organic solvents with a higher polarity could better dissolve the S units (Tring et al., 1996).
3.5. Thermal analysis of SCL and fractionated lignins Thermal decomposition of lignin is known to occur by the cleavage of various bonds at different temperatures, depending on the bond energies (Jakab et al., 1997). For this reason, TG analysis has been widely used to investigate the thermal decomposition behavior and thermal stability of lignin at high temperature. In addition, characterization of the thermal decomposition of lignin is critical for its application in carbon fibre, lignin-polymer composites, and etc. Fig. 1(a) shows the thermograms of the SCL and fractionated lignins. As fractionation proceeded, the char residue of the respective fractions increased (i.e., from F1 (28%) to F4 (36%)), and the thermal stability of the lignin increased with an increase in the MW. The thermal stability may be influenced by the structure and the degree of condensation and branching (Yoshida et al., 1987). Jakab et al. (1997) reported that lignin with a low methoxyl content produced the highest char yield and hardwood lignin had the lowest char yield, where the latter has a high methoxyl content. Furthermore, Wang et al. (2015) investigated the pyrolysis behaviour of various lignins and reported that the final char residue decreased with an increase in the methoxyl content. Therefore, it is considered that lignin fractions such as
F3 and F4 have higher thermal stability because of their high MW and low content of functional groups. However, FIN generated 5.9% char residue, and based on the Py-GC/MS and FT-IR data, most of the carbohydrate-derived material was thermally decomposed, leading to low residue. DTG curves of the lignins in Fig. 1(b) provided the corresponding rate of weight loss. The maximum degradation temperatures (TM) and glass transition temperatures (Tg) of the SCL and fractionated lignins are listed in Table 4. All lignins began to deteriorate around 180℃, and the TM of the lignins, except for F1, was observed around 360℃, related to cleavage of the methyl-aryl ether bonds (Sun et al., 2000). The thermogram of F1 showed a TM of about 386℃ with a shoulder near 360℃. According to Park et al. (2018), carbon-carbon linkages are cleaved at 370−400℃. The shift of the TM to high temperature may be attributed to a more stable condensed structure. Therefore, it is assumed that F1 has a relatively larger percentage of condensed structures. Although the β-O-4 structure is the most abundant linkage, the number of C-C bonds is very important in the structure of lignin. In addition, the C-C linkages, which involve the C5 in the aromatic ring, are the most abundant among the C-C bonds (Tejado et al., 2007). In the case of the G units, it is possible to form this C-C linkage; however, this linkage is rarely present in the S units because of the methoxyl group at the C5 position (Tejado et al., 2007). Therefore, it is considered that the F1 fraction with a lower MW had a higher TM, as a relatively larger amount of the G units formed condensed C-C linkages, corresponding to the Py-GC/MS data. DSC is the method most widely used to determine the Tg of lignin. Generally, the Tg is affected by various factors, including the MW, water content, and crystalline phase (Gregorova, 2013). Fig. 2 and Table 4 show the DSC thermograms and Tg of the SCL and fractionated lignins. The Tg of F1 was observed around 83℃, whereas the Tg of the other fractions was higher than 120℃. Thus, the Tg increased with increasing MW. Gregorova
(2013) reported that polymers with low MW may have a lower Tg due to the increase in the polymer free volume. In the polymer, the interior units within the chain are restrained by two covalent bonds, whereas the chain ends are tied down on one end (Bair et al., 2014). Thus, the chain ends have more free volume than the covalently bonded units within the chain because of their higher mobility (Bair et al., 2014). This means that a polymer with lower MW, which has more chain ends, has greater free volume and a lower Tg. Therefore, it is considered that F1 had the lowest Tg because the polymer free volume increased due to the lower MW.
4. Conclusions In this study, crude SCL extracted from the residue generated after SCW pretreatment of hardwood was fractionated using organic solvents and characterized. The functional group (methoxyl and phenolic hydroxyl groups) content and S/G ratio differed for each fraction, indicative of different chemical compositions. The fraction with a higher molecular weight exhibited improved thermal stability, associated with the lignin structure in the narrow MW range. Therefore, it can be proposed that more homogeneous lignin has great potential for application in lignin-based materials with specific chemical and physical properties.
E-supplementary data for this work can be found in e-version of this paper online.
Acknowledgements This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No. 2018R1A6A1A0325582).
References
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List of captions Scheme 1. Schematic diagram of sequential solvent fractionation of crude SCL. Figure 1. TG (a) and DTG (b) thermograms of SCL and fractionated lignins. Figure 2. DSC thermograms of SCL and fractionated lignins.
Table 1. Yield and molecular weight distribution of SCL and fractionated lignins Table 2. Content of methoxyl and hydroxyl groups in SCL and fractionated lignins Table 3. Pyrolysis products of SCL and fractionated lignins Table 4. Maximum degradation temperature and glass transition temperature of SCL and fractionated lignins
Scheme 1.
Figure 1.
Figure 2.
Table 1. Samples
Yield (%)
Mn (g mol-1)
Mw (g mol-1)
PDI (Mw/Mn)
SCL
-
2280
5400
2.4
F1
3.7
1090
1420
1.3
F2
2.0
1780
2380
1.3
F3
41.7
3480
6750
1.9
F4
4.3
9740
18840
1.9
FIN a
Not available
30.6
a
N.A
a
N.A
N.Aa
Table 2. Samples
Methoxyl groups
Phenolic OH
-1
Total OH
(mmol g )
(mmol g )
(mmol g-1)
SCL
3.1
1.1
2.4
3.5
F1
5.0
3.2
1.9
5.1
F2
4.5
2.9
1.9
4.8
F3
4.0
1.3
2.7
4.0
F4
3.5
0.8
2.5
3.3
FIN
0.6
N.Aa
N.Aa
N.Aa
Not available
-1
Aliphatic OH
(mmol g )
a
-1
Table 3. Types
Compounds
Distribution of pyrolysis products (peak area, × 106) SCL
Hydroxyacetone
26.3
F1
F2
F3
F4
FIN
-
-
-
-
109.0
Furfural
-
-
-
-
-
23.7
1,2-Cyclopentanedione
-
-
-
-
-
102.3
-
-
-
-
-
23.8
375.8
-
-
-
-
946.6
-
-
-
-
-
1,2-Cyclopentanedione, 3methylLevoglucosan H-lignin Phenol
8.0
Guaiacol
103.0
81.0
81.7
72.0
74.9
46.5
4-Methylguaiacol
118.7
234.7
248.4
201.9
182.6
13.6
4-Ethylguaiacol
51.9
92.8
93.7
78.2
85.9
4-Vinylguaiacol
112.9
128.5
147.8
116.4
98.3
Orcinol, monomethyl ether
-
-
-
136.2
143.0
-
-
21.0
-
25.4 -
G-lignin Isoeugenol 4-Propylguaiacol
S-lignin
79.9 -
120.2 25.1
132.6 -
15.3 -
Acetoguaiacone
34.1
45.1
44.9
38.5
29.7
-
Vanillin
58.4
66.9
66.6
54.4
55.2
-
Vanillic acid
248.1
388.5
415.6
448.2
424.2
26.5
3,4-Dimethoxyphenol
364.3
330.4
346.3
364.3
361.0
48.9
4-Allylsyringol
332.2
321.8
351.0
461.1
383.1
44.5
Syringylaldehyde
145.7
151.2
159.3
173.1
189.8
83.0
112.2
117.8
124.4
112.3
15.3
-
-
-
-
11.7
Acetosyringone Benzeneacetic acid
-
Sinapinaldehyde
59.3
Phenol, 4,4'-methylenebis [2,6-dimethoxyC-lignin Pyrocatechol
17.6
27.1
77.2
-
94.0
-
26.2
-
-
-
-
-
93.7
107.9
118.9
108.0
110.4
-
-
-
-
Sum(H type)
-
Sum(G type)
807.0
1173.7
1241.7
1175.9
1083.4
127.3
Sum(S type)
1010.7
933.2
1001.5
1200.1
1140.2
120.4
Total
2313.5
2214.8
2370.1
2484.0
2334.0
1453.1
1.25
0.80
0.81
1.02
1.05
0.95
S/G ratio
-
8.0
Table 4. Samples
Max. degradation temp. (℃)
Glass transition temp. (℃)
SCL
367
119
F1
386
83
F2
368
123
F3
361
123
F4
363
137
FIN
373
N.Da
a
Not detected
Highlights Supercritical lignin was sequenced into five lignin fractions using four organic solvents. Each fraction represent different chemical compositions. The fraction with a higher MW exhibited improved thermal stability.