Structural Characterization of Lignin Isolated from Wheat-Straw during the Alkali Cooking Process
Xiaojun Tian,a,# Bing Wang,b,# Bin Wang,a,* Jinpeng Li,a and Kefu Chen a
To investigate the behavior of lignin during the alkali cooking process with different alkali doses, this work demonstrated the structural characteristics illustrated by spectroscopic analyses. Gel permutation chromatography (GPC) and nuclear magnetic resonance (NMR) indicated that the lignin was composed of typical structures for a grass, generally with S and G units and small amounts of H units. The main substructures present were β-O-4 aryl ether linkages, and there were lower amounts of β-β and β-5 linkages. Alkali treatment conditions had evident effects on the chemical structures and properties of lignin. Moreover, NMR indicated that alkali cooking caused lignin to depolymerize more easily with increasing severity and to condense.
Keywords: Alkali cooking; Lignin; Structural characterization; Wheat-straw
Contact information: a: Plant Micro/Nano Fiber Research Center, State Key Laboratory of Pulp and Paper Engineering, School of Light Industry and Engineering, South China University of Technology, Guangzhou 516640, China; and b: Beijing Key Laboratory of Lignocellulosic Chemistry, Beijing Forestry University, Beijing, 100083, China;
* Corresponding author: firstname.lastname@example.org; # Contributed equally to this work
With the inevitable depletion of fossil fuels and increasing environmental concerns, lignocelluloses, such as agricultural and forest residues, are viewed as sustainable alterative resources for producing liquid fuels and chemicals because of their abundance and sustainability (Balat and Balat 2009; Lu and Ralph 2011). The main components of lignocellulosic biomass are cellulose, hemicelluloses, and lignin, and these biopolymers can be converted to biofuels (bioethanol) and bio-based chemicals by employing either a thermo-chemical or biological approach (Boateng et al. 2008).
Traditionally, cellulose fibers are used to produce pulp, paper, and bioethanol. Cellulose fibers are chemically or mechanically separated from lignocelluloses such as wheat straw. It is considered as a suitable raw material for pulp and biofuels production, owing to its characteristics, such as annual, abundance, renewability, and high-sugar content (Saini et al. 2015). Meanwhile, plentiful amounts of lignin are generated as a by-product with modified and degradative structures in the pulping technology, for example: alkali cooking, which is one important process (Stark 2011). As the second most abundant natural polymer, lignin accounts for 25% to 30% of biomass. It is the only renewable source of aromatic compounds for the chemical industry, and it presents both a significant opportunity and many challenges for enhancing the operation of a lignocellulosic biorefinery (Tuck et al. 2012; Upton and Kasko 2015). Its potential, however, is currently underutilized. Approximately 2% of the lignin extracted from pulping processes is used for high-value products, while the rest is burned as a low-value fuel (Stewart 2008; Liu et al. 2012; Kumar and Murthy 2013).
Structural information of lignin is particularly important for evaluating its utilization. Understanding of lignin structural features will facilitate lignin recovery and enable catalytic modifications to obtain desired chemicals and physical properties (Duval and Lawoko 2014). Lignin is a structurally complex polyphenolic biomacromolecule consisting primarily of three basic structural phenylpropane units: syringyl (S), guaiacyl (G), and p-hydroxyphenyl (H). Lignin is composed of different substructures, including the ether (β-O-4, α-O-4, and 4-O-5) and C-C linkages (β-β, β-5, β-1, and 5-5), and the proportion of β–O-4 linkages of the lignin obviously affects the distribution of degraded products and yield of depolymerized monomeric products (Bouxin et al. 2015). Each of these substructures varies according to the biological species and the chemical method used for its extraction (Zhang et al. 2016).
Lignin plays an important role in defense and water transport in vascular terrestrial plants (Zeng et al. 2013). Agricultural residues, such as cereal straws, are potential materials for the pulp and paper industry because their major constituents are comparable to wood species. In particular, wheat straw is used extensively as a raw material for pulp and paper. Generally, a large amount of lignin is generated during the pulping process, and both the structure and the chemical properties are modified. Therefore, it is essential to analyze the structure of lignin that is extracted from the pulp residues of wheat straw.
Several methods have been developed to separate lignin from lignocellulosic biomass, including chemical (dilute acid or alkali, organosolv, and ionic liquid), physical (mechanical extrusion), physico-chemical (steam explosion), and biological (white and soft rot fungi) (Kapoor et al. 2015). Alkaline treatment is the most widespread and cost-effective method for isolating lignin (mainly alkali lignin). Generally, alkali pretreatment involves the use of bases, such as sodium, potassium, calcium, and ammonium hydroxide, leading to lignin extraction by breaking hydrogen and other covalent bonds. Mild alkaline treatment, such as aqueous NaOH, causes the slight destruction of lignin structures that have relatively small molecular weight and still contain appreciable amounts of phenolic hydroxyl groups. Although the lignins in wood and non-wood plants share the basic chemical structure, the structural properties of gramineae lignin differ from those of softwood or hardwood lignins. For instance, the lignins isolated from graminaceous plants contain hydroxycinnamic acids, such as p-coumaric and ferulic acids (Sun et al. 2000b; Sun and Tomkinson 2002).
In most of the current pulping technologies, delignification consists of degradation and dissolution, which cleaves the native lignin into soluble fragments to separate it from the fibers (Gandini 2008). The important reactions during the alkaline pulping include the cleavage of phenolic α-O-4 linkages and non-phenolic β-O-4 linkages, and the removal of residual lignin fractions by cleavage of carbon-carbon linkages or by carbohydrate degradation, releasing lignin-carbohydrate fractions (Sun et al. 2000a). Though lignin has a negative impact on paper manufacturing, it has shown potential in other areas, such as a constituent of polymeric materials and in chemicals (Wang et al. 2016).
This study investigated the detailed chemical changes taking place in wheat straw lignin when wheat straw was cooked with varying concentrations of sodium hydroxide and sodium sulfide. The effect of temperature on the structural features of lignin was also analyzed. The structural information of the lignin fractions obtained was investigated by gel permeation chromatography (GPC), quantitative 13C, two-dimensional heteronuclear single-quantum coherence (2D-HSQC), and 31P nuclear magnetic resonance (NMR) spectroscopy.
Wheat straw was provided by Shandong Tranlin Group Co., Ltd. (Shandong, China). The material was cut into pieces (approximately 20 mm long) and washed with tap water to remove soil particles. The washed wheat straw was air-dried and stored in a self-sealing plastic bag. Solid alkalis of sodium hydroxide and sodium sulphide powder with a purity of over 96.0% were purchased from Tianjin Kermel Reagent Co. Ltd., Tianjin, China. Deuterated dimethyl sulfoxide (DMSO- d6) with a purity of over 99.9% was obtained from Sigma-Aldrich (Shanghai, China).
Water (18.2 MΩ) was purified with a Millipore Milli-Q system (Millipore Direct-Q5, Billerica, MA, USA). All other analytical reagent grade chemicals were obtained from Sinopharm Chemical Reagent Co. (Shanghai, China) and used without further purification.
Alkali Cooking Process
Twenty-five g dried wheat straw were loaded into a horizontal rotary digester (KRK 2611, Kyoraku Co. Ltd., Osaka, Japan) with a liquid/solid ratio of 10:1 (w/w). The added proportion of sodium hydroxide and sodium sulphide (based on oven-dried (od) raw material) was according to the cooking conditions as listed in Table 1.
The cooker was sealed, heated to 155 °C with a heating rate of 1 °C/min, and held at the target temperature (Table 1) for 30 min. When the cooking process completed, the black liquor was collected by filtration with an 80-mesh screen.
Table 1. Parameters of Alkali Cooking
Precipitation of Lignin
The black liquor was concentrated to approximately 30 mL using rotary evaporation, and 90 mL ethanol was added into the concentrated black liquor to ensure the ethanol concentration up to 70%. The sediment was removed by a G2 glass sand funnel filter, and the filtrate was concentrated to 10 mL. To completely precipitate the lignin, the solution was adjusted to pH 2 by slowly adding hydrochloric acid (1 mol/L). The precipitate was washed with deionized water to a neutral pH and collected by centrifugation at 10000 rpm for 10 min. After separation and purification, the resultant lignin was dried into powder at 40 °C for 24 h.
The associated polysaccharides in lignin were determined using high performance anion exchange chromatography (HPAEC) as reported previously (Sun et al. 2014). The weight-average (Mw) and number-average (Mn) molecular weights of the lignin fractions were determined by GPC, which were based on the procedures described in a previous literature (Sun et al. 2014). A PL-gel 10 mm mixed-B 7.5 mm i.d. column (Polymer Laboratories, Amherst, MA, USA) was calibrated with PL polystyrene standards. The lignin samples (2 mg) without acetylation were dissolved in 4 mL of tetrahydrofuran (THF), and 20 μL lignin solutions were injected. The solution-state NMR spectra of the lignin samples were acquired on a 400 MHz NMR spectrometer (AVIII 400, Bruker, Karlsruhe, Germany) at 25 °C using 140 mg of lignin in 0.5 mL of DMSO-d6 for 13C NMR spectra and 40 mg of lignin in 0.5 mL of DMSO-d6 for HSQC NMR spectra (Holtman et al. 2006; Rencoret et al. 2008). 31P-NMR spectra were used to detect the functional groups (aliphatic hydroxyl, phenolic hydroxyl, and carboxyl groups) of the lignin as previously described (Granata and Argyropoulos 1995).
RESULTS AND DISCUSSION
Associated Polysaccharides of the Lignin Fractions
To verify the purity of the lignin extracted from the wheat straw, the associated polysaccharides of the lignin samples were determined by HPAEC (Table 2). The five lignin samples contained a low amount of associated polysaccharides, ranging from 0.47% to 1.22%, implying that the lignin had a higher purity. Glucose (0.26% to 0.65%) and xylose (0.15% to 0.65%) were the major sugar constituents of the five lignin fractions. Arabinose was identified as a minor sugar constituent, and galactose was only detected in L3. In addition, glucuronic and galacturonic acids were not detected in any lignin fractions. The glucose content decreased with increasing temperature. In contrast, xylose content increased with elevating temperature. Furthermore, the amount of total sugars decreased as the alkali concentration and temperature increased, suggesting that the ether and ester linkages between lignin and hemicelluloses in the cell walls of wheat straw were cleaved under the conditions given.
Table 2. Contents of Associated Polysaccharides of the Lignin Fractions Obtained from Wheat Straw
a Represents the total associated carbohydrates in the lignin fractions (% dried lignin)
b ND, not detected
Molecular Weight Analysis of the Lignin Fractions
The weight-average (Mw), number-average (Mn) molecular weights, and polydispersity (Mw/Mn) of the lignin fractions were obtained by GPC (Table 3). The increase of alkaline concentration from 6 to 10% led to the increase of Mw from 2140 to 2510 g/mol. The reason for this increase may be that higher content of S-type lignin fractions were extracted, as evidenced by the 2D-HSQC NMR spectra. However, as the alkaline concentration further increased to 14%, the Mw of lignin (L5) then decreased to 1740 g/mol, suggesting that the depolymerization possibly occurred at a relatively high alkaline concentration. With the extension of time from 60 to 100 min, the Mw of the lignin increased from 2150 to 3080 g/mol. However, the Mw of lignin (L3) then decreased to 2510 g/mol as the time extended to 120 min. The diversified reason was possibly that the depolymerization and repolymerisation occurred under the special conditions. In other words, depolymerization and repolymerisation of lignin were competitive reactions. The depolymerization was the dominant reaction at relatively mild conditions. In contrast, the critical role of the repolymerization on the extraction of lignin occurred at severe conditions (high temperature and high alkali concentration).
Table 3. Weight-Average Molecular Weight (Mw), Number-Average (Mn) Molecular Weight, and Polydispersity (Mw/Mn) of Lignin Fractions
Quantitative 13C-NMR Analysis
To understand the structural transformation of the lignin fractions during the pretreatment process, quantitative 13C-NMR spectra of L1, L3, and L9 were recorded (Fig. 1), and the detail assignments were conducted according to previous publications about the NMR characterization of lignin (Martínez et al. 2008; del Río et al. 2009). The region between 104.4 and 168.2 ppm represents the aromatic group of the lignin, displaying the basic lignin units (G, S, and H units). Specifically, the basic units of lignin appeared at 152.7 ppm (etherified, S3,5), 138.1 ppm (etherified, S4), 134.4 to 134.7 ppm (etherified, S1 or G1), 133.4 ppm (nonetherified, S1 and G1), 106.8 ppm (oxidized, S’2,6), 104.3 ppm (S2,6), 149.7 (etherified, G3), 148.0 ppm (nonetherified G3 or nonetherified S3,5), 145.5 ppm (nonetherified G4), 119.8 (etherified and nonetherified G6), 114.8 (etherified and nonetherified G5), and 128.1 ppm (H2,6). In addition, the 13C-NMR spectrum of the lignin was almost absent of typical polysaccharides (90.0 to 102.0 ppm), suggesting that the lignin fractions contained small amounts of associated polysaccharides. Furthermore, other strong signals in the 13C-NMR spectrum were also detected. For instance, the signals at 168.1, 160.0, 144.3, 130.2, 125.3, 115.9, and 115.4 ppm were corresponded to C-9 (γ), C-4, C-7, C-2/C-6, C-1, C-3/C-5, and C (β) in p- coumaric acid (PCA), respectively. Moreover, the signals of ferulic acid (FA) at 168.1, 144.3, and 121.9 ppm were C-9, C-7, and C-6, respectively. In general, the spectra of the three lignin samples were similar except for different intensities of these signals. The intensity of etherified S3,5 signals in L1 was strong as compared with that of L3, implying that more S-type lignin were extracted with 18% Na2S for 140 min. Moreover, the bands between 120.0 and 140.0 ppm became weak in L5 as compared with those in L3, indicating that some depolymerization between lignin units appeared at the higher degree sodium hydroxide dosage. More importantly, in this study, the C–β, C–α, and C–γ of β–O–4 units in the lignin fractions could be distinguished by 86.0, 71.7, and 60.1 ppm, respectively, which indicated that β–O–4 units were not cleaved under the given alkaline condition.
Fig. 1. 13C-NMR of the lignin fractions obtained from wheat straw under different pretreatment conditions. (a) L1 : sodium hydroxide dosage 10%, sulfidity 18%, heating time 140 min; (b) L3 : sodium hydroxide dosage 10%, sulfidity 10%, heating time 140 min; (c) L5 : sodium hydroxide dosage 12%, sulfidity 14%, heating time 140 min
Quantification of the lignin fractions gave more precise structural features of lignin from wheat straw (Table 4) (Martínez et al. 2008). Different pretreatment conditions had a slight effect on the content of the C–O linkage (integration region from 155.0 to 154.0 ppm). However, the contents of the C–C linkage (integration region from 140.0 to 124.0 ppm) were 1.88/100Ar and 1.81/100Ar in L1 and L5, respectively, implying that the lignin was depolymerized under a relatively high alkali concentration. In addition, the breakage of C–C linkages generated more C–H linkages. The content of β-O-4 linkage increased under the severe pretreatment conditions (high concentration of alkali or high degree of vulcanization) because the higher ratio of S-type lignin was extracted. In other words, the extracted S-type lignin contained more β–O–4 linkage content. In contrast, the content of OCH3 decreased from 1.35/Ar in L1 to 1.12/Ar in L3, suggesting that demethoxylation occurred at the higher alkaline concentration. Considering the higher S content, which was calculated from 2D-HSQC of L3, it was concluded that demethoxylation preferentially took place in G units at 10% NaOH conditions.
Table 4. Quantification of the Lignin Fractions by the Quantitative 13C-NMR Method
a Results expressed per one Ar based on quantitative 13C-NMR spectra
2D-HSQC NMR Spectra
2D-HSQC spectroscopy is a powerful tool for the determination and quantitative analysis of lignin structures. To further illustrate the structural features of lignins, the lignin samples of L1, L3, L5, L11, and L13 were investigated by 2D-HSQC NMR spectra. As shown in Figs. 2 and 3, the HSQC NMR spectra of the lignin fractions exhibited aliphatic (C/H 10–40/0.5–2.5) and aromatic 13C–1H correlation regions (C/H 95–150/5.5–8.0). The main cross signals of the lignin fractions in the HSQC spectra are assigned in Table 5 according to previous reports (Rencoret et al. 2011; Sun et al. 2013).
The obvious signal of Cα–Hα correlations in β–O–4 substructures was observed in all the lignin factions at δC/δH 71.6/4.86, whereas the Cγ–Hγ correlations in β–O–4 substructures were located at δC/δH 60.3/3.38. The Cβ–Hβ correlations at δC/δH 83.8/4.29 and 85.8/4.12 linked to G/H and S lignin units in β–O–4 substructures, respectively. In addition, the β–β substructures with different intensity signals were also observed in the HSQC spectra. The signals were detected with their Cα–Hα, Cβ–Hβ, and double Cγ–Hγ correlations at δC/δH 84.9/4.62, 53.5/3.07, and 71.0/4.15 (3.79), respectively. However, the signal of phenylcoumaran (β–5) substructures for Cα–α correlations was only detected at δC/δH 87.1/5.50 in L13.
In the aromatic region, syringyl (S), guaiacyl (G), and p-hydroxyphenyl (H) units showed prominent correlations at δC/δH 103.9/6.72 (S2,6), 111.0/6.98 (G2), 115.0/6.75 (G5), 118.7/6.81 (G6), and 128.5/7.20 (H2,6), respectively (Fig. 3). Varying degrees of oxidized syringyl units were detected due to the presence of a correlation at δC/δH 105.8/7.25 (S′2,6). In addition, the dissociated PCA8 (C8–H8) correlations were located at δC/δH 115.4/6.30. However, the signals corresponding to PCA8 were not detected in the lignin fractions of L1 and L11. Furthermore, the cross-signals of correlations C2,6–H2,6, C3,5–H3,5, and C7-–H7 in dissociated PCA were observed at δC/δH 129.8/7.51, 115.4/6.79, and 143.8/7.49, respectively. Likewise, the FA2 (C2–H2), FA8 (C8–H8), and FA7 (C7–H7) correlations were observed at δC/δH 111.0/7.28, 116.4/6.40, and 143.7/7.50.
Table 5. Assignments of 13C–1H Cross-Signals in HSQC Spectra of Lignin Fraction from Wheat Straw
Based on the quantification methodology (Rencoret et al. 2011; Sun et al. 2013), the main substructures among the lignin fractions and their abundances were estimated quantitatively. As shown in Table 6, the ratio of S/G/H was used to estimate the difference of lignin fractions in different conditions. For example, the ratios of L1 and L3 were 43/48/9 and 56/41/3, respectively.
Table 6. Quantification of the Lignin Fractions by Quantitative 2D-HSQC NMR
a Results expressed per 100 Ar based on quantitative 2D-HSQC spectra
b S/G/H ratio obtained by the equation: S/G/H ratio = 0.5I (S2,6) / I (G2)/ 0.5I (H2,6)
c Not detectable
Fig. 2. 2D-HSQC spectra (side-chain region) of the lignin samples obtained from wheat straw
Fig. 3. 2D-HSQC spectra (aromatic regions) of the lignin sample
The high S/G/H ratio indicated that the S-type lignin fractions were more easily released than the G-type lignin fractions (Sun et al. 2013). For the S/G/H ratio, it could be also concluded that the lignin extracted from wheat straw was an HGS type, which had predominant amounts of G and S units together with a minor proportion of H units.
For the β–O–4 linkage, the relative contents in L1 and L3 were 8.76/100Ar and 13.94/100Ar, respectively. The reason for this increase was possibly that more S-type lignin fractions were released under the condition of high alkali concentration, which could be reflected by the high S/G/H ratio (56/41/3 versus 43/48/9). However, the content of β–O–4 linkages decreased to 5.76/100Ar in L5, suggesting that the β–O–4 linkage was more easily cleaved in the alkali concentration of 14%. In addition, the extension of the pretreatment time from 60 to 120 min resulted in an obvious decrease in the content of β–O–4 linkages. Therefore, it can be concluded that the β–O–4 linkage was more easily ruptured under harsh pretreatment conditions.
Besides the cleavage of the β–O–4 aryl ether linkage, the prominent structural alterations of β–β and β–5 linkages were also observed after the pretreatment process. The relative content of β–β linkages in L3 was enhanced as compared to that of L1, suggesting that more carbon-carbon linkages were formed in the higher alkaline concentration (10%). However, the content of β–β linkages decreased to 1.38/100 Ar in the alkaline concentration of 14% (L5), indicating that a partial depolymerization of lignin occurred during the treatment with a higher concentration of alkaline. In addition, the content of β–β linkages decreased from 3.07 to 1.35/100/Ar with an increase in time from 60 to 100 min. The β–5 linkage, however, seemed unstable as compared with β–5 and β–O–4 linkages, and it was not easily detected in the 2D-HSQC spectra of L3, L5, and L13. The reason for the decrease of the signal intensity in β–5 linkages was probably that the resolution of 2D-HSQC NMR was largely reduced for condensed lignin so that newly formed β–5 linkages were generally less detectable in 2D-HSQC NMR than that in C-13 NMR.
In addition to the cleavage of β–O–4 aryl ether linkages and the degradation or condensation of carbon-carbon linkages, the PCA/FA ratio was also an important parameter when investigating the structural transformation of lignin. The differences in the ratios of PCA/FA were probably caused by the different treatment conditions, and the ratio of PCA/FA rapidly increased from 0.08 (L1) to 0.74 (L3). This increase suggested that most of the etherified FA was removed and the increment of alkali concentration increased from 6% to 10%. As the concentration of alkali further increased to 14%, however, the PCA/FA ratio increased to 0.94 (L5). In contrast, the ratio of PCA/FA decreased when the treated time increased from 60 to 100 min, and increased again when the treated time was 120 min.
31P-NMR Spectral Analysis
To further investigate the functional hydroxyl of the lignin obtained from wheat straw, a quantitative 31P-NMR technique was applied (Fig. 4 and Table 7) (Pu et al. 2011). The signals of the aliphatic hydroxyl and carboxyl group in the lignin were located in the ranges 146.0 to 149.0 and 134.5 to 134.2 ppm, respectively. The peaks of the guaiacyl phenolic OH (G–OH) and the syringyl phenolic OH (S–OH) were separately located from 138.8 to 140.2 and 142.2 to 143 ppm, respectively. The signals of condensed phenolic OH (5-substitued OH) for G-type lignin located from 141.4 to 142.2 ppm. The content of aliphatic OH of the L5 decreased from 2.22 (L3) to 2.18 mmol/g (L5) as the alkali concentration increased from 6% to 14%, which could have resulted from the dehydration reaction in the side-chain of lignin. The decrease of aliphatic OH content in L5 likely occurred because the oxidization happened under harsh conditions, reflected by the increasing of carboxylic group content. In addition, the content of S-type OH was less than that of the corresponding G-type OH in all the lignin fractions, implying that the majority of S-type lignin was involved in forming the β–O–4 linkages, and only a small amount of S-type OH could be detected by the 31P-NMR technique. Additionally, the low content of total phenolic OH in L5 and L13 suggested that the lignin extracted under the harsh conditions may have more etherified lignin units owing to the formation of phenyl glycoside linkages (Granata and Argyropoulos 1995).
Table 7. Quantitative 31P-NMR of the Lignin Fractions (mmol/g)
PCA-OH, free p-coumaric acid
Fig. 4. 31P spectra of the lignin samples obtained from wheat straw
- The lignin fractions obtained from wheat straw were typical grass lignin that contained primarily S and G units, as well as small amounts of H units. The lignin fractions were mainly composed of β–O–4 aryl ether linkages, followed by lower amounts of β–β and β–5 linkages.
- The lignin fractions obtained at mild conditions were rich in β–O–4 linkages, which were particularly important for their subsequent utilization, such as degradation into chemicals and modification into materials. Molecular weight and NMR analysis indicated that the different treatment conditions not only enhanced the lignin depolymerization but also promoted lignin condensation. The alkali concentration, sulfurizing degree, and treatment temperature were synergistically critical in the depolymerization of lignin.
The authors are grateful for the support of the China Postdoctoral Science Foundation (2016M602472), the National Natural Science Foundation of China (No. 31600471), the Natural Science Foundation of Guangdong Province (2015A030310369), the Foundation (NO. 2016QN02) of State Key Laboratory of Pulp and Paper Engineering (South China University of Technology), and 111 Plan and Guangdong Provincial Science and Technology Plan Projects (Number: 2015B020241001, Name: Research and Application of Biomass Pretreatment and Ethanol Production Technology).
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Article submitted: November 14, 2016; Peer review completed: January 7, 2017; Revised version received: January 16, 2017; Accepted: January 17, 2017; Published: February 10, 2017.