Abstract
The soluble lignin present in the pre-hydrolysis liquor (PHL) is detrimental for value-added utilization of the PHL from production of kraft-based dissolving pulp. In this paper, the soluble lignin was separated from PHL by activated carbon adsorption and subsequent desorption with methanol. The structural characteristics of the soluble and dioxane lignins (from bamboo-willow material) were analyzed by Fourier transform infrared (FTIR) spectroscopy, nuclear magnetic resonance (NMR), and thermogravimetric analysis (TGA). The FTIR and proton-NMR results showed that cleavage of β-O-4 aryl-ether linkages and demethylation occurred in the lignin structure during the pre-hydrolysis. The main linkages between structural monomeric units of the soluble lignin were β-β, β-5, β-1, and β-O-4. Additionally, the TGA results showed that the residual char yield at 800 °C of the soluble and dioxane lignins were 33.4% and 35.3%, respectively. Therefore, the soluble lignin obtained from PHL possessed thermal stability comparable to that of dioxane lignin, and can be used as a renewable source for carbon fiber.
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Structural Characterization of Soluble Lignin in the Pre-Hydrolysis Liquor of Bamboo-willow Dissolving Pulp
Jinchao Zhang,a Chaojun Wu,a,b,* Dongmei Yu,a and Yachong Zhu a
The soluble lignin present in the pre-hydrolysis liquor (PHL) is detrimental for value-added utilization of the PHL from production of kraft-based dissolving pulp. In this paper, the soluble lignin was separated from PHL by activated carbon adsorption and subsequent desorption with methanol. The structural characteristics of the soluble and dioxane lignins (from bamboo-willow material) were analyzed by Fourier transform infrared (FTIR) spectroscopy, nuclear magnetic resonance (NMR), and thermogravimetric analysis (TGA). The FTIR and proton-NMR results showed that cleavage of β-O-4 aryl-ether linkages and demethylation occurred in the lignin structure during the pre-hydrolysis. The main linkages between structural monomeric units of the soluble lignin were β–β, β-5, β-1, and β-O-4. Additionally, the TGA results showed that the residual char yield at 800 °C of the soluble and dioxane lignins were 33.4% and 35.3%, respectively. Therefore, the soluble lignin obtained from PHL possessed thermal stability comparable to that of dioxane lignin, and can be used as a renewable source for carbon fiber.
Keywords: Dissolving pulp; Pre-hydrolysis liquor; Soluble lignin; Structural characteristics
Contact information: a: State Key Laboratory of Biobased Material and Green Papermaking, Qilu University of Technology, Shandong Academy of Sciences, Jinan, 250353; b: State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou, 510640;
*Corresponding author: chaojunwu2007@163.com
INTRODUCTION
Pre-hydrolysis liquor (PHL) is considered a waste liquor produced in the first step of the kraft-based dissolving pulp production process. The PHL contains a substantial amount of degradation products derived from lignocellulosic materials, such as saccharides formed from hemicelluloses, lignin derivatives, and acetic acid. The saccharides present in the PHL can be further processed to produce biofuels, biochemicals, functional oligosaccharides, etc. (Kumar and Christopher 2017). The lignin present in the PHL can be used to produce biofuel, plastics, phenols, etc. (Schorr et al. 2014). However, the presence of lignin in the PHL is detrimental for value-added utilization of saccharides. Lignin adversely affects the fermentation of xylose/xylan in the PHL into ethanol or xylitol (Lee et al. 2013). In addition, the presence of lignin in the PHL may hinder the separation and concentration of acetic acid (Yang et al. 2013a). Therefore, the removal of soluble lignin from the PHL is essential for the further resource utilization of the PHL. Such as approach not only conforms to the forest biorefinery concept, but also can add additional revenue to the dissolving pulp industry.
In the pre-hydrolysis stage, the lignin macromolecules are depolymerized and degraded under high temperature and acidic reaction conditions. During this step, the main β-O-4 aryl ether linkages in lignin undergo homolytic cleavage, resulting in an increase in the content of free phenolic hydroxyl groups (Evtuguin et al. 2001).
After depolymerization, the lignin is converted into oligomers, dimers, and monomers rich in phenolic hydroxyl groups (Bardet et al. 1985). As the pre-hydrolysis step proceeds, the lignin moieties further undergo repolymerization or condensation reactions as shown by the increase in the contents of -C-C- bonds and the decrease of β-O-4 linkages (Capanema et al. 2005). Chua and Wayman (1979) studied the hot water pre-hydrolysis of poplar wood and observed the formation of lignin condensation products, which was attributed to a series of reactions occurring between lignin aromatic rings, furfural, and hydroxymethyl-furfural. Under strong acidic conditions, the diphenylmethane structure is the main product of the lignin condensation reaction. During the pre-hydrolysis, a stilbene structure also can be formed from the β-1 structure and the cleavage of β-O-4 bonds (Leschinsky et al. 2008). Mašura (1987) showed that lignin would be re-deposited in the later stage of pre-hydrolysis, causing re-adsorption to the surface of wood chips. The condensation and re-deposition of lignin during the pre-hydrolysis process will impart adverse effects on subsequent cooking and bleaching.
The structural characteristics of soluble lignin in the PHL have a great influence on its specific removal. Therefore, it is important to study the structure and properties of soluble lignin in the PHL for the selective removal of soluble lignin. Various investigations have been conducted on separation and structural characterization of lignin in the PHL. Yang et al. (2013b) separated lignin from the PHL via acidification, and characterized its structure. Results showed that PHL lignin has a lower methoxyl content and molecular weight compared to dioxane lignins. Chen et al. (2019) studied the structure of lignin after separating lignin from PHL by hydrothermal acid hydrolysis, the results showed that the lignin samples obtained revealed relatively high molecular weight, surface hydrophilicity, and thermal stability. Tong et al. (2017) studied the structure of lignin in the PHL via acidification treatment (AT) and rotary vacuum evaporation treatment (RVET), and the results showed that the RVET lignin could be identified as p-hydroxyphenyl (H), syringyl (S), guaiacyl (G) lignins, and the AT lignin was mainly composed of S-lignin that had β-1, β-5, β-O-4, and β–β bonds. Wang et al. (2014a) separated lignin samples from the PHL by activated carbon adsorption, and studied the effect of laccase treatment on the molecular weight of lignin in the PHL.
However, the structure of soluble lignin in PHL has not been clearly studied. To enhance the removal of soluble lignin from PHL, the structural characteristic of soluble lignin should be further explored. The study of Wang et al. (2014b) demonstrated that phenolic hydroxyl groups (PhOH) in lignin played an important role in the removal lignin from PHL.
In the present study, the PHL of bamboo-willow dissolving pulp was prepared by using the optimized parameters in our previous research (Zhang et al. 2019). The soluble lignin was separated from PHL via activated carbon adsorption and subsequent desorption using methanol, and then the soluble lignin samples thus obtained were purified. For comparison, the dioxane lignin was also prepared and separated from the bamboo-willow material. The isolated lignin samples were characterized by Fourier transform infrared (FTIR), proton nuclear magnetic resonance (1H-NMR), two dimensional heteronuclear single quantum correlation (2D-HSQC) spectroscopies, and thermogravimetric analysis (TGA).
EXPERIMENTAL
Materials
The raw material of bamboo-willow (Salix sp or zhuliu in Chinese) was provided by one of the forestry centers in the Xinjiang autonomous region of Wulumuqi, China. The chemical compositional analysis of the raw material revealed 77.2% holocellulose, 40.4% α-cellulose, 24.1% pentosan, 20.3% acid-insoluble lignin, and 3.46% acid-soluble lignin (Zhang et al. 2019). Chipped bamboo-willow was screened to obtain particles with the dimensions of 15 mm to 20 mm (length) × 10 mm to 20 mm (width) × 3 mm to 5 mm (thickness). A portion of the wood chips was taken for moisture content determination for subsequent experiments.
Methods
Preparation of PHL
Pre-hydrolysis was performed in an electrically heated stainless-steel digester (15-L) with 1.0 kg oven-dried chips at a reaction temperature of 160 °C for 90 min (Zhang et al. 2019). Deionized water was added to reach a 6:1 liquid-to-wood ratio. The dosage of phosphoric acid used was 0.5% (based on the oven-dried weight of the materials). Upon completion of pre-hydrolysis, the digester was cooled by decompressing the exhaust valve, which subsequently separated the PHL from the solid mass. The collected PHL was slowly filtered using two-tiers of quantitative filter papers and then refrigerated for experimental use. The contents of soluble lignin, arabinose, galactose, glucose, xylose, mannose, and total sugars in original PHL were 4.84, 0.91, 1.00, 4.02, 8.67, 1.17, and 15.8 g/L, respectively.
Analytical methods
The soluble lignin content in the PHL was measured via ultraviolet visible (UV-vis) spectroscopy (Agilent 8453; Agilent Technologies Inc., Palo Alto, CA, USA) at a wavelength of 205 nm, according to TAPPI um 250 (2000) (Wang et al. 2014a).
The sugars content in the PHL was measured using an indirect method based on quantitative acid hydrolysis of the liquid sample. To convert the oligomeric sugars in the PHL to monomeric sugars, the PHL was acid hydrolyzed using 4 wt% sulfuric acid at 121 °C for 1 h in an oil bath according to a technical report from the National Renewable Energy Laboratory (NREL) (Sluiter et al. 2006; Shen et al. 2011). The acid-hydrolyzed PHL was then diluted, and the content of monomeric sugars was measured. The monomeric sugars were measured using high-performance anion-exchange chromatography coupled with a pulsed amperometric detector (HPAEC-PAD) and an HPAEC-PAD system (ICS-5000; Thermo Fisher Scientific, Sunnyvale, CA, USA) equipped with a CarboPac PA20 analytical column (3 mm × 150 mm) (Thermo Fisher Scientific, Sunnyvale, CA, USA) and guard column (3 mm × 30 mm). The samples were filtered through 0.22-μm syringe filters prior to injection. The concentration of the sugars after this additional hydrolysis stage reflected the total sugars content of the PHL (Shen et al. 2011).
Lignin isolation from PHL
The method of separating lignin from the PHL was achieved by following the method of Shen et al. (2013) and Wang et al. (2014a). The mixture of PHL and activated carbon at a ratio of 30:1 (m/m) was shaken at 250 rpm at room temperature for 5 h. Then, the mixture was filtered with Whatman (Q8) filter paper and air-dried at room temperature for 48 h. The air-dried lignin/activated carbon mixture was then mixed with methanol at a ratio of 30:1 at room temperature for 5 h at 250 rpm for three replicates. Finally, the lignin sample was obtained by filtration followed by vacuum rotary evaporation (IKA, Staufen, Germany), and dried in a vacuum oven with P2O5 at 50 °C.
Lignin isolation from raw material
The wood meal of 40/60-mesh was extracted with benzene-ethanol for 8 h, and vacuum-dried over P2O5. The extracted wood meals were extracted with an acidic aqueous dioxane (90% v/v) solution for 1 h, in N2 atmosphere. The HCl concentration in the dioxane solution was 0.2 mol/L. The dioxane solution-to-wood meal ratio was kept at 8:1. Upon completion of refluxing, the mixture was filtered with a Buchner funnel, and the residue was washed with an aqueous dioxane solution (90% v/v). The filtrate was neutralized with solid Na2CO3, and then the filtrate was collected by filtration. The filtrate was concentrated in a rotary vacuum evaporator at 40 °C, and the concentrated filtrate was slowly dropped into deionized water to precipitate lignin, the lignin precipitate was washed, and vacuum-dried with P2O5.
Lignin purification
Lignin purification was achieved by following the method of Yang et al. (2013b). The crude lignin separated from PHL and raw material were dissolved in dioxane solution (90% v/v), and precipitated in ether by magnetic stirrer. The precipitated lignin was vacuum-dried with P2O5.
Analysis Methods
FTIR analysis
The FTIR spectra of the samples were recorded with an IR-Prestige-21 spectrometer (Shimadzu, Kyoto, Japan). The dried lignin samples were embedded in KBr pellets at concentrations of approximately 1 mg/100 mg KBr. The spectra was recorded in the absorption mode in the 4000 cm-1 to 400 cm-1 range.
1H-NMR analysis
The 1H-NMR spectra of the samples were recorded on an AVANCE II 400 MHz NMR spectrometer (Bruker, Karlsruhe, Germany). Samples of approximately 30 mg were completely dissolved in 0.6 mL of dimethyl sulphoxide (DMSO-d6, 99.99%; Cambridge Isotope Laboratories, Andover, MA, USA). The chemical shifts were calibrated relative to the signals of DMSO solvent, which was used as an internal standard at 2.49 ppm for the 1H-NMR spectra. The acquiring time (AQ) was 3.98 s, and the relaxation time used was 1.0 s.
2D-HSQC analysis
The 2D-HSQC analysis of the samples were recorded on an AVANCE II 400 MHz NMR spectrometer (Bruker, Karlsruhe, Germany). Samples of approximately 40 mg was dissolved in 0.6 mL of DMSO-d6 was used for NMR measurement.
TGA
The thermal stability of the samples was measured using a Q50 thermogravimetric analyzer (TA Instruments, New Castle DE, USA). Approximately 3.0 mg of sample was heated from room temperature to 800 °C at a heating rate of 10 °C/min under N2 atmosphere.
RESULTS AND DISCUSSION
FTIR Spectroscopy
The FTIR spectra of both the soluble and dioxane lignins were recorded, and are shown in Fig. 1. The peak assignments are presented in Table 1 following previous studies (Jahan and Mun 2007; Zhao et al. 2014; Constant et al. 2015). The absorption peaks at 1515 cm-1 and 1605 cm-1 were assigned to the skeletal vibrations of the benzene ring (aromatic units) and were selected as qualitative absorption of lignin (Buta et al. 1989; Kang et al. 2012). The absorption peaks at approximately 3410 cm-1 and 3460 cm-1 were assigned to the stretching of O-H groups, inclusive of alcoholic and phenolic hydroxyl groups (Yang et al. 2013b).
Fig. 1. FTIR spectra of soluble and dioxane lignins
The absorbance peak at 890 cm-1 was assigned to isolated aromatic C-H bond vibrations (Lora and Wayman 1980), which indicated that there was a condensation structure in the soluble lignin obtained from PHL. The band at 1505 cm-1 was shifted to 1515 cm-1 in soluble lignin fraction, which was probably caused by the decreased amount of β-O-4 linkages (Yang et al. 2013b). The band at 1325 cm-1 to 1330 cm-1 was related to the syringyl (S-type) structural unit of lignin, and the band at 1030 cm-1 to 1035 cm-1 belongs to the C-H bending of guaiacyl (G-type) structural units. These results indicated that there were mainly syringyl (S-type) and guaiacyl (G-type) structural units in both soluble (1328 cm-1 and 1035 cm-1) and dioxane (1330 cm-1 and 1030 cm-1) lignin fractions. A C=O structure in an unconjugated ketone group (β-carbonyl group) was observed at 1730 cm-1 (soluble lignin) and 1715 cm-1 (dioxane lignin). In addition, the band at 1465 cm-1 indicated C-H deformation and aromatic vibration. Further, the infrared spectra of the obtained soluble lignin showed that there was a weak absorbance peak at 1107 cm-1. The band at 1107 cm-1 denoted the C-O vibration in carbohydrates (Tong et al. 2017), indicating that the soluble lignin obtained by activated carbon adsorption from PHL contained carbohydrates component.
Table 1. Assignments of FTIR Spectra of Soluble and Dioxane Lignins
1H-NMR Analysis
The integrated 1H-NMR spectra for the soluble and dioxane lignins are shown in Fig. 2. Table 2 lists the positions of signals assigned from previous reports (Jahan and Mun 2007; Kang and Zhou 2012; Yang et al. 2013b). In combination with the above FTIR analysis and the 1H-NMR signals of the p-hydroxyphenyl aromatic protons (δ 7.48 ppm and 7.49 ppm), the soluble lignin from PHL could be justified as HSG lignin. According to Table 2, the phenolic hydroxyl (Ar-OH) content of the soluble lignin was lower than that of dioxane lignin, which indicated that the rupture of aryl-ether bond occurred during pre-hydrolysis (Jahan et al. 2012; Yang et al. 2013b).
Fig. 2. 1H-NMR spectra of the soluble and dioxane lignins
It is noteworthy that both soluble and dioxane lignins possessed absorbance peaks at the range of δ 3.00 ppm to 4.15 ppm (methoxyl groups). Further, the methoxyl content of soluble lignin was lower than that of dioxane lignin, indicating the demethylation of the aromatic methoxyl groups in the lignin structure occurred during pre-hydrolysis, this was consistent with the results of Ibrahim et al. (2010). In addition to this observation, the peak strength at 2.49 ppm was also strong, which denotes the absorbance peak for solvent DMSO-d6 (Tong 2017). The absorbance peaks at δ 1.23 ppm and 1.24 ppm correspond to the hydrogen atom in aliphatic groups, which may be caused by carbohydrates. Additionally, the cinnamyl alcohol structure was detected in both soluble (δ 4.34 ppm to 4.84 ppm) and dioxane (δ 4.31 ppm to 4.91 ppm) lignins.
Table 2. Assignments of Signals in the 1H-NMR Spectra of Soluble and Dioxane Lignins
2D-HSQC Analysis
To further understand the structural characteristics of the soluble lignin obtained from PHL, qualitative 2D-HSQC spectra of the soluble and dioxane lignins were recorded and are shown in Fig. 3, and the detailed peak assignments are presented in Table 3 (Del Rio et al. 2009; Xiao et al. 2012; Chen et al. 2019). The structure of the soluble lignin obtained from PHL can be analyzed more intuitively by combining the structure diagrams of several dimers, such as β-ether (β-O-4, A), and H, S, G-monomers of lignin as shown in Fig. 3 (Xiao et al. 2012).
In the side-chain region, the signal of C-H in methoxy group in the spectrum showed a strong correlation signal at δC/δH 56.0/3.7 ppm. The signals at δC/δH 63.5/4.2 ppm were assigned to the Cγ-Hγ in γ-acetylated β-O-4′ substructures (A′/A′′), indicating that the β-O-4′ structure of the soluble lignin obtained from PHL underwent an acetylation reaction at the γ position. The signals of the methoxy groups and β-O-4′ substructures (substructure A) were the most prominent ones in both soluble and dioxane lignins. The signals corresponding to spirodienone (β-1′) substructures (D) could also be confirmed in the spectra of the soluble lignin, its Cβ-Hβ correlation being at δC/δH 61.1/3.2 ppm, and the schematic is shown in Fig. 3D. In the aromatic region, the signals of C2-H2 in guaiacyl units (G) were found at δC/δH 110/6.7 ppm in both soluble and dioxane lignin, suggesting that both soluble and dioxane lignins possessed a G-type structural unit, which is in agreement with the results of FTIR spectroscopy. Additionally, the signals at δC/δH 123.0/7.5 ppm were assigned to C6-H6 in ferulic acid ester, indicating that there was a ferulic acid ester structure in the soluble lignin from PHL.
Fig. 3. 2D-HSQC spectra of the soluble (a); and dioxane (b) lignins. The lignin substructures are also shown: (A) β-O-4′ linkages; (A′, A”) β-O-4′ linkages with acetylated γ-carbon; (D) C-H coupling structure of spiro structure; and (G(S)) guaiacyl unit or syringyl unit
Table 3. Assignments of Signals in the 2D-HSQC Spectra of Soluble and Dioxane Lignin
Thermogravimetric Analysis
The thermal stability is a vital parameter for thermosetting or thermoplastic materials. Figure 4 shows the TG and derivative TG (DTG) curves of the soluble lignin obtained from PHL and dioxane lignin. The obtained soluble lignin sample had a maximum mass loss peak at 200 °C, which indicated the temperature at which hemicellulose begins to decompose was at approximately 200 °C. The maximum thermal decomposition of lignin was observed at 350 °C to 450 °C (Liu 2009), consequently, hemicellulose has completed most of the decomposition when lignin begins to decompose. It can be seen that the maximum mass loss rate at 200 °C was caused by the decomposition of hemicellulose, which was due to the hemicellulose contained in the soluble lignin sample obtained by the activated carbon adsorption method.
Fig. 4. TG/DTG curves of the soluble and dioxane lignins
The maximum thermal decomposition rate temperatures of soluble and dioxane lignins were at 370 °C and 417 °C, respectively. Previous literature also reported that the aromatic ring of lignin molecules began to disintegrate at temperatures above 400 °C (Sun et al. 2001). Above 500 °C, the mass loss and mass loss rate of soluble lignin and dioxane lignin basically converge. The residual char yields determined at 800 °C of soluble and dioxane lignins were 33.36% and 35.30%, respectively. Approximately 2% of yield gap between the two samples can be ascribed to the existence of hemicellulose component in the obtained soluble lignin from PHL. Notably, the residual char yields of the two samples turned out to be almost the same, which exactly indicated that the main component of the sample obtained from PHL by activated carbon adsorption was lignin. Therefore, the soluble lignin obtained from PHL possessed thermal stability comparable to dioxane lignin, and the mixture of lignin and hemicelluloses can be used to produce carbon fiber by a combination process of electrospinning, carbonization and activation.
CONCLUSIONS
- The present study compared the structural characteristics of soluble lignin in the PHL and dioxane-extracted lignin from bamboo-willow dissolving pulp. Results showed that the cleavage of β-O-4 aryl-ether linkages and demethylation occurred in the lignin structure during the pre-hydrolysis step.
- The soluble lignin isolated from PHL could be identified as p-hydroxyphenyl-syringyl-guaiacyl (HSG) lignin, and the main linker bonds between structural units of soluble lignin were β–β, β-5, β-1, and β-O-4.
- The soluble lignin obtained from PHL possessed thermal stability comparable to dioxane lignin from bamboo-willow material. This study will hopefully provide some guidance for the resource utilization and selective removal of soluble lignin in the PHL.
ACKNOWLEDGMENTS
The authors gratefully acknowledge the financial support from the National Science Foundation of Shandong Province (No. ZR2017LC016), the Open Fund of National Key Laboratory of Pulp and Paper Engineering, and the South China University of Technology (201817). The authors also appreciate the financial support from a project of Shandong Province Higher Educational Science and Technology Program (No. J17KA142) and the National Key R&D Program of China (2017YFB0307900).
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Article submitted: September 3, 2019; Peer review completed: November 17, 2019; Revised version received and accepted: December 9, 2019; Published: December 11, 2019.
DOI: 10.15376/biores.15.1.825-839