Abstract
This study was conducted to determine the lignin substructures, hydroxyl (phenolic + aliphatic) contents, and carboxyl contents in kraft lignin (KL) prepared from mixed hardwoods by using 2D Heteronuclear Multiple Quantum Coherence Nuclear Magnetic Resonance (HMQC NMR) and 31P NMR techniques. Based on 2D HMQC NMR analysis of KL, stilbene and vanillin substructures were present in the aromatic region, while trace amounts of β-O-4 and β-β moieties were detected in the oxygenated aliphatic region. The total hydroxyl content calculated from 31P NMR was 5.24 mmol/g KL. The aliphatic hydroxyl content was 1.04 mmol/g KL, and phenolic hydroxyl content was 4.20 mmol/g KL. Of the phenolic hydroxyl groups, the contribution of syringyl (S) and guaiacyl (G) was 1.02 and 0.97 mmol/g KL, respectively. The S/G molar ratio of KL calculated from 31P NMR was 1.05. The carboxyl content was 0.44 mmol/g KL.
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Characterization of Kraft Lignin Prepared from Mixed Hardwoods by 2D HMQC and 31P NMR Analyses
Ji Sun Mun,a Justin Alfred Pe III,b and Sung Phil Mun b,*
This study was conducted to determine the lignin substructures, hydroxyl (phenolic + aliphatic) contents, and carboxyl contents in kraft lignin (KL) prepared from mixed hardwoods by using 2D Heteronuclear Multiple Quantum Coherence Nuclear Magnetic Resonance (HMQC NMR) and 31P NMR techniques. Based on 2D HMQC NMR analysis of KL, stilbene and vanillin substructures were present in the aromatic region, while trace amounts of β-O-4 and β-β moieties were detected in the oxygenated aliphatic region. The total hydroxyl content calculated from 31P NMR was 5.24 mmol/g KL. The aliphatic hydroxyl content was 1.04 mmol/g KL, and phenolic hydroxyl content was 4.20 mmol/g KL. Of the phenolic hydroxyl groups, the contribution of syringyl (S) and guaiacyl (G) was 1.02 and 0.97 mmol/g KL, respectively. The S/G molar ratio of KL calculated from 31P NMR was 1.05. The carboxyl content was 0.44 mmol/g KL.
DOI: 10.15376/biores.17.4.6626-6637
Keywords: Kraft lignin; Mixed hardwood; HMQC; 31P NMR; Hydroxyl content
Contact information: a: Department of Carbon Materials and Fiber Engineering, Jeonbuk National University, 54896, Jeonju, Korea; b: Department of Wood Science and Technology, Jeonbuk National University, 54896, Jeonju, Korea; *Corresponding author: msp@jbnu.ac.kr
INTRODUCTION
The road to carbon neutrality by 2050 is considered as the world’s most urgent mission (Guterres 2020). Interest in the utilization of bioresources such as lignin, the second most abundant organic materials next to cellulose, are growing and actively studied. Among all types of lignin, kraft lignin (KL) is considered as an attractive yet underutilized bioresource to researchers and pulping companies. Valorization of lignin for industrial applications has been challenging due to their heterogeneity, modified structure, presence of sulfur, and poor quality of the final product (Vishtal and Kraslawski 2011; Jardim et al. 2020). Despite these drawbacks, KL still has potential to be converted into value-added materials, as it is cheap, renewable, and available in large amounts (Mun et al. 2021).
Previously, commercial KL produced from South Korea was characterized by elemental analysis, gel permeation chromatography, infrared spectroscopy, and 1H and 13C NMR spectroscopy. The structural changes that occurred in KL have been compared to milled wood lignins (MWLs), which were prepared from the same hardwood species used in the production of KL. In general, 1D NMR provides structural information such as the bond between atoms and functional groups. In the case of lignin, different types of linkages present in lignin can be identified from 1H and 13C NMR. Because lignin is an amorphous polymer, severe overlapping of peaks is inevitable. The advantage of 2D NMR methods such as HMQC is that they can detect the direct chemical bonds between hydrogen and carbon in a molecule by detecting the correlation between 1H and directly spin-bonded, heterogeneous nucleus, 13C (Yu et al. 2003). With this, HMQC analysis of KL from mixed hardwoods was performed since the linkages in lignin substructures can be determined more clearly through 2D NMR analysis (Wen et al. 2013a).
In lignin chemistry, the structural features in lignin that indicate reactivity are the phenolic hydroxyl groups (Cateto et al. 2008). Thus, the determination of phenolic hydroxyl groups in KL is necessary for their future application and utilization. Techniques such as ultraviolet (UV), infrared (IR), and 1H NMR spectroscopy have been reported for the analysis of hydroxyl groups in lignin (Argyropoulos et al. 2021). Through UV spectroscopy, only phenolic hydroxyl groups can be determined (Goldschmid 1954). Quantitative information on hydroxyl groups cannot be obtained from IR spectroscopy due to the signal overlapping issues. 1H NMR can quantify the aliphatic and phenolic hydroxyl groups after acetylation of lignin. 31P NMR is capable of precisely detecting and quantifying the different hydroxyl groups such as syringyl (S), guaiacyl (G), p-hydroxyphenyl (H), and even carboxyl groups in lignins (Argyropoulos et al. 2021).
In this work, the lignin substructures and hydroxyl groups of KL, prepared from mixed hardwoods, were determined via 2D HMQC and 31P NMR techniques. The outcomes gave a better understanding of the structural features of hardwood KL, which can be useful for chemical modification, functionalization, and utilization of KL in the near future. In addition, this work adds to the literature for hardwood KLs since most of studies focused on softwood KL.
EXPERIMENTAL
Materials
The KL used in this study was provided by Moorim P&P Co., Ltd. located in Ulsan, Korea. The wood chips used for kraft pulping were 50% Acacia spp. from Vietnam and 50% mixed hardwood (Quercus spp. + other hardwood, 1:1) from Korea. A detailed description about the cooking conditions and purification process of KL can be found in Mun et al. (2021).
The reagents used for acetylation were anhydrous pyridine (99.5%, Kanto Chemical, Tokyo, Japan) and acetic anhydride (93%, Duksan Pure Chemical, Ansan, Korea). The solvents used for 2D HMQC NMR were acetone-d6 (Cambridge Isotope Laboratories, Andover, USA) and D2O (Merck, Darmstadt, Germany). The reagents used for 31P NMR were anhydrous pyridine (99.5%, Kanto Chemical, Tokyo, Japan), CDCl3 (Eurisotop, Saint-Aubin, France), N-hydroxy-5-norbornene-2,3-dicarboximide (97%, AlfaAesar, Lancashire, UK), chromium (III) acetylacetonate (97%, AlfaAesar, Ward Hill, USA), and 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane (95%, Sigma-Aldrich, St. Louis, USA). All reagents and solvents were used without further purification. Molecular sieves with pore diameter 4Å (1.6 mm pellet, Yakuri Pure Chemicals, Osaka, Japan) were used for all experiments.
2D HMQC NMR Analysis
For the HMQC NMR analysis, the sample was firstly acetylated with anhydrous pyridine and acetic anhydride (1:1 v/v) at room temperature for 48 h according to the authors’ previous paper (Mun et al. 2021). A 100 mg acetylated KL (Ac-KL) was dissolved in 150 μL acetone-d6 and 300 μL D2O (1:2 v/v) solvent in a 10-mL conical beaker. The conical beaker was sonicated for 1 to 2 min to dissolve the sample. The mixture was filtered through a fine glass wool suspended inside a Pasteur pipette, which was directly connected to an NMR tube. The conical beaker was rinsed with solvent, and the contents were transferred as described in the previous filtration method. The measurement was conducted using an NMR spectrometer (500 MHz, JEOL, Tokyo, Japan) at the Center for University-wide Research Facility (CURF), Jeonbuk National University (JBNU), Jeonju, Korea.
31P NMR Analysis
The hydroxyl content of KL was determined by 31P NMR analysis according to the method of Argyropoulos et al. (2021). The pyridine/CDCl3 (1.6:1 v/v) solvent was prepared by mixing 6.40 mL anhydrous pyridine and 4.00 mL CDCl3 in a pre-dried 20-mL vial. Molecular sieves were added into the vial to completely remove moisture. The vial was sealed with a septum cap and stored in the dark. The internal standard, N-hydroxy-5-norbornene-2,3-dicarboximide (NHND, 35.8 mg), and the relaxation agent, chromium (III) acetylacetonate (10 mg), were dissolved in 280 μL pyridine/CDCl3 solvent in a pre-dried 2-mL vial. Molecular sieves were added into the internal standard (IS) solution. The vial was sealed with a septum cap and wrapped with an Al foil. A 30 mg vacuum-dried moisture-free KL was dissolved in 500 μL pyridine/CDCl3 solvent in a pre-dried 2-mL vial. This was followed by the addition of 100 μL IS solution via syringe. The vial was sonicated for 90 s and then stirred at around 500 rpm for 18 h using a magnetic stirrer (RCN-7, Eyela, Tokyo, Japan). After dissolution of KL, 100 μL phosphitylating agent, 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane (TMDP), was added into the vial using a syringe and then stirred vigorously under magnetic stirring. The sample was transferred to an NMR tube using the same filtration method above. The 31P NMR analysis was conducted using an NMR spectrometer (600 MHz, JEOL, Tokyo, Japan) at the CURF, JBNU. The spectrum was acquired through an inverse-gated decoupling pulse sequence, 10 s relaxation delay, and 64 scans.
RESULTS AND DISCUSSION
2D HMQC NMR Analysis
The 13C NMR spectra of acetylated KL (Ac-KL) and non-acetylated KL were compared. The 13C NMR spectra shown in Fig. 1 were obtained from HMQC analysis (Ac-KL) and the previous study (KL). The chemical shifts are listed in Table 1 along with their assignments in acetylated lignins based on Nimz et al. (1981). In Ac-KL, two strong signals at 171 to 168 and 20.6 ppm correspond to the C=O and CH3 in acetyl groups, respectively. Since these two signals were absent in the spectrum of KL, these confirmed that the hydroxyl groups in KL were well acetylated.
The 160 to 100 ppm in 13C NMR spectrum corresponds to the aromatic region and 90 to 50 ppm (excluding the methoxyl group signal) to the aliphatic region (Nimz et al. 1981; Zhao et al. 2017). In the aromatic region, syringyl (S) and guaiacyl (G) related signals were present as anticipated for hardwood lignins (Ralph et al. 2007; Katahira et al. 2018). The number assignments in the 13C NMR of Ac-KL refers to the substructures shown in Fig. 2. In case of Ac-KL, one of the most remarkable signals in the aromatic region was 3; this signal was assigned to the S3,5 and G3 units. This corresponded to the carbon containing the methoxyl groups. Also, relatively strong signals 8 and 11 were assigned to G5 and S2,6 units, respectively, which corresponded to the free aromatic C–H bonds. Meanwhile, the weak signals found in the aliphatic region of both Ac-KL and KL were indications of aliphatic sidechain cleavage.
In 13C NMR spectrum of KL, two distinct signals at 148.2 ppm and 115.5 ppm were not detected in Ac-KL. The intense signal at 148.2 ppm corresponded to S3,5 and G3/G3‘ units; the biphenyl (5-53) was also assigned to this signal (Mun et al. 2021, Fig. 2). Thus, the signal at 148.2 ppm in KL shifted to 153.2 ppm in Ac-KL. The signal at 115.5 ppm was attributed to G5 overlapping with H3,5. This suggested that the signal at 115.5 ppm in KL shifted to 123.6 ppm in Ac-KL.
Fig. 1. 13C NMR spectra of Ac-KL and KL
Table 1. 13C NMR Assignments of Ac-KL
Fig. 2. Substructures of Ac-KL and KL corresponding to 13C NMR assignments
Figure 3 shows the aromatic (140–90/7.9–6.4 ppm) and oxygenated aliphatic (120–70/6.1–4.6 ppm) regions of Ac-KL in 2D HMQC spectrum. The chemical shifts and assignments listed in Table 2 were based on the works of Nimz et al. (1981), Balakshin et al. (2003), del Río et al. (2009), Wen et al. (2013b), Eugenio et al. (2021), Lahtinen et al. (2021), and Wang et al. (2022). The substructures present in Ac-KL with their corresponding notations are shown in Fig. 4.
The aromatic region (Fig. 3a) shows S and G related structures, as well as stilbene. The S moieties (S2,6, S2,6‘, and S2,6“) were in the range 106–103/7.3–6.7 ppm. The G moieties were situated in 115–110/7.7–6.9 ppm for C2–H2 in G related structures and 125–118/7.6–6.9 ppm corresponded to the C6–H6 of G units. The Cα-oxidized guaiacyl related structures such as vanillin and acetovanillone were also detected in the spectrum. In addition, the signal around 124.8/7.3–6.8 ppm was assigned to C6–H6 of conjugated carbonyl or carboxyl and 129.2/7.4–7.1 ppm to C–Hα/β of the G unit.
The Cα–Hα in stilbene structures with β-1 (SB1) linkage, and Cα–Hα and Cβ–Hβ with β-5 (SB5) linkage were identified (Table 2). During kraft pulping, the β-1 moieties (e.g. diphenylethanes, spirodienones) generates SB1, while the β-5 moieties can produce SB5 via retro-aldol addition reaction (Crestini et al. 2017; Giummarella et al. 2020).
In the oxygenated aliphatic region (Fig. 3b), β-O-4 moieties (A) were detected. In general, the β-O-4 linkage, comprising about 60% in hardwood lignins, is extensively cleaved due to the nucleophilic attack of SH– and OH– during kraft pulping. However, the Cα–Hα (75.4/6.02 ppm) and Cβ–Hβ (80.4/4.6 ppm) of A were detected in HMQC spectrum. This suggests that there were small amounts of β-O-4 bonds uncleaved during kraft pulping. The β-β moieties (B) in 86.6/4.76 ppm were also detected.
Fig. 3. Aromatic (a) and oxygenated aliphatic (b) region of the HMQC spectra of Ac-KL
Table 2. Assignments of 13C–1H Correlation Signals in HMQC Spectra of Ac-KL
The structural changes caused by kraft pulping were confirmed by 2D HMQC NMR
result of Ac-KL. Some structures such as stilbene and vanillin, which were not observed previously by 1D NMR analysis, were confirmed by 2D HMQC NMR.
Fig. 4. Substructures present in Ac-KL from 2D HMQC NMR
31P NMR Analysis
The hydroxyl and carboxyl content of KL were determined through the 31P NMR protocol based on Meng et al. (2019) and Argyropoulos et al. (2021). Based on qualitative and quantitative analysis of hydroxyl groups in lignin model compounds (Archipov et al. 1991) and various types of lignin (Argyropoulos et al. 1993a), different types of hydroxyl groups such as aliphatic and phenolic hydroxyl groups – even the guaiacyl (G), syringyl (S), p-hydroxyphenyl (H), and C5 condensed phenolic hydroxyl groups – can be quantified by simply comparing the integrals of sample peaks to an internal standard (Argyropoulos et al. 2021). N-hydroxy-5-norbornene-2,3-dicarboximide (NHND) was used as an internal standard since NHND is baseline resolved from lignin-derived resonances (Zawadzki and Ragauskas 2001). Simply, its lone hydroxyl group does not overlap with any of the hydroxyl groups originating from the sample. In addition, the quantification of carboxylic acid group in KL is possible with NHND. Chromium (III) acetylacetonate was also added as a relaxation agent to speed up the 31P spin-lattice relaxation behavior of phosphitylated samples (Argyropoulos et al. 1993b).
For a reliable determination of hydroxyl groups present in KL by 31P NMR, there are two important factors to be considered. First, the KL sample must completely be dissolved in the pyridine/CDCl3 solvent. Second, there should be no presence of water in the sample. The presence of water in the sample destroys the phosphitylating agent, 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane, and produces 2-hydroxy-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane, which is a yellow precipitate. Any formation of precipitate leads to non-homogeneity of the sample being analyzed, which makes the sample not suitable for analysis. In this study, KL was completely dissolved in the solvent and was ensured to be free of moisture.
Figure 5 (a) shows the full 31P NMR spectrum of phosphitylated KL and (b) shows the enlarged hydroxyl group region of interest (150–134 ppm). A sharp peak at 174 ppm was due to the excess amount of unreacted TMDP. The presence of an excess amount of TMDP indicated that all hydroxyl groups in KL were completely derivatized.
Fig. 5. 31P NMR spectrum (a) and enlarged spectrum (b) of KL. S: syringyl, G: guaiacyl, H: p-hydroxyphenyl
The aliphatic and phenolic (S, G, H) hydroxyl, and carboxyl contents of KL calculated from the 31P NMR result are listed in Table 3. The hydroxyl content was compared to modified and commercial KLs from published data (Cateto et al. 2008; Sameni et al. 2016; Antonino et al. 2021). For the KL used in this study, which was provided by Moorim P&P – the only kraft mill in Korea, the total hydroxyl content was found to be 5.24 mmol/g KL. The total hydroxyl content of KL in this study was similar to the KL derived from hardwood in alkaline condition (5.14 mmol/g KL). The total hydroxyl content was remarkably lower compared to Indulin AT, a commercial KL from softwoods. This suggests that softwood KL has higher hydroxyl content than hardwood KL. The phenolic hydroxyl content was not remarkably different between hardwood and softwood KLs, however the aliphatic hydroxyl content was relatively lower in hardwood KLs than softwood.
The phenolic hydroxyl content was higher than aliphatic hydroxyl content for each of the KLs listed. This result agreed with the result of 1H NMR analysis from the study of Mun et al. (2021) (Fig. 6). This indicated that the cleavage of β-O-4 and α-O-4 bonds during the kraft pulping process and creation of new phenolic hydroxyl groups in etherified lignin (Sameni et al. 2016).
Table 3. Hydroxyl and Carboxyl Content of KL
Fig. 6. Comparison of aromatic/aliphatic regions of KL and MWL from 1H NMR (Mun et al. 2021).
Of the phenolic hydroxyl groups, the contribution of S and G was 1.02 and 0.97 mmol/g KL, respectively. The S/G molar ratio of KL determined via 31P NMR method was 1.05, which was similar to the hardwood acid (1.09) and alkali (1.04) KL (Antonino et al. 2021). In addition, it had similar tendency with the S/G ratio obtained from 1H NMR (1.13) in the authors’ previous study (Mun et al. 2021).
The region around 143.0–140.2 ppm is for C5 substituted phenolic hydroxyl group (Balakshin and Capanema 2015). Due to the signal overlapping between 5-substituted phenolics (S units and various 5-condensed G units), overestimation of S and underestimation of condensed units can occur in hardwood KLs (Meng et al. 2019). In KL, 5-substituted phenolics not only represent 4-O-5, β-5, 5-5 but also stilbenes with β-5 linkage (Lancefield et al. 2018). The C5 substituted hydroxyl content of mixed hardwood KL was 2.21 mmol/g KL. The S hydroxyl content (1.02 mmol/g KL) was subtracted from C5 substituted phenolic hydroxyl content (2.21 mmol/g), which gives 5-condensed hydroxyl content (1.19 mmol/g KL).
In mixed hardwood KL, the H hydroxyl content (0.30 mmol/g KL) and carboxyl content (0.44 mmol/g KL) were relatively lower than their aliphatic, C5 substituted phenolic, S and G hydroxyl counterparts.
CONCLUSIONS
- From 2D HMQC NMR analysis of kraft lignin (KL) from mixed hardwoods, lignin substructures such as stilbene (β-1 and β-5) and vanillin were confirmed. These substructures were previously not detected in 1D NMR but were confirmed in 2D NMR.
- The syringyl and guaiacyl moieties in KL were detected in the aromatic region, while trace amounts of β-O-4 and β-β moieties were detected in the oxygenated aliphatic region.
- The total hydroxyl content calculated from 31P NMR was 5.24 mmol/g KL, and the phenolic hydroxyl content (4.20 mmol/g KL) was higher than aliphatic hydroxyl content (1.04 mmol/g KL).
- The S/G molar ratio of KL calculated from 31P NMR was 1.05 (S: 1.02 mmol/g KL, G: 0.97 mmol/g KL).
- The carboxyl content was 0.44 mmol/g KL.
ACKNOWLEDGMENTS
This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (No. 2020R1A2C2012356). The authors would like to appreciate Moorim P&P for providing kraft lignin and CURF, JBNU for the technical assistance.
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Article submitted: July 19, 2022; Peer review completed: August 21, 2022; Revised version received: August 26, 2022; Accepted: October 7, 2022; Published: October 14, 2022.
DOI: 10.15376/biores.17.4.6626-6637