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. 2024 Jun 20;9(26):28167–28175. doi: 10.1021/acsomega.4c01267

A Novel Method of Self-Cross-Linking of Syringaldehyde with Activated Methoxy Groups via Cross-Coupling for Lignin-Based Wood Adhesives

Ega Cyntia Watumlawar 1, Byung-Dae Park 1,*
PMCID: PMC11223239  PMID: 38973923

Abstract

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As steric hindrance, methoxy groups are limiting the valorization of hardwood lignin. This paper reports a novel method of self-cross-linking of the syringaldehyde with activated methoxy groups (−OCH3) via cross-coupling reaction to obtain thermosetting polymers for lignin-based wood adhesives. The methoxy groups of syringaldehyde have been activated via cross-coupling reaction by substituting Ar–OCH3 with Ar–CH2–SiMe3, and dichloromethane, leading to cross-linking via methylene bridges to build a thermosetting polymer. FTIR spectra showed a decrease in the intensity of a −CH3 and −OH group, owing to the substitution of the methoxy group. 13C NMR spectra also supported these results with the −SiMe3 signal that disappeared after the cross-linking reaction. Furthermore, cross-linking between the activated methoxy groups was confirmed with a strong exothermic peak at 130 °C, resulting in an increase in the adhesion strength as hot-pressing temperature increased from 160 to 180 °C. These results suggest that the cross-linking between the activated methoxy groups of syringaldehyde is an important understanding of valorizing hardwood lignin via building thermosetting polymers for lignin-based adhesives.

1. Introduction

Various modifications of lignin have been widely investigated for lignin-based wood adhesives.1−4 Lignin modification utilizes the main structural units of lignin such as p-coumaryl alcohol (H-unit), guaiacyl alcohol (G-unit), and sinapyl alcohol (S-unit).5−7 However, a difference in the proportion of structural units is observed between hardwood and softwood lignin (Table 1). Softwood lignin that has a higher proportion of guaiacyl lignin has been investigated by using various modification methods to build lignin-based adhesives. For example, phenolation,8−10 demethylation,2,11 hydroxymethylation,12−14 glyoxalation,15,16 and allylation5,17−19 have been used to modify softwood lignin and to build a cross-linked network with three-dimensional linkages for the synthesis of adhesives. However, hardwood lignin with a high proportion of the syringyl unit is known as a weakness to create a cross-linked network structure due to the less reactive methoxy group in the unit.20 In recent studies, hardwood lignin modifications have been mainly based on exploiting their para and ortho positions.7,21 Thus, a novel approach for the meta position of aromatic hardwood lignin was noticed for this study.

Table 1. Proportion of the Structural Units in Hardwood and Softwood Lignin.

1.

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a

The data was cited from ref (22).

The methoxy group is a functional group that contains a methyl group bound to oxygen and has high application potential in the medical field23,24 but not in the adhesive application. The methoxy group has been utilized as a protecting group in many substrates.25−28 Moreover, the methoxy groups of lignin have been used to prepare amides via carbonylation29 and have been converted to obtain pure chemicals.30 However, the modification of the methoxy group with a substitution method has become a promising alternative strategy.31 The substitution of the methoxy group has been conducted to produce cyclized products via ipso substitution through nucleophilic and electrophilic aromatic substitutions.25,32 Furthermore, a substitution via cross-coupling reaction has attracted considerable interest because of the potential reaction of substrate and reagent with the aid of a catalyst under mild conditions.31 Previous studies employed cross-coupling reactions for the synthesis of several chemicals to incorporate an alkyl group into aromatic rings with metal-catalyzed cross-coupling,33 to synthesize two chemical compounds with the addition of organometallic species,34 and to substitute the methoxy group.31,35 However, the activation of methoxy groups of syringaldehyde (SA) via cross-coupling reaction has not been reported yet for the self-cross-linking of SA. This methoxy group activation makes it possible to generate activated carbon atoms of the methoxy groups of SA for their self-cross-linking to build a network structure as adhesives.31

SA is a naturally occurring compound with various bioactive characteristics that occur in lignin.36 Lignin provides a continuous, renewable, and cheap supply of SA.37 Hence, the SA model compound is a potential lignin compound with a methoxy group in two meta positions, due to numerous studies of SA depolymerization from lignin with stable conditions and high yield.38 SA has been studied for its biological effects using a semisynthetic method,39 and the excellent performance of SA has indicated the application potential of this compound in thermoset polymer.40−42

The objectives of this work are to activate the methoxy groups of the SA model compound and to cross-link the activated SA to build thermosetting polymers with a network structure as lignin-based adhesives for bonding wood. Thus, this study investigated a novel method of self-cross-linking through the activation of methoxy groups and their cross-linking to build a network structure by forming methylene bonds.

2. Materials and Methods

2.1. Materials

The SA model compound and Ni(COD)dq were obtained from Sigma-Aldrich, USA. LiCH2SiMe3 and toluene were purchased from Thermo Fisher Scientific, USA. Methyl alcohol (methanol) was obtained from Duksan Chemical, Korea. Potassium hydroxide (KOH), dimethyl sulfoxide (DMSO), and dichloromethane (DCM) were obtained from Daejung Chemicals, Korea.

2.2. Methods

2.2.1. Synthesis of the Activated Methoxy Group of SA

The method used for synthesizing the methoxy group of SA was adapted from the method published in the literature;31 the method is schematized in Figure 1. An oven-dried, nitrogen-flushed Schlenk tube was charged and was ready to use. First, 52 mg of SA (0.279 mmol) and 0.926 mg of Ni(COD)dq (1 mol %) were added in the tube. Subsequently, the tube was immediately sealed and flushed with nitrogen. Thereafter, 1.5 mL of toluene and 1.178 mL of a LiCH2SiMe3 solution in hexane (0.7 M, 3.3 equiv) were added and the mixture was stirred for 2 h at 80 °C. The activated SA (A-SA) product was vacuumed to remove the solvent and then characterized.

Figure 1.

Figure 1

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Schematic diagram of the reaction setup for the activation of the methoxy group of SA under nitrogen gas.

In this study, an SA model compound with two methoxy groups in the meta position was reacted with activated Li and nickel catalysts, where a cross-coupling reaction might have occurred. In addition, the reaction with −OH groups also occurred during the activation. However, the product is expected to substitute for −OCH3 and activate it into −CH2–SiMe3 (Scheme 1).

Scheme 1. Schematic Reactions of the Activation of Methoxy Groups of SA.

Scheme 1

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2.2.2. Self-Cross-Linking of SA with the Activated Methoxy Groups

Initially, 10 mL of 1 N KOH solutions was prepared in a round-bottom flask and mixed with 20 mL of methanol for 30 min under room temperature. Subsequently, 546.5 mg of A-SA was added to the flask and the mixture was stirred for 1 h at 60 °C. Thereafter, 2 mL of DCM was added slowly, followed by stirring at 60 °C for 4 h. The mixture was evaporated under a vacuum to remove the solvent before being analyzed. The procedure is shown in Figure 2.

Figure 2.

Figure 2

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Schematic procedures of the self-cross-linking of A-SA.

In addition, further modification to accomplish cross-linking between substrates can occur via self-cross-linking. We developed a new reaction in the form of self-cross-linking to build a methylene bridge among activated products. Dichloromethane (DCM) functioned as a reagent to afford a methylene bridge among activated products (Scheme 2). Fourier transform infrared (FTIR) spectroscopy, 13C nuclear magnetic resonance (13C NMR) spectroscopy, and thermal behavior analysis were performed to investigate self-cross-linking of A-SA molecules.

Scheme 2. Self-Cross-Linking Reaction of SA with the Activated Methoxy Groups.

Scheme 2

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2.2.3. FTIR Spectroscopy

The functional groups of SA, A-SA, and cross-linked SA (C-SA) after the cross-linking reaction were characterized via attenuated total reflectance infrared spectroscopy (ALPHA, Bruker Optics GmbH, Ettlingen, Germany). The FTIR spectra were recorded in the 4000–400 cm–1 range with a resolution of 4 cm–1 and 32 scans.

2.2.4. Differential Scanning Calorimetry (DSC)

A DSC instrument (Discovery 25, TA Instruments, New Castle, USA) was used to determine the thermal behavior of SA, A-SA, and C-SA. A dried sample (3–5 mg) was sealed in an aluminum pan with a lid. The samples were equilibrated for 30 min and then heated to 250 °C at a rate of 10 °C/min under a nitrogen flow of 50 mL/min.

2.2.5. Solid-State 13C NMR Spectroscopy

Solid-state 13C cross-polarization/magic-angle-spinning (CP/MAS) NMR spectra were acquired using NMR spectroscopy (Bruker Avance III HD, Rheinstetten, Germany; 400 MHz) to characterize the chemical structure of all samples.43 The powder sample was investigated using a 4 mm CP-MAS probe at a frequency of 100.6 MHz and a spinning rate of 10 kHz. Thus, structural information that could not be obtained via infrared spectroscopy was addressed.

2.2.6. Determination of Adhesion Strength

The adhesion strength was determined at room temperature using a Universal Testing Machine (OTT-005, Oriental TM, Republic of Korea). The test was carried out with thin rectangular wood strips (30 mm wide × 30 mm long), and adhesives were spread over the overlapping area (10 mm × 10 mm) with 400 g/m2 glue spread. Then, the bonded wood stripes were hot-pressed at 160 or 180 °C for 8 min. The lap shear specimens were tested in tensile conditions at a crosshead speed of 0.5 mm/s. At least three replicates were tested for each sample, and their average values were reported.

3. Result and Discussion

The synthesis of the methoxy group of SA and self-cross-linking reaction have succeeded providing outcomes. Cross-coupling reactions in the form of oxidative addition, transmetalation, and reductive elimination might be assumed to have occurred in the transformation between −OCH3 into activated −SiMe3. The reaction of LiCH2SiMe3 as an activating agent with the methoxy group of SA in the presence of a nickel catalyst has occurred in this study. Certainly, the concentration of reactants, temperature, and the presence of catalysts influence the reaction rates. Temperature is influential because of sufficiently energetic collisions among reactants. Moreover, Figure 3 shows color transformations in each synthesis. SA manifested in the form of a white cream, while the color of A-SA changed to yellow. Furthermore, the self-cross-linking of C-SA displayed a change to orange. The color change during chemical reactions commonly occurs probably because of the changes in energy emitted by the electron while the reaction occurs.44,45

Figure 3.

Figure 3

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Samples of (a) syringaldehyde (SA), (b) activated syringaldehyde (A-SA), and (c) self-cross-linked SA (C-SA).

In addition, DCM has been selected as a solvent in various organic applications.46−48 As a reagent, DCM has shown a good performance. During electron bond formation, DCM reacted by acquiring electrons or sharing electrons that previously belonged to A-SA49 (Scheme 2). The activation of DCM has been studied to build methylene-bridged bisamines, which are usually used as ligands in chemical applications.50 In addition, DCM has been used in combination with pyridine under ambient conditions.51 DCM has also been used as a reagent in the synthesis of pyrrole to obtain high yields under phase-transfer conditions.52 The methylene bridge that formed during the A-SA reaction provides a self-cross-linking reaction that could generate cohesion for adhesives. Therefore, chemical analyses were conducted.

3.1. FTIR Spectroscopy

FTIR analysis was conducted to understand the substitution of the functional methoxy groups of SA, A-SA, and C-SA. Table 2 shows the SA spectra consisting of a common linkage, as reported in the literature.53Figure 4 shows that transformations among spectra have occurred during every reaction. Substituting the methoxy group with LiCH2SiMe3 revealed the disappearance of peaks from the A-SA spectra, particularly those belonging to −OCH3 at 1277 cm–1. Furthermore, after methoxy group activation, C-SA exhibits other peaks, such as those at 1403 and 1366 cm–1, indicating a reduction in the spectral intensity of the C–C reaction. However, an interesting phenomenon has shown that the band intensities at 3250 cm–1 corresponding to −OH of A-SA have decreased. This result indicates that LiCH2SiMe3 reacts with −OH groups and is converted to O–Li+, besides the methoxy groups. Moreover, the self-cross-linking reaction indicated that two molecules of A-SA serve as a nucleophile and DCM serves as an electrophile to combine and form a single molecule that builds a methylene bridge. During the reaction, water is usually released as a secondary product. In addition, the Si–C network suggests that incorporation is formed with a large amount of H.54 Therefore, the spectra of A-SA at 2940 cm–1 corresponding to −CH indicates the possible occurrence of Si–CH3 stretching.54

Table 2. FTIR Band Assignment of SA.

wavenumber (cm–1) vibration assignmentsa
3250 ν OH and CH
3032 νas CH3
2967, 2940 ν CH, νs CH3
2863, 2838 νs CH3
1667 ν C=O, ν CC
1606 ν CC
1583 ν CC
1511 δas CH3
1451 δas CH3, β CH
1422 δs CH3, β CH
1403 ν CC, β CH
1366, 1327 ν CC, δs CH3, β CH
1277 ν CO, β CH, δs CH3
1249 ν CO, β CH, β OH
1202 β OH, β CH
1137 ν CO, β CH
1099 ν CO, β CH
1036 ν CO, β CH
907 γ CH
828 γ CH
767 γ CH, δ C–OH, δas CH3
723 γ CH, γ OH
665 γ OH, δ C–OH, δas CH3
633 γ OH, δ C–OH, δas CH3
583 γ OH
522 γ OH, δ C–OH
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a

ν, stretching; νs, symmetric stretching; νas, asymmetric stretching; δ, bending; δs, symmetric bending; δas, asymmetric bending; β, in-plane bending; γ, out-of-plane bending.

Figure 4.

Figure 4

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FTIR spectra of SA, A-SA, and C-SA.

Based on the self-cross-linking of A-SA, Figure 4 shows the FTIR spectra of C-SA. The activated methoxy group and the −CH2–SiMe3 network led to the formation of a methylene bridge via a reaction between −CH2– groups and −CH2+ of CH2Cl2 under room temperature, causing the reaction to accelerate and cause cross-linking to form a polymer–particle network. The graph revealed that the self-cross-linking of C-SA might have succeeded by the presence of a 1422 cm–1 band known as the methylene bridge.

3.2. 13C NMR Spectral Analysis

13C CP/MAS NMR analysis was conducted to investigate the chemical spectra of SA, A-SA, and C-SA. Figure 5 shows SA’s chemical shifts, whereas the number of chemical shift assignments is shown in Table 3. The SA shows the chemical shifts that are corresponding well with the peaks reported in the literature.53

Figure 5.

Figure 5

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13C CP/MAS NMR spectra and chemical structures of SA, A-SA, and C-SA.

Table 3. Chemical Shift Assignments of 13C NMR Spectra.

sample peak number δc (ppm)
SA 1 192.23
2 126.01
3 140.91
4 99.33
5 146.61
6 55.19
A-SA 1 187.45
2 120.49
3 128.03
4 102.09
5 148.82
6 55.74
7 0.37
C-SA 1 188.55
2 149.19
3 163.72
4 113.87
5 168.33
6 53.90
7 100.81
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Furthermore, NMR provides precise information about the interatomic interaction that cannot be obtained using other structural techniques.55,56 SA shifts are for the alkyl groups (0–50 ppm), O-alkyl groups (50–110), aromatic groups (110–165 ppm), and carbonyl groups (165–200 ppm).57,58 In particular, the spectra of A-SA and C-SA revealed peak changes corresponding to alkyl groups. A-SA exhibited a new peak around δ = 0.37 ppm attributed to the Si–Me3 linkage. However, the results confirmed that corrosion after synthesis might occur, as made evident by the reduction of spectral intensity. A catalyst of high activity might affect degradation and discoloration.59 The self-cross-linking reaction or C-SA can build a methylene bridge between −CH2–SiMe3 of the C-SA and CH2Cl2 of DCM. Spectral analysis of C-SA has proven the disappearance of the −SiMe3 linkage in δ = 0 ppm; instead, a new strong peak showed around δ = 100.81 ppm, indicating that a new linkage has formed, which could serve as a methylene bridge.

3.3. Thermal Behavior Using DSC

Thermal behaviors of SA, A-SA, and C-SA were investigated using DSC. DSC analysis detects thermal transitions that display endothermic or exothermic peaks in a sample. In other words, this analysis monitors heat-induced phase transition changes that determine the temperature and heat flow associated with the material transition as a function of time and temperature, respectively.60 In polymers, a phase transition has been studied to understand the relationship between properties and microstructure.61 Furthermore, in the case of thermoset polymers, DSC measurement is important to understand the phase transition of cross-linking and the curing temperature and time.62 As shown in Figure 6a, the SA has a strong endothermic peak starting at approximately 106 °C, indicating the melting process that occurred in SA. This result is compatible with the reported result that the SA-based polymer showed an endothermic peak of SA.37 Moreover, lignin with the methoxy group has considerably lower melting temperatures.63 However, the A-SA reaction involving a nickel catalyst showed an endothermic peak at approximately 40 °C (Figure 6b), which is decreased compared to that of SA before modification. The activation of the methoxy group has indicated the changes in thermal properties. The endothermic peak of A-SA revealed the melting process, indicating that decomposition might have occurred during the reaction. However, partial crystallization at approximately 108 °C might have occurred.

Figure 6.

Figure 6

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A DSC thermograms of (a) SA, (b) A-SA, and (c) C-SA.

Furthermore, the self-cross-linking reaction of C-SA (Figure 6c) has been proven by the presence of a strong exothermic peak at approximately 130 °C. The C-SA peak indicated that such cross-linking has developed after the reaction. However, the self-cross-linking reaction has been confirmed by the presence of a new peak in FTIR and 13C–NMR. The formation of a methylene bridge can occur by causing reactions between −CH2–OH groups and −CH2+ groups under ambient conditions where it will start to grow and cross-link to form a polymer–particle network.64 The result of C-SA showed the potential to cross-link A-SA with DCM as a reagent.

4. Adhesion Strength

To evaluate the adhesive strength of the novel path of SA self-cross-linking, C-SA has been tested for lap shear strength in the thin wooden board. Figure 7 shows the lap shear adhesion strengths of A-SA and C-SA at different hot-pressing temperatures of 160 and 180 °C, as shown in Table 3. However, the hot-pressing temperatures were selected on the basis of the exothermic temperature displayed in DSC. Ten lap shear adhesion strengths gradually increased from 0.07 to 0.12 MPa when the hot-pressing temperature increased from 160 to 180 °C. The results suggest that the cross-linking of the C-SA sample is advanced further in the curing process at a high hot-pressing temperature. Regardless of the adhesion performance, these results provided evidence for the self-cross-linking of C-SA for lignin-based wood adhesives, which is a new way of cross-linking of SA by the activation of its methoxy groups. In addition, the self-cross-linking of C-SA could be improved under higher hot-pressing temperatures in the future.

Figure 7.

Figure 7

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Shear strength and fractured specimens of C-SA.

5. Conclusions

The results of this study show a novel way of self-cross-linking of SA by activating the methoxy group of SA for lignin-based wood adhesives. After the activation reaction induced a change in its color changes, functional group analysis such as FTIR of SA has shown reduction spectra corresponding to peaks of −OCH3, indicating the occurrence of methoxy modification of activated syringaldehyde (A-SA). The A-SA results were supported by chemical structure analysis of the 13C–NMR spectrum that revealed the appearance of a peak belonging to −SiMe3; this peak disappeared after the self-cross-linking reaction. In addition, the characterization of thermal behavior via DSC has revealed the appearance of a strong exothermic peak around 130 °C after the A-SA reaction, which led to a cross-linking reaction. The cross-linking reaction was strengthened with adhesion strength as the temperature increased from 160 to 180 °C. This study has opened a new path to develop a novel approach of methoxy groups for creating lignin-based adhesives. Thus, the results could lead to employment of the cross-linking method of hardwood lignin for the application of advanced wood adhesives in the future.

Acknowledgments

This work was supported by the National Research Foundation (NRF) of Korea, funded by the Korean Government (MSIT) (Grant No. RS-2023-00240043)

The authors declare no competing financial interest.

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