Study on the Solubility of Industrial Lignin in Choline Chloride-Based Deep Eutectic Solvents

Abstract

Deep eutectic solvents (DESs) have emerged as a promising class of solvents for lignin dissolution, which could significantly expand the application potential of lignin. In this study, over forty ChCl-based DESs of three major types were synthesized and subjected to investigate the solubilisation of two industrial lignins in DES. The results showed that higher HBD content, shorter carbon chain length in the HBD, and fewer functional groups favored lignin solubilization. DESs containing carboxyl groups were found to be more favorable for breaking β-O-4 bonds and solubilizing lignin. Additionally, high temperature and appropriate water content were observed to promote lignin solubilization. The pretreatment of poplar and maize straw with ChC/FA(1:3), ChC/LA(1:3), ChC/Gly(1:3), ChC/EG(1:3), ChC/Urea(1:3), and ChC/TEOA(1:3) showed good solubilization of lignin, with ChCl/FA(1:3) being particularly effective in solubilizing poplar lignin and maize straw lignin, achieving lignin solubilization of 82% and 57%, respectively. Overall, these findings suggest that DESs have great potential as solvents for lignin dissolution.

1. Introduction

Lignin, as the sole renewable aromatic natural biopolymer found in nature, holds immense potential in the production of valuable aromatic compounds, fuel products, and functional materials [1]. The lignin, along with cellulose and hemicellulose, forms the structural backbone of plants, with the three components forming a complex polymer fortified by hydrogen bonds and van der Waals forces [2]. The complex structure of lignin makes it difficult for existing solvents to effectively dissolve it, thus limiting its wide application [3]. Currently, the high value utilization rate of lignin is only about 2%, with the majority being used for combustion to provide heat, which results in significant waste of these renewable resources [4]. While ionic liquids have shown promise in effectively dissolving lignin, their use poses logistical challenges due to poor stability, high cost, and the volatility of organic solvents [5,6]. Consequently, the quest for environmentally-friendly deep eutectic solvent (DES) with high lignin solubility continues to be a popular research topic.

DES, composed of a hydrogen bond donor (HBD) and a hydrogen bond acceptor (HBA), are being investigated as a more feasible alternative to ionic liquids, owing to their impressive ability to effectively dissolve lignin [7,8,9]. Liu et al. developed two novel types of DES utilizing lactic acid and N-methylthiourea as HBDs, and four different quaternary ammonium salts as HBAs, demonstrating their effectiveness in dissolving various types of lignin [10]. Choline chloride (ChCl) is currently one of the more commonly utilized HBAs, known for its biodegradability, low cost, and low toxicity, and has shown great potential in processing lignocellulosic biomass [11,12,13]. Lou et al. investigated the preparation of lignin nanoparticles from herbaceous biomass (wheatgrass) using choline chloride-lactic acid (ChCl-LA) [14]. The study found that DES was capable of extracting lignin with a high purity of 94.8% and a yield of 81.5%, resulting in well-dispersed nanoparticles with a narrow size distribution and a peak of 70–90 nm. ChCl-based DES demonstrated a superior extraction effect on lignin, as Cl⁻ formed hydrogen bonds with the hydroxyl groups in lignin [15,16]. Studies have demonstrated that X⁻ in HBA aids in breaking the β-O-4 bond in lignin, while also preventing self-condensation of lignin [17]. The complex polymer structure and intricate bonding patterns of lignin require an optimal degradation system, as different types of DES can have varying effects on its degradation [18]. Commonly used hydrogen bonding donors for DES synthesis can be classified into three types: carboxylic acid, hydroxyl, and amine/amide, which correspond to acidic, neutral, and basic pH, respectively. By combining these HBDs with ChCl, DES with varying properties can be synthesized, which can have different impacts on lignin solubilization. Acid-based DES is particularly of interest due to its ability to break the β-O-4 bond in lignin [19,20]. Furthermore, the solubilization of lignin is also affected by the molar ratio, temperature, and water content [18]. Exploring the relationship between the structure of HBDs and lignin solubilization is beneficial for achieving selective extraction and utilization of lignin.

This study aimed to further explore the solubilization of alkaline lignin and sodium lignosulfonate in ChCl-based DES by selecting three major types of HBDs. The relationship between the hydrogen bond length, the number of functional groups, and the amount of lignin dissolved was investigated. Two DESs of each of the three types with good lignin solubilization effects were selected to pretreat poplar wood and straw to investigate their separation effects on natural lignin. The structures of different low co-solvents were analyzed using ¹H NMR, FT-IR, and thermogravimetry. In addition, the effect of water on the structure of DES was also explored.

2. Materials and Methods

2.1. Materials

Alkali lignin (AL), sodium lignosulfonate (SL), choline chloride (ChCl), formic acid (FA), acetic acid (HAC), propionic acid (Pro), butyric acid (But), lactic acid (LA), citric acid (CA), p-toluene sulfonic acid (PTS), malic acid (MA), Benzoic acid (BA), glycol (EG), 1, 3-propylene glycol (1,3-PG), 1, 2-propylene glycol (1,2-PG), 1, 4-butanediol (BDO), 1,5-butyl glycol (1,5-Pen), phenol (Phe), benzyl alcohol (BP), phenylethanol (β-Phe), glycerin (Gly), formamide (Form), acetamide (Ace), and triethanolamine (TEOA) were obtained from commercial suppliers and used without further purification. Poplar (Y) and corn straw (J) were produced in China’s Shandong province.

2.2. Preparation of DES

To select DES that remained liquid, a fixed amount of ChCl was mixed with a representative HBD, such as formic acid, lactic acid, glycerol, urea, etc., from the three major categories of acidic, near-neutral, and basic HBDs in molar ratios of 1:1, 1:2, and 1:3, respectively, at 80 °C with stirring until a clear and transparent liquid was obtained. The resulting mixture was left overnight to observe its state, and the DES that remained liquid was selected for further analysis.

The prepared DES was then mixed with water at different weight ratios, ranging from 10 wt% to 60 wt%, and thoroughly mixed.

2.3. Lignin Solubility Determination

The first step involves drying alkaline lignin or sodium lignosulfonate at 80 °C for 6 h. Then, the excess amount of the dried lignin is weighed and dissolved in different DES at 40 °C, 80 °C, and 120 °C for 6 h. After dissolution, the mixture is centrifuged and the resulting precipitate is washed with anhydrous ethanol, dried, and labeled as U-XT-Z (where X represents the DES species, T represents the temperature, and Z represents the lignin species). The supernatant is treated with an excess amount of anhydrous ethanol, left to stand overnight, and then washed by centrifugation and dried. This is labeled as XT-Z.

Lignin solubility was calculated using the following equation.

$$\mathrm{Lignin}\mathrm{solubility}\left(\mathrm{wt}\%\right)=\frac{{\mathrm{m}}_0-{\mathrm{m}}_U}{{\mathrm{m}}_{\mathrm{DES}}}\times 100\%$$

where m₀ represents the initial mass of lignin; m_U represents the mass of undissolved lignin; m_DES represents the mass of DES.

2.4. Characterization of DES

Fourier transform infrared spectroscopy (FT-IR) was conducted using a Thermo Fisher Nicolet Model IN10 FT-IR spectrometer in the range of 400–4000 cm⁻¹, with 32 scans per spectrum and a resolution of 4 cm⁻¹, to characterize the DES. A suitable amount of DES was mixed with 0.6 mL of DESO-d6, thoroughly mixed, and quickly transferred to an NMR tube for characterization by ¹H NMR on a German Bruker NMR mass spectrometer (sampling frequency 400 MHz). The thermal weight loss analysis of DES was performed on a TG/DTA 8122 thermal weight loss instrument manufactured by Rigaku, Japan, under nitrogen protection, increasing from 30 °C to 400 °C at a rate of 10 °C/min.

2.5. Characterization of Lignin

Fully dried lignin (1 mg) was mixed with KBr (100 mg) and compressed into tablets using the same testing conditions as for DES.

2.6. Pretreatment

The poplar wood and corn straw are soaked in water to remove the water-soluble matter, dried, and sieved. A certain amount of 40–60 mesh dried corn straw or poplar wood is weighed and added to the DES at a solid-liquid ratio of 1:30 and then reacted at 120 °C for 3 h. After the reaction is complete, cool to room temperature, filter, filter residue with deionized water, and anhydrous ethanol wash three times.

2.7. Component Analysis

The components of poplar and corn straw before and after pretreatment were analyzed according to the standard method of the National Renewable Energy Laboratory (NREL) [21].

3. Results and Discussion

3.1. Initial Screening of DES

This study utilized choline chloride as the constant HBAs and explored the relationship between different types of HBDs, dissolution conditions, and the solubilization effect on alkaline lignin and sodium lignosulfonate, as illustrated in Scheme 1. Among the three major types of DES, two were selected for their effective solubilization effects on lignin and were used for the pretreatment of poplar wood and corn stover, respectively. The hydrogen bonding effect of the DES was characterized by FT-IR and ¹H NMR, and their thermal stability was analyzed through TG.

The type and proportion of HBDs in DES have a significant impact on their properties. This study mainly investigates whether they can maintain a stable liquid state at room temperature and screens out stable DES for subsequent work, as shown in Table S1. The presence of hydroxyl groups seems to facilitate DES formation, as lactic acid can form a transparent liquid with ChCl in mole ratios of 1:1, 1:2, and 1:3. On the other hand, when propionic acid was utilized as the HBD, only the DES formed in a 1:3 ratio could sustain a stable liquid state at room temperature. In comparison to HBDs containing carboxyl and hydroxyl groups, amino/acrylamide-containing HBDs had a lower content and a lesser likelihood of developing DES with ChCl.

3.2. Solubility of Lignin in DES

The solubility of two industrial lignins in DES consisting of various types of HBDs was investigated. As illustrated in Figure 1, an augmented concentration of most HBDs led to enhanced solubility of lignins, suggesting that a rise in HBD content facilitated the solubilization of lignin through the creation of more opportunities for hydrogen bond formation [22]. In general, HBDs containing carboxyl groups exhibited greater lignin solubilization capacity compared to those containing hydroxyl, amino/amide groups. As observed from Figure 1a,b, p-toluene sulphonic acid (p-TSA) was the most effective carboxyl-containing HBD, as ChCl/p-TSA(1:2) was able to dissolve up to 7.48 wt% and 8.21 wt% of alkaline lignin and sodium lignosulphonate, respectively. It could be observed that the solubility of lignin, including both alkaline lignin and sodium lignosulfonate, was reduced with an increase in the carbon chain length of the carboxyl-containing HBDs, such as formic, acetic, propionic, and butyric acids. This indicates that longer carbon chains tend to have an inhibiting effect on the dissolution ability of these HBDs for lignin. Additionally, the presence of hydroxyl groups enhanced the lignin solubility for the same carbon chain length. The solubility of lignin in ChCl/LA(1:3) was found to be significantly higher than ChCl/Pro(1:3) due to the presence of a hydroxyl group in lactic acid. This hydroxyl group provides an additional hydrogen bonding site between the DES and lignin, thus allowing for enhanced solubility of the DES and facilitating the dissolution of lignin [23]. The dissolution of lignin is negatively affected by an increase in the number of carboxyl groups. This could be due to the fact that an increase in viscosity leads to reduced contact between DES and lignin, thereby weakening the solubilization effect of lignin. Another reason for this could be the decarboxylation of the carboxyl groups in polyacids at high temperatures, which results in the release of CO₂ and further reduces the solubility of lignin [22].

The hydroxyl-containing HBD forms DES with ChCl at a near-neutral pH and shows good biocompatibility, allowing the intended experimental results to be obtained under mild conditions. As shown in Figure 1b,e, ethylene glycol is the HBD that contains hydroxyl groups and exhibits the most superior solubility. ChCl/EG(1:3) dissolved 6.57 wt% of alkali lignin and 6.50 wt% of lignosulfonate. The next best was ChCl/Gly(1:3), which dissolved 5.91 wt% of alkali lignin and 5.59 wt% of lignosulfonate. Similar to DESs based on carboxylic acids, the solubility of lignin decreased with the increasing length of the carbon chain. The number of hydroxyl groups is inversely proportional to the ability of hydroxyl-containing HBDs to dissolve lignin. ChCl/EG(1:3) had a stronger ability to dissolve lignin than ChCl/Gly(1:3). Excess hydroxyl groups in the HBD can lead to a strengthening of the hydrogen bond network between HBDs, which will weaken the hydrogen bond between DES and lignin, resulting in the reduction of lignin solubility [18]. The number of hydroxyl groups is inversely proportional to the ability of hydroxyl-containing HBDs to dissolve lignin. ChCl/EG(1:3) had a stronger ability to dissolve lignin than ChCl/Gly(1:3). Excess hydroxyl groups in the HBD can lead to a strengthening of the hydrogen bond network between HBDs, which will weaken the hydrogen bond between DES and lignin, resulting in reduced solubility.

DESs formed by HBDs containing amino or amide groups and choline chloride (ChCl) tend to be basic in nature. The solubility of lignin in ChCl/Form(1:2) was higher than that in ChCl/Urea(1:2) and, similar to hydroxyl-containing HBDs, the number of amino groups was negatively correlated with the solubility of lignin. The other amide/amino group in ChCl can form additional hydrogen bonds with ChCl, occupying the hydrogen bonding position of DES and reducing the hydrogen bond between DES and lignin, thus weakening the solubility of lignin [18].

The optimal proportion of each DES with the best lignin dissolution effect was selected to investigate the dissolution temperature. As shown in Figure 2, most DESs exhibited the highest lignin dissolution efficiency at 120 °C, indicating that higher temperatures promoted the dissolution of lignin, possibly due to the thermal breakdown of lignin bonds [9]. In addition, high temperature can decrease the viscosity of DES and increase the possibility of contact between DES and lignin, resulting in an improvement of the dissolution rate [24]. Considering both the dissolution effect and cost of lignin, we selected ChCl/FA (1:3), ChCl/LA (1:3), ChCl/Gly (1:3), ChCl/EG (1:3), ChCl/Urea (1:2), and ChCl/TEOA (1:3) for further investigation. At 120 °C, the solubility of alkaline lignin in ChCl/FA (1:3), ChCl/LA (1:3), ChCl/Gly (1:3), ChCl/EG (1:3), ChCl/Urea (1:2), and ChCl/TEOA (1:3) was 4.54 wt%, 6.34 wt%, 7.55 wt%, 7.60 wt%, 7.49 wt%, and 6.14 wt%, respectively. For sodium lignosulfonate, the dissolution rates were 4.35 wt%, 6.76 wt%, 6.34 wt%, 8.25 wt%, 7.79 wt%, and 6.00 wt% for ChCl/FA (1:3), ChCl/LA (1:3), ChCl/Gly (1:3), ChCl/EG (1:3), ChCl/Urea (1:2), and ChCl/TEOA (1:3), respectively.

Research has demonstrated that the addition of water can decrease viscosity, increase polarity and conductivity, and therefore enhance processes such as electrodeposition, dissolution, and extraction [25,26,27]. Belinda Soares and colleagues conducted a comprehensive study on the solubility of lignin monomeric model compounds in pure DES and aqueous DES solutions [28]. They first revealed the ability of DES to enhance the solubility of poorly soluble lignin derivatives and organic solvent lignin itself in DES aqueous solutions through a hydrophilic mechanism, mainly due to dispersion interactions. In addition, Zeynep Sumer used molecular dynamics simulations to study the solubility trend of lignin in DES-water mixtures, and proposed a method to explain its solubility through a hydrophilic mechanism [29]. Among them, hydrogen bonding between solute and solvent molecules is one of the key factors in enhancing solubility. The highest solubility is achieved when a wide network of hydrogen bonds is formed, allowing nonpolar groups in the DES containing acidic hydrogen bonding donors to interact with both the solute and hydrogen bonding acceptor molecules in water. Therefore, a series of in depth experiments were conducted to investigate the dissolution ability of DES on lignin. Through a systematic variation of the water content in the DES, the ability of the solvent system to dissolve lignin was progressively examined, aiming to identify the most effective DES solvent and enhance the lignin solubility. As seen from Figure 3, adding an appropriate amount of water to the DES can enhance the solubilization of lignin, with different DESs showing different effects on water content. ChCl/FA(1:3) showed the best solubilization of alkaline lignin at 30 wt% water, reaching 8.99 wt%, while sodium lignosulfonate reached a maximum value of 8.92 wt% at 40 wt% water. The solubility of ChCl/LA(1:3) reached its highest value at 20–30 wt% water content and decreased as the water content continued to increase. ChCl/Urea(1:2) showed good solubility for both lignins when 40 wt% water was added. ChCl/EG(1:3) reached its maximum solubility for sodium lignosulfonate at 40 wt% water content, while the solubility of alkaline lignin increased with increasing water content. In the range of 20–60 wt%, the water content of ChCl/Gly(1:3) and ChCl/TEOA(1:3) was proportional to the solubility of lignin.

While adding an appropriate amount of water to DES can effectively enhance lignin solubilization, excessive water can have a significant impact on the structure of DES. The excess water acts as an additional HBD and acceptor, participating in the intermolecular network and gradually weakening the intermolecular interactions between all ionic and neutral species. As a result, at high dilution, all components of DES become completely soluble [30]. Therefore, ensuring the stability of DES in an aqueous solution is of utmost importance.

3.3. Characterization of DES

The formation of hydrogen bonds was characterized using FT-IR and ¹H NMR. When hydrogen bonds are established between molecules, the electron cloud density around protons typically decreases. This is primarily due to the presence of strongly electronegative atoms that form hydrogen bonds with protons. This leads to a significant shift of proton signals in hydrogen bonds towards lower fields, and the displacement of active hydrogens can characterize the formation of hydrogen bonds between DES molecules [31,32]. From Figure 4, it can be observed that the -OH peak of ChCl appeared at 5.668 ppm, while the hydrogen signal of -CH₃ was at 3.147 ppm, and the hydrogen signal of the two methylene groups were at 3.433 and 3.808 ppm, respectively. The signal of water appeared near 3.392 ppm. Furthermore, no new bonds were detected in the ¹H NMR of DES, indicating that DES was formed due to the interaction between HBDs and HBAs. As shown in Figure 4a, the -COOH peak of FA appeared at 12.723 ppm. After the formation of DES with ChCl, the peak shifted to 12.892 ppm. This is due to the hydrogen bond formation between the carboxyl group and Cl⁻ of ChCl, which reduces the electron shielding effect and transforms resonance absorption to a higher chemical shift. It has been shown that the carboxyl group can influence the nuclear magnetic resonance expression of the hydroxyl group, causing a smaller -OH signal from ChCl in ChCl/FA(1:3). Similarly, the active hydrogen from lactic acid in ChCl/LA(1:3) also experienced a low field shift. After the formation of DES, the H chemical shift of -OH in glycerol changed from 4.391 ppm to 4.414 ppm, indicating that the -OH in glycerol formed a hydrogen bond with Cl⁻ in ChCl. Simultaneously, the chemical shift of -OH in ChCl shifted to 5.441 ppm because the hydroxyl group in ChCl can form hydrogen bonds with both glycerol and Cl⁻. The hydrogen bond strength formed with Cl⁻ was higher than that formed with glycerol, resulting in the observed chemical shift [31]. Additionally, previous studies have demonstrated that the formation of hydrogen bonds with HBD was more favorable as the ChCl content in DES increases. When the molar ratio of ChCl:Gly is 1:3, ChCl tended to form hydrogen bonds with glycerol. In ChCl/EG(1:3), the OH from ethylene glycol also underwent a low field displacement. Similarly, it can be inferred that there was a hydrogen bond between ChCl and HBD in ChCl/Urea(1:2) and ChCl/TEOA(1:3).

As depicted in Figure 5, it can be observed that the FT-IR spectrum of DES exhibited significant broadening and shift when compared to pure HBD. Notably, a strong and broad spectrum at 3600–3100 cm⁻¹ can be observed, which is attributed to the stretching vibration of -OH and the formation of a large number of hydrogen bonds [33]. The DES spectra showed the presence of the characteristic peak C-N of choline chloride (near 955 cm⁻¹). The peak position of the hydroxyl group in formic acid was observed to be 3427 cm⁻¹ in ChCl/FA(1:3), while the peak position shifted red to 3388 cm⁻¹ after DES formation, indicating the formation of hydrogen bonds between formic acid and ChCl. Additionally, the stretching vibrational absorption peak of the carboxyl group (C=O) in formic acid shifted from 1728 cm⁻¹ to 1724 cm⁻¹ after DES formation, confirming the formation of hydrogen bonds between the two. Figure 5b shows that the -OH stretching vibration absorption peak, the C=O stretching vibration absorption peak, and the C-O stretching vibration absorption peak in ChCl/LA(1:3) were shifted. The C-O bond in glycerol was found at 1042 cm⁻¹ for C₁ and C₃, at 1110 cm⁻¹ for C₂, and the -OH stretching vibration absorption peak occurred at 3373 cm⁻¹. The location of the peak remained almost unchanged after DES formation, but the range of the peak significantly increased. In Figure 5d, the range of the -OH stretching vibration peak became significantly larger after DES formation compared to EG. In addition, the C=O (1685 cm⁻¹) and N-H (3369 cm⁻¹) peaks of urea both shifted to lower wave numbers when forming DES. This is because the -NH₂ and -C=O groups of urea form N-H…Cl and O-H…O hydrogen bonds, respectively, with the -Cl⁻ and -OH in choline chloride [30]. In the ChCl/TEOA(1:3) spectrum, the C-O peak (1072 cm⁻¹) exhibited a blue shift compared to triethanolamine, and the -OH peak range broadened, possibly due to the formation of O-H^…Cl hydrogen bonds between the two. The absence of new peaks in the FT-IR spectra of all the DES formed indicated that DES formation resulted from the bonding between the HBD and HBA, and no new compounds were formed. Additionally, the small peak near 1635 cm⁻¹ in part of the spectrum can be attributed to the presence of a small amount of water in the sample, which is consistent with the ¹H NMR results.

Figure 6 illustrates that DES undergoes gradual degradation in stages as the temperature increases. The mass loss can be divided into three main parts. Initially, the mass loss at around 100 °C is primarily due to the evaporation of water. As the temperature continues to rise, the hydrogen bond of DES is broken down, and it is decomposed into its constituent HBA and HBD components. The second part involves the decomposition of the HBD with poor thermal stability, and the decomposition temperature varies depending on the specific HBD. The thermal stability of DES largely depends on the HBD component. Finally, the HBAs with better thermal stability undergo decomposition. The ChCl used in this study begins to decompose at 250 °C and is almost completely decomposed by 300 °C. In Figure 6a, the decomposition of formic acid in ChCl/FA(1:3) occurred between 100–250 °C. Similarly, in Figure 6b, ChCl/LA(1:3) showed rapid degradation of lactic acid at 200–250 °C, resulting in a mass loss of 41.4%. Figure 6c demonstrates that ChCl/Gly(1:3) remained mostly stable between 100–200 °C, and the decomposition of glycerol primarily occurred at 200–235 °C. The degradation of ChCl/Urea(1:2) and ChCl/TEOA(1:3) at high temperatures was comparable to ChCl/Gly(1:3), with better stability observed at 200 °C. The degradation of glycols in Figure 6d mainly occurred between 100–200 °C, with a mass loss of 40.87%, while remaining stable between 200–250 °C.

It has been observed that water, acting as an additional HBD and HBA, participates in the intermolecular network and gradually weakens the intermolecular interactions, ultimately resulting in the complete dissolution of DES [30]. In order to further elucidate the effect of water on DES, ¹H NMR characterization of aqueous DES was carried out, as shown in Figure S2. To more clearly represent the change in hydrogen bonding, Δδ was defined as the relative shift. Δδ = δ − δ₀, with δ₀ representing the shift in the ¹H NMR spectrum of pure DES, and if Δδ > 0, then a high-frequency shift occurred; i.e., a stronger hydrogen bond was formed, as shown in Figure 7. The addition of water had the greatest impact on the HBD and the active hydrogen in the HBD, leading to varying degrees of low field shifts in the -OH in ChCl. This suggested that some degree of water association with ChCl occurred, which was supported by Figure S2g. However, since the ChCl cation had a weak interaction with DES molecules, the addition of water may not form an interaction with it, resulting in displacement [34]. As shown in Figure 7, the effect of water on DES varied depending on the type of DES. In Figure 7a, the low-frequency shift of -COOH in formic acid gradually increased as the water content increased, and when the water content exceeded 40 wt%, the -COOH signal completely disappeared, indicating the complete disruption of the ChCl/FA(1:3) hydrogen bond structure. In Figure 7b, the changes in the NMR of the mixture of ChCl/LA(1:3) and water were mainly reflected in the low-frequency shift of water, indicating that the hydrogen bonds formed by water and DES were weaker. In Figure 7c, a high-frequency shift in H₂O occurred due to the stronger bond formed between water and Cl⁻ compared to other water molecules in the mixture of DES and water [34]. This suggested that the effect of water on DES depended on the specific components and their interaction with water. There was a Cl-EG supramolecular complex in ChCl/EG(1:3). However, upon the addition of water, the hydroxyl groups in glycol and ChCl underwent a low-field shift and gradually became hydrated through H₂O to form new hydrogen bonds, resulting in the deshielding of the hydroxyl proton. This indicated an exchange between -OH and water [25,35]. The supramolecular complex structure was partially retained even after the addition of water up to 60 wt%, as shown in Figure 7d. Previous studies have indicated that the addition of trace amounts of water to DES can enhance the interaction between choline and urea, but further dilution can result in the disruption of the DES structure [36]. The high-frequency shift of water in the NMR spectra indicated the formation of hydrogen bonds between water and DES [37]. Figure 7e shows that water formed NH…O bonds to replace the bond formed by ChCl with urea, which led to a low-frequency shift of the -NH group. In Figure 7f, the mixture of ChCl/TEOA(1:3) with water resulted in a high-frequency shift of the hydroxyl groups of water and TEOA, indicating the formation of a stronger hydrogen bond between the two. Based on the stability of the aqueous DES solution and the solubility of lignin, the water content of ChCl/FA(1:3), ChCl/LA(1:3) was determined to be 30 wt%, and that of ChCl/EG(1:3), ChCl/TEOA(1:3) was determined to be 50 wt%. The water content of ChCl/Urea(1:2) and ChCl/Gly(1:3) were determined to be 40 wt% and 20 wt%, respectively.

3.4. Structural Change of Lignin

To characterize the structural changes of lignin before and after dissolution, FT-IR spectral analysis was performed, and the results are shown in Figure 8. It can be observed that the structure of lignin remained mostly unchanged after DES dissolution. The absorption peak at 3440cm⁻¹ corresponds to the stretching vibration of -OH, while 2935 cm⁻¹ represents the C-H stretching vibration peak of methyl and methylene. The characteristic absorption peaks of the lignin benzene ring were observed at 1629 cm⁻¹ and 1579 cm⁻¹. The absorption peaks at 1411 cm⁻¹ and 1416 cm⁻¹ correspond to the methyl absorption peaks in the side chain of lignin structural units or the methyl in the methoxy group of the benzene ring. Additionally, the absorption peak at 1126 cm⁻¹ corresponds to the characteristic absorption peak of guaiac-based (S) lignin structural units. The stretching vibration of C-O-C was observed at 1055 cm⁻¹. The benzene ring structure was not significantly affected in all lignin structures after treatment, indicating that the dissolution process did not lead to the destruction of the benzene ring structure. However, the vibration intensity of the -OH near 3440 cm⁻¹ decreased, which could be attributed to the breaking of the ether bond during the dissolution process and the generation of new -OH.

3.5. The Separation of Lignin from Poplar and Corn Stover Using DES

Referring to Figure 9a,c, it can be observed that the pretreatment of poplar and corn stover at 120 °C by DES resulted in an increase in the content of glucan, as well as changes in the fractional contents of acid-soluble lignin, acid-insoluble lignin, cellulose, hemicellulose, and ash of the residues. As shown in Figure 9b, all six DESs had a certain degree of solubilization effect on poplar lignin, with the acidic DESs being more effective. Specifically, ChCl/FA(1:3) achieved 82% solubility of lignin, and ChCl/LA(1:3) reached 76%, indicating that they could effectively break the β-O-4 bond of lignin. In contrast, neutral and basic DESs showed lower effectiveness in solubilizing lignin under the same conditions. The acidic DESs were more capable of breaking the β-O-4 bond. In addition, Xu et al. investigated the mechanism of lignin removal in ten different ChCl-based DES using molecular dynamics (MD) simulations. The results showed that DES containing an acidic HBD exhibited the best lignin removal rate due to the presence of free protons in the solvent [38]. This finding is in agreement with the outcomes obtained in this research. Moreover, acidic DESs (Figure 9d) were also good solvents for maize stover lignin, with ChCl/FA(1:3) achieving 57% solubility. Alkaline DESs also demonstrated effective extraction of lignin, with ChCl/Urea(1:2) reaching 46%.

4. Conclusions

The solubilization of lignin by DES is primarily facilitated by the breaking of hydrogen bonds, which is influenced by the high content of HBD in the DES. The number of functional groups and the carbon chain length in HBD has an inverse relationship with lignin solubilization. The position of hydroxyl groups also plays a role, with the presence of such groups promoting the lignin solubilization in DES with carboxyl groups. Furthermore, high temperatures and the addition of water can enhance the ability of DES to dissolve lignin. Finally, ChCl/FA(1:3), ChCl/LA(1:3), ChCl/Gly(1:3), ChCl/EG(1:3), ChCl/Urea(1:2), and ChCl/TEOA(1:3) exhibited excellent lignin solubility. The characterization of DES by FT-IR and ¹H NMR demonstrated that the formation of DES was due to the formation of hydrogen bonds between HBA and HBD. In addition, water acted as an additional HBD or HBA, which gradually weakened the interactions of DES itself, ultimately leading to the complete dissolution of DES. Pretreatment of poplar and maize straw with DES revealed that ChCl/FA(1:3) pretreatments at 120 °C for 3 h could achieve 82% solubility of poplar lignin and 57% solubility of corn stover lignin.