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Article

Depolymerization of Kraft Lignin Using a Metal Chloride-Based Deep Eutectic Solvent: Pathways to Sustainable Lignin Valorization

by
Shubho Ghosh
,
Masud Rana
and
Jeong-Hun Park
*
Department of Environment and Energy Engineering, Chonnam National University, Gwangju 61186, Republic of Korea
*
Author to whom correspondence should be addressed.
Appl. Sci. 2024, 14(24), 11571; https://doi.org/10.3390/app142411571
Submission received: 25 October 2024 / Revised: 30 November 2024 / Accepted: 9 December 2024 / Published: 11 December 2024
(This article belongs to the Special Issue Industrial Chemical Engineering and Organic Chemical Technology)
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Abstract

:
Driven by the urgent need for sustainable alternatives to fossil fuels, the focus on the exploration of lignocellulosic biomass, particularly lignin, as a promising renewable feedstock for biofuels and high-value chemicals has intensified. This study investigated the depolymerization of KL using a DES comprising ChCl and ZnCl2. Our analysis systematically focused on the effects of reaction temperature, time, and the DES-to-lignin ratio on the yields and characteristics of the products. Optimal KL depolymerization was observed at a temperature of 190 °C and a duration of 8 h, yielding a maximum liquid product yield of 54.44% and RL yield of 45.56%. The results revealed that increasing the reaction temperature enhanced the depolymerization process owing to a reduction in the viscosity of the DES, which improved mass transfer and interactions with lignin. Under these optimal conditions, the molecular weight of the bio-oil was considerably lower (Mw = 1498 g/mol and Mn = 1061 g/mol) than that of the bio-oil obtained without DES treatment (Mw = 1872 g/mol and Mn = 1259 g/mol), indicating a more favorable molecular weight distribution with DES treatment. Furthermore, elemental analysis revealed a reduction in the O, N, and S contents of the RL following DES treatment, increasing the high heating value from 24.82 MJ kg−1 for the non-DES-treated RL to 26.44 MJ kg−1 for the DES-treated RL. These findings underscore the potential of the (ChCl:ZnCl2) DES as a sustainable and effective medium for lignin valorization, paving the way for the synthesis of high-quality biofuels and chemicals from lignocellulosic biomass.

1. Introduction

With the depletion of fossil fuels and the rise in global energy demand, renewable energy resources have garnered extensive attention [1]. Under these circumstances, lignocellulosic biomass has emerged as a critical renewable feedstock for synthesizing fuels and chemicals and harnessing energy [2]. Lignocellulosic biomass is composed of three major constituents: cellulose, hemicellulose and lignin. Among these constituents, lignin, a complex and highly branched three-dimensional polymer, is composed of different phenolic units, including guaiacyl, syringyl, and p-hydroxyphenyl, which are interconnected through C–C and C–O linkages [3]. As a major component of lignocellulosic biomass, lignin accounts for approximately 15–30% of the dry weight of wood and other plant materials. Lignin, alongside cellulose and hemicellulose, holds significant potential as a source of sustainable aromatic compounds [4] and alkane derivatives suitable for liquid fuels [5]. In the context of the biorefinery concept, lignin is a valuable source of aromatics and a key candidate for replacing fossil-based feedstocks in the production of fuels, chemicals, and materials [6]. However, despite its abundance and importance in nature, lignin remains underutilized owing to its unidentified molecular structures, molecular weight fluctuations, complex structure, and robust C–C bonds, which impede efficient conversion processes [7].
To address this, several approaches for lignin depolymerization have been proposed to date, including fast pyrolysis [8], oxidation [9], solvothermal methods [10], and hydrothermal liquefaction [11,12,13,14]. Among these techniques, solvothermal methods have garnered particular attention for their effectiveness in lignin degradation, utilizing either external hydrogen or hydrogen donor solvents with or without metal catalysts [15]. Both heterogeneous [16] and homogeneous catalysis approaches have been explored for the disruption of lignin’s chemical bonds to produce aromatic chemicals [17]. Heterogeneous catalysis typically uses metal catalysts, which can be highly efficient but often involve expensive noble metals such as Pd, Pt, and Ru, or less-efficient metal-based catalysts such as Zn or Cu, which can become saturated with lignin fragments. While homogeneous catalysis overcomes some of these shortcomings, it often requires strong mineral acids or alkaline solutions, posing technical challenges in downstream processing. Hence, green solvents or catalysts are being explored as more sustainable solutions to these environmental and health concerns [18,19]. In this context, ionic liquids (ILs) are considered a promising class of green solvents [20], particularly for biomass fractionation [21] and lignin depolymerization [22]. Previous studies have demonstrated that acidic ILs can effectively dissolve lignin. For instance, Cox et al. [23] achieved successful lignin depolymerization using 1-H-3-methylimidazolium chloride at temperatures ranging from 110 to 150 °C, which led to the disintegration of alkyl–aryl ether linkages. Jia et al. [24] reported a yield of over 70% in hydrolyzing β-O-4 bonds in lignin model compounds at 150 °C. However, despite the effectiveness of ILs, their high costs and limited availability have constrained their applications in large-scale lignin depolymerization, prompting the exploration of other green alternatives. Furthermore, given that ILs are entirely composed of ions, their use requires high-purity solvents, which can drastically alter their characteristics owing to ionic or molecular impurities [25].
Recent advancements in deep eutectic solvents (DESs) have considerably improved the efficiency of lignin extraction and biomass fractionation. A deep eutectic solvent is a mixture of two or more compounds that form a eutectic mixture, which exhibits a melting point significantly lower than that of the individual components. Notably, DESs offer numerous advantages, including cost-effectiveness, non-toxicity, and biocompatibility [26,27]. Furthermore, they can selectively dissolve lignin from plant materials through acid-based catalytic mechanisms [28]. DESs are also generally less expensive than numerous ILs owing to their low precursor costs, straightforward synthesis methods, and recyclability. Among the wide variety of DESs, metal halide-based DESs, particularly those containing zinc chloride (ZnCl2), have emerged as viable and affordable alternatives to conventional solvents. These DESs are particularly valued for their remarkable solvation capabilities, rendering them useful as solvents in biodiesel applications, reusable or homogenous catalysts, and high-temperature electrolytes [29].
Despite the successful applications of ZnCl2-based DESs as solvents in biodiesel applications, related fundamental research and further explorations in biomass conversion remain limited. Inspired by previous studies, this study investigated the combined effects of ethanol and (ChCl:ZnCl2) DESs on the depolymerization of Kraft lignin (KL). Key variables, including the molar ratio of ChCl to ZnCl2, reaction temperature, and reaction duration, were examined to assess their impacts on lignin depolymerization. Analytical techniques such as gas chromatography–mass spectrometry (GC–MS), gel permeation chromatography (GPC), and Fourier transform infrared spectroscopy (FT-IR) were employed to analyze the formation of different products and the cleavage of bonds in lignin.

2. Experimental Section

2.1. Chemicals and Materials

In this study, KL derived from pine wood (Source: Sigma-Aldrich, Saint Louis, MO, USA, CAS no. 67-48-1, Product number: 102496383) served as the primary material for analysis. All chemicals and reagents (with a purity level of ≥99.5%), including ethanol (≥98%), ethyl acetate (CH3COOC2H5), choline chloride (ChCl), and ZnCl2, were also obtained from Sigma-Aldrich (Saint Louis, MO, USA). All chemicals and solvents were used as received without further purification.

2.2. DES Preparation

The DES was synthesized following a previous method [30]. Specifically, ChCl and ZnCl2 were mixed in a 1:1 molar ratio. Varying amounts of the ChCl:ZnCl2 DES (3 g, 9 g, 15 g, and 21 g) were prepared. These mixtures were prepared by combining the following amounts of each component: 1.52 g, 4.55 g, 7.59 g, and 10.63 g of ChCl and 1.48 g, 4.45 g, 7.41 g, and 10.37 g of ZnCl2, corresponding to the respective total DES weights. Subsequently, the mixture was then heated to 90–100 °C in an oil bath under continuous agitation until a homogeneous, transparent liquid phase was achieved, as illustrated in Figure 1.

2.3. Lignin Depolymerization

KL depolymerization experiments were conducted using an HR-8300 batch-type high-pressure Hastelloy reactor (Hanwoul Engineering, Gunpo-City, Republic of Korea). The digital system of the reactor regulated pressure, temperature, and time, and these parameters were monitored using a thermocouple and gauge. In each experiment, 3.0 g of KL and 90 mL of ethanol were taken into the reactor, along with varying amounts of the ChCl:ZnCl2 DES (3, 9, 15, and 21 g). Following sample loading, the reactor was sealed and purged three times with N2 gas to eliminate air. The reaction was initiated with a constant stirring speed of 220 rpm under the specified experimental conditions, which included treatment at different temperatures (110, 130, 150, 170, 190, and 210 °C) for varying durations. Once the reaction was complete, the reactor was rapidly cooled in an ice bath to prevent repolymerization. The gas by-products were vented to the atmosphere to release internal pressure, following which the reactor was opened to extract the reaction mixture for further processing.

2.4. Product Separation

After cooling the reactor, the depolymerized product mixture containing both solid and liquid components was collected and then separated as shown in Figure 2. These solid and liquid phases were separated using filter paper. The solid phase, comprising the recovered lignin (RL) and DES, was washed with water to remove any residual DES and was then dried in an oven for 24 h. The liquid phase, containing the DES and bio-oil, was evaporated in a rotary evaporator to remove ethanol. Ethyl acetate was then added to the liquid phase to form a layer between the DES/water and bio-oil components. The organic (bio-oil) phase was then isolated using a separating funnel and further evaporated to remove the ethyl acetate. The yields of the liquid phase and RL were calculated using Equations (1) and (2), respectively:
Yield liquid part (wt.%) = (Weight liquid part/Weight initial lignin) × 100%,
Yield recovered lignin (wt.%) = (Weight recovered lignin/Weight initial lignin) × 100%.
Each experiment was repeated three times, and the average values were calculated.

2.5. Product Analyses

The extracted bio-oil was analyzed both qualitatively and quantitatively using a GC–MS (Agilent 6890, Santa Clara, CA, USA) apparatus equipped with an HP-5MS capillary column (30 m × 0.25 μm × 0.25 mm). High-purity He was used as the carrier gas at a flow rate of 1 mL min−1. The column temperature was initially maintained at 40 °C for 2 min and subsequently ramped up to 170 °C at a heating rate of 10 °C min−1, where it was held for 5 min. Thereafter, the temperature was increased to 300 °C at the same heating rate (10 °C min−1) and held for an additional 2 min. The individual components of the bio-oil were identified by comparing their GC–MS spectra and retention times with those of reference compounds cataloged in the mass spectral library of the National Institute of Standards and Technology. Selective phenolic compounds (such as phenol, guaiacol, 4-methyl guaiacol, 4-ethyl guaiacol, 4-propyl guaiacol, eugenol, vanillin, acetovanillone, isoeugenol, and homovanillic acid) were quantified using an external calibration method, and their yields were determined using Equation (3):
Yield monomer (wt.%) = (Weight specific monomer/Weight initial lignin) × 100.
FT-IR analysis was performed using a PerkinElmer 400 FT-IR spectrometer (Waltham, MA, USA) operating at a resolution of 1 cm−1 across a wavelength range of 500–4000 cm−1. The molecular weight distribution of the bio-oil was determined using a GPC apparatus (Waters-2695, Milford, MA, USA) equipped with three styragel columns (styragel HR 3, HR 4, and HR 5E). Polystyrene was used as the calibration standard. The GPC analysis was conducted at 35 °C, using tetrahydrofuran as the eluent at a flow rate of 1 mL/min. Each sample was subjected to GPC analysis twice, and the average values of the results were calculated. Elemental analysis of the KL feedstock and RL was conducted using a Vario MACRO cube analyzer (Hanau, Germany). The C, H, and N contents of the RL and lignin-derived bio-oil were directly measured using an elemental analyzer, while the O content was calculated based on the resulting difference. The high heating values (HHVs), serving as an indicator of the potential energy yield from a material, of both the KL and RL materials were calculated using their elemental compositions, including C, H, N, S, and O contents, according to the DIN 51900 standard [31,32], as indicated in Equation (4):
HHV (MJ kg−1) = [(34 × C) + (124.3 × H) + (6.3 × N) + (19.3 × S) − (9.8 × O)]/100.

3. Results and Discussion

3.1. Effect of Reaction Temperature and Time

Reaction temperature plays a critical role in determining the yields of the depolymerized lignin products formed during the hydrothermal depolymerization process. Hence, the effect of reaction temperature on the depolymerization of KL using the (ChCl:ZnCl2) DES was evaluated. These experiments were conducted with a KL:DES ratio of 1:1 at various temperatures of 110, 130, 150, 170, 190, and 210 °C, each for a duration of 8 h (Figure 3A). At 110 °C, the yield of the liquid phase was 31%, while that of the RL was 69%. As the temperature increased to 130, 150, and 170 °C, the liquid phase yields increased to 36.2%, 38%, and 43.06%, respectively, while the RL yields correspondingly decreased to 63.8%, 62%, and 54.94%. Interestingly, at 190 °C, the liquid yield dramatically increased to 54.44%, while the RL yield decreased to 45.56%, indicating that higher reaction temperatures favor lignin depolymerization. This is because the elevated temperatures lead to a reduction in the viscosity of the DES [33], thus minimizing mass transfer limitations and consequently enhancing interactions between the DES and lignin. However, at 210 °C, the liquid yield substantially decreased to 26.33%, while the RL yield increased to 73.67%, suggesting the simultaneous occurrence of depolymerization and repolymerization, with repolymerization predominating at this temperature [34]. Hence, to maximize the liquid product yield, further reactions were conducted at 190 °C, as this temperature yielded the highest amount of the liquid product. To assess the impact of reaction time on lignin depolymerization, reactions were conducted at 190 °C for durations ranging from 4 to 10 h (Figure 3B). The reaction conducted at 190 °C for 4 h resulted in liquid and RL yields of 37% and 63%, respectively. Extending the reaction time to 6 h substantially increased the liquid product yield to 42.22% and decreased the RL yield to 57.78%. After 8 h, the liquid product yield increased dramatically to a peak value of 54.44%, while the RL yield dropped to 45.56%. However, after 10 h, the liquid phase yield declined to 38.70%, while the RL yield increased to 61.30%, likely due to the repolymerization of the degraded products. These results indicate that prolonged reaction times can hinder lignin degradation [35].

Effect of DES Ratio

To investigate the role of the (ChCl:ZnCl2) DES in the depolymerization of KL, experiments were conducted with varying DES-to-lignin ratios (1:1, 3:1, 5:1, and 7:1) at 190 °C for 8 h, as illustrated in Figure 3C. At a DES-to-lignin ratio of 1:1, the liquid yield reached its peak at 54.44%, while the RL yield reached 45.56%. However, further increasing the DES-to-lignin ratio to 3:1 resulted in a considerable decline in the liquid product yield to 25.15%, while the RL yield increased to 74.85%. This trend of decreasing liquid product yield and increasing RL yield was also observed for DES-to-lignin ratios of 5:1 and 7:1. At a DES-to-lignin ratio of 7:1, the liquid product yield dropped to 7.21%, while the RL yield increased to 92.79%, indicating insufficient interaction between the DES and lignin. This is because higher DES-to-lignin ratios led to increased viscosity of the DES, which reduced the efficiency of energy conversion and the solubility of lignin during its depolymerization [36].

3.2. Characterization of the RL

3.2.1. FT-IR Analysis of the RL

FT-IR analyses were performed on the RL samples to examine potential chemical modifications induced by the DES during depolymerization. Figure 4 illustrates the FT-IR spectra of the KL feedstock and RL obtained from the DES treatment of KL. Notably, the peaks at 3388 cm−1 correspond to the hydroxyl groups (O–H) of aliphatic and aromatic compounds. Compared to the untreated residue and original KL, the peak intensity appears diminished for the RL obtained from the DES treatment, indicating a reduction in hydroxyl groups following DES treatment [37]. The twin peaks at 2937 cm−1 correspond to the C–H bonds of methyl and methylene groups [38]. The intensities of these peaks also decrease for the DES-treated RL, indicating a weakening of C–H bonds during DES processing. The carbonyl peak at 1670 cm−1 corresponds to the stretching vibrations of the C=O functional group, which is associated with the lignin side chain conjugated with aromatic compounds [39]. The peaks at 1372 cm−1 are attributed to phenolic hydroxyl groups [40]. Compared to the non-DES-treated RL, the peak at 1038 cm−1 diminished for the DES-treated lignin, suggesting effective lignin depolymerization. Furthermore, the peaks at 1217 cm−1 correspond to guaiacyl rings [41]. The reduction in the intensity of the guaiacyl peak for the DES-treated lignin sample indicates that lignin depolymerization decreases the guaiacyl unit concentration. Figure 5A,B illustrate the FT-IR spectra of the raw KL and RL obtained from the DES treatment of KL at different reaction temperatures and times. The most notable changes were observed in the RL obtained from KL depolymerization at 190 °C with a DES-to-lignin ratio of 1:1 for 8 h. Noticeably, the decrease in the 1038 cm−1 peak for the regenerated lignin sample after the DES treatment in ethanol at 190 °C for 8 h suggests that ether bonds in the lignin structure have been cleaved or significantly altered. This could be due to the solubilizing action of the DES and ethanol, combined with the high temperature, which leads to the depolymerization or degradation of lignin.

3.2.2. Elemental Analysis of the RL

The KL feedstock, untreated RL sample, and RL sample obtained from the DES treatment of KL at varying temperatures, times, and DES-to-lignin ratios were subjected to elemental analysis, and the obtained results are summarized in Table 1. Notably, the KL feedstock exhibited higher levels of O, N, and S compared to the untreated RL sample obtained from KL. For instance, the O, N, and S contents of KL were 32.26%, 1.02%, and 1.22%, respectively. Following KL depolymerization at 190 °C for 8 h in ethanol, these values decreased to 32.10%, 0.61%, and 1.13%, respectively, in the RL, resulting in a relatively high HHV of 24.82 MJ kg−1. Following DES treatment under the same reaction conditions, the O, N, and S content further decreased, while the C and H contents increased, leading to an increase in the HHV to 26.44 MJ kg−1. These findings indicate that the DES effectively catalyzes hydrogenation, hydrodesulfurization, and deoxygenation during lignin depolymerization in ethanol. The elevated levels of O and N in the raw KL impact its energy density, chemical reactivity, material properties, and stability. In particular, high O levels can lower energy density and increase the likelihood of harmful emissions during combustion, while higher N contents can lead to the formation of nitrogen oxides (NOxs). In contrast, the RL samples with reduced O and N levels demonstrate improved energy values, low environmental impact, enhanced stability, and improved material properties, rendering them favorable for various applications.
Figure 6A illustrates the O/C and H/C ratios of the KL feedstock, untreated RL sample, and RL sample obtained from DES treatments, while the corresponding S/C and H/C ratios are depicted in Figure 6B. Notably, the O/C and H/C ratios of the KL and RL samples exhibit considerable variations. Initially, KL exhibited O/C and H/C ratios of 0.016 and 0.094, respectively. After depolymerization at 190 °C for 8 h, the O/C ratio of the untreated RL sample slightly decreased to 0.015, while its H/C ratio increased to 0.095. Following DES treatment, the O/C ratio further decreased to 0.014, while the H/C ratio increased to 0.099, suggesting that the DES enhances the hydrogenation of unsaturated bonds in lignin and its depolymerized products. Furthermore, the S/C ratio of KL was 0.020, which decreased to 0.011 for the RL sample obtained from the DES treatment. These results imply that the DES facilitates the hydrogenolysis of lignin into various monomeric products, followed by their hydrogenation and deoxygenation, yielding saturated phenolic monomers.

3.3. Characterization of the Liquid Product

3.3.1. GC–MS Analysis of the Liquid Product

A GC–MS analysis was performed to identify the depolymerization products present in the liquid phase derived from the DES-assisted KL depolymerization conducted in ethanol at 190 °C for 8 h. Initially, 80 compounds were identified; however, among these, the 10 most representative ones were selected to compare the performance of various DES systems. The identified products primarily included aromatic monomers derived from syringyl and guaiacyl units. In the liquid fraction of the non-DES-treated bio-oil (Figure 7), several phenolic derivatives were detected, including guaiacol (5.59 wt%), 2-methoxyphenol (2.54 wt%), vanillin (7.77 wt%), acetovanillone (2.2 wt%), ethyl vanillate (1.81 wt%), homovanillic acid (2.01 wt%), and 1,2-benzenediol-4-methyl (1.6 wt%). Notably, these compounds originate from the cleavage of β-O-4 linkages in the guaiacyl units of lignin [13]. In addition to these, a substantial amount of isoeugenol (0.43 wt%) was detected. Interestingly, following treatment with the (ChCl:ZnCl2) DES, the yields of all detected compounds increased considerably in the liquid fraction. This suggests that the (ChCl:ZnCl2) DES enhances the alkylation, hydrogenation, and decarboxylation reactions of the unsaturated and oxygenated monomers in the liquid fraction, thereby increasing the yields of the resulting compounds.

3.3.2. GPC Analysis of the Raw KL and Depolymerized Liquid Product

A GPC analysis was conducted to determine the molecular weights of the raw KL and liquid fractions. Table 2 details the weight-averaged molecular weight (Mw), number-averaged molecular weight (Mn), and polydispersity index (PDI) of the KL, non-DES-treated liquid fraction, and liquid fraction obtained from DES treatments under various conditions. Notably, the molecular weight distributions of the KL and liquid fractions exhibit substantial differences. For instance, the raw KL exhibits an Mw value of 2450 g/mol and a Mn value of 1493 g/mol. After depolymerization, the resulting liquid fraction exhibited considerably lower molecular weights, indicating effective lignin disintegration. This reduction in molecular weight is attributed to the cleavage of α-O-4 and β-O-4 linkages during depolymerization [42]. Specifically, the liquid fraction obtained from the DES treatment of KL at 190 °C for 8 h exhibited the lowest molecular weight, with an Mw of 1498 g/mol and a Mn of 1061 g/mol. This result suggests that DES significantly enhances the depolymerization process, yielding a more favorable molecular weight distribution. Furthermore, the PDI of the DES-treated liquid fraction under optimal conditions was 1.41, indicating that this liquid fraction exhibited a more uniform molecular weight distribution compared to the untreated KL sample, along with improved homogeneity [43]. In contrast, at the same reaction temperature and duration, the non-DES-treated liquid fraction exhibited an Mw of 1872 g/mol and a Mn of 1259 g/mol, highlighting the substantial improvement in the quality of the liquid fraction subjected to DES treatment. These findings underscore the effectiveness of the DES in reducing the molecular weight of lignin and enhancing the quality of the liquid fraction through improved depolymerization.

3.4. Reaction Routes and Mechanisms

Compared to KL depolymerization without DES treatment, KL depolymerization using a ZnCl2-based DES solvent under the same conditions improved the yields of phenolic compounds, the molecular weight of the bio-oil, and the HHV of the RL (Figure 7, Table 1 and Table 2). This improvement is attributed to ZnCl2, a Lewis acid catalyst, which facilitates the cleavage of lignin linkages during depolymerization. For instance, during the cleavage of β-O-4, Zn2+ ions coordinate with the O atom in the β-O-4 linkage, weakening the Cα–Cβ bond. This bond weakening leads to its cleavage, resulting in the formation of vanillin—a process already documented in the literature [44]. FT-IR results suggest that the content of phenolic hydroxyl groups in the RL increased over time, while the content of alcoholic hydroxy groups remained almost constant (Figure 5B). The cleavage of the β-O-4 aryl ether bonds in lignin contributes to the formation of phenolic hydroxyl groups, while that of the Cα–Cβ bond leads to an increase in the content of alcohol hydroxyl groups in the RL [45]. Additionally, the molecular weight of the bio-oil and its higher HHV are enhanced, supporting the notion that ZnCl2-induced bond cleavage generates reactive intermediates that undergo further reactions, contributing to larger molecular weight products and a higher energy content. The presence of aromatic aldehydes and alcohols in the GC–MS analysis further corroborates the mechanism of lignin breakdown into valuable small molecules. In contrast, control experiments without ZnCl2 show lower yields of phenolic compounds and a lower molecular weight of bio-oil, confirming the critical role of ZnCl2 in promoting the depolymerization process. These experimental observations align with the reaction pathway where ZnCl2 acts as a catalyst to break key lignin linkages, improving both the quantity and quality of the bio-oil.
Figure 8 illustrates a possible degradation pathway for lignin in the acidic (ChCl:ZnCl2) DES based on the chemical structure analysis of the RL and the GC–MS analysis of small-molecule degradation products. Initially, under the acidic conditions created by the DES, protons attack the α-position of the hydroxyl groups in the lignin alkyl side chains, leading to the formation of carbocations. These unstable carbocations are then deprotonated to form enol ethers, which, in the future, can degrade into small molecules under the acidic conditions. Concurrently, the generated carbocations can react with small lignin degradation products to form new C–C bonds at the C5 position of the aromatic rings (Du et al., 2013; Pielhop et al., 2015) [46,47].

4. Conclusions

This study underscores the considerable potential of DESs, specifically the (ChCl:ZnCl2) DES, for the effective depolymerization of KL. Our results reveal that both reaction temperature and time are crucial for optimizing the yield and quality of the generated bio-oil. Under optimal conditions of 190 °C and 8 h, the depolymerization process not only increased the liquid product yield but also substantially reduced the molecular weight of the bio-oil, indicating enhanced depolymerization efficiency. Notably, the Mw value of the bio-oil derived from the DES-assisted depolymerization of KL was 1498 g/mol, which was considerably lower compared to the corresponding value of 1872 g/mol for the bio-oil obtained without the DES treatment. This reduction in molecular weight, coupled with improved homogeneity, highlights the effectiveness of the DES in facilitating lignin breakdown and enhancing product quality. Furthermore, the increase in the HHV of the RL sample from 24.82 MJ kg−1 to 26.44 MJ kg−1 demonstrates that the (ChCl:ZnCl2) DES-assisted depolymerization treatment not only enhances lignin valorization but also contributes to the production of sustainable biofuels with higher energy contents. These findings pave the way for further explorations of (ChCl:ZnCl2) DESs in biomass conversion processes, emphasizing their potential as cost-effective, non-toxic, and efficient alternatives to traditional solvents.

Author Contributions

Conceptualization, S.G.; Methodology, S.G.; Software, S.G.; Formal analysis, M.R.; Investigation, S.G.; Resources, J.-H.P.; Data curation, M.R.; Writing—original draft, S.G.; Writing—review and editing, M.R.; Visualization, M.R.; Supervision, J.-H.P.; Project administration, J.-H.P.; Funding acquisition, J.-H.P. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by [National Research Foundation, South Korea] grant number [2022R1A2B501002469].

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

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Applsci 14 11571 g001
Figure 1. DES preparation.
Figure 1. DES preparation.
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Figure 2. Procedures for DES-assisted lignin depolymerization and depolymerized product separation.
Figure 2. Procedures for DES-assisted lignin depolymerization and depolymerized product separation.
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Figure 3. Effect of reaction (A) temperature, (B) time, and (C) DES-to-lignin ratio on the yield of depolymerized products. Reaction conditions for (A): 3.0 g of lignin, 8 h, DES-to-lignin ratio of 1:1, and 90 mL of ethanol; reaction conditions for (B): 3.0 g of lignin, DES-to-lignin ratio of 1:1, 190 °C, and 90 mL of ethanol; reaction conditions for (C): 3.0 g of lignin, 190 °C, 8 h, and 90 mL of ethanol.
Figure 3. Effect of reaction (A) temperature, (B) time, and (C) DES-to-lignin ratio on the yield of depolymerized products. Reaction conditions for (A): 3.0 g of lignin, 8 h, DES-to-lignin ratio of 1:1, and 90 mL of ethanol; reaction conditions for (B): 3.0 g of lignin, DES-to-lignin ratio of 1:1, 190 °C, and 90 mL of ethanol; reaction conditions for (C): 3.0 g of lignin, 190 °C, 8 h, and 90 mL of ethanol.
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Figure 4. FT-IR spectra of the raw KL (a), RL sample after DES treatment at 190 °C for 8 h (b), and RL sample without DES treatment at 190 °C for 8 h (c) in ethanol.
Figure 4. FT-IR spectra of the raw KL (a), RL sample after DES treatment at 190 °C for 8 h (b), and RL sample without DES treatment at 190 °C for 8 h (c) in ethanol.
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Figure 5. FT-IR spectra of the RL sample at different reaction temperatures (A) and times (B). In (A) raw KL (a) and RL sample obtained after DES treatment in ethanol at 110 (b), 170 (c), 190 (d), and 210 °C (e) for 8 h. In (B) raw KL (a) and RL sample obtained after DES treatment in ethanol at 190 °C for 4 (b), 8 (c), and 10 h (d).
Figure 5. FT-IR spectra of the RL sample at different reaction temperatures (A) and times (B). In (A) raw KL (a) and RL sample obtained after DES treatment in ethanol at 110 (b), 170 (c), 190 (d), and 210 °C (e) for 8 h. In (B) raw KL (a) and RL sample obtained after DES treatment in ethanol at 190 °C for 4 (b), 8 (c), and 10 h (d).
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Figure 6. Van Krevelen diagram comparing KL and RL (A,B). The O/C vs. H/C and S/C vs. H/C ratios of (a, a’) KL. The O/C vs. H/C and S/C vs. H/C ratios of RL obtained at (b, b′) 190 °C for 8 h without DES, and (c, c’) 190 °C for 8 h using DES.
Figure 6. Van Krevelen diagram comparing KL and RL (A,B). The O/C vs. H/C and S/C vs. H/C ratios of (a, a’) KL. The O/C vs. H/C and S/C vs. H/C ratios of RL obtained at (b, b′) 190 °C for 8 h without DES, and (c, c’) 190 °C for 8 h using DES.
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Figure 7. Yields of lignin depolymerization products in the liquid phase after treating KL with DES and without DES at 190 °C for 8 h.
Figure 7. Yields of lignin depolymerization products in the liquid phase after treating KL with DES and without DES at 190 °C for 8 h.
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Figure 8. Mechanism of ChCl:ZnCl2 DES-assisted KL depolymerization.
Figure 8. Mechanism of ChCl:ZnCl2 DES-assisted KL depolymerization.
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Table 1. Elemental analysis of the raw KL and RL (RL = recovered lignin).
Table 1. Elemental analysis of the raw KL and RL (RL = recovered lignin).
SampleN (%)C (%)H (%)S (%)O (%)HHV (MJ kg−1) *
Kraft lignin1.0259.885.621.2232.2624.48
RL at 110 °C/8 h0.4660.655.931.0631.925.10
RL at 170 °C/8 h0.6262.175.911.0430.2625.76
RL at 190 °C/8 h0.7162.896.240.7229.4426.44
RL at 210 °C/8 h0.8861.755.890.9930.4925.57
RL at 190 °C/4 h0.9360.515.851.0131.7024.99
RL at 190 °C/8 h0.7162.896.240.7229.4426.44
RL at 190 °C/10 h0.8362.036.140.8530.1525.98
RL at 190 °C/8 h (no DES)0.6160.395.771.1332.1024.82
D:L of 1:1 at 190 °C/8 h0.7162.896.240.7229.4426.44
D:L of 3:3 at 190 °C/8 h0.7461.945.861.0930.3725.62
D:L of 5:5 at 190 °C/8 h1.1260.015.701.1831.9924.65
* HHV (MJ kg−1) = [(34 × C) + (124.3 × H) + (6.3 × N) + (19.3 × S) − (9.8 × O)]/100.
Table 2. Molecular weights of the bio-oil (Mn: number-averaged molecular weight; Mw: weight-averaged molecular weight; and PDI: polydispersity index).
Table 2. Molecular weights of the bio-oil (Mn: number-averaged molecular weight; Mw: weight-averaged molecular weight; and PDI: polydispersity index).
SampleReaction ConditionsMwMnPDI (Mw/Mn)
Kraft lignin 245014931.64
DES-treated bio-oil110 °C for 8 h185612611.47
150 °C for 8 h181312351.46
170 °C for 8 h165311551.43
190 °C for 8 h149810611.41
210 °C for 8 h165111461.44
4 h at 190 °C173411421.52
6 h at 190 °C165211121.48
8 h at 190 °C149810611.41
10 h at 190 °C181712541.45
Without DES190 °C for 8 h187212591.49
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Ghosh, S.; Rana, M.; Park, J.-H. Depolymerization of Kraft Lignin Using a Metal Chloride-Based Deep Eutectic Solvent: Pathways to Sustainable Lignin Valorization. Appl. Sci. 2024, 14, 11571. https://doi.org/10.3390/app142411571

AMA Style

Ghosh S, Rana M, Park J-H. Depolymerization of Kraft Lignin Using a Metal Chloride-Based Deep Eutectic Solvent: Pathways to Sustainable Lignin Valorization. Applied Sciences. 2024; 14(24):11571. https://doi.org/10.3390/app142411571

Chicago/Turabian Style

Ghosh, Shubho, Masud Rana, and Jeong-Hun Park. 2024. "Depolymerization of Kraft Lignin Using a Metal Chloride-Based Deep Eutectic Solvent: Pathways to Sustainable Lignin Valorization" Applied Sciences 14, no. 24: 11571. https://doi.org/10.3390/app142411571

APA Style

Ghosh, S., Rana, M., & Park, J.-H. (2024). Depolymerization of Kraft Lignin Using a Metal Chloride-Based Deep Eutectic Solvent: Pathways to Sustainable Lignin Valorization. Applied Sciences, 14(24), 11571. https://doi.org/10.3390/app142411571

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