Microwave-Assisted Oxidative Degradation of Lignin Catalyzed by Hydrogen Peroxide–Alkaline Ionic Liquid System

Abstract

In recent years, various green solvents have played more and more important roles in catalysis and biomass studies. In this work, three imidazolium anion-based alkaline ionic liquids (ILs, including [BMIM]Im, [Ch]Im, and [N₄₂₂₂]Im) were selected to catalyze the oxidative degradation of alkaline lignin by a microwave-assisted hydrogen peroxide–alkaline ionic liquid system for the first time, which aimed to promote the depolymerization and high-value conversion of lignin and increase the number of alcohol hydroxyl groups and the reactivity of lignin. The changes in the number of the alcohol hydroxyl groups of lignin before and after degradation were taken as the primary indices. As the main conditions, the influence of the microwave exposure time, microwave power, ionic liquid concentration, and hydrogen peroxide concentration on the degradation efficacy was subsequently examined for the ionic liquid that exhibited the most effective degradation performance. In addition, the extracted lignin degradation reaction solution was analyzed in combination with gas chromatography–mass spectrometry (GC–MS), and the degraded lignin solids were characterized by scanning electron microscopy (SEM), ultraviolet and visible (UV–Vis) spectroscopy, Fourier-transform infrared spectroscopy (FT–IR), and thermogravimetric and derivative thermogravimetric (TG–DTG) methods, which determined the composition of the degradation products, the degradation mechanism, and the intuitive structural changes in the lignin, thereby providing insights into the extent of lignin degradation with green solvents.

1. Introduction

The escalating environmental pollution and the depletion of fossil fuels as a source of fuels, chemicals, and energy have led to the increasing significance of utilizing renewable resources as a solution for the sustainable production of fuels and bulk chemicals to reduce our dependence on fossil fuels [1,2,3]. Biomass energy is environmentally friendly, cost-effective, and carbon-neutral, making it a valuable resource for sustainable development. Most importantly, biomass is the most abundant renewable energy source in nature, which contains aromatic units [1]. Biomass resources include triglycerides, lignocellulose, chitin, starch, etc. Among the various types of biomass energy, lignocellulose is the most abundant on Earth. It is primarily composed of hemicellulose, cellulose, and lignin [4,5]. As the second most abundant terrestrial polymer on earth after cellulose, lignin exists as an economical, readily accessible, renewable carbon-based material found extensively in the cell walls of lignified plants to reinforce plant tissues [6]. Lignin is a natural polymer characterized by a three-dimensional network structure and a significant number of hydroxyl groups [7]. It is synthesized in plants through the shikimate–cinnamate pathways [8] and primarily consists of three elements: carbon, hydrogen, and oxygen [9,10]. It is generally believed that lignin is a three-dimensional highly branched heteropolymer composed of three types of phenylpropane units, a syringyl unit (S), guaiacyl unit (G), and p-hydroxyphenyl unit (H), corresponding to three precursors, sinapyl alcohol, coniferyl alcohol, and p-coumarol [10]. The types and proportions of the phenylpropane units constituting lignin vary among plant species [1,10]. These units are irregularly arranged by carbon–carbon (e.g., 5-5′, β-5, β-β, and β-1) and ether bonds (e.g., β-O-4, α-O-4, 4-O-5, and α-O-γ), with the ether bonds accounting for 40~65% and the carbon–carbon bonds accounting for 20~40% [11]. Upon depolymerization, lignin, which is rich in rigid structures, can produce various reactive sites, such as phenolic structures and aliphatic hydroxyl groups, making lignin a promising polymer monomer for the preparation of economical and environmentally friendly lignin-based polymers. For instance, lignin could be used as a replacement for polyols in the synthesis or blending of thermoplastic polyurethanes (TPUs) to produce a new generation of sustainable lignin-based polyurethanes with improved performance and unique properties [12], such as radiation protection [13], degradability [14], antibacterial activity [15], and so on.

Lignin is the only renewable resource in nature containing an aromatic structure, with significant potential for exploitation as an ideal raw material in the eco-friendly manufacturing of a wide range of bulk and fine chemicals, particularly aromatic compounds [16]. However, its complex three-dimensional network structure, along with its chemical and hydrogen bonding of lignin macromolecules greatly increase the difficulty of its degradation and limit its high-value-added utilization [17,18]. Actually, most of the 50~70 million tons of lignin annually produced from pulp and paper facilities world-wide is burned as a low-value fuel to generate electricity and heat, and only less than 2% is used for producing specialty chemicals, such as dispersants, adhesives, surfactants, and other value-added products [6,19,20,21]. In the face of the abundant lignin resources in nature and the generation of large quantities of industrial waste, the high-value conversion and utilization of lignin has great development prospects and potential.

As for its valorization, the modification methods for lignin are mainly divided into two categories: one is to modify lignin by generating new chemically active sites to increase the reactivity of the lignin functional groups, including hydroxylation modification, demethylation modification, amination modification, etc.; the second is to transform natural lignin into low-molecular-weight lignin monomers by depolymerization, while simultaneously obtaining high-value small-molecule aromatic compounds [2,9]. The fragmentation reactions can be principally divided into lignin cracking or hydrolysis reactions, catalytic reduction reactions, and catalytic oxidation reactions [2]. The catalytic oxidation method has the advantages of low energy consumption, mild reaction conditions, and high product yield. It can be divided into photocatalysis, electrochemical catalysis, metal-free catalysis, organometallic catalysis, and other methods. The metal-free catalytic degradation of lignin is commonly carried out with oxygen and hydrogen peroxide (H₂O₂) to break down lignin, resulting in the production of various classes of chemical monomers, depending on the degree of oxidation [22]. In acidic or alkaline environments, the reduction product of H₂O₂ is water; therefore, H₂O₂ can be employed as a green oxidant in the catalytic oxidation degradation process, aligning with growing demands for stricter environmental protection regulations. Meanwhile, the exceptional ability of ionic liquids (ILs) to dissolve biomass and lignin makes their combination with external catalysts or the utilization of bi-functional (solvent and catalyst) ILs promising for effectively depolymerizing lignins under comparatively milder reaction conditions [1,23,24]. The current methods for lignin conversion using ILs consist of acid/base depolymerization, hydroprocessing, oxidative depolymerization, pyrolysis, and biocatalysis [25]. For instance, Das et al. [26] investigated the depolymerization of alkali lignin in aqueous [EMIM]OAc through a catalytic oxidation process. Seven different transition metal catalysts were evaluated for their catalytic activity in the presence of hydrogen peroxide (H₂O₂) as the oxidizing agent. Among these catalysts, CoCl₂ and Nb₂O₅ demonstrated the highest efficacy for producing degradation products, including guaiacol, syringol, vanillin, acetovanillone, and homovanillic acid. In 2020, Tolesa et al. [23] synthesized two novel ammonium-based alkaline ionic liquids ([DIPEA][Cl] and [DIPEA][Bn]) with bi-functionality for the degradation of alkali lignin while obtaining some essential poly-hydroxy or phenolic compounds. The results showed that at favorable operation conditions (170 °C for 60 min), a significant reduction was observed in the average molecular weights, by 84.8% and 71.1%, by using [DIPEA][Cl] and [DIPEA][Bn], respectively. Likewise, Li and colleagues [27] reported that [EMIM]OAc can serve as an effective medium for alkali lignin depolymerization without an external catalyst. For the enhanced conversion of alkali lignin, the efficacies of the ILs are ranked as follows: [EMIM]OAc (72.3 wt%) > [BMIM]OAc (68 wt%) > [HMIM]OAc (65 wt%) > [OMIM]OAc (60.4 wt%). In addition, microwave irradiation can also efficiently facilitate the degradation of lignin in conjunction with catalytic degradation. In 2010, Ouyang et al. [28] investigated the oxidative degradation of soda lignin using hydrogen peroxide. It was found that microwave irradiation could efficiently facilitate the degradation of lignin in comparison to conventionally heated oxidation at a low oxidant dosage, low temperature, or short oxidation time. Furthermore, the microwave-assisted oxidation of lignin could effectively facilitate the degradation of high-molecular-weight (Mw) lignin and the re-condensation of low-Mw lignin, resulting in lignin with a narrower Mw distribution and relatively lower Mw. Hamzah et al. [29] optimized the parametric microwave conditions for the catalytic degradation of lignin model compounds in 41 types of imidazolium-based ILs. Microwave irradiation was found to be more efficient than conventional heating for achieving the desired conversion percentages and yield in a short time (less than 30 min of reaction time).

To the best of our knowledge, the microwave-assisted catalytic oxidative degradation of lignin using alkaline ionic liquids has been rarely reported on in the literature. Integrating the strengths of oxidative degradation, alkali catalysis, and microwave irradiation, the microwave-assisted catalytic oxidative degradation of lignin by a green system of hydrogen peroxide and three alkaline imidazolium-based ILs, [BMIM]Im, [Ch]Im, and [N₄₂₂₂]Im, was investigated to promote the depolymerization of lignin, reduce its molecular weight, and increase the number of alcohol hydroxyl groups and the reactivity of lignin, with a view to promoting the conversion and utilization of lignin for high-value applications.

2. Results and Discussion

2.1. Effect of Reaction Conditions on the Degradation of Lignin

Three ionic liquids—[BMIM]Im, [Ch]Im, and [N₄₂₂₂]Im (purity beyond 95%)—were provided by Qiyue Biotechnology Co., Ltd. (Xi’an, China) together with Alardin Reagent company (Shanghai, China), and their synthetic routes can be found in a previous report [30]. Their structures and pH values are shown in

Table 1. The abbreviations, structures, and pH values of the ionic liquids (2 mmol/L, 30 °C).

No.	ILs	Cations	Anions	pH
1	[BMIM]Im			12.24
2	[Ch]Im			12.74
3	[N4222]Im			13.06

. The overall experimental procedure was as follows: 2.0 g of lignin was weighed and loaded into a flask containing 60 mL of distilled water, and a quantitative amount of ionic liquids together with an H₂O₂ solution were added to the flask. The reaction system was subjected to a specified amount of microwave radiation for a preset duration, and the reaction was terminated by rapid cooling upon reaching the designated reaction time. The pH of the solution was adjusted to 1 using a 1 mol/L HCl solution, then separated by centrifugation at 4000 rpm, and dried under a vacuum at 50 °C until a constant weight was reached to determine the hydroxyl degree of the lignin degradation product.

2.1.1. Microwave Time

The effect of the different microwave times on the hydroxyl values of the lignin degradation products was firstly investigated using three different ionic liquids under the same conditions. The hydroxyl values of the degradation products were determined after 0.5 h, 1 h, 1.5 h, 2 h, 2.5 h, and 3 h of microwave reaction time by adding 0.1 mol/L of ionic liquids and 0.5 mol/L of H₂O₂ solution to the flask under a microwave power of 300 W. The variations in the alcohol hydroxyl values and phenol hydroxyl values are shown in Figure 1.

As shown in Figure 1a, the trends of the alcohol hydroxyl content of the degradation products after treatment with the three ionic liquids were consistent. Initially, there was a gradual increase in the alcohol hydroxyl content with the microwave time, but after a certain reaction time, a further increase in the reaction time led to a decrease in the hydroxyl content. This indicates the occurrence of deep oxidation with a prolonged reaction time, during which the hydroxyl groups were further converted into other functional groups. Among them, the phenolic hydroxyl groups were oxidized to α-carbonyl or quinone groups, and the aliphatic hydroxyl groups were oxidized to carboxyl or ketone groups. Among the three ionic liquids, [Ch]Im demonstrated the most effective degradation of lignin during the microwave-assisted treatment, resulting in a maximum conversion of the alcohol hydroxyl groups on the lignin surface of 280.75 mg KOH/g, and a phenol hydroxyl content of 1.30 mmol/g. The number of alcohol hydroxyl groups was doubled compared to the untreated microwave treatment [31], while the phenol hydroxyl group content did not show a significant increase relative to that of the raw lignin material.

Furthermore, the increase in the hydroxyl values after microwave assistance was significantly higher than that in the product degraded at atmospheric pressure, highlighting the ability of microwave treatment to expedite reactions and enhance the degradation efficacy. Under microwave radiation, the hydroxyl content of the lignin products degraded by [Ch]Im was the highest among the three ionic liquids, although the degradation using the [N₄₂₂₂]Im ionic liquid was the best without microwave assistance. However, the results showed similar results for the alcohol hydroxyl values using [N₄₂₂₂]Im and [Ch]Im, with or without microwave assistance. This similarity can be attributed to their comparable alkalinity and smaller cationic spatial site resistance, which collectively contributed to their more effective degradation compared to [BMIM]Im.

As shown in Figure 1b, the trend of the phenol hydroxyl values in the degradation products with respect to the microwave time was not obvious. However, the highest phenolic hydroxyl value of the lignin degraded by [Ch]Im was observed at 2 h of microwave reaction time. After a comprehensive analysis, subsequent single-factor experiments were carried out using the ionic liquid [Ch]Im, and 2 h of microwave radiation was selected as the optimal time for the following experiments.

2.1.2. Microwave Power

The impact of increasing the microwave power on the acceleration of the chemical reaction mechanism has been a key subject of exploration, and has been primarily studied from the perspective of the microwave non-thermal and thermal effects. The microwave non-thermal effect attributes the rise in microwave power to the enhanced stability of the special molecules, intermediates, or transition states within the reaction system due to interactions with the microwave electromagnetic field, rather than to significant changes in the macro-reaction temperature. Conversely, the microwave thermal effect enables rapid and uniform heating without delays, and its contribution to chemical reactions is limited to increased temperature [32]. Here, the degradation of lignin was determined by varying the microwave power. A total of 0.1 mol/L of [Ch]Im ionic liquid and 0.5 mol/L of H₂O₂ solution were added into a flask, and a microwave reaction was carried out for 2 h. The range of microwave power levels tested included 100 W, 200 W, 300 W, 400 W, 500 W, and 600 W.

Obviously, the alcohol hydroxyl content of the degraded lignin initially increased and subsequently decreased with rising microwave power, peaking when the power reached 300 W (see Figure 2a). On the one hand, high-power microwaves facilitated the further degradation of the hydroxyl groups on the surface of the lignin oligomers, leading to a reduction in the hydroxyl content. On the other hand, prolonged exposure to high-power microwaves prompted C-H bond cleavage and deep oxidation, where the hydroxyl groups were further converted into other functional groups.

Both the phenolic hydroxyl value and alcohol hydroxyl content exhibited a similar trend in their degradation products, reaching peak values under 300 W power (see Figure 2b). These findings suggest that microwaves can facilitate the cleavage of the β-O-4 ether bond in lignin at specific power levels, resulting in an increase in the content of alcohol and phenol hydroxyls. However, when a certain microwave power threshold is surpassed, it leads to the breakage of the C-C bond in lignin, generating additional alcohol and phenol small molecules, thereby reducing the hydroxyl content in lignin oligomers.

2.1.3. The Concentration of Ionic Liquid

To further explore the impact of varying concentrations of the ionic liquid on lignin degradation, a 0.5 mol/L H₂O₂ solution and different concentrations of [Ch]Im were added into the reactive system, and experiments were carried out under reaction conditions with microwave power of 300 W and microwave time of 2 h, with sequential ionic liquid concentrations of 0.01 mol/L, 0.05 mol/L, 0.1 mol/L, 0.15 mol/L, and 0.2 mol/L.

As depicted in Figure 3a, the alcohol hydroxyl content demonstrated a similar trend, where the impact of the ionic liquid concentration initially rose and then declined. With an escalation in the [Ch]Im concentration, the pH of the reaction solution increased. Simultaneously, the presence of the electrophilic group OOH^– also intensified, aiding in lignin degradation and promoting the augmentation of the hydroxyl content. However, escalating the ionic liquid concentration further resulted in an increase in the electrophilic groups, leading to the oxidation of the alcohol hydroxyl groups and a subsequent reduction in their content. Consequently, the selection of an ionic liquid concentration of 0.1 mol/L was deemed appropriate.

The phenolic hydroxyl content of the degradation products from the microwave-assisted degradation showed minimal variation (see Figure 3b). However, the phenolic hydroxyl content also peaked at 1.300 mmol/g with an ionic liquid concentration of 0.1 mol/L, aligning closely with the trend for the alcohol hydroxyl content. Both exhibited an initial increase followed by a decline. This consistency can be attributed to similar mechanisms. As the [Ch]Im concentration rose, more electrophilic groups targeted the carbon in the β-position, causing β-O-4 bond cleavage and enhancing the phenolic and alcohol hydroxyl groups contents. The continued escalation of the ionic liquid concentration might have resulted in the further degradation of the hydroxyl groups.

2.1.4. The Concentration of Hydrogen Peroxide

Regarding the role of H₂O₂, the hydroxyl radicals (·OH) and superoxide radicals (·OOH) generated by it can attack the aromatic rings and side chains of lignin, breaking the β-O-4 bond and other bonds, and causing lignin to dissociate into low-molecular-weight fragments. H₂O₂ can also oxidize the aromatic structure of lignin benzene rings, leading to ring opening and the formation of water-soluble products, such as carboxylic acids and ketones. In order to investigate the effect of the H₂O₂ concentration on the degradation of lignin, 0.1 mol/L of the ionic liquid [Ch]Im and different concentrations of H₂O₂ solutions were added to flasks under reaction conditions with a microwave power of 300 W, microwave time of 2 h, and concentrations of H₂O₂ of 0.1 mol/L, 0.3 mol/L, 0.5 mol/L, 0.7 mol/L, and 0.9 mol/L in sequence.

Based on the findings depicted in Figure 4a, the impact of the H₂O₂ concentration on the alcohol hydroxyl content of lignin demonstrated an initial rise followed by a subsequent decrease with an increasing H₂O₂ concentration. With further increases in the H₂O₂ concentration, an abundance of OOH- ions were present, and thus had a higher reactivity, which continued to oxidize the alcohol hydroxyl groups on the surface of the degradation products, ultimately leading to a reduction in the measured alcohol hydroxyl content.

The results in Figure 4b illustrate a similar trend in the phenolic hydroxyl content of the lignin degradation products corresponding to the changes observed in the alcohol hydroxyl content. The H₂O₂ primarily targeted the β-O-4 bond within the lignin structure, leading to an increase in both the alcohol and phenolic hydroxyl contents in the hydrolysis products. However, an excessive oxidant concentration could result in overoxidation, diminishing the hydroxyl groups on the surface of the degradation products. Consequently, the hydrogen peroxide concentration was regulated at a level of 0.5 mol/L as a suitable condition.

2.2. GC–MS Analysis of Catalytic Degradation Products of Lignin

During the catalytic oxidative degradation process, the degradation products of lignin primarily consist of degraded macromolecular lignin oligomers and small-molecular degradation products in the reaction solution. The composition of the extracted degradation solution was analyzed in combination with GC–MS, and the results can be found in Figure 5 and

Table 2. Components of lignin degradation products with microwave assistance.

No.	Retention Time (Min)	Area (%)	Molecular Formula	Compound Name	Structure
1	5.86	13.77	C3H6O3	Lactic acid
2	6.07	2.09	C5H6O4	2-Methyl-2-butenedioic acid
3	6.81	1.59	C6H6O	Phenol
4	7.41	2.73	C4H8O2	2-Butene-1,4-diol
5	8.54	0.43	C6H12O2	Butyl acetate
6	8.68	27.15	C7H8O2	Guaiacol
7	9.90	0.20	C5H10O	4-Penten-1-ol
8	9.19	0.38	C5H8O	Dimethyl succinate
9	9.63	0.48	C5H6O5	2-Methoxy-2-butenedioic acid
10	10.24	3.20	C7H6O2	Benzoic acid
11	10.34	0.19	C8H10O2	2-Methoxy-6-methylphenol
12	11.38	0.35	C8H8O2	Phenylacetic acid
13	11.59	0.38	C7H8O3	2-Methoxybenzene-1,3-diol
14	12.18	0.89	C9H10O2	4-Hydroxy-3-methoxystyrene
15	12.23	0.69	C8H6O4	Phthalic acid
16	12.73	1.73	C8H10O3	2,6-Dimethoxyphenol
17	13.48	0.81	C7H8O3	3-Methoxybenzene-1,2-diol
18	14.48	0.43	C8H10O3	2-Hydroxy-3-methoxybenzyl alcohol
19	14.63	1.10	C9H10O3	Acetovanillone
20	15.02	0.23	C11H12O5	Dimethyl 4-Methoxyisophthalate
21	15.21	0.37	C10H12O3	4-Hydroxy-3-methoxyphenylacetone
22	16.30	14.05	C8H8O4	3-Hydroxy-4-methoxybenzoic acid
23	16.79	0.36	C13H18O	2-Methyl-5-phenylhex-1-en-3-ol
24	16.98	8.50	C9H10O4	Methyl 3-Hydroxy-4-methoxybenzoate
25	17.14	0.21	C10H14O	2-Phenylbutan-1-ol
26	17.28	2.11	C9H10O4	4-Hydroxy-3-methoxybenzeneacetic acid
27	17.74	3.28	C11H14O5	Ethyl 2-(4-Hydroxy-3-methoxyphenyl) acetate
28	19.29	1.03	C10H12O2	2-Methoxy-5-(prop-2-en-1-yl) phenol
29	20.43	0.16	C10H12O3	3-Ethoxy-4-methoxybenzaldehyde
30	22.14	0.13	C11H14O3	4-Methoxy-3-propoxybenzaldehyde
31	23.23	0.28	C10H14O	2,3,5,6-Tetramethyl phenol
32	23.72	0.28	C10H12O2	2-Methoxy-5-(prop-2-en-1-yl) phenol
33	24.13	0.14	C10H14O2	4-Propylguaiacol
34	24.22	0.15	C11H10O4	4-Hydroxymethyl-7-methoxycoumarin
35	24.35	0.23	C10H10O2	6-Methoxy-3-methylbenzofuran
36	25.28	0.18	C10H14O2	2-Methoxy-4-propylphenol
37	26.90	0.43	C10H14O	2,4,6-Trimethylanisole
38	27.53	0.74	C8H8O3	4-Hydroxy-2-methoxybenzaldehyde
39	28.03	1.49	C10H14O2	3-Methoxy-2,5,6-trimethyl-phenol
40	29.90	0.10	C11H14O3	3-Isopropoxy-4-methoxy-benzaldehyde
41	30.51	0.27	C24H38O4	Bis(2-ethylhexyl) phthalate
42	34.55	0.61	C20H26O7	2,3-Bis[(4-hydroxy-3-methoxyphenyl) methyl] butane-1,2,4-triol

.

Based on the data presented in Table 2, the small-molecule degradation products of lignin are predominantly acids, alcohols, aldehydes, and esters, primarily consisting of aromatic substances, representing approximately 73.84% of the total. Among them, around 62.86% of the degradation products feature a guaiacol group structure, with guaiacol alone contributing to approximately 27.15% of the content. The fatty acids and esters in the lignin degradation products exhibited a low boiling point, concentrated in peaks before 10 min. As the column temperature exceeded 130 °C after 10 min, the products shifted towards monoaromatic cyclic compounds, like aromatic alcohols, aldehydes, acids, and esters. The aromatic alcohols could undergo further oxidation into aldehydes and acids in the presence of oxidants, though most of the degradation products retained their phenol structure. Following a 30 min duration, the column temperature was elevated to 280 °C, leading to peaks mainly comprising aromatic compounds, like dimers, with a higher boiling point.

2.3. Study of Degradation Mechanisms

Upon analyzing the structure of the products, it is evident that lignin degradation primarily occurs through the degradation of β-O-4 and α-O-4 linkages and other pathways, leading to a heightened hydroxyl content. The degradation of lignin into low-molecular-weight aromatic compounds proceeds in a stochastic manner, and the pathway for each product is non-unique, thus contributing to the complexity of its degradation mechanism. The potential degradation mechanism is conjectured based on the aforementioned degradation products, which are illustrated in Figure 6 and Figure 7.

Figure 6a illustrates the breakdown of lignin, highlighting two main types of bond breakage: C-C bond breakage and C-O bond breakage. Specifically, there are two main forms of C-O bond breakage identified, α-O-4 and β-O-4. The C-C bond in lignin is considered stable, whereas the C-O bond is characterized by a low bond energy, resulting in degradation primarily through ether bond breakage. The proximity of the C atom at the α position to the benzene ring makes the β-O-4 bond more susceptible to breakage due to the conjugation effect. In Figure 6b, the formation process of guaiacol-based small-molecule compounds is depicted, mainly through dealkylation reactions that eliminate the branched chains from the aromatic ring to obtain lignin monomeric structures. The compounds, such as 2-methoxybenzene-1,3-diol, 3-methoxybenzene-1,2-diol, 2,6-dimethoxyphenol, and 2-hydroxy-3-methoxybenzyl alcohol, in Table 2 may have resulted from degradation via this pathway.

Figure 7 presents the hydrolysis reaction of lignin under strongly alkaline conditions. The presence of the strong nucleophilic OOH- group leads to the removal of H from the hydroxyl group, resulting in a negatively charged system. Subsequently, with the assistance of H₂O molecules, the β-O-4 ether bond is further cleaved, leading to an increase in the phenolic and alcoholic hydroxyl groups contents throughout the lignin molecule. Furthermore, aldehydes, ketones, and carboxylic acids featuring the structure of the guaiacol group presented in Table 2 can be generated through the additional oxidation of the phenolic and alcoholic hydroxyl groups of the lignin degradation monomers produced via the hydrolysis reaction.

2.4. Characterization of Degraded Lignin Solids

2.4.1. Scanning Electron Microscopy (SEM)

A scanning electron microscope was employed to examine the solid lignin oligomers following the microwave-assisted degradation by the ionic liquid [Ch]Im. The objective was to assess the alterations to the lignin surface before and after degradation at various magnifications, thereby confirming the effectiveness of the lignin degradation.

Figure 8 presents a visual comparison of the images of the lignin pre- and post-degradation under a scanning electron microscope at 1000×, 2000×, and 5000× magnification, from which the more desirable degradation effects of lignin can be visualized. Specifically, the surface of the lignin in Figure 8(a1–a3) before degradation is relatively dense and smooth, while the surface of the degradation products in Figure 8(b1–b3) after degradation is obviously fluffy and enlarged. This visual evidence suggests the destruction of the chemical structure of the lignin surface. However, despite this surface alteration, clustered aggregates are still visible in the degradation products, indicating that the solid products primarily consist of lignin oligomers, and the overarching structural integrity of the lignin remains largely intact.

2.4.2. Ultraviolet and Visible (UV–Vis) Spectral Analysis

The solutions derived from the oligomer molecules of the catalytically oxidatively degraded lignin underwent analysis via full-wavelength scanning in the UV–visible range at identical concentrations. The UV spectra of these solutions were subsequently compared with those generated from non-degraded lignin feedstocks to evaluate the variations in the UV spectra pre- and post-degradation (see Figure 9a).

The absorption peak at 280 nm in the UV spectrum is attributed to the existence of delocalized π-electrons within the aromatic ring structure [33]. The reduction in absorbance at 280 nm suggests a partial breakdown of the aromatic structure, confirming the occurrence of ring opening in the aromatic ring during the catalytic oxidation of lignin, which aligns with the results obtained from the compositional analysis using GC–MS.

2.4.3. Fourier-Transform Infrared Spectroscopy (FT–IR)

In order to understand the structural changes in lignin polymers at the molecular level, FT–IR was utilized to study the structure of the pre- and post-catalytic oxidative degradation products. In Figure 9b, various characteristic vibrations are observable: an O-H stretching vibration peak of the hydroxyl group at around 3448 cm⁻¹, a C-H stretching vibration peak of the aliphatic chain between 3000 and 2850 cm⁻¹, a C-H stretching vibration of the methyl group or methylidene group at 2969 cm⁻¹, a C=O stretching vibration of the carbonyl group at 1718 cm⁻¹, and a distinctive absorption peak of the benzene ring within 1460~1600 cm⁻¹. Additional peaks are observable at 1214 cm⁻¹ for C-O stretching vibrations, 1019 cm⁻¹ for C-O stretching vibrations related to O-CH₃ or C-OH, and 900 cm⁻¹ for the bending vibrations of the C-H group on the aromatic ring.

The infrared spectrum revealed that the main structure of the oligomer products after microwave-assisted lignin degradation was basically unchanged. Nevertheless, compared to the peaks of the original lignin material, the lignin oligomers degraded catalytically by [Ch]Im exhibited new absorption peaks primarily at 1716 cm⁻¹, 1089 cm⁻¹, and 960 cm⁻¹. The broader peak observed at around 3448 cm⁻¹ in the spectral band post-lignin degradation indicates the stretching vibration of alcohol hydroxyl and phenol hydroxyl O-H bonds, signifying the presence of numerous alcohol or phenol groups in the degraded lignin oligomers. Following the catalytic oxidative degradation, a fresh peak emerged near 1716 cm⁻¹, primarily associated with a C=O stretching vibration, confirming the generation of aldehydes and ketones during the catalytic oxidation process. Moreover, a distinct absorption peak at 1089 cm⁻¹ appeared, representing a C-O stretching vibration in the hydroxyl C-OH group, further suggesting an increased content of alcohol hydroxyl groups in the degradation products.

2.4.4. Thermogravimetric and Derivative Thermogravimetric (TG–DTG) Analysis

A thermogravimetric analysis is commonly utilized in the study of polymer properties. The differences in the thermal stability of lignin before and after degradation can be analyzed by a thermogravimetric analyzer. Figure 10 presents the TG/DTG curves of the microwave-assisted lignin before and after degradation.

From Figure 10, it can be found that the thermal stability of the lignin before degradation was better than that of the lignin oligomers after degradation. Both of the lignin solids, before and after degradation, showed weight loss peaks of free water, bound water, and small molecules at about 100 °C. The undegraded lignin raw material started to decompose further at about 165 °C, and reached its maximum decomposition rate at about 305 °C. Meanwhile, the degradation rate decreased at 500 °C, and the residue was still 49.58% of the original weight at 800 °C, which had a good thermal stability. While the degraded lignin started to decompose at 119 °C, the decomposition rate reached its maximum at 245 °C, while the weight loss was about 92.39% at 800 °C. This diminished thermal stability can be attributed to the disruption of specific chemical bonds and cross-linking networks during degradation.

3. Materials and Methods

3.1. Experimental Reagents

Unless specified, the following reagents were analytical grade and used without purification. 1-Bromobutane, triethylamine, choline chloride, imidazole, anhydrous ethanol, methanol, pyridine, 1,4-dioxane, phthalic anhydride, 4-aminobenzenesulfonate, and concentrated hydrochloric acid (36% (w/w)) were supplied by Chengdu Kelong Chemical Co., Ltd. (Chengdu, China). Ether, 30% hydrogen peroxide, and sodium hydroxide were purchased from Sichuan Hangjia Biomedical Technology Co., Ltd. (Chengdu, China). Chemically pure methylimidazole was purchased from Shanghai Hanhong Chemical Technology Co., Ltd. (Shanghai, China). Alkaline lignin was supplied by TCI (Shanghai) Development Co., Ltd. (Shanghai, China).

3.2. Experimental Instruments and Analytical Methods

Deionized water utilized in the experiments was prepared by UPH-I-10T UPH ultrapure water manufacturing system (produced by Chengdu Ultrapure Technology Co., Ltd., Chengdu, China). Microwave irradiation was provided by XH-300UL microwave reactor (Beijing Xiangbao Science and Technology Co., Ltd., Beijing, China). A JSM-7500F scanning electron microscope (abbreviated SEM, JEOL, Nara, Japan) was employed for observing the surface morphology of lignin before and after degradation. Small quantities of dried samples were weighed, and the surfaces of lignin and degraded lignin solids were examined using SEM at 1000×, 2000×, and 5000× magnification to compare the changes in surface morphology. A Spectrum Two FT–IR spectrometer (abbreviated as FT–IR; instrument number: L1600300; Perkin Elmer, Waltham, MA, USA) was used to characterize the structure of the lignin polymers. Small quantities of dried samples were weighed and processed using the potassium bromide tablet-pressing method, with a mass ratio of 1:150 potassium bromide, and analyzed within the wavenumber range of 4500~400 cm⁻¹. A TU-1810 UV–Vis spectrophotometer (Beijing Pudian General Instrument Co., Ltd., Beijing, China) was used to analyze the structural changes in the conjugated system. Sample solutions of 24 mg/L were prepared, and UV–Vis spectra were generated through full-wavelength scanning within 190~600 nm. Thermal stability assessments were conducted using thermogravimetric and derivative thermogravimetric (TG–DTG) analysis performed with a TG 209 F1 Libra^® thermogravimetric instrument (NETZSCH-Gerätebau GmbH, Selb, Germany) to evaluate the thermal stability of lignin pre- and post-degradation. A microcomputer differential thermal balance was used to weigh about 20 mg of dry samples, and the thermal stability curves of the samples were determined for a temperature range of 20~800 °C under nitrogen atmosphere at a heating rate of 10 °C/min. A GC/MS-QP 2010 Plus (Shimadzu, Kyoto, Japan) was utilized to analyze the composition of lignin small-molecule degradation products. The chromatogram was equipped with a Rxi-5ms (30 m × 0.25 mm × 0.25 μm) capillary column, where helium served as the carrier gas at a flow rate of 1 mL/min. The initial column temperature was maintained at 50 °C for 2 min, then increased to 180 °C at a rate of 10 °C/min and held for 5 min before increasing to 280 °C at 10 °C/min and maintained for an additional 5 min. The temperature within the gasification chamber was set at 280 °C. The mass spectrometric conditions comprised an electron bombardment ion source at 70 eV and 200 °C, with an interface temperature of 220 °C, FTD detector, and NIST search library.

3.3. Catalytic Oxidative Degradation of Lignin

The experimental procedure for the catalytic oxidative degradation of lignin in this study essentially adhered to the following protocol, as shown in Figure 11. Firstly, the hydroxyl values of the degraded lignin solid were determined, including the content of alcohol hydroxyl and phenol hydroxyl. Subsequently, SEM, UV, FT–IR, and TG–DTG methods were selected for the characterization to reveal the changes in the number of hydroxyl groups and the structural changes in the intuition of the lignin. Afterwards, the extracted lignin degradation reaction solution was analyzed in combination with GC–MS, to study the composition of the small-molecule degradation products and the degradation mechanism, to further reflect the degree of lignin degradation.

3.3.1. Determination of Alcohol Hydroxyl Groups in Lignin

In this study, the alcohol hydroxyl content of degraded lignin was determined according to a modified phthalic anhydride–pyridine method [34]. An appropriate amount of lignin sample (m₀) was first weighed into a round-bottomed flask, and 25.00 mL of 0.16 g/mL of phthalic anhydride–pyridine solution was added to dissolve the sample, then the flask was refluxed in an oil bath at 115 ± 2 °C for 1 h, and the flask was shaken 1~2 times during the refluxing process. The flask was taken out of the oil bath and had cooled to room temperature after 1 h. The flask was rinsed with about 25 mL of distilled water. The condenser tube and flask were rinsed with about 25 mL of distilled water in batches and transferred to a 250 mL conical flask together with the reaction solution. This was followed by titration with 1 mol/L standard solution of sodium hydroxide (NaOH) using phenolphthalein as an indicator. In addition, a blank test was carried out in the same way and the alcohol hydroxyl value (x) was calculated according to Equation (1).

After that, an appropriate amount of sample (m₁) was weighed in a 100 mL beaker; 25 mL of anhydrous ethanol and 25 mL of distilled water were added sequentially and shaken until the sample was completely dissolved. Then, the sample was titrated according to the same method as described above. Additionally, the same method was used for the blank test, and the acid value (a) was calculated according to Equation (2). Afterwards, an appropriate amount of sample (m₂) was weighed in a 100 mL beaker, and potassium hydrogen phthalate was weighed accurately and added to the beaker as well. A total of 25 mL of anhydrous ethanol and 25 mL of distilled water were added and shaken until the sample was completely dissolved. The titration was carried out in the same way and another blank test was performed to calculate the alkali value (b) according to Equation (3). Finally, the calibration hydroxyl value (x’) was calculated according to Equation (4):

x = ((V₀ − V₁) × C × 56.1)/m₀

(1)

a = ((V₂ − V₃) × C × 56.1)/m₁

(2)

b = ((V₄ − V₅) × C × 56.1)/m₂

(3)

x’ = x + a − b

(4)

where x and x’ (mg KOH/g) represent the alcohol hydroxyl value and calibrated alcohol hydroxyl value of the sample, respectively; V₀ and V₁ (mL) represent the volume of NaOH standard solution consumed in blank experiment and sample titration for hydroxyl value determination, respectively; a (mg KOH/g) represents the acid value of the sample; V₃ and V₂ (mL) represent the volume of NaOH standard solution consumed in blank experiment and sample titration for acid value determination, respectively; b (mg KOH/g) represents the alkali value of the sample; V₅ and V₄ (mL) represent the volume of NaOH standard solution consumed in blank experiment and sample titration for alkali value determination, respectively; C (mol/L) represents the concentration of standard solution of sodium hydroxide; and 56.1 (g/mol) is the molar mass of potassium hydroxide.

3.3.2. Determination of Phenolic Hydroxyl Groups in Lignin

The phenolic hydroxyl content of lignin was determined according to a modified ionization difference ultraviolet spectrophotometry (∆ε-IDUS) method [35]. Accurately weighed 10 mg sample was placed in a 10 mL test tube and 5 mL of 1,4-dioxane and 5 mL of 0.2 mol/L NaOH solution were added and shaken to dissolve the sample completely. Two separate 2 mL aliquots were subsequently distributed into two 25 mL test tubes; one tube was filled to 25 mL with a 0.2 mol/L NaOH solution, while the other was brought up to 25 mL with a citric acid–sodium hydroxide buffer with a pH of 6, serving as a blank for determining the absorbance of the sample solution at 300 nm and 350 nm wavelengths. The total amounts of various types of phenolic hydroxyl (x₀), as shown in Figure 12, was calculated using Equation (5).

x₀ = OH (tot) = OH (I + II + III + IV) = (0.25 × Δε₁ + 0.107 × Δε₂) × 1/c

(5)

where x₀ (mmol/g) represents the phenolic hydroxyl value of the sample; c (g/L) represents the concentration of sample solution; and Δε₁ and Δε₂ represent the absorbance of the sample solution at 300 nm and 350 nm, respectively.

4. Conclusions

In this study, three alkaline imidazolium anion-based ionic liquids—[BMIM]Im, [Ch]Im, and [N₄₂₂₂]Im—were selected for the catalytic oxidative degradation of lignin with microwave assistance, and this work provides a useful and effective comparison of the catalytic degradation performance of the three alkaline ionic liquids under uniform conditions for the first time. The experimental results show that the [Ch]Im was the most effective for degrading lignin under microwave radiation. Afterwards, a single-factor experiment was conducted on the reaction conditions for lignin degradation by the [Ch]Im to investigate the effects of four factors, including the microwave time, microwave power, ionic liquid concentration, and hydrogen peroxide concentration, on the hydroxyl content of the lignin degradation products. The optimal reaction conditions were determined as a microwave time of 2 h, microwave power of 300 W, ionic liquid concentration of 0.1 mol/L, and hydrogen peroxide concentration of 0.5 mol/L. The alcohol hydroxyl content of the degraded lignin solid could be converted from 48.62 mg KOH/g to 280.75 mg KOH/g, and the phenol hydroxyl content from 1.223 mmol/g to 1.30 mmol/g.

The extracted reaction solution from the lignin degradation was analyzed by GC–MS for small-molecule degradation products, and it was found that 73.84% of the small-molecule degradation products were aromatic substances. Among these, around 62.86% of the degradation products featured a guaiacol group structure, with guaiacol alone contributing to approximately 27.15% of the content. This study of the degradation mechanism found that the lignin degradation process was mainly dominated by the breaking of the β-O-4 and α-O-4 ether bonds, which led to an increase in the hydroxyl content of the degraded lignin oligomers, while the dealkylation reaction led to the predominance of the guaiacol group structure in the aromatic products.

The degraded lignin solids were then characterized using scanning electron microscopy (SEM), ultraviolet–visible (UV–Vis) spectroscopy, Fourier-transform infrared (FT–IR) spectroscopy, and thermogravimetric and derivative thermogravimetric (TG–DTG) analysis, which visually presented the changes in the surface structure of the degraded lignin and reflected the degree of degradation by analyzing the changes in the structure and thermal stability of the lignin before and after degradation.