Recent Advances in Characterization and Valorization of Lignin and Its Value-Added Products: Challenges and Future Perspectives

Abstract

Lignin, the earth’s second-most abundant biopolymer after cellulose, has long been relegated to low-value byproducts in the pulp and paper industry. However, recent advancements in valorization are transforming lignin into a sustainable and versatile feedstock for producing high-value biofuels, bioplastics, and specialty chemicals. This review explores the conversion of lignin’s complex structure, composed of syringyl (S), guaiacyl (G), and p-hydroxyphenyl (H) units, into value-added products. We critically assess various biochemical and analytical techniques employed for comprehensive lignin characterization. Additionally, we explore strategies for lignin upgrading and functionalization to enhance its suitability for advanced biomaterials. The review emphasizes key areas of lignin valorization, including catalytic depolymerization methods, along with the associated challenges and advancements. We discuss its potential as a feedstock for diverse products such as biofuels, bioplastics, carbon fibers, adhesives, and phenolic compounds. Furthermore, the review briefly explores lignin’s inherent properties as a UV protectant and antioxidant, alongside its potential for incorporation into polymer blends and composites. By presenting recent advancements and case studies from the literature, this review highlights the significant economic and environmental benefits of lignin valorization, including waste reduction, lower greenhouse gas emissions, and decreased reliance on non-renewable resources. Finally, we address future perspectives and challenges associated with achieving large-scale, techno-economically feasible, and environmentally sustainable lignin valorization.

1. Introduction

Lignin, being the second most abundant natural biopolymer on Earth, constitutes about 15–30% of the total lignocellulosic biomass of plants and trees [1,2]. Lignin is the most abundant non-carbohydrate component that imparts rigidity and hydrophobicity to the plant cell wall [3]. It possesses a complex aromatic heteropolymer structure and strong binding to cellulose and hemicellulose [4,5]. These properties position lignin as the principal contributor to lignocellulose recalcitrance. Structurally, lignin is an amorphous polymer composed of oxygenated p-propyl phenol units, such as syringyl (S), guaiacyl (G), and p-hydrophenyl (H), that form a complex three-dimensional (3D) network. These polymers are produced as a result of the linkage of ester bonds of the monolingols, a class of three basic building blocks of alcohols, i.e., p-courmaryl, coniferyl, and sinapyl alcohols [6]. Figure 1 shows the complex structure of lignin, highlighting its heterogeneous nature and strong interactions with other biomass components in plant cell walls. Understanding these interactions can unlock the potential for lignin characterization, improved bioconversion technologies, and novel applications. Within the bioeconomy, lignin’s potential pivots on its key building blocks, monolignols. These phenylpropanoid molecules (coniferyl, p-coumaryl, and sinapyl alcohol) share a core structure with aromatic rings. These aromatic rings are essential for forming strong bonds during lignin polymerization, leading to its strength and stability [7,8], as shown in Figure 2. By probing into monolignol structures, particularly the aromatic rings, researchers unlock doors to improved lignin characterization, enabling targeted modifications and, ultimately, the development of novel bio-based applications for a more sustainable future.

Native lignin (wood) differs from technical lignins due to its unique structure, which is formed from three monolignols (p-coumaryl, coniferyl, and sinapyl alcohol). Their proportions vary by plant type, affecting properties. Softwoods have mainly guaiacyl units (coniferyl alcohol), while hardwoods have a mix of guaiacyl and syringyl units (sinapyl alcohol). Grasses contain all three types [9]. However, technical lignins are modified during the pulping processes, resulting in changes to their structure and properties. These modifications include the introduction of sulfur groups in Kraft lignin and lignosulfonates and the reduction of molecular weight in organosolv lignin [9,10]. Understanding these differences is crucial for optimizing the valorization of technical lignins for various industrial applications [10,11]. Similarly, Table 1 summarizes the properties and applications of technical lignins derived from the three main pulping processes: Kraft, lignosulfonate, and organosolv.

Despite its abundance and potential, lignin remains largely underutilized, with a major portion of it currently burned for energy [2,12]. Unlocking its full potential necessitates innovative approaches to overcome the degradation challenges, thus transforming this underutilized resource into a bioeconomy game-changer [5,13,14]. The bioeconomy aims to utilize renewable biological resources to replace fossil-based materials and fuels, thereby contributing to sustainability and reducing environmental impact. In this regard, lignin represents a promising resource due to its aromatic structure, which can be harnessed to produce a wide range of high-value products, including biofuels, bioplastics, carbon fibers, adhesives, and phenolic compounds [3]. However, the complex and heterogeneous nature of lignin poses significant challenges to its efficient utilization and valorization. The complex and energy-intensive extraction process, such as the Kraft process, along with the structural variability of lignin, make it difficult to develop standardized conversion methods. The unfavorable properties of lignin, including its complex three-dimensional structure and limited solubility, further complicate its utilization [15,16]. To overcome these challenges, current research focuses on efficient conversion methods, such as catalytic depolymerization and enzymatic breakdown, as well as exploring the environmental impact through life cycle assessments. Successfully unlocking lignin’s potential could lead to a sustainable, bio-based economy, with lignin serving as a renewable resource for various industries, including coatings, plastics, and liquid fuels [15,16]. Ongoing research aims to develop efficient and economically viable processes for lignin valorization, offering significant environmental and economic benefits.

Table 1. Properties and applications of technical lignins derived from different pulping processes.

Feature	Kraft Lignin	Lignosulfonates	Organosolv Lignin
Production Process	Kraft pulping (NaOH & Na2S)	Sulfite pulping (SO2 & salts)	Organosolv pulping (Organic solvents)
Sulfur Content	High	High	Low/Sulfur-free
Molecular Weight	High	Low	Low/Medium
Solubility	Low (water)	High (water)	Variable (solvent dependent)
Applications	Adhesives, dispersants, chemicals/materials precursor	Concrete additives, animal feed binders, dispersants	High-purity lignin derivatives, specialty chemicals, carbon fibers, resins, composites
Advantages	Most widely produced	Water-soluble, versatile	Relatively pure, sulfur-free
Disadvantages	High sulfur content, complex processing	High sulfur content, environmental challenges	Variable solubility
References	[17,18]	[19,20]	[21,22]

Table 1. Properties and applications of technical lignins derived from different pulping processes.

Feature	Kraft Lignin	Lignosulfonates	Organosolv Lignin
Production Process	Kraft pulping (NaOH & Na2S)	Sulfite pulping (SO2 & salts)	Organosolv pulping (Organic solvents)
Sulfur Content	High	High	Low/Sulfur-free
Molecular Weight	High	Low	Low/Medium
Solubility	Low (water)	High (water)	Variable (solvent dependent)
Applications	Adhesives, dispersants, chemicals/materials precursor	Concrete additives, animal feed binders, dispersants	High-purity lignin derivatives, specialty chemicals, carbon fibers, resins, composites
Advantages	Most widely produced	Water-soluble, versatile	Relatively pure, sulfur-free
Disadvantages	High sulfur content, complex processing	High sulfur content, environmental challenges	Variable solubility
References	[17,18]	[19,20]	[21,22]

Driven by the bioeconomy’s demand for sustainable materials, research is now focused on transforming lignin from a low-value fuel into high-value products like bioplastics and resins [23,24]. This shift signifies a significant leap toward more sustainable and economically viable biorefineries. The key to successful lignin valorization lies in achieving a multi-pronged approach. Integrating lignin valorization with existing processes in the paper and pulp industry promotes a circular bioeconomy by maximizing resource utilization and minimizing waste generation [25,26]. The benefits of successfully upgraded lignin are far-reaching. It offers a renewable source of aromatic building blocks, reducing our dependence on fossil fuels. Upgraded lignin can be transformed into biofuels and bio-based chemicals, creating sustainable alternatives to traditional petroleum-derived products [27,28]. Furthermore, its unique properties make it a valuable resource for the development of sustainable materials like resins, insulating materials, and specialty chemicals. By achieving the optimal balance during the upgrading process, lignin can transform from a byproduct into a game-changer, paving the way for a more sustainable future [3,29]. This review examines lignin’s complexities including characterization, upgrading strategies, and diverse applications for a sustainable future. However, achieving large-scale adoption depends on techno-economic feasibility and unwavering environmental sustainability. Lignin, a plentiful but underutilized byproduct of the paper and pulp industry, is undergoing a transformative shift toward becoming a game-changer in the bioeconomy. This comprehensive review explores the intricate nature of lignin, covering characterization methods, upgrading strategies, and diverse applications that position it as a crucial contributor to a sustainable future.

Research Gap and Objective of the Study

However, achieving large-scale adoption depends on overcoming significant research gaps. These include the development of more efficient, selective, and robust catalytic depolymerization processes, cost-effective and economically viable methods, comprehensive life cycle assessments for environmental sustainability, and advanced analytical techniques for precise characterization and standardization. Additionally, integrating lignin-derived materials into commercial applications poses challenges due to performance limitations and compatibility issues. The aim of this study is to explore and address these gaps by providing a detailed analysis of current lignin valorization methods, highlighting recent technological advancements, and proposing future research directions. By presenting a comprehensive overview of lignin’s potential and the innovative approaches needed to fully utilize this resource, this review seeks to promote sustainability and innovation in various industrial applications, ultimately contributing to a more sustainable and circular bioeconomy.

2. Methodology

For this study, a comprehensive literature survey was conducted using the PubMed (https://pubmed.ncbi.nlm.nih.gov/, accessed on 5 June 2024) and Scopus (http://www.scopus.com/, accessed on 5 June 2024) databases. These databases are renowned and widely preferred for bibliometric analysis. The search terms employed included “Lignin valorization”, “Lignin biorefinery”, “Value-added products from lignin”, and “Bioplastics”, covering research papers from the year 2000 onwards (Figure 3). This search yielded a total of 1322 relevant research/review papers, which have garnered over 34,880 citations. The most productive years to date are 2021, 2022, and 2023, each reported over 202, 240, and 260 publications, respectively. Figure 3A illustrates the evolution of this research topic over the past two decades, highlighting the significant attention from the scientific community in recent years, particularly after the year 2010. Figure 3B represents a word cloud analysis of the lignin valorization, with organic chemistry being the dominant subject depicting the characterization of the compound. The identified subject domains primarily relate to the chemistry, engineering, and valorization/utilization of lignin for the derivation of various commodity chemicals and products from lignin biomass.

Using data from PubMed and Scopus, a VOSviewer version 1.6.20 bibliometric analysis of lignin studies was developed (Figure 4). The findings from the VOSviewer analysis indicated a significant expansion in the body of knowledge surrounding the characterization of lignin and its valorization, with recent research focusing on the production of biofuels and other products from it (Figure 4B). Figure 4A demonstrates a collaborative analysis of lignin valorization by examining the co-occurrence of author keywords in the literature. Out of the total keywords (1482), over 101 keywords met the criteria of five keyword occurrences, forming 7 different clusters. Each of the first and second clusters (red and green, respectively) contains 20 keywords. The third and fourth clusters (Blue and pale) had 18 and 15 items, respectively, while the fifth cluster (magenta) had 14 items. Each cluster is represented by a distinct color to illustrate the relationships between its constituents. Clusters of identical terms typically have stronger relationships.

2.1. Understanding Lignin: Characterization as the Foundation for Sustainable Valorization

Lignin holds massive potential as a sustainable resource for biofuels, bioplastics, and high-value products. However, unlocking this potential requires thorough characterization to understand its unique structural variations across plant sources. This knowledge is essential for developing tailored valorization strategies. Overcoming the limitations of a “one-size-fits-all” approach, the characterization provides a toolbox of techniques to address lignin’s diverse behavior and breakdown.

2.2. Chemical Analysis (Compositional Methods)

Quantifying the building blocks and functional groups of lignin is essential for understanding its composition and reactivity. Analytical techniques such as gas chromatography-mass spectrometry (GC-MS) and liquid chromatography-mass spectrometry (LC-MS) play a pivotal role in identifying and quantifying monomeric units and low molecular weight compounds in lignin. This knowledge is akin to understanding the ingredients and their relative abundance in a complex recipe, guiding the optimization of lignin transformation for specific pathways [30]. The GC-MS analyses have been widely employed for the identification and quantification of the monomeric units (e.g., guaiacyl, syringyl, p-hydroxyphenyl) and low molecular weight compounds present in lignin, providing insights into their distribution and influence on reactivity [30]. On the other hand, LC-MS is a powerful tool for characterizing both native and degraded lignin samples, including high molecular weight fractions, distinguishing various structural features such as monomeric units, interunit linkages, and functional groups [30]. These techniques are often complemented by pyrolysis-GC/MS to gain a comprehensive understanding of lignin’s structural complexity [30,31]. By elucidating the intricate composition and functional group distribution, researchers can optimize valorization processes, develop targeted modification strategies, and unlock lignin’s full potential for producing valuable chemicals and materials tailored to specific applications [30,31].

Gel Permeation Chromatography (GPC) and Size Exclusion Chromatography (SEC) are widely used techniques for determining the molecular weight distribution of lignin, separating molecules based on size. GPC provides detailed information on the molecular weight distribution profile, while SEC is applicable to various types of lignin [32]. Matrix-Assisted Laser Desorption/Ionization-Time of Flight (MALDI-TOF) mass spectrometry offers high-resolution mass spectra for precise molecular weight determination but requires specialized equipment and complex sample preparation [24,33]. Nuclear Magnetic Resonance (NMR) spectroscopy, particularly 2D-NMR, provides insights into lignin’s molecular weight distribution by analyzing its structural characteristics, although it is less commonly used solely for molecular weight determination [34]. These techniques play crucial roles in characterizing lignin and furthering its utilization. Researchers often combine these methods to gain a comprehensive understanding of lignin’s molecular characteristics. For instance, integrating GPC data with 2D-NMR results allows for quantitative molecular weight distribution analysis alongside qualitative structural insights [32,33,34].

2.3. Spectroscopic Techniques for Characterization of Lignin

Nuclear Magnetic Resonance spectroscopy (NMR) and Fourier Transform Infrared Spectroscopy (FTIR) provide powerful insights into lignin’s molecular composition, enabling sustainable utilization. NMR spectroscopy serves as a powerful tool for uncovering the intricate architecture of lignin, providing detailed information about its composition and structure. By manipulating the magnetic properties of atomic nuclei, NMR acts as a high-resolution microscope, revealing the types and arrangements of atoms within the lignin molecule [35]. This enables the identification of monomeric units (e.g., p-hydroxyphenylpropane, guaiacylpropane, and syringylpropane) and quantification of their relative abundance. NMR also allows for the quantification of functional groups (e.g., phenolic hydroxyls, methoxyls, and aliphatic hydroxyls) and the investigation of chemical bonds (e.g., β-O-4 and α-ether linkages) that connect individual monomers in the lignin structure [3,27,29,36]. By integrating this information, researchers gain a comprehensive understanding of lignin’s architecture, facilitating the development of effective strategies for converting lignin into valuable products.

NMR spectroscopy serves as a powerful tool for identifying new chemical entities across different organic substrates. Various advanced 2D NMR techniques have been extensively used to study lignin, focusing on its composition, degradation pathways, and the formation of phenolic compounds and organic acids [37]. The one-dimensional ¹³C heteronuclear single-quantum coherence (HSQC) method, in particular, stands out for its superior sensitivity and ability to explore a wide range of chemical shifts [38,39]. To enhance its capabilities, optimizing spectral parameters, employing sparse sampling for faster data acquisition, and utilizing high-field NMR instruments are crucial. Despite its qualitative nature, 2D NMR can be made quantitative through calibration and specialized pulse sequences [3]. Integrating these advancements with other analytical methods will enable a comprehensive understanding of lignin’s behavior in various processes and its impact on product outcomes.

FTIR spectroscopy is also a powerful tool for rapid and informative analysis of lignin’s functional groups, crucial for sustainable valorization. FTIR analyzes how lignin interacts with infrared light, revealing a “fingerprint” of its functional groups like phenolics, aromatics, and carbonyls [40,41]. By identifying reactive groups, researchers can assess lignin’s suitability for different conversion processes and monitor changes during treatment [41,42]. FTIR acts as a quality control tool, ensuring the desired functionality in the final product. The presence of specific bands corresponding to aromatic O-H and methoxy groups confirms the integrity of the lignin structure, while their intensity indicates the abundance of reactive sites, guiding the selection of appropriate valorization pathways [41,43]. Additionally, FTIR can detect changes in functional groups during pretreatment or conversion processes, enabling real-time monitoring and optimization of reaction conditions. As a non-destructive and cost-effective technique, FTIR can rapidly analyze small amounts of lignin, making it suitable for high-throughput screening and process optimization [41,42]. By FTIR spectroscopy, researchers can gain a deeper understanding of lignin’s reactivity and develop tailored valorization strategies that maximize the potential of this abundant biopolymer, paving the way for sustainable production of biofuels, biochemicals, and value-added products from lignin [41,42]. Recently, Blindheim et al. 2023 evaluated three FTIR techniques for lignin, highlighting differences in spectral quality and quantification of functional groups, such as phenolic OH [25]. The authors revealed that ATR of native lignin offers convenience; however, the KBr pelleting method provides smoother spectra with higher resolution, correlating strongly with phenolic OH content determined via titration and size-exclusion chromatography (SEC) analyses [40]. Similarly, the FTIR analysis of the milled wood lignin (MWL) extracted from Dendrocalamus sinicus bamboo exhibited a high concentration of β-O-4′ aryl ethers (~79.5% per 100 Ar) and a syringyl/guaiacyl (S/G) ratio of approximately 1.6 [36].

The integration of NMR and FTIR data in lignin valorization allows for the optimization of conditions, including catalysts, temperature, and time, to selectively target and preserve specific structural features. This synergistic approach enhances product selectivity and yield by identifying the most reactive functional groups. Both provide a comprehensive understanding of the structure-reactivity relationships of lignin, facilitating the development of customized valorization strategies. This knowledge aids in the design of efficient processes for the selective depolymerization of lignin into valuable phenolic compounds, aromatics, or platform chemicals, thereby maximizing the utilization of this renewable resource [44,45]. New analytical tools like NMR and FTIR are providing a deeper understanding of lignin’s structure and how it interacts with other biomass components [13]. This knowledge is key to developing targeted pretreatment methods that reveal lignin’s potential for bioconversion. NMR, particularly solution-state NMR, has become the most widely used technique for the structural characterization of lignin due to its versatility in illustrating structural features and transformations [46,47].

2.4. Microscopic Techniques for Characterization of Lignin

While compositional and spectroscopic techniques provide valuable information, transmission electron microscopy (TEM) and scanning electron microscopies (SEM) act as powerful magnifying tools, revealing how lignin interacts within the cell wall at finer levels [48]. TEM allows visualization of lignin at the nanoscale, providing high-resolution images detailing atomic structures. It uncovers lignin distribution within the cell wall, showcasing its role in forming a complex network around cellulose microfibrils for structural support. TEM also reveals the ultrastructure of lignin, including different density domains and pores, shedding light on lignin’s behavior during valorization processes [49,50]. The synergy of SEM with TEM and other techniques provides comprehensive insights into lignin’s role, spatial distribution, ultrastructure, and surface features within the cell wall, aiding in tailored valorization strategies [48,49]. These microscopy methods complementarily identify lignin’s pores, surface topography, and morphological features, guiding the use of solvents or pretreatments to enhance depolymerization accessibility [51]. Understanding lignin’s interactions with cellulose and hemicellulose through TEM and SEM assists in selectively targeting lignin bonds while minimizing the degradation of other components during valorization processes [51,52]. These microscopic characterization techniques augment to optimize pretreatment strategies, maximizing valuable product yield from lignin by identifying concentrated or tightly bound lignin regions for specific pretreatments or catalysts for selective depolymerization or modification [51,52].

Table 2. An overview of the key techniques and their specific features concerning the characterization and valorization of lignin for value-added products.

Technique	Uses/Key Features	References
Functional group characterization
31P NMR	Quantitative determination of different types of hydroxyl groups present in lignin, including aliphatic (α-OH, β-OH), phenolic (OH ph), and carboxylic acid groups	[53,54,55]
FTIR	i. Identification of functional groups like alcohols (O-H stretch around 3300 cm−1), carbonyls (C=O stretch around 1700 cm−1), alkenes (C=C stretch around 1650 cm−1), etc. ii. The fingerprint region (600–1500 cm−1) of an FTIR spectrum contains a unique pattern of absorption bands that can be used to identify structural features and distinguish between isomers. iii. The S/G ratio, which is the molar ratio of syringyl (S) to guaiacyl (G) units, is an important parameter for characterizing lignin structure. Intensities of bands around 1330 cm−1 (S ring breathing) and 1270 cm−1 (G ring breathing) can be used to estimate the S/G ratio.	[56,57,58]
Morphological analysis
SEM	Visualization of lignin morphology on cell wall surfaces, interaction with other components, surface topography, and modifications after pretreatment i. SEM imaging can reveal the surface morphology of cell walls and the distribution of lignin before and after pretreatments. ii. SEM showed the highest lignin concentration in the cell corner middle lamella regions, followed by the compound middle lamella (CML), with the lowest levels in the secondary cell wall in untreated Eucalyptus samples.	[59,60]
	Determination of lignin distribution across cell wall layers, interaction with cellulose microfibrils. i. TEM imaging of ultra-thin sections stained with uranyl acetate and lead citrate reveals lignin distribution across the primary (P) and secondary (S1, S2, S3) cell wall layers in various plant fibers. The middle lamella region shows the highest level of lignification in all studied fibers. High-voltage TEM imaging confirms higher lignin concentrations in the compound middle lamella and S1 layer compared to the S2 layer in wood samples. ii. TEM provides insights into the interaction between lignin and cellulose microfibrils within the cell wall layers. In the S2 layer, lignin forms a matrix surrounding the parallel cellulose microfibrils, creating a cellulose-lignin composite structure. The spatial distribution and interactions between cellulose microfibrils and lignin matrix vary due to the differences in orientation and packing across layers (S1, S2, S3). High-resolution TEM techniques, such as electron tomography, offer the potential for 3D visualization of cellulose microfibril arrangement and encasement by lignin within the cell wall layers.	[61]
AFM	Nanoscopic mapping of lignin location and distribution on cellulose nanofibers: i. Thiophenol-coated tip interacts with lignin through hydrophobic and π-π interactions. ii. FCM data from force–distance curves indicate lignin-rich regions with higher interactive forces. iii. AFM + FCM provides comprehensive info on surface morphology and nanoscopic lignin distribution. iv. Lignin covers cellulose fibers in a grained structure, not uniformly distributed. v. AFM-FCM determines nanoscopic lignin location, surpassing the capabilities of STEM. vi. AFM is a powerful tool for nanoscale analysis of lignin in lignocellulosic materials.	[62]
Structural elements characterization
Py-GC-MS	Identification of Monomers: i. Pyrolysis breaks down polymers into monomers or smaller fragments for GC-MS analysis. ii. Pyrolyzates from common polymers exhibit characteristic monomer patterns for identification. iii. Polyethylene pyrolysis produces paraffin triplets, allowing the identification of ethylene monomers. iv. Complex polymers like lignin and proteins yield phenolic compounds, aromatic hydrocarbons, and nitrogen-containing fragments for monomer inference. Identification of Inter-unit Linkages: i. Specific pyrolysis products indicate inter-unit linkages in polymers. ii. Lignin pyrolyzate compounds like vinyl phenols indicate ether and carbon-carbon linkages. iii. Abundances of these products estimate proportions of inter-unit linkages in lignin. iv. Proteins’ pyrolysis products provide insights into peptide linkages and amino acid sequences. Py-GC-MS is a powerful technique for identifying monomers and linkages in polymers.	[63,64]
NMR (1H, 13C, 2D)	Elucidation of structural elements and inter-unit linkages.	[65]
Molar mass distribution analysis
SEC	Determination of weight-average (Mw), number-average (Mn), and peak molar mass (Mp)	[66]
Other Techniques
XRD	Evaluation of crystallinity and amorphous regions	[66]
Thermal Analysis (TGA, DSC)	Thermal stability and phase transitions	[66]
Elemental Analysis	Determination of elemental composition (C, H, O, S, etc.)	[66]
LC-MS	Analysis of lignin degradation products, identification of monomers, dimers, and oligomers, structural elucidation	[66,67,68]
GC-MS	Analysis of volatile lignin degradation products, identification of monomers and dimers	[59,66]

summarizes various techniques used for lignin characterization and their specific applications.

Table 1 Overcoming the hurdle: Advancements in lignin upgradation methods.

Lignin, being the largest source of aromatic building blocks on the planet, has great potential to be used as a platform for large-scale chemical production. The products obtained from lignin can be classified into three groups: aromatic compounds, macromolecule group, and carbon fiber group [31]. The former group includes smaller aromatic compounds obtained by fragmentation of lignin, e.g., phenol, benzene, xylene, toluene, etc. The second group employs lignin macromolecules as additives or polymer blends. The last group encompasses carbon materials derived from lignin, such as carbon fiber [69]. However, the major challenges for the lignin platform industry are high-quality lignin isolation from lignocellulose due to the variation in the composition of lignin polymers that primarily depend on the lignin source and lignin extraction method [70]. Lignin, once a roadblock in the bioconversion of lignocellulose, is now being transformed into a valuable resource. Recent breakthroughs in lignin upgrading methods have unlocked its potential, paving the way for its full utilization.

3. Pretreatment: Loosening Bonds with Other Biomass Components

Pretreatment methods aim to disrupt the intricate network of lignin-carbohydrate complexes, facilitating the subsequent fractionation and depolymerization processes [71]. Techniques such as ionic liquid-assisted fractionation, γ-valerolactone-assisted hydrolysis, and mild organosolv processes have shown promise in selectively removing lignin while preserving its native structure and valuable inter-unit linkages like β-O-4 [71,72].

3.1. Thermal Depolymerization of Lignin

Thermal depolymerization techniques, namely pyrolysis, gasification, and hydrothermal liquefaction, are employed to break down the resilient lignin polymer into diverse products. Pyrolysis involves subjecting lignin to high temperatures (typically 400–800 °C) in the absence of oxygen, resulting in the formation of monomeric and oligomeric phenols present in liquid oil (e.g., phenol, catechol, syringol, guaiacol), volatile compounds (e.g., methanol, acetone), gaseous substances (H₂, CO₂, CH₄, C₂H₄), and biochar [72,73]. Gasification, on the other hand, occurs at extremely high temperatures (>800 °C) in the presence of an oxidizing agent such as air or oxygen, leading to the production of synthesis gas (syngas) primarily composed of H₂, CO, CO₂, and CH₄ [72]. Hydrothermal liquefaction (HTL) involves treating lignin at moderate temperatures (200–400 °C) and high pressure (5–28 MPa) in a solvent like water, yielding low-oxygen liquid bio-oil and/or phenolics such as syringol, vanillin, guaiacol, along with phenolic oligomers [72,73,74]. These thermal depolymerization techniques effectively cleave the robust bonds within lignin, resulting in the generation of valuable phenolic monomers, dimers, oligomers, aromatic hydrocarbons, and char. The distribution of products is influenced by factors including the source of lignin, temperature, pressure, and residence time [72,74]. Analyzing the depolymerization methods (thermochemical and chemical) and product utilization pathways (biochemicals, biofuels, bioelectricity) presented in Figure 5 offers valuable insights into future advancements in lignin conversion. Consulting recent literature on these areas will be critical for identifying promising strategies to enhance lignin’s value in bioconversion processes. This knowledge can pave the way for the development of next-generation methods, ultimately maximizing lignin’s potential within a sustainable bio-based economy.

Thermal pyrolysis of lignin often yields a complex mixture with low monomer yield due to poor selectivity and the propensity for repolymerization [75,76]. To enhance product selectivity and efficiency, an effective catalytic system is essential. Zeolites, in particular, have garnered significant attention for lignin pyrolysis. Their unique pore structure and shape selectivity stabilize reactive intermediates and facilitate the conversion of depolymerized monomers into desired products, such as aromatics [69,77,78].

3.2. Catalytic Depolymerization

This method offers several advantages, including the selective cleavage of lignin bonds by targeting ether and C-C linkages, resulting in the breakdown of the polymer into smaller, valuable compounds [79,80,81]. Moreover, catalytic processes improve the efficiency of lignin depolymerization, leading to higher yields of desired products and better selectivity compared to non-catalytic methods [79,80,81,82]. Acid/base catalysis involves the utilization of bases like sodium hydroxide (NaOH) for depolymerizing lignin, effectively converting it into low molecular weight, water-soluble species. An example includes the base-catalyzed depolymerization (BCD) of biorefinery lignins using NaOH at temperatures of 270–330 °C, which resulted in significant depolymerization and the production of methoxyphenols and benzenediols [80]. Oxidative depolymerization, on the other hand, employs oxidants such as hydrogen peroxide (H₂O₂) or oxygen (O₂) alongside metal catalysts (e.g., Cu, Mn, V) to cleave C-C and C-O bonds in lignin [72,81]. For example, oxidative depolymerization of lignin using formic acid (FA) and sulfuric acid (H₂SO₄) under mild conditions produces low molecular weight aromatics [72]. Microwave-assisted depolymerization applies microwave irradiation in the presence of catalysts and solvents to rapidly and efficiently break down lignin. An example of a metal catalyst involves the depolymerization of Kraft lignin in ethanol using Pt/C, Ni/C, and Ru/C catalysts at 250 °C, resulting in high yields of alkylated phenolic monomers. These examples illustrate the efficacy of catalytic depolymerization in valorizing lignin [82].

Solid acid catalysts, particularly zeolites (HZSM-5 and hierarchical Hβ) and sulfonated carbon, are emerging as game-changers in lignin depolymerization. These reusable and environment-friendly catalysts act like microscopic factories with well-defined pores [83,84]. They selectively break down lignin’s complex structure under milder conditions compared to traditional methods [85,86]. Zeolites, with their precise acidity and pore structure, promote targeted cleavage of specific bonds within lignin. Hierarchical zeolites, like Hβ, are particularly effective due to enhanced mass transfer for bulky lignin molecules. This process yields valuable products like aromatic monomers—building blocks for polymers—and phenolic oils useful for fuel additives or resins. However, some gas and char byproducts are also formed [83,84,86]. Sulfonated carbon, on the other hand, excels at producing high yields of phenols, crucial precursors for various industrial chemicals. It also generates aromatic aldehydes used in fragrances and dyes, as well as carboxylic acids, versatile molecules with applications in plastic and biofuel production [87]. Notably, sulfonated carbon operates at even lower temperatures compared to zeolites [87,88]. Overall, these solid acid catalysts offer a sustainable approach for biorefineries. By transforming abundant lignin waste into valuable resources, they pave the way for a greener future in the chemical industry.

3.3. Ionic Liquid Pretreatment for Lignin Depolymerization

Ionic liquids (ILs) offer a customizable approach to lignin depolymerization. Protic ILs (PILs) cleave bonds efficiently with their acidic nature, yielding aromatic monomers and phenolics [89,90]. Cellulose ILs (CILs) like 1-Butyl-3-methylimidazolium chloride (BMIMCl) dissolve and depolymerize lignin in one step, producing a range of valuable products [90,91]. Metal-containing ILs (M-ILs) act as selective catalysts, allowing control over the desired products like aromatic hydrocarbons or phenolics [90,91]. This tailored approach using PILs, CILs, and M-ILs reveals a sustainable path for transforming lignin into valuable bio-based products [90,91]. This depolymerization process unlocks a pathway for transforming lignin, a by-product in pulping processes, into valuable bio-based products. Furthermore, the combination of ILs with enzymes is presented as a promising, sustainable, and efficient strategy for lignin depolymerization. This enzymatic approach offers an attractive alternative to traditional methods, paving the way for the development of novel, value-added products from lignin [72,92]. The ILs emerge as powerful tools for sustainable lignin conversion, enabling depolymerization into valuable chemicals. This powerful technology has its sights set on conquering mixed lignin sources and even integrating seamlessly with other processes to achieve single-step, high-value product conversion. With economic hurdles overcome, lignin depolymerization has the potential to become the cornerstone of a thriving, sustainable bioeconomy built upon this versatile bioresource. All these methods have been summarized in Figure 6, which outlines catalyst depolymerization for lignin valorization. This integrated knowledge is crucial for developing efficient depolymerization, maximizing product yield, and potentially generating bioenergy, ultimately unlocking lignin’s potential in a sustainable bio-economy. While chemical methods currently offer the most established approach, they often generate harmful byproducts and require significant resources. Researchers are actively exploring greener alternatives like electrochemical depolymerization, which utilizes milder conditions and avoids hazardous chemicals [93]. Biological methods hold promise for a more sustainable approach, but limitations exist in terms of efficiency and scalability, hindering large-scale adoption. Thermochemical techniques offer efficient breakdown of lignin, but the high energy input and potential for environmental impact necessitate further optimization [94].

3.4. Biological Depolymerization of Lignin

Biological depolymerization of lignin involves the utilization of ligninolytic enzymes produced by fungi, bacteria and some animals. Biological depolymerization offers several advantages compared to other methods. It operates under milder conditions, including ambient temperatures and pressures, leading to reduced energy consumption and environmental impact. The specificity of ligninolytic enzymes allows for selective targeting of specific bonds within lignin, resulting in controlled depolymerization and higher yields of desired products. Furthermore, biological approaches are environment-friendly, as they avoid the use of harsh chemicals and generate fewer pollutants compared to chemical and thermal methods [95,96,97]. Hitherto, several microorganisms, including fungi and bacteria, are involved in lignin depolymerization. White-rot fungi are renowned for their ability to completely degrade lignin using enzymes like laccases and peroxidases [65,66,67]. Brown-rot and soft-rot fungi, on the other hand, partially degrade lignin, resulting in modified lignin residues.

The discovery of bacteria with robust lignin degradation capabilities, along with the characterization of their associated enzymes, holds substantial promise for lignin valorization. However, the isolation source is crucial for identifying high-performing lignin-degrading bacteria. Lignin-degrading bacteria are typically prevalent in lignin-rich environments, both natural and man-made, such as leaf litter, pulp and paper mill sludge, compost soils, decomposing wood, and activated sludge [98,99]. To date, several ligninolytic bacteria have been isolated from the steeping fluid of eroded bamboo slips, which were excavated from an ancient tomb over 1700 years old [100]. The wood-eating termites digest most lignocellulosic materials in their hindguts, making the termite gut a rich source for isolating lignin-degrading bacteria [1,101,102,103]. Hitherto, the successful isolation of bacteria responsible for lignin degradation from termite guts has been reported by several studies [104,105,106]. Additionally, certain endophytic bacteria, which can rapidly decompose plant residues upon the plant’s death due to their lignocellulose degradation capabilities, have also been identified and isolated for lignin degradation [107]. A substantial number of lignin-degrading bacteria have been isolated to date, predominantly from the phyla Proteobacteria, Actinobacteria, and Firmicutes [69]. Recent studies have also highlighted the role of the archaeal phylum Bathyarchaeota in lignin degradation [108]. Actinobacteria, particularly those belonging to the Solirubrobacterales and Thermoleophilaceae families, have demonstrated lignin degradation capabilities in thermal environments. Additionally, certain bacterial species employ enzymes like DyPs and laccase-like multi-copper oxidases (LCMOs) to break down lignin [66,67]. Notably, certain strains, such as Pseudomonas putida KT2440, Sphingobium sp. SYK-6, and Rhodococcus opacus PD630, have been utilized as model organisms for producing value-added products from lignin and for mining new lignin-degrading enzymes [109,110,111].

Ongoing research has led to the characterization of inherent enzymes and metabolic pathways involved in lignin degradation by these bacteria [112]. These include enzymes and pathways that catalyze oxidative and hydroxylation reactions, depolymerize phenolic and non-phenolic lignin polymers, perform demethylation reactions, and open the aromatic rings of lignin-based compounds. Some of these bacteria are now employed in treating sewage from the pulp and paper industry, degrading heterogeneous compounds, and pre-treating lignocellulosic biomass [113]. Biological depolymerization of lignin holds promise for lignin valorization, benefiting from the involvement of ligninolytic enzymes produced by fungi and bacteria. These enzymes can be classified into two main categories: lignin-modifying enzymes (LME) and lignin-degrading auxiliary (LDA) enzymes [97]. LMEs, such as laccases and peroxidases, play a crucial role in the breakdown of lignin by catalyzing the oxidation of phenolic substrates and utilizing hydrogen peroxide to cleave the complex lignin structure [96,97,114]. LDA enzymes, including heme-thiolate haloperoxidases and dye-decolorizing peroxidases (DyPs), also contribute to lignin degradation, although their precise functions are not yet fully understood [97,115].

The use of these enzymes, coupled with the mild conditions and enzymatic specificity of biological processes, enables the effective breakdown of lignin into valuable compounds [95,96]. Enhancing lignin depolymerization efficiency can be achieved by combining enzymes like aryl alcohol oxidase and lignin peroxidase in biocompatible ionic liquids [97]. Figure 7 depicts biological depolymerization for lignin valorization. Analyzing this alongside advancements in enzymes, microbes, and process optimization can guide research toward efficient, sustainable methods, maximizing product yield and bioenergy potential, ultimately unlocking lignin’s value in a bio-based economy. Furthermore, researchers are exploring eco-friendly approaches using microbes like fungi and bacteria. Processes like PIPOL demonstrate the promise of these microbes in efficiently breaking down and converting lignin [2,23,116].

3.5. Emerging Techniques for Valorization of Lignin

Lignin depolymerization gets a boost from eco-friendly electrochemical depolymerization. This promising approach harnesses renewable energy, precisely targets desired products, and avoids harsh conditions. Electrochemical depolymerization of lignin involves two interconnected mechanisms. The first mechanism is the direct electro-oxidation of lignin on the anode, which cleaves the C-C and C-O-C linkages within the lignin polymer [117]. Simultaneously, the second mechanism occurs through chemical oxidation facilitated by in-situ electro-generated hydrogen peroxide (H₂O₂) formed on the cathode. This H₂O₂ further decomposes into reactive oxygen species (ROS) in the alkaline electrolyte, enhancing lignin depolymerization [118]. These dual oxidation processes generate over 20 different low-molecular-weight (LMW) aromatic compounds with diverse functional groups, such as aldehydes, ketones, phenols, and acids. Analytical techniques like GC-MS and ESI-MS/MS are employed to identify these LMW products. To achieve optimal results, it is recommended to perform the electrolysis under specific conditions, including a current density of 8 mA cm⁻² at 80 °C with the addition of extra oxygen (O₂). Under these conditions, after one hour, a significant 59.2% depolymerization of lignin into LMW products can be achieved [118,119].

The electrocatalytic upgrading technique employing a PtNiB/CMK-3 catalyst involves the electrocatalytic hydrogenation (ECH) of guaiacol and related lignin model monomers. This process leads to the formation of valuable products with a high faradaic efficiency of 86.2%. In comparison, the faradaic efficiency of PtNi/CMK-3 without boron doping is significantly lower at 6.3%. The mechanism behind this improvement lies in the optimization of the PtNiB alloy surface electron structure through boron doping. This optimization enhances the adsorption of the substrate and intermediates, ultimately promoting the efficiency of the electrocatalytic hydrogenation process [120].

The utilization of Deep Eutectic Solvents (DES) in electrochemical oxidative depolymerization presents an effective process for breaking down lignin. This method involves dissolving lignin in DES and subjecting it to electrochemical treatment. The successful depolymerization of lignin in DES has been demonstrated by the detection of guaiacol and vanillin as the most abundant products. The use of DES offers several advantages in this process. Firstly, it eliminates the requirement for strong alkaline conditions, which can result in undesired oxidations and restrict the choice of electrode materials. This enables a more flexible and controlled depolymerization process, making DES an attractive option for lignin valorization [121].

The utilization of a dodecyl sulfate-intercalated cobalt sulfide (DS-CoS) nanocone catalyst enables the electrocatalytic cleavage of Cα-Cβ bonds in β-O-4 lignin models under mild conditions. This process results in the selective oxidation of the lignin model, leading to the formation of aryl aldehydes or carboxylates as the main products. The yields obtained for aryl aldehydes and carboxylates are 21.4% and 23.9%, respectively. The mechanism behind this transformation involves the generation of hydroxyl radicals on the surface of the SCo³⁺OH intermediate. These hydroxyl radicals induce the cleavage of the targeted Cα-Cβ bond. Subsequently, the resulting intermediates are further oxidized to carboxylic acids by superoxide radicals. This electrocatalytic process offers a promising approach for the selective conversion of lignin models into valuable compounds under mild reaction conditions [122].

The emerging techniques in lignin depolymerization, particularly electrochemical methods, offer several advantages that contribute to their potential as sustainable and efficient approaches. Firstly, these techniques allow for the integration of renewable energy sources, reducing reliance on fossil fuels and enhancing the overall sustainability of the process [122,123]. Secondly, electrochemical methods provide fine-tuned selectivity, granting precise control over the reaction conditions. By adjusting the electrical potential, it becomes possible to selectively produce desired compounds, offering versatility and customization in product formation [122,123]. Lastly, these processes typically occur under milder conditions compared to traditional thermal or chemical methods. This characteristic not only reduces energy consumption but also minimizes the generation of harmful by-products, promoting a more environment-friendly approach to lignin depolymerization. The combination of sustainable integration, fine-tuned selectivity, and mild reaction conditions makes electrochemical methods a promising avenue for the efficient valorization of lignin and the transition toward a more sustainable bio-based economy [119,120,121,122].

4. Challenges and Readiness of Lignin Depolymerization

Lignin depolymerization faces challenges across various methods. Thermal depolymerization, while simple, suffers from high energy needs and low selectivity (TRL 4-5) that leads to the formation of unwanted by-products like char and tar [72,124]. These issues complicate product separation and purification, limiting its industrial applications to pilot-scale demonstrations. The Technology Readiness Level (TRL) for thermal depolymerization is estimated at 4-5, indicating that the technology has been validated in laboratory and relevant environments but is not yet ready for full-scale commercial deployment. Catalytic depolymerization offers targeted product formation but struggles with catalyst cost and scalability. It faces drawbacks such as high catalyst costs, catalyst deactivation, and the need for harsh reaction conditions [72,79,82]. Challenges in catalyst recovery and recycling, as well as scalability issues due to high costs, represent significant gaps in this technology. Despite these challenges, some catalytic processes have reached early commercial application, particularly in biorefineries, with a TRL of 5-6. Biological approaches (TRL 3-4) are limited by slow reaction rates and sensitivity to environmental conditions [96,97,101]. Maintaining optimal conditions for microbial or enzymatic activity on an industrial scale remains a significant challenge. This method is mostly at the research and pilot-scale stages, with some niche applications in bioremediation, and has a TRL of 3-4.

Both oxidative and reductive depolymerization offer promising avenues for lignin valorization, yet each faces its own limitations. Oxidative depolymerization, while effective at cleaving lignin bonds, struggles with unwanted byproducts, high oxidant demands, and potential environmental concerns (TRL 4). Controlling reaction pathways and minimizing byproducts remains a significant hurdle [94,117,125]. Reductive depolymerization, on the other hand, offers selective bond cleavage but faces challenges due to the high cost and complex reaction conditions required for the reducing agents (TRL 3-4) [126]. Scalability and efficient recovery/reuse of these agents are also significant gaps that need to be addressed. Despite the above promise, all lignin depolymerization methods face hurdles in efficiency, selectivity, cost, and scalability. Catalytic approaches, though most advanced for industrial use (TRL 5-6), still require development. Continued research is crucial to improve efficiency, selectivity, and cost-effectiveness across all methods for a future biorefinery sector built on sustainable lignin valorization.

5. Advancements in the Valorization of Lignin

Lignin’s valorization has gained significance due to advancements driven by the increasing demand for sustainable materials and reduced dependence on fossil fuels. Recent research and technological innovations in lignin valorization have yielded key advancements with significant implications for the industry.

5.1. Lignin as a Source of Biofuels

Lignin valorization into biofuels shows great promise for sustainable energy production. Recent biotechnological advancements have significantly improved the efficiency of lignin breakdown into simpler molecules, enabling the production of bioethanol and biodiesel. These developments are driven by the pressing need for sustainable energy sources and the desire to maximize the utilization of lignocellulosic biomass, aligning with key UN Sustainable Development Goals such as clean energy (SDG 7) and climate action (SDG 13) [3,6,14]. Enzymatic degradation techniques have significantly improved lignin conversion rates. Novel enzymatic cocktails have been developed, increasing lignin degradation efficiency by up to 40% compared to traditional methods, leading to higher bioethanol yields. This advancement highlights the potential for increased biofuel production from lignin-rich biomass [127]. Advancements in microbial degradation approaches have yielded significant results. By utilizing engineered microbes, researchers have achieved improvements in converting lignin to biodiesel, with studies reporting up to a 50% increase in yield [3]. This progress represents a substantial step toward enhancing the economic feasibility of biodiesel production from lignin. Moreover, the development of microbial consortia has improved lignin breakdown, resulting in notable reductions in processing time and costs [128]. The integration of chemical and biological approaches has been an effective strategy in lignin valorization. Integrated methods have shown an increase in lignin-to-bioethanol conversion efficiency of up to 35%. This synergistic approach highlights the potential of combining multiple technologies to overcome the challenges related to lignin’s resistance and enhance biofuel yields [128]. This synergistic approach not only overcomes the challenges associated with lignin’s resistance but also enhances biofuel yields. These improvements in overall biofuel production efficiency highlight the potential for lignin to become a valuable resource in current biofuel production facilities [3].

Recent advances in enzymatic and microbial degradation techniques, along with process integrations, have significantly improved the economic viability of lignin-based biofuel production, addressed challenges, and enhanced conversion rates, yields, and processing efficiencies [3,6,127,128]. Therefore, it is expected that lignin will play an increasingly vital role in sustainable biofuel production, supporting global efforts to reduce reliance on fossil fuels and combat climate change. Ongoing research in lignin valorization holds promise for promoting biorefinery sustainability and for contributing to carbon neutrality targets worldwide [3,6]. Future directions involve optimizing degradation processes, exploring novel catalysts, and scaling up production while improving cost-effectiveness. Continued advancements in lignin valorization will drive the transition toward a more sustainable and renewable energy future.

5.2. Lignin-Derived Chemicals and Materials

Recent advances in depolymerization processes have greatly improved the conversion of lignin into valuable aromatic compounds, serving as precursors for various chemicals and materials. Catalytic hydrogenolysis, in particular, has made significant advancements in enhancing the yield of targeted aromatic compounds from lignin. Du et al. (2023) achieved remarkable lignin conversion rates of up to 91.2% and a total yield of monomer products reaching 44.9 wt.% under optimal conditions [129]. This improvement highlights the potential of lignin as a renewable source of valuable aromatic compounds. Similarly, oxidation methods in lignin valorization have seen significant advancements. These methods produce phenolic aldehydes and acids through side chain cleavage and have the potential to cleave aromatic rings in lignin. The versatility in product formation further underscores the potential of oxidation methods for effective lignin valorization [130].

Furthermore, pyrolysis processes have been optimized to generate valuable aromatic monomers from lignin. Recent research has focused on catalytic pyrolysis for enhanced production of aromatic hydrocarbons. For example, Zhou et al. achieved a carbon yield of 8.65% for aromatic hydrocarbons through catalytic fast pyrolysis of herbaceous lignin using HZSM-5 zeolite catalyst [131]. While the yield may appear modest, it signifies a noteworthy advancement in the selective production of valuable aromatics from lignin. The development of lignin-derived bioplastics and bio-based materials has made significant progress. Research demonstrates that lignocellulosic biomass from agricultural waste can be utilized to produce bioplastics, contributing to the circular economy [132]. Moreover, lignin has been effectively employed in the production of biocomposites and as a dispersant in diverse applications, such as bioasphalt, concrete, and dyes for textiles and agricultural chemicals [133]. The conversion of lignin into bio-based resins for eco-friendly adhesives has shown promising outcomes. A recent study focused on developing a fully bio-based adhesive using lignin and cellulose, aligning with the concept of “take from wood, back to wood [134]. A study highlighted the potential of lignin valorization for producing aromatic chemicals and polymeric materials, with values above USD 1000–2000 per ton, which is ten times higher than when used as a solid fuel [135]. This approach highlights the potential for lignin to substitute petroleum-based products in diverse applications, including adhesives.

Therefore, its depolymerization and conversion techniques have made lignin-derived chemicals and materials economically viable and environmentally sustainable. These improvements in yield, selectivity, and product quality address previous challenges in lignin valorization. Its increasing role in sustainable production aligns with global efforts to reduce dependence on fossil resources and mitigate climate change. Ongoing developments in lignin valorization hold promise for promoting sustainability and contributing to circular economy goals worldwide.

5.3. Lignin in Polymer Blends and Composites

Lignin has emerged as a promising component in polymer blends and composites, offering numerous advantages due to its unique properties and environmental benefits. Lignin possesses several desirable properties that make it suitable for incorporation into polymer materials, including high thermal stability, antioxidant properties, biodegradability, and antimicrobial actions [136]. These characteristics have led to its extensive use in a wide range of applications. In polymer blends and composites, lignin can serve multiple functions: as a filler, enhancing the mechanical properties of the composite materials; as a stabilizer, contributing to improved stability of the polymer matrix; as a compatibilizer, improving the compatibility between different polymer components in blends; and as a biodegradability enhancer, increasing the biodegradability of synthetic polymers [137,138]. The development of lignin-based thermoplastics often involves altering the viscoelastic properties of lignin through chemical modification or polymer blending, leading to the creation of various lignin-polymer composites with enhanced properties [139]. Notable applications of lignin-based polymer blends and composites include packaging materials, automotive components, construction materials, 3D printing, and energy storage and electronic devices. Recent advancements in this field include the development of poly (lactic acid) (PLA)/lignin nanoparticle composites, poly (butylene adipate-co-terephthalate) (PBAT)/lignin composites, and the exploration of lignin as a filler in various polymeric matrices [137,138]. Despite these advancements, challenges remain in optimizing the uniform distribution of lignin in polymer matrices and further improving the performance of lignin-based composites [138]. Ongoing research is focused on refining processing techniques and exploring new applications to fully harness the potential of lignin in polymer blends and composites.

5.4. Lignin as a UV Protector and Antioxidant

Lignin’s antioxidant and UV-absorbing properties are utilized for UV protection. In polymer composites, as little as 1% lignin content in poly (methyl methacrylate) films achieves 60% UV-blocking capacity at 400 nanometers. Cellulose films containing 2 wt.% lignin provide full UVB protection (280–320 nm) and over 90% UVA protection (320–400 nm) [140]. It is highly effective as a UV blocker in biodegradable films. Cellulose-lignin films regenerated in acetone provide complete UV blocking up to 400 nanometers with just 2% lignin content. These films retain their UV protection after thermal treatment and UV irradiation. In poly (butylene adipate-co-terephthalate) (PBAT) films, 10% lignin content ensures excellent UV-barrier properties in the 280–400 nanometer range, even after 50 h of UV exposure [140].

Its antioxidant properties find application in food packaging, where lignin-containing biodegradable films extend the shelf life of perishable goods [141]. These films provide UV protection and contribute to packaging sustainability. Recent advancements involve chemically modified lignin nanoparticles in polymer composites. Modified lignin nanoparticles in polylactic acid (PLA) nanocomposites enhance UV-blocking properties while preserving transparency, addressing the dark color challenge associated with lignin in food packaging where aesthetics matter [142].

5.5. Functionalization and Modification of Lignin

Functionalization and modification techniques have expanded lignin’s applications by enhancing its properties and compatibility with other materials. Acetylation has proven effective in increasing lignin’s solubility in organic solvents. Researchers developed an acetylation technique that improved lignin’s solubility by 60%, enabling its use in high-performance coatings and adhesives. This enhanced solubility opens up new possibilities for lignin in industries relying on organic solvent-based processes [143,144]. Sulfonation is another notable technique that improves lignin’s dispersibility in water, making it valuable for applications in surfactants and detergents. Researchers have developed a sulfonation technique that significantly enhances lignin’s water dispersibility, opening up new possibilities for its use in water-based products and processes. This modification offers sustainable alternatives to petroleum-based surfactants, particularly for industries seeking environmentally friendly solutions [145,146]. Grafting processes integrate lignin into biodegradable polymer blends, enhancing their mechanical properties and environmental profile. This technique improves strength and durability while maintaining biodegradability. It is particularly significant for the packaging and consumer goods industries, facilitating the development of eco-friendly materials [147]. Recent work has developed effective amination techniques using dimethylamine, ethylenediamine, or diethylenetriamine, achieving nitrogen contents up to 10.18%. This modification is particularly relevant for fertilizer applications [148].

In pharmaceuticals, functionalized lignin shows potential as a drug delivery agent; for instance, its nanoparticles (LNPs) are being investigated for safe use in drug and gene delivery [149]. To enhance sustainability, green chemistry approaches like the use of deep eutectic solvents (DES) enable recyclable and tunable lignin modifications. Green metrics such as the E-factor evaluate and improve the sustainability of these processes [149]. Challenges include standardizing lignin sources and characterization methods, scaling up modification processes cost-effectively, enhancing compatibility with diverse polymer matrices, and developing efficient and environmentally friendly modification techniques. Future research aims to address these challenges and explore new applications for functionalized lignin in advanced materials and sustainable technologies, promoting innovative and eco-friendly solutions.

5.6. Economic and Environmental Benefits of Lignin Valorization

Advancements in lignin valorization significantly contribute to the circular economy and offer substantial environmental benefits. By converting lignin into valuable products, industries can reduce waste, lower greenhouse gas emissions, and decrease reliance on non-renewable resources. Economically, lignin-based products have become more viable due to technological innovations and growing market demand for sustainable solutions. For instance, lignin-based bioplastics can reduce production costs by up to 20% compared to conventional plastics [150], and the development of lignin-derived products opens new markets and revenue streams [151]. Environmentally, lignin valorization minimizes industrial waste [152], reduces CO₂ emissions by 30–40% [153], and promotes sustainable resource utilization, aligning with circular economy principles [154]. These advancements not only enhance profitability and market competitiveness but also support ecosystem health and biodiversity preservation, illustrating the profound impact of lignin valorization on both the economy and the environment [155].

6. Exploring the Expanding Applications of Upgraded Lignin

Lignin has long been an underutilized byproduct of the paper and pulp industry. However, recent advancements are launching a greener future with the rise of upgraded lignin. This revolution is transforming lignin from waste material into a valuable resource with the potential to reshape the bioeconomy. Upgraded lignin, with its unique chemical structure, offers diverse applications for a sustainable future. Some of the key areas of lignin utilization are discussed in the following sections.

6.1. Lignin as Precursors for Biofuels and Bio-Based Chemicals

The advancements in lignin upgrading methods have opened up exciting possibilities in the fields of biofuels and bio-based chemicals. Upgraded lignin can be strategically broken down into its constituent building blocks, enabling the production of advanced biofuels like ethanol. This offers a renewable and eco-friendly alternative to petroleum-derived fuels, reducing our dependence on fossil resources [156,157]. Additionally, lignin-derived aromatic compounds hold great potential for the production of various bio-based chemicals, such as phenols and vanillin, which can serve as precursors for polymers, resins, and industrial products. Leveraging lignin as a feedstock for bio-based chemicals not only contributes to the development of sustainable alternatives but also promotes a circular and resource-efficient economy. The continuous research in and development of lignin upgrading methods will further unlock the full potential of this abundant and renewable resource, driving us toward a more sustainable and environmentally conscious future [156,157].

6.2. Role of Lignin as a Functional Additive in Biocomposites

Upgraded lignin is poised to revolutionize the world of biocomposites, offering a sustainable and high-performance alternative to traditional materials. By incorporating upgraded lignin as a functional additive, biocomposites can experience a significant boost in mechanical strength, making them more durable and robust for applications in construction and automotive parts [158]. Additionally, the inherent fire-resistant properties of lignin enhance the flame retardancy of biocomposites, ensuring safer building materials and improved fire safety. Furthermore, the water-repellent nature of lignin enables the creation of biocomposites with superior hydrophobicity, making them ideal for moisture-resistant applications like packaging materials [158,159]. This shift toward utilizing upgraded lignin in biocomposites not only enhances their performance but also promotes a circular and eco-friendly approach to material production by minimizing waste and reducing environmental impact. As research continues to advance lignin upgrading methods, we can expect even more innovative applications to emerge, solidifying lignin’s role as a key component in the development of sustainable materials for a greener future [158,159].

Carbon fiber and carbon fiber reinforced polymer matrix composites (CFRPs), with their light weight, high tensile strength, and high resistance to corrosion, have applications in aerospace, automotive, rail transport, wind turbines, etc. [160]. The global demand for CFRPs is projected to increase from 181 Kt in 2014 to 285 in 2025 [160,161]. Carbon Fiber Market size is forecast to reach USD 11.2 billion by 2026 [106]. Carbon fibers are currently produced using polyacrylonitrile (PAN) due to its excellent mechanical strength properties. These properties for lignin-based carbon fibers are poorer compared to PAN-based carbon fibers [162]. The properties of lignin-based carbon fibers depend on the properties of lignin as well as manufacturing processes. The advantageous features of using lignin for carbon fiber are its high carbon content and low cost. Lignin, as a precursor for carbon fiber, has, compared to PAN, a lower melting temperature and faster stabilization capability.

6.3. Source of Aromatic Building Blocks for Novel Biomaterials

Upgraded lignin is proving to be a game changer in the realm of biomaterials, offering a wealth of possibilities for sustainable material innovation. Through the strategic utilization of lignin-derived aromatic building blocks, researchers are unlocking the potential to create high-performance biomaterials. These building blocks can be harnessed to develop bio-based plastics that rival traditional counterparts in strength and durability while significantly reducing the environmental impact [156,163]. Moreover, lignin-derived building blocks hold promise for the production of bio-based pharmaceuticals, enabling more sustainable drug delivery systems and therapeutic agents. The versatility of lignin extends further, as it can be used to create advanced materials such as biosensors and biocompatible materials for 3D printing and tissue engineering. As research progresses, the focus will be on refining lignin upgrading methods and maximizing its potential, paving the way for a future where biomaterials are both high-performing and environmentally responsible [163].

6.4. Reduced Reliance on Fossil Resources

Upgraded lignin offers a compelling solution to our reliance on fossil fuels, paving the way for a future driven by bio-based alternatives. By replacing petroleum-derived materials, lignin can significantly reduce carbon footprints and minimize environmental degradation. This renewable resource has the potential to revolutionize various sectors, from construction materials to batteries and carbon fibers [27,164]. Moreover, lignin-derived biofuels and chemicals provide a sustainable approach to energy production, reducing our dependence on fossil fuels and enhancing energy security [27]. As research progresses, optimizing lignin upgrading methods and exploring novel applications will further solidify its role in replacing traditional materials. This transition to bio-based alternatives promises not only environmental responsibility but also economic viability, creating a future where renewable resources drive sustainable solutions across industries [165].

6.5. Efficient Use of Natural Resources

Lignin, a once-overlooked byproduct, is now emerging as a valuable and sustainable resource. Recent advancements in lignin upgrading methods have transformed this abundant natural resource into high-value products, maximizing resource utilization and promoting sustainability [166]. Advanced extraction and refinement techniques allow for selective extraction of specific lignin fractions, improving purity and enabling tailored chemical modifications. These innovations create cost-competitive and efficient lignin-based materials, unlocking their full potential [166,167]. The utilization of lignin through upgrading methods brings numerous sustainability benefits, including reduced waste generation, enhanced resource efficiency, and the development of environmentally friendly products. Continuous innovation in extraction, refinement, and integration with biorefineries will further enhance lignin utilization. By harnessing the power of lignin, we can drive a sustainable future in the paper and pulp industry and beyond [94,168].

6.6. Lignin Valorization for Polyurethane

Polyurethane is a versatile and widely used synthetic polymer that is currently produced by the reaction of petrochemically-derived polyols and isocyanates. Lignin is a polyol polymer that can be used in reaction with other reagents, such as isocyanate, to produce polyurethane. These polymers have a wide range of applications such as foam, coatings, adhesives, elastomers, rigid and flexible plastics, footwear, etc. The global revenue of polyurethane is estimated to be USD 78.90 billion and is forecasted to reach USD 105.99 by 2028 [169]. Quinsaat, J.E.Q., et al. employed lignin hydrogenolysis oil (LHO) obtained from depolymerizing native softwood lignin for the synthesis of thermoplastic polyurethanes (TPU) that can be used as the hot-melt adhesive for wood [170]. Mechanical properties of the thermoplastic polyurethanes are remarkably enhanced using the high lignin content (38–46% by wt.%) and polyols having high molecular mass (e.g., polypropylene glycol with Mn = 4 Kg/mol). For lignin-based polyurethane synthesis, significant progress is evident from the fact that around 157 patents related to lignin-based polyurethanes had been registered until 2021 [171]. A joint venture of West Fraser Timber Company and Hexion has been producing lignin-based resins for plywood. Polyurethane foams with lignin as an additive (20–25 wt.%) are commercially produced. Significant efforts have also been made to replace the toxic isocyanate and develop non-isocyanate polyurethane. A patent is filed by Hexion for using lignin as a building block in the synthesis of cyclic carbonates for the development of new isocyanate-free polyurethanes. Lignin or modified lignin impart the polyurethane material with additional features, i.e., UV-blocking ability, hydrophobicity, and flame retardancy [172]. Though there are still some challenges (i.e., well-defined oligomer production, lignin heterogeneity, steric hindrance, and low activity, etc.) in lignin-based polyurethane production significant development as evident from patented work, provides the potential route for using technical lignin as feedstock for commercial production in future.

6.7. Lignin for Bioplastics

Recently, the lignin industry AB has launched lignin-based Renol production at an industrial scale with a capacity of 1000 tonnes/year. Insulating foams and molding compounds that can be used as a base material for molded plastic products. These products, such as phenol-based plastics, have good mechanical strength (compression for breaking even point and tension tests for elongation). Lignin resin in insulating foams provides long-lasting protection in case of building fire [173,174]. A patent for lignin-based bioplastic with its applications in agriculture has been granted by the World Intellectual Property Organization (WIPO) to Green Innovation, Austria, in 2021 [175,176]. According to the patent, the bioplastic material comprising lignin and soil enhancer (e.g., K₂CO₃) is highly effective for plant growth due to the conversion of lignin into humic acid and protection against phytopathogens. Biodegradable plastic materials having superior properties to conventional polypropylene and polyethylene can be fabricated by incorporation of lignin into poly butylene succinate [176].

7. Conclusions

In the bioeconomy, lignin has evolved from being regarded as an underutilized fraction in conventional biorefinery processes to a valuable resource. It possesses sufficient potential to produce biobased chemicals, biofuels, and useful biocomposite additives. Understanding the complex structure of lignin is crucial in order to fully utilize its potential, which can be achieved through sophisticated characterization techniques. This information aids in the customization and effectiveness of degradation strategies. These customized and upgraded processes have improved energy consumption and saved costs, and involve environmentally friendly catalysts and solvents. Additionally, the upgraded lignin can serve as a renewable feedstock, reducing dependence on fossil fuels and permitting the production of sustainable materials. This shift to a circular bioeconomy reduces waste production and encourages the effective use of resources, all of which contribute to a greener and more sustainable future.

Future Prospects and Recommendations

In the emerging landscape of the biobased economy, lignin holds significant promise as a renewable resource for sustainable development. With advancements in biorefinery technologies and the increasing demand for eco-friendly alternatives to fossil fuels and petrochemicals, lignin valorization is poised to play a pivotal role. Through innovative processes such as depolymerization, catalysis, and microbial conversion, lignin can be transformed into a diverse array of high-value products, including biofuels, bioplastics, adhesives, and antioxidants. Moreover, the integration of lignin-derived materials into various industrial sectors not only reduces reliance on finite resources but also contributes to mitigating greenhouse gas emissions and fostering circular economic models. As research continues to unlock the full potential of lignin as a versatile feedstock, its widespread utilization is anticipated to drive the transition toward a more sustainable and resource-efficient biobased economy.

In recent years, innovative “lignin-first” strategies have emerged as promising alternatives. These approaches utilize physical and chemical methods to protect reactive lignin derivatives during extraction, allowing for efficient lignin recovery while leaving behind a carbohydrate fraction suitable for further refinement. This holistic approach not only enhances the overall valorization of lignocellulosic biomass compared to traditional methods but also opens up new avenues for the production of high-value products. Advances in greener upgradation techniques present promising pathways toward realizing lignin’s full potential in a more efficient and environmentally sustainable manner. By enabling targeted and responsible conversion processes, these methodologies pave the way for a greener future. Integration of lignin valorization into existing biorefineries through innovative designs and “lignin-first” approaches offers a means to maximize resource efficiency and minimize waste production. Moreover, a deeper understanding of lignin’s composition opens doors to customized lignin products with unique characteristics, fostering innovation across industries such as advanced materials and bioplastics. Life cycle assessment methods may play a crucial role in ensuring the sustainability of lignin valorization, emphasizing waste reduction, bio-based resource utilization, and energy efficiency. Standardization of characterization methods and upgrading processes is essential for successful market entry, enabling consistent quality and efficient penetration across diverse industries. These prospects position lignin as a cornerstone of a sustainable future, transforming it from a waste product into a valuable resource and driving the transition toward a circular and environmentally conscious bio-based economy. The journey of lignin promises to redefine sustainability in the years ahead, shaping a greener and more resilient world.

The complexity of lignin and its derived product mixtures poses another hurdle, limiting insights from individual analytical techniques. Developing comprehensive analytical methods will be crucial for identifying suitable pathways and products effectively. Furthermore, insufficient attention has been given to the efficient separation of depolymerized lignin products, a critical aspect that must be addressed alongside depolymerization to optimize both upstream and downstream processes for oxidative lignin valorization. The recovery of aromatic products, such as vanillin and syringaldehyde, from oxidative lignin depolymerization involves intricate separation and purification processes due to the mixture’s similarity in physical and chemical properties with other compounds. Current separation methods are labor-intensive, energy-demanding, and rely on environmentally harmful solvents, leading to significant material losses. Therefore, developing efficient and eco-friendly separation processes is essential for the industrial application of lignin-derived aromatics. In summary, overcoming these challenges—enhanced catalyst stability, development of advanced analytical methods, and improved separation processes—is crucial for realizing the full potential of lignin depolymerization in sustainable biorefinery applications.