Lignin Valorization from Lignocellulosic Biomass: Extraction, Depolymerization, and Applications in the Circular Bioeconomy

Abstract

Lignocellulosic biomass—the non-edible fraction of plants composed of cellulose, hemicellulose, and lignin—is the most abundant renewable carbon resource and a key lever for shifting from fossil to bio-based production. Agro-industrial residues (straws, cobs, shells, bagasse, brewery spent grains, etc.) offer low-cost, widely available feedstocks but are difficult to process because their polymers form a tightly integrated, three-dimensional matrix. Within this matrix, lignin provides rigidity, hydrophobicity, and defense, yet its heterogeneity and recalcitrance impede saccharification and upgrading. Today, most technical lignin from pulping and emerging biorefineries is burned for energy, despite growing opportunities to valorize it directly as a macromolecule (e.g., adhesives, foams, carbon precursors, UV/antioxidant additives) or via depolymerization to low-molecular-weight aromatics for fuels and chemicals. Extraction route and severity strongly condition lignin structure linkages (coumaryl-, coniferyl-, and sinapyl-alcohol ratios), determining reactivity, solubility, and product selectivity. Advances in selective fractionation, reductive/oxidative catalysis, and hybrid chemo-biological routes are improving yields while limiting condensation. Remaining barriers include feedstock variability, solvent and catalyst recovery, hydrogen and energy intensity, and market adoption (e.g., low-emission adhesives). Elevating lignin from fuel to product within integrated biorefineries can unlock significant environmental and economic benefits.

1. Introduction

Fossil resources remain the primary feedstock for energy and organic compounds, yet their utilization drives greenhouse gas emissions that contribute to climate change. The looming climate crisis demands an urgent shift from a fossil-based economy to a bio-based one, where lignocellulosic biomass, rather than petroleum, is used to produce fuels, chemicals, and materials [1]. This feedstock offers key advantages over other biomass sources, as it represents the non-edible fraction of plants and therefore does not compete with food supplies [2]. Lignocellulosic biomass is the most abundant renewable source of organic carbon on Earth, making it a highly promising pathway toward future sustainable biorefineries [3]. As a major structural component of terrestrial plants, lignocellulose is widely available and chemically suitable as a feedstock in bio-based production [4]. It’s abundance, low cost, and independence from food systems make it one of the most promising resources for a circular bioeconomy [5].

Structurally, lignocellulosic biomass consists of three major polymers—cellulose, hemicellulose, and lignin, along with smaller amounts of components such as extractives, pectins, and minerals [5,6,7,8,9,10]. These polymers form the complex architecture of plant cell walls, providing rigidity, flexibility, and resistance to microbial degradation [11]. Both cellulose (Figure 1) and hemicellulose (Figure 2) are essential structural elements of the cell wall [12,13]. Cellulose is the main constituent, accounting for 40–50% of the wall, and occurs mainly as fibrils in crystalline or amorphous form; a higher proportion of crystalline cellulose contributes to greater hardness [8,14,15]. Hemicellulose, in contrast, is more amorphous and easier to degrade by xylanases and glycosidases, producing fermentable sugars [15]. It also contains small amounts of uronic acids and acetyl groups [5]. Hemicellulose makes up about 20–25% of the cell wall [14], with its primary function being to link cellulose with lignin; however, it is more vulnerable to degradation by pathogenic fungi. Lignin, on the other hand, is more resistant to microbial attack and typically represents 20–25% of the cell wall [8,15,16,17,18]. Understanding this structure is essential for developing efficient biomass pretreatment and valorization technologies.

From an economic and resource-efficiency perspective, lignocellulosic biomass offers considerable advantages. The agri-food sector generates vast quantities of residues and by-products, many of which are currently undervalued. In the United States, food processing and agriculture generate approximately 300 million tons of organic waste annually, while in China the amount reaches nearly 0.9 billion tons [19]. By-products from industries such as fruit processing, brewing, oil production, and grain processing include seeds, peels, stalks, pomace, oil cakes, brewer’s grains, bran, and germ [20,21]. These materials are often considered low-value waste and are typically discarded or incinerated on-site for economic and logistical reasons [20]. However, they represent a significant source of lignocellulosic material that can be converted into renewable fuels and value-added products. Agricultural residues represent an abundant, inexpensive, and easily accessible source of lignocellulose, particularly in agriculturally intensive countries such as Brazil, China, India, and the United States [5]. Consequently, lignocellulosic biomass is regarded as the most realistic and sustainable resource for large-scale renewable fuel production [2,14].

Within this context, lignin—one of the main components of lignocellulosic biomass—plays a crucial role. It is located in the secondary cell wall of plants, where it fills the spaces between cellulose, hemicellulose, and pectin components, thereby increasing the rigidity and hydrophobicity of the cell wall [11]. Currently, lignin is primarily used for bioenergy production, such as electricity and heat [22]. Nevertheless, it possesses significant potential to serve as a key renewable feedstock that can reduce reliance on fossil-based resources. Its valorization is expected to play a crucial role in the biorefinery sector, enabling the production of bio-based chemicals, fuels, and materials with wide-ranging applications in polymers, cosmetics and personal care, biomedicine and pharmaceuticals, as well as the energy industry [16,23,24,25]. Lignin exhibits several advantageous characteristics, including antioxidant and antibacterial activity, biodegradability, compatibility, high thermal reactivity, and strong adhesive properties [21]. These features make it a highly versatile chemical with considerable economic potential for industry. Furthermore, its bioactive properties—such as anti-tumor, antiviral, anti-diabetic, antioxidant, and antimicrobial effects—are attracting increasing attention in biomedical research [25].

The global demand for plant-based resources for both food and fuel is projected to rise by 50% by 2050 [23]. The growing interest in the use of lignin is mainly related to its availability in industry, as it is produced in large quantities during the processing of cellulose pulp (accounting for almost 80% of available lignin) and other primary processing of lignocellulosic biomass [24,26]. In Europe, sulphite pulp production represents 4.4% of the total output, yielding 1.7 tons of pulp, with most production facilities concentrated in Sweden (31.2%) and Finland (30.2%). Overall, Europe contributes around 25.3% of global pulp production [22]. The global lignin market was valued at USD 1.08 billion in 2023 and is expected to grow at a compound annual growth rate of 4.5% between 2024 and 2030 [27]. Lignin valorization generally follows two main pathways: it can either be used directly as a macromolecule to develop high-value materials or it can be depolymerized into low-molecular-weight monomers [3]. This dual approach underlines its strategic importance as a cornerstone of the emerging bioeconomy.

The aim of this study is to provide an overview of the latest advances in lignin research, with a focus on methods for its extraction, conversion, and valorization in lignocellulosic biorefineries, as well as to integrate information from experimental studies, industry reports, and comprehensive reviews in order to describe the current state of knowledge and new directions for the use of lignin. The specific objectives of the study are as follows:

• To analyze general research trends. To summarize the developments in lignin extraction and conversion technologies described in the latest literature, emphasizing the evolution of chemical, biological, and thermochemical processes.
• Identify recognized methodologies and types of lignin. Categorize the main methods of lignin isolation (e.g., Kraft, sulfite, alkaline, organosolv) and relate them to structural and chemical properties associated with valorization potential.
• Explore opportunities and limitations. Discuss industrial and environmental aspects of lignin utilization, identify key opportunities in the circular bioeconomy, as well as challenges related to process efficiency, scale-up, and product selectivity.

To achieve these objectives, this review aims to establish a coherent conceptual framework that reflects the dynamic progress in lignin utilization research and provides insights into future advances in sustainable biorefining.

2. Sources, Composition, and Valorization Potential of Lignocellulosic Biomass

2.1. Composition of Lignocellulosic Biomass

Wood is the prototypical lignified tissue [9]. One of the main challenges in valorizing lignocellulosic biomass lies in its highly complex matrix structure. The plant cell walls that constitute this biomass are primarily built from three macromolecular polymers—cellulose, hemicellulose, and lignin—interconnected in a three-dimensional network, which complicates extraction and conversion.

Recovering biopolymers such as lignin from these residues through biorefinery processes offers opportunities for the comprehensive valorization of lignocellulosic biomass [28]. For lignin structure and properties, see Section 3; for extraction, fractionation, and industrial lignin types, see Section 4.

2.2. Representative Sources of Lignocellulosic Biomass

Lignocellulosic biowaste represents a significant portion of total waste, not only from industrial processes but also from agricultural and household sources, accounting for 7–14% of waste in Europe. In Europe, Spain and the Netherlands are leading producers of agricultural waste, generating 6,254,000 tons and 4,679,000 tons respectively in 2018 [29]. Agricultural residues include cobs, husks, leaves, pods, prunings, roots, seeds, shells, stalks, stems, straw, stubbles, and stover [30]. These materials are abundant, renewable, and widely available, offering significant potential for industrial applications [28]. However, in many cases, they are left in the field, creating disposal challenges for local agro-industries [31].

Lignin can also be sourced from municipal and industrial by-products, including residues from the food and beverage industry, such as pomace, peels, bran, pulp, brewery spent grains, sugarcane bagasse, and papermaking sludge [32].

2.2.1. Cereal Residues

Corn is the most widely produced grain globally, followed by wheat and rice. Notably, wheat straw and rice straw contain higher amounts of lignin compared to corn stover [5]. Corn is also a universally traded commodity, with Africa contributing about 7.42% of global production. Corncobs represent 27–30% of total maize residues and are considered a lignocellulosic material with a particularly high cellulose content (69.2%), while hemicellulose and lignin account for 22.8% and 8%, respectively [30].

Straw is an agricultural byproduct generated during harvesting when grains are separated, either manually or mechanically, leaving the stalk residues that are often spread or piled in the field [33]. Compared with herbaceous feedstocks, woody feedstocks offer several advantages for biorefinery applications, including lower ash content, higher lignin concentration, greater bulk density, and independence from seasonal cycles [5]. Wheat and rice straws typically contain 17.5–30% and 15.6–25% lignin (dry weight), respectively [33]. Corn stover is a valuable source of cell wall material; however, the presence of lignin complicates its deconstruction and limits the efficient recovery of polysaccharides for conversion into biofuels and other coproducts [34].

2.2.2. Industrial Residues

Sugarcane bagasse, an abundant byproduct of both agriculture and industrial processing, is the most widely produced agricultural residue, with an estimated 1044.8 million tons annually. Its composition—rich in cellulose (35–45%), hemicellulose (26–35%), lignin (11–25%), and extractives (3–14%)—makes it an attractive feedstock for the generation of diverse value-added products [35]. Furthermore, it is estimated that an additional 730 million tons of lignin per year could be recovered from stalks and straws generated as agricultural byproducts, with less than half required to be returned to the soil for maintaining fertility [36].

Brewery spent grains fiber, which accounts for about 85% of total waste generated in the brewing process [37], is another important source of lignocellulosic material. Its composition includes cellulose (16–25%), hemicellulose (28–35%), protein (15–24%), lignin (11–27%), and various solvent extractives. Brewery spent grains have been widely applied in the production of bioethanol, C₅ and C₆ sugars, xylitol, polylactic acid, and in uses such as bread formulation, charcoal, and brick manufacturing [38].

2.2.3. Nut and Fruit Residues

The growing production of nuts, including peanuts, almonds, cashews, and walnuts, is leading to an increase in nutshell waste, which, depending on the variety, accounts for 20–80% of the nut’s weight [19]. Dense lignocellulosic feedstocks, such as drupe endocarps (shells) from olives, eastern black walnuts, and coconuts, have the highest lignin content of any known plant organ, with an energy potential comparable to coal [23]. On a dry weight basis, coconut waste contains 20–30% cellulose, 15–30% hemicellulose, and nearly 50% lignin [39]. Similarly, spent coffee grounds and other natural fibers are rich in cellulose, hemicellulose, and lignin, with hydrophilic hydroxyl groups embedded in their polysaccharide structures [40]. Coffee husk fibers from coffee Arabica typically contain 30–35% cellulose, 18–21% hemicellulose, 19–22% lignin, and 25–28% of other components, such as waxes and inorganic matter [38,41]. Soybean hulls, a by-product of soybean crushing, are mainly composed of cellulose (29–51%), hemicellulose (10–25%), lignin (1–18%), pectin (4–30%), and proteins (11–15%) [42].

Other agro-industrial residues include pistachio shells and cherry tree prunings, both abundant renewable biomasses. Cherry tree prunings contain primarily cellulose (37.6%) and smaller amounts of hemicellulose (16.9%), while pistachio shells have roughly equal amounts of cellulose and hemicellulose (~31% each). Lignin content is similar in both materials, at about 21–23% [43]. Almond shells, with cellulose levels as high as 45.9%, represent an excellent source of fibrocellulosic material for cellulose extraction and sustainable material development. Hazelnut shells consist mainly of cellulose (15.4%), lignin (25.9%), and hemicellulose (22.4%), forming the bulk of their fiber structure [44]. Cocoa pod husks and cocoa bean shells contain up to 30% lignin, 3–18% hemicellulose, and 23–39% cellulose [45].

Table 1. Representative lignocellulosic biomass sources.

Biomass Source	Origin	Cellulose (%)	Hemicellulose (%)	Lignin (%)	Other Major Components	Notes/Potential Applications
Corncob	Cereal residue	69.2	22.8	8.0	-	High cellulose content; bioethanol feedstock [30]
Wheat straw	Cereal residue	33–45	26–32	17.5–30	-	Common agricultural byproduct [33]
Rice straw	Cereal residue	35–40	30	15.6–25	-	High lignin limits digestibility [33]
Sugarcane bagasse	Industrial residue	35–45	26–35	11–25	3–14% other extractives	Abundant; used for power and bioproducts [35]
Brewery spent grain	Industrial residue	16–25	28–35	11–27	15–24% protein	Used for ethanol, xylitol, food additives [38]
Coffee husk	Industrial residue	30–35	18–21	19–22	25–28% waxes and inorganic matter	Biochar, composting, polymers [38,41]
Coconut shells	Nut residue	20–30	15–30	~50	-	Very high lignin; energy applications [39]
Almond shells	Nut residue	~46	23	21	-	Source of fibrous material [44]
Hazelnut shells	Nut residue	15.4	22.4	25.9	-	Dense lignocellulosic biomass [44]

presents representative agricultural and industrial biomass sources with their typical polymer contents.

2.3. Structure of Plant Cell Walls

To understand the complexity of these materials and the challenges associated with their processing, it is important to examine the structural organization of plant cell walls. The wall comprises the middle lamella, primary, and secondary layers; ~90% of dry mass consists of cellulose, hemicellulose, and lignin [46].

In general, lignocellulosic fibers consist of cellulose fibrils embedded in a matrix of hemicellulose and either lignin or pectin, arranged in one or more layers. The proportion and orientation of cellulose fibrils vary across layers, contributing to structural diversity [17]. Hemicellulose refers to a diverse group of matrix polysaccharides, including xylan, glucuronoxylan, arabinoxylan, glucomannan, and xyloglucan. Unlike cellulose, hemicellulose is characterized by a heterogeneous and amorphous structure [47]. Its content can be quantified using the following formula [48]:

$$hemicellulose, \% = \left({\displaystyle \frac{W_H}{W_W}}\right)·100,$$

(1)

where W_H is the sum of the extraction stages (H_0.6 to H_2.5) (g), and W_W is the extractive-free wood weight (g).

Hemicelluloses are often described as the internal “glue” that binds together cellulose microfibrils and lignin aggregates in plant cell walls [9]. Both primary and secondary cell walls are microfibril-based nanocomposites, yet they differ in polymer arrangement, structural organization, hydration levels, and mechanical properties.

Primary cell walls, synthesized during plant growth, are thin, pliable, and highly hydrated. They consist of 15–40% cellulose, 30–50% pectic polysaccharides, 20–30% xyloglucans, and smaller amounts of arabinoxylans and structural proteins (dry weight basis), organized into one or more lamellae. Secondary cell walls, by contrast, provide rigidity and mechanical strength in tissues that have ceased growing. These walls contain 3-nm cellulose microfibrils embedded in a matrix of lignin, xylans, and glucomannans, which largely replace the xyloglucans and pectins found in primary walls. Secondary walls are less hydrated than primary ones, with only about 30% water at saturation. In coniferous wood, cellulose microfibrils form loose bundles 10–20 nm wide, with direct lateral adhesion occurring along parts of their length. Most lignin and hemicellulose are located outside these bundles, although glucomannans remain closely associated with cellulose [47].

In addition, cellulose microfibrils have been observed in many seed coat mucilages [49]. The mechanical behaviour of wood under tensile stress is influenced by the orientation of cellulose microfibrils. At low microfibril angles, cellulose largely determines the wood’s mechanical performance. However, as the angle of the microfibrils increases, the contribution of the surrounding matrix, consisting of hemicellulose and lignin, becomes increasingly important [50]. Overall, hemicelluloses represent a heterogeneous class of polysaccharides, present in both woody (20–30% dry weight) and non-woody plants (up to 40% dry weight). By interacting with cellulose and lignin, they play a vital role in providing strength and rigidity to plant cell walls [9,51].

Cellulose is one of the most abundant organic materials on Earth, present in forestry and agricultural residues as well as in various types of waste streams [52]. The term residue refers to materials that are not intentionally generated during production processes, but which are not necessarily classified as waste [21].

As a polysaccharide, cellulose is a key structural component of plant cell walls (Figure 1), consisting of both crystalline and amorphous regions [47]. Cellulose polymers form a rigid, insoluble, and highly ordered network, making them resistant to both biological and chemical degradation [52].

The cellulose content of a sample can be quantified using the following formula [48]:

$$cellulose, \%=\left({\displaystyle \frac{W_C}{W_W}}\right)·100,$$

(2)

where W_C is the cellulose weight (g), and W_W is the extractive-free wood weight (g).

3. Lignin as a Multifunctional Biopolymer

Lignin is the most abundant aromatic polymer in nature [28,53] and the second most plentiful biopolymer on Earth after cellulose [9,54]. It is also the main by-product of the wood pulping industry [55]. Functionally, lignin serves as a natural adhesive in plants, providing structural cohesion and mechanical strength [7,8,18,54]. Its relatively low cost, high phenolic content, and environmental benefits make it an attractive substitute for phenol in the production of phenol–formaldehyde resins and adhesives [55].

Beyond tracheary elements, lignin is also deposited in other tissues such as fibers and seed coats [56]. Lignin accumulation in seed coats remains a relatively underexplored field, yet it holds potential for identifying new regulatory mechanisms of lignin biosynthesis, which may foster the development of bio-based products [57]. Evolutionarily, lignins are considered essential to the adaptation of land plants, although evidence suggests that their biosynthetic pathways may have originated earlier [58].

As an organic polymer, lignin plays a crucial role in the formation of plant cell walls [59]. Its content can be determined using the following formula [48]:

$$lignin, \%=\left({\displaystyle \frac{W_L}{W_W}}\right)·100,$$

(3)

where W_L is the lignin weight (g), and W_W is the extractive-free wood weight (g).

Within the cell wall structure, lignin is closely associated with hemicellulose and cellulose. Once lignin is removed, hemicellulose surrounds cellulose fibrils, forming a physical barrier to the external environment. Within this structure, hemicellulose establishes hydrogen bonds with cellulose and covalent bonds with lignin, often through hydroxycinnamic acid dimerization and internal covalent linkages [29].

Despite its importance, lignin presents challenges for the valorization of lignocellulosic biomass, as its presence hinders the efficient production of high-value-added products such as ethanol, xylitol, butanol, and paper pulp [28]. The molecular weight of lignin, ranging from 1000 to 20,000 (g·mol⁻¹), is a key parameter influencing its properties. Due to fragmentation during extraction and the random repetition of its subunits, determining its degree of polymerization is difficult [17]. High-molecular-weight and polydisperse lignins generally exhibit low reactivity toward formaldehyde or isocyanates; however, solvent fractionation can enhance their reactivity by increasing hydroxyl group availability [24].

The distribution of lignin varies among plant species and even across different cell wall layers [16]. Its chemical characterization is complex due to its three-dimensional network, multiple functional groups, diverse linkages, difficult isolation, and limited solubility in organic solvents [60]. Both the source of lignin and the isolation method strongly influence its chemical structure [28,53]. Consequently, selecting the appropriate type of lignin for a given application requires careful consideration of its origin and properties [16,54].

Lignin structural analysis requires gentle extraction to preserve the native polymer, while compositional analysis demands complete depolymerization into monomers. Degradative techniques such as oxidation, reduction, hydrolysis, and acidolysis are widely applied for compositional studies, whereas derivatization methods, including thioglycolic acid and acetyl bromide, are employed for structural analysis. The degradation products of lignin are typically examined using gas chromatography, mass spectrometry, and proton nuclear magnetic resonance (¹H-NMR) spectroscopy [23].

Structurally, lignin consists of cross-linked polyphenols with methoxy groups and terminal aldehyde functionalities [6]. Its architecture depends on the types and ratios of monomers as well as the linkages that form between them (e.g., β–O–4, β–β, and β–5 bonds), resulting in a heterogeneous, irregular polymer network adapted to diverse functions. Lignin provides rigidity and compressive strength to cell walls, enabling plants to grow tall and withstand environmental stresses [27]. Once synthesized, lignin monomers are transported to the cell wall, where they undergo radical coupling reactions mediated by peroxidases, producing complex and variable polymeric structures. The proportion of lignin monomers differs among species and tissues and can change in response to environmental conditions. This chemical flexibility allows plants to tailor lignin composition to specific structural and protective requirements [61].

Functionally, lignin serves multiple roles, including mechanical reinforcement and antimicrobial defense [18]. In nature, lignin degradation occurs in two stages: depolymerization into low-molecular-weight aromatics, followed by mineralization of these compounds [62]. Unlike thermochemical pretreatments, biological degradation overcomes the structural barriers of lignocellulosic biomass through microbial production of carbohydrate-hydrolyzing enzymes and ligninolytic enzymes [63].

The deposition of lignin in vascular plants was key to their successful colonization of terrestrial environments. It provides rigidity to fibers and xylem cells, enabling them to withstand the negative pressure created during water transport. Furthermore, lignin forms an effective physical and chemical barrier, protecting vital tissues against herbivory and microbial attack [56,64].

Structure, Composition, and Tissue-Specific Variability

Lignin is a complex, cross-linked polymer composed of three phenylpropanoid precursors: p-coumaryl, coniferyl, and sinapyl alcohols (Figure 3) [65,66,67]. These monolignols, often referred to as phenylpropanoids, differ in the substitutions at the 3-C and 5-C positions of the aromatic ring [67,68]. They are synthesized in the cytoplasm and subsequently transported to the apoplast, where laccases and peroxidases dehydrogenate them into monolignol radicals [1].

These monolignol units are randomly linked through various ether (C–O) and carbon–carbon (C–C) bonds. Aryl ether linkages are generally easier to cleave compared to the more chemically stable C–C linkages, which are highly resistant to depolymerization [17,69]. As the primary chemical component responsible for the recalcitrance of lignocellulosic biomass, lignin hinders both biomass deconstruction and subsequent saccharification. Lignin has three main aromatic substructures: p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) (Figure 4), and features various internal bonds, such as β-O-4, β-β, and β-5 (Figure 5) [4,26]. The relative abundance of these monomers determines the type of lignin formed:

• Guaiacyl (G) lignin: Predominantly coniferyl alcohol, typical of gymnosperms (softwoods).
• Syringyl (S) lignin: Predominantly sinapyl alcohol, commonly found in angiosperms (hardwoods).
• p-Hydroxyphenyl (H) lignin: Contains more p-coumaryl alcohol, often found in grasses and some other plants [27].

Generally, softwoods have a larger percentage of lignin than hardwoods, accounting for 23–33% in softwoods and 16–25% in hardwoods, respectively [71]. In angiosperms, lignin primarily consists of guaiacyl (G) and syringyl (S) units polymerized through multiple linkage types. Gymnosperm lignin, with a high G-unit composition, is more condensed and contains fewer S units, making it less amenable to chemical and physical treatments such as pulping. Increasing the S/G ratio via genetic modification can improve pulping efficiency [72]. Lignification is a flexible mechanism, and the plant can use a variety of phenolic compounds for the formation of the lignin polymers [73,74]. Phenolic compounds are important bioactive antioxidants of plant origin, whose structure consists of an aromatic ring with hydroxyl groups. The number and position of hydroxyl groups determine their physiological properties, such as antioxidant, anti-inflammatory, antimicrobial, and cardiovascular protective effects [21]. Lignin generally contains phenolic hydroxyl, methoxyl, and terminal aldehyde groups. Softwood lignin is mostly guaiacyl-based, hardwood lignin contains both guaiacyl and syringyl units, and grasses incorporate significant amounts of p-coumaryl alcohol [7]. Guaiacyl and syringyl units are present in the cell walls of fibers, parenchyma, and vessels [75].

The presence of lignin in cork has also been demonstrated, predominantly as guaiacyl units with minor syringyl and p-hydroxyphenyl content [76]. Lourenço et al. (2016) analyzed Quercus suber L. samples from a 6-year-old tree and reported tissue-specific differences in lignin content: cork (27.1%), phloem (38.4%), and xylem (23.6%) [61]. Cork lignin was enriched in G units (H:G:S = 2:85:13), phloem lignin had fewer G units (H:G:S = 1:58:41), and xylem lignin was enriched in S units (H:G:S = 1:45:55). Comprehensive analysis using Pyrolysis-Gas Chromatography/Mass Spectrometry (Py-GC/MS), Nuclear Magnetic Resonance (NMR), and DFRC revealed structural differences among tissues: cork lignin was highly condensed with phenylcoumarans (20%) and dibenzodioxocins (5%), while phloem and xylem lignins contained predominantly alkyl-aryl ether (β–O–4) linkages (71% and 77%, respectively) [61]. These findings highlight that lignin composition and structure vary depending on cell type, tissue proportion, and lignification kinetics, including secondary wall deposition rates.

Beyond canonical H, G, and S units, lignin exhibits compositional plasticity. For example, the flavone tricin is incorporated into lignin in some monocots and in the dicot alfalfa (Medicago sativa). Tricin can react with monolignols under radical coupling conditions, forming tricin-oligolignol metabolites, and acts as a nucleation site for lignification, even in high-molecular-weight lignin fractions [77].

The relative proportions of each monolignol depend on plant type (softwood, hardwood, or non-wood) and the extraction method used [28]. Lignin content varies with species, tissue type, developmental stage, and environmental conditions [78]. Based on monomer composition, lignins are classified as softwood, hardwood, or grass lignins [79]. Softwoods contain 23–33% lignin, primarily G units (>95%) with low H content (<5%), while hardwoods contain 16–25% lignin, composed of G (25–50%) and S (45–75%) units with traces of H (0–8%). Grass lignin contains all three monolignols (H: 5–35%; G: 35–80%; S: 20–55%), being the only biomass with a significant H content [11,48,65,80]. Variations among species and within plant tissues are common [68].

Monolignols are derived from the general phenylpropanoid pathway, beginning with the deamination of phenylalanine (or tyrosine), followed by sequential aromatic hydroxylation, O-methylation, and reduction of the side chain carboxyl group to an aldehyde and then an alcohol by specific enzymes [57,81,82]. A higher proportion of syringyl units indicates efficient delignification during paper production (e.g., Kraft process) and suggests potential for lignin valorization in biofuel production. Species with high S/G ratios are considered more suitable for paper production [48].

4. Lignin Extraction, Fractionation and Industrial Lignin Types

Within the framework of a circular bioeconomy, lignin is increasingly viewed as a renewable feedstock rather than waste. Enzymatic modifications and bioprocessing strategies align with sustainable practices, reducing reliance on toxic solvents and enabling cradle-to-cradle integration of lignin into biological cycles [22,36]. In this context, lignin-derived compounds can act as renewable substrates, preventing accumulation of waste in ecosystems and contributing to global carbon cycling. Extracting lignin from lignocellulosic agro-industrial wastes involves a series of stepwise treatments designed to achieve four main objectives: (i) conditioning of the raw biomass, (ii) solubilization of lignin, (iii) lignin recovery, and (iv) depolymerization [28]. These steps transform lignin into fractions suitable for applications in chemicals, fuels, and biomaterials.

4.1. Lignin Fractionation

The separation of lignin from cellulose and hemicellulose is known as fractionation. The most common product of fractionation is pulping liquor or Kraft lignin, generated during paper manufacturing. High-purity low molecular weight lignin can be isolated using enzymes or special solvents, but this is expensive. More recently, membrane-based separation by molecular weight has gained interest due to its specificity [23].

Lignin products are typically classified into five categories based on their isolation method [7,26]:

• Technical lignin: derived from industrial paper, pulp, and cellulose production.
• Kraft lignin: obtained via the sulfate-based pulping process.
• Lignosulfonate: produced through sulfite pulping.
• Alkali lignin: extracted from biomass using alkaline solutions.
• Organosolv lignin: isolated with organic solvents.

These types differ in molecular weight, polydispersity, homogeneity, and functional groups [83]. The following section discusses the main industrial lignin types obtained though these fractionation processes and their characteristic chemical features.

4.2. Industrially Relevant Lignin Types

As outlined in the previous section, lignin obtained through industrial pulping or chemical treatments is generally referred to as technical lignin [54]. These materials, produced mainly as by-products of the pulp and paper industry, differ substantially from native lignin [28,84]. Harsh extraction conditions—such as those applied in Kraft, sulfite, or organosolv pulping—alter the balance of ether and carbon—carbon linkages, leading to changes in molecular weight, functional groups, and overall reactivity [54,84].

The Kraft process is the most prominent industrial method for lignin isolation [5]. This process uses a solution of sodium hydroxide and sodium sulfide, commonly referred to as white liquor, to dissolve lignin at high temperatures (around 170 °C) and high pH levels (typically between 13 and 14). The reaction usually takes about two hours, after which the lignin can be separated from the remaining pulp by sedimentation, often using sulfuric acid [85]. During the Kraft pulping process, phenolic groups are initially converted into quinone methide intermediates by sodium hydroxide. Subsequently, hydrogen sulfide ions attack the α-carbon atoms of the ether linkages in lignin, leading to the formation of benzylthiolate anions and cleavage of the ether bonds. The benzylthiolate anion then releases a β-phenolate anion, which in turn generates free phenolic groups through the action of sodium sulfide. These free phenolic groups regenerate quinone methide intermediates, allowing the cycle to repeat and progressively cleave ether bonds, ultimately producing lower-molecular-weight fragments [60]. Owing to the combined effects of high temperature and strongly alkaline conditions, the lignin structure undergoes significant modification. As a result, Kraft lignin is obtained as a recalcitrant material with reduced reactivity compared to native lignin [54].

Kraft lignin is produced at large scale as a papermaking by-product, making it attractive for high-volume applications such as adhesives. However, Kraft exhibits between 1 and 3% sulfur content in H-bonds compared to 3.5–8% in lignosulfonate [83].

The sulfite pulping process is another major method of lignin extraction, in which chemical reactions involving lignin, sulfur dioxide (SO₂), and metal sulfites, such as calcium sulfite (CaSO₃) or magnesium sulfite (MgSO₃), produce lignosulfonates. This reaction also breaks down the α-ether and β-ether linkages in the lignin structure, facilitating the separation of lignin from the wood matrix. The process usually takes place at a temperature of 120–180 °C for 1–5 h and can be adapted to operate under alkaline, neutral, or acidic conditions, depending on the desired results [54,85]. These water-soluble lignins, containing 3.5–8% sulfur [83], account for ~90% of commercial lignin, with a global annual production of ~1.8 million tons [86]. They typically have higher molecular weight than Kraft lignin and are widely used in adhesives, dispersants, and binders [83].

Chemical pretreatment with thermal heating is widely applied to fractionate biomass into its main components—lignin, cellulose, and hemicellulose. At low acid concentrations, the treatment disrupts the outer surface of the biomass, releasing extractives and a portion of hemicellulose. In contrast, concentrated acids under high-temperature conditions break down the rigid biomass structure, simultaneously releasing both lignin and hemicellulosic sugars. Commonly used acids in this method include sulfuric, hydrochloric, and nitric acids at concentrations of 1–10% (w/v) [7]. Acid pretreatment is generally performed under high temperatures of 135–200 °C, with short retention times of 30–120 °C. Reportedly, the range of temperature in the process can be minimized to 110 °C by adding sulfuric acid (H₂SO₄) [87].

Organosolv processes use organic solvents such as ethanol, methanol, formic acid, or acetic acid—often in 40–70% (v/v) concentration—combined with acid (e.g., (H₂SO₄, HCl) or alkaline (e.g., NaOH) catalysts, typically operated at 150–200 °C for 1–3 h under autogenous pressure. This method produces lignin that is structurally closer to native lignin compared to Kraft or lignosulfonate lignins [17,54,87]. Organosolv lignin is especially effective when processing annual or hardwood plants [60].

Major lignin extraction methods are given in

Table 2. Comparative summary of major lignin extraction methods.

Extraction Method	Main Reagents/Conditions	Lignin Characteristics	Advantages	Limitations	References
Kraft process	NaOH + Na2S, 160–180 °C	Sulfur-rich, condensed	Industrially dominant, robust	Odorous, limited reactivity	[54,60,83,85]
Sulfite process	SO2 + CaSO3/MgSO3, 120–180 °C	Lignosulfonates, water-soluble	Produces sulfonated lignin, easy to handle	Low purity, sulfur content	[54,83,85,86]
Organosolv process	Organic solvent (ethanol, acetic acid), 150–200 °C	Low-sulfur, high-purity lignin	Easy to depolymerize, suitable for fine chemicals	Higher cost, solvent recovery needed	[7,17,87]
Alkali process	H2SO4, HCl or HNO3, 100–150 °C	Phenolic, partially degraded	Simple and inexpensive	Structural modification, lower yield	[7,87]

.

Because technical lignins are often inert and structurally altered, the “lignin-first” approach has been proposed. Here, lignin is targeted as a primary product rather than a residue, with processes designed to generate low-molecular-weight hydrocarbons suitable for fuels or high-value chemicals [36,54]. This can be achieved through organosolv-based solubilization strategies.

5. Lignin Depolymerization and Upgrading

5.1. Chemical and Catalytic Depolymerization

Lignin depolymerization breaks down the complex aromatic polymer into smaller molecules of commercial value, such as chemicals and biofuels. Chemically, depolymerization can be achieved through:

• Pyrolysis is a process in which oxygen is not required for the thermal treatment of lignin. The temperature can range from 300 °C to 600 °C. The absence of oxygen is necessary to prevent the reaction from continuing and CO₂ from forming. The extent to which this process transforms the ring structures of lignin depends on the type of raw material, temperature, and heating rate [23].
• Hydrogenolysis. In a typical lignin reduction depolymerization system, lignin or lignocellulose is treated at a temperature of 180–300 °C and a pressure of 0–5 MPa H₂ (at room temperature). The solvent is usually a polar alcohol solvent or another hydrogen-donating solvent. Solvolysis breaks down the lignin in the matrix and depolymerizes it into small fragments. Some of these fragments interact with the catalyst and are hydrogenated to form stable phenolic monomers [88].
• Oxidative depolymerization is attractive due to its relatively mild operating conditions and the possibility of producing targeted products with multiple functionalities. During oxidative depolymerization, lignin is converted in the presence of an oxidant, typically O₂ or H₂O₂. Oxidation can cause the breakdown of side chains, forming phenolic aldehydes and acids, but it can also break down aromatic rings in lignin, forming aliphatic carboxylic acids [89].
• Gasification requires higher temperatures (between 700 °C and 1000 °C) compared to the other thermochemical methods, and it focuses primarily on the production of non-condensable gases, such as H₂, CO, CO₂, and CH₄. Once syngas is produced, cleaned and filtered to remove problematic chemical compounds, it can then be used to generate energy [90].
• Combustion takes place in the presence of oxygen and at extremely high temperatures (around 800 °C to 1000 °C) and can be used to produce heat, electricity, gas, and solid carbon residue [23].

A comparative summary of lignin depolymerization routes is given in

Table 3. Comparison of lignin depolymerization routes.

Depolymerization Method	Catalyst/Conditions	Products	References
Pyrolysis	300–600 °C (no O2)	Bio-oil, phenols, gases, biochar	[23]
Hydrogenolysis	180–300 °C	Phenolics, aromatics	[88]
Oxidative depolymerization	O2 or H2O2	Phenolic acids, aldehydes	[89]
Gasification	High temperature (700–1000 °C)	H2, CO, CO2, and CH4	[90]

.

The efficiency of these methods depends both on the source of lignin and the method used to separate it from cellulose and hemicellulose [23]. Efficient catalytic depolymerization is critical for using lignin as a renewable feedstock [26]. These depolymerization processes, when applied to extracted lignin, aim to dismantle its complex polymeric structure into smaller and more valuable molecules. To be effective, depolymerization must yield low-molecular-weight lignins (monomers and oligomers), since only compounds of this size can act as substrates for subsequent cellular assimilation [84]. The specific properties of lignin, which are determined by the extraction method and processing conditions, ultimately determine its suitability for future applications [28,69]. Ideally, the process yields low-molecular-weight lignins (mono- and oligomers), which can serve as substrates for microbial assimilation [84]. Since lignin properties vary according to extraction conditions, its applications are highly dependent on the chosen method [28,69].

Despite significant progress in lignin extraction, depolymerization, and valorization, several knowledge gaps and technical challenges remain unresolved. One of the main limitations is the structural heterogeneity of lignin, which varies greatly depending on the type of biomass and extraction method. This variability complicates process optimization and limits reproducibility. Therefore, future research should focus on the development of standardized analytical methods that allow the structure of lignin to be linked to its chemical reactivity and valorization potential. Another problem is the limited selectivity of catalytic and thermochemical processes. Although recent advances in metal and hybrid catalysts have improved conversion efficiency [91], problems such as catalyst deactivation, carbon loss, and unwanted condensation reactions remain. The development of more robust, recyclable, and substrate-resistant catalysts remains a key priority for the large-scale utilization of lignin.

5.2. Biological Depolymerization and Hybrid Routes

Biological delignification relies on microorganisms, mainly bacteria and fungi, that can produce enzymes that break down lignocellulosic agricultural waste [14,23,28,92]. These processes are attractive due to their specificity, which enables microorganisms to target lignin while leaving other valuable biomass components intact. They also operate under mild conditions and have a low environmental impact. However, challenges remain with respect to reaction rates and overall cost.

Enzymatic hydrolysis is typically applied to biomass that has undergone pretreatment in order to enhance enzyme accessibility. Typical enzymatic hydrolysis of cellulose-enriched substrates is carried out in a heterogeneous, multi-step process using enzymes called cellulases under mild, specific conditions (e.g., pH 4.8, 50 °C). First, the cellulases adsorb onto the surface of the cellulose, then break it down into shorter chains and cellobiose. Finally, another enzyme, beta-glucosidase, hydrolyses the intermediate products into glucose [93]. In this context, cellulose-enriched solids serve as the primary substrates. The efficiency of enzymatic hydrolysis depends strongly on the crystallinity index and degree of polymerization of the cellulose: the lower both parameters are, the more readily the cellulose can be hydrolysed [28]. Through enzymatic hydrolysis, cellulose is broken down into its monosaccharide building blocks, which can then be converted into a variety of products via microbial fermentation. However, enzymatic hydrolysis is often incomplete, largely due to the supramolecular arrangement of cellulose, which varies depending on biomass origin and pretreatment method [52].

Currently, fungal cellulolytic enzymes are commercially applied to degrade non-lignin biomass, whereas the use of laccases and peroxidases in lignin depolymerization remains mostly at the laboratory stage [23]. Laccase and peroxidase activities are generally optimal at 25–40 °C and pH 4–6, in the presence of molecular oxygen or low concentrations of H₂O₂ as oxidant [94]. Based on their redox potential, laccases are divided into low- (bacteria, plants, insects) and high- (fungi) redox-potential groups. They catalyze anabolic and catabolic reactions: fungal laccases break down lignin and humus, and participate in the synthesis of pigments, polyflavonoids, and lignification [2]. In addition to lignin modification, laccases are involved in the formation of fungal spore pigments, plant pathogenesis, fungal virulence, iron metabolism, and plant kernel browning processes [85]. It should be noted that the redox potential of plants, bacteria, and insects allows radicals to combine without additional chemicals [2].

Increasing evidence suggests that bacterial laccases and peroxidases, structurally and functionally similar to their fungal counterparts, also participate in lignin degradation [23,86]. These enzymes have been identified in bacterial species inhabiting forest soils [23]. One well-characterized example is the CotA protein, a bacterial laccase located on the outer coat of Bacillus subtilis and related species [2]. Together, fungal and bacterial ligninolytic systems demonstrate the biological versatility and ecological importance of enzymatic lignin breakdown.

Following lignin removal, the enzymatic hydrolysis of cellulose becomes a key step in biomass conversion. Cellulose hydrolysis involves three main families of enzymes: endoglucanases, exoglucanases, and β-glucosidases. Each plays a distinct role: endoglucanases randomly cleave internal β-1,4-glycosidic bonds within cellulose chains; exoglucanases act on chain ends to release cellobiose units; and β-glucosidases convert cellobiose into glucose monomers. Depending on the pretreatment strategy, the lignin-rich fraction generated during hydrolysis may be either solid or liquid. In liquid fractions, lignin can be recovered through precipitation, while in solid fractions, enzymatic hydrolysis yields lignin-enriched residues that can subsequently be depolymerized into lignin-derived products [28].

From both economic and environmental perspectives, pretreatment with lignocellulolytic fungi can replace or at least supplement mechanical, thermal, or chemical pretreatment in lignocellulosic biorefineries [4]. Since lignin depolymerization is critical for lignin utilization, a wide range of lignin-degrading enzymes and microbial metabolic systems have evolved to facilitate lignin breakdown and conversion. Among microorganisms, fungi are the most effective lignin degraders, capable of secreting multiple classes of ligninolytic enzymes. Based on their degradation mechanisms, lignin-degrading fungi are broadly divided into three groups: white-rot, brown-rot, and soft-rot fungi [92]. In general, wood-rotting fungi are the primary lignin degraders and represent the only group of microorganisms able to mineralize this abundant biopolymer, which accounts for roughly 25% of removable organic matter in the biosphere [95].

White-rot fungi are the most effective lignin degraders in nature, with superior capabilities compared to brown-rot and soft-rot fungi [92]. In laboratory and bioreactor studies, white-rot fungal delignification is typically performed under aerobic, static or agitated conditions at 25–30 °C and pH 4.5–5.5, with incubation periods ranging from several days up to two weeks depending on the substrate [68,96]. Wood colonized by white-rot fungi often appears pale due to oxidative bleaching and lignin removal, while retaining a fibrous texture [68]. These filamentous decay fungi are common in forest litter and fallen trees and hold great promise for biotechnological applications, including hazardous waste remediation and industrial processing of paper and textiles.

All white-rot fungi belong to the Basidiomycota phylum type—a very diverse group consisting of edible mushrooms, plant pathogens (e.g., rusts and spots), mycorrhizal symbionts, and even opportunistic human pathogens [97]. Their oxidative degradation of lignin is an important process in global carbon cycling, although the underlying mechanisms remain only partly understood [98].

White-rot fungi secrete a range of hydrolytic and oxidative enzymes, including cellulases, hemicellulases, and ligninolytic enzymes such as laccase, manganese peroxidase, and lignin peroxidase [4,99]. Lignin peroxidases, members of the oxidoreductase family, catalyse lignin degradation in the presence of H₂O₂ [99]. Laccases (benzenediol oxygen oxidoreductases, EC 1.10.3.2) are multicopper enzymes that oxidize phenols and anilines while reducing molecular oxygen to water [100,101,102]. The resulting radicals may undergo further oxidation, hydration, disproportionation, or polymerization reactions [92]. Laccases are widespread in nature and occur not only in fungi but also in plants, bacteria, and insects [22,99,102]. Typically secreted as extracellular enzymes, they mediate both polymerization and depolymerization processes [102].

White-rot fungi can be further classified according to their degradation patterns [4]. Some species, such as Ceriporiopsis subvermispora, Phellinus pini, Ganoderma australe, and Phlebia tremellosa, selectively degrade lignin and hemicellulose without damaging cellulose. Others, including Phanerochaete chrysosporium, Trametes versicolor, Heterobasidion annosum, and Irpex lacteus, simultaneously degrade cellulose, hemicellulose, and lignin. Their extracellular enzyme arsenal primarily consists of oxidases and peroxidases [92].

Peroxidases, like laccases, are abundant and widely distributed across all domains of life, including plants, fungi, bacteria, and humans (e.g., glutathione peroxidase) [22]. The lignin-degrading potential of white-rot fungi underpins their industrial applications in bio-pulping, as well as in bioremediation of contaminated soils and waters and biorefining of lignocellulosic feedstocks [92].

Brown-rot fungi primarily colonize softwoods and represent approximately 7% of wood-rotting basidiomycetes. Brown-rot fungal degradation commonly proceeds at 20–30 °C and pH 4–5 under aerobic conditions, driven by Fenton-type reactions generating hydroxyl radicals. Unlike white-rot fungi, they rapidly depolymerize cellulose and hemicellulose while only partially oxidizing lignin. For example, Gloeophyllum trabeum non-selectively cleaves intermonomer side-chain linkages in lignin, with fermentation causing up to 16% lignin loss in spruce wood [92].

Soft-rot fungi, mainly belonging to the Ascomycota and Deuteromycota groups, preferentially degrade hardwoods. They attack syringyl units in lignin and are also capable of degrading compounds such as vanillic acid and phenols. Species including Aspergillus niger and Penicillium chrysogenum have been reported to degrade pine and sycamore wood. Although their enzymatic systems remain less well understood, evidence suggests that soft-rot fungi often modify rather than fully mineralize lignin [92].

In addition to fungi, several bacterial species with lignin-degrading capabilities have been identified in diverse environments, including soil, decayed wood, wastewater treatment plants, and animal gastrointestinal tracts [92]. Although bacteria generally exhibit lower lignin-degrading efficiency compared to fungi, they demonstrate greater environmental adaptability. Under aerobic conditions, lignin-degrading bacteria secrete oxidative enzymes that facilitate lignin breakdown in the presence of oxygen. However, lignin degradation can also occur under strictly anaerobic conditions [92].

Among aerobic lignin-degrading bacteria, members of the Actinobacteria phylum are well known, especially Streptomyces and Rhodococcus. Streptomyces viridosporus T7A, for example, secretes extracellular enzymes from its filamentous form to degrade lignin, including that of native wheat straw, resulting in the reduction of guaiacyl units [92].

Anaerobic bacteria have also been reported to convert lignin and lignin-derived aromatics into methane and carbon dioxide. Interestingly, modified lignin is more readily degraded under anaerobic conditions than native lignin, which is typically more methoxylated. The methoxy groups serve as primary attack sites during bacterial degradation of methoxylated aromatics. Anaerobic lignin degradation involves three major steps: demethoxylation, aromatic ring cleavage, and methanogenesis, whereby lignin-derived aromatics are ultimately transformed into methane by methanogenic microbial consortia [92].

Extremophilic and thermophilic bacteria show particular promise for lignin degradation due to their specialized enzymes and metabolic pathways. For instance, Thermobifida fusca produces a DyP-type peroxidase capable of degrading Kraft lignin and oxidizing β-aryl ether model compounds [92]. DyP peroxidases function at acidic pH, oxidizing various substrates, but their natural physiological substrates are still unknown [102].

From a biotechnological perspective, ligninolytic enzymes and microbial systems are promising for environmentally friendly lignin conversion. However, their low reaction rate, instability under process conditions, and limited substrate availability hinder industrial application. Further research in the areas of enzyme engineering, genetic modification, and process integration could significantly improve the efficiency of bioconversion.

6. Lignin Properties and Applications

Lignin, unlike polysaccharides, is distinguished by its aromatic functionality [6,23,66]. In its native state, lignin appears colourless to pale yellow but typically darkens when exposed to acidic or alkaline treatments [65]. Because it is a non-edible, carbon-neutral biopolymer, lignin has attracted growing attention as a sustainable raw material, especially since it is abundantly available in agricultural and forestry residues [81,103]. In natural systems, its persistence and resistance to microbial attack make lignin an important contributor to soil organic matter and humus formation, with significant implications for carbon sequestration and soil fertility [67].

From a structural perspective, lignin is a highly complex and heterogeneous aromatic network. In the late stages of plant cell wall deposition—p-coumaric, coniferyl, and sinapyl alcohols—polymerize into p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) subunits [104]. The resulting three-dimensional matrix binds cellulose and hemicellulose, conferring rigidity and hydrophobicity but at the same time creating a barrier to enzymatic hydrolysis and biomass deconstruction [7]. Its precise composition is not fixed: environmental conditions such as ultraviolet exposure, drought, or pathogen attack can induce significant changes in monomer ratios and cross-linking patterns, reflecting lignin’s adaptive role in plant evolution and stress tolerance [21,27]. This variability, while beneficial for the plant, complicates industrial recovery and utilization.

Most of the lignin recovered from pulping processes is combusted on-site to generate energy for the mill; however, a considerable fraction is also separated and commercialized as an independent product [81]. In Europe, the leading producers of paper waste in 2018 were Germany (3,413,000 tonnes), Finland (3,275,000 tonnes), and France (2,617,000 tonnes) [29]. On a global scale, the amount of lignin extracted annually by industry represents only about 1.5–1.8% of the total lignin present in the biosphere, yet projections suggest that industrial lignin generation could reach 225 Mt by 2030 [28]. At present, approximately 70 Mt of technical lignin are estimated to be available each year from wood pulping operations and the growing cellulosic ethanol sector [18,28]. Recent geopolitical and economic developments, especially energy price fluctuations, have further shaped the trade-off between burning lignin for energy versus valorizing it as a renewable resource for chemicals and advanced materials [22,105]. Beyond direct combustion, lignin can also serve as a feedstock for advanced biofuel production. Recent studies emphasize catalytic depolymerization and hydrodeoxygenation as effective routes for producing lignin-derived bio-oils suitable for aviation and marine fuels [106]. Pyrolysis and gasification followed via Fischer–Tropsch synthesis can yield drop-in hydrocarbons with reduced lifecycle CO₂ emissions compared to fossil analogues. Integrating lignin-to-biofuel pathways into existing biorefineries could substantially enhance overall carbon efficiency and energy output while reducing process waste [107].

These structural features underpin lignin’s material relevance. Rich in phenolic hydroxyl groups, lignin displays antioxidant activity, ultraviolet absorbance, and thermal stabilization potential [24]. Such characteristics explain why lignin has long been considered for incorporation into polymer matrices. Its abundance, low cost, high carbon content, and biodegradability add to its attractiveness as a candidate for sustainable composites and bio-based polymers [108]. Recent studies show that chemically modified lignins (by esterification, alkylation, or grafting with reactive polymers) can significantly improve interfacial adhesion and compatibility in thermoplastic and thermosetting matrices [109,110]. The incorporation of lignin into biodegradable polymers such as polylactic acid (PLA) and polyhydroxyalkanoates (PHA) increases tensile strength, UV resistance, and antioxidant capacity, providing an environmentally friendly alternative to synthetic stabilizers. Currently, customized lignin-based copolymers and reactive blends are being investigated for use in packaging, coatings, and the automotive industry, where partial replacement of petroleum-based resins could reduce both costs and carbon dioxide emissions [109]. However, effective utilization requires careful control of compatibility and dispersion to realize these benefits.

Lignin exhibits an intrinsic property of autofluorescence, arising from fluorophores within its structure. These molecules absorb energy at specific excitation wavelengths, enter a short-lived excited electronic singlet state (lasting only a few nanoseconds), and release part of this energy as light upon returning to the ground state at a longer emission wavelength [8,46]. Although autofluorescence is inherent to lignin [111], its quantum yield is relatively low, which can make imaging challenging depending on the sensitivity of the available microscopy equipment [112]. Biomass fluorescence varies with local composition, representing the combined emission of its components [59]. As a result, different lignin types can be distinguished by their characteristic fluorescence spectra or by colour variations observed under fluorescence microscopy. The fluorescence lifetime of lignin-bound fluorophores is shorter than that of fluorophores bound to more ordered structural polymers, such as cellulose. This is due to lignin’s irregular architecture, characterized by random linkages and extensive branching [8]. In addition, lignin fluorescence is sensitive to environmental conditions: for example, brightness increases significantly at pH 9 compared to lower values.

Applications of lignin are remarkably diverse and have been studied for decades. In polymeric systems, lignin can act directly as a UV stabilizer, antioxidant, flame retardant, or plasticizer [108,113]. When blended with other biopolymers, it can improve mechanical properties, and high-purity lignin has been identified as a potential precursor for carbon fiber composites [16,17]. Lignin-derived carbon fibers are attracting strong research interest as sustainable alternatives to polyacrylonitrile (PAN) precursors. Advances in fractionation and melt-spinning of softwood Kraft lignin have yielded fibers with tensile strengths exceeding 1.5 GPa and carbon yields above 40% [89]. Process optimization, including stabilization under controlled oxidative atmospheres and blending with cellulose or polyolefins, continues to improve mechanical performance and spinnability. These developments indicate that lignin could soon support large-scale carbon-fiber production for automotive and aerospace composites [110,114].

Adhesives and foams represent another promising domain. Numerous studies have explored lignin as a renewable replacement for phenol and polyols in phenol–formaldehyde resins and polyurethane foams [24,115]. Although issues such as high viscosity, reduced reactivity, and undesirable odour limit the extent of substitution, unmodified lignin can still function as both filler and cross-linker in polyurethane formulations [116]. More recently, alkoxylated lignin derivatives have been investigated as polyols for rigid foams, and isocyanurate foams have shown greater tolerance to the dark colouration typical of lignin-based materials [53]. Beyond polymer systems, lignin has also entered niche markets: in cosmetics, it is employed as a natural sunscreen agent, replacing some synthetic UV absorbers [25,117], while in coatings, resins, paints, and even food applications, its aromatic structure offers both functionality and sustainability [16]. In the cosmetics sector, lignin’s strong UV absorbance and antioxidant activity have spurred the development of lignin-based sunscreens and anti-aging formulations that combine high photostability with biodegradability [109]. Current formulations seek to mitigate colour and odour through nanoscale fractionation or chemical bleaching. In biomedical materials, lignin-derived nanoparticles, hydrogels, and bio-adhesives have shown promise for wound healing, drug delivery, and tissue engineering, owing to their biocompatibility, radical-scavenging ability, and low cytotoxicity [114]. These emerging applications exemplify lignin’s transition from an industrial by-product to a multifunctional bio-based material platform.

A particularly well-documented case of industrial integration is the use of lignin in oriented strand boards. Oriented strand board production has grown to dominate the construction panel market in North America, where wood strands are typically bonded with phenolic or isocyanate resins. As early as the 1990s, trials demonstrated that organosolv lignin could replace 5–25% of phenolic resin in oriented strand board formulations without compromising panel performance [53,118]. In fact, some products exceeded control specifications while simultaneously improving workplace conditions by reducing dust emissions in production facilities. This case highlights both the technical feasibility and the environmental benefits of lignin substitution.

Health and regulatory concerns provide an additional driver for lignin valorization. Formaldehyde-based adhesives, long a staple of the panel industry, have come under scrutiny for their adverse health effects. Formaldehyde has been classified as a human carcinogen (Category 1B) by the International Agency for Research on Cancer and under EU regulations [83,118]. In response, emission limits have been progressively tightened in Europe, Japan, and the USA, which has accelerated the search for bio-based, low-emission alternatives. Lignin, as a renewable substitute for phenol in phenol–formaldehyde resins, is therefore positioned not only as a cost-saving additive but also as a safer material aligned with regulatory expectations [118].

In modern biorefineries, lignin is increasingly viewed not as waste but as a co-product with strategic value [28]. The United States and Brazil are the world’s leading producers of bioethanol, together accounting for more than 85% of global output [119]. The most widely produced renewable fuel today is first-generation ethanol, typically derived from food-based crops such as corn, wheat, sugar beet, and sugar cane [119,120]. In contrast, second-generation biofuels are considered more sustainable because they are produced from lignocellulose [121]. The production of second-generation bioethanol from lignocellulosic feedstocks consistently yields substantial amounts of lignin, with estimates of 0.5–1.5 kg generated per liter of ethanol [28]. Feedstocks for second-generation ethanol include agricultural residues such as maize and wheat stalks, rice straw and husks, sawdust, cotton stalks, and other non-edible plant biomass, including wood and pasture grasses. A third generation of bioethanol is also emerging, based on microalgae and macroalgae as renewable sources of fermentable carbon [119].

In lignocellulosic biorefineries, bioethanol production from the carbohydrate fraction generates lignin-rich residues as a major by-product. Rather than being burned for low-value heat and power, this lignin stream can be valorized into high-value products such as aromatic chemicals, adhesives, carbon fibers, and biopolymers. Integrating lignin valorization with sugar-to-ethanol conversion is therefore crucial to improving the overall energy efficiency, carbon footprint, and economic feasibility of lignocellulosic biofuel processes [14,28].

Despite decades of research, several obstacles still limit large-scale exploitation. Lignin’s structural heterogeneity, variable molecular weight distribution, and limited reactivity make processing difficult and reproducibility uncertain [9]. Its thermo-mechanical properties, often considered unattractive, complicate direct incorporation into polymer systems. Moreover, from an economic perspective, lignin must still compete with its role as an internal energy source in pulp mills. Nevertheless, advances in selective fractionation, chemical modification, and the development of “lignin-first” biorefining strategies are opening new pathways to increase its value. As these technologies mature, lignin is increasingly seen not as an inconvenient residue but as a cornerstone of the circular bioeconomy, capable of contributing to sustainable materials, fuels, and chemicals.

Methods for lignin valorization routes according to [122] are shown in Figure 6.

7. Conclusions

Lignocellulosic biomass, as the most abundant renewable carbon resource, offers significant opportunities to replace fossil-based feedstocks for fuels, chemicals, and advanced materials. Within this framework, lignin stands out as both a challenge and an opportunity. Its complex and heterogeneous structure hinders efficient biomass deconstruction, yet at the same time provides unique aromatic functionalities that are not found in polysaccharides. Current industrial practice still relies heavily on burning lignin for energy recovery, but advances in extraction, fractionation, and depolymerization are gradually shifting perspectives toward its valorization as a high-value co-product in biorefineries.

Both chemical and biological depolymerization strategies demonstrate potential to convert lignin into low-molecular-weight monomers and oligomers suitable for use in biofuels, polymers, adhesives, carbon fibers, cosmetics, and biomedical applications. However, structural variability, processing inefficiencies, and economic barriers remain major obstacles to large-scale implementation. Overcoming these challenges will require integrated approaches that combine selective catalytic and enzymatic pathways with emerging “lignin-first” biorefining concepts, supported by life-cycle assessments and techno-economic analyses.

It is necessary to bridge the gap between laboratory and industrial scale implementation. Although many uses of lignin are promising, only a few have been validated by technical and economic studies and life cycle assessments. Future efforts should focus on pilot projects, integration into existing biorefinery systems, and interdisciplinary collaboration involving expertise in chemistry, biology, and engineering.

Ultimately, lignin valorization is pivotal to realizing the full potential of lignocellulosic biomass in a sustainable circular bioeconomy. Its successful integration into industrial processes could reduce dependence on fossil resources, enhance waste utilization, and contribute to climate change mitigation while generating new avenues for value-added bioproducts.