Delignification as a Key Strategy for Advanced Wood-Based Materials: Chemistry, Delignification Parameters, and Emerging Applications

Abstract

Wood is a naturally abundant, biodegradable, and renewable material with significant potential as an alternative to petroleum-based materials. However, its inherent heterogeneity, anisotropy, and modest mechanical properties limit its application in high-performance structural uses. Delignification, a critical process in papermaking and biorefining, has emerged as a promising pretreatment technique to enhance the properties of wood for advanced subsequent applications. This process selectively removes lignin while preserving the aligned cellulose structure, thereby improving mechanical strength, dimensional stability, and potential for functionalization. Various delignification methods, including alkaline, acidic, and reductive catalytic fractionation, have been explored to optimize the wood’s structural and chemical properties. When combined with densification or impregnation, delignified wood exhibits superior mechanical performance, making it suitable for a range of applications, including structural materials, optical devices, biomedical applications, and energy storage. This detailed review examines the chemistry and mechanisms of delignification, its impact on the physical and mechanical properties of wood, and its role in developing sustainable, high-performance bio-based materials. Furthermore, challenges and future opportunities in delignification research are discussed, highlighting its potential for next-generation wood-based innovative applications.

1. Introduction

Wood as a biodegradable, renewable, and abundant natural material stands as a great alternative to petroleum-based materials. Its low-cost, good mechanical performance, and lightweight design result from a “wisely”-optimized structure for water transportation within and stiffness in living trees. However, wood’s inherent heterogeneity, anisotropy, and moderate strength limit its use in high-end structural applications [1,2,3]. Conventional modification methods, such as thermal treatments, impregnation, chemical modifications, and coatings [4,5,6,7,8,9], only partially improve wood’s final properties. Treatments like steam, heat, and NH₃ fuming, followed by densification, often result in low dimensional stability under moist conditions [10,11]. These prompts increased research into new wood-based materials with enhanced performance [12] and greater possibilities. Lignin, the structural material that trees consist of, is a complex organic polymer, rich in aromatic content, which of course varies from species to species, as is the case in all natural substrates. It is a cross-linked phenolic polymer (Figure 1) in fibrous microstructure.

Delignification, originally used in papermaking and biorefining, involves the selective removal and modification of lignin from wood, while preserving its aligned cellulose structure. Lignin, a complex, cross-linked polymer synthesized from phenylpropanoid-derived monolignols, is key to wood’s rigidity, water transport, and defense, but also impedes further functionalization [14]. By removing lignin from the middle lamella and secondary layers, delignification increases the accessibility of cellulose microfibers, facilitating effective surface modifications or chemical treatments that enhance mechanical strength, water resistance, and biodegradability [12,15,16,17,18,19,20,21,22]. This process produces an open-pored structure with larger lumina and smaller pits improving the penetration of functional materials and an unobstructed pathway for flow. Although natural wood’s hydrophobic lignin minimizes water uptake, its removal makes delignified wood more moisture-absorbent and dimensionally sensitive. However, subsequent densification or chemical modification (e.g., acetylation) can reduce porosity and free hydroxyl groups, thereby enhancing dimensional stability and mechanical properties [23,24,25]. Techniques such as hot pressing, vacuum-assisted densification, or resin impregnation further collapse cell walls into a dense, high-fiber-volume structure [5,22,26,27,28,29,30,31,32,33,34,35,36].

Modern research designs delignified wood for diverse applications—structural materials [5,18,19,22,32,37,38,39,40,41,42], optical devices [5,43,44], biomedical scaffolds [45,46], and energy storage/electronics [30,31,34,42,47,48,49,50,51]. Notably, the studies by Hou [52], Burgert [53], and Guan [54] have pioneered applications such as super-strong low-density materials, interpenetrating composites, “transparent” wood composites, and conductive, “smart” wood composites [40,42,46,53,54,55,56]. Yano et al. [57] demonstrated that partial lignin removal using NaClO₂ followed by NaOH treatment and resin impregnation can produce high-strength wood, while subsequent studies have confirmed the high mechanical anisotropy of delignified and densified wood [17,58]. The first optically “transparent” wood was produced by Fink in 1992; since then, it has been refined using NaClO₂ delignification with pre-polymerized methyl methacrylate impregnation, along with other methods such as solar-assisted chemical brushing and UV irradiation [48]. Recent advances include the creation of robust, mesoporous, hydrophobic bio-sourced composites for selective oil/water separation [59,60].

This review focuses on the delignification process as a pretreatment fundamental process for subsequent manufacturing of advanced wood-based products. It examines the wood cell-wall structure and chemistry, the mechanisms of lignin removal, and how these factors interplay to influence the physical, mechanical, chemical, and thermal properties of wood produced. The review also discusses how different delignification parameters and drying conditions affect the final product, concluding with the potential challenges and opportunities for developing sustainable, wood-based materials for subsequent interesting applications.

2. Materials and Methods

This review adopts a structured, narrative approach to comprehensively examine the current state of research on the delignification of solid wood. The methodology consists of four key stages: literature search, selection criteria, data extraction, and synthesis of findings. First, a comprehensive literature search was conducted across multiple academic databases, including ScienceDirect, Scopus, and Google Scholar, to ensure a broad understanding of the subject. The search was limited to publications from 2000 to 2024 to capture contemporary methods and trends of this century. A combination of keywords and Boolean operators was employed to maximize coverage. Search terms included “Delignification” AND “solid wood”, “Lignin removal” AND (“hardwood” OR “softwood”), “Chemical treatment” AND “lignin”, “Biological delignification” OR “enzymatic delignification”, “Pretreatment of wood” AND “lignin dissolution”. Next, the selection criteria for choosing the publications for further studying and reviewing were the following:

• Inclusion criteria: Studies focused specifically on solid wood, rather than wood pulp or fiber, articles presenting experimental, theoretical or review-based findings on delignification methods, mechanisms, or effects. Only peer-reviewed journal articles, conference proceedings, and patents were considered.
• Exclusion criteria: Studies focused exclusively on paper production or lignocellulosic biomass for biofuels, research on lignin post-extraction valorization, unless directly linked to delignification, non-English publications (due to translation limitations), or non-peer-reviewed materials (e.g., magazine articles and blogs).

As for data extraction, from each selected study, key data were extracted, including wood species used (hardwood vs. softwood), delignification method (chemical, biological, physical, or combined), process conditions (temperature, time, and reagent concentrations), degree of lignin removal, impact on wood structure, and mechanical integrity. Finally, the synthesis of findings was executed to facilitate comparison and thematic analysis and building of tables. The studies were categorized based on the delignification method employed, i.e., chemical methods (e.g., acid/base treatments, organosolv, and oxidative processes), biological methods (e.g., fungal or enzymatic treatments), and emerging and hybrid methods (e.g., microwave-assisted techniques and deep eutectic solvents).

All in all, the board and precise research on the delignification literature is testified, apart from the quality of the discussion, from the great number of references cited, as well.

3. Delignification Process as Fundamental Treatment

Delignification removes lignin from lignocellulosic materials via chemical, physical, or biological treatments [61]. Lignin, constituting 20%–30% of wood’s dry weight [5,27,35,62], is a stiff, cross-linked polymer that binds cellulose. Its removal creates numerous mesopores in wood cell walls [19], enhancing the infusion of modifiers and functional chemicals. Although reducing lignin may soften the matrix and decrease rigidity, it also improves fracture toughness [5,62,63,64] and promotes cellulose fibril aggregation. Most delignification processes employ alkaline methods such as Kraft pulping—with NaOH combined with Na₂SO₃, Na₂S, or alone—often supplemented by stabilizers like CH₃COOH and subsequent H₂O₂ bleaching. Recently, “greener” alternatives like ionic liquids or deep eutectic solvents have emerged [65]; for example, Chen et al. [66] used a choline chloride with lactic acid to process wood for a solar steam generating system, something very different to NaOH.

Delignification increases wood’s flexibility and formability, critical for defect-free densification that can achieve fiber volume contents up to 80%. A “green” system of H₂O₂/CH₃COOH steam-modified technique dissolves more lignin, generating additional pores for subsequent resin impregnation or even manufacturing of “transparent” wood composites incorporating polymers [5]. The inherent network of natural lignin and the presence of resins, enable the production of such composites. Steam’s high penetration enables delignification across various wood species and orientations, with total or partial lignin removal, chosen to either maximize porosity or retain some binding capacity during hot pressing [56,67]. This pretreatment reduces the pressure required for densification by enhancing hydrogen bonding, thus lowering processing costs and minimizing defects from high-pressure cracking [68]. Dimensional stability is also crucial. Hot pressing induces some plastic deformation in wood cells, which can partly reverse—known as the “spring-back” effect [1,10,12,19,34,69,70]. To fix this, strategies such as resin impregnation [47,48], cross-linking [10,22,34,36,47,71], and heat treatments (using vacuum, nitrogen, steam, or oil) are applied. Hydrothermal treatment at high temperatures enhances anti-weathering resistance and dimensional stability by relieving internal tensions via microcracks [49,59,60,72].

Figure 2 lists the most crucial factors that affect the delignification process, by indicating various parameters that lead to differentiations in the lignin resulted after treatment. Those factors, that will be discussed later, are divided into two categories: those that concern the milieu conditions the wood faces when treated, where the process evolves, and the characteristics/properties/qualities of the wood itself applied, thus its original nature. Treatments externally pose the chemicals and conditions for delignification to go, but the substrate—the wood—may react differently towards them, depending further on its own qualities. Figure 2 summarizes those two influential groups. More particularly, the chemical solution and the choice of chemicals used in the delignification process can affect its final efficiency. Common delignification agents include alkalis (e.g., NaOH), acids (e.g., H₂SO₄), and oxygen-based agents (e.g., H₂O₂). The concentration, temperature, and duration of exposure to these agents can also impact the effectiveness of the delignification process. At low temperatures, NaOH, Na₂S, or Na₂CO₃ are utilized for delignification. The “Kraft process”, also known as the sulfate process for the conversion of wood into wood pulp, which consists of almost pure cellulose fibers, combines cooking in a combination of hot H₂O, NaOH, and Na₂S with mechanical processing processes.

Delignification with structure preservation maintains the cellulose arrangement and hierarchical architecture of natural wood, making the wood’s natural cellulose framework a matrix, available for functionalization. Delignified wood can be chemically modified to give additional functions or may be further densified to produce cellulose materials with improved mechanical properties; the degree of delignification determines which alkali delignification is a widely used method for delignifying solid wood. Alkalis, as strong bases that can react with lignin, break down its chemical structure and dissolve it from the wood fibers. The delignification efficiency of alkalis can be affected by factors such as the concentration of the alkali, temperature, and reaction time, so higher alkali concentrations, higher temperatures, and longer reaction times generally result in higher delignification rates [73,74]. Acid delignification is another commonly used method for solid wood delignification. Acids can break down the bonds between the lignin and the wood fibers, resulting in its removal. The delignification efficiency of acids can also be affected by factors such as the concentration of the acid, temperature, and reaction time. Lower acid concentrations, higher temperatures, and longer reaction times generally result in higher delignification rates [29,48,75,76]. On the other hand, oxygen-based agents, such as H₂O₂, used for solid wood delignification, can break down lignin by oxidizing its chemical bonds. The delignification efficiency of oxygen-based agents can be affected by factors such as the concentration of the agent, temperature, and reaction time, as seen in Figure 2. Higher concentrations, higher temperatures, and longer reaction times generally result in higher delignification rates [77].

Yano et al. [57] use a combination of NaClO₂ and NaOH under mild conditions to remove lignin before polymer infiltration. The NaClO₂ partly delignifies the wood and the NaOH chemical removes hemicelluloses. This procedure is performed at mild conditions and has been proven a promising way to remove lignin without affecting the strength of microfibrils; this procedure was adapted by Li et al. [78] for the manufacturing of “transparent” wood composites. The research team of Song et al. [79] performed a partial lignin and hemicellulose removal under boiling process in an aqueous solution mixture of NaOH and Na₂SO₃ to produce delignified wood following densification. This enabled lignin to better dissolve in alkaline solution by its sulfonation when the reaction time is sped up. For the production of flexible wood membranes and “transparent” wood composites the same delignification method was used by Zhu et al. [46].

The group of Segmehl et al. [56] compared acidic bleaching by a H₂O₂/CH₃COOH treatment and soda pulping with NaOH (10 wt%). They found that acidic bleaching was an appropriate method for bulk wood delignification, as the reaction is initiated at an elevated temperature, then allows an infiltration of the solution (uptake) into the wood tissue at room temperature, prior to activation. A treatment at 80 °C led to the complete removal of lignin with low dimensional stability in wet state. On the other hand, the soda pulping mainly affected the middle lamella region, while in the secondary cell wall region the aromatic content did not decrease. Also, other reactants including H₂O₂, organic solvents, ionic liquids, or deep eutectic solvents are potential alternatives to traditional pulping and bleaching delignification processes are yet to be used, but are known to promote hydrolysis [76,80,81]. However, an additional bleaching treatment in H₂O₂ had to be performed prior to matrix infiltration to obtain transparency.

Lignin has effectively been removed from wood using NaClO in an acidic environment [78,82] and Na₂SO₃ in an alkaline environment [78,82,83]. Lignin provides the strong woody mechanical qualities—this is why it is a significant adhesive for attaching cellulose fibers [84]. The mechanical properties of wood are considerably degraded once a large amount of lignin is removed. Based on this, Li et al. [27] presented a novel approach to retaining lignin while preparing “Transparent” Wood composites. They preserved 80% of the lignin components and accomplished the bleaching effect by removing only the chromogenic group in lignin with basic H₂O₂. Xia et al. [85] used H₂O₂ and UV light for the photocatalytic oxidation of lignin to selectively remove the chromogenic group of lignin while retaining its whole aromatic ring structure and considerably improving the mechanical characteristics of bleached wood templates to create a type of photonic wood.

It is important to note that the choice of chemicals and the specific conditions used for delignification can also affect the properties of the resulting wood product, as illustrated in

Table 1. Delignification systems and functions.

Delignification System	Function
ClO2	Degrades lignin’s unsaturated structure, including aromatic ring components and olefinic side chains
NaClO	Deconstructs the quinone type of lignin, as well as decomposes the aliphatic side chain
H2O2	Breaks down the ether bond in the lignin structure, producing aromatic compounds and side chains that are then oxidized into carbonyl and carboxyl compounds
Na2S	The ionized OH− and S2− break the lignin macromolecules

. For example, higher delignification rates can lead to reduced strength and stiffness of the wood. So, it is important to carefully balance delignification efficiency with maintaining the desired properties of the final product.

3.1. The Alkaline Systems for Delignification

The alkaline system induces the depolymerization of cellulose molecules, comprising the peeling reactions starting from the reducing end group and hydrolysis reactions happening at the randomly glycosidic linkage. In the alkaline system, a mixture of NaOH and Na₂S is used to break down the lignin in the wood. The process is carried out under high temperature and pressure conditions to facilitate the chemical reactions and increase the efficiency of delignification. During the process, the alkaline chemicals react with the lignin in the wood, causing it to break down and dissolve into the cooking liquor. The resulting product is then washed and bleached to remove any remaining lignin and other impurities. The alkaline system has several advantages over other pulping processes. It is highly efficient, producing a high-quality final product. It is also relatively environmentally friendly, as the chemicals used in the process can be recovered and recycled, reducing waste and pollution. In the alkaline Na₂S system, the ionized OH⁻ and S₂⁻ break the lignin macromolecules nucleophilic reactions. The thiol structure can be produced by the breaking of the α-ether bonds within the phenolic β-aryl ether unit through the use of hydroxyl and sulfite ions. Such mercaptide anion can attack the β-carbon forming the thiirane complex and eliminate the β-aryloxy group. This reaction is often referred to as a neighboring group participation reaction. In addition, the element S in the thiirane complex can further dissociate, which leads to the degradation of side chain of lignin molecules. The speed and efficiency of reactions that break down phenolic β-aryl ether bonds play a crucial role in determining the overall rate of the reaction. While the β-aryl ether bonds found in non-phenolic lignin structures that have a hydroxyl group at the α-position can also be broken down in an alkaline Na₂S system, the reaction rate is typically slow. In these lignin structures, the hydroxyl group becomes ionized under the highly alkaline conditions, resulting in the formation of an alkoxide anion that can cause the cleavage of the β-aryl ether bonds and subsequent elimination of the β-aryl substituent [86,87]. The alkaline aqueous environment induces an equilibrium between the hemiacetal and aldehyde types, and a further equilibrium between the aldehyde and ketone forms on the reducing end group of cellulose molecule chains. Once forming the keto structure, subsequent β-alkoxy elimination reaction takes place even at mild temperature. More specifically, the glycosidic bond at the 4-carbon atom is cleaved, which results in the liberated glucose unit and new reducing end unit of cellulose chains. The dissociated glucose then goes through a benzylic acid rearrangement reaction to produce iso-saccharinic acid. Simultaneously, the formed reducing end units of cellulose chains continuously undergo the same cleavage reaction, thus constructing the peeling reaction of cellulose molecule [29]. On the other hand, a dehydration reaction occurs in the aldehyde types of cellulose molecules, leading to a stable meta-saccharinic acid structure and finally stopping the degradation reaction of cellulose chain. Under high temperatures, a hydrolysis reaction of the glycosidic linkage takes place. This reaction is initiated by the ionization of the C-2 hydroxyl group, which attracts the C-1 carbon atom, eventually resulting in the detachment of the glucose unit from the cellulose chain. It is important to note that both the peeling and hydrolysis reactions cause the depolymerization of the cellulose chain, leading to a decrease in its weight and alteration of its properties [29,86,87].

The NaClO is an alternative alkaline delignification system. Lignin, with the anionic phenol and cleaved β-ether bonds owing to the existing NaOH in the system, is attacked by the ClO⁻ ions, leading to the hydrochloride ester structure of lignin. Via the liberation of ClO⁻ ions, lignin is oxidated, hydrolyzed, and further converted into the carboxyl- and carbonyl-based fragmentations, as well as CO₂. During the process of NaClO-based delignification, the chemical reactions of cellulose also take place, mainly involving the conversion of functional groups from hydroxyl into carbonyl and further carboxyl types, and the degradation of polysaccharides into oligosaccharides and related saccharic and organic acids [88].

Solid wood is typically treated with alkaline solutions, such as NaOH or Ca(OH)₂, to remove lignin and other non-cellulosic materials. The alkaline treatment helps to break down the lignin and separate the cellulose fibers, leaving behind a cleaner, more refined product. While NaClO can be used as a bleaching agent to remove residual color from the wood after delignification, it is not typically used as a primary delignification agent due to its strong oxidizing properties and potential to damage the wood fibers. As a result, solid wood is typically delignified using steam explosion or organosolv pulping (“organosolv” describes a process involving solvents as acetone, alcohols, phenols, or acids in 40%–80% concentration at 140–220 °C) or acid hydrolysis. These methods are better suited for solid wood because they can penetrate deeper into the material and effectively remove the lignin while preserving the structural integrity of the wood [89].

However, there are also some disadvantages to the alkaline system. It requires a high initial investment in equipment and infrastructure, while the process can be energy-intensive due to the high temperature and pressure requirements. Additionally, the chemicals used in the process can be hazardous if not handled properly, requiring strict safety regulations. Overall, the alkaline system is a widely used and effective method of delignification, particularly for high-quality paper products. However, it is important to carefully consider the advantages and disadvantages of the process before selecting it for use in a specific application [86,87].

3.2. The Acidic Systems for Delignification

The acid chlorite delignification is a commonly used method for delignifying lignocellulosic materials, such as wood pulp, but it is not typically used for solid wood. The reason for this is that solid wood is a more complex and heterogeneous material, which makes it difficult to control the delignification process using acid chlorite. The acid chlorite delignification process involves treating lignocellulosic materials with a mixture of NaClO₂ and an acid, typically CH₃COOH or HCl. The acid activates the NaClO₂, which reacts with the lignin in the material and breaks it down into smaller, more soluble fragments that can be easily removed. However, with solid wood, the acid chlorite delignification process is often ineffective due to the heterogeneous nature of the material. The acid and NaClO₂ may not penetrate deep enough into the wood to break down the lignin effectively, resulting in incomplete delignification and a lower quality final product.

The mechanism of acid chlorite delignification can be summarized in the following steps: (i) Activation of NaClO₂: The acid is added, which generates HClO₂ and ClO₂. The last one is a powerful oxidizing agent that can selectively react with lignin. (ii) Oxidation of lignin: The ClO₂ then reacts with the lignin in the wood, oxidizing it and breaking it down into smaller, more soluble fragments. This reaction can occur through various pathways but generally involves the cleavage of lignin’s aryl–aryl ether bonds and oxidation of the lignin monomers. (iii) Solubilization of lignin fragments: The oxidized lignin fragments are then solubilized in the liquid phase and can be easily removed from the wood fibers. (iv) Neutralization and washing: The delignified wood is then typically neutralized with a base, such as NaOH, and washed to remove any remaining acid or lignin fragments.

Thus, the exact mechanism of acid chlorite delignification can be complex and depend on several factors such as the type of acid used, the concentration of the NaClO₂ solution, and the temperature and duration of the reaction. However, the general process involves the oxidation of lignin using ClO₂ and the solubilization of the resulting fragments in the liquid phase [12,16,17,18,29,44,69,70,81,90,91].

In acid chlorite delignification, ClO₂ acts as the main active radical which forms out of NaClO₂ in acidic solution. ClO₂ oxidizes the phenolic structure by one-electron oxidation to a phenoxy radical, reducing the ClO₂ to chlorite. Then, a three-electron oxidation of the phenoxyl-radical leads to the monomethylester of the muconic acid derivative including a ring opening. In a second option, one-electron oxidation leads to the respective o-quinone structure [89]. Depending on the oxidative agent, these reactions involve nucleophilic reactions, electrophilic, radical, and oxidation reactions that also function as the effective pathways toward the in situ delignification of wood. Chemicals like ClO₂, NaClO₂, and NaClO show strong competitiveness [89]. Two chemicals, ClO₂ and NaClO₂, are known for their ability to selectively break down lignin while preserving most of the cellulose and producing fewer chlorinated compounds. ClO₂ is particularly effective at breaking down unsaturated structures in lignin, such as aromatic ring moieties and olefinic side chains. The reaction begins with the transfer of a phenolic hydroxyl group to form a phenoxy radical, which then reacts with ClO₂ to form hypochlorous esters. Hydrolysis then occurs at the aromatic ring moieties, producing HClO and a muconic acid structure. The C–C double bond of the olefinic side chains is cleaved to form epoxide units, which are then oxidized along with the muconic acid structure to degrade lignin into smaller molecules. Non-phenolic structures can also undergo similar reactions with ClO₂, but at a much slower rate. Additionally, ClO₂ can cause fragmentation of the benzyl alcohol structures in lignin by disrupting the bonds between the aromatic rings and side chains [12,16,17,18,29,44,69,70,74,77,81,89,90,91].

3.3. Other Systems for Delignification

Delignification has been carried out using harsh chemicals such as Cl₂ and NaOH. However, these chemicals are not environmentally friendly and can generate harmful byproducts [5,80,81]. Instead, delignification of solid wood may also be achieved through other methods such as steam explosion, organosolv pulping, or acid hydrolysis. These methods involve treating the wood with steam, organic solvents or acid solutions to remove lignin, and other non-cellulosic materials, leaving behind a purified cellulose product. Several “green” chemicals have been developed to replace these harsh chemicals for delignification. Some examples of this category are as follows:

• Ionic Liquids: Ionic liquids are salts that exist as liquids at room temperature and are effective in the delignification of plant material. They are considered “green” solvents because they have low toxicity, are non-volatile, and can be recycled [76].
• Peroxides: Peroxides, such as H₂O₂ and Na₂O₂, can be used for delignification. They are environmentally friendly because they break down into H₂O and O₂ and they do not generate toxic byproducts [55].
• Organic Acids: Organic acids such as CH₃COOH, HCHO, and citric acid can be used for delignification. They are considered green chemicals because they are naturally occurring, biodegradable, and non-toxic.
• Enzymes: Enzymes, such as laccase, manganese peroxidase, and lignin peroxidase, can be used for delignification. They are considered “green” as well because they are biodegradable, have low toxicity, and can be used under mild conditions.

Overall, the use of “green” chemical systems for delignification can reduce the environmental impact of the process while still achieving high levels of delignification [5,80,81]. However, it is worth noting that the use of NaClO can also have negative environmental impacts if not handled properly, as it can generate toxic byproducts and harm aquatic life if released into waterways. Therefore, alternative bleaching agents, such as H₂O₂ or O₃, are increasingly being used in the industry to minimize environmental impacts [12,16,17,18,29,44,69,70,74,77,81,90,91].

3.4. Oxygen-Based Agents as Delignification Systems

In delignification procedures of peracetic acid and CH₃COOH/H₂O₂—an environmentally friendly chemical because the decomposition products of H₂O₂ are H₂O and O₂ —hydroxonium ions (HO⁺) are used as a reactive component. The two common delignification procedures that use CH₃COOOH or CH₃COOH/H₂O₂ act as follows: preparation of solutions by mixing H₂O₂ and CH₃COOH in a suitable ratio, treatment of lignocellulosic material, and maintenance at a suitable temperature for a specific period. During this time, the acid reacts with lignin and breaks down its chemical structure, resulting in the removal of lignin from the material. After treatment, the material is washed thoroughly with H₂O to remove any remaining acid. Then, it is neutralized with an alkaline solution to stop the delignification process.

Both of these delignification procedures are effective in removing lignin from lignocellulosic materials and can be used in various industrial applications such as pulp and paper production, biofuel production, and food and beverage industries [12,16,17,18,29,44,69,70,74,77,81,90,91].

Peroxy acid oxidation is a chemical process used to remove lignin from wood and other plant materials. The process involves the use of a peroxy acid, such as CH₃COOOH or H₂SO₅, to oxidize and break down the lignin. The main reactions that occur during peroxy acid oxidations include the following: (i) formation of peroxy acids by the reaction of H₂O₂ with CH₃COOH or H₂SO₄; (ii) attack on lignin when breaking down the aromatic rings and creating reactive intermediates; (iii) chain reactions, producing additional reactive intermediates and breaking down the lignin into smaller fragments; (iv) next hydrolysis when fragments form water-soluble compounds, which can be easily removed from the wood; (v) side reactions that occur, such as the oxidation of hemicellulose and cellulose. It is important to carefully control the reaction conditions to ensure that the desired reactions occur and to minimize any side reactions. The electrophilic property of HO⁺ is due to the heterolytic breakage of the peroxy bond. As a result, it interacts with the various electron-rich sites in lignin, such as the aromatic ring or the olephinic side chains.

It should be noted that the hydroxyl groups found in wood polysaccharides do not afford adequate reaction sites for the H₃O⁺ ion, and hence carbohydrates remain mostly undegraded. To be more exact some active anions and radicals easily form from the H₂O₂ molecules, like HOO⁻ ions, O₂^−•, and HO^• radicals. HOO⁻ ions react with lignin by cutting the bonds between aromatic rings and side chains and by breaking unsaturated bonds in lignin molecules’ side chains (such as carbonyl groups and olefine aldehyde structures), resulting in the production, and subsequent deconstruction of oxirane radicals into tiny molecular aliphatic compounds [15]. Meanwhile, the aromatic compounds are targeted by HOO⁻ ions to build epoxide radicals, which are then oxidatively degraded into the final products, which are mostly constituted of carbonyl and carboxylic acids. HO ions, HO^•, and O₂^• radials, contribute to the oxidation, hydrolysis, and destruction of lignin toward the delignification of wood [5]. The breakup of lignin macromolecules and the decomposition of lignin-linked types show that these chemicals are effective at in situ delignification to alter the structure of the wood at the micro- and nanoscale. It should be noted that transition metal ions cause the breakdown of H₂O₂ to generate O₂, which reduces the overall rate of delignification. Ethylenediamine-tetra-acetic acid and diethylenetriamine-penta-acetic acid are commonly added into the H₂O₂ delignification system to inhibit metal catalyzed H₂O₂ breakdown. MgSO₄ and Na₂SiO₃, on the other hand, can act as stabilizers, slowing the breakdown of H₂O₂ and, as a result, increasing the pace and efficiency of delignification. As a result, in partial delignification, tunable removes the lignin from wood (resulting in a more porous, softer structure of a partially delignified wood, with well-preserved original hierarchical wood structure). Near-complete delignification refers to a control lignin level of less than 1.5 wt% [15], which makes cellulose more accessible while preserving the hierarchical structure of wood [15].

3.5. Other Conditions Affecting Treatment

The temperature and duration of the delignification process can affect the efficiency of lignin removal. Generally, higher temperatures and longer reaction times can increase the rate of delignification but may also lead to the greater degradation of the cellulose and hemicellulose components of the wood [5,12,19,29,44,69,70,74,75,77,81,91,92]. In all cases (alkali, acidic, and oxygen-based delignification), too high temperatures may result in the degradation of the cellulose and hemicellulose components of the wood, leading to the reduced strength and stiffness of the final product. The optimal temperature for alkali delignification depends on the specific alkali used but is typically in the range of 80–120 °C [74]. The optimal temperature for acid delignification depends on the specific acid used but is typically in the range of 60–100 °C [74]. The optimal temperature for H₂O₂ delignification depends on the specific conditions used but is typically in the range of 40–80 °C. Overall, the choice of temperature for solid wood delignification depends on the specific conditions used and the desired properties of the final product. The careful optimization of temperature, along with other parameters, such as chemical concentration and reaction time, is necessary to achieve efficient and effective delignification without causing significant degradation of the wood [5,12,19,29,44,69,70,74,75,77,81,91,92].

Lignin can be categorized as either glassy, stretchy, or molten. In the glassy state, only certain parts of the molecule can move. As temperature rises, the internal energy of the polymer increases, allowing individual segments to deform and move, but not altering the overall location of the molecule’s center of mass. This state is highly adaptable and can undergo reversible deformation. At higher internal energy, the molecules can move relative to each other, causing the polymer to flow and enter a plasticized state. The polymer transitions between these states at specific temperatures. Although polymers do not have a defined melting point, they can display a wide range of physical properties depending on the conditions they are in. When moist lignin is present in wood, it softens at around 100 °C, allowing the molecules to deform in the cell walls. If the temperature of the cooking process is reduced to 90 °C, the amount of lignin acquired will decrease. Delignification at high temperatures can also remove organic solvents, ethers, and C–C bonds present in lignin [5,27,35,62,93].

Several factors can influence the temperature required for lignin to undergo a glass transition phase, including the presence of rigid phenolic side groups on the main chain, cross-linking, the number of bonds between chains, H bonds, MW, isolation method, species, lignin conformation, and thermal prehistory [94,95,96]. When dry, the difference in T_g temperature of cellulose, hemicellulose, and lignin is not significant, with values ranging from 200 to 250 °C for the amorphous region of cellulose, 150 to 220 °C for hemicelluloses, and 205 °C for lignin [97]. However, these values are difficult to confirm due to hemicellulose and lignin degradation near these temperatures [98,99]. Lignin appears to be the most thermally stable component of wood, but various changes still occur below 200 °C [100]. Lignin decomposes over a broader temperature range (200–500 °C) than cellulose and hemicellulose. Degradation studies on various types of lignin have shown an endothermic peak at 100–180 °C, resulting from the elimination of humidity, followed by two exothermic peaks, the first from 280 to 390 °C and the second around 420 °C [101].

Moreover, the delignification process is affected by various factors, including the type of wood, the type of delignification process used, and the conditions under which the wood is stored before treatment. Storage conditions can affect delignification by influencing the chemical composition and physical properties of the wood. For example, if wood is stored in a wet environment, it can absorb water. Wet wood may also contain more extractives, such as tannins and natural resins, which can interfere with the delignification process. Conversely, if wood is stored in a dry environment, it may become more brittle and prone to cracking, which can also affect the process. Temperature and humidity can also affect the chemical composition of the wood, as well as the growth of fungi and bacteria that can degrade the wood and weaken it, by making it more difficult to pulp. In general, it is important to store wood in a controlled and well-ventilated area, avoiding exposure to direct sunlight or moisture [5,12,15,16,17,29,41,44,58,59,69,70,75,77,81,102].

Moisture content is one of the most important factors that affects the delignification of solid wood. Moisture content is calculated on the dry mass of wood, at each circumstance. The amount of moisture in the wood can influence the rate and efficiency of delignification, as well as the quality of the resulting product. When wood is delignified, water and chemicals are used to break down the lignin. If the wood is too wet, the excess water can dilute the chemicals and slow down the delignification process. Wet wood may also absorb more chemicals than necessary, which can increase the cost of the process. On the other hand, if the wood is too dry, it may be more difficult to break down the lignin. Dry wood may also be more brittle and prone to cracking, which can create problems. Therefore, the moisture content of the wood should be carefully controlled to optimize the delignification process. Typically, the optimal moisture content for delignification is between 25 and 60%. This can be achieved by storing the wood in a controlled environment with a relative humidity of 40%–60%. To achieve the optimal moisture conditions for delignification, the wood can be conditioned before treatment. Conditioning involves adjusting the moisture content of the wood to a specific level by soaking or steaming the wood in water or by drying it to a desired moisture content. The conditioning process can help to ensure that the wood is at the ideal moisture content for efficient and high-quality delignification. Overall, it is important to carefully manage moisture conditions during the processing of solid wood to ensure efficient and high-quality delignification [5,12,15,16,17,29,41,44,58,59,69,70,75,77,81,102].

The time period during which the wood is exposed to the delignifying chemicals is another critical parameter that can significantly affect the rate and efficiency of delignification in solid wood. Generally, longer exposure times can lead to more complete delignification, but there are limits to the time that can be used before it causes significant degradation of the wood. In alkali delignification, longer exposure times can increase the reaction rate between the alkali and lignin, resulting in a more complete delignification process. However, too long an exposure time can also lead to degradation of the cellulose and hemicellulose components of the wood, leading to reduced strength and stiffness of the final product. The optimal exposure time for alkali delignification depends on the specific alkali used but is typically in the range of 1–6 h. In acid delignification, longer exposure times can also increase the reaction rate between the acid and lignin, resulting in a more complete delignification process. However, too long an exposure time can lead to significant degradation of the wood and a decrease in the strength and stiffness of the final product. The optimal exposure time for acid delignification depends on the specific acid used but is typically in the range of 1–3 h. In oxygen-based delignification, such as H₂O₂ delignification, longer exposure times can also increase the reaction rate between the peroxide and lignin, resulting in a more complete delignification process. However, too long an exposure time can lead to significant degradation of the wood and reduced strength and stiffness of the final product. The optimal exposure time for H₂O₂ delignification depends on the specific conditions used but is typically in the range of 4–24 h. Overall, the choice of exposure time for solid wood delignification depends on the specific conditions used and the desired properties of the final product. Careful optimization of exposure time, along with other parameters, such as chemical concentration and temperature, is necessary to achieve efficient and effective delignification without causing significant degradation of the wood [5,12,15,16,17,29,41,44,58,59,69,70,75,77,81,102].

The pH of the delignification reaction can affect the efficiency of the process and various pHs may be applied. Table 2 provides a great range of details on that.

4. Characteristics of Wood (And Its Interactions)

4.1. Structure (Vessels, Fiber Cells, and Tracheid)

The macroscale trunk of trees is constituted of cells (vessels, fiber cells, and tracheid), which are composed of cellulose fibrils, hemicelluloses, and lignin. The structure changes depending on the wood species, which are classified as softwoods or hardwoods [103]. Tracheids are the primary constituents of softwood (∼90%−95%), providing multiple functions, such as transferring H₂O and nutrients mainly by thin-walled tracheids, as well as providing mechanical strength mainly by thick-walled tracheids. The tracheid wall consists of a middle lamella, a primary wall, a multilayered secondary wall, and open lumen gaps. The thickness of the cell walls varies with cell type (i.e., tracheids and parenchyma cells) and wood species, as do the thickenings and depositions of the growth rings. Middle lamella is an intercellular amorphous zone that largely sits in the middle of cell walls. Middle lamella is formed of pectic chemicals but progressively becomes highly lignified; lignin plays a significant role in cell wall structural integrity as a cementing component [104]. The outermost layer of the main cell wall of tracheids is relatively thin (0.1–0.2 m), yet it is stronger in nature to withstand the tensile forces arising from turgor pressure, extensible to allow wall stress relaxation, which motivates cell water uptake and physical enlargement of the cell [105]. The secondary wall of softwood tracheids consists of three distinct layers (S1, S2 and S3) with highly ordered cellulose microfibrils. The S2 layer is regarded as the most critical layer for delivering structural properties to a living tree. It is 5–10 μm thick and 70%–90% of the total cell volume. It is also responsible for the strength properties of wood since it carries most of the axial loading in softwood tracheid. Pits (small gaps in the walls of neighboring cells) allow H₂O to travel between the tracheids. In many species, axial and radial parenchyma occupy more than 50% of the volume. Softwood tracheid pits are bordered, which indicates the secondary wall overarches the pit membrane [104].

Hardwoods, on the other hand, are mostly made up of vessels and fiber cells. Hardwood fibers are exclusively responsible for mechanical support. Hardwood fibers usually are 0.2–1.2 mm long and have lower lumens than softwood tracheids. The cell walls of hardwood fibers are thicker than those of softwood and earlywood tracheids. This is because the fibers do not transport H₂O, and hence, their pits are less in size as compared to softwood. Some hardwood fibers differ from softwood tracheids in their capacity to create an additional gelatinous wall layer in place of a regular S3 layer [106]. The cell wall structure of hardwood vessels differs from that of softwood tracheids and hardwood fibers (Figure 3). To some extent, softwoods and hardwoods have differing wood structures and chemical components. Certain hardwoods, such as fast-growing poplar, have a lower density and a looser structure than softwoods due to large-diameter fiber cells. Because of this structural difference, many hardwoods are very reactive and easily modified by chemicals. Furthermore, most hardwoods have a reduced lignin concentration. Despite the structural and compositional differences mentioned above, softwoods and hardwoods share compositional and structural characteristics (i.e., hierarchically porous structure and consisting of cellulose, hemicellulose, and lignin) [106].

The units of phenylpropane in lignin form chains that are cross-linked in an amorphous, three-dimensional structure, and connected to cellulose fibrils via hemicelluloses. The chemical structure of lignin’s monomer units allows it to be categorized at three types of lignin: grass lignin, softwood lignin, and hardwood lignin, depending on the arrangement of guaiacyl, syringyl, and p-hydroxyphenylpropane units. The lignin content varies across species as well as between particular tissues such as bark, earlywood, latewood, normal wood and compressed wood, branch wood, also by cell type (parenchyma or fibers) and cell wall layers, such as the middle lamella, main, and secondary wall layers and cell corners. The compound middle lamella and cell corners contained the highest quantities of lignin, whereas secondary cell walls contained the lowest concentrations.

Factors like the delignification degree and the choice of the wood species, influence the porosity of the delignified wood. Softwoods with an alternating latewood–earlywood pattern, for example, show bands of different strength and flow conductivity, which can lead to desired modifications in functional or mechanical properties. Flow-through devices for filter or membrane applications, however, are often based on hardwoods. Their long pipes for water transportation in the wood tissue, so-called vessels, can be better utilized for the directed flow [20].

Due to the anisotropy of wood (Figure 3), bulk wood delignification is intrinsically inhomogeneous, with uneven chemical changes. As a result, it is critical to monitor lignin distribution in the cell wall and gain a better understanding of the local property changes in delignified and changed wood. Since lignin fluoresces over a wide range of wavelengths, wood cell walls are naturally luminous due to the presence of lignin, the location of which may be determined using confocal laser scanning microscopy [37,40,73]. The wood cell wall structure can significantly affect the delignification of solid wood. The wood cell wall is composed of three main layers: the middle lamella, the primary cell wall, and the secondary cell wall. The secondary cell wall, which is the thickest and most complex layer, contains the majority of the lignin in wood [12,19,20,34,35,91,107,108].

The following are some ways in which the wood cell wall structure can affect the delignification of solid wood:

• Lignin distribution: The distribution of lignin within the wood cell wall can affect the efficiency of the delignification process. For example, if the lignin is mainly concentrated in the middle lamella and primary cell wall layers, it may be more easily removed than if it is mainly concentrated in the secondary cell wall layer.
• Lignin content: The amount of lignin present in the wood cell wall can also affect the delignification process. High lignin content can make the wood more difficult to delignify and may require more severe delignification conditions to achieve the desired level of lignin removal.
• Lignin composition: The composition of the lignin in the wood cell wall can affect the delignification process. For example, lignin that is more condensed and cross-linked may be more difficult to remove than lignin that is more linear and less condensed.
• Cell wall structure: The overall structure and composition of the wood cell wall can also affect the delignification process. For example, wood with a high proportion of hardwood fibers, which have a more complex and rigid cell wall structure than softwood fibers, may require more severe delignification conditions to achieve the desired level of lignin removal.

The wood cell wall structure plays a crucial role in the delignification of solid wood. By understanding the properties and characteristics of the wood cell wall, it is possible to optimize the delignification process and achieve the efficient and effective removal of lignin from the wood.

4.2. Moisture Content and Drying Conditions

Numerous researchers have investigated how moisture content affects the T_g of extracted or in situ hemicelluloses and lignin. Moisture content has a significant impact on the T_g, with several studies reporting a decrease in T_g as moisture content increases. This is because the structure of lignin contains phenolic hydroxyl, which can create H bonds between molecules. H₂O molecules can break these bonds, leading to increased segmental motion. The amount of water bound to lignin is influenced by the number of OH groups. The discrepancies in the results of various studies could be due to differences in moisture conditions, such as equilibrium and non-equilibrium moisture levels during testing. For instance, while some researchers maintained strict moisture control during temperature changes, others only conditioned the samples to a specific initial moisture content without regulating it during the tests [27,93].

Han et al. [72] investigate the way of each other interaction aligned cellulose fibrils by hydrogen bonding to generate compressed ultra-strong bulk materials based on delignified wood. To modify H bonding formation, they used three different drying processes (air-drying, solvent exchange-drying, and freeze-drying). They also looked at the impact of drying conditions on the mechanical characteristics and H₂O vapor sorption behavior of manufactured bulk materials. The air-dried sample lost 62% of its volume, had a density of 0.5 g/cm³, and had a porosity of 66.1%. The solvent exchange-dried sample lost 20% of its volume, had a basic density of 0.22 g/cm³, and 85.5% porosity. The freeze-dried sample exhibited very different properties and experienced a volume increase of ~15%, had a density of 0.15 g/cm³, and ~90% porosity. Due to air-drying, the aligned nanocellulose structure of delignified wood collapsed. Furthermore, freeze-drying preserved the structural integrity of the delignified wood as well as the porosity structure created as a result of lignin removal. Moreover, volume change was caused by different drying methods [72]. After drying, unbound OH groups create H-bonds with one another, allowing the delignified wood to be shaped. Because of the intricate nanofibril structure, the fibril aggregates in delignified wood retain their cell configurations after delignification. The OH groups have weak interactions immediately after delignification, so the cellulose is easily distorted during wet pressing conditions, and after the H₂O is removed from the densified wood, the cellulose fibrils form permanent H-bonds [72].

4.3. Chemical Structure

The chemical structure of lignin affects its delignification from solid wood. Lignin is a complex and heterogeneous polymer made up of three main units: p-hydroxyphenyl, guaiacyl, and syringyl that have different chemical structures and properties. The delignification process involves breaking the bonds between the lignin and the cellulose fibers in the wood. So, the structure of lignin influences how easily these bonds can be broken and how efficiently the lignin can be removed. For example, the presence of syringyl units in the lignin structure can make it more difficult to remove, as they have more ether bonds and are more resistant to chemical reactions than other units. Additionally, lignin that is highly condensed or cross-linked may be more difficult to remove than less condensed or cross-linked lignin. The chemical structure of the wood itself can also affect its delignification. For example, softwoods such as pine and spruce have a higher proportion of guaiacyl units in their lignin compared to hardwoods, which have a higher proportion of syringyl units. This can affect the efficiency of delignification and the properties of the resulting product.

Overall, the chemical structure of lignin and wood can have a significant impact on the delignification process. It is important to carefully consider the properties of the wood and lignin, as well as the specific pulping process being used, to achieve the desired results [27,93].

4.4. Species and Dimensions

The size and shape of the wood particles can affect the delignification process. Smaller particles may have a larger surface area and may be more susceptible to delignification, while irregularly shaped particles may have areas that are more difficult to access.

Different species of wood origins have varying levels of lignin content, which can affect the delignification process. Generally, woods with higher lignin contents are more difficult to delignify. Softwoods and hardwoods have different structures and chemical compositions, with softwoods containing more lignin and having higher glass transition temperatures (138–160 °C) compared to hardwoods (110–130 °C) under dry conditions. Hardwood lignins have a lower T_g range and a higher content of methoxyl groups than softwood lignins and they are also less cross-linked [109]. However, the modification of lignin can alter its softening behavior, with sulfonation lowering T_g and lignin esterification decreasing it further. Research on genetically modified aspen trees with reduced lignin content and/or increased syringyl/guaiacyl ratio suggests that the higher content of methoxyl groups and less cross-linked lignin did not alter the softening behavior of lignin [110]. The modification of lignin can also shift the T_g. Hemicellulose has a high softening temperature under dry conditions, but H₂O acts as a plasticizer and decreases the T_g temperature. Elevated temperature softens both hemicellulose and lignin and pressure affects the T_g. Lignin softening is important in several industrial processes, such as pellet manufacturing, binderless panel manufacturing, wood welding, wood bonding, wood surface compacting, and veneer manufacturing by peeling. These processes involve other physical and chemical reactions that contribute to the result [94,95,96,111].

Various wood species have been used as a material for delignification according to the desired applications of produced delignified wood. For example, when delignified wood is utilized to develop “transparent” wood, the most often delignified wood species have been balsa wood (Ochroma pyramidale), different poplar species, basswood species (Tilia), beech wood, and pine wood. Moreover, when delignified wood is utilized to develop bulk wood, birch (Betula spp.), spruce, basswood species (Tilia spp.), oak (Quercus spp.), poplar (Populus spp., Thuja plicata, Pinus strobus), spruce and hoop pine are applied, as seen in Table 2.

Finally, certain substances present in the wood, such as extractives, can inhibit the delignification process. Pretreatment steps, such as washing or solvent extraction, may be necessary to remove these inhibitors before delignification. Overall, the delignification process is complex and dependent on a variety of factors and optimizing the process requires careful consideration of these.

Table 2. Delignification’s methods described in the literature with experimental details such as wood species, dimensions, delignification process, lignin content, further modification, and application (2017–today).

Ref.	Wood Species	Sample Dimensions	Chemicals/Delignification Conditions	Moisture/Storage Conditions	Lignin Content (Initial → Final)	Post-Delignification Process	Application
Van Hai et al. [112]	Pinus koraiensis	150 × 50 × 15 mm3	5-step treatment: (i) 1% NaClO2 at 80 °C for 12 h (wash 2 h, water bath 1 d, oven-dry at 40 °C for 1–2 d); (ii) 2% NaClO2 at 80 °C for 12 h; (iii) Repeat step i; (iv) 0.1% NaOH suspension for 1 d (wash, water bath, oven-dry); (v) Repeat step (i)	Pre-dried in oven at 103 °C; post-densification stored in desiccator (15% RH, 23 °C)	30.3% (native) → progressively reduced to 5%	Hot pressing at 13 MPa (4 h), 26 MPa (8 h), 52 MPa (16 h) at 100 °C	Bulk wood
Mai et al. [113]	Balsa	15 × 15 × 5 mm	Boiling in 2.5 M NaOH and 0.4 M Na2SO3 at 100 °C for 12 h; then treatment with 30% H2O2	–	–	In situ polymerization (using CTAB, CaCl2, NaCl, HMA, AAM, KPS, TEMED; aged at 50 °C for 24 h)	Ultra-flexible flame-retardant composites
Gao et al. [48]	Balsa	Length: 20 mm; Diameter: 2–5 mm	Soaking in a solution of 50 wt% glacial CH3COOH and 50 wt% H2O2; cooked 10–12 h at 80 °C until white and soft	Solvent exchange (acetone) to replace moisture	Treated fiber: lignin reduced to ~8%; cellulose increased to ~59%	–	“Transparent” wood composite
Samanta et al. [114]	Balsa and Birch	Veneers: 20 × 20 × 1 mm or 200 × 100 × 1 mm	Immersion in acetate buffer (pH ≈ 4.6) containing 1.0 wt% NaClO2 at 80 °C until color changes from brown to white; then thoroughly washed; solvent exchanged to EtOH and acetone under vacuum	–	–	Infiltration with melamine–formaldehyde resin under vacuum for ≥24 h	Fire-retardant “transparent” wood composite
Sun et al. [49]	Balsa (Ochromapyramidale)	50 × 50 × 1 mm (L × R × T)	Immersion in 1 wt% NaClO2 solution (buffered with CH3COOH, pH 4.6) at 80 °C for 12 h	–	–	TEMPO-mediated oxidation, in situ lignin deposition and mechanical hot pressing	Reconstructed wood with high strength, water resistance and excellent optical properties
Liang et al. [91]	Poplar	50 × 50 × 5 mm	Steam treatment with a 1:1 mixture of 30% H2O2 and glacial CH3COOH at 160 °C for 2–8 h (vapor phase)	Pre-dried in oven at 60 °C; post-treatment: freeze-dried at −60 °C overnight	26.7% → 1.45%	Vacuum-assisted resin transfer molding with bisphenol A-type epoxy resin; densification at 23 MPa, 100 °C for 1 h	Porous material with improved compressibility
Ruan et al. [115]	Basswood	–	NaOH (≥96%), Na2SO3 (≥97%), H2O2 (30%), and absolute EtOH in ratio 1:0.16:20; specimens dried under supercritical CO2 (101 bar, 5 L/min)	–	–	Immersion, boiling for 5 h, ultrasonic cleaning; then treated in H2O2 and boiled; immersed in EtOH at 60 °C for 12 h	–
Han et al. [59]	Poplar (Populus)	20 × 20 × 5 mm	Treatment with NaClO2 and glacial CH3COOH at 80 °C for 18 h (using CH3COOH, pH 4–5)	Post-treatment: slices frozen at −20 °C for 12 h, then freeze-dried for 24 h	–	Pyrolysis at 1000 °C for 2 h under argon	Hydrophobic, porous, flame-resistant lignocellulosic carbon material
Zou et al. [22]	Birch (Betula spp.)	120 × 80 × 0.7 mm (rotary-cut)	Immersion in 2.5 M NaOH and 0.4 M Na2SO3 at 100 °C for 2 h; then immersion in boiling H2O2 for 30 min	Dried in oven at 103 °C until moisture reached 5%; densified samples cooled to 20–30 °C	Reduced from 19.8% to 11.6%	Infiltration with phenol–formaldehyde resin; densification pressing at 150 °C, 15 MPa for 15 min	Structural applications
Niu et al. [42]	Poplar	Not specified (wood tubes molded)	Treated with NaClO2 and CH3COOH; tubes poured into mold, frozen, then freeze-dried at −50 °C	–	Removal ~56 ± 5%	Pressing under 20 MPa, followed by carbonization at 600, 800, 1000, 1200 °C	Energy storage applications
Wu et al. [116]	New Zealand pine and Basswood	Veneers: 20 × 20 × 0.50 mm	Treatment with MMA, NaClO2, EtOH, glacial CH3COOH, NaOH and AIBN; samples dried at 103 °C for 24 h and stored in EtOH	–	New Zealand pine: 27.64% → 22.80%; Basswood: 23.04% → 19.11%	MMA polymerization and impregnation	“Transparent” wood composite
Wang et al. [12]	Poplar tree	100 × 20 × 10 mm	Delignification using aq. NaOH (0–6 wt%) and 50 wt% MA at 155 °C for 30 min in a 1 L bomb reactor	After densification: dried in a climate chamber at 20 °C, 65% RH for 2 weeks	–	Acid hydrotropic delignification using MA hydrotropic fractionation at 100 °C for 30 min; densification from 10 mm to 8 mm at 1.0 MPa, 15 min at 150 °C	Broad applications
Liu et al. [21]	Balsa (Ochromalagopus Swartz)	40 × 20 × 2 mm	Immersion in 2.5 M NaOH and 0.4 M Na2SO3, boiled for 36 h	–	–	Impregnation with solid–solid phase change materials	Energy-saving building materials
Yang et al. [70]	Populus Euramericana	50 × 10 × 3 mm (L × T × R)	Hot H2O extraction with ethanol:benzene (1:2 v/v) for 48 h, then water bath at 60 °C for 3 h; delignification under −0.1 MPa vacuum for 5 h (mixture: 483.5 mL H2O, 10 g NaClO2, 6.5 mL CH3COOH); water bath at 40 °C for 30 h	Air-dried 48 h, then vacuum-dried at 80 °C until constant weight	Native wood: 22.89%; delignification rates: 1L = 12.81%, 2L = 39.12%, 3L = 52.24%	–	Investigation of dynamic mechanical and sorption behavior
Foster et al. [55]	Balsa	100 × 100 mm	Two approaches: (a) Chlorite-based: 1 wt% NaClO2 in 1 N acetate buffer (pH ≈ 4.6) at 80 °C for 8–12 h; (b) Peroxide-based: Solution with 3 wt% sodium silicate, 3 wt% NaOH, 0.1 wt% MgSO4, 1 wt% EDTA, 4 wt% H2O2 at ~70 °C for ~2 h	–	Lignin content: chlorite: 11.4%; peroxide: 20.3% (vs. natural 24.8%)	Acetylation and methacrylation of delignified wood	Modified wood for enhanced properties
Liang et al. [19]	Hybrid poplar clones	2 × 2 cm (cross-section)	Treatment in a 1:1 mixture of 30% H2O2 and glacial CH3COOH; boiled at 40 °C for durations of 0, 1, 6, 10, 32, 34 h	Pre-dried in oven at 60 °C; post-freeze-dried at −60 °C overnight	Reduced from 27.3% to 22.6%, 14.8%, 5.6%, 1.40%, 0.2% with increased time	Vacuum-assisted resin transfer molding with bisphenol A-type epoxy resin; densification via hot pressing at 23 MPa, 100 °C for 1 h	Porous material with improved compressibility
Chen et al. [73]	Balsa	30 × 30 × 1 mm	Immersion in 2 wt% NaClO2 in CH3COOH (pH 4.6) for 2 h; preserved in EtOH; then immersed in 15 wt% NaOH at room temperature for 2 h	–	–	In situ chemical polymerization with polyacrylamide	Strong, flexible hydrogel reinforced by wood skeleton
Wu et al. [116]	New Zealand pine and Basswood	Veneers: 20 × 20 × 0.50 mm	Treatment with MMA, NaClO2, EtOH, glacial CH3COOH, NaOH and AIBN; samples dried at 103 °C for 24 h and stored in EtOH	–	New Zealand pine: 27.64% → 22.80%; Basswood: 23.04% → 19.11%	MMA polymerization and impregnation	“Transparent” wood composite
Jakob et al. [58]	Spruce veneer	50 × 50 mm	Soaking in a 2:1 EtOH–DI H2O mixture with 1.5% CH3COOH in a 5 L pressure reactor at 170 °C, 14 bar for 180 min (heating 75 min, cooling 120 min); or immersion in 0.4 M Na2SO3 and 2.5 M NaOH at 98.5 °C for 240 min	Stored in a climate chamber at 20 °C and 65% RH	–	Densification at 120 °C under 20 MPa for 15 min (held overnight)	Densified plywood
Jakob et al. [17]	Spruce	100 × 50 × 1.47 mm (axial × radial × tangential)	20 veneers immersed in 1 L solution of 2.5 M NaOH and 0.4 M Na2SO3 at ~119 °C (0.19 MPa) for 4 h; repeatedly washed	After cooling (to 60 °C), equilibrated at 20 °C and 65% RH for 2 weeks	–	Veneers densified in tangential direction in a hot press at 120 °C, 35 MPa for 7.5 min	Load-bearing applications
Mi et al. [117]	Balsa	100 × 50 × 0.8 mm	Bleaching in 5% NaClO solution for 3, 8, 12, 24 h at room temperature; rinsing with DI H2O	Samples freeze-dried, then dried in a hot oven at 60 °C for 48 h	Final lignin content ~0.8%	Infiltration with poly(vinyl alcohol) solution (8 wt% at 90 °C) with degassing under 200 Pa	“Transparent” wood composites and thermal insulators
He et al. [40]	Basswood	20 mm thickness	Immersion in 2.5 M H2O2 (in DI H2O and EtOH); boiled until wood turned white; rinsed in EtOH/H2O three times	Preserved in EtOH/H2O overnight and air-dried	Final lignin content ~0.6%	Partial densification by hot pressing (thickness reduced by half, density ≈ 0.7 g/cm3)	Bulk materials via stacking delignified blocks
Mania et al. [118]	Poplar (Populus alba L.) and Birch (Betula pendula)	20 (T) × 30 (R) × 20 (L) mm	Immersion in boiling 2.5 M NaOH and 0.4 M Na2SO3 solution for 7 h at 110 °C	–	Birch: 21.6% → 17.2%; Poplar: 25.2% → 16.6%	Plasticization in boiling water (1 h); then tangential pressing with clamping	Partial delignification and densification
Fang et al. [119]	Pinewood	100 × 35 × 3 mm	In a 1 L autoclave: 5 g NaOH, 15 g Na2SO3, 0.03 g anthraquinone dissolved in 600 mL of 20 wt% MeOH solution (MeOH/H2O = 1:4, wood/solution = 1:20); processed for 1–7 h at 170 °C; quenched	–	–	Immersed in 20 wt% MeOH solution for 24 h; pressed at 8 MPa for 3 h, then at 90 °C for 3 min	Super-strong nanocellulose films
Li et al. [5]	Basswood	Various thicknesses: 0.8, 5, 40 mm; sizes: 210 × 190 mm, 100 × 50 mm, 50 × 50 mm	H2O2 (30 wt%) steam delignification: steamed until yellow color disappeared (≈2–12 h); rinsed with ultrapure H2O and EtOH	Dried at 105 °C for 24 h pre-treatment	Final lignin contents: 0.84, 0.96, 0.94 (approx.)	Infiltration with MA and epoxy polymer	“Transparent” wood composites
Khakalo et al. [120]	Birch	Veneers: 1.5 ± 0.1 mm; Dimensions: 10 × 10 cm	1 wt% NaClO2 in acetate buffer (pH 4.6) in 3 L at 80 °C for 12 h	Stored at 23 °C and 50% RH	Final lignin content: 8.6%	Infiltration with ionic liquid ([EMIM]OAc) activated at 95 °C; hot pressing at 5 MPa for 16 h at 100 °C	Bulk hot-pressed high-performance wood via ionic liquid treatment
Wu et al. [44]	Basswood (Tilia)	Veneers: 20 × 20 × 0.42 mm	2 wt% NaClO2, 0.1 wt% glacial CH3COOH, 97.9 wt% ultrapure H2O; treated in water bath at 80 °C (40 rpm) for 30–150 min	Dried at 103 °C for 24 h pre-treatment; then stored in EtOH	Lignin content decreases gradually (e.g., from 24% to 9%)	MMA polymerization and impregnation	“Transparent” wood composites for optical applications
Yang et al. [121]	Poplar	Not specified	Hot water extraction using EtOH:benzene (1:2 v/v) for 48 h; water bath at 60 °C for 3 h; then delignification: 967 mL water, 20 g NaClO2, 13 mL CH3COOH under −0.1 MPa vacuum for 5 h; water bath at 40 °C for 30 h	–	–	Wood furfurylation	Wood furfurylation
Han et al. [72]	Basswood	50 × 50 × 10 mm	Bleaching in 2 wt% NaClO2 (buffered with CH3COOH at pH 3.5) for 12 h at 70 °C (repeated 3 times), followed by treatment in 5 wt% NaOH at 90 °C for 7 h	Pre-dried at 105 °C for 12 h; equilibrated in desiccator (saturated K2SO4, ~97.6% RH at 20 °C)	Final lignin content: 2.3%	Densification along radial direction: compression from 10 mm to 3.5 mm (LF) or 2.5 mm (HF); conditioned at 72 °C for 24 h then cooled	Ultra-strong and tough bulk materials via hydrogen bonding
Huang et al. [122]	Balsa	No tspecified	Immersion in 2 wt% NaClO2 (buffered with CH3COOH, pH 3.8) at 105 °C for 12 h (solution refreshed every 2 h); then transferred to 8 wt% NaOH at 80 °C for 12 h	–	–	–	Magnetic wood sponge for crude oil cleanup via electrothermal processes
Vitas et al. [20]	Beech	2.5 cm × 1 mm (RT × L)	CH3COOH (>99.8%) and 35 wt% H2O2; samples oven-dried at 65 °C pre-treatment; post-delignification drying in oven (65 °C, 24 h) or freeze-drying (with liquid N2, then high vacuum for 5 days)	–	–	Characterization by mercury intrusion porosimetry	Porosity characterization of delignified beech wood
Gan et al. [123]	Basswood	10 × 10 × 25 mm; 10 × 5 × 25 mm	Immersion in 1000 mL of mixed 2.5 M NaOH and 0.4 M Na2SO3 solution; boiled for 24 h; rinsed with distilled H2O	–	–	Densification at 100 °C under 5 MPa for ≈24 h	Fire-retardant wood structural material
Frey et al. [124]	Norway spruce	100 × 10 × 20 mm	Immersion in equal-volume mixture of 35 wt% H2O2 and glacial CH3COOH at 80 °C for 6 h; repeated once with fresh solution	Stored at 20 °C/65% RH; conditioned until constant mass	–	Densification in radial direction (compression from 10 mm to 3.5 mm for LF, 2.5 mm for HF)	High-strength bulk material
Segmehl et al. [56]	Spruce	Not specified	Acidic bleaching with 1:1 mixture of 30% H2O2 and CH3COOH; various treatment times (0.5–4 h at 40, 60, 80 °C); also soda pulping with 10 wt% NaOH at 40 and 80 °C for 4–8 h	–	–	–	Tunable wood and functional materials
Song et al. [79]	Basswood, Oak, Poplar, Western Red Cedar, Eastern White Pine	120 × 44 × 44 mm	Immersion in boiling aqueous solution of 2.5 M NaOH and 0.4 M Na2SO3 for 7 h	–	Achieved lignin removal ~45%	Pressed at 100 °C under ~5 MPa for 24 h	“Super” wood materials
Gan et al. [82]	Cathay poplar (Populus cathayana Rehd)	20 × 20 × 0.5 mm	Treated with 2 wt% NaClO2 in CH3COOH (pH 4.6) at 80 °C for 12 h; rinsed; then immersed in 5 M H2O2 and boiled for 4 h	–	–	Infiltration with prepolymerized MMA and Fe3O4 nanoparticles (ratios 1000:1, 1000:2, 1000:5) under vacuum (30 min, repeated 3×); dried at 50 °C for 6 h	“Transparent” wood composites with magnetic function
Song et al. [125]	Balsa	102 × 51 × 1 mm	Boiling in aqueous solution of NaOH and Na2SO3 for 1 h; then quickly transferred into a vacuum chamber	–	Final lignin content: 19.7%	–	“Super flexible” wood materials
Li et al. [27]	Pine, Birch, Ash	100 × 100 × 1.5 mm	Immersion in solution of DI H2O, sodium silicate (3.0 wt%), NaOH (3.0 wt%), MgSO4 (0.1 wt%), DTPA (0.1 wt%), and H2O2 (4.0 wt%)	Samples dried at 105 °C for 24 h pre-treatment	Pine: 32.5% → 5.2%; Birch: 24.2% → 3.3%; Balsa: 23.5% → 2.2%; Ash: 27.1% → 5.3%	Immersion at 70 °C until white; washed and stored in water	“Transparent” wood composites
Li et al. [78]	Balsa	20 × 20 mm; thickness: 0.6 ± 0.1, 1.0, 2.5 ± 0.1, 5.0, 8.0 mm	1 wt% NaClO2 with acetate buffer (pH 4.6) at 80 °C; reaction time: 6 h (<3 mm) and 12 h (5 and 8 mm)	Samples dried at 105 °C for 24 h pre-treatment	Reduced from 24.9% to 2.9%	Sequential washing (DI H2O, then EtOH, then 1:1 EtOH/acetone, then pure acetone; 3 cycles); infiltration with prepolymerized MMA; polymerization at 70 °C for 4 h	“Transparent” wood composites for optical applications
Zhu et al. [46]	Basswood	Not specified	Boiling in NaOH (2.5 M) and Na2SO3 (0.4 M) in DI H2O for 12 h; followed by bleaching in H2O2 (2.5 M)	–	–	–	“Transparent” wood composites for solar cells (using PVP)
Zhang et al. [126]	Chinesefir (Cunninghamialanceolata)	100 μm (tangential) × 10 mm (radial) × 35 mm (longitudinal)	Aqueous solution of 0.3% NaClO2 buffered with glacial CH3COOH (pH 4.4–4.8) for 4–8 h at 80 °C; then treatment with a solution (pH 2.7) containing NaClO2 (1.0 g in 150 mL water and 2.0 mL glacial acid) for 8 h at 80 °C	Samples washed and stored in refrigerator at 4–8 °C	Lignin content: 29.76, 25.37, 0.40 (values provided)	–	Analyzing mechanical properties
Yano et al. [57]	Hooppine veneers	80 × 60 × 1 mm	Partial removal using 1% NaClO2 followed by 0.1% NaOH at 20 °C for 24 h; then treatment at 45 °C for 12 h with 2000 mL 1–2% NaClO2 (pH 4.5), repeated up to 3 times	–	Weight loss: 14% (1% NaClO2) or 24% (2% NaClO2)	Impregnation with low-molecular-weight PF resin; nine-ply lamination; compression at 80 MPa and 160 °C for 1 h	High-strength wood

Table 2. Delignification’s methods described in the literature with experimental details such as wood species, dimensions, delignification process, lignin content, further modification, and application (2017–today).

Ref.	Wood Species	Sample Dimensions	Chemicals/Delignification Conditions	Moisture/Storage Conditions	Lignin Content (Initial → Final)	Post-Delignification Process	Application
Van Hai et al. [112]	Pinus koraiensis	150 × 50 × 15 mm3	5-step treatment: (i) 1% NaClO2 at 80 °C for 12 h (wash 2 h, water bath 1 d, oven-dry at 40 °C for 1–2 d); (ii) 2% NaClO2 at 80 °C for 12 h; (iii) Repeat step i; (iv) 0.1% NaOH suspension for 1 d (wash, water bath, oven-dry); (v) Repeat step (i)	Pre-dried in oven at 103 °C; post-densification stored in desiccator (15% RH, 23 °C)	30.3% (native) → progressively reduced to 5%	Hot pressing at 13 MPa (4 h), 26 MPa (8 h), 52 MPa (16 h) at 100 °C	Bulk wood
Mai et al. [113]	Balsa	15 × 15 × 5 mm	Boiling in 2.5 M NaOH and 0.4 M Na2SO3 at 100 °C for 12 h; then treatment with 30% H2O2	–	–	In situ polymerization (using CTAB, CaCl2, NaCl, HMA, AAM, KPS, TEMED; aged at 50 °C for 24 h)	Ultra-flexible flame-retardant composites
Gao et al. [48]	Balsa	Length: 20 mm; Diameter: 2–5 mm	Soaking in a solution of 50 wt% glacial CH3COOH and 50 wt% H2O2; cooked 10–12 h at 80 °C until white and soft	Solvent exchange (acetone) to replace moisture	Treated fiber: lignin reduced to ~8%; cellulose increased to ~59%	–	“Transparent” wood composite
Samanta et al. [114]	Balsa and Birch	Veneers: 20 × 20 × 1 mm or 200 × 100 × 1 mm	Immersion in acetate buffer (pH ≈ 4.6) containing 1.0 wt% NaClO2 at 80 °C until color changes from brown to white; then thoroughly washed; solvent exchanged to EtOH and acetone under vacuum	–	–	Infiltration with melamine–formaldehyde resin under vacuum for ≥24 h	Fire-retardant “transparent” wood composite
Sun et al. [49]	Balsa (Ochromapyramidale)	50 × 50 × 1 mm (L × R × T)	Immersion in 1 wt% NaClO2 solution (buffered with CH3COOH, pH 4.6) at 80 °C for 12 h	–	–	TEMPO-mediated oxidation, in situ lignin deposition and mechanical hot pressing	Reconstructed wood with high strength, water resistance and excellent optical properties
Liang et al. [91]	Poplar	50 × 50 × 5 mm	Steam treatment with a 1:1 mixture of 30% H2O2 and glacial CH3COOH at 160 °C for 2–8 h (vapor phase)	Pre-dried in oven at 60 °C; post-treatment: freeze-dried at −60 °C overnight	26.7% → 1.45%	Vacuum-assisted resin transfer molding with bisphenol A-type epoxy resin; densification at 23 MPa, 100 °C for 1 h	Porous material with improved compressibility
Ruan et al. [115]	Basswood	–	NaOH (≥96%), Na2SO3 (≥97%), H2O2 (30%), and absolute EtOH in ratio 1:0.16:20; specimens dried under supercritical CO2 (101 bar, 5 L/min)	–	–	Immersion, boiling for 5 h, ultrasonic cleaning; then treated in H2O2 and boiled; immersed in EtOH at 60 °C for 12 h	–
Han et al. [59]	Poplar (Populus)	20 × 20 × 5 mm	Treatment with NaClO2 and glacial CH3COOH at 80 °C for 18 h (using CH3COOH, pH 4–5)	Post-treatment: slices frozen at −20 °C for 12 h, then freeze-dried for 24 h	–	Pyrolysis at 1000 °C for 2 h under argon	Hydrophobic, porous, flame-resistant lignocellulosic carbon material
Zou et al. [22]	Birch (Betula spp.)	120 × 80 × 0.7 mm (rotary-cut)	Immersion in 2.5 M NaOH and 0.4 M Na2SO3 at 100 °C for 2 h; then immersion in boiling H2O2 for 30 min	Dried in oven at 103 °C until moisture reached 5%; densified samples cooled to 20–30 °C	Reduced from 19.8% to 11.6%	Infiltration with phenol–formaldehyde resin; densification pressing at 150 °C, 15 MPa for 15 min	Structural applications
Niu et al. [42]	Poplar	Not specified (wood tubes molded)	Treated with NaClO2 and CH3COOH; tubes poured into mold, frozen, then freeze-dried at −50 °C	–	Removal ~56 ± 5%	Pressing under 20 MPa, followed by carbonization at 600, 800, 1000, 1200 °C	Energy storage applications
Wu et al. [116]	New Zealand pine and Basswood	Veneers: 20 × 20 × 0.50 mm	Treatment with MMA, NaClO2, EtOH, glacial CH3COOH, NaOH and AIBN; samples dried at 103 °C for 24 h and stored in EtOH	–	New Zealand pine: 27.64% → 22.80%; Basswood: 23.04% → 19.11%	MMA polymerization and impregnation	“Transparent” wood composite
Wang et al. [12]	Poplar tree	100 × 20 × 10 mm	Delignification using aq. NaOH (0–6 wt%) and 50 wt% MA at 155 °C for 30 min in a 1 L bomb reactor	After densification: dried in a climate chamber at 20 °C, 65% RH for 2 weeks	–	Acid hydrotropic delignification using MA hydrotropic fractionation at 100 °C for 30 min; densification from 10 mm to 8 mm at 1.0 MPa, 15 min at 150 °C	Broad applications
Liu et al. [21]	Balsa (Ochromalagopus Swartz)	40 × 20 × 2 mm	Immersion in 2.5 M NaOH and 0.4 M Na2SO3, boiled for 36 h	–	–	Impregnation with solid–solid phase change materials	Energy-saving building materials
Yang et al. [70]	Populus Euramericana	50 × 10 × 3 mm (L × T × R)	Hot H2O extraction with ethanol:benzene (1:2 v/v) for 48 h, then water bath at 60 °C for 3 h; delignification under −0.1 MPa vacuum for 5 h (mixture: 483.5 mL H2O, 10 g NaClO2, 6.5 mL CH3COOH); water bath at 40 °C for 30 h	Air-dried 48 h, then vacuum-dried at 80 °C until constant weight	Native wood: 22.89%; delignification rates: 1L = 12.81%, 2L = 39.12%, 3L = 52.24%	–	Investigation of dynamic mechanical and sorption behavior
Foster et al. [55]	Balsa	100 × 100 mm	Two approaches: (a) Chlorite-based: 1 wt% NaClO2 in 1 N acetate buffer (pH ≈ 4.6) at 80 °C for 8–12 h; (b) Peroxide-based: Solution with 3 wt% sodium silicate, 3 wt% NaOH, 0.1 wt% MgSO4, 1 wt% EDTA, 4 wt% H2O2 at ~70 °C for ~2 h	–	Lignin content: chlorite: 11.4%; peroxide: 20.3% (vs. natural 24.8%)	Acetylation and methacrylation of delignified wood	Modified wood for enhanced properties
Liang et al. [19]	Hybrid poplar clones	2 × 2 cm (cross-section)	Treatment in a 1:1 mixture of 30% H2O2 and glacial CH3COOH; boiled at 40 °C for durations of 0, 1, 6, 10, 32, 34 h	Pre-dried in oven at 60 °C; post-freeze-dried at −60 °C overnight	Reduced from 27.3% to 22.6%, 14.8%, 5.6%, 1.40%, 0.2% with increased time	Vacuum-assisted resin transfer molding with bisphenol A-type epoxy resin; densification via hot pressing at 23 MPa, 100 °C for 1 h	Porous material with improved compressibility
Chen et al. [73]	Balsa	30 × 30 × 1 mm	Immersion in 2 wt% NaClO2 in CH3COOH (pH 4.6) for 2 h; preserved in EtOH; then immersed in 15 wt% NaOH at room temperature for 2 h	–	–	In situ chemical polymerization with polyacrylamide	Strong, flexible hydrogel reinforced by wood skeleton
Wu et al. [116]	New Zealand pine and Basswood	Veneers: 20 × 20 × 0.50 mm	Treatment with MMA, NaClO2, EtOH, glacial CH3COOH, NaOH and AIBN; samples dried at 103 °C for 24 h and stored in EtOH	–	New Zealand pine: 27.64% → 22.80%; Basswood: 23.04% → 19.11%	MMA polymerization and impregnation	“Transparent” wood composite
Jakob et al. [58]	Spruce veneer	50 × 50 mm	Soaking in a 2:1 EtOH–DI H2O mixture with 1.5% CH3COOH in a 5 L pressure reactor at 170 °C, 14 bar for 180 min (heating 75 min, cooling 120 min); or immersion in 0.4 M Na2SO3 and 2.5 M NaOH at 98.5 °C for 240 min	Stored in a climate chamber at 20 °C and 65% RH	–	Densification at 120 °C under 20 MPa for 15 min (held overnight)	Densified plywood
Jakob et al. [17]	Spruce	100 × 50 × 1.47 mm (axial × radial × tangential)	20 veneers immersed in 1 L solution of 2.5 M NaOH and 0.4 M Na2SO3 at ~119 °C (0.19 MPa) for 4 h; repeatedly washed	After cooling (to 60 °C), equilibrated at 20 °C and 65% RH for 2 weeks	–	Veneers densified in tangential direction in a hot press at 120 °C, 35 MPa for 7.5 min	Load-bearing applications
Mi et al. [117]	Balsa	100 × 50 × 0.8 mm	Bleaching in 5% NaClO solution for 3, 8, 12, 24 h at room temperature; rinsing with DI H2O	Samples freeze-dried, then dried in a hot oven at 60 °C for 48 h	Final lignin content ~0.8%	Infiltration with poly(vinyl alcohol) solution (8 wt% at 90 °C) with degassing under 200 Pa	“Transparent” wood composites and thermal insulators
He et al. [40]	Basswood	20 mm thickness	Immersion in 2.5 M H2O2 (in DI H2O and EtOH); boiled until wood turned white; rinsed in EtOH/H2O three times	Preserved in EtOH/H2O overnight and air-dried	Final lignin content ~0.6%	Partial densification by hot pressing (thickness reduced by half, density ≈ 0.7 g/cm3)	Bulk materials via stacking delignified blocks
Mania et al. [118]	Poplar (Populus alba L.) and Birch (Betula pendula)	20 (T) × 30 (R) × 20 (L) mm	Immersion in boiling 2.5 M NaOH and 0.4 M Na2SO3 solution for 7 h at 110 °C	–	Birch: 21.6% → 17.2%; Poplar: 25.2% → 16.6%	Plasticization in boiling water (1 h); then tangential pressing with clamping	Partial delignification and densification
Fang et al. [119]	Pinewood	100 × 35 × 3 mm	In a 1 L autoclave: 5 g NaOH, 15 g Na2SO3, 0.03 g anthraquinone dissolved in 600 mL of 20 wt% MeOH solution (MeOH/H2O = 1:4, wood/solution = 1:20); processed for 1–7 h at 170 °C; quenched	–	–	Immersed in 20 wt% MeOH solution for 24 h; pressed at 8 MPa for 3 h, then at 90 °C for 3 min	Super-strong nanocellulose films
Li et al. [5]	Basswood	Various thicknesses: 0.8, 5, 40 mm; sizes: 210 × 190 mm, 100 × 50 mm, 50 × 50 mm	H2O2 (30 wt%) steam delignification: steamed until yellow color disappeared (≈2–12 h); rinsed with ultrapure H2O and EtOH	Dried at 105 °C for 24 h pre-treatment	Final lignin contents: 0.84, 0.96, 0.94 (approx.)	Infiltration with MA and epoxy polymer	“Transparent” wood composites
Khakalo et al. [120]	Birch	Veneers: 1.5 ± 0.1 mm; Dimensions: 10 × 10 cm	1 wt% NaClO2 in acetate buffer (pH 4.6) in 3 L at 80 °C for 12 h	Stored at 23 °C and 50% RH	Final lignin content: 8.6%	Infiltration with ionic liquid ([EMIM]OAc) activated at 95 °C; hot pressing at 5 MPa for 16 h at 100 °C	Bulk hot-pressed high-performance wood via ionic liquid treatment
Wu et al. [44]	Basswood (Tilia)	Veneers: 20 × 20 × 0.42 mm	2 wt% NaClO2, 0.1 wt% glacial CH3COOH, 97.9 wt% ultrapure H2O; treated in water bath at 80 °C (40 rpm) for 30–150 min	Dried at 103 °C for 24 h pre-treatment; then stored in EtOH	Lignin content decreases gradually (e.g., from 24% to 9%)	MMA polymerization and impregnation	“Transparent” wood composites for optical applications
Yang et al. [121]	Poplar	Not specified	Hot water extraction using EtOH:benzene (1:2 v/v) for 48 h; water bath at 60 °C for 3 h; then delignification: 967 mL water, 20 g NaClO2, 13 mL CH3COOH under −0.1 MPa vacuum for 5 h; water bath at 40 °C for 30 h	–	–	Wood furfurylation	Wood furfurylation
Han et al. [72]	Basswood	50 × 50 × 10 mm	Bleaching in 2 wt% NaClO2 (buffered with CH3COOH at pH 3.5) for 12 h at 70 °C (repeated 3 times), followed by treatment in 5 wt% NaOH at 90 °C for 7 h	Pre-dried at 105 °C for 12 h; equilibrated in desiccator (saturated K2SO4, ~97.6% RH at 20 °C)	Final lignin content: 2.3%	Densification along radial direction: compression from 10 mm to 3.5 mm (LF) or 2.5 mm (HF); conditioned at 72 °C for 24 h then cooled	Ultra-strong and tough bulk materials via hydrogen bonding
Huang et al. [122]	Balsa	No tspecified	Immersion in 2 wt% NaClO2 (buffered with CH3COOH, pH 3.8) at 105 °C for 12 h (solution refreshed every 2 h); then transferred to 8 wt% NaOH at 80 °C for 12 h	–	–	–	Magnetic wood sponge for crude oil cleanup via electrothermal processes
Vitas et al. [20]	Beech	2.5 cm × 1 mm (RT × L)	CH3COOH (>99.8%) and 35 wt% H2O2; samples oven-dried at 65 °C pre-treatment; post-delignification drying in oven (65 °C, 24 h) or freeze-drying (with liquid N2, then high vacuum for 5 days)	–	–	Characterization by mercury intrusion porosimetry	Porosity characterization of delignified beech wood
Gan et al. [123]	Basswood	10 × 10 × 25 mm; 10 × 5 × 25 mm	Immersion in 1000 mL of mixed 2.5 M NaOH and 0.4 M Na2SO3 solution; boiled for 24 h; rinsed with distilled H2O	–	–	Densification at 100 °C under 5 MPa for ≈24 h	Fire-retardant wood structural material
Frey et al. [124]	Norway spruce	100 × 10 × 20 mm	Immersion in equal-volume mixture of 35 wt% H2O2 and glacial CH3COOH at 80 °C for 6 h; repeated once with fresh solution	Stored at 20 °C/65% RH; conditioned until constant mass	–	Densification in radial direction (compression from 10 mm to 3.5 mm for LF, 2.5 mm for HF)	High-strength bulk material
Segmehl et al. [56]	Spruce	Not specified	Acidic bleaching with 1:1 mixture of 30% H2O2 and CH3COOH; various treatment times (0.5–4 h at 40, 60, 80 °C); also soda pulping with 10 wt% NaOH at 40 and 80 °C for 4–8 h	–	–	–	Tunable wood and functional materials
Song et al. [79]	Basswood, Oak, Poplar, Western Red Cedar, Eastern White Pine	120 × 44 × 44 mm	Immersion in boiling aqueous solution of 2.5 M NaOH and 0.4 M Na2SO3 for 7 h	–	Achieved lignin removal ~45%	Pressed at 100 °C under ~5 MPa for 24 h	“Super” wood materials
Gan et al. [82]	Cathay poplar (Populus cathayana Rehd)	20 × 20 × 0.5 mm	Treated with 2 wt% NaClO2 in CH3COOH (pH 4.6) at 80 °C for 12 h; rinsed; then immersed in 5 M H2O2 and boiled for 4 h	–	–	Infiltration with prepolymerized MMA and Fe3O4 nanoparticles (ratios 1000:1, 1000:2, 1000:5) under vacuum (30 min, repeated 3×); dried at 50 °C for 6 h	“Transparent” wood composites with magnetic function
Song et al. [125]	Balsa	102 × 51 × 1 mm	Boiling in aqueous solution of NaOH and Na2SO3 for 1 h; then quickly transferred into a vacuum chamber	–	Final lignin content: 19.7%	–	“Super flexible” wood materials
Li et al. [27]	Pine, Birch, Ash	100 × 100 × 1.5 mm	Immersion in solution of DI H2O, sodium silicate (3.0 wt%), NaOH (3.0 wt%), MgSO4 (0.1 wt%), DTPA (0.1 wt%), and H2O2 (4.0 wt%)	Samples dried at 105 °C for 24 h pre-treatment	Pine: 32.5% → 5.2%; Birch: 24.2% → 3.3%; Balsa: 23.5% → 2.2%; Ash: 27.1% → 5.3%	Immersion at 70 °C until white; washed and stored in water	“Transparent” wood composites
Li et al. [78]	Balsa	20 × 20 mm; thickness: 0.6 ± 0.1, 1.0, 2.5 ± 0.1, 5.0, 8.0 mm	1 wt% NaClO2 with acetate buffer (pH 4.6) at 80 °C; reaction time: 6 h (<3 mm) and 12 h (5 and 8 mm)	Samples dried at 105 °C for 24 h pre-treatment	Reduced from 24.9% to 2.9%	Sequential washing (DI H2O, then EtOH, then 1:1 EtOH/acetone, then pure acetone; 3 cycles); infiltration with prepolymerized MMA; polymerization at 70 °C for 4 h	“Transparent” wood composites for optical applications
Zhu et al. [46]	Basswood	Not specified	Boiling in NaOH (2.5 M) and Na2SO3 (0.4 M) in DI H2O for 12 h; followed by bleaching in H2O2 (2.5 M)	–	–	–	“Transparent” wood composites for solar cells (using PVP)
Zhang et al. [126]	Chinesefir (Cunninghamialanceolata)	100 μm (tangential) × 10 mm (radial) × 35 mm (longitudinal)	Aqueous solution of 0.3% NaClO2 buffered with glacial CH3COOH (pH 4.4–4.8) for 4–8 h at 80 °C; then treatment with a solution (pH 2.7) containing NaClO2 (1.0 g in 150 mL water and 2.0 mL glacial acid) for 8 h at 80 °C	Samples washed and stored in refrigerator at 4–8 °C	Lignin content: 29.76, 25.37, 0.40 (values provided)	–	Analyzing mechanical properties
Yano et al. [57]	Hooppine veneers	80 × 60 × 1 mm	Partial removal using 1% NaClO2 followed by 0.1% NaOH at 20 °C for 24 h; then treatment at 45 °C for 12 h with 2000 mL 1–2% NaClO2 (pH 4.5), repeated up to 3 times	–	Weight loss: 14% (1% NaClO2) or 24% (2% NaClO2)	Impregnation with low-molecular-weight PF resin; nine-ply lamination; compression at 80 MPa and 160 °C for 1 h	High-strength wood

5. Influence of Delignification on Physical–Mechanical Properties of Resulting Wood

Delignification alters both the physical and mechanical characteristics of wood, with effects largely dependent on the extent of lignin removal and any subsequent post-treatment modifications.

Regarding physical properties, it is noted that removing lignin lightens wood color from its natural brown to a yellowish or whitish tone [127]. Moreover, lignin removal reduces density by eliminating part of the wood’s structural mass. However, densification (e.g., hot pressing) can restore or even increase density [102]. Delignification softens wood by reducing its hardness, making it more pliable. Increased porosity enhances moisture permeability, which can both reduce warping and risk dimensional instability [102,113]. Although lignin removal may weaken wood by reducing its inherent rigidity, improved cellulose fibril aggregation can counteract some strength losses when combined with densification.

As far as mechanical properties and behavior, lignin provides rigidity and fracture resistance; its removal generally decreases tensile strength and toughness [55,124]. Eliminating lignin increases elasticity and reduces weight, offering advantages for applications where flexibility is needed [127,128]. Post-treatment densification or polymer infiltration collapses the wood’s porous structure, significantly enhancing mechanical performance. For instance, partial delignification followed by hot pressing has yielded tensile strengths up to 580 MPa and moduli around 50 GPa [32,44]. The hierarchical, well-aligned cellulose nanofibers in densified wood form extensive interfibrillar hydrogen bonds, which substantially improve both stiffness and toughness. However, these benefits are moisture-sensitive and may diminish under high humidity [60,70,112,129].

Wood’s orthotropic, multi-scale structure—comprising cellulose, hemicellulose, and lignin—plays a pivotal role in its mechanical behavior. Moisture variations significantly impact H-bond dynamics: under dry conditions, densified wood exhibits excellent fiber interlocking and strength, but high moisture can lead to reduced performance and increased deformation [130].

Delignification lightens color and reduces density and hardness but may initially decrease mechanical strength and toughness due to the loss of lignin’s binding function. When properly optimized and followed by densification or chemical modification; however, the increased cellulose fibril aggregation and enhanced hydrogen bonding can lead to superior mechanical properties. Balancing the degree of delignification is thus essential to produce high-performance, sustainable wood-based materials [15,32,44,55,60,70,102,112,123,124,127,128,129,130]

6. Influence of Delignification on Wood Chemical, Microstructural, Optical, and Thermal Properties, Along with the Role of Drying Conditions

Delignification significantly alters wood’s chemical composition; for example, lignin removal lowers wood’s lignin percentage, profoundly affecting its chemical structure [19,29,44,74,75]. On the other hand, with lignin removed, the relative cellulose content rises, enhancing wood’s inherent strength and rigidity [19,29]. Non-structural compounds such as resins, tannins, and oils are also partially removed and lowered, altering color, odor, and biological resistance [29,75]. Removing acidic lignin increases wood pH and modifies chemical reactivity, impacting subsequent chemical treatments [129,131]. Overall, delignification alters wood’s chemical framework, affecting its subsequent functionalization and application.

Delignification also influences wood’s physical microstructure and moisture interactions. Removal of lignin loosens cell wall integrity, increases porosity, and may alter the orientation of cellulose fibers [50,127,132]. The exposure of additional hydroxyl groups enhances water absorption; however, full delignification may cause pore collapse upon drying, altering moisture uptake behavior [70]. Initial delignification increases meso- and macropore formation, though excessive treatment may lead to collapse of micro-porosity, depending on drying method. SEM and BET analyses show significant changes in pore size distribution, with freeze-drying helping to preserve the porous structure [20,48,59]. These microstructural changes directly affect wood’s functional performance in applications such as filtration, catalysis, and composite fabrication.

Delignification impacts wood’s appearance and light-transmitting characteristics: removal of light-absorbing lignin lightens wood, yielding a yellowish to nearly colorless, white wood. When thickness is also low, it is frequently called “transparent” material [33,36,85,133,134]. With altered composition and microstructure, the refractive index increases, affecting how light is scattered and transmitted. The wood’s inherent grain and pore structure determine the extent of light scattering, making the optical properties highly dependent on both chemical removal and microstructural alterations [44]. Optimizing delignification can balance enhanced transparency with maintaining adequate mechanical integrity for decorative and functional applications.

Delignification modifies wood’s heat transfer characteristics. Removing lignin, a poor heat conductor, generally increases thermal conductivity, allowing faster heat transfer [28,31,47,48]. With increased diffusivity, delignified wood heats and cools more rapidly, which is critical in applications requiring efficient thermal management [12,51,134,135]. Such thermal modifications must be considered when selecting materials for insulation versus applications requiring rapid heat dissipation.

Drying methods post-delignification greatly affect wood’s final properties. Although simple, it may cause uneven drying and collapse of the porous network. Drying provides uniform moisture reduction but risks thermal damage. It also preserves the microstructure and porosity, maintaining higher surface area and more robust mechanical properties [59].

Regarding drying conditions, standardized drying conditions (as per ASTM D143, ISO 13061) ensure consistency in mechanical testing and optimal retention of the desired wood structure [12,19,20,28,29,31,33,36,44,47,48,50,51,59,70,72,74,75,85,127,129,131,132,134,135,136,137].

7. Applications of Delignified Wood

Delignified wood, especially when combined with densification or impregnation processes, exhibits enhanced strength, reduced weight, and improved dimensional stability, making it attractive for a wide range of applications. Some are summarized below [15,28,31,35,47,48,52,79,138,139].

• Structural and Construction Materials: Delignified and densified wood can be used for flooring, decking, siding, and building components, where increased strength and stability are critical for outdoor and load-bearing applications [15,32]. Furthermore, delignified wood treated with hydrophobic agents or impregnated with polymers improves resistance to decay and moisture, making it suitable for marine structures (e.g., piers and docks) and utility poles [39,128].
• Furniture and Decorative Applications: The material’s unique esthetic—lightened color and increased transparency—combined with enhanced mechanical performance enables the design of thinner, more delicate furniture while maintaining durability [72,85].
• Automotive and Aerospace: Owing to its reduced weight and high strength, delignified and densified wood is a promising alternative in automotive and aerospace industries, contributing to improved fuel efficiency and performance [34,68].
• Packaging: The sustainability, low weight, and high strength of modified wood make it a viable substitute for traditional, non-degradable packaging materials [12].
• Musical Instruments: Enhanced mechanical properties and dimensional stability offer potential for manufacturing high-quality, durable musical instruments with improved acoustic performance [55].
• Biomaterials and Tissue Engineering: Delignified and impregnated wood can serve as a biocompatible scaffold for medical implants, bone grafts, and tissue engineering. Its hierarchical porous structure promotes cell adhesion, proliferation, and nutrient transport. For example, anisotropic hydrogels derived from delignified wood have been used to mimic muscle tissue, while wood-based scaffolds infused with hydroxyapatite and polycaprolactone simulate bone structure and support osteogenic differentiation [79,88,140].
• Solar Cells: Modified delignified wood has been explored as a substrate for various types of solar cells—dye-sensitized, perovskite, organic, and flexible—due to its high surface area, sustainability, and ability to support crystal growth or enhance active layer adhesion [72,128].
• Thermal Management and Insulation: Delignified wood can function both as a thermal conductor and an insulator. Densified wood may serve as an efficient heat sink or structural cooling material in buildings, while modified wood with low thermal conductivity is ideal for insulation in construction, refrigeration, and industrial high-temperature applications [28,31,47,48].

Overall, the unique combination of low density, high strength, and tailored chemical and microstructural properties makes delignified wood a versatile material with significant potential across construction, transportation, consumer goods, renewable energy, and biomedical sectors. As research progresses, additional applications and further enhancements in performance are expected to emerge [12,15,21,28,31,32,34,39,47,48,55,68,72,85,88,128].

Utilization of Lignin After Delignification

Lignin, traditionally seen as a by-product of pulp and paper processes, is now recognized as a valuable renewable resource. Its abundant functional groups (alcohol, phenol, and carbonyl) enable chemical modifications for advanced applications. Key utilization avenues include the following:

• High-Value Chemicals: Lignin can be depolymerized into compounds, such as vanillin, syringol, and guaiacol, for use in flavors, fragrances, pharmaceuticals, and polymeric synthesis [138].
• Biofuels: Processes like pyrolysis, gasification, and fermentation convert lignin into biofuels, offering sustainable alternatives to fossil fuels.
• Advanced Materials: Lignin serves as a precursor for carbon fibers, graphene, and nanocellulose composites. When combined with binder polymers (e.g., PAN, PVA, and PVP), lignin-based electrospun nanofibers can be carbonized to yield materials for solar cells, catalysis, energy storage, and biomedical applications [35,139].
• Agricultural Applications: Lignin can function as a soil conditioner and fertilizer, improving soil structure and water retention, or be transformed into bio-based herbicides and pesticides.
• Adhesives and Binders: Currently, lignin is used as a binder in the chemical industry. Recent studies, such as those by Hou et al. [52], have demonstrated that lignin can reinforce cellulose paper, improving its dry and wet strength, thermostability, and UV-blocking ability.

Despite these promising applications, challenges persist due to lignin’s low purity, heterogeneous molecular weight, and complex chemical structure. Purification and further functionalization are needed to fully harness lignin’s potential.

8. Conclusions

Wood is an abundant sustainable, bio-based material with a hierarchical structure where cellulose fibrils are embedded in a matrix of hemicelluloses and lignin. Delignification—when carefully controlled to preserve the aligned cellulose architecture—opens new avenues for producing high-performance, functional materials. By removing or modifying lignin, it is possible to tailor wood’s mechanical, thermal, optical, and chemical properties through subsequent treatments, such as densification, polymer infiltration, and carbonization.

However, complete or partial delignification also poses challenges. It increases moisture sorption by exposing hydroxyl groups and may alter microfibril angles, potentially reducing mechanical strength if hemicelluloses are excessively removed. Furthermore, scaling up delignification from laboratory-scale veneers to large wood blocks introduces issues of chemical diffusion and inhomogeneity due to wood’s inherent anisotropy. Advanced process control—potentially incorporating artificial intelligence—and “green” chemical design are needed to address these obstacles.

Overall, delignification offers a promising route for developing advanced wood-based materials for applications ranging from lightweight construction and thermal management to renewable energy and biomedical devices, contributing to a more sustainable future.