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Suota, M. J., Kochepka, D. M., Ganter Moura, M. G., Pirich, C. L., Matos, M., Magalhães, W. L. E., and Ramos, L. P. (2021). "Lignin functionalization strategies and the potential applications of its derivatives – A Review," BioResources 16(3), 6471-6511.

Abstract

Lignin is one of the most important and widespread carbon sources on Earth. Significant amounts of lignin are delivered to the market by pulp mills and biorefineries, and there have been many efforts to develop routes for its valorization. Over the years, lignin has been used to produce biobased chemicals, materials, and advanced biofuels on the basis of its variable functionalities and physicochemical properties. Today, lignin’s applications are still limited by its heterogeneity, variability, and low reactivity. Thus, modification technologies have been developed to allow lignin to be suitable for a wider range of attractive industrial applications. The most common modifications used for this purpose include amination, methylation, demethylation, phenolation, sulfomethylation, oxyalkylation, acylation or esterification, epoxidation, phosphorylation, nitration, and sulfonation. This article reviews the chemistry involved in these lignin modification technologies, discussing their effect on the finished product while presenting some market perspectives and future outlook to utilize lignin in sustainable biorefineries.


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Lignin Functionalization Strategies and the Potential Applications of Its Derivatives – A Review

Maria Juliane Suota,a Débora Merediane Kochepka,a Marlon Gualberto Ganter Moura,a Cleverton Luiz Pirich,a Mailson Matos,b Washington Luiz Esteves Magalhães,b,c and Luiz Pereira Ramos a,*

Lignin is one of the most important and widespread carbon sources on Earth. Significant amounts of lignin are delivered to the market by pulp mills and biorefineries, and there have been many efforts to develop routes for its valorization. Over the years, lignin has been used to produce biobased chemicals, materials, and advanced biofuels on the basis of its variable functionalities and physicochemical properties. Today, lignin’s applications are still limited by its heterogeneity, variability, and low reactivity. Thus, modification technologies have been developed to allow lignin to be suitable for a wider range of attractive industrial applications. The most common modifications used for this purpose include amination, methylation, demethylation, phenolation, sulfomethylation, oxyalkylation, acylation or esterification, epoxidation, phosphorylation, nitration, and sulfonation. This article reviews the chemistry involved in these lignin modification technologies, discussing their effect on the finished product while presenting some market perspectives and future outlook to utilize lignin in sustainable biorefineries.

Keywords: Lignin functionalization; Derivatized lignin; Biobased products; Value-added materials

Contact information: a: Department of Chemistry, Federal University of Paraná (UFPR), 81531-980 Curitiba, PR, Brazil; b: Graduate Program in Materials Science and Engineering; Federal University of Paraná (UFPR), 81531-980 Curitiba, PR, Brazil; c: Embrapa Florestas, 83411-000 Colombo, PR, Brazil;

* Corresponding author: luiz.ramos@ufpr.br

GRAPHICAL ABSTRACT

INTRODUCTION

Since pulping and paper industries were established at the end of the nineteenth century, a massive amount of residual lignin has been incinerated to recover chemicals and produce power and steam at the pulp mill sites. Such processing recovers only the fuel-equivalent value of the lignin. However, lignin may be a promising source of biobased materials such as polyols, antioxidants, binders, coatings, reinforcing agents, and biofuels. Besides being one of the most important and widespread carbon source on Earth, lignin is capable of delivering renewable aromatic compounds on an extensive scale (Bajwa et al. 2019). Pulp mills demonstrate an increasing interest in alternative lignin applications to diversify their product portfolio and generate additional sources of revenue. Other drivers to this end are improvements in pulping efficiency, the development of cost-effective technologies for lignin isolation, and the possibility of using underutilized biomass such as branches and bark for cogeneration. With this, in the mid-to-long term, more lignin will be made available for conversion to biobased fuels, chemicals and materials (Dessbesell et al. 2020).

Lignin is a natural macromolecule deposited in the plant cell wall as a result of condensation reactions involving phenylpropanoid radicals generated from p-coumaryl, coniferyl, and sinapyl alcohols during the lignification process (Figs. 1A, B). These cinnamic alcohols are the basis of the primary lignin structure, forming monolignols that are named p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) units, respectively. These are distributed in variable ratios in the plant kingdom (Fig. 1C).

Fig. 1. Phenylpropanoid structure (A), types of monolignol precursors and their appearance in the lignin structure (B), and lignin types that are found in the plant kingdom (C)

Gymnosperm (softwoods) lignins have more than 90% of G units and low levels of H units in their structure. By contrast, angiosperm dicot (hardwoods) lignins contain a mixture of S and G units in different ratios, while angiosperm monocot (grasses or herbaceous species) lignins contain the three types of lignin monomers in variable quantities (Boerjan et al. 2003). In general, softwood species have lignin contents slightly higher than those of hardwood species. For instance, Mutturi et al. (2014) reported acid-insoluble (Klason) lignin contents in the range of 16.7 to 24.9 wt% and 27.1 to 31.4 wt% for seven hardwood and eight softwood species, respectively.

Sulfite, soda, kraft, and organosolv pulping technologies (please refer to the next section for basic descriptions) were developed to remove lignin from lignocellulosic materials (Hu et al. 2018). The industrial interest in these technical lignins has increased considerably over the past years, prompting people at companies and research institutes to find cost-effective strategies to isolate lignin from black liquors, spent liquors, and process slurries. After its isolation, lignin can be applied as a macromolecule or source of monomers for organic synthesis, with the chemical functionality resulting from separation processes or further chemical modification (Schutyser et al. 2018). However, the inherent heterogeneity of lignin is potentially increased during isolation, and its high degree of structural variability is considered a bottleneck for its valorization. At first glance, the lack of regularity might hinder the control and final quality of the lignin derivatives. By contrast, the full knowledge of lignin structure allows for its selective modification toward unique properties that enlarge the portfolio of possible lignin applications (Eraghi Kazzaz et al. 2019).

The development of alternative lignin applications is limited by its heterogeneity, variability, and low reactivity (Bajwa et al. 2019). Thereby, lignin fractionation, assisted or not by modification strategies such as amination, demethylation, demethoxylation, phenolation, sulfonation, methylation, and depolymerization, among others, are needed to tailor lignin into specific structure and properties. This review dwells with (I) the chemical properties of currently available lignins, (II) the reactions that lignins are prone to undertake, (III) the main structures and properties resulting after modification, and (IV) the targeted applications for each type of modified lignin.

Lignin Production Processes and Types of Technical Lignins

Brauns (1939) defined native lignin as the lignin isolated in such way that the isolation process does not cause any change in its structure and chemical composition. Therefore, lignins that incorporate features from the isolation process cannot be named native. This applies to technical lignins derived from sulfite (lignosulfonate), soda, kraft, organosolv pulping, and those originated from acid hydrolysis, enzyme treatments and, extraction procedures using ionic liquids and deep eutectic solvents.

Sulfite pulping, the first established chemical route to produce wood pulp in large scale, employs sulfite and bisulfite salts to obtain cellulose from lignocellulosic materials. As a result of sulfite pulping, lignosulfonates are produced by sulfonation of the alpha carbon atom, making lignin water-soluble and higher in its average molecular mass (Li and Takkellapati 2018). Today, due to its low efficiency in chemical recovery and high environmental impact to produce pulps with relatively low fiber strength for paper making, sulfite pulping accounts for less than 10% of the pulp mills worldwide. However, the interest in lignosulfonates grew over time because lignosulfonates are an important bulk material for vanillin production, in addition to applications as a plasticizer for concrete, flocculating agent, metal adsorbent, binder for composites, and dust suppressant (Aro and Fatehi 2017).

Kraft pulping is the leading pulping process worldwide. Sodium hydroxide and sodium sulfide act on lignocellulosic materials to release cellulose fibers, and a severely depolymerized lignin is recovered in a liquid phase named black liquor (Demuner et al. 2019). This process is advantageous due to the chemical recovery step, whereby the lignin-containing black liquor is concentrated and burned while reagents of the spent liquor are recovered and reactivated. Just 2% of kraft lignin is designated for applications other than burning. However, the composition of this renewable aromatic carbon source and the potential of kraft pulping to deliver vast amounts of lignin to the market have incentivized the use of kraft lignin as a source of biobased chemicals, fuels, and advanced materials (Hu et al. 2018; Li and Takkellapati 2018). Kraft pulping results in changes to lignin’s structure and composition. Sulfur is introduced as thiol groups, and residual carbohydrates (mostly from hemicelluloses) remain as contaminants in different levels. Besides, aryl-ether bonds are broken, and condensation reactions occur primarily through C5 of the aromatic ring of guaiacyl substructures (Demuner et al. 2019).

Soda pulping is very similar to kraft pulping except for the absence of Na2S in the pulping liquor. Most soda pulp mills were converted to kraft pulping when the latter was made commercially available. Nevertheless, soda pulping is more suitable for non-woody biomasses due to their lower lignin content and higher accessibility to the impregnating chemicals. The resulting soda lignin is free from sulfur despite being as condensed as kraft lignin. In the soda pulping process, anthraquinone (AQ) can be used as an additive that protects carbohydrates against degradation and improves pulping efficiency (Francis et al. 2008). AQ is active in the reductive cleavage of ether bonds and produces soda-AQ lignins deficient in -O-4′ linkages. However, the industrial use of AQ has been banned in several countries due to its toxicity and harsh environmental impact (Hart and Rudie 2014).

Delignification with organic solvents (organosolv pulping) is another example of sulfur-free pulping technologies. It combines solvent systems (usually a mixture of alcohol and water), temperature, pressure, and sometimes the use of an exogenous acid catalyst for the selective removal of hemicelluloses and lignin. Organosolv lignins are recovered by precipitation from the spent liquor in high-quality, since they do not contain ash and sulfur. Also, these lignins usually have higher apparent molecular masses, lower dispersities, and lower condensation degrees compared to kraft and sulfite lignins. These characteristics make organosolv lignins interesting for conversion into chemicals and value-added products (Li and Takkellapati 2018). Also, organosolv pulping is less polluting than kraft and sulfite processes, and it does not release foul odors. Besides, the solvents used in the extraction procedure can be recyclable and renewable, such as in the case of ethanol and water.

Another emergent and large-scale lignin source is hydrolysis lignin (HL) (Bajwa et al. 2019). Cellulosic ethanol mills generate several tons of lignin as a coproduct from pretreatment and enzymatic hydrolysis. In the end, a solid residue containing 65 to 80% lignin is obtained, with its chemistry strongly dependent on the technique used for pretreatment and some nitrogen content due to enzyme adsorption (Li and Takkellapati 2018; Bajwa et al. 2019).

In addition to the aforementioned technical lignins, the ionosolv pulping technology delivers ionic liquid (IL) extracted lignins with properties and composition that depend on the applied process parameters (Rashid et al. 2021). Some ILs are very selective in lignin removal, while others remove plant polysaccharides or lead to complete lignocellulose dissolution. Ionosolv is similar to organosolv in operating temperatures, and the solvent applied is an IL with the ability to dissolve lignin (Schutyser et al. 2018). Lignin can be precipitated from this liquor, and its properties strongly depend on the severity used for delignification (Rashid et al. 2021).

Other green chemicals such as deep eutectic solvents and supercritical carbon dioxide (scCO2) have also been used for lignin isolation (Chen and Wan 2018; Jiang et al. 2020). These processes are friendly alternatives compared to isolation routes that employ high charges of harsh inorganic chemicals (Ramos et al. 2020). The scCO2 medium combined with a co-solvent such as ethanol and water penetrates the fiber cell walls, facilitating hemicellulose hydrolysis and lignin removal (Jiang et al. 2020). Carbonic acid formed from CO2 and water at high temperatures and pressures assists the partial depolymerization of hemicelluloses and lignin, along with other organic acids that are released in situ (e.g., acetic acid). Lignin isolated by scCO2 may be rich in ether linkages, making it suitable to produce polyols for further processing into polyurethane foams and adhesives (Jiang et al. 2020).

The increasing interest in lignin chemistry encouraged the development of lignin isolation strategies from pulping liquors, such as LignoForceTM and LignoBoostTM. Both technologies employ acid precipitation using carbon dioxide (CO2) for recovering lignin from the kraft pulping black liquor. Lignin is precipitated, washed, and dried to yield a solid powder with low carbohydrate and ash contents. The advantage of LignoForceTM over LignoBoostTM and conventional acid precipitation is a previous black liquor oxidization step that improves lignin filterability, reduces chemical consumption and minimizes the emission of volatile sulfur derivatives throughout the production process (Kouisni et al. 2016).

Excellent reviews can be found in the literature containing detailed information about pulping and pretreatment processes, lignin chemistry, and the prospects for use of technical lignins (Li and Takkellapati 2018; Schutyser et al. 2018; Bajwa et al. 2019).

LIGNIN FUNCTIONALIZATION

The presence of multiple functional groups represents the level of lignin’s complexity, besides its macromolecular structure. Functional groups such as alcohols, phenols, and ethers are distributed throughout the phenylpropanoid structure, decorating the biopolymer and making it susceptible to chemical modification (Fig. 2).

Fig. 2. Reactive sites of a lignin model structure and reactions that may be used to develop ideal properties for specific applications

Fig. 3. Main lignin transformations in aromatic and aliphatic (side-chain) moieties

Likewise, the original functional groups themselves may act as acids, nucleophiles, or electrophiles against a series of organic compounds. As a result, many transformations in lignin structure are possible, providing an increase or decrease in molecular mass distribution, an increase in reactivity due to the augmentation of highly reactive functional groups (e.g., phenolic and aliphatic hydroxyls), and an improvement of its compatibility with some synthetic polymeric matrices (Fig. 3). These include amination, methylation, demethylation, phenolation, sulfomethylation, oxyalkylation, acylation or esterification, epoxidation, phosphorylation, nitration, and sulfonation. Besides, improved solubility and thermal stability may be achieved, depending on the new functional groups attached to the lignin macromolecule.

Amination

Nitrogen is not an abundant element in lignin composition, as demonstrated by its elemental analysis (Wang et al. 2014; Ge et al. 2015). However, the use of amines to graft nitrogen into the lignin structure seems to be an exciting option for adding reactive sites capable of being used as intermediaries for lignin functionalization. In acidic conditions, amino groups are ionizable and positively charged, making lignin highly reactive in aqueous media (Wang et al. 2018a). Amino groups may also convert hydrophobic technical lignins such as kraft lignin into a highly hydrophilic material, improving its foamability, emulsifying properties, aging resistance, and mechanical strength (Liu et al. 2016). Also, aminated lignins may be used to remove dyes and heavy metals from aqueous systems due to their cationic and anionic absorbing capacities (Wang et al. 2014; Xu et al. 2017; An et al. 2020). Several studies related to the synthesis and applications of aminated lignin are listed in Table 1.

Table 1. Reactional Conditions for Lignin Amination and Potential Applications of the Final Products

Perhaps the Mannich reaction is the main way to introduce amine groups in lignin. As a traditional reaction to produce nitrogen-containing compounds, amination may occur in alkaline, neutral, and acidic media. This reaction has been reported with both macromolecular and depolymerized lignin (Ge et al. 2015; Xu et al. 2017; Wang et al. 2018a). The Mannich reaction requires formaldehyde and dialkylaminomethyl groups that react to produce in N,N-dialkylaminomethanol under alkaline conditions (Fig. 4a), and N,N-dialkyl(methylene) ammonium in acid media (Fig. 4b). These amination intermediaries are selective for the H3,5 and G5 positions in lignins and deliver the same reaction product. However, the addition can also occur in phenolates, depending on the reaction pH (Du et al. 2014; Wang et al. 2018a). Figures 4a and 4b show the Mannich reaction for free phenolic OH substructures. However, this reaction can also proceed when lignin is etherified at the C4 position.

Fig. 4. Lignin amination routes based on (a) Mannich in basic media, (b) Mannich under acidic media, (c) use of an amino-silanizating reactant, (d) use of a three-step catalytic synthesis, and e) chlorination followed by amination. Ts stands for the tosyl group.

The Mannich amination has some limitations for S-lignins, which possess less availability of free C5 positions due to their higher methoxylation degree. Besides, Mannich-aminated lignins contain more secondary and tertiary amine groups compared to primary amine groups, limiting further modification of the lignin structure (Pan et al. 2013). Mannich-aminated lignins have excellent potential to be applied as adsorbents for heavy metals and dyes (Liu et al. 2013) and curing agents (Nikafshar et al. 2017), besides value-added applications based on their colloidal properties (Du et al. 2014).

Wang et al. (2018a) improved the reactivity of industrial alkali lignins under acidic conditions using alcoholic depolymerization followed by Mannich reaction with dimethylamine and formaldehyde. Reactions were carried out in an aqueous solution at 60 °C for 4 h under constant stirring with acetic acid as catalyst. These authors were able to anchor 3.40 to 4.17% of N in the lignin structure. Du et al. (2014) used the same conditions to modify industrial softwood kraft lignins using the Mannich reaction, reaching 4.8% N content in the aminated lignins that formed very stable colloidal suspensions in water.

Ge et al. (2015) prepared grafted amine groups in alkali lignin with formaldehyde and methylamine under several conditions. The optimum parameters for achieving 8.32% of N involved the use of 0.16 methylamine/lignin mass ratio and 0.8 formaldehyde/ methylamine molar ratio at 90 °C and pH 12 for 4 h. Aminated lignin was assessed as a biosorbent for lead ions and used as surfactants, polycationic materials, and slow-releasing fertilizers. Liu et al. (2016) prepared a cationic amine emulsifier from a phenolated kraft lignin using the Mannich reaction. Dehydroabietyl and diethylenetriamine groups were used as hydrophobic and hydrophilic moieties, respectively. The resulting material was applied as a cationic asphalt emulsifier in road construction and maintenance due to its foamability, emulsifiability, and surface tension properties.

Another strategy to functionalize lignin with amine groups requires an amine-silanization reagent such as 3-aminopropyl triethoxysilane (APTES). This compound is capable of silanizing and aminating lignin in one step (Fig. 4c). An et al. (2020) reacted a hardwood kraft lignin with APTES under an inert atmosphere and reflux at 70 °C for 24 h. The resulting material containing 6.1% of N was used to remove Congo red and Cu (II) ions from aqueous solutions, showing high adsorption performances by electrostatic attraction and complexation, respectively. APTES reacts with lignin hydroxyl groups, forming an alkoxysilane linkage and exposing a primary amine group at the chain end. The remaining APTES molecules may also react with the first layer, forming a crosslinked APTES multi-layer around lignin. This study deserves attention because it was the first demonstration of a successful amine-silanization of lignin.

Lignin can be aminated using 25% aqueous ammonia and nanoalumina as a catalyst. However, besides the need for catalyst activation, lignin must be demethylated to release free hydroxyl groups that ought to be tosylated prior to amination (Fig. 4d). Nikafshar et al. (2017) obtained aminated lignin with 4.63% of N by this three-step organic synthesis and applied it as a curing agent for epoxy resins. This lignin-based curing agent enhanced the crosslinking density of the epoxy resins and improved their thermal stability (higher glass transition temperature – Tg) and mechanical properties. The authors claimed that this aminated lignin could compete with commercially available curing agents.

Zhang et al. (2011) developed a biobased adsorbent selective to 2,4,6-trinitrotoluene (TNT) from aminated lignin. First, lignin was modified by Friedel-Crafts alkylation followed by amination with a diamine through an SN2 reaction. For alkylation, 1,2-dichloroethane and anhydrous aluminum chloride reacted under reflux at 65 °C and stirring for 6 h to produce chlorinated lignin (ChL). After purification and drying, ChL was solubilized in N,N-dimethylformamide (DMF), and reacted with ethylenediamine at 80 °C for 7 h. Aminated lignin was demonstrated as a suitable adsorbent for TNT at a neutral pH (Fig. 4e).

Lignin can also be aminated after its epoxidation using a diamine. Pan et al. (2013) used propane diamine in different epoxy/diamine molar ratios. Higher diamine loadings favored the obtainment of primary and secondary amines in the lignin structure. Aminated lignins with 5.31 to 6.95% N in their composition were obtained, and these biobased materials were proposed to be used as curing agents or reactant in polyurethane production.

The lignin functionalization with amine groups increases its molecular mass and lowers its Tg. Properties such as reactivity, solubility, and surface tension are increased, tailoring technical lignins for several value-added applications.

Methylation

Methyl groups are very abundant in lignins as part of methoxyl (-O-CH3) groups, and methylation enhances the methoxylation degree of lignins without significantly affecting their molecular mass distribution (Sen et al. 2015; Shen et al. 2020). As a result, methylation increases the solubility in organic solvents and renders lignin more available to specific polymer synthesis applications (Xiong et al. 2020). Lignin methylation was first performed by Brauns (1939) as a method for lignin characterization. Methylation of native spruce lignin was carried out with diazomethane in dioxane and further with dimethyl sulfate in sodium hydroxide. Methylation occurred both in phenolic and enolic hydroxyl groups, rendering lignin insoluble in bisulfite. Today, methylation in dimethyl sulfoxide (DMSO) is one of the most common strategies to modify lignin, making the derivatives less reactive and less thermally stable than the original hydroxylated lignin (Alwadani and Fatehi 2019; Shen et al. 2020; Wang et al. 2015). Furthermore, methylated lignins are more compatible with polymeric materials such as polyethers, polyesters, polyethylene, and natural rubber (Wang et al. 2015) (Table 2).

Table 2. Reactional Conditions for Lignin Methylation and Potential Applications of the Resulting Materials

Methyl groups can be introduced into the lignin structure by nucleophilic aromatic substitution, bimolecular alkyl cleavage nucleophilic substitution, and bimolecular acyl cleavage nucleophilic substitution mechanisms depending on the reactants and reaction conditions (Sadeghifar et al. 2012; Sen et al. 2015). Methylation is more selective for phenolic hydroxyl groups because they are more acidic than aliphatic hydroxyl groups. As a stronger nucleophile, phenolate attacks the methylic carbon atoms in methylating agents, taking the methylic carbon atom for itself (Fig. 5).

Fig. 5. Methylating agents and reactional conditions for the partial, complete, or selective methylation of lignin

Sen et al. (2015) methylated a softwood kraft lignin with dimethyl carbonate (DMC) using a greener protocol compared to other methylation agents considered toxic and hazardous, such as methanol and dimethyl sulfate (DMS). However, the reaction mechanism for DMC methylation under basic conditions was temperature dependent. At 90 °C, DMC acted as a carboxymethylation reagent by a bimolecular acyl cleavage nucleophilic substitution. By contrast, at temperatures above 120 °C, lignin methylation occurred by a bimolecular alkyl cleavage nucleophilic substitution. Therefore, high temperatures were crucial for lignin methylation with DMC. The lignin methylation degree could also be controlled by setting up the reaction time and the DMC to phenolic hydroxyl ratio. The reactional conditions in Fig 5a methylated 100% phenolic OH, 99% carboxylic OH, and 75% aliphatic OH using 10 DMC equivalents to phenolic OH groups (Fig. 5a). However, the use of a base weaker than NaOH at a milder temperature and reaction time led to partial lignin methylation using the same amount of DMC (88.5% phenolic OH, 91% carboxylic OH, and 82% aliphatic OH) (Fig. 5b).

Duval and Avérous (2020) proposed a controlled protocol for lignin methylation using trimethyl phosphate as the methylation agent. Their findings showed that lignin could be partially methylated with precise control and even fully methylated by varying the catalyst amount, temperature, and reaction time. Using 1 equivalent of K2CO3 as the catalyst, trimethyl phosphate acts selectively on phenolic and carboxylic OH, methylating all of them in just 1 h at 120 °C. Otherwise, 20 to 80% of phenolic OH groups were converted when K2CO3 amounts were in the range of 0.125 to 0.5 equivalents. Aliphatic OH groups remained intact under these reaction conditions, ending up as polyols in methylated lignin (Fig. 5c).

Methylation is also a way to protect hydroxyl groups against undesired reactions during lignin conversion. Hence, methylation can be a useful pretreatment strategy for lignin before pyrolysis and thermal conversion. Kim et al. (2019) produced pyrolytic lignin (lignin oil) by solvent liquefaction and noticed that the previous methylation improved the crude oil storage stability in comparison to non-premethylated lignin. In the study, both DMS and DMC were tested as methylating agents. While DMS reacted with lignin at 80 °C for 2 h in 0.7 mol L-1 aqueous NaOH, DMC required more severe reaction conditions of 20 mL g-1 lignin in DMSO plus 0.25 g g-1 NaOH at 130 °C for 15 h. The former methylating condition was selective for phenolic OH, while the latter completely methylated phenolic OH and almost all initial amounts of aliphatic OH (Figd and Fige). Also, pre-methylation increased process yields and decreased char formation after liquefaction. Similarly, Zhu et al. (2016) methylated lignin model compounds prior to a microwave-assisted (400 W) depolymerization in methanol at 100 to 160 °C for 0.5 to 120 min. Pre-methylation facilitated lignin depolymerization, yielding aromatic monomers with reduced oxygen content.

Overall, lignin becomes more hydrophobic upon methylation, with a slightly higher apparent molecular mass and carbon content, lower Tg, higher reactivity, and more extensive intermolecular interactions (Duval and Avérous 2020). Methylated lignin applications are summarized in polymer science as copolymerization components, acting as a source of monomers and intermediates in formulations for lignin-based polymeric materials (Table 2).

Demethylation

Biological, thermo-catalytic, and chemical routes are capable of demethylating lignin, replacing their natural methoxyl groups with hydroxyls, and cleaving aryl-ether linkages in the lignin macromolecule. As a result, more hydroxylated and reactive lignin is achieved with a slightly lower molecular mass distribution. Hydroxyl groups play a primordial role in lignin reactivity because they are easily switchable to new functional groups (Podschun et al. 2017). The conversion of methoxyl into hydroxyl groups renders several potential lignin applications in polymer science to produce polyurethanes, polyesters, and epoxy resins (Table 3).

Table 3. Reactional Conditions for Lignin Demethylation and Potential Applications of the Final Products

Chemical demethylation may be carried out with several reagents and reaction conditions, with sulfur compounds having a great potential to demethylate lignin (Hu et al. 2011). Li et al. (2016) tested several sulfur-demethylating reagents to prepare fast curing agents for phenolic resins. Among them, Na2SO3 produced demethylated lignin in 10 wt% aqueous NaOH at 90 °C for 1 h (Fig. 6).

Fig. 6. Reactional conditions for lignin demethylation according to a) Li et al. (2017), b) Hu et al. (2014), c) Podschun et al. (2017), d) Ma et al. (2021)

Similarly, deprotonated 1-dodecanethiol was used to demethylate pine kraft lignin (Indulin AT – Ingevity, North Charleston, SC, USA) in DMF. The reaction was performed at 130 °C for 1 h under N2 and reflux following an SN2 mechanism. Besides demethylation, partial depolymerization of lignin was evidenced by the lower molecular mass, higher UV absorption spectra, and absence of β-O-4′ ether linkages after demethylation, as demonstrated by proton nuclear magnetic resonance (1H NMR) (Hu et al. 2014).

Podschun et al. (2017) compared conventional and microwave-assisted heating to demethylate organosolv lignin with indium triflate ((In(OTf)3) in water and water/ sulfolane (tetramethylene sulfone, C4H8O2S) mixture. Organosolv lignin was better demethylated under microwave-assisted heating in the presence of the water/sulfolane solvent system. The number of aromatic hydroxyl groups was increased from 2.1 to 4.4 mmol g−1. However, an increase in lignin molecular mass distribution and a decrease in its number of aromatic protons was observed, demonstrating that some level of condensation reactions had occurred. While high molecular mass lignins may help to produce carbon fibers, a great aromatic hydroxyl content opens the prospect for applying lignin as starting material for phenolic and epoxy resins and polyurethanes.

Nikafshar et al. (2017) used Na2SO3 in distilled water to demethylate lignin prior to amination, as aforementioned in the amination section. Chen et al. (2020a) applied Na2SO3 to demethylate corn lignin in aqueous NaOH (33 wt%) for 2 h at 90 °C. The resulting material had its bonding performance increased by oxidation with 20% NaIO4. NaIO4 oxidation produced aldehydes to react with free aromatic sites on demethylated lignin, improving its overall performance as an adhesive for wood-based materials. This chemical demethylation was confirmed by Fourier-transform infrared spectroscopy (FTIR) and by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry. Chen et al. (2020b) demethylated lignin to apply it as polyols for polyurethane synthesis. This functionalization was performed with HBr (48 wt% aqueous) and hexadecyltributylphosphonium bromide (TBHDPB) as a phase-transfer agent in DMF under heating. Demethylation improved the reactivity of lignin and caused a decrease in its apparent molecular mass.

Using a guaiacyl-type structure as an aromatic system to represent lignin, Sawamura et al. (2017) synthesized tannin-like derivatives via demethylation, producing materials with properties and functionality adequate for application as antioxidants, curing agents, adsorbents, and protein precipitation agents. Three substances (1-dodecanethiol, DSH; hydroiodic acid, HI; and iodocyclohexane, ICH) were tested in DMF under reflux for demethylation. HI was the best demethylation reagent compared to DSH and ICH, with the latter leading to recondensation.

Biological demethylation is conducted by fungal (soft-rot, white-rot, and brown-rot) and bacterial enzymes. Among them, bacteria such as Pseudomonas putida, Rhodococcus jostii RHA1, and Sphingomonas sp. SYK6 and fungi such as Penicillium simplicissimum, P. cinnabarinus A-360, Chaetomium piluliferum, and Xerocomus badius are some O-demethylase secreting organisms able to remove the O-methyl groups from lignin, with variable effects on the hydrolysis of aryl ether bonds (Kamimura et al. 2017; Venkatesagowda 2019). By contrast, ligninolytic enzymes such as laccases are known to depolymerize lignin into small fragments besides demethylating it (Kamimura et al. 2019). Venkatesagowda and Dekker (2021) observed that biological demethylation requires long residence times to be effective and that purified O-demethylases are not yet available for industrial applications. Hence, it seems that biological demethylation cannot yet compete with chemical demethylation. For comprehensive reviews on this topic, please refer to Venkatesagowda (2019) and Venkatesagowda and Dekker (2021).

Phenolation

Aromatic condensation (phenolation) in an acid medium modifies lignin by altering its chemical reactivity (Taleb et al. 2020; Thébault et al. 2020). This is because the concentration of phenolic hydroxyl groups is increased, augmenting the number of reactive sites for polymer synthesis and functionalization (Mou et al. 2020). However, for bioprocessing applications, an increment in free phenolic hydroxyl groups may be harmful because these reactive sites can be inhibitory to hydrolases such as cellulases (Mou et al. 2020). Lignin phenolation takes place through different reaction mechanisms (Zhang et al. 2019a). Dehydration in an acidic medium generates reactive sites at Cα and Cγ of lignin side-chains, forming stable benzylic carbocations that may undergo condensation by direct phenol addition. The other routes involve Cγ formaldehyde elimination, enol formation, and addition of phenol in para or ortho positions (Jiang et al. 2018; Londoño Zuluaga et al. 2018). Some of the recent phenolation studies are shown in Table 4.

Table 4. Phenolation of Lignin under Different Conditions

Jiang et al. (2018) conducted a more in-depth study about lignin phenolation with probable structures formed due to this reaction route. According to these authors, apart from breaking most of the ether bonds and secondary reactions, a significant decrease in aliphatic hydroxyl groups was observed due to the phenol incorporation in lignin substructures such as β-O-4′, β-5’/α-O-4′, β-β,’ α-carbonyl, and others. The phenolation process used phenol as a reactant and solvent, and lower acid concentrations (e.g., 5%) were used to improve the viability of the production process.

Taleb et al. (2020) studied both phenolation and acetylation of spent coffee ground lignin after pretreatment with dilute sulfuric acid. Compared to acetylation, phenolation led to more thermally stable (430 °C) lignin streams, having a higher availability of OH sites. Moreover, phenolated lignin had a superior adsorption performance of methylene blue (recovery of 99.6%), a compound used to evaluate the cationic adsorbent capacity of dyes. This study demonstrated a different application for lignin and offered a low-cost material to treat textile effluents. Although relatively inexpensive, the recovery and reuse of phenolated lignin were not demonstrated in this study.

Recently, phenolated Eucalyptus sp. alkaline-extracted lignin was studied as a matrix for cellulase adsorption. Lignoresorcinol (LigR) and lignopyrogallol (LigP) phenolated lignins (Fig. 7) had maximum cellulase adsorption capacities of 842.1 and 911.4 mg g-1, respectively, compared to 76.5 mg g-1 of the starting material. The enzymes were removed from both phenolated lignins by changing the pH from 10 to 4.8, with LigP adsorbing ten times more cellulases than lignin without modification (Mou et al. 2020). Compared to LigR, LigP provided a better enzyme migration to fresh cellulosic materials during the enzymatic hydrolysis stage, with LigP-desorbed enzymes displaying a higher total cellulase activity and a better hydrolysis performance.

Fig. 7. Structure of pyrogallol and resorcinol phenolated lignins

Thébault et al. (2020) evaluated the effects of lignin type and substitution degree on the properties of phenol-formaldehyde resins based on phenolated lignins. A factorial design was carried out using kraft and lignosulfonate lignins, unmodified and phenolated lignins, and level of phenolation (30 or 50%) as independent variables. Phenolation increased the number of reactive sites and decreased the average molecular mass of lignin. As a result, phenol-formaldehyde resins based on phenolated lignins had surface tension, viscosity, molecular mass distribution, and reactivity higher than those derived from unmodified lignins.

Wang et al. (2020) synthesized lignin-containing phenol-formaldehyde wood adhesives (LPF) from fractions of an industrial birch alkaline lignin that was previously submitted to sequential solvent extraction with isopropyl alcohol (i-PrOH), ethanol (EtOH), and methanol (MeOH). All these fractions were characterized to elucidate LPF structure/performance correlations. Carbohydrate, ash, and Klason lignin contents increased along this solvent extraction sequence and lignin apparent weight-average molecular mass (Mw). Lignin, phenol, and formaldehyde were used as reactants using a one-pot reaction system to produce LPF. Lignin was integrated covalently into the phenol-formaldehyde resin, and this was correlated to its adhesive strength. Also, the incorporation of lignin with high Mw and a high degree of condensation (MeOH lignin) affected the resin adhesiveness. This work was the first to demonstrate the feasibility of fractionating industrial birch alkaline technical lignins to produce thermoset LPF wood adhesives.

Depolymerization followed by phenolation was carried out by Zhou et al. (2020) to synthesize lignin-based phenolic foams. Under optimal reaction conditions, both Mw and number-average molecular mass (Mn) decreased from 12.600 and 6.480 g mol-1 in alkaline lignin (AL) to 6.100 and 1.500 g mol-1 in phenolated alkaline lignin (PAL). In addition, lignin phenolic hydroxyl groups increased from 2.4 in AL to 3.3 mmol g-1 in PAL. Foams were also characterized for their physical, mechanical, thermal, and morphological properties. Both macro and micro images of PAL-based and AL-based phenolic foams revealed that the foam structure remained uniform for lignin incorporation up to 30%. Both samples had better thermal stability, lower volumetric water absorption, and lower slag rate than foams synthesized in the absence of lignin. Also, for materials with the same degree of substitution, PAL-based phenolic foams had higher compressive strength and a more uniform structure than AL-based foams.

Sulfomethylation

Low water solubility is a limiting factor for the valorization of kraft lignins. In this context, lignin sulfomethylation produces water-soluble derivatives by introducing a methylene sulfonate group in aromatic rings (Aro and Fatehi 2017). This process differs from sulfite pulping, whereby sulfonic acid groups are placed in the lignin aliphatic side-chains. Figure 8 shows that lignin sulfomethylation occurs by adding sodium sulfite anions into alkaline media, preferably at the unsubstituted C5 of lignin substructures. This reaction takes place at various pH (7 to 13), temperatures (60 to 160 °C), reaction times (0.5 to 9 h), and sulfite/lignin and formaldehyde/lignin mass ratios of 0.1 to 1.0 and 0.01 to 1.0, respectively (Aro and Fatehi 2017; Eraghi Kazzaz et al. 2019; Konduri and Fatehi 2015). He and Fatehi (2015) studied the sulfomethylation of LignoForceTM kraft lignins using formaldehyde (HCHO) and Na2S2O5. The maximum estimated sulfonation degree was achieved at 97.1 °C for 3.2 h using 0.97/1 HCHO-to-lignin and 0.48/1 Na2S2O5-to-lignin molar ratios.

Qin et al. (2015) investigated the use of grafted sulfonated alkali lignin (GSAL) as a dispersant for coal-water slurries. GSAL was synthesized by sulfomethylation followed by etherification and polycondensation to obtain high levels of sulfomethylation and high molecular mass distributions. In this application, sulfomethylation was carried out for 1 h at pH 10 and 60°C.

Fig. 8. Scheme of sulfomethylation of lignin with sodium sulfite (adapted from Eraghi Kazzaz et al. 2019)

Different conditions in lignin sulfomethylation allow the obtainment of sulfomethyl-derivatives with various degrees of substitution, leading to materials with different average molecular masses and degrees of sulfonation. This reaction has been applied to kraft lignins by the MeadWestvaco Corporation (now Ingevity) to produce dye dispersants marketed as Reax® since the 70’s (Meister 2002). Sulfomethylated lignins (SML) have also been used as water reducer for cement admixtures (Kamoun et al. 2003), flocculant in water purification systems (Bolto and Gregory 2007), dispersant in pesticide formulations (Li and Ge 2011), and also as potential corrosion inhibitors for iron-based materials (Abu-Dalo et al. 2013).

Sulfomethylation side reactions include sodium thiosulfate formation, which may be avoided using high temperatures (100 to 150 °C) and sodium sulfite in excess to improve SML yields. Another hindrance is found in the lower reactivity of hardwood lignins because, unlike guaiacyl, the nucleophilic attack of phenolic hydroxyl groups is hindered by its high degree of methoxylation.

Konduri and Fatehi (2015) studied the sulfomethylation of hardwood kraft lignins. The main objective of the work was to produce water-soluble kraft lignin with an anionic charge. The optimal reaction conditions involved a lignin concentration of 20 g L-1, 0.5 mol L-1 of NaOH(aq), 0.9 sodium hydroxymethyl sulfonate/lignin molar ratio, 100 °C, and 3 h of reaction time. These conditions provided an SML with a charge density of −1.6 meq g-1 and 1.48 mmol g-1 sulfonate groups, while the unmodified lignin had a negligible charge density and 0.03 mmol g-1 sulfonate groups. The SML solubility in water at neutral pH was successfully improved to 40 g L-1, in contrast to the insolubility of the unmodified lignin. Further increments in the degree of sulfomethylation were not possible due to the formation of undesirable byproducts such as sodium thiosulfate. However, the density of sulfonate groups achieved in this study was higher than that obtained by Wu et al. (2012) after sulfomethylation of corn stalk alkaline lignin, reaching 1.29 mmol g-1 as reported in the literature.

Huang et al. (2018) performed the sulfomethylation of alkaline lignin (AL), and enzymatic hydrolysis lignin (EHL) derived from alkali-pretreated bamboo fibers. SML yields of circa 95% were achieved from AL sulfomethylation after 3 h at 110 °C using a sodium hydroxymethylsulfonate/lignin molar ratio of 1.0. The maximum lignosulfonate yield from EHL was only 68.9% when the reaction was carried out for 4 h at 110 °C with a sodium hydroxymethylsulfonate/lignin molar ratio of 0.8. The largest sulfomethylation of AL was attributed to the availability of more reaction sites (free C5 position of lignin), possibly due to its lower average molecular mass and the presence of residual carbohydrates in EHL.

One additional drawback of sulfomethylation is the use of formaldehyde, which is carcinogenic, mutagenic, and environmentally unfriendly. Hence, recent studies are focused on developing alternative sulfoalkylation reaction routes to alleviate its environmental impact. For instance, lignin sulfobutylation provides water-soluble lignin derivatives similar to conventional sulfomethylation with the advantage of being carried out in aqueous media (Eraghi Kazzaz et al. 2019; Hopa and Fatehi 2020; Huang et al. 2018).