**3. Lignin isolation procedures and** *lignin-first* **concept**

Most often, lignin valorization starts with its isolation from the lignocellulosic material (biomass) with two perspectives: (i) to gain insights into lignin's

chemical and structural characterization, which involves methods such as Björkman, dioxane, and cellulolytic enzyme lignin [32–34], where the obtained lignin presents a structure close to the lignin in the cell wall matrix (*native lignin*); or (ii) aiming to obtain lignin in a high yield, in which case the biomass is delignified (pulp process), obtaining a solid fraction rich in cellulose and a liquid one mainly constituted by lignin and hemicelluloses. Afterward, the lignin is isolated by precipitation from the spent liquor (Kraft, organosolv, or lignosulfonate) in yields dependent on the precipitation agent, pH, etc. In these cases, the pulping process leads to an extensive chemical change in the lignin, such as the cleavage of β—Od4<sup>0</sup> linkages and the formation of recalcitrant CdC bonds, which results in difficult conditions for lignin depolymerization. This lignin presents different characteristics (related to the isolation process) from those of *native lignin* and is called *technical lignin* [35].

There are several pulping processes from which different technical lignins can be obtained [8, 36–38]. The conditions applied depend on the biomass chemical characteristics, particularly the lignin content and composition, but these issues will not be discussed here. Instead, a brief description of the processes is presented in the following lines.

**Soda pulping** started in 1853 using non-woody biomass such as straw or flax, but now the process has been applied to soft and hardwoods. The delignification is done with sodium hydroxide (NaOH) liquor under temperatures around 160°C. Lignin depolymerization is promoted, starting with the cleavage of βdOd4<sup>0</sup> linkages, which allows the ionization of phenolic groups, but condensation reactions also occur that are negative for the further lignin valorization. Besides, the lignin-carbohydrate complexes (LCCs) are also broken [18].

**Kraft pulping** was developed later than soda pulping (around 1890) and has become the most common process involving the delignification of either softwood or hardwood, but also non-woody species, with a white liquor containing sodium hydroxide (NaOH) and sodium hydrosulfide (Na2S) at temperatures around 170°C [39, 40]. Delignification occurs by the hydroxyl groups (OH�) but also by hydrogen sulfide (HS�), which, due to its nucleophilic character, enhances delignification. Lignin has been extracted industrially from the black liquor by *LignoBoost* or *LignoForce* technology [41]. It is the most commonly used market process to produce technical lignins [18]. The isolated lignin presents sulfur as thiol groups (which make lignin valorization difficult), is highly condensed, and contains a low amount of ether bonds [8].

**Sulfite pulping** started in 1930. Biomass is pulped under the attack of sulfite or bisulfite salt of sodium, ammonium, or magnesium at temperatures ranging from 140 to 170°C. The sulfonic groups are incorporated in the aliphatic chain of the lignin monomers, and water-soluble lignosulfonate salt is formed. The *technical lignin* produced is called lignosulfonate, representing 90% of the commercialized lignin [34]. Still, since Kraft pulping is more efficient, the sulfite process has decreased (<5%) [8]. The lignosulfonate: (i) can be isolated by ultrafiltration, extraction, or precipitation; attained as a salt (Na+ , NH3 + , Mg<sup>+</sup> , Ca2 + ); (ii) presents a large amount of sulfur (4–8% wt compared with Kraft lignin), and has a high number of degraded oligomers and a low amount of βdOd4<sup>0</sup> [8].

**Organosolv pulping** appeared in the late twentieth century as a promising process to selectively attain lignin using, for example, methanol or ethanol. The lignin extraction is done by solvolysis with or without the presence of acids (*Brønsted*, e.g., H3PO4, *Lignin as Feedstock for Nanoparticles Production DOI: http://dx.doi.org/10.5772/intechopen.109267*

H2SO4, AcOH, CF3SO3H; or *Lewis acids* such as FeCl2, ZnCl2). If the acid is not used, the pH will decrease from 7 to 4 due to the hemicelluloses deacetylation; on the contrary, if the acid is present, the pH can decrease to between 4 and 2; thus, the solvolysis of the biomass is accelerated, and both hemicelluloses and lignin are removed [17].

**Ionic liquids** (IL) have been used for lignin extraction due to their characteristics (e.g., green and recyclable). Overejo-Pérez and coworkers [42] used the protic ionic liquid 1-methylimidazolium chloride to isolate lignin from *E. globulus* under different conditions to study the treatment severity. More severe conditions promoted: (i) the cleavage of ether linkages, (ii) lignin fractions with higher molecular weight (MW) and more stable due to condensation reactions, and (iii) preferential degradation of S-units, contrasting to mild conditions where the degradation is of G-units.

**Deep eutectic solvents** (DESs) are promising alternative solvents for IL because they can also be recycled, are biodegradable, easy to produce, and reuse several times without losing strength, depending on the chosen system. Lignin from corncob residues was extracted by DES and used to make LNP by self-assembly [43].

Therefore, there is a great diversity of the *technical lignins* obtained due to the intrinsic variability of the biomasses used and the different extraction processes. As a result, the heterogeneity of the *technical lignins* is a negative point for their valorization. As an example, **Table 2** presents the characteristics of Kraft lignin and lignosulfonates.

Consequently, upgrading solutions must be found so that the concepts of biorefinery, the circular economy, and the zero-waste philosophy can be fully applied. Having in mind these aspects, researchers have treated biomass under mild conditions to attain more uniform lignin products in the so-called *lignin-first* approach (also known as reductive catalytic fractionation (RCF) or catalytic upstream biorefining (CUB)). Under this new biorefinery concept, the lignin is firstly removed from the biomass (instead of the carbohydrates fraction), and the cellulose and hemicellulose fractions are almost intact [44, 45]. Abu-Omar and coworkers [7] present the concept: "*The lignin-first biorefining is not a synonym for lignin valorization, but rather an integral approach that derives value from both lignin and polysaccharides, towards an atom-efficient and more sustainable utilization of lignocellulosic biomass*". This approach is more selective for lignin; hence, it prevents undesirable and irreversible condensation reactions, eliminates the need for purification steps, and reduces production costs [45]. The traditional methods promote the cleavage of the βdOd4<sup>0</sup> linkages and the formation of CdC bonds,


#### **Table 2.**

*Properties of Kraft lignin and lignosulfonates. Data from Bozell et al. [34].*

producing a condensed lignin that is harder to depolymerize in successive steps (**Figure 4**, fragments E and F). To overcome these negative aspects, the *lignin-first* approach involves the application of mild conditions to stabilize the βdOd4<sup>0</sup> linkages and the low lignin MW products (e.g., monomers, dimers, and short oligomers) formed during the biomass fractionation, preventing lignin condensation [46].

In summary, the *lignin-first* approach has three steps: (i) the lignin is extracted from the biomass by an organic solvent through solvolysis or acid-catalyzed reactions (similarly to organosolv pretreatment); (ii) the intermediates formed are stabilized to prevent condensation, and (iii) the lignin can be further depolymerized during the stabilization stage [7]. The stabilization approaches deliver the target chemical molecules without requiring further chemical modifications [47].

Some biomass fractionation methods applied to obtain uncondensed lignin include (i) ammonium-based fractionation—which can solubilize the lignin at room temperature under a pressure of 7–10 bars [48]; (ii) mild organosolv—using organic solvents (e.g., ethanol, methanol, acetic and formic acids) that are recycled during the recovery of the lignin [49]; (iii) γ-valerolactone-assisted hydrolysis—using γ-valerolactone (GVL, a green solvent that can be produced from glucose) and water (cosolvent), for example, pine wood fractionated by 80% γvalerolactone at 140–180°C, yields lignin of 33% [50]; or (iv) ionic-liquid-assisted fractionation [51].

Other methods are more focused on stabilizing the lignin monomers; the most studied one is catalytic hydrogenolysis, which combines lignin depolymerization with solvolytic extraction under the reductive stabilization of the intermediates. The hydrogenolysis cleaves the CdO bond at the βdOd4<sup>0</sup> moieties, generating lignin monomers; the reaction requires a solvent that will be responsible for the lignin

#### **Figure 5.**

*Lignin depolymerization by acid hydrolysis: (a) hydrolysis and dehydration of the β—O—4*<sup>0</sup> *lignin fragment A, leads to fragments B (benzylic carbocation intermediate), C (alkene products from the acid hydrolysis). All these fragments can react, and condensation reactions occur, leading to products E (includes an α-2*<sup>0</sup> *linkage) and F (includes a 5-5*<sup>0</sup> *linkage). (b) The β—O—4*<sup>0</sup> *lignin fragment can be involved in the following reactions: (i) oxidation leading to fragment G (oxidized lignin fragment) that can form fragment H (oxidized monomer), (ii) solvolysis leads to products that can be transformed by reductive catalytic fractionation into fragment K (a reduced monomer), and (iii) stabilization with aldehydes forms the acetal-stabilized lignin fragment (I). Adapt from Questello-Santiago et al. [17].*

*Lignin as Feedstock for Nanoparticles Production DOI: http://dx.doi.org/10.5772/intechopen.109267*

depolymerization (e.g., methanol or formic acid that will be the H2 donor), and a metal-containing catalyst such as Pt, Ru, Pd, Ni, or Rh, to prevent lignin condensation [17].

Shuai et al. [52] and other researchers proposed another *lignin-first* strategy to preserve the Caryl-O ether bonds, particularly the βdOd4<sup>0</sup> moieties, to prevent the formation of a Benzylic carbocation intermediate (**Figure 5a**, fragment B) due to its condensation ability. During lignin extraction, the βdOd4<sup>0</sup> linkage is stabilized with an aldehyde (e.g., formaldehydedCH2O or acetaldehyde—MeCHO) to form the stable acetals, that is, cyclic 1,3-dioxanes (**Figure 5b**, fragment I) that entrap the diol, preventing its dehydration and degradation. This method permits the dissociation between the biomass fractionation from the subsequent depolymerization step, enabling: (i) a broader range of depolymerization methods; and (ii) the optimization of the fractionation and depolymerization steps since they are independent. The advantage is that this produces insignificant condensation reactions, a high yield of monomers, and high selectivity. However, the method presents some limitations for large-scale implementation: (i) it is challenging to find the right balance between the removal of lignin from the biomass and the preservation of the carbohydrates in the pulp; (ii) the separation of the catalyst from the pulp. Both points are seen as a downside of this method. Questello et al. [17] attained a lignin monomeric yield of 42–50%, but it was structurally more complex (**Figure 5b**, fragment G).

Other critical reviews were published in 2020 focused on developments in the *lignin-first* approach [18, 35, 44, 53, 54], in 2021 [7, 38] and in 2022 [45, 55]. One of them emphasizes the fundamental catalytic reactions of the extraction and depolymerization of lignin and posterior stabilization of the phenolic units; the authors also present a brief overview of the possible modifications of the lignin-derived phenols and monolignols, focused on added-value chemicals, polymers, and other developments [53]. Gigli and Crestine [35] present different methodologies for


**Table 3.**

*Methods for the fractionation of lignin. Adapted from Sadeghifar and Ragasukas [56].*

lignin fractionation by (i) solvents (most common)—using solvent mixtures or under a sequence of solvents; (ii) membranes—based on ultra- and nanofiltration using ceramic or polymeric membranes; (iii) precipitation of the re-dissolved lignin in a binary solvent system; and (iv) pH-mediated gradient precipitation (**Table 3**). The lignin-membrane-based approach makes the separation into cuts of lignin with a defined molecular weight (MW) and narrow polydispersity, while the goal of the solvent-based approach is to obtain fractions with distinctive physicochemical properties. However, both parameters (MW and structure/functionalities) appear to be closely interdependent, since low MW fractions: (i) are typically soluble in polar solvents; (ii) possess high phenolic content and condensed structures; and (iii) have increased antioxidant activity. On the other hand, the higher MW fractions present fewer modified lignins but have a lower amount of aromatic hydroxyl groups and condensing units. The authors also highlight the applications of fractionated lignin, keeping in mind their characteristics. For example, lignin with high MW attained from *P. radiata* Kraft black liquor was successfully used to produce phenol-formaldehyde resins after methylolation and demethylation (to increase the lignin reactivity) [57].

The main lignin applications studied so far are: (i) polymers production such as composites (17%), thermosets (8%) and thermoplastics (4%); (ii) properties improvement involving antioxidant activity (15%), standards generation (6%), antimicrobial activity (4%) and color reduction (2%); (iii) micro and nano-structures such as fibers (12%) and particles (6%); (iv) others (14%) and finally (v) structure modification (12%) [35].

A state of the art on lignin valorization strategy was presented by Liu et al. [18], who discuss the production of *technical lignins* (giving a resumé of the fractionation methods such as Kraft or organosolv delignification), describe conventional methods for lignin catalytic depolymerization (e.g., pyrolysis, solvolysis, catalytic oxidative and reductive depolymerizations), plus the emerging strategy of the *lignin-first* approach, and the methods to improve the yield of the lignin phenolic products. Karlsson et al. [54] present a study where green solvents are used to obtain high yields of the fraction's hemicelluloses lignin and cellulose. The biomass was treated by supercritical water extraction to remove the hemicelluloses (liquid fraction, characterized by a partial hydrolysate of the glycosidic bonds, but preserving the native structure of the oligomers). Then the solid fraction was treated with aqueous ethanol supercritical extraction catalyzed with H2SO4 at 160°C to remove the lignin (liquid) and the solid rich in cellulose (with crystallinity preserved after both treatments). Overall, the proposed method combines active stabilization with physical lignin protection, minimizing the lignin condensation reactions. The potential uses of each fraction were discussed: hemicelluloses and cellulose could be used for ethanol production or hydrolyzed to monomeric sugars for chemical applications; the lignin could be used directly for polymers production or be further depolymerized to the monomeric platform [54]. Abu-Omar and coworkers [7] discuss the importance of biomass diversity and the analytical methods used for its characterization (e.g., analytical pyrolysis, NMR), but also present the big picture of the *lignin-first* approach, from feedstock to reactor design, the importance of the catalyst, the mass balance and yields, and finish with an outlook on the development of *lignin-first* biorefinery [7]. Korányi et al. [44] presented an extensive list of the works done under the *lignin-first* approach from 2018 to 2020, focusing also on the studies on wood digestion/conversion from 1940 to 2014, mentioning the different feedstock, conditions, and products obtained. As mentioned before, the


#### **Table 4.**

*Possible chemical reactions during lignin fractionation by base solvents. Adapted from Schutyser et al. [8].*

characteristics of the technical lignins depend on the methods used to isolate or depolymerize the lignin. Therefore, **Tables 4**–**6** present a resumé of the lignin chemistry during the different processes.


Reductive reactions


**Advantage**: prevents the formation of reactive functional groups ) avoiding condensations or repolymerization.

**Disadvantage**: inability to cleave CdC bonds ) degree of depolymerization is linked to the amount of ether bonds that can be cleaved.

#### **Table 5.**

*Possible chemical reactions during lignin fractionation by acid solvents and reductive reactions. Adapted from Schutyser et al. [8].*

Despite all the work developed so far, there are still some issues to be overcome for the large-scale implementation of the *lignin-first* approach, such as that (i) the lignin fractions are correlated with the native lignin, that is, depend on the biomass used, so obtaining fractions with high reproducibility is an issue; (ii) the fractioning process is usually done in a batch, but a continuous process is more favorable for industrialization and to use the black liquor directly from the pulping industry [35].

*Lignin as Feedstock for Nanoparticles Production DOI: http://dx.doi.org/10.5772/intechopen.109267*

