A detailed understanding of the structure of technical lignins is critically important in order to direct the efforts toward their valorization (Glasser, 2000). Not surpris­ingly, there is in the literature significant evidence suggesting that the performance of purified technical lignins can be linked to their chemical structure (Gosselink et al., 2010; Berlin, 2011, 2012a, b; Chung and Washburn, 2012). However, it is recognized that there is a fundamental lack of knowledge in the under­standing of technical lignins as a polymer and their con­version to materials, so targeted modifications via refining, chemical modifications, or fractionation can be pursued to maximize their performance in formu­lated products (Baker and Rials, 2013). Hence, the importance of the lignin analytical methods employed
to study the structure of these lignins will be discussed in detail below.

Native lignin is an irregular heterogeneous polymer. The same applies to technical lignins with the particular­ity that the lignin heterogeneity is typically increased by the biomass processing. It is widely believed that the lignin structure is tridimensional; however, there is no solid evidence supporting this hypothesis. Some scien­tists question the latter claim (Ralph et al., 2004). Lignin is optically inactive. The repeated (monomeric) unit in lignin is the phenylpropane unit (or so-called the "C9-unit") of the p-hydroxyphenyl (H), guaiacyl (G) and syringyl (S) types (Figure 18.2). Coniferous lignins are predominantly of G-type. Hardwood lignins contain both G — and S-units. The H-unit content in woody lignin is usually low; however, they can significantly contribute to the structure of nonwoody lignins, for instance, from annual fibers. In addition, annual fiber lignins contain significant amounts of cinnamic and ferrulic acid derivatives attached to lignin predomi­nantly via ester linkages with the gamma hydroxyl of C9-units (Adler, 1977; Sakakibara, 1991; Ralph et al., 2004). The lignin C9-units contain different functional groups. The most common ones are aromatic methoxyl and phenolic hydroxyl, primary and secondary aliphatic hydroxyl, small amounts of carbonyl groups (of the aldehyde and ketone types) and carboxyl groups. The monomeric C9 lignin units are linked to form a polymer by C—O—C and C—C linkages. The most abundant lignin interunit linkage is the b-O-4 type of linkage (structures 1—4, and 7; Figure 18.2) comprising about 50% of the interunit linkages in lignin (ca. 45% in soft­woods and up to 60—65% in hardwoods). Other com­mon lignin interunit linkages are the resinol (P~P) (structure 6; Figure 18.2), phenylcoumaran (b-5) (struc­ture 5; Figure 18.2), 5-5′ (structure 12; Figure 18.2), and

FIGURE 18.2

4- O-5 (structure 11; Figure 18.2) moieties. The number of these structures varies in different lignins, but rarely ex­ceeds 10% of the total lignin moieties. The number of other lignin moieties is usually below 5% (Adler, 1977; Sakakibara, 1991; Balakshin et al., 2008).

The degree of lignin condensation (DC) is an impor­tant lignin characteristic as it is often correlated (nega­tively) with lignin reactivity. The definition of condensed lignin moieties found in the literature is not always clear. Most commonly, condensed lignin struc­tures are lignin moieties linked to other lignin units
via 2, 5 or 6 positions of the aromatic ring (in H-units also C-3 position). The most common condensed struc­tures are 5-50, b-5, and 4-O-5′ structures. Since the C-5 position of the syringyl aromatic ring is occupied by a methoxyl group and therefore it cannot be involved in condensation, hardwood lignins are less condensed than softwood lignins.

According to recent findings, almost all lignin in soft­wood and softwood pulps is linked to polysaccharides, mainly via hemicelluloses (Lawoko et al., 2005). The main types of lignin—carbohydrate (LC) linkages in
wood are phenyl glycoside bonds (A), esters (B) and benzyl ethers (C) (Figure 18.2; Helm, 2000; Koshijima and Watanabe, 2003; Balakshin et al., 2007; Balakshin et al., 2011; Balakshin et al., 2014). The occurrence of sta­ble LC bonds in native lignins is one of the main reasons preventing selective separation of the wood components in biorefining processes.

Technical lignins are obtained as a result of lignocellu — losic biomass processing. Technical lignins differ dramatically from the corresponding native ones as a result of a combination of multiple reactions including catalyzed biomass hydrolysis, condensation of lignin fragments, elimination of native lignin functional groups, formation of new functional groups, and others. They are appreciably more heterogeneous (in terms of chemical structure and molecular mass) than the native lignins. Technical lignins have a high variety of structural moieties present in rather small amounts (Balakshin et al., 2003; Liitia et al., 2003).

Technical lignins can be classified from different points of view (Table 18.2). From a practical point of view, there are technical lignins originated from pulp and paper industrial processes which are considered mostly waste products without controllable chemical properties. These are kraft and soda lignins (kraft and soda pulping, correspondingly) and lignosulfonates (sulfite pulping). On the other hand, there is a big group of technical lignins from various emerging biomass bio­refining processes such as different variations of AH, steam explosion (SE), and OS pretreatment, in particular.

In terms of the process chemistry, and, correspond­ingly, the lignin chemical structure, lignins can be derived from acid — or alkali-based processes. The former includes most of the emerging biomass biorefinery pre­treatments, such as AH, SE (except AFEX) and most of OS processes as well as lignosulfonates. Alkaline pro­cesses are kraft and soda pulping, AFEX pretreatment, and some OS processes. In addition to the process na­ture, the feedstock source has naturally an important impact on the structure of technical lignins.

Another consideration which can be used to classify technical lignins, especially in view of their application, is the presence or absence of sulfur in their structure. Therefore, kraft lignin, and, especially lignosulfonates, are sulfur-containing lignins whereas soda, OS, AH and SE lignin are sulfur-free or low-sulfur-containing lignins.

In terms of the chemical structure, native lignins un­dergo significant degradation/modification during biomass processing. Lignin degradation occurs predom­inantly via cleavage of b-O-4 linkages (although the mechanisms are different for different processes), which results in an increase of phenolic hydroxyls and a decrease in lignin molecular mass. The lignin degrada­tion also leads to a decrease in aliphatic hydroxyls, oxygenated aliphatic moieties and the formation of carboxyl groups and saturated aliphatic structures. In contrast to lignin degradation, some reversed reactions, such as lignin repolymerization/condensation, take place to some degree resulting in increase of lignin mo­lecular mass and decrease of its reactivity. These changes are common for most of the technical lignins although the degree of transformation varies significantly de­pending on the process conditions (temperature, time, pH, and others) and feedstock origin.

Each process provides the lignin with specific chemi­cal characteristics. First, the reaction mechanism is quite different in acidic and basic media. The cleavage of b-O — 4 linkages under alkaline conditions occurs via a quinone methide intermediate which results in the for­mation of coniferyl alcohol-type moieties as a primary reaction product (Figure 18.3). They are not accumulated in the lignin; however, they undergo further secondary rearrangement reactions forming various (aryl-) aliphatic acids. b-5 and b-1 type of linkages of the native lignin cannot be cleaved during the process but are transformed into stilbene-type structures (structure 30; Figure 18.2). The latter are stable and are accumulated in alkaline lignins. In addition, a significant amount of vinyl ether structures (structure 29; Figure 18.2) forms

during soda pulping and accumulates in lignin, in contrast to kraft lignin. Another relevant structural dif­ference between soda and kraft lignins, resulting from differences in the reaction mechanism, is the presence of aryl-glycerol type structures (structure 20; Figure 18.2) in the former. On the other hand, lignin undergoes demethylation reactions which result in formation of

o-quinone structures during kraft pulping (but not in the case of soda pulping). In addition, kraft lignins contain small amounts of organically bound sulfur, likely in the form of thiol compounds (Marton, 1971; Gierer, 1980; Gellerstedt, 1996; Balakshin et al., 2003). Kraft and soda lignins show significantly higher degree of condensation than the corresponding native lignins. However, this is the result of accumulation of condensed moieties of original native lignin rather than the result of extensive condensation reactions during pulping (Balak­shin et al., 2003). Kraft and soda lignins contain small amounts of carbohydrate and ash impurities. The amounts of these contaminants are dependent on feed­stock origin and are significantly higher in annual fiber lignins than in woody lignins.

The lignin chemistry originated from the emerging acid-based biomass biorefinery processes is very diverse (Glasser et al., 1983). The acid-based biomass biorefining can be catalyzed by addition of mineral or organic acids (from catalytic amounts to the use of organic acids as the reaction media) or without acid addition (autohydroly­sis) when organic acids are generated due to cleavage of acetyl groups of lignocellulosics as well as due to the formation of acidic reaction products. Technical lig­nins derived from biomass biorefinery processes have
been much less investigated than kraft lignins. More­over, a high diversity of lignins is expected in the future given the large number of technical biomass pretreat­ment processes under either R&D or industrial deploy­ment and the high variety of potential raw materials (softwoods, hardwoods, annual fibers, agricultural resi­dues, etc.) as compared to the relative uniformity of pulping processes.

The main pathway of lignin degradation under acidic conditions is the acidic hydrolysis of b-O-4 linkages (Figure 18.4). The major products of this reaction are the so-called Hibbert ketones (Wallis, 1971). The accu­mulation of Hibbert ketones in lignin results in relatively high content, as it compares to alkaline lignins, of carbonyl groups and the corresponding saturated aliphatic structures (Berlin et al., 2006). Although degra­dation of lignin under acidic condition occurs via vinyl ether intermediates, they do not accumulate in the lignin since vinyl ether structures are very unstable in acid me­dia. Significant amounts of olefinic moieties were observed in lignin obtained under acidic conditions, but their nature is different from the olefinic structures of kraft and soda lignins, their exact structure is still not well understood (Berlin et al., 2006). Moreover, lignin condensation reactions under acidic conditions are more significant than those occurring in alkaline pro­cesses. Acidic lignin condensation occurs predomi­nantly via 2 and 6 positions of the aromatic ring, in contrast to alkaline condensation which occurs predom­inantly at the C-5 position of the aromatic ring (Glasser et al., 1983). The DC is dependent on the acidity of the reaction media (pH and solvent used) and the process

Hibbert ketones

severity (temperature and time). An extreme example of highly modified technical lignins is the industrial AH lignin produced in Russia or Belarus which is obtained at 170-190 °C, 2-3 h with 1% H2SO4. AH lignin is insol­uble in polar organic solvents and alkaline solutions due to the strong condensation/polymerization occurred during the AH process. Hydrolysis lignin has high con­tent of phenolic hydroxyl groups and olefinic structures. In addition, it contains 10-30% residual carbohydrates and up to 20% lipophilic extractives (Chudakov, 1983). In contrast, a significant fraction of AH lignin obtained at very high reaction temperature (>220 °C) but short re­action time (<1 min) was soluble in 1 N NaOH (70-90% of AH lignin) and dioxane (50%); the amounts of carbo­hydrates in these soluble lignins were significantly lower, 2-4% (Glasser et al., 1983; Lora and Glasser,

2002) . SE lignin is also quite degraded in terms of cleav­age of b-O-4 linkages, but apparently much less condensed than AH lignins (Glasser et al., 1983; Robert et al., 1988; Li et al., 2009).

OS methods of lignin production are very diverse, where different organic solvents and reaction pH can be varied during the process. Each of these processes produces lignins that are very different in their physico­chemical and biochemical properties. Furthermore, the physical conditions (e. g. temperature, time, and pres­sure) and chemical conditions (e. g. pH and concentra­tion of solvents) under which these processes are conducted can drastically affect the molecular weight (Mw), chemical structure, and functional groups distri­bution in the generated lignin derivatives (Abdelkafi et al., 2011; Balakshin et al., 2013a, b). The most investi­gated OS delignification technology is the Alcell process
deployed at industrial scale in Eastern Canada during the 1980s. The Alcell process can be carried out in aqueous ethanol liquor at moderate acidity (no exoge­nous acid is added; the acidic pH results from formation of organic acids during the process). Alcell lignin is practically sulfur-free and has significantly lower amounts of carbohydrate and ash impurities compared to kraft lignins.

Lignosulfonates are a special class of technical lignins and they constitute the bulk of the commercial lignins for materials and chemical applications. Lignosulfonates are primarily isolated from sulfite spent liquors. Howev­er, sulfonated or sulfomethylated kraft lignins are often used in similar to lignosulfonates applications but they have different chemical properties, in particular the sul­fonic acid groups in lignosulfonates are located in the side alkyl chains, whereas in sulfonated kraft lignins they are found in the aromatic ring. Sulfur-containing lignins, in form of lignosulfonates or sulfomethylated kraft lignins, are commercialized by Borregaard (Sarpsborg, Norway), MeadWestvaco (Richmond, VA, USA), Tembec (Montreal, Quebec, Canada) and other smaller players for a wide variety of applications including dispersants for wettable powders, binders for granules and seed coatings, additives, etc. Although sulfur-containing lignins are generated during acid sul­fite pulping or are produced by sulfonation of kraft lignin, the chemistry of the process and correspondingly the lignin structure is quite specific. The main reaction in sulfite pulping is sulfonation of lignin side chain, pre­dominantly at the a-position of the propane chain as well as at the conjugated a-hydroxyl group. In addition, carbonyl groups also undergo sulfonation although the

mechanism is different from that for hydroxyl groups. Introduction of highly polar sulfonate groups into the lignin structure strongly increases its solubility in aqueous solutions. Most of the lignosulfonates contain about 1 sulfonate group per 2 monomeric units. Although strong degradation of lignin is not needed to transfer sulfonated lignin from solid phase into solution, it still takes place and the degree of degradation is dependent on the reaction conditions. Therefore, the molecular mass of lignosulfonates is very high and varies strongly. The average Mw of lignosulfonates has been reported in the range of 10,000—40,000 Da and a fraction with Mw up to 100,000 Da has been isolated. An increased number of phenolic hydroxyl groups can be observed in lignosulfonates and it is strongly depen­dent on reaction conditions (Glennie, 1971).