A review on chemical mechanisms of kraft pulping

1 Introduction

Chemical pulping is today’s dominating technique for making pulp from wood due to its volume and price and also the most favorable source of lignin for technical purposes (Chakar and Raguskas 2004; Vishtal and Kraslawski 2011). The processes are based on the wood cells (fibers) that are liberated from each other by the dissolution of lignin. Of the “more traditional” chemical pulping concepts, we have two main groups of processes, sulfite pulping processes (Annergren and Germgård 2014), and alkaline processes, where kraft pulping is dominant (Chakar and Raguskas 2004; Ragnar et al. 2013; Sharma et al. 2020). Sulfite processes remove the lignin by producing a sulfonated water-soluble lignin derivate i.e. lignosulfonates, that have many valuable applications, and these have been technically established for a long time (Aro and Fathei 2017). However, the kraft process has such large advantages in operation costs, tolerance for raw material, scalability, and quality of the pulp, that it has become the most dominant process, not only for chemical pulping but also for pulping of different wood raw materials (Chakar and Raguska 2004). In this process, the lignin degrades into fragments with rather different structures than the native lignin. i.e. kraft lignin, which is fully solubilized in the strongly alkalic pulping solution (but not at neutral and low pH) (Gierer 1980; Ragnar et al. 2013; Guimarella et al. 2016). This solution, the black liquor, is transferred to a chemical recovery system, that not only generates the chemicals needed for pulping (white liquor) but also generates enough energy for the whole process, often with a certain surplus (Dogardis et al. 2019; Reeve 2002). However, some, not negligible, quantities of lignin can be taken out from the process without disturbing the chemical recovery process (Jönsson et al. 2008; Tomani et al. 2011), and this lignin has recently gained much interest (Liao et al. 2020; Vishtal and Kraslawski 2011).

Why is then the kraft process so successful? It combines mildness against the cellulose polymers, with an efficient delignification (Chakar and Raguskas 2004; Gierer 1980; Hartler 1997), that is based on depolymerization of lignin to molecules that are soluble in the conditions of kraft pulping, i.e., high temperature and high pH (Mattsson et al. 2017).

In this critical mini-review, we will discuss the chemical bases for lignin and polysaccharide degradation and their solubilization in kraft pulping, and aim to present a coherent picture of the most important chemical reactions in kraft pulping. However, firstly we briefly discuss the natural structure of lignin and polysaccharides.

2 Natural lignin structure

Although lignin from different types of plants differs in its composition of monomers (monolignols) (Figure 1), the basic properties of lignin are relatively similar in all vascular plants (Boerjan et al. 2003; Henriksson 2017). The most important inter-monolignol bond is the β-O-4 bond (Figure 2) (Ralph et al. 2019), and it is also this bond that is the main target that gets cleaved in kraft pulping (Gierer et al. 1965; Gierer 1980; Ljunggren 1980). Other naturally occurring inter-monolignol bonds are virtually unbreakable during kraft pulping conditions and are often called condensed bonds (Balakshin et al. 2003, 2011; Ertman 1957; Gierer 1980). The structure of the most important condensed bonds is shown in Figure 3. The size of the lignin polymer is not known with certainty, but some studies suggest a rather low degree of polymerization – around 20, but these data are based on heavily milled wood, and other studies suggest much higher weights in the wood (Boerjan et al. 2003; Crestini et al. 2011; Guerra et al. 2006; Henriksson 2017; Lebo et al. 2001). It is also a matter of discussion if lignin is a linear or a network-shaped polymer (Balakshin et al. 2020; Crestini et al. 2011; Davin and Lewis 2005). Furthermore, multiple covalent bonds between lignin and the polysaccharides of the cell wall are present, and the most important LCC (“lignin carbohydrate complexes, α-ethers, γ-esters and phenyl glucosides bonds”) (Guimarella et al. 2019; Henriksson 2017; Tarasov et al. 2018) are shown in Figure 4. The frequency of LCC on lignin is not known with certainty, but it is known that individual lignin molecules can bind covalently to at least two different polysaccharide molecules, in this way forming hybrid-molecules that are both lignin and polysaccharides and may even crosslink different types of polysaccharides (Lawoko et al. 2003), which might result in a cross liked structure (Figure 4b). For a further description of the structure of natural lignin, as well as the biosynthesis and biology of the biopolymer, see recent review articles (Guimarella et al. 2019; Henriksson 2017; Ralph et al. 2019; Tarasov et al. 2018).

Figure 1:

The most important lignin monomers (monolignols).

Figure 2:

The most common inter monolignol bond in natural lignin, the β-O-4 bond.

Figure 3:

Examples of the most important condensed inter-monolignol bonds.

Figure 4:

Lignin carbohydrate complexes (LCC): (a) the most common LCCs, i.e., covalent bonds between lignin and polysaccharides are ether, ester, and phenyl glucosides. (b) LCC crosslinks different polysaccharides in wood – schematic presentation of such crosslinked structure in the cell wall.

About 40–60 % of the inter-monolignol bonds are believed to be β-O-4 bonds, and the rest are different types of condensed bonds. The distribution of different bonds in polymer appears to be random (Ralph et al. 2019), and condensed bonds appear seldom in clusters with more than three after each other (Önnerud and Gellerstedt 2003). This means that if one could break al β-O-4 bonds and al LCC: s, the lignin should be degraded into fragments corresponding to monomers up to tetramers of monolignols, and such small molecules should be soluble in the strongly alkaline pulping liquid (Mattson et al. 2017).

3 Natural polysaccharide structure

The polysaccharides of wood are divided into cellulose and non-cellulose polysaccharides with usually relatively equal proportion, although there are normally slightly more cellulose than the non-cellulose polysaccharides, especially in some eucalypt species where the cellulose content can be as high as 54 % (Coté 1968; Pereira 1988). Cellulose has a simple molecular structure, consisting of straight, flat, and unbranched chains of anhydrous β-glucopyranoside connected with 1,4 glycosidic bonds (O’Sullivan 1997). This simple structure and a high degree (several thousand) of polymerization – allow the cellulose chains to form more or less crystalline microfibrils, held together by hydrogen bonds and hydrophobic interactions (Medrono et al. 2012), that can be described as strong enforcing fibers in the plant cell wall (Figure 5).

Figure 5:

Structure of cellulose. (a) Cellulose consists of linear and unbranched chains of glucose residues connected by β 1,4 glycosidic bonds, that are stabilized by hydrogen bonds – one on each side of the glycosidic bond. The degree of polymerization can be several thousand. (b) Parallel cellulose chains form hydrogen bonds between the chains forming cellulose sheets. (c) The sheets are packed “sandwich style” either as I_α or I_β, and are held together by hydrophobic interactions. (d) This crystalline structure forms extended crystals called microfibrils, which are suggested to contain around 28 chains and have a diamond shape.

The non-cellulose polysaccharides vary considerably between different kinds of wood and can be divided into two kinds, hemicelluloses, which is the main chain that can adapt to a similar straight and flat structure as cellulose, and pectins that cannot adapt to this structure (Henriksson et al. 2018; Pauly et al. 2013; Thakur et al. 1997). Both groups often have side groups/chains and modifications of monosaccharide residues, such as acetylations and methylations. Normally hemicelluloses are more abundant than pectins in most woods. In hardwood, xylan-based hemicellulose is dominant, and in softwood, glucomannan is the most common group although xylan also exists in considerable amounts (Calvahero et al. 2008; Zhou et al. 2016). The degree of polymerization is relatively low – and believed to be around 200, but covalent bonds to lignin by LCC are common thereby forming hybrid molecules with both hemicellulose and lignin functionality (Henriksson 2017; Lawoko et al. 2003). In Figure 6, the structure of some important non-cellulose polysaccharides is shown.

Figure 6:

Structure of some non-cellulose cell wall polysaccharides. (a) The hemicelluloses, xylan, glucomannan, and xyloglucan. (b) The pectins, polygalacturonic acid, rhamnogalacturonan, and β-galactan. Note that some variations in the structures can occur due to origin, for instance, most xylans from hardwood lack arabinose side groups. The xylan and glucomannan structures shown here are of softwood type.

3 Lignin degradation during kraft pulping

β-O-4 bonds are broken mainly by two different mechanisms shown in Figure 7; the ‘slow sulfide independent reaction’ that works both on phenolic and non-phenolic structures (Ljunggren 1980), and the ‘fast sulfide dependent reaction’ that requires a phenolic residue for the degradation (Gierer et al. 1965). Two interesting observations can be made here; firstly, the first reaction (Figure 7a) can ‘cut up’ lignin polymer in the middle, and thereafter the second reaction can one-by-one remove monomers from the created ends (Figures 7b and 8). As a result, the sulfide-independent reaction is much slower than the sulfide-dependent reaction, and it performs a key reaction, so it can be considered a rate-limiting reaction (Ljunggren 1980; Ragnar et al. 2013), although others consider that soluble lignin diffusion from fiber is the limiting factor (Mattson et al. 2017). Secondly, the sulfur-dependent reaction, except for removing one monomer from the lignin polymer, also breaks LCC bonds of the α-ether type (Figure 4a and b), but note that this breakage requires a phenolic end group.

Figure 7:

Lignin degradation during kraft pulping. Breakage of non-phenolic (a) and phenolic (b) β-O-4 bonds during kraft pulping. Note that in reaction (b) the sulfur depends on the degradation of a phenolic structure and also a covalent bond to polysaccharides might be broken.

Figure 8:

Schematic presentation of the role of the lignin degrading reaction in kraft pulping.

How are other LCC bonds cleaved during kraft pulping? The matter has not been intensively studied, but it appears that the γ-esters are cleaved, probably by alkaline hydrolysis (Figure 9a) (Deshpande et al. 2020). Phenyl glucoside bonds may be relatively stable, but might also be partly broken by alkaline hydrolysis (Figure 9b) (Lawoko et al. 2009).

Figure 9:

Possible mechanisms of breakage of LCC bonds during kraft pulping.

The above description so far indicates that all lignin could be removed from the wood by these types of reactions. Unfortunately, this is not the case in reality, since unfavorable side reactions will also occur. The most important result of this is the formation of enol ether from phenolic end groups (Figure 10) (Gellerstedt and Lindfors 1987). The enol ether is relatively stable during kraft pulping (see below for its reaction with polysulfide anion), and can therefore be regarded as a ‘dead end’ that is an obstacle for further delignification (Figure 8). If the mechanism in Figure 7b and 10 are studied, it is obvious that there are two pathways from the quinone methide intermediate; one leading to favorable depolymerization, and the other to enol ether and formaldehyde. Higher hydrogen sulfide content will therefore favor the depolymerizing reaction which of course is positive. Unfortunately, hydrogen sulfide ions will also react with the methoxy groups of lignin creating methyl mercaptan and dimethyl sulfide, both toxic gases with very bad odor, which lead to high content of sulfur in pulping liquors is problematic (Tormund and Teder 1987).

Figure 10:

Formation of enol ethers (“dead ends”).

Reactions similar to the enol ether formation can also occur on two types of condensed bonds in lignin, the β-1 bond and the β-5 bond (Figure 11). In neither case any depolymerization occurs, although formaldehyde is created (Balakshin et al. 2003; Gierer 1980). Nevertheless, the reactions are interesting, since a stilbene structure is created; in reality, these reactions fuse the conjugated systems of two aromatic rings into a large conjugated system; this might be a large part of the explanation for the strong color of both unbleached kraft pulp and the consumed pulping liquor, i.e. the black liquor. Stilbenes might also react with other components in the pulping system (Lahtinen et al. 2021).

Figure 11:

Formation of stilbene structures from β-1 and β-5 structures.

Another reaction that can fuse conjugated systems and contribute to the color of kraft lignin/residual lignin is radical couplings of phenolic radicals of lignin fragments (Figure 12) (Gellerstedt et al. 2004; Giumarella et al. 2020). Radicals can be created by either the homolytic cleavage of inter-monolignol bonds directly, or by oxidation by sulfur radicals created by homolytic cleavage of elementary sulfur or polysulfide (Crestini et al. 2017; Gellerstedt et al. 2004). During the pulping process, the latter components are created. The polysulfide ions do not only create problems, but may also contribute to efficient lignin degradation, and even have the potential for cleaving enol ether structures (Figure 13) (Berthold and Lindström 1997).

Figure 12:

Radical coupling of lignin. The oxidation might be performed by sulfur radicals.

Figure 13:

Polysulfide in delignification reaction. Here the suggested mechanisms are shown by the tetra sulfide anion, but also other polysulfides can react in similar ways.

4 Polysaccharide degradation during kraft pulping

In kraft pulping carbohydrate losses can be considerable, which is an economical problem with the kraft process, due to yield losses. However, it is mainly the non-cellulose polysaccharides, and especially pectin and glucomannan, that are degraded, whereas xylan, and especially cellulose, are rather stable. There are three types of reactions responsible for the degradation, ß-eliminations, alkaline hydrolysis, and peeling reactions. In addition to these chemical reactions’ loss of carbohydrates by solubilization does occur, and the degrading reaction will increase the solubility of remaining fragments, similar to lignin degradation.

1. β-Eliminations are alkaline-catalyzed reactions that have a high reaction rate during kraft pulping. They are responsible for the degradation of pectin structures (Figure 14a), which is so efficient that normally no trace of pectin is left in the kraft pulp (Keijbets and Pilnik 1974). Another important β-elimination occurs on the O-methyl glucuronic acid residue that is the side chain on xylan (Figure 14b); this reaction does not lead to depolymerization, but it is important as well, since an unsaturated structure, hexenuronic acid, will be formed on the xylan chain, and thereby remain in the pulp (Teleman et al. 1995). This structure is normally regarded as undesired and can cause problems with the consumption of bleaching chemicals, post-yellowing, and lowered selectivity of bleaching (Sevastyanova et al. 2006).

2. Alkaline hydrolysis occurs on all polysaccharides in wood, and a suggested mechanism is shown in Figure 15. In the figure, an epoxide and a deprotonated alcohol form a reactive intermediate. As seen in the figure, the mechanism has a reactive intermediate consisting of an epoxide and a deprotonated alcohol. This can give a stable depolymerization if it reacts with water, but can also recreate the original structure (Figure 15). This gives a mechanistic explanation for the high stability of cellulose since in a crystalline structure the reactive groups are held close to each other which facilitates relegation (Figure 16a). For more disordered or even soluble hemicelluloses on the other hand, the epoxide and alcohol anion easily move away from each other which favors stable depolymerization, and this is even more the case for side groups (Figure 16b), which explains that non-cellulose polysaccharides are more sensitive than cellulose to alkaline hydrolysis, and that side groups often are lost. If this cutting reaction is intensive enough, the material will be lost by solubilization, but also fewer intensive splits will have negative consequences, i.e., lowering the degree of polymerization of cellulose (“viscosity losses”) that decrease the strength of the fiber (Berglund et al. 2019; Nieminen et al. 2014; Sjöström 1977).

3. The “peeling reaction” is a multistep reaction that leads to monosaccharides in modified forms being cleaved one after another from the reducing end of a polysaccharide chain producing a new reducing end and the reaction can therefore be continued (Figure 17). Peeling can be carried out both by reducing ends already present in the wood (primary peeling), but also by reducing ends created by alkaline hydrolysis (Figure 15) (i.e., secondary peeling). The peeling reaction is believed to be the main reason for the yield losses of polysaccharides during kraft pulping, but it is a worse problem for non-cellulose polysaccharides, simply because the degree of polymerization of cellulose is so high, that there are not so many starting points for peeling in this case (Da Silva Perez and Van Heinigen 2015; Nieminen et al. 2014). Fortunately, the peeling reaction competes with a “stopping reaction” where a group is leaving from the 3-carbon-position of the polysaccharide (instead of the 4-position as in peeling) (Figure 18). If this is done in glucomannan, the leaving group is a hydroxyl ion, which is not favorable, but on xylan, the leaving group could be an arabinose anion which is better (Ragnar et al. 2013; Sjöström 1977, 1993). This leads to the xylans that are much more stable than glucomannans in kraft pulping. However, there are probably other reasons for xylan’s relative stability in kraft pulping; two possibilities are shown in Figure 19, that the negative charges of the carboxylic acids of xylan create a protection for alkaline hydrolysis (Figure 19a), and that the substitution of C2 – O-methyl glucuronic acid, simply blocks the peeling reaction (Figure 19b) (Hartler and Svensson 1965). Glucomannan is uncharged, and has only acetylation as a partial substituent on the C2 position which quickly is removed by hydrolysis in kraft pulping, and is therefore more sensitive to peeling (Fearon et al. 2020; Gierer 1980).

It shall be noted that the polysaccharide degrading products from these reactions are not “normal” monosaccharides (Figure 17). Furthermore, they undergo a series of reactions in the black liquor, forming unsaturated structures, that may be strongly colored. These degradation products are opposite to the soluble sugars formed by sulfate pulping that are difficult to ferment.

Figure 14:

β-Elimination reaction in polysaccharide degradation during kraft pulping. The reaction mechanisms here are suggested to be of the carbanionic type (McLennan 1967), with resonance stabilization of the intermediate, something that requires a uronic acid. (a) Cutting of pectin chains by β-elimination and (b) reaction on O-methyl glucuronic acid side chain on xylan. This reaction does not lead to depolymerization but to the presence of an oxidable group in the pulp, i.e. hexenuronic acid.

Figure 15:

Alkaline hydrolysis of glycosidic bonds during kraft pulping. Note that a reactive intermediate is formed that might recouple, or react with water to a permanent cleavage.

Figure 16:

Different polysaccharides vary in sensitivity towards alkaline hydrolysis. (a) Cellulose is relatively insensitive to alkaline hydrolysis, since the reactive intermediate (iii) often goes back to the original form (i), since the chain ends are held together by adjacent cellulose chains, i.e., the crystalline structure. (b) Glycosidic bonds in hemicellulose chains are more sensitive to alkaline hydrolysis, since the chain ends in the reactive intermediate (iii) can more easily move away from each other and get time to react with water forming stable cleavage products (iv). (c) Side groups of hemicelluloses are even more sensitive, sine the reactive intermediate groups (iii) easily can move away from each other forming the stable end product (iv).

Figure 17:

“Peeling” reaction. This reaction led to monosaccharide residues that are lost one by one from the reducing end of the polysaccharide. Here the reaction is shown on glucomannan/cellulose, but similar reaction can also happen on xylan.

Figure 18:

“Stopping” reaction. This reaction can happen on both cellulose and hemicelluloses (xylan and glucomannan). However, since the tendency of this reaction is higher the better the leaving group (OR_B ⁻) is, and therefore xylan, where the leaving group is deprotonated arabinose, is more stable than cellulose and glucomannan, where the leaving group is a hydroxyl ion.

Figure 19:

Other reasons than the stopping reaction why xylan is relatively stable in kraft pulping. (a) The negative charges on xylan repel the hydroxyl ions creating a local lower pH. (b) Substitution on the 2-position, – O-methyl glucuronic acid – blocks the peeling reaction.

Except for lignin and polysaccharides, extractives and inorganic content also may play an unfavorable role in kraft pulping. Extractives that have a highly conjugated structure, such as pinosylvin, might couple with lignin to diminish defibrillation; this problem is often severe in acid sulfite pulping (Annegren and Germgård 2014; Koljonen et al. 2004). The high content of minerals, especially calcium in wood, may decrease the delignification and stimulate carbohydrate degradation, thereby decreasing the selectivity of the pulping. The mechanisms for this are unclear, but decreased solubility of lignin fragments and even catalytic properties of the calcium have been suggested (Lindgren 1997; Saltberg et al. 2009; Vegunta et al. 2022).

5 Concluding remarks

In summary, the regarded lignin degrading reactions compete with unwanted side reactions, and for lignin, this means that it will not be possible to remove all lignin from the pulp fibers (Figure 6); residual lignin that is strongly colored and in general covalently bond to the polysaccharides in the pulp (Lawoko et al. 2003) by LCC bonds are stable for the pulping conditions, maybe “protected” by enol ether structures or condensed bonds (Figure 20). For polysaccharides, these reactions mean that certain loss will be inevitable, however, the yield losses are more intensive on hemicellulose than on cellulose, and glucomannan is more lost than xylan. However, hexenuronic acid will be formed by side groups on the latter polysaccharide (Teleman et al. 1995).

Figure 20:

A hypothetical example of how a lignin structure (with an LCC to xylan) could react in kraft pulping producing a residual lignin molecule covalently attached to xylan in a pulp fiber. The LCC is “protected” by a stable condensed bond and will not be easily broken in kraft pulping. Note the large conjugated system (marked with a dotted line) – such structures might be partly responsible for the strong color of unbleached kraft pulp.

There are nevertheless possibilities to choose conditions of pulping that favor lignin degradation over carbohydrate depolymerization; the degradation of carbohydrates during the kraft cook can be divided into two phases of first order. In the first phase, the loss of easily dissolved hemicellulose has an activation energy of 146 kJ/mol and this phase involves physical dissolution and primary peeling. The value for the second phase is 169 kJ/mol and the second phase involves an alkaline cleavage reaction (Lindgren 1997). On the other hand, the activation energy for the bulk delignification of hardwood is 117 kJ/mol (Lindgren and Lindström 1997), and for softwood 127 kJ/mol (Lindgren and Lindström 1996). This means that it is favorable to decrease the cooking temperature in the kraft cook to improve yield, and the slower delignification can be compensated with an increased hydrogen sulfide concentration. The problem might also be attacked by chemical means. The polysaccharide yield losses are to a large extent due to the peeling reaction (Figure 17), and stabilization of the reducing end of polysaccharides by oxidation or reduction has been demonstrated to increase the yield of pulping while the peeling reaction (Figure 17) is strongly diminished (Wang et al. 2015). Anthraquinone pulping was a pulping concept that successfully used oxidation for improved yield, but due to potential health risks with the chemical, the method has lost popularity (Hart and Rudie 2014). Novel concepts for oxidation or reduction are therefore needed; polysulphide pulping is a relatively old technique for stabilizing polysaccharides by oxidation (Colodette et al. 2001), but suffers sometimes from technical problems. However, there are also other chemical treatments suggested, such as pretreatment of chips with sodium borohydride (Gulsoy and Eroglu 2011) and with dithionite (Lindén et al. 2020). Adding chemicals to kraft pulping is however an intricate technical problem since it must be compatible with the chemical recovery system (Ulmgren 1997).