1
General Background and Introduction

Abstract

Lignin, a complicated organic polymer, plays a significant structural function in the support tissues of vascular plants. It is particularly prevalent in woody plants and is highly polymerized. Lignin is one of the three crucial elements of wood, along with extractives and carbohydrates. Lignin, a three-dimensional amorphous polymer made of methoxylated phenylpropane structures, is important for the survival of vascular plants. In nature, lignin polymer generally forms ether or ester linkages with hemicellulose which is also connected with cellulose. The structural and chemical composition of lignin; representative linkages in lignin molecules and types of lignin are presented in this chapter.

Keywords Lignin; Phenylpropane units; Monolignols; Sinapyl alcohol; Coniferyl alcohol; p-coumaryl alcohol; Hardwood lignin; Softwood lignin;

With the rapid growth of populations and rising living standards in developing nations, global energy demand is rapidly rising. To meet this rising energy demand, fossil resources alone will not be sufficient. At the same time, there are significant concerns regarding the impact of climate change, which may be linked to the combustion of fossil fuels that are not renewable. As a result, it is crucial to develop technologies that can use new energy solutions on a large scale and provide more environmentally friendly alternatives to the current economy based on fossil fuels. Because this renewable feedstock can theoretically be incorporated into a carbon dioxide-neutral energy cycle, biomass is an option for the production of sustainable fuels and chemicals. Cellulose, hemicellulose, and lignin are the three main components of biomass.

Aromatic compounds, which can be used as fuel or as intermediate chemicals in the industry, can be obtained from lignin, the organic biopolymer that is found in the second highest concentration anywhere on the planet. Biomass conversion technology's viability can be improved by incorporating lignin into biorefineries. The recalcitrant and complicated nature of the lignin feedstock presents the primary obstacle in this situation. It is a huge challenge to properly convert lignin into functional polymers, but this is a fascinating area of research in both industry and academia (Guvenatam, 2015).

1.1 Structural and Chemical Composition of Lignin

The Swiss botanist Augustin Pyramus de Candolle was the first person to use the term lignin, which comes from the Latin word lignum, which means wood (Candolle et al., 1821). Lignin, a complicated organic polymer, plays a significant structural role in the support tissues of vascular plants. It is particularly prevalent in woody plants and is highly polymerized. Lignin is one of the three crucial elements of wood, along with extractives and carbohydrates (Sarkanen and Ludwig, 1971; Sjöström, 1982). Protolignin is the name given to lignin when it is in its natural state, as it is in plants. Lignin, a three-dimensional amorphous polymer made of methoxylated phenylpropane structures, is essential for the survival of vascular plants. In nature, lignin polymer usually forms ether or ester linkages with hemicellulose which is also associated with cellulose. Therefore, these natural polymers construct a complicated and valuable lignocellulose polymer (Figure 1.1).

Figure 1.1 Lignocellulose in biomass and its composition. Chonlong Chio et al. 2019/ Reproduced with permission from Elsevier.

1.2 Major Backbone Units and Representative Linkages in Lignin Molecules

It is generally acknowledged that the polymerization of three types of phenylpropane units, also known as monolignols, initiates the biosynthesis of lignin (Freudenberg and Neish, 1968; Lewis, 1999; Ralph, 1999; Sarkanen and Ludwig, 1971). These units, sinapyl, coniferyl, and p-coumaryl alcohol, are linked by the chemical bonds of aryl ether (ß-O-4), phenylcoumaran (ß-5), resinol (ß-ß), biphenyl ether (5-O-4), and dibenzodioxocin (5-5) (Figure 1.2). In Figure 1.3, the three structures are shown. The most typical linkage among the various typical linkages (ß-O-4, ß-5, ß-1, 5-5, a-O-4, 4-O-5, ß-ß) (Figure 1.4) is the ß-aryl ether (ß-O-4), which accounts for more than half of the structure of lignin (Dutta et al., 2014; Rinaldi et al., 2016). Figure 1.5 shows model lignin structures: A softwood, B hardwood, and C grass (Lu and Gu, 2022).

Figure 1.2 Major backbone units and representative linkages in lignin molecules. (a) The building blocks of lignin consist of three primary types of monolignols, namely p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol. The alcohols form the corresponding phenylpropanoid units like p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) in lignin polymer, respectively. (b) Backbone units are conjugated via different chemical bonds (e.g., ß-O-4, ß-ß, 5-5, and ß-5) resulting in high resistance to lignin depolymerisation Weng et al. (2021) / Springer Nature / Public Domain CC BY 4.0.

Figure 1.3 The three building blocks of lignin. Chakar and Ragauskas, 2004 / with permission of ELSEVIER.

Figure 1.4 Typical linkages present in lignin. Agarwal et al. (2018) / with permission of ELSEVIER.

Figure 1.5 Model lignin structures: (A) softwood, (B) hardwood, and (C) grass. Lu and Gu (2022) / Springer Nature / Public Domain CC BY 4.0.

Figure 1.6 demonstrates the phenoxy radicals that are resonance-stabilized and the dehydrogenation of coniferyl alcohol (Chakar and Ragauskas, 2004). The polymerization interaction is set up by the oxidation of the monolignol phenolic hydroxyl groups. It has been demonstrated that an enzymatic pathway catalyzes the oxidation itself. An electron transfer initiates the enzymatic dehydrogenation, resulting in reactive monolignol species and free radicals, which are able to pair with one another. The aromaticity of the benzene ring will be restored by a subsequent nucleophilic attack by water, alcohols, or phenolic hydroxyl groups on the benzyl carbon of the quinone methide intermediate. Polymerization will continue on the produced dilignols.

Figure 1.6 Dehydrogenation of coniferyl alcohol and the mesomeric radicals. Chakar and Ragauskas, 2004 / with permission of ELSEVIER.

Architecturally, lignin forms a complex, three-dimensional heterogeneous network thanks to the chemical bonds it forms with cellulose and hemicellulose via covalent and non-covalent bonds (Figure 1.7) (Agarwal et al., 2018). Due to lignin's irregular, heterogeneous structure, it remains difficult to produce commodity chemicals with added value.

Figure 1.7 Structure of lignin in lignocellulosic material. Agarwal et al. (2018) / with permission of ELSEVIER.

The primary sources of lignin that can be utilized on a larger scale are spent cooking liquor and the chemical extraction of wood fibers from the pulp and paper industry. Over 50 million tons of lignin-based materials and chemicals are produced annually worldwide. Despite the fact that most lignin in the world is still used as boiler fuel in facilities that process carbohydrates, its low value and abundance indicate that it might be used to create high-value new products.

If converted into chemical compounds, bioproducts based on lignin could lead to a multibillion-dollar industry. Several million tonnes of lignin are produced as a low-value byproduct of industrial cellulosic bioethanol production. It is anticipated that the US bioethanol industry alone will produce up to 60 Mt./year of lignin by the end of 2022 (Holladay et al., 2007a; Joffres et al., 2014).

1.3 Types of Lignin

Sinapyl alcohol, coniferyl alcohol and p-coumaryl alcohol, are frequently joined by non-hydrolysable linkages during the dehydrogenation of phenylpropanoid precursors that is carried out by free radicals with the assistance of peroxidase and results in the formation of lignin. The aromatic amorphous heteropolymer lignin lacks any optical activity. These three monoolignols are present in varying amounts in various plant species. For instance, softwood lignin contains a lot of coniferyl alcohol, whereas hardwood lignin contains both sinapyl and coniferyl alcohols and grass lignin contains all the three monolignols (Duval and Lawoko, 2014). The extracted lignin has been divided into four major categories-lignosulfonates, kraft lignin, soda lignin, and organosolv lignin based on the chemical pre-treatment method used: sulfur-free soda lignin is produced when biomass is treated with sodium hydroxide, whereas kraft lignin is produced when biomass is treated with sodium sulfide and sodium hydroxide. The ethanol-water extraction method and the pre-treatment of biomass with aqueous sulfur dioxide produce lignosulfonates and organosolv lignin, respectively. In addition to the four primary types of lignin, ionic liquid lignin, which is produced by treating biomass with ionic liquid is attracting a lot of interest because of its condensed structure and a low ß-O-4 content (Wen et al., 2013). Due to the structural changes that take place when lignin is separated from lignocellulose biomass, chemical properties vary between lignin types. Contrary to kraft lignin and lignosulfonates, organosolv lignin, which is...