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A Brief Introduction of Lignin

Yuhe Liao

Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, No. 2 Nengyuan Road, Tianhe, Guangzhou, 510640, China

1.1 Introduction

To reduce greenhouse gas (GHG) emissions, society should use renewable carbon resources in a sustainable way to produce chemicals, materials, and fuels next to circular use of currently available carbon feedstock. Lignocellulosic biomass has been considered as an abundant, carbon dioxide neutral, and renewable carbon resource [1]. The major compositions in lignocellulosic biomass are three oxygen-containing biopolymers: cellulose (40-60%), hemicellulose (10-40%), and lignin (15-30%), and the minor compositions include proteins, fats, pectins, inorganic matter, and others [2]. Cellulose and hemicellulose are carbohydrate-based biopolymers, whereas lignin is a complex aromatic biopolymer with a high carbon content (Figure 1.1) [3, 4]. Typically, cellulose determines the structure of cell walls in the form of microfibrils. The cross-linked lignin and hemicellulose wrap around the cellulose microfibrils (Figure 1.1). The lignin can provide additional rigidity and cause the cell walls to be hydrophobic and water impermeable. Therefore, these three main fractions are intertwined to yield the complex structure, contributing to biomass recalcitrance, which hampers the effective valorization of lignocellulosic biomass toward high-value products such as chemical and liquid fuels [5].

Different approaches have been developed to overcome the recalcitrance and valorize lignocellulosic biomass over the past decades [2, 6]. Classically, the lignocellulosic biomass is utilized to produce high-quality pulps for paper production. Emerging approaches are conversion of lignocellulose via thermal cracking (e.g. pyrolysis), biocatalysis, chemocatalysis, and integration of them toward chemicals and fuels such as bioethanol, furfurals, and levulinic acid [1, 3, 4, 7-13]. It is clear that these products are usually derived from cellulose and hemicellulose, whereas lignin, either left as a solid residue after conversion of cellulose and hemicellulose (such as via hydrolysis) or extracted from lignocellulose with cellulose and hemicellulose as solid residue, is considered as a waste or a low-value product for energy use. Currently, the pulping and biorefinery processes such as bioethanol production generate more than 50 million tons of lignin annually with ca. 95% used as fuel for heat and power generation due to the recalcitrance of lignin [14, 15]. The utilized 5% of lignin have several applications such as additives, surfactants, and adhesives. Although delignification (i.e. removal of lignin) of lignocellulosic biomass can facilitate the utilization of cellulose and hemicellulose to improve the economics of biorefineries, the value of biomass may not be maximized without utilization of lignin toward high-value products. Techno-economic analysis (TEA) and life-cycle assessment (LCA) have shown that valorize lignin can improve both economics and sustainability of biorefineries [16]. Hence, it is paramount to valorization of lignin with novel strategies to explore the potential of all carbon constituents of lignocellulose.

Figure 1.1 General structure of lignocellulose in plant and representative structure of cellulose, hemicellulose, and lignin.

Source: From Liao et al. [2].

Over the past years, some progresses were achieved in the aspects of lignin characterizations to reveal the structure of lignin, isolation to obtain lignin with different properties (even native like), and valorization toward chemicals, fuels, and materials with novel approaches. Hence, this book aims to introduce the most recent advancements in these aspects, particularly the valorization methods such as oxidation, photocatalysis, electrocatalysis, and valorization of native lignin. As a preface to the following chapters, this introductory chapter will briefly introduce the structure of lignin from the point of view of monomeric units, inter-unit linkages, and biosynthesis.

1.2 The Building Blocks of Lignin

Lignin, a phenolic biopolymer, is derived primarily from three kinds of 4-hydroxyphenylpropanoids (i.e. monolignols, e.g. p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol, Figure 1.2). These monolignols differ in substitution degree of methoxylation on the aromatic ring (i.e. 2 and 6 positions) and incorporate into lignin chain to produce corresponding p-hydroxyphenyl (), guaiacyl (G), and syringyl (S) units, respectively [17]. Besides, it is found that lignin is derived from numerous other building blocks, such as p-coumarates, ferulates, caffeyl alcohol, p-hydroxybenzoates, hydroxycinnamaldehdyes, tricin, and hydroxystilbenes [18, 19]. These building blocks are shown in Figure 1.2.

Currently, the only known route to form these building blocks is the phenylpropanoid pathway starting from phenylalanine via multiple steps in all plants with different enzymes [20, 21]. Whereas tyrosine can be an additional starting substrate for grasses. Figure 1.3 overviews the complete pathway of phenylpropanoid pathway for these building blocks. The phenylalanine was first deaminated in the presence of phenylalanine ammonia-lyase (PAL) toward cinnamic acid, which is then hydroxylated toward p-coumaric acid in the presence of cinnamate 4-hydroxylase (C4H). While in the case of tyrosine, p-coumaric acid can be produced from a shortcut pathway, direct deamination with tyrosine ammonia-lyase (TAL) or PAL [14, 23].

Then, p-coumaric acid is enzymatically converted toward either p-coumaroyl-CoA via 4-coumarate: CoA ligase (4CL) or caffeic acid through hydroxylation via p-coumarate 3-hydroxylase (C3H). p-Coumaryl alcohol is produced from reduction of p-coumaroyl-CoA via cinnamoyl-CoA reductase (CCR) and cinnamyl alcohol dehydrogenase (CAD). For caffeic acid, methylation via caffeic acid O-methyltransferase (COMT) forms ferulic acid, which can be transformed into feruloyl-CoA. Meanwhile, caffeic acid can be converted toward cafferoyl-CoA, which can be further methylated to feruloyl-CoA via caffeoyl-CoA O-methyltransferase (CCoAOMT). In addition, conversion of p-coumaroyl-CoA in the presence of shikimate/quinate hydroxycinnamoyl transferase (HCT) yields p-coumaroyl shikimate. Hydroxylation of p-coumaroyl shikimate via C3H can produce p-caffeoyl shikimate, which can be transformed toward caffeic acid and caffeoyl-CoA in the presence of caffeoyl shikimate esterase (CSE) and HCT, respectively. The shikimate intermediates are currently recognized as the favored substrates for hydroxylation [22].

Reduction of feruloyl-CoA via CCR yields coniferaldehyde, which can be transformed to coniferyl alcohol and 5-hydroxyconiferaldehyde through reduction (via CAD) and hydroxylation (via ferulate 5-hydroxylase, F5H), respectively (Figure 1.3). The main pathway to produce sinapyl alcohol is hydroxylation (via F5H) of coniferyl aldehyde followed by tandem methylation (via COMT) and reduction. Hydroxylation (via F5H) of coniferyl alcohol followed by methylation can form sinapyl alcohol as well. Oxidation of coniferylaldehyde by hydroxycinnamaldehyde dehydrogenase (HCALDH) forms ferulic acid. These synthesized monolignols are transported to the cell wall and integrated into a growing lignin chain.

Figure 1.2 Lignin building blocks. Three main monolignols (i.e. p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol) are shown in bold.

Figure 1.3 Summary of the metabolic pathway toward lignin building blocks. PAL, phenylalanine ammonia-lyase (PAL); cinnamate 4-hydroxylase (C4H); tyrosine ammonia-lyase (TAL); p-coumarate 3-hydroxylase (C3H); caffeic acid O-methyltransferase (COMT); 4-coumarate:CoA ligase (4CL); cinnamoyl-CoA reductase (CCR); cinnamyl alcohol dehydrogenase (CAD); p-hydroxycinnamoyl-CoA: quinate/shikimate p-hydroxycinnamoyltransferase (HCT); caffeoyl-CoA O-methyltransferase (CCoAOMT); caffeoyl shikimate esterase (CSE); ferulate 5-hydroxylase (F5H); p-coumaroyl-CoA monolignol transferase (PMT); feruloyl-CoA monolignol transferase (FMT); chalcone synthase (CHS); chalcone isomerase (CHI); flavone synthase (FNS); flavonoid 3'-hydroxylase (F3'H); flavonoid 5'-hydroxylase (F5'H); stilbene synthase (STS); hydroxycinnamaldehyde dehydrogenase (HCALDH);...