**3.2. Lignin: chemical and biological conversion**

commonly involves degradation of the materials using microorganisms or fermentation. Meanwhile, woody plants or dry biomass involves physical techniques such as gasification,

Lignin is one of the major components in lignocellulosic biomass together with cellulose and hemicellulose. It intertwined with the cellulose-hemicellulose structural linkage that poses barriers for any physical or biological disturbance through the strong structure of lignin. The structural chemistry of lignin has been discussed and detailed of each of its monomers also presented. In addition, lignin conversion to fine chemicals and fuel has been added and

Through the binding of arrays of carbon-carbon and ether linkages, a single intermonomeric bonding scaffold was formed which is also known as lignin matrix [37]. This complex heterogenous structure of lignin consists essentially from three aromatic alcohols: p-coumaryl, coniferyl and sinapyl alcohols. These monolignols form phenolic substructures such as guaiacyl (G, from coniferyl alcohol), p-hydroxyphenyl (H, from coumaryl alcohol) and syringyl (S, from sinapyl alcohol) as shown in **Figure 2**. Each chemical structure confers a distinctive characteristic to lignin. Lignins composed mostly of G-units are usually softwood lignins, while lignins with different ratios of G- and S-units are hard-

**Figure 2.** A segment of lignin polymer structure with monolignols involved in lignin biosynthesis: p-coumaryl alcohol (1), coniferyl alcohol (2) and sinapyl alcohol (3). Possible phenolic structures: guaiacyl (G), p-hydroxyphenyl (H) and

syringyl (S). Reprinted from Ref. [38] with permission from Hindawi.

**3. Lignin: structural chemistry and route to conversion**

294 New Advances in Hydrogenation Processes - Fundamentals and Applications

pyrolysis or physical combustion.

mostly focuses on the present available data.

**3.1. Lignin: structural chemistry**

wood lignins [37–40].

High value compound can be produced through biological and chemical conversion of lignin. Processes such as gasification, hydrolysis, oxidation and pyrolysis are a well-known chemical conversion process. A wide range of polymers, chemicals and building blocks can be synthesised from chemically converted lignin [40]. Hydrodeoxygenation (HDO) is an example of chemical conversion method. It involves the removal of oxygen from oxygen-containing molecules in the presence of catalysts with high-pressure hydrogen at moderate temperature. Oxygen is removed to form water via hydrogenolysis reaction and then saturated by hydrogenation reaction [37]. Catalytic HDO of guaiacol (2-methoxyphenol), an oxygen-rich lignin model compound, has been investigated by Aqsha et al. for production of deoxygenated products. Guaiacol conversion products are mainly determined by methoxy, hydroxyl and benzene ring [43].

Alternatively, lignin converted from biological process such as enzymatic oxidation and microbial conversion involves living organisms [40]. Lignin-degrading microbes such as ligninolytic peroxidase enzymes or laccase enzymes have been exploited to oxidise aromatic units within lignin complex molecules [44].

#### **3.3. Examples of lignin to value-added materials**

Yi-Lin Chung et al. developed a catalytic and solvent-free method for synthesis of a lignin-poly (lactic acid) copolymer. The g-type poly(lactic acid) (PLA) copolymer, synthesised from graft polymerisation of lactide onto lignin, is catalysed by triazabicyclodecene (TBD) as depicted in **Figure 4**. It displays a glass transition temperature range from 45 to 85°C with multiphase melting behaviour. It also can be used to enhance UV absorption and reduce brittleness without sacrificing its elasticity [45].

**Figure 4.** Ring-opening polymerisation of lactide (LA) on lignin using triazabicyclodecene (TBD) catalyst scheme. Reprinted with permission from Ref. [45]. Copyright (2013) American Chemical Society.

**Figure 5.** Quantitative analysis of product streams from organosolv lignin substrate. Reprinted with permission from Ref. [46]. Copyright (2014) American Chemical Society.

Barta and Ford devised a novel catalytic system to produce organic liquids from renewable lignocellulose feedstock. Supercritical methanol was used as reaction medium in a single-stage reactor at operating temperature of 300–320°C at 160–220 bar using copper-doped porous metal oxide. This system was tested on organosolv lignin solution (deep brown colour). As summarised in **Figure 5**, no char was formed during reaction, and the gaseous products were mainly hydrogen gas. The average molecular weight of liquid-phase product is within range of the monomer, and dimer units indicate that organosolv lignin was fully converted [46].
