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

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 mostly focuses on the present available data.

#### **3.1. Lignin: structural chemistry**

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 hardwood lignins [37–40].

**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.

There are few types of native lignin substructures and interunit for different biomass sources as seen in **Figure 3**. The β-O-4′ (aryl ether) interunit linkage has been identified as a major lignin substructure where it provides a foundation for polymeric framework deconstruction in industrial processes. Some are known to have some resistant to chemical degradation such as β-5′, β-β′, 5-5′, 5-O-4′ and β-1′ [37, 41]. Thus, lignin usually appears in a complicated macromolecule form due to the diversity functional groups and various kinds of linkage of lignin substructures [42]. Lignin usually exists in the form of lignin-carbohydrate complexes (LCCs) resulted from dehydrogenation of G, S and H monolignols. Hence, the covalently linked to hemicellulose lignin and the occurrence of lignin cross-linked to polysaccharides provide mechanical strength to plant cell wall and additional protection to other structures from external [37].

**Figure 3.** Common interunit linkages in lignin polymeric framework. Reprinted from Ref. [41] with permission of The Royal Society of Chemistry.
