Biorefining of Lignin Wastes: Modularized Production of Value-Added Compounds

Abstract

Lignin, an aromatic polymer present in lignocellulosic biomasses, is conventionally viewed as a waste by-product of the pulp, paper, and other industries that use plant biomass as feedstocks. More recently, lignin has been reported as a renewable feedstock whose valorization generates several renewable aromatic intermediates, which can be used as carbon sources to synthesize other value-added chemicals, polymers, fuels, and other oxidized products. In this endeavor, the lignin’s recalcitrance, complex structure, and heterogeneity are major impediments that not only make microbial depolymerization exceptionally challenging but also generate phenolic compounds which inhibit microbial fermentation. Furthermore, during lignin’s breakdown, a heterogeneous complex mixture of low-molecular-weight aromatic monomers is released, representing additional hurdles when striving to recover the desired compound selectively. Nevertheless, in nature, some microorganisms are competent to funnel the heterogeneous stream of central aromatic intermediates into a single, pure compound. Centered on lignin, this chapter starts with a generalized description of the structure and types of commercially available lignin (e.g., lignosulphonates or sulfonated lignin, kraft lignin, soda lignin, organosolv lignin, and biorefinery lignin). Next, the slate of aromatic intermediatory compounds formed down the lignin-degrading β-ketoadipate pathway is briefly presented. Finally, this chapter summarizes few case studies related to the production of a high value-added chemical, vanillin, and a biopolymer, polyhydroxyalkanoates (PHAs), using lignin or its derivatives.KeywordsBiorefiningLigninPolyhydroxyalkanoatesValorizationVanillin