Nonwoody Lignin for Vanillin and Vanillic Acid Production

Lignin, the second plentiful organic polymer after cellulose, is an unexploited bioresource in biomass conversion processes. Despite conventionally lignin is considered as the hindrance in carbohydrate valorization and often downgraded as a fuel source for heat generation, lignin shows a sustainable feedstock of bioproducts, especially aromatic compounds. Nonwoody lignin that is sourced from a variety of herbaceous plants and crop waste have offered several advantages including readily available feedstock, not competing with food resources, and low sulfur impurities compared with woody lignin. These make them an attractive bioresource for selective production of valuable chemicals, namely vanillin and vanillic acid, which are primary applications in food, pharmaceutical, cosmetics, and chemical synthesis. This chapter discusses the isolation and purification methods for nonwoody lignin, ranging from conventional techniques (i.e., alkaline and organosolv extractions) to eco-friendly solvents (i.e., ionic liquids and deep eutectic solvents). The purity, molecular weight distribution, and structural integrity for downstream applications are mainly focused. Furthermore, pathways for nonwoody lignin conversion mainly chemical oxidations and bioconversions (enzymatic and microbial transformation, and metabolic engineering) are investigated with underlining on reaction mechanisms, yields, and limitations. Specific consideration is highlighted to technological innovations for catalytic oxidation such as alkaline chemical, electrochemical oxidation, biological (enzymatic), and photocatalytic addressing slow industrial production, severe processing conditions, low yields, and catalyst deactivation. Besides, this chapter reviews industrial case studies, techno-economic analysis, and sustainability that emphasize the practicability of lignin-derived vanillin and vanillic acid productions. Nonwoody lignin valorization not only alleviates environmental concerns correlated with crops residue disposal but also lowers the dependency on fossil- and petrochemical-based raw materials. Although the challenge of scaling up the process, process improvement, and product recovery, recent developments in catalysis innovation, integrated bioprocessing and biorefinery design strengthen the lignin as a main platform in sustainable productions of valuable aromatic chemicals.