Chemicals from lignin: Recent depolymerization techniques and upgrading extended pathways

Lignin is a complex aromatic heteropolymer that harbors great potential to serve as a basis for the production of chemicals, biofuels, and materials. In a bio-economy, lignin valorization through depolymerization to monomers and upgrading of these monomers to targeted chemicals is highly challenging, but attractive and important for the chemical industry. To address current limitations of lignin valorization and further upgrading, we will summarize the recent lignin depolymerization techniques that yield phenolic aldehydes and phenolics such as alkaline oxidation, fast pyrolysis, hydrogenolysis, and hydrolysis. In addition, this review describes novel upgrading approaches via biological and chemo-catalytic pathways from lignin monomers to targeted bulk chemicals, including cis, cis-muconic acid, adipic acid, and terephthalic acid, and to fine chemicals, and then discuss future prospects.