The driving force of biomass value-addition: Selective catalytic depolymerization of lignin to high-value chemicals

Lignin is a cross linked aromatic polymer and it is abundant in nature. It is regarded as a potential substitute of fossil resource and industrial raw materials due to its highly aromatic character via lignin valorization and commercialization. Lignin depolymerization is an effective solution to achieve lignin valorization by yielding value-added fine chemicals (such as monophenols and biofuels). Due to their stable structure, complex spatial configurations and electronic effect, lignin depolymerization often suffered from serious condensation and complexity of products. Commonly, this results in low yield and selectivity that limit large scale of lignin depolymerization. A challenge exists, however, on how to develop high selective catalytic strategies/reactors, which would enable efficient and selective catalysis by controlling the bond breaking mode and inhibiting the condensation of active intermedia. In this review, we begin by introducing the potential of lignin as an emerging star for lignin commercialization. And then catalytic depolymerization strategy (e.g., catalytic pyrolysis, catalytic hydrolysis, catalytic oxidation, catalytic reduction and two-step redox) and also their selective bond breaking mechanism are reviewed. Particularly, we report latest vision about lignin highly efficient valorization targeting at reaching market, by controlling of reaction condensation and using continuous-flow reactor depolymerization. Most importantly, the limitations and potential solutions for lignin depolymerization are proposed to present opportunities for future industrialization applications.