Lignin derived from lignocellulose is a renewable resource for the production of chemicals and fuels. However

due to its highly irregular polymeric structure and the carbon based inactive property

lignin valorization is very difficult. Lignin is usually viewed as a waste by-product in the current biorefinery processes and most of the lignin is burned to produce heat and power for the biorefinery processes. There were a series of studies on the lignin conversion such as depolymerization over acid or base

pyrolysis

hydroprocessing and oxygenation. The degradation of lignin over hydroprocessing was the most efficient method to produce alkane fuels and high value-added chemicals such as phenols. However

there were some problems remained to be solved such as catalyst deactivation and low yield. This review focuses on the catalytic systems for lignin hydroprocessing and current challenges in order to provide a reference for efficient and large-scale application of lignin.