Recent developments in chemical degradation of lignin: catalytic oxidation and ionic liquids

Lignin, considered as the second most abundant resource of aromatic chemicals in nature, has attracted a great deal of attention in academia and industry. Recently, numerous investigations have been carried out to consider the production of aromatic chemicals with high value, sourced from lignin. Among all these methods, oxidative treatments of the lignin show a promising way to generate highly functionalised monomeric or oligomeric chemicals, which can be further applied in chemical industries. Moreover, ionic liquids, known as salts with melting point below 100 °C, have also been introduced into oxidative lignin depolymerisation as these are known to disrupt the lignocellulose structure as well as partially dissolving the lignin. Herein, we reviewed various significant pathways of catalytic lignin oxidation and also discussed strengths and weaknesses between different lignin oxidation routes reported in the recent literature. Several innovative strategies for oxidative lignin depolymerisation in ionic liquid are also discussed, along with some recommendations and pathways in the field of catalytic lignin oxidation and depolymerisation involving ionic liquids.