Review: Oxidation of Lignin Using Ionic Liquids-An Innovative Strategy To Produce Renewable Chemicals

Lignin, one of the three subcomponents of lignocellulosic biomass (along with cellulose and hemicellulose), represents more than 20% of the total mass of the Earth's biosphere. However, essentially due to its complex structure, this renewable polymer derived from biomass is mainly burned as a source of energy in the pulp and paper industry. Today, the valorization of lignin into the production of chemical feedstocks represents a real challenge in terms of both sustainability and environmental protection. This review first briefly outlines the main points of this challenge and compares the different methods investigated by chemists over the past several decades, pointing out the major difficulties met. Next, the review highlights the recent use of ionic liquids (ILs) as solvents that have provided some new opportunities to efficiently convert lignin and lignin model compounds into value-added aromatic chemicals. Particular focus is given to these new strategics in terms of selectivity, separation and the unique compounds obtained for the oxidation of lignin using ILs. Finally, an assessment of the challenges that must be resolved in order for ILs to become an eco-friendly way of producing chemicals from biomass, including lignin, is proposed.