Lignin transformations for high value applications: towards targeted modifications using green chemistry

Lignin represents a considerable source of renewable and bio-based carbon. Pulping processes enable lignin, together with all components of the lignocellulosic biomass, to enter valorizable streams. A current key objective is to further valorize this versatile aromatic biopolymer, and for that, to go beyond its mere energy use. Despite the emergence of numerous proposals for value-added products coming from lignin, most of them remain at the research stage. The main challenges arise from the complexity and heterogeneity of the lignin structure and resulting molecular properties, the variability of the biomass source, pre-treatment processes, and the growing environment. Keeping in mind that future integrated bio-refineries must take into account environmental concerns, lignin processing in accordance with green chemistry principles should first be favoured. From this very perspective, this work proposes to review the most promising current routes towards fractionation and/or depolymerization of lignin. Those should represent sustainable treatment technologies potentially leading to a broad spectrum of marketable lignin-based molecules and products. First, lignin fractionation by selective precipitation using pH as well as green solvents, or by using membrane technologies, will be addressed. Then lignin depolymerization will be discussed at length, notably from a catalytic point of view and by hydrogenolysis; the knowledge about the fundamental chemistry stemming from the use of model compounds will be described. Substitution of organic solvents with environmentally harmless supercritical fluids or with negligible vapour pressure ionic liquids is of great interest to modify lignin, and is finally reviewed. Lastly, challenges for integrated biorefineries and for launching new lignin-based compounds and products will be discussed.