A guide to lignin valorization in biorefineries: traditional, recent, and forthcoming approaches to convert raw lignocellulose into valuable materials and chemicals

Lignin is the most abundant source of renewable aromatics on Earth, yet its enormous potential remains underexploited in current biorefinery and pulping processes. The extensive degree of condensation of the lignin fractions produced via the most widely adopted biomass pretreatments (i.e. “technical lignin”) poses a prominent limitation to their subsequent conversion toward valuable products. In this work, a broad range of methods for biomass pretreatment are reviewed, illustrating the impact of each strategy on the properties of the isolated lignin and carbohydrate fractions. The main pathways for the valorization of the obtained lignin streams (i.e. toward polymeric materials or chemicals) are critically discussed, and the relationship existing between (i) native lignin structure, (ii) pretreatment conditions, and (iii) lignin processability is rationalized. A key aspect for producing lignin streams amenable to further upgrading is the prevention of condensation reactions between lignin fragments during biomass fractionation. In this respect, a class of so-called “lignin-first” pretreatments, targeting the prompt stabilization of reactive lignin intermediates to minimize lignin condensation, has recently gained momentum. Herein, lignin-first approaches are reviewed, discussing in detail the fate of lignin, cellulose, and hemicellulose for each strategy. The potential of lignin-first biorefineries to realize a more complete valorization of lignocellulose and the current limitations of each method are highlighted. Overall, this work provides a comprehensive overview of the technologies that are available or currently emerging for lignin isolation and subsequent valorization.