Lignin Refinery: Towards the Preparation of Renewable Aromatic Building Blocks

With its wide availability and its low cost, lignocellulosic biomass represents a very convenient and sustainable alternative to fossil fuels for the preparation of energy, materials, and chemical building blocks. While processes for cellulose and hemicellulose conversion into valuable products are already advanced, adding value to lignin is still a challenge, due to its complex structure and its relatively high stability towards a variety of chemical transformations. The pathway for the valorization of lignin from biomass often includes an initial extraction step, which in some cases can affect the native structure of the lignin backbone. At this stage, a further catalytic depolymerization step can follow, leading to the production of bio-based small aromatic building blocks. In this review we describe the most important lignin extraction methods applied in industry (Kraft, Sulfite and Soda processes) as well as more recent processes based on so-called organosolv and ionosolv protocols. In addition, the most important approaches for the heterogeneous catalytic depolymerization of lignin are critically discussed.