Fast and Selective Production of Aromatics via Efficient Lignin Depolymerization: Critical Factors and Mechanism Studies

Efficient conversion of renewable lignin into value-added chemicals and biofuel is of great importance for the sustainable development of biorefineries. However, lignin valorization is highly restricted by its structural variation and complexity. In this regard, we have produced a series of lignins extracted from grass, hardwood, and softwood and focused on scrutinizing their structural variability and key characteristics to provide structural insights into fast and selective production of aromatics. As an energy-efficient approach, microwave-assisted hydrothermal liquefaction without introducing an external oxygen was performed to facilitate the base-catalyzed depolymerization of lignin under moderate conditions. The general applicability of this approach was proved on five different lignins. High yields (58.5–78.6%) of low-molecular-weight bio-oil from all lignins were obtained only after 1 min at 220 °C. In particular, a high ratio of cleavable β–O–4 linkage of grass lignin enabled a fast monomer production, while its specific p-coumaric acid moieties allowed a high monomer selectivity to 4-vinylphenol of 66%. A difference in the product distribution was systematically investigated through a comprehensive analysis of lignin characteristics (e.g., interlinkage, S/G/H unit, and molecular weight) and stability studies of main monomeric products. These results revealed the fast degradation of reactive intermediates (aldehydes and S-type products) and how the monomer's stability/reactivity affects the selective production. Alongside reaction parameters, several critical factors governing the lignin conversion were further identified by calculating the Pearson correlation coefficient between lignin characteristics and the conversion efficiency. The lignin structure–property–degradability relationships and reaction mechanisms are expounded and updated to foster efficient lignin utilization.