Optimum phenolic monomer production by competing catalytic depolymerization and repolymerization of oak-extracted organosolv lignin

Abstract

Lignin, a byproduct of pulping and lignocellulosic biorefineries, holds promise as a feedstock for producing aromatic chemicals that can replace petroleum-derived counterparts. Reductive catalytic depolymerization of lignin has been proposed as a sustainable approach to generate phenolic monomers. However, achieving high yields of these monomers is challenging because of the complexity of the product mixture and process deactivation. Additionally, the interplay between lignin depolymerization and repolymerization remains poorly understood. In this study, organosolv lignin extracted from oak was depolymerized using a hydrogen-form zeolite β-supported ruthenium catalyst. By optimizing the catalyst-to-lignin ratio (0.25 w/w), a maximum phenolic monomer yield of 15.9 % (at 280 °C in 75 % (v/v) aqueous methanol) was achieved, independent of other reaction conditions. This finding highlights the catalyst-to-lignin ratio as a critical determinant of lignin conversion efficiency. Furthermore, the study emphasizes the need to optimize reaction conditions to mitigate repolymerization, which leads to the formation of non-degradable polymers and suppresses phenolic molecule production.