Depolymerization of Methylene Linkage in Condensed Lignin with Commercial Zeolite in Water

Lignin, an aromatic polymer, is a crucial component of lignocellulosic biomass and is a significant source of renewable aromatics found in nature. Lignin constitutes up to 30 wt % of lignocellulosic biomass and is generated in excess of 100 million tons per year from global paper and bio-ethanol industries. Unfortunately, most technical lignin produced in this manner is burned as a heat source, without any value-added valorization. The conversion of technical lignin into valuable compounds can significantly increase the profitability of biorefineries. However, the productivity of this process is constrained by the severe lignin condensation that occurs during extraction procedures, resulting in the formation of robust C–C bonds, such as methylene linkages, which lead to monomer yields of only 8–15%. Herein, we present an approach to overcome this limitation by producing well-defined aromatic monomers from condensed lignin fractions. The approach employs commercial zeolite-catalyzed cleavage of methylene linkages with water as both the reacting medium and the reactant. Up to 7–10% of additional aromatic monomers can be obtained from various types of technical lignin, thus increasing the total monomer yields by 59–102%. The findings of this study present a promising avenue for the valorization of technical lignin and offer potential for the development of circular economy.