Lignin Valorization via Catalytic Modification: Ruthenium-catalyzed hydrogen transfer approach for modification of lignin model compounds and lignin

The valorization of lignin, the most abundant natural aromatic polymer, represents a critical step toward sustainable biomass utilization and the production of value-added chemicals, fuels, and functional materials. However, structural complexity and heterogeneity of lignin significantly complicates valorization process making essential the development of selective and robust methodologies for successful lignin valorization towards value-added products. Ruthenium-based catalysts, particularly the Shvo catalyst, have emerged as promising tools for promoting hydrogen transfer reactions due to their high activity, tunable selectivity, and tolerance to diverse functional groups. In this work, selective oxidation and degradation of lignin model compounds and lignin were explored using the Shvo catalyst hydrogen transfer approach. Benzylic hydroxyl groups in lignin model diols and extracted spruce milled wood lignin were selectively dehydrogenated under aerobic conditions. Detailed kinetic studies of the Shvo-catalyzed dehydrogenation of 1-phenyl-1,3propanediol revealed a half-order dependence on catalyst concentration, an apparent activation energy of 77 kJ/mol, and pseudo-first-order behavior under excess 2-butanone, enabling the development of a kinetic model that takes into account catalyst loading, temperature, and solvent effects. The applicability of Shvo-catalyzed hydrogen transfer approach was evaluated for bond cleavage of β-O-4 lignin model compounds, where Cβ-Cγ, Cγ-Oγ and CβOβ bond cleavage was observed. Major degradation products were isolated and characterized. The application of Shvo-catalyzed degradation methodology to extracted ethanosolve lignin resulted in thermal degradation and formation of new substructures that could not be characterized due to poor solubility, complexity and unknown reaction pathway. Together, these results highlight the potential of Shvo-catalyzed hydrogen transfer methodologies for selective dehydrogenation, kinetic modeling, and targeted bond cleavage in lignin chemistry, while also underscoring the challenges associated with extending these methods to native lignin.