Unraveling the Mechanism of Dual Hydrogen Sources in Reductive Lignin Depolymerization over Ru/C

Lignin valorization is a critical pillar for the economic feasibility of modern biorefineries and sustainable renewable energy production. However, lignin’s inherent chemical recalcitrance poses a fundamental barrier to its efficient conversion. While catalytic transfer hydrogenation (CTH) is highly effective for reductive depolymerization, the mechanistic role and necessity of co-feeding external molecular H2 remain ambiguous. Herein, we use milled wood lignin (MWL) to elucidate the mechanism of Ru/C-catalyzed depolymerization involving dual hydrogen sources. Comparative evaluations reveal that secondary alcohols serve as superior hydrogen donors compared to primary alcohols. Introducing pressurized H2 successfully suppressed solid char formation to ∼1% but provided only a marginal enhancement in phenolic monomer yields. Monitoring 2-propanol conversion revealed a lack of synergy between the hydrogen pools. The presence of external H2 decreased in situ alcohol dehydrogenation from 26.6 to 20.2%. Density functional theory (DFT) computations validate these experimental observations, indicating a competitive adsorption regime wherein molecular H2 outcompetes the bulkier donor solvent for active metallic Ru sites, hindering the primary CTH cycle. Ultimately, these insights reveal that the hydrogen donor solvent provides an entirely adequate hydrogen pool to effectively drive depolymerization, negating the need for additional pressurized H2 and guiding highly efficient, H2-free biorefinery designs.