Direct Production of Cyclohexanones from Lignin via Chlorine-Mediated Catalytic Hydroprocessing

Cyclohexanones, as critical biopolymer precursors, could be produced from lignin yet rarely reported due to the formidable challenge of simultaneously removing oxygen-containing functional groups (e.g., methoxy, hydroxyl) and achieving selective C═O bond retention during catalytic hydrogenation. Herein, we demonstrate a chlorine-modified ZrO₂ supported Pd catalyst (Pd-Cl/ZrO₂) efficiently converting lignin to cyclohexanones under optimized reaction conditions, based on poplar RCF (reductive catalytic fractionation) lignin oil. Notably, the addition of trace HCl enables the catalytic process to proceed under milder conditions (200 °C), achieving a cyclohexanones yield of 34.3 wt% (relative to lignin content in poplar biomass). The acidity (H⁺) from HCl promotes dehydroxylation of lignin oil. Moreover, studies on model compound guaiacol reveal that Cl species may partially block Pd to suppress aromatic ring hydrogenation and promote electron transfer from Pd to the ZrO₂ support, collectively promoting ketone yield. This study presents a novel catalytic approach for the efficient and selective conversion of lignin, advancing biorefineries toward the direct synthesis of ketone derivatives.