Highly efficient oxidative cleavage of lignin β-O-4 linkages via synergistic Co-CoO_x/N-doped carbon and recyclable hexaniobate catalysis

The oxidative depolymerization of β-O-4 linkages into value-added aromatic chemicals is of great importance for lignin valorization. However, achieving highly selective and rapid cleavage of β-O-4 linkages under mild conditions remains a challenge. Herein, cobalt-cobalt oxides (Co-CoO_x) supported on N-doped carbon (CoMA/C₉₀₀) combined with a green and recyclable base K₇HNb₆O₁₉ (KNb₆) have shown remarkable activity for the one-step oxidative cleavage of lignin β-O-4 linkages. Under relatively mild conditions (100 °C, 0.2 MPa O₂), a lignin β-O-4 alcohol model compound was almost completely converted within 3 h, affording a narrow product distribution of phenol (yield: 99%) and methyl benzoate (yield: 98%). Based on the control experiments, kinetic study, and spectroscopic analysis, a synergistic oxidative cleavage mechanism was proposed: CoMA/C₉₀₀ activates molecular oxygen to form a superoxide radical, while basic KNb₆ promotes deprotonation of secondary alcohol, and they synergistically catalyze the oxidation of β-O-4 alcohol and the rapid cleavage of C_β-O and C_α-C_β bonds. During the reaction, the oxidation of β-O-4 alcohol to β-O-4 ketone is the rate-determining step, while the cleavage of PP-one to phenol and methyl benzoate can be completed within 15 min. Moreover, the CoMA/C₉₀₀-KNb₆ catalyst is recyclable at least three times and highly active for the oxidative cleavages of other lignin models and organosolv birch lignin.